entity: refine requires on entt_traits for enum

I made it btw, love this project

.bazelrc

@@ -5,8 +5,8 @@ build --enable_runfiles
 build --incompatible_strict_action_env
 
 # required for googletest
-build:linux --cxxopt=-std=c++17
-build:macos --cxxopt=-std=c++17
+build:linux --cxxopt=-std=c++20
+build:macos --cxxopt=-std=c++20
 
 common:ci --announce_rc
 common:ci --verbose_failures

.clang-format

@@ -3,6 +3,7 @@ BasedOnStyle: llvm
 AccessModifierOffset: -4
 AlignEscapedNewlines: DontAlign
 AllowShortBlocksOnASingleLine: Always
+AllowShortCompoundRequirementOnASingleLine: true
 AllowShortEnumsOnASingleLine: true
 AllowShortFunctionsOnASingleLine: Empty
 AllowShortIfStatementsOnASingleLine: WithoutElse
@@ -10,6 +11,8 @@ AllowShortLambdasOnASingleLine: All
 AllowShortLoopsOnASingleLine: true
 AlwaysBreakTemplateDeclarations: Yes
 BreakBeforeBinaryOperators: NonAssignment
+BreakBeforeBraces: Attach
+BreakBeforeConceptDeclarations: Always
 BreakBeforeTernaryOperators: true
 ColumnLimit: 0
 DerivePointerAlignment: false
@@ -26,17 +29,33 @@ IncludeCategories:
     Priority: 5
 IncludeIsMainRegex: "^$"
 IndentPPDirectives: AfterHash
+IndentRequiresClause: false
 IndentWidth: 4
+InsertBraces: true
+InsertNewlineAtEOF: true
 KeepEmptyLinesAtTheStartOfBlocks: false
 Language: Cpp
 PointerAlignment: Right
+RequiresClausePosition: OwnLineWithBrace
+RequiresExpressionIndentation: OuterScope
 SpaceAfterCStyleCast: false
 SpaceAfterTemplateKeyword: false
 SpaceAroundPointerQualifiers: After
 SpaceBeforeCaseColon: false
 SpaceBeforeCtorInitializerColon: false
 SpaceBeforeInheritanceColon: false
-SpaceBeforeParens: Never
+SpaceBeforeParens: Custom
+SpaceBeforeParensOptions:
+  AfterControlStatements: false
+  AfterForeachMacros: false
+  AfterFunctionDeclarationName: false
+  AfterFunctionDefinitionName: false
+  AfterIfMacros: false
+  AfterOverloadedOperator: false
+  AfterPlacementOperator: false
+  AfterRequiresInClause: true
+  AfterRequiresInExpression: false
+  BeforeNonEmptyParentheses: false
 SpaceBeforeRangeBasedForLoopColon: false
 Standard: Latest
 TabWidth: 4

.clang-tidy

@@ -1,5 +1,7 @@
 Checks: >
     bugprone-*,
+    clang-analyzer-*,
+    -clang-analyzer-optin.core.EnumCastOutOfRange,
     concurrency-*,
     cppcoreguidelines-*,
     -cppcoreguidelines-owning-memory,
@@ -16,14 +18,20 @@ Checks: >
     performance-*,
     portability-*,
     readability-*,
+    -readability-else-after-return,
     -readability-function-cognitive-complexity,
     -readability-named-parameter,
+    -readability-redundant-member-init,
     -readability-uppercase-literal-suffix,
 CheckOptions:
     - key: cppcoreguidelines-avoid-magic-numbers.IgnoreAllFloatingPointValues
       value: true
     - key: cppcoreguidelines-avoid-magic-numbers.IgnorePowersOf2IntegerValues
       value: true
+    - key: cppcoreguidelines-rvalue-reference-param-not-moved.AllowPartialMove
+      value: true
+    - key: cppcoreguidelines-rvalue-reference-param-not-moved.IgnoreUnnamedParams
+      value: true
     - key: cppcoreguidelines-special-member-functions.AllowMissingMoveFunctions
       value: true
     - key: cppcoreguidelines-special-member-functions.AllowMissingMoveFunctionsWhenCopyIsDeleted
@@ -36,6 +44,8 @@ CheckOptions:
       value: true
     - key: modernize-avoid-c-arrays.AllowStringArrays
       value: true
+    - key: performance-enum-size.EnumIgnoreList
+      value: meta_traits
     - key: readability-function-cognitive-complexity.IgnoreMacros
       value: true
     - key: readability-identifier-length.MinimumParameterNameLength

.github/workflows/analyzer.yml

@@ -1,9 +1,9 @@
-name: tools
+name: analyzer
 
 on:
   push:
     branches:
-      - tools
+      - analyzer
 
 jobs:
 
@@ -11,8 +11,8 @@ jobs:
     timeout-minutes: 60
 
     env:
-      IWYU: "0.22"
-      LLVM: "18"
+      IWYU: "0.24"
+      LLVM: "20"
 
     runs-on: ubuntu-latest
     continue-on-error: true
@@ -23,10 +23,10 @@ jobs:
       # see: https://apt.llvm.org/
       run: |
         wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
-        sudo add-apt-repository "deb http://apt.llvm.org/focal/ llvm-toolchain-focal-$LLVM main"
+        sudo add-apt-repository "deb http://apt.llvm.org/noble/ llvm-toolchain-noble-$LLVM main"
         sudo apt update
         sudo apt remove -y "llvm*"
-        sudo apt remove -y "libclang-dev*"
+        sudo apt remove -y "libclang*"
         sudo apt remove -y "clang*"
         sudo apt install -y llvm-$LLVM-dev
         sudo apt install -y libclang-$LLVM-dev
@@ -55,6 +55,7 @@ jobs:
               -DENTT_BUILD_EXAMPLE=ON \
               -DENTT_BUILD_LIB=ON \
               -DENTT_BUILD_SNAPSHOT=ON \
+              -DENTT_BUILD_TESTBED=ON \
               -DCMAKE_CXX_INCLUDE_WHAT_YOU_USE="include-what-you-use;-Xiwyu;--mapping_file=${GITHUB_WORKSPACE}/entt.imp;-Xiwyu;--no_fwd_decls;-Xiwyu;--verbose=1" \
               ..
         make -j4
@@ -72,10 +73,12 @@ jobs:
       env:
         CXX: clang++
       run: |
-        cmake -DENTT_BUILD_TESTING=ON -DENTT_BUILD_LIB=ON -DENTT_BUILD_EXAMPLE=ON -DENTT_USE_CLANG_TIDY=ON ..
+        cmake -DENTT_BUILD_TESTING=ON \
+              -DENTT_BUILD_BENCHMARK=ON \
+              -DENTT_BUILD_EXAMPLE=ON \
+              -DENTT_BUILD_LIB=ON \
+              -DENTT_BUILD_SNAPSHOT=ON \
+              -DENTT_BUILD_TESTBED=ON \
+              -DENTT_USE_CLANG_TIDY=ON \
+              ..
         make -j4
-    - name: Run tests
-      working-directory: build
-      env:
-        CTEST_OUTPUT_ON_FAILURE: 1
-      run: ctest -C Debug -j4

.github/workflows/build.yml

@@ -5,51 +5,18 @@ on: [push, pull_request]
 jobs:
 
   linux:
-    timeout-minutes: 15
-
     strategy:
       matrix:
-        os: [ubuntu-latest, ubuntu-24.04]
         compiler:
-          - { pkg: g++, exe: 'g++', version: 9 }
-          - { pkg: g++, exe: 'g++', version: 10 }
-          - { pkg: g++, exe: 'g++', version: 11 }
           - { pkg: g++, exe: 'g++', version: 12 }
           - { pkg: g++, exe: 'g++', version: 13 }
           - { pkg: g++, exe: 'g++', version: 14 }
-          - { pkg: clang, exe: 'clang++', version: 13 }
-          - { pkg: clang, exe: 'clang++', version: 14 }
-          - { pkg: clang, exe: 'clang++', version: 15 }
           - { pkg: clang, exe: 'clang++', version: 16 }
           - { pkg: clang, exe: 'clang++', version: 17 }
           - { pkg: clang, exe: 'clang++', version: 18 }
-        exclude:
-          - os: ubuntu-latest
-            compiler: { pkg: g++, exe: 'g++', version: 12 }
-          - os: ubuntu-latest
-            compiler: { pkg: g++, exe: 'g++', version: 13 }
-          - os: ubuntu-latest
-            compiler: { pkg: g++, exe: 'g++', version: 14 }
-          - os: ubuntu-latest
-            compiler: { pkg: clang, exe: 'clang++', version: 16 }
-          - os: ubuntu-latest
-            compiler: { pkg: clang, exe: 'clang++', version: 17 }
-          - os: ubuntu-latest
-            compiler: { pkg: clang, exe: 'clang++', version: 18 }
-          - os: ubuntu-24.04
-            compiler: { pkg: g++, exe: 'g++', version: 9 }
-          - os: ubuntu-24.04
-            compiler: { pkg: g++, exe: 'g++', version: 10 }
-          - os: ubuntu-24.04
-            compiler: { pkg: g++, exe: 'g++', version: 11 }
-          - os: ubuntu-24.04
-            compiler: { pkg: clang, exe: 'clang++', version: 13 }
-          - os: ubuntu-24.04
-            compiler: { pkg: clang, exe: 'clang++', version: 14 }
-          - os: ubuntu-24.04
-            compiler: { pkg: clang, exe: 'clang++', version: 15 }
 
-    runs-on: ${{ matrix.os }}
+    timeout-minutes: 15
+    runs-on: ubuntu-latest
 
     steps:
     - uses: actions/checkout@v4
@@ -71,17 +38,16 @@ jobs:
       run: ctest -C Debug -j4
 
   windows:
-    timeout-minutes: 15
-
     strategy:
       matrix:
-        toolset: [default, v142, clang-cl]
+        toolset: [default, v143, clang-cl]
         include:
-          - toolset: v142
-            toolset_option: -T"v142"
+          - toolset: v143
+            toolset_option: -T"v143"
           - toolset: clang-cl
             toolset_option: -T"ClangCl"
 
+    timeout-minutes: 15
     runs-on: windows-latest
 
     steps:
@@ -115,14 +81,13 @@ jobs:
       run: ctest -C Debug -j4
 
   extra:
-    timeout-minutes: 15
-
     strategy:
       matrix:
         os: [windows-latest, macOS-latest, ubuntu-latest]
         id_type: ["std::uint32_t", "std::uint64_t"]
-        cxx_std: [cxx_std_17, cxx_std_20]
+        cxx_std: [cxx_std_20, cxx_std_23]
 
+    timeout-minutes: 15
     runs-on: ${{ matrix.os }}
 
     steps:

.github/workflows/coverage.yml

@@ -27,7 +27,7 @@ jobs:
       working-directory: build
       run: |
         sudo apt install lcov
-        lcov -c -d . -o coverage.info
+        lcov -c -d . -o coverage.info --ignore-errors gcov,gcov,mismatch,mismatch
         lcov -l coverage.info
     - name: Upload coverage to Codecov
       uses: codecov/codecov-action@v3

.github/workflows/deploy.yml

@@ -2,7 +2,7 @@ name: deploy
 
 on:
   release:
-    types: published
+    types: [published]
 
 jobs:
 

.github/workflows/sanitizer.yml

@@ -11,7 +11,7 @@ jobs:
       matrix:
         compiler: [clang++]
         id_type: ["std::uint32_t", "std::uint64_t"]
-        cxx_std: [cxx_std_17, cxx_std_20]
+        cxx_std: [cxx_std_20, cxx_std_23]
 
     runs-on: ubuntu-latest
 

.github/workflows/testbed.yml

@@ -0,0 +1,79 @@
+name: testbed
+
+on: [push]
+
+jobs:
+
+  linux:
+    timeout-minutes: 15
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v4
+    - name: Install required packages
+      run: |
+        sudo apt update
+        sudo apt install -y \
+            build-essential \
+            git \
+            make \
+            pkg-config \
+            cmake \
+            ninja-build \
+            gnome-desktop-testing \
+            libasound2-dev \
+            libpulse-dev \
+            libaudio-dev \
+            libjack-dev \
+            libsndio-dev \
+            libx11-dev \
+            libxext-dev \
+            libxrandr-dev \
+            libxcursor-dev \
+            libxfixes-dev \
+            libxi-dev \
+            libxss-dev \
+            libxtst-dev \
+            libxkbcommon-dev \
+            libdrm-dev \
+            libgbm-dev \
+            libgl1-mesa-dev \
+            libgles2-mesa-dev \
+            libegl1-mesa-dev \
+            libdbus-1-dev \
+            libibus-1.0-dev \
+            libudev-dev \
+            libpipewire-0.3-dev \
+            libwayland-dev \
+            libdecor-0-dev \
+            liburing-dev
+    - name: Compile testbed
+      working-directory: build
+      run: |
+        cmake -DENTT_BUILD_TESTBED=ON ..
+        make -j4
+
+  windows:
+    timeout-minutes: 15
+    runs-on: windows-latest
+
+    steps:
+    - uses: actions/checkout@v4
+    - uses: seanmiddleditch/gha-setup-ninja@master
+    - name: Compile testbed
+      working-directory: build
+      run: |
+        cmake -DENTT_BUILD_TESTBED=ON .. -G Ninja
+        cmake --build . -j 4
+
+  macos:
+    timeout-minutes: 15
+    runs-on: macOS-latest
+
+    steps:
+    - uses: actions/checkout@v4
+    - name: Compile testbed
+      working-directory: build
+      run: |
+        cmake -DENTT_BUILD_TESTBED=ON ..
+        make -j4

CMakeLists.txt

@@ -1,6 +1,6 @@
 # EnTT
 
-cmake_minimum_required(VERSION 3.15.7)
+cmake_minimum_required(VERSION 3.28)
 
 # Read project version
 
@@ -22,10 +22,10 @@ project(
 if(NOT CMAKE_BUILD_TYPE)
     set(CMAKE_BUILD_TYPE Debug)
 endif()
-
+    
 message(VERBOSE "*")
 message(VERBOSE "* ${PROJECT_NAME} v${PROJECT_VERSION} (${CMAKE_BUILD_TYPE})")
-message(VERBOSE "* Copyright (c) 2017-2024 Michele Caini <michele.caini@gmail.com>")
+message(VERBOSE "* Copyright (c) 2017-2026 Michele Caini <michele.caini@gmail.com>")
 message(VERBOSE "*")
 
 # CMake stuff
@@ -81,20 +81,6 @@ endif()
 
 # Add EnTT target
 
-option(ENTT_INCLUDE_HEADERS "Add all EnTT headers to the EnTT target." OFF)
-option(ENTT_INCLUDE_NATVIS "Add EnTT natvis files to the EnTT target." OFF)
-
-if(ENTT_INCLUDE_NATVIS)
-    if(MSVC)
-        set(ENTT_HAS_NATVIS TRUE CACHE BOOL "" FORCE)
-        mark_as_advanced(ENTT_HAS_NATVIS)
-    endif()
-
-    if(NOT ENTT_HAS_NATVIS)
-        message(VERBOSE "The option ENTT_INCLUDE_NATVIS is set but natvis files are not supported. They will not be added to the target.")
-    endif()
-endif()
-
 include(GNUInstallDirs)
 
 add_library(EnTT INTERFACE)
@@ -107,107 +93,10 @@ target_include_directories(
         $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
 )
 
-target_compile_features(EnTT INTERFACE cxx_std_17)
+target_compile_features(EnTT INTERFACE cxx_std_20)
 
-if(ENTT_INCLUDE_HEADERS)
-    target_sources(
-        EnTT
-        INTERFACE
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/config/config.h>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/config/macro.h>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/config/version.h>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/container/dense_map.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/container/dense_set.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/container/table.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/container/fwd.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/algorithm.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/any.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/attribute.h>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/bit.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/compressed_pair.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/enum.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/family.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/fwd.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/hashed_string.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/ident.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/iterator.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/memory.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/monostate.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/ranges.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/tuple.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/type_info.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/type_traits.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/utility.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/component.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/entity.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/fwd.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/group.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/handle.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/mixin.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/helper.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/observer.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/organizer.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/ranges.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/registry.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/runtime_view.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/snapshot.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/sparse_set.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/storage.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/view.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/graph/adjacency_matrix.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/graph/dot.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/graph/flow.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/graph/fwd.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/locator/locator.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/adl_pointer.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/container.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/context.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/factory.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/fwd.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/meta.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/node.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/pointer.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/policy.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/range.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/resolve.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/template.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/type_traits.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/utility.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/poly/fwd.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/poly/poly.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/process/fwd.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/process/process.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/process/scheduler.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/resource/cache.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/resource/fwd.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/resource/loader.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/resource/resource.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/signal/delegate.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/signal/dispatcher.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/signal/emitter.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/signal/fwd.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/signal/sigh.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entt.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/fwd.hpp>
-    )
-endif()
-
-if(ENTT_HAS_NATVIS)
-    target_sources(
-        EnTT
-        INTERFACE
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/config.natvis>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/container.natvis>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/core.natvis>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/entity.natvis>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/graph.natvis>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/locator.natvis>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/meta.natvis>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/poly.natvis>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/process.natvis>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/resource.natvis>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/signal.natvis>
-    )
+if(ENTT_HAS_LIBCPP)
+    target_compile_options(EnTT BEFORE INTERFACE -stdlib=libc++)
 endif()
 
 if(ENTT_HAS_SANITIZER)
@@ -216,109 +105,252 @@ if(ENTT_HAS_SANITIZER)
 endif()
 
 if(ENTT_CLANG_TIDY_EXECUTABLE)
-    set(CMAKE_CXX_CLANG_TIDY "${ENTT_CLANG_TIDY_EXECUTABLE};--config-file=${EnTT_SOURCE_DIR}/.clang-tidy;--header-filter=${EnTT_SOURCE_DIR}/src/entt/.*")
+    set(ENTT_CLANG_TIDY_OPTIONS ";--config-file=${EnTT_SOURCE_DIR}/.clang-tidy;--header-filter=${EnTT_SOURCE_DIR}/src/entt/.*")
+
+    if(MSVC AND NOT (${CMAKE_CXX_COMPILER_ID} STREQUAL "Clang"))
+        set(ENTT_CLANG_TIDY_OPTIONS "${ENTT_CLANG_TIDY_OPTIONS};--extra-arg=/EHsc;--extra-arg=/wd4996")
+    endif()
+
+    set(CMAKE_CXX_CLANG_TIDY "${ENTT_CLANG_TIDY_EXECUTABLE}${ENTT_CLANG_TIDY_OPTIONS}")
+ endif()
+
+# Add EnTT goodies
+
+option(ENTT_INCLUDE_HEADERS "Add all EnTT headers to the EnTT target." OFF)
+option(ENTT_INCLUDE_NATVIS "Add EnTT natvis files to the EnTT target." OFF)
+
+if(ENTT_INCLUDE_HEADERS)
+    set(
+        HEADERS_FILES
+        config/config.h
+        config/macro.h
+        config/version.h
+        container/dense_map.hpp
+        container/dense_set.hpp
+        container/table.hpp
+        container/fwd.hpp
+        core/algorithm.hpp
+        core/any.hpp
+        core/bit.hpp
+        core/compressed_pair.hpp
+        core/concepts.hpp
+        core/enum.hpp
+        core/family.hpp
+        core/fwd.hpp
+        core/hashed_string.hpp
+        core/ident.hpp
+        core/iterator.hpp
+        core/memory.hpp
+        core/monostate.hpp
+        core/ranges.hpp
+        core/tuple.hpp
+        core/type_info.hpp
+        core/type_traits.hpp
+        core/utility.hpp
+        entity/component.hpp
+        entity/entity.hpp
+        entity/fwd.hpp
+        entity/group.hpp
+        entity/handle.hpp
+        entity/mixin.hpp
+        entity/helper.hpp
+        entity/organizer.hpp
+        entity/ranges.hpp
+        entity/registry.hpp
+        entity/runtime_view.hpp
+        entity/snapshot.hpp
+        entity/sparse_set.hpp
+        entity/storage.hpp
+        entity/view.hpp
+        graph/adjacency_matrix.hpp
+        graph/dot.hpp
+        graph/flow.hpp
+        graph/fwd.hpp
+        locator/locator.hpp
+        meta/adl_pointer.hpp
+        meta/container.hpp
+        meta/context.hpp
+        meta/factory.hpp
+        meta/fwd.hpp
+        meta/meta.hpp
+        meta/node.hpp
+        meta/pointer.hpp
+        meta/policy.hpp
+        meta/range.hpp
+        meta/resolve.hpp
+        meta/template.hpp
+        meta/type_traits.hpp
+        meta/utility.hpp
+        poly/fwd.hpp
+        poly/poly.hpp
+        process/fwd.hpp
+        process/process.hpp
+        process/scheduler.hpp
+        resource/cache.hpp
+        resource/fwd.hpp
+        resource/loader.hpp
+        resource/resource.hpp
+        signal/delegate.hpp
+        signal/dispatcher.hpp
+        signal/emitter.hpp
+        signal/fwd.hpp
+        signal/sigh.hpp
+        stl/functional.hpp
+        stl/iterator.hpp
+        stl/memory.hpp
+        tools/davey.hpp
+        entt.hpp
+        fwd.hpp
+        tools.hpp
+    )
+
+    list(TRANSFORM HEADERS_FILES APPEND ">" OUTPUT_VARIABLE HEADERS_BUILD_INTERFACE)
+    list(TRANSFORM HEADERS_BUILD_INTERFACE PREPEND "$<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/")
+
+    list(TRANSFORM HEADERS_FILES APPEND ">" OUTPUT_VARIABLE HEADERS_INSTALL_INTERFACE)
+    list(TRANSFORM HEADERS_INSTALL_INTERFACE PREPEND "$<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}/entt/")
+
+    target_sources(EnTT INTERFACE ${HEADERS_BUILD_INTERFACE} ${HEADERS_INSTALL_INTERFACE})
 endif()
 
-if(ENTT_HAS_LIBCPP)
-    target_compile_options(EnTT BEFORE INTERFACE -stdlib=libc++)
+if(ENTT_INCLUDE_NATVIS)
+    if(MSVC)
+        set(ENTT_HAS_NATVIS TRUE CACHE BOOL "" FORCE)
+        mark_as_advanced(ENTT_HAS_NATVIS)
+    endif()
+
+    if(NOT ENTT_HAS_NATVIS)
+        message(VERBOSE "The option ENTT_INCLUDE_NATVIS is set but natvis files are not supported. They will not be added to the target.")
+    endif()
 endif()
 
-# Install pkg-config file
+if(ENTT_HAS_NATVIS)
+    set(
+        NATVIS_FILES
+        config.natvis
+        container.natvis
+        core.natvis
+        entity.natvis
+        graph.natvis
+        locator.natvis
+        meta.natvis
+        poly.natvis
+        process.natvis
+        resource.natvis
+        signal.natvis
+    )
 
-include(JoinPaths)
+    list(TRANSFORM NATVIS_FILES APPEND ">" OUTPUT_VARIABLE NATVIS_BUILD_INTERFACE)
+    list(TRANSFORM NATVIS_BUILD_INTERFACE PREPEND "$<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/natvis/")
 
-set(EnTT_PKGCONFIG ${CMAKE_CURRENT_BINARY_DIR}/entt.pc)
+    list(TRANSFORM NATVIS_FILES APPEND ">" OUTPUT_VARIABLE NATVIS_INSTALL_INTERFACE)
+    list(TRANSFORM NATVIS_INSTALL_INTERFACE PREPEND "$<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}/entt/natvis/")
 
-join_paths(EnTT_PKGCONFIG_INCLUDEDIR "\${prefix}" "${CMAKE_INSTALL_INCLUDEDIR}")
+    target_sources(EnTT INTERFACE ${NATVIS_BUILD_INTERFACE} ${NATVIS_INSTALL_INTERFACE})
+endif()
 
-configure_file(
-    ${EnTT_SOURCE_DIR}/cmake/in/entt.pc.in
-    ${EnTT_PKGCONFIG}
-    @ONLY
-)
+# Install EnTT and all related files
 
-install(
-    FILES ${EnTT_PKGCONFIG}
-    DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig
-)
+option(ENTT_INSTALL "Install EnTT and all related files." OFF)
 
-# Install EnTT
+if(ENTT_INSTALL)
+    # Install pkg-config file
 
-include(CMakePackageConfigHelpers)
+    include(JoinPaths)
 
-install(
-    TARGETS EnTT
-    EXPORT EnTTTargets
-    ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
-)
+    set(EnTT_PKGCONFIG ${CMAKE_CURRENT_BINARY_DIR}/entt.pc)
 
-write_basic_package_version_file(
-    EnTTConfigVersion.cmake
-    VERSION ${PROJECT_VERSION}
-    COMPATIBILITY AnyNewerVersion
-)
+    join_paths(EnTT_PKGCONFIG_INCLUDEDIR "\${prefix}" "${CMAKE_INSTALL_INCLUDEDIR}")
 
-configure_package_config_file(
-    ${EnTT_SOURCE_DIR}/cmake/in/EnTTConfig.cmake.in
-    EnTTConfig.cmake
-    INSTALL_DESTINATION ${CMAKE_INSTALL_LIBDIR}/EnTT/cmake
-)
+    configure_file(
+        ${EnTT_SOURCE_DIR}/cmake/in/entt.pc.in
+        ${EnTT_PKGCONFIG}
+        @ONLY
+    )
 
-export(
-    EXPORT EnTTTargets
-    FILE ${CMAKE_CURRENT_BINARY_DIR}/EnTTTargets.cmake
-    NAMESPACE EnTT::
-)
+    install(
+        FILES ${EnTT_PKGCONFIG}
+        DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig
+    )
 
-install(
-    EXPORT EnTTTargets
-    FILE EnTTTargets.cmake
-    DESTINATION ${CMAKE_INSTALL_LIBDIR}/EnTT/cmake
-    NAMESPACE EnTT::
-)
+    # Install EnTT
 
-install(
-    FILES
-        ${PROJECT_BINARY_DIR}/EnTTConfig.cmake
-        ${PROJECT_BINARY_DIR}/EnTTConfigVersion.cmake
-    DESTINATION ${CMAKE_INSTALL_LIBDIR}/EnTT/cmake
-)
+    include(CMakePackageConfigHelpers)
 
-install(
-    DIRECTORY src/
-    DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
-    FILES_MATCHING
-    PATTERN "*.h"
-    PATTERN "*.hpp"
-)
+    install(
+        TARGETS EnTT
+        EXPORT EnTTTargets
+        ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
+    )
 
-export(PACKAGE EnTT)
+    write_basic_package_version_file(
+        EnTTConfigVersion.cmake
+        VERSION ${PROJECT_VERSION}
+        COMPATIBILITY AnyNewerVersion
+    )
 
-# Tests
+    configure_package_config_file(
+        ${EnTT_SOURCE_DIR}/cmake/in/EnTTConfig.cmake.in
+        EnTTConfig.cmake
+        INSTALL_DESTINATION ${CMAKE_INSTALL_LIBDIR}/EnTT/cmake
+    )
+
+    export(
+        EXPORT EnTTTargets
+        FILE ${CMAKE_CURRENT_BINARY_DIR}/EnTTTargets.cmake
+        NAMESPACE EnTT::
+    )
+
+    install(
+        EXPORT EnTTTargets
+        FILE EnTTTargets.cmake
+        DESTINATION ${CMAKE_INSTALL_LIBDIR}/EnTT/cmake
+        NAMESPACE EnTT::
+    )
+
+    install(
+        FILES
+            ${PROJECT_BINARY_DIR}/EnTTConfig.cmake
+            ${PROJECT_BINARY_DIR}/EnTTConfigVersion.cmake
+        DESTINATION ${CMAKE_INSTALL_LIBDIR}/EnTT/cmake
+    )
+
+    install(
+        DIRECTORY src/
+        DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
+        FILES_MATCHING
+        PATTERN "*.h"
+        PATTERN "*.hpp"
+        PATTERN "*.natvis"
+    )
+
+    export(PACKAGE EnTT)
+endif()
+
+# Tests and testbed
 
 option(ENTT_BUILD_TESTING "Enable building tests." OFF)
+option(ENTT_BUILD_TESTBED "Enable building testbed." OFF)
 
-if(ENTT_BUILD_TESTING)
-    option(ENTT_FIND_GTEST_PACKAGE "Enable finding gtest package." OFF)
-    option(ENTT_BUILD_BENCHMARK "Build benchmark." OFF)
-    option(ENTT_BUILD_EXAMPLE "Build examples." OFF)
-    option(ENTT_BUILD_LIB "Build lib tests." OFF)
-    option(ENTT_BUILD_SNAPSHOT "Build snapshot test with Cereal." OFF)
+if(ENTT_BUILD_TESTING OR ENTT_BUILD_TESTBED)
+    set(ENTT_ID_TYPE std::uint32_t CACHE STRING "Type of identifiers to use for tests and testbed")
+    set(ENTT_CXX_STD cxx_std_20 CACHE STRING "C++ standard revision to use for tests and testbed")
 
-    set(ENTT_ID_TYPE std::uint32_t CACHE STRING "Type of identifiers to use for the tests")
-    set(ENTT_CXX_STD cxx_std_17 CACHE STRING "C++ standard revision to use for the tests")
-
-    include(CTest)
-    enable_testing()
-    add_subdirectory(test)
-endif()
-
-# Tools
-
-option(ENTT_BUILD_TOOLS "Enable building tools." OFF)
-
-if(ENTT_BUILD_TOOLS)
-    add_subdirectory(tools)
+    # Tests and tesetbed do not work together because SDL gets confused with EnTT tests
+    if(ENTT_BUILD_TESTING)
+        option(ENTT_FIND_GTEST_PACKAGE "Enable finding gtest package." OFF)
+    
+        option(ENTT_BUILD_BENCHMARK "Build benchmark." OFF)
+        option(ENTT_BUILD_EXAMPLE "Build examples." OFF)
+        option(ENTT_BUILD_LIB "Build lib tests." OFF)
+        option(ENTT_BUILD_SNAPSHOT "Build snapshot test with Cereal." OFF)
+    
+        include(CTest)
+        enable_testing()
+        add_subdirectory(test)
+    elseif(ENTT_BUILD_TESTBED)
+        add_subdirectory(testbed)
+    endif()
 endif()
 
 # Documentation
@@ -326,9 +358,5 @@ endif()
 option(ENTT_BUILD_DOCS "Enable building with documentation." OFF)
 
 if(ENTT_BUILD_DOCS)
-    find_package(Doxygen 1.10)
-
-    if(DOXYGEN_FOUND)
-        add_subdirectory(docs)
-    endif()
+    add_subdirectory(docs)
 endif()

LICENSE

@@ -1,6 +1,6 @@
 The MIT License (MIT)
 
-Copyright (c) 2017-2024 Michele Caini, author of EnTT
+Copyright (c) 2017-2026 Michele Caini, author of EnTT
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal

README.md

@@ -217,12 +217,12 @@ the include paths.
 ## Requirements
 
 To be able to use `EnTT`, users must provide a full-featured compiler that
-supports at least C++17.<br/>
+supports at least C++20.<br/>
 The requirements below are mandatory to compile the tests and to extract the
 documentation:
 
-* `CMake` version 3.7 or later.
-* `Doxygen` version 1.8 or later.
+* `CMake` version 3.28 or later.
+* `Doxygen` version 1.14 or later.
 
 Alternatively, [Bazel](https://bazel.build) is also supported as a build system
 (credits to [zaucy](https://github.com/zaucy) who offered to maintain it).<br/>
@@ -232,7 +232,8 @@ build system of the library.
 ## CMake
 
 To use `EnTT` from a `CMake` project, just link an existing target to the
-`EnTT::EnTT` alias.<br/>
+`EnTT::EnTT` alias.
+
 The library offers everything you need for locating (as in `find_package`),
 embedding (as in `add_subdirectory`), fetching (as in `FetchContent`) or using
 it in many of the ways that you can think of and that involve `CMake`.<br/>
@@ -240,12 +241,17 @@ Covering all possible cases would require a treatise and not a simple README
 file, but I'm confident that anyone reading this section also knows what it's
 about and can use `EnTT` from a `CMake` project without problems.
 
+Note that all `install` calls are guarded by the `ENTT_INSTALL` option to allow
+using `EnTT` as a submodule without conflicting with user logic.<br/>
+It is therefore necessary to set the option to true to take advantage of the
+installation logic provided by this library.
+
 ## Natvis support
 
 When using `CMake`, just enable the option `ENTT_INCLUDE_NATVIS` and enjoy
 it.<br/>
 Otherwise, most of the tools are covered via Natvis and all files can be found
-in the `natvis` directory, divided by module.<br/>
+in the `natvis` subdirectory, divided by module.<br/>
 If you spot errors or have suggestions, any contribution is welcome!
 
 ## Packaging Tools
@@ -319,7 +325,7 @@ If you spot errors or have suggestions, any contribution is welcome!
   `bazel` project, add the following to your `MODULE.bazel` file:
 
   ```starlark
-  bazel_dep(name = "entt", version = "3.12.2")
+  bazel_dep(name = "entt", version = "3.16.0")
   ```
 
   EnTT will now be available as `@entt` (short for `@entt//:entt`) to be used
@@ -403,7 +409,7 @@ know who has participated so far.
 
 # License
 
-Code and documentation Copyright (c) 2017-2024 Michele Caini.<br/>
+Code and documentation Copyright (c) 2017-2026 Michele Caini.<br/>
 Colorful logo Copyright (c) 2018-2021 Richard Caseres.
 
 Code released under

TODO

@@ -10,35 +10,29 @@ DOC:
 * bump entities, reserved bits on identifiers
 
 TODO:
-* deprecate non-owning groups in favor of owning views and view packs, introduce lazy owning views
+* review all NOLINT
 * bring nested groups back in place (see bd34e7f)
 * work stealing job system (see #100) + mt scheduler based on const awareness for types
-* view: reduce inst due to/improve perf with index-based approach in dispatch_get/pick_and_each/each (single type too, define storage ::at and ::at_as_tuple)
-* view: update natvis as needed after the last rework, merge pools/filter in the same array, drop check (?) and turn view into a position
-* view: type-only view_iterator (dyn get/excl sizes), type-only basic_common_view (dyn get/excl sizes with pointer to array from derived)
 * combine version-mask-vs-version-bits tricks with reserved bits to allow things like enabling/disabling
 * self contained entity traits to avoid explicit specializations (ie enum constants)
 * auto type info data from types if present
-* test: push sharing types further
-* after non-continuous generation for entity storage:
-  - get/reset placeholder to position after saving/loading (avoid long lookup)
-  - allow skipping/reserving entity identifiers
-  - documentation for reserved entities
-* storage entity: no emplace/insert, rename and add a fast range-push from above
-* view: propagate tombstone check request to iterator
+* storage entity: fast range-push from above
 * table: pop back to support swap and pop, single column access, empty type optimization
-* checkout tools workflow
-* improve front (no multiple checks) and back (ie no contains) for multi-type view
-* cleanup common view from tricks to handle single swap-only and in-place, if constexpr branches
-* entity based component_traits
-* review cmake warning about FetchContent_Populate (need .28 and EXCLUDE_FROM_ALL for FetchContent)
-* after removing meta prop vectors, copy meta objects in their handles directly
 * suppress -Wself-move on CI with g++13
-* view and view iterator specializations for multi, single and filtered elements
-* organizer support to groups
-* meta range: move id to meta objects and return plain types (?), then remove id from meta base and meta ctor too
-* refine the storage fallback mechanism for views (ie alloc?)
-* sigh_mixin: automatic signal registration
-* sigh_mixin: change cb signature from reg/entt to storage/entt (breaking for the good)
-* don't pass reactive storage by default to callback
 * runtime types support for meta for types that aren't backed by C++ types
+* built-in no-pagination storage - no_pagination page size as limits::max
+* any cdynamic to support const ownership construction
+* allow passing arguments to meta setter/getter (we can fallback on meta invoke probably)
+* meta non-const allow_cast overloads: (const int &) to (int &) is not allowed, but (const int &) to (double &) is allowed (support only for convertibles)
+* review build process for testbed (i.e. tests first due to SDL)
+* use unique_ptr or any for meta_custom_node
+* paged vector as a standalone class
+* resource: shared_from_this?
+* finish the imgui viewer/editor!
+* archetype-like a-là EnTT support (see my own notes)
+* organizer: view/storage only based model, no registry
+* introduce a way to inject stl from outside too
+* redesign snapshot as a whole
+* explore "runtime" mode for hashed string where the source is copied internally
+* storage: shrink_to_fit does not work with reentrant destructor?
+* test trivially_destructible optimization

bazel/copts.bzl

@@ -2,11 +2,11 @@ load("@bazel_skylib//lib:selects.bzl", "selects")
 
 COPTS = selects.with_or({
     ("//conditions:default", "@rules_cc//cc/compiler:clang", "@rules_cc//cc/compiler:gcc", "@rules_cc//cc/compiler:mingw-gcc"): [
-        "-std=c++17",
+        "-std=c++20",
         "-w",
     ],
     ("@rules_cc//cc/compiler:msvc-cl", "@rules_cc//cc/compiler:clang-cl"): [
-        "/std:c++17",
+        "/std:c++20",
         "/permissive-",
         "/w",
     ],

conan/test_package/CMakeLists.txt

@@ -1,9 +1,9 @@
-cmake_minimum_required(VERSION 3.7.2)
+cmake_minimum_required(VERSION 3.28)
 project(test_package)
 
 set(CMAKE_VERBOSE_MAKEFILE TRUE)
 
-set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
 include(${CMAKE_BINARY_DIR}/conanbuildinfo.cmake)

docs/CMakeLists.txt

@@ -1,54 +1,54 @@
 # Doxygen configuration (documentation)
 
-include(FetchContent)
+find_package(Doxygen 1.14)
 
-FetchContent_Declare(
-    doxygen-awesome-css
-    GIT_REPOSITORY https://github.com/jothepro/doxygen-awesome-css
-    GIT_TAG main
-    GIT_SHALLOW 1
-)
+if(DOXYGEN_FOUND)
+    include(FetchContent)
 
-FetchContent_GetProperties(doxygen-awesome-css)
+    FetchContent_Declare(
+        doxygen-awesome-css
+        GIT_REPOSITORY https://github.com/jothepro/doxygen-awesome-css
+        GIT_TAG main
+        GIT_SHALLOW 1
+    )
 
-if(NOT doxygen-awesome-css_POPULATED)
-    FetchContent_Populate(doxygen-awesome-css)
-    set(doxygen-awesome-css_INCLUDE_DIR ${doxygen-awesome-css_SOURCE_DIR})
+    FetchContent_MakeAvailable(doxygen-awesome-css)
+    
+    set(DOXY_SOURCE_DIRECTORY ${EnTT_SOURCE_DIR}/src)
+    set(DOXY_CSS_DIRECTORY ${doxygen-awesome-css_SOURCE_DIR})
+    set(DOXY_DOCS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})
+    set(DOXY_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
+
+    configure_file(doxy.in doxy.cfg @ONLY)
+
+    add_custom_target(
+        docs ALL
+        COMMAND ${DOXYGEN_EXECUTABLE} ${CMAKE_CURRENT_BINARY_DIR}/doxy.cfg
+        WORKING_DIRECTORY ${EnTT_SOURCE_DIR}
+        VERBATIM
+        SOURCES
+            md/config.md
+            md/container.md
+            md/core.md
+            md/entity.md
+            md/faq.md
+            md/graph.md
+            md/lib.md
+            md/links.md
+            md/locator.md
+            md/meta.md
+            md/poly.md
+            md/process.md
+            md/reference.md
+            md/resource.md
+            md/signal.md
+            doxy.in
+    )
+
+    if(ENTT_INSTALL)
+        install(
+            DIRECTORY ${DOXY_OUTPUT_DIRECTORY}/html
+            DESTINATION share/${PROJECT_NAME}-${PROJECT_VERSION}/
+        )
+    endif()
 endif()
-
-set(DOXY_SOURCE_DIRECTORY ${EnTT_SOURCE_DIR}/src)
-set(DOXY_CSS_DIRECTORY ${doxygen-awesome-css_INCLUDE_DIR})
-set(DOXY_DOCS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})
-set(DOXY_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
-
-configure_file(doxy.in doxy.cfg @ONLY)
-
-add_custom_target(
-    docs ALL
-    COMMAND ${DOXYGEN_EXECUTABLE} ${CMAKE_CURRENT_BINARY_DIR}/doxy.cfg
-    WORKING_DIRECTORY ${EnTT_SOURCE_DIR}
-    VERBATIM
-    SOURCES
-        dox/extra.dox
-        md/config.md
-        md/container.md
-        md/core.md
-        md/entity.md
-        md/faq.md
-        md/lib.md
-        md/links.md
-        md/locator.md
-        md/meta.md
-        md/poly.md
-        md/process.md
-        md/reference.md
-        md/resource.md
-        md/signal.md
-        md/unreal.md
-        doxy.in
-)
-
-install(
-    DIRECTORY ${DOXY_OUTPUT_DIRECTORY}/html
-    DESTINATION share/${PROJECT_NAME}-${PROJECT_VERSION}/
-)

docs/dox/extra.dox

@@ -1,5 +0,0 @@
-/**
- * @namespace entt
- *
- * @brief `EnTT` default namespace.
- */

docs/md/config.md

@@ -4,7 +4,8 @@
 
 * [Introduction](#introduction)
 * [Definitions](#definitions)
-  * [ENTT_NOEXCEPTION](#entt_noexception)
+  * [ENTT_USE_STL](#entt_use_stl) 
+  * [ENTT_NO_EXCEPTION](#entt_no_exception)
   * [ENTT_USE_ATOMIC](#entt_use_atomic)
   * [ENTT_ID_TYPE](#entt_id_type)
   * [ENTT_SPARSE_PAGE](#entt_sparse_page)
@@ -13,7 +14,9 @@
     * [ENTT_ASSERT_CONSTEXPR](#entt_assert_constexpr)
     * [ENTT_DISABLE_ASSERT](#entt_disable_assert)
   * [ENTT_NO_ETO](#entt_no_eto)
+  * [ENTT_NO_MIXIN](#entt_no_mixin)
   * [ENTT_STANDARD_CPP](#entt_standard_cpp)
+* [Configuration injection](#configuration-injection)
 
 # Introduction
 
@@ -22,18 +25,25 @@ respects. These variables are just one of the many ways to customize how it
 works.<br/>
 In the vast majority of cases, users will have no interest in changing the
 default parameters. For all other cases, the list of possible configurations
-with which it's possible to adjust the behavior of the library at runtime can be
-found below.
+with which it is possible to adjust the behavior of the library at runtime can
+be found below.
 
 # Definitions
 
 All options are intended as parameters to the compiler (or user-defined macros
 within the compilation units, if preferred).<br/>
-Each parameter can result in internal library definitions. It's not recommended
+Each parameter can result in internal library definitions. It is not recommended
 to try to also modify these definitions, since there is no guarantee that they
 will remain stable over time unlike the options below.
 
-## ENTT_NOEXCEPTION
+## ENTT_USE_STL
+
+Intended for testing purposes, it forces the use of built-in replacements of
+some parts of the standard library that aren't always available otherwise.<br/>
+`EnTT` _detects_ these cases on its own, and users should never define this
+variable explicitly. However, it's still possible if desired.
+
+## ENTT_NO_EXCEPTION
 
 Define this variable without assigning any value to it to turn off exception
 handling in `EnTT`.<br/>
@@ -42,11 +52,12 @@ also limited to this library only.
 
 ## ENTT_USE_ATOMIC
 
-In general, `EnTT` doesn't offer primitives to support multi-threading. Many of
+In general, `EnTT` does not offer primitives to support multi-threading. Many of
 the features can be split over multiple threads without any explicit control and
 the user is the one who knows if a synchronization point is required.<br/>
-However, some features aren't easily accessible to users and are made
-thread-safe by means of this definition.
+However, some internal static data shared between threads should be atomic when
+using `EnTT` from multiple threads, even when dealing with local storage. Define
+this macro without assigning any value to it to get the job done.
 
 ## ENTT_ID_TYPE
 
@@ -57,24 +68,25 @@ default type if necessary.
 
 ## ENTT_SPARSE_PAGE
 
-It's known that the ECS module of `EnTT` is based on _sparse sets_. What is less
-known perhaps is that the sparse arrays are paged to reduce memory usage.<br/>
+It is known that the ECS module of `EnTT` is based on _sparse sets_. What is
+less known perhaps is that the sparse arrays are paged to reduce memory
+usage.<br/>
 Default size of pages (that is, the number of elements they contain) is 4096 but
-users can adjust it if appropriate. In all case, the chosen value **must** be a
+users can adjust it if appropriate. In all cases, the chosen value **must** be a
 power of 2.
 
 ## ENTT_PACKED_PAGE
 
 As it happens with sparse arrays, packed arrays are also paginated. However, in
-this case the aim isn't to reduce memory usage but to have pointer stability
+this case the aim is not to reduce memory usage but to have pointer stability
 upon component creation.<br/>
 Default size of pages (that is, the number of elements they contain) is 1024 but
-users can adjust it if appropriate. In all case, the chosen value **must** be a
+users can adjust it if appropriate. In all cases, the chosen value **must** be a
 power of 2.
 
 ## ENTT_ASSERT
 
-For performance reasons, `EnTT` doesn't use exceptions or any other control
+For performance reasons, `EnTT` does not use exceptions or any other control
 structures. In fact, it offers many features that result in undefined behavior
 if not used correctly.<br/>
 To get around this, the library relies on a lot of asserts for the purpose of
@@ -83,7 +95,7 @@ are allowed to overwrite its behavior by setting this variable.
 
 ### ENTT_ASSERT_CONSTEXPR
 
-Usually, an assert within a `constexpr` function isn't a big deal. However, in
+Usually, an assert within a `constexpr` function is not a big deal. However, in
 case of extreme customizations, it might be useful to differentiate.<br/>
 For this purpose, `EnTT` introduces an admittedly badly named variable to make
 the job easier in this regard. By default, this variable forwards its arguments
@@ -104,11 +116,29 @@ never instantiated nor stored by the ECS module of `EnTT`.<br/>
 Use this variable to treat these types like all others and therefore to create a
 dedicated storage for them.
 
+## ENTT_NO_MIXIN
+
+`EnTT` automatically assigns mixins to all storage types to support signaling
+when creating, destroying, and modifying elements.<br/>
+Mixins can have a (most likely negligible) cost in terms of performance and
+compilation time. If unwanted, this macro suppresses automatic generation.
+
 ## ENTT_STANDARD_CPP
 
 `EnTT` mixes non-standard language features with others that are perfectly
 compliant to offer some of its functionalities.<br/>
 This definition prevents the library from using non-standard techniques, that
-is, functionalities that aren't fully compliant with the standard C++.<br/>
+is, functionalities that are not fully compliant with the standard C++.<br/>
 While there are no known portability issues at the time of this writing, this
 should make the library fully portable anyway if needed.
+
+# Configuration injection
+
+Configuration variables are provided via code or injected directly from the
+outside via a dedicated file.<br/>
+`EnTT` uses `__has_include` internally and looks for a specific path, namely
+`<entt/ext/config.h>`. This can be provided by the user by setting the include
+paths appropriately.<br/>
+For example, `CMake` allows users to _bind_ additional include directories to a
+target with `target_include_directories`. See the test suite, and in particular
+the `config_ext` test for a practical example.

docs/md/container.md

@@ -12,16 +12,17 @@
 # Introduction
 
 The standard C++ library offers a wide range of containers and adaptors already.
-It's really difficult to do better (although it's very easy to do worse, as many
-examples available online demonstrate).<br/>
-`EnTT` doesn't try in any way to replace what is offered by the standard. Quite
+It is really difficult to do better (although it is very easy to do worse, as
+many examples available online demonstrate).<br/>
+`EnTT` does not try in any way to replace what is offered by the standard. Quite
 the opposite, given the widespread use that is made of standard containers.<br/>
 However, the library also tries to fill a gap in features and functionalities by
 making available some containers and adaptors initially developed for internal
 use.
 
 This section of the library is likely to grow larger over time. However, for the
-moment it's quite small and mainly aimed at satisfying some internal needs.<br/>
+moment it is quite small and mainly aimed at satisfying some internal
+needs.<br/>
 For all containers and adaptors made available, full test coverage and stability
 over time is guaranteed as usual.
 
@@ -78,7 +79,7 @@ The internal implementation is purposely supported by a tuple of containers
 rather than a container of tuples. The purpose is to allow efficient access to
 single columns and not just access to the entire data set of the table.
 
-When a row is accessed, all data are returned in the form of a tuple containing
+When a row is accessed, all data is returned in the form of a tuple containing
 (possibly const) references to the elements of the row itself.<br/>
 Similarly, when a table is iterated, tuples of references to table elements are
 returned for each row.

docs/md/core.md

@@ -10,7 +10,7 @@
 * [Compressed pair](#compressed-pair)
 * [Enum as bitmask](#enum-as-bitmask)
 * [Hashed strings](#hashed-strings)
-  * [Wide characters](wide-characters)
+  * [Wide characters](#wide-characters)
   * [Conflicts](#conflicts)
 * [Iterators](#iterators)
   * [Input iterator pointer](#input-iterator-pointer)
@@ -41,7 +41,7 @@
 
 `EnTT` comes with a bunch of core functionalities mostly used by the other parts
 of the library.<br/>
-Many of these tools are also useful in everyday work. Therefore, it's worth
+Many of these tools are also useful in everyday work. Therefore, it is worth
 describing them so as not to reinvent the wheel in case of need.
 
 # Any as in any type
@@ -49,7 +49,7 @@ describing them so as not to reinvent the wheel in case of need.
 `EnTT` offers its own `any` type. It may seem redundant considering that C++17
 introduced `std::any`, but it is not (hopefully).<br/>
 First of all, the _type_ returned by an `std::any` is a const reference to an
-`std::type_info`, an implementation defined class that's not something everyone
+`std::type_info`, an implementation defined class that is not something everyone
 wants to see in a software. Furthermore, there is no way to bind it to the type
 system of the library and therefore with its integrated RTTI support.
 
@@ -68,22 +68,25 @@ entt::any empty{};
 // a container for an int
 entt::any any{0};
 
-// in place construction
-entt::any in_place{std::in_place_type<int>, 42};
+// in place type construction
+entt::any in_place_type{std::in_place_type<int>, 42};
+
+// take ownership of already existing, dynamically allocated objects
+entt::any in_place{std::in_place, std::make_unique<int>(42).release()};
 ```
 
-Alternatively, the `make_any` function serves the same purpose but requires to
-always be explicit about the type:
+Alternatively, the `make_any` function serves the same purpose. It requires to
+always be explicit about the type and does not support taking ownership:
 
 ```cpp
 entt::any any = entt::make_any<int>(42);
 ```
 
-In both cases, the `any` class takes the burden of destroying the contained
+In all cases, the `any` class takes the burden of destroying the contained
 element when required, regardless of the storage strategy used for the specific
 object.<br/>
-Furthermore, an instance of `any` isn't tied to an actual type. Therefore, the
-wrapper is reconfigured when it's assigned a new object of a type other than
+Furthermore, an instance of `any` is not tied to an actual type. Therefore, the
+wrapper is reconfigured when it is assigned a new object of a type other than
 the one it contains.
 
 There is also a way to directly assign a value to the variable contained by an
@@ -115,8 +118,8 @@ The type is also used internally when comparing two `any` objects:
 if(any == empty) { /* ... */ }
 ```
 
-In this case, before proceeding with a comparison, it's verified that the _type_
-of the two objects is actually the same.<br/>
+In this case, before proceeding with a comparison, it is verified that the
+_type_ of the two objects is actually the same.<br/>
 Refer to the `EnTT` type system documentation for more details about how
 `type_info` works and the possible risks of a comparison.
 
@@ -135,9 +138,9 @@ any.emplace<const int &>(value);
 
 In other words, whenever `any` is explicitly told to construct an _alias_, it
 acts as a pointer to the original instance rather than making a copy of it or
-moving it internally. The contained object is never destroyed and users must
+moving it internally. The contained object is never destroyed, and users must
 ensure that its lifetime exceeds that of the container.<br/>
-Similarly, it's possible to create non-owning copies of `any` from an existing
+Similarly, it is possible to create non-owning copies of `any` from an existing
 object:
 
 ```cpp
@@ -145,12 +148,12 @@ object:
 entt::any ref = other.as_ref();
 ```
 
-In this case, it doesn't matter if the original container actually holds an
+In this case, it does not matter if the original container actually holds an
 object or is as a reference for unmanaged elements already. The new instance
-thus created doesn't create copies and only serves as a reference for the
+thus created does not create copies and only serves as a reference for the
 original item.
 
-It's worth mentioning that, while everything works transparently when it comes
+It is worth mentioning that, while everything works transparently when it comes
 to non-const references, there are some exceptions when it comes to const
 references.<br/>
 In particular, the `data` member function invoked on a non-const instance of
@@ -158,9 +161,9 @@ In particular, the `data` member function invoked on a non-const instance of
 
 To cast an instance of `any` to a type, the library offers a set of `any_cast`
 functions in all respects similar to their most famous counterparts.<br/>
-The only difference is that, in the case of `EnTT`, they won't raise exceptions
-but will only trigger an assert in debug mode, otherwise resulting in undefined
-behavior in case of misuse in release mode.
+The only difference is that, in the case of `EnTT`, they will not raise
+exceptions but will only trigger an assert in debug mode, otherwise resulting in
+undefined behavior in case of misuse in release mode.
 
 ## Small buffer optimization
 
@@ -185,8 +188,8 @@ and always dynamically allocates objects (except for aliasing cases).
 
 The alignment requirement is optional and by default the most stringent (the
 largest) for any object whose size is at most equal to the one provided.<br/>
-It's provided as an optional second parameter following the desired size for the
-internal storage:
+It is provided as an optional second parameter following the desired size for
+the internal storage:
 
 ```cpp
 using my_any = entt::basic_any<sizeof(double[4]), alignof(double[4])>;
@@ -198,19 +201,15 @@ in order to meet them.
 
 # Bit
 
-Finding out the population count of an unsigned integral value (`popcount`),
-whether a number is a power of two or not (`has_single_bit`) as well as the next
-power of two given a random value (`next_power_of_two`) can be useful.<br/>
-For example, it helps to allocate memory in pages having a size suitable for the
-fast modulus:
+Some general purpose utilities, such as the fast module function:
 
 ```cpp
 const std::size_t result = entt::fast_mod(value, modulus);
 ```
 
-Where `modulus` is necessarily a power of two. Perhaps not everyone knows that
-this type of operation is far superior in terms of performance to the basic
-modulus and for this reason preferred in many areas.
+Where `modulus` is necessarily a power of two. This type of operation is far
+superior in terms of performance to the basic modulus and for this reason
+preferred in many areas.
 
 # Compressed pair
 
@@ -231,16 +230,16 @@ entt::compressed_pair pair{0, 3.};
 pair.first() = 42;
 ```
 
-There isn't much to describe then. It's recommended to rely on documentation and
-intuition. At the end of the day, it's just a pair and nothing more.
+There is not much to describe then. It is recommended to rely on documentation
+and intuition. At the end of the day, it is just a pair and nothing more.
 
 # Enum as bitmask
 
-Sometimes it's useful to be able to use enums as bitmasks. However, enum classes
-aren't really suitable for the purpose. Main problem is that they don't convert
-implicitly to their underlying type.<br/>
+Sometimes it is useful to be able to use enums as bitmasks. However, enum
+classes are not really suitable for the purpose. Main problem is that they do
+not convert implicitly to their underlying type.<br/>
 The choice is then between using old-fashioned enums (with all their problems
-that I don't want to discuss here) or writing _ugly_ code.
+that I do not want to discuss here) or writing _ugly_ code.
 
 Fortunately, there is also a third way: adding enough operators in the global
 scope to treat enum classes as bitmasks transparently.<br/>
@@ -273,7 +272,7 @@ struct entt::enum_as_bitmask<my_flag>
 ```
 
 This is handy when dealing with enum classes defined by third party libraries
-and over which the user has no control. However, it's also verbose and can be
+and over which the user has no control. However, it is also verbose and can be
 avoided by adding a specific value to the enum class itself:
 
 ```cpp
@@ -336,7 +335,7 @@ entt::hashed_string str{orig.c_str()};
 const auto hash = entt::hashed_string::value(orig.c_str());
 ```
 
-This possibility shouldn't be exploited in tight loops, since the computation
+This possibility should not be exploited in tight loops, since the computation
 takes place at runtime and no longer at compile-time. It could therefore affect
 performance to some degrees.
 
@@ -359,16 +358,16 @@ The hash type of `hashed_wstring` is the same as its counterpart.
 The hashed string class uses FNV-1a internally to hash strings. Because of the
 _pigeonhole principle_, conflicts are possible. This is a fact.<br/>
 There is no silver bullet to solve the problem of conflicts when dealing with
-hashing functions. In this case, the best solution is likely to give up. That's
+hashing functions. In this case, the best solution is likely to give up. That is
 all.<br/>
-After all, human-readable unique identifiers aren't something strictly defined
+After all, human-readable unique identifiers are not something strictly defined
 and over which users have not the control. Choosing a slightly different
 identifier is probably the best solution to make the conflict disappear in this
 case.
 
 # Iterators
 
-Writing and working with iterators isn't always easy. More often than not it
+Writing and working with iterators is not always easy. More often than not it
 also leads to duplicated code.<br/>
 `EnTT` tries to overcome this problem by offering some utilities designed to
 make this hard work easier.
@@ -376,12 +375,12 @@ make this hard work easier.
 ## Input iterator pointer
 
 When writing an input iterator that returns in-place constructed values if
-dereferenced, it's not always straightforward to figure out what `value_type` is
-and how to make it behave like a full-fledged pointer.<br/>
+dereferenced, it is not always straightforward to figure out what `value_type`
+is and how to make it behave like a full-fledged pointer.<br/>
 Conversely, it would be very useful to have an `operator->` available on the
 iterator itself that always works without too much complexity.
 
-The input iterator pointer is meant for this. It's a small class that wraps the
+The input iterator pointer is meant for this. It is a small class that wraps the
 in-place constructed value and adds some functions on top of it to make it
 suitable for use with input iterators: 
 
@@ -433,7 +432,7 @@ _iterable_ object with all the expected methods like `begin`, `end` and whatnot.
 
 The library uses this class extensively.<br/>
 Think for example of views, which can be iterated to access entities but also
-offer a method of obtaining an iterable object that returns tuples of entities
+offer a method for obtaining an iterable object that returns tuples of entities
 and components at once.<br/>
 Another example is the registry class which allows users to iterate its storage
 by returning an iterable object for the purpose.
@@ -446,16 +445,16 @@ Some are geared towards simplifying the implementation of (internal or external)
 allocator aware containers. Others are designed to help the developer with
 everyday problems.
 
-The former are very specific and for niche problems. These are tools designed to
-unwrap fancy or plain pointers (`to_address`) or to help forget the meaning of
-acronyms like _POCCA_, _POCMA_ or _POCS_.<br/>
-I won't describe them here in detail. Instead, I recommend reading the inline
+The former are very specific and for niche problems. For example, there are
+tools designed to help forget the meaning of acronyms like _POCCA_, _POCMA_ or
+_POCS_.<br/>
+I will not describe them here in detail. Instead, I recommend reading the inline
 documentation to those interested in the subject.
 
 ## Allocator aware unique pointers
 
 A nasty thing in C++ (at least up to C++20) is the fact that shared pointers
-support allocators while unique pointers don't.<br/>
+support allocators while unique pointers do not.<br/>
 There is a proposal at the moment that also shows (among the other things) how
 this can be implemented without any compiler support.
 
@@ -463,7 +462,7 @@ The `allocate_unique` function follows this proposal, making a virtue out of
 necessity:
 
 ```cpp
-std::unique_ptr<my_type, entt::allocation_deleter<my_type>> ptr = entt::allocate_unique<my_type>(allocator, arguments);
+std::unique_ptr<my_type, entt::allocation_deleter<allocator_type>> ptr = entt::allocate_unique<my_type>(allocator, arguments);
 ```
 
 Although the internal implementation is slightly different from what is proposed
@@ -506,9 +505,9 @@ library or that will never be.
 
 Runtime type identification support (or RTTI) is one of the most frequently
 disabled features in the C++ world, especially in the gaming sector. Regardless
-of the reasons for this, it's often a shame not to be able to rely on opaque
+of the reasons for this, it is often a shame not to be able to rely on opaque
 type information at runtime.<br/>
-The library tries to fill this gap by offering a built-in system that doesn't
+The library tries to fill this gap by offering a built-in system that does not
 serve as a replacement but comes very close to being one and offers similar
 information to that provided by its counterpart.
 
@@ -520,17 +519,18 @@ Basically, the whole system relies on a handful of classes. In particular:
   auto index = entt::type_index<a_type>::value();
   ```
 
-  The returned value isn't guaranteed to be stable across different runs.<br/>
+  The returned value is not guaranteed to be stable across different runs.<br/>
   However, it can be very useful as index in associative and unordered
   associative containers or for positional accesses in a vector or an array.
   
   An external generator can also be used if needed. In fact, `type_index` can be
-  specialized by type and is also _sfinae-friendly_ in order to allow more
+  specialized by type or constrained with a concept in order to allow more
   refined specializations such as:
   
   ```cpp
   template<typename Type>
-  struct entt::type_index<Type, std::void_d<decltype(Type::index())>> {
+  requires requires { { Type::index() } -> std::same_as<entt::id_type>; }
+  struct entt::type_index<Type> {
       static entt::id_type value() noexcept {
           return Type::index();
       }
@@ -548,8 +548,8 @@ Basically, the whole system relies on a handful of classes. In particular:
   ```
 
   In general, the `value` function exposed by `type_hash` is also `constexpr`
-  but this isn't guaranteed for all compilers and platforms (although it's valid
-  with the most well-known and popular ones).
+  but this is not guaranteed for all compilers and platforms (although it is
+  valid with the most well-known and popular ones).
 
   This function **can** use non-standard features of the language for its own
   purposes. This makes it possible to provide compile-time identifiers that
@@ -559,8 +559,8 @@ Basically, the whole system relies on a handful of classes. In particular:
   identifiers remain stable across executions. Moreover, they are generated
   at runtime and are no longer a compile-time thing.
 
-  As it happens with `type_index`, also `type_hash` is a _sfinae-friendly_ class
-  that can be specialized in order to customize its behavior globally or on a
+  As it happens with `type_index`, also `type_hash` can be specialized or
+  constrained with a concept in order to customize its behavior globally or on a
   per-type or per-traits basis.
 
 * The name associated with a given type:
@@ -571,7 +571,7 @@ Basically, the whole system relies on a handful of classes. In particular:
 
   This value is extracted from some information generally made available by the
   compiler in use. Therefore, it may differ depending on the compiler and may be
-  empty in the event that this information isn't available.<br/>
+  empty in the event that this information is not available.<br/>
   For example, given the following class:
 
   ```cpp
@@ -583,15 +583,15 @@ Basically, the whole system relies on a handful of classes. In particular:
   Most of the time the name is also retrieved at compile-time and is therefore
   always returned through an `std::string_view`. Users can easily access it and
   modify it as needed, for example by removing the word `struct` to normalize
-  the result. `EnTT` doesn't do this for obvious reasons, since it would be
+  the result. `EnTT` does not do this for obvious reasons, since it would be
   creating a new string at runtime otherwise.
 
   This function **can** use non-standard features of the language for its own
   purposes. Users can prevent the library from using these features by means of
   the `ENTT_STANDARD_CPP` definition. In this case, the name is just empty.
 
-  As it happens with `type_index`, also `type_name` is a _sfinae-friendly_ class
-  that can be specialized in order to customize its behavior globally or on a
+  As it happens with `type_index`, also `type_name` can be specialized or
+  constrained with a concept in order to customize its behavior globally or on a
   per-type or per-traits basis.
 
 These are then combined into utilities that aim to offer an API that is somewhat
@@ -599,8 +599,8 @@ similar to that made available by the standard library.
 
 ### Type info
 
-The `type_info` class isn't a drop-in replacement for `std::type_info` but can
-provide similar information which are not implementation defined and don't
+The `type_info` class is not a drop-in replacement for `std::type_info` but can
+provide similar information which are not implementation defined and do not
 require to enable RTTI.<br/>
 Therefore, they can sometimes be even more reliable than those obtained
 otherwise.
@@ -636,7 +636,7 @@ These are the information made available by `type_info`:
   This is also an alias for the following:
 
   ```cpp
-  auto idx = entt::type_index<std::remove_cv_t<std::remove_reference_t<a_type>>>::value();
+  auto idx = entt::type_index<std::remove_cvref_t<a_type>>::value();
   ```
 
 * The hash value associated with a given type:
@@ -648,7 +648,7 @@ These are the information made available by `type_info`:
   This is also an alias for the following:
 
   ```cpp
-  auto hash = entt::type_hash<std::remove_cv_t<std::remove_reference_t<a_type>>>::value();
+  auto hash = entt::type_hash<std::remove_cvref_t<a_type>>::value();
   ```
 
 * The name associated with a given type:
@@ -660,7 +660,7 @@ These are the information made available by `type_info`:
   This is also an alias for the following:
 
   ```cpp
-  auto name = entt::type_name<std::remove_cv_t<std::remove_reference_t<a_type>>>::value();
+  auto name = entt::type_name<std::remove_cvref_t<a_type>>::value();
   ```
 
 Where all accessed features are available at compile-time, the `type_info` class
@@ -673,7 +673,7 @@ described above.
 Since the default non-standard, compile-time implementation of `type_hash` makes
 use of hashed strings, it may happen that two types are assigned the same hash
 value.<br/>
-In fact, although this is quite rare, it's not entirely excluded.
+In fact, although this is quite rare, it is not entirely excluded.
 
 Another case where two types are assigned the same identifier is when classes
 from different contexts (for example two or more libraries loaded at runtime)
@@ -682,8 +682,9 @@ value for the two types.<br/>
 Fortunately, there are several easy ways to deal with this:
 
 * The most trivial one is to define the `ENTT_STANDARD_CPP` macro. Runtime
-  identifiers don't suffer from the same problem in fact. However, this solution
-  doesn't work well with a plugin system, where the libraries aren't linked.
+  identifiers do not suffer from the same problem in fact. However, this
+  solution does not work well with a plugin system, where the libraries are not
+  linked.
 
 * Another possibility is to specialize the `type_name` class for one of the
   conflicting types, in order to assign it a custom identifier. This is probably
@@ -692,8 +693,8 @@ Fortunately, there are several easy ways to deal with this:
 * A fully customized identifier generation policy (based for example on enum
   classes or preprocessing steps) may represent yet another option.
 
-These are just some examples of possible approaches to the problem but there are
-many others. As already mentioned above, since users have full control over
+These are just some examples of possible approaches to the problem, but there
+are many others. As already mentioned above, since users have full control over
 their types, this problem is in any case easy to solve and should not worry too
 much.<br/>
 In all likelihood, it will never happen to run into a conflict anyway.
@@ -709,10 +710,10 @@ offered by this module.
 ### Size of
 
 The standard operator `sizeof` complains if users provide it with functions or
-incomplete types. On the other hand, it's guaranteed that its result is always
+incomplete types. On the other hand, it is guaranteed that its result is always
 non-zero, even if applied to an empty class type.<br/>
 This small class combines the two and offers an alternative to `sizeof` that
-works under all circumstances, returning zero if the type isn't supported:
+works under all circumstances, returning zero if the type is not supported:
 
 ```cpp
 const auto size = entt::size_of_v<void>;
@@ -747,7 +748,7 @@ using type = entt::constness_as_t<dst_type, const src_type>;
 ```
 
 The trait is subject to the rules of the language. For example, _transferring_
-constness between references won't give the desired effect.
+constness between references will not give the desired effect.
 
 ### Member class type
 
@@ -805,7 +806,7 @@ where types would be required otherwise. As an example:
 registry.emplace<entt::tag<"enemy"_hs>>(entity);
 ```
 
-However, this isn't the only permitted use. Literally any value convertible to
+However, this is not the only permitted use. Literally any value convertible to
 `id_type` is a good candidate, such as the named constants of an unscoped enum.
 
 ### Type list and value list
@@ -827,7 +828,7 @@ type list:
 * `type_list_diff[_t]` to remove types from type lists.
 * `type_list_transform[_t]` to _transform_ a range and create another type list.
 
-I'm also pretty sure that more and more utilities will be added over time as
+I am also pretty sure that more and more utilities will be added over time as
 needs become apparent.<br/>
 Many of these functionalities also exist in their version dedicated to value
 lists. We therefore have `value_list_element[_v]` as well as
@@ -835,11 +836,11 @@ lists. We therefore have `value_list_element[_v]` as well as
 
 # Unique sequential identifiers
 
-Sometimes it's useful to be able to give unique, sequential numeric identifiers
+Sometimes it is useful to be able to give unique, sequential numeric identifiers
 to types either at compile-time or runtime.<br/>
-There are plenty of different solutions for this out there and I could have used
-one of them. However, I decided to spend my time to define a couple of tools
-that fully embraces what the modern C++ has to offer.
+There are plenty of different solutions for this out there, and I could have
+used one of them. However, I decided to spend my time to define a couple of
+tools that fully embrace what modern C++ has to offer.
 
 ## Compile-time generator
 
@@ -869,12 +870,13 @@ contains a numeric identifier for the given type. It can be used in any context
 where constant expressions are required.
 
 As long as the list remains unchanged, identifiers are also guaranteed to be
-stable across different runs. In case they have been used in a production
-environment and a type has to be removed, one can just use a placeholder to
-leave the other identifiers unchanged:
+stable across different runs. If used in a production environment where a type
+needs to be removed, a placeholder can help to leave the other identifiers the
+same:
 
 ```cpp
-template<typename> struct ignore_type {};
+template<typename>
+struct ignore_type {};
 
 using id = entt::ident<
     a_type_still_valid,
@@ -883,7 +885,7 @@ using id = entt::ident<
 >;
 ```
 
-Perhaps a bit ugly to see in a codebase but it gets the job done at least.
+Perhaps a bit ugly to see in a codebase, but it gets the job done at least.
 
 ## Runtime generator
 
@@ -905,19 +907,15 @@ numeric identifier for the given type.<br/>
 The generator is customizable, so as to get different _sequences_ for different
 purposes if needed.
 
-Identifiers aren't guaranteed to be stable across different runs. Indeed it
+Identifiers are not guaranteed to be stable across different runs. Indeed it
 mostly depends on the flow of execution.
 
 # Utilities
 
-It's not possible to escape the temptation to add utilities of some kind to a
+It is not possible to escape the temptation to add utilities of some kind to a
 library. In fact, `EnTT` also provides a handful of tools to simplify the
 life of developers:
 
-* `entt::identity`: the identity function object that will be available with
-  C++20. It returns its argument unchanged and nothing more. It's useful as a
-  sort of _do nothing_ function in template programming.
-
 * `entt::overload`: a tool to disambiguate different overloads from their
   function type. It works with both free and member functions.<br/>
   Consider the following definition:
@@ -961,7 +959,8 @@ life of developers:
   callable object that supports multiple types at once.
 
 * `entt::y_combinator`: this is a C++ implementation of **the** _y-combinator_.
-  If it's not clear what it is, there is probably no need for this utility.<br/>
+  If it is not clear what it is, there is probably no need for this
+  utility.<br/>
   Below is a small example to show its use:
 
   ```cpp
@@ -972,9 +971,9 @@ life of developers:
   const auto result = gauss(3u);
   ```
 
-  Maybe convoluted at a first glance but certainly effective. Unfortunately,
-  the language doesn't make it possible to do much better.
+  Maybe convoluted at first glance but certainly effective. Unfortunately,
+  the language does not make it possible to do much better.
 
 This is a rundown of the (actually few) utilities made available by `EnTT`. The
-list will probably grow over time but the size of each will remain rather small,
-as has been the case so far.
+list will probably grow over time, but the size of each will remain rather
+small, as has been the case so far.

docs/md/faq.md

@@ -14,10 +14,10 @@
 
 # Introduction
 
-This is a constantly updated section where I'm trying to put the answers to the
+This is a constantly updated section where I am trying to put the answers to the
 most frequently asked questions.<br/>
-If you don't find your answer here, there are two cases: nobody has done it yet
-or this section needs updating. In both cases, you can
+If you do not find your answer here, there are two cases: nobody has done it
+yet, or this section needs updating. In both cases, you can
 [open a new issue](https://github.com/skypjack/entt/issues/new) or enter either
 the [gitter channel](https://gitter.im/skypjack/entt) or the
 [discord server](https://discord.gg/5BjPWBd) to ask for help.<br/>
@@ -29,12 +29,12 @@ part of the documentation.
 ## Why is my debug build on Windows so slow?
 
 `EnTT` is an experimental project that I also use to keep me up-to-date with the
-latest revision of the language and the standard library. For this reason, it's
-likely that some classes you're working with are using standard containers under
-the hood.<br/>
-Unfortunately, it's known that the standard containers aren't particularly
+latest revision of the language and the standard library. For this reason, it is
+likely that some classes you are working with are using standard containers
+under the hood.<br/>
+Unfortunately, it is known that the standard containers are not particularly
 performing in debugging (the reasons for this go beyond this document) and are
-even less so on Windows apparently. Fortunately this can also be mitigated a
+even less so on Windows, apparently. Fortunately, this can also be mitigated a
 lot, achieving good results in many cases.
 
 First of all, there are two things to do in a Windows project:
@@ -53,7 +53,7 @@ macro to disable internal debug checks in `EnTT`:
 #define ENTT_ASSERT(...) ((void)0)
 ```
 
-These asserts are introduced to help the users but they require to access to the
+These asserts are introduced to help the users, but they require access to the
 underlying containers and therefore risk ruining the performance in some cases.
 
 With these changes, debug performance should increase enough in most cases. If
@@ -64,15 +64,15 @@ preferably `O1`.
 
 This is one of the first questions that anyone makes when starting to work with
 the entity-component-system architectural pattern.<br/>
-There are several approaches to the problem and the best one depends mainly on
-the real problem one is facing. In all cases, how to do it doesn't strictly
+There are several approaches to the problem, and the best one depends mainly on
+the real problem one is facing. In all cases, how to do it does not strictly
 depend on the library in use, but the latter certainly allows or not different
 techniques depending on how the data are laid out.
 
 I tried to describe some of the approaches that fit well with the model of
 `EnTT`. [This](https://skypjack.github.io/2019-06-25-ecs-baf-part-4/) is the
 first post of a series that tries to _explore_ the problem. More will probably
-come in future.<br/>
+come in the future.<br/>
 In addition, `EnTT` also offers the possibility to create stable storage types
 and therefore have pointer stability for one, all or some components. This is by
 far the most convenient solution when it comes to creating hierarchies and
@@ -83,7 +83,7 @@ what concerns the `component_traits` class for further details.
 
 Custom entity identifiers are definitely a good idea in two cases at least:
 
-* If `std::uint32_t` isn't large enough for your purposes, since this is the
+* If `std::uint32_t` is not large enough for your purposes, since this is the
   underlying type of `entt::entity`.
 
 * If you want to avoid conflicts when using multiple registries.
@@ -104,8 +104,8 @@ On Windows, a header file defines two macros `min` and `max` which may result in
 conflicts with their counterparts in the standard library and therefore in
 errors during compilation.
 
-It's a pretty big problem but fortunately it's not a problem of `EnTT` and there
-is a fairly simple solution to it.<br/>
+It is a pretty big problem. However, fortunately it is not a problem of `EnTT`
+and there is a fairly simple solution to it.<br/>
 It consists in defining the `NOMINMAX` macro before including any other header
 so as to get rid of the extra definitions:
 
@@ -119,9 +119,9 @@ more details.
 ## The standard and the non-copyable types
 
 `EnTT` uses internally the trait `std::is_copy_constructible_v` to check if a
-component is actually copyable. However, this trait doesn't really check whether
-a type is actually copyable. Instead, it just checks that a suitable copy
-constructor and copy operator exist.<br/>
+component is actually copyable. However, this trait does not really check
+whether a type is actually copyable. Instead, it just checks that a suitable
+copy constructor and copy operator exist.<br/>
 This can lead to surprising results due to some idiosyncrasies of the standard.
 
 For example, `std::vector` defines a copy constructor that is conditionally
@@ -163,7 +163,7 @@ to mitigate the problem makes it manageable.
 
 Storage classes offer three _signals_ that are emitted following specific
 operations. Maybe not everyone knows what these operations are, though.<br/>
-If this isn't clear, below you can find a _vademecum_ for this purpose:
+If this is not clear, below you can find a _vademecum_ for this purpose:
 
 * `on_created` is invoked when a component is first added (neither modified nor 
   replaced) to an entity.
@@ -174,7 +174,7 @@ If this isn't clear, below you can find a _vademecum_ for this purpose:
   from an entity.
 
 Among the most controversial functions can be found `emplace_or_replace` and
-`destroy`. However, following the above rules, it's quite simple to know what 
+`destroy`. However, following the above rules, it is quite simple to know what 
 will happen.<br/>
 In the first case, `on_created` is invoked if the entity has not the component,
 otherwise the latter is replaced and therefore `on_update` is triggered. As for
@@ -184,9 +184,9 @@ owned by the entity that is destroyed.
 
 ## Duplicate storage for the same component
 
-It's rare but you can see double sometimes, especially when it comes to storage.
-This can be caused by a conflict in the hash assigned to the various component
-types (one of a kind) or by bugs in your compiler
+It is rare, but you can see double sometimes, especially when it comes to
+storage. This can be caused by a conflict in the hash assigned to the various
+component types (one of a kind) or by bugs in your compiler
 ([more common](https://github.com/skypjack/entt/issues/1063) apparently).<br/>
 Regardless of the cause, `EnTT` offers a customization point that also serves as
 a solution in this case:
@@ -194,11 +194,11 @@ a solution in this case:
 ```cpp
 template<>
 struct entt::type_hash<Type> final {
-    [[nodiscard]] static constexpr id_type value() noexcept {
+    [[nodiscard]] static consteval id_type value() noexcept {
         return hashed_string::value("Type");
     }
 
-    [[nodiscard]] constexpr operator id_type() const noexcept {
+    [[nodiscard]] consteval operator id_type() const noexcept {
         return value();
     }
 };

docs/md/graph.md

@@ -14,20 +14,21 @@
 
 # Introduction
 
-`EnTT` doesn't aim to offer everything one needs to work with graphs. Therefore,
-anyone looking for this in the _graph_ submodule will be disappointed.<br/>
+`EnTT` does not aim to offer everything one needs to work with graphs.
+Therefore, anyone looking for this in the _graph_ submodule will be
+disappointed.<br/>
 Quite the opposite is true though. This submodule is minimal and contains only
 the data structures and algorithms strictly necessary for the development of
 some tools such as the _flow builder_.
 
 # Data structures
 
-As anticipated in the introduction, the aim isn't to offer all possible data
+As anticipated in the introduction, the aim is not to offer all possible data
 structures suitable for representing and working with graphs. Many will likely
-be added or refined over time. However I want to discourage anyone expecting
+be added or refined over time. However, I want to discourage anyone expecting
 tight scheduling on the subject.<br/>
 The data structures presented in this section are mainly useful for the
-development and support of some tools which are also part of the same submodule.
+development and support of some tools that are also part of the same submodule.
 
 ## Adjacency matrix
 
@@ -108,11 +109,11 @@ Both the functions expect the vertex to visit (that is, to return the in- or
 out-edges for) as an argument.<br/>
 Finally, the adjacency matrix is an allocator-aware container and offers most of
 the functionalities one would expect from this type of containers, such as
-`clear` or 'get_allocator` and so on.
+`clear` or `get_allocator` and so on.
 
 ## Graphviz dot language
 
-As it's one of the most popular formats, the library offers minimal support for
+As it is one of the most popular formats, the library offers minimal support for
 converting a graph to a Graphviz dot snippet.<br/>
 The simplest way is to pass both an output stream and a graph to the `dot`
 function:
@@ -122,7 +123,7 @@ std::ostringstream output{};
 entt::dot(output, adjacency_matrix);
 ```
 
-It's also possible to provide a callback to which the vertices are passed and
+It is also possible to provide a callback to which the vertices are passed and
 which can be used to add (`dot`) properties to the output as needed:
 
 ```cpp
@@ -139,7 +140,7 @@ externally managed data to the graph being converted.
 # Flow builder
 
 A flow builder is used to create execution graphs from tasks and resources.<br/>
-The implementation is as generic as possible and doesn't bind to any other part
+The implementation is as generic as possible and does not bind to any other part
 of the library.
 
 This class is designed as a sort of _state machine_ to which a specific task is
@@ -165,7 +166,7 @@ particular data structure. On the other hand, it requires the user to keep track
 of the association between identifiers and operations or actual data.
 
 Once a flow builder is created (which requires no constructor arguments), the
-first thing to do is to bind a task. This tells to the builder _who_ intends to
+first thing to do is to bind a task. This tells the builder _who_ intends to
 consume the resources that are specified immediately after:
 
 ```cpp
@@ -175,15 +176,15 @@ builder.bind("task_1"_hs);
 
 The example uses the `EnTT` hashed string to generate an identifier for the
 task.<br/>
-Indeed, the use of `id_type` as an identifier type isn't by accident. In fact,
-it matches well with the internal hashed string class. Moreover, it's also the
+Indeed, the use of `id_type` as an identifier type is not by accident. In fact,
+it matches well with the internal hashed string class. Moreover, it is also the
 same type returned by the hash function of the internal RTTI system, in case the
 user wants to rely on that.<br/>
 However, being an integral value, it leaves the user full freedom to rely on his
 own tools if necessary.
 
-Once a task is associated with the flow builder, it's also assigned read-only or
-read-write resources as appropriate:
+Once a task is associated with the flow builder, it has also assigned read-only
+or read-write resources as appropriate:
 
 ```cpp
 builder
@@ -194,7 +195,7 @@ builder
         .rw("resource_2"_hs)
 ```
 
-As mentioned, many functions return the builder itself and it's therefore easy
+As mentioned, many functions return the builder itself, and it is therefore easy
 to concatenate the different calls.<br/>
 Also in the case of resources, they are identified by numeric values of type
 `id_type`. As above, the choice is not entirely random. This goes well with the
@@ -208,13 +209,13 @@ pair of iterators, so that one can pass a range of resources in one go.
 The `flow` class is resource based rather than task based. This means that graph
 generation is driven by resources and not by the order of _appearance_ of tasks
 during flow definition.<br/>
-Although this concept is particularly important, it's almost irrelevant for the
+Although this concept is particularly important, it is almost irrelevant for the
 vast majority of cases. However, it becomes relevant when _rebinding_ resources
 or tasks.
 
 In fact, nothing prevents rebinding elements to a flow.<br/>
 However, the behavior changes slightly from case to case and has some nuances
-that it's worth knowing about.
+that it is worth knowing about.
 
 Directly rebinding a resource without the task being replaced trivially results
 in the task's access mode for that resource being updated:
@@ -225,7 +226,7 @@ builder.bind("task"_hs).rw("resource"_hs).ro("resource"_hs)
 
 In this case, the resource is accessed in read-only mode, regardless of the
 first call to `rw`.<br/>
-Behind the scenes, the call doesn't actually _replace_ the previous one but is
+Behind the scenes, the call does not actually _replace_ the previous one but is
 queued after it instead, overwriting it when generating the graph. Thus, a large
 number of resource rebindings may even impact processing times (very difficult
 to observe but theoretically possible).
@@ -246,12 +247,12 @@ builder
 ```
 
 What happens here is that the resource first _sees_ a read-only access request
-from the first task, then a read-write request from the second task and finally
-a new read-only request from the first task.<br/>
+from the first task, then a read-only request from the second task and finally
+a read-write request from the first task.<br/>
 Although this definition would probably be counted as an error, the resulting
-graph may be unexpected. This in fact consists of a single edge outgoing from
+graph may be unexpected. In fact, this consists of a single edge outgoing from
 the second task and directed to the first task.<br/>
-To intuitively understand what happens, it's enough to think of the fact that a
+To intuitively understand what happens, it is enough to think of the fact that a
 task never has an edge pointing to itself.
 
 While not obvious, this approach has its pros and cons like any other solution.
@@ -272,22 +273,22 @@ As expected, this definition leads to the creation of two edges that define a
 loop between the two tasks.
 
 As a general rule, rebinding resources and tasks is highly discouraged because
-it could lead to subtle bugs if users don't know what they're doing.<br/>
+it could lead to subtle bugs if users do not know what they are doing.<br/>
 However, once the mechanisms of resource-based graph generation are understood,
-it can offer to the expert user a flexibility and a range of possibilities
+it can offer to the expert user flexibility and a range of possibilities
 otherwise inaccessible.
 
 ## Fake resources and order of execution
 
-The flow builder doesn't offer the ability to specify when a task should execute
+The flow builder does not offer the ability to specify when a task should run
 before or after another task.<br/>
 In fact, the order of _registration_ on the resources also determines the order
 in which the tasks are processed during the generation of the execution graph.
 
 However, there is a way to _force_ the execution order of two processes.<br/>
 Briefly, since accessing a resource in opposite modes requires sequential rather
-than parallel scheduling, it's possible to make use of fake resources to rule on
-the execution order:
+than parallel scheduling, it is possible to make use of fake resources to rule
+on the execution order:
 
 ```cpp
 builder
@@ -305,7 +306,7 @@ builder
 This snippet forces the execution of `task_1` **before** `task_2` and `task_3`.
 This is due to the fact that the former sets a read-write requirement on a fake
 resource that the other tasks also want to access in read-only mode.<br/>
-Similarly, it's possible to force a task to run **after** a certain group:
+Similarly, it is possible to force a task to run **after** a certain group:
 
 ```cpp
 builder
@@ -322,22 +323,22 @@ builder
 
 In this case, since there are a number of processes that want to read a specific
 resource, they will do so in parallel by forcing `task_3` to run after all the
-others tasks.
+other tasks.
 
 ## Sync points
 
-Sometimes it's useful to assign the role of _sync point_ to a node.<br/>
+Sometimes it is useful to assign the role of _sync point_ to a node.<br/>
 Whether it accesses new resources or is simply a watershed, the procedure for
-assigning this role to a vertex is always the same. First it's tied to the flow
+assigning this role to a vertex is always the same. First it is tied to the flow
 builder, then the `sync` function is invoked:
 
 ```cpp
 builder.bind("sync_point"_hs).sync();
 ```
 
-The choice to assign an _identity_ to this type of nodes lies in the fact that,
+The choice to assign an _identity_ to this type of node lies in the fact that,
 more often than not, they also perform operations on resources.<br/>
-If this isn't the case, it will still be possible to create no-op vertices to
+If this is not the case, it will still be possible to create no-op vertices to
 which empty tasks are assigned.
 
 ## Execution graph
@@ -361,6 +362,6 @@ for(auto &&vertex: graph) {
 }
 ```
 
-Then it's possible to instantiate an execution graph by means of other functions
-such as `out_edges` to retrieve the children of a given task or `edges` to get
-the identifiers.
+Then it is possible to instantiate an execution graph by means of other
+functions such as `out_edges` to retrieve the children of a given task or
+`edges` to get the identifiers.

docs/md/lib.md

@@ -20,31 +20,31 @@ Fortunately, nowadays `EnTT` works smoothly across boundaries.
 Many classes in `EnTT` make extensive use of type erasure for their purposes.
 This raises the need to identify objects whose type has been erased.<br/>
 The `type_hash` class template is how identifiers are generated and thus made
-available to the rest of the library. In general, this class doesn't arouse much
+available to the rest of the library. In general, this class arouses little
 interest. The only exception is when a conflict between identifiers occurs
-(definitely uncommon though) or when the default solution proposed by `EnTT`
-isn't suitable for the user's purposes.<br/>
+(definitely uncommon though) or when the default solution proposed by `EnTT` is
+not suitable for the user's purposes.<br/>
 The section dedicated to `type_info` contains all the details to get around the
 issue in a concise and elegant way. Please refer to the specific documentation.
 
 When working with linked libraries, compile definitions `ENTT_API_EXPORT` and
-`ENTT_API_IMPORT` are to import or export symbols, so as to make everything work
-nicely across boundaries.<br/>
+`ENTT_API_IMPORT` are there to import or export symbols, so as to make
+everything work nicely across boundaries.<br/>
 On the other hand, everything should run smoothly when working with plugins or
-shared libraries that don't export any symbols.
+shared libraries that do not export any symbols.
 
 For those who need more details, the test suite contains many examples covering
 the most common cases (see the `lib` directory for all details).<br/>
-It goes without saying that it's impossible to cover **all** possible cases.
+It goes without saying that it is impossible to cover **all** possible cases.
 However, what is offered should hopefully serve as a basis for all of them.
 
 ## Meta context
 
 The runtime reflection system deserves a special mention when it comes to using
 it across boundaries.<br/>
-Since it's linked already to a static context to which the elements are attached
-and different contexts don't relate to each other, they must be _shared_ to
-allow the use of meta types across boundaries.
+Since it is linked already to a static context to which the elements are
+attached and different contexts do not relate to each other, they must be
+_shared_ to allow the use of meta types across boundaries.
 
 Fortunately, sharing a context is also trivial to do. First of all, the local
 one is acquired in the main space:
@@ -53,16 +53,16 @@ one is acquired in the main space:
 auto handle = entt::locator<entt::meta_ctx>::handle();
 ```
 
-Then, it's passed to the receiving space that sets it as its default context,
+Then, it is passed to the receiving space that sets it as its default context,
 thus discarding or storing aside the local one:
 
 ```cpp
 entt::locator<entt::meta_ctx>::reset(handle);
 ```
 
-From now on, both spaces refer to the same context and on it are attached all
-new meta types, no matter where they are created.<br/>
-Note that _replacing_ the main context doesn't also propagate changes across
+From now on, both spaces refer to the same context and to it are all new meta
+types attached, no matter where they are created.<br/>
+Note that _replacing_ the main context does not also propagate changes across
 boundaries. In other words, replacing a context results in the decoupling of the
 two sides and therefore a divergence in the contents.
 
@@ -81,9 +81,10 @@ is unknown to the former, a dedicated pool is created within the registry on
 first use.<br/>
 As one can guess, this pool is instantiated on a different side of the boundary
 from the `registry`. Therefore, the instance is now managing memory from
-different spaces and this can quickly lead to crashes if not properly addressed.
+different spaces, and this can quickly lead to crashes if not properly
+addressed.
 
-To overcome the risk, it's recommended to use well-defined interfaces that make
+To overcome the risk, it is recommended to use well-defined interfaces that make
 fundamental types pass through the boundaries, isolating the instances of the
 `EnTT` classes from time to time and as appropriate.<br/>
 Refer to the test suite for some examples, read the documentation available

docs/md/links.md

@@ -14,16 +14,16 @@
 `EnTT` is widely used in private and commercial applications. I cannot even
 mention most of them because of some signatures I put on some documents time
 ago. Fortunately, there are also people who took the time to implement open
-source projects based on `EnTT` and didn't hold back when it came to documenting
-them.
+source projects based on `EnTT` and did not hold back when it came to
+documenting them.
 
 Below an incomplete list of games, applications and articles that can be used as
 a reference.<br/>
 Where I put the word _apparently_ means that the use of `EnTT` is documented but
-the authors didn't make explicit announcements or contacted me directly.
+the authors did not make explicit announcements or contacted me directly.
 
 If you know of other resources out there that are about `EnTT`, feel free to
-open an issue or a PR and I'll be glad to add them to this page.<br/>
+open an issue or a PR. I will be glad to add them to this page.<br/>
 I hope the following lists can grow much more in the future.
 
 # EnTT in Action
@@ -48,7 +48,7 @@ I hope the following lists can grow much more in the future.
   * [Apparently](https://www.youtube.com/watch?v=P8xvOA3ikrQ&t=1105s)
     [Call of Duty: Vanguard](https://www.callofduty.com/vanguard) by
     [Sledgehammer Games](https://www.sledgehammergames.com/): I can neither
-    confirm nor deny but there is a license I know in the credits.
+    confirm nor deny, but there is a license I know in the credits.
   * Apparently [D&D Dark Alliance](https://darkalliance.wizards.com) by
     [Wizards of the Coast](https://company.wizards.com): your party, their
     funeral.
@@ -97,20 +97,20 @@ I hope the following lists can grow much more in the future.
     by Quake.
   * [Destroid](https://github.com/tyrannicaltoucan/destroid): _one-bazillionth_
     arcade game about shooting dirty rocks in space, inspired by Asteroids.
-  * [Wanderer](https://github.com/albin-johansson/wanderer): a 2D exploration
-    based indie game.
-  * [Spelunky® Classic remake](https://github.com/dbeef/spelunky-psp): a truly
+  * [Wanderer](https://github.com/albin-johansson/wanderer): a 2D
+    exploration-based indie game.
+  * [Spelunky® Classic remake](https://github.com/dbeef/spelunky-psp): a truly
     multiplatform experience with a rewrite from scratch.
   * [CubbyTower](https://github.com/utilForever/CubbyTower): a simple tower
     defense game using C++ with Entity Component System (ECS).
   * [Runeterra](https://github.com/utilForever/Runeterra): Legends of Runeterra
     simulator using C++ with some reinforcement learning.
   * [Black Sun](https://store.steampowered.com/app/1670930/Black_Sun/): fly your
-    space ship through a large 2D open world.
-  * [PokeMaster](https://github.com/utilForever/PokeMaster): Pokemon Battle
+    spaceship through a large 2D open world.
+  * [PokeMaster](https://github.com/utilForever/PokeMaster): Pokémon Battle
     simulator using C++ with some reinforcement learning.
   * [HomeHearth](https://youtu.be/GrEWl8npL9Y): choose your hero, protect the
-    town, before it's too late.
+    town, before it is too late.
   * [City Builder Game](https://github.com/PhiGei2000/CityBuilderGame): a simple
     city-building game using C++ and OpenGL.
   * [BattleSub](https://github.com/bfeldpw/battlesub): two player 2D submarine
@@ -137,8 +137,18 @@ I hope the following lists can grow much more in the future.
     build your own car and go racing.
   * [Lichgate](https://buas.itch.io/lichgate): top-down action rogue-like where
     users unlock abilities to fight horde of enemies in an endless world.
-  * [Letalka](https://github.com/dviglo2d-learn/mini_games/tree/main/letalka):
+  * [Letalka](https://github.com/dviglo2d/dviglo2d/tree/main/games/letalka):
     small demo game with ships and bullets flying everywhere on the screen.
+  * [Lichgate](https://buas.itch.io/lichgate): step into the robes of a powerful
+    mage determined to halt the relentless hordes of undead.
+  * [You Are Circle](https://store.steampowered.com/app/3578190/You_Are_Circle/):
+    a roguelite top-down shooter with a high-contrast vector line aesthetic.
+  * [EnTT Dino](https://github.com/omgitsaheadcrab/entt_dino): a Dinosaur Game
+    clone in C++ using only `SDL2` and `EnTT`.
+  * [Bim!](https://github.com/j-jorge/bim): a last-man-standing arcade online
+    game for Android.
+  * [MonsterWar](https://github.com/WispSnow/MonsterWar): a tower defense game
+    developed in C++ with `SDL3`, `EnTT`, and a few other libraries.
 
 ## Engines and the like:
 
@@ -190,7 +200,7 @@ I hope the following lists can grow much more in the future.
     framework in C++17 for backend development.
   * [Unity/EnTT](https://github.com/TongTungGiang/unity-entt): tech demo of a
     native simulation layer using `EnTT` and `Unity` as a rendering engine.
-  * [OverEngine](https://github.com/OverShifted/OverEngine): an over-engineered
+  * [OverEngine](https://github.com/OverShifted/OverEngine): an overengineered
     game engine.
   * [Electro](https://github.com/Electro-Technologies/Electro): high performance
     3D game engine with a high emphasis on rendering.
@@ -199,13 +209,13 @@ I hope the following lists can grow much more in the future.
   * [Becketron](https://github.com/Doctor-Foxling/Becketron): a game engine
     written mostly in C++.
   * [Spatial Engine](https://github.com/luizgabriel/Spatial.Engine): a
-    cross-platform engine created on top of google's filament rendering engine.
+    cross-platform engine created on top of Google's filament rendering engine.
   * [Kaguya](https://github.com/KaiH0717/Kaguya): D3D12 Rendering Engine.
   * [OpenAWE](https://github.com/OpenAWE-Project/OpenAWE): open implementation
     of the Alan Wake Engine.
   * [Nazara Engine](https://github.com/DigitalPulseSoftware/NazaraEngine): fast,
     cross-platform, object-oriented API to help in daily developer life.
-  * [Billy Engine](https://github.com/billy4479/BillyEngine): some kind of a 2D
+  * [Billy Engine](https://github.com/billy4479/BillyEngine): some kind of 2D
     engine based on `SDL2` and `EnTT`.
   * [Ducktape](https://github.com/DucktapeEngine/Ducktape): an open source C++
     2D & 3D game engine that focuses on being fast and powerful.
@@ -234,6 +244,16 @@ I hope the following lists can grow much more in the future.
     API with CSS3.
   * [Coral Engine](https://github.com/GuusKemperman/CoralEngine): open-source
     student engine with the tools to make games in C++ and Visual scripting.
+  * [Star Engine](https://github.com/HODAKdev/StarEngine): an Advanced C++
+    DirectX 11 game engine.
+  * [Darmok](https://github.com/miguelibero/darmok): another C++ game engine.
+  * [Magique](https://github.com/gk646/magique): 2D game engine for programmers
+    (or those yet to be).
+  * [Physecs](https://github.com/thfProjects/Physecs): real-time 3D rigid body
+    physics simulation built on `EnTT`.
+  * [KODZA](https://gitlab.com/arqitek/kodza/): A work in progress game engine.
+  * [Omnix](https://github.com/Ace-codes-swift/Omnix): An under-development,
+    multi-purpose 3D engine for `macOS`, using `EnTT` for the ECS.
 
 ## Articles, videos and blog posts:
 
@@ -275,7 +295,7 @@ I hope the following lists can grow much more in the future.
     by [linkdd](https://github.com/linkdd): an interesting walkthrough of
     developing a game (also) with EnTT.
   * [Use EnTT When You Need An ECS](https://www.codingwiththomas.com/blog/use-entt-when-you-need-an-ecs)
-    by [Thomas](https://www.codingwiththomas.com/): I couldn't have said it
+    by [Thomas](https://www.codingwiththomas.com/): I could not have said it
     better.
   * [Space Battle: Huge edition](http://victor.madtriangles.com/code%20experiment/2018/06/11/post-ecs-battle-huge.html):
     huge space battle built entirely from scratch.
@@ -307,13 +327,16 @@ I hope the following lists can grow much more in the future.
 
   * [ArcGIS Runtime SDKs](https://developers.arcgis.com/arcgis-runtime/) by
     [Esri](https://www.esri.com/): they use `EnTT` for the internal ECS and the
-    cross platform C++ rendering engine. The SDKs are utilized by a lot of
+    cross-platform C++ rendering engine. The SDKs are used by a lot of
     enterprise custom apps, as well as by Esri for its own public applications
     such as
     [Explorer](https://play.google.com/store/apps/details?id=com.esri.explorer),
     [Collector](https://play.google.com/store/apps/details?id=com.esri.arcgis.collector)
     and
     [Navigator](https://play.google.com/store/apps/details?id=com.esri.navigator).
+  * [OneArc](https://onearc.com/): [licenses](https://onearc.com/third-party-licensors/)
+    do not lie. Their products use EnTT in some way, but it is not known _what_
+    way.
   * [FASTSUITE Edition 2](https://www.fastsuite.com/en_EN/fastsuite/fastsuite-edition-2.html)
     by [Cenit](http://www.cenit.com/en_EN/about-us/overview.html): they use
     `EnTT` to drive their simulation, that is, the communication between robot
@@ -323,7 +346,7 @@ I hope the following lists can grow much more in the future.
   * [Project Lagrange](https://github.com/adobe/lagrange): a robust geometry
     processing library by [Adobe](https://github.com/adobe).
   * [AtomicDEX](https://github.com/KomodoPlatform/atomicDEX-Desktop): a secure
-    wallet and non-custodial decentralized exchange rolled into one application.
+    wallet and noncustodial decentralized exchange rolled into one application.
   * [Apparently](https://www.linkedin.com/in/skypjack/)
     [NIO](https://www.nio.io/): there was a collaboration to make some changes
     to `EnTT`, at the time used for internal projects.

docs/md/locator.md

@@ -8,7 +8,7 @@
 
 # Introduction
 
-Usually, service locators are tightly bound to the services they expose and it's
+Usually, service locators are tightly bound to the services they expose. It is
 hard to define a general purpose solution.<br/>
 This tiny class tries to fill the gap and gets rid of the burden of defining a
 different specific locator for each application.
@@ -27,7 +27,7 @@ entt::locator<interface>::allocate_emplace<service>(allocator, argument);
 
 The difference is that the latter expects an allocator as the first argument and
 uses it to allocate the service itself.<br/>
-Once a service is set up, it's retrieved using the `value` function:
+Once a service is set up, it is retrieved using the `value` function:
 
 ```cpp
 interface &service = entt::locator<interface>::value();
@@ -45,17 +45,17 @@ if(entt::locator<interface>::has_value()) {
 interface &service = entt::locator<interface>::value_or<fallback_impl>(argument);
 ```
 
-All arguments are used only if necessary, that is, if a service doesn't already
+All arguments are used only if necessary, that is, if a service does not already
 exist and therefore the fallback service is constructed and returned. In all
 other cases, they are discarded.<br/>
 Finally, to reset a service, use the `reset` function.
 
 ## Opaque handles
 
-Sometimes it's useful to _transfer_ a copy of a service to another locator. For
-example, when working across boundaries it's common to _share_ a service with a
+Sometimes it is useful to _transfer_ a copy of a service to another locator. For
+example, when working across boundaries it is common to _share_ a service with a
 dynamically loaded module.<br/>
-Options aren't much in this case. Among these is the possibility of _exporting_
+Options are not much in this case. Among these is the possibility of _exporting_
 services and assigning them to a different locator.
 
 This is what the `handle` and `reset` functions are meant for.<br/>
@@ -69,14 +69,14 @@ auto handle = entt::locator<interface>::handle();
 entt::locator<interface>::reset(handle);
 ```
 
-It's worth noting that it's possible to get handles for uninitialized services
+It is worth noting that it is possible to get handles for uninitialized services
 and use them with other locators. Of course, all a user will get is to have an
 uninitialized service elsewhere as well.
 
 Note that exporting a service allows users to _share_ the object currently set
-in a locator. Replacing it won't replace the element even where a service has
-been configured with a handle to the previous item.<br/>
+in a locator. Replacing it will not replace the element, even where a service
+has been configured with a handle to the previous item.<br/>
 In other words, if an audio service is replaced with a null object to silence an
-application and the original service was shared, this operation won't propagate
-to the other locators. Therefore, a module that share the ownership of the
-original audio service is still able to emit sounds.
+application and the original service was shared, this operation will not
+propagate to the other locators. Therefore, a module that shares the ownership
+of the original audio service is still able to emit sounds.

docs/md/meta.md

@@ -3,10 +3,11 @@
 # Table of Contents
 
 * [Introduction](#introduction)
-* [Names and identifiers](#names-and-identifiers)
+* [Identifiers](#identifiers)
 * [Reflection in a nutshell](#reflection-in-a-nutshell)
   * [Any to the rescue](#any-to-the-rescue)
   * [Enjoy the runtime](#enjoy-the-runtime)
+    * [Tell me your name](#tell-me-your-name)
   * [Container support](#container-support)
   * [Pointer-like types](#pointer-like-types)
   * [Template information](#template-information)
@@ -26,36 +27,36 @@
 Reflection (or rather, its lack) is a trending topic in the C++ world and a tool
 that can unlock a lot of interesting features in the specific case of `EnTT`. I
 looked for a third-party library that met my needs on the subject, but I always
-came across some details that I didn't like: macros, being intrusive, too many
+came across some details that I did not like: macros, being intrusive, too many
 allocations, and so on.<br/>
 I finally decided to write a built-in, non-intrusive and macro-free runtime
-reflection system for `EnTT`. Maybe I didn't do better than others or maybe yes,
-time will tell me, but at least I can model this tool around the library to
+reflection system for `EnTT`. Maybe I did not do better than others or maybe
+yes, time will tell me, but at least I can model this tool around the library to
 which it belongs and not the opposite.
 
-# Names and identifiers
+# Identifiers
 
-The meta system doesn't force users to rely on the tools provided by the library
-when it comes to working with names and identifiers. It does this by offering an
-API that works with opaque identifiers that may or may not be generated by means
-of a hashed string.<br/>
+The meta system does not force users to rely on the tools provided by the
+library when it comes to working with identifiers. It does this by offering an
+API that works with integral values that may or may not be generated by means of
+a hashed string.<br/>
 This means that users can assign any type of identifier to the meta objects, as
-long as they're numeric. It doesn't matter if they're generated at runtime, at
-compile-time or with custom functions.
+long as they are numeric. It does not matter if they are generated at runtime,
+at compile-time or with custom functions.
 
-That being said, the examples in the following sections are all based on the
-`hashed_string` class as provided by this library. Therefore, where an
-identifier is required, it's likely that a user defined literal is used as
+That being said, some of the examples in the following sections are based on the
+`hashed_string` class as provided by this library. Therefore, where an integral
+identifier is provided, it is likely that a user defined literal is used as
 follows:
 
 ```cpp
-auto factory = entt::meta<my_type>().type("reflected_type"_hs);
+entt::meta_factory<my_type>{}.type("reflected_type"_hs);
 ```
 
-For what it's worth, this is completely equivalent to:
+For what it is worth, this is completely equivalent to:
 
 ```cpp
-auto factory = entt::meta<my_type>().type(42u);
+entt::meta_factory<my_type>{}.type(42u);
 ```
 
 Obviously, human-readable identifiers are more convenient to use and highly
@@ -65,10 +66,10 @@ recommended.
 
 Reflection always starts from actual C++ types. Users cannot reflect _imaginary_
 types.<br/>
-The `meta` function is where it all starts:
+The `meta_factory` class is where it all starts:
 
 ```cpp
-auto factory = entt::meta<my_type>();
+entt::meta_factory<my_type> factory{};
 ```
 
 The returned value is a _factory object_ to use to continue building the meta
@@ -76,70 +77,53 @@ type.
 
 By default, a meta type is associated with the identifier returned by the
 runtime type identification system built-in in `EnTT`.<br/>
-However, it's also possible to assign custom identifiers to meta types:
+However, it is also possible to assign custom identifiers to meta types:
 
 ```cpp
-auto factory = entt::meta<my_type>().type("reflected_type"_hs);
+entt::meta_factory<my_type>{}.type("reflected_type"_hs);
 ```
 
-Identifiers are used to _retrieve_ meta types at runtime by _name_ other than by
-type.<br/>
+Identifiers are used instead of the type to _retrieve_ meta types at runtime, if
+necessary.<br/>
 However, users can be interested in adding features to a reflected type so that
-the reflection system can use it correctly under the hood, while they don't want
-to also make the type _searchable_. In this case, it's sufficient not to invoke
-`type`.
+the reflection system can use it correctly under the hood, while they do not
+want to also make the type _searchable_. In this case, it is sufficient not to
+invoke `type`.
 
-A factory is such that all its member functions return the factory itself. It's
+A factory is such that all its member functions return the factory itself. It is
 generally used to create the following:
 
-* _Constructors_. A constructors is assigned to a reflected type by specifying
+* _Constructors_. A constructor is assigned to a reflected type by specifying
   its _list of arguments_. Free functions are also accepted if the return type
   is the expected one. From a client perspective, nothing changes between a free
   function or an actual constructor:
 
   ```cpp
-  entt::meta<my_type>().ctor<int, char>().ctor<&factory>();
+  entt::meta_factory<my_type>{}.ctor<int, char>().ctor<&factory>();
   ```
 
   Meta default constructors are implicitly generated, if possible.
 
-* _Destructors_. Both free functions and member functions are valid destructors:
-
-  ```cpp
-  entt::meta<my_type>().dtor<&destroy>();
-  ```
-
-  The purpose is to offer the possibility to free up resources that require
-  _special treatment_ before an object is actually destroyed.<br/>
-  A function should neither delete nor explicitly invoke the destructor of a
-  given instance.
-
 * _Data members_. Meta data members are actual data members of the underlying
   type but also static and global variables or constants of any kind. From the
   point of view of the client, all the variables associated with the reflected
   type appear as if they were part of the type itself:
 
   ```cpp
-  entt::meta<my_type>()
+  entt::meta_factory<my_type>{}
       .data<&my_type::static_variable>("static"_hs)
       .data<&my_type::data_member>("member"_hs)
       .data<&global_variable>("global"_hs);
   ```
 
   The `data` function requires the identifier to use for the meta data member.
-  Users can then access it by _name_ at runtime.<br/>
+  This is then used for runtime access.<br/>
   Data members are also defined by means of a setter and getter pair. These are
   either free functions, class members or a mix of them. This approach is also
   convenient to create read-only properties from a non-const data member:
 
   ```cpp
-  entt::meta<my_type>().data<nullptr, &my_type::data_member>("member"_hs);
-  ```
-
-  Multiple setters are also supported by means of a `value_list` object:
-
-  ```cpp
-  entt::meta<my_type>().data<entt::value_list<&from_int, &from_string>, &my_type::data_member>("member"_hs);
+  entt::meta_factory<my_type>{}.data<nullptr, &my_type::data_member>("member"_hs);
   ```
 
 * _Member functions_. Meta member functions are actual member functions of the
@@ -148,14 +132,14 @@ generally used to create the following:
   were part of the type itself:
 
   ```cpp
-  entt::meta<my_type>()
+  entt::meta_factory<my_type>{}
       .func<&my_type::static_function>("static"_hs)
       .func<&my_type::member_function>("member"_hs)
       .func<&free_function>("free"_hs);
   ```
 
   The `func` function requires the identifier to use for the meta data function.
-  Users can then access it by _name_ at runtime.<br/>
+  This is then used for runtime access.<br/>
   Overloading of meta functions is supported. Overloaded functions are resolved
   at runtime by the reflection system according to the types of the arguments.
 
@@ -163,7 +147,7 @@ generally used to create the following:
   derived from it:
 
   ```cpp
-  entt::meta<derived_type>().base<base_type>();
+  entt::meta_factory<derived_type>{}.base<base_type>();
   ```
 
   The reflection system tracks the relationship and allows for implicit casts at
@@ -174,7 +158,7 @@ generally used to create the following:
   that are implicitly performed by the reflection system when required:
 
   ```cpp
-  entt::meta<double>().conv<int>();
+  entt::meta_factory<double>{}.conv<int>();
   ```
 
 This is everything users need to create meta types. Refer to the inline
@@ -191,11 +175,12 @@ The API is very similar to that of the `any` type. The class `meta_any` _wraps_
 many of the feature to infer a meta node, before forwarding some or all of the
 arguments to the underlying storage.<br/>
 Among the few relevant differences, `meta_any` adds support for containers and
-pointer-like types, while `any` doesn't.<br/>
+pointer-like types, while `any` does not.<br/>
 Similar to `any`, this class is also used to create _aliases_ for unmanaged
 objects either with `forward_as_meta` or using the `std::in_place_type<T &>`
 disambiguation tag, as well as from an existing object by means of the `as_ref`
-member function.<br/>
+member function. Additionally, it can take ownership of pointers passed as
+arguments along with `std::in_place`.<br/>
 Unlike `any` instead, `meta_any` treats an empty instance and one initialized
 with `void` differently:
 
@@ -205,7 +190,7 @@ entt::meta_any other{std::in_place_type<void>};
 ```
 
 While `any` considers both as empty, `meta_any` treats objects initialized with
-`void` as if they were _valid_ ones. This allows to differentiate between failed
+`void` as if they were _valid_ ones. This allows differentiating between failed
 function calls and function calls that are successful but return nothing.
 
 Finally, the member functions `try_cast`, `cast` and `allow_cast` are used to
@@ -216,7 +201,7 @@ There is in fact no `any_cast` equivalent for `meta_any`.
 
 ## Enjoy the runtime
 
-Once the web of reflected types is constructed, it's a matter of using it at
+Once the web of reflected types is constructed, it is a matter of using it at
 runtime where required.<br/>
 There are a few options to search for a reflected type:
 
@@ -242,10 +227,10 @@ for(auto &&[id, type]: entt::resolve()) {
 ```
 
 In all cases, the returned value is an instance of `meta_type` (possibly with
-its id). This kind of objects offer an API to know their _runtime identifiers_,
-to iterate all the meta objects associated with them and even to build instances
-of the underlying type.<br/>
-Meta data members and functions are accessed by name:
+its id). These objects offer an API to know their _runtime identifiers_, to
+iterate all the meta objects associated with them and even to build instances of
+the underlying type.<br/>
+Meta data members and functions are accessed by means of their identifiers:
 
 * Meta data members:
 
@@ -256,7 +241,7 @@ Meta data members and functions are accessed by name:
   The returned type is `meta_data` and may be invalid if there is no meta data
   object associated with the given identifier.<br/>
   A meta data object offers an API to query the underlying type (for example, to
-  know if it's a const or a static one), to get the meta type of the variable
+  know if it is a const or a static one), to get the meta type of the variable
   and to set or get the contained value.
 
 * Meta function members:
@@ -268,11 +253,14 @@ Meta data members and functions are accessed by name:
   The returned type is `meta_func` and may be invalid if there is no meta
   function object associated with the given identifier.<br/>
   A meta function object offers an API to query the underlying type (for
-  example, to know if it's a const or a static function), to know the number of
+  example, to know if it is a const or a static function), to know the number of
   arguments, the meta return type and the meta types of the parameters. In
   addition, a meta function object is used to invoke the underlying function and
   then get the return value in the form of a `meta_any` object.
 
+Both functions search for the elements throughout the meta type hierarchy.
+However, they offer the option of passing a second boolean argument to stop the
+search at the top-level meta type.<br/>
 All the meta objects thus obtained as well as the meta types explicitly convert
 to a boolean value to check for validity:
 
@@ -282,9 +270,9 @@ if(auto func = entt::resolve<my_type>().func("member"_hs); func) {
 }
 ```
 
-Furthermore, all them (and a few more, like meta basis) are returned by a bunch
-of overloads that provide the caller with iterable ranges of top-level elements.
-As an example:
+Furthermore, all of them (and a few more, like meta basis) are returned by a
+bunch of overloads that provide the caller with iterable ranges of top-level
+elements. As an example:
 
 ```cpp
 for(auto &&[id, type]: entt::resolve<my_type>().base()) {
@@ -292,56 +280,82 @@ for(auto &&[id, type]: entt::resolve<my_type>().base()) {
 }
 ```
 
-Meta type are also used to `construct` actual instances of the underlying
+Meta types are also used to `construct` actual instances of the underlying
 type.<br/>
 In particular, the `construct` member function accepts a variable number of
 arguments and searches for a match. It then returns a `meta_any` object that may
 or may not be initialized, depending on whether a suitable constructor was found
-or not.
-
-There is no object that wraps the destructor of a meta type nor a `destroy`
-member function in its API. Destructors are invoked implicitly by `meta_any`
-behind the scenes and users have not to deal with them explicitly. Furthermore,
-they've no name, cannot be searched and wouldn't have member functions to expose
-anyway.<br/>
-Similarly, conversion functions aren't directly accessible. They're used
-internally by `meta_any` and the meta objects when needed.
+or not.<br/>
+Conversion functions are not accessible instead. They are used internally by
+`meta_any` and the meta objects when needed.
 
 Meta types and meta objects in general contain much more than what was said.
 Refer to the inline documentation for further details.
 
+### Tell me your name
+
+For meta types, data and functions, users can also provide custom _names_:
+
+```cpp
+entt::meta_factory<my_type>{}
+    .type("type"_hs, "my_type")
+    .data<&variable>("data"_hs, "variable")
+    .func<&function>("func"_hs, "function");
+```
+
+The _label_ provided **should** be a string literal. The library does not make
+copies. It is up to the user to guarantee the lifetime of the name itself.<br/>
+String identifiers are returned from the meta objects via the `name` function:
+
+```cpp
+const char *name = entt::resolve<my_type>().name();
+```
+
+Since most of the time there is no need to differentiate the _name_ from the
+numeric identifier associated with a meta object, `EnTT` also offers a more
+compact version of these functions:
+
+```cpp
+entt::meta_factory<my_type>{}
+    .type("my_type")
+    .data<&variable>("variable")
+    .func<&function>("function");
+```
+
+Again, the name provided **should** be a string literal. The string is then used
+to generate a numeric identifier with the `hashed_string` class.<br/>
+Despite support for names, there are no string-based lookup functions available.
+That is, types (`resolve`) as well as data members (`data`) and function members
+(`func`) are only _searchable_ by numeric identifiers.
+
 ## Container support
 
 The runtime reflection system also supports containers of all types.<br/>
-Moreover, _containers_ doesn't necessarily mean those offered by the C++
+Moreover, _containers_ does not necessarily mean those offered by the C++
 standard library. In fact, user defined data structures can also work with the
 meta system in many cases.
 
-To make a container be recognized as such by the meta system, users are required
-to provide specializations for either the `meta_sequence_container_traits` class
-or the `meta_associative_container_traits` class, according to the actual _type_
-of the container.<br/>
-`EnTT` already exports the specializations for some common classes. In
-particular:
+Containers are automatically _detected_ based on some common _traits_.<br/>
+For example, and not limited to, a sequence container must have a `begin`/`end`
+pair that returns a forward iterator, while an associative container must also
+provide a `key_type` member and a `find` function.<br/>
+If a container is not recognized as such, it is still possible to provide an
+_adapter_ by specializing the template classes `meta_sequence_container_traits`
+and `meta_associative_container_traits`. Similarly, users can _inhibit_ the
+detection of a type as a meta container by specializing the right traits class
+but without providing any definition.
 
-* `std::vector`, `std::array`, `std::deque` and `std::list` (but not
-  `std::forward_list`) are supported as _sequence containers_.
-
-* `std::map`, `std::set` and their unordered counterparts are supported as
-  _associative containers_.
-
-It's important to include the header file `container.hpp` to make these
-specializations available to the compiler when needed.<br/>
-The same file also contains many examples for the users that are interested in
+Standard library containers are generally exported as meta containers out of the
+box (with the exception of `std::string`, which is not considered a sequence
+container on purpose).<br/>
+However, it is important to include the header file `container.hpp` to make
+the right specializations available to the compiler when needed.<br/>
+The same file also contains some examples for the users that are interested in
 making their own containers available to the meta system.
 
-When a specialization of the `meta_sequence_container_traits` class exists, the
-meta system treats the wrapped type as a sequence container. In a similar way,
-a type is treated as an associative container if a specialization of the
-`meta_associative_container_traits` class is found for it.<br/>
-Proxy objects are returned by dedicated members of the `meta_any` class. The
-following is a deliberately verbose example of how users can access a proxy
-object for a sequence container:
+For meta containers, the `meta_any` class returns properly initialized proxy
+objects for ease of use. The following is a deliberately verbose example of how
+users can access a proxy object for a sequence container:
 
 ```cpp
 std::vector<int> vec{1, 2, 3};
@@ -354,14 +368,14 @@ if(any.type().is_sequence_container()) {
 }
 ```
 
-The method to use to get a proxy object for associative containers is
+Proxy object for associative containers are accessed in the same way by calling
 `as_associative_container` instead.<br/>
-It's not necessary to perform a double check actually. Instead, it's enough to
+It is not necessary to perform a double check actually. Instead, it is enough to
 query the meta type or verify that the proxy object is valid. In fact, proxies
 are contextually convertible to bool to check for validity. For example, invalid
-proxies are returned when the wrapped object isn't a container.<br/>
-In all cases, users aren't expected to _reflect_ containers explicitly. It's
-sufficient to assign a container for which a specialization of the traits
+proxies are returned when the wrapped object is not a container.<br/>
+In all cases, users are not expected to _reflect_ containers explicitly. It is
+sufficient to assign a container for which a specialization of the _traits_
 classes exists to a `meta_any` object to be able to get its proxy object.
 
 The interface of the `meta_sequence_container` proxy object is the same for all
@@ -373,17 +387,17 @@ to case. In particular:
 * The `size` member function returns the number of elements in the container as
   an unsigned integer value.
 
-* The `resize` member function allows to resize the wrapped container and
-  returns true in case of success.<br/>
-  For example, it's not possible to resize fixed size containers.
-
-* The `clear` member function allows to clear the wrapped container and returns
+* The `resize` member function allows resizing the wrapped container and returns
   true in case of success.<br/>
-  For example, it's not possible to clear fixed size containers.
+  For example, it is not possible to resize fixed size containers.
 
-* The `reserve` member function allows to increase the capacity of the wrapped
+* The `clear` member function allows clearing the wrapped container and returns
+  true in case of success.<br/>
+  For example, it is not possible to clear fixed size containers.
+
+* The `reserve` member function allows increasing the capacity of the wrapped
   container and returns true in case of success.<br/>
-  For example, it's not possible to increase capacity of fixed size containers.
+  For example, it is not possible to increase capacity of fixed size containers.
 
 * The `begin` and `end` member functions return opaque iterators that is used to
   iterate the container directly:
@@ -397,8 +411,8 @@ to case. In particular:
   In all cases, given an underlying container of type `C`, the returned element
   contains an object of type `C::value_type` which therefore depends on the
   actual container.<br/>
-  All meta iterators are input iterators and don't offer an indirection operator
-  on purpose.
+  All meta iterators are input iterators and do not offer an indirection
+  operator on purpose.
 
 * The `insert` member function is used to add elements to the container. It
   accepts a meta iterator and the element to insert:
@@ -412,7 +426,7 @@ to case. In particular:
   This function returns a meta iterator pointing to the inserted element and a
   boolean value to indicate whether the operation was successful or not. A call
   to `insert` may silently fail in case of fixed size containers or whether the
-  arguments aren't at least convertible to the required types.<br/>
+  arguments are not at least convertible to the required types.<br/>
   Since meta iterators are contextually convertible to bool, users can rely on
   them to know if the operation failed on the actual container or upstream, for
   example due to an argument conversion problem.
@@ -469,12 +483,12 @@ differences in behavior in the case of key-only containers. In particular:
 * The `size` member function returns the number of elements in the container as
   an unsigned integer value.
 
-* The `clear` member function allows to clear the wrapped container and returns
+* The `clear` member function allows clearing the wrapped container and returns
   true in case of success.
 
-* The `reserve` member function allows to increase the capacity of the wrapped
+* The `reserve` member function allows increasing the capacity of the wrapped
   container and returns true in case of success.<br/>
-  For example, it's not possible to increase capacity of standard maps.
+  For example, it is not possible to increase capacity of standard maps.
 
 * The `begin` and `end` member functions return opaque iterators that are used
   to iterate the container directly:
@@ -487,10 +501,10 @@ differences in behavior in the case of key-only containers. In particular:
 
   In all cases, given an underlying container of type `C`, the returned element
   is a key-value pair where the key has type `C::key_type` and the value has
-  type `C::mapped_type`. Since key-only containers don't have a mapped type,
+  type `C::mapped_type`. Since key-only containers do not have a mapped type,
   their _value_ is nothing more than an invalid `meta_any` object.<br/>
-  All meta iterators are input iterators and don't offer an indirection operator
-  on purpose.
+  All meta iterators are input iterators and do not offer an indirection
+  operator on purpose.
 
   While the accessed key is usually constant in the associative containers and
   is therefore returned by copy, the value (if any) is wrapped by an instance of
@@ -498,7 +512,7 @@ differences in behavior in the case of key-only containers. In particular:
   modifies the element inside the container.
 
 * The `insert` member function is used to add elements to a container. It gets
-  two arguments, respectively the key and the value to insert:
+  two arguments, the key and the value to insert:
 
   ```cpp
   auto last = view.end();
@@ -507,7 +521,7 @@ differences in behavior in the case of key-only containers. In particular:
   ```
 
   This function returns a boolean value to indicate whether the operation was
-  successful or not. A call to `insert` may fail when the arguments aren't at
+  successful or not. A call to `insert` may fail when the arguments are not at
   least convertible to the required types.
 
 * The `erase` member function is used to remove elements from a container. It
@@ -518,8 +532,8 @@ differences in behavior in the case of key-only containers. In particular:
   ```
 
   This function returns a boolean value to indicate whether the operation was
-  successful or not. A call to `erase` may fail when the argument isn't at least
-  convertible to the required type.
+  successful or not. A call to `erase` may fail when the argument is not at
+  least convertible to the required type.
 
 * The `operator[]` is used to access elements in a container. It gets a single
   argument, the key of the element to return:
@@ -536,7 +550,7 @@ Container support is minimal but likely sufficient to satisfy all needs.
 
 ## Pointer-like types
 
-As with containers, it's also possible to _tell_ to the meta system which types
+As with containers, it is also possible to _tell_ to the meta system which types
 are _pointers_. This makes it possible to dereference instances of `meta_any`,
 thus obtaining light _references_ to pointed objects that are also correctly
 associated with their meta types.<br/>
@@ -546,13 +560,21 @@ some common classes. In particular:
 
 * All types of raw pointers.
 * `std::unique_ptr` and `std::shared_ptr`.
+* All classes that _export_ a type member called `is_meta_pointer_like`:
+  ```cpp
+  struct smart_pointer {
+      using is_meta_pointer_like = void;
+      // ...
+  };
+  ```
+  The actual type is irrelevant and will not be used in any way.
 
-It's important to include the header file `pointer.hpp` to make these
+It is important to include the header file `pointer.hpp` to make these
 specializations available to the compiler when needed.<br/>
 The same file also contains many examples for the users that are interested in
 making their own pointer-like types available to the meta system.
 
-When a type is recognized as a pointer-like one by the meta system, it's
+When a type is recognized as a pointer-like one by the meta system, it is
 possible to dereference the instances of `meta_any` that contain these objects.
 The following is a deliberately verbose example to show how to use this feature:
 
@@ -569,18 +591,18 @@ if(any.type().is_pointer_like()) {
 }
 ```
 
-It's not necessary to perform a double check. Instead, it's enough to query the
-meta type or verify that the returned object is valid. For example, invalid
-instances are returned when the wrapped object isn't a pointer-like type.<br/>
+It is not necessary to perform a double check. Instead, it is enough to query
+the meta type or verify that the returned object is valid. For example, invalid
+instances are returned when the wrapped object is not a pointer-like type.<br/>
 Dereferencing a pointer-like object returns an instance of `meta_any` which
 _refers_ to the pointed object. Modifying it means modifying the pointed object
 directly (unless the returned element is const).
 
 In general, _dereferencing_ a pointer-like type boils down to a `*ptr`. However,
-`EnTT` also supports classes that don't offer an `operator*`. In particular:
+`EnTT` also supports classes that do not offer an `operator*`. In particular:
 
-* It's possible to exploit a solution based on ADL lookup by offering a function
-  (also a template one) named `dereference_meta_pointer_like`:
+* It is possible to exploit a solution based on ADL lookup by implementing a
+  function (also a template one) named `dereference_meta_pointer_like`:
 
   ```cpp
   template<typename Type>
@@ -589,7 +611,7 @@ In general, _dereferencing_ a pointer-like type boils down to a `*ptr`. However,
   }
   ```
 
-* When not in control of the type's namespace, it's possible to inject into the
+* When not in control of the type's namespace, it is possible to inject into the
   `entt` namespace a specialization of the `adl_meta_pointer_like` class
   template to bypass the adl lookup as a whole:
 
@@ -608,12 +630,12 @@ of the pointed type, no user intervention is required.
 ## Template information
 
 Meta types also provide a minimal set of information about the _nature_ of the
-original type in case it's a class template.<br/>
-By default, this works out of the box and requires no user action. However, it's
-important to include the header file `template.hpp` to make this information
+original type in case it is a class template.<br/>
+By default, this works out of the box and requires no user action. However, it
+is important to include the header file `template.hpp` to make this information
 available to the compiler when needed.
 
-Meta template information are easily found:
+Meta template information is easily found:
 
 ```cpp
 // this method returns true if the type is recognized as a class template specialization
@@ -629,7 +651,7 @@ if(auto type = entt::resolve<std::shared_ptr<my_type>>(); type.is_template_speci
 }
 ```
 
-Typically, when template information for a type are required, what the library
+Typically, when template information for a type is required, what the library
 provides is sufficient. However, there are some cases where a user may want more
 details or a different set of information.<br/>
 Consider the case of a class template that is meant to wrap function types:
@@ -656,8 +678,8 @@ struct entt::meta_template_traits<function_type<Ret(Args...)>> {
 };
 ```
 
-The reflection system doesn't verify the accuracy of the information nor infer a
-correspondence between real types and meta types.<br/>
+The reflection system does not verify the accuracy of the information nor infer
+a correspondence between real types and meta types.<br/>
 Therefore, the specialization is used as is and the information it contains is
 associated with the appropriate type when required.
 
@@ -669,7 +691,7 @@ If this were to be translated into explicit registrations with the reflection
 system, it would result in a long series of instructions such as the following:
 
 ```cpp
-entt::meta<int>()
+entt::meta_factory<int>{}
     .conv<bool>()
     .conv<char>()
     // ...
@@ -683,7 +705,7 @@ underlying types and offers what it takes to do the same for scoped enums. It
 would result in the following if it were to be done explicitly:
 
 ```cpp
-entt::meta<my_enum>()
+entt::meta_factory<my_enum>{}
     .conv<std::underlying_type_t<my_enum>>();
 ```
 
@@ -708,8 +730,8 @@ int value = any.cast<int>();
 
 This makes working with arithmetic types and scoped or unscoped enums as easy as
 it is in C++.<br/>
-It's still possible to set up conversion functions manually and these are always
-preferred over the automatic ones.
+It is still possible to set up conversion functions manually, and these are
+always preferred over the automatic ones.
 
 ## Implicitly generated default constructor
 
@@ -722,13 +744,13 @@ them.
 For default constructible types only, default constructors are automatically
 defined and associated with their meta types, whether they are explicitly or
 implicitly generated.<br/>
-Therefore, this is all is needed to construct an integer from its meta type:
+Therefore, this is all needed to construct an integer from its meta type:
 
 ```cpp
 entt::resolve<int>().construct();
 ```
 
-Where the meta type is for example the one returned from a meta container,
+Where the meta type is, for example, the one returned from a meta container,
 useful for building keys without knowing or having to register the actual types.
 
 In all cases, when users register default constructors, they are preferred both
@@ -746,7 +768,7 @@ designed to convert an opaque pointer into a `meta_any`:
 entt::meta_any any = entt::resolve(id).from_void(element);
 ```
 
-Unfortunately, it's not possible to do a check on the actual type. Therefore,
+Unfortunately, it is not possible to do a check on the actual type. Therefore,
 this call can be considered as a _static cast_ with all its _problems_.<br/>
 On the other hand, the ability to construct a `meta_any` from an opaque pointer
 opens the door to some pretty interesting uses that are worth exploring.
@@ -759,14 +781,14 @@ Their purpose is to require slightly different behavior than the default in some
 specific cases. For example, when reading a given data member, its value is
 returned wrapped in a `meta_any` object which, by default, makes a copy of it.
 For large objects or if the caller wants to access the original instance, this
-behavior isn't desirable. Policies are there to offer a solution to this and
+behavior is not desirable. Policies are there to offer a solution to this and
 other problems.
 
 There are a few alternatives available at the moment:
 
-* The _as-is_ policy, associated with the type `entt::as_is_t`.<br/>
+* The _as-value_ policy, associated with the type `entt::as_value_t`.<br/>
   This is the default policy. In general, it should never be used explicitly,
-  since it's implicitly selected if no other policy is specified.<br/>
+  since it is implicitly selected if no other policy is specified.<br/>
   In this case, the return values of the functions as well as the properties
   exposed as data members are always returned by copy in a dedicated wrapper and
   therefore associated with their original meta types.
@@ -776,13 +798,13 @@ There are a few alternatives available at the moment:
   thus making it appear as if its type were `void`:
 
   ```cpp
-  entt::meta<my_type>().func<&my_type::member_function, entt::as_void_t>("member"_hs);
+  entt::meta_factory<my_type>{}.func<&my_type::member_function, entt::as_void_t>("member"_hs);
   ```
 
   If the use with functions is obvious, perhaps less so is use with constructors
   and data members. In the first case, the returned wrapper is always empty even
   though the constructor is still invoked. In the second case, the property
-  isn't accessible for reading instead.
+  is not accessible for reading instead.
 
 * The _as-ref_ and _as-cref_ policies, associated with the types
   `entt::as_ref_t` and `entt::as_cref_t`.<br/>
@@ -792,7 +814,7 @@ There are a few alternatives available at the moment:
   the wrapper itself:
 
   ```cpp
-  entt::meta<my_type>().data<&my_type::data_member, entt::as_ref_t>("member"_hs);
+  entt::meta_factory<my_type>{}.data<&my_type::data_member, entt::as_ref_t>("member"_hs);
   ```
 
   These policies work with constructors (for example, when objects are taken
@@ -802,7 +824,19 @@ There are a few alternatives available at the moment:
   `as_ref_t` _adapts_ to the constness of the passed object and to that of the
   return type if any.
 
-Some uses are rather trivial, but it's useful to note that there are some less
+* The _as-is_ policy, associated with the type `entt::as_is_t`.<br/>
+  Useful for decoupling meta type creation code from calling code while still
+  preserving the behavior of data members and member functions as defined:
+
+  ```cpp
+  entt::meta_factory<my_type>{}.func<&my_type::any_member, entt::as_is_t>("member"_hs);
+  ```
+
+  For data members or member functions that return a reference type, the value
+  is returned by reference with the same constness. In all other cases, the
+  value is returned by copy.
+
+Some uses are rather trivial, but it is useful to note that there are some less
 obvious corner cases that can in turn be solved with the use of policies.
 
 ## Named constants and enums
@@ -819,15 +853,15 @@ between enums and classes in C++ directly in the space of the reflected types.
 Exposing constant values or elements from an enum is quite simple:
 
 ```cpp
-entt::meta<my_enum>()
+entt::meta_factory<my_enum>{}
     .data<my_enum::a_value>("a_value"_hs)
     .data<my_enum::another_value>("another_value"_hs);
 
-entt::meta<int>().data<2048>("max_int"_hs);
+entt::meta_factory<int>{}.data<2048>("max_int"_hs);
 ```
 
-Accessing them is trivial as well. It's a matter of doing the following, as with
-any other data member of a meta type:
+Accessing them is trivial as well. It is a matter of doing the following, as
+with any other data member of a meta type:
 
 ```cpp
 auto value = entt::resolve<my_enum>().data("a_value"_hs).get({}).cast<my_enum>();
@@ -859,20 +893,17 @@ and meta functions.
 User-defined traits are set via a meta factory:
 
 ```cpp
-entt::meta<my_type>().traits(my_traits::required | my_traits::hidden);
+entt::meta_factory<my_type>{}.traits(my_traits::required | my_traits::hidden);
 ```
 
-In the example above, `EnTT` bitmask enum support is used but any integral value
-is fine, as long as it doesn't exceed 16 bits.<br/>
-It's not possible to assign traits at different times. Therefore, multiple calls
-to the `traits` function overwrite previous values. However, traits can be read
-from meta objects and used to update existing data with a factory, effectively
-extending them as needed.<br/>
-Likewise, users can also set traits on meta objects later if needed, as long as
-the factory is reset to the meta object of interest:
+In the example above, `EnTT` bitmask enum support is used, but any integral
+value is fine, as long as it does not exceed 16 bits.<br/>
+Traits can be assigned at different times. Subsequent calls to the `traits`
+function do not reset previously set values. However, users must reset the
+factory to the meta object of interest:
 
 ```cpp
-entt::meta<my_type>()
+entt::meta_factory<my_type>{}
     .data<&my_type::data_member, entt::as_ref_t>("member"_hs)
     .traits(my_traits::internal);
 ```
@@ -893,12 +924,12 @@ correctly.
 Custom arbitrary data are set via a meta factory:
 
 ```cpp
-entt::meta<my_type>().custom<type_data>("name");
+entt::meta_factory<my_type>{}.custom<type_data>("name");
 ```
 
 The way to do this is by specifying the data type to the `custom` function and
 passing the necessary arguments to construct it correctly.<br/>
-It's not possible to assign custom data at different times. Therefore, multiple
+It is not possible to assign custom data at different times. Therefore, multiple
 calls to the `custom` function overwrite previous values. However, this value
 can be read from meta objects and used to update existing data with a factory,
 effectively updating them as needed.<br/>
@@ -906,7 +937,7 @@ Likewise, users can also set custom data on meta objects later if needed, as
 long as the factory is reset to the meta object of interest:
 
 ```cpp
-entt::meta<my_type>()
+entt::meta_factory<my_type>{}
     .func<&my_type::member_function>("member"_hs)
     .custom<function_data>("tooltip");
 ```
@@ -927,7 +958,7 @@ null pointer is returned to inform the user of the failed attempt.
 
 A type registered with the reflection system can also be _unregistered_. This
 means unregistering all its data members, member functions, conversion functions
-and so on. However, base classes aren't unregistered as well, since they don't
+and so on. However, base classes are not unregistered as well, since they do not
 necessarily depend on it.<br/>
 Roughly speaking, unregistering a type means disconnecting all associated meta
 objects from it and making its identifier no longer available:
@@ -936,14 +967,14 @@ objects from it and making its identifier no longer available:
 entt::meta_reset<my_type>();
 ```
 
-It's also possible to reset types by their unique identifiers:
+It is also possible to reset types by their unique identifiers:
 
 ```cpp
 entt::meta_reset("my_type"_hs);
 ```
 
 Finally, there exists a non-template overload of the `meta_reset` function that
-doesn't accept arguments and resets all meta types at once:
+does not accept arguments and resets all meta types at once:
 
 ```cpp
 entt::meta_reset();
@@ -960,7 +991,7 @@ _context_. This is obtained via a service locator as:
 auto &&context = entt::locator<entt::meta_context>::value_or();
 ```
 
-By itself, a context is an opaque object that the user cannot do much with.
+By itself, a context is an opaque object that the user can do little with.
 However, users can replace an existing context with another at any time:
 
 ```cpp
@@ -970,20 +1001,20 @@ std::swap(context, other);
 ```
 
 This is useful for testing purposes or to define multiple context objects with
-different meta type to use as appropriate.
+different meta types to use as appropriate.
 
-If _replacing_ the default context isn't enough, `EnTT` also offers the ability
+If _replacing_ the default context is not enough, `EnTT` also offers the ability
 to use multiple and externally managed contexts with the runtime reflection
 system.<br/>
 For example, to create new meta types within a context other than the default
-one, simply pass it as an argument to the `meta` call:
+one, simply pass it as an argument to the `meta_factory` constructor:
 
 ```cpp
 entt::meta_ctx context{};
-auto factory = entt::meta<my_type>(context).type("reflected_type"_hs);
+entt::meta_factory<my_type>{context}.type("reflected_type"_hs);
 ```
 
-By doing so, the new meta type isn't available in the default context but is
+By doing so, the new meta type is not available in the default context but is
 usable by passing around the new context when needed, such as when creating a
 new `meta_any` object:
 
@@ -992,17 +1023,17 @@ entt::meta_any any{context, std::in_place_type<my_type>};
 ```
 
 Similarly, to search for meta types in a context other than the default one,
-it's necessary to pass it to the `resolve` function:
+it is necessary to pass it to the `resolve` function:
 
 ```cpp
 entt::meta_type type = entt::resolve(context, "reflected_type"_hs)
 ```
 
-More generally, when using externally managed contexts, it's always required to
+More generally, when using externally managed contexts, it is always required to
 provide the system with the context to use, at least at the _entry point_.<br/>
-For example, once the `meta_type` instant is obtained, it's no longer necessary
+For example, once the `meta_type` instant is obtained, it is no longer necessary
 to pass the context around as the meta type takes the information with it and
 eventually propagates it to all its parts.<br/>
-On the other hand, it's necessary to instruct the library on where meta types
+On the other hand, it is necessary to instruct the library on where meta types
 are to be fetched when `meta_any`s and `meta_handle`s are constructed, a factory
 created or a meta type resolved.

docs/md/poly.md

@@ -45,12 +45,12 @@ The ones that I like more are:
   object wrapper.
 
 The former is admittedly an experimental library, with many interesting ideas.
-I've some doubts about the usefulness of some feature in real world projects,
+I have some doubts about the usefulness of some features in real world projects,
 but perhaps my lack of experience comes into play here. In my opinion, its only
-flaw is the API which I find slightly more cumbersome than other solutions.<br/>
-The latter was undoubtedly a source of inspiration for this module, although I
+flaw is the API that I find slightly more cumbersome than other solutions.<br/>
+The latter was undoubtedly a source of inspiration for this module. Although I
 opted for different choices in the implementation of both the final API and some
-feature.
+features.
 
 Either way, the authors are gurus of the C++ community, people I only have to
 learn from.
@@ -63,7 +63,7 @@ types will have to adhere to.<br/>
 For this purpose, the library offers a single class that supports both deduced
 and fully defined interfaces. Although having interfaces deduced automatically
 is convenient and allows users to write less code in most cases, it has some
-limitations and it's therefore useful to be able to get around the deduction by
+limitations. It is therefore useful to be able to get around the deduction by
 providing a custom definition for the static virtual table.
 
 Once the interface is defined, a generic implementation is needed to fulfill the
@@ -86,7 +86,7 @@ struct Drawable: entt::type_list<> {
 };
 ```
 
-It's recognizable by the fact that it inherits from an empty type list.<br/>
+It is recognizable by the fact that it inherits from an empty type list.<br/>
 Functions can also be const, accept any number of parameters and return a type
 other than `void`:
 
@@ -104,7 +104,7 @@ struct Drawable: entt::type_list<> {
 In this case, all parameters are passed to `invoke` after the reference to
 `this` and the return value is whatever the internal call returns.<br/>
 As for `invoke`, this is a name that is injected into the _concept_ through
-`Base`, from which one must necessarily inherit. Since it's also a dependent
+`Base`, from which one must necessarily inherit. Since it is also a dependent
 name, the `this-> template` form is unfortunately necessary due to the rules of
 the language. However, there also exists an alternative that goes through an
 external call:
@@ -171,9 +171,9 @@ If the concept exposes a member function called `draw` with function type
 * Or through a lambda that makes use of existing member functions from the
   interface itself.
 
-In other words, it's not possible to make use of functions not belonging to the
-interface, even if they're part of the types that fulfill the concept.<br/>
-Similarly, it's not possible to deduce a function in the static virtual table
+In other words, it is not possible to make use of functions not belonging to the
+interface, even if they are part of the types that fulfill the concept.<br/>
+Similarly, it is not possible to deduce a function in the static virtual table
 with a function type different from that of the associated member function in
 the interface itself.
 
@@ -182,7 +182,7 @@ allows maximum flexibility when providing the implementation for a concept.
 
 ## Fulfill a concept
 
-The `impl` alias template of a concept is used to define how it's fulfilled:
+The `impl` alias template of a concept is used to define how it is fulfilled:
 
 ```cpp
 struct Drawable: entt::type_list<> {
@@ -193,7 +193,7 @@ struct Drawable: entt::type_list<> {
 };
 ```
 
-In this case, it's stated that the `draw` method of a generic type is enough to
+In this case, it is stated that the `draw` method of a generic type is enough to
 satisfy the requirements of the `Drawable` concept.<br/>
 Both member functions and free functions are supported to fulfill concepts:
 
@@ -211,9 +211,9 @@ struct Drawable: entt::type_list<void()> {
 
 Likewise, as long as the parameter types and return type support conversions to
 and from those of the function type referenced in the static virtual table, the
-actual implementation may differ in its function type since it's erased
+actual implementation may differ in its function type since it is erased
 internally.<br/>
-Moreover, the `self` parameter isn't strictly required by the system and can be
+Moreover, the `self` parameter is not strictly required by the system and can be
 left out for free functions if not required.
 
 Refer to the inline documentation for more details.
@@ -221,7 +221,7 @@ Refer to the inline documentation for more details.
 # Inheritance
 
 _Concept inheritance_ is straightforward due to how poly looks like in `EnTT`.
-Therefore, it's quite easy to build hierarchies of concepts if necessary.<br/>
+Therefore, it is quite easy to build hierarchies of concepts if necessary.<br/>
 The only constraint is that all concepts in a hierarchy must belong to the same
 _family_, that is, they must be either all deduced or all defined.
 
@@ -230,14 +230,14 @@ For a deduced concept, inheritance is achieved in a few steps:
 ```cpp
 struct DrawableAndErasable: entt::type_list<> {
     template<typename Base>
-    struct type: typename Drawable::type<Base> {
+    struct type: Drawable::type<Base> {
         static constexpr auto base = Drawable::impl<Drawable::type<entt::poly_inspector>>::size;
         void erase() { entt::poly_call<base + 0>(*this); }
     };
 
     template<typename Type>
     using impl = entt::value_list_cat_t<
-        typename Drawable::impl<Type>,
+        Drawable::impl<Type>,
         entt::value_list<&Type::erase>
     >;
 };
@@ -253,15 +253,15 @@ in appending the new functions to the previous list.
 
 As for a defined concept instead, the list of types is _extended_ in a similar
 way to what is shown for the implementation of the above concept.<br/>
-To do this, it's useful to declare a function that allows to convert a _concept_
-into its underlying `type_list` object:
+To do this, it is useful to declare a function that allows to convert a
+_concept_ into its underlying `type_list` object:
 
 ```cpp
 template<typename... Type>
 entt::type_list<Type...> as_type_list(const entt::type_list<Type...> &);
 ```
 
-The definition isn't strictly required, since the function is only used through
+The definition is not strictly required, since the function is only used through
 a `decltype` as it follows:
 
 ```cpp
@@ -278,7 +278,7 @@ Everything else is the same as already shown instead.
 
 # Static polymorphism in the wild
 
-Once the _concept_ and implementation are defined, it's possible to use the
+Once the _concept_ and implementation are defined, it is possible to use the
 `poly` class template to _wrap_ instances that meet the requirements:
 
 ```cpp
@@ -312,15 +312,16 @@ circle shape;
 drawable instance{std::in_place_type<circle &>, shape};
 ```
 
-Similarly, it's possible to create non-owning copies of `poly` from an existing
+Similarly, it is possible to create non-owning copies of `poly` from an existing
 object:
 
 ```cpp
 drawable other = instance.as_ref();
 ```
 
-In both cases, although the interface of the `poly` object doesn't change, it
-doesn't construct any element or take care of destroying the referenced objects.
+In both cases, although the interface of the `poly` object does not change, it
+does not construct any element or take care of destroying the referenced
+objects.
 
 Note also how the underlying concept is accessed via a call to `operator->` and
 not directly as `instance.draw()`.<br/>
@@ -342,9 +343,9 @@ entt::basic_poly<Drawable, sizeof(double[4]), alignof(double[4])>
 
 The default size is `sizeof(double[2])`, which seems like a good compromise
 between a buffer that is too large and one unable to hold anything larger than
-an integer. The alignment requirement is optional and by default such that it's
-the most stringent (the largest) for any object whose size is at most equal to
-the one provided.<br/>
-It's worth noting that providing a size of 0 (which is an accepted value in all
+an integer. The alignment requirement is optional, and by default such that it
+is the most stringent (the largest) for any object whose size is at most equal
+to the one provided.<br/>
+It is worth noting that providing a size of 0 (which is an accepted value in all
 respects) will force the system to dynamically allocate the contained objects in
 all cases.

docs/md/process.md

@@ -4,7 +4,8 @@
 
 * [Introduction](#introduction)
 * [The process](#the-process)
-  * [Adaptor](#adaptor)
+  * [Continuation](#continuation)
+  * [Shared process](#shared-process)
 * [The scheduler](#the-scheduler)
 
 # Introduction
@@ -17,27 +18,19 @@ to define and execute cooperative processes.
 
 # The process
 
-A typical task inherits from the `process` class template that stays true to the
-CRTP idiom. Moreover, derived classes specify what the intended type for elapsed
-times is.
+A typical task inherits from the `process` class template. Derived classes also
+specify what the intended type for elapsed times is.
 
-A process should expose publicly the following member functions whether needed
-(note that it isn't required to define a function unless the derived class wants
-to _override_ the default behavior):
+A process should implement the following member functions whether needed (note
+that it is not required to define a function unless the derived class wants to
+_override_ the default behavior):
 
 * `void update(Delta, void *);`
 
   This is invoked once per tick until a process is explicitly aborted or ends
-  either with or without errors. Even though it's not mandatory to declare this
-  member function, as a rule of thumb each process should at least define it to
-  work _properly_. The `void *` parameter is an opaque pointer to user data (if
-  any) forwarded directly to the process during an update.
-
-* `void init();`
-
-  This is invoked when the process joins the running queue of a scheduler. It
-  happens usually as soon as the process is attached to the scheduler if it's a
-  top level one, otherwise when it replaces its parent if it's a _continuation_.
+  either with or without errors. Each process should at least define it to work
+  _properly_. The `void *` parameter is an opaque pointer to user data (if any)
+  forwarded directly to the process during an update.
 
 * `void succeeded();`
 
@@ -52,22 +45,24 @@ to _override_ the default behavior):
 * `void aborted();`
 
   This is invoked only if a process is explicitly aborted. There is no guarantee
-  that it executes in the same tick, it depends solely on whether the process is
+  that it executes in the same tick. It depends solely on whether the process is
   aborted immediately or not.
 
-Derived classes can also change the internal state of a process by invoking
-`succeed` and `fail`, as well as `pause` and `unpause` the process itself.<br/>
-All these are protected member functions made available to manage the life cycle
-of a process from a derived class.
+A class can also change the state of a process by invoking `succeed` and `fail`,
+as well as `pause` and `unpause` the process itself.<br/>
+All these are public member functions made available to manage the life cycle of
+a process easily.
 
 Here is a minimal example for the sake of curiosity:
 
 ```cpp
-struct my_process: entt::process<my_process, std::uint32_t> {
-    using delta_type = std::uint32_t;
+struct my_process: entt::process {
+    using allocator_type = entt::process::allocator_type;
+    using delta_type = entt::process::delta_type;
 
-    my_process(delta_type delay)
-        : remaining{delay}
+    my_process(const allocator_type &allocator, delta_type delay)
+        : entt::process{allocator},
+          remaining{delay}
     {}
 
     void update(delta_type delta, void *) {
@@ -85,46 +80,62 @@ private:
 };
 ```
 
-## Adaptor
+## Continuation
 
-Lambdas and functors can't be used directly with a scheduler because they aren't
-properly defined processes with managed life cycles.<br/>
-This class helps in filling the gap and turning lambdas and functors into
-full-featured processes usable by a scheduler.
-
-The function call operator has a signature similar to the one of the `update`
-function of a process but for the fact that it receives two extra callbacks to
-invoke whenever a process terminates with success or with an error:
+A process may be followed by other processes upon successful termination.<br/>
+This pairing can be set up at creation time, keeping the processes conceptually
+separate from each other while still combining them at runtime:
 
 ```cpp
-void(Delta delta, void *data, auto succeed, auto fail);
+my_process process{};
+process.then<my_other_process>();
 ```
 
-Parameters have the following meaning:
+This approach allows processes to be developed in isolation and combined to
+define complex actions.<br/>
+For example, a delayed operation where a parent process (such as a timer)
+_schedules_ a child process (the deferred task) once the time is over.
 
-* `delta` is the elapsed time.
-* `data` is an opaque pointer to user data if any, `nullptr` otherwise.
-* `succeed` is a function to call when a process terminates with success.
-* `fail` is a function to call when a process terminates with errors.
+The `then` function also accepts lambdas, which are associated with a dedicated
+process internally:
 
-Both `succeed` and `fail` accept no parameters at all.
+```cpp
+process.then([](entt::process &proc, std::uint32_t delta, void *data) {
+    // ...
+})
+```
 
-Note that usually users shouldn't worry about creating adaptors at all. A
-scheduler creates them internally each and every time a lambda or a functor is
-used as a process.
+The lambda function is such that it accepts a reference to the process that
+manages it (to be able to terminate it, pause it and so on), plus the usual
+values also passed to the `update` function.
+
+## Shared process
+
+All processes inherit from `std::enable_shared_from_this` to allow sharing with
+the caller.<br/>
+The returned smart pointer was created using the allocator associated with the
+scheduler and therefore all its processes. This same allocator is available by
+invoking `get_allocator` on the process itself.
+
+As far as possible, sharing a process is not intended to allow the caller to
+manage it. This could actually compromise the proper functioning of the
+scheduler and the process itself.<br/>
+Rather, the purpose is to allow the callers to save a valid reference to the
+process, allowing them to intervene in its lifecycle through calls like `pause`
+and the like.
 
 # The scheduler
 
-A cooperative scheduler runs different processes and helps managing their life
+A cooperative scheduler runs different processes and helps manage their life
 cycles.
 
-Each process is invoked once per tick. If it terminates, it's removed
-automatically from the scheduler and it's never invoked again. Otherwise, it's
-a good candidate to run one more time the next tick.<br/>
+Each process is invoked once per tick. If it terminates, it is removed
+automatically from the scheduler, and it is never invoked again. Otherwise,
+it is a good candidate to run one more time the next tick.<br/>
 A process can also have a _child_. In this case, the parent process is replaced
 with its child when it terminates and only if it returns with success. In case
-of errors, both the parent process and its child are discarded. This way, it's
-easy to create chain of processes to run sequentially.
+of errors, both the parent process and its child are discarded. This way, it is
+easy to create a _chain of processes_ to run sequentially.
 
 Using a scheduler is straightforward. To create it, users must provide only the
 type for the elapsed times and no arguments at all:
@@ -137,7 +148,7 @@ Otherwise, the `scheduler` alias is also available for the most common cases. It
 uses `std::uint32_t` as a default type:
 
 ```cpp
-entt::scheduler scheduler;
+entt::scheduler scheduler{};
 ```
 
 The class has member functions to query its internal data structures, like
@@ -154,34 +165,33 @@ entt::scheduler::size_type size = scheduler.size();
 scheduler.clear();
 ```
 
-To attach a process to a scheduler there are mainly two ways:
+To attach a process to a scheduler, invoke the `attach` function with a process
+type and the arguments to use to construct it:
 
-* If the process inherits from the `process` class template, it's enough to
-  indicate its type and submit all the parameters required to construct it to
-  the `attach` member function:
+```cpp
+scheduler.attach<my_process>(_1000u);
+```
 
-  ```cpp
-  scheduler.attach<my_process>(1000u);
-  ```
+The scheduler will also provide the process with its allocator as the first
+argument.<br>
+In case of lambdas or functors, the required signature is the one already seen
+for the `then` function of a process:
 
-* Otherwise, in case of a lambda or a functor, it's enough to provide an
-  instance of the class to the `attach` member function:
+```cpp
+scheduler.attach([](entt::process &, std::uint32_t, void *){ /* ... */ });
+```
 
-  ```cpp
-  scheduler.attach([](auto...){ /* ... */ });
-  ```
-
-In both cases, the scheduler is returned and its `then` member function can be
-used to create chains of processes to run sequentially.<br/>
+In both cases, the newly created process is returned by reference and its `then`
+member function is used to create chains of processes to run sequentially.<br/>
 As a minimal example of use:
 
 ```cpp
 // schedules a task in the form of a lambda function
-scheduler.attach([](auto delta, void *, auto succeed, auto fail) {
+scheduler.attach([](entt::process &, std::uint32_t, void *) {
     // ...
 })
 // appends a child in the form of another lambda function
-.then([](auto delta, void *, auto succeed, auto fail) {
+.then([](entt::process &, std::uint32_t, void *) {
     // ...
 })
 // appends a child in the form of a process class
@@ -209,3 +219,6 @@ scheduler.abort(true);
 // ... or gracefully during the next tick
 scheduler.abort();
 ```
+
+The argument passed to the `abort` function indicates whether execution should
+be stopped immediately or processes should be notified on the next tick.

docs/md/reference.md

@@ -14,13 +14,14 @@ Others developed different architectures from scratch and therefore offer
 alternative solutions with their pros and cons.
 
 If you know of other similar projects out there, feel free to open an issue or a
-PR and I'll be glad to add them to this page.<br/>
+PR, and I will be glad to add them to this page.<br/>
 I hope the following lists can grow much more in the future.
 
 # Similar projects
 
-Below an incomplete list of similar projects that I've come across so far.<br/>
-If some terms or designs aren't clear, I recommend referring to the
+Below an incomplete list of similar projects that I have come across so
+far.<br/>
+If some terms or designs are not clear, I recommend referring to the
 [_ECS Back and Forth_](https://skypjack.github.io/tags/#ecs) series for all the
 details.
 
@@ -34,12 +35,12 @@ details.
   * [lent](https://github.com/nem0/lent): the Donald Trump of the ECS libraries.
 
 * C++:
-  * [decs](https://github.com/vblanco20-1/decs): a chunk based archetype ECS.
+  * [decs](https://github.com/vblanco20-1/decs): a chunk-based archetype ECS.
   * [ecst](https://github.com/SuperV1234/ecst): a multithreaded compile-time
     ECS that uses sparse sets to keep track of entities in systems.
   * [EntityX](https://github.com/alecthomas/entityx): a bitset based ECS that
     uses a single large matrix of components indexed with entities.
-  * [Gaia-ECS](https://github.com/richardbiely/gaia-ecs): a chunk based
+  * [Gaia-ECS](https://github.com/richardbiely/gaia-ecs): a chunk-based
     archetype ECS.
   * [Polypropylene](https://github.com/pmbittner/Polypropylene): a hybrid
     solution between an ECS and dynamic mixins.
@@ -49,14 +50,16 @@ details.
     inspired archetype ECS with optional multithreading. 
   * [Entitas](https://github.com/sschmid/Entitas-CSharp): the ECS framework for
     C# and Unity, where _reactive systems_ were invented.
-  * [Friflo Engine ECS](https://github.com/friflo/Friflo.Json.Fliox/blob/main/Engine/README.md):
-    an archetype ECS with focus on performance, cache locality and DX.
+  * [Fennecs](https://github.com/outfox/fennecs): the little archetype ECS that
+    loves you back.
+  * [Friflo ECS](https://github.com/friflo/Friflo.Engine.ECS): an archetype ECS
+    with focus on performance and minimal GC allocations.
   * [LeoECS](https://github.com/Leopotam/ecs): simple lightweight C# Entity
     Component System framework.
   * [Massive ECS](https://github.com/nilpunch/massive): sparse set based ECS
     featuring rollbacks.
   * [Svelto.ECS](https://github.com/sebas77/Svelto.ECS): a very interesting
-    platform agnostic and table based ECS framework.
+    platform-agnostic and table-based ECS framework.
 
 * Go:
   * [gecs](https://github.com/tutumagi/gecs): a sparse sets based ECS inspired 
@@ -65,7 +68,7 @@ details.
 * Javascript:
   * [\@javelin/ecs](https://github.com/3mcd/javelin/tree/master/packages/ecs):
     an archetype ECS in TypeScript.
-  * [ecsy](https://github.com/MozillaReality/ecsy): I haven't had the time to
+  * [ecsy](https://github.com/MozillaReality/ecsy): I have not had the time to
     investigate the underlying design of `ecsy` but it looks cool anyway.
 
 * Perl:

docs/md/resource.md

@@ -17,7 +17,7 @@ requirements of the piece of software in which they are used.<br/>
 Examples are loading everything on start, loading on request, predictive
 loading, and so on.
 
-`EnTT` doesn't pretend to offer a _one-fits-all_ solution for the different
+`EnTT` does not pretend to offer a _one-fits-all_ solution for the different
 cases.<br/>
 Instead, the library comes with a minimal, general purpose resource cache that
 might be useful in many cases.
@@ -31,7 +31,8 @@ The _resource_ is an image, an audio, a video or any other type:
 struct my_resource { const int value; };
 ```
 
-The _loader_ is a callable type the aim of which is to load a specific resource:
+The _loader_ is a callable type, the aim of which is to load a specific
+resource:
 
 ```cpp
 struct my_loader final {
@@ -68,9 +69,9 @@ discarded when a player leaves it.
 
 ## Resource handle
 
-Resources aren't returned directly to the caller. Instead, they are wrapped in a
-_resource handle_, an instance of the `entt::resource` class template.<br/>
-For those who know the _flyweight design pattern_ already, that's exactly what
+Resources are not returned directly to the caller. Instead, they are wrapped in
+a _resource handle_, an instance of the `entt::resource` class template.<br/>
+For those who know the _flyweight design pattern_ already, that is exactly what
 it is. To all others, this is the time to brush up on some notions instead.
 
 A shared pointer could have been used as a resource handle. In fact, the default
@@ -84,22 +85,22 @@ more or all resource types could help over time.
 ## Loaders
 
 A loader is responsible for _loading_ resources (quite obviously).<br/>
-By default, it's just a callable object that forwards its arguments to the
+By default, it is just a callable object that forwards its arguments to the
 resource itself. That is, a _passthrough type_. All the work is demanded to the
 constructor(s) of the resource itself.<br/>
 Loaders also are fully customizable as expected.
 
 A custom loader is a class with at least one function call operator and a member
 type named `result_type`.<br/>
-The loader isn't required to return a resource handle. As long as `return_type`
-is suitable for constructing a handle, that's fine.
+The loader is not required to return a resource handle. As long as `return_type`
+is suitable for constructing a handle, that is fine.
 
 When using the default handle, it expects a resource type which is convertible
 to or suitable for constructing an `std::shared_ptr<Type>` (where `Type` is the
 actual resource type).<br/>
 In other terms, the loader should return shared pointers to the given resource
-type. However, this isn't mandatory. Users can easily get around this constraint
-by specializing both the handle and the loader.
+type. However, this is not mandatory. Users can easily get around this
+constraint by specializing both the handle and the loader.
 
 A cache forwards all its arguments to the loader if required. This means that
 loaders can also support tag dispatching to offer different loading policies:
@@ -111,13 +112,13 @@ struct my_loader {
     struct from_disk_tag{};
     struct from_network_tag{};
 
-    template<typename Args>
+    template<typename... Args>
     result_type operator()(from_disk_tag, Args&&... args) {
         // ...
         return std::make_shared<my_resource>(std::forward<Args>(args)...);
     }
 
-    template<typename Args>
+    template<typename... Args>
     result_type operator()(from_network_tag, Args&&... args) {
         // ...
         return std::make_shared<my_resource>(std::forward<Args>(args)...);
@@ -139,7 +140,7 @@ entt::resource_cache<my_resource, my_loader> cache{};
 Under the hood, a cache is nothing more than a map where the key value has type
 `entt::id_type` while the mapped value is whatever type its loader returns.<br/>
 For this reason, it offers most of the functionalities a user would expect from
-a map, such as `empty` or `size` and so on. Similarly, it's an iterable type
+a map, such as `empty` or `size` and so on. Similarly, it is an iterable type
 that also supports indexing by resource id:
 
 ```cpp
@@ -158,9 +159,9 @@ functions (such as `contains` or `erase`).
 Set aside the part of the API that this class _shares_ with a map, it also adds
 something on top of it in order to address the most common requirements of a
 resource cache.<br/>
-In particular, it doesn't have an `emplace` member function which is replaced by
-`load` and `force_load` instead (where the former loads a new resource only if
-not present while the second triggers a forced loading in any case):
+In particular, it does not have an `emplace` member function which is replaced
+by `load` and `force_load` instead (where the former loads a new resource only
+if not present while the second triggers a forced loading in any case):
 
 ```cpp
 auto ret = cache.load("resource/id"_hs);
@@ -176,10 +177,10 @@ Note that the hashed string is used for convenience in the example above.<br/>
 Resource identifiers are nothing more than integral values. Therefore, plain
 numbers as well as non-class enum value are accepted.
 
-It's worth mentioning that the iterators of a cache as well as its indexing
+It is worth mentioning that the iterators of a cache as well as its indexing
 operators return resource handles rather than instances of the mapped type.<br/>
-Since the cache has no control over the loader and a resource isn't required to
+Since the cache has no control over the loader and a resource is not required to
 also be convertible to bool, these handles can be invalid. This usually means an
-error in the user logic but it may also be an _expected_ event.<br/>
-It's therefore recommended to verify handles validity with a check in debug (for
-example, when loading) or an appropriate logic in retail.
+error in the user logic, but it may also be an _expected_ event.<br/>
+It is therefore recommended to verify handles validity with a check in debug
+(for example, when loading) or an appropriate logic in retail.

docs/md/signal.md

@@ -9,6 +9,7 @@
   * [Raw access](#raw-access)
 * [Signals](#signals)
 * [Event dispatcher](#event-dispatcher)
+  * [Connect, disconnect, publish](#connect-disconnect-publish)]
   * [Named queues](#named-queues)
 * [Event emitter](#event-emitter)
 
@@ -19,14 +20,14 @@ in general.<br/>
 They help to decouple the various parts of a system while allowing them to
 communicate with each other somehow.
 
-The so called _modern C++_ comes with a tool that can be useful in this regard,
+The so-called _modern C++_ comes with a tool that can be useful in this regard,
 the `std::function`. As an example, it can be used to create delegates.<br/>
-However, there is no guarantee that an `std::function` doesn't perform
+However, there is no guarantee that an `std::function` does not perform
 allocations under the hood and this could be problematic sometimes. Furthermore,
 it solves a problem but may not adapt well to other requirements that may arise
 from time to time.
 
-In case that the flexibility and power of an `std::function` isn't required or
+In case that the flexibility and power of an `std::function` is not required or
 if the price to pay for them is too high, `EnTT` offers a complete set of
 lightweight classes to solve the same and many other problems.
 
@@ -35,8 +36,8 @@ lightweight classes to solve the same and many other problems.
 A delegate can be used as a general purpose invoker with no memory overhead for
 free functions, lambdas and members provided along with an instance on which to
 invoke them.<br/>
-It doesn't claim to be a drop-in replacement for an `std::function`, so don't
-expect to use it whenever an `std::function` fits well. That said, it's most
+It does not claim to be a drop-in replacement for an `std::function`, so do not
+expect to use it whenever an `std::function` fits well. That said, it is most
 likely even a better fit than an `std::function` in a lot of cases, so expect to
 use it quite a lot anyway.
 
@@ -75,8 +76,8 @@ function type of the delegate is such that the parameter list is empty and the
 value of the data member is at least convertible to the return type.
 
 Free functions having type equivalent to `void(T &, args...)` are accepted as
-well. The first argument `T &` is considered a payload and the function will
-receive it back every time it's invoked. In other terms, this works just fine
+well. The first argument `T &` is considered a payload, and the function will
+receive it back every time it is invoked. In other terms, this works just fine
 with the above definition:
 
 ```cpp
@@ -106,8 +107,8 @@ is used as a building block of a signal-slot system.<br/>
 In fact, this filtering works both ways. The class tries to pass its first
 _count_ arguments **first**, then the last _count_. Watch out for conversion
 rules if in doubt when connecting a listener!<br/>
-Arbitrary functions that pull random arguments from the delegate list aren't
-supported instead. Other feature were preferred, such as support for functions
+Arbitrary functions that pull random arguments from the delegate list are not
+supported instead. Other features were preferred, such as support for functions
 with compatible argument lists although not equal to those of the delegate.
 
 To create and initialize a delegate at once, there are a few specialized
@@ -118,7 +119,7 @@ means of the `entt::connect_arg` variable template:
 entt::delegate<int(int)> func{entt::connect_arg<&f>};
 ```
 
-Aside `connect`, a `disconnect` counterpart isn't provided. Instead, there
+Aside `connect`, a `disconnect` counterpart is not provided. Instead, there
 exists a `reset` member function to use to clear a delegate.<br/>
 To know if a delegate is empty, it can be used explicitly in every conditional
 statement:
@@ -136,14 +137,14 @@ already shown in the examples above:
 auto ret = delegate(42);
 ```
 
-In all cases, listeners don't have to strictly follow the signature of the
+In all cases, listeners do not have to strictly follow the signature of the
 delegate. As long as a listener can be invoked with the given arguments to yield
 a result that is convertible to the given result type, everything works just
 fine.
 
 As a side note, members of classes may or may not be associated with instances.
 If they are not, the first argument of the function type must be that of the
-class on which the members operate and an instance of this class must obviously
+class on which the members operate, and an instance of this class must obviously
 be passed when invoking the delegate:
 
 ```cpp
@@ -154,8 +155,8 @@ my_struct instance;
 delegate(instance, 42);
 ```
 
-In this case, it's not possible to _deduce_ the function type since the first
-argument doesn't necessarily have to be a reference (for example, it can be a
+In this case, it is not possible to _deduce_ the function type since the first
+argument does not necessarily have to be a reference (for example, it can be a
 pointer, as well as a const reference).<br/>
 Therefore, the function type must be declared explicitly for unbound members.
 
@@ -164,13 +165,13 @@ Therefore, the function type must be declared explicitly for unbound members.
 The `delegate` class is meant to be used primarily with template arguments.
 However, as a consequence of its design, it also offers minimal support for
 runtime arguments.<br/>
-When used like this, some features aren't supported though. In particular:
+When used like this, some features are not supported though. In particular:
 
-* Curried functions aren't accepted.
-* Functions with an argument list that differs from that of the delegate aren't
+* Curried functions are not accepted.
+* Functions with an argument list that differs from that of the delegate are not
   supported.
 * Return type and types of arguments **must** coincide with those of the
-  delegate and _being at least convertible_ isn't enough anymore.
+  delegate and _being at least convertible_ is not enough anymore.
 
 Moreover, for a given function type `Ret(Args...)`, the signature of the
 functions connected at runtime must necessarily be `Ret(const void *, Args...)`.
@@ -200,12 +201,12 @@ explained.
 
 ## Lambda support
 
-In general, the `delegate` class doesn't fully support lambda functions in all
-their nuances. The reason is pretty simple: a `delegate` isn't a drop-in
+In general, the `delegate` class does not fully support lambda functions in all
+their nuances. The reason is pretty simple: a `delegate` is not a drop-in
 replacement for an `std::function`. Instead, it tries to overcome the problems
 with the latter.<br/>
 That being said, non-capturing lambda functions are supported, even though some
-features aren't available in this case.
+features are not available in this case.
 
 This is a logical consequence of the support for connecting functions at
 runtime. Therefore, lambda functions undergo the same rules and
@@ -228,7 +229,7 @@ delegate.connect([](const void *ptr, int value) {
 }, &instance);
 ```
 
-As above, the first parameter (`const void *`) isn't part of the function type
+As above, the first parameter (`const void *`) is not part of the function type
 of the delegate and is used to dispatch arbitrary user data back and forth. In
 other terms, the function type of the delegate above is `int(int)`.
 
@@ -252,12 +253,12 @@ particular delegate through its descriptive _traits_ instead.
 
 # Signals
 
-Signal handlers work with references to classes, function pointers and pointers
-to members. Listeners can be any kind of objects and users are in charge of
+Signal handlers work with references to classes, function pointers, and pointers
+to members. Listeners can be any kind of objects, and users are in charge of
 connecting and disconnecting them from a signal to avoid crashes due to
-different lifetimes. On the other side, performance shouldn't be affected that
+different lifetimes. On the other side, performance should not be affected that
 much by the presence of such a signal handler.<br/>
-Signals make use of delegates internally and therefore they undergo the same
+Signals make use of delegates internally, and therefore they undergo the same
 rules and offer similar functionalities. It may be a good idea to consult the
 documentation of the `delegate` class for further information.
 
@@ -271,7 +272,7 @@ The API of a signal handler is straightforward. If a collector is supplied to
 the signal when something is published, all the values returned by its listeners
 are literally _collected_ and used later by the caller. Otherwise, the class
 works just like a plain signal that emits events from time to time.<br/>
-To create instances of signal handlers it's sufficient to provide the type of
+To create instances of signal handlers it is sufficient to provide the type of
 function to which they refer:
 
 ```cpp
@@ -315,7 +316,7 @@ sink.disconnect(&instance);
 sink.disconnect();
 ```
 
-As shown above, listeners don't have to strictly follow the signature of the
+As shown above, listeners do not have to strictly follow the signature of the
 signal. As long as a listener can be invoked with the given arguments to yield a
 result that is convertible to the given return type, everything works just
 fine.<br/>
@@ -357,7 +358,7 @@ assert(vec[1] == 1);
 A collector must expose a function operator that accepts as an argument a type
 to which the return type of the listeners can be converted. Moreover, it can
 optionally return a boolean value that is true to stop collecting data, false
-otherwise. This way one can avoid calling all the listeners in case it isn't
+otherwise. This way one can avoid calling all the listeners in case it is not
 necessary.<br/>
 Functors can also be used in place of a lambda. Since the collector is copied
 when invoking the `collect` member function, `std::ref` is the way to go in this
@@ -382,34 +383,45 @@ signal.collect(std::ref(collector));
 # Event dispatcher
 
 The event dispatcher class allows users to trigger immediate events or to queue
-and publish them all together later.<br/>
-This class lazily instantiates its queues. Therefore, it's not necessary to
-_announce_ the event types in advance:
+and publish them all together later:
 
 ```cpp
 // define a general purpose dispatcher
 entt::dispatcher dispatcher{};
 ```
 
-A listener registered with a dispatcher is such that its type offers one or more
-member functions that take arguments of type `Event &` for any type of event,
-regardless of the return value.<br/>
-These functions are linked directly via `connect` to a _sink_:
+This class lazily instantiates its queues. Therefore, it is not necessary to
+_announce_ the event types in advance.
+
+## Connect, disconnect, publish
+
+Listeners registered with a dispatcher are mainly of two types: free and member
+functions. Lambdas as template functions are also accepted and belong to the
+first group.<br/>
+In all cases, a listener accepts an argument of type `Event &` for any type
+of event, regardless of the return value.
+
+Listeners are linked directly via `connect` to a _sink_ object:
 
 ```cpp
 struct an_event { int value; };
 struct another_event {};
 
+void on_event(const an_event &event) { /* ... */ }
+
 struct listener {
-    void receive(const an_event &) { /* ... */ }
-    void method(const another_event &) { /* ... */ }
+    // Member function listener
+    void on_event(const another_event &) { /* ... */ }
 };
 
 // ...
 
+// free function listener
+dispatcher.sink<an_event>().connect<&on_event>();
+
 listener listener;
-dispatcher.sink<an_event>().connect<&listener::receive>(listener);
-dispatcher.sink<another_event>().connect<&listener::method>(listener);
+// member function listener
+dispatcher.sink<another_event>().connect<&listener::on_event>(listener);
 ```
 
 Note that connecting listeners within event handlers can result in undefined
@@ -418,7 +430,13 @@ The `disconnect` member function is used to remove one listener at a time or all
 of them at once:
 
 ```cpp
-dispatcher.sink<an_event>().disconnect<&listener::receive>(listener);
+// disconnects a free function
+dispatcher.sink<an_event>().disconnect<&on_event>();
+
+// disconnect a member function of an instance
+dispatcher.sink<another_event>().disconnect<&listener::on_event>(listener);
+
+// disconnect all member functions of an instance, if any
 dispatcher.sink<another_event>().disconnect(&listener);
 ```
 
@@ -427,10 +445,10 @@ to all the listeners registered so far:
 
 ```cpp
 dispatcher.trigger(an_event{42});
-dispatcher.trigger<another_event>();
+dispatcher.trigger(another_event{});
 ```
 
-Listeners are invoked immediately, order of execution isn't guaranteed. This
+Listeners are invoked immediately, order of execution is not guaranteed. This
 method can be used to push around urgent messages like an _is terminating_
 notification on a mobile app.
 
@@ -459,7 +477,7 @@ once per tick to their systems.
 
 All queues within a dispatcher are associated by default with an event type and
 then retrieved from it.<br/>
-However, it's possible to create queues with different _names_ (and therefore
+However, it is possible to create queues with different _names_ (and therefore
 also multiple queues for a single type). In fact, more or less all functions
 also take an additional parameter. As an example:
 
@@ -476,8 +494,8 @@ parameter for it but rather a different function:
 dispatcher.enqueue_hint<an_event>("custom"_hs, 42);
 ```
 
-This is mainly due to the template argument deduction rules and unfortunately
-there is no real (elegant) way to avoid it.
+This is mainly due to the template argument deduction rules, and there is no
+real (elegant) way to avoid it.
 
 # Event emitter
 
@@ -495,7 +513,7 @@ struct my_emitter: emitter<my_emitter> {
 }
 ```
 
-Handlers for the different events are created internally on the fly. It's not
+Handlers for the different events are created internally on the fly. It is not
 required to specify in advance the full list of accepted events.<br/>
 Moreover, whenever an event is published, an emitter also passes a reference
 to itself to its listeners.
@@ -555,5 +573,5 @@ if(emitter.contains<my_event>()) {
 ```
 
 This class introduces a _nice-to-have_ model based on events and listeners.<br/>
-More in general, it's a handy tool when the derived classes _wrap_ asynchronous
-operations but it's not limited to such uses.
+More in general, it is a handy tool when the derived classes _wrap_ asynchronous
+operations, but it is not limited to such uses.

docs/md/unreal.md

@@ -1,103 +0,0 @@
-# EnTT and Unreal Engine
-
-# Table of Contents
-
-* [Enable Cpp17](#enable-cpp17)
-* [EnTT as a third party module](#entt-as-a-third-party-module)
-* [Include EnTT](#include-entt)
-
-## Enable Cpp17
-
-> Skip this part if you are working with UE5, Since UE5 uses cpp17 by default.
-
-As of writing (Unreal Engine v4.25), the default C++ standard of Unreal Engine
-is C++14.<br/>
-On the other hand, note that `EnTT` requires C++17 to compile. To enable it, in
-the main module of the project there should be a `<Game Name>.Build.cs` file,
-the constructor of which must contain the following lines:
-
-```cs
-PCHUsage = PCHUsageMode.NoSharedPCHs;
-PrivatePCHHeaderFile = "<PCH filename>.h";
-CppStandard = CppStandardVersion.Cpp17;
-```
-
-Replace `<PCH filename>.h` with the name of the already existing PCH header
-file, if any.<br/>
-In case the project doesn't already contain a file of this type, it's possible
-to create one with the following content:
-
-```cpp
-#pragma once
-#include "CoreMinimal.h"
-```
-
-Remember to remove any old `PCHUsage = <...>` line that was previously there. At
-this point, C++17 support should be in place.<br/>
-Try to compile the project to ensure it works as expected before following
-further steps.
-
-Note that updating a *project* to C++17 doesn't necessarily mean that the IDE in
-use will also start to recognize its syntax.<br/>
-If the plan is to use C++17 in the project too, check the specific instructions
-for the IDE in use.
-
-## EnTT as a third party module
-
-Once this point is reached, the `Source` directory should look like this:
-
-```
-Source
-|  MyGame.Target.cs
-|  MyGameEditor.Target.cs
-|
-+---MyGame
-|  |  MyGame.Build.cs
-|  |  MyGame.h (PCH Header file)
-|
-\---ThirdParty
-   \---EnTT
-      |   EnTT.Build.cs
-      |
-      \---entt (GitHub repository content inside)
-```
-
-To make this happen, create the folder `ThirdParty` under `Source` if it doesn't
-exist already. Then, add an `EnTT` folder under `ThirdParty`.<br/>
-Within the latter, create a new file `EnTT.Build.cs` with the following content:
-
-```cs
-using System.IO;
-using UnrealBuildTool;
-
-public class EnTT: ModuleRules {
-    public EnTT(ReadOnlyTargetRules Target) : base(Target) {
-        Type = ModuleType.External;
-        PublicIncludePaths.Add(Path.Combine(ModuleDirectory, "entt", "src", "entt"));
-    }
-}
-```
-
-The last line indicates that the actual files will be found in the directory
-`EnTT/entt/src/entt`.<br/>
-Download the repository for `EnTT` and place it next to `EnTT.Build.cs` or
-update the path above accordingly.
-
-Finally, open the `<Game Name>.Build.cs` file and add `EnTT` as a dependency at
-the end of the list:
-
-```cs
-PublicDependencyModuleNames.AddRange(new[] {
-    "Core", "CoreUObject", "Engine", "InputCore", [...], "EnTT"
-});
-```
-
-Note that some IDEs might require a restart to start recognizing the new module
-for code-highlighting features and such.
-
-## Include EnTT
-
-In any source file of the project, add `#include "entt.hpp"` or any other path
-to the file from `EnTT` to use it.<br/>
-Try to create a registry as `entt::registry registry;` to make sure everything
-compiles fine.

entt.imp

@@ -20,7 +20,6 @@
   { "include": [ "@[\"<].*/entity/fwd\\.hpp[\">]", "private", "<entt/entity/handle.hpp>", "public" ] },
   { "include": [ "@[\"<].*/entity/fwd\\.hpp[\">]", "private", "<entt/entity/helper.hpp>", "public" ] },
   { "include": [ "@[\"<].*/entity/fwd\\.hpp[\">]", "private", "<entt/entity/mixin.hpp>", "public" ] },
-  { "include": [ "@[\"<].*/entity/fwd\\.hpp[\">]", "private", "<entt/entity/observer.hpp>", "public" ] },
   { "include": [ "@[\"<].*/entity/fwd\\.hpp[\">]", "private", "<entt/entity/organizer.hpp>", "public" ] },
   { "include": [ "@[\"<].*/entity/fwd\\.hpp[\">]", "private", "<entt/entity/ranges.hpp>", "public" ] },
   { "include": [ "@[\"<].*/entity/fwd\\.hpp[\">]", "private", "<entt/entity/registry.hpp>", "public" ] },

natvis/entt/process.natvis

@@ -1,3 +0,0 @@
-<?xml version="1.0" encoding="utf-8"?>
-<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
-</AutoVisualizer>

src/entt/config/config.h

@@ -1,22 +1,40 @@
 #ifndef ENTT_CONFIG_CONFIG_H
 #define ENTT_CONFIG_CONFIG_H
 
+#if __has_include(<entt/ext/config.h>)
+#    include <entt/ext/config.h>
+#endif
+
 #include "version.h"
 
 // NOLINTBEGIN(cppcoreguidelines-macro-usage)
 
-#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
-#    define ENTT_CONSTEXPR
+#ifdef ENTT_USE_STL
+#    define ENTT_FORCE_STL
+#endif
+
+#if defined(__cpp_exceptions) && !defined(ENTT_NO_EXCEPTION)
 #    define ENTT_THROW throw
 #    define ENTT_TRY try
 #    define ENTT_CATCH catch(...)
 #else
-#    define ENTT_CONSTEXPR constexpr // use only with throwing functions (waiting for C++20)
 #    define ENTT_THROW
 #    define ENTT_TRY if(true)
 #    define ENTT_CATCH if(false)
 #endif
 
+#if __has_include(<version>)
+#    include <version>
+#
+#    if defined(__cpp_consteval)
+#        define ENTT_CONSTEVAL consteval
+#    endif
+#endif
+
+#ifndef ENTT_CONSTEVAL
+#    define ENTT_CONSTEVAL constexpr
+#endif
+
 #ifdef ENTT_USE_ATOMIC
 #    include <atomic>
 #    define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
@@ -83,6 +101,32 @@
 #    endif
 #endif
 
+#ifndef ENTT_EXPORT
+#    if defined _WIN32 || defined __CYGWIN__ || defined _MSC_VER
+#        define ENTT_EXPORT __declspec(dllexport)
+#        define ENTT_IMPORT __declspec(dllimport)
+#        define ENTT_HIDDEN
+#    elif defined __GNUC__ && __GNUC__ >= 4
+#        define ENTT_EXPORT __attribute__((visibility("default")))
+#        define ENTT_IMPORT __attribute__((visibility("default")))
+#        define ENTT_HIDDEN __attribute__((visibility("hidden")))
+#    else /* Unsupported compiler */
+#        define ENTT_EXPORT
+#        define ENTT_IMPORT
+#        define ENTT_HIDDEN
+#    endif
+#endif
+
+#ifndef ENTT_API
+#    if defined ENTT_API_EXPORT
+#        define ENTT_API ENTT_EXPORT
+#    elif defined ENTT_API_IMPORT
+#        define ENTT_API ENTT_IMPORT
+#    else /* No API */
+#        define ENTT_API
+#    endif
+#endif
+
 #if defined _MSC_VER
 #    pragma detect_mismatch("entt.version", ENTT_VERSION)
 #    pragma detect_mismatch("entt.noexcept", ENTT_XSTR(ENTT_TRY))

src/entt/config/version.h

@@ -3,16 +3,16 @@
 
 #include "macro.h"
 
-// NOLINTBEGIN(cppcoreguidelines-macro-usage)
+// NOLINTBEGIN(cppcoreguidelines-macro-*,modernize-macro-*)
 
-#define ENTT_VERSION_MAJOR 3
-#define ENTT_VERSION_MINOR 14
+#define ENTT_VERSION_MAJOR 4
+#define ENTT_VERSION_MINOR 0
 #define ENTT_VERSION_PATCH 0
 
 #define ENTT_VERSION \
     ENTT_XSTR(ENTT_VERSION_MAJOR) \
     "." ENTT_XSTR(ENTT_VERSION_MINOR) "." ENTT_XSTR(ENTT_VERSION_PATCH)
 
-// NOLINTEND(cppcoreguidelines-macro-usage)
+// NOLINTEND(cppcoreguidelines-macro-*,modernize-macro-*)
 
 #endif

src/entt/container/dense_map.hpp

@@ -1,7 +1,10 @@
 #ifndef ENTT_CONTAINER_DENSE_MAP_HPP
 #define ENTT_CONTAINER_DENSE_MAP_HPP
 
+#include <bit>
 #include <cmath>
+#include <compare>
+#include <concepts>
 #include <cstddef>
 #include <functional>
 #include <iterator>
@@ -17,13 +20,16 @@
 #include "../core/iterator.hpp"
 #include "../core/memory.hpp"
 #include "../core/type_traits.hpp"
+#include "../stl/iterator.hpp"
 #include "fwd.hpp"
 
 namespace entt {
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
+static constexpr std::size_t dense_map_placeholder_position = (std::numeric_limits<std::size_t>::max)();
+
 template<typename Key, typename Type>
 struct dense_map_node final {
     using value_type = std::pair<Key, Type>;
@@ -33,18 +39,16 @@ struct dense_map_node final {
         : next{pos},
           element{std::forward<Args>(args)...} {}
 
-    template<typename Allocator, typename... Args>
-    dense_map_node(std::allocator_arg_t, const Allocator &allocator, const std::size_t pos, Args &&...args)
+    template<typename... Args>
+    dense_map_node(std::allocator_arg_t, const auto &allocator, const std::size_t pos, Args &&...args)
         : next{pos},
           element{entt::make_obj_using_allocator<value_type>(allocator, std::forward<Args>(args)...)} {}
 
-    template<typename Allocator>
-    dense_map_node(std::allocator_arg_t, const Allocator &allocator, const dense_map_node &other)
+    dense_map_node(std::allocator_arg_t, const auto &allocator, const dense_map_node &other)
         : next{other.next},
           element{entt::make_obj_using_allocator<value_type>(allocator, other.element)} {}
 
-    template<typename Allocator>
-    dense_map_node(std::allocator_arg_t, const Allocator &allocator, dense_map_node &&other)
+    dense_map_node(std::allocator_arg_t, const auto &allocator, dense_map_node &&other)
         : next{other.next},
           element{entt::make_obj_using_allocator<value_type>(allocator, std::move(other.element))} {}
 
@@ -57,6 +61,7 @@ class dense_map_iterator final {
     template<typename>
     friend class dense_map_iterator;
 
+    static_assert(std::is_pointer_v<It>, "Not a pointer type");
     using first_type = decltype(std::as_const(std::declval<It>()->element.first));
     using second_type = decltype((std::declval<It>()->element.second));
 
@@ -74,7 +79,8 @@ public:
     constexpr dense_map_iterator(const It iter) noexcept
         : it{iter} {}
 
-    template<typename Other, typename = std::enable_if_t<!std::is_same_v<It, Other> && std::is_constructible_v<It, Other>>>
+    template<typename Other>
+    requires (!std::same_as<It, Other> && std::constructible_from<It, Other>)
     constexpr dense_map_iterator(const dense_map_iterator<Other> &other) noexcept
         : it{other.it} {}
 
@@ -83,7 +89,7 @@ public:
     }
 
     constexpr dense_map_iterator operator++(int) noexcept {
-        dense_map_iterator orig = *this;
+        const dense_map_iterator orig = *this;
         return ++(*this), orig;
     }
 
@@ -92,7 +98,7 @@ public:
     }
 
     constexpr dense_map_iterator operator--(int) noexcept {
-        dense_map_iterator orig = *this;
+        const dense_map_iterator orig = *this;
         return operator--(), orig;
     }
 
@@ -126,59 +132,31 @@ public:
         return operator[](0);
     }
 
-    template<typename Lhs, typename Rhs>
-    friend constexpr std::ptrdiff_t operator-(const dense_map_iterator<Lhs> &, const dense_map_iterator<Rhs> &) noexcept;
+    template<typename Other>
+    [[nodiscard]] constexpr std::ptrdiff_t operator-(const dense_map_iterator<Other> &other) const noexcept {
+        return it - other.it;
+    }
 
-    template<typename Lhs, typename Rhs>
-    friend constexpr bool operator==(const dense_map_iterator<Lhs> &, const dense_map_iterator<Rhs> &) noexcept;
+    template<typename Other>
+    [[nodiscard]] constexpr bool operator==(const dense_map_iterator<Other> &other) const noexcept {
+        return it == other.it;
+    }
 
-    template<typename Lhs, typename Rhs>
-    friend constexpr bool operator<(const dense_map_iterator<Lhs> &, const dense_map_iterator<Rhs> &) noexcept;
+    template<typename Other>
+    [[nodiscard]] constexpr auto operator<=>(const dense_map_iterator<Other> &other) const noexcept {
+        return it <=> other.it;
+    }
 
 private:
     It it;
 };
 
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr std::ptrdiff_t operator-(const dense_map_iterator<Lhs> &lhs, const dense_map_iterator<Rhs> &rhs) noexcept {
-    return lhs.it - rhs.it;
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator==(const dense_map_iterator<Lhs> &lhs, const dense_map_iterator<Rhs> &rhs) noexcept {
-    return lhs.it == rhs.it;
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator!=(const dense_map_iterator<Lhs> &lhs, const dense_map_iterator<Rhs> &rhs) noexcept {
-    return !(lhs == rhs);
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator<(const dense_map_iterator<Lhs> &lhs, const dense_map_iterator<Rhs> &rhs) noexcept {
-    return lhs.it < rhs.it;
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator>(const dense_map_iterator<Lhs> &lhs, const dense_map_iterator<Rhs> &rhs) noexcept {
-    return rhs < lhs;
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator<=(const dense_map_iterator<Lhs> &lhs, const dense_map_iterator<Rhs> &rhs) noexcept {
-    return !(lhs > rhs);
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator>=(const dense_map_iterator<Lhs> &lhs, const dense_map_iterator<Rhs> &rhs) noexcept {
-    return !(lhs < rhs);
-}
-
 template<typename It>
 class dense_map_local_iterator final {
     template<typename>
     friend class dense_map_local_iterator;
 
+    static_assert(std::is_pointer_v<It>, "Not a pointer type");
     using first_type = decltype(std::as_const(std::declval<It>()->element.first));
     using second_type = decltype((std::declval<It>()->element.second));
 
@@ -190,25 +168,24 @@ public:
     using iterator_category = std::input_iterator_tag;
     using iterator_concept = std::forward_iterator_tag;
 
-    constexpr dense_map_local_iterator() noexcept
-        : it{},
-          offset{} {}
+    constexpr dense_map_local_iterator() noexcept = default;
 
     constexpr dense_map_local_iterator(It iter, const std::size_t pos) noexcept
         : it{iter},
           offset{pos} {}
 
-    template<typename Other, typename = std::enable_if_t<!std::is_same_v<It, Other> && std::is_constructible_v<It, Other>>>
+    template<typename Other>
+    requires (!std::same_as<It, Other> && std::constructible_from<It, Other>)
     constexpr dense_map_local_iterator(const dense_map_local_iterator<Other> &other) noexcept
         : it{other.it},
           offset{other.offset} {}
 
     constexpr dense_map_local_iterator &operator++() noexcept {
-        return offset = it[offset].next, *this;
+        return (offset = it[static_cast<difference_type>(offset)].next), *this;
     }
 
     constexpr dense_map_local_iterator operator++(int) noexcept {
-        dense_map_local_iterator orig = *this;
+        const dense_map_local_iterator orig = *this;
         return ++(*this), orig;
     }
 
@@ -217,7 +194,13 @@ public:
     }
 
     [[nodiscard]] constexpr reference operator*() const noexcept {
-        return {it[offset].element.first, it[offset].element.second};
+        const auto idx = static_cast<difference_type>(offset);
+        return {it[idx].element.first, it[idx].element.second};
+    }
+
+    template<typename Other>
+    [[nodiscard]] constexpr bool operator==(const dense_map_local_iterator<Other> &other) const noexcept {
+        return offset == other.offset;
     }
 
     [[nodiscard]] constexpr std::size_t index() const noexcept {
@@ -225,20 +208,10 @@ public:
     }
 
 private:
-    It it;
-    std::size_t offset;
+    It it{};
+    std::size_t offset{dense_map_placeholder_position};
 };
 
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator==(const dense_map_local_iterator<Lhs> &lhs, const dense_map_local_iterator<Rhs> &rhs) noexcept {
-    return lhs.index() == rhs.index();
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator!=(const dense_map_local_iterator<Lhs> &lhs, const dense_map_local_iterator<Rhs> &rhs) noexcept {
-    return !(lhs == rhs);
-}
-
 } // namespace internal
 /*! @endcond */
 
@@ -259,6 +232,7 @@ template<typename Key, typename Type, typename Hash, typename KeyEqual, typename
 class dense_map {
     static constexpr float default_threshold = 0.875f;
     static constexpr std::size_t minimum_capacity = 8u;
+    static constexpr std::size_t placeholder_position = internal::dense_map_placeholder_position;
 
     using node_type = internal::dense_map_node<Key, Type>;
     using alloc_traits = std::allocator_traits<Allocator>;
@@ -266,28 +240,25 @@ class dense_map {
     using sparse_container_type = std::vector<std::size_t, typename alloc_traits::template rebind_alloc<std::size_t>>;
     using packed_container_type = std::vector<node_type, typename alloc_traits::template rebind_alloc<node_type>>;
 
-    template<typename Other>
-    [[nodiscard]] std::size_t key_to_bucket(const Other &key) const noexcept {
+    [[nodiscard]] std::size_t key_to_bucket(const auto &key) const noexcept {
         // NOLINTNEXTLINE(cppcoreguidelines-pro-bounds-array-to-pointer-decay)
         return fast_mod(static_cast<size_type>(sparse.second()(key)), bucket_count());
     }
 
-    template<typename Other>
-    [[nodiscard]] auto constrained_find(const Other &key, std::size_t bucket) {
-        for(auto it = begin(bucket), last = end(bucket); it != last; ++it) {
-            if(packed.second()(it->first, key)) {
-                return begin() + static_cast<typename iterator::difference_type>(it.index());
+    [[nodiscard]] auto constrained_find(const auto &key, const std::size_t bucket) {
+        for(auto offset = sparse.first()[bucket]; offset != placeholder_position; offset = packed.first()[offset].next) {
+            if(packed.second()(packed.first()[offset].element.first, key)) {
+                return begin() + static_cast<iterator::difference_type>(offset);
             }
         }
 
         return end();
     }
 
-    template<typename Other>
-    [[nodiscard]] auto constrained_find(const Other &key, std::size_t bucket) const {
-        for(auto it = cbegin(bucket), last = cend(bucket); it != last; ++it) {
-            if(packed.second()(it->first, key)) {
-                return cbegin() + static_cast<typename iterator::difference_type>(it.index());
+    [[nodiscard]] auto constrained_find(const auto &key, const std::size_t bucket) const {
+        for(auto offset = sparse.first()[bucket]; offset != placeholder_position; offset = packed.first()[offset].next) {
+            if(packed.second()(packed.first()[offset].element.first, key)) {
+                return cbegin() + static_cast<const_iterator::difference_type>(offset);
             }
         }
 
@@ -337,8 +308,8 @@ class dense_map {
     }
 
     void rehash_if_required() {
-        if(size() > (bucket_count() * max_load_factor())) {
-            rehash(bucket_count() * 2u);
+        if(const auto bc = bucket_count(); size() > static_cast<size_type>(static_cast<float>(bc) * max_load_factor())) {
+            rehash(bc * 2u);
         }
     }
 
@@ -353,18 +324,20 @@ public:
     using value_type = std::pair<const Key, Type>;
     /*! @brief Unsigned integer type. */
     using size_type = std::size_t;
+    /*! @brief Signed integer type. */
+    using difference_type = std::ptrdiff_t;
     /*! @brief Type of function to use to hash the keys. */
     using hasher = Hash;
     /*! @brief Type of function to use to compare the keys for equality. */
     using key_equal = KeyEqual;
     /*! @brief Input iterator type. */
-    using iterator = internal::dense_map_iterator<typename packed_container_type::iterator>;
+    using iterator = internal::dense_map_iterator<typename packed_container_type::pointer>;
     /*! @brief Constant input iterator type. */
-    using const_iterator = internal::dense_map_iterator<typename packed_container_type::const_iterator>;
+    using const_iterator = internal::dense_map_iterator<typename packed_container_type::const_pointer>;
     /*! @brief Input iterator type. */
-    using local_iterator = internal::dense_map_local_iterator<typename packed_container_type::iterator>;
+    using local_iterator = internal::dense_map_local_iterator<typename packed_container_type::pointer>;
     /*! @brief Constant input iterator type. */
-    using const_local_iterator = internal::dense_map_local_iterator<typename packed_container_type::const_iterator>;
+    using const_local_iterator = internal::dense_map_local_iterator<typename packed_container_type::const_pointer>;
 
     /*! @brief Default constructor. */
     dense_map()
@@ -451,6 +424,17 @@ public:
      */
     dense_map &operator=(dense_map &&) noexcept = default;
 
+    /**
+     * @brief Exchanges the contents with those of a given container.
+     * @param other Container to exchange the content with.
+     */
+    void swap(dense_map &other) noexcept {
+        using std::swap;
+        swap(sparse, other.sparse);
+        swap(packed, other.packed);
+        swap(threshold, other.threshold);
+    }
+
     /**
      * @brief Returns the associated allocator.
      * @return The associated allocator.
@@ -467,7 +451,7 @@ public:
      * @return An iterator to the first instance of the internal array.
      */
     [[nodiscard]] const_iterator cbegin() const noexcept {
-        return packed.first().begin();
+        return packed.first().data();
     }
 
     /*! @copydoc cbegin */
@@ -477,7 +461,7 @@ public:
 
     /*! @copydoc begin */
     [[nodiscard]] iterator begin() noexcept {
-        return packed.first().begin();
+        return packed.first().data();
     }
 
     /**
@@ -486,7 +470,7 @@ public:
      * internal array.
      */
     [[nodiscard]] const_iterator cend() const noexcept {
-        return packed.first().end();
+        return packed.first().data() + packed.first().size();
     }
 
     /*! @copydoc cend */
@@ -496,7 +480,7 @@ public:
 
     /*! @copydoc end */
     [[nodiscard]] iterator end() noexcept {
-        return packed.first().end();
+        return packed.first().data() + packed.first().size();
     }
 
     /**
@@ -551,19 +535,17 @@ public:
      * @tparam Arg Type of the key-value pair to insert into the container.
      */
     template<typename Arg>
-    std::enable_if_t<std::is_constructible_v<value_type, Arg &&>, std::pair<iterator, bool>>
-    insert(Arg &&value) {
+    requires std::constructible_from<value_type, Arg &&>
+    std::pair<iterator, bool> insert(Arg &&value) {
         return insert_or_do_nothing(std::forward<Arg>(value).first, std::forward<Arg>(value).second);
     }
 
     /**
      * @brief Inserts elements into the container, if their keys do not exist.
-     * @tparam It Type of input iterator.
      * @param first An iterator to the first element of the range of elements.
      * @param last An iterator past the last element of the range of elements.
      */
-    template<typename It>
-    void insert(It first, It last) {
+    void insert(stl::input_iterator auto first, stl::input_iterator auto last) {
         for(; first != last; ++first) {
             insert(*first);
         }
@@ -669,7 +651,7 @@ public:
         const auto dist = first - cbegin();
 
         for(auto from = last - cbegin(); from != dist; --from) {
-            erase(packed.first()[from - 1u].element.first);
+            erase(packed.first()[static_cast<size_type>(from) - 1u].element.first);
         }
 
         return (begin() + dist);
@@ -681,7 +663,7 @@ public:
      * @return Number of elements removed (either 0 or 1).
      */
     size_type erase(const key_type &key) {
-        for(size_type *curr = &sparse.first()[key_to_bucket(key)]; *curr != (std::numeric_limits<size_type>::max)(); curr = &packed.first()[*curr].next) {
+        for(size_type *curr = &sparse.first()[key_to_bucket(key)]; *curr != placeholder_position; curr = &packed.first()[*curr].next) {
             if(packed.second()(packed.first()[*curr].element.first, key)) {
                 const auto index = *curr;
                 *curr = packed.first()[*curr].next;
@@ -693,17 +675,6 @@ public:
         return 0u;
     }
 
-    /**
-     * @brief Exchanges the contents with those of a given container.
-     * @param other Container to exchange the content with.
-     */
-    void swap(dense_map &other) noexcept {
-        using std::swap;
-        swap(sparse, other.sparse);
-        swap(packed, other.packed);
-        swap(threshold, other.threshold);
-    }
-
     /**
      * @brief Accesses a given element with bounds checking.
      * @param key A key of an element to find.
@@ -722,6 +693,26 @@ public:
         return it->second;
     }
 
+    /**
+     * @brief Accesses a given element with bounds checking.
+     * @param key A key of an element to find.
+     * @return A reference to the mapped value of the requested element.
+     */
+    [[nodiscard]] mapped_type const &at(const auto &key) const
+    requires is_transparent_v<hasher> && is_transparent_v<key_equal> {
+        auto it = find(key);
+        ENTT_ASSERT(it != cend(), "Invalid key");
+        return it->second;
+    }
+
+    /*! @copydoc at */
+    [[nodiscard]] mapped_type &at(const auto &key)
+    requires is_transparent_v<hasher> && is_transparent_v<key_equal> {
+        auto it = find(key);
+        ENTT_ASSERT(it != end(), "Invalid key");
+        return it->second;
+    }
+
     /**
      * @brief Accesses or inserts a given element.
      * @param key A key of an element to find or insert.
@@ -751,13 +742,11 @@ public:
 
     /**
      * @brief Returns the number of elements matching a key (either 1 or 0).
-     * @tparam Other Type of the key value of an element to search for.
      * @param key Key value of an element to search for.
      * @return Number of elements matching the key (either 1 or 0).
      */
-    template<typename Other>
-    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, size_type>>
-    count(const Other &key) const {
+    [[nodiscard]] size_type count(const auto &key) const
+    requires is_transparent_v<hasher> && is_transparent_v<key_equal> {
         return find(key) != end();
     }
 
@@ -779,21 +768,18 @@ public:
     /**
      * @brief Finds an element with a key that compares _equivalent_ to a given
      * key.
-     * @tparam Other Type of the key value of an element to search for.
      * @param key Key value of an element to search for.
      * @return An iterator to an element with the given key. If no such element
      * is found, a past-the-end iterator is returned.
      */
-    template<typename Other>
-    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, iterator>>
-    find(const Other &key) {
+    [[nodiscard]] iterator find(const auto &key)
+    requires is_transparent_v<hasher> && is_transparent_v<key_equal> {
         return constrained_find(key, key_to_bucket(key));
     }
 
     /*! @copydoc find */
-    template<typename Other>
-    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, const_iterator>>
-    find(const Other &key) const {
+    [[nodiscard]] const_iterator find(const auto &key) const
+    requires is_transparent_v<hasher> && is_transparent_v<key_equal> {
         return constrained_find(key, key_to_bucket(key));
     }
 
@@ -817,22 +803,19 @@ public:
     /**
      * @brief Returns a range containing all elements that compare _equivalent_
      * to a given key.
-     * @tparam Other Type of an element to search for.
      * @param key Key value of an element to search for.
      * @return A pair of iterators pointing to the first element and past the
      * last element of the range.
      */
-    template<typename Other>
-    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, std::pair<iterator, iterator>>>
-    equal_range(const Other &key) {
+    [[nodiscard]] std::pair<iterator, iterator> equal_range(const auto &key)
+    requires is_transparent_v<hasher> && is_transparent_v<key_equal> {
         const auto it = find(key);
         return {it, it + !(it == end())};
     }
 
     /*! @copydoc equal_range */
-    template<typename Other>
-    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, std::pair<const_iterator, const_iterator>>>
-    equal_range(const Other &key) const {
+    [[nodiscard]] std::pair<const_iterator, const_iterator> equal_range(const auto &key) const
+    requires is_transparent_v<hasher> && is_transparent_v<key_equal> {
         const auto it = find(key);
         return {it, it + !(it == cend())};
     }
@@ -849,13 +832,11 @@ public:
     /**
      * @brief Checks if the container contains an element with a key that
      * compares _equivalent_ to a given value.
-     * @tparam Other Type of the key value of an element to search for.
      * @param key Key value of an element to search for.
      * @return True if there is such an element, false otherwise.
      */
-    template<typename Other>
-    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, bool>>
-    contains(const Other &key) const {
+    [[nodiscard]] bool contains(const auto &key) const
+    requires is_transparent_v<hasher> && is_transparent_v<key_equal> {
         return (find(key) != cend());
     }
 
@@ -865,7 +846,7 @@ public:
      * @return An iterator to the beginning of the given bucket.
      */
     [[nodiscard]] const_local_iterator cbegin(const size_type index) const {
-        return {packed.first().begin(), sparse.first()[index]};
+        return {packed.first().data(), sparse.first()[index]};
     }
 
     /**
@@ -883,7 +864,7 @@ public:
      * @return An iterator to the beginning of the given bucket.
      */
     [[nodiscard]] local_iterator begin(const size_type index) {
-        return {packed.first().begin(), sparse.first()[index]};
+        return {packed.first().data(), sparse.first()[index]};
     }
 
     /**
@@ -892,7 +873,7 @@ public:
      * @return An iterator to the end of the given bucket.
      */
     [[nodiscard]] const_local_iterator cend([[maybe_unused]] const size_type index) const {
-        return {packed.first().begin(), (std::numeric_limits<size_type>::max)()};
+        return {};
     }
 
     /**
@@ -910,7 +891,7 @@ public:
      * @return An iterator to the end of the given bucket.
      */
     [[nodiscard]] local_iterator end([[maybe_unused]] const size_type index) {
-        return {packed.first().begin(), (std::numeric_limits<size_type>::max)()};
+        return {};
     }
 
     /**
@@ -952,7 +933,7 @@ public:
      * @return The average number of elements per bucket.
      */
     [[nodiscard]] float load_factor() const {
-        return size() / static_cast<float>(bucket_count());
+        return static_cast<float>(size()) / static_cast<float>(bucket_count());
     }
 
     /**
@@ -980,14 +961,14 @@ public:
      */
     void rehash(const size_type cnt) {
         auto value = cnt > minimum_capacity ? cnt : minimum_capacity;
-        const auto cap = static_cast<size_type>(size() / max_load_factor());
+        const auto cap = static_cast<size_type>(static_cast<float>(size()) / max_load_factor());
         value = value > cap ? value : cap;
 
-        if(const auto sz = next_power_of_two(value); sz != bucket_count()) {
+        if(const auto sz = std::bit_ceil(value); sz != bucket_count()) {
             sparse.first().resize(sz);
 
             for(auto &&elem: sparse.first()) {
-                elem = (std::numeric_limits<size_type>::max)();
+                elem = placeholder_position;
             }
 
             for(size_type pos{}, last = size(); pos < last; ++pos) {
@@ -1004,7 +985,7 @@ public:
      */
     void reserve(const size_type cnt) {
         packed.first().reserve(cnt);
-        rehash(static_cast<size_type>(std::ceil(cnt / max_load_factor())));
+        rehash(static_cast<size_type>(std::ceil(static_cast<float>(cnt) / max_load_factor())));
     }
 
     /**
@@ -1031,7 +1012,7 @@ private:
 
 } // namespace entt
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace std {
 
 template<typename Key, typename Value, typename Allocator>

src/entt/container/dense_set.hpp

@@ -1,7 +1,10 @@
 #ifndef ENTT_CONTAINER_DENSE_SET_HPP
 #define ENTT_CONTAINER_DENSE_SET_HPP
 
+#include <bit>
 #include <cmath>
+#include <compare>
+#include <concepts>
 #include <cstddef>
 #include <functional>
 #include <iterator>
@@ -15,20 +18,25 @@
 #include "../core/bit.hpp"
 #include "../core/compressed_pair.hpp"
 #include "../core/type_traits.hpp"
+#include "../stl/iterator.hpp"
 #include "fwd.hpp"
 
 namespace entt {
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
+static constexpr std::size_t dense_set_placeholder_position = (std::numeric_limits<std::size_t>::max)();
+
 template<typename It>
 class dense_set_iterator final {
     template<typename>
     friend class dense_set_iterator;
 
+    static_assert(std::is_pointer_v<It>, "Not a pointer type");
+
 public:
-    using value_type = typename It::value_type::second_type;
+    using value_type = std::remove_const_t<std::remove_pointer_t<It>>::second_type;
     using pointer = const value_type *;
     using reference = const value_type &;
     using difference_type = std::ptrdiff_t;
@@ -40,7 +48,8 @@ public:
     constexpr dense_set_iterator(const It iter) noexcept
         : it{iter} {}
 
-    template<typename Other, typename = std::enable_if_t<!std::is_same_v<It, Other> && std::is_constructible_v<It, Other>>>
+    template<typename Other>
+    requires (!std::same_as<It, Other> && std::constructible_from<It, Other>)
     constexpr dense_set_iterator(const dense_set_iterator<Other> &other) noexcept
         : it{other.it} {}
 
@@ -49,7 +58,7 @@ public:
     }
 
     constexpr dense_set_iterator operator++(int) noexcept {
-        dense_set_iterator orig = *this;
+        const dense_set_iterator orig = *this;
         return ++(*this), orig;
     }
 
@@ -58,7 +67,7 @@ public:
     }
 
     constexpr dense_set_iterator operator--(int) noexcept {
-        dense_set_iterator orig = *this;
+        const dense_set_iterator orig = *this;
         return operator--(), orig;
     }
 
@@ -92,115 +101,82 @@ public:
         return operator[](0);
     }
 
-    template<typename Lhs, typename Rhs>
-    friend constexpr std::ptrdiff_t operator-(const dense_set_iterator<Lhs> &, const dense_set_iterator<Rhs> &) noexcept;
+    template<typename Other>
+    [[nodiscard]] constexpr std::ptrdiff_t operator-(const dense_set_iterator<Other> &other) const noexcept {
+        return it - other.it;
+    }
 
-    template<typename Lhs, typename Rhs>
-    friend constexpr bool operator==(const dense_set_iterator<Lhs> &, const dense_set_iterator<Rhs> &) noexcept;
+    template<typename Other>
+    [[nodiscard]] constexpr bool operator==(const dense_set_iterator<Other> &other) const noexcept {
+        return it == other.it;
+    }
 
-    template<typename Lhs, typename Rhs>
-    friend constexpr bool operator<(const dense_set_iterator<Lhs> &, const dense_set_iterator<Rhs> &) noexcept;
+    template<typename Other>
+    [[nodiscard]] constexpr auto operator<=>(const dense_set_iterator<Other> &other) const noexcept {
+        return it <=> other.it;
+    }
 
 private:
     It it;
 };
 
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr std::ptrdiff_t operator-(const dense_set_iterator<Lhs> &lhs, const dense_set_iterator<Rhs> &rhs) noexcept {
-    return lhs.it - rhs.it;
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator==(const dense_set_iterator<Lhs> &lhs, const dense_set_iterator<Rhs> &rhs) noexcept {
-    return lhs.it == rhs.it;
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator!=(const dense_set_iterator<Lhs> &lhs, const dense_set_iterator<Rhs> &rhs) noexcept {
-    return !(lhs == rhs);
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator<(const dense_set_iterator<Lhs> &lhs, const dense_set_iterator<Rhs> &rhs) noexcept {
-    return lhs.it < rhs.it;
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator>(const dense_set_iterator<Lhs> &lhs, const dense_set_iterator<Rhs> &rhs) noexcept {
-    return rhs < lhs;
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator<=(const dense_set_iterator<Lhs> &lhs, const dense_set_iterator<Rhs> &rhs) noexcept {
-    return !(lhs > rhs);
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator>=(const dense_set_iterator<Lhs> &lhs, const dense_set_iterator<Rhs> &rhs) noexcept {
-    return !(lhs < rhs);
-}
-
 template<typename It>
 class dense_set_local_iterator final {
     template<typename>
     friend class dense_set_local_iterator;
 
+    static_assert(std::is_pointer_v<It>, "Not a pointer type");
+
 public:
-    using value_type = typename It::value_type::second_type;
+    using value_type = std::remove_const_t<std::remove_pointer_t<It>>::second_type;
     using pointer = const value_type *;
     using reference = const value_type &;
     using difference_type = std::ptrdiff_t;
     using iterator_category = std::forward_iterator_tag;
 
-    constexpr dense_set_local_iterator() noexcept
-        : it{},
-          offset{} {}
+    constexpr dense_set_local_iterator() noexcept = default;
 
     constexpr dense_set_local_iterator(It iter, const std::size_t pos) noexcept
         : it{iter},
           offset{pos} {}
 
-    template<typename Other, typename = std::enable_if_t<!std::is_same_v<It, Other> && std::is_constructible_v<It, Other>>>
+    template<typename Other>
+    requires (!std::same_as<It, Other> && std::constructible_from<It, Other>)
     constexpr dense_set_local_iterator(const dense_set_local_iterator<Other> &other) noexcept
         : it{other.it},
           offset{other.offset} {}
 
     constexpr dense_set_local_iterator &operator++() noexcept {
-        return offset = it[offset].first, *this;
+        return offset = it[static_cast<difference_type>(offset)].first, *this;
     }
 
     constexpr dense_set_local_iterator operator++(int) noexcept {
-        dense_set_local_iterator orig = *this;
+        const dense_set_local_iterator orig = *this;
         return ++(*this), orig;
     }
 
     [[nodiscard]] constexpr pointer operator->() const noexcept {
-        return std::addressof(it[offset].second);
+        return std::addressof(it[static_cast<difference_type>(offset)].second);
     }
 
     [[nodiscard]] constexpr reference operator*() const noexcept {
         return *operator->();
     }
 
+    template<typename Other>
+    [[nodiscard]] constexpr bool operator==(const dense_set_local_iterator<Other> &other) const noexcept {
+        return offset == other.offset;
+    }
+
     [[nodiscard]] constexpr std::size_t index() const noexcept {
         return offset;
     }
 
 private:
-    It it;
-    std::size_t offset;
+    It it{};
+    std::size_t offset{dense_set_placeholder_position};
 };
 
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator==(const dense_set_local_iterator<Lhs> &lhs, const dense_set_local_iterator<Rhs> &rhs) noexcept {
-    return lhs.index() == rhs.index();
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator!=(const dense_set_local_iterator<Lhs> &lhs, const dense_set_local_iterator<Rhs> &rhs) noexcept {
-    return !(lhs == rhs);
-}
-
 } // namespace internal
 /*! @endcond */
 
@@ -220,6 +196,7 @@ template<typename Type, typename Hash, typename KeyEqual, typename Allocator>
 class dense_set {
     static constexpr float default_threshold = 0.875f;
     static constexpr std::size_t minimum_capacity = 8u;
+    static constexpr std::size_t placeholder_position = internal::dense_set_placeholder_position;
 
     using node_type = std::pair<std::size_t, Type>;
     using alloc_traits = std::allocator_traits<Allocator>;
@@ -227,27 +204,24 @@ class dense_set {
     using sparse_container_type = std::vector<std::size_t, typename alloc_traits::template rebind_alloc<std::size_t>>;
     using packed_container_type = std::vector<node_type, typename alloc_traits::template rebind_alloc<node_type>>;
 
-    template<typename Other>
-    [[nodiscard]] std::size_t value_to_bucket(const Other &value) const noexcept {
+    [[nodiscard]] std::size_t value_to_bucket(const auto &value) const noexcept {
         return fast_mod(static_cast<size_type>(sparse.second()(value)), bucket_count());
     }
 
-    template<typename Other>
-    [[nodiscard]] auto constrained_find(const Other &value, std::size_t bucket) {
-        for(auto it = begin(bucket), last = end(bucket); it != last; ++it) {
-            if(packed.second()(*it, value)) {
-                return begin() + static_cast<typename iterator::difference_type>(it.index());
+    [[nodiscard]] auto constrained_find(const auto &value, const std::size_t bucket) {
+        for(auto offset = sparse.first()[bucket]; offset != placeholder_position; offset = packed.first()[offset].first) {
+            if(packed.second()(packed.first()[offset].second, value)) {
+                return begin() + static_cast<iterator::difference_type>(offset);
             }
         }
 
         return end();
     }
 
-    template<typename Other>
-    [[nodiscard]] auto constrained_find(const Other &value, std::size_t bucket) const {
-        for(auto it = cbegin(bucket), last = cend(bucket); it != last; ++it) {
-            if(packed.second()(*it, value)) {
-                return cbegin() + static_cast<typename iterator::difference_type>(it.index());
+    [[nodiscard]] auto constrained_find(const auto &value, const std::size_t bucket) const {
+        for(auto offset = sparse.first()[bucket]; offset != placeholder_position; offset = packed.first()[offset].first) {
+            if(packed.second()(packed.first()[offset].second, value)) {
+                return cbegin() + static_cast<const_iterator::difference_type>(offset);
             }
         }
 
@@ -281,8 +255,8 @@ class dense_set {
     }
 
     void rehash_if_required() {
-        if(size() > (bucket_count() * max_load_factor())) {
-            rehash(bucket_count() * 2u);
+        if(const auto bc = bucket_count(); size() > static_cast<size_type>(static_cast<float>(bc) * max_load_factor())) {
+            rehash(bc * 2u);
         }
     }
 
@@ -295,22 +269,24 @@ public:
     using value_type = Type;
     /*! @brief Unsigned integer type. */
     using size_type = std::size_t;
+    /*! @brief Signed integer type. */
+    using difference_type = std::ptrdiff_t;
     /*! @brief Type of function to use to hash the elements. */
     using hasher = Hash;
     /*! @brief Type of function to use to compare the elements for equality. */
     using key_equal = KeyEqual;
     /*! @brief Random access iterator type. */
-    using iterator = internal::dense_set_iterator<typename packed_container_type::iterator>;
+    using iterator = internal::dense_set_iterator<typename packed_container_type::pointer>;
     /*! @brief Constant random access iterator type. */
-    using const_iterator = internal::dense_set_iterator<typename packed_container_type::const_iterator>;
+    using const_iterator = internal::dense_set_iterator<typename packed_container_type::const_pointer>;
     /*! @brief Reverse iterator type. */
     using reverse_iterator = std::reverse_iterator<iterator>;
     /*! @brief Constant reverse iterator type. */
     using const_reverse_iterator = std::reverse_iterator<const_iterator>;
     /*! @brief Forward iterator type. */
-    using local_iterator = internal::dense_set_local_iterator<typename packed_container_type::iterator>;
+    using local_iterator = internal::dense_set_local_iterator<typename packed_container_type::pointer>;
     /*! @brief Constant forward iterator type. */
-    using const_local_iterator = internal::dense_set_local_iterator<typename packed_container_type::const_iterator>;
+    using const_local_iterator = internal::dense_set_local_iterator<typename packed_container_type::const_pointer>;
 
     /*! @brief Default constructor. */
     dense_set()
@@ -397,6 +373,17 @@ public:
      */
     dense_set &operator=(dense_set &&) noexcept = default;
 
+    /**
+     * @brief Exchanges the contents with those of a given container.
+     * @param other Container to exchange the content with.
+     */
+    void swap(dense_set &other) noexcept {
+        using std::swap;
+        swap(sparse, other.sparse);
+        swap(packed, other.packed);
+        swap(threshold, other.threshold);
+    }
+
     /**
      * @brief Returns the associated allocator.
      * @return The associated allocator.
@@ -413,7 +400,7 @@ public:
      * @return An iterator to the first instance of the internal array.
      */
     [[nodiscard]] const_iterator cbegin() const noexcept {
-        return packed.first().begin();
+        return packed.first().data();
     }
 
     /*! @copydoc cbegin */
@@ -423,7 +410,7 @@ public:
 
     /*! @copydoc begin */
     [[nodiscard]] iterator begin() noexcept {
-        return packed.first().begin();
+        return packed.first().data();
     }
 
     /**
@@ -432,7 +419,7 @@ public:
      * internal array.
      */
     [[nodiscard]] const_iterator cend() const noexcept {
-        return packed.first().end();
+        return packed.first().data() + packed.first().size();
     }
 
     /*! @copydoc cend */
@@ -442,7 +429,7 @@ public:
 
     /*! @copydoc end */
     [[nodiscard]] iterator end() noexcept {
-        return packed.first().end();
+        return packed.first().data() + packed.first().size();
     }
 
     /**
@@ -534,12 +521,10 @@ public:
 
     /**
      * @brief Inserts elements into the container, if they do not exist.
-     * @tparam It Type of input iterator.
      * @param first An iterator to the first element of the range of elements.
      * @param last An iterator past the last element of the range of elements.
      */
-    template<typename It>
-    void insert(It first, It last) {
+    void insert(stl::input_iterator auto first, stl::input_iterator auto last) {
         for(; first != last; ++first) {
             insert(*first);
         }
@@ -599,7 +584,7 @@ public:
         const auto dist = first - cbegin();
 
         for(auto from = last - cbegin(); from != dist; --from) {
-            erase(packed.first()[from - 1u].second);
+            erase(packed.first()[static_cast<size_type>(from) - 1u].second);
         }
 
         return (begin() + dist);
@@ -611,7 +596,7 @@ public:
      * @return Number of elements removed (either 0 or 1).
      */
     size_type erase(const value_type &value) {
-        for(size_type *curr = &sparse.first()[value_to_bucket(value)]; *curr != (std::numeric_limits<size_type>::max)(); curr = &packed.first()[*curr].first) {
+        for(size_type *curr = &sparse.first()[value_to_bucket(value)]; *curr != placeholder_position; curr = &packed.first()[*curr].first) {
             if(packed.second()(packed.first()[*curr].second, value)) {
                 const auto index = *curr;
                 *curr = packed.first()[*curr].first;
@@ -623,17 +608,6 @@ public:
         return 0u;
     }
 
-    /**
-     * @brief Exchanges the contents with those of a given container.
-     * @param other Container to exchange the content with.
-     */
-    void swap(dense_set &other) noexcept {
-        using std::swap;
-        swap(sparse, other.sparse);
-        swap(packed, other.packed);
-        swap(threshold, other.threshold);
-    }
-
     /**
      * @brief Returns the number of elements matching a value (either 1 or 0).
      * @param key Key value of an element to search for.
@@ -645,13 +619,11 @@ public:
 
     /**
      * @brief Returns the number of elements matching a key (either 1 or 0).
-     * @tparam Other Type of the key value of an element to search for.
      * @param key Key value of an element to search for.
      * @return Number of elements matching the key (either 1 or 0).
      */
-    template<typename Other>
-    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, size_type>>
-    count(const Other &key) const {
+    [[nodiscard]] size_type count(const auto &key) const
+    requires is_transparent_v<hasher> && is_transparent_v<key_equal> {
         return find(key) != end();
     }
 
@@ -672,21 +644,18 @@ public:
 
     /**
      * @brief Finds an element that compares _equivalent_ to a given value.
-     * @tparam Other Type of an element to search for.
      * @param value Value of an element to search for.
      * @return An iterator to an element with the given value. If no such
      * element is found, a past-the-end iterator is returned.
      */
-    template<typename Other>
-    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, iterator>>
-    find(const Other &value) {
+    [[nodiscard]] iterator find(const auto &value)
+    requires is_transparent_v<hasher> && is_transparent_v<key_equal> {
         return constrained_find(value, value_to_bucket(value));
     }
 
     /*! @copydoc find */
-    template<typename Other>
-    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, const_iterator>>
-    find(const Other &value) const {
+    [[nodiscard]] const_iterator find(const auto &value) const
+    requires is_transparent_v<hasher> && is_transparent_v<key_equal> {
         return constrained_find(value, value_to_bucket(value));
     }
 
@@ -710,22 +679,19 @@ public:
     /**
      * @brief Returns a range containing all elements that compare _equivalent_
      * to a given value.
-     * @tparam Other Type of an element to search for.
      * @param value Value of an element to search for.
      * @return A pair of iterators pointing to the first element and past the
      * last element of the range.
      */
-    template<typename Other>
-    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, std::pair<iterator, iterator>>>
-    equal_range(const Other &value) {
+    [[nodiscard]] std::pair<iterator, iterator> equal_range(const auto &value)
+    requires is_transparent_v<hasher> && is_transparent_v<key_equal> {
         const auto it = find(value);
         return {it, it + !(it == end())};
     }
 
     /*! @copydoc equal_range */
-    template<typename Other>
-    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, std::pair<const_iterator, const_iterator>>>
-    equal_range(const Other &value) const {
+    [[nodiscard]] std::pair<const_iterator, const_iterator> equal_range(const auto &value) const
+    requires is_transparent_v<hasher> && is_transparent_v<key_equal> {
         const auto it = find(value);
         return {it, it + !(it == cend())};
     }
@@ -742,13 +708,11 @@ public:
     /**
      * @brief Checks if the container contains an element that compares
      * _equivalent_ to a given value.
-     * @tparam Other Type of an element to search for.
      * @param value Value of an element to search for.
      * @return True if there is such an element, false otherwise.
      */
-    template<typename Other>
-    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, bool>>
-    contains(const Other &value) const {
+    [[nodiscard]] bool contains(const auto &value) const
+    requires is_transparent_v<hasher> && is_transparent_v<key_equal> {
         return (find(value) != cend());
     }
 
@@ -758,7 +722,7 @@ public:
      * @return An iterator to the beginning of the given bucket.
      */
     [[nodiscard]] const_local_iterator cbegin(const size_type index) const {
-        return {packed.first().begin(), sparse.first()[index]};
+        return {packed.first().data(), sparse.first()[index]};
     }
 
     /**
@@ -776,7 +740,7 @@ public:
      * @return An iterator to the beginning of the given bucket.
      */
     [[nodiscard]] local_iterator begin(const size_type index) {
-        return {packed.first().begin(), sparse.first()[index]};
+        return {packed.first().data(), sparse.first()[index]};
     }
 
     /**
@@ -785,7 +749,7 @@ public:
      * @return An iterator to the end of the given bucket.
      */
     [[nodiscard]] const_local_iterator cend([[maybe_unused]] const size_type index) const {
-        return {packed.first().begin(), (std::numeric_limits<size_type>::max)()};
+        return {};
     }
 
     /**
@@ -803,7 +767,7 @@ public:
      * @return An iterator to the end of the given bucket.
      */
     [[nodiscard]] local_iterator end([[maybe_unused]] const size_type index) {
-        return {packed.first().begin(), (std::numeric_limits<size_type>::max)()};
+        return {};
     }
 
     /**
@@ -845,7 +809,7 @@ public:
      * @return The average number of elements per bucket.
      */
     [[nodiscard]] float load_factor() const {
-        return size() / static_cast<float>(bucket_count());
+        return static_cast<float>(size()) / static_cast<float>(bucket_count());
     }
 
     /**
@@ -873,14 +837,14 @@ public:
      */
     void rehash(const size_type cnt) {
         auto value = cnt > minimum_capacity ? cnt : minimum_capacity;
-        const auto cap = static_cast<size_type>(size() / max_load_factor());
+        const auto cap = static_cast<size_type>(static_cast<float>(size()) / max_load_factor());
         value = value > cap ? value : cap;
 
-        if(const auto sz = next_power_of_two(value); sz != bucket_count()) {
+        if(const auto sz = std::bit_ceil(value); sz != bucket_count()) {
             sparse.first().resize(sz);
 
             for(auto &&elem: sparse.first()) {
-                elem = (std::numeric_limits<size_type>::max)();
+                elem = placeholder_position;
             }
 
             for(size_type pos{}, last = size(); pos < last; ++pos) {
@@ -897,7 +861,7 @@ public:
      */
     void reserve(const size_type cnt) {
         packed.first().reserve(cnt);
-        rehash(static_cast<size_type>(std::ceil(cnt / max_load_factor())));
+        rehash(static_cast<size_type>(std::ceil(static_cast<float>(cnt) / max_load_factor())));
     }
 
     /**

src/entt/container/table.hpp

@@ -1,10 +1,10 @@
 #ifndef ENTT_CONTAINER_TABLE_HPP
 #define ENTT_CONTAINER_TABLE_HPP
 
+#include <concepts>
 #include <cstddef>
 #include <iterator>
 #include <tuple>
-#include <type_traits>
 #include <utility>
 #include "../config/config.h"
 #include "../core/iterator.hpp"
@@ -12,7 +12,7 @@
 
 namespace entt {
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
 template<typename... It>
@@ -34,7 +34,8 @@ public:
     constexpr table_iterator(It... from) noexcept
         : it{from...} {}
 
-    template<typename... Other, typename = std::enable_if_t<(std::is_constructible_v<It, Other> && ...)>>
+    template<typename... Other>
+    requires (std::constructible_from<It, Other> && ...)
     constexpr table_iterator(const table_iterator<Other...> &other) noexcept
         : table_iterator{std::get<Other>(other.it)...} {}
 
@@ -43,7 +44,7 @@ public:
     }
 
     constexpr table_iterator operator++(int) noexcept {
-        table_iterator orig = *this;
+        const table_iterator orig = *this;
         return ++(*this), orig;
     }
 
@@ -52,7 +53,7 @@ public:
     }
 
     constexpr table_iterator operator--(int) noexcept {
-        table_iterator orig = *this;
+        const table_iterator orig = *this;
         return operator--(), orig;
     }
 
@@ -85,54 +86,25 @@ public:
         return operator[](0);
     }
 
-    template<typename... Lhs, typename... Rhs>
-    friend constexpr std::ptrdiff_t operator-(const table_iterator<Lhs...> &, const table_iterator<Rhs...> &) noexcept;
+    template<typename... Other>
+    [[nodiscard]] constexpr std::ptrdiff_t operator-(const table_iterator<Other...> &other) const noexcept {
+        return std::get<0>(it) - std::get<0>(other.it);
+    }
 
-    template<typename... Lhs, typename... Rhs>
-    friend constexpr bool operator==(const table_iterator<Lhs...> &, const table_iterator<Rhs...> &) noexcept;
+    template<typename... Other>
+    [[nodiscard]] constexpr bool operator==(const table_iterator<Other...> &other) const noexcept {
+        return std::get<0>(it) == std::get<0>(other.it);
+    }
 
-    template<typename... Lhs, typename... Rhs>
-    friend constexpr bool operator<(const table_iterator<Lhs...> &, const table_iterator<Rhs...> &) noexcept;
+    template<typename... Other>
+    [[nodiscard]] constexpr auto operator<=>(const table_iterator<Other...> &other) const noexcept {
+        return std::get<0>(it) <=> std::get<0>(other.it);
+    }
 
 private:
     std::tuple<It...> it;
 };
 
-template<typename... Lhs, typename... Rhs>
-[[nodiscard]] constexpr std::ptrdiff_t operator-(const table_iterator<Lhs...> &lhs, const table_iterator<Rhs...> &rhs) noexcept {
-    return std::get<0>(lhs.it) - std::get<0>(rhs.it);
-}
-
-template<typename... Lhs, typename... Rhs>
-[[nodiscard]] constexpr bool operator==(const table_iterator<Lhs...> &lhs, const table_iterator<Rhs...> &rhs) noexcept {
-    return std::get<0>(lhs.it) == std::get<0>(rhs.it);
-}
-
-template<typename... Lhs, typename... Rhs>
-[[nodiscard]] constexpr bool operator!=(const table_iterator<Lhs...> &lhs, const table_iterator<Rhs...> &rhs) noexcept {
-    return !(lhs == rhs);
-}
-
-template<typename... Lhs, typename... Rhs>
-[[nodiscard]] constexpr bool operator<(const table_iterator<Lhs...> &lhs, const table_iterator<Rhs...> &rhs) noexcept {
-    return std::get<0>(lhs.it) < std::get<0>(rhs.it);
-}
-
-template<typename... Lhs, typename... Rhs>
-[[nodiscard]] constexpr bool operator>(const table_iterator<Lhs...> &lhs, const table_iterator<Rhs...> &rhs) noexcept {
-    return rhs < lhs;
-}
-
-template<typename... Lhs, typename... Rhs>
-[[nodiscard]] constexpr bool operator<=(const table_iterator<Lhs...> &lhs, const table_iterator<Rhs...> &rhs) noexcept {
-    return !(lhs > rhs);
-}
-
-template<typename... Lhs, typename... Rhs>
-[[nodiscard]] constexpr bool operator>=(const table_iterator<Lhs...> &lhs, const table_iterator<Rhs...> &rhs) noexcept {
-    return !(lhs < rhs);
-}
-
 } // namespace internal
 /*! @endcond */
 
@@ -152,6 +124,8 @@ class basic_table {
 public:
     /*! @brief Unsigned integer type. */
     using size_type = std::size_t;
+    /*! @brief Signed integer type. */
+    using difference_type = std::ptrdiff_t;
     /*! @brief Input iterator type. */
     using iterator = internal::table_iterator<typename Container::iterator...>;
     /*! @brief Constant input iterator type. */
@@ -196,11 +170,9 @@ public:
 
     /**
      * @brief Constructs the underlying containers using a given allocator.
-     * @tparam Allocator Type of allocator.
      * @param allocator A valid allocator.
      */
-    template<typename Allocator>
-    explicit basic_table(const Allocator &allocator)
+    explicit basic_table(const auto &allocator)
         : payload{Container{allocator}...} {}
 
     /**
@@ -417,7 +389,7 @@ public:
      */
     void erase(const size_type pos) {
         ENTT_ASSERT(pos < size(), "Index out of bounds");
-        erase(begin() + static_cast<typename const_iterator::difference_type>(pos));
+        erase(begin() + static_cast<difference_type>(pos));
     }
 
     /**
@@ -447,7 +419,7 @@ private:
 
 } // namespace entt
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace std {
 
 template<typename... Container, typename Allocator>

src/entt/core/algorithm.hpp

@@ -2,11 +2,13 @@
 #define ENTT_CORE_ALGORITHM_HPP
 
 #include <algorithm>
+#include <concepts>
 #include <functional>
 #include <iterator>
 #include <utility>
 #include <vector>
-#include "utility.hpp"
+#include "../stl/functional.hpp"
+#include "../stl/iterator.hpp"
 
 namespace entt {
 
@@ -24,7 +26,6 @@ struct std_sort {
      *
      * Sorts the elements in a range using the given binary comparison function.
      *
-     * @tparam It Type of random access iterator.
      * @tparam Compare Type of comparison function object.
      * @tparam Args Types of arguments to forward to the sort function.
      * @param first An iterator to the first element of the range to sort.
@@ -32,8 +33,8 @@ struct std_sort {
      * @param compare A valid comparison function object.
      * @param args Arguments to forward to the sort function, if any.
      */
-    template<typename It, typename Compare = std::less<>, typename... Args>
-    void operator()(It first, It last, Compare compare = Compare{}, Args &&...args) const {
+    template<typename Compare = std::less<>, typename... Args>
+    void operator()(stl::random_access_iterator auto first, stl::random_access_iterator auto last, Compare compare = Compare{}, Args &&...args) const {
         std::sort(std::forward<Args>(args)..., std::move(first), std::move(last), std::move(compare));
     }
 };
@@ -45,14 +46,13 @@ struct insertion_sort {
      *
      * Sorts the elements in a range using the given binary comparison function.
      *
-     * @tparam It Type of random access iterator.
      * @tparam Compare Type of comparison function object.
      * @param first An iterator to the first element of the range to sort.
      * @param last An iterator past the last element of the range to sort.
      * @param compare A valid comparison function object.
      */
-    template<typename It, typename Compare = std::less<>>
-    void operator()(It first, It last, Compare compare = Compare{}) const {
+    template<typename Compare = std::less<>>
+    void operator()(stl::random_access_iterator auto first, stl::random_access_iterator auto last, Compare compare = Compare{}) const {
         if(first < last) {
             for(auto it = first + 1; it < last; ++it) {
                 auto value = std::move(*it);
@@ -76,9 +76,8 @@ struct insertion_sort {
  * @tparam N Maximum number of bits to sort.
  */
 template<std::size_t Bit, std::size_t N>
+requires ((N % Bit) == 0) // The maximum number of bits to sort must be a multiple of the number of bits processed per pass
 struct radix_sort {
-    static_assert((N % Bit) == 0, "The maximum number of bits to sort must be a multiple of the number of bits processed per pass");
-
     /**
      * @brief Sorts the elements in a range.
      *
@@ -94,19 +93,20 @@ struct radix_sort {
      * @param last An iterator past the last element of the range to sort.
      * @param getter A valid _getter_ function object.
      */
-    template<typename It, typename Getter = identity>
+    template<stl::random_access_iterator It, typename Getter = stl::identity>
     void operator()(It first, It last, Getter getter = Getter{}) const {
         if(first < last) {
             constexpr auto passes = N / Bit;
 
-            using value_type = typename std::iterator_traits<It>::value_type;
-            std::vector<value_type> aux(std::distance(first, last));
+            using value_type = std::iterator_traits<It>::value_type;
+            using difference_type = std::iterator_traits<It>::difference_type;
+            std::vector<value_type> aux(static_cast<std::size_t>(std::distance(first, last)));
 
             auto part = [getter = std::move(getter)](auto from, auto to, auto out, auto start) {
                 constexpr auto mask = (1 << Bit) - 1;
                 constexpr auto buckets = 1 << Bit;
 
-                // NOLINTNEXTLINE(cppcoreguidelines-avoid-c-arrays, modernize-avoid-c-arrays)
+                // NOLINTNEXTLINE(cppcoreguidelines-avoid-c-arrays, modernize-avoid-c-arrays, misc-const-correctness)
                 std::size_t count[buckets]{};
 
                 for(auto it = from; it != to; ++it) {
@@ -121,11 +121,12 @@ struct radix_sort {
                 }
 
                 for(auto it = from; it != to; ++it) {
-                    out[index[(getter(*it) >> start) & mask]++] = std::move(*it);
+                    const auto pos = index[(getter(*it) >> start) & mask]++;
+                    out[static_cast<difference_type>(pos)] = std::move(*it);
                 }
             };
 
-            for(std::size_t pass = 0; pass < (passes & ~1); pass += 2) {
+            for(std::size_t pass = 0; pass < (passes & ~1u); pass += 2) {
                 part(first, last, aux.begin(), pass * Bit);
                 part(aux.begin(), aux.end(), first, (pass + 1) * Bit);
             }

src/entt/core/any.hpp

@@ -1,161 +1,185 @@
 #ifndef ENTT_CORE_ANY_HPP
 #define ENTT_CORE_ANY_HPP
 
+#include <concepts>
 #include <cstddef>
 #include <memory>
 #include <type_traits>
 #include <utility>
 #include "../config/config.h"
-#include "../core/utility.hpp"
+#include "../core/concepts.hpp"
 #include "fwd.hpp"
 #include "type_info.hpp"
 #include "type_traits.hpp"
+#include "utility.hpp"
 
 namespace entt {
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
-enum class any_operation : std::uint8_t {
-    copy,
-    move,
+enum class any_request : std::uint8_t {
+    info,
     transfer,
     assign,
-    destroy,
     compare,
-    get
+    copy,
+    move
 };
 
+template<std::size_t Len, std::size_t Align>
+struct basic_any_storage {
+    static constexpr bool has_buffer = true;
+    union {
+        const void *instance{};
+        // NOLINTNEXTLINE(cppcoreguidelines-avoid-c-arrays, modernize-avoid-c-arrays)
+        alignas(Align) std::byte buffer[Len];
+    };
+};
+
+template<std::size_t Align>
+struct basic_any_storage<0u, Align> {
+    static constexpr bool has_buffer = false;
+    const void *instance{};
+};
+
+template<typename Type, std::size_t Len, std::size_t Align>
+// NOLINTNEXTLINE(bugprone-sizeof-expression)
+struct in_situ: std::bool_constant<(Len != 0u) && alignof(Type) <= Align && sizeof(Type) <= Len && std::is_nothrow_move_constructible_v<Type>> {};
+
+template<std::size_t Len, std::size_t Align>
+struct in_situ<void, Len, Align>: std::false_type {};
+
 } // namespace internal
 /*! @endcond */
 
-/*! @brief Possible modes of an any object. */
-enum class any_policy : std::uint8_t {
-    /*! @brief Default mode, the object owns the contained element. */
-    owner,
-    /*! @brief Aliasing mode, the object _points_ to a non-const element. */
-    ref,
-    /*! @brief Const aliasing mode, the object _points_ to a const element. */
-    cref
-};
-
 /**
  * @brief A SBO friendly, type-safe container for single values of any type.
- * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Len Size of the buffer reserved for the small buffer optimization.
  * @tparam Align Optional alignment requirement.
  */
 template<std::size_t Len, std::size_t Align>
-class basic_any {
-    using operation = internal::any_operation;
-    using vtable_type = const void *(const operation, const basic_any &, const void *);
-
-    struct storage_type {
-        // NOLINTNEXTLINE(cppcoreguidelines-avoid-c-arrays, modernize-avoid-c-arrays)
-        alignas(Align) std::byte data[Len + static_cast<std::size_t>(Len == 0u)];
-    };
+class basic_any: private internal::basic_any_storage<Len, Align> {
+    using request = internal::any_request;
+    using base_type = internal::basic_any_storage<Len, Align>;
+    using vtable_type = const void *(const request, const basic_any &, const void *);
+    using deleter_type = void(const basic_any &);
 
     template<typename Type>
-    static constexpr bool in_situ = (Len != 0u) && alignof(Type) <= Align && sizeof(Type) <= Len && std::is_nothrow_move_constructible_v<Type>;
+    static constexpr bool in_situ_v = internal::in_situ<Type, Len, Align>::value;
 
-    template<typename Type>
-    static const void *basic_vtable(const operation op, const basic_any &value, const void *other) {
-        static_assert(!std::is_void_v<Type> && std::is_same_v<std::remove_cv_t<std::remove_reference_t<Type>>, Type>, "Invalid type");
-        const Type *elem = nullptr;
-
-        if constexpr(in_situ<Type>) {
-            elem = (value.mode == any_policy::owner) ? reinterpret_cast<const Type *>(&value.storage) : static_cast<const Type *>(value.instance);
-        } else {
-            elem = static_cast<const Type *>(value.instance);
-        }
-
-        switch(op) {
-        case operation::copy:
-            if constexpr(std::is_copy_constructible_v<Type>) {
-                static_cast<basic_any *>(const_cast<void *>(other))->initialize<Type>(*elem);
-            }
-            break;
-        case operation::move:
-            if constexpr(in_situ<Type>) {
-                if(value.mode == any_policy::owner) {
-                    return ::new(&static_cast<basic_any *>(const_cast<void *>(other))->storage) Type{std::move(*const_cast<Type *>(elem))};
-                }
-            }
-
-            return (static_cast<basic_any *>(const_cast<void *>(other))->instance = std::exchange(const_cast<basic_any &>(value).instance, nullptr));
-        case operation::transfer:
+    template<cvref_unqualified Type>
+    static const void *basic_vtable(const request req, const basic_any &value, const void *other) {
+        switch(const auto *elem = static_cast<const Type *>(value.data()); req) {
+            using enum internal::any_request;
+        case info:
+            return &type_id<Type>();
+        case transfer:
             if constexpr(std::is_move_assignable_v<Type>) {
+                // NOLINTNEXTLINE(bugprone-casting-through-void)
                 *const_cast<Type *>(elem) = std::move(*static_cast<Type *>(const_cast<void *>(other)));
                 return other;
             }
             [[fallthrough]];
-        case operation::assign:
+        case assign:
             if constexpr(std::is_copy_assignable_v<Type>) {
                 *const_cast<Type *>(elem) = *static_cast<const Type *>(other);
                 return other;
             }
             break;
-        case operation::destroy:
-            if constexpr(in_situ<Type>) {
-                elem->~Type();
-            } else if constexpr(std::is_array_v<Type>) {
-                delete[] elem;
-            } else {
-                delete elem;
-            }
-            break;
-        case operation::compare:
+        case compare:
             if constexpr(!std::is_function_v<Type> && !std::is_array_v<Type> && is_equality_comparable_v<Type>) {
-                return *elem == *static_cast<const Type *>(other) ? other : nullptr;
+                return (*elem == *static_cast<const Type *>(other)) ? other : nullptr;
             } else {
                 return (elem == other) ? other : nullptr;
             }
-        case operation::get:
-            return elem;
+        case copy:
+            if constexpr(std::is_copy_constructible_v<Type>) {
+                // NOLINTNEXTLINE(bugprone-casting-through-void)
+                static_cast<basic_any *>(const_cast<void *>(other))->initialize<Type>(*elem);
+            }
+            break;
+        case move:
+            ENTT_ASSERT(value.mode == any_policy::embedded, "Unexpected policy");
+            if constexpr(in_situ_v<Type>) {
+                // NOLINTNEXTLINE(bugprone-casting-through-void, bugprone-multi-level-implicit-pointer-conversion)
+                return ::new(&static_cast<basic_any *>(const_cast<void *>(other))->buffer) Type{std::move(*const_cast<Type *>(elem))};
+            }
         }
 
         return nullptr;
     }
 
+    template<cvref_unqualified Type>
+    static void basic_deleter(const basic_any &value) {
+        ENTT_ASSERT((value.mode == any_policy::dynamic) || ((value.mode == any_policy::embedded) && !std::is_trivially_destructible_v<Type>), "Unexpected policy");
+
+        const auto *elem = static_cast<const Type *>(value.data());
+
+        if constexpr(in_situ_v<Type>) {
+            (value.mode == any_policy::embedded) ? elem->~Type() : (delete elem);
+        } else if constexpr(std::is_array_v<Type>) {
+            delete[] elem;
+        } else {
+            delete elem;
+        }
+    }
+
     template<typename Type, typename... Args>
     void initialize([[maybe_unused]] Args &&...args) {
-        using plain_type = std::remove_cv_t<std::remove_reference_t<Type>>;
-        info = &type_id<plain_type>();
+        using plain_type = std::remove_cvref_t<Type>;
 
-        if constexpr(!std::is_void_v<Type>) {
-            vtable = basic_vtable<plain_type>;
+        vtable = basic_vtable<plain_type>;
+        underlying_type = type_hash<plain_type>::value();
 
-            if constexpr(std::is_lvalue_reference_v<Type>) {
-                static_assert((std::is_lvalue_reference_v<Args> && ...) && (sizeof...(Args) == 1u), "Invalid arguments");
-                mode = std::is_const_v<std::remove_reference_t<Type>> ? any_policy::cref : any_policy::ref;
-                instance = (std::addressof(args), ...);
-            } else if constexpr(in_situ<plain_type>) {
-                if constexpr(std::is_aggregate_v<plain_type> && (sizeof...(Args) != 0u || !std::is_default_constructible_v<plain_type>)) {
-                    ::new(&storage) plain_type{std::forward<Args>(args)...};
-                } else {
-                    // NOLINTNEXTLINE(cppcoreguidelines-pro-bounds-array-to-pointer-decay)
-                    ::new(&storage) plain_type(std::forward<Args>(args)...);
-                }
+        if constexpr(std::is_void_v<Type>) {
+            deleter = nullptr;
+            mode = any_policy::empty;
+            this->instance = nullptr;
+        } else if constexpr(std::is_lvalue_reference_v<Type>) {
+            deleter = nullptr;
+            mode = std::is_const_v<std::remove_reference_t<Type>> ? any_policy::cref : any_policy::ref;
+            static_assert((std::is_lvalue_reference_v<Args> && ...) && (sizeof...(Args) == 1u), "Invalid arguments");
+            // NOLINTNEXTLINE(bugprone-multi-level-implicit-pointer-conversion)
+            this->instance = (std::addressof(args), ...);
+        } else if constexpr(in_situ_v<plain_type>) {
+            if constexpr(std::is_trivially_destructible_v<plain_type>) {
+                deleter = nullptr;
             } else {
-                if constexpr(std::is_aggregate_v<plain_type> && (sizeof...(Args) != 0u || !std::is_default_constructible_v<plain_type>)) {
-                    instance = new plain_type{std::forward<Args>(args)...};
-                } else if constexpr(std::is_array_v<plain_type>) {
-                    static_assert(sizeof...(Args) == 0u, "Invalid arguments");
-                    instance = new plain_type[std::extent_v<plain_type>]();
-                } else {
-                    instance = new plain_type(std::forward<Args>(args)...);
-                }
+                deleter = &basic_deleter<plain_type>;
+            }
+
+            mode = any_policy::embedded;
+
+            if constexpr(std::is_aggregate_v<plain_type> && (sizeof...(Args) != 0u || !std::is_default_constructible_v<plain_type>)) {
+                ::new(&this->buffer) plain_type{std::forward<Args>(args)...};
+            } else {
+                // NOLINTNEXTLINE(cppcoreguidelines-pro-bounds-array-to-pointer-decay)
+                ::new(&this->buffer) plain_type(std::forward<Args>(args)...);
+            }
+        } else {
+            deleter = &basic_deleter<plain_type>;
+            mode = any_policy::dynamic;
+
+            if constexpr(std::is_aggregate_v<plain_type> && (sizeof...(Args) != 0u || !std::is_default_constructible_v<plain_type>)) {
+                this->instance = new plain_type{std::forward<Args>(args)...};
+            } else if constexpr(std::is_array_v<plain_type>) {
+                static_assert(sizeof...(Args) == 0u, "Invalid arguments");
+                this->instance = new plain_type[std::extent_v<plain_type>]();
+            } else {
+                this->instance = new plain_type(std::forward<Args>(args)...);
             }
         }
     }
 
-    basic_any(const basic_any &other, const any_policy pol) noexcept
-        : instance{other.data()},
-          info{other.info},
-          vtable{other.vtable},
-          mode{pol} {}
+    void invoke_deleter_if_exists() {
+        if(deleter != nullptr) {
+            deleter(*this);
+        }
+    }
 
 public:
-    /*! @brief Size of the internal storage. */
+    /*! @brief Size of the internal buffer. */
     static constexpr auto length = Len;
     /*! @brief Alignment requirement. */
     static constexpr auto alignment = Align;
@@ -172,19 +196,35 @@ public:
      */
     template<typename Type, typename... Args>
     explicit basic_any(std::in_place_type_t<Type>, Args &&...args)
-        : instance{},
-          info{},
-          vtable{},
-          mode{any_policy::owner} {
+        : base_type{} {
         initialize<Type>(std::forward<Args>(args)...);
     }
 
+    /**
+     * @brief Constructs a wrapper taking ownership of the passed object.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value A pointer to an object to take ownership of.
+     */
+    template<typename Type>
+    requires (!std::is_const_v<Type> && !std::is_void_v<Type>)
+    explicit basic_any(std::in_place_t, Type *value)
+        : base_type{} {
+        if(value == nullptr) {
+            initialize<void>();
+        } else {
+            initialize<Type &>(*value);
+            deleter = &basic_deleter<Type>;
+            mode = any_policy::dynamic;
+        }
+    }
+
     /**
      * @brief Constructs a wrapper from a given value.
      * @tparam Type Type of object to use to initialize the wrapper.
      * @param value An instance of an object to use to initialize the wrapper.
      */
-    template<typename Type, typename = std::enable_if_t<!std::is_same_v<std::decay_t<Type>, basic_any>>>
+    template<typename Type>
+    requires (!std::same_as<std::remove_cvref_t<Type>, basic_any>)
     basic_any(Type &&value)
         : basic_any{std::in_place_type<std::decay_t<Type>>, std::forward<Type>(value)} {}
 
@@ -194,9 +234,7 @@ public:
      */
     basic_any(const basic_any &other)
         : basic_any{} {
-        if(other.vtable) {
-            other.vtable(operation::copy, other, this);
-        }
+        other.vtable(request::copy, other, this);
     }
 
     /**
@@ -204,20 +242,21 @@ public:
      * @param other The instance to move from.
      */
     basic_any(basic_any &&other) noexcept
-        : instance{},
-          info{other.info},
+        : base_type{},
           vtable{other.vtable},
+          deleter{other.deleter},
+          underlying_type{other.underlying_type},
           mode{other.mode} {
-        if(other.vtable) {
-            other.vtable(operation::move, other, this);
+        if(other.mode == any_policy::embedded) {
+            other.vtable(request::move, other, this);
+        } else if(other.mode != any_policy::empty) {
+            this->instance = std::exchange(other.instance, nullptr);
         }
     }
 
-    /*! @brief Frees the internal storage, whatever it means. */
+    /*! @brief Frees the internal buffer, whatever it means. */
     ~basic_any() {
-        if(vtable && (mode == any_policy::owner)) {
-            vtable(operation::destroy, *this, nullptr);
-        }
+        invoke_deleter_if_exists();
     }
 
     /**
@@ -227,10 +266,12 @@ public:
      */
     basic_any &operator=(const basic_any &other) {
         if(this != &other) {
-            reset();
+            invoke_deleter_if_exists();
 
-            if(other.vtable) {
-                other.vtable(operation::copy, other, this);
+            if(other) {
+                other.vtable(request::copy, other, this);
+            } else {
+                initialize<void>();
             }
         }
 
@@ -243,14 +284,18 @@ public:
      * @return This any object.
      */
     basic_any &operator=(basic_any &&other) noexcept {
-        ENTT_ASSERT(this != &other, "Self move assignment");
+        if(this != &other) {
+            invoke_deleter_if_exists();
 
-        reset();
+            if(other.mode == any_policy::embedded) {
+                other.vtable(request::move, other, this);
+            } else if(other.mode != any_policy::empty) {
+                this->instance = std::exchange(other.instance, nullptr);
+            }
 
-        if(other.vtable) {
-            other.vtable(operation::move, other, this);
-            info = other.info;
             vtable = other.vtable;
+            deleter = other.deleter;
+            underlying_type = other.underlying_type;
             mode = other.mode;
         }
 
@@ -263,18 +308,50 @@ public:
      * @param value An instance of an object to use to initialize the wrapper.
      * @return This any object.
      */
-    template<typename Type, typename = std::enable_if_t<!std::is_same_v<std::decay_t<Type>, basic_any>>>
+    template<typename Type>
+    requires (!std::same_as<std::remove_cvref_t<Type>, basic_any>)
     basic_any &operator=(Type &&value) {
         emplace<std::decay_t<Type>>(std::forward<Type>(value));
         return *this;
     }
 
     /**
-     * @brief Returns the object type if any, `type_id<void>()` otherwise.
-     * @return The object type if any, `type_id<void>()` otherwise.
+     * @brief Returns false if a wrapper is empty, true otherwise.
+     * @return False if the wrapper is empty, true otherwise.
      */
-    [[nodiscard]] const type_info &type() const noexcept {
-        return *info;
+    [[nodiscard]] bool has_value() const noexcept {
+        return (mode != any_policy::empty);
+    }
+
+    /**
+     * @brief Returns false if the wrapper does not contain the expected type,
+     * true otherwise.
+     * @param req Expected type.
+     * @return False if the wrapper does not contain the expected type, true
+     * otherwise.
+     */
+    [[nodiscard]] bool has_value(const type_info &req) const noexcept {
+        return (underlying_type == req.hash());
+    }
+
+    /**
+     * @brief Returns false if the wrapper does not contain the expected type,
+     * true otherwise.
+     * @tparam Type Expected type.
+     * @return False if the wrapper does not contain the expected type, true
+     * otherwise.
+     */
+    template<cvref_unqualified Type>
+    [[nodiscard]] bool has_value() const noexcept {
+        return (underlying_type == type_hash<Type>::value());
+    }
+
+    /**
+     * @brief Returns the object type info if any, `type_id<void>()` otherwise.
+     * @return The object type info if any, `type_id<void>()` otherwise.
+     */
+    [[nodiscard]] const type_info &info() const noexcept {
+        return *static_cast<const type_info *>(vtable(request::info, *this, nullptr));
     }
 
     /**
@@ -282,7 +359,11 @@ public:
      * @return An opaque pointer the contained instance, if any.
      */
     [[nodiscard]] const void *data() const noexcept {
-        return vtable ? vtable(operation::get, *this, nullptr) : nullptr;
+        if constexpr(base_type::has_buffer) {
+            return (mode == any_policy::embedded) ? &this->buffer : this->instance;
+        } else {
+            return this->instance;
+        }
     }
 
     /**
@@ -291,7 +372,17 @@ public:
      * @return An opaque pointer the contained instance, if any.
      */
     [[nodiscard]] const void *data(const type_info &req) const noexcept {
-        return *info == req ? data() : nullptr;
+        return has_value(req) ? data() : nullptr;
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @tparam Type Expected type.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    template<typename Type>
+    [[nodiscard]] const Type *data() const noexcept {
+        return has_value<std::remove_const_t<Type>>() ? static_cast<const Type *>(data()) : nullptr;
     }
 
     /**
@@ -299,7 +390,7 @@ public:
      * @return An opaque pointer the contained instance, if any.
      */
     [[nodiscard]] void *data() noexcept {
-        return mode == any_policy::cref ? nullptr : const_cast<void *>(std::as_const(*this).data());
+        return (mode == any_policy::cref) ? nullptr : const_cast<void *>(std::as_const(*this).data());
     }
 
     /**
@@ -308,7 +399,21 @@ public:
      * @return An opaque pointer the contained instance, if any.
      */
     [[nodiscard]] void *data(const type_info &req) noexcept {
-        return mode == any_policy::cref ? nullptr : const_cast<void *>(std::as_const(*this).data(req));
+        return (mode == any_policy::cref) ? nullptr : const_cast<void *>(std::as_const(*this).data(req));
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @tparam Type Expected type.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    template<typename Type>
+    [[nodiscard]] Type *data() noexcept {
+        if constexpr(std::is_const_v<Type>) {
+            return std::as_const(*this).template data<std::remove_const_t<Type>>();
+        } else {
+            return (mode == any_policy::cref) ? nullptr : const_cast<Type *>(std::as_const(*this).template data<std::remove_const_t<Type>>());
+        }
     }
 
     /**
@@ -319,7 +424,7 @@ public:
      */
     template<typename Type, typename... Args>
     void emplace(Args &&...args) {
-        reset();
+        invoke_deleter_if_exists();
         initialize<Type>(std::forward<Args>(args)...);
     }
 
@@ -329,21 +434,18 @@ public:
      * @return True in case of success, false otherwise.
      */
     bool assign(const basic_any &other) {
-        if(vtable && mode != any_policy::cref && *info == *other.info) {
-            return (vtable(operation::assign, *this, other.data()) != nullptr);
+        if(other && (mode != any_policy::cref) && (underlying_type == other.underlying_type)) {
+            return (vtable(request::assign, *this, other.data()) != nullptr);
         }
 
         return false;
     }
 
     /*! @copydoc assign */
+    // NOLINTNEXTLINE(cppcoreguidelines-rvalue-reference-param-not-moved)
     bool assign(basic_any &&other) {
-        if(vtable && mode != any_policy::cref && *info == *other.info) {
-            if(auto *val = other.data(); val) {
-                return (vtable(operation::transfer, *this, val) != nullptr);
-            }
-
-            return (vtable(operation::assign, *this, std::as_const(other).data()) != nullptr);
+        if(other && (mode != any_policy::cref) && (underlying_type == other.underlying_type)) {
+            return (other.mode == any_policy::cref) ? (vtable(request::assign, *this, std::as_const(other).data()) != nullptr) : (vtable(request::transfer, *this, other.data()) != nullptr);
         }
 
         return false;
@@ -351,15 +453,8 @@ public:
 
     /*! @brief Destroys contained object */
     void reset() {
-        if(vtable && (mode == any_policy::owner)) {
-            vtable(operation::destroy, *this, nullptr);
-        }
-
-        // unnecessary but it helps to detect nasty bugs
-        ENTT_ASSERT((instance = nullptr) == nullptr, "");
-        info = &type_id<void>();
-        vtable = nullptr;
-        mode = any_policy::owner;
+        invoke_deleter_if_exists();
+        initialize<void>();
     }
 
     /**
@@ -367,7 +462,7 @@ public:
      * @return False if the wrapper is empty, true otherwise.
      */
     [[nodiscard]] explicit operator bool() const noexcept {
-        return vtable != nullptr;
+        return has_value();
     }
 
     /**
@@ -376,20 +471,11 @@ public:
      * @return False if the two objects differ in their content, true otherwise.
      */
     [[nodiscard]] bool operator==(const basic_any &other) const noexcept {
-        if(vtable && *info == *other.info) {
-            return (vtable(operation::compare, *this, other.data()) != nullptr);
+        if(other && (underlying_type == other.underlying_type)) {
+            return (vtable(request::compare, *this, other.data()) != nullptr);
         }
 
-        return (!vtable && !other.vtable);
-    }
-
-    /**
-     * @brief Checks if two wrappers differ in their content.
-     * @param other Wrapper with which to compare.
-     * @return True if the two objects differ in their content, false otherwise.
-     */
-    [[nodiscard]] bool operator!=(const basic_any &other) const noexcept {
-        return !(*this == other);
+        return (!*this && !other);
     }
 
     /**
@@ -397,12 +483,38 @@ public:
      * @return A wrapper that shares a reference to an unmanaged object.
      */
     [[nodiscard]] basic_any as_ref() noexcept {
-        return basic_any{*this, (mode == any_policy::cref ? any_policy::cref : any_policy::ref)};
+        basic_any other = std::as_const(*this).as_ref();
+
+        switch(mode) {
+            using enum any_policy;
+        case cref:
+        case empty:
+            other.mode = mode;
+            break;
+        default:
+            other.mode = any_policy::ref;
+            break;
+        }
+
+        return other;
     }
 
     /*! @copydoc as_ref */
     [[nodiscard]] basic_any as_ref() const noexcept {
-        return basic_any{*this, any_policy::cref};
+        basic_any other{};
+        other.instance = data();
+        other.vtable = vtable;
+        other.underlying_type = underlying_type;
+        other.mode = any_policy::cref;
+        return other;
+    }
+
+    /**
+     * @brief Returns true if a wrapper owns its object, false otherwise.
+     * @return True if the wrapper owns its object, false otherwise.
+     */
+    [[nodiscard]] bool owner() const noexcept {
+        return (mode == any_policy::dynamic || mode == any_policy::embedded);
     }
 
     /**
@@ -414,19 +526,16 @@ public:
     }
 
 private:
-    union {
-        const void *instance;
-        storage_type storage;
-    };
-    const type_info *info;
-    vtable_type *vtable;
-    any_policy mode;
+    vtable_type *vtable{};
+    deleter_type *deleter{};
+    id_type underlying_type{};
+    any_policy mode{};
 };
 
 /**
  * @brief Performs type-safe access to the contained object.
  * @tparam Type Type to which conversion is required.
- * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Len Size of the buffer reserved for the small buffer optimization.
  * @tparam Align Alignment requirement.
  * @param data Target any object.
  * @return The element converted to the requested type.
@@ -449,8 +558,9 @@ template<typename Type, std::size_t Len, std::size_t Align>
 
 /*! @copydoc any_cast */
 template<typename Type, std::size_t Len, std::size_t Align>
+// NOLINTNEXTLINE(cppcoreguidelines-rvalue-reference-param-not-moved)
 [[nodiscard]] std::remove_const_t<Type> any_cast(basic_any<Len, Align> &&data) noexcept {
-    if constexpr(std::is_copy_constructible_v<std::remove_cv_t<std::remove_reference_t<Type>>>) {
+    if constexpr(std::is_copy_constructible_v<std::remove_cvref_t<Type>>) {
         if(auto *const instance = any_cast<std::remove_reference_t<Type>>(&data); instance) {
             return static_cast<Type>(std::move(*instance));
         }
@@ -466,8 +576,7 @@ template<typename Type, std::size_t Len, std::size_t Align>
 /*! @copydoc any_cast */
 template<typename Type, std::size_t Len, std::size_t Align>
 [[nodiscard]] const Type *any_cast(const basic_any<Len, Align> *data) noexcept {
-    const auto &info = type_id<std::remove_cv_t<Type>>();
-    return static_cast<const Type *>(data->data(info));
+    return data->template data<std::remove_const_t<Type>>();
 }
 
 /*! @copydoc any_cast */
@@ -477,15 +586,14 @@ template<typename Type, std::size_t Len, std::size_t Align>
         // last attempt to make wrappers for const references return their values
         return any_cast<Type>(&std::as_const(*data));
     } else {
-        const auto &info = type_id<std::remove_cv_t<Type>>();
-        return static_cast<Type *>(data->data(info));
+        return data->template data<Type>();
     }
 }
 
 /**
  * @brief Constructs a wrapper from a given type, passing it all arguments.
  * @tparam Type Type of object to use to initialize the wrapper.
- * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Len Size of the buffer reserved for the small buffer optimization.
  * @tparam Align Optional alignment requirement.
  * @tparam Args Types of arguments to use to construct the new instance.
  * @param args Parameters to use to construct the instance.
@@ -498,7 +606,7 @@ template<typename Type, std::size_t Len = basic_any<>::length, std::size_t Align
 
 /**
  * @brief Forwards its argument and avoids copies for lvalue references.
- * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Len Size of the buffer reserved for the small buffer optimization.
  * @tparam Align Optional alignment requirement.
  * @tparam Type Type of argument to use to construct the new instance.
  * @param value Parameter to use to construct the instance.

src/entt/core/attribute.h

@@ -1,30 +0,0 @@
-#ifndef ENTT_CORE_ATTRIBUTE_H
-#define ENTT_CORE_ATTRIBUTE_H
-
-#ifndef ENTT_EXPORT
-#    if defined _WIN32 || defined __CYGWIN__ || defined _MSC_VER
-#        define ENTT_EXPORT __declspec(dllexport)
-#        define ENTT_IMPORT __declspec(dllimport)
-#        define ENTT_HIDDEN
-#    elif defined __GNUC__ && __GNUC__ >= 4
-#        define ENTT_EXPORT __attribute__((visibility("default")))
-#        define ENTT_IMPORT __attribute__((visibility("default")))
-#        define ENTT_HIDDEN __attribute__((visibility("hidden")))
-#    else /* Unsupported compiler */
-#        define ENTT_EXPORT
-#        define ENTT_IMPORT
-#        define ENTT_HIDDEN
-#    endif
-#endif
-
-#ifndef ENTT_API
-#    if defined ENTT_API_EXPORT
-#        define ENTT_API ENTT_EXPORT
-#    elif defined ENTT_API_IMPORT
-#        define ENTT_API ENTT_IMPORT
-#    else /* No API */
-#        define ENTT_API
-#    endif
-#endif
-
-#endif

src/entt/core/bit.hpp

@@ -1,56 +1,13 @@
 #ifndef ENTT_CORE_BIT_HPP
 #define ENTT_CORE_BIT_HPP
 
+#include <bit>
+#include <concepts>
 #include <cstddef>
-#include <limits>
-#include <type_traits>
 #include "../config/config.h"
 
 namespace entt {
 
-/**
- * @brief Returns the number of set bits in a value (waiting for C++20 and
- * `std::popcount`).
- * @tparam Type Unsigned integer type.
- * @param value A value of unsigned integer type.
- * @return The number of set bits in the value.
- */
-template<typename Type>
-[[nodiscard]] constexpr std::enable_if_t<std::is_unsigned_v<Type>, int> popcount(const Type value) noexcept {
-    return value ? (int(value & 1) + popcount(static_cast<Type>(value >> 1))) : 0;
-}
-
-/**
- * @brief Checks whether a value is a power of two or not (waiting for C++20 and
- * `std::has_single_bit`).
- * @tparam Type Unsigned integer type.
- * @param value A value of unsigned integer type.
- * @return True if the value is a power of two, false otherwise.
- */
-template<typename Type>
-[[nodiscard]] constexpr std::enable_if_t<std::is_unsigned_v<Type>, bool> has_single_bit(const Type value) noexcept {
-    return value && ((value & (value - 1)) == 0);
-}
-
-/**
- * @brief Computes the smallest power of two greater than or equal to a value
- * (waiting for C++20 and `std::bit_ceil`).
- * @tparam Type Unsigned integer type.
- * @param value A value of unsigned integer type.
- * @return The smallest power of two greater than or equal to the given value.
- */
-template<typename Type>
-[[nodiscard]] constexpr std::enable_if_t<std::is_unsigned_v<Type>, Type> next_power_of_two(const Type value) noexcept {
-    ENTT_ASSERT_CONSTEXPR(value < (Type{1u} << (std::numeric_limits<Type>::digits - 1)), "Numeric limits exceeded");
-    Type curr = value - (value != 0u);
-
-    for(int next = 1; next < std::numeric_limits<Type>::digits; next = next * 2) {
-        curr |= (curr >> next);
-    }
-
-    return ++curr;
-}
-
 /**
  * @brief Fast module utility function (powers of two only).
  * @tparam Type Unsigned integer type.
@@ -58,10 +15,10 @@ template<typename Type>
  * @param mod _Modulus_, it must be a power of two.
  * @return The common remainder.
  */
-template<typename Type>
-[[nodiscard]] constexpr std::enable_if_t<std::is_unsigned_v<Type>, Type> fast_mod(const Type value, const std::size_t mod) noexcept {
-    ENTT_ASSERT_CONSTEXPR(has_single_bit(mod), "Value must be a power of two");
-    return value & (mod - 1u);
+template<std::unsigned_integral Type>
+[[nodiscard]] constexpr Type fast_mod(const Type value, const std::size_t mod) noexcept {
+    ENTT_ASSERT_CONSTEXPR(std::has_single_bit(mod), "Value must be a power of two");
+    return static_cast<Type>(value & (mod - 1u));
 }
 
 } // namespace entt

src/entt/core/compressed_pair.hpp

@@ -1,6 +1,7 @@
 #ifndef ENTT_CORE_COMPRESSED_PAIR_HPP
 #define ENTT_CORE_COMPRESSED_PAIR_HPP
 
+#include <concepts>
 #include <cstddef>
 #include <tuple>
 #include <type_traits>
@@ -10,20 +11,21 @@
 
 namespace entt {
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
-template<typename Type, std::size_t, typename = void>
+template<typename Type, std::size_t>
 struct compressed_pair_element {
     using reference = Type &;
     using const_reference = const Type &;
 
-    template<typename Dummy = Type, typename = std::enable_if_t<std::is_default_constructible_v<Dummy>>>
     // NOLINTNEXTLINE(modernize-use-equals-default)
-    constexpr compressed_pair_element() noexcept(std::is_nothrow_default_constructible_v<Type>) {}
+    constexpr compressed_pair_element() noexcept(std::is_nothrow_default_constructible_v<Type>)
+    requires std::default_initializable<Type> {}
 
-    template<typename Arg, typename = std::enable_if_t<!std::is_same_v<std::remove_cv_t<std::remove_reference_t<Arg>>, compressed_pair_element>>>
+    template<typename Arg>
     constexpr compressed_pair_element(Arg &&arg) noexcept(std::is_nothrow_constructible_v<Type, Arg>)
+    requires (!std::same_as<std::remove_cvref_t<Arg>, compressed_pair_element>)
         : value{std::forward<Arg>(arg)} {}
 
     template<typename... Args, std::size_t... Index>
@@ -43,17 +45,19 @@ private:
 };
 
 template<typename Type, std::size_t Tag>
-struct compressed_pair_element<Type, Tag, std::enable_if_t<is_ebco_eligible_v<Type>>>: Type {
+requires is_ebco_eligible_v<Type>
+struct compressed_pair_element<Type, Tag>: Type {
     using reference = Type &;
     using const_reference = const Type &;
     using base_type = Type;
 
-    template<typename Dummy = Type, typename = std::enable_if_t<std::is_default_constructible_v<Dummy>>>
     constexpr compressed_pair_element() noexcept(std::is_nothrow_default_constructible_v<base_type>)
+    requires std::default_initializable<Type>
         : base_type{} {}
 
-    template<typename Arg, typename = std::enable_if_t<!std::is_same_v<std::remove_cv_t<std::remove_reference_t<Arg>>, compressed_pair_element>>>
+    template<typename Arg>
     constexpr compressed_pair_element(Arg &&arg) noexcept(std::is_nothrow_constructible_v<base_type, Arg>)
+    requires (!std::same_as<std::remove_cvref_t<Arg>, compressed_pair_element>)
         : base_type{std::forward<Arg>(arg)} {}
 
     template<typename... Args, std::size_t... Index>
@@ -99,11 +103,9 @@ public:
      *
      * This constructor is only available when the types that the pair stores
      * are both at least default constructible.
-     *
-     * @tparam Dummy Dummy template parameter used for internal purposes.
      */
-    template<bool Dummy = true, typename = std::enable_if_t<Dummy && std::is_default_constructible_v<first_type> && std::is_default_constructible_v<second_type>>>
     constexpr compressed_pair() noexcept(std::is_nothrow_default_constructible_v<first_base> && std::is_nothrow_default_constructible_v<second_base>)
+    requires std::default_initializable<first_type> && std::default_initializable<second_type>
         : first_base{},
           second_base{} {}
 
@@ -203,22 +205,22 @@ public:
      * reference to the second element if `Index` is 1.
      */
     template<std::size_t Index>
+    requires (Index <= 1u)
     [[nodiscard]] constexpr decltype(auto) get() noexcept {
         if constexpr(Index == 0u) {
             return first();
         } else {
-            static_assert(Index == 1u, "Index out of bounds");
             return second();
         }
     }
 
     /*! @copydoc get */
     template<std::size_t Index>
+    requires (Index <= 1u)
     [[nodiscard]] constexpr decltype(auto) get() const noexcept {
         if constexpr(Index == 0u) {
             return first();
         } else {
-            static_assert(Index == 1u, "Index out of bounds");
             return second();
         }
     }
@@ -240,7 +242,7 @@ compressed_pair(Type &&, Other &&) -> compressed_pair<std::decay_t<Type>, std::d
  * @param rhs A valid compressed pair object.
  */
 template<typename First, typename Second>
-inline constexpr void swap(compressed_pair<First, Second> &lhs, compressed_pair<First, Second> &rhs) {
+constexpr void swap(compressed_pair<First, Second> &lhs, compressed_pair<First, Second> &rhs) noexcept {
     lhs.swap(rhs);
 }
 
@@ -263,9 +265,8 @@ struct tuple_size<entt::compressed_pair<First, Second>>: integral_constant<size_
  * @tparam Second The type of the second element that the pair stores.
  */
 template<size_t Index, typename First, typename Second>
-struct tuple_element<Index, entt::compressed_pair<First, Second>>: conditional<Index == 0u, First, Second> {
-    static_assert(Index < 2u, "Index out of bounds");
-};
+requires (Index <= 1u)
+struct tuple_element<Index, entt::compressed_pair<First, Second>>: conditional<Index == 0u, First, Second> {};
 
 } // namespace std
 

src/entt/core/concepts.hpp

@@ -0,0 +1,17 @@
+#ifndef ENTT_CORE_CONCEPTS_HPP
+#define ENTT_CORE_CONCEPTS_HPP
+
+#include <type_traits>
+
+namespace entt {
+
+/**
+ * @brief Specifies that a type is not a cv-qualified reference.
+ * @tparam Type Type to check.
+ */
+template<typename Type>
+concept cvref_unqualified = std::is_same_v<std::remove_cvref_t<Type>, Type>;
+
+} // namespace entt
+
+#endif

src/entt/core/enum.hpp

@@ -1,6 +1,7 @@
 #ifndef ENTT_CORE_ENUM_HPP
 #define ENTT_CORE_ENUM_HPP
 
+#include <concepts>
 #include <type_traits>
 
 namespace entt {
@@ -9,12 +10,16 @@ namespace entt {
  * @brief Enable bitmask support for enum classes.
  * @tparam Type The enum type for which to enable bitmask support.
  */
-template<typename Type, typename = void>
+template<typename Type>
 struct enum_as_bitmask: std::false_type {};
 
 /*! @copydoc enum_as_bitmask */
 template<typename Type>
-struct enum_as_bitmask<Type, std::void_t<decltype(Type::_entt_enum_as_bitmask)>>: std::is_enum<Type> {};
+requires requires {
+    requires std::is_enum_v<Type>;
+    { Type::_entt_enum_as_bitmask } -> std::same_as<Type>;
+}
+struct enum_as_bitmask<Type>: std::true_type {};
 
 /**
  * @brief Helper variable template.
@@ -23,6 +28,14 @@ struct enum_as_bitmask<Type, std::void_t<decltype(Type::_entt_enum_as_bitmask)>>
 template<typename Type>
 inline constexpr bool enum_as_bitmask_v = enum_as_bitmask<Type>::value;
 
+/**
+ * @brief Specifies that an enum class supports bitmask operations.
+ * @tparam Type Enum class type.
+ */
+template<typename Type>
+// check again that it is an enum to deal with incorrect specializations
+concept enum_bitmask = std::is_enum_v<Type> && enum_as_bitmask_v<Type>;
+
 } // namespace entt
 
 /**
@@ -33,23 +46,20 @@ inline constexpr bool enum_as_bitmask_v = enum_as_bitmask<Type>::value;
  * @return The result of invoking the operator on the underlying types of the
  * two values provided.
  */
-template<typename Type>
-[[nodiscard]] constexpr std::enable_if_t<entt::enum_as_bitmask_v<Type>, Type>
-operator|(const Type lhs, const Type rhs) noexcept {
+template<entt::enum_bitmask Type>
+[[nodiscard]] constexpr Type operator|(const Type lhs, const Type rhs) noexcept {
     return static_cast<Type>(static_cast<std::underlying_type_t<Type>>(lhs) | static_cast<std::underlying_type_t<Type>>(rhs));
 }
 
 /*! @copydoc operator| */
-template<typename Type>
-[[nodiscard]] constexpr std::enable_if_t<entt::enum_as_bitmask_v<Type>, Type>
-operator&(const Type lhs, const Type rhs) noexcept {
+template<entt::enum_bitmask Type>
+[[nodiscard]] constexpr Type operator&(const Type lhs, const Type rhs) noexcept {
     return static_cast<Type>(static_cast<std::underlying_type_t<Type>>(lhs) & static_cast<std::underlying_type_t<Type>>(rhs));
 }
 
 /*! @copydoc operator| */
-template<typename Type>
-[[nodiscard]] constexpr std::enable_if_t<entt::enum_as_bitmask_v<Type>, Type>
-operator^(const Type lhs, const Type rhs) noexcept {
+template<entt::enum_bitmask Type>
+[[nodiscard]] constexpr Type operator^(const Type lhs, const Type rhs) noexcept {
     return static_cast<Type>(static_cast<std::underlying_type_t<Type>>(lhs) ^ static_cast<std::underlying_type_t<Type>>(rhs));
 }
 
@@ -60,37 +70,32 @@ operator^(const Type lhs, const Type rhs) noexcept {
  * @return The result of invoking the operator on the underlying types of the
  * value provided.
  */
-template<typename Type>
-[[nodiscard]] constexpr std::enable_if_t<entt::enum_as_bitmask_v<Type>, Type>
-operator~(const Type value) noexcept {
+template<entt::enum_bitmask Type>
+[[nodiscard]] constexpr Type operator~(const Type value) noexcept {
     return static_cast<Type>(~static_cast<std::underlying_type_t<Type>>(value));
 }
 
 /*! @copydoc operator~ */
-template<typename Type>
-[[nodiscard]] constexpr std::enable_if_t<entt::enum_as_bitmask_v<Type>, bool>
-operator!(const Type value) noexcept {
+template<entt::enum_bitmask Type>
+[[nodiscard]] constexpr bool operator!(const Type value) noexcept {
     return !static_cast<std::underlying_type_t<Type>>(value);
 }
 
 /*! @copydoc operator| */
-template<typename Type>
-constexpr std::enable_if_t<entt::enum_as_bitmask_v<Type>, Type &>
-operator|=(Type &lhs, const Type rhs) noexcept {
+template<entt::enum_bitmask Type>
+constexpr Type &operator|=(Type &lhs, const Type rhs) noexcept {
     return (lhs = (lhs | rhs));
 }
 
 /*! @copydoc operator| */
-template<typename Type>
-constexpr std::enable_if_t<entt::enum_as_bitmask_v<Type>, Type &>
-operator&=(Type &lhs, const Type rhs) noexcept {
+template<entt::enum_bitmask Type>
+constexpr Type &operator&=(Type &lhs, const Type rhs) noexcept {
     return (lhs = (lhs & rhs));
 }
 
 /*! @copydoc operator| */
-template<typename Type>
-constexpr std::enable_if_t<entt::enum_as_bitmask_v<Type>, Type &>
-operator^=(Type &lhs, const Type rhs) noexcept {
+template<entt::enum_bitmask Type>
+constexpr Type &operator^=(Type &lhs, const Type rhs) noexcept {
     return (lhs = (lhs ^ rhs));
 }
 

src/entt/core/family.hpp

@@ -15,8 +15,10 @@ namespace entt {
  */
 template<typename...>
 class family {
-    // NOLINTNEXTLINE(cppcoreguidelines-avoid-non-const-global-variables)
-    inline static ENTT_MAYBE_ATOMIC(id_type) identifier{};
+    static auto identifier() noexcept {
+        static ENTT_MAYBE_ATOMIC(id_type) value{};
+        return value++;
+    }
 
 public:
     /*! @brief Unsigned integer type. */
@@ -24,8 +26,8 @@ public:
 
     /*! @brief Statically generated unique identifier for the given type. */
     template<typename... Type>
-    // at the time I'm writing, clang crashes during compilation if auto is used instead of family_type
-    inline static const value_type value = identifier++;
+    // at the time I'm writing, clang crashes during compilation if auto is used instead of value_type
+    inline static const value_type value = identifier();
 };
 
 } // namespace entt

src/entt/core/fwd.hpp

@@ -2,10 +2,25 @@
 #define ENTT_CORE_FWD_HPP
 
 #include <cstddef>
+#include <cstdint>
 #include "../config/config.h"
 
 namespace entt {
 
+/*! @brief Possible modes of an any object. */
+enum class any_policy : std::uint8_t {
+    /*! @brief Default mode, no element available. */
+    empty,
+    /*! @brief Owning mode, dynamically allocated element. */
+    dynamic,
+    /*! @brief Owning mode, embedded element. */
+    embedded,
+    /*! @brief Aliasing mode, non-const reference. */
+    ref,
+    /*! @brief Const aliasing mode, const reference. */
+    cref
+};
+
 // NOLINTNEXTLINE(cppcoreguidelines-avoid-c-arrays, modernize-avoid-c-arrays)
 template<std::size_t Len = sizeof(double[2]), std::size_t = alignof(double[2])>
 class basic_any;

src/entt/core/hashed_string.hpp

@@ -3,12 +3,11 @@
 
 #include <cstddef>
 #include <cstdint>
-#include <string_view>
 #include "fwd.hpp"
 
 namespace entt {
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
 template<typename = id_type>
@@ -32,9 +31,9 @@ struct basic_hashed_string {
     using size_type = std::size_t;
     using hash_type = id_type;
 
-    const value_type *repr;
-    size_type length;
-    hash_type hash;
+    const value_type *repr{};
+    hash_type hash{fnv_1a_params<>::offset};
+    size_type length{};
 };
 
 } // namespace internal
@@ -62,30 +61,19 @@ class basic_hashed_string: internal::basic_hashed_string<Char> {
 
     struct const_wrapper {
         // non-explicit constructor on purpose
-        constexpr const_wrapper(const Char *str) noexcept
+        constexpr const_wrapper(const base_type::value_type *str) noexcept
             : repr{str} {}
 
-        const Char *repr;
+        const base_type::value_type *repr;
     };
 
-    // Fowler–Noll–Vo hash function v. 1a - the good
-    [[nodiscard]] static constexpr auto helper(const std::basic_string_view<Char> view) noexcept {
-        base_type base{view.data(), view.size(), params::offset};
-
-        for(auto &&curr: view) {
-            base.hash = (base.hash ^ static_cast<id_type>(curr)) * params::prime;
-        }
-
-        return base;
-    }
-
 public:
     /*! @brief Character type. */
-    using value_type = typename base_type::value_type;
+    using value_type = base_type::value_type;
     /*! @brief Unsigned integer type. */
-    using size_type = typename base_type::size_type;
+    using size_type = base_type::size_type;
     /*! @brief Unsigned integer type. */
-    using hash_type = typename base_type::hash_type;
+    using hash_type = base_type::hash_type;
 
     /**
      * @brief Returns directly the numeric representation of a string view.
@@ -105,7 +93,7 @@ public:
      */
     template<std::size_t N>
     // NOLINTNEXTLINE(cppcoreguidelines-avoid-c-arrays, modernize-avoid-c-arrays)
-    [[nodiscard]] static constexpr hash_type value(const value_type (&str)[N]) noexcept {
+    [[nodiscard]] static ENTT_CONSTEVAL hash_type value(const value_type (&str)[N]) noexcept {
         return basic_hashed_string{str};
     }
 
@@ -128,7 +116,14 @@ public:
      * @param len Length of the string to hash.
      */
     constexpr basic_hashed_string(const value_type *str, const size_type len) noexcept
-        : base_type{helper({str, len})} {}
+        // NOLINTNEXTLINE(cppcoreguidelines-pro-bounds-array-to-pointer-decay)
+        : base_type{str} {
+        // NOLINTBEGIN(cppcoreguidelines-pro-bounds-pointer-arithmetic)
+        for(; base_type::length < len; ++base_type::length) {
+            base_type::hash = (base_type::hash ^ static_cast<id_type>(str[base_type::length])) * params::prime;
+        }
+        // NOLINTEND(cppcoreguidelines-pro-bounds-pointer-arithmetic)
+    }
 
     /**
      * @brief Constructs a hashed string from an array of const characters.
@@ -137,8 +132,13 @@ public:
      */
     template<std::size_t N>
     // NOLINTNEXTLINE(cppcoreguidelines-avoid-c-arrays, modernize-avoid-c-arrays)
-    constexpr basic_hashed_string(const value_type (&str)[N]) noexcept
-        : base_type{helper({static_cast<const value_type *>(str)})} {}
+    ENTT_CONSTEVAL basic_hashed_string(const value_type (&str)[N]) noexcept
+        // NOLINTNEXTLINE(cppcoreguidelines-pro-bounds-array-to-pointer-decay)
+        : base_type{str} {
+        for(; str[base_type::length]; ++base_type::length) {
+            base_type::hash = (base_type::hash ^ static_cast<id_type>(str[base_type::length])) * params::prime;
+        }
+    }
 
     /**
      * @brief Explicit constructor on purpose to avoid constructing a hashed
@@ -150,10 +150,16 @@ public:
      * @param wrapper Helps achieving the purpose by relying on overloading.
      */
     explicit constexpr basic_hashed_string(const_wrapper wrapper) noexcept
-        : base_type{helper({wrapper.repr})} {}
+        : base_type{wrapper.repr} {
+        // NOLINTBEGIN(cppcoreguidelines-pro-bounds-pointer-arithmetic)
+        for(; wrapper.repr[base_type::length]; ++base_type::length) {
+            base_type::hash = (base_type::hash ^ static_cast<id_type>(wrapper.repr[base_type::length])) * params::prime;
+        }
+        // NOLINTEND(cppcoreguidelines-pro-bounds-pointer-arithmetic)
+    }
 
     /**
-     * @brief Returns the size a hashed string.
+     * @brief Returns the size of a hashed string.
      * @return The size of the hashed string.
      */
     [[nodiscard]] constexpr size_type size() const noexcept {
@@ -177,7 +183,7 @@ public:
     }
 
     /*! @copydoc data */
-    [[nodiscard]] constexpr operator const value_type *() const noexcept {
+    [[nodiscard]] explicit constexpr operator const value_type *() const noexcept {
         return data();
     }
 
@@ -188,6 +194,24 @@ public:
     [[nodiscard]] constexpr operator hash_type() const noexcept {
         return value();
     }
+
+    /**
+     * @brief Compares two hashed strings.
+     * @param other A valid hashed string.
+     * @return True if the two hashed strings are identical, false otherwise.
+     */
+    [[nodiscard]] constexpr bool operator==(const basic_hashed_string &other) const noexcept {
+        return value() == other.value();
+    }
+
+    /**
+     * @brief Lexicographically compares two hashed strings.
+     * @param other A valid hashed string.
+     * @return The relative order between the two hashed strings.
+     */
+    [[nodiscard]] constexpr auto operator<=>(const basic_hashed_string &other) const noexcept {
+        return value() <=> other.value();
+    }
 };
 
 /**
@@ -209,81 +233,6 @@ template<typename Char, std::size_t N>
 // NOLINTNEXTLINE(cppcoreguidelines-avoid-c-arrays, modernize-avoid-c-arrays)
 basic_hashed_string(const Char (&str)[N]) -> basic_hashed_string<Char>;
 
-/**
- * @brief Compares two hashed strings.
- * @tparam Char Character type.
- * @param lhs A valid hashed string.
- * @param rhs A valid hashed string.
- * @return True if the two hashed strings are identical, false otherwise.
- */
-template<typename Char>
-[[nodiscard]] constexpr bool operator==(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) noexcept {
-    return lhs.value() == rhs.value();
-}
-
-/**
- * @brief Compares two hashed strings.
- * @tparam Char Character type.
- * @param lhs A valid hashed string.
- * @param rhs A valid hashed string.
- * @return True if the two hashed strings differ, false otherwise.
- */
-template<typename Char>
-[[nodiscard]] constexpr bool operator!=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) noexcept {
-    return !(lhs == rhs);
-}
-
-/**
- * @brief Compares two hashed strings.
- * @tparam Char Character type.
- * @param lhs A valid hashed string.
- * @param rhs A valid hashed string.
- * @return True if the first element is less than the second, false otherwise.
- */
-template<typename Char>
-[[nodiscard]] constexpr bool operator<(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) noexcept {
-    return lhs.value() < rhs.value();
-}
-
-/**
- * @brief Compares two hashed strings.
- * @tparam Char Character type.
- * @param lhs A valid hashed string.
- * @param rhs A valid hashed string.
- * @return True if the first element is less than or equal to the second, false
- * otherwise.
- */
-template<typename Char>
-[[nodiscard]] constexpr bool operator<=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) noexcept {
-    return !(rhs < lhs);
-}
-
-/**
- * @brief Compares two hashed strings.
- * @tparam Char Character type.
- * @param lhs A valid hashed string.
- * @param rhs A valid hashed string.
- * @return True if the first element is greater than the second, false
- * otherwise.
- */
-template<typename Char>
-[[nodiscard]] constexpr bool operator>(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) noexcept {
-    return rhs < lhs;
-}
-
-/**
- * @brief Compares two hashed strings.
- * @tparam Char Character type.
- * @param lhs A valid hashed string.
- * @param rhs A valid hashed string.
- * @return True if the first element is greater than or equal to the second,
- * false otherwise.
- */
-template<typename Char>
-[[nodiscard]] constexpr bool operator>=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) noexcept {
-    return !(lhs < rhs);
-}
-
 inline namespace literals {
 
 /**
@@ -291,7 +240,7 @@ inline namespace literals {
  * @param str The literal without its suffix.
  * @return A properly initialized hashed string.
  */
-[[nodiscard]] constexpr hashed_string operator"" _hs(const char *str, std::size_t) noexcept {
+[[nodiscard]] ENTT_CONSTEVAL hashed_string operator""_hs(const char *str, std::size_t) noexcept {
     return hashed_string{str};
 }
 
@@ -300,7 +249,7 @@ inline namespace literals {
  * @param str The literal without its suffix.
  * @return A properly initialized hashed wstring.
  */
-[[nodiscard]] constexpr hashed_wstring operator"" _hws(const wchar_t *str, std::size_t) noexcept {
+[[nodiscard]] ENTT_CONSTEVAL hashed_wstring operator""_hws(const wchar_t *str, std::size_t) noexcept {
     return hashed_wstring{str};
 }
 

src/entt/core/ident.hpp

@@ -16,8 +16,7 @@ namespace entt {
 template<typename... Type>
 class ident {
     template<typename Curr, std::size_t... Index>
-    [[nodiscard]] static constexpr id_type get(std::index_sequence<Index...>) noexcept {
-        static_assert((std::is_same_v<Curr, Type> || ...), "Invalid type");
+    [[nodiscard]] static ENTT_CONSTEVAL id_type get(std::index_sequence<Index...>) noexcept {
         return (0 + ... + (std::is_same_v<Curr, type_list_element_t<Index, type_list<std::decay_t<Type>...>>> ? id_type{Index} : id_type{}));
     }
 
@@ -27,7 +26,8 @@ public:
 
     /*! @brief Statically generated unique identifier for the given type. */
     template<typename Curr>
-    static constexpr value_type value = get<std::decay_t<Curr>>(std::index_sequence_for<Type...>{});
+    requires (std::is_same_v<std::remove_cvref_t<Curr>, Type> || ...)
+    static constexpr value_type value = get<std::remove_cvref_t<Curr>>(std::index_sequence_for<Type...>{});
 };
 
 } // namespace entt

src/entt/core/iterator.hpp

@@ -1,10 +1,12 @@
 #ifndef ENTT_CORE_ITERATOR_HPP
 #define ENTT_CORE_ITERATOR_HPP
 
+#include <concepts>
 #include <iterator>
 #include <memory>
 #include <type_traits>
 #include <utility>
+#include "../stl/iterator.hpp"
 
 namespace entt {
 
@@ -52,11 +54,8 @@ private:
  * @brief Plain iota iterator (waiting for C++20).
  * @tparam Type Value type.
  */
-template<typename Type>
-class iota_iterator final {
-    static_assert(std::is_integral_v<Type>, "Not an integral type");
-
-public:
+template<std::integral Type>
+struct iota_iterator final {
     /*! @brief Value type, likely an integral one. */
     using value_type = Type;
     /*! @brief Invalid pointer type. */
@@ -92,7 +91,7 @@ public:
      * @return This iota iterator.
      */
     constexpr iota_iterator operator++(int) noexcept {
-        iota_iterator orig = *this;
+        const iota_iterator orig = *this;
         return ++(*this), orig;
     }
 
@@ -104,43 +103,28 @@ public:
         return current;
     }
 
+    /**
+     * @brief Comparison operator.
+     * @param other A properly initialized iota iterator.
+     * @return True if the two iterators are identical, false otherwise.
+     */
+    [[nodiscard]] constexpr bool operator==(const iota_iterator &other) const noexcept {
+        return current == other.current;
+    }
+
 private:
     value_type current;
 };
 
-/**
- * @brief Comparison operator.
- * @tparam Type Value type of the iota iterator.
- * @param lhs A properly initialized iota iterator.
- * @param rhs A properly initialized iota iterator.
- * @return True if the two iterators are identical, false otherwise.
- */
-template<typename Type>
-[[nodiscard]] constexpr bool operator==(const iota_iterator<Type> &lhs, const iota_iterator<Type> &rhs) noexcept {
-    return *lhs == *rhs;
-}
-
-/**
- * @brief Comparison operator.
- * @tparam Type Value type of the iota iterator.
- * @param lhs A properly initialized iota iterator.
- * @param rhs A properly initialized iota iterator.
- * @return True if the two iterators differ, false otherwise.
- */
-template<typename Type>
-[[nodiscard]] constexpr bool operator!=(const iota_iterator<Type> &lhs, const iota_iterator<Type> &rhs) noexcept {
-    return !(lhs == rhs);
-}
-
 /**
  * @brief Utility class to create an iterable object from a pair of iterators.
  * @tparam It Type of iterator.
  * @tparam Sentinel Type of sentinel.
  */
-template<typename It, typename Sentinel = It>
+template<stl::input_or_output_iterator It, stl::sentinel_for<It> Sentinel = It>
 struct iterable_adaptor final {
     /*! @brief Value type. */
-    using value_type = typename std::iterator_traits<It>::value_type;
+    using value_type = std::iterator_traits<It>::value_type;
     /*! @brief Iterator type. */
     using iterator = It;
     /*! @brief Sentinel type. */

src/entt/core/memory.hpp

@@ -7,24 +7,10 @@
 #include <type_traits>
 #include <utility>
 #include "../config/config.h"
+#include "../stl/memory.hpp"
 
 namespace entt {
 
-/**
- * @brief Unwraps fancy pointers, does nothing otherwise (waiting for C++20).
- * @tparam Type Pointer type.
- * @param ptr Fancy or raw pointer.
- * @return A raw pointer that represents the address of the original pointer.
- */
-template<typename Type>
-[[nodiscard]] constexpr auto to_address(Type &&ptr) noexcept {
-    if constexpr(std::is_pointer_v<std::decay_t<Type>>) {
-        return ptr;
-    } else {
-        return to_address(std::forward<Type>(ptr).operator->());
-    }
-}
-
 /**
  * @brief Utility function to design allocation-aware containers.
  * @tparam Allocator Type of allocator.
@@ -76,7 +62,7 @@ struct allocation_deleter: private Allocator {
     /*! @brief Allocator type. */
     using allocator_type = Allocator;
     /*! @brief Pointer type. */
-    using pointer = typename std::allocator_traits<Allocator>::pointer;
+    using pointer = std::allocator_traits<Allocator>::pointer;
 
     /**
      * @brief Inherited constructors.
@@ -91,7 +77,7 @@ struct allocation_deleter: private Allocator {
      */
     constexpr void operator()(pointer ptr) noexcept(std::is_nothrow_destructible_v<typename allocator_type::value_type>) {
         using alloc_traits = std::allocator_traits<Allocator>;
-        alloc_traits::destroy(*this, to_address(ptr));
+        alloc_traits::destroy(*this, stl::to_address(ptr));
         alloc_traits::deallocate(*this, ptr, 1u);
     }
 };
@@ -106,17 +92,17 @@ struct allocation_deleter: private Allocator {
  * @return A properly initialized unique pointer with a custom deleter.
  */
 template<typename Type, typename Allocator, typename... Args>
-ENTT_CONSTEXPR auto allocate_unique(Allocator &allocator, Args &&...args) {
+constexpr auto allocate_unique(Allocator &allocator, Args &&...args) {
     static_assert(!std::is_array_v<Type>, "Array types are not supported");
 
-    using alloc_traits = typename std::allocator_traits<Allocator>::template rebind_traits<Type>;
-    using allocator_type = typename alloc_traits::allocator_type;
+    using alloc_traits = std::allocator_traits<Allocator>::template rebind_traits<Type>;
+    using allocator_type = alloc_traits::allocator_type;
 
     allocator_type alloc{allocator};
     auto ptr = alloc_traits::allocate(alloc, 1u);
 
     ENTT_TRY {
-        alloc_traits::construct(alloc, to_address(ptr), std::forward<Args>(args)...);
+        alloc_traits::construct(alloc, stl::to_address(ptr), std::forward<Args>(args)...);
     }
     ENTT_CATCH {
         alloc_traits::deallocate(alloc, ptr, 1u);
@@ -126,7 +112,7 @@ ENTT_CONSTEXPR auto allocate_unique(Allocator &allocator, Args &&...args) {
     return std::unique_ptr<Type, allocation_deleter<allocator_type>>{ptr, alloc};
 }
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
 template<typename Type>
@@ -152,31 +138,30 @@ template<typename Type, typename Other>
 struct uses_allocator_construction<std::pair<Type, Other>> {
     using type = std::pair<Type, Other>;
 
-    template<typename Allocator, typename First, typename Second>
-    static constexpr auto args(const Allocator &allocator, std::piecewise_construct_t, First &&first, Second &&second) noexcept {
+    template<typename First, typename Second>
+    static constexpr auto args(const auto &allocator, std::piecewise_construct_t, First &&first, Second &&second) noexcept {
         return std::make_tuple(
             std::piecewise_construct,
             std::apply([&allocator](auto &&...curr) { return uses_allocator_construction<Type>::args(allocator, std::forward<decltype(curr)>(curr)...); }, std::forward<First>(first)),
             std::apply([&allocator](auto &&...curr) { return uses_allocator_construction<Other>::args(allocator, std::forward<decltype(curr)>(curr)...); }, std::forward<Second>(second)));
     }
 
-    template<typename Allocator>
-    static constexpr auto args(const Allocator &allocator) noexcept {
+    static constexpr auto args(const auto &allocator) noexcept {
         return uses_allocator_construction<type>::args(allocator, std::piecewise_construct, std::tuple<>{}, std::tuple<>{});
     }
 
-    template<typename Allocator, typename First, typename Second>
-    static constexpr auto args(const Allocator &allocator, First &&first, Second &&second) noexcept {
+    template<typename First, typename Second>
+    static constexpr auto args(const auto &allocator, First &&first, Second &&second) noexcept {
         return uses_allocator_construction<type>::args(allocator, std::piecewise_construct, std::forward_as_tuple(std::forward<First>(first)), std::forward_as_tuple(std::forward<Second>(second)));
     }
 
-    template<typename Allocator, typename First, typename Second>
-    static constexpr auto args(const Allocator &allocator, const std::pair<First, Second> &value) noexcept {
+    template<typename First, typename Second>
+    static constexpr auto args(const auto &allocator, const std::pair<First, Second> &value) noexcept {
         return uses_allocator_construction<type>::args(allocator, std::piecewise_construct, std::forward_as_tuple(value.first), std::forward_as_tuple(value.second));
     }
 
-    template<typename Allocator, typename First, typename Second>
-    static constexpr auto args(const Allocator &allocator, std::pair<First, Second> &&value) noexcept {
+    template<typename First, typename Second>
+    static constexpr auto args(const auto &allocator, std::pair<First, Second> &&value) noexcept {
         return uses_allocator_construction<type>::args(allocator, std::piecewise_construct, std::forward_as_tuple(std::move(value.first)), std::forward_as_tuple(std::move(value.second)));
     }
 };
@@ -191,14 +176,13 @@ struct uses_allocator_construction<std::pair<Type, Other>> {
  * create an object of a given type by means of uses-allocator construction.
  *
  * @tparam Type Type to return arguments for.
- * @tparam Allocator Type of allocator used to manage memory and elements.
  * @tparam Args Types of arguments to use to construct the object.
  * @param allocator The allocator to use.
  * @param args Parameters to use to construct the object.
  * @return The arguments needed to create an object of the given type.
  */
-template<typename Type, typename Allocator, typename... Args>
-constexpr auto uses_allocator_construction_args(const Allocator &allocator, Args &&...args) noexcept {
+template<typename Type, typename... Args>
+constexpr auto uses_allocator_construction_args(const auto &allocator, Args &&...args) noexcept {
     return internal::uses_allocator_construction<Type>::args(allocator, std::forward<Args>(args)...);
 }
 
@@ -209,14 +193,13 @@ constexpr auto uses_allocator_construction_args(const Allocator &allocator, Args
  * means of uses-allocator construction.
  *
  * @tparam Type Type of object to create.
- * @tparam Allocator Type of allocator used to manage memory and elements.
  * @tparam Args Types of arguments to use to construct the object.
  * @param allocator The allocator to use.
  * @param args Parameters to use to construct the object.
  * @return A newly created object of the given type.
  */
-template<typename Type, typename Allocator, typename... Args>
-constexpr Type make_obj_using_allocator(const Allocator &allocator, Args &&...args) {
+template<typename Type, typename... Args>
+constexpr Type make_obj_using_allocator(const auto &allocator, Args &&...args) {
     return std::make_from_tuple<Type>(internal::uses_allocator_construction<Type>::args(allocator, std::forward<Args>(args)...));
 }
 
@@ -227,15 +210,14 @@ constexpr Type make_obj_using_allocator(const Allocator &allocator, Args &&...ar
  * means of uses-allocator construction at an uninitialized memory location.
  *
  * @tparam Type Type of object to create.
- * @tparam Allocator Type of allocator used to manage memory and elements.
  * @tparam Args Types of arguments to use to construct the object.
  * @param value Memory location in which to place the object.
  * @param allocator The allocator to use.
  * @param args Parameters to use to construct the object.
  * @return A pointer to the newly created object of the given type.
  */
-template<typename Type, typename Allocator, typename... Args>
-constexpr Type *uninitialized_construct_using_allocator(Type *value, const Allocator &allocator, Args &&...args) {
+template<typename Type, typename... Args>
+constexpr Type *uninitialized_construct_using_allocator(Type *value, const auto &allocator, Args &&...args) {
     return std::apply([value](auto &&...curr) { return ::new(value) Type(std::forward<decltype(curr)>(curr)...); }, internal::uses_allocator_construction<Type>::args(allocator, std::forward<Args>(args)...));
 }
 

src/entt/core/tuple.hpp

@@ -7,25 +7,20 @@
 
 namespace entt {
 
-/*! @cond TURN_OFF_DOXYGEN */
-namespace internal {
-
-template<typename>
-struct is_tuple_impl: std::false_type {};
-
-template<typename... Args>
-struct is_tuple_impl<std::tuple<Args...>>: std::true_type {};
-
-} // namespace internal
-/*! @endcond */
-
 /**
  * @brief Provides the member constant `value` to true if a given type is a
  * tuple, false otherwise.
  * @tparam Type The type to test.
  */
 template<typename Type>
-struct is_tuple: internal::is_tuple_impl<std::remove_cv_t<Type>> {};
+struct is_tuple: std::false_type {};
+
+/**
+ * @copybrief is_tuple
+ * @tparam Args Tuple template arguments.
+ */
+template<typename... Args>
+struct is_tuple<std::tuple<Args...>>: std::true_type {};
 
 /**
  * @brief Helper variable template.
@@ -88,7 +83,7 @@ struct forward_apply: private Func {
  * @tparam Func Type of underlying invocable object.
  */
 template<typename Func>
-forward_apply(Func) -> forward_apply<std::remove_reference_t<std::remove_cv_t<Func>>>;
+forward_apply(Func) -> forward_apply<std::remove_cvref_t<Func>>;
 
 } // namespace entt
 

src/entt/core/type_info.hpp

@@ -1,17 +1,17 @@
 #ifndef ENTT_CORE_TYPE_INFO_HPP
 #define ENTT_CORE_TYPE_INFO_HPP
 
+#include <compare>
 #include <string_view>
 #include <type_traits>
 #include <utility>
 #include "../config/config.h"
-#include "../core/attribute.h"
 #include "fwd.hpp"
 #include "hashed_string.hpp"
 
 namespace entt {
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
 struct ENTT_API type_index final {
@@ -21,20 +21,29 @@ struct ENTT_API type_index final {
     }
 };
 
+template<typename Type>
+[[nodiscard]] constexpr const char *pretty_function() noexcept {
+#if defined ENTT_PRETTY_FUNCTION
+    return static_cast<const char *>(ENTT_PRETTY_FUNCTION);
+#else
+    return "";
+#endif
+}
+
 template<typename Type>
 [[nodiscard]] constexpr auto stripped_type_name() noexcept {
 #if defined ENTT_PRETTY_FUNCTION
-    std::string_view pretty_function{static_cast<const char *>(ENTT_PRETTY_FUNCTION)};
-    auto first = pretty_function.find_first_not_of(' ', pretty_function.find_first_of(ENTT_PRETTY_FUNCTION_PREFIX) + 1);
-    auto value = pretty_function.substr(first, pretty_function.find_last_of(ENTT_PRETTY_FUNCTION_SUFFIX) - first);
+    const std::string_view full_name{pretty_function<Type>()};
+    auto first = full_name.find_first_not_of(' ', full_name.find_first_of(ENTT_PRETTY_FUNCTION_PREFIX) + 1);
+    auto value = full_name.substr(first, full_name.find_last_of(ENTT_PRETTY_FUNCTION_SUFFIX) - first);
     return value;
 #else
-    return std::string_view{""};
+    return std::string_view{};
 #endif
 }
 
 template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
-[[nodiscard]] constexpr std::string_view type_name(int) noexcept {
+[[nodiscard]] ENTT_CONSTEVAL std::string_view type_name(int) noexcept {
     constexpr auto value = stripped_type_name<Type>();
     return value;
 }
@@ -46,7 +55,7 @@ template<typename Type>
 }
 
 template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
-[[nodiscard]] constexpr id_type type_hash(int) noexcept {
+[[nodiscard]] ENTT_CONSTEVAL id_type type_hash(int) noexcept {
     constexpr auto stripped = stripped_type_name<Type>();
     constexpr auto value = hashed_string::value(stripped.data(), stripped.size());
     return value;
@@ -67,7 +76,7 @@ template<typename Type>
  * @brief Type sequential identifier.
  * @tparam Type Type for which to generate a sequential identifier.
  */
-template<typename Type, typename = void>
+template<typename Type>
 struct ENTT_API type_index final {
     /**
      * @brief Returns the sequential identifier of a given type.
@@ -88,7 +97,7 @@ struct ENTT_API type_index final {
  * @brief Type hash.
  * @tparam Type Type for which to generate a hash value.
  */
-template<typename Type, typename = void>
+template<typename Type>
 struct type_hash final {
     /**
      * @brief Returns the numeric representation of a given type.
@@ -113,7 +122,7 @@ struct type_hash final {
  * @brief Type name.
  * @tparam Type Type for which to generate a name.
  */
-template<typename Type, typename = void>
+template<typename Type>
 struct type_name final {
     /**
      * @brief Returns the name of a given type.
@@ -138,9 +147,9 @@ struct type_info final {
     template<typename Type>
     // NOLINTBEGIN(modernize-use-transparent-functors)
     constexpr type_info(std::in_place_type_t<Type>) noexcept
-        : seq{type_index<std::remove_cv_t<std::remove_reference_t<Type>>>::value()},
-          identifier{type_hash<std::remove_cv_t<std::remove_reference_t<Type>>>::value()},
-          alias{type_name<std::remove_cv_t<std::remove_reference_t<Type>>>::value()} {}
+        : seq{type_index<std::remove_cvref_t<Type>>::value()},
+          identifier{type_hash<std::remove_cvref_t<Type>>::value()},
+          alias{type_name<std::remove_cvref_t<Type>>::value()} {}
     // NOLINTEND(modernize-use-transparent-functors)
 
     /**
@@ -167,75 +176,30 @@ struct type_info final {
         return alias;
     }
 
+    /**
+     * @brief Compares two type info objects.
+     * @param other A type info object.
+     * @return True if the two type info objects are identical, false otherwise.
+     */
+    [[nodiscard]] constexpr bool operator==(const type_info &other) const noexcept {
+        return identifier == other.identifier;
+    }
+
+    /**
+     * @brief Lexicographically compares two type info objects.
+     * @param other A type info object.
+     * @return The relative order between the two type info objects.
+     */
+    [[nodiscard]] constexpr auto operator<=>(const type_info &other) const noexcept {
+        return seq <=> other.seq;
+    }
+
 private:
     id_type seq;
     id_type identifier;
     std::string_view alias;
 };
 
-/**
- * @brief Compares the contents of two type info objects.
- * @param lhs A type info object.
- * @param rhs A type info object.
- * @return True if the two type info objects are identical, false otherwise.
- */
-[[nodiscard]] inline constexpr bool operator==(const type_info &lhs, const type_info &rhs) noexcept {
-    return lhs.hash() == rhs.hash();
-}
-
-/**
- * @brief Compares the contents of two type info objects.
- * @param lhs A type info object.
- * @param rhs A type info object.
- * @return True if the two type info objects differ, false otherwise.
- */
-[[nodiscard]] inline constexpr bool operator!=(const type_info &lhs, const type_info &rhs) noexcept {
-    return !(lhs == rhs);
-}
-
-/**
- * @brief Compares two type info objects.
- * @param lhs A valid type info object.
- * @param rhs A valid type info object.
- * @return True if the first element is less than the second, false otherwise.
- */
-[[nodiscard]] inline constexpr bool operator<(const type_info &lhs, const type_info &rhs) noexcept {
-    return lhs.index() < rhs.index();
-}
-
-/**
- * @brief Compares two type info objects.
- * @param lhs A valid type info object.
- * @param rhs A valid type info object.
- * @return True if the first element is less than or equal to the second, false
- * otherwise.
- */
-[[nodiscard]] inline constexpr bool operator<=(const type_info &lhs, const type_info &rhs) noexcept {
-    return !(rhs < lhs);
-}
-
-/**
- * @brief Compares two type info objects.
- * @param lhs A valid type info object.
- * @param rhs A valid type info object.
- * @return True if the first element is greater than the second, false
- * otherwise.
- */
-[[nodiscard]] inline constexpr bool operator>(const type_info &lhs, const type_info &rhs) noexcept {
-    return rhs < lhs;
-}
-
-/**
- * @brief Compares two type info objects.
- * @param lhs A valid type info object.
- * @param rhs A valid type info object.
- * @return True if the first element is greater than or equal to the second,
- * false otherwise.
- */
-[[nodiscard]] inline constexpr bool operator>=(const type_info &lhs, const type_info &rhs) noexcept {
-    return !(lhs < rhs);
-}
-
 /**
  * @brief Returns the type info object associated to a given type.
  *
@@ -249,18 +213,18 @@ private:
  */
 template<typename Type>
 [[nodiscard]] const type_info &type_id() noexcept {
-    if constexpr(std::is_same_v<Type, std::remove_cv_t<std::remove_reference_t<Type>>>) {
-        static type_info instance{std::in_place_type<Type>};
+    if constexpr(std::is_same_v<Type, std::remove_cvref_t<Type>>) {
+        static const type_info instance{std::in_place_type<Type>};
         return instance;
     } else {
-        return type_id<std::remove_cv_t<std::remove_reference_t<Type>>>();
+        return type_id<std::remove_cvref_t<Type>>();
     }
 }
 
 /*! @copydoc type_id */
 template<typename Type>
-[[nodiscard]] const type_info &type_id(Type &&) noexcept {
-    return type_id<std::remove_cv_t<std::remove_reference_t<Type>>>();
+[[nodiscard]] const type_info &type_id(const Type &) noexcept {
+    return type_id<std::remove_cvref_t<Type>>();
 }
 
 } // namespace entt

src/entt/core/type_traits.hpp

@@ -1,6 +1,7 @@
 #ifndef ENTT_CORE_TYPE_TRAITS_HPP
 #define ENTT_CORE_TYPE_TRAITS_HPP
 
+#include <concepts>
 #include <cstddef>
 #include <iterator>
 #include <tuple>
@@ -18,7 +19,7 @@ namespace entt {
 template<std::size_t N>
 struct choice_t
     // unfortunately, doxygen cannot parse such a construct
-    : /*! @cond TURN_OFF_DOXYGEN */ choice_t<N - 1> /*! @endcond */
+    : /*! @cond ENTT_INTERNAL */ choice_t<N - 1> /*! @endcond */
 {};
 
 /*! @copybrief choice_t */
@@ -32,37 +33,18 @@ struct choice_t<0> {};
 template<std::size_t N>
 inline constexpr choice_t<N> choice{};
 
-/**
- * @brief Identity type trait.
- *
- * Useful to establish non-deduced contexts in template argument deduction
- * (waiting for C++20) or to provide types through function arguments.
- *
- * @tparam Type A type.
- */
-template<typename Type>
-struct type_identity {
-    /*! @brief Identity type. */
-    using type = Type;
-};
-
-/**
- * @brief Helper type.
- * @tparam Type A type.
- */
-template<typename Type>
-using type_identity_t = typename type_identity<Type>::type;
-
 /**
  * @brief A type-only `sizeof` wrapper that returns 0 where `sizeof` complains.
  * @tparam Type The type of which to return the size.
  */
-template<typename Type, typename = void>
+template<typename Type>
 struct size_of: std::integral_constant<std::size_t, 0u> {};
 
 /*! @copydoc size_of */
 template<typename Type>
-struct size_of<Type, std::void_t<decltype(sizeof(Type))>>
+requires requires { sizeof(Type); }
+struct size_of<Type>
+    // NOLINTNEXTLINE(bugprone-sizeof-expression)
     : std::integral_constant<std::size_t, sizeof(Type)> {};
 
 /**
@@ -145,7 +127,7 @@ struct type_list_element<0u, type_list<First, Other...>> {
  * @tparam List Type list to search into.
  */
 template<std::size_t Index, typename List>
-using type_list_element_t = typename type_list_element<Index, List>::type;
+using type_list_element_t = type_list_element<Index, List>::type;
 
 /*! @brief Primary template isn't defined on purpose. */
 template<typename, typename>
@@ -206,7 +188,7 @@ inline constexpr std::size_t type_list_index_v = type_list_index<Type, List>::va
  * @return A type list composed by the types of both the type lists.
  */
 template<typename... Type, typename... Other>
-constexpr type_list<Type..., Other...> operator+(type_list<Type...>, type_list<Other...>) {
+ENTT_CONSTEVAL type_list<Type..., Other...> operator+(type_list<Type...>, type_list<Other...>) {
     return {};
 }
 
@@ -230,7 +212,7 @@ struct type_list_cat<> {
 template<typename... Type, typename... Other, typename... List>
 struct type_list_cat<type_list<Type...>, type_list<Other...>, List...> {
     /*! @brief A type list composed by the types of all the type lists. */
-    using type = typename type_list_cat<type_list<Type..., Other...>, List...>::type;
+    using type = type_list_cat<type_list<Type..., Other...>, List...>::type;
 };
 
 /**
@@ -248,9 +230,9 @@ struct type_list_cat<type_list<Type...>> {
  * @tparam List Type lists to concatenate.
  */
 template<typename... List>
-using type_list_cat_t = typename type_list_cat<List...>::type;
+using type_list_cat_t = type_list_cat<List...>::type;
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
 template<typename...>
@@ -275,7 +257,7 @@ struct type_list_unique<type_list<>, Type...> {
 template<typename List>
 struct type_list_unique {
     /*! @brief A type list without duplicate types. */
-    using type = typename internal::type_list_unique<List>::type;
+    using type = internal::type_list_unique<List>::type;
 };
 
 /**
@@ -283,7 +265,7 @@ struct type_list_unique {
  * @tparam List Type list.
  */
 template<typename List>
-using type_list_unique_t = typename type_list_unique<List>::type;
+using type_list_unique_t = type_list_unique<List>::type;
 
 /**
  * @brief Provides the member constant `value` to true if a type list contains a
@@ -331,7 +313,7 @@ struct type_list_diff<type_list<Type...>, type_list<Other...>> {
  * @tparam List Type lists between which to compute the difference.
  */
 template<typename... List>
-using type_list_diff_t = typename type_list_diff<List...>::type;
+using type_list_diff_t = type_list_diff<List...>::type;
 
 /*! @brief Primary template isn't defined on purpose. */
 template<typename, template<typename...> class>
@@ -345,6 +327,7 @@ struct type_list_transform;
 template<typename... Type, template<typename...> class Op>
 struct type_list_transform<type_list<Type...>, Op> {
     /*! @brief Resulting type list after applying the transform function. */
+    // NOLINTNEXTLINE(modernize-type-traits)
     using type = type_list<typename Op<Type>::type...>;
 };
 
@@ -354,7 +337,7 @@ struct type_list_transform<type_list<Type...>, Op> {
  * @tparam Op Unary operation as template class with a type member named `type`.
  */
 template<typename List, template<typename...> class Op>
-using type_list_transform_t = typename type_list_transform<List, Op>::type;
+using type_list_transform_t = type_list_transform<List, Op>::type;
 
 /**
  * @brief A class to use to push around lists of constant values, nothing more.
@@ -401,7 +384,7 @@ struct value_list_element<0u, value_list<Value, Other...>> {
  * @tparam List Value list to search into.
  */
 template<std::size_t Index, typename List>
-using value_list_element_t = typename value_list_element<Index, List>::type;
+using value_list_element_t = value_list_element<Index, List>::type;
 
 /**
  * @brief Helper type.
@@ -470,7 +453,7 @@ inline constexpr std::size_t value_list_index_v = value_list_index<Value, List>:
  * @return A value list composed by the values of both the value lists.
  */
 template<auto... Value, auto... Other>
-constexpr value_list<Value..., Other...> operator+(value_list<Value...>, value_list<Other...>) {
+ENTT_CONSTEVAL value_list<Value..., Other...> operator+(value_list<Value...>, value_list<Other...>) {
     return {};
 }
 
@@ -494,7 +477,7 @@ struct value_list_cat<> {
 template<auto... Value, auto... Other, typename... List>
 struct value_list_cat<value_list<Value...>, value_list<Other...>, List...> {
     /*! @brief A value list composed by the values of all the value lists. */
-    using type = typename value_list_cat<value_list<Value..., Other...>, List...>::type;
+    using type = value_list_cat<value_list<Value..., Other...>, List...>::type;
 };
 
 /**
@@ -512,7 +495,7 @@ struct value_list_cat<value_list<Value...>> {
  * @tparam List Value lists to concatenate.
  */
 template<typename... List>
-using value_list_cat_t = typename value_list_cat<List...>::type;
+using value_list_cat_t = value_list_cat<List...>::type;
 
 /*! @brief Primary template isn't defined on purpose. */
 template<typename>
@@ -544,7 +527,7 @@ struct value_list_unique<value_list<>> {
  * @tparam Type A value list.
  */
 template<typename Type>
-using value_list_unique_t = typename value_list_unique<Type>::type;
+using value_list_unique_t = value_list_unique<Type>::type;
 
 /**
  * @brief Provides the member constant `value` to true if a value list contains
@@ -592,7 +575,7 @@ struct value_list_diff<value_list<Value...>, value_list<Other...>> {
  * @tparam List Value lists between which to compute the difference.
  */
 template<typename... List>
-using value_list_diff_t = typename value_list_diff<List...>::type;
+using value_list_diff_t = value_list_diff<List...>::type;
 
 /*! @brief Same as std::is_invocable, but with tuples. */
 template<typename, typename>
@@ -653,12 +636,13 @@ inline constexpr bool is_applicable_r_v = is_applicable_r<Ret, Func, Args>::valu
  * complete, false otherwise.
  * @tparam Type The type to test.
  */
-template<typename Type, typename = void>
+template<typename Type>
 struct is_complete: std::false_type {};
 
 /*! @copydoc is_complete */
 template<typename Type>
-struct is_complete<Type, std::void_t<decltype(sizeof(Type))>>: std::true_type {};
+requires requires { sizeof(Type); }
+struct is_complete<Type>: std::true_type {};
 
 /**
  * @brief Helper variable template.
@@ -672,25 +656,26 @@ inline constexpr bool is_complete_v = is_complete<Type>::value;
  * iterator, false otherwise.
  * @tparam Type The type to test.
  */
-template<typename Type, typename = void>
+template<typename Type>
 struct is_iterator: std::false_type {};
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
-template<typename, typename = void>
+template<typename>
 struct has_iterator_category: std::false_type {};
 
 template<typename Type>
-struct has_iterator_category<Type, std::void_t<typename std::iterator_traits<Type>::iterator_category>>: std::true_type {};
+requires requires { typename std::iterator_traits<Type>::iterator_category; }
+struct has_iterator_category<Type>: std::true_type {};
 
 } // namespace internal
 /*! @endcond */
 
 /*! @copydoc is_iterator */
 template<typename Type>
-struct is_iterator<Type, std::enable_if_t<!std::is_void_v<std::remove_cv_t<std::remove_pointer_t<Type>>>>>
-    : internal::has_iterator_category<Type> {};
+requires (!std::is_void_v<std::remove_const_t<std::remove_pointer_t<Type>>>)
+struct is_iterator<Type>: internal::has_iterator_category<Type> {};
 
 /**
  * @brief Helper variable template.
@@ -705,8 +690,7 @@ inline constexpr bool is_iterator_v = is_iterator<Type>::value;
  * @tparam Type The type to test
  */
 template<typename Type>
-struct is_ebco_eligible
-    : std::bool_constant<std::is_empty_v<Type> && !std::is_final_v<Type>> {};
+struct is_ebco_eligible: std::bool_constant<std::is_empty_v<Type> && !std::is_final_v<Type>> {};
 
 /**
  * @brief Helper variable template.
@@ -720,12 +704,13 @@ inline constexpr bool is_ebco_eligible_v = is_ebco_eligible<Type>::value;
  * is valid and denotes a type, false otherwise.
  * @tparam Type The type to test.
  */
-template<typename Type, typename = void>
+template<typename Type>
 struct is_transparent: std::false_type {};
 
 /*! @copydoc is_transparent */
 template<typename Type>
-struct is_transparent<Type, std::void_t<typename Type::is_transparent>>: std::true_type {};
+requires requires { typename Type::is_transparent; }
+struct is_transparent<Type>: std::true_type {};
 
 /**
  * @brief Helper variable template.
@@ -734,56 +719,58 @@ struct is_transparent<Type, std::void_t<typename Type::is_transparent>>: std::tr
 template<typename Type>
 inline constexpr bool is_transparent_v = is_transparent<Type>::value;
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
-template<typename, typename = void>
+template<typename>
 struct has_tuple_size_value: std::false_type {};
 
 template<typename Type>
-struct has_tuple_size_value<Type, std::void_t<decltype(std::tuple_size<const Type>::value)>>: std::true_type {};
+requires is_complete_v<std::tuple_size<const Type>>
+struct has_tuple_size_value<Type>: std::true_type {};
 
-template<typename, typename = void>
+template<typename>
 struct has_value_type: std::false_type {};
 
 template<typename Type>
-struct has_value_type<Type, std::void_t<typename Type::value_type>>: std::true_type {};
+requires requires { typename Type::value_type; }
+struct has_value_type<Type>: std::true_type {};
 
 template<typename>
-[[nodiscard]] constexpr bool dispatch_is_equality_comparable();
+[[nodiscard]] ENTT_CONSTEVAL bool dispatch_is_equality_comparable();
 
 template<typename Type, std::size_t... Index>
-[[nodiscard]] constexpr bool unpack_maybe_equality_comparable(std::index_sequence<Index...>) {
+[[nodiscard]] ENTT_CONSTEVAL bool unpack_maybe_equality_comparable(std::index_sequence<Index...>) {
     return (dispatch_is_equality_comparable<std::tuple_element_t<Index, Type>>() && ...);
 }
 
 template<typename>
-[[nodiscard]] constexpr bool maybe_equality_comparable(char) {
+[[nodiscard]] ENTT_CONSTEVAL bool maybe_equality_comparable(char) {
     return false;
 }
 
 template<typename Type>
-[[nodiscard]] constexpr auto maybe_equality_comparable(int) -> decltype(std::declval<Type>() == std::declval<Type>()) {
+[[nodiscard]] ENTT_CONSTEVAL auto maybe_equality_comparable(int) -> decltype(std::declval<Type>() == std::declval<Type>()) {
     return true;
 }
 
 template<typename Type>
-[[nodiscard]] constexpr bool dispatch_is_equality_comparable() {
+[[nodiscard]] ENTT_CONSTEVAL bool dispatch_is_equality_comparable() {
     // NOLINTBEGIN(modernize-use-transparent-functors)
     if constexpr(std::is_array_v<Type>) {
         return false;
-    } else if constexpr(!is_iterator_v<Type> && has_value_type<Type>::value) {
-        if constexpr(std::is_same_v<typename Type::value_type, Type> || dispatch_is_equality_comparable<typename Type::value_type>()) {
-            return maybe_equality_comparable<Type>(0);
-        } else {
-            return false;
-        }
-    } else if constexpr(is_complete_v<std::tuple_size<std::remove_cv_t<Type>>>) {
+    } else if constexpr(is_complete_v<std::tuple_size<std::remove_const_t<Type>>>) {
         if constexpr(has_tuple_size_value<Type>::value) {
             return maybe_equality_comparable<Type>(0) && unpack_maybe_equality_comparable<Type>(std::make_index_sequence<std::tuple_size<Type>::value>{});
         } else {
             return maybe_equality_comparable<Type>(0);
         }
+    } else if constexpr(has_value_type<Type>::value) {
+        if constexpr(is_iterator_v<Type> || std::is_same_v<typename Type::value_type, Type> || dispatch_is_equality_comparable<typename Type::value_type>()) {
+            return maybe_equality_comparable<Type>(0);
+        } else {
+            return false;
+        }
     } else {
         return maybe_equality_comparable<Type>(0);
     }
@@ -836,7 +823,7 @@ struct constness_as<To, const From> {
  * @tparam From The type from which to transcribe the constness.
  */
 template<typename To, typename From>
-using constness_as_t = typename constness_as<To, From>::type;
+using constness_as_t = constness_as<To, From>::type;
 
 /**
  * @brief Extracts the class of a non-static member object or function.
@@ -865,7 +852,7 @@ public:
  * @tparam Member A pointer to a non-static member object or function.
  */
 template<typename Member>
-using member_class_t = typename member_class<Member>::type;
+using member_class_t = member_class<Member>::type;
 
 /**
  * @brief Extracts the n-th argument of a _callable_ type.
@@ -875,19 +862,19 @@ using member_class_t = typename member_class<Member>::type;
 template<std::size_t Index, typename Candidate>
 class nth_argument {
     template<typename Ret, typename... Args>
-    static constexpr type_list<Args...> pick_up(Ret (*)(Args...));
+    static ENTT_CONSTEVAL type_list<Args...> pick_up(Ret (*)(Args...));
 
     template<typename Ret, typename Class, typename... Args>
-    static constexpr type_list<Args...> pick_up(Ret (Class ::*)(Args...));
+    static ENTT_CONSTEVAL type_list<Args...> pick_up(Ret (Class ::*)(Args...));
 
     template<typename Ret, typename Class, typename... Args>
-    static constexpr type_list<Args...> pick_up(Ret (Class ::*)(Args...) const);
+    static ENTT_CONSTEVAL type_list<Args...> pick_up(Ret (Class ::*)(Args...) const);
 
     template<typename Type, typename Class>
-    static constexpr type_list<Type> pick_up(Type Class ::*);
+    static ENTT_CONSTEVAL type_list<Type> pick_up(Type Class ::*);
 
     template<typename Type>
-    static constexpr decltype(pick_up(&Type::operator())) pick_up(Type &&);
+    static ENTT_CONSTEVAL decltype(pick_up(&Type::operator())) pick_up(Type &&);
 
 public:
     /*! @brief N-th argument of the _callable_ type. */
@@ -900,7 +887,7 @@ public:
  * @tparam Candidate A valid function, member function or data member type.
  */
 template<std::size_t Index, typename Candidate>
-using nth_argument_t = typename nth_argument<Index, Candidate>::type;
+using nth_argument_t = nth_argument<Index, Candidate>::type;
 
 } // namespace entt
 

src/entt/core/utility.hpp

@@ -6,23 +6,6 @@
 
 namespace entt {
 
-/*! @brief Identity function object (waiting for C++20). */
-struct identity {
-    /*! @brief Indicates that this is a transparent function object. */
-    using is_transparent = void;
-
-    /**
-     * @brief Returns its argument unchanged.
-     * @tparam Type Type of the argument.
-     * @param value The actual argument.
-     * @return The submitted value as-is.
-     */
-    template<typename Type>
-    [[nodiscard]] constexpr Type &&operator()(Type &&value) const noexcept {
-        return std::forward<Type>(value);
-    }
-};
-
 /**
  * @brief Constant utility to disambiguate overloaded members of a class.
  * @tparam Type Type of the desired overload.

src/entt/entity/component.hpp

@@ -1,49 +1,52 @@
 #ifndef ENTT_ENTITY_COMPONENT_HPP
 #define ENTT_ENTITY_COMPONENT_HPP
 
+#include <concepts>
 #include <cstddef>
 #include <type_traits>
 #include "../config/config.h"
+#include "../core/concepts.hpp"
 #include "fwd.hpp"
 
 namespace entt {
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
-template<typename Type, typename = void>
+template<typename Type>
 struct in_place_delete: std::bool_constant<!(std::is_move_constructible_v<Type> && std::is_move_assignable_v<Type>)> {};
 
 template<>
 struct in_place_delete<void>: std::false_type {};
 
 template<typename Type>
-struct in_place_delete<Type, std::enable_if_t<Type::in_place_delete>>
-    : std::true_type {};
+requires Type::in_place_delete
+struct in_place_delete<Type>: std::true_type {};
 
-template<typename Type, typename = void>
+template<typename Type>
 struct page_size: std::integral_constant<std::size_t, !std::is_empty_v<ENTT_ETO_TYPE(Type)> * ENTT_PACKED_PAGE> {};
 
 template<>
 struct page_size<void>: std::integral_constant<std::size_t, 0u> {};
 
 template<typename Type>
-struct page_size<Type, std::void_t<decltype(Type::page_size)>>
-    : std::integral_constant<std::size_t, Type::page_size> {};
+requires std::is_convertible_v<decltype(Type::page_size), std::size_t>
+struct page_size<Type>: std::integral_constant<std::size_t, Type::page_size> {};
 
 } // namespace internal
 /*! @endcond */
 
 /**
  * @brief Common way to access various properties of components.
- * @tparam Type Type of component.
+ * @tparam Type Element type.
+ * @tparam Entity A valid entity type.
  */
-template<typename Type, typename = void>
+template<cvref_unqualified Type, typename Entity>
 struct component_traits {
-    static_assert(std::is_same_v<std::decay_t<Type>, Type>, "Unsupported type");
-
-    /*! @brief Component type. */
-    using type = Type;
+    /*! @brief Element type. */
+    using element_type = Type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
 
     /*! @brief Pointer stability, default is `false`. */
     static constexpr bool in_place_delete = internal::in_place_delete<Type>::value;

src/entt/entity/entity.hpp

@@ -1,6 +1,8 @@
 #ifndef ENTT_ENTITY_ENTITY_HPP
 #define ENTT_ENTITY_ENTITY_HPP
 
+#include <bit>
+#include <concepts>
 #include <cstddef>
 #include <cstdint>
 #include <type_traits>
@@ -10,20 +12,24 @@
 
 namespace entt {
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
-template<typename, typename = void>
+template<typename>
 struct entt_traits;
 
 template<typename Type>
-struct entt_traits<Type, std::enable_if_t<std::is_enum_v<Type>>>
-    : entt_traits<std::underlying_type_t<Type>> {
+requires requires {
+    requires std::is_enum_v<Type>;
+    typename internal::entt_traits<std::underlying_type_t<Type>>::value_type;
+}
+struct entt_traits<Type>: entt_traits<std::underlying_type_t<Type>> {
     using value_type = Type;
 };
 
 template<typename Type>
-struct entt_traits<Type, std::enable_if_t<std::is_class_v<Type>>>
+requires requires { typename Type::entity_type; }
+struct entt_traits<Type>
     : entt_traits<typename Type::entity_type> {
     using value_type = Type;
 };
@@ -53,24 +59,33 @@ struct entt_traits<std::uint64_t> {
 } // namespace internal
 /*! @endcond */
 
+/**
+ * @brief Specifies that a type is an entity-like type.
+ * @tparam Type Type to check.
+ */
+template<typename Type>
+concept entity_like = requires {
+    typename internal::entt_traits<Type>::value_type;
+};
+
 /**
  * @brief Common basic entity traits implementation.
  * @tparam Traits Actual entity traits to use.
  */
 template<typename Traits>
 class basic_entt_traits {
-    static constexpr auto length = popcount(Traits::entity_mask);
+    static constexpr auto length = std::popcount(Traits::entity_mask);
 
     static_assert(Traits::entity_mask && ((Traits::entity_mask & (Traits::entity_mask + 1)) == 0), "Invalid entity mask");
     static_assert((Traits::version_mask & (Traits::version_mask + 1)) == 0, "Invalid version mask");
 
 public:
     /*! @brief Value type. */
-    using value_type = typename Traits::value_type;
+    using value_type = Traits::value_type;
     /*! @brief Underlying entity type. */
-    using entity_type = typename Traits::entity_type;
+    using entity_type = Traits::entity_type;
     /*! @brief Underlying version type. */
-    using version_type = typename Traits::version_type;
+    using version_type = Traits::version_type;
 
     /*! @brief Entity mask size. */
     static constexpr entity_type entity_mask = Traits::entity_mask;
@@ -159,7 +174,7 @@ public:
  * @brief Entity traits.
  * @tparam Type Type of identifier.
  */
-template<typename Type>
+template<entity_like Type>
 struct entt_traits: basic_entt_traits<internal::entt_traits<Type>> {
     /*! @brief Base type. */
     using base_type = basic_entt_traits<internal::entt_traits<Type>>;
@@ -174,7 +189,7 @@ struct entt_traits: basic_entt_traits<internal::entt_traits<Type>> {
  * @return The integral representation of the given value.
  */
 template<typename Entity>
-[[nodiscard]] constexpr typename entt_traits<Entity>::entity_type to_integral(const Entity value) noexcept {
+[[nodiscard]] constexpr entt_traits<Entity>::entity_type to_integral(const Entity value) noexcept {
     return entt_traits<Entity>::to_integral(value);
 }
 
@@ -185,7 +200,7 @@ template<typename Entity>
  * @return The integral representation of the entity part.
  */
 template<typename Entity>
-[[nodiscard]] constexpr typename entt_traits<Entity>::entity_type to_entity(const Entity value) noexcept {
+[[nodiscard]] constexpr entt_traits<Entity>::entity_type to_entity(const Entity value) noexcept {
     return entt_traits<Entity>::to_entity(value);
 }
 
@@ -196,7 +211,7 @@ template<typename Entity>
  * @return The integral representation of the version part.
  */
 template<typename Entity>
-[[nodiscard]] constexpr typename entt_traits<Entity>::version_type to_version(const Entity value) noexcept {
+[[nodiscard]] constexpr entt_traits<Entity>::version_type to_version(const Entity value) noexcept {
     return entt_traits<Entity>::to_version(value);
 }
 
@@ -207,11 +222,10 @@ struct null_t {
      * @tparam Entity Type of identifier.
      * @return The null representation for the given type.
      */
-    template<typename Entity>
+    template<entity_like Entity>
     [[nodiscard]] constexpr operator Entity() const noexcept {
         using traits_type = entt_traits<Entity>;
-        constexpr auto value = traits_type::construct(traits_type::entity_mask, traits_type::version_mask);
-        return value;
+        return traits_type::construct(traits_type::entity_mask, traits_type::version_mask);
     }
 
     /**
@@ -223,63 +237,19 @@ struct null_t {
         return true;
     }
 
-    /**
-     * @brief Compares two null objects.
-     * @param other A null object.
-     * @return False in all cases.
-     */
-    [[nodiscard]] constexpr bool operator!=([[maybe_unused]] const null_t other) const noexcept {
-        return false;
-    }
-
     /**
      * @brief Compares a null object and an identifier of any type.
      * @tparam Entity Type of identifier.
      * @param entity Identifier with which to compare.
      * @return False if the two elements differ, true otherwise.
      */
-    template<typename Entity>
+    template<entity_like Entity>
     [[nodiscard]] constexpr bool operator==(const Entity entity) const noexcept {
         using traits_type = entt_traits<Entity>;
         return traits_type::to_entity(entity) == traits_type::to_entity(*this);
     }
-
-    /**
-     * @brief Compares a null object and an identifier of any type.
-     * @tparam Entity Type of identifier.
-     * @param entity Identifier with which to compare.
-     * @return True if the two elements differ, false otherwise.
-     */
-    template<typename Entity>
-    [[nodiscard]] constexpr bool operator!=(const Entity entity) const noexcept {
-        return !(entity == *this);
-    }
 };
 
-/**
- * @brief Compares a null object and an identifier of any type.
- * @tparam Entity Type of identifier.
- * @param lhs Identifier with which to compare.
- * @param rhs A null object yet to be converted.
- * @return False if the two elements differ, true otherwise.
- */
-template<typename Entity>
-[[nodiscard]] constexpr bool operator==(const Entity lhs, const null_t rhs) noexcept {
-    return rhs.operator==(lhs);
-}
-
-/**
- * @brief Compares a null object and an identifier of any type.
- * @tparam Entity Type of identifier.
- * @param lhs Identifier with which to compare.
- * @param rhs A null object yet to be converted.
- * @return True if the two elements differ, false otherwise.
- */
-template<typename Entity>
-[[nodiscard]] constexpr bool operator!=(const Entity lhs, const null_t rhs) noexcept {
-    return !(rhs == lhs);
-}
-
 /*! @brief Tombstone object for all identifiers.  */
 struct tombstone_t {
     /**
@@ -287,11 +257,10 @@ struct tombstone_t {
      * @tparam Entity Type of identifier.
      * @return The tombstone representation for the given type.
      */
-    template<typename Entity>
+    template<entity_like Entity>
     [[nodiscard]] constexpr operator Entity() const noexcept {
         using traits_type = entt_traits<Entity>;
-        constexpr auto value = traits_type::construct(traits_type::entity_mask, traits_type::version_mask);
-        return value;
+        return traits_type::construct(traits_type::entity_mask, traits_type::version_mask);
     }
 
     /**
@@ -303,22 +272,13 @@ struct tombstone_t {
         return true;
     }
 
-    /**
-     * @brief Compares two tombstone objects.
-     * @param other A tombstone object.
-     * @return False in all cases.
-     */
-    [[nodiscard]] constexpr bool operator!=([[maybe_unused]] const tombstone_t other) const noexcept {
-        return false;
-    }
-
     /**
      * @brief Compares a tombstone object and an identifier of any type.
      * @tparam Entity Type of identifier.
      * @param entity Identifier with which to compare.
      * @return False if the two elements differ, true otherwise.
      */
-    template<typename Entity>
+    template<entity_like Entity>
     [[nodiscard]] constexpr bool operator==(const Entity entity) const noexcept {
         using traits_type = entt_traits<Entity>;
 
@@ -328,43 +288,8 @@ struct tombstone_t {
             return (traits_type::to_version(entity) == traits_type::to_version(*this));
         }
     }
-
-    /**
-     * @brief Compares a tombstone object and an identifier of any type.
-     * @tparam Entity Type of identifier.
-     * @param entity Identifier with which to compare.
-     * @return True if the two elements differ, false otherwise.
-     */
-    template<typename Entity>
-    [[nodiscard]] constexpr bool operator!=(const Entity entity) const noexcept {
-        return !(entity == *this);
-    }
 };
 
-/**
- * @brief Compares a tombstone object and an identifier of any type.
- * @tparam Entity Type of identifier.
- * @param lhs Identifier with which to compare.
- * @param rhs A tombstone object yet to be converted.
- * @return False if the two elements differ, true otherwise.
- */
-template<typename Entity>
-[[nodiscard]] constexpr bool operator==(const Entity lhs, const tombstone_t rhs) noexcept {
-    return rhs.operator==(lhs);
-}
-
-/**
- * @brief Compares a tombstone object and an identifier of any type.
- * @tparam Entity Type of identifier.
- * @param lhs Identifier with which to compare.
- * @param rhs A tombstone object yet to be converted.
- * @return True if the two elements differ, false otherwise.
- */
-template<typename Entity>
-[[nodiscard]] constexpr bool operator!=(const Entity lhs, const tombstone_t rhs) noexcept {
-    return !(rhs == lhs);
-}
-
 /**
  * @brief Compile-time constant for null entities.
  *

src/entt/entity/fwd.hpp

@@ -5,6 +5,7 @@
 #include <memory>
 #include <type_traits>
 #include "../config/config.h"
+#include "../core/concepts.hpp"
 #include "../core/fwd.hpp"
 #include "../core/type_traits.hpp"
 
@@ -20,13 +21,18 @@ enum class deletion_policy : std::uint8_t {
     /*! @brief In-place deletion policy. */
     in_place = 1u,
     /*! @brief Swap-only deletion policy. */
-    swap_only = 2u
+    swap_only = 2u,
+    /*! @brief Unspecified deletion policy. */
+    unspecified = swap_and_pop
 };
 
+template<cvref_unqualified Type, typename Entity = entity>
+struct component_traits;
+
 template<typename Entity = entity, typename = std::allocator<Entity>>
 class basic_sparse_set;
 
-template<typename Type, typename = entity, typename = std::allocator<Type>, typename = void>
+template<typename Type, typename = entity, typename = std::allocator<Type>>
 class basic_storage;
 
 template<typename, typename>
@@ -38,7 +44,7 @@ class basic_reactive_mixin;
 template<typename Entity = entity, typename = std::allocator<Entity>>
 class basic_registry;
 
-template<typename, typename, typename = void>
+template<typename, typename>
 class basic_view;
 
 template<typename Type, typename = std::allocator<Type *>>
@@ -47,9 +53,6 @@ class basic_runtime_view;
 template<typename, typename, typename>
 class basic_group;
 
-template<typename, typename = std::allocator<void>>
-class basic_observer;
-
 template<typename>
 class basic_organizer;
 
@@ -92,9 +95,6 @@ using reactive_mixin = basic_reactive_mixin<Type, basic_registry<typename Type::
 /*! @brief Alias declaration for the most common use case. */
 using registry = basic_registry<>;
 
-/*! @brief Alias declaration for the most common use case. */
-using observer = basic_observer<registry>;
-
 /*! @brief Alias declaration for the most common use case. */
 using organizer = basic_organizer<registry>;
 
@@ -140,7 +140,7 @@ using const_runtime_view = basic_runtime_view<const sparse_set>;
 template<typename... Type>
 struct exclude_t final: type_list<Type...> {
     /*! @brief Default constructor. */
-    explicit constexpr exclude_t() = default;
+    explicit ENTT_CONSTEVAL exclude_t() = default;
 };
 
 /**
@@ -157,7 +157,7 @@ inline constexpr exclude_t<Type...> exclude{};
 template<typename... Type>
 struct get_t final: type_list<Type...> {
     /*! @brief Default constructor. */
-    explicit constexpr get_t() = default;
+    explicit ENTT_CONSTEVAL get_t() = default;
 };
 
 /**
@@ -174,7 +174,7 @@ inline constexpr get_t<Type...> get{};
 template<typename... Type>
 struct owned_t final: type_list<Type...> {
     /*! @brief Default constructor. */
-    explicit constexpr owned_t() = default;
+    explicit ENTT_CONSTEVAL owned_t() = default;
 };
 
 /**
@@ -223,7 +223,7 @@ struct type_list_transform<owned_t<Type...>, Op> {
  * @tparam Entity A valid entity type.
  * @tparam Allocator Type of allocator used to manage memory and elements.
  */
-template<typename Type, typename Entity = entity, typename Allocator = std::allocator<Type>, typename = void>
+template<typename Type, typename Entity = entity, typename Allocator = std::allocator<Type>>
 struct storage_type {
     /*! @brief Type-to-storage conversion result. */
     using type = ENTT_STORAGE(sigh_mixin, basic_storage<Type, Entity, Allocator>);
@@ -248,7 +248,7 @@ struct storage_type<reactive, Entity, Allocator> {
  * @tparam Args Arguments to forward.
  */
 template<typename... Args>
-using storage_type_t = typename storage_type<Args...>::type;
+using storage_type_t = storage_type<Args...>::type;
 
 /**
  * Type-to-storage conversion utility that preserves constness.
@@ -267,7 +267,7 @@ struct storage_for {
  * @tparam Args Arguments to forward.
  */
 template<typename... Args>
-using storage_for_t = typename storage_for<Args...>::type;
+using storage_for_t = storage_for<Args...>::type;
 
 /**
  * @brief Alias declaration for the most common use case.
@@ -283,7 +283,7 @@ using view = basic_view<type_list_transform_t<Get, storage_for>, type_list_trans
  * @tparam Get Types of storage _observed_ by the group.
  * @tparam Exclude Types of storage used to filter the group.
  */
-template<typename Owned, typename Get, typename Exclude>
+template<typename Owned, typename Get = get_t<>, typename Exclude = exclude_t<>>
 using group = basic_group<type_list_transform_t<Owned, storage_for>, type_list_transform_t<Get, storage_for>, type_list_transform_t<Exclude, storage_for>>;
 
 } // namespace entt

src/entt/entity/group.hpp

@@ -2,6 +2,7 @@
 #define ENTT_ENTITY_GROUP_HPP
 
 #include <array>
+#include <concepts>
 #include <cstddef>
 #include <iterator>
 #include <tuple>
@@ -13,12 +14,13 @@
 #include "../core/iterator.hpp"
 #include "../core/type_info.hpp"
 #include "../core/type_traits.hpp"
+#include "../stl/iterator.hpp"
 #include "entity.hpp"
 #include "fwd.hpp"
 
 namespace entt {
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
 template<typename, typename, typename>
@@ -57,7 +59,7 @@ public:
     }
 
     extended_group_iterator operator++(int) noexcept {
-        extended_group_iterator orig = *this;
+        const extended_group_iterator orig = *this;
         return ++(*this), orig;
     }
 
@@ -73,24 +75,16 @@ public:
         return it;
     }
 
-    template<typename... Lhs, typename... Rhs>
-    friend constexpr bool operator==(const extended_group_iterator<Lhs...> &, const extended_group_iterator<Rhs...> &) noexcept;
+    template<typename... Args>
+    [[nodiscard]] constexpr bool operator==(const extended_group_iterator<Args...> &other) const noexcept {
+        return it == other.it;
+    }
 
 private:
     It it;
     std::tuple<Owned *..., Get *...> pools;
 };
 
-template<typename... Lhs, typename... Rhs>
-[[nodiscard]] constexpr bool operator==(const extended_group_iterator<Lhs...> &lhs, const extended_group_iterator<Rhs...> &rhs) noexcept {
-    return lhs.it == rhs.it;
-}
-
-template<typename... Lhs, typename... Rhs>
-[[nodiscard]] constexpr bool operator!=(const extended_group_iterator<Lhs...> &lhs, const extended_group_iterator<Rhs...> &rhs) noexcept {
-    return !(lhs == rhs);
-}
-
 struct group_descriptor {
     using size_type = std::size_t;
     virtual ~group_descriptor() = default;
@@ -101,7 +95,7 @@ struct group_descriptor {
 
 template<typename Type, std::size_t Owned, std::size_t Get, std::size_t Exclude>
 class group_handler final: public group_descriptor {
-    using entity_type = typename Type::entity_type;
+    using entity_type = Type::entity_type;
 
     void swap_elements(const std::size_t pos, const entity_type entt) {
         for(size_type next{}; next < Owned; ++next) {
@@ -131,14 +125,14 @@ class group_handler final: public group_descriptor {
 
     void common_setup() {
         // we cannot iterate backwards because we want to leave behind valid entities in case of owned types
-        for(auto first = pools[0u]->rbegin(), last = first + pools[0u]->size(); first != last; ++first) {
+        for(auto first = pools[0u]->rbegin(), last = first + static_cast<decltype(pools)::difference_type>(pools[0u]->size()); first != last; ++first) {
             push_on_construct(*first);
         }
     }
 
 public:
     using common_type = Type;
-    using size_type = typename Type::size_type;
+    using size_type = Type::size_type;
 
     template<typename... OGType, typename... EType>
     group_handler(std::tuple<OGType &...> ogpool, std::tuple<EType &...> epool)
@@ -151,7 +145,7 @@ public:
 
     [[nodiscard]] bool owned(const id_type hash) const noexcept override {
         for(size_type pos{}; pos < Owned; ++pos) {
-            if(pools[pos]->type().hash() == hash) {
+            if(pools[pos]->info().hash() == hash) {
                 return true;
             }
         }
@@ -180,7 +174,7 @@ private:
 
 template<typename Type, std::size_t Get, std::size_t Exclude>
 class group_handler<Type, 0u, Get, Exclude> final: public group_descriptor {
-    using entity_type = typename Type::entity_type;
+    using entity_type = Type::entity_type;
 
     void push_on_construct(const entity_type entt) {
         if(!elem.contains(entt)
@@ -281,7 +275,7 @@ class basic_group;
 template<typename... Get, typename... Exclude>
 class basic_group<owned_t<>, get_t<Get...>, exclude_t<Exclude...>> {
     using base_type = std::common_type_t<typename Get::base_type..., typename Exclude::base_type...>;
-    using underlying_type = typename base_type::entity_type;
+    using underlying_type = base_type::entity_type;
 
     template<typename Type>
     static constexpr std::size_t index_of = type_list_index_v<std::remove_const_t<Type>, type_list<typename Get::element_type..., typename Exclude::element_type...>>;
@@ -297,12 +291,14 @@ public:
     using entity_type = underlying_type;
     /*! @brief Unsigned integer type. */
     using size_type = std::size_t;
+    /*! @brief Signed integer type. */
+    using difference_type = std::ptrdiff_t;
     /*! @brief Common type among all storage types. */
     using common_type = base_type;
     /*! @brief Random access iterator type. */
-    using iterator = typename common_type::iterator;
+    using iterator = common_type::iterator;
     /*! @brief Reverse iterator type. */
-    using reverse_iterator = typename common_type::reverse_iterator;
+    using reverse_iterator = common_type::reverse_iterator;
     /*! @brief Iterable group type. */
     using iterable = iterable_adaptor<internal::extended_group_iterator<iterator, owned_t<>, get_t<Get...>>>;
     /*! @brief Group handler type. */
@@ -465,7 +461,7 @@ public:
      * @return The identifier that occupies the given position.
      */
     [[nodiscard]] entity_type operator[](const size_type pos) const {
-        return begin()[pos];
+        return begin()[static_cast<difference_type>(pos)];
     }
 
     /**
@@ -644,12 +640,10 @@ public:
      * The shared pool of entities and thus its order is affected by the changes
      * to each and every pool that it tracks.
      *
-     * @tparam It Type of input iterator.
      * @param first An iterator to the first element of the range of entities.
      * @param last An iterator past the last element of the range of entities.
      */
-    template<typename It>
-    void sort_as(It first, It last) const {
+    void sort_as(stl::input_iterator auto first, stl::input_iterator auto last) const {
         if(*this) {
             descriptor->handle().sort_as(first, last);
         }
@@ -695,7 +689,7 @@ class basic_group<owned_t<Owned...>, get_t<Get...>, exclude_t<Exclude...>> {
     static_assert(((Owned::storage_policy != deletion_policy::in_place) && ...), "Groups do not support in-place delete");
 
     using base_type = std::common_type_t<typename Owned::base_type..., typename Get::base_type..., typename Exclude::base_type...>;
-    using underlying_type = typename base_type::entity_type;
+    using underlying_type = base_type::entity_type;
 
     template<typename Type>
     static constexpr std::size_t index_of = type_list_index_v<std::remove_const_t<Type>, type_list<typename Owned::element_type..., typename Get::element_type..., typename Exclude::element_type...>>;
@@ -711,12 +705,14 @@ public:
     using entity_type = underlying_type;
     /*! @brief Unsigned integer type. */
     using size_type = std::size_t;
+    /*! @brief Signed integer type. */
+    using difference_type = std::ptrdiff_t;
     /*! @brief Common type among all storage types. */
     using common_type = base_type;
     /*! @brief Random access iterator type. */
-    using iterator = typename common_type::iterator;
+    using iterator = common_type::iterator;
     /*! @brief Reverse iterator type. */
-    using reverse_iterator = typename common_type::reverse_iterator;
+    using reverse_iterator = common_type::reverse_iterator;
     /*! @brief Iterable group type. */
     using iterable = iterable_adaptor<internal::extended_group_iterator<iterator, owned_t<Owned...>, get_t<Get...>>>;
     /*! @brief Group handler type. */
@@ -794,7 +790,7 @@ public:
      * @return An iterator to the first entity of the group.
      */
     [[nodiscard]] iterator begin() const noexcept {
-        return *this ? (handle().end() - descriptor->length()) : iterator{};
+        return *this ? (handle().end() - static_cast<difference_type>(descriptor->length())) : iterator{};
     }
 
     /**
@@ -824,7 +820,7 @@ public:
      * reversed group.
      */
     [[nodiscard]] reverse_iterator rend() const noexcept {
-        return *this ? (handle().rbegin() + descriptor->length()) : reverse_iterator{};
+        return *this ? (handle().rbegin() + static_cast<difference_type>(descriptor->length())) : reverse_iterator{};
     }
 
     /**
@@ -864,7 +860,7 @@ public:
      * @return The identifier that occupies the given position.
      */
     [[nodiscard]] entity_type operator[](const size_type pos) const {
-        return begin()[pos];
+        return begin()[static_cast<difference_type>(pos)];
     }
 
     /**

src/entt/entity/handle.hpp

@@ -13,19 +13,19 @@
 
 namespace entt {
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
 template<typename It>
 class handle_storage_iterator final {
-    template<typename Other>
+    template<typename>
     friend class handle_storage_iterator;
 
     using underlying_type = std::remove_reference_t<typename It::value_type::second_type>;
-    using entity_type = typename underlying_type::entity_type;
+    using entity_type = underlying_type::entity_type;
 
 public:
-    using value_type = typename std::iterator_traits<It>::value_type;
+    using value_type = std::iterator_traits<It>::value_type;
     using pointer = input_iterator_pointer<value_type>;
     using reference = value_type;
     using difference_type = std::ptrdiff_t;
@@ -47,12 +47,12 @@ public:
     }
 
     constexpr handle_storage_iterator &operator++() noexcept {
-        while(++it != last && !it->second.contains(entt)) {}
+        for(++it; it != last && !it->second.contains(entt); ++it) {}
         return *this;
     }
 
     constexpr handle_storage_iterator operator++(int) noexcept {
-        handle_storage_iterator orig = *this;
+        const handle_storage_iterator orig = *this;
         return ++(*this), orig;
     }
 
@@ -64,8 +64,10 @@ public:
         return operator*();
     }
 
-    template<typename ILhs, typename IRhs>
-    friend constexpr bool operator==(const handle_storage_iterator<ILhs> &, const handle_storage_iterator<IRhs> &) noexcept;
+    template<typename Other>
+    [[nodiscard]] constexpr bool operator==(const handle_storage_iterator<Other> &other) const noexcept {
+        return it == other.it;
+    }
 
 private:
     entity_type entt;
@@ -73,16 +75,6 @@ private:
     It last;
 };
 
-template<typename ILhs, typename IRhs>
-[[nodiscard]] constexpr bool operator==(const handle_storage_iterator<ILhs> &lhs, const handle_storage_iterator<IRhs> &rhs) noexcept {
-    return lhs.it == rhs.it;
-}
-
-template<typename ILhs, typename IRhs>
-[[nodiscard]] constexpr bool operator!=(const handle_storage_iterator<ILhs> &lhs, const handle_storage_iterator<IRhs> &rhs) noexcept {
-    return !(lhs == rhs);
-}
-
 } // namespace internal
 /*! @endcond */
 
@@ -107,9 +99,9 @@ public:
     /*! @brief Type of registry accepted by the handle. */
     using registry_type = Registry;
     /*! @brief Underlying entity identifier. */
-    using entity_type = typename traits_type::value_type;
+    using entity_type = traits_type::value_type;
     /*! @brief Underlying version type. */
-    using version_type = typename traits_type::version_type;
+    using version_type = traits_type::version_type;
     /*! @brief Unsigned integer type. */
     using size_type = std::size_t;
     /*! @brief Iterable handle type. */
@@ -331,6 +323,27 @@ public:
         return owner_or_assert().orphan(entt);
     }
 
+    /**
+     * @brief Compares two handles.
+     * @tparam Other Scope of the other handle.
+     * @param other A valid handle.
+     * @return True if both handles refer to the same registry and the same
+     * entity, false otherwise.
+     */
+    template<typename... Other>
+    [[nodiscard]] bool operator==(const basic_handle<Other...> &other) const noexcept {
+        return owner == other.registry() && entt == other.entity();
+    }
+
+    /**
+     * @brief Compares a handle with the null object.
+     * @param other A null object yet to be converted.
+     * @return False if the two elements differ, true otherwise.
+     */
+    [[nodiscard]] constexpr bool operator==(const null_t other) const noexcept {
+        return (entt == other);
+    }
+
     /**
      * @brief Returns a const handle from a non-const one.
      * @tparam Other A valid entity type.
@@ -350,82 +363,6 @@ private:
     entity_type entt;
 };
 
-/**
- * @brief Compares two handles.
- * @tparam Args Scope of the first handle.
- * @tparam Other Scope of the second handle.
- * @param lhs A valid handle.
- * @param rhs A valid handle.
- * @return True if both handles refer to the same registry and the same
- * entity, false otherwise.
- */
-template<typename... Args, typename... Other>
-[[nodiscard]] bool operator==(const basic_handle<Args...> &lhs, const basic_handle<Other...> &rhs) noexcept {
-    return lhs.registry() == rhs.registry() && lhs.entity() == rhs.entity();
-}
-
-/**
- * @brief Compares two handles.
- * @tparam Args Scope of the first handle.
- * @tparam Other Scope of the second handle.
- * @param lhs A valid handle.
- * @param rhs A valid handle.
- * @return False if both handles refer to the same registry and the same
- * entity, true otherwise.
- */
-template<typename... Args, typename... Other>
-[[nodiscard]] bool operator!=(const basic_handle<Args...> &lhs, const basic_handle<Other...> &rhs) noexcept {
-    return !(lhs == rhs);
-}
-
-/**
- * @brief Compares a handle with the null object.
- * @tparam Args Scope of the handle.
- * @param lhs A valid handle.
- * @param rhs A null object yet to be converted.
- * @return False if the two elements differ, true otherwise.
- */
-template<typename... Args>
-[[nodiscard]] constexpr bool operator==(const basic_handle<Args...> &lhs, const null_t rhs) noexcept {
-    return (lhs.entity() == rhs);
-}
-
-/**
- * @brief Compares a handle with the null object.
- * @tparam Args Scope of the handle.
- * @param lhs A null object yet to be converted.
- * @param rhs A valid handle.
- * @return False if the two elements differ, true otherwise.
- */
-template<typename... Args>
-[[nodiscard]] constexpr bool operator==(const null_t lhs, const basic_handle<Args...> &rhs) noexcept {
-    return (rhs == lhs);
-}
-
-/**
- * @brief Compares a handle with the null object.
- * @tparam Args Scope of the handle.
- * @param lhs A valid handle.
- * @param rhs A null object yet to be converted.
- * @return True if the two elements differ, false otherwise.
- */
-template<typename... Args>
-[[nodiscard]] constexpr bool operator!=(const basic_handle<Args...> &lhs, const null_t rhs) noexcept {
-    return (lhs.entity() != rhs);
-}
-
-/**
- * @brief Compares a handle with the null object.
- * @tparam Args Scope of the handle.
- * @param lhs A null object yet to be converted.
- * @param rhs A valid handle.
- * @return True if the two elements differ, false otherwise.
- */
-template<typename... Args>
-[[nodiscard]] constexpr bool operator!=(const null_t lhs, const basic_handle<Args...> &rhs) noexcept {
-    return (rhs != lhs);
-}
-
 } // namespace entt
 
 #endif

src/entt/entity/helper.hpp

@@ -22,21 +22,21 @@ template<typename Registry>
 class as_view {
     template<typename... Get, typename... Exclude>
     [[nodiscard]] auto dispatch(get_t<Get...>, exclude_t<Exclude...>) const {
-        return reg.template view<constness_as_t<typename Get::element_type, Get>...>(exclude_t<constness_as_t<typename Exclude::element_type, Exclude>...>{});
+        return reg->template view<constness_as_t<typename Get::element_type, Get>...>(exclude_t<constness_as_t<typename Exclude::element_type, Exclude>...>{});
     }
 
 public:
     /*! @brief Type of registry to convert. */
     using registry_type = Registry;
     /*! @brief Underlying entity identifier. */
-    using entity_type = typename registry_type::entity_type;
+    using entity_type = registry_type::entity_type;
 
     /**
      * @brief Constructs a converter for a given registry.
      * @param source A valid reference to a registry.
      */
     as_view(registry_type &source) noexcept
-        : reg{source} {}
+        : reg{&source} {}
 
     /**
      * @brief Conversion function from a registry to a view.
@@ -50,7 +50,7 @@ public:
     }
 
 private:
-    registry_type &reg;
+    registry_type *reg;
 };
 
 /**
@@ -62,9 +62,9 @@ class as_group {
     template<typename... Owned, typename... Get, typename... Exclude>
     [[nodiscard]] auto dispatch(owned_t<Owned...>, get_t<Get...>, exclude_t<Exclude...>) const {
         if constexpr(std::is_const_v<registry_type>) {
-            return reg.template group_if_exists<typename Owned::element_type...>(get_t<typename Get::element_type...>{}, exclude_t<typename Exclude::element_type...>{});
+            return reg->template group_if_exists<typename Owned::element_type...>(get_t<typename Get::element_type...>{}, exclude_t<typename Exclude::element_type...>{});
         } else {
-            return reg.template group<constness_as_t<typename Owned::element_type, Owned>...>(get_t<constness_as_t<typename Get::element_type, Get>...>{}, exclude_t<constness_as_t<typename Exclude::element_type, Exclude>...>{});
+            return reg->template group<constness_as_t<typename Owned::element_type, Owned>...>(get_t<constness_as_t<typename Get::element_type, Get>...>{}, exclude_t<constness_as_t<typename Exclude::element_type, Exclude>...>{});
         }
     }
 
@@ -72,14 +72,14 @@ public:
     /*! @brief Type of registry to convert. */
     using registry_type = Registry;
     /*! @brief Underlying entity identifier. */
-    using entity_type = typename registry_type::entity_type;
+    using entity_type = registry_type::entity_type;
 
     /**
      * @brief Constructs a converter for a given registry.
      * @param source A valid reference to a registry.
      */
     as_group(registry_type &source) noexcept
-        : reg{source} {}
+        : reg{&source} {}
 
     /**
      * @brief Conversion function from a registry to a group.
@@ -94,7 +94,7 @@ public:
     }
 
 private:
-    registry_type &reg;
+    registry_type *reg;
 };
 
 /**
@@ -123,17 +123,18 @@ void invoke(Registry &reg, const typename Registry::entity_type entt) {
  * @return The entity associated with the given element.
  */
 template<typename... Args>
-typename basic_storage<Args...>::entity_type to_entity(const basic_storage<Args...> &storage, const typename basic_storage<Args...>::value_type &instance) {
-    using traits_type = component_traits<typename basic_storage<Args...>::value_type>;
+basic_storage<Args...>::entity_type to_entity(const basic_storage<Args...> &storage, const typename basic_storage<Args...>::value_type &instance) {
+    using traits_type = component_traits<typename basic_storage<Args...>::value_type, typename basic_storage<Args...>::entity_type>;
     static_assert(traits_type::page_size != 0u, "Unexpected page size");
-    const typename basic_storage<Args...>::base_type &base = storage;
-    const auto *addr = std::addressof(instance);
+    const auto *page = storage.raw();
 
-    for(auto it = base.rbegin(), last = base.rend(); it < last; it += traits_type::page_size) {
-        if(const auto dist = (addr - std::addressof(storage.get(*it))); dist >= 0 && dist < static_cast<decltype(dist)>(traits_type::page_size)) {
-            return *(it + dist);
+    // NOLINTBEGIN(cppcoreguidelines-pro-bounds-pointer-arithmetic)
+    for(std::size_t pos{}, count = storage.size(); pos < count; pos += traits_type::page_size, ++page) {
+        if(const auto dist = (std::addressof(instance) - *page); dist >= 0 && dist < static_cast<decltype(dist)>(traits_type::page_size)) {
+            return *(static_cast<const basic_storage<Args...>::base_type &>(storage).rbegin() + static_cast<decltype(dist)>(pos) + dist);
         }
     }
+    // NOLINTEND(cppcoreguidelines-pro-bounds-pointer-arithmetic)
 
     return null;
 }

src/entt/entity/mixin.hpp

@@ -1,54 +1,42 @@
 #ifndef ENTT_ENTITY_MIXIN_HPP
 #define ENTT_ENTITY_MIXIN_HPP
 
+#include <concepts>
 #include <type_traits>
 #include <utility>
 #include "../config/config.h"
 #include "../core/any.hpp"
 #include "../core/type_info.hpp"
 #include "../signal/sigh.hpp"
+#include "../stl/iterator.hpp"
 #include "entity.hpp"
 #include "fwd.hpp"
 
 namespace entt {
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
-template<typename, typename, typename = void>
+template<typename, typename>
 struct has_on_construct final: std::false_type {};
 
 template<typename Type, typename Registry>
-struct has_on_construct<Type, Registry, std::void_t<decltype(Type::on_construct(std::declval<Registry &>(), std::declval<Registry>().create()))>>
-    : std::true_type {};
+requires std::invocable<decltype(&Type::on_construct), Registry &, typename Registry::entity_type>
+struct has_on_construct<Type, Registry>: std::true_type {};
 
-template<typename, typename, typename = void>
+template<typename, typename>
 struct has_on_update final: std::false_type {};
 
 template<typename Type, typename Registry>
-struct has_on_update<Type, Registry, std::void_t<decltype(Type::on_update(std::declval<Registry &>(), std::declval<Registry>().create()))>>
-    : std::true_type {};
+requires std::invocable<decltype(&Type::on_update), Registry &, typename Registry::entity_type>
+struct has_on_update<Type, Registry>: std::true_type {};
 
-template<typename, typename, typename = void>
+template<typename, typename>
 struct has_on_destroy final: std::false_type {};
 
 template<typename Type, typename Registry>
-struct has_on_destroy<Type, Registry, std::void_t<decltype(Type::on_destroy(std::declval<Registry &>(), std::declval<Registry>().create()))>>
-    : std::true_type {};
-
-template<typename Type>
-auto *any_to_owner(any &value) noexcept {
-    using base_type = basic_registry<typename Type::entity_type, typename Type::allocator_type>;
-    auto *reg = any_cast<base_type>(&value);
-
-    if constexpr(!std::is_same_v<Type, base_type>) {
-        if(!reg) {
-            reg = any_cast<Type>(&value);
-        }
-    }
-
-    return reg;
-}
+requires std::invocable<decltype(&Type::on_destroy), Registry &, typename Registry::entity_type>
+struct has_on_destroy<Type, Registry>: std::true_type {};
 
 } // namespace internal
 /*! @endcond */
@@ -74,7 +62,7 @@ class basic_sigh_mixin final: public Type {
 
     using basic_registry_type = basic_registry<typename owner_type::entity_type, typename owner_type::allocator_type>;
     using sigh_type = sigh<void(owner_type &, const typename underlying_type::entity_type), typename underlying_type::allocator_type>;
-    using underlying_iterator = typename underlying_type::base_type::basic_iterator;
+    using underlying_iterator = underlying_type::base_type::basic_iterator;
 
     static_assert(std::is_base_of_v<basic_registry_type, owner_type>, "Invalid registry type");
 
@@ -104,7 +92,7 @@ private:
                     destruction.publish(reg, underlying_type::base_type::operator[](pos));
                 }
             } else {
-                for(auto entt: static_cast<typename underlying_type::base_type &>(*this)) {
+                for(auto entt: static_cast<underlying_type::base_type &>(*this)) {
                     if constexpr(underlying_type::storage_policy == deletion_policy::in_place) {
                         if(entt != tombstone) {
                             destruction.publish(reg, entt);
@@ -119,7 +107,7 @@ private:
         underlying_type::pop_all();
     }
 
-    underlying_iterator try_emplace(const typename underlying_type::entity_type entt, const bool force_back, const void *value) final {
+    underlying_iterator try_emplace(const underlying_type::entity_type entt, const bool force_back, const void *value) final {
         const auto it = underlying_type::try_emplace(entt, force_back, value);
 
         if(auto &reg = owner_or_assert(); it != underlying_type::base_type::end()) {
@@ -130,15 +118,22 @@ private:
     }
 
     void bind_any(any value) noexcept final {
-        owner = internal::any_to_owner<registry_type>(value);
+        owner = any_cast<basic_registry_type>(&value);
+
+        if constexpr(!std::is_same_v<registry_type, basic_registry_type>) {
+            if(owner == nullptr) {
+                owner = any_cast<registry_type>(&value);
+            }
+        }
+
         underlying_type::bind_any(std::move(value));
     }
 
 public:
     /*! @brief Allocator type. */
-    using allocator_type = typename underlying_type::allocator_type;
+    using allocator_type = underlying_type::allocator_type;
     /*! @brief Underlying entity identifier. */
-    using entity_type = typename underlying_type::entity_type;
+    using entity_type = underlying_type::entity_type;
     /*! @brief Expected registry type. */
     using registry_type = owner_type;
 
@@ -157,15 +152,15 @@ public:
           destruction{allocator},
           update{allocator} {
         if constexpr(internal::has_on_construct<typename underlying_type::element_type, Registry>::value) {
-            entt::sink{construction}.template connect<&underlying_type::element_type::on_construct>();
+            sink{construction}.template connect<&underlying_type::element_type::on_construct>();
         }
 
         if constexpr(internal::has_on_update<typename underlying_type::element_type, Registry>::value) {
-            entt::sink{update}.template connect<&underlying_type::element_type::on_update>();
+            sink{update}.template connect<&underlying_type::element_type::on_update>();
         }
 
         if constexpr(internal::has_on_destroy<typename underlying_type::element_type, Registry>::value) {
-            entt::sink{destruction}.template connect<&underlying_type::element_type::on_destroy>();
+            sink{destruction}.template connect<&underlying_type::element_type::on_destroy>();
         }
     }
 
@@ -177,7 +172,7 @@ public:
      * @param other The instance to move from.
      */
     basic_sigh_mixin(basic_sigh_mixin &&other) noexcept
-        : underlying_type{std::move(other)},
+        : underlying_type{static_cast<underlying_type &&>(other)},
           owner{other.owner},
           construction{std::move(other.construction)},
           destruction{std::move(other.destruction)},
@@ -189,7 +184,7 @@ public:
      * @param allocator The allocator to use.
      */
     basic_sigh_mixin(basic_sigh_mixin &&other, const allocator_type &allocator)
-        : underlying_type{std::move(other), allocator},
+        : underlying_type{static_cast<underlying_type &&>(other), allocator},
           owner{other.owner},
           construction{std::move(other.construction), allocator},
           destruction{std::move(other.destruction), allocator},
@@ -294,48 +289,59 @@ public:
     }
 
     /**
-     * @brief Emplace elements into a storage.
-     *
-     * The behavior of this operation depends on the underlying storage type
-     * (for example, components vs entities).<br/>
-     * Refer to the specific documentation for more details.
-     *
-     * @return Whatever the underlying storage returns.
+     * @brief Creates a new identifier or recycles a destroyed one.
+     * @return A valid identifier.
      */
-    auto emplace() {
-        const auto entt = underlying_type::emplace();
+    auto generate() {
+        const auto entt = underlying_type::generate();
         construction.publish(owner_or_assert(), entt);
         return entt;
     }
 
     /**
-     * @brief Emplace elements into a storage.
-     *
-     * The behavior of this operation depends on the underlying storage type
-     * (for example, components vs entities).<br/>
-     * Refer to the specific documentation for more details.
-     *
-     * @tparam Args Types of arguments to forward to the underlying storage.
-     * @param hint A valid identifier.
-     * @param args Parameters to forward to the underlying storage.
-     * @return Whatever the underlying storage returns.
+     * @brief Creates a new identifier or recycles a destroyed one.
+     * @param hint Required identifier.
+     * @return A valid identifier.
      */
-    template<typename... Args>
-    std::conditional_t<std::is_same_v<typename underlying_type::element_type, entity_type>, entity_type, decltype(std::declval<underlying_type>().get({}))>
-    emplace(const entity_type hint, Args &&...args) {
-        if constexpr(std::is_same_v<typename underlying_type::element_type, entity_type>) {
-            const auto entt = underlying_type::emplace(hint, std::forward<Args>(args)...);
-            construction.publish(owner_or_assert(), entt);
-            return entt;
-        } else {
-            underlying_type::emplace(hint, std::forward<Args>(args)...);
-            construction.publish(owner_or_assert(), hint);
-            return this->get(hint);
+    entity_type generate(const entity_type hint) {
+        const auto entt = underlying_type::generate(hint);
+        construction.publish(owner_or_assert(), entt);
+        return entt;
+    }
+
+    /**
+     * @brief Assigns each element in a range an identifier.
+     * @tparam It Type of output iterator.
+     * @param first An iterator to the first element of the range to generate.
+     * @param last An iterator past the last element of the range to generate.
+     */
+    template<stl::output_iterator<entity_type> It>
+    void generate(It first, It last) {
+        underlying_type::generate(first, last);
+
+        if(auto &reg = owner_or_assert(); !construction.empty()) {
+            for(; first != last; ++first) {
+                construction.publish(reg, *first);
+            }
         }
     }
 
     /**
-     * @brief Patches the given instance for an entity.
+     * @brief Assigns an entity to a storage and constructs its object.
+     * @tparam Args Types of arguments to forward to the underlying storage.
+     * @param entt A valid identifier.
+     * @param args Parameters to forward to the underlying storage.
+     * @return A reference to the newly created object.
+     */
+    template<typename... Args>
+    decltype(auto) emplace(const entity_type entt, Args &&...args) {
+        underlying_type::emplace(entt, std::forward<Args>(args)...);
+        construction.publish(owner_or_assert(), entt);
+        return this->get(entt);
+    }
+
+    /**
+     * @brief Updates the instance assigned to a given entity in-place.
      * @tparam Func Types of the function objects to invoke.
      * @param entt A valid identifier.
      * @param func Valid function objects.
@@ -349,24 +355,20 @@ public:
     }
 
     /**
-     * @brief Emplace elements into a storage.
-     *
-     * The behavior of this operation depends on the underlying storage type
-     * (for example, components vs entities).<br/>
-     * Refer to the specific documentation for more details.
-     *
-     * @tparam It Iterator type (as required by the underlying storage type).
+     * @brief Assigns one or more entities to a storage and constructs their
+     * objects from a given instance.
      * @tparam Args Types of arguments to forward to the underlying storage.
-     * @param first An iterator to the first element of the range.
-     * @param last An iterator past the last element of the range.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
      * @param args Parameters to use to forward to the underlying storage.
      */
-    template<typename It, typename... Args>
-    void insert(It first, It last, Args &&...args) {
+    template<typename... Args>
+    void insert(stl::input_iterator auto first, stl::input_iterator auto last, Args &&...args) {
         auto from = underlying_type::size();
         underlying_type::insert(first, last, std::forward<Args>(args)...);
 
         if(auto &reg = owner_or_assert(); !construction.empty()) {
+            // fine as long as insert passes force_back true to try_emplace
             for(const auto to = underlying_type::size(); from != to; ++from) {
                 construction.publish(reg, underlying_type::operator[](from));
             }
@@ -390,7 +392,9 @@ class basic_reactive_mixin final: public Type {
     using underlying_type = Type;
     using owner_type = Registry;
 
+    using alloc_traits = std::allocator_traits<typename underlying_type::allocator_type>;
     using basic_registry_type = basic_registry<typename owner_type::entity_type, typename owner_type::allocator_type>;
+    using container_type = std::vector<connection, typename alloc_traits::template rebind_alloc<connection>>;
 
     static_assert(std::is_base_of_v<basic_registry_type, owner_type>, "Invalid registry type");
 
@@ -399,7 +403,7 @@ class basic_reactive_mixin final: public Type {
         return static_cast<owner_type &>(*owner);
     }
 
-    void emplace_element(const Registry &, typename underlying_type::entity_type entity) {
+    void emplace_element(const Registry &, underlying_type::entity_type entity) {
         if(!underlying_type::contains(entity)) {
             underlying_type::emplace(entity);
         }
@@ -407,15 +411,22 @@ class basic_reactive_mixin final: public Type {
 
 private:
     void bind_any(any value) noexcept final {
-        owner = internal::any_to_owner<registry_type>(value);
+        owner = any_cast<basic_registry_type>(&value);
+
+        if constexpr(!std::is_same_v<registry_type, basic_registry_type>) {
+            if(owner == nullptr) {
+                owner = any_cast<registry_type>(&value);
+            }
+        }
+
         underlying_type::bind_any(std::move(value));
     }
 
 public:
     /*! @brief Allocator type. */
-    using allocator_type = typename underlying_type::allocator_type;
+    using allocator_type = underlying_type::allocator_type;
     /*! @brief Underlying entity identifier. */
-    using entity_type = typename underlying_type::entity_type;
+    using entity_type = underlying_type::entity_type;
     /*! @brief Expected registry type. */
     using registry_type = owner_type;
 
@@ -429,7 +440,8 @@ public:
      */
     explicit basic_reactive_mixin(const allocator_type &allocator)
         : underlying_type{allocator},
-          owner{} {
+          owner{},
+          conn{allocator} {
     }
 
     /*! @brief Default copy constructor, deleted on purpose. */
@@ -440,8 +452,10 @@ public:
      * @param other The instance to move from.
      */
     basic_reactive_mixin(basic_reactive_mixin &&other) noexcept
-        : underlying_type{std::move(other)},
-          owner{other.owner} {}
+        : underlying_type{static_cast<underlying_type &&>(other)},
+          owner{other.owner},
+          conn{std::move(other.conn)} {
+    }
 
     /**
      * @brief Allocator-extended move constructor.
@@ -449,8 +463,10 @@ public:
      * @param allocator The allocator to use.
      */
     basic_reactive_mixin(basic_reactive_mixin &&other, const allocator_type &allocator)
-        : underlying_type{std::move(other), allocator},
-          owner{other.owner} {}
+        : underlying_type{static_cast<underlying_type &&>(other), allocator},
+          owner{other.owner},
+          conn{std::move(other.conn), allocator} {
+    }
 
     /*! @brief Default destructor. */
     ~basic_reactive_mixin() override = default;
@@ -467,18 +483,8 @@ public:
      * @return This mixin.
      */
     basic_reactive_mixin &operator=(basic_reactive_mixin &&other) noexcept {
-        swap(other);
-        return *this;
-    }
-
-    /**
-     * @brief Exchanges the contents with those of a given storage.
-     * @param other Storage to exchange the content with.
-     */
-    void swap(basic_reactive_mixin &other) noexcept {
-        using std::swap;
-        swap(owner, other.owner);
         underlying_type::swap(other);
+        return *this;
     }
 
     /**
@@ -490,7 +496,8 @@ public:
      */
     template<typename Clazz, auto Candidate = &basic_reactive_mixin::emplace_element>
     basic_reactive_mixin &on_construct(const id_type id = type_hash<Clazz>::value()) {
-        owner_or_assert().template storage<Clazz>(id).on_construct().template connect<Candidate>(*this);
+        auto curr = owner_or_assert().template storage<Clazz>(id).on_construct().template connect<Candidate>(*this);
+        conn.push_back(std::move(curr));
         return *this;
     }
 
@@ -503,7 +510,8 @@ public:
      */
     template<typename Clazz, auto Candidate = &basic_reactive_mixin::emplace_element>
     basic_reactive_mixin &on_update(const id_type id = type_hash<Clazz>::value()) {
-        owner_or_assert().template storage<Clazz>(id).on_update().template connect<Candidate>(*this);
+        auto curr = owner_or_assert().template storage<Clazz>(id).on_update().template connect<Candidate>(*this);
+        conn.push_back(std::move(curr));
         return *this;
     }
 
@@ -516,7 +524,8 @@ public:
      */
     template<typename Clazz, auto Candidate = &basic_reactive_mixin::emplace_element>
     basic_reactive_mixin &on_destroy(const id_type id = type_hash<Clazz>::value()) {
-        owner_or_assert().template storage<Clazz>(id).on_destroy().template connect<Candidate>(*this);
+        auto curr = owner_or_assert().template storage<Clazz>(id).on_destroy().template connect<Candidate>(*this);
+        conn.push_back(std::move(curr));
         return *this;
     }
 
@@ -560,12 +569,22 @@ public:
     template<typename... Get, typename... Exclude>
     [[nodiscard]] basic_view<get_t<const basic_reactive_mixin, typename basic_registry_type::template storage_for_type<Get>...>, exclude_t<typename basic_registry_type::template storage_for_type<Exclude>...>>
     view(exclude_t<Exclude...> = exclude_t{}) {
-        owner_type &parent = owner_or_assert();
+        std::conditional_t<((std::is_const_v<Get> && ...) && (std::is_const_v<Exclude> && ...)), const owner_type, owner_type> &parent = owner_or_assert();
         return {*this, parent.template storage<std::remove_const_t<Get>>()..., parent.template storage<std::remove_const_t<Exclude>>()...};
     }
 
+    /*! @brief Releases all connections to the underlying registry, if any. */
+    void reset() {
+        for(auto &&curr: conn) {
+            curr.release();
+        }
+
+        conn.clear();
+    }
+
 private:
     basic_registry_type *owner;
+    container_type conn;
 };
 
 } // namespace entt

src/entt/entity/observer.hpp

@@ -1,438 +0,0 @@
-#ifndef ENTT_ENTITY_OBSERVER_HPP
-#define ENTT_ENTITY_OBSERVER_HPP
-
-#include <cstddef>
-#include <cstdint>
-#include <limits>
-#include <type_traits>
-#include <utility>
-#include "../core/type_traits.hpp"
-#include "fwd.hpp"
-#include "storage.hpp"
-
-namespace entt {
-
-/*! @brief Grouping matcher. */
-template<typename...>
-struct matcher {};
-
-/**
- * @brief Collector.
- *
- * Primary template isn't defined on purpose. All the specializations give a
- * compile-time error, but for a few reasonable cases.
- */
-template<typename...>
-struct basic_collector;
-
-/**
- * @brief Collector.
- *
- * A collector contains a set of rules (literally, matchers) to use to track
- * entities.<br/>
- * Its main purpose is to generate a descriptor that allows an observer to know
- * how to connect to a registry.
- */
-template<>
-struct basic_collector<> {
-    /**
-     * @brief Adds a grouping matcher to the collector.
-     * @tparam AllOf Types of elements tracked by the matcher.
-     * @tparam NoneOf Types of elements used to filter out entities.
-     * @return The updated collector.
-     */
-    template<typename... AllOf, typename... NoneOf>
-    static constexpr auto group(exclude_t<NoneOf...> = exclude_t{}) noexcept {
-        return basic_collector<matcher<type_list<>, type_list<>, type_list<NoneOf...>, AllOf...>>{};
-    }
-
-    /**
-     * @brief Adds an observing matcher to the collector.
-     * @tparam AnyOf Type of element for which changes should be detected.
-     * @return The updated collector.
-     */
-    template<typename AnyOf>
-    static constexpr auto update() noexcept {
-        return basic_collector<matcher<type_list<>, type_list<>, AnyOf>>{};
-    }
-};
-
-/**
- * @brief Collector.
- * @copydetails basic_collector<>
- * @tparam Reject Untracked types used to filter out entities.
- * @tparam Require Untracked types required by the matcher.
- * @tparam Rule Specific details of the current matcher.
- * @tparam Other Other matchers.
- */
-template<typename... Reject, typename... Require, typename... Rule, typename... Other>
-struct [[deprecated("use reactive mixin instead")]] basic_collector<matcher<type_list<Reject...>, type_list<Require...>, Rule...>, Other...> {
-    /*! @brief Current matcher. */
-    using current_type = matcher<type_list<Reject...>, type_list<Require...>, Rule...>;
-
-    /**
-     * @brief Adds a grouping matcher to the collector.
-     * @tparam AllOf Types of elements tracked by the matcher.
-     * @tparam NoneOf Types of elements used to filter out entities.
-     * @return The updated collector.
-     */
-    template<typename... AllOf, typename... NoneOf>
-    static constexpr auto group(exclude_t<NoneOf...> = exclude_t{}) noexcept {
-        return basic_collector<matcher<type_list<>, type_list<>, type_list<NoneOf...>, AllOf...>, current_type, Other...>{};
-    }
-
-    /**
-     * @brief Adds an observing matcher to the collector.
-     * @tparam AnyOf Type of element for which changes should be detected.
-     * @return The updated collector.
-     */
-    template<typename AnyOf>
-    static constexpr auto update() noexcept {
-        return basic_collector<matcher<type_list<>, type_list<>, AnyOf>, current_type, Other...>{};
-    }
-
-    /**
-     * @brief Updates the filter of the last added matcher.
-     * @tparam AllOf Types of elements required by the matcher.
-     * @tparam NoneOf Types of elements used to filter out entities.
-     * @return The updated collector.
-     */
-    template<typename... AllOf, typename... NoneOf>
-    static constexpr auto where(exclude_t<NoneOf...> = exclude_t{}) noexcept {
-        using extended_type = matcher<type_list<Reject..., NoneOf...>, type_list<Require..., AllOf...>, Rule...>;
-        return basic_collector<extended_type, Other...>{};
-    }
-};
-
-/*! @brief Variable template used to ease the definition of collectors. */
-inline constexpr basic_collector<> collector{};
-
-/**
- * @brief Observer.
- *
- * An observer returns all the entities and only the entities that fit the
- * requirements of at least one matcher. Moreover, it's guaranteed that the
- * entity list is tightly packed in memory for fast iterations.<br/>
- * In general, observers don't stay true to the order of any set of elements.
- *
- * Observers work mainly with two types of matchers, provided through a
- * collector:
- *
- * * Observing matcher: an observer will return at least all the living entities
- *   for which one or more of the given elements have been updated and not yet
- *   destroyed.
- * * Grouping matcher: an observer will return at least all the living entities
- *   that would have entered the given group if it existed and that would have
- *   not yet left it.
- *
- * If an entity respects the requirements of multiple matchers, it will be
- * returned once and only once by the observer in any case.
- *
- * Matchers support also filtering by means of a _where_ clause that accepts
- * both a list of types and an exclusion list.<br/>
- * Whenever a matcher finds that an entity matches its requirements, the
- * condition of the filter is verified before to register the entity itself.
- * Moreover, a registered entity isn't returned by the observer if the condition
- * set by the filter is broken in the meantime.
- *
- * @b Important
- *
- * Iterators aren't invalidated if:
- *
- * * New instances of the given elements are created and assigned to entities.
- * * The entity currently pointed is modified (as an example, if one of the
- *   given elements is removed from the entity to which the iterator points).
- * * The entity currently pointed is destroyed.
- *
- * In all the other cases, modifying the pools of the given elements in any way
- * invalidates all the iterators.
- *
- * @warning
- * Lifetime of an observer doesn't necessarily have to overcome that of the
- * registry to which it is connected. However, the observer must be disconnected
- * from the registry before being destroyed to avoid crashes due to dangling
- * pointers.
- *
- * @tparam Registry Basic registry type.
- * @tparam Allocator Type of allocator used to manage memory and elements.
- */
-template<typename Registry, typename Allocator>
-class basic_observer {
-    using mask_type = std::uint64_t;
-    using storage_type = basic_storage<mask_type, typename Registry::entity_type, typename std::allocator_traits<Allocator>::template rebind_alloc<mask_type>>;
-
-    template<std::size_t Index>
-    static void discard_if(storage_type &storage, Registry &, const typename Registry::entity_type entt) {
-        if(storage.contains(entt) && !(storage.get(entt) &= (~(1 << Index)))) {
-            storage.erase(entt);
-        }
-    }
-
-    template<typename>
-    struct matcher_handler;
-
-    template<typename... Reject, typename... Require, typename AnyOf>
-    struct matcher_handler<matcher<type_list<Reject...>, type_list<Require...>, AnyOf>> {
-        template<std::size_t Index>
-        static void maybe_valid_if(storage_type &storage, Registry &parent, const typename Registry::entity_type entt) {
-            if(parent.template all_of<Require...>(entt) && !parent.template any_of<Reject...>(entt)) {
-                if(!storage.contains(entt)) {
-                    storage.emplace(entt);
-                }
-
-                storage.get(entt) |= (1 << Index);
-            }
-        }
-
-        template<std::size_t Index>
-        static void connect(storage_type &storage, Registry &parent) {
-            (parent.template on_destroy<Require>().template connect<&discard_if<Index>>(storage), ...);
-            (parent.template on_construct<Reject>().template connect<&discard_if<Index>>(storage), ...);
-            parent.template on_update<AnyOf>().template connect<&maybe_valid_if<Index>>(storage);
-            parent.template on_destroy<AnyOf>().template connect<&discard_if<Index>>(storage);
-        }
-
-        static void disconnect(storage_type &storage, Registry &parent) {
-            (parent.template on_destroy<Require>().disconnect(&storage), ...);
-            (parent.template on_construct<Reject>().disconnect(&storage), ...);
-            parent.template on_update<AnyOf>().disconnect(&storage);
-            parent.template on_destroy<AnyOf>().disconnect(&storage);
-        }
-    };
-
-    template<typename... Reject, typename... Require, typename... NoneOf, typename... AllOf>
-    struct matcher_handler<matcher<type_list<Reject...>, type_list<Require...>, type_list<NoneOf...>, AllOf...>> {
-        template<std::size_t Index, typename... Ignore>
-        static void maybe_valid_if(storage_type &storage, Registry &parent, const typename Registry::entity_type entt) {
-            bool guard{};
-
-            if constexpr(sizeof...(Ignore) == 0) {
-                guard = parent.template all_of<AllOf..., Require...>(entt) && !parent.template any_of<NoneOf..., Reject...>(entt);
-            } else {
-                guard = parent.template all_of<AllOf..., Require...>(entt) && ((std::is_same_v<Ignore..., NoneOf> || !parent.template any_of<NoneOf>(entt)) && ...) && !parent.template any_of<Reject...>(entt);
-            }
-
-            if(guard) {
-                if(!storage.contains(entt)) {
-                    storage.emplace(entt);
-                }
-
-                storage.get(entt) |= (1 << Index);
-            }
-        }
-
-        template<std::size_t Index>
-        static void connect(storage_type &storage, Registry &parent) {
-            (parent.template on_destroy<Require>().template connect<&discard_if<Index>>(storage), ...);
-            (parent.template on_construct<Reject>().template connect<&discard_if<Index>>(storage), ...);
-            (parent.template on_construct<AllOf>().template connect<&maybe_valid_if<Index>>(storage), ...);
-            (parent.template on_destroy<NoneOf>().template connect<&maybe_valid_if<Index, NoneOf>>(storage), ...);
-            (parent.template on_destroy<AllOf>().template connect<&discard_if<Index>>(storage), ...);
-            (parent.template on_construct<NoneOf>().template connect<&discard_if<Index>>(storage), ...);
-        }
-
-        static void disconnect(storage_type &storage, Registry &parent) {
-            (parent.template on_destroy<Require>().disconnect(&storage), ...);
-            (parent.template on_construct<Reject>().disconnect(&storage), ...);
-            (parent.template on_construct<AllOf>().disconnect(&storage), ...);
-            (parent.template on_destroy<NoneOf>().disconnect(&storage), ...);
-            (parent.template on_destroy<AllOf>().disconnect(&storage), ...);
-            (parent.template on_construct<NoneOf>().disconnect(&storage), ...);
-        }
-    };
-
-    template<typename... Matcher>
-    static void disconnect(Registry &parent, storage_type &storage) {
-        (matcher_handler<Matcher>::disconnect(storage, parent), ...);
-    }
-
-    template<typename... Matcher, std::size_t... Index>
-    static void connect(Registry &parent, storage_type &storage, std::index_sequence<Index...>) {
-        static_assert(sizeof...(Matcher) < std::numeric_limits<mask_type>::digits, "Too many matchers");
-        (matcher_handler<Matcher>::template connect<Index>(storage, parent), ...);
-    }
-
-public:
-    /*! @brief Allocator type. */
-    using allocator_type = Allocator;
-    /*! Basic registry type. */
-    using registry_type = Registry;
-    /*! @brief Underlying entity identifier. */
-    using entity_type = typename registry_type::entity_type;
-    /*! @brief Unsigned integer type. */
-    using size_type = std::size_t;
-    /*! @brief Random access iterator type. */
-    using iterator = typename registry_type::common_type::iterator;
-
-    /*! @brief Default constructor. */
-    basic_observer()
-        : basic_observer{allocator_type{}} {}
-
-    /**
-     * @brief Constructs an empty storage with a given allocator.
-     * @param allocator The allocator to use.
-     */
-    explicit basic_observer(const allocator_type &allocator)
-        : release{},
-          parent{},
-          storage{allocator} {}
-
-    /*! @brief Default copy constructor, deleted on purpose. */
-    basic_observer(const basic_observer &) = delete;
-
-    /*! @brief Default move constructor, deleted on purpose. */
-    basic_observer(basic_observer &&) = delete;
-
-    /**
-     * @brief Creates an observer and connects it to a given registry.
-     * @tparam Matcher Types of matchers to use to initialize the observer.
-     * @param reg A valid reference to a registry.
-     * @param allocator The allocator to use.
-     */
-    template<typename... Matcher>
-    basic_observer(registry_type &reg, basic_collector<Matcher...>, const allocator_type &allocator = allocator_type{})
-        : release{&basic_observer::disconnect<Matcher...>},
-          parent{&reg},
-          storage{allocator} {
-        connect<Matcher...>(reg, storage, std::index_sequence_for<Matcher...>{});
-    }
-
-    /*! @brief Default destructor. */
-    ~basic_observer() = default;
-
-    /**
-     * @brief Default copy assignment operator, deleted on purpose.
-     * @return This observer.
-     */
-    basic_observer &operator=(const basic_observer &) = delete;
-
-    /**
-     * @brief Default move assignment operator, deleted on purpose.
-     * @return This observer.
-     */
-    basic_observer &operator=(basic_observer &&) = delete;
-
-    /**
-     * @brief Connects an observer to a given registry.
-     * @tparam Matcher Types of matchers to use to initialize the observer.
-     * @param reg A valid reference to a registry.
-     */
-    template<typename... Matcher>
-    void connect(registry_type &reg, basic_collector<Matcher...>) {
-        disconnect();
-        storage.clear();
-
-        parent = &reg;
-        release = &basic_observer::disconnect<Matcher...>;
-        connect<Matcher...>(reg, storage, std::index_sequence_for<Matcher...>{});
-    }
-
-    /*! @brief Disconnects an observer from the registry it keeps track of. */
-    void disconnect() {
-        if(release) {
-            release(*parent, storage);
-            release = nullptr;
-        }
-    }
-
-    /**
-     * @brief Returns the number of elements in an observer.
-     * @return Number of elements.
-     */
-    [[nodiscard]] size_type size() const noexcept {
-        return storage.size();
-    }
-
-    /**
-     * @brief Checks whether an observer is empty.
-     * @return True if the observer is empty, false otherwise.
-     */
-    [[nodiscard]] bool empty() const noexcept {
-        return storage.empty();
-    }
-
-    /**
-     * @brief Direct access to the list of entities of the observer.
-     *
-     * The returned pointer is such that range `[data(), data() + size())` is
-     * always a valid range, even if the container is empty.
-     *
-     * @note
-     * Entities are in the reverse order as returned by the `begin`/`end`
-     * iterators.
-     *
-     * @return A pointer to the array of entities.
-     */
-    [[nodiscard]] const entity_type *data() const noexcept {
-        return storage.data();
-    }
-
-    /**
-     * @brief Returns an iterator to the first entity of the observer.
-     *
-     * If the observer is empty, the returned iterator will be equal to `end()`.
-     *
-     * @return An iterator to the first entity of the observer.
-     */
-    [[nodiscard]] iterator begin() const noexcept {
-        return storage.storage_type::base_type::begin();
-    }
-
-    /**
-     * @brief Returns an iterator that is past the last entity of the observer.
-     * @return An iterator to the entity following the last entity of the
-     * observer.
-     */
-    [[nodiscard]] iterator end() const noexcept {
-        return storage.storage_type::base_type::end();
-    }
-
-    /*! @brief Clears the underlying container. */
-    void clear() noexcept {
-        storage.clear();
-    }
-
-    /**
-     * @brief Iterates entities and applies the given function object to them.
-     *
-     * The function object is invoked for each entity.<br/>
-     * The signature of the function must be equivalent to the following form:
-     *
-     * @code{.cpp}
-     * void(const entity_type);
-     * @endcode
-     *
-     * @tparam Func Type of the function object to invoke.
-     * @param func A valid function object.
-     */
-    template<typename Func>
-    void each(Func func) const {
-        for(const auto entity: *this) {
-            func(entity);
-        }
-    }
-
-    /**
-     * @brief Iterates entities and applies the given function object to them,
-     * then clears the observer.
-     *
-     * @sa each
-     *
-     * @tparam Func Type of the function object to invoke.
-     * @param func A valid function object.
-     */
-    template<typename Func>
-    void each(Func func) {
-        std::as_const(*this).each(std::move(func));
-        clear();
-    }
-
-private:
-    void (*release)(registry_type &, storage_type &);
-    registry_type *parent;
-    storage_type storage;
-};
-
-} // namespace entt
-
-#endif

src/entt/entity/organizer.hpp

@@ -15,7 +15,7 @@
 
 namespace entt {
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
 template<typename>
@@ -27,6 +27,15 @@ struct is_view<basic_view<Args...>>: std::true_type {};
 template<typename Type>
 inline constexpr bool is_view_v = is_view<Type>::value;
 
+template<typename>
+struct is_group: std::false_type {};
+
+template<typename... Args>
+struct is_group<basic_group<Args...>>: std::true_type {};
+
+template<typename Type>
+inline constexpr bool is_group_v = is_group<Type>::value;
+
 template<typename Type, typename Override>
 struct unpack_type {
     using ro = std::conditional_t<
@@ -60,27 +69,38 @@ template<typename... Get, typename... Exclude, typename... Override>
 struct unpack_type<const basic_view<get_t<Get...>, exclude_t<Exclude...>>, type_list<Override...>>
     : unpack_type<basic_view<get_t<Get...>, exclude_t<Exclude...>>, type_list<Override...>> {};
 
-template<typename, typename>
+template<typename... Owned, typename... Get, typename... Exclude, typename... Override>
+struct unpack_type<basic_group<owned_t<Owned...>, get_t<Get...>, exclude_t<Exclude...>>, type_list<Override...>> {
+    using ro = type_list_cat_t<type_list<typename Exclude::element_type...>, typename unpack_type<constness_as_t<typename Get::element_type, Get>, type_list<Override...>>::ro..., typename unpack_type<constness_as_t<typename Owned::element_type, Owned>, type_list<Override...>>::ro...>;
+    using rw = type_list_cat_t<typename unpack_type<constness_as_t<typename Get::element_type, Get>, type_list<Override...>>::rw..., typename unpack_type<constness_as_t<typename Owned::element_type, Owned>, type_list<Override...>>::rw...>;
+};
+
+template<typename... Owned, typename... Get, typename... Exclude, typename... Override>
+struct unpack_type<const basic_group<owned_t<Owned...>, get_t<Get...>, exclude_t<Exclude...>>, type_list<Override...>>
+    : unpack_type<basic_group<owned_t<Owned...>, get_t<Get...>, exclude_t<Exclude...>>, type_list<Override...>> {};
+
+template<typename, typename, typename>
 struct resource_traits;
 
-template<typename... Args, typename... Req>
-struct resource_traits<type_list<Args...>, type_list<Req...>> {
+template<typename Registry, typename... Args, typename... Req>
+struct resource_traits<Registry, type_list<Args...>, type_list<Req...>> {
     using args = type_list<std::remove_const_t<Args>...>;
     using ro = type_list_cat_t<typename unpack_type<Args, type_list<Req...>>::ro..., typename unpack_type<Req, type_list<>>::ro...>;
     using rw = type_list_cat_t<typename unpack_type<Args, type_list<Req...>>::rw..., typename unpack_type<Req, type_list<>>::rw...>;
+    static constexpr auto sync_point = (std::is_same_v<Args, Registry> || ...);
 };
 
-template<typename... Req, typename Ret, typename... Args>
-resource_traits<type_list<std::remove_reference_t<Args>...>, type_list<Req...>> free_function_to_resource_traits(Ret (*)(Args...));
+template<typename Registry, typename... Req, typename Ret, typename... Args>
+resource_traits<Registry, type_list<std::remove_reference_t<Args>...>, type_list<Req...>> free_function_to_resource_traits(Ret (*)(Args...));
 
-template<typename... Req, typename Ret, typename Type, typename... Args>
-resource_traits<type_list<std::remove_reference_t<Args>...>, type_list<Req...>> constrained_function_to_resource_traits(Ret (*)(Type &, Args...));
+template<typename Registry, typename... Req, typename Ret, typename Type, typename... Args>
+resource_traits<Registry, type_list<std::remove_reference_t<Args>...>, type_list<Req...>> constrained_function_to_resource_traits(Ret (*)(Type &, Args...));
 
-template<typename... Req, typename Ret, typename Class, typename... Args>
-resource_traits<type_list<std::remove_reference_t<Args>...>, type_list<Req...>> constrained_function_to_resource_traits(Ret (Class::*)(Args...));
+template<typename Registry, typename... Req, typename Ret, typename Class, typename... Args>
+resource_traits<Registry, type_list<std::remove_reference_t<Args>...>, type_list<Req...>> constrained_function_to_resource_traits(Ret (Class::*)(Args...));
 
-template<typename... Req, typename Ret, typename Class, typename... Args>
-resource_traits<type_list<std::remove_reference_t<Args>...>, type_list<Req...>> constrained_function_to_resource_traits(Ret (Class::*)(Args...) const);
+template<typename Registry, typename... Req, typename Ret, typename Class, typename... Args>
+resource_traits<Registry, type_list<std::remove_reference_t<Args>...>, type_list<Req...>> constrained_function_to_resource_traits(Ret (Class::*)(Args...) const);
 
 } // namespace internal
 /*! @endcond */
@@ -119,6 +139,8 @@ class basic_organizer final {
             return reg;
         } else if constexpr(internal::is_view_v<Type>) {
             return static_cast<Type>(as_view{reg});
+        } else if constexpr(internal::is_group_v<Type>) {
+            return static_cast<Type>(as_group{reg});
         } else {
             return reg.ctx().template emplace<std::remove_reference_t<Type>>();
         }
@@ -148,9 +170,9 @@ class basic_organizer final {
     }
 
     template<typename... RO, typename... RW>
-    void track_dependencies(std::size_t index, const bool requires_registry, type_list<RO...>, type_list<RW...>) {
+    void track_dependencies(std::size_t index, const bool sync_point, type_list<RO...>, type_list<RW...>) {
         builder.bind(static_cast<id_type>(index));
-        builder.set(type_hash<Registry>::value(), requires_registry || (sizeof...(RO) + sizeof...(RW) == 0u));
+        builder.set(type_hash<Registry>::value(), sync_point || (sizeof...(RO) + sizeof...(RW) == 0u));
         (builder.ro(type_hash<RO>::value()), ...);
         (builder.rw(type_hash<RW>::value()), ...);
     }
@@ -159,7 +181,7 @@ public:
     /*! Basic registry type. */
     using registry_type = Registry;
     /*! @brief Underlying entity identifier. */
-    using entity_type = typename registry_type::entity_type;
+    using entity_type = registry_type::entity_type;
     /*! @brief Unsigned integer type. */
     using size_type = std::size_t;
     /*! @brief Raw task function type. */
@@ -272,14 +294,6 @@ public:
             return out;
         }
 
-        /**
-         * @brief Returns the list of nodes reachable from a given vertex.
-         * @return The list of nodes reachable from the vertex.
-         */
-        [[deprecated("use ::out_edges")]] [[nodiscard]] const std::vector<std::size_t> &children() const noexcept {
-            return out_edges();
-        }
-
         /**
          * @brief Prepares a registry and assures that all required resources
          * are properly instantiated before using them.
@@ -303,8 +317,7 @@ public:
      */
     template<auto Candidate, typename... Req>
     void emplace(const char *name = nullptr) {
-        using resource_type = decltype(internal::free_function_to_resource_traits<Req...>(Candidate));
-        constexpr auto requires_registry = type_list_contains_v<typename resource_type::args, registry_type>;
+        using resource_type = decltype(internal::free_function_to_resource_traits<registry_type, Req...>(Candidate));
 
         callback_type *callback = +[](const void *, registry_type &reg) {
             std::apply(Candidate, to_args(reg, typename resource_type::args{}));
@@ -320,7 +333,7 @@ public:
             +[](registry_type &reg) { void(to_args(reg, typename resource_type::args{})); },
             &type_id<std::integral_constant<decltype(Candidate), Candidate>>()};
 
-        track_dependencies(vertices.size(), requires_registry, typename resource_type::ro{}, typename resource_type::rw{});
+        track_dependencies(vertices.size(), resource_type::sync_point, typename resource_type::ro{}, typename resource_type::rw{});
         vertices.push_back(std::move(vdata));
     }
 
@@ -335,8 +348,7 @@ public:
      */
     template<auto Candidate, typename... Req, typename Type>
     void emplace(Type &value_or_instance, const char *name = nullptr) {
-        using resource_type = decltype(internal::constrained_function_to_resource_traits<Req...>(Candidate));
-        constexpr auto requires_registry = type_list_contains_v<typename resource_type::args, registry_type>;
+        using resource_type = decltype(internal::constrained_function_to_resource_traits<registry_type, Req...>(Candidate));
 
         callback_type *callback = +[](const void *payload, registry_type &reg) {
             Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
@@ -353,7 +365,7 @@ public:
             +[](registry_type &reg) { void(to_args(reg, typename resource_type::args{})); },
             &type_id<std::integral_constant<decltype(Candidate), Candidate>>()};
 
-        track_dependencies(vertices.size(), requires_registry, typename resource_type::ro{}, typename resource_type::rw{});
+        track_dependencies(vertices.size(), resource_type::sync_point, typename resource_type::ro{}, typename resource_type::rw{});
         vertices.push_back(std::move(vdata));
     }
 
@@ -367,7 +379,7 @@ public:
      */
     template<typename... Req>
     void emplace(function_type *func, const void *payload = nullptr, const char *name = nullptr) {
-        using resource_type = internal::resource_traits<type_list<>, type_list<Req...>>;
+        using resource_type = internal::resource_traits<registry_type, type_list<>, type_list<Req...>>;
         track_dependencies(vertices.size(), true, typename resource_type::ro{}, typename resource_type::rw{});
 
         vertex_data vdata{
@@ -387,12 +399,11 @@ public:
      * @brief Generates a task graph for the current content.
      * @return The adjacency list of the task graph.
      */
-    [[nodiscard]] std::vector<vertex> graph() {
+    [[nodiscard]] std::vector<vertex> graph() const {
         std::vector<vertex> adjacency_list{};
         adjacency_list.reserve(vertices.size());
-        auto adjacency_matrix = builder.graph();
 
-        for(auto curr: adjacency_matrix.vertices()) {
+        for(auto adjacency_matrix = builder.graph(); auto curr: adjacency_matrix.vertices()) {
             std::vector<std::size_t> in{};
             std::vector<std::size_t> out{};
 

src/entt/entity/registry.hpp

@@ -3,6 +3,8 @@
 
 #include <algorithm>
 #include <array>
+#include <compare>
+#include <concepts>
 #include <cstddef>
 #include <functional>
 #include <iterator>
@@ -15,12 +17,14 @@
 #include "../container/dense_map.hpp"
 #include "../core/algorithm.hpp"
 #include "../core/any.hpp"
+#include "../core/concepts.hpp"
 #include "../core/fwd.hpp"
 #include "../core/iterator.hpp"
 #include "../core/memory.hpp"
 #include "../core/type_info.hpp"
 #include "../core/type_traits.hpp"
-#include "../core/utility.hpp"
+#include "../stl/functional.hpp"
+#include "../stl/iterator.hpp"
 #include "entity.hpp"
 #include "fwd.hpp"
 #include "group.hpp"
@@ -31,12 +35,12 @@
 
 namespace entt {
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
 template<typename It>
 class registry_storage_iterator final {
-    template<typename Other>
+    template<typename>
     friend class registry_storage_iterator;
 
     using mapped_type = std::remove_reference_t<decltype(std::declval<It>()->second)>;
@@ -55,7 +59,8 @@ public:
     constexpr registry_storage_iterator(It iter) noexcept
         : it{iter} {}
 
-    template<typename Other, typename = std::enable_if_t<!std::is_same_v<It, Other> && std::is_constructible_v<It, Other>>>
+    template<typename Other>
+    requires (!std::same_as<It, Other> && std::constructible_from<It, Other>)
     constexpr registry_storage_iterator(const registry_storage_iterator<Other> &other) noexcept
         : registry_storage_iterator{other.it} {}
 
@@ -64,7 +69,7 @@ public:
     }
 
     constexpr registry_storage_iterator operator++(int) noexcept {
-        registry_storage_iterator orig = *this;
+        const registry_storage_iterator orig = *this;
         return ++(*this), orig;
     }
 
@@ -73,7 +78,7 @@ public:
     }
 
     constexpr registry_storage_iterator operator--(int) noexcept {
-        registry_storage_iterator orig = *this;
+        const registry_storage_iterator orig = *this;
         return operator--(), orig;
     }
 
@@ -107,63 +112,38 @@ public:
         return operator*();
     }
 
-    template<typename Lhs, typename Rhs>
-    friend constexpr std::ptrdiff_t operator-(const registry_storage_iterator<Lhs> &, const registry_storage_iterator<Rhs> &) noexcept;
+    template<typename Other>
+    [[nodiscard]] constexpr std::ptrdiff_t operator-(const registry_storage_iterator<Other> &other) const noexcept {
+        return it - other.it;
+    }
 
-    template<typename Lhs, typename Rhs>
-    friend constexpr bool operator==(const registry_storage_iterator<Lhs> &, const registry_storage_iterator<Rhs> &) noexcept;
+    template<typename Other>
+    [[nodiscard]] constexpr bool operator==(const registry_storage_iterator<Other> &other) const noexcept {
+        return it == other.it;
+    }
 
-    template<typename Lhs, typename Rhs>
-    friend constexpr bool operator<(const registry_storage_iterator<Lhs> &, const registry_storage_iterator<Rhs> &) noexcept;
+    template<typename Other>
+    [[nodiscard]] constexpr auto operator<=>(const registry_storage_iterator<Other> &other) const noexcept {
+        return it <=> other.it;
+    }
 
 private:
     It it;
 };
 
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr std::ptrdiff_t operator-(const registry_storage_iterator<Lhs> &lhs, const registry_storage_iterator<Rhs> &rhs) noexcept {
-    return lhs.it - rhs.it;
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator==(const registry_storage_iterator<Lhs> &lhs, const registry_storage_iterator<Rhs> &rhs) noexcept {
-    return lhs.it == rhs.it;
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator!=(const registry_storage_iterator<Lhs> &lhs, const registry_storage_iterator<Rhs> &rhs) noexcept {
-    return !(lhs == rhs);
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator<(const registry_storage_iterator<Lhs> &lhs, const registry_storage_iterator<Rhs> &rhs) noexcept {
-    return lhs.it < rhs.it;
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator>(const registry_storage_iterator<Lhs> &lhs, const registry_storage_iterator<Rhs> &rhs) noexcept {
-    return rhs < lhs;
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator<=(const registry_storage_iterator<Lhs> &lhs, const registry_storage_iterator<Rhs> &rhs) noexcept {
-    return !(lhs > rhs);
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator>=(const registry_storage_iterator<Lhs> &lhs, const registry_storage_iterator<Rhs> &rhs) noexcept {
-    return !(lhs < rhs);
-}
-
 template<typename Allocator>
 class registry_context {
     using alloc_traits = std::allocator_traits<Allocator>;
-    using allocator_type = typename alloc_traits::template rebind_alloc<std::pair<const id_type, basic_any<0u>>>;
+    using allocator_type = alloc_traits::template rebind_alloc<std::pair<const id_type, basic_any<0u>>>;
 
 public:
     explicit registry_context(const allocator_type &allocator)
         : ctx{allocator} {}
 
+    void clear() noexcept {
+        ctx.clear();
+    }
+
     template<typename Type, typename... Args>
     Type &emplace_as(const id_type id, Args &&...args) {
         return any_cast<Type &>(ctx.try_emplace(id, std::in_place_type<Type>, std::forward<Args>(args)...).first->second);
@@ -176,7 +156,7 @@ public:
 
     template<typename Type>
     Type &insert_or_assign(const id_type id, Type &&value) {
-        return any_cast<std::remove_cv_t<std::remove_reference_t<Type>> &>(ctx.insert_or_assign(id, std::forward<Type>(value)).first->second);
+        return any_cast<std::remove_cvref_t<Type> &>(ctx.insert_or_assign(id, std::forward<Type>(value)).first->second);
     }
 
     template<typename Type>
@@ -187,7 +167,7 @@ public:
     template<typename Type>
     bool erase(const id_type id = type_id<Type>().hash()) {
         const auto it = ctx.find(id);
-        return it != ctx.end() && it->second.type() == type_id<Type>() ? (ctx.erase(it), true) : false;
+        return it != ctx.end() && it->second.info() == type_id<Type>() ? (ctx.erase(it), true) : false;
     }
 
     template<typename Type>
@@ -215,11 +195,11 @@ public:
     template<typename Type>
     [[nodiscard]] bool contains(const id_type id = type_id<Type>().hash()) const {
         const auto it = ctx.find(id);
-        return it != ctx.cend() && it->second.type() == type_id<Type>();
+        return it != ctx.cend() && it->second.info() == type_id<Type>();
     }
 
 private:
-    dense_map<id_type, basic_any<0u>, identity, std::equal_to<>, allocator_type> ctx;
+    dense_map<id_type, basic_any<0u>, stl::identity, std::equal_to<>, allocator_type> ctx;
 };
 
 } // namespace internal
@@ -236,14 +216,12 @@ class basic_registry {
     using alloc_traits = std::allocator_traits<Allocator>;
     static_assert(std::is_same_v<typename alloc_traits::value_type, Entity>, "Invalid value type");
     // std::shared_ptr because of its type erased allocator which is useful here
-    using pool_container_type = dense_map<id_type, std::shared_ptr<base_type>, identity, std::equal_to<>, typename alloc_traits::template rebind_alloc<std::pair<const id_type, std::shared_ptr<base_type>>>>;
-    using group_container_type = dense_map<id_type, std::shared_ptr<internal::group_descriptor>, identity, std::equal_to<>, typename alloc_traits::template rebind_alloc<std::pair<const id_type, std::shared_ptr<internal::group_descriptor>>>>;
+    using pool_container_type = dense_map<id_type, std::shared_ptr<base_type>, stl::identity, std::equal_to<>, typename alloc_traits::template rebind_alloc<std::pair<const id_type, std::shared_ptr<base_type>>>>;
+    using group_container_type = dense_map<id_type, std::shared_ptr<internal::group_descriptor>, stl::identity, std::equal_to<>, typename alloc_traits::template rebind_alloc<std::pair<const id_type, std::shared_ptr<internal::group_descriptor>>>>;
     using traits_type = entt_traits<Entity>;
 
-    template<typename Type>
+    template<cvref_unqualified Type>
     [[nodiscard]] auto &assure([[maybe_unused]] const id_type id = type_hash<Type>::value()) {
-        static_assert(std::is_same_v<Type, std::decay_t<Type>>, "Non-decayed types not allowed");
-
         if constexpr(std::is_same_v<Type, entity_type>) {
             ENTT_ASSERT(id == type_hash<Type>::value(), "User entity storage not allowed");
             return entities;
@@ -251,20 +229,11 @@ class basic_registry {
             using storage_type = storage_for_type<Type>;
 
             if(auto it = pools.find(id); it != pools.cend()) {
-                ENTT_ASSERT(it->second->type() == type_id<Type>(), "Unexpected type");
+                ENTT_ASSERT(it->second->info() == type_id<Type>(), "Unexpected type");
                 return static_cast<storage_type &>(*it->second);
             }
 
-            using alloc_type = typename storage_type::allocator_type;
-            typename pool_container_type::mapped_type cpool{};
-
-            if constexpr(std::is_void_v<Type> && !std::is_constructible_v<alloc_type, allocator_type>) {
-                // std::allocator<void> has no cross constructors (waiting for C++20)
-                cpool = std::allocate_shared<storage_type>(get_allocator(), alloc_type{});
-            } else {
-                cpool = std::allocate_shared<storage_type>(get_allocator(), get_allocator());
-            }
-
+            typename pool_container_type::mapped_type cpool = std::allocate_shared<storage_type>(get_allocator(), get_allocator());
             pools.emplace(id, cpool);
             cpool->bind(*this);
 
@@ -272,16 +241,14 @@ class basic_registry {
         }
     }
 
-    template<typename Type>
+    template<cvref_unqualified Type>
     [[nodiscard]] const auto *assure([[maybe_unused]] const id_type id = type_hash<Type>::value()) const {
-        static_assert(std::is_same_v<Type, std::decay_t<Type>>, "Non-decayed types not allowed");
-
         if constexpr(std::is_same_v<Type, entity_type>) {
             ENTT_ASSERT(id == type_hash<Type>::value(), "User entity storage not allowed");
             return &entities;
         } else {
             if(const auto it = pools.find(id); it != pools.cend()) {
-                ENTT_ASSERT(it->second->type() == type_id<Type>(), "Unexpected type");
+                ENTT_ASSERT(it->second->info() == type_id<Type>(), "Unexpected type");
                 return static_cast<const storage_for_type<Type> *>(it->second.get());
             }
 
@@ -301,9 +268,9 @@ public:
     /*! @brief Allocator type. */
     using allocator_type = Allocator;
     /*! @brief Underlying entity identifier. */
-    using entity_type = typename traits_type::value_type;
+    using entity_type = traits_type::value_type;
     /*! @brief Underlying version type. */
-    using version_type = typename traits_type::version_type;
+    using version_type = traits_type::version_type;
     /*! @brief Unsigned integer type. */
     using size_type = std::size_t;
     /*! @brief Common type among all storage types. */
@@ -320,7 +287,7 @@ public:
      * @tparam Type Storage value type, eventually const.
      */
     template<typename Type>
-    using storage_for_type = typename storage_for<Type, Entity, typename alloc_traits::template rebind_alloc<std::remove_const_t<Type>>>::type;
+    using storage_for_type = storage_for<Type, Entity, typename alloc_traits::template rebind_alloc<std::remove_const_t<Type>>>::type;
 
     /*! @brief Default constructor. */
     basic_registry()
@@ -449,7 +416,7 @@ public:
      */
     template<typename Type>
     storage_for_type<Type> &storage(const id_type id = type_hash<Type>::value()) {
-        return assure<Type>(id);
+        return assure<std::remove_const_t<Type>>(id);
     }
 
     /**
@@ -460,7 +427,7 @@ public:
      */
     template<typename Type>
     [[nodiscard]] const storage_for_type<Type> *storage(const id_type id = type_hash<Type>::value()) const {
-        return assure<Type>(id);
+        return assure<std::remove_const_t<Type>>(id);
     }
 
     /**
@@ -497,7 +464,7 @@ public:
      * @return A valid identifier.
      */
     [[nodiscard]] entity_type create() {
-        return entities.emplace();
+        return entities.generate();
     }
 
     /**
@@ -510,7 +477,7 @@ public:
      * @return A valid identifier.
      */
     [[nodiscard]] entity_type create(const entity_type hint) {
-        return entities.emplace(hint);
+        return entities.generate(hint);
     }
 
     /**
@@ -518,13 +485,13 @@ public:
      *
      * @sa create
      *
-     * @tparam It Type of forward iterator.
+     * @tparam It Type of output iterator.
      * @param first An iterator to the first element of the range to generate.
      * @param last An iterator past the last element of the range to generate.
      */
-    template<typename It>
+    template<stl::output_iterator<entity_type> It>
     void create(It first, It last) {
-        entities.insert(std::move(first), std::move(last));
+        entities.generate(std::move(first), std::move(last));
     }
 
     /**
@@ -539,7 +506,7 @@ public:
      */
     version_type destroy(const entity_type entt) {
         for(size_type pos = pools.size(); pos != 0u; --pos) {
-            pools.begin()[pos - 1u].second->remove(entt);
+            pools.begin()[static_cast<pool_container_type::difference_type>(pos - 1u)].second->remove(entt);
         }
 
         entities.erase(entt);
@@ -569,14 +536,12 @@ public:
      *
      * @sa destroy
      *
-     * @tparam It Type of input iterator.
      * @param first An iterator to the first element of the range of entities.
      * @param last An iterator past the last element of the range of entities.
      */
-    template<typename It>
-    void destroy(It first, It last) {
+    void destroy(stl::input_iterator auto first, stl::input_iterator auto last) {
         const auto to = entities.sort_as(first, last);
-        const auto from = entities.cend() - entities.free_list();
+        const auto from = entities.cend() - static_cast<common_type::difference_type>(entities.free_list());
 
         for(auto &&curr: pools) {
             curr.second->remove(from, to);
@@ -612,13 +577,12 @@ public:
      * @sa emplace
      *
      * @tparam Type Type of element to create.
-     * @tparam It Type of input iterator.
      * @param first An iterator to the first element of the range of entities.
      * @param last An iterator past the last element of the range of entities.
      * @param value An instance of the element to assign.
      */
-    template<typename Type, typename It>
-    void insert(It first, It last, const Type &value = {}) {
+    template<typename Type>
+    void insert(stl::input_iterator auto first, stl::input_iterator auto last, const Type &value = {}) {
         ENTT_ASSERT(std::all_of(first, last, [this](const auto entt) { return valid(entt); }), "Invalid entity");
         assure<Type>().insert(std::move(first), std::move(last), value);
     }
@@ -635,7 +599,8 @@ public:
      * @param last An iterator past the last element of the range of entities.
      * @param from An iterator to the first element of the range of elements.
      */
-    template<typename Type, typename EIt, typename CIt, typename = std::enable_if_t<std::is_same_v<typename std::iterator_traits<CIt>::value_type, Type>>>
+    template<typename Type, typename EIt, typename CIt>
+    requires std::same_as<typename std::iterator_traits<CIt>::value_type, Type>
     void insert(EIt first, EIt last, CIt from) {
         ENTT_ASSERT(std::all_of(first, last, [this](const auto entt) { return valid(entt); }), "Invalid entity");
         assure<Type>().insert(first, last, from);
@@ -728,7 +693,7 @@ public:
      * @param last An iterator past the last element of the range of entities.
      * @return The number of elements actually removed.
      */
-    template<typename Type, typename... Other, typename It>
+    template<typename Type, typename... Other, stl::input_iterator It>
     size_type remove(It first, It last) {
         size_type count{};
 
@@ -781,7 +746,7 @@ public:
      * @param first An iterator to the first element of the range of entities.
      * @param last An iterator past the last element of the range of entities.
      */
-    template<typename Type, typename... Other, typename It>
+    template<typename Type, typename... Other, stl::input_iterator It>
     void erase(It first, It last) {
         if constexpr(std::is_same_v<It, typename common_type::iterator>) {
             std::array cpools{static_cast<common_type *>(&assure<Type>()), static_cast<common_type *>(&assure<Other>())...};
@@ -960,7 +925,7 @@ public:
     void clear() {
         if constexpr(sizeof...(Type) == 0u) {
             for(size_type pos = pools.size(); pos; --pos) {
-                pools.begin()[pos - 1u].second->clear();
+                pools.begin()[static_cast<pool_container_type::difference_type>(pos - 1u)].second->clear();
             }
 
             const auto elem = entities.each();
@@ -1084,7 +1049,7 @@ public:
     basic_group<owned_t<storage_for_type<Owned>...>, get_t<storage_for_type<Get>...>, exclude_t<storage_for_type<Exclude>...>>
     group(get_t<Get...> = get_t{}, exclude_t<Exclude...> = exclude_t{}) {
         using group_type = basic_group<owned_t<storage_for_type<Owned>...>, get_t<storage_for_type<Get>...>, exclude_t<storage_for_type<Exclude>...>>;
-        using handler_type = typename group_type::handler;
+        using handler_type = group_type::handler;
 
         if(auto it = groups.find(group_type::group_id()); it != groups.cend()) {
             return {*std::static_pointer_cast<handler_type>(it->second)};
@@ -1108,7 +1073,7 @@ public:
     [[nodiscard]] basic_group<owned_t<storage_for_type<const Owned>...>, get_t<storage_for_type<const Get>...>, exclude_t<storage_for_type<const Exclude>...>>
     group_if_exists(get_t<Get...> = get_t{}, exclude_t<Exclude...> = exclude_t{}) const {
         using group_type = basic_group<owned_t<storage_for_type<const Owned>...>, get_t<storage_for_type<const Get>...>, exclude_t<storage_for_type<const Exclude>...>>;
-        using handler_type = typename group_type::handler;
+        using handler_type = group_type::handler;
 
         if(auto it = groups.find(group_type::group_id()); it != groups.cend()) {
             return {*std::static_pointer_cast<handler_type>(it->second)};

src/entt/entity/runtime_view.hpp

@@ -11,12 +11,12 @@
 
 namespace entt {
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
 template<typename Set>
 class runtime_view_iterator final {
-    using iterator_type = typename Set::iterator;
+    using iterator_type = Set::iterator;
     using iterator_traits = std::iterator_traits<iterator_type>;
 
     [[nodiscard]] bool valid() const {
@@ -26,10 +26,10 @@ class runtime_view_iterator final {
     }
 
 public:
-    using value_type = typename iterator_traits::value_type;
-    using pointer = typename iterator_traits::pointer;
-    using reference = typename iterator_traits::reference;
-    using difference_type = typename iterator_traits::difference_type;
+    using value_type = iterator_traits::value_type;
+    using pointer = iterator_traits::pointer;
+    using reference = iterator_traits::reference;
+    using difference_type = iterator_traits::difference_type;
     using iterator_category = std::bidirectional_iterator_tag;
 
     constexpr runtime_view_iterator() noexcept
@@ -38,8 +38,7 @@ public:
           it{},
           tombstone_check{} {}
 
-    // NOLINTNEXTLINE(bugprone-easily-swappable-parameters)
-    runtime_view_iterator(const std::vector<Set *> &cpools, const std::vector<Set *> &ignore, iterator_type curr) noexcept
+    runtime_view_iterator(const std::vector<Set *> &cpools, iterator_type curr, const std::vector<Set *> &ignore) noexcept
         : pools{&cpools},
           filter{&ignore},
           it{curr},
@@ -56,7 +55,7 @@ public:
     }
 
     runtime_view_iterator operator++(int) {
-        runtime_view_iterator orig = *this;
+        const runtime_view_iterator orig = *this;
         return ++(*this), orig;
     }
 
@@ -67,7 +66,7 @@ public:
     }
 
     runtime_view_iterator operator--(int) {
-        runtime_view_iterator orig = *this;
+        const runtime_view_iterator orig = *this;
         return operator--(), orig;
     }
 
@@ -83,10 +82,6 @@ public:
         return it == other.it;
     }
 
-    [[nodiscard]] constexpr bool operator!=(const runtime_view_iterator &other) const noexcept {
-        return !(*this == other);
-    }
-
 private:
     const std::vector<Set *> *pools;
     const std::vector<Set *> *filter;
@@ -126,13 +121,21 @@ class basic_runtime_view {
     static_assert(std::is_same_v<typename alloc_traits::value_type, Type *>, "Invalid value type");
     using container_type = std::vector<Type *, Allocator>;
 
+    [[nodiscard]] auto offset() const noexcept {
+        ENTT_ASSERT(!pools.empty(), "Invalid view");
+        const auto &leading = *pools.front();
+        return (leading.policy() == deletion_policy::swap_only) ? leading.free_list() : leading.size();
+    }
+
 public:
     /*! @brief Allocator type. */
     using allocator_type = Allocator;
     /*! @brief Underlying entity identifier. */
-    using entity_type = typename Type::entity_type;
+    using entity_type = Type::entity_type;
     /*! @brief Unsigned integer type. */
     using size_type = std::size_t;
+    /*! @brief Signed integer type. */
+    using difference_type = std::ptrdiff_t;
     /*! @brief Common type among all storage types. */
     using common_type = Type;
     /*! @brief Bidirectional iterator type. */
@@ -219,10 +222,10 @@ public:
      * @return This runtime view.
      */
     basic_runtime_view &iterate(common_type &base) {
-        if(pools.empty() || !(base.size() < pools[0u]->size())) {
+        if(pools.empty() || !(base.size() < pools.front()->size())) {
             pools.push_back(&base);
         } else {
-            pools.push_back(std::exchange(pools[0u], &base));
+            pools.push_back(std::exchange(pools.front(), &base));
         }
 
         return *this;
@@ -243,7 +246,7 @@ public:
      * @return Estimated number of entities iterated by the view.
      */
     [[nodiscard]] size_type size_hint() const {
-        return pools.empty() ? size_type{} : pools.front()->size();
+        return pools.empty() ? size_type{} : offset();
     }
 
     /**
@@ -255,7 +258,7 @@ public:
      * @return An iterator to the first entity that has the given elements.
      */
     [[nodiscard]] iterator begin() const {
-        return pools.empty() ? iterator{} : iterator{pools, filter, pools[0]->begin()};
+        return pools.empty() ? iterator{} : iterator{pools, pools.front()->end() - static_cast<difference_type>(offset()), filter};
     }
 
     /**
@@ -265,7 +268,7 @@ public:
      * given elements.
      */
     [[nodiscard]] iterator end() const {
-        return pools.empty() ? iterator{} : iterator{pools, filter, pools[0]->end()};
+        return pools.empty() ? iterator{} : iterator{pools, pools.front()->end(), filter};
     }
 
     /**
@@ -284,7 +287,8 @@ public:
     [[nodiscard]] bool contains(const entity_type entt) const {
         return !pools.empty()
                && std::all_of(pools.cbegin(), pools.cend(), [entt](const auto *curr) { return curr->contains(entt); })
-               && std::none_of(filter.cbegin(), filter.cend(), [entt](const auto *curr) { return curr && curr->contains(entt); });
+               && std::none_of(filter.cbegin(), filter.cend(), [entt](const auto *curr) { return curr && curr->contains(entt); })
+               && pools.front()->index(entt) < offset();
     }
 
     /**

src/entt/entity/snapshot.hpp

@@ -1,6 +1,7 @@
 #ifndef ENTT_ENTITY_SNAPSHOT_HPP
 #define ENTT_ENTITY_SNAPSHOT_HPP
 
+#include <concepts>
 #include <cstddef>
 #include <iterator>
 #include <tuple>
@@ -10,20 +11,19 @@
 #include "../config/config.h"
 #include "../container/dense_map.hpp"
 #include "../core/type_traits.hpp"
+#include "../stl/iterator.hpp"
 #include "entity.hpp"
 #include "fwd.hpp"
 #include "view.hpp"
 
 namespace entt {
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
 template<typename Registry>
 void orphans(Registry &registry) {
-    auto &storage = registry.template storage<typename Registry::entity_type>();
-
-    for(auto entt: storage) {
+    for(auto &storage = registry.template storage<typename Registry::entity_type>(); auto entt: storage) {
         if(registry.orphan(entt)) {
             storage.erase(entt);
         }
@@ -52,7 +52,7 @@ public:
     /*! Basic registry type. */
     using registry_type = Registry;
     /*! @brief Underlying entity identifier. */
-    using entity_type = typename registry_type::entity_type;
+    using entity_type = registry_type::entity_type;
 
     /**
      * @brief Constructs an instance that is bound to a given registry.
@@ -95,10 +95,10 @@ public:
         if(const auto *storage = reg->template storage<Type>(id); storage) {
             const typename registry_type::common_type &base = *storage;
 
-            archive(static_cast<typename traits_type::entity_type>(storage->size()));
+            archive(static_cast<traits_type::entity_type>(storage->size()));
 
             if constexpr(std::is_same_v<Type, entity_type>) {
-                archive(static_cast<typename traits_type::entity_type>(storage->free_list()));
+                archive(static_cast<traits_type::entity_type>(storage->free_list()));
 
                 for(auto first = base.rbegin(), last = base.rend(); first != last; ++first) {
                     archive(*first);
@@ -129,19 +129,18 @@ public:
      * the entities in a range.
      * @tparam Type Type of elements to serialize.
      * @tparam Archive Type of output archive.
-     * @tparam It Type of input iterator.
      * @param archive A valid reference to an output archive.
      * @param first An iterator to the first element of the range to serialize.
      * @param last An iterator past the last element of the range to serialize.
      * @param id Optional name used to map the storage within the registry.
      * @return An object of this type to continue creating the snapshot.
      */
-    template<typename Type, typename Archive, typename It>
-    const basic_snapshot &get(Archive &archive, It first, It last, const id_type id = type_hash<Type>::value()) const {
+    template<typename Type, typename Archive>
+    const basic_snapshot &get(Archive &archive, stl::input_iterator auto first, stl::input_iterator auto last, const id_type id = type_hash<Type>::value()) const {
         static_assert(!std::is_same_v<Type, entity_type>, "Entity types not supported");
 
         if(const auto *storage = reg->template storage<Type>(id); storage && !storage->empty()) {
-            archive(static_cast<typename traits_type::entity_type>(std::distance(first, last)));
+            archive(static_cast<traits_type::entity_type>(std::distance(first, last)));
 
             for(; first != last; ++first) {
                 if(const auto entt = *first; storage->contains(entt)) {
@@ -181,7 +180,7 @@ public:
     /*! Basic registry type. */
     using registry_type = Registry;
     /*! @brief Underlying entity identifier. */
-    using entity_type = typename registry_type::entity_type;
+    using entity_type = registry_type::entity_type;
 
     /**
      * @brief Constructs an instance that is bound to a given registry.
@@ -231,15 +230,18 @@ public:
 
         if constexpr(std::is_same_v<Type, entity_type>) {
             typename traits_type::entity_type count{};
+            entity_type placeholder{};
 
             storage.reserve(length);
             archive(count);
 
             for(entity_type entity = null; length; --length) {
                 archive(entity);
-                storage.emplace(entity);
+                storage.generate(entity);
+                placeholder = (entity > placeholder) ? entity : placeholder;
             }
 
+            storage.start_from(traits_type::next(placeholder));
             storage.free_list(count);
         } else {
             auto &other = reg->template storage<entity_type>();
@@ -247,7 +249,7 @@ public:
 
             while(length--) {
                 if(archive(entt); entt != null) {
-                    const auto entity = other.contains(entt) ? entt : other.emplace(entt);
+                    const auto entity = other.contains(entt) ? entt : other.generate(entt);
                     ENTT_ASSERT(entity == entt, "Entity not available for use");
 
                     if constexpr(std::tuple_size_v<decltype(storage.get_as_tuple({}))> == 0u) {
@@ -304,7 +306,7 @@ class basic_continuous_loader {
     static_assert(!std::is_const_v<Registry>, "Non-const registry type required");
     using traits_type = entt_traits<typename Registry::entity_type>;
 
-    void restore(typename Registry::entity_type entt) {
+    void restore(Registry::entity_type entt) {
         if(const auto entity = to_entity(entt); remloc.contains(entity) && remloc[entity].first == entt) {
             if(!reg->valid(remloc[entity].second)) {
                 remloc[entity].second = reg->create();
@@ -321,7 +323,7 @@ class basic_continuous_loader {
 
         for(auto &&pair: container) {
             using first_type = std::remove_const_t<typename std::decay_t<decltype(pair)>::first_type>;
-            using second_type = typename std::decay_t<decltype(pair)>::second_type;
+            using second_type = std::decay_t<decltype(pair)>::second_type;
 
             if constexpr(std::is_same_v<first_type, entity_type> && std::is_same_v<second_type, entity_type>) {
                 other.emplace(map(pair.first), map(pair.second));
@@ -363,7 +365,7 @@ public:
     /*! Basic registry type. */
     using registry_type = Registry;
     /*! @brief Underlying entity identifier. */
-    using entity_type = typename registry_type::entity_type;
+    using entity_type = registry_type::entity_type;
 
     /**
      * @brief Constructs an instance that is bound to a given registry.

src/entt/entity/sparse_set.hpp

@@ -1,6 +1,8 @@
 #ifndef ENTT_ENTITY_SPARSE_SET_HPP
 #define ENTT_ENTITY_SPARSE_SET_HPP
 
+#include <compare>
+#include <concepts>
 #include <cstddef>
 #include <iterator>
 #include <memory>
@@ -12,20 +14,21 @@
 #include "../core/any.hpp"
 #include "../core/bit.hpp"
 #include "../core/type_info.hpp"
+#include "../stl/iterator.hpp"
 #include "entity.hpp"
 #include "fwd.hpp"
 
 namespace entt {
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
 template<typename Container>
 struct sparse_set_iterator final {
-    using value_type = typename Container::value_type;
-    using pointer = typename Container::const_pointer;
-    using reference = typename Container::const_reference;
-    using difference_type = typename Container::difference_type;
+    using value_type = Container::value_type;
+    using pointer = Container::const_pointer;
+    using reference = Container::const_reference;
+    using difference_type = Container::difference_type;
     using iterator_category = std::random_access_iterator_tag;
 
     constexpr sparse_set_iterator() noexcept
@@ -41,7 +44,7 @@ struct sparse_set_iterator final {
     }
 
     constexpr sparse_set_iterator operator++(int) noexcept {
-        sparse_set_iterator orig = *this;
+        const sparse_set_iterator orig = *this;
         return ++(*this), orig;
     }
 
@@ -50,7 +53,7 @@ struct sparse_set_iterator final {
     }
 
     constexpr sparse_set_iterator operator--(int) noexcept {
-        sparse_set_iterator orig = *this;
+        const sparse_set_iterator orig = *this;
         return operator--(), orig;
     }
 
@@ -73,7 +76,7 @@ struct sparse_set_iterator final {
     }
 
     [[nodiscard]] constexpr reference operator[](const difference_type value) const noexcept {
-        return (*packed)[index() - value];
+        return (*packed)[static_cast<Container::size_type>(index() - value)];
     }
 
     [[nodiscard]] constexpr pointer operator->() const noexcept {
@@ -84,6 +87,20 @@ struct sparse_set_iterator final {
         return operator[](0);
     }
 
+    [[nodiscard]] constexpr std::ptrdiff_t operator-(const sparse_set_iterator &other) const noexcept {
+        // intentionally reversed due to backward iteration
+        return other.offset - offset;
+    }
+
+    [[nodiscard]] constexpr bool operator==(const sparse_set_iterator &other) const noexcept {
+        return offset == other.offset;
+    }
+
+    [[nodiscard]] constexpr auto operator<=>(const sparse_set_iterator &other) const noexcept {
+        // intentionally reversed due to backward iteration
+        return other.offset <=> offset;
+    }
+
     [[nodiscard]] constexpr pointer data() const noexcept {
         return packed ? packed->data() : nullptr;
     }
@@ -97,41 +114,6 @@ private:
     difference_type offset;
 };
 
-template<typename Container>
-[[nodiscard]] constexpr std::ptrdiff_t operator-(const sparse_set_iterator<Container> &lhs, const sparse_set_iterator<Container> &rhs) noexcept {
-    return rhs.index() - lhs.index();
-}
-
-template<typename Container>
-[[nodiscard]] constexpr bool operator==(const sparse_set_iterator<Container> &lhs, const sparse_set_iterator<Container> &rhs) noexcept {
-    return lhs.index() == rhs.index();
-}
-
-template<typename Container>
-[[nodiscard]] constexpr bool operator!=(const sparse_set_iterator<Container> &lhs, const sparse_set_iterator<Container> &rhs) noexcept {
-    return !(lhs == rhs);
-}
-
-template<typename Container>
-[[nodiscard]] constexpr bool operator<(const sparse_set_iterator<Container> &lhs, const sparse_set_iterator<Container> &rhs) noexcept {
-    return lhs.index() > rhs.index();
-}
-
-template<typename Container>
-[[nodiscard]] constexpr bool operator>(const sparse_set_iterator<Container> &lhs, const sparse_set_iterator<Container> &rhs) noexcept {
-    return rhs < lhs;
-}
-
-template<typename Container>
-[[nodiscard]] constexpr bool operator<=(const sparse_set_iterator<Container> &lhs, const sparse_set_iterator<Container> &rhs) noexcept {
-    return !(lhs > rhs);
-}
-
-template<typename Container>
-[[nodiscard]] constexpr bool operator>=(const sparse_set_iterator<Container> &lhs, const sparse_set_iterator<Container> &rhs) noexcept {
-    return !(lhs < rhs);
-}
-
 } // namespace internal
 /*! @endcond */
 
@@ -164,29 +146,38 @@ class basic_sparse_set {
 
     static constexpr auto max_size = static_cast<std::size_t>(traits_type::to_entity(null));
 
-    [[nodiscard]] auto policy_to_head() const noexcept {
-        return static_cast<size_type>(max_size * static_cast<decltype(max_size)>(mode != deletion_policy::swap_only));
+    // it could be auto but gcc complains and emits a warning due to a false positive
+    [[nodiscard]] std::size_t policy_to_head() const noexcept {
+        return static_cast<size_type>(max_size * static_cast<std::remove_const_t<decltype(max_size)>>(mode != deletion_policy::swap_only));
+    }
+
+    [[nodiscard]] auto entity_to_pos(const Entity entt) const noexcept {
+        return static_cast<size_type>(traits_type::to_entity(entt));
+    }
+
+    [[nodiscard]] auto pos_to_page(const std::size_t pos) const noexcept {
+        return static_cast<size_type>(pos / traits_type::page_size);
     }
 
     [[nodiscard]] auto sparse_ptr(const Entity entt) const {
-        const auto pos = static_cast<size_type>(traits_type::to_entity(entt));
-        const auto page = pos / traits_type::page_size;
+        const auto pos = entity_to_pos(entt);
+        const auto page = pos_to_page(pos);
         return (page < sparse.size() && sparse[page]) ? (sparse[page] + fast_mod(pos, traits_type::page_size)) : nullptr;
     }
 
     [[nodiscard]] auto &sparse_ref(const Entity entt) const {
         ENTT_ASSERT(sparse_ptr(entt), "Invalid element");
-        const auto pos = static_cast<size_type>(traits_type::to_entity(entt));
-        return sparse[pos / traits_type::page_size][fast_mod(pos, traits_type::page_size)];
+        const auto pos = entity_to_pos(entt);
+        return sparse[pos_to_page(pos)][fast_mod(pos, traits_type::page_size)];
     }
 
     [[nodiscard]] auto to_iterator(const Entity entt) const {
-        return --(end() - index(entt));
+        return --(end() - static_cast<difference_type>(index(entt)));
     }
 
     [[nodiscard]] auto &assure_at_least(const Entity entt) {
-        const auto pos = static_cast<size_type>(traits_type::to_entity(entt));
-        const auto page = pos / traits_type::page_size;
+        const auto pos = entity_to_pos(entt);
+        const auto page = pos_to_page(pos);
 
         if(!(page < sparse.size())) {
             sparse.resize(page + 1u, nullptr);
@@ -203,9 +194,7 @@ class basic_sparse_set {
     }
 
     void release_sparse_pages() {
-        auto page_allocator{packed.get_allocator()};
-
-        for(auto &&page: sparse) {
+        for(auto page_allocator{packed.get_allocator()}; auto &&page: sparse) {
             if(page != nullptr) {
                 std::destroy(page, page + traits_type::page_size);
                 alloc_traits::deallocate(page_allocator, page, traits_type::page_size);
@@ -218,8 +207,8 @@ class basic_sparse_set {
         auto &from = packed[lhs];
         auto &to = packed[rhs];
 
-        sparse_ref(from) = traits_type::combine(static_cast<typename traits_type::entity_type>(rhs), traits_type::to_integral(from));
-        sparse_ref(to) = traits_type::combine(static_cast<typename traits_type::entity_type>(lhs), traits_type::to_integral(to));
+        sparse_ref(from) = traits_type::combine(static_cast<traits_type::entity_type>(rhs), traits_type::to_integral(from));
+        sparse_ref(to) = traits_type::combine(static_cast<traits_type::entity_type>(lhs), traits_type::to_integral(to));
 
         std::swap(from, to);
     }
@@ -239,26 +228,27 @@ protected:
 
     /**
      * @brief Erases an entity from a sparse set.
-     * @param it An iterator to the element to pop.
+     * @param entt A valid identifier for the element to pop.
      */
-    void swap_only(const basic_iterator it) {
+    void swap_only(const Entity entt) {
         ENTT_ASSERT(mode == deletion_policy::swap_only, "Deletion policy mismatch");
-        const auto pos = index(*it);
-        bump(traits_type::next(*it));
+        const auto pos = index(entt);
+        bump(traits_type::next(entt));
         swap_at(pos, head -= (pos < head));
     }
 
     /**
      * @brief Erases an entity from a sparse set.
-     * @param it An iterator to the element to pop.
+     * @param entt A valid identifier for the element to pop.
      */
-    void swap_and_pop(const basic_iterator it) {
+    void swap_and_pop(const Entity entt) {
         ENTT_ASSERT(mode == deletion_policy::swap_and_pop, "Deletion policy mismatch");
-        auto &self = sparse_ref(*it);
-        const auto entt = traits_type::to_entity(self);
-        sparse_ref(packed.back()) = traits_type::combine(entt, traits_type::to_integral(packed.back()));
-        packed[static_cast<size_type>(entt)] = packed.back();
+        auto &self = sparse_ref(entt);
+        const auto pos = traits_type::to_entity(self);
+        sparse_ref(packed.back()) = traits_type::combine(pos, traits_type::to_integral(packed.back()));
+        packed[static_cast<size_type>(pos)] = packed.back();
         // unnecessary but it helps to detect nasty bugs
+        // NOLINTNEXTLINE(bugprone-assert-side-effect)
         ENTT_ASSERT((packed.back() = null, true), "");
         // lazy self-assignment guard
         self = null;
@@ -267,12 +257,12 @@ protected:
 
     /**
      * @brief Erases an entity from a sparse set.
-     * @param it An iterator to the element to pop.
+     * @param entt A valid identifier for the element to pop.
      */
-    void in_place_pop(const basic_iterator it) {
+    void in_place_pop(const Entity entt) {
         ENTT_ASSERT(mode == deletion_policy::in_place, "Deletion policy mismatch");
-        const auto pos = static_cast<size_type>(traits_type::to_entity(std::exchange(sparse_ref(*it), null)));
-        packed[pos] = traits_type::combine(static_cast<typename traits_type::entity_type>(std::exchange(head, pos)), tombstone);
+        const auto pos = entity_to_pos(std::exchange(sparse_ref(entt), null));
+        packed[pos] = traits_type::combine(static_cast<traits_type::entity_type>(std::exchange(head, pos)), tombstone);
     }
 
     /**
@@ -284,17 +274,17 @@ protected:
         switch(mode) {
         case deletion_policy::swap_and_pop:
             for(; first != last; ++first) {
-                swap_and_pop(first);
+                swap_and_pop(*first);
             }
             break;
         case deletion_policy::in_place:
             for(; first != last; ++first) {
-                in_place_pop(first);
+                in_place_pop(*first);
             }
             break;
         case deletion_policy::swap_only:
             for(; first != last; ++first) {
-                swap_only(first);
+                swap_only(*first);
             }
             break;
         }
@@ -302,23 +292,15 @@ protected:
 
     /*! @brief Erases all entities of a sparse set. */
     virtual void pop_all() {
-        switch(mode) {
-        case deletion_policy::in_place:
-            if(head != max_size) {
-                for(auto first = begin(); !(first.index() < 0); ++first) {
-                    if(*first != tombstone) {
-                        sparse_ref(*first) = null;
+        if(!packed.empty()) {
+            // suboptimal with few entities, but exploits cache way more with many
+            for(auto &&elem: sparse) {
+                if(elem) {
+                    for(size_type pos{}; pos < traits_type::page_size; ++pos) {
+                        elem[pos] = null;
                     }
                 }
-                break;
             }
-            [[fallthrough]];
-        case deletion_policy::swap_only:
-        case deletion_policy::swap_and_pop:
-            for(auto first = begin(); !(first.index() < 0); ++first) {
-                sparse_ref(*first) = null;
-            }
-            break;
         }
 
         head = policy_to_head();
@@ -341,31 +323,31 @@ protected:
             if(head != max_size && !force_back) {
                 pos = head;
                 ENTT_ASSERT(elem == null, "Slot not available");
-                elem = traits_type::combine(static_cast<typename traits_type::entity_type>(head), traits_type::to_integral(entt));
-                head = static_cast<size_type>(traits_type::to_entity(std::exchange(packed[pos], entt)));
+                elem = traits_type::combine(static_cast<traits_type::entity_type>(head), traits_type::to_integral(entt));
+                head = entity_to_pos(std::exchange(packed[pos], entt));
                 break;
             }
             [[fallthrough]];
         case deletion_policy::swap_and_pop:
             packed.push_back(entt);
             ENTT_ASSERT(elem == null, "Slot not available");
-            elem = traits_type::combine(static_cast<typename traits_type::entity_type>(packed.size() - 1u), traits_type::to_integral(entt));
+            elem = traits_type::combine(static_cast<traits_type::entity_type>(packed.size() - 1u), traits_type::to_integral(entt));
             break;
         case deletion_policy::swap_only:
             if(elem == null) {
                 packed.push_back(entt);
-                elem = traits_type::combine(static_cast<typename traits_type::entity_type>(packed.size() - 1u), traits_type::to_integral(entt));
+                elem = traits_type::combine(static_cast<traits_type::entity_type>(packed.size() - 1u), traits_type::to_integral(entt));
             } else {
-                ENTT_ASSERT(!(static_cast<size_type>(traits_type::to_entity(elem)) < head), "Slot not available");
+                ENTT_ASSERT(!(entity_to_pos(elem) < head), "Slot not available");
                 bump(entt);
             }
 
             pos = head++;
-            swap_at(static_cast<size_type>(traits_type::to_entity(elem)), pos);
+            swap_at(entity_to_pos(elem), pos);
             break;
         }
 
-        return --(end() - static_cast<typename iterator::difference_type>(pos));
+        return iterator{packed, static_cast<difference_type>(++pos)};
     }
 
     /*! @brief Forwards variables to derived classes, if any. */
@@ -376,13 +358,15 @@ public:
     /*! @brief Allocator type. */
     using allocator_type = Allocator;
     /*! @brief Underlying entity identifier. */
-    using entity_type = typename traits_type::value_type;
+    using entity_type = traits_type::value_type;
     /*! @brief Underlying version type. */
-    using version_type = typename traits_type::version_type;
+    using version_type = traits_type::version_type;
     /*! @brief Unsigned integer type. */
     using size_type = std::size_t;
+    /*! @brief Signed integer type. */
+    using difference_type = std::ptrdiff_t;
     /*! @brief Pointer type to contained entities. */
-    using pointer = typename packed_container_type::const_pointer;
+    using pointer = packed_container_type::const_pointer;
     /*! @brief Random access iterator type. */
     using iterator = basic_iterator;
     /*! @brief Constant random access iterator type. */
@@ -421,7 +405,7 @@ public:
     explicit basic_sparse_set(const type_info &elem, deletion_policy pol = deletion_policy::swap_and_pop, const allocator_type &allocator = {})
         : sparse{allocator},
           packed{allocator},
-          info{&elem},
+          descriptor{&elem},
           mode{pol},
           head{policy_to_head()} {
         ENTT_ASSERT(traits_type::version_mask || mode != deletion_policy::in_place, "Policy does not support zero-sized versions");
@@ -437,7 +421,7 @@ public:
     basic_sparse_set(basic_sparse_set &&other) noexcept
         : sparse{std::move(other.sparse)},
           packed{std::move(other.packed)},
-          info{other.info},
+          descriptor{other.descriptor},
           mode{other.mode},
           head{std::exchange(other.head, policy_to_head())} {}
 
@@ -449,7 +433,7 @@ public:
     basic_sparse_set(basic_sparse_set &&other, const allocator_type &allocator)
         : sparse{std::move(other.sparse), allocator},
           packed{std::move(other.packed), allocator},
-          info{other.info},
+          descriptor{other.descriptor},
           mode{other.mode},
           head{std::exchange(other.head, policy_to_head())} {
         ENTT_ASSERT(alloc_traits::is_always_equal::value || get_allocator() == other.get_allocator(), "Copying a sparse set is not allowed");
@@ -485,7 +469,7 @@ public:
         using std::swap;
         swap(sparse, other.sparse);
         swap(packed, other.packed);
-        swap(info, other.info);
+        swap(descriptor, other.descriptor);
         swap(mode, other.mode);
         swap(head, other.head);
     }
@@ -546,6 +530,31 @@ public:
 
     /*! @brief Requests the removal of unused capacity. */
     virtual void shrink_to_fit() {
+        sparse_container_type other{sparse.get_allocator()};
+        const auto len = sparse.size();
+        other.reserve(len);
+
+        for(size_type cnt{}; auto &&elem: std::as_const(packed)) {
+            if(elem != tombstone) {
+                if(const auto page = pos_to_page(entity_to_pos(elem)); sparse[page] != nullptr) {
+                    if(const auto sz = page + 1u; sz > other.size()) {
+                        other.resize(sz, nullptr);
+                    }
+
+                    other[page] = std::exchange(sparse[page], nullptr);
+
+                    if(++cnt == len) {
+                        // early exit due to lack of pages
+                        break;
+                    }
+                }
+            }
+        }
+
+        release_sparse_pages();
+        sparse.swap(other);
+
+        sparse.shrink_to_fit();
         packed.shrink_to_fit();
     }
 
@@ -553,9 +562,8 @@ public:
      * @brief Returns the extent of a sparse set.
      *
      * The extent of a sparse set is also the size of the internal sparse array.
-     * There is no guarantee that the internal packed array has the same size.
-     * Usually the size of the internal sparse array is equal or greater than
-     * the one of the internal packed array.
+     * There is no guarantee that all pages have been allocated, nor that the
+     * internal packed array is be the same size.
      *
      * @return Extent of the sparse set.
      */
@@ -610,7 +618,7 @@ public:
      * @return An iterator to the first entity of the sparse set.
      */
     [[nodiscard]] iterator begin() const noexcept {
-        const auto pos = static_cast<typename iterator::difference_type>(packed.size());
+        const auto pos = static_cast<difference_type>(packed.size());
         return iterator{packed, pos};
     }
 
@@ -712,7 +720,7 @@ public:
      */
     [[nodiscard]] size_type index(const entity_type entt) const noexcept {
         ENTT_ASSERT(contains(entt), "Set does not contain entity");
-        return static_cast<size_type>(traits_type::to_entity(sparse_ref(entt)));
+        return entity_to_pos(sparse_ref(entt));
     }
 
     /**
@@ -767,14 +775,12 @@ public:
      * Attempting to assign an entity that already belongs to the sparse set
      * results in undefined behavior.
      *
-     * @tparam It Type of input iterator.
      * @param first An iterator to the first element of the range of entities.
      * @param last An iterator past the last element of the range of entities.
      * @return Iterator pointing to the first element inserted in case of
      * success, the `end()` iterator otherwise.
      */
-    template<typename It>
-    iterator push(It first, It last) {
+    iterator push(stl::input_iterator auto first, stl::input_iterator auto last) {
         auto curr = end();
 
         for(; first != last; ++first) {
@@ -798,7 +804,7 @@ public:
         auto &elem = sparse_ref(entt);
         ENTT_ASSERT(entt != null && elem != tombstone, "Cannot set the required version");
         elem = traits_type::combine(traits_type::to_integral(elem), traits_type::to_integral(entt));
-        packed[static_cast<size_type>(traits_type::to_entity(elem))] = entt;
+        packed[entity_to_pos(elem)] = entt;
         return traits_type::to_version(entt);
     }
 
@@ -825,7 +831,7 @@ public:
      * @param first An iterator to the first element of the range of entities.
      * @param last An iterator past the last element of the range of entities.
      */
-    template<typename It>
+    template<stl::input_iterator It>
     void erase(It first, It last) {
         if constexpr(std::is_same_v<It, basic_iterator>) {
             pop(first, last);
@@ -852,7 +858,7 @@ public:
      * @param last An iterator past the last element of the range of entities.
      * @return The number of entities actually removed.
      */
-    template<typename It>
+    template<stl::input_iterator It>
     size_type remove(It first, It last) {
         size_type count{};
 
@@ -868,7 +874,7 @@ public:
                     ++first;
                 }
 
-                count += std::distance(it, first);
+                count += static_cast<size_type>(std::distance(it, first));
                 erase(it, first);
             }
         } else {
@@ -889,19 +895,19 @@ public:
             for(; from && packed[from - 1u] == tombstone; --from) {}
 
             while(pos != max_size) {
-                if(const auto to = std::exchange(pos, static_cast<size_type>(traits_type::to_entity(packed[pos]))); to < from) {
+                if(const auto to = std::exchange(pos, entity_to_pos(packed[pos])); to < from) {
                     --from;
                     swap_or_move(from, to);
 
                     packed[to] = packed[from];
-                    const auto elem = static_cast<typename traits_type::entity_type>(to);
+                    const auto elem = static_cast<traits_type::entity_type>(to);
                     sparse_ref(packed[to]) = traits_type::combine(elem, traits_type::to_integral(packed[to]));
 
                     for(; from && packed[from - 1u] == tombstone; --from) {}
                 }
             }
 
-            packed.erase(packed.begin() + from, packed.end());
+            packed.erase(packed.begin() + static_cast<difference_type>(from), packed.end());
         }
     }
 
@@ -962,7 +968,7 @@ public:
         ENTT_ASSERT((mode != deletion_policy::in_place) || (head == max_size), "Sorting with tombstones not allowed");
         ENTT_ASSERT(!(length > packed.size()), "Length exceeds the number of elements");
 
-        algo(packed.rend() - length, packed.rend(), std::move(compare), std::forward<Args>(args)...);
+        algo(packed.rend() - static_cast<difference_type>(length), packed.rend(), std::move(compare), std::forward<Args>(args)...);
 
         for(size_type pos{}; pos < length; ++pos) {
             auto curr = pos;
@@ -973,7 +979,7 @@ public:
                 const auto entt = packed[curr];
 
                 swap_or_move(next, idx);
-                const auto elem = static_cast<typename traits_type::entity_type>(curr);
+                const auto elem = static_cast<traits_type::entity_type>(curr);
                 sparse_ref(entt) = traits_type::combine(elem, traits_type::to_integral(packed[curr]));
                 curr = std::exchange(next, idx);
             }
@@ -1011,11 +1017,11 @@ public:
      * @param last An iterator past the last element of the range of entities.
      * @return An iterator past the last of the elements actually shared.
      */
-    template<typename It>
+    template<stl::input_iterator It>
     iterator sort_as(It first, It last) {
         ENTT_ASSERT((mode != deletion_policy::in_place) || (head == max_size), "Sorting with tombstones not allowed");
         const size_type len = (mode == deletion_policy::swap_only) ? head : packed.size();
-        auto it = end() - static_cast<typename iterator::difference_type>(len);
+        auto it = end() - static_cast<difference_type>(len);
 
         for(const auto other = end(); (it != other) && (first != last); ++first) {
             if(const auto curr = *first; contains(curr)) {
@@ -1041,42 +1047,27 @@ public:
     }
 
     /**
-     * @brief Returned value type, if any.
-     * @return Returned value type, if any.
+     * @brief Returns a type info object for the value type, if any.
+     * @return A type info object for the value type, if any.
      */
-    [[nodiscard]] const type_info &type() const noexcept {
-        return *info;
+    [[nodiscard]] const type_info &info() const noexcept {
+        return *descriptor;
     }
 
     /**
      * @brief Forwards variables to derived classes, if any.
      * @tparam Type Type of the element to forward.
      * @param value The element to forward.
-     * @return Nothing.
      */
     template<typename Type>
-    [[deprecated("avoid wrapping elements with basic_any")]] std::enable_if_t<std::is_same_v<std::remove_const_t<std::remove_reference_t<Type>>, basic_any<>>>
-    bind(Type &&value) noexcept {
-        // backward compatibility
-        bind_any(std::forward<Type>(value));
-    }
-
-    /**
-     * @brief Forwards variables to derived classes, if any.
-     * @tparam Type Type of the element to forward.
-     * @param value The element to forward.
-     * @return Nothing.
-     */
-    template<typename Type>
-    std::enable_if_t<!std::is_same_v<std::remove_const_t<std::remove_reference_t<Type>>, basic_any<>>>
-    bind(Type &&value) noexcept {
+    void bind(Type &&value) noexcept {
         bind_any(forward_as_any(std::forward<Type>(value)));
     }
 
 private:
     sparse_container_type sparse;
     packed_container_type packed;
-    const type_info *info;
+    const type_info *descriptor;
     deletion_policy mode;
     size_type head;
 };

src/entt/entity/storage.hpp

@@ -1,6 +1,8 @@
 #ifndef ENTT_ENTITY_STORAGE_HPP
 #define ENTT_ENTITY_STORAGE_HPP
 
+#include <compare>
+#include <concepts>
 #include <cstddef>
 #include <iterator>
 #include <memory>
@@ -13,6 +15,8 @@
 #include "../core/iterator.hpp"
 #include "../core/memory.hpp"
 #include "../core/type_info.hpp"
+#include "../stl/iterator.hpp"
+#include "../stl/memory.hpp"
 #include "component.hpp"
 #include "entity.hpp"
 #include "fwd.hpp"
@@ -20,12 +24,13 @@
 
 namespace entt {
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
-template<typename Container>
+template<typename Container, auto Page>
 class storage_iterator final {
-    friend storage_iterator<const Container>;
+    template<typename, auto>
+    friend class storage_iterator;
 
     using container_type = std::remove_const_t<Container>;
     using alloc_traits = std::allocator_traits<typename container_type::allocator_type>;
@@ -36,10 +41,10 @@ class storage_iterator final {
         typename alloc_traits::template rebind_traits<typename std::pointer_traits<typename container_type::value_type>::element_type>::pointer>>;
 
 public:
-    using value_type = typename iterator_traits::value_type;
-    using pointer = typename iterator_traits::pointer;
-    using reference = typename iterator_traits::reference;
-    using difference_type = typename iterator_traits::difference_type;
+    using value_type = iterator_traits::value_type;
+    using pointer = iterator_traits::pointer;
+    using reference = iterator_traits::reference;
+    using difference_type = iterator_traits::difference_type;
     using iterator_category = std::random_access_iterator_tag;
 
     constexpr storage_iterator() noexcept = default;
@@ -48,8 +53,9 @@ public:
         : payload{ref},
           offset{idx} {}
 
-    template<bool Const = std::is_const_v<Container>, typename = std::enable_if_t<Const>>
-    constexpr storage_iterator(const storage_iterator<std::remove_const_t<Container>> &other) noexcept
+    template<std::same_as<std::remove_const_t<Container>> Other>
+    requires std::is_const_v<Container>
+    constexpr storage_iterator(const storage_iterator<Other, Page> &other) noexcept
         : storage_iterator{other.payload, other.offset} {}
 
     constexpr storage_iterator &operator++() noexcept {
@@ -57,7 +63,7 @@ public:
     }
 
     constexpr storage_iterator operator++(int) noexcept {
-        storage_iterator orig = *this;
+        const storage_iterator orig = *this;
         return ++(*this), orig;
     }
 
@@ -66,7 +72,7 @@ public:
     }
 
     constexpr storage_iterator operator--(int) noexcept {
-        storage_iterator orig = *this;
+        const storage_iterator orig = *this;
         return operator--(), orig;
     }
 
@@ -89,9 +95,8 @@ public:
     }
 
     [[nodiscard]] constexpr reference operator[](const difference_type value) const noexcept {
-        const auto pos = index() - value;
-        constexpr auto page_size = component_traits<value_type>::page_size;
-        return (*payload)[pos / page_size][fast_mod(static_cast<std::size_t>(pos), page_size)];
+        const auto pos = static_cast<Container::size_type>(index() - value);
+        return (*payload)[pos / Page][fast_mod(static_cast<std::size_t>(pos), Page)];
     }
 
     [[nodiscard]] constexpr pointer operator->() const noexcept {
@@ -102,6 +107,23 @@ public:
         return operator[](0);
     }
 
+    template<typename Other, auto Arg>
+    [[nodiscard]] constexpr std::ptrdiff_t operator-(const storage_iterator<Other, Arg> &other) const noexcept {
+        // intentionally reversed due to backward iteration
+        return other.offset - offset;
+    }
+
+    template<typename Other, auto Arg>
+    [[nodiscard]] constexpr bool operator==(const storage_iterator<Other, Arg> &other) const noexcept {
+        return offset == other.offset;
+    }
+
+    template<typename Other, auto Arg>
+    [[nodiscard]] constexpr auto operator<=>(const storage_iterator<Other, Arg> &other) const noexcept {
+        // intentionally reversed due to backward iteration
+        return other.offset <=> offset;
+    }
+
     [[nodiscard]] constexpr difference_type index() const noexcept {
         return offset - 1;
     }
@@ -111,44 +133,9 @@ private:
     difference_type offset;
 };
 
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr std::ptrdiff_t operator-(const storage_iterator<Lhs> &lhs, const storage_iterator<Rhs> &rhs) noexcept {
-    return rhs.index() - lhs.index();
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator==(const storage_iterator<Lhs> &lhs, const storage_iterator<Rhs> &rhs) noexcept {
-    return lhs.index() == rhs.index();
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator!=(const storage_iterator<Lhs> &lhs, const storage_iterator<Rhs> &rhs) noexcept {
-    return !(lhs == rhs);
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator<(const storage_iterator<Lhs> &lhs, const storage_iterator<Rhs> &rhs) noexcept {
-    return lhs.index() > rhs.index();
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator>(const storage_iterator<Lhs> &lhs, const storage_iterator<Rhs> &rhs) noexcept {
-    return rhs < lhs;
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator<=(const storage_iterator<Lhs> &lhs, const storage_iterator<Rhs> &rhs) noexcept {
-    return !(lhs > rhs);
-}
-
-template<typename Lhs, typename Rhs>
-[[nodiscard]] constexpr bool operator>=(const storage_iterator<Lhs> &lhs, const storage_iterator<Rhs> &rhs) noexcept {
-    return !(lhs < rhs);
-}
-
 template<typename It, typename... Other>
 class extended_storage_iterator final {
-    template<typename Iter, typename... Args>
+    template<typename, typename...>
     friend class extended_storage_iterator;
 
 public:
@@ -166,7 +153,8 @@ public:
     constexpr extended_storage_iterator(iterator_type base, Other... other)
         : it{base, other...} {}
 
-    template<typename... Args, typename = std::enable_if_t<(!std::is_same_v<Other, Args> && ...) && (std::is_constructible_v<Other, Args> && ...)>>
+    template<typename... Args>
+    requires (!std::same_as<Other, Args> && ...) && (std::constructible_from<Other, Args> && ...)
     constexpr extended_storage_iterator(const extended_storage_iterator<It, Args...> &other)
         : it{other.it} {}
 
@@ -175,7 +163,7 @@ public:
     }
 
     constexpr extended_storage_iterator operator++(int) noexcept {
-        extended_storage_iterator orig = *this;
+        const extended_storage_iterator orig = *this;
         return ++(*this), orig;
     }
 
@@ -191,23 +179,15 @@ public:
         return std::get<It>(it);
     }
 
-    template<typename... Lhs, typename... Rhs>
-    friend constexpr bool operator==(const extended_storage_iterator<Lhs...> &, const extended_storage_iterator<Rhs...> &) noexcept;
+    template<typename... Args>
+    [[nodiscard]] constexpr bool operator==(const extended_storage_iterator<Args...> &other) const noexcept {
+        return std::get<0>(it) == std::get<0>(other.it);
+    }
 
 private:
     std::tuple<It, Other...> it;
 };
 
-template<typename... Lhs, typename... Rhs>
-[[nodiscard]] constexpr bool operator==(const extended_storage_iterator<Lhs...> &lhs, const extended_storage_iterator<Rhs...> &rhs) noexcept {
-    return std::get<0>(lhs.it) == std::get<0>(rhs.it);
-}
-
-template<typename... Lhs, typename... Rhs>
-[[nodiscard]] constexpr bool operator!=(const extended_storage_iterator<Lhs...> &lhs, const extended_storage_iterator<Rhs...> &rhs) noexcept {
-    return !(lhs == rhs);
-}
-
 } // namespace internal
 /*! @endcond */
 
@@ -226,14 +206,14 @@ template<typename... Lhs, typename... Rhs>
  * @tparam Entity A valid entity type.
  * @tparam Allocator Type of allocator used to manage memory and elements.
  */
-template<typename Type, typename Entity, typename Allocator, typename>
+template<typename Type, typename Entity, typename Allocator>
 class basic_storage: public basic_sparse_set<Entity, typename std::allocator_traits<Allocator>::template rebind_alloc<Entity>> {
     using alloc_traits = std::allocator_traits<Allocator>;
     static_assert(std::is_same_v<typename alloc_traits::value_type, Type>, "Invalid value type");
     using container_type = std::vector<typename alloc_traits::pointer, typename alloc_traits::template rebind_alloc<typename alloc_traits::pointer>>;
     using underlying_type = basic_sparse_set<Entity, typename alloc_traits::template rebind_alloc<Entity>>;
-    using underlying_iterator = typename underlying_type::basic_iterator;
-    using traits_type = component_traits<Type>;
+    using underlying_iterator = underlying_type::basic_iterator;
+    using traits_type = component_traits<Type, Entity>;
 
     [[nodiscard]] auto &element_at(const std::size_t pos) const {
         return payload[pos / traits_type::page_size][fast_mod(pos, traits_type::page_size)];
@@ -266,7 +246,7 @@ class basic_storage: public basic_sparse_set<Entity, typename std::allocator_tra
         const auto it = base_type::try_emplace(entt, force_back);
 
         ENTT_TRY {
-            auto *elem = to_address(assure_at_least(static_cast<size_type>(it.index())));
+            auto *elem = stl::to_address(assure_at_least(static_cast<size_type>(it.index())));
             entt::uninitialized_construct_using_allocator(elem, get_allocator(), std::forward<Args>(args)...);
         }
         ENTT_CATCH {
@@ -281,13 +261,15 @@ class basic_storage: public basic_sparse_set<Entity, typename std::allocator_tra
         const auto from = (sz + traits_type::page_size - 1u) / traits_type::page_size;
         allocator_type allocator{get_allocator()};
 
-        for(auto pos = sz, length = base_type::size(); pos < length; ++pos) {
-            if constexpr(traits_type::in_place_delete) {
-                if(base_type::data()[pos] != tombstone) {
+        if constexpr(!std::is_trivially_destructible_v<element_type>) {
+            for(auto pos = sz, length = base_type::size(); pos < length; ++pos) {
+                if constexpr(traits_type::in_place_delete) {
+                    if(base_type::data()[pos] != tombstone) {
+                        alloc_traits::destroy(allocator, std::addressof(element_at(pos)));
+                    }
+                } else {
                     alloc_traits::destroy(allocator, std::addressof(element_at(pos)));
                 }
-            } else {
-                alloc_traits::destroy(allocator, std::addressof(element_at(pos)));
             }
         }
 
@@ -296,6 +278,19 @@ class basic_storage: public basic_sparse_set<Entity, typename std::allocator_tra
         }
 
         payload.resize(from);
+        payload.shrink_to_fit();
+    }
+
+    void swap_at(const std::size_t lhs, const std::size_t rhs) {
+        using std::swap;
+        swap(element_at(lhs), element_at(rhs));
+    }
+
+    void move_to(const std::size_t lhs, const std::size_t rhs) {
+        auto &elem = element_at(lhs);
+        allocator_type allocator{get_allocator()};
+        entt::uninitialized_construct_using_allocator(stl::to_address(assure_at_least(rhs)), allocator, std::move(elem));
+        alloc_traits::destroy(allocator, std::addressof(elem));
     }
 
 private:
@@ -304,24 +299,16 @@ private:
     }
 
     void swap_or_move([[maybe_unused]] const std::size_t from, [[maybe_unused]] const std::size_t to) override {
-        static constexpr bool is_pinned_type_v = !(std::is_move_constructible_v<Type> && std::is_move_assignable_v<Type>);
+        static constexpr bool is_pinned_type = !(std::is_move_constructible_v<Type> && std::is_move_assignable_v<Type>);
         // use a runtime value to avoid compile-time suppression that drives the code coverage tool crazy
-        ENTT_ASSERT((from + 1u) && !is_pinned_type_v, "Pinned type");
-
-        if constexpr(!is_pinned_type_v) {
-            auto &elem = element_at(from);
+        ENTT_ASSERT((from + 1u) && !is_pinned_type, "Pinned type");
 
+        if constexpr(!is_pinned_type) {
             if constexpr(traits_type::in_place_delete) {
-                if(base_type::operator[](to) == tombstone) {
-                    allocator_type allocator{get_allocator()};
-                    entt::uninitialized_construct_using_allocator(to_address(assure_at_least(to)), allocator, std::move(elem));
-                    alloc_traits::destroy(allocator, std::addressof(elem));
-                    return;
-                }
+                (base_type::operator[](to) == tombstone) ? move_to(from, to) : swap_at(from, to);
+            } else {
+                swap_at(from, to);
             }
-
-            using std::swap;
-            swap(elem, element_at(to));
         }
     }
 
@@ -337,31 +324,38 @@ protected:
             auto &elem = element_at(base_type::index(*first));
 
             if constexpr(traits_type::in_place_delete) {
-                base_type::in_place_pop(first);
+                base_type::in_place_pop(*first);
                 alloc_traits::destroy(allocator, std::addressof(elem));
+            } else if constexpr(std::is_trivially_destructible_v<element_type>) {
+                elem = std::move(element_at(base_type::size() - 1u));
+                base_type::swap_and_pop(*first);
             } else {
                 auto &other = element_at(base_type::size() - 1u);
                 // destroying on exit allows reentrant destructors
                 [[maybe_unused]] auto unused = std::exchange(elem, std::move(other));
                 alloc_traits::destroy(allocator, std::addressof(other));
-                base_type::swap_and_pop(first);
+                base_type::swap_and_pop(*first);
             }
         }
     }
 
     /*! @brief Erases all entities of a storage. */
     void pop_all() override {
-        allocator_type allocator{get_allocator()};
+        if constexpr(std::is_trivially_destructible_v<element_type>) {
+            base_type::pop_all();
+        } else {
+            allocator_type allocator{get_allocator()};
 
-        for(auto first = base_type::begin(); !(first.index() < 0); ++first) {
-            if constexpr(traits_type::in_place_delete) {
-                if(*first != tombstone) {
-                    base_type::in_place_pop(first);
+            for(auto first = base_type::begin(); !(first.index() < 0); ++first) {
+                if constexpr(traits_type::in_place_delete) {
+                    if(*first != tombstone) {
+                        base_type::in_place_pop(*first);
+                        alloc_traits::destroy(allocator, std::addressof(element_at(static_cast<size_type>(first.index()))));
+                    }
+                } else {
+                    base_type::swap_and_pop(*first);
                     alloc_traits::destroy(allocator, std::addressof(element_at(static_cast<size_type>(first.index()))));
                 }
-            } else {
-                base_type::swap_and_pop(first);
-                alloc_traits::destroy(allocator, std::addressof(element_at(static_cast<size_type>(first.index()))));
             }
         }
     }
@@ -402,14 +396,16 @@ public:
     using entity_type = Entity;
     /*! @brief Unsigned integer type. */
     using size_type = std::size_t;
+    /*! @brief Signed integer type. */
+    using difference_type = std::ptrdiff_t;
     /*! @brief Pointer type to contained elements. */
-    using pointer = typename container_type::pointer;
+    using pointer = container_type::pointer;
     /*! @brief Constant pointer type to contained elements. */
-    using const_pointer = typename alloc_traits::template rebind_traits<typename alloc_traits::const_pointer>::const_pointer;
+    using const_pointer = alloc_traits::template rebind_traits<typename alloc_traits::const_pointer>::const_pointer;
     /*! @brief Random access iterator type. */
-    using iterator = internal::storage_iterator<container_type>;
+    using iterator = internal::storage_iterator<container_type, traits_type::page_size>;
     /*! @brief Constant random access iterator type. */
-    using const_iterator = internal::storage_iterator<const container_type>;
+    using const_iterator = internal::storage_iterator<const container_type, traits_type::page_size>;
     /*! @brief Reverse iterator type. */
     using reverse_iterator = std::reverse_iterator<iterator>;
     /*! @brief Constant reverse iterator type. */
@@ -445,7 +441,7 @@ public:
      * @param other The instance to move from.
      */
     basic_storage(basic_storage &&other) noexcept
-        : base_type{std::move(other)},
+        : base_type{static_cast<base_type &&>(other)},
           payload{std::move(other.payload)} {}
 
     /**
@@ -454,9 +450,8 @@ public:
      * @param allocator The allocator to use.
      */
     basic_storage(basic_storage &&other, const allocator_type &allocator)
-        : base_type{std::move(other), allocator},
+        : base_type{static_cast<base_type &&>(other), allocator},
           payload{std::move(other.payload), allocator} {
-        // NOLINTNEXTLINE(bugprone-use-after-move)
         ENTT_ASSERT(alloc_traits::is_always_equal::value || get_allocator() == other.get_allocator(), "Copying a storage is not allowed");
     }
 
@@ -552,7 +547,7 @@ public:
      * @return An iterator to the first instance of the internal array.
      */
     [[nodiscard]] const_iterator cbegin() const noexcept {
-        const auto pos = static_cast<typename iterator::difference_type>(base_type::size());
+        const auto pos = static_cast<difference_type>(base_type::size());
         return const_iterator{&payload, pos};
     }
 
@@ -563,7 +558,7 @@ public:
 
     /*! @copydoc begin */
     [[nodiscard]] iterator begin() noexcept {
-        const auto pos = static_cast<typename iterator::difference_type>(base_type::size());
+        const auto pos = static_cast<difference_type>(base_type::size());
         return iterator{&payload, pos};
     }
 
@@ -705,14 +700,12 @@ public:
      * Attempting to assign an entity that already belongs to the storage
      * results in undefined behavior.
      *
-     * @tparam It Type of input iterator.
      * @param first An iterator to the first element of the range of entities.
      * @param last An iterator past the last element of the range of entities.
      * @param value An instance of the object to construct.
      * @return Iterator pointing to the first element inserted, if any.
      */
-    template<typename It>
-    iterator insert(It first, It last, const value_type &value = {}) {
+    iterator insert(stl::input_iterator auto first, stl::input_iterator auto last, const value_type &value = {}) {
         for(; first != last; ++first) {
             emplace_element(*first, true, value);
         }
@@ -726,15 +719,15 @@ public:
      *
      * @sa construct
      *
-     * @tparam EIt Type of input iterator.
-     * @tparam CIt Type of input iterator.
+     * @tparam It Type of input iterator.
      * @param first An iterator to the first element of the range of entities.
      * @param last An iterator past the last element of the range of entities.
      * @param from An iterator to the first element of the range of objects.
      * @return Iterator pointing to the first element inserted, if any.
      */
-    template<typename EIt, typename CIt, typename = std::enable_if_t<std::is_same_v<typename std::iterator_traits<CIt>::value_type, value_type>>>
-    iterator insert(EIt first, EIt last, CIt from) {
+    template<stl::input_iterator It>
+    requires std::same_as<typename std::iterator_traits<It>::value_type, value_type>
+    iterator insert(stl::input_iterator auto first, stl::input_iterator auto last, It from) {
         for(; first != last; ++first, ++from) {
             emplace_element(*first, true, *from);
         }
@@ -781,11 +774,12 @@ private:
 
 /*! @copydoc basic_storage */
 template<typename Type, typename Entity, typename Allocator>
-class basic_storage<Type, Entity, Allocator, std::enable_if_t<component_traits<Type>::page_size == 0u>>
+requires (component_traits<Type, Entity>::page_size == 0u)
+class basic_storage<Type, Entity, Allocator>
     : public basic_sparse_set<Entity, typename std::allocator_traits<Allocator>::template rebind_alloc<Entity>> {
     using alloc_traits = std::allocator_traits<Allocator>;
     static_assert(std::is_same_v<typename alloc_traits::value_type, Type>, "Invalid value type");
-    using traits_type = component_traits<Type>;
+    using traits_type = component_traits<Type, Entity>;
 
 public:
     /*! @brief Allocator type. */
@@ -800,6 +794,8 @@ public:
     using entity_type = Entity;
     /*! @brief Unsigned integer type. */
     using size_type = std::size_t;
+    /*! @brief Signed integer type. */
+    using difference_type = std::ptrdiff_t;
     /*! @brief Extended iterable storage proxy. */
     using iterable = iterable_adaptor<internal::extended_storage_iterator<typename base_type::iterator>>;
     /*! @brief Constant extended iterable storage proxy. */
@@ -860,12 +856,7 @@ public:
      * @return The associated allocator.
      */
     [[nodiscard]] constexpr allocator_type get_allocator() const noexcept {
-        // std::allocator<void> has no cross constructors (waiting for C++20)
-        if constexpr(std::is_void_v<element_type> && !std::is_constructible_v<allocator_type, typename base_type::allocator_type>) {
-            return allocator_type{};
-        } else {
-            return allocator_type{base_type::get_allocator()};
-        }
+        return allocator_type{base_type::get_allocator()};
     }
 
     /**
@@ -898,11 +889,9 @@ public:
      * Attempting to use an entity that already belongs to the storage results
      * in undefined behavior.
      *
-     * @tparam Args Types of arguments to use to construct the object.
      * @param entt A valid identifier.
      */
-    template<typename... Args>
-    void emplace(const entity_type entt, Args &&...) {
+    void emplace(const entity_type entt, const auto &...) {
         base_type::try_emplace(entt, false);
     }
 
@@ -920,13 +909,10 @@ public:
 
     /**
      * @brief Assigns entities to a storage.
-     * @tparam It Type of input iterator.
-     * @tparam Args Types of optional arguments.
      * @param first An iterator to the first element of the range of entities.
      * @param last An iterator past the last element of the range of entities.
      */
-    template<typename It, typename... Args>
-    void insert(It first, It last, Args &&...) {
+    void insert(stl::input_iterator auto first, stl::input_iterator auto last, const auto &...) {
         for(; first != last; ++first) {
             base_type::try_emplace(*first, true);
         }
@@ -975,16 +961,22 @@ class basic_storage<Entity, Entity, Allocator>
     : public basic_sparse_set<Entity, Allocator> {
     using alloc_traits = std::allocator_traits<Allocator>;
     static_assert(std::is_same_v<typename alloc_traits::value_type, Entity>, "Invalid value type");
-    using underlying_iterator = typename basic_sparse_set<Entity, Allocator>::basic_iterator;
+    using underlying_iterator = basic_sparse_set<Entity, Allocator>::basic_iterator;
     using traits_type = entt_traits<Entity>;
 
-    auto next() noexcept {
-        entity_type entt = null;
+    auto from_placeholder() noexcept {
+        const auto entt = traits_type::combine(static_cast<traits_type::entity_type>(placeholder), {});
+        ENTT_ASSERT(entt != null, "No more entities available");
+        placeholder += static_cast<size_type>(entt != null);
+        return entt;
+    }
 
-        do {
-            ENTT_ASSERT(placeholder < traits_type::to_entity(null), "No more entities available");
-            entt = traits_type::combine(static_cast<typename traits_type::entity_type>(placeholder++), {});
-        } while(base_type::current(entt) != traits_type::to_version(tombstone) && entt != null);
+    auto next() noexcept {
+        entity_type entt = from_placeholder();
+
+        while(base_type::current(entt) != traits_type::to_version(tombstone) && entt != null) {
+            entt = from_placeholder();
+        }
 
         return entt;
     }
@@ -1002,7 +994,7 @@ protected:
      * @return Iterator pointing to the emplaced element.
      */
     underlying_iterator try_emplace(const Entity hint, const bool, const void *) override {
-        return base_type::find(emplace(hint));
+        return base_type::find(generate(hint));
     }
 
 public:
@@ -1018,6 +1010,8 @@ public:
     using entity_type = Entity;
     /*! @brief Unsigned integer type. */
     using size_type = std::size_t;
+    /*! @brief Signed integer type. */
+    using difference_type = std::ptrdiff_t;
     /*! @brief Extended iterable storage proxy. */
     using iterable = iterable_adaptor<internal::extended_storage_iterator<typename base_type::iterator>>;
     /*! @brief Constant extended iterable storage proxy. */
@@ -1048,8 +1042,9 @@ public:
      * @brief Move constructor.
      * @param other The instance to move from.
      */
+    // NOLINTNEXTLINE(cppcoreguidelines-rvalue-reference-param-not-moved)
     basic_storage(basic_storage &&other) noexcept
-        : base_type{std::move(other)},
+        : base_type{static_cast<base_type &&>(other)},
           placeholder{other.placeholder} {}
 
     /**
@@ -1057,8 +1052,9 @@ public:
      * @param other The instance to move from.
      * @param allocator The allocator to use.
      */
+    // NOLINTNEXTLINE(cppcoreguidelines-rvalue-reference-param-not-moved)
     basic_storage(basic_storage &&other, const allocator_type &allocator)
-        : base_type{std::move(other), allocator},
+        : base_type{static_cast<base_type &&>(other), allocator},
           placeholder{other.placeholder} {}
 
     /*! @brief Default destructor. */
@@ -1081,6 +1077,16 @@ public:
         return *this;
     }
 
+    /**
+     * @brief Exchanges the contents with those of a given storage.
+     * @param other Storage to exchange the content with.
+     */
+    void swap(basic_storage &other) noexcept {
+        using std::swap;
+        swap(placeholder, other.placeholder);
+        base_type::swap(other);
+    }
+
     /**
      * @brief Returns the object assigned to an entity, that is `void`.
      *
@@ -1108,7 +1114,7 @@ public:
      * @brief Creates a new identifier or recycles a destroyed one.
      * @return A valid identifier.
      */
-    entity_type emplace() {
+    entity_type generate() {
         const auto len = base_type::free_list();
         const auto entt = (len == base_type::size()) ? next() : base_type::data()[len];
         return *base_type::try_emplace(entt, true);
@@ -1123,14 +1129,31 @@ public:
      * @param hint Required identifier.
      * @return A valid identifier.
      */
-    entity_type emplace(const entity_type hint) {
+    entity_type generate(const entity_type hint) {
         if(hint != null && hint != tombstone) {
             if(const auto curr = traits_type::construct(traits_type::to_entity(hint), base_type::current(hint)); curr == tombstone || !(base_type::index(curr) < base_type::free_list())) {
                 return *base_type::try_emplace(hint, true);
             }
         }
 
-        return emplace();
+        return generate();
+    }
+
+    /**
+     * @brief Assigns each element in a range an identifier.
+     * @tparam It Type of output iterator.
+     * @param first An iterator to the first element of the range to generate.
+     * @param last An iterator past the last element of the range to generate.
+     */
+    template<stl::output_iterator<entity_type> It>
+    void generate(It first, It last) {
+        for(const auto sz = base_type::size(); first != last && base_type::free_list() != sz; ++first) {
+            *first = *base_type::try_emplace(base_type::data()[base_type::free_list()], true);
+        }
+
+        for(; first != last; ++first) {
+            *first = *base_type::try_emplace(next(), true);
+        }
     }
 
     /**
@@ -1145,23 +1168,6 @@ public:
         (std::forward<Func>(func)(), ...);
     }
 
-    /**
-     * @brief Assigns each element in a range an identifier.
-     * @tparam It Type of mutable forward iterator.
-     * @param first An iterator to the first element of the range to generate.
-     * @param last An iterator past the last element of the range to generate.
-     */
-    template<typename It>
-    void insert(It first, It last) {
-        for(const auto sz = base_type::size(); first != last && base_type::free_list() != sz; ++first) {
-            *first = *base_type::try_emplace(base_type::data()[base_type::free_list()], true);
-        }
-
-        for(; first != last; ++first) {
-            *first = *base_type::try_emplace(next(), true);
-        }
-    }
-
     /**
      * @brief Returns an iterable object to use to _visit_ a storage.
      *
@@ -1176,7 +1182,8 @@ public:
     /*! @copydoc each */
     [[nodiscard]] const_iterable each() const noexcept {
         const auto it = base_type::cend();
-        return const_iterable{it - base_type::free_list(), it};
+        const auto offset = static_cast<difference_type>(base_type::free_list());
+        return const_iterable{it - offset, it};
     }
 
     /**
@@ -1193,7 +1200,19 @@ public:
     /*! @copydoc reach */
     [[nodiscard]] const_reverse_iterable reach() const noexcept {
         const auto it = base_type::crbegin();
-        return const_reverse_iterable{it, it + base_type::free_list()};
+        const auto offset = static_cast<difference_type>(base_type::free_list());
+        return const_reverse_iterable{it, it + offset};
+    }
+
+    /**
+     * @brief Sets the starting identifier for generation.
+     *
+     * The version is ignored, regardless of the value.
+     *
+     * @param hint A valid identifier.
+     */
+    void start_from(const entity_type hint) {
+        placeholder = static_cast<size_type>(traits_type::to_entity(hint));
     }
 
 private:

src/entt/entity/view.hpp

@@ -2,12 +2,14 @@
 #define ENTT_ENTITY_VIEW_HPP
 
 #include <array>
+#include <concepts>
 #include <cstddef>
 #include <iterator>
 #include <tuple>
 #include <type_traits>
 #include <utility>
 #include "../config/config.h"
+#include "../core/concepts.hpp"
 #include "../core/iterator.hpp"
 #include "../core/type_traits.hpp"
 #include "entity.hpp"
@@ -15,31 +17,32 @@
 
 namespace entt {
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
-template<typename Type>
+template<typename... Type>
+// NOLINTNEXTLINE(misc-redundant-expression)
+static constexpr bool tombstone_check_v = ((sizeof...(Type) == 1u) && ... && (Type::storage_policy == deletion_policy::in_place));
+
+template<cvref_unqualified Type>
 const Type *view_placeholder() {
-    static_assert(std::is_same_v<std::remove_const_t<std::remove_reference_t<Type>>, Type>, "Unexpected type");
     static const Type placeholder{};
     return &placeholder;
 }
 
-template<typename It, typename Entity>
-[[nodiscard]] bool all_of(It first, const It last, const Entity entt) noexcept {
+[[nodiscard]] bool all_of(auto first, const auto last, const auto entt) noexcept {
     for(; (first != last) && (*first)->contains(entt); ++first) {}
     return first == last;
 }
 
-template<typename It, typename Entity>
-[[nodiscard]] bool none_of(It first, const It last, const Entity entt) noexcept {
+[[nodiscard]] bool none_of(auto first, const auto last, const auto entt) noexcept {
     for(; (first != last) && !(*first)->contains(entt); ++first) {}
     return first == last;
 }
 
 template<typename It>
-[[nodiscard]] bool fully_initialized(It first, const It last) noexcept {
-    for(const auto *placeholder = view_placeholder<std::remove_const_t<std::remove_pointer_t<typename std::iterator_traits<It>::value_type>>>(); (first != last) && *first != placeholder; ++first) {}
+[[nodiscard]] bool fully_initialized(It first, const It last, const std::remove_pointer_t<typename std::iterator_traits<It>::value_type> *placeholder) noexcept {
+    for(; (first != last) && *first != placeholder; ++first) {}
     return first == last;
 }
 
@@ -48,23 +51,24 @@ template<typename Result, typename View, typename Other, std::size_t... GLhs, st
     Result elem{};
     // friend-initialization, avoid multiple calls to refresh
     elem.pools = {view.template storage<GLhs>()..., other.template storage<GRhs>()...};
-    elem.filter = {view.template storage<sizeof...(GLhs) + ELhs>()..., other.template storage<sizeof...(GRhs) + ERhs>()...};
+    [[maybe_unused]] const auto filter_or_placeholder = [placeholder = elem.placeholder](auto *value) { return (value == nullptr) ? placeholder : value; };
+    elem.filter = {filter_or_placeholder(view.template storage<sizeof...(GLhs) + ELhs>())..., filter_or_placeholder(other.template storage<sizeof...(GRhs) + ERhs>())...};
     elem.refresh();
     return elem;
 }
 
-template<typename Type, std::size_t Get, std::size_t Exclude>
+template<typename Type, bool Checked, std::size_t Get, std::size_t Exclude>
 class view_iterator final {
     template<typename, typename...>
-    friend class extended_view_iterator;
+    friend struct extended_view_iterator;
 
-    using iterator_type = typename Type::const_iterator;
+    using iterator_type = Type::const_iterator;
     using iterator_traits = std::iterator_traits<iterator_type>;
 
-    [[nodiscard]] bool valid(const typename iterator_traits::value_type entt) const noexcept {
-        return ((Get != 1u) || (entt != tombstone))
-               && internal::all_of(pools.begin(), pools.begin() + index, entt) && internal::all_of(pools.begin() + index + 1, pools.end(), entt)
-               && internal::none_of(filter.begin(), filter.end(), entt);
+    [[nodiscard]] bool valid(const iterator_traits::value_type entt) const noexcept {
+        return (!Checked || (entt != tombstone))
+               && ((Get == 1u) || (internal::all_of(pools.begin(), pools.begin() + index, entt) && internal::all_of(pools.begin() + index + 1, pools.end(), entt)))
+               && ((Exclude == 0u) || internal::none_of(filter.begin(), filter.end(), entt));
     }
 
     void seek_next() {
@@ -72,10 +76,10 @@ class view_iterator final {
     }
 
 public:
-    using value_type = typename iterator_traits::value_type;
-    using pointer = typename iterator_traits::pointer;
-    using reference = typename iterator_traits::reference;
-    using difference_type = typename iterator_traits::difference_type;
+    using value_type = iterator_traits::value_type;
+    using pointer = iterator_traits::pointer;
+    using reference = iterator_traits::reference;
+    using difference_type = iterator_traits::difference_type;
     using iterator_category = std::forward_iterator_tag;
 
     constexpr view_iterator() noexcept
@@ -88,7 +92,8 @@ public:
         : it{first},
           pools{value},
           filter{excl},
-          index{idx} {
+          index{static_cast<difference_type>(idx)} {
+        ENTT_ASSERT((Get != 1u) || (Exclude != 0u) || pools[0u]->policy() == deletion_policy::in_place, "Non in-place storage view iterator");
         seek_next();
     }
 
@@ -99,7 +104,7 @@ public:
     }
 
     view_iterator operator++(int) noexcept {
-        view_iterator orig = *this;
+        const view_iterator orig = *this;
         return ++(*this), orig;
     }
 
@@ -111,34 +116,20 @@ public:
         return *operator->();
     }
 
-    template<typename LhsType, auto... LhsArgs, typename RhsType, auto... RhsArgs>
-    friend constexpr bool operator==(const view_iterator<LhsType, LhsArgs...> &, const view_iterator<RhsType, RhsArgs...> &) noexcept;
+    template<typename Other, auto... Args>
+    [[nodiscard]] constexpr bool operator==(const view_iterator<Other, Args...> &other) const noexcept {
+        return it == other.it;
+    }
 
 private:
     iterator_type it;
     std::array<const Type *, Get> pools;
     std::array<const Type *, Exclude> filter;
-    std::size_t index;
+    difference_type index;
 };
 
-template<typename LhsType, auto... LhsArgs, typename RhsType, auto... RhsArgs>
-[[nodiscard]] constexpr bool operator==(const view_iterator<LhsType, LhsArgs...> &lhs, const view_iterator<RhsType, RhsArgs...> &rhs) noexcept {
-    return lhs.it == rhs.it;
-}
-
-template<typename LhsType, auto... LhsArgs, typename RhsType, auto... RhsArgs>
-[[nodiscard]] constexpr bool operator!=(const view_iterator<LhsType, LhsArgs...> &lhs, const view_iterator<RhsType, RhsArgs...> &rhs) noexcept {
-    return !(lhs == rhs);
-}
-
 template<typename It, typename... Get>
-class extended_view_iterator final {
-    template<std::size_t... Index>
-    [[nodiscard]] auto dereference(std::index_sequence<Index...>) const noexcept {
-        return std::tuple_cat(std::make_tuple(*it), static_cast<Get *>(const_cast<constness_as_t<typename Get::base_type, Get> *>(std::get<Index>(it.pools)))->get_as_tuple(*it)...);
-    }
-
-public:
+struct extended_view_iterator final {
     using iterator_type = It;
     using value_type = decltype(std::tuple_cat(std::make_tuple(*std::declval<It>()), std::declval<Get>().get_as_tuple({})...));
     using pointer = input_iterator_pointer<value_type>;
@@ -158,12 +149,14 @@ public:
     }
 
     extended_view_iterator operator++(int) noexcept {
-        extended_view_iterator orig = *this;
+        const extended_view_iterator orig = *this;
         return ++(*this), orig;
     }
 
     [[nodiscard]] reference operator*() const noexcept {
-        return dereference(std::index_sequence_for<Get...>{});
+        return [this]<auto... Index>(std::index_sequence<Index...>) {
+            return std::tuple_cat(std::make_tuple(*it), static_cast<Get *>(const_cast<constness_as_t<typename Get::base_type, Get> *>(std::get<Index>(it.pools)))->get_as_tuple(*it)...);
+        }(std::index_sequence_for<Get...>{});
     }
 
     [[nodiscard]] pointer operator->() const noexcept {
@@ -174,23 +167,15 @@ public:
         return it;
     }
 
-    template<typename... Lhs, typename... Rhs>
-    friend bool constexpr operator==(const extended_view_iterator<Lhs...> &, const extended_view_iterator<Rhs...> &) noexcept;
+    template<typename... Other>
+    [[nodiscard]] constexpr bool operator==(const extended_view_iterator<Other...> &other) const noexcept {
+        return it == other.it;
+    }
 
 private:
     It it;
 };
 
-template<typename... Lhs, typename... Rhs>
-[[nodiscard]] constexpr bool operator==(const extended_view_iterator<Lhs...> &lhs, const extended_view_iterator<Rhs...> &rhs) noexcept {
-    return lhs.it == rhs.it;
-}
-
-template<typename... Lhs, typename... Rhs>
-[[nodiscard]] constexpr bool operator!=(const extended_view_iterator<Lhs...> &lhs, const extended_view_iterator<Rhs...> &rhs) noexcept {
-    return !(lhs == rhs);
-}
-
 } // namespace internal
 /*! @endcond */
 
@@ -212,20 +197,19 @@ template<typename... Lhs, typename... Rhs>
  * In all other cases, modifying the storage iterated by a view in any way can
  * invalidate all iterators.
  */
-template<typename, typename, typename>
+template<typename, typename>
 class basic_view;
 
 /**
  * @brief Basic storage view implementation.
  * @warning For internal use only, backward compatibility not guaranteed.
  * @tparam Type Common type among all storage types.
+ * @tparam Checked True to enable the tombstone check, false otherwise.
  * @tparam Get Number of storage iterated by the view.
  * @tparam Exclude Number of storage used to filter the view.
  */
-template<typename Type, std::size_t Get, std::size_t Exclude>
+template<cvref_unqualified Type, bool Checked, std::size_t Get, std::size_t Exclude>
 class basic_common_view {
-    static_assert(std::is_same_v<std::remove_const_t<std::remove_reference_t<Type>>, Type>, "Unexpected type");
-
     template<typename Return, typename View, typename Other, std::size_t... GLhs, std::size_t... ELhs, std::size_t... GRhs, std::size_t... ERhs>
     friend Return internal::view_pack(const View &, const Other &, std::index_sequence<GLhs...>, std::index_sequence<ELhs...>, std::index_sequence<GRhs...>, std::index_sequence<ERhs...>);
 
@@ -247,10 +231,10 @@ class basic_common_view {
     }
 
 protected:
-    /*! @cond TURN_OFF_DOXYGEN */
+    /*! @cond ENTT_INTERNAL */
     basic_common_view() noexcept {
-        for(size_type pos{}; pos < Exclude; ++pos) {
-            filter[pos] = internal::view_placeholder<Type>();
+        for(size_type pos{}, last = filter.size(); pos < last; ++pos) {
+            filter[pos] = placeholder;
         }
     }
 
@@ -265,30 +249,22 @@ protected:
         return pools[pos];
     }
 
-    [[nodiscard]] const Type *storage(const std::size_t pos) const noexcept {
-        if(pos < Get) {
-            return pools[pos];
-        }
-
-        if(const auto idx = pos - Get; filter[idx] != internal::view_placeholder<Type>()) {
-            return filter[idx];
-        }
-
-        return nullptr;
-    }
-
-    void storage(const std::size_t pos, const Type *elem) noexcept {
+    void pool_at(const std::size_t pos, const Type *elem) noexcept {
         ENTT_ASSERT(elem != nullptr, "Unexpected element");
-
-        if(pos < Get) {
-            pools[pos] = elem;
-            refresh();
-        } else {
-            filter[pos - Get] = elem;
-        }
+        pools[pos] = elem;
+        refresh();
     }
 
-    [[nodiscard]] bool none_of(const typename Type::entity_type entt) const noexcept {
+    [[nodiscard]] const Type *filter_at(const std::size_t pos) const noexcept {
+        return (filter[pos] == placeholder) ? nullptr : filter[pos];
+    }
+
+    void filter_at(const std::size_t pos, const Type *elem) noexcept {
+        ENTT_ASSERT(elem != nullptr, "Unexpected element");
+        filter[pos] = elem;
+    }
+
+    [[nodiscard]] bool none_of(const Type::entity_type entt) const noexcept {
         return internal::none_of(filter.begin(), filter.end(), entt);
     }
 
@@ -301,11 +277,13 @@ public:
     /*! @brief Common type among all storage types. */
     using common_type = Type;
     /*! @brief Underlying entity identifier. */
-    using entity_type = typename Type::entity_type;
+    using entity_type = Type::entity_type;
     /*! @brief Unsigned integer type. */
     using size_type = std::size_t;
+    /*! @brief Signed integer type. */
+    using difference_type = std::ptrdiff_t;
     /*! @brief Forward iterator type. */
-    using iterator = internal::view_iterator<common_type, Get, Exclude>;
+    using iterator = internal::view_iterator<common_type, Checked, Get, Exclude>;
 
     /*! @brief Updates the internal leading view if required. */
     void refresh() noexcept {
@@ -341,7 +319,7 @@ public:
      * @return An iterator to the first entity of the view.
      */
     [[nodiscard]] iterator begin() const noexcept {
-        return (index != Get) ? iterator{pools[index]->end() - static_cast<typename iterator::difference_type>(offset()), pools, filter, index} : iterator{};
+        return (index != Get) ? iterator{pools[index]->end() - static_cast<difference_type>(offset()), pools, filter, index} : iterator{};
     }
 
     /**
@@ -370,8 +348,8 @@ public:
     [[nodiscard]] entity_type back() const noexcept {
         if(index != Get) {
             auto it = pools[index]->rbegin();
-            const auto last = it + static_cast<typename iterator::difference_type>(offset());
-            for(; it != last && !contains(*it); ++it) {}
+            const auto last = it + static_cast<difference_type>(offset());
+            for(const auto idx = static_cast<difference_type>(index); it != last && !(internal::all_of(pools.begin(), pools.begin() + idx, *it) && internal::all_of(pools.begin() + idx + 1, pools.end(), *it) && internal::none_of(filter.begin(), filter.end(), *it)); ++it) {}
             return it == last ? null : *it;
         }
 
@@ -393,7 +371,7 @@ public:
      * @return True if the view is fully initialized, false otherwise.
      */
     [[nodiscard]] explicit operator bool() const noexcept {
-        return (index != Get) && internal::fully_initialized(filter.begin(), filter.end());
+        return (index != Get) && internal::fully_initialized(filter.begin(), filter.end(), placeholder);
     }
 
     /**
@@ -411,6 +389,7 @@ public:
 private:
     std::array<const common_type *, Get> pools{};
     std::array<const common_type *, Exclude> filter{};
+    const common_type *placeholder{internal::view_placeholder<common_type>()};
     size_type index{Get};
 };
 
@@ -427,18 +406,17 @@ private:
  * @tparam Exclude Types of storage used to filter the view.
  */
 template<typename... Get, typename... Exclude>
-class basic_view<get_t<Get...>, exclude_t<Exclude...>, std::enable_if_t<(sizeof...(Get) != 0u)>>
-    : public basic_common_view<std::common_type_t<typename Get::base_type..., typename Exclude::base_type...>, sizeof...(Get), sizeof...(Exclude)> {
-    using base_type = basic_common_view<std::common_type_t<typename Get::base_type..., typename Exclude::base_type...>, sizeof...(Get), sizeof...(Exclude)>;
+requires (sizeof...(Get) != 0u)
+class basic_view<get_t<Get...>, exclude_t<Exclude...>>
+    : public basic_common_view<std::common_type_t<typename Get::base_type...>, internal::tombstone_check_v<Get...>, sizeof...(Get), sizeof...(Exclude)> {
+    using base_type = basic_common_view<std::common_type_t<typename Get::base_type...>, internal::tombstone_check_v<Get...>, sizeof...(Get), sizeof...(Exclude)>;
+
+    template<std::size_t Index>
+    using element_at = type_list_element_t<Index, type_list<Get..., Exclude...>>;
 
     template<typename Type>
     static constexpr std::size_t index_of = type_list_index_v<std::remove_const_t<Type>, type_list<typename Get::element_type..., typename Exclude::element_type...>>;
 
-    template<std::size_t... Index>
-    [[nodiscard]] auto get(const typename base_type::entity_type entt, std::index_sequence<Index...>) const noexcept {
-        return std::tuple_cat(storage<Index>()->get_as_tuple(entt)...);
-    }
-
     template<std::size_t Curr, std::size_t Other, typename... Args>
     [[nodiscard]] auto dispatch_get(const std::tuple<typename base_type::entity_type, Args...> &curr) const {
         if constexpr(Curr == Other) {
@@ -449,11 +427,9 @@ class basic_view<get_t<Get...>, exclude_t<Exclude...>, std::enable_if_t<(sizeof.
     }
 
     template<std::size_t Curr, typename Func, std::size_t... Index>
-    void each(Func &func, std::index_sequence<Index...>) const {
-        static constexpr bool tombstone_check_required = ((sizeof...(Get) == 1u) && ... && (Get::storage_policy == deletion_policy::in_place));
-
+    void each(Func func, std::index_sequence<Index...>) const {
         for(const auto curr: storage<Curr>()->each()) {
-            if(const auto entt = std::get<0>(curr); (!tombstone_check_required || (entt != tombstone)) && ((Curr == Index || base_type::pool_at(Index)->contains(entt)) && ...) && base_type::none_of(entt)) {
+            if(const auto entt = std::get<0>(curr); (!internal::tombstone_check_v<Get...> || (entt != tombstone)) && ((Curr == Index || base_type::pool_at(Index)->contains(entt)) && ...) && base_type::none_of(entt)) {
                 if constexpr(is_applicable_v<Func, decltype(std::tuple_cat(std::tuple<entity_type>{}, std::declval<basic_view>().get({})))>) {
                     std::apply(func, std::tuple_cat(std::make_tuple(entt), dispatch_get<Curr, Index>(curr)...));
                 } else {
@@ -463,22 +439,24 @@ class basic_view<get_t<Get...>, exclude_t<Exclude...>, std::enable_if_t<(sizeof.
         }
     }
 
-    template<typename Func, std::size_t... Index>
-    void pick_and_each(Func &func, std::index_sequence<Index...> seq) const {
-        if(const auto *view = base_type::handle(); view != nullptr) {
-            ((view == base_type::pool_at(Index) ? each<Index>(func, seq) : void()), ...);
+    template<typename Type>
+    void storage_if(Type *elem) noexcept {
+        if(elem != nullptr) {
+            storage<index_of<typename Type::element_type>>(*elem);
         }
     }
 
 public:
     /*! @brief Common type among all storage types. */
-    using common_type = typename base_type::common_type;
+    using common_type = base_type::common_type;
     /*! @brief Underlying entity identifier. */
-    using entity_type = typename base_type::entity_type;
+    using entity_type = base_type::entity_type;
     /*! @brief Unsigned integer type. */
-    using size_type = typename base_type::size_type;
+    using size_type = base_type::size_type;
+    /*! @brief Signed integer type. */
+    using difference_type = std::ptrdiff_t;
     /*! @brief Forward iterator type. */
-    using iterator = typename base_type::iterator;
+    using iterator = base_type::iterator;
     /*! @brief Iterable view type. */
     using iterable = iterable_adaptor<internal::extended_view_iterator<iterator, Get...>>;
 
@@ -503,6 +481,19 @@ public:
     basic_view(std::tuple<Get &...> value, std::tuple<Exclude &...> excl = {}) noexcept
         : basic_view{std::make_from_tuple<basic_view>(std::tuple_cat(value, excl))} {}
 
+    /**
+     * @brief Constructs a view from a convertible counterpart.
+     * @tparam Args Storage types managed by the other view.
+     * @param other A view to convert from.
+     */
+    template<typename... Args>
+    requires (!std::same_as<basic_view, basic_view<Args...>>)
+    basic_view(const basic_view<Args...> &other) noexcept
+        : basic_view{} {
+        (storage_if(other.template storage<typename Get::element_type>()), ...);
+        (storage_if(other.template storage<typename Exclude::element_type>()), ...);
+    }
+
     /**
      * @brief Forces a view to use a given element to drive iterations
      * @tparam Type Type of element to use to drive iterations.
@@ -538,8 +529,11 @@ public:
      */
     template<std::size_t Index>
     [[nodiscard]] auto *storage() const noexcept {
-        using type = type_list_element_t<Index, type_list<Get..., Exclude...>>;
-        return static_cast<type *>(const_cast<constness_as_t<common_type, type> *>(base_type::storage(Index)));
+        if constexpr(Index < sizeof...(Get)) {
+            return static_cast<element_at<Index> *>(const_cast<constness_as_t<common_type, element_at<Index>> *>(base_type::pool_at(Index)));
+        } else {
+            return static_cast<element_at<Index> *>(const_cast<constness_as_t<common_type, element_at<Index>> *>(base_type::filter_at(Index - sizeof...(Get))));
+        }
     }
 
     /**
@@ -560,8 +554,13 @@ public:
      */
     template<std::size_t Index, typename Type>
     void storage(Type &elem) noexcept {
-        static_assert(std::is_convertible_v<Type &, type_list_element_t<Index, type_list<Get..., Exclude...>> &>, "Unexpected type");
-        base_type::storage(Index, &elem);
+        static_assert(std::is_convertible_v<Type &, element_at<Index> &>, "Unexpected type");
+
+        if constexpr(Index < sizeof...(Get)) {
+            base_type::pool_at(Index, &elem);
+        } else {
+            base_type::filter_at(Index - sizeof...(Get), &elem);
+        }
     }
 
     /**
@@ -594,7 +593,9 @@ public:
     template<std::size_t... Index>
     [[nodiscard]] decltype(auto) get(const entity_type entt) const {
         if constexpr(sizeof...(Index) == 0) {
-            return get(entt, std::index_sequence_for<Get...>{});
+            return [this, entt]<auto... Idx>(std::index_sequence<Idx...>) {
+                return std::tuple_cat(this->storage<Idx>()->get_as_tuple(entt)...);
+            }(std::index_sequence_for<Get...>{});
         } else if constexpr(sizeof...(Index) == 1) {
             return (storage<Index>()->get(entt), ...);
         } else {
@@ -619,7 +620,11 @@ public:
      */
     template<typename Func>
     void each(Func func) const {
-        pick_and_each(func, std::index_sequence_for<Get...>{});
+        [this, &func]<auto... Index>(std::index_sequence<Index...> seq) {
+            if(const auto *view = base_type::handle(); view != nullptr) {
+                ((view == base_type::pool_at(Index) ? each<Index>(std::move(func), seq) : void()), ...);
+            }
+        }(std::index_sequence_for<Get...>{});
     }
 
     /**
@@ -635,6 +640,17 @@ public:
         return iterable{base_type::begin(), base_type::end()};
     }
 
+    /**
+     * @brief Combines a view and a storage in _more specific_ view.
+     * @tparam OGet Type of storage to combine the view with.
+     * @param other The storage for the type to combine the view with.
+     * @return A more specific view.
+     */
+    template<std::derived_from<common_type> OGet>
+    [[nodiscard]] basic_view<get_t<Get..., OGet>, exclude_t<Exclude...>> operator|(OGet &other) const noexcept {
+        return *this | basic_view<get_t<OGet>, exclude_t<>>{other};
+    }
+
     /**
      * @brief Combines two views in a _more specific_ one.
      * @tparam OGet Element list of the view to combine with.
@@ -642,7 +658,7 @@ public:
      * @param other The view to combine with.
      * @return A more specific view.
      */
-    template<typename... OGet, typename... OExclude>
+    template<std::derived_from<common_type>... OGet, std::derived_from<common_type>... OExclude>
     [[nodiscard]] auto operator|(const basic_view<get_t<OGet...>, exclude_t<OExclude...>> &other) const noexcept {
         return internal::view_pack<basic_view<get_t<Get..., OGet...>, exclude_t<Exclude..., OExclude...>>>(
             *this, other, std::index_sequence_for<Get...>{}, std::index_sequence_for<Exclude...>{}, std::index_sequence_for<OGet...>{}, std::index_sequence_for<OExclude...>{});
@@ -655,12 +671,10 @@ public:
  * @tparam Type Common type among all storage types.
  * @tparam Policy Storage policy.
  */
-template<typename Type, deletion_policy Policy>
+template<cvref_unqualified Type, deletion_policy Policy>
 class basic_storage_view {
-    static_assert(std::is_same_v<std::remove_const_t<std::remove_reference_t<Type>>, Type>, "Unexpected type");
-
 protected:
-    /*! @cond TURN_OFF_DOXYGEN */
+    /*! @cond ENTT_INTERNAL */
     basic_storage_view() noexcept = default;
 
     basic_storage_view(const Type *value) noexcept
@@ -673,11 +687,13 @@ public:
     /*! @brief Common type among all storage types. */
     using common_type = Type;
     /*! @brief Underlying entity identifier. */
-    using entity_type = typename common_type::entity_type;
+    using entity_type = common_type::entity_type;
     /*! @brief Unsigned integer type. */
     using size_type = std::size_t;
+    /*! @brief Signed integer type. */
+    using difference_type = std::ptrdiff_t;
     /*! @brief Random access iterator type. */
-    using iterator = std::conditional_t<Policy == deletion_policy::in_place, internal::view_iterator<common_type, 1u, 0u>, typename common_type::iterator>;
+    using iterator = std::conditional_t<Policy == deletion_policy::in_place, internal::view_iterator<common_type, true, 1u, 0u>, typename common_type::iterator>;
     /*! @brief Reverse iterator type. */
     using reverse_iterator = std::conditional_t<Policy == deletion_policy::in_place, void, typename common_type::reverse_iterator>;
 
@@ -691,11 +707,10 @@ public:
 
     /**
      * @brief Returns the number of entities that have the given element.
-     * @tparam Pol Dummy template parameter used for sfinae purposes only.
      * @return Number of entities that have the given element.
      */
-    template<typename..., deletion_policy Pol = Policy>
-    [[nodiscard]] std::enable_if_t<Pol != deletion_policy::in_place, size_type> size() const noexcept {
+    [[nodiscard]] size_type size() const noexcept
+    requires (Policy != deletion_policy::in_place) {
         if constexpr(Policy == deletion_policy::swap_and_pop) {
             return leading ? leading->size() : size_type{};
         } else {
@@ -706,21 +721,19 @@ public:
 
     /**
      * @brief Estimates the number of entities iterated by the view.
-     * @tparam Pol Dummy template parameter used for sfinae purposes only.
      * @return Estimated number of entities iterated by the view.
      */
-    template<typename..., deletion_policy Pol = Policy>
-    [[nodiscard]] std::enable_if_t<Pol == deletion_policy::in_place, size_type> size_hint() const noexcept {
+    [[nodiscard]] size_type size_hint() const noexcept
+    requires (Policy == deletion_policy::in_place) {
         return leading ? leading->size() : size_type{};
     }
 
     /**
      * @brief Checks whether a view is empty.
-     * @tparam Pol Dummy template parameter used for sfinae purposes only.
      * @return True if the view is empty, false otherwise.
      */
-    template<typename..., deletion_policy Pol = Policy>
-    [[nodiscard]] std::enable_if_t<Pol != deletion_policy::in_place, bool> empty() const noexcept {
+    [[nodiscard]] bool empty() const noexcept
+    requires (Policy != deletion_policy::in_place) {
         if constexpr(Policy == deletion_policy::swap_and_pop) {
             return !leading || leading->empty();
         } else {
@@ -740,7 +753,7 @@ public:
         if constexpr(Policy == deletion_policy::swap_and_pop) {
             return leading ? leading->begin() : iterator{};
         } else if constexpr(Policy == deletion_policy::swap_only) {
-            return leading ? (leading->end() - leading->free_list()) : iterator{};
+            return leading ? (leading->end() - static_cast<difference_type>(leading->free_list())) : iterator{};
         } else {
             static_assert(Policy == deletion_policy::in_place, "Unexpected storage policy");
             return leading ? iterator{leading->begin(), {leading}, {}, 0u} : iterator{};
@@ -765,28 +778,26 @@ public:
      *
      * If the view is empty, the returned iterator will be equal to `rend()`.
      *
-     * @tparam Pol Dummy template parameter used for sfinae purposes only.
      * @return An iterator to the first entity of the reversed view.
      */
-    template<typename..., deletion_policy Pol = Policy>
-    [[nodiscard]] std::enable_if_t<Pol != deletion_policy::in_place, reverse_iterator> rbegin() const noexcept {
+    [[nodiscard]] reverse_iterator rbegin() const noexcept
+    requires (Policy != deletion_policy::in_place) {
         return leading ? leading->rbegin() : reverse_iterator{};
     }
 
     /**
      * @brief Returns an iterator that is past the last entity of the reversed
      * view.
-     * @tparam Pol Dummy template parameter used for sfinae purposes only.
      * @return An iterator to the entity following the last entity of the
      * reversed view.
      */
-    template<typename..., deletion_policy Pol = Policy>
-    [[nodiscard]] std::enable_if_t<Pol != deletion_policy::in_place, reverse_iterator> rend() const noexcept {
+    [[nodiscard]] reverse_iterator rend() const noexcept
+    requires (Policy != deletion_policy::in_place) {
         if constexpr(Policy == deletion_policy::swap_and_pop) {
             return leading ? leading->rend() : reverse_iterator{};
         } else {
             static_assert(Policy == deletion_policy::swap_only, "Unexpected storage policy");
-            return leading ? (leading->rbegin() + leading->free_list()) : reverse_iterator{};
+            return leading ? (leading->rbegin() + static_cast<difference_type>(leading->free_list())) : reverse_iterator{};
         }
     }
 
@@ -799,7 +810,7 @@ public:
         if constexpr(Policy == deletion_policy::swap_and_pop) {
             return empty() ? null : *leading->begin();
         } else if constexpr(Policy == deletion_policy::swap_only) {
-            return empty() ? null : *(leading->end() - leading->free_list());
+            return empty() ? null : *(leading->end() - static_cast<difference_type>(leading->free_list()));
         } else {
             static_assert(Policy == deletion_policy::in_place, "Unexpected storage policy");
             const auto it = begin();
@@ -842,8 +853,7 @@ public:
             const auto it = leading ? leading->find(entt) : iterator{};
             return leading && (static_cast<size_type>(it.index()) < leading->free_list()) ? it : iterator{};
         } else {
-            const auto it = leading ? leading->find(entt) : typename common_type::iterator{};
-            return iterator{it, {leading}, {}, 0u};
+            return leading ? iterator{leading->find(entt), {leading}, {}, 0u} : iterator{};
         }
     }
 
@@ -888,17 +898,25 @@ class basic_view<get_t<Get>, exclude_t<>>
     : public basic_storage_view<typename Get::base_type, Get::storage_policy> {
     using base_type = basic_storage_view<typename Get::base_type, Get::storage_policy>;
 
+    void storage_if(Get *value) noexcept {
+        if(value != nullptr) {
+            storage(*value);
+        }
+    }
+
 public:
     /*! @brief Common type among all storage types. */
-    using common_type = typename base_type::common_type;
+    using common_type = base_type::common_type;
     /*! @brief Underlying entity identifier. */
-    using entity_type = typename base_type::entity_type;
+    using entity_type = base_type::entity_type;
     /*! @brief Unsigned integer type. */
-    using size_type = typename base_type::size_type;
+    using size_type = base_type::size_type;
+    /*! @brief Signed integer type. */
+    using difference_type = std::ptrdiff_t;
     /*! @brief Random access iterator type. */
-    using iterator = typename base_type::iterator;
+    using iterator = base_type::iterator;
     /*! @brief Reverse iterator type. */
-    using reverse_iterator = typename base_type::reverse_iterator;
+    using reverse_iterator = base_type::reverse_iterator;
     /*! @brief Iterable view type. */
     using iterable = std::conditional_t<Get::storage_policy == deletion_policy::in_place, iterable_adaptor<internal::extended_view_iterator<iterator, Get>>, decltype(std::declval<Get>().each())>;
 
@@ -921,12 +939,24 @@ public:
     basic_view(std::tuple<Get &> value, std::tuple<> = {}) noexcept
         : basic_view{std::get<0>(value)} {}
 
+    /**
+     * @brief Constructs a view from a convertible counterpart.
+     * @tparam Args Storage types managed by the other view.
+     * @param other A view to convert from.
+     */
+    template<typename... Args>
+    requires (!std::same_as<basic_view, basic_view<Args...>>)
+    basic_view(const basic_view<Args...> &other) noexcept
+        : base_type{} {
+        storage_if(other.template storage<typename Get::element_type>());
+    }
+
     /**
      * @brief Returns the storage for a given element type, if any.
      * @tparam Type Type of element of which to return the storage.
      * @return The storage for the given element type.
      */
-    template<typename Type = typename Get::element_type>
+    template<typename Type = Get::element_type>
     [[nodiscard]] auto *storage() const noexcept {
         static_assert(std::is_same_v<std::remove_const_t<Type>, typename Get::element_type>, "Invalid element type");
         return storage<0>();
@@ -1015,7 +1045,7 @@ public:
      *
      * @code{.cpp}
      * void(const entity_type, Type &);
-     * void(typename Type &);
+     * void(Type &);
      * @endcode
      *
      * @tparam Func Type of the function object to invoke.
@@ -1033,7 +1063,7 @@ public:
                     func();
                 }
             } else {
-                if(const auto len = base_type::size(); len != 0u) {
+                if(const auto len = static_cast<difference_type>(base_type::size()); len != 0) {
                     for(auto last = storage()->end(), first = last - len; first != last; ++first) {
                         func(*first);
                     }
@@ -1066,6 +1096,17 @@ public:
         }
     }
 
+    /**
+     * @brief Combines a view and a storage in _more specific_ view.
+     * @tparam OGet Type of storage to combine the view with.
+     * @param other The storage for the type to combine the view with.
+     * @return A more specific view.
+     */
+    template<std::derived_from<common_type> OGet>
+    [[nodiscard]] basic_view<get_t<Get, OGet>, exclude_t<>> operator|(OGet &other) const noexcept {
+        return *this | basic_view<get_t<OGet>, exclude_t<>>{other};
+    }
+
     /**
      * @brief Combines two views in a _more specific_ one.
      * @tparam OGet Element list of the view to combine with.
@@ -1073,7 +1114,7 @@ public:
      * @param other The view to combine with.
      * @return A more specific view.
      */
-    template<typename... OGet, typename... OExclude>
+    template<std::derived_from<common_type>... OGet, std::derived_from<common_type>... OExclude>
     [[nodiscard]] auto operator|(const basic_view<get_t<OGet...>, exclude_t<OExclude...>> &other) const noexcept {
         return internal::view_pack<basic_view<get_t<Get, OGet...>, exclude_t<OExclude...>>>(
             *this, other, std::index_sequence_for<Get>{}, std::index_sequence_for<>{}, std::index_sequence_for<OGet...>{}, std::index_sequence_for<OExclude...>{});

src/entt/entt.hpp

@@ -1,3 +1,6 @@
+/*! @brief `EnTT` default namespace. */
+namespace entt {}
+
 // IWYU pragma: begin_exports
 #include "config/config.h"
 #include "config/macro.h"
@@ -7,9 +10,9 @@
 #include "container/table.hpp"
 #include "core/algorithm.hpp"
 #include "core/any.hpp"
-#include "core/attribute.h"
 #include "core/bit.hpp"
 #include "core/compressed_pair.hpp"
+#include "core/concepts.hpp"
 #include "core/enum.hpp"
 #include "core/family.hpp"
 #include "core/hashed_string.hpp"
@@ -28,7 +31,6 @@
 #include "entity/handle.hpp"
 #include "entity/helper.hpp"
 #include "entity/mixin.hpp"
-#include "entity/observer.hpp"
 #include "entity/organizer.hpp"
 #include "entity/ranges.hpp"
 #include "entity/registry.hpp"
@@ -64,4 +66,7 @@
 #include "signal/dispatcher.hpp"
 #include "signal/emitter.hpp"
 #include "signal/sigh.hpp"
+#include "stl/functional.hpp"
+#include "stl/iterator.hpp"
+#include "stl/memory.hpp"
 // IWYU pragma: end_exports

src/entt/graph/adjacency_matrix.hpp

@@ -1,6 +1,7 @@
 #ifndef ENTT_GRAPH_ADJACENCY_MATRIX_HPP
 #define ENTT_GRAPH_ADJACENCY_MATRIX_HPP
 
+#include <concepts>
 #include <cstddef>
 #include <iterator>
 #include <memory>
@@ -13,13 +14,17 @@
 
 namespace entt {
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
 template<typename It>
 class edge_iterator {
     using size_type = std::size_t;
 
+    void find_next() noexcept {
+        for(; pos != last && !it[static_cast<It::difference_type>(pos)]; pos += offset) {}
+    }
+
 public:
     using value_type = std::pair<size_type, size_type>;
     using pointer = input_iterator_pointer<value_type>;
@@ -37,16 +42,17 @@ public:
           pos{from},
           last{to},
           offset{step} {
-        for(; pos != last && !it[pos]; pos += offset) {}
+        find_next();
     }
 
     constexpr edge_iterator &operator++() noexcept {
-        for(pos += offset; pos != last && !it[pos]; pos += offset) {}
+        pos += offset;
+        find_next();
         return *this;
     }
 
     constexpr edge_iterator operator++(int) noexcept {
-        edge_iterator orig = *this;
+        const edge_iterator orig = *this;
         return ++(*this), orig;
     }
 
@@ -58,8 +64,9 @@ public:
         return std::make_pair<size_type>(pos / vert, pos % vert);
     }
 
-    template<typename Type>
-    friend constexpr bool operator==(const edge_iterator<Type> &, const edge_iterator<Type> &) noexcept;
+    [[nodiscard]] constexpr bool operator==(const edge_iterator &other) const noexcept {
+        return pos == other.pos;
+    }
 
 private:
     It it{};
@@ -69,16 +76,6 @@ private:
     size_type offset{};
 };
 
-template<typename Container>
-[[nodiscard]] inline constexpr bool operator==(const edge_iterator<Container> &lhs, const edge_iterator<Container> &rhs) noexcept {
-    return lhs.pos == rhs.pos;
-}
-
-template<typename Container>
-[[nodiscard]] inline constexpr bool operator!=(const edge_iterator<Container> &lhs, const edge_iterator<Container> &rhs) noexcept {
-    return !(lhs == rhs);
-}
-
 } // namespace internal
 /*! @endcond */
 
@@ -87,10 +84,9 @@ template<typename Container>
  * @tparam Category Either a directed or undirected category tag.
  * @tparam Allocator Type of allocator used to manage memory and elements.
  */
-template<typename Category, typename Allocator>
+template<std::derived_from<directed_tag> Category, typename Allocator>
 class adjacency_matrix {
     using alloc_traits = std::allocator_traits<Allocator>;
-    static_assert(std::is_base_of_v<directed_tag, Category>, "Invalid graph category");
     static_assert(std::is_same_v<typename alloc_traits::value_type, std::size_t>, "Invalid value type");
     using container_type = std::vector<std::size_t, typename alloc_traits::template rebind_alloc<std::size_t>>;
 
@@ -175,6 +171,16 @@ public:
      */
     adjacency_matrix &operator=(adjacency_matrix &&) noexcept = default;
 
+    /**
+     * @brief Exchanges the contents with those of a given adjacency matrix.
+     * @param other Adjacency matrix to exchange the content with.
+     */
+    void swap(adjacency_matrix &other) noexcept {
+        using std::swap;
+        swap(matrix, other.matrix);
+        swap(vert, other.vert);
+    }
+
     /**
      * @brief Returns the associated allocator.
      * @return The associated allocator.
@@ -189,16 +195,6 @@ public:
         vert = {};
     }
 
-    /**
-     * @brief Exchanges the contents with those of a given adjacency matrix.
-     * @param other Adjacency matrix to exchange the content with.
-     */
-    void swap(adjacency_matrix &other) noexcept {
-        using std::swap;
-        swap(matrix, other.matrix);
-        swap(vert, other.vert);
-    }
-
     /**
      * @brief Returns true if an adjacency matrix is empty, false otherwise.
      *

src/entt/graph/dot.hpp

@@ -1,8 +1,8 @@
 #ifndef ENTT_GRAPH_DOT_HPP
 #define ENTT_GRAPH_DOT_HPP
 
+#include <concepts>
 #include <ostream>
-#include <type_traits>
 #include "fwd.hpp"
 
 namespace entt {
@@ -10,16 +10,14 @@ namespace entt {
 /**
  * @brief Outputs a graph in dot format.
  * @tparam Graph Graph type, valid as long as it exposes edges and vertices.
- * @tparam Writer Vertex decorator type.
  * @param out A standard output stream.
  * @param graph The graph to output.
  * @param writer Vertex decorator object.
  */
-template<typename Graph, typename Writer>
-void dot(std::ostream &out, const Graph &graph, Writer writer) {
-    static_assert(std::is_base_of_v<directed_tag, typename Graph::graph_category>, "Invalid graph category");
-
-    if constexpr(std::is_same_v<typename Graph::graph_category, undirected_tag>) {
+template<typename Graph>
+requires std::derived_from<typename Graph::graph_category, directed_tag>
+void dot(std::ostream &out, const Graph &graph, std::invocable<std::ostream &, typename Graph::vertex_type> auto writer) {
+    if constexpr(std::same_as<typename Graph::graph_category, undirected_tag>) {
         out << "graph{";
     } else {
         out << "digraph{";
@@ -32,7 +30,7 @@ void dot(std::ostream &out, const Graph &graph, Writer writer) {
     }
 
     for(auto [lhs, rhs]: graph.edges()) {
-        if constexpr(std::is_same_v<typename Graph::graph_category, undirected_tag>) {
+        if constexpr(std::same_as<typename Graph::graph_category, undirected_tag>) {
             out << lhs << "--" << rhs << ";";
         } else {
             out << lhs << "->" << rhs << ";";

src/entt/graph/flow.hpp

@@ -2,6 +2,7 @@
 #define ENTT_GRAPH_FLOW_HPP
 
 #include <algorithm>
+#include <concepts>
 #include <cstddef>
 #include <functional>
 #include <iterator>
@@ -15,7 +16,8 @@
 #include "../core/compressed_pair.hpp"
 #include "../core/fwd.hpp"
 #include "../core/iterator.hpp"
-#include "../core/utility.hpp"
+#include "../stl/functional.hpp"
+#include "../stl/iterator.hpp"
 #include "adjacency_matrix.hpp"
 #include "fwd.hpp"
 
@@ -29,9 +31,9 @@ template<typename Allocator>
 class basic_flow {
     using alloc_traits = std::allocator_traits<Allocator>;
     static_assert(std::is_same_v<typename alloc_traits::value_type, id_type>, "Invalid value type");
-    using task_container_type = dense_set<id_type, identity, std::equal_to<>, typename alloc_traits::template rebind_alloc<id_type>>;
+    using task_container_type = dense_set<id_type, stl::identity, std::equal_to<>, typename alloc_traits::template rebind_alloc<id_type>>;
     using ro_rw_container_type = std::vector<std::pair<std::size_t, bool>, typename alloc_traits::template rebind_alloc<std::pair<std::size_t, bool>>>;
-    using deps_container_type = dense_map<id_type, ro_rw_container_type, identity, std::equal_to<>, typename alloc_traits::template rebind_alloc<std::pair<const id_type, ro_rw_container_type>>>;
+    using deps_container_type = dense_map<id_type, ro_rw_container_type, stl::identity, std::equal_to<>, typename alloc_traits::template rebind_alloc<std::pair<const id_type, ro_rw_container_type>>>;
     using adjacency_matrix_type = adjacency_matrix<directed_tag, typename alloc_traits::template rebind_alloc<std::size_t>>;
 
     void emplace(const id_type res, const bool is_rw) {
@@ -180,6 +182,18 @@ public:
      */
     basic_flow &operator=(basic_flow &&) noexcept = default;
 
+    /**
+     * @brief Exchanges the contents with those of a given flow builder.
+     * @param other Flow builder to exchange the content with.
+     */
+    void swap(basic_flow &other) noexcept {
+        using std::swap;
+        std::swap(index, other.index);
+        std::swap(vertices, other.vertices);
+        std::swap(deps, other.deps);
+        std::swap(sync_on, other.sync_on);
+    }
+
     /**
      * @brief Returns the associated allocator.
      * @return The associated allocator.
@@ -194,7 +208,7 @@ public:
      * @return The requested identifier.
      */
     [[nodiscard]] id_type operator[](const size_type pos) const {
-        return vertices.cbegin()[pos];
+        return vertices.cbegin()[static_cast<task_container_type::difference_type>(pos)];
     }
 
     /*! @brief Clears the flow builder. */
@@ -205,18 +219,6 @@ public:
         sync_on = {};
     }
 
-    /**
-     * @brief Exchanges the contents with those of a given flow builder.
-     * @param other Flow builder to exchange the content with.
-     */
-    void swap(basic_flow &other) noexcept {
-        using std::swap;
-        std::swap(index, other.index);
-        std::swap(vertices, other.vertices);
-        std::swap(deps, other.deps);
-        std::swap(sync_on, other.sync_on);
-    }
-
     /**
      * @brief Returns true if a flow builder contains no tasks, false otherwise.
      * @return True if the flow builder contains no tasks, false otherwise.
@@ -283,14 +285,11 @@ public:
 
     /**
      * @brief Assigns a range of read-only resources to the current task.
-     * @tparam It Type of input iterator.
      * @param first An iterator to the first element of the range of elements.
      * @param last An iterator past the last element of the range of elements.
      * @return This flow builder.
      */
-    template<typename It>
-    std::enable_if_t<std::is_same_v<std::remove_const_t<typename std::iterator_traits<It>::value_type>, id_type>, basic_flow &>
-    ro(It first, It last) {
+    basic_flow &ro(stl::input_iterator auto first, stl::input_iterator auto last) {
         for(; first != last; ++first) {
             emplace(*first, false);
         }
@@ -310,14 +309,11 @@ public:
 
     /**
      * @brief Assigns a range of writable resources to the current task.
-     * @tparam It Type of input iterator.
      * @param first An iterator to the first element of the range of elements.
      * @param last An iterator past the last element of the range of elements.
      * @return This flow builder.
      */
-    template<typename It>
-    std::enable_if_t<std::is_same_v<std::remove_const_t<typename std::iterator_traits<It>::value_type>, id_type>, basic_flow &>
-    rw(It first, It last) {
+    basic_flow &rw(stl::input_iterator auto first, stl::input_iterator auto last) {
         for(; first != last; ++first) {
             emplace(*first, true);
         }

src/entt/graph/fwd.hpp

@@ -1,6 +1,7 @@
 #ifndef ENTT_GRAPH_FWD_HPP
 #define ENTT_GRAPH_FWD_HPP
 
+#include <concepts>
 #include <cstddef>
 #include <memory>
 #include "../core/fwd.hpp"
@@ -13,7 +14,7 @@ struct directed_tag {};
 /*! @brief Directed graph category tag. */
 struct undirected_tag: directed_tag {};
 
-template<typename, typename = std::allocator<std::size_t>>
+template<std::derived_from<directed_tag>, typename = std::allocator<std::size_t>>
 class adjacency_matrix;
 
 template<typename = std::allocator<id_type>>

src/entt/locator/locator.hpp

@@ -1,6 +1,7 @@
 #ifndef ENTT_LOCATOR_LOCATOR_HPP
 #define ENTT_LOCATOR_LOCATOR_HPP
 
+#include <concepts>
 #include <memory>
 #include <utility>
 #include "../config/config.h"
@@ -84,7 +85,8 @@ public:
      * @param args Parameters to use to construct the fallback service.
      * @return A reference to a valid service.
      */
-    template<typename Type = Service, typename... Args>
+    template<std::derived_from<Service> Type = Service, typename... Args>
+    requires std::constructible_from<Type, Args...>
     [[nodiscard]] static Service &value_or(Args &&...args) {
         return service ? *service : emplace<Type>(std::forward<Args>(args)...);
     }
@@ -96,7 +98,8 @@ public:
      * @param args Parameters to use to construct the service.
      * @return A reference to a valid service.
      */
-    template<typename Type = Service, typename... Args>
+    template<std::derived_from<Service> Type = Service, typename... Args>
+    requires std::constructible_from<Type, Args...>
     static Service &emplace(Args &&...args) {
         service = std::make_shared<Type>(std::forward<Args>(args)...);
         return *service;
@@ -105,14 +108,14 @@ public:
     /**
      * @brief Sets or replaces a service using a given allocator.
      * @tparam Type Service type.
-     * @tparam Allocator Type of allocator used to manage memory and elements.
      * @tparam Args Types of arguments to use to construct the service.
      * @param alloc The allocator to use.
      * @param args Parameters to use to construct the service.
      * @return A reference to a valid service.
      */
-    template<typename Type = Service, typename Allocator, typename... Args>
-    static Service &emplace(std::allocator_arg_t, Allocator alloc, Args &&...args) {
+    template<std::derived_from<Service> Type = Service, typename... Args>
+    requires std::constructible_from<Type, Args...>
+    static Service &emplace(std::allocator_arg_t, auto alloc, Args &&...args) {
         service = std::allocate_shared<Type>(alloc, std::forward<Args>(args)...);
         return *service;
     }
@@ -142,7 +145,7 @@ public:
      * @param elem A pointer to a service to manage.
      * @param deleter A deleter to use to destroy the service.
      */
-    template<typename Type, typename Deleter = std::default_delete<Type>>
+    template<std::derived_from<Service> Type, typename Deleter = std::default_delete<Type>>
     static void reset(Type *elem, Deleter deleter = {}) {
         service = std::shared_ptr<Service>{elem, std::move(deleter)};
     }

src/entt/meta/container.hpp

@@ -3,53 +3,55 @@
 #ifndef ENTT_META_CONTAINER_HPP
 #define ENTT_META_CONTAINER_HPP
 
-#include <array>
-#include <deque>
+#include <concepts>
+#include <cstddef>
 #include <iterator>
-#include <list>
-#include <map>
-#include <set>
 #include <type_traits>
-#include <unordered_map>
-#include <unordered_set>
-#include <vector>
-#include "../container/dense_map.hpp"
-#include "../container/dense_set.hpp"
+#include <utility>
+#include "../core/concepts.hpp"
+#include "../core/type_traits.hpp"
+#include "../stl/iterator.hpp"
 #include "context.hpp"
+#include "fwd.hpp"
 #include "meta.hpp"
 #include "type_traits.hpp"
 
 namespace entt {
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
-template<typename, typename = void>
-struct fixed_size_sequence_container: std::true_type {};
+template<typename Type>
+struct sequence_container_extent: integral_constant<meta_dynamic_extent> {};
 
 template<typename Type>
-struct fixed_size_sequence_container<Type, std::void_t<decltype(&Type::clear)>>: std::false_type {};
+requires is_complete_v<std::tuple_size<Type>>
+struct sequence_container_extent<Type>: integral_constant<std::tuple_size_v<Type>> {};
 
 template<typename Type>
-inline constexpr bool fixed_size_sequence_container_v = fixed_size_sequence_container<Type>::value;
-
-template<typename, typename = void>
-struct key_only_associative_container: std::true_type {};
+inline constexpr std::size_t sequence_container_extent_v = sequence_container_extent<Type>::value;
 
 template<typename Type>
-struct key_only_associative_container<Type, std::void_t<typename Type::mapped_type>>: std::false_type {};
+concept meta_sequence_container_like = requires(Type elem) {
+    typename Type::value_type;
+    typename Type::iterator;
+    requires entt::stl::forward_iterator<typename Type::iterator>;
+    { elem.begin() } -> std::same_as<typename Type::iterator>;
+    { elem.end() } -> std::same_as<typename Type::iterator>;
+    requires !requires { typename Type::key_type; };
+    requires !requires { elem.substr(); };
+};
 
 template<typename Type>
-inline constexpr bool key_only_associative_container_v = key_only_associative_container<Type>::value;
-
-template<typename, typename = void>
-struct reserve_aware_container: std::false_type {};
-
-template<typename Type>
-struct reserve_aware_container<Type, std::void_t<decltype(&Type::reserve)>>: std::true_type {};
-
-template<typename Type>
-inline constexpr bool reserve_aware_container_v = reserve_aware_container<Type>::value;
+concept meta_associative_container_like = requires(Type value) {
+    typename Type::key_type;
+    typename Type::value_type;
+    typename Type::iterator;
+    requires entt::stl::forward_iterator<typename Type::iterator>;
+    { value.begin() } -> std::same_as<typename Type::iterator>;
+    { value.end() } -> std::same_as<typename Type::iterator>;
+    value.find(std::declval<typename Type::key_type>());
+};
 
 } // namespace internal
 /*! @endcond */
@@ -58,17 +60,15 @@ inline constexpr bool reserve_aware_container_v = reserve_aware_container<Type>:
  * @brief General purpose implementation of meta sequence container traits.
  * @tparam Type Type of underlying sequence container.
  */
-template<typename Type>
+template<cvref_unqualified Type>
 struct basic_meta_sequence_container_traits {
-    static_assert(std::is_same_v<Type, std::remove_cv_t<std::remove_reference_t<Type>>>, "Unexpected type");
-
     /*! @brief Unsigned integer type. */
-    using size_type = typename meta_sequence_container::size_type;
+    using size_type = meta_sequence_container::size_type;
     /*! @brief Meta iterator type. */
-    using iterator = typename meta_sequence_container::iterator;
+    using iterator = meta_sequence_container::iterator;
 
-    /*! @brief True in case of key-only containers, false otherwise. */
-    static constexpr bool fixed_size = internal::fixed_size_sequence_container_v<Type>;
+    /*! @brief Number of elements, or `meta_dynamic_extent` if dynamic. */
+    static constexpr std::size_t extent = internal::sequence_container_extent_v<Type>;
 
     /**
      * @brief Returns the number of elements in a container.
@@ -85,11 +85,11 @@ struct basic_meta_sequence_container_traits {
      * @return True in case of success, false otherwise.
      */
     [[nodiscard]] static bool clear([[maybe_unused]] void *container) {
-        if constexpr(fixed_size) {
-            return false;
-        } else {
+        if constexpr(requires(Type elem) { elem.clear(); }) {
             static_cast<Type *>(container)->clear();
             return true;
+        } else {
+            return false;
         }
     }
 
@@ -100,7 +100,7 @@ struct basic_meta_sequence_container_traits {
      * @return True in case of success, false otherwise.
      */
     [[nodiscard]] static bool reserve([[maybe_unused]] void *container, [[maybe_unused]] const size_type sz) {
-        if constexpr(internal::reserve_aware_container_v<Type>) {
+        if constexpr(requires(Type elem) { elem.reserve(sz); }) {
             static_cast<Type *>(container)->reserve(sz);
             return true;
         } else {
@@ -115,36 +115,27 @@ struct basic_meta_sequence_container_traits {
      * @return True in case of success, false otherwise.
      */
     [[nodiscard]] static bool resize([[maybe_unused]] void *container, [[maybe_unused]] const size_type sz) {
-        if constexpr(fixed_size || !std::is_default_constructible_v<typename Type::value_type>) {
-            return false;
-        } else {
+        if constexpr(std::is_default_constructible_v<typename Type::value_type> && requires(Type elem) { elem.resize(sz); }) {
             static_cast<Type *>(container)->resize(sz);
             return true;
+        } else {
+            return false;
         }
     }
 
     /**
-     * @brief Returns a possibly const iterator to the beginning.
+     * @brief Returns a possibly const iterator to the beginning or the end.
      * @param area The context to pass to the newly created iterator.
      * @param container Opaque pointer to a container of the given type.
      * @param as_const Const opaque pointer fallback.
-     * @return An iterator to the first element of the container.
+     * @param end False to get a pointer that is past the last element.
+     * @return An iterator to the first or past the last element of the
+     * container.
      */
-    static iterator begin(const meta_ctx &area, void *container, const void *as_const) {
-        return (container != nullptr) ? iterator{area, static_cast<Type *>(container)->begin()}
-                                      : iterator{area, static_cast<const Type *>(as_const)->begin()};
-    }
-
-    /**
-     * @brief Returns a possibly const iterator to the end.
-     * @param area The context to pass to the newly created iterator.
-     * @param container Opaque pointer to a container of the given type.
-     * @param as_const Const opaque pointer fallback.
-     * @return An iterator that is past the last element of the container.
-     */
-    static iterator end(const meta_ctx &area, void *container, const void *as_const) {
-        return (container != nullptr) ? iterator{area, static_cast<Type *>(container)->end()}
-                                      : iterator{area, static_cast<const Type *>(as_const)->end()};
+    static iterator iter(const meta_ctx &area, void *container, const void *as_const, const bool end) {
+        return (container == nullptr)
+                   ? iterator{area, end ? static_cast<const Type *>(as_const)->cend() : static_cast<const Type *>(as_const)->cbegin()}
+                   : iterator{area, end ? static_cast<Type *>(container)->end() : static_cast<Type *>(container)->begin()};
     }
 
     /**
@@ -160,13 +151,13 @@ struct basic_meta_sequence_container_traits {
      * @return A possibly invalid iterator to the inserted element.
      */
     [[nodiscard]] static iterator insert([[maybe_unused]] const meta_ctx &area, [[maybe_unused]] void *container, [[maybe_unused]] const void *value, [[maybe_unused]] const void *cref, [[maybe_unused]] const iterator &it) {
-        if constexpr(fixed_size) {
-            return iterator{};
-        } else {
+        if constexpr(requires(Type elem, typename Type::const_iterator iter, Type::value_type instance) { elem.insert(iter, instance); }) {
             auto *const non_const = any_cast<typename Type::iterator>(&it.base());
             return {area, static_cast<Type *>(container)->insert(
                               non_const ? *non_const : any_cast<const typename Type::const_iterator &>(it.base()),
-                              (value != nullptr) ? *static_cast<const typename Type::value_type *>(value) : *static_cast<const std::remove_reference_t<typename Type::const_reference> *>(cref))};
+                              (value != nullptr) ? *static_cast<const Type::value_type *>(value) : *static_cast<const std::remove_reference_t<typename Type::const_reference> *>(cref))};
+        } else {
+            return iterator{};
         }
     }
 
@@ -178,11 +169,11 @@ struct basic_meta_sequence_container_traits {
      * @return A possibly invalid iterator following the last removed element.
      */
     [[nodiscard]] static iterator erase([[maybe_unused]] const meta_ctx &area, [[maybe_unused]] void *container, [[maybe_unused]] const iterator &it) {
-        if constexpr(fixed_size) {
-            return iterator{};
-        } else {
+        if constexpr(requires(Type elem, typename Type::const_iterator iter) { elem.erase(iter); }) {
             auto *const non_const = any_cast<typename Type::iterator>(&it.base());
             return {area, static_cast<Type *>(container)->erase(non_const ? *non_const : any_cast<const typename Type::const_iterator &>(it.base()))};
+        } else {
+            return iterator{};
         }
     }
 };
@@ -191,17 +182,15 @@ struct basic_meta_sequence_container_traits {
  * @brief General purpose implementation of meta associative container traits.
  * @tparam Type Type of underlying associative container.
  */
-template<typename Type>
+template<cvref_unqualified Type>
 struct basic_meta_associative_container_traits {
-    static_assert(std::is_same_v<Type, std::remove_cv_t<std::remove_reference_t<Type>>>, "Unexpected type");
-
     /*! @brief Unsigned integer type. */
-    using size_type = typename meta_associative_container::size_type;
+    using size_type = meta_associative_container::size_type;
     /*! @brief Meta iterator type. */
-    using iterator = typename meta_associative_container::iterator;
+    using iterator = meta_associative_container::iterator;
 
     /*! @brief True in case of key-only containers, false otherwise. */
-    static constexpr bool key_only = internal::key_only_associative_container_v<Type>;
+    static constexpr bool key_only = !requires { typename Type::mapped_type; };
 
     /**
      * @brief Returns the number of elements in a container.
@@ -229,7 +218,7 @@ struct basic_meta_associative_container_traits {
      * @return True in case of success, false otherwise.
      */
     [[nodiscard]] static bool reserve([[maybe_unused]] void *container, [[maybe_unused]] const size_type sz) {
-        if constexpr(internal::reserve_aware_container_v<Type>) {
+        if constexpr(requires(Type elem) { elem.reserve(sz); }) {
             static_cast<Type *>(container)->reserve(sz);
             return true;
         } else {
@@ -238,27 +227,18 @@ struct basic_meta_associative_container_traits {
     }
 
     /**
-     * @brief Returns a possibly const iterator to the beginning.
+     * @brief Returns a possibly const iterator to the beginning or the end.
      * @param area The context to pass to the newly created iterator.
      * @param container Opaque pointer to a container of the given type.
      * @param as_const Const opaque pointer fallback.
-     * @return An iterator to the first element of the container.
+     * @param end False to get a pointer that is past the last element.
+     * @return An iterator to the first or past the last element of the
+     * container.
      */
-    static iterator begin(const meta_ctx &area, void *container, const void *as_const) {
-        return (container != nullptr) ? iterator{area, std::bool_constant<key_only>{}, static_cast<Type *>(container)->begin()}
-                                      : iterator{area, std::bool_constant<key_only>{}, static_cast<const Type *>(as_const)->begin()};
-    }
-
-    /**
-     * @brief Returns a possibly const iterator to the end.
-     * @param area The context to pass to the newly created iterator.
-     * @param container Opaque pointer to a container of the given type.
-     * @param as_const Const opaque pointer fallback.
-     * @return An iterator that is past the last element of the container.
-     */
-    static iterator end(const meta_ctx &area, void *container, const void *as_const) {
-        return (container != nullptr) ? iterator{area, std::bool_constant<key_only>{}, static_cast<Type *>(container)->end()}
-                                      : iterator{area, std::bool_constant<key_only>{}, static_cast<const Type *>(as_const)->end()};
+    static iterator iter(const meta_ctx &area, void *container, const void *as_const, const bool end) {
+        return (container == nullptr)
+                   ? iterator{area, std::bool_constant<key_only>{}, end ? static_cast<const Type *>(as_const)->cend() : static_cast<const Type *>(as_const)->cbegin()}
+                   : iterator{area, std::bool_constant<key_only>{}, end ? static_cast<Type *>(container)->end() : static_cast<Type *>(container)->begin()};
     }
 
     /**
@@ -270,9 +250,9 @@ struct basic_meta_associative_container_traits {
      */
     [[nodiscard]] static bool insert(void *container, const void *key, [[maybe_unused]] const void *value) {
         if constexpr(key_only) {
-            return static_cast<Type *>(container)->insert(*static_cast<const typename Type::key_type *>(key)).second;
+            return static_cast<Type *>(container)->insert(*static_cast<const Type::key_type *>(key)).second;
         } else {
-            return static_cast<Type *>(container)->emplace(*static_cast<const typename Type::key_type *>(key), *static_cast<const typename Type::mapped_type *>(value)).second;
+            return static_cast<Type *>(container)->emplace(*static_cast<const Type::key_type *>(key), *static_cast<const Type::mapped_type *>(value)).second;
         }
     }
 
@@ -283,7 +263,7 @@ struct basic_meta_associative_container_traits {
      * @return Number of elements removed (either 0 or 1).
      */
     [[nodiscard]] static size_type erase(void *container, const void *key) {
-        return static_cast<Type *>(container)->erase(*static_cast<const typename Type::key_type *>(key));
+        return static_cast<Type *>(container)->erase(*static_cast<const Type::key_type *>(key));
     }
 
     /**
@@ -295,93 +275,24 @@ struct basic_meta_associative_container_traits {
      * @return An iterator to the element with the given key, if any.
      */
     static iterator find(const meta_ctx &area, void *container, const void *as_const, const void *key) {
-        return (container != nullptr) ? iterator{area, std::bool_constant<key_only>{}, static_cast<Type *>(container)->find(*static_cast<const typename Type::key_type *>(key))}
-                                      : iterator{area, std::bool_constant<key_only>{}, static_cast<const Type *>(as_const)->find(*static_cast<const typename Type::key_type *>(key))};
+        return (container != nullptr) ? iterator{area, std::bool_constant<key_only>{}, static_cast<Type *>(container)->find(*static_cast<const Type::key_type *>(key))}
+                                      : iterator{area, std::bool_constant<key_only>{}, static_cast<const Type *>(as_const)->find(*static_cast<const Type::key_type *>(key))};
     }
 };
 
 /**
- * @brief Meta sequence container traits for `std::vector`s of any type.
- * @tparam Args Template arguments for the container.
+ * @brief Traits meta sequence container like types.
+ * @tparam Type Container type to inspect.
  */
-template<typename... Args>
-struct meta_sequence_container_traits<std::vector<Args...>>
-    : basic_meta_sequence_container_traits<std::vector<Args...>> {};
+template<internal::meta_sequence_container_like Type>
+struct meta_sequence_container_traits<Type>: basic_meta_sequence_container_traits<Type> {};
 
 /**
- * @brief Meta sequence container traits for `std::array`s of any type.
- * @tparam Type Template arguments for the container.
- * @tparam N Template arguments for the container.
+ * @brief Traits for meta associative container like types.
+ * @tparam Type Container type to inspect.
  */
-template<typename Type, auto N>
-struct meta_sequence_container_traits<std::array<Type, N>>
-    : basic_meta_sequence_container_traits<std::array<Type, N>> {};
-
-/**
- * @brief Meta sequence container traits for `std::list`s of any type.
- * @tparam Args Template arguments for the container.
- */
-template<typename... Args>
-struct meta_sequence_container_traits<std::list<Args...>>
-    : basic_meta_sequence_container_traits<std::list<Args...>> {};
-
-/**
- * @brief Meta sequence container traits for `std::deque`s of any type.
- * @tparam Args Template arguments for the container.
- */
-template<typename... Args>
-struct meta_sequence_container_traits<std::deque<Args...>>
-    : basic_meta_sequence_container_traits<std::deque<Args...>> {};
-
-/**
- * @brief Meta associative container traits for `std::map`s of any type.
- * @tparam Args Template arguments for the container.
- */
-template<typename... Args>
-struct meta_associative_container_traits<std::map<Args...>>
-    : basic_meta_associative_container_traits<std::map<Args...>> {};
-
-/**
- * @brief Meta associative container traits for `std::unordered_map`s of any
- * type.
- * @tparam Args Template arguments for the container.
- */
-template<typename... Args>
-struct meta_associative_container_traits<std::unordered_map<Args...>>
-    : basic_meta_associative_container_traits<std::unordered_map<Args...>> {};
-
-/**
- * @brief Meta associative container traits for `std::set`s of any type.
- * @tparam Args Template arguments for the container.
- */
-template<typename... Args>
-struct meta_associative_container_traits<std::set<Args...>>
-    : basic_meta_associative_container_traits<std::set<Args...>> {};
-
-/**
- * @brief Meta associative container traits for `std::unordered_set`s of any
- * type.
- * @tparam Args Template arguments for the container.
- */
-template<typename... Args>
-struct meta_associative_container_traits<std::unordered_set<Args...>>
-    : basic_meta_associative_container_traits<std::unordered_set<Args...>> {};
-
-/**
- * @brief Meta associative container traits for `dense_map`s of any type.
- * @tparam Args Template arguments for the container.
- */
-template<typename... Args>
-struct meta_associative_container_traits<dense_map<Args...>>
-    : basic_meta_associative_container_traits<dense_map<Args...>> {};
-
-/**
- * @brief Meta associative container traits for `dense_set`s of any type.
- * @tparam Args Template arguments for the container.
- */
-template<typename... Args>
-struct meta_associative_container_traits<dense_set<Args...>>
-    : basic_meta_associative_container_traits<dense_set<Args...>> {};
+template<internal::meta_associative_container_like Type>
+struct meta_associative_container_traits<Type>: basic_meta_associative_container_traits<Type> {};
 
 } // namespace entt
 

src/entt/meta/context.hpp

@@ -1,24 +1,26 @@
 #ifndef ENTT_META_CTX_HPP
 #define ENTT_META_CTX_HPP
 
+#include <memory>
 #include "../container/dense_map.hpp"
 #include "../core/fwd.hpp"
-#include "../core/utility.hpp"
+#include "../stl/functional.hpp"
+#include "fwd.hpp"
 
 namespace entt {
 
-class meta_ctx;
-
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
 struct meta_type_node;
 
 struct meta_context {
-    dense_map<id_type, meta_type_node, identity> value{};
+    using container_type = dense_map<id_type, std::unique_ptr<meta_type_node>, stl::identity>;
 
-    [[nodiscard]] inline static meta_context &from(meta_ctx &ctx);
-    [[nodiscard]] inline static const meta_context &from(const meta_ctx &ctx);
+    container_type bucket;
+
+    [[nodiscard]] inline static meta_context &from(meta_ctx &);
+    [[nodiscard]] inline static const meta_context &from(const meta_ctx &);
 };
 
 } // namespace internal
@@ -36,7 +38,7 @@ class meta_ctx: private internal::meta_context {
     friend struct internal::meta_context;
 };
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 [[nodiscard]] inline internal::meta_context &internal::meta_context::from(meta_ctx &ctx) {
     return ctx;
 }

src/entt/meta/factory.hpp

@@ -1,6 +1,7 @@
 #ifndef ENTT_META_FACTORY_HPP
 #define ENTT_META_FACTORY_HPP
 
+#include <concepts>
 #include <cstddef>
 #include <cstdint>
 #include <functional>
@@ -11,10 +12,12 @@
 #include "../config/config.h"
 #include "../core/bit.hpp"
 #include "../core/fwd.hpp"
+#include "../core/hashed_string.hpp"
 #include "../core/type_info.hpp"
 #include "../core/type_traits.hpp"
 #include "../locator/locator.hpp"
 #include "context.hpp"
+#include "fwd.hpp"
 #include "meta.hpp"
 #include "node.hpp"
 #include "policy.hpp"
@@ -24,135 +27,131 @@
 
 namespace entt {
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
 class basic_meta_factory {
     using invoke_type = std::remove_pointer_t<decltype(meta_func_node::invoke)>;
 
-    auto *find_member_or_assert() {
-        auto *member = find_member<&meta_data_node::id>(details->data, bucket);
+    enum class mode {
+        type,
+        data,
+        func
+    };
+
+    [[nodiscard]] auto *find_member_or_assert() {
+        auto *member = find_member(parent->details->data, bucket);
         ENTT_ASSERT(member != nullptr, "Cannot find member");
         return member;
     }
 
-    auto *find_overload_or_assert() {
-        auto *overload = find_overload(find_member<&meta_func_node::id>(details->func, bucket), invoke);
+    [[nodiscard]] auto *find_overload_or_assert() {
+        ENTT_ASSERT(invoke != nullptr, "Invoke function not available");
+        auto *overload = find_overload(find_member(parent->details->func, bucket), invoke);
         ENTT_ASSERT(overload != nullptr, "Cannot find overload");
         return overload;
     }
 
-    void reset_bucket(const id_type id, invoke_type *const ref = nullptr) {
-        invoke = ref;
-        bucket = id;
-    }
-
 protected:
-    void type(const id_type id) noexcept {
-        reset_bucket(parent);
-        auto &&elem = meta_context::from(*ctx).value[parent];
-        ENTT_ASSERT(elem.id == id || !resolve(*ctx, id), "Duplicate identifier");
-        elem.id = id;
+    void type(const id_type id, const char *name) noexcept {
+        state = mode::type;
+        ENTT_ASSERT(parent->id == id || !resolve(*ctx, id), "Duplicate identifier");
+        parent->name = name;
+        parent->id = id;
     }
 
     template<typename Type>
     void insert_or_assign(Type node) {
-        reset_bucket(parent);
+        state = mode::type;
 
         if constexpr(std::is_same_v<Type, meta_base_node>) {
-            auto *member = find_member<&meta_base_node::type>(details->base, node.type);
-            member ? (*member = node) : details->base.emplace_back(node);
+            auto *member = find_member(parent->details->base, node.id);
+            member ? (*member = node) : parent->details->base.emplace_back(node);
         } else if constexpr(std::is_same_v<Type, meta_conv_node>) {
-            auto *member = find_member<&meta_conv_node::type>(details->conv, node.type);
-            member ? (*member = node) : details->conv.emplace_back(node);
+            auto *member = find_member(parent->details->conv, node.id);
+            member ? (*member = node) : parent->details->conv.emplace_back(node);
         } else {
             static_assert(std::is_same_v<Type, meta_ctor_node>, "Unexpected type");
-            auto *member = find_member<&meta_ctor_node::id>(details->ctor, node.id);
-            member ? (*member = node) : details->ctor.emplace_back(node);
+            auto *member = find_member(parent->details->ctor, node.id);
+            member ? (*member = node) : parent->details->ctor.emplace_back(node);
         }
     }
 
-    void dtor(meta_dtor_node node) {
-        reset_bucket(parent);
-        meta_context::from(*ctx).value[parent].dtor = node;
-    }
-
     void data(meta_data_node node) {
-        reset_bucket(node.id);
+        state = mode::data;
+        bucket = node.id;
 
-        if(auto *member = find_member<&meta_data_node::id>(details->data, node.id); member == nullptr) {
-            details->data.emplace_back(std::move(node));
+        if(auto *member = find_member(parent->details->data, node.id); member == nullptr) {
+            parent->details->data.emplace_back(std::move(node));
         } else if(member->set != node.set || member->get != node.get) {
             *member = std::move(node);
         }
     }
 
     void func(meta_func_node node) {
-        reset_bucket(node.id, node.invoke);
+        state = mode::func;
+        bucket = node.id;
+        invoke = node.invoke;
 
-        if(auto *member = find_member<&meta_func_node::id>(details->func, node.id); member == nullptr) {
-            details->func.emplace_back(std::move(node));
+        if(auto *member = find_member(parent->details->func, node.id); member == nullptr) {
+            parent->details->func.emplace_back(std::move(node));
         } else if(auto *overload = find_overload(member, node.invoke); overload == nullptr) {
             while(member->next != nullptr) { member = member->next.get(); }
-            member->next = std::make_shared<meta_func_node>(std::move(node));
+            member->next = std::make_unique<meta_func_node>(std::move(node));
         }
     }
 
-    void prop(meta_prop_node node) {
-        std::vector<meta_prop_node> *container = nullptr;
+    void traits(const meta_traits value, const bool unset) {
+        const auto set_or_unset_on = [=](auto &node) {
+            node.traits = (unset ? (node.traits & ~value) : (node.traits | value));
+        };
 
-        if(bucket == parent) {
-            container = &details->prop;
-        } else if(invoke == nullptr) {
-            container = &find_member_or_assert()->prop;
-        } else {
-            container = &find_overload_or_assert()->prop;
-        }
-
-        auto *member = find_member<&meta_prop_node::id>(*container, node.id);
-        (member != nullptr) ? (*member = std::move(node)) : container->emplace_back(std::move(node));
-    }
-
-    void traits(const meta_traits value) {
-        if(bucket == parent) {
-            meta_context::from(*ctx).value[bucket].traits |= value;
-        } else if(invoke == nullptr) {
-            find_member_or_assert()->traits |= value;
-        } else {
-            find_overload_or_assert()->traits |= value;
+        switch(state) {
+        case mode::type:
+            set_or_unset_on(*parent);
+            break;
+        case mode::data:
+            set_or_unset_on(*find_member_or_assert());
+            break;
+        case mode::func:
+            set_or_unset_on(*find_overload_or_assert());
+            break;
         }
     }
 
     void custom(meta_custom_node node) {
-        if(bucket == parent) {
-            meta_context::from(*ctx).value[bucket].custom = std::move(node);
-        } else if(invoke == nullptr) {
+        switch(state) {
+        case mode::type:
+            parent->custom = std::move(node);
+            break;
+        case mode::data:
             find_member_or_assert()->custom = std::move(node);
-        } else {
+            break;
+        case mode::func:
             find_overload_or_assert()->custom = std::move(node);
+            break;
         }
     }
 
 public:
-    basic_meta_factory(const id_type id, meta_ctx &area)
+    basic_meta_factory(meta_ctx &area, meta_type_node node)
         : ctx{&area},
-          parent{id},
-          bucket{id} {
-        auto &&elem = meta_context::from(*ctx).value[parent];
-
-        if(!elem.details) {
-            elem.details = std::make_shared<meta_type_descriptor>();
+          bucket{node.info->hash()},
+          state{mode::type} {
+        if(const auto it = meta_context::from(*ctx).bucket.find(bucket); it == meta_context::from(*ctx).bucket.cend()) {
+            parent = meta_context::from(*ctx).bucket.emplace(node.info->hash(), std::make_unique<meta_type_node>(std::move(node))).first->second.get();
+            parent->details = std::make_unique<meta_type_descriptor>();
+        } else {
+            parent = it->second.get();
         }
-
-        details = elem.details.get();
     }
 
 private:
     meta_ctx *ctx{};
-    id_type parent{};
     id_type bucket{};
     invoke_type *invoke{};
-    meta_type_descriptor *details{};
+    meta_type_node *parent{};
+    mode state{};
 };
 
 } // namespace internal
@@ -160,49 +159,44 @@ private:
 
 /**
  * @brief Meta factory to be used for reflection purposes.
- * @tparam Type Reflected type for which the factory was created.
+ * @tparam Type Type for which the factory was created.
  */
 template<typename Type>
 class meta_factory: private internal::basic_meta_factory {
     using base_type = internal::basic_meta_factory;
 
-    template<typename Setter, auto Getter, typename Policy, std::size_t... Index>
-    void data(const id_type id, std::index_sequence<Index...>) noexcept {
-        using data_type = std::invoke_result_t<decltype(Getter), Type &>;
-        using args_type = type_list<typename meta_function_helper_t<Type, decltype(value_list_element_v<Index, Setter>)>::args_type...>;
-        static_assert(Policy::template value<data_type>, "Invalid return type for the given policy");
-
-        base_type::data(
-            internal::meta_data_node{
-                id,
-                /* this is never static */
-                (std::is_member_object_pointer_v<decltype(value_list_element_v<Index, Setter>)> && ... && std::is_const_v<std::remove_reference_t<data_type>>) ? internal::meta_traits::is_const : internal::meta_traits::is_none,
-                Setter::size,
-                &internal::resolve<std::remove_cv_t<std::remove_reference_t<data_type>>>,
-                &meta_arg<type_list<type_list_element_t<type_list_element_t<Index, args_type>::size != 1u, type_list_element_t<Index, args_type>>...>>,
-                +[](meta_handle instance, meta_any value) { return (meta_setter<Type, value_list_element_v<Index, Setter>>(*instance.operator->(), value.as_ref()) || ...); },
-                &meta_getter<Type, Getter, Policy>});
-    }
-
 public:
+    /*! @brief Type of object for which this factory builds a meta type. */
+    using element_type = Type;
+
     /*! @brief Default constructor. */
     meta_factory() noexcept
-        : internal::basic_meta_factory{type_id<Type>(), locator<meta_ctx>::value_or()} {}
+        : meta_factory{locator<meta_ctx>::value_or()} {}
 
     /**
      * @brief Context aware constructor.
      * @param area The context into which to construct meta types.
      */
     meta_factory(meta_ctx &area) noexcept
-        : internal::basic_meta_factory{type_id<Type>().hash(), area} {}
+        : internal::basic_meta_factory{area, internal::setup_node_for<Type>()} {}
+
+    /**
+     * @brief Assigns a custom unique identifier to a meta type.
+     * @param name A custom unique identifier as a **string literal**.
+     * @return A meta factory for the given type.
+     */
+    meta_factory type(const char *name) noexcept {
+        return type(hashed_string::value(name), name);
+    }
 
     /**
      * @brief Assigns a custom unique identifier to a meta type.
      * @param id A custom unique identifier.
+     * @param name An optional name for the type as a **string literal**.
      * @return A meta factory for the given type.
      */
-    meta_factory type(const id_type id) noexcept {
-        base_type::type(id);
+    meta_factory type(const id_type id, const char *name = nullptr) noexcept {
+        base_type::type(id, name);
         return *this;
     }
 
@@ -215,10 +209,18 @@ public:
      * @return A meta factory for the parent type.
      */
     template<typename Base>
+    requires std::derived_from<Type, Base>
     meta_factory base() noexcept {
-        static_assert(!std::is_same_v<Type, Base> && std::is_base_of_v<Base, Type>, "Invalid base type");
-        auto *const op = +[](const void *instance) noexcept { return static_cast<const void *>(static_cast<const Base *>(static_cast<const Type *>(instance))); };
-        base_type::insert_or_assign(internal::meta_base_node{type_id<Base>().hash(), &internal::resolve<Base>, op});
+        if constexpr(!std::same_as<Type, Base>) {
+            auto *const op = +[](const void *instance) noexcept { return static_cast<const void *>(static_cast<const Base *>(static_cast<const Type *>(instance))); };
+
+            base_type::insert_or_assign(
+                internal::meta_base_node{
+                    type_id<Base>().hash(),
+                    &internal::resolve<Base>,
+                    op});
+        }
+
         return *this;
     }
 
@@ -236,9 +238,14 @@ public:
      */
     template<auto Candidate>
     auto conv() noexcept {
-        using conv_type = std::remove_cv_t<std::remove_reference_t<std::invoke_result_t<decltype(Candidate), Type &>>>;
+        using conv_type = std::remove_cvref_t<std::invoke_result_t<decltype(Candidate), Type &>>;
         auto *const op = +[](const meta_ctx &area, const void *instance) { return forward_as_meta(area, std::invoke(Candidate, *static_cast<const Type *>(instance))); };
-        base_type::insert_or_assign(internal::meta_conv_node{type_id<conv_type>().hash(), op});
+
+        base_type::insert_or_assign(
+            internal::meta_conv_node{
+                type_id<conv_type>().hash(),
+                op});
+
         return *this;
     }
 
@@ -253,9 +260,14 @@ public:
      */
     template<typename To>
     meta_factory conv() noexcept {
-        using conv_type = std::remove_cv_t<std::remove_reference_t<To>>;
+        using conv_type = std::remove_cvref_t<To>;
         auto *const op = +[](const meta_ctx &area, const void *instance) { return forward_as_meta(area, static_cast<To>(*static_cast<const Type *>(instance))); };
-        base_type::insert_or_assign(internal::meta_conv_node{type_id<conv_type>().hash(), op});
+
+        base_type::insert_or_assign(
+            internal::meta_conv_node{
+                type_id<conv_type>().hash(),
+                op});
+
         return *this;
     }
 
@@ -272,12 +284,19 @@ public:
      * @tparam Policy Optional policy (no policy set by default).
      * @return A meta factory for the parent type.
      */
-    template<auto Candidate, typename Policy = as_is_t>
+    template<auto Candidate, typename Policy = as_value_t>
     meta_factory ctor() noexcept {
         using descriptor = meta_function_helper_t<Type, decltype(Candidate)>;
         static_assert(Policy::template value<typename descriptor::return_type>, "Invalid return type for the given policy");
-        static_assert(std::is_same_v<std::remove_cv_t<std::remove_reference_t<typename descriptor::return_type>>, Type>, "The function doesn't return an object of the required type");
-        base_type::insert_or_assign(internal::meta_ctor_node{type_id<typename descriptor::args_type>().hash(), descriptor::args_type::size, &meta_arg<typename descriptor::args_type>, &meta_construct<Type, Candidate, Policy>});
+        static_assert(std::is_same_v<std::remove_cvref_t<typename descriptor::return_type>, Type>, "The function doesn't return an object of the required type");
+
+        base_type::insert_or_assign(
+            internal::meta_ctor_node{
+                type_id<typename descriptor::args_type>().hash(),
+                descriptor::args_type::size,
+                &meta_arg<typename descriptor::args_type>,
+                &meta_construct<Type, Candidate, Policy>});
+
         return *this;
     }
 
@@ -296,36 +315,28 @@ public:
         // default constructor is already implicitly generated, no need for redundancy
         if constexpr(sizeof...(Args) != 0u) {
             using descriptor = meta_function_helper_t<Type, Type (*)(Args...)>;
-            base_type::insert_or_assign(internal::meta_ctor_node{type_id<typename descriptor::args_type>().hash(), descriptor::args_type::size, &meta_arg<typename descriptor::args_type>, &meta_construct<Type, Args...>});
+
+            base_type::insert_or_assign(
+                internal::meta_ctor_node{
+                    type_id<typename descriptor::args_type>().hash(),
+                    descriptor::args_type::size,
+                    &meta_arg<typename descriptor::args_type>,
+                    &meta_construct<Type, Args...>});
         }
 
         return *this;
     }
 
     /**
-     * @brief Assigns a meta destructor to a meta type.
-     *
-     * Both free functions and member functions can be assigned to meta types in
-     * the role of destructors.<br/>
-     * The signature of a free function should be identical to the following:
-     *
-     * @code{.cpp}
-     * void(Type &);
-     * @endcode
-     *
-     * Member functions should not take arguments instead.<br/>
-     * The purpose is to give users the ability to free up resources that
-     * require special treatment before an object is actually destroyed.
-     *
-     * @tparam Func The actual function to use as a destructor.
-     * @return A meta factory for the parent type.
+     * @brief Assigns a meta data to a meta type.
+     * @tparam Data The actual variable to attach to the meta type.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @param name A custom unique identifier as a **string literal**.
+     * @return A meta factory for the given type.
      */
-    template<auto Func>
-    meta_factory dtor() noexcept {
-        static_assert(std::is_invocable_v<decltype(Func), Type &>, "The function doesn't accept an object of the type provided");
-        auto *const op = +[](void *instance) { std::invoke(Func, *static_cast<Type *>(instance)); };
-        base_type::dtor(internal::meta_dtor_node{op});
-        return *this;
+    template<auto Data, typename Policy = as_value_t>
+    meta_factory data(const char *name) noexcept {
+        return data<Data, Policy>(hashed_string::value(name), name);
     }
 
     /**
@@ -339,10 +350,11 @@ public:
      * @tparam Data The actual variable to attach to the meta type.
      * @tparam Policy Optional policy (no policy set by default).
      * @param id Unique identifier.
+     * @param name An optional name for the meta data as a **string literal**.
      * @return A meta factory for the parent type.
      */
-    template<auto Data, typename Policy = as_is_t>
-    meta_factory data(const id_type id) noexcept {
+    template<auto Data, typename Policy = as_value_t>
+    meta_factory data(const id_type id, const char *name = nullptr) noexcept {
         if constexpr(std::is_member_object_pointer_v<decltype(Data)>) {
             using data_type = std::invoke_result_t<decltype(Data), Type &>;
             static_assert(Policy::template value<data_type>, "Invalid return type for the given policy");
@@ -350,11 +362,12 @@ public:
             base_type::data(
                 internal::meta_data_node{
                     id,
+                    name,
                     /* this is never static */
                     std::is_const_v<std::remove_reference_t<data_type>> ? internal::meta_traits::is_const : internal::meta_traits::is_none,
                     1u,
-                    &internal::resolve<std::remove_cv_t<std::remove_reference_t<data_type>>>,
-                    &meta_arg<type_list<std::remove_cv_t<std::remove_reference_t<data_type>>>>,
+                    &internal::resolve<std::remove_cvref_t<data_type>>,
+                    &meta_arg<type_list<std::remove_cvref_t<data_type>>>,
                     &meta_setter<Type, Data>,
                     &meta_getter<Type, Data, Policy>});
         } else {
@@ -369,10 +382,11 @@ public:
             base_type::data(
                 internal::meta_data_node{
                     id,
-                    ((std::is_same_v<Type, std::remove_cv_t<std::remove_reference_t<data_type>>> || std::is_const_v<std::remove_reference_t<data_type>>) ? internal::meta_traits::is_const : internal::meta_traits::is_none) | internal::meta_traits::is_static,
+                    name,
+                    ((!std::is_pointer_v<decltype(Data)> || std::is_const_v<data_type>) ? internal::meta_traits::is_const : internal::meta_traits::is_none) | internal::meta_traits::is_static,
                     1u,
-                    &internal::resolve<std::remove_cv_t<std::remove_reference_t<data_type>>>,
-                    &meta_arg<type_list<std::remove_cv_t<std::remove_reference_t<data_type>>>>,
+                    &internal::resolve<std::remove_cvref_t<data_type>>,
+                    &meta_arg<type_list<std::remove_cvref_t<data_type>>>,
                     &meta_setter<Type, Data>,
                     &meta_getter<Type, Data, Policy>});
         }
@@ -380,6 +394,20 @@ public:
         return *this;
     }
 
+    /**
+     * @brief Assigns a meta data to a meta type by means of its setter and
+     * getter.
+     * @tparam Setter The actual function to use as a setter.
+     * @tparam Getter The actual function to use as a getter.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @param name A custom unique identifier as a **string literal**.
+     * @return A meta factory for the given type.
+     */
+    template<auto Setter, auto Getter, typename Policy = as_value_t>
+    meta_factory data(const char *name) noexcept {
+        return data<Setter, Getter, Policy>(hashed_string::value(name), name);
+    }
+
     /**
      * @brief Assigns a meta data to a meta type by means of its setter and
      * getter.
@@ -398,35 +426,38 @@ public:
      * @tparam Getter The actual function to use as a getter.
      * @tparam Policy Optional policy (no policy set by default).
      * @param id Unique identifier.
+     * @param name An optional name for the meta data as a **string literal**.
      * @return A meta factory for the parent type.
      */
-    template<auto Setter, auto Getter, typename Policy = as_is_t>
-    meta_factory data(const id_type id) noexcept {
-        using data_type = std::invoke_result_t<decltype(Getter), Type &>;
-        static_assert(Policy::template value<data_type>, "Invalid return type for the given policy");
+    template<auto Setter, auto Getter, typename Policy = as_value_t>
+    meta_factory data(const id_type id, const char *name = nullptr) noexcept {
+        using descriptor = meta_function_helper_t<Type, decltype(Getter)>;
+        static_assert(Policy::template value<typename descriptor::return_type>, "Invalid return type for the given policy");
 
         if constexpr(std::is_same_v<decltype(Setter), std::nullptr_t>) {
             base_type::data(
                 internal::meta_data_node{
                     id,
+                    name,
                     /* this is never static */
                     internal::meta_traits::is_const,
                     0u,
-                    &internal::resolve<std::remove_cv_t<std::remove_reference_t<data_type>>>,
+                    &internal::resolve<std::remove_cvref_t<typename descriptor::return_type>>,
                     &meta_arg<type_list<>>,
                     &meta_setter<Type, Setter>,
                     &meta_getter<Type, Getter, Policy>});
         } else {
-            using args_type = typename meta_function_helper_t<Type, decltype(Setter)>::args_type;
+            using args_type = meta_function_helper_t<Type, decltype(Setter)>::args_type;
 
             base_type::data(
                 internal::meta_data_node{
                     id,
+                    name,
                     /* this is never static nor const */
                     internal::meta_traits::is_none,
                     1u,
-                    &internal::resolve<std::remove_cv_t<std::remove_reference_t<data_type>>>,
-                    &meta_arg<type_list<type_list_element_t<args_type::size != 1u, args_type>>>,
+                    &internal::resolve<std::remove_cvref_t<typename descriptor::return_type>>,
+                    &meta_arg<type_list<type_list_element_t<static_cast<std::size_t>(args_type::size != 1u), args_type>>>,
                     &meta_setter<Type, Setter>,
                     &meta_getter<Type, Getter, Policy>});
         }
@@ -435,26 +466,15 @@ public:
     }
 
     /**
-     * @brief Assigns a meta data to a meta type by means of its setters and
-     * getter.
-     *
-     * Multi-setter support for meta data members. All setters are tried in the
-     * order of definition before returning to the caller.<br/>
-     * Setters can be either free functions, member functions or a mix of them
-     * and are provided via a `value_list` type.
-     *
-     * @sa data
-     *
-     * @tparam Setter The actual functions to use as setters.
-     * @tparam Getter The actual getter function.
+     * @brief Assigns a meta function to a meta type.
+     * @tparam Candidate The actual function to attach to the meta function.
      * @tparam Policy Optional policy (no policy set by default).
-     * @param id Unique identifier.
-     * @return A meta factory for the parent type.
+     * @param name A custom unique identifier as a **string literal**.
+     * @return A meta factory for the given type.
      */
-    template<typename Setter, auto Getter, typename Policy = as_is_t>
-    meta_factory data(const id_type id) noexcept {
-        data<Setter, Getter, Policy>(id, std::make_index_sequence<Setter::size>{});
-        return *this;
+    template<auto Candidate, typename Policy = as_value_t>
+    meta_factory func(const char *name) noexcept {
+        return func<Candidate, Policy>(hashed_string::value(name), name);
     }
 
     /**
@@ -468,46 +488,27 @@ public:
      * @tparam Candidate The actual function to attach to the meta type.
      * @tparam Policy Optional policy (no policy set by default).
      * @param id Unique identifier.
+     * @param name An optional name for the function as a **string literal**.
      * @return A meta factory for the parent type.
      */
-    template<auto Candidate, typename Policy = as_is_t>
-    meta_factory func(const id_type id) noexcept {
+    template<auto Candidate, typename Policy = as_value_t>
+    meta_factory func(const id_type id, const char *name = nullptr) noexcept {
         using descriptor = meta_function_helper_t<Type, decltype(Candidate)>;
         static_assert(Policy::template value<typename descriptor::return_type>, "Invalid return type for the given policy");
 
         base_type::func(
             internal::meta_func_node{
                 id,
+                name,
                 (descriptor::is_const ? internal::meta_traits::is_const : internal::meta_traits::is_none) | (descriptor::is_static ? internal::meta_traits::is_static : internal::meta_traits::is_none),
                 descriptor::args_type::size,
-                &internal::resolve<std::conditional_t<std::is_same_v<Policy, as_void_t>, void, std::remove_cv_t<std::remove_reference_t<typename descriptor::return_type>>>>,
+                &internal::resolve<std::conditional_t<std::is_same_v<Policy, as_void_t>, void, std::remove_cvref_t<typename descriptor::return_type>>>,
                 &meta_arg<typename descriptor::args_type>,
                 &meta_invoke<Type, Candidate, Policy>});
 
         return *this;
     }
 
-    /**
-     * @brief Assigns a property to the last created meta object.
-     *
-     * Both the key and the value (if any) must be at least copy constructible.
-     *
-     * @tparam Value Optional type of the property value.
-     * @param id Property key.
-     * @param value Optional property value.
-     * @return A meta factory for the parent type.
-     */
-    template<typename... Value>
-    [[deprecated("use ::custom() instead")]] meta_factory prop(id_type id, [[maybe_unused]] Value &&...value) {
-        if constexpr(sizeof...(Value) == 0u) {
-            base_type::prop(internal::meta_prop_node{id, &internal::resolve<void>});
-        } else {
-            base_type::prop(internal::meta_prop_node{id, &internal::resolve<std::decay_t<Value>>..., std::make_shared<std::decay_t<Value>>(std::forward<Value>(value))...});
-        }
-
-        return *this;
-    }
-
     /**
      * @brief Sets traits on the last created meta object.
      *
@@ -515,12 +516,13 @@ public:
      *
      * @tparam Value Type of the traits value.
      * @param value Traits value.
+     * @param unset True to unset the given traits, false otherwise.
      * @return A meta factory for the parent type.
      */
     template<typename Value>
-    meta_factory traits(const Value value) {
+    meta_factory traits(const Value value, const bool unset = false) {
         static_assert(std::is_enum_v<Value>, "Invalid enum type");
-        base_type::traits(internal::user_to_meta_traits(value));
+        base_type::traits(internal::user_to_meta_traits(value), unset);
         return *this;
     }
 
@@ -538,42 +540,6 @@ public:
     }
 };
 
-/**
- * @brief Utility function to use for reflection.
- *
- * This is the point from which everything starts.<br/>
- * By invoking this function with a type that is not yet reflected, a meta type
- * is created to which it will be possible to attach meta objects through a
- * dedicated factory.
- *
- * @tparam Type Type to reflect.
- * @param ctx The context into which to construct meta types.
- * @return A meta factory for the given type.
- */
-template<typename Type>
-[[nodiscard]] auto meta(meta_ctx &ctx) noexcept {
-    auto &&context = internal::meta_context::from(ctx);
-    // make sure the type exists in the context before returning a factory
-    context.value.try_emplace(type_id<Type>().hash(), internal::resolve<Type>(context));
-    return meta_factory<Type>{ctx};
-}
-
-/**
- * @brief Utility function to use for reflection.
- *
- * This is the point from which everything starts.<br/>
- * By invoking this function with a type that is not yet reflected, a meta type
- * is created to which it will be possible to attach meta objects through a
- * dedicated factory.
- *
- * @tparam Type Type to reflect.
- * @return A meta factory for the given type.
- */
-template<typename Type>
-[[nodiscard]] auto meta() noexcept {
-    return meta<Type>(locator<meta_ctx>::value_or());
-}
-
 /**
  * @brief Resets a type and all its parts.
  *
@@ -587,11 +553,11 @@ template<typename Type>
  * @param ctx The context from which to reset meta types.
  */
 inline void meta_reset(meta_ctx &ctx, const id_type id) noexcept {
-    auto &&context = internal::meta_context::from(ctx);
+    auto &context = internal::meta_context::from(ctx);
 
-    for(auto it = context.value.begin(); it != context.value.end();) {
-        if(it->second.id == id) {
-            it = context.value.erase(it);
+    for(auto it = context.bucket.begin(); it != context.bucket.end();) {
+        if(it->second->id == id) {
+            it = context.bucket.erase(it);
         } else {
             ++it;
         }
@@ -623,7 +589,7 @@ inline void meta_reset(const id_type id) noexcept {
  */
 template<typename Type>
 void meta_reset(meta_ctx &ctx) noexcept {
-    internal::meta_context::from(ctx).value.erase(type_id<Type>().hash());
+    internal::meta_context::from(ctx).bucket.erase(type_id<Type>().hash());
 }
 
 /**
@@ -646,7 +612,7 @@ void meta_reset() noexcept {
  * @param ctx The context from which to reset meta types.
  */
 inline void meta_reset(meta_ctx &ctx) noexcept {
-    internal::meta_context::from(ctx).value.clear();
+    internal::meta_context::from(ctx).bucket.clear();
 }
 
 /**

src/entt/meta/fwd.hpp

@@ -1,17 +1,20 @@
 #ifndef ENTT_META_FWD_HPP
 #define ENTT_META_FWD_HPP
 
+#include <cstddef>
+#include <limits>
+
 namespace entt {
 
+class meta_ctx;
+
 class meta_sequence_container;
 
 class meta_associative_container;
 
 class meta_any;
 
-struct meta_handle;
-
-struct meta_prop;
+class meta_handle;
 
 struct meta_custom;
 
@@ -19,8 +22,16 @@ struct meta_data;
 
 struct meta_func;
 
+struct meta_base;
+
 class meta_type;
 
+template<typename>
+class meta_factory;
+
+/*! @brief Used to identicate that a sequence container has not a fixed size. */
+inline constexpr std::size_t meta_dynamic_extent = (std::numeric_limits<std::size_t>::max)();
+
 } // namespace entt
 
 #endif

src/entt/meta/node.hpp

@@ -1,29 +1,28 @@
 #ifndef ENTT_META_NODE_HPP
 #define ENTT_META_NODE_HPP
 
+#include <array>
+#include <bit>
 #include <cstddef>
 #include <memory>
 #include <type_traits>
 #include <utility>
 #include <vector>
 #include "../config/config.h"
-#include "../core/attribute.h"
 #include "../core/bit.hpp"
+#include "../core/concepts.hpp"
 #include "../core/enum.hpp"
 #include "../core/fwd.hpp"
 #include "../core/type_info.hpp"
 #include "../core/type_traits.hpp"
 #include "../core/utility.hpp"
 #include "context.hpp"
+#include "fwd.hpp"
 #include "type_traits.hpp"
 
 namespace entt {
 
-class meta_any;
-class meta_type;
-struct meta_handle;
-
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
 enum class meta_traits : std::uint32_t {
@@ -37,24 +36,24 @@ enum class meta_traits : std::uint32_t {
     is_enum = 0x0040,
     is_class = 0x0080,
     is_pointer = 0x0100,
-    is_meta_pointer_like = 0x0200,
-    is_meta_sequence_container = 0x0400,
-    is_meta_associative_container = 0x0800,
+    is_pointer_like = 0x0200,
+    is_sequence_container = 0x0400,
+    is_associative_container = 0x0800,
     _user_defined_traits = 0xFFFF,
     _entt_enum_as_bitmask = 0xFFFF
 };
 
 template<typename Type>
+requires std::is_enum_v<Type>
 [[nodiscard]] auto meta_to_user_traits(const meta_traits traits) noexcept {
-    static_assert(std::is_enum_v<Type>, "Invalid enum type");
-    constexpr auto shift = popcount(static_cast<std::underlying_type_t<meta_traits>>(meta_traits::_user_defined_traits));
+    constexpr auto shift = std::popcount(static_cast<std::underlying_type_t<meta_traits>>(meta_traits::_user_defined_traits));
     return Type{static_cast<std::underlying_type_t<Type>>(static_cast<std::underlying_type_t<meta_traits>>(traits) >> shift)};
 }
 
 template<typename Type>
+requires std::is_enum_v<Type>
 [[nodiscard]] auto user_to_meta_traits(const Type value) noexcept {
-    static_assert(std::is_enum_v<Type>, "Invalid enum type");
-    constexpr auto shift = popcount(static_cast<std::underlying_type_t<meta_traits>>(meta_traits::_user_defined_traits));
+    constexpr auto shift = std::popcount(static_cast<std::underlying_type_t<meta_traits>>(meta_traits::_user_defined_traits));
     const auto traits = static_cast<std::underlying_type_t<internal::meta_traits>>(static_cast<std::underlying_type_t<Type>>(value));
     ENTT_ASSERT(traits < ((~static_cast<std::underlying_type_t<meta_traits>>(meta_traits::_user_defined_traits)) >> shift), "Invalid traits");
     return meta_traits{traits << shift};
@@ -63,24 +62,18 @@ template<typename Type>
 struct meta_type_node;
 
 struct meta_custom_node {
-    id_type type{};
-    std::shared_ptr<void> value{};
-};
-
-struct meta_prop_node {
     id_type id{};
-    meta_type_node (*type)(const meta_context &) noexcept {};
     std::shared_ptr<void> value{};
 };
 
 struct meta_base_node {
-    id_type type{};
-    meta_type_node (*resolve)(const meta_context &) noexcept {};
+    id_type id{};
+    const meta_type_node &(*type)(const meta_context &) noexcept {};
     const void *(*cast)(const void *) noexcept {};
 };
 
 struct meta_conv_node {
-    id_type type{};
+    id_type id{};
     meta_any (*conv)(const meta_ctx &, const void *){};
 };
 
@@ -93,44 +86,40 @@ struct meta_ctor_node {
     meta_any (*invoke)(const meta_ctx &, meta_any *const){};
 };
 
-struct meta_dtor_node {
-    void (*dtor)(void *){};
-};
-
 struct meta_data_node {
     using size_type = std::size_t;
 
     id_type id{};
+    const char *name{};
     meta_traits traits{meta_traits::is_none};
     size_type arity{0u};
-    meta_type_node (*type)(const meta_context &) noexcept {};
+    const meta_type_node &(*type)(const meta_context &) noexcept {};
     meta_type (*arg)(const meta_ctx &, const size_type) noexcept {};
     bool (*set)(meta_handle, meta_any){};
-    meta_any (*get)(const meta_ctx &, meta_handle){};
+    meta_any (*get)(meta_handle){};
     meta_custom_node custom{};
-    std::vector<meta_prop_node> prop{};
 };
 
 struct meta_func_node {
     using size_type = std::size_t;
 
     id_type id{};
+    const char *name{};
     meta_traits traits{meta_traits::is_none};
     size_type arity{0u};
-    meta_type_node (*ret)(const meta_context &) noexcept {};
+    const meta_type_node &(*ret)(const meta_context &) noexcept {};
     meta_type (*arg)(const meta_ctx &, const size_type) noexcept {};
-    meta_any (*invoke)(const meta_ctx &, meta_handle, meta_any *const){};
-    std::shared_ptr<meta_func_node> next{};
+    meta_any (*invoke)(meta_handle, meta_any *const){};
+    std::unique_ptr<meta_func_node> next;
     meta_custom_node custom{};
-    std::vector<meta_prop_node> prop{};
 };
 
 struct meta_template_node {
     using size_type = std::size_t;
 
     size_type arity{0u};
-    meta_type_node (*resolve)(const meta_context &) noexcept {};
-    meta_type_node (*arg)(const meta_context &, const size_type) noexcept {};
+    const meta_type_node &(*resolve)(const meta_context &) noexcept {};
+    const meta_type_node &(*arg)(const meta_context &, const size_type) noexcept {};
 };
 
 struct meta_type_descriptor {
@@ -139,7 +128,6 @@ struct meta_type_descriptor {
     std::vector<meta_conv_node> conv{};
     std::vector<meta_data_node> data{};
     std::vector<meta_func_node> func{};
-    std::vector<meta_prop_node> prop{};
 };
 
 struct meta_type_node {
@@ -147,28 +135,27 @@ struct meta_type_node {
 
     const type_info *info{};
     id_type id{};
+    const char *name{};
     meta_traits traits{meta_traits::is_none};
     size_type size_of{0u};
-    meta_type_node (*resolve)(const meta_context &) noexcept {};
-    meta_type_node (*remove_pointer)(const meta_context &) noexcept {};
+    const meta_type_node &(*remove_pointer)(const meta_context &) noexcept {};
     meta_any (*default_constructor)(const meta_ctx &){};
     double (*conversion_helper)(void *, const void *){};
     meta_any (*from_void)(const meta_ctx &, void *, const void *){};
     meta_template_node templ{};
-    meta_dtor_node dtor{};
     meta_custom_node custom{};
-    std::shared_ptr<meta_type_descriptor> details{};
+    std::unique_ptr<meta_type_descriptor> details{};
 };
 
-template<auto Member, typename Type, typename Value>
+template<typename Type, typename Value>
 [[nodiscard]] auto *find_member(Type &from, const Value value) {
     for(auto &&elem: from) {
-        if((elem.*Member) == value) {
+        if(elem.id == value) {
             return &elem;
         }
     }
 
-    return static_cast<typename Type::value_type *>(nullptr);
+    return static_cast<Type::value_type *>(nullptr);
 }
 
 [[nodiscard]] inline auto *find_overload(meta_func_node *curr, std::remove_pointer_t<decltype(meta_func_node::invoke)> *const ref) {
@@ -177,17 +164,19 @@ template<auto Member, typename Type, typename Value>
 }
 
 template<auto Member>
-[[nodiscard]] auto *look_for(const meta_context &context, const meta_type_node &node, const id_type id) {
-    using value_type = typename std::remove_reference_t<decltype((node.details.get()->*Member))>::value_type;
+[[nodiscard]] auto *look_for(const meta_context &context, const meta_type_node &node, const id_type id, bool recursive) {
+    using value_type = std::remove_reference_t<decltype((node.details.get()->*Member))>::value_type;
 
     if(node.details) {
-        if(auto *member = find_member<&value_type::id>((node.details.get()->*Member), id); member != nullptr) {
+        if(auto *member = find_member((node.details.get()->*Member), id); member != nullptr) {
             return member;
         }
 
-        for(auto &&curr: node.details->base) {
-            if(auto *elem = look_for<Member>(context, curr.resolve(context), id); elem) {
-                return elem;
+        if(recursive) {
+            for(auto &&curr: node.details->base) {
+                if(auto *elem = look_for<Member>(context, curr.type(context), id, recursive); elem) {
+                    return elem;
+                }
             }
         }
     }
@@ -195,26 +184,23 @@ template<auto Member>
     return static_cast<value_type *>(nullptr);
 }
 
-template<typename Type>
-meta_type_node resolve(const meta_context &) noexcept;
+template<cvref_unqualified Type>
+const meta_type_node &resolve(const meta_context &) noexcept;
 
 template<typename... Args>
-[[nodiscard]] auto meta_arg_node(const meta_context &context, type_list<Args...>, [[maybe_unused]] const std::size_t index) noexcept {
-    [[maybe_unused]] std::size_t pos{};
-    meta_type_node (*value)(const meta_context &) noexcept = nullptr;
-    ((value = (pos++ == index ? &resolve<std::remove_cv_t<std::remove_reference_t<Args>>> : value)), ...);
-    ENTT_ASSERT(value != nullptr, "Out of bounds");
-    return value(context);
+[[nodiscard]] const meta_type_node &meta_arg_node(const meta_context &context, type_list<Args...>, const std::size_t index) noexcept {
+    using resolve_type = const meta_type_node &(*)(const meta_context &) noexcept;
+    constexpr std::array<resolve_type, sizeof...(Args)> list{&resolve<std::remove_cvref_t<Args>>...};
+    ENTT_ASSERT(index < sizeof...(Args), "Out of bounds");
+    return list[index](context);
 }
 
-[[nodiscard]] inline const void *try_cast(const meta_context &context, const meta_type_node &from, const meta_type_node &to, const void *instance) noexcept {
-    if((from.info != nullptr) && (to.info != nullptr) && *from.info == *to.info) {
-        return instance;
-    }
-
+[[nodiscard]] inline const void *try_cast(const meta_context &context, const meta_type_node &from, const id_type to, const void *instance) noexcept {
     if(from.details) {
         for(auto &&curr: from.details->base) {
-            if(const void *elem = try_cast(context, curr.resolve(context), to, curr.cast(instance)); elem) {
+            if(const void *other = curr.cast(instance); curr.id == to) {
+                return other;
+            } else if(const void *elem = try_cast(context, curr.type(context), to, other); elem) {
                 return elem;
             }
         }
@@ -223,49 +209,12 @@ template<typename... Args>
     return nullptr;
 }
 
-template<typename Func>
-[[nodiscard]] inline auto try_convert(const meta_context &context, const meta_type_node &from, const type_info &to, const bool arithmetic_or_enum, const void *instance, Func func) {
-    if(from.info && *from.info == to) {
-        return func(instance, from);
-    }
-
-    if(from.details) {
-        for(auto &&elem: from.details->conv) {
-            if(elem.type == to.hash()) {
-                return func(instance, elem);
-            }
-        }
-
-        for(auto &&curr: from.details->base) {
-            if(auto other = try_convert(context, curr.resolve(context), to, arithmetic_or_enum, curr.cast(instance), func); other) {
-                return other;
-            }
-        }
-    }
-
-    if(from.conversion_helper && arithmetic_or_enum) {
-        return func(instance, from.conversion_helper);
-    }
-
-    return func(instance);
-}
-
-[[nodiscard]] inline const meta_type_node *try_resolve(const meta_context &context, const type_info &info) noexcept {
-    const auto it = context.value.find(info.hash());
-    return it != context.value.end() ? &it->second : nullptr;
-}
-
 template<typename Type>
-[[nodiscard]] meta_type_node resolve(const meta_context &context) noexcept {
-    static_assert(std::is_same_v<Type, std::remove_const_t<std::remove_reference_t<Type>>>, "Invalid type");
-
-    if(auto *elem = try_resolve(context, type_id<Type>()); elem) {
-        return *elem;
-    }
-
+auto setup_node_for() noexcept {
     meta_type_node node{
         &type_id<Type>(),
         type_id<Type>().hash(),
+        nullptr,
         (std::is_arithmetic_v<Type> ? meta_traits::is_arithmetic : meta_traits::is_none)
             | (std::is_integral_v<Type> ? meta_traits::is_integral : meta_traits::is_none)
             | (std::is_signed_v<Type> ? meta_traits::is_signed : meta_traits::is_none)
@@ -273,12 +222,11 @@ template<typename Type>
             | (std::is_enum_v<Type> ? meta_traits::is_enum : meta_traits::is_none)
             | (std::is_class_v<Type> ? meta_traits::is_class : meta_traits::is_none)
             | (std::is_pointer_v<Type> ? meta_traits::is_pointer : meta_traits::is_none)
-            | (is_meta_pointer_like_v<Type> ? meta_traits::is_meta_pointer_like : meta_traits::is_none)
-            | (is_complete_v<meta_sequence_container_traits<Type>> ? meta_traits::is_meta_sequence_container : meta_traits::is_none)
-            | (is_complete_v<meta_associative_container_traits<Type>> ? meta_traits::is_meta_associative_container : meta_traits::is_none),
+            | (is_meta_pointer_like_v<Type> ? meta_traits::is_pointer_like : meta_traits::is_none)
+            | (is_complete_v<meta_sequence_container_traits<Type>> ? meta_traits::is_sequence_container : meta_traits::is_none)
+            | (is_complete_v<meta_associative_container_traits<Type>> ? meta_traits::is_associative_container : meta_traits::is_none),
         size_of_v<Type>,
-        &resolve<Type>,
-        &resolve<std::remove_cv_t<std::remove_pointer_t<Type>>>};
+        &resolve<std::remove_const_t<std::remove_pointer_t<Type>>>};
 
     if constexpr(std::is_default_constructible_v<Type>) {
         node.default_constructor = +[](const meta_ctx &ctx) {
@@ -298,10 +246,15 @@ template<typename Type>
 
     if constexpr(!std::is_void_v<Type> && !std::is_function_v<Type>) {
         node.from_void = +[](const meta_ctx &ctx, void *elem, const void *celem) {
-            if(elem) {
+            if(elem && celem) { // ownership construction request
+                return meta_any{ctx, std::in_place, static_cast<std::decay_t<Type> *>(elem)};
+            }
+
+            if(elem) { // non-const reference construction request
                 return meta_any{ctx, std::in_place_type<std::decay_t<Type> &>, *static_cast<std::decay_t<Type> *>(elem)};
             }
 
+            // const reference construction request
             return meta_any{ctx, std::in_place_type<const std::decay_t<Type> &>, *static_cast<const std::decay_t<Type> *>(celem)};
         };
     }
@@ -310,12 +263,24 @@ template<typename Type>
         node.templ = meta_template_node{
             meta_template_traits<Type>::args_type::size,
             &resolve<typename meta_template_traits<Type>::class_type>,
-            +[](const meta_context &area, const std::size_t index) noexcept { return meta_arg_node(area, typename meta_template_traits<Type>::args_type{}, index); }};
+            +[](const meta_context &area, const std::size_t index) noexcept -> decltype(auto) { return meta_arg_node(area, typename meta_template_traits<Type>::args_type{}, index); }};
     }
 
     return node;
 }
 
+[[nodiscard]] inline const meta_type_node *try_resolve(const meta_context &context, const type_info &info) noexcept {
+    const auto it = context.bucket.find(info.hash());
+    return (it != context.bucket.end()) ? it->second.get() : nullptr;
+}
+
+template<cvref_unqualified Type>
+[[nodiscard]] const meta_type_node &resolve(const meta_context &context) noexcept {
+    static const meta_type_node node = setup_node_for<Type>();
+    const auto *elem = try_resolve(context, *node.info);
+    return (elem == nullptr) ? node : *elem;
+}
+
 } // namespace internal
 /*! @endcond */
 

src/entt/meta/pointer.hpp

@@ -9,24 +9,6 @@
 
 namespace entt {
 
-/**
- * @brief Makes plain pointers pointer-like types for the meta system.
- * @tparam Type Element type.
- */
-template<typename Type>
-struct is_meta_pointer_like<Type *>
-    : std::true_type {};
-
-/**
- * @brief Partial specialization used to reject pointers to arrays.
- * @tparam Type Type of elements of the array.
- * @tparam N Number of elements of the array.
- */
-template<typename Type, std::size_t N>
-// NOLINTNEXTLINE(cppcoreguidelines-avoid-c-arrays, modernize-avoid-c-arrays)
-struct is_meta_pointer_like<Type (*)[N]>
-    : std::false_type {};
-
 /**
  * @brief Makes `std::shared_ptr`s of any type pointer-like types for the meta
  * system.
@@ -46,6 +28,15 @@ template<typename Type, typename... Args>
 struct is_meta_pointer_like<std::unique_ptr<Type, Args...>>
     : std::true_type {};
 
+/**
+ * @brief Specialization for self-proclaimed meta pointer like types.
+ * @tparam Type Element type.
+ */
+template<typename Type>
+requires requires { typename Type::is_meta_pointer_like; }
+struct is_meta_pointer_like<Type>
+    : std::true_type {};
+
 } // namespace entt
 
 #endif

src/entt/meta/policy.hpp

@@ -5,33 +5,49 @@
 
 namespace entt {
 
-/*! @brief Empty class type used to request the _as ref_ policy. */
-struct as_ref_t final {
-    /*! @cond TURN_OFF_DOXYGEN */
-    template<typename Type>
-    static constexpr bool value = std::is_reference_v<Type> && !std::is_const_v<std::remove_reference_t<Type>>;
-    /*! @endcond */
-};
+/*! @cond ENTT_INTERNAL */
+namespace internal {
 
-/*! @brief Empty class type used to request the _as cref_ policy. */
-struct as_cref_t final {
-    /*! @cond TURN_OFF_DOXYGEN */
-    template<typename Type>
-    static constexpr bool value = std::is_reference_v<Type>;
-    /*! @endcond */
-};
+struct meta_policy {};
+
+} // namespace internal
+/*! @endcond */
 
 /*! @brief Empty class type used to request the _as-is_ policy. */
-struct as_is_t final {
-    /*! @cond TURN_OFF_DOXYGEN */
+struct as_value_t final: private internal::meta_policy {
+    /*! @cond ENTT_INTERNAL */
     template<typename>
     static constexpr bool value = true;
     /*! @endcond */
 };
 
 /*! @brief Empty class type used to request the _as void_ policy. */
-struct as_void_t final {
-    /*! @cond TURN_OFF_DOXYGEN */
+struct as_void_t final: private internal::meta_policy {
+    /*! @cond ENTT_INTERNAL */
+    template<typename>
+    static constexpr bool value = true;
+    /*! @endcond */
+};
+
+/*! @brief Empty class type used to request the _as ref_ policy. */
+struct as_ref_t final: private internal::meta_policy {
+    /*! @cond ENTT_INTERNAL */
+    template<typename Type>
+    static constexpr bool value = std::is_reference_v<Type> && !std::is_const_v<std::remove_reference_t<Type>>;
+    /*! @endcond */
+};
+
+/*! @brief Empty class type used to request the _as cref_ policy. */
+struct as_cref_t final: private internal::meta_policy {
+    /*! @cond ENTT_INTERNAL */
+    template<typename Type>
+    static constexpr bool value = std::is_reference_v<Type>;
+    /*! @endcond */
+};
+
+/*! @brief Empty class type used to request the _as auto_ policy. */
+struct as_is_t final: private internal::meta_policy {
+    /*! @cond ENTT_INTERNAL */
     template<typename>
     static constexpr bool value = true;
     /*! @endcond */
@@ -44,7 +60,7 @@ struct as_void_t final {
  */
 template<typename Type>
 struct is_meta_policy
-    : std::bool_constant<std::is_same_v<Type, as_ref_t> || std::is_same_v<Type, as_cref_t> || std::is_same_v<Type, as_is_t> || std::is_same_v<Type, as_void_t>> {};
+    : std::bool_constant<std::is_base_of_v<internal::meta_policy, Type>> {};
 
 /**
  * @brief Helper variable template.
@@ -53,6 +69,13 @@ struct is_meta_policy
 template<typename Type>
 inline constexpr bool is_meta_policy_v = is_meta_policy<Type>::value;
 
+/**
+ * @brief Specifies whether a type is a meta policy.
+ * @tparam Type Type to check.
+ */
+template<typename Type>
+concept meta_policy = is_meta_policy_v<Type>;
+
 } // namespace entt
 
 #endif

src/entt/meta/range.hpp

@@ -1,16 +1,19 @@
 #ifndef ENTT_META_RANGE_HPP
 #define ENTT_META_RANGE_HPP
 
+#include <compare>
+#include <concepts>
 #include <cstddef>
 #include <iterator>
 #include <utility>
 #include "../core/fwd.hpp"
 #include "../core/iterator.hpp"
+#include "../stl/iterator.hpp"
 #include "context.hpp"
 
 namespace entt {
 
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
 struct meta_base_node;
@@ -37,7 +40,7 @@ struct meta_range_iterator final {
     }
 
     constexpr meta_range_iterator operator++(int) noexcept {
-        meta_range_iterator orig = *this;
+        const meta_range_iterator orig = *this;
         return ++(*this), orig;
     }
 
@@ -46,7 +49,7 @@ struct meta_range_iterator final {
     }
 
     constexpr meta_range_iterator operator--(int) noexcept {
-        meta_range_iterator orig = *this;
+        const meta_range_iterator orig = *this;
         return operator--(), orig;
     }
 
@@ -69,10 +72,8 @@ struct meta_range_iterator final {
     }
 
     [[nodiscard]] constexpr reference operator[](const difference_type value) const noexcept {
-        if constexpr(std::is_same_v<It, typename decltype(meta_context::value)::const_iterator>) {
-            return {it[value].first, Type{*ctx, it[value].second}};
-        } else if constexpr(std::is_same_v<typename std::iterator_traits<It>::value_type, meta_base_node>) {
-            return {it[value].type, Type{*ctx, it[value]}};
+        if constexpr(std::is_same_v<It, typename meta_context::container_type::const_iterator>) {
+            return {it[value].first, Type{*ctx, *it[value].second}};
         } else {
             return {it[value].id, Type{*ctx, it[value]}};
         }
@@ -86,55 +87,23 @@ struct meta_range_iterator final {
         return operator[](0);
     }
 
-    template<typename... Args>
-    friend constexpr std::ptrdiff_t operator-(const meta_range_iterator<Args...> &, const meta_range_iterator<Args...> &) noexcept;
+    [[nodiscard]] constexpr std::ptrdiff_t operator-(const meta_range_iterator &other) const noexcept {
+        return it - other.it;
+    }
 
-    template<typename... Args>
-    friend constexpr bool operator==(const meta_range_iterator<Args...> &, const meta_range_iterator<Args...> &) noexcept;
+    [[nodiscard]] constexpr bool operator==(const meta_range_iterator &other) const noexcept {
+        return it == other.it;
+    }
 
-    template<typename... Args>
-    friend constexpr bool operator<(const meta_range_iterator<Args...> &, const meta_range_iterator<Args...> &) noexcept;
+    [[nodiscard]] constexpr auto operator<=>(const meta_range_iterator &other) const noexcept {
+        return it <=> other.it;
+    }
 
 private:
     It it;
     const meta_ctx *ctx;
 };
 
-template<typename... Args>
-[[nodiscard]] constexpr std::ptrdiff_t operator-(const meta_range_iterator<Args...> &lhs, const meta_range_iterator<Args...> &rhs) noexcept {
-    return lhs.it - rhs.it;
-}
-
-template<typename... Args>
-[[nodiscard]] constexpr bool operator==(const meta_range_iterator<Args...> &lhs, const meta_range_iterator<Args...> &rhs) noexcept {
-    return lhs.it == rhs.it;
-}
-
-template<typename... Args>
-[[nodiscard]] constexpr bool operator!=(const meta_range_iterator<Args...> &lhs, const meta_range_iterator<Args...> &rhs) noexcept {
-    return !(lhs == rhs);
-}
-
-template<typename... Args>
-[[nodiscard]] constexpr bool operator<(const meta_range_iterator<Args...> &lhs, const meta_range_iterator<Args...> &rhs) noexcept {
-    return lhs.it < rhs.it;
-}
-
-template<typename... Args>
-[[nodiscard]] constexpr bool operator>(const meta_range_iterator<Args...> &lhs, const meta_range_iterator<Args...> &rhs) noexcept {
-    return rhs < lhs;
-}
-
-template<typename... Args>
-[[nodiscard]] constexpr bool operator<=(const meta_range_iterator<Args...> &lhs, const meta_range_iterator<Args...> &rhs) noexcept {
-    return !(lhs > rhs);
-}
-
-template<typename... Args>
-[[nodiscard]] constexpr bool operator>=(const meta_range_iterator<Args...> &lhs, const meta_range_iterator<Args...> &rhs) noexcept {
-    return !(lhs < rhs);
-}
-
 } // namespace internal
 /*! @endcond */
 
@@ -143,7 +112,7 @@ template<typename... Args>
  * @tparam Type Type of meta objects returned.
  * @tparam It Type of forward iterator.
  */
-template<typename Type, typename It>
+template<typename Type, stl::forward_iterator It>
 using meta_range = iterable_adaptor<internal::meta_range_iterator<Type, It>>;
 
 } // namespace entt

src/entt/meta/resolve.hpp

@@ -19,8 +19,8 @@ namespace entt {
  */
 template<typename Type>
 [[nodiscard]] meta_type resolve(const meta_ctx &ctx) noexcept {
-    auto &&context = internal::meta_context::from(ctx);
-    return {ctx, internal::resolve<std::remove_cv_t<std::remove_reference_t<Type>>>(context)};
+    const auto &context = internal::meta_context::from(ctx);
+    return {ctx, internal::resolve<std::remove_cvref_t<Type>>(context)};
 }
 
 /**
@@ -38,16 +38,16 @@ template<typename Type>
  * @param ctx The context from which to search for meta types.
  * @return An iterable range to use to visit all meta types.
  */
-[[nodiscard]] inline meta_range<meta_type, typename decltype(internal::meta_context::value)::const_iterator> resolve(const meta_ctx &ctx) noexcept {
-    auto &&context = internal::meta_context::from(ctx);
-    return {{ctx, context.value.cbegin()}, {ctx, context.value.cend()}};
+[[nodiscard]] inline meta_range<meta_type, typename internal::meta_context::container_type::const_iterator> resolve(const meta_ctx &ctx) noexcept {
+    const auto &context = internal::meta_context::from(ctx);
+    return {{ctx, context.bucket.cbegin()}, {ctx, context.bucket.cend()}};
 }
 
 /**
  * @brief Returns a range to use to visit all meta types.
  * @return An iterable range to use to visit all meta types.
  */
-[[nodiscard]] inline meta_range<meta_type, typename decltype(internal::meta_context::value)::const_iterator> resolve() noexcept {
+[[nodiscard]] inline meta_range<meta_type, typename internal::meta_context::container_type::const_iterator> resolve() noexcept {
     return resolve(locator<meta_ctx>::value_or());
 }
 
@@ -83,7 +83,7 @@ template<typename Type>
  * @return The meta type associated with the given type info object, if any.
  */
 [[nodiscard]] inline meta_type resolve(const meta_ctx &ctx, const type_info &info) noexcept {
-    auto &&context = internal::meta_context::from(ctx);
+    const auto &context = internal::meta_context::from(ctx);
     const auto *elem = internal::try_resolve(context, info);
     return (elem != nullptr) ? meta_type{ctx, *elem} : meta_type{};
 }

src/entt/meta/utility.hpp

@@ -93,11 +93,11 @@ struct meta_function_descriptor<Type, Ret (*)(MaybeType, Args...)>
     : meta_function_descriptor_traits<
           Ret,
           std::conditional_t<
-              std::is_same_v<std::remove_cv_t<std::remove_reference_t<MaybeType>>, Type> || std::is_base_of_v<std::remove_cv_t<std::remove_reference_t<MaybeType>>, Type>,
+              std::is_same_v<std::remove_cvref_t<MaybeType>, Type> || std::is_base_of_v<std::remove_cvref_t<MaybeType>, Type>,
               type_list<Args...>,
               type_list<MaybeType, Args...>>,
-          !(std::is_same_v<std::remove_cv_t<std::remove_reference_t<MaybeType>>, Type> || std::is_base_of_v<std::remove_cv_t<std::remove_reference_t<MaybeType>>, Type>),
-          std::is_const_v<std::remove_reference_t<MaybeType>> && (std::is_same_v<std::remove_cv_t<std::remove_reference_t<MaybeType>>, Type> || std::is_base_of_v<std::remove_cv_t<std::remove_reference_t<MaybeType>>, Type>)> {};
+          !(std::is_same_v<std::remove_cvref_t<MaybeType>, Type> || std::is_base_of_v<std::remove_cvref_t<MaybeType>, Type>),
+          std::is_const_v<std::remove_reference_t<MaybeType>> && (std::is_same_v<std::remove_cvref_t<MaybeType>, Type> || std::is_base_of_v<std::remove_cvref_t<MaybeType>, Type>)> {};
 
 /**
  * @brief Meta function descriptor.
@@ -124,19 +124,20 @@ struct meta_function_descriptor<Type, Ret (*)()>
 template<typename Type, typename Candidate>
 class meta_function_helper {
     template<typename Ret, typename... Args, typename Class>
-    static constexpr meta_function_descriptor<Type, Ret (Class::*)(Args...) const> get_rid_of_noexcept(Ret (Class::*)(Args...) const);
+    static meta_function_descriptor<Type, Ret (Class::*)(Args...) const> get_rid_of_noexcept(Ret (Class::*)(Args...) const);
 
     template<typename Ret, typename... Args, typename Class>
-    static constexpr meta_function_descriptor<Type, Ret (Class::*)(Args...)> get_rid_of_noexcept(Ret (Class::*)(Args...));
+    static meta_function_descriptor<Type, Ret (Class::*)(Args...)> get_rid_of_noexcept(Ret (Class::*)(Args...));
 
-    template<typename Ret, typename Class, typename = std::enable_if_t<std::is_member_object_pointer_v<Ret Class::*>>>
-    static constexpr meta_function_descriptor<Type, Ret Class::*> get_rid_of_noexcept(Ret Class::*);
+    template<typename Ret, typename Class>
+    requires std::is_member_object_pointer_v<Ret Class::*>
+    static meta_function_descriptor<Type, Ret Class::*> get_rid_of_noexcept(Ret Class::*);
 
     template<typename Ret, typename... Args>
-    static constexpr meta_function_descriptor<Type, Ret (*)(Args...)> get_rid_of_noexcept(Ret (*)(Args...));
+    static meta_function_descriptor<Type, Ret (*)(Args...)> get_rid_of_noexcept(Ret (*)(Args...));
 
     template<typename Class>
-    static constexpr meta_function_descriptor<Class, decltype(&Class::operator())> get_rid_of_noexcept(Class);
+    static meta_function_descriptor<Class, decltype(&Class::operator())> get_rid_of_noexcept(Class);
 
 public:
     /*! @brief The meta function descriptor of the given function. */
@@ -149,7 +150,7 @@ public:
  * @tparam Candidate The actual function to associate with the reflected type.
  */
 template<typename Type, typename Candidate>
-using meta_function_helper_t = typename meta_function_helper<Type, Candidate>::type;
+using meta_function_helper_t = meta_function_helper<Type, Candidate>::type;
 
 /**
  * @brief Wraps a value depending on the given policy.
@@ -164,15 +165,15 @@ using meta_function_helper_t = typename meta_function_helper<Type, Candidate>::t
  * @param value Value to wrap.
  * @return A meta any containing the returned value, if any.
  */
-template<typename Policy = as_is_t, typename Type>
-[[nodiscard]] std::enable_if_t<is_meta_policy_v<Policy>, meta_any> meta_dispatch(const meta_ctx &ctx, [[maybe_unused]] Type &&value) {
-    if constexpr(std::is_same_v<Policy, as_void_t>) {
-        return meta_any{ctx, std::in_place_type<void>};
-    } else if constexpr(std::is_same_v<Policy, as_ref_t>) {
-        return meta_any{ctx, std::in_place_type<Type>, value};
-    } else if constexpr(std::is_same_v<Policy, as_cref_t>) {
+template<meta_policy Policy = as_value_t, typename Type>
+[[nodiscard]] meta_any meta_dispatch(const meta_ctx &ctx, [[maybe_unused]] Type &&value) {
+    if constexpr(std::is_same_v<Policy, as_cref_t>) {
         static_assert(std::is_lvalue_reference_v<Type>, "Invalid type");
         return meta_any{ctx, std::in_place_type<const std::remove_reference_t<Type> &>, std::as_const(value)};
+    } else if constexpr(std::is_same_v<Policy, as_ref_t> || (std::is_same_v<Policy, as_is_t> && std::is_lvalue_reference_v<Type>)) {
+        return meta_any{ctx, std::in_place_type<Type>, value};
+    } else if constexpr(std::is_same_v<Policy, as_void_t>) {
+        return meta_any{ctx, std::in_place_type<void>};
     } else {
         return meta_any{ctx, std::forward<Type>(value)};
     }
@@ -185,135 +186,12 @@ template<typename Policy = as_is_t, typename Type>
  * @param value Value to wrap.
  * @return A meta any containing the returned value, if any.
  */
-template<typename Policy = as_is_t, typename Type>
-[[nodiscard]] std::enable_if_t<is_meta_policy_v<Policy>, meta_any> meta_dispatch(Type &&value) {
+template<meta_policy Policy = as_value_t, typename Type>
+[[nodiscard]] meta_any meta_dispatch(Type &&value) {
     return meta_dispatch<Policy, Type>(locator<meta_ctx>::value_or(), std::forward<Type>(value));
 }
 
-/**
- * @brief Returns the meta type of the i-th element of a list of arguments.
- * @tparam Type Type list of the actual types of arguments.
- * @param ctx The context from which to search for meta types.
- * @param index The index of the element for which to return the meta type.
- * @return The meta type of the i-th element of the list of arguments.
- */
-template<typename Type>
-[[nodiscard]] static meta_type meta_arg(const meta_ctx &ctx, const std::size_t index) noexcept {
-    auto &&context = internal::meta_context::from(ctx);
-    return {ctx, internal::meta_arg_node(context, Type{}, index)};
-}
-
-/**
- * @brief Returns the meta type of the i-th element of a list of arguments.
- * @tparam Type Type list of the actual types of arguments.
- * @param index The index of the element for which to return the meta type.
- * @return The meta type of the i-th element of the list of arguments.
- */
-template<typename Type>
-[[nodiscard]] static meta_type meta_arg(const std::size_t index) noexcept {
-    return meta_arg<Type>(locator<meta_ctx>::value_or(), index);
-}
-
-/**
- * @brief Sets the value of a given variable.
- * @tparam Type Reflected type to which the variable is associated.
- * @tparam Data The actual variable to set.
- * @param instance An opaque instance of the underlying type, if required.
- * @param value Parameter to use to set the variable.
- * @return True in case of success, false otherwise.
- */
-template<typename Type, auto Data>
-[[nodiscard]] bool meta_setter([[maybe_unused]] meta_handle instance, [[maybe_unused]] meta_any value) {
-    if constexpr(!std::is_same_v<decltype(Data), Type> && !std::is_same_v<decltype(Data), std::nullptr_t>) {
-        if constexpr(std::is_member_function_pointer_v<decltype(Data)> || std::is_function_v<std::remove_reference_t<std::remove_pointer_t<decltype(Data)>>>) {
-            using descriptor = meta_function_helper_t<Type, decltype(Data)>;
-            using data_type = type_list_element_t<descriptor::is_static, typename descriptor::args_type>;
-
-            if(auto *const clazz = instance->try_cast<Type>(); clazz && value.allow_cast<data_type>()) {
-                std::invoke(Data, *clazz, value.cast<data_type>());
-                return true;
-            }
-        } else if constexpr(std::is_member_object_pointer_v<decltype(Data)>) {
-            using data_type = std::remove_reference_t<typename meta_function_helper_t<Type, decltype(Data)>::return_type>;
-
-            if constexpr(!std::is_array_v<data_type> && !std::is_const_v<data_type>) {
-                if(auto *const clazz = instance->try_cast<Type>(); clazz && value.allow_cast<data_type>()) {
-                    std::invoke(Data, *clazz) = value.cast<data_type>();
-                    return true;
-                }
-            }
-        } else {
-            using data_type = std::remove_reference_t<decltype(*Data)>;
-
-            if constexpr(!std::is_array_v<data_type> && !std::is_const_v<data_type>) {
-                if(value.allow_cast<data_type>()) {
-                    *Data = value.cast<data_type>();
-                    return true;
-                }
-            }
-        }
-    }
-
-    return false;
-}
-
-/**
- * @brief Gets the value of a given variable.
- *
- * @warning
- * The context provided is used only for the return type.<br/>
- * It's up to the caller to bind the arguments to the right context(s).
- *
- * @tparam Type Reflected type to which the variable is associated.
- * @tparam Data The actual variable to get.
- * @tparam Policy Optional policy (no policy set by default).
- * @param ctx The context from which to search for meta types.
- * @param instance An opaque instance of the underlying type, if required.
- * @return A meta any containing the value of the underlying variable.
- */
-template<typename Type, auto Data, typename Policy = as_is_t>
-[[nodiscard]] std::enable_if_t<is_meta_policy_v<Policy>, meta_any> meta_getter(const meta_ctx &ctx, [[maybe_unused]] meta_handle instance) {
-    if constexpr(std::is_member_pointer_v<decltype(Data)> || std::is_function_v<std::remove_reference_t<std::remove_pointer_t<decltype(Data)>>>) {
-        if constexpr(!std::is_array_v<std::remove_cv_t<std::remove_reference_t<std::invoke_result_t<decltype(Data), Type &>>>>) {
-            if constexpr(std::is_invocable_v<decltype(Data), Type &>) {
-                if(auto *clazz = instance->try_cast<Type>(); clazz) {
-                    return meta_dispatch<Policy>(ctx, std::invoke(Data, *clazz));
-                }
-            }
-
-            if constexpr(std::is_invocable_v<decltype(Data), const Type &>) {
-                if(auto *fallback = instance->try_cast<const Type>(); fallback) {
-                    return meta_dispatch<Policy>(ctx, std::invoke(Data, *fallback));
-                }
-            }
-        }
-
-        return meta_any{meta_ctx_arg, ctx};
-    } else if constexpr(std::is_pointer_v<decltype(Data)>) {
-        if constexpr(std::is_array_v<std::remove_pointer_t<decltype(Data)>>) {
-            return meta_any{meta_ctx_arg, ctx};
-        } else {
-            return meta_dispatch<Policy>(ctx, *Data);
-        }
-    } else {
-        return meta_dispatch<Policy>(ctx, Data);
-    }
-}
-
-/**
- * @brief Gets the value of a given variable.
- * @tparam Type Reflected type to which the variable is associated.
- * @tparam Data The actual variable to get.
- * @tparam Policy Optional policy (no policy set by default).
- * @param instance An opaque instance of the underlying type, if required.
- * @return A meta any containing the value of the underlying variable.
- */
-template<typename Type, auto Data, typename Policy = as_is_t>
-[[nodiscard]] std::enable_if_t<is_meta_policy_v<Policy>, meta_any> meta_getter(meta_handle instance) {
-    return meta_getter<Type, Data, Policy>(locator<meta_ctx>::value_or(), std::move(instance));
-}
-
-/*! @cond TURN_OFF_DOXYGEN */
+/*! @cond ENTT_INTERNAL */
 namespace internal {
 
 template<typename Policy, typename Candidate, typename... Args>
@@ -327,26 +205,26 @@ template<typename Policy, typename Candidate, typename... Args>
 }
 
 template<typename Type, typename Policy, typename Candidate, std::size_t... Index>
-[[nodiscard]] meta_any meta_invoke(const meta_ctx &ctx, [[maybe_unused]] meta_handle instance, Candidate &&candidate, [[maybe_unused]] meta_any *const args, std::index_sequence<Index...>) {
+[[nodiscard]] meta_any meta_invoke(meta_any &instance, Candidate &&candidate, [[maybe_unused]] meta_any *const args, std::index_sequence<Index...>) {
     using descriptor = meta_function_helper_t<Type, std::remove_reference_t<Candidate>>;
 
     // NOLINTBEGIN(cppcoreguidelines-pro-bounds-pointer-arithmetic) - waiting for C++20 (and std::span)
     if constexpr(std::is_invocable_v<std::remove_reference_t<Candidate>, const Type &, type_list_element_t<Index, typename descriptor::args_type>...>) {
-        if(const auto *const clazz = instance->try_cast<const Type>(); clazz && ((args + Index)->allow_cast<type_list_element_t<Index, typename descriptor::args_type>>() && ...)) {
-            return meta_invoke_with_args<Policy>(ctx, std::forward<Candidate>(candidate), *clazz, (args + Index)->cast<type_list_element_t<Index, typename descriptor::args_type>>()...);
+        if(const auto *const clazz = instance.try_cast<const Type>(); clazz && ((args + Index)->allow_cast<type_list_element_t<Index, typename descriptor::args_type>>() && ...)) {
+            return meta_invoke_with_args<Policy>(instance.context(), std::forward<Candidate>(candidate), *clazz, (args + Index)->cast<type_list_element_t<Index, typename descriptor::args_type>>()...);
         }
     } else if constexpr(std::is_invocable_v<std::remove_reference_t<Candidate>, Type &, type_list_element_t<Index, typename descriptor::args_type>...>) {
-        if(auto *const clazz = instance->try_cast<Type>(); clazz && ((args + Index)->allow_cast<type_list_element_t<Index, typename descriptor::args_type>>() && ...)) {
-            return meta_invoke_with_args<Policy>(ctx, std::forward<Candidate>(candidate), *clazz, (args + Index)->cast<type_list_element_t<Index, typename descriptor::args_type>>()...);
+        if(auto *const clazz = instance.try_cast<Type>(); clazz && ((args + Index)->allow_cast<type_list_element_t<Index, typename descriptor::args_type>>() && ...)) {
+            return meta_invoke_with_args<Policy>(instance.context(), std::forward<Candidate>(candidate), *clazz, (args + Index)->cast<type_list_element_t<Index, typename descriptor::args_type>>()...);
         }
     } else {
         if(((args + Index)->allow_cast<type_list_element_t<Index, typename descriptor::args_type>>() && ...)) {
-            return meta_invoke_with_args<Policy>(ctx, std::forward<Candidate>(candidate), (args + Index)->cast<type_list_element_t<Index, typename descriptor::args_type>>()...);
+            return meta_invoke_with_args<Policy>(instance.context(), std::forward<Candidate>(candidate), (args + Index)->cast<type_list_element_t<Index, typename descriptor::args_type>>()...);
         }
     }
     // NOLINTEND(cppcoreguidelines-pro-bounds-pointer-arithmetic)
 
-    return meta_any{meta_ctx_arg, ctx};
+    return meta_any{meta_ctx_arg, instance.context()};
 }
 
 template<typename Type, typename... Args, std::size_t... Index>
@@ -364,24 +242,105 @@ template<typename Type, typename... Args, std::size_t... Index>
 /*! @endcond */
 
 /**
- * @brief Tries to _invoke_ an object given a list of erased parameters.
- *
- * @warning
- * The context provided is used only for the return type.<br/>
- * It's up to the caller to bind the arguments to the right context(s).
- *
- * @tparam Type Reflected type to which the object to _invoke_ is associated.
- * @tparam Policy Optional policy (no policy set by default).
+ * @brief Returns the meta type of the i-th element of a list of arguments.
+ * @tparam Type Type list of the actual types of arguments.
  * @param ctx The context from which to search for meta types.
- * @tparam Candidate The type of the actual object to _invoke_.
- * @param instance An opaque instance of the underlying type, if required.
- * @param candidate The actual object to _invoke_.
- * @param args Parameters to use to _invoke_ the object.
- * @return A meta any containing the returned value, if any.
+ * @param index The index of the element for which to return the meta type.
+ * @return The meta type of the i-th element of the list of arguments.
  */
-template<typename Type, typename Policy = as_is_t, typename Candidate>
-[[nodiscard]] std::enable_if_t<is_meta_policy_v<Policy>, meta_any> meta_invoke(const meta_ctx &ctx, meta_handle instance, Candidate &&candidate, meta_any *const args) {
-    return internal::meta_invoke<Type, Policy>(ctx, std::move(instance), std::forward<Candidate>(candidate), args, std::make_index_sequence<meta_function_helper_t<Type, std::remove_reference_t<Candidate>>::args_type::size>{});
+template<typename Type>
+[[nodiscard]] meta_type meta_arg(const meta_ctx &ctx, const std::size_t index) noexcept {
+    const auto &context = internal::meta_context::from(ctx);
+    return {ctx, internal::meta_arg_node(context, Type{}, index)};
+}
+
+/**
+ * @brief Returns the meta type of the i-th element of a list of arguments.
+ * @tparam Type Type list of the actual types of arguments.
+ * @param index The index of the element for which to return the meta type.
+ * @return The meta type of the i-th element of the list of arguments.
+ */
+template<typename Type>
+[[nodiscard]] meta_type meta_arg(const std::size_t index) noexcept {
+    return meta_arg<Type>(locator<meta_ctx>::value_or(), index);
+}
+
+/**
+ * @brief Sets the value of a given variable.
+ * @tparam Type Reflected type to which the variable is associated.
+ * @tparam Data The actual variable to set.
+ * @param instance An opaque instance of the underlying type, if required.
+ * @param value Parameter to use to set the variable.
+ * @return True in case of success, false otherwise.
+ */
+template<typename Type, auto Data>
+[[nodiscard]] bool meta_setter([[maybe_unused]] meta_handle instance, [[maybe_unused]] meta_any value) {
+    if constexpr(std::is_member_function_pointer_v<decltype(Data)> || std::is_function_v<std::remove_reference_t<std::remove_pointer_t<decltype(Data)>>>) {
+        using descriptor = meta_function_helper_t<Type, decltype(Data)>;
+        using data_type = type_list_element_t<descriptor::is_static, typename descriptor::args_type>;
+
+        if(auto *const clazz = instance->try_cast<Type>(); clazz && value.allow_cast<data_type>()) {
+            std::invoke(Data, *clazz, value.cast<data_type>());
+            return true;
+        }
+    } else if constexpr(std::is_member_object_pointer_v<decltype(Data)>) {
+        using data_type = std::remove_reference_t<typename meta_function_helper_t<Type, decltype(Data)>::return_type>;
+
+        if constexpr(!std::is_array_v<data_type> && !std::is_const_v<data_type>) {
+            if(auto *const clazz = instance->try_cast<Type>(); clazz && value.allow_cast<data_type>()) {
+                std::invoke(Data, *clazz) = value.cast<data_type>();
+                return true;
+            }
+        }
+    } else if constexpr(std::is_pointer_v<decltype(Data)>) {
+        using data_type = std::remove_reference_t<decltype(*Data)>;
+
+        if constexpr(!std::is_array_v<data_type> && !std::is_const_v<data_type>) {
+            if(value.allow_cast<data_type>()) {
+                *Data = value.cast<data_type>();
+                return true;
+            }
+        }
+    }
+
+    return false;
+}
+
+/**
+ * @brief Gets the value of a given variable.
+ * @tparam Type Reflected type to which the variable is associated.
+ * @tparam Data The actual variable to get.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @param instance An opaque instance of the underlying type, if required.
+ * @return A meta any containing the value of the underlying variable.
+ */
+template<typename Type, auto Data, meta_policy Policy = as_value_t>
+[[nodiscard]] meta_any meta_getter(meta_handle instance) {
+    if constexpr(std::is_member_pointer_v<decltype(Data)> || std::is_function_v<std::remove_reference_t<std::remove_pointer_t<decltype(Data)>>>) {
+        if constexpr(!std::is_array_v<std::remove_cvref_t<std::invoke_result_t<decltype(Data), Type &>>>) {
+            if constexpr(std::is_invocable_v<decltype(Data), Type &>) {
+                if(auto *clazz = instance->try_cast<Type>(); clazz) {
+                    return meta_dispatch<Policy>(instance->context(), std::invoke(Data, *clazz));
+                }
+            }
+
+            if constexpr(std::is_invocable_v<decltype(Data), const Type &>) {
+                if(auto *fallback = instance->try_cast<const Type>(); fallback) {
+                    return meta_dispatch<Policy>(instance->context(), std::invoke(Data, *fallback));
+                }
+            }
+        }
+
+        return meta_any{meta_ctx_arg, instance->context()};
+    } else if constexpr(std::is_pointer_v<decltype(Data)>) {
+        if constexpr(std::is_array_v<std::remove_pointer_t<decltype(Data)>>) {
+            return meta_any{meta_ctx_arg, instance->context()};
+        } else {
+            return meta_dispatch<Policy>(instance->context(), *Data);
+        }
+    } else {
+        return meta_dispatch<Policy>(instance->context(), Data);
+    }
 }
 
 /**
@@ -394,29 +353,9 @@ template<typename Type, typename Policy = as_is_t, typename Candidate>
  * @param args Parameters to use to _invoke_ the object.
  * @return A meta any containing the returned value, if any.
  */
-template<typename Type, typename Policy = as_is_t, typename Candidate>
-[[nodiscard]] std::enable_if_t<is_meta_policy_v<Policy>, meta_any> meta_invoke(meta_handle instance, Candidate &&candidate, meta_any *const args) {
-    return meta_invoke<Type, Policy>(locator<meta_ctx>::value_or(), std::move(instance), std::forward<Candidate>(candidate), args);
-}
-
-/**
- * @brief Tries to invoke a function given a list of erased parameters.
- *
- * @warning
- * The context provided is used only for the return type.<br/>
- * It's up to the caller to bind the arguments to the right context(s).
- *
- * @tparam Type Reflected type to which the function is associated.
- * @tparam Candidate The actual function to invoke.
- * @tparam Policy Optional policy (no policy set by default).
- * @param ctx The context from which to search for meta types.
- * @param instance An opaque instance of the underlying type, if required.
- * @param args Parameters to use to invoke the function.
- * @return A meta any containing the returned value, if any.
- */
-template<typename Type, auto Candidate, typename Policy = as_is_t>
-[[nodiscard]] std::enable_if_t<is_meta_policy_v<Policy>, meta_any> meta_invoke(const meta_ctx &ctx, meta_handle instance, meta_any *const args) {
-    return internal::meta_invoke<Type, Policy>(ctx, std::move(instance), Candidate, args, std::make_index_sequence<meta_function_helper_t<Type, std::remove_reference_t<decltype(Candidate)>>::args_type::size>{});
+template<typename Type, meta_policy Policy = as_value_t, typename Candidate>
+[[nodiscard]] meta_any meta_invoke(meta_handle instance, Candidate &&candidate, meta_any *const args) {
+    return internal::meta_invoke<Type, Policy>(*instance.operator->(), std::forward<Candidate>(candidate), args, std::make_index_sequence<meta_function_helper_t<Type, std::remove_reference_t<Candidate>>::args_type::size>{});
 }
 
 /**
@@ -428,9 +367,9 @@ template<typename Type, auto Candidate, typename Policy = as_is_t>
  * @param args Parameters to use to invoke the function.
  * @return A meta any containing the returned value, if any.
  */
-template<typename Type, auto Candidate, typename Policy = as_is_t>
-[[nodiscard]] std::enable_if_t<is_meta_policy_v<Policy>, meta_any> meta_invoke(meta_handle instance, meta_any *const args) {
-    return meta_invoke<Type, Candidate, Policy>(locator<meta_ctx>::value_or(), std::move(instance), args);
+template<typename Type, auto Candidate, meta_policy Policy = as_value_t>
+[[nodiscard]] meta_any meta_invoke(meta_handle instance, meta_any *const args) {
+    return internal::meta_invoke<Type, Policy>(*instance.operator->(), Candidate, args, std::make_index_sequence<meta_function_helper_t<Type, std::remove_reference_t<decltype(Candidate)>>::args_type::size>{});
 }
 
 /**
@@ -478,13 +417,14 @@ template<typename Type, typename... Args>
  * @param args Parameters to use to _invoke_ the object.
  * @return A meta any containing the returned value, if any.
  */
-template<typename Type, typename Policy = as_is_t, typename Candidate>
+template<typename Type, typename Policy = as_value_t, typename Candidate>
 [[nodiscard]] meta_any meta_construct(const meta_ctx &ctx, Candidate &&candidate, meta_any *const args) {
-    if constexpr(meta_function_helper_t<Type, Candidate>::is_static || std::is_class_v<std::remove_cv_t<std::remove_reference_t<Candidate>>>) {
-        return internal::meta_invoke<Type, Policy>(ctx, {}, std::forward<Candidate>(candidate), args, std::make_index_sequence<meta_function_helper_t<Type, std::remove_reference_t<Candidate>>::args_type::size>{});
+    if constexpr(meta_function_helper_t<Type, Candidate>::is_static || std::is_class_v<std::remove_cvref_t<Candidate>>) {
+        meta_any placeholder{meta_ctx_arg, ctx};
+        return internal::meta_invoke<Type, Policy>(placeholder, std::forward<Candidate>(candidate), args, std::make_index_sequence<meta_function_helper_t<Type, std::remove_reference_t<Candidate>>::args_type::size>{});
     } else {
         // NOLINTNEXTLINE(cppcoreguidelines-pro-bounds-pointer-arithmetic) - waiting for C++20 (and std::span)
-        return internal::meta_invoke<Type, Policy>(ctx, *args, std::forward<Candidate>(candidate), args + 1u, std::make_index_sequence<meta_function_helper_t<Type, std::remove_reference_t<Candidate>>::args_type::size>{});
+        return internal::meta_invoke<Type, Policy>(*args, std::forward<Candidate>(candidate), args + 1u, std::make_index_sequence<meta_function_helper_t<Type, std::remove_reference_t<Candidate>>::args_type::size>{});
     }
 }
 
@@ -497,8 +437,8 @@ template<typename Type, typename Policy = as_is_t, typename Candidate>
  * @param args Parameters to use to _invoke_ the object.
  * @return A meta any containing the returned value, if any.
  */
-template<typename Type, typename Policy = as_is_t, typename Candidate>
-[[nodiscard]] std::enable_if_t<is_meta_policy_v<Policy>, meta_any> meta_construct(Candidate &&candidate, meta_any *const args) {
+template<typename Type, meta_policy Policy = as_value_t, typename Candidate>
+[[nodiscard]] meta_any meta_construct(Candidate &&candidate, meta_any *const args) {
     return meta_construct<Type, Policy>(locator<meta_ctx>::value_or(), std::forward<Candidate>(candidate), args);
 }
 
@@ -516,8 +456,8 @@ template<typename Type, typename Policy = as_is_t, typename Candidate>
  * @param args Parameters to use to invoke the function.
  * @return A meta any containing the returned value, if any.
  */
-template<typename Type, auto Candidate, typename Policy = as_is_t>
-[[nodiscard]] std::enable_if_t<is_meta_policy_v<Policy>, meta_any> meta_construct(const meta_ctx &ctx, meta_any *const args) {
+template<typename Type, auto Candidate, meta_policy Policy = as_value_t>
+[[nodiscard]] meta_any meta_construct(const meta_ctx &ctx, meta_any *const args) {
     return meta_construct<Type, Policy>(ctx, Candidate, args);
 }
 
@@ -529,8 +469,8 @@ template<typename Type, auto Candidate, typename Policy = as_is_t>
  * @param args Parameters to use to invoke the function.
  * @return A meta any containing the returned value, if any.
  */
-template<typename Type, auto Candidate, typename Policy = as_is_t>
-[[nodiscard]] std::enable_if_t<is_meta_policy_v<Policy>, meta_any> meta_construct(meta_any *const args) {
+template<typename Type, auto Candidate, meta_policy Policy = as_value_t>
+[[nodiscard]] meta_any meta_construct(meta_any *const args) {
     return meta_construct<Type, Candidate, Policy>(locator<meta_ctx>::value_or(), args);
 }
 

src/entt/natvis/core.natvis

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
     <Type Name="entt::basic_any&lt;*&gt;">
-		<DisplayString>{{ type={ info->alias,na }, policy={ mode,en } }}</DisplayString>
+		<DisplayString>{{ policy={ mode,en } }}</DisplayString>
     </Type>
 	<Type Name="entt::compressed_pair&lt;*&gt;">
 		<Intrinsic Name="first" Optional="true" Expression="((first_base*)this)->value"/>

src/entt/natvis/entity.natvis

@@ -47,7 +47,7 @@
 		<Intrinsic Name="is_valid_entity" Expression="!traits_type::version_mask || (*((traits_type::entity_type *)&amp;entity) &lt; (traits_type::version_mask &lt;&lt; traits_type::length))">
 			<Parameter Name="entity" Type="const traits_type::value_type &amp;"/>
 		</Intrinsic>
-		<DisplayString>{{ size={ packed.size() }, type={ info->alias,na } }}</DisplayString>
+		<DisplayString>{{ size={ packed.size() }, type={ descriptor->alias,na } }}</DisplayString>
 		<Expand>
 			<Item Name="[capacity]" ExcludeView="simple">packed.capacity()</Item>
 			<Item Name="[policy]" ExcludeView="simple">mode,en</Item>
@@ -94,7 +94,7 @@
 		<Intrinsic Name="is_valid_entity" Expression="!base_type::traits_type::version_mask || (*((base_type::traits_type::entity_type *)&amp;entity) &lt; (base_type::traits_type::version_mask &lt;&lt; base_type::traits_type::length))">
 			<Parameter Name="entity" Type="const base_type::traits_type::value_type &amp;"/>
 		</Intrinsic>
-		<DisplayString>{{ size={ base_type::packed.size() }, type={ base_type::info->alias,na } }}</DisplayString>
+		<DisplayString>{{ size={ base_type::packed.size() }, type={ base_type::descriptor->alias,na } }}</DisplayString>
 		<Expand>
 			<Item Name="[capacity]" Optional="true" ExcludeView="simple">payload.capacity() * traits_type::page_size</Item>
 			<Item Name="[page size]" Optional="true" ExcludeView="simple">traits_type::page_size</Item>
@@ -163,7 +163,7 @@
 						<Loop>
 							<Break Condition="pos == last"/>
 							<If Condition="pool_at(pos)->sparse.size() &gt; page &amp;&amp; pool_at(pos)->sparse[page] != nullptr &amp;&amp; ((*((traits_type::entity_type *)&amp;pool_at(pos)->sparse[page][offset])) &amp; entity_mask) != entity_mask">
-								<Item Name="[{ pool_at(pos)->info->alias,na }:{ ((*((traits_type::entity_type *)&amp;pool_at(pos)->sparse[page][offset])) &amp; entity_mask) != entity_mask }]">pool_at(pos),view(simple)nanr</Item>
+								<Item Name="[{ pool_at(pos)->descriptor->alias,na }:{ ((*((traits_type::entity_type *)&amp;pool_at(pos)->sparse[page][offset])) &amp; entity_mask) != entity_mask }]">pool_at(pos),view(simple)nanr</Item>
 							</If>
 							<Exec>++pos</Exec>
 						</Loop>

src/entt/natvis/meta.natvis

@@ -1,17 +1,17 @@
 <?xml version="1.0" encoding="utf-8"?>
 <AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
 	<Type Name="entt::internal::meta_base_node">
-		<DisplayString Condition="resolve != nullptr">{{ type={ type } }}</DisplayString>
+		<DisplayString Condition="type != nullptr">{{ id={ id } }}</DisplayString>
 		<DisplayString>{{}}</DisplayString>
 		<Expand>
-			<Item Name="[type]">type</Item>
+			<Item Name="[id]">id</Item>
 		</Expand>
 	</Type>
 	<Type Name="entt::internal::meta_conv_node">
-		<DisplayString Condition="conv != nullptr">{{ type={ type } }}</DisplayString>
+		<DisplayString Condition="conv != nullptr">{{ id={ id } }}</DisplayString>
 		<DisplayString>{{}}</DisplayString>
 		<Expand>
-			<Item Name="[type]">type</Item>
+			<Item Name="[type]">id</Item>
 		</Expand>
 	</Type>
 	<Type Name="entt::internal::meta_ctor_node">
@@ -23,10 +23,10 @@
 		</Expand>
 	</Type>
 	<Type Name="entt::internal::meta_custom_node">
-		<DisplayString Condition="value != nullptr">{{ type={ type } }}</DisplayString>
+		<DisplayString Condition="value != nullptr">{{ id={ id } }}</DisplayString>
 		<DisplayString>{{}}</DisplayString>
 		<Expand>
-			<Item Name="[type]">type</Item>
+			<Item Name="[id]">id</Item>
 			<Item Name="[value]">value</Item>
 		</Expand>
 	</Type>
@@ -34,43 +34,33 @@
 		<Intrinsic Name="has_trait" Expression="!!(traits &amp; property)">
 			<Parameter Name="property" Type="int"/>
 		</Intrinsic>
+		<DisplayString Condition="name != nullptr">{{ id={ name,na } }}</DisplayString>
 		<DisplayString Condition="get != nullptr">{{ id={ id } }}</DisplayString>
 		<DisplayString>{{}}</DisplayString>
 		<Expand>
 			<Item Name="[id]">id</Item>
+			<Item Name="[name]" Condition="name != nullptr">name,na</Item>
 			<Item Name="[arity]">arity</Item>
 			<Item Name="[is_const]">has_trait(entt::internal::meta_traits::is_const)</Item>
 			<Item Name="[is_static]">has_trait(entt::internal::meta_traits::is_static)</Item>
-			<Item Name="[prop]">prop,view(simple)</Item>
-			<Item Name="[custom]">custom</Item>
+			<Item Name="[custom]" Condition="custom.value != nullptr">custom</Item>
 		</Expand>
 	</Type>
-	<Type Name="entt::internal::meta_dtor_node">
-		<DisplayString>{{}}</DisplayString>
-		<Expand/>
-	</Type>	
 	<Type Name="entt::internal::meta_func_node"	>
 		<Intrinsic Name="has_trait" Expression="!!(traits &amp; property)">
 			<Parameter Name="property" Type="int"/>
 		</Intrinsic>
+		<DisplayString Condition="name != nullptr">{{ id={ name,na } }}</DisplayString>
 		<DisplayString Condition="invoke != nullptr">{{ id={ id } }}</DisplayString>
 		<DisplayString>{{}}</DisplayString>
 		<Expand>
 			<Item Name="[id]">id</Item>
+			<Item Name="[name]" Condition="name != nullptr">name,na</Item>
 			<Item Name="[arity]">arity</Item>
 			<Item Name="[is_const]">has_trait(entt::internal::meta_traits::is_const)</Item>
 			<Item Name="[is_static]">has_trait(entt::internal::meta_traits::is_static)</Item>
 			<Item Name="[next]" Condition="next != nullptr">*next</Item>
-			<Item Name="[prop]">prop,view(simple)</Item>
-			<Item Name="[custom]">custom</Item>
-		</Expand>
-	</Type>
-	<Type Name="entt::internal::meta_prop_node">
-		<DisplayString Condition="value != nullptr">{{ key={ id } }}</DisplayString>
-		<DisplayString>{{}}</DisplayString>
-		<Expand>
-			<Item Name="[key]">id</Item>
-			<Item Name="[value]">value</Item>
+			<Item Name="[custom]" Condition="custom.value != nullptr">custom</Item>
 		</Expand>
 	</Type>
 	<Type Name="entt::internal::meta_template_node">
@@ -88,17 +78,18 @@
 			<Item Name="[conv]">conv,view(simple)</Item>
 			<Item Name="[data]">data,view(simple)</Item>
 			<Item Name="[func]">func,view(simple)</Item>
-			<Item Name="[prop]">prop,view(simple)</Item>
 		</Expand>
 	</Type>
 	<Type Name="entt::internal::meta_type_node">
 		<Intrinsic Name="has_trait" Expression="!!(traits &amp; property)">
 			<Parameter Name="property" Type="int"/>
 		</Intrinsic>
+		<DisplayString Condition="name != nullptr">{{ type={ name,na } }}</DisplayString>
 		<DisplayString Condition="info != nullptr">{{ type={ info->alias,na } }}</DisplayString>
 		<DisplayString>{{}}</DisplayString>
 		<Expand>
 			<Item Name="[id]">id</Item>
+			<Item Name="[name]" Condition="name != nullptr">name,na</Item>
 			<Item Name="[sizeof]">size_of</Item>
 			<Item Name="[is_arithmetic]">has_trait(entt::internal::meta_traits::is_arithmetic)</Item>
 			<Item Name="[is_integral]">has_trait(entt::internal::meta_traits::is_integral)</Item>
@@ -107,23 +98,23 @@
 			<Item Name="[is_enum]">has_trait(entt::internal::meta_traits::is_enum)</Item>
 			<Item Name="[is_class]">has_trait(entt::internal::meta_traits::is_class)</Item>
 			<Item Name="[is_pointer]">has_trait(entt::internal::meta_traits::is_pointer)</Item>
-			<Item Name="[is_meta_pointer_like]">has_trait(entt::internal::meta_traits::is_meta_pointer_like)</Item>
-			<Item Name="[is_meta_sequence_container]">has_trait(entt::internal::meta_traits::is_meta_sequence_container)</Item>
-			<Item Name="[is_meta_associative_container]">has_trait(entt::internal::meta_traits::is_meta_associative_container)</Item>
+			<Item Name="[is_pointer_like]">has_trait(entt::internal::meta_traits::is_pointer_like)</Item>
+			<Item Name="[is_sequence_container]">has_trait(entt::internal::meta_traits::is_sequence_container)</Item>
+			<Item Name="[is_associative_container]">has_trait(entt::internal::meta_traits::is_associative_container)</Item>
 			<Item Name="[default_constructor]">default_constructor != nullptr</Item>
 			<Item Name="[conversion_helper]">conversion_helper != nullptr</Item>
 			<Item Name="[from_void]">from_void != nullptr</Item>
-			<Item Name="[template_info]">templ</Item>
-			<Item Name="[custom]">custom</Item>
-			<Item Name="[details]" Condition="!(details == nullptr)">*details</Item>
+			<Item Name="[template_info]" Condition="templ.arity != 0u">templ</Item>
+			<Item Name="[custom]" Condition="custom.value != nullptr">custom</Item>
+			<Item Name="[details]" Condition="details != nullptr">*details</Item>
 		</Expand>
 	</Type>
 	<Type Name="entt::meta_any">
-		<DisplayString Condition="node.info != nullptr">{{ type={ node.info->alias,na }, policy={ storage.mode,en } }}</DisplayString>
+		<DisplayString>{ storage }</DisplayString>
 		<DisplayString>{{}}</DisplayString>
 		<Expand>
-			<ExpandedItem>node</ExpandedItem>
-			<Item Name="[context]" Condition="ctx != nullptr">ctx->value</Item>
+			<ExpandedItem Condition="node != nullptr">node,na</ExpandedItem>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx,na</Item>
 		</Expand>
 	</Type>
 	<Type Name="entt::meta_handle">
@@ -136,7 +127,7 @@
 		<DisplayString Condition="data != nullptr">{{ const={ const_only } }}</DisplayString>
 		<DisplayString>{{}}</DisplayString>
 		<Expand>
-			<Item Name="[context]" Condition="ctx != nullptr">ctx->value</Item>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx,na</Item>
 			<Item Name="[const]">const_only</Item>
 			<Item Name="[data]">data</Item>
 		</Expand>
@@ -145,51 +136,59 @@
 		<DisplayString Condition="data != nullptr">{{ const={ const_only } }}</DisplayString>
 		<DisplayString>{{}}</DisplayString>
 		<Expand>
-			<Item Name="[context]" Condition="ctx != nullptr">ctx->value</Item>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx,na</Item>
 			<Item Name="[const]">const_only</Item>
 			<Item Name="[data]">data</Item>
 		</Expand>
 	</Type>
-	<Type Name="entt::meta_data">
-		<DisplayString Condition="node != nullptr">{ *node }</DisplayString>
+	<Type Name="entt::meta_custom">
+		<DisplayString Condition="node != nullptr">{ node,na }</DisplayString>
 		<DisplayString>{{}}</DisplayString>
 		<Expand>
 			<ExpandedItem Condition="node != nullptr">node</ExpandedItem>
-			<Item Name="[context]" Condition="ctx != nullptr">ctx->value</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_data">
+		<DisplayString Condition="node != nullptr">{ node,na }</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<ExpandedItem Condition="node != nullptr">node</ExpandedItem>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx,na</Item>
 		</Expand>
 	</Type>
 	<Type Name="entt::meta_func">
-		<DisplayString Condition="node != nullptr">{ *node }</DisplayString>
+		<DisplayString Condition="node != nullptr">{ node,na }</DisplayString>
 		<DisplayString>{{}}</DisplayString>
 		<Expand>
 			<ExpandedItem Condition="node != nullptr">node</ExpandedItem>
-			<Item Name="[context]" Condition="ctx != nullptr">ctx->value</Item>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx,na</Item>
 		</Expand>
 	</Type>
-	<Type Name="entt::meta_prop">
-		<DisplayString Condition="node.type != nullptr">{ node }</DisplayString>
+	<Type Name="entt::meta_base">
+		<DisplayString Condition="node != nullptr">{ node,na }</DisplayString>
 		<DisplayString>{{}}</DisplayString>
 		<Expand>
-			<ExpandedItem Condition="node.type != nullptr">node</ExpandedItem>
-			<Item Name="[context]" Condition="ctx != nullptr">ctx->value</Item>
+			<ExpandedItem Condition="node != nullptr">node</ExpandedItem>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx,na</Item>
 		</Expand>
 	</Type>
 	<Type Name="entt::meta_type">
-		<DisplayString>{ node }</DisplayString>
+		<DisplayString Condition="node != nullptr">{ node,na }</DisplayString>
+		<DisplayString>{{}}</DisplayString>
 		<Expand>
-			<ExpandedItem>node</ExpandedItem>
-			<Item Name="[context]" Condition="ctx != nullptr">ctx->value</Item>
+			<ExpandedItem Condition="node != nullptr">node</ExpandedItem>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx,na</Item>
 		</Expand>
 	</Type>
 	<Type Name="entt::meta_ctx">
-		<Intrinsic Name="element_at" Expression="value.packed.first_base::value[pos].element">
+		<Intrinsic Name="element_at" Expression="bucket.packed.first_base::value[pos].element">
 			<Parameter Name="pos" Type="int"/>
 		</Intrinsic>
-		<DisplayString>{ value }</DisplayString>
+		<DisplayString>{ bucket }</DisplayString>
 		<Expand>
 			<CustomListItems>
 				<Variable Name="pos" InitialValue="0"/>
-				<Variable Name="last" InitialValue="value.size()"/>
+				<Variable Name="last" InitialValue="bucket.size()"/>
 				<Loop>
 					<Break Condition="pos == last"/>
 					<Item Name="[{ element_at(pos).first }]">element_at(pos).second</Item>