update single include file

* drop any from meta containers rebind
* further reduce the size of the meta any vtable for containers

.bazelignore

@@ -0,0 +1 @@
+test

.bazeliskrc

@@ -0,0 +1 @@
+USE_BAZEL_VERSION=6.x

.bazelrc

@@ -0,0 +1,17 @@
+common --enable_bzlmod
+build --enable_platform_specific_config
+build --incompatible_use_platforms_repo_for_constraints
+build --incompatible_enable_cc_toolchain_resolution
+build --enable_runfiles
+build --incompatible_strict_action_env
+
+# required for googletest 
+build:linux --cxxopt=-std=c++17
+build:macos --cxxopt=-std=c++17
+
+common:ci --announce_rc
+common:ci --verbose_failures
+common:ci --keep_going
+test:ci --test_output=errors
+
+try-import %workspace%/user.bazelrc

.clang-tidy

@@ -0,0 +1,26 @@
+Checks: >
+    bugprone-*,
+    concurrency-*,
+    cppcoreguidelines-*,
+    misc-*,
+    -misc-no-recursion,
+    modernize-*,
+    -modernize-use-trailing-return-type,
+    performance-*,
+    portability-*,
+    readibility-*
+CheckOptions:
+    - key: cppcoreguidelines-special-member-functions.AllowSoleDefaultDtor
+      value: true
+    - key: cppcoreguidelines-special-member-functions.AllowMissingMoveFunctions
+      value: true
+    - key: cppcoreguidelines-special-member-functions.AllowMissingMoveFunctionsWhenCopyIsDeleted
+      value: true
+    - key: misc-non-private-member-variables-in-classes.IgnoreClassesWithAllMemberVariablesBeingPublic
+      value: true
+    - key: misc-non-private-member-variables-in-classes.IgnorePublicMemberVariables
+      value: true
+    - key: cppcoreguidelines-avoid-magic-numbers.IgnorePowersOf2IntegerValues
+      value: true
+    - key: cppcoreguidelines-avoid-magic-numbers.IgnoreAllFloatingPointValues
+      value: true

.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/analyzer.yml

@@ -0,0 +1,61 @@
+name: analyzer
+
+on:
+  push:
+    branches:
+      - master
+      - wip
+
+jobs:
+
+  iwyu:
+    timeout-minutes: 30
+
+    env:
+      IWYU: "0.20"
+      LLVM: "16"
+
+    runs-on: ubuntu-latest
+    continue-on-error: true
+
+    steps:
+    - uses: actions/checkout@v4
+    - name: Install llvm/clang
+      # see: https://apt.llvm.org/
+      run: |
+        wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
+        sudo add-apt-repository "deb http://apt.llvm.org/focal/ llvm-toolchain-focal-$LLVM main"
+        sudo apt update
+        sudo apt remove -y "llvm*"
+        sudo apt remove -y "libclang-dev*"
+        sudo apt remove -y "clang*"
+        sudo apt install -y llvm-$LLVM-dev
+        sudo apt install -y libclang-$LLVM-dev
+        sudo apt install -y clang-$LLVM
+    - name: Compile iwyu
+      # see: https://github.com/include-what-you-use/include-what-you-use
+      working-directory: build
+      run: |
+        git clone https://github.com/include-what-you-use/include-what-you-use.git --branch $IWYU --depth 1
+        mkdir include-what-you-use/build
+        cd include-what-you-use/build
+        cmake -DCMAKE_C_COMPILER=clang-$LLVM \
+              -DCMAKE_CXX_COMPILER=clang++-$LLVM \
+              -DCMAKE_INSTALL_PREFIX=./ \
+              ..
+        make -j4
+        bin/include-what-you-use --version
+    - name: Compile tests
+      working-directory: build
+      run: |
+        export PATH=$PATH:${GITHUB_WORKSPACE}/build/include-what-you-use/build/bin
+        cmake -DCMAKE_C_COMPILER=clang-$LLVM \
+              -DCMAKE_CXX_COMPILER=clang++-$LLVM \
+              -DENTT_BUILD_TESTING=ON \
+              -DENTT_BUILD_BENCHMARK=ON \
+              -DENTT_BUILD_EXAMPLE=ON \
+              -DENTT_BUILD_LIB=ON \
+              -DENTT_BUILD_SNAPSHOT=ON \
+              -DCMAKE_CXX_INCLUDE_WHAT_YOU_USE="include-what-you-use;-Xiwyu;--mapping_file=${GITHUB_WORKSPACE}/entt.imp;-Xiwyu;--no_fwd_decls;-Xiwyu;--verbose=1" \
+              ..
+        make -j4

.github/workflows/bazel-release-archive.yml

@@ -0,0 +1,20 @@
+name: Bazel Release
+
+on:
+  release:
+    types: [published]
+
+jobs:
+  # A release archive is required for bzlmod
+  # See: https://blog.bazel.build/2023/02/15/github-archive-checksum.html
+  bazel-release-archive:
+    runs-on: ubuntu-latest
+    continue-on-error: true
+    permissions:
+      contents: write
+    steps:
+      - uses: actions/checkout@v4
+      - run: git archive $GITHUB_REF -o "entt-${GITHUB_REF:10}.tar.gz"
+      - run: gh release upload ${GITHUB_REF:10} "entt-${GITHUB_REF:10}.tar.gz"
+        env:
+          GH_TOKEN: ${{ github.token }}

.github/workflows/bazel.yml

@@ -0,0 +1,20 @@
+name: bazel
+
+on: [push, pull_request]
+
+jobs:
+  test:
+    strategy:
+      matrix:
+        os:
+          - ubuntu-latest
+          - windows-latest
+          - macos-latest
+    runs-on: ${{ matrix.os }}
+    continue-on-error: true
+    steps:
+    - uses: actions/checkout@v4
+    - run: bazelisk test --config=ci ...
+      working-directory: test
+      env:
+        USE_BAZEL_VERSION: 6.x

.github/workflows/build.yml

@@ -5,40 +5,65 @@ on: [push, pull_request]
 jobs:
 
   linux:
-    timeout-minutes: 10
+    timeout-minutes: 15
 
     strategy:
       matrix:
+        os: [ubuntu-latest, ubuntu-20.04]
         compiler:
-          - pkg: g++-7
-            exe: g++-7
-          - pkg: g++-8
-            exe: g++-8
-          - pkg: g++-9
-            exe: g++-9
-          - pkg: g++
-            exe: g++
-          - pkg: clang-8
-            exe: clang++-8
-          - pkg: clang-9
-            exe: clang++-9
-          - pkg: clang-10
-            exe: clang++-10
-          - pkg: clang
-            exe: clang++
+          - { pkg: g++, exe: 'g++', version: 7 }
+          - { pkg: g++, exe: 'g++', version: 8 }
+          - { pkg: g++, exe: 'g++', version: 9 }
+          - { pkg: g++, exe: 'g++', version: 10 }
+          - { pkg: g++, exe: 'g++', version: 11 }
+          - { pkg: g++, exe: 'g++', version: 12 }
+          - { pkg: clang, exe: 'clang++', version: 8 }
+          - { pkg: clang, exe: 'clang++', version: 9 }
+          - { pkg: clang, exe: 'clang++', version: 10 }
+          - { pkg: clang, exe: 'clang++', version: 11 }
+          - { pkg: clang, exe: 'clang++', version: 12 }
+          - { pkg: clang, exe: 'clang++', version: 13 }
+          - { pkg: clang, exe: 'clang++', version: 14 }
+        exclude:
+          - os: ubuntu-latest
+            compiler: { pkg: g++, exe: 'g++', version: 7 }
+          - os: ubuntu-latest
+            compiler: { pkg: g++, exe: 'g++', version: 8 }
+          - os: ubuntu-latest
+            compiler: { pkg: g++, exe: 'g++', version: 9 }
+          - os: ubuntu-latest
+            compiler: { pkg: clang, exe: 'clang++', version: 8 }
+          - os: ubuntu-latest
+            compiler: { pkg: clang, exe: 'clang++', version: 9 }
+          - os: ubuntu-latest
+            compiler: { pkg: clang, exe: 'clang++', version: 10 }
+          - os: ubuntu-latest
+            compiler: { pkg: clang, exe: 'clang++', version: 11 }
+          - os: ubuntu-20.04
+            compiler: { pkg: g++, exe: 'g++', version: 10 }
+          - os: ubuntu-20.04
+            compiler: { pkg: g++, exe: 'g++', version: 11 }
+          - os: ubuntu-20.04
+            compiler: { pkg: g++, exe: 'g++', version: 12 }
+          - os: ubuntu-20.04
+            compiler: { pkg: clang, exe: 'clang++', version: 12 }
+          - os: ubuntu-20.04
+            compiler: { pkg: clang, exe: 'clang++', version: 13 }
+          - os: ubuntu-20.04
+            compiler: { pkg: clang, exe: 'clang++', version: 14 }
 
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - name: Install compiler
       run: |
-        sudo apt-get update
-        sudo apt-get install ${{ matrix.compiler.pkg }} -y
+        sudo apt update
+        sudo apt install -y ${{ matrix.compiler.pkg }}-${{ matrix.compiler.version }}
     - name: Compile tests
       working-directory: build
       env:
-        CXX: ${{ matrix.compiler.exe }}
+        CXX: ${{ matrix.compiler.exe }}-${{ matrix.compiler.version }}
       run: |
         cmake -DENTT_BUILD_TESTING=ON -DENTT_BUILD_LIB=ON -DENTT_BUILD_EXAMPLE=ON ..
         make -j4
@@ -46,55 +71,26 @@ jobs:
       working-directory: build
       env:
         CTEST_OUTPUT_ON_FAILURE: 1
-      run: ctest --timeout 30 -C Debug -j4
-
-  linux-extra:
-    timeout-minutes: 10
-
-    strategy:
-      matrix:
-        compiler: [g++, clang++]
-        id_type: [uint32, uint64]
-        cxx_std: [cxx_std_17, cxx_std_20]
-        include:
-          - id_type: uint64
-            id_type_variable: -DENTT_ID_TYPE=std::uint64_t
-          - cxx_std: cxx_std_20
-            cxx_std_variable: -DENTT_CXX_STD=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 ${{ matrix.id_type_variable }} ${{ matrix.cxx_std_variable }} ..
-        make -j4
-    - name: Run tests
-      working-directory: build
-      env:
-        CTEST_OUTPUT_ON_FAILURE: 1
-      run: ctest --timeout 30 -C Debug -j4
+      run: ctest -C Debug -j4
 
   windows:
-    timeout-minutes: 10
+    timeout-minutes: 15
 
     strategy:
       matrix:
-        toolset: [clang-cl, default, v141]
+        toolset: [default, v141, v142, clang-cl]
         include:
-          - toolset: clang-cl
-            toolset_option: -T"ClangCl"
           - toolset: v141
             toolset_option: -T"v141"
+          - toolset: v142
+            toolset_option: -T"v142"
+          - toolset: clang-cl
+            toolset_option: -T"ClangCl"
 
     runs-on: windows-latest
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - name: Compile tests
       working-directory: build
       run: |
@@ -104,42 +100,14 @@ jobs:
       working-directory: build
       env:
         CTEST_OUTPUT_ON_FAILURE: 1
-      run: ctest --timeout 30 -C Debug -j4
-
-  windows-extra:
-    timeout-minutes: 10
-
-    strategy:
-      matrix:
-        id_type: [uint32, uint64]
-        cxx_std: [cxx_std_17, cxx_std_20]
-        include:
-          - id_type: uint64
-            id_type_variable: -DENTT_ID_TYPE=std::uint64_t
-          - cxx_std: cxx_std_20
-            cxx_std_variable: -DENTT_CXX_STD=cxx_std_20
-
-    runs-on: windows-latest
-
-    steps:
-    - uses: actions/checkout@v2
-    - name: Compile tests
-      working-directory: build
-      run: |
-        cmake -DENTT_BUILD_TESTING=ON ${{ matrix.id_type_variable }} ${{ matrix.cxx_std_variable }} ..
-        cmake --build . -j 4
-    - name: Run tests
-      working-directory: build
-      env:
-        CTEST_OUTPUT_ON_FAILURE: 1
-      run: ctest --timeout 30 -C Debug -j4
+      run: ctest -C Debug -j4
 
   macos:
-    timeout-minutes: 10
+    timeout-minutes: 15
     runs-on: macOS-latest
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - name: Compile tests
       working-directory: build
       run: |
@@ -149,32 +117,28 @@ jobs:
       working-directory: build
       env:
         CTEST_OUTPUT_ON_FAILURE: 1
-      run: ctest --timeout 30 -C Debug -j4
+      run: ctest -C Debug -j4
 
-  macos-extra:
-    timeout-minutes: 10
+  extra:
+    timeout-minutes: 15
 
     strategy:
       matrix:
-        id_type: [uint32, uint64]
+        os: [windows-latest, macOS-latest, ubuntu-latest]
+        id_type: ["std::uint32_t", "std::uint64_t"]
         cxx_std: [cxx_std_17, cxx_std_20]
-        include:
-          - id_type: uint64
-            id_type_variable: -DENTT_ID_TYPE=std::uint64_t
-          - cxx_std: cxx_std_20
-            cxx_std_variable: -DENTT_CXX_STD=cxx_std_20
 
-    runs-on: macOS-latest
+    runs-on: ${{ matrix.os }}
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - name: Compile tests
       working-directory: build
       run: |
-        cmake -DENTT_BUILD_TESTING=ON ${{ matrix.id_type_variable }} ${{ matrix.cxx_std_variable }} ..
-        make -j4
+        cmake -DENTT_BUILD_TESTING=ON -DENTT_CXX_STD=${{ 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
+      run: ctest -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 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
+    - uses: actions/checkout@v4
+    - name: Compile tests
+      working-directory: build
+      env:
+        CXXFLAGS: "--coverage -fno-elide-constructors -fno-inline -fno-default-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 -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@v3
+      with:
+        token: ${{ secrets.CODECOV_TOKEN }}
+        files: build/coverage.info
+        name: EnTT
+        fail_ci_if_error: true

.github/workflows/deploy.yml

@@ -15,7 +15,7 @@ jobs:
       FORMULA: entt.rb
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - name: Clone repository
       working-directory: build
       env:

.github/workflows/sanitizer.yml

@@ -4,33 +4,30 @@ on: [push, pull_request]
 
 jobs:
 
-  linux:
-    timeout-minutes: 10
+  clang:
+    timeout-minutes: 15
 
     strategy:
       matrix:
         compiler: [clang++]
-        id_type: [uint32, uint64]
+        id_type: ["std::uint32_t", "std::uint64_t"]
         cxx_std: [cxx_std_17, cxx_std_20]
-        include:
-          - id_type: uint64
-            id_type_variable: -DENTT_ID_TYPE=std::uint64_t
-          - cxx_std: cxx_std_20
-            cxx_std_variable: -DENTT_CXX_STD=cxx_std_20
 
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
+    # temporary workaround for https://github.com/actions/runner-images/issues/8659
+    - uses: mjp41/workaround8649@c8550b715ccdc17f89c8d5c28d7a48eeff9c94a8
     - name: Compile tests
       working-directory: build
       env:
         CXX: ${{ matrix.compiler }}
       run: |
-        cmake -DENTT_USE_SANITIZER=ON -DENTT_BUILD_TESTING=ON -DENTT_BUILD_LIB=ON -DENTT_BUILD_EXAMPLE=ON ${{ matrix.id_type_variable }} ${{ matrix.cxx_std_variable }} ..
+        cmake -DENTT_USE_SANITIZER=ON -DENTT_BUILD_TESTING=ON -DENTT_BUILD_LIB=ON -DENTT_BUILD_EXAMPLE=ON -DENTT_CXX_STD=${{ 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 30 -C Debug -j4
+      run: ctest -C Debug -j4

.gitignore

@@ -11,3 +11,5 @@ cpp.hint
 
 # Bazel
 /bazel-*
+/test/bazel-*
+/user.bazelrc

AUTHORS

@@ -11,10 +11,12 @@ ceeac
 ColinH
 corystegel
 Croydon
+cschreib
 cugone
 dbacchet
 dBagrat
 djarek
+DNKpp
 DonKult
 drglove
 eliasdaler
@@ -31,6 +33,7 @@ Lawrencemm
 markand
 mhammerc
 Milerius
+Minimonium
 morbo84
 m-waka
 netpoetica
@@ -39,6 +42,7 @@ Oortonaut
 Paolo-Oliverio
 pgruenbacher
 prowolf
+Qix-
 stefanofiorentino
 suVrik
 szunhammer
@@ -46,6 +50,7 @@ The5-1
 vblanco20-1
 willtunnels
 WizardIke
+WoLfulus 
 w1th0utnam3
 xissburg
 zaucy

BUILD.bazel

@@ -1,14 +1,6 @@
-_msvc_copts = ["/std:c++17"]	
-_gcc_copts = ["-std=c++17"]
+package(default_visibility = ["//visibility:public"])
 
-cc_library(
+alias(
     name = "entt",
-    visibility = ["//visibility:public"],
-    strip_include_prefix = "src",
-    hdrs = glob(["src/**/*.h", "src/**/*.hpp"]),
-    copts = select({
-      "@bazel_tools//src/conditions:windows": _msvc_copts,
-      "@bazel_tools//src/conditions:windows_msvc": _msvc_copts,
-      "//conditions:default": _gcc_copts,
-    }),
+    actual = "//src:entt",
 )

CMakeLists.txt

@@ -1,29 +1,15 @@
-#
 # EnTT
-#
 
-cmake_minimum_required(VERSION 3.12.4)
+cmake_minimum_required(VERSION 3.15.7)
 
-#
-# 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
-#
 
 set(ENTT_VERSION_REGEX "#define ENTT_VERSION_.*[ \t]+(.+)")
 file(STRINGS "${CMAKE_CURRENT_SOURCE_DIR}/src/entt/config/version.h" ENTT_VERSION REGEX ${ENTT_VERSION_REGEX})
 list(TRANSFORM ENTT_VERSION REPLACE ${ENTT_VERSION_REGEX} "\\1")
 string(JOIN "." ENTT_VERSION ${ENTT_VERSION})
 
-#
 # Project configuration
-#
 
 project(
     EnTT
@@ -39,15 +25,18 @@ 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) 2017-2023 Michele Caini <michele.caini@gmail.com>")
 message(VERBOSE "*")
 
-#
-# Compiler stuff
-#
+# CMake stuff
 
-option(ENTT_USE_LIBCPP "Use libc++ by adding -stdlib=libc++ flag if availbale." OFF)
-option(ENTT_USE_SANITIZER "Enable sanitizers by adding -fsanitize=address -fno-omit-frame-pointer -fsanitize=undefined flags if availbale." OFF)
+list(INSERT CMAKE_MODULE_PATH 0 ${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules)
+
+# Compiler stuff
+
+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)
+option(ENTT_USE_CLANG_TIDY "Enable static analysis with clang-tidy" OFF)
 
 if(ENTT_USE_LIBCPP)
     if(NOT WIN32)
@@ -67,7 +56,7 @@ if(ENTT_USE_LIBCPP)
     endif()
 
     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.")
+        message(VERBOSE "The option ENTT_USE_LIBCPP is set but libc++ is not available.")
     endif()
 endif()
 
@@ -78,13 +67,19 @@ if(ENTT_USE_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.")
+        message(VERBOSE "The option ENTT_USE_SANITIZER is set but sanitizer support is not available.")
+    endif()
+endif()
+
+if(ENTT_USE_CLANG_TIDY)
+    find_program(ENTT_CLANG_TIDY_EXECUTABLE "clang-tidy")
+
+    if(NOT ENTT_CLANG_TIDY_EXECUTABLE)
+        message(VERBOSE "The option ENTT_USE_CLANG_TIDY is set but clang-tidy executable is not available.")
     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)
@@ -119,9 +114,10 @@ if(ENTT_INCLUDE_HEADERS)
         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_hash_map.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/container/dense_hash_set.hpp>
+            $<BUILD_INTERFACE:${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>
@@ -144,6 +140,7 @@ if(ENTT_INCLUDE_HEADERS)
             $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/fwd.hpp>
             $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/group.hpp>
             $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/handle.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/mixin.hpp>
             $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/helper.hpp>
             $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/observer.hpp>
             $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/organizer.hpp>
@@ -152,12 +149,15 @@ if(ENTT_INCLUDE_HEADERS)
             $<BUILD_INTERFACE:${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/graph/adjacency_matrix.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/graph/dot.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/graph/flow.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/graph/fwd.hpp>
             $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/locator/locator.hpp>
             $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/adl_pointer.hpp>
             $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/container.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/ctx.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/context.hpp>
             $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/factory.hpp>
             $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/fwd.hpp>
             $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/meta.hpp>
@@ -172,12 +172,13 @@ if(ENTT_INCLUDE_HEADERS)
             $<BUILD_INTERFACE:${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/fwd.hpp>
             $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/process/process.hpp>
             $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/process/scheduler.hpp>
             $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/resource/cache.hpp>
             $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/resource/fwd.hpp>
-            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/resource/handle.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>
@@ -196,6 +197,7 @@ if(ENTT_HAS_NATVIS)
             $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/container.natvis>
             $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/core.natvis>
             $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/entity.natvis>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/graph.natvis>
             $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/locator.natvis>
             $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/meta.natvis>
             $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/platform.natvis>
@@ -211,16 +213,22 @@ if(ENTT_HAS_SANITIZER)
     target_link_libraries(EnTT INTERFACE $<$<CONFIG:Debug>:-fsanitize=address -fno-omit-frame-pointer -fsanitize=undefined>)
 endif()
 
+if(ENTT_CLANG_TIDY_EXECUTABLE)
+    set(CMAKE_CXX_CLANG_TIDY "${ENTT_CLANG_TIDY_EXECUTABLE};--config-file=${EnTT_SOURCE_DIR}/.clang-tidy;--header-filter=${EnTT_SOURCE_DIR}/src/entt/.*;--extra-arg=/EHsc")
+endif()
+
 if(ENTT_HAS_LIBCPP)
     target_compile_options(EnTT BEFORE INTERFACE -stdlib=libc++)
 endif()
 
-#
 # Install pkg-config file
-#
+
+include(JoinPaths)
 
 set(EnTT_PKGCONFIG ${CMAKE_CURRENT_BINARY_DIR}/entt.pc)
 
+join_paths(EnTT_PKGCONFIG_INCLUDEDIR "\${prefix}" "${CMAKE_INSTALL_INCLUDEDIR}")
+
 configure_file(
     ${EnTT_SOURCE_DIR}/cmake/in/entt.pc.in
     ${EnTT_PKGCONFIG}
@@ -232,9 +240,7 @@ install(
     DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig
 )
 
-#
 # Install EnTT
-#
 
 include(CMakePackageConfigHelpers)
 
@@ -276,13 +282,17 @@ install(
     DESTINATION ${CMAKE_INSTALL_LIBDIR}/EnTT/cmake
 )
 
-install(DIRECTORY src/ DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
+install(
+    DIRECTORY src/
+    DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
+    FILES_MATCHING
+    PATTERN "*.h"
+    PATTERN "*.hpp"
+)
 
 export(PACKAGE EnTT)
 
-#
 # Tests
-#
 
 option(ENTT_BUILD_TESTING "Enable building tests." OFF)
 
@@ -301,9 +311,15 @@ if(ENTT_BUILD_TESTING)
     add_subdirectory(test)
 endif()
 
-#
+# Tools
+
+option(ENTT_BUILD_TOOLS "Enable building tools." OFF)
+
+if(ENTT_BUILD_TOOLS)
+    add_subdirectory(tools)
+endif()
+
 # Documentation
-#
 
 option(ENTT_BUILD_DOCS "Enable building with documentation." OFF)
 
@@ -314,22 +330,3 @@ if(ENTT_BUILD_DOCS)
         add_subdirectory(docs)
     endif()
 endif()
-
-#
-# AOB
-#
-
-add_custom_target(
-    aob
-    SOURCES
-        .github/workflows/build.yml
-        .github/workflows/coverage.yml
-        .github/workflows/deploy.yml
-        .github/workflows/sanitizer.yml
-        .github/FUNDING.yml
-        AUTHORS
-        CONTRIBUTING.md
-        LICENSE
-        README.md
-        TODO
-)

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, author of EnTT
+Copyright (c) 2017-2023 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

MODULE.bazel

@@ -0,0 +1,8 @@
+module(
+    name = "entt",
+    version = "3.12.2",
+    compatibility_level = 3012,
+)
+
+bazel_dep(name = "rules_cc", version = "0.0.8")
+bazel_dep(name = "bazel_skylib", version = "1.4.2")

README.md

@@ -6,10 +6,16 @@
 [![Build Status](https://github.com/skypjack/entt/workflows/build/badge.svg)](https://github.com/skypjack/entt/actions)
 [![Coverage](https://codecov.io/gh/skypjack/entt/branch/master/graph/badge.svg)](https://codecov.io/gh/skypjack/entt)
 [![Try online](https://img.shields.io/badge/try-online-brightgreen)](https://godbolt.org/z/zxW73f)
-[![Documentation](https://img.shields.io/badge/docs-docsforge-blue)](http://entt.docsforge.com/)
+[![Documentation](https://img.shields.io/badge/docs-doxygen-blue)](https://skypjack.github.io/entt/)
+[![Vcpkg port](https://img.shields.io/vcpkg/v/entt)](https://vcpkg.link/ports/entt)
+[![Conan Center](https://img.shields.io/conan/v/entt)](https://conan.io/center/recipes/entt)
 [![Gitter chat](https://badges.gitter.im/skypjack/entt.png)](https://gitter.im/skypjack/entt)
 [![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
+>
+> -- Every EnTT User Ever
 
 `EnTT` is a header-only, tiny and easy to use library for game programming and
 much more written in **modern C++**.<br/>
@@ -33,7 +39,8 @@ Don't forget to check the
 there.
 
 Do you want to support `EnTT`? Consider becoming a
-[**sponsor**](https://github.com/users/skypjack/sponsorship).
+[**sponsor**](https://github.com/users/skypjack/sponsorship) or making a
+donation via [**PayPal**](https://www.paypal.me/skypjack).<br/>
 Many thanks to [these people](https://skypjack.github.io/sponsorship/) and
 **special** thanks to:
 
@@ -45,7 +52,7 @@ Many thanks to [these people](https://skypjack.github.io/sponsorship/) and
 * [Introduction](#introduction)
   * [Code Example](#code-example)
   * [Motivation](#motivation)
-  * [Performance](#performance)
+  * [Benchmark](#benchmark)
 * [Integration](#integration)
   * [Requirements](#requirements)
   * [CMake](#cmake)
@@ -75,18 +82,20 @@ 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:
 
 * Built-in **RTTI system** mostly similar to the standard one.
-* A `constexpr` utility for human readable **resource names**.
+* A `constexpr` utility for human-readable **resource names**.
 * Minimal **configuration system** built using the monostate pattern.
 * Incredibly fast **entity-component system** with its own _pay for what you
-  use_ policy.
+  use_ policy, unconstrained component types with optional pointer stability and
+  hooks for storage customization.
 * 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.
+* General purpose **execution graph builder** for optimal scheduling.
 * 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 **dense hash map**.
+* 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** and a tiny event dispatcher.
@@ -94,16 +103,12 @@ Here is a brief, yet incomplete list of what it offers today:
 * And **much more**! Check out the
   [**wiki**](https://github.com/skypjack/entt/wiki).
 
-**Breaking news**:
+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.<br/>
+Please, do note that all tools are also DLL-friendly now and run smoothly across
+boundaries.
 
-* The ECS allows attaching multiple components of the same type to an entity.
-* All tools work perfectly across boundaries (DLL-friendly)!!
-  :slightly_smiling_face:
-
-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.
-
-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.
@@ -160,7 +165,7 @@ 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
@@ -171,37 +176,28 @@ Nowadays, `EnTT` is finally what I was looking for: still faster than its
 _competitors_, lower memory usage in the average case, a really good API and an
 amazing set of features. And even more, of course.
 
-## Performance
+## Benchmark
 
-The proposed entity-component system is incredibly fast to iterate entities and
-components, this is a fact. Some compilers make a lot of optimizations because
-of how `EnTT` works, some others aren't that good. In general, if we consider
-real world cases, `EnTT` is somewhere between a bit and much faster than many of
-the other solutions around, although I couldn't check them all for obvious
-reasons.
+For what it's worth, you'll **never** see me trying to make other projects look
+bad or offer dubious comparisons just to make this library seem cooler.<br/>
+I leave this activity to others, if they enjoy it (and it seems that some people
+actually like it). I prefer to make better use of my time.
 
 If you are interested, you can compile the `benchmark` test in release mode (to
 enable compiler optimizations, otherwise it would make little sense) by setting
 the `ENTT_BUILD_BENCHMARK` option of `CMake` to `ON`, then evaluate yourself
 whether you're satisfied with the results or not.
 
-Honestly I got tired of updating the README file whenever there is an
-improvement.<br/>
-There are already a lot of projects out there that use `EnTT` as a basis for
+There are also a lot of projects out there that use `EnTT` as a basis for
 comparison (this should already tell you a lot). Many of these benchmarks are
 completely wrong, many others are simply incomplete, good at omitting some
 information and using the wrong function to compare a given feature. Certainly
 there are also good ones but they age quickly if nobody updates them, especially
-when the library they are dealing with is actively developed.
-
-The choice to use `EnTT` should be based on its carefully designed API, its
-set of features and the general performance, **not** because some single
-benchmark shows it to be the fastest tool available.
-
-In the future I'll likely try to get even better performance while still adding
-new features, mainly for fun.<br/>
-If you want to contribute and/or have suggestions, feel free to make a PR or
-open an issue to discuss your idea.
+when the library they are dealing with is actively developed.<br/>
+Out of all of them, [this](https://github.com/abeimler/ecs_benchmark) seems like
+the most up-to-date project and also covers a certain number of libraries. I
+can't say exactly whether `EnTT` is used correctly or not. However, even if used
+poorly, it should still give the reader an idea of where it's going to operate.
 
 # Integration
 
@@ -323,6 +319,18 @@ If you spot errors or have suggestions, any contribution is welcome!
   [documentation](https://build2.org/build2-toolchain/doc/build2-toolchain-intro.xhtml#guide-repositories)
   for more details.
 
+* [`bzlmod`](https://bazel.build/external/overview#bzlmod), Bazel's external
+  dependency management system.<br/>
+  To use the [`entt`](https://registry.bazel.build/modules/entt) module in a
+  `bazel` project, add the following to your `MODULE.bazel` file:
+
+  ```starlark
+  bazel_dep(name = "entt", version = "3.12.2")
+  ```
+
+  EnTT will now be available as `@entt` (short for `@entt//:entt`) to be used
+  in your `cc_*` rule `deps`. 
+
 Consider this list a work in progress and help me to make it longer if you like.
 
 ## pkg-config
@@ -340,8 +348,8 @@ The documentation is based on [doxygen](http://www.doxygen.nl/). To build it:
     $ cmake .. -DENTT_BUILD_DOCS=ON
     $ make
 
-The API reference will be created in HTML format within the directory
-`build/docs/html`. To navigate it with your favorite browser:
+The API reference is created in HTML format in the `build/docs/html` directory.
+To navigate it with your favorite browser:
 
     $ cd build
     $ your_favorite_browser docs/html/index.html
@@ -350,10 +358,7 @@ The API reference will be created in HTML format within the directory
 @cond TURN_OFF_DOXYGEN
 -->
 The same version is also available [online](https://skypjack.github.io/entt/)
-for the latest release, that is the last stable tag. If you are looking for
-something more pleasing to the eye, consider reading the nice-looking version
-available on [docsforge](https://entt.docsforge.com/): same documentation, much
-more pleasant to read.<br/>
+for the latest release, that is the last stable tag.<br/>
 Moreover, there exists a [wiki](https://github.com/skypjack/entt/wiki) dedicated
 to the project where users can find all related documentation pages.
 <!--
@@ -363,9 +368,8 @@ to the project where users can find all related documentation pages.
 # Tests
 
 To compile and run the tests, `EnTT` requires *googletest*.<br/>
-`cmake` will download and compile the library before compiling anything else.
-In order to build the tests, set the `CMake` option `ENTT_BUILD_TESTING` to
-`ON`.
+`cmake` downloads and compiles the library before compiling anything else. In
+order to build the tests, set the `CMake` option `ENTT_BUILD_TESTING` to `ON`.
 
 To build the most basic set of tests:
 
@@ -417,7 +421,7 @@ know who has participated so far.
 
 # License
 
-Code and documentation Copyright (c) 2017-2021 Michele Caini.<br/>
+Code and documentation Copyright (c) 2017-2023 Michele Caini.<br/>
 Colorful logo Copyright (c) 2018-2021 Richard Caseres.
 
 Code released under

TODO

@@ -1,25 +1,31 @@
-* 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
-* add examples (and credits) from @alanjfs :)
+EXAMPLES
+* filter on runtime values/variables (not only types)
+* support to polymorphic types (see #859)
 
-WIP:
-* add the possibility of disabling entities without deleting components thanks to the new full check
-* add registry::storage id -> basic_sparse_set (no template arg), add example of use
-* fast-contains for sparse sets (low prio but nice-to-have)
-* runtime events (dispatcher/emitter), runtime context variables...
-* runtime_view/registry, remove reference to basic_sparse_set<E>
-* dedicated entity storage, in-place O(1) release/destroy for non-orphaned entities, out-of-sync model
-* custom allocators all over
+DOC:
+* custom storage/view
+* update entity doc when the storage based model is in place
+* in-place O(1) release/destroy for non-orphaned entities, out-of-sync model
+* view: single vs multi type views are no longer a thing actually
+* bump entities, reserved bits on identifiers
 
-WIP:
-* customizable any_vtable, sfinae-friendly definition and op::custom for user-def
-* resource, forward the id to the loader from the cache and if constexpr the call to load, update doc and describe customization points
-* add user data to type_info
-* 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
+TODO:
+* resource cache: avoid using shared ptr with loader and the others
+* further optimize exclusion lists in multi type views (no existence check)
+* further improve meta resolve function by id (bimap)
+* get rid of observers, storage based views made them pointless - document alternatives
 * deprecate non-owning groups in favor of owning views and view packs, introduce lazy owning views
-* snapshot: support for range-based archives
+* bring nested groups back in place (see bd34e7f)
+* work stealing job system (see #100) + mt scheduler based on const awareness for types
+* view: reduce inst due to/improve perf with index-based approach in dispatch_get/pick_and_each/each (single type too, define storage ::at and ::at_as_tuple)
+* view: update natvis as needed after the last rework, merge pools/filter in the same array, drop check (?) and turn view into a position
+* view: type-only view_iterator (dyn get/excl sizes), type-only basic_common_view (dyn get/excl sizes with pointer to array from derived)
+* combine version-mask-vs-version-bits tricks with reserved bits to allow things like enabling/disabling
+* basic_sparse_set(basic_sparse_set &&other, const allocator_type &allocator) uses moved-from packed in assert (same for storage)
+* maybe drop begin(v)/end(v) from sparse set and only use scoped begin/end at call sites
+* view unchecked_refresh loop is never executed if Get is 1u
+* review all // NOLINT + linter workflow + review clang-tidy file
+* self contained entity traits to avoid explicit specializations (ie enum constants)
+* review assure conflicts check, hash doesn't fit the purpose maybe
+* allow to zero-sized versions (with non-regression tests)
+* auto type info data from types if present

WORKSPACE

@@ -1 +0,0 @@
-workspace(name = "com_github_skypjack_entt")

WORKSPACE.bazel

@@ -0,0 +1 @@
+# SEE MODULE.bazel

bazel/copts.bzl

@@ -0,0 +1,13 @@
+load("@bazel_skylib//lib:selects.bzl", "selects")
+
+COPTS = selects.with_or({
+    ("//conditions:default", "@rules_cc//cc/compiler:clang", "@rules_cc//cc/compiler:gcc", "@rules_cc//cc/compiler:mingw-gcc"): [
+        "-std=c++17",
+        "-w",
+    ],
+    ("@rules_cc//cc/compiler:msvc-cl", "@rules_cc//cc/compiler:clang-cl"): [
+        "/std:c++17",
+        "/permissive-",
+        "/w",
+    ],
+})

cmake/in/entt.pc.in

@@ -1,5 +1,5 @@
 prefix=@CMAKE_INSTALL_PREFIX@
-includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@
+includedir=@EnTT_PKGCONFIG_INCLUDEDIR@
 
 Name: EnTT
 Description: Gaming meets modern C++

cmake/modules/JoinPaths.cmake

@@ -0,0 +1,23 @@
+# This module provides function for joining paths
+# known from most languages
+#
+# SPDX-License-Identifier: (MIT OR CC0-1.0)
+# Copyright 2020 Jan Tojnar
+# https://github.com/jtojnar/cmake-snips
+#
+# Modelled after Python’s os.path.join
+# https://docs.python.org/3.7/library/os.path.html#os.path.join
+# Windows not supported
+function(join_paths joined_path first_path_segment)
+    set(temp_path "${first_path_segment}")
+    foreach(current_segment IN LISTS ARGN)
+        if(NOT ("${current_segment}" STREQUAL ""))
+            if(IS_ABSOLUTE "${current_segment}")
+                set(temp_path "${current_segment}")
+            else()
+                set(temp_path "${temp_path}/${current_segment}")
+            endif()
+        endif()
+    endforeach()
+    set(${joined_path} "${temp_path}" PARENT_SCOPE)
+endfunction()

docs/CMakeLists.txt

@@ -1,9 +1,23 @@
-#
 # Doxygen configuration (documentation)
-#
 
-set(DOXY_DEPS_DIRECTORY ${EnTT_SOURCE_DIR}/deps)
+include(FetchContent)
+
+FetchContent_Declare(
+    doxygen-awesome-css
+    GIT_REPOSITORY https://github.com/jothepro/doxygen-awesome-css
+    GIT_TAG main
+    GIT_SHALLOW 1
+)
+
+FetchContent_GetProperties(doxygen-awesome-css)
+
+if(NOT doxygen-awesome-css_POPULATED)
+    FetchContent_Populate(doxygen-awesome-css)
+    set(doxygen-awesome-css_INCLUDE_DIR ${doxygen-awesome-css_SOURCE_DIR})
+endif()
+
 set(DOXY_SOURCE_DIRECTORY ${EnTT_SOURCE_DIR}/src)
+set(DOXY_CSS_DIRECTORY ${doxygen-awesome-css_INCLUDE_DIR})
 set(DOXY_DOCS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})
 set(DOXY_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
 

docs/md/config.md

@@ -7,24 +7,25 @@
 
 * [Introduction](#introduction)
 * [Definitions](#definitions)
-  * [ENTT_NOEXCEPTION](#entt_noexcept)
+  * [ENTT_NOEXCEPTION](#entt_noexception)
   * [ENTT_USE_ATOMIC](#entt_use_atomic)
   * [ENTT_ID_TYPE](#entt_id_type)
   * [ENTT_SPARSE_PAGE](#entt_sparse_page)
   * [ENTT_PACKED_PAGE](#entt_packed_page)
   * [ENTT_ASSERT](#entt_assert)
+    * [ENTT_ASSERT_CONSTEXPR](#entt_assert_constexpr)
     * [ENTT_DISABLE_ASSERT](#entt_disable_assert)
   * [ENTT_NO_ETO](#entt_no_eto)
   * [ENTT_STANDARD_CPP](#entt_standard_cpp)
-
 <!--
 @endcond TURN_OFF_DOXYGEN
 -->
 
 # Introduction
 
-`EnTT` doesn't offer many hooks for customization but it certainly offers
-some.<br/>
+`EnTT` has become almost completely customizable over time, in many
+respects. These variables are just one of the many ways to customize how it
+works.<br/>
 In the vast majority of cases, users will have no interest in changing the
 default parameters. For all other cases, the list of possible configurations
 with which it's possible to adjust the behavior of the library at runtime can be
@@ -40,17 +41,17 @@ will remain stable over time unlike the options below.
 
 ## ENTT_NOEXCEPTION
 
-This parameter can be used to switch off exception handling in `EnTT`.<br/>
-To do this, simply define the variable without assigning any value to it. This
-is roughly equivalent to setting the compiler flag `-ff-noexceptions`.
+Define this variable without assigning any value to it to turn off exception
+handling in `EnTT`.<br/>
+This is roughly equivalent to setting the compiler flag `-fno-exceptions` but is
+also limited to this library only.
 
 ## ENTT_USE_ATOMIC
 
 In general, `EnTT` doesn't offer primitives to support multi-threading. Many of
 the features can be split over multiple threads without any explicit control and
-the user is the only one who knows if and when a synchronization point is
-required.<br/>
-However, some features aren't easily accessible to users and can be made
+the user is the one who knows if a synchronization point is required.<br/>
+However, some features aren't easily accessible to users and are made
 thread-safe by means of this definition.
 
 ## ENTT_ID_TYPE
@@ -70,9 +71,9 @@ power of 2.
 
 ## ENTT_PACKED_PAGE
 
-Similar to sparse arrays, packed arrays of components are paginated as well. In
-However, int this case the aim isn't to reduce memory usage but to have pointer
-stability upon component creation.<br/>
+As it happens with sparse arrays, packed arrays are also paginated. However, in
+this case the aim isn't to reduce memory usage but to have pointer stability
+upon component creation.<br/>
 Default size of pages (that is, the number of elements they contain) is 1024 but
 users can adjust it if appropriate. In all case, the chosen value **must** be a
 power of 2.
@@ -83,21 +84,29 @@ For performance reasons, `EnTT` doesn't use exceptions or any other control
 structures. In fact, it offers many features that result in undefined behavior
 if not used correctly.<br/>
 To get around this, the library relies on a lot of asserts for the purpose of
-detecting errors in debug builds. By default, it uses `assert` internally, but
-users are allowed to overwrite its behavior by setting this variable.
+detecting errors in debug builds. By default, it uses `assert` internally. Users
+are allowed to overwrite its behavior by setting this variable.
+
+### ENTT_ASSERT_CONSTEXPR
+
+Usually, an assert within a `constexpr` function isn't a big deal. However, in
+case of extreme customizations, it might be useful to differentiate.<br/>
+For this purpose, `EnTT` introduces an admittedly badly named variable to make
+the job easier in this regard. By default, this variable forwards its arguments
+to `ENTT_ASSERT`.
 
 ### ENTT_DISABLE_ASSERT
 
 Assertions may in turn affect performance to an extent when enabled. Whether
-`ENTT_ASSERT` is redefined or not, all asserts can be disabled at once by means
-of this definition.<br/>
-Note that `ENTT_DISABLE_ASSERT` takes precedence over the redefinition of
-`ENTT_ASSERT` and is therefore meant to disable all controls no matter what.
+`ENTT_ASSERT` and `ENTT_ASSERT_CONSTEXPR` are redefined or not, all asserts can
+be disabled at once by means of this definition.<br/>
+Note that `ENTT_DISABLE_ASSERT` takes precedence over the redefinition of the
+other variables and is therefore meant to disable all controls no matter what.
 
 ## ENTT_NO_ETO
 
 In order to reduce memory consumption and increase performance, empty types are
-never stored by the ECS module of `EnTT`.<br/>
+never instantiated nor stored by the ECS module of `EnTT`.<br/>
 Use this variable to treat these types like all others and therefore to create a
 dedicated storage for them.
 
@@ -105,7 +114,7 @@ dedicated storage for them.
 
 `EnTT` mixes non-standard language features with others that are perfectly
 compliant to offer some of its functionalities.<br/>
-This definition will prevent the library from using non-standard techniques,
-that is, functionalities that aren't fully compliant with the standard C++.<br/>
+This definition prevents the library from using non-standard techniques, that
+is, functionalities that aren't fully compliant with the standard C++.<br/>
 While there are no known portability issues at the time of this writing, this
 should make the library fully portable anyway if needed.

docs/md/container.md

@@ -7,9 +7,8 @@
 
 * [Introduction](#introduction)
 * [Containers](#containers)
-  * [Dense hash map](#dense-hash-map)
-  * [Dense hash set](#dense-hash-set)
-
+  * [Dense map](#dense-map)
+  * [Dense set](#dense-set)
 <!--
 @endcond TURN_OFF_DOXYGEN
 -->
@@ -21,7 +20,7 @@ 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
+However, the library also tries to fill a gap in features and functionalities 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
@@ -31,26 +30,38 @@ guaranteed as usual.
 
 # Containers
 
-## Dense hash map
+## Dense map
 
-The dense hash map made available in `EnTT` is a map that aims to return a
+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
-the iteration.<br/>
+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,
+the `std::unordered_map` class.<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_map`.<br/>
-Therefore, there is no need to go into the API description.
-
-## Dense hash set
-
-The dense hash set made available in `EnTT` is a set that aims to return a
-packed array of elements, so as to reduce the number of jumps in memory during
-the iteration.<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.
+library, that is, the `std::unordered_set` class.<br/>
+However, this type of set also supports reverse iteration and therefore offers
+all the functions necessary for the purpose (such as `rbegin` and `rend`).

docs/md/faq.md

@@ -14,18 +14,20 @@
   * [Warning C4003: the min, the max and the macro](#warning-C4003-the-min-the-max-and-the-macro)
   * [The standard and the non-copyable types](#the-standard-and-the-non-copyable-types)
   * [Which functions trigger which signals](#which-functions-trigger-which-signals)
+  * [Duplicate storage for the same component](#duplicate-storage-for-the-same-component)
 <!--
 @endcond TURN_OFF_DOXYGEN
 -->
 
 # Introduction
 
-This is a constantly updated section where I'll try to put the answers to the
+This is a constantly updated section where I'm trying to put the answers to the
 most frequently asked questions.<br/>
 If you don't find your answer here, there are two cases: nobody has done it yet
-or this section needs updating. In both cases, try to
-[open a new issue](https://github.com/skypjack/entt/issues/new) or enter the
-[gitter channel](https://gitter.im/skypjack/entt) and ask your question.
+or this section needs updating. In both cases, you can
+[open a new issue](https://github.com/skypjack/entt/issues/new) or enter either
+the [gitter channel](https://gitter.im/skypjack/entt) or the
+[discord server](https://discord.gg/5BjPWBd) to ask for help.<br/>
 Probably someone already has an answer for you and we can then integrate this
 part of the documentation.
 
@@ -45,14 +47,14 @@ lot, achieving good results in many cases.
 First of all, there are two things to do in a Windows project:
 
 * Disable the [`/JMC`](https://docs.microsoft.com/cpp/build/reference/jmc)
-  option (_Just My Code_ debugging), available starting in Visual Studio 2017
+  option (_Just My Code_ debugging), available starting with Visual Studio 2017
   version 15.8.
 
 * Set the [`_ITERATOR_DEBUG_LEVEL`](https://docs.microsoft.com/cpp/standard-library/iterator-debug-level)
   macro to 0. This will disable checked iterators and iterator debugging.
 
-Moreover, the macro `ENTT_ASSERT` should be redefined to disable internal checks
-made by `EnTT` in debug:
+Moreover, set the `ENTT_DISABLE_ASSERT` variable or redefine the `ENTT_ASSERT`
+macro to disable internal debug checks in `EnTT`:
 
 ```cpp
 #define ENTT_ASSERT(...) ((void)0)
@@ -61,22 +63,22 @@ made by `EnTT` in debug:
 These asserts are introduced to help the users but they require to access to the
 underlying containers and therefore risk ruining the performance in some cases.
 
-With these changes, debug performance should increase enough for most cases. If
-you want something more, you can can also switch to an optimization level `O0`
-or preferably `O1`.
+With these changes, debug performance should increase enough in most cases. If
+you want something more, you can also switch to an optimization level `O0` or
+preferably `O1`.
 
 ## How can I represent hierarchies with my components?
 
 This is one of the first questions that anyone makes when starting to work with
 the entity-component-system architectural pattern.<br/>
-There are several approaches to the problem and what’s the best one depends
-mainly on the real problem one is facing. In all cases, how to do it doesn't
-strictly depend on the library in use, but the latter can certainly allow or
-not different techniques depending on how the data are laid out.
+There are several approaches to the problem and the best one depends mainly on
+the real problem one is facing. In all cases, how to do it doesn't strictly
+depend on the library in use, but the latter certainly allows or not different
+techniques depending on how the data are laid out.
 
-I tried to describe some of the techniques that fit well with the model of
-`EnTT`. [Here](https://skypjack.github.io/2019-06-25-ecs-baf-part-4/) is the
-first post of a series that tries to explore the problem. More will probably
+I tried to describe some of the approaches that fit well with the model of
+`EnTT`. [This](https://skypjack.github.io/2019-06-25-ecs-baf-part-4/) is the
+first post of a series that tries to _explore_ the problem. More will probably
 come in future.<br/>
 In addition, `EnTT` also offers the possibility to create stable storage types
 and therefore have pointer stability for one, all or some components. This is by
@@ -90,6 +92,7 @@ Custom entity identifiers are definitely a good idea in two cases at least:
 
 * If `std::uint32_t` isn't large enough for your purposes, since this is the
   underlying type of `entt::entity`.
+
 * If you want to avoid conflicts when using multiple registries.
 
 Identifiers can be defined through enum classes and class types that define an
@@ -105,7 +108,7 @@ There is no limit to the number of identifiers that can be defined.
 ## Warning C4307: integral constant overflow
 
 According to [this](https://github.com/skypjack/entt/issues/121) issue, using a
-hashed string under VS could generate a warning.<br/>
+hashed string under VS (toolset v141) could generate a warning.<br/>
 First of all, I want to reassure you: it's expected and harmless. However, it
 can be annoying.
 
@@ -136,7 +139,7 @@ errors during compilation.
 
 It's a pretty big problem but fortunately it's not a problem of `EnTT` and there
 is a fairly simple solution to it.<br/>
-It consists in defining the `NOMINMAX` macro before to include any other header
+It consists in defining the `NOMINMAX` macro before including any other header
 so as to get rid of the extra definitions:
 
 ```cpp
@@ -149,15 +152,14 @@ 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 {
@@ -165,31 +167,42 @@ 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
 
-The `registry` class offers three signals that are emitted following specific
+Storage classes offer three _signals_ that are emitted following specific
 operations. Maybe not everyone knows what these operations are, though.<br/>
 If this isn't clear, below you can find a _vademecum_ for this purpose:
 
 * `on_created` is invoked when a component is first added (neither modified nor 
   replaced) to an entity.
+
 * `on_update` is called whenever an existing component is modified or replaced.
+
 * `on_destroyed` is called when a component is explicitly or implicitly removed 
   from an entity.
 
@@ -201,3 +214,28 @@ otherwise the latter is replaced and therefore `on_update` is triggered. As for
 the second case, components are removed from their entities and thus freed when
 they are recycled. It means that `on_destroyed` is triggered for every component 
 owned by the entity that is destroyed.
+
+## Duplicate storage for the same component
+
+It's rare but you can see double sometimes, especially when it comes to storage.
+This can be caused by a conflict in the hash assigned to the various component
+types (one of a kind) or by bugs in your compiler
+([more common](https://github.com/skypjack/entt/issues/1063) apparently).<br/>
+Regardless of the cause, `EnTT` offers a customization point that also serves as
+a solution in this case:
+
+```cpp
+template<>
+struct entt::type_hash<Type> final {
+    [[nodiscard]] static constexpr id_type value() noexcept {
+        return hashed_string::value("Type");
+    }
+
+    [[nodiscard]] constexpr operator id_type() const noexcept {
+        return value();
+    }
+};
+```
+
+Specializing `type_hash` directly bypasses the default implementation offered by
+`EnTT`, thus avoiding any possible conflicts or compiler bugs.

docs/md/graph.md

@@ -0,0 +1,372 @@
+# Crash Course: graph
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [Data structures](#data-structures)
+  * [Adjacency matrix](#adjacency-matrix)
+  * [Graphviz dot language](#graphviz-dot-language)
+* [Flow builder](#flow-builder)
+  * [Tasks and resources](#tasks-and-resources)
+  * [Fake resources and order of execution](#fake-resources-and-order-of-execution)
+  * [Sync points](#sync-points)
+  * [Execution graph](#execution-graph)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+`EnTT` doesn't aim to offer everything one needs to work with graphs. Therefore,
+anyone looking for this in the _graph_ submodule will be disappointed.<br/>
+Quite the opposite is true though. This submodule is minimal and contains only
+the data structures and algorithms strictly necessary for the development of
+some tools such as the _flow builder_.
+
+# Data structures
+
+As anticipated in the introduction, the aim isn't to offer all possible data
+structures suitable for representing and working with graphs. Many will likely
+be added or refined over time. However I want to discourage anyone expecting
+tight scheduling on the subject.<br/>
+The data structures presented in this section are mainly useful for the
+development and support of some tools which are also part of the same submodule.
+
+## Adjacency matrix
+
+The adjacency matrix is designed to represent either a directed or an undirected
+graph:
+
+```cpp
+entt::adjacency_matrix<entt::directed_tag> adjacency_matrix{};
+```
+
+The `directed_tag` type _creates_ the graph as directed. There is also an
+`undirected_tag` counterpart which creates it as undirected.<br/>
+The interface deviates slightly from the typical double indexing of C and offers
+an API that is perhaps more familiar to a C++ programmer. Therefore, the access
+and modification of an element takes place via the `contains`, `insert` and
+`erase` functions rather than a double call to an `operator[]`:
+
+```cpp
+if(adjacency_matrix.contains(0u, 1u)) {
+    adjacency_matrix.erase(0u, 1u);
+} else {
+    adjacency_matrix.insert(0u, 1u);
+}
+```
+
+Both `insert` and` erase` are _idempotent_ functions which have no effect if the
+element already exists or has already been deleted.<br/>
+The first one returns an `std::pair` containing the iterator to the element and
+a boolean value indicating whether the element was newly inserted or not. The
+second one returns the number of deleted elements (0 or 1).
+
+An adjacency matrix is initialized with the number of elements (vertices) when
+constructing it but can also be resized later using the `resize` function:
+
+```cpp
+entt::adjacency_matrix<entt::directed_tag> adjacency_matrix{3u};
+```
+
+To visit all vertices, the class offers a function named `vertices` that returns
+an iterable object suitable for the purpose:
+
+```cpp
+for(auto &&vertex: adjacency_matrix.vertices()) {
+    // ...
+}
+```
+
+The same result is obtained with the following snippet, since the vertices are
+plain unsigned integral values:
+
+```cpp
+for(auto last = adjacency_matrix.size(), pos = {}; pos < last; ++pos) {
+    // ...
+}
+```
+
+As for visiting the edges, a few functions are available.<br/>
+When the purpose is to visit all the edges of a given adjacency matrix, the
+`edges` function returns an iterable object that is used to get them as pairs of
+vertices:
+
+```cpp
+for(auto [lhs, rhs]: adjacency_matrix.edges()) {
+    // ...
+}
+```
+
+If the goal is to visit all the in- or out-edges of a given vertex instead, the
+`in_edges` and `out_edges` functions are meant for that:
+
+```cpp
+for(auto [lhs, rhs]: adjacency_matrix.out_edges(3u)) {
+    // ...
+}
+```
+
+Both the functions expect the vertex to visit (that is, to return the in- or
+out-edges for) as an argument.<br/>
+Finally, the adjacency matrix is an allocator-aware container and offers most of
+the functionalities one would expect from this type of containers, such as
+`clear` or 'get_allocator` and so on.
+
+## Graphviz dot language
+
+As it's one of the most popular formats, the library offers minimal support for
+converting a graph to a Graphviz dot snippet.<br/>
+The simplest way is to pass both an output stream and a graph to the `dot`
+function:
+
+```cpp
+std::ostringstream output{};
+entt::dot(output, adjacency_matrix);
+```
+
+It's also possible to provide a callback to which the vertices are passed and
+which can be used to add (`dot`) properties to the output as needed:
+
+```cpp
+std::ostringstream output{};
+
+entt::dot(output, adjacency_matrix, [](auto &output, auto vertex) {
+    out << "label=\"v\"" << vertex << ",shape=\"box\"";
+});
+```
+
+This second mode is particularly convenient when the user wants to associate
+externally managed data to the graph being converted.
+
+# Flow builder
+
+A flow builder is used to create execution graphs from tasks and resources.<br/>
+The implementation is as generic as possible and doesn't bind to any other part
+of the library.
+
+This class is designed as a sort of _state machine_ to which a specific task is
+attached for which the resources accessed in read-only or read-write mode are
+specified.<br/>
+Most of the functions in the API also return the flow builder itself, according
+to what is the common sense API when it comes to builder classes.
+
+Once all tasks are registered and resources assigned to them, an execution graph
+in the form of an adjacency matrix is returned to the user.<br/>
+This graph contains all the tasks assigned to the flow builder in the form of
+_vertices_. The _vertex_ itself is used as an index to get the identifier passed
+during registration.
+
+## Tasks and resources
+
+Although these terms are used extensively in the documentation, the flow builder
+has no real concept of tasks and resources.<br/>
+This class works mainly with _identifiers_, that is, values of type `id_type`.
+In other terms, both tasks and resources are identified by integral values.<br/>
+This allows not to couple the class itself to the rest of the library or to any
+particular data structure. On the other hand, it requires the user to keep track
+of the association between identifiers and operations or actual data.
+
+Once a flow builder is created (which requires no constructor arguments), the
+first thing to do is to bind a task. This tells to the builder _who_ intends to
+consume the resources that are specified immediately after:
+
+```cpp
+entt::flow builder{};
+builder.bind("task_1"_hs);
+```
+
+The example uses the `EnTT` hashed string to generate an identifier for the
+task.<br/>
+Indeed, the use of `id_type` as an identifier type isn't by accident. In fact,
+it matches well with the internal hashed string class. Moreover, it's also the
+same type returned by the hash function of the internal RTTI system, in case the
+user wants to rely on that.<br/>
+However, being an integral value, it leaves the user full freedom to rely on his
+own tools if necessary.
+
+Once a task is associated with the flow builder, it's also assigned read-only or
+read-write resources as appropriate:
+
+```cpp
+builder
+    .bind("task_1"_hs)
+        .ro("resource_1"_hs)
+        .ro("resource_2"_hs)
+    .bind("task_2"_hs)
+        .rw("resource_2"_hs)
+```
+
+As mentioned, many functions return the builder itself and it's therefore easy
+to concatenate the different calls.<br/>
+Also in the case of resources, they are identified by numeric values of type
+`id_type`. As above, the choice is not entirely random. This goes well with the
+tools offered by the library while leaving room for maximum flexibility.
+
+Finally, both the `ro` and` rw` functions also offer an overload that accepts a
+pair of iterators, so that one can pass a range of resources in one go.
+
+### Rebinding
+
+The `flow` class is resource based rather than task based. This means that graph
+generation is driven by resources and not by the order of _appearance_ of tasks
+during flow definition.<br/>
+Although this concept is particularly important, it's almost irrelevant for the
+vast majority of cases. However, it becomes relevant when _rebinding_ resources
+or tasks.
+
+In fact, nothing prevents rebinding elements to a flow.<br/>
+However, the behavior changes slightly from case to case and has some nuances
+that it's worth knowing about.
+
+Directly rebinding a resource without the task being replaced trivially results
+in the task's access mode for that resource being updated:
+
+```cpp
+builder.bind("task"_hs).rw("resource"_hs).ro("resource"_hs)
+```
+
+In this case, the resource is accessed in read-only mode, regardless of the
+first call to `rw`.<br/>
+Behind the scenes, the call doesn't actually _replace_ the previous one but is
+queued after it instead, overwriting it when generating the graph. Thus, a large
+number of resource rebindings may even impact processing times (very difficult
+to observe but theoretically possible).
+
+Rebinding resources and also combining it with changes to tasks has far more
+implications instead.<br/>
+As mentioned, graph generation takes place starting from resources and not from
+tasks. Therefore, the result may not be as expected:
+
+```cpp
+builder
+    .bind("task_1"_hs)
+        .ro("resource"_hs)
+    .bind("task_2"_hs)
+        .ro("resource"_hs)
+    .bind("task_1"_hs)
+        .rw("resource"_hs);
+```
+
+What happens here is that the resource first _sees_ a read-only access request
+from the first task, then a read-write request from the second task and finally
+a new read-only request from the first task.<br/>
+Although this definition would probably be counted as an error, the resulting
+graph may be unexpected. This in fact consists of a single edge outgoing from
+the second task and directed to the first task.<br/>
+To intuitively understand what happens, it's enough to think of the fact that a
+task never has an edge pointing to itself.
+
+While not obvious, this approach has its pros and cons like any other solution.
+For example, creating loops is actually simple in the context of resource-based
+graph generations:
+
+```cpp
+builder
+    .bind("task_1"_hs)
+        .rw("resource"_hs)
+    .bind("task_2"_hs)
+        .rw("resource"_hs)
+    .bind("task_1"_hs)
+        .rw("resource"_hs);
+```
+
+As expected, this definition leads to the creation of two edges that define a
+loop between the two tasks.
+
+As a general rule, rebinding resources and tasks is highly discouraged because
+it could lead to subtle bugs if users don't know what they're doing.<br/>
+However, once the mechanisms of resource-based graph generation are understood,
+it can offer to the expert user a flexibility and a range of possibilities
+otherwise inaccessible.
+
+## Fake resources and order of execution
+
+The flow builder doesn't offer the ability to specify when a task should execute
+before or after another task.<br/>
+In fact, the order of _registration_ on the resources also determines the order
+in which the tasks are processed during the generation of the execution graph.
+
+However, there is a way to _force_ the execution order of two processes.<br/>
+Briefly, since accessing a resource in opposite modes requires sequential rather
+than parallel scheduling, it's possible to make use of fake resources to rule on
+the execution order:
+
+```cpp
+builder
+    .bind("task_1"_hs)
+        .ro("resource_1"_hs)
+        .rw("fake"_hs)
+    .bind("task_2"_hs)
+        .ro("resource_2"_hs)
+        .ro("fake"_hs)
+    .bind("task_3"_hs)
+        .ro("resource_2"_hs)
+        .ro("fake"_hs)
+```
+
+This snippet forces the execution of `task_1` **before** `task_2` and `task_3`.
+This is due to the fact that the former sets a read-write requirement on a fake
+resource that the other tasks also want to access in read-only mode.<br/>
+Similarly, it's possible to force a task to run **after** a certain group:
+
+```cpp
+builder
+    .bind("task_1"_hs)
+        .ro("resource_1"_hs)
+        .ro("fake"_hs)
+    .bind("task_2"_hs)
+        .ro("resource_1"_hs)
+        .ro("fake"_hs)
+    .bind("task_3"_hs)
+        .ro("resource_2"_hs)
+        .rw("fake"_hs)
+```
+
+In this case, since there are a number of processes that want to read a specific
+resource, they will do so in parallel by forcing `task_3` to run after all the
+others tasks.
+
+## Sync points
+
+Sometimes it's useful to assign the role of _sync point_ to a node.<br/>
+Whether it accesses new resources or is simply a watershed, the procedure for
+assigning this role to a vertex is always the same. First it's tied to the flow
+builder, then the `sync` function is invoked:
+
+```cpp
+builder.bind("sync_point"_hs).sync();
+```
+
+The choice to assign an _identity_ to this type of nodes lies in the fact that,
+more often than not, they also perform operations on resources.<br/>
+If this isn't the case, it will still be possible to create no-op vertices to
+which empty tasks are assigned.
+
+## Execution graph
+
+Once both the resources and their consumers have been properly registered, the
+purpose of this tool is to generate an execution graph that takes into account
+all specified constraints to return the best scheduling for the vertices:
+
+```cpp
+entt::adjacency_matrix<entt::directed_tag> graph = builder.graph();
+```
+
+Searching for the main vertices (that is, those without in-edges) is usually the
+first thing required:
+
+```cpp
+for(auto &&vertex: graph) {
+    if(auto in_edges = graph.in_edges(vertex); in_edges.begin() == in_edges.end()) {
+        // ...
+    }
+}
+```
+
+Then it's possible to instantiate an execution graph by means of other functions
+such as `out_edges` to retrieve the children of a given task or `edges` to get
+the identifiers.

docs/md/lib.md

@@ -19,14 +19,12 @@
 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 much easier.
+Fortunately, nowadays `EnTT` works smoothly across boundaries.
 
 ## Smooth until proven otherwise
 
 Many classes in `EnTT` make extensive use of type erasure for their purposes.
-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/>
+This raises the need to identify objects whose type has been erased.<br/>
 The `type_hash` class template is how identifiers are generated and thus made
 available to the rest of the library. In general, this class doesn't arouse much
 interest. The only exception is when a conflict between identifiers occurs
@@ -36,13 +34,13 @@ The section dedicated to `type_info` contains all the details to get around the
 issue in a concise and elegant way. Please refer to the specific documentation.
 
 When working with linked libraries, compile definitions `ENTT_API_EXPORT` and
-`ENTT_API_IMPORT` can be used where there is a need to import or export symbols,
-so as to make everything work nicely across boundaries.<br/>
+`ENTT_API_IMPORT` are 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/>
+For those who need more details, the test suite contains many examples covering
+the most common cases (see the `lib` directory for all details).<br/>
 It goes without saying that it's impossible to cover **all** possible cases.
 However, what is offered should hopefully serve as a basis for all of them.
 
@@ -50,40 +48,29 @@ However, what is offered should hopefully serve as a basis for all of them.
 
 The runtime reflection system deserves a special mention when it comes to using
 it across boundaries.<br/>
-Since it's linked already to a static context to which the visible components
-are attached and different contexts don't relate to each other, they must be
-_shared_ to allow the use of meta types across boundaries.
+Since it's linked already to a static context to which the elements are attached
+and different contexts don't relate to each other, they must be _shared_ to
+allow the use of meta types across boundaries.
 
-Sharing a context is trivial though. First of all, the local one must be
-acquired in the main space:
+Fortunately, sharing a context is also trivial to do. First of all, the local
+one is acquired in the main space:
 
 ```cpp
-entt::meta_ctx ctx{};
+auto handle = entt::locator<entt::meta_ctx>::handle();
 ```
 
-Then, it must passed to the receiving space that will set it as its global
-context, thus releasing the local one that remains available but is no longer
-referred to by the runtime reflection system:
+Then, it's passed to the receiving space that sets it as its default context,
+thus discarding or storing aside the local one:
 
 ```cpp
-entt::meta_ctx::bind(ctx);
+entt::locator<entt::meta_ctx>::reset(handle);
 ```
 
-From now on, both spaces will refer to the same context and on it will be
-attached the new visible meta types, no matter where they are created.<br/>
-A context can also be reset and then associated again locally as:
-
-```cpp
-entt::meta_ctx::bind(entt::meta_ctx{});
-```
-
-This is allowed because local and global contexts are separated. Therefore, it's
-always possible to make the local context the current one again.
-
-Before to release a context, all locally registered types should be reset to
-avoid dangling references. Otherwise, if a type is accessed from another space
-by name, there could be an attempt to address its parts that are no longer
-available.
+From now on, both spaces refer to the same context and on it are attached all
+new meta types, no matter where they are created.<br/>
+Note that _replacing_ the main context doesn't also propagate changes across
+boundaries. In other words, replacing a context results in the decoupling of the
+two sides and therefore a divergence in the contents.
 
 ## Memory Management
 

docs/md/links.md

@@ -1,5 +1,22 @@
 # EnTT in Action
 
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [EnTT in Action](#entt-in-action)
+  * [Games](#games)
+  * [Engines and the like](#engines-and-the-like)
+  * [Articles, videos and blog posts](#articles-videos-and-blog-posts)
+  * [Any Other Business](#any-other-business)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
 `EnTT` is widely used in private and commercial applications. I cannot even
 mention most of them because of some signatures I put on some documents time
 ago. Fortunately, there are also people who took the time to implement open
@@ -7,25 +24,43 @@ source projects based on `EnTT` and didn't hold back when it came to documenting
 them.
 
 Below an incomplete list of games, applications and articles that can be used as
-a reference. Where I put the word _apparently_ means that the use of `EnTT` is
-documented but the authors didn't make explicit announcements or contacted me
-directly.
+a reference.<br/>
+Where I put the word _apparently_ means that the use of `EnTT` is documented but
+the authors didn't make explicit announcements or contacted me directly.
 
-I hope this list can grow much more in the future:
+If you know of other resources out there that are about `EnTT`, feel free to
+open an issue or a PR and I'll be glad to add them to this page.<br/>
+I hope the following lists can grow much more in the future.
+
+# EnTT in Action
+
+## Games
 
-* Games:
   * [Minecraft](https://minecraft.net/en-us/attribution/) by
     [Mojang](https://mojang.com/): of course, **that** Minecraft, see the
     open source attributions page for more details.
+  * [Minecraft Legends](https://www.minecraft.net/it-it/about-legends) by
+    [Mojang](https://mojang.com/): an action strategy game where users have to
+    fight to defend the Overworld.
   * [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.
-  * [Apparently](https://www.youtube.com/watch?v=P8xvOA3ikrQ&t=1105s)
-    [Call of Duty: Vanguard](https://www.callofduty.com/vanguard): I can neither
-    confirm nor deny but there is a license I know in the credits.
   * [Antkeeper](https://github.com/antkeeper/antkeeper-source): an ant colony
     simulation [game](https://antkeeper.com/).
   * [Openblack](https://github.com/openblack/openblack): open source
@@ -44,7 +79,7 @@ I hope this list can grow much more in the future:
     puzzler with logic gates and other cool stuff.
     [Check it out](https://indi-kernick.itch.io/the-machine-web-version).
   * [EnTTPong](https://github.com/DomRe/EnttPong): a basic game made to showcase
-    different parts of EnTT and C++17.
+    different parts of `EnTT` and C++17.
   * [Randballs](https://github.com/gale93/randballs): simple `SFML` and `EnTT`
     playground.
   * [EnTT Tower Defense](https://github.com/Daivuk/tddod): a data oriented tower
@@ -80,8 +115,31 @@ I hope this list can grow much more in the future:
     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.
+  * [BattleSub](https://github.com/bfeldpw/battlesub): two player 2D submarine
+    game with some fluid dynamics.
+  * [Crimson Rush](https://github.com/WilKam01/LuaCGame): a dungeon-crawler and
+    rougelike inspired game about exploring and surviving as long as possible.
+  * [Space Fight](https://github.com/cholushkin/SpaceFight): one screen
+    multi-player arcade shooter game prototype.
+  * [Confetti Party](https://github.com/hexerei/entt-confetti): C++ sample
+    application as a starting point using `EnTT` and `SDL2`.
+  * [Hellbound](https://buas.itch.io/hellbound): a top-down action rogue-like
+    where to fight colossal demons in procedurally generated levels of hell.
+  * [Saurian Sorcery](https://github.com/cajallen/spellbook): a tower defense
+    game where to assemble a tribe of lizards to defend against robot invaders.
+  * [robotfindskitten](https://github.com/autogalkin/robotfindskitten): a clone
+    of `robotfindskitten` inside `Notepad.exe`, powered by `EnTT`.
+  * [Orion](https://github.com/alekskoloch/Orion): Outer-space Research and
+    Interstellar Observation Network (a space shooter game).
+  * [EnTT Boids](https://github.com/DanielEliasib/entt_boids): a simple boids
+    implementation using `EnTT` and `Raylib`.
+
+## Engines and the like:
 
-* Engines and the like:
   * [Aether Engine](https://hadean.com/spatial-simulation/)
     [v1.1+](https://docs.hadean.com/v1.1/Licenses/) by
     [Hadean](https://hadean.com/): a library designed for spatially partitioning
@@ -135,13 +193,37 @@ I hope this list can grow much more in the future:
     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
+  * [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
+  * [OpenAWE](https://github.com/OpenAWE-Project/OpenAWE): open implementation
     of the Alan Wake Engine.
+  * [Nazara Engine](https://github.com/DigitalPulseSoftware/NazaraEngine): fast,
+    cross-platform, object-oriented API to help in daily developer life.
+  * [Billy Engine](https://github.com/billy4479/BillyEngine): some kind of a 2D
+    engine based on `SDL2` and `EnTT`.
+  * [Ducktape](https://github.com/DucktapeEngine/Ducktape): an open source C++
+    2D & 3D game engine that focuses on being fast and powerful.
+  * [The Worst Engine](https://github.com/Parasik72/TWE): a game engine based on
+    OpenGL.
+  * [Ecsact](https://ecsact.dev/): a language aimed at describing ECS, with a
+    [runtime implementation](https://github.com/ecsact-dev/ecsact_rt_entt) based
+    on `EnTT`.
+  * [AGE (Arc Game Engine)](https://github.com/MohitSethi99/ArcGameEngine): an
+    open-source engine for building 2D & 3D real-time rendering and interactive
+    contents.
+  * [Kengine](https://github.com/phisko/kengine): the _Koala engine_ is a game
+    engine entirely implemented as an entity-component-system.
+  * [Scion2D](https://github.com/dwjclark11/Scion2D): 2D game engine with
+    [YouTube series](https://www.youtube.com/playlist?list=PL3HUvSWOJR7XRDwVVQqqWO-zyyscb8L-v)
+    included.
+  * [EnTT Editor](https://github.com/TheDimin/EnttEditor): an editor for `EnTT`
+    libary that combines its built-in reflection system with `ImGui`.
+  * [Era Game Engine](https://github.com/EldarMuradov/EraGameEngine): a modern
+    ECS-based game engine.
+
+## Articles, videos and blog posts:
 
-* Articles, videos and blog posts:
   * [Some posts](https://skypjack.github.io/tags/#entt) on my personal
     [blog](https://skypjack.github.io/) are about `EnTT`, for those who want to
     know **more** on this project.
@@ -165,6 +247,23 @@ I hope this list can grow much more in the future:
     - ... And so on.
       [Check out](https://www.youtube.com/channel/UCQ-W1KE9EYfdxhL6S4twUNw) the
       _Game Engine Series_ by The Cherno for more videos.
+  * [Game Engine series](https://www.youtube.com/@JADE-iteGames/videos) by
+    [dwjclark11](https://github.com/dwjclark11) (not just `EnTT` but a lot of
+    it):
+    - [Getting into ECS](https://youtu.be/k9CbonLopJU?si=za3Tisyc96_92DWM)
+    - [Creating ECS Wrapper Classes](https://youtu.be/yetyuMJRdbo?si=PJTkmap4Ysqbzb_M)
+    - [Runtime Reflection using EnTT meta](https://youtu.be/GrXV5A07GTY?si=fKdWTj9AOhnhtiXq)
+    - [Adding entt::meta and Sol2 bindings](https://youtu.be/IM55JgxOqFA?si=rsbb4AG_NVh4IUmD)
+      (with [part two](https://youtu.be/-PTt-b1tzRw?si=zPJ4vEluyheMcNgO) too)
+    - ... And so on.
+      [Check it out](https://www.youtube.com/playlist?list=PL3HUvSWOJR7XRDwVVQqqWO-zyyscb8L-v)
+      for more videos.
+  * [Warmonger Dynasty devlog series](https://david-delassus.medium.com/list/warmonger-dynasty-devlogs-f64b71f556de)
+    by [linkdd](https://github.com/linkdd): an interesting walkthrough of
+    developing a game (also) with EnTT.
+  * [Use EnTT When You Need An ECS](https://www.codingwiththomas.com/blog/use-entt-when-you-need-an-ecs)
+    by [Thomas](https://www.codingwiththomas.com/): I couldn't have said it
+    better.
   * [Space Battle: Huge edition](http://victor.madtriangles.com/code%20experiment/2018/06/11/post-ecs-battle-huge.html):
     huge space battle built entirely from scratch.
   * [Space Battle](https://github.com/vblanco20-1/ECS_SpaceBattle): huge space
@@ -187,8 +286,12 @@ I hope this list can grow much more in the future:
     project retrospect by [Eric Hildebrand](https://www.erichildebrand.net/).
   * [EnTT Entity Component System Gaming Library](https://gamefromscratch.com/entt-entity-component-system-gaming-library/):
     `EnTT` on GameFromScratch.com.
+  * [Custom C++ server for UE5](https://youtu.be/fbXZVNCOvjM) optimized for
+    MMO(RPG)s and its [follow-up](https://youtu.be/yGlZeopx2hU) episode about
+    player bots and full external ECS: a series definitely worth looking at.
+
+## Any Other Business:
 
-* Any Other Business:
   * [ArcGIS Runtime SDKs](https://developers.arcgis.com/arcgis-runtime/) by
     [Esri](https://www.esri.com/): they use `EnTT` for the internal ECS and the
     cross platform C++ rendering engine. The SDKs are utilized by a lot of
@@ -230,6 +333,3 @@ I hope this list can grow much more in the future:
   * GitHub contains also
     [many other examples](https://github.com/search?o=desc&q=%22skypjack%2Fentt%22&s=indexed&type=Code)
     of use of `EnTT` from which to take inspiration if interested.
-
-If you know of other resources out there that are about `EnTT`, feel free to
-open an issue or a PR and I'll be glad to add them to this page.

docs/md/locator.md

@@ -7,69 +7,82 @@
 
 * [Introduction](#introduction)
 * [Service locator](#service-locator)
+  * [Opaque handles](#opaque-handles)
 <!--
 @endcond TURN_OFF_DOXYGEN
 -->
 
 # 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.
+Usually, service locators are tightly bound to the services they expose and it's
+hard to define a general purpose solution.<br/>
+This tiny class tries to fill the gap and gets rid of the burden of defining a
+different specific locator for each application.
 
 # 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 is 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.
+
+## Opaque handles
+
+Sometimes it's useful to _transfer_ a copy of a service to another locator. For
+example, when working across boundaries it's common to _share_ a service with a
+dynamically loaded module.<br/>
+Options aren't much in this case. Among these is the possibility of _exporting_
+services and assigning them to a different locator.
+
+This is what the `handle` and `reset` functions are meant for.<br/>
+The former returns an opaque object useful for _exporting_ (or rather, obtaining
+a reference to) a service. The latter also accepts an optional argument to a
+handle which then allows users to reset a service by initializing it with an
+opaque handle:
+
+```cpp
+auto handle = entt::locator<interface>::handle();
+entt::locator<interface>::reset(handle);
+```
+
+It's worth noting that it's possible to get handles for uninitialized services
+and use them with other locators. Of course, all a user will get is to have an
+uninitialized service elsewhere as well.
+
+Note that exporting a service allows users to _share_ the object currently set
+in a locator. Replacing it won't replace the element even where a service has
+been configured with a handle to the previous item.<br/>
+In other words, if an audio service is replaced with a null object to silence an
+application and the original service was shared, this operation won't propagate
+to the other locators. Therefore, a module that share the ownership of the
+original audio service is still able to emit sounds.

docs/md/meta.md

@@ -15,10 +15,12 @@
   * [Template information](#template-information)
   * [Automatic conversions](#automatic-conversions)
   * [Implicitly generated default constructor](#implicitly-generated-default-constructor)
+  * [From void to any](#from-void-to-any)
   * [Policies: the more, the less](#policies-the-more-the-less)
   * [Named constants and enums](#named-constants-and-enums)
   * [Properties and meta objects](#properties-and-meta-objects)
   * [Unregister types](#unregister-types)
+  * [Meta context](#meta-context)
 <!--
 @endcond TURN_OFF_DOXYGEN
 -->
@@ -26,7 +28,7 @@
 # Introduction
 
 Reflection (or rather, its lack) is a trending topic in the C++ world and a tool
-that can unlock a lot of interesting feature in the specific case of `EnTT`. I
+that can unlock a lot of interesting features in the specific case of `EnTT`. I
 looked for a third-party library that met my needs on the subject, but I always
 came across some details that I didn't like: macros, being intrusive, too many
 allocations, and so on.<br/>
@@ -42,22 +44,22 @@ when it comes to working with names and identifiers. It does this by offering an
 API that works with opaque identifiers that may or may not be generated by means
 of a hashed string.<br/>
 This means that users can assign any type of identifier to the meta objects, as
-long as they are numeric. It doesn't matter if they are generated at runtime, at
+long as they're numeric. It doesn't matter if they're generated at runtime, at
 compile-time or with custom functions.
 
 That being said, the examples in the following sections are all based on the
 `hashed_string` class as provided by this library. Therefore, where an
-identifier is required, it's likely that an user defined literal is used as
+identifier is required, it's likely that a user defined literal is used as
 follows:
 
 ```cpp
 auto factory = entt::meta<my_type>().type("reflected_type"_hs);
 ```
 
-For what it's worth, this is likely completely equivalent to:
+For what it's worth, this is completely equivalent to:
 
 ```cpp
-auto factory = entt::meta<my_type>().type(42);
+auto factory = entt::meta<my_type>().type(42u);
 ```
 
 Obviously, human-readable identifiers are more convenient to use and highly
@@ -65,69 +67,61 @@ recommended.
 
 # Reflection in a nutshell
 
-Reflection always starts from real types (users cannot reflect imaginary types
-and it would not make much sense, we wouldn't be talking about reflection
-anymore).<br/>
-To create a meta node, the library provides the `meta` function that accepts a
-type to reflect as a template parameter:
+Reflection always starts from actual C++ types. Users cannot reflect _imaginary_
+types.<br/>
+The `meta` function is where it all starts:
 
 ```cpp
 auto factory = entt::meta<my_type>();
 ```
 
-This isn't enough to _export_ the given type and make it visible though.<br/>
-The returned value is a factory object to use to continue building the meta
-type. In order to make the type _visible_, users can assign it an identifier:
+The returned value is a _factory object_ to use to continue building the meta
+type.
+
+By default, a meta type is associated with the identifier returned by the
+runtime type identification system built-in in `EnTT`.<br/>
+However, it's also possible to assign custom identifiers to meta types:
 
 ```cpp
 auto factory = entt::meta<my_type>().type("reflected_type"_hs);
 ```
 
-Or use the default one, that is, the built-in identifier for the given type:
+Identifiers are used to _retrieve_ meta types at runtime by _name_ other than by
+type.<br/>
+However, users can be interested in adding features to a reflected type so that
+the reflection system can use it correctly under the hood, while they don't want
+to also make the type _searchable_. In this case, it's sufficient not to invoke
+`type`.
 
-```cpp
-auto factory = entt::meta<my_type>().type();
-```
+A factory is such that all its member functions return the factory itself. It's
+generally used to create the following:
 
-Identifiers are important because users can retrieve meta types at runtime by
-searching for them by _name_ other than by type.<br/>
-On the other hand, there are cases in which users can be interested in adding
-features to a reflected type so that the reflection system can use it correctly
-under the hood, but they don't want to also make the type _searchable_. In this
-case, it's sufficient not to invoke `type`.
-
-A factory is such that all its member functions return the factory itself or a
-decorated version of it. This object can be used to add the following:
-
-* _Constructors_. Actual constructors can be assigned to a reflected type by
-  specifying their list of arguments. Free functions (namely, factories) can be
-  used as well, as long as the return type is the expected one. From a client's
-  point of view, nothing changes if a constructor is a free function or an
-  actual constructor.<br/>
-  Use the `ctor` member function for this purpose:
+* _Constructors_. A constructors is assigned to a reflected type by specifying
+  its _list of arguments_. Free functions are also accepted if the return type
+  is the expected one. From a client perspective, nothing changes between a free
+  function or an actual constructor:
 
   ```cpp
   entt::meta<my_type>().ctor<int, char>().ctor<&factory>();
   ```
 
-* _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:
+  Meta default constructors are implicitly generated, if possible.
+
+* _Destructors_. Both free functions and member functions are valid destructors:
 
   ```cpp
   entt::meta<my_type>().dtor<&destroy>();
   ```
 
+  The purpose is to offer the possibility to free up resources that require
+  _special treatment_ before an object is actually destroyed.<br/>
   A function should neither delete nor explicitly invoke the destructor of a
   given instance.
 
-* _Data members_. Both real data members of the underlying type and static and
-  global variables, as well as constants of any kind, can be attached to a meta
-  type. From the point of view of the client, all the variables associated with
-  the reflected type will appear as if they were part of the type itself.<br/>
-  Use the `data` member function for this purpose:
+* _Data members_. Meta data members are actual data members of the underlying
+  type but also static and global variables or constants of any kind. From the
+  point of view of the client, all the variables associated with the reflected
+  type appear as if they were part of the type itself:
 
   ```cpp
   entt::meta<my_type>()
@@ -136,13 +130,11 @@ decorated version of it. This object can be used to add the following:
       .data<&global_variable>("global"_hs);
   ```
 
-  The function requires as an argument the identifier to give to the meta data
-  once created. Users can then access meta data at runtime by searching for them
-  by _name_.<br/>
-  Data members can also be defined by means of a setter and getter pair. Setters
-  and getters can be either free functions, class members or a mix of them, as
-  long as they respect the required signatures. This approach is also convenient
-  to create a read-only variable from a non-const data member:
+  The `data` function requires the identifier to use for the meta data member.
+  Users can then access it by _name_ at runtime.<br/>
+  Data members are also defined by means of a setter and getter pair. These are
+  either free functions, class members or a mix of them. This approach is also
+  convenient to create read-only properties from a non-const data member:
 
   ```cpp
   entt::meta<my_type>().data<nullptr, &my_type::data_member>("member"_hs);
@@ -154,13 +146,10 @@ decorated version of it. This object can be used to add the following:
   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
-  functions can be attached to a meta type. From the point of view of the
-  client, all the functions associated with the reflected type will appear as if
-  they were part of the type itself.<br/>
-  Use the `func` member function for this purpose:
+* _Member functions_. Meta member functions are actual member functions of the
+  underlying type but also plain free functions. From the point of view of the
+  client, all the functions associated with the reflected type appear as if they
+  were part of the type itself:
 
   ```cpp
   entt::meta<my_type>()
@@ -169,40 +158,31 @@ decorated version of it. This object can be used to add the following:
       .func<&free_function>("free"_hs);
   ```
 
-  The function requires as an argument the identifier to give to the meta
-  function once created. Users can then access meta functions at runtime by
-  searching for them by _name_.<br/>
+  The `func` function requires the identifier to use for the meta data function.
+  Users can then access it by _name_ at runtime.<br/>
   Overloading of meta functions is supported. Overloaded functions are resolved
   at runtime by the reflection system according to the types of the arguments.
 
 * _Base classes_. A base class is such that the underlying type is actually
-  derived from it. In this case, the reflection system tracks the relationship
-  and allows for implicit casts at runtime when required.<br/>
-  Use the `base` member function for this purpose:
+  derived from it:
 
   ```cpp
   entt::meta<derived_type>().base<base_type>();
   ```
 
-  From now on, wherever a `base_type` is required, an instance of `derived_type`
-  will also be accepted.
+  The reflection system tracks the relationship and allows for implicit casts at
+  runtime when required. In other terms, wherever a `base_type` is required, an
+  instance of `derived_type` is also accepted.
 
-* _Conversion functions_. Actual types can be converted, this is a fact. Just
-  think of the relationship between a `double` and an `int` to see it. Similar
-  to bases, conversion functions allow users to define conversions that will be
-  implicitly performed by the reflection system when required.<br/>
-  Use the `conv` member function for this purpose:
+* _Conversion functions_. Conversion functions allow users to define conversions
+  that are implicitly performed by the reflection system when required:
 
   ```cpp
   entt::meta<double>().conv<int>();
   ```
 
-That's all, everything users need to create meta types and enjoy the reflection
-system. At first glance it may not seem that much, but users usually learn to
-appreciate it over time.<br/>
-Also, do not forget what these few lines hide under the hood: a built-in,
-non-intrusive and macro-free system for reflection in C++. Features that are
-definitely worth the price, at least for me.
+This is everything users need to create meta types. Refer to the inline
+documentation for further details.
 
 ## Any to the rescue
 
@@ -215,13 +195,13 @@ The API is very similar to that of the `any` type. The class `meta_any` _wraps_
 many of the feature to infer a meta node, before forwarding some or all of the
 arguments to the underlying storage.<br/>
 Among the few relevant differences, `meta_any` adds support for containers and
-pointer-like types (see the following sections for more details), while `any`
-does not.<br/>
-Similar to `any`, this class can also be used to create _aliases_ for unmanaged
+pointer-like types, while `any` doesn't.<br/>
+Similar to `any`, this class is also used to create _aliases_ for unmanaged
 objects either with `forward_as_meta` or using the `std::in_place_type<T &>`
 disambiguation tag, as well as from an existing object by means of the `as_ref`
-member function. However, unlike `any`, `meta_any` treats an empty instance and
-one initialized with `void` differently:
+member function.<br/>
+Unlike `any` instead, `meta_any` treats an empty instance and one initialized
+with `void` differently:
 
 ```cpp
 entt::meta_any empty{};
@@ -230,21 +210,19 @@ entt::meta_any other{std::in_place_type<void>};
 
 While `any` considers both as empty, `meta_any` treats objects initialized with
 `void` as if they were _valid_ ones. This allows to differentiate between failed
-function calls and function calls that are successful but return nothing.<br/>
+function calls and function calls that are successful but return nothing.
+
 Finally, the member functions `try_cast`, `cast` and `allow_cast` are used to
 cast the underlying object to a given type (either a reference or a value type)
 or to _convert_ a `meta_any` in such a way that a cast becomes viable for the
-resulting object. There is in fact no `any_cast` equivalent for `meta_any`.
+resulting object.<br/>
+There is in fact no `any_cast` equivalent for `meta_any`.
 
 ## Enjoy the runtime
 
-Once the web of reflected types has been constructed, it's a matter of using it
-at runtime where required.<br/>
-All this has the great merit that the reflection system stands in fact as a
-non-intrusive tool for the runtime, unlike the vast majority of the things
-offered by this library and closely linked to the compile-time.
-
-To search for a reflected type there are a few options:
+Once the web of reflected types is constructed, it's a matter of using it at
+runtime where required.<br/>
+There are a few options to search for a reflected type:
 
 ```cpp
 // direct access to a reflected type
@@ -257,25 +235,23 @@ auto by_id = entt::resolve("reflected_type"_hs);
 auto by_type_id = entt::resolve(entt::type_id<my_type>());
 ```
 
-There exits also an overload of the `resolve` function to use to iterate all the
-reflected types at once. It returns an iterable object that can be used in a
-range-for loop:
+There exists also an overload of the `resolve` function to use to iterate all
+reflected types at once. It returns an iterable object to be used in a range-for
+loop:
 
 ```cpp
-for(auto type: entt::resolve()) {
+for(auto &&[id, type]: entt::resolve()) {
     // ...
 }
 ```
 
-In all cases, the returned value is an instance of `meta_type`. This kind of
-objects offer an API to know their _runtime identifiers_, to iterate all the
-meta objects associated with them and even to build instances of the underlying
-type.<br/>
-Refer to the inline documentation for all the details.
+In all cases, the returned value is an instance of `meta_type` (possibly with
+its id). This kind of objects offer an API to know their _runtime identifiers_,
+to iterate all the meta objects associated with them and even to build instances
+of the underlying type.<br/>
+Meta data members and functions are accessed by name:
 
-The meta objects that compose a meta type are accessed in the following ways:
-
-* _Meta data_. They are accessed by _name_:
+* Meta data members:
 
   ```cpp
   auto data = entt::resolve<my_type>().data("member"_hs);
@@ -287,7 +263,7 @@ The meta objects that compose a meta type are accessed in the following ways:
   know if it's a const or a static one), to get the meta type of the variable
   and to set or get the contained value.
 
-* _Meta functions_. They are accessed by _name_:
+* Meta function members:
 
   ```cpp
   auto func = entt::resolve<my_type>().func("member"_hs);
@@ -298,20 +274,11 @@ The meta objects that compose a meta type are accessed in the following ways:
   A meta function object offers an API to query the underlying type (for
   example, to know if it's a const or a static function), to know the number of
   arguments, the meta return type and the meta types of the parameters. In
-  addition, a meta function object can be used to invoke the underlying function
-  and then get the return value in the form of a `meta_any` object.
+  addition, a meta function object is used to invoke the underlying function and
+  then get the return value in the form of a `meta_any` object.
 
-* _Meta bases_. They are accessed through the _name_ of the base types:
-
-  ```cpp
-  auto base = entt::resolve<derived_type>().base("base"_hs);
-  ```
-
-  The returned type is `meta_type` and may be invalid if there is no meta base
-  object associated with the given identifier.
-
-All the objects thus obtained as well as the meta types can be explicitly
-converted to a boolean value to check if they are valid:
+All the meta objects thus obtained as well as the meta types explicitly convert
+to a boolean value to check for validity:
 
 ```cpp
 if(auto func = entt::resolve<my_type>().func("member"_hs); func) {
@@ -319,35 +286,33 @@ if(auto func = entt::resolve<my_type>().func("member"_hs); func) {
 }
 ```
 
-Furthermore, all them are also returned by specific overloads that provide the
-caller with iterable ranges of top-level elements. As an example:
+Furthermore, all them (and a few more, like meta basis) are returned by a bunch
+of overloads that provide the caller with iterable ranges of top-level elements.
+As an example:
 
 ```cpp
-for(auto data: entt::resolve<my_type>().data()) {
+for(auto &&[id, type]: entt::resolve<my_type>().base()) {
     // ...
 }
 ```
 
-A meta type can be used to `construct` actual instances of the underlying
+Meta type are also used to `construct` actual instances of the underlying
 type.<br/>
 In particular, the `construct` member function accepts a variable number of
 arguments and searches for a match. It then returns a `meta_any` object that may
-or may not be initialized, depending on whether a suitable constructor has been
-found or not.
+or may not be initialized, depending on whether a suitable constructor was found
+or not.
 
 There is no object that wraps the destructor of a meta type nor a `destroy`
 member function in its API. Destructors are invoked implicitly by `meta_any`
 behind the scenes and users have not to deal with them explicitly. Furthermore,
-they have no name, cannot be searched and wouldn't have member functions to
-expose anyway.<br/>
-Similarly, conversion functions aren't directly accessible. They are used
+they've no name, cannot be searched and wouldn't have member functions to expose
+anyway.<br/>
+Similarly, conversion functions aren't directly accessible. They're used
 internally by `meta_any` and the meta objects when needed.
 
-Meta types and meta objects in general contain much more than what is said: a
-plethora of functions in addition to those listed whose purposes and uses go
-unfortunately beyond the scope of this document.<br/>
-I invite anyone interested in the subject to look at the code, experiment and
-read the inline documentation to get the best out of this powerful tool.
+Meta types and meta objects in general contain much more than what was said.
+Refer to the inline documentation for further details.
 
 ## Container support
 
@@ -358,13 +323,15 @@ meta system in many cases.
 
 To make a container be recognized as such by the meta system, users are required
 to provide specializations for either the `meta_sequence_container_traits` class
-or the `meta_associative_container_traits` class, according with the actual type
+or the `meta_associative_container_traits` class, according to the actual _type_
 of the container.<br/>
 `EnTT` already exports the specializations for some common classes. In
 particular:
 
-* `std::vector` and `std::array` are exported as _sequence containers_.
-* `std::map`, `std::set` and their unordered counterparts are exported as
+* `std::vector`, `std::array`, `std::deque` and `std::list` (but not
+  `std::forward_list`) are supported as _sequence containers_.
+
+* `std::map`, `std::set` and their unordered counterparts are supported as
   _associative containers_.
 
 It's important to include the header file `container.hpp` to make these
@@ -393,11 +360,10 @@ if(any.type().is_sequence_container()) {
 
 The method to use to get a proxy object for associative containers is
 `as_associative_container` instead.<br/>
-It goes without saying that it's not necessary to perform a double check.
-Instead, it's sufficient to query the meta type or verify that the proxy object
-is valid. In fact, proxies are contextually convertible to bool to know if they
-are valid. For example, invalid proxies are returned when the wrapped object
-isn't a container.<br/>
+It's not necessary to perform a double check actually. Instead, it's enough to
+query the meta type or verify that the proxy object is valid. In fact, proxies
+are contextually convertible to bool to check for validity. For example, invalid
+proxies are returned when the wrapped object isn't a container.<br/>
 In all cases, users aren't expected to _reflect_ containers explicitly. It's
 sufficient to assign a container for which a specialization of the traits
 classes exists to a `meta_any` object to be able to get its proxy object.
@@ -409,32 +375,22 @@ to case. In particular:
 * The `value_type` member function returns the meta type of the elements.
 
 * The `size` member function returns the number of elements in the container as
-  an unsigned integer value:
-
-  ```cpp
-  const auto size = view.size();
-  ```
+  an unsigned integer value.
 
 * The `resize` member function allows to resize the wrapped container and
-  returns true in case of succes:
-
-  ```cpp
-  const bool ok = view.resize(3u);
-  ```
-
+  returns true in case of success.<br/>
   For example, it's not possible to resize fixed size containers.
 
 * The `clear` member function allows to clear the wrapped container and returns
-  true in case of success:
-
-  ```cpp
-  const bool ok = view.clear();
-  ```
-
+  true in case of success.<br/>
   For example, it's not possible to clear fixed size containers.
 
-* The `begin` and `end` member functions return opaque iterators that can be
-  used to iterate the container directly:
+* The `reserve` member function allows to increase the capacity of the wrapped
+  container and returns true in case of success.<br/>
+  For example, it's not possible to increase capacity of fixed size containers.
+
+* The `begin` and `end` member functions return opaque iterators that is used to
+  iterate the container directly:
 
   ```cpp
   for(entt::meta_any element: view) {
@@ -448,7 +404,7 @@ to case. In particular:
   All meta iterators are input iterators and don't offer an indirection operator
   on purpose.
 
-* The `insert` member function can be used to add elements to the container. It
+* The `insert` member function is used to add elements to the container. It
   accepts a meta iterator and the element to insert:
 
   ```cpp
@@ -458,15 +414,15 @@ to case. In particular:
   ```
 
   This function returns a meta iterator pointing to the inserted element and a
-  boolean value to indicate whether the operation was successful or not. Note
-  that a call to `insert` may silently fail in case of fixed size containers or
-  whether the arguments aren't at least convertible to the required types.<br/>
-  Since the meta iterators are contextually convertible to bool, users can rely
-  on them to know if the operation has failed on the actual container or
-  upstream, for example for an argument conversion problem.
+  boolean value to indicate whether the operation was successful or not. A call
+  to `insert` may silently fail in case of fixed size containers or whether the
+  arguments aren't at least convertible to the required types.<br/>
+  Since meta iterators are contextually convertible to bool, users can rely on
+  them to know if the operation failed on the actual container or upstream, for
+  example due to an argument conversion problem.
 
-* The `erase` member function can be used to remove elements from the container.
-  It accepts a meta iterator to the element to remove:
+* The `erase` member function is used to remove elements from the container. It
+  accepts a meta iterator to the element to remove:
 
   ```cpp
   auto first = view.begin();
@@ -475,11 +431,11 @@ to case. In particular:
   ```
 
   This function returns a meta iterator following the last removed element and a
-  boolean value to indicate whether the operation was successful or not. Note
-  that a call to `erase` may silently fail in case of fixed size containers.
+  boolean value to indicate whether the operation was successful or not. A call
+  to `erase` may silently fail in case of fixed size containers.
 
-* The `operator[]` can be used to access elements in a container. It accepts a
-  single argument, that is the position of the element to return:
+* The `operator[]` is used to access container elements. It accepts a single
+  argument, the position of the element to return:
 
   ```cpp
   for(std::size_t pos{}, last = view.size(); pos < last; ++pos) {
@@ -489,8 +445,12 @@ to case. In particular:
   ```
 
   The function returns instances of `meta_any` that directly refer to the actual
-  elements. Modifying the returned object will then directly modify the element
-  inside the container.
+  elements. Modifying the returned object directly modifies the element inside
+  the container.<br/>
+  Depending on the underlying sequence container, this operation may not be as
+  efficient. For example, in the case of an `std::list`, a positional access
+  translates to a linear visit of the list itself (probably not what the user
+  expects).
 
 Similarly, also the interface of the `meta_associative_container` proxy object
 is the same for all types of associative containers. However, there are some
@@ -511,21 +471,17 @@ differences in behavior in the case of key-only containers. In particular:
   `std::map<int, char>`.
 
 * The `size` member function returns the number of elements in the container as
-  an unsigned integer value:
-
-  ```cpp
-  const auto size = view.size();
-  ```
+  an unsigned integer value.
 
 * The `clear` member function allows to clear the wrapped container and returns
-  true in case of success:
+  true in case of success.
 
-  ```cpp
-  const bool ok = view.clear();
-  ```
+* The `reserve` member function allows to increase the capacity of the wrapped
+  container and returns true in case of success.<br/>
+  For example, it's not possible to increase capacity of standard maps.
 
-* The `begin` and `end` member functions return opaque iterators that can be
-  used to iterate the container directly:
+* The `begin` and `end` member functions return opaque iterators that are used
+  to iterate the container directly:
 
   ```cpp
   for(std::pair<entt::meta_any, entt::meta_any> element: view) {
@@ -542,11 +498,11 @@ differences in behavior in the case of key-only containers. In particular:
 
   While the accessed key is usually constant in the associative containers and
   is therefore returned by copy, the value (if any) is wrapped by an instance of
-  `meta_any` that directly refers to the actual element. Modifying it will then
-  directly modify the element inside the container.
+  `meta_any` that directly refers to the actual element. Modifying it directly
+  modifies the element inside the container.
 
-* The `insert` member function can be used to add elements to the container. It
-  accepts two arguments, respectively the key and the value to be inserted:
+* The `insert` member function is used to add elements to a container. It gets
+  two arguments, respectively the key and the value to insert:
 
   ```cpp
   auto last = view.end();
@@ -555,39 +511,39 @@ differences in behavior in the case of key-only containers. In particular:
   ```
 
   This function returns a boolean value to indicate whether the operation was
-  successful or not. Note that a call to `insert` may fail when the arguments
-  aren't at least convertible to the required types.
+  successful or not. A call to `insert` may fail when the arguments aren't at
+  least convertible to the required types.
 
-* The `erase` member function can be used to remove elements from the container.
-  It accepts a single argument, that is the key to be removed:
+* The `erase` member function is used to remove elements from a container. It
+  gets a single argument, the key to remove:
 
   ```cpp
   view.erase(42);
   ```
 
   This function returns a boolean value to indicate whether the operation was
-  successful or not. Note that a call to `erase` may fail when the argument
-  isn't at least convertible to the required type.
+  successful or not. A call to `erase` may fail when the argument isn't at least
+  convertible to the required type.
 
-* The `operator[]` can be used to access elements in a container. It accepts a
-  single argument, that is the key of the element to return:
+* The `operator[]` is used to access elements in a container. It gets a single
+  argument, the key of the element to return:
 
   ```cpp
   entt::meta_any value = view[42];
   ```
 
   The function returns instances of `meta_any` that directly refer to the actual
-  elements. Modifying the returned object will then directly modify the element
-  inside the container.
+  elements. Modifying the returned object directly modifies the element inside
+  the container.
 
 Container support is minimal but likely sufficient to satisfy all needs.
 
 ## Pointer-like types
 
-As with containers, it's also possible to communicate to the meta system which
-types to consider _pointers_. This will allow to dereference instances of
-`meta_any`, thus obtaining light _references_ to the pointed objects that are
-also correctly associated with their meta types.<br/>
+As with containers, it's also possible to _tell_ to the meta system which types
+are _pointers_. This makes it possible to dereference instances of `meta_any`,
+thus obtaining light _references_ to pointed objects that are also correctly
+associated with their meta types.<br/>
 To make the meta system recognize a type as _pointer-like_, users can specialize
 the `is_meta_pointer_like` class. `EnTT` already exports the specializations for
 some common classes. In particular:
@@ -617,13 +573,12 @@ if(any.type().is_pointer_like()) {
 }
 ```
 
-Of course, it's not necessary to perform a double check. Instead, it's enough to
-query the meta type or verify that the returned object is valid. For example,
-invalid instances are returned when the wrapped object isn't a pointer-like
-type.<br/>
-Note that dereferencing a pointer-like object returns an instance of `meta_any`
-which refers to the pointed object and allows users to modify it directly
-(unless the returned element is const, of course).
+It's not necessary to perform a double check. Instead, it's enough to query the
+meta type or verify that the returned object is valid. For example, invalid
+instances are returned when the wrapped object isn't a pointer-like type.<br/>
+Dereferencing a pointer-like object returns an instance of `meta_any` which
+_refers_ to the pointed object. Modifying it means modifying the pointed object
+directly (unless the returned element is const).
 
 In general, _dereferencing_ a pointer-like type boils down to a `*ptr`. However,
 `EnTT` also supports classes that don't offer an `operator*`. In particular:
@@ -651,15 +606,15 @@ In general, _dereferencing_ a pointer-like type boils down to a `*ptr`. However,
   };
   ```
 
-In all other cases, that is, when dereferencing a pointer works as expected and
-regardless of the pointed type, no user intervention is required.
+In all other cases and when dereferencing a pointer works as expected regardless
+of the pointed type, no user intervention is required.
 
 ## Template information
 
-Meta types also provide a minimal set of information about the nature of the
+Meta types also provide a minimal set of information about the _nature_ of the
 original type in case it's a class template.<br/>
 By default, this works out of the box and requires no user action. However, it's
-important to include the header file `template.hpp` to make these information
+important to include the header file `template.hpp` to make this information
 available to the compiler when needed.
 
 Meta template information are easily found:
@@ -691,9 +646,9 @@ template<typename Ret, typename... Args>
 struct function_type<Ret(Args...)> {};
 ```
 
-In this case, rather than the function type, the user might want the return type
-and unpacked arguments as if they were different template parameters for the
-original class template.<br/>
+In this case, rather than the function type, it might be useful to provide the
+return type and unpacked arguments as if they were different template parameters
+for the original class template.<br/>
 To achieve this, users must enter the library internals and provide their own
 specialization for the class template `entt::meta_template_traits`, such as:
 
@@ -707,8 +662,8 @@ struct entt::meta_template_traits<function_type<Ret(Args...)>> {
 
 The reflection system doesn't verify the accuracy of the information nor infer a
 correspondence between real types and meta types.<br/>
-Therefore, the specialization will be used as is and the information it contains
-will be associated with the appropriate type when required.
+Therefore, the specialization is used as is and the information it contains is
+associated with the appropriate type when required.
 
 ## Automatic conversions
 
@@ -726,7 +681,7 @@ entt::meta<int>()
 ```
 
 Repeated for each type eligible to undergo this type of conversions. This is
-both error prone and repetitive.<br/>
+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:
@@ -755,34 +710,51 @@ 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.
+This makes working with arithmetic types and scoped or unscoped enums as easy as
+it is in C++.<br/>
+It's still possible to set up conversion functions manually and these are always
+preferred over the automatic ones.
 
 ## Implicitly generated default constructor
 
-In many cases, it's useful to be able to create objects of default constructible
-types through the reflection system, while not having to explicitly register the
-meta type or the default constructor.<br/>
+Creating objects of default constructible types through the reflection system
+while not having to explicitly register the meta type or its default constructor
+is also possible.<br/>
 For example, in the case of primitive types like `int` or `char`, but not just
 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/>
+For default constructible types only, default constructors are automatically
+defined and associated with their meta types, whether they are explicitly or
+implicitly generated.<br/>
 Therefore, this is all is needed to construct an integer from its meta type:
 
 ```cpp
 entt::resolve<int>().construct();
 ```
 
-Where the meta type can be for example the one returned from a meta container,
+Where the meta type is for example the one returned from a meta container,
 useful for building keys without knowing or having to register the actual types.
 
 In all cases, when users register default constructors, they are preferred both
 during searches and when the `construct` member function is invoked.
 
+## From void to any
+
+Sometimes all a user has is an opaque pointer to an object of a known meta type.
+It would be handy in this case to be able to construct a `meta_any` element from
+it.<br/>
+For this purpose, the `meta_type` class offers a `from_void` member function
+designed to convert an opaque pointer into a `meta_any`:
+
+```cpp
+entt::meta_any any = entt::resolve(id).from_void(element);
+```
+
+Unfortunately, it's not possible to do a check on the actual type. Therefore,
+this call can be considered as a _static cast_ with all its _problems_.<br/>
+On the other hand, the ability to construct a `meta_any` from an opaque pointer
+opens the door to some pretty interesting uses that are worth exploring.
+
 ## Policies: the more, the less
 
 Policies are a kind of compile-time directives that can be used when registering
@@ -811,17 +783,17 @@ There are a few alternatives available at the moment:
   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.
-  In the second case, instead, the property will not be accessible for reading.
+  If the use with functions is obvious, perhaps less so is use with constructors
+  and data members. In the first case, the returned wrapper is always empty even
+  though the constructor is still invoked. In the second case, the property
+  isn't accessible for reading instead.
 
 * The _as-ref_ and _as-cref_ policies, associated with the types
   `entt::as_ref_t` and `entt::as_cref_t`.<br/>
   They allow to build wrappers that act as references to unmanaged objects.
   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:
+  requested makes 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);
@@ -839,21 +811,16 @@ obvious corner cases that can in turn be solved with the use of policies.
 
 ## Named constants and enums
 
-A special mention should be made for constant values and enums. It wouldn't be
-necessary, but it will help distracted readers.
-
-As mentioned, the `data` member function can be used to reflect constants of any
-type among the other things.<br/>
-This allows users to create meta types for enums that will work exactly like any
-other meta type built from a class. Similarly, arithmetic types can be enriched
+As mentioned, the `data` member function is used to reflect constants of any
+type.<br/>
+This allows users to create meta types for enums that work exactly like any
+other meta type built from a class. Similarly, arithmetic types are _enriched_
 with constants of special meaning where required.<br/>
-Personally, I find it very useful not to export what is the difference between
-enums and classes in C++ directly in the space of the reflected types.
+All values thus exported appear to users as if they were constant data members
+of the reflected types. This avoids the need to _export_ what is the difference
+between enums and classes in C++ directly in the space of the reflected types.
 
-All the values thus exported will appear to users as if they were constant data
-members of the reflected types.
-
-Exporting constant values or elements from an enum is as simple as ever:
+Exposing constant values or elements from an enum is quite simple:
 
 ```cpp
 entt::meta<my_enum>()
@@ -863,68 +830,40 @@ entt::meta<my_enum>()
 entt::meta<int>().data<2048>("max_int"_hs);
 ```
 
-It goes without saying that accessing them is trivial as well. It's a matter of
-doing the following, as with any other data member of a meta type:
+Accessing them is trivial as well. It's a matter of doing the following, as with
+any other data member of a meta type:
 
 ```cpp
 auto value = entt::resolve<my_enum>().data("a_value"_hs).get({}).cast<my_enum>();
 auto max = entt::resolve<int>().data("max_int"_hs).get({}).cast<int>();
 ```
 
-As a side note, remember that all this happens behind the scenes without any
-allocation because of the small object optimization performed by the `meta_any`
-class.
+All this happens behind the scenes without any allocation because of the small
+object optimization performed by the `meta_any` class.
 
 ## Properties and meta objects
 
 Sometimes (for example, when it comes to creating an editor) it might be useful
 to attach properties to the meta objects created. Fortunately, this is possible
-for most of them.<br/>
-For the meta objects that support properties, the member functions of the
-factory used for registering them will return a decorated version of the factory
-itself. The latter can be used to attach properties to the last created meta
-object.<br/>
-Apparently, it's more difficult to say than to do:
+for most of them:
 
 ```cpp
 entt::meta<my_type>().type("reflected_type"_hs).prop("tooltip"_hs, "message");
 ```
 
-Properties are always in the key/value form. There are no restrictions on the
-type of the key or value, as long as they are copy constructible objects.<br/>
-Multiple formats are supported when it comes to defining a property:
+Properties are always in the key/value form. The key is a numeric identifier,
+mostly similar to the identifier used to register meta objects. There are no
+restrictions on the type of the value instead, as long as it's movable.<br/>
+Key only properties are also supported out of the box:
 
-* Properties as key/value pairs:
+```cpp
+entt::meta<my_type>().type("reflected_type"_hs).prop(my_enum::key_only);
+```
 
-  ```cpp
-  entt::meta<my_type>().type("reflected_type"_hs).prop("tooltip"_hs, "message");
-  ```
-
-* Properties as `std::pair`s:
-
-  ```cpp
-  entt::meta<my_type>().type("reflected_type"_hs).prop(std::make_pair("tooltip"_hs, "message"));
-  ```
-
-* Key only properties:
-
-  ```cpp
-  entt::meta<my_type>().type("reflected_type"_hs).prop(my_enum::key_only);
-  ```
-
-* Properties as `std::tuple`s:
-
-  ```cpp
-  entt::meta<my_type>().type("reflected_type"_hs).prop(std::make_tuple(std::make_pair("tooltip"_hs, "message"), my_enum::key_only));
-  ```
-
-  A tuple contains one or more properties. All of them are treated individually.
-
-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.
+To attach multiple properties to a meta object, just invoke `prop` more than
+once.<br/>
+It's also possible to call `prop` at different times, as long as the factory is
+reset to the meta object of interest.
 
 The meta objects for which properties are supported are currently meta types,
 meta data and meta functions.<br/>
@@ -933,7 +872,7 @@ properties at once or to search a specific property by key:
 
 ```cpp
 // iterate all properties of a meta type
-for(auto prop: entt::resolve<my_type>().prop()) {
+for(auto &&[id, prop]: entt::resolve<my_type>().prop()) {
     // ...
 }
 
@@ -942,37 +881,89 @@ auto prop = entt::resolve<my_type>().prop("tooltip"_hs);
 ```
 
 Meta properties are objects having a fairly poor interface, all in all. They
-only provide the `key` and the `value` member functions to be used to retrieve
-the key and the value contained in the form of `meta_any` objects, respectively.
+only provide the `value` member function to retrieve the contained value in the
+form of a `meta_any` object.
 
 ## Unregister types
 
-A type registered with the reflection system can also be unregistered. This
+A type registered with the reflection system can also be _unregistered_. This
 means unregistering all its data members, member functions, conversion functions
 and so on. However, base classes aren't unregistered as well, since they don't
-necessarily depend on it. Similarly, implicitly generated types (as an example,
-the meta types implicitly generated for function parameters when needed) aren't
-unregistered.<br/>
+necessarily depend on it.<br/>
 Roughly speaking, unregistering a type means disconnecting all associated meta
-objects from it and making its identifier no longer visible. The underlying node
-will remain available though, as if it were implicitly generated:
+objects from it and making its identifier no longer available:
 
 ```cpp
 entt::meta_reset<my_type>();
 ```
 
-It's also possible to reset types by their unique identifiers if required:
+It's also possible to reset types by their unique identifiers:
 
 ```cpp
 entt::meta_reset("my_type"_hs);
 ```
 
 Finally, there exists a non-template overload of the `meta_reset` function that
-doesn't accept argument and resets all searchable types (that is, all types that
-were assigned an unique identifier):
+doesn't accept arguments and resets all meta types at once:
 
 ```cpp
 entt::meta_reset();
 ```
 
-All types can be re-registered later with a completely different name and form.
+A type can be re-registered later with a completely different name and form.
+
+## Meta context
+
+All meta types and their parts are created at runtime and stored in a default
+_context_. This is obtained via a service locator as:
+
+```cpp
+auto &&context = entt::locator<entt::meta_context>::value_or();
+```
+
+By itself, a context is an opaque object that the user cannot do much with.
+However, users can replace an existing context with another at any time:
+
+```cpp
+entt::meta_context other{};
+auto &&context = entt::locator<entt::meta_context>::value_or();
+std::swap(context, other);
+```
+
+This is useful for testing purposes or to define multiple context objects with
+different meta type to use as appropriate.
+
+If _replacing_ the default context isn't enough, `EnTT` also offers the ability
+to use multiple and externally managed contexts with the runtime reflection
+system.<br/>
+For example, to create new meta types within a context other than the default
+one, simply pass it as an argument to the `meta` call:
+
+```cpp
+entt::meta_ctx context{};
+auto factory = entt::meta<my_type>(context).type("reflected_type"_hs);
+```
+
+By doing so, the new meta type isn't available in the default context but is
+usable by passing around the new context when needed, such as when creating a
+new `meta_any` object:
+
+```cpp
+entt::meta_any any{context, std::in_place_type<my_type>};
+```
+
+Similarly, to search for meta types in a context other than the default one,
+it's necessary to pass it to the `resolve` function:
+
+```cpp
+entt::meta_type type = entt::resolve(context, "reflected_type"_hs)
+```
+
+More generally, when using externally managed contexts, it's always required to
+provide the system with the context to use, at least at the _entry point_.<br/>
+For example, once the `meta_type` instant is obtained, it's no longer necessary
+to pass the context around as the meta type takes the information with it and
+eventually propagates it to all its parts.<br/>
+On the other hand, it's necessary to instruct the library on where meta types
+are to be fetched when `meta_any`s and `meta_handle`s are constructed, a factory
+created or a meta type resolved.

docs/md/poly.md

@@ -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)
@@ -24,19 +24,18 @@ 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/>
-This is, among others, one of the advantages of static polymorphism in general
+The library allows to define _concepts_ as interfaces to fulfill with concrete
+classes without having to inherit from a common base.<br/>
+Among others, this is 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/>
-What users get is an object that can be passed around as such and not through a
-reference or a pointer, as happens when it comes to working with dynamic
-polymorphism.
+The result is an object to pass around as such and not through a reference or a
+pointer, as it happens when it comes to working with dynamic polymorphism.
 
 Since the `poly` class template makes use of `entt::any` internally, it also
-supports most of its feature. Among the most important, the possibility to
-create aliases to existing and thus unmanaged objects. This allows users to
-exploit the static polymorphism while maintaining ownership of objects.<br/>
+supports most of its feature. For example, the possibility to create aliases to
+existing and thus unmanaged objects. This allows users to exploit the static
+polymorphism while maintaining ownership of objects.<br/>
 Likewise, the `poly` class template also benefits from the small buffer
 optimization offered by the `entt::any` class and therefore minimizes the number
 of allocations, avoiding them altogether where possible.
@@ -44,7 +43,7 @@ of allocations, avoiding them altogether where possible.
 ## Other libraries
 
 There are some very interesting libraries regarding static polymorphism.<br/>
-Among all, the two that I prefer are:
+The ones that I like more are:
 
 * [`dyno`](https://github.com/ldionne/dyno): runtime polymorphism done right.
 * [`Poly`](https://github.com/facebook/folly/blob/master/folly/docs/Poly.md):
@@ -69,18 +68,18 @@ use the terminology introduced by Eric Niebler) is to define a _concept_ that
 types will have to adhere to.<br/>
 For this purpose, the library offers a single class that supports both deduced
 and fully defined interfaces. Although having interfaces deduced automatically
-is convenient and allows users to write less code in most cases, this has some
+is convenient and allows users to write less code in most cases, it has some
 limitations and it's therefore useful to be able to get around the deduction by
 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/>
+Once the interface is defined, a generic implementation is needed to fulfill the
+concept itself.<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.
 
 ## Deduced interface
 
-This is how a concept with a deduced interface is introduced:
+This is how a concept with a deduced interface is defined:
 
 ```cpp
 struct Drawable: entt::type_list<> {
@@ -94,7 +93,7 @@ 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
@@ -108,12 +107,12 @@ struct Drawable: entt::type_list<> {
 };
 ```
 
-In this case, all parameters must be passed to `invoke` after the reference to
+In this case, all parameters are passed to `invoke` after the reference to
 `this` and the return value is whatever the internal call returns.<br/>
 As for `invoke`, this is a name that is injected into the _concept_ through
 `Base`, from which one must necessarily inherit. Since it's also a dependent
 name, the `this-> template` form is unfortunately necessary due to the rules of
-the language. However, there exists also an alternative that goes through an
+the language. However, there also exists an alternative that goes through an
 external call:
 
 ```cpp
@@ -165,12 +164,12 @@ struct Drawable: entt::type_list<bool(int) const> {
 
 Why should a user fully define a concept if the function types are the same as
 the deduced ones?<br>
-Because, in fact, this is exactly the limitation that can be worked around by
-manually defining the static virtual table.
+In fact, this is the limitation that can be worked around by manually defining
+the static virtual table.
 
 When things are deduced, there is an implicit constraint.<br/>
 If the concept exposes a member function called `draw` with function type
-`void()`, a concept can be satisfied:
+`void()`, a concept is satisfied:
 
 * Either by a class that exposes a member function with the same name and the
   same signature.
@@ -179,7 +178,7 @@ If the concept exposes a member function called `draw` with function type
   interface itself.
 
 In other words, it's not possible to make use of functions not belonging to the
-interface, even if they are present in the types that fulfill the concept.<br/>
+interface, even if they're part of the types that fulfill the concept.<br/>
 Similarly, it's not possible to deduce a function in the static virtual table
 with a function type different from that of the associated member function in
 the interface itself.
@@ -187,7 +186,7 @@ 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:
 
@@ -200,9 +199,9 @@ 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:
+In this case, it's stated that the `draw` method of a generic type is enough to
+satisfy the requirements of the `Drawable` concept.<br/>
+Both member functions and free functions are supported to fulfill concepts:
 
 ```cpp
 template<typename Type>
@@ -251,15 +250,15 @@ struct DrawableAndErasable: entt::type_list<> {
 ```
 
 The static virtual table is empty and must remain so.<br/>
-On the other hand, `type` no longer inherits from `Base` and instead forwards
+On the other hand, `type` no longer inherits from `Base`. Instead, it forwards
 its template parameter to the type exposed by the _base class_. Internally, the
-size of the static virtual table of the base class is used as an offset for the
-local indexes.<br/>
+_size_ of the static virtual table of the base class is used as an offset for
+the local indexes.<br/>
 Finally, by means of the `value_list_cat_t` utility, the implementation consists
 in appending the new functions to the previous list.
 
-As for a defined concept instead, also the list of types must be extended, in a
-similar way to what is shown for the implementation of the above concept.<br/>
+As for a defined concept instead, the list of types is _extended_ in a similar
+way to what is shown for the implementation of the above concept.<br/>
 To do this, it's useful to declare a function that allows to convert a _concept_
 into its underlying `type_list` object:
 
@@ -268,8 +267,8 @@ template<typename... Type>
 entt::type_list<Type...> as_type_list(const entt::type_list<Type...> &);
 ```
 
-The definition isn't strictly required, since the function will only be used
-through a `decltype` as it follows:
+The definition isn't strictly required, since the function is only used through
+a `decltype` as it follows:
 
 ```cpp
 struct DrawableAndErasable: entt::type_list_cat_t<
@@ -286,9 +285,8 @@ Everything else is the same as already shown instead.
 
 # Static polymorphism in the wild
 
-Once the _concept_ and implementation have been introduced, it will be possible
-to use the `poly` class template to contain instances that meet the
-requirements:
+Once the _concept_ and implementation are defined, it's possible to use the
+`poly` class template to _wrap_ instances that meet the requirements:
 
 ```cpp
 using drawable = entt::poly<Drawable>;
@@ -310,9 +308,9 @@ instance = square{};
 instance->draw();
 ```
 
-The `poly` class template offers a wide range of constructors, from the default
-one (which will return an uninitialized `poly` object) to the copy and move
-constructors, as well as the ability to create objects in-place.<br/>
+This class offers a wide range of constructors, from the default one (which
+returns an uninitialized `poly` object) to the copy and move constructors, as
+well as the ability to create objects in-place.<br/>
 Among others, there is also a constructor that allows users to wrap unmanaged
 objects in a `poly` instance (either const or non-const ones):
 
@@ -329,14 +327,14 @@ drawable other = instance.as_ref();
 ```
 
 In both cases, although the interface of the `poly` object doesn't change, it
-won't construct any element or take care of destroying the referenced objects.
+doesn't construct any element or take care of destroying the referenced objects.
 
 Note also how the underlying concept is accessed via a call to `operator->` and
 not directly as `instance.draw()`.<br/>
 This allows users to decouple the API of the wrapper from that of the concept.
-Therefore, where `instance.data()` will invoke the `data` member function of the
-poly object, `instance->data()` will map directly to the functionality exposed
-by the underlying concept.
+Therefore, where `instance.data()` invokes the `data` member function of the
+poly object, `instance->data()` maps directly to the functionality exposed by
+the underlying concept.
 
 # Storage size and alignment requirement
 
@@ -351,9 +349,9 @@ entt::basic_poly<Drawable, sizeof(double[4]), alignof(double[4])>
 
 The default size is `sizeof(double[2])`, which seems like a good compromise
 between a buffer that is too large and one unable to hold anything larger than
-an integer. The alignment requirement is optional instead and by default such
-that it's the most stringent (the largest) for any object whose size is at most
-equal to the one provided.<br/>
+an integer. The alignment requirement is optional and by default such that it's
+the most stringent (the largest) for any object whose size is at most equal to
+the one provided.<br/>
 It's worth noting that providing a size of 0 (which is an accepted value in all
 respects) will force the system to dynamically allocate the contained objects in
 all cases.

docs/md/process.md

@@ -15,18 +15,17 @@
 
 # Introduction
 
-Sometimes processes are a useful tool to work around the strict definition of a
-system and introduce logic in a different way, usually without resorting to the
-introduction of other components.
-
-`EnTT` offers a minimal support to this paradigm by introducing a few classes
-that users can use to define and execute cooperative processes.
+Processes are a useful tool to work around the strict definition of a system and
+introduce logic in a different way, usually without resorting to other component
+types.<br/>
+`EnTT` offers minimal support to this paradigm by introducing a few classes used
+to define and execute cooperative processes.
 
 # The process
 
-A typical process must inherit from the `process` class template that stays true
-to the CRTP idiom. Moreover, derived classes must specify what's the intended
-type for elapsed times.
+A typical task inherits from the `process` class template that stays true to the
+CRTP idiom. Moreover, derived classes specify what the intended type for elapsed
+times is.
 
 A process should expose publicly the following member functions whether needed
 (note that it isn't required to define a function unless the derived class wants
@@ -34,39 +33,38 @@ to _override_ the default behavior):
 
 * `void update(Delta, void *);`
 
-  It's invoked once per tick until a process is explicitly aborted or it
-  terminates either with or without errors. Even though it's not mandatory to
-  declare this member function, as a rule of thumb each process should at
-  least define it to work properly. The `void *` parameter is an opaque pointer
-  to user data (if any) forwarded directly to the process during an update.
+  This is invoked once per tick until a process is explicitly aborted or ends
+  either with or without errors. Even though it's not mandatory to declare this
+  member function, as a rule of thumb each process should at least define it to
+  work _properly_. The `void *` parameter is an opaque pointer to user data (if
+  any) forwarded directly to the process during an update.
 
 * `void init();`
 
-  It's invoked when the process joins the running queue of a scheduler. This
-  happens as soon as it's attached to the scheduler if the process is a top
-  level one, otherwise when it replaces its parent if the process is a
-  continuation.
+  This is invoked when the process joins the running queue of a scheduler. It
+  happens usually as soon as the process is attached to the scheduler if it's a
+  top level one, otherwise when it replaces its parent if it's a _continuation_.
 
 * `void succeeded();`
 
-  It's invoked in case of success, immediately after an update and during the
+  This is invoked in case of success, immediately after an update and during the
   same tick.
 
 * `void failed();`
 
-  It's invoked in case of errors, immediately after an update and during the
+  This is invoked in case of errors, immediately after an update and during the
   same tick.
 
 * `void aborted();`
 
-  It's invoked only if a process is explicitly aborted. There is no guarantee
-  that it executes in the same tick, this depends solely on whether the
-  process is aborted immediately or not.
+  This is invoked only if a process is explicitly aborted. There is no guarantee
+  that it executes in the same tick, it depends solely on whether the process is
+  aborted immediately or not.
 
 Derived classes can also change the internal state of a process by invoking
-`succeed` and `fail`, as well as `pause` and `unpause` the process itself. All
-these are protected member functions made available to be able to manage the
-life cycle of a process from a derived class.
+`succeed` and `fail`, as well as `pause` and `unpause` the process itself.<br/>
+All these are protected member functions made available to manage the life cycle
+of a process from a derived class.
 
 Here is a minimal example for the sake of curiosity:
 
@@ -95,14 +93,14 @@ private:
 
 ## Adaptor
 
-Lambdas and functors can't be used directly with a scheduler for they are not
+Lambdas and functors can't be used directly with a scheduler because they aren't
 properly defined processes with managed life cycles.<br/>
 This class helps in filling the gap and turning lambdas and functors into
-full featured processes usable by a scheduler.
+full-featured processes usable by a scheduler.
 
 The function call operator has a signature similar to the one of the `update`
-function of a process but for the fact that it receives two extra arguments to
-call whenever a process is terminated with success or with an error:
+function of a process but for the fact that it receives two extra callbacks to
+invoke whenever a process terminates with success or with an error:
 
 ```cpp
 void(Delta delta, void *data, auto succeed, auto fail);
@@ -127,9 +125,9 @@ A cooperative scheduler runs different processes and helps managing their life
 cycles.
 
 Each process is invoked once per tick. If it terminates, it's removed
-automatically from the scheduler and it's never invoked again. Otherwise it's
+automatically from the scheduler and it's never invoked again. Otherwise, it's
 a good candidate to run one more time the next tick.<br/>
-A process can also have a child. In this case, the parent process is replaced
+A process can also have a _child_. In this case, the parent process is replaced
 with its child when it terminates and only if it returns with success. In case
 of errors, both the parent process and its child are discarded. This way, it's
 easy to create chain of processes to run sequentially.
@@ -138,18 +136,25 @@ Using a scheduler is straightforward. To create it, users must provide only the
 type for the elapsed times and no arguments at all:
 
 ```cpp
-entt::scheduler<std::uint32_t> scheduler;
+entt::basic_scheduler<std::uint64_t> scheduler;
 ```
 
-It has member functions to query its internal data structures, like `empty` or
-`size`, as well as a `clear` utility to reset it to a clean state:
+Otherwise, the `scheduler` alias is also available for the most common cases. It
+uses `std::uint32_t` as a default type:
+
+```cpp
+entt::scheduler scheduler;
+```
+
+The class has member functions to query its internal data structures, like
+`empty` or `size`, as well as a `clear` utility to reset it to a clean state:
 
 ```cpp
 // checks if there are processes still running
 const auto empty = scheduler.empty();
 
 // gets the number of processes still running
-entt::scheduler<std::uint32_t>::size_type size = scheduler.size();
+entt::scheduler::size_type size = scheduler.size();
 
 // resets the scheduler to its initial state and discards all the processes
 scheduler.clear();
@@ -172,8 +177,8 @@ To attach a process to a scheduler there are mainly two ways:
   scheduler.attach([](auto...){ /* ... */ });
   ```
 
-In both cases, the return value is an opaque object that offers a `then` member
-function to use to create chains of processes to run sequentially.<br/>
+In both cases, the scheduler is returned and its `then` member function can be
+used to create chains of processes to run sequentially.<br/>
 As a minimal example of use:
 
 ```cpp
@@ -201,7 +206,7 @@ scheduler.update(delta, &data);
 ```
 
 In addition to these functions, the scheduler offers an `abort` member function
-that can be used to discard all the running processes at once:
+that is used to discard all the running processes at once:
 
 ```cpp
 // aborts all the processes abruptly ...

docs/md/reference.md

@@ -1,18 +1,35 @@
 # Similar projects
 
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [Similar projects](#similar-projects)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
 There are many projects similar to `EnTT`, both open source and not.<br/>
 Some even borrowed some ideas from this library and expressed them in different
 languages.<br/>
 Others developed different architectures from scratch and therefore offer
 alternative solutions with their pros and cons.
 
-Below an incomplete list of those that I've come across so far.<br/>
+If you know of other similar projects out there, feel free to open an issue or a
+PR and I'll be glad to add them to this page.<br/>
+I hope the following lists can grow much more in the future.
+
+# Similar projects
+
+Below an incomplete list of similar projects that I've come across so far.<br/>
 If some terms or designs aren't clear, I recommend referring to the
 [_ECS Back and Forth_](https://skypjack.github.io/tags/#ecs) series for all the
 details.
 
-I hope this list can grow much more in the future:
-
 * C:
   * [destral_ecs](https://github.com/roig/destral_ecs): a single-file ECS based
     on sparse sets.
@@ -28,10 +45,14 @@ 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.
 
 * C#
+  * [Arch](https://github.com/genaray/Arch): a simple, fast and _unity entities_
+    inspired archetype ECS with optional multithreading. 
   * [Entitas](https://github.com/sschmid/Entitas-CSharp): the ECS framework for
     C# and Unity, where _reactive systems_ were invented.
   * [LeoECS](https://github.com/Leopotam/ecs): simple lightweight C# Entity
@@ -61,11 +82,10 @@ I hope this list can grow much more in the future:
   * [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.
 
 * Zig
   * [zig-ecs](https://github.com/prime31/zig-ecs): a _zig-ification_ of `EnTT`.
-
-If you know of other resources out there that can be of interest for the reader,
-feel free to open an issue or a PR and I'll be glad to add them to this page.

docs/md/resource.md

@@ -7,225 +7,185 @@
 
 * [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
 -->
 
 # Introduction
 
-Resource management is usually one of the most critical part of a software like
-a game. Solutions are often tuned to the particular application. There exist
-several approaches and all of them are perfectly fine as long as they fit the
+Resource management is usually one of the most critical parts of a game.
+Solutions are often tuned to the particular application. There exist several
+approaches and all of them are perfectly fine as long as they fit the
 requirements of the piece of software in which they are used.<br/>
 Examples are loading everything on start, loading on request, predictive
 loading, and so on.
 
 `EnTT` doesn't pretend to offer a _one-fits-all_ solution for the different
-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 comes with 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 class is designed to create different caches for different resource types
+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_, an instance of 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
+implementation mostly maps the interface of its standard counterpart and only
+adds a few things on top of it.<br/>
+However, the handle in `EnTT` is designed as a standalone class template. This
+is due to the fact that specializing a class in the standard library 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 responsible for _loading_ resources (quite obviously).<br/>
+By default, it's just a callable object that forwards its arguments to the
+resource itself. That is, a _passthrough type_. All the work is demanded to the
+constructor(s) of the resource itself.<br/>
+Loaders also are fully customizable as expected.
+
+A custom loader is a class with at least one function call operator and a member
+type named `result_type`.<br/>
+The loader isn't required to return a resource handle. As long as `return_type`
+is suitable for constructing a handle, that's fine.
+
+When using the default handle, it expects a resource type which is convertible
+to or suitable for constructing an `std::shared_ptr<Type>` (where `Type` is the
+actual resource type).<br/>
+In other terms, the loader should return shared pointers to the given resource
+type. However, this isn't mandatory. Users can easily get around this constraint
+by specializing both the handle and the loader.
+
+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, stores 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 functionalities 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(auto [id, res]: 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 (such as `contains` or `erase`).
+
+Set aside the part of the API that this class _shares_ with a map, it also adds
+something on top of it in order to address the most common requirements of a
+resource cache.<br/>
+In particular, it doesn't have an `emplace` member function which is replaced by
+`load` and `force_load` instead (where the former loads a new resource only if
+not present while the second triggers a forced loading in any case):
 
 ```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->second;
 ```
 
-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.
+It's worth mentioning that the iterators of a cache as well as its indexing
+operators return resource handles rather than instances of the mapped type.<br/>
+Since the cache has no control over the loader and a resource isn't required to
+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

@@ -9,8 +9,10 @@
 * [Delegate](#delegate)
   * [Runtime arguments](#runtime-arguments)
   * [Lambda support](#lambda-support)
+  * [Raw access](#raw-access)
 * [Signals](#signals)
 * [Event dispatcher](#event-dispatcher)
+  * [Named queues](#named-queues)
 * [Event emitter](#event-emitter)
 <!--
 @endcond TURN_OFF_DOXYGEN
@@ -18,27 +20,27 @@
 
 # Introduction
 
-Signals are usually a core part of games and software architectures in
-general.<br/>
-Roughly speaking, they help to decouple the various parts of a system while
-allowing them to communicate with each other somehow.
+Signals are more often than not a core part of games and software architectures
+in general.<br/>
+They help to decouple the various parts of a system while allowing them to
+communicate with each other somehow.
 
-The so called _modern C++_ comes with a tool that can be useful in these terms,
+The so called _modern C++_ comes with a tool that can be useful in this regard,
 the `std::function`. As an example, it can be used to create delegates.<br/>
-However, there is no guarantee that an `std::function` does not perform
+However, there is no guarantee that an `std::function` doesn't perform
 allocations under the hood and this could be problematic sometimes. Furthermore,
 it solves a problem but may not adapt well to other requirements that may arise
 from time to time.
 
 In case that the flexibility and power of an `std::function` isn't required or
-if the price to pay for them is too high,` EnTT` offers a complete set of
+if the price to pay for them is too high, `EnTT` offers a complete set of
 lightweight classes to solve the same and many other problems.
 
 # Delegate
 
 A delegate can be used as a general purpose invoker with no memory overhead for
-free functions and members provided along with an instance on which to invoke
-them.<br/>
+free functions, lambdas and members provided along with an instance on which to
+invoke them.<br/>
 It doesn't claim to be a drop-in replacement for an `std::function`, so don't
 expect to use it whenever an `std::function` fits well. That said, it's most
 likely even a better fit than an `std::function` in a lot of cases, so expect to
@@ -51,15 +53,13 @@ delegates:
 entt::delegate<int(int)> delegate{};
 ```
 
-All what is needed to create an instance is to specify the type of the function
-the delegate will _contain_, that is the signature of the free function or the
-member one wants to assign to it.
+What is needed to create an instance is to specify the type of the function the
+delegate _accepts_, that is the signature of the functions it models.<br/>
+However, attempting to use an empty delegate by invoking its function call
+operator results in undefined behavior or most likely a crash.
 
-Attempting to use an empty delegate by invoking its function call operator
-results in undefined behavior or most likely a crash. Before to use a delegate,
-it must be initialized.<br/>
-There exists a bunch of overloads of the `connect` member function to do that.
-As an example of use:
+There exist a few overloads of the `connect` member function to initialize a
+delegate:
 
 ```cpp
 int f(int i) { return i; }
@@ -76,7 +76,7 @@ my_struct instance;
 delegate.connect<&my_struct::f>(instance);
 ```
 
-The delegate class accepts also data members, if needed. In this case, the
+The delegate class also accepts data members, if needed. In this case, the
 function type of the delegate is such that the parameter list is empty and the
 value of the data member is at least convertible to the return type.
 
@@ -93,14 +93,11 @@ delegate.connect<&g>(c);
 delegate(42);
 ```
 
-The function `g` will be invoked with a reference to `c` and `42`. However, the
-function type of the delegate is still `void(int)`. This is also the signature
-of its function call operator.
-
-Another interesting aspect of the delegate class is that it accepts also
-functions with a list of parameters that is shorter than that of the function
-type used to specialize the delegate itself.<br/>
-The following code is therefore perfectly valid:
+Function `g` is invoked with a reference to `c` and `42`. However, the function
+type of the delegate is still `void(int)`. This is also the signature of its
+function call operator.<br/>
+Another interesting aspect of the delegate class is that it accepts functions
+with a list of parameters that is shorter than that of its function type:
 
 ```cpp
 void g() { /* ... */ }
@@ -109,9 +106,15 @@ delegate(42);
 ```
 
 Where the function type of the delegate is `void(int)` as above. It goes without
-saying that the extra arguments are silently discarded internally.<br/>
-This is a nice-to-have feature in a lot of cases, as an example when the
-`delegate` class is used as a building block of a signal-slot system.
+saying that the extra arguments are silently discarded internally. This is a
+nice-to-have feature in a lot of cases, as an example when the `delegate` class
+is used as a building block of a signal-slot system.<br/>
+In fact, this filtering works both ways. The class tries to pass its first
+_count_ arguments **first**, then the last _count_. Watch out for conversion
+rules if in doubt when connecting a listener!<br/>
+Arbitrary functions that pull random arguments from the delegate list aren't
+supported instead. Other feature were preferred, such as support for functions
+with compatible argument lists although not equal to those of the delegate.
 
 To create and initialize a delegate at once, there are a few specialized
 constructors. Because of the rules of the language, the listener is provided by
@@ -139,7 +142,7 @@ already shown in the examples above:
 auto ret = delegate(42);
 ```
 
-In all cases, the listeners don't have to strictly follow the signature of the
+In all cases, listeners don't have to strictly follow the signature of the
 delegate. As long as a listener can be invoked with the given arguments to yield
 a result that is convertible to the given result type, everything works just
 fine.
@@ -157,7 +160,7 @@ my_struct instance;
 delegate(instance, 42);
 ```
 
-In this case, it's not possible to deduce the function type since the first
+In this case, it's not possible to _deduce_ the function type since the first
 argument doesn't necessarily have to be a reference (for example, it can be a
 pointer, as well as a const reference).<br/>
 Therefore, the function type must be declared explicitly for unbound members.
@@ -165,9 +168,9 @@ Therefore, the function type must be declared explicitly for unbound members.
 ## Runtime arguments
 
 The `delegate` class is meant to be used primarily with template arguments.
-However, as a consequence of its design, it can also offer minimal support for
+However, as a consequence of its design, it also offers minimal support for
 runtime arguments.<br/>
-When used in this modality, some feature aren't supported though. In particular:
+When used like this, some features aren't supported though. In particular:
 
 * Curried functions aren't accepted.
 * Functions with an argument list that differs from that of the delegate aren't
@@ -208,7 +211,7 @@ their nuances. The reason is pretty simple: a `delegate` isn't a drop-in
 replacement for an `std::function`. Instead, it tries to overcome the problems
 with the latter.<br/>
 That being said, non-capturing lambda functions are supported, even though some
-feature aren't available in this case.
+features aren't available in this case.
 
 This is a logical consequence of the support for connecting functions at
 runtime. Therefore, lambda functions undergo the same rules and
@@ -235,6 +238,24 @@ As above, the first parameter (`const void *`) isn't part of the function type
 of the delegate and is used to dispatch arbitrary user data back and forth. In
 other terms, the function type of the delegate above is `int(int)`.
 
+## Raw access
+
+While not recommended, a delegate also allows direct access to the stored
+callable function target and underlying data, if any.<br/>
+This makes it possible to bypass the behavior of the delegate itself and force
+calls on different instances:
+
+```cpp
+my_struct other;
+delegate.target(&other, 42);
+```
+
+It goes without saying that this type of approach is **very** risky, especially
+since there is no way of knowing whether the contained function was originally a
+member function of some class, a free function or a lambda.<br/>
+Another possible (and meaningful) use of this feature is that of identifying a
+particular delegate through its descriptive _traits_ instead.
+
 # Signals
 
 Signal handlers work with references to classes, function pointers and pointers
@@ -246,16 +267,17 @@ Signals make use of delegates internally and therefore they undergo the same
 rules and offer similar functionalities. It may be a good idea to consult the
 documentation of the `delegate` class for further information.
 
-A signal handler can be used as a private data member without exposing any
-_publish_ functionality to the clients of a class. The basic idea is to impose a
-clear separation between the signal itself and the `sink` class, that is a tool
-to be used to connect and disconnect listeners on the fly.
+A signal handler is can be used as a private data member without exposing any
+_publish_ functionality to the clients of a class.<br/>
+The basic idea is to impose a clear separation between the signal itself and the
+`sink` class, that is a tool to be used to connect and disconnect listeners on
+the fly.
 
 The API of a signal handler is straightforward. If a collector is supplied to
-the signal when something is published, all the values returned by the listeners
-can be literally _collected_ and used later by the caller. Otherwise, the class
+the signal when something is published, all the values returned by its listeners
+are literally _collected_ and used later by the caller. Otherwise, the class
 works just like a plain signal that emits events from time to time.<br/>
-To create instances of signal handlers it is sufficient to provide the type of
+To create instances of signal handlers it's sufficient to provide the type of
 function to which they refer:
 
 ```cpp
@@ -293,41 +315,31 @@ sink.disconnect<&foo>();
 sink.disconnect<&listener::bar>(instance);
 
 // disconnect all member functions of an instance, if any
-sink.disconnect(instance);
+sink.disconnect(&instance);
 
 // discards all listeners at once
 sink.disconnect();
 ```
 
-As shown above, the listeners don't have to strictly follow the signature of the
+As shown above, listeners don't have to strictly follow the signature of the
 signal. As long as a listener can be invoked with the given arguments to yield a
 result that is convertible to the given return type, everything works just
 fine.<br/>
-It's also possible to connect a listener before other listeners already
-contained by the signal. The `before` function returns a `sink` object correctly
-initialized for the purpose that can be used to connect one or more listeners in
-order and in the desired position:
-
-```cpp
-sink.before<&foo>().connect<&listener::bar>(instance);
-```
-
 In all cases, the `connect` member function returns by default a `connection`
 object to be used as an alternative to break a connection by means of its
-`release` member function. A `scoped_connection` can also be created from a
-connection. In this case, the link is broken automatically as soon as the object
-goes out of scope.
+`release` member function.<br/>
+A `scoped_connection` can also be created from a connection. In this case, the
+link is broken automatically as soon as the object goes out of scope.
 
 Once listeners are attached (or even if there are no listeners at all), events
-and data in general can be published through a signal by means of the `publish`
+and data in general are published through a signal by means of the `publish`
 member function:
 
 ```cpp
 signal.publish(42, 'c');
 ```
 
-To collect data, the `collect` member function should be used instead. Below is
-a minimal example to show how to use it:
+To collect data, the `collect` member function is used instead:
 
 ```cpp
 int f() { return 0; }
@@ -361,7 +373,7 @@ 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 +387,20 @@ 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; };
@@ -409,23 +418,21 @@ 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:
+Note that connecting listeners within event handlers can result in undefined
+behavior.<br/>
+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);
-dispatcher.sink<another_event>().disconnect(listener);
+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 +441,14 @@ 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 +461,30 @@ 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
@@ -464,8 +493,7 @@ Originally designed to fit the requirements of
 [`uvw`](https://github.com/skypjack/uvw) (a wrapper for `libuv` written in
 modern C++), it was adapted later to be included in this library.
 
-To create a custom emitter type, derived classes must inherit directly from the
-base class as:
+To create an emitter type, derived classes must inherit from the base as:
 
 ```cpp
 struct my_emitter: emitter<my_emitter> {
@@ -473,18 +501,10 @@ struct my_emitter: emitter<my_emitter> {
 }
 ```
 
-The full list of accepted types of events isn't required. Handlers are created
-internally on the fly and thus each type of event is accepted by default.
-
-Whenever an event is published, an emitter provides the listeners with a
-reference to itself along with a reference to the event. Therefore listeners
-have an handy way to work with it without incurring in the need of capturing a
-reference to the emitter itself.<br/>
-In addition, an opaque object is returned each time a connection is established
-between an emitter and a listener, allowing the caller to disconnect them at a
-later time.<br/>
-The opaque object used to handle connections is both movable and copyable. On
-the other side, an event emitter is movable but not copyable by default.
+Handlers for the different events are created internally on the fly. It's not
+required to specify in advance the full list of accepted events.<br/>
+Moreover, whenever an event is published, an emitter also passes a reference
+to itself to its listeners.
 
 To create new instances of an emitter, no arguments are required:
 
@@ -492,90 +512,54 @@ To create new instances of an emitter, no arguments are required:
 my_emitter emitter{};
 ```
 
-Listeners must be movable and callable objects (free functions, lambdas,
-functors, `std::function`s, whatever) whose function type is compatible with:
+Listeners are movable and callable objects (free functions, lambdas, functors,
+`std::function`s, whatever) whose function type is compatible with:
 
 ```cpp
-void(Event &, my_emitter &)
+void(Type &, my_emitter &)
 ```
 
-Where `Event` is the type of event they want to listen.<br/>
-There are two ways to attach a listener to an event emitter that differ
-slightly from each other:
-
-* To register a long-lived listener, use the `on` member function. It is meant
-  to register a listener designed to be invoked more than once for the given
-  event type.<br/>
-  As an example:
-
-  ```cpp
-  auto conn = emitter.on<my_event>([](const my_event &event, my_emitter &emitter) {
-      // ...
-  });
-  ```
-
-  The connection object can be freely discarded. Otherwise, it can be used later
-  to disconnect the listener if required.
-
-* To register a short-lived listener, use the `once` member function. It is
-  meant to register a listener designed to be invoked only once for the given
-  event type. The listener is automatically disconnected after the first
-  invocation.<br/>
-  As an example:
-
-  ```cpp
-  auto conn = emitter.once<my_event>([](const my_event &event, my_emitter &emitter) {
-      // ...
-  });
-  ```
-
-  The connection object can be freely discarded. Otherwise, it can be used later
-  to disconnect the listener if required.
-
-In both cases, the connection object can be used with the `erase` member
-function:
+Where `Type` is the type of event they want to receive.<br/>
+To attach a listener to an emitter, there exists the `on` member function:
 
 ```cpp
-emitter.erase(conn);
+emitter.on<my_event>([](const my_event &event, my_emitter &emitter) {
+    // ...
+});
 ```
 
-There are also two member functions to use either to disconnect all the
-listeners for a given type of event or to clear the emitter:
+Similarly, the `reset` member function is used to disconnect listeners given a
+type while `clear` is used to disconnect all listeners at once:
 
 ```cpp
-// removes all the listener for the specific event
-emitter.clear<my_event>();
+// resets the listener for my_event
+emitter.erase<my_event>();
 
-// removes all the listeners registered so far
-emitter.clear();
+// resets all listeners
+emitter.clear()
 ```
 
-To send an event to all the listeners that are interested in it, the `publish`
-member function offers a convenient approach that relieves users from having to
-create the event:
+To send an event to the listener registered on a given type, the `publish`
+function is the way to go:
 
 ```cpp
 struct my_event { int i; };
 
 // ...
 
-emitter.publish<my_event>(42);
+emitter.publish(my_event{42});
 ```
 
-Finally, the `empty` member function tests if there exists at least either a
-listener registered with the event emitter or to a given type of event:
+Finally, the `empty` member function tests if there exists at least a listener
+registered with the event emitter while `contains` is used to check if a given
+event type is associated with a valid listener:
 
 ```cpp
-bool empty;
-
-// checks if there is any listener registered for the specific event
-empty = emitter.empty<my_event>();
-
-// checks it there are listeners registered with the event emitter
-empty = emitter.empty();
+if(emitter.contains<my_event>()) {
+    // ...
+}
 ```
 
-In general, the event emitter is a handy tool when the derived classes _wrap_
-asynchronous operations, because it introduces a _nice-to-have_ model based on
-events and listeners that kindly hides the complexity behind the scenes. However
-it is not limited to such uses.
+This class introduces a _nice-to-have_ model based on events and listeners.<br/>
+More in general, it's a handy tool when the derived classes _wrap_ asynchronous
+operations but it's not limited to such uses.

docs/md/unreal.md

@@ -14,6 +14,8 @@
 
 ## Enable Cpp17
 
+> Skip this part if you are working with UE5, Since UE5 uses cpp17 by default.
+
 As of writing (Unreal Engine v4.25), the default C++ standard of Unreal Engine
 is C++14.<br/>
 On the other hand, note that `EnTT` requires C++17 to compile. To enable it, in

entt.imp

@@ -0,0 +1,42 @@
+[
+  # gtest only
+  { "include": [ "@<gtest/internal/.*>", "private", "<gtest/gtest.h>", "public" ] },
+  { "include": [ "@<gtest/gtest-.*>", "private", "<gtest/gtest.h>", "public" ] },
+  # forward files
+  { "include": [ "@[\"<].*/container/fwd\\.hpp[\">]", "private", "<entt/container/dense_map.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/container/fwd\\.hpp[\">]", "private", "<entt/container/dense_set.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/core/fwd\\.hpp[\">]", "private", "<entt/core/any.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/core/fwd\\.hpp[\">]", "private", "<entt/core/family.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/core/fwd\\.hpp[\">]", "private", "<entt/core/hashed_string.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/core/fwd\\.hpp[\">]", "private", "<entt/core/ident.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/core/fwd\\.hpp[\">]", "private", "<entt/core/monostate.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/core/fwd\\.hpp[\">]", "private", "<entt/core/type_info.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/core/fwd\\.hpp[\">]", "private", "<entt/core/type_traits.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd\\.hpp[\">]", "private", "<entt/entity/component.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd\\.hpp[\">]", "private", "<entt/entity/entity.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd\\.hpp[\">]", "private", "<entt/entity/group.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd\\.hpp[\">]", "private", "<entt/entity/handle.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd\\.hpp[\">]", "private", "<entt/entity/helper.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd\\.hpp[\">]", "private", "<entt/entity/observer.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd\\.hpp[\">]", "private", "<entt/entity/organizer.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd\\.hpp[\">]", "private", "<entt/entity/registry.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd\\.hpp[\">]", "private", "<entt/entity/runtime_view.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd\\.hpp[\">]", "private", "<entt/entity/snapshot.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd\\.hpp[\">]", "private", "<entt/entity/sparse_set.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd\\.hpp[\">]", "private", "<entt/entity/storage.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd\\.hpp[\">]", "private", "<entt/entity/view.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/graph/fwd\\.hpp[\">]", "private", "<entt/graph/adjacency_matrix.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/graph/fwd\\.hpp[\">]", "private", "<entt/graph/dot.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/graph/fwd\\.hpp[\">]", "private", "<entt/graph/flow.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/meta/fwd\\.hpp[\">]", "private", "<entt/meta/meta.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/poly/fwd\\.hpp[\">]", "private", "<entt/poly/poly.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/process/fwd\\.hpp[\">]", "private", "<entt/process/process.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/process/fwd\\.hpp[\">]", "private", "<entt/process/scheduler.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/resource/fwd\\.hpp[\">]", "private", "<entt/resource/cache.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/resource/fwd\\.hpp[\">]", "private", "<entt/resource/loader.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/resource/fwd\\.hpp[\">]", "private", "<entt/resource/resource.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/signal/fwd\\.hpp[\">]", "private", "<entt/signal/delegate.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/signal/fwd\\.hpp[\">]", "private", "<entt/signal/dispatcher.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/signal/fwd\\.hpp[\">]", "private", "<entt/signal/emitter.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/signal/fwd\\.hpp[\">]", "private", "<entt/signal/sigh.hpp>", "public" ] }
+]

natvis/entt/container.natvis

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
-	<Type Name="entt::dense_hash_map&lt;*&gt;">
+	<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>
@@ -15,7 +15,7 @@
 			</IndexListItems>
 		</Expand>
 	</Type>
-	<Type Name="entt::dense_hash_set&lt;*&gt;">
+	<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>
@@ -26,7 +26,7 @@
 			<Item Name="[max_load_factor]" ExcludeView="simple">threshold</Item>
 			<IndexListItems>
 				<Size>size()</Size>
-				<ValueNode>packed.first_base::value[$i].element</ValueNode>
+				<ValueNode>packed.first_base::value[$i].second</ValueNode>
 			</IndexListItems>
 		</Expand>
 	</Type>

natvis/entt/core.natvis

@@ -15,10 +15,11 @@
 		</Expand>
 	</Type>
 	<Type Name="entt::basic_hashed_string&lt;*&gt;">
-		<DisplayString Condition="str != nullptr">{{ hash={ hash } }}</DisplayString>
+		<DisplayString Condition="base_type::repr != nullptr">{{ hash={ base_type::hash } }}</DisplayString>
 		<DisplayString>{{}}</DisplayString>
 		<Expand>
-			<Item Name="[data]">str,na</Item>
+			<Item Name="[data]">base_type::repr,na</Item>
+			<Item Name="[length]">base_type::length</Item>
 		</Expand>
 	</Type>
 	<Type Name="entt::type_info">

natvis/entt/entity.natvis

@@ -1,102 +1,97 @@
 <?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.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>
+		<DisplayString>{{ pools={ pools.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>
+			<Item Name="[entities]">entities</Item>
 			<Synthetic Name="[pools]">
-				<DisplayString>{ pools_size() }</DisplayString>
+				<DisplayString>{ pools.size() }</DisplayString>
 				<Expand>
 					<IndexListItems ExcludeView="simple">
-						<Size>pools_size()</Size>
+						<Size>pools.size()</Size>
 						<ValueNode>*pools.packed.first_base::value[$i].element.second</ValueNode>
 					</IndexListItems>
 					<IndexListItems IncludeView="simple">
-						<Size>pools_size()</Size>
+						<Size>pools.size()</Size>
 						<ValueNode>*pools.packed.first_base::value[$i].element.second,view(simple)</ValueNode>
 					</IndexListItems>
 				</Expand>
 			</Synthetic>
 			<Item Name="[groups]" ExcludeView="simple">groups.size()</Item>
 			<Synthetic Name="[vars]">
-				<DisplayString>{ vars_size() }</DisplayString>
+				<DisplayString>{ vars.ctx.size() }</DisplayString>
 				<Expand>
 					<IndexListItems>
-						<Size>vars_size()</Size>
-						<ValueNode>vars.packed.first_base::value[$i].element.second</ValueNode>
+						<Size>vars.ctx.size()</Size>
+						<ValueNode>vars.ctx.packed.first_base::value[$i].element.second</ValueNode>
 					</IndexListItems>
 				</Expand>
 			</Synthetic>
 		</Expand>
 	</Type>
 	<Type Name="entt::basic_sparse_set&lt;*&gt;">
+		<Intrinsic Name="cap" Expression="(traits_type::version_mask &lt;&lt; traits_type::length)"/>
 		<DisplayString>{{ size={ packed.size() }, type={ info->alias,na } }}</DisplayString>
 		<Expand>
 			<Item Name="[capacity]" ExcludeView="simple">packed.capacity()</Item>
 			<Item Name="[policy]">mode,en</Item>
-			<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>
+			<Item Name="[free_list]">head</Item>
+			<Synthetic Name="[sparse]">
+				<DisplayString>{ sparse.size() * traits_type::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() * traits_type::page_size"/>
+						<Loop>
+							<Break Condition="pos == last"/>
+							<Exec>page = pos / traits_type::page_size</Exec>
+							<Exec>offset = pos &amp; (traits_type::page_size - 1)</Exec>
+							<If Condition="sparse[page] &amp;&amp; (*((traits_type::entity_type *)&amp;sparse[page][offset]) &lt; cap())">
+								<Item Name="[{ pos }]">*((traits_type::entity_type *)&amp;sparse[page][offset]) &amp; traits_type::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="*((traits_type::entity_type *)&amp;packed[pos]) &lt; cap()">
+								<Item Name="[{ pos }]">packed[pos]</Item>
+							</If>
+							<Exec>++pos</Exec>
+						</Loop>
+					</CustomListItems>
+				</Expand>
+			</Synthetic>
 		</Expand>
 	</Type>
 	<Type Name="entt::basic_storage&lt;*&gt;">
+		<Intrinsic Name="cap" Expression="(base_type::traits_type::version_mask &lt;&lt; base_type::traits_type::length)"/>
 		<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="[capacity]" Optional="true" ExcludeView="simple">payload.capacity() * traits_type::page_size</Item>
+			<Item Name="[page size]" Optional="true" ExcludeView="simple">traits_type::page_size</Item>
 			<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">
+			<CustomListItems Condition="payload.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 Condition="*((base_type::traits_type::entity_type *)&amp;base_type::packed[pos]) &lt; cap()">
+						<Item Name="[{ pos }:{ base_type::packed[pos] }]">payload[pos / traits_type::page_size][pos &amp; (traits_type::page_size - 1)]</Item>
 					</If>
 					<Exec>++pos</Exec>
 				</Loop>
@@ -104,7 +99,17 @@
 		</Expand>
 	</Type>
 	<Type Name="entt::basic_view&lt;*&gt;">
-		<DisplayString>{{ size_hint={ view->packed.size() } }}</DisplayString>
+		<DisplayString Condition="leading != nullptr">{{ size_hint={ leading->packed.size() } }}</DisplayString>
+		<DisplayString>{{ size_hint=0 }}</DisplayString>
+		<Expand>
+			<Item Name="[pools]" Optional="true">pools,na</Item>
+			<Item Name="[filter]" Optional="true">filter,na</Item>
+			<Item Name="[handle]" Condition="leading != nullptr">leading,na</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_runtime_view&lt;*&gt;">
+		<DisplayString Condition="pools.size() != 0u">{{ size_hint={ pools[0]->packed.size() } }}</DisplayString>
+		<DisplayString>{{ size_hint=0 }}</DisplayString>
 		<Expand>
 			<Item Name="[pools]">pools,na</Item>
 			<Item Name="[filter]">filter,na</Item>

natvis/entt/graph.natvis

@@ -0,0 +1,19 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+    <Type Name="entt::adjacency_matrix&lt;*&gt;">
+		<DisplayString>{{ size={ vert } }}</DisplayString>
+        <Expand>
+			<CustomListItems>
+				<Variable Name="pos" InitialValue="0" />
+				<Variable Name="last" InitialValue="vert * vert"/>
+				<Loop>
+					<Break Condition="pos == last"/>
+					<If Condition="matrix[pos] != 0u">
+						<Item Name="{pos / vert}">pos % vert</Item>
+					</If>
+					<Exec>++pos</Exec>
+				</Loop>
+			</CustomListItems>
+        </Expand>
+    </Type>
+</AutoVisualizer>

natvis/entt/meta.natvis

@@ -1,115 +1,43 @@
 <?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>
+		<DisplayString>{{ arity={ arity } }}</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>
+		<DisplayString>{{ arity={ arity } }}</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>
+			<Item Name="[prop]">prop</Item>
 		</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>
+		<DisplayString>{{ arity={ arity } }}</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>
+			<Item Name="[next]" Condition="next != nullptr">*next</Item>
+			<Item Name="[prop]">prop</Item>
 		</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>
+		<DisplayString>{ value }</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>
+		<DisplayString>{{ arity={ arity } }}</DisplayString>
 	</Type>
 	<Type Name="entt::internal::meta_type_node">
 		<Intrinsic Name="has_property" Expression="!!(traits &amp; property)">
@@ -121,77 +49,73 @@
 			<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_integral]">has_property(entt::internal::meta_traits::is_integral)</Item>
+			<Item Name="[is_signed]">has_property(entt::internal::meta_traits::is_signed)</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>
+			<Item Name="[from_void]">from_void != nullptr</Item>
+			<Item Name="[template_info]">templ</Item>
+			<Item Name="[details]" Condition="details != nullptr">*details</Item>
 		</Expand>
 	</Type>
-	<Type Name="entt::meta_type">
+	<Type Name="entt::meta_any">
+		<DisplayString Condition="node.info != nullptr">{{ type={ node.info->alias,na }, policy={ storage.mode,en } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<ExpandedItem>node</ExpandedItem>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx->value</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_handle">
+		<DisplayString>{ any }</DisplayString>
+	</Type>
+	<Type Name="entt::meta_associative_container">
+		<DisplayString>{ storage }</DisplayString>
+		<Expand>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx->value</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_sequence_container">
+		<DisplayString>{ storage }</DisplayString>
+		<Expand>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx->value</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_data">
 		<DisplayString Condition="node != nullptr">{ *node }</DisplayString>
 		<DisplayString>{{}}</DisplayString>
 		<Expand>
 			<ExpandedItem Condition="node != nullptr">node</ExpandedItem>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx->value</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_func">
+		<DisplayString Condition="node != nullptr">{ *node }</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<ExpandedItem Condition="node != nullptr">node</ExpandedItem>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx->value</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_prop">
+		<DisplayString Condition="node != nullptr">{ *node }</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<ExpandedItem Condition="node != nullptr">node</ExpandedItem>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx->value</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_type">
+		<DisplayString>{ node }</DisplayString>
+		<Expand>
+			<ExpandedItem>node</ExpandedItem>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx->value</Item>
 		</Expand>
 	</Type>
 </AutoVisualizer>

natvis/entt/resource.natvis

@@ -1,3 +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

@@ -1,35 +1,39 @@
 <?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={ "$T1" } }}</DisplayString>
 		<Expand>
 			<Item Name="[empty]">fn == nullptr</Item>
-			<Item Name="[data]">data</Item>
+			<Item Name="[data]">instance</Item>
 		</Expand>
 	</Type>
-	<Type Name="entt::dispatcher">
-		<DisplayString>{{ size={ pools.size() } }}</DisplayString>
+	<Type Name="entt::basic_dispatcher&lt;*&gt;">
+		<Intrinsic Name="size" Expression="pools.first_base::value.size()"/>
+		<DisplayString>{{ size={ size() } }}</DisplayString>
 		<Expand>
 			<Synthetic Name="[pools]">
-				<DisplayString>{ pools.size() }</DisplayString>
+				<DisplayString>{ size() }</DisplayString>
 				<Expand>
 					<IndexListItems>
-						<Size>pools.size()</Size>
-						<ValueNode>*pools.packed.first_base::value[$i].element.second</ValueNode>
+						<Size>size()</Size>
+						<ValueNode>*pools.first_base::value.packed.first_base::value[$i].element.second</ValueNode>
 					</IndexListItems>
 				</Expand>
 			</Synthetic>
 		</Expand>
 	</Type>
-	<Type Name="entt::dispatcher::pool_handler&lt;*&gt;">
+	<Type Name="entt::internal::dispatcher_handler&lt;*&gt;">
 		<DisplayString>{{ size={ events.size() }, event={ "$T1" } }}</DisplayString>
 		<Expand>
 			<Item Name="[signal]">signal</Item>
 		</Expand>
 	</Type>
+	<Type Name="entt::emitter&lt;*&gt;">
+		<DisplayString>{{ size={ handlers.first_base::value.packed.first_base::value.size() } }}</DisplayString>
+	</Type>
+	<Type Name="entt::connection">
+		<DisplayString>{{ bound={ signal != nullptr } }}</DisplayString>
+	</Type>
 	<Type Name="entt::scoped_connection">
 		<DisplayString>{ conn }</DisplayString>
 	</Type>
@@ -46,7 +50,6 @@
 		<DisplayString>{{ type={ "$T1" } }}</DisplayString>
 		<Expand>
 			<Item Name="[signal]">signal,na</Item>
-			<Item Name="[offset]">offset</Item>
 		</Expand>
 	</Type>
 </AutoVisualizer>

src/BUILD.bazel

@@ -0,0 +1,11 @@
+load("@bazel_skylib//lib:selects.bzl", "selects")
+load("//bazel:copts.bzl", "COPTS")
+
+package(default_visibility = ["//:__subpackages__"])
+
+cc_library(
+    name = "entt",
+    includes = ["."],
+    hdrs = glob(["**/*.h", "**/*.hpp"]),
+    copts = COPTS,
+)

src/entt/config/config.h

@@ -1,35 +1,32 @@
 #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_CONSTEXPR
 #    define ENTT_THROW throw
 #    define ENTT_TRY try
 #    define ENTT_CATCH catch(...)
 #else
-#    define ENTT_NOEXCEPT
+#    define ENTT_CONSTEXPR constexpr // use only with throwing functions (waiting for C++20)
 #    define ENTT_THROW
 #    define ENTT_TRY if(true)
 #    define ENTT_CATCH if(false)
 #endif
 
-#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
-#    include <new>
-#    define ENTT_LAUNDER(expr) std::launder(expr)
-#else
-#    define ENTT_LAUNDER(expr) expr
-#endif
-
-#ifndef ENTT_USE_ATOMIC
-#    define ENTT_MAYBE_ATOMIC(Type) Type
-#else
+#ifdef ENTT_USE_ATOMIC
 #    include <atomic>
 #    define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#else
+#    define ENTT_MAYBE_ATOMIC(Type) Type
 #endif
 
 #ifndef ENTT_ID_TYPE
 #    include <cstdint>
 #    define ENTT_ID_TYPE std::uint32_t
+#else
+#    include <cstdint> // provides coverage for types in the std namespace
 #endif
 
 #ifndef ENTT_SPARSE_PAGE
@@ -42,19 +39,31 @@
 
 #ifdef ENTT_DISABLE_ASSERT
 #    undef ENTT_ASSERT
-#    define ENTT_ASSERT(...) (void(0))
+#    define ENTT_ASSERT(condition, msg) (void(0))
 #elif !defined ENTT_ASSERT
 #    include <cassert>
-#    define ENTT_ASSERT(condition, ...) assert(condition)
+#    define ENTT_ASSERT(condition, msg) assert(condition)
 #endif
 
+#ifdef ENTT_DISABLE_ASSERT
+#    undef ENTT_ASSERT_CONSTEXPR
+#    define ENTT_ASSERT_CONSTEXPR(condition, msg) (void(0))
+#elif !defined ENTT_ASSERT_CONSTEXPR
+#    define ENTT_ASSERT_CONSTEXPR(condition, msg) ENTT_ASSERT(condition, msg)
+#endif
+
+#define ENTT_FAIL(msg) ENTT_ASSERT(false, msg);
+
 #ifdef ENTT_NO_ETO
-#    define ENTT_IGNORE_IF_EMPTY false
+#    define ENTT_ETO_TYPE(Type) void
 #else
-#    define ENTT_IGNORE_IF_EMPTY true
+#    define ENTT_ETO_TYPE(Type) Type
 #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 '='
@@ -66,4 +75,11 @@
 #    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

@@ -1,8 +1,14 @@
 #ifndef ENTT_CONFIG_VERSION_H
 #define ENTT_CONFIG_VERSION_H
 
+#include "macro.h"
+
 #define ENTT_VERSION_MAJOR 3
-#define ENTT_VERSION_MINOR 9
-#define ENTT_VERSION_PATCH 0
+#define ENTT_VERSION_MINOR 13
+#define ENTT_VERSION_PATCH 2
+
+#define ENTT_VERSION \
+    ENTT_XSTR(ENTT_VERSION_MAJOR) \
+    "." ENTT_XSTR(ENTT_VERSION_MINOR) "." ENTT_XSTR(ENTT_VERSION_PATCH)
 
 #endif

src/entt/container/dense_set.hpp

@@ -1,7 +1,6 @@
-#ifndef ENTT_CONTAINER_DENSE_HASH_SET_HPP
-#define ENTT_CONTAINER_DENSE_HASH_SET_HPP
+#ifndef ENTT_CONTAINER_DENSE_SET_HPP
+#define ENTT_CONTAINER_DENSE_SET_HPP
 
-#include <algorithm>
 #include <cmath>
 #include <cstddef>
 #include <functional>
@@ -20,184 +19,170 @@
 
 namespace entt {
 
-/**
- * @cond TURN_OFF_DOXYGEN
- * Internal details not to be documented.
- */
-
+/*! @cond TURN_OFF_DOXYGEN */
 namespace internal {
 
-template<typename Type>
-struct dense_hash_set_node final {
-    template<typename... Args>
-    dense_hash_set_node(const std::size_t pos, Args &&...args)
-        : next{pos},
-          element{std::forward<Args>(args)...} {}
-
-    std::size_t next;
-    Type element;
-};
-
 template<typename It>
-class dense_hash_set_iterator {
-    friend dense_hash_set_iterator<const std::remove_pointer_t<It> *>;
-
-    using iterator_traits = std::iterator_traits<decltype(std::addressof(std::as_const(std::declval<It>()->element)))>;
+class dense_set_iterator final {
+    template<typename>
+    friend class dense_set_iterator;
 
 public:
-    using value_type = typename iterator_traits::value_type;
-    using pointer = typename iterator_traits::pointer;
-    using reference = typename iterator_traits::reference;
-    using difference_type = typename iterator_traits::difference_type;
+    using value_type = 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_hash_set_iterator() ENTT_NOEXCEPT = default;
+    constexpr dense_set_iterator() noexcept
+        : it{} {}
 
-    dense_hash_set_iterator(const It iter) ENTT_NOEXCEPT
+    constexpr dense_set_iterator(const It iter) noexcept
         : it{iter} {}
 
-    template<bool Const = std::is_const_v<std::remove_pointer_t<It>>, typename = std::enable_if_t<Const>>
-    dense_hash_set_iterator(const dense_hash_set_iterator<std::remove_const_t<std::remove_pointer_t<It>> *> &other)
+    template<typename Other, typename = std::enable_if_t<!std::is_same_v<It, Other> && std::is_constructible_v<It, Other>>>
+    constexpr dense_set_iterator(const dense_set_iterator<Other> &other) noexcept
         : it{other.it} {}
 
-    dense_hash_set_iterator &operator++() ENTT_NOEXCEPT {
+    constexpr dense_set_iterator &operator++() noexcept {
         return ++it, *this;
     }
 
-    dense_hash_set_iterator operator++(int) ENTT_NOEXCEPT {
-        dense_hash_set_iterator orig = *this;
+    constexpr dense_set_iterator operator++(int) noexcept {
+        dense_set_iterator orig = *this;
         return ++(*this), orig;
     }
 
-    dense_hash_set_iterator &operator--() ENTT_NOEXCEPT {
+    constexpr dense_set_iterator &operator--() noexcept {
         return --it, *this;
     }
 
-    dense_hash_set_iterator operator--(int) ENTT_NOEXCEPT {
-        dense_hash_set_iterator orig = *this;
+    constexpr dense_set_iterator operator--(int) noexcept {
+        dense_set_iterator orig = *this;
         return operator--(), orig;
     }
 
-    dense_hash_set_iterator &operator+=(const difference_type value) ENTT_NOEXCEPT {
+    constexpr dense_set_iterator &operator+=(const difference_type value) noexcept {
         it += value;
         return *this;
     }
 
-    dense_hash_set_iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
-        dense_hash_set_iterator copy = *this;
+    constexpr dense_set_iterator operator+(const difference_type value) const noexcept {
+        dense_set_iterator copy = *this;
         return (copy += value);
     }
 
-    dense_hash_set_iterator &operator-=(const difference_type value) ENTT_NOEXCEPT {
+    constexpr dense_set_iterator &operator-=(const difference_type value) noexcept {
         return (*this += -value);
     }
 
-    dense_hash_set_iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
+    constexpr dense_set_iterator operator-(const difference_type value) const noexcept {
         return (*this + -value);
     }
 
-    [[nodiscard]] reference operator[](const difference_type value) const {
-        return it[value].element;
+    [[nodiscard]] constexpr reference operator[](const difference_type value) const noexcept {
+        return it[value].second;
     }
 
-    [[nodiscard]] pointer operator->() const {
-        return std::addressof(it->element);
+    [[nodiscard]] constexpr pointer operator->() const noexcept {
+        return std::addressof(it->second);
     }
 
-    [[nodiscard]] reference operator*() const {
+    [[nodiscard]] constexpr reference operator*() const noexcept {
         return *operator->();
     }
 
-    template<typename ILhs, typename IRhs>
-    friend auto operator-(const dense_hash_set_iterator<ILhs> &, const dense_hash_set_iterator<IRhs> &) ENTT_NOEXCEPT;
+    template<typename Lhs, typename Rhs>
+    friend constexpr std::ptrdiff_t operator-(const dense_set_iterator<Lhs> &, const dense_set_iterator<Rhs> &) noexcept;
 
-    template<typename ILhs, typename IRhs>
-    friend bool operator==(const dense_hash_set_iterator<ILhs> &, const dense_hash_set_iterator<IRhs> &) ENTT_NOEXCEPT;
+    template<typename Lhs, typename Rhs>
+    friend constexpr bool operator==(const dense_set_iterator<Lhs> &, const dense_set_iterator<Rhs> &) noexcept;
 
-    template<typename ILhs, typename IRhs>
-    friend bool operator<(const dense_hash_set_iterator<ILhs> &, const dense_hash_set_iterator<IRhs> &) ENTT_NOEXCEPT;
+    template<typename Lhs, typename Rhs>
+    friend constexpr bool operator<(const dense_set_iterator<Lhs> &, const dense_set_iterator<Rhs> &) noexcept;
 
 private:
     It it;
 };
 
-template<typename ILhs, typename IRhs>
-[[nodiscard]] auto operator-(const dense_hash_set_iterator<ILhs> &lhs, const dense_hash_set_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+template<typename Lhs, typename Rhs>
+[[nodiscard]] constexpr std::ptrdiff_t operator-(const dense_set_iterator<Lhs> &lhs, const dense_set_iterator<Rhs> &rhs) noexcept {
     return lhs.it - rhs.it;
 }
 
-template<typename ILhs, typename IRhs>
-[[nodiscard]] bool operator==(const dense_hash_set_iterator<ILhs> &lhs, const dense_hash_set_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+template<typename Lhs, typename Rhs>
+[[nodiscard]] constexpr bool operator==(const dense_set_iterator<Lhs> &lhs, const dense_set_iterator<Rhs> &rhs) noexcept {
     return lhs.it == rhs.it;
 }
 
-template<typename ILhs, typename IRhs>
-[[nodiscard]] bool operator!=(const dense_hash_set_iterator<ILhs> &lhs, const dense_hash_set_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+template<typename Lhs, typename Rhs>
+[[nodiscard]] constexpr bool operator!=(const dense_set_iterator<Lhs> &lhs, const dense_set_iterator<Rhs> &rhs) noexcept {
     return !(lhs == rhs);
 }
 
-template<typename ILhs, typename IRhs>
-[[nodiscard]] bool operator<(const dense_hash_set_iterator<ILhs> &lhs, const dense_hash_set_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+template<typename Lhs, typename Rhs>
+[[nodiscard]] constexpr bool operator<(const dense_set_iterator<Lhs> &lhs, const dense_set_iterator<Rhs> &rhs) noexcept {
     return lhs.it < rhs.it;
 }
 
-template<typename ILhs, typename IRhs>
-[[nodiscard]] bool operator>(const dense_hash_set_iterator<ILhs> &lhs, const dense_hash_set_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+template<typename Lhs, typename Rhs>
+[[nodiscard]] constexpr bool operator>(const dense_set_iterator<Lhs> &lhs, const dense_set_iterator<Rhs> &rhs) noexcept {
     return rhs < lhs;
 }
 
-template<typename ILhs, typename IRhs>
-[[nodiscard]] bool operator<=(const dense_hash_set_iterator<ILhs> &lhs, const dense_hash_set_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+template<typename Lhs, typename Rhs>
+[[nodiscard]] constexpr bool operator<=(const dense_set_iterator<Lhs> &lhs, const dense_set_iterator<Rhs> &rhs) noexcept {
     return !(lhs > rhs);
 }
 
-template<typename ILhs, typename IRhs>
-[[nodiscard]] bool operator>=(const dense_hash_set_iterator<ILhs> &lhs, const dense_hash_set_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+template<typename Lhs, typename Rhs>
+[[nodiscard]] constexpr bool operator>=(const dense_set_iterator<Lhs> &lhs, const dense_set_iterator<Rhs> &rhs) noexcept {
     return !(lhs < rhs);
 }
 
 template<typename It>
-class dense_hash_set_local_iterator {
-    friend dense_hash_set_local_iterator<const std::remove_pointer_t<It> *>;
-
-    using iterator_traits = std::iterator_traits<decltype(std::addressof(std::as_const(std::declval<It>()->element)))>;
+class dense_set_local_iterator final {
+    template<typename>
+    friend class dense_set_local_iterator;
 
 public:
-    using value_type = typename iterator_traits::value_type;
-    using pointer = typename iterator_traits::pointer;
-    using reference = typename iterator_traits::reference;
-    using difference_type = typename iterator_traits::difference_type;
+    using value_type = 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_hash_set_local_iterator() ENTT_NOEXCEPT = default;
+    constexpr dense_set_local_iterator() noexcept
+        : it{},
+          offset{} {}
 
-    dense_hash_set_local_iterator(It iter, const std::size_t pos) ENTT_NOEXCEPT
+    constexpr dense_set_local_iterator(It iter, const std::size_t pos) noexcept
         : it{iter},
           offset{pos} {}
 
-    template<bool Const = std::is_const_v<std::remove_pointer_t<It>>, typename = std::enable_if_t<Const>>
-    dense_hash_set_local_iterator(const dense_hash_set_local_iterator<std::remove_const_t<std::remove_pointer_t<It>> *> &other)
+    template<typename Other, typename = std::enable_if_t<!std::is_same_v<It, Other> && std::is_constructible_v<It, Other>>>
+    constexpr dense_set_local_iterator(const dense_set_local_iterator<Other> &other) noexcept
         : it{other.it},
           offset{other.offset} {}
 
-    dense_hash_set_local_iterator &operator++() ENTT_NOEXCEPT {
-        return offset = it[offset].next, *this;
+    constexpr dense_set_local_iterator &operator++() noexcept {
+        return offset = it[offset].first, *this;
     }
 
-    dense_hash_set_local_iterator operator++(int) ENTT_NOEXCEPT {
-        dense_hash_set_local_iterator orig = *this;
+    constexpr dense_set_local_iterator operator++(int) noexcept {
+        dense_set_local_iterator orig = *this;
         return ++(*this), orig;
     }
 
-    [[nodiscard]] pointer operator->() const {
-        return std::addressof(it[offset].element);
+    [[nodiscard]] constexpr pointer operator->() const noexcept {
+        return std::addressof(it[offset].second);
     }
 
-    [[nodiscard]] reference operator*() const {
+    [[nodiscard]] constexpr reference operator*() const noexcept {
         return *operator->();
     }
 
-    [[nodiscard]] std::size_t index() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr std::size_t index() const noexcept {
         return offset;
     }
 
@@ -206,22 +191,18 @@ private:
     std::size_t offset;
 };
 
-template<typename ILhs, typename IRhs>
-[[nodiscard]] bool operator==(const dense_hash_set_local_iterator<ILhs> &lhs, const dense_hash_set_local_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+template<typename Lhs, typename Rhs>
+[[nodiscard]] constexpr bool operator==(const dense_set_local_iterator<Lhs> &lhs, const dense_set_local_iterator<Rhs> &rhs) noexcept {
     return lhs.index() == rhs.index();
 }
 
-template<typename ILhs, typename IRhs>
-[[nodiscard]] bool operator!=(const dense_hash_set_local_iterator<ILhs> &lhs, const dense_hash_set_local_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+template<typename Lhs, typename Rhs>
+[[nodiscard]] constexpr bool operator!=(const dense_set_local_iterator<Lhs> &lhs, const dense_set_local_iterator<Rhs> &rhs) noexcept {
     return !(lhs == rhs);
 }
 
 } // namespace internal
-
-/**
- * Internal details not to be documented.
- * @endcond
- */
+/*! @endcond */
 
 /**
  * @brief Associative container for unique objects of a given type.
@@ -236,27 +217,26 @@ template<typename ILhs, typename IRhs>
  * @tparam Allocator Type of allocator used to manage memory and elements.
  */
 template<typename Type, typename Hash, typename KeyEqual, typename Allocator>
-class dense_hash_set final {
+class dense_set {
     static constexpr float default_threshold = 0.875f;
     static constexpr std::size_t minimum_capacity = 8u;
 
-    using allocator_traits = std::allocator_traits<Allocator>;
-    using alloc = typename allocator_traits::template rebind_alloc<Type>;
-    using alloc_traits = typename std::allocator_traits<alloc>;
-
-    using node_type = internal::dense_hash_set_node<Type>;
+    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>>;
 
-    [[nodiscard]] std::size_t hash_to_bucket(const std::size_t hash) const ENTT_NOEXCEPT {
-        return fast_mod(hash, bucket_count());
+    template<typename Other>
+    [[nodiscard]] std::size_t value_to_bucket(const Other &value) const noexcept {
+        return fast_mod(static_cast<size_type>(sparse.second()(value)), bucket_count());
     }
 
     template<typename Other>
     [[nodiscard]] auto constrained_find(const Other &value, std::size_t bucket) {
         for(auto it = begin(bucket), last = end(bucket); it != last; ++it) {
             if(packed.second()(*it, value)) {
-                return begin() + it.index();
+                return begin() + static_cast<typename iterator::difference_type>(it.index());
             }
         }
 
@@ -267,61 +247,45 @@ class dense_hash_set final {
     [[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() + it.index();
+                return cbegin() + static_cast<typename iterator::difference_type>(it.index());
             }
         }
 
         return cend();
     }
 
-    template<typename Arg>
-    [[nodiscard]] auto get_or_emplace(Arg &&arg) {
-        const auto hash = sparse.second()(arg);
-        auto index = hash_to_bucket(hash);
+    template<typename Other>
+    [[nodiscard]] auto insert_or_do_nothing(Other &&value) {
+        const auto index = value_to_bucket(value);
 
-        if(auto it = constrained_find(arg, index); it != end()) {
+        if(auto it = constrained_find(value, index); it != end()) {
             return std::make_pair(it, false);
         }
 
-        if(const auto count = size() + 1u; count > (bucket_count() * max_load_factor())) {
-            rehash(bucket_count() * 2u);
-            index = hash_to_bucket(hash);
-        }
-
-        packed.first().emplace_back(sparse.first()[index], std::forward<Arg>(arg));
-        // update goes after emplace to enforce exception guarantees
-        sparse.first()[index] = size() - 1u;
+        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);
     }
 
-    template<typename Other>
-    bool do_erase(const Other &value) {
-        for(size_type *curr = sparse.first().data() + bucket(value); *curr != std::numeric_limits<size_type>::max(); curr = &packed.first()[*curr].next) {
-            if(packed.second()(packed.first()[*curr].element, value)) {
-                const auto index = *curr;
-                *curr = packed.first()[*curr].next;
-                move_and_pop(index);
-                return true;
-            }
-        }
-
-        return false;
-    }
-
     void move_and_pop(const std::size_t pos) {
         if(const auto last = size() - 1u; pos != last) {
-            size_type *curr = sparse.first().data() + bucket(packed.first().back().element);
-            for(; *curr != last; curr = &packed.first()[*curr].next) {}
-            *curr = pos;
-
-            // basic exception guarantees when value type has a throwing move constructor
+            size_type *curr = sparse.first().data() + value_to_bucket(packed.first().back().second);
             packed.first()[pos] = std::move(packed.first().back());
+            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;
@@ -336,179 +300,194 @@ public:
     /*! @brief Allocator type. */
     using allocator_type = Allocator;
     /*! @brief Random access iterator type. */
-    using iterator = internal::dense_hash_set_iterator<typename packed_container_type::pointer>;
+    using iterator = internal::dense_set_iterator<typename packed_container_type::iterator>;
     /*! @brief Constant random access iterator type. */
-    using const_iterator = internal::dense_hash_set_iterator<typename packed_container_type::const_pointer>;
+    using const_iterator = internal::dense_set_iterator<typename packed_container_type::const_iterator>;
+    /*! @brief Reverse iterator type. */
+    using reverse_iterator = std::reverse_iterator<iterator>;
+    /*! @brief Constant reverse iterator type. */
+    using const_reverse_iterator = std::reverse_iterator<const_iterator>;
     /*! @brief Forward iterator type. */
-    using local_iterator = internal::dense_hash_set_local_iterator<typename packed_container_type::pointer>;
+    using local_iterator = internal::dense_set_local_iterator<typename packed_container_type::iterator>;
     /*! @brief Constant forward iterator type. */
-    using const_local_iterator = internal::dense_hash_set_local_iterator<typename packed_container_type::const_pointer>;
+    using const_local_iterator = internal::dense_set_local_iterator<typename packed_container_type::const_iterator>;
 
     /*! @brief Default constructor. */
-    dense_hash_set()
-        : dense_hash_set(minimum_capacity) {}
+    dense_set()
+        : dense_set{minimum_capacity} {}
 
     /**
      * @brief Constructs an empty container with a given allocator.
      * @param allocator The allocator to use.
      */
-    explicit dense_hash_set(const allocator_type &allocator)
-        : dense_hash_set{minimum_capacity, hasher{}, key_equal{}, allocator} {}
+    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 cnt Minimal number of buckets.
      * @param allocator The allocator to use.
      */
-    dense_hash_set(const size_type bucket_count, const allocator_type &allocator)
-        : dense_hash_set{bucket_count, hasher{}, key_equal{}, allocator} {}
+    dense_set(const size_type cnt, const allocator_type &allocator)
+        : dense_set{cnt, 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 cnt Minimal number of buckets.
      * @param hash Hash function to use.
      * @param allocator The allocator to use.
      */
-    dense_hash_set(const size_type bucket_count, const hasher &hash, const allocator_type &allocator)
-        : dense_hash_set{bucket_count, hash, key_equal{}, allocator} {}
+    dense_set(const size_type cnt, const hasher &hash, const allocator_type &allocator)
+        : dense_set{cnt, 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 cnt Minimal number of buckets.
      * @param hash Hash function to use.
      * @param equal Compare function to use.
      * @param allocator The allocator to use.
      */
-    explicit dense_hash_set(const size_type bucket_count, const hasher &hash = hasher{}, const key_equal &equal = key_equal{}, const allocator_type &allocator = allocator_type())
+    explicit dense_set(const size_type cnt, 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);
+        rehash(cnt);
     }
 
-    /**
-     * @brief Copy constructor.
-     * @param other The instance to copy from.
-     */
-    dense_hash_set(const dense_hash_set &other)
-        : dense_hash_set{other, alloc_traits::select_on_container_copy_construction(other.get_allocator())} {}
+    /*! @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_hash_set(const dense_hash_set &other, const allocator_type &allocator)
-        : sparse{sparse_container_type{other.sparse.first(), allocator}, other.sparse.second()},
-          // cannot copy the container directly due to a nasty issue of apple clang :(
-          packed{packed_container_type{other.packed.first().begin(), other.packed.first().end(), allocator}, other.packed.second()},
-          threshold{other.threshold} {
-    }
+    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.
-     * @param other The instance to move from.
-     */
-    dense_hash_set(dense_hash_set &&other) ENTT_NOEXCEPT = default;
+    /*! @brief Default move constructor. */
+    dense_set(dense_set &&) noexcept(std::is_nothrow_move_constructible_v<compressed_pair<sparse_container_type, hasher>> &&std::is_nothrow_move_constructible_v<compressed_pair<packed_container_type, key_equal>>) = default;
 
     /**
      * @brief Allocator-extended move constructor.
      * @param other The instance to move from.
      * @param allocator The allocator to use.
      */
-    dense_hash_set(dense_hash_set &&other, const allocator_type &allocator) ENTT_NOEXCEPT
-        : sparse{sparse_container_type{std::move(other.sparse.first()), allocator}, std::move(other.sparse.second())},
-          // cannot move the container directly due to a nasty issue of apple clang :(
-          packed{packed_container_type{std::make_move_iterator(other.packed.first().begin()), std::make_move_iterator(other.packed.first().end()), allocator}, std::move(other.packed.second())},
+    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 destructor. */
-    ~dense_hash_set() = default;
-
     /**
-     * @brief Copy assignment operator.
-     * @param other The instance to copy from.
+     * @brief Default copy assignment operator.
      * @return This container.
      */
-    dense_hash_set &operator=(const dense_hash_set &other) {
-        threshold = other.threshold;
-        sparse.first().clear();
-        packed.first().clear();
-        rehash(other.bucket_count());
-        packed.first().reserve(other.packed.first().size());
-        insert(other.cbegin(), other.cend());
-        return *this;
-    }
+    dense_set &operator=(const dense_set &) = default;
 
     /**
      * @brief Default move assignment operator.
-     * @param other The instance to move from.
      * @return This container.
      */
-    dense_hash_set &operator=(dense_hash_set &&other) ENTT_NOEXCEPT = default;
+    dense_set &operator=(dense_set &&) noexcept(std::is_nothrow_move_assignable_v<compressed_pair<sparse_container_type, hasher>> &&std::is_nothrow_move_assignable_v<compressed_pair<packed_container_type, key_equal>>) = default;
 
     /**
      * @brief Returns the associated allocator.
      * @return The associated allocator.
      */
-    [[nodiscard]] constexpr allocator_type get_allocator() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr allocator_type get_allocator() const 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().data();
+    [[nodiscard]] const_iterator cbegin() const noexcept {
+        return packed.first().begin();
     }
 
     /*! @copydoc cbegin */
-    [[nodiscard]] const_iterator begin() const ENTT_NOEXCEPT {
+    [[nodiscard]] const_iterator begin() const noexcept {
         return cbegin();
     }
 
     /*! @copydoc begin */
-    [[nodiscard]] iterator begin() ENTT_NOEXCEPT {
-        return packed.first().data();
+    [[nodiscard]] iterator begin() 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().data() + size();
+    [[nodiscard]] const_iterator cend() const noexcept {
+        return packed.first().end();
     }
 
     /*! @copydoc cend */
-    [[nodiscard]] const_iterator end() const ENTT_NOEXCEPT {
+    [[nodiscard]] const_iterator end() const noexcept {
         return cend();
     }
 
     /*! @copydoc end */
-    [[nodiscard]] iterator end() ENTT_NOEXCEPT {
-        return packed.first().data() + size();
+    [[nodiscard]] iterator end() noexcept {
+        return packed.first().end();
+    }
+
+    /**
+     * @brief Returns a reverse iterator to the beginning.
+     *
+     * If the array is empty, the returned iterator will be equal to `rend()`.
+     *
+     * @return An iterator to the first instance of the reversed internal array.
+     */
+    [[nodiscard]] const_reverse_iterator crbegin() const noexcept {
+        return std::make_reverse_iterator(cend());
+    }
+
+    /*! @copydoc crbegin */
+    [[nodiscard]] const_reverse_iterator rbegin() const noexcept {
+        return crbegin();
+    }
+
+    /*! @copydoc rbegin */
+    [[nodiscard]] reverse_iterator rbegin() noexcept {
+        return std::make_reverse_iterator(end());
+    }
+
+    /**
+     * @brief Returns a reverse iterator to the end.
+     * @return An iterator to the element following the last instance of the
+     * reversed internal array.
+     */
+    [[nodiscard]] const_reverse_iterator crend() const noexcept {
+        return std::make_reverse_iterator(cbegin());
+    }
+
+    /*! @copydoc crend */
+    [[nodiscard]] const_reverse_iterator rend() const noexcept {
+        return crend();
+    }
+
+    /*! @copydoc rend */
+    [[nodiscard]] reverse_iterator rend() noexcept {
+        return std::make_reverse_iterator(begin());
     }
 
     /**
      * @brief Checks whether a container is empty.
      * @return True if the container is empty, false otherwise.
      */
-    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+    [[nodiscard]] bool empty() const noexcept {
         return packed.first().empty();
     }
 
@@ -516,12 +495,20 @@ public:
      * @brief Returns the number of elements in a container.
      * @return Number of elements in a container.
      */
-    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+    [[nodiscard]] size_type size() const noexcept {
         return packed.first().size();
     }
 
+    /**
+     * @brief Returns the maximum possible number of elements.
+     * @return Maximum possible number of elements.
+     */
+    [[nodiscard]] size_type max_size() const noexcept {
+        return packed.first().max_size();
+    }
+
     /*! @brief Clears the container. */
-    void clear() ENTT_NOEXCEPT {
+    void clear() noexcept {
         sparse.first().clear();
         packed.first().clear();
         rehash(0u);
@@ -535,12 +522,12 @@ public:
      * insertion took place.
      */
     std::pair<iterator, bool> insert(const value_type &value) {
-        return emplace(value);
+        return insert_or_do_nothing(value);
     }
 
     /*! @copydoc insert */
     std::pair<iterator, bool> insert(value_type &&value) {
-        return emplace(std::move(value));
+        return insert_or_do_nothing(std::move(value));
     }
 
     /**
@@ -552,12 +539,16 @@ public:
     template<typename It>
     void insert(It first, It last) {
         for(; first != last; ++first) {
-            emplace(*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.
@@ -567,10 +558,21 @@ public:
      */
     template<typename... Args>
     std::pair<iterator, bool> emplace(Args &&...args) {
-        if constexpr(((sizeof...(Args) == 1u) && ... && std::is_same_v<std::remove_const_t<std::remove_reference_t<Args>>, value_type>)) {
-            return get_or_emplace(std::forward<Args>(args)...);
+        if constexpr(((sizeof...(Args) == 1u) && ... && std::is_same_v<std::decay_t<Args>, value_type>)) {
+            return insert_or_do_nothing(std::forward<Args>(args)...);
         } else {
-            return get_or_emplace(value_type{std::forward<Args>(args)...});
+            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);
         }
     }
 
@@ -580,9 +582,9 @@ public:
      * @return An iterator following the removed element.
      */
     iterator erase(const_iterator pos) {
-        const auto dist = std::distance(cbegin(), pos);
+        const auto diff = pos - cbegin();
         erase(*pos);
-        return begin() + dist;
+        return begin() + diff;
     }
 
     /**
@@ -592,13 +594,13 @@ public:
      * @return An iterator following the last removed element.
      */
     iterator erase(const_iterator first, const_iterator last) {
-        const auto dist = std::distance(cbegin(), first);
+        const auto dist = first - cbegin();
 
-        for(auto rfirst = std::make_reverse_iterator(last), rlast = std::make_reverse_iterator(first); rfirst != rlast; ++rfirst) {
-            erase(*rfirst);
+        for(auto from = last - cbegin(); from != dist; --from) {
+            erase(packed.first()[from - 1u].second);
         }
 
-        return dist > static_cast<decltype(dist)>(size()) ? end() : (begin() + dist);
+        return (begin() + dist);
     }
 
     /**
@@ -607,20 +609,50 @@ public:
      * @return Number of elements removed (either 0 or 1).
      */
     size_type erase(const value_type &value) {
-        return do_erase(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_hash_set &other) {
+    void swap(dense_set &other) {
         using std::swap;
         swap(sparse, other.sparse);
         swap(packed, other.packed);
         swap(threshold, other.threshold);
     }
 
+    /**
+     * @brief Returns the number of elements matching a value (either 1 or 0).
+     * @param key Key value of an element to search for.
+     * @return Number of elements matching the key (either 1 or 0).
+     */
+    [[nodiscard]] size_type count(const value_type &key) const {
+        return find(key) != end();
+    }
+
+    /**
+     * @brief Returns the number of elements matching a key (either 1 or 0).
+     * @tparam Other Type of the key value of an element to search for.
+     * @param key Key value of an element to search for.
+     * @return Number of elements matching the key (either 1 or 0).
+     */
+    template<typename Other>
+    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, size_type>>
+    count(const Other &key) const {
+        return find(key) != end();
+    }
+
     /**
      * @brief Finds an element with a given value.
      * @param value Value of an element to search for.
@@ -628,12 +660,12 @@ public:
      * element is found, a past-the-end iterator is returned.
      */
     [[nodiscard]] iterator find(const value_type &value) {
-        return constrained_find(value, bucket(value));
+        return constrained_find(value, value_to_bucket(value));
     }
 
     /*! @copydoc find */
     [[nodiscard]] const_iterator find(const value_type &value) const {
-        return constrained_find(value, bucket(value));
+        return constrained_find(value, value_to_bucket(value));
     }
 
     /**
@@ -646,14 +678,54 @@ public:
     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, bucket(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, bucket(value));
+        return constrained_find(value, value_to_bucket(value));
+    }
+
+    /**
+     * @brief Returns a range containing all elements with a given value.
+     * @param value Value of an element to search for.
+     * @return A pair of iterators pointing to the first element and past the
+     * last element of the range.
+     */
+    [[nodiscard]] std::pair<iterator, iterator> equal_range(const value_type &value) {
+        const auto it = find(value);
+        return {it, it + !(it == end())};
+    }
+
+    /*! @copydoc equal_range */
+    [[nodiscard]] std::pair<const_iterator, const_iterator> equal_range(const value_type &value) const {
+        const auto it = find(value);
+        return {it, it + !(it == cend())};
+    }
+
+    /**
+     * @brief Returns a range containing all elements that compare _equivalent_
+     * to a given value.
+     * @tparam Other Type of an element to search for.
+     * @param value Value of an element to search for.
+     * @return A pair of iterators pointing to the first element and past the
+     * last element of the range.
+     */
+    template<typename Other>
+    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, std::pair<iterator, iterator>>>
+    equal_range(const Other &value) {
+        const auto it = find(value);
+        return {it, it + !(it == end())};
+    }
+
+    /*! @copydoc equal_range */
+    template<typename Other>
+    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, std::pair<const_iterator, const_iterator>>>
+    equal_range(const Other &value) const {
+        const auto it = find(value);
+        return {it, it + !(it == cend())};
     }
 
     /**
@@ -684,7 +756,7 @@ public:
      * @return An iterator to the beginning of the given bucket.
      */
     [[nodiscard]] const_local_iterator cbegin(const size_type index) const {
-        return {packed.first().data(), sparse.first()[index]};
+        return {packed.first().begin(), sparse.first()[index]};
     }
 
     /**
@@ -702,7 +774,7 @@ public:
      * @return An iterator to the beginning of the given bucket.
      */
     [[nodiscard]] local_iterator begin(const size_type index) {
-        return {packed.first().data(), sparse.first()[index]};
+        return {packed.first().begin(), sparse.first()[index]};
     }
 
     /**
@@ -711,7 +783,7 @@ public:
      * @return An iterator to the end of the given bucket.
      */
     [[nodiscard]] const_local_iterator cend([[maybe_unused]] const size_type index) const {
-        return {packed.first().data(), std::numeric_limits<size_type>::max()};
+        return {packed.first().begin(), (std::numeric_limits<size_type>::max)()};
     }
 
     /**
@@ -719,7 +791,7 @@ public:
      * @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 {
+    [[nodiscard]] const_local_iterator end(const size_type index) const {
         return cend(index);
     }
 
@@ -729,7 +801,7 @@ public:
      * @return An iterator to the end of the given bucket.
      */
     [[nodiscard]] local_iterator end([[maybe_unused]] const size_type index) {
-        return {packed.first().data(), std::numeric_limits<size_type>::max()};
+        return {packed.first().begin(), (std::numeric_limits<size_type>::max)()};
     }
 
     /**
@@ -763,7 +835,7 @@ public:
      * @return The bucket for the given element.
      */
     [[nodiscard]] size_type bucket(const value_type &value) const {
-        return hash_to_bucket(sparse.second()(value));
+        return value_to_bucket(value);
     }
 
     /**
@@ -795,19 +867,23 @@ public:
     /**
      * @brief Reserves at least the specified number of buckets and regenerates
      * the hash table.
-     * @param count New number of buckets.
+     * @param cnt 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()));
+    void rehash(const size_type cnt) {
+        auto value = cnt > minimum_capacity ? cnt : minimum_capacity;
+        const auto cap = static_cast<size_type>(size() / max_load_factor());
+        value = value > cap ? value : cap;
 
         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(auto &&elem: sparse.first()) {
+                elem = (std::numeric_limits<size_type>::max)();
+            }
 
             for(size_type pos{}, last = size(); pos < last; ++pos) {
-                const auto index = bucket(packed.first()[pos].element);
-                packed.first()[pos].next = std::exchange(sparse.first()[index], pos);
+                const auto index = value_to_bucket(packed.first()[pos].second);
+                packed.first()[pos].first = std::exchange(sparse.first()[index], pos);
             }
         }
     }
@@ -815,11 +891,11 @@ public:
     /**
      * @brief Reserves space for at least the specified number of elements and
      * regenerates the hash table.
-     * @param count New number of elements.
+     * @param cnt 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())));
+    void reserve(const size_type cnt) {
+        packed.first().reserve(cnt);
+        rehash(static_cast<size_type>(std::ceil(cnt / max_load_factor())));
     }
 
     /**

src/entt/container/fwd.hpp

@@ -3,22 +3,24 @@
 
 #include <functional>
 #include <memory>
+#include <utility>
 
 namespace entt {
 
 template<
-    typename Key, typename Type,
+    typename Key,
+    typename Type,
     typename = std::hash<Key>,
     typename = std::equal_to<Key>,
     typename = std::allocator<std::pair<const Key, Type>>>
-class dense_hash_map;
+class dense_map;
 
 template<
     typename Type,
     typename = std::hash<Type>,
     typename = std::equal_to<Type>,
     typename = std::allocator<Type>>
-class dense_hash_set;
+class dense_set;
 
 } // namespace entt
 

src/entt/core/algorithm.hpp

@@ -95,14 +95,15 @@ struct radix_sort {
     template<typename It, typename Getter = identity>
     void operator()(It first, It last, Getter getter = Getter{}) const {
         if(first < last) {
-            static constexpr auto mask = (1 << Bit) - 1;
-            static constexpr auto buckets = 1 << Bit;
-            static constexpr auto passes = N / Bit;
+            constexpr auto passes = N / Bit;
 
             using value_type = typename std::iterator_traits<It>::value_type;
             std::vector<value_type> aux(std::distance(first, last));
 
             auto part = [getter = std::move(getter)](auto from, auto to, auto out, auto start) {
+                constexpr auto mask = (1 << Bit) - 1;
+                constexpr auto buckets = 1 << Bit;
+
                 std::size_t index[buckets]{};
                 std::size_t count[buckets]{};
 

src/entt/core/any.hpp

@@ -13,6 +13,32 @@
 
 namespace entt {
 
+/*! @cond TURN_OFF_DOXYGEN */
+namespace internal {
+
+enum class any_operation : std::uint8_t {
+    copy,
+    move,
+    transfer,
+    assign,
+    destroy,
+    compare,
+    get
+};
+
+} // namespace internal
+/*! @endcond */
+
+/*! @brief Possible modes of an any object. */
+enum class any_policy : std::uint8_t {
+    /*! @brief Default mode, the object owns the contained element. */
+    owner,
+    /*! @brief Aliasing mode, the object _points_ to a non-const element. */
+    ref,
+    /*! @brief Const aliasing mode, the object _points_ to a const element. */
+    cref
+};
+
 /**
  * @brief A SBO friendly, type-safe container for single values of any type.
  * @tparam Len Size of the storage reserved for the small buffer optimization.
@@ -20,82 +46,70 @@ namespace entt {
  */
 template<std::size_t Len, std::size_t Align>
 class basic_any {
-    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 operation = internal::any_operation;
     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>;
+    struct storage_type {
+        alignas(Align) std::byte data[Len + !Len];
+    };
 
     template<typename Type>
-    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_reference_t<std::remove_const_t<Type>>, Type>, "Invalid type");
-        const Type *instance = nullptr;
+    static constexpr bool in_situ = Len && alignof(Type) <= Align && sizeof(Type) <= Len && std::is_nothrow_move_constructible_v<Type>;
+
+    template<typename Type>
+    static const void *basic_vtable(const operation op, const basic_any &value, const void *other) {
+        static_assert(!std::is_void_v<Type> && std::is_same_v<std::remove_cv_t<std::remove_reference_t<Type>>, Type>, "Invalid type");
+        const Type *element = nullptr;
 
         if constexpr(in_situ<Type>) {
-            instance = (value.mode == policy::owner) ? ENTT_LAUNDER(reinterpret_cast<const Type *>(&value.storage)) : static_cast<const Type *>(value.instance);
+            element = (value.mode == any_policy::owner) ? reinterpret_cast<const Type *>(&value.storage) : static_cast<const Type *>(value.instance);
         } else {
-            instance = static_cast<const Type *>(value.instance);
+            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>(*instance);
+                static_cast<basic_any *>(const_cast<void *>(other))->initialize<Type>(*element);
             }
             break;
         case operation::move:
             if constexpr(in_situ<Type>) {
-                if(value.mode == policy::owner) {
-                    return new(&static_cast<basic_any *>(const_cast<void *>(other))->storage) Type{std::move(*const_cast<Type *>(instance))};
+                if(value.mode == any_policy::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 *>(instance) = std::move(*static_cast<Type *>(const_cast<void *>(other)));
+                *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 *>(instance) = *static_cast<const Type *>(other);
+                *const_cast<Type *>(element) = *static_cast<const Type *>(other);
                 return other;
             }
             break;
         case operation::destroy:
             if constexpr(in_situ<Type>) {
-                instance->~Type();
+                element->~Type();
             } else if constexpr(std::is_array_v<Type>) {
-                delete[] instance;
+                delete[] element;
             } else {
-                delete instance;
+                delete element;
             }
             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 *>(instance) == *static_cast<const Type *>(other) ? other : nullptr;
+                return *element == *static_cast<const Type *>(other) ? other : nullptr;
             } else {
-                return (instance == other) ? other : nullptr;
+                return (element == other) ? other : nullptr;
             }
         case operation::get:
-            return instance;
+            return element;
         }
 
         return nullptr;
@@ -103,31 +117,32 @@ class basic_any {
 
     template<typename Type, typename... Args>
     void initialize([[maybe_unused]] Args &&...args) {
+        info = &type_id<std::remove_cv_t<std::remove_reference_t<Type>>>();
+
         if constexpr(!std::is_void_v<Type>) {
-            vtable = basic_vtable<std::remove_const_t<std::remove_reference_t<Type>>>;
-            info = &type_id<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;
+                static_assert((std::is_lvalue_reference_v<Args> && ...) && (sizeof...(Args) == 1u), "Invalid arguments");
+                mode = std::is_const_v<std::remove_reference_t<Type>> ? any_policy::cref : any_policy::ref;
                 instance = (std::addressof(args), ...);
-            } else if constexpr(in_situ<Type>) {
-                if constexpr(sizeof...(Args) != 0u && std::is_aggregate_v<Type>) {
-                    new(&storage) Type{std::forward<Args>(args)...};
+            } else if constexpr(in_situ<std::remove_cv_t<std::remove_reference_t<Type>>>) {
+                if constexpr(std::is_aggregate_v<std::remove_cv_t<std::remove_reference_t<Type>>> && (sizeof...(Args) != 0u || !std::is_default_constructible_v<std::remove_cv_t<std::remove_reference_t<Type>>>)) {
+                    new(&storage) std::remove_cv_t<std::remove_reference_t<Type>>{std::forward<Args>(args)...};
                 } else {
-                    new(&storage) Type(std::forward<Args>(args)...);
+                    new(&storage) std::remove_cv_t<std::remove_reference_t<Type>>(std::forward<Args>(args)...);
                 }
             } else {
-                if constexpr(sizeof...(Args) != 0u && std::is_aggregate_v<Type>) {
-                    instance = new Type{std::forward<Args>(args)...};
+                if constexpr(std::is_aggregate_v<std::remove_cv_t<std::remove_reference_t<Type>>> && (sizeof...(Args) != 0u || !std::is_default_constructible_v<std::remove_cv_t<std::remove_reference_t<Type>>>)) {
+                    instance = new std::remove_cv_t<std::remove_reference_t<Type>>{std::forward<Args>(args)...};
                 } else {
-                    instance = new Type(std::forward<Args>(args)...);
+                    instance = new std::remove_cv_t<std::remove_reference_t<Type>>(std::forward<Args>(args)...);
                 }
             }
         }
     }
 
-    basic_any(const basic_any &other, const policy pol) ENTT_NOEXCEPT
+    basic_any(const basic_any &other, const any_policy pol) noexcept
         : instance{other.data()},
           info{other.info},
           vtable{other.vtable},
@@ -140,11 +155,8 @@ public:
     static constexpr auto alignment = Align;
 
     /*! @brief Default constructor. */
-    basic_any() ENTT_NOEXCEPT
-        : instance{},
-          info{&type_id<void>()},
-          vtable{},
-          mode{policy::owner} {}
+    constexpr basic_any() noexcept
+        : basic_any{std::in_place_type<void>} {}
 
     /**
      * @brief Constructs a wrapper by directly initializing the new object.
@@ -154,7 +166,10 @@ public:
      */
     template<typename Type, typename... Args>
     explicit basic_any(std::in_place_type_t<Type>, Args &&...args)
-        : basic_any{} {
+        : instance{},
+          info{},
+          vtable{},
+          mode{any_policy::owner} {
         initialize<Type>(std::forward<Args>(args)...);
     }
 
@@ -165,9 +180,7 @@ public:
      */
     template<typename Type, typename = std::enable_if_t<!std::is_same_v<std::decay_t<Type>, basic_any>>>
     basic_any(Type &&value)
-        : basic_any{} {
-        initialize<std::decay_t<Type>>(std::forward<Type>(value));
-    }
+        : basic_any{std::in_place_type<std::decay_t<Type>>, std::forward<Type>(value)} {}
 
     /**
      * @brief Copy constructor.
@@ -184,7 +197,7 @@ public:
      * @brief Move constructor.
      * @param other The instance to move from.
      */
-    basic_any(basic_any &&other) ENTT_NOEXCEPT
+    basic_any(basic_any &&other) noexcept
         : instance{},
           info{other.info},
           vtable{other.vtable},
@@ -196,7 +209,7 @@ public:
 
     /*! @brief Frees the internal storage, whatever it means. */
     ~basic_any() {
-        if(vtable && mode == policy::owner) {
+        if(vtable && (mode == any_policy::owner)) {
             vtable(operation::destroy, *this, nullptr);
         }
     }
@@ -221,7 +234,7 @@ public:
      * @param other The instance to move from.
      * @return This any object.
      */
-    basic_any &operator=(basic_any &&other) ENTT_NOEXCEPT {
+    basic_any &operator=(basic_any &&other) noexcept {
         reset();
 
         if(other.vtable) {
@@ -251,7 +264,7 @@ public:
      * @brief Returns the object type if any, `type_id<void>()` otherwise.
      * @return The object type if any, `type_id<void>()` otherwise.
      */
-    [[nodiscard]] const type_info &type() const ENTT_NOEXCEPT {
+    [[nodiscard]] const type_info &type() const noexcept {
         return *info;
     }
 
@@ -259,7 +272,7 @@ 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 noexcept {
         return vtable ? vtable(operation::get, *this, nullptr) : nullptr;
     }
 
@@ -268,7 +281,7 @@ public:
      * @param req Expected type.
      * @return An opaque pointer the contained instance, if any.
      */
-    [[nodiscard]] const void *data(const type_info &req) const ENTT_NOEXCEPT {
+    [[nodiscard]] const void *data(const type_info &req) const noexcept {
         return *info == req ? data() : nullptr;
     }
 
@@ -276,8 +289,8 @@ public:
      * @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));
+    [[nodiscard]] void *data() noexcept {
+        return mode == any_policy::cref ? nullptr : const_cast<void *>(std::as_const(*this).data());
     }
 
     /**
@@ -285,8 +298,8 @@ public:
      * @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;
+    [[nodiscard]] void *data(const type_info &req) noexcept {
+        return mode == any_policy::cref ? nullptr : const_cast<void *>(std::as_const(*this).data(req));
     }
 
     /**
@@ -302,25 +315,21 @@ public:
     }
 
     /**
-     * @brief Copy assigns a value to the contained object without replacing it.
+     * @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) {
+    bool assign(const basic_any &other) {
+        if(vtable && mode != any_policy::cref && *info == *other.info) {
             return (vtable(operation::assign, *this, other.data()) != nullptr);
         }
 
         return false;
     }
 
-    /**
-     * @brief Move 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(any &&other) {
-        if(vtable && mode != policy::cref && *info == *other.info) {
+    /*! @copydoc assign */
+    bool assign(basic_any &&other) {
+        if(vtable && mode != any_policy::cref && *info == *other.info) {
             if(auto *val = other.data(); val) {
                 return (vtable(operation::transfer, *this, val) != nullptr);
             } else {
@@ -333,20 +342,22 @@ public:
 
     /*! @brief Destroys contained object */
     void reset() {
-        if(vtable && mode == policy::owner) {
+        if(vtable && (mode == any_policy::owner)) {
             vtable(operation::destroy, *this, nullptr);
         }
 
+        // unnecessary but it helps to detect nasty bugs
+        ENTT_ASSERT((instance = nullptr) == nullptr, "");
         info = &type_id<void>();
         vtable = nullptr;
-        mode = policy::owner;
+        mode = any_policy::owner;
     }
 
     /**
      * @brief Returns false if a wrapper is empty, true otherwise.
      * @return False if the wrapper is empty, true otherwise.
      */
-    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+    [[nodiscard]] explicit operator bool() const noexcept {
         return vtable != nullptr;
     }
 
@@ -355,7 +366,7 @@ public:
      * @param other Wrapper with which to compare.
      * @return False if the two objects differ in their content, true otherwise.
      */
-    bool operator==(const basic_any &other) const ENTT_NOEXCEPT {
+    [[nodiscard]] bool operator==(const basic_any &other) const noexcept {
         if(vtable && *info == *other.info) {
             return (vtable(operation::compare, *this, other.data()) != nullptr);
         }
@@ -363,25 +374,42 @@ public:
         return (!vtable && !other.vtable);
     }
 
+    /**
+     * @brief Checks if two wrappers differ in their content.
+     * @param other Wrapper with which to compare.
+     * @return True if the two objects differ in their content, false otherwise.
+     */
+    [[nodiscard]] bool operator!=(const basic_any &other) const noexcept {
+        return !(*this == other);
+    }
+
     /**
      * @brief Aliasing constructor.
      * @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)};
+    [[nodiscard]] basic_any as_ref() noexcept {
+        return basic_any{*this, (mode == any_policy::cref ? any_policy::cref : any_policy::ref)};
     }
 
     /*! @copydoc as_ref */
-    [[nodiscard]] basic_any as_ref() const ENTT_NOEXCEPT {
-        return basic_any{*this, policy::cref};
+    [[nodiscard]] basic_any as_ref() const noexcept {
+        return basic_any{*this, any_policy::cref};
     }
 
     /**
      * @brief Returns true if a wrapper owns its object, false otherwise.
      * @return True if the wrapper owns its object, false otherwise.
      */
-    [[nodiscard]] bool owner() const ENTT_NOEXCEPT {
-        return (mode == policy::owner);
+    [[deprecated("use policy() and any_policy instead")]] [[nodiscard]] bool owner() const noexcept {
+        return (mode == any_policy::owner);
+    }
+
+    /**
+     * @brief Returns the current mode of an any object.
+     * @return The current mode of the any object.
+     */
+    [[nodiscard]] any_policy policy() const noexcept {
+        return mode;
     }
 
 private:
@@ -391,22 +419,9 @@ private:
     };
     const type_info *info;
     vtable_type *vtable;
-    policy mode;
+    any_policy mode;
 };
 
-/**
- * @brief Checks if two wrappers differ in their content.
- * @tparam Len Size of the storage reserved for the small buffer optimization.
- * @tparam Align Alignment requirement.
- * @param lhs A wrapper, either empty or not.
- * @param rhs A wrapper, either empty or not.
- * @return True if the two wrappers differ in their content, false otherwise.
- */
-template<std::size_t Len, std::size_t Align>
-[[nodiscard]] inline bool operator!=(const basic_any<Len, Align> &lhs, const basic_any<Len, Align> &rhs) ENTT_NOEXCEPT {
-    return !(lhs == rhs);
-}
-
 /**
  * @brief Performs type-safe access to the contained object.
  * @tparam Type Type to which conversion is required.
@@ -416,7 +431,7 @@ template<std::size_t Len, std::size_t Align>
  * @return The element converted to the requested type.
  */
 template<typename Type, std::size_t Len, std::size_t Align>
-Type any_cast(const basic_any<Len, Align> &data) ENTT_NOEXCEPT {
+[[nodiscard]] Type any_cast(const basic_any<Len, Align> &data) noexcept {
     const auto *const instance = any_cast<std::remove_reference_t<Type>>(&data);
     ENTT_ASSERT(instance, "Invalid instance");
     return static_cast<Type>(*instance);
@@ -424,7 +439,7 @@ Type any_cast(const basic_any<Len, Align> &data) ENTT_NOEXCEPT {
 
 /*! @copydoc any_cast */
 template<typename Type, std::size_t Len, std::size_t Align>
-Type any_cast(basic_any<Len, Align> &data) ENTT_NOEXCEPT {
+[[nodiscard]] Type any_cast(basic_any<Len, Align> &data) 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");
@@ -433,8 +448,8 @@ Type any_cast(basic_any<Len, Align> &data) ENTT_NOEXCEPT {
 
 /*! @copydoc any_cast */
 template<typename Type, std::size_t Len, std::size_t Align>
-Type any_cast(basic_any<Len, Align> &&data) ENTT_NOEXCEPT {
-    if constexpr(std::is_copy_constructible_v<std::remove_const_t<std::remove_reference_t<Type>>>) {
+[[nodiscard]] Type any_cast(basic_any<Len, Align> &&data) noexcept {
+    if constexpr(std::is_copy_constructible_v<std::remove_cv_t<std::remove_reference_t<Type>>>) {
         if(auto *const instance = any_cast<std::remove_reference_t<Type>>(&data); instance) {
             return static_cast<Type>(std::move(*instance));
         } else {
@@ -449,17 +464,21 @@ Type any_cast(basic_any<Len, Align> &&data) ENTT_NOEXCEPT {
 
 /*! @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 {
-    const auto &info = type_id<std::remove_const_t<std::remove_reference_t<Type>>>();
+[[nodiscard]] const Type *any_cast(const basic_any<Len, Align> *data) noexcept {
+    const auto &info = type_id<std::remove_cv_t<Type>>();
     return static_cast<const Type *>(data->data(info));
 }
 
 /*! @copydoc any_cast */
 template<typename Type, std::size_t Len, std::size_t Align>
-Type *any_cast(basic_any<Len, Align> *data) ENTT_NOEXCEPT {
-    const auto &info = type_id<std::remove_const_t<std::remove_reference_t<Type>>>();
-    // last attempt to make wrappers for const references return their values
-    return static_cast<Type *>(static_cast<constness_as_t<basic_any<Len, Align>, Type> *>(data)->data(info));
+[[nodiscard]] Type *any_cast(basic_any<Len, Align> *data) noexcept {
+    if constexpr(std::is_const_v<Type>) {
+        // last attempt to make wrappers for const references return their values
+        return any_cast<Type>(&std::as_const(*data));
+    } else {
+        const auto &info = type_id<std::remove_cv_t<Type>>();
+        return static_cast<Type *>(data->data(info));
+    }
 }
 
 /**
@@ -472,7 +491,7 @@ 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) {
+[[nodiscard]] basic_any<Len, Align> make_any(Args &&...args) {
     return basic_any<Len, Align>{std::in_place_type<Type>, std::forward<Args>(args)...};
 }
 
@@ -485,8 +504,8 @@ basic_any<Len, Align> make_any(Args &&...args) {
  * @return A properly initialized and not necessarily owning wrapper.
  */
 template<std::size_t Len = basic_any<>::length, std::size_t Align = basic_any<Len>::alignment, typename Type>
-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)};
+[[nodiscard]] basic_any<Len, Align> forward_as_any(Type &&value) {
+    return basic_any<Len, Align>{std::in_place_type<Type &&>, std::forward<Type>(value)};
 }
 
 } // namespace entt

src/entt/core/compressed_pair.hpp

@@ -5,16 +5,11 @@
 #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.
- */
-
+/*! @cond TURN_OFF_DOXYGEN */
 namespace internal {
 
 template<typename Type, std::size_t, typename = void>
@@ -23,22 +18,22 @@ struct compressed_pair_element {
     using const_reference = const Type &;
 
     template<bool Dummy = true, typename = std::enable_if_t<Dummy && std::is_default_constructible_v<Type>>>
-    compressed_pair_element()
+    constexpr compressed_pair_element() noexcept(std::is_nothrow_default_constructible_v<Type>)
         : value{} {}
 
-    template<typename Args, typename = std::enable_if_t<!std::is_same_v<std::remove_const_t<std::remove_reference_t<Args>>, compressed_pair_element>>>
-    compressed_pair_element(Args &&args)
-        : value{std::forward<Args>(args)} {}
+    template<typename Arg, typename = std::enable_if_t<!std::is_same_v<std::remove_cv_t<std::remove_reference_t<Arg>>, compressed_pair_element>>>
+    constexpr compressed_pair_element(Arg &&arg) noexcept(std::is_nothrow_constructible_v<Type, Arg>)
+        : value{std::forward<Arg>(arg)} {}
 
     template<typename... Args, std::size_t... Index>
-    compressed_pair_element(std::tuple<Args...> args, std::index_sequence<Index...>)
-        : value{std::get<Index>(args)...} {}
+    constexpr compressed_pair_element(std::tuple<Args...> args, std::index_sequence<Index...>) noexcept(std::is_nothrow_constructible_v<Type, Args...>)
+        : value{std::forward<Args>(std::get<Index>(args))...} {}
 
-    [[nodiscard]] reference get() ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr reference get() noexcept {
         return value;
     }
 
-    [[nodiscard]] const_reference get() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr const_reference get() const noexcept {
         return value;
     }
 
@@ -53,32 +48,28 @@ struct compressed_pair_element<Type, Tag, std::enable_if_t<is_ebco_eligible_v<Ty
     using base_type = Type;
 
     template<bool Dummy = true, typename = std::enable_if_t<Dummy && std::is_default_constructible_v<base_type>>>
-    compressed_pair_element()
+    constexpr compressed_pair_element() noexcept(std::is_nothrow_default_constructible_v<base_type>)
         : base_type{} {}
 
-    template<typename Args, typename = std::enable_if_t<!std::is_same_v<std::remove_const_t<std::remove_reference_t<Args>>, compressed_pair_element>>>
-    compressed_pair_element(Args &&args)
-        : base_type{std::forward<Args>(args)} {}
+    template<typename Arg, typename = std::enable_if_t<!std::is_same_v<std::remove_cv_t<std::remove_reference_t<Arg>>, compressed_pair_element>>>
+    constexpr compressed_pair_element(Arg &&arg) noexcept(std::is_nothrow_constructible_v<base_type, Arg>)
+        : base_type{std::forward<Arg>(arg)} {}
 
     template<typename... Args, std::size_t... Index>
-    compressed_pair_element(std::tuple<Args...> args, std::index_sequence<Index...>)
-        : base_type{std::get<Index>(args)...} {}
+    constexpr compressed_pair_element(std::tuple<Args...> args, std::index_sequence<Index...>) noexcept(std::is_nothrow_constructible_v<base_type, Args...>)
+        : base_type{std::forward<Args>(std::get<Index>(args))...} {}
 
-    [[nodiscard]] reference get() ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr reference get() noexcept {
         return *this;
     }
 
-    [[nodiscard]] const_reference get() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr const_reference get() const noexcept {
         return *this;
     }
 };
 
 } // namespace internal
-
-/**
- * Internal details not to be documented.
- * @endcond
- */
+/*! @endcond */
 
 /**
  * @brief A compressed pair.
@@ -111,7 +102,7 @@ public:
      * @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()
+    constexpr compressed_pair() noexcept(std::is_nothrow_default_constructible_v<first_base> &&std::is_nothrow_default_constructible_v<second_base>)
         : first_base{},
           second_base{} {}
 
@@ -119,13 +110,13 @@ public:
      * @brief Copy constructor.
      * @param other The instance to copy from.
      */
-    constexpr compressed_pair(const compressed_pair &other) = default;
+    constexpr compressed_pair(const compressed_pair &other) noexcept(std::is_nothrow_copy_constructible_v<first_base> &&std::is_nothrow_copy_constructible_v<second_base>) = default;
 
     /**
      * @brief Move constructor.
      * @param other The instance to move from.
      */
-    constexpr compressed_pair(compressed_pair &&other) ENTT_NOEXCEPT = default;
+    constexpr compressed_pair(compressed_pair &&other) noexcept(std::is_nothrow_move_constructible_v<first_base> &&std::is_nothrow_move_constructible_v<second_base>) = default;
 
     /**
      * @brief Constructs a pair from its values.
@@ -135,7 +126,7 @@ public:
      * @param other Value to use to initialize the second element.
      */
     template<typename Arg, typename Other>
-    constexpr compressed_pair(Arg &&arg, Other &&other)
+    constexpr compressed_pair(Arg &&arg, Other &&other) noexcept(std::is_nothrow_constructible_v<first_base, Arg> &&std::is_nothrow_constructible_v<second_base, Other>)
         : first_base{std::forward<Arg>(arg)},
           second_base{std::forward<Other>(other)} {}
 
@@ -147,7 +138,7 @@ public:
      * @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)
+    constexpr compressed_pair(std::piecewise_construct_t, std::tuple<Args...> args, std::tuple<Other...> other) noexcept(std::is_nothrow_constructible_v<first_base, Args...> &&std::is_nothrow_constructible_v<second_base, Other...>)
         : first_base{std::move(args), std::index_sequence_for<Args...>{}},
           second_base{std::move(other), std::index_sequence_for<Other...>{}} {}
 
@@ -156,25 +147,25 @@ public:
      * @param other The instance to copy from.
      * @return This compressed pair object.
      */
-    constexpr compressed_pair &operator=(const compressed_pair &other) = default;
+    constexpr compressed_pair &operator=(const compressed_pair &other) noexcept(std::is_nothrow_copy_assignable_v<first_base> &&std::is_nothrow_copy_assignable_v<second_base>) = default;
 
     /**
      * @brief Move assignment operator.
      * @param other The instance to move from.
      * @return This compressed pair object.
      */
-    constexpr compressed_pair &operator=(compressed_pair &&other) ENTT_NOEXCEPT = default;
+    constexpr compressed_pair &operator=(compressed_pair &&other) noexcept(std::is_nothrow_move_assignable_v<first_base> &&std::is_nothrow_move_assignable_v<second_base>) = default;
 
     /**
      * @brief Returns the first element that a pair stores.
      * @return The first element that a pair stores.
      */
-    [[nodiscard]] first_type &first() ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr first_type &first() noexcept {
         return static_cast<first_base &>(*this).get();
     }
 
     /*! @copydoc first */
-    [[nodiscard]] const first_type &first() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr const first_type &first() const noexcept {
         return static_cast<const first_base &>(*this).get();
     }
 
@@ -182,12 +173,12 @@ public:
      * @brief Returns the second element that a pair stores.
      * @return The second element that a pair stores.
      */
-    [[nodiscard]] second_type &second() ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr second_type &second() noexcept {
         return static_cast<second_base &>(*this).get();
     }
 
     /*! @copydoc second */
-    [[nodiscard]] const second_type &second() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr const second_type &second() const noexcept {
         return static_cast<const second_base &>(*this).get();
     }
 
@@ -195,7 +186,7 @@ public:
      * @brief Swaps two compressed pair objects.
      * @param other The compressed pair to swap with.
      */
-    void swap(compressed_pair &other) {
+    constexpr void swap(compressed_pair &other) noexcept(std::is_nothrow_swappable_v<first_type> &&std::is_nothrow_swappable_v<second_type>) {
         using std::swap;
         swap(first(), other.first());
         swap(second(), other.second());
@@ -208,7 +199,7 @@ public:
      * reference to the second element if `Index` is 1.
      */
     template<std::size_t Index>
-    decltype(auto) get() ENTT_NOEXCEPT {
+    constexpr decltype(auto) get() noexcept {
         if constexpr(Index == 0u) {
             return first();
         } else {
@@ -219,7 +210,7 @@ public:
 
     /*! @copydoc get */
     template<std::size_t Index>
-    decltype(auto) get() const ENTT_NOEXCEPT {
+    constexpr decltype(auto) get() const noexcept {
         if constexpr(Index == 0u) {
             return first();
         } else {
@@ -245,7 +236,7 @@ compressed_pair(Type &&, Other &&) -> compressed_pair<std::decay_t<Type>, std::d
  * @param rhs A valid compressed pair object.
  */
 template<typename First, typename Second>
-inline void swap(compressed_pair<First, Second> &lhs, compressed_pair<First, Second> &rhs) {
+inline constexpr void swap(compressed_pair<First, Second> &lhs, compressed_pair<First, Second> &rhs) {
     lhs.swap(rhs);
 }
 
@@ -255,9 +246,20 @@ inline void swap(compressed_pair<First, Second> &lhs, compressed_pair<First, Sec
 #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");

src/entt/core/enum.hpp

@@ -1,8 +1,7 @@
-#ifndef ENTT_CORE_FLAG_HPP
-#define ENTT_CORE_FLAG_HPP
+#ifndef ENTT_CORE_ENUM_HPP
+#define ENTT_CORE_ENUM_HPP
 
 #include <type_traits>
-#include "../config/config.h"
 
 namespace entt {
 
@@ -15,7 +14,7 @@ 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::true_type {};
+struct enum_as_bitmask<Type, std::void_t<decltype(Type::_entt_enum_as_bitmask)>>: std::is_enum<Type> {};
 
 /**
  * @brief Helper variable template.
@@ -35,23 +34,23 @@ inline constexpr bool enum_as_bitmask_v = enum_as_bitmask<Type>::value;
  * two values provided.
  */
 template<typename Type>
-[[nodiscard]] constexpr std::enable_if_t<std::is_enum_v<Type> && entt::enum_as_bitmask_v<Type>, Type>
-operator|(const Type lhs, const Type rhs) ENTT_NOEXCEPT {
-    return Type{static_cast<std::underlying_type_t<Type>>(lhs) | static_cast<std::underlying_type_t<Type>>(rhs)};
+[[nodiscard]] constexpr std::enable_if_t<entt::enum_as_bitmask_v<Type>, Type>
+operator|(const Type lhs, const Type rhs) noexcept {
+    return static_cast<Type>(static_cast<std::underlying_type_t<Type>>(lhs) | static_cast<std::underlying_type_t<Type>>(rhs));
 }
 
 /*! @copydoc operator| */
 template<typename Type>
-[[nodiscard]] constexpr std::enable_if_t<std::is_enum_v<Type> && entt::enum_as_bitmask_v<Type>, Type>
-operator&(const Type lhs, const Type rhs) ENTT_NOEXCEPT {
-    return Type{static_cast<std::underlying_type_t<Type>>(lhs) & static_cast<std::underlying_type_t<Type>>(rhs)};
+[[nodiscard]] constexpr std::enable_if_t<entt::enum_as_bitmask_v<Type>, Type>
+operator&(const Type lhs, const Type rhs) noexcept {
+    return static_cast<Type>(static_cast<std::underlying_type_t<Type>>(lhs) & static_cast<std::underlying_type_t<Type>>(rhs));
 }
 
 /*! @copydoc operator| */
 template<typename Type>
-[[nodiscard]] constexpr std::enable_if_t<std::is_enum_v<Type> && entt::enum_as_bitmask_v<Type>, Type>
-operator^(const Type lhs, const Type rhs) ENTT_NOEXCEPT {
-    return Type{static_cast<std::underlying_type_t<Type>>(lhs) ^ static_cast<std::underlying_type_t<Type>>(rhs)};
+[[nodiscard]] constexpr std::enable_if_t<entt::enum_as_bitmask_v<Type>, Type>
+operator^(const Type lhs, const Type rhs) noexcept {
+    return static_cast<Type>(static_cast<std::underlying_type_t<Type>>(lhs) ^ static_cast<std::underlying_type_t<Type>>(rhs));
 }
 
 /**
@@ -62,51 +61,37 @@ operator^(const Type lhs, const Type rhs) ENTT_NOEXCEPT {
  * value provided.
  */
 template<typename Type>
-[[nodiscard]] constexpr std::enable_if_t<std::is_enum_v<Type> && entt::enum_as_bitmask_v<Type>, Type>
-operator~(const Type value) ENTT_NOEXCEPT {
-    return Type{~static_cast<std::underlying_type_t<Type>>(value)};
+[[nodiscard]] constexpr std::enable_if_t<entt::enum_as_bitmask_v<Type>, Type>
+operator~(const Type value) noexcept {
+    return static_cast<Type>(~static_cast<std::underlying_type_t<Type>>(value));
 }
 
 /*! @copydoc operator~ */
 template<typename Type>
-[[nodiscard]] constexpr std::enable_if_t<std::is_enum_v<Type> && entt::enum_as_bitmask_v<Type>, bool>
-operator!(const Type value) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr std::enable_if_t<entt::enum_as_bitmask_v<Type>, bool>
+operator!(const Type value) noexcept {
     return !static_cast<std::underlying_type_t<Type>>(value);
 }
 
 /*! @copydoc operator| */
 template<typename Type>
-constexpr std::enable_if_t<std::is_enum_v<Type> && entt::enum_as_bitmask_v<Type>, Type &>
-operator|=(Type &lhs, const Type rhs) ENTT_NOEXCEPT {
+constexpr std::enable_if_t<entt::enum_as_bitmask_v<Type>, Type &>
+operator|=(Type &lhs, const Type rhs) noexcept {
     return (lhs = (lhs | rhs));
 }
 
 /*! @copydoc operator| */
 template<typename Type>
-constexpr std::enable_if_t<std::is_enum_v<Type> && entt::enum_as_bitmask_v<Type>, Type &>
-operator&=(Type &lhs, const Type rhs) ENTT_NOEXCEPT {
+constexpr std::enable_if_t<entt::enum_as_bitmask_v<Type>, Type &>
+operator&=(Type &lhs, const Type rhs) noexcept {
     return (lhs = (lhs & rhs));
 }
 
 /*! @copydoc operator| */
 template<typename Type>
-constexpr std::enable_if_t<std::is_enum_v<Type> && entt::enum_as_bitmask_v<Type>, Type &>
-operator^=(Type &lhs, const Type rhs) ENTT_NOEXCEPT {
+constexpr std::enable_if_t<entt::enum_as_bitmask_v<Type>, Type &>
+operator^=(Type &lhs, const Type rhs) noexcept {
     return (lhs = (lhs ^ rhs));
 }
 
-/*! @copydoc operator| */
-template<typename Type>
-[[nodiscard]] constexpr std::enable_if_t<std::is_enum_v<Type> && entt::enum_as_bitmask_v<Type>, Type>
-operator==(const Type lhs, const Type rhs) ENTT_NOEXCEPT {
-    return (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<std::is_enum_v<Type> && entt::enum_as_bitmask_v<Type>, Type>
-operator!=(const Type lhs, const Type rhs) ENTT_NOEXCEPT {
-    return !(lhs == rhs);
-}
-
 #endif

src/entt/core/family.hpp

@@ -19,12 +19,12 @@ class family {
 
 public:
     /*! @brief Unsigned integer type. */
-    using family_type = id_type;
+    using value_type = id_type;
 
     /*! @brief Statically generated unique identifier for the given type. */
     template<typename... Type>
     // at the time I'm writing, clang crashes during compilation if auto is used instead of family_type
-    inline static const family_type type = identifier++;
+    inline static const value_type value = identifier++;
 };
 
 } // namespace entt

src/entt/core/fwd.hpp

@@ -1,12 +1,12 @@
 #ifndef ENTT_CORE_FWD_HPP
 #define ENTT_CORE_FWD_HPP
 
-#include <type_traits>
+#include <cstddef>
 #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>)>
+template<std::size_t Len = sizeof(double[2]), std::size_t = alignof(double[2])>
 class basic_any;
 
 /*! @brief Alias declaration for type identifiers. */

src/entt/core/hashed_string.hpp

@@ -3,16 +3,11 @@
 
 #include <cstddef>
 #include <cstdint>
-#include "../config/config.h"
 #include "fwd.hpp"
 
 namespace entt {
 
-/**
- * @cond TURN_OFF_DOXYGEN
- * Internal details not to be documented.
- */
-
+/*! @cond TURN_OFF_DOXYGEN */
 namespace internal {
 
 template<typename>
@@ -32,18 +27,25 @@ struct fnv1a_traits<std::uint64_t> {
     static constexpr std::uint64_t prime = 1099511628211ull;
 };
 
-} // namespace internal
+template<typename Char>
+struct basic_hashed_string {
+    using value_type = Char;
+    using size_type = std::size_t;
+    using hash_type = id_type;
 
-/**
- * Internal details not to be documented.
- * @endcond
- */
+    const value_type *repr;
+    size_type length;
+    hash_type hash;
+};
+
+} // namespace internal
+/*! @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.
@@ -55,62 +57,67 @@ struct fnv1a_traits<std::uint64_t> {
  * @tparam Char Character type.
  */
 template<typename Char>
-class basic_hashed_string {
-    using hs_traits = internal::fnv1a_traits<id_type>;
+class basic_hashed_string: internal::basic_hashed_string<Char> {
+    using base_type = internal::basic_hashed_string<Char>;
+    using traits_type = 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) 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 = hs_traits::offset;
+    [[nodiscard]] static constexpr auto helper(const Char *str) noexcept {
+        base_type base{str, 0u, traits_type::offset};
 
-        while(*curr != 0) {
-            value = (value ^ static_cast<hs_traits::type>(*(curr++))) * hs_traits::prime;
+        for(; str[base.length]; ++base.length) {
+            base.hash = (base.hash ^ static_cast<traits_type::type>(str[base.length])) * traits_type::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) noexcept {
+        base_type base{str, len, traits_type::offset};
+
+        for(size_type pos{}; pos < len; ++pos) {
+            base.hash = (base.hash ^ static_cast<traits_type::type>(str[pos])) * traits_type::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{hs_traits::offset};
-        while(size--) { partial = (partial ^ (str++)[0]) * hs_traits::prime; }
-        return partial;
+    [[nodiscard]] static constexpr hash_type value(const value_type *str, const size_type len) 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>::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);
+    [[nodiscard]] static constexpr hash_type value(const value_type (&str)[N]) noexcept {
+        return basic_hashed_string{str};
     }
 
     /**
@@ -118,33 +125,30 @@ 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) noexcept {
+        return basic_hashed_string{wrapper};
     }
 
     /*! @brief Constructs an empty hashed string. */
-    constexpr basic_hashed_string() ENTT_NOEXCEPT
-        : str{nullptr},
-          hash{} {}
+    constexpr basic_hashed_string() noexcept
+        : 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) 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]) noexcept
+        : base_type{helper(str)} {}
 
     /**
      * @brief Explicit constructor on purpose to avoid constructing a hashed
@@ -155,53 +159,61 @@ public:
      *
      * @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)} {}
+    explicit constexpr basic_hashed_string(const_wrapper wrapper) noexcept
+        : 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 noexcept {
+        return base_type::length; // NOLINT
+    }
 
     /**
      * @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 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;
+    [[nodiscard]] constexpr hash_type value() const noexcept {
+        return base_type::hash;
     }
 
     /*! @copydoc data */
-    [[nodiscard]] constexpr operator const value_type *() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr operator const value_type *() const 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 {
+    [[nodiscard]] constexpr operator hash_type() const 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>;
@@ -214,7 +226,7 @@ basic_hashed_string(const Char (&str)[N]) -> basic_hashed_string<Char>;
  * @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 {
+[[nodiscard]] constexpr bool operator==(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) noexcept {
     return lhs.value() == rhs.value();
 }
 
@@ -226,7 +238,7 @@ template<typename Char>
  * @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 {
+[[nodiscard]] constexpr bool operator!=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) noexcept {
     return !(lhs == rhs);
 }
 
@@ -238,7 +250,7 @@ template<typename Char>
  * @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 {
+[[nodiscard]] constexpr bool operator<(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) noexcept {
     return lhs.value() < rhs.value();
 }
 
@@ -251,7 +263,7 @@ template<typename Char>
  * otherwise.
  */
 template<typename Char>
-[[nodiscard]] constexpr bool operator<=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr bool operator<=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) noexcept {
     return !(rhs < lhs);
 }
 
@@ -264,7 +276,7 @@ template<typename Char>
  * otherwise.
  */
 template<typename Char>
-[[nodiscard]] constexpr bool operator>(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr bool operator>(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) noexcept {
     return rhs < lhs;
 }
 
@@ -277,7 +289,7 @@ template<typename Char>
  * false otherwise.
  */
 template<typename Char>
-[[nodiscard]] constexpr bool operator>=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr bool operator>=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) noexcept {
     return !(lhs < rhs);
 }
 
@@ -294,8 +306,8 @@ inline namespace literals {
  * @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) noexcept {
+    return hashed_string{str};
 }
 
 /**
@@ -303,8 +315,8 @@ inline namespace literals {
  * @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) noexcept {
+    return hashed_wstring{str};
 }
 
 } // namespace literals

src/entt/core/ident.hpp

@@ -4,54 +4,30 @@
 #include <cstddef>
 #include <type_traits>
 #include <utility>
-#include "../config/config.h"
 #include "fwd.hpp"
 #include "type_traits.hpp"
 
 namespace entt {
 
 /**
- * @brief Types identifiers.
- *
- * Variable template used to generate identifiers at compile-time for the given
- * types. Use the `get` member function to know what's the identifier associated
- * to the specific type.
- *
- * @note
- * Identifiers are constant expression and can be used in any context where such
- * an expression is required. As an example:
- * @code{.cpp}
- * 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:
- *     // ...
- * }
- * @endcode
- *
- * @tparam Types List of types for which to generate identifiers.
+ * @brief Type integral identifiers.
+ * @tparam Type List of types for which to generate identifiers.
  */
-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");
-        return (0 + ... + (std::is_same_v<Type, type_list_element_t<Index, type_list<std::decay_t<Types>...>>> ? id_type{Index} : id_type{}));
+template<typename... Type>
+class ident {
+    template<typename Curr, std::size_t... Index>
+    [[nodiscard]] static constexpr id_type get(std::index_sequence<Index...>) noexcept {
+        static_assert((std::is_same_v<Curr, Type> || ...), "Invalid type");
+        return (0 + ... + (std::is_same_v<Curr, type_list_element_t<Index, type_list<std::decay_t<Type>...>>> ? id_type{Index} : id_type{}));
     }
 
 public:
     /*! @brief Unsigned integer type. */
-    using identifier_type = id_type;
+    using value_type = id_type;
 
     /*! @brief Statically generated unique identifier for the given type. */
-    template<typename Type>
-    static constexpr identifier_type type = get<std::decay_t<Type>>(std::index_sequence_for<Types...>{});
+    template<typename Curr>
+    static constexpr value_type value = get<std::decay_t<Curr>>(std::index_sequence_for<Type...>{});
 };
 
 } // namespace entt

src/entt/core/iterator.hpp

@@ -3,8 +3,8 @@
 
 #include <iterator>
 #include <memory>
+#include <type_traits>
 #include <utility>
-#include "../config/config.h"
 
 namespace entt {
 
@@ -14,76 +14,157 @@ namespace entt {
  */
 template<typename Type>
 struct input_iterator_pointer final {
+    /*! @brief Value type. */
+    using value_type = Type;
     /*! @brief Pointer type. */
-    using pointer = decltype(std::addressof(std::declval<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;
+    using pointer = Type *;
+    /*! @brief Reference type. */
+    using reference = Type &;
 
     /**
      * @brief Constructs a proxy object by move.
      * @param val Value to use to initialize the proxy object.
      */
-    input_iterator_pointer(Type &&val)
+    constexpr input_iterator_pointer(value_type &&val) noexcept(std::is_nothrow_move_constructible_v<value_type>)
         : 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 {
+    [[nodiscard]] constexpr pointer operator->() noexcept {
         return std::addressof(value);
     }
 
+    /**
+     * @brief Dereference operator for accessing wrapped values.
+     * @return A reference to the wrapped value.
+     */
+    [[nodiscard]] constexpr reference operator*() noexcept {
+        return value;
+    }
+
 private:
     Type value;
 };
 
 /**
- * @brief Utility class to create an iterable object from a pair of iterators.
- * @tparam It Type of iterators.
+ * @brief Plain iota iterator (waiting for C++20).
+ * @tparam Type Value type.
  */
-template<typename It>
+template<typename Type>
+class iota_iterator final {
+    static_assert(std::is_integral_v<Type>, "Not an integral type");
+
+public:
+    /*! @brief Value type, likely an integral one. */
+    using value_type = Type;
+    /*! @brief Invalid pointer type. */
+    using pointer = void;
+    /*! @brief Non-reference type, same as value type. */
+    using reference = value_type;
+    /*! @brief Difference type. */
+    using difference_type = std::ptrdiff_t;
+    /*! @brief Iterator category. */
+    using iterator_category = std::input_iterator_tag;
+
+    /*! @brief Default constructor. */
+    constexpr iota_iterator() noexcept
+        : current{} {}
+
+    /**
+     * @brief Constructs an iota iterator from a given value.
+     * @param init The initial value assigned to the iota iterator.
+     */
+    constexpr iota_iterator(const value_type init) noexcept
+        : current{init} {}
+
+    /**
+     * @brief Pre-increment operator.
+     * @return This iota iterator.
+     */
+    constexpr iota_iterator &operator++() noexcept {
+        return ++current, *this;
+    }
+
+    /**
+     * @brief Post-increment operator.
+     * @return This iota iterator.
+     */
+    constexpr iota_iterator operator++(int) noexcept {
+        iota_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    /**
+     * @brief Dereference operator.
+     * @return The underlying value.
+     */
+    [[nodiscard]] constexpr reference operator*() const noexcept {
+        return current;
+    }
+
+private:
+    value_type current;
+};
+
+/**
+ * @brief Comparison operator.
+ * @tparam Type Value type of the iota iterator.
+ * @param lhs A properly initialized iota iterator.
+ * @param rhs A properly initialized iota iterator.
+ * @return True if the two iterators are identical, false otherwise.
+ */
+template<typename Type>
+[[nodiscard]] constexpr bool operator==(const iota_iterator<Type> &lhs, const iota_iterator<Type> &rhs) noexcept {
+    return *lhs == *rhs;
+}
+
+/**
+ * @brief Comparison operator.
+ * @tparam Type Value type of the iota iterator.
+ * @param lhs A properly initialized iota iterator.
+ * @param rhs A properly initialized iota iterator.
+ * @return True if the two iterators differ, false otherwise.
+ */
+template<typename Type>
+[[nodiscard]] constexpr bool operator!=(const iota_iterator<Type> &lhs, const iota_iterator<Type> &rhs) noexcept {
+    return !(lhs == rhs);
+}
+
+/**
+ * @brief Utility class to create an iterable object from a pair of iterators.
+ * @tparam It Type of iterator.
+ * @tparam Sentinel Type of sentinel.
+ */
+template<typename It, typename Sentinel = It>
 struct iterable_adaptor final {
-    /*! @brief Type of the objects returned during iteration. */
+    /*! @brief Value type. */
     using value_type = typename std::iterator_traits<It>::value_type;
     /*! @brief Iterator type. */
     using iterator = It;
-    /*! @brief Const iterator type. */
-    using const_iterator = iterator;
+    /*! @brief Sentinel type. */
+    using sentinel = Sentinel;
 
     /*! @brief Default constructor. */
-    iterable_adaptor() = default;
+    constexpr iterable_adaptor() noexcept(std::is_nothrow_default_constructible_v<iterator> &&std::is_nothrow_default_constructible_v<sentinel>)
+        : first{},
+          last{} {}
 
     /**
      * @brief Creates an iterable object from a pair of iterators.
      * @param from Begin iterator.
      * @param to End iterator.
      */
-    iterable_adaptor(It from, It to)
-        : first{from},
-          last{to} {}
+    constexpr iterable_adaptor(iterator from, sentinel to) noexcept(std::is_nothrow_move_constructible_v<iterator> &&std::is_nothrow_move_constructible_v<sentinel>)
+        : first{std::move(from)},
+          last{std::move(to)} {}
 
     /**
      * @brief Returns an iterator to the beginning.
      * @return An iterator to the first element of the range.
      */
-    [[nodiscard]] const_iterator begin() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr iterator begin() const noexcept {
         return first;
     }
 
@@ -92,23 +173,23 @@ struct iterable_adaptor final {
      * @return An iterator to the element following the last element of the
      * range.
      */
-    [[nodiscard]] const_iterator end() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr sentinel end() const noexcept {
         return last;
     }
 
     /*! @copydoc begin */
-    [[nodiscard]] const_iterator cbegin() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr iterator cbegin() const noexcept {
         return begin();
     }
 
     /*! @copydoc end */
-    [[nodiscard]] const_iterator cend() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr sentinel cend() const noexcept {
         return end();
     }
 
 private:
     It first;
-    It last;
+    Sentinel last;
 };
 
 } // namespace entt

src/entt/core/memory.hpp

@@ -4,6 +4,7 @@
 #include <cstddef>
 #include <limits>
 #include <memory>
+#include <tuple>
 #include <type_traits>
 #include <utility>
 #include "../config/config.h"
@@ -11,78 +12,23 @@
 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_const_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.
+ * @brief Checks whether a value is a power of two or not (waiting for C++20 and
+ * `std::has_single_bit`).
  * @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 {
+[[nodiscard]] inline constexpr bool is_power_of_two(const std::size_t value) noexcept {
     return value && ((value & (value - 1)) == 0);
 }
 
 /**
- * @brief Computes the smallest power of two greater than or equal to a value.
+ * @brief Computes the smallest power of two greater than or equal to a value
+ * (waiting for C++20 and `std::bit_ceil`).
  * @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");
+[[nodiscard]] inline constexpr std::size_t next_power_of_two(const std::size_t value) noexcept {
+    ENTT_ASSERT_CONSTEXPR(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) {
@@ -98,11 +44,240 @@ constexpr void propagate_on_container_swap([[maybe_unused]] Allocator &lhs, [[ma
  * @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");
+[[nodiscard]] inline constexpr std::size_t fast_mod(const std::size_t value, const std::size_t mod) noexcept {
+    ENTT_ASSERT_CONSTEXPR(is_power_of_two(mod), "Value must be a power of two");
     return value & (mod - 1u);
 }
 
+/**
+ * @brief Unwraps fancy pointers, does nothing otherwise (waiting for C++20).
+ * @tparam Type Pointer type.
+ * @param ptr Fancy or raw pointer.
+ * @return A raw pointer that represents the address of the original pointer.
+ */
+template<typename Type>
+[[nodiscard]] constexpr auto to_address(Type &&ptr) noexcept {
+    if constexpr(std::is_pointer_v<std::decay_t<Type>>) {
+        return ptr;
+    } else {
+        return to_address(std::forward<Type>(ptr).operator->());
+    }
+}
+
+/**
+ * @brief Utility function to design allocation-aware containers.
+ * @tparam Allocator Type of allocator.
+ * @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) 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) 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) noexcept {
+    if constexpr(std::allocator_traits<Allocator>::propagate_on_container_swap::value) {
+        using std::swap;
+        swap(lhs, rhs);
+    } else {
+        ENTT_ASSERT_CONSTEXPR(lhs == rhs, "Cannot swap the containers");
+    }
+}
+
+/**
+ * @brief Deleter for allocator-aware unique pointers (waiting for C++20).
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+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.
+     */
+    constexpr allocation_deleter(const allocator_type &alloc) noexcept(std::is_nothrow_copy_constructible_v<allocator_type>)
+        : Allocator{alloc} {}
+
+    /**
+     * @brief Destroys the pointed object and deallocates its memory.
+     * @param ptr A valid pointer to an object of the given type.
+     */
+    constexpr void operator()(pointer ptr) noexcept(std::is_nothrow_destructible_v<typename allocator_type::value_type>) {
+        using alloc_traits = std::allocator_traits<Allocator>;
+        alloc_traits::destroy(*this, to_address(ptr));
+        alloc_traits::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>
+ENTT_CONSTEXPR auto allocate_unique(Allocator &allocator, Args &&...args) {
+    static_assert(!std::is_array_v<Type>, "Array types are not supported");
+
+    using alloc_traits = typename std::allocator_traits<Allocator>::template rebind_traits<Type>;
+    using allocator_type = typename alloc_traits::allocator_type;
+
+    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 */
+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) 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) noexcept {
+        return std::make_tuple(
+            std::piecewise_construct,
+            std::apply([&allocator](auto &&...curr) { return uses_allocator_construction<Type>::args(allocator, std::forward<decltype(curr)>(curr)...); }, std::forward<First>(first)),
+            std::apply([&allocator](auto &&...curr) { return uses_allocator_construction<Other>::args(allocator, std::forward<decltype(curr)>(curr)...); }, std::forward<Second>(second)));
+    }
+
+    template<typename Allocator>
+    static constexpr auto args(const Allocator &allocator) noexcept {
+        return uses_allocator_construction<type>::args(allocator, std::piecewise_construct, std::tuple<>{}, std::tuple<>{});
+    }
+
+    template<typename Allocator, typename First, typename Second>
+    static constexpr auto args(const Allocator &allocator, First &&first, Second &&second) noexcept {
+        return uses_allocator_construction<type>::args(allocator, std::piecewise_construct, std::forward_as_tuple(std::forward<First>(first)), std::forward_as_tuple(std::forward<Second>(second)));
+    }
+
+    template<typename Allocator, typename First, typename Second>
+    static constexpr auto args(const Allocator &allocator, const std::pair<First, Second> &value) noexcept {
+        return uses_allocator_construction<type>::args(allocator, std::piecewise_construct, std::forward_as_tuple(value.first), std::forward_as_tuple(value.second));
+    }
+
+    template<typename Allocator, typename First, typename Second>
+    static constexpr auto args(const Allocator &allocator, std::pair<First, Second> &&value) noexcept {
+        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
+/*! @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) 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([value](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

@@ -25,7 +25,7 @@ struct monostate {
      * @param val User data to assign to the given key.
      */
     template<typename Type>
-    void operator=(Type val) const ENTT_NOEXCEPT {
+    void operator=(Type val) const noexcept {
         value<Type> = val;
     }
 
@@ -35,7 +35,7 @@ struct monostate {
      * @return Stored value, if any.
      */
     template<typename Type>
-    operator Type() const ENTT_NOEXCEPT {
+    operator Type() const noexcept {
         return value<Type>;
     }
 

src/entt/core/tuple.hpp

@@ -4,10 +4,36 @@
 #include <tuple>
 #include <type_traits>
 #include <utility>
-#include "../config/config.h"
 
 namespace entt {
 
+/*! @cond TURN_OFF_DOXYGEN */
+namespace internal {
+
+template<typename>
+struct is_tuple_impl: std::false_type {};
+
+template<typename... Args>
+struct is_tuple_impl<std::tuple<Args...>>: std::true_type {};
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is a
+ * tuple, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+struct is_tuple: internal::is_tuple_impl<std::remove_cv_t<Type>> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_tuple_v = is_tuple<Type>::value;
+
 /**
  * @brief Utility function to unwrap tuples of a single element.
  * @tparam Type Tuple type of any sizes.
@@ -16,7 +42,7 @@ namespace entt {
  * element otherwise.
  */
 template<typename Type>
-constexpr decltype(auto) unwrap_tuple(Type &&value) ENTT_NOEXCEPT {
+constexpr decltype(auto) unwrap_tuple(Type &&value) noexcept {
     if constexpr(std::tuple_size_v<std::remove_reference_t<Type>> == 1u) {
         return std::get<0>(std::forward<Type>(value));
     } else {
@@ -24,6 +50,46 @@ constexpr decltype(auto) unwrap_tuple(Type &&value) ENTT_NOEXCEPT {
     }
 }
 
+/**
+ * @brief Utility class to forward-and-apply tuple objects.
+ * @tparam Func Type of underlying invocable object.
+ */
+template<typename Func>
+struct forward_apply: private Func {
+    /**
+     * @brief Constructs a forward-and-apply object.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename... Args>
+    constexpr forward_apply(Args &&...args) noexcept(std::is_nothrow_constructible_v<Func, Args...>)
+        : Func{std::forward<Args>(args)...} {}
+
+    /**
+     * @brief Forwards and applies the arguments with the underlying function.
+     * @tparam Type Tuple-like type to forward to the underlying function.
+     * @param args Parameters to forward to the underlying function.
+     * @return Return value of the underlying function, if any.
+     */
+    template<typename Type>
+    constexpr decltype(auto) operator()(Type &&args) noexcept(noexcept(std::apply(std::declval<Func &>(), args))) {
+        return std::apply(static_cast<Func &>(*this), std::forward<Type>(args));
+    }
+
+    /*! @copydoc operator()() */
+    template<typename Type>
+    constexpr decltype(auto) operator()(Type &&args) const noexcept(noexcept(std::apply(std::declval<const Func &>(), args))) {
+        return std::apply(static_cast<const Func &>(*this), std::forward<Type>(args));
+    }
+};
+
+/**
+ * @brief Deduction guide.
+ * @tparam Func Type of underlying invocable object.
+ */
+template<typename Func>
+forward_apply(Func) -> forward_apply<std::remove_reference_t<std::remove_cv_t<Func>>>;
+
 } // namespace entt
 
 #endif

src/entt/core/type_info.hpp

@@ -11,22 +11,18 @@
 
 namespace entt {
 
-/**
- * @cond TURN_OFF_DOXYGEN
- * Internal details not to be documented.
- */
-
+/*! @cond TURN_OFF_DOXYGEN */
 namespace internal {
 
 struct ENTT_API type_index final {
-    [[nodiscard]] static id_type next() ENTT_NOEXCEPT {
+    [[nodiscard]] static id_type next() noexcept {
         static ENTT_MAYBE_ATOMIC(id_type) value{};
         return value++;
     }
 };
 
 template<typename Type>
-[[nodiscard]] constexpr auto stripped_type_name() ENTT_NOEXCEPT {
+[[nodiscard]] constexpr auto stripped_type_name() 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);
@@ -38,26 +34,26 @@ template<typename Type>
 }
 
 template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
-[[nodiscard]] static constexpr std::string_view type_name(int) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr std::string_view type_name(int) noexcept {
     constexpr auto value = stripped_type_name<Type>();
     return value;
 }
 
 template<typename Type>
-[[nodiscard]] static std::string_view type_name(char) ENTT_NOEXCEPT {
+[[nodiscard]] std::string_view type_name(char) 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 {
+[[nodiscard]] constexpr id_type type_hash(int) noexcept {
     constexpr auto stripped = stripped_type_name<Type>();
     constexpr auto value = hashed_string::value(stripped.data(), stripped.size());
     return value;
 }
 
 template<typename Type>
-[[nodiscard]] static id_type type_hash(char) ENTT_NOEXCEPT {
+[[nodiscard]] id_type type_hash(char) noexcept {
     static const auto value = [](const auto stripped) {
         return hashed_string::value(stripped.data(), stripped.size());
     }(stripped_type_name<Type>());
@@ -65,11 +61,7 @@ template<typename Type>
 }
 
 } // namespace internal
-
-/**
- * Internal details not to be documented.
- * @endcond
- */
+/*! @endcond */
 
 /**
  * @brief Type sequential identifier.
@@ -81,13 +73,13 @@ 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 {
+    [[nodiscard]] static id_type value() noexcept {
         static const id_type value = internal::type_index::next();
         return value;
     }
 
     /*! @copydoc value */
-    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr operator id_type() const noexcept {
         return value();
     }
 };
@@ -103,16 +95,16 @@ struct type_hash final {
      * @return The numeric representation of the given type.
      */
 #if defined ENTT_PRETTY_FUNCTION
-    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+    [[nodiscard]] static constexpr id_type value() noexcept {
         return internal::type_hash<Type>(0);
 #else
-    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+    [[nodiscard]] static constexpr id_type value() noexcept {
         return type_index<Type>::value();
 #endif
     }
 
     /*! @copydoc value */
-    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr operator id_type() const noexcept {
         return value();
     }
 };
@@ -127,33 +119,33 @@ struct type_name final {
      * @brief Returns the name of a given type.
      * @return The name of the given type.
      */
-    [[nodiscard]] static constexpr std::string_view value() ENTT_NOEXCEPT {
+    [[nodiscard]] static constexpr std::string_view value() noexcept {
         return internal::type_name<Type>(0);
     }
 
     /*! @copydoc value */
-    [[nodiscard]] constexpr operator std::string_view() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr operator std::string_view() const noexcept {
         return value();
     }
 };
 
 /*! @brief Implementation specific information about a type. */
-class type_info final {
+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>
-    friend const type_info &type_id() ENTT_NOEXCEPT;
+    constexpr type_info(std::in_place_type_t<Type>) noexcept
+        : seq{type_index<std::remove_cv_t<std::remove_reference_t<Type>>>::value()},
+          identifier{type_hash<std::remove_cv_t<std::remove_reference_t<Type>>>::value()},
+          alias{type_name<std::remove_cv_t<std::remove_reference_t<Type>>>::value()} {}
 
-    template<typename Type>
-    constexpr type_info(std::in_place_type_t<Type>) ENTT_NOEXCEPT
-        : seq{type_index<std::remove_reference_t<std::remove_const_t<Type>>>::value()},
-          identifier{type_hash<std::remove_reference_t<std::remove_const_t<Type>>>::value()},
-          alias{type_name<std::remove_reference_t<std::remove_const_t<Type>>>::value()} {}
-
-public:
     /**
      * @brief Type index.
      * @return Type index.
      */
-    [[nodiscard]] constexpr id_type index() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr id_type index() const noexcept {
         return seq;
     }
 
@@ -161,7 +153,7 @@ public:
      * @brief Type hash.
      * @return Type hash.
      */
-    [[nodiscard]] constexpr id_type hash() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr id_type hash() const noexcept {
         return identifier;
     }
 
@@ -169,7 +161,7 @@ public:
      * @brief Type name.
      * @return Type name.
      */
-    [[nodiscard]] constexpr std::string_view name() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr std::string_view name() const noexcept {
         return alias;
     }
 
@@ -185,7 +177,7 @@ private:
  * @param rhs A type info object.
  * @return True if the two type info objects are identical, false otherwise.
  */
-[[nodiscard]] inline constexpr bool operator==(const type_info &lhs, const type_info &rhs) ENTT_NOEXCEPT {
+[[nodiscard]] inline constexpr bool operator==(const type_info &lhs, const type_info &rhs) noexcept {
     return lhs.hash() == rhs.hash();
 }
 
@@ -195,7 +187,7 @@ private:
  * @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 {
+[[nodiscard]] inline constexpr bool operator!=(const type_info &lhs, const type_info &rhs) noexcept {
     return !(lhs == rhs);
 }
 
@@ -205,7 +197,7 @@ private:
  * @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 {
+[[nodiscard]] constexpr bool operator<(const type_info &lhs, const type_info &rhs) noexcept {
     return lhs.index() < rhs.index();
 }
 
@@ -216,7 +208,7 @@ private:
  * @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 {
+[[nodiscard]] constexpr bool operator<=(const type_info &lhs, const type_info &rhs) noexcept {
     return !(rhs < lhs);
 }
 
@@ -227,7 +219,7 @@ private:
  * @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 {
+[[nodiscard]] constexpr bool operator>(const type_info &lhs, const type_info &rhs) noexcept {
     return rhs < lhs;
 }
 
@@ -238,30 +230,37 @@ private:
  * @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 {
+[[nodiscard]] constexpr bool operator>=(const type_info &lhs, const type_info &rhs) noexcept {
     return !(lhs < rhs);
 }
 
 /**
  * @brief Returns the type info object associated to a given type.
  *
+ * 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 A properly initialized type info object.
+ * @return A reference to a properly initialized type info object.
  */
 template<typename Type>
-[[nodiscard]] const type_info &type_id() ENTT_NOEXCEPT {
+[[nodiscard]] const type_info &type_id() noexcept {
     if constexpr(std::is_same_v<Type, std::remove_cv_t<std::remove_reference_t<Type>>>) {
-        static type_info instance{std::in_place_type<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 &&) noexcept {
+    return type_id<std::remove_cv_t<std::remove_reference_t<Type>>>();
+}
+
 } // namespace entt
 
 #endif

src/entt/core/type_traits.hpp

@@ -3,6 +3,7 @@
 
 #include <cstddef>
 #include <iterator>
+#include <tuple>
 #include <type_traits>
 #include <utility>
 #include "../config/config.h"
@@ -16,7 +17,7 @@ namespace entt {
  */
 template<std::size_t N>
 struct choice_t
-    // Unfortunately, doxygen cannot parse such a construct.
+    // unfortunately, doxygen cannot parse such a construct
     : /*! @cond TURN_OFF_DOXYGEN */ choice_t<N - 1> /*! @endcond */
 {};
 
@@ -55,7 +56,6 @@ 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.
- * @tparam The size of the type if `sizeof` accepts it, 0 otherwise.
  */
 template<typename Type, typename = void>
 struct size_of: std::integral_constant<std::size_t, 0u> {};
@@ -121,22 +121,22 @@ 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.
- * @tparam Type First type provided by the type list.
+ * @tparam First First type provided by the type list.
  * @tparam Other Other types provided by the type list.
  */
-template<std::size_t Index, typename Type, typename... Other>
-struct type_list_element<Index, type_list<Type, Other...>>
+template<std::size_t Index, typename First, typename... Other>
+struct type_list_element<Index, type_list<First, Other...>>
     : type_list_element<Index - 1u, type_list<Other...>> {};
 
 /**
  * @brief Provides compile-time indexed access to the types of a type list.
- * @tparam Type First type provided by the type list.
+ * @tparam First First type provided by the type list.
  * @tparam Other Other types provided by the type list.
  */
-template<typename Type, typename... Other>
-struct type_list_element<0u, type_list<Type, Other...>> {
+template<typename First, typename... Other>
+struct type_list_element<0u, type_list<First, Other...>> {
     /*! @brief Searched type. */
-    using type = Type;
+    using type = First;
 };
 
 /**
@@ -147,6 +147,58 @@ 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 Primary template isn't defined on purpose. */
+template<typename, typename>
+struct type_list_index;
+
+/**
+ * @brief Provides compile-time type access to the types of a type list.
+ * @tparam Type Type to look for and for which to return the index.
+ * @tparam First First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<typename Type, typename First, typename... Other>
+struct type_list_index<Type, type_list<First, Other...>> {
+    /*! @brief Unsigned integer type. */
+    using value_type = std::size_t;
+    /*! @brief Compile-time position of the given type in the sublist. */
+    static constexpr value_type value = 1u + type_list_index<Type, type_list<Other...>>::value;
+};
+
+/**
+ * @brief Provides compile-time type access to the types of a type list.
+ * @tparam Type Type to look for and for which to return the index.
+ * @tparam Other Other types provided by the type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_index<Type, type_list<Type, Other...>> {
+    static_assert(type_list_index<Type, type_list<Other...>>::value == sizeof...(Other), "Non-unique type");
+    /*! @brief Unsigned integer type. */
+    using value_type = std::size_t;
+    /*! @brief Compile-time position of the given type in the sublist. */
+    static constexpr value_type value = 0u;
+};
+
+/**
+ * @brief Provides compile-time type access to the types of a type list.
+ * @tparam Type Type to look for and for which to return the index.
+ */
+template<typename Type>
+struct type_list_index<Type, type_list<>> {
+    /*! @brief Unsigned integer type. */
+    using value_type = std::size_t;
+    /*! @brief Compile-time position of the given type in the sublist. */
+    static constexpr value_type value = 0u;
+};
+
+/**
+ * @brief Helper variable template.
+ * @tparam List Type list.
+ * @tparam Type Type to look for and for which to return the index.
+ */
+template<typename Type, typename List>
+inline constexpr std::size_t type_list_index_v = type_list_index<Type, List>::value;
+
 /**
  * @brief Concatenates multiple type lists.
  * @tparam Type Types provided by the first type list.
@@ -198,37 +250,40 @@ 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>
+/*! @cond TURN_OFF_DOXYGEN */
+namespace internal {
+
+template<typename...>
 struct type_list_unique;
 
+template<typename First, typename... Other, typename... Type>
+struct type_list_unique<type_list<First, Other...>, Type...>
+    : std::conditional_t<(std::is_same_v<First, Type> || ...), type_list_unique<type_list<Other...>, Type...>, type_list_unique<type_list<Other...>, Type..., First>> {};
+
+template<typename... Type>
+struct type_list_unique<type_list<>, Type...> {
+    using type = type_list<Type...>;
+};
+
+} // namespace internal
+/*! @endcond */
+
 /**
  * @brief Removes duplicates types from a type list.
- * @tparam Type One of the types provided by the given type list.
- * @tparam Other The other types provided by the given type list.
+ * @tparam List Type list.
  */
-template<typename Type, typename... Other>
-struct type_list_unique<type_list<Type, Other...>> {
+template<typename List>
+struct type_list_unique {
     /*! @brief A type list without duplicate types. */
-    using type = std::conditional_t<
-        std::disjunction_v<std::is_same<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>>;
-};
-
-/*! @brief Removes duplicates types from a type list. */
-template<>
-struct type_list_unique<type_list<>> {
-    /*! @brief A type list without duplicate types. */
-    using type = type_list<>;
+    using type = typename internal::type_list_unique<List>::type;
 };
 
 /**
  * @brief Helper type.
- * @tparam Type A type list.
+ * @tparam List Type list.
  */
-template<typename Type>
-using type_list_unique_t = typename type_list_unique<Type>::type;
+template<typename List>
+using type_list_unique_t = typename type_list_unique<List>::type;
 
 /**
  * @brief Provides the member constant `value` to true if a type list contains a
@@ -245,7 +300,8 @@ struct type_list_contains;
  * @tparam Other Type to look for.
  */
 template<typename... Type, typename Other>
-struct type_list_contains<type_list<Type...>, Other>: std::disjunction<std::is_same<Type, Other>...> {};
+struct type_list_contains<type_list<Type...>, Other>
+    : std::bool_constant<(std::is_same_v<Type, Other> || ...)> {};
 
 /**
  * @brief Helper variable template.
@@ -277,6 +333,29 @@ 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 Primary template isn't defined on purpose. */
+template<typename, template<typename...> class>
+struct type_list_transform;
+
+/**
+ * @brief Applies a given _function_ to a type list and generate a new list.
+ * @tparam Type Types provided by the type list.
+ * @tparam Op Unary operation as template class with a type member named `type`.
+ */
+template<typename... Type, template<typename...> class Op>
+struct type_list_transform<type_list<Type...>, Op> {
+    /*! @brief Resulting type list after applying the transform function. */
+    using type = type_list<typename Op<Type>::type...>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam List Type list.
+ * @tparam Op Unary operation as template class with a type member named `type`.
+ */
+template<typename List, template<typename...> class Op>
+using type_list_transform_t = typename type_list_transform<List, Op>::type;
+
 /**
  * @brief A class to use to push around lists of constant values, nothing more.
  * @tparam Value Values provided by the value list.
@@ -310,10 +389,20 @@ struct value_list_element<Index, value_list<Value, Other...>>
  */
 template<auto Value, auto... Other>
 struct value_list_element<0u, value_list<Value, Other...>> {
+    /*! @brief Searched type. */
+    using type = decltype(Value);
     /*! @brief Searched value. */
     static constexpr auto value = Value;
 };
 
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the type to return.
+ * @tparam List Value list to search into.
+ */
+template<std::size_t Index, typename List>
+using value_list_element_t = typename value_list_element<Index, List>::type;
+
 /**
  * @brief Helper type.
  * @tparam Index Index of the value to return.
@@ -322,6 +411,58 @@ 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 Primary template isn't defined on purpose. */
+template<auto, typename>
+struct value_list_index;
+
+/**
+ * @brief Provides compile-time type access to the values of a value list.
+ * @tparam Value Value to look for and for which to return the index.
+ * @tparam First First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<auto Value, auto First, auto... Other>
+struct value_list_index<Value, value_list<First, Other...>> {
+    /*! @brief Unsigned integer type. */
+    using value_type = std::size_t;
+    /*! @brief Compile-time position of the given value in the sublist. */
+    static constexpr value_type value = 1u + value_list_index<Value, value_list<Other...>>::value;
+};
+
+/**
+ * @brief Provides compile-time type access to the values of a value list.
+ * @tparam Value Value to look for and for which to return the index.
+ * @tparam Other Other values provided by the value list.
+ */
+template<auto Value, auto... Other>
+struct value_list_index<Value, value_list<Value, Other...>> {
+    static_assert(value_list_index<Value, value_list<Other...>>::value == sizeof...(Other), "Non-unique type");
+    /*! @brief Unsigned integer type. */
+    using value_type = std::size_t;
+    /*! @brief Compile-time position of the given value in the sublist. */
+    static constexpr value_type value = 0u;
+};
+
+/**
+ * @brief Provides compile-time type access to the values of a value list.
+ * @tparam Value Value to look for and for which to return the index.
+ */
+template<auto Value>
+struct value_list_index<Value, value_list<>> {
+    /*! @brief Unsigned integer type. */
+    using value_type = std::size_t;
+    /*! @brief Compile-time position of the given type in the sublist. */
+    static constexpr value_type value = 0u;
+};
+
+/**
+ * @brief Helper variable template.
+ * @tparam List Value list.
+ * @tparam Value Value to look for and for which to return the index.
+ */
+template<auto Value, typename List>
+inline constexpr std::size_t value_list_index_v = value_list_index<Value, List>::value;
+
 /**
  * @brief Concatenates multiple value lists.
  * @tparam Value Values provided by the first value list.
@@ -373,6 +514,89 @@ struct value_list_cat<value_list<Value...>> {
 template<typename... List>
 using value_list_cat_t = typename value_list_cat<List...>::type;
 
+/*! @brief Primary template isn't defined on purpose. */
+template<typename>
+struct value_list_unique;
+
+/**
+ * @brief Removes duplicates values from a value list.
+ * @tparam Value One of the values provided by the given value list.
+ * @tparam Other The other values provided by the given value list.
+ */
+template<auto Value, auto... Other>
+struct value_list_unique<value_list<Value, Other...>> {
+    /*! @brief A value list without duplicate types. */
+    using type = std::conditional_t<
+        ((Value == Other) || ...),
+        typename value_list_unique<value_list<Other...>>::type,
+        value_list_cat_t<value_list<Value>, typename value_list_unique<value_list<Other...>>::type>>;
+};
+
+/*! @brief Removes duplicates values from a value list. */
+template<>
+struct value_list_unique<value_list<>> {
+    /*! @brief A value list without duplicate types. */
+    using type = value_list<>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam Type A value list.
+ */
+template<typename Type>
+using value_list_unique_t = typename value_list_unique<Type>::type;
+
+/**
+ * @brief Provides the member constant `value` to true if a value list contains
+ * a given value, false otherwise.
+ * @tparam List Value list.
+ * @tparam Value Value to look for.
+ */
+template<typename List, auto Value>
+struct value_list_contains;
+
+/**
+ * @copybrief value_list_contains
+ * @tparam Value Values provided by the value list.
+ * @tparam Other Value to look for.
+ */
+template<auto... Value, auto Other>
+struct value_list_contains<value_list<Value...>, Other>
+    : std::bool_constant<((Value == Other) || ...)> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam List Value list.
+ * @tparam Value Value to look for.
+ */
+template<typename List, auto Value>
+inline constexpr bool value_list_contains_v = value_list_contains<List, Value>::value;
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+class value_list_diff;
+
+/**
+ * @brief Computes the difference between two value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ */
+template<auto... Value, auto... Other>
+class value_list_diff<value_list<Value...>, value_list<Other...>> {
+    using v141_toolset_workaround = value_list<Other...>;
+
+public:
+    /*! @brief A value list that is the difference between the two value lists. */
+    using type = value_list_cat_t<std::conditional_t<value_list_contains_v<v141_toolset_workaround, Value>, value_list<>, value_list<Value>>...>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam List Value lists between which to compute the difference.
+ */
+template<typename... List>
+using value_list_diff_t = typename value_list_diff<List...>::type;
+
 /*! @brief Same as std::is_invocable, but with tuples. */
 template<typename, typename>
 struct is_applicable: std::false_type {};
@@ -454,11 +678,7 @@ 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.
- */
-
+/*! @cond TURN_OFF_DOXYGEN */
 namespace internal {
 
 template<typename, typename = void>
@@ -468,15 +688,11 @@ 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
- */
+/*! @endcond */
 
 /*! @copydoc is_iterator */
 template<typename Type>
-struct is_iterator<Type, std::enable_if_t<!std::is_same_v<std::remove_const_t<std::remove_pointer_t<Type>>, void>>>
+struct is_iterator<Type, std::enable_if_t<!std::is_void_v<std::remove_cv_t<std::remove_pointer_t<Type>>>>>
     : internal::has_iterator_category<Type> {};
 
 /**
@@ -486,33 +702,6 @@ struct is_iterator<Type, std::enable_if_t<!std::is_same_v<std::remove_const_t<st
 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.
- */
-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> {};
-
-/**
- * @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;
-
 /**
  * @brief Provides the member constant `value` to true if a given type is both
  * an empty and non-final class, false otherwise.
@@ -520,7 +709,7 @@ inline constexpr bool is_iterator_type_v = is_iterator_type<Type, It>::value;
  */
 template<typename Type>
 struct is_ebco_eligible
-    : std::conjunction<std::is_empty<Type>, std::negation<std::is_final<Type>>> {};
+    : std::bool_constant<std::is_empty_v<Type> && !std::is_final_v<Type>> {};
 
 /**
  * @brief Helper variable template.
@@ -548,19 +737,7 @@ struct is_transparent<Type, std::void_t<typename Type::is_transparent>>: std::tr
 template<typename Type>
 inline constexpr bool is_transparent_v = is_transparent<Type>::value;
 
-/**
- * @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.
- */
-
+/*! @cond TURN_OFF_DOXYGEN */
 namespace internal {
 
 template<typename, typename = void>
@@ -569,47 +746,69 @@ 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, typename = void>
+struct has_value_type: std::false_type {};
+
+template<typename Type>
+struct has_value_type<Type, std::void_t<typename Type::value_type>>: std::true_type {};
+
+template<typename>
+[[nodiscard]] constexpr bool dispatch_is_equality_comparable();
+
 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 && ...);
+    return (dispatch_is_equality_comparable<std::tuple_element_t<Index, Type>>() && ...);
 }
 
 template<typename>
-[[nodiscard]] constexpr bool maybe_equality_comparable(choice_t<0>) {
+[[nodiscard]] constexpr bool maybe_equality_comparable(char) {
+    return false;
+}
+
+template<typename Type>
+[[nodiscard]] constexpr auto maybe_equality_comparable(int) -> decltype(std::declval<Type>() == std::declval<Type>()) {
     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>);
+[[nodiscard]] constexpr bool dispatch_is_equality_comparable() {
+    if constexpr(std::is_array_v<Type>) {
+        return false;
+    } else if constexpr(is_iterator_v<Type>) {
+        return maybe_equality_comparable<Type>(0);
+    } else if constexpr(has_value_type<Type>::value) {
+        if constexpr(std::is_same_v<typename Type::value_type, Type>) {
+            return maybe_equality_comparable<Type>(0);
+        } else if constexpr(dispatch_is_equality_comparable<typename Type::value_type>()) {
+            return maybe_equality_comparable<Type>(0);
+        } else {
+            return false;
+        }
+    } else if constexpr(is_complete_v<std::tuple_size<std::remove_cv_t<Type>>>) {
+        if constexpr(has_tuple_size_value<Type>::value) {
+            return maybe_equality_comparable<Type>(0) && unpack_maybe_equality_comparable<Type>(std::make_index_sequence<std::tuple_size<Type>::value>{});
+        } else {
+            return maybe_equality_comparable<Type>(0);
+        }
     } else {
-        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>);
+        return maybe_equality_comparable<Type>(0);
     }
 }
 
 } // namespace internal
+/*! @endcond */
 
 /**
- * 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>
+struct is_equality_comparable: std::bool_constant<internal::dispatch_is_equality_comparable<Type>()> {};
 
 /*! @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>)> {};
+struct is_equality_comparable<const Type>: is_equality_comparable<Type> {};
 
 /**
  * @brief Helper variable template.
@@ -633,7 +832,7 @@ struct constness_as {
 template<typename To, typename From>
 struct constness_as<To, const From> {
     /*! @brief The type resulting from the transcription of the constness. */
-    using type = std::add_const_t<To>;
+    using type = const To;
 };
 
 /**
@@ -673,6 +872,50 @@ public:
 template<typename Member>
 using member_class_t = typename member_class<Member>::type;
 
+/**
+ * @brief Extracts the n-th argument of a given function or member function.
+ * @tparam Index The index of the argument to extract.
+ * @tparam Candidate A valid function, member function or data member type.
+ */
+template<std::size_t Index, typename Candidate>
+class nth_argument {
+    template<typename Ret, typename... Args>
+    static constexpr type_list<Args...> pick_up(Ret (*)(Args...));
+
+    template<typename Ret, typename Class, typename... Args>
+    static constexpr type_list<Args...> pick_up(Ret (Class ::*)(Args...));
+
+    template<typename Ret, typename Class, typename... Args>
+    static constexpr type_list<Args...> pick_up(Ret (Class ::*)(Args...) const);
+
+    template<typename Type, typename Class>
+    static constexpr type_list<Type> pick_up(Type Class ::*);
+
+public:
+    /*! @brief N-th argument of the given function or member function. */
+    using type = type_list_element_t<Index, decltype(pick_up(std::declval<Candidate>()))>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam Index The index of the argument to extract.
+ * @tparam Candidate A valid function, member function or data member type.
+ */
+template<std::size_t Index, typename Candidate>
+using nth_argument_t = typename nth_argument<Index, Candidate>::type;
+
 } // namespace entt
 
+template<typename... Type>
+struct std::tuple_size<entt::type_list<Type...>>: std::integral_constant<std::size_t, entt::type_list<Type...>::size> {};
+
+template<std::size_t Index, typename... Type>
+struct std::tuple_element<Index, entt::type_list<Type...>>: entt::type_list_element<Index, entt::type_list<Type...>> {};
+
+template<auto... Value>
+struct std::tuple_size<entt::value_list<Value...>>: std::integral_constant<std::size_t, entt::value_list<Value...>::size> {};
+
+template<std::size_t Index, auto... Value>
+struct std::tuple_element<Index, entt::value_list<Value...>>: entt::value_list_element<Index, entt::value_list<Value...>> {};
+
 #endif

src/entt/core/utility.hpp

@@ -1,8 +1,8 @@
 #ifndef ENTT_CORE_UTILITY_HPP
 #define ENTT_CORE_UTILITY_HPP
 
+#include <type_traits>
 #include <utility>
-#include "../config/config.h"
 
 namespace entt {
 
@@ -17,8 +17,8 @@ struct identity {
      * @param value The actual argument.
      * @return The submitted value as-is.
      */
-    template<class Type>
-    [[nodiscard]] constexpr Type &&operator()(Type &&value) const ENTT_NOEXCEPT {
+    template<typename Type>
+    [[nodiscard]] constexpr Type &&operator()(Type &&value) const noexcept {
         return std::forward<Type>(value);
     }
 };
@@ -31,7 +31,7 @@ struct identity {
  * @return Pointer to the member.
  */
 template<typename Type, typename Class>
-[[nodiscard]] constexpr auto overload(Type Class::*member) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr auto overload(Type Class::*member) noexcept {
     return member;
 }
 
@@ -42,7 +42,7 @@ template<typename Type, typename Class>
  * @return Pointer to the function.
  */
 template<typename Func>
-[[nodiscard]] constexpr auto overload(Func *func) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr auto overload(Func *func) noexcept {
     return func;
 }
 
@@ -50,7 +50,7 @@ template<typename Func>
  * @brief Helper type for visitors.
  * @tparam Func Types of function objects.
  */
-template<class... Func>
+template<typename... Func>
 struct overloaded: Func... {
     using Func::operator()...;
 };
@@ -59,20 +59,20 @@ struct overloaded: Func... {
  * @brief Deduction guide.
  * @tparam Func Types of function objects.
  */
-template<class... Func>
+template<typename... Func>
 overloaded(Func...) -> overloaded<Func...>;
 
 /**
  * @brief Basic implementation of a y-combinator.
  * @tparam Func Type of a potentially recursive function.
  */
-template<class Func>
+template<typename Func>
 struct y_combinator {
     /**
      * @brief Constructs a y-combinator from a given function.
      * @param recursive A potentially recursive function.
      */
-    y_combinator(Func recursive)
+    constexpr y_combinator(Func recursive) noexcept(std::is_nothrow_move_constructible_v<Func>)
         : func{std::move(recursive)} {}
 
     /**
@@ -81,14 +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<typename... Args>
+    constexpr decltype(auto) operator()(Args &&...args) const noexcept(std::is_nothrow_invocable_v<Func, const y_combinator &, Args...>) {
         return func(*this, std::forward<Args>(args)...);
     }
 
     /*! @copydoc operator()() */
-    template<class... Args>
-    decltype(auto) operator()(Args &&...args) {
+    template<typename... Args>
+    constexpr decltype(auto) operator()(Args &&...args) noexcept(std::is_nothrow_invocable_v<Func, y_combinator &, Args...>) {
         return func(*this, std::forward<Args>(args)...);
     }
 

src/entt/entity/component.hpp

@@ -1,36 +1,55 @@
 #ifndef ENTT_ENTITY_COMPONENT_HPP
 #define ENTT_ENTITY_COMPONENT_HPP
 
+#include <cstddef>
 #include <type_traits>
 #include "../config/config.h"
+#include "fwd.hpp"
 
 namespace entt {
 
-/*! @brief Commonly used default traits for all types. */
-struct basic_component_traits {
-    /*! @brief Pointer stability, default is `false`. */
-    static constexpr auto in_place_delete = false;
-    /*! @brief Empty type optimization, default is `ENTT_IGNORE_IF_EMPTY`. */
-    static constexpr auto ignore_if_empty = ENTT_IGNORE_IF_EMPTY;
-    /*! @brief Page size, default is `ENTT_PACKED_PAGE`. */
-    static constexpr auto page_size = ENTT_PACKED_PAGE;
-};
+/*! @cond TURN_OFF_DOXYGEN */
+namespace internal {
+
+template<typename Type, typename = void>
+struct in_place_delete: std::bool_constant<!(std::is_move_constructible_v<Type> && std::is_move_assignable_v<Type>)> {};
+
+template<>
+struct in_place_delete<void>: std::false_type {};
+
+template<typename Type>
+struct in_place_delete<Type, std::enable_if_t<Type::in_place_delete>>
+    : std::true_type {};
+
+template<typename Type, typename = void>
+struct page_size: std::integral_constant<std::size_t, !std::is_empty_v<ENTT_ETO_TYPE(Type)> * ENTT_PACKED_PAGE> {};
+
+template<>
+struct page_size<void>: std::integral_constant<std::size_t, 0u> {};
+
+template<typename Type>
+struct page_size<Type, std::void_t<decltype(Type::page_size)>>
+    : std::integral_constant<std::size_t, Type::page_size> {};
+
+} // namespace internal
+/*! @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 Helper variable template.
- * @tparam Type Type of component.
- */
-template<class Type>
-inline constexpr bool ignore_as_empty_v = component_traits<Type>::ignore_if_empty &&std::is_empty_v<Type>;
+    /*! @brief Component type. */
+    using type = 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;
+};
 
 } // namespace entt
 

src/entt/entity/entity.hpp

@@ -9,77 +9,85 @@
 
 namespace entt {
 
-/**
- * @cond TURN_OFF_DOXYGEN
- * Internal details not to be documented.
- */
-
+/*! @cond TURN_OFF_DOXYGEN */
 namespace internal {
 
+// waiting for C++20 and std::popcount
+template<typename Type>
+constexpr int popcount(Type value) noexcept {
+    return value ? (int(value & 1) + popcount(value >> 1)) : 0;
+}
+
 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>> {
+    using value_type = 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> {
+    using value_type = Type;
+};
 
 template<>
 struct entt_traits<std::uint32_t> {
+    using value_type = std::uint32_t;
+
     using entity_type = std::uint32_t;
     using version_type = std::uint16_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 value_type = std::uint64_t;
+
     using entity_type = std::uint64_t;
     using version_type = std::uint32_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
+/*! @endcond */
 
 /**
- * Internal details not to be documented.
- * @endcond
+ * @brief Common basic entity traits implementation.
+ * @tparam Traits Actual entity traits to use.
  */
+template<typename Traits>
+class basic_entt_traits {
+    static constexpr auto length = internal::popcount(Traits::entity_mask);
 
-/**
- * @brief Entity traits.
- * @tparam Type Type of identifier.
- */
-template<typename Type>
-class entt_traits: internal::entt_traits<Type> {
-    using base_type = internal::entt_traits<Type>;
+    static_assert(Traits::entity_mask && ((typename Traits::entity_type{1} << length) == (Traits::entity_mask + 1)), "Invalid entity mask");
+    static_assert((typename Traits::entity_type{1} << internal::popcount(Traits::version_mask)) == (Traits::version_mask + 1), "Invalid version mask");
 
 public:
     /*! @brief Value type. */
-    using value_type = Type;
+    using value_type = typename Traits::value_type;
     /*! @brief Underlying entity type. */
-    using entity_type = typename base_type::entity_type;
+    using entity_type = typename Traits::entity_type;
     /*! @brief Underlying version 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;
+    using version_type = typename Traits::version_type;
+
+    /*! @brief Entity mask size. */
+    static constexpr entity_type entity_mask = Traits::entity_mask;
+    /*! @brief Version mask size */
+    static constexpr entity_type version_mask = Traits::version_mask;
 
     /**
      * @brief Converts an entity to its underlying type.
      * @param value The value to convert.
      * @return The integral representation of the given value.
      */
-    [[nodiscard]] static constexpr entity_type to_integral(const value_type value) ENTT_NOEXCEPT {
+    [[nodiscard]] static constexpr entity_type to_integral(const value_type value) noexcept {
         return static_cast<entity_type>(value);
     }
 
@@ -88,8 +96,8 @@ public:
      * @param value The value to convert.
      * @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) & base_type::entity_mask);
+    [[nodiscard]] static constexpr entity_type to_entity(const value_type value) noexcept {
+        return (to_integral(value) & entity_mask);
     }
 
     /**
@@ -97,8 +105,18 @@ public:
      * @param value The value to convert.
      * @return The integral representation of the version part.
      */
-    [[nodiscard]] static constexpr version_type to_version(const value_type value) ENTT_NOEXCEPT {
-        return (to_integral(value) >> base_type::entity_shift);
+    [[nodiscard]] static constexpr version_type to_version(const value_type value) noexcept {
+        return (static_cast<version_type>(to_integral(value) >> length) & version_mask);
+    }
+
+    /**
+     * @brief Returns the successor of a given identifier.
+     * @param value The identifier of which to return the successor.
+     * @return The successor of the given identifier.
+     */
+    [[nodiscard]] static constexpr value_type next(const value_type value) noexcept {
+        const auto vers = to_version(value) + 1;
+        return construct(to_integral(value), static_cast<version_type>(vers + (vers == version_mask)));
     }
 
     /**
@@ -111,8 +129,8 @@ public:
      * @param version The version part of the identifier.
      * @return A properly constructed identifier.
      */
-    [[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)};
+    [[nodiscard]] static constexpr value_type construct(const entity_type entity, const version_type version) noexcept {
+        return value_type{(entity & entity_mask) | (static_cast<entity_type>(version & version_mask) << length)};
     }
 
     /**
@@ -125,36 +143,53 @@ public:
      * @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)};
+    [[nodiscard]] static constexpr value_type combine(const entity_type lhs, const entity_type rhs) noexcept {
+        return value_type{(lhs & entity_mask) | (rhs & (version_mask << length))};
     }
 };
 
 /**
- * @copydoc entt_traits<Entity>::to_integral
+ * @brief Entity traits.
+ * @tparam Type Type of identifier.
+ */
+template<typename Type>
+struct entt_traits: basic_entt_traits<internal::entt_traits<Type>> {
+    /*! @brief Base type. */
+    using base_type = basic_entt_traits<internal::entt_traits<Type>>;
+    /*! @brief Page size, default is `ENTT_SPARSE_PAGE`. */
+    static constexpr std::size_t page_size = ENTT_SPARSE_PAGE;
+};
+
+/**
+ * @brief Converts an entity to its underlying type.
  * @tparam Entity The value type.
+ * @param value The value to convert.
+ * @return The integral representation of the given value.
  */
 template<typename Entity>
-[[nodiscard]] constexpr typename entt_traits<Entity>::entity_type to_integral(const Entity value) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr typename entt_traits<Entity>::entity_type to_integral(const Entity value) noexcept {
     return entt_traits<Entity>::to_integral(value);
 }
 
 /**
- * @copydoc entt_traits<Entity>::to_entity
+ * @brief Returns the entity part once converted to the underlying type.
  * @tparam Entity The value type.
+ * @param value The value to convert.
+ * @return The integral representation of the entity part.
  */
 template<typename Entity>
-[[nodiscard]] constexpr typename entt_traits<Entity>::entity_type to_entity(const Entity value) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr typename entt_traits<Entity>::entity_type to_entity(const Entity value) noexcept {
     return entt_traits<Entity>::to_entity(value);
 }
 
 /**
- * @copydoc entt_traits<Entity>::to_version
+ * @brief Returns the version part once converted to the underlying type.
  * @tparam Entity The value type.
+ * @param value The value to convert.
+ * @return The integral representation of the version part.
  */
 template<typename Entity>
-[[nodiscard]] constexpr typename entt_traits<Entity>::version_type to_version(const Entity value) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr typename entt_traits<Entity>::version_type to_version(const Entity value) noexcept {
     return entt_traits<Entity>::to_version(value);
 }
 
@@ -166,9 +201,10 @@ struct null_t {
      * @return The null representation for the given type.
      */
     template<typename Entity>
-    [[nodiscard]] constexpr operator Entity() const ENTT_NOEXCEPT {
-        using entity_traits = entt_traits<Entity>;
-        return entity_traits::combine(entity_traits::reserved, entity_traits::reserved);
+    [[nodiscard]] constexpr operator Entity() const noexcept {
+        using traits_type = entt_traits<Entity>;
+        constexpr auto value = traits_type::construct(traits_type::entity_mask, traits_type::version_mask);
+        return value;
     }
 
     /**
@@ -176,7 +212,7 @@ struct null_t {
      * @param other A null object.
      * @return True in all cases.
      */
-    [[nodiscard]] constexpr bool operator==([[maybe_unused]] const null_t other) const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr bool operator==([[maybe_unused]] const null_t other) const noexcept {
         return true;
     }
 
@@ -185,7 +221,7 @@ struct null_t {
      * @param other A null object.
      * @return False in all cases.
      */
-    [[nodiscard]] constexpr bool operator!=([[maybe_unused]] const null_t other) const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr bool operator!=([[maybe_unused]] const null_t other) const noexcept {
         return false;
     }
 
@@ -196,9 +232,9 @@ struct null_t {
      * @return False if the two elements differ, true otherwise.
      */
     template<typename Entity>
-    [[nodiscard]] constexpr bool operator==(const Entity entity) const ENTT_NOEXCEPT {
-        using entity_traits = entt_traits<Entity>;
-        return entity_traits::to_entity(entity) == entity_traits::to_entity(*this);
+    [[nodiscard]] constexpr bool operator==(const Entity entity) const noexcept {
+        using traits_type = entt_traits<Entity>;
+        return traits_type::to_entity(entity) == traits_type::to_entity(*this);
     }
 
     /**
@@ -208,7 +244,7 @@ struct null_t {
      * @return True if the two elements differ, false otherwise.
      */
     template<typename Entity>
-    [[nodiscard]] constexpr bool operator!=(const Entity entity) const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr bool operator!=(const Entity entity) const noexcept {
         return !(entity == *this);
     }
 };
@@ -221,7 +257,7 @@ struct null_t {
  * @return False if the two elements differ, true otherwise.
  */
 template<typename Entity>
-[[nodiscard]] constexpr bool operator==(const Entity entity, const null_t other) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr bool operator==(const Entity entity, const null_t other) noexcept {
     return other.operator==(entity);
 }
 
@@ -233,7 +269,7 @@ template<typename Entity>
  * @return True if the two elements differ, false otherwise.
  */
 template<typename Entity>
-[[nodiscard]] constexpr bool operator!=(const Entity entity, const null_t other) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr bool operator!=(const Entity entity, const null_t other) noexcept {
     return !(other == entity);
 }
 
@@ -245,9 +281,10 @@ struct tombstone_t {
      * @return The tombstone representation for the given type.
      */
     template<typename Entity>
-    [[nodiscard]] constexpr operator Entity() const ENTT_NOEXCEPT {
-        using entity_traits = entt_traits<Entity>;
-        return entity_traits::combine(entity_traits::reserved, entity_traits::reserved);
+    [[nodiscard]] constexpr operator Entity() const noexcept {
+        using traits_type = entt_traits<Entity>;
+        constexpr auto value = traits_type::construct(traits_type::entity_mask, traits_type::version_mask);
+        return value;
     }
 
     /**
@@ -255,7 +292,7 @@ struct tombstone_t {
      * @param other A tombstone object.
      * @return True in all cases.
      */
-    [[nodiscard]] constexpr bool operator==([[maybe_unused]] const tombstone_t other) const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr bool operator==([[maybe_unused]] const tombstone_t other) const noexcept {
         return true;
     }
 
@@ -264,7 +301,7 @@ struct tombstone_t {
      * @param other A tombstone object.
      * @return False in all cases.
      */
-    [[nodiscard]] constexpr bool operator!=([[maybe_unused]] const tombstone_t other) const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr bool operator!=([[maybe_unused]] const tombstone_t other) const noexcept {
         return false;
     }
 
@@ -275,9 +312,9 @@ struct tombstone_t {
      * @return False if the two elements differ, true otherwise.
      */
     template<typename Entity>
-    [[nodiscard]] constexpr bool operator==(const Entity entity) const ENTT_NOEXCEPT {
-        using entity_traits = entt_traits<Entity>;
-        return entity_traits::to_version(entity) == entity_traits::to_version(*this);
+    [[nodiscard]] constexpr bool operator==(const Entity entity) const noexcept {
+        using traits_type = entt_traits<Entity>;
+        return traits_type::to_version(entity) == traits_type::to_version(*this);
     }
 
     /**
@@ -287,7 +324,7 @@ struct tombstone_t {
      * @return True if the two elements differ, false otherwise.
      */
     template<typename Entity>
-    [[nodiscard]] constexpr bool operator!=(const Entity entity) const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr bool operator!=(const Entity entity) const noexcept {
         return !(entity == *this);
     }
 };
@@ -300,7 +337,7 @@ struct tombstone_t {
  * @return False if the two elements differ, true otherwise.
  */
 template<typename Entity>
-[[nodiscard]] constexpr bool operator==(const Entity entity, const tombstone_t other) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr bool operator==(const Entity entity, const tombstone_t other) noexcept {
     return other.operator==(entity);
 }
 
@@ -312,7 +349,7 @@ template<typename Entity>
  * @return True if the two elements differ, false otherwise.
  */
 template<typename Entity>
-[[nodiscard]] constexpr bool operator!=(const Entity entity, const tombstone_t other) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr bool operator!=(const Entity entity, const tombstone_t other) noexcept {
     return !(other == entity);
 }
 
@@ -320,7 +357,7 @@ template<typename Entity>
  * @brief Compile-time constant for null entities.
  *
  * There exist implicit conversions from this variable to identifiers of any
- * allowed type. Similarly, there exist comparision operators between the null
+ * allowed type. Similarly, there exist comparison operators between the null
  * entity and any other identifier.
  */
 inline constexpr null_t null{};
@@ -329,7 +366,7 @@ inline constexpr null_t null{};
  * @brief Compile-time constant for tombstone entities.
  *
  * There exist implicit conversions from this variable to identifiers of any
- * allowed type. Similarly, there exist comparision operators between the
+ * allowed type. Similarly, there exist comparison operators between the
  * tombstone entity and any other identifier.
  */
 inline constexpr tombstone_t tombstone{};

src/entt/entity/fwd.hpp

@@ -1,31 +1,49 @@
 #ifndef ENTT_ENTITY_FWD_HPP
 #define ENTT_ENTITY_FWD_HPP
 
+#include <cstdint>
 #include <memory>
+#include <type_traits>
 #include "../core/fwd.hpp"
-#include "utility.hpp"
+#include "../core/type_traits.hpp"
 
 namespace entt {
 
-template<typename Entity, typename = std::allocator<Entity>>
+/*! @brief Default entity identifier. */
+enum class entity : id_type {};
+
+/*! @brief Storage deletion policy. */
+enum class deletion_policy : std::uint8_t {
+    /*! @brief Swap-and-pop deletion policy. */
+    swap_and_pop = 0u,
+    /*! @brief In-place deletion policy. */
+    in_place = 1u,
+    /*! @brief Swap-only deletion policy. */
+    swap_only = 2u
+};
+
+template<typename Entity = entity, typename = std::allocator<Entity>>
 class basic_sparse_set;
 
-template<typename, typename Type, typename = std::allocator<Type>, typename = void>
+template<typename Type, typename = entity, typename = std::allocator<Type>, typename = void>
 class basic_storage;
 
-template<typename>
+template<typename, typename>
+class basic_sigh_mixin;
+
+template<typename Entity = entity, typename = std::allocator<Entity>>
 class basic_registry;
 
-template<typename, typename, typename, typename = void>
+template<typename, typename, typename = void>
 class basic_view;
 
-template<typename>
+template<typename Type, typename = std::allocator<Type *>>
 class basic_runtime_view;
 
-template<typename, typename, typename, typename>
+template<typename, typename, typename>
 class basic_group;
 
-template<typename>
+template<typename, typename Mask = std::uint32_t, typename = std::allocator<Mask>>
 class basic_observer;
 
 template<typename>
@@ -43,74 +61,205 @@ class basic_snapshot_loader;
 template<typename>
 class basic_continuous_loader;
 
-/*! @brief Default entity identifier. */
-enum class entity : id_type {};
+/*! @brief Alias declaration for the most common use case. */
+using sparse_set = basic_sparse_set<>;
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Type Type of objects assigned to the entities.
+ */
+template<typename Type>
+using storage = basic_storage<Type>;
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Type Underlying storage type.
+ */
+template<typename Type>
+using sigh_mixin = basic_sigh_mixin<Type, basic_registry<typename Type::entity_type, typename Type::base_type::allocator_type>>;
 
 /*! @brief Alias declaration for the most common use case. */
-using sparse_set = basic_sparse_set<entity>;
+using registry = basic_registry<>;
+
+/*! @brief Alias declaration for the most common use case. */
+using observer = basic_observer<registry>;
+
+/*! @brief Alias declaration for the most common use case. */
+using organizer = basic_organizer<registry>;
+
+/*! @brief Alias declaration for the most common use case. */
+using handle = basic_handle<registry>;
+
+/*! @brief Alias declaration for the most common use case. */
+using const_handle = basic_handle<const registry>;
 
 /**
  * @brief Alias declaration for the most common use case.
  * @tparam Args Other template parameters.
  */
 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>;
+using handle_view = basic_handle<registry, Args...>;
 
 /**
  * @brief Alias declaration for the most common use case.
  * @tparam Args Other template parameters.
  */
 template<typename... Args>
-using handle_view = basic_handle<entity, Args...>;
+using const_handle_view = basic_handle<const registry, Args...>;
+
+/*! @brief Alias declaration for the most common use case. */
+using snapshot = basic_snapshot<registry>;
+
+/*! @brief Alias declaration for the most common use case. */
+using snapshot_loader = basic_snapshot_loader<registry>;
+
+/*! @brief Alias declaration for the most common use case. */
+using continuous_loader = basic_continuous_loader<registry>;
+
+/*! @brief Alias declaration for the most common use case. */
+using runtime_view = basic_runtime_view<sparse_set>;
+
+/*! @brief Alias declaration for the most common use case. */
+using const_runtime_view = basic_runtime_view<const sparse_set>;
 
 /**
- * @brief Alias declaration for the most common use case.
- * @tparam Args Other template parameters.
+ * @brief Alias for exclusion lists.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+struct exclude_t final: type_list<Type...> {
+    /*! @brief Default constructor. */
+    explicit constexpr exclude_t() {}
+};
+
+/**
+ * @brief Variable template for exclusion lists.
+ * @tparam Type List of types.
+ */
+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 final: type_list<Type...> {
+    /*! @brief Default constructor. */
+    explicit constexpr get_t() {}
+};
+
+/**
+ * @brief Variable template for lists of observed components.
+ * @tparam Type List of types.
+ */
+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 final: type_list<Type...> {
+    /*! @brief Default constructor. */
+    explicit constexpr owned_t() {}
+};
+
+/**
+ * @brief Variable template for lists of owned components.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+inline constexpr owned_t<Type...> owned{};
+
+/**
+ * @brief Applies a given _function_ to a get list and generate a new list.
+ * @tparam Type Types provided by the get list.
+ * @tparam Op Unary operation as template class with a type member named `type`.
+ */
+template<typename... Type, template<typename...> class Op>
+struct type_list_transform<get_t<Type...>, Op> {
+    /*! @brief Resulting get list after applying the transform function. */
+    using type = get_t<typename Op<Type>::type...>;
+};
+
+/**
+ * @brief Applies a given _function_ to an exclude list and generate a new list.
+ * @tparam Type Types provided by the exclude list.
+ * @tparam Op Unary operation as template class with a type member named `type`.
+ */
+template<typename... Type, template<typename...> class Op>
+struct type_list_transform<exclude_t<Type...>, Op> {
+    /*! @brief Resulting exclude list after applying the transform function. */
+    using type = exclude_t<typename Op<Type>::type...>;
+};
+
+/**
+ * @brief Applies a given _function_ to an owned list and generate a new list.
+ * @tparam Type Types provided by the owned list.
+ * @tparam Op Unary operation as template class with a type member named `type`.
+ */
+template<typename... Type, template<typename...> class Op>
+struct type_list_transform<owned_t<Type...>, Op> {
+    /*! @brief Resulting owned list after applying the transform function. */
+    using type = owned_t<typename Op<Type>::type...>;
+};
+
+/**
+ * @brief Provides a common way to define storage types.
+ * @tparam Type Storage value type.
+ * @tparam Entity A valid entity type.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Type, typename Entity = entity, typename Allocator = std::allocator<Type>, typename = void>
+struct storage_type {
+    /*! @brief Type-to-storage conversion result. */
+    using type = sigh_mixin<basic_storage<Type, Entity, Allocator>>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam Args Arguments to forward.
  */
 template<typename... Args>
-using const_handle_view = basic_handle<const entity, Args...>;
+using storage_type_t = typename storage_type<Args...>::type;
 
-/*! @brief Alias declaration for the most common use case. */
-using snapshot = basic_snapshot<entity>;
+/**
+ * Type-to-storage conversion utility that preserves constness.
+ * @tparam Type Storage value type, eventually const.
+ * @tparam Entity A valid entity type.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Type, typename Entity = entity, typename Allocator = std::allocator<std::remove_const_t<Type>>>
+struct storage_for {
+    /*! @brief Type-to-storage conversion result. */
+    using type = constness_as_t<storage_type_t<std::remove_const_t<Type>, Entity, Allocator>, Type>;
+};
 
-/*! @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 Helper type.
+ * @tparam Args Arguments to forward.
+ */
+template<typename... Args>
+using storage_for_t = typename storage_for<Args...>::type;
 
 /**
  * @brief Alias declaration for the most common use case.
- * @tparam Get Types of components iterated by the view.
- * @tparam Exclude Types of components used to filter the view.
+ * @tparam Get Types of storage iterated by the view.
+ * @tparam Exclude Types of storage used to filter the view.
  */
 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 view = basic_view<type_list_transform_t<Get, storage_for>, type_list_transform_t<Exclude, storage_for>>;
 
 /**
  * @brief Alias declaration for the most common use case.
- * @tparam Args Other template parameters.
+ * @tparam Owned Types of storage _owned_ by the group.
+ * @tparam Get Types of storage _observed_ by the group.
+ * @tparam Exclude Types of storage used to filter the group.
  */
-template<typename... Args>
-using group = basic_group<entity, Args...>;
+template<typename Owned, typename Get, typename Exclude>
+using group = basic_group<type_list_transform_t<Owned, storage_for>, type_list_transform_t<Get, storage_for>, type_list_transform_t<Exclude, storage_for>>;
 
 } // namespace entt
 

src/entt/entity/handle.hpp

@@ -1,28 +1,102 @@
 #ifndef ENTT_ENTITY_HANDLE_HPP
 #define ENTT_ENTITY_HANDLE_HPP
 
+#include <iterator>
 #include <tuple>
 #include <type_traits>
 #include <utility>
-#include "../config/config.h"
+#include "../core/iterator.hpp"
 #include "../core/type_traits.hpp"
+#include "entity.hpp"
 #include "fwd.hpp"
-#include "registry.hpp"
 
 namespace entt {
 
+/*! @cond TURN_OFF_DOXYGEN */
+namespace internal {
+
+template<typename It>
+class handle_storage_iterator final {
+    template<typename Other>
+    friend class handle_storage_iterator;
+
+    using underlying_type = std::remove_reference_t<typename It::value_type::second_type>;
+    using entity_type = typename underlying_type::entity_type;
+
+public:
+    using value_type = typename std::iterator_traits<It>::value_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;
+    using iterator_concept = std::forward_iterator_tag;
+
+    constexpr handle_storage_iterator() noexcept
+        : entt{null},
+          it{},
+          last{} {}
+
+    constexpr handle_storage_iterator(entity_type value, It from, It to) noexcept
+        : entt{value},
+          it{from},
+          last{to} {
+        while(it != last && !it->second.contains(entt)) {
+            ++it;
+        }
+    }
+
+    constexpr handle_storage_iterator &operator++() noexcept {
+        while(++it != last && !it->second.contains(entt)) {}
+        return *this;
+    }
+
+    constexpr handle_storage_iterator operator++(int) noexcept {
+        handle_storage_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    [[nodiscard]] constexpr reference operator*() const noexcept {
+        return *it;
+    }
+
+    [[nodiscard]] constexpr pointer operator->() const noexcept {
+        return operator*();
+    }
+
+    template<typename ILhs, typename IRhs>
+    friend constexpr bool operator==(const handle_storage_iterator<ILhs> &, const handle_storage_iterator<IRhs> &) noexcept;
+
+private:
+    entity_type entt;
+    It it;
+    It last;
+};
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] constexpr bool operator==(const handle_storage_iterator<ILhs> &lhs, const handle_storage_iterator<IRhs> &rhs) noexcept {
+    return lhs.it == rhs.it;
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] constexpr bool operator!=(const handle_storage_iterator<ILhs> &lhs, const handle_storage_iterator<IRhs> &rhs) noexcept {
+    return !(lhs == rhs);
+}
+
+} // namespace internal
+/*! @endcond */
+
 /**
  * @brief Non-owning handle to an entity.
  *
  * Tiny wrapper around a registry and an entity.
  *
- * @tparam Entity A valid entity type (see entt_traits for more details).
- * @tparam Type Types to which to restrict the scope of a handle.
+ * @tparam Registry Basic registry type.
+ * @tparam Scope Types to which to restrict the scope of a handle.
  */
-template<typename Entity, typename... Type>
+template<typename Registry, typename... Scope>
 struct basic_handle {
     /*! @brief Type of registry accepted by the handle. */
-    using registry_type = constness_as_t<basic_registry<std::remove_const_t<Entity>>, Entity>;
+    using registry_type = Registry;
     /*! @brief Underlying entity identifier. */
     using entity_type = typename registry_type::entity_type;
     /*! @brief Underlying version type. */
@@ -31,7 +105,7 @@ struct basic_handle {
     using size_type = typename registry_type::size_type;
 
     /*! @brief Constructs an invalid handle. */
-    basic_handle() ENTT_NOEXCEPT
+    basic_handle() noexcept
         : reg{},
           entt{null} {}
 
@@ -40,21 +114,37 @@ struct basic_handle {
      * @param ref An instance of the registry class.
      * @param value A valid identifier.
      */
-    basic_handle(registry_type &ref, entity_type value) ENTT_NOEXCEPT
+    basic_handle(registry_type &ref, entity_type value) noexcept
         : reg{&ref},
           entt{value} {}
 
+    /**
+     * @brief Returns an iterable object to use to _visit_ a handle.
+     *
+     * The iterable object returns a pair that contains the name and a reference
+     * to the current storage.<br/>
+     * Returned storage are those that contain the entity associated with the
+     * handle.
+     *
+     * @return An iterable object to use to _visit_ the handle.
+     */
+    [[nodiscard]] auto storage() const noexcept {
+        auto iterable = reg->storage();
+        using iterator_type = internal::handle_storage_iterator<typename decltype(iterable)::iterator>;
+        return iterable_adaptor{iterator_type{entt, iterable.begin(), iterable.end()}, iterator_type{entt, iterable.end(), iterable.end()}};
+    }
+
     /**
      * @brief Constructs a const handle from a non-const one.
-     * @tparam Other A valid entity type (see entt_traits for more details).
+     * @tparam Other A valid entity type.
      * @tparam Args Scope of the handle to construct.
      * @return A const handle referring to the same registry and the same
      * entity.
      */
     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>, "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");
+    operator basic_handle<Other, Args...>() const noexcept {
+        static_assert(std::is_same_v<Other, Registry> || std::is_same_v<std::remove_const_t<Other>, Registry>, "Invalid conversion between different handles");
+        static_assert((sizeof...(Scope) == 0 || ((sizeof...(Args) != 0 && sizeof...(Args) <= sizeof...(Scope)) && ... && (type_list_contains_v<type_list<Scope...>, Args>))), "Invalid conversion between different handles");
 
         return reg ? basic_handle<Other, Args...>{*reg, entt} : basic_handle<Other, Args...>{};
     }
@@ -63,7 +153,7 @@ struct basic_handle {
      * @brief Converts a handle to its underlying entity.
      * @return The contained identifier.
      */
-    [[nodiscard]] operator entity_type() const ENTT_NOEXCEPT {
+    [[nodiscard]] operator entity_type() const noexcept {
         return entity();
     }
 
@@ -71,7 +161,7 @@ struct basic_handle {
      * @brief Checks if a handle refers to non-null registry pointer and entity.
      * @return True if the handle refers to non-null registry and entity, false otherwise.
      */
-    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+    [[nodiscard]] explicit operator bool() const noexcept {
         return reg && reg->valid(entt);
     }
 
@@ -87,7 +177,7 @@ 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 noexcept {
         return reg;
     }
 
@@ -95,30 +185,25 @@ struct basic_handle {
      * @brief Returns the entity associated with a handle.
      * @return The entity associated with the handle.
      */
-    [[nodiscard]] entity_type entity() const ENTT_NOEXCEPT {
+    [[nodiscard]] entity_type entity() const noexcept {
         return entt;
     }
 
-    /**
-     * @brief Destroys the entity associated with a handle.
-     * @sa basic_registry::destroy
-     */
+    /*! @brief Destroys the entity associated with a handle. */
     void destroy() {
-        reg->destroy(entt);
+        reg->destroy(std::exchange(entt, null));
     }
 
     /**
      * @brief Destroys the entity associated with a handle.
-     * @sa basic_registry::destroy
      * @param version A desired version upon destruction.
      */
     void destroy(const version_type version) {
-        reg->destroy(entt, version);
+        reg->destroy(std::exchange(entt, null), version);
     }
 
     /**
      * @brief Assigns the given component to a handle.
-     * @sa basic_registry::emplace
      * @tparam Component Type of component to create.
      * @tparam Args Types of arguments to use to construct the component.
      * @param args Parameters to use to initialize the component.
@@ -126,13 +211,12 @@ struct basic_handle {
      */
     template<typename Component, typename... Args>
     decltype(auto) emplace(Args &&...args) const {
-        static_assert(((sizeof...(Type) == 0) || ... || std::is_same_v<Component, Type>), "Invalid type");
+        static_assert(((sizeof...(Scope) == 0) || ... || std::is_same_v<Component, Scope>), "Invalid type");
         return reg->template emplace<Component>(entt, std::forward<Args>(args)...);
     }
 
     /**
      * @brief Assigns or replaces the given component for a handle.
-     * @sa basic_registry::emplace_or_replace
      * @tparam Component Type of component to assign or replace.
      * @tparam Args Types of arguments to use to construct the component.
      * @param args Parameters to use to initialize the component.
@@ -140,13 +224,12 @@ struct basic_handle {
      */
     template<typename Component, typename... Args>
     decltype(auto) emplace_or_replace(Args &&...args) const {
-        static_assert(((sizeof...(Type) == 0) || ... || std::is_same_v<Component, Type>), "Invalid type");
+        static_assert(((sizeof...(Scope) == 0) || ... || std::is_same_v<Component, Scope>), "Invalid type");
         return reg->template emplace_or_replace<Component>(entt, std::forward<Args>(args)...);
     }
 
     /**
      * @brief Patches the given component for a handle.
-     * @sa basic_registry::patch
      * @tparam Component Type of component to patch.
      * @tparam Func Types of the function objects to invoke.
      * @param func Valid function objects.
@@ -154,13 +237,12 @@ struct basic_handle {
      */
     template<typename Component, typename... Func>
     decltype(auto) patch(Func &&...func) const {
-        static_assert(((sizeof...(Type) == 0) || ... || std::is_same_v<Component, Type>), "Invalid type");
+        static_assert(((sizeof...(Scope) == 0) || ... || std::is_same_v<Component, Scope>), "Invalid type");
         return reg->template patch<Component>(entt, std::forward<Func>(func)...);
     }
 
     /**
      * @brief Replaces the given component for a handle.
-     * @sa basic_registry::replace
      * @tparam Component Type of component to replace.
      * @tparam Args Types of arguments to use to construct the component.
      * @param args Parameters to use to initialize the component.
@@ -168,36 +250,33 @@ struct basic_handle {
      */
     template<typename Component, typename... Args>
     decltype(auto) replace(Args &&...args) const {
-        static_assert(((sizeof...(Type) == 0) || ... || std::is_same_v<Component, Type>), "Invalid type");
+        static_assert(((sizeof...(Scope) == 0) || ... || std::is_same_v<Component, Scope>), "Invalid type");
         return reg->template replace<Component>(entt, std::forward<Args>(args)...);
     }
 
     /**
      * @brief Removes the given components from a handle.
-     * @sa basic_registry::remove
      * @tparam Component Types of components to remove.
      * @return The number of components actually removed.
      */
     template<typename... Component>
     size_type remove() const {
-        static_assert(sizeof...(Type) == 0 || (type_list_contains_v<type_list<Type...>, Component> && ...), "Invalid type");
+        static_assert(sizeof...(Scope) == 0 || (type_list_contains_v<type_list<Scope...>, Component> && ...), "Invalid type");
         return reg->template remove<Component...>(entt);
     }
 
     /**
      * @brief Erases the given components from a handle.
-     * @sa basic_registry::erase
      * @tparam Component Types of components to erase.
      */
     template<typename... Component>
     void erase() const {
-        static_assert(sizeof...(Type) == 0 || (type_list_contains_v<type_list<Type...>, Component> && ...), "Invalid type");
+        static_assert(sizeof...(Scope) == 0 || (type_list_contains_v<type_list<Scope...>, Component> && ...), "Invalid type");
         reg->template erase<Component...>(entt);
     }
 
     /**
      * @brief Checks if a handle has all the given components.
-     * @sa basic_registry::all_of
      * @tparam Component Components for which to perform the check.
      * @return True if the handle has all the components, false otherwise.
      */
@@ -208,7 +287,6 @@ struct basic_handle {
 
     /**
      * @brief Checks if a handle has at least one of the given components.
-     * @sa basic_registry::any_of
      * @tparam Component Components for which to perform the check.
      * @return True if the handle has at least one of the given components,
      * false otherwise.
@@ -220,19 +298,17 @@ struct basic_handle {
 
     /**
      * @brief Returns references to the given components for a handle.
-     * @sa basic_registry::get
      * @tparam Component Types of components to get.
      * @return References to the components owned by the handle.
      */
     template<typename... Component>
     [[nodiscard]] decltype(auto) get() const {
-        static_assert(sizeof...(Type) == 0 || (type_list_contains_v<type_list<Type...>, Component> && ...), "Invalid type");
+        static_assert(sizeof...(Scope) == 0 || (type_list_contains_v<type_list<Scope...>, Component> && ...), "Invalid type");
         return reg->template get<Component...>(entt);
     }
 
     /**
      * @brief Returns a reference to the given component for a handle.
-     * @sa basic_registry::get_or_emplace
      * @tparam Component Type of component to get.
      * @tparam Args Types of arguments to use to construct the component.
      * @param args Parameters to use to initialize the component.
@@ -240,19 +316,18 @@ struct basic_handle {
      */
     template<typename Component, typename... Args>
     [[nodiscard]] decltype(auto) get_or_emplace(Args &&...args) const {
-        static_assert(((sizeof...(Type) == 0) || ... || std::is_same_v<Component, Type>), "Invalid type");
+        static_assert(((sizeof...(Scope) == 0) || ... || std::is_same_v<Component, Scope>), "Invalid type");
         return reg->template get_or_emplace<Component>(entt, std::forward<Args>(args)...);
     }
 
     /**
      * @brief Returns pointers to the given components for a handle.
-     * @sa basic_registry::try_get
      * @tparam Component Types of components to get.
      * @return Pointers to the components owned by the handle.
      */
     template<typename... Component>
     [[nodiscard]] auto try_get() const {
-        static_assert(sizeof...(Type) == 0 || (type_list_contains_v<type_list<Type...>, Component> && ...), "Invalid type");
+        static_assert(sizeof...(Scope) == 0 || (type_list_contains_v<type_list<Scope...>, Component> && ...), "Invalid type");
         return reg->template try_get<Component...>(entt);
     }
 
@@ -264,30 +339,6 @@ struct basic_handle {
         return reg->orphan(entt);
     }
 
-    /**
-     * @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 {
-        for(auto [id, storage]: reg->storage()) {
-            if(storage.contains(entt)) {
-                func(id, storage);
-            }
-        }
-    }
-
 private:
     registry_type *reg;
     entity_type entt;
@@ -303,7 +354,7 @@ private:
  * entity, false otherwise.
  */
 template<typename... Args, typename... Other>
-[[nodiscard]] bool operator==(const basic_handle<Args...> &lhs, const basic_handle<Other...> &rhs) ENTT_NOEXCEPT {
+[[nodiscard]] bool operator==(const basic_handle<Args...> &lhs, const basic_handle<Other...> &rhs) noexcept {
     return lhs.registry() == rhs.registry() && lhs.entity() == rhs.entity();
 }
 
@@ -317,24 +368,10 @@ template<typename... Args, typename... Other>
  * entity, true otherwise.
  */
 template<typename... Args, typename... Other>
-[[nodiscard]] bool operator!=(const basic_handle<Args...> &lhs, const basic_handle<Other...> &rhs) ENTT_NOEXCEPT {
+[[nodiscard]] bool operator!=(const basic_handle<Args...> &lhs, const basic_handle<Other...> &rhs) 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(const basic_registry<Entity> &, Entity) -> basic_handle<const Entity>;
-
 } // namespace entt
 
 #endif

src/entt/entity/helper.hpp

@@ -1,124 +1,113 @@
 #ifndef ENTT_ENTITY_HELPER_HPP
 #define ENTT_ENTITY_HELPER_HPP
 
+#include <memory>
 #include <type_traits>
-#include "../config/config.h"
+#include <utility>
 #include "../core/fwd.hpp"
 #include "../core/type_traits.hpp"
 #include "../signal/delegate.hpp"
 #include "fwd.hpp"
-#include "registry.hpp"
+#include "group.hpp"
+#include "storage.hpp"
+#include "view.hpp"
 
 namespace entt {
 
 /**
  * @brief Converts a registry to a view.
- * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Registry Basic registry type.
  */
-template<typename Entity>
-struct as_view {
-    /*! @brief Underlying entity identifier. */
-    using entity_type = std::remove_const_t<Entity>;
+template<typename Registry>
+class as_view {
+    template<typename... Get, typename... Exclude>
+    auto dispatch(get_t<Get...>, exclude_t<Exclude...>) const {
+        return reg.template view<constness_as_t<typename Get::value_type, Get>...>(exclude_t<constness_as_t<typename Exclude::value_type, Exclude>...>{});
+    }
+
+public:
     /*! @brief Type of registry to convert. */
-    using registry_type = constness_as_t<basic_registry<entity_type>, Entity>;
+    using registry_type = Registry;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename registry_type::entity_type;
 
     /**
      * @brief Constructs a converter for a given registry.
      * @param source A valid reference to a registry.
      */
-    as_view(registry_type &source) ENTT_NOEXCEPT: reg{source} {}
+    as_view(registry_type &source) noexcept
+        : reg{source} {}
 
     /**
      * @brief Conversion function from a registry to a view.
-     * @tparam Exclude Types of components used to filter the view.
-     * @tparam Component Type of components used to construct the view.
+     * @tparam Get Type of storage used to construct the view.
+     * @tparam Exclude Types of storage used to filter the view.
      * @return A newly created view.
      */
-    template<typename Exclude, typename... Component>
-    operator basic_view<entity_type, get_t<Component...>, Exclude>() const {
-        return reg.template view<Component...>(Exclude{});
+    template<typename Get, typename Exclude>
+    operator basic_view<Get, Exclude>() const {
+        return dispatch(Get{}, Exclude{});
     }
 
 private:
     registry_type &reg;
 };
 
-/**
- * @brief Deduction guide.
- * @tparam Entity A valid entity type (see entt_traits for more details).
- */
-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).
- */
-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).
+ * @tparam Registry Basic registry type.
  */
-template<typename Entity>
-struct as_group {
-    /*! @brief Underlying entity identifier. */
-    using entity_type = std::remove_const_t<Entity>;
+template<typename Registry>
+class as_group {
+    template<typename... Owned, typename... Get, typename... Exclude>
+    auto dispatch(owned_t<Owned...>, get_t<Get...>, exclude_t<Exclude...>) const {
+        if constexpr(std::is_const_v<registry_type>) {
+            return reg.template group_if_exists<typename Owned::value_type...>(get_t<typename Get::value_type...>{}, exclude_t<typename Exclude::value_type...>{});
+        } else {
+            return reg.template group<constness_as_t<typename Owned::value_type, Owned>...>(get_t<constness_as_t<typename Get::value_type, Get>...>{}, exclude_t<constness_as_t<typename Exclude::value_type, Exclude>...>{});
+        }
+    }
+
+public:
     /*! @brief Type of registry to convert. */
-    using registry_type = constness_as_t<basic_registry<entity_type>, Entity>;
+    using registry_type = Registry;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename registry_type::entity_type;
 
     /**
      * @brief Constructs a converter for a given registry.
      * @param source A valid reference to a registry.
      */
-    as_group(registry_type &source) ENTT_NOEXCEPT: reg{source} {}
+    as_group(registry_type &source) noexcept
+        : reg{source} {}
 
     /**
      * @brief Conversion function from a registry to a 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.
+     * @tparam Owned Types of _owned_ by the group.
+     * @tparam Get Types of storage _observed_ by the group.
+     * @tparam Exclude Types of storage used to filter the group.
      * @return A newly created group.
      */
-    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 {
-            return reg.template group<Owned...>(Get{}, Exclude{});
-        }
+    template<typename Owned, typename Get, typename Exclude>
+    operator basic_group<Owned, Get, Exclude>() const {
+        return dispatch(Owned{}, Get{}, Exclude{});
     }
 
 private:
     registry_type &reg;
 };
 
-/**
- * @brief Deduction guide.
- * @tparam Entity A valid entity type (see entt_traits for more details).
- */
-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).
- */
-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.
- * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Registry Basic registry type.
  * @param reg A registry that contains the given entity and its components.
  * @param entt Entity from which to get the component.
  */
-template<auto Member, typename Entity = entity>
-void invoke(basic_registry<Entity> &reg, const Entity entt) {
+template<auto Member, typename Registry = std::decay_t<nth_argument_t<0u, decltype(Member)>>>
+void invoke(Registry &reg, const typename Registry::entity_type entt) {
     static_assert(std::is_member_function_pointer_v<decltype(Member)>, "Invalid pointer to non-static member function");
-    delegate<void(basic_registry<Entity> &, const Entity)> func;
+    delegate<void(Registry &, const typename Registry::entity_type)> func;
     func.template connect<Member>(reg.template get<member_class_t<decltype(Member)>>(entt));
     func(reg, entt);
 }
@@ -127,23 +116,22 @@ void invoke(basic_registry<Entity> &reg, const Entity entt) {
  * @brief Returns the entity associated with a given component.
  *
  * @warning
- * Currently, this function only works correctly with the default pool as it
+ * Currently, this function only works correctly with the default storage as it
  * makes assumptions about how the components are laid out.
  *
- * @tparam Entity A valid entity type (see entt_traits for more details).
- * @tparam Component Type of component.
- * @param reg A registry that contains the given entity and its components.
+ * @tparam Args Storage type template parameters.
+ * @param storage A storage that contains the given component.
  * @param instance A valid component instance.
  * @return The entity associated with the given component.
  */
-template<typename Entity, typename Component>
-Entity to_entity(const basic_registry<Entity> &reg, const Component &instance) {
-    const auto &storage = reg.template storage<Component>();
-    const typename basic_registry<Entity>::base_type &base = storage;
+template<typename... Args>
+auto to_entity(const basic_storage<Args...> &storage, const typename basic_storage<Args...>::value_type &instance) -> typename basic_storage<Args...>::entity_type {
+    constexpr auto page_size = basic_storage<Args...>::traits_type::page_size;
+    const typename basic_storage<Args...>::base_type &base = storage;
     const auto *addr = std::addressof(instance);
 
-    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) {
+    for(auto it = base.rbegin(), last = base.rend(); it < last; it += page_size) {
+        if(const auto dist = (addr - std::addressof(storage.get(*it))); dist >= 0 && dist < static_cast<decltype(dist)>(page_size)) {
             return *(it + dist);
         }
     }
@@ -151,6 +139,135 @@ Entity to_entity(const basic_registry<Entity> &reg, const Component &instance) {
     return null;
 }
 
+/**
+ * @copybrief to_entity
+ * @tparam Args Registry type template parameters.
+ * @tparam Component Type of component.
+ * @param reg A registry that contains the given entity and its components.
+ * @param instance A valid component instance.
+ * @return The entity associated with the given component.
+ */
+template<typename... Args, typename Component>
+[[deprecated("use storage based to_entity instead")]] typename basic_registry<Args...>::entity_type to_entity(const basic_registry<Args...> &reg, const Component &instance) {
+    if(const auto *storage = reg.template storage<Component>(); storage) {
+        return to_entity(*storage, instance);
+    }
+
+    return null;
+}
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct sigh_helper;
+
+/**
+ * @brief Signal connection helper for registries.
+ * @tparam Registry Basic registry type.
+ */
+template<typename Registry>
+struct sigh_helper<Registry> {
+    /*! @brief Registry type. */
+    using registry_type = Registry;
+
+    /**
+     * @brief Constructs a helper for a given registry.
+     * @param ref A valid reference to a registry.
+     */
+    sigh_helper(registry_type &ref)
+        : bucket{&ref} {}
+
+    /**
+     * @brief Binds a properly initialized helper to a given signal type.
+     * @tparam Type Type of signal to bind the helper to.
+     * @param id Optional name for the underlying storage to use.
+     * @return A helper for a given registry and signal type.
+     */
+    template<typename Type>
+    auto with(const id_type id = type_hash<Type>::value()) noexcept {
+        return sigh_helper<registry_type, Type>{*bucket, id};
+    }
+
+    /**
+     * @brief Returns a reference to the underlying registry.
+     * @return A reference to the underlying registry.
+     */
+    [[nodiscard]] registry_type &registry() noexcept {
+        return *bucket;
+    }
+
+private:
+    registry_type *bucket;
+};
+
+/**
+ * @brief Signal connection helper for registries.
+ * @tparam Registry Basic registry type.
+ * @tparam Type Type of signal to connect listeners to.
+ */
+template<typename Registry, typename Type>
+struct sigh_helper<Registry, Type> final: sigh_helper<Registry> {
+    /*! @brief Registry type. */
+    using registry_type = Registry;
+
+    /**
+     * @brief Constructs a helper for a given registry.
+     * @param ref A valid reference to a registry.
+     * @param id Optional name for the underlying storage to use.
+     */
+    sigh_helper(registry_type &ref, const id_type id = type_hash<Type>::value())
+        : sigh_helper<Registry>{ref},
+          name{id} {}
+
+    /**
+     * @brief Forwards the call to `on_construct` on the underlying storage.
+     * @tparam Candidate Function or member to connect.
+     * @tparam Args Type of class or type of payload, if any.
+     * @param args A valid object that fits the purpose, if any.
+     * @return This helper.
+     */
+    template<auto Candidate, typename... Args>
+    auto on_construct(Args &&...args) {
+        this->registry().template on_construct<Type>(name).template connect<Candidate>(std::forward<Args>(args)...);
+        return *this;
+    }
+
+    /**
+     * @brief Forwards the call to `on_update` on the underlying storage.
+     * @tparam Candidate Function or member to connect.
+     * @tparam Args Type of class or type of payload, if any.
+     * @param args A valid object that fits the purpose, if any.
+     * @return This helper.
+     */
+    template<auto Candidate, typename... Args>
+    auto on_update(Args &&...args) {
+        this->registry().template on_update<Type>(name).template connect<Candidate>(std::forward<Args>(args)...);
+        return *this;
+    }
+
+    /**
+     * @brief Forwards the call to `on_destroy` on the underlying storage.
+     * @tparam Candidate Function or member to connect.
+     * @tparam Args Type of class or type of payload, if any.
+     * @param args A valid object that fits the purpose, if any.
+     * @return This helper.
+     */
+    template<auto Candidate, typename... Args>
+    auto on_destroy(Args &&...args) {
+        this->registry().template on_destroy<Type>(name).template connect<Candidate>(std::forward<Args>(args)...);
+        return *this;
+    }
+
+private:
+    id_type name;
+};
+
+/**
+ * @brief Deduction guide.
+ * @tparam Registry Basic registry type.
+ */
+template<typename Registry>
+sigh_helper(Registry &) -> sigh_helper<Registry>;
+
 } // namespace entt
 
 #endif

src/entt/entity/mixin.hpp

@@ -0,0 +1,303 @@
+#ifndef ENTT_ENTITY_MIXIN_HPP
+#define ENTT_ENTITY_MIXIN_HPP
+
+#include <type_traits>
+#include <utility>
+#include "../config/config.h"
+#include "../core/any.hpp"
+#include "../signal/sigh.hpp"
+#include "entity.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 Underlying storage type.
+ * @tparam Registry Basic registry type.
+ */
+template<typename Type, typename Registry>
+class basic_sigh_mixin final: public Type {
+    using underlying_type = Type;
+    using owner_type = Registry;
+
+    using basic_registry_type = basic_registry<typename underlying_type::entity_type, typename underlying_type::base_type::allocator_type>;
+    using sigh_type = sigh<void(owner_type &, const typename underlying_type::entity_type), typename underlying_type::allocator_type>;
+    using underlying_iterator = typename underlying_type::base_type::basic_iterator;
+
+    static_assert(std::is_base_of_v<basic_registry_type, owner_type>, "Invalid registry type");
+
+    owner_type &owner_or_assert() const noexcept {
+        ENTT_ASSERT(owner != nullptr, "Invalid pointer to registry");
+        return static_cast<owner_type &>(*owner);
+    }
+
+    void pop(underlying_iterator first, underlying_iterator last) final {
+        if(auto &reg = owner_or_assert(); destruction.empty()) {
+            underlying_type::pop(first, last);
+        } else {
+            for(; first != last; ++first) {
+                const auto entt = *first;
+                destruction.publish(reg, entt);
+                const auto it = underlying_type::find(entt);
+                underlying_type::pop(it, it + 1u);
+            }
+        }
+    }
+
+    void pop_all() final {
+        if(auto &reg = owner_or_assert(); !destruction.empty()) {
+            for(auto it = underlying_type::base_type::begin(0), last = underlying_type::base_type::end(0); it != last; ++it) {
+                if constexpr(std::is_same_v<typename underlying_type::value_type, typename underlying_type::entity_type>) {
+                    destruction.publish(reg, *it);
+                } else {
+                    if constexpr(underlying_type::traits_type::in_place_delete) {
+                        if(const auto entt = *it; entt != tombstone) {
+                            destruction.publish(reg, entt);
+                        }
+                    } else {
+                        destruction.publish(reg, *it);
+                    }
+                }
+            }
+        }
+
+        underlying_type::pop_all();
+    }
+
+    underlying_iterator try_emplace(const typename underlying_type::entity_type entt, const bool force_back, const void *value) final {
+        const auto it = underlying_type::try_emplace(entt, force_back, value);
+
+        if(auto &reg = owner_or_assert(); it != underlying_type::base_type::end()) {
+            construction.publish(reg, *it);
+        }
+
+        return it;
+    }
+
+public:
+    /*! @brief Allocator type. */
+    using allocator_type = typename underlying_type::allocator_type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename underlying_type::entity_type;
+    /*! @brief Expected registry type. */
+    using registry_type = owner_type;
+
+    /*! @brief Default constructor. */
+    basic_sigh_mixin()
+        : basic_sigh_mixin{allocator_type{}} {}
+
+    /**
+     * @brief Constructs an empty storage with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit basic_sigh_mixin(const allocator_type &allocator)
+        : underlying_type{allocator},
+          owner{},
+          construction{allocator},
+          destruction{allocator},
+          update{allocator} {}
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    basic_sigh_mixin(basic_sigh_mixin &&other) noexcept
+        : underlying_type{std::move(other)},
+          owner{other.owner},
+          construction{std::move(other.construction)},
+          destruction{std::move(other.destruction)},
+          update{std::move(other.update)} {}
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    basic_sigh_mixin(basic_sigh_mixin &&other, const allocator_type &allocator) noexcept
+        : underlying_type{std::move(other), allocator},
+          owner{other.owner},
+          construction{std::move(other.construction), allocator},
+          destruction{std::move(other.destruction), allocator},
+          update{std::move(other.update), allocator} {}
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This storage.
+     */
+    basic_sigh_mixin &operator=(basic_sigh_mixin &&other) noexcept {
+        underlying_type::operator=(std::move(other));
+        owner = other.owner;
+        construction = std::move(other.construction);
+        destruction = std::move(other.destruction);
+        update = std::move(other.update);
+        return *this;
+    }
+
+    /**
+     * @brief Exchanges the contents with those of a given storage.
+     * @param other Storage to exchange the content with.
+     */
+    void swap(basic_sigh_mixin &other) {
+        using std::swap;
+        underlying_type::swap(other);
+        swap(owner, other.owner);
+        swap(construction, other.construction);
+        swap(destruction, other.destruction);
+        swap(update, other.update);
+    }
+
+    /**
+     * @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() 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() 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() noexcept {
+        return sink{destruction};
+    }
+
+    /**
+     * @brief Emplace elements into a storage.
+     *
+     * The behavior of this operation depends on the underlying storage type
+     * (for example, components vs entities).<br/>
+     * Refer to the specific documentation for more details.
+     *
+     * @return A return value as returned by the underlying storage.
+     */
+    auto emplace() {
+        const auto entt = underlying_type::emplace();
+        construction.publish(owner_or_assert(), entt);
+        return entt;
+    }
+
+    /**
+     * @brief Emplace elements into a storage.
+     *
+     * The behavior of this operation depends on the underlying storage type
+     * (for example, components vs entities).<br/>
+     * Refer to the specific documentation for more details.
+     *
+     * @tparam Args Types of arguments to forward to the underlying storage.
+     * @param hint A valid identifier.
+     * @param args Parameters to forward to the underlying storage.
+     * @return A return value as returned by the underlying storage.
+     */
+    template<typename... Args>
+    decltype(auto) emplace(const entity_type hint, Args &&...args) {
+        if constexpr(std::is_same_v<typename underlying_type::value_type, typename underlying_type::entity_type>) {
+            const auto entt = underlying_type::emplace(hint, std::forward<Args>(args)...);
+            construction.publish(owner_or_assert(), entt);
+            return entt;
+        } else {
+            underlying_type::emplace(hint, std::forward<Args>(args)...);
+            construction.publish(owner_or_assert(), hint);
+            return this->get(hint);
+        }
+    }
+
+    /**
+     * @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) {
+        underlying_type::patch(entt, std::forward<Func>(func)...);
+        update.publish(owner_or_assert(), entt);
+        return this->get(entt);
+    }
+
+    /**
+     * @brief Emplace elements into a storage.
+     *
+     * The behavior of this operation depends on the underlying storage type
+     * (for example, components vs entities).<br/>
+     * Refer to the specific documentation for more details.
+     *
+     * @tparam It Iterator type (as required by the underlying storage type).
+     * @tparam Args Types of arguments to forward to the underlying storage.
+     * @param first An iterator to the first element of the range.
+     * @param last An iterator past the last element of the range.
+     * @param args Parameters to use to forward to the underlying storage.
+     */
+    template<typename It, typename... Args>
+    void insert(It first, It last, Args &&...args) {
+        auto from = underlying_type::size();
+        underlying_type::insert(first, last, std::forward<Args>(args)...);
+
+        if(auto &reg = owner_or_assert(); !construction.empty()) {
+            for(const auto to = underlying_type::size(); from != to; ++from) {
+                construction.publish(reg, underlying_type::operator[](from));
+            }
+        }
+    }
+
+    /**
+     * @brief Forwards variables to derived classes, if any.
+     * @param value A variable wrapped in an opaque container.
+     */
+    void bind(any value) noexcept final {
+        auto *reg = any_cast<basic_registry_type>(&value);
+        owner = reg ? reg : owner;
+        underlying_type::bind(std::move(value));
+    }
+
+private:
+    basic_registry_type *owner;
+    sigh_type construction;
+    sigh_type destruction;
+    sigh_type update;
+};
+
+} // namespace entt
+
+#endif

src/entt/entity/observer.hpp

@@ -6,14 +6,10 @@
 #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"
 
 namespace entt {
 
@@ -47,7 +43,7 @@ struct basic_collector<> {
      * @return The updated collector.
      */
     template<typename... AllOf, typename... NoneOf>
-    static constexpr auto group(exclude_t<NoneOf...> = {}) ENTT_NOEXCEPT {
+    static constexpr auto group(exclude_t<NoneOf...> = exclude_t{}) noexcept {
         return basic_collector<matcher<type_list<>, type_list<>, type_list<NoneOf...>, AllOf...>>{};
     }
 
@@ -57,7 +53,7 @@ struct basic_collector<> {
      * @return The updated collector.
      */
     template<typename AnyOf>
-    static constexpr auto update() ENTT_NOEXCEPT {
+    static constexpr auto update() noexcept {
         return basic_collector<matcher<type_list<>, type_list<>, AnyOf>>{};
     }
 };
@@ -82,7 +78,7 @@ struct basic_collector<matcher<type_list<Reject...>, type_list<Require...>, Rule
      * @return The updated collector.
      */
     template<typename... AllOf, typename... NoneOf>
-    static constexpr auto group(exclude_t<NoneOf...> = {}) ENTT_NOEXCEPT {
+    static constexpr auto group(exclude_t<NoneOf...> = exclude_t{}) noexcept {
         return basic_collector<matcher<type_list<>, type_list<>, type_list<NoneOf...>, AllOf...>, current_type, Other...>{};
     }
 
@@ -92,7 +88,7 @@ struct basic_collector<matcher<type_list<Reject...>, type_list<Require...>, Rule
      * @return The updated collector.
      */
     template<typename AnyOf>
-    static constexpr auto update() ENTT_NOEXCEPT {
+    static constexpr auto update() noexcept {
         return basic_collector<matcher<type_list<>, type_list<>, AnyOf>, current_type, Other...>{};
     }
 
@@ -103,7 +99,7 @@ struct basic_collector<matcher<type_list<Reject...>, type_list<Require...>, Rule
      * @return The updated collector.
      */
     template<typename... AllOf, typename... NoneOf>
-    static constexpr auto where(exclude_t<NoneOf...> = {}) ENTT_NOEXCEPT {
+    static constexpr auto where(exclude_t<NoneOf...> = exclude_t{}) noexcept {
         using extended_type = matcher<type_list<Reject..., NoneOf...>, type_list<Require..., AllOf...>, Rule...>;
         return basic_collector<extended_type, Other...>{};
     }
@@ -150,8 +146,7 @@ inline constexpr basic_collector<> collector{};
  * * The entity currently pointed is destroyed.
  *
  * In all the other cases, modifying the pools of the given components in any
- * way invalidates all the iterators and using them results in undefined
- * behavior.
+ * way invalidates all the iterators.
  *
  * @warning
  * Lifetime of an observer doesn't necessarily have to overcome that of the
@@ -159,11 +154,13 @@ inline constexpr basic_collector<> collector{};
  * from the registry before being destroyed to avoid crashes due to dangling
  * pointers.
  *
- * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Registry Basic registry type.
+ * @tparam Mask Mask type.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
  */
-template<typename Entity>
-class basic_observer {
-    using payload_type = std::uint32_t;
+template<typename Registry, typename Mask, typename Allocator>
+class basic_observer: private basic_storage<Mask, typename Registry::entity_type, Allocator> {
+    using base_type = basic_storage<Mask, typename Registry::entity_type, Allocator>;
 
     template<typename>
     struct matcher_handler;
@@ -171,43 +168,43 @@ class basic_observer {
     template<typename... Reject, typename... Require, typename AnyOf>
     struct matcher_handler<matcher<type_list<Reject...>, type_list<Require...>, AnyOf>> {
         template<std::size_t Index>
-        static void maybe_valid_if(basic_observer &obs, basic_registry<Entity> &reg, const Entity entt) {
+        static void maybe_valid_if(basic_observer &obs, Registry &reg, const typename Registry::entity_type entt) {
             if(reg.template all_of<Require...>(entt) && !reg.template any_of<Reject...>(entt)) {
-                if(!obs.storage.contains(entt)) {
-                    obs.storage.emplace(entt);
+                if(!obs.contains(entt)) {
+                    obs.emplace(entt);
                 }
 
-                obs.storage.get(entt) |= (1 << Index);
+                obs.get(entt) |= (1 << Index);
             }
         }
 
         template<std::size_t Index>
-        static void discard_if(basic_observer &obs, basic_registry<Entity> &, const Entity entt) {
-            if(obs.storage.contains(entt) && !(obs.storage.get(entt) &= (~(1 << Index)))) {
-                obs.storage.erase(entt);
+        static void discard_if(basic_observer &obs, Registry &, const typename Registry::entity_type entt) {
+            if(obs.contains(entt) && !(obs.get(entt) &= (~(1 << Index)))) {
+                obs.erase(entt);
             }
         }
 
         template<std::size_t Index>
-        static void connect(basic_observer &obs, basic_registry<Entity> &reg) {
+        static void connect(basic_observer &obs, Registry &reg) {
             (reg.template on_destroy<Require>().template connect<&discard_if<Index>>(obs), ...);
             (reg.template on_construct<Reject>().template connect<&discard_if<Index>>(obs), ...);
             reg.template on_update<AnyOf>().template connect<&maybe_valid_if<Index>>(obs);
             reg.template on_destroy<AnyOf>().template connect<&discard_if<Index>>(obs);
         }
 
-        static void disconnect(basic_observer &obs, basic_registry<Entity> &reg) {
-            (reg.template on_destroy<Require>().disconnect(obs), ...);
-            (reg.template on_construct<Reject>().disconnect(obs), ...);
-            reg.template on_update<AnyOf>().disconnect(obs);
-            reg.template on_destroy<AnyOf>().disconnect(obs);
+        static void disconnect(basic_observer &obs, Registry &reg) {
+            (reg.template on_destroy<Require>().disconnect(&obs), ...);
+            (reg.template on_construct<Reject>().disconnect(&obs), ...);
+            reg.template on_update<AnyOf>().disconnect(&obs);
+            reg.template on_destroy<AnyOf>().disconnect(&obs);
         }
     };
 
     template<typename... Reject, typename... Require, typename... NoneOf, typename... AllOf>
     struct matcher_handler<matcher<type_list<Reject...>, type_list<Require...>, type_list<NoneOf...>, AllOf...>> {
         template<std::size_t Index, typename... Ignore>
-        static void maybe_valid_if(basic_observer &obs, basic_registry<Entity> &reg, const Entity entt) {
+        static void maybe_valid_if(basic_observer &obs, Registry &reg, const typename Registry::entity_type 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);
@@ -217,23 +214,23 @@ class basic_observer {
             };
 
             if(condition()) {
-                if(!obs.storage.contains(entt)) {
-                    obs.storage.emplace(entt);
+                if(!obs.contains(entt)) {
+                    obs.emplace(entt);
                 }
 
-                obs.storage.get(entt) |= (1 << Index);
+                obs.get(entt) |= (1 << Index);
             }
         }
 
         template<std::size_t Index>
-        static void discard_if(basic_observer &obs, basic_registry<Entity> &, const Entity entt) {
-            if(obs.storage.contains(entt) && !(obs.storage.get(entt) &= (~(1 << Index)))) {
-                obs.storage.erase(entt);
+        static void discard_if(basic_observer &obs, Registry &, const typename Registry::entity_type entt) {
+            if(obs.contains(entt) && !(obs.get(entt) &= (~(1 << Index)))) {
+                obs.erase(entt);
             }
         }
 
         template<std::size_t Index>
-        static void connect(basic_observer &obs, basic_registry<Entity> &reg) {
+        static void connect(basic_observer &obs, Registry &reg) {
             (reg.template on_destroy<Require>().template connect<&discard_if<Index>>(obs), ...);
             (reg.template on_construct<Reject>().template connect<&discard_if<Index>>(obs), ...);
             (reg.template on_construct<AllOf>().template connect<&maybe_valid_if<Index>>(obs), ...);
@@ -242,43 +239,55 @@ class basic_observer {
             (reg.template on_construct<NoneOf>().template connect<&discard_if<Index>>(obs), ...);
         }
 
-        static void disconnect(basic_observer &obs, basic_registry<Entity> &reg) {
-            (reg.template on_destroy<Require>().disconnect(obs), ...);
-            (reg.template on_construct<Reject>().disconnect(obs), ...);
-            (reg.template on_construct<AllOf>().disconnect(obs), ...);
-            (reg.template on_destroy<NoneOf>().disconnect(obs), ...);
-            (reg.template on_destroy<AllOf>().disconnect(obs), ...);
-            (reg.template on_construct<NoneOf>().disconnect(obs), ...);
+        static void disconnect(basic_observer &obs, Registry &reg) {
+            (reg.template on_destroy<Require>().disconnect(&obs), ...);
+            (reg.template on_construct<Reject>().disconnect(&obs), ...);
+            (reg.template on_construct<AllOf>().disconnect(&obs), ...);
+            (reg.template on_destroy<NoneOf>().disconnect(&obs), ...);
+            (reg.template on_destroy<AllOf>().disconnect(&obs), ...);
+            (reg.template on_construct<NoneOf>().disconnect(&obs), ...);
         }
     };
 
     template<typename... Matcher>
-    static void disconnect(basic_registry<Entity> &reg, basic_observer &obs) {
+    static void disconnect(Registry &reg, basic_observer &obs) {
         (matcher_handler<Matcher>::disconnect(obs, reg), ...);
     }
 
     template<typename... Matcher, std::size_t... Index>
-    void connect(basic_registry<Entity> &reg, std::index_sequence<Index...>) {
-        static_assert(sizeof...(Matcher) < std::numeric_limits<payload_type>::digits, "Too many matchers");
+    void connect(Registry &reg, std::index_sequence<Index...>) {
+        static_assert(sizeof...(Matcher) < std::numeric_limits<typename base_type::value_type>::digits, "Too many matchers");
         (matcher_handler<Matcher>::template connect<Index>(*this, reg), ...);
         release.template connect<&basic_observer::disconnect<Matcher...>>(reg);
     }
 
 public:
+    /*! Basic registry type. */
+    using registry_type = Registry;
     /*! @brief Underlying entity identifier. */
-    using entity_type = Entity;
+    using entity_type = typename registry_type::entity_type;
     /*! @brief Unsigned integer type. */
     using size_type = std::size_t;
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
     /*! @brief Random access iterator type. */
-    using iterator = typename basic_sparse_set<Entity>::iterator;
+    using iterator = typename registry_type::common_type::iterator;
 
     /*! @brief Default constructor. */
     basic_observer()
-        : release{},
-          storage{} {}
+        : basic_observer{allocator_type{}} {}
+
+    /**
+     * @brief Constructs an empty storage with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit basic_observer(const allocator_type &allocator)
+        : base_type{allocator},
+          release{} {}
 
     /*! @brief Default copy constructor, deleted on purpose. */
     basic_observer(const basic_observer &) = delete;
+
     /*! @brief Default move constructor, deleted on purpose. */
     basic_observer(basic_observer &&) = delete;
 
@@ -286,16 +295,14 @@ public:
      * @brief Creates an observer and connects it to a given registry.
      * @tparam Matcher Types of matchers to use to initialize the observer.
      * @param reg A valid reference to a registry.
+     * @param allocator The allocator to use.
      */
     template<typename... Matcher>
-    basic_observer(basic_registry<entity_type> &reg, basic_collector<Matcher...>)
-        : basic_observer{} {
+    basic_observer(registry_type &reg, basic_collector<Matcher...>, const allocator_type &allocator = allocator_type{})
+        : basic_observer{allocator} {
         connect<Matcher...>(reg, std::index_sequence_for<Matcher...>{});
     }
 
-    /*! @brief Default destructor. */
-    ~basic_observer() = default;
-
     /**
      * @brief Default copy assignment operator, deleted on purpose.
      * @return This observer.
@@ -314,10 +321,10 @@ public:
      * @param reg A valid reference to a registry.
      */
     template<typename... Matcher>
-    void connect(basic_registry<entity_type> &reg, basic_collector<Matcher...>) {
+    void connect(registry_type &reg, basic_collector<Matcher...>) {
         disconnect();
         connect<Matcher...>(reg, std::index_sequence_for<Matcher...>{});
-        storage.clear();
+        base_type::clear();
     }
 
     /*! @brief Disconnects an observer from the registry it keeps track of. */
@@ -332,16 +339,16 @@ public:
      * @brief Returns the number of elements in an observer.
      * @return Number of elements.
      */
-    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
-        return storage.size();
+    [[nodiscard]] size_type size() const noexcept {
+        return base_type::size();
     }
 
     /**
      * @brief Checks whether an observer is empty.
      * @return True if the observer is empty, false otherwise.
      */
-    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
-        return storage.empty();
+    [[nodiscard]] bool empty() const noexcept {
+        return base_type::empty();
     }
 
     /**
@@ -356,39 +363,33 @@ public:
      *
      * @return A pointer to the array of entities.
      */
-    [[nodiscard]] const entity_type *data() const ENTT_NOEXCEPT {
-        return storage.data();
+    [[nodiscard]] const entity_type *data() const noexcept {
+        return base_type::data();
     }
 
     /**
      * @brief Returns an iterator to the first entity of the observer.
      *
-     * The returned iterator points to the first entity of the observer. If the
-     * container is empty, the returned iterator will be equal to `end()`.
+     * If the observer is empty, the returned iterator will be equal to `end()`.
      *
      * @return An iterator to the first entity of the observer.
      */
-    [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
-        return storage.basic_sparse_set<entity_type>::begin();
+    [[nodiscard]] iterator begin() const noexcept {
+        return base_type::base_type::begin();
     }
 
     /**
      * @brief Returns an iterator that is past the last entity of the observer.
-     *
-     * The returned iterator points to the entity following the last entity of
-     * the observer. Attempting to dereference the returned iterator results in
-     * undefined behavior.
-     *
      * @return An iterator to the entity following the last entity of the
      * observer.
      */
-    [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
-        return storage.basic_sparse_set<entity_type>::end();
+    [[nodiscard]] iterator end() const noexcept {
+        return base_type::base_type::end();
     }
 
     /*! @brief Clears the underlying container. */
-    void clear() ENTT_NOEXCEPT {
-        storage.clear();
+    void clear() noexcept {
+        base_type::clear();
     }
 
     /**
@@ -428,7 +429,6 @@ public:
 
 private:
     delegate<void(basic_observer &)> release;
-    basic_storage<entity_type, payload_type> storage;
 };
 
 } // namespace entt

src/entt/entity/organizer.hpp

@@ -1,32 +1,28 @@
 #ifndef ENTT_ENTITY_ORGANIZER_HPP
 #define ENTT_ENTITY_ORGANIZER_HPP
 
-#include <algorithm>
 #include <cstddef>
 #include <type_traits>
 #include <utility>
 #include <vector>
-#include "../container/dense_hash_map.hpp"
 #include "../core/type_info.hpp"
 #include "../core/type_traits.hpp"
 #include "../core/utility.hpp"
+#include "../graph/adjacency_matrix.hpp"
+#include "../graph/flow.hpp"
 #include "fwd.hpp"
 #include "helper.hpp"
 
 namespace entt {
 
-/**
- * @cond TURN_OFF_DOXYGEN
- * Internal details not to be documented.
- */
-
+/*! @cond TURN_OFF_DOXYGEN */
 namespace internal {
 
 template<typename>
 struct is_view: std::false_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... Args>
+struct is_view<basic_view<Args...>>: std::true_type {};
 
 template<typename Type>
 inline constexpr bool is_view_v = is_view<Type>::value;
@@ -34,67 +30,60 @@ 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_contains_v<Override, const 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<>>;
 
     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_contains_v<Override, std::remove_const_t<Type>> || (!std::is_const_v<Type> && !type_list_contains_v<Override, const Type>),
         type_list<Type>,
         type_list<>>;
 };
 
-template<typename Entity, typename... Override>
-struct unpack_type<basic_registry<Entity>, type_list<Override...>> {
+template<typename... Args, typename... Override>
+struct unpack_type<basic_registry<Args...>, type_list<Override...>> {
     using ro = type_list<>;
     using rw = type_list<>;
 };
 
-template<typename Entity, typename... Override>
-struct unpack_type<const basic_registry<Entity>, type_list<Override...>>
-    : unpack_type<basic_registry<Entity>, type_list<Override...>> {};
+template<typename... Args, typename... Override>
+struct unpack_type<const basic_registry<Args...>, type_list<Override...>>
+    : unpack_type<basic_registry<Args...>, 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... Get, typename... Exclude, typename... Override>
+struct unpack_type<basic_view<get_t<Get...>, exclude_t<Exclude...>>, type_list<Override...>> {
+    using ro = type_list_cat_t<type_list<typename Exclude::value_type...>, typename unpack_type<constness_as_t<typename Get::value_type, Get>, type_list<Override...>>::ro...>;
+    using rw = type_list_cat_t<typename unpack_type<constness_as_t<typename Get::value_type, Get>, type_list<Override...>>::rw...>;
 };
 
-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... Get, typename... Exclude, typename... Override>
+struct unpack_type<const basic_view<get_t<Get...>, exclude_t<Exclude...>>, type_list<Override...>>
+    : unpack_type<basic_view<get_t<Get...>, exclude_t<Exclude...>>, type_list<Override...>> {};
 
 template<typename, typename>
-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
- */
+/*! @endcond */
 
 /**
  * @brief Utility class for creating a static task graph.
@@ -105,12 +94,12 @@ resource<type_list<>, type_list<Req...>> to_resource();
  * goal of the tool. Instead, they are returned to the user in the form of a
  * graph that allows for safe execution.
  *
- * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Registry Basic registry type.
  */
-template<typename Entity>
+template<typename Registry>
 class basic_organizer final {
-    using callback_type = void(const void *, basic_registry<Entity> &);
-    using prepare_type = void(basic_registry<Entity> &);
+    using callback_type = void(const void *, Registry &);
+    using prepare_type = void(Registry &);
     using dependency_type = std::size_t(const bool, const type_info **, const std::size_t);
 
     struct vertex_data final {
@@ -125,18 +114,18 @@ class basic_organizer final {
     };
 
     template<typename Type>
-    [[nodiscard]] static decltype(auto) extract(basic_registry<Entity> &reg) {
-        if constexpr(std::is_same_v<Type, basic_registry<Entity>>) {
+    [[nodiscard]] static decltype(auto) extract(Registry &reg) {
+        if constexpr(std::is_same_v<Type, Registry>) {
             return reg;
         } else if constexpr(internal::is_view_v<Type>) {
-            return as_view{reg};
+            return static_cast<Type>(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>>();
         }
     }
 
     template<typename... Args>
-    [[nodiscard]] static auto to_args(basic_registry<Entity> &reg, type_list<Args...>) {
+    [[nodiscard]] static auto to_args(Registry &reg, type_list<Args...>) {
         return std::tuple<decltype(extract<Args>(reg))...>(extract<Args>(reg)...);
     }
 
@@ -146,92 +135,29 @@ class basic_organizer final {
             return {};
         } else {
             const type_info *info[sizeof...(Type)]{&type_id<Type>()...};
-            const auto length = (std::min)(count, sizeof...(Type));
-            std::copy_n(info, length, buffer);
+            const auto length = count < sizeof...(Type) ? count : sizeof...(Type);
+
+            for(std::size_t pos{}; pos < length; ++pos) {
+                buffer[pos] = info[pos];
+            }
+
             return length;
         }
     }
 
     template<typename... RO, typename... RW>
     void track_dependencies(std::size_t index, const bool requires_registry, type_list<RO...>, type_list<RW...>) {
-        dependencies[type_hash<basic_registry<Entity>>::value()].emplace_back(index, requires_registry || (sizeof...(RO) + sizeof...(RW) == 0u));
-        (dependencies[type_hash<RO>::value()].emplace_back(index, false), ...);
-        (dependencies[type_hash<RW>::value()].emplace_back(index, true), ...);
-    }
-
-    [[nodiscard]] std::vector<bool> adjacency_matrix() {
-        const auto length = vertices.size();
-        std::vector<bool> edges(length * length, false);
-
-        // creates the ajacency matrix
-        for(const auto &deps: dependencies) {
-            const auto last = deps.second.cend();
-            auto it = deps.second.cbegin();
-
-            while(it != last) {
-                if(it->second) {
-                    // rw item
-                    if(auto curr = it++; it != last) {
-                        if(it->second) {
-                            edges[curr->first * length + it->first] = true;
-                        } else {
-                            if(const auto next = std::find_if(it, last, [](const auto &elem) { return elem.second; }); next != last) {
-                                for(; it != next; ++it) {
-                                    edges[curr->first * length + it->first] = true;
-                                    edges[it->first * length + next->first] = true;
-                                }
-                            } else {
-                                for(; it != next; ++it) {
-                                    edges[curr->first * length + it->first] = true;
-                                }
-                            }
-                        }
-                    }
-                } else {
-                    // ro item, possibly only on first iteration
-                    if(const auto next = std::find_if(it, last, [](const auto &elem) { return elem.second; }); next != last) {
-                        for(; it != next; ++it) {
-                            edges[it->first * length + next->first] = true;
-                        }
-                    } else {
-                        it = last;
-                    }
-                }
-            }
-        }
-
-        // computes the transitive closure
-        for(std::size_t vk{}; vk < length; ++vk) {
-            for(std::size_t vi{}; vi < length; ++vi) {
-                for(std::size_t vj{}; vj < length; ++vj) {
-                    edges[vi * length + vj] = edges[vi * length + vj] || (edges[vi * length + vk] && edges[vk * length + vj]);
-                }
-            }
-        }
-
-        // applies the transitive reduction
-        for(std::size_t vert{}; vert < length; ++vert) {
-            edges[vert * length + vert] = false;
-        }
-
-        for(std::size_t vj{}; vj < length; ++vj) {
-            for(std::size_t vi{}; vi < length; ++vi) {
-                if(edges[vi * length + vj]) {
-                    for(std::size_t vk{}; vk < length; ++vk) {
-                        if(edges[vj * length + vk]) {
-                            edges[vi * length + vk] = false;
-                        }
-                    }
-                }
-            }
-        }
-
-        return edges;
+        builder.bind(static_cast<id_type>(index));
+        builder.set(type_hash<Registry>::value(), requires_registry || (sizeof...(RO) + sizeof...(RW) == 0u));
+        (builder.ro(type_hash<RO>::value()), ...);
+        (builder.rw(type_hash<RW>::value()), ...);
     }
 
 public:
+    /*! Basic registry type. */
+    using registry_type = Registry;
     /*! @brief Underlying entity identifier. */
-    using entity_type = Entity;
+    using entity_type = typename registry_type::entity_type;
     /*! @brief Unsigned integer type. */
     using size_type = std::size_t;
     /*! @brief Raw task function type. */
@@ -257,7 +183,7 @@ 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(const 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 noexcept {
             return node.dependency(false, buffer, length);
         }
 
@@ -268,7 +194,7 @@ 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(const 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 noexcept {
             return node.dependency(true, buffer, length);
         }
 
@@ -276,7 +202,7 @@ public:
          * @brief Returns the number of read-only resources of a vertex.
          * @return The number of read-only resources of the vertex.
          */
-        size_type ro_count() const ENTT_NOEXCEPT {
+        size_type ro_count() const noexcept {
             return node.ro_count;
         }
 
@@ -284,7 +210,7 @@ public:
          * @brief Returns the number of writable resources of a vertex.
          * @return The number of writable resources of the vertex.
          */
-        size_type rw_count() const ENTT_NOEXCEPT {
+        size_type rw_count() const noexcept {
             return node.rw_count;
         }
 
@@ -292,7 +218,7 @@ public:
          * @brief Checks if a vertex is also a top-level one.
          * @return True if the vertex is a top-level one, false otherwise.
          */
-        bool top_level() const ENTT_NOEXCEPT {
+        bool top_level() const noexcept {
             return is_top_level;
         }
 
@@ -300,7 +226,7 @@ public:
          * @brief Returns a type info object associated with a vertex.
          * @return A properly initialized type info object.
          */
-        const type_info &info() const ENTT_NOEXCEPT {
+        const type_info &info() const noexcept {
             return *node.info;
         }
 
@@ -308,7 +234,7 @@ public:
          * @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 noexcept {
             return node.name;
         }
 
@@ -316,7 +242,7 @@ 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 noexcept {
             return node.callback;
         }
 
@@ -324,7 +250,7 @@ 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 noexcept {
             return node.payload;
         }
 
@@ -332,7 +258,7 @@ 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 noexcept {
             return reachable;
         }
 
@@ -341,7 +267,7 @@ public:
          * are properly instantiated before using them.
          * @param reg A valid registry.
          */
-        void prepare(basic_registry<entity_type> &reg) const {
+        void prepare(registry_type &reg) const {
             node.prepare ? node.prepare(reg) : void();
         }
 
@@ -359,10 +285,10 @@ public:
      */
     template<auto Candidate, typename... Req>
     void emplace(const char *name = nullptr) {
-        using resource_type = decltype(internal::free_function_to_resource<Req...>(Candidate));
-        constexpr auto requires_registry = type_list_contains_v<typename resource_type::args, basic_registry<entity_type>>;
+        using resource_type = decltype(internal::free_function_to_resource_traits<Req...>(Candidate));
+        constexpr auto requires_registry = type_list_contains_v<typename resource_type::args, registry_type>;
 
-        callback_type *callback = +[](const void *, basic_registry<entity_type> &reg) {
+        callback_type *callback = +[](const void *, registry_type &reg) {
             std::apply(Candidate, to_args(reg, typename resource_type::args{}));
         };
 
@@ -373,7 +299,7 @@ public:
             nullptr,
             callback,
             +[](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{})); },
+            +[](registry_type &reg) { void(to_args(reg, typename resource_type::args{})); },
             &type_id<std::integral_constant<decltype(Candidate), Candidate>>()};
 
         track_dependencies(vertices.size(), requires_registry, typename resource_type::ro{}, typename resource_type::rw{});
@@ -391,10 +317,10 @@ 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));
-        constexpr auto requires_registry = type_list_contains_v<typename resource_type::args, basic_registry<entity_type>>;
+        using resource_type = decltype(internal::constrained_function_to_resource_traits<Req...>(Candidate));
+        constexpr auto requires_registry = type_list_contains_v<typename resource_type::args, registry_type>;
 
-        callback_type *callback = +[](const void *payload, basic_registry<entity_type> &reg) {
+        callback_type *callback = +[](const void *payload, registry_type &reg) {
             Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
             std::apply(Candidate, std::tuple_cat(std::forward_as_tuple(*curr), to_args(reg, typename resource_type::args{})));
         };
@@ -406,7 +332,7 @@ public:
             &value_or_instance,
             callback,
             +[](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{})); },
+            +[](registry_type &reg) { void(to_args(reg, typename resource_type::args{})); },
             &type_id<std::integral_constant<decltype(Candidate), Candidate>>()};
 
         track_dependencies(vertices.size(), requires_registry, typename resource_type::ro{}, typename resource_type::rw{});
@@ -423,7 +349,7 @@ 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{});
 
         vertex_data vdata{
@@ -444,28 +370,19 @@ public:
      * @return The adjacency list of the task graph.
      */
     std::vector<vertex> graph() {
-        const auto edges = adjacency_matrix();
-
-        // creates the adjacency list
         std::vector<vertex> adjacency_list{};
         adjacency_list.reserve(vertices.size());
+        auto adjacency_matrix = builder.graph();
 
-        for(std::size_t col{}, length = vertices.size(); col < length; ++col) {
+        for(auto curr: adjacency_matrix.vertices()) {
+            const auto iterable = adjacency_matrix.in_edges(curr);
             std::vector<std::size_t> reachable{};
-            const auto row = col * length;
-            bool is_top_level = true;
 
-            for(std::size_t next{}; next < length; ++next) {
-                if(edges[row + next]) {
-                    reachable.push_back(next);
-                }
+            for(auto &&edge: adjacency_matrix.out_edges(curr)) {
+                reachable.push_back(edge.second);
             }
 
-            for(std::size_t next{}; next < length && is_top_level; ++next) {
-                is_top_level = !edges[next * length + col];
-            }
-
-            adjacency_list.emplace_back(is_top_level, vertices[col], std::move(reachable));
+            adjacency_list.emplace_back(iterable.cbegin() == iterable.cend(), vertices[curr], std::move(reachable));
         }
 
         return adjacency_list;
@@ -473,13 +390,13 @@ public:
 
     /*! @brief Erases all elements from a container. */
     void clear() {
-        dependencies.clear();
+        builder.clear();
         vertices.clear();
     }
 
 private:
-    dense_hash_map<id_type, std::vector<std::pair<std::size_t, bool>>, identity> dependencies;
     std::vector<vertex_data> vertices;
+    flow builder;
 };
 
 } // namespace entt

src/entt/entity/runtime_view.hpp

@@ -2,45 +2,46 @@
 #define ENTT_ENTITY_RUNTIME_VIEW_HPP
 
 #include <algorithm>
+#include <cstddef>
 #include <iterator>
-#include <type_traits>
 #include <utility>
 #include <vector>
-#include "../config/config.h"
 #include "entity.hpp"
 #include "fwd.hpp"
-#include "sparse_set.hpp"
 
 namespace entt {
 
-/**
- * @cond TURN_OFF_DOXYGEN
- * Internal details not to be documented.
- */
-
+/*! @cond TURN_OFF_DOXYGEN */
 namespace internal {
 
-template<typename Type>
+template<typename Set>
 class runtime_view_iterator final {
+    using iterator_type = typename Set::iterator;
+
     [[nodiscard]] bool valid() const {
-        return std::all_of(pools->begin(), pools->end(), [entt = *it](const auto *curr) { return curr->contains(entt); })
+        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 iterator_type = typename Type::iterator;
     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 = default;
+    constexpr runtime_view_iterator() noexcept
+        : pools{},
+          filter{},
+          it{},
+          tombstone_check{} {}
 
-    runtime_view_iterator(const std::vector<const Type *> &cpools, const std::vector<const Type *> &ignore, iterator_type curr) ENTT_NOEXCEPT
+    runtime_view_iterator(const std::vector<Set *> &cpools, const std::vector<Set *> &ignore, iterator_type curr) noexcept
         : pools{&cpools},
           filter{&ignore},
-          it{curr} {
+          it{curr},
+          tombstone_check{pools->size() == 1u && (*pools)[0u]->policy() == deletion_policy::in_place} {
         if(it != (*pools)[0]->end() && !valid()) {
             ++(*this);
         }
@@ -56,118 +57,178 @@ public:
         return ++(*this), orig;
     }
 
-    runtime_view_iterator &operator--() ENTT_NOEXCEPT {
+    runtime_view_iterator &operator--() {
         while(--it != (*pools)[0]->begin() && !valid()) {}
         return *this;
     }
 
-    runtime_view_iterator operator--(int) ENTT_NOEXCEPT {
+    runtime_view_iterator operator--(int) {
         runtime_view_iterator orig = *this;
         return operator--(), orig;
     }
 
-    [[nodiscard]] pointer operator->() const {
+    [[nodiscard]] pointer operator->() const noexcept {
         return it.operator->();
     }
 
-    [[nodiscard]] reference operator*() const {
+    [[nodiscard]] reference operator*() const noexcept {
         return *operator->();
     }
 
-    [[nodiscard]] bool operator==(const runtime_view_iterator &other) const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr bool operator==(const runtime_view_iterator &other) const noexcept {
         return it == other.it;
     }
 
-    [[nodiscard]] bool operator!=(const runtime_view_iterator &other) const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr bool operator!=(const runtime_view_iterator &other) const noexcept {
         return !(*this == other);
     }
 
 private:
-    const std::vector<const Type *> *pools;
-    const std::vector<const Type *> *filter;
+    const std::vector<Set *> *pools;
+    const std::vector<Set *> *filter;
     iterator_type it;
+    bool tombstone_check;
 };
 
 } // namespace internal
+/*! @endcond */
 
 /**
- * Internal details not to be documented.
- * @endcond
- */
-
-/**
- * @brief 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
- * number of entities available for each component and picks up a reference to
- * the smallest set of candidate entities in order to get a performance boost
- * when iterate.<br/>
- * Order of elements during iterations are highly dependent on the order of the
- * underlying data structures. See sparse_set and its specializations for more
- * details.
+ * Runtime views iterate over those entities that are at least in the given
+ * storage. During initialization, a runtime view looks at the number of
+ * entities available for each component and uses the smallest set in order to
+ * get a performance boost when iterating.
  *
  * @b Important
  *
  * Iterators aren't invalidated if:
  *
- * * New instances of the given components are created and assigned to entities.
- * * The entity currently pointed is modified (as an example, if one of the
- *   given components is removed from the entity to which the iterator points).
+ * * New elements are added to the storage.
+ * * The entity currently pointed is modified (for example, components are added
+ *   or removed from it).
  * * The entity currently pointed is destroyed.
  *
- * In all the other cases, modifying the pools of the given components in any
- * way invalidates all the iterators and using them results in undefined
- * behavior.
+ * In all other cases, modifying the storage iterated by the view in any way
+ * invalidates all the iterators.
  *
- * @note
- * Views share references to the underlying data structures of the registry that
- * generated them. Therefore any change to the entities and to the components
- * made by means of the registry are immediately reflected by the views, unless
- * a pool was missing when the view was built (in this case, the view won't
- * have a valid reference and won't be updated accordingly).
- *
- * @warning
- * Lifetime of a view must not overcome that of the registry that generated it.
- * 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 Type Common base type.
+ * @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>;
-
-    [[nodiscard]] bool valid() const {
-        return !pools.empty() && pools.front();
-    }
+template<typename Type, typename Allocator>
+class basic_runtime_view {
+    using alloc_traits = std::allocator_traits<Allocator>;
+    static_assert(std::is_same_v<typename alloc_traits::value_type, Type *>, "Invalid value type");
+    using container_type = std::vector<Type *, Allocator>;
 
 public:
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
     /*! @brief Underlying entity identifier. */
-    using entity_type = Entity;
+    using entity_type = typename Type::entity_type;
     /*! @brief Unsigned integer type. */
     using size_type = std::size_t;
+    /*! @brief Common type among all storage types. */
+    using common_type = Type;
     /*! @brief Bidirectional iterator type. */
-    using iterator = internal::runtime_view_iterator<basic_common_type>;
+    using iterator = internal::runtime_view_iterator<common_type>;
 
     /*! @brief Default constructor to use to create empty, invalid views. */
-    basic_runtime_view() ENTT_NOEXCEPT
-        : pools{},
-          filter{} {}
+    basic_runtime_view() noexcept
+        : basic_runtime_view{allocator_type{}} {}
 
     /**
-     * @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 Constructs an empty, invalid view with a given allocator.
+     * @param allocator The allocator to use.
      */
-    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)} {
-        auto candidate = std::min_element(pools.begin(), pools.end(), [](const auto *lhs, const auto *rhs) {
-            return (!lhs && rhs) || (lhs && rhs && lhs->size() < rhs->size());
-        });
+    explicit basic_runtime_view(const allocator_type &allocator)
+        : pools{allocator},
+          filter{allocator} {}
 
-        // brings the best candidate (if any) on front of the vector
-        std::rotate(pools.begin(), candidate, pools.end());
+    /*! @brief Default copy constructor. */
+    basic_runtime_view(const basic_runtime_view &) = default;
+
+    /**
+     * @brief Allocator-extended copy constructor.
+     * @param other The instance to copy from.
+     * @param allocator The allocator to use.
+     */
+    basic_runtime_view(const basic_runtime_view &other, const allocator_type &allocator)
+        : pools{other.pools, allocator},
+          filter{other.filter, allocator} {}
+
+    /*! @brief Default move constructor. */
+    basic_runtime_view(basic_runtime_view &&) noexcept(std::is_nothrow_move_constructible_v<container_type>) = default;
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    basic_runtime_view(basic_runtime_view &&other, const allocator_type &allocator)
+        : pools{std::move(other.pools), allocator},
+          filter{std::move(other.filter), allocator} {}
+
+    /**
+     * @brief Default copy assignment operator.
+     * @return This container.
+     */
+    basic_runtime_view &operator=(const basic_runtime_view &) = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This container.
+     */
+    basic_runtime_view &operator=(basic_runtime_view &&) noexcept(std::is_nothrow_move_assignable_v<container_type>) = default;
+
+    /**
+     * @brief Exchanges the contents with those of a given view.
+     * @param other View to exchange the content with.
+     */
+    void swap(basic_runtime_view &other) {
+        using std::swap;
+        swap(pools, other.pools);
+        swap(filter, other.filter);
+    }
+
+    /**
+     * @brief Returns the associated allocator.
+     * @return The associated allocator.
+     */
+    [[nodiscard]] constexpr allocator_type get_allocator() const noexcept {
+        return pools.get_allocator();
+    }
+
+    /*! @brief Clears the view. */
+    void clear() {
+        pools.clear();
+        filter.clear();
+    }
+
+    /**
+     * @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 &iterate(common_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(common_type &base) {
+        filter.push_back(&base);
+        return *this;
     }
 
     /**
@@ -175,36 +236,29 @@ 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();
     }
 
     /**
      * @brief Returns an iterator to the first entity that has the given
      * components.
      *
-     * The returned iterator points to the first entity that has the given
-     * components. If the view is empty, the returned iterator will be equal to
-     * `end()`.
+     * If the view is empty, the returned iterator will be equal to `end()`.
      *
      * @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()};
     }
 
     /**
      * @brief Returns an iterator that is past the last entity that has the
      * given components.
-     *
-     * The returned iterator points to the entity following the last entity that
-     * has the given components. Attempting to dereference the returned iterator
-     * results in undefined behavior.
-     *
      * @return An iterator to the entity following the last entity that has the
      * 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()};
     }
 
     /**
@@ -213,7 +267,8 @@ public:
      * @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); })
+        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); });
     }
 
@@ -221,8 +276,7 @@ public:
      * @brief Iterates entities and applies the given function object to them.
      *
      * The function object is invoked for each entity. It is provided only with
-     * the entity itself. To get the components, users can use the registry with
-     * which the view was built.<br/>
+     * the entity itself.<br/>
      * The signature of the function should be equivalent to the following:
      *
      * @code{.cpp}
@@ -240,8 +294,8 @@ public:
     }
 
 private:
-    std::vector<const basic_common_type *> pools;
-    std::vector<const basic_common_type *> filter;
+    container_type pools;
+    container_type filter;
 };
 
 } // namespace entt

src/entt/entity/snapshot.hpp

@@ -1,7 +1,6 @@
 #ifndef ENTT_ENTITY_SNAPSHOT_HPP
 #define ENTT_ENTITY_SNAPSHOT_HPP
 
-#include <array>
 #include <cstddef>
 #include <iterator>
 #include <tuple>
@@ -9,15 +8,31 @@
 #include <utility>
 #include <vector>
 #include "../config/config.h"
-#include "../container/dense_hash_map.hpp"
+#include "../container/dense_map.hpp"
 #include "../core/type_traits.hpp"
-#include "component.hpp"
 #include "entity.hpp"
 #include "fwd.hpp"
-#include "registry.hpp"
+#include "view.hpp"
 
 namespace entt {
 
+/*! @cond TURN_OFF_DOXYGEN */
+namespace internal {
+
+template<typename Registry>
+void orphans(Registry &registry) {
+    auto &storage = registry.template storage<typename Registry::entity_type>();
+
+    for(auto entt: storage) {
+        if(registry.orphan(entt)) {
+            storage.erase(entt);
+        }
+    }
+}
+
+} // namespace internal
+/*! @endcond */
+
 /**
  * @brief Utility class to create snapshots from a registry.
  *
@@ -26,125 +41,110 @@ namespace entt {
  * This type can be used in both cases if provided with a correctly configured
  * output archive.
  *
- * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Registry Basic registry type.
  */
-template<typename Entity>
+template<typename Registry>
 class basic_snapshot {
-    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 entity_traits::entity_type(sz));
-
-        while(first != last) {
-            const auto entt = *(first++);
-
-            if(reg->template all_of<Component>(entt)) {
-                std::apply(archive, std::tuple_cat(std::make_tuple(entt), view.get(entt)));
-            }
-        }
-    }
-
-    template<typename... Component, typename Archive, typename It, std::size_t... Index>
-    void component(Archive &archive, It first, It last, std::index_sequence<Index...>) const {
-        std::array<std::size_t, sizeof...(Index)> size{};
-        auto begin = first;
-
-        while(begin != last) {
-            const auto entt = *(begin++);
-            ((reg->template all_of<Component>(entt) ? ++size[Index] : 0u), ...);
-        }
-
-        (get<Component>(archive, size[Index], first, last), ...);
-    }
+    static_assert(!std::is_const_v<Registry>, "Non-const registry type required");
+    using traits_type = typename Registry::traits_type;
 
 public:
+    /*! Basic registry type. */
+    using registry_type = Registry;
     /*! @brief Underlying entity identifier. */
-    using entity_type = Entity;
+    using entity_type = typename registry_type::entity_type;
 
     /**
      * @brief Constructs an instance that is bound to a given registry.
      * @param source A valid reference to a registry.
      */
-    basic_snapshot(const basic_registry<entity_type> &source) ENTT_NOEXCEPT
+    basic_snapshot(const registry_type &source) noexcept
         : reg{&source} {}
 
     /*! @brief Default move constructor. */
-    basic_snapshot(basic_snapshot &&) = default;
+    basic_snapshot(basic_snapshot &&) noexcept = default;
 
     /*! @brief Default move assignment operator. @return This snapshot. */
-    basic_snapshot &operator=(basic_snapshot &&) = default;
+    basic_snapshot &operator=(basic_snapshot &&) noexcept = default;
 
     /**
-     * @brief Puts aside all the entities from the underlying registry.
-     *
-     * Entities are serialized along with their versions. Destroyed entities are
-     * taken in consideration as well by this function.
-     *
+     * @brief Serializes all elements of a type with associated identifiers.
+     * @tparam Type Type of elements to serialize.
      * @tparam Archive Type of output archive.
      * @param archive A valid reference to an output archive.
+     * @param id Optional name used to map the storage within the registry.
      * @return An object of this type to continue creating the snapshot.
      */
-    template<typename Archive>
-    const basic_snapshot &entities(Archive &archive) const {
-        const auto sz = reg->size();
+    template<typename Type, typename Archive>
+    const basic_snapshot &get(Archive &archive, const id_type id = type_hash<Type>::value()) const {
+        if(const auto *storage = reg->template storage<Type>(id); storage) {
+            archive(static_cast<typename traits_type::entity_type>(storage->size()));
 
-        archive(typename entity_traits::entity_type(sz + 1u));
-        archive(reg->released());
+            if constexpr(std::is_same_v<Type, entity_type>) {
+                archive(static_cast<typename traits_type::entity_type>(storage->free_list()));
 
-        for(auto first = reg->data(), last = first + sz; first != last; ++first) {
-            archive(*first);
+                for(auto first = storage->data(), last = first + storage->size(); first != last; ++first) {
+                    archive(*first);
+                }
+            } else if constexpr(component_traits<Type>::in_place_delete) {
+                const typename registry_type::common_type &base = *storage;
+
+                for(auto it = base.rbegin(), last = base.rend(); it != last; ++it) {
+                    if(const auto entt = *it; entt == tombstone) {
+                        archive(static_cast<entity_type>(null));
+                    } else {
+                        archive(entt);
+                        std::apply([&archive](auto &&...args) { (archive(std::forward<decltype(args)>(args)), ...); }, storage->get_as_tuple(entt));
+                    }
+                }
+            } else {
+                for(auto elem: storage->reach()) {
+                    std::apply([&archive](auto &&...args) { (archive(std::forward<decltype(args)>(args)), ...); }, elem);
+                }
+            }
+        } else {
+            archive(typename traits_type::entity_type{});
         }
 
         return *this;
     }
 
     /**
-     * @brief Puts aside the given components.
-     *
-     * Each instance is serialized together with the entity to which it belongs.
-     * Entities are serialized along with their versions.
-     *
-     * @tparam Component Types of components to serialize.
-     * @tparam Archive Type of output archive.
-     * @param archive A valid reference to an output archive.
-     * @return An object of this type to continue creating the snapshot.
-     */
-    template<typename... Component, typename Archive>
-    const basic_snapshot &component(Archive &archive) const {
-        if constexpr(sizeof...(Component) == 1u) {
-            const auto view = reg->template view<const Component...>();
-            (component<Component>(archive, view.rbegin(), view.rend()), ...);
-            return *this;
-        } else {
-            (component<Component>(archive), ...);
-            return *this;
-        }
-    }
-
-    /**
-     * @brief Puts aside the given components for the entities in a range.
-     *
-     * Each instance is serialized together with the entity to which it belongs.
-     * Entities are serialized along with their versions.
-     *
-     * @tparam Component Types of components to serialize.
+     * @brief Serializes all elements of a type with associated identifiers for
+     * the entities in a range.
+     * @tparam Type Type of elements to serialize.
      * @tparam Archive Type of output archive.
      * @tparam It Type of input iterator.
      * @param archive A valid reference to an output archive.
      * @param first An iterator to the first element of the range to serialize.
      * @param last An iterator past the last element of the range to serialize.
+     * @param id Optional name used to map the storage within the registry.
      * @return An object of this type to continue creating the snapshot.
      */
-    template<typename... Component, typename Archive, typename It>
-    const basic_snapshot &component(Archive &archive, It first, It last) const {
-        component<Component...>(archive, first, last, std::index_sequence_for<Component...>{});
+    template<typename Type, typename Archive, typename It>
+    const basic_snapshot &get(Archive &archive, It first, It last, const id_type id = type_hash<Type>::value()) const {
+        static_assert(!std::is_same_v<Type, entity_type>, "Entity types not supported");
+
+        if(const auto *storage = reg->template storage<Type>(id); storage && !storage->empty()) {
+            archive(static_cast<typename traits_type::entity_type>(std::distance(first, last)));
+
+            for(; first != last; ++first) {
+                if(const auto entt = *first; storage->contains(entt)) {
+                    archive(entt);
+                    std::apply([&archive](auto &&...args) { (archive(std::forward<decltype(args)>(args)), ...); }, storage->get_as_tuple(entt));
+                } else {
+                    archive(static_cast<entity_type>(null));
+                }
+            }
+        } else {
+            archive(typename traits_type::entity_type{});
+        }
+
         return *this;
     }
 
 private:
-    const basic_registry<entity_type> *reg;
+    const registry_type *reg;
 };
 
 /**
@@ -155,100 +155,82 @@ private:
  * originally had.<br/>
  * An example of use is the implementation of a save/restore utility.
  *
- * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Registry Basic registry type.
  */
-template<typename Entity>
+template<typename Registry>
 class basic_snapshot_loader {
-    using entity_traits = entt_traits<Entity>;
-
-    template<typename Type, typename Archive>
-    void assign(Archive &archive) const {
-        typename entity_traits::entity_type length{};
-        archive(length);
-
-        entity_type entt{};
-
-        if constexpr(ignore_as_empty_v<std::remove_const_t<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);
-            }
-        } else {
-            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));
-            }
-        }
-    }
+    static_assert(!std::is_const_v<Registry>, "Non-const registry type required");
+    using traits_type = typename Registry::traits_type;
 
 public:
+    /*! Basic registry type. */
+    using registry_type = Registry;
     /*! @brief Underlying entity identifier. */
-    using entity_type = Entity;
+    using entity_type = typename registry_type::entity_type;
 
     /**
      * @brief Constructs an instance that is bound to a given registry.
      * @param source A valid reference to a registry.
      */
-    basic_snapshot_loader(basic_registry<entity_type> &source) ENTT_NOEXCEPT
+    basic_snapshot_loader(registry_type &source) noexcept
         : reg{&source} {
         // restoring a snapshot as a whole requires a clean registry
-        ENTT_ASSERT(reg->empty(), "Registry must be empty");
+        ENTT_ASSERT(reg->template storage<entity_type>().free_list() == 0u, "Registry must be empty");
     }
 
     /*! @brief Default move constructor. */
-    basic_snapshot_loader(basic_snapshot_loader &&) = default;
+    basic_snapshot_loader(basic_snapshot_loader &&) 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 &&) noexcept = default;
 
     /**
-     * @brief Restores entities that were in use during serialization.
-     *
-     * This function restores the entities that were in use during serialization
-     * and gives them the versions they originally had.
-     *
+     * @brief Restores all elements of a type with associated identifiers.
+     * @tparam Type Type of elements to restore.
      * @tparam Archive Type of input archive.
      * @param archive A valid reference to an input archive.
+     * @param id Optional name used to map the storage within the registry.
      * @return A valid loader to continue restoring data.
      */
-    template<typename Archive>
-    const basic_snapshot_loader &entities(Archive &archive) const {
-        typename entity_traits::entity_type length{};
+    template<typename Type, typename Archive>
+    basic_snapshot_loader &get(Archive &archive, const id_type id = type_hash<Type>::value()) {
+        auto &storage = reg->template storage<Type>(id);
+        typename traits_type::entity_type length{};
 
         archive(length);
-        std::vector<entity_type> all(length);
 
-        for(std::size_t pos{}; pos < length; ++pos) {
-            archive(all[pos]);
+        if constexpr(std::is_same_v<Type, entity_type>) {
+            typename traits_type::entity_type count{};
+
+            storage.reserve(length);
+            archive(count);
+
+            for(entity_type entity = null; length; --length) {
+                archive(entity);
+                storage.emplace(entity);
+            }
+
+            storage.free_list(count);
+        } else {
+            auto &other = reg->template storage<entity_type>();
+            entity_type entt{null};
+
+            while(length--) {
+                if(archive(entt); entt != null) {
+                    const auto entity = other.contains(entt) ? entt : other.emplace(entt);
+                    ENTT_ASSERT(entity == entt, "Entity not available for use");
+
+                    if constexpr(std::tuple_size_v<decltype(storage.get_as_tuple({}))> == 0u) {
+                        storage.emplace(entity);
+                    } else {
+                        Type elem{};
+                        archive(elem);
+                        storage.emplace(entity, std::move(elem));
+                    }
+                }
+            }
         }
 
-        reg->assign(++all.cbegin(), all.cend(), all[0u]);
-
-        return *this;
-    }
-
-    /**
-     * @brief Restores components and assigns them to the right entities.
-     *
-     * The template parameter list must be exactly the same used during
-     * serialization. In the event that the entity to which the component is
-     * assigned doesn't exist yet, the loader will take care to create it with
-     * the version it originally had.
-     *
-     * @tparam Component Types of components to restore.
-     * @tparam Archive Type of input archive.
-     * @param archive A valid reference to an input archive.
-     * @return A valid loader to continue restoring data.
-     */
-    template<typename... Component, typename Archive>
-    const basic_snapshot_loader &component(Archive &archive) const {
-        (assign<Component>(archive), ...);
         return *this;
     }
 
@@ -258,22 +240,17 @@ public:
      * In case all the entities were serialized but only part of the components
      * was saved, it could happen that some of the entities have no components
      * once restored.<br/>
-     * This functions helps to identify and destroy those entities.
+     * This function helps to identify and destroy those entities.
      *
      * @return A valid loader to continue restoring data.
      */
-    const basic_snapshot_loader &orphans() const {
-        reg->each([this](const auto entt) {
-            if(reg->orphan(entt)) {
-                reg->release(entt);
-            }
-        });
-
+    basic_snapshot_loader &orphans() {
+        internal::orphans(*reg);
         return *this;
     }
 
 private:
-    basic_registry<entity_type> *reg;
+    registry_type *reg;
 };
 
 /**
@@ -286,43 +263,29 @@ private:
  * Identifiers that entities originally had are not transferred to the target.
  * Instead, the loader maps remote identifiers to local ones while restoring a
  * snapshot.<br/>
- * An example of use is the implementation of a client-server applications with
+ * An example of use is the implementation of a client-server application with
  * the requirement of transferring somehow parts of the representation side to
  * side.
  *
- * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Registry Basic registry type.
  */
-template<typename Entity>
+template<typename Registry>
 class basic_continuous_loader {
-    using entity_traits = entt_traits<Entity>;
+    static_assert(!std::is_const_v<Registry>, "Non-const registry type required");
+    using traits_type = typename Registry::traits_type;
 
-    void destroy(Entity entt) {
-        if(const auto it = remloc.find(entt); it == remloc.cend()) {
-            const auto local = reg->create();
-            remloc.emplace(entt, std::make_pair(local, true));
-            reg->destroy(local);
-        }
-    }
-
-    void restore(Entity entt) {
-        const auto it = remloc.find(entt);
-
-        if(it == remloc.cend()) {
-            const auto local = reg->create();
-            remloc.emplace(entt, std::make_pair(local, true));
-        } else {
-            if(!reg->valid(remloc[entt].first)) {
-                remloc[entt].first = reg->create();
+    void restore(typename Registry::entity_type entt) {
+        if(const auto entity = to_entity(entt); remloc.contains(entity) && remloc[entity].first == entt) {
+            if(!reg->valid(remloc[entity].second)) {
+                remloc[entity].second = reg->create();
             }
-
-            // set the dirty flag
-            remloc[entt].second = true;
+        } else {
+            remloc.insert_or_assign(entity, std::make_pair(entt, reg->create()));
         }
     }
 
     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;
 
@@ -340,12 +303,12 @@ 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");
 
@@ -354,9 +317,9 @@ class basic_continuous_loader {
         }
     }
 
-    template<typename Other, typename Type, typename Member>
-    void update([[maybe_unused]] Other &instance, [[maybe_unused]] Member Type::*member) {
-        if constexpr(!std::is_same_v<Other, Type>) {
+    template<typename Component, typename Other, typename Member>
+    void update([[maybe_unused]] Component &instance, [[maybe_unused]] Member Other::*member) {
+        if constexpr(!std::is_same_v<Component, Other>) {
             return;
         } else if constexpr(std::is_same_v<Member, entity_type>) {
             instance.*member = map(instance.*member);
@@ -366,52 +329,19 @@ class basic_continuous_loader {
         }
     }
 
-    template<typename Component>
-    void remove_if_exists() {
-        for(auto &&ref: remloc) {
-            const auto local = ref.second.first;
-
-            if(reg->valid(local)) {
-                reg->template remove<Component>(local);
-            }
-        }
-    }
-
-    template<typename Other, typename Archive, typename... Type, typename... Member>
-    void assign(Archive &archive, [[maybe_unused]] Member Type::*...member) {
-        typename entity_traits::entity_type length{};
-        archive(length);
-
-        entity_type entt{};
-
-        if constexpr(ignore_as_empty_v<std::remove_const_t<Other>>) {
-            while(length--) {
-                archive(entt);
-                restore(entt);
-                reg->template emplace_or_replace<Other>(map(entt));
-            }
-        } else {
-            Other instance{};
-
-            while(length--) {
-                archive(entt, instance);
-                (update(instance, member), ...);
-                restore(entt);
-                reg->template emplace_or_replace<Other>(map(entt), std::move(instance));
-            }
-        }
-    }
-
 public:
+    /*! Basic registry type. */
+    using registry_type = Registry;
     /*! @brief Underlying entity identifier. */
-    using entity_type = Entity;
+    using entity_type = typename registry_type::entity_type;
 
     /**
      * @brief Constructs an instance that is bound to a given registry.
      * @param source A valid reference to a registry.
      */
-    basic_continuous_loader(basic_registry<entity_type> &source) ENTT_NOEXCEPT
-        : reg{&source} {}
+    basic_continuous_loader(registry_type &source) noexcept
+        : remloc{source.get_allocator()},
+          reg{&source} {}
 
     /*! @brief Default move constructor. */
     basic_continuous_loader(basic_continuous_loader &&) = default;
@@ -420,87 +350,66 @@ public:
     basic_continuous_loader &operator=(basic_continuous_loader &&) = default;
 
     /**
-     * @brief Restores entities that were in use during serialization.
+     * @brief Restores all elements of a type with associated identifiers.
      *
-     * This function restores the entities that were in use during serialization
-     * and creates local counterparts for them if required.
+     * It creates local counterparts for remote elements as needed.<br/>
+     * Members are either data members of type entity_type or containers of
+     * entities. In both cases, a loader visits them and replaces entities with
+     * their local counterpart.
      *
+     * @tparam Type Type of elements to restore.
      * @tparam Archive Type of input archive.
      * @param archive A valid reference to an input archive.
-     * @return A non-const reference to this loader.
+     * @param id Optional name used to map the storage within the registry.
+     * @return A valid loader to continue restoring data.
      */
-    template<typename Archive>
-    basic_continuous_loader &entities(Archive &archive) {
-        typename entity_traits::entity_type length{};
-        entity_type entt{};
+    template<typename Type, typename Archive>
+    basic_continuous_loader &get(Archive &archive, const id_type id = type_hash<Type>::value()) {
+        auto &storage = reg->template storage<Type>(id);
+        typename traits_type::entity_type length{};
+        entity_type entt{null};
 
         archive(length);
-        // discards the head of the list of destroyed entities
-        archive(entt);
 
-        for(std::size_t pos{}, last = length - 1u; pos < last; ++pos) {
-            archive(entt);
+        if constexpr(std::is_same_v<Type, entity_type>) {
+            typename traits_type::entity_type in_use{};
 
-            if(const auto entity = entity_traits::to_entity(entt); entity == pos) {
+            storage.reserve(length);
+            archive(in_use);
+
+            for(std::size_t pos{}; pos < in_use; ++pos) {
+                archive(entt);
                 restore(entt);
-            } else {
-                destroy(entt);
             }
-        }
 
-        return *this;
-    }
+            for(std::size_t pos = in_use; pos < length; ++pos) {
+                archive(entt);
 
-    /**
-     * @brief Restores components and assigns them to the right entities.
-     *
-     * The template parameter list must be exactly the same used during
-     * serialization. In the event that the entity to which the component is
-     * assigned doesn't exist yet, the loader will take care to create a local
-     * counterpart for it.<br/>
-     * Members can be either data members of type entity_type or containers of
-     * entities. In both cases, the loader will visit them and update the
-     * entities by replacing each one with its local counterpart.
-     *
-     * @tparam Component Type of component to restore.
-     * @tparam Archive Type of input archive.
-     * @tparam Type Types of components to update with local counterparts.
-     * @tparam Member Types of members to update with their local counterparts.
-     * @param archive A valid reference to an input archive.
-     * @param member Members to update with their local counterparts.
-     * @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) {
-        (remove_if_exists<Component>(), ...);
-        (assign<Component>(archive, member...), ...);
-        return *this;
-    }
+                if(const auto entity = to_entity(entt); remloc.contains(entity)) {
+                    if(reg->valid(remloc[entity].second)) {
+                        reg->destroy(remloc[entity].second);
+                    }
 
-    /**
-     * @brief Helps to purge entities that no longer have a conterpart.
-     *
-     * Users should invoke this member function after restoring each snapshot,
-     * unless they know exactly what they are doing.
-     *
-     * @return A non-const reference to this loader.
-     */
-    basic_continuous_loader &shrink() {
-        auto it = remloc.begin();
-
-        while(it != remloc.cend()) {
-            const auto local = it->second.first;
-            bool &dirty = it->second.second;
-
-            if(dirty) {
-                dirty = false;
-                ++it;
-            } else {
-                if(reg->valid(local)) {
-                    reg->destroy(local);
+                    remloc.erase(entity);
                 }
+            }
+        } else {
+            for(auto &&ref: remloc) {
+                storage.remove(ref.second.second);
+            }
 
-                it = remloc.erase(it);
+            while(length--) {
+                if(archive(entt); entt != null) {
+                    restore(entt);
+
+                    if constexpr(std::tuple_size_v<decltype(storage.get_as_tuple({}))> == 0u) {
+                        storage.emplace(map(entt));
+                    } else {
+                        Type elem{};
+                        archive(elem);
+                        storage.emplace(map(entt), std::move(elem));
+                    }
+                }
             }
         }
 
@@ -513,17 +422,12 @@ public:
      * In case all the entities were serialized but only part of the components
      * was saved, it could happen that some of the entities have no components
      * once restored.<br/>
-     * This functions helps to identify and destroy those entities.
+     * This function helps to identify and destroy those entities.
      *
      * @return A non-const reference to this loader.
      */
     basic_continuous_loader &orphans() {
-        reg->each([this](const auto entt) {
-            if(reg->orphan(entt)) {
-                reg->release(entt);
-            }
-        });
-
+        internal::orphans(*reg);
         return *this;
     }
 
@@ -532,8 +436,9 @@ public:
      * @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 {
-        return (remloc.find(entt) != remloc.cend());
+    [[nodiscard]] bool contains(entity_type entt) const noexcept {
+        const auto it = remloc.find(to_entity(entt));
+        return it != remloc.cend() && it->second.first == entt;
     }
 
     /**
@@ -541,20 +446,17 @@ public:
      * @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 {
-        const auto it = remloc.find(entt);
-        entity_type other = null;
-
-        if(it != remloc.cend()) {
-            other = it->second.first;
+    [[nodiscard]] entity_type map(entity_type entt) const noexcept {
+        if(const auto it = remloc.find(to_entity(entt)); it != remloc.cend() && it->second.first == entt) {
+            return it->second.second;
         }
 
-        return other;
+        return null;
     }
 
 private:
-    dense_hash_map<entity_type, std::pair<entity_type, bool>> remloc;
-    basic_registry<entity_type> *reg;
+    dense_map<typename traits_type::entity_type, std::pair<entity_type, entity_type>> remloc;
+    registry_type *reg;
 };
 
 } // namespace entt

src/entt/entity/utility.hpp

@@ -1,52 +0,0 @@
-#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.
- */
-template<typename... Type>
-struct exclude_t: type_list<Type...> {};
-
-/**
- * @brief Variable template for exclusion lists.
- * @tparam Type List of types.
- */
-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...> {};
-
-/**
- * @brief Variable template for lists of observed components.
- * @tparam Type List of types.
- */
-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

@@ -1,6 +1,9 @@
+// IWYU pragma: begin_exports
+#include "config/config.h"
+#include "config/macro.h"
 #include "config/version.h"
-#include "container/dense_hash_map.hpp"
-#include "container/dense_hash_set.hpp"
+#include "container/dense_map.hpp"
+#include "container/dense_set.hpp"
 #include "core/algorithm.hpp"
 #include "core/any.hpp"
 #include "core/attribute.h"
@@ -21,6 +24,7 @@
 #include "entity/group.hpp"
 #include "entity/handle.hpp"
 #include "entity/helper.hpp"
+#include "entity/mixin.hpp"
 #include "entity/observer.hpp"
 #include "entity/organizer.hpp"
 #include "entity/registry.hpp"
@@ -28,12 +32,14 @@
 #include "entity/snapshot.hpp"
 #include "entity/sparse_set.hpp"
 #include "entity/storage.hpp"
-#include "entity/utility.hpp"
 #include "entity/view.hpp"
+#include "graph/adjacency_matrix.hpp"
+#include "graph/dot.hpp"
+#include "graph/flow.hpp"
 #include "locator/locator.hpp"
 #include "meta/adl_pointer.hpp"
 #include "meta/container.hpp"
-#include "meta/ctx.hpp"
+#include "meta/context.hpp"
 #include "meta/factory.hpp"
 #include "meta/meta.hpp"
 #include "meta/node.hpp"
@@ -49,9 +55,10 @@
 #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"
 #include "signal/sigh.hpp"
+// IWYU pragma: end_exports

src/entt/fwd.hpp

@@ -1,7 +1,11 @@
+// IWYU pragma: begin_exports
 #include "container/fwd.hpp"
 #include "core/fwd.hpp"
 #include "entity/fwd.hpp"
+#include "graph/fwd.hpp"
 #include "meta/fwd.hpp"
 #include "poly/fwd.hpp"
+#include "process/fwd.hpp"
 #include "resource/fwd.hpp"
 #include "signal/fwd.hpp"
+// IWYU pragma: end_exports

src/entt/graph/adjacency_matrix.hpp

@@ -0,0 +1,341 @@
+#ifndef ENTT_GRAPH_ADJACENCY_MATRIX_HPP
+#define ENTT_GRAPH_ADJACENCY_MATRIX_HPP
+
+#include <cstddef>
+#include <iterator>
+#include <memory>
+#include <type_traits>
+#include <utility>
+#include <vector>
+#include "../config/config.h"
+#include "../core/iterator.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/*! @cond TURN_OFF_DOXYGEN */
+namespace internal {
+
+template<typename It>
+class edge_iterator {
+    using size_type = std::size_t;
+
+public:
+    using value_type = std::pair<size_type, size_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;
+    using iterator_concept = std::forward_iterator_tag;
+
+    constexpr edge_iterator() noexcept
+        : it{},
+          vert{},
+          pos{},
+          last{},
+          offset{} {}
+
+    constexpr edge_iterator(It base, const size_type vertices, const size_type from, const size_type to, const size_type step) noexcept
+        : it{std::move(base)},
+          vert{vertices},
+          pos{from},
+          last{to},
+          offset{step} {
+        for(; pos != last && !it[pos]; pos += offset) {}
+    }
+
+    constexpr edge_iterator &operator++() noexcept {
+        for(pos += offset; pos != last && !it[pos]; pos += offset) {}
+        return *this;
+    }
+
+    constexpr edge_iterator operator++(int) noexcept {
+        edge_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    [[nodiscard]] constexpr reference operator*() const noexcept {
+        return *operator->();
+    }
+
+    [[nodiscard]] constexpr pointer operator->() const noexcept {
+        return std::make_pair<size_type>(pos / vert, pos % vert);
+    }
+
+    template<typename Type>
+    friend constexpr bool operator==(const edge_iterator<Type> &, const edge_iterator<Type> &) noexcept;
+
+private:
+    It it;
+    size_type vert;
+    size_type pos;
+    size_type last;
+    size_type offset{};
+};
+
+template<typename Container>
+[[nodiscard]] inline constexpr bool operator==(const edge_iterator<Container> &lhs, const edge_iterator<Container> &rhs) noexcept {
+    return lhs.pos == rhs.pos;
+}
+
+template<typename Container>
+[[nodiscard]] inline constexpr bool operator!=(const edge_iterator<Container> &lhs, const edge_iterator<Container> &rhs) noexcept {
+    return !(lhs == rhs);
+}
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief Basic implementation of a directed adjacency matrix.
+ * @tparam Category Either a directed or undirected category tag.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Category, typename Allocator>
+class adjacency_matrix {
+    using alloc_traits = std::allocator_traits<Allocator>;
+    static_assert(std::is_base_of_v<directed_tag, Category>, "Invalid graph category");
+    static_assert(std::is_same_v<typename alloc_traits::value_type, std::size_t>, "Invalid value type");
+    using container_type = std::vector<std::size_t, typename alloc_traits::template rebind_alloc<std::size_t>>;
+
+public:
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Vertex type. */
+    using vertex_type = size_type;
+    /*! @brief Edge type. */
+    using edge_type = std::pair<vertex_type, vertex_type>;
+    /*! @brief Vertex iterator type. */
+    using vertex_iterator = iota_iterator<vertex_type>;
+    /*! @brief Edge iterator type. */
+    using edge_iterator = internal::edge_iterator<typename container_type::const_iterator>;
+    /*! @brief Out edge iterator type. */
+    using out_edge_iterator = edge_iterator;
+    /*! @brief In edge iterator type. */
+    using in_edge_iterator = edge_iterator;
+    /*! @brief Graph category tag. */
+    using graph_category = Category;
+
+    /*! @brief Default constructor. */
+    adjacency_matrix() noexcept(noexcept(allocator_type{}))
+        : adjacency_matrix{0u} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit adjacency_matrix(const allocator_type &allocator) noexcept
+        : adjacency_matrix{0u, allocator} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator and user
+     * supplied number of vertices.
+     * @param vertices Number of vertices.
+     * @param allocator The allocator to use.
+     */
+    adjacency_matrix(const size_type vertices, const allocator_type &allocator = allocator_type{})
+        : matrix{vertices * vertices, allocator},
+          vert{vertices} {}
+
+    /**
+     * @brief Copy constructor.
+     * @param other The instance to copy from.
+     */
+    adjacency_matrix(const adjacency_matrix &other)
+        : adjacency_matrix{other, other.get_allocator()} {}
+
+    /**
+     * @brief Allocator-extended copy constructor.
+     * @param other The instance to copy from.
+     * @param allocator The allocator to use.
+     */
+    adjacency_matrix(const adjacency_matrix &other, const allocator_type &allocator)
+        : matrix{other.matrix, allocator},
+          vert{other.vert} {}
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    adjacency_matrix(adjacency_matrix &&other) noexcept
+        : adjacency_matrix{std::move(other), other.get_allocator()} {}
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    adjacency_matrix(adjacency_matrix &&other, const allocator_type &allocator)
+        : matrix{std::move(other.matrix), allocator},
+          vert{std::exchange(other.vert, 0u)} {}
+
+    /**
+     * @brief Default copy assignment operator.
+     * @param other The instance to copy from.
+     * @return This container.
+     */
+    adjacency_matrix &operator=(const adjacency_matrix &other) {
+        matrix = other.matrix;
+        vert = other.vert;
+        return *this;
+    }
+
+    /**
+     * @brief Default move assignment operator.
+     * @param other The instance to move from.
+     * @return This container.
+     */
+    adjacency_matrix &operator=(adjacency_matrix &&other) noexcept {
+        matrix = std::move(other.matrix);
+        vert = std::exchange(other.vert, 0u);
+        return *this;
+    }
+
+    /**
+     * @brief Returns the associated allocator.
+     * @return The associated allocator.
+     */
+    [[nodiscard]] constexpr allocator_type get_allocator() const noexcept {
+        return matrix.get_allocator();
+    }
+
+    /*! @brief Clears the adjacency matrix. */
+    void clear() noexcept {
+        matrix.clear();
+        vert = {};
+    }
+
+    /**
+     * @brief Exchanges the contents with those of a given adjacency matrix.
+     * @param other Adjacency matrix to exchange the content with.
+     */
+    void swap(adjacency_matrix &other) {
+        using std::swap;
+        swap(matrix, other.matrix);
+        swap(vert, other.vert);
+    }
+
+    /**
+     * @brief Returns the number of vertices.
+     * @return The number of vertices.
+     */
+    [[nodiscard]] size_type size() const noexcept {
+        return vert;
+    }
+
+    /**
+     * @brief Returns an iterable object to visit all vertices of a matrix.
+     * @return An iterable object to visit all vertices of a matrix.
+     */
+    [[nodiscard]] iterable_adaptor<vertex_iterator> vertices() const noexcept {
+        return {0u, vert};
+    }
+
+    /**
+     * @brief Returns an iterable object to visit all edges of a matrix.
+     * @return An iterable object to visit all edges of a matrix.
+     */
+    [[nodiscard]] iterable_adaptor<edge_iterator> edges() const noexcept {
+        const auto it = matrix.cbegin();
+        const auto sz = matrix.size();
+        return {{it, vert, 0u, sz, 1u}, {it, vert, sz, sz, 1u}};
+    }
+
+    /**
+     * @brief Returns an iterable object to visit all out edges of a vertex.
+     * @param vertex The vertex of which to return all out edges.
+     * @return An iterable object to visit all out edges of a vertex.
+     */
+    [[nodiscard]] iterable_adaptor<out_edge_iterator> out_edges(const vertex_type vertex) const noexcept {
+        const auto it = matrix.cbegin();
+        const auto from = vertex * vert;
+        const auto to = from + vert;
+        return {{it, vert, from, to, 1u}, {it, vert, to, to, 1u}};
+    }
+
+    /**
+     * @brief Returns an iterable object to visit all in edges of a vertex.
+     * @param vertex The vertex of which to return all in edges.
+     * @return An iterable object to visit all in edges of a vertex.
+     */
+    [[nodiscard]] iterable_adaptor<in_edge_iterator> in_edges(const vertex_type vertex) const noexcept {
+        const auto it = matrix.cbegin();
+        const auto from = vertex;
+        const auto to = vert * vert + from;
+        return {{it, vert, from, to, vert}, {it, vert, to, to, vert}};
+    }
+
+    /**
+     * @brief Resizes an adjacency matrix.
+     * @param vertices The new number of vertices.
+     */
+    void resize(const size_type vertices) {
+        adjacency_matrix other{vertices, get_allocator()};
+
+        for(auto [lhs, rhs]: edges()) {
+            other.insert(lhs, rhs);
+        }
+
+        other.swap(*this);
+    }
+
+    /**
+     * @brief Inserts an edge into the adjacency matrix, if it does not exist.
+     * @param lhs The left hand vertex of the edge.
+     * @param rhs The right hand vertex of the edge.
+     * @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<edge_iterator, bool> insert(const vertex_type lhs, const vertex_type rhs) {
+        const auto pos = lhs * vert + rhs;
+
+        if constexpr(std::is_same_v<graph_category, undirected_tag>) {
+            const auto rev = rhs * vert + lhs;
+            ENTT_ASSERT(matrix[pos] == matrix[rev], "Something went really wrong");
+            matrix[rev] = 1u;
+        }
+
+        const auto inserted = !std::exchange(matrix[pos], 1u);
+        return {edge_iterator{matrix.cbegin(), vert, pos, matrix.size(), 1u}, inserted};
+    }
+
+    /**
+     * @brief Removes the edge associated with a pair of given vertices.
+     * @param lhs The left hand vertex of the edge.
+     * @param rhs The right hand vertex of the edge.
+     * @return Number of elements removed (either 0 or 1).
+     */
+    size_type erase(const vertex_type lhs, const vertex_type rhs) {
+        const auto pos = lhs * vert + rhs;
+
+        if constexpr(std::is_same_v<graph_category, undirected_tag>) {
+            const auto rev = rhs * vert + lhs;
+            ENTT_ASSERT(matrix[pos] == matrix[rev], "Something went really wrong");
+            matrix[rev] = 0u;
+        }
+
+        return std::exchange(matrix[pos], 0u);
+    }
+
+    /**
+     * @brief Checks if an adjacency matrix contains a given edge.
+     * @param lhs The left hand vertex of the edge.
+     * @param rhs The right hand vertex of the edge.
+     * @return True if there is such an edge, false otherwise.
+     */
+    [[nodiscard]] bool contains(const vertex_type lhs, const vertex_type rhs) const {
+        const auto pos = lhs * vert + rhs;
+        return pos < matrix.size() && matrix[pos];
+    }
+
+private:
+    container_type matrix;
+    size_type vert;
+};
+
+} // namespace entt
+
+#endif

src/entt/graph/dot.hpp

@@ -0,0 +1,58 @@
+#ifndef ENTT_GRAPH_DOT_HPP
+#define ENTT_GRAPH_DOT_HPP
+
+#include <ostream>
+#include <type_traits>
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @brief Outputs a graph in dot format.
+ * @tparam Graph Graph type, valid as long as it exposes edges and vertices.
+ * @tparam Writer Vertex decorator type.
+ * @param out A standard output stream.
+ * @param graph The graph to output.
+ * @param writer Vertex decorator object.
+ */
+template<typename Graph, typename Writer>
+void dot(std::ostream &out, const Graph &graph, Writer writer) {
+    static_assert(std::is_base_of_v<directed_tag, typename Graph::graph_category>, "Invalid graph category");
+
+    if constexpr(std::is_same_v<typename Graph::graph_category, undirected_tag>) {
+        out << "graph{";
+    } else {
+        out << "digraph{";
+    }
+
+    for(auto &&vertex: graph.vertices()) {
+        out << vertex << "[";
+        writer(out, vertex);
+        out << "];";
+    }
+
+    for(auto [lhs, rhs]: graph.edges()) {
+        if constexpr(std::is_same_v<typename Graph::graph_category, undirected_tag>) {
+            out << lhs << "--" << rhs << ";";
+        } else {
+            out << lhs << "->" << rhs << ";";
+        }
+    }
+
+    out << "}";
+}
+
+/**
+ * @brief Outputs a graph in dot format.
+ * @tparam Graph Graph type, valid as long as it exposes edges and vertices.
+ * @param out A standard output stream.
+ * @param graph The graph to output.
+ */
+template<typename Graph>
+void dot(std::ostream &out, const Graph &graph) {
+    return dot(out, graph, [](auto &&...) {});
+}
+
+} // namespace entt
+
+#endif

src/entt/graph/flow.hpp

@@ -0,0 +1,341 @@
+#ifndef ENTT_GRAPH_FLOW_HPP
+#define ENTT_GRAPH_FLOW_HPP
+
+#include <algorithm>
+#include <cstddef>
+#include <functional>
+#include <iterator>
+#include <memory>
+#include <type_traits>
+#include <utility>
+#include <vector>
+#include "../config/config.h"
+#include "../container/dense_map.hpp"
+#include "../container/dense_set.hpp"
+#include "../core/compressed_pair.hpp"
+#include "../core/fwd.hpp"
+#include "../core/iterator.hpp"
+#include "../core/utility.hpp"
+#include "adjacency_matrix.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @brief Utility class for creating task graphs.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Allocator>
+class basic_flow {
+    using alloc_traits = std::allocator_traits<Allocator>;
+    static_assert(std::is_same_v<typename alloc_traits::value_type, id_type>, "Invalid value type");
+    using task_container_type = dense_set<id_type, identity, std::equal_to<id_type>, typename alloc_traits::template rebind_alloc<id_type>>;
+    using ro_rw_container_type = std::vector<std::pair<std::size_t, bool>, typename alloc_traits::template rebind_alloc<std::pair<std::size_t, bool>>>;
+    using deps_container_type = dense_map<id_type, ro_rw_container_type, identity, std::equal_to<id_type>, typename alloc_traits::template rebind_alloc<std::pair<const id_type, ro_rw_container_type>>>;
+    using adjacency_matrix_type = adjacency_matrix<directed_tag, typename alloc_traits::template rebind_alloc<std::size_t>>;
+
+    void emplace(const id_type res, const bool is_rw) {
+        ENTT_ASSERT(index.first() < vertices.size(), "Invalid node");
+
+        if(!deps.contains(res) && sync_on != vertices.size()) {
+            deps[res].emplace_back(sync_on, true);
+        }
+
+        deps[res].emplace_back(index.first(), is_rw);
+    }
+
+    void setup_graph(adjacency_matrix_type &matrix) const {
+        for(const auto &elem: deps) {
+            const auto last = elem.second.cend();
+            auto it = elem.second.cbegin();
+
+            while(it != last) {
+                if(it->second) {
+                    // rw item
+                    if(auto curr = it++; it != last) {
+                        if(it->second) {
+                            matrix.insert(curr->first, it->first);
+                        } else if(const auto next = std::find_if(it, last, [](const auto &value) { return value.second; }); next != last) {
+                            for(; it != next; ++it) {
+                                matrix.insert(curr->first, it->first);
+                                matrix.insert(it->first, next->first);
+                            }
+                        } else {
+                            for(; it != next; ++it) {
+                                matrix.insert(curr->first, it->first);
+                            }
+                        }
+                    }
+                } else {
+                    // ro item (first iteration only)
+                    if(const auto next = std::find_if(it, last, [](const auto &value) { return value.second; }); next != last) {
+                        for(; it != next; ++it) {
+                            matrix.insert(it->first, next->first);
+                        }
+                    } else {
+                        it = last;
+                    }
+                }
+            }
+        }
+    }
+
+    void transitive_closure(adjacency_matrix_type &matrix) const {
+        const auto length = matrix.size();
+
+        for(std::size_t vk{}; vk < length; ++vk) {
+            for(std::size_t vi{}; vi < length; ++vi) {
+                for(std::size_t vj{}; vj < length; ++vj) {
+                    if(matrix.contains(vi, vk) && matrix.contains(vk, vj)) {
+                        matrix.insert(vi, vj);
+                    }
+                }
+            }
+        }
+    }
+
+    void transitive_reduction(adjacency_matrix_type &matrix) const {
+        const auto length = matrix.size();
+
+        for(std::size_t vert{}; vert < length; ++vert) {
+            matrix.erase(vert, vert);
+        }
+
+        for(std::size_t vj{}; vj < length; ++vj) {
+            for(std::size_t vi{}; vi < length; ++vi) {
+                if(matrix.contains(vi, vj)) {
+                    for(std::size_t vk{}; vk < length; ++vk) {
+                        if(matrix.contains(vj, vk)) {
+                            matrix.erase(vi, vk);
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+public:
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Iterable task list. */
+    using iterable = iterable_adaptor<typename task_container_type::const_iterator>;
+    /*! @brief Adjacency matrix type. */
+    using graph_type = adjacency_matrix_type;
+
+    /*! @brief Default constructor. */
+    basic_flow()
+        : basic_flow{allocator_type{}} {}
+
+    /**
+     * @brief Constructs a flow builder with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit basic_flow(const allocator_type &allocator)
+        : index{0u, allocator},
+          vertices{allocator},
+          deps{allocator},
+          sync_on{} {}
+
+    /*! @brief Default copy constructor. */
+    basic_flow(const basic_flow &) = default;
+
+    /**
+     * @brief Allocator-extended copy constructor.
+     * @param other The instance to copy from.
+     * @param allocator The allocator to use.
+     */
+    basic_flow(const basic_flow &other, const allocator_type &allocator)
+        : index{other.index.first(), allocator},
+          vertices{other.vertices, allocator},
+          deps{other.deps, allocator},
+          sync_on{other.sync_on} {}
+
+    /*! @brief Default move constructor. */
+    basic_flow(basic_flow &&) noexcept = default;
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    basic_flow(basic_flow &&other, const allocator_type &allocator)
+        : index{other.index.first(), allocator},
+          vertices{std::move(other.vertices), allocator},
+          deps{std::move(other.deps), allocator},
+          sync_on{other.sync_on} {}
+
+    /**
+     * @brief Default copy assignment operator.
+     * @return This flow builder.
+     */
+    basic_flow &operator=(const basic_flow &) = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This flow builder.
+     */
+    basic_flow &operator=(basic_flow &&) noexcept = default;
+
+    /**
+     * @brief Returns the associated allocator.
+     * @return The associated allocator.
+     */
+    [[nodiscard]] constexpr allocator_type get_allocator() const noexcept {
+        return allocator_type{index.second()};
+    }
+
+    /**
+     * @brief Returns the identifier at specified location.
+     * @param pos Position of the identifier to return.
+     * @return The requested identifier.
+     */
+    [[nodiscard]] id_type operator[](const size_type pos) const {
+        return vertices.cbegin()[pos];
+    }
+
+    /*! @brief Clears the flow builder. */
+    void clear() noexcept {
+        index.first() = {};
+        vertices.clear();
+        deps.clear();
+        sync_on = {};
+    }
+
+    /**
+     * @brief Exchanges the contents with those of a given flow builder.
+     * @param other Flow builder to exchange the content with.
+     */
+    void swap(basic_flow &other) {
+        using std::swap;
+        std::swap(index, other.index);
+        std::swap(vertices, other.vertices);
+        std::swap(deps, other.deps);
+        std::swap(sync_on, other.sync_on);
+    }
+
+    /**
+     * @brief Returns the number of tasks.
+     * @return The number of tasks.
+     */
+    [[nodiscard]] size_type size() const noexcept {
+        return vertices.size();
+    }
+
+    /**
+     * @brief Binds a task to a flow builder.
+     * @param value Task identifier.
+     * @return This flow builder.
+     */
+    basic_flow &bind(const id_type value) {
+        sync_on += (sync_on == vertices.size());
+        const auto it = vertices.emplace(value).first;
+        index.first() = size_type(it - vertices.begin());
+        return *this;
+    }
+
+    /**
+     * @brief Turns the current task into a sync point.
+     * @return This flow builder.
+     */
+    basic_flow &sync() {
+        ENTT_ASSERT(index.first() < vertices.size(), "Invalid node");
+        sync_on = index.first();
+
+        for(const auto &elem: deps) {
+            elem.second.emplace_back(sync_on, true);
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a resource to the current task with a given access mode.
+     * @param res Resource identifier.
+     * @param is_rw Access mode.
+     * @return This flow builder.
+     */
+    basic_flow &set(const id_type res, bool is_rw = false) {
+        emplace(res, is_rw);
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a read-only resource to the current task.
+     * @param res Resource identifier.
+     * @return This flow builder.
+     */
+    basic_flow &ro(const id_type res) {
+        emplace(res, false);
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a range of read-only resources to the current task.
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of elements.
+     * @param last An iterator past the last element of the range of elements.
+     * @return This flow builder.
+     */
+    template<typename It>
+    std::enable_if_t<std::is_same_v<std::remove_const_t<typename std::iterator_traits<It>::value_type>, id_type>, basic_flow &>
+    ro(It first, It last) {
+        for(; first != last; ++first) {
+            emplace(*first, false);
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a writable resource to the current task.
+     * @param res Resource identifier.
+     * @return This flow builder.
+     */
+    basic_flow &rw(const id_type res) {
+        emplace(res, true);
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a range of writable resources to the current task.
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of elements.
+     * @param last An iterator past the last element of the range of elements.
+     * @return This flow builder.
+     */
+    template<typename It>
+    std::enable_if_t<std::is_same_v<std::remove_const_t<typename std::iterator_traits<It>::value_type>, id_type>, basic_flow &>
+    rw(It first, It last) {
+        for(; first != last; ++first) {
+            emplace(*first, true);
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Generates a task graph for the current content.
+     * @return The adjacency matrix of the task graph.
+     */
+    [[nodiscard]] graph_type graph() const {
+        graph_type matrix{vertices.size(), get_allocator()};
+
+        setup_graph(matrix);
+        transitive_closure(matrix);
+        transitive_reduction(matrix);
+
+        return matrix;
+    }
+
+private:
+    compressed_pair<size_type, allocator_type> index;
+    task_container_type vertices;
+    deps_container_type deps;
+    size_type sync_on;
+};
+
+} // namespace entt
+
+#endif

src/entt/graph/fwd.hpp

@@ -0,0 +1,27 @@
+#ifndef ENTT_GRAPH_FWD_HPP
+#define ENTT_GRAPH_FWD_HPP
+
+#include <cstddef>
+#include <memory>
+#include "../core/fwd.hpp"
+
+namespace entt {
+
+/*! @brief Undirected graph category tag. */
+struct directed_tag {};
+
+/*! @brief Directed graph category tag. */
+struct undirected_tag: directed_tag {};
+
+template<typename, typename = std::allocator<std::size_t>>
+class adjacency_matrix;
+
+template<typename = std::allocator<id_type>>
+class basic_flow;
+
+/*! @brief Alias declaration for the most common use case. */
+using flow = basic_flow<>;
+
+} // namespace entt
+
+#endif

src/entt/locator/locator.hpp

@@ -10,95 +10,136 @@ 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;
+class locator final {
+    class service_handle {
+        friend class locator<Service>;
+        std::shared_ptr<Service> value{};
+    };
+
+public:
+    /*! @brief Service type. */
+    using type = Service;
+    /*! @brief Service node type. */
+    using node_type = service_handle;
 
     /*! @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() 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() noexcept {
+        ENTT_ASSERT(has_value(), "Service not available");
         return *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 Type Fallback service type.
+     * @param args Parameters to use to construct the fallback service.
+     * @return A reference to a valid service.
+     */
+    template<typename Type = Service, typename... Args>
+    [[nodiscard]] static Service &value_or(Args &&...args) {
+        return service ? *service : emplace<Type>(std::forward<Args>(args)...);
+    }
+
     /**
      * @brief Sets or replaces a service.
-     * @tparam Impl Type of the new service to use.
+     * @tparam Type 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.
      */
-    template<typename Impl = Service, typename... Args>
-    static void set(Args &&...args) {
-        service = std::make_shared<Impl>(std::forward<Args>(args)...);
+    template<typename Type = Service, typename... Args>
+    static Service &emplace(Args &&...args) {
+        service = std::make_shared<Type>(std::forward<Args>(args)...);
+        return *service;
     }
 
     /**
-     * @brief Sets or replaces a service.
-     * @param ptr Service to use to replace the current one.
+     * @brief Sets or replaces a service using a given allocator.
+     * @tparam Type Service type.
+     * @tparam Allocator Type of allocator used to manage memory and elements.
+     * @tparam Args Types of arguments to use to construct the service.
+     * @param alloc The allocator to use.
+     * @param args Parameters to use to construct the service.
+     * @return A reference to a valid service.
      */
-    static void set(std::shared_ptr<Service> ptr) {
-        ENTT_ASSERT(static_cast<bool>(ptr), "Null service not allowed");
-        service = std::move(ptr);
+    template<typename Type = Service, typename Allocator, typename... Args>
+    static Service &emplace(std::allocator_arg_t, Allocator alloc, Args &&...args) {
+        service = std::allocate_shared<Type>(alloc, std::forward<Args>(args)...);
+        return *service;
     }
 
     /**
-     * @brief Resets a service.
-     *
-     * The service is no longer valid after a reset.
+     * @brief Returns a handle to the underlying service.
+     * @return A handle to the underlying service.
      */
-    static void reset() {
-        service.reset();
+    static node_type handle() noexcept {
+        node_type node{};
+        node.value = service;
+        return node;
+    }
+
+    /**
+     * @brief Resets or replaces a service.
+     * @param other Optional handle with which to replace the service.
+     */
+    static void reset(const node_type &other = {}) noexcept {
+        service = other.value;
+    }
+
+    /**
+     * @brief Resets or replaces a service.
+     * @tparam Type Service type.
+     * @tparam Deleter Deleter type.
+     * @param elem A pointer to a service to manage.
+     * @param deleter A deleter to use to destroy the service.
+     */
+    template<typename Type, typename Deleter = std::default_delete<Type>>
+    static void reset(Type *elem, Deleter deleter = {}) {
+        service = std::shared_ptr<Service>{elem, std::move(deleter)};
     }
 
 private:
-    inline static std::shared_ptr<Service> service = nullptr;
+    // std::shared_ptr because of its type erased allocator which is useful here
+    inline static std::shared_ptr<Service> service{};
 };
 
 } // namespace entt

src/entt/meta/container.hpp

@@ -2,272 +2,384 @@
 #define ENTT_META_CONTAINER_HPP
 
 #include <array>
+#include <deque>
+#include <iterator>
+#include <list>
 #include <map>
 #include <set>
 #include <type_traits>
 #include <unordered_map>
 #include <unordered_set>
 #include <vector>
-#include "../container/dense_hash_map.hpp"
-#include "../container/dense_hash_set.hpp"
+#include "../container/dense_map.hpp"
+#include "../container/dense_set.hpp"
+#include "context.hpp"
 #include "meta.hpp"
 #include "type_traits.hpp"
 
 namespace entt {
 
-/**
- * @cond TURN_OFF_DOXYGEN
- * Internal details not to be documented.
- */
-
+/*! @cond TURN_OFF_DOXYGEN */
 namespace internal {
 
 template<typename, typename = void>
-struct is_dynamic_sequence_container: std::false_type {};
+struct fixed_size_sequence_container: std::true_type {};
 
 template<typename Type>
-struct is_dynamic_sequence_container<Type, std::void_t<decltype(&Type::reserve)>>: std::true_type {};
+struct fixed_size_sequence_container<Type, std::void_t<decltype(&Type::clear)>>: std::false_type {};
+
+template<typename Type>
+inline constexpr bool fixed_size_sequence_container_v = fixed_size_sequence_container<Type>::value;
 
 template<typename, typename = void>
-struct is_key_only_meta_associative_container: std::true_type {};
+struct key_only_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 {};
+struct key_only_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;
+inline constexpr bool key_only_associative_container_v = key_only_associative_container<Type>::value;
 
-    [[nodiscard]] static size_type size(const any &container) ENTT_NOEXCEPT {
-        return any_cast<const Type &>(container).size();
-    }
-
-    [[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;
-            }
-        }
-
-        return false;
-    }
-
-    [[nodiscard]] static bool clear([[maybe_unused]] any &container) {
-        if constexpr(is_dynamic_sequence_container<Type>::value) {
-            if(auto *const cont = any_cast<Type>(&container); cont) {
-                cont->clear();
-                return true;
-            }
-        }
-
-        return false;
-    }
-
-    [[nodiscard]] static iterator begin(any &container) {
-        if(auto *const cont = any_cast<Type>(&container); cont) {
-            return iterator{std::begin(*cont)};
-        }
-
-        return iterator{std::begin(any_cast<const Type &>(container))};
-    }
-
-    [[nodiscard]] static iterator end(any &container) {
-        if(auto *const cont = any_cast<Type>(&container); cont) {
-            return iterator{std::end(*cont)};
-        }
-
-        return iterator{std::end(any_cast<const Type &>(container))};
-    }
-
-    [[nodiscard]] static iterator insert([[maybe_unused]] any &container, [[maybe_unused]] iterator it, [[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>>();
-                    return iterator{cont->insert(any_cast<const typename Type::iterator &>(it.base()), element ? *element : value.cast<typename Type::value_type>())};
-                }
-            }
-        }
-
-        return {};
-    }
-
-    [[nodiscard]] static iterator erase([[maybe_unused]] any &container, [[maybe_unused]] iterator it) {
-        if constexpr(is_dynamic_sequence_container<Type>::value) {
-            if(auto *const cont = any_cast<Type>(&container); cont) {
-                return iterator{cont->erase(any_cast<const typename Type::iterator &>(it.base()))};
-            }
-        }
-
-        return {};
-    }
-
-    [[nodiscard]] static meta_any get(any &container, size_type pos) {
-        if(auto *const cont = any_cast<Type>(&container); cont) {
-            return meta_any{std::in_place_type<typename Type::reference>, (*cont)[pos]};
-        }
-
-        return meta_any{std::in_place_type<typename Type::const_reference>, any_cast<const Type &>(container)[pos]};
-    }
-};
+template<typename, typename = void>
+struct reserve_aware_container: std::false_type {};
 
 template<typename Type>
-struct basic_meta_associative_container_traits {
-    using iterator = meta_associative_container::iterator;
-    using size_type = std::size_t;
+struct reserve_aware_container<Type, std::void_t<decltype(&Type::reserve)>>: std::true_type {};
 
-    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 begin(any &container) {
-        if(auto *const cont = any_cast<Type>(&container); cont) {
-            return iterator{std::integral_constant<bool, key_only>{}, cont->begin()};
-        }
-
-        return iterator{std::integral_constant<bool, key_only>{}, std::begin(any_cast<const Type &>(container))};
-    }
-
-    [[nodiscard]] static iterator end(any &container) {
-        if(auto *const cont = any_cast<Type>(&container); cont) {
-            return iterator{std::integral_constant<bool, key_only>{}, cont->end()};
-        }
-
-        return iterator{std::integral_constant<bool, key_only>{}, std::end(any_cast<const Type &>(container))};
-    }
-
-    [[nodiscard]] static bool insert(any &container, meta_any &key, [[maybe_unused]] meta_any &value) {
-        if(auto *const cont = any_cast<Type>(&container); cont && key.allow_cast<const typename Type::key_type &>()) {
-            if constexpr(is_key_only_meta_associative_container<Type>::value) {
-                return cont->insert(key.cast<const typename Type::key_type &>()).second;
-            } else {
-                if(value.allow_cast<const typename Type::mapped_type &>()) {
-                    return cont->emplace(key.cast<const typename Type::key_type &>(), value.cast<const typename Type::mapped_type &>()).second;
-                }
-            }
-        }
-
-        return false;
-    }
-
-    [[nodiscard]] static bool erase(any &container, meta_any &key) {
-        if(auto *const cont = any_cast<Type>(&container); cont && key.allow_cast<const typename Type::key_type &>()) {
-            return (cont->erase(key.cast<const typename Type::key_type &>()) != cont->size());
-        }
-
-        return false;
-    }
-
-    [[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 {};
-    }
-};
+template<typename Type>
+inline constexpr bool reserve_aware_container_v = reserve_aware_container<Type>::value;
 
 } // namespace internal
+/*! @endcond */
 
 /**
- * Internal details not to be documented.
- * @endcond
+ * @brief General purpose implementation of meta sequence container traits.
+ * @tparam Type Type of underlying sequence container.
  */
+template<typename Type>
+struct basic_meta_sequence_container_traits {
+    static_assert(std::is_same_v<Type, std::remove_cv_t<std::remove_reference_t<Type>>>, "Unexpected type");
+
+    /*! @brief True in case of key-only containers, false otherwise. */
+    static constexpr bool fixed_size = internal::fixed_size_sequence_container_v<Type>;
+
+    /*! @brief Unsigned integer type. */
+    using size_type = typename meta_sequence_container::size_type;
+    /*! @brief Meta iterator type. */
+    using iterator = typename meta_sequence_container::iterator;
+
+    /**
+     * @brief Returns the number of elements in a container.
+     * @param container Opaque pointer to a container of the given type.
+     * @return Number of elements.
+     */
+    [[nodiscard]] static size_type size(const void *container) {
+        return static_cast<const Type *>(container)->size();
+    }
+
+    /**
+     * @brief Clears a container.
+     * @param container Opaque pointer to a container of the given type.
+     * @return True in case of success, false otherwise.
+     */
+    [[nodiscard]] static bool clear([[maybe_unused]] void *container) {
+        if constexpr(fixed_size) {
+            return false;
+        } else {
+            static_cast<Type *>(container)->clear();
+            return true;
+        }
+    }
+
+    /**
+     * @brief Increases the capacity of a container.
+     * @param container Opaque pointer to a container of the given type.
+     * @param sz Desired capacity.
+     * @return True in case of success, false otherwise.
+     */
+    [[nodiscard]] static bool reserve([[maybe_unused]] void *container, [[maybe_unused]] const size_type sz) {
+        if constexpr(internal::reserve_aware_container_v<Type>) {
+            static_cast<Type *>(container)->reserve(sz);
+            return true;
+        } else {
+            return false;
+        }
+    }
+
+    /**
+     * @brief Resizes a container.
+     * @param container Opaque pointer to a container of the given type.
+     * @param sz The new number of elements.
+     * @return True in case of success, false otherwise.
+     */
+    [[nodiscard]] static bool resize([[maybe_unused]] void *container, [[maybe_unused]] const size_type sz) {
+        if constexpr(fixed_size || !std::is_default_constructible_v<typename Type::value_type>) {
+            return false;
+        } else {
+            static_cast<Type *>(container)->resize(sz);
+            return true;
+        }
+    }
+
+    /**
+     * @brief Returns a possibly const iterator to the beginning.
+     * @param area The context to pass to the newly created iterator.
+     * @param container Opaque pointer to a container of the given type.
+     * @param as_const Const opaque pointer fallback.
+     * @return An iterator to the first element of the container.
+     */
+    static iterator begin(const meta_ctx &area, void *container, const void *as_const) {
+        return container ? iterator{area, static_cast<Type *>(container)->begin()}
+                         : iterator{area, static_cast<const Type *>(as_const)->begin()};
+    }
+
+    /**
+     * @brief Returns a possibly const iterator to the end.
+     * @param area The context to pass to the newly created iterator.
+     * @param container Opaque pointer to a container of the given type.
+     * @param as_const Const opaque pointer fallback.
+     * @return An iterator that is past the last element of the container.
+     */
+    static iterator end(const meta_ctx &area, void *container, const void *as_const) {
+        return container ? iterator{area, static_cast<Type *>(container)->end()}
+                         : iterator{area, static_cast<const Type *>(as_const)->end()};
+    }
+
+    /**
+     * @brief Assigns one element to a container and constructs its object from
+     * a given opaque instance.
+     * @param area The context to pass to the newly created iterator.
+     * @param container Opaque pointer to a container of the given type.
+     * @param value Optional opaque instance of the object to construct (as
+     * value type).
+     * @param cref Optional opaque instance of the object to construct (as
+     * decayed const reference type).
+     * @param it Iterator before which the element will be inserted.
+     * @return A possibly invalid iterator to the inserted element.
+     */
+    [[nodiscard]] static iterator insert(const meta_ctx &area, [[maybe_unused]] void *container, [[maybe_unused]] const void *value, [[maybe_unused]] const void *cref, [[maybe_unused]] const iterator &it) {
+        if constexpr(fixed_size) {
+            return iterator{area};
+        } else {
+            auto *const non_const = any_cast<typename Type::iterator>(&it.base());
+            return {area, static_cast<Type *>(container)->insert(
+                              non_const ? *non_const : any_cast<const typename Type::const_iterator &>(it.base()),
+                              value ? *static_cast<const typename Type::value_type *>(value) : *static_cast<const std::remove_reference_t<typename Type::const_reference> *>(cref))};
+        }
+    }
+
+    /**
+     * @brief Erases an element from a container.
+     * @param area The context to pass to the newly created iterator.
+     * @param container Opaque pointer to a container of the given type.
+     * @param it An opaque iterator to the element to erase.
+     * @return A possibly invalid iterator following the last removed element.
+     */
+    [[nodiscard]] static iterator erase(const meta_ctx &area, [[maybe_unused]] void *container, [[maybe_unused]] const iterator &it) {
+        if constexpr(fixed_size) {
+            return iterator{area};
+        } else {
+            auto *const non_const = any_cast<typename Type::iterator>(&it.base());
+            return {area, static_cast<Type *>(container)->erase(non_const ? *non_const : any_cast<const typename Type::const_iterator &>(it.base()))};
+        }
+    }
+};
+
+/**
+ * @brief General purpose implementation of meta associative container traits.
+ * @tparam Type Type of underlying associative container.
+ */
+template<typename Type>
+struct basic_meta_associative_container_traits {
+    static_assert(std::is_same_v<Type, std::remove_cv_t<std::remove_reference_t<Type>>>, "Unexpected type");
+
+    /*! @brief True in case of key-only containers, false otherwise. */
+    static constexpr bool key_only = internal::key_only_associative_container_v<Type>;
+
+    /*! @brief Unsigned integer type. */
+    using size_type = typename meta_associative_container::size_type;
+    /*! @brief Meta iterator type. */
+    using iterator = typename meta_associative_container::iterator;
+
+    /**
+     * @brief Returns the number of elements in a container.
+     * @param container Opaque pointer to a container of the given type.
+     * @return Number of elements.
+     */
+    [[nodiscard]] static size_type size(const void *container) {
+        return static_cast<const Type *>(container)->size();
+    }
+
+    /**
+     * @brief Clears a container.
+     * @param container Opaque pointer to a container of the given type.
+     * @return True in case of success, false otherwise.
+     */
+    [[nodiscard]] static bool clear(void *container) {
+        static_cast<Type *>(container)->clear();
+        return true;
+    }
+
+    /**
+     * @brief Increases the capacity of a container.
+     * @param container Opaque pointer to a container of the given type.
+     * @param sz Desired capacity.
+     * @return True in case of success, false otherwise.
+     */
+    [[nodiscard]] static bool reserve([[maybe_unused]] void *container, [[maybe_unused]] const size_type sz) {
+        if constexpr(internal::reserve_aware_container_v<Type>) {
+            static_cast<Type *>(container)->reserve(sz);
+            return true;
+        } else {
+            return false;
+        }
+    }
+
+    /**
+     * @brief Returns a possibly const iterator to the beginning.
+     * @param area The context to pass to the newly created iterator.
+     * @param container Opaque pointer to a container of the given type.
+     * @param as_const Const opaque pointer fallback.
+     * @return An iterator to the first element of the container.
+     */
+    static iterator begin(const meta_ctx &area, void *container, const void *as_const) {
+        return container ? iterator{area, std::bool_constant<key_only>{}, static_cast<Type *>(container)->begin()}
+                         : iterator{area, std::bool_constant<key_only>{}, static_cast<const Type *>(as_const)->begin()};
+    }
+
+    /**
+     * @brief Returns a possibly const iterator to the end.
+     * @param area The context to pass to the newly created iterator.
+     * @param container Opaque pointer to a container of the given type.
+     * @param as_const Const opaque pointer fallback.
+     * @return An iterator that is past the last element of the container.
+     */
+    static iterator end(const meta_ctx &area, void *container, const void *as_const) {
+        return container ? iterator{area, std::bool_constant<key_only>{}, static_cast<Type *>(container)->end()}
+                         : iterator{area, std::bool_constant<key_only>{}, static_cast<const Type *>(as_const)->end()};
+    }
+
+    /**
+     * @brief Inserts an element into a container, if the key does not exist.
+     * @param container Opaque pointer to a container of the given type.
+     * @param key An opaque key value of an element to insert.
+     * @param value Optional opaque value to insert (key-value containers).
+     * @return True if the insertion took place, false otherwise.
+     */
+    [[nodiscard]] static bool insert(void *container, const void *key, [[maybe_unused]] const void *value) {
+        if constexpr(key_only) {
+            return static_cast<Type *>(container)->insert(*static_cast<const typename Type::key_type *>(key)).second;
+        } else {
+            return static_cast<Type *>(container)->emplace(*static_cast<const typename Type::key_type *>(key), *static_cast<const typename Type::mapped_type *>(value)).second;
+        }
+    }
+
+    /**
+     * @brief Removes an element from a container.
+     * @param container Opaque pointer to a container of the given type.
+     * @param key An opaque key value of an element to remove.
+     * @return Number of elements removed (either 0 or 1).
+     */
+    [[nodiscard]] static size_type erase(void *container, const void *key) {
+        return static_cast<Type *>(container)->erase(*static_cast<const typename Type::key_type *>(key));
+    }
+
+    /**
+     * @brief Finds an element with a given key.
+     * @param area The context to pass to the newly created iterator.
+     * @param container Opaque pointer to a container of the given type.
+     * @param as_const Const opaque pointer fallback.
+     * @param key Opaque key value of an element to search for.
+     * @return An iterator to the element with the given key, if any.
+     */
+    static iterator find(const meta_ctx &area, void *container, const void *as_const, const void *key) {
+        return container ? iterator{area, std::bool_constant<key_only>{}, static_cast<Type *>(container)->find(*static_cast<const typename Type::key_type *>(key))}
+                         : iterator{area, std::bool_constant<key_only>{}, static_cast<const Type *>(as_const)->find(*static_cast<const typename Type::key_type *>(key))};
+    }
+};
 
 /**
  * @brief Meta sequence container traits for `std::vector`s of any type.
- * @tparam Type The type of elements.
- * @tparam Args Other arguments.
+ * @tparam Args Template arguments for the container.
  */
-template<typename Type, typename... Args>
-struct meta_sequence_container_traits<std::vector<Type, Args...>>
-    : internal::basic_meta_sequence_container_traits<std::vector<Type, Args...>> {};
+template<typename... Args>
+struct meta_sequence_container_traits<std::vector<Args...>>
+    : basic_meta_sequence_container_traits<std::vector<Args...>> {};
 
 /**
  * @brief Meta sequence container traits for `std::array`s of any type.
- * @tparam Type The type of elements.
- * @tparam N The number of elements.
+ * @tparam Type Template arguments for the container.
+ * @tparam N Template arguments for the container.
  */
 template<typename Type, auto N>
 struct meta_sequence_container_traits<std::array<Type, N>>
-    : internal::basic_meta_sequence_container_traits<std::array<Type, N>> {};
+    : basic_meta_sequence_container_traits<std::array<Type, N>> {};
+
+/**
+ * @brief Meta sequence container traits for `std::list`s of any type.
+ * @tparam Args Template arguments for the container.
+ */
+template<typename... Args>
+struct meta_sequence_container_traits<std::list<Args...>>
+    : basic_meta_sequence_container_traits<std::list<Args...>> {};
+
+/**
+ * @brief Meta sequence container traits for `std::deque`s of any type.
+ * @tparam Args Template arguments for the container.
+ */
+template<typename... Args>
+struct meta_sequence_container_traits<std::deque<Args...>>
+    : basic_meta_sequence_container_traits<std::deque<Args...>> {};
 
 /**
  * @brief Meta associative container traits for `std::map`s of any type.
- * @tparam Key The key type of elements.
- * @tparam Value The value type of elements.
- * @tparam Args Other arguments.
+ * @tparam Args Template arguments for the container.
  */
-template<typename Key, typename Value, typename... Args>
-struct meta_associative_container_traits<std::map<Key, Value, Args...>>
-    : internal::basic_meta_associative_container_traits<std::map<Key, Value, Args...>> {};
+template<typename... Args>
+struct meta_associative_container_traits<std::map<Args...>>
+    : basic_meta_associative_container_traits<std::map<Args...>> {};
 
 /**
  * @brief Meta associative container traits for `std::unordered_map`s of any
  * type.
- * @tparam Key The key type of elements.
- * @tparam Value The value type of elements.
- * @tparam Args Other arguments.
+ * @tparam Args Template arguments for the container.
  */
-template<typename Key, typename Value, typename... Args>
-struct meta_associative_container_traits<std::unordered_map<Key, Value, Args...>>
-    : internal::basic_meta_associative_container_traits<std::unordered_map<Key, Value, Args...>> {};
+template<typename... Args>
+struct meta_associative_container_traits<std::unordered_map<Args...>>
+    : basic_meta_associative_container_traits<std::unordered_map<Args...>> {};
 
 /**
  * @brief Meta associative container traits for `std::set`s of any type.
- * @tparam Key The type of elements.
- * @tparam Args Other arguments.
+ * @tparam Args Template arguments for the container.
  */
-template<typename Key, typename... Args>
-struct meta_associative_container_traits<std::set<Key, Args...>>
-    : internal::basic_meta_associative_container_traits<std::set<Key, Args...>> {};
+template<typename... Args>
+struct meta_associative_container_traits<std::set<Args...>>
+    : basic_meta_associative_container_traits<std::set<Args...>> {};
 
 /**
  * @brief Meta associative container traits for `std::unordered_set`s of any
  * type.
- * @tparam Key The type of elements.
- * @tparam Args Other arguments.
+ * @tparam Args Template arguments for the container.
  */
-template<typename Key, typename... Args>
-struct meta_associative_container_traits<std::unordered_set<Key, Args...>>
-    : internal::basic_meta_associative_container_traits<std::unordered_set<Key, Args...>> {};
+template<typename... Args>
+struct meta_associative_container_traits<std::unordered_set<Args...>>
+    : basic_meta_associative_container_traits<std::unordered_set<Args...>> {};
 
 /**
- * @brief Meta associative container traits for `dense_hash_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.
+ * @brief Meta associative container traits for `dense_map`s of any type.
+ * @tparam Args Template arguments for the container.
  */
-template<typename Key, typename Type, typename... Args>
-struct meta_associative_container_traits<dense_hash_map<Key, Type, Args...>>
-    : internal::basic_meta_associative_container_traits<dense_hash_map<Key, Type, Args...>> {};
+template<typename... Args>
+struct meta_associative_container_traits<dense_map<Args...>>
+    : basic_meta_associative_container_traits<dense_map<Args...>> {};
 
 /**
- * @brief Meta associative container traits for `dense_hash_set`s of any type.
- * @tparam Type The value type of the elements.
- * @tparam Args Other arguments.
+ * @brief Meta associative container traits for `dense_set`s of any type.
+ * @tparam Args Template arguments for the container.
  */
-template<typename Type, typename... Args>
-struct meta_associative_container_traits<dense_hash_set<Type, Args...>>
-    : internal::basic_meta_associative_container_traits<dense_hash_set<Type, Args...>> {};
+template<typename... Args>
+struct meta_associative_container_traits<dense_set<Args...>>
+    : basic_meta_associative_container_traits<dense_set<Args...>> {};
 
 } // namespace entt