update single include file

* container traits don't really support plain arrays anymore (if they ever did)
* fix an issue with insert/erase of meta sequence containers
* add non-regression tests

.clang-format

@@ -0,0 +1,41 @@
+BasedOnStyle: llvm
+---
+AccessModifierOffset: -4
+AlignEscapedNewlines: DontAlign
+AllowShortBlocksOnASingleLine: Empty
+AllowShortEnumsOnASingleLine: true
+AllowShortFunctionsOnASingleLine: Empty
+AllowShortIfStatementsOnASingleLine: WithoutElse
+AllowShortLoopsOnASingleLine: true
+AlwaysBreakTemplateDeclarations: Yes
+BreakBeforeBinaryOperators: NonAssignment
+BreakBeforeTernaryOperators: true
+ColumnLimit: 0
+DerivePointerAlignment: false
+IncludeCategories:
+  - Regex: '<[[:alnum:]_]+>'
+    Priority: 1
+  - Regex: '<(gtest|gmock)/'
+    Priority: 2
+  - Regex: '<[[:alnum:]_./]+>'
+    Priority: 3
+  - Regex: '<entt/'
+    Priority: 4
+  - Regex: '.*'
+    Priority: 5
+IndentPPDirectives: AfterHash
+IndentWidth: 4
+KeepEmptyLinesAtTheStartOfBlocks: false
+Language: Cpp
+PointerAlignment: Right
+SpaceAfterCStyleCast: false
+SpaceAfterTemplateKeyword: false
+SpaceAroundPointerQualifiers: After
+SpaceBeforeCaseColon: false
+SpaceBeforeCtorInitializerColon: false
+SpaceBeforeInheritanceColon: false
+SpaceBeforeParens: Never
+SpaceBeforeRangeBasedForLoopColon: false
+Standard: Latest
+TabWidth: 4
+UseTab: Never

.github/FUNDING.yml

@@ -1,12 +1,4 @@
 # These are supported funding model platforms
 
 github: skypjack
-patreon:
-open_collective:
-ko_fi:
-tidelift:
-community_bridge:
-liberapay:
-issuehunt:
-otechie:
 custom: https://www.paypal.me/skypjack

.github/workflows/build.yml

@@ -5,48 +5,42 @@ on: [push, pull_request]
 jobs:
 
   linux:
-    timeout-minutes: 10
+    timeout-minutes: 15
 
     strategy:
       matrix:
-        compiler: [
-            g++-7, g++-8, g++-9, g++,
-            clang++-8, clang++-9, clang++-10, clang++
-        ]
+        compiler:
+          - pkg: g++-7
+            exe: g++-7
+          - pkg: g++-8
+            exe: g++-8
+          - pkg: g++-9
+            exe: g++-9
+          - pkg: g++-10
+            exe: g++-10
+          - pkg: clang-8
+            exe: clang++-8
+          - pkg: clang-9
+            exe: clang++-9
+          - pkg: clang-10
+            exe: clang++-10
+          - pkg: clang-11
+            exe: clang++-11
+          - pkg: clang-12
+            exe: clang++-12
 
     runs-on: ubuntu-latest
 
     steps:
     - uses: actions/checkout@v2
-    - name: Install g++-7
-      if: ${{ matrix.compiler == 'g++-7' }}
+    - name: Install compiler
       run: |
-        sudo apt-get update
-        sudo apt-get install g++-7 -y
-    - name: Install g++-8
-      if: ${{ matrix.compiler == 'g++-8' }}
-      run: |
-        sudo apt-get update
-        sudo apt-get install g++-8 -y
-    - name: Install clang-8
-      if: ${{ matrix.compiler == 'clang++-8' }}
-      run: |
-        sudo apt-get update
-        sudo apt-get install clang-8 -y
-    - name: Install clang-9
-      if: ${{ matrix.compiler == 'clang++-9' }}
-      run: |
-        sudo apt-get update
-        sudo apt-get install clang-9 -y
-    - name: Install clang-10
-      if: ${{ matrix.compiler == 'clang++-10' }}
-      run: |
-        sudo apt-get update
-        sudo apt-get install clang-10 -y
+        sudo apt update
+        sudo apt install -y ${{ matrix.compiler.pkg }}
     - name: Compile tests
       working-directory: build
       env:
-        CXX: ${{ matrix.compiler }}
+        CXX: ${{ matrix.compiler.exe }}
       run: |
         cmake -DENTT_BUILD_TESTING=ON -DENTT_BUILD_LIB=ON -DENTT_BUILD_EXAMPLE=ON ..
         make -j4
@@ -54,27 +48,49 @@ jobs:
       working-directory: build
       env:
         CTEST_OUTPUT_ON_FAILURE: 1
-      run: ctest --timeout 10 -C Debug -j4
+      run: ctest --timeout 30 -C Debug -j4
 
-  windows:
-    timeout-minutes: 10
+  linux-extra:
+    timeout-minutes: 15
 
     strategy:
       matrix:
-        os: [windows-latest, windows-2016]
-        toolset: [clang-cl, default, v141]
+        compiler: [g++, clang++]
+        id_type: ["std::uint32_t", "std::uint64_t"]
+        cxx_std: [cxx_std_17, cxx_std_20]
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Compile tests
+      working-directory: build
+      env:
+        CXX: ${{ matrix.compiler }}
+      run: |
+        cmake -DENTT_BUILD_TESTING=ON -DENTT_CXX_STD=${{ matrix.cxx_std }} -DENTT_ID_TYPE=${{ matrix.id_type }} ..
+        make -j4
+    - name: Run tests
+      working-directory: build
+      env:
+        CTEST_OUTPUT_ON_FAILURE: 1
+      run: ctest --timeout 30 -C Debug -j4
+
+  windows:
+    timeout-minutes: 15
+
+    strategy:
+      matrix:
+        toolset: [default, v141, v142, clang-cl]
         include:
-          - toolset: clang-cl
-            toolset_option: -T"ClangCl"
           - toolset: v141
             toolset_option: -T"v141"
-        exclude:
-          - os: windows-2016
-            toolset: clang-cl
-          - os: windows-2016
-            toolset: v141
+          - toolset: v142
+            toolset_option: -T"v142"
+          - toolset: clang-cl
+            toolset_option: -T"ClangCl"
 
-    runs-on: ${{ matrix.os }}
+    runs-on: windows-latest
 
     steps:
     - uses: actions/checkout@v2
@@ -87,10 +103,33 @@ jobs:
       working-directory: build
       env:
         CTEST_OUTPUT_ON_FAILURE: 1
-      run: ctest --timeout 10 -C Debug -j4
+      run: ctest --timeout 30 -C Debug -j4
+
+  windows-extra:
+    timeout-minutes: 15
+
+    strategy:
+      matrix:
+        id_type: ["std::uint32_t", "std::uint64_t"]
+        cxx_std: [cxx_std_17, cxx_std_20]
+
+    runs-on: windows-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Compile tests
+      working-directory: build
+      run: |
+        cmake -DENTT_BUILD_TESTING=ON -DENTT_CXX_STD=${{ matrix.cxx_std }} -DENTT_ID_TYPE=${{ matrix.id_type }} ..
+        cmake --build . -j 4
+    - name: Run tests
+      working-directory: build
+      env:
+        CTEST_OUTPUT_ON_FAILURE: 1
+      run: ctest --timeout 30 -C Debug -j4
 
   macos:
-    timeout-minutes: 10
+    timeout-minutes: 15
     runs-on: macOS-latest
 
     steps:
@@ -104,4 +143,27 @@ jobs:
       working-directory: build
       env:
         CTEST_OUTPUT_ON_FAILURE: 1
-      run: ctest --timeout 10 -C Debug -j4
+      run: ctest --timeout 30 -C Debug -j4
+
+  macos-extra:
+    timeout-minutes: 15
+
+    strategy:
+      matrix:
+        id_type: ["std::uint32_t", "std::uint64_t"]
+        cxx_std: [cxx_std_17, cxx_std_20]
+
+    runs-on: macOS-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Compile tests
+      working-directory: build
+      run: |
+        cmake -DENTT_BUILD_TESTING=ON -DENTT_CXX_STD=${{ matrix.cxx_std }} -DENTT_ID_TYPE=${{ matrix.id_type }} ..
+        make -j4
+    - name: Run tests
+      working-directory: build
+      env:
+        CTEST_OUTPUT_ON_FAILURE: 1
+      run: ctest --timeout 30 -C Debug -j4

.github/workflows/coverage.yml

@@ -5,34 +5,34 @@ on: [push, pull_request]
 jobs:
 
   codecov:
-    timeout-minutes: 10
+    timeout-minutes: 15
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v2
-      - name: Compile tests
-        working-directory: build
-        env:
-          CXXFLAGS: "--coverage -fno-inline"
-          CXX: g++
-        run: |
-          cmake -DENTT_BUILD_TESTING=ON -DENTT_BUILD_LIB=ON -DENTT_BUILD_EXAMPLE=ON ..
-          make -j4
-      - name: Run tests
-        working-directory: build
-        env:
-          CTEST_OUTPUT_ON_FAILURE: 1
-        run: ctest --timeout 10 -C Debug -j4
-      - name: Collect data
-        working-directory: build
-        run: |
-          sudo apt install lcov
-          lcov -c -d . -o coverage.info
-          lcov -l coverage.info
-      - name: Upload coverage to Codecov
-        uses: codecov/codecov-action@v1
-        with:
-          token: ${{ secrets.CODECOV_TOKEN }}
-          file: build/coverage.info
-          name: EnTT
-          fail_ci_if_error: true
+    - uses: actions/checkout@v2
+    - name: Compile tests
+      working-directory: build
+      env:
+        CXXFLAGS: "--coverage -fno-inline"
+        CXX: g++
+      run: |
+        cmake -DENTT_BUILD_TESTING=ON -DENTT_BUILD_LIB=ON -DENTT_BUILD_EXAMPLE=ON ..
+        make -j4
+    - name: Run tests
+      working-directory: build
+      env:
+        CTEST_OUTPUT_ON_FAILURE: 1
+      run: ctest --timeout 30 -C Debug -j4
+    - name: Collect data
+      working-directory: build
+      run: |
+        sudo apt install lcov
+        lcov -c -d . -o coverage.info
+        lcov -l coverage.info
+    - name: Upload coverage to Codecov
+      uses: codecov/codecov-action@v2
+      with:
+        token: ${{ secrets.CODECOV_TOKEN }}
+        files: build/coverage.info
+        name: EnTT
+        fail_ci_if_error: true

.github/workflows/sanitizer.yml

@@ -4,12 +4,14 @@ on: [push, pull_request]
 
 jobs:
 
-  linux:
-    timeout-minutes: 10
+  clang:
+    timeout-minutes: 15
 
     strategy:
       matrix:
         compiler: [clang++]
+        id_type: ["std::uint32_t", "std::uint64_t"]
+        cxx_std: [cxx_std_17, cxx_std_20]
 
     runs-on: ubuntu-latest
 
@@ -20,10 +22,10 @@ jobs:
       env:
         CXX: ${{ matrix.compiler }}
       run: |
-        cmake -DENTT_USE_SANITIZER=ON -DENTT_BUILD_TESTING=ON -DENTT_BUILD_LIB=ON -DENTT_BUILD_EXAMPLE=ON ..
+        cmake -DENTT_USE_SANITIZER=ON -DENTT_BUILD_TESTING=ON -DENTT_BUILD_LIB=ON -DENTT_BUILD_EXAMPLE=ON -DENTT_CXX_STD=${{ matrix.cxx_std }} -DENTT_ID_TYPE=${{ matrix.id_type }} ..
         make -j4
     - name: Run tests
       working-directory: build
       env:
         CTEST_OUTPUT_ON_FAILURE: 1
-      run: ctest --timeout 10 -C Debug -j4
+      run: ctest --timeout 30 -C Debug -j4

AUTHORS

@@ -11,6 +11,7 @@ ceeac
 ColinH
 corystegel
 Croydon
+cschreib
 cugone
 dbacchet
 dBagrat
@@ -31,6 +32,7 @@ Lawrencemm
 markand
 mhammerc
 Milerius
+Minimonium
 morbo84
 m-waka
 netpoetica
@@ -46,6 +48,7 @@ The5-1
 vblanco20-1
 willtunnels
 WizardIke
+WoLfulus 
 w1th0utnam3
 xissburg
 zaucy

CMakeLists.txt

@@ -4,14 +4,6 @@
 
 cmake_minimum_required(VERSION 3.12.4)
 
-#
-# Building in-tree is not allowed (we take care of your craziness).
-#
-
-if(${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_BINARY_DIR})
-    message(FATAL_ERROR "Prevented in-tree built. Please create a build directory outside of the source code and call cmake from there. Thank you.")
-endif()
-
 #
 # Read project version
 #
@@ -30,7 +22,7 @@ project(
     VERSION ${ENTT_VERSION}
     DESCRIPTION "Gaming meets modern C++ - a fast and reliable entity-component system (ECS) and much more"
     HOMEPAGE_URL "https://github.com/skypjack/entt"
-    LANGUAGES CXX
+    LANGUAGES C CXX
 )
 
 if(NOT CMAKE_BUILD_TYPE)
@@ -39,40 +31,67 @@ endif()
 
 message(VERBOSE "*")
 message(VERBOSE "* ${PROJECT_NAME} v${PROJECT_VERSION} (${CMAKE_BUILD_TYPE})")
-message(VERBOSE "* Copyright (c) 2017-2021 Michele Caini <michele.caini@gmail.com>")
+message(VERBOSE "* Copyright (c) 2017-2022 Michele Caini <michele.caini@gmail.com>")
 message(VERBOSE "*")
 
-option(ENTT_USE_LIBCPP "Use libc++ by adding -stdlib=libc++ flag if availbale." ON)
-option(ENTT_USE_SANITIZER "Enable sanitizers by adding -fsanitize=address -fno-omit-frame-pointer -fsanitize=undefined flags" OFF)
-
 #
 # Compiler stuff
 #
 
-if(NOT WIN32 AND ENTT_USE_LIBCPP)
-    include(CheckCXXSourceCompiles)
-    include(CMakePushCheckState)
+option(ENTT_USE_LIBCPP "Use libc++ by adding -stdlib=libc++ flag if available." OFF)
+option(ENTT_USE_SANITIZER "Enable sanitizers by adding -fsanitize=address -fno-omit-frame-pointer -fsanitize=undefined flags if available." OFF)
 
-    cmake_push_check_state()
+if(ENTT_USE_LIBCPP)
+    if(NOT WIN32)
+        include(CheckCXXSourceCompiles)
+        include(CMakePushCheckState)
 
-    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -stdlib=libc++")
+        cmake_push_check_state()
 
-    check_cxx_source_compiles("
-        #include<type_traits>
-        int main() { return std::is_same_v<int, char>; }
-    " ENTT_HAS_LIBCPP)
+        set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -stdlib=libc++")
 
-    if(NOT ENTT_HAS_LIBCPP)
-        message(VERBOSE "The option ENTT_USE_LIBCPP is set (by default) but libc++ is not available. The flag will not be added to the target.")
+        check_cxx_source_compiles("
+            #include<type_traits>
+            int main() { return std::is_same_v<int, char>; }
+        " ENTT_HAS_LIBCPP)
+
+        cmake_pop_check_state()
     endif()
 
-    cmake_pop_check_state()
+    if(NOT ENTT_HAS_LIBCPP)
+        message(VERBOSE "The option ENTT_USE_LIBCPP is set but libc++ is not available. The flag will not be added to the target.")
+    endif()
+endif()
+
+if(ENTT_USE_SANITIZER)
+    if(CMAKE_CXX_COMPILER_ID MATCHES "Clang|GNU")
+        set(ENTT_HAS_SANITIZER TRUE CACHE BOOL "" FORCE)
+        mark_as_advanced(ENTT_HAS_SANITIZER)
+    endif()
+
+    if(NOT ENTT_HAS_SANITIZER)
+        message(VERBOSE "The option ENTT_USE_SANITIZER is set but sanitizer support is not available. The flags will not be added to the target.")
+    endif()
 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)
@@ -85,7 +104,103 @@ target_include_directories(
         $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
 )
 
-if(ENTT_USE_SANITIZER)
+target_compile_features(EnTT INTERFACE cxx_std_17)
+
+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/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/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/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/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/registry.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/runtime_view.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/sigh_storage_mixin.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/utility.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/view.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/ctx.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/platform/android-ndk-r17.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/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/locator.natvis>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/meta.natvis>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/platform.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>
+    )
+endif()
+
+if(ENTT_HAS_SANITIZER)
     target_compile_options(EnTT INTERFACE $<$<CONFIG:Debug>:-fsanitize=address -fno-omit-frame-pointer -fsanitize=undefined>)
     target_link_libraries(EnTT INTERFACE $<$<CONFIG:Debug>:-fsanitize=address -fno-omit-frame-pointer -fsanitize=undefined>)
 endif()
@@ -94,8 +209,6 @@ if(ENTT_HAS_LIBCPP)
     target_compile_options(EnTT BEFORE INTERFACE -stdlib=libc++)
 endif()
 
-target_compile_features(EnTT INTERFACE cxx_std_17)
-
 #
 # Install pkg-config file
 #
@@ -174,6 +287,9 @@ if(ENTT_BUILD_TESTING)
     option(ENTT_BUILD_LIB "Build lib tests." OFF)
     option(ENTT_BUILD_SNAPSHOT "Build snapshot test with Cereal." OFF)
 
+    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)

CONTRIBUTING.md

@@ -1,7 +1,7 @@
 # Contributing
 
 First of all, thank you very much for taking the time to contribute to the
-`EnTT` framework.<br/>
+`EnTT` library.<br/>
 How to do it mostly depends on the type of contribution:
 
 * If you have a question, **please** ensure there isn't already an answer for
@@ -28,7 +28,7 @@ How to do it mostly depends on the type of contribution:
 * If you found a bug and you wrote a patch to fix it, open a new
   [pull request](https://github.com/skypjack/entt/pulls) with your code.
   **Please**, add some tests to avoid regressions in future if possible, it
-  would be really appreciated. Note that the `EnTT` framework has a
+  would be really appreciated. Note that the `EnTT` library has a
   [coverage at 100%](https://coveralls.io/github/skypjack/entt?branch=master)
   (at least it was at 100% at the time I wrote this file) and this is the reason
   for which you can be confident with using it in a production environment.

LICENSE

@@ -1,6 +1,6 @@
 The MIT License (MIT)
 
-Copyright (c) 2017-2021 Michele Caini
+Copyright (c) 2017-2022 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

@@ -11,6 +11,9 @@
 [![Discord channel](https://img.shields.io/discord/707607951396962417?logo=discord)](https://discord.gg/5BjPWBd)
 [![Donate](https://img.shields.io/badge/donate-paypal-blue.svg)](https://www.paypal.me/skypjack)
 
+> `EnTT` has been a dream so far, we haven't found a single bug to date and it's
+> super easy to work with
+
 `EnTT` is a header-only, tiny and easy to use library for game programming and
 much more written in **modern C++**.<br/>
 [Among others](https://github.com/skypjack/entt/wiki/EnTT-in-Action), it's used
@@ -49,6 +52,7 @@ Many thanks to [these people](https://skypjack.github.io/sponsorship/) and
 * [Integration](#integration)
   * [Requirements](#requirements)
   * [CMake](#cmake)
+  * [Natvis support](#natvis-support)
   * [Packaging Tools](#packaging-tools)
   * [pkg-config](#pkg-config)
 * [Documentation](#documentation)
@@ -73,33 +77,32 @@ This project started off as a pure entity-component system. Over time the
 codebase has grown as more and more classes and functionalities were added.<br/>
 Here is a brief, yet incomplete list of what it offers today:
 
-* Statically generated integer **identifiers** for types (assigned either at
-  compile-time or at runtime).
+* Built-in **RTTI system** mostly similar to the standard one.
 * A `constexpr` utility for human readable **resource names**.
-* A minimal **configuration system** built using the monostate pattern.
-* An incredibly fast **entity-component system** based on sparse sets, with its
-  own _pay for what you use_ policy to adjust performance and memory usage
-  according to the users' requirements.
+* Minimal **configuration system** built using the monostate pattern.
+* Incredibly fast **entity-component system** with its own _pay for what you
+  use_ policy.
 * Views and groups to iterate entities and components and allow different access
   patterns, from **perfect SoA** to fully random.
 * A lot of **facilities** built on top of the entity-component system to help
-  the users and avoid reinventing the wheel (dependencies, snapshot, handles,
-  support for **reactive systems** and so on).
+  the users and avoid reinventing the wheel.
 * The smallest and most basic implementation of a **service locator** ever seen.
 * A built-in, non-intrusive and macro-free runtime **reflection system**.
 * **Static polymorphism** made simple and within everyone's reach.
+* A few homemade containers, like a sparse set based **hash map**.
 * A **cooperative scheduler** for processes of any type.
 * All that is needed for **resource management** (cache, loaders, handles).
-* Delegates, **signal handlers** (with built-in support for collectors) and a
-  tiny event dispatcher for immediate and delayed events to integrate in loops.
+* Delegates, **signal handlers** and a tiny event dispatcher.
 * A general purpose **event emitter** as a CRTP idiom based class template.
 * And **much more**! Check out the
   [**wiki**](https://github.com/skypjack/entt/wiki).
 
-Consider this list a work in progress as well as the project. The whole API is
-fully documented in-code for those who are brave enough to read it.
+Consider these lists a work in progress as well as the project. The whole API is
+fully documented in-code for those who are brave enough to read it.<br/>
+Please, do note that all tools are also DLL-friendly now and run smoothly across
+boundaries.
 
-It is also known that `EnTT` is used in **Minecraft**.<br/>
+One thing known to most is that `EnTT` is also used in **Minecraft**.<br/>
 Given that the game is available literally everywhere, I can confidently say 
 that the library has been sufficiently tested on every platform that can come to 
 mind.
@@ -156,7 +159,7 @@ int main() {
 ## Motivation
 
 I started developing `EnTT` for the _wrong_ reason: my goal was to design an
-entity-component system to beat another well known open source solution both in
+entity-component system to beat another well known open source library both in
 terms of performance and possibly memory usage.<br/>
 In the end, I did it, but it wasn't very satisfying. Actually it wasn't
 satisfying at all. The fastest and nothing more, fairly little indeed. When I
@@ -246,6 +249,14 @@ Covering all possible cases would require a treaty 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.
 
+## 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/>
+If you spot errors or have suggestions, any contribution is welcome!
+
 ## Packaging Tools
 
 `EnTT` is available for some of the most known packaging tools. In particular:
@@ -265,6 +276,12 @@ and can use `EnTT` from a `CMake` project without problems.
   $ vcpkg install entt
   ```
 
+  Or you can use the `experimental` feature to test the latest changes:
+
+  ```
+  vcpkg install entt[experimental] --head
+  ```
+
   The `EnTT` port in `vcpkg` is kept up to date by Microsoft team members and
   community contributors.<br/>
   If the version is out of date, please
@@ -399,7 +416,7 @@ know who has participated so far.
 
 # License
 
-Code and documentation Copyright (c) 2017-2021 Michele Caini.<br/>
+Code and documentation Copyright (c) 2017-2022 Michele Caini.<br/>
 Colorful logo Copyright (c) 2018-2021 Richard Caseres.
 
 Code released under

TODO

@@ -1,36 +1,26 @@
-* long term feature: shared_ptr less locator and resource cache
 * debugging tools (#60): the issue online already contains interesting tips on this, look at it
 * work stealing job system (see #100) + mt scheduler based on const awareness for types
-* allow to replace std:: with custom implementations
 * add examples (and credits) from @alanjfs :)
-* custom pools example (multi instance, tables, enable/disable, and so on...)
+
+EXAMPLES
+* filter on runtime values/variables (not only types)
+* support to polymorphic types (see #859)
 
 WIP:
-* remove view/storage dispatcher, add support to relax policy constraints on user request (eg view.use<T>())
-* improve perf for sparse_set/storage::insert/emplace/destroy/remove/...
-* custom allocators all over
+* view/group: no storage_traits dependency -> use storage instead of components for the definition
+* basic_storage::bind for cross-registry setups
+* uses-allocator construction: any (with allocator support), poly, ...
+* process scheduler: reviews, use free lists internally
+* iterator based try_emplace vs try_insert for perf reasons
+* dedicated entity storage, in-place O(1) release/destroy for non-orphaned entities, out-of-sync model
+* entity-only and exclude-only views
+* custom allocators all over (sigh storage mixin, registry, ...)
+* consider removing ENTT_NOEXCEPT, use ENTT_NOEXCEPT_IF (or noexcept(...)) as appropriate in any case (ie make compressed_pair conditionally noexcept)
+* add test for maximum number of entities reached
 
 WIP:
-* make value_type available from meta container types, otherwise we have to default construct a container to get it
-* make it possible to register externally managed pools with the registry (allow for system centric mode)
-* registry: switch to the udata/mixin model and get rid of poly storage, use pointer to sparse set only for pools, discard pool_data type.
-* it's now possible to have 0 as null entity/version, so we can finally switch to it
-* make pools available (registry/view/group), review operator| for views
-* page size: add per-pool size, allow for 0 sizes (old fully packed array)
-* compressed pair to exploit ebo in sparse set and the others
-* isolate view iterator, unwrap iterators in registry ::remove/::erase/::destroy to use the faster solution for non-view iterators
-* remove view each<T>(F), each<T>(), make view::use return a view and remove the mutable data member
-* resource, forward the id to the loader from the cache and if constexpr the call to load, update doc and describe customization points
-* make it possible to create views of the type `view<T, T>`, add get by index and such, allow to register custom pools by name with the registry
 * add user data to type_info
-* any_vector for context variables
-* make const registry::view thread safe, switch to a view<T...>{registry} model (long term goal)
-* weak reference wrapper example with custom storage
-* headless (sparse set only) view
 * write documentation for custom storages and views!!
 * make runtime views use opaque storage and therefore return also elements.
-* add exclude-only views to combine with packs
 * entity-aware observer, add observer functions aside observer class
 * deprecate non-owning groups in favor of owning views and view packs, introduce lazy owning views
-* snapshot: support for range-based archives
-* add example: 64 bit ids with 32 bits reserved for users' purposes

docs/CMakeLists.txt

@@ -17,6 +17,7 @@ add_custom_target(
     SOURCES
         dox/extra.dox
         md/config.md
+        md/container.md
         md/core.md
         md/entity.md
         md/faq.md

docs/md/container.md

@@ -0,0 +1,67 @@
+# Crash Course: containers
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [Containers](#containers)
+  * [Dense map](#dense-map)
+  * [Dense set](#dense-set)
+
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+The standard C++ library offers a wide range of containers and 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
+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 functionality by
+making available some containers 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/>
+For all containers made available, full test coverage and stability over time is
+guaranteed as usual.
+
+# Containers
+
+## Dense map
+
+The dense map made available in `EnTT` is a hash map that aims to return a
+packed array of elements, so as to reduce the number of jumps in memory during
+iterations.<br/>
+The implementation is based on _sparse sets_ and each bucket is identified by an
+implicit list within the packed array itself.
+
+The interface is very close to its counterpart in the standard library, that is,
+`std::unordered_map`.<br/>
+However, both local and non-local iterators returned by a dense map belong to
+the input iterator category although they respectively model the concepts of a
+_forward iterator_ type and a _random access iterator_ type.<br/>
+This is because they return a pair of references rather than a reference to a
+pair. In other words, dense maps return a so called _proxy iterator_ the value
+type of which is:
+
+* `std::pair<const Key &, Type &>` for non-const iterator types.
+* `std::pair<const Key &, const Type &>` for const iterator types.
+
+This is quite different from what any standard library map returns and should be
+taken into account when looking for a drop-in replacement.
+
+## Dense set
+
+The dense set made available in `EnTT` is a hash set that aims to return a
+packed array of elements, so as to reduce the number of jumps in memory during
+iterations.<br/>
+The implementation is based on _sparse sets_ and each bucket is identified by an
+implicit list within the packed array itself.
+
+The interface is in all respects similar to its counterpart in the standard
+library, that is, `std::unordered_set`.<br/>
+Therefore, there is no need to go into the API description.

docs/md/core.md

@@ -6,18 +6,21 @@
 # Table of Contents
 
 * [Introduction](#introduction)
-* [Unique sequential identifiers](#unique-sequential-identifiers)
-  * [Compile-time generator](#compile-time-generator)
-  * [Runtime generator](#runtime-generator)
-* [Hashed strings](#hashed-strings)
-  * [Wide characters](wide-characters)
-  * [Conflicts](#conflicts)
-* [Monostate](#monostate)
 * [Any as in any type](#any-as-in-any-type)
   * [Small buffer optimization](#small-buffer-optimization)
   * [Alignment requirement](#alignment-requirement)
+* [Compressed pair](#compressed-pair)
+* [Enum as bitmask](#enum-as-bitmask)
+* [Hashed strings](#hashed-strings)
+  * [Wide characters](wide-characters)
+  * [Conflicts](#conflicts)
+* [Memory](#memory)
+  * [Power of two and fast modulus](#power-of-two-and-fast-modulus)
+  * [Allocator aware unique pointers](#allocator-aware-unique-pointers)
+* [Monostate](#monostate)
 * [Type support](#type-support)
-  * [Type info](#type-info)
+  * [Built-in RTTI support](#built-in-rtti-support)
+    * [Type info](#type-info)
     * [Almost unique identifiers](#almost-unique-identifiers)
   * [Type traits](#type-traits)
     * [Size of](#size-of)
@@ -27,6 +30,9 @@
     * [Integral constant](#integral-constant)
     * [Tag](#tag)
     * [Type list and value list](#type-list-and-value-list)
+* [Unique sequential identifiers](#unique-sequential-identifiers)
+  * [Compile-time generator](#compile-time-generator)
+  * [Runtime generator](#runtime-generator)
 * [Utilities](#utilities)
 <!--
 @endcond TURN_OFF_DOXYGEN
@@ -39,80 +45,249 @@ of the library itself.<br/>
 Hardly users will include these features in their code, but it's worth
 describing what `EnTT` offers so as not to reinvent the wheel in case of need.
 
-# Unique sequential identifiers
+# Any as in any type
 
-Sometimes it's 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.
+`EnTT` comes with 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
+wants to see in a software. Furthermore, there is no way to connect it with the
+type system of the library and therefore with its integrated RTTI support.<br/>
+Note that this class is largely used internally by the library itself.
 
-## Compile-time generator
+The API is very similar to that of its most famous counterpart, mainly because
+this class serves the same purpose of being an opaque container for any type of
+value.<br/>
+Instances of `any` also minimize the number of allocations by relying on a well
+known technique called _small buffer optimization_ and a fake vtable.
 
-To generate sequential numeric identifiers at compile-time, `EnTT` offers the
-`identifier` class template:
+Creating an object of the `any` type, whether empty or not, is trivial:
 
 ```cpp
-// defines the identifiers for the given types
-using id = entt::identifier<a_type, another_type>;
+// an empty container
+entt::any empty{};
 
-// ...
+// a container for an int
+entt::any any{0};
 
-switch(a_type_identifier) {
-case id::type<a_type>:
-    // ...
-    break;
-case id::type<another_type>:
-    // ...
-    break;
-default:
-    // ...
-}
+// in place construction
+entt::any in_place{std::in_place_type<int>, 42};
 ```
 
-This is all what this class template has to offer: a `type` inline variable that
-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 left
-the other identifiers unchanged:
+Alternatively, the `make_any` function serves the same purpose but requires to
+always be explicit about the type:
 
 ```cpp
-template<typename> struct ignore_type {};
-
-using id = entt::identifier<
-    a_type_still_valid,
-    ignore_type<a_type_no_longer_valid>,
-    another_type_still_valid
->;
+entt::any any = entt::make_any<int>(42);
 ```
 
-Perhaps a bit ugly to see in a codebase but it gets the job done at least.
+In both 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
+the one it contains.
 
-## Runtime generator
-
-To generate sequential numeric identifiers at runtime, `EnTT` offers the
-`family` class template:
+There exists also a way to directly assign a value to the variable contained by
+an `entt::any`, without necessarily replacing it. This is especially useful when
+the object is used in _aliasing mode_, as described below:
 
 ```cpp
-// defines a custom generator
-using id = entt::family<struct my_tag>;
+entt::any any{42};
+entt::any value{3};
 
-// ...
+// assigns by copy
+any.assign(value);
 
-const auto a_type_id = id::type<a_type>;
-const auto another_type_id = id::type<another_type>;
+// assigns by move
+any.assign(std::move(value));
 ```
 
-This is all what a _family_ has to offer: a `type` inline variable that contains
-a numeric identifier for the given type.<br/>
-The generator is customizable, so as to get different _sequences_ for different
-purposes if needed.
+The `any` class will also perform a check on the type information and whether or
+not the original type was copy or move assignable, as appropriate.<br/>
+In all cases, the `assign` function returns a boolean value to indicate the
+success or failure of the operation.
 
-Please, note that identifiers aren't guaranteed to be stable across different
-runs. Indeed it mostly depends on the flow of execution.
+When in doubt about the type of object contained, the `type` member function of
+`any` returns a const reference to the `type_info` associated with its element,
+or `type_id<void>()` if the container is empty. The type is also used internally
+when comparing two `any` objects:
+
+```cpp
+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/>
+Refer to the `EnTT` type system documentation for more details about how
+`type_info` works and on possible risks of a comparison.
+
+A particularly interesting feature of this class is that it can also be used as
+an opaque container for const and non-const references:
+
+```cpp
+int value = 42;
+
+entt::any any{std::in_place_type<int &>(value)};
+entt::any cany = entt::make_any<const int &>(value);
+entt::any fwd = entt::forward_as_any(value);
+
+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
+ensure that its lifetime exceeds that of the container.<br/>
+Similarly, it's possible to create non-owning copies of `any` from an existing
+object:
+
+```cpp
+// aliasing constructor
+entt::any ref = other.as_ref();
+```
+
+In this case, it doesn't matter if the original container actually holds an
+object or acts already as a reference for unmanaged elements, the new instance
+thus created won't create copies and will only serve as a reference for the
+original item.<br/>
+This means that, starting from the example above, both `ref` and `other` will
+point to the same object, whether it's initially contained in `other` or already
+an unmanaged element.
+
+As a side note, it's 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
+`any` that wraps a const reference will return a null pointer in all cases.
+
+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`, these 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.
+
+## Small buffer optimization
+
+The `any` class uses a technique called _small buffer optimization_ to reduce
+the number of allocations where possible.<br/>
+The default reserved size for an instance of `any` is `sizeof(double[2])`.
+However, this is also configurable if needed. In fact, `any` is defined as an
+alias for `basic_any<Len>`, where `Len` is the size above.<br/>
+Users can easily set a custom size or define their own aliases:
+
+```cpp
+using my_any = entt::basic_any<sizeof(double[4])>;
+```
+
+This feature, in addition to allowing the choice of a size that best suits the
+needs of an application, also offers the possibility of forcing dynamic creation
+of objects during construction.<br/>
+In other terms, if the size is 0, `any` avoids the use of any optimization and
+always dynamically allocates objects (except for aliasing cases).
+
+Note that the size of the internal storage as well as the alignment requirements
+are directly part of the type and therefore contribute to define different types
+that won't be able to interoperate with each other.
+
+## Alignment requirement
+
+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/>
+The `basic_any` class template inspects the alignment requirements in each case,
+even when not provided and may decide not to use the small buffer optimization
+in order to meet them.
+
+The alignment requirement 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])>;
+```
+
+Note that the alignment requirements as well as the size of the internal storage
+are directly part of the type and therefore contribute to define different types
+that won't be able to interoperate with each other.
+
+# Compressed pair
+
+Primarily designed for internal use and far from being feature complete, the
+`compressed_pair` class does exactly what it promises: it tries to reduce the
+size of a pair by exploiting _Empty Base Class Optimization_ (or _EBCO_).<br/>
+This class **is not** a drop-in replacement for `std::pair`. However, it offers
+enough functionalities to be a good alternative for when reducing memory usage
+is more important than having some cool and probably useless feature.
+
+Although the API is very close to that of `std::pair` (apart from the fact that
+the template parameters are inferred from the constructor and therefore there is
+no` entt::make_compressed_pair`), the major difference is that `first` and
+`second` are functions for implementation needs:
+
+```cpp
+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.
+
+# 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 out of the box. Main problem is that they
+don't convert implicitly to their underlying type.<br/>
+All that remains is to make a choice between using old-fashioned enums (with all
+their problems that I don't 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 bitmask transparently.<br/>
+The ultimate goal is to be able to write code like the following (or maybe
+something more meaningful, but this should give a grasp and remain simple at the
+same time):
+
+```cpp
+enum class my_flag {
+    unknown = 0x01,
+    enabled = 0x02,
+    disabled = 0x04
+};
+
+const my_flag flags = my_flag::enabled;
+const bool is_enabled = !!(flags & my_flag::enabled);
+```
+
+The problem with adding all operators to the global scope is that these will
+come into play even when not required, with the risk of introducing errors that
+are difficult to deal with.<br/>
+However, C++ offers enough tools to get around this problem. In particular, the
+library requires users to register all enum classes for which bitmask support
+should be enabled:
+
+```cpp
+template<>
+struct entt::enum_as_bitmask<my_flag>
+    : std::true_type
+{};
+```
+
+This is handy when dealing with enum classes defined by third party libraries
+and over which the users have no control. However, it's also verbose and can be
+avoided by adding a specific value to the enum class itself:
+
+```cpp
+enum class my_flag {
+    unknown = 0x01,
+    enabled = 0x02,
+    disabled = 0x04,
+    _entt_enum_as_bitmask
+};
+```
+
+In this case, there is no need to specialize the `enum_as_bitmask` traits, since
+`EnTT` will automatically detect the flag and enable the bitmask support.<br/>
+Once the enum class has been registered (in one way or the other) all the most
+common operators will be available, such as `&`, `|` but also `&=` and `|=`.
+Refer to the official documentation for the full list of operators.
 
 # Hashed strings
 
@@ -195,6 +370,54 @@ 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.
 
+# Memory
+
+There are a handful of tools within EnTT to interact with memory in one way or
+another.<br/>
+Some are geared towards simplifying the implementation of (internal or external)
+allocator aware containers. Others, on the other hand, 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
+documentation to those interested in the subject.
+
+## Power of two and fast modulus
+
+Finding out if a number is a power of two (`is_power_of_two`) or what the next
+power of two is given a random value (`next_power_of_two`) is very useful at
+times.<br/>
+For example, it helps to allocate memory in pages having a size suitable for the
+fast modulus:
+
+```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.
+
+## 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/>
+There is a proposal at the moment that also shows among the other things how
+this can be implemented without any compiler support.
+
+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);
+```
+
+Although the internal implementation is slightly different from what is proposed
+for the standard, this function offers an API that is a drop-in replacement for
+the same feature.
+
 # Monostate
 
 The monostate pattern is often presented as an alternative to a singleton based
@@ -218,182 +441,28 @@ const bool b = entt::monostate<"mykey"_hs>{};
 const int i = entt::monostate<entt::hashed_string{"mykey"}>{};
 ```
 
-# Any as in any type
-
-`EnTT` comes with its own `any` type. It may seem redundant considering that
-C++17 introduced `std::any`, but it is not (hopefully).<br/>
-In fact, 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
-wants to see in a software. Furthermore, there is no way to connect it with the
-type system of the library and therefore with its integrated RTTI support.<br/>
-Note that this class is largely used internally by the library itself.
-
-The API is very similar to that of its most famous counterpart, mainly because
-this class serves the same purpose of being an opaque container for any type of
-value.<br/>
-Instances of `any` also minimize the number of allocations by relying on a well
-known technique called _small buffer optimization_ and a fake vtable.
-
-Creating an object of the `any` type, whether empty or not, is trivial:
-
-```cpp
-// an empty container
-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};
-```
-
-Alternatively, the `make_any` function serves the same purpose but requires to
-always be explicit about the type:
-
-```cpp
-entt::any any = entt::make_any<int>(42);
-```
-
-In both 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` is not tied to an actual type. Therefore, the
-wrapper will be reconfigured by assigning it an object of a different type than
-the one contained, so as to be able to handle the new instance.<br/>
-When in doubt about the type of object contained, the `type` member function of
-`any` returns an instance of `type_info` associated with its element, or an
-invalid `type_info` object if the container is empty. The type is also used
-internally when comparing two `any` objects:
-
-```cpp
-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/>
-Refer to the `EnTT` type system documentation for more details on how
-`type_info` works and on possible risks of a comparison.
-
-A particularly interesting feature of this class is that it can also be used as
-an opaque container for const and non-const references:
-
-```cpp
-int value = 42;
-
-entt::any any{std::in_place_type<int &>(value)};
-entt::any cany = entt::make_any<const int &>(value);
-entt::any fwd = entt::forward_as_any(value);
-
-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
-ensure that its lifetime exceeds that of the container.<br/>
-Similarly, it's possible to create non-owning copies of `any` from an existing
-object:
-
-```cpp
-// aliasing constructor
-entt::any ref = other.as_ref();
-```
-
-In this case, it doesn't matter if the original container actually holds an
-object or acts already as a reference for unmanaged elements, the new instance
-thus created won't create copies and will only serve as a reference for the
-original item.<br/>
-This means that, starting from the example above, both `ref` and` other` will
-point to the same object, whether it's initially contained in `other` or already
-an unmanaged element.
-
-As a side note, it's 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
-`any` that wraps a const reference will return a null pointer in all cases.
-
-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`, these 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.
-
-## Small buffer optimization
-
-The `any` class uses a technique called _small buffer optimization_ to reduce
-the number of allocations where possible.<br/>
-The default reserved size for an instance of `any` is `sizeof(double[2])`.
-However, this is also configurable if needed. In fact, `any` is defined as an
-alias for `basic_any<Len>`, where `Len` is the size above.<br/>
-Users can easily set a custom size or define their own aliases:
-
-```cpp
-using my_any = entt::basic_any<sizeof(double[4])>;
-```
-
-This feature, in addition to allowing the choice of a size that best suits the
-needs of an application, also offers the possibility of forcing dynamic creation
-of objects during construction.<br/>
-In other terms, if the size is 0, `any` avoids the use of any optimization and
-always dynamically allocates objects (except for aliasing cases).
-
-Note that the size of the internal storage as well as the alignment requirements
-are directly part of the type and therefore contribute to define different types
-that won't be able to interoperate with each other.
-
-## Alignment requirement
-
-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/>
-The `basic_any` class template inspects the alignment requirements in each case,
-even when not provided and may decide not to use the small buffer optimization
-in order to meet them.
-
-The alignment requirement 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])>;
-```
-
-Note that the alignment requirements as well as the size of the internal storage
-are directly part of the type and therefore contribute to define different types
-that won't be able to interoperate with each other.
-
 # Type support
 
 `EnTT` provides some basic information about types of all kinds.<br/>
 It also offers additional features that are not yet available in the standard
 library or that will never be.
 
-## Type info
+## Built-in RTTI support
 
-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
-require to enable RTTI.<br/>
-Therefore, they can sometimes be even more reliable than those obtained
-otherwise.
+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
+type information at runtime.<br/>
+The library tries to fill this gap by offering a built-in system that doesn't
+serve as a replacement but comes very close to being one and offers similar
+information to that provided by its counterpart.
 
-A type info object is an opaque class that is also copy and move constructible.
-This class is returned by the `type_id` function template:
+Basically, the whole system relies on a handful of classes. In particular:
 
-```cpp
-auto info = entt::type_id<a_type>();
-```
-
-These are the information made available by this object:
-
-* The unique, sequential identifier associated with a given type:
+* The unique sequential identifier associated with a given type:
 
   ```cpp
-  auto index = entt::type_id<a_type>().seq();
-  ```
-
-  This is also an alias for the following:
-
-  ```cpp
-  auto index = entt::type_seq<a_type>::value();
+  auto index = entt::type_index<a_type>::value();
   ```
 
   The returned value isn't guaranteed to be stable across different runs.
@@ -407,13 +476,13 @@ These are the information made available by this object:
   and therefore the generation of custom runtime sequences of indices for their
   own purposes, if necessary.
   
-  An external generator can also be used if needed. In fact, `type_seq` can be
+  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
   refined specializations such as:
   
   ```cpp
   template<typename Type>
-  struct entt::type_seq<Type, std::void_d<decltype(Type::index())>> {
+  struct entt::type_index<Type, std::void_d<decltype(Type::index())>> {
       static entt::id_type value() ENTT_NOEXCEPT {
           return Type::index();
       }
@@ -426,21 +495,13 @@ These are the information made available by this object:
 
 * The hash value associated with a given type:
 
-  ```cpp
-  auto hash = entt::type_id<a_type>().hash();
-  ```
-
-  This is also an alias for the following:
-
   ```cpp
   auto hash = entt::type_hash<a_type>::value();
   ```
 
   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).<br/>
-  The `hash` function offered by the type info object isn't `constexpr` in any
-  case instead.
+  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
@@ -450,18 +511,12 @@ These are the information made available by this object:
   that identifiers remain stable across executions. Moreover, they are generated
   at runtime and are no longer a compile-time thing.
 
-  As for `type_seq`, also `type_hash` is a _sfinae-friendly_ class that can be
+  As for `type_index`, also `type_hash` is a _sfinae-friendly_ class that can be
   specialized in order to customize its behavior globally or on a per-type or
   per-traits basis.
 
 * The name associated with a given type:
 
-  ```cpp
-  auto name = entt::type_id<my_type>().name();
-  ```
-
-  This is also an alias for the following:
-
   ```cpp
   auto name = entt::type_name<a_type>::value();
   ```
@@ -489,10 +544,84 @@ These are the information made available by this object:
   means of the `ENTT_STANDARD_CPP` definition. In this case, the name will be
   empty by default.
 
-  As for `type_seq`, also `type_name` is a _sfinae-friendly_ class that can be
+  As for `type_index`, also `type_name` is a _sfinae-friendly_ class that can be
   specialized 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
+similar to that offered by the language.
+
+### 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
+require to enable RTTI.<br/>
+Therefore, they can sometimes be even more reliable than those obtained
+otherwise.
+
+Its type defines an opaque class that is also copyable and movable.<br/>
+Objects of this type are generally returned by the `type_id` functions:
+
+```cpp
+// by type
+auto info = entt::type_id<a_type>();
+
+// by value
+auto other = entt::type_id(42);
+```
+
+All elements thus received are nothing more than const references to instances
+of `type_info` with static storage duration.<br/>
+This is convenient for saving the entire object aside for the cost of a pointer.
+However, nothing prevents from constructing `type_info` objects directly:
+
+```cpp
+entt::type_info info{std::in_place_type<int>};
+```
+
+These are the information made available by `type_info`:
+
+* The index associated with a given type:
+
+  ```cpp
+  auto idx = entt::type_id<a_type>().index();
+  ```
+
+  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();
+  ```
+
+* The hash value associated with a given type:
+
+  ```cpp
+  auto hash = entt::type_id<a_type>().hash();
+  ```
+
+  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();
+  ```
+
+* The name associated with a given type:
+
+  ```cpp
+  auto name = entt::type_id<my_type>().name();
+  ```
+
+  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();
+  ```
+
+Where all accessed features are available at compile-time, the `type_info` class
+is also fully `constexpr`. However, this cannot be guaranteed in advance and
+depends mainly on the compiler in use and any specializations of the classes
+described above.
+
 ### Almost unique identifiers
 
 Since the default non-standard, compile-time implementation of `type_hash` makes
@@ -633,7 +762,7 @@ Here is a (possibly incomplete) list of the functionalities that come with a
 type list:
 
 * `type_list_element[_t]` to get the N-th element of a type list.
-* `type_list_cast[_t]` and a handy `operator+` to concatenate type lists.
+* `type_list_cat[_t]` and a handy `operator+` to concatenate type lists.
 * `type_list_unique[_t]` to remove duplicate types from a type list.
 * `type_list_contains[_v]` to know if a type list contains a given type.
 * `type_list_diff[_t]` to remove types from type lists.
@@ -644,6 +773,81 @@ Many of these functionalities also exist in their version dedicated to value
 lists. We therefore have `value_list_element[_v]` as well as
 `value_list_cat[_t]`and so on.
 
+# Unique sequential identifiers
+
+Sometimes it's 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.
+
+## Compile-time generator
+
+To generate sequential numeric identifiers at compile-time, `EnTT` offers the
+`identifier` class template:
+
+```cpp
+// defines the identifiers for the given types
+using id = entt::identifier<a_type, another_type>;
+
+// ...
+
+switch(a_type_identifier) {
+case id::type<a_type>:
+    // ...
+    break;
+case id::type<another_type>:
+    // ...
+    break;
+default:
+    // ...
+}
+```
+
+This is all what this class template has to offer: a `type` inline variable that
+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 left
+the other identifiers unchanged:
+
+```cpp
+template<typename> struct ignore_type {};
+
+using id = entt::identifier<
+    a_type_still_valid,
+    ignore_type<a_type_no_longer_valid>,
+    another_type_still_valid
+>;
+```
+
+Perhaps a bit ugly to see in a codebase but it gets the job done at least.
+
+## Runtime generator
+
+To generate sequential numeric identifiers at runtime, `EnTT` offers the
+`family` class template:
+
+```cpp
+// defines a custom generator
+using id = entt::family<struct my_tag>;
+
+// ...
+
+const auto a_type_id = id::type<a_type>;
+const auto another_type_id = id::type<another_type>;
+```
+
+This is all what a _family_ has to offer: a `type` inline variable that contains
+a numeric identifier for the given type.<br/>
+The generator is customizable, so as to get different _sequences_ for different
+purposes if needed.
+
+Please, note that identifiers aren't 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

docs/md/faq.md

@@ -149,15 +149,14 @@ 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. This trait doesn't check if an object can
-actually be copied but only verifies if there is a copy constructor
-available.<br/>
-This can lead to surprising results due to some idiosyncrasies of the standard
-mainly related to the need to guarantee backward compatibility.
+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/>
+This can lead to surprising results due to some idiosyncrasies of the standard.
 
-For example, `std::vector` defines a copy constructor no matter if its value
-type is copyable or not. As a result, `std::is_copy_constructible_v` is true
-for the following specialization:
+For example, `std::vector` defines a copy constructor that is conditionally
+enabled depending on whether the value type is copyable or not. As a result,
+`std::is_copy_constructible_v` returns true for the following specialization:
 
 ```cpp
 struct type {
@@ -165,21 +164,30 @@ struct type {
 };
 ```
 
-When trying to assign an instance of this type to an entity in the ECS part,
-this may trigger a compilation error because we cannot really make a copy of
-it.<br/>
-As a workaround, users can mark the type explicitly as non-copyable:
+However, the copy constructor is effectively disabled upon specialization.
+Therefore, trying to assign an instance of this type to an entity may trigger a
+compilation error.<br/>
+As a workaround, users can mark the type explicitly as non-copyable. This also
+suppresses the implicit generation of the move constructor and operator, which
+will therefore have to be defaulted accordingly:
 
 ```cpp
 struct type {
     type(const type &) = delete;
+    type(type &&) = default;
+
     type & operator=(const type &) = delete;
+    type & operator=(type &&) = default;
 
     std::vector<std::unique_ptr<action>> vec;
 };
 ```
 
-Unfortunately, this will also disable aggregate initialization.
+Note that aggregate initialization is also disabled as a consequence.<br/>
+Fortunately, this type of trick is quite rare. The bad news is that there is no
+way to deal with it at the library level, this being due to the design of the
+language. On the other hand, the fact that the language itself also offers a way
+to mitigate the problem makes it manageable.
 
 ## Which functions trigger which signals
 

docs/md/lib.md

@@ -6,7 +6,7 @@
 # Table of Contents
 
 * [Working across boundaries](#working-across-boundaries)
-  * [The EnTT way](#the-entt-way)
+  * [Smooth until proven otherwise](#smooth-until-proven-otherwise)
   * [Meta context](#meta-context)
   * [Memory management](#memory-management)
 <!--
@@ -19,48 +19,31 @@
 general and on GNU/Linux when default visibility was set to hidden. The
 limitation was mainly due to a custom utility used to assign unique, sequential
 identifiers with different types.<br/>
-Fortunately, nowadays using `EnTT` across boundaries is easier. However, use in
-standalone applications is favored and user intervention is otherwise required.
+Fortunately, nowadays using `EnTT` across boundaries is much easier.
 
-## The EnTT way
+## Smooth until proven otherwise
 
 Many classes in `EnTT` make extensive use of type erasure for their purposes.
-This isn't a problem in itself (in fact, it's the basis of an API so convenient
+This isn't a problem on itself (in fact, it's the basis of an API so convenient
 to use). However, a way is needed to recognize the objects whose type has been
 erased on the other side of a boundary.<br/>
 The `type_hash` class template is how identifiers are generated and thus made
-available to the rest of the library. The `type_seq` class template makes all
-types _indexable_ instead, so as to speed up the lookup.
+available to the rest of the library. In general, this class doesn't arouse much
+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/>
+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.
 
-In general, these classes don't arouse much interest. The only exceptions are:
-
-* 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/>
-  The section dedicated to `type_info` contains all the details to get around
-  the problem in a concise and elegant way. Please refer to the specific
-  documentation.
-
-* When working with linked libraries that also export all required symbols.<br/>
-  Compile definitions `ENTT_API_EXPORT` and `ENTT_API_IMPORT` should be passed
-  respectively where there is a need to import or export the symbols defined by
-  `EnTT`, so as to make everything work nicely across boundaries.
-
-* When working with plugins or shared libraries that don't export any symbol. In
-  this case, `type_seq` confuses the other classes by giving potentially wrong
-  information to them.<br/>
-  To avoid problems, it's required to provide a custom generator. Briefly, it's
-  necessary to specialize the `type_seq` class and make it point to a context
-  that is also shared between the main application and the dynamically loaded
-  libraries or plugins.<br/>
-  This will make the type system available to the whole application, not just to
-  a particular tool such as the registry or the dispatcher. It means that a call
-  to `type_seq::value()` will return the same identifier for the same type from
-  both sides of a boundary and can be used reliably for any purpose.
+When working with linked libraries, compile definitions `ENTT_API_EXPORT` and
+`ENTT_API_IMPORT` can be used where there is a need 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.
 
 For anyone who needs more details, the test suite contains multiple 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 the possible cases.
+It goes without saying that it's impossible to cover **all** possible cases.
 However, what is offered should hopefully serve as a basis for all of them.
 
 ## Meta context

docs/md/links.md

@@ -20,14 +20,24 @@ I hope this list can grow much more in the future:
   * [Minecraft Earth](https://www.minecraft.net/en-us/about-earth) by
     [Mojang](https://mojang.com/): an augmented reality game for mobile, that
     lets users bring Minecraft into the real world.
+  * [Ember Sword](https://embersword.com/): a modern Free-to-Play MMORPG with a
+    player-driven economy, a classless combat system, and scarce, tradable
+    cosmetic collectibles.
+  * Apparently [Diablo II: Resurrected](https://diablo2.blizzard.com/) by
+    [Blizzard](https://www.blizzard.com/): monsters, heroes, items, spells, all
+    resurrected. Thanks unknown insider.
+  * [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.
+  * Apparently [D&D Dark Alliance](https://darkalliance.wizards.com) by
+    [Wizards of the Coast](https://company.wizards.com): your party, their
+    funeral.
   * [TiltedOnline](https://github.com/tiltedphoques/TiltedOnline) by
     [Tilted Phoques](https://github.com/tiltedphoques): Skyrim and Fallout 4 mod
     to play online.
   * [Antkeeper](https://github.com/antkeeper/antkeeper-source): an ant colony
     simulation [game](https://antkeeper.com/).
-  * [War of Rights](https://store.steampowered.com/app/424030/War_of_Rights/): a
-    multiplayer game set during the perilous days of the American Civil War, by
-    Campfire Games.
   * [Openblack](https://github.com/openblack/openblack): open source
     reimplementation of the game _Black & White_ (2001).
   * [Land of the Rair](https://github.com/LandOfTheRair/core2): the new backend
@@ -70,8 +80,20 @@ I hope this list can grow much more in the future:
     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
+  * [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
+    simulator using C++ with some reinforcement learning.
+  * [HomeHearth](https://youtu.be/GrEWl8npL9Y): choose your hero, protect the
+    town, before it's too late.
+  * [City Builder Game](https://github.com/PhiGei2000/CityBuilderGame): a simple
+    city-building game using C++ and OpenGL.
 
 * Engines and the like:
   * [Aether Engine](https://hadean.com/spatial-simulation/)
@@ -92,6 +114,9 @@ I hope this list can grow much more in the future:
     vrooooommm.
   * [Antara Gaming SDK](https://github.com/KomodoPlatform/antara-gaming-sdk):
     the Komodo Gaming Software Development Kit.
+  * [XVP](https://ravingbots.com/xvp-expansive-vehicle-physics-for-unreal-engine/):
+    [_eXpansive Vehicle Physics_](https://github.com/raving-bots/xvp/wiki/Plugin-integration-guide)
+    plugin for Unreal Engine.
   * [Apparently](https://teamwisp.github.io/credits/)
     [Wisp](https://teamwisp.github.io/product/) by
     [Team Wisp](https://teamwisp.github.io/): an advanced real-time ray tracing
@@ -116,6 +141,21 @@ I hope this list 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
+    game engine.
+  * [Electro](https://github.com/Electro-Technologies/Electro): high performance
+    3D game engine with a high emphasis on rendering.
+  * [Kawaii](https://github.com/Mathieu-Lala/Kawaii_Engine): a modern data
+    oriented game engine.
+  * [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.
+  * [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.
 
 * Articles, videos and blog posts:
   * [Some posts](https://skypjack.github.io/tags/#entt) on my personal
@@ -180,6 +220,8 @@ I hope this list can grow much more in the future:
     controller emulator and renderer.
   * [Ragdoll](https://ragdolldynamics.com/): real-time physics for Autodesk Maya
     2020.
+  * [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.
   * [Apparently](https://www.linkedin.com/in/skypjack/)

docs/md/locator.md

@@ -14,62 +14,43 @@
 # Introduction
 
 Usually service locators are tightly bound to the services they expose and it's
-hard to define a general purpose solution. This template based implementation
-tries to fill the gap and to get rid of the burden of defining a different
-specific locator for each application.<br/>
-This class is tiny, partially unsafe and thus risky to use. Moreover it doesn't
-fit probably most of the scenarios in which a service locator is required. Look
-at it as a small tool that can sometimes be useful if users know how to handle
-it.
+hard to define a general purpose solution.<br/>
+This tiny class tries to fill the gap and to get rid of the burden of defining a
+different specific locator for each application.
 
 # Service locator
 
-The API is straightforward. The basic idea is that services are implemented by
-means of interfaces and rely on polymorphism.<br/>
-The locator is instantiated with the base type of the service if any and a
-concrete implementation is provided along with all the parameters required to
-initialize it. As an example:
+The service locator API tries to mimic that of `std::optional` and adds some
+extra functionalities on top of it such as allocator support.<br/>
+There are a couple of functions to set up a service, namely `emplace` and
+`allocate_emplace`:
 
 ```cpp
-// the service has no base type, a locator is used to treat it as a kind of singleton
-entt::service_locator<my_service>::set(params...);
-
-// sets up an opaque service
-entt::service_locator<audio_interface>::set<audio_implementation>(params...);
-
-// resets (destroys) the service
-entt::service_locator<audio_interface>::reset();
+entt::locator<interface>::emplace<service>(argument);
+entt::locator<interface>::allocate_emplace<service>(allocator, argument);
 ```
 
-The locator can also be queried to know if an active service is currently set
-and to retrieve it if necessary (either as a pointer or as a reference):
+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 has been set up, it's retrieved using the value function:
 
 ```cpp
-// no service currently set
-auto empty = entt::service_locator<audio_interface>::empty();
-
-// gets a (possibly empty) shared pointer to the service ...
-std::shared_ptr<audio_interface> ptr = entt::service_locator<audio_interface>::get();
-
-// ... or a reference, but it's undefined behaviour if the service isn't set yet
-audio_interface &ref = entt::service_locator<audio_interface>::ref();
+interface &service = entt::locator<interface>::value();
 ```
 
-A common use is to wrap the different locators in a container class, creating
-aliases for the various services:
+Since the service may not be set (and therefore this function may result in an
+undefined behavior), the `has_value` and `value_or` functions are also available
+to test a service locator and to get a fallback service in case there is none:
 
 ```cpp
-struct locator {
-    using camera = entt::service_locator<camera_interface>;
-    using audio = entt::service_locator<audio_interface>;
-    // ...
-};
-
-// ...
-
-void init() {
-    locator::camera::set<camera_null>();
-    locator::audio::set<audio_implementation>(params...);
+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
+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.

docs/md/meta.md

@@ -13,6 +13,7 @@
   * [Container support](#container-support)
   * [Pointer-like types](#pointer-like-types)
   * [Template information](#template-information)
+  * [Automatic conversions](#automatic-conversions)
   * [Implicitly generated default constructor](#implicitly-generated-default-constructor)
   * [Policies: the more, the less](#policies-the-more-the-less)
   * [Named constants and enums](#named-constants-and-enums)
@@ -109,9 +110,10 @@ decorated version of it. This object can be used to add the following:
   entt::meta<my_type>().ctor<int, char>().ctor<&factory>();
   ```
 
-* _Destructors_. Free functions can be set as destructors of reflected types.
-  The purpose is to give users the ability to free up resources that require
-  special treatment before an object is actually destroyed.<br/>
+* _Destructors_. Free functions and member functions can be used as destructors
+  of reflected types. The purpose is to give users the ability to free up
+  resources that require special treatment before an object is actually
+  destroyed.<br/>
   Use the `dtor` member function for this purpose:
 
   ```cpp
@@ -146,6 +148,12 @@ decorated version of it. This object can be used to add the following:
   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);
+  ```
+
   Refer to the inline documentation for all the details.
 
 * _Member functions_. Both real member functions of the underlying type and free
@@ -267,19 +275,6 @@ Refer to the inline documentation for all the details.
 
 The meta objects that compose a meta type are accessed in the following ways:
 
-* _Meta constructors_. They are accessed by types of arguments:
-
-  ```cpp
-  auto ctor = entt::resolve<my_type>().ctor<int, char>();
-  ```
-
-  The returned type is `meta_ctor` and may be invalid if there is no constructor
-  that accepts the supplied arguments or at least some types from which they are
-  derived or to which they can be converted.<br/>
-  A meta constructor offers an API to know the number of its arguments and their
-  expected meta types. Furthermor, it's possible to invoke it and therefore to
-  construct new instances of the underlying type.
-
 * _Meta data_. They are accessed by _name_:
 
   ```cpp
@@ -328,7 +323,7 @@ Furthermore, all them are also returned by specific overloads that provide the
 caller with iterable ranges of top-level elements. As an example:
 
 ```cpp
-for(auto data = entt::resolve<my_type>().data()) {
+for(auto data: entt::resolve<my_type>().data()) {
     // ...
 }
 ```
@@ -421,7 +416,7 @@ to case. In particular:
   ```
 
 * The `resize` member function allows to resize the wrapped container and
-  returns true in case of succes:
+  returns true in case of success:
 
   ```cpp
   const bool ok = view.resize(3u);
@@ -715,6 +710,56 @@ correspondence between real types and meta types.<br/>
 Therefore, the specialization will be used as is and the information it contains
 will be associated with the appropriate type when required.
 
+## Automatic conversions
+
+In C++, there are a number of conversions allowed between arithmetic types that
+make it convenient to work with this kind of data.<br/>
+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>()
+    .conv<bool>()
+    .conv<char>()
+    // ...
+    .conv<double>();
+```
+
+Repeated for each type eligible to undergo this type of conversions. This is
+both error prone and repetitive.<br/>
+Similarly, the language allows users to silently convert unscoped enums to their
+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>()
+    .conv<std::underlying_type_t<my_enum>>();
+```
+
+Fortunately, all of this can also be avoided. `EnTT` offers implicit support for
+these types of conversions:
+
+```cpp
+entt::meta_any any{42};
+any.allow_cast<double>();
+double value = any.cast<double>();
+```
+
+With no need for registration, the conversion takes place automatically under
+the hood. The same goes for a call to `allow_cast` involving a meta type:
+
+```cpp
+entt::meta_type type = entt::resolve<int>();
+entt::meta_any any{my_enum::a_value};
+any.allow_cast(type);
+int value = any.cast<int>();
+```
+
+This should make working with arithmetic types and scoped or unscoped enums as
+easy as it is in C++.<br/>
+It's also worth noting that it's still possible to set up conversion functions
+manually and these will always be preferred over the automatic ones.
+
 ## Implicitly generated default constructor
 
 In many cases, it's useful to be able to create objects of default constructible
@@ -726,14 +771,7 @@ them.
 For this reason and only for default constructible types, default constructors
 are automatically defined and associated with their meta types, whether they are
 explicitly or implicitly generated.<br/>
-Therefore, it won't be necessary to do this in order to construct an integer
-from its meta type:
-
-```cpp
-entt::meta<int>().ctor<>();
-```
-
-Instead, just do this:
+Therefore, this is all is needed to construct an integer from its meta type:
 
 ```cpp
 entt::resolve<int>().construct();
@@ -742,11 +780,8 @@ entt::resolve<int>().construct();
 Where the meta type can be 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 custom defaul constructors, they are preferred
-both during searches and when the `construct` member function is invoked.<br/>
-However, the implicitly generated default constructor will always be returned,
-either if one is not explicitly specified or if all constructors are iterated
-for some reason (in this case, it will always be the last element).
+In all cases, when users register default constructors, they are preferred both
+during searches and when the `construct` member function is invoked.
 
 ## Policies: the more, the less
 
@@ -771,9 +806,11 @@ There are a few alternatives available at the moment:
 * The _as-void_ policy, associated with the type `entt::as_void_t`.<br/>
   Its purpose is to discard the return value of a meta object, whatever it is,
   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);
   ```
+
   If the use with functions is obvious, it must be said that it's also possible
   to use this policy with constructors and data members. In the first case, the
   constructor will be invoked but the returned wrapper will actually be empty.
@@ -785,9 +822,11 @@ There are a few alternatives available at the moment:
   Accessing the object contained in the wrapper for which the _reference_ was
   requested will make it possible to directly access the instance used to
   initialize the wrapper itself:
+
   ```cpp
   entt::meta<my_type>().data<&my_type::data_member, entt::as_ref_t>("member"_hs);
   ```
+
   These policies work with constructors (for example, when objects are taken
   from an external container rather than created on demand), data members and
   functions in general.<br/>
@@ -881,38 +920,15 @@ Multiple formats are supported when it comes to defining a property:
 
   A tuple contains one or more properties. All of them are treated individually.
 
-* Annotations:
+Note that it's not possible to invoke `prop` multiple times for the same meta
+object and trying to do that will result in a compilation error.<br/>
+However, the `props` function is available to associate several properties at
+once. In this case, properties in the key/value form aren't allowed, since they
+would be interpreted as two different properties rather than a single one.
 
-  ```cpp
-  entt::meta<my_type>().type("reflected_type"_hs).prop(&property_generator);
-  ```
-
-  An annotation is an invocable object that returns one or more properties. All
-  of them are treated individually.
-
-It's possible to invoke the `prop` function several times if needed, one for
-each property to associate with the last meta object created:
-
-```cpp
-entt::meta<my_type>()
-    .type("reflected_type"_hs)
-        .prop(entt::hashed_string{"Name"}, "Reflected Type")
-    .data<&my_type::data_member>("member"_hs)
-        .prop(std::make_pair("tooltip"_hs, "Member"))
-        .prop(my_enum::a_value, 42);
-```
-
-Alternatively, the `props` function is available to associate several properties
-at a time. However, in this case properties in the key/value form aren't
-allowed, since they would be interpreted as two different properties rather than
-a single one.
-
-The meta objects for which properties are supported are currently the meta
-types, meta constructors, meta data and meta functions. It's not possible to
-attach properties to other types of meta objects and the factory returned as a
-result of their construction won't allow such an operation.
-
-These types offer a couple of member functions named `prop` to iterate all
+The meta objects for which properties are supported are currently meta types,
+meta data and meta functions.<br/>
+These types also offer a couple of member functions named `prop` to iterate all
 properties at once or to search a specific property by key:
 
 ```cpp
@@ -942,7 +958,21 @@ objects from it and making its identifier no longer visible. The underlying node
 will remain available though, as if it were implicitly generated:
 
 ```cpp
-entt::resolve<my_type>().reset();
+entt::meta_reset<my_type>();
 ```
 
-The type can be re-registered later with a completely different name and form.
+It's also possible to reset types by their unique identifiers if required:
+
+```cpp
+entt::meta_reset("my_type"_hs);
+```
+
+Finally, there exists a non-template overload of the `meta_reset` function that
+doesn't accept argument and resets all searchable types (that is, all types that
+were assigned an unique identifier):
+
+```cpp
+entt::meta_reset();
+```
+
+All types can be re-registered later with a completely different name and form.

docs/md/poly.md

@@ -10,7 +10,7 @@
 * [Concept and implementation](#concept-and-implementation)
   * [Deduced interface](#deduced-interface)
   * [Defined interface](#defined-interface)
-  * [Fullfill a concept](#fullfill-a-concept)
+  * [Fulfill a concept](#fulfill-a-concept)
 * [Inheritance](#inheritance)
 * [Static polymorphism in the wild](#static-polymorphism-in-the-wild)
 * [Storage size and alignment requirement](#storage-size-and-alignment-requirement)
@@ -24,8 +24,8 @@ Static polymorphism is a very powerful tool in C++, albeit sometimes cumbersome
 to obtain.<br/>
 This module aims to make it simple and easy to use.
 
-The library allows to define _concepts_ as interfaces to fullfill with concrete
-classes withouth having to inherit from a common base.<br/>
+The library allows to define _concepts_ as interfaces to fulfill with concrete
+classes without having to inherit from a common base.<br/>
 This is, among others, one of the advantages of static polymorphism in general
 and of a generic wrapper like that offered by the `poly` class template in
 particular.<br/>
@@ -74,7 +74,7 @@ limitations and it's 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, it will be sufficient to provide a generic
-implementation to fullfill the concept.<br/>
+implementation to fulfill the concept.<br/>
 Also in this case, the library allows customizations based on types or families
 of types, so as to be able to go beyond the generic case where necessary.
 
@@ -94,7 +94,7 @@ struct Drawable: entt::type_list<> {
 ```
 
 It's recognizable by the fact that it inherits from an empty type list.<br/>
-Functions can also be const, accept any number of paramters and return a type
+Functions can also be const, accept any number of parameters and return a type
 other than `void`:
 
 ```cpp
@@ -120,7 +120,7 @@ external call:
 struct Drawable: entt::type_list<> {
     template<typename Base>
     struct type: Base {
-        bool draw() const { entt::poly_call<0>(*this); }
+        void draw() const { entt::poly_call<0>(*this); }
     };
 
     // ...
@@ -187,7 +187,7 @@ the interface itself.
 Explicitly defining a static virtual table suppresses the deduction step and
 allows maximum flexibility when providing the implementation for a concept.
 
-## Fullfill a concept
+## Fulfill a concept
 
 The `impl` alias template of a concept is used to define how it's fulfilled:
 
@@ -202,7 +202,7 @@ struct Drawable: entt::type_list<> {
 
 In this case, it's stated that the `draw` method of a generic type will be
 enough to satisfy the requirements of the `Drawable` concept.<br/>
-Both member functions and free functions are supported to fullfill concepts:
+Both member functions and free functions are supported to fulfill concepts:
 
 ```cpp
 template<typename Type>

docs/md/reference.md

@@ -28,6 +28,8 @@ I hope this list can grow much more in the future:
     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
+    archetype ECS.
   * [Polypropylene](https://github.com/pmbittner/Polypropylene): a hybrid
     solution between an ECS and dynamic mixins.
 
@@ -44,15 +46,25 @@ I hope this list can grow much more in the future:
     by `EnTT`.
 
 * Javascript:
-  * [\@javelin/ecs](https://github.com/3mcd/javelin/tree/master/packages/ecs): an
-    archetype ECS in TypeScript.
+  * [\@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
     investigate the underlying design of `ecsy` but it looks cool anyway.
 
+* Perl:
+  * [Game::Entities](https://gitlab.com/jjatria/perl-game-entities): a simple
+    entity registry for ECS designs inspired by `EnTT`.
+
+* Raku:
+  * [Game::Entities](https://gitlab.com/jjatria/raku-game-entities): a simple
+    entity registry for ECS designs inspired by `EnTT`.
+
 * Rust:
   * [Legion](https://github.com/TomGillen/legion): a chunk based archetype ECS.
   * [Shipyard](https://github.com/leudz/shipyard): it borrows some ideas from
     `EnTT` and offers a sparse sets based ECS with grouping functionalities.
+  * [Sparsey](https://github.com/LechintanTudor/sparsey): sparse set based ECS
+    written in Rust.
   * [Specs](https://github.com/amethyst/specs): a parallel ECS based mainly on
     hierarchical bitsets that allows different types of storage as needed.
 

docs/md/resource.md

@@ -7,6 +7,9 @@
 
 * [Introduction](#introduction)
 * [The resource, the loader and the cache](#the-resource-the-loader-and-the-cache)
+  * [Resource handle](#resource-handle)
+  * [Loaders](#loader)
+  * [The cache class](#the-cache)
 <!--
 @endcond TURN_OFF_DOXYGEN
 -->
@@ -21,211 +24,169 @@ 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
-cases. Instead, it offers a minimal and perhaps trivial cache that can be useful
-most of the time during prototyping and sometimes even in a production
-environments.<br/>
-For those interested in the subject, the plan is to improve it considerably over
-time in terms of performance, memory usage and functionalities. Hoping to make
-it, of course, one step at a time.
+cases.<br/>
+Instead, the library offers a minimal, general purpose resource cache that might
+be useful in many cases.
 
 # The resource, the loader and the cache
 
-There are three main actors in the model: the resource, the loader and the
-cache.
-
-The _resource_ is whatever users want it to be. An image, a video, an audio,
-whatever. There are no limits.<br/>
-As a minimal example:
+Resource, loader and cache are the three main actors for the purpose.<br/>
+The _resource_ is an image, an audio, a video or any other type:
 
 ```cpp
 struct my_resource { const int value; };
 ```
 
-A _loader_ is a class the aim of which is to load a specific resource. It has to
-inherit directly from a dedicated base class as in the following example:
+The _loader_ is a callable type the aim of which is to load a specific resource:
 
 ```cpp
-struct my_loader final: entt::resource_loader<my_loader, my_resource> {
-    // ...
-};
-```
+struct my_loader final {
+    using result_type = std::shared_ptr<my_resource>;
 
-Where `my_resource` is the type of resources it creates.<br/>
-A resource loader must also expose a public const member function named `load`
-that accepts a variable number of arguments and returns a shared pointer to a
-resource.<br/>
-As an example:
-
-```cpp
-struct my_loader: entt::resource_loader<my_loader, my_resource> {
-    std::shared_ptr<my_resource> load(int value) const {
+    result_type operator()(int value) const {
         // ...
-        return std::shared_ptr<my_resource>(new my_resource{ value });
+        return std::make_shared<my_resource>(value);
     }
 };
 ```
 
-In general, resource loaders should not have a state or retain data of any type.
-They should let the cache manage their resources instead.<br/>
-As a side note, base class and CRTP idiom aren't strictly required with the
-current implementation. One could argue that a cache can easily work with
-loaders of any type. However, future changes won't be breaking ones by forcing
-the use of a base class today and that's why the model is already in its place.
+Its function operator can accept any arguments and should return a value of the
+declared result type (`std::shared_ptr<my_resource>` in the example).<br/>
+A loader can also overload its function call operator to make it possible to
+construct the same or another resource from different lists of arguments.
 
 Finally, a cache is a specialization of a class template tailored to a specific
-resource:
+resource and (optionally) a loader:
 
 ```cpp
-using my_cache = entt::resource_cache<my_resource>;
+using my_cache = entt::resource_cache<my_resource, my_loader>;
 
 // ...
 
 my_cache cache{};
 ```
 
-The idea is to create different caches for different types of resources and to
-manage each one independently in the most appropriate way.<br/>
+The cache is meant to be used to create different caches for different types of
+resources and to manage each one independently in the most appropriate way.<br/>
 As a (very) trivial example, audio tracks can survive in most of the scenes of
-an application while meshes can be associated with a single scene and then
-discarded when users leave it.
+an application while meshes can be associated with a single scene only, then
+discarded when a player leaves it.
 
-A cache offers a set of basic functionalities to query its internal state and to
-_organize_ it:
+## Resource handle
+
+Resources aren't returned directly to the caller. Instead, they are wrapped in a
+_resource handle_ identified by the `entt::resource` class template.<br/>
+For those who know the _flyweight design pattern_ already, that's 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
+handle mostly maps the interface of its standard counterpart and only adds a few
+things to it.<br/>
+However, the handle in `EnTT` is designed as a standalone class template named
+`resource`. It boils down to the fact that specializing a class in the standard
+is often undefined behavior while having the ability to specialize the handle
+for one, more or all resource types could help over time.
+
+## Loaders
+
+A loader is a class that is responsible for _loading_ the resources.<br/>
+By default, it's just a callable object which 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.
+
+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, it isn't 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:
 
 ```cpp
-// gets the number of resources managed by a cache
-const auto size = cache.size();
+struct my_loader {
+    using result_type = std::shared_ptr<my_resource>;
 
-// checks if a cache contains at least a valid resource
-const auto empty = cache.empty();
+    struct from_disk_tag{};
+    struct from_network_tag{};
 
-// clears a cache and discards its content
-cache.clear();
+    template<typename Args>
+    result_type operator()(from_disk_tag, Args&&... args) {
+        // ...
+        return std::make_shared<my_resource>(std::forward<Args>(args)...);
+    }
+
+    template<typename Args>
+    result_type operator()(from_network_tag, Args&&... args) {
+        // ...
+        return std::make_shared<my_resource>(std::forward<Args>(args)...);
+    }
+}
 ```
 
-Besides these member functions, a cache contains what is needed to load, use and
-discard resources of the given type.<br/>
-Before exploring this part of the interface, it makes sense to mention how
-resources are identified. They have type `id_type` and therefore they can be
-created explicitly as in the following example:
+This makes the whole loading logic quite flexible and easy to extend over time.
+
+## The cache class
+
+The cache is the class that is asked to _connect the dots_.<br/>
+It loads the resources, store them aside and returns handles as needed:
 
 ```cpp
-constexpr auto identifier = "my/resource/identifier"_hs;
-// this is equivalent to the following
-constexpr entt::id_type hs = entt::hashed_string{"my/resource/identifier"};
+entt::resource_cache<my_resource, my_loader> cache{};
 ```
 
-The class `hashed_string` is described in a dedicated section, so I won't go in
-details here.
-
-Resources are loaded and thus stored in a cache through the `load` member
-function. It accepts the loader to use as a template parameter, the resource
-identifier and the parameters used to construct the resource as arguments:
+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 functionality a user would expect from a
+map, such as `empty` or `size` and so on. Similarly, it's an iterable type that
+also supports indexing by resource id:
 
 ```cpp
-// uses the identifier declared above
-cache.load<my_loader>(identifier, 0);
+for(entt::resource<my_resource> curr: cache) {
+    // ...
+}
 
-// uses a hashed string directly
-cache.load<my_loader>("another/identifier"_hs, 42);
-```
-
-The function returns a handle to the resource, whether it already exists or is
-loaded. In case the loader returns an invalid pointer, the handle is invalid as
-well and therefore it can be easily used with an `if` statement:
-
-```cpp
-if(entt::resource_handle handle = cache.load<my_loader>("another/identifier"_hs, 42); handle) {
+if(entt::resource<my_resource> res = cache["resource/id"_hs]; res) {
     // ...
 }
 ```
 
-Before trying to load a resource, the `contains` member function can be used to
-know if a cache already contains a specific resource:
+Please, refer to the inline documentation for all the details about the other
+functions (for example `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):
 
 ```cpp
-auto exists = cache.contains("my/identifier"_hs);
+auto ret = cache.load("resource/id"_hs);
+
+// true only if the resource was not already present
+const bool loaded = ret.second;
+
+// takes the resource handle pointed to by the returned iterator
+entt::resource<my_resource> res = *ret.first;
 ```
 
-There exists also a member function to use to force a reload of an already
-existing resource if needed:
+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.
 
-```cpp
-auto handle = cache.reload<my_loader>("another/identifier"_hs, 42);
-```
-
-As above, the function returns a handle to the resource that is invalid in case
-of errors. The `reload` member function is a kind of alias of the following
-snippet:
-
-```cpp
-cache.discard(identifier);
-cache.load<my_loader>(identifier, 42);
-```
-
-Where the `discard` member function is used to get rid of a resource if loaded.
-In case the cache doesn't contain a resource for the given identifier, `discard`
-does nothing and returns immediately.
-
-So far, so good. Resources are finally loaded and stored within the cache.<br/>
-They are returned to users in the form of handles. To get one of them later on:
-
-```cpp
-auto handle = cache.handle("my/identifier"_hs);
-```
-
-The idea behind a handle is the same of the flyweight pattern. In other terms,
-resources aren't copied around. Instead, instances are shared between handles.
-Users of a resource own a handle that guarantees that a resource isn't destroyed
-until all the handles are destroyed, even if the resource itself is removed from
-the cache.<br/>
-Handles are tiny objects both movable and copyable. They return the contained
-resource as a (possibly const) reference on request:
-
-* By means of the `get` member function:
-
-  ```cpp
-  auto &resource = handle.get();
-  ```
-
-* Using the proper cast operator:
-
-  ```cpp
-  auto &resource = handle;
-  ```
-
-* Through the dereference operator:
-
-  ```cpp
-  auto &resource = *handle;
-  ```
-
-The resource can also be accessed directly using the arrow operator if required:
-
-```cpp
-auto value = handle->value;
-```
-
-To test if a handle is still valid, the cast operator to `bool` allows users to
-use it in a guard:
-
-```cpp
-if(handle) {
-    // ...
-}
-```
-
-Finally, in case there is the need to load a resource and thus to get a handle
-without storing the resource itself in the cache, users can rely on the `temp`
-member function template.<br/>
-The declaration is similar to that of `load`, a (possibly invalid) handle for
-the resource is returned also in this case:
-
-```cpp
-if(auto handle = cache.temp<my_loader>(42); handle) {
-    // ...
-}
-```
-
-Do not forget to test the handle for validity. Otherwise, getting a reference to
-the resource it points may result in undefined behavior.
+Moreover, it's worth mentioning that both the iterators of a cache and 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
+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.

docs/md/signal.md

@@ -11,6 +11,7 @@
   * [Lambda support](#lambda-support)
 * [Signals](#signals)
 * [Event dispatcher](#event-dispatcher)
+  * [Named queues](#named-queues)
 * [Event emitter](#event-emitter)
 <!--
 @endcond TURN_OFF_DOXYGEN
@@ -361,7 +362,7 @@ case:
 struct my_collector {
     std::vector<int> vec{};
 
-    bool operator()(int v) noexcept {
+    bool operator()(int v) {
         vec.push_back(v);
         return true;
     }
@@ -375,23 +376,20 @@ signal.collect(std::ref(collector));
 
 # Event dispatcher
 
-The event dispatcher class is designed so as to be used in a loop. It allows
-users both to trigger immediate events or to queue events to be published all
-together once per tick.<br/>
-This class shares part of its API with the one of the signal handler, but it
-doesn't require that all the types of events are specified when declared:
+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:
 
 ```cpp
 // define a general purpose dispatcher
 entt::dispatcher dispatcher{};
 ```
 
-In order to register an instance of a class to a dispatcher, its type must
-expose one or more member functions the arguments of which are such that `E &`
-can be converted to them for each type of event `E`, no matter what the return
-value is.<br/>
-The name of the member function aimed to receive the event must be provided to
-the `connect` member function of the sink in charge for the specific event:
+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_:
 
 ```cpp
 struct an_event { int value; };
@@ -409,8 +407,8 @@ dispatcher.sink<an_event>().connect<&listener::receive>(listener);
 dispatcher.sink<another_event>().connect<&listener::method>(listener);
 ```
 
-The `disconnect` member function follows the same pattern and can be used to
-remove one listener at a time or all of them at once:
+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);
@@ -418,14 +416,10 @@ dispatcher.sink<another_event>().disconnect(listener);
 ```
 
 The `trigger` member function serves the purpose of sending an immediate event
-to all the listeners registered so far. It offers a convenient approach that
-relieves users from having to create the event itself. Instead, it's enough to
-specify the type of event and provide all the parameters required to construct
-it.<br/>
-As an example:
+to all the listeners registered so far:
 
 ```cpp
-dispatcher.trigger<an_event>(42);
+dispatcher.trigger(an_event{42});
 dispatcher.trigger<another_event>();
 ```
 
@@ -434,16 +428,14 @@ method can be used to push around urgent messages like an _is terminating_
 notification on a mobile app.
 
 On the other hand, the `enqueue` member function queues messages together and
-allows to maintain control over the moment they are sent to listeners. The
-signature of this method is more or less the same of `trigger`:
+helps to maintain control over the moment they are sent to listeners:
 
 ```cpp
 dispatcher.enqueue<an_event>(42);
-dispatcher.enqueue<another_event>();
+dispatcher.enqueue(another_event{});
 ```
 
-Events are stored aside until the `update` member function is invoked, then all
-the messages that are still pending are sent to the listeners at once:
+Events are stored aside until the `update` member function is invoked:
 
 ```cpp
 // emits all the events of the given type at once
@@ -456,6 +448,30 @@ dispatcher.update();
 This way users can embed the dispatcher in a loop and literally dispatch events
 once per tick to their systems.
 
+## Named queues
+
+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
+also multiple queues for a single type). In fact, more or less all functions
+also take an additional parameter. As an example:
+
+```cpp
+dispatcher.sink<an_event>("custom"_hs).connect<&listener::receive>(listener);
+```
+
+In this case, the term _name_ is misused as these are actual numeric identifiers
+of type `id_type`.<br/>
+An exception to this rule is the `enqueue` function. There is no additional
+parameter for it but rather a different function:
+
+```cpp
+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.
+
 # Event emitter
 
 A general purpose event emitter thought mainly for those cases where it comes to

natvis/entt/config.natvis

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

natvis/entt/container.natvis

@@ -0,0 +1,33 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+	<Type Name="entt::dense_map&lt;*&gt;">
+		<Intrinsic Name="size" Expression="packed.first_base::value.size()"/>
+		<Intrinsic Name="bucket_count" Expression="sparse.first_base::value.size()"/>
+		<DisplayString>{{ size={ size() } }}</DisplayString>
+		<Expand>
+			<Item Name="[capacity]" ExcludeView="simple">packed.first_base::value.capacity()</Item>
+			<Item Name="[bucket_count]" ExcludeView="simple">bucket_count()</Item>
+			<Item Name="[load_factor]" ExcludeView="simple">(float)size() / (float)bucket_count()</Item>
+			<Item Name="[max_load_factor]" ExcludeView="simple">threshold</Item>
+			<IndexListItems>
+				<Size>size()</Size>
+				<ValueNode>packed.first_base::value[$i].element</ValueNode>
+			</IndexListItems>
+		</Expand>
+	</Type>
+	<Type Name="entt::dense_set&lt;*&gt;">
+		<Intrinsic Name="size" Expression="packed.first_base::value.size()"/>
+		<Intrinsic Name="bucket_count" Expression="sparse.first_base::value.size()"/>
+		<DisplayString>{{ size={ size() } }}</DisplayString>
+		<Expand>
+			<Item Name="[capacity]" ExcludeView="simple">packed.first_base::value.capacity()</Item>
+			<Item Name="[bucket_count]" ExcludeView="simple">bucket_count()</Item>
+			<Item Name="[load_factor]" ExcludeView="simple">(float)size() / (float)bucket_count()</Item>
+			<Item Name="[max_load_factor]" ExcludeView="simple">threshold</Item>
+			<IndexListItems>
+				<Size>size()</Size>
+				<ValueNode>packed.first_base::value[$i].second</ValueNode>
+			</IndexListItems>
+		</Expand>
+	</Type>
+</AutoVisualizer>

natvis/entt/core.natvis

@@ -0,0 +1,33 @@
+<?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>
+    </Type>
+	<Type Name="entt::compressed_pair&lt;*&gt;">
+		<Intrinsic Name="first" Optional="true" Expression="((first_base*)this)->value"/>
+		<Intrinsic Name="first" Optional="true" Expression="*(first_base::base_type*)this"/>
+		<Intrinsic Name="second" Optional="true" Expression="((second_base*)this)->value"/>
+		<Intrinsic Name="second" Optional="true" Expression="*(second_base::base_type*)this"/>
+		<DisplayString >({ first() }, { second() })</DisplayString>
+		<Expand>
+			<Item Name="[first]">first()</Item>
+			<Item Name="[second]">second()</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_hashed_string&lt;*&gt;">
+		<DisplayString Condition="base_type::repr != nullptr">{{ hash={ base_type::hash } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<Item Name="[data]">base_type::repr,na</Item>
+			<Item Name="[length]">base_type::length</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::type_info">
+		<DisplayString Condition="seq != 0u">{{ name={ alias,na } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<Item Name="[hash]">identifier</Item>
+			<Item Name="[index]">seq</Item>
+		</Expand>
+	</Type>
+</AutoVisualizer>

natvis/entt/entity.natvis

@@ -0,0 +1,145 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+	<Type Name="entt::basic_registry&lt;*&gt;">
+		<Intrinsic Name="pools_size" Expression="pools.packed.first_base::value.size()"/>
+		<Intrinsic Name="vars_size" Expression="vars.data.packed.first_base::value.size()"/>
+		<Intrinsic Name="to_entity" Expression="*((entity_traits::entity_type *)&amp;entity) &amp; entity_traits::entity_mask">
+			<Parameter Name="entity" Type="entity_traits::value_type &amp;"/>
+		</Intrinsic>
+		<DisplayString>{{ size={ entities.size() } }}</DisplayString>
+		<Expand>
+			<Item IncludeView="simple" Name="[entities]">entities,view(simple)nr</Item>
+			<Synthetic Name="[entities]" ExcludeView="simple">
+				<DisplayString>{ entities.size() }</DisplayString>
+				<Expand>
+					<CustomListItems>
+						<Variable Name="pos" InitialValue="0" />
+						<Variable Name="last" InitialValue="entities.size()"/>
+						<Loop>
+							<Break Condition="pos == last"/>
+							<If Condition="to_entity(entities[pos]) == pos">
+								<Item Name="[{ pos }]">entities[pos]</Item>
+							</If>
+							<Exec>++pos</Exec>
+						</Loop>
+					</CustomListItems>
+				</Expand>
+			</Synthetic>
+			<Synthetic Name="[destroyed]" ExcludeView="simple">
+				<DisplayString>{ to_entity(free_list) != entity_traits::entity_mask }</DisplayString>
+				<Expand>
+					<CustomListItems>
+						<Variable Name="it" InitialValue="to_entity(free_list)" />
+						<Loop>
+							<Break Condition="it == entity_traits::entity_mask"/>
+							<Item Name="[{ it }]">entities[it]</Item>
+							<Exec>it = to_entity(entities[it])</Exec>
+						</Loop>
+					</CustomListItems>
+				</Expand>
+			</Synthetic>
+			<Synthetic Name="[pools]">
+				<DisplayString>{ pools_size() }</DisplayString>
+				<Expand>
+					<IndexListItems ExcludeView="simple">
+						<Size>pools_size()</Size>
+						<ValueNode>*pools.packed.first_base::value[$i].element.second</ValueNode>
+					</IndexListItems>
+					<IndexListItems IncludeView="simple">
+						<Size>pools_size()</Size>
+						<ValueNode>*pools.packed.first_base::value[$i].element.second,view(simple)</ValueNode>
+					</IndexListItems>
+				</Expand>
+			</Synthetic>
+			<Item Name="[groups]" ExcludeView="simple">groups.size()</Item>
+			<Synthetic Name="[vars]">
+				<DisplayString>{ vars_size() }</DisplayString>
+				<Expand>
+					<IndexListItems>
+						<Size>vars_size()</Size>
+						<ValueNode>vars.data.packed.first_base::value[$i].element.second</ValueNode>
+					</IndexListItems>
+				</Expand>
+			</Synthetic>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_sparse_set&lt;*&gt;">
+		<DisplayString>{{ size={ packed.size() }, type={ info->alias,na } }}</DisplayString>
+		<Expand>
+			<Item Name="[capacity]" ExcludeView="simple">packed.capacity()</Item>
+			<Item Name="[policy]">mode,en</Item>
+			<Synthetic Name="[sparse]">
+				<DisplayString>{ sparse.size() * entity_traits::page_size }</DisplayString>
+				<Expand>
+					<ExpandedItem IncludeView="simple">sparse,view(simple)</ExpandedItem>
+					<CustomListItems ExcludeView="simple">
+						<Variable Name="pos" InitialValue="0"/>
+						<Variable Name="page" InitialValue="0"/>
+						<Variable Name="offset" InitialValue="0"/>
+						<Variable Name="last" InitialValue="sparse.size() * entity_traits::page_size"/>
+						<Loop>
+							<Break Condition="pos == last"/>
+							<Exec>page = pos / entity_traits::page_size</Exec>
+							<Exec>offset = pos &amp; (entity_traits::page_size - 1)</Exec>
+							<If Condition="sparse[page] &amp;&amp; (*((entity_traits::entity_type *)&amp;sparse[page][offset]) &lt; ~entity_traits::entity_mask)">
+								<Item Name="[{ pos }]">*((entity_traits::entity_type *)&amp;sparse[page][offset]) &amp; entity_traits::entity_mask</Item>
+							</If>
+							<Exec>++pos</Exec>
+						</Loop>
+					</CustomListItems>
+				</Expand>
+			</Synthetic>
+			<Synthetic Name="[packed]">
+				<DisplayString>{ packed.size() }</DisplayString>
+				<Expand>
+					<ExpandedItem IncludeView="simple">packed,view(simple)</ExpandedItem>
+					<CustomListItems ExcludeView="simple">
+						<Variable Name="pos" InitialValue="0"/>
+						<Variable Name="last" InitialValue="packed.size()"/>
+						<Loop>
+							<Break Condition="pos == last"/>
+							<If Condition="*((entity_traits::entity_type *)&amp;packed[pos]) &lt; ~entity_traits::entity_mask">
+								<Item Name="[{ pos }]">packed[pos]</Item>
+							</If>
+							<Exec>++pos</Exec>
+						</Loop>
+					</CustomListItems>
+				</Expand>
+			</Synthetic>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_storage&lt;*&gt;">
+		<DisplayString>{{ size={ base_type::packed.size() }, type={ base_type::info->alias,na } }}</DisplayString>
+		<Expand>
+			<Item Name="[capacity]" Optional="true" ExcludeView="simple">packed.first_base::value.capacity() * comp_traits::page_size</Item>
+			<Item Name="[page size]" Optional="true" ExcludeView="simple">comp_traits::page_size</Item>
+			<Item Name="[base]" ExcludeView="simple">(base_type*)this,nand</Item>
+			<Item Name="[base]" IncludeView="simple">(base_type*)this,view(simple)nand</Item>
+			<!-- having SFINAE-like techniques in natvis is priceless :) -->
+			<CustomListItems Condition="packed.first_base::value.size() != 0" Optional="true">
+				<Variable Name="pos" InitialValue="0" />
+				<Variable Name="last" InitialValue="base_type::packed.size()"/>
+				<Loop>
+					<Break Condition="pos == last"/>
+					<If Condition="*((base_type::entity_traits::entity_type *)&amp;base_type::packed[pos]) &lt; ~base_type::entity_traits::entity_mask">
+						<Item Name="[{ pos }]">packed.first_base::value[pos / comp_traits::page_size][pos &amp; (comp_traits::page_size - 1)]</Item>
+					</If>
+					<Exec>++pos</Exec>
+				</Loop>
+			</CustomListItems>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_view&lt;*&gt;">
+		<DisplayString>{{ size_hint={ view->packed.size() } }}</DisplayString>
+		<Expand>
+			<Item Name="[pools]">pools,na</Item>
+			<Item Name="[filter]">filter,na</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::null_t">
+		<DisplayString>&lt;null&gt;</DisplayString>
+	</Type>
+	<Type Name="entt::tombstone_t">
+		<DisplayString>&lt;tombstone&gt;</DisplayString>
+	</Type>
+</AutoVisualizer>

natvis/entt/locator.natvis

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

natvis/entt/meta.natvis

@@ -0,0 +1,197 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+	<Type Name="entt::meta_any">
+		<DisplayString Condition="node != nullptr">{{ type={ node->info->alias,na }, policy={ storage.mode,en } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<ExpandedItem Condition="node != nullptr">*node</ExpandedItem>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_associative_container">
+		<DisplayString Condition="mapped_type_node != nullptr">{{ key_type={ key_type_node->info->alias,na }, mapped_type={ mapped_type_node->info->alias,na } }}</DisplayString>
+		<DisplayString Condition="key_type_node != nullptr">{{ key_type={ key_type_node->info->alias,na } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+	</Type>
+	<Type Name="entt::internal::meta_base_node">
+		<DisplayString Condition="type != nullptr">{{ type={ type->info->alias,na } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+	</Type>
+	<Type Name="entt::internal::meta_conv_node">
+		<DisplayString Condition="type != nullptr">{{ type={ type->info->alias,na } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+	</Type>
+	<Type Name="entt::internal::meta_ctor_node">
+		<DisplayString Condition="arg != nullptr">{{ arity={ arity } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+	</Type>
+	<Type Name="entt::internal::meta_data_node">
+		<Intrinsic Name="has_property" Expression="!!(traits &amp; property)">
+			<Parameter Name="property" Type="int"/>
+		</Intrinsic>
+		<DisplayString Condition="type != nullptr">{{ type={ type->info->alias,na } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<Item Name="[id]">id</Item>
+			<Item Name="[arity]">arity</Item>
+			<Item Name="[is_const]">has_property(entt::internal::meta_traits::is_const)</Item>
+			<Item Name="[is_static]">has_property(entt::internal::meta_traits::is_static)</Item>
+			<Synthetic Name="[prop]" Condition="prop != nullptr">
+				<Expand>
+					<LinkedListItems>
+						<HeadPointer>prop</HeadPointer>
+						<NextPointer>next</NextPointer>
+						<ValueNode>*this</ValueNode>
+					</LinkedListItems>
+				</Expand>
+			</Synthetic>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_data">
+		<DisplayString Condition="node != nullptr">{ *node }</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<ExpandedItem Condition="node != nullptr">node</ExpandedItem>
+		</Expand>
+	</Type>
+	<Type Name="entt::internal::meta_func_node"	>
+		<Intrinsic Name="has_property" Expression="!!(traits &amp; property)">
+			<Parameter Name="property" Type="int"/>
+		</Intrinsic>
+		<DisplayString Condition="ret != nullptr">{{ arity={ arity }, ret={ ret->info->alias,na } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<Item Name="[id]">id</Item>
+			<Item Name="[is_const]">has_property(entt::internal::meta_traits::is_const)</Item>
+			<Item Name="[is_static]">has_property(entt::internal::meta_traits::is_static)</Item>
+			<Synthetic Name="[prop]" Condition="prop != nullptr">
+				<Expand>
+					<LinkedListItems>
+						<HeadPointer>prop</HeadPointer>
+						<NextPointer>next</NextPointer>
+						<ValueNode>*this</ValueNode>
+					</LinkedListItems>
+				</Expand>
+			</Synthetic>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_func">
+		<DisplayString Condition="node != nullptr">{ *node }</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<ExpandedItem Condition="node != nullptr">node</ExpandedItem>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_handle">
+		<DisplayString>{ any }</DisplayString>
+	</Type>
+	<Type Name="entt::internal::meta_prop_node">
+		<DisplayString Condition="value.node != nullptr">{{ key_type={ id.node->info->alias,na }, mapped_type={ value.node->info->alias,na } }}</DisplayString>
+		<DisplayString Condition="id.node != nullptr">{{ key_type={ id.node->info->alias,na } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<Item Name="[key]">id</Item>
+			<Item Name="[value]">value</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_prop">
+		<DisplayString Condition="node != nullptr">{ *node }</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<ExpandedItem Condition="node != nullptr">node</ExpandedItem>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_sequence_container">
+		<DisplayString Condition="value_type_node != nullptr">{{ value_type={ value_type_node->info->alias,na } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+	</Type>
+	<Type Name="entt::internal::meta_template_node">
+		<DisplayString Condition="type != nullptr">{{ type={ type->info->alias,na } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<Item Name="[arity]">arity</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::internal::meta_type_node">
+		<Intrinsic Name="has_property" Expression="!!(traits &amp; property)">
+			<Parameter Name="property" Type="int"/>
+		</Intrinsic>
+		<DisplayString Condition="info != nullptr">{{ type={ info->alias,na } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<Item Name="[id]">id</Item>
+			<Item Name="[sizeof]">size_of</Item>
+			<Item Name="[is_arithmetic]">has_property(entt::internal::meta_traits::is_arithmetic)</Item>
+			<Item Name="[is_array]">has_property(entt::internal::meta_traits::is_array)</Item>
+			<Item Name="[is_enum]">has_property(entt::internal::meta_traits::is_enum)</Item>
+			<Item Name="[is_class]">has_property(entt::internal::meta_traits::is_class)</Item>
+			<Item Name="[is_pointer]">has_property(entt::internal::meta_traits::is_pointer)</Item>
+			<Item Name="[is_meta_pointer_like]">has_property(entt::internal::meta_traits::is_meta_pointer_like)</Item>
+			<Item Name="[is_meta_sequence_container]">has_property(entt::internal::meta_traits::is_meta_sequence_container)</Item>
+			<Item Name="[is_meta_associative_container]">has_property(entt::internal::meta_traits::is_meta_associative_container)</Item>
+			<Item Name="[default_constructor]">default_constructor != nullptr</Item>
+			<Item Name="[conversion_helper]">conversion_helper != nullptr</Item>
+			<Item Name="[template_info]" Condition="templ != nullptr">*templ</Item>
+			<Synthetic Name="[ctor]" Condition="ctor != nullptr">
+				<Expand>
+					<LinkedListItems>
+						<HeadPointer>ctor</HeadPointer>
+						<NextPointer>next</NextPointer>
+						<ValueNode>*this</ValueNode>
+					</LinkedListItems>
+				</Expand>
+			</Synthetic>
+			<Synthetic Name="[base]" Condition="base != nullptr">
+				<Expand>
+					<LinkedListItems>
+						<HeadPointer>base</HeadPointer>
+						<NextPointer>next</NextPointer>
+						<ValueNode>*this</ValueNode>
+					</LinkedListItems>
+				</Expand>
+			</Synthetic>
+			<Synthetic Name="[conv]" Condition="conv != nullptr">
+				<Expand>
+					<LinkedListItems>
+						<HeadPointer>conv</HeadPointer>
+						<NextPointer>next</NextPointer>
+						<ValueNode>*this</ValueNode>
+					</LinkedListItems>
+				</Expand>
+			</Synthetic>
+			<Synthetic Name="[data]" Condition="data != nullptr">
+				<Expand>
+					<LinkedListItems>
+						<HeadPointer>data</HeadPointer>
+						<NextPointer>next</NextPointer>
+						<ValueNode>*this</ValueNode>
+					</LinkedListItems>
+				</Expand>
+			</Synthetic>
+			<Synthetic Name="[func]" Condition="func != nullptr">
+				<Expand>
+					<LinkedListItems>
+						<HeadPointer>func</HeadPointer>
+						<NextPointer>next</NextPointer>
+						<ValueNode>*this</ValueNode>
+					</LinkedListItems>
+				</Expand>
+			</Synthetic>
+			<Synthetic Name="[prop]" Condition="prop != nullptr">
+				<Expand>
+					<LinkedListItems>
+						<HeadPointer>prop</HeadPointer>
+						<NextPointer>next</NextPointer>
+						<ValueNode>*this</ValueNode>
+					</LinkedListItems>
+				</Expand>
+			</Synthetic>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_type">
+		<DisplayString Condition="node != nullptr">{ *node }</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<ExpandedItem Condition="node != nullptr">node</ExpandedItem>
+		</Expand>
+	</Type>
+</AutoVisualizer>

natvis/entt/platform.natvis

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

natvis/entt/poly.natvis

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+	<Type Name="entt::basic_poly&lt;*&gt;">
+		<DisplayString>{ storage }</DisplayString>
+	</Type>
+</AutoVisualizer>

natvis/entt/process.natvis

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

natvis/entt/resource.natvis

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+	<Type Name="entt::resource&lt;*&gt;">
+		<DisplayString>{ value }</DisplayString>
+		<Expand>
+			<ExpandedItem>value</ExpandedItem>
+		</Expand>
+	</Type>
+	<Type Name="entt::resource_cache&lt;*&gt;">
+		<DisplayString>{ pool.first_base::value }</DisplayString>
+		<Expand>
+			<ExpandedItem>pool.first_base::value</ExpandedItem>
+		</Expand>
+	</Type>
+</AutoVisualizer>

natvis/entt/signal.natvis

@@ -0,0 +1,52 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+	<Type Name="entt::connection">
+		<DisplayString>{{ bound={ signal != nullptr } }}</DisplayString>
+	</Type>
+	<Type Name="entt::delegate&lt;*&gt;">
+		<DisplayString>{{ type={ "$T1" } }}</DisplayString>
+		<Expand>
+			<Item Name="[empty]">fn == nullptr</Item>
+			<Item Name="[data]">instance</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::dispatcher">
+		<DisplayString>{{ size={ pools.size() } }}</DisplayString>
+		<Expand>
+			<Synthetic Name="[pools]">
+				<DisplayString>{ pools.size() }</DisplayString>
+				<Expand>
+					<IndexListItems>
+						<Size>pools.size()</Size>
+						<ValueNode>*pools.packed.first_base::value[$i].element.second</ValueNode>
+					</IndexListItems>
+				</Expand>
+			</Synthetic>
+		</Expand>
+	</Type>
+	<Type Name="entt::internal::dispatcher_handler&lt;*&gt;">
+		<DisplayString>{{ size={ events.size() }, event={ "$T1" } }}</DisplayString>
+		<Expand>
+			<Item Name="[signal]">signal</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::scoped_connection">
+		<DisplayString>{ conn }</DisplayString>
+	</Type>
+	<Type Name="entt::sigh&lt;*&gt;">
+		<DisplayString>{{ size={ calls.size() }, type={ "$T1" } }}</DisplayString>
+		<Expand>
+			<IndexListItems>
+				<Size>calls.size()</Size>
+				<ValueNode>calls[$i]</ValueNode>
+			</IndexListItems>
+		</Expand>
+	</Type>
+	<Type Name="entt::sink&lt;*&gt;">
+		<DisplayString>{{ type={ "$T1" } }}</DisplayString>
+		<Expand>
+			<Item Name="[signal]">signal,na</Item>
+			<Item Name="[offset]">offset</Item>
+		</Expand>
+	</Type>
+</AutoVisualizer>

src/entt/config/config.h

@@ -1,85 +1,79 @@
 #ifndef ENTT_CONFIG_CONFIG_H
 #define ENTT_CONFIG_CONFIG_H
 
+#include "version.h"
 
 #if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
-#   define ENTT_NOEXCEPT noexcept
-#   define ENTT_THROW throw
-#   define ENTT_TRY try
-#   define ENTT_CATCH catch(...)
+#    define ENTT_THROW throw
+#    define ENTT_TRY try
+#    define ENTT_CATCH catch(...)
 #else
-#   define ENTT_NOEXCEPT
-#   define ENTT_THROW
-#   define ENTT_TRY if(true)
-#   define ENTT_CATCH if(false)
+#    define ENTT_THROW
+#    define ENTT_TRY if(true)
+#    define ENTT_CATCH if(false)
 #endif
 
-
-#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
-#   include <new>
-#   define ENTT_LAUNDER(expr) std::launder(expr)
-#else
-#   define ENTT_LAUNDER(expr) expr
+#ifndef ENTT_NOEXCEPT
+#    define ENTT_NOEXCEPT noexcept
+#    define ENTT_NOEXCEPT_IF(expr) noexcept(expr)
+# else
+#    define ENTT_NOEXCEPT_IF(...)
 #endif
 
-
-#ifndef ENTT_USE_ATOMIC
-#   define ENTT_MAYBE_ATOMIC(Type) Type
+#ifdef ENTT_USE_ATOMIC
+#    include <atomic>
+#    define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
 #else
-#   include <atomic>
-#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#    define ENTT_MAYBE_ATOMIC(Type) Type
 #endif
 
-
 #ifndef ENTT_ID_TYPE
-#   include <cstdint>
-#   define ENTT_ID_TYPE std::uint32_t
+#    include <cstdint>
+#    define ENTT_ID_TYPE std::uint32_t
 #endif
 
-
-#ifdef ENTT_SPARSE_PAGE
-	static_assert(ENTT_SPARSE_PAGE && ((ENTT_SPARSE_PAGE & (ENTT_SPARSE_PAGE - 1)) == 0), "ENTT_SPARSE_PAGE must be a power of two");
-#else
-#   define ENTT_SPARSE_PAGE 4096
+#ifndef ENTT_SPARSE_PAGE
+#    define ENTT_SPARSE_PAGE 4096
 #endif
 
-
-#ifdef ENTT_PACKED_PAGE
-static_assert(ENTT_PACKED_PAGE && ((ENTT_PACKED_PAGE & (ENTT_PACKED_PAGE - 1)) == 0), "ENTT_PACKED_PAGE must be a power of two");
-#else
-#   define ENTT_PACKED_PAGE 1024
+#ifndef ENTT_PACKED_PAGE
+#    define ENTT_PACKED_PAGE 1024
 #endif
 
-
 #ifdef ENTT_DISABLE_ASSERT
-#   undef ENTT_ASSERT
-#   define ENTT_ASSERT(...) (void(0))
+#    undef ENTT_ASSERT
+#    define ENTT_ASSERT(...) (void(0))
 #elif !defined ENTT_ASSERT
-#   include <cassert>
-#   define ENTT_ASSERT(condition, ...) assert(condition)
+#    include <cassert>
+#    define ENTT_ASSERT(condition, ...) assert(condition)
 #endif
 
-
 #ifdef ENTT_NO_ETO
-#   include <type_traits>
-#   define ENTT_IGNORE_IF_EMPTY std::false_type
+#    define ENTT_IGNORE_IF_EMPTY false
 #else
-#   include <type_traits>
-#   define ENTT_IGNORE_IF_EMPTY std::true_type
+#    define ENTT_IGNORE_IF_EMPTY true
 #endif
 
-
-#ifndef ENTT_STANDARD_CPP
+#ifdef ENTT_STANDARD_CPP
+#    define ENTT_NONSTD false
+#else
+#    define ENTT_NONSTD true
 #    if defined __clang__ || defined __GNUC__
-#       define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
-#       define ENTT_PRETTY_FUNCTION_PREFIX '='
-#       define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#        define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#        define ENTT_PRETTY_FUNCTION_PREFIX '='
+#        define ENTT_PRETTY_FUNCTION_SUFFIX ']'
 #    elif defined _MSC_VER
-#       define ENTT_PRETTY_FUNCTION __FUNCSIG__
-#       define ENTT_PRETTY_FUNCTION_PREFIX '<'
-#       define ENTT_PRETTY_FUNCTION_SUFFIX '>'
-#   endif
+#        define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#        define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#        define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#    endif
 #endif
 
+#if defined _MSC_VER
+#    pragma detect_mismatch("entt.version", ENTT_VERSION)
+#    pragma detect_mismatch("entt.noexcept", ENTT_XSTR(ENTT_TRY))
+#    pragma detect_mismatch("entt.id", ENTT_XSTR(ENTT_ID_TYPE))
+#    pragma detect_mismatch("entt.nonstd", ENTT_XSTR(ENTT_NONSTD))
+#endif
 
 #endif

src/entt/config/macro.h

@@ -0,0 +1,7 @@
+#ifndef ENTT_CONFIG_MACRO_H
+#define ENTT_CONFIG_MACRO_H
+
+#define ENTT_STR(arg) #arg
+#define ENTT_XSTR(arg) ENTT_STR(arg)
+
+#endif

src/entt/config/version.h

@@ -1,10 +1,14 @@
 #ifndef ENTT_CONFIG_VERSION_H
 #define ENTT_CONFIG_VERSION_H
 
+#include "macro.h"
 
 #define ENTT_VERSION_MAJOR 3
-#define ENTT_VERSION_MINOR 8
-#define ENTT_VERSION_PATCH 0
+#define ENTT_VERSION_MINOR 10
+#define ENTT_VERSION_PATCH 1
 
+#define ENTT_VERSION \
+    ENTT_XSTR(ENTT_VERSION_MAJOR) \
+    "." ENTT_XSTR(ENTT_VERSION_MINOR) "." ENTT_XSTR(ENTT_VERSION_PATCH)
 
 #endif

src/entt/container/dense_map.hpp

@@ -0,0 +1,988 @@
+#ifndef ENTT_CONTAINER_DENSE_MAP_HPP
+#define ENTT_CONTAINER_DENSE_MAP_HPP
+
+#include <algorithm>
+#include <cmath>
+#include <cstddef>
+#include <functional>
+#include <iterator>
+#include <limits>
+#include <memory>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include <vector>
+#include "../config/config.h"
+#include "../core/compressed_pair.hpp"
+#include "../core/iterator.hpp"
+#include "../core/memory.hpp"
+#include "../core/type_traits.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename Key, typename Type>
+struct dense_map_node final {
+    using value_type = std::pair<Key, Type>;
+
+    template<typename... Args>
+    dense_map_node(const std::size_t pos, Args &&...args)
+        : 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)
+        : 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)
+        : 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)
+        : next{other.next},
+          element{entt::make_obj_using_allocator<value_type>(allocator, std::move(other.element))} {}
+
+    std::size_t next;
+    value_type element;
+};
+
+template<typename It>
+class dense_map_iterator final {
+    template<typename>
+    friend class dense_map_iterator;
+
+    using first_type = decltype(std::as_const(std::declval<It>()->element.first));
+    using second_type = decltype((std::declval<It>()->element.second));
+
+public:
+    using value_type = std::pair<first_type, second_type>;
+    using pointer = input_iterator_pointer<value_type>;
+    using reference = value_type;
+    using difference_type = std::ptrdiff_t;
+    using iterator_category = std::input_iterator_tag;
+
+    dense_map_iterator() ENTT_NOEXCEPT
+        : it{} {}
+
+    dense_map_iterator(const It iter) ENTT_NOEXCEPT
+        : it{iter} {}
+
+    template<typename Other, typename = std::enable_if_t<!std::is_same_v<It, Other> && std::is_constructible_v<It, Other>>>
+    dense_map_iterator(const dense_map_iterator<Other> &other) ENTT_NOEXCEPT
+        : it{other.it} {}
+
+    dense_map_iterator &operator++() ENTT_NOEXCEPT {
+        return ++it, *this;
+    }
+
+    dense_map_iterator operator++(int) ENTT_NOEXCEPT {
+        dense_map_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    dense_map_iterator &operator--() ENTT_NOEXCEPT {
+        return --it, *this;
+    }
+
+    dense_map_iterator operator--(int) ENTT_NOEXCEPT {
+        dense_map_iterator orig = *this;
+        return operator--(), orig;
+    }
+
+    dense_map_iterator &operator+=(const difference_type value) ENTT_NOEXCEPT {
+        it += value;
+        return *this;
+    }
+
+    dense_map_iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
+        dense_map_iterator copy = *this;
+        return (copy += value);
+    }
+
+    dense_map_iterator &operator-=(const difference_type value) ENTT_NOEXCEPT {
+        return (*this += -value);
+    }
+
+    dense_map_iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
+        return (*this + -value);
+    }
+
+    [[nodiscard]] reference operator[](const difference_type value) const ENTT_NOEXCEPT {
+        return {it[value].element.first, it[value].element.second};
+    }
+
+    [[nodiscard]] pointer operator->() const ENTT_NOEXCEPT {
+        return operator*();
+    }
+
+    [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
+        return {it->element.first, it->element.second};
+    }
+
+    template<typename ILhs, typename IRhs>
+    friend std::ptrdiff_t operator-(const dense_map_iterator<ILhs> &, const dense_map_iterator<IRhs> &) ENTT_NOEXCEPT;
+
+    template<typename ILhs, typename IRhs>
+    friend bool operator==(const dense_map_iterator<ILhs> &, const dense_map_iterator<IRhs> &) ENTT_NOEXCEPT;
+
+    template<typename ILhs, typename IRhs>
+    friend bool operator<(const dense_map_iterator<ILhs> &, const dense_map_iterator<IRhs> &) ENTT_NOEXCEPT;
+
+private:
+    It it;
+};
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] std::ptrdiff_t operator-(const dense_map_iterator<ILhs> &lhs, const dense_map_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return lhs.it - rhs.it;
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator==(const dense_map_iterator<ILhs> &lhs, const dense_map_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return lhs.it == rhs.it;
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator!=(const dense_map_iterator<ILhs> &lhs, const dense_map_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator<(const dense_map_iterator<ILhs> &lhs, const dense_map_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return lhs.it < rhs.it;
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator>(const dense_map_iterator<ILhs> &lhs, const dense_map_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return rhs < lhs;
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator<=(const dense_map_iterator<ILhs> &lhs, const dense_map_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return !(lhs > rhs);
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator>=(const dense_map_iterator<ILhs> &lhs, const dense_map_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return !(lhs < rhs);
+}
+
+template<typename It>
+class dense_map_local_iterator final {
+    template<typename>
+    friend class dense_map_local_iterator;
+
+    using first_type = decltype(std::as_const(std::declval<It>()->element.first));
+    using second_type = decltype((std::declval<It>()->element.second));
+
+public:
+    using value_type = std::pair<first_type, second_type>;
+    using pointer = input_iterator_pointer<value_type>;
+    using reference = value_type;
+    using difference_type = std::ptrdiff_t;
+    using iterator_category = std::input_iterator_tag;
+
+    dense_map_local_iterator() ENTT_NOEXCEPT
+        : it{},
+          offset{} {}
+
+    dense_map_local_iterator(It iter, const std::size_t pos) ENTT_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>>>
+    dense_map_local_iterator(const dense_map_local_iterator<Other> &other) ENTT_NOEXCEPT
+        : it{other.it},
+          offset{other.offset} {}
+
+    dense_map_local_iterator &operator++() ENTT_NOEXCEPT {
+        return offset = it[offset].next, *this;
+    }
+
+    dense_map_local_iterator operator++(int) ENTT_NOEXCEPT {
+        dense_map_local_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    [[nodiscard]] pointer operator->() const ENTT_NOEXCEPT {
+        return operator*();
+    }
+
+    [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
+        return {it[offset].element.first, it[offset].element.second};
+    }
+
+    [[nodiscard]] std::size_t index() const ENTT_NOEXCEPT {
+        return offset;
+    }
+
+private:
+    It it;
+    std::size_t offset;
+};
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator==(const dense_map_local_iterator<ILhs> &lhs, const dense_map_local_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return lhs.index() == rhs.index();
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator!=(const dense_map_local_iterator<ILhs> &lhs, const dense_map_local_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief Associative container for key-value pairs with unique keys.
+ *
+ * Internally, elements are organized into buckets. Which bucket an element is
+ * placed into depends entirely on the hash of its key. Keys with the same hash
+ * code appear in the same bucket.
+ *
+ * @tparam Key Key type of the associative container.
+ * @tparam Type Mapped type of the associative container.
+ * @tparam Hash Type of function to use to hash the keys.
+ * @tparam KeyEqual Type of function to use to compare the keys for equality.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Key, typename Type, typename Hash, typename KeyEqual, typename Allocator>
+class dense_map {
+    static constexpr float default_threshold = 0.875f;
+    static constexpr std::size_t minimum_capacity = 8u;
+
+    using node_type = internal::dense_map_node<Key, Type>;
+    using alloc_traits = typename std::allocator_traits<Allocator>;
+    static_assert(std::is_same_v<typename alloc_traits::value_type, std::pair<const Key, Type>>, "Invalid value type");
+    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 ENTT_NOEXCEPT {
+        return fast_mod(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());
+            }
+        }
+
+        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());
+            }
+        }
+
+        return cend();
+    }
+
+    template<typename Other, typename... Args>
+    [[nodiscard]] auto insert_or_do_nothing(Other &&key, Args &&...args) {
+        const auto index = key_to_bucket(key);
+
+        if(auto it = constrained_find(key, index); it != end()) {
+            return std::make_pair(it, false);
+        }
+
+        packed.first().emplace_back(sparse.first()[index], std::piecewise_construct, std::forward_as_tuple(std::forward<Other>(key)), std::forward_as_tuple(std::forward<Args>(args)...));
+        sparse.first()[index] = packed.first().size() - 1u;
+        rehash_if_required();
+
+        return std::make_pair(--end(), true);
+    }
+
+    template<typename Other, typename Arg>
+    [[nodiscard]] auto insert_or_overwrite(Other &&key, Arg &&value) {
+        const auto index = key_to_bucket(key);
+
+        if(auto it = constrained_find(key, index); it != end()) {
+            it->second = std::forward<Arg>(value);
+            return std::make_pair(it, false);
+        }
+
+        packed.first().emplace_back(sparse.first()[index], std::forward<Other>(key), std::forward<Arg>(value));
+        sparse.first()[index] = packed.first().size() - 1u;
+        rehash_if_required();
+
+        return std::make_pair(--end(), true);
+    }
+
+    void move_and_pop(const std::size_t pos) {
+        if(const auto last = size() - 1u; pos != last) {
+            packed.first()[pos] = std::move(packed.first().back());
+            size_type *curr = sparse.first().data() + key_to_bucket(packed.first().back().element.first);
+            for(; *curr != last; curr = &packed.first()[*curr].next) {}
+            *curr = pos;
+        }
+
+        packed.first().pop_back();
+    }
+
+    void rehash_if_required() {
+        if(size() > (bucket_count() * max_load_factor())) {
+            rehash(bucket_count() * 2u);
+        }
+    }
+
+public:
+    /*! @brief Key type of the container. */
+    using key_type = Key;
+    /*! @brief Mapped type of the container. */
+    using mapped_type = Type;
+    /*! @brief Key-value type of the container. */
+    using value_type = std::pair<const Key, Type>;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_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 Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Input iterator type. */
+    using iterator = internal::dense_map_iterator<typename packed_container_type::iterator>;
+    /*! @brief Constant input iterator type. */
+    using const_iterator = internal::dense_map_iterator<typename packed_container_type::const_iterator>;
+    /*! @brief Input iterator type. */
+    using local_iterator = internal::dense_map_local_iterator<typename packed_container_type::iterator>;
+    /*! @brief Constant input iterator type. */
+    using const_local_iterator = internal::dense_map_local_iterator<typename packed_container_type::const_iterator>;
+
+    /*! @brief Default constructor. */
+    dense_map()
+        : dense_map(minimum_capacity) {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit dense_map(const allocator_type &allocator)
+        : dense_map{minimum_capacity, hasher{}, key_equal{}, allocator} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator and user
+     * supplied minimal number of buckets.
+     * @param bucket_count Minimal number of buckets.
+     * @param allocator The allocator to use.
+     */
+    dense_map(const size_type bucket_count, const allocator_type &allocator)
+        : dense_map{bucket_count, hasher{}, key_equal{}, allocator} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator, hash
+     * function and user supplied minimal number of buckets.
+     * @param bucket_count Minimal number of buckets.
+     * @param hash Hash function to use.
+     * @param allocator The allocator to use.
+     */
+    dense_map(const size_type bucket_count, const hasher &hash, const allocator_type &allocator)
+        : dense_map{bucket_count, hash, key_equal{}, allocator} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator, hash
+     * function, compare function and user supplied minimal number of buckets.
+     * @param bucket_count Minimal number of buckets.
+     * @param hash Hash function to use.
+     * @param equal Compare function to use.
+     * @param allocator The allocator to use.
+     */
+    explicit dense_map(const size_type bucket_count, const hasher &hash = hasher{}, const key_equal &equal = key_equal{}, const allocator_type &allocator = allocator_type{})
+        : sparse{allocator, hash},
+          packed{allocator, equal},
+          threshold{default_threshold} {
+        rehash(bucket_count);
+    }
+
+    /*! @brief Default copy constructor. */
+    dense_map(const dense_map &) = default;
+
+    /**
+     * @brief Allocator-extended copy constructor.
+     * @param other The instance to copy from.
+     * @param allocator The allocator to use.
+     */
+    dense_map(const dense_map &other, const allocator_type &allocator)
+        : sparse{std::piecewise_construct, std::forward_as_tuple(other.sparse.first(), allocator), std::forward_as_tuple(other.sparse.second())},
+          packed{std::piecewise_construct, std::forward_as_tuple(other.packed.first(), allocator), std::forward_as_tuple(other.packed.second())},
+          threshold{other.threshold} {}
+
+    /*! @brief Default move constructor. */
+    dense_map(dense_map &&) = default;
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    dense_map(dense_map &&other, const allocator_type &allocator)
+        : sparse{std::piecewise_construct, std::forward_as_tuple(std::move(other.sparse.first()), allocator), std::forward_as_tuple(std::move(other.sparse.second()))},
+          packed{std::piecewise_construct, std::forward_as_tuple(std::move(other.packed.first()), allocator), std::forward_as_tuple(std::move(other.packed.second()))},
+          threshold{other.threshold} {}
+
+    /**
+     * @brief Default copy assignment operator.
+     * @return This container.
+     */
+    dense_map &operator=(const dense_map &) = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This container.
+     */
+    dense_map &operator=(dense_map &&) = default;
+
+    /**
+     * @brief Returns the associated allocator.
+     * @return The associated allocator.
+     */
+    [[nodiscard]] constexpr allocator_type get_allocator() const ENTT_NOEXCEPT {
+        return sparse.first().get_allocator();
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * The returned iterator points to the first instance of the internal array.
+     * If the array is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first instance of the internal array.
+     */
+    [[nodiscard]] const_iterator cbegin() const ENTT_NOEXCEPT {
+        return packed.first().begin();
+    }
+
+    /*! @copydoc cbegin */
+    [[nodiscard]] const_iterator begin() const ENTT_NOEXCEPT {
+        return cbegin();
+    }
+
+    /*! @copydoc begin */
+    [[nodiscard]] iterator begin() ENTT_NOEXCEPT {
+        return packed.first().begin();
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     *
+     * The returned iterator points to the element following the last instance
+     * of the internal array. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @return An iterator to the element following the last instance of the
+     * internal array.
+     */
+    [[nodiscard]] const_iterator cend() const ENTT_NOEXCEPT {
+        return packed.first().end();
+    }
+
+    /*! @copydoc cend */
+    [[nodiscard]] const_iterator end() const ENTT_NOEXCEPT {
+        return cend();
+    }
+
+    /*! @copydoc end */
+    [[nodiscard]] iterator end() ENTT_NOEXCEPT {
+        return packed.first().end();
+    }
+
+    /**
+     * @brief Checks whether a container is empty.
+     * @return True if the container is empty, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return packed.first().empty();
+    }
+
+    /**
+     * @brief Returns the number of elements in a container.
+     * @return Number of elements in a container.
+     */
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return packed.first().size();
+    }
+
+    /*! @brief Clears the container. */
+    void clear() ENTT_NOEXCEPT {
+        sparse.first().clear();
+        packed.first().clear();
+        rehash(0u);
+    }
+
+    /**
+     * @brief Inserts an element into the container, if the key does not exist.
+     * @param value A key-value pair eventually convertible to the value type.
+     * @return A pair consisting of an iterator to the inserted element (or to
+     * the element that prevented the insertion) and a bool denoting whether the
+     * insertion took place.
+     */
+    std::pair<iterator, bool> insert(const value_type &value) {
+        return insert_or_do_nothing(value.first, value.second);
+    }
+
+    /*! @copydoc insert */
+    std::pair<iterator, bool> insert(value_type &&value) {
+        return insert_or_do_nothing(std::move(value.first), std::move(value.second));
+    }
+
+    /**
+     * @copydoc insert
+     * @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) {
+        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) {
+        for(; first != last; ++first) {
+            insert(*first);
+        }
+    }
+
+    /**
+     * @brief Inserts an element into the container or assigns to the current
+     * element if the key already exists.
+     * @tparam Arg Type of the value to insert or assign.
+     * @param key A key used both to look up and to insert if not found.
+     * @param value A value to insert or assign.
+     * @return A pair consisting of an iterator to the element and a bool
+     * denoting whether the insertion took place.
+     */
+    template<typename Arg>
+    std::pair<iterator, bool> insert_or_assign(const key_type &key, Arg &&value) {
+        return insert_or_overwrite(key, std::forward<Arg>(value));
+    }
+
+    /*! @copydoc insert_or_assign */
+    template<typename Arg>
+    std::pair<iterator, bool> insert_or_assign(key_type &&key, Arg &&value) {
+        return insert_or_overwrite(std::move(key), std::forward<Arg>(value));
+    }
+
+    /**
+     * @brief Constructs an element in-place, if the key does not exist.
+     *
+     * The element is also constructed when the container already has the key,
+     * in which case the newly constructed object is destroyed immediately.
+     *
+     * @tparam Args Types of arguments to forward to the constructor of the
+     * element.
+     * @param args Arguments to forward to the constructor of the element.
+     * @return A pair consisting of an iterator to the inserted element (or to
+     * the element that prevented the insertion) and a bool denoting whether the
+     * insertion took place.
+     */
+    template<typename... Args>
+    std::pair<iterator, bool> emplace([[maybe_unused]] Args &&...args) {
+        if constexpr(sizeof...(Args) == 0u) {
+            return insert_or_do_nothing(key_type{});
+        } else if constexpr(sizeof...(Args) == 1u) {
+            return insert_or_do_nothing(std::forward<Args>(args).first..., std::forward<Args>(args).second...);
+        } else if constexpr(sizeof...(Args) == 2u) {
+            return insert_or_do_nothing(std::forward<Args>(args)...);
+        } else {
+            auto &node = packed.first().emplace_back(packed.first().size(), std::forward<Args>(args)...);
+            const auto index = key_to_bucket(node.element.first);
+
+            if(auto it = constrained_find(node.element.first, index); it != end()) {
+                packed.first().pop_back();
+                return std::make_pair(it, false);
+            }
+
+            std::swap(node.next, sparse.first()[index]);
+            rehash_if_required();
+
+            return std::make_pair(--end(), true);
+        }
+    }
+
+    /**
+     * @brief Inserts in-place if the key does not exist, does nothing if the
+     * key exists.
+     * @tparam Args Types of arguments to forward to the constructor of the
+     * element.
+     * @param key A key used both to look up and to insert if not found.
+     * @param args Arguments to forward to the constructor of the element.
+     * @return A pair consisting of an iterator to the inserted element (or to
+     * the element that prevented the insertion) and a bool denoting whether the
+     * insertion took place.
+     */
+    template<typename... Args>
+    std::pair<iterator, bool> try_emplace(const key_type &key, Args &&...args) {
+        return insert_or_do_nothing(key, std::forward<Args>(args)...);
+    }
+
+    /*! @copydoc try_emplace */
+    template<typename... Args>
+    std::pair<iterator, bool> try_emplace(key_type &&key, Args &&...args) {
+        return insert_or_do_nothing(std::move(key), std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Removes an element from a given position.
+     * @param pos An iterator to the element to remove.
+     * @return An iterator following the removed element.
+     */
+    iterator erase(const_iterator pos) {
+        const auto diff = pos - cbegin();
+        erase(pos->first);
+        return begin() + diff;
+    }
+
+    /**
+     * @brief Removes the given elements from a container.
+     * @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 An iterator following the last removed element.
+     */
+    iterator erase(const_iterator first, const_iterator last) {
+        const auto dist = first - cbegin();
+
+        for(auto from = last - cbegin(); from != dist; --from) {
+            erase(packed.first()[from - 1u].element.first);
+        }
+
+        return (begin() + dist);
+    }
+
+    /**
+     * @brief Removes the element associated with a given key.
+     * @param key A key value of an element to remove.
+     * @return Number of elements removed (either 0 or 1).
+     */
+    size_type erase(const key_type &key) {
+        for(size_type *curr = sparse.first().data() + key_to_bucket(key); *curr != (std::numeric_limits<size_type>::max)(); curr = &packed.first()[*curr].next) {
+            if(packed.second()(packed.first()[*curr].element.first, key)) {
+                const auto index = *curr;
+                *curr = packed.first()[*curr].next;
+                move_and_pop(index);
+                return 1u;
+            }
+        }
+
+        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) {
+        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.
+     * @return A reference to the mapped value of the requested element.
+     */
+    [[nodiscard]] mapped_type &at(const key_type &key) {
+        auto it = find(key);
+        ENTT_ASSERT(it != end(), "Invalid key");
+        return it->second;
+    }
+
+    /*! @copydoc at */
+    [[nodiscard]] const mapped_type &at(const key_type &key) const {
+        auto it = find(key);
+        ENTT_ASSERT(it != cend(), "Invalid key");
+        return it->second;
+    }
+
+    /**
+     * @brief Accesses or inserts a given element.
+     * @param key A key of an element to find or insert.
+     * @return A reference to the mapped value of the requested element.
+     */
+    [[nodiscard]] mapped_type &operator[](const key_type &key) {
+        return insert_or_do_nothing(key).first->second;
+    }
+
+    /**
+     * @brief Accesses or inserts a given element.
+     * @param key A key of an element to find or insert.
+     * @return A reference to the mapped value of the requested element.
+     */
+    [[nodiscard]] mapped_type &operator[](key_type &&key) {
+        return insert_or_do_nothing(std::move(key)).first->second;
+    }
+
+    /**
+     * @brief Finds an element with a given key.
+     * @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.
+     */
+    [[nodiscard]] iterator find(const key_type &key) {
+        return constrained_find(key, key_to_bucket(key));
+    }
+
+    /*! @copydoc find */
+    [[nodiscard]] const_iterator find(const key_type &key) const {
+        return constrained_find(key, key_to_bucket(key));
+    }
+
+    /**
+     * @brief Finds 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 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) {
+        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 {
+        return constrained_find(key, key_to_bucket(key));
+    }
+
+    /**
+     * @brief Checks if the container contains an element with a given key.
+     * @param key Key value of an element to search for.
+     * @return True if there is such an element, false otherwise.
+     */
+    [[nodiscard]] bool contains(const key_type &key) const {
+        return (find(key) != cend());
+    }
+
+    /**
+     * @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 {
+        return (find(key) != cend());
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning of a given bucket.
+     * @param index An index of a bucket to access.
+     * @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]};
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the beginning of the given bucket.
+     */
+    [[nodiscard]] const_local_iterator begin(const size_type index) const {
+        return cbegin(index);
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning of a given bucket.
+     * @param index An index of a bucket to access.
+     * @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]};
+    }
+
+    /**
+     * @brief Returns an iterator to the end of a given bucket.
+     * @param index An index of a bucket to access.
+     * @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)()};
+    }
+
+    /**
+     * @brief Returns an iterator to the end of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the end of the given bucket.
+     */
+    [[nodiscard]] const_local_iterator end([[maybe_unused]] const size_type index) const {
+        return cend(index);
+    }
+
+    /**
+     * @brief Returns an iterator to the end of a given bucket.
+     * @param index An index of a bucket to access.
+     * @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)()};
+    }
+
+    /**
+     * @brief Returns the number of buckets.
+     * @return The number of buckets.
+     */
+    [[nodiscard]] size_type bucket_count() const {
+        return sparse.first().size();
+    }
+
+    /**
+     * @brief Returns the maximum number of buckets.
+     * @return The maximum number of buckets.
+     */
+    [[nodiscard]] size_type max_bucket_count() const {
+        return sparse.first().max_size();
+    }
+
+    /**
+     * @brief Returns the number of elements in a given bucket.
+     * @param index The index of the bucket to examine.
+     * @return The number of elements in the given bucket.
+     */
+    [[nodiscard]] size_type bucket_size(const size_type index) const {
+        return static_cast<size_type>(std::distance(begin(index), end(index)));
+    }
+
+    /**
+     * @brief Returns the bucket for a given key.
+     * @param key The value of the key to examine.
+     * @return The bucket for the given key.
+     */
+    [[nodiscard]] size_type bucket(const key_type &key) const {
+        return key_to_bucket(key);
+    }
+
+    /**
+     * @brief Returns the average number of elements per bucket.
+     * @return The average number of elements per bucket.
+     */
+    [[nodiscard]] float load_factor() const {
+        return size() / static_cast<float>(bucket_count());
+    }
+
+    /**
+     * @brief Returns the maximum average number of elements per bucket.
+     * @return The maximum average number of elements per bucket.
+     */
+    [[nodiscard]] float max_load_factor() const {
+        return threshold;
+    }
+
+    /**
+     * @brief Sets the desired maximum average number of elements per bucket.
+     * @param value A desired maximum average number of elements per bucket.
+     */
+    void max_load_factor(const float value) {
+        ENTT_ASSERT(value > 0.f, "Invalid load factor");
+        threshold = value;
+        rehash(0u);
+    }
+
+    /**
+     * @brief Reserves at least the specified number of buckets and regenerates
+     * the hash table.
+     * @param count New number of buckets.
+     */
+    void rehash(const size_type count) {
+        auto value = (std::max)(count, minimum_capacity);
+        value = (std::max)(value, static_cast<size_type>(size() / max_load_factor()));
+
+        if(const auto sz = next_power_of_two(value); sz != bucket_count()) {
+            sparse.first().resize(sz);
+            std::fill(sparse.first().begin(), sparse.first().end(), (std::numeric_limits<size_type>::max)());
+
+            for(size_type pos{}, last = size(); pos < last; ++pos) {
+                const auto index = key_to_bucket(packed.first()[pos].element.first);
+                packed.first()[pos].next = std::exchange(sparse.first()[index], pos);
+            }
+        }
+    }
+
+    /**
+     * @brief Reserves space for at least the specified number of elements and
+     * regenerates the hash table.
+     * @param count New number of elements.
+     */
+    void reserve(const size_type count) {
+        packed.first().reserve(count);
+        rehash(static_cast<size_type>(std::ceil(count / max_load_factor())));
+    }
+
+    /**
+     * @brief Returns the function used to hash the keys.
+     * @return The function used to hash the keys.
+     */
+    [[nodiscard]] hasher hash_function() const {
+        return sparse.second();
+    }
+
+    /**
+     * @brief Returns the function used to compare keys for equality.
+     * @return The function used to compare keys for equality.
+     */
+    [[nodiscard]] key_equal key_eq() const {
+        return packed.second();
+    }
+
+private:
+    compressed_pair<sparse_container_type, hasher> sparse;
+    compressed_pair<packed_container_type, key_equal> packed;
+    float threshold;
+};
+
+} // namespace entt
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace std {
+
+template<typename Key, typename Value, typename Allocator>
+struct uses_allocator<entt::internal::dense_map_node<Key, Value>, Allocator>
+    : std::true_type {};
+
+} // namespace std
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+#endif

src/entt/container/dense_set.hpp

@@ -0,0 +1,823 @@
+#ifndef ENTT_CONTAINER_DENSE_SET_HPP
+#define ENTT_CONTAINER_DENSE_SET_HPP
+
+#include <algorithm>
+#include <cmath>
+#include <cstddef>
+#include <functional>
+#include <iterator>
+#include <limits>
+#include <memory>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include <vector>
+#include "../config/config.h"
+#include "../core/compressed_pair.hpp"
+#include "../core/memory.hpp"
+#include "../core/type_traits.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename It>
+class dense_set_iterator final {
+    template<typename>
+    friend class dense_set_iterator;
+
+public:
+    using value_type = typename It::value_type::second_type;
+    using pointer = const value_type *;
+    using reference = const value_type &;
+    using difference_type = std::ptrdiff_t;
+    using iterator_category = std::random_access_iterator_tag;
+
+    dense_set_iterator() ENTT_NOEXCEPT
+        : it{} {}
+
+    dense_set_iterator(const It iter) ENTT_NOEXCEPT
+        : it{iter} {}
+
+    template<typename Other, typename = std::enable_if_t<!std::is_same_v<It, Other> && std::is_constructible_v<It, Other>>>
+    dense_set_iterator(const dense_set_iterator<Other> &other) ENTT_NOEXCEPT
+        : it{other.it} {}
+
+    dense_set_iterator &operator++() ENTT_NOEXCEPT {
+        return ++it, *this;
+    }
+
+    dense_set_iterator operator++(int) ENTT_NOEXCEPT {
+        dense_set_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    dense_set_iterator &operator--() ENTT_NOEXCEPT {
+        return --it, *this;
+    }
+
+    dense_set_iterator operator--(int) ENTT_NOEXCEPT {
+        dense_set_iterator orig = *this;
+        return operator--(), orig;
+    }
+
+    dense_set_iterator &operator+=(const difference_type value) ENTT_NOEXCEPT {
+        it += value;
+        return *this;
+    }
+
+    dense_set_iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
+        dense_set_iterator copy = *this;
+        return (copy += value);
+    }
+
+    dense_set_iterator &operator-=(const difference_type value) ENTT_NOEXCEPT {
+        return (*this += -value);
+    }
+
+    dense_set_iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
+        return (*this + -value);
+    }
+
+    [[nodiscard]] reference operator[](const difference_type value) const ENTT_NOEXCEPT {
+        return it[value].second;
+    }
+
+    [[nodiscard]] pointer operator->() const ENTT_NOEXCEPT {
+        return std::addressof(it->second);
+    }
+
+    [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
+        return *operator->();
+    }
+
+    template<typename ILhs, typename IRhs>
+    friend std::ptrdiff_t operator-(const dense_set_iterator<ILhs> &, const dense_set_iterator<IRhs> &) ENTT_NOEXCEPT;
+
+    template<typename ILhs, typename IRhs>
+    friend bool operator==(const dense_set_iterator<ILhs> &, const dense_set_iterator<IRhs> &) ENTT_NOEXCEPT;
+
+    template<typename ILhs, typename IRhs>
+    friend bool operator<(const dense_set_iterator<ILhs> &, const dense_set_iterator<IRhs> &) ENTT_NOEXCEPT;
+
+private:
+    It it;
+};
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] std::ptrdiff_t operator-(const dense_set_iterator<ILhs> &lhs, const dense_set_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return lhs.it - rhs.it;
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator==(const dense_set_iterator<ILhs> &lhs, const dense_set_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return lhs.it == rhs.it;
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator!=(const dense_set_iterator<ILhs> &lhs, const dense_set_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator<(const dense_set_iterator<ILhs> &lhs, const dense_set_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return lhs.it < rhs.it;
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator>(const dense_set_iterator<ILhs> &lhs, const dense_set_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return rhs < lhs;
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator<=(const dense_set_iterator<ILhs> &lhs, const dense_set_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return !(lhs > rhs);
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator>=(const dense_set_iterator<ILhs> &lhs, const dense_set_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return !(lhs < rhs);
+}
+
+template<typename It>
+class dense_set_local_iterator final {
+    template<typename>
+    friend class dense_set_local_iterator;
+
+public:
+    using value_type = typename It::value_type::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;
+
+    dense_set_local_iterator() ENTT_NOEXCEPT
+        : it{},
+          offset{} {}
+
+    dense_set_local_iterator(It iter, const std::size_t pos) ENTT_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>>>
+    dense_set_local_iterator(const dense_set_local_iterator<Other> &other) ENTT_NOEXCEPT
+        : it{other.it},
+          offset{other.offset} {}
+
+    dense_set_local_iterator &operator++() ENTT_NOEXCEPT {
+        return offset = it[offset].first, *this;
+    }
+
+    dense_set_local_iterator operator++(int) ENTT_NOEXCEPT {
+        dense_set_local_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    [[nodiscard]] pointer operator->() const ENTT_NOEXCEPT {
+        return std::addressof(it[offset].second);
+    }
+
+    [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
+        return *operator->();
+    }
+
+    [[nodiscard]] std::size_t index() const ENTT_NOEXCEPT {
+        return offset;
+    }
+
+private:
+    It it;
+    std::size_t offset;
+};
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator==(const dense_set_local_iterator<ILhs> &lhs, const dense_set_local_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return lhs.index() == rhs.index();
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator!=(const dense_set_local_iterator<ILhs> &lhs, const dense_set_local_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief Associative container for unique objects of a given type.
+ *
+ * Internally, elements are organized into buckets. Which bucket an element is
+ * placed into depends entirely on its hash. Elements with the same hash code
+ * appear in the same bucket.
+ *
+ * @tparam Type Value type of the associative container.
+ * @tparam Hash Type of function to use to hash the values.
+ * @tparam KeyEqual Type of function to use to compare the values for equality.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+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;
+
+    using node_type = std::pair<std::size_t, Type>;
+    using alloc_traits = std::allocator_traits<Allocator>;
+    static_assert(std::is_same_v<typename alloc_traits::value_type, Type>, "Invalid value type");
+    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 ENTT_NOEXCEPT {
+        return fast_mod(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());
+            }
+        }
+
+        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());
+            }
+        }
+
+        return cend();
+    }
+
+    template<typename Other>
+    [[nodiscard]] auto insert_or_do_nothing(Other &&value) {
+        const auto index = value_to_bucket(value);
+
+        if(auto it = constrained_find(value, index); it != end()) {
+            return std::make_pair(it, false);
+        }
+
+        packed.first().emplace_back(sparse.first()[index], std::forward<Other>(value));
+        sparse.first()[index] = packed.first().size() - 1u;
+        rehash_if_required();
+
+        return std::make_pair(--end(), true);
+    }
+
+    void move_and_pop(const std::size_t pos) {
+        if(const auto last = size() - 1u; pos != last) {
+            packed.first()[pos] = std::move(packed.first().back());
+            size_type *curr = sparse.first().data() + value_to_bucket(packed.first().back().second);
+            for(; *curr != last; curr = &packed.first()[*curr].first) {}
+            *curr = pos;
+        }
+
+        packed.first().pop_back();
+    }
+
+    void rehash_if_required() {
+        if(size() > (bucket_count() * max_load_factor())) {
+            rehash(bucket_count() * 2u);
+        }
+    }
+
+public:
+    /*! @brief Key type of the container. */
+    using key_type = Type;
+    /*! @brief Value type of the container. */
+    using value_type = Type;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_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 Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Random access iterator type. */
+    using iterator = internal::dense_set_iterator<typename packed_container_type::iterator>;
+    /*! @brief Constant random access iterator type. */
+    using const_iterator = internal::dense_set_iterator<typename packed_container_type::const_iterator>;
+    /*! @brief Forward iterator type. */
+    using local_iterator = internal::dense_set_local_iterator<typename packed_container_type::iterator>;
+    /*! @brief Constant forward iterator type. */
+    using const_local_iterator = internal::dense_set_local_iterator<typename packed_container_type::const_iterator>;
+
+    /*! @brief Default constructor. */
+    dense_set()
+        : dense_set(minimum_capacity) {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit dense_set(const allocator_type &allocator)
+        : dense_set{minimum_capacity, hasher{}, key_equal{}, allocator} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator and user
+     * supplied minimal number of buckets.
+     * @param bucket_count Minimal number of buckets.
+     * @param allocator The allocator to use.
+     */
+    dense_set(const size_type bucket_count, const allocator_type &allocator)
+        : dense_set{bucket_count, hasher{}, key_equal{}, allocator} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator, hash
+     * function and user supplied minimal number of buckets.
+     * @param bucket_count Minimal number of buckets.
+     * @param hash Hash function to use.
+     * @param allocator The allocator to use.
+     */
+    dense_set(const size_type bucket_count, const hasher &hash, const allocator_type &allocator)
+        : dense_set{bucket_count, hash, key_equal{}, allocator} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator, hash
+     * function, compare function and user supplied minimal number of buckets.
+     * @param bucket_count Minimal number of buckets.
+     * @param hash Hash function to use.
+     * @param equal Compare function to use.
+     * @param allocator The allocator to use.
+     */
+    explicit dense_set(const size_type bucket_count, const hasher &hash = hasher{}, const key_equal &equal = key_equal{}, const allocator_type &allocator = allocator_type{})
+        : sparse{allocator, hash},
+          packed{allocator, equal},
+          threshold{default_threshold} {
+        rehash(bucket_count);
+    }
+
+    /*! @brief Default copy constructor. */
+    dense_set(const dense_set &) = default;
+
+    /**
+     * @brief Allocator-extended copy constructor.
+     * @param other The instance to copy from.
+     * @param allocator The allocator to use.
+     */
+    dense_set(const dense_set &other, const allocator_type &allocator)
+        : sparse{std::piecewise_construct, std::forward_as_tuple(other.sparse.first(), allocator), std::forward_as_tuple(other.sparse.second())},
+          packed{std::piecewise_construct, std::forward_as_tuple(other.packed.first(), allocator), std::forward_as_tuple(other.packed.second())},
+          threshold{other.threshold} {}
+
+    /*! @brief Default move constructor. */
+    dense_set(dense_set &&) = default;
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    dense_set(dense_set &&other, const allocator_type &allocator)
+        : sparse{std::piecewise_construct, std::forward_as_tuple(std::move(other.sparse.first()), allocator), std::forward_as_tuple(std::move(other.sparse.second()))},
+          packed{std::piecewise_construct, std::forward_as_tuple(std::move(other.packed.first()), allocator), std::forward_as_tuple(std::move(other.packed.second()))},
+          threshold{other.threshold} {}
+
+    /**
+     * @brief Default copy assignment operator.
+     * @return This container.
+     */
+    dense_set &operator=(const dense_set &) = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This container.
+     */
+    dense_set &operator=(dense_set &&) = default;
+
+    /**
+     * @brief Returns the associated allocator.
+     * @return The associated allocator.
+     */
+    [[nodiscard]] constexpr allocator_type get_allocator() const ENTT_NOEXCEPT {
+        return sparse.first().get_allocator();
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * The returned iterator points to the first instance of the internal array.
+     * If the array is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first instance of the internal array.
+     */
+    [[nodiscard]] const_iterator cbegin() const ENTT_NOEXCEPT {
+        return packed.first().begin();
+    }
+
+    /*! @copydoc cbegin */
+    [[nodiscard]] const_iterator begin() const ENTT_NOEXCEPT {
+        return cbegin();
+    }
+
+    /*! @copydoc begin */
+    [[nodiscard]] iterator begin() ENTT_NOEXCEPT {
+        return packed.first().begin();
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     *
+     * The returned iterator points to the element following the last instance
+     * of the internal array. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @return An iterator to the element following the last instance of the
+     * internal array.
+     */
+    [[nodiscard]] const_iterator cend() const ENTT_NOEXCEPT {
+        return packed.first().end();
+    }
+
+    /*! @copydoc cend */
+    [[nodiscard]] const_iterator end() const ENTT_NOEXCEPT {
+        return cend();
+    }
+
+    /*! @copydoc end */
+    [[nodiscard]] iterator end() ENTT_NOEXCEPT {
+        return packed.first().end();
+    }
+
+    /**
+     * @brief Checks whether a container is empty.
+     * @return True if the container is empty, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return packed.first().empty();
+    }
+
+    /**
+     * @brief Returns the number of elements in a container.
+     * @return Number of elements in a container.
+     */
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return packed.first().size();
+    }
+
+    /*! @brief Clears the container. */
+    void clear() ENTT_NOEXCEPT {
+        sparse.first().clear();
+        packed.first().clear();
+        rehash(0u);
+    }
+
+    /**
+     * @brief Inserts an element into the container, if it does not exist.
+     * @param value An element to insert into the container.
+     * @return A pair consisting of an iterator to the inserted element (or to
+     * the element that prevented the insertion) and a bool denoting whether the
+     * insertion took place.
+     */
+    std::pair<iterator, bool> insert(const value_type &value) {
+        return insert_or_do_nothing(value);
+    }
+
+    /*! @copydoc insert */
+    std::pair<iterator, bool> insert(value_type &&value) {
+        return insert_or_do_nothing(std::move(value));
+    }
+
+    /**
+     * @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) {
+        for(; first != last; ++first) {
+            insert(*first);
+        }
+    }
+
+    /**
+     * @brief Constructs an element in-place, if it does not exist.
+     *
+     * The element is also constructed when the container already has the key,
+     * in which case the newly constructed object is destroyed immediately.
+     *
+     * @tparam Args Types of arguments to forward to the constructor of the
+     * element.
+     * @param args Arguments to forward to the constructor of the element.
+     * @return A pair consisting of an iterator to the inserted element (or to
+     * the element that prevented the insertion) and a bool denoting whether the
+     * insertion took place.
+     */
+    template<typename... Args>
+    std::pair<iterator, bool> emplace(Args &&...args) {
+        if constexpr(((sizeof...(Args) == 1u) && ... && std::is_same_v<std::remove_cv_t<std::remove_reference_t<Args>>, value_type>)) {
+            return insert_or_do_nothing(std::forward<Args>(args)...);
+        } else {
+            auto &node = packed.first().emplace_back(std::piecewise_construct, std::make_tuple(packed.first().size()), std::forward_as_tuple(std::forward<Args>(args)...));
+            const auto index = value_to_bucket(node.second);
+
+            if(auto it = constrained_find(node.second, index); it != end()) {
+                packed.first().pop_back();
+                return std::make_pair(it, false);
+            }
+
+            std::swap(node.first, sparse.first()[index]);
+            rehash_if_required();
+
+            return std::make_pair(--end(), true);
+        }
+    }
+
+    /**
+     * @brief Removes an element from a given position.
+     * @param pos An iterator to the element to remove.
+     * @return An iterator following the removed element.
+     */
+    iterator erase(const_iterator pos) {
+        const auto diff = pos - cbegin();
+        erase(*pos);
+        return begin() + diff;
+    }
+
+    /**
+     * @brief Removes the given elements from a container.
+     * @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 An iterator following the last removed element.
+     */
+    iterator erase(const_iterator first, const_iterator last) {
+        const auto dist = first - cbegin();
+
+        for(auto from = last - cbegin(); from != dist; --from) {
+            erase(packed.first()[from - 1u].second);
+        }
+
+        return (begin() + dist);
+    }
+
+    /**
+     * @brief Removes the element associated with a given value.
+     * @param value Value of an element to remove.
+     * @return Number of elements removed (either 0 or 1).
+     */
+    size_type erase(const value_type &value) {
+        for(size_type *curr = sparse.first().data() + value_to_bucket(value); *curr != (std::numeric_limits<size_type>::max)(); curr = &packed.first()[*curr].first) {
+            if(packed.second()(packed.first()[*curr].second, value)) {
+                const auto index = *curr;
+                *curr = packed.first()[*curr].first;
+                move_and_pop(index);
+                return 1u;
+            }
+        }
+
+        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) {
+        using std::swap;
+        swap(sparse, other.sparse);
+        swap(packed, other.packed);
+        swap(threshold, other.threshold);
+    }
+
+    /**
+     * @brief Finds an element with a given value.
+     * @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.
+     */
+    [[nodiscard]] iterator find(const value_type &value) {
+        return constrained_find(value, value_to_bucket(value));
+    }
+
+    /*! @copydoc find */
+    [[nodiscard]] const_iterator find(const value_type &value) const {
+        return constrained_find(value, value_to_bucket(value));
+    }
+
+    /**
+     * @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) {
+        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 {
+        return constrained_find(value, value_to_bucket(value));
+    }
+
+    /**
+     * @brief Checks if the container contains an element with a given value.
+     * @param value Value of an element to search for.
+     * @return True if there is such an element, false otherwise.
+     */
+    [[nodiscard]] bool contains(const value_type &value) const {
+        return (find(value) != cend());
+    }
+
+    /**
+     * @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 {
+        return (find(value) != cend());
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning of a given bucket.
+     * @param index An index of a bucket to access.
+     * @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]};
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the beginning of the given bucket.
+     */
+    [[nodiscard]] const_local_iterator begin(const size_type index) const {
+        return cbegin(index);
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning of a given bucket.
+     * @param index An index of a bucket to access.
+     * @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]};
+    }
+
+    /**
+     * @brief Returns an iterator to the end of a given bucket.
+     * @param index An index of a bucket to access.
+     * @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)()};
+    }
+
+    /**
+     * @brief Returns an iterator to the end of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the end of the given bucket.
+     */
+    [[nodiscard]] const_local_iterator end([[maybe_unused]] const size_type index) const {
+        return cend(index);
+    }
+
+    /**
+     * @brief Returns an iterator to the end of a given bucket.
+     * @param index An index of a bucket to access.
+     * @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)()};
+    }
+
+    /**
+     * @brief Returns the number of buckets.
+     * @return The number of buckets.
+     */
+    [[nodiscard]] size_type bucket_count() const {
+        return sparse.first().size();
+    }
+
+    /**
+     * @brief Returns the maximum number of buckets.
+     * @return The maximum number of buckets.
+     */
+    [[nodiscard]] size_type max_bucket_count() const {
+        return sparse.first().max_size();
+    }
+
+    /**
+     * @brief Returns the number of elements in a given bucket.
+     * @param index The index of the bucket to examine.
+     * @return The number of elements in the given bucket.
+     */
+    [[nodiscard]] size_type bucket_size(const size_type index) const {
+        return static_cast<size_type>(std::distance(begin(index), end(index)));
+    }
+
+    /**
+     * @brief Returns the bucket for a given element.
+     * @param value The value of the element to examine.
+     * @return The bucket for the given element.
+     */
+    [[nodiscard]] size_type bucket(const value_type &value) const {
+        return value_to_bucket(value);
+    }
+
+    /**
+     * @brief Returns the average number of elements per bucket.
+     * @return The average number of elements per bucket.
+     */
+    [[nodiscard]] float load_factor() const {
+        return size() / static_cast<float>(bucket_count());
+    }
+
+    /**
+     * @brief Returns the maximum average number of elements per bucket.
+     * @return The maximum average number of elements per bucket.
+     */
+    [[nodiscard]] float max_load_factor() const {
+        return threshold;
+    }
+
+    /**
+     * @brief Sets the desired maximum average number of elements per bucket.
+     * @param value A desired maximum average number of elements per bucket.
+     */
+    void max_load_factor(const float value) {
+        ENTT_ASSERT(value > 0.f, "Invalid load factor");
+        threshold = value;
+        rehash(0u);
+    }
+
+    /**
+     * @brief Reserves at least the specified number of buckets and regenerates
+     * the hash table.
+     * @param count New number of buckets.
+     */
+    void rehash(const size_type count) {
+        auto value = (std::max)(count, minimum_capacity);
+        value = (std::max)(value, static_cast<size_type>(size() / max_load_factor()));
+
+        if(const auto sz = next_power_of_two(value); sz != bucket_count()) {
+            sparse.first().resize(sz);
+            std::fill(sparse.first().begin(), sparse.first().end(), (std::numeric_limits<size_type>::max)());
+
+            for(size_type pos{}, last = size(); pos < last; ++pos) {
+                const auto index = value_to_bucket(packed.first()[pos].second);
+                packed.first()[pos].first = std::exchange(sparse.first()[index], pos);
+            }
+        }
+    }
+
+    /**
+     * @brief Reserves space for at least the specified number of elements and
+     * regenerates the hash table.
+     * @param count New number of elements.
+     */
+    void reserve(const size_type count) {
+        packed.first().reserve(count);
+        rehash(static_cast<size_type>(std::ceil(count / max_load_factor())));
+    }
+
+    /**
+     * @brief Returns the function used to hash the elements.
+     * @return The function used to hash the elements.
+     */
+    [[nodiscard]] hasher hash_function() const {
+        return sparse.second();
+    }
+
+    /**
+     * @brief Returns the function used to compare elements for equality.
+     * @return The function used to compare elements for equality.
+     */
+    [[nodiscard]] key_equal key_eq() const {
+        return packed.second();
+    }
+
+private:
+    compressed_pair<sparse_container_type, hasher> sparse;
+    compressed_pair<packed_container_type, key_equal> packed;
+    float threshold;
+};
+
+} // namespace entt
+
+#endif

src/entt/container/fwd.hpp

@@ -0,0 +1,26 @@
+#ifndef ENTT_CONTAINER_FWD_HPP
+#define ENTT_CONTAINER_FWD_HPP
+
+#include <functional>
+#include <memory>
+
+namespace entt {
+
+template<
+    typename Key,
+    typename Type,
+    typename = std::hash<Key>,
+    typename = std::equal_to<Key>,
+    typename = std::allocator<std::pair<const Key, Type>>>
+class dense_map;
+
+template<
+    typename Type,
+    typename = std::hash<Type>,
+    typename = std::equal_to<Type>,
+    typename = std::allocator<Type>>
+class dense_set;
+
+} // namespace entt
+
+#endif

src/entt/core/algorithm.hpp

@@ -1,18 +1,15 @@
 #ifndef ENTT_CORE_ALGORITHM_HPP
 #define ENTT_CORE_ALGORITHM_HPP
 
-
-#include <vector>
-#include <utility>
-#include <iterator>
 #include <algorithm>
 #include <functional>
+#include <iterator>
+#include <utility>
+#include <vector>
 #include "utility.hpp"
 
-
 namespace entt {
 
-
 /**
  * @brief Function object to wrap `std::sort` in a class type.
  *
@@ -36,12 +33,11 @@ struct std_sort {
      * @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 {
+    void operator()(It first, It last, Compare compare = Compare{}, Args &&...args) const {
         std::sort(std::forward<Args>(args)..., std::move(first), std::move(last), std::move(compare));
     }
 };
 
-
 /*! @brief Function object for performing insertion sort. */
 struct insertion_sort {
     /**
@@ -62,8 +58,8 @@ struct insertion_sort {
                 auto value = std::move(*it);
                 auto pre = it;
 
-                for(; pre > first && compare(value, *(pre-1)); --pre) {
-                    *pre = std::move(*(pre-1));
+                for(; pre > first && compare(value, *(pre - 1)); --pre) {
+                    *pre = std::move(*(pre - 1));
                 }
 
                 *pre = std::move(value);
@@ -72,7 +68,6 @@ struct insertion_sort {
     }
 };
 
-
 /**
  * @brief Function object for performing LSD radix sort.
  * @tparam Bit Number of bits processed per pass.
@@ -137,8 +132,6 @@ struct radix_sort {
     }
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/core/any.hpp

@@ -1,9 +1,7 @@
 #ifndef ENTT_CORE_ANY_HPP
 #define ENTT_CORE_ANY_HPP
 
-
 #include <cstddef>
-#include <functional>
 #include <memory>
 #include <type_traits>
 #include <utility>
@@ -13,10 +11,8 @@
 #include "type_info.hpp"
 #include "type_traits.hpp"
 
-
 namespace entt {
 
-
 /**
  * @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.
@@ -24,100 +20,102 @@ namespace entt {
  */
 template<std::size_t Len, std::size_t Align>
 class basic_any {
-    enum class operation: std::uint8_t { COPY, MOVE, DTOR, COMP, ADDR, CADDR, TYPE };
-    enum class policy: std::uint8_t { OWNER, REF, CREF };
+    enum class operation : std::uint8_t {
+        copy,
+        move,
+        transfer,
+        assign,
+        destroy,
+        compare,
+        get
+    };
+
+    enum class policy : std::uint8_t {
+        owner,
+        ref,
+        cref
+    };
 
     using storage_type = std::aligned_storage_t<Len + !Len, Align>;
-    using vtable_type = const void *(const operation, const basic_any &, void *);
+    using vtable_type = const void *(const operation, const basic_any &, const void *);
 
     template<typename Type>
     static constexpr bool in_situ = Len && alignof(Type) <= alignof(storage_type) && sizeof(Type) <= sizeof(storage_type) && std::is_nothrow_move_constructible_v<Type>;
 
     template<typename Type>
-    [[nodiscard]] static constexpr policy type_to_policy() {
-        if constexpr(std::is_lvalue_reference_v<Type>) {
-            if constexpr(std::is_const_v<std::remove_reference_t<Type>>) {
-                return policy::CREF;
+    static const void *basic_vtable([[maybe_unused]] const operation op, [[maybe_unused]] const basic_any &value, [[maybe_unused]] const void *other) {
+        static_assert(!std::is_same_v<Type, void> && std::is_same_v<std::remove_cv_t<std::remove_reference_t<Type>>, Type>, "Invalid type");
+        const Type *element = nullptr;
+
+        if constexpr(in_situ<Type>) {
+            element = value.owner() ? reinterpret_cast<const Type *>(&value.storage) : static_cast<const Type *>(value.instance);
+        } else {
+            element = 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>(*element);
+            }
+            break;
+        case operation::move:
+            if constexpr(in_situ<Type>) {
+                if(value.owner()) {
+                    return new(&static_cast<basic_any *>(const_cast<void *>(other))->storage) Type{std::move(*const_cast<Type *>(element))};
+                }
+            }
+
+            return (static_cast<basic_any *>(const_cast<void *>(other))->instance = std::exchange(const_cast<basic_any &>(value).instance, nullptr));
+        case operation::transfer:
+            if constexpr(std::is_move_assignable_v<Type>) {
+                *const_cast<Type *>(element) = std::move(*static_cast<Type *>(const_cast<void *>(other)));
+                return other;
+            }
+            [[fallthrough]];
+        case operation::assign:
+            if constexpr(std::is_copy_assignable_v<Type>) {
+                *const_cast<Type *>(element) = *static_cast<const Type *>(other);
+                return other;
+            }
+            break;
+        case operation::destroy:
+            if constexpr(in_situ<Type>) {
+                element->~Type();
+            } else if constexpr(std::is_array_v<Type>) {
+                delete[] element;
             } else {
-                return policy::REF;
+                delete element;
             }
-        } else {
-            return policy::OWNER;
-        }
-    }
-
-    template<typename Type>
-    [[nodiscard]] static bool compare(const void *lhs, const void *rhs) {
-        if constexpr(!std::is_function_v<Type> && is_equality_comparable_v<Type>) {
-            return *static_cast<const Type *>(lhs) == *static_cast<const Type *>(rhs);
-        } else {
-            return lhs == rhs;
-        }
-    }
-
-    template<typename Type>
-    static const void * basic_vtable([[maybe_unused]] const operation op, [[maybe_unused]] const basic_any &from, [[maybe_unused]] void *to) {
-        static_assert(std::is_same_v<std::remove_reference_t<std::remove_const_t<Type>>, Type>, "Invalid type");
-
-        if constexpr(!std::is_void_v<Type>) {
-            const Type *instance = (in_situ<Type> && from.mode == policy::OWNER)
-                ? ENTT_LAUNDER(reinterpret_cast<const Type *>(&from.storage))
-                : static_cast<const Type *>(from.instance);
-
-            switch(op) {
-            case operation::COPY:
-                if constexpr(std::is_copy_constructible_v<Type>) {
-                    static_cast<basic_any *>(to)->emplace<Type>(*instance);
-                }
-                break;
-            case operation::MOVE:
-                if constexpr(in_situ<Type>) {
-                    if(from.mode == policy::OWNER) {
-                        return new (&static_cast<basic_any *>(to)->storage) Type{std::move(*const_cast<Type *>(instance))};
-                    }
-                }
-
-                return (static_cast<basic_any *>(to)->instance = std::exchange(const_cast<basic_any &>(from).instance, nullptr));
-            case operation::DTOR:
-                if(from.mode == policy::OWNER) {
-                    if constexpr(in_situ<Type>) {
-                        instance->~Type();
-                    } else if constexpr(std::is_array_v<Type>) {
-                        delete[] instance;
-                    } else {
-                        delete instance;
-                    }
-                }
-                break;
-            case operation::COMP:
-                return compare<Type>(instance, (*static_cast<const basic_any **>(to))->data()) ? to : nullptr;
-            case operation::ADDR:
-                if(from.mode == policy::CREF) {
-                    return nullptr;
-                }
-                [[fallthrough]];
-            case operation::CADDR:
-                return instance;
-            case operation::TYPE:
-                *static_cast<type_info *>(to) = type_id<Type>();
-                break;
+            break;
+        case operation::compare:
+            if constexpr(!std::is_function_v<Type> && !std::is_array_v<Type> && is_equality_comparable_v<Type>) {
+                return *static_cast<const Type *>(element) == *static_cast<const Type *>(other) ? other : nullptr;
+            } else {
+                return (element == other) ? other : nullptr;
             }
+        case operation::get:
+            return element;
         }
 
         return nullptr;
     }
 
     template<typename Type, typename... Args>
-    void initialize([[maybe_unused]] Args &&... args) {
+    void initialize([[maybe_unused]] Args &&...args) {
         if constexpr(!std::is_void_v<Type>) {
+            info = &type_id<std::remove_cv_t<std::remove_reference_t<Type>>>();
+            vtable = basic_vtable<std::remove_cv_t<std::remove_reference_t<Type>>>;
+
             if constexpr(std::is_lvalue_reference_v<Type>) {
                 static_assert(sizeof...(Args) == 1u && (std::is_lvalue_reference_v<Args> && ...), "Invalid arguments");
+                mode = std::is_const_v<std::remove_reference_t<Type>> ? policy::cref : policy::ref;
                 instance = (std::addressof(args), ...);
             } else if constexpr(in_situ<Type>) {
                 if constexpr(sizeof...(Args) != 0u && std::is_aggregate_v<Type>) {
-                    new (&storage) Type{std::forward<Args>(args)...};
+                    new(&storage) Type{std::forward<Args>(args)...};
                 } else {
-                    new (&storage) Type(std::forward<Args>(args)...);
+                    new(&storage) Type(std::forward<Args>(args)...);
                 }
             } else {
                 if constexpr(sizeof...(Args) != 0u && std::is_aggregate_v<Type>) {
@@ -131,9 +129,9 @@ class basic_any {
 
     basic_any(const basic_any &other, const policy pol) ENTT_NOEXCEPT
         : instance{other.data()},
+          info{other.info},
           vtable{other.vtable},
-          mode{pol}
-    {}
+          mode{pol} {}
 
 public:
     /*! @brief Size of the internal storage. */
@@ -142,11 +140,11 @@ public:
     static constexpr auto alignment = Align;
 
     /*! @brief Default constructor. */
-    basic_any() ENTT_NOEXCEPT
+    constexpr basic_any() ENTT_NOEXCEPT
         : instance{},
-          vtable{&basic_vtable<void>},
-          mode{policy::OWNER}
-    {}
+          info{&type_id<void>()},
+          vtable{},
+          mode{policy::owner} {}
 
     /**
      * @brief Constructs a wrapper by directly initializing the new object.
@@ -155,27 +153,11 @@ public:
      * @param args Parameters to use to construct the instance.
      */
     template<typename Type, typename... Args>
-    explicit basic_any(std::in_place_type_t<Type>, Args &&... args)
-        : instance{},
-          vtable{&basic_vtable<std::remove_const_t<std::remove_reference_t<Type>>>},
-          mode{type_to_policy<Type>()}
-    {
+    explicit basic_any(std::in_place_type_t<Type>, Args &&...args)
+        : basic_any{} {
         initialize<Type>(std::forward<Args>(args)...);
     }
 
-    /**
-     * @brief Constructs a wrapper that holds an unmanaged object.
-     * @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>
-    basic_any(std::reference_wrapper<Type> value) ENTT_NOEXCEPT
-        : basic_any{}
-    {
-        // invokes deprecated assignment operator (and avoids issues with vs2017)
-        *this = value;
-    }
-
     /**
      * @brief Constructs a wrapper from a given value.
      * @tparam Type Type of object to use to initialize the wrapper.
@@ -183,10 +165,7 @@ public:
      */
     template<typename Type, typename = std::enable_if_t<!std::is_same_v<std::decay_t<Type>, basic_any>>>
     basic_any(Type &&value)
-        : instance{},
-          vtable{&basic_vtable<std::decay_t<Type>>},
-          mode{policy::OWNER}
-    {
+        : basic_any{} {
         initialize<std::decay_t<Type>>(std::forward<Type>(value));
     }
 
@@ -195,11 +174,10 @@ public:
      * @param other The instance to copy from.
      */
     basic_any(const basic_any &other)
-        : instance{},
-          vtable{&basic_vtable<void>},
-          mode{policy::OWNER}
-    {
-        other.vtable(operation::COPY, other, this);
+        : basic_any{} {
+        if(other.vtable) {
+            other.vtable(operation::copy, other, this);
+        }
     }
 
     /**
@@ -208,15 +186,19 @@ public:
      */
     basic_any(basic_any &&other) ENTT_NOEXCEPT
         : instance{},
+          info{other.info},
           vtable{other.vtable},
-          mode{other.mode}
-    {
-        vtable(operation::MOVE, other, this);
+          mode{other.mode} {
+        if(other.vtable) {
+            other.vtable(operation::move, other, this);
+        }
     }
 
     /*! @brief Frees the internal storage, whatever it means. */
     ~basic_any() {
-        vtable(operation::DTOR, *this, nullptr);
+        if(vtable && owner()) {
+            vtable(operation::destroy, *this, nullptr);
+        }
     }
 
     /**
@@ -224,9 +206,13 @@ public:
      * @param other The instance to copy from.
      * @return This any object.
      */
-    basic_any & operator=(const basic_any &other) {
+    basic_any &operator=(const basic_any &other) {
         reset();
-        other.vtable(operation::COPY, other, this);
+
+        if(other.vtable) {
+            other.vtable(operation::copy, other, this);
+        }
+
         return *this;
     }
 
@@ -235,23 +221,16 @@ public:
      * @param other The instance to move from.
      * @return This any object.
      */
-    basic_any & operator=(basic_any &&other) ENTT_NOEXCEPT {
-        std::exchange(vtable, other.vtable)(operation::DTOR, *this, nullptr);
-        other.vtable(operation::MOVE, other, this);
-        mode = other.mode;
-        return *this;
-    }
+    basic_any &operator=(basic_any &&other) ENTT_NOEXCEPT {
+        reset();
+
+        if(other.vtable) {
+            other.vtable(operation::move, other, this);
+            info = other.info;
+            vtable = other.vtable;
+            mode = other.mode;
+        }
 
-    /**
-     * @brief Value assignment operator.
-     * @tparam Type Type of object to use to initialize the wrapper.
-     * @param value An instance of an object to use to initialize the wrapper.
-     * @return This any object.
-     */
-    template<typename Type>
-    [[deprecated("Use std::in_place_type<T &>, entt::make_any<T &>, emplace<Type &> or forward_as_any instead")]]
-    basic_any & operator=(std::reference_wrapper<Type> value) ENTT_NOEXCEPT {
-        emplace<Type &>(value.get());
         return *this;
     }
 
@@ -269,26 +248,45 @@ public:
     }
 
     /**
-     * @brief Returns the type of the contained object.
-     * @return The type of the contained object, if any.
+     * @brief Returns the object type if any, `type_id<void>()` otherwise.
+     * @return The object type if any, `type_id<void>()` otherwise.
      */
-    [[nodiscard]] type_info type() const ENTT_NOEXCEPT {
-        type_info info{};
-        vtable(operation::TYPE, *this, &info);
-        return info;
+    [[nodiscard]] const type_info &type() const ENTT_NOEXCEPT {
+        return *info;
     }
 
     /**
      * @brief Returns an opaque pointer to the contained instance.
      * @return An opaque pointer the contained instance, if any.
      */
-    [[nodiscard]] const void * data() const ENTT_NOEXCEPT {
-        return vtable(operation::CADDR, *this, nullptr);
+    [[nodiscard]] const void *data() const ENTT_NOEXCEPT {
+        return vtable ? vtable(operation::get, *this, nullptr) : nullptr;
     }
 
-    /*! @copydoc data */
-    [[nodiscard]] void * data() ENTT_NOEXCEPT {
-        return const_cast<void *>(vtable(operation::ADDR, *this, nullptr));
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @param req Expected type.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] const void *data(const type_info &req) const ENTT_NOEXCEPT {
+        return *info == req ? data() : nullptr;
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] void *data() ENTT_NOEXCEPT {
+        return (!vtable || mode == policy::cref) ? nullptr : const_cast<void *>(vtable(operation::get, *this, nullptr));
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @param req Expected type.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] void *data(const type_info &req) ENTT_NOEXCEPT {
+        return *info == req ? data() : nullptr;
     }
 
     /**
@@ -298,16 +296,46 @@ public:
      * @param args Parameters to use to construct the instance.
      */
     template<typename Type, typename... Args>
-    void emplace(Args &&... args) {
-        std::exchange(vtable, &basic_vtable<std::remove_const_t<std::remove_reference_t<Type>>>)(operation::DTOR, *this, nullptr);
-        mode = type_to_policy<Type>();
+    void emplace(Args &&...args) {
+        reset();
         initialize<Type>(std::forward<Args>(args)...);
     }
 
+    /**
+     * @brief Assigns a value to the contained object without replacing it.
+     * @param other The value to assign to the contained object.
+     * @return True in case of success, false otherwise.
+     */
+    bool assign(const any &other) {
+        if(vtable && mode != policy::cref && *info == *other.info) {
+            return (vtable(operation::assign, *this, other.data()) != nullptr);
+        }
+
+        return false;
+    }
+
+    /*! @copydoc assign */
+    bool assign(any &&other) {
+        if(vtable && mode != policy::cref && *info == *other.info) {
+            if(auto *val = other.data(); val) {
+                return (vtable(operation::transfer, *this, val) != nullptr);
+            } else {
+                return (vtable(operation::assign, *this, std::as_const(other).data()) != nullptr);
+            }
+        }
+
+        return false;
+    }
+
     /*! @brief Destroys contained object */
     void reset() {
-        std::exchange(vtable, &basic_vtable<void>)(operation::DTOR, *this, nullptr);
-        mode = policy::OWNER;
+        if(vtable && owner()) {
+            vtable(operation::destroy, *this, nullptr);
+        }
+
+        info = &type_id<void>();
+        vtable = nullptr;
+        mode = policy::owner;
     }
 
     /**
@@ -315,7 +343,7 @@ public:
      * @return False if the wrapper is empty, true otherwise.
      */
     [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
-        return !(vtable(operation::CADDR, *this, nullptr) == nullptr);
+        return vtable != nullptr;
     }
 
     /**
@@ -324,8 +352,11 @@ public:
      * @return False if the two objects differ in their content, true otherwise.
      */
     bool operator==(const basic_any &other) const ENTT_NOEXCEPT {
-        const basic_any *trampoline = &other;
-        return type() == other.type() && (vtable(operation::COMP, *this, &trampoline) || !other.data());
+        if(vtable && *info == *other.info) {
+            return (vtable(operation::compare, *this, other.data()) != nullptr);
+        }
+
+        return (!vtable && !other.vtable);
     }
 
     /**
@@ -333,12 +364,12 @@ public:
      * @return A wrapper that shares a reference to an unmanaged object.
      */
     [[nodiscard]] basic_any as_ref() ENTT_NOEXCEPT {
-        return basic_any{*this, (mode == policy::CREF ? policy::CREF : policy::REF)};
+        return basic_any{*this, (mode == policy::cref ? policy::cref : policy::ref)};
     }
 
     /*! @copydoc as_ref */
     [[nodiscard]] basic_any as_ref() const ENTT_NOEXCEPT {
-        return basic_any{*this, policy::CREF};
+        return basic_any{*this, policy::cref};
     }
 
     /**
@@ -346,16 +377,19 @@ public:
      * @return True if the wrapper owns its object, false otherwise.
      */
     [[nodiscard]] bool owner() const ENTT_NOEXCEPT {
-        return (mode == policy::OWNER);
+        return (mode == policy::owner);
     }
 
 private:
-    union { const void *instance; storage_type storage; };
+    union {
+        const void *instance;
+        storage_type storage;
+    };
+    const type_info *info;
     vtable_type *vtable;
     policy mode;
 };
 
-
 /**
  * @brief Checks if two wrappers differ in their content.
  * @tparam Len Size of the storage reserved for the small buffer optimization.
@@ -369,7 +403,6 @@ template<std::size_t Len, std::size_t Align>
     return !(lhs == rhs);
 }
 
-
 /**
  * @brief Performs type-safe access to the contained object.
  * @tparam Type Type to which conversion is required.
@@ -380,47 +413,51 @@ template<std::size_t Len, std::size_t Align>
  */
 template<typename Type, std::size_t Len, std::size_t Align>
 Type any_cast(const basic_any<Len, Align> &data) ENTT_NOEXCEPT {
-    const auto * const instance = any_cast<std::remove_reference_t<Type>>(&data);
+    const auto *const instance = any_cast<std::remove_reference_t<Type>>(&data);
     ENTT_ASSERT(instance, "Invalid instance");
     return static_cast<Type>(*instance);
 }
 
-
 /*! @copydoc any_cast */
 template<typename Type, std::size_t Len, std::size_t Align>
 Type any_cast(basic_any<Len, Align> &data) ENTT_NOEXCEPT {
     // forces const on non-reference types to make them work also with wrappers for const references
-    auto * const instance = any_cast<std::remove_reference_t<const Type>>(&data);
+    auto *const instance = any_cast<std::remove_reference_t<const Type>>(&data);
     ENTT_ASSERT(instance, "Invalid instance");
     return static_cast<Type>(*instance);
 }
 
-
 /*! @copydoc any_cast */
 template<typename Type, std::size_t Len, std::size_t Align>
 Type any_cast(basic_any<Len, Align> &&data) ENTT_NOEXCEPT {
-    // forces const on non-reference types to make them work also with wrappers for const references
-    auto * const instance = any_cast<std::remove_reference_t<const Type>>(&data);
-    ENTT_ASSERT(instance, "Invalid instance");
-    return static_cast<Type>(std::move(*instance));
+    if constexpr(std::is_copy_constructible_v<std::remove_cv_t<std::remove_reference_t<Type>>>) {
+        if(auto *const instance = any_cast<std::remove_reference_t<Type>>(&data); instance) {
+            return static_cast<Type>(std::move(*instance));
+        } else {
+            return any_cast<Type>(data);
+        }
+    } else {
+        auto *const instance = any_cast<std::remove_reference_t<Type>>(&data);
+        ENTT_ASSERT(instance, "Invalid instance");
+        return static_cast<Type>(std::move(*instance));
+    }
 }
 
-
 /*! @copydoc any_cast */
 template<typename Type, std::size_t Len, std::size_t Align>
-const Type * any_cast(const basic_any<Len, Align> *data) ENTT_NOEXCEPT {
-    return (data->type() == type_id<Type>() ? static_cast<const Type *>(data->data()) : nullptr);
+const Type *any_cast(const basic_any<Len, Align> *data) ENTT_NOEXCEPT {
+    const auto &info = type_id<std::remove_cv_t<std::remove_reference_t<Type>>>();
+    return static_cast<const Type *>(data->data(info));
 }
 
-
 /*! @copydoc any_cast */
 template<typename Type, std::size_t Len, std::size_t Align>
-Type * any_cast(basic_any<Len, Align> *data) ENTT_NOEXCEPT {
+Type *any_cast(basic_any<Len, Align> *data) ENTT_NOEXCEPT {
+    const auto &info = type_id<std::remove_cv_t<std::remove_reference_t<Type>>>();
     // last attempt to make wrappers for const references return their values
-    return (data->type() == type_id<Type>() ? static_cast<Type *>(static_cast<constness_as_t<basic_any<Len, Align>, Type> *>(data)->data()) : nullptr);
+    return static_cast<Type *>(static_cast<constness_as_t<basic_any<Len, Align>, Type> *>(data)->data(info));
 }
 
-
 /**
  * @brief Constructs a wrapper from a given type, passing it all arguments.
  * @tparam Type Type of object to use to initialize the wrapper.
@@ -431,11 +468,10 @@ Type * any_cast(basic_any<Len, Align> *data) ENTT_NOEXCEPT {
  * @return A properly initialized wrapper for an object of the given type.
  */
 template<typename Type, std::size_t Len = basic_any<>::length, std::size_t Align = basic_any<Len>::alignment, typename... Args>
-basic_any<Len, Align> make_any(Args &&... args) {
+basic_any<Len, Align> make_any(Args &&...args) {
     return basic_any<Len, Align>{std::in_place_type<Type>, std::forward<Args>(args)...};
 }
 
-
 /**
  * @brief Forwards its argument and avoids copies for lvalue references.
  * @tparam Len Size of the storage reserved for the small buffer optimization.
@@ -449,8 +485,6 @@ basic_any<Len, Align> forward_as_any(Type &&value) {
     return basic_any<Len, Align>{std::in_place_type<std::conditional_t<std::is_rvalue_reference_v<Type>, std::decay_t<Type>, Type>>, std::forward<Type>(value)};
 }
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/core/attribute.h

@@ -1,33 +1,30 @@
 #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
+#    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
+#    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/compressed_pair.hpp

@@ -0,0 +1,280 @@
+#ifndef ENTT_CORE_COMPRESSED_PAIR_HPP
+#define ENTT_CORE_COMPRESSED_PAIR_HPP
+
+#include <cstddef>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include "../config/config.h"
+#include "type_traits.hpp"
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename Type, std::size_t, typename = void>
+struct compressed_pair_element {
+    using reference = Type &;
+    using const_reference = const Type &;
+
+    template<bool Dummy = true, typename = std::enable_if_t<Dummy && std::is_default_constructible_v<Type>>>
+    compressed_pair_element()
+        : value{} {}
+
+    template<typename Args, typename = std::enable_if_t<!std::is_same_v<std::remove_cv_t<std::remove_reference_t<Args>>, compressed_pair_element>>>
+    compressed_pair_element(Args &&args)
+        : value{std::forward<Args>(args)} {}
+
+    template<typename... Args, std::size_t... Index>
+    compressed_pair_element(std::tuple<Args...> args, std::index_sequence<Index...>)
+        : value{std::forward<Args>(std::get<Index>(args))...} {}
+
+    [[nodiscard]] reference get() ENTT_NOEXCEPT {
+        return value;
+    }
+
+    [[nodiscard]] const_reference get() const ENTT_NOEXCEPT {
+        return value;
+    }
+
+private:
+    Type value;
+};
+
+template<typename Type, std::size_t Tag>
+struct compressed_pair_element<Type, Tag, std::enable_if_t<is_ebco_eligible_v<Type>>>: Type {
+    using reference = Type &;
+    using const_reference = const Type &;
+    using base_type = Type;
+
+    template<bool Dummy = true, typename = std::enable_if_t<Dummy && std::is_default_constructible_v<base_type>>>
+    compressed_pair_element()
+        : base_type{} {}
+
+    template<typename Args, typename = std::enable_if_t<!std::is_same_v<std::remove_cv_t<std::remove_reference_t<Args>>, compressed_pair_element>>>
+    compressed_pair_element(Args &&args)
+        : base_type{std::forward<Args>(args)} {}
+
+    template<typename... Args, std::size_t... Index>
+    compressed_pair_element(std::tuple<Args...> args, std::index_sequence<Index...>)
+        : base_type{std::forward<Args>(std::get<Index>(args))...} {}
+
+    [[nodiscard]] reference get() ENTT_NOEXCEPT {
+        return *this;
+    }
+
+    [[nodiscard]] const_reference get() const ENTT_NOEXCEPT {
+        return *this;
+    }
+};
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief A compressed pair.
+ *
+ * A pair that exploits the _Empty Base Class Optimization_ (or _EBCO_) to
+ * reduce its final size to a minimum.
+ *
+ * @tparam First The type of the first element that the pair stores.
+ * @tparam Second The type of the second element that the pair stores.
+ */
+template<typename First, typename Second>
+class compressed_pair final
+    : internal::compressed_pair_element<First, 0u>,
+      internal::compressed_pair_element<Second, 1u> {
+    using first_base = internal::compressed_pair_element<First, 0u>;
+    using second_base = internal::compressed_pair_element<Second, 1u>;
+
+public:
+    /*! @brief The type of the first element that the pair stores. */
+    using first_type = First;
+    /*! @brief The type of the second element that the pair stores. */
+    using second_type = Second;
+
+    /**
+     * @brief Default constructor, conditionally enabled.
+     *
+     * 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()
+        : first_base{},
+          second_base{} {}
+
+    /**
+     * @brief Copy constructor.
+     * @param other The instance to copy from.
+     */
+    constexpr compressed_pair(const compressed_pair &other) = default;
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    constexpr compressed_pair(compressed_pair &&other) = default;
+
+    /**
+     * @brief Constructs a pair from its values.
+     * @tparam Arg Type of value to use to initialize the first element.
+     * @tparam Other Type of value to use to initialize the second element.
+     * @param arg Value to use to initialize the first element.
+     * @param other Value to use to initialize the second element.
+     */
+    template<typename Arg, typename Other>
+    constexpr compressed_pair(Arg &&arg, Other &&other)
+        : first_base{std::forward<Arg>(arg)},
+          second_base{std::forward<Other>(other)} {}
+
+    /**
+     * @brief Constructs a pair by forwarding the arguments to its parts.
+     * @tparam Args Types of arguments to use to initialize the first element.
+     * @tparam Other Types of arguments to use to initialize the second element.
+     * @param args Arguments to use to initialize the first element.
+     * @param other Arguments to use to initialize the second element.
+     */
+    template<typename... Args, typename... Other>
+    constexpr compressed_pair(std::piecewise_construct_t, std::tuple<Args...> args, std::tuple<Other...> other)
+        : first_base{std::move(args), std::index_sequence_for<Args...>{}},
+          second_base{std::move(other), std::index_sequence_for<Other...>{}} {}
+
+    /**
+     * @brief Copy assignment operator.
+     * @param other The instance to copy from.
+     * @return This compressed pair object.
+     */
+    constexpr compressed_pair &operator=(const compressed_pair &other) = default;
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This compressed pair object.
+     */
+    constexpr compressed_pair &operator=(compressed_pair &&other) = default;
+
+    /**
+     * @brief Returns the first element that a pair stores.
+     * @return The first element that a pair stores.
+     */
+    [[nodiscard]] first_type &first() ENTT_NOEXCEPT {
+        return static_cast<first_base &>(*this).get();
+    }
+
+    /*! @copydoc first */
+    [[nodiscard]] const first_type &first() const ENTT_NOEXCEPT {
+        return static_cast<const first_base &>(*this).get();
+    }
+
+    /**
+     * @brief Returns the second element that a pair stores.
+     * @return The second element that a pair stores.
+     */
+    [[nodiscard]] second_type &second() ENTT_NOEXCEPT {
+        return static_cast<second_base &>(*this).get();
+    }
+
+    /*! @copydoc second */
+    [[nodiscard]] const second_type &second() const ENTT_NOEXCEPT {
+        return static_cast<const second_base &>(*this).get();
+    }
+
+    /**
+     * @brief Swaps two compressed pair objects.
+     * @param other The compressed pair to swap with.
+     */
+    void swap(compressed_pair &other) {
+        using std::swap;
+        swap(first(), other.first());
+        swap(second(), other.second());
+    }
+
+    /**
+     * @brief Extracts an element from the compressed pair.
+     * @tparam Index An integer value that is either 0 or 1.
+     * @return Returns a reference to the first element if `Index` is 0 and a
+     * reference to the second element if `Index` is 1.
+     */
+    template<std::size_t Index>
+    decltype(auto) get() ENTT_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>
+    decltype(auto) get() const ENTT_NOEXCEPT {
+        if constexpr(Index == 0u) {
+            return first();
+        } else {
+            static_assert(Index == 1u, "Index out of bounds");
+            return second();
+        }
+    }
+};
+
+/**
+ * @brief Deduction guide.
+ * @tparam Type Type of value to use to initialize the first element.
+ * @tparam Other Type of value to use to initialize the second element.
+ */
+template<typename Type, typename Other>
+compressed_pair(Type &&, Other &&) -> compressed_pair<std::decay_t<Type>, std::decay_t<Other>>;
+
+/**
+ * @brief Swaps two compressed pair objects.
+ * @tparam First The type of the first element that the pairs store.
+ * @tparam Second The type of the second element that the pairs store.
+ * @param lhs A valid compressed pair object.
+ * @param rhs A valid compressed pair object.
+ */
+template<typename First, typename Second>
+inline void swap(compressed_pair<First, Second> &lhs, compressed_pair<First, Second> &rhs) {
+    lhs.swap(rhs);
+}
+
+} // namespace entt
+
+// disable structured binding support for clang 6, it messes when specializing tuple_size
+#if !defined __clang_major__ || __clang_major__ > 6
+namespace std {
+
+/**
+ * @brief `std::tuple_size` specialization for `compressed_pair`s.
+ * @tparam First The type of the first element that the pair stores.
+ * @tparam Second The type of the second element that the pair stores.
+ */
+template<typename First, typename Second>
+struct tuple_size<entt::compressed_pair<First, Second>>: integral_constant<size_t, 2u> {};
+
+/**
+ * @brief `std::tuple_element` specialization for `compressed_pair`s.
+ * @tparam Index The index of the type to return.
+ * @tparam First The type of the first element that the pair stores.
+ * @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");
+};
+
+} // namespace std
+#endif
+
+#endif

src/entt/core/enum.hpp

@@ -0,0 +1,98 @@
+#ifndef ENTT_CORE_ENUM_HPP
+#define ENTT_CORE_ENUM_HPP
+
+#include <type_traits>
+#include "../config/config.h"
+
+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>
+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> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The enum class type for which to enable bitmask support.
+ */
+template<typename Type>
+inline constexpr bool enum_as_bitmask_v = enum_as_bitmask<Type>::value;
+
+} // namespace entt
+
+/**
+ * @brief Operator available for enums for which bitmask support is enabled.
+ * @tparam Type Enum class type.
+ * @param lhs The first value to use.
+ * @param rhs The second value to use.
+ * @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) ENTT_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) ENTT_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) ENTT_NOEXCEPT {
+    return static_cast<Type>(static_cast<std::underlying_type_t<Type>>(lhs) ^ static_cast<std::underlying_type_t<Type>>(rhs));
+}
+
+/**
+ * @brief Operator available for enums for which bitmask support is enabled.
+ * @tparam Type Enum class type.
+ * @param value The value to use.
+ * @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) ENTT_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) ENTT_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) ENTT_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) ENTT_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) ENTT_NOEXCEPT {
+    return (lhs = (lhs ^ rhs));
+}
+
+#endif

src/entt/core/family.hpp

@@ -1,14 +1,11 @@
 #ifndef ENTT_CORE_FAMILY_HPP
 #define ENTT_CORE_FAMILY_HPP
 
-
 #include "../config/config.h"
 #include "fwd.hpp"
 
-
 namespace entt {
 
-
 /**
  * @brief Dynamic identifier generator.
  *
@@ -30,8 +27,6 @@ public:
     inline static const family_type type = identifier++;
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/core/fwd.hpp

@@ -1,27 +1,21 @@
 #ifndef ENTT_CORE_FWD_HPP
 #define ENTT_CORE_FWD_HPP
 
-
+#include <cstdint>
 #include <type_traits>
 #include "../config/config.h"
 
-
 namespace entt {
 
-
 template<std::size_t Len = sizeof(double[2]), std::size_t = alignof(typename std::aligned_storage_t<Len + !Len>)>
 class basic_any;
 
-
 /*! @brief Alias declaration for type identifiers. */
 using id_type = ENTT_ID_TYPE;
 
-
 /*! @brief Alias declaration for the most common use case. */
 using any = basic_any<>;
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/core/hashed_string.hpp

@@ -1,29 +1,23 @@
 #ifndef ENTT_CORE_HASHED_STRING_HPP
 #define ENTT_CORE_HASHED_STRING_HPP
 
-
 #include <cstddef>
 #include <cstdint>
 #include "../config/config.h"
 #include "fwd.hpp"
 
-
 namespace entt {
 
-
 /**
  * @cond TURN_OFF_DOXYGEN
  * Internal details not to be documented.
  */
 
-
 namespace internal {
 
-
 template<typename>
 struct fnv1a_traits;
 
-
 template<>
 struct fnv1a_traits<std::uint32_t> {
     using type = std::uint32_t;
@@ -31,7 +25,6 @@ struct fnv1a_traits<std::uint32_t> {
     static constexpr std::uint32_t prime = 16777619;
 };
 
-
 template<>
 struct fnv1a_traits<std::uint64_t> {
     using type = std::uint64_t;
@@ -39,84 +32,99 @@ struct fnv1a_traits<std::uint64_t> {
     static constexpr std::uint64_t prime = 1099511628211ull;
 };
 
+template<typename Char>
+struct basic_hashed_string {
+    using value_type = Char;
+    using size_type = std::size_t;
+    using hash_type = id_type;
 
-}
+    const value_type *repr;
+    size_type length;
+    hash_type hash;
+};
 
+} // namespace internal
 
 /**
  * Internal details not to be documented.
  * @endcond
  */
 
-
 /**
  * @brief Zero overhead unique identifier.
  *
  * A hashed string is a compile-time tool that allows users to use
- * human-readable identifers in the codebase while using their numeric
+ * human-readable identifiers in the codebase while using their numeric
  * counterparts at runtime.<br/>
  * Because of that, a hashed string can also be used in constant expressions if
  * required.
  *
+ * @warning
+ * This class doesn't take ownership of user-supplied strings nor does it make a
+ * copy of them.
+ *
  * @tparam Char Character type.
  */
 template<typename Char>
-class basic_hashed_string {
-    using traits_type = internal::fnv1a_traits<id_type>;
+class basic_hashed_string: internal::basic_hashed_string<Char> {
+    using base_type = internal::basic_hashed_string<Char>;
+    using hs_traits = internal::fnv1a_traits<id_type>;
 
     struct const_wrapper {
         // non-explicit constructor on purpose
-        constexpr const_wrapper(const Char *curr) ENTT_NOEXCEPT: str{curr} {}
-        const Char *str;
+        constexpr const_wrapper(const Char *str) ENTT_NOEXCEPT: repr{str} {}
+        const Char *repr;
     };
 
     // Fowler–Noll–Vo hash function v. 1a - the good
-    [[nodiscard]] static constexpr id_type helper(const Char *curr) ENTT_NOEXCEPT {
-        auto value = traits_type::offset;
+    [[nodiscard]] static constexpr auto helper(const Char *str) ENTT_NOEXCEPT {
+        base_type base{str, 0u, hs_traits::offset};
 
-        while(*curr != 0) {
-            value = (value ^ static_cast<traits_type::type>(*(curr++))) * traits_type::prime;
+        for(; str[base.length]; ++base.length) {
+            base.hash = (base.hash ^ static_cast<hs_traits::type>(str[base.length])) * hs_traits::prime;
         }
 
-        return value;
+        return base;
+    }
+
+    // Fowler–Noll–Vo hash function v. 1a - the good
+    [[nodiscard]] static constexpr auto helper(const Char *str, const std::size_t len) ENTT_NOEXCEPT {
+        base_type base{str, len, hs_traits::offset};
+
+        for(size_type pos{}; pos < len; ++pos) {
+            base.hash = (base.hash ^ static_cast<hs_traits::type>(str[pos])) * hs_traits::prime;
+        }
+
+        return base;
     }
 
 public:
     /*! @brief Character type. */
-    using value_type = Char;
+    using value_type = typename base_type::value_type;
     /*! @brief Unsigned integer type. */
-    using hash_type = id_type;
+    using size_type = typename base_type::size_type;
+    /*! @brief Unsigned integer type. */
+    using hash_type = typename base_type::hash_type;
 
     /**
      * @brief Returns directly the numeric representation of a string view.
-     * @param str Human-readable identifer.
-     * @param size Length of the string to hash.
+     * @param str Human-readable identifier.
+     * @param len Length of the string to hash.
      * @return The numeric representation of the string.
      */
-    [[nodiscard]] static constexpr hash_type value(const value_type *str, std::size_t size) ENTT_NOEXCEPT {
-        id_type partial{traits_type::offset};
-        while(size--) { partial = (partial^(str++)[0])*traits_type::prime; }
-        return partial;
+    [[nodiscard]] static constexpr hash_type value(const value_type *str, const size_type len) ENTT_NOEXCEPT {
+        return basic_hashed_string{str, len};
     }
 
     /**
      * @brief Returns directly the numeric representation of a string.
-     *
-     * Forcing template resolution avoids implicit conversions. An
-     * human-readable identifier can be anything but a plain, old bunch of
-     * characters.<br/>
-     * Example of use:
-     * @code{.cpp}
-     * const auto value = basic_hashed_string<char>::to_value("my.png");
-     * @endcode
-     *
      * @tparam N Number of characters of the identifier.
-     * @param str Human-readable identifer.
+     * @param str Human-readable identifier.
      * @return The numeric representation of the string.
      */
     template<std::size_t N>
     [[nodiscard]] static constexpr hash_type value(const value_type (&str)[N]) ENTT_NOEXCEPT {
-        return helper(str);
+        return basic_hashed_string{str};
     }
 
     /**
@@ -124,97 +132,98 @@ public:
      * @param wrapper Helps achieving the purpose by relying on overloading.
      * @return The numeric representation of the string.
      */
-    [[nodiscard]] static hash_type value(const_wrapper wrapper) ENTT_NOEXCEPT {
-        return helper(wrapper.str);
+    [[nodiscard]] static constexpr hash_type value(const_wrapper wrapper) ENTT_NOEXCEPT {
+        return basic_hashed_string{wrapper};
     }
 
     /*! @brief Constructs an empty hashed string. */
     constexpr basic_hashed_string() ENTT_NOEXCEPT
-        : str{nullptr}, hash{}
-    {}
+        : base_type{} {}
+
+    /**
+     * @brief Constructs a hashed string from a string view.
+     * @param str Human-readable identifier.
+     * @param len Length of the string to hash.
+     */
+    constexpr basic_hashed_string(const value_type *str, const size_type len) ENTT_NOEXCEPT
+        : base_type{helper(str, len)} {}
 
     /**
      * @brief Constructs a hashed string from an array of const characters.
-     *
-     * Forcing template resolution avoids implicit conversions. An
-     * human-readable identifier can be anything but a plain, old bunch of
-     * characters.<br/>
-     * Example of use:
-     * @code{.cpp}
-     * basic_hashed_string<char> hs{"my.png"};
-     * @endcode
-     *
      * @tparam N Number of characters of the identifier.
-     * @param curr Human-readable identifer.
+     * @param str Human-readable identifier.
      */
     template<std::size_t N>
-    constexpr basic_hashed_string(const value_type (&curr)[N]) ENTT_NOEXCEPT
-        : str{curr}, hash{helper(curr)}
-    {}
+    constexpr basic_hashed_string(const value_type (&str)[N]) ENTT_NOEXCEPT
+        : base_type{helper(str)} {}
 
     /**
      * @brief Explicit constructor on purpose to avoid constructing a hashed
      * string directly from a `const value_type *`.
+     *
+     * @warning
+     * The lifetime of the string is not extended nor is it copied.
+     *
      * @param wrapper Helps achieving the purpose by relying on overloading.
      */
     explicit constexpr basic_hashed_string(const_wrapper wrapper) ENTT_NOEXCEPT
-        : str{wrapper.str}, hash{helper(wrapper.str)}
-    {}
+        : base_type{helper(wrapper.repr)} {}
+
+    /**
+     * @brief Returns the size a hashed string.
+     * @return The size of the hashed string.
+     */
+    [[nodiscard]] constexpr size_type size() const ENTT_NOEXCEPT {
+        return base_type::length;
+    }
 
     /**
      * @brief Returns the human-readable representation of a hashed string.
-     * @return The string used to initialize the instance.
+     * @return The string used to initialize the hashed string.
      */
-    [[nodiscard]] constexpr const value_type * data() const ENTT_NOEXCEPT {
-        return str;
+    [[nodiscard]] constexpr const value_type *data() const ENTT_NOEXCEPT {
+        return base_type::repr;
     }
 
     /**
      * @brief Returns the numeric representation of a hashed string.
-     * @return The numeric representation of the instance.
+     * @return The numeric representation of the hashed string.
      */
     [[nodiscard]] constexpr hash_type value() const ENTT_NOEXCEPT {
-        return hash;
+        return base_type::hash;
     }
 
     /*! @copydoc data */
-    [[nodiscard]] constexpr operator const value_type *() const ENTT_NOEXCEPT { return data(); }
+    [[nodiscard]] constexpr operator const value_type *() const ENTT_NOEXCEPT {
+        return data();
+    }
 
     /**
      * @brief Returns the numeric representation of a hashed string.
-     * @return The numeric representation of the instance.
+     * @return The numeric representation of the hashed string.
      */
-    [[nodiscard]] constexpr operator hash_type() const ENTT_NOEXCEPT { return value(); }
-
-    /**
-     * @brief Compares two hashed strings.
-     * @param other Hashed string with which to compare.
-     * @return True if the two hashed strings are identical, false otherwise.
-     */
-    [[nodiscard]] constexpr bool operator==(const basic_hashed_string &other) const ENTT_NOEXCEPT {
-        return hash == other.hash;
+    [[nodiscard]] constexpr operator hash_type() const ENTT_NOEXCEPT {
+        return value();
     }
-
-private:
-    const value_type *str;
-    hash_type hash;
 };
 
-
 /**
  * @brief Deduction guide.
- *
- * It allows to deduce the character type of the hashed string directly from a
- * human-readable identifer provided to the constructor.
- *
+ * @tparam Char Character type.
+ * @param str Human-readable identifier.
+ * @param len Length of the string to hash.
+ */
+template<typename Char>
+basic_hashed_string(const Char *str, const std::size_t len) -> basic_hashed_string<Char>;
+
+/**
+ * @brief Deduction guide.
  * @tparam Char Character type.
  * @tparam N Number of characters of the identifier.
- * @param str Human-readable identifer.
+ * @param str Human-readable identifier.
  */
 template<typename Char, std::size_t N>
-basic_hashed_string(const Char (&str)[N])
--> basic_hashed_string<Char>;
-
+basic_hashed_string(const Char (&str)[N]) -> basic_hashed_string<Char>;
 
 /**
  * @brief Compares two hashed strings.
@@ -224,46 +233,101 @@ basic_hashed_string(const Char (&str)[N])
  * @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) ENTT_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) ENTT_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) ENTT_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) ENTT_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) ENTT_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) ENTT_NOEXCEPT {
+    return !(lhs < rhs);
+}
 
 /*! @brief Aliases for common character types. */
 using hashed_string = basic_hashed_string<char>;
 
-
 /*! @brief Aliases for common character types. */
 using hashed_wstring = basic_hashed_string<wchar_t>;
 
-
 inline namespace literals {
 
-
 /**
  * @brief User defined literal for hashed strings.
  * @param str The literal without its suffix.
  * @return A properly initialized hashed string.
  */
-[[nodiscard]] constexpr entt::hashed_string operator"" _hs(const char *str, std::size_t) ENTT_NOEXCEPT {
-    return entt::hashed_string{str};
+[[nodiscard]] constexpr hashed_string operator"" _hs(const char *str, std::size_t) ENTT_NOEXCEPT {
+    return hashed_string{str};
 }
 
-
 /**
  * @brief User defined literal for hashed wstrings.
  * @param str The literal without its suffix.
  * @return A properly initialized hashed wstring.
  */
-[[nodiscard]] constexpr entt::hashed_wstring operator"" _hws(const wchar_t *str, std::size_t) ENTT_NOEXCEPT {
-    return entt::hashed_wstring{str};
+[[nodiscard]] constexpr hashed_wstring operator"" _hws(const wchar_t *str, std::size_t) ENTT_NOEXCEPT {
+    return hashed_wstring{str};
 }
 
+} // namespace literals
 
-}
-
-
-}
-
+} // namespace entt
 
 #endif

src/entt/core/ident.hpp

@@ -1,18 +1,15 @@
 #ifndef ENTT_CORE_IDENT_HPP
 #define ENTT_CORE_IDENT_HPP
 
-
 #include <cstddef>
-#include <utility>
 #include <type_traits>
+#include <utility>
 #include "../config/config.h"
 #include "fwd.hpp"
 #include "type_traits.hpp"
 
-
 namespace entt {
 
-
 /**
  * @brief Types identifiers.
  *
@@ -43,8 +40,8 @@ namespace entt {
 template<typename... Types>
 class identifier {
     template<typename Type, std::size_t... Index>
-    [[nodiscard]] static constexpr id_type get(std::index_sequence<Index...>) {
-        static_assert(std::disjunction_v<std::is_same<Type, Types>...>, "Invalid type");
+    [[nodiscard]] static constexpr id_type get(std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        static_assert((std::is_same_v<Type, Types> || ...), "Invalid type");
         return (0 + ... + (std::is_same_v<Type, type_list_element_t<Index, type_list<std::decay_t<Types>...>>> ? id_type{Index} : id_type{}));
     }
 
@@ -57,8 +54,6 @@ public:
     static constexpr identifier_type type = get<std::decay_t<Type>>(std::index_sequence_for<Types...>{});
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/core/iterator.hpp

@@ -0,0 +1,117 @@
+#ifndef ENTT_CORE_ITERATOR_HPP
+#define ENTT_CORE_ITERATOR_HPP
+
+#include <iterator>
+#include <memory>
+#include <utility>
+#include "../config/config.h"
+
+namespace entt {
+
+/**
+ * @brief Helper type to use as pointer with input iterators.
+ * @tparam Type of wrapped value.
+ */
+template<typename Type>
+struct input_iterator_pointer final {
+    /*! @brief Pointer type. */
+    using pointer = Type *;
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    input_iterator_pointer(const input_iterator_pointer &) = delete;
+
+    /*! @brief Default move constructor. */
+    input_iterator_pointer(input_iterator_pointer &&) = default;
+
+    /**
+     * @brief Constructs a proxy object by move.
+     * @param val Value to use to initialize the proxy object.
+     */
+    input_iterator_pointer(Type &&val)
+        : value{std::move(val)} {}
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This proxy object.
+     */
+    input_iterator_pointer &operator=(const input_iterator_pointer &) = delete;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This proxy object.
+     */
+    input_iterator_pointer &operator=(input_iterator_pointer &&) = default;
+
+    /**
+     * @brief Access operator for accessing wrapped values.
+     * @return A pointer to the wrapped value.
+     */
+    [[nodiscard]] pointer operator->() ENTT_NOEXCEPT {
+        return std::addressof(value);
+    }
+
+private:
+    Type value;
+};
+
+/**
+ * @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>
+struct iterable_adaptor final {
+    /*! @brief Value type. */
+    using value_type = typename std::iterator_traits<It>::value_type;
+    /*! @brief Iterator type. */
+    using iterator = It;
+    /*! @brief Sentinel type. */
+    using sentinel = Sentinel;
+
+    /*! @brief Default constructor. */
+    iterable_adaptor() = default;
+
+    /**
+     * @brief Creates an iterable object from a pair of iterators.
+     * @param from Begin iterator.
+     * @param to End iterator.
+     */
+    iterable_adaptor(iterator from, sentinel to)
+        : first{from},
+          last{to} {}
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     * @return An iterator to the first element of the range.
+     */
+    [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+        return first;
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     * @return An iterator to the element following the last element of the
+     * range.
+     */
+    [[nodiscard]] sentinel end() const ENTT_NOEXCEPT {
+        return last;
+    }
+
+    /*! @copydoc begin */
+    [[nodiscard]] iterator cbegin() const ENTT_NOEXCEPT {
+        return begin();
+    }
+
+    /*! @copydoc end */
+    [[nodiscard]] sentinel cend() const ENTT_NOEXCEPT {
+        return end();
+    }
+
+private:
+    It first;
+    Sentinel last;
+};
+
+} // namespace entt
+
+#endif

src/entt/core/memory.hpp

@@ -0,0 +1,289 @@
+#ifndef ENTT_CORE_MEMORY_HPP
+#define ENTT_CORE_MEMORY_HPP
+
+#include <cstddef>
+#include <limits>
+#include <memory>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include "../config/config.h"
+
+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) ENTT_NOEXCEPT {
+    if constexpr(std::is_pointer_v<std::remove_cv_t<std::remove_reference_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.
+ * @param lhs A valid allocator.
+ * @param rhs Another valid allocator.
+ */
+template<typename Allocator>
+constexpr void propagate_on_container_copy_assignment([[maybe_unused]] Allocator &lhs, [[maybe_unused]] Allocator &rhs) ENTT_NOEXCEPT {
+    if constexpr(std::allocator_traits<Allocator>::propagate_on_container_copy_assignment::value) {
+        lhs = rhs;
+    }
+}
+
+/**
+ * @brief Utility function to design allocation-aware containers.
+ * @tparam Allocator Type of allocator.
+ * @param lhs A valid allocator.
+ * @param rhs Another valid allocator.
+ */
+template<typename Allocator>
+constexpr void propagate_on_container_move_assignment([[maybe_unused]] Allocator &lhs, [[maybe_unused]] Allocator &rhs) ENTT_NOEXCEPT {
+    if constexpr(std::allocator_traits<Allocator>::propagate_on_container_move_assignment::value) {
+        lhs = std::move(rhs);
+    }
+}
+
+/**
+ * @brief Utility function to design allocation-aware containers.
+ * @tparam Allocator Type of allocator.
+ * @param lhs A valid allocator.
+ * @param rhs Another valid allocator.
+ */
+template<typename Allocator>
+constexpr void propagate_on_container_swap([[maybe_unused]] Allocator &lhs, [[maybe_unused]] Allocator &rhs) ENTT_NOEXCEPT {
+    ENTT_ASSERT(std::allocator_traits<Allocator>::propagate_on_container_swap::value || lhs == rhs, "Cannot swap the containers");
+
+    if constexpr(std::allocator_traits<Allocator>::propagate_on_container_swap::value) {
+        using std::swap;
+        swap(lhs, rhs);
+    }
+}
+
+/**
+ * @brief Checks whether a value is a power of two or not.
+ * @param value A value that may or may not be a power of two.
+ * @return True if the value is a power of two, false otherwise.
+ */
+[[nodiscard]] inline constexpr bool is_power_of_two(const std::size_t value) ENTT_NOEXCEPT {
+    return value && ((value & (value - 1)) == 0);
+}
+
+/**
+ * @brief Computes the smallest power of two greater than or equal to a value.
+ * @param value The value to use.
+ * @return The smallest power of two greater than or equal to the given value.
+ */
+[[nodiscard]] inline constexpr std::size_t next_power_of_two(const std::size_t value) ENTT_NOEXCEPT {
+    ENTT_ASSERT(value < (std::size_t{1u} << (std::numeric_limits<std::size_t>::digits - 1)), "Numeric limits exceeded");
+    std::size_t curr = value - (value != 0u);
+
+    for(int next = 1; next < std::numeric_limits<std::size_t>::digits; next = next * 2) {
+        curr |= curr >> next;
+    }
+
+    return ++curr;
+}
+
+/**
+ * @brief Fast module utility function (powers of two only).
+ * @param value A value for which to calculate the modulus.
+ * @param mod _Modulus_, it must be a power of two.
+ * @return The common remainder.
+ */
+[[nodiscard]] inline constexpr std::size_t fast_mod(const std::size_t value, const std::size_t mod) ENTT_NOEXCEPT {
+    ENTT_ASSERT(is_power_of_two(mod), "Value must be a power of two");
+    return value & (mod - 1u);
+}
+
+/**
+ * @brief Deleter for allocator-aware unique pointers (waiting for C++20).
+ * @tparam Args Types of arguments to use to construct the object.
+ */
+template<typename Allocator>
+struct allocation_deleter: private Allocator {
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Pointer type. */
+    using pointer = typename std::allocator_traits<Allocator>::pointer;
+
+    /**
+     * @brief Inherited constructors.
+     * @param alloc The allocator to use.
+     */
+    allocation_deleter(const allocator_type &alloc)
+        : Allocator{alloc} {}
+
+    /**
+     * @brief Destroys the pointed object and deallocates its memory.
+     * @param ptr A valid pointer to an object of the given type.
+     */
+    void operator()(pointer ptr) {
+        using alloc_traits = typename std::allocator_traits<Allocator>;
+        alloc_traits::destroy(*this, to_address(ptr));
+        alloc_traits::deallocate(*this, ptr, 1u);
+    }
+};
+
+/**
+ * @brief Allows `std::unique_ptr` to use allocators (waiting for C++20).
+ * @tparam Type Type of object to allocate for and to construct.
+ * @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 properly initialized unique pointer with a custom deleter.
+ */
+template<typename Type, typename Allocator, typename... Args>
+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;
+
+    allocator_type alloc{allocator};
+    auto ptr = alloc_traits::allocate(alloc, 1u);
+
+    ENTT_TRY {
+        alloc_traits::construct(alloc, to_address(ptr), std::forward<Args>(args)...);
+    }
+    ENTT_CATCH {
+        alloc_traits::deallocate(alloc, ptr, 1u);
+        ENTT_THROW;
+    }
+
+    return std::unique_ptr<Type, allocation_deleter<allocator_type>>{ptr, alloc};
+}
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename Type>
+struct uses_allocator_construction {
+    template<typename Allocator, typename... Params>
+    static constexpr auto args([[maybe_unused]] const Allocator &allocator, Params &&...params) ENTT_NOEXCEPT {
+        if constexpr(!std::uses_allocator_v<Type, Allocator> && std::is_constructible_v<Type, Params...>) {
+            return std::forward_as_tuple(std::forward<Params>(params)...);
+        } else {
+            static_assert(std::uses_allocator_v<Type, Allocator>, "Ill-formed request");
+
+            if constexpr(std::is_constructible_v<Type, std::allocator_arg_t, const Allocator &, Params...>) {
+                return std::tuple<std::allocator_arg_t, const Allocator &, Params &&...>(std::allocator_arg, allocator, std::forward<Params>(params)...);
+            } else {
+                static_assert(std::is_constructible_v<Type, Params..., const Allocator &>, "Ill-formed request");
+                return std::forward_as_tuple(std::forward<Params>(params)..., allocator);
+            }
+        }
+    }
+};
+
+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) ENTT_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) ENTT_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) ENTT_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) ENTT_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) ENTT_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)));
+    }
+};
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief Uses-allocator construction utility (waiting for C++20).
+ *
+ * Primarily intended for internal use. Prepares the argument list needed to
+ * 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) ENTT_NOEXCEPT {
+    return internal::uses_allocator_construction<Type>::args(allocator, std::forward<Args>(args)...);
+}
+
+/**
+ * @brief Uses-allocator construction utility (waiting for C++20).
+ *
+ * Primarily intended for internal use. Creates an object of a given type by
+ * 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) {
+    return std::make_from_tuple<Type>(internal::uses_allocator_construction<Type>::args(allocator, std::forward<Args>(args)...));
+}
+
+/**
+ * @brief Uses-allocator construction utility (waiting for C++20).
+ *
+ * Primarily intended for internal use. Creates an object of a given type by
+ * 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) {
+    return std::apply([&](auto &&...curr) { return new(value) Type(std::forward<decltype(curr)>(curr)...); }, internal::uses_allocator_construction<Type>::args(allocator, std::forward<Args>(args)...));
+}
+
+} // namespace entt
+
+#endif

src/entt/core/monostate.hpp

@@ -1,14 +1,11 @@
 #ifndef ENTT_CORE_MONOSTATE_HPP
 #define ENTT_CORE_MONOSTATE_HPP
 
-
 #include "../config/config.h"
 #include "fwd.hpp"
 
-
 namespace entt {
 
-
 /**
  * @brief Minimal implementation of the monostate pattern.
  *
@@ -47,7 +44,6 @@ private:
     inline static ENTT_MAYBE_ATOMIC(Type) value{};
 };
 
-
 /**
  * @brief Helper variable template.
  * @tparam Value Value used to differentiate between different variables.
@@ -55,8 +51,6 @@ private:
 template<id_type Value>
 inline monostate<Value> monostate_v = {};
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/core/tuple.hpp

@@ -0,0 +1,29 @@
+#ifndef ENTT_CORE_TUPLE_HPP
+#define ENTT_CORE_TUPLE_HPP
+
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include "../config/config.h"
+
+namespace entt {
+
+/**
+ * @brief Utility function to unwrap tuples of a single element.
+ * @tparam Type Tuple type of any sizes.
+ * @param value A tuple object of the given type.
+ * @return The tuple itself if it contains more than one element, the first
+ * element otherwise.
+ */
+template<typename Type>
+constexpr decltype(auto) unwrap_tuple(Type &&value) ENTT_NOEXCEPT {
+    if constexpr(std::tuple_size_v<std::remove_reference_t<Type>> == 1u) {
+        return std::get<0>(std::forward<Type>(value));
+    } else {
+        return std::forward<Type>(value);
+    }
+}
+
+} // namespace entt
+
+#endif

src/entt/core/type_info.hpp

@@ -1,40 +1,35 @@
 #ifndef ENTT_CORE_TYPE_INFO_HPP
 #define ENTT_CORE_TYPE_INFO_HPP
 
-
 #include <string_view>
 #include <type_traits>
+#include <utility>
 #include "../config/config.h"
 #include "../core/attribute.h"
-#include "hashed_string.hpp"
 #include "fwd.hpp"
-
+#include "hashed_string.hpp"
 
 namespace entt {
 
-
 /**
  * @cond TURN_OFF_DOXYGEN
  * Internal details not to be documented.
  */
 
-
 namespace internal {
 
-
-struct ENTT_API type_seq final {
+struct ENTT_API type_index final {
     [[nodiscard]] static id_type next() ENTT_NOEXCEPT {
         static ENTT_MAYBE_ATOMIC(id_type) value{};
         return value++;
     }
 };
 
-
 template<typename Type>
 [[nodiscard]] constexpr auto stripped_type_name() ENTT_NOEXCEPT {
 #if defined ENTT_PRETTY_FUNCTION
     std::string_view pretty_function{ENTT_PRETTY_FUNCTION};
-    auto first = pretty_function.find_first_not_of(' ', pretty_function.find_first_of(ENTT_PRETTY_FUNCTION_PREFIX)+1);
+    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);
     return value;
 #else
@@ -42,21 +37,18 @@ template<typename Type>
 #endif
 }
 
-
 template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
 [[nodiscard]] static constexpr std::string_view type_name(int) ENTT_NOEXCEPT {
     constexpr auto value = stripped_type_name<Type>();
     return value;
 }
 
-
 template<typename Type>
 [[nodiscard]] static std::string_view type_name(char) ENTT_NOEXCEPT {
     static const auto value = stripped_type_name<Type>();
     return value;
 }
 
-
 template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
 [[nodiscard]] static constexpr id_type type_hash(int) ENTT_NOEXCEPT {
     constexpr auto stripped = stripped_type_name<Type>();
@@ -64,7 +56,6 @@ template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
     return value;
 }
 
-
 template<typename Type>
 [[nodiscard]] static id_type type_hash(char) ENTT_NOEXCEPT {
     static const auto value = [](const auto stripped) {
@@ -73,36 +64,34 @@ template<typename Type>
     return value;
 }
 
-
-}
-
+} // namespace internal
 
 /**
  * Internal details not to be documented.
  * @endcond
  */
 
-
 /**
  * @brief Type sequential identifier.
  * @tparam Type Type for which to generate a sequential identifier.
  */
 template<typename Type, typename = void>
-struct ENTT_API type_seq final {
+struct ENTT_API type_index final {
     /**
      * @brief Returns the sequential identifier of a given type.
      * @return The sequential identifier of a given type.
      */
     [[nodiscard]] static id_type value() ENTT_NOEXCEPT {
-        static const id_type value = internal::type_seq::next();
+        static const id_type value = internal::type_index::next();
         return value;
     }
 
     /*! @copydoc value */
-    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT { return value(); }
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT {
+        return value();
+    }
 };
 
-
 /**
  * @brief Type hash.
  * @tparam Type Type for which to generate a hash value.
@@ -118,15 +107,16 @@ struct type_hash final {
         return internal::type_hash<Type>(0);
 #else
     [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
-        return type_seq<Type>::value();
+        return type_index<Type>::value();
 #endif
     }
 
     /*! @copydoc value */
-    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT { return value(); }
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT {
+        return value();
+    }
 };
 
-
 /**
  * @brief Type name.
  * @tparam Type Type for which to generate a name.
@@ -142,119 +132,143 @@ struct type_name final {
     }
 
     /*! @copydoc value */
-    [[nodiscard]] constexpr operator std::string_view() const ENTT_NOEXCEPT { return value(); }
+    [[nodiscard]] constexpr operator std::string_view() const ENTT_NOEXCEPT {
+        return value();
+    }
 };
 
-
 /*! @brief Implementation specific information about a type. */
-class type_info final {
-    template<typename>
-    friend type_info type_id() ENTT_NOEXCEPT;
-
-    type_info(id_type seq_v, id_type hash_v, std::string_view name_v) ENTT_NOEXCEPT
-        : seq_value{seq_v},
-          hash_value{hash_v},
-          name_value{name_v}
-    {}
-
-public:
-    /*! @brief Default constructor. */
-    type_info() ENTT_NOEXCEPT
-        : type_info({}, {}, {})
-    {}
-
-    /*! @brief Default copy constructor. */
-    type_info(const type_info &) ENTT_NOEXCEPT = default;
-    /*! @brief Default move constructor. */
-    type_info(type_info &&) ENTT_NOEXCEPT = default;
+struct type_info final {
+    /**
+     * @brief Constructs a type info object for a given type.
+     * @tparam Type Type for which to construct a type info object.
+     */
+    template<typename Type>
+    constexpr type_info(std::in_place_type_t<Type>) ENTT_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()} {}
 
     /**
-     * @brief Default copy assignment operator.
-     * @return This type info object.
+     * @brief Type index.
+     * @return Type index.
      */
-    type_info & operator=(const type_info &) ENTT_NOEXCEPT = default;
-
-    /**
-     * @brief Default move assignment operator.
-     * @return This type info object.
-     */
-    type_info & operator=(type_info &&) ENTT_NOEXCEPT = default;
-
-    /**
-     * @brief Checks if a type info object is properly initialized.
-     * @return True if the object is properly initialized, false otherwise.
-     */
-    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
-        return name_value.data() != nullptr;
-    }
-
-    /**
-     * @brief Type sequential identifier.
-     * @return Type sequential identifier.
-     */
-    [[nodiscard]] id_type seq() const ENTT_NOEXCEPT {
-        return seq_value;
+    [[nodiscard]] constexpr id_type index() const ENTT_NOEXCEPT {
+        return seq;
     }
 
     /**
      * @brief Type hash.
      * @return Type hash.
      */
-    [[nodiscard]] id_type hash() const ENTT_NOEXCEPT {
-        return hash_value;
+    [[nodiscard]] constexpr id_type hash() const ENTT_NOEXCEPT {
+        return identifier;
     }
 
     /**
      * @brief Type name.
      * @return Type name.
      */
-    [[nodiscard]] std::string_view name() const ENTT_NOEXCEPT {
-        return name_value;
-    }
-
-    /**
-     * @brief Compares the contents of two type info objects.
-     * @param other Object with which to compare.
-     * @return False if the two contents differ, true otherwise.
-     */
-    [[nodiscard]] bool operator==(const type_info &other) const ENTT_NOEXCEPT {
-        return hash_value == other.hash_value;
+    [[nodiscard]] constexpr std::string_view name() const ENTT_NOEXCEPT {
+        return alias;
     }
 
 private:
-    id_type seq_value;
-    id_type hash_value;
-    std::string_view name_value;
+    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 contents differ, false otherwise.
+ * @return True if the two type info objects are identical, false otherwise.
  */
-[[nodiscard]] inline bool operator!=(const type_info &lhs, const type_info &rhs) ENTT_NOEXCEPT {
+[[nodiscard]] inline constexpr bool operator==(const type_info &lhs, const type_info &rhs) ENTT_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) ENTT_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]] constexpr bool operator<(const type_info &lhs, const type_info &rhs) ENTT_NOEXCEPT {
+    return lhs.index() < rhs.index();
+}
 
 /**
- * @brief Returns the type info object for a given type.
+ * @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]] constexpr bool operator<=(const type_info &lhs, const type_info &rhs) ENTT_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]] constexpr bool operator>(const type_info &lhs, const type_info &rhs) ENTT_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]] constexpr bool operator>=(const type_info &lhs, const type_info &rhs) ENTT_NOEXCEPT {
+    return !(lhs < rhs);
+}
+
+/**
+ * @brief Returns the type info object associated to a given type.
+ *
+ * The returned element refers to an object with static storage duration.<br/>
+ * The type doesn't need to be a complete type. If the type is a reference, the
+ * result refers to the referenced type. In all cases, top-level cv-qualifiers
+ * are ignored.
+ *
  * @tparam Type Type for which to generate a type info object.
- * @return The type info object for the given type.
+ * @return A reference to a properly initialized type info object.
  */
 template<typename Type>
-[[nodiscard]] type_info type_id() ENTT_NOEXCEPT {
-    return type_info{
-        type_seq<std::remove_cv_t<std::remove_reference_t<Type>>>::value(),
-        type_hash<std::remove_cv_t<std::remove_reference_t<Type>>>::value(),
-        type_name<std::remove_cv_t<std::remove_reference_t<Type>>>::value()
-    };
+[[nodiscard]] const type_info &type_id() ENTT_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>};
+        return instance;
+    } else {
+        return type_id<std::remove_cv_t<std::remove_reference_t<Type>>>();
+    }
 }
 
-
+/*! @copydoc type_id */
+template<typename Type>
+[[nodiscard]] const type_info &type_id(Type &&) ENTT_NOEXCEPT {
+    return type_id<std::remove_cv_t<std::remove_reference_t<Type>>>();
 }
 
+} // namespace entt
 
 #endif

src/entt/core/type_traits.hpp

@@ -1,7 +1,6 @@
 #ifndef ENTT_CORE_TYPE_TRAITS_HPP
 #define ENTT_CORE_TYPE_TRAITS_HPP
 
-
 #include <cstddef>
 #include <iterator>
 #include <type_traits>
@@ -9,10 +8,8 @@
 #include "../config/config.h"
 #include "fwd.hpp"
 
-
 namespace entt {
 
-
 /**
  * @brief Utility class to disambiguate overloaded functions.
  * @tparam N Number of choices available.
@@ -20,17 +17,13 @@ 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 TURN_OFF_DOXYGEN */ choice_t<N - 1> /*! @endcond */
 {};
 
-
 /*! @copybrief choice_t */
 template<>
 struct choice_t<0> {};
 
-
 /**
  * @brief Variable template for the choice trick.
  * @tparam N Number of choices available.
@@ -38,7 +31,6 @@ struct choice_t<0> {};
 template<std::size_t N>
 inline constexpr choice_t<N> choice{};
 
-
 /**
  * @brief Identity type trait.
  *
@@ -53,7 +45,6 @@ struct type_identity {
     using type = Type;
 };
 
-
 /**
  * @brief Helper type.
  * @tparam Type A type.
@@ -61,7 +52,6 @@ struct type_identity {
 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.
@@ -70,30 +60,25 @@ using type_identity_t = typename type_identity<Type>::type;
 template<typename Type, typename = void>
 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))>>
-    : std::integral_constant<std::size_t, sizeof(Type)>
-{};
-
+    : std::integral_constant<std::size_t, sizeof(Type)> {};
 
 /**
  * @brief Helper variable template.
  * @tparam Type The type of which to return the size.
  */
-template<class Type>
+template<typename Type>
 inline constexpr std::size_t size_of_v = size_of<Type>::value;
 
-
 /**
  * @brief Using declaration to be used to _repeat_ the same type a number of
  * times equal to the size of a given parameter pack.
  * @tparam Type A type to repeat.
  */
 template<typename Type, typename>
-using unpack_as_t = Type;
-
+using unpack_as_type = Type;
 
 /**
  * @brief Helper variable template to be used to _repeat_ the same value a
@@ -101,8 +86,7 @@ using unpack_as_t = Type;
  * @tparam Value A value to repeat.
  */
 template<auto Value, typename>
-inline constexpr auto unpack_as_v = Value;
-
+inline constexpr auto unpack_as_value = Value;
 
 /**
  * @brief Wraps a static constant.
@@ -111,7 +95,6 @@ inline constexpr auto unpack_as_v = Value;
 template<auto Value>
 using integral_constant = std::integral_constant<decltype(Value), Value>;
 
-
 /**
  * @brief Alias template to facilitate the creation of named values.
  * @tparam Value A constant value at least convertible to `id_type`.
@@ -119,7 +102,6 @@ using integral_constant = std::integral_constant<decltype(Value), Value>;
 template<id_type Value>
 using tag = integral_constant<Value>;
 
-
 /**
  * @brief A class to use to push around lists of types, nothing more.
  * @tparam Type Types provided by the type list.
@@ -132,12 +114,10 @@ struct type_list {
     static constexpr auto size = sizeof...(Type);
 };
 
-
 /*! @brief Primary template isn't defined on purpose. */
 template<std::size_t, typename>
 struct type_list_element;
 
-
 /**
  * @brief Provides compile-time indexed access to the types of a type list.
  * @tparam Index Index of the type to return.
@@ -146,9 +126,7 @@ struct type_list_element;
  */
 template<std::size_t Index, typename Type, typename... Other>
 struct type_list_element<Index, type_list<Type, Other...>>
-    : type_list_element<Index - 1u, type_list<Other...>>
-{};
-
+    : type_list_element<Index - 1u, type_list<Other...>> {};
 
 /**
  * @brief Provides compile-time indexed access to the types of a type list.
@@ -161,7 +139,6 @@ struct type_list_element<0u, type_list<Type, Other...>> {
     using type = Type;
 };
 
-
 /**
  * @brief Helper type.
  * @tparam Index Index of the type to return.
@@ -170,7 +147,6 @@ struct type_list_element<0u, type_list<Type, Other...>> {
 template<std::size_t Index, typename List>
 using type_list_element_t = typename type_list_element<Index, List>::type;
 
-
 /**
  * @brief Concatenates multiple type lists.
  * @tparam Type Types provided by the first type list.
@@ -178,14 +154,14 @@ using type_list_element_t = typename type_list_element<Index, List>::type;
  * @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...>) { return {}; }
-
+constexpr type_list<Type..., Other...> operator+(type_list<Type...>, type_list<Other...>) {
+    return {};
+}
 
 /*! @brief Primary template isn't defined on purpose. */
 template<typename...>
 struct type_list_cat;
 
-
 /*! @brief Concatenates multiple type lists. */
 template<>
 struct type_list_cat<> {
@@ -193,7 +169,6 @@ struct type_list_cat<> {
     using type = type_list<>;
 };
 
-
 /**
  * @brief Concatenates multiple type lists.
  * @tparam Type Types provided by the first type list.
@@ -206,7 +181,6 @@ struct type_list_cat<type_list<Type...>, type_list<Other...>, List...> {
     using type = typename type_list_cat<type_list<Type..., Other...>, List...>::type;
 };
 
-
 /**
  * @brief Concatenates multiple type lists.
  * @tparam Type Types provided by the type list.
@@ -217,7 +191,6 @@ struct type_list_cat<type_list<Type...>> {
     using type = type_list<Type...>;
 };
 
-
 /**
  * @brief Helper type.
  * @tparam List Type lists to concatenate.
@@ -225,12 +198,10 @@ struct type_list_cat<type_list<Type...>> {
 template<typename... List>
 using type_list_cat_t = typename type_list_cat<List...>::type;
 
-
 /*! @brief Primary template isn't defined on purpose. */
 template<typename>
 struct type_list_unique;
 
-
 /**
  * @brief Removes duplicates types from a type list.
  * @tparam Type One of the types provided by the given type list.
@@ -240,13 +211,11 @@ template<typename Type, typename... Other>
 struct type_list_unique<type_list<Type, Other...>> {
     /*! @brief A type list without duplicate types. */
     using type = std::conditional_t<
-        std::disjunction_v<std::is_same<Type, Other>...>,
+        (std::is_same_v<Type, Other> || ...),
         typename type_list_unique<type_list<Other...>>::type,
-        type_list_cat_t<type_list<Type>, typename type_list_unique<type_list<Other...>>::type>
-    >;
+        type_list_cat_t<type_list<Type>, typename type_list_unique<type_list<Other...>>::type>>;
 };
 
-
 /*! @brief Removes duplicates types from a type list. */
 template<>
 struct type_list_unique<type_list<>> {
@@ -254,7 +223,6 @@ struct type_list_unique<type_list<>> {
     using type = type_list<>;
 };
 
-
 /**
  * @brief Helper type.
  * @tparam Type A type list.
@@ -262,7 +230,6 @@ struct type_list_unique<type_list<>> {
 template<typename Type>
 using type_list_unique_t = typename type_list_unique<Type>::type;
 
-
 /**
  * @brief Provides the member constant `value` to true if a type list contains a
  * given type, false otherwise.
@@ -272,7 +239,6 @@ using type_list_unique_t = typename type_list_unique<Type>::type;
 template<typename List, typename Type>
 struct type_list_contains;
 
-
 /**
  * @copybrief type_list_contains
  * @tparam Type Types provided by the type list.
@@ -281,21 +247,18 @@ struct type_list_contains;
 template<typename... Type, typename Other>
 struct type_list_contains<type_list<Type...>, Other>: std::disjunction<std::is_same<Type, Other>...> {};
 
-
 /**
  * @brief Helper variable template.
  * @tparam List Type list.
  * @tparam Type Type to look for.
  */
-template<class List, typename Type>
+template<typename List, typename Type>
 inline constexpr bool type_list_contains_v = type_list_contains<List, Type>::value;
 
-
 /*! @brief Primary template isn't defined on purpose. */
 template<typename...>
 struct type_list_diff;
 
-
 /**
  * @brief Computes the difference between two type lists.
  * @tparam Type Types provided by the first type list.
@@ -307,7 +270,6 @@ struct type_list_diff<type_list<Type...>, type_list<Other...>> {
     using type = type_list_cat_t<std::conditional_t<type_list_contains_v<type_list<Other...>, Type>, type_list<>, type_list<Type>>...>;
 };
 
-
 /**
  * @brief Helper type.
  * @tparam List Type lists between which to compute the difference.
@@ -315,7 +277,6 @@ struct type_list_diff<type_list<Type...>, type_list<Other...>> {
 template<typename... List>
 using type_list_diff_t = typename type_list_diff<List...>::type;
 
-
 /**
  * @brief A class to use to push around lists of constant values, nothing more.
  * @tparam Value Values provided by the value list.
@@ -328,12 +289,10 @@ struct value_list {
     static constexpr auto size = sizeof...(Value);
 };
 
-
 /*! @brief Primary template isn't defined on purpose. */
 template<std::size_t, typename>
 struct value_list_element;
 
-
 /**
  * @brief Provides compile-time indexed access to the values of a value list.
  * @tparam Index Index of the value to return.
@@ -342,9 +301,7 @@ struct value_list_element;
  */
 template<std::size_t Index, auto Value, auto... Other>
 struct value_list_element<Index, value_list<Value, Other...>>
-    : value_list_element<Index - 1u, value_list<Other...>>
-{};
-
+    : value_list_element<Index - 1u, value_list<Other...>> {};
 
 /**
  * @brief Provides compile-time indexed access to the types of a type list.
@@ -357,7 +314,6 @@ struct value_list_element<0u, value_list<Value, Other...>> {
     static constexpr auto value = Value;
 };
 
-
 /**
  * @brief Helper type.
  * @tparam Index Index of the value to return.
@@ -366,7 +322,6 @@ struct value_list_element<0u, value_list<Value, Other...>> {
 template<std::size_t Index, typename List>
 inline constexpr auto value_list_element_v = value_list_element<Index, List>::value;
 
-
 /**
  * @brief Concatenates multiple value lists.
  * @tparam Value Values provided by the first value list.
@@ -374,14 +329,14 @@ inline constexpr auto value_list_element_v = value_list_element<Index, List>::va
  * @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...>) { return {}; }
-
+constexpr value_list<Value..., Other...> operator+(value_list<Value...>, value_list<Other...>) {
+    return {};
+}
 
 /*! @brief Primary template isn't defined on purpose. */
 template<typename...>
 struct value_list_cat;
 
-
 /*! @brief Concatenates multiple value lists. */
 template<>
 struct value_list_cat<> {
@@ -389,7 +344,6 @@ struct value_list_cat<> {
     using type = value_list<>;
 };
 
-
 /**
  * @brief Concatenates multiple value lists.
  * @tparam Value Values provided by the first value list.
@@ -402,7 +356,6 @@ struct value_list_cat<value_list<Value...>, value_list<Other...>, List...> {
     using type = typename value_list_cat<value_list<Value..., Other...>, List...>::type;
 };
 
-
 /**
  * @brief Concatenates multiple value lists.
  * @tparam Value Values provided by the value list.
@@ -413,7 +366,6 @@ struct value_list_cat<value_list<Value...>> {
     using type = value_list<Value...>;
 };
 
-
 /**
  * @brief Helper type.
  * @tparam List Value lists to concatenate.
@@ -421,74 +373,10 @@ struct value_list_cat<value_list<Value...>> {
 template<typename... List>
 using value_list_cat_t = typename value_list_cat<List...>::type;
 
-
-/**
- * @cond TURN_OFF_DOXYGEN
- * Internal details not to be documented.
- */
-
-
-namespace internal {
-
-
-template<typename>
-[[nodiscard]] constexpr bool is_equality_comparable(...) { return false; }
-
-
-template<typename Type>
-[[nodiscard]] constexpr auto is_equality_comparable(choice_t<0>)
--> decltype(std::declval<Type>() == std::declval<Type>()) { return true; }
-
-
-template<typename Type>
-[[nodiscard]] constexpr auto is_equality_comparable(choice_t<1>)
--> decltype(std::declval<typename Type::value_type>(), std::declval<Type>() == std::declval<Type>()) {
-    if constexpr(std::is_same_v<typename Type::value_type, Type>) {
-        return is_equality_comparable<Type>(choice<0>);
-    } else {
-        return is_equality_comparable<typename Type::value_type>(choice<2>);
-    }
-}
-
-
-template<typename Type>
-[[nodiscard]] constexpr auto is_equality_comparable(choice_t<2>)
--> decltype(std::declval<typename Type::mapped_type>(), std::declval<Type>() == std::declval<Type>()) {
-    return is_equality_comparable<typename Type::key_type>(choice<2>) && is_equality_comparable<typename Type::mapped_type>(choice<2>);
-}
-
-
-}
-
-
-/**
- * Internal details not to be documented.
- * @endcond
- */
-
-
-/**
- * @brief Provides the member constant `value` to true if a given type is
- * equality comparable, false otherwise.
- * @tparam Type The type to test.
- */
-template<typename Type, typename = void>
-struct is_equality_comparable: std::bool_constant<internal::is_equality_comparable<Type>(choice<2>)> {};
-
-
-/**
- * @brief Helper variable template.
- * @tparam Type The type to test.
- */
-template<class Type>
-inline constexpr bool is_equality_comparable_v = is_equality_comparable<Type>::value;
-
-
 /*! @brief Same as std::is_invocable, but with tuples. */
 template<typename, typename>
 struct is_applicable: std::false_type {};
 
-
 /**
  * @copybrief is_applicable
  * @tparam Func A valid function type.
@@ -498,7 +386,6 @@ struct is_applicable: std::false_type {};
 template<typename Func, template<typename...> class Tuple, typename... Args>
 struct is_applicable<Func, Tuple<Args...>>: std::is_invocable<Func, Args...> {};
 
-
 /**
  * @copybrief is_applicable
  * @tparam Func A valid function type.
@@ -508,7 +395,6 @@ struct is_applicable<Func, Tuple<Args...>>: std::is_invocable<Func, Args...> {};
 template<typename Func, template<typename...> class Tuple, typename... Args>
 struct is_applicable<Func, const Tuple<Args...>>: std::is_invocable<Func, Args...> {};
 
-
 /**
  * @brief Helper variable template.
  * @tparam Func A valid function type.
@@ -517,12 +403,10 @@ struct is_applicable<Func, const Tuple<Args...>>: std::is_invocable<Func, Args..
 template<typename Func, typename Args>
 inline constexpr bool is_applicable_v = is_applicable<Func, Args>::value;
 
-
 /*! @brief Same as std::is_invocable_r, but with tuples for arguments. */
 template<typename, typename, typename>
 struct is_applicable_r: std::false_type {};
 
-
 /**
  * @copybrief is_applicable_r
  * @tparam Ret The type to which the return type of the function should be
@@ -533,7 +417,6 @@ struct is_applicable_r: std::false_type {};
 template<typename Ret, typename Func, typename... Args>
 struct is_applicable_r<Ret, Func, std::tuple<Args...>>: std::is_invocable_r<Ret, Func, Args...> {};
 
-
 /**
  * @brief Helper variable template.
  * @tparam Ret The type to which the return type of the function should be
@@ -544,7 +427,6 @@ struct is_applicable_r<Ret, Func, std::tuple<Args...>>: std::is_invocable_r<Ret,
 template<typename Ret, typename Func, typename Args>
 inline constexpr bool is_applicable_r_v = is_applicable_r<Ret, Func, Args>::value;
 
-
 /**
  * @brief Provides the member constant `value` to true if a given type is
  * complete, false otherwise.
@@ -553,12 +435,10 @@ inline constexpr bool is_applicable_r_v = is_applicable_r<Ret, Func, Args>::valu
 template<typename Type, typename = void>
 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 {};
 
-
 /**
  * @brief Helper variable template.
  * @tparam Type The type to test.
@@ -566,7 +446,6 @@ struct is_complete<Type, std::void_t<decltype(sizeof(Type))>>: std::true_type {}
 template<typename Type>
 inline constexpr bool is_complete_v = is_complete<Type>::value;
 
-
 /**
  * @brief Provides the member constant `value` to true if a given type is an
  * iterator, false otherwise.
@@ -575,13 +454,30 @@ inline constexpr bool is_complete_v = is_complete<Type>::value;
 template<typename Type, typename = void>
 struct is_iterator: std::false_type {};
 
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename, typename = void>
+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 {};
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
 
 /*! @copydoc is_iterator */
 template<typename Type>
-struct is_iterator<Type, std::void_t<typename std::iterator_traits<Type>::iterator_category>>
-    : std::true_type
-{};
-
+struct is_iterator<Type, std::enable_if_t<!std::is_same_v<std::remove_const_t<std::remove_pointer_t<Type>>, void>>>
+    : internal::has_iterator_category<Type> {};
 
 /**
  * @brief Helper variable template.
@@ -590,39 +486,110 @@ struct is_iterator<Type, std::void_t<typename std::iterator_traits<Type>::iterat
 template<typename Type>
 inline constexpr bool is_iterator_v = is_iterator<Type>::value;
 
-
 /**
- * @brief Provides the member constant `value` to true if a given type is of the
- * required iterator type, false otherwise.
- * @tparam Type The type to test.
- * @tparam It Required iterator type.
+ * @brief Provides the member constant `value` to true if a given type is both
+ * an empty and non-final class, false otherwise.
+ * @tparam Type The type to test
  */
-template<typename Type, typename It, typename = void>
-struct is_iterator_type: std::false_type {};
-
-
-/*! @copydoc is_iterator_type */
-template<typename Type, typename It>
-struct is_iterator_type<Type, It, std::enable_if_t<is_iterator_v<Type> && std::is_same_v<Type, It>>>
-    : std::true_type
-{};
-
-
-/*! @copydoc is_iterator_type */
-template<typename Type, typename It>
-struct is_iterator_type<Type, It, std::enable_if_t<!std::is_same_v<Type, It>, std::void_t<typename It::iterator_type>>>
-    : is_iterator_type<Type, typename It::iterator_type>
-{};
-
+template<typename Type>
+struct is_ebco_eligible
+    : std::conjunction<std::is_empty<Type>, std::negation<std::is_final<Type>>> {};
 
 /**
  * @brief Helper variable template.
  * @tparam Type The type to test.
- * @tparam It Required iterator type.
  */
-template<typename Type, typename It>
-inline constexpr bool is_iterator_type_v = is_iterator_type<Type, It>::value;
+template<typename Type>
+inline constexpr bool is_ebco_eligible_v = is_ebco_eligible<Type>::value;
 
+/**
+ * @brief Provides the member constant `value` to true if `Type::is_transparent`
+ * is valid and denotes a type, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+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 {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_transparent_v = is_transparent<Type>::value;
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * equality comparable, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_equality_comparable: std::false_type {};
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename, typename = void>
+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 {};
+
+template<typename Type, std::size_t... Index>
+[[nodiscard]] constexpr bool unpack_maybe_equality_comparable(std::index_sequence<Index...>) {
+    return (is_equality_comparable<std::tuple_element_t<Index, Type>>::value && ...);
+}
+
+template<typename>
+[[nodiscard]] constexpr bool maybe_equality_comparable(choice_t<0>) {
+    return true;
+}
+
+template<typename Type>
+[[nodiscard]] constexpr auto maybe_equality_comparable(choice_t<1>) -> decltype(std::declval<typename Type::value_type>(), bool{}) {
+    if constexpr(is_iterator_v<Type>) {
+        return true;
+    } else if constexpr(std::is_same_v<typename Type::value_type, Type>) {
+        return maybe_equality_comparable<Type>(choice<0>);
+    } else {
+        return is_equality_comparable<typename Type::value_type>::value;
+    }
+}
+
+template<typename Type>
+[[nodiscard]] constexpr std::enable_if_t<is_complete_v<std::tuple_size<std::remove_const_t<Type>>>, bool> maybe_equality_comparable(choice_t<2>) {
+    if constexpr(has_tuple_size_value<Type>::value) {
+        return unpack_maybe_equality_comparable<Type>(std::make_index_sequence<std::tuple_size<Type>::value>{});
+    } else {
+        return maybe_equality_comparable<Type>(choice<1>);
+    }
+}
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/*! @copydoc is_equality_comparable */
+template<typename Type>
+struct is_equality_comparable<Type, std::void_t<decltype(std::declval<Type>() == std::declval<Type>())>>
+    : std::bool_constant<internal::maybe_equality_comparable<Type>(choice<2>)> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_equality_comparable_v = is_equality_comparable<Type>::value;
 
 /**
  * @brief Transcribes the constness of a type to another type.
@@ -635,7 +602,6 @@ struct constness_as {
     using type = std::remove_const_t<To>;
 };
 
-
 /*! @copydoc constness_as */
 template<typename To, typename From>
 struct constness_as<To, const From> {
@@ -643,7 +609,6 @@ struct constness_as<To, const From> {
     using type = std::add_const_t<To>;
 };
 
-
 /**
  * @brief Alias template to facilitate the transcription of the constness.
  * @tparam To The type to which to transcribe the constness.
@@ -652,7 +617,6 @@ struct constness_as<To, const From> {
 template<typename To, typename From>
 using constness_as_t = typename constness_as<To, From>::type;
 
-
 /**
  * @brief Extracts the class of a non-static member object or function.
  * @tparam Member A pointer to a non-static member object or function.
@@ -662,20 +626,19 @@ class member_class {
     static_assert(std::is_member_pointer_v<Member>, "Invalid pointer type to non-static member object or function");
 
     template<typename Class, typename Ret, typename... Args>
-    static Class * clazz(Ret(Class:: *)(Args...));
+    static Class *clazz(Ret (Class::*)(Args...));
 
     template<typename Class, typename Ret, typename... Args>
-    static Class * clazz(Ret(Class:: *)(Args...) const);
+    static Class *clazz(Ret (Class::*)(Args...) const);
 
     template<typename Class, typename Type>
-    static Class * clazz(Type Class:: *);
+    static Class *clazz(Type Class::*);
 
 public:
     /*! @brief The class of the given non-static member object or function. */
     using type = std::remove_pointer_t<decltype(clazz(std::declval<Member>()))>;
 };
 
-
 /**
  * @brief Helper type.
  * @tparam Member A pointer to a non-static member object or function.
@@ -683,8 +646,6 @@ public:
 template<typename Member>
 using member_class_t = typename member_class<Member>::type;
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/core/utility.hpp

@@ -1,16 +1,16 @@
 #ifndef ENTT_CORE_UTILITY_HPP
 #define ENTT_CORE_UTILITY_HPP
 
-
 #include <utility>
 #include "../config/config.h"
 
-
 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.
@@ -18,12 +18,11 @@ struct identity {
      * @return The submitted value as-is.
      */
     template<class Type>
-    [[nodiscard]] constexpr Type && operator()(Type &&value) const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr Type &&operator()(Type &&value) const ENTT_NOEXCEPT {
         return std::forward<Type>(value);
     }
 };
 
-
 /**
  * @brief Constant utility to disambiguate overloaded members of a class.
  * @tparam Type Type of the desired overload.
@@ -32,8 +31,9 @@ struct identity {
  * @return Pointer to the member.
  */
 template<typename Type, typename Class>
-[[nodiscard]] constexpr auto overload(Type Class:: *member) ENTT_NOEXCEPT { return member; }
-
+[[nodiscard]] constexpr auto overload(Type Class::*member) ENTT_NOEXCEPT {
+    return member;
+}
 
 /**
  * @brief Constant utility to disambiguate overloaded functions.
@@ -42,8 +42,9 @@ template<typename Type, typename Class>
  * @return Pointer to the function.
  */
 template<typename Func>
-[[nodiscard]] constexpr auto overload(Func *func) ENTT_NOEXCEPT { return func; }
-
+[[nodiscard]] constexpr auto overload(Func *func) ENTT_NOEXCEPT {
+    return func;
+}
 
 /**
  * @brief Helper type for visitors.
@@ -54,15 +55,12 @@ struct overloaded: Func... {
     using Func::operator()...;
 };
 
-
 /**
  * @brief Deduction guide.
  * @tparam Func Types of function objects.
  */
 template<class... Func>
-overloaded(Func...)
--> overloaded<Func...>;
-
+overloaded(Func...) -> overloaded<Func...>;
 
 /**
  * @brief Basic implementation of a y-combinator.
@@ -74,9 +72,8 @@ struct y_combinator {
      * @brief Constructs a y-combinator from a given function.
      * @param recursive A potentially recursive function.
      */
-    y_combinator(Func recursive):
-        func{std::move(recursive)}
-    {}
+    y_combinator(Func recursive)
+        : func{std::move(recursive)} {}
 
     /**
      * @brief Invokes a y-combinator and therefore its underlying function.
@@ -84,14 +81,14 @@ struct y_combinator {
      * @param args Parameters to use to invoke the underlying function.
      * @return Return value of the underlying function, if any.
      */
-    template <class... Args>
-    decltype(auto) operator()(Args &&... args) const {
+    template<class... Args>
+    decltype(auto) operator()(Args &&...args) const {
         return func(*this, std::forward<Args>(args)...);
     }
 
     /*! @copydoc operator()() */
-    template <class... Args>
-    decltype(auto) operator()(Args &&... args) {
+    template<class... Args>
+    decltype(auto) operator()(Args &&...args) {
         return func(*this, std::forward<Args>(args)...);
     }
 
@@ -99,8 +96,6 @@ private:
     Func func;
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/entity/component.hpp

@@ -1,34 +1,61 @@
 #ifndef ENTT_ENTITY_COMPONENT_HPP
 #define ENTT_ENTITY_COMPONENT_HPP
 
-
+#include <cstddef>
 #include <type_traits>
 #include "../config/config.h"
 
-
 namespace entt {
 
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
 
-/*! @brief Commonly used default traits for all types. */
-struct basic_component_traits {
-    /*! @brief Pointer stability, default is `std::false_type`. */
-    using in_place_delete = std::false_type;
-    /*! @brief Empty type optimization, default is `ENTT_IGNORE_IF_EMPTY`. */
-    using ignore_if_empty = ENTT_IGNORE_IF_EMPTY;
-};
+namespace internal {
 
+template<typename, typename = void>
+struct in_place_delete: std::false_type {};
+
+template<typename Type>
+struct in_place_delete<Type, std::enable_if_t<Type::in_place_delete>>
+    : std::true_type {};
+
+template<typename Type, typename = void>
+struct page_size: std::integral_constant<std::size_t, (ENTT_IGNORE_IF_EMPTY && std::is_empty_v<Type>) ? 0u : ENTT_PACKED_PAGE> {};
+
+template<typename Type>
+struct page_size<Type, std::enable_if_t<std::is_convertible_v<decltype(Type::page_size), std::size_t>>>
+    : std::integral_constant<std::size_t, Type::page_size> {};
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
 
 /**
  * @brief Common way to access various properties of components.
  * @tparam Type Type of component.
  */
 template<typename Type, typename = void>
-struct component_traits: basic_component_traits {
+struct component_traits {
     static_assert(std::is_same_v<std::decay_t<Type>, Type>, "Unsupported type");
+
+    /*! @brief Pointer stability, default is `false`. */
+    static constexpr bool in_place_delete = internal::in_place_delete<Type>::value;
+    /*! @brief Page size, default is `ENTT_PACKED_PAGE` for non-empty types. */
+    static constexpr std::size_t page_size = internal::page_size<Type>::value;
 };
 
+/**
+ * @brief Helper variable template.
+ * @tparam Type Type of component.
+ */
+template<class Type>
+inline constexpr bool ignore_as_empty_v = (component_traits<Type>::page_size == 0u);
 
-}
-
+} // namespace entt
 
 #endif

src/entt/entity/entity.hpp

@@ -1,91 +1,78 @@
 #ifndef ENTT_ENTITY_ENTITY_HPP
 #define ENTT_ENTITY_ENTITY_HPP
 
-
 #include <cstddef>
 #include <cstdint>
 #include <type_traits>
 #include "../config/config.h"
-
+#include "fwd.hpp"
 
 namespace entt {
 
-
 /**
  * @cond TURN_OFF_DOXYGEN
  * Internal details not to be documented.
  */
 
-
 namespace internal {
 
-
 template<typename, typename = void>
 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>>
-{};
-
+    : entt_traits<std::underlying_type_t<Type>> {};
 
 template<typename Type>
 struct entt_traits<Type, std::enable_if_t<std::is_class_v<Type>>>
-    : entt_traits<typename Type::entity_type>
-{};
-
+    : entt_traits<typename Type::entity_type> {};
 
 template<>
 struct entt_traits<std::uint32_t> {
     using entity_type = std::uint32_t;
     using version_type = std::uint16_t;
-    using difference_type = std::int64_t;
 
     static constexpr entity_type entity_mask = 0xFFFFF;
     static constexpr entity_type version_mask = 0xFFF;
     static constexpr std::size_t entity_shift = 20u;
 };
 
-
 template<>
 struct entt_traits<std::uint64_t> {
     using entity_type = std::uint64_t;
     using version_type = std::uint32_t;
-    using difference_type = std::int64_t;
 
     static constexpr entity_type entity_mask = 0xFFFFFFFF;
     static constexpr entity_type version_mask = 0xFFFFFFFF;
     static constexpr std::size_t entity_shift = 32u;
 };
 
-
-}
-
+} // namespace internal
 
 /**
  * Internal details not to be documented.
  * @endcond
  */
 
-
 /**
  * @brief Entity traits.
  * @tparam Type Type of identifier.
  */
 template<typename Type>
-class entt_traits: private internal::entt_traits<Type> {
-    using traits_type = internal::entt_traits<Type>;
+class entt_traits: internal::entt_traits<Type> {
+    using base_type = internal::entt_traits<Type>;
 
 public:
     /*! @brief Value type. */
     using value_type = Type;
     /*! @brief Underlying entity type. */
-    using entity_type = typename traits_type::entity_type;
+    using entity_type = typename base_type::entity_type;
     /*! @brief Underlying version type. */
-    using version_type = typename traits_type::version_type;
-    /*! @brief Difference type. */
-    using difference_type = typename traits_type::difference_type;
+    using version_type = typename base_type::version_type;
+    /*! @brief Reserved identifier. */
+    static constexpr entity_type reserved = base_type::entity_mask | (base_type::version_mask << base_type::entity_shift);
+    /*! @brief Page size, default is `ENTT_SPARSE_PAGE`. */
+    static constexpr auto page_size = ENTT_SPARSE_PAGE;
 
     /**
      * @brief Converts an entity to its underlying type.
@@ -102,7 +89,7 @@ public:
      * @return The integral representation of the entity part.
      */
     [[nodiscard]] static constexpr entity_type to_entity(const value_type value) ENTT_NOEXCEPT {
-        return (to_integral(value) & traits_type::entity_mask);
+        return (to_integral(value) & base_type::entity_mask);
     }
 
     /**
@@ -111,8 +98,7 @@ public:
      * @return The integral representation of the version part.
      */
     [[nodiscard]] static constexpr version_type to_version(const value_type value) ENTT_NOEXCEPT {
-        constexpr auto mask = (traits_type::version_mask << traits_type::entity_shift);
-        return ((to_integral(value) & mask) >> traits_type::entity_shift);
+        return (to_integral(value) >> base_type::entity_shift);
     }
 
     /**
@@ -125,34 +111,64 @@ public:
      * @param version The version part of the identifier.
      * @return A properly constructed identifier.
      */
-    [[nodiscard]] static constexpr value_type construct(const entity_type entity = traits_type::entity_mask, const version_type version = traits_type::version_mask) ENTT_NOEXCEPT {
-        return value_type{(entity & traits_type::entity_mask) | (version << traits_type::entity_shift)};
+    [[nodiscard]] static constexpr value_type construct(const entity_type entity, const version_type version) ENTT_NOEXCEPT {
+        return value_type{(entity & base_type::entity_mask) | (static_cast<entity_type>(version) << base_type::entity_shift)};
+    }
+
+    /**
+     * @brief Combines two identifiers in a single one.
+     *
+     * The returned identifier is a copy of the first element except for its
+     * version, which is taken from the second element.
+     *
+     * @param lhs The identifier from which to take the entity part.
+     * @param rhs The identifier from which to take the version part.
+     * @return A properly constructed identifier.
+     */
+    [[nodiscard]] static constexpr value_type combine(const entity_type lhs, const entity_type rhs) ENTT_NOEXCEPT {
+        constexpr auto mask = (base_type::version_mask << base_type::entity_shift);
+        return value_type{(lhs & base_type::entity_mask) | (rhs & mask)};
     }
 };
 
-
 /**
- * @brief Converts an entity to its underlying type.
+ * @copydoc entt_traits<Entity>::to_integral
  * @tparam Entity The value type.
- * @param entity The value to convert.
- * @return The integral representation of the given value.
  */
 template<typename Entity>
-[[nodiscard]] constexpr auto to_integral(const Entity entity) ENTT_NOEXCEPT {
-    return entt_traits<Entity>::to_integral(entity);
+[[nodiscard]] constexpr typename entt_traits<Entity>::entity_type to_integral(const Entity value) ENTT_NOEXCEPT {
+    return entt_traits<Entity>::to_integral(value);
 }
 
+/**
+ * @copydoc entt_traits<Entity>::to_entity
+ * @tparam Entity The value type.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr typename entt_traits<Entity>::entity_type to_entity(const Entity value) ENTT_NOEXCEPT {
+    return entt_traits<Entity>::to_entity(value);
+}
 
-/*! @brief Null object for all entity identifiers.  */
+/**
+ * @copydoc entt_traits<Entity>::to_version
+ * @tparam Entity The value type.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr typename entt_traits<Entity>::version_type to_version(const Entity value) ENTT_NOEXCEPT {
+    return entt_traits<Entity>::to_version(value);
+}
+
+/*! @brief Null object for all identifiers.  */
 struct null_t {
     /**
      * @brief Converts the null object to identifiers of any type.
-     * @tparam Entity Type of entity identifier.
+     * @tparam Entity Type of identifier.
      * @return The null representation for the given type.
      */
     template<typename Entity>
     [[nodiscard]] constexpr operator Entity() const ENTT_NOEXCEPT {
-        return entt_traits<Entity>::construct();
+        using entity_traits = entt_traits<Entity>;
+        return entity_traits::combine(entity_traits::reserved, entity_traits::reserved);
     }
 
     /**
@@ -174,44 +190,33 @@ struct null_t {
     }
 
     /**
-     * @brief Compares a null object and an entity identifier of any type.
-     * @tparam Entity Type of entity identifier.
-     * @param entity Entity identifier with which to compare.
+     * @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>
     [[nodiscard]] constexpr bool operator==(const Entity entity) const ENTT_NOEXCEPT {
-        return entt_traits<Entity>::to_entity(entity) == entt_traits<Entity>::to_entity(*this);
+        using entity_traits = entt_traits<Entity>;
+        return entity_traits::to_entity(entity) == entity_traits::to_entity(*this);
     }
 
     /**
-     * @brief Compares a null object and an entity identifier of any type.
-     * @tparam Entity Type of entity identifier.
-     * @param entity Entity identifier with which to compare.
+     * @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 ENTT_NOEXCEPT {
         return !(entity == *this);
     }
-
-    /**
-     * @brief Creates a null object from an entity identifier of any type.
-     * @tparam Entity Type of entity identifier.
-     * @param entity Entity identifier to turn into a null object.
-     * @return The null representation for the given identifier.
-     */
-    template<typename Entity>
-    [[nodiscard]] constexpr Entity operator|(const Entity entity) const ENTT_NOEXCEPT {
-        return entt_traits<Entity>::construct(entt_traits<Entity>::to_entity(*this), entt_traits<Entity>::to_version(entity));
-    }
 };
 
-
 /**
- * @brief Compares a null object and an entity identifier of any type.
- * @tparam Entity Type of entity identifier.
- * @param entity Entity identifier with which to compare.
+ * @brief Compares a null object and an identifier of any type.
+ * @tparam Entity Type of identifier.
+ * @param entity Identifier with which to compare.
  * @param other A null object yet to be converted.
  * @return False if the two elements differ, true otherwise.
  */
@@ -220,11 +225,10 @@ template<typename Entity>
     return other.operator==(entity);
 }
 
-
 /**
- * @brief Compares a null object and an entity identifier of any type.
- * @tparam Entity Type of entity identifier.
- * @param entity Entity identifier with which to compare.
+ * @brief Compares a null object and an identifier of any type.
+ * @tparam Entity Type of identifier.
+ * @param entity Identifier with which to compare.
  * @param other A null object yet to be converted.
  * @return True if the two elements differ, false otherwise.
  */
@@ -233,17 +237,17 @@ template<typename Entity>
     return !(other == entity);
 }
 
-
-/*! @brief Tombstone object for all entity identifiers.  */
+/*! @brief Tombstone object for all identifiers.  */
 struct tombstone_t {
     /**
      * @brief Converts the tombstone object to identifiers of any type.
-     * @tparam Entity Type of entity identifier.
+     * @tparam Entity Type of identifier.
      * @return The tombstone representation for the given type.
      */
     template<typename Entity>
     [[nodiscard]] constexpr operator Entity() const ENTT_NOEXCEPT {
-        return entt_traits<Entity>::construct();
+        using entity_traits = entt_traits<Entity>;
+        return entity_traits::combine(entity_traits::reserved, entity_traits::reserved);
     }
 
     /**
@@ -265,44 +269,33 @@ struct tombstone_t {
     }
 
     /**
-     * @brief Compares a tombstone object and an entity identifier of any type.
-     * @tparam Entity Type of entity identifier.
-     * @param entity Entity identifier with which to compare.
+     * @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>
     [[nodiscard]] constexpr bool operator==(const Entity entity) const ENTT_NOEXCEPT {
-        return entt_traits<Entity>::to_version(entity) == entt_traits<Entity>::to_version(*this);
+        using entity_traits = entt_traits<Entity>;
+        return entity_traits::to_version(entity) == entity_traits::to_version(*this);
     }
 
     /**
-     * @brief Compares a tombstone object and an entity identifier of any type.
-     * @tparam Entity Type of entity identifier.
-     * @param entity Entity identifier with which to compare.
+     * @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 ENTT_NOEXCEPT {
         return !(entity == *this);
     }
-
-    /**
-     * @brief Creates a tombstone object from an entity identifier of any type.
-     * @tparam Entity Type of entity identifier.
-     * @param entity Entity identifier to turn into a tombstone object.
-     * @return The tombstone representation for the given identifier.
-     */
-    template<typename Entity>
-    [[nodiscard]] constexpr Entity operator|(const Entity entity) const ENTT_NOEXCEPT {
-        return entt_traits<Entity>::construct(entt_traits<Entity>::to_entity(entity));
-    }
 };
 
-
 /**
- * @brief Compares a tombstone object and an entity identifier of any type.
- * @tparam Entity Type of entity identifier.
- * @param entity Entity identifier with which to compare.
+ * @brief Compares a tombstone object and an identifier of any type.
+ * @tparam Entity Type of identifier.
+ * @param entity Identifier with which to compare.
  * @param other A tombstone object yet to be converted.
  * @return False if the two elements differ, true otherwise.
  */
@@ -311,11 +304,10 @@ template<typename Entity>
     return other.operator==(entity);
 }
 
-
 /**
- * @brief Compares a tombstone object and an entity identifier of any type.
- * @tparam Entity Type of entity identifier.
- * @param entity Entity identifier with which to compare.
+ * @brief Compares a tombstone object and an identifier of any type.
+ * @tparam Entity Type of identifier.
+ * @param entity Identifier with which to compare.
  * @param other A tombstone object yet to be converted.
  * @return True if the two elements differ, false otherwise.
  */
@@ -324,28 +316,24 @@ template<typename Entity>
     return !(other == entity);
 }
 
-
 /**
  * @brief Compile-time constant for null entities.
  *
- * There exist implicit conversions from this variable to entity identifiers of
- * any allowed type. Similarly, there exist comparision operators between the
- * null entity and any other entity identifier.
+ * There exist implicit conversions from this variable to identifiers of any
+ * allowed type. Similarly, there exist comparison operators between the null
+ * entity and any other identifier.
  */
 inline constexpr null_t null{};
 
-
 /**
  * @brief Compile-time constant for tombstone entities.
  *
- * There exist implicit conversions from this variable to entity identifiers of
- * any allowed type. Similarly, there exist comparision operators between the
- * tombstone entity and any other entity identifier.
+ * There exist implicit conversions from this variable to identifiers of any
+ * allowed type. Similarly, there exist comparison operators between the
+ * tombstone entity and any other identifier.
  */
 inline constexpr tombstone_t tombstone{};
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/entity/fwd.hpp

@@ -1,70 +1,54 @@
 #ifndef ENTT_ENTITY_FWD_HPP
 #define ENTT_ENTITY_FWD_HPP
 
-
 #include <memory>
 #include "../core/fwd.hpp"
-
+#include "utility.hpp"
 
 namespace entt {
 
-
 template<typename Entity, typename = std::allocator<Entity>>
 class basic_sparse_set;
 
-
-template<typename, typename Type, typename = std::allocator<Type>>
-struct basic_storage;
-
+template<typename, typename Type, typename = std::allocator<Type>, typename = void>
+class basic_storage;
 
 template<typename>
 class basic_registry;
 
-
-template<typename...>
-struct basic_view;
-
+template<typename, typename, typename, typename = void>
+class basic_view;
 
 template<typename>
-class basic_runtime_view;
+struct basic_runtime_view;
 
-
-template<typename...>
+template<typename, typename, typename, typename>
 class basic_group;
 
-
 template<typename>
 class basic_observer;
 
-
 template<typename>
 class basic_organizer;
 
-
 template<typename, typename...>
 struct basic_handle;
 
-
 template<typename>
 class basic_snapshot;
 
-
 template<typename>
 class basic_snapshot_loader;
 
-
 template<typename>
 class basic_continuous_loader;
 
-
 /*! @brief Default entity identifier. */
-enum class entity: id_type {};
-
+enum class entity : id_type {};
 
 /*! @brief Alias declaration for the most common use case. */
 using sparse_set = basic_sparse_set<entity>;
 
-
 /**
  * @brief Alias declaration for the most common use case.
  * @tparam Args Other template parameters.
@@ -72,27 +56,21 @@ using sparse_set = basic_sparse_set<entity>;
 template<typename... Args>
 using storage = basic_storage<entity, Args...>;
 
-
 /*! @brief Alias declaration for the most common use case. */
 using registry = basic_registry<entity>;
 
-
 /*! @brief Alias declaration for the most common use case. */
 using observer = basic_observer<entity>;
 
-
 /*! @brief Alias declaration for the most common use case. */
 using organizer = basic_organizer<entity>;
 
-
 /*! @brief Alias declaration for the most common use case. */
 using handle = basic_handle<entity>;
 
-
 /*! @brief Alias declaration for the most common use case. */
 using const_handle = basic_handle<const entity>;
 
-
 /**
  * @brief Alias declaration for the most common use case.
  * @tparam Args Other template parameters.
@@ -100,7 +78,6 @@ using const_handle = basic_handle<const entity>;
 template<typename... Args>
 using handle_view = basic_handle<entity, Args...>;
 
-
 /**
  * @brief Alias declaration for the most common use case.
  * @tparam Args Other template parameters.
@@ -108,30 +85,25 @@ using handle_view = basic_handle<entity, Args...>;
 template<typename... Args>
 using const_handle_view = basic_handle<const entity, Args...>;
 
-
 /*! @brief Alias declaration for the most common use case. */
 using snapshot = basic_snapshot<entity>;
 
-
 /*! @brief Alias declaration for the most common use case. */
 using snapshot_loader = basic_snapshot_loader<entity>;
 
-
 /*! @brief Alias declaration for the most common use case. */
 using continuous_loader = basic_continuous_loader<entity>;
 
-
 /**
  * @brief Alias declaration for the most common use case.
- * @tparam Args Other template parameters.
+ * @tparam Get Types of components iterated by the view.
+ * @tparam Exclude Types of components used to filter the view.
  */
-template<typename... Args>
-using view = basic_view<entity, Args...>;
-
+template<typename Get, typename Exclude = exclude_t<>>
+using view = basic_view<entity, Get, Exclude>;
 
 /*! @brief Alias declaration for the most common use case. */
-using runtime_view = basic_runtime_view<entity>;
-
+using runtime_view = basic_runtime_view<sparse_set>;
 
 /**
  * @brief Alias declaration for the most common use case.
@@ -140,8 +112,6 @@ using runtime_view = basic_runtime_view<entity>;
 template<typename... Args>
 using group = basic_group<entity, Args...>;
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/entity/group.hpp

@@ -1,32 +1,30 @@
 #ifndef ENTT_ENTITY_GROUP_HPP
 #define ENTT_ENTITY_GROUP_HPP
 
-
 #include <tuple>
-#include <utility>
 #include <type_traits>
+#include <utility>
 #include "../config/config.h"
+#include "../core/iterator.hpp"
 #include "../core/type_traits.hpp"
+#include "component.hpp"
 #include "entity.hpp"
 #include "fwd.hpp"
 #include "sparse_set.hpp"
 #include "storage.hpp"
 #include "utility.hpp"
 
-
 namespace entt {
 
-
 /**
  * @brief Group.
  *
  * Primary template isn't defined on purpose. All the specializations give a
  * compile-time error, but for a few reasonable cases.
  */
-template<typename...>
+template<typename, typename, typename, typename>
 class basic_group;
 
-
 /**
  * @brief Non-owning group.
  *
@@ -60,112 +58,112 @@ class basic_group;
  * In any other case, attempting to use a group results in undefined behavior.
  *
  * @tparam Entity A valid entity type (see entt_traits for more details).
- * @tparam Exclude Types of components used to filter the group.
  * @tparam Get Type of components observed by the group.
+ * @tparam Exclude Types of components used to filter the group.
  */
-template<typename Entity, typename... Exclude, typename... Get>
-class basic_group<Entity, exclude_t<Exclude...>, get_t<Get...>> final {
+template<typename Entity, typename... Get, typename... Exclude>
+class basic_group<Entity, owned_t<>, get_t<Get...>, exclude_t<Exclude...>> {
     /*! @brief A registry is allowed to create groups. */
     friend class basic_registry<Entity>;
 
-    using basic_common_type = basic_sparse_set<Entity>;
+    template<typename Comp>
+    using storage_type = constness_as_t<typename storage_traits<Entity, std::remove_const_t<Comp>>::storage_type, Comp>;
 
-    template<typename Component>
-    using storage_type = constness_as_t<typename storage_traits<Entity, std::remove_const_t<Component>>::storage_type, Component>;
+    using basic_common_type = std::common_type_t<typename storage_type<Get>::base_type...>;
 
-    class iterable final {
-        template<typename It>
-        struct iterable_iterator final {
-            using difference_type = std::ptrdiff_t;
-            using value_type = decltype(std::tuple_cat(std::tuple<Entity>{}, std::declval<basic_group>().get({})));
-            using pointer = void;
-            using reference = value_type;
-            using iterator_category = std::input_iterator_tag;
+    struct extended_group_iterator final {
+        using difference_type = std::ptrdiff_t;
+        using value_type = decltype(std::tuple_cat(std::tuple<Entity>{}, std::declval<basic_group>().get({})));
+        using pointer = input_iterator_pointer<value_type>;
+        using reference = value_type;
+        using iterator_category = std::input_iterator_tag;
 
-            template<typename... Args>
-            iterable_iterator(It from, const std::tuple<storage_type<Get> *...> &args) ENTT_NOEXCEPT
-                : it{from},
-                  pools{args}
-            {}
+        extended_group_iterator() = default;
 
-            iterable_iterator & operator++() ENTT_NOEXCEPT {
-                return ++it, *this;
-            }
+        extended_group_iterator(typename basic_common_type::iterator from, const std::tuple<storage_type<Get> *...> &args)
+            : it{from},
+              pools{args} {}
 
-            iterable_iterator operator++(int) ENTT_NOEXCEPT {
-                iterable_iterator orig = *this;
-                return ++(*this), orig;
-            }
-
-            [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
-                const auto entt = *it;
-                return std::tuple_cat(std::make_tuple(entt), get_as_tuple(*std::get<storage_type<Get> *>(pools), entt)...);
-            }
-
-            [[nodiscard]] bool operator==(const iterable_iterator &other) const ENTT_NOEXCEPT {
-                return other.it == it;
-            }
-
-            [[nodiscard]] bool operator!=(const iterable_iterator &other) const ENTT_NOEXCEPT {
-                return !(*this == other);
-            }
-
-        private:
-            It it;
-            std::tuple<storage_type<Get> *...> pools;
-        };
-
-    public:
-        using iterator = iterable_iterator<typename basic_common_type::iterator>;
-        using reverse_iterator = iterable_iterator<typename basic_common_type::reverse_iterator>;
-
-        iterable(basic_common_type * const ref, const std::tuple<storage_type<Get> *...> &cpools)
-            : handler{ref},
-              pools{cpools}
-        {}
-
-        [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
-            return handler ? iterator{handler->begin(), pools} : iterator{{}, pools};
+        extended_group_iterator &operator++() ENTT_NOEXCEPT {
+            return ++it, *this;
         }
 
-        [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
-            return handler ? iterator{handler->end(), pools} : iterator{{}, pools};
+        extended_group_iterator operator++(int) ENTT_NOEXCEPT {
+            extended_group_iterator orig = *this;
+            return ++(*this), orig;
         }
 
-        [[nodiscard]] reverse_iterator rbegin() const ENTT_NOEXCEPT {
-            return handler ? reverse_iterator{handler->rbegin(), pools} : reverse_iterator{{}, pools};
+        [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
+            const auto entt = *it;
+            return std::tuple_cat(std::make_tuple(entt), std::get<storage_type<Get> *>(pools)->get_as_tuple(entt)...);
         }
 
-        [[nodiscard]] reverse_iterator rend() const ENTT_NOEXCEPT {
-            return handler ? reverse_iterator{handler->rend(), pools} : reverse_iterator{{}, pools};
+        [[nodiscard]] pointer operator->() const ENTT_NOEXCEPT {
+            return operator*();
+        }
+
+        [[nodiscard]] bool operator==(const extended_group_iterator &other) const ENTT_NOEXCEPT {
+            return other.it == it;
+        }
+
+        [[nodiscard]] bool operator!=(const extended_group_iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
         }
 
     private:
-        basic_common_type * const handler;
-        const std::tuple<storage_type<Get> *...> pools;
+        typename basic_common_type::iterator it;
+        std::tuple<storage_type<Get> *...> pools;
     };
 
-    basic_group(basic_common_type &ref, storage_type<Get> &... gpool) ENTT_NOEXCEPT
+    basic_group(basic_common_type &ref, storage_type<Get> &...gpool) ENTT_NOEXCEPT
         : handler{&ref},
-          pools{&gpool...}
-    {}
+          pools{&gpool...} {}
 
 public:
     /*! @brief Underlying entity identifier. */
     using entity_type = Entity;
     /*! @brief Unsigned integer type. */
     using size_type = std::size_t;
+    /*! @brief Common type among all storage types. */
+    using base_type = basic_common_type;
     /*! @brief Random access iterator type. */
-    using iterator = typename basic_common_type::iterator;
+    using iterator = typename base_type::iterator;
     /*! @brief Reversed iterator type. */
-    using reverse_iterator = typename basic_common_type::reverse_iterator;
+    using reverse_iterator = typename base_type::reverse_iterator;
     /*! @brief Iterable group type. */
-    using iterable_group = iterable;
+    using iterable = iterable_adaptor<extended_group_iterator>;
 
     /*! @brief Default constructor to use to create empty, invalid groups. */
     basic_group() ENTT_NOEXCEPT
-        : handler{}
-    {}
+        : handler{} {}
+
+    /**
+     * @brief Returns a const reference to the underlying handler.
+     * @return A const reference to the underlying handler.
+     */
+    const base_type &handle() const ENTT_NOEXCEPT {
+        return *handler;
+    }
+
+    /**
+     * @brief Returns the storage for a given component type.
+     * @tparam Comp Type of component of which to return the storage.
+     * @return The storage for the given component type.
+     */
+    template<typename Comp>
+    [[nodiscard]] decltype(auto) storage() const ENTT_NOEXCEPT {
+        return *std::get<storage_type<Comp> *>(pools);
+    }
+
+    /**
+     * @brief Returns the storage for a given component type.
+     * @tparam Comp Index of component of which to return the storage.
+     * @return The storage for the given component type.
+     */
+    template<std::size_t Comp>
+    [[nodiscard]] decltype(auto) storage() const ENTT_NOEXCEPT {
+        return *std::get<Comp>(pools);
+    }
 
     /**
      * @brief Returns the number of entities that have the given components.
@@ -199,18 +197,6 @@ public:
         return !*this || handler->empty();
     }
 
-    /**
-     * @brief Direct access to the list of entities.
-     *
-     * The returned pointer is such that range `[data(), data() + size())` is
-     * always a valid range, even if the container is empty.
-     *
-     * @return A pointer to the array of entities.
-     */
-    [[nodiscard]] auto data() const ENTT_NOEXCEPT {
-        return *this ? handler->data() : nullptr;
-    }
-
     /**
      * @brief Returns an iterator to the first entity of the group.
      *
@@ -269,7 +255,7 @@ public:
      * @return The first entity of the group if one exists, the null entity
      * otherwise.
      */
-    [[nodiscard]] entity_type front() const {
+    [[nodiscard]] entity_type front() const ENTT_NOEXCEPT {
         const auto it = begin();
         return it != end() ? *it : null;
     }
@@ -279,18 +265,18 @@ public:
      * @return The last entity of the group if one exists, the null entity
      * otherwise.
      */
-    [[nodiscard]] entity_type back() const {
+    [[nodiscard]] entity_type back() const ENTT_NOEXCEPT {
         const auto it = rbegin();
         return it != rend() ? *it : null;
     }
 
     /**
      * @brief Finds an entity.
-     * @param entt A valid entity identifier.
+     * @param entt A valid identifier.
      * @return An iterator to the given entity if it's found, past the end
      * iterator otherwise.
      */
-    [[nodiscard]] iterator find(const entity_type entt) const {
+    [[nodiscard]] iterator find(const entity_type entt) const ENTT_NOEXCEPT {
         const auto it = *this ? handler->find(entt) : iterator{};
         return it != end() && *it == entt ? it : end();
     }
@@ -314,10 +300,10 @@ public:
 
     /**
      * @brief Checks if a group contains an entity.
-     * @param entt A valid entity identifier.
+     * @param entt A valid identifier.
      * @return True if the group contains the given entity, false otherwise.
      */
-    [[nodiscard]] bool contains(const entity_type entt) const {
+    [[nodiscard]] bool contains(const entity_type entt) const ENTT_NOEXCEPT {
         return *this && handler->contains(entt);
     }
 
@@ -332,20 +318,20 @@ public:
      * error. Attempting to use an entity that doesn't belong to the group
      * results in undefined behavior.
      *
-     * @tparam Component Types of components to get.
-     * @param entt A valid entity identifier.
+     * @tparam Comp Types of components to get.
+     * @param entt A valid identifier.
      * @return The components assigned to the entity.
      */
-    template<typename... Component>
+    template<typename... Comp>
     [[nodiscard]] decltype(auto) get(const entity_type entt) const {
         ENTT_ASSERT(contains(entt), "Group does not contain entity");
 
-        if constexpr(sizeof...(Component) == 0) {
-            return std::tuple_cat(get_as_tuple(*std::get<storage_type<Get> *>(pools), entt)...);
-        } else if constexpr(sizeof...(Component) == 1) {
-            return (std::get<storage_type<Component> *>(pools)->get(entt), ...);
+        if constexpr(sizeof...(Comp) == 0) {
+            return std::tuple_cat(std::get<storage_type<Get> *>(pools)->get_as_tuple(entt)...);
+        } else if constexpr(sizeof...(Comp) == 1) {
+            return (std::get<storage_type<Comp> *>(pools)->get(entt), ...);
         } else {
-            return std::tuple_cat(get_as_tuple(*std::get<storage_type<Component> *>(pools), entt)...);
+            return std::tuple_cat(std::get<storage_type<Comp> *>(pools)->get_as_tuple(entt)...);
         }
     }
 
@@ -383,7 +369,7 @@ public:
     }
 
     /**
-     * @brief Returns an iterable object to use to _visit_ the group.
+     * @brief Returns an iterable object to use to _visit_ a group.
      *
      * The iterable object returns tuples that contain the current entity and a
      * set of references to its non-empty components. The _constness_ of the
@@ -395,8 +381,9 @@ public:
      *
      * @return An iterable object to use to _visit_ the group.
      */
-    [[nodiscard]] iterable_group each() const ENTT_NOEXCEPT {
-        return iterable_group{handler, pools};
+    [[nodiscard]] iterable each() const ENTT_NOEXCEPT {
+        return handler ? iterable{extended_group_iterator{handler->begin(), pools}, extended_group_iterator{handler->end(), pools}}
+                       : iterable{extended_group_iterator{{}, pools}, extended_group_iterator{{}, pools}};
     }
 
     /**
@@ -420,14 +407,14 @@ public:
      * Moreover, the comparison function object shall induce a
      * _strict weak ordering_ on the values.
      *
-     * The sort function oject must offer a member function template
+     * The sort function object must offer a member function template
      * `operator()` that accepts three arguments:
      *
      * * An iterator to the first element of the range to sort.
      * * An iterator past the last element of the range to sort.
      * * A comparison function to use to compare the elements.
      *
-     * @tparam Component Optional types of components to compare.
+     * @tparam Comp Optional types of components to compare.
      * @tparam Compare Type of comparison function object.
      * @tparam Sort Type of sort function object.
      * @tparam Args Types of arguments to forward to the sort function object.
@@ -435,20 +422,22 @@ public:
      * @param algo A valid sort function object.
      * @param args Arguments to forward to the sort function object, if any.
      */
-    template<typename... Component, typename Compare, typename Sort = std_sort, typename... Args>
-    void sort(Compare compare, Sort algo = Sort{}, Args &&... args) {
+    template<typename... Comp, typename Compare, typename Sort = std_sort, typename... Args>
+    void sort(Compare compare, Sort algo = Sort{}, Args &&...args) {
         if(*this) {
-            if constexpr(sizeof...(Component) == 0) {
+            if constexpr(sizeof...(Comp) == 0) {
                 static_assert(std::is_invocable_v<Compare, const entity_type, const entity_type>, "Invalid comparison function");
                 handler->sort(std::move(compare), std::move(algo), std::forward<Args>(args)...);
-            }  else if constexpr(sizeof...(Component) == 1) {
-                handler->sort([this, compare = std::move(compare)](const entity_type lhs, const entity_type rhs) {
-                    return compare((std::get<storage_type<Component> *>(pools)->get(lhs), ...), (std::get<storage_type<Component> *>(pools)->get(rhs), ...));
-                }, std::move(algo), std::forward<Args>(args)...);
             } else {
-                handler->sort([this, compare = std::move(compare)](const entity_type lhs, const entity_type rhs) {
-                    return compare(std::forward_as_tuple(std::get<storage_type<Component> *>(pools)->get(lhs)...), std::forward_as_tuple(std::get<storage_type<Component> *>(pools)->get(rhs)...));
-                }, std::move(algo), std::forward<Args>(args)...);
+                auto comp = [this, &compare](const entity_type lhs, const entity_type rhs) {
+                    if constexpr(sizeof...(Comp) == 1) {
+                        return compare((std::get<storage_type<Comp> *>(pools)->get(lhs), ...), (std::get<storage_type<Comp> *>(pools)->get(rhs), ...));
+                    } else {
+                        return compare(std::forward_as_tuple(std::get<storage_type<Comp> *>(pools)->get(lhs)...), std::forward_as_tuple(std::get<storage_type<Comp> *>(pools)->get(rhs)...));
+                    }
+                };
+
+                handler->sort(std::move(comp), std::move(algo), std::forward<Args>(args)...);
             }
         }
     }
@@ -467,21 +456,20 @@ public:
      * can quickly ruin the order imposed to the pool of entities shared between
      * the non-owning groups.
      *
-     * @tparam Component Type of component to use to impose the order.
+     * @tparam Comp Type of component to use to impose the order.
      */
-    template<typename Component>
+    template<typename Comp>
     void sort() const {
         if(*this) {
-            handler->respect(*std::get<storage_type<Component> *>(pools));
+            handler->respect(*std::get<storage_type<Comp> *>(pools));
         }
     }
 
 private:
-    basic_common_type * const handler;
+    base_type *const handler;
     const std::tuple<storage_type<Get> *...> pools;
 };
 
-
 /**
  * @brief Owning group.
  *
@@ -524,143 +512,118 @@ private:
  * In any other case, attempting to use a group results in undefined behavior.
  *
  * @tparam Entity A valid entity type (see entt_traits for more details).
- * @tparam Exclude Types of components used to filter the group.
- * @tparam Get Types of components observed by the group.
  * @tparam Owned Types of components owned by the group.
+ * @tparam Get Types of components observed by the group.
+ * @tparam Exclude Types of components used to filter the group.
  */
-template<typename Entity, typename... Exclude, typename... Get, typename... Owned>
-class basic_group<Entity, exclude_t<Exclude...>, get_t<Get...>, Owned...> final {
+template<typename Entity, typename... Owned, typename... Get, typename... Exclude>
+class basic_group<Entity, owned_t<Owned...>, get_t<Get...>, exclude_t<Exclude...>> {
     /*! @brief A registry is allowed to create groups. */
     friend class basic_registry<Entity>;
 
-    using basic_common_type = basic_sparse_set<Entity>;
+    template<typename Comp>
+    using storage_type = constness_as_t<typename storage_traits<Entity, std::remove_const_t<Comp>>::storage_type, Comp>;
 
-    template<typename Component>
-    using storage_type = constness_as_t<typename storage_traits<Entity, std::remove_const_t<Component>>::storage_type, Component>;
+    using basic_common_type = std::common_type_t<typename storage_type<Owned>::base_type..., typename storage_type<Get>::base_type...>;
 
-    class iterable final {
-        template<typename, typename>
-        struct iterable_iterator;
-
-        template<typename It, typename... OIt>
-        struct iterable_iterator<It, type_list<OIt...>> final {
-            using difference_type = std::ptrdiff_t;
-            using value_type = decltype(std::tuple_cat(std::tuple<Entity>{}, std::declval<basic_group>().get({})));
-            using pointer = void;
-            using reference = value_type;
-            using iterator_category = std::input_iterator_tag;
-
-            template<typename... Other>
-            iterable_iterator(It from, const std::tuple<Other...> &other, const std::tuple<storage_type<Get> *...> &cpools) ENTT_NOEXCEPT
-                : it{from},
-                  owned{std::get<OIt>(other)...},
-                  get{cpools}
-            {}
-
-            iterable_iterator & operator++() ENTT_NOEXCEPT {
-                return ++it, (++std::get<OIt>(owned), ...), *this;
+    class extended_group_iterator final {
+        template<typename Type>
+        auto index_to_element(storage_type<Type> &cpool) const {
+            if constexpr(ignore_as_empty_v<std::remove_const_t<Type>>) {
+                return std::make_tuple();
+            } else {
+                return std::forward_as_tuple(cpool.rbegin()[it.index()]);
             }
-
-            iterable_iterator operator++(int) ENTT_NOEXCEPT {
-                iterable_iterator orig = *this;
-                return ++(*this), orig;
-            }
-
-            [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
-                return std::tuple_cat(
-                    std::make_tuple(*it),
-                    std::forward_as_tuple(*std::get<OIt>(owned)...),
-                    get_as_tuple(*std::get<storage_type<Get> *>(get), *it)...
-                );
-            }
-
-            [[nodiscard]] bool operator==(const iterable_iterator &other) const ENTT_NOEXCEPT {
-                return other.it == it;
-            }
-
-            [[nodiscard]] bool operator!=(const iterable_iterator &other) const ENTT_NOEXCEPT {
-                return !(*this == other);
-            }
-
-        private:
-            It it;
-            std::tuple<OIt...> owned;
-            std::tuple<storage_type<Get> *...> get;
-        };
+        }
 
     public:
-        using iterator = iterable_iterator<
-            typename basic_common_type::iterator,
-            type_list_cat_t<std::conditional_t<std::is_void_v<decltype(std::declval<storage_type<Owned>>().get({}))>, type_list<>, type_list<decltype(std::declval<storage_type<Owned>>().end())>>...>
-        >;
-        using reverse_iterator = iterable_iterator<
-            typename basic_common_type::reverse_iterator,
-            type_list_cat_t<std::conditional_t<std::is_void_v<decltype(std::declval<storage_type<Owned>>().get({}))>, type_list<>, type_list<decltype(std::declval<storage_type<Owned>>().rbegin())>>...>
-        >;
+        using difference_type = std::ptrdiff_t;
+        using value_type = decltype(std::tuple_cat(std::tuple<Entity>{}, std::declval<basic_group>().get({})));
+        using pointer = input_iterator_pointer<value_type>;
+        using reference = value_type;
+        using iterator_category = std::input_iterator_tag;
 
-        iterable(std::tuple<storage_type<Owned> *..., storage_type<Get> *...> cpools, const std::size_t * const extent)
-            : pools{cpools},
-              length{extent}
-        {}
+        extended_group_iterator() = default;
 
-        [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
-            return length ? iterator{
-                std::get<0>(pools)->basic_common_type::end() - *length,
-                std::make_tuple((std::get<storage_type<Owned> *>(pools)->end() - *length)...),
-                std::make_tuple(std::get<storage_type<Get> *>(pools)...)
-            } : iterator{{}, std::make_tuple(decltype(std::get<storage_type<Owned> *>(pools)->end()){}...), std::make_tuple(std::get<storage_type<Get> *>(pools)...)};
+        template<typename... Other>
+        extended_group_iterator(typename basic_common_type::iterator from, const std::tuple<storage_type<Owned> *..., storage_type<Get> *...> &cpools)
+            : it{from},
+              pools{cpools} {}
+
+        extended_group_iterator &operator++() ENTT_NOEXCEPT {
+            return ++it, *this;
         }
 
-        [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
-            return length ? iterator{
-                std::get<0>(pools)->basic_common_type::end(),
-                std::make_tuple((std::get<storage_type<Owned> *>(pools)->end())...),
-                std::make_tuple(std::get<storage_type<Get> *>(pools)...)
-            } : iterator{{}, std::make_tuple(decltype(std::get<storage_type<Owned> *>(pools)->end()){}...), std::make_tuple(std::get<storage_type<Get> *>(pools)...)};
+        extended_group_iterator operator++(int) ENTT_NOEXCEPT {
+            extended_group_iterator orig = *this;
+            return ++(*this), orig;
         }
 
-        [[nodiscard]] reverse_iterator rbegin() const ENTT_NOEXCEPT {
-            return length ? reverse_iterator{
-                std::get<0>(pools)->basic_common_type::rbegin(),
-                std::make_tuple((std::get<storage_type<Owned> *>(pools)->rbegin())...),
-                std::make_tuple(std::get<storage_type<Get> *>(pools)...)
-            } : reverse_iterator{{}, std::make_tuple(decltype(std::get<storage_type<Owned> *>(pools)->rbegin()){}...), std::make_tuple(std::get<storage_type<Get> *>(pools)...)};
+        [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
+            return std::tuple_cat(
+                std::make_tuple(*it),
+                index_to_element<Owned>(*std::get<storage_type<Owned> *>(pools))...,
+                std::get<storage_type<Get> *>(pools)->get_as_tuple(*it)...);
         }
 
-        [[nodiscard]] reverse_iterator rend() const ENTT_NOEXCEPT {
-            return length ? reverse_iterator{
-                std::get<0>(pools)->basic_common_type::rbegin() + *length,
-                std::make_tuple((std::get<storage_type<Owned> *>(pools)->rbegin() + *length)...),
-                std::make_tuple(std::get<storage_type<Get> *>(pools)...)
-            } : reverse_iterator{{}, std::make_tuple(decltype(std::get<storage_type<Owned> *>(pools)->rbegin()){}...), std::make_tuple(std::get<storage_type<Get> *>(pools)...)};
+        [[nodiscard]] pointer operator->() const ENTT_NOEXCEPT {
+            return operator*();
+        }
+
+        [[nodiscard]] bool operator==(const extended_group_iterator &other) const ENTT_NOEXCEPT {
+            return other.it == it;
+        }
+
+        [[nodiscard]] bool operator!=(const extended_group_iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
         }
 
     private:
-        const std::tuple<storage_type<Owned> *..., storage_type<Get> *...> pools;
-        const std::size_t * const length;
+        typename basic_common_type::iterator it;
+        std::tuple<storage_type<Owned> *..., storage_type<Get> *...> pools;
     };
 
-    basic_group(const std::size_t &extent, storage_type<Owned> &... opool, storage_type<Get> &... gpool) ENTT_NOEXCEPT
+    basic_group(const std::size_t &extent, storage_type<Owned> &...opool, storage_type<Get> &...gpool) ENTT_NOEXCEPT
         : pools{&opool..., &gpool...},
-          length{&extent}
-    {}
+          length{&extent} {}
 
 public:
     /*! @brief Underlying entity identifier. */
     using entity_type = Entity;
     /*! @brief Unsigned integer type. */
     using size_type = std::size_t;
+    /*! @brief Common type among all storage types. */
+    using base_type = basic_common_type;
     /*! @brief Random access iterator type. */
-    using iterator = typename basic_common_type::iterator;
+    using iterator = typename base_type::iterator;
     /*! @brief Reversed iterator type. */
-    using reverse_iterator = typename basic_common_type::reverse_iterator;
+    using reverse_iterator = typename base_type::reverse_iterator;
     /*! @brief Iterable group type. */
-    using iterable_group = iterable;
+    using iterable = iterable_adaptor<extended_group_iterator>;
 
     /*! @brief Default constructor to use to create empty, invalid groups. */
     basic_group() ENTT_NOEXCEPT
-        : length{}
-    {}
+        : length{} {}
+
+    /**
+     * @brief Returns the storage for a given component type.
+     * @tparam Comp Type of component of which to return the storage.
+     * @return The storage for the given component type.
+     */
+    template<typename Comp>
+    [[nodiscard]] decltype(auto) storage() const ENTT_NOEXCEPT {
+        return *std::get<storage_type<Comp> *>(pools);
+    }
+
+    /**
+     * @brief Returns the storage for a given component type.
+     * @tparam Comp Index of component of which to return the storage.
+     * @return The storage for the given component type.
+     */
+    template<std::size_t Comp>
+    [[nodiscard]] decltype(auto) storage() const ENTT_NOEXCEPT {
+        return *std::get<Comp>(pools);
+    }
 
     /**
      * @brief Returns the number of entities that have the given components.
@@ -678,34 +641,6 @@ public:
         return !*this || !*length;
     }
 
-    /**
-     * @brief Direct access to the raw representation offered by the storage.
-     *
-     * @warning
-     * This function is only available for owned types.
-     *
-     * @tparam Component Type of component in which one is interested.
-     * @return A pointer to the array of components.
-     */
-    template<typename Component>
-    [[nodiscard]] auto raw() const ENTT_NOEXCEPT {
-        static_assert((std::is_same_v<Component, Owned> || ...), "Non-owned type");
-        auto *cpool = std::get<storage_type<Component> *>(pools);
-        return cpool ? cpool->raw() : decltype(cpool->raw()){};
-    }
-
-    /**
-     * @brief Direct access to the list of entities.
-     *
-     * The returned pointer is such that range `[data(), data() + size())` is
-     * always a valid range, even if the container is empty.
-     *
-     * @return A pointer to the array of entities.
-     */
-    [[nodiscard]] auto data() const ENTT_NOEXCEPT {
-        return *this ? std::get<0>(pools)->data() : nullptr;
-    }
-
     /**
      * @brief Returns an iterator to the first entity of the group.
      *
@@ -715,7 +650,7 @@ public:
      * @return An iterator to the first entity of the group.
      */
     [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
-        return *this ? (std::get<0>(pools)->basic_common_type::end() - *length) : iterator{};
+        return *this ? (std::get<0>(pools)->base_type::end() - *length) : iterator{};
     }
 
     /**
@@ -729,7 +664,7 @@ public:
      * group.
      */
     [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
-        return *this ? std::get<0>(pools)->basic_common_type::end() : iterator{};
+        return *this ? std::get<0>(pools)->base_type::end() : iterator{};
     }
 
     /**
@@ -741,7 +676,7 @@ public:
      * @return An iterator to the first entity of the reversed group.
      */
     [[nodiscard]] reverse_iterator rbegin() const ENTT_NOEXCEPT {
-        return *this ? std::get<0>(pools)->basic_common_type::rbegin() : reverse_iterator{};
+        return *this ? std::get<0>(pools)->base_type::rbegin() : reverse_iterator{};
     }
 
     /**
@@ -756,7 +691,7 @@ public:
      * reversed group.
      */
     [[nodiscard]] reverse_iterator rend() const ENTT_NOEXCEPT {
-        return *this ? (std::get<0>(pools)->basic_common_type::rbegin() + *length) : reverse_iterator{};
+        return *this ? (std::get<0>(pools)->base_type::rbegin() + *length) : reverse_iterator{};
     }
 
     /**
@@ -764,7 +699,7 @@ public:
      * @return The first entity of the group if one exists, the null entity
      * otherwise.
      */
-    [[nodiscard]] entity_type front() const {
+    [[nodiscard]] entity_type front() const ENTT_NOEXCEPT {
         const auto it = begin();
         return it != end() ? *it : null;
     }
@@ -774,18 +709,18 @@ public:
      * @return The last entity of the group if one exists, the null entity
      * otherwise.
      */
-    [[nodiscard]] entity_type back() const {
+    [[nodiscard]] entity_type back() const ENTT_NOEXCEPT {
         const auto it = rbegin();
         return it != rend() ? *it : null;
     }
 
     /**
      * @brief Finds an entity.
-     * @param entt A valid entity identifier.
+     * @param entt A valid identifier.
      * @return An iterator to the given entity if it's found, past the end
      * iterator otherwise.
      */
-    [[nodiscard]] iterator find(const entity_type entt) const {
+    [[nodiscard]] iterator find(const entity_type entt) const ENTT_NOEXCEPT {
         const auto it = *this ? std::get<0>(pools)->find(entt) : iterator{};
         return it != end() && it >= begin() && *it == entt ? it : end();
     }
@@ -809,10 +744,10 @@ public:
 
     /**
      * @brief Checks if a group contains an entity.
-     * @param entt A valid entity identifier.
+     * @param entt A valid identifier.
      * @return True if the group contains the given entity, false otherwise.
      */
-    [[nodiscard]] bool contains(const entity_type entt) const {
+    [[nodiscard]] bool contains(const entity_type entt) const ENTT_NOEXCEPT {
         return *this && std::get<0>(pools)->contains(entt) && (std::get<0>(pools)->index(entt) < (*length));
     }
 
@@ -827,20 +762,20 @@ public:
      * error. Attempting to use an entity that doesn't belong to the group
      * results in undefined behavior.
      *
-     * @tparam Component Types of components to get.
-     * @param entt A valid entity identifier.
+     * @tparam Comp Types of components to get.
+     * @param entt A valid identifier.
      * @return The components assigned to the entity.
      */
-    template<typename... Component>
+    template<typename... Comp>
     [[nodiscard]] decltype(auto) get(const entity_type entt) const {
         ENTT_ASSERT(contains(entt), "Group does not contain entity");
 
-        if constexpr(sizeof...(Component) == 0) {
-            return std::tuple_cat(get_as_tuple(*std::get<storage_type<Owned> *>(pools), entt)..., get_as_tuple(*std::get<storage_type<Get> *>(pools), entt)...);
-        } else if constexpr(sizeof...(Component) == 1) {
-            return (std::get<storage_type<Component> *>(pools)->get(entt), ...);
+        if constexpr(sizeof...(Comp) == 0) {
+            return std::tuple_cat(std::get<storage_type<Owned> *>(pools)->get_as_tuple(entt)..., std::get<storage_type<Get> *>(pools)->get_as_tuple(entt)...);
+        } else if constexpr(sizeof...(Comp) == 1) {
+            return (std::get<storage_type<Comp> *>(pools)->get(entt), ...);
         } else {
-            return std::tuple_cat(get_as_tuple(*std::get<storage_type<Component> *>(pools), entt)...);
+            return std::tuple_cat(std::get<storage_type<Comp> *>(pools)->get_as_tuple(entt)...);
         }
     }
 
@@ -872,13 +807,13 @@ public:
             if constexpr(is_applicable_v<Func, decltype(std::tuple_cat(std::tuple<entity_type>{}, std::declval<basic_group>().get({})))>) {
                 std::apply(func, args);
             } else {
-                std::apply([&func](auto, auto &&... less) { func(std::forward<decltype(less)>(less)...); }, args);
+                std::apply([&func](auto, auto &&...less) { func(std::forward<decltype(less)>(less)...); }, args);
             }
         }
     }
 
     /**
-     * @brief Returns an iterable object to use to _visit_ the group.
+     * @brief Returns an iterable object to use to _visit_ a group.
      *
      * The iterable object returns tuples that contain the current entity and a
      * set of references to its non-empty components. The _constness_ of the
@@ -890,8 +825,9 @@ public:
      *
      * @return An iterable object to use to _visit_ the group.
      */
-    [[nodiscard]] iterable_group each() const ENTT_NOEXCEPT {
-        return iterable_group{pools, length};
+    [[nodiscard]] iterable each() const ENTT_NOEXCEPT {
+        iterator last = length ? std::get<0>(pools)->basic_common_type::end() : iterator{};
+        return {extended_group_iterator{last - *length, pools}, extended_group_iterator{last, pools}};
     }
 
     /**
@@ -916,14 +852,14 @@ public:
      * Moreover, the comparison function object shall induce a
      * _strict weak ordering_ on the values.
      *
-     * The sort function oject must offer a member function template
+     * The sort function object must offer a member function template
      * `operator()` that accepts three arguments:
      *
      * * An iterator to the first element of the range to sort.
      * * An iterator past the last element of the range to sort.
      * * A comparison function to use to compare the elements.
      *
-     * @tparam Component Optional types of components to compare.
+     * @tparam Comp Optional types of components to compare.
      * @tparam Compare Type of comparison function object.
      * @tparam Sort Type of sort function object.
      * @tparam Args Types of arguments to forward to the sort function object.
@@ -931,39 +867,39 @@ public:
      * @param algo A valid sort function object.
      * @param args Arguments to forward to the sort function object, if any.
      */
-    template<typename... Component, typename Compare, typename Sort = std_sort, typename... Args>
-    void sort(Compare compare, Sort algo = Sort{}, Args &&... args) const {
+    template<typename... Comp, typename Compare, typename Sort = std_sort, typename... Args>
+    void sort(Compare compare, Sort algo = Sort{}, Args &&...args) const {
         auto *cpool = std::get<0>(pools);
 
-        if constexpr(sizeof...(Component) == 0) {
+        if constexpr(sizeof...(Comp) == 0) {
             static_assert(std::is_invocable_v<Compare, const entity_type, const entity_type>, "Invalid comparison function");
             cpool->sort_n(*length, std::move(compare), std::move(algo), std::forward<Args>(args)...);
-        } else if constexpr(sizeof...(Component) == 1) {
-            cpool->sort_n(*length, [this, compare = std::move(compare)](const entity_type lhs, const entity_type rhs) {
-                return compare((std::get<storage_type<Component> *>(pools)->get(lhs), ...), (std::get<storage_type<Component> *>(pools)->get(rhs), ...));
-            }, std::move(algo), std::forward<Args>(args)...);
         } else {
-            cpool->sort_n(*length, [this, compare = std::move(compare)](const entity_type lhs, const entity_type rhs) {
-                return compare(std::forward_as_tuple(std::get<storage_type<Component> *>(pools)->get(lhs)...), std::forward_as_tuple(std::get<storage_type<Component> *>(pools)->get(rhs)...));
-            }, std::move(algo), std::forward<Args>(args)...);
+            auto comp = [this, &compare](const entity_type lhs, const entity_type rhs) {
+                if constexpr(sizeof...(Comp) == 1) {
+                    return compare((std::get<storage_type<Comp> *>(pools)->get(lhs), ...), (std::get<storage_type<Comp> *>(pools)->get(rhs), ...));
+                } else {
+                    return compare(std::forward_as_tuple(std::get<storage_type<Comp> *>(pools)->get(lhs)...), std::forward_as_tuple(std::get<storage_type<Comp> *>(pools)->get(rhs)...));
+                }
+            };
+
+            cpool->sort_n(*length, std::move(comp), std::move(algo), std::forward<Args>(args)...);
         }
 
-        [this](auto *head, auto *... other) {
+        [this](auto *head, auto *...other) {
             for(auto next = *length; next; --next) {
                 const auto pos = next - 1;
                 [[maybe_unused]] const auto entt = head->data()[pos];
-                (other->swap(other->data()[pos], entt), ...);
+                (other->swap_elements(other->data()[pos], entt), ...);
             }
         }(std::get<storage_type<Owned> *>(pools)...);
     }
 
 private:
     const std::tuple<storage_type<Owned> *..., storage_type<Get> *...> pools;
-    const size_type * const length;
+    const size_type *const length;
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/entity/handle.hpp

@@ -1,7 +1,6 @@
 #ifndef ENTT_ENTITY_HANDLE_HPP
 #define ENTT_ENTITY_HANDLE_HPP
 
-
 #include <tuple>
 #include <type_traits>
 #include <utility>
@@ -10,10 +9,8 @@
 #include "fwd.hpp"
 #include "registry.hpp"
 
-
 namespace entt {
 
-
 /**
  * @brief Non-owning handle to an entity.
  *
@@ -35,29 +32,17 @@ struct basic_handle {
 
     /*! @brief Constructs an invalid handle. */
     basic_handle() ENTT_NOEXCEPT
-        : reg{}, entt{null}
-    {}
+        : reg{},
+          entt{null} {}
 
     /**
      * @brief Constructs a handle from a given registry and entity.
      * @param ref An instance of the registry class.
-     * @param value An entity identifier.
+     * @param value A valid identifier.
      */
     basic_handle(registry_type &ref, entity_type value) ENTT_NOEXCEPT
-        : reg{&ref}, entt{value}
-    {}
-
-    /**
-     * @brief Compares two handles.
-     * @tparam Args Template parameters of the handle with which to compare.
-     * @param other Handle with which to compare.
-     * @return True if both handles refer to the same registry and the same
-     * entity, false otherwise.
-     */
-    template<typename... Args>
-    [[nodiscard]] bool operator==(const basic_handle<Args...> &other) const ENTT_NOEXCEPT {
-        return reg == other.registry() && entt == other.entity();
-    }
+        : reg{&ref},
+          entt{value} {}
 
     /**
      * @brief Constructs a const handle from a non-const one.
@@ -68,18 +53,15 @@ struct basic_handle {
      */
     template<typename Other, typename... Args>
     operator basic_handle<Other, Args...>() const ENTT_NOEXCEPT {
-        static_assert(
-            (std::is_same_v<Other, Entity> || std::is_same_v<std::remove_const_t<Other>, Entity>)
-            && (sizeof...(Type) == 0 || ((sizeof...(Args) != 0 && sizeof...(Args) <= sizeof...(Type)) && ... && (type_list_contains_v<type_list<Type...>, Args>))),
-            "Invalid conversion between different handles"
-        );
+        static_assert(std::is_same_v<Other, Entity> || std::is_same_v<std::remove_const_t<Other>, Entity>, "Invalid conversion between different handles");
+        static_assert((sizeof...(Type) == 0 || ((sizeof...(Args) != 0 && sizeof...(Args) <= sizeof...(Type)) && ... && (type_list_contains_v<type_list<Type...>, Args>))), "Invalid conversion between different handles");
 
         return reg ? basic_handle<Other, Args...>{*reg, entt} : basic_handle<Other, Args...>{};
     }
 
     /**
      * @brief Converts a handle to its underlying entity.
-     * @return An entity identifier.
+     * @return The contained identifier.
      */
     [[nodiscard]] operator entity_type() const ENTT_NOEXCEPT {
         return entity();
@@ -105,7 +87,7 @@ struct basic_handle {
      * @brief Returns a pointer to the underlying registry, if any.
      * @return A pointer to the underlying registry, if any.
      */
-    [[nodiscard]] registry_type * registry() const ENTT_NOEXCEPT {
+    [[nodiscard]] registry_type *registry() const ENTT_NOEXCEPT {
         return reg;
     }
 
@@ -143,7 +125,7 @@ struct basic_handle {
      * @return A reference to the newly created component.
      */
     template<typename Component, typename... Args>
-    decltype(auto) emplace(Args &&... args) const {
+    decltype(auto) emplace(Args &&...args) const {
         static_assert(((sizeof...(Type) == 0) || ... || std::is_same_v<Component, Type>), "Invalid type");
         return reg->template emplace<Component>(entt, std::forward<Args>(args)...);
     }
@@ -157,7 +139,7 @@ struct basic_handle {
      * @return A reference to the newly created component.
      */
     template<typename Component, typename... Args>
-    decltype(auto) emplace_or_replace(Args &&... args) const {
+    decltype(auto) emplace_or_replace(Args &&...args) const {
         static_assert(((sizeof...(Type) == 0) || ... || std::is_same_v<Component, Type>), "Invalid type");
         return reg->template emplace_or_replace<Component>(entt, std::forward<Args>(args)...);
     }
@@ -171,7 +153,7 @@ struct basic_handle {
      * @return A reference to the patched component.
      */
     template<typename Component, typename... Func>
-    decltype(auto) patch(Func &&... func) const {
+    decltype(auto) patch(Func &&...func) const {
         static_assert(((sizeof...(Type) == 0) || ... || std::is_same_v<Component, Type>), "Invalid type");
         return reg->template patch<Component>(entt, std::forward<Func>(func)...);
     }
@@ -185,7 +167,7 @@ struct basic_handle {
      * @return A reference to the component being replaced.
      */
     template<typename Component, typename... Args>
-    decltype(auto) replace(Args &&... args) const {
+    decltype(auto) replace(Args &&...args) const {
         static_assert(((sizeof...(Type) == 0) || ... || std::is_same_v<Component, Type>), "Invalid type");
         return reg->template replace<Component>(entt, std::forward<Args>(args)...);
     }
@@ -213,23 +195,6 @@ struct basic_handle {
         reg->template erase<Component...>(entt);
     }
 
-    /*! @copydoc remove */
-    template<typename... Component>
-    [[deprecated("Use ::remove instead")]]
-    size_type remove_if_exists() const {
-        return remove<Component...>();
-    }
-
-    /**
-     * @brief Removes all the components from a handle and makes it orphaned.
-     * @sa basic_registry::remove_all
-     */
-    [[deprecated("No longer supported")]]
-    void remove_all() const {
-        static_assert(sizeof...(Type) == 0, "Invalid operation");
-        reg->remove_all(entt);
-    }
-
     /**
      * @brief Checks if a handle has all the given components.
      * @sa basic_registry::all_of
@@ -274,7 +239,7 @@ struct basic_handle {
      * @return Reference to the component owned by the handle.
      */
     template<typename Component, typename... Args>
-    [[nodiscard]] decltype(auto) get_or_emplace(Args &&... args) const {
+    [[nodiscard]] decltype(auto) get_or_emplace(Args &&...args) const {
         static_assert(((sizeof...(Type) == 0) || ... || std::is_same_v<Component, Type>), "Invalid type");
         return reg->template get_or_emplace<Component>(entt, std::forward<Args>(args)...);
     }
@@ -300,14 +265,27 @@ struct basic_handle {
     }
 
     /**
-     * @brief Visits a handle and returns the types for its components.
-     * @sa basic_registry::visit
+     * @brief Visits a handle and returns the pools for its components.
+     *
+     * The signature of the function should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * void(id_type, const basic_sparse_set<entity_type> &);
+     * @endcode
+     *
+     * Returned pools are those that contain the entity associated with the
+     * handle.
+     *
      * @tparam Func Type of the function object to invoke.
      * @param func A valid function object.
      */
     template<typename Func>
     void visit(Func &&func) const {
-        reg->visit(entt, std::forward<Func>(func));
+        for(auto [id, storage]: reg->storage()) {
+            if(storage.contains(entt)) {
+                func(id, storage);
+            }
+        }
     }
 
 private:
@@ -315,41 +293,48 @@ 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) ENTT_NOEXCEPT {
+    return lhs.registry() == rhs.registry() && lhs.entity() == rhs.entity();
+}
 
 /**
  * @brief Compares two handles.
- * @tparam Type A valid entity type (see entt_traits for more details).
- * @tparam Other A valid entity type (see entt_traits for more details).
+ * @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 Type, typename Other>
-bool operator!=(const basic_handle<Type> &lhs, const basic_handle<Other> &rhs) ENTT_NOEXCEPT {
+template<typename... Args, typename... Other>
+[[nodiscard]] bool operator!=(const basic_handle<Args...> &lhs, const basic_handle<Other...> &rhs) ENTT_NOEXCEPT {
     return !(lhs == rhs);
 }
 
+/**
+ * @brief Deduction guide.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+basic_handle(basic_registry<Entity> &, Entity) -> basic_handle<Entity>;
 
 /**
  * @brief Deduction guide.
  * @tparam Entity A valid entity type (see entt_traits for more details).
  */
 template<typename Entity>
-basic_handle(basic_registry<Entity> &, Entity)
--> basic_handle<Entity>;
-
-
-/**
- * @brief Deduction guide.
- * @tparam Entity A valid entity type (see entt_traits for more details).
- */
-template<typename Entity>
-basic_handle(const basic_registry<Entity> &, Entity)
--> basic_handle<const Entity>;
-
-
-}
+basic_handle(const basic_registry<Entity> &, Entity) -> basic_handle<const Entity>;
 
+} // namespace entt
 
 #endif

src/entt/entity/helper.hpp

@@ -1,19 +1,16 @@
 #ifndef ENTT_ENTITY_HELPER_HPP
 #define ENTT_ENTITY_HELPER_HPP
 
-
 #include <type_traits>
 #include "../config/config.h"
 #include "../core/fwd.hpp"
 #include "../core/type_traits.hpp"
 #include "../signal/delegate.hpp"
-#include "registry.hpp"
 #include "fwd.hpp"
-
+#include "registry.hpp"
 
 namespace entt {
 
-
 /**
  * @brief Converts a registry to a view.
  * @tparam Entity A valid entity type (see entt_traits for more details).
@@ -38,7 +35,7 @@ struct as_view {
      * @return A newly created view.
      */
     template<typename Exclude, typename... Component>
-    operator basic_view<entity_type, Exclude, Component...>() const {
+    operator basic_view<entity_type, get_t<Component...>, Exclude>() const {
         return reg.template view<Component...>(Exclude{});
     }
 
@@ -46,7 +43,6 @@ private:
     registry_type &reg;
 };
 
-
 /**
  * @brief Deduction guide.
  * @tparam Entity A valid entity type (see entt_traits for more details).
@@ -54,7 +50,6 @@ private:
 template<typename Entity>
 as_view(basic_registry<Entity> &) -> as_view<Entity>;
 
-
 /**
  * @brief Deduction guide.
  * @tparam Entity A valid entity type (see entt_traits for more details).
@@ -62,7 +57,6 @@ as_view(basic_registry<Entity> &) -> as_view<Entity>;
 template<typename Entity>
 as_view(const basic_registry<Entity> &) -> as_view<const Entity>;
 
-
 /**
  * @brief Converts a registry to a group.
  * @tparam Entity A valid entity type (see entt_traits for more details).
@@ -82,13 +76,13 @@ struct as_group {
 
     /**
      * @brief Conversion function from a registry to a group.
-     * @tparam Exclude Types of components used to filter the group.
      * @tparam Get Types of components observed by the group.
+     * @tparam Exclude Types of components used to filter the group.
      * @tparam Owned Types of components owned by the group.
      * @return A newly created group.
      */
-    template<typename Exclude, typename Get, typename... Owned>
-    operator basic_group<entity_type, Exclude, Get, Owned...>() const {
+    template<typename Get, typename Exclude, typename... Owned>
+    operator basic_group<entity_type, owned_t<Owned...>, Get, Exclude>() const {
         if constexpr(std::is_const_v<registry_type>) {
             return reg.template group_if_exists<Owned...>(Get{}, Exclude{});
         } else {
@@ -100,7 +94,6 @@ private:
     registry_type &reg;
 };
 
-
 /**
  * @brief Deduction guide.
  * @tparam Entity A valid entity type (see entt_traits for more details).
@@ -108,7 +101,6 @@ private:
 template<typename Entity>
 as_group(basic_registry<Entity> &) -> as_group<Entity>;
 
-
 /**
  * @brief Deduction guide.
  * @tparam Entity A valid entity type (see entt_traits for more details).
@@ -116,8 +108,6 @@ as_group(basic_registry<Entity> &) -> as_group<Entity>;
 template<typename Entity>
 as_group(const basic_registry<Entity> &) -> as_group<const Entity>;
 
-
-
 /**
  * @brief Helper to create a listener that directly invokes a member function.
  * @tparam Member Member function to invoke on a component of the given type.
@@ -133,7 +123,6 @@ void invoke(basic_registry<Entity> &reg, const Entity entt) {
     func(reg, entt);
 }
 
-
 /**
  * @brief Returns the entity associated with a given component.
  *
@@ -149,20 +138,19 @@ void invoke(basic_registry<Entity> &reg, const Entity entt) {
  */
 template<typename Entity, typename Component>
 Entity to_entity(const basic_registry<Entity> &reg, const Component &instance) {
-    const auto view = reg.template view<const Component>();
+    const auto &storage = reg.template storage<Component>();
+    const typename basic_registry<Entity>::base_type &base = storage;
     const auto *addr = std::addressof(instance);
 
-    for(auto it = view.rbegin(), last = view.rend(); it < last; it += ENTT_PACKED_PAGE) {
-        if(const auto dist = (addr - std::addressof(view.template get<const Component>(*it))); dist >= 0 && dist < ENTT_PACKED_PAGE) {
+    for(auto it = base.rbegin(), last = base.rend(); it < last; it += ENTT_PACKED_PAGE) {
+        if(const auto dist = (addr - std::addressof(storage.get(*it))); dist >= 0 && dist < ENTT_PACKED_PAGE) {
             return *(it + dist);
         }
     }
 
-    return entt::null;
-}
-
-
+    return null;
 }
 
+} // namespace entt
 
 #endif

src/entt/entity/observer.hpp

@@ -1,30 +1,26 @@
 #ifndef ENTT_ENTITY_OBSERVER_HPP
 #define ENTT_ENTITY_OBSERVER_HPP
 
-
-#include <limits>
 #include <cstddef>
 #include <cstdint>
-#include <utility>
+#include <limits>
 #include <type_traits>
+#include <utility>
 #include "../config/config.h"
 #include "../core/type_traits.hpp"
 #include "../signal/delegate.hpp"
+#include "entity.hpp"
+#include "fwd.hpp"
 #include "registry.hpp"
 #include "storage.hpp"
 #include "utility.hpp"
-#include "entity.hpp"
-#include "fwd.hpp"
-
 
 namespace entt {
 
-
 /*! @brief Grouping matcher. */
 template<typename...>
 struct matcher {};
 
-
 /**
  * @brief Collector.
  *
@@ -34,7 +30,6 @@ struct matcher {};
 template<typename...>
 struct basic_collector;
 
-
 /**
  * @brief Collector.
  *
@@ -114,11 +109,9 @@ struct basic_collector<matcher<type_list<Reject...>, type_list<Require...>, Rule
     }
 };
 
-
 /*! @brief Variable template used to ease the definition of collectors. */
 inline constexpr basic_collector<> collector{};
 
-
 /**
  * @brief Observer.
  *
@@ -215,14 +208,15 @@ class basic_observer {
     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(basic_observer &obs, basic_registry<Entity> &reg, const Entity entt) {
-            if([&reg, entt]() {
+            auto condition = [&reg, entt]() {
                 if constexpr(sizeof...(Ignore) == 0) {
                     return reg.template all_of<AllOf..., Require...>(entt) && !reg.template any_of<NoneOf..., Reject...>(entt);
                 } else {
                     return reg.template all_of<AllOf..., Require...>(entt) && ((std::is_same_v<Ignore..., NoneOf> || !reg.template any_of<NoneOf>(entt)) && ...) && !reg.template any_of<Reject...>(entt);
                 }
-            }())
-            {
+            };
+
+            if(condition()) {
                 if(!obs.storage.contains(entt)) {
                     obs.storage.emplace(entt);
                 }
@@ -281,8 +275,7 @@ public:
     /*! @brief Default constructor. */
     basic_observer()
         : release{},
-          storage{}
-    {}
+          storage{} {}
 
     /*! @brief Default copy constructor, deleted on purpose. */
     basic_observer(const basic_observer &) = delete;
@@ -296,8 +289,7 @@ public:
      */
     template<typename... Matcher>
     basic_observer(basic_registry<entity_type> &reg, basic_collector<Matcher...>)
-        : basic_observer{}
-    {
+        : basic_observer{} {
         connect<Matcher...>(reg, std::index_sequence_for<Matcher...>{});
     }
 
@@ -308,13 +300,13 @@ public:
      * @brief Default copy assignment operator, deleted on purpose.
      * @return This observer.
      */
-    basic_observer & operator=(const basic_observer &) = delete;
+    basic_observer &operator=(const basic_observer &) = delete;
 
     /**
      * @brief Default move assignment operator, deleted on purpose.
      * @return This observer.
      */
-    basic_observer & operator=(basic_observer &&) = delete;
+    basic_observer &operator=(basic_observer &&) = delete;
 
     /**
      * @brief Connects an observer to a given registry.
@@ -364,7 +356,7 @@ public:
      *
      * @return A pointer to the array of entities.
      */
-    [[nodiscard]] const entity_type * data() const ENTT_NOEXCEPT {
+    [[nodiscard]] const entity_type *data() const ENTT_NOEXCEPT {
         return storage.data();
     }
 
@@ -439,8 +431,6 @@ private:
     basic_storage<entity_type, payload_type> storage;
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/entity/organizer.hpp

@@ -1,54 +1,47 @@
 #ifndef ENTT_ENTITY_ORGANIZER_HPP
 #define ENTT_ENTITY_ORGANIZER_HPP
 
-
-#include <cstddef>
 #include <algorithm>
+#include <cstddef>
 #include <type_traits>
-#include <unordered_map>
 #include <utility>
 #include <vector>
+#include "../container/dense_map.hpp"
 #include "../core/type_info.hpp"
 #include "../core/type_traits.hpp"
+#include "../core/utility.hpp"
 #include "fwd.hpp"
 #include "helper.hpp"
 
-
 namespace entt {
 
-
 /**
  * @cond TURN_OFF_DOXYGEN
  * Internal details not to be documented.
  */
 
-
 namespace internal {
 
-
 template<typename>
 struct is_view: std::false_type {};
 
-template<typename Entity, typename... Exclude, typename... Component>
-struct is_view<basic_view<Entity, exclude_t<Exclude...>, Component...>>: std::true_type {};
+template<typename Entity, typename... Component, typename... Exclude>
+struct is_view<basic_view<Entity, get_t<Component...>, exclude_t<Exclude...>>>: std::true_type {};
 
 template<typename Type>
 inline constexpr bool is_view_v = is_view<Type>::value;
 
-
 template<typename Type, typename Override>
 struct unpack_type {
     using ro = std::conditional_t<
         type_list_contains_v<Override, std::add_const_t<Type>> || (std::is_const_v<Type> && !type_list_contains_v<Override, std::remove_const_t<Type>>),
         type_list<std::remove_const_t<Type>>,
-        type_list<>
-    >;
+        type_list<>>;
 
     using rw = std::conditional_t<
         type_list_contains_v<Override, std::remove_const_t<Type>> || (!std::is_const_v<Type> && !type_list_contains_v<Override, std::add_const_t<Type>>),
         type_list<Type>,
-        type_list<>
-    >;
+        type_list<>>;
 };
 
 template<typename Entity, typename... Override>
@@ -59,57 +52,47 @@ struct unpack_type<basic_registry<Entity>, type_list<Override...>> {
 
 template<typename Entity, typename... Override>
 struct unpack_type<const basic_registry<Entity>, type_list<Override...>>
-    : unpack_type<basic_registry<Entity>, type_list<Override...>>
-{};
+    : unpack_type<basic_registry<Entity>, type_list<Override...>> {};
 
-template<typename Entity, typename... Exclude, typename... Component, typename... Override>
-struct unpack_type<basic_view<Entity, exclude_t<Exclude...>, Component...>, type_list<Override...>> {
+template<typename Entity, typename... Component, typename... Exclude, typename... Override>
+struct unpack_type<basic_view<Entity, get_t<Component...>, exclude_t<Exclude...>>, type_list<Override...>> {
     using ro = type_list_cat_t<type_list<Exclude...>, typename unpack_type<Component, type_list<Override...>>::ro...>;
     using rw = type_list_cat_t<typename unpack_type<Component, type_list<Override...>>::rw...>;
 };
 
-template<typename Entity, typename... Exclude, typename... Component, typename... Override>
-struct unpack_type<const basic_view<Entity, exclude_t<Exclude...>, Component...>, type_list<Override...>>
-    : unpack_type<basic_view<Entity, exclude_t<Exclude...>, Component...>, type_list<Override...>>
-{};
-
+template<typename Entity, typename... Component, typename... Exclude, typename... Override>
+struct unpack_type<const basic_view<Entity, get_t<Component...>, exclude_t<Exclude...>>, type_list<Override...>>
+    : unpack_type<basic_view<Entity, get_t<Component...>, exclude_t<Exclude...>>, type_list<Override...>> {};
 
 template<typename, typename>
-struct resource;
+struct resource_traits;
 
 template<typename... Args, typename... Req>
-struct resource<type_list<Args...>, type_list<Req...>> {
+struct resource_traits<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...>;
 };
 
-
 template<typename... Req, typename Ret, typename... Args>
-resource<type_list<std::remove_reference_t<Args>...>, type_list<Req...>> free_function_to_resource(Ret(*)(Args...));
+resource_traits<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<type_list<std::remove_reference_t<Args>...>, type_list<Req...>> constrained_function_to_resource(Ret(*)(Type &, Args...));
+resource_traits<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<type_list<std::remove_reference_t<Args>...>, type_list<Req...>> constrained_function_to_resource(Ret(Class:: *)(Args...));
+resource_traits<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<type_list<std::remove_reference_t<Args>...>, type_list<Req...>> constrained_function_to_resource(Ret(Class:: *)(Args...) const);
-
-template<typename... Req>
-resource<type_list<>, type_list<Req...>> to_resource();
-
-
-}
+resource_traits<type_list<std::remove_reference_t<Args>...>, type_list<Req...>> constrained_function_to_resource_traits(Ret (Class::*)(Args...) const);
 
+} // namespace internal
 
 /**
  * Internal details not to be documented.
  * @endcond
  */
 
-
 /**
  * @brief Utility class for creating a static task graph.
  *
@@ -123,9 +106,9 @@ resource<type_list<>, type_list<Req...>> to_resource();
  */
 template<typename Entity>
 class basic_organizer final {
-    using callback_type = void(const void *, entt::basic_registry<Entity> &);
-    using prepare_type = void(entt::basic_registry<Entity> &);
-    using dependency_type = std::size_t(const bool, type_info *, const std::size_t);
+    using callback_type = void(const void *, basic_registry<Entity> &);
+    using prepare_type = void(basic_registry<Entity> &);
+    using dependency_type = std::size_t(const bool, const type_info **, const std::size_t);
 
     struct vertex_data final {
         std::size_t ro_count{};
@@ -135,7 +118,7 @@ class basic_organizer final {
         callback_type *callback{};
         dependency_type *dependency;
         prepare_type *prepare{};
-        type_info info{};
+        const type_info *info{};
     };
 
     template<typename Type>
@@ -145,7 +128,7 @@ class basic_organizer final {
         } else if constexpr(internal::is_view_v<Type>) {
             return as_view{reg};
         } else {
-            return reg.template ctx_or_set<std::remove_reference_t<Type>>();
+            return reg.ctx().template emplace<std::remove_reference_t<Type>>();
         }
     }
 
@@ -155,11 +138,11 @@ class basic_organizer final {
     }
 
     template<typename... Type>
-    static std::size_t fill_dependencies(type_list<Type...>, [[maybe_unused]] type_info *buffer, [[maybe_unused]] const std::size_t count) {
+    static std::size_t fill_dependencies(type_list<Type...>, [[maybe_unused]] const type_info **buffer, [[maybe_unused]] const std::size_t count) {
         if constexpr(sizeof...(Type) == 0u) {
             return {};
         } else {
-            type_info info[sizeof...(Type)]{type_id<Type>()...};
+            const type_info *info[sizeof...(Type)]{&type_id<Type>()...};
             const auto length = (std::min)(count, sizeof...(Type));
             std::copy_n(info, length, buffer);
             return length;
@@ -177,7 +160,7 @@ class basic_organizer final {
         const auto length = vertices.size();
         std::vector<bool> edges(length * length, false);
 
-        // creates the ajacency matrix
+        // creates the adjacency matrix
         for(const auto &deps: dependencies) {
             const auto last = deps.second.cend();
             auto it = deps.second.cbegin();
@@ -262,8 +245,7 @@ public:
         vertex(const bool vtype, vertex_data data, std::vector<std::size_t> edges)
             : is_top_level{vtype},
               node{std::move(data)},
-              reachable{std::move(edges)}
-        {}
+              reachable{std::move(edges)} {}
 
         /**
          * @brief Fills a buffer with the type info objects for the writable
@@ -272,7 +254,7 @@ public:
          * @param length The length of the user-supplied buffer.
          * @return The number of type info objects written to the buffer.
          */
-        size_type ro_dependency(type_info *buffer, const std::size_t length) const ENTT_NOEXCEPT {
+        size_type ro_dependency(const type_info **buffer, const std::size_t length) const ENTT_NOEXCEPT {
             return node.dependency(false, buffer, length);
         }
 
@@ -283,7 +265,7 @@ public:
          * @param length The length of the user-supplied buffer.
          * @return The number of type info objects written to the buffer.
          */
-        size_type rw_dependency(type_info *buffer, const std::size_t length) const ENTT_NOEXCEPT {
+        size_type rw_dependency(const type_info **buffer, const std::size_t length) const ENTT_NOEXCEPT {
             return node.dependency(true, buffer, length);
         }
 
@@ -315,15 +297,15 @@ public:
          * @brief Returns a type info object associated with a vertex.
          * @return A properly initialized type info object.
          */
-        type_info info() const ENTT_NOEXCEPT {
-            return node.info;
+        const type_info &info() const ENTT_NOEXCEPT {
+            return *node.info;
         }
 
         /**
          * @brief Returns a user defined name associated with a vertex, if any.
          * @return The user defined name associated with the vertex, if any.
          */
-        const char * name() const ENTT_NOEXCEPT {
+        const char *name() const ENTT_NOEXCEPT {
             return node.name;
         }
 
@@ -331,7 +313,7 @@ public:
          * @brief Returns the function associated with a vertex.
          * @return The function associated with the vertex.
          */
-        function_type * callback() const ENTT_NOEXCEPT {
+        function_type *callback() const ENTT_NOEXCEPT {
             return node.callback;
         }
 
@@ -339,7 +321,7 @@ public:
          * @brief Returns the payload associated with a vertex, if any.
          * @return The payload associated with the vertex, if any.
          */
-        const void * data() const ENTT_NOEXCEPT {
+        const void *data() const ENTT_NOEXCEPT {
             return node.payload;
         }
 
@@ -347,7 +329,7 @@ public:
          * @brief Returns the list of nodes reachable from a given vertex.
          * @return The list of nodes reachable from the vertex.
          */
-        const std::vector<std::size_t> & children() const ENTT_NOEXCEPT {
+        const std::vector<std::size_t> &children() const ENTT_NOEXCEPT {
             return reachable;
         }
 
@@ -374,25 +356,25 @@ public:
      */
     template<auto Candidate, typename... Req>
     void emplace(const char *name = nullptr) {
-        using resource_type = decltype(internal::free_function_to_resource<Req...>(Candidate));
+        using resource_type = decltype(internal::free_function_to_resource_traits<Req...>(Candidate));
         constexpr auto requires_registry = type_list_contains_v<typename resource_type::args, basic_registry<entity_type>>;
 
         callback_type *callback = +[](const void *, basic_registry<entity_type> &reg) {
             std::apply(Candidate, to_args(reg, typename resource_type::args{}));
         };
 
-        track_dependencies(vertices.size(), requires_registry, typename resource_type::ro{}, typename resource_type::rw{});
-
-        vertices.push_back({
+        vertex_data vdata{
             resource_type::ro::size,
             resource_type::rw::size,
             name,
             nullptr,
             callback,
-            +[](const bool rw, type_info *buffer, const std::size_t length) { return rw ? fill_dependencies(typename resource_type::rw{}, buffer, length) : fill_dependencies(typename resource_type::ro{}, buffer, length); },
+            +[](const bool rw, const type_info **buffer, const std::size_t length) { return rw ? fill_dependencies(typename resource_type::rw{}, buffer, length) : fill_dependencies(typename resource_type::ro{}, buffer, length); },
             +[](basic_registry<entity_type> &reg) { void(to_args(reg, typename resource_type::args{})); },
-            type_id<std::integral_constant<decltype(Candidate), Candidate>>()
-        });
+            &type_id<std::integral_constant<decltype(Candidate), Candidate>>()};
+
+        track_dependencies(vertices.size(), requires_registry, typename resource_type::ro{}, typename resource_type::rw{});
+        vertices.push_back(std::move(vdata));
     }
 
     /**
@@ -406,7 +388,7 @@ 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<Req...>(Candidate));
+        using resource_type = decltype(internal::constrained_function_to_resource_traits<Req...>(Candidate));
         constexpr auto requires_registry = type_list_contains_v<typename resource_type::args, basic_registry<entity_type>>;
 
         callback_type *callback = +[](const void *payload, basic_registry<entity_type> &reg) {
@@ -414,22 +396,18 @@ public:
             std::apply(Candidate, std::tuple_cat(std::forward_as_tuple(*curr), to_args(reg, typename resource_type::args{})));
         };
 
-        track_dependencies(vertices.size(), requires_registry, typename resource_type::ro{}, typename resource_type::rw{});
-
-        vertices.push_back({
+        vertex_data vdata{
             resource_type::ro::size,
             resource_type::rw::size,
             name,
             &value_or_instance,
             callback,
-            +[](const bool rw, type_info *buffer, const std::size_t length) {
-                return rw ? fill_dependencies(typename resource_type::rw{}, buffer, length) : fill_dependencies(typename resource_type::ro{}, buffer, length);
-            },
-            +[](basic_registry<entity_type> &reg) {
-                void(to_args(reg, typename resource_type::args{}));
-            },
-            type_id<std::integral_constant<decltype(Candidate), Candidate>>()
-        });
+            +[](const bool rw, const type_info **buffer, const std::size_t length) { return rw ? fill_dependencies(typename resource_type::rw{}, buffer, length) : fill_dependencies(typename resource_type::ro{}, buffer, length); },
+            +[](basic_registry<entity_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{});
+        vertices.push_back(std::move(vdata));
     }
 
     /**
@@ -442,21 +420,20 @@ public:
      */
     template<typename... Req>
     void emplace(function_type *func, const void *payload = nullptr, const char *name = nullptr) {
-        using resource_type = internal::resource<type_list<>, type_list<Req...>>;
+        using resource_type = internal::resource_traits<type_list<>, type_list<Req...>>;
         track_dependencies(vertices.size(), true, typename resource_type::ro{}, typename resource_type::rw{});
 
-        vertices.push_back({
+        vertex_data vdata{
             resource_type::ro::size,
             resource_type::rw::size,
             name,
             payload,
             func,
-            +[](const bool rw, type_info *buffer, const std::size_t length) {
-                return rw ? fill_dependencies(typename resource_type::rw{}, buffer, length) : fill_dependencies(typename resource_type::ro{}, buffer, length);
-            },
+            +[](const bool rw, const type_info **buffer, const std::size_t length) { return rw ? fill_dependencies(typename resource_type::rw{}, buffer, length) : fill_dependencies(typename resource_type::ro{}, buffer, length); },
             nullptr,
-            type_info{}
-        });
+            &type_id<void>()};
+
+        vertices.push_back(std::move(vdata));
     }
 
     /**
@@ -498,12 +475,10 @@ public:
     }
 
 private:
-    std::unordered_map<entt::id_type, std::vector<std::pair<std::size_t, bool>>> dependencies;
+    dense_map<id_type, std::vector<std::pair<std::size_t, bool>>, identity> dependencies;
     std::vector<vertex_data> vertices;
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/entity/poly_storage.hpp

@@ -1,60 +0,0 @@
-#ifndef ENTT_ENTITY_POLY_STORAGE_HPP
-#define ENTT_ENTITY_POLY_STORAGE_HPP
-
-
-#include <cstddef>
-#include <tuple>
-#include "../core/type_info.hpp"
-#include "../core/type_traits.hpp"
-#include "../poly/poly.hpp"
-#include "fwd.hpp"
-
-
-namespace entt {
-
-
-/**
- * @brief Basic poly storage implementation.
- * @tparam Entity A valid entity type (see entt_traits for more details).
- */
-template<typename Entity>
-struct Storage: type_list<type_info() const ENTT_NOEXCEPT> {
-    /**
-     * @brief Concept definition.
-     * @tparam Base Opaque base class from which to inherit.
-     */
-    template<typename Base>
-    struct type: Base {
-        /**
-         * @brief Returns a type info for the contained objects.
-         * @return The type info for the contained objects.
-         */
-        type_info value_type() const ENTT_NOEXCEPT {
-            return poly_call<0>(*this);
-        }
-    };
-
-    /**
-     * @brief Concept implementation.
-     * @tparam Type Type for which to generate an implementation.
-     */
-    template<typename Type>
-    using impl = value_list<&type_id<typename Type::value_type>>;
-};
-
-
-/**
- * @brief Defines the poly storage type associate with a given entity type.
- * @tparam Entity A valid entity type (see entt_traits for more details).
- */
-template<typename Entity, typename = void>
-struct poly_storage_traits {
-    /*! @brief Poly storage type for the given entity type. */
-    using storage_type = poly<Storage<Entity>>;
-};
-
-
-}
-
-
-#endif

src/entt/entity/runtime_view.hpp

@@ -1,23 +1,119 @@
 #ifndef ENTT_ENTITY_RUNTIME_VIEW_HPP
 #define ENTT_ENTITY_RUNTIME_VIEW_HPP
 
-
-#include <iterator>
-#include <vector>
-#include <utility>
 #include <algorithm>
+#include <iterator>
 #include <type_traits>
+#include <utility>
+#include <vector>
 #include "../config/config.h"
 #include "entity.hpp"
-#include "sparse_set.hpp"
 #include "fwd.hpp"
-
+#include "sparse_set.hpp"
 
 namespace entt {
 
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename Set>
+class runtime_view_iterator final {
+    using iterator_type = typename Set::iterator;
+
+    [[nodiscard]] bool valid() const {
+        return (!tombstone_check || *it != tombstone)
+               && std::all_of(++pools->begin(), pools->end(), [entt = *it](const auto *curr) { return curr->contains(entt); })
+               && std::none_of(filter->cbegin(), filter->cend(), [entt = *it](const auto *curr) { return curr && curr->contains(entt); });
+    }
+
+public:
+    using difference_type = typename iterator_type::difference_type;
+    using value_type = typename iterator_type::value_type;
+    using pointer = typename iterator_type::pointer;
+    using reference = typename iterator_type::reference;
+    using iterator_category = std::bidirectional_iterator_tag;
+
+    runtime_view_iterator() ENTT_NOEXCEPT
+        : pools{},
+          filter{},
+          it{},
+          tombstone_check{} {}
+
+    runtime_view_iterator(const std::vector<const Set *> &cpools, const std::vector<const Set *> &ignore, iterator_type curr) ENTT_NOEXCEPT
+        : pools{&cpools},
+          filter{&ignore},
+          it{curr},
+          tombstone_check{pools->size() == 1u && (*pools)[0u]->policy() == deletion_policy::in_place} {
+        if(it != (*pools)[0]->end() && !valid()) {
+            ++(*this);
+        }
+    }
+
+    runtime_view_iterator &operator++() {
+        while(++it != (*pools)[0]->end() && !valid()) {}
+        return *this;
+    }
+
+    runtime_view_iterator operator++(int) {
+        runtime_view_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    runtime_view_iterator &operator--() {
+        while(--it != (*pools)[0]->begin() && !valid()) {}
+        return *this;
+    }
+
+    runtime_view_iterator operator--(int) {
+        runtime_view_iterator orig = *this;
+        return operator--(), orig;
+    }
+
+    [[nodiscard]] pointer operator->() const ENTT_NOEXCEPT {
+        return it.operator->();
+    }
+
+    [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
+        return *operator->();
+    }
+
+    [[nodiscard]] bool operator==(const runtime_view_iterator &other) const ENTT_NOEXCEPT {
+        return it == other.it;
+    }
+
+    [[nodiscard]] bool operator!=(const runtime_view_iterator &other) const ENTT_NOEXCEPT {
+        return !(*this == other);
+    }
+
+private:
+    const std::vector<const Set *> *pools;
+    const std::vector<const Set *> *filter;
+    iterator_type it;
+    bool tombstone_check;
+};
+
+} // namespace internal
 
 /**
- * @brief Runtime view.
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief Runtime view implementation.
+ *
+ * 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_runtime_view;
+
+/**
+ * @brief Generic runtime view.
  *
  * Runtime views iterate over those entities that have at least all the given
  * components in their bags. During initialization, a runtime view looks at the
@@ -53,115 +149,47 @@ namespace entt {
  * In any other case, attempting to use a view results in undefined behavior.
  *
  * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Allocator Type of allocator used to manage memory and elements.
  */
-template<typename Entity>
-class basic_runtime_view final {
-    using basic_common_type = basic_sparse_set<Entity>;
-    using underlying_iterator = typename basic_common_type::iterator;
-
-    class view_iterator final {
-        [[nodiscard]] bool valid() const {
-            const auto entt = *it;
-
-            return (!stable_storage || (entt != tombstone))
-                && std::all_of(pools->begin()++, pools->end(), [entt](const auto *curr) { return curr->contains(entt); })
-                && std::none_of(filter->cbegin(), filter->cend(), [entt](const auto *curr) { return curr && curr->contains(entt); });
-        }
-
-    public:
-        using difference_type = typename underlying_iterator::difference_type;
-        using value_type = typename underlying_iterator::value_type;
-        using pointer = typename underlying_iterator::pointer;
-        using reference = typename underlying_iterator::reference;
-        using iterator_category = std::bidirectional_iterator_tag;
-
-        view_iterator() ENTT_NOEXCEPT = default;
-
-        view_iterator(const std::vector<const basic_common_type *> &cpools, const std::vector<const basic_common_type *> &ignore, underlying_iterator curr) ENTT_NOEXCEPT
-            : pools{&cpools},
-              filter{&ignore},
-              it{curr},
-              stable_storage{std::any_of(pools->cbegin(), pools->cend(), [](const basic_common_type *cpool) { return (cpool->policy() == deletion_policy::in_place); })}
-        {
-            if(it != (*pools)[0]->end() && !valid()) {
-                ++(*this);
-            }
-        }
-
-        view_iterator & operator++() {
-            while(++it != (*pools)[0]->end() && !valid());
-            return *this;
-        }
-
-        view_iterator operator++(int) {
-            view_iterator orig = *this;
-            return ++(*this), orig;
-        }
-
-        view_iterator & operator--() ENTT_NOEXCEPT {
-            while(--it != (*pools)[0]->begin() && !valid());
-            return *this;
-        }
-
-        view_iterator operator--(int) ENTT_NOEXCEPT {
-            view_iterator orig = *this;
-            return operator--(), orig;
-        }
-
-        [[nodiscard]] bool operator==(const view_iterator &other) const ENTT_NOEXCEPT {
-            return other.it == it;
-        }
-
-        [[nodiscard]] bool operator!=(const view_iterator &other) const ENTT_NOEXCEPT {
-            return !(*this == other);
-        }
-
-        [[nodiscard]] pointer operator->() const {
-            return it.operator->();
-        }
-
-        [[nodiscard]] reference operator*() const {
-            return *operator->();
-        }
-
-    private:
-        const std::vector<const basic_common_type *> *pools;
-        const std::vector<const basic_common_type *> *filter;
-        underlying_iterator it;
-        bool stable_storage;
-    };
-
-    [[nodiscard]] bool valid() const {
-        return !pools.empty() && pools.front();
-    }
-
-public:
+template<typename Entity, typename Allocator>
+struct basic_runtime_view<basic_sparse_set<Entity, Allocator>> {
     /*! @brief Underlying entity identifier. */
     using entity_type = Entity;
     /*! @brief Unsigned integer type. */
     using size_type = std::size_t;
+    /*! @brief Common type among all storage types. */
+    using base_type = basic_sparse_set<Entity, Allocator>;
     /*! @brief Bidirectional iterator type. */
-    using iterator = view_iterator;
+    using iterator = internal::runtime_view_iterator<base_type>;
 
     /*! @brief Default constructor to use to create empty, invalid views. */
     basic_runtime_view() ENTT_NOEXCEPT
         : pools{},
-          filter{}
-    {}
+          filter{} {}
 
     /**
-     * @brief Constructs a runtime view from a set of storage classes.
-     * @param cpools The storage for the types to iterate.
-     * @param epools The storage for the types used to filter the view.
+     * @brief Appends an opaque storage object to a runtime view.
+     * @param base An opaque reference to a storage object.
+     * @return This runtime view.
      */
-    basic_runtime_view(std::vector<const basic_common_type *> cpools, std::vector<const basic_common_type *> epools) ENTT_NOEXCEPT
-        : pools{std::move(cpools)},
-          filter{std::move(epools)}
-    {
-        // brings the best candidate (if any) on front of the vector
-        std::rotate(pools.begin(), std::min_element(pools.begin(), pools.end(), [](const auto *lhs, const auto *rhs) {
-            return (!lhs && rhs) || (lhs && rhs && lhs->size() < rhs->size());
-        }), pools.end());
+    basic_runtime_view &iterate(const base_type &base) {
+        if(pools.empty() || !(base.size() < pools[0u]->size())) {
+            pools.push_back(&base);
+        } else {
+            pools.push_back(std::exchange(pools[0u], &base));
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Adds an opaque storage object as a filter of a runtime view.
+     * @param base An opaque reference to a storage object.
+     * @return This runtime view.
+     */
+    basic_runtime_view &exclude(const base_type &base) {
+        filter.push_back(&base);
+        return *this;
     }
 
     /**
@@ -169,7 +197,7 @@ public:
      * @return Estimated number of entities iterated by the view.
      */
     [[nodiscard]] size_type size_hint() const {
-        return valid() ? pools.front()->size() : size_type{};
+        return pools.empty() ? size_type{} : pools.front()->size();
     }
 
     /**
@@ -183,7 +211,7 @@ public:
      * @return An iterator to the first entity that has the given components.
      */
     [[nodiscard]] iterator begin() const {
-        return valid() ? iterator{pools, filter, pools[0]->begin()} : iterator{};
+        return pools.empty() ? iterator{} : iterator{pools, filter, pools[0]->begin()};
     }
 
     /**
@@ -198,17 +226,18 @@ public:
      * given components.
      */
     [[nodiscard]] iterator end() const {
-        return valid() ? iterator{pools, filter, pools[0]->end()} : iterator{};
+        return pools.empty() ? iterator{} : iterator{pools, filter, pools[0]->end()};
     }
 
     /**
      * @brief Checks if a view contains an entity.
-     * @param entt A valid entity identifier.
+     * @param entt A valid identifier.
      * @return True if the view contains the given entity, false otherwise.
      */
     [[nodiscard]] bool contains(const entity_type entt) const {
-        return valid() && 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); });
+        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); });
     }
 
     /**
@@ -234,12 +263,10 @@ public:
     }
 
 private:
-    std::vector<const basic_common_type *> pools;
-    std::vector<const basic_common_type *> filter;
+    std::vector<const base_type *> pools;
+    std::vector<const base_type *> filter;
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/entity/sigh_storage_mixin.hpp

@@ -0,0 +1,177 @@
+#ifndef ENTT_ENTITY_SIGH_STORAGE_MIXIN_HPP
+#define ENTT_ENTITY_SIGH_STORAGE_MIXIN_HPP
+
+#include <utility>
+#include "../config/config.h"
+#include "../core/any.hpp"
+#include "../signal/sigh.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @brief Mixin type used to add signal support to storage types.
+ *
+ * The function type of a listener is equivalent to:
+ *
+ * @code{.cpp}
+ * void(basic_registry<entity_type> &, entity_type);
+ * @endcode
+ *
+ * This applies to all signals made available.
+ *
+ * @tparam Type The type of the underlying storage.
+ */
+template<typename Type>
+class sigh_storage_mixin final: public Type {
+    using basic_iterator = typename Type::basic_iterator;
+
+    template<typename Func>
+    void notify_destruction(basic_iterator first, basic_iterator last, Func func) {
+        ENTT_ASSERT(owner != nullptr, "Invalid pointer to registry");
+
+        for(; first != last; ++first) {
+            const auto entt = *first;
+            destruction.publish(*owner, entt);
+            const auto it = Type::find(entt);
+            func(it, it + 1u);
+        }
+    }
+
+    void swap_and_pop(basic_iterator first, basic_iterator last) final {
+        notify_destruction(std::move(first), std::move(last), [this](auto... args) { Type::swap_and_pop(args...); });
+    }
+
+    void in_place_pop(basic_iterator first, basic_iterator last) final {
+        notify_destruction(std::move(first), std::move(last), [this](auto... args) { Type::in_place_pop(args...); });
+    }
+
+    basic_iterator try_emplace(const typename Type::entity_type entt, const bool force_back, const void *value) final {
+        ENTT_ASSERT(owner != nullptr, "Invalid pointer to registry");
+        Type::try_emplace(entt, force_back, value);
+        construction.publish(*owner, entt);
+        return Type::find(entt);
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename Type::entity_type;
+
+    /*! @brief Inherited constructors. */
+    using Type::Type;
+
+    /**
+     * @brief Returns a sink object.
+     *
+     * The sink returned by this function can be used to receive notifications
+     * whenever a new instance is created and assigned to an entity.<br/>
+     * Listeners are invoked after the object has been assigned to the entity.
+     *
+     * @sa sink
+     *
+     * @return A temporary sink object.
+     */
+    [[nodiscard]] auto on_construct() ENTT_NOEXCEPT {
+        return sink{construction};
+    }
+
+    /**
+     * @brief Returns a sink object.
+     *
+     * The sink returned by this function can be used to receive notifications
+     * whenever an instance is explicitly updated.<br/>
+     * Listeners are invoked after the object has been updated.
+     *
+     * @sa sink
+     *
+     * @return A temporary sink object.
+     */
+    [[nodiscard]] auto on_update() ENTT_NOEXCEPT {
+        return sink{update};
+    }
+
+    /**
+     * @brief Returns a sink object.
+     *
+     * The sink returned by this function can be used to receive notifications
+     * whenever an instance is removed from an entity and thus destroyed.<br/>
+     * Listeners are invoked before the object has been removed from the entity.
+     *
+     * @sa sink
+     *
+     * @return A temporary sink object.
+     */
+    [[nodiscard]] auto on_destroy() ENTT_NOEXCEPT {
+        return sink{destruction};
+    }
+
+    /**
+     * @brief Assigns entities to a storage.
+     * @tparam Args Types of arguments to use to construct the object.
+     * @param entt A valid identifier.
+     * @param args Parameters to use to initialize the object.
+     * @return A reference to the newly created object.
+     */
+    template<typename... Args>
+    decltype(auto) emplace(const entity_type entt, Args &&...args) {
+        ENTT_ASSERT(owner != nullptr, "Invalid pointer to registry");
+        Type::emplace(entt, std::forward<Args>(args)...);
+        construction.publish(*owner, entt);
+        return this->get(entt);
+    }
+
+    /**
+     * @brief Patches the given instance for an entity.
+     * @tparam Func Types of the function objects to invoke.
+     * @param entt A valid identifier.
+     * @param func Valid function objects.
+     * @return A reference to the patched instance.
+     */
+    template<typename... Func>
+    decltype(auto) patch(const entity_type entt, Func &&...func) {
+        ENTT_ASSERT(owner != nullptr, "Invalid pointer to registry");
+        Type::patch(entt, std::forward<Func>(func)...);
+        update.publish(*owner, entt);
+        return this->get(entt);
+    }
+
+    /**
+     * @brief Assigns entities to a storage.
+     * @tparam It Type of input iterator.
+     * @tparam Args Types of arguments to use to construct the objects assigned
+     * to the entities.
+     * @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 initialize the objects assigned to the
+     * entities.
+     */
+    template<typename It, typename... Args>
+    void insert(It first, It last, Args &&...args) {
+        ENTT_ASSERT(owner != nullptr, "Invalid pointer to registry");
+        Type::insert(first, last, std::forward<Args>(args)...);
+
+        for(auto it = construction.empty() ? last : first; it != last; ++it) {
+            construction.publish(*owner, *it);
+        }
+    }
+
+    /**
+     * @brief Forwards variables to mixins, if any.
+     * @param value A variable wrapped in an opaque container.
+     */
+    void bind(any value) ENTT_NOEXCEPT final {
+        auto *reg = any_cast<basic_registry<entity_type>>(&value);
+        owner = reg ? reg : owner;
+        Type::bind(std::move(value));
+    }
+
+private:
+    sigh<void(basic_registry<entity_type> &, const entity_type)> construction{};
+    sigh<void(basic_registry<entity_type> &, const entity_type)> destruction{};
+    sigh<void(basic_registry<entity_type> &, const entity_type)> update{};
+    basic_registry<entity_type> *owner{};
+};
+
+} // namespace entt
+
+#endif

src/entt/entity/snapshot.hpp

@@ -1,25 +1,23 @@
 #ifndef ENTT_ENTITY_SNAPSHOT_HPP
 #define ENTT_ENTITY_SNAPSHOT_HPP
 
-
 #include <array>
 #include <cstddef>
 #include <iterator>
 #include <tuple>
 #include <type_traits>
-#include <unordered_map>
 #include <utility>
 #include <vector>
 #include "../config/config.h"
+#include "../container/dense_map.hpp"
 #include "../core/type_traits.hpp"
+#include "component.hpp"
 #include "entity.hpp"
 #include "fwd.hpp"
 #include "registry.hpp"
 
-
 namespace entt {
 
-
 /**
  * @brief Utility class to create snapshots from a registry.
  *
@@ -32,12 +30,12 @@ namespace entt {
  */
 template<typename Entity>
 class basic_snapshot {
-    using traits_type = entt_traits<Entity>;
+    using entity_traits = entt_traits<Entity>;
 
     template<typename Component, typename Archive, typename It>
     void get(Archive &archive, std::size_t sz, It first, It last) const {
         const auto view = reg->template view<std::add_const_t<Component>>();
-        archive(typename traits_type::entity_type(sz));
+        archive(typename entity_traits::entity_type(sz));
 
         while(first != last) {
             const auto entt = *(first++);
@@ -55,7 +53,7 @@ class basic_snapshot {
 
         while(begin != last) {
             const auto entt = *(begin++);
-            ((reg->template all_of<Component>(entt) ? ++size[Index] : size[Index]), ...);
+            ((reg->template all_of<Component>(entt) ? ++size[Index] : 0u), ...);
         }
 
         (get<Component>(archive, size[Index], first, last), ...);
@@ -70,14 +68,13 @@ public:
      * @param source A valid reference to a registry.
      */
     basic_snapshot(const basic_registry<entity_type> &source) ENTT_NOEXCEPT
-        : reg{&source}
-    {}
+        : reg{&source} {}
 
     /*! @brief Default move constructor. */
-    basic_snapshot(basic_snapshot &&) = default;
+    basic_snapshot(basic_snapshot &&) ENTT_NOEXCEPT = default;
 
     /*! @brief Default move assignment operator. @return This snapshot. */
-    basic_snapshot & operator=(basic_snapshot &&) = default;
+    basic_snapshot &operator=(basic_snapshot &&) ENTT_NOEXCEPT = default;
 
     /**
      * @brief Puts aside all the entities from the underlying registry.
@@ -90,17 +87,16 @@ public:
      * @return An object of this type to continue creating the snapshot.
      */
     template<typename Archive>
-    const basic_snapshot & entities(Archive &archive) const {
+    const basic_snapshot &entities(Archive &archive) const {
         const auto sz = reg->size();
 
-        archive(typename traits_type::entity_type(sz));
+        archive(typename entity_traits::entity_type(sz + 1u));
+        archive(reg->released());
 
         for(auto first = reg->data(), last = first + sz; first != last; ++first) {
             archive(*first);
         }
 
-        archive(reg->released());
-
         return *this;
     }
 
@@ -116,10 +112,10 @@ public:
      * @return An object of this type to continue creating the snapshot.
      */
     template<typename... Component, typename Archive>
-    const basic_snapshot & component(Archive &archive) const {
+    const basic_snapshot &component(Archive &archive) const {
         if constexpr(sizeof...(Component) == 1u) {
             const auto view = reg->template view<const Component...>();
-            (component<Component>(archive, view.data(), view.data() + view.size()), ...);
+            (component<Component>(archive, view.rbegin(), view.rend()), ...);
             return *this;
         } else {
             (component<Component>(archive), ...);
@@ -142,7 +138,7 @@ public:
      * @return An object of this type to continue creating the snapshot.
      */
     template<typename... Component, typename Archive, typename It>
-    const basic_snapshot & component(Archive &archive, It first, It last) const {
+    const basic_snapshot &component(Archive &archive, It first, It last) const {
         component<Component...>(archive, first, last, std::index_sequence_for<Component...>{});
         return *this;
     }
@@ -151,7 +147,6 @@ private:
     const basic_registry<entity_type> *reg;
 };
 
-
 /**
  * @brief Utility class to restore a snapshot as a whole.
  *
@@ -164,30 +159,30 @@ private:
  */
 template<typename Entity>
 class basic_snapshot_loader {
-    using traits_type = entt_traits<Entity>;
+    using entity_traits = entt_traits<Entity>;
 
     template<typename Type, typename Archive>
     void assign(Archive &archive) const {
-        typename traits_type::entity_type length{};
+        typename entity_traits::entity_type length{};
+        entity_type entt;
+
         archive(length);
 
-        entity_type entt{};
-
-        if constexpr(std::tuple_size_v<decltype(reg->template view<Type>().get({}))> == 0) {
+        if constexpr(ignore_as_empty_v<Type>) {
             while(length--) {
                 archive(entt);
                 const auto entity = reg->valid(entt) ? entt : reg->create(entt);
                 ENTT_ASSERT(entity == entt, "Entity not available for use");
-                reg->template emplace<Type>(entity);
+                reg->template emplace<Type>(entt);
             }
         } else {
-            Type instance{};
+            Type instance;
 
             while(length--) {
                 archive(entt, instance);
                 const auto entity = reg->valid(entt) ? entt : reg->create(entt);
                 ENTT_ASSERT(entity == entt, "Entity not available for use");
-                reg->template emplace<Type>(entity, std::move(instance));
+                reg->template emplace<Type>(entt, std::move(instance));
             }
         }
     }
@@ -201,17 +196,16 @@ public:
      * @param source A valid reference to a registry.
      */
     basic_snapshot_loader(basic_registry<entity_type> &source) ENTT_NOEXCEPT
-        : reg{&source}
-    {
+        : reg{&source} {
         // restoring a snapshot as a whole requires a clean registry
         ENTT_ASSERT(reg->empty(), "Registry must be empty");
     }
 
     /*! @brief Default move constructor. */
-    basic_snapshot_loader(basic_snapshot_loader &&) = default;
+    basic_snapshot_loader(basic_snapshot_loader &&) ENTT_NOEXCEPT = default;
 
     /*! @brief Default move assignment operator. @return This loader. */
-    basic_snapshot_loader & operator=(basic_snapshot_loader &&) = default;
+    basic_snapshot_loader &operator=(basic_snapshot_loader &&) ENTT_NOEXCEPT = default;
 
     /**
      * @brief Restores entities that were in use during serialization.
@@ -224,20 +218,17 @@ public:
      * @return A valid loader to continue restoring data.
      */
     template<typename Archive>
-    const basic_snapshot_loader & entities(Archive &archive) const {
-        typename traits_type::entity_type length{};
+    const basic_snapshot_loader &entities(Archive &archive) const {
+        typename entity_traits::entity_type length{};
 
         archive(length);
         std::vector<entity_type> all(length);
 
-        for(decltype(length) pos{}; pos < length; ++pos) {
+        for(std::size_t pos{}; pos < length; ++pos) {
             archive(all[pos]);
         }
 
-        entity_type destroyed;
-        archive(destroyed);
-
-        reg->assign(all.cbegin(), all.cend(), destroyed);
+        reg->assign(++all.cbegin(), all.cend(), all[0u]);
 
         return *this;
     }
@@ -256,7 +247,7 @@ public:
      * @return A valid loader to continue restoring data.
      */
     template<typename... Component, typename Archive>
-    const basic_snapshot_loader & component(Archive &archive) const {
+    const basic_snapshot_loader &component(Archive &archive) const {
         (assign<Component>(archive), ...);
         return *this;
     }
@@ -271,9 +262,11 @@ public:
      *
      * @return A valid loader to continue restoring data.
      */
-    const basic_snapshot_loader & orphans() const {
-        reg->orphans([this](const auto entt) {
-            reg->release(entt);
+    const basic_snapshot_loader &orphans() const {
+        reg->each([this](const auto entt) {
+            if(reg->orphan(entt)) {
+                reg->release(entt);
+            }
         });
 
         return *this;
@@ -283,7 +276,6 @@ private:
     basic_registry<entity_type> *reg;
 };
 
-
 /**
  * @brief Utility class for _continuous loading_.
  *
@@ -302,7 +294,7 @@ private:
  */
 template<typename Entity>
 class basic_continuous_loader {
-    using traits_type = entt_traits<Entity>;
+    using entity_traits = entt_traits<Entity>;
 
     void destroy(Entity entt) {
         if(const auto it = remloc.find(entt); it == remloc.cend()) {
@@ -329,8 +321,7 @@ class basic_continuous_loader {
     }
 
     template<typename Container>
-    auto update(int, Container &container)
-    -> decltype(typename Container::mapped_type{}, void()) {
+    auto update(int, Container &container) -> decltype(typename Container::mapped_type{}, void()) {
         // map like container
         Container other;
 
@@ -348,12 +339,12 @@ class basic_continuous_loader {
             }
         }
 
-        std::swap(container, other);
+        using std::swap;
+        swap(container, other);
     }
 
     template<typename Container>
-    auto update(char, Container &container)
-    -> decltype(typename Container::value_type{}, void()) {
+    auto update(char, Container &container) -> decltype(typename Container::value_type{}, void()) {
         // vector like container
         static_assert(std::is_same_v<typename Container::value_type, entity_type>, "Invalid value type");
 
@@ -363,7 +354,7 @@ class basic_continuous_loader {
     }
 
     template<typename Other, typename Type, typename Member>
-    void update([[maybe_unused]] Other &instance, [[maybe_unused]] Member Type:: *member) {
+    void update([[maybe_unused]] Other &instance, [[maybe_unused]] Member Type::*member) {
         if constexpr(!std::is_same_v<Other, Type>) {
             return;
         } else if constexpr(std::is_same_v<Member, entity_type>) {
@@ -386,20 +377,20 @@ class basic_continuous_loader {
     }
 
     template<typename Other, typename Archive, typename... Type, typename... Member>
-    void assign(Archive &archive, [[maybe_unused]] Member Type:: *... member) {
-        typename traits_type::entity_type length{};
+    void assign(Archive &archive, [[maybe_unused]] Member Type::*...member) {
+        typename entity_traits::entity_type length{};
+        entity_type entt;
+
         archive(length);
 
-        entity_type entt{};
-
-        if constexpr(std::tuple_size_v<decltype(reg->template view<Other>().get({}))> == 0) {
+        if constexpr(ignore_as_empty_v<Other>) {
             while(length--) {
                 archive(entt);
                 restore(entt);
                 reg->template emplace_or_replace<Other>(map(entt));
             }
         } else {
-            Other instance{};
+            Other instance;
 
             while(length--) {
                 archive(entt, instance);
@@ -419,14 +410,13 @@ public:
      * @param source A valid reference to a registry.
      */
     basic_continuous_loader(basic_registry<entity_type> &source) ENTT_NOEXCEPT
-        : reg{&source}
-    {}
+        : reg{&source} {}
 
     /*! @brief Default move constructor. */
     basic_continuous_loader(basic_continuous_loader &&) = default;
 
     /*! @brief Default move assignment operator. @return This loader. */
-    basic_continuous_loader & operator=(basic_continuous_loader &&) = default;
+    basic_continuous_loader &operator=(basic_continuous_loader &&) = default;
 
     /**
      * @brief Restores entities that were in use during serialization.
@@ -439,25 +429,24 @@ public:
      * @return A non-const reference to this loader.
      */
     template<typename Archive>
-    basic_continuous_loader & entities(Archive &archive) {
-        typename traits_type::entity_type length{};
+    basic_continuous_loader &entities(Archive &archive) {
+        typename entity_traits::entity_type length{};
         entity_type entt{};
 
         archive(length);
+        // discards the head of the list of destroyed entities
+        archive(entt);
 
-        for(decltype(length) pos{}; pos < length; ++pos) {
+        for(std::size_t pos{}, last = length - 1u; pos < last; ++pos) {
             archive(entt);
 
-            if(const auto entity = traits_type::to_entity(entt); entity == pos) {
+            if(const auto entity = entity_traits::to_entity(entt); entity == pos) {
                 restore(entt);
             } else {
                 destroy(entt);
             }
         }
 
-        // discards the head of the list of destroyed entities
-        archive(entt);
-
         return *this;
     }
 
@@ -481,7 +470,7 @@ public:
      * @return A non-const reference to this loader.
      */
     template<typename... Component, typename Archive, typename... Type, typename... Member>
-    basic_continuous_loader & component(Archive &archive, Member Type:: *... member) {
+    basic_continuous_loader &component(Archive &archive, Member Type::*...member) {
         (remove_if_exists<Component>(), ...);
         (assign<Component>(archive, member...), ...);
         return *this;
@@ -495,7 +484,7 @@ public:
      *
      * @return A non-const reference to this loader.
      */
-    basic_continuous_loader & shrink() {
+    basic_continuous_loader &shrink() {
         auto it = remloc.begin();
 
         while(it != remloc.cend()) {
@@ -527,9 +516,11 @@ public:
      *
      * @return A non-const reference to this loader.
      */
-    basic_continuous_loader & orphans() {
-        reg->orphans([this](const auto entt) {
-            reg->release(entt);
+    basic_continuous_loader &orphans() {
+        reg->each([this](const auto entt) {
+            if(reg->orphan(entt)) {
+                reg->release(entt);
+            }
         });
 
         return *this;
@@ -537,7 +528,7 @@ public:
 
     /**
      * @brief Tests if a loader knows about a given entity.
-     * @param entt An entity identifier.
+     * @param entt A valid identifier.
      * @return True if `entity` is managed by the loader, false otherwise.
      */
     [[nodiscard]] bool contains(entity_type entt) const ENTT_NOEXCEPT {
@@ -546,7 +537,7 @@ public:
 
     /**
      * @brief Returns the identifier to which an entity refers.
-     * @param entt An entity identifier.
+     * @param entt A valid identifier.
      * @return The local identifier if any, the null entity otherwise.
      */
     [[nodiscard]] entity_type map(entity_type entt) const ENTT_NOEXCEPT {
@@ -561,12 +552,10 @@ public:
     }
 
 private:
-    std::unordered_map<entity_type, std::pair<entity_type, bool>> remloc;
+    dense_map<entity_type, std::pair<entity_type, bool>> remloc;
     basic_registry<entity_type> *reg;
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/entity/utility.hpp

@@ -1,13 +1,10 @@
 #ifndef ENTT_ENTITY_UTILITY_HPP
 #define ENTT_ENTITY_UTILITY_HPP
 
-
 #include "../core/type_traits.hpp"
 
-
 namespace entt {
 
-
 /**
  * @brief Alias for exclusion lists.
  * @tparam Type List of types.
@@ -15,7 +12,6 @@ namespace entt {
 template<typename... Type>
 struct exclude_t: type_list<Type...> {};
 
-
 /**
  * @brief Variable template for exclusion lists.
  * @tparam Type List of types.
@@ -23,14 +19,12 @@ struct exclude_t: type_list<Type...> {};
 template<typename... Type>
 inline constexpr exclude_t<Type...> exclude{};
 
-
 /**
  * @brief Alias for lists of observed components.
  * @tparam Type List of types.
  */
 template<typename... Type>
-struct get_t: type_list<Type...>{};
-
+struct get_t: type_list<Type...> {};
 
 /**
  * @brief Variable template for lists of observed components.
@@ -39,8 +33,20 @@ struct get_t: type_list<Type...>{};
 template<typename... Type>
 inline constexpr get_t<Type...> get{};
 
+/**
+ * @brief Alias for lists of owned components.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+struct owned_t: type_list<Type...> {};
 
-}
+/**
+ * @brief Variable template for lists of owned components.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+inline constexpr owned_t<Type...> owned{};
 
+} // namespace entt
 
 #endif

src/entt/entt.hpp

@@ -1,11 +1,20 @@
+#include "config/config.h"
+#include "config/macro.h"
 #include "config/version.h"
+#include "container/dense_map.hpp"
+#include "container/dense_set.hpp"
 #include "core/algorithm.hpp"
 #include "core/any.hpp"
 #include "core/attribute.h"
+#include "core/compressed_pair.hpp"
+#include "core/enum.hpp"
 #include "core/family.hpp"
 #include "core/hashed_string.hpp"
 #include "core/ident.hpp"
+#include "core/iterator.hpp"
+#include "core/memory.hpp"
 #include "core/monostate.hpp"
+#include "core/tuple.hpp"
 #include "core/type_info.hpp"
 #include "core/type_traits.hpp"
 #include "core/utility.hpp"
@@ -16,9 +25,9 @@
 #include "entity/helper.hpp"
 #include "entity/observer.hpp"
 #include "entity/organizer.hpp"
-#include "entity/poly_storage.hpp"
 #include "entity/registry.hpp"
 #include "entity/runtime_view.hpp"
+#include "entity/sigh_storage_mixin.hpp"
 #include "entity/snapshot.hpp"
 #include "entity/sparse_set.hpp"
 #include "entity/storage.hpp"
@@ -43,8 +52,8 @@
 #include "process/process.hpp"
 #include "process/scheduler.hpp"
 #include "resource/cache.hpp"
-#include "resource/handle.hpp"
 #include "resource/loader.hpp"
+#include "resource/resource.hpp"
 #include "signal/delegate.hpp"
 #include "signal/dispatcher.hpp"
 #include "signal/emitter.hpp"

src/entt/fwd.hpp

@@ -1,5 +1,7 @@
+#include "container/fwd.hpp"
 #include "core/fwd.hpp"
 #include "entity/fwd.hpp"
+#include "meta/fwd.hpp"
 #include "poly/fwd.hpp"
 #include "resource/fwd.hpp"
 #include "signal/fwd.hpp"

src/entt/locator/locator.hpp

@@ -1,111 +1,114 @@
 #ifndef ENTT_LOCATOR_LOCATOR_HPP
 #define ENTT_LOCATOR_LOCATOR_HPP
 
-
 #include <memory>
 #include <utility>
 #include "../config/config.h"
 
-
 namespace entt {
 
-
 /**
  * @brief Service locator, nothing more.
  *
- * A service locator can be used to do what it promises: locate services.<br/>
+ * A service locator is used to do what it promises: locate services.<br/>
  * Usually service locators are tightly bound to the services they expose and
- * thus it's hard to define a general purpose class to do that. This template
- * based implementation tries to fill the gap and to get rid of the burden of
- * defining a different specific locator for each application.
+ * thus it's hard to define a general purpose class to do that. This tiny class
+ * tries to fill the gap and to get rid of the burden of defining a different
+ * specific locator for each application.
  *
- * @tparam Service Type of service managed by the locator.
+ * @note
+ * Users shouldn't retain references to a service. The recommended way is to
+ * retrieve the service implementation currently set each and every time the
+ * need for it arises. The risk is to incur in unexpected behaviors otherwise.
+ *
+ * @tparam Service Service type.
  */
 template<typename Service>
-struct service_locator {
-    /*! @brief Type of service offered. */
-    using service_type = Service;
+struct locator final {
+    /*! @brief Service type. */
+    using type = Service;
 
     /*! @brief Default constructor, deleted on purpose. */
-    service_locator() = delete;
+    locator() = delete;
     /*! @brief Default destructor, deleted on purpose. */
-    ~service_locator() = delete;
+    ~locator() = delete;
 
     /**
-     * @brief Tests if a valid service implementation is set.
-     * @return True if the service is set, false otherwise.
+     * @brief Checks whether a service locator contains a value.
+     * @return True if the service locator contains a value, false otherwise.
      */
-    [[nodiscard]] static bool empty() ENTT_NOEXCEPT {
-        return !static_cast<bool>(service);
+    [[nodiscard]] static bool has_value() ENTT_NOEXCEPT {
+        return (service != nullptr);
     }
 
     /**
-     * @brief Returns a weak pointer to a service implementation, if any.
-     *
-     * Clients of a service shouldn't retain references to it. The recommended
-     * way is to retrieve the service implementation currently set each and
-     * every time the need of using it arises. Otherwise users can incur in
-     * unexpected behaviors.
-     *
-     * @return A reference to the service implementation currently set, if any.
-     */
-    [[nodiscard]] static std::weak_ptr<Service> get() ENTT_NOEXCEPT {
-        return service;
-    }
-
-    /**
-     * @brief Returns a weak reference to a service implementation, if any.
-     *
-     * Clients of a service shouldn't retain references to it. The recommended
-     * way is to retrieve the service implementation currently set each and
-     * every time the need of using it arises. Otherwise users can incur in
-     * unexpected behaviors.
+     * @brief Returns a reference to a valid service, if any.
      *
      * @warning
-     * In case no service implementation has been set, a call to this function
-     * results in undefined behavior.
+     * Invoking this function can result in undefined behavior if the service
+     * hasn't been set yet.
      *
-     * @return A reference to the service implementation currently set, if any.
+     * @return A reference to the service currently set, if any.
      */
-    [[nodiscard]] static Service & ref() ENTT_NOEXCEPT {
+    [[nodiscard]] static Service &value() ENTT_NOEXCEPT {
+        ENTT_ASSERT(has_value(), "Service not available");
         return *service;
     }
 
     /**
-     * @brief Sets or replaces a service.
-     * @tparam Impl Type of the new service to use.
-     * @tparam Args Types of arguments to use to construct the service.
-     * @param args Parameters to use to construct the service.
+     * @brief Returns a service if available or sets it from a fallback type.
+     *
+     * Arguments are used only if a service doesn't already exist. In all other
+     * cases, they are discarded.
+     *
+     * @tparam Args Types of arguments to use to construct the fallback service.
+     * @tparam Impl Fallback service type.
+     * @param args Parameters to use to construct the fallback service.
+     * @return A reference to a valid service.
      */
     template<typename Impl = Service, typename... Args>
-    static void set(Args &&... args) {
-        service = std::make_shared<Impl>(std::forward<Args>(args)...);
+    [[nodiscard]] static Service &value_or(Args &&...args) {
+        return service ? *service : emplace<Impl>(std::forward<Args>(args)...);
     }
 
     /**
      * @brief Sets or replaces a service.
-     * @param ptr Service to use to replace the current one.
+     * @tparam Impl Service type.
+     * @tparam Args Types of arguments to use to construct the service.
+     * @param args Parameters to use to construct the service.
+     * @return A reference to a valid service.
      */
-    static void set(std::shared_ptr<Service> ptr) {
-        ENTT_ASSERT(static_cast<bool>(ptr), "Null service not allowed");
-        service = std::move(ptr);
+    template<typename Impl = Service, typename... Args>
+    static Service &emplace(Args &&...args) {
+        service = std::make_shared<Impl>(std::forward<Args>(args)...);
+        return *service;
     }
 
     /**
-     * @brief Resets a service.
-     *
-     * The service is no longer valid after a reset.
+     * @brief Sets or replaces a service using a given allocator.
+     * @tparam Impl 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.
      */
-    static void reset() {
+    template<typename Impl = Service, typename Allocator, typename... Args>
+    static Service &allocate_emplace(Allocator alloc, Args &&...args) {
+        service = std::allocate_shared<Impl>(alloc, std::forward<Args>(args)...);
+        return *service;
+    }
+
+    /*! @brief Resets a service. */
+    static void reset() ENTT_NOEXCEPT {
         service.reset();
     }
 
 private:
+    // std::shared_ptr because of its type erased allocator which is pretty useful here
     inline static std::shared_ptr<Service> service = nullptr;
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/meta/adl_pointer.hpp

@@ -1,10 +1,8 @@
 #ifndef ENTT_META_ADL_POINTER_HPP
 #define ENTT_META_ADL_POINTER_HPP
 
-
 namespace entt {
 
-
 /**
  * @brief ADL based lookup function for dereferencing meta pointer-like types.
  * @tparam Type Element type.
@@ -16,7 +14,6 @@ decltype(auto) dereference_meta_pointer_like(const Type &value) {
     return *value;
 }
 
-
 /**
  * @brief Fake ADL based lookup function for meta pointer-like types.
  * @tparam Type Element type.
@@ -33,8 +30,6 @@ struct adl_meta_pointer_like {
     }
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/meta/container.hpp

@@ -1,295 +1,161 @@
 #ifndef ENTT_META_CONTAINER_HPP
 #define ENTT_META_CONTAINER_HPP
 
-
 #include <array>
 #include <map>
 #include <set>
 #include <type_traits>
 #include <unordered_map>
 #include <unordered_set>
-#include <utility>
 #include <vector>
-#include "../config/config.h"
-#include "../core/type_traits.hpp"
+#include "../container/dense_map.hpp"
+#include "../container/dense_set.hpp"
+#include "meta.hpp"
 #include "type_traits.hpp"
 
-
 namespace entt {
 
-
 /**
- * @brief Container traits.
- * @tparam Container Type of the underlying container.
- * @tparam Trait Traits associated with the underlying container.
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
  */
-template<typename Container, template<typename> class... Trait>
-struct meta_container_traits: public Trait<Container>... {
-    /*! @brief Type of container. */
-    using type = Container;
-};
 
+namespace internal {
 
-/**
- * @brief Basic STL-compatible container traits
- * @tparam Container The type of the container.
- */
-template<typename Container>
-struct basic_container {
-    /**
-     * @brief Returns the size of the given container.
-     * @param cont The container for which to return the size.
-     * @return The size of the given container.
-     */
-    [[nodiscard]] static typename Container::size_type size(const Container &cont) ENTT_NOEXCEPT {
-        return cont.size();
+template<typename, typename = void>
+struct is_dynamic_sequence_container: std::false_type {};
+
+template<typename Type>
+struct is_dynamic_sequence_container<Type, std::void_t<decltype(&Type::reserve)>>: std::true_type {};
+
+template<typename, typename = void>
+struct is_key_only_meta_associative_container: std::true_type {};
+
+template<typename Type>
+struct is_key_only_meta_associative_container<Type, std::void_t<typename Type::mapped_type>>: std::false_type {};
+
+template<typename Type>
+struct basic_meta_sequence_container_traits {
+    using iterator = meta_sequence_container::iterator;
+    using size_type = std::size_t;
+
+    [[nodiscard]] static size_type size(const any &container) ENTT_NOEXCEPT {
+        return any_cast<const Type &>(container).size();
     }
 
-    /**
-     * @brief Returns an iterator to the first element of the given container.
-     * @param cont The container for which to return the iterator.
-     * @return An iterator to the first element of the given container.
-     */
-    [[nodiscard]] static typename Container::iterator begin(Container &cont) {
-        return cont.begin();
-    }
+    [[nodiscard]] static bool resize([[maybe_unused]] any &container, [[maybe_unused]] size_type sz) {
+        if constexpr(is_dynamic_sequence_container<Type>::value) {
+            if(auto *const cont = any_cast<Type>(&container); cont) {
+                cont->resize(sz);
+                return true;
+            }
+        }
 
-    /**
-     * @brief Returns an iterator to the first element of the given container.
-     * @param cont The container for which to return the iterator.
-     * @return An iterator to the first element of the given container.
-     */
-    [[nodiscard]] static typename Container::const_iterator cbegin(const Container &cont) {
-        return cont.begin();
-    }
-
-    /**
-     * @brief Returns an iterator past the last element of the given container.
-     * @param cont The container for which to return the iterator.
-     * @return An iterator past the last element of the given container.
-     */
-    [[nodiscard]] static typename Container::iterator end(Container &cont) {
-        return cont.end();
-    }
-
-    /**
-     * @brief Returns an iterator past the last element of the given container.
-     * @param cont The container for which to return the iterator.
-     * @return An iterator past the last element of the given container.
-     */
-    [[nodiscard]] static typename Container::const_iterator cend(const Container &cont) {
-        return cont.end();
-    }
-};
-
-
-/**
- * @brief Basic STL-compatible associative container traits
- * @tparam Container The type of the container.
- */
-template<typename Container>
-struct basic_associative_container {
-    /**
-     * @brief Returns an iterator to the element with key equivalent to the
-     * given one, if any.
-     * @param cont The container in which to search for the element.
-     * @param key The key of the element to search.
-     * @return An iterator to the element with the given key, if any.
-     */
-    [[nodiscard]] static typename Container::iterator find(Container &cont, const typename Container::key_type &key) {
-        return cont.find(key);
-    }
-
-    /*! @copydoc find */
-    [[nodiscard]] static typename Container::const_iterator cfind(const Container &cont, const typename Container::key_type &key) {
-        return cont.find(key);
-    }
-};
-
-
-/**
- * @brief Basic STL-compatible dynamic container traits
- * @tparam Container The type of the container.
- */
-template<typename Container>
-struct basic_dynamic_container {
-    /**
-     * @brief Clears the content of the given container.
-     * @param cont The container for which to clear the content.
-     * @return True in case of success, false otherwise.
-     */
-    [[nodiscard]] static bool clear([[maybe_unused]] Container &cont) {
-        return cont.clear(), true;
-    }
-};
-
-
-/**
- * @brief Basic STL-compatible dynamic associative container traits
- * @tparam Container The type of the container.
- */
-template<typename Container>
-struct basic_dynamic_associative_container {
-    /**
-     * @brief Removes the specified element from the given container.
-     * @param cont The container from which to remove the element.
-     * @param key The element to remove.
-     * @return A bool denoting whether the removal took place.
-     */
-    [[nodiscard]] static bool erase([[maybe_unused]] Container &cont, [[maybe_unused]] const typename Container::key_type &key) {
-        const auto sz = cont.size();
-        return cont.erase(key) != sz;
-    }
-};
-
-
-/**
- * @brief Basic STL-compatible sequence container traits
- * @tparam Container The type of the container.
- */
-template<typename Container>
-struct basic_sequence_container {
-    /**
-     * @brief Returns a reference to the element at the specified location of
-     * the given container (no bounds checking is performed).
-     * @param cont The container from which to get the element.
-     * @param pos The position of the element to return.
-     * @return A reference to the requested element.
-     */
-    [[nodiscard]] static typename Container::reference get(Container &cont, typename Container::size_type pos) {
-        return cont[pos];
-    }
-
-    /*! @copydoc get */
-    [[nodiscard]] static typename Container::const_reference cget(const Container &cont, typename Container::size_type pos) {
-        return cont[pos];
-    }
-};
-
-
-/**
- * @brief STL-compatible dynamic associative key-only container traits
- * @tparam Container The type of the container.
- */
-template<typename Container>
-struct dynamic_associative_key_only_container {
-    /**
-     * @brief Inserts an element into the given container.
-     * @param cont The container in which to insert the element.
-     * @param key The element to insert.
-     * @return A bool denoting whether the insertion took place.
-     */
-    [[nodiscard]] static bool insert([[maybe_unused]] Container &cont, [[maybe_unused]] const typename Container::key_type &key) {
-        return cont.insert(key).second;
-    }
-};
-
-
-/**
- * @brief STL-compatible dynamic key-value associative container traits
- * @tparam Container The type of the container.
- */
-template<typename Container>
-struct dynamic_associative_key_value_container {
-    /**
-     * @brief Inserts an element (a key/value pair) into the given container.
-     * @param cont The container in which to insert the element.
-     * @param key The key of the element to insert.
-     * @param value The value of the element to insert.
-     * @return A bool denoting whether the insertion took place.
-     */
-    [[nodiscard]] static bool insert([[maybe_unused]] Container &cont, [[maybe_unused]] const typename Container::key_type &key, [[maybe_unused]] const typename Container::mapped_type &value) {
-        return cont.insert(std::make_pair(key, value)).second;
-    }
-};
-
-
-/**
- * @brief STL-compatible dynamic sequence container traits
- * @tparam Container The type of the container.
- */
-template<typename Container>
-struct dynamic_sequence_container {
-    /**
-     * @brief Resizes the given container to contain the given number of
-     * elements.
-     * @param cont The container to resize.
-     * @param sz The new size of the container.
-     * @return True in case of success, false otherwise.
-     */
-    [[nodiscard]] static bool resize([[maybe_unused]] Container &cont, [[maybe_unused]] typename Container::size_type sz) {
-        return cont.resize(sz), true;
-    }
-
-    /**
-     * @brief Inserts an element at the specified location of the given
-     * container.
-     * @param cont The container into which to insert the element.
-     * @param it Iterator before which the element will be inserted.
-     * @param value Element value to insert.
-     * @return A pair consisting of an iterator to the inserted element (in case
-     * of success) and a bool denoting whether the insertion took place.
-     */
-    [[nodiscard]] static std::pair<typename Container::iterator, bool> insert([[maybe_unused]] Container &cont, [[maybe_unused]] typename Container::const_iterator it, [[maybe_unused]] const typename Container::value_type &value) {
-        return { cont.insert(it, value), true };
-    }
-
-    /**
-     * @brief Removes the element at the specified location from the given
-     * container.
-     * @param cont The container from which to remove the element.
-     * @param it Iterator to the element to remove.
-     * @return A pair consisting of an iterator following the last removed
-     * element (in case of success) and a bool denoting whether the insertion
-     * took place.
-     */
-    [[nodiscard]] static std::pair<typename Container::iterator, bool> erase([[maybe_unused]] Container &cont, [[maybe_unused]] typename Container::const_iterator it) {
-        return { cont.erase(it), true };
-    }
-};
-
-
-/**
- * @brief STL-compatible fixed sequence container traits
- * @tparam Container The type of the container.
- */
-template<typename Container>
-struct fixed_sequence_container {
-    /**
-     * @brief Does nothing.
-     * @return False to indicate failure in all cases.
-     */
-    [[nodiscard]] static bool resize(const Container &, typename Container::size_type) {
         return false;
     }
 
-    /**
-     * @brief Does nothing.
-     * @return False to indicate failure in all cases.
-     */
-    [[nodiscard]] static bool clear(const Container &) {
-        return false;
+    [[nodiscard]] static iterator iter(any &container, const bool as_end) {
+        if(auto *const cont = any_cast<Type>(&container); cont) {
+            return iterator{*cont, static_cast<typename iterator::difference_type>(as_end * cont->size())};
+        }
+
+        const Type &as_const = any_cast<const Type &>(container);
+        return iterator{as_const, static_cast<typename iterator::difference_type>(as_end * as_const.size())};
     }
 
-    /**
-     * @brief Does nothing.
-     * @return A pair consisting of an invalid iterator and a false value to
-     * indicate failure in all cases.
-     */
-    [[nodiscard]] static std::pair<typename Container::iterator, bool> insert(const Container &, typename Container::const_iterator, const typename Container::value_type &) {
-        return { {}, false };
+    [[nodiscard]] static iterator insert([[maybe_unused]] any &container, [[maybe_unused]] const std::ptrdiff_t offset, [[maybe_unused]] meta_any &value) {
+        if constexpr(is_dynamic_sequence_container<Type>::value) {
+            if(auto *const cont = any_cast<Type>(&container); cont) {
+                // this abomination is necessary because only on macos value_type and const_reference are different types for std::vector<bool>
+                if(value.allow_cast<typename Type::const_reference>() || value.allow_cast<typename Type::value_type>()) {
+                    const auto *element = value.try_cast<std::remove_reference_t<typename Type::const_reference>>();
+                    const auto curr = cont->insert(cont->begin() + offset, element ? *element : value.cast<typename Type::value_type>());
+                    return iterator{*cont, curr - cont->begin()};
+                }
+            }
+        }
+
+        return {};
     }
 
-    /**
-     * @brief Does nothing.
-     * @return A pair consisting of an invalid iterator and a false value to
-     * indicate failure in all cases.
-     */
-    [[nodiscard]] static std::pair<typename Container::iterator, bool> erase(const Container &, typename Container::const_iterator) {
-        return { {}, false };
+    [[nodiscard]] static iterator erase([[maybe_unused]] any &container, [[maybe_unused]] const std::ptrdiff_t offset) {
+        if constexpr(is_dynamic_sequence_container<Type>::value) {
+            if(auto *const cont = any_cast<Type>(&container); cont) {
+                const auto curr = cont->erase(cont->begin() + offset);
+                return iterator{*cont, curr - cont->begin()};
+            }
+        }
+
+        return {};
     }
 };
 
+template<typename Type>
+struct basic_meta_associative_container_traits {
+    using iterator = meta_associative_container::iterator;
+    using size_type = std::size_t;
+
+    static constexpr auto key_only = is_key_only_meta_associative_container<Type>::value;
+
+    [[nodiscard]] static size_type size(const any &container) ENTT_NOEXCEPT {
+        return any_cast<const Type &>(container).size();
+    }
+
+    [[nodiscard]] static bool clear(any &container) {
+        if(auto *const cont = any_cast<Type>(&container); cont) {
+            cont->clear();
+            return true;
+        }
+
+        return false;
+    }
+
+    [[nodiscard]] static iterator iter(any &container, const bool as_end) {
+        if(auto *const cont = any_cast<Type>(&container); cont) {
+            return iterator{std::integral_constant<bool, key_only>{}, as_end ? cont->end() : cont->begin()};
+        }
+
+        const auto &as_const = any_cast<const Type &>(container);
+        return iterator{std::integral_constant<bool, key_only>{}, as_end ? as_const.end() : as_const.begin()};
+    }
+
+    [[nodiscard]] static bool insert(any &container, meta_any &key, [[maybe_unused]] meta_any &value) {
+        auto *const cont = any_cast<Type>(&container);
+
+        if constexpr(is_key_only_meta_associative_container<Type>::value) {
+            return cont && key.allow_cast<const typename Type::key_type &>()
+                   && cont->insert(key.cast<const typename Type::key_type &>()).second;
+        } else {
+            return cont && key.allow_cast<const typename Type::key_type &>() && value.allow_cast<const typename Type::mapped_type &>()
+                   && cont->emplace(key.cast<const typename Type::key_type &>(), value.cast<const typename Type::mapped_type &>()).second;
+        }
+    }
+
+    [[nodiscard]] static bool erase(any &container, meta_any &key) {
+        auto *const cont = any_cast<Type>(&container);
+        return cont && key.allow_cast<const typename Type::key_type &>()
+               && (cont->erase(key.cast<const typename Type::key_type &>()) != cont->size());
+    }
+
+    [[nodiscard]] static iterator find(any &container, meta_any &key) {
+        if(key.allow_cast<const typename Type::key_type &>()) {
+            if(auto *const cont = any_cast<Type>(&container); cont) {
+                return iterator{std::integral_constant<bool, key_only>{}, cont->find(key.cast<const typename Type::key_type &>())};
+            }
+
+            return iterator{std::integral_constant<bool, key_only>{}, any_cast<const Type &>(container).find(key.cast<const typename Type::key_type &>())};
+        }
+
+        return {};
+    }
+};
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
 
 /**
  * @brief Meta sequence container traits for `std::vector`s of any type.
@@ -298,15 +164,7 @@ struct fixed_sequence_container {
  */
 template<typename Type, typename... Args>
 struct meta_sequence_container_traits<std::vector<Type, Args...>>
-        : meta_container_traits<
-              std::vector<Type, Args...>,
-              basic_container,
-              basic_dynamic_container,
-              basic_sequence_container,
-              dynamic_sequence_container
-          >
-{};
-
+    : internal::basic_meta_sequence_container_traits<std::vector<Type, Args...>> {};
 
 /**
  * @brief Meta sequence container traits for `std::array`s of any type.
@@ -315,14 +173,7 @@ struct meta_sequence_container_traits<std::vector<Type, Args...>>
  */
 template<typename Type, auto N>
 struct meta_sequence_container_traits<std::array<Type, N>>
-        : meta_container_traits<
-              std::array<Type, N>,
-              basic_container,
-              basic_sequence_container,
-              fixed_sequence_container
-          >
-{};
-
+    : internal::basic_meta_sequence_container_traits<std::array<Type, N>> {};
 
 /**
  * @brief Meta associative container traits for `std::map`s of any type.
@@ -332,16 +183,7 @@ struct meta_sequence_container_traits<std::array<Type, N>>
  */
 template<typename Key, typename Value, typename... Args>
 struct meta_associative_container_traits<std::map<Key, Value, Args...>>
-        : meta_container_traits<
-              std::map<Key, Value, Args...>,
-              basic_container,
-              basic_associative_container,
-              basic_dynamic_container,
-              basic_dynamic_associative_container,
-              dynamic_associative_key_value_container
-          >
-{};
-
+    : internal::basic_meta_associative_container_traits<std::map<Key, Value, Args...>> {};
 
 /**
  * @brief Meta associative container traits for `std::unordered_map`s of any
@@ -352,16 +194,7 @@ struct meta_associative_container_traits<std::map<Key, Value, Args...>>
  */
 template<typename Key, typename Value, typename... Args>
 struct meta_associative_container_traits<std::unordered_map<Key, Value, Args...>>
-        : meta_container_traits<
-              std::unordered_map<Key, Value, Args...>,
-              basic_container,
-              basic_associative_container,
-              basic_dynamic_container,
-              basic_dynamic_associative_container,
-              dynamic_associative_key_value_container
-          >
-{};
-
+    : internal::basic_meta_associative_container_traits<std::unordered_map<Key, Value, Args...>> {};
 
 /**
  * @brief Meta associative container traits for `std::set`s of any type.
@@ -370,16 +203,7 @@ struct meta_associative_container_traits<std::unordered_map<Key, Value, Args...>
  */
 template<typename Key, typename... Args>
 struct meta_associative_container_traits<std::set<Key, Args...>>
-        : meta_container_traits<
-              std::set<Key, Args...>,
-              basic_container,
-              basic_associative_container,
-              basic_dynamic_container,
-              basic_dynamic_associative_container,
-              dynamic_associative_key_only_container
-          >
-{};
-
+    : internal::basic_meta_associative_container_traits<std::set<Key, Args...>> {};
 
 /**
  * @brief Meta associative container traits for `std::unordered_set`s of any
@@ -389,18 +213,27 @@ struct meta_associative_container_traits<std::set<Key, Args...>>
  */
 template<typename Key, typename... Args>
 struct meta_associative_container_traits<std::unordered_set<Key, Args...>>
-        : meta_container_traits<
-              std::unordered_set<Key, Args...>,
-              basic_container,
-              basic_associative_container,
-              basic_dynamic_container,
-              basic_dynamic_associative_container,
-              dynamic_associative_key_only_container
-          >
-{};
+    : internal::basic_meta_associative_container_traits<std::unordered_set<Key, Args...>> {};
 
+/**
+ * @brief Meta associative container traits for `dense_map`s of any type.
+ * @tparam Key The key type of the elements.
+ * @tparam Type The value type of the elements.
+ * @tparam Args Other arguments.
+ */
+template<typename Key, typename Type, typename... Args>
+struct meta_associative_container_traits<dense_map<Key, Type, Args...>>
+    : internal::basic_meta_associative_container_traits<dense_map<Key, Type, Args...>> {};
 
-}
+/**
+ * @brief Meta associative container traits for `dense_set`s of any type.
+ * @tparam Type The value type of the elements.
+ * @tparam Args Other arguments.
+ */
+template<typename Type, typename... Args>
+struct meta_associative_container_traits<dense_set<Type, Args...>>
+    : internal::basic_meta_associative_container_traits<dense_set<Type, Args...>> {};
 
+} // namespace entt
 
 #endif

src/entt/meta/ctx.hpp

@@ -1,52 +1,43 @@
 #ifndef ENTT_META_CTX_HPP
 #define ENTT_META_CTX_HPP
 
-
-#include "../core/attribute.h"
 #include "../config/config.h"
-
+#include "../core/attribute.h"
 
 namespace entt {
 
-
 /**
  * @cond TURN_OFF_DOXYGEN
  * Internal details not to be documented.
  */
 
-
 namespace internal {
 
-
 struct meta_type_node;
 
-
 struct ENTT_API meta_context {
     // we could use the lines below but VS2017 returns with an ICE if combined with ENTT_API despite the code being valid C++
     //     inline static meta_type_node *local = nullptr;
     //     inline static meta_type_node **global = &local;
 
-    [[nodiscard]] static meta_type_node * & local() ENTT_NOEXCEPT {
+    [[nodiscard]] static meta_type_node *&local() ENTT_NOEXCEPT {
         static meta_type_node *chain = nullptr;
         return chain;
     }
 
-    [[nodiscard]] static meta_type_node ** & global() ENTT_NOEXCEPT {
+    [[nodiscard]] static meta_type_node **&global() ENTT_NOEXCEPT {
         static meta_type_node **chain = &local();
         return chain;
     }
 };
 
-
-}
-
+} // namespace internal
 
 /**
  * Internal details not to be documented.
  * @endcond
  */
 
-
 /*! @brief Opaque container for a meta context. */
 struct meta_ctx {
     /**
@@ -61,8 +52,6 @@ private:
     internal::meta_type_node **ctx{&internal::meta_context::local()};
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/meta/factory.hpp

@@ -1,8 +1,9 @@
 #ifndef ENTT_META_FACTORY_HPP
 #define ENTT_META_FACTORY_HPP
 
-
+#include <algorithm>
 #include <cstddef>
+#include <functional>
 #include <tuple>
 #include <type_traits>
 #include <utility>
@@ -13,46 +14,11 @@
 #include "meta.hpp"
 #include "node.hpp"
 #include "policy.hpp"
+#include "range.hpp"
 #include "utility.hpp"
 
-
 namespace entt {
 
-
-/**
- * @cond TURN_OFF_DOXYGEN
- * Internal details not to be documented.
- */
-
-
-namespace internal {
-
-
-template<typename Node>
-[[nodiscard]] bool find_if(const Node *candidate, const Node *node) ENTT_NOEXCEPT {
-    return node && (node == candidate || find_if(candidate, node->next));
-}
-
-
-template<typename Id, typename Node>
-[[nodiscard]] bool find_if_not(const Id id, Node *node, const Node *owner) ENTT_NOEXCEPT {
-    if constexpr(std::is_pointer_v<Id>) {
-        return node && ((*node->id == *id && node != owner) || find_if_not(id, node->next, owner));
-    } else {
-        return node && ((node->id == id && node != owner) || find_if_not(id, node->next, owner));
-    }
-}
-
-
-}
-
-
-/**
- * Internal details not to be documented.
- * @endcond
- */
-
-
 /**
  * @brief Meta factory to be used for reflection purposes.
  *
@@ -62,8 +28,7 @@ template<typename Id, typename Node>
  * there are no subtle errors at runtime.
  */
 template<typename...>
-struct meta_factory;
-
+class meta_factory;
 
 /**
  * @brief Extended meta factory to be used for reflection purposes.
@@ -71,58 +36,51 @@ struct meta_factory;
  * @tparam Spec Property specialization pack used to disambiguate overloads.
  */
 template<typename Type, typename... Spec>
-struct meta_factory<Type, Spec...>: public meta_factory<Type> {
-private:
-    template<std::size_t Step = 0, std::size_t... Index, typename... Property, typename... Other>
-    void unpack(std::index_sequence<Index...>, std::tuple<Property...> property, Other &&... other) {
-        unroll<Step>(choice<3>, std::move(std::get<Index>(property))..., std::forward<Other>(other)...);
+class meta_factory<Type, Spec...>: public meta_factory<Type> {
+    void link_prop_if_required(internal::meta_prop_node &node) ENTT_NOEXCEPT {
+        if(meta_range<internal::meta_prop_node *, internal::meta_prop_node> range{*ref}; std::find(range.cbegin(), range.cend(), &node) == range.cend()) {
+            ENTT_ASSERT(std::find_if(range.cbegin(), range.cend(), [&node](const auto *curr) { return curr->id == node.id; }) == range.cend(), "Duplicate identifier");
+            node.next = *ref;
+            *ref = &node;
+        }
     }
 
     template<std::size_t Step = 0, typename... Property, typename... Other>
-    void unroll(choice_t<3>, std::tuple<Property...> property, Other &&... other) {
-        unpack<Step>(std::index_sequence_for<Property...>{}, std::move(property), std::forward<Other>(other)...);
+    void unroll(choice_t<2>, std::tuple<Property...> property, Other &&...other) ENTT_NOEXCEPT {
+        std::apply([this](auto &&...curr) { (this->unroll<Step>(choice<2>, std::forward<Property>(curr)...)); }, property);
+        unroll<Step + sizeof...(Property)>(choice<2>, std::forward<Other>(other)...);
     }
 
     template<std::size_t Step = 0, typename... Property, typename... Other>
-    void unroll(choice_t<2>, std::pair<Property...> property, Other &&... other) {
+    void unroll(choice_t<1>, std::pair<Property...> property, Other &&...other) ENTT_NOEXCEPT {
         assign<Step>(std::move(property.first), std::move(property.second));
-        unroll<Step+1>(choice<3>, std::forward<Other>(other)...);
+        unroll<Step + 1>(choice<2>, std::forward<Other>(other)...);
     }
 
     template<std::size_t Step = 0, typename Property, typename... Other>
-    std::enable_if_t<!std::is_invocable_v<Property>>
-    unroll(choice_t<1>, Property &&property, Other &&... other) {
+    void unroll(choice_t<0>, Property &&property, Other &&...other) ENTT_NOEXCEPT {
         assign<Step>(std::forward<Property>(property));
-        unroll<Step+1>(choice<3>, std::forward<Other>(other)...);
-    }
-
-    template<std::size_t Step = 0, typename Func, typename... Other>
-    void unroll(choice_t<0>, Func &&invocable, Other &&... other) {
-        unroll<Step>(choice<3>, std::forward<Func>(invocable)(), std::forward<Other>(other)...);
+        unroll<Step + 1>(choice<2>, std::forward<Other>(other)...);
     }
 
     template<std::size_t>
-    void unroll(choice_t<0>) {}
+    void unroll(choice_t<0>) ENTT_NOEXCEPT {}
 
-    template<std::size_t = 0, typename Key>
-    void assign(Key &&key, meta_any value = {}) {
+    template<std::size_t = 0>
+    void assign(meta_any key, meta_any value = {}) {
         static meta_any property[2u]{};
 
         static internal::meta_prop_node node{
             nullptr,
             property[0u],
             property[1u]
+            // tricks clang-format
         };
 
-        entt::meta_any instance{std::forward<Key>(key)};
-        ENTT_ASSERT(!internal::find_if_not(&instance, *curr, &node), "Duplicate key");
-        property[0u] = std::move(instance);
+        property[0u] = std::move(key);
         property[1u] = std::move(value);
 
-        if(!internal::find_if(&node, *curr)) {
-            node.next = *curr;
-            *curr = &node;
-        }
+        link_prop_if_required(node);
     }
 
 public:
@@ -131,8 +89,7 @@ public:
      * @param target The underlying node to which to assign the properties.
      */
     meta_factory(internal::meta_prop_node **target) ENTT_NOEXCEPT
-        : curr{target}
-    {}
+        : ref{target} {}
 
     /**
      * @brief Assigns a property to the last meta object created.
@@ -146,60 +103,127 @@ public:
      * @return A meta factory for the parent type.
      */
     template<typename PropertyOrKey, typename... Value>
-    auto prop(PropertyOrKey &&property_or_key, Value &&... value) && {
+    meta_factory<Type> prop(PropertyOrKey &&property_or_key, Value &&...value) {
         if constexpr(sizeof...(Value) == 0) {
-            unroll(choice<3>, std::forward<PropertyOrKey>(property_or_key));
+            unroll(choice<2>, std::forward<PropertyOrKey>(property_or_key));
         } else {
             assign(std::forward<PropertyOrKey>(property_or_key), std::forward<Value>(value)...);
         }
 
-        return meta_factory<Type, Spec..., PropertyOrKey, Value...>{curr};
+        return {};
     }
 
     /**
      * @brief Assigns properties to the last meta object created.
      *
-     * Both the keys and the values (if any) must be at least copy
-     * constructible.
+     * Both key and value (if any) must be at least copy constructible.
      *
      * @tparam Property Types of the properties.
      * @param property Properties to assign to the last meta object created.
      * @return A meta factory for the parent type.
      */
-    template <typename... Property>
-    auto props(Property... property) && {
-        unroll(choice<3>, std::forward<Property>(property)...);
-        return meta_factory<Type, Spec..., Property...>{curr};
+    template<typename... Property>
+    meta_factory<Type> props(Property... property) {
+        unroll(choice<2>, std::forward<Property>(property)...);
+        return {};
     }
 
 private:
-    internal::meta_prop_node **curr;
+    internal::meta_prop_node **ref;
 };
 
-
 /**
  * @brief Basic meta factory to be used for reflection purposes.
  * @tparam Type Reflected type for which the factory was created.
  */
 template<typename Type>
-struct meta_factory<Type> {
+class meta_factory<Type> {
+    void link_base_if_required(internal::meta_base_node &node) ENTT_NOEXCEPT {
+        if(meta_range<internal::meta_base_node *, internal::meta_base_node> range{owner->base}; std::find(range.cbegin(), range.cend(), &node) == range.cend()) {
+            node.next = owner->base;
+            owner->base = &node;
+        }
+    }
+
+    void link_conv_if_required(internal::meta_conv_node &node) ENTT_NOEXCEPT {
+        if(meta_range<internal::meta_conv_node *, internal::meta_conv_node> range{owner->conv}; std::find(range.cbegin(), range.cend(), &node) == range.cend()) {
+            node.next = owner->conv;
+            owner->conv = &node;
+        }
+    }
+
+    void link_ctor_if_required(internal::meta_ctor_node &node) ENTT_NOEXCEPT {
+        if(meta_range<internal::meta_ctor_node *, internal::meta_ctor_node> range{owner->ctor}; std::find(range.cbegin(), range.cend(), &node) == range.cend()) {
+            node.next = owner->ctor;
+            owner->ctor = &node;
+        }
+    }
+
+    void link_data_if_required(const id_type id, internal::meta_data_node &node) ENTT_NOEXCEPT {
+        meta_range<internal::meta_data_node *, internal::meta_data_node> range{owner->data};
+        ENTT_ASSERT(std::find_if(range.cbegin(), range.cend(), [id, &node](const auto *curr) { return curr != &node && curr->id == id; }) == range.cend(), "Duplicate identifier");
+        node.id = id;
+
+        if(std::find(range.cbegin(), range.cend(), &node) == range.cend()) {
+            node.next = owner->data;
+            owner->data = &node;
+        }
+    }
+
+    void link_func_if_required(const id_type id, internal::meta_func_node &node) ENTT_NOEXCEPT {
+        node.id = id;
+
+        if(meta_range<internal::meta_func_node *, internal::meta_func_node> range{owner->func}; std::find(range.cbegin(), range.cend(), &node) == range.cend()) {
+            node.next = owner->func;
+            owner->func = &node;
+        }
+    }
+
+    template<typename Setter, auto Getter, typename Policy, std::size_t... Index>
+    auto data(const id_type id, std::index_sequence<Index...>) ENTT_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");
+
+        static internal::meta_data_node node{
+            {},
+            /* 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,
+            nullptr,
+            nullptr,
+            Setter::size,
+            internal::meta_node<std::remove_cv_t<std::remove_reference_t<data_type>>>::resolve(),
+            &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) -> bool { return (meta_setter<Type, value_list_element_v<Index, Setter>>(*instance.operator->(), value.as_ref()) || ...); },
+            &meta_getter<Type, Getter, Policy>
+            // tricks clang-format
+        };
+
+        link_data_if_required(id, node);
+        return meta_factory<Type, Setter, std::integral_constant<decltype(Getter), Getter>>{&node.prop};
+    }
+
+public:
+    /*! @brief Default constructor. */
+    meta_factory() ENTT_NOEXCEPT
+        : owner{internal::meta_node<Type>::resolve()} {}
+
     /**
      * @brief Makes a meta type _searchable_.
      * @param id Optional unique identifier.
      * @return An extended meta factory for the given type.
      */
-    auto type(const id_type id = type_hash<Type>::value()) {
-        auto * const node = internal::meta_info<Type>::resolve();
+    auto type(const id_type id = type_hash<Type>::value()) ENTT_NOEXCEPT {
+        meta_range<internal::meta_type_node *, internal::meta_type_node> range{*internal::meta_context::global()};
+        ENTT_ASSERT(std::find_if(range.cbegin(), range.cend(), [id, this](const auto *curr) { return curr != owner && curr->id == id; }) == range.cend(), "Duplicate identifier");
+        owner->id = id;
 
-        ENTT_ASSERT(!internal::find_if_not(id, *internal::meta_context::global(), node), "Duplicate identifier");
-        node->id = id;
-
-        if(!internal::find_if(node, *internal::meta_context::global())) {
-            node->next = *internal::meta_context::global();
-            *internal::meta_context::global() = node;
+        if(std::find(range.cbegin(), range.cend(), owner) == range.cend()) {
+            owner->next = *internal::meta_context::global();
+            *internal::meta_context::global() = owner;
         }
 
-        return meta_factory<Type, Type>{&node->prop};
+        return meta_factory<Type, Type>{&owner->prop};
     }
 
     /**
@@ -212,23 +236,22 @@ struct meta_factory<Type> {
      */
     template<typename Base>
     auto base() ENTT_NOEXCEPT {
-        static_assert(std::is_base_of_v<Base, Type>, "Invalid base type");
-        auto * const type = internal::meta_info<Type>::resolve();
+        static_assert(!std::is_same_v<Type, Base> && std::is_base_of_v<Base, Type>, "Invalid base type");
 
         static internal::meta_base_node node{
-            type,
             nullptr,
-            &internal::meta_info<Base>::resolve,
-            [](const void *instance) ENTT_NOEXCEPT -> const void * {
-                return static_cast<const Base *>(static_cast<const Type *>(instance));
+            internal::meta_node<Base>::resolve(),
+            [](meta_any other) ENTT_NOEXCEPT -> meta_any {
+                if(auto *ptr = other.data(); ptr) {
+                    return forward_as_meta(*static_cast<Base *>(static_cast<Type *>(ptr)));
+                }
+
+                return forward_as_meta(*static_cast<const Base *>(static_cast<const Type *>(std::as_const(other).data())));
             }
+            // tricks clang-format
         };
 
-        if(!internal::find_if(&node, type->base)) {
-            node.next = type->base;
-            type->base = &node;
-        }
-
+        link_base_if_required(node);
         return meta_factory<Type>{};
     }
 
@@ -245,47 +268,17 @@ struct meta_factory<Type> {
      * @return A meta factory for the parent type.
      */
     template<auto Candidate>
-    std::enable_if_t<std::is_member_function_pointer_v<decltype(Candidate)>, meta_factory<Type>> conv() ENTT_NOEXCEPT {
-        using conv_type = std::invoke_result_t<decltype(Candidate), Type &>;
-        auto * const type = internal::meta_info<Type>::resolve();
-
+    auto conv() ENTT_NOEXCEPT {
         static internal::meta_conv_node node{
-            type,
             nullptr,
-            &internal::meta_info<conv_type>::resolve,
-            [](const void *instance) -> meta_any {
-                return (static_cast<const Type *>(instance)->*Candidate)();
+            internal::meta_node<std::remove_cv_t<std::remove_reference_t<std::invoke_result_t<decltype(Candidate), Type &>>>>::resolve(),
+            [](const meta_any &instance) -> meta_any {
+                return forward_as_meta(std::invoke(Candidate, *static_cast<const Type *>(instance.data())));
             }
+            // tricks clang-format
         };
 
-        if(!internal::find_if(&node, type->conv)) {
-            node.next = type->conv;
-            type->conv = &node;
-        }
-
-        return meta_factory<Type>{};
-    }
-
-    /*! @copydoc conv */
-    template<auto Candidate>
-    std::enable_if_t<!std::is_member_function_pointer_v<decltype(Candidate)>, meta_factory<Type>> conv() ENTT_NOEXCEPT {
-        using conv_type = std::invoke_result_t<decltype(Candidate), Type &>;
-        auto * const type = internal::meta_info<Type>::resolve();
-
-        static internal::meta_conv_node node{
-            type,
-            nullptr,
-            &internal::meta_info<conv_type>::resolve,
-            [](const void *instance) -> meta_any {
-                return Candidate(*static_cast<const Type *>(instance));
-            }
-        };
-
-        if(!internal::find_if(&node, type->conv)) {
-            node.next = type->conv;
-            type->conv = &node;
-        }
-
+        link_conv_if_required(node);
         return meta_factory<Type>{};
     }
 
@@ -300,23 +293,14 @@ struct meta_factory<Type> {
      */
     template<typename To>
     auto conv() ENTT_NOEXCEPT {
-        static_assert(std::is_convertible_v<Type, To>, "Could not convert to the required type");
-        auto * const type = internal::meta_info<Type>::resolve();
-
         static internal::meta_conv_node node{
-            type,
             nullptr,
-            &internal::meta_info<To>::resolve,
-            [](const void *instance) -> meta_any {
-                return static_cast<To>(*static_cast<const Type *>(instance));
-            }
+            internal::meta_node<std::remove_cv_t<std::remove_reference_t<To>>>::resolve(),
+            [](const meta_any &instance) -> meta_any { return forward_as_meta(static_cast<To>(*static_cast<const Type *>(instance.data()))); }
+            // tricks clang-format
         };
 
-        if(!internal::find_if(&node, type->conv)) {
-            node.next = type->conv;
-            type->conv = &node;
-        }
-
+        link_conv_if_required(node);
         return meta_factory<Type>{};
     }
 
@@ -336,28 +320,19 @@ struct meta_factory<Type> {
     template<auto Candidate, typename Policy = as_is_t>
     auto ctor() ENTT_NOEXCEPT {
         using descriptor = meta_function_helper_t<Type, decltype(Candidate)>;
-        static_assert(std::is_same_v<std::decay_t<typename descriptor::return_type>, Type>, "The function doesn't return an object of the required type");
-        auto * const type = internal::meta_info<Type>::resolve();
+        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");
 
         static internal::meta_ctor_node node{
-            type,
-            nullptr,
             nullptr,
             descriptor::args_type::size,
-            [](const typename internal::meta_ctor_node::size_type index) ENTT_NOEXCEPT {
-                return meta_arg(typename descriptor::args_type{}, index);
-            },
-            [](meta_any * const args) {
-                return meta_invoke<Type, Candidate, Policy>({}, args, std::make_index_sequence<descriptor::args_type::size>{});
-            }
+            &meta_arg<typename descriptor::args_type>,
+            &meta_construct<Type, Candidate, Policy>
+            // tricks clang-format
         };
 
-        if(!internal::find_if(&node, type->ctor)) {
-            node.next = type->ctor;
-            type->ctor = &node;
-        }
-
-        return meta_factory<Type, std::integral_constant<decltype(Candidate), Candidate>>{&node.prop};
+        link_ctor_if_required(node);
+        return meta_factory<Type>{};
     }
 
     /**
@@ -372,40 +347,32 @@ struct meta_factory<Type> {
      */
     template<typename... Args>
     auto ctor() ENTT_NOEXCEPT {
-        using descriptor = meta_function_helper_t<Type, Type(*)(Args...)>;
-        auto * const type = internal::meta_info<Type>::resolve();
+        using descriptor = meta_function_helper_t<Type, Type (*)(Args...)>;
 
         static internal::meta_ctor_node node{
-            type,
-            nullptr,
             nullptr,
             descriptor::args_type::size,
-            [](const typename internal::meta_ctor_node::size_type index) ENTT_NOEXCEPT {
-                return meta_arg(typename descriptor::args_type{}, index);
-            },
-            [](meta_any * const args) {
-                return meta_construct<Type, Args...>(args, std::make_index_sequence<descriptor::args_type::size>{});
-            }
+            &meta_arg<typename descriptor::args_type>,
+            &meta_construct<Type, Args...>
+            // tricks clang-format
         };
 
-        if(!internal::find_if(&node, type->ctor)) {
-            node.next = type->ctor;
-            type->ctor = &node;
-        }
-
-        return meta_factory<Type, Type(Args...)>{&node.prop};
+        link_ctor_if_required(node);
+        return meta_factory<Type>{};
     }
 
     /**
      * @brief Assigns a meta destructor to a meta type.
      *
-     * Free functions can be assigned to meta types in the role of destructors.
-     * The signature of the function should identical to the following:
+     * 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.
      *
@@ -415,12 +382,7 @@ struct meta_factory<Type> {
     template<auto Func>
     auto dtor() ENTT_NOEXCEPT {
         static_assert(std::is_invocable_v<decltype(Func), Type &>, "The function doesn't accept an object of the type provided");
-        auto * const type = internal::meta_info<Type>::resolve();
-
-        type->dtor = [](void *instance) {
-            Func(*static_cast<Type *>(instance));
-        };
-
+        owner->dtor = [](void *instance) { std::invoke(Func, *static_cast<Type *>(instance)); };
         return meta_factory<Type>{};
     }
 
@@ -440,31 +402,41 @@ struct meta_factory<Type> {
     template<auto Data, typename Policy = as_is_t>
     auto data(const id_type id) ENTT_NOEXCEPT {
         if constexpr(std::is_member_object_pointer_v<decltype(Data)>) {
-            return data<Data, Data, Policy>(id);
-        } else {
-            using data_type = std::remove_pointer_t<decltype(Data)>;
-            auto * const type = internal::meta_info<Type>::resolve();
+            using data_type = std::remove_reference_t<std::invoke_result_t<decltype(Data), Type &>>;
 
             static internal::meta_data_node node{
                 {},
-                type,
+                /* this is never static */
+                std::is_const_v<data_type> ? internal::meta_traits::is_const : internal::meta_traits::is_none,
                 nullptr,
                 nullptr,
-                std::is_same_v<Type, data_type> || std::is_const_v<data_type>,
-                true,
-                &internal::meta_info<data_type>::resolve,
+                1u,
+                internal::meta_node<std::remove_const_t<data_type>>::resolve(),
+                &meta_arg<type_list<std::remove_const_t<data_type>>>,
                 &meta_setter<Type, Data>,
                 &meta_getter<Type, Data, Policy>
+                // tricks clang-format
             };
 
-            ENTT_ASSERT(!internal::find_if_not(id, type->data, &node), "Duplicate identifier");
-            node.id = id;
+            link_data_if_required(id, node);
+            return meta_factory<Type, std::integral_constant<decltype(Data), Data>, std::integral_constant<decltype(Data), Data>>{&node.prop};
+        } else {
+            using data_type = std::remove_reference_t<std::remove_pointer_t<decltype(Data)>>;
 
-            if(!internal::find_if(&node, type->data)) {
-                node.next = type->data;
-                type->data = &node;
-            }
+            static internal::meta_data_node node{
+                {},
+                ((std::is_same_v<Type, std::remove_const_t<data_type>> || std::is_const_v<data_type>) ? internal::meta_traits::is_const : internal::meta_traits::is_none) | internal::meta_traits::is_static,
+                nullptr,
+                nullptr,
+                1u,
+                internal::meta_node<std::remove_const_t<data_type>>::resolve(),
+                &meta_arg<type_list<std::remove_const_t<data_type>>>,
+                &meta_setter<Type, Data>,
+                &meta_getter<Type, Data, Policy>
+                // tricks clang-format
+            };
 
+            link_data_if_required(id, node);
             return meta_factory<Type, std::integral_constant<decltype(Data), Data>>{&node.prop};
         }
     }
@@ -491,34 +463,72 @@ struct meta_factory<Type> {
      */
     template<auto Setter, auto Getter, typename Policy = as_is_t>
     auto data(const id_type id) ENTT_NOEXCEPT {
-        using underlying_type = std::remove_reference_t<std::invoke_result_t<decltype(Getter), Type &>>;
-        auto * const type = internal::meta_info<Type>::resolve();
+        using data_type = std::invoke_result_t<decltype(Getter), Type &>;
+        static_assert(Policy::template value<data_type>, "Invalid return type for the given policy");
 
-        static internal::meta_data_node node{
-            {},
-            type,
-            nullptr,
-            nullptr,
-            std::is_same_v<decltype(Setter), std::nullptr_t> || (std::is_member_object_pointer_v<decltype(Setter)> && std::is_const_v<underlying_type>),
-            false,
-            &internal::meta_info<underlying_type>::resolve,
-            &meta_setter<Type, Setter>,
-            &meta_getter<Type, Getter, Policy>
-        };
+        if constexpr(std::is_same_v<decltype(Setter), std::nullptr_t>) {
+            static internal::meta_data_node node{
+                {},
+                /* this is never static */
+                internal::meta_traits::is_const,
+                nullptr,
+                nullptr,
+                0u,
+                internal::meta_node<std::remove_cv_t<std::remove_reference_t<data_type>>>::resolve(),
+                &meta_arg<type_list<>>,
+                &meta_setter<Type, Setter>,
+                &meta_getter<Type, Getter, Policy>
+                // tricks clang-format
+            };
 
-        ENTT_ASSERT(!internal::find_if_not(id, type->data, &node), "Duplicate identifier");
-        node.id = id;
+            link_data_if_required(id, node);
+            return meta_factory<Type, std::integral_constant<decltype(Setter), Setter>, std::integral_constant<decltype(Getter), Getter>>{&node.prop};
+        } else {
+            using args_type = typename meta_function_helper_t<Type, decltype(Setter)>::args_type;
 
-        if(!internal::find_if(&node, type->data)) {
-            node.next = type->data;
-            type->data = &node;
+            static internal::meta_data_node node{
+                {},
+                /* this is never static nor const */
+                internal::meta_traits::is_none,
+                nullptr,
+                nullptr,
+                1u,
+                internal::meta_node<std::remove_cv_t<std::remove_reference_t<data_type>>>::resolve(),
+                &meta_arg<type_list<type_list_element_t<args_type::size != 1u, args_type>>>,
+                &meta_setter<Type, Setter>,
+                &meta_getter<Type, Getter, Policy>
+                // tricks clang-format
+            };
+
+            link_data_if_required(id, node);
+            return meta_factory<Type, std::integral_constant<decltype(Setter), Setter>, std::integral_constant<decltype(Getter), Getter>>{&node.prop};
         }
-
-        return meta_factory<Type, std::integral_constant<decltype(Setter), Setter>, std::integral_constant<decltype(Getter), Getter>>{&node.prop};
     }
 
     /**
-     * @brief Assigns a meta funcion to a meta type.
+     * @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.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @param id Unique identifier.
+     * @return An extended meta factory for the parent type.
+     */
+    template<typename Setter, auto Getter, typename Policy = as_is_t>
+    auto data(const id_type id) ENTT_NOEXCEPT {
+        return data<Setter, Getter, Policy>(id, std::make_index_sequence<Setter::size>{});
+    }
+
+    /**
+     * @brief Assigns a meta function to a meta type.
      *
      * Both member functions and free functions can be assigned to a meta
      * type.<br/>
@@ -533,44 +543,27 @@ struct meta_factory<Type> {
     template<auto Candidate, typename Policy = as_is_t>
     auto func(const id_type id) ENTT_NOEXCEPT {
         using descriptor = meta_function_helper_t<Type, decltype(Candidate)>;
-        auto * const type = internal::meta_info<Type>::resolve();
+        static_assert(Policy::template value<typename descriptor::return_type>, "Invalid return type for the given policy");
 
         static internal::meta_func_node node{
             {},
-            type,
+            (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),
             nullptr,
             nullptr,
             descriptor::args_type::size,
-            descriptor::is_const,
-            descriptor::is_static,
-            &internal::meta_info<std::conditional_t<std::is_same_v<Policy, as_void_t>, void, typename descriptor::return_type>>::resolve,
-            [](const typename internal::meta_func_node::size_type index) ENTT_NOEXCEPT {
-                return meta_arg(typename descriptor::args_type{}, index);
-            },
-            [](meta_handle instance, meta_any *args) {
-                return meta_invoke<Type, Candidate, Policy>(std::move(instance), args, std::make_index_sequence<descriptor::args_type::size>{});
-            }
+            internal::meta_node<std::conditional_t<std::is_same_v<Policy, as_void_t>, void, std::remove_cv_t<std::remove_reference_t<typename descriptor::return_type>>>>::resolve(),
+            &meta_arg<typename descriptor::args_type>,
+            &meta_invoke<Type, Candidate, Policy>
+            // tricks clang-format
         };
 
-        for(auto *it = &type->func; *it; it = &(*it)->next) {
-            if(*it == &node) {
-                *it = node.next;
-                break;
-            }
-        }
-
-        internal::meta_func_node **it = &type->func;
-        for(; *it && (*it)->id != id; it = &(*it)->next);
-        for(; *it && (*it)->id == id && (*it)->arity < node.arity; it = &(*it)->next);
-
-        node.id = id;
-        node.next = *it;
-        *it = &node;
-
+        link_func_if_required(id, node);
         return meta_factory<Type, std::integral_constant<decltype(Candidate), Candidate>>{&node.prop};
     }
-};
 
+private:
+    internal::meta_type_node *owner;
+};
 
 /**
  * @brief Utility function to use for reflection.
@@ -585,13 +578,73 @@ struct meta_factory<Type> {
  */
 template<typename Type>
 [[nodiscard]] auto meta() ENTT_NOEXCEPT {
-    auto * const node = internal::meta_info<Type>::resolve();
+    auto *const node = internal::meta_node<Type>::resolve();
     // extended meta factory to allow assigning properties to opaque meta types
     return meta_factory<Type, Type>{&node->prop};
 }
 
+/**
+ * @brief Resets a type and all its parts.
+ *
+ * Resets a type and all its data members, member functions and properties, as
+ * well as its constructors, destructors and conversion functions if any.<br/>
+ * Base classes aren't reset but the link between the two types is removed.
+ *
+ * The type is also removed from the list of searchable types.
+ *
+ * @param id Unique identifier.
+ */
+inline void meta_reset(const id_type id) ENTT_NOEXCEPT {
+    auto clear_chain = [](auto **curr, auto... member) {
+        for(; *curr; *curr = std::exchange((*curr)->next, nullptr)) {
+            if constexpr(sizeof...(member) != 0u) {
+                static_assert(sizeof...(member) == 1u, "Assert in defense of the future me");
+                for(auto **sub = (&((*curr)->*member), ...); *sub; *sub = std::exchange((*sub)->next, nullptr)) {}
+            }
+        }
+    };
 
+    for(auto **it = internal::meta_context::global(); *it; it = &(*it)->next) {
+        if(auto *node = *it; node->id == id) {
+            clear_chain(&node->prop);
+            clear_chain(&node->base);
+            clear_chain(&node->conv);
+            clear_chain(&node->ctor);
+            clear_chain(&node->data, &internal::meta_data_node::prop);
+            clear_chain(&node->func, &internal::meta_func_node::prop);
+
+            node->id = {};
+            node->dtor = nullptr;
+            *it = std::exchange(node->next, nullptr);
+
+            break;
+        }
+    }
 }
 
+/**
+ * @brief Resets a type and all its parts.
+ *
+ * @sa meta_reset
+ *
+ * @tparam Type Type to reset.
+ */
+template<typename Type>
+void meta_reset() ENTT_NOEXCEPT {
+    meta_reset(internal::meta_node<Type>::resolve()->id);
+}
+
+/**
+ * @brief Resets all searchable types.
+ *
+ * @sa meta_reset
+ */
+inline void meta_reset() ENTT_NOEXCEPT {
+    while(*internal::meta_context::global()) {
+        meta_reset((*internal::meta_context::global())->id);
+    }
+}
+
+} // namespace entt
 
 #endif

src/entt/meta/fwd.hpp

@@ -0,0 +1,24 @@
+#ifndef ENTT_META_FWD_HPP
+#define ENTT_META_FWD_HPP
+
+namespace entt {
+
+class meta_sequence_container;
+
+class meta_associative_container;
+
+class meta_any;
+
+struct meta_handle;
+
+struct meta_prop;
+
+struct meta_data;
+
+struct meta_func;
+
+class meta_type;
+
+} // namespace entt
+
+#endif

src/entt/meta/node.hpp

@@ -1,190 +1,161 @@
 #ifndef ENTT_META_NODE_HPP
 #define ENTT_META_NODE_HPP
 
-
 #include <cstddef>
 #include <type_traits>
 #include <utility>
 #include "../config/config.h"
 #include "../core/attribute.h"
+#include "../core/enum.hpp"
 #include "../core/fwd.hpp"
 #include "../core/type_info.hpp"
 #include "../core/type_traits.hpp"
 #include "type_traits.hpp"
 
-
 namespace entt {
 
-
 class meta_any;
 class meta_type;
 struct meta_handle;
 
-
 /**
  * @cond TURN_OFF_DOXYGEN
  * Internal details not to be documented.
  */
 
-
 namespace internal {
 
+enum class meta_traits : std::uint32_t {
+    is_none = 0x0000,
+    is_const = 0x0001,
+    is_static = 0x0002,
+    is_arithmetic = 0x0004,
+    is_array = 0x0008,
+    is_enum = 0x0010,
+    is_class = 0x0020,
+    is_pointer = 0x0040,
+    is_meta_pointer_like = 0x0080,
+    is_meta_sequence_container = 0x0100,
+    is_meta_associative_container = 0x0200,
+    _entt_enum_as_bitmask
+};
 
 struct meta_type_node;
 
-
 struct meta_prop_node {
-    meta_prop_node * next;
+    meta_prop_node *next;
     const meta_any &id;
     meta_any &value;
 };
 
-
 struct meta_base_node {
-    meta_type_node * const parent;
-    meta_base_node * next;
-    meta_type_node *(* const type)() ENTT_NOEXCEPT;
-    const void *(* const cast)(const void *) ENTT_NOEXCEPT;
+    meta_base_node *next;
+    meta_type_node *const type;
+    meta_any (*const cast)(meta_any) ENTT_NOEXCEPT;
 };
 
-
 struct meta_conv_node {
-    meta_type_node * const parent;
-    meta_conv_node * next;
-    meta_type_node *(* const type)() ENTT_NOEXCEPT;
-    meta_any(* const conv)(const void *);
+    meta_conv_node *next;
+    meta_type_node *const type;
+    meta_any (*const conv)(const meta_any &);
 };
 
-
 struct meta_ctor_node {
     using size_type = std::size_t;
-    meta_type_node * const parent;
-    meta_ctor_node * next;
-    meta_prop_node * prop;
+    meta_ctor_node *next;
     const size_type arity;
-    meta_type(* const arg)(const size_type) ENTT_NOEXCEPT;
-    meta_any(* const invoke)(meta_any * const);
+    meta_type (*const arg)(const size_type) ENTT_NOEXCEPT;
+    meta_any (*const invoke)(meta_any *const);
 };
 
-
 struct meta_data_node {
+    using size_type = std::size_t;
     id_type id;
-    meta_type_node * const parent;
-    meta_data_node * next;
-    meta_prop_node * prop;
-    const bool is_const;
-    const bool is_static;
-    meta_type_node *(* const type)() ENTT_NOEXCEPT;
-    bool(* const set)(meta_handle, meta_any);
-    meta_any(* const get)(meta_handle);
+    const meta_traits traits;
+    meta_data_node *next;
+    meta_prop_node *prop;
+    const size_type arity;
+    meta_type_node *const type;
+    meta_type (*const arg)(const size_type) ENTT_NOEXCEPT;
+    bool (*const set)(meta_handle, meta_any);
+    meta_any (*const get)(meta_handle);
 };
 
-
 struct meta_func_node {
     using size_type = std::size_t;
     id_type id;
-    meta_type_node * const parent;
-    meta_func_node * next;
-    meta_prop_node * prop;
+    const meta_traits traits;
+    meta_func_node *next;
+    meta_prop_node *prop;
     const size_type arity;
-    const bool is_const;
-    const bool is_static;
-    meta_type_node *(* const ret)() ENTT_NOEXCEPT;
-    meta_type(* const arg)(const size_type) ENTT_NOEXCEPT;
-    meta_any(* const invoke)(meta_handle, meta_any *);
+    meta_type_node *const ret;
+    meta_type (*const arg)(const size_type) ENTT_NOEXCEPT;
+    meta_any (*const invoke)(meta_handle, meta_any *const);
 };
 
-
-struct meta_template_info {
+struct meta_template_node {
     using size_type = std::size_t;
-    const bool is_template_specialization;
     const size_type arity;
-    meta_type_node *(* const type)() ENTT_NOEXCEPT;
-    meta_type_node *(* const arg)(const size_type) ENTT_NOEXCEPT;
+    meta_type_node *const type;
+    meta_type_node *(*const arg)(const size_type)ENTT_NOEXCEPT;
 };
 
-
 struct meta_type_node {
     using size_type = std::size_t;
-    const type_info info;
+    const type_info *info;
     id_type id;
-    meta_type_node * next;
-    meta_prop_node * prop;
+    const meta_traits traits;
+    meta_type_node *next;
+    meta_prop_node *prop;
     const size_type size_of;
-    const bool is_void;
-    const bool is_integral;
-    const bool is_floating_point;
-    const bool is_array;
-    const bool is_enum;
-    const bool is_union;
-    const bool is_class;
-    const bool is_pointer;
-    const bool is_function_pointer;
-    const bool is_member_object_pointer;
-    const bool is_member_function_pointer;
-    const bool is_pointer_like;
-    const bool is_sequence_container;
-    const bool is_associative_container;
-    const meta_template_info template_info;
-    const size_type rank;
-    size_type(* const extent)(const size_type) ENTT_NOEXCEPT ;
-    meta_type_node *(* const remove_pointer)() ENTT_NOEXCEPT;
-    meta_type_node *(* const remove_extent)() ENTT_NOEXCEPT;
-    meta_ctor_node * const def_ctor;
+    meta_type_node *(*const remove_pointer)() ENTT_NOEXCEPT;
+    meta_any (*const default_constructor)();
+    double (*const conversion_helper)(void *, const void *);
+    const meta_template_node *const templ;
     meta_ctor_node *ctor{nullptr};
     meta_base_node *base{nullptr};
     meta_conv_node *conv{nullptr};
     meta_data_node *data{nullptr};
     meta_func_node *func{nullptr};
-    void(* dtor)(void *){nullptr};
+    void (*dtor)(void *){nullptr};
 };
 
-
-template<auto Member, typename Op, typename Node>
-auto meta_visit(const Op &op, const Node *node)
--> std::decay_t<decltype(node->*Member)> {
-    for(auto *curr = node->*Member; curr; curr = curr->next) {
-        if(op(curr)) {
-            return curr;
-        }
-    }
-
-    if constexpr(std::is_same_v<Node, meta_type_node>) {
-        for(auto *curr = node->base; curr; curr = curr->next) {
-            if(auto *ret = meta_visit<Member>(op, curr->type()); ret) {
-                return ret;
-            }
-        }
-    }
-
-    return nullptr;
-}
-
-
 template<typename... Args>
-meta_type_node * meta_arg_node(type_list<Args...>, const std::size_t index) ENTT_NOEXCEPT;
-
+meta_type_node *meta_arg_node(type_list<Args...>, const std::size_t index) ENTT_NOEXCEPT;
 
 template<typename Type>
 class ENTT_API meta_node {
     static_assert(std::is_same_v<Type, std::remove_cv_t<std::remove_reference_t<Type>>>, "Invalid type");
 
-    template<std::size_t... Index>
-    [[nodiscard]] static auto extent(const meta_type_node::size_type dim, std::index_sequence<Index...>) ENTT_NOEXCEPT {
-        meta_type_node::size_type ext{};
-        ((ext = (dim == Index ? std::extent_v<Type, Index> : ext)), ...);
-        return ext;
+    [[nodiscard]] static auto *meta_default_constructor() ENTT_NOEXCEPT {
+        if constexpr(std::is_default_constructible_v<Type>) {
+            return +[]() { return meta_any{std::in_place_type<Type>}; };
+        } else {
+            return static_cast<std::decay_t<decltype(meta_type_node::default_constructor)>>(nullptr);
+        }
     }
 
-    [[nodiscard]] static meta_ctor_node * meta_default_constructor([[maybe_unused]] meta_type_node *type) ENTT_NOEXCEPT {
-        if constexpr(std::is_default_constructible_v<Type>) {
-            static meta_ctor_node node{
-                type,
-                nullptr,
-                nullptr,
-                0u,
-                nullptr,
-                [](meta_any * const) { return meta_any{std::in_place_type<Type>}; }
+    [[nodiscard]] static auto *meta_conversion_helper() ENTT_NOEXCEPT {
+        if constexpr(std::is_arithmetic_v<Type>) {
+            return +[](void *bin, const void *value) {
+                return bin ? static_cast<double>(*static_cast<Type *>(bin) = static_cast<Type>(*static_cast<const double *>(value))) : static_cast<double>(*static_cast<const Type *>(value));
+            };
+        } else if constexpr(std::is_enum_v<Type>) {
+            return +[](void *bin, const void *value) {
+                return bin ? static_cast<double>(*static_cast<Type *>(bin) = static_cast<Type>(static_cast<std::underlying_type_t<Type>>(*static_cast<const double *>(value)))) : static_cast<double>(*static_cast<const Type *>(value));
+            };
+        } else {
+            return static_cast<std::decay_t<decltype(meta_type_node::conversion_helper)>>(nullptr);
+        }
+    }
+
+    [[nodiscard]] static meta_template_node *meta_template_info() ENTT_NOEXCEPT {
+        if constexpr(is_complete_v<meta_template_traits<Type>>) {
+            static meta_template_node node{
+                meta_template_traits<Type>::args_type::size,
+                meta_node<typename meta_template_traits<Type>::class_type>::resolve(),
+                [](const std::size_t index) ENTT_NOEXCEPT { return meta_arg_node(typename meta_template_traits<Type>::args_type{}, index); }
+                // tricks clang-format
             };
 
             return &node;
@@ -193,78 +164,74 @@ class ENTT_API meta_node {
         }
     }
 
-    [[nodiscard]] static meta_template_info meta_template_descriptor() ENTT_NOEXCEPT {
-        if constexpr(is_complete_v<meta_template_traits<Type>>) {
-            return {
-                true,
-                meta_template_traits<Type>::args_type::size,
-                &meta_node<typename meta_template_traits<Type>::class_type>::resolve,
-                [](const std::size_t index) ENTT_NOEXCEPT {
-                    return meta_arg_node(typename meta_template_traits<Type>::args_type{}, index);
-                }
-            };
-        } else {
-            return { false, 0u, nullptr, nullptr };
-        }
-    }
-
 public:
-    [[nodiscard]] static meta_type_node * resolve() ENTT_NOEXCEPT {
+    [[nodiscard]] static meta_type_node *resolve() ENTT_NOEXCEPT {
         static meta_type_node node{
-            type_id<Type>(),
+            &type_id<Type>(),
             {},
+            internal::meta_traits::is_none
+                | (std::is_arithmetic_v<Type> ? internal::meta_traits::is_arithmetic : internal::meta_traits::is_none)
+                | (std::is_array_v<Type> ? internal::meta_traits::is_array : internal::meta_traits::is_none)
+                | (std::is_enum_v<Type> ? internal::meta_traits::is_enum : internal::meta_traits::is_none)
+                | (std::is_class_v<Type> ? internal::meta_traits::is_class : internal::meta_traits::is_none)
+                | (std::is_pointer_v<Type> ? internal::meta_traits::is_pointer : internal::meta_traits::is_none)
+                | (is_meta_pointer_like_v<Type> ? internal::meta_traits::is_meta_pointer_like : internal::meta_traits::is_none)
+                | (is_complete_v<meta_sequence_container_traits<Type>> ? internal::meta_traits::is_meta_sequence_container : internal::meta_traits::is_none)
+                | (is_complete_v<meta_associative_container_traits<Type>> ? internal::meta_traits::is_meta_associative_container : internal::meta_traits::is_none),
             nullptr,
             nullptr,
             size_of_v<Type>,
-            std::is_void_v<Type>,
-            std::is_integral_v<Type>,
-            std::is_floating_point_v<Type>,
-            std::is_array_v<Type>,
-            std::is_enum_v<Type>,
-            std::is_union_v<Type>,
-            std::is_class_v<Type>,
-            std::is_pointer_v<Type>,
-            std::is_pointer_v<Type> && std::is_function_v<std::remove_pointer_t<Type>>,
-            std::is_member_object_pointer_v<Type>,
-            std::is_member_function_pointer_v<Type>,
-            is_meta_pointer_like_v<Type>,
-            is_complete_v<meta_sequence_container_traits<Type>>,
-            is_complete_v<meta_associative_container_traits<Type>>,
-            meta_template_descriptor(),
-            std::rank_v<Type>,
-            [](meta_type_node::size_type dim) ENTT_NOEXCEPT { return extent(dim, std::make_index_sequence<std::rank_v<Type>>{}); },
             &meta_node<std::remove_cv_t<std::remove_reference_t<std::remove_pointer_t<Type>>>>::resolve,
-            &meta_node<std::remove_cv_t<std::remove_reference_t<std::remove_extent_t<Type>>>>::resolve,
-            meta_default_constructor(&node),
-            meta_default_constructor(&node)
+            meta_default_constructor(),
+            meta_conversion_helper(),
+            meta_template_info()
+            // tricks clang-format
         };
 
         return &node;
     }
 };
 
-
-template<typename Type>
-struct meta_info: meta_node<std::remove_cv_t<std::remove_reference_t<Type>>> {};
-
-
 template<typename... Args>
-meta_type_node * meta_arg_node(type_list<Args...>, const std::size_t index) ENTT_NOEXCEPT {
-    meta_type_node *args[sizeof...(Args) + 1u]{nullptr, internal::meta_info<Args>::resolve()...};
+[[nodiscard]] meta_type_node *meta_arg_node(type_list<Args...>, const std::size_t index) ENTT_NOEXCEPT {
+    meta_type_node *args[sizeof...(Args) + 1u]{nullptr, internal::meta_node<std::remove_cv_t<std::remove_reference_t<Args>>>::resolve()...};
     return args[index + 1u];
 }
 
+template<auto Member, typename Type>
+[[nodiscard]] static std::decay_t<decltype(std::declval<internal::meta_type_node>().*Member)> find_by(const Type &info_or_id, const internal::meta_type_node *node) ENTT_NOEXCEPT {
+    for(auto *curr = node->*Member; curr; curr = curr->next) {
+        if constexpr(std::is_same_v<Type, type_info>) {
+            if(*curr->type->info == info_or_id) {
+                return curr;
+            }
+        } else if constexpr(std::is_same_v<decltype(curr), meta_base_node *>) {
+            if(curr->type->id == info_or_id) {
+                return curr;
+            }
+        } else {
+            if(curr->id == info_or_id) {
+                return curr;
+            }
+        }
+    }
 
+    for(auto *curr = node->base; curr; curr = curr->next) {
+        if(auto *ret = find_by<Member>(info_or_id, curr->type); ret) {
+            return ret;
+        }
+    }
+
+    return nullptr;
 }
 
+} // namespace internal
 
 /**
  * Internal details not to be documented.
  * @endcond
  */
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/meta/pointer.hpp

@@ -1,24 +1,19 @@
 #ifndef ENTT_META_POINTER_HPP
 #define ENTT_META_POINTER_HPP
 
-
 #include <memory>
 #include <type_traits>
 #include "type_traits.hpp"
 
-
 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
-{};
-
+    : std::true_type {};
 
 /**
  * @brief Partial specialization used to reject pointers to arrays.
@@ -26,10 +21,8 @@ struct is_meta_pointer_like<Type *>
  * @tparam N Number of elements of the array.
  */
 template<typename Type, std::size_t N>
-struct is_meta_pointer_like<Type(*)[N]>
-    : std::false_type
-{};
-
+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
@@ -38,9 +31,7 @@ struct is_meta_pointer_like<Type(*)[N]>
  */
 template<typename Type>
 struct is_meta_pointer_like<std::shared_ptr<Type>>
-        : std::true_type
-{};
-
+    : std::true_type {};
 
 /**
  * @brief Makes `std::unique_ptr`s of any type pointer-like types for the meta
@@ -50,11 +41,8 @@ struct is_meta_pointer_like<std::shared_ptr<Type>>
  */
 template<typename Type, typename... Args>
 struct is_meta_pointer_like<std::unique_ptr<Type, Args...>>
-        : std::true_type
-{};
-
-
-}
+    : std::true_type {};
 
+} // namespace entt
 
 #endif

src/entt/meta/policy.hpp

@@ -1,27 +1,66 @@
 #ifndef ENTT_META_POLICY_HPP
 #define ENTT_META_POLICY_HPP
 
+#include <type_traits>
 
 namespace entt {
 
-
 /*! @brief Empty class type used to request the _as ref_ policy. */
-struct as_ref_t {};
-
+struct as_ref_t {
+    /**
+     * @cond TURN_OFF_DOXYGEN
+     * Internal details not to be documented.
+     */
+    template<typename Type>
+    static constexpr bool value = std::is_reference_v<Type> && !std::is_const_v<std::remove_reference_t<Type>>;
+    /**
+     * Internal details not to be documented.
+     * @endcond
+     */
+};
 
 /*! @brief Empty class type used to request the _as cref_ policy. */
-struct as_cref_t {};
-
+struct as_cref_t {
+    /**
+     * @cond TURN_OFF_DOXYGEN
+     * Internal details not to be documented.
+     */
+    template<typename Type>
+    static constexpr bool value = std::is_reference_v<Type>;
+    /**
+     * Internal details not to be documented.
+     * @endcond
+     */
+};
 
 /*! @brief Empty class type used to request the _as-is_ policy. */
-struct as_is_t {};
-
+struct as_is_t {
+    /**
+     * @cond TURN_OFF_DOXYGEN
+     * Internal details not to be documented.
+     */
+    template<typename>
+    static constexpr bool value = true;
+    /**
+     * Internal details not to be documented.
+     * @endcond
+     */
+};
 
 /*! @brief Empty class type used to request the _as void_ policy. */
-struct as_void_t {};
-
-
-}
+struct as_void_t {
+    /**
+     * @cond TURN_OFF_DOXYGEN
+     * Internal details not to be documented.
+     */
+    template<typename>
+    static constexpr bool value = true;
+    /**
+     * Internal details not to be documented.
+     * @endcond
+     */
+};
 
+} // namespace entt
 
 #endif

src/entt/meta/range.hpp

@@ -1,13 +1,69 @@
 #ifndef ENTT_META_RANGE_HPP
 #define ENTT_META_RANGE_HPP
 
-
 #include <cstddef>
 #include <iterator>
-
+#include "../core/iterator.hpp"
 
 namespace entt {
 
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename Type, typename Node>
+struct meta_range_iterator final {
+    using difference_type = std::ptrdiff_t;
+    using value_type = Type;
+    using pointer = input_iterator_pointer<value_type>;
+    using reference = value_type;
+    using iterator_category = std::input_iterator_tag;
+    using node_type = Node;
+
+    meta_range_iterator() ENTT_NOEXCEPT
+        : it{} {}
+
+    meta_range_iterator(node_type *head) ENTT_NOEXCEPT
+        : it{head} {}
+
+    meta_range_iterator &operator++() ENTT_NOEXCEPT {
+        return (it = it->next), *this;
+    }
+
+    meta_range_iterator operator++(int) ENTT_NOEXCEPT {
+        meta_range_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
+        return it;
+    }
+
+    [[nodiscard]] pointer operator->() const ENTT_NOEXCEPT {
+        return operator*();
+    }
+
+    [[nodiscard]] bool operator==(const meta_range_iterator &other) const ENTT_NOEXCEPT {
+        return it == other.it;
+    }
+
+    [[nodiscard]] bool operator!=(const meta_range_iterator &other) const ENTT_NOEXCEPT {
+        return !(*this == other);
+    }
+
+private:
+    node_type *it;
+};
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
 
 /**
  * @brief Iterable range to use to iterate all types of meta objects.
@@ -15,51 +71,13 @@ namespace entt {
  * @tparam Node Type of meta nodes iterated.
  */
 template<typename Type, typename Node = typename Type::node_type>
-class meta_range {
-    struct range_iterator {
-        using difference_type = std::ptrdiff_t;
-        using value_type = Type;
-        using pointer = void;
-        using reference = value_type;
-        using iterator_category = std::input_iterator_tag;
-        using node_type = Node;
-
-        range_iterator() ENTT_NOEXCEPT = default;
-
-        range_iterator(node_type *head) ENTT_NOEXCEPT
-            : it{head}
-        {}
-
-        range_iterator & operator++() ENTT_NOEXCEPT {
-            return (it = it->next), *this;
-        }
-
-        range_iterator operator++(int) ENTT_NOEXCEPT {
-            range_iterator orig = *this;
-            return ++(*this), orig;
-        }
-
-        [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
-            return it;
-        }
-
-        [[nodiscard]] bool operator==(const range_iterator &other) const ENTT_NOEXCEPT {
-            return other.it == it;
-        }
-
-        [[nodiscard]] bool operator!=(const range_iterator &other) const ENTT_NOEXCEPT {
-            return !(*this == other);
-        }
-
-    private:
-        node_type *it{};
-    };
-
-public:
+struct meta_range final {
     /*! @brief Node type. */
     using node_type = Node;
     /*! @brief Input iterator type. */
-    using iterator = range_iterator;
+    using iterator = internal::meta_range_iterator<Type, Node>;
+    /*! @brief Constant input iterator type. */
+    using const_iterator = iterator;
 
     /*! @brief Default constructor. */
     meta_range() ENTT_NOEXCEPT = default;
@@ -68,33 +86,40 @@ public:
      * @brief Constructs a meta range from a given node.
      * @param head The underlying node with which to construct the range.
      */
-    meta_range(node_type *head)
-        : node{head}
-    {}
+    meta_range(node_type *head) ENTT_NOEXCEPT
+        : node{head} {}
 
     /**
      * @brief Returns an iterator to the beginning.
      * @return An iterator to the first meta object of the range.
      */
-    [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+    [[nodiscard]] const_iterator cbegin() const ENTT_NOEXCEPT {
         return iterator{node};
     }
 
+    /*! @copydoc cbegin */
+    [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+        return cbegin();
+    }
+
     /**
      * @brief Returns an iterator to the end.
      * @return An iterator to the element following the last meta object of the
      * range.
      */
-    [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+    [[nodiscard]] const_iterator cend() const ENTT_NOEXCEPT {
         return iterator{};
     }
 
+    /*! @copydoc cend */
+    [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+        return cend();
+    }
+
 private:
     node_type *node{nullptr};
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/meta/resolve.hpp

@@ -1,7 +1,6 @@
 #ifndef ENTT_META_RESOLVE_HPP
 #define ENTT_META_RESOLVE_HPP
 
-
 #include <algorithm>
 #include "../core/type_info.hpp"
 #include "ctx.hpp"
@@ -9,10 +8,8 @@
 #include "node.hpp"
 #include "range.hpp"
 
-
 namespace entt {
 
-
 /**
  * @brief Returns the meta type associated with a given type.
  * @tparam Type Type to use to search for a meta type.
@@ -20,27 +17,25 @@ namespace entt {
  */
 template<typename Type>
 [[nodiscard]] meta_type resolve() ENTT_NOEXCEPT {
-    return internal::meta_info<Type>::resolve();
+    return internal::meta_node<std::remove_cv_t<std::remove_reference_t<Type>>>::resolve();
 }
 
-
 /**
  * @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> resolve() {
+[[nodiscard]] inline meta_range<meta_type> resolve() ENTT_NOEXCEPT {
     return *internal::meta_context::global();
 }
 
-
 /**
  * @brief Returns the meta type associated with a given identifier, if any.
  * @param id Unique identifier.
  * @return The meta type associated with the given identifier, if any.
  */
 [[nodiscard]] inline meta_type resolve(const id_type id) ENTT_NOEXCEPT {
-    for(auto *curr = *internal::meta_context::global(); curr; curr = curr->next) {
-        if(curr->id == id) {
+    for(auto &&curr: resolve()) {
+        if(curr.id() == id) {
             return curr;
         }
     }
@@ -48,16 +43,14 @@ template<typename Type>
     return {};
 }
 
-
 /**
- * @brief Returns the meta type associated with a given type info object, if
- * any.
+ * @brief Returns the meta type associated with a given type info object.
  * @param info The type info object of the requested type.
  * @return The meta type associated with the given type info object, if any.
  */
-[[nodiscard]] inline meta_type resolve(const type_info info) ENTT_NOEXCEPT {
-    for(auto *curr = *internal::meta_context::global(); curr; curr = curr->next) {
-        if(curr->info == info) {
+[[nodiscard]] inline meta_type resolve(const type_info &info) ENTT_NOEXCEPT {
+    for(auto &&curr: resolve()) {
+        if(curr.info() == info) {
             return curr;
         }
     }
@@ -65,8 +58,6 @@ template<typename Type>
     return {};
 }
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/meta/template.hpp

@@ -1,24 +1,20 @@
 #ifndef ENTT_META_TEMPLATE_HPP
 #define ENTT_META_TEMPLATE_HPP
 
-
 #include "../core/type_traits.hpp"
 
-
 namespace entt {
 
-
 /*! @brief Utility class to disambiguate class templates. */
-template<template<typename...> typename>
+template<template<typename...> class>
 struct meta_class_template_tag {};
 
-
 /**
  * @brief General purpose traits class for generating meta template information.
  * @tparam Clazz Type of class template.
  * @tparam Args Types of template arguments.
  */
-template<template<typename...> typename Clazz, typename... Args>
+template<template<typename...> class Clazz, typename... Args>
 struct meta_template_traits<Clazz<Args...>> {
     /*! @brief Wrapped class template. */
     using class_type = meta_class_template_tag<Clazz>;
@@ -26,8 +22,6 @@ struct meta_template_traits<Clazz<Args...>> {
     using args_type = type_list<Args...>;
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/meta/type_traits.hpp

@@ -1,13 +1,11 @@
 #ifndef ENTT_META_TYPE_TRAITS_HPP
 #define ENTT_META_TYPE_TRAITS_HPP
 
-
 #include <type_traits>
-
+#include <utility>
 
 namespace entt {
 
-
 /**
  * @brief Traits class template to be specialized to enable support for meta
  * template information.
@@ -15,7 +13,6 @@ namespace entt {
 template<typename>
 struct meta_template_traits;
 
-
 /**
  * @brief Traits class template to be specialized to enable support for meta
  * sequence containers.
@@ -23,7 +20,6 @@ struct meta_template_traits;
 template<typename>
 struct meta_sequence_container_traits;
 
-
 /**
  * @brief Traits class template to be specialized to enable support for meta
  * associative containers.
@@ -31,31 +27,6 @@ struct meta_sequence_container_traits;
 template<typename>
 struct meta_associative_container_traits;
 
-
-/**
- * @brief Provides the member constant `value` to true if a meta associative
- * container claims to wrap a key-only type, false otherwise.
- * @tparam Type Potentially key-only meta associative container type.
- */
-template<typename, typename = void>
-struct is_key_only_meta_associative_container: std::true_type {};
-
-
-/*! @copydoc is_key_only_meta_associative_container */
-template<typename Type>
-struct is_key_only_meta_associative_container<Type, std::void_t<typename meta_associative_container_traits<Type>::type::mapped_type>>
-    : std::false_type
-{};
-
-
-/**
- * @brief Helper variable template.
- * @tparam Type Potentially key-only meta associative container type.
- */
-template<typename Type>
-inline constexpr auto is_key_only_meta_associative_container_v = is_key_only_meta_associative_container<Type>::value;
-
-
 /**
  * @brief Provides the member constant `value` to true if a given type is a
  * pointer-like type from the point of view of the meta system, false otherwise.
@@ -64,7 +35,6 @@ inline constexpr auto is_key_only_meta_associative_container_v = is_key_only_met
 template<typename>
 struct is_meta_pointer_like: std::false_type {};
 
-
 /**
  * @brief Partial specialization to ensure that const pointer-like types are
  * also accepted.
@@ -73,7 +43,6 @@ struct is_meta_pointer_like: std::false_type {};
 template<typename Type>
 struct is_meta_pointer_like<const Type>: is_meta_pointer_like<Type> {};
 
-
 /**
  * @brief Helper variable template.
  * @tparam Type Potentially pointer-like type.
@@ -81,8 +50,6 @@ struct is_meta_pointer_like<const Type>: is_meta_pointer_like<Type> {};
 template<typename Type>
 inline constexpr auto is_meta_pointer_like_v = is_meta_pointer_like<Type>::value;
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/meta/utility.hpp

@@ -1,7 +1,6 @@
 #ifndef ENTT_META_UTILITY_HPP
 #define ENTT_META_UTILITY_HPP
 
-
 #include <cstddef>
 #include <functional>
 #include <type_traits>
@@ -12,15 +11,12 @@
 #include "node.hpp"
 #include "policy.hpp"
 
-
 namespace entt {
 
-
 /*! @brief Primary template isn't defined on purpose. */
 template<typename, typename>
 struct meta_function_descriptor;
 
-
 /**
  * @brief Meta function descriptor.
  * @tparam Type Reflected type to which the meta function is associated.
@@ -29,19 +25,18 @@ struct meta_function_descriptor;
  * @tparam Args Function arguments.
  */
 template<typename Type, typename Ret, typename Class, typename... Args>
-struct meta_function_descriptor<Type, Ret(Class:: *)(Args...) const> {
+struct meta_function_descriptor<Type, Ret (Class::*)(Args...) const> {
     /*! @brief Meta function return type. */
     using return_type = Ret;
     /*! @brief Meta function arguments. */
-    using args_type = std::conditional_t<std::is_same_v<Type, Class>, type_list<Args...>, type_list<const Class &, Args...>>;
+    using args_type = std::conditional_t<std::is_base_of_v<Class, Type>, type_list<Args...>, type_list<const Class &, Args...>>;
 
     /*! @brief True if the meta function is const, false otherwise. */
     static constexpr auto is_const = true;
     /*! @brief True if the meta function is static, false otherwise. */
-    static constexpr auto is_static = !std::is_same_v<Type, Class>;
+    static constexpr auto is_static = !std::is_base_of_v<Class, Type>;
 };
 
-
 /**
  * @brief Meta function descriptor.
  * @tparam Type Reflected type to which the meta function is associated.
@@ -50,31 +45,68 @@ struct meta_function_descriptor<Type, Ret(Class:: *)(Args...) const> {
  * @tparam Args Function arguments.
  */
 template<typename Type, typename Ret, typename Class, typename... Args>
-struct meta_function_descriptor<Type, Ret(Class:: *)(Args...)> {
+struct meta_function_descriptor<Type, Ret (Class::*)(Args...)> {
     /*! @brief Meta function return type. */
     using return_type = Ret;
     /*! @brief Meta function arguments. */
-    using args_type = std::conditional_t<std::is_same_v<Type, Class>, type_list<Args...>, type_list<Class &, Args...>>;
+    using args_type = std::conditional_t<std::is_base_of_v<Class, Type>, type_list<Args...>, type_list<Class &, Args...>>;
 
     /*! @brief True if the meta function is const, false otherwise. */
     static constexpr auto is_const = false;
     /*! @brief True if the meta function is static, false otherwise. */
-    static constexpr auto is_static = !std::is_same_v<Type, Class>;
+    static constexpr auto is_static = !std::is_base_of_v<Class, Type>;
 };
 
+/**
+ * @brief Meta function descriptor.
+ * @tparam Type Reflected type to which the meta data is associated.
+ * @tparam Class Actual owner of the data member.
+ * @tparam Ret Data member type.
+ */
+template<typename Type, typename Ret, typename Class>
+struct meta_function_descriptor<Type, Ret Class::*> {
+    /*! @brief Meta data return type. */
+    using return_type = Ret &;
+    /*! @brief Meta data arguments. */
+    using args_type = std::conditional_t<std::is_base_of_v<Class, Type>, type_list<>, type_list<Class &>>;
+
+    /*! @brief True if the meta data is const, false otherwise. */
+    static constexpr auto is_const = false;
+    /*! @brief True if the meta data is static, false otherwise. */
+    static constexpr auto is_static = !std::is_base_of_v<Class, Type>;
+};
 
 /**
  * @brief Meta function descriptor.
  * @tparam Type Reflected type to which the meta function is associated.
  * @tparam Ret Function return type.
- * @tparam Args Function arguments.
+ * @tparam MaybeType First function argument.
+ * @tparam Args Other function arguments.
  */
-template<typename Type, typename Ret, typename... Args>
-struct meta_function_descriptor<Type, Ret(*)(Args...)> {
+template<typename Type, typename Ret, typename MaybeType, typename... Args>
+struct meta_function_descriptor<Type, Ret (*)(MaybeType, Args...)> {
     /*! @brief Meta function return type. */
     using return_type = Ret;
     /*! @brief Meta function arguments. */
-    using args_type = type_list<Args...>;
+    using args_type = std::conditional_t<std::is_base_of_v<std::remove_cv_t<std::remove_reference_t<MaybeType>>, Type>, type_list<Args...>, type_list<MaybeType, Args...>>;
+
+    /*! @brief True if the meta function is const, false otherwise. */
+    static constexpr auto is_const = std::is_base_of_v<std::remove_cv_t<std::remove_reference_t<MaybeType>>, Type> && std::is_const_v<std::remove_reference_t<MaybeType>>;
+    /*! @brief True if the meta function is static, false otherwise. */
+    static constexpr auto is_static = !std::is_base_of_v<std::remove_cv_t<std::remove_reference_t<MaybeType>>, Type>;
+};
+
+/**
+ * @brief Meta function descriptor.
+ * @tparam Type Reflected type to which the meta function is associated.
+ * @tparam Ret Function return type.
+ */
+template<typename Type, typename Ret>
+struct meta_function_descriptor<Type, Ret (*)()> {
+    /*! @brief Meta function return type. */
+    using return_type = Ret;
+    /*! @brief Meta function arguments. */
+    using args_type = type_list<>;
 
     /*! @brief True if the meta function is const, false otherwise. */
     static constexpr auto is_const = false;
@@ -82,7 +114,6 @@ struct meta_function_descriptor<Type, Ret(*)(Args...)> {
     static constexpr auto is_static = true;
 };
 
-
 /**
  * @brief Meta function helper.
  *
@@ -95,20 +126,25 @@ struct meta_function_descriptor<Type, Ret(*)(Args...)> {
 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 constexpr 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 constexpr meta_function_descriptor<Type, Ret (Class::*)(Args...)> get_rid_of_noexcept(Ret (Class::*)(Args...));
+
+    template<typename Ret, typename Class>
+    static constexpr 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 constexpr 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);
 
 public:
     /*! @brief The meta function descriptor of the given function. */
     using type = decltype(get_rid_of_noexcept(std::declval<Candidate>()));
 };
 
-
 /**
  * @brief Helper type.
  * @tparam Type Reflected type to which the meta function is associated.
@@ -117,36 +153,38 @@ public:
 template<typename Type, typename Candidate>
 using meta_function_helper_t = typename meta_function_helper<Type, Candidate>::type;
 
+/**
+ * @brief Wraps a value depending on the given policy.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @tparam Type Type of value to wrap.
+ * @param value Value to wrap.
+ * @return A meta any containing the returned value, if any.
+ */
+template<typename Policy = as_is_t, typename Type>
+meta_any meta_dispatch([[maybe_unused]] Type &&value) {
+    if constexpr(std::is_same_v<Policy, as_void_t>) {
+        return meta_any{std::in_place_type<void>};
+    } else if constexpr(std::is_same_v<Policy, as_ref_t>) {
+        return meta_any{std::in_place_type<Type>, std::forward<Type>(value)};
+    } else if constexpr(std::is_same_v<Policy, as_cref_t>) {
+        static_assert(std::is_lvalue_reference_v<Type>, "Invalid type");
+        return meta_any{std::in_place_type<const std::remove_reference_t<Type> &>, std::as_const(value)};
+    } else {
+        static_assert(std::is_same_v<Policy, as_is_t>, "Policy not supported");
+        return meta_any{std::forward<Type>(value)};
+    }
+}
 
 /**
  * @brief Returns the meta type of the i-th element of a list of arguments.
- * @tparam Args Actual types of arguments.
+ * @tparam Type Type list of the actual types of arguments.
  * @return The meta type of the i-th element of the list of arguments.
  */
-template<typename... Args>
-[[nodiscard]] static meta_type meta_arg(type_list<Args...>, const std::size_t index) ENTT_NOEXCEPT {
-    return internal::meta_arg_node(type_list<Args...>{}, index);
+template<typename Type>
+[[nodiscard]] static meta_type meta_arg(const std::size_t index) ENTT_NOEXCEPT {
+    return internal::meta_arg_node(Type{}, index);
 }
 
-
-/**
- * @brief Constructs an instance given a list of erased parameters, if possible.
- * @tparam Type Actual type of the instance to construct.
- * @tparam Args Types of arguments expected.
- * @tparam Index Indexes to use to extract erased arguments from their list.
- * @param args Parameters to use to construct the instance.
- * @return A meta any containing the new instance, if any.
- */
-template<typename Type, typename... Args, std::size_t... Index>
-[[nodiscard]] meta_any meta_construct(meta_any * const args, std::index_sequence<Index...>) {
-    if(((args+Index)->allow_cast<Args>() && ...)) {
-        return Type{(args+Index)->cast<Args>()...};
-    }
-
-    return {};
-}
-
-
 /**
  * @brief Sets the value of a given variable.
  * @tparam Type Reflected type to which the variable is associated.
@@ -158,26 +196,20 @@ template<typename Type, typename... Args, std::size_t... Index>
 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_function_v<std::remove_reference_t<std::remove_pointer_t<decltype(Data)>>>) {
-            using data_type = type_list_element_t<1u, typename meta_function_helper_t<Type, decltype(Data)>::args_type>;
+        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>()) {
-                Data(*clazz, value.cast<data_type>());
-                return true;
-            }
-        } else if constexpr(std::is_member_function_pointer_v<decltype(Data)>) {
-            using data_type = type_list_element_t<0u, typename meta_function_helper_t<Type, decltype(Data)>::args_type>;
-
-            if(auto * const clazz = instance->try_cast<Type>(); clazz && value.allow_cast<data_type>()) {
-                (clazz->*Data)(value.cast<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_member_object_pointer_v<decltype(Data)>) {
-            using data_type = std::remove_reference_t<decltype(std::declval<Type>().*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>()) {
-                    clazz->*Data = value.cast<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;
                 }
             }
@@ -196,30 +228,6 @@ template<typename Type, auto Data>
     return false;
 }
 
-
-/**
- * @brief Wraps a value depending on the given policy.
- * @tparam Policy Optional policy (no policy set by default).
- * @tparam Type Type of value to wrap.
- * @param value Value to wrap.
- * @return A meta any containing the returned value.
- */
-template<typename Policy = as_is_t, typename Type>
-meta_any meta_dispatch(Type &&value) {
-    if constexpr(std::is_same_v<Policy, as_void_t>) {
-        return meta_any{std::in_place_type<void>, std::forward<Type>(value)};
-    } else if constexpr(std::is_same_v<Policy, as_ref_t>) {
-        return meta_any{std::in_place_type<Type>, std::forward<Type>(value)};
-    } else if constexpr(std::is_same_v<Policy, as_cref_t>) {
-        static_assert(std::is_lvalue_reference_v<Type>, "Invalid type");
-        return meta_any{std::in_place_type<const std::remove_reference_t<Type> &>, std::as_const(value)};
-    } else {
-        static_assert(std::is_same_v<Policy, as_is_t>, "Policy not supported");
-        return meta_any{std::forward<Type>(value)};
-    }
-}
-
-
 /**
  * @brief Gets the value of a given variable.
  * @tparam Type Reflected type to which the variable is associated.
@@ -230,18 +238,18 @@ meta_any meta_dispatch(Type &&value) {
  */
 template<typename Type, auto Data, typename Policy = as_is_t>
 [[nodiscard]] meta_any meta_getter([[maybe_unused]] meta_handle instance) {
-    if constexpr(std::is_function_v<std::remove_reference_t<std::remove_pointer_t<decltype(Data)>>>) {
-        auto * const clazz = instance->try_cast<std::conditional_t<std::is_invocable_v<decltype(Data), const Type &>, const Type, Type>>();
-        return clazz ? meta_dispatch<Policy>(Data(*clazz)) : meta_any{};
-    } else if constexpr(std::is_member_function_pointer_v<decltype(Data)>) {
-        auto * const clazz = instance->try_cast<std::conditional_t<std::is_invocable_v<decltype(Data), const Type &>, const Type, Type>>();
-        return clazz ? meta_dispatch<Policy>((clazz->*Data)()) : meta_any{};
-    } else if constexpr(std::is_member_object_pointer_v<decltype(Data)>) {
-        if constexpr(!std::is_array_v<std::remove_cv_t<std::remove_reference_t<decltype(std::declval<Type>().*Data)>>>) {
-            if(auto * clazz = instance->try_cast<Type>(); clazz) {
-                return meta_dispatch<Policy>(clazz->*Data);
-            } else if(auto * fallback = instance->try_cast<const Type>(); fallback) {
-                return meta_dispatch<Policy>(fallback->*Data);
+    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>(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>(std::invoke(Data, *fallback));
+                }
             }
         }
 
@@ -257,77 +265,132 @@ template<typename Type, auto Data, typename Policy = as_is_t>
     }
 }
 
-
 /**
- * @brief Invokes a function given a list of erased parameters, if possible.
- * @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).
- * @tparam Index Indexes to use to extract erased arguments from their list.
- * @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.
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
  */
-template<typename Type, auto Candidate, typename Policy = as_is_t, std::size_t... Index>
-[[nodiscard]] std::enable_if_t<!std::is_invocable_v<decltype(Candidate)>, meta_any> meta_invoke([[maybe_unused]] meta_handle instance, meta_any *args, std::index_sequence<Index...>) {
-    using descriptor = meta_function_helper_t<Type, decltype(Candidate)>;
 
-    const auto invoke = [](auto &&maybe_clazz, auto &&... other) {
-        if constexpr(std::is_member_function_pointer_v<decltype(Candidate)>) {
-            if constexpr(std::is_void_v<typename descriptor::return_type>) {
-                (std::forward<decltype(maybe_clazz)>(maybe_clazz).*Candidate)(std::forward<decltype(other)>(other)...);
-                return meta_any{std::in_place_type<void>};
-            } else {
-                return meta_dispatch<Policy>((std::forward<decltype(maybe_clazz)>(maybe_clazz).*Candidate)(std::forward<decltype(other)>(other)...));
-            }
-        } else {
-            if constexpr(std::is_void_v<typename descriptor::return_type>) {
-                Candidate(std::forward<decltype(maybe_clazz)>(maybe_clazz), std::forward<decltype(other)>(other)...);
-                return meta_any{std::in_place_type<void>};
-            } else {
-                return meta_dispatch<Policy>(Candidate(std::forward<decltype(maybe_clazz)>(maybe_clazz), std::forward<decltype(other)>(other)...));
-            }
-        }
-    };
+namespace internal {
 
-    if constexpr(std::is_invocable_v<decltype(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 invoke(*clazz, (args+Index)->cast<type_list_element_t<Index, typename descriptor::args_type>>()...);
+template<typename Type, typename Policy, typename Candidate, typename... Args>
+[[nodiscard]] meta_any meta_invoke_with_args(Candidate &&candidate, Args &&...args) {
+    if constexpr(std::is_same_v<std::invoke_result_t<decltype(candidate), Args...>, void>) {
+        std::invoke(candidate, args...);
+        return meta_any{std::in_place_type<void>};
+    } else {
+        return meta_dispatch<Policy>(std::invoke(candidate, args...));
+    }
+}
+
+template<typename Type, typename Policy, typename Candidate, std::size_t... Index>
+[[nodiscard]] meta_any meta_invoke([[maybe_unused]] meta_handle instance, Candidate &&candidate, [[maybe_unused]] meta_any *args, std::index_sequence<Index...>) {
+    using descriptor = meta_function_helper_t<Type, std::remove_reference_t<Candidate>>;
+
+    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<Type, Policy>(std::forward<Candidate>(candidate), *clazz, (args + Index)->cast<type_list_element_t<Index, typename descriptor::args_type>>()...);
         }
-    } else if constexpr(std::is_invocable_v<decltype(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 invoke(*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<Type, Policy>(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 invoke((args+Index)->cast<type_list_element_t<Index, typename descriptor::args_type>>()...);
+        if(((args + Index)->allow_cast<type_list_element_t<Index, typename descriptor::args_type>>() && ...)) {
+            return meta_invoke_with_args<Type, Policy>(std::forward<Candidate>(candidate), (args + Index)->cast<type_list_element_t<Index, typename descriptor::args_type>>()...);
         }
     }
 
     return meta_any{};
 }
 
+template<typename Type, typename... Args, std::size_t... Index>
+[[nodiscard]] meta_any meta_construct(meta_any *const args, std::index_sequence<Index...>) {
+    if(((args + Index)->allow_cast<Args>() && ...)) {
+        return meta_any{std::in_place_type<Type>, (args + Index)->cast<Args>()...};
+    }
+
+    return meta_any{};
+}
+
+} // namespace internal
 
 /**
- * @brief Invokes a function given a list of erased parameters, if possible.
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief Tries to _invoke_ an object given a list of erased parameters.
+ * @tparam Type Reflected type to which the object to _invoke_ is associated.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @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.
+ */
+template<typename Type, typename Policy = as_is_t, typename Candidate>
+[[nodiscard]] meta_any meta_invoke([[maybe_unused]] meta_handle instance, Candidate &&candidate, [[maybe_unused]] meta_any *const args) {
+    return internal::meta_invoke<Type, Policy>(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>{});
+}
+
+/**
+ * @brief Tries to invoke a function given a list of erased parameters.
  * @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).
- * @tparam Index Indexes to use to extract erased arguments from their list.
+ * @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, std::size_t... Index>
-[[nodiscard]] std::enable_if_t<std::is_invocable_v<decltype(Candidate)>, meta_any> meta_invoke(meta_handle, meta_any *, std::index_sequence<Index...>) {
-    if constexpr(std::is_void_v<decltype(Candidate())>) {
-        Candidate();
-        return meta_any{std::in_place_type<void>};
+template<typename Type, auto Candidate, typename Policy = as_is_t>
+[[nodiscard]] meta_any meta_invoke(meta_handle instance, meta_any *const args) {
+    return internal::meta_invoke<Type, Policy>(std::move(instance), Candidate, args, std::make_index_sequence<meta_function_helper_t<Type, std::remove_reference_t<decltype(Candidate)>>::args_type::size>{});
+}
+
+/**
+ * @brief Tries to construct an instance given a list of erased parameters.
+ * @tparam Type Actual type of the instance to construct.
+ * @tparam Args Types of arguments expected.
+ * @param args Parameters to use to construct the instance.
+ * @return A meta any containing the new instance, if any.
+ */
+template<typename Type, typename... Args>
+[[nodiscard]] meta_any meta_construct(meta_any *const args) {
+    return internal::meta_construct<Type, Args...>(args, std::index_sequence_for<Args...>{});
+}
+
+/**
+ * @brief Tries to construct an instance given a list of erased parameters.
+ * @tparam Type Reflected type to which the object to _invoke_ is associated.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @tparam Candidate The type of the actual object to _invoke_.
+ * @param args Parameters to use to _invoke_ the object.
+ * @param candidate The actual object to _invoke_.
+ * @return A meta any containing the returned value, if any.
+ */
+template<typename Type, typename Policy = as_is_t, typename Candidate>
+[[nodiscard]] meta_any meta_construct(Candidate &&candidate, meta_any *const args) {
+    if constexpr(meta_function_helper_t<Type, Candidate>::is_static) {
+        return internal::meta_invoke<Type, Policy>({}, std::forward<Candidate>(candidate), args, std::make_index_sequence<meta_function_helper_t<Type, std::remove_reference_t<Candidate>>::args_type::size>{});
     } else {
-        return meta_dispatch<Policy>(Candidate());
+        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>{});
     }
 }
 
-
+/**
+ * @brief Tries to construct an instance given a list of erased parameters.
+ * @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 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]] meta_any meta_construct(meta_any *const args) {
+    return meta_construct<Type, Policy>(Candidate, args);
 }
 
+} // namespace entt
 
 #endif

src/entt/platform/android-ndk-r17.hpp

@@ -1,37 +1,29 @@
 #ifndef ENTT_PLATFORM_ANDROID_NDK_R17_HPP
 #define ENTT_PLATFORM_ANDROID_NDK_R17_HPP
 
-
 /**
  * @cond TURN_OFF_DOXYGEN
  * Internal details not to be documented.
  */
 
-
 #ifdef __ANDROID__
-#include <android/ndk-version.h>
-#if __NDK_MAJOR__ == 17
-
-
-#include <functional>
-#include <type_traits>
-#include <utility>
+#    include <android/ndk-version.h>
+#    if __NDK_MAJOR__ == 17
 
+#        include <functional>
+#        include <type_traits>
+#        include <utility>
 
 namespace std {
 
-
 namespace internal {
 
-
 template<typename Func, typename... Args>
 constexpr auto is_invocable(int) -> decltype(std::invoke(std::declval<Func>(), std::declval<Args>()...), std::true_type{});
 
-
 template<typename, typename...>
 constexpr std::false_type is_invocable(...);
 
-
 template<typename Ret, typename Func, typename... Args>
 constexpr auto is_invocable_r(int)
 -> std::enable_if_t<decltype(std::is_convertible_v<decltype(std::invoke(std::declval<Func>(), std::declval<Args>()...)), Ret>, std::true_type>;
@@ -40,47 +32,36 @@ constexpr auto is_invocable_r(int)
 template<typename, typename, typename...>
 constexpr std::false_type is_invocable_r(...);
 
-
-}
-
+} // namespace internal
 
 template<typename Func, typename... Args>
 struct is_invocable: decltype(internal::is_invocable<Func, Args...>(0)) {};
 
-
 template<typename Func, typename... Argsv>
 inline constexpr bool is_invocable_v = std::is_invocable<Func, Args...>::value;
 
-
 template<typename Ret, typename Func, typename... Args>
 struct is_invocable_r: decltype(internal::is_invocable_r<Ret, Func, Args...>(0)) {};
 
-
 template<typename Ret, typename Func, typename... Args>
 inline constexpr bool is_invocable_r_v = std::is_invocable_r<Ret, Func, Args...>::value;
 
-
-template<typename Func, typename...Args>
+template<typename Func, typename... Args>
 struct invoke_result {
     using type = decltype(std::invoke(std::declval<Func>(), std::declval<Args>()...));
 };
 
-
 template<typename Func, typename... Args>
 using invoke_result_t = typename std::invoke_result<Func, Args...>::type;
 
+} // namespace std
 
-}
-
-
+#    endif
 #endif
-#endif
-
 
 /**
  * Internal details not to be documented.
  * @endcond
  */
 
-
 #endif

src/entt/poly/fwd.hpp

@@ -1,26 +1,21 @@
 #ifndef ENTT_POLY_FWD_HPP
 #define ENTT_POLY_FWD_HPP
 
-
+#include <cstdint>
 #include <type_traits>
 
-
 namespace entt {
 
-
-template<typename, std::size_t Len, std::size_t = alignof(typename std::aligned_storage_t<Len + !Len>)>
+template<typename, std::size_t Len = sizeof(double[2]), std::size_t = alignof(typename std::aligned_storage_t<Len + !Len>)>
 class basic_poly;
 
-
 /**
  * @brief Alias declaration for the most common use case.
  * @tparam Concept Concept descriptor.
  */
 template<typename Concept>
-using poly = basic_poly<Concept, sizeof(double[2])>;
-
-
-}
+using poly = basic_poly<Concept>;
 
+} // namespace entt
 
 #endif

src/entt/poly/poly.hpp

@@ -1,7 +1,6 @@
 #ifndef ENTT_POLY_POLY_HPP
 #define ENTT_POLY_POLY_HPP
 
-
 #include <cstddef>
 #include <functional>
 #include <tuple>
@@ -13,17 +12,15 @@
 #include "../core/type_traits.hpp"
 #include "fwd.hpp"
 
-
 namespace entt {
 
-
 /*! @brief Inspector class used to infer the type of the virtual table. */
 struct poly_inspector {
     /**
      * @brief Generic conversion operator (definition only).
      * @tparam Type Type to which conversion is requested.
      */
-    template <class Type>
+    template<class Type>
     operator Type &&() const;
 
     /**
@@ -33,15 +30,14 @@ struct poly_inspector {
      * @param args The arguments to pass to the function.
      * @return A poly inspector convertible to any type.
      */
-    template<auto Member, typename... Args>
-    poly_inspector invoke(Args &&... args) const;
+    template<std::size_t Member, typename... Args>
+    poly_inspector invoke(Args &&...args) const;
 
     /*! @copydoc invoke */
-    template<auto Member, typename... Args>
-    poly_inspector invoke(Args &&... args);
+    template<std::size_t Member, typename... Args>
+    poly_inspector invoke(Args &&...args);
 };
 
-
 /**
  * @brief Static virtual table factory.
  * @tparam Concept Concept descriptor.
@@ -53,35 +49,35 @@ class poly_vtable {
     using inspector = typename Concept::template type<poly_inspector>;
 
     template<typename Ret, typename... Args>
-    static auto vtable_entry(Ret(*)(inspector &, Args...)) -> Ret(*)(basic_any<Len, Align> &, Args...);
+    static auto vtable_entry(Ret (*)(inspector &, Args...)) -> Ret (*)(basic_any<Len, Align> &, Args...);
 
     template<typename Ret, typename... Args>
-    static auto vtable_entry(Ret(*)(const inspector &, Args...)) -> Ret(*)(const basic_any<Len, Align> &, Args...);
+    static auto vtable_entry(Ret (*)(const inspector &, Args...)) -> Ret (*)(const basic_any<Len, Align> &, Args...);
 
     template<typename Ret, typename... Args>
-    static auto vtable_entry(Ret(*)(Args...)) -> Ret(*)(const basic_any<Len, Align> &, Args...);
+    static auto vtable_entry(Ret (*)(Args...)) -> Ret (*)(const basic_any<Len, Align> &, Args...);
 
     template<typename Ret, typename... Args>
-    static auto vtable_entry(Ret(inspector:: *)(Args...)) -> Ret(*)(basic_any<Len, Align> &, Args...);
+    static auto vtable_entry(Ret (inspector::*)(Args...)) -> Ret (*)(basic_any<Len, Align> &, Args...);
 
     template<typename Ret, typename... Args>
-    static auto vtable_entry(Ret(inspector:: *)(Args...) const) -> Ret(*)(const basic_any<Len, Align> &, Args...);
+    static auto vtable_entry(Ret (inspector::*)(Args...) const) -> Ret (*)(const basic_any<Len, Align> &, Args...);
 
     template<auto... Candidate>
-    static auto make_vtable(value_list<Candidate...>)
-    -> decltype(std::make_tuple(vtable_entry(Candidate)...));
+    static auto make_vtable(value_list<Candidate...>) ENTT_NOEXCEPT
+        -> decltype(std::make_tuple(vtable_entry(Candidate)...));
 
     template<typename... Func>
-    [[nodiscard]] static constexpr auto make_vtable(type_list<Func...>)  {
-        if constexpr(sizeof...(Func) == 0) {
+    [[nodiscard]] static constexpr auto make_vtable(type_list<Func...>) ENTT_NOEXCEPT {
+        if constexpr(sizeof...(Func) == 0u) {
             return decltype(make_vtable(typename Concept::template impl<inspector>{})){};
         } else if constexpr((std::is_function_v<Func> && ...)) {
-            return decltype(std::make_tuple(vtable_entry(std::declval<Func inspector:: *>())...)){};
+            return decltype(std::make_tuple(vtable_entry(std::declval<Func inspector::*>())...)){};
         }
     }
 
     template<typename Type, auto Candidate, typename Ret, typename Any, typename... Args>
-    static void fill_vtable_entry(Ret(* &entry)(Any &, Args...)) {
+    static void fill_vtable_entry(Ret (*&entry)(Any &, Args...)) ENTT_NOEXCEPT {
         if constexpr(std::is_invocable_r_v<Ret, decltype(Candidate), Args...>) {
             entry = +[](Any &, Args... args) -> Ret {
                 return std::invoke(Candidate, std::forward<Args>(args)...);
@@ -94,15 +90,18 @@ class poly_vtable {
     }
 
     template<typename Type, auto... Index>
-    [[nodiscard]] static auto fill_vtable(std::index_sequence<Index...>) {
-        type impl{};
+    [[nodiscard]] static auto fill_vtable(std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        vtable_type impl{};
         (fill_vtable_entry<Type, value_list_element_v<Index, typename Concept::template impl<Type>>>(std::get<Index>(impl)), ...);
         return impl;
     }
 
+    using vtable_type = decltype(make_vtable(Concept{}));
+    static constexpr bool is_mono_v = std::tuple_size_v<vtable_type> == 1u;
+
 public:
     /*! @brief Virtual table type. */
-    using type = decltype(make_vtable(Concept{}));
+    using type = std::conditional_t<is_mono_v, std::tuple_element_t<0u, vtable_type>, const vtable_type *>;
 
     /**
      * @brief Returns a static virtual table for a specific concept and type.
@@ -110,14 +109,18 @@ public:
      * @return A static virtual table for the given concept and type.
      */
     template<typename Type>
-    [[nodiscard]] static const auto * instance() {
+    [[nodiscard]] static type instance() ENTT_NOEXCEPT {
         static_assert(std::is_same_v<Type, std::decay_t<Type>>, "Type differs from its decayed form");
-        static const auto vtable = fill_vtable<Type>(std::make_index_sequence<Concept::template impl<Type>::size>{});
-        return &vtable;
+        static const vtable_type vtable = fill_vtable<Type>(std::make_index_sequence<Concept::template impl<Type>::size>{});
+
+        if constexpr(is_mono_v) {
+            return std::get<0>(vtable);
+        } else {
+            return &vtable;
+        }
     }
 };
 
-
 /**
  * @brief Poly base class used to inject functionalities into concepts.
  * @tparam Poly The outermost poly class.
@@ -132,21 +135,31 @@ struct poly_base {
      * @param args The arguments to pass to the function.
      * @return The return value of the invoked function, if any.
      */
-    template<auto Member, typename... Args>
-    [[nodiscard]] decltype(auto) invoke(const poly_base &self, Args &&... args) const {
+    template<std::size_t Member, typename... Args>
+    [[nodiscard]] decltype(auto) invoke(const poly_base &self, Args &&...args) const {
         const auto &poly = static_cast<const Poly &>(self);
-        return std::get<Member>(*poly.vtable)(poly.storage, std::forward<Args>(args)...);
+
+        if constexpr(std::is_function_v<std::remove_pointer_t<decltype(poly.vtable)>>) {
+            return poly.vtable(poly.storage, std::forward<Args>(args)...);
+        } else {
+            return std::get<Member>(*poly.vtable)(poly.storage, std::forward<Args>(args)...);
+        }
     }
 
     /*! @copydoc invoke */
-    template<auto Member, typename... Args>
-    [[nodiscard]] decltype(auto) invoke(poly_base &self, Args &&... args) {
+    template<std::size_t Member, typename... Args>
+    [[nodiscard]] decltype(auto) invoke(poly_base &self, Args &&...args) {
         auto &poly = static_cast<Poly &>(self);
-        return std::get<Member>(*poly.vtable)(poly.storage, std::forward<Args>(args)...);
+
+        if constexpr(std::is_function_v<std::remove_pointer_t<decltype(poly.vtable)>>) {
+            static_assert(Member == 0u, "Unknown member");
+            return poly.vtable(poly.storage, std::forward<Args>(args)...);
+        } else {
+            return std::get<Member>(*poly.vtable)(poly.storage, std::forward<Args>(args)...);
+        }
     }
 };
 
-
 /**
  * @brief Shortcut for calling `poly_base<Type>::invoke`.
  * @tparam Member Index of the function to invoke.
@@ -156,12 +169,11 @@ struct poly_base {
  * @param args The arguments to pass to the function.
  * @return The return value of the invoked function, if any.
  */
-template<auto Member, typename Poly, typename... Args>
-decltype(auto) poly_call(Poly &&self, Args &&... args) {
+template<std::size_t Member, typename Poly, typename... Args>
+decltype(auto) poly_call(Poly &&self, Args &&...args) {
     return std::forward<Poly>(self).template invoke<Member>(self, std::forward<Args>(args)...);
 }
 
-
 /**
  * @brief Static polymorphism made simple and within everyone's reach.
  *
@@ -182,17 +194,16 @@ class basic_poly: private Concept::template type<poly_base<basic_poly<Concept, L
     /*! @brief A poly base is allowed to snoop into a poly object. */
     friend struct poly_base<basic_poly>;
 
-    using vtable_type = typename poly_vtable<Concept, Len, Align>::type;
-
 public:
     /*! @brief Concept type. */
     using concept_type = typename Concept::template type<poly_base<basic_poly>>;
+    /*! @brief Virtual table type. */
+    using vtable_type = typename poly_vtable<Concept, Len, Align>::type;
 
     /*! @brief Default constructor. */
     basic_poly() ENTT_NOEXCEPT
         : storage{},
-          vtable{}
-    {}
+          vtable{} {}
 
     /**
      * @brief Constructs a poly by directly initializing the new object.
@@ -201,10 +212,9 @@ public:
      * @param args Parameters to use to construct the instance.
      */
     template<typename Type, typename... Args>
-    explicit basic_poly(std::in_place_type_t<Type>, Args &&... args)
+    explicit basic_poly(std::in_place_type_t<Type>, Args &&...args)
         : storage{std::in_place_type<Type>, std::forward<Args>(args)...},
-          vtable{poly_vtable<Concept, Len, Align>::template instance<std::remove_const_t<std::remove_reference_t<Type>>>()}
-    {}
+          vtable{poly_vtable<Concept, Len, Align>::template instance<std::remove_cv_t<std::remove_reference_t<Type>>>()} {}
 
     /**
      * @brief Constructs a poly from a given value.
@@ -213,40 +223,13 @@ public:
      */
     template<typename Type, typename = std::enable_if_t<!std::is_same_v<std::remove_cv_t<std::remove_reference_t<Type>>, basic_poly>>>
     basic_poly(Type &&value) ENTT_NOEXCEPT
-        : basic_poly{std::in_place_type<std::remove_cv_t<std::remove_reference_t<Type>>>, std::forward<Type>(value)}
-    {}
+        : basic_poly{std::in_place_type<std::remove_cv_t<std::remove_reference_t<Type>>>, std::forward<Type>(value)} {}
 
     /**
-     * @brief Copy constructor.
-     * @param other The instance to copy from.
+     * @brief Returns the object type if any, `type_id<void>()` otherwise.
+     * @return The object type if any, `type_id<void>()` otherwise.
      */
-    basic_poly(const basic_poly &other) = default;
-
-    /**
-     * @brief Move constructor.
-     * @param other The instance to move from.
-     */
-    basic_poly(basic_poly &&other) ENTT_NOEXCEPT
-        : basic_poly{}
-    {
-        swap(*this, other);
-    }
-
-    /**
-     * @brief Assignment operator.
-     * @param other The instance to assign from.
-     * @return This poly object.
-     */
-    basic_poly & operator=(basic_poly other) {
-        swap(other, *this);
-        return *this;
-    }
-
-    /**
-     * @brief Returns the type of the contained object.
-     * @return The type of the contained object, if any.
-     */
-    [[nodiscard]] type_info type() const ENTT_NOEXCEPT {
+    [[nodiscard]] const type_info &type() const ENTT_NOEXCEPT {
         return storage.type();
     }
 
@@ -254,12 +237,12 @@ public:
      * @brief Returns an opaque pointer to the contained instance.
      * @return An opaque pointer the contained instance, if any.
      */
-    [[nodiscard]] const void * data() const ENTT_NOEXCEPT {
+    [[nodiscard]] const void *data() const ENTT_NOEXCEPT {
         return storage.data();
     }
 
     /*! @copydoc data */
-    [[nodiscard]] void * data() ENTT_NOEXCEPT {
+    [[nodiscard]] void *data() ENTT_NOEXCEPT {
         return storage.data();
     }
 
@@ -270,13 +253,15 @@ public:
      * @param args Parameters to use to construct the instance.
      */
     template<typename Type, typename... Args>
-    void emplace(Args &&... args) {
-        *this = basic_poly{std::in_place_type<Type>, std::forward<Args>(args)...};
+    void emplace(Args &&...args) {
+        storage.template emplace<Type>(std::forward<Args>(args)...);
+        vtable = poly_vtable<Concept, Len, Align>::template instance<std::remove_cv_t<std::remove_reference_t<Type>>>();
     }
 
     /*! @brief Destroys contained object */
     void reset() {
-        *this = basic_poly{};
+        storage.reset();
+        vtable = {};
     }
 
     /**
@@ -284,40 +269,30 @@ public:
      * @return False if the poly is empty, true otherwise.
      */
     [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
-        return !(vtable == nullptr);
+        return static_cast<bool>(storage);
     }
 
     /**
      * @brief Returns a pointer to the underlying concept.
      * @return A pointer to the underlying concept.
      */
-    [[nodiscard]] concept_type * operator->() ENTT_NOEXCEPT {
+    [[nodiscard]] concept_type *operator->() ENTT_NOEXCEPT {
         return this;
     }
 
     /*! @copydoc operator-> */
-    [[nodiscard]] const concept_type * operator->() const ENTT_NOEXCEPT {
+    [[nodiscard]] const concept_type *operator->() const ENTT_NOEXCEPT {
         return this;
     }
 
-    /**
-     * @brief Swaps two poly objects.
-     * @param lhs A valid poly object.
-     * @param rhs A valid poly object.
-     */
-    friend void swap(basic_poly &lhs, basic_poly &rhs) {
-        using std::swap;
-        swap(lhs.storage, rhs.storage);
-        swap(lhs.vtable, rhs.vtable);
-    }
-
     /**
      * @brief Aliasing constructor.
      * @return A poly that shares a reference to an unmanaged object.
      */
     [[nodiscard]] basic_poly as_ref() ENTT_NOEXCEPT {
-        basic_poly ref = std::as_const(*this).as_ref();
+        basic_poly ref{};
         ref.storage = storage.as_ref();
+        ref.vtable = vtable;
         return ref;
     }
 
@@ -331,11 +306,9 @@ public:
 
 private:
     basic_any<Len, Align> storage;
-    const vtable_type *vtable;
+    vtable_type vtable;
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/process/process.hpp

@@ -1,15 +1,13 @@
 #ifndef ENTT_PROCESS_PROCESS_HPP
 #define ENTT_PROCESS_PROCESS_HPP
 
-
-#include <utility>
+#include <cstdint>
 #include <type_traits>
+#include <utility>
 #include "../config/config.h"
 
-
 namespace entt {
 
-
 /**
  * @brief Base class for processes.
  *
@@ -71,44 +69,44 @@ namespace entt {
  */
 template<typename Derived, typename Delta>
 class process {
-    enum class state: unsigned int {
-        UNINITIALIZED = 0,
-        RUNNING,
-        PAUSED,
-        SUCCEEDED,
-        FAILED,
-        ABORTED,
-        FINISHED,
-        REJECTED
+    enum class state : std::uint8_t {
+        uninitialized = 0,
+        running,
+        paused,
+        succeeded,
+        failed,
+        aborted,
+        finished,
+        rejected
     };
 
     template<typename Target = Derived>
-    auto next(std::integral_constant<state, state::UNINITIALIZED>)
-    -> decltype(std::declval<Target>().init(), void()) {
+    auto next(std::integral_constant<state, state::uninitialized>)
+        -> decltype(std::declval<Target>().init(), void()) {
         static_cast<Target *>(this)->init();
     }
 
     template<typename Target = Derived>
-    auto next(std::integral_constant<state, state::RUNNING>, Delta delta, void *data)
-    -> decltype(std::declval<Target>().update(delta, data), void()) {
+    auto next(std::integral_constant<state, state::running>, Delta delta, void *data)
+        -> decltype(std::declval<Target>().update(delta, data), void()) {
         static_cast<Target *>(this)->update(delta, data);
     }
 
     template<typename Target = Derived>
-    auto next(std::integral_constant<state, state::SUCCEEDED>)
-    -> decltype(std::declval<Target>().succeeded(), void()) {
+    auto next(std::integral_constant<state, state::succeeded>)
+        -> decltype(std::declval<Target>().succeeded(), void()) {
         static_cast<Target *>(this)->succeeded();
     }
 
     template<typename Target = Derived>
-    auto next(std::integral_constant<state, state::FAILED>)
-    -> decltype(std::declval<Target>().failed(), void()) {
+    auto next(std::integral_constant<state, state::failed>)
+        -> decltype(std::declval<Target>().failed(), void()) {
         static_cast<Target *>(this)->failed();
     }
 
     template<typename Target = Derived>
-    auto next(std::integral_constant<state, state::ABORTED>)
-    -> decltype(std::declval<Target>().aborted(), void()) {
+    auto next(std::integral_constant<state, state::aborted>)
+        -> decltype(std::declval<Target>().aborted(), void()) {
         static_cast<Target *>(this)->aborted();
     }
 
@@ -123,7 +121,7 @@ protected:
      */
     void succeed() ENTT_NOEXCEPT {
         if(alive()) {
-            current = state::SUCCEEDED;
+            current = state::succeeded;
         }
     }
 
@@ -135,7 +133,7 @@ protected:
      */
     void fail() ENTT_NOEXCEPT {
         if(alive()) {
-            current = state::FAILED;
+            current = state::failed;
         }
     }
 
@@ -146,8 +144,8 @@ protected:
      * running.
      */
     void pause() ENTT_NOEXCEPT {
-        if(current == state::RUNNING) {
-            current = state::PAUSED;
+        if(current == state::running) {
+            current = state::paused;
         }
     }
 
@@ -158,8 +156,8 @@ protected:
      * paused.
      */
     void unpause() ENTT_NOEXCEPT {
-        if(current  == state::PAUSED) {
-            current  = state::RUNNING;
+        if(current == state::paused) {
+            current = state::running;
         }
     }
 
@@ -168,7 +166,7 @@ public:
     using delta_type = Delta;
 
     /*! @brief Default destructor. */
-    virtual ~process() {
+    virtual ~process() ENTT_NOEXCEPT {
         static_assert(std::is_base_of_v<process, Derived>, "Incorrect use of the class template");
     }
 
@@ -182,7 +180,7 @@ public:
      */
     void abort(const bool immediately = false) {
         if(alive()) {
-            current = state::ABORTED;
+            current = state::aborted;
 
             if(immediately) {
                 tick({});
@@ -195,7 +193,7 @@ public:
      * @return True if the process is still alive, false otherwise.
      */
     [[nodiscard]] bool alive() const ENTT_NOEXCEPT {
-        return current == state::RUNNING || current == state::PAUSED;
+        return current == state::running || current == state::paused;
     }
 
     /**
@@ -203,7 +201,7 @@ public:
      * @return True if the process is terminated, false otherwise.
      */
     [[nodiscard]] bool finished() const ENTT_NOEXCEPT {
-        return current == state::FINISHED;
+        return current == state::finished;
     }
 
     /**
@@ -211,7 +209,7 @@ public:
      * @return True if the process is paused, false otherwise.
      */
     [[nodiscard]] bool paused() const ENTT_NOEXCEPT {
-        return current == state::PAUSED;
+        return current == state::paused;
     }
 
     /**
@@ -219,7 +217,7 @@ public:
      * @return True if the process terminated with errors, false otherwise.
      */
     [[nodiscard]] bool rejected() const ENTT_NOEXCEPT {
-        return current == state::REJECTED;
+        return current == state::rejected;
     }
 
     /**
@@ -228,13 +226,13 @@ public:
      * @param data Optional data.
      */
     void tick(const Delta delta, void *data = nullptr) {
-        switch (current) {
-        case state::UNINITIALIZED:
-            next(std::integral_constant<state, state::UNINITIALIZED>{});
-            current = state::RUNNING;
+        switch(current) {
+        case state::uninitialized:
+            next(std::integral_constant<state, state::uninitialized>{});
+            current = state::running;
             break;
-        case state::RUNNING:
-            next(std::integral_constant<state, state::RUNNING>{}, delta, data);
+        case state::running:
+            next(std::integral_constant<state, state::running>{}, delta, data);
             break;
         default:
             // suppress warnings
@@ -243,17 +241,17 @@ public:
 
         // if it's dead, it must be notified and removed immediately
         switch(current) {
-        case state::SUCCEEDED:
-            next(std::integral_constant<state, state::SUCCEEDED>{});
-            current = state::FINISHED;
+        case state::succeeded:
+            next(std::integral_constant<state, state::succeeded>{});
+            current = state::finished;
             break;
-        case state::FAILED:
-            next(std::integral_constant<state, state::FAILED>{});
-            current = state::REJECTED;
+        case state::failed:
+            next(std::integral_constant<state, state::failed>{});
+            current = state::rejected;
             break;
-        case state::ABORTED:
-            next(std::integral_constant<state, state::ABORTED>{});
-            current = state::REJECTED;
+        case state::aborted:
+            next(std::integral_constant<state, state::aborted>{});
+            current = state::rejected;
             break;
         default:
             // suppress warnings
@@ -262,10 +260,9 @@ public:
     }
 
 private:
-    state current{state::UNINITIALIZED};
+    state current{state::uninitialized};
 };
 
-
 /**
  * @brief Adaptor for lambdas and functors to turn them into processes.
  *
@@ -313,9 +310,8 @@ struct process_adaptor: process<process_adaptor<Func, Delta>, Delta>, private Fu
      * @param args Parameters to use to initialize the actual process.
      */
     template<typename... Args>
-    process_adaptor(Args &&... args)
-        : Func{std::forward<Args>(args)...}
-    {}
+    process_adaptor(Args &&...args)
+        : Func{std::forward<Args>(args)...} {}
 
     /**
      * @brief Updates a process and its internal state if required.
@@ -323,12 +319,14 @@ struct process_adaptor: process<process_adaptor<Func, Delta>, Delta>, private Fu
      * @param data Optional data.
      */
     void update(const Delta delta, void *data) {
-        Func::operator()(delta, data, [this]() { this->succeed(); }, [this]() { this->fail(); });
+        Func::operator()(
+            delta,
+            data,
+            [this]() { this->succeed(); },
+            [this]() { this->fail(); });
     }
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/process/scheduler.hpp

@@ -1,19 +1,17 @@
 #ifndef ENTT_PROCESS_SCHEDULER_HPP
 #define ENTT_PROCESS_SCHEDULER_HPP
 
-
-#include <vector>
-#include <memory>
-#include <utility>
 #include <algorithm>
+#include <iterator>
+#include <memory>
 #include <type_traits>
+#include <utility>
+#include <vector>
 #include "../config/config.h"
 #include "process.hpp"
 
-
 namespace entt {
 
-
 /**
  * @brief Cooperative scheduler for processes.
  *
@@ -43,9 +41,9 @@ namespace entt {
 template<typename Delta>
 class scheduler {
     struct process_handler {
-        using instance_type = std::unique_ptr<void, void(*)(void *)>;
-        using update_fn_type = bool(process_handler &, Delta, void *);
-        using abort_fn_type = void(process_handler &, bool);
+        using instance_type = std::unique_ptr<void, void (*)(void *)>;
+        using update_fn_type = bool(scheduler &, std::size_t, Delta, void *);
+        using abort_fn_type = void(scheduler &, std::size_t, bool);
         using next_type = std::unique_ptr<process_handler>;
 
         instance_type instance;
@@ -55,12 +53,11 @@ class scheduler {
     };
 
     struct continuation {
-        continuation(process_handler *ref)
-            : handler{ref}
-        {}
+        continuation(process_handler *ref) ENTT_NOEXCEPT
+            : handler{ref} {}
 
         template<typename Proc, typename... Args>
-        continuation then(Args &&... args) {
+        continuation then(Args &&...args) {
             static_assert(std::is_base_of_v<process<Proc, Delta>, Proc>, "Invalid process type");
             auto proc = typename process_handler::instance_type{new Proc{std::forward<Args>(args)...}, &scheduler::deleter<Proc>};
             handler->next.reset(new process_handler{std::move(proc), &scheduler::update<Proc>, &scheduler::abort<Proc>, nullptr});
@@ -78,17 +75,17 @@ class scheduler {
     };
 
     template<typename Proc>
-    [[nodiscard]] static bool update(process_handler &handler, const Delta delta, void *data) {
-        auto *process = static_cast<Proc *>(handler.instance.get());
+    [[nodiscard]] static bool update(scheduler &owner, std::size_t pos, const Delta delta, void *data) {
+        auto *process = static_cast<Proc *>(owner.handlers[pos].instance.get());
         process->tick(delta, data);
 
         if(process->rejected()) {
             return true;
         } else if(process->finished()) {
-            if(handler.next) {
+            if(auto &&handler = owner.handlers[pos]; handler.next) {
                 handler = std::move(*handler.next);
                 // forces the process to exit the uninitialized state
-                return handler.update(handler, {}, nullptr);
+                return handler.update(owner, pos, {}, nullptr);
             }
 
             return true;
@@ -98,8 +95,8 @@ class scheduler {
     }
 
     template<typename Proc>
-    static void abort(process_handler &handler, const bool immediately) {
-        static_cast<Proc *>(handler.instance.get())->abort(immediately);
+    static void abort(scheduler &owner, std::size_t pos, const bool immediately) {
+        static_cast<Proc *>(owner.handlers[pos].instance.get())->abort(immediately);
     }
 
     template<typename Proc>
@@ -118,7 +115,7 @@ public:
     scheduler(scheduler &&) = default;
 
     /*! @brief Default move assignment operator. @return This scheduler. */
-    scheduler & operator=(scheduler &&) = default;
+    scheduler &operator=(scheduler &&) = default;
 
     /**
      * @brief Number of processes currently scheduled.
@@ -172,13 +169,13 @@ public:
      * @return An opaque object to use to concatenate processes.
      */
     template<typename Proc, typename... Args>
-    auto attach(Args &&... args) {
+    auto attach(Args &&...args) {
         static_assert(std::is_base_of_v<process<Proc, Delta>, Proc>, "Invalid process type");
         auto proc = typename process_handler::instance_type{new Proc{std::forward<Args>(args)...}, &scheduler::deleter<Proc>};
-        process_handler handler{std::move(proc), &scheduler::update<Proc>, &scheduler::abort<Proc>, nullptr};
+        auto &&ref = handlers.emplace_back(process_handler{std::move(proc), &scheduler::update<Proc>, &scheduler::abort<Proc>, nullptr});
         // forces the process to exit the uninitialized state
-        handler.update(handler, {}, nullptr);
-        return continuation{&handlers.emplace_back(std::move(handler))};
+        ref.update(*this, handlers.size() - 1u, {}, nullptr);
+        return continuation{&handlers.back()};
     }
 
     /**
@@ -250,17 +247,14 @@ public:
      * @param data Optional data.
      */
     void update(const Delta delta, void *data = nullptr) {
-        auto sz = handlers.size();
-
         for(auto pos = handlers.size(); pos; --pos) {
-            auto &handler = handlers[pos-1];
+            const auto curr = pos - 1u;
 
-            if(const auto dead = handler.update(handler, delta, data); dead) {
-                std::swap(handler, handlers[--sz]);
+            if(const auto dead = handlers[curr].update(*this, curr, delta, data); dead) {
+                std::swap(handlers[curr], handlers.back());
+                handlers.pop_back();
             }
         }
-
-        handlers.erase(handlers.begin() + sz, handlers.end());
     }
 
     /**
@@ -274,23 +268,16 @@ public:
      * @param immediately Requests an immediate operation.
      */
     void abort(const bool immediately = false) {
-        decltype(handlers) exec;
-        exec.swap(handlers);
-
-        for(auto &&handler: exec) {
-            handler.abort(handler, immediately);
+        for(auto pos = handlers.size(); pos; --pos) {
+            const auto curr = pos - 1u;
+            handlers[curr].abort(*this, curr, immediately);
         }
-
-        std::move(handlers.begin(), handlers.end(), std::back_inserter(exec));
-        handlers.swap(exec);
     }
 
 private:
     std::vector<process_handler> handlers{};
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/resource/cache.hpp

@@ -1,52 +1,288 @@
-#ifndef ENTT_RESOURCE_CACHE_HPP
-#define ENTT_RESOURCE_CACHE_HPP
-
+#ifndef ENTT_RESOURCE_RESOURCE_CACHE_HPP
+#define ENTT_RESOURCE_RESOURCE_CACHE_HPP
 
+#include <cstddef>
+#include <functional>
+#include <iterator>
+#include <memory>
+#include <tuple>
 #include <type_traits>
-#include <unordered_map>
 #include <utility>
 #include "../config/config.h"
+#include "../container/dense_map.hpp"
+#include "../core/compressed_pair.hpp"
 #include "../core/fwd.hpp"
-#include "handle.hpp"
-#include "loader.hpp"
+#include "../core/iterator.hpp"
+#include "../core/utility.hpp"
 #include "fwd.hpp"
-
+#include "loader.hpp"
+#include "resource.hpp"
 
 namespace entt {
 
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename Type, typename It>
+class resource_cache_iterator final {
+    template<typename, typename>
+    friend class resource_cache_iterator;
+
+public:
+    using value_type = std::pair<id_type, resource<Type>>;
+    using pointer = input_iterator_pointer<value_type>;
+    using reference = value_type;
+    using difference_type = std::ptrdiff_t;
+    using iterator_category = std::input_iterator_tag;
+
+    resource_cache_iterator() ENTT_NOEXCEPT = default;
+
+    resource_cache_iterator(const It iter) ENTT_NOEXCEPT
+        : it{iter} {}
+
+    template<typename Other, typename = std::enable_if_t<!std::is_same_v<It, Other> && std::is_constructible_v<It, Other>>>
+    resource_cache_iterator(const resource_cache_iterator<std::remove_const_t<Type>, Other> &other) ENTT_NOEXCEPT
+        : it{other.it} {}
+
+    resource_cache_iterator &operator++() ENTT_NOEXCEPT {
+        return ++it, *this;
+    }
+
+    resource_cache_iterator operator++(int) ENTT_NOEXCEPT {
+        resource_cache_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    resource_cache_iterator &operator--() ENTT_NOEXCEPT {
+        return --it, *this;
+    }
+
+    resource_cache_iterator operator--(int) ENTT_NOEXCEPT {
+        resource_cache_iterator orig = *this;
+        return operator--(), orig;
+    }
+
+    resource_cache_iterator &operator+=(const difference_type value) ENTT_NOEXCEPT {
+        it += value;
+        return *this;
+    }
+
+    resource_cache_iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
+        resource_cache_iterator copy = *this;
+        return (copy += value);
+    }
+
+    resource_cache_iterator &operator-=(const difference_type value) ENTT_NOEXCEPT {
+        return (*this += -value);
+    }
+
+    resource_cache_iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
+        return (*this + -value);
+    }
+
+    [[nodiscard]] reference operator[](const difference_type value) const ENTT_NOEXCEPT {
+        return {it[value].first, resource<Type>{it[value].second}};
+    }
+
+    [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
+        return (*this)[0];
+    }
+
+    [[nodiscard]] pointer operator->() const ENTT_NOEXCEPT {
+        return operator*();
+    }
+
+    template<typename TLhs, typename ILhs, typename TRhs, typename IRhs>
+    friend std::ptrdiff_t operator-(const resource_cache_iterator<TLhs, ILhs> &, const resource_cache_iterator<TRhs, IRhs> &) ENTT_NOEXCEPT;
+
+    template<typename TLhs, typename ILhs, typename TRhs, typename IRhs>
+    friend bool operator==(const resource_cache_iterator<TLhs, ILhs> &, const resource_cache_iterator<TRhs, IRhs> &) ENTT_NOEXCEPT;
+
+    template<typename TLhs, typename ILhs, typename TRhs, typename IRhs>
+    friend bool operator<(const resource_cache_iterator<TLhs, ILhs> &, const resource_cache_iterator<TRhs, IRhs> &) ENTT_NOEXCEPT;
+
+private:
+    It it;
+};
+
+template<typename TLhs, typename ILhs, typename TRhs, typename IRhs>
+[[nodiscard]] std::ptrdiff_t operator-(const resource_cache_iterator<TLhs, ILhs> &lhs, const resource_cache_iterator<TRhs, IRhs> &rhs) ENTT_NOEXCEPT {
+    return lhs.it - rhs.it;
+}
+
+template<typename TLhs, typename ILhs, typename TRhs, typename IRhs>
+[[nodiscard]] bool operator==(const resource_cache_iterator<TLhs, ILhs> &lhs, const resource_cache_iterator<TRhs, IRhs> &rhs) ENTT_NOEXCEPT {
+    return lhs.it == rhs.it;
+}
+
+template<typename TLhs, typename ILhs, typename TRhs, typename IRhs>
+[[nodiscard]] bool operator!=(const resource_cache_iterator<TLhs, ILhs> &lhs, const resource_cache_iterator<TRhs, IRhs> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+template<typename TLhs, typename ILhs, typename TRhs, typename IRhs>
+[[nodiscard]] bool operator<(const resource_cache_iterator<TLhs, ILhs> &lhs, const resource_cache_iterator<TRhs, IRhs> &rhs) ENTT_NOEXCEPT {
+    return lhs.it < rhs.it;
+}
+
+template<typename TLhs, typename ILhs, typename TRhs, typename IRhs>
+[[nodiscard]] bool operator>(const resource_cache_iterator<TLhs, ILhs> &lhs, const resource_cache_iterator<TRhs, IRhs> &rhs) ENTT_NOEXCEPT {
+    return rhs < lhs;
+}
+
+template<typename TLhs, typename ILhs, typename TRhs, typename IRhs>
+[[nodiscard]] bool operator<=(const resource_cache_iterator<TLhs, ILhs> &lhs, const resource_cache_iterator<TRhs, IRhs> &rhs) ENTT_NOEXCEPT {
+    return !(lhs > rhs);
+}
+
+template<typename TLhs, typename ILhs, typename TRhs, typename IRhs>
+[[nodiscard]] bool operator>=(const resource_cache_iterator<TLhs, ILhs> &lhs, const resource_cache_iterator<TRhs, IRhs> &rhs) ENTT_NOEXCEPT {
+    return !(lhs < rhs);
+}
+
+} // namespace internal
 
 /**
- * @brief Simple cache for resources of a given type.
- *
- * Minimal implementation of a cache for resources of a given type. It doesn't
- * offer much functionalities but it's suitable for small or medium sized
- * applications and can be freely inherited to add targeted functionalities for
- * large sized applications.
- *
- * @tparam Resource Type of resources managed by a cache.
+ * Internal details not to be documented.
+ * @endcond
  */
-template<typename Resource>
-struct resource_cache {
+
+/**
+ * @brief Basic cache for resources of any type.
+ * @tparam Type Type of resources managed by a cache.
+ * @tparam Loader Type of loader used to create the resources.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Type, typename Loader, typename Allocator>
+class resource_cache {
+    using alloc_traits = typename std::allocator_traits<Allocator>;
+    static_assert(std::is_same_v<typename alloc_traits::value_type, Type>, "Invalid value type");
+    using container_allocator = typename alloc_traits::template rebind_alloc<std::pair<const id_type, typename Loader::result_type>>;
+    using container_type = dense_map<id_type, typename Loader::result_type, identity, std::equal_to<id_type>, container_allocator>;
+
+public:
+    /*! @brief Resource type. */
+    using value_type = Type;
     /*! @brief Unsigned integer type. */
     using size_type = std::size_t;
-    /*! @brief Type of resources managed by a cache. */
-    using resource_type = Resource;
+    /*! @brief Loader type. */
+    using loader_type = Loader;
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Input iterator type. */
+    using iterator = internal::resource_cache_iterator<Type, typename container_type::iterator>;
+    /*! @brief Constant input iterator type. */
+    using const_iterator = internal::resource_cache_iterator<const Type, typename container_type::const_iterator>;
 
     /*! @brief Default constructor. */
-    resource_cache() = default;
+    resource_cache()
+        : resource_cache{loader_type{}} {}
+
+    /**
+     * @brief Constructs an empty cache with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit resource_cache(const allocator_type &allocator)
+        : resource_cache{loader_type{}, allocator} {}
+
+    /**
+     * @brief Constructs an empty cache with a given allocator and loader.
+     * @param callable The loader to use.
+     * @param allocator The allocator to use.
+     */
+    explicit resource_cache(const loader_type &callable, const allocator_type &allocator = allocator_type{})
+        : pool{container_type{allocator}, callable} {}
+
+    /*! @brief Default copy constructor. */
+    resource_cache(const resource_cache &) = default;
+
+    /**
+     * @brief Allocator-extended copy constructor.
+     * @param other The instance to copy from.
+     * @param allocator The allocator to use.
+     */
+    resource_cache(const resource_cache &other, const allocator_type &allocator)
+        : pool{std::piecewise_construct, std::forward_as_tuple(other.pool.first(), allocator), std::forward_as_tuple(other.pool.second())} {}
 
     /*! @brief Default move constructor. */
     resource_cache(resource_cache &&) = default;
 
-    /*! @brief Default move assignment operator. @return This cache. */
-    resource_cache & operator=(resource_cache &&) = default;
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    resource_cache(resource_cache &&other, const allocator_type &allocator)
+        : pool{std::piecewise_construct, std::forward_as_tuple(std::move(other.pool.first()), allocator), std::forward_as_tuple(std::move(other.pool.second()))} {}
 
     /**
-     * @brief Number of resources managed by a cache.
-     * @return Number of resources currently stored.
+     * @brief Default copy assignment operator.
+     * @return This cache.
      */
-    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
-        return resources.size();
+    resource_cache &operator=(const resource_cache &) = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This cache.
+     */
+    resource_cache &operator=(resource_cache &&) = default;
+
+    /**
+     * @brief Returns the associated allocator.
+     * @return The associated allocator.
+     */
+    [[nodiscard]] constexpr allocator_type get_allocator() const ENTT_NOEXCEPT {
+        return pool.first().get_allocator();
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * The returned iterator points to the first instance of the cache. If the
+     * cache is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first instance of the internal cache.
+     */
+    [[nodiscard]] const_iterator cbegin() const ENTT_NOEXCEPT {
+        return pool.first().begin();
+    }
+
+    /*! @copydoc cbegin */
+    [[nodiscard]] const_iterator begin() const ENTT_NOEXCEPT {
+        return cbegin();
+    }
+
+    /*! @copydoc begin */
+    [[nodiscard]] iterator begin() ENTT_NOEXCEPT {
+        return pool.first().begin();
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     *
+     * The returned iterator points to the element following the last instance
+     * of the cache. Attempting to dereference the returned iterator results in
+     * undefined behavior.
+     *
+     * @return An iterator to the element following the last instance of the
+     * internal cache.
+     */
+    [[nodiscard]] const_iterator cend() const ENTT_NOEXCEPT {
+        return pool.first().end();
+    }
+
+    /*! @copydoc cend */
+    [[nodiscard]] const_iterator end() const ENTT_NOEXCEPT {
+        return cend();
+    }
+
+    /*! @copydoc end */
+    [[nodiscard]] iterator end() ENTT_NOEXCEPT {
+        return pool.first().end();
     }
 
     /**
@@ -54,115 +290,79 @@ struct resource_cache {
      * @return True if the cache contains no resources, false otherwise.
      */
     [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
-        return resources.empty();
+        return pool.first().empty();
     }
 
     /**
-     * @brief Clears a cache and discards all its resources.
-     *
-     * Handles are not invalidated and the memory used by a resource isn't
-     * freed as long as at least a handle keeps the resource itself alive.
+     * @brief Number of resources managed by a cache.
+     * @return Number of resources currently stored.
      */
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return pool.first().size();
+    }
+
+    /*! @brief Clears a cache. */
     void clear() ENTT_NOEXCEPT {
-        resources.clear();
+        pool.first().clear();
     }
 
     /**
-     * @brief Loads the resource that corresponds to a given identifier.
+     * @brief Loads a resource, if its identifier does not exist.
      *
-     * In case an identifier isn't already present in the cache, it loads its
-     * resource and stores it aside for future uses. Arguments are forwarded
-     * directly to the loader in order to construct properly the requested
-     * resource.
-     *
-     * @note
-     * If the identifier is already present in the cache, this function does
-     * nothing and the arguments are simply discarded.
+     * Arguments are forwarded directly to the loader and _consumed_ only if the
+     * resource doesn't already exist.
      *
      * @warning
-     * If the resource cannot be loaded correctly, the returned handle will be
+     * If the resource isn't loaded correctly, the returned handle could be
      * invalid and any use of it will result in undefined behavior.
      *
-     * @tparam Loader Type of loader to use to load the resource if required.
      * @tparam Args Types of arguments to use to load the resource if required.
      * @param id Unique resource identifier.
      * @param args Arguments to use to load the resource if required.
+     * @return A pair consisting of an iterator to the inserted element (or to
+     * the element that prevented the insertion) and a bool denoting whether the
+     * insertion took place.
+     */
+    template<typename... Args>
+    std::pair<iterator, bool> load(const id_type id, Args &&...args) {
+        if(auto it = pool.first().find(id); it != pool.first().end()) {
+            return {it, false};
+        }
+
+        return pool.first().emplace(id, pool.second()(std::forward<Args>(args)...));
+    }
+
+    /**
+     * @brief Force loads a resource, if its identifier does not exist.
+     * @copydetails load
+     */
+    template<typename... Args>
+    std::pair<iterator, bool> force_load(const id_type id, Args &&...args) {
+        return {pool.first().insert_or_assign(id, pool.second()(std::forward<Args>(args)...)).first, true};
+    }
+
+    /**
+     * @brief Returns a handle for a given resource identifier.
+     *
+     * @warning
+     * There is no guarantee that the returned handle is valid.<br/>
+     * If it is not, any use will result in indefinite behavior.
+     *
+     * @param id Unique resource identifier.
      * @return A handle for the given resource.
      */
-    template<typename Loader, typename... Args>
-    resource_handle<Resource> load(const id_type id, Args &&... args) {
-        if(auto it = resources.find(id); it == resources.cend()) {
-            if(auto handle = temp<Loader>(std::forward<Args>(args)...); handle) {
-                return (resources[id] = std::move(handle));
-            }
-        } else {
-            return it->second;
+    [[nodiscard]] resource<const value_type> operator[](const id_type id) const {
+        if(auto it = pool.first().find(id); it != pool.first().cend()) {
+            return resource<const value_type>{it->second};
         }
 
         return {};
     }
 
-    /**
-     * @brief Reloads a resource or loads it for the first time if not present.
-     *
-     * Equivalent to the following snippet (pseudocode):
-     *
-     * @code{.cpp}
-     * cache.discard(id);
-     * cache.load(id, args...);
-     * @endcode
-     *
-     * Arguments are forwarded directly to the loader in order to construct
-     * properly the requested resource.
-     *
-     * @warning
-     * If the resource cannot be loaded correctly, the returned handle will be
-     * invalid and any use of it will result in undefined behavior.
-     *
-     * @tparam Loader Type of loader to use to load the resource.
-     * @tparam Args Types of arguments to use to load the resource.
-     * @param id Unique resource identifier.
-     * @param args Arguments to use to load the resource.
-     * @return A handle for the given resource.
-     */
-    template<typename Loader, typename... Args>
-    resource_handle<Resource> reload(const id_type id, Args &&... args) {
-        return (discard(id), load<Loader>(id, std::forward<Args>(args)...));
-    }
-
-    /**
-     * @brief Creates a temporary handle for a resource.
-     *
-     * Arguments are forwarded directly to the loader in order to construct
-     * properly the requested resource. The handle isn't stored aside and the
-     * cache isn't in charge of the lifetime of the resource itself.
-     *
-     * @tparam Loader Type of loader to use to load the resource.
-     * @tparam Args Types of arguments to use to load the resource.
-     * @param args Arguments to use to load the resource.
-     * @return A handle for the given resource.
-     */
-    template<typename Loader, typename... Args>
-    [[nodiscard]] resource_handle<Resource> temp(Args &&... args) const {
-        return Loader{}.get(std::forward<Args>(args)...);
-    }
-
-    /**
-     * @brief Creates a handle for a given resource identifier.
-     *
-     * A resource handle can be in a either valid or invalid state. In other
-     * terms, a resource handle is properly initialized with a resource if the
-     * cache contains the resource itself. Otherwise the returned handle is
-     * uninitialized and accessing it results in undefined behavior.
-     *
-     * @sa resource_handle
-     *
-     * @param id Unique resource identifier.
-     * @return A handle for the given resource.
-     */
-    [[nodiscard]] resource_handle<Resource> handle(const id_type id) const {
-        if(auto it = resources.find(id); it != resources.cend()) {
-            return it->second;
+    /*! @copydoc operator[] */
+    [[nodiscard]] resource<value_type> operator[](const id_type id) {
+        if(auto it = pool.first().find(id); it != pool.first().end()) {
+            return resource<value_type>{it->second};
         }
 
         return {};
@@ -174,64 +374,51 @@ struct resource_cache {
      * @return True if the cache contains the resource, false otherwise.
      */
     [[nodiscard]] bool contains(const id_type id) const {
-        return (resources.find(id) != resources.cend());
+        return pool.first().contains(id);
     }
 
     /**
-     * @brief Discards the resource that corresponds to a given identifier.
-     *
-     * Handles are not invalidated and the memory used by the resource isn't
-     * freed as long as at least a handle keeps the resource itself alive.
-     *
+     * @brief Removes an element from a given position.
+     * @param pos An iterator to the element to remove.
+     * @return An iterator following the removed element.
+     */
+    iterator erase(const_iterator pos) {
+        const auto it = pool.first().begin();
+        return pool.first().erase(it + (pos - const_iterator{it}));
+    }
+
+    /**
+     * @brief Removes the given elements from a cache.
+     * @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 An iterator following the last removed element.
+     */
+    iterator erase(const_iterator first, const_iterator last) {
+        const auto it = pool.first().begin();
+        return pool.first().erase(it + (first - const_iterator{it}), it + (last - const_iterator{it}));
+    }
+
+    /**
+     * @brief Removes the given elements from a cache.
      * @param id Unique resource identifier.
+     * @return Number of resources erased (either 0 or 1).
      */
-    void discard(const id_type id) {
-        if(auto it = resources.find(id); it != resources.end()) {
-            resources.erase(it);
-        }
+    size_type erase(const id_type id) {
+        return pool.first().erase(id);
     }
 
     /**
-     * @brief Iterates all resources.
-     *
-     * The function object is invoked for each element. It is provided with
-     * either the resource identifier, the resource handle or both of them.<br/>
-     * The signature of the function must be equivalent to one of the following
-     * forms:
-     *
-     * @code{.cpp}
-     * void(const entt::id_type);
-     * void(entt::resource_handle<Resource>);
-     * void(const entt::id_type, entt::resource_handle<Resource>);
-     * @endcode
-     *
-     * @tparam Func Type of the function object to invoke.
-     * @param func A valid function object.
+     * @brief Returns the loader used to create resources.
+     * @return The loader used to create resources.
      */
-    template <typename Func>
-    void each(Func func) const {
-        auto begin = resources.begin();
-        auto end = resources.end();
-
-        while(begin != end) {
-            auto curr = begin++;
-
-            if constexpr(std::is_invocable_v<Func, id_type>) {
-                func(curr->first);
-            } else if constexpr(std::is_invocable_v<Func, resource_handle<Resource>>) {
-                func(curr->second);
-            } else {
-                func(curr->first, curr->second);
-            }
-        }
+    [[nodiscard]] loader_type loader() const {
+        return pool.second();
     }
 
 private:
-    std::unordered_map<id_type, resource_handle<Resource>> resources;
+    compressed_pair<container_type, loader_type> pool;
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/resource/fwd.hpp

@@ -1,23 +1,19 @@
 #ifndef ENTT_RESOURCE_FWD_HPP
 #define ENTT_RESOURCE_FWD_HPP
 
+#include <memory>
 
 namespace entt {
 
+template<typename>
+struct resource_loader;
+
+template<typename Type, typename = resource_loader<Type>, typename = std::allocator<Type>>
+class resource_cache;
 
 template<typename>
-struct resource_cache;
-
-
-template<typename>
-class resource_handle;
-
-
-template<typename, typename>
-class resource_loader;
-
-
-}
+class resource;
 
+} // namespace entt
 
 #endif

src/entt/resource/handle.hpp

@@ -1,189 +0,0 @@
-#ifndef ENTT_RESOURCE_HANDLE_HPP
-#define ENTT_RESOURCE_HANDLE_HPP
-
-
-#include <memory>
-#include <type_traits>
-#include <utility>
-#include "../config/config.h"
-#include "fwd.hpp"
-
-
-namespace entt {
-
-
-/**
- * @brief Shared resource handle.
- *
- * A shared resource handle is a small class that wraps a resource and keeps it
- * alive even if it's deleted from the cache. It can be either copied or
- * moved. A handle shares a reference to the same resource with all the other
- * handles constructed for the same identifier.<br/>
- * As a rule of thumb, resources should never be copied nor moved. Handles are
- * the way to go to keep references to them.
- *
- * @tparam Resource Type of resource managed by a handle.
- */
-template<typename Resource>
-class resource_handle {
-    /*! @brief Resource handles are friends with each other. */
-    template<typename>
-    friend class resource_handle;
-
-public:
-    /*! @brief Default constructor. */
-    resource_handle() ENTT_NOEXCEPT = default;
-
-    /**
-     * @brief Creates a handle from a shared pointer, namely a resource.
-     * @param res A pointer to a properly initialized resource.
-     */
-    resource_handle(std::shared_ptr<Resource> res) ENTT_NOEXCEPT
-        : resource{std::move(res)}
-    {}
-
-    /**
-     * @brief Copy constructor.
-     * @param other The instance to copy from.
-     */
-    resource_handle(const resource_handle<Resource> &other) ENTT_NOEXCEPT = default;
-
-    /**
-     * @brief Move constructor.
-     * @param other The instance to move from.
-     */
-    resource_handle(resource_handle<Resource> &&other) ENTT_NOEXCEPT = default;
-
-    /**
-     * @brief Copy constructs a handle which shares ownership of the resource.
-     * @tparam Other Type of resource managed by the received handle.
-     * @param other The handle to copy from.
-     */
-    template<typename Other, typename = std::enable_if_t<!std::is_same_v<Other, Resource> && std::is_base_of_v<Resource, Other>>>
-    resource_handle(const resource_handle<Other> &other) ENTT_NOEXCEPT
-        : resource{other.resource}
-    {}
-
-    /**
-     * @brief Move constructs a handle which takes ownership of the resource.
-     * @tparam Other Type of resource managed by the received handle.
-     * @param other The handle to move from.
-     */
-    template<typename Other, typename = std::enable_if_t<!std::is_same_v<Other, Resource> && std::is_base_of_v<Resource, Other>>>
-    resource_handle(resource_handle<Other> &&other) ENTT_NOEXCEPT
-        : resource{std::move(other.resource)}
-    {}
-
-    /**
-     * @brief Copy assignment operator.
-     * @param other The instance to copy from.
-     * @return This resource handle.
-     */
-    resource_handle & operator=(const resource_handle<Resource> &other) ENTT_NOEXCEPT = default;
-
-    /**
-     * @brief Move assignment operator.
-     * @param other The instance to move from.
-     * @return This resource handle.
-     */
-    resource_handle & operator=(resource_handle<Resource> &&other) ENTT_NOEXCEPT = default;
-
-    /**
-     * @brief Copy assignment operator from foreign handle.
-     * @tparam Other Type of resource managed by the received handle.
-     * @param other The handle to copy from.
-     * @return This resource handle.
-     */
-    template<typename Other>
-    std::enable_if_t<!std::is_same_v<Other, Resource> && std::is_base_of_v<Resource, Other>, resource_handle &>
-    operator=(const resource_handle<Other> &other) ENTT_NOEXCEPT {
-        resource = other.resource;
-        return *this;
-    }
-
-    /**
-     * @brief Move assignment operator from foreign handle.
-     * @tparam Other Type of resource managed by the received handle.
-     * @param other The handle to move from.
-     * @return This resource handle.
-     */
-    template<typename Other>
-    std::enable_if_t<!std::is_same_v<Other, Resource> && std::is_base_of_v<Resource, Other>, resource_handle &>
-    operator=(resource_handle<Other> &&other) ENTT_NOEXCEPT {
-        resource = std::move(other.resource);
-        return *this;
-    }
-
-    /**
-     * @brief Gets a reference to the managed resource.
-     *
-     * @warning
-     * The behavior is undefined if the handle doesn't contain a resource.
-     *
-     * @return A reference to the managed resource.
-     */
-    [[nodiscard]] const Resource & get() const ENTT_NOEXCEPT {
-        ENTT_ASSERT(static_cast<bool>(resource), "Invalid resource");
-        return *resource;
-    }
-
-    /*! @copydoc get */
-    [[nodiscard]] Resource & get() ENTT_NOEXCEPT {
-        return const_cast<Resource &>(std::as_const(*this).get());
-    }
-
-    /*! @copydoc get */
-    [[nodiscard]] operator const Resource & () const ENTT_NOEXCEPT {
-        return get();
-    }
-
-    /*! @copydoc get */
-    [[nodiscard]] operator Resource & () ENTT_NOEXCEPT {
-        return get();
-    }
-
-    /*! @copydoc get */
-    [[nodiscard]] const Resource & operator *() const ENTT_NOEXCEPT {
-        return get();
-    }
-
-    /*! @copydoc get */
-    [[nodiscard]] Resource & operator *() ENTT_NOEXCEPT {
-        return get();
-    }
-
-    /**
-     * @brief Gets a pointer to the managed resource.
-     *
-     * @warning
-     * The behavior is undefined if the handle doesn't contain a resource.
-     *
-     * @return A pointer to the managed resource or `nullptr` if the handle
-     * contains no resource at all.
-     */
-    [[nodiscard]] const Resource * operator->() const ENTT_NOEXCEPT {
-        return resource.get();
-    }
-
-    /*! @copydoc operator-> */
-    [[nodiscard]] Resource * operator->() ENTT_NOEXCEPT {
-        return const_cast<Resource *>(std::as_const(*this).operator->());
-    }
-
-    /**
-     * @brief Returns true if a handle contains a resource, false otherwise.
-     * @return True if the handle contains a resource, false otherwise.
-     */
-    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
-        return static_cast<bool>(resource);
-    }
-
-private:
-    std::shared_ptr<Resource> resource;
-};
-
-
-}
-
-
-#endif