updated single include file

* remove visit
* added storage_proxy_iterator (input iterator category, modeled as a random access iterator)
* added ::storage() (const and non-const) to return an iterable object over all pools

.clang-format

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

.github/FUNDING.yml

@@ -1,12 +1,12 @@
 # These are supported funding model platforms
 
 github: skypjack
-patreon: skypjack
-open_collective: # Replace with a single Open Collective username
-ko_fi: # Replace with a single Ko-fi username
-tidelift: # Replace with a single Tidelift platform-name/package-name e.g., npm/babel
-community_bridge: # Replace with a single Community Bridge project-name e.g., cloud-foundry
-liberapay: # Replace with a single Liberapay username
-issuehunt: # Replace with a single IssueHunt username
-otechie: # Replace with a single Otechie username
+patreon:
+open_collective:
+ko_fi:
+tidelift:
+community_bridge:
+liberapay:
+issuehunt:
+otechie:
 custom: https://www.paypal.me/skypjack

.github/workflows/build.yml

@@ -7,9 +7,60 @@ jobs:
   linux:
     timeout-minutes: 10
 
+    strategy:
+      matrix:
+        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++
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Install compiler
+      run: |
+        sudo apt-get update
+        sudo apt-get install ${{ matrix.compiler.pkg }} -y
+    - name: Compile tests
+      working-directory: build
+      env:
+        CXX: ${{ matrix.compiler.exe }}
+      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
+
+  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
 
@@ -20,42 +71,68 @@ jobs:
       env:
         CXX: ${{ matrix.compiler }}
       run: |
-        cmake -DBUILD_TESTING=ON -DBUILD_LIB=ON ..
+        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 5 -C Debug -j4
+      run: ctest --timeout 30 -C Debug -j4
 
   windows:
     timeout-minutes: 10
 
     strategy:
       matrix:
-        os: [windows-latest, windows-2016]
-        toolset: [clang-cl, default]
+        toolset: [clang-cl, default, v141]
         include:
           - toolset: clang-cl
             toolset_option: -T"ClangCl"
-        exclude:
-          - os: windows-2016
-            toolset: clang-cl
+          - toolset: v141
+            toolset_option: -T"v141"
 
-    runs-on: ${{ matrix.os }}
+    runs-on: windows-latest
 
     steps:
     - uses: actions/checkout@v2
     - name: Compile tests
       working-directory: build
       run: |
-        cmake -DBUILD_TESTING=ON -DBUILD_LIB=ON ${{ matrix.toolset_option }} ..
+        cmake -DENTT_BUILD_TESTING=ON -DENTT_BUILD_LIB=ON -DENTT_BUILD_EXAMPLE=ON ${{ matrix.toolset_option }} ..
         cmake --build . -j 4
     - name: Run tests
       working-directory: build
       env:
         CTEST_OUTPUT_ON_FAILURE: 1
-      run: ctest --timeout 5 -C Debug -j4
+      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
 
   macos:
     timeout-minutes: 10
@@ -66,10 +143,38 @@ jobs:
     - name: Compile tests
       working-directory: build
       run: |
-        cmake -DBUILD_TESTING=ON -DBUILD_LIB=ON ..
+        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 5 -C Debug -j4
+      run: ctest --timeout 30 -C Debug -j4
+
+  macos-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: macOS-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 }} ..
+        make -j4
+    - name: Run tests
+      working-directory: build
+      env:
+        CTEST_OUTPUT_ON_FAILURE: 1
+      run: ctest --timeout 30 -C Debug -j4

.github/workflows/coverage.yml

@@ -9,25 +9,30 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v2
-    - name: Compile tests
-      working-directory: build
-      env:
-        CXXFLAGS: "-O0 --coverage -fno-inline -fno-inline-small-functions -fno-default-inline"
-        CXX: g++
-      run: |
-        cmake -DBUILD_TESTING=ON -DBUILD_LIB=ON ..
-        make -j4
-    - name: Run tests
-      working-directory: build
-      env:
-        CTEST_OUTPUT_ON_FAILURE: 1
-      run: ctest --timeout 5 -C Debug -j4
-    - name: Upload coverage to Codecov
-      working-directory: build
-      env:
-        CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
-      run: |
-        wget https://codecov.io/bash -O codecov
-        chmod +x codecov
-        ./codecov -t $CODECOV_TOKEN -B $GITHUB_REF -s test/
+      - uses: actions/checkout@v2
+      - name: Compile tests
+        working-directory: build
+        env:
+          CXXFLAGS: "--coverage -fno-inline"
+          CXX: g++
+        run: |
+          cmake -DENTT_BUILD_TESTING=ON -DENTT_BUILD_LIB=ON -DENTT_BUILD_EXAMPLE=ON ..
+          make -j4
+      - name: Run tests
+        working-directory: build
+        env:
+          CTEST_OUTPUT_ON_FAILURE: 1
+        run: ctest --timeout 30 -C Debug -j4
+      - name: Collect data
+        working-directory: build
+        run: |
+          sudo apt install lcov
+          lcov -c -d . -o coverage.info
+          lcov -l coverage.info
+      - name: Upload coverage to Codecov
+        uses: codecov/codecov-action@v2
+        with:
+          token: ${{ secrets.CODECOV_TOKEN }}
+          files: build/coverage.info
+          name: EnTT
+          fail_ci_if_error: true

.github/workflows/deploy.yml

@@ -20,8 +20,7 @@ jobs:
       working-directory: build
       env:
         PERSONAL_ACCESS_TOKEN: ${{ secrets.PERSONAL_ACCESS_TOKEN }}
-      run: |
-        git clone https://$GITHUB_ACTOR:$PERSONAL_ACCESS_TOKEN@github.com/$GITHUB_ACTOR/$GH_REPO.git
+      run: git clone https://$GITHUB_ACTOR:$PERSONAL_ACCESS_TOKEN@github.com/$GITHUB_ACTOR/$GH_REPO.git
     - name: Prepare formula
       working-directory: build
       run: |
@@ -37,4 +36,4 @@ jobs:
         git config --local user.name "$GITHUB_ACTOR"
         git add $FORMULA
         git commit -m "Update to ${{ github.ref }}"
-        git push --dry-run origin master
+        git push origin master

.github/workflows/sanitizer.yml

@@ -0,0 +1,36 @@
+name: sanitizer
+
+on: [push, pull_request]
+
+jobs:
+
+  linux:
+    timeout-minutes: 10
+
+    strategy:
+      matrix:
+        compiler: [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_USE_SANITIZER=ON -DENTT_BUILD_TESTING=ON -DENTT_BUILD_LIB=ON -DENTT_BUILD_EXAMPLE=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

.gitignore

@@ -5,6 +5,9 @@ conan/test_package/build
 *.user
 .idea
 .vscode
+.vs
+CMakeSettings.json
+cpp.hint
 
 # Bazel
 /bazel-*

AUTHORS

@@ -4,17 +4,21 @@ skypjack
 
 # Contributors
 
+alexames
 BenediktConze
 bjadamson
 ceeac
 ColinH
 corystegel
 Croydon
+cugone
 dbacchet
 dBagrat
 djarek
 DonKult
 drglove
+eliasdaler
+erez-o
 eugeneko
 gale83
 ghost
@@ -22,13 +26,16 @@ grdowns
 Green-Sky
 Innokentiy-Alaytsev
 Kerndog73
+Koward
 Lawrencemm
 markand
 mhammerc
 Milerius
 morbo84
 m-waka
+netpoetica
 NixAJ
+Oortonaut
 Paolo-Oliverio
 pgruenbacher
 prowolf

BUILD.bazel

@@ -9,7 +9,6 @@ cc_library(
     copts = select({
       "@bazel_tools//src/conditions:windows": _msvc_copts,
       "@bazel_tools//src/conditions:windows_msvc": _msvc_copts,
-      "@bazel_tools//src/conditions:windows_msys": _msvc_copts,
       "//conditions:default": _gcc_copts,
     }),
 )

CMakeLists.txt

@@ -30,49 +30,76 @@ project(
     VERSION ${ENTT_VERSION}
     DESCRIPTION "Gaming meets modern C++ - a fast and reliable entity-component system (ECS) and much more"
     HOMEPAGE_URL "https://github.com/skypjack/entt"
-    LANGUAGES CXX
+    LANGUAGES C CXX
 )
 
 if(NOT CMAKE_BUILD_TYPE)
     set(CMAKE_BUILD_TYPE Debug)
 endif()
 
-message("*")
-message("* ${PROJECT_NAME} v${PROJECT_VERSION} (${CMAKE_BUILD_TYPE})")
-message("* Copyright (c) 2017-2020 Michele Caini <michele.caini@gmail.com>")
-message("*")
-
-option(USE_LIBCPP "Use libc++ by adding -stdlib=libc++ flag if availbale." ON)
-option(USE_ASAN "Use address sanitizer by adding -fsanitize=address -fno-omit-frame-pointer flags" OFF)
+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 "*")
 
 #
 # Compiler stuff
 #
 
-if(NOT WIN32 AND USE_LIBCPP)
-    include(CheckCXXSourceCompiles)
-    include(CMakePushCheckState)
+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)
 
-    cmake_push_check_state()
+if(ENTT_USE_LIBCPP)
+    if(NOT WIN32)
+        include(CheckCXXSourceCompiles)
+        include(CMakePushCheckState)
 
-    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -stdlib=libc++")
+        cmake_push_check_state()
 
-    check_cxx_source_compiles("
-        #include<type_traits>
-        int main() { return std::is_same_v<int, char>; }
-    " HAS_LIBCPP)
+        set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -stdlib=libc++")
 
-    if(NOT HAS_LIBCPP)
-        message(WARNING "The option USE_LIBCPP is set (by default) but libc++ is not available. The flag will not be added to the target.")
+        check_cxx_source_compiles("
+            #include<type_traits>
+            int main() { return std::is_same_v<int, char>; }
+        " ENTT_HAS_LIBCPP)
+
+        cmake_pop_check_state()
     endif()
 
-    cmake_pop_check_state()
+    if(NOT ENTT_HAS_LIBCPP)
+        message(VERBOSE "The option ENTT_USE_LIBCPP is set but libc++ is not available. The flag will not be added to the target.")
+    endif()
+endif()
+
+if(ENTT_USE_SANITIZER)
+    if(CMAKE_CXX_COMPILER_ID MATCHES "Clang|GNU")
+        set(ENTT_HAS_SANITIZER TRUE CACHE BOOL "" FORCE)
+        mark_as_advanced(ENTT_HAS_SANITIZER)
+    endif()
+
+    if(NOT ENTT_HAS_SANITIZER)
+        message(VERBOSE "The option ENTT_USE_SANITIZER is set but sanitizer support is not available. The flags will not be added to the target.")
+    endif()
 endif()
 
 #
 # Add EnTT target
 #
 
+option(ENTT_INCLUDE_HEADERS "Add all EnTT headers to the EnTT target." OFF)
+option(ENTT_INCLUDE_NATVIS "Add EnTT natvis files to the EnTT target." OFF)
+
+if(ENTT_INCLUDE_NATVIS)
+    if(MSVC)
+        set(ENTT_HAS_NATVIS TRUE CACHE BOOL "" FORCE)
+        mark_as_advanced(ENTT_HAS_NATVIS)
+    endif()
+
+    if(NOT ENTT_HAS_NATVIS)
+        message(VERBOSE "The option ENTT_INCLUDE_NATVIS is set but natvis files are not supported. They will not be added to the target.")
+    endif()
+endif()
+
 include(GNUInstallDirs)
 
 add_library(EnTT INTERFACE)
@@ -85,16 +112,125 @@ target_include_directories(
         $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
 )
 
-if(USE_ASAN)
-    target_compile_options(EnTT INTERFACE $<$<CONFIG:Debug>:-fsanitize=address -fno-omit-frame-pointer>)
-    target_link_libraries(EnTT INTERFACE $<$<CONFIG:Debug>:-fsanitize=address -fno-omit-frame-pointer>)
+target_compile_features(EnTT INTERFACE cxx_std_17)
+
+if(ENTT_INCLUDE_HEADERS)
+    target_sources(
+        EnTT
+        INTERFACE
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/config/config.h>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/config/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/fwd.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/algorithm.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/any.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/attribute.h>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/compressed_pair.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/enum.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/family.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/fwd.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/hashed_string.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/ident.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/iterator.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/memory.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/monostate.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/tuple.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/type_info.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/type_traits.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/utility.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/component.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/entity.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/fwd.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/group.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/handle.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/helper.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/observer.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/organizer.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/registry.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/runtime_view.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/snapshot.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/sparse_set.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/storage.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/utility.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/view.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/locator/locator.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/adl_pointer.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/container.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/ctx.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/factory.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/fwd.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/meta.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/node.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/pointer.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/policy.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/range.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/resolve.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/template.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/type_traits.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/utility.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/platform/android-ndk-r17.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/poly/fwd.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/poly/poly.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/process/process.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/process/scheduler.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/resource/cache.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/resource/fwd.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/resource/handle.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/resource/loader.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/signal/delegate.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/signal/dispatcher.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/signal/emitter.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/signal/fwd.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/signal/sigh.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entt.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/fwd.hpp>
+    )
 endif()
 
-if(HAS_LIBCPP)
+if(ENTT_HAS_NATVIS)
+    target_sources(
+        EnTT
+        INTERFACE
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/config.natvis>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/container.natvis>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/core.natvis>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/entity.natvis>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/locator.natvis>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/meta.natvis>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/platform.natvis>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/poly.natvis>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/process.natvis>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/resource.natvis>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/signal.natvis>
+    )
+endif()
+
+if(ENTT_HAS_SANITIZER)
+    target_compile_options(EnTT INTERFACE $<$<CONFIG:Debug>:-fsanitize=address -fno-omit-frame-pointer -fsanitize=undefined>)
+    target_link_libraries(EnTT INTERFACE $<$<CONFIG:Debug>:-fsanitize=address -fno-omit-frame-pointer -fsanitize=undefined>)
+endif()
+
+if(ENTT_HAS_LIBCPP)
     target_compile_options(EnTT BEFORE INTERFACE -stdlib=libc++)
 endif()
 
-target_compile_features(EnTT INTERFACE cxx_std_17)
+#
+# Install pkg-config file
+#
+
+set(EnTT_PKGCONFIG ${CMAKE_CURRENT_BINARY_DIR}/entt.pc)
+
+configure_file(
+    ${EnTT_SOURCE_DIR}/cmake/in/entt.pc.in
+    ${EnTT_PKGCONFIG}
+    @ONLY
+)
+
+install(
+    FILES ${EnTT_PKGCONFIG}
+    DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig
+)
 
 #
 # Install EnTT
@@ -102,19 +238,10 @@ target_compile_features(EnTT INTERFACE cxx_std_17)
 
 include(CMakePackageConfigHelpers)
 
-if(${CMAKE_SYSTEM_NAME} STREQUAL "Windows")
-    set(CUSTOM_INSTALL_CONFIGDIR cmake)
-else()
-    set(CUSTOM_INSTALL_CONFIGDIR ${CMAKE_INSTALL_LIBDIR}/cmake/entt)
-endif()
-
-install(TARGETS EnTT EXPORT EnTTTargets)
-
-configure_package_config_file(
-    ${EnTT_SOURCE_DIR}/cmake/in/EnTTConfig.cmake.in
-    EnTTConfig.cmake
-    INSTALL_DESTINATION ${CUSTOM_INSTALL_CONFIGDIR}
-    PATH_VARS CMAKE_INSTALL_INCLUDEDIR
+install(
+    TARGETS EnTT
+    EXPORT EnTTTargets
+    ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
 )
 
 write_basic_package_version_file(
@@ -123,10 +250,22 @@ write_basic_package_version_file(
     COMPATIBILITY AnyNewerVersion
 )
 
+configure_package_config_file(
+    ${EnTT_SOURCE_DIR}/cmake/in/EnTTConfig.cmake.in
+    EnTTConfig.cmake
+    INSTALL_DESTINATION ${CMAKE_INSTALL_LIBDIR}/EnTT/cmake
+)
+
+export(
+    EXPORT EnTTTargets
+    FILE ${CMAKE_CURRENT_BINARY_DIR}/EnTTTargets.cmake
+    NAMESPACE EnTT::
+)
+
 install(
     EXPORT EnTTTargets
     FILE EnTTTargets.cmake
-    DESTINATION ${CUSTOM_INSTALL_CONFIGDIR}
+    DESTINATION ${CMAKE_INSTALL_LIBDIR}/EnTT/cmake
     NAMESPACE EnTT::
 )
 
@@ -134,23 +273,30 @@ install(
     FILES
         ${PROJECT_BINARY_DIR}/EnTTConfig.cmake
         ${PROJECT_BINARY_DIR}/EnTTConfigVersion.cmake
-    DESTINATION ${CUSTOM_INSTALL_CONFIGDIR}
+    DESTINATION ${CMAKE_INSTALL_LIBDIR}/EnTT/cmake
 )
 
 install(DIRECTORY src/ DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
 
+export(PACKAGE EnTT)
+
 #
 # Tests
 #
 
-option(BUILD_TESTING "Enable testing with ctest." OFF)
+option(ENTT_BUILD_TESTING "Enable building tests." OFF)
 
-if(BUILD_TESTING)
-    option(FIND_GTEST_PACKAGE "Enable finding gtest package." OFF)
-    option(BUILD_BENCHMARK "Build benchmark." OFF)
-    option(BUILD_LIB "Build lib example." OFF)
-    option(BUILD_SNAPSHOT "Build snapshot example." OFF)
+if(ENTT_BUILD_TESTING)
+    option(ENTT_FIND_GTEST_PACKAGE "Enable finding gtest package." OFF)
+    option(ENTT_BUILD_BENCHMARK "Build benchmark." OFF)
+    option(ENTT_BUILD_EXAMPLE "Build examples." OFF)
+    option(ENTT_BUILD_LIB "Build lib tests." OFF)
+    option(ENTT_BUILD_SNAPSHOT "Build snapshot test with Cereal." OFF)
 
+    set(ENTT_ID_TYPE std::uint32_t CACHE STRING "Type of identifiers to use for the tests")
+    set(ENTT_CXX_STD cxx_std_17 CACHE STRING "C++ standard revision to use for the tests")
+
+    include(CTest)
     enable_testing()
     add_subdirectory(test)
 endif()
@@ -159,9 +305,9 @@ endif()
 # Documentation
 #
 
-option(BUILD_DOCS "Enable building with documentation." OFF)
+option(ENTT_BUILD_DOCS "Enable building with documentation." OFF)
 
-if(BUILD_DOCS)
+if(ENTT_BUILD_DOCS)
     find_package(Doxygen 1.8)
 
     if(DOXYGEN_FOUND)
@@ -179,6 +325,7 @@ add_custom_target(
         .github/workflows/build.yml
         .github/workflows/coverage.yml
         .github/workflows/deploy.yml
+        .github/workflows/sanitizer.yml
         .github/FUNDING.yml
         AUTHORS
         CONTRIBUTING.md

LICENSE

@@ -1,6 +1,6 @@
 The MIT License (MIT)
 
-Copyright (c) 2017-2020 Michele Caini
+Copyright (c) 2017-2021 Michele Caini, author of EnTT
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal

README.md

@@ -1,23 +1,22 @@
-![EnTT: Gaming meets modern C++](https://user-images.githubusercontent.com/1812216/42513718-ee6e98d0-8457-11e8-9baf-8d83f61a3097.png)
+![EnTT: Gaming meets modern C++](https://user-images.githubusercontent.com/1812216/103550016-90752280-4ea8-11eb-8667-12ed2219e137.png)
 
 <!--
 @cond TURN_OFF_DOXYGEN
 -->
-[![GitHub version](https://badge.fury.io/gh/skypjack%2Fentt.svg)](https://github.com/skypjack/entt/releases)
 [![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/v8txVr)
+[![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/)
 [![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)
-[![Patreon](https://img.shields.io/badge/become-patron-red.svg)](https://www.patreon.com/bePatron?c=1772573)
 
 `EnTT` is a header-only, tiny and easy to use library for game programming and
-much more written in **modern C++**, mainly known for its innovative
-**entity-component-system (ECS)** model.<br/>
+much more written in **modern C++**.<br/>
 [Among others](https://github.com/skypjack/entt/wiki/EnTT-in-Action), it's used
-in [**Minecraft**](https://minecraft.net/en-us/attribution/) by Mojang and the
-[**ArcGIS Runtime SDKs**](https://developers.arcgis.com/arcgis-runtime/) by
-Esri.<br/>
+in [**Minecraft**](https://minecraft.net/en-us/attribution/) by Mojang, the
+[**ArcGIS Runtime SDKs**](https://developers.arcgis.com/arcgis-runtime/) by Esri
+and the amazing [**Ragdoll**](https://ragdolldynamics.com/).<br/>
 If you don't see your project in the list, please open an issue, submit a PR or
 add the [#entt](https://github.com/topics/entt) tag to your _topics_! :+1:
 
@@ -25,22 +24,21 @@ add the [#entt](https://github.com/topics/entt) tag to your _topics_! :+1:
 
 Do you want to **keep up with changes** or do you have a **question** that
 doesn't require you to open an issue?<br/>
-Join the [gitter channel](https://gitter.im/skypjack/entt) and meet other users
-like you. The more we are, the better for everyone.
+Join the [gitter channel](https://gitter.im/skypjack/entt) and the
+[discord server](https://discord.gg/5BjPWBd), meet other users like you. The
+more we are, the better for everyone.<br/>
+Don't forget to check the
+[FAQs](https://github.com/skypjack/entt/wiki/Frequently-Asked-Questions) and the
+[wiki](https://github.com/skypjack/entt/wiki) too. Your answers may already be
+there.
 
-Wondering why your **debug build** is so slow on Windows or how to represent a
-**hierarchy** with components?<br/>
-Check out the
-[FAQ](https://github.com/skypjack/entt/wiki/Frequently-Asked-Questions) and the
-[wiki](https://github.com/skypjack/entt/wiki) if you have these or other doubts,
-your answers may already be there.
+Do you want to support `EnTT`? Consider becoming a
+[**sponsor**](https://github.com/users/skypjack/sponsorship).
+Many thanks to [these people](https://skypjack.github.io/sponsorship/) and
+**special** thanks to:
 
-If you use `EnTT` and you want to say thanks or support the project, please
-**consider becoming a
-[sponsor](https://github.com/users/skypjack/sponsorship)**.<br/>
-You can help me make the difference.
-[Many thanks](https://skypjack.github.io/sponsorship/) to those who supported me
-and still support me today.
+[![mojang](https://user-images.githubusercontent.com/1812216/106253145-67ca1980-6217-11eb-9c0b-d93561b37098.png)](https://mojang.com)
+[![imgly](https://user-images.githubusercontent.com/1812216/106253726-271ed000-6218-11eb-98e0-c9c681925770.png)](https://img.ly/)
 
 # Table of Contents
 
@@ -48,16 +46,17 @@ and still support me today.
   * [Code Example](#code-example)
   * [Motivation](#motivation)
   * [Performance](#performance)
-* [Build Instructions](#build-instructions)
+* [Integration](#integration)
   * [Requirements](#requirements)
-  * [Library](#library)
-  * [Documentation](#documentation)
-  * [Tests](#tests)
-* [Packaging Tools](#packaging-tools)
+  * [CMake](#cmake)
+  * [Natvis support](#natvis-support)
+  * [Packaging Tools](#packaging-tools)
+  * [pkg-config](#pkg-config)
+* [Documentation](#documentation)
+* [Tests](#tests)
 * [EnTT in Action](#entt-in-action)
 * [Contributors](#contributors)
 * [License](#license)
-* [Support](#support)
 <!--
 @endcond TURN_OFF_DOXYGEN
 -->
@@ -75,41 +74,44 @@ This project started off as a pure entity-component system. Over time the
 codebase has grown as more and more classes and functionalities were added.<br/>
 Here is a brief, yet incomplete list of what it offers today:
 
-* Statically generated integer **identifiers** for types (assigned either at
-  compile-time or at runtime).
+* Built-in **RTTI system** mostly similar to the standard one.
 * A `constexpr` utility for human readable **resource names**.
-* A minimal **configuration system** built using the monostate pattern.
-* An incredibly fast **entity-component system** based on sparse sets, with its
-  own _pay for what you use_ policy to adjust performance and memory usage
-  according to the users' requirements.
+* Minimal **configuration system** built using the monostate pattern.
+* Incredibly fast **entity-component system** with its own _pay for what you
+  use_ policy.
 * Views and groups to iterate entities and components and allow different access
   patterns, from **perfect SoA** to fully random.
 * A lot of **facilities** built on top of the entity-component system to help
-  the users and avoid reinventing the wheel (dependencies, snapshot, actor
-  class, support for **reactive systems** and so on).
+  the users and avoid reinventing the wheel.
 * The smallest and most basic implementation of a **service locator** ever seen.
 * A built-in, non-intrusive and macro-free runtime **reflection system**.
+* **Static polymorphism** made simple and within everyone's reach.
+* A few homemade containers, like a sparse set based **dense hash map**.
 * A **cooperative scheduler** for processes of any type.
 * All that is needed for **resource management** (cache, loaders, handles).
-* Delegates, **signal handlers** (with built-in support for collectors) and a
-  tiny event dispatcher for immediate and delayed events to integrate in loops.
+* Delegates, **signal handlers** and a tiny event dispatcher.
 * A general purpose **event emitter** as a CRTP idiom based class template.
 * And **much more**! Check out the
   [**wiki**](https://github.com/skypjack/entt/wiki).
 
-Consider this list a work in progress as well as the project. The whole API is
+**Breaking news**:
+
+* 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.
 
-Currently, `EnTT` is tested on Linux, Microsoft Windows and OSX. It has proven
-to work also on both Android and iOS.<br/>
-Most likely it won't be problematic on other systems as well, but it hasn't been
-sufficiently tested so far.
+It is also known that `EnTT` is 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.
 
 ## Code Example
 
 ```cpp
 #include <entt/entt.hpp>
-#include <cstdint>
 
 struct position {
     float x;
@@ -122,45 +124,36 @@ struct velocity {
 };
 
 void update(entt::registry &registry) {
-    auto view = registry.view<position, velocity>();
+    auto view = registry.view<const position, velocity>();
 
-    for(auto entity: view) {
-        // gets only the components that are going to be used ...
+    // use a callback
+    view.each([](const auto &pos, auto &vel) { /* ... */ });
 
-        auto &vel = view.get<velocity>(entity);
-
-        vel.dx = 0.;
-        vel.dy = 0.;
+    // use an extended callback
+    view.each([](const auto entity, const auto &pos, auto &vel) { /* ... */ });
 
+    // use a range-for
+    for(auto [entity, pos, vel]: view.each()) {
         // ...
     }
-}
-
-void update(std::uint64_t dt, entt::registry &registry) {
-    registry.view<position, velocity>().each([dt](auto &pos, auto &vel) {
-        // gets all the components of the view at once ...
-
-        pos.x += vel.dx * dt;
-        pos.y += vel.dy * dt;
 
+    // use forward iterators and get only the components of interest
+    for(auto entity: view) {
+        auto &vel = view.get<velocity>(entity);
         // ...
-    });
+    }
 }
 
 int main() {
     entt::registry registry;
-    std::uint64_t dt = 16;
 
-    for(auto i = 0; i < 10; ++i) {
-        auto entity = registry.create();
-        registry.assign<position>(entity, i * 1.f, i * 1.f);
-        if(i % 2 == 0) { registry.assign<velocity>(entity, i * .1f, i * .1f); }
+    for(auto i = 0u; i < 10u; ++i) {
+        const auto entity = registry.create();
+        registry.emplace<position>(entity, i * 1.f, i * 1.f);
+        if(i % 2 == 0) { registry.emplace<velocity>(entity, i * .1f, i * .1f); }
     }
 
-    update(dt, registry);
     update(registry);
-
-    // ...
 }
 ```
 
@@ -189,8 +182,8 @@ reasons.
 
 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 `BUILD_BENCHMARK` option of `CMake` to `ON`, then evaluate yourself whether
-you're satisfied with the results or not.
+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/>
@@ -210,26 +203,7 @@ 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.
 
-# Build Instructions
-
-## Requirements
-
-To be able to use `EnTT`, users must provide a full-featured compiler that
-supports at least C++17.<br/>
-The requirements below are mandatory to compile the tests and to extract the
-documentation:
-
-* `CMake` version 3.7 or later.
-* `Doxygen` version 1.8 or later.
-
-Alternatively, [Bazel](https://bazel.build) is also supported as a build system
-(credits to [zaucy](https://github.com/zaucy) who offered to maintain it).<br/>
-In the documentation below I'll still refer to `CMake`, this being the official
-build system of the library.
-
-If you are looking for a C++14 version of `EnTT`, check out the git tag `cpp14`.
-
-## Library
+# Integration
 
 `EnTT` is a header-only library. This means that including the `entt.hpp` header
 is enough to include the library as a whole and use it. For those who are
@@ -250,48 +224,41 @@ Use the line below to include only the entity-component system instead:
 Then pass the proper `-I` argument to the compiler to add the `src` directory to
 the include paths.
 
-## Documentation
+## Requirements
 
-The documentation is based on [doxygen](http://www.doxygen.nl/).
-To build it:
+To be able to use `EnTT`, users must provide a full-featured compiler that
+supports at least C++17.<br/>
+The requirements below are mandatory to compile the tests and to extract the
+documentation:
 
-    $ cd build
-    $ cmake .. -DBUILD_DOCS=ON
-    $ make
+* `CMake` version 3.7 or later.
+* `Doxygen` version 1.8 or later.
 
-The API reference will be created in HTML format within the directory
-`build/docs/html`. To navigate it with your favorite browser:
+Alternatively, [Bazel](https://bazel.build) is also supported as a build system
+(credits to [zaucy](https://github.com/zaucy) who offered to maintain it).<br/>
+In the documentation below I'll still refer to `CMake`, this being the official
+build system of the library.
 
-    $ cd build
-    $ your_favorite_browser docs/html/index.html
+## CMake
 
-<!--
-@cond TURN_OFF_DOXYGEN
--->
-It's also available [online](https://skypjack.github.io/entt/) for the latest
-version, 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.
-<!--
-@endcond TURN_OFF_DOXYGEN
--->
+To use `EnTT` from a `CMake` project, just link an existing target to the
+`EnTT::EnTT` alias.<br/>
+The library offers everything you need for locating (as in `find_package`),
+embedding (as in `add_subdirectory`), fetching (as in `FetchContent`) or using
+it in many of the ways that you can think of and that involve `CMake`.<br/>
+Covering all possible cases would require a treaty and not a simple README file,
+but I'm confident that anyone reading this section also knows what it's about
+and can use `EnTT` from a `CMake` project without problems.
 
-## Tests
+## Natvis support
 
-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 `BUILD_TESTING` to `ON`.
+When using `CMake`, just enable the option `ENTT_INCLUDE_NATVIS` and enjoy
+it.<br/>
+Otherwise, most of the tools are covered via Natvis and all files can be found
+in the `natvis` directory, divided by module.<br/>
+If you spot errors or have suggestions, any contribution is welcome!
 
-To build the most basic set of tests:
-
-* `$ cd build`
-* `$ cmake -DBUILD_TESTING=ON ..`
-* `$ make`
-* `$ make test`
-
-Note that benchmarks are not part of this set.
-
-# Packaging Tools
+## Packaging Tools
 
 `EnTT` is available for some of the most known packaging tools. In particular:
 
@@ -310,6 +277,12 @@ Note that benchmarks are not part of this set.
   $ vcpkg install entt
   ```
 
+  Or you can use the `experimental` feature to test the latest changes:
+
+  ```
+  vcpkg install entt[experimental] --head
+  ```
+
   The `EnTT` port in `vcpkg` is kept up to date by Microsoft team members and
   community contributors.<br/>
   If the version is out of date, please
@@ -324,7 +297,84 @@ Note that benchmarks are not part of this set.
   brew install skypjack/entt/entt
   ```
 
-Consider this list a work in progress and help me to make it longer.
+* [`build2`](https://build2.org), build toolchain for developing and packaging C
+  and C++ code.<br/>
+  In order to use the [`entt`](https://cppget.org/entt) package in a `build2`
+  project, add the following line or a similar one to the `manifest` file:
+
+  ```
+  depends: entt ^3.0.0
+  ```
+
+  Also check that the configuration refers to a valid repository, so that the
+  package can be found by `build2`:
+
+  * [`cppget.org`](https://cppget.org), the open-source community central
+    repository, accessible as `https://pkg.cppget.org/1/stable`.
+
+  * [Package source repository](https://github.com/build2-packaging/entt):
+    accessible as either `https://github.com/build2-packaging/entt.git` or
+    `ssh://git@github.com/build2-packaging/entt.git`.
+    Feel free to [report issues](https://github.com/build2-packaging/entt) with
+    this package.
+
+  Both can be used with `bpkg add-repo` or added in a project
+  `repositories.manifest`. See the official
+  [documentation](https://build2.org/build2-toolchain/doc/build2-toolchain-intro.xhtml#guide-repositories)
+  for more details.
+
+Consider this list a work in progress and help me to make it longer if you like.
+
+## pkg-config
+
+`EnTT` also supports `pkg-config` (for some definition of _supports_ at least).
+A suitable file called `entt.pc` is generated and installed in a proper
+directory when running `CMake`.<br/>
+This should also make it easier to use with tools such as `Meson` or similar.
+
+# Documentation
+
+The documentation is based on [doxygen](http://www.doxygen.nl/). To build it:
+
+    $ cd build
+    $ 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:
+
+    $ cd build
+    $ your_favorite_browser docs/html/index.html
+
+<!--
+@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/>
+Moreover, there exists a [wiki](https://github.com/skypjack/entt/wiki) dedicated
+to the project where users can find all related documentation pages.
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# 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`.
+
+To build the most basic set of tests:
+
+* `$ cd build`
+* `$ cmake -DENTT_BUILD_TESTING=ON ..`
+* `$ make`
+* `$ make test`
+
+Note that benchmarks are not part of this set.
 
 <!--
 @cond TURN_OFF_DOXYGEN
@@ -346,18 +396,14 @@ open an issue or a PR and I'll be glad to add them to the list.
 
 # Contributors
 
-`EnTT` was written initially as a faster alternative to other well known and
-open source entity-component systems. Nowadays this library is moving its first
-steps. Much more will come in the future and hopefully I'm going to work on it
-for a long time.<br/>
-Requests for features, PR, suggestions ad feedback are highly appreciated.
+Requests for features, PRs, suggestions ad feedback are highly appreciated.
 
-If you find you can help me and want to contribute to the project with your
-experience or you do want to get part of the project for some other reasons,
-feel free to contact me directly (you can find the mail in the
+If you find you can help and want to contribute to the project with your
+experience or you do want to get part of the project for some other reason, feel
+free to contact me directly (you can find the mail in the
 [profile](https://github.com/skypjack)).<br/>
 I can't promise that each and every contribution will be accepted, but I can
-assure that I'll do my best to take them all seriously.
+assure that I'll do my best to take them all as soon as possible.
 
 If you decide to participate, please see the guidelines for
 [contributing](CONTRIBUTING.md) before to create issues or pull
@@ -371,25 +417,12 @@ know who has participated so far.
 
 # License
 
-Code and documentation Copyright (c) 2017-2020 Michele Caini.<br/>
-Logo Copyright (c) 2018-2020 Richard Caseres.
+Code and documentation Copyright (c) 2017-2021 Michele Caini.<br/>
+Colorful logo Copyright (c) 2018-2021 Richard Caseres.
 
 Code released under
-[the MIT license](https://github.com/skypjack/entt/blob/master/LICENSE).
+[the MIT license](https://github.com/skypjack/entt/blob/master/LICENSE).<br/>
 Documentation released under
 [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/).<br/>
-Logo released under
+All logos released under
 [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0/).
-
-<!--
-@cond TURN_OFF_DOXYGEN
--->
-# Support
-
-If you want to support this project, you can
-[offer me](https://github.com/users/skypjack/sponsorship) an espresso.<br/>
-If you find that it's not enough, feel free to
-[help me](https://www.paypal.me/skypjack) the way you prefer.
-<!--
-@endcond TURN_OFF_DOXYGEN
--->

TODO

@@ -1,33 +1,25 @@
 * long term feature: shared_ptr less locator and resource cache
-* custom allocators and EnTT allocator-aware in general (long term feature, I don't actually need it at the moment) - see #22
 * 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
-* meta: sort of meta view based on meta stuff to iterate entities, void * and meta info objects (remove runtime views, welcome reflection)
-* add opaque input iterators to views and groups that return tuples <entity, T &...> (proxy), multi-pass guaranteed
-* allow to replace std:: with custom implementations
-* custom (decoupled) pools ==> N-buffering, shared components, multi-model, hibitsets, and so on
 * add examples (and credits) from @alanjfs :)
-* static reflection, hint: template<> meta_type_t<Type>: meta_descriptor<name, func..., props..., etc...> (see #342)
-* observer: user defined filters (eg .replace<T, &function> or .group<T, U, &func>)
-* can we write a bool conv func for entt::entity that silently compares it to null?
-* reset... reset everywhere...
-* is it possible to make 0 the entity null?
-* document undocumented parts (entt::overload and a few others)
-* any-of rule for views/groups (eg entity has A and any of B/C/D)
-  - get -> all, exclude -> none
-* review prepare after clone and the others have been removed
-* unlock deploy.yml
 
-Next:
-* review pool<T>::remove, ::assign
-* replace observer class with observer functions
-* workflow to update the single include file automatically
-* workflow to update the doc automatically
+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
 
-* WIP:
- - deprecate snapshot, loader, ...
- - provide documentation to describe alternatives
-
-* WIP: snapshot rework/deprecation
- - remove snapshot/loader from registry, make them external (faster) tools
- - deprecate snapshot classes, update documentation to describe alternatives
+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
+* deprecate non-owning groups in favor of owning views and view packs, introduce lazy owning views
+* snapshot: support for range-based archives

cmake/in/EnTTConfig.cmake.in

@@ -1,11 +1,5 @@
-set(ENTT_VERSION "@PROJECT_VERSION@")
-
 @PACKAGE_INIT@
 
-set_and_check(ENTT_INCLUDE_DIRS "@PACKAGE_CMAKE_INSTALL_INCLUDEDIR@")
-
-if(NOT CMAKE_VERSION VERSION_LESS "3.0")
-    include("${CMAKE_CURRENT_LIST_DIR}/EnTTTargets.cmake")
-endif()
-
+set(EnTT_VERSION "@PROJECT_VERSION@")
+include("${CMAKE_CURRENT_LIST_DIR}/EnTTTargets.cmake")
 check_required_components("@PROJECT_NAME@")

cmake/in/entt.pc.in

@@ -0,0 +1,8 @@
+prefix=@CMAKE_INSTALL_PREFIX@
+includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@
+
+Name: EnTT
+Description: Gaming meets modern C++
+Url: https://github.com/skypjack/entt
+Version: @ENTT_VERSION@
+Cflags: -I${includedir}

conan/test_package/test_package.cpp

@@ -43,8 +43,8 @@ int main() {
 
     for(auto i = 0; i < 10; ++i) {
         auto entity = registry.create();
-        registry.assign<position>(entity, i * 1.f, i * 1.f);
-        if(i % 2 == 0) { registry.assign<velocity>(entity, i * .1f, i * .1f); }
+        registry.emplace<position>(entity, i * 1.f, i * 1.f);
+        if(i % 2 == 0) { registry.emplace<velocity>(entity, i * .1f, i * .1f); }
     }
 
     update(dt, registry);

docs/CMakeLists.txt

@@ -16,6 +16,8 @@ add_custom_target(
     VERBATIM
     SOURCES
         dox/extra.dox
+        md/config.md
+        md/container.md
         md/core.md
         md/entity.md
         md/faq.md
@@ -23,9 +25,12 @@ add_custom_target(
         md/links.md
         md/locator.md
         md/meta.md
+        md/poly.md
         md/process.md
+        md/reference.md
         md/resource.md
         md/signal.md
+        md/unreal.md
         doxy.in
 )
 

docs/doxy.in

@@ -1,4 +1,4 @@
-# Doxyfile 1.8.16
+# Doxyfile 1.9.1
 
 # This file describes the settings to be used by the documentation system
 # doxygen (www.doxygen.org) for a project.
@@ -217,6 +217,14 @@ QT_AUTOBRIEF           = NO
 
 MULTILINE_CPP_IS_BRIEF = NO
 
+# By default Python docstrings are displayed as preformatted text and doxygen's
+# special commands cannot be used. By setting PYTHON_DOCSTRING to NO the
+# doxygen's special commands can be used and the contents of the docstring
+# documentation blocks is shown as doxygen documentation.
+# The default value is: YES.
+
+PYTHON_DOCSTRING       = YES
+
 # If the INHERIT_DOCS tag is set to YES then an undocumented member inherits the
 # documentation from any documented member that it re-implements.
 # The default value is: YES.
@@ -253,12 +261,6 @@ TAB_SIZE               = 4
 
 ALIASES                =
 
-# This tag can be used to specify a number of word-keyword mappings (TCL only).
-# A mapping has the form "name=value". For example adding "class=itcl::class"
-# will allow you to use the command class in the itcl::class meaning.
-
-TCL_SUBST              =
-
 # Set the OPTIMIZE_OUTPUT_FOR_C tag to YES if your project consists of C sources
 # only. Doxygen will then generate output that is more tailored for C. For
 # instance, some of the names that are used will be different. The list of all
@@ -299,19 +301,22 @@ OPTIMIZE_OUTPUT_SLICE  = NO
 # parses. With this tag you can assign which parser to use for a given
 # extension. Doxygen has a built-in mapping, but you can override or extend it
 # using this tag. The format is ext=language, where ext is a file extension, and
-# language is one of the parsers supported by doxygen: IDL, Java, Javascript,
-# Csharp (C#), C, C++, D, PHP, md (Markdown), Objective-C, Python, Slice,
+# language is one of the parsers supported by doxygen: IDL, Java, JavaScript,
+# Csharp (C#), C, C++, D, PHP, md (Markdown), Objective-C, Python, Slice, VHDL,
 # Fortran (fixed format Fortran: FortranFixed, free formatted Fortran:
 # FortranFree, unknown formatted Fortran: Fortran. In the later case the parser
 # tries to guess whether the code is fixed or free formatted code, this is the
-# default for Fortran type files), VHDL, tcl. For instance to make doxygen treat
-# .inc files as Fortran files (default is PHP), and .f files as C (default is
-# Fortran), use: inc=Fortran f=C.
+# default for Fortran type files). For instance to make doxygen treat .inc files
+# as Fortran files (default is PHP), and .f files as C (default is Fortran),
+# use: inc=Fortran f=C.
 #
 # Note: For files without extension you can use no_extension as a placeholder.
 #
 # Note that for custom extensions you also need to set FILE_PATTERNS otherwise
-# the files are not read by doxygen.
+# the files are not read by doxygen. When specifying no_extension you should add
+# * to the FILE_PATTERNS.
+#
+# Note see also the list of default file extension mappings.
 
 EXTENSION_MAPPING      =
 
@@ -445,6 +450,19 @@ TYPEDEF_HIDES_STRUCT   = NO
 
 LOOKUP_CACHE_SIZE      = 0
 
+# The NUM_PROC_THREADS specifies the number threads doxygen is allowed to use
+# during processing. When set to 0 doxygen will based this on the number of
+# cores available in the system. You can set it explicitly to a value larger
+# than 0 to get more control over the balance between CPU load and processing
+# speed. At this moment only the input processing can be done using multiple
+# threads. Since this is still an experimental feature the default is set to 1,
+# which efficively disables parallel processing. Please report any issues you
+# encounter. Generating dot graphs in parallel is controlled by the
+# DOT_NUM_THREADS setting.
+# Minimum value: 0, maximum value: 32, default value: 1.
+
+NUM_PROC_THREADS       = 1
+
 #---------------------------------------------------------------------------
 # Build related configuration options
 #---------------------------------------------------------------------------
@@ -508,6 +526,13 @@ EXTRACT_LOCAL_METHODS  = NO
 
 EXTRACT_ANON_NSPACES   = NO
 
+# If this flag is set to YES, the name of an unnamed parameter in a declaration
+# will be determined by the corresponding definition. By default unnamed
+# parameters remain unnamed in the output.
+# The default value is: YES.
+
+RESOLVE_UNNAMED_PARAMS = YES
+
 # If the HIDE_UNDOC_MEMBERS tag is set to YES, doxygen will hide all
 # undocumented members inside documented classes or files. If set to NO these
 # members will be included in the various overviews, but no documentation
@@ -525,8 +550,8 @@ HIDE_UNDOC_MEMBERS     = NO
 HIDE_UNDOC_CLASSES     = NO
 
 # If the HIDE_FRIEND_COMPOUNDS tag is set to YES, doxygen will hide all friend
-# (class|struct|union) declarations. If set to NO, these declarations will be
-# included in the documentation.
+# declarations. If set to NO, these declarations will be included in the
+# documentation.
 # The default value is: NO.
 
 HIDE_FRIEND_COMPOUNDS  = NO
@@ -545,11 +570,18 @@ HIDE_IN_BODY_DOCS      = NO
 
 INTERNAL_DOCS          = NO
 
-# If the CASE_SENSE_NAMES tag is set to NO then doxygen will only generate file
-# names in lower-case letters. If set to YES, upper-case letters are also
-# allowed. This is useful if you have classes or files whose names only differ
-# in case and if your file system supports case sensitive file names. Windows
-# (including Cygwin) ands Mac users are advised to set this option to NO.
+# With the correct setting of option CASE_SENSE_NAMES doxygen will better be
+# able to match the capabilities of the underlying filesystem. In case the
+# filesystem is case sensitive (i.e. it supports files in the same directory
+# whose names only differ in casing), the option must be set to YES to properly
+# deal with such files in case they appear in the input. For filesystems that
+# are not case sensitive the option should be be set to NO to properly deal with
+# output files written for symbols that only differ in casing, such as for two
+# classes, one named CLASS and the other named Class, and to also support
+# references to files without having to specify the exact matching casing. On
+# Windows (including Cygwin) and MacOS, users should typically set this option
+# to NO, whereas on Linux or other Unix flavors it should typically be set to
+# YES.
 # The default value is: system dependent.
 
 CASE_SENSE_NAMES       = YES
@@ -788,7 +820,10 @@ WARN_IF_DOC_ERROR      = YES
 WARN_NO_PARAMDOC       = YES
 
 # If the WARN_AS_ERROR tag is set to YES then doxygen will immediately stop when
-# a warning is encountered.
+# a warning is encountered. If the WARN_AS_ERROR tag is set to FAIL_ON_WARNINGS
+# then doxygen will continue running as if WARN_AS_ERROR tag is set to NO, but
+# at the end of the doxygen process doxygen will return with a non-zero status.
+# Possible values are: NO, YES and FAIL_ON_WARNINGS.
 # The default value is: NO.
 
 WARN_AS_ERROR          = NO
@@ -826,8 +861,8 @@ INPUT                  = @DOXY_SOURCE_DIRECTORY@ \
 # This tag can be used to specify the character encoding of the source files
 # that doxygen parses. Internally doxygen uses the UTF-8 encoding. Doxygen uses
 # libiconv (or the iconv built into libc) for the transcoding. See the libiconv
-# documentation (see: https://www.gnu.org/software/libiconv/) for the list of
-# possible encodings.
+# documentation (see:
+# https://www.gnu.org/software/libiconv/) for the list of possible encodings.
 # The default value is: UTF-8.
 
 INPUT_ENCODING         = UTF-8
@@ -840,11 +875,15 @@ INPUT_ENCODING         = UTF-8
 # need to set EXTENSION_MAPPING for the extension otherwise the files are not
 # read by doxygen.
 #
+# Note the list of default checked file patterns might differ from the list of
+# default file extension mappings.
+#
 # If left blank the following patterns are tested:*.c, *.cc, *.cxx, *.cpp,
 # *.c++, *.java, *.ii, *.ixx, *.ipp, *.i++, *.inl, *.idl, *.ddl, *.odl, *.h,
 # *.hh, *.hxx, *.hpp, *.h++, *.cs, *.d, *.php, *.php4, *.php5, *.phtml, *.inc,
-# *.m, *.markdown, *.md, *.mm, *.dox, *.py, *.pyw, *.f90, *.f95, *.f03, *.f08,
-# *.f, *.for, *.tcl, *.vhd, *.vhdl, *.ucf, *.qsf and *.ice.
+# *.m, *.markdown, *.md, *.mm, *.dox (to be provided as doxygen C comment),
+# *.py, *.pyw, *.f90, *.f95, *.f03, *.f08, *.f18, *.f, *.for, *.vhd, *.vhdl,
+# *.ucf, *.qsf and *.ice.
 
 FILE_PATTERNS          = *.h \
                          *.hpp \
@@ -1062,16 +1101,22 @@ USE_HTAGS              = NO
 VERBATIM_HEADERS       = YES
 
 # If the CLANG_ASSISTED_PARSING tag is set to YES then doxygen will use the
-# clang parser (see: http://clang.llvm.org/) for more accurate parsing at the
-# cost of reduced performance. This can be particularly helpful with template
-# rich C++ code for which doxygen's built-in parser lacks the necessary type
-# information.
+# clang parser (see:
+# http://clang.llvm.org/) for more accurate parsing at the cost of reduced
+# performance. This can be particularly helpful with template rich C++ code for
+# which doxygen's built-in parser lacks the necessary type information.
 # Note: The availability of this option depends on whether or not doxygen was
 # generated with the -Duse_libclang=ON option for CMake.
 # The default value is: NO.
 
 CLANG_ASSISTED_PARSING = NO
 
+# If clang assisted parsing is enabled and the CLANG_ADD_INC_PATHS tag is set to
+# YES then doxygen will add the directory of each input to the include path.
+# The default value is: YES.
+
+CLANG_ADD_INC_PATHS    = YES
+
 # If clang assisted parsing is enabled you can provide the compiler with command
 # line options that you would normally use when invoking the compiler. Note that
 # the include paths will already be set by doxygen for the files and directories
@@ -1081,10 +1126,13 @@ CLANG_ASSISTED_PARSING = NO
 CLANG_OPTIONS          =
 
 # If clang assisted parsing is enabled you can provide the clang parser with the
-# path to the compilation database (see:
-# http://clang.llvm.org/docs/HowToSetupToolingForLLVM.html) used when the files
-# were built. This is equivalent to specifying the "-p" option to a clang tool,
-# such as clang-check. These options will then be passed to the parser.
+# path to the directory containing a file called compile_commands.json. This
+# file is the compilation database (see:
+# http://clang.llvm.org/docs/HowToSetupToolingForLLVM.html) containing the
+# options used when the source files were built. This is equivalent to
+# specifying the -p option to a clang tool, such as clang-check. These options
+# will then be passed to the parser. Any options specified with CLANG_OPTIONS
+# will be added as well.
 # Note: The availability of this option depends on whether or not doxygen was
 # generated with the -Duse_libclang=ON option for CMake.
 
@@ -1101,13 +1149,6 @@ CLANG_DATABASE_PATH    =
 
 ALPHABETICAL_INDEX     = YES
 
-# The COLS_IN_ALPHA_INDEX tag can be used to specify the number of columns in
-# which the alphabetical index list will be split.
-# Minimum value: 1, maximum value: 20, default value: 5.
-# This tag requires that the tag ALPHABETICAL_INDEX is set to YES.
-
-COLS_IN_ALPHA_INDEX    = 5
-
 # In case all classes in a project start with a common prefix, all classes will
 # be put under the same header in the alphabetical index. The IGNORE_PREFIX tag
 # can be used to specify a prefix (or a list of prefixes) that should be ignored
@@ -1246,9 +1287,9 @@ HTML_TIMESTAMP         = NO
 
 # If the HTML_DYNAMIC_MENUS tag is set to YES then the generated HTML
 # documentation will contain a main index with vertical navigation menus that
-# are dynamically created via Javascript. If disabled, the navigation index will
+# are dynamically created via JavaScript. If disabled, the navigation index will
 # consists of multiple levels of tabs that are statically embedded in every HTML
-# page. Disable this option to support browsers that do not have Javascript,
+# page. Disable this option to support browsers that do not have JavaScript,
 # like the Qt help browser.
 # The default value is: YES.
 # This tag requires that the tag GENERATE_HTML is set to YES.
@@ -1278,10 +1319,11 @@ HTML_INDEX_NUM_ENTRIES = 100
 
 # If the GENERATE_DOCSET tag is set to YES, additional index files will be
 # generated that can be used as input for Apple's Xcode 3 integrated development
-# environment (see: https://developer.apple.com/xcode/), introduced with OSX
-# 10.5 (Leopard). To create a documentation set, doxygen will generate a
-# Makefile in the HTML output directory. Running make will produce the docset in
-# that directory and running make install will install the docset in
+# environment (see:
+# https://developer.apple.com/xcode/), introduced with OSX 10.5 (Leopard). To
+# create a documentation set, doxygen will generate a Makefile in the HTML
+# output directory. Running make will produce the docset in that directory and
+# running make install will install the docset in
 # ~/Library/Developer/Shared/Documentation/DocSets so that Xcode will find it at
 # startup. See https://developer.apple.com/library/archive/featuredarticles/Doxy
 # genXcode/_index.html for more information.
@@ -1323,8 +1365,8 @@ DOCSET_PUBLISHER_NAME  = Publisher
 # If the GENERATE_HTMLHELP tag is set to YES then doxygen generates three
 # additional HTML index files: index.hhp, index.hhc, and index.hhk. The
 # index.hhp is a project file that can be read by Microsoft's HTML Help Workshop
-# (see: https://www.microsoft.com/en-us/download/details.aspx?id=21138) on
-# Windows.
+# (see:
+# https://www.microsoft.com/en-us/download/details.aspx?id=21138) on Windows.
 #
 # The HTML Help Workshop contains a compiler that can convert all HTML output
 # generated by doxygen into a single compiled HTML file (.chm). Compiled HTML
@@ -1354,7 +1396,7 @@ CHM_FILE               =
 HHC_LOCATION           =
 
 # The GENERATE_CHI flag controls if a separate .chi index file is generated
-# (YES) or that it should be included in the master .chm file (NO).
+# (YES) or that it should be included in the main .chm file (NO).
 # The default value is: NO.
 # This tag requires that the tag GENERATE_HTMLHELP is set to YES.
 
@@ -1399,7 +1441,8 @@ QCH_FILE               =
 
 # The QHP_NAMESPACE tag specifies the namespace to use when generating Qt Help
 # Project output. For more information please see Qt Help Project / Namespace
-# (see: https://doc.qt.io/archives/qt-4.8/qthelpproject.html#namespace).
+# (see:
+# https://doc.qt.io/archives/qt-4.8/qthelpproject.html#namespace).
 # The default value is: org.doxygen.Project.
 # This tag requires that the tag GENERATE_QHP is set to YES.
 
@@ -1407,8 +1450,8 @@ QHP_NAMESPACE          = org.doxygen.Project
 
 # The QHP_VIRTUAL_FOLDER tag specifies the namespace to use when generating Qt
 # Help Project output. For more information please see Qt Help Project / Virtual
-# Folders (see: https://doc.qt.io/archives/qt-4.8/qthelpproject.html#virtual-
-# folders).
+# Folders (see:
+# https://doc.qt.io/archives/qt-4.8/qthelpproject.html#virtual-folders).
 # The default value is: doc.
 # This tag requires that the tag GENERATE_QHP is set to YES.
 
@@ -1416,16 +1459,16 @@ QHP_VIRTUAL_FOLDER     = doc
 
 # If the QHP_CUST_FILTER_NAME tag is set, it specifies the name of a custom
 # filter to add. For more information please see Qt Help Project / Custom
-# Filters (see: https://doc.qt.io/archives/qt-4.8/qthelpproject.html#custom-
-# filters).
+# Filters (see:
+# https://doc.qt.io/archives/qt-4.8/qthelpproject.html#custom-filters).
 # This tag requires that the tag GENERATE_QHP is set to YES.
 
 QHP_CUST_FILTER_NAME   =
 
 # The QHP_CUST_FILTER_ATTRS tag specifies the list of the attributes of the
 # custom filter to add. For more information please see Qt Help Project / Custom
-# Filters (see: https://doc.qt.io/archives/qt-4.8/qthelpproject.html#custom-
-# filters).
+# Filters (see:
+# https://doc.qt.io/archives/qt-4.8/qthelpproject.html#custom-filters).
 # This tag requires that the tag GENERATE_QHP is set to YES.
 
 QHP_CUST_FILTER_ATTRS  =
@@ -1437,9 +1480,9 @@ QHP_CUST_FILTER_ATTRS  =
 
 QHP_SECT_FILTER_ATTRS  =
 
-# The QHG_LOCATION tag can be used to specify the location of Qt's
-# qhelpgenerator. If non-empty doxygen will try to run qhelpgenerator on the
-# generated .qhp file.
+# The QHG_LOCATION tag can be used to specify the location (absolute path
+# including file name) of Qt's qhelpgenerator. If non-empty doxygen will try to
+# run qhelpgenerator on the generated .qhp file.
 # This tag requires that the tag GENERATE_QHP is set to YES.
 
 QHG_LOCATION           =
@@ -1516,6 +1559,17 @@ TREEVIEW_WIDTH         = 250
 
 EXT_LINKS_IN_WINDOW    = NO
 
+# If the HTML_FORMULA_FORMAT option is set to svg, doxygen will use the pdf2svg
+# tool (see https://github.com/dawbarton/pdf2svg) or inkscape (see
+# https://inkscape.org) to generate formulas as SVG images instead of PNGs for
+# the HTML output. These images will generally look nicer at scaled resolutions.
+# Possible values are: png (the default) and svg (looks nicer but requires the
+# pdf2svg or inkscape tool).
+# The default value is: png.
+# This tag requires that the tag GENERATE_HTML is set to YES.
+
+HTML_FORMULA_FORMAT    = png
+
 # Use this tag to change the font size of LaTeX formulas included as images in
 # the HTML documentation. When you change the font size after a successful
 # doxygen run you need to manually remove any form_*.png images from the HTML
@@ -1536,8 +1590,14 @@ FORMULA_FONTSIZE       = 10
 
 FORMULA_TRANSPARENT    = YES
 
+# The FORMULA_MACROFILE can contain LaTeX \newcommand and \renewcommand commands
+# to create new LaTeX commands to be used in formulas as building blocks. See
+# the section "Including formulas" for details.
+
+FORMULA_MACROFILE      =
+
 # Enable the USE_MATHJAX option to render LaTeX formulas using MathJax (see
-# https://www.mathjax.org) which uses client side Javascript for the rendering
+# https://www.mathjax.org) which uses client side JavaScript for the rendering
 # instead of using pre-rendered bitmaps. Use this if you do not have LaTeX
 # installed or if you want to formulas look prettier in the HTML output. When
 # enabled you may also need to install MathJax separately and configure the path
@@ -1549,7 +1609,7 @@ USE_MATHJAX            = NO
 
 # When MathJax is enabled you can set the default output format to be used for
 # the MathJax output. See the MathJax site (see:
-# http://docs.mathjax.org/en/latest/output.html) for more details.
+# http://docs.mathjax.org/en/v2.7-latest/output.html) for more details.
 # Possible values are: HTML-CSS (which is slower, but has the best
 # compatibility), NativeMML (i.e. MathML) and SVG.
 # The default value is: HTML-CSS.
@@ -1565,7 +1625,7 @@ MATHJAX_FORMAT         = HTML-CSS
 # Content Delivery Network so you can quickly see the result without installing
 # MathJax. However, it is strongly recommended to install a local copy of
 # MathJax from https://www.mathjax.org before deployment.
-# The default value is: https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.5/.
+# The default value is: https://cdn.jsdelivr.net/npm/mathjax@2.
 # This tag requires that the tag USE_MATHJAX is set to YES.
 
 MATHJAX_RELPATH        = https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.5/
@@ -1579,7 +1639,8 @@ MATHJAX_EXTENSIONS     =
 
 # The MATHJAX_CODEFILE tag can be used to specify a file with javascript pieces
 # of code that will be used on startup of the MathJax code. See the MathJax site
-# (see: http://docs.mathjax.org/en/latest/output.html) for more details. For an
+# (see:
+# http://docs.mathjax.org/en/v2.7-latest/output.html) for more details. For an
 # example see the documentation.
 # This tag requires that the tag USE_MATHJAX is set to YES.
 
@@ -1607,7 +1668,7 @@ MATHJAX_CODEFILE       =
 SEARCHENGINE           = YES
 
 # When the SERVER_BASED_SEARCH tag is enabled the search engine will be
-# implemented using a web server instead of a web client using Javascript. There
+# implemented using a web server instead of a web client using JavaScript. There
 # are two flavors of web server based searching depending on the EXTERNAL_SEARCH
 # setting. When disabled, doxygen will generate a PHP script for searching and
 # an index file used by the script. When EXTERNAL_SEARCH is enabled the indexing
@@ -1626,7 +1687,8 @@ SERVER_BASED_SEARCH    = NO
 #
 # Doxygen ships with an example indexer (doxyindexer) and search engine
 # (doxysearch.cgi) which are based on the open source search engine library
-# Xapian (see: https://xapian.org/).
+# Xapian (see:
+# https://xapian.org/).
 #
 # See the section "External Indexing and Searching" for details.
 # The default value is: NO.
@@ -1639,8 +1701,9 @@ EXTERNAL_SEARCH        = NO
 #
 # Doxygen ships with an example indexer (doxyindexer) and search engine
 # (doxysearch.cgi) which are based on the open source search engine library
-# Xapian (see: https://xapian.org/). See the section "External Indexing and
-# Searching" for details.
+# Xapian (see:
+# https://xapian.org/). See the section "External Indexing and Searching" for
+# details.
 # This tag requires that the tag SEARCHENGINE is set to YES.
 
 SEARCHENGINE_URL       =
@@ -1804,9 +1867,11 @@ LATEX_EXTRA_FILES      =
 
 PDF_HYPERLINKS         = YES
 
-# If the USE_PDFLATEX tag is set to YES, doxygen will use pdflatex to generate
-# the PDF file directly from the LaTeX files. Set this option to YES, to get a
-# higher quality PDF documentation.
+# If the USE_PDFLATEX tag is set to YES, doxygen will use the engine as
+# specified with LATEX_CMD_NAME to generate the PDF file directly from the LaTeX
+# files. Set this option to YES, to get a higher quality PDF documentation.
+#
+# See also section LATEX_CMD_NAME for selecting the engine.
 # The default value is: YES.
 # This tag requires that the tag GENERATE_LATEX is set to YES.
 
@@ -2100,7 +2165,7 @@ ENABLE_PREPROCESSING   = YES
 # The default value is: NO.
 # This tag requires that the tag ENABLE_PREPROCESSING is set to YES.
 
-MACRO_EXPANSION        = NO
+MACRO_EXPANSION        = YES
 
 # If the EXPAND_ONLY_PREDEF and MACRO_EXPANSION tags are both set to YES then
 # the macro expansion is limited to the macros specified with the PREDEFINED and
@@ -2317,10 +2382,32 @@ UML_LOOK               = NO
 # but if the number exceeds 15, the total amount of fields shown is limited to
 # 10.
 # Minimum value: 0, maximum value: 100, default value: 10.
-# This tag requires that the tag HAVE_DOT is set to YES.
+# This tag requires that the tag UML_LOOK is set to YES.
 
 UML_LIMIT_NUM_FIELDS   = 10
 
+# If the DOT_UML_DETAILS tag is set to NO, doxygen will show attributes and
+# methods without types and arguments in the UML graphs. If the DOT_UML_DETAILS
+# tag is set to YES, doxygen will add type and arguments for attributes and
+# methods in the UML graphs. If the DOT_UML_DETAILS tag is set to NONE, doxygen
+# will not generate fields with class member information in the UML graphs. The
+# class diagrams will look similar to the default class diagrams but using UML
+# notation for the relationships.
+# Possible values are: NO, YES and NONE.
+# The default value is: NO.
+# This tag requires that the tag UML_LOOK is set to YES.
+
+DOT_UML_DETAILS        = NO
+
+# The DOT_WRAP_THRESHOLD tag can be used to set the maximum number of characters
+# to display on a single line. If the actual line length exceeds this threshold
+# significantly it will wrapped across multiple lines. Some heuristics are apply
+# to avoid ugly line breaks.
+# Minimum value: 0, maximum value: 1000, default value: 17.
+# This tag requires that the tag HAVE_DOT is set to YES.
+
+DOT_WRAP_THRESHOLD     = 17
+
 # If the TEMPLATE_RELATIONS tag is set to YES then the inheritance and
 # collaboration graphs will show the relations between templates and their
 # instances.
@@ -2512,9 +2599,11 @@ DOT_MULTI_TARGETS      = NO
 
 GENERATE_LEGEND        = YES
 
-# If the DOT_CLEANUP tag is set to YES, doxygen will remove the intermediate dot
+# If the DOT_CLEANUP tag is set to YES, doxygen will remove the intermediate
 # files that are used to generate the various graphs.
+#
+# Note: This setting is not only used for dot files but also for msc and
+# plantuml temporary files.
 # The default value is: YES.
-# This tag requires that the tag HAVE_DOT is set to YES.
 
 DOT_CLEANUP            = YES

docs/md/config.md

@@ -0,0 +1,111 @@
+# Crash Course: configuration
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [Definitions](#definitions)
+  * [ENTT_NOEXCEPTION](#entt_noexcept)
+  * [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_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/>
+In the vast majority of cases, users will have no interest in changing the
+default parameters. For all other cases, the list of possible configurations
+with which it's possible to adjust the behavior of the library at runtime can be
+found below.
+
+# Definitions
+
+All options are intended as parameters to the compiler (or user-defined macros
+within the compilation units, if preferred).<br/>
+Each parameter can result in internal library definitions. It's not recommended
+to try to also modify these definitions, since there is no guarantee that they
+will remain stable over time unlike the options below.
+
+## ENTT_NOEXCEPTION
+
+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`.
+
+## 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
+thread-safe by means of this definition.
+
+## ENTT_ID_TYPE
+
+`entt::id_type` is directly controlled by this definition and widely used within
+the library.<br/>
+By default, its type is `std::uint32_t`. However, users can define a different
+default type if necessary.
+
+## ENTT_SPARSE_PAGE
+
+It's known that the ECS module of `EnTT` is based on _sparse sets_. What is less
+known perhaps is that the sparse arrays are paged to reduce memory usage.<br/>
+Default size of pages (that is, the number of elements they contain) is 4096 but
+users can adjust it if appropriate. In all case, the chosen value **must** be a
+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/>
+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.
+
+## ENTT_ASSERT
+
+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.
+
+### 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_NO_ETO
+
+In order to reduce memory consumption and increase performance, empty types are
+never stored by the ECS module of `EnTT`.<br/>
+Use this variable to treat these types like all others and therefore to create a
+dedicated storage for them.
+
+## ENTT_STANDARD_CPP
+
+`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/>
+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

@@ -0,0 +1,56 @@
+# Crash Course: containers
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [Containers](#containers)
+  * [Dense hash map](#dense-hash-map)
+  * [Dense hash set](#dense-hash-set)
+
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+The standard C++ library offers a wide range of containers and it's really
+difficult to do better (although it's very easy to do worse, as many examples
+available online demonstrate).<br/>
+`EnTT` doesn't try in any way to replace what is offered by the standard. Quite
+the opposite, given the widespread use that is made of standard containers.<br/>
+However, the library also tries to fill a gap in features and functionality by
+making available some containers initially developed for internal use.
+
+This section of the library is likely to grow larger over time. However, for the
+moment it's quite small and mainly aimed at satisfying some internal needs.<br/>
+For all containers made available, full test coverage and stability over time is
+guaranteed as usual.
+
+# Containers
+
+## Dense hash map
+
+The dense hash map made available in `EnTT` is a map 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_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.

docs/md/core.md

@@ -6,17 +6,31 @@
 # Table of Contents
 
 * [Introduction](#introduction)
-* [Compile-time identifiers](#compile-time-identifiers)
-* [Runtime identifiers](#runtime-identifiers)
+* [Unique sequential identifiers](#unique-sequential-identifiers)
+  * [Compile-time generator](#compile-time-generator)
+  * [Runtime generator](#runtime-generator)
 * [Hashed strings](#hashed-strings)
   * [Wide characters](wide-characters)
   * [Conflicts](#conflicts)
 * [Monostate](#monostate)
-* [Type info](#type-info)
-  * [Almost unique identifiers](#almost-unique-identifiers)
-* [Traits](#traits)
-  * [Member class type](#member-class-type)
-  * [Tags](#tags)
+* [Any as in any type](#any-as-in-any-type)
+  * [Small buffer optimization](#small-buffer-optimization)
+  * [Alignment requirement](#alignment-requirement)
+* [Type support](#type-support)
+  * [Built-in RTTI support](#built-in-rtti-support)
+    * [Type info](#type-info)
+    * [Almost unique identifiers](#almost-unique-identifiers)
+  * [Type traits](#type-traits)
+    * [Size of](#size-of)
+    * [Is applicable](#is-applicable)
+    * [Constness as](#constness-as)
+    * [Member class type](#member-class-type)
+    * [Integral constant](#integral-constant)
+    * [Tag](#tag)
+    * [Type list and value list](#type-list-and-value-list)
+* [Compressed pair](#compressed-pair)
+* [Enum as bitmask](#enum-as-bitmask)
+* [Utilities](#utilities)
 <!--
 @endcond TURN_OFF_DOXYGEN
 -->
@@ -28,19 +42,20 @@ of the library itself.<br/>
 Hardly users will include these features in their code, but it's worth
 describing what `EnTT` offers so as not to reinvent the wheel in case of need.
 
-# Compile-time identifiers
+# Unique sequential identifiers
 
-Sometimes it's useful to be able to give unique identifiers to types at
-compile-time.<br/>
-There are plenty of different solutions out there and I could have used one of
-them. However, I decided to spend my time to define a compact and versatile tool
+Sometimes it's useful to be able to give unique, sequential numeric identifiers
+to types either at compile-time or runtime.<br/>
+There are plenty of different solutions for this out there and I could have used
+one of them. However, I decided to spend my time to define a couple of tools
 that fully embraces what the modern C++ has to offer.
 
-The _result of my efforts_ is the `identifier` class template:
+## Compile-time generator
+
+To generate sequential numeric identifiers at compile-time, `EnTT` offers the
+`identifier` class template:
 
 ```cpp
-#include <ident.hpp>
-
 // defines the identifiers for the given types
 using id = entt::identifier<a_type, another_type>;
 
@@ -58,14 +73,14 @@ default:
 }
 ```
 
-This is all what the class template has to offer: a `type` inline variable that
-contains a numerical identifier for the given type. It can be used in any
-context where constant expressions are required.
+This is all what this class template has to offer: a `type` inline variable that
+contains a numeric identifier for the given type. It can be used in any context
+where constant expressions are required.
 
-As long as the list remains unchanged, identifiers are also guaranteed to be the
-same for every run. In case they have been used in a production environment and
-a type has to be removed, one can just use a placeholder to left the other
-identifiers unchanged:
+As long as the list remains unchanged, identifiers are also guaranteed to be
+stable across different runs. In case they have been used in a production
+environment and a type has to be removed, one can just use a placeholder to left
+the other identifiers unchanged:
 
 ```cpp
 template<typename> struct ignore_type {};
@@ -77,35 +92,30 @@ using id = entt::identifier<
 >;
 ```
 
-A bit ugly to see, but it works at least.
+Perhaps a bit ugly to see in a codebase but it gets the job done at least.
 
-# Runtime identifiers
+## Runtime generator
 
-Sometimes it's useful to be able to give unique identifiers to types at
-runtime.<br/>
-There are plenty of different solutions out there and I could have used one of
-them. In fact, I adapted the most common one to my requirements and used it
-extensively within the entire library.
-
-It's the `family` class. Here is an example of use directly from the
-entity-component system:
+To generate sequential numeric identifiers at runtime, `EnTT` offers the
+`family` class template:
 
 ```cpp
-using component_family = entt::family<struct internal_registry_component_family>;
+// defines a custom generator
+using id = entt::family<struct my_tag>;
 
 // ...
 
-template<typename Component>
-component_type component() const noexcept {
-    return component_family::type<Component>;
-}
+const auto a_type_id = id::type<a_type>;
+const auto another_type_id = id::type<another_type>;
 ```
 
 This is all what a _family_ has to offer: a `type` inline variable that contains
-a numerical identifier for the given type.
+a numeric identifier for the given type.<br/>
+The generator is customizable, so as to get different _sequences_ for different
+purposes if needed.
 
-Please, note that identifiers aren't guaranteed to be the same for every run.
-Indeed it mostly depends on the flow of execution.
+Please, note that identifiers aren't guaranteed to be stable across different
+runs. Indeed it mostly depends on the flow of execution.
 
 # Hashed strings
 
@@ -114,7 +124,8 @@ human-readable identifiers in the codebase while using their numeric
 counterparts at runtime, thus without affecting performance.<br/>
 The class has an implicit `constexpr` constructor that chews a bunch of
 characters. Once created, all what one can do with it is getting back the
-original string or converting it into a number.<br/>
+original string through the `data` member function or converting the instance
+into a number.<br/>
 The good part is that a hashed string can be used wherever a constant expression
 is required and no _string-to-number_ conversion will take place at runtime if
 used carefully.
@@ -133,9 +144,31 @@ There is also a _user defined literal_ dedicated to hashed strings to make them
 more user-friendly:
 
 ```cpp
+using namespace entt::literals;
 constexpr auto str = "text"_hs;
 ```
 
+To use it, remember that all user defined literals in `EnTT` are enclosed in the
+`entt::literals` namespace. Therefore, the entire namespace or selectively the
+literal of interest must be explicitly included before each use, a bit like
+`std::literals`.<br/>
+Finally, in case users need to create hashed strings at runtime, this class also
+offers the necessary functionalities:
+
+```cpp
+std::string orig{"text"};
+
+// create a full-featured hashed string...
+entt::hashed_string str{orig.c_str()};
+
+// ... or compute only the unique identifier
+const auto hash = entt::hashed_string::value(orig.c_str());
+```
+
+This possibility shouldn't be exploited in tight loops, since the computation
+takes place at runtime and no longer at compile-time and could therefore impact
+performance to some degrees.
+
 ## Wide characters
 
 The hashed string has a design that is close to that of an `std::basic_string`.
@@ -147,7 +180,7 @@ In this case, the user defined literal to use to create hashed strings on the
 fly is `_hws`:
 
 ```cpp
-constexpr auto str = "text"_hws;
+constexpr auto str = L"text"_hws;
 ```
 
 Note that the hash type of the `hashed_wstring` is the same of its counterpart.
@@ -188,66 +221,356 @@ const bool b = entt::monostate<"mykey"_hs>{};
 const int i = entt::monostate<entt::hashed_string{"mykey"}>{};
 ```
 
-# Type info
+# Any as in any type
 
-The `type_info` class template is meant to provide some basic information about
-types of all kinds.<br/>
-Currently, the only information available is the numeric identifier associated
-with a given type:
+`EnTT` comes with its own `any` type. It may seem redundant considering that
+C++17 introduced `std::any`, but it is not (hopefully).<br/>
+In fact, the _type_ returned by an `std::any` is a const reference to an
+`std::type_info`, an implementation defined class that's not something everyone
+wants to see in a software. Furthermore, there is no way to connect it with the
+type system of the library and therefore with its integrated RTTI support.<br/>
+Note that this class is largely used internally by the library itself.
+
+The API is very similar to that of its most famous counterpart, mainly because
+this class serves the same purpose of being an opaque container for any type of
+value.<br/>
+Instances of `any` also minimize the number of allocations by relying on a well
+known technique called _small buffer optimization_ and a fake vtable.
+
+Creating an object of the `any` type, whether empty or not, is trivial:
 
 ```cpp
-auto id = entt::type_info<my_type>::id();
+// an empty container
+entt::any empty{};
+
+// a container for an int
+entt::any any{0};
+
+// in place construction
+entt::any in_place{std::in_place_type<int>, 42};
 ```
 
-In general, the `id` function is also `constexpr` but this isn't guaranteed for
-all compilers and platforms (although it's valid with the most well-known and
-popular compilers).<br/>
-This function **can** use non-standard features of the language for its own
-purposes. This allows it to provide compile-time identifiers that remain stable
-between different runs. However, it's possible to force the use of standard
-features only by defining the macro `ENTT_STANDARD_CPP`. In this case, there is
-no guarantee that the identifiers are stable across executions though. Moreover,
-identifiers are generated at runtime and are no longer a compile-time thing.
-
-An external type system can also be used if needed. In fact, `type_info` can be
-specialized by type and is also _sfinae-friendly_ in order to allow more refined
-specializations such as:
+Alternatively, the `make_any` function serves the same purpose but requires to
+always be explicit about the type:
 
 ```cpp
-template<typename Type>
-struct entt::type_info<Type, std::void_d<decltype(Type::custom_id())>> {
-    static constexpr ENTT_ID_TYPE id() ENTT_NOEXCEPT {
-        return Type::custom_id();
-    }
-};
+entt::any any = entt::make_any<int>(42);
 ```
 
-Note that this class template and its specializations are widely used within
-`EnTT`. It also plays a very important role in making `EnTT` work transparently
-across boundaries.<br/>
-Please refer to the dedicated section for more details.
+In both cases, the `any` class takes the burden of destroying the contained
+element when required, regardless of the storage strategy used for the specific
+object.<br/>
+Furthermore, an instance of `any` is not tied to an actual type. Therefore, the
+wrapper will be reconfigured by assigning it an object of a different type than
+the one contained, so as to be able to handle the new instance.
 
-## Almost unique identifiers
+There exists also a way to directly assign a value to the variable contained by
+an `entt::any`, without necessarily replacing it. This is especially useful when
+the object is used in _aliasing mode_, as described below:
 
-Since the default non-standard, compile-time implementation makes use of hashed
-strings, it may happen that two types are assigned the same numeric
-identifier.<br/>
+```cpp
+entt::any any{42};
+entt::any value{3};
+
+// assigns by copy
+any.assign(value);
+
+// assigns by move
+any.assign(std::move(value));
+```
+
+The `any` class will also perform a check on the type information and whether or
+not the original type was copy or move assignable, as appropriate.<br/>
+In all cases, the `assign` function returns a boolean value to indicate the
+success or failure of the operation.
+
+When in doubt about the type of object contained, the `type` member function of
+`any` returns a const reference to the `type_info` associated with its element,
+or `type_id<void>()` if the container is empty. The type is also used internally
+when comparing two `any` objects:
+
+```cpp
+if(any == empty) { /* ... */ }
+```
+
+In this case, before proceeding with a comparison, it's verified that the _type_
+of the two objects is actually the same.<br/>
+Refer to the `EnTT` type system documentation for more details about how
+`type_info` works and on possible risks of a comparison.
+
+A particularly interesting feature of this class is that it can also be used as
+an opaque container for const and non-const references:
+
+```cpp
+int value = 42;
+
+entt::any any{std::in_place_type<int &>(value)};
+entt::any cany = entt::make_any<const int &>(value);
+entt::any fwd = entt::forward_as_any(value);
+
+any.emplace<const int &>(value);
+```
+
+In other words, whenever `any` is explicitly told to construct an _alias_, it
+acts as a pointer to the original instance rather than making a copy of it or
+moving it internally. The contained object is never destroyed and users must
+ensure that its lifetime exceeds that of the container.<br/>
+Similarly, it's possible to create non-owning copies of `any` from an existing
+object:
+
+```cpp
+// aliasing constructor
+entt::any ref = other.as_ref();
+```
+
+In this case, it doesn't matter if the original container actually holds an
+object or acts already as a reference for unmanaged elements, the new instance
+thus created won't create copies and will only serve as a reference for the
+original item.<br/>
+This means that, starting from the example above, both `ref` and` other` will
+point to the same object, whether it's initially contained in `other` or already
+an unmanaged element.
+
+As a side note, it's worth mentioning that, while everything works transparently
+when it comes to non-const references, there are some exceptions when it comes
+to const references.<br/>
+In particular, the `data` member function invoked on a non-const instance of
+`any` that wraps a const reference will return a null pointer in all cases.
+
+To cast an instance of `any` to a type, the library offers a set of `any_cast`
+functions in all respects similar to their most famous counterparts.<br/>
+The only difference is that, in the case of `EnTT`, these won't raise exceptions
+but will only trigger an assert in debug mode, otherwise resulting in undefined
+behavior in case of misuse in release mode.
+
+## Small buffer optimization
+
+The `any` class uses a technique called _small buffer optimization_ to reduce
+the number of allocations where possible.<br/>
+The default reserved size for an instance of `any` is `sizeof(double[2])`.
+However, this is also configurable if needed. In fact, `any` is defined as an
+alias for `basic_any<Len>`, where `Len` is the size above.<br/>
+Users can easily set a custom size or define their own aliases:
+
+```cpp
+using my_any = entt::basic_any<sizeof(double[4])>;
+```
+
+This feature, in addition to allowing the choice of a size that best suits the
+needs of an application, also offers the possibility of forcing dynamic creation
+of objects during construction.<br/>
+In other terms, if the size is 0, `any` avoids the use of any optimization and
+always dynamically allocates objects (except for aliasing cases).
+
+Note that the size of the internal storage as well as the alignment requirements
+are directly part of the type and therefore contribute to define different types
+that won't be able to interoperate with each other.
+
+## Alignment requirement
+
+The alignment requirement is optional and by default the most stringent (the
+largest) for any object whose size is at most equal to the one provided.<br/>
+The `basic_any` class template inspects the alignment requirements in each case,
+even when not provided and may decide not to use the small buffer optimization
+in order to meet them.
+
+The alignment requirement is provided as an optional second parameter following
+the desired size for the internal storage:
+
+```cpp
+using my_any = entt::basic_any<sizeof(double[4]), alignof(double[4])>;
+```
+
+Note that the alignment requirements as well as the size of the internal storage
+are directly part of the type and therefore contribute to define different types
+that won't be able to interoperate with each other.
+
+# Type support
+
+`EnTT` provides some basic information about types of all kinds.<br/>
+It also offers additional features that are not yet available in the standard
+library or that will never be.
+
+## Built-in RTTI support
+
+Runtime type identification support (or RTTI) is one of the most frequently
+disabled features in the C++ world, especially in the gaming sector. Regardless
+of the reasons for this, it's often a shame not to be able to rely on opaque
+type information at runtime.<br/>
+The library tries to fill this gap by offering a built-in system that doesn't
+serve as a replacement but comes very close to being one and offers similar
+information to that provided by its counterpart.
+
+Basically, the whole system relies on a handful of classes. In particular:
+
+* The unique sequential identifier associated with a given type:
+
+  ```cpp
+  auto index = entt::type_index<a_type>::value();
+  ```
+
+  The returned value isn't guaranteed to be stable across different runs.
+  However, it can be very useful as index in associative and unordered
+  associative containers or for positional accesses in a vector or an array.
+  
+  So as not to conflict with the other tools available, the `family` class isn't
+  used to generate these indexes. Therefore, the numeric identifiers returned by
+  the two tools may differ.<br/>
+  On the other hand, this leaves users with full powers over the `family` class
+  and therefore the generation of custom runtime sequences of indices for their
+  own purposes, if necessary.
+  
+  An external generator can also be used if needed. In fact, `type_index` can be
+  specialized by type and is also _sfinae-friendly_ in order to allow more
+  refined specializations such as:
+  
+  ```cpp
+  template<typename Type>
+  struct entt::type_index<Type, std::void_d<decltype(Type::index())>> {
+      static entt::id_type value() ENTT_NOEXCEPT {
+          return Type::index();
+      }
+  };
+  ```
+  
+  Note that indexes **must** still be generated sequentially in this case.<br/>
+  The tool is widely used within `EnTT`. Generating indices not sequentially
+  would break an assumption and would likely lead to undesired behaviors.
+
+* The hash value associated with a given type:
+
+  ```cpp
+  auto hash = entt::type_hash<a_type>::value();
+  ```
+
+  In general, the `value` function exposed by `type_hash` is also `constexpr`
+  but this isn't guaranteed for all compilers and platforms (although it's valid
+  with the most well-known and popular ones).
+
+  This function **can** use non-standard features of the language for its own
+  purposes. This makes it possible to provide compile-time identifiers that
+  remain stable across different runs.<br/>
+  In all cases, users can prevent the library from using these features by means
+  of the `ENTT_STANDARD_CPP` definition. In this case, there is no guarantee
+  that identifiers remain stable across executions. Moreover, they are generated
+  at runtime and are no longer a compile-time thing.
+
+  As for `type_index`, also `type_hash` is a _sfinae-friendly_ class that can be
+  specialized in order to customize its behavior globally or on a per-type or
+  per-traits basis.
+
+* The name associated with a given type:
+
+  ```cpp
+  auto name = entt::type_name<a_type>::value();
+  ```
+
+  The name associated with a type is extracted from some information generally
+  made available by the compiler in use. Therefore, it may differ depending on
+  the compiler and may be empty in the event that this information isn't
+  available.<br/>
+  For example, given the following class:
+
+  ```cpp
+  struct my_type { /* ... */ };
+  ```
+
+  The name is `my_type` when compiled with GCC or CLang and `struct my_type`
+  when MSVC is in use.<br/>
+  Most of the time the name is also retrieved at compile-time and is therefore
+  always returned through an `std::string_view`. Users can easily access it and
+  modify it as needed, for example by removing the word `struct` to standardize
+  the result. `EnTT` won't do this for obvious reasons, since it requires
+  copying and creating a new string potentially at runtime.
+
+  This function **can** use non-standard features of the language for its own
+  purposes. Users can prevent the library from using non-standard features by
+  means of the `ENTT_STANDARD_CPP` definition. In this case, the name will be
+  empty by default.
+
+  As for `type_index`, also `type_name` is a _sfinae-friendly_ class that can be
+  specialized in order to customize its behavior globally or on a per-type or
+  per-traits basis.
+
+These are then combined into utilities that aim to offer an API that is somewhat
+similar to that offered by the language.
+
+### Type info
+
+The `type_info` class isn't a drop-in replacement for `std::type_info` but can
+provide similar information which are not implementation defined and don't
+require to enable RTTI.<br/>
+Therefore, they can sometimes be even more reliable than those obtained
+otherwise.
+
+A type info object is an opaque class that is also copy and move constructible.
+Objects of this class are returned by the `type_id` function template:
+
+```cpp
+auto info = entt::type_id<a_type>();
+```
+
+These are the information made available by a `type_info` object:
+
+* The index associated with a given type:
+
+  ```cpp
+  auto idx = entt::type_id<a_type>().index();
+  ```
+
+  This is also an alias for the following:
+
+  ```cpp
+  auto idx = entt::type_index<std::remove_const_t<std::remove_reference_t<a_type>>>::value();
+  ```
+
+* The hash value associated with a given type:
+
+  ```cpp
+  auto hash = entt::type_id<a_type>().hash();
+  ```
+
+  This is also an alias for the following:
+
+  ```cpp
+  auto hash = entt::type_hash<std::remove_const_t<std::remove_reference_t<a_type>>>::value();
+  ```
+
+* The name associated with a given type:
+
+  ```cpp
+  auto name = entt::type_id<my_type>().name();
+  ```
+
+  This is also an alias for the following:
+
+  ```cpp
+  auto name = entt::type_name<std::remove_const_t<std::remove_reference_t<a_type>>>::value();
+  ```
+
+Where all accessed features are available at compile-time, the `type_info` class
+is also fully `constexpr`. However, this cannot be guaranteed in advance and
+depends mainly on the compiler in use and any specializations of the classes
+described above.
+
+### Almost unique identifiers
+
+Since the default non-standard, compile-time implementation of `type_hash` makes
+use of hashed strings, it may happen that two types are assigned the same hash
+value.<br/>
 In fact, although this is quite rare, it's not entirely excluded.
 
 Another case where two types are assigned the same identifier is when classes
 from different contexts (for example two or more libraries loaded at runtime)
-have the same fully qualified name.<br/>
-Since the default model is based on the name of the classes, if the types belong
-to the same namespace then their identifiers _could_ be identical (they won't
-necessarily be the same though).
-
+have the same fully qualified name. In this case, also `type_name` will return
+the same value for the two types.<br/>
 Fortunately, there are several easy ways to deal with this:
 
-* The most trivial one is to define the `ENTT_STANDARD_CPP` macro. Note that
-  runtime identifiers don't suffer from the sam problem. However, this solution
+* The most trivial one is to define the `ENTT_STANDARD_CPP` macro. Runtime
+  identifiers don't suffer from the same problem in fact. However, this solution
   doesn't work well with a plugin system, where the libraries aren't linked.
 
-* Another possibility is to specialize the `type_info` class for one of the
+* Another possibility is to specialize the `type_name` class for one of the
   conflicting types, in order to assign it a custom identifier. This is probably
   the easiest solution that also preserves the feature of the tool.
 
@@ -260,12 +583,58 @@ their types, this problem is in any case easy to solve and should not worry too
 much.<br/>
 In all likelihood, it will never happen to run into a conflict anyway.
 
-# Traits
+## Type traits
 
-This section contains a handful of utilities and traits not present in the
-standard template library but which can be useful in everyday life.
+A handful of utilities and traits not present in the standard template library
+but which can be useful in everyday life.<br/>
+This list **is not** exhaustive and contains only some of the most useful
+classes. Refer to the inline documentation for more information on the features
+offered by this module.
 
-## Member class type
+### Size of
+
+The standard operator `sizeof` complains when users provide it for example with
+function or incomplete types. On the other hand, it's guaranteed that its result
+is always nonzero, even if applied to an empty class type.<br/>
+This small class combines the two and offers an alternative to `sizeof` that
+works under all circumstances, returning zero if the type isn't supported:
+
+```cpp
+const auto size = entt::size_of_v<void>;
+```
+
+### Is applicable
+
+The standard library offers the great `std::is_invocable` trait in several
+forms. This takes a function type and a series of arguments and returns true if
+the condition is satisfied.<br/>
+Moreover, users are also provided with `std::apply`, a tool for combining
+invocable elements and tuples of arguments.
+
+It would therefore be a good idea to have a variant of `std::is_invocable` that
+also accepts its arguments in the form of a tuple-like type, so as to complete
+the offer:
+
+```cpp
+constexpr bool result = entt::is_applicable<Func, std::tuple<a_type, another_type>>;
+```
+
+This trait is built on top of `std::is_invocable` and does nothing but unpack a
+tuple-like type and simplify the code at the call site.
+
+### Constness as
+
+An utility to easily transfer the constness of a type to another type:
+
+```cpp
+// type is const dst_type because of the constness of src_type
+using type = entt::constness_as_t<dst_type, const src_type>;
+```
+
+The trait is subject to the rules of the language. Therefore, for example,
+transferring constness between references won't give the desired effect.
+
+### Member class type
 
 The `auto` template parameter introduced with C++17 made it possible to simplify
 many class templates and template functions but also made the class type opaque
@@ -277,20 +646,211 @@ template<typename Member>
 using clazz = entt::member_class_t<Member>;
 ```
 
-## Tags
+### Integral constant
 
-Since in `EnTT` the type identified by `ENTT_ID_TYPE` is very important and
-widely used, there is a more user-friendly shortcut for the creation of integral
-constants based on it.<br/>
+Since `std::integral_constant` may be annoying because of its form that requires
+to specify both a type and a value of that type, there is a more user-friendly
+shortcut for the creation of integral constants.<br/>
+This shortcut is the alias template `entt::integral_constant`:
+
+```cpp
+constexpr auto constant = entt::integral_constant<42>;
+```
+
+Among the other uses, when combined with a hashed string it helps to define tags
+as human-readable _names_ where actual types would be required otherwise:
+
+```cpp
+constexpr auto enemy_tag = entt::integral_constant<"enemy"_hs>;
+registry.emplace<enemy_tag>(entity);
+```
+
+### Tag
+
+Since `id_type` is very important and widely used in `EnTT`, there is a more
+user-friendly shortcut for the creation of integral constants based on it.<br/>
 This shortcut is the alias template `entt::tag`.
 
 If used in combination with hashed strings, it helps to use human-readable names
 where types would be required otherwise. As an example:
 
 ```cpp
-registry.assign<entt::tag<"enemy"_hs>>(entity);
+registry.emplace<entt::tag<"enemy"_hs>>(entity);
 ```
 
 However, this isn't the only permitted use. Literally any value convertible to
-`ENTT_ID_TYPE` is a good candidate, such as the named constants of an unscoped
-enum.
+`id_type` is a good candidate, such as the named constants of an unscoped enum.
+
+### Type list and value list
+
+There is no respectable library where the much desired _type list_ can be
+missing.<br/>
+`EnTT` is no exception and provides (making extensive use of it internally) the
+`type_list` type, in addition to its `value_list` counterpart dedicated to
+non-type template parameters.
+
+Here is a (possibly incomplete) list of the functionalities that come with a
+type list:
+
+* `type_list_element[_t]` to get the N-th element of a type list.
+* `type_list_cat[_t]` and a handy `operator+` to concatenate type lists.
+* `type_list_unique[_t]` to remove duplicate types from a type list.
+* `type_list_contains[_v]` to know if a type list contains a given type.
+* `type_list_diff[_t]` to remove types from type lists.
+
+I'm also pretty sure that more and more utilities will be added over time as
+needs become apparent.<br/>
+Many of these functionalities also exist in their version dedicated to value
+lists. We therefore have `value_list_element[_v]` as well as
+`value_list_cat[_t]`and so on.
+
+# Compressed pair
+
+Primarily designed for internal use and far from being feature complete, the
+`compressed_pair` class does exactly what it promises: it tries to reduce the
+size of a pair by exploiting _Empty Base Class Optimization_ (or _EBCO_).<br/>
+This class **is not** a drop-in replacement for `std::pair`. However, it offers
+enough functionalities to be a good alternative for when reducing memory usage
+is more important than having some cool and probably useless feature.
+
+Although the API is very close to that of `std::pair` (apart from the fact that
+the template parameters are inferred from the constructor and therefore there is
+no` entt::make_compressed_pair`), the major difference is that `first` and
+`second` are functions for implementation needs:
+
+```cpp
+entt::compressed_pair pair{0, 3.};
+pair.first() = 42;
+```
+
+There isn't much to describe then. It's recommended to rely on documentation and
+intuition. At the end of the day, it's just a pair and nothing more.
+
+# Enum as bitmask
+
+Sometimes it's useful to be able to use enums as bitmasks. However, enum classes
+aren't really suitable for the purpose out of the box. Main problem is that they
+don't convert implicitly to their underlying type.<br/>
+All that remains is to make a choice between using old-fashioned enums (with all
+their problems that I don't want to discuss here) or writing _ugly_ code.
+
+Fortunately, there is also a third way: adding enough operators in the global
+scope to treat enum classes as bitmask transparently.<br/>
+The ultimate goal is to be able to write code like the following (or maybe
+something more meaningful, but this should give a grasp and remain simple at the
+same time):
+
+```cpp
+enum class my_flag {
+    unknown = 0x01,
+    enabled = 0x02,
+    disabled = 0x04
+};
+
+const my_flag flags = my_flag::enabled;
+const bool is_enabled = !!(flags & my_flag::enabled);
+```
+
+The problem with adding all operators to the global scope is that these will
+come into play even when not required, with the risk of introducing errors that
+are difficult to deal with.<br/>
+However, C++ offers enough tools to get around this problem. In particular, the
+library requires users to register all enum classes for which bitmask support
+should be enabled:
+
+```cpp
+template<>
+struct entt::enum_as_bitmask<my_flag>
+    : std::true_type
+{};
+```
+
+This is handy when dealing with enum classes defined by third party libraries
+and over which the users have no control. However, it's also verbose and can be
+avoided by adding a specific value to the enum class itself:
+
+```cpp
+enum class my_flag {
+    unknown = 0x01,
+    enabled = 0x02,
+    disabled = 0x04,
+    _entt_enum_as_bitmask
+};
+```
+
+In this case, there is no need to specialize the `enum_as_bitmask` traits, since
+`EnTT` will automatically detect the flag and enable the bitmask support.<br/>
+Once the enum class has been registered (in one way or the other) all the most
+common operators will be available, such as `&`, `|` but also `&=` and `|=`.
+Refer to the official documentation for the full list of operators.
+
+# Utilities
+
+It's not possible to escape the temptation to add utilities of some kind to a
+library. In fact, `EnTT` also provides a handful of tools to simplify the
+life of developers:
+
+* `entt::identity`: the identity function object that will be available with
+  C++20. It returns its argument unchanged and nothing more. It's useful as a
+  sort of _do nothing_ function in template programming.
+
+* `entt::overload`: a tool to disambiguate different overloads from their
+  function type. It works with both free and member functions.<br/>
+  Consider the following definition:
+
+  ```cpp
+  struct clazz {
+      void bar(int) {}
+      void bar() {}
+  };
+  ```
+
+  This utility can be used to get the _right_ overload as:
+
+  ```cpp
+  auto *member = entt::overload<void(int)>(&clazz::bar);
+  ```
+
+  The line above is literally equivalent to:
+
+  ```cpp
+  auto *member = static_cast<void(clazz:: *)(int)>(&clazz::bar);
+  ```
+
+  Just easier to read and shorter to type.
+
+* `entt::overloaded`: a small class template used to create a new type with an
+  overloaded `operator()` from a bunch of lambdas or functors.<br/>
+  As an example:
+
+  ```cpp
+  entt::overloaded func{
+      [](int value) { /* ... */ },
+      [](char value) { /* ... */ }
+  };
+
+  func(42);
+  func('c');
+  ```
+
+  Rather useful when doing metaprogramming and having to pass to a function a
+  callable object that supports multiple types at once.
+
+* `entt::y_combinator`: this is a C++ implementation of **the** _y-combinator_.
+  If it's not clear what it is, there is probably no need for this utility.<br/>
+  Below is a small example to show its use:
+
+  ```cpp
+  entt::y_combinator gauss([](const auto &self, auto value) -> unsigned int {
+      return value ? (value + self(value-1u)) : 0;
+  });
+
+  const auto result = gauss(3u);
+  ```
+
+  Maybe convoluted at a first glance but certainly effective. Unfortunately,
+  the language doesn't make it possible to do much better.
+
+This is a rundown of the (actually few) utilities made available by `EnTT`. The
+list will probably grow over time but the size of each will remain rather small,
+as has been the case so far.

docs/md/faq.md

@@ -13,6 +13,7 @@
   * [Warning C4307: integral constant overflow](#warning-C4307-integral-constant-overflow)
   * [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)
 <!--
 @endcond TURN_OFF_DOXYGEN
 -->
@@ -76,49 +77,30 @@ 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
-come in future.
-
-Long story short, you can always define a tree where the nodes expose implicit
-lists of children by means of the following type:
-
-```cpp
-struct relationship {
-    std::size_t children{};
-    entt::entity first{entt::null};
-    entt::entity prev{entt::null};
-    entt::entity next{entt::null};
-    entt::entity parent{entt::null};
-    // ... other data members ...
-};
-```
-
-The sort functionalities of `EnTT`, the groups and all the other features of the
-library can help then to get the best in terms of data locality and therefore
-performance from this component.
+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
+far the most convenient solution when it comes to creating hierarchies and
+whatnot. See the documentation for the ECS part of the library and in particular
+what concerns the `component_traits` class for further details.
 
 ## Custom entity identifiers: yay or nay?
 
 Custom entity identifiers are definitely a good idea in two cases at least:
 
-* If `std::uint32_t` isn't large enough as an underlying type.
+* 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.
 
-These identifiers are nothing more than enum classes with some salt.<br/>
-To simplify the creation of new identifiers, `EnTT` provides the macro
-`ENTT_OPAQUE_TYPE` that accepts two arguments:
-
-* The name you want to give to the new identifier (watch out for namespaces).
-* The underlying type to use (either `std::uint16_t`, `std::uint32_t`
-  or `std::uint64_t`).
-
-In fact, this is the definition of `entt::entity`:
+Identifiers can be defined through enum classes and class types that define an
+`entity_type` member of type `std::uint32_t` or `std::uint64_t`.<br/>
+In fact, this is a definition equivalent to that of `entt::entity`:
 
 ```cpp
-ENTT_OPAQUE_TYPE(entity, std::uint32_t)
+enum class entity: std::uint32_t {};
 ```
 
-The use of this macro is highly recommended, so as not to run into problems if
-the requirements for the identifiers should change in the future.
+There is no limit to the number of identifiers that can be defined.
 
 ## Warning C4307: integral constant overflow
 
@@ -132,7 +114,7 @@ here is a workaround in the form of a macro:
 
 ```cpp
 #if defined(_MSC_VER)
-#define HS(str) __pragma(warning(suppress:4307)) entt::hashed_string{str}\
+#define HS(str) __pragma(warning(suppress:4307)) entt::hashed_string{str}
 #else
 #define HS(str) entt::hashed_string{str}
 #endif
@@ -198,3 +180,24 @@ struct type {
 ```
 
 Unfortunately, this will also disable aggregate initialization.
+
+## Which functions trigger which signals
+
+The `registry` class offers 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.
+
+Among the most controversial functions can be found `emplace_or_replace` and
+`destroy`. However, following the above rules, it's quite simple to know what 
+will happen.<br/>
+In the first case, `on_created` is invoked if the entity has not the component,
+otherwise the latter is replaced and therefore `on_update` is triggered. As for
+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.

docs/md/lib.md

@@ -5,46 +5,53 @@
 -->
 # Table of Contents
 
-* [Introduction](#introduction)
-* [The EnTT way](#the-entt-way)
-* [Meta context](#meta-context)
-* [Memory management](#memory-management)
+* [Working across boundaries](#working-across-boundaries)
+  * [Smooth until proven otherwise](#smooth-until-proven-otherwise)
+  * [Meta context](#meta-context)
+  * [Memory management](#memory-management)
 <!--
 @endcond TURN_OFF_DOXYGEN
 -->
 
-# Introduction
+# Working across boundaries
 
 `EnTT` has historically had a limit when used across boundaries on Windows in
 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 to different types.<br/>
-Fortunately, nowadays using `EnTT` across boundaries is straightforward. In
-fact, everything just works transparently in almost all cases. There are only a
-few exceptions, easy to deal with anyway.
+identifiers with different types.<br/>
+Fortunately, nowadays using `EnTT` across boundaries is much easier.
 
-# The EnTT way
+## Smooth until proven otherwise
 
 Many classes in `EnTT` make extensive use of type erasure for their purposes.
-This isn't a problem in itself (in fact, it's the basis of an API so convenient
+This isn't a problem on itself (in fact, it's the basis of an API so convenient
 to use). However, a way is needed to recognize the objects whose type has been
 erased on the other side of a boundary.<br/>
-The `type_info` class template is how identifiers are generated and thus made
-available to the rest of the library.
+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
+(definitely uncommon though) or when the default solution proposed by `EnTT`
+isn't suitable for the user's purposes.<br/>
+The section dedicated to `type_info` contains all the details to get around the
+issue in a concise and elegant way. Please refer to the specific documentation.
 
-In general, this class doesn't arouse much interest. The only exception is in
-case of conflicts between identifiers (definitely uncommon though) or where the
-default solution proposed by `EnTT` isn't suitable for the user's purposes.<br/>
-The section dedicated to this core class contains all the details to get around
-the problem in a concise and elegant way. Please refer to the specific
-documentation.
+When working with linked libraries, compile definitions `ENTT_API_EXPORT` and
+`ENTT_API_IMPORT` can be used where there is a need to import or export symbols,
+so as to make everything work nicely across boundaries.<br/>
+On the other hand, everything should run smoothly when working with plugins or
+shared libraries that don't export any symbols.
 
-# Meta context
+For anyone who needs more details, the test suite contains multiple examples
+covering the most common cases (see the `lib` directory for all details).<br/>
+It goes without saying that it's impossible to cover **all** possible cases.
+However, what is offered should hopefully serve as a basis for all of them.
+
+## Meta context
 
 The runtime reflection system deserves a special mention when it comes to using
 it across boundaries.<br/>
-Since it's linked to a static context to which the visible components are
-attached and different contexts don't relate to each other, they must be
+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.
 
 Sharing a context is trivial though. First of all, the local one must be
@@ -67,7 +74,7 @@ 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{});
+entt::meta_ctx::bind(entt::meta_ctx{});
 ```
 
 This is allowed because local and global contexts are separated. Therefore, it's
@@ -78,7 +85,7 @@ 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.
 
-# Memory Management
+## Memory Management
 
 There is another subtle problem due to memory management that can lead to
 headaches.<br/>

docs/md/links.md

@@ -3,8 +3,8 @@
 `EnTT` is widely used in private and commercial applications. I cannot even
 mention most of them because of some signatures I put on some documents time
 ago. Fortunately, there are also people who took the time to implement open
-source projects based on `EnTT` and did not hold back when it came to
-documenting them.
+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
@@ -17,10 +17,21 @@ I hope this list can grow much more in the future:
   * [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.
-  * [Land of the Rair](https://github.com/LandOfTheRair/core2): the new backend
-    of [a retro-style MUD](https://rair.land/) for the new age.
+  * [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.
+  * [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
     reimplementation of the game _Black & White_ (2001).
+  * [Land of the Rair](https://github.com/LandOfTheRair/core2): the new backend
+    of [a retro-style MUD](https://rair.land/) for the new age.
   * [Face Smash](https://play.google.com/store/apps/details?id=com.gamee.facesmash):
     a game to play with your face.
   * [EnTT Pacman](https://github.com/Kerndog73/EnTT-Pacman): an example of how
@@ -32,8 +43,8 @@ I hope this list can grow much more in the future:
   * [The Machine](https://github.com/Kerndog73/The-Machine): a box pushing
     puzzler with logic gates and other cool stuff.
     [Check it out](https://indi-kernick.itch.io/the-machine-web-version).
-  * [EnttPong](https://github.com/reworks/EnttPong): an example of how to make
-    Pong with `EnTT`.
+  * [EnTTPong](https://github.com/DomRe/EnttPong): a basic game made to showcase
+    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
@@ -52,22 +63,52 @@ I hope this list can grow much more in the future:
     football game.
   * [DungeonSlayer](https://github.com/alohaeee/DungeonSlayer): 2D game made
     from scratch in C++.
+  * [3DGame](https://github.com/kwarkGorny/3DGame): 2.5D top-down space shooter.
+  * [Pulcher](https://github.com/AODQ/pulcher): 2D cross-platform game inspired
+    by Quake.
+  * [Destroid](https://github.com/tyrannicaltoucan/destroid): _one-bazillionth_
+    arcade game about shooting dirty rocks in space, inspired by Asteroids.
+  * [Wanderer](https://github.com/albin-johansson/wanderer): a 2D exploration
+    based indie game.
+  * [Spelunky® Classic remake](https://github.com/dbeef/spelunky-psp): a truly
+    multiplatform experience with a rewrite from scratch.
+  * [CubbyTower](https://github.com/utilForever/CubbyTower): a simple tower
+    defense game using C++ with Entity Component System (ECS).
+  * [Runeterra](https://github.com/utilForever/Runeterra): Legends of Runeterra
+    simulator using C++ with some reinforcement learning.
+  * [Black Sun](https://store.steampowered.com/app/1670930/Black_Sun/): fly your
+    space ship through a large 2D open world.
+  * [PokeMaster](https://github.com/utilForever/PokeMaster): Pokemon Battle
+    simulator using C++ with some reinforcement learning.
 
 * 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
+    agent-based simulations.
   * [Fling Engine](https://github.com/flingengine/FlingEngine): a Vulkan game
     engine with a focus on data oriented design.
+  * [NovusCore](https://github.com/novuscore/NovusCore): a modern take on World
+    of Warcraft emulation.
+  * [Chrysalis](https://github.com/ivanhawkes/Chrysalis): action RPG SDK for
+    CRYENGINE games.
+  * [LM-Engine](https://github.com/Lawrencemm/LM-Engine): the Vim of game
+    engines.
+  * [Edyn](https://github.com/xissburg/edyn): a real-time physics engine
+    organized as an ECS.
+  * [MushMachine](https://github.com/MadeOfJelly/MushMachine): engine...
+    vrooooommm.
+  * [Antara Gaming SDK](https://github.com/KomodoPlatform/antara-gaming-sdk):
+    the Komodo Gaming Software Development Kit.
+  * [XVP](https://ravingbots.com/xvp-expansive-vehicle-physics-for-unreal-engine/):
+    [_eXpansive Vehicle Physics_](https://github.com/raving-bots/xvp/wiki/Plugin-integration-guide)
+    plugin for Unreal Engine.
   * [Apparently](https://teamwisp.github.io/credits/)
     [Wisp](https://teamwisp.github.io/product/) by
     [Team Wisp](https://teamwisp.github.io/): an advanced real-time ray tracing
     renderer built for the demands of video game artists.
-  * [starlight](https://github.com/DomRe/starlight): game programming framework
-    using `Allegro`, `Lua` and modern C++.
-  * [Apparently](https://github.com/JosiahWI/qub3d-libdeps)
-    [Qub3d](https://qub3d.org/): because blocks should be open source.
   * [shiva](https://github.com/Milerius/shiva): modern C++ engine with
     modularity.
-  * [NovusCore](https://github.com/novuscore/NovusCore): a modern take on World
-    of Warcraft emulation.
   * [ImGui/EnTT editor](https://github.com/Green-Sky/imgui_entt_entity_editor):
     a drop-in, single-file entity editor for `EnTT` that uses `ImGui` as
     graphical backend (with
@@ -76,13 +117,54 @@ I hope this list can grow much more in the future:
     developed for educational purposes.
   * [Lumos](https://github.com/jmorton06/Lumos): game engine written in C++
     using OpenGL and Vulkan.
-  * [Chrysalis](https://github.com/ivanhawkes/Chrysalis): action RPG SDK for
-    CRYENGINE games.
+  * [Silvanus](https://github.com/hobbyistmaker/silvanus): Silvanus Fusion 360
+    Box Generator.
+  * [Lina Engine](https://github.com/inanevin/LinaEngine): an open-source,
+    modular, tiny and fast C++ game engine, aimed to develop 3D desktop games.
+  * [Spike](https://github.com/FahimFuad/Spike): a powerful game engine which
+    can run on a toaster.
+  * [Helena Framework](https://github.com/NIKEA-SOFT/HelenaFramework): a modern
+    framework in C++17 for backend development.
+  * [Unity/EnTT](https://github.com/TongTungGiang/unity-entt): tech demo of a
+    native simulation layer using `EnTT` and `Unity` as a rendering engine.
+  * [OverEngine](https://github.com/OverShifted/OverEngine): an over-engineered
+    game engine.
+  * [Electro](https://github.com/Electro-Technologies/Electro): high performance
+    3D game engine with a high emphasis on rendering.
+  * [Kawaii](https://github.com/Mathieu-Lala/Kawaii_Engine): a modern data
+    oriented game engine.
+  * [Becketron](https://github.com/Doctor-Foxling/Becketron): a game engine
+    written mostly in C++.
+  * [Spatial Engine](https://github.com/luizgabriel/Spatial.Engine): A
+    cross-platform engine created on top of google's filament rendering engine.
+  * [Kaguya](https://github.com/KaiH0717/Kaguya): D3D12 Rendering Engine.
+  * [OpenAWE](https://github.com/OpenAWE-Project/OpenAWE): Open implementation
+    of the Alan Wake Engine.
 
-* Articles 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.
+  * [Game Engine series](https://www.youtube.com/c/TheChernoProject/videos) by
+    [The Cherno](https://github.com/TheCherno) (not only about `EnTT` but also
+    on the use of an ECS in general):
+    - [Intro to EnTT](https://www.youtube.com/watch?v=D4hz0wEB978).
+    - [Entities and Components](https://www.youtube.com/watch?v=-B1iu4QJTUc).
+    - [The ENTITY Class](https://www.youtube.com/watch?v=GfSzeAcsBb0).
+    - [Camera Systems](https://www.youtube.com/watch?v=ubZn7BlrnTU).
+    - [Scene Camera](https://www.youtube.com/watch?v=UKVFRRufKzo).
+    - [Native Scripting](https://www.youtube.com/watch?v=iIUhg88MK5M).
+    - [Native Scripting (now with virtual functions!)](https://www.youtube.com/watch?v=1cHEcrIn8IQ).
+    - [Scene Hierarchy Panel](https://www.youtube.com/watch?v=wziDnE8guvI).
+    - [Properties Panel](https://www.youtube.com/watch?v=NBpB0qscF3E).
+    - [Camera Component UI](https://www.youtube.com/watch?v=RIMt_6agUiU).
+    - [Drawing Component UI](https://www.youtube.com/watch?v=u3yq8s3KuSE).
+    - [Transform Component UI](https://www.youtube.com/watch?v=8JqcXYbzPJc).
+    - [Adding/Removing Entities and Components UI](https://www.youtube.com/watch?v=PsyGmsIgp9M).
+    - [Saving and Loading Scenes](https://www.youtube.com/watch?v=IEiOP7Y-Mbc).
+    - ... And so on.
+      [Check out](https://www.youtube.com/channel/UCQ-W1KE9EYfdxhL6S4twUNw) the
+      _Game Engine Series_ by The Cherno for more videos.
   * [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
@@ -94,27 +176,48 @@ I hope this list can grow much more in the future:
   * [Conan Adventures (SFML and EnTT in C++)](https://leinnan.github.io/blog/conan-adventuressfml-and-entt-in-c.html):
     create projects in modern C++ using `SFML`, `EnTT`, `Conan` and `CMake`.
   * [Adding EnTT ECS to Chrysalis](https://www.tauradius.com/post/adding-an-ecs-to-chrysalis/):
-    a blog entry about the process followed and the results of the integration
-    of `EnTT` into `Chrysalis`.
+    a blog entry (and its 
+    [follow-up](https://www.tauradius.com/post/chrysalis-update-2020-08-02/)) 
+    about the integration of `EnTT` into `Chrysalis`, an action RPG SDK for
+    CRYENGINE games.
+  * [Creating Minecraft in One Week with C++ and Vulkan](https://vazgriz.com/189/creating-minecraft-in-one-week-with-c-and-vulkan/):
+    a crack at recreating Minecraft in one week using a custom C++ engine and
+    Vulkan ([code included](https://github.com/vazgriz/VoxelGame)).
+  * [Ability Creator](https://www.erichildebrand.net/blog/ability-creator-project-retrospect):
+    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.
 
 * Any Other Business:
-  * The [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
+  * [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
     enterprise custom apps, as well as by Esri for its own public applications
     such as
     [Explorer](https://play.google.com/store/apps/details?id=com.esri.explorer),
     [Collector](https://play.google.com/store/apps/details?id=com.esri.arcgis.collector)
     and
     [Navigator](https://play.google.com/store/apps/details?id=com.esri.navigator).
-  * [Sequentity](https://github.com/alanjfs/sequentity): A MIDI-like
-    sequencer/tracker for C++ and `ImGui` (with `Magnum` and `EnTT`).
+  * [FASTSUITE Edition 2](https://www.fastsuite.com/en_EN/fastsuite/fastsuite-edition-2.html)
+    by [Cenit](http://www.cenit.com/en_EN/about-us/overview.html): they use
+    `EnTT` to drive their simulation, that is, the communication between robot
+    controller emulator and renderer.
+  * [Ragdoll](https://ragdolldynamics.com/): real-time physics for Autodesk Maya
+    2020.
+  * [Project Lagrange](https://github.com/adobe/lagrange): a robust geometry
+    processing library by [Adobe](https://github.com/adobe).
+  * [AtomicDEX](https://github.com/KomodoPlatform/atomicDEX-Desktop): a secure
+    wallet and non-custodial decentralized exchange rolled into one application.
   * [Apparently](https://www.linkedin.com/in/skypjack/)
     [NIO](https://www.nio.io/): there was a collaboration to make some changes
     to `EnTT`, at the time used for internal projects.
   * [Apparently](https://www.linkedin.com/jobs/view/architekt-c%2B%2B-at-tieto-1219512333/)
     [Tieto](https://www.tieto.com/): they published a job post where `EnTT` was
     listed on their software stack.
+  * [Sequentity](https://github.com/alanjfs/sequentity): A MIDI-like
+    sequencer/tracker for C++ and `ImGui` (with `Magnum` and `EnTT`).
+  * [EnTT meets Sol2](https://github.com/skaarj1989/entt-meets-sol2): freely
+    available examples of how to combine `EnTT` and `Sol2`.
   * [Godot meets EnTT](https://github.com/portaloffreedom/godot_entt_example/):
     a simple example on how to use `EnTT` within
     [`Godot`](https://godotengine.org/).

docs/md/meta.md

@@ -8,8 +8,13 @@
 * [Introduction](#introduction)
 * [Names and identifiers](#names-and-identifiers)
 * [Reflection in a nutshell](#reflection-in-a-nutshell)
-  * [Any as in any type](#any-as-in-any-type)
+  * [Any to the rescue](#any-to-the-rescue)
   * [Enjoy the runtime](#enjoy-the-runtime)
+  * [Container support](#container-support)
+  * [Pointer-like types](#pointer-like-types)
+  * [Template information](#template-information)
+  * [Automatic conversions](#automatic-conversions)
+  * [Implicitly generated default constructor](#implicitly-generated-default-constructor)
   * [Policies: the more, the less](#policies-the-more-the-less)
   * [Named constants and enums](#named-constants-and-enums)
   * [Properties and meta objects](#properties-and-meta-objects)
@@ -20,15 +25,15 @@
 
 # Introduction
 
-Reflection (or rather, its lack) is a trending topic in the C++ world and, in
-the specific case of `EnTT`, a tool that can unlock a lot of other features. I
+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
 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. In one word: unsatisfactory.<br/>
+allocations, and so on.<br/>
 I finally decided to write a built-in, non-intrusive and macro-free runtime
 reflection system for `EnTT`. Maybe I didn't do better than others or maybe yes,
 time will tell me, but at least I can model this tool around the library to
-which it belongs and not vice versa.
+which it belongs and not the opposite.
 
 # Names and identifiers
 
@@ -40,9 +45,9 @@ 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
 compile-time or with custom functions.
 
-However, the examples in the following sections are all based on the
+That being said, the examples in the following sections are all based on the
 `hashed_string` class as provided by this library. Therefore, where an
-identifier is required, it's likely that a user defined literal is used as
+identifier is required, it's likely that an user defined literal is used as
 follows:
 
 ```cpp
@@ -78,18 +83,21 @@ type. In order to make the type _visible_, users can assign it an identifier:
 auto factory = entt::meta<my_type>().type("reflected_type"_hs);
 ```
 
-When working with named types, it isn't even necessary to specify the
-identifier. In fact, it isn't allowed and it will trigger a compilation
-error.<br/>
-Identifiers are important because users can retrieve meta types at runtime by
-searching for them by _name_ other than by type. 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 allow searching the type by _name_. In this case, it's sufficient not
-to invoke `type` and the type will not be searchable _by name_.
+Or use the default one, that is, the built-in identifier for the given type:
 
-A factory is such that all its member functions returns the factory itself or
-a decorated version of it. This object can be used to add the following:
+```cpp
+auto factory = entt::meta<my_type>().type();
+```
+
+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
@@ -102,9 +110,10 @@ a decorated version of it. This object can be used to add the following:
   entt::meta<my_type>().ctor<int, char>().ctor<&factory>();
   ```
 
-* _Destructors_. Free functions can be set as destructors of reflected types.
-  The purpose is to give users the ability to free up resources that require
-  special treatment before an object is actually destroyed.<br/>
+* _Destructors_. Free functions and member functions can be used as destructors
+  of reflected types. The purpose is to give users the ability to free up
+  resources that require special treatment before an object is actually
+  destroyed.<br/>
   Use the `dtor` member function for this purpose:
 
   ```cpp
@@ -116,8 +125,8 @@ a decorated version of it. This object can be used to add the following:
 
 * _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 a client's point of view, all the variables associated with the
-  reflected type will appear as if they were part of the type itself.<br/>
+  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:
 
   ```cpp
@@ -127,18 +136,30 @@ a decorated version of it. This object can be used to add the following:
       .data<&global_variable>("global"_hs);
   ```
 
-  This function requires as an argument the identifier to give to the meta data
+  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 be set also by means of a couple of functions, namely a
-  setter and a getter. Setters and getters can be either free functions, member
-  functions or mixed ones, as long as they respect the required signatures.<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:
+
+  ```cpp
+  entt::meta<my_type>().data<nullptr, &my_type::data_member>("member"_hs);
+  ```
+
+  Multiple setters are also supported by means of a `value_list` object:
+
+  ```cpp
+  entt::meta<my_type>().data<entt::value_list<&from_int, &from_string>, &my_type::data_member>("member"_hs);
+  ```
+
   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 a client's point of view, all
-  the functions associated with the reflected type will appear as if they were
-  part of the type itself.<br/>
+  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:
 
   ```cpp
@@ -148,9 +169,11 @@ a decorated version of it. This object can be used to add the following:
       .func<&free_function>("free"_hs);
   ```
 
-  This function requires as an argument the identifier to give to the meta
+  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_.
+  searching for them by _name_.<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
@@ -181,100 +204,67 @@ 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.
 
-## Any as in any type
+## Any to the rescue
 
-The reflection system comes with its own `meta_any` type. It may seem redundant
-since C++17 introduced `std::any`, but it is not.<br/>
-In fact, the _type_ returned by an `std::any` is a const reference to an
-`std::type_info`, an implementation defined class that's not something everyone
-wants to see in a software. Furthermore, the class `std::type_info` suffers from
-some design flaws and there is even no way to _convert_ an `std::type_info` into
-a meta type, thus linking the two worlds.
+The reflection system offers a kind of _extended version_ of the `entt::any`
+class (see the core module for more details).<br/>
+The purpose is to add some feature on top of those already present, so as to
+integrate it with the meta type system without having to duplicate the code.
 
-The class `meta_any` offers an API similar to that of its most famous
-counterpart and serves the same purpose of being an opaque container for any
-type of value.<br/>
-It minimizes the allocations required, which are almost absent thanks to _SBO_
-techniques. In fact, unless users deal with _fat types_ and create instances of
-them through the reflection system, allocations are at zero.
-
-Creating instances of `meta_any`, whether empty or from existing objects, is
-trivial:
+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
+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:
 
 ```cpp
-// a container for an int
-entt::meta_any any{0};
-
-// an empty container
 entt::meta_any empty{};
+entt::meta_any other{std::in_place_type<void>};
 ```
 
-The `meta_any` class takes also the burden of destroying the contained object
-when required.<br/>
-Furthermore, an instance of `meta_any` is not tied to a specific type.
-Therefore, the wrapper will be reconfigured by assigning it an object of a
-different type than the one contained, so as to be able to handle the new
-instance.
-
-A particularly interesting feature of this class is that it can also be used as
-an opaque container for unmanaged objects:
-
-```cpp
-int value;
-entt::meta_any any{std::ref(value)};
-```
-
-In other words, whenever `meta_any` intercepts a `reference_wrapper`, it acts as
-a reference to the original instance rather than making a copy of it. The
-contained object is never destroyed and users must ensure that its lifetime
-exceeds that of the container.<br/>
-Similarly, to create a copy that works as a light reference for the managed
-object, it's possible to dereference a given `meta_any`:
-
-```cpp
-entt::meta_any ref = *any;
-```
-
-It doesn't matter if the starting container actually holds an object or acts as
-a reference for unmanaged elements, the new instance thus created won't create
-copies and will only serve as a reference for the original item.<br/>
-It means that, starting from the example above, both `ref` and` any` will point
-to the same object, whether it's initially contained in `any` or already an
-unmanaged one. This is particularly useful for passing instances of `meta_any`
-belonging to the external context by reference to a function or a constructor
-rather than making copies of them.
-
-The `meta_any` class has also a `type` member function that returns the meta
-type of the contained value, if any. The member functions `try_cast`, `cast` and
-`convert` are then used to know if the underlying object has a given type as a
-base or if it can be converted implicitly to it.
+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/>
+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`.
 
 ## 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, unlike the vast majority of the things
-present in this library and closely linked to the compile-time, the reflection
-system stands in fact as a non-intrusive tool for the runtime.
+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 two options: by type or by _name_. In
-both cases, the search can be done by means of the `resolve` function:
+To search for a reflected type there are a few options:
 
 ```cpp
-// search for a reflected type by type
+// direct access to a reflected type
 auto by_type = entt::resolve<my_type>();
 
-// search for a reflected type by name
-auto by_name = entt::resolve("reflected_type"_hs);
+// look up a reflected type by identifier
+auto by_id = entt::resolve("reflected_type"_hs);
+
+// look up a reflected type by type info
+auto by_type_id = entt::resolve(entt::type_id<my_type>());
 ```
 
-There exits also a third overload of the `resolve` function to use to iterate
-all the reflected types at once:
+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:
 
 ```cpp
-resolve([](auto type) {
+for(auto type: entt::resolve()) {
     // ...
-});
+}
 ```
 
 In all cases, the returned value is an instance of `meta_type`. This kind of
@@ -285,19 +275,6 @@ Refer to the inline documentation for all the details.
 
 The meta objects that compose a meta type are accessed in the following ways:
 
-* _Meta constructors_. They are accessed by types of arguments:
-
-  ```cpp
-  auto ctor = entt::resolve<my_type>().ctor<int, char>();
-  ```
-
-  The returned type is `meta_ctor` and may be invalid if there is no constructor
-  that accepts the supplied arguments or at least some types from which they are
-  derived or to which they can be converted.<br/>
-  A meta constructor offers an API to know the number of its arguments and their
-  expected meta types. Furthermor, it's possible to invoke it and therefore to
-  construct new instances of the underlying type.
-
 * _Meta data_. They are accessed by _name_:
 
   ```cpp
@@ -330,24 +307,8 @@ The meta objects that compose a meta type are accessed in the following ways:
   auto base = entt::resolve<derived_type>().base("base"_hs);
   ```
 
-  The returned type is `meta_base` and may be invalid if there is no meta base
-  object associated with the given identifier.<br/>
-  Meta bases aren't meant to be used directly, even though they are freely
-  accessible. They expose only a few methods to use to know the meta type of the
-  base class and to convert a raw pointer between types.
-
-* _Meta conversion functions_. They are accessed by type:
-
-  ```cpp
-  auto conv = entt::resolve<double>().conv<int>();
-  ```
-
-  The returned type is `meta_conv` and may be invalid if there is no meta
-  conversion function associated with the given type.<br/>
-  The meta conversion functions are as thin as the meta bases and with a very
-  similar interface. The sole difference is that they return a newly created
-  instance wrapped in a `meta_any` object when they convert between different
-  types.
+  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:
@@ -358,13 +319,13 @@ if(auto func = entt::resolve<my_type>().func("member"_hs); func) {
 }
 ```
 
-Furthermore, all meta objects can be iterated through an overload that accepts a
-callback through which to return them. As an example:
+Furthermore, all them are also returned by specific overloads that provide the
+caller with iterable ranges of top-level elements. As an example:
 
 ```cpp
-entt::resolve<my_type>().data([](auto data) {
+for(auto data: entt::resolve<my_type>().data()) {
     // ...
-});
+}
 ```
 
 A meta type can be used to `construct` actual instances of the underlying
@@ -375,12 +336,12 @@ or may not be initialized, depending on whether a suitable constructor has been
 found or not.
 
 There is no object that wraps the destructor of a meta type nor a `destroy`
-member function in its API. The reason is quickly explained: 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/>
-Therefore, exposing destructors would be pointless and would add nothing to the
-library itself.
+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
+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
@@ -388,9 +349,443 @@ 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.
 
+## Container support
+
+The runtime reflection system also supports containers of all types.<br/>
+Moreover, _containers_ doesn't necessarily mean those offered by the C++
+standard library. In fact, user defined data structures can also work with the
+meta system in many cases.
+
+To make a container be recognized as such by the meta system, users are required
+to provide specializations for either the `meta_sequence_container_traits` class
+or the `meta_associative_container_traits` class, according with 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
+  _associative containers_.
+
+It's important to include the header file `container.hpp` to make these
+specializations available to the compiler when needed.<br/>
+The same file also contains many examples for the users that are interested in
+making their own containers available to the meta system.
+
+When a specialization of the `meta_sequence_container_traits` class exists, the
+meta system treats the wrapped type as a sequence container. In a similar way,
+a type is treated as an associative container if a specialization of the
+`meta_associative_container_traits` class is found for it.<br/>
+Proxy objects are returned by dedicated members of the `meta_any` class. The
+following is a deliberately verbose example of how users can access a proxy
+object for a sequence container:
+
+```cpp
+std::vector<int> vec{1, 2, 3};
+entt::meta_any any = entt::forward_as_meta(vec);
+
+if(any.type().is_sequence_container()) {
+    if(auto view = any.as_sequence_container(); view) {
+        // ...
+    }
+}
+```
+
+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/>
+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.
+
+The interface of the `meta_sequence_container` proxy object is the same for all
+types of sequence containers, although the available features differ from case
+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();
+  ```
+
+* The `resize` member function allows to resize the wrapped container and
+  returns true in case of succes:
+
+  ```cpp
+  const bool ok = view.resize(3u);
+  ```
+
+  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();
+  ```
+
+  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:
+
+  ```cpp
+  for(entt::meta_any element: view) {
+      // ...
+  }
+  ```
+
+  In all cases, given an underlying container of type `C`, the returned element
+  contains an object of type `C::value_type` which therefore depends on the
+  actual container.<br/>
+  All meta iterators are input iterators and don't offer an indirection operator
+  on purpose.
+
+* The `insert` member function can be used to add elements to the container. It
+  accepts a meta iterator and the element to insert:
+
+  ```cpp
+  auto last = view.end();
+  // appends an integer to the container
+  view.insert(last, 42);
+  ```
+
+  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.
+
+* The `erase` member function can be used to remove elements from the container.
+  It accepts a meta iterator to the element to remove:
+
+  ```cpp
+  auto first = view.begin();
+  // removes the first element from the container
+  view.erase(first);
+  ```
+
+  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.
+
+* 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:
+
+  ```cpp
+  for(std::size_t pos{}, last = view.size(); pos < last; ++pos) {
+      entt::meta_any value = view[pos];
+      // ...
+  }
+  ```
+
+  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.
+
+Similarly, also the interface of the `meta_associative_container` proxy object
+is the same for all types of associative containers. However, there are some
+differences in behavior in the case of key-only containers. In particular:
+
+* The `key_only` member function returns true if the wrapped container is a
+  key-only one.
+
+* The `key_type` member function returns the meta type of the keys.
+
+* The `mapped_type` member function returns an invalid meta type for key-only
+  containers and the meta type of the mapped values for all other types of
+  containers.
+
+* The `value_type` member function returns the meta type of the elements.<br/>
+  For example, it returns the meta type of `int` for `std::set<int>` while it
+  returns the meta type of `std::pair<const int, char>` for
+  `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();
+  ```
+
+* The `clear` member function allows to clear the wrapped container and returns
+  true in case of success:
+
+  ```cpp
+  const bool ok = view.clear();
+  ```
+
+* The `begin` and `end` member functions return opaque iterators that can be
+  used to iterate the container directly:
+
+  ```cpp
+  for(std::pair<entt::meta_any, entt::meta_any> element: view) {
+      // ...
+  }
+  ```
+
+  In all cases, given an underlying container of type `C`, the returned element
+  is a key-value pair where the key has type `C::key_type` and the value has
+  type `C::mapped_type`. Since key-only containers don't have a mapped type,
+  their _value_ is nothing more than an invalid `meta_any` object.<br/>
+  All meta iterators are input iterators and don't offer an indirection operator
+  on purpose.
+
+  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.
+
+* 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:
+
+  ```cpp
+  auto last = view.end();
+  // appends an integer to the container
+  view.insert(last.handle(), 42, 'c');
+  ```
+
+  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.
+
+* 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:
+
+  ```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.
+
+* 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:
+
+  ```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.
+
+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/>
+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:
+
+* All types of raw pointers.
+* `std::unique_ptr` and `std::shared_ptr`.
+
+It's important to include the header file `pointer.hpp` to make these
+specializations available to the compiler when needed.<br/>
+The same file also contains many examples for the users that are interested in
+making their own pointer-like types available to the meta system.
+
+When a type is recognized as a pointer-like one by the meta system, it's
+possible to dereference the instances of `meta_any` that contain these objects.
+The following is a deliberately verbose example to show how to use this feature:
+
+```cpp
+int value = 42;
+// meta type equivalent to that of int *
+entt::meta_any any{&value};
+
+if(any.type().is_pointer_like()) {
+    // meta type equivalent to that of int
+    if(entt::meta_any ref = *any; ref) {
+        // ...
+    }
+}
+```
+
+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).
+
+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:
+
+* It's possible to exploit a solution based on ADL lookup by offering a function
+  (also a template one) named `dereference_meta_pointer_like`:
+
+  ```cpp
+  template<typename Type>
+  Type & dereference_meta_pointer_like(const custom_pointer_type<Type> &ptr) {
+      return ptr.deref();
+  }
+  ```
+
+* When not in control of the type's namespace, it's possible to inject into the
+  `entt` namespace a specialization of the `adl_meta_pointer_like` class
+  template to bypass the adl lookup as a whole:
+
+  ```cpp
+  template<typename Type>
+  struct entt::adl_meta_pointer_like<custom_pointer_type<Type>> {
+      static decltype(auto) dereference(const custom_pointer_type<Type> &ptr) {
+          return ptr.deref();
+      }
+  };
+  ```
+
+In all other cases, that is, when dereferencing a pointer works as expected and
+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
+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
+available to the compiler when needed.
+
+Meta template information are easily found:
+
+```cpp
+// this method returns true if the type is recognized as a class template specialization
+if(auto type = entt::resolve<std::shared_ptr<my_type>>(); type.is_template_specialization()) {
+    // meta type of the class template conveniently wrapped by entt::meta_class_template_tag
+    auto class_type = type.template_type();
+
+    // number of template arguments
+    std::size_t arity = type.template_arity();
+
+    // meta type of the i-th argument
+    auto arg_type = type.template_arg(0u);
+}
+```
+
+Typically, when template information for a type are required, what the library
+provides is sufficient. However, there are some cases where a user may want more
+details or a different set of information.<br/>
+Consider the case of a class template that is meant to wrap function types:
+
+```cpp
+template<typename>
+struct function_type;
+
+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/>
+To achieve this, users must enter the library internals and provide their own
+specialization for the class template `entt::meta_template_traits`, such as:
+
+```cpp
+template<typename Ret, typename... Args>
+struct entt::meta_template_traits<function_type<Ret(Args...)>> {
+    using class_type = meta_class_template_tag<function_type>;
+    using args_type = type_list<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.
+
+## Automatic conversions
+
+In C++, there are a number of conversions allowed between arithmetic types that
+make it convenient to work with this kind of data.<br/>
+If this were to be translated into explicit registrations with the reflection
+system, it would result in a long series of instructions such as the following:
+
+```cpp
+entt::meta<int>()
+    .conv<bool>()
+    .conv<char>()
+    // ...
+    .conv<double>();
+```
+
+Repeated for each type eligible to undergo this type of conversions. This is
+both error prone and repetitive.<br/>
+Similarly, the language allows users to silently convert unscoped enums to their
+underlying types and offers what it takes to do the same for scoped enums. It
+would result in the following if it were to be done explicitly:
+
+```cpp
+entt::meta<my_enum>()
+    .conv<std::underlying_type_t<my_enum>>();
+```
+
+Fortunately, all of this can also be avoided. `EnTT` offers implicit support for
+these types of conversions:
+
+```cpp
+entt::meta_any any{42};
+any.allow_cast<double>();
+double value = any.cast<double>();
+```
+
+With no need for registration, the conversion takes place automatically under
+the hood. The same goes for a call to `allow_cast` involving a meta type:
+
+```cpp
+entt::meta_type type = entt::resolve<int>();
+entt::meta_any any{my_enum::a_value};
+any.allow_cast(type);
+int value = any.cast<int>();
+```
+
+This should make working with arithmetic types and scoped or unscoped enums as
+easy as it is in C++.<br/>
+It's also worth noting that it's still possible to set up conversion functions
+manually and these will always be preferred over the automatic ones.
+
+## Implicitly generated default constructor
+
+In many cases, it's useful to be able to create objects of default constructible
+types through the reflection system, while not having to explicitly register the
+meta type or the default constructor.<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/>
+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,
+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.
+
 ## Policies: the more, the less
 
-Policies are a kind of compile-time directives that can be used when recording
+Policies are a kind of compile-time directives that can be used when registering
 reflection information.<br/>
 Their purpose is to require slightly different behavior than the default in some
 specific cases. For example, when reading a given data member, its value is
@@ -410,33 +805,35 @@ There are a few alternatives available at the moment:
 
 * The _as-void_ policy, associated with the type `entt::as_void_t`.<br/>
   Its purpose is to discard the return value of a meta object, whatever it is,
-  thus making it appear as if its type were `void`.<br/>
-  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.
-
-  As an example of use:
+  thus making it appear as if its type were `void`:
 
   ```cpp
   entt::meta<my_type>().func<&my_type::member_function, entt::as_void_t>("member"_hs);
   ```
 
-* The _as-alias_ policy, associated with the type `entt::as_alias_t`.<br/>
-  It allows to build wrappers that act as aliases for the objects that generated
-  them. Modifying the object contained in the wrapper for which the _aliasing_
-  was requested will make it possible to directly modify the instance used to
-  initialize the wrapper itself.<br/>
-  This policy works with constructors (for example, when objects are taken from
-  an external container rather than created on demand), data members and
-  functions in general (as long as their return types are lvalue references).
+  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.
 
-  As an example of use:
+* 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:
 
   ```cpp
-  entt::meta<my_type>().data<&my_type::data_member, entt::as_alias_t>("member"_hs);
+  entt::meta<my_type>().data<&my_type::data_member, entt::as_ref_t>("member"_hs);
   ```
 
+  These policies work with constructors (for example, when objects are taken
+  from an external container rather than created on demand), data members and
+  functions in general.<br/>
+  If on the one hand `as_cref_t` always forces the return type to be const,
+  `as_ref_t` _adapts_ to the constness of the passed object and to that of the
+  return type if any.
+
 Some uses are rather trivial, but it's useful to note that there are some less
 obvious corner cases that can in turn be solved with the use of policies.
 
@@ -460,8 +857,8 @@ Exporting constant values or elements from an enum is as simple as ever:
 
 ```cpp
 entt::meta<my_enum>()
-        .data<my_enum::a_value>("a_value"_hs)
-        .data<my_enum::another_value>("another_value"_hs);
+    .data<my_enum::a_value>("a_value"_hs)
+    .data<my_enum::another_value>("another_value"_hs);
 
 entt::meta<int>().data<2048>("max_int"_hs);
 ```
@@ -518,51 +915,27 @@ Multiple formats are supported when it comes to defining a property:
 * 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));
+  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.
 
-* Annotations:
+Note that it's not possible to invoke `prop` multiple times for the same meta
+object and trying to do that will result in a compilation error.<br/>
+However, the `props` function is available to associate several properties at
+once. In this case, properties in the key/value form aren't allowed, since they
+would be interpreted as two different properties rather than a single one.
 
-  ```cpp
-  entt::meta<my_type>().type("reflected_type"_hs).prop(&property_generator);
-  ```
-
-  An annotation is an invocable object that returns one or more properties. All
-  of them are treated individually.
-
-It's possible to invoke the `prop` function several times if needed, one for
-each property to associate with the last meta object created:
-
-```cpp
-entt::meta<my_type>()
-        .type("reflected_type"_hs)
-            .prop(entt::hashed_string{"Name"}, "Reflected Type")
-        .data<&my_type::data_member>("member"_hs)
-            .prop(std::make_pair("tooltip"_hs, "Member"))
-            .prop(my_enum::a_value, 42);
-```
-
-Alternatively, the `props` function is available to associate several properties
-at a time. However, in this case properties in the key/value form aren't
-allowed, since they would be interpreted as two different properties rather than
-a single one.
-
-The meta objects for which properties are supported are currently the meta
-types, meta constructors, meta data and meta functions. It's not possible to
-attach properties to other types of meta objects and the factory returned as a
-result of their construction won't allow such an operation.
-
-These types offer a couple of member functions named `prop` to iterate all
+The meta objects for which properties are supported are currently meta types,
+meta data and meta functions.<br/>
+These types also offer a couple of member functions named `prop` to iterate all
 properties at once or to search a specific property by key:
 
 ```cpp
 // iterate all properties of a meta type
-entt::resolve<my_type>().prop([](auto prop) {
+for(auto prop: entt::resolve<my_type>().prop()) {
     // ...
-});
+}
 
 // search for a given property by name
 auto prop = entt::resolve<my_type>().prop("tooltip"_hs);
@@ -576,16 +949,30 @@ the key and the value contained in the form of `meta_any` objects, respectively.
 
 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, the base classes won't be unregistered, since they don't
+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) won't
-be unregistered.<br/>
+the meta types implicitly generated for function parameters when needed) aren't
+unregistered.<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:
 
 ```cpp
-entt::meta<my_type>().reset();
+entt::meta_reset<my_type>();
 ```
 
-The type can be re-registered later with a completely different name and form.
+It's also possible to reset types by their unique identifiers if required:
+
+```cpp
+entt::meta_reset("my_type"_hs);
+```
+
+Finally, there exists a non-template overload of the `meta_reset` function that
+doesn't accept argument and resets all searchable types (that is, all types that
+were assigned an unique identifier):
+
+```cpp
+entt::meta_reset();
+```
+
+All types can be re-registered later with a completely different name and form.

docs/md/poly.md

@@ -0,0 +1,359 @@
+# Crash Course: poly
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+  * [Other libraries](#other-libraries)
+* [Concept and implementation](#concept-and-implementation)
+  * [Deduced interface](#deduced-interface)
+  * [Defined interface](#defined-interface)
+  * [Fullfill a concept](#fullfill-a-concept)
+* [Inheritance](#inheritance)
+* [Static polymorphism in the wild](#static-polymorphism-in-the-wild)
+* [Storage size and alignment requirement](#storage-size-and-alignment-requirement)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+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
+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.
+
+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/>
+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.
+
+## Other libraries
+
+There are some very interesting libraries regarding static polymorphism.<br/>
+Among all, the two that I prefer are:
+
+* [`dyno`](https://github.com/ldionne/dyno): runtime polymorphism done right.
+* [`Poly`](https://github.com/facebook/folly/blob/master/folly/docs/Poly.md):
+  a class template that makes it easy to define a type-erasing polymorphic
+  object wrapper.
+
+The former is admittedly an experimental library, with many interesting ideas.
+I've some doubts about the usefulness of some feature in real world projects,
+but perhaps my lack of experience comes into play here. In my opinion, its only
+flaw is the API which I find slightly more cumbersome than other solutions.<br/>
+The latter was undoubtedly a source of inspiration for this module, although I
+opted for different choices in the implementation of both the final API and some
+feature.
+
+Either way, the authors are gurus of the C++ community, people I only have to
+learn from.
+
+# Concept and implementation
+
+The first thing to do to create a _type-erasing polymorphic object wrapper_ (to
+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
+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/>
+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:
+
+```cpp
+struct Drawable: entt::type_list<> {
+    template<typename Base>
+    struct type: Base {
+        void draw() { this->template invoke<0>(*this); }
+    };
+
+    // ...
+};
+```
+
+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
+other than `void`:
+
+```cpp
+struct Drawable: entt::type_list<> {
+    template<typename Base>
+    struct type: Base {
+        bool draw(int pt) const { return this->template invoke<0>(*this, pt); }
+    };
+
+    // ...
+};
+```
+
+In this case, all parameters must be 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
+external call:
+
+```cpp
+struct Drawable: entt::type_list<> {
+    template<typename Base>
+    struct type: Base {
+        void draw() const { entt::poly_call<0>(*this); }
+    };
+
+    // ...
+};
+```
+
+Once the _concept_ is defined, users must provide a generic implementation of it
+in order to tell the system how any type can satisfy its requirements. This is
+done via an alias template within the concept itself.<br/>
+The index passed as a template parameter to either `invoke` or `poly_call`
+refers to how this alias is defined.
+
+## Defined interface
+
+A fully defined concept is no different to one for which the interface is
+deduced, with the only difference that the list of types is not empty this time:
+
+```cpp
+struct Drawable: entt::type_list<void()> {
+    template<typename Base>
+    struct type: Base {
+        void draw() { entt::poly_call<0>(*this); }
+    };
+
+    // ...
+};
+```
+
+Again, parameters and return values other than `void` are allowed. Also, the
+function type must be const when the method to bind to it is const:
+
+```cpp
+struct Drawable: entt::type_list<bool(int) const> {
+    template<typename Base>
+    struct type: Base {
+        bool draw(int pt) const { return entt::poly_call<0>(*this, pt); }
+    };
+
+    // ...
+};
+```
+
+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.
+
+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:
+
+* Either by a class that exposes a member function with the same name and the
+  same signature.
+
+* Or through a lambda that makes use of existing member functions from the
+  interface itself.
+
+In other words, it's not possible to make use of functions not belonging to the
+interface, even if they are present in 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.
+
+Explicitly defining a static virtual table suppresses the deduction step and
+allows maximum flexibility when providing the implementation for a concept.
+
+## Fullfill a concept
+
+The `impl` alias template of a concept is used to define how it's fulfilled:
+
+```cpp
+struct Drawable: entt::type_list<> {
+    // ...
+
+    template<typename Type>
+    using impl = entt::value_list<&Type::draw>;
+};
+```
+
+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:
+
+```cpp
+template<typename Type>
+void print(Type &self) { self.print(); }
+
+struct Drawable: entt::type_list<void()> {
+    // ...
+
+    template<typename Type>
+    using impl = entt::value_list<&print<Type>>;
+};
+```
+
+Likewise, as long as the parameter types and return type support conversions to
+and from those of the function type referenced in the static virtual table, the
+actual implementation may differ in its function type since it's erased
+internally.<br/>
+Moreover, the `self` parameter isn't strictly required by the system and can be
+left out for free functions if not required.
+
+Refer to the inline documentation for more details.
+
+# Inheritance
+
+_Concept inheritance_ is straightforward due to how poly looks like in `EnTT`.
+Therefore, it's quite easy to build hierarchies of concepts if necessary.<br/>
+The only constraint is that all concepts in a hierarchy must belong to the same
+_family_, that is, they must be either all deduced or all defined.
+
+For a deduced concept, inheritance is achieved in a few steps:
+
+```cpp
+struct DrawableAndErasable: entt::type_list<> {
+    template<typename Base>
+    struct type: typename Drawable::template type<Base> {
+        static constexpr auto base = std::tuple_size_v<typename entt::poly_vtable<Drawable>::type>;
+        void erase() { entt::poly_call<base + 0>(*this); }
+    };
+
+    template<typename Type>
+    using impl = entt::value_list_cat_t<
+        typename Drawable::impl<Type>,
+        entt::value_list<&Type::erase>
+    >;
+};
+```
+
+The static virtual table is empty and must remain so.<br/>
+On the other hand, `type` no longer inherits from `Base` and instead 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/>
+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/>
+To do this, it's useful to declare a function that allows to convert a _concept_
+into its underlying `type_list` object:
+
+```cpp
+template<typename... Type>
+entt::type_list<Type...> as_type_list(const entt::type_list<Type...> &);
+```
+
+The definition isn't strictly required, since the function will only be used
+through a `decltype` as it follows:
+
+```cpp
+struct DrawableAndErasable: entt::type_list_cat_t<
+    decltype(as_type_list(std::declval<Drawable>())),
+    entt::type_list<void()>
+> {
+    // ...
+};
+```
+
+Similar to above, `type_list_cat_t` is used to concatenate the underlying static
+virtual table with the new function types.<br/>
+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:
+
+```cpp
+using drawable = entt::poly<Drawable>;
+
+struct circle {
+    void draw() { /* ... */ }
+};
+
+struct square {
+    void draw() { /* ... */ }
+};
+
+// ...
+
+drawable instance{circle{}};
+instance->draw();
+
+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/>
+Among others, there is also a constructor that allows users to wrap unmanaged
+objects in a `poly` instance (either const or non-const ones):
+
+```cpp
+circle shape;
+drawable instance{std::in_place_type<circle &>, shape};
+```
+
+Similarly, it's possible to create non-owning copies of `poly` from an existing
+object:
+
+```cpp
+drawable other = instance.as_ref();
+```
+
+In both cases, although the interface of the `poly` object doesn't change, it
+won'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.
+
+# Storage size and alignment requirement
+
+Under the hood, the `poly` class template makes use of `entt::any`. Therefore,
+it can take advantage of the possibility of defining at compile-time the size of
+the storage suitable for the small buffer optimization as well as the alignment
+requirements:
+
+```cpp
+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/>
+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

@@ -28,9 +28,9 @@ 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 process should expose publicly the following member functions whether
-required (note that it isn't required to define a function unless the derived
-class wants to _override_ the default behavior):
+A process should expose publicly the following member functions whether needed
+(note that it isn't required to define a function unless the derived class wants
+to _override_ the default behavior):
 
 * `void update(Delta, void *);`
 

docs/md/reference.md

@@ -0,0 +1,71 @@
+# Similar projects
+
+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 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.
+  * [Diana](https://github.com/discoloda/Diana): an ECS that uses sparse sets to
+    keep track of entities in systems.
+  * [Flecs](https://github.com/SanderMertens/flecs): a multithreaded archetype
+    ECS based on semi-contiguous arrays rather than chunks.
+  * [lent](https://github.com/nem0/lent): the Donald Trump of the ECS libraries.
+
+* C++:
+  * [decs](https://github.com/vblanco20-1/decs): a chunk based archetype ECS.
+  * [ecst](https://github.com/SuperV1234/ecst): a multithreaded compile-time
+    ECS that uses sparse sets to keep track of entities in systems.
+  * [EntityX](https://github.com/alecthomas/entityx): a bitset based ECS that
+    uses a single large matrix of components indexed with entities.
+  * [Polypropylene](https://github.com/pmbittner/Polypropylene): a hybrid
+    solution between an ECS and dynamic mixins.
+
+* C#
+  * [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
+    Component System framework.
+  * [Svelto.ECS](https://github.com/sebas77/Svelto.ECS): a very interesting
+    platform agnostic and table based ECS framework.
+
+* Go:
+  * [gecs](https://github.com/tutumagi/gecs): a sparse sets based ECS inspired 
+    by `EnTT`.
+
+* Javascript:
+  * [\@javelin/ecs](https://github.com/3mcd/javelin/tree/master/packages/ecs):
+    an archetype ECS in TypeScript.
+  * [ecsy](https://github.com/MozillaReality/ecsy): I haven't had the time to
+    investigate the underlying design of `ecsy` but it looks cool anyway.
+
+* Perl:
+  * [Game::Entities](https://gitlab.com/jjatria/perl-game-entities): a simple
+    entity registry for ECS designs inspired by `EnTT`.
+
+* Raku:
+  * [Game::Entities](https://gitlab.com/jjatria/raku-game-entities): a simple
+    entity registry for ECS designs inspired by `EnTT`.
+
+* Rust:
+  * [Legion](https://github.com/TomGillen/legion): a chunk based archetype ECS.
+  * [Shipyard](https://github.com/leudz/shipyard): it borrows some ideas from
+    `EnTT` and offers a sparse sets based ECS with grouping functionalities.
+  * [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

@@ -23,7 +23,7 @@ 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
-environment.<br/>
+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.
@@ -42,10 +42,10 @@ 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 the dedicated base class as in the following example:
+inherit directly from a dedicated base class as in the following example:
 
 ```cpp
-struct my_loader final: entt::loader<my_loader, my_resource> {
+struct my_loader final: entt::resource_loader<my_loader, my_resource> {
     // ...
 };
 ```
@@ -57,7 +57,7 @@ resource.<br/>
 As an example:
 
 ```cpp
-struct my_loader: entt::loader<my_loader, my_resource> {
+struct my_loader: entt::resource_loader<my_loader, my_resource> {
     std::shared_ptr<my_resource> load(int value) const {
         // ...
         return std::shared_ptr<my_resource>(new my_resource{ value });
@@ -76,7 +76,7 @@ Finally, a cache is a specialization of a class template tailored to a specific
 resource:
 
 ```cpp
-using my_cache = entt::cache<my_resource>;
+using my_cache = entt::resource_cache<my_resource>;
 
 // ...
 
@@ -105,20 +105,14 @@ cache.clear();
 
 Besides these member functions, a cache contains what is needed to load, use and
 discard resources of the given type.<br/>
-Before to explore this part of the interface, it makes sense to mention how
-resources are identified. The type of the identifiers to use is defined as:
+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:
 
 ```cpp
-entt::cache<resource>::id_type
-```
-
-Where `id_type` is an alias for `entt::hashed_string::hash_type`. Therefore,
-resource identifiers are created explicitly as in the following example:
-
-```cpp
-constexpr auto identifier = entt::cache<resource>::id_type{"my/resource/identifier"_hs};
+constexpr auto identifier = "my/resource/identifier"_hs;
 // this is equivalent to the following
-constexpr auto hs = entt::hashed_string{"my/resource/identifier"};
+constexpr entt::id_type hs = entt::hashed_string{"my/resource/identifier"};
 ```
 
 The class `hashed_string` is described in a dedicated section, so I won't go in
@@ -132,7 +126,7 @@ identifier and the parameters used to construct the resource as arguments:
 // uses the identifier declared above
 cache.load<my_loader>(identifier, 0);
 
-// uses a const char * directly as an identifier
+// uses a hashed string directly
 cache.load<my_loader>("another/identifier"_hs, 42);
 ```
 
@@ -141,7 +135,7 @@ 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(auto handle = cache.load<my_loader>("another/identifier"_hs, 42); handle) {
+if(entt::resource_handle handle = cache.load<my_loader>("another/identifier"_hs, 42); handle) {
     // ...
 }
 ```
@@ -186,24 +180,24 @@ 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 const reference on request:
+resource as a (possibly const) reference on request:
 
 * By means of the `get` member function:
 
   ```cpp
-  const auto &resource = handle.get();
+  auto &resource = handle.get();
   ```
 
 * Using the proper cast operator:
 
   ```cpp
-  const auto &resource = handle;
+  auto &resource = handle;
   ```
 
 * Through the dereference operator:
 
   ```cpp
-  const auto &resource = *handle;
+  auto &resource = *handle;
   ```
 
 The resource can also be accessed directly using the arrow operator if required:

docs/md/signal.md

@@ -7,6 +7,8 @@
 
 * [Introduction](#introduction)
 * [Delegate](#delegate)
+  * [Runtime arguments](#runtime-arguments)
+  * [Lambda support](#lambda-support)
 * [Signals](#signals)
 * [Event dispatcher](#event-dispatcher)
 * [Event emitter](#event-emitter)
@@ -63,7 +65,7 @@ As an example of use:
 int f(int i) { return i; }
 
 struct my_struct {
-    int f(const int &i) { return i }
+    int f(const int &i) const { return i; }
 };
 
 // bind a free function to the delegate
@@ -142,12 +144,12 @@ delegate. As long as a listener can be invoked with the given arguments to yield
 a result that is convertible to the given result type, everything works just
 fine.
 
-As a side note, note that members of a class may or may not be associated with
-instances. If they are not, the first argument of the function type must be that
-of the class on which the members operate and an instance of this class must
-obviously be passed when invoking the delegate:
+As a side note, members of classes may or may not be associated with instances.
+If they are not, the first argument of the function type must be that of the
+class on which the members operate and an instance of this class must obviously
+be passed when invoking the delegate:
 
-```
+```cpp
 entt::delegate<void(my_struct &, int)> delegate;
 delegate.connect<&my_struct::f>();
 
@@ -160,6 +162,79 @@ 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.
 
+## 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
+runtime arguments.<br/>
+When used in this modality, some feature 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
+  supported.
+* Return type and types of arguments **must** coincide with those of the
+  delegate and _being at least convertible_ isn't enough anymore.
+
+Moreover, for a given function type `Ret(Args...)`, the signature of the
+functions connected at runtime must necessarily be `Ret(const void *, Args...)`.
+
+Runtime arguments can be passed both to the constructor of a delegate and to the
+`connect` member function. An optional parameter is also accepted in both cases.
+This argument is used to pass arbitrary user data back and forth as a
+`const void *` upon invocation.<br/>
+To connect a function to a delegate _in the hard way_:
+
+```cpp
+int func(const void *ptr, int i) { return *static_cast<const int *>(ptr) * i; }
+const int value = 42;
+
+// use the constructor ...
+entt::delegate delegate{&func, &value};
+
+// ... or the connect member function
+delegate.connect(&func, &value);
+```
+
+The type of the delegate is deduced from the function if possible. In this case,
+since the first argument is an implementation detail, the deduced function type
+is `int(int)`.<br/>
+Invoking a delegate built in this way follows the same rules as previously
+explained.
+
+## Lambda support
+
+In general, the `delegate` class doesn't fully support lambda functions in all
+their nuances. The reason is pretty simple: a `delegate` isn't a drop-in
+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.
+
+This is a logical consequence of the support for connecting functions at
+runtime. Therefore, lambda functions undergo the same rules and
+limitations.<br/>
+In fact, since non-capturing lambda functions decay to pointers to functions,
+they can be used with a `delegate` as if they were _normal functions_ with
+optional payload:
+
+```cpp
+my_struct instance;
+
+// use the constructor ...
+entt::delegate delegate{+[](const void *ptr, int value) {
+    return static_cast<const my_struct *>(ptr)->f(value);
+}, &instance};
+
+// ... or the connect member function
+delegate.connect([](const void *ptr, int value) {
+    return static_cast<const my_struct *>(ptr)->f(value);
+}, &instance);
+```
+
+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)`.
+
 # Signals
 
 Signal handlers work with references to classes, function pointers and pointers
@@ -312,9 +387,9 @@ 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
-`const E &` can be converted to them for each type of event `E`, no matter what
-the return value is.<br/>
+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:
 
@@ -372,7 +447,7 @@ the messages that are still pending are sent to the listeners at once:
 
 ```cpp
 // emits all the events of the given type at once
-dispatcher.update<my_event>();
+dispatcher.update<an_event>();
 
 // emits all the events queued so far at once
 dispatcher.update();
@@ -402,9 +477,9 @@ 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 const 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/>
+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/>
@@ -418,10 +493,10 @@ my_emitter emitter{};
 ```
 
 Listeners must be movable and callable objects (free functions, lambdas,
-functors, `std::function`s, whatever) whose function type is:
+functors, `std::function`s, whatever) whose function type is compatible with:
 
 ```cpp
-void(const Event &, my_emitter &)
+void(Event &, my_emitter &)
 ```
 
 Where `Event` is the type of event they want to listen.<br/>

docs/md/unreal.md

@@ -0,0 +1,107 @@
+# EnTT and Unreal Engine
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Enable Cpp17](#enable-cpp17)
+* [EnTT as a third party module](#entt-as-a-third-party-module)
+* [Include EnTT](#include-entt)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+## Enable Cpp17
+
+As of writing (Unreal Engine v4.25), the default C++ standard of Unreal Engine
+is C++14.<br/>
+On the other hand, note that `EnTT` requires C++17 to compile. To enable it, in
+the main module of the project there should be a `<Game Name>.Build.cs` file,
+the constructor of which must contain the following lines:
+
+```cs
+PCHUsage = PCHUsageMode.NoSharedPCHs;
+PrivatePCHHeaderFile = "<PCH filename>.h";
+CppStandard = CppStandardVersion.Cpp17;
+```
+
+Replace `<PCH filename>.h` with the name of the already existing PCH header
+file, if any.<br/>
+In case the project doesn't already contain a file of this type, it's possible
+to create one with the following content:
+
+```cpp
+#pragma once
+#include "CoreMinimal.h"
+```
+
+Remember to remove any old `PCHUsage = <...>` line that was previously there. At
+this point, C++17 support should be in place.<br/>
+Try to compile the project to ensure it works as expected before following
+further steps.
+
+Note that updating a *project* to C++17 doesn't necessarily mean that the IDE in
+use will also start to recognize its syntax.<br/>
+If the plan is to use C++17 in the project too, check the specific instructions
+for the IDE in use.
+
+## EnTT as a third party module
+
+Once this point is reached, the `Source` directory should look like this:
+
+```
+Source
+|  MyGame.Target.cs
+|  MyGameEditor.Target.cs
+|
++---MyGame
+|  |  MyGame.Build.cs
+|  |  MyGame.h (PCH Header file)
+|
+\---ThirdParty
+   \---EnTT
+      |   EnTT.Build.cs
+      |
+      \---entt (GitHub repository content inside)
+```
+
+To make this happen, create the folder `ThirdParty` under `Source` if it doesn't
+exist already. Then, add an `EnTT` folder under `ThirdParty`.<br/>
+Within the latter, create a new file `EnTT.Build.cs` with the following content:
+
+```cs
+using System.IO;
+using UnrealBuildTool;
+
+public class EnTT: ModuleRules {
+    public EnTT(ReadOnlyTargetRules Target) : base(Target) {
+        Type = ModuleType.External;
+        PublicIncludePaths.Add(Path.Combine(ModuleDirectory, "entt", "src", "entt"));
+    }
+}
+```
+
+The last line indicates that the actual files will be found in the directory
+`EnTT/entt/src/entt`.<br/>
+Download the repository for `EnTT` and place it next to `EnTT.Build.cs` or
+update the path above accordingly.
+
+Finally, open the `<Game Name>.Build.cs` file and add `EnTT` as a dependency at
+the end of the list:
+
+```cs
+PublicDependencyModuleNames.AddRange(new[] {
+    "Core", "CoreUObject", "Engine", "InputCore", [...], "EnTT"
+});
+```
+
+Note that some IDEs might require a restart to start recognizing the new module
+for code-highlighting features and such.
+
+## Include EnTT
+
+In any source file of the project, add `#include "entt.hpp"` or any other path
+to the file from `EnTT` to use it.<br/>
+Try to create a registry as `entt::registry registry;` to make sure everything
+compiles fine.

natvis/entt/config.natvis

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

natvis/entt/container.natvis

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

natvis/entt/core.natvis

@@ -0,0 +1,31 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+    <Type Name="entt::basic_any&lt;*&gt;">
+		<DisplayString>{{ type={ info->alias,na }, policy={ mode,en } }}</DisplayString>
+    </Type>
+	<Type Name="entt::compressed_pair&lt;*&gt;">
+		<Intrinsic Name="first" Optional="true" Expression="((first_base*)this)->value"/>
+		<Intrinsic Name="first" Optional="true" Expression="*(first_base::base_type*)this"/>
+		<Intrinsic Name="second" Optional="true" Expression="((second_base*)this)->value"/>
+		<Intrinsic Name="second" Optional="true" Expression="*(second_base::base_type*)this"/>
+		<DisplayString >({ first() }, { second() })</DisplayString>
+		<Expand>
+			<Item Name="[first]">first()</Item>
+			<Item Name="[second]">second()</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_hashed_string&lt;*&gt;">
+		<DisplayString Condition="str != nullptr">{{ hash={ hash } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<Item Name="[data]">str,na</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::type_info">
+		<DisplayString>{{ name={ alias,na } }}</DisplayString>
+		<Expand>
+			<Item Name="[hash]">identifier</Item>
+			<Item Name="[index]">seq</Item>
+		</Expand>
+	</Type>
+</AutoVisualizer>

natvis/entt/entity.natvis

@@ -0,0 +1,119 @@
+<?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>
+		<Expand>
+			<Item IncludeView="simple" Name="[entities]">entities,view(simple)nr</Item>
+			<Synthetic Name="[entities]" ExcludeView="simple">
+				<DisplayString>{ entities.size() }</DisplayString>
+				<Expand>
+					<CustomListItems>
+						<Variable Name="pos" InitialValue="0" />
+						<Variable Name="last" InitialValue="entities.size()"/>
+						<Loop>
+							<Break Condition="pos == last"/>
+							<If Condition="to_entity(entities[pos]) == pos">
+								<Item Name="[{ pos }]">entities[pos]</Item>
+							</If>
+							<Exec>++pos</Exec>
+						</Loop>
+					</CustomListItems>
+				</Expand>
+			</Synthetic>
+			<Synthetic Name="[destroyed]" ExcludeView="simple">
+				<DisplayString>{ to_entity(free_list) != entity_traits::entity_mask }</DisplayString>
+				<Expand>
+					<CustomListItems>
+						<Variable Name="it" InitialValue="to_entity(free_list)" />
+						<Loop>
+							<Break Condition="it == entity_traits::entity_mask"/>
+							<Item Name="[{ it }]">entities[it]</Item>
+							<Exec>it = to_entity(entities[it])</Exec>
+						</Loop>
+					</CustomListItems>
+				</Expand>
+			</Synthetic>
+			<Synthetic Name="[pools]">
+				<DisplayString>{ pools_size() }</DisplayString>
+				<Expand>
+					<IndexListItems ExcludeView="simple">
+						<Size>pools_size()</Size>
+						<ValueNode>*pools.packed.first_base::value[$i].element.second</ValueNode>
+					</IndexListItems>
+					<IndexListItems IncludeView="simple">
+						<Size>pools_size()</Size>
+						<ValueNode>*pools.packed.first_base::value[$i].element.second,view(simple)</ValueNode>
+					</IndexListItems>
+				</Expand>
+			</Synthetic>
+			<Item Name="[groups]" ExcludeView="simple">groups.size()</Item>
+			<Synthetic Name="[vars]">
+				<DisplayString>{ vars_size() }</DisplayString>
+				<Expand>
+					<IndexListItems>
+						<Size>vars_size()</Size>
+						<ValueNode>vars.packed.first_base::value[$i].element.second</ValueNode>
+					</IndexListItems>
+				</Expand>
+			</Synthetic>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_sparse_set&lt;*&gt;">
+		<DisplayString>{{ size={ packed.size() }, type={ info->alias,na } }}</DisplayString>
+		<Expand>
+			<Item Name="[capacity]" ExcludeView="simple">packed.capacity()</Item>
+			<Item Name="[policy]">mode,en</Item>
+			<ExpandedItem IncludeView="simple">packed,view(simple)</ExpandedItem>
+			<CustomListItems ExcludeView="simple">
+				<Variable Name="pos" InitialValue="0" />
+				<Variable Name="last" InitialValue="packed.size()"/>
+				<Loop>
+					<Break Condition="pos == last"/>
+					<If Condition="*((entity_traits::entity_type *)&amp;packed[pos]) &lt; ~entity_traits::entity_mask">
+						<Item Name="[{ pos }]">packed[pos]</Item>
+					</If>
+					<Exec>++pos</Exec>
+				</Loop>
+			</CustomListItems>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_storage&lt;*&gt;">
+		<DisplayString>{{ size={ base_type::packed.size() }, type={ base_type::info->alias,na } }}</DisplayString>
+		<Expand>
+			<Item Name="[capacity]" Optional="true" ExcludeView="simple">packed.first_base::value.capacity() * comp_traits::page_size</Item>
+			<Item Name="[page size]" Optional="true" ExcludeView="simple">comp_traits::page_size</Item>
+			<Item Name="[base]" ExcludeView="simple">(base_type*)this,nand</Item>
+			<Item Name="[base]" IncludeView="simple">(base_type*)this,view(simple)nand</Item>
+			<!-- having SFINAE-like techniques in natvis is priceless :) -->
+			<CustomListItems Condition="packed.first_base::value.size() != 0" Optional="true">
+				<Variable Name="pos" InitialValue="0" />
+				<Variable Name="last" InitialValue="base_type::packed.size()"/>
+				<Loop>
+					<Break Condition="pos == last"/>
+					<If Condition="*((base_type::entity_traits::entity_type *)&amp;base_type::packed[pos]) &lt; ~base_type::entity_traits::entity_mask">
+						<Item Name="[{ pos }]">packed.first_base::value[pos / comp_traits::page_size][pos &amp; (comp_traits::page_size - 1)]</Item>
+					</If>
+					<Exec>++pos</Exec>
+				</Loop>
+			</CustomListItems>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_view&lt;*&gt;">
+		<DisplayString>{{ size_hint={ view->packed.size() } }}</DisplayString>
+		<Expand>
+			<Item Name="[pools]">pools,na</Item>
+			<Item Name="[filter]">filter,na</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::null_t">
+		<DisplayString>&lt;null&gt;</DisplayString>
+	</Type>
+	<Type Name="entt::tombstone_t">
+		<DisplayString>&lt;tombstone&gt;</DisplayString>
+	</Type>
+</AutoVisualizer>

natvis/entt/locator.natvis

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

natvis/entt/meta.natvis

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

natvis/entt/platform.natvis

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

natvis/entt/poly.natvis

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

natvis/entt/process.natvis

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

natvis/entt/resource.natvis

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

natvis/entt/signal.natvis

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

src/entt/config/config.h

@@ -1,67 +1,69 @@
 #ifndef ENTT_CONFIG_CONFIG_H
 #define ENTT_CONFIG_CONFIG_H
 
-
-#ifndef ENTT_NOEXCEPT
-#   define ENTT_NOEXCEPT noexcept
+#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
+#    define ENTT_NOEXCEPT noexcept
+#    define ENTT_THROW throw
+#    define ENTT_TRY try
+#    define ENTT_CATCH catch(...)
+#else
+#    define ENTT_NOEXCEPT
+#    define ENTT_THROW
+#    define ENTT_TRY if(true)
+#    define ENTT_CATCH if(false)
 #endif
 
-
-#ifndef ENTT_HS_SUFFIX
-#   define ENTT_HS_SUFFIX _hs
+#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_HWS_SUFFIX
-#   define ENTT_HWS_SUFFIX _hws
-#endif
-
-
 #ifndef ENTT_USE_ATOMIC
-#   define ENTT_MAYBE_ATOMIC(Type) Type
+#    define ENTT_MAYBE_ATOMIC(Type) Type
 #else
-#   include <atomic>
-#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#    include <atomic>
+#    define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
 #endif
 
-
 #ifndef ENTT_ID_TYPE
-#   include <cstdint>
-#   define ENTT_ID_TYPE std::uint32_t
+#    include <cstdint>
+#    define ENTT_ID_TYPE std::uint32_t
 #endif
 
-
-#ifndef ENTT_PAGE_SIZE
-#   define ENTT_PAGE_SIZE 32768
+#ifndef ENTT_SPARSE_PAGE
+#    define ENTT_SPARSE_PAGE 4096
 #endif
 
-
-#ifndef ENTT_ASSERT
-#   include <cassert>
-#   define ENTT_ASSERT(condition) assert(condition)
+#ifndef ENTT_PACKED_PAGE
+#    define ENTT_PACKED_PAGE 1024
 #endif
 
+#ifdef ENTT_DISABLE_ASSERT
+#    undef ENTT_ASSERT
+#    define ENTT_ASSERT(...) (void(0))
+#elif !defined ENTT_ASSERT
+#    include <cassert>
+#    define ENTT_ASSERT(condition, ...) assert(condition)
+#endif
 
-#ifndef ENTT_DISABLE_ETO
-#   include <type_traits>
-#   define ENTT_ENABLE_ETO(Type) (std::is_default_constructible_v<Type> && std::is_empty_v<Type>)
+#ifdef ENTT_NO_ETO
+#    define ENTT_IGNORE_IF_EMPTY false
 #else
-#   // sfinae-friendly definition
-#   define ENTT_ENABLE_ETO(Type) (false && std::is_void_v<Type>)
+#    define ENTT_IGNORE_IF_EMPTY true
 #endif
 
-
 #ifndef ENTT_STANDARD_CPP
-#   if defined _MSC_VER
-#      define ENTT_PRETTY_FUNCTION __FUNCSIG__
-#      define ENTT_PRETTY_FUNCTION_CONSTEXPR ENTT_PRETTY_FUNCTION
-#   elif defined __clang__ || (defined __GNUC__ && __GNUC__ > 8)
-#      define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
-#      define ENTT_PRETTY_FUNCTION_CONSTEXPR ENTT_PRETTY_FUNCTION
-#   elif defined __GNUC__
-#      define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
-#   endif
+#    if defined __clang__ || defined __GNUC__
+#        define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#        define ENTT_PRETTY_FUNCTION_PREFIX '='
+#        define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#        define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#        define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#        define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#    endif
 #endif
 
-
 #endif

src/entt/config/version.h

@@ -1,10 +1,8 @@
 #ifndef ENTT_CONFIG_VERSION_H
 #define ENTT_CONFIG_VERSION_H
 
-
 #define ENTT_VERSION_MAJOR 3
-#define ENTT_VERSION_MINOR 3
-#define ENTT_VERSION_PATCH 2
-
+#define ENTT_VERSION_MINOR 9
+#define ENTT_VERSION_PATCH 0
 
 #endif

src/entt/container/dense_hash_map.hpp

@@ -0,0 +1,959 @@
+#ifndef ENTT_CONTAINER_DENSE_HASH_MAP_HPP
+#define ENTT_CONTAINER_DENSE_HASH_MAP_HPP
+
+#include <algorithm>
+#include <cmath>
+#include <cstddef>
+#include <functional>
+#include <iterator>
+#include <limits>
+#include <memory>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include <vector>
+#include "../config/config.h"
+#include "../core/compressed_pair.hpp"
+#include "../core/memory.hpp"
+#include "../core/type_traits.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename Key, typename Type>
+struct dense_hash_map_node final {
+    template<typename... Args>
+    dense_hash_map_node(const std::size_t pos, Args &&...args)
+        : next{pos},
+          element{std::forward<Args>(args)...} {}
+
+    std::size_t next;
+    std::pair<Key, Type> element;
+};
+
+template<typename It>
+class dense_hash_map_iterator {
+    friend dense_hash_map_iterator<const std::remove_pointer_t<It> *>;
+
+    using iterator_traits = std::iterator_traits<decltype(std::addressof(std::declval<It>()->element))>;
+
+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 iterator_category = std::random_access_iterator_tag;
+
+    dense_hash_map_iterator() ENTT_NOEXCEPT = default;
+
+    dense_hash_map_iterator(const It iter) ENTT_NOEXCEPT
+        : it{iter} {}
+
+    template<bool Const = std::is_const_v<std::remove_pointer_t<It>>, typename = std::enable_if_t<Const>>
+    dense_hash_map_iterator(const dense_hash_map_iterator<std::remove_const_t<std::remove_pointer_t<It>> *> &other)
+        : it{other.it} {}
+
+    dense_hash_map_iterator &operator++() ENTT_NOEXCEPT {
+        return ++it, *this;
+    }
+
+    dense_hash_map_iterator operator++(int) ENTT_NOEXCEPT {
+        dense_hash_map_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    dense_hash_map_iterator &operator--() ENTT_NOEXCEPT {
+        return --it, *this;
+    }
+
+    dense_hash_map_iterator operator--(int) ENTT_NOEXCEPT {
+        dense_hash_map_iterator orig = *this;
+        return operator--(), orig;
+    }
+
+    dense_hash_map_iterator &operator+=(const difference_type value) ENTT_NOEXCEPT {
+        it += value;
+        return *this;
+    }
+
+    dense_hash_map_iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
+        dense_hash_map_iterator copy = *this;
+        return (copy += value);
+    }
+
+    dense_hash_map_iterator &operator-=(const difference_type value) ENTT_NOEXCEPT {
+        return (*this += -value);
+    }
+
+    dense_hash_map_iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
+        return (*this + -value);
+    }
+
+    [[nodiscard]] reference operator[](const difference_type value) const {
+        return it[value].element;
+    }
+
+    [[nodiscard]] pointer operator->() const {
+        return std::addressof(it->element);
+    }
+
+    [[nodiscard]] reference operator*() const {
+        return *operator->();
+    }
+
+    template<typename ILhs, typename IRhs>
+    friend auto operator-(const dense_hash_map_iterator<ILhs> &, const dense_hash_map_iterator<IRhs> &) ENTT_NOEXCEPT;
+
+    template<typename ILhs, typename IRhs>
+    friend bool operator==(const dense_hash_map_iterator<ILhs> &, const dense_hash_map_iterator<IRhs> &) ENTT_NOEXCEPT;
+
+    template<typename ILhs, typename IRhs>
+    friend bool operator<(const dense_hash_map_iterator<ILhs> &, const dense_hash_map_iterator<IRhs> &) ENTT_NOEXCEPT;
+
+private:
+    It it;
+};
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] auto operator-(const dense_hash_map_iterator<ILhs> &lhs, const dense_hash_map_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return lhs.it - rhs.it;
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator==(const dense_hash_map_iterator<ILhs> &lhs, const dense_hash_map_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return lhs.it == rhs.it;
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator!=(const dense_hash_map_iterator<ILhs> &lhs, const dense_hash_map_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator<(const dense_hash_map_iterator<ILhs> &lhs, const dense_hash_map_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return lhs.it < rhs.it;
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator>(const dense_hash_map_iterator<ILhs> &lhs, const dense_hash_map_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return rhs < lhs;
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator<=(const dense_hash_map_iterator<ILhs> &lhs, const dense_hash_map_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return !(lhs > rhs);
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator>=(const dense_hash_map_iterator<ILhs> &lhs, const dense_hash_map_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return !(lhs < rhs);
+}
+
+template<typename It>
+class dense_hash_map_local_iterator {
+    friend dense_hash_map_local_iterator<const std::remove_pointer_t<It> *>;
+
+    using iterator_traits = std::iterator_traits<decltype(std::addressof(std::declval<It>()->element))>;
+
+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 iterator_category = std::forward_iterator_tag;
+
+    dense_hash_map_local_iterator() ENTT_NOEXCEPT = default;
+
+    dense_hash_map_local_iterator(It iter, const std::size_t pos) ENTT_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_map_local_iterator(const dense_hash_map_local_iterator<std::remove_const_t<std::remove_pointer_t<It>> *> &other)
+        : it{other.it},
+          offset{other.offset} {}
+
+    dense_hash_map_local_iterator &operator++() ENTT_NOEXCEPT {
+        return offset = it[offset].next, *this;
+    }
+
+    dense_hash_map_local_iterator operator++(int) ENTT_NOEXCEPT {
+        dense_hash_map_local_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    [[nodiscard]] pointer operator->() const {
+        return std::addressof(it[offset].element);
+    }
+
+    [[nodiscard]] reference operator*() const {
+        return *operator->();
+    }
+
+    [[nodiscard]] std::size_t index() const ENTT_NOEXCEPT {
+        return offset;
+    }
+
+private:
+    It it;
+    std::size_t offset;
+};
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator==(const dense_hash_map_local_iterator<ILhs> &lhs, const dense_hash_map_local_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return lhs.index() == rhs.index();
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator!=(const dense_hash_map_local_iterator<ILhs> &lhs, const dense_hash_map_local_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief Associative container for key-value pairs with unique keys.
+ *
+ * Internally, elements are organized into buckets. Which bucket an element is
+ * placed into depends entirely on the hash of its key. Keys with the same hash
+ * code appear in the same bucket.
+ *
+ * @tparam Key Key type of the associative container.
+ * @tparam Type Mapped type of the associative container.
+ * @tparam Hash Type of function to use to hash the keys.
+ * @tparam KeyEqual Type of function to use to compare the keys for equality.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Key, typename Type, typename Hash, typename KeyEqual, typename Allocator>
+class dense_hash_map final {
+    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<std::pair<const Key, Type>>;
+    using alloc_traits = typename std::allocator_traits<alloc>;
+
+    using node_type = internal::dense_hash_map_node<const Key, 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]] auto constrained_find(const Other &key, std::size_t bucket) {
+        for(auto it = begin(bucket), last = end(bucket); it != last; ++it) {
+            if(packed.second()(it->first, key)) {
+                return begin() + it.index();
+            }
+        }
+
+        return end();
+    }
+
+    template<typename Other>
+    [[nodiscard]] auto constrained_find(const Other &key, std::size_t bucket) const {
+        for(auto it = cbegin(bucket), last = cend(bucket); it != last; ++it) {
+            if(packed.second()(it->first, key)) {
+                return cbegin() + it.index();
+            }
+        }
+
+        return cend();
+    }
+
+    template<typename... Args>
+    [[nodiscard]] auto get_or_emplace(const Key &key, Args &&...args) {
+        const auto hash = sparse.second()(key);
+        auto index = hash_to_bucket(hash);
+
+        if(auto it = constrained_find(key, 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<Args>(args)...);
+        // update goes after emplace to enforce exception guarantees
+        sparse.first()[index] = size() - 1u;
+
+        return std::make_pair(--end(), true);
+    }
+
+    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.first);
+            for(; *curr != last; curr = &packed.first()[*curr].next) {}
+            *curr = pos;
+
+            using node_allocator_traits = typename alloc_traits::template rebind_traits<decltype(node_type::element)>;
+            typename node_allocator_traits::allocator_type allocator = packed.first().get_allocator();
+            auto *ptr = std::addressof(packed.first()[pos].element);
+
+            std::destroy_at(ptr);
+            packed.first()[pos].next = packed.first().back().next;
+            // no exception guarantees when mapped type has a throwing move constructor (we're technically doomed)
+            node_allocator_traits::construct(allocator, ptr, std::move(packed.first().back().element));
+        }
+
+        packed.first().pop_back();
+    }
+
+public:
+    /*! @brief Key type of the container. */
+    using key_type = Key;
+    /*! @brief Mapped type of the container. */
+    using mapped_type = Type;
+    /*! @brief Key-value type of the container. */
+    using value_type = std::pair<const Key, Type>;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Type of function to use to hash the keys. */
+    using hasher = Hash;
+    /*! @brief Type of function to use to compare the keys for equality. */
+    using key_equal = KeyEqual;
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Random access iterator type. */
+    using iterator = internal::dense_hash_map_iterator<typename packed_container_type::pointer>;
+    /*! @brief Constant random access iterator type. */
+    using const_iterator = internal::dense_hash_map_iterator<typename packed_container_type::const_pointer>;
+    /*! @brief Forward iterator type. */
+    using local_iterator = internal::dense_hash_map_local_iterator<typename packed_container_type::pointer>;
+    /*! @brief Constant forward iterator type. */
+    using const_local_iterator = internal::dense_hash_map_local_iterator<typename packed_container_type::const_pointer>;
+
+    /*! @brief Default constructor. */
+    dense_hash_map()
+        : dense_hash_map(minimum_capacity) {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit dense_hash_map(const allocator_type &allocator)
+        : dense_hash_map{minimum_capacity, hasher{}, key_equal{}, allocator} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator and user
+     * supplied minimal number of buckets.
+     * @param bucket_count Minimal number of buckets.
+     * @param allocator The allocator to use.
+     */
+    dense_hash_map(const size_type bucket_count, const allocator_type &allocator)
+        : dense_hash_map{bucket_count, hasher{}, key_equal{}, allocator} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator, hash
+     * function and user supplied minimal number of buckets.
+     * @param bucket_count Minimal number of buckets.
+     * @param hash Hash function to use.
+     * @param allocator The allocator to use.
+     */
+    dense_hash_map(const size_type bucket_count, const hasher &hash, const allocator_type &allocator)
+        : dense_hash_map{bucket_count, hash, key_equal{}, allocator} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator, hash
+     * function, compare function and user supplied minimal number of buckets.
+     * @param bucket_count Minimal number of buckets.
+     * @param hash Hash function to use.
+     * @param equal Compare function to use.
+     * @param allocator The allocator to use.
+     */
+    explicit dense_hash_map(const size_type bucket_count, const hasher &hash = hasher{}, const key_equal &equal = key_equal{}, const allocator_type &allocator = allocator_type())
+        : sparse{allocator, hash},
+          packed{allocator, equal},
+          threshold{default_threshold} {
+        rehash(bucket_count);
+    }
+
+    /**
+     * @brief Copy constructor.
+     * @param other The instance to copy from.
+     */
+    dense_hash_map(const dense_hash_map &other)
+        : dense_hash_map{other, alloc_traits::select_on_container_copy_construction(other.get_allocator())} {}
+
+    /**
+     * @brief Allocator-extended copy constructor.
+     * @param other The instance to copy from.
+     * @param allocator The allocator to use.
+     */
+    dense_hash_map(const dense_hash_map &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} {
+    }
+
+    /**
+     * @brief Default move constructor.
+     * @param other The instance to move from.
+     */
+    dense_hash_map(dense_hash_map &&other) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    dense_hash_map(dense_hash_map &&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())},
+          threshold{other.threshold} {}
+
+    /*! @brief Default destructor. */
+    ~dense_hash_map() = default;
+
+    /**
+     * @brief Copy assignment operator.
+     * @param other The instance to copy from.
+     * @return This container.
+     */
+    dense_hash_map &operator=(const dense_hash_map &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;
+    }
+
+    /**
+     * @brief Default move assignment operator.
+     * @param other The instance to move from.
+     * @return This container.
+     */
+    dense_hash_map &operator=(dense_hash_map &&other) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Returns the associated allocator.
+     * @return The associated allocator.
+     */
+    [[nodiscard]] constexpr allocator_type get_allocator() const ENTT_NOEXCEPT {
+        return sparse.first().get_allocator();
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * The returned iterator points to the first instance of the internal array.
+     * If the array is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first instance of the internal array.
+     */
+    [[nodiscard]] const_iterator cbegin() const ENTT_NOEXCEPT {
+        return packed.first().data();
+    }
+
+    /*! @copydoc cbegin */
+    [[nodiscard]] const_iterator begin() const ENTT_NOEXCEPT {
+        return cbegin();
+    }
+
+    /*! @copydoc begin */
+    [[nodiscard]] iterator begin() ENTT_NOEXCEPT {
+        return packed.first().data();
+    }
+
+    /**
+     * @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();
+    }
+
+    /*! @copydoc cend */
+    [[nodiscard]] const_iterator end() const ENTT_NOEXCEPT {
+        return cend();
+    }
+
+    /*! @copydoc end */
+    [[nodiscard]] iterator end() ENTT_NOEXCEPT {
+        return packed.first().data() + size();
+    }
+
+    /**
+     * @brief Checks whether a container is empty.
+     * @return True if the container is empty, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return packed.first().empty();
+    }
+
+    /**
+     * @brief Returns the number of elements in a container.
+     * @return Number of elements in a container.
+     */
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return packed.first().size();
+    }
+
+    /*! @brief Clears the container. */
+    void clear() ENTT_NOEXCEPT {
+        sparse.first().clear();
+        packed.first().clear();
+        rehash(0u);
+    }
+
+    /**
+     * @brief Inserts an element into the container, if the key does not exist.
+     * @param value A key-value pair eventually convertible to the value type.
+     * @return A pair consisting of an iterator to the inserted element (or to
+     * the element that prevented the insertion) and a bool denoting whether the
+     * insertion took place.
+     */
+    std::pair<iterator, bool> insert(const value_type &value) {
+        return emplace(value);
+    }
+
+    /*! @copydoc insert */
+    std::pair<iterator, bool> insert(value_type &&value) {
+        return emplace(std::move(value));
+    }
+
+    /**
+     * @copydoc insert
+     * @tparam Arg Type of the key-value pair to insert into the container.
+     */
+    template<typename Arg>
+    std::enable_if_t<std::is_constructible_v<value_type, Arg &&>, std::pair<iterator, bool>>
+    insert(Arg &&value) {
+        return emplace(std::forward<Arg>(value));
+    }
+
+    /**
+     * @brief Inserts elements into the container, if their keys do not exist.
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of elements.
+     * @param last An iterator past the last element of the range of elements.
+     */
+    template<typename It>
+    void insert(It first, It last) {
+        for(; first != last; ++first) {
+            emplace(*first);
+        }
+    }
+
+    /**
+     * @brief Inserts an element into the container or assigns to the current
+     * element if the key already exists.
+     * @tparam Arg Type of the value to insert or assign.
+     * @param key A key used both to look up and to insert if not found.
+     * @param value A value to insert or assign.
+     * @return A pair consisting of an iterator to the element and a bool
+     * denoting whether the insertion took place.
+     */
+    template<typename Arg>
+    std::pair<iterator, bool> insert_or_assign(const key_type &key, Arg &&value) {
+        auto result = try_emplace(key, std::forward<Arg>(value));
+
+        if(!result.second) {
+            result.first->second = std::forward<Arg>(value);
+        }
+
+        return result;
+    }
+
+    /*! @copydoc insert_or_assign */
+    template<typename Arg>
+    std::pair<iterator, bool> insert_or_assign(key_type &&key, Arg &&value) {
+        auto result = try_emplace(std::move(key), std::forward<Arg>(value));
+
+        if(!result.second) {
+            result.first->second = std::forward<Arg>(value);
+        }
+
+        return result;
+    }
+
+    /**
+     * @brief Constructs an element in-place, if the key does not exist.
+     * @tparam Args Types of arguments to forward to the constructor of the
+     * element.
+     * @param args Arguments to forward to the constructor of the element.
+     * @return A pair consisting of an iterator to the inserted element (or to
+     * the element that prevented the insertion) and a bool denoting whether the
+     * insertion took place.
+     */
+    template<typename... Args>
+    std::pair<iterator, bool> emplace(Args &&...args) {
+        if constexpr(sizeof...(Args) == 0u) {
+            return get_or_emplace(key_type{});
+        } else if constexpr(sizeof...(Args) == 1u) {
+            return get_or_emplace(args.first..., std::forward<Args>(args)...);
+        } else if constexpr(sizeof...(Args) == 2u) {
+            return get_or_emplace(std::get<0u>(std::tie(args...)), std::forward<Args>(args)...);
+        } else {
+            static_assert(sizeof...(Args) == 3u, "Invalid arguments");
+            return emplace(std::pair<key_type, mapped_type>{std::forward<Args>(args)...});
+        }
+    }
+
+    /**
+     * @brief Inserts in-place if the key does not exist, does nothing if the
+     * key exists.
+     * @tparam Args Types of arguments to forward to the constructor of the
+     * element.
+     * @param key A key used both to look up and to insert if not found.
+     * @param args Arguments to forward to the constructor of the element.
+     * @return A pair consisting of an iterator to the inserted element (or to
+     * the element that prevented the insertion) and a bool denoting whether the
+     * insertion took place.
+     */
+    template<typename... Args>
+    std::pair<iterator, bool> try_emplace(const key_type &key, Args &&...args) {
+        return get_or_emplace(key, std::piecewise_construct, std::forward_as_tuple(key), std::forward_as_tuple(std::forward<Args>(args)...));
+    }
+
+    /*! @copydoc try_emplace */
+    template<typename... Args>
+    std::pair<iterator, bool> try_emplace(key_type &&key, Args &&...args) {
+        return get_or_emplace(key, std::piecewise_construct, std::forward_as_tuple(std::move(key)), std::forward_as_tuple(std::forward<Args>(args)...));
+    }
+
+    /**
+     * @brief Removes an element from a given position.
+     * @param pos An iterator to the element to remove.
+     * @return An iterator following the removed element.
+     */
+    iterator erase(const_iterator pos) {
+        const auto dist = std::distance(cbegin(), pos);
+        erase(pos->first);
+        return begin() + dist;
+    }
+
+    /**
+     * @brief Removes the given elements from a container.
+     * @param first An iterator to the first element of the range of elements.
+     * @param last An iterator past the last element of the range of elements.
+     * @return An iterator following the last removed element.
+     */
+    iterator erase(const_iterator first, const_iterator last) {
+        const auto dist = std::distance(cbegin(), first);
+
+        for(auto rfirst = std::make_reverse_iterator(last), rlast = std::make_reverse_iterator(first); rfirst != rlast; ++rfirst) {
+            erase(rfirst->first);
+        }
+
+        return dist > static_cast<decltype(dist)>(size()) ? end() : (begin() + dist);
+    }
+
+    /**
+     * @brief Removes the element associated with a given key.
+     * @param key A key value of an element to remove.
+     * @return Number of elements removed (either 0 or 1).
+     */
+    size_type erase(const key_type &key) {
+        for(size_type *curr = sparse.first().data() + bucket(key); *curr != std::numeric_limits<size_type>::max(); curr = &packed.first()[*curr].next) {
+            if(packed.second()(packed.first()[*curr].element.first, key)) {
+                const auto index = *curr;
+                *curr = packed.first()[*curr].next;
+                move_and_pop(index);
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    /**
+     * @brief Exchanges the contents with those of a given container.
+     * @param other Container to exchange the content with.
+     */
+    void swap(dense_hash_map &other) {
+        using std::swap;
+        swap(sparse, other.sparse);
+        swap(packed, other.packed);
+        swap(threshold, other.threshold);
+    }
+
+    /**
+     * @brief Accesses a given element with bounds checking.
+     * @param key A key of an element to find.
+     * @return A reference to the mapped value of the requested element.
+     */
+    [[nodiscard]] Type &at(const key_type &key) {
+        auto it = find(key);
+        ENTT_ASSERT(it != end(), "Invalid key");
+        return it->second;
+    }
+
+    /*! @copydoc at */
+    [[nodiscard]] const Type &at(const key_type &key) const {
+        auto it = find(key);
+        ENTT_ASSERT(it != cend(), "Invalid key");
+        return it->second;
+    }
+
+    /**
+     * @brief Accesses or inserts a given element.
+     * @param key A key of an element to find or insert.
+     * @return A reference to the mapped value of the requested element.
+     */
+    [[nodiscard]] Type &operator[](const key_type &key) {
+        return try_emplace(key).first->second;
+    }
+
+    /**
+     * @brief Accesses or inserts a given element.
+     * @param key A key of an element to find or insert.
+     * @return A reference to the mapped value of the requested element.
+     */
+    [[nodiscard]] Type &operator[](key_type &&key) {
+        return try_emplace(std::move(key)).first->second;
+    }
+
+    /**
+     * @brief Finds an element with a given key.
+     * @param key Key value of an element to search for.
+     * @return An iterator to an element with the given key. If no such element
+     * is found, a past-the-end iterator is returned.
+     */
+    [[nodiscard]] iterator find(const key_type &key) {
+        return constrained_find(key, bucket(key));
+    }
+
+    /*! @copydoc find */
+    [[nodiscard]] const_iterator find(const key_type &key) const {
+        return constrained_find(key, bucket(key));
+    }
+
+    /**
+     * @brief Finds an element with a key that compares _equivalent_ to a given
+     * value.
+     * @tparam Other Type of the key value of an element to search for.
+     * @param key Key value of an element to search for.
+     * @return An iterator to an element with the given key. If no such element
+     * is found, a past-the-end iterator is returned.
+     */
+    template<typename Other>
+    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, iterator>>
+    find(const Other &key) {
+        return constrained_find(key, bucket(key));
+    }
+
+    /*! @copydoc find */
+    template<typename Other>
+    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, const_iterator>>
+    find(const Other &key) const {
+        return constrained_find(key, bucket(key));
+    }
+
+    /**
+     * @brief Checks if the container contains an element with a given key.
+     * @param key Key value of an element to search for.
+     * @return True if there is such an element, false otherwise.
+     */
+    [[nodiscard]] bool contains(const key_type &key) const {
+        return (find(key) != cend());
+    }
+
+    /**
+     * @brief Checks if the container contains an element with a key that
+     * compares _equivalent_ to a given value.
+     * @tparam Other Type of the key value of an element to search for.
+     * @param key Key value of an element to search for.
+     * @return True if there is such an element, false otherwise.
+     */
+    template<typename Other>
+    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, bool>>
+    contains(const Other &key) const {
+        return (find(key) != cend());
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the beginning of the given bucket.
+     */
+    [[nodiscard]] const_local_iterator cbegin(const size_type index) const {
+        return {packed.first().data(), sparse.first()[index]};
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the beginning of the given bucket.
+     */
+    [[nodiscard]] const_local_iterator begin(const size_type index) const {
+        return cbegin(index);
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the beginning of the given bucket.
+     */
+    [[nodiscard]] local_iterator begin(const size_type index) {
+        return {packed.first().data(), sparse.first()[index]};
+    }
+
+    /**
+     * @brief Returns an iterator to the end of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the end of the given bucket.
+     */
+    [[nodiscard]] const_local_iterator cend([[maybe_unused]] const size_type index) const {
+        return {packed.first().data(), std::numeric_limits<size_type>::max()};
+    }
+
+    /**
+     * @brief Returns an iterator to the end of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the end of the given bucket.
+     */
+    [[nodiscard]] const_local_iterator end([[maybe_unused]] const size_type index) const {
+        return cend(index);
+    }
+
+    /**
+     * @brief Returns an iterator to the end of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the end of the given bucket.
+     */
+    [[nodiscard]] local_iterator end([[maybe_unused]] const size_type index) {
+        return {packed.first().data(), std::numeric_limits<size_type>::max()};
+    }
+
+    /**
+     * @brief Returns the number of buckets.
+     * @return The number of buckets.
+     */
+    [[nodiscard]] size_type bucket_count() const {
+        return sparse.first().size();
+    }
+
+    /**
+     * @brief Returns the maximum number of buckets.
+     * @return The maximum number of buckets.
+     */
+    [[nodiscard]] size_type max_bucket_count() const {
+        return sparse.first().max_size();
+    }
+
+    /**
+     * @brief Returns the number of elements in a given bucket.
+     * @param index The index of the bucket to examine.
+     * @return The number of elements in the given bucket.
+     */
+    [[nodiscard]] size_type bucket_size(const size_type index) const {
+        return static_cast<size_type>(std::distance(begin(index), end(index)));
+    }
+
+    /**
+     * @brief Returns the bucket for a given key.
+     * @param key The value of the key to examine.
+     * @return The bucket for the given key.
+     */
+    [[nodiscard]] size_type bucket(const key_type &key) const {
+        return hash_to_bucket(sparse.second()(key));
+    }
+
+    /**
+     * @brief Returns the average number of elements per bucket.
+     * @return The average number of elements per bucket.
+     */
+    [[nodiscard]] float load_factor() const {
+        return size() / static_cast<float>(bucket_count());
+    }
+
+    /**
+     * @brief Returns the maximum average number of elements per bucket.
+     * @return The maximum average number of elements per bucket.
+     */
+    [[nodiscard]] float max_load_factor() const {
+        return threshold;
+    }
+
+    /**
+     * @brief Sets the desired maximum average number of elements per bucket.
+     * @param value A desired maximum average number of elements per bucket.
+     */
+    void max_load_factor(const float value) {
+        ENTT_ASSERT(value > 0.f, "Invalid load factor");
+        threshold = value;
+        rehash(0u);
+    }
+
+    /**
+     * @brief Reserves at least the specified number of buckets and regenerates
+     * the hash table.
+     * @param count New number of buckets.
+     */
+    void rehash(const size_type count) {
+        auto value = (std::max)(count, minimum_capacity);
+        value = (std::max)(value, static_cast<size_type>(size() / max_load_factor()));
+
+        if(const auto sz = next_power_of_two(value); sz != bucket_count()) {
+            sparse.first().resize(sz);
+            std::fill(sparse.first().begin(), sparse.first().end(), std::numeric_limits<size_type>::max());
+
+            for(size_type pos{}, last = size(); pos < last; ++pos) {
+                const auto index = bucket(packed.first()[pos].element.first);
+                packed.first()[pos].next = std::exchange(sparse.first()[index], pos);
+            }
+        }
+    }
+
+    /**
+     * @brief Reserves space for at least the specified number of elements and
+     * regenerates the hash table.
+     * @param count New number of elements.
+     */
+    void reserve(const size_type count) {
+        packed.first().reserve(count);
+        rehash(static_cast<size_type>(std::ceil(count / max_load_factor())));
+    }
+
+    /**
+     * @brief Returns the function used to hash the keys.
+     * @return The function used to hash the keys.
+     */
+    [[nodiscard]] hasher hash_function() const {
+        return sparse.second();
+    }
+
+    /**
+     * @brief Returns the function used to compare keys for equality.
+     * @return The function used to compare keys for equality.
+     */
+    [[nodiscard]] key_equal key_eq() const {
+        return packed.second();
+    }
+
+private:
+    compressed_pair<sparse_container_type, hasher> sparse;
+    compressed_pair<packed_container_type, key_equal> packed;
+    float threshold;
+};
+
+} // namespace entt
+
+#endif

src/entt/container/dense_hash_set.hpp

@@ -0,0 +1,849 @@
+#ifndef ENTT_CONTAINER_DENSE_HASH_SET_HPP
+#define ENTT_CONTAINER_DENSE_HASH_SET_HPP
+
+#include <algorithm>
+#include <cmath>
+#include <cstddef>
+#include <functional>
+#include <iterator>
+#include <limits>
+#include <memory>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include <vector>
+#include "../config/config.h"
+#include "../core/compressed_pair.hpp"
+#include "../core/memory.hpp"
+#include "../core/type_traits.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename 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)))>;
+
+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 iterator_category = std::random_access_iterator_tag;
+
+    dense_hash_set_iterator() ENTT_NOEXCEPT = default;
+
+    dense_hash_set_iterator(const It iter) ENTT_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)
+        : it{other.it} {}
+
+    dense_hash_set_iterator &operator++() ENTT_NOEXCEPT {
+        return ++it, *this;
+    }
+
+    dense_hash_set_iterator operator++(int) ENTT_NOEXCEPT {
+        dense_hash_set_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    dense_hash_set_iterator &operator--() ENTT_NOEXCEPT {
+        return --it, *this;
+    }
+
+    dense_hash_set_iterator operator--(int) ENTT_NOEXCEPT {
+        dense_hash_set_iterator orig = *this;
+        return operator--(), orig;
+    }
+
+    dense_hash_set_iterator &operator+=(const difference_type value) ENTT_NOEXCEPT {
+        it += value;
+        return *this;
+    }
+
+    dense_hash_set_iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
+        dense_hash_set_iterator copy = *this;
+        return (copy += value);
+    }
+
+    dense_hash_set_iterator &operator-=(const difference_type value) ENTT_NOEXCEPT {
+        return (*this += -value);
+    }
+
+    dense_hash_set_iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
+        return (*this + -value);
+    }
+
+    [[nodiscard]] reference operator[](const difference_type value) const {
+        return it[value].element;
+    }
+
+    [[nodiscard]] pointer operator->() const {
+        return std::addressof(it->element);
+    }
+
+    [[nodiscard]] reference operator*() const {
+        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 ILhs, typename IRhs>
+    friend bool operator==(const dense_hash_set_iterator<ILhs> &, const dense_hash_set_iterator<IRhs> &) ENTT_NOEXCEPT;
+
+    template<typename ILhs, typename IRhs>
+    friend bool operator<(const dense_hash_set_iterator<ILhs> &, const dense_hash_set_iterator<IRhs> &) ENTT_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 {
+    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 {
+    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 {
+    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 {
+    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 {
+    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 {
+    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 {
+    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)))>;
+
+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 iterator_category = std::forward_iterator_tag;
+
+    dense_hash_set_local_iterator() ENTT_NOEXCEPT = default;
+
+    dense_hash_set_local_iterator(It iter, const std::size_t pos) ENTT_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)
+        : it{other.it},
+          offset{other.offset} {}
+
+    dense_hash_set_local_iterator &operator++() ENTT_NOEXCEPT {
+        return offset = it[offset].next, *this;
+    }
+
+    dense_hash_set_local_iterator operator++(int) ENTT_NOEXCEPT {
+        dense_hash_set_local_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    [[nodiscard]] pointer operator->() const {
+        return std::addressof(it[offset].element);
+    }
+
+    [[nodiscard]] reference operator*() const {
+        return *operator->();
+    }
+
+    [[nodiscard]] std::size_t index() const ENTT_NOEXCEPT {
+        return offset;
+    }
+
+private:
+    It it;
+    std::size_t offset;
+};
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] bool operator==(const dense_hash_set_local_iterator<ILhs> &lhs, const dense_hash_set_local_iterator<IRhs> &rhs) ENTT_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 {
+    return !(lhs == rhs);
+}
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief Associative container for unique objects of a given type.
+ *
+ * Internally, elements are organized into buckets. Which bucket an element is
+ * placed into depends entirely on its hash. Elements with the same hash code
+ * appear in the same bucket.
+ *
+ * @tparam Type Value type of the associative container.
+ * @tparam Hash Type of function to use to hash the values.
+ * @tparam KeyEqual Type of function to use to compare the values for equality.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Type, typename Hash, typename KeyEqual, typename Allocator>
+class dense_hash_set final {
+    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 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]] 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 end();
+    }
+
+    template<typename Other>
+    [[nodiscard]] auto constrained_find(const Other &value, std::size_t bucket) const {
+        for(auto it = cbegin(bucket), last = cend(bucket); it != last; ++it) {
+            if(packed.second()(*it, value)) {
+                return cbegin() + 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);
+
+        if(auto it = constrained_find(arg, 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;
+
+        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
+            packed.first()[pos] = std::move(packed.first().back());
+        }
+
+        packed.first().pop_back();
+    }
+
+public:
+    /*! @brief Key type of the container. */
+    using key_type = Type;
+    /*! @brief Value type of the container. */
+    using value_type = Type;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Type of function to use to hash the elements. */
+    using hasher = Hash;
+    /*! @brief Type of function to use to compare the elements for equality. */
+    using key_equal = KeyEqual;
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Random access iterator type. */
+    using iterator = internal::dense_hash_set_iterator<typename packed_container_type::pointer>;
+    /*! @brief Constant random access iterator type. */
+    using const_iterator = internal::dense_hash_set_iterator<typename packed_container_type::const_pointer>;
+    /*! @brief Forward iterator type. */
+    using local_iterator = internal::dense_hash_set_local_iterator<typename packed_container_type::pointer>;
+    /*! @brief Constant forward iterator type. */
+    using const_local_iterator = internal::dense_hash_set_local_iterator<typename packed_container_type::const_pointer>;
+
+    /*! @brief Default constructor. */
+    dense_hash_set()
+        : dense_hash_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} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator and user
+     * supplied minimal number of buckets.
+     * @param bucket_count Minimal number of buckets.
+     * @param allocator The allocator to use.
+     */
+    dense_hash_set(const size_type bucket_count, const allocator_type &allocator)
+        : dense_hash_set{bucket_count, hasher{}, key_equal{}, allocator} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator, hash
+     * function and user supplied minimal number of buckets.
+     * @param bucket_count Minimal number of buckets.
+     * @param hash Hash function to use.
+     * @param allocator The allocator to use.
+     */
+    dense_hash_set(const size_type bucket_count, const hasher &hash, const allocator_type &allocator)
+        : dense_hash_set{bucket_count, hash, key_equal{}, allocator} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator, hash
+     * function, compare function and user supplied minimal number of buckets.
+     * @param bucket_count Minimal number of buckets.
+     * @param hash Hash function to use.
+     * @param equal Compare function to use.
+     * @param allocator The allocator to use.
+     */
+    explicit dense_hash_set(const size_type bucket_count, const hasher &hash = hasher{}, const key_equal &equal = key_equal{}, const allocator_type &allocator = allocator_type())
+        : sparse{allocator, hash},
+          packed{allocator, equal},
+          threshold{default_threshold} {
+        rehash(bucket_count);
+    }
+
+    /**
+     * @brief 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 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} {
+    }
+
+    /**
+     * @brief Default move constructor.
+     * @param other The instance to move from.
+     */
+    dense_hash_set(dense_hash_set &&other) ENTT_NOEXCEPT = 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())},
+          threshold{other.threshold} {}
+
+    /*! @brief Default destructor. */
+    ~dense_hash_set() = default;
+
+    /**
+     * @brief Copy assignment operator.
+     * @param other The instance to copy from.
+     * @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;
+    }
+
+    /**
+     * @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;
+
+    /**
+     * @brief Returns the associated allocator.
+     * @return The associated allocator.
+     */
+    [[nodiscard]] constexpr allocator_type get_allocator() const ENTT_NOEXCEPT {
+        return sparse.first().get_allocator();
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * The returned iterator points to the first instance of the internal array.
+     * If the array is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first instance of the internal array.
+     */
+    [[nodiscard]] const_iterator cbegin() const ENTT_NOEXCEPT {
+        return packed.first().data();
+    }
+
+    /*! @copydoc cbegin */
+    [[nodiscard]] const_iterator begin() const ENTT_NOEXCEPT {
+        return cbegin();
+    }
+
+    /*! @copydoc begin */
+    [[nodiscard]] iterator begin() ENTT_NOEXCEPT {
+        return packed.first().data();
+    }
+
+    /**
+     * @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();
+    }
+
+    /*! @copydoc cend */
+    [[nodiscard]] const_iterator end() const ENTT_NOEXCEPT {
+        return cend();
+    }
+
+    /*! @copydoc end */
+    [[nodiscard]] iterator end() ENTT_NOEXCEPT {
+        return packed.first().data() + size();
+    }
+
+    /**
+     * @brief Checks whether a container is empty.
+     * @return True if the container is empty, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return packed.first().empty();
+    }
+
+    /**
+     * @brief Returns the number of elements in a container.
+     * @return Number of elements in a container.
+     */
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return packed.first().size();
+    }
+
+    /*! @brief Clears the container. */
+    void clear() ENTT_NOEXCEPT {
+        sparse.first().clear();
+        packed.first().clear();
+        rehash(0u);
+    }
+
+    /**
+     * @brief Inserts an element into the container, if it does not exist.
+     * @param value An element to insert into the container.
+     * @return A pair consisting of an iterator to the inserted element (or to
+     * the element that prevented the insertion) and a bool denoting whether the
+     * insertion took place.
+     */
+    std::pair<iterator, bool> insert(const value_type &value) {
+        return emplace(value);
+    }
+
+    /*! @copydoc insert */
+    std::pair<iterator, bool> insert(value_type &&value) {
+        return emplace(std::move(value));
+    }
+
+    /**
+     * @brief Inserts elements into the container, if they do not exist.
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of elements.
+     * @param last An iterator past the last element of the range of elements.
+     */
+    template<typename It>
+    void insert(It first, It last) {
+        for(; first != last; ++first) {
+            emplace(*first);
+        }
+    }
+
+    /**
+     * @brief Constructs an element in-place, if it does not exist.
+     * @tparam Args Types of arguments to forward to the constructor of the
+     * element.
+     * @param args Arguments to forward to the constructor of the element.
+     * @return A pair consisting of an iterator to the inserted element (or to
+     * the element that prevented the insertion) and a bool denoting whether the
+     * insertion took place.
+     */
+    template<typename... Args>
+    std::pair<iterator, bool> emplace(Args &&...args) {
+        if constexpr(((sizeof...(Args) == 1u) && ... && std::is_same_v<std::remove_const_t<std::remove_reference_t<Args>>, value_type>)) {
+            return get_or_emplace(std::forward<Args>(args)...);
+        } else {
+            return get_or_emplace(value_type{std::forward<Args>(args)...});
+        }
+    }
+
+    /**
+     * @brief Removes an element from a given position.
+     * @param pos An iterator to the element to remove.
+     * @return An iterator following the removed element.
+     */
+    iterator erase(const_iterator pos) {
+        const auto dist = std::distance(cbegin(), pos);
+        erase(*pos);
+        return begin() + dist;
+    }
+
+    /**
+     * @brief Removes the given elements from a container.
+     * @param first An iterator to the first element of the range of elements.
+     * @param last An iterator past the last element of the range of elements.
+     * @return An iterator following the last removed element.
+     */
+    iterator erase(const_iterator first, const_iterator last) {
+        const auto dist = std::distance(cbegin(), first);
+
+        for(auto rfirst = std::make_reverse_iterator(last), rlast = std::make_reverse_iterator(first); rfirst != rlast; ++rfirst) {
+            erase(*rfirst);
+        }
+
+        return dist > static_cast<decltype(dist)>(size()) ? end() : (begin() + dist);
+    }
+
+    /**
+     * @brief Removes the element associated with a given value.
+     * @param value Value of an element to remove.
+     * @return Number of elements removed (either 0 or 1).
+     */
+    size_type erase(const value_type &value) {
+        return do_erase(value);
+    }
+
+    /**
+     * @brief Exchanges the contents with those of a given container.
+     * @param other Container to exchange the content with.
+     */
+    void swap(dense_hash_set &other) {
+        using std::swap;
+        swap(sparse, other.sparse);
+        swap(packed, other.packed);
+        swap(threshold, other.threshold);
+    }
+
+    /**
+     * @brief Finds an element with a given value.
+     * @param value Value of an element to search for.
+     * @return An iterator to an element with the given value. If no such
+     * element is found, a past-the-end iterator is returned.
+     */
+    [[nodiscard]] iterator find(const value_type &value) {
+        return constrained_find(value, bucket(value));
+    }
+
+    /*! @copydoc find */
+    [[nodiscard]] const_iterator find(const value_type &value) const {
+        return constrained_find(value, bucket(value));
+    }
+
+    /**
+     * @brief Finds an element that compares _equivalent_ to a given value.
+     * @tparam Other Type of an element to search for.
+     * @param value Value of an element to search for.
+     * @return An iterator to an element with the given value. If no such
+     * element is found, a past-the-end iterator is returned.
+     */
+    template<typename Other>
+    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, iterator>>
+    find(const Other &value) {
+        return constrained_find(value, 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));
+    }
+
+    /**
+     * @brief Checks if the container contains an element with a given value.
+     * @param value Value of an element to search for.
+     * @return True if there is such an element, false otherwise.
+     */
+    [[nodiscard]] bool contains(const value_type &value) const {
+        return (find(value) != cend());
+    }
+
+    /**
+     * @brief Checks if the container contains an element that compares
+     * _equivalent_ to a given value.
+     * @tparam Other Type of an element to search for.
+     * @param value Value of an element to search for.
+     * @return True if there is such an element, false otherwise.
+     */
+    template<typename Other>
+    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, bool>>
+    contains(const Other &value) const {
+        return (find(value) != cend());
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the beginning of the given bucket.
+     */
+    [[nodiscard]] const_local_iterator cbegin(const size_type index) const {
+        return {packed.first().data(), sparse.first()[index]};
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the beginning of the given bucket.
+     */
+    [[nodiscard]] const_local_iterator begin(const size_type index) const {
+        return cbegin(index);
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the beginning of the given bucket.
+     */
+    [[nodiscard]] local_iterator begin(const size_type index) {
+        return {packed.first().data(), sparse.first()[index]};
+    }
+
+    /**
+     * @brief Returns an iterator to the end of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the end of the given bucket.
+     */
+    [[nodiscard]] const_local_iterator cend([[maybe_unused]] const size_type index) const {
+        return {packed.first().data(), std::numeric_limits<size_type>::max()};
+    }
+
+    /**
+     * @brief Returns an iterator to the end of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the end of the given bucket.
+     */
+    [[nodiscard]] const_local_iterator end([[maybe_unused]] const size_type index) const {
+        return cend(index);
+    }
+
+    /**
+     * @brief Returns an iterator to the end of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the end of the given bucket.
+     */
+    [[nodiscard]] local_iterator end([[maybe_unused]] const size_type index) {
+        return {packed.first().data(), std::numeric_limits<size_type>::max()};
+    }
+
+    /**
+     * @brief Returns the number of buckets.
+     * @return The number of buckets.
+     */
+    [[nodiscard]] size_type bucket_count() const {
+        return sparse.first().size();
+    }
+
+    /**
+     * @brief Returns the maximum number of buckets.
+     * @return The maximum number of buckets.
+     */
+    [[nodiscard]] size_type max_bucket_count() const {
+        return sparse.first().max_size();
+    }
+
+    /**
+     * @brief Returns the number of elements in a given bucket.
+     * @param index The index of the bucket to examine.
+     * @return The number of elements in the given bucket.
+     */
+    [[nodiscard]] size_type bucket_size(const size_type index) const {
+        return static_cast<size_type>(std::distance(begin(index), end(index)));
+    }
+
+    /**
+     * @brief Returns the bucket for a given element.
+     * @param value The value of the element to examine.
+     * @return The bucket for the given element.
+     */
+    [[nodiscard]] size_type bucket(const value_type &value) const {
+        return hash_to_bucket(sparse.second()(value));
+    }
+
+    /**
+     * @brief Returns the average number of elements per bucket.
+     * @return The average number of elements per bucket.
+     */
+    [[nodiscard]] float load_factor() const {
+        return size() / static_cast<float>(bucket_count());
+    }
+
+    /**
+     * @brief Returns the maximum average number of elements per bucket.
+     * @return The maximum average number of elements per bucket.
+     */
+    [[nodiscard]] float max_load_factor() const {
+        return threshold;
+    }
+
+    /**
+     * @brief Sets the desired maximum average number of elements per bucket.
+     * @param value A desired maximum average number of elements per bucket.
+     */
+    void max_load_factor(const float value) {
+        ENTT_ASSERT(value > 0.f, "Invalid load factor");
+        threshold = value;
+        rehash(0u);
+    }
+
+    /**
+     * @brief Reserves at least the specified number of buckets and regenerates
+     * the hash table.
+     * @param count New number of buckets.
+     */
+    void rehash(const size_type count) {
+        auto value = (std::max)(count, minimum_capacity);
+        value = (std::max)(value, static_cast<size_type>(size() / max_load_factor()));
+
+        if(const auto sz = next_power_of_two(value); sz != bucket_count()) {
+            sparse.first().resize(sz);
+            std::fill(sparse.first().begin(), sparse.first().end(), std::numeric_limits<size_type>::max());
+
+            for(size_type pos{}, last = size(); pos < last; ++pos) {
+                const auto index = bucket(packed.first()[pos].element);
+                packed.first()[pos].next = std::exchange(sparse.first()[index], pos);
+            }
+        }
+    }
+
+    /**
+     * @brief Reserves space for at least the specified number of elements and
+     * regenerates the hash table.
+     * @param count New number of elements.
+     */
+    void reserve(const size_type count) {
+        packed.first().reserve(count);
+        rehash(static_cast<size_type>(std::ceil(count / max_load_factor())));
+    }
+
+    /**
+     * @brief Returns the function used to hash the elements.
+     * @return The function used to hash the elements.
+     */
+    [[nodiscard]] hasher hash_function() const {
+        return sparse.second();
+    }
+
+    /**
+     * @brief Returns the function used to compare elements for equality.
+     * @return The function used to compare elements for equality.
+     */
+    [[nodiscard]] key_equal key_eq() const {
+        return packed.second();
+    }
+
+private:
+    compressed_pair<sparse_container_type, hasher> sparse;
+    compressed_pair<packed_container_type, key_equal> packed;
+    float threshold;
+};
+
+} // namespace entt
+
+#endif

src/entt/container/fwd.hpp

@@ -0,0 +1,25 @@
+#ifndef ENTT_CONTAINER_FWD_HPP
+#define ENTT_CONTAINER_FWD_HPP
+
+#include <functional>
+#include <memory>
+
+namespace entt {
+
+template<
+    typename Key, typename Type,
+    typename = std::hash<Key>,
+    typename = std::equal_to<Key>,
+    typename = std::allocator<std::pair<const Key, Type>>>
+class dense_hash_map;
+
+template<
+    typename Type,
+    typename = std::hash<Type>,
+    typename = std::equal_to<Type>,
+    typename = std::allocator<Type>>
+class dense_hash_set;
+
+} // namespace entt
+
+#endif

src/entt/core/algorithm.hpp

@@ -1,18 +1,15 @@
 #ifndef ENTT_CORE_ALGORITHM_HPP
 #define ENTT_CORE_ALGORITHM_HPP
 
-
-#include <vector>
-#include <utility>
-#include <iterator>
 #include <algorithm>
 #include <functional>
+#include <iterator>
+#include <utility>
+#include <vector>
 #include "utility.hpp"
 
-
 namespace entt {
 
-
 /**
  * @brief Function object to wrap `std::sort` in a class type.
  *
@@ -36,12 +33,11 @@ struct std_sort {
      * @param args Arguments to forward to the sort function, if any.
      */
     template<typename It, typename Compare = std::less<>, typename... Args>
-    void operator()(It first, It last, Compare compare = Compare{}, Args &&... args) const {
+    void operator()(It first, It last, Compare compare = Compare{}, Args &&...args) const {
         std::sort(std::forward<Args>(args)..., std::move(first), std::move(last), std::move(compare));
     }
 };
 
-
 /*! @brief Function object for performing insertion sort. */
 struct insertion_sort {
     /**
@@ -58,12 +54,12 @@ struct insertion_sort {
     template<typename It, typename Compare = std::less<>>
     void operator()(It first, It last, Compare compare = Compare{}) const {
         if(first < last) {
-            for(auto it = first+1; it < last; ++it) {
+            for(auto it = first + 1; it < last; ++it) {
                 auto value = std::move(*it);
                 auto pre = it;
 
-                for(; pre > first && compare(value, *(pre-1)); --pre) {
-                    *pre = std::move(*(pre-1));
+                for(; pre > first && compare(value, *(pre - 1)); --pre) {
+                    *pre = std::move(*(pre - 1));
                 }
 
                 *pre = std::move(value);
@@ -72,7 +68,6 @@ struct insertion_sort {
     }
 };
 
-
 /**
  * @brief Function object for performing LSD radix sort.
  * @tparam Bit Number of bits processed per pass.
@@ -80,7 +75,7 @@ struct insertion_sort {
  */
 template<std::size_t Bit, std::size_t N>
 struct radix_sort {
-    static_assert((N % Bit) == 0);
+    static_assert((N % Bit) == 0, "The maximum number of bits to sort must be a multiple of the number of bits processed per pass");
 
     /**
      * @brief Sorts the elements in a range.
@@ -111,17 +106,17 @@ struct radix_sort {
                 std::size_t index[buckets]{};
                 std::size_t count[buckets]{};
 
-                std::for_each(from, to, [&getter, &count, start](const value_type &item) {
-                    ++count[(getter(item) >> start) & mask];
-                });
+                for(auto it = from; it != to; ++it) {
+                    ++count[(getter(*it) >> start) & mask];
+                }
 
-                std::for_each(std::next(std::begin(index)), std::end(index), [index = std::begin(index), count = std::begin(count)](auto &item) mutable {
-                    item = *(index++) + *(count++);
-                });
+                for(std::size_t pos{}, end = buckets - 1u; pos < end; ++pos) {
+                    index[pos + 1u] = index[pos] + count[pos];
+                }
 
-                std::for_each(from, to, [&getter, &out, &index, start](value_type &item) {
-                    out[index[(getter(item) >> start) & mask]++] = std::move(item);
-                });
+                for(auto it = from; it != to; ++it) {
+                    out[index[(getter(*it) >> start) & mask]++] = std::move(*it);
+                }
             };
 
             for(std::size_t pass = 0; pass < (passes & ~1); pass += 2) {
@@ -137,8 +132,6 @@ struct radix_sort {
     }
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/core/any.hpp

@@ -0,0 +1,494 @@
+#ifndef ENTT_CORE_ANY_HPP
+#define ENTT_CORE_ANY_HPP
+
+#include <cstddef>
+#include <memory>
+#include <type_traits>
+#include <utility>
+#include "../config/config.h"
+#include "../core/utility.hpp"
+#include "fwd.hpp"
+#include "type_info.hpp"
+#include "type_traits.hpp"
+
+namespace entt {
+
+/**
+ * @brief A SBO friendly, type-safe container for single values of any type.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ */
+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 vtable_type = const void *(const operation, const basic_any &, const void *);
+
+    template<typename Type>
+    static constexpr bool in_situ = Len && alignof(Type) <= alignof(storage_type) && sizeof(Type) <= sizeof(storage_type) && std::is_nothrow_move_constructible_v<Type>;
+
+    template<typename Type>
+    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;
+
+        if constexpr(in_situ<Type>) {
+            instance = (value.mode == policy::owner) ? ENTT_LAUNDER(reinterpret_cast<const Type *>(&value.storage)) : static_cast<const Type *>(value.instance);
+        } else {
+            instance = 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);
+            }
+            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))};
+                }
+            }
+
+            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)));
+                return other;
+            }
+            [[fallthrough]];
+        case operation::assign:
+            if constexpr(std::is_copy_assignable_v<Type>) {
+                *const_cast<Type *>(instance) = *static_cast<const Type *>(other);
+                return other;
+            }
+            break;
+        case operation::destroy:
+            if constexpr(in_situ<Type>) {
+                instance->~Type();
+            } else if constexpr(std::is_array_v<Type>) {
+                delete[] instance;
+            } else {
+                delete instance;
+            }
+            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;
+            } else {
+                return (instance == other) ? other : nullptr;
+            }
+        case operation::get:
+            return instance;
+        }
+
+        return nullptr;
+    }
+
+    template<typename Type, typename... Args>
+    void initialize([[maybe_unused]] Args &&...args) {
+        if constexpr(!std::is_void_v<Type>) {
+            vtable = basic_vtable<std::remove_const_t<std::remove_reference_t<Type>>>;
+            info = &type_id<Type>();
+
+            if constexpr(std::is_lvalue_reference_v<Type>) {
+                static_assert(sizeof...(Args) == 1u && (std::is_lvalue_reference_v<Args> && ...), "Invalid arguments");
+                mode = std::is_const_v<std::remove_reference_t<Type>> ? policy::cref : policy::ref;
+                instance = (std::addressof(args), ...);
+            } else if constexpr(in_situ<Type>) {
+                if constexpr(sizeof...(Args) != 0u && std::is_aggregate_v<Type>) {
+                    new(&storage) Type{std::forward<Args>(args)...};
+                } else {
+                    new(&storage) Type(std::forward<Args>(args)...);
+                }
+            } else {
+                if constexpr(sizeof...(Args) != 0u && std::is_aggregate_v<Type>) {
+                    instance = new Type{std::forward<Args>(args)...};
+                } else {
+                    instance = new Type(std::forward<Args>(args)...);
+                }
+            }
+        }
+    }
+
+    basic_any(const basic_any &other, const policy pol) ENTT_NOEXCEPT
+        : instance{other.data()},
+          info{other.info},
+          vtable{other.vtable},
+          mode{pol} {}
+
+public:
+    /*! @brief Size of the internal storage. */
+    static constexpr auto length = Len;
+    /*! @brief Alignment requirement. */
+    static constexpr auto alignment = Align;
+
+    /*! @brief Default constructor. */
+    basic_any() ENTT_NOEXCEPT
+        : instance{},
+          info{&type_id<void>()},
+          vtable{},
+          mode{policy::owner} {}
+
+    /**
+     * @brief Constructs a wrapper by directly initializing the new object.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    explicit basic_any(std::in_place_type_t<Type>, Args &&...args)
+        : basic_any{} {
+        initialize<Type>(std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Constructs a wrapper from a given value.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     */
+    template<typename Type, typename = std::enable_if_t<!std::is_same_v<std::decay_t<Type>, basic_any>>>
+    basic_any(Type &&value)
+        : basic_any{} {
+        initialize<std::decay_t<Type>>(std::forward<Type>(value));
+    }
+
+    /**
+     * @brief Copy constructor.
+     * @param other The instance to copy from.
+     */
+    basic_any(const basic_any &other)
+        : basic_any{} {
+        if(other.vtable) {
+            other.vtable(operation::copy, other, this);
+        }
+    }
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    basic_any(basic_any &&other) ENTT_NOEXCEPT
+        : instance{},
+          info{other.info},
+          vtable{other.vtable},
+          mode{other.mode} {
+        if(other.vtable) {
+            other.vtable(operation::move, other, this);
+        }
+    }
+
+    /*! @brief Frees the internal storage, whatever it means. */
+    ~basic_any() {
+        if(vtable && mode == policy::owner) {
+            vtable(operation::destroy, *this, nullptr);
+        }
+    }
+
+    /**
+     * @brief Copy assignment operator.
+     * @param other The instance to copy from.
+     * @return This any object.
+     */
+    basic_any &operator=(const basic_any &other) {
+        reset();
+
+        if(other.vtable) {
+            other.vtable(operation::copy, other, this);
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This any object.
+     */
+    basic_any &operator=(basic_any &&other) ENTT_NOEXCEPT {
+        reset();
+
+        if(other.vtable) {
+            other.vtable(operation::move, other, this);
+            info = other.info;
+            vtable = other.vtable;
+            mode = other.mode;
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Value assignment operator.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     * @return This any object.
+     */
+    template<typename Type>
+    std::enable_if_t<!std::is_same_v<std::decay_t<Type>, basic_any>, basic_any &>
+    operator=(Type &&value) {
+        emplace<std::decay_t<Type>>(std::forward<Type>(value));
+        return *this;
+    }
+
+    /**
+     * @brief Returns the object type if any, `type_id<void>()` otherwise.
+     * @return The object type if any, `type_id<void>()` otherwise.
+     */
+    [[nodiscard]] const type_info &type() const ENTT_NOEXCEPT {
+        return *info;
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] const void *data() const ENTT_NOEXCEPT {
+        return vtable ? vtable(operation::get, *this, nullptr) : nullptr;
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @param req Expected type.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] const void *data(const type_info &req) const ENTT_NOEXCEPT {
+        return *info == req ? data() : nullptr;
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] void *data() ENTT_NOEXCEPT {
+        return (!vtable || mode == policy::cref) ? nullptr : const_cast<void *>(vtable(operation::get, *this, nullptr));
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @param req Expected type.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] void *data(const type_info &req) ENTT_NOEXCEPT {
+        return *info == req ? data() : nullptr;
+    }
+
+    /**
+     * @brief Replaces the contained object by creating a new instance directly.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    void emplace(Args &&...args) {
+        reset();
+        initialize<Type>(std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Copy assigns a value to the contained object without replacing it.
+     * @param other The value to assign to the contained object.
+     * @return True in case of success, false otherwise.
+     */
+    bool assign(const any &other) {
+        if(vtable && mode != policy::cref && *info == *other.info) {
+            return (vtable(operation::assign, *this, other.data()) != nullptr);
+        }
+
+        return false;
+    }
+
+    /**
+     * @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) {
+            if(auto *val = other.data(); val) {
+                return (vtable(operation::transfer, *this, val) != nullptr);
+            } else {
+                return (vtable(operation::assign, *this, std::as_const(other).data()) != nullptr);
+            }
+        }
+
+        return false;
+    }
+
+    /*! @brief Destroys contained object */
+    void reset() {
+        if(vtable && mode == policy::owner) {
+            vtable(operation::destroy, *this, nullptr);
+        }
+
+        info = &type_id<void>();
+        vtable = nullptr;
+        mode = 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 {
+        return vtable != nullptr;
+    }
+
+    /**
+     * @brief Checks if two wrappers differ in their content.
+     * @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 {
+        if(vtable && *info == *other.info) {
+            return (vtable(operation::compare, *this, other.data()) != nullptr);
+        }
+
+        return (!vtable && !other.vtable);
+    }
+
+    /**
+     * @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)};
+    }
+
+    /*! @copydoc as_ref */
+    [[nodiscard]] basic_any as_ref() const ENTT_NOEXCEPT {
+        return basic_any{*this, 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);
+    }
+
+private:
+    union {
+        const void *instance;
+        storage_type storage;
+    };
+    const type_info *info;
+    vtable_type *vtable;
+    policy mode;
+};
+
+/**
+ * @brief Checks if two wrappers differ in their content.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @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.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Alignment requirement.
+ * @param data Target any object.
+ * @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 {
+    const auto *const instance = any_cast<std::remove_reference_t<Type>>(&data);
+    ENTT_ASSERT(instance, "Invalid instance");
+    return static_cast<Type>(*instance);
+}
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type any_cast(basic_any<Len, Align> &data) ENTT_NOEXCEPT {
+    // forces const on non-reference types to make them work also with wrappers for const references
+    auto *const instance = any_cast<std::remove_reference_t<const Type>>(&data);
+    ENTT_ASSERT(instance, "Invalid instance");
+    return static_cast<Type>(*instance);
+}
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type any_cast(basic_any<Len, Align> &&data) ENTT_NOEXCEPT {
+    if constexpr(std::is_copy_constructible_v<std::remove_const_t<std::remove_reference_t<Type>>>) {
+        if(auto *const instance = any_cast<std::remove_reference_t<Type>>(&data); instance) {
+            return static_cast<Type>(std::move(*instance));
+        } else {
+            return any_cast<Type>(data);
+        }
+    } else {
+        auto *const instance = any_cast<std::remove_reference_t<Type>>(&data);
+        ENTT_ASSERT(instance, "Invalid instance");
+        return static_cast<Type>(std::move(*instance));
+    }
+}
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+const Type *any_cast(const basic_any<Len, Align> *data) ENTT_NOEXCEPT {
+    const auto &info = type_id<std::remove_const_t<std::remove_reference_t<Type>>>();
+    return static_cast<const Type *>(data->data(info));
+}
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type *any_cast(basic_any<Len, Align> *data) ENTT_NOEXCEPT {
+    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));
+}
+
+/**
+ * @brief Constructs a wrapper from a given type, passing it all arguments.
+ * @tparam Type Type of object to use to initialize the wrapper.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ * @tparam Args Types of arguments to use to construct the new instance.
+ * @param args Parameters to use to construct the instance.
+ * @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) {
+    return basic_any<Len, Align>{std::in_place_type<Type>, std::forward<Args>(args)...};
+}
+
+/**
+ * @brief Forwards its argument and avoids copies for lvalue references.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ * @tparam Type Type of argument to use to construct the new instance.
+ * @param value Parameter to use to construct the instance.
+ * @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)};
+}
+
+} // namespace entt
+
+#endif

src/entt/core/attribute.h

@@ -1,33 +1,30 @@
 #ifndef ENTT_CORE_ATTRIBUTE_H
 #define ENTT_CORE_ATTRIBUTE_H
 
-
 #ifndef ENTT_EXPORT
-#   if defined _WIN32 || defined __CYGWIN__ || defined _MSC_VER
-#       define ENTT_EXPORT __declspec(dllexport)
-#       define ENTT_IMPORT __declspec(dllimport)
-#       define ENTT_HIDDEN
-#   elif defined __GNUC__ && __GNUC__ >= 4
-#       define ENTT_EXPORT __attribute__((visibility("default")))
-#       define ENTT_IMPORT __attribute__((visibility("default")))
-#       define ENTT_HIDDEN __attribute__((visibility("hidden")))
-#   else /* Unsupported compiler */
-#       define ENTT_EXPORT
-#       define ENTT_IMPORT
-#       define ENTT_HIDDEN
-#   endif
+#    if defined _WIN32 || defined __CYGWIN__ || defined _MSC_VER
+#        define ENTT_EXPORT __declspec(dllexport)
+#        define ENTT_IMPORT __declspec(dllimport)
+#        define ENTT_HIDDEN
+#    elif defined __GNUC__ && __GNUC__ >= 4
+#        define ENTT_EXPORT __attribute__((visibility("default")))
+#        define ENTT_IMPORT __attribute__((visibility("default")))
+#        define ENTT_HIDDEN __attribute__((visibility("hidden")))
+#    else /* Unsupported compiler */
+#        define ENTT_EXPORT
+#        define ENTT_IMPORT
+#        define ENTT_HIDDEN
+#    endif
 #endif
 
-
 #ifndef ENTT_API
-#   if defined ENTT_API_EXPORT
-#       define ENTT_API ENTT_EXPORT
-#   elif defined ENTT_API_IMPORT
-#       define ENTT_API ENTT_IMPORT
-#   else /* No API */
-#       define ENTT_API
-#   endif
+#    if defined ENTT_API_EXPORT
+#        define ENTT_API ENTT_EXPORT
+#    elif defined ENTT_API_IMPORT
+#        define ENTT_API ENTT_IMPORT
+#    else /* No API */
+#        define ENTT_API
+#    endif
 #endif
 
-
 #endif

src/entt/core/compressed_pair.hpp

@@ -0,0 +1,269 @@
+#ifndef ENTT_CORE_COMPRESSED_PAIR_HPP
+#define ENTT_CORE_COMPRESSED_PAIR_HPP
+
+#include <cstddef>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include "../config/config.h"
+#include "type_traits.hpp"
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename Type, std::size_t, typename = void>
+struct compressed_pair_element {
+    using reference = Type &;
+    using const_reference = const Type &;
+
+    template<bool Dummy = true, typename = std::enable_if_t<Dummy && std::is_default_constructible_v<Type>>>
+    compressed_pair_element()
+        : value{} {}
+
+    template<typename Args, typename = std::enable_if_t<!std::is_same_v<std::remove_const_t<std::remove_reference_t<Args>>, compressed_pair_element>>>
+    compressed_pair_element(Args &&args)
+        : value{std::forward<Args>(args)} {}
+
+    template<typename... Args, std::size_t... Index>
+    compressed_pair_element(std::tuple<Args...> args, std::index_sequence<Index...>)
+        : value{std::get<Index>(args)...} {}
+
+    [[nodiscard]] reference get() ENTT_NOEXCEPT {
+        return value;
+    }
+
+    [[nodiscard]] const_reference get() const ENTT_NOEXCEPT {
+        return value;
+    }
+
+private:
+    Type value;
+};
+
+template<typename Type, std::size_t Tag>
+struct compressed_pair_element<Type, Tag, std::enable_if_t<is_ebco_eligible_v<Type>>>: Type {
+    using reference = Type &;
+    using const_reference = const Type &;
+    using base_type = Type;
+
+    template<bool Dummy = true, typename = std::enable_if_t<Dummy && std::is_default_constructible_v<base_type>>>
+    compressed_pair_element()
+        : base_type{} {}
+
+    template<typename Args, typename = std::enable_if_t<!std::is_same_v<std::remove_const_t<std::remove_reference_t<Args>>, compressed_pair_element>>>
+    compressed_pair_element(Args &&args)
+        : base_type{std::forward<Args>(args)} {}
+
+    template<typename... Args, std::size_t... Index>
+    compressed_pair_element(std::tuple<Args...> args, std::index_sequence<Index...>)
+        : base_type{std::get<Index>(args)...} {}
+
+    [[nodiscard]] reference get() ENTT_NOEXCEPT {
+        return *this;
+    }
+
+    [[nodiscard]] const_reference get() const ENTT_NOEXCEPT {
+        return *this;
+    }
+};
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief A compressed pair.
+ *
+ * A pair that exploits the _Empty Base Class Optimization_ (or _EBCO_) to
+ * reduce its final size to a minimum.
+ *
+ * @tparam First The type of the first element that the pair stores.
+ * @tparam Second The type of the second element that the pair stores.
+ */
+template<typename First, typename Second>
+class compressed_pair final
+    : internal::compressed_pair_element<First, 0u>,
+      internal::compressed_pair_element<Second, 1u> {
+    using first_base = internal::compressed_pair_element<First, 0u>;
+    using second_base = internal::compressed_pair_element<Second, 1u>;
+
+public:
+    /*! @brief The type of the first element that the pair stores. */
+    using first_type = First;
+    /*! @brief The type of the second element that the pair stores. */
+    using second_type = Second;
+
+    /**
+     * @brief Default constructor, conditionally enabled.
+     *
+     * This constructor is only available when the types that the pair stores
+     * are both at least default constructible.
+     *
+     * @tparam Dummy Dummy template parameter used for internal purposes.
+     */
+    template<bool Dummy = true, typename = std::enable_if_t<Dummy && std::is_default_constructible_v<first_type> && std::is_default_constructible_v<second_type>>>
+    constexpr compressed_pair()
+        : first_base{},
+          second_base{} {}
+
+    /**
+     * @brief Copy constructor.
+     * @param other The instance to copy from.
+     */
+    constexpr compressed_pair(const compressed_pair &other) = default;
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    constexpr compressed_pair(compressed_pair &&other) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Constructs a pair from its values.
+     * @tparam Arg Type of value to use to initialize the first element.
+     * @tparam Other Type of value to use to initialize the second element.
+     * @param arg Value to use to initialize the first element.
+     * @param other Value to use to initialize the second element.
+     */
+    template<typename Arg, typename Other>
+    constexpr compressed_pair(Arg &&arg, Other &&other)
+        : first_base{std::forward<Arg>(arg)},
+          second_base{std::forward<Other>(other)} {}
+
+    /**
+     * @brief Constructs a pair by forwarding the arguments to its parts.
+     * @tparam Args Types of arguments to use to initialize the first element.
+     * @tparam Other Types of arguments to use to initialize the second element.
+     * @param args Arguments to use to initialize the first element.
+     * @param other Arguments to use to initialize the second element.
+     */
+    template<typename... Args, typename... Other>
+    constexpr compressed_pair(std::piecewise_construct_t, std::tuple<Args...> args, std::tuple<Other...> other)
+        : first_base{std::move(args), std::index_sequence_for<Args...>{}},
+          second_base{std::move(other), std::index_sequence_for<Other...>{}} {}
+
+    /**
+     * @brief Copy assignment operator.
+     * @param other The instance to copy from.
+     * @return This compressed pair object.
+     */
+    constexpr compressed_pair &operator=(const compressed_pair &other) = default;
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This compressed pair object.
+     */
+    constexpr compressed_pair &operator=(compressed_pair &&other) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Returns the first element that a pair stores.
+     * @return The first element that a pair stores.
+     */
+    [[nodiscard]] first_type &first() ENTT_NOEXCEPT {
+        return static_cast<first_base &>(*this).get();
+    }
+
+    /*! @copydoc first */
+    [[nodiscard]] const first_type &first() const ENTT_NOEXCEPT {
+        return static_cast<const first_base &>(*this).get();
+    }
+
+    /**
+     * @brief Returns the second element that a pair stores.
+     * @return The second element that a pair stores.
+     */
+    [[nodiscard]] second_type &second() ENTT_NOEXCEPT {
+        return static_cast<second_base &>(*this).get();
+    }
+
+    /*! @copydoc second */
+    [[nodiscard]] const second_type &second() const ENTT_NOEXCEPT {
+        return static_cast<const second_base &>(*this).get();
+    }
+
+    /**
+     * @brief Swaps two compressed pair objects.
+     * @param other The compressed pair to swap with.
+     */
+    void swap(compressed_pair &other) {
+        using std::swap;
+        swap(first(), other.first());
+        swap(second(), other.second());
+    }
+
+    /**
+     * @brief Extracts an element from the compressed pair.
+     * @tparam Index An integer value that is either 0 or 1.
+     * @return Returns a reference to the first element if `Index` is 0 and a
+     * reference to the second element if `Index` is 1.
+     */
+    template<std::size_t Index>
+    decltype(auto) get() ENTT_NOEXCEPT {
+        if constexpr(Index == 0u) {
+            return first();
+        } else {
+            static_assert(Index == 1u, "Index out of bounds");
+            return second();
+        }
+    }
+
+    /*! @copydoc get */
+    template<std::size_t Index>
+    decltype(auto) get() const ENTT_NOEXCEPT {
+        if constexpr(Index == 0u) {
+            return first();
+        } else {
+            static_assert(Index == 1u, "Index out of bounds");
+            return second();
+        }
+    }
+};
+
+/**
+ * @brief Deduction guide.
+ * @tparam Type Type of value to use to initialize the first element.
+ * @tparam Other Type of value to use to initialize the second element.
+ */
+template<typename Type, typename Other>
+compressed_pair(Type &&, Other &&) -> compressed_pair<std::decay_t<Type>, std::decay_t<Other>>;
+
+/**
+ * @brief Swaps two compressed pair objects.
+ * @tparam First The type of the first element that the pairs store.
+ * @tparam Second The type of the second element that the pairs store.
+ * @param lhs A valid compressed pair object.
+ * @param rhs A valid compressed pair object.
+ */
+template<typename First, typename Second>
+inline void swap(compressed_pair<First, Second> &lhs, compressed_pair<First, Second> &rhs) {
+    lhs.swap(rhs);
+}
+
+} // namespace entt
+
+// disable structured binding support for clang 6, it messes when specializing tuple_size
+#if !defined __clang_major__ || __clang_major__ > 6
+namespace std {
+
+template<typename First, typename Second>
+struct tuple_size<entt::compressed_pair<First, Second>>: integral_constant<size_t, 2u> {};
+
+template<size_t Index, typename First, typename Second>
+struct tuple_element<Index, entt::compressed_pair<First, Second>>: conditional<Index == 0u, First, Second> {
+    static_assert(Index < 2u, "Index out of bounds");
+};
+
+} // namespace std
+#endif
+
+#endif

src/entt/core/enum.hpp

@@ -0,0 +1,112 @@
+#ifndef ENTT_CORE_FLAG_HPP
+#define ENTT_CORE_FLAG_HPP
+
+#include <type_traits>
+#include "../config/config.h"
+
+namespace entt {
+
+/**
+ * @brief Enable bitmask support for enum classes.
+ * @tparam Type The enum type for which to enable bitmask support.
+ */
+template<typename Type, typename = void>
+struct enum_as_bitmask: std::false_type {};
+
+/*! @copydoc enum_as_bitmask */
+template<typename Type>
+struct enum_as_bitmask<Type, std::void_t<decltype(Type::_entt_enum_as_bitmask)>>: std::true_type {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The enum class type for which to enable bitmask support.
+ */
+template<typename Type>
+inline constexpr bool enum_as_bitmask_v = enum_as_bitmask<Type>::value;
+
+} // namespace entt
+
+/**
+ * @brief Operator available for enums for which bitmask support is enabled.
+ * @tparam Type Enum class type.
+ * @param lhs The first value to use.
+ * @param rhs The second value to use.
+ * @return The result of invoking the operator on the underlying types of the
+ * two values provided.
+ */
+template<typename Type>
+[[nodiscard]] constexpr std::enable_if_t<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)};
+}
+
+/*! @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)};
+}
+
+/*! @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)};
+}
+
+/**
+ * @brief Operator available for enums for which bitmask support is enabled.
+ * @tparam Type Enum class type.
+ * @param value The value to use.
+ * @return The result of invoking the operator on the underlying types of the
+ * value provided.
+ */
+template<typename Type>
+[[nodiscard]] constexpr std::enable_if_t<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)};
+}
+
+/*! @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 {
+    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 {
+    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 {
+    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 {
+    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

@@ -1,13 +1,11 @@
 #ifndef ENTT_CORE_FAMILY_HPP
 #define ENTT_CORE_FAMILY_HPP
 
-
 #include "../config/config.h"
-
+#include "fwd.hpp"
 
 namespace entt {
 
-
 /**
  * @brief Dynamic identifier generator.
  *
@@ -17,11 +15,11 @@ namespace entt {
  */
 template<typename...>
 class family {
-    inline static ENTT_MAYBE_ATOMIC(ENTT_ID_TYPE) identifier{};
+    inline static ENTT_MAYBE_ATOMIC(id_type) identifier{};
 
 public:
     /*! @brief Unsigned integer type. */
-    using family_type = ENTT_ID_TYPE;
+    using family_type = id_type;
 
     /*! @brief Statically generated unique identifier for the given type. */
     template<typename... Type>
@@ -29,8 +27,6 @@ public:
     inline static const family_type type = identifier++;
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/core/fwd.hpp

@@ -0,0 +1,20 @@
+#ifndef ENTT_CORE_FWD_HPP
+#define ENTT_CORE_FWD_HPP
+
+#include <type_traits>
+#include "../config/config.h"
+
+namespace entt {
+
+template<std::size_t Len = sizeof(double[2]), std::size_t = alignof(typename std::aligned_storage_t<Len + !Len>)>
+class basic_any;
+
+/*! @brief Alias declaration for type identifiers. */
+using id_type = ENTT_ID_TYPE;
+
+/*! @brief Alias declaration for the most common use case. */
+using any = basic_any<>;
+
+} // namespace entt
+
+#endif

src/entt/core/hashed_string.hpp

@@ -1,28 +1,23 @@
 #ifndef ENTT_CORE_HASHED_STRING_HPP
 #define ENTT_CORE_HASHED_STRING_HPP
 
-
 #include <cstddef>
 #include <cstdint>
 #include "../config/config.h"
-
+#include "fwd.hpp"
 
 namespace entt {
 
-
 /**
  * @cond TURN_OFF_DOXYGEN
  * Internal details not to be documented.
  */
 
-
 namespace internal {
 
-
 template<typename>
 struct fnv1a_traits;
 
-
 template<>
 struct fnv1a_traits<std::uint32_t> {
     using type = std::uint32_t;
@@ -30,7 +25,6 @@ struct fnv1a_traits<std::uint32_t> {
     static constexpr std::uint32_t prime = 16777619;
 };
 
-
 template<>
 struct fnv1a_traits<std::uint64_t> {
     using type = std::uint64_t;
@@ -38,16 +32,13 @@ struct fnv1a_traits<std::uint64_t> {
     static constexpr std::uint64_t prime = 1099511628211ull;
 };
 
-
-}
-
+} // namespace internal
 
 /**
  * Internal details not to be documented.
- * @endcond TURN_OFF_DOXYGEN
+ * @endcond
  */
 
-
 /**
  * @brief Zero overhead unique identifier.
  *
@@ -57,11 +48,15 @@ struct fnv1a_traits<std::uint64_t> {
  * Because of that, a hashed string can also be used in constant expressions if
  * required.
  *
+ * @warning
+ * This class doesn't take ownership of user-supplied strings nor does it make a
+ * copy of them.
+ *
  * @tparam Char Character type.
  */
 template<typename Char>
 class basic_hashed_string {
-    using traits_type = internal::fnv1a_traits<ENTT_ID_TYPE>;
+    using hs_traits = internal::fnv1a_traits<id_type>;
 
     struct const_wrapper {
         // non-explicit constructor on purpose
@@ -70,11 +65,11 @@ class basic_hashed_string {
     };
 
     // Fowler–Noll–Vo hash function v. 1a - the good
-    static constexpr ENTT_ID_TYPE helper(const Char *curr) ENTT_NOEXCEPT {
-        auto value = traits_type::offset;
+    [[nodiscard]] static constexpr id_type helper(const Char *curr) ENTT_NOEXCEPT {
+        auto value = hs_traits::offset;
 
         while(*curr != 0) {
-            value = (value ^ static_cast<traits_type::type>(*(curr++))) * traits_type::prime;
+            value = (value ^ static_cast<hs_traits::type>(*(curr++))) * hs_traits::prime;
         }
 
         return value;
@@ -84,7 +79,19 @@ public:
     /*! @brief Character type. */
     using value_type = Char;
     /*! @brief Unsigned integer type. */
-    using hash_type = ENTT_ID_TYPE;
+    using hash_type = id_type;
+
+    /**
+     * @brief Returns directly the numeric representation of a string view.
+     * @param str Human-readable identifer.
+     * @param size 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;
+    }
 
     /**
      * @brief Returns directly the numeric representation of a string.
@@ -94,7 +101,7 @@ public:
      * characters.<br/>
      * Example of use:
      * @code{.cpp}
-     * const auto value = basic_hashed_string<char>::to_value("my.png");
+     * const auto value = basic_hashed_string<char>::value("my.png");
      * @endcode
      *
      * @tparam N Number of characters of the identifier.
@@ -102,7 +109,7 @@ public:
      * @return The numeric representation of the string.
      */
     template<std::size_t N>
-    static constexpr hash_type value(const value_type (&str)[N]) ENTT_NOEXCEPT {
+    [[nodiscard]] static constexpr hash_type value(const value_type (&str)[N]) ENTT_NOEXCEPT {
         return helper(str);
     }
 
@@ -111,26 +118,14 @@ public:
      * @param wrapper Helps achieving the purpose by relying on overloading.
      * @return The numeric representation of the string.
      */
-    static hash_type value(const_wrapper wrapper) ENTT_NOEXCEPT {
+    [[nodiscard]] static hash_type value(const_wrapper wrapper) ENTT_NOEXCEPT {
         return helper(wrapper.str);
     }
 
-    /**
-     * @brief Returns directly the numeric representation of a string view.
-     * @param str Human-readable identifer.
-     * @param size Length of the string to hash.
-     * @return The numeric representation of the string.
-     */
-    static hash_type value(const value_type *str, std::size_t size) ENTT_NOEXCEPT {
-        ENTT_ID_TYPE partial{traits_type::offset};
-        while(size--) { partial = (partial^(str++)[0])*traits_type::prime; }
-        return partial;
-    }
-
     /*! @brief Constructs an empty hashed string. */
     constexpr basic_hashed_string() ENTT_NOEXCEPT
-        : str{nullptr}, hash{}
-    {}
+        : str{nullptr},
+          hash{} {}
 
     /**
      * @brief Constructs a hashed string from an array of const characters.
@@ -148,23 +143,27 @@ public:
      */
     template<std::size_t N>
     constexpr basic_hashed_string(const value_type (&curr)[N]) ENTT_NOEXCEPT
-        : str{curr}, hash{helper(curr)}
-    {}
+        : str{curr},
+          hash{helper(curr)} {}
 
     /**
      * @brief Explicit constructor on purpose to avoid constructing a hashed
      * string directly from a `const value_type *`.
+     *
+     * @warning
+     * The lifetime of the string is not extended nor is it copied.
+     *
      * @param wrapper Helps achieving the purpose by relying on overloading.
      */
     explicit constexpr basic_hashed_string(const_wrapper wrapper) ENTT_NOEXCEPT
-        : str{wrapper.str}, hash{helper(wrapper.str)}
-    {}
+        : str{wrapper.str},
+          hash{helper(wrapper.str)} {}
 
     /**
      * @brief Returns the human-readable representation of a hashed string.
      * @return The string used to initialize the instance.
      */
-    constexpr const value_type * data() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr const value_type *data() const ENTT_NOEXCEPT {
         return str;
     }
 
@@ -172,26 +171,21 @@ public:
      * @brief Returns the numeric representation of a hashed string.
      * @return The numeric representation of the instance.
      */
-    constexpr hash_type value() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr hash_type value() const ENTT_NOEXCEPT {
         return hash;
     }
 
     /*! @copydoc data */
-    constexpr operator const value_type *() const ENTT_NOEXCEPT { return data(); }
+    [[nodiscard]] constexpr operator const value_type *() const ENTT_NOEXCEPT {
+        return data();
+    }
 
     /**
      * @brief Returns the numeric representation of a hashed string.
      * @return The numeric representation of the instance.
      */
-    constexpr operator hash_type() const ENTT_NOEXCEPT { return value(); }
-
-    /**
-     * @brief Compares two hashed strings.
-     * @param other Hashed string with which to compare.
-     * @return True if the two hashed strings are identical, false otherwise.
-     */
-    constexpr bool operator==(const basic_hashed_string &other) const ENTT_NOEXCEPT {
-        return hash == other.hash;
+    [[nodiscard]] constexpr operator hash_type() const ENTT_NOEXCEPT {
+        return value();
     }
 
 private:
@@ -199,7 +193,6 @@ private:
     hash_type hash;
 };
 
-
 /**
  * @brief Deduction guide.
  *
@@ -211,9 +204,7 @@ private:
  * @param str Human-readable identifer.
  */
 template<typename Char, std::size_t N>
-basic_hashed_string(const Char (&str)[N]) ENTT_NOEXCEPT
--> basic_hashed_string<Char>;
-
+basic_hashed_string(const Char (&str)[N]) -> basic_hashed_string<Char>;
 
 /**
  * @brief Compares two hashed strings.
@@ -223,40 +214,101 @@ basic_hashed_string(const Char (&str)[N]) ENTT_NOEXCEPT
  * @return True if the two hashed strings are identical, false otherwise.
  */
 template<typename Char>
-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) ENTT_NOEXCEPT {
+    return lhs.value() == rhs.value();
+}
+
+/**
+ * @brief Compares two hashed strings.
+ * @tparam Char Character type.
+ * @param lhs A valid hashed string.
+ * @param rhs A valid hashed string.
+ * @return True if the two hashed strings differ, false otherwise.
+ */
+template<typename Char>
+[[nodiscard]] constexpr bool operator!=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) ENTT_NOEXCEPT {
     return !(lhs == rhs);
 }
 
+/**
+ * @brief Compares two hashed strings.
+ * @tparam Char Character type.
+ * @param lhs A valid hashed string.
+ * @param rhs A valid hashed string.
+ * @return True if the first element is less than the second, false otherwise.
+ */
+template<typename Char>
+[[nodiscard]] constexpr bool operator<(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) ENTT_NOEXCEPT {
+    return lhs.value() < rhs.value();
+}
+
+/**
+ * @brief Compares two hashed strings.
+ * @tparam Char Character type.
+ * @param lhs A valid hashed string.
+ * @param rhs A valid hashed string.
+ * @return True if the first element is less than or equal to the second, false
+ * otherwise.
+ */
+template<typename Char>
+[[nodiscard]] constexpr bool operator<=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) ENTT_NOEXCEPT {
+    return !(rhs < lhs);
+}
+
+/**
+ * @brief Compares two hashed strings.
+ * @tparam Char Character type.
+ * @param lhs A valid hashed string.
+ * @param rhs A valid hashed string.
+ * @return True if the first element is greater than the second, false
+ * otherwise.
+ */
+template<typename Char>
+[[nodiscard]] constexpr bool operator>(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) ENTT_NOEXCEPT {
+    return rhs < lhs;
+}
+
+/**
+ * @brief Compares two hashed strings.
+ * @tparam Char Character type.
+ * @param lhs A valid hashed string.
+ * @param rhs A valid hashed string.
+ * @return True if the first element is greater than or equal to the second,
+ * false otherwise.
+ */
+template<typename Char>
+[[nodiscard]] constexpr bool operator>=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) ENTT_NOEXCEPT {
+    return !(lhs < rhs);
+}
 
 /*! @brief Aliases for common character types. */
 using hashed_string = basic_hashed_string<char>;
 
-
 /*! @brief Aliases for common character types. */
 using hashed_wstring = basic_hashed_string<wchar_t>;
 
-
-}
-
+inline namespace literals {
 
 /**
  * @brief User defined literal for hashed strings.
  * @param str The literal without its suffix.
  * @return A properly initialized hashed string.
  */
-constexpr entt::hashed_string operator"" ENTT_HS_SUFFIX(const char *str, std::size_t) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr entt::hashed_string operator"" _hs(const char *str, std::size_t) ENTT_NOEXCEPT {
     return entt::hashed_string{str};
 }
 
-
 /**
  * @brief User defined literal for hashed wstrings.
  * @param str The literal without its suffix.
  * @return A properly initialized hashed wstring.
  */
-constexpr entt::hashed_wstring operator"" ENTT_HWS_SUFFIX(const wchar_t *str, std::size_t) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr entt::hashed_wstring operator"" _hws(const wchar_t *str, std::size_t) ENTT_NOEXCEPT {
     return entt::hashed_wstring{str};
 }
 
+} // namespace literals
+
+} // namespace entt
 
 #endif

src/entt/core/ident.hpp

@@ -1,17 +1,15 @@
 #ifndef ENTT_CORE_IDENT_HPP
 #define ENTT_CORE_IDENT_HPP
 
-
-#include <tuple>
 #include <cstddef>
-#include <utility>
 #include <type_traits>
+#include <utility>
 #include "../config/config.h"
-
+#include "fwd.hpp"
+#include "type_traits.hpp"
 
 namespace entt {
 
-
 /**
  * @brief Types identifiers.
  *
@@ -41,25 +39,21 @@ namespace entt {
  */
 template<typename... Types>
 class identifier {
-    using tuple_type = std::tuple<std::decay_t<Types>...>;
-
-    template<typename Type, std::size_t... Indexes>
-    static constexpr ENTT_ID_TYPE get(std::index_sequence<Indexes...>) {
-        static_assert(std::disjunction_v<std::is_same<Type, Types>...>);
-        return (0 + ... + (std::is_same_v<Type, std::tuple_element_t<Indexes, tuple_type>> ? ENTT_ID_TYPE(Indexes) : ENTT_ID_TYPE{}));
+    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{}));
     }
 
 public:
     /*! @brief Unsigned integer type. */
-    using identifier_type = ENTT_ID_TYPE;
+    using identifier_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...>{});
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/core/iterator.hpp

@@ -0,0 +1,116 @@
+#ifndef ENTT_CORE_ITERATOR_HPP
+#define ENTT_CORE_ITERATOR_HPP
+
+#include <iterator>
+#include <memory>
+#include <utility>
+#include "../config/config.h"
+
+namespace entt {
+
+/**
+ * @brief Helper type to use as pointer with input iterators.
+ * @tparam Type of wrapped value.
+ */
+template<typename Type>
+struct input_iterator_pointer final {
+    /*! @brief Pointer type. */
+    using pointer = 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;
+
+    /**
+     * @brief Constructs a proxy object by move.
+     * @param val Value to use to initialize the proxy object.
+     */
+    input_iterator_pointer(Type &&val)
+        : value{std::move(val)} {}
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This proxy object.
+     */
+    input_iterator_pointer &operator=(const input_iterator_pointer &) = delete;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This proxy object.
+     */
+    input_iterator_pointer &operator=(input_iterator_pointer &&) = default;
+
+    /**
+     * @brief Access operator for accessing wrapped values.
+     * @return A pointer to the wrapped value.
+     */
+    [[nodiscard]] pointer operator->() ENTT_NOEXCEPT {
+        return std::addressof(value);
+    }
+
+private:
+    Type value;
+};
+
+/**
+ * @brief Utility class to create an iterable object from a pair of iterators.
+ * @tparam It Type of iterators.
+ */
+template<typename It>
+struct iterable_adaptor final {
+    /*! @brief Type of the objects returned during iteration. */
+    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 Default constructor. */
+    iterable_adaptor() = default;
+
+    /**
+     * @brief Creates an iterable object from a pair of iterators.
+     * @param from Begin iterator.
+     * @param to End iterator.
+     */
+    iterable_adaptor(It from, It to)
+        : first{from},
+          last{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 {
+        return first;
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     * @return An iterator to the element following the last element of the
+     * range.
+     */
+    [[nodiscard]] const_iterator end() const ENTT_NOEXCEPT {
+        return last;
+    }
+
+    /*! @copydoc begin */
+    [[nodiscard]] const_iterator cbegin() const ENTT_NOEXCEPT {
+        return begin();
+    }
+
+    /*! @copydoc end */
+    [[nodiscard]] const_iterator cend() const ENTT_NOEXCEPT {
+        return end();
+    }
+
+private:
+    It first;
+    It last;
+};
+
+} // namespace entt
+
+#endif

src/entt/core/memory.hpp

@@ -0,0 +1,108 @@
+#ifndef ENTT_CORE_MEMORY_HPP
+#define ENTT_CORE_MEMORY_HPP
+
+#include <cstddef>
+#include <limits>
+#include <memory>
+#include <type_traits>
+#include <utility>
+#include "../config/config.h"
+
+namespace entt {
+
+/**
+ * @brief Unwraps fancy pointers, does nothing otherwise (waiting for C++20).
+ * @tparam Type Pointer type.
+ * @param ptr Fancy or raw pointer.
+ * @return A raw pointer that represents the address of the original pointer.
+ */
+template<typename Type>
+[[nodiscard]] constexpr auto to_address(Type &&ptr) ENTT_NOEXCEPT {
+    if constexpr(std::is_pointer_v<std::remove_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.
+ * @param value A value that may or may not be a power of two.
+ * @return True if the value is a power of two, false otherwise.
+ */
+[[nodiscard]] inline constexpr bool is_power_of_two(const std::size_t value) ENTT_NOEXCEPT {
+    return value && ((value & (value - 1)) == 0);
+}
+
+/**
+ * @brief Computes the smallest power of two greater than or equal to a value.
+ * @param value The value to use.
+ * @return The smallest power of two greater than or equal to the given value.
+ */
+[[nodiscard]] inline constexpr std::size_t next_power_of_two(const std::size_t value) ENTT_NOEXCEPT {
+    ENTT_ASSERT(value < (std::size_t{1u} << (std::numeric_limits<std::size_t>::digits - 1)), "Numeric limits exceeded");
+    std::size_t curr = value - (value != 0u);
+
+    for(int next = 1; next < std::numeric_limits<std::size_t>::digits; next = next * 2) {
+        curr |= curr >> next;
+    }
+
+    return ++curr;
+}
+
+/**
+ * @brief Fast module utility function (powers of two only).
+ * @param value A value for which to calculate the modulus.
+ * @param mod _Modulus_, it must be a power of two.
+ * @return The common remainder.
+ */
+[[nodiscard]] inline constexpr std::size_t fast_mod(const std::size_t value, const std::size_t mod) ENTT_NOEXCEPT {
+    ENTT_ASSERT(is_power_of_two(mod), "Value must be a power of two");
+    return value & (mod - 1u);
+}
+
+} // namespace entt
+
+#endif

src/entt/core/monostate.hpp

@@ -1,13 +1,11 @@
 #ifndef ENTT_CORE_MONOSTATE_HPP
 #define ENTT_CORE_MONOSTATE_HPP
 
-
 #include "../config/config.h"
-
+#include "fwd.hpp"
 
 namespace entt {
 
-
 /**
  * @brief Minimal implementation of the monostate pattern.
  *
@@ -19,7 +17,7 @@ namespace entt {
  * both during an assignment and when they try to read back their data.
  * Otherwise, they can incur in unexpected results.
  */
-template<ENTT_ID_TYPE>
+template<id_type>
 struct monostate {
     /**
      * @brief Assigns a value of a specific type to a given key.
@@ -46,16 +44,13 @@ private:
     inline static ENTT_MAYBE_ATOMIC(Type) value{};
 };
 
-
 /**
  * @brief Helper variable template.
  * @tparam Value Value used to differentiate between different variables.
  */
-template<ENTT_ID_TYPE Value>
+template<id_type Value>
 inline monostate<Value> monostate_v = {};
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/core/tuple.hpp

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

src/entt/core/type_info.hpp

@@ -1,80 +1,267 @@
 #ifndef ENTT_CORE_TYPE_INFO_HPP
 #define ENTT_CORE_TYPE_INFO_HPP
 
-
+#include <string_view>
+#include <type_traits>
+#include <utility>
 #include "../config/config.h"
 #include "../core/attribute.h"
+#include "fwd.hpp"
 #include "hashed_string.hpp"
 
-
-#ifndef ENTT_PRETTY_FUNCTION
-#   define ENTT_TYPE_ID_API ENTT_API
-#else
-#   define ENTT_TYPE_ID_API
-#endif
-
-
 namespace entt {
 
-
-#ifndef ENTT_PRETTY_FUNCTION
 /**
  * @cond TURN_OFF_DOXYGEN
  * Internal details not to be documented.
  */
 
-
 namespace internal {
 
-
-struct ENTT_API type_id_generator {
-    static ENTT_ID_TYPE next() ENTT_NOEXCEPT {
-        static ENTT_ID_TYPE value{};
+struct ENTT_API type_index final {
+    [[nodiscard]] static id_type next() ENTT_NOEXCEPT {
+        static ENTT_MAYBE_ATOMIC(id_type) value{};
         return value++;
     }
 };
 
-
+template<typename Type>
+[[nodiscard]] constexpr auto stripped_type_name() ENTT_NOEXCEPT {
+#if defined ENTT_PRETTY_FUNCTION
+    std::string_view pretty_function{ENTT_PRETTY_FUNCTION};
+    auto first = pretty_function.find_first_not_of(' ', pretty_function.find_first_of(ENTT_PRETTY_FUNCTION_PREFIX) + 1);
+    auto value = pretty_function.substr(first, pretty_function.find_last_of(ENTT_PRETTY_FUNCTION_SUFFIX) - first);
+    return value;
+#else
+    return std::string_view{""};
+#endif
 }
 
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr std::string_view type_name(int) ENTT_NOEXCEPT {
+    constexpr auto value = stripped_type_name<Type>();
+    return value;
+}
+
+template<typename Type>
+[[nodiscard]] static std::string_view type_name(char) ENTT_NOEXCEPT {
+    static const auto value = stripped_type_name<Type>();
+    return value;
+}
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr id_type type_hash(int) ENTT_NOEXCEPT {
+    constexpr auto stripped = stripped_type_name<Type>();
+    constexpr auto value = hashed_string::value(stripped.data(), stripped.size());
+    return value;
+}
+
+template<typename Type>
+[[nodiscard]] static id_type type_hash(char) ENTT_NOEXCEPT {
+    static const auto value = [](const auto stripped) {
+        return hashed_string::value(stripped.data(), stripped.size());
+    }(stripped_type_name<Type>());
+    return value;
+}
+
+} // namespace internal
 
 /**
  * Internal details not to be documented.
- * @endcond TURN_OFF_DOXYGEN
+ * @endcond
  */
-#endif
-
 
 /**
- * @brief Types identifiers.
- * @tparam Type Type for which to generate an identifier.
+ * @brief Type sequential identifier.
+ * @tparam Type Type for which to generate a sequential identifier.
  */
 template<typename Type, typename = void>
-struct ENTT_TYPE_ID_API type_info {
+struct ENTT_API type_index final {
+    /**
+     * @brief Returns the sequential identifier of a given type.
+     * @return The sequential identifier of a given type.
+     */
+    [[nodiscard]] static id_type value() ENTT_NOEXCEPT {
+        static const id_type value = internal::type_index::next();
+        return value;
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT {
+        return value();
+    }
+};
+
+/**
+ * @brief Type hash.
+ * @tparam Type Type for which to generate a hash value.
+ */
+template<typename Type, typename = void>
+struct type_hash final {
     /**
      * @brief Returns the numeric representation of a given type.
      * @return The numeric representation of the given type.
      */
-#if defined ENTT_PRETTY_FUNCTION_CONSTEXPR
-    static constexpr ENTT_ID_TYPE id() ENTT_NOEXCEPT {
-        constexpr auto value = entt::hashed_string::value(ENTT_PRETTY_FUNCTION_CONSTEXPR);
-        return value;
-    }
-#elif defined ENTT_PRETTY_FUNCTION
-    static ENTT_ID_TYPE id() ENTT_NOEXCEPT {
-        static const auto value = entt::hashed_string::value(ENTT_PRETTY_FUNCTION);
-        return value;
-    }
+#if defined ENTT_PRETTY_FUNCTION
+    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+        return internal::type_hash<Type>(0);
 #else
-    static ENTT_ID_TYPE id() ENTT_NOEXCEPT {
-        static const ENTT_ID_TYPE value = internal::type_id_generator::next();
-        return value;
-    }
+    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+        return type_index<Type>::value();
 #endif
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT {
+        return value();
+    }
 };
 
+/**
+ * @brief Type name.
+ * @tparam Type Type for which to generate a name.
+ */
+template<typename Type, typename = void>
+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 {
+        return internal::type_name<Type>(0);
+    }
 
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator std::string_view() const ENTT_NOEXCEPT {
+        return value();
+    }
+};
+
+/*! @brief Implementation specific information about a type. */
+class type_info final {
+    template<typename Type>
+    friend const type_info &type_id() ENTT_NOEXCEPT;
+
+    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 {
+        return seq;
+    }
+
+    /**
+     * @brief Type hash.
+     * @return Type hash.
+     */
+    [[nodiscard]] constexpr id_type hash() const ENTT_NOEXCEPT {
+        return identifier;
+    }
+
+    /**
+     * @brief Type name.
+     * @return Type name.
+     */
+    [[nodiscard]] constexpr std::string_view name() const ENTT_NOEXCEPT {
+        return alias;
+    }
+
+private:
+    id_type seq;
+    id_type identifier;
+    std::string_view alias;
+};
+
+/**
+ * @brief Compares the contents of two type info objects.
+ * @param lhs A type info object.
+ * @param rhs A type info object.
+ * @return True if the two type info objects are identical, false otherwise.
+ */
+[[nodiscard]] inline constexpr bool operator==(const type_info &lhs, const type_info &rhs) ENTT_NOEXCEPT {
+    return lhs.hash() == rhs.hash();
 }
 
+/**
+ * @brief Compares the contents of two type info objects.
+ * @param lhs A type info object.
+ * @param rhs A type info object.
+ * @return True if the two type info objects differ, false otherwise.
+ */
+[[nodiscard]] inline constexpr bool operator!=(const type_info &lhs, const type_info &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+/**
+ * @brief Compares two type info objects.
+ * @param lhs A valid type info object.
+ * @param rhs A valid type info object.
+ * @return True if the first element is less than the second, false otherwise.
+ */
+[[nodiscard]] constexpr bool operator<(const type_info &lhs, const type_info &rhs) ENTT_NOEXCEPT {
+    return lhs.index() < rhs.index();
+}
+
+/**
+ * @brief Compares two type info objects.
+ * @param lhs A valid type info object.
+ * @param rhs A valid type info object.
+ * @return True if the first element is less than or equal to the second, false
+ * otherwise.
+ */
+[[nodiscard]] constexpr bool operator<=(const type_info &lhs, const type_info &rhs) ENTT_NOEXCEPT {
+    return !(rhs < lhs);
+}
+
+/**
+ * @brief Compares two type info objects.
+ * @param lhs A valid type info object.
+ * @param rhs A valid type info object.
+ * @return True if the first element is greater than the second, false
+ * otherwise.
+ */
+[[nodiscard]] constexpr bool operator>(const type_info &lhs, const type_info &rhs) ENTT_NOEXCEPT {
+    return rhs < lhs;
+}
+
+/**
+ * @brief Compares two type info objects.
+ * @param lhs A valid type info object.
+ * @param rhs A valid type info object.
+ * @return True if the first element is greater than or equal to the second,
+ * false otherwise.
+ */
+[[nodiscard]] constexpr bool operator>=(const type_info &lhs, const type_info &rhs) ENTT_NOEXCEPT {
+    return !(lhs < rhs);
+}
+
+/**
+ * @brief Returns the type info object associated to a given type.
+ *
+ * The 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.
+ */
+template<typename Type>
+[[nodiscard]] const type_info &type_id() ENTT_NOEXCEPT {
+    if constexpr(std::is_same_v<Type, std::remove_cv_t<std::remove_reference_t<Type>>>) {
+        static type_info instance{std::in_place_type<std::remove_cv_t<std::remove_reference_t<Type>>>};
+        return instance;
+    } else {
+        return type_id<std::remove_cv_t<std::remove_reference_t<Type>>>();
+    }
+}
+
+} // namespace entt
 
 #endif

src/entt/core/type_traits.hpp

@@ -1,76 +1,167 @@
 #ifndef ENTT_CORE_TYPE_TRAITS_HPP
 #define ENTT_CORE_TYPE_TRAITS_HPP
 
-
 #include <cstddef>
-#include <utility>
+#include <iterator>
 #include <type_traits>
+#include <utility>
 #include "../config/config.h"
-#include "../core/hashed_string.hpp"
-
+#include "fwd.hpp"
 
 namespace entt {
 
-
 /**
  * @brief Utility class to disambiguate overloaded functions.
  * @tparam N Number of choices available.
  */
 template<std::size_t N>
 struct choice_t
-        // Unfortunately, doxygen cannot parse such a construct.
-        /*! @cond TURN_OFF_DOXYGEN */
-        : choice_t<N-1>
-        /*! @endcond TURN_OFF_DOXYGEN */
+    // Unfortunately, doxygen cannot parse such a construct.
+    : /*! @cond TURN_OFF_DOXYGEN */ choice_t<N - 1> /*! @endcond */
 {};
 
-
 /*! @copybrief choice_t */
 template<>
 struct choice_t<0> {};
 
-
 /**
  * @brief Variable template for the choice trick.
  * @tparam N Number of choices available.
  */
 template<std::size_t N>
-constexpr choice_t<N> choice{};
-
-
-/*! @brief A class to use to push around lists of types, nothing more. */
-template<typename...>
-struct type_list {};
-
-
-/*! @brief Primary template isn't defined on purpose. */
-template<typename>
-struct type_list_size;
-
+inline constexpr choice_t<N> choice{};
 
 /**
- * @brief Compile-time number of elements in a type list.
- * @tparam Type Types provided by the type list.
+ * @brief Identity type trait.
+ *
+ * Useful to establish non-deduced contexts in template argument deduction
+ * (waiting for C++20) or to provide types through function arguments.
+ *
+ * @tparam Type A type.
  */
-template<typename... Type>
-struct type_list_size<type_list<Type...>>
-        : std::integral_constant<std::size_t, sizeof...(Type)>
-{};
+template<typename Type>
+struct type_identity {
+    /*! @brief Identity type. */
+    using type = Type;
+};
 
+/**
+ * @brief Helper type.
+ * @tparam Type A type.
+ */
+template<typename Type>
+using type_identity_t = typename type_identity<Type>::type;
+
+/**
+ * @brief A type-only `sizeof` wrapper that returns 0 where `sizeof` complains.
+ * @tparam Type The type of which to return the size.
+ * @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> {};
+
+/*! @copydoc size_of */
+template<typename Type>
+struct size_of<Type, std::void_t<decltype(sizeof(Type))>>
+    : std::integral_constant<std::size_t, sizeof(Type)> {};
 
 /**
  * @brief Helper variable template.
- * @tparam List Type list.
+ * @tparam Type The type of which to return the size.
  */
-template<class List>
-constexpr auto type_list_size_v = type_list_size<List>::value;
+template<typename Type>
+inline constexpr std::size_t size_of_v = size_of<Type>::value;
 
+/**
+ * @brief Using declaration to be used to _repeat_ the same type a number of
+ * times equal to the size of a given parameter pack.
+ * @tparam Type A type to repeat.
+ */
+template<typename Type, typename>
+using unpack_as_type = Type;
+
+/**
+ * @brief Helper variable template to be used to _repeat_ the same value a
+ * number of times equal to the size of a given parameter pack.
+ * @tparam Value A value to repeat.
+ */
+template<auto Value, typename>
+inline constexpr auto unpack_as_value = Value;
+
+/**
+ * @brief Wraps a static constant.
+ * @tparam Value A static constant.
+ */
+template<auto Value>
+using integral_constant = std::integral_constant<decltype(Value), Value>;
+
+/**
+ * @brief Alias template to facilitate the creation of named values.
+ * @tparam Value A constant value at least convertible to `id_type`.
+ */
+template<id_type Value>
+using tag = integral_constant<Value>;
+
+/**
+ * @brief A class to use to push around lists of types, nothing more.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list {
+    /*! @brief Type list type. */
+    using type = type_list;
+    /*! @brief Compile-time number of elements in the type list. */
+    static constexpr auto size = sizeof...(Type);
+};
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct type_list_element;
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Index Index of the type to return.
+ * @tparam Type 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...>>
+    : 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 Other Other types provided by the type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_element<0u, type_list<Type, Other...>> {
+    /*! @brief Searched type. */
+    using type = Type;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the type to return.
+ * @tparam List Type list to search into.
+ */
+template<std::size_t Index, typename List>
+using type_list_element_t = typename type_list_element<Index, List>::type;
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @return A type list composed by the types of both the type lists.
+ */
+template<typename... Type, typename... Other>
+constexpr type_list<Type..., Other...> operator+(type_list<Type...>, type_list<Other...>) {
+    return {};
+}
 
 /*! @brief Primary template isn't defined on purpose. */
 template<typename...>
 struct type_list_cat;
 
-
 /*! @brief Concatenates multiple type lists. */
 template<>
 struct type_list_cat<> {
@@ -78,7 +169,6 @@ struct type_list_cat<> {
     using type = type_list<>;
 };
 
-
 /**
  * @brief Concatenates multiple type lists.
  * @tparam Type Types provided by the first type list.
@@ -91,7 +181,6 @@ struct type_list_cat<type_list<Type...>, type_list<Other...>, List...> {
     using type = typename type_list_cat<type_list<Type..., Other...>, List...>::type;
 };
 
-
 /**
  * @brief Concatenates multiple type lists.
  * @tparam Type Types provided by the type list.
@@ -102,7 +191,6 @@ struct type_list_cat<type_list<Type...>> {
     using type = type_list<Type...>;
 };
 
-
 /**
  * @brief Helper type.
  * @tparam List Type lists to concatenate.
@@ -110,12 +198,10 @@ struct type_list_cat<type_list<Type...>> {
 template<typename... List>
 using type_list_cat_t = typename type_list_cat<List...>::type;
 
-
 /*! @brief Primary template isn't defined on purpose. */
 template<typename>
 struct type_list_unique;
 
-
 /**
  * @brief Removes duplicates types from a type list.
  * @tparam Type One of the types provided by the given type list.
@@ -127,11 +213,9 @@ struct type_list_unique<type_list<Type, Other...>> {
     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>
-    >;
+        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<>> {
@@ -139,7 +223,6 @@ struct type_list_unique<type_list<>> {
     using type = type_list<>;
 };
 
-
 /**
  * @brief Helper type.
  * @tparam Type A type list.
@@ -147,28 +230,419 @@ struct type_list_unique<type_list<>> {
 template<typename Type>
 using type_list_unique_t = typename type_list_unique<Type>::type;
 
+/**
+ * @brief Provides the member constant `value` to true if a type list contains a
+ * given type, false otherwise.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<typename List, typename Type>
+struct type_list_contains;
+
+/**
+ * @copybrief type_list_contains
+ * @tparam Type Types provided by the type list.
+ * @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>...> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<typename List, typename Type>
+inline constexpr bool type_list_contains_v = type_list_contains<List, Type>::value;
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_diff;
+
+/**
+ * @brief Computes the difference between two type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ */
+template<typename... Type, typename... Other>
+struct type_list_diff<type_list<Type...>, type_list<Other...>> {
+    /*! @brief A type list that is the difference between the two type lists. */
+    using type = type_list_cat_t<std::conditional_t<type_list_contains_v<type_list<Other...>, Type>, type_list<>, type_list<Type>>...>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists between which to compute the difference.
+ */
+template<typename... List>
+using type_list_diff_t = typename type_list_diff<List...>::type;
+
+/**
+ * @brief A class to use to push around lists of constant values, nothing more.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list {
+    /*! @brief Value list type. */
+    using type = value_list;
+    /*! @brief Compile-time number of elements in the value list. */
+    static constexpr auto size = sizeof...(Value);
+};
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct value_list_element;
+
+/**
+ * @brief Provides compile-time indexed access to the values of a value list.
+ * @tparam Index Index of the value to return.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<std::size_t Index, auto Value, auto... Other>
+struct value_list_element<Index, value_list<Value, Other...>>
+    : value_list_element<Index - 1u, value_list<Other...>> {};
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<auto Value, auto... Other>
+struct value_list_element<0u, value_list<Value, Other...>> {
+    /*! @brief Searched value. */
+    static constexpr auto value = Value;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the value to return.
+ * @tparam List Value list to search into.
+ */
+template<std::size_t Index, typename List>
+inline constexpr auto value_list_element_v = value_list_element<Index, List>::value;
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @return A value list composed by the values of both the value lists.
+ */
+template<auto... Value, auto... Other>
+constexpr value_list<Value..., Other...> operator+(value_list<Value...>, value_list<Other...>) {
+    return {};
+}
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct value_list_cat;
+
+/*! @brief Concatenates multiple value lists. */
+template<>
+struct value_list_cat<> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<>;
+};
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @tparam List Other value lists, if any.
+ */
+template<auto... Value, auto... Other, typename... List>
+struct value_list_cat<value_list<Value...>, value_list<Other...>, List...> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = typename value_list_cat<value_list<Value..., Other...>, List...>::type;
+};
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list_cat<value_list<Value...>> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<Value...>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam List Value lists to concatenate.
+ */
+template<typename... List>
+using value_list_cat_t = typename value_list_cat<List...>::type;
+
+/*! @brief Same as std::is_invocable, but with tuples. */
+template<typename, typename>
+struct is_applicable: std::false_type {};
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, const Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, typename Args>
+inline constexpr bool is_applicable_v = is_applicable<Func, Args>::value;
+
+/*! @brief Same as std::is_invocable_r, but with tuples for arguments. */
+template<typename, typename, typename>
+struct is_applicable_r: std::false_type {};
+
+/**
+ * @copybrief is_applicable_r
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename... Args>
+struct is_applicable_r<Ret, Func, std::tuple<Args...>>: std::is_invocable_r<Ret, Func, Args...> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename Args>
+inline constexpr bool is_applicable_r_v = is_applicable_r<Ret, Func, Args>::value;
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * complete, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_complete: std::false_type {};
+
+/*! @copydoc is_complete */
+template<typename Type>
+struct is_complete<Type, std::void_t<decltype(sizeof(Type))>>: std::true_type {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_complete_v = is_complete<Type>::value;
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is an
+ * iterator, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_iterator: std::false_type {};
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename, typename = void>
+struct has_iterator_category: std::false_type {};
+
+template<typename Type>
+struct has_iterator_category<Type, std::void_t<typename std::iterator_traits<Type>::iterator_category>>: std::true_type {};
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/*! @copydoc is_iterator */
+template<typename Type>
+struct is_iterator<Type, std::enable_if_t<!std::is_same_v<std::remove_const_t<std::remove_pointer_t<Type>>, void>>>
+    : internal::has_iterator_category<Type> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+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.
+ * @tparam Type The type to test
+ */
+template<typename Type>
+struct is_ebco_eligible
+    : std::conjunction<std::is_empty<Type>, std::negation<std::is_final<Type>>> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_ebco_eligible_v = is_ebco_eligible<Type>::value;
+
+/**
+ * @brief Provides the member constant `value` to true if `Type::is_transparent`
+ * is valid and denotes a type, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_transparent: std::false_type {};
+
+/*! @copydoc is_transparent */
+template<typename Type>
+struct is_transparent<Type, std::void_t<typename Type::is_transparent>>: std::true_type {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_transparent_v = is_transparent<Type>::value;
 
 /**
  * @brief Provides the member constant `value` to true if a given type is
  * equality comparable, false otherwise.
- * @tparam Type Potentially equality comparable type.
+ * @tparam Type The type to test.
  */
-template<typename Type, typename = std::void_t<>>
+template<typename Type, typename = void>
 struct is_equality_comparable: std::false_type {};
 
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename, typename = void>
+struct has_tuple_size_value: std::false_type {};
+
+template<typename Type>
+struct has_tuple_size_value<Type, std::void_t<decltype(std::tuple_size<const Type>::value)>>: std::true_type {};
+
+template<typename Type, std::size_t... Index>
+[[nodiscard]] constexpr bool unpack_maybe_equality_comparable(std::index_sequence<Index...>) {
+    return (is_equality_comparable<std::tuple_element_t<Index, Type>>::value && ...);
+}
+
+template<typename>
+[[nodiscard]] constexpr bool maybe_equality_comparable(choice_t<0>) {
+    return true;
+}
+
+template<typename Type>
+[[nodiscard]] constexpr auto maybe_equality_comparable(choice_t<1>) -> decltype(std::declval<typename Type::value_type>(), bool{}) {
+    if constexpr(is_iterator_v<Type>) {
+        return true;
+    } else if constexpr(std::is_same_v<typename Type::value_type, Type>) {
+        return maybe_equality_comparable<Type>(choice<0>);
+    } else {
+        return is_equality_comparable<typename Type::value_type>::value;
+    }
+}
+
+template<typename Type>
+[[nodiscard]] constexpr std::enable_if_t<is_complete_v<std::tuple_size<std::remove_const_t<Type>>>, bool> maybe_equality_comparable(choice_t<2>) {
+    if constexpr(has_tuple_size_value<Type>::value) {
+        return unpack_maybe_equality_comparable<Type>(std::make_index_sequence<std::tuple_size<Type>::value>{});
+    } else {
+        return maybe_equality_comparable<Type>(choice<1>);
+    }
+}
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
 
 /*! @copydoc is_equality_comparable */
 template<typename Type>
-struct is_equality_comparable<Type, std::void_t<decltype(std::declval<Type>() == std::declval<Type>())>>: std::true_type {};
-
+struct is_equality_comparable<Type, std::void_t<decltype(std::declval<Type>() == std::declval<Type>())>>
+    : std::bool_constant<internal::maybe_equality_comparable<Type>(choice<2>)> {};
 
 /**
  * @brief Helper variable template.
- * @tparam Type Potentially equality comparable type.
+ * @tparam Type The type to test.
  */
-template<class Type>
-constexpr auto is_equality_comparable_v = is_equality_comparable<Type>::value;
+template<typename Type>
+inline constexpr bool is_equality_comparable_v = is_equality_comparable<Type>::value;
 
+/**
+ * @brief Transcribes the constness of a type to another type.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+struct constness_as {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::remove_const_t<To>;
+};
+
+/*! @copydoc 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>;
+};
+
+/**
+ * @brief Alias template to facilitate the transcription of the constness.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+using constness_as_t = typename constness_as<To, From>::type;
 
 /**
  * @brief Extracts the class of a non-static member object or function.
@@ -176,23 +650,22 @@ constexpr auto is_equality_comparable_v = is_equality_comparable<Type>::value;
  */
 template<typename Member>
 class member_class {
-    static_assert(std::is_member_pointer_v<Member>);
+    static_assert(std::is_member_pointer_v<Member>, "Invalid pointer type to non-static member object or function");
 
     template<typename Class, typename Ret, typename... Args>
-    static Class * clazz(Ret(Class:: *)(Args...));
+    static Class *clazz(Ret (Class::*)(Args...));
 
     template<typename Class, typename Ret, typename... Args>
-    static Class * clazz(Ret(Class:: *)(Args...) const);
+    static Class *clazz(Ret (Class::*)(Args...) const);
 
     template<typename Class, typename Type>
-    static Class * clazz(Type Class:: *);
+    static Class *clazz(Type Class::*);
 
 public:
     /*! @brief The class of the given non-static member object or function. */
     using type = std::remove_pointer_t<decltype(clazz(std::declval<Member>()))>;
 };
 
-
 /**
  * @brief Helper type.
  * @tparam Member A pointer to a non-static member object or function.
@@ -200,30 +673,6 @@ public:
 template<typename Member>
 using member_class_t = typename member_class<Member>::type;
 
-
-/**
- * @brief Alias template to ease the creation of named values.
- * @tparam Value A constant value at least convertible to `ENTT_ID_TYPE`.
- */
-template<ENTT_ID_TYPE Value>
-using tag = std::integral_constant<ENTT_ID_TYPE, Value>;
-
-
-}
-
-
-/**
- * @brief Defines an enum class to use for opaque identifiers and a dedicate
- * `to_integer` function to convert the identifiers to their underlying type.
- * @param clazz The name to use for the enum class.
- * @param type The underlying type for the enum class.
- */
-#define ENTT_OPAQUE_TYPE(clazz, type)\
-    enum class clazz: type {};\
-    constexpr auto to_integral(const clazz id) ENTT_NOEXCEPT {\
-        return static_cast<std::underlying_type_t<clazz>>(id);\
-    }\
-    static_assert(true)
-
+} // namespace entt
 
 #endif

src/entt/core/utility.hpp

@@ -1,16 +1,16 @@
 #ifndef ENTT_CORE_UTILITY_HPP
 #define ENTT_CORE_UTILITY_HPP
 
-
 #include <utility>
 #include "../config/config.h"
 
-
 namespace entt {
 
-
 /*! @brief Identity function object (waiting for C++20). */
 struct identity {
+    /*! @brief Indicates that this is a transparent function object. */
+    using is_transparent = void;
+
     /**
      * @brief Returns its argument unchanged.
      * @tparam Type Type of the argument.
@@ -18,12 +18,11 @@ struct identity {
      * @return The submitted value as-is.
      */
     template<class Type>
-    constexpr Type && operator()(Type &&value) const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr Type &&operator()(Type &&value) const ENTT_NOEXCEPT {
         return std::forward<Type>(value);
     }
 };
 
-
 /**
  * @brief Constant utility to disambiguate overloaded members of a class.
  * @tparam Type Type of the desired overload.
@@ -32,8 +31,9 @@ struct identity {
  * @return Pointer to the member.
  */
 template<typename Type, typename Class>
-constexpr auto overload(Type Class:: *member) ENTT_NOEXCEPT { return member; }
-
+[[nodiscard]] constexpr auto overload(Type Class::*member) ENTT_NOEXCEPT {
+    return member;
+}
 
 /**
  * @brief Constant utility to disambiguate overloaded functions.
@@ -42,8 +42,9 @@ constexpr auto overload(Type Class:: *member) ENTT_NOEXCEPT { return member; }
  * @return Pointer to the function.
  */
 template<typename Func>
-constexpr auto overload(Func *func) ENTT_NOEXCEPT { return func; }
-
+[[nodiscard]] constexpr auto overload(Func *func) ENTT_NOEXCEPT {
+    return func;
+}
 
 /**
  * @brief Helper type for visitors.
@@ -54,7 +55,6 @@ struct overloaded: Func... {
     using Func::operator()...;
 };
 
-
 /**
  * @brief Deduction guide.
  * @tparam Func Types of function objects.
@@ -62,7 +62,6 @@ struct overloaded: Func... {
 template<class... Func>
 overloaded(Func...) -> overloaded<Func...>;
 
-
 /**
  * @brief Basic implementation of a y-combinator.
  * @tparam Func Type of a potentially recursive function.
@@ -73,9 +72,8 @@ struct y_combinator {
      * @brief Constructs a y-combinator from a given function.
      * @param recursive A potentially recursive function.
      */
-    y_combinator(Func recursive):
-        func{std::move(recursive)}
-    {}
+    y_combinator(Func recursive)
+        : func{std::move(recursive)} {}
 
     /**
      * @brief Invokes a y-combinator and therefore its underlying function.
@@ -83,14 +81,14 @@ struct y_combinator {
      * @param args Parameters to use to invoke the underlying function.
      * @return Return value of the underlying function, if any.
      */
-    template <class... Args>
-    decltype(auto) operator()(Args &&... args) const {
+    template<class... Args>
+    decltype(auto) operator()(Args &&...args) const {
         return func(*this, std::forward<Args>(args)...);
     }
 
     /*! @copydoc operator()() */
-    template <class... Args>
-    decltype(auto) operator()(Args &&... args) {
+    template<class... Args>
+    decltype(auto) operator()(Args &&...args) {
         return func(*this, std::forward<Args>(args)...);
     }
 
@@ -98,8 +96,6 @@ private:
     Func func;
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/entity/actor.hpp

@@ -1,206 +0,0 @@
-#ifndef ENTT_ENTITY_ACTOR_HPP
-#define ENTT_ENTITY_ACTOR_HPP
-
-
-#include <utility>
-#include <type_traits>
-#include "../config/config.h"
-#include "registry.hpp"
-#include "entity.hpp"
-#include "fwd.hpp"
-
-
-namespace entt {
-
-
-/**
- * @brief Dedicated to those who aren't confident with the
- * entity-component-system architecture.
- *
- * Tiny wrapper around a registry, for all those users that aren't confident
- * with entity-component-system architecture and prefer to iterate objects
- * directly.
- *
- * @tparam Entity A valid entity type (see entt_traits for more details).
- */
-template<typename Entity>
-struct basic_actor {
-    /*! @brief Type of registry used internally. */
-    using registry_type = basic_registry<Entity>;
-    /*! @brief Underlying entity identifier. */
-    using entity_type = Entity;
-
-    basic_actor() ENTT_NOEXCEPT
-        : entt{entt::null}, reg{nullptr}
-    {}
-
-    /**
-     * @brief Move constructor.
-     *
-     * After actor move construction, instances that have been moved from are
-     * placed in a valid but unspecified state. It's highly discouraged to
-     * continue using them.
-     *
-     * @param other The instance to move from.
-     */
-    basic_actor(basic_actor &&other) ENTT_NOEXCEPT
-        : entt{other.entt}, reg{other.reg}
-    {
-        other.entt = null;
-    }
-
-    /**
-     * @brief Constructs an actor from a given registry.
-     * @param ref An instance of the registry class.
-     */
-    explicit basic_actor(registry_type &ref)
-        : entt{ref.create()}, reg{&ref}
-    {}
-
-    /**
-     * @brief Constructs an actor from a given entity.
-     * @param entity A valid entity identifier.
-     * @param ref An instance of the registry class.
-     */
-    explicit basic_actor(entity_type entity, registry_type &ref) ENTT_NOEXCEPT
-        : entt{entity}, reg{&ref}
-    {
-        ENTT_ASSERT(ref.valid(entity));
-    }
-
-    /*! @brief Default destructor. */
-    virtual ~basic_actor() {
-        if(*this) {
-            reg->destroy(entt);
-        }
-    }
-
-    /**
-     * @brief Move assignment operator.
-     *
-     * After actor move assignment, instances that have been moved from are
-     * placed in a valid but unspecified state. It's highly discouraged to
-     * continue using them.
-     *
-     * @param other The instance to move from.
-     * @return This actor.
-     */
-    basic_actor & operator=(basic_actor &&other) ENTT_NOEXCEPT {
-        if(this != &other) {
-            auto tmp{std::move(other)};
-            std::swap(reg, tmp.reg);
-            std::swap(entt, tmp.entt);
-        }
-
-        return *this;
-    }
-
-    /**
-     * @brief Assigns the given component to an actor.
-     *
-     * A new instance of the given component is created and initialized with the
-     * arguments provided (the component must have a proper constructor or be of
-     * aggregate type). Then the component is assigned to the actor.<br/>
-     * In case the actor already has a component of the given type, it's
-     * replaced with the new one.
-     *
-     * @tparam Component Type of the component to create.
-     * @tparam Args Types of arguments to use to construct the component.
-     * @param args Parameters to use to initialize the component.
-     * @return A reference to the newly created component.
-     */
-    template<typename Component, typename... Args>
-    decltype(auto) assign(Args &&... args) {
-        return reg->template assign_or_replace<Component>(entt, std::forward<Args>(args)...);
-    }
-
-    /**
-     * @brief Removes the given component from an actor.
-     * @tparam Component Type of the component to remove.
-     */
-    template<typename Component>
-    void remove() {
-        reg->template remove<Component>(entt);
-    }
-
-    /**
-     * @brief Checks if an actor has the given components.
-     * @tparam Component Components for which to perform the check.
-     * @return True if the actor has all the components, false otherwise.
-     */
-    template<typename... Component>
-    bool has() const {
-        return reg->template has<Component...>(entt);
-    }
-
-    /**
-     * @brief Returns references to the given components for an actor.
-     * @tparam Component Types of components to get.
-     * @return References to the components owned by the actor.
-     */
-    template<typename... Component>
-    decltype(auto) get() const {
-        return std::as_const(*reg).template get<Component...>(entt);
-    }
-
-    /*! @copydoc get */
-    template<typename... Component>
-    decltype(auto) get() {
-        return reg->template get<Component...>(entt);
-    }
-
-    /**
-     * @brief Returns pointers to the given components for an actor.
-     * @tparam Component Types of components to get.
-     * @return Pointers to the components owned by the actor.
-     */
-    template<typename... Component>
-    auto try_get() const {
-        return std::as_const(*reg).template try_get<Component...>(entt);
-    }
-
-    /*! @copydoc try_get */
-    template<typename... Component>
-    auto try_get() {
-        return reg->template try_get<Component...>(entt);
-    }
-
-    /**
-     * @brief Returns a reference to the underlying registry.
-     * @return A reference to the underlying registry.
-     */
-    const registry_type & backend() const ENTT_NOEXCEPT {
-        return *reg;
-    }
-
-    /*! @copydoc backend */
-    registry_type & backend() ENTT_NOEXCEPT {
-        return const_cast<registry_type &>(std::as_const(*this).backend());
-    }
-
-    /**
-     * @brief Returns the entity associated with an actor.
-     * @return The entity associated with the actor.
-     */
-    entity_type entity() const ENTT_NOEXCEPT {
-        return entt;
-    }
-
-    /**
-     * @brief Checks if an actor refers to a valid entity or not.
-     * @return True if the actor refers to a valid entity, false otherwise.
-     */
-    explicit operator bool() const {
-        return reg && reg->valid(entt);
-    }
-
-private:
-    entity_type entt;
-    registry_type *reg;
-};
-
-
-}
-
-
-#endif

src/entt/entity/component.hpp

@@ -0,0 +1,37 @@
+#ifndef ENTT_ENTITY_COMPONENT_HPP
+#define ENTT_ENTITY_COMPONENT_HPP
+
+#include <type_traits>
+#include "../config/config.h"
+
+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;
+};
+
+/**
+ * @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 {
+    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>;
+
+} // namespace entt
+
+#endif

src/entt/entity/entity.hpp

@@ -1,175 +1,339 @@
 #ifndef ENTT_ENTITY_ENTITY_HPP
 #define ENTT_ENTITY_ENTITY_HPP
 
-
+#include <cstddef>
 #include <cstdint>
 #include <type_traits>
 #include "../config/config.h"
-#include "../core/type_traits.hpp"
-
+#include "fwd.hpp"
 
 namespace entt {
 
-
-/**
- * @brief Entity traits.
- *
- * Primary template isn't defined on purpose. All the specializations give a
- * compile-time error unless the template parameter is an accepted entity type.
- */
-template<typename>
-struct entt_traits;
-
-
-/**
- * @brief Entity traits for a 16 bits entity identifier.
- *
- * A 16 bits entity identifier guarantees:
- *
- * * 12 bits for the entity number (up to 4k entities).
- * * 4 bit for the version (resets in [0-15]).
- */
-template<>
-struct entt_traits<std::uint16_t> {
-    /*! @brief Underlying entity type. */
-    using entity_type = std::uint16_t;
-    /*! @brief Underlying version type. */
-    using version_type = std::uint8_t;
-    /*! @brief Difference type. */
-    using difference_type = std::int32_t;
-
-    /*! @brief Mask to use to get the entity number out of an identifier. */
-    static constexpr std::uint16_t entity_mask = 0xFFF;
-    /*! @brief Mask to use to get the version out of an identifier. */
-    static constexpr std::uint16_t version_mask = 0xF;
-    /*! @brief Extent of the entity number within an identifier. */
-    static constexpr auto entity_shift = 12;
-};
-
-
-/**
- * @brief Entity traits for a 32 bits entity identifier.
- *
- * A 32 bits entity identifier guarantees:
- *
- * * 20 bits for the entity number (suitable for almost all the games).
- * * 12 bit for the version (resets in [0-4095]).
- */
-template<>
-struct entt_traits<std::uint32_t> {
-    /*! @brief Underlying entity type. */
-    using entity_type = std::uint32_t;
-    /*! @brief Underlying version type. */
-    using version_type = std::uint16_t;
-    /*! @brief Difference type. */
-    using difference_type = std::int64_t;
-
-    /*! @brief Mask to use to get the entity number out of an identifier. */
-    static constexpr std::uint32_t entity_mask = 0xFFFFF;
-    /*! @brief Mask to use to get the version out of an identifier. */
-    static constexpr std::uint32_t version_mask = 0xFFF;
-    /*! @brief Extent of the entity number within an identifier. */
-    static constexpr auto entity_shift = 20;
-};
-
-
-/**
- * @brief Entity traits for a 64 bits entity identifier.
- *
- * A 64 bits entity identifier guarantees:
- *
- * * 32 bits for the entity number (an indecently large number).
- * * 32 bit for the version (an indecently large number).
- */
-template<>
-struct entt_traits<std::uint64_t> {
-    /*! @brief Underlying entity type. */
-    using entity_type = std::uint64_t;
-    /*! @brief Underlying version type. */
-    using version_type = std::uint32_t;
-    /*! @brief Difference type. */
-    using difference_type = std::int64_t;
-
-    /*! @brief Mask to use to get the entity number out of an identifier. */
-    static constexpr std::uint64_t entity_mask = 0xFFFFFFFF;
-    /*! @brief Mask to use to get the version out of an identifier. */
-    static constexpr std::uint64_t version_mask = 0xFFFFFFFF;
-    /*! @brief Extent of the entity number within an identifier. */
-    static constexpr auto entity_shift = 32;
-};
-
-
 /**
  * @cond TURN_OFF_DOXYGEN
  * Internal details not to be documented.
  */
 
-
 namespace internal {
 
+template<typename, typename = void>
+struct entt_traits;
 
-class null {
-    template<typename Entity>
-    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
+template<typename Type>
+struct entt_traits<Type, std::enable_if_t<std::is_enum_v<Type>>>
+    : entt_traits<std::underlying_type_t<Type>> {};
+
+template<typename Type>
+struct entt_traits<Type, std::enable_if_t<std::is_class_v<Type>>>
+    : entt_traits<typename Type::entity_type> {};
+
+template<>
+struct entt_traits<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 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
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @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>;
 
 public:
-    template<typename Entity>
-    constexpr operator Entity() const ENTT_NOEXCEPT {
-        return Entity{traits_type<Entity>::entity_mask};
+    /*! @brief Value type. */
+    using value_type = Type;
+    /*! @brief Underlying entity type. */
+    using entity_type = typename base_type::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;
+
+    /**
+     * @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 {
+        return static_cast<entity_type>(value);
     }
 
-    constexpr bool operator==(null) const ENTT_NOEXCEPT {
+    /**
+     * @brief Returns the entity part once converted to the underlying type.
+     * @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);
+    }
+
+    /**
+     * @brief Returns the version part once converted to the underlying type.
+     * @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);
+    }
+
+    /**
+     * @brief Constructs an identifier from its parts.
+     *
+     * If the version part is not provided, a tombstone is returned.<br/>
+     * If the entity part is not provided, a null identifier is returned.
+     *
+     * @param entity The entity part of the identifier.
+     * @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)};
+    }
+
+    /**
+     * @brief Combines two identifiers in a single one.
+     *
+     * The returned identifier is a copy of the first element except for its
+     * version, which is taken from the second element.
+     *
+     * @param lhs The identifier from which to take the entity part.
+     * @param rhs The identifier from which to take the version part.
+     * @return A properly constructed identifier.
+     */
+    [[nodiscard]] static constexpr value_type combine(const entity_type lhs, const entity_type rhs) ENTT_NOEXCEPT {
+        constexpr auto mask = (base_type::version_mask << base_type::entity_shift);
+        return value_type{(lhs & base_type::entity_mask) | (rhs & mask)};
+    }
+};
+
+/**
+ * @copydoc entt_traits<Entity>::to_integral
+ * @tparam Entity The value type.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr typename entt_traits<Entity>::entity_type to_integral(const Entity value) ENTT_NOEXCEPT {
+    return entt_traits<Entity>::to_integral(value);
+}
+
+/**
+ * @copydoc entt_traits<Entity>::to_entity
+ * @tparam Entity The value type.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr typename entt_traits<Entity>::entity_type to_entity(const Entity value) ENTT_NOEXCEPT {
+    return entt_traits<Entity>::to_entity(value);
+}
+
+/**
+ * @copydoc entt_traits<Entity>::to_version
+ * @tparam Entity The value type.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr typename entt_traits<Entity>::version_type to_version(const Entity value) ENTT_NOEXCEPT {
+    return entt_traits<Entity>::to_version(value);
+}
+
+/*! @brief Null object for all identifiers.  */
+struct null_t {
+    /**
+     * @brief Converts the null object to identifiers of any type.
+     * @tparam Entity Type of identifier.
+     * @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);
+    }
+
+    /**
+     * @brief Compares two null objects.
+     * @param other A null object.
+     * @return True in all cases.
+     */
+    [[nodiscard]] constexpr bool operator==([[maybe_unused]] const null_t other) const ENTT_NOEXCEPT {
         return true;
     }
 
-    constexpr bool operator!=(null) const ENTT_NOEXCEPT {
+    /**
+     * @brief Compares two null objects.
+     * @param other A null object.
+     * @return False in all cases.
+     */
+    [[nodiscard]] constexpr bool operator!=([[maybe_unused]] const null_t other) const ENTT_NOEXCEPT {
         return false;
     }
 
+    /**
+     * @brief Compares a null object and an identifier of any type.
+     * @tparam Entity Type of identifier.
+     * @param entity Identifier with which to compare.
+     * @return False if the two elements differ, true otherwise.
+     */
     template<typename Entity>
-    constexpr bool operator==(const Entity entity) const ENTT_NOEXCEPT {
-        return (to_integral(entity) & traits_type<Entity>::entity_mask) == to_integral(static_cast<Entity>(*this));
+    [[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);
     }
 
+    /**
+     * @brief Compares a null object and an identifier of any type.
+     * @tparam Entity Type of identifier.
+     * @param entity Identifier with which to compare.
+     * @return True if the two elements differ, false otherwise.
+     */
     template<typename Entity>
-    constexpr bool operator!=(const Entity entity) const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr bool operator!=(const Entity entity) const ENTT_NOEXCEPT {
         return !(entity == *this);
     }
 };
 
-
+/**
+ * @brief Compares a null object and an identifier of any type.
+ * @tparam Entity Type of identifier.
+ * @param entity Identifier with which to compare.
+ * @param other A null object yet to be converted.
+ * @return False if the two elements differ, true otherwise.
+ */
 template<typename Entity>
-constexpr bool operator==(const Entity entity, null other) ENTT_NOEXCEPT {
-    return other == entity;
+[[nodiscard]] constexpr bool operator==(const Entity entity, const null_t other) ENTT_NOEXCEPT {
+    return other.operator==(entity);
 }
 
-
-template<typename Entity>
-constexpr bool operator!=(const Entity entity, null other) ENTT_NOEXCEPT {
-    return other != entity;
-}
-
-
-}
-
-
 /**
- * Internal details not to be documented.
- * @endcond TURN_OFF_DOXYGEN
+ * @brief Compares a null object and an identifier of any type.
+ * @tparam Entity Type of identifier.
+ * @param entity Identifier with which to compare.
+ * @param other A null object yet to be converted.
+ * @return True if the two elements differ, false otherwise.
  */
+template<typename Entity>
+[[nodiscard]] constexpr bool operator!=(const Entity entity, const null_t other) ENTT_NOEXCEPT {
+    return !(other == entity);
+}
 
+/*! @brief Tombstone object for all identifiers.  */
+struct tombstone_t {
+    /**
+     * @brief Converts the tombstone object to identifiers of any type.
+     * @tparam Entity Type of identifier.
+     * @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);
+    }
+
+    /**
+     * @brief Compares two tombstone objects.
+     * @param other A tombstone object.
+     * @return True in all cases.
+     */
+    [[nodiscard]] constexpr bool operator==([[maybe_unused]] const tombstone_t other) const ENTT_NOEXCEPT {
+        return true;
+    }
+
+    /**
+     * @brief Compares two tombstone objects.
+     * @param other A tombstone object.
+     * @return False in all cases.
+     */
+    [[nodiscard]] constexpr bool operator!=([[maybe_unused]] const tombstone_t other) const ENTT_NOEXCEPT {
+        return false;
+    }
+
+    /**
+     * @brief Compares a tombstone object and an identifier of any type.
+     * @tparam Entity Type of identifier.
+     * @param entity Identifier with which to compare.
+     * @return False if the two elements differ, true otherwise.
+     */
+    template<typename Entity>
+    [[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);
+    }
+
+    /**
+     * @brief Compares a tombstone object and an identifier of any type.
+     * @tparam Entity Type of identifier.
+     * @param entity Identifier with which to compare.
+     * @return True if the two elements differ, false otherwise.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr bool operator!=(const Entity entity) const ENTT_NOEXCEPT {
+        return !(entity == *this);
+    }
+};
+
+/**
+ * @brief Compares a tombstone object and an identifier of any type.
+ * @tparam Entity Type of identifier.
+ * @param entity Identifier with which to compare.
+ * @param other A tombstone object yet to be converted.
+ * @return False if the two elements differ, true otherwise.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr bool operator==(const Entity entity, const tombstone_t other) ENTT_NOEXCEPT {
+    return other.operator==(entity);
+}
+
+/**
+ * @brief Compares a tombstone object and an identifier of any type.
+ * @tparam Entity Type of identifier.
+ * @param entity Identifier with which to compare.
+ * @param other A tombstone object yet to be converted.
+ * @return True if the two elements differ, false otherwise.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr bool operator!=(const Entity entity, const tombstone_t other) ENTT_NOEXCEPT {
+    return !(other == entity);
+}
 
 /**
  * @brief Compile-time constant for null entities.
  *
- * There exist implicit conversions from this variable to entity identifiers of
- * any allowed type. Similarly, there exist comparision operators between the
- * null entity and any other entity identifier.
+ * There exist implicit conversions from this variable to identifiers of any
+ * allowed type. Similarly, there exist comparision operators between the null
+ * entity and any other identifier.
  */
-constexpr auto null = internal::null{};
+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
+ * tombstone entity and any other identifier.
+ */
+inline constexpr tombstone_t tombstone{};
 
-}
-
+} // namespace entt
 
 #endif

src/entt/entity/fwd.hpp

@@ -1,84 +1,60 @@
 #ifndef ENTT_ENTITY_FWD_HPP
 #define ENTT_ENTITY_FWD_HPP
 
-
-#include "../config/config.h"
-#include "../core/type_traits.hpp"
-
+#include <memory>
+#include "../core/fwd.hpp"
+#include "utility.hpp"
 
 namespace entt {
 
+template<typename Entity, typename = std::allocator<Entity>>
+class basic_sparse_set;
 
-/**
- * @brief Alias for exclusion lists.
- * @tparam Type List of types.
- */
-template<typename... Type>
-struct exclude_t: type_list<Type...> {};
+template<typename, typename Type, typename = std::allocator<Type>, typename = void>
+class basic_storage;
 
-
-/**
- * @brief Variable template for exclusion lists.
- * @tparam Type List of types.
- */
-template<typename... Type>
-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>
-constexpr get_t<Type...> get{};
-
-
-/*! @class basic_registry */
-template <typename>
+template<typename>
 class basic_registry;
 
-/*! @class basic_view */
-template<typename...>
+template<typename, typename, typename, typename = void>
 class basic_view;
 
-/*! @class basic_runtime_view */
 template<typename>
 class basic_runtime_view;
 
-/*! @class basic_group */
-template<typename...>
+template<typename, typename, typename, typename>
 class basic_group;
 
-/*! @class basic_observer */
 template<typename>
 class basic_observer;
 
-/*! @struct basic_actor */
-template <typename>
-struct basic_actor;
+template<typename>
+class basic_organizer;
+
+template<typename, typename...>
+struct basic_handle;
 
-/*! @class basic_snapshot */
 template<typename>
 class basic_snapshot;
 
-/*! @class basic_snapshot_loader */
 template<typename>
 class basic_snapshot_loader;
 
-/*! @class basic_continuous_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. */
-ENTT_OPAQUE_TYPE(entity, ENTT_ID_TYPE);
+using sparse_set = basic_sparse_set<entity>;
+
+/**
+ * @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>;
@@ -87,7 +63,27 @@ using registry = basic_registry<entity>;
 using observer = basic_observer<entity>;
 
 /*! @brief Alias declaration for the most common use case. */
-using actor = basic_actor<entity>;
+using organizer = basic_organizer<entity>;
+
+/*! @brief Alias declaration for the most common use case. */
+using handle = basic_handle<entity>;
+
+/*! @brief Alias declaration for the most common use case. */
+using const_handle = basic_handle<const entity>;
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Args Other template parameters.
+ */
+template<typename... Args>
+using handle_view = basic_handle<entity, Args...>;
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Args Other template parameters.
+ */
+template<typename... Args>
+using const_handle_view = basic_handle<const entity, Args...>;
 
 /*! @brief Alias declaration for the most common use case. */
 using snapshot = basic_snapshot<entity>;
@@ -100,23 +96,22 @@ using continuous_loader = basic_continuous_loader<entity>;
 
 /**
  * @brief Alias declaration for the most common use case.
- * @tparam Types Types of components iterated by the view.
+ * @tparam Get Types of components iterated by the view.
+ * @tparam Exclude Types of components used to filter the view.
  */
-template<typename... Types>
-using view = basic_view<entity, Types...>;
+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>;
 
 /**
  * @brief Alias declaration for the most common use case.
- * @tparam Types Types of components iterated by the group.
+ * @tparam Args Other template parameters.
  */
-template<typename... Types>
-using group = basic_group<entity, Types...>;
-
-
-}
+template<typename... Args>
+using group = basic_group<entity, Args...>;
 
+} // namespace entt
 
 #endif

src/entt/entity/handle.hpp

@@ -0,0 +1,340 @@
+#ifndef ENTT_ENTITY_HANDLE_HPP
+#define ENTT_ENTITY_HANDLE_HPP
+
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include "../config/config.h"
+#include "../core/type_traits.hpp"
+#include "fwd.hpp"
+#include "registry.hpp"
+
+namespace entt {
+
+/**
+ * @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.
+ */
+template<typename Entity, typename... Type>
+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>;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename registry_type::entity_type;
+    /*! @brief Underlying version type. */
+    using version_type = typename registry_type::version_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = typename registry_type::size_type;
+
+    /*! @brief Constructs an invalid handle. */
+    basic_handle() ENTT_NOEXCEPT
+        : reg{},
+          entt{null} {}
+
+    /**
+     * @brief Constructs a handle from a given registry and entity.
+     * @param ref An instance of the registry class.
+     * @param value A valid identifier.
+     */
+    basic_handle(registry_type &ref, entity_type value) ENTT_NOEXCEPT
+        : reg{&ref},
+          entt{value} {}
+
+    /**
+     * @brief Constructs a const handle from a non-const one.
+     * @tparam Other A valid entity type (see entt_traits for more details).
+     * @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");
+
+        return reg ? basic_handle<Other, Args...>{*reg, entt} : basic_handle<Other, Args...>{};
+    }
+
+    /**
+     * @brief Converts a handle to its underlying entity.
+     * @return The contained identifier.
+     */
+    [[nodiscard]] operator entity_type() const ENTT_NOEXCEPT {
+        return entity();
+    }
+
+    /**
+     * @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 {
+        return reg && reg->valid(entt);
+    }
+
+    /**
+     * @brief Checks if a handle refers to a valid entity or not.
+     * @return True if the handle refers to a valid entity, false otherwise.
+     */
+    [[nodiscard]] bool valid() const {
+        return reg->valid(entt);
+    }
+
+    /**
+     * @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 {
+        return reg;
+    }
+
+    /**
+     * @brief Returns the entity associated with a handle.
+     * @return The entity associated with the handle.
+     */
+    [[nodiscard]] entity_type entity() const ENTT_NOEXCEPT {
+        return entt;
+    }
+
+    /**
+     * @brief Destroys the entity associated with a handle.
+     * @sa basic_registry::destroy
+     */
+    void destroy() {
+        reg->destroy(entt);
+    }
+
+    /**
+     * @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);
+    }
+
+    /**
+     * @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.
+     * @return A reference to the newly created component.
+     */
+    template<typename Component, typename... Args>
+    decltype(auto) emplace(Args &&...args) const {
+        static_assert(((sizeof...(Type) == 0) || ... || std::is_same_v<Component, Type>), "Invalid type");
+        return reg->template emplace<Component>(entt, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @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.
+     * @return A reference to the newly created component.
+     */
+    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");
+        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.
+     * @return A reference to the patched component.
+     */
+    template<typename Component, typename... Func>
+    decltype(auto) patch(Func &&...func) const {
+        static_assert(((sizeof...(Type) == 0) || ... || std::is_same_v<Component, Type>), "Invalid type");
+        return reg->template patch<Component>(entt, std::forward<Func>(func)...);
+    }
+
+    /**
+     * @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.
+     * @return A reference to the component being replaced.
+     */
+    template<typename Component, typename... Args>
+    decltype(auto) replace(Args &&...args) const {
+        static_assert(((sizeof...(Type) == 0) || ... || std::is_same_v<Component, Type>), "Invalid type");
+        return reg->template replace<Component>(entt, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @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");
+        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");
+        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.
+     */
+    template<typename... Component>
+    [[nodiscard]] decltype(auto) all_of() const {
+        return reg->template all_of<Component...>(entt);
+    }
+
+    /**
+     * @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.
+     */
+    template<typename... Component>
+    [[nodiscard]] decltype(auto) any_of() const {
+        return reg->template any_of<Component...>(entt);
+    }
+
+    /**
+     * @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");
+        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.
+     * @return Reference to the component owned by the 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");
+        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");
+        return reg->template try_get<Component...>(entt);
+    }
+
+    /**
+     * @brief Checks if a handle has components assigned.
+     * @return True if the handle has no components assigned, false otherwise.
+     */
+    [[nodiscard]] bool orphan() const {
+        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;
+};
+
+/**
+ * @brief Compares two handles.
+ * @tparam Args Scope of the first handle.
+ * @tparam Other Scope of the second handle.
+ * @param lhs A valid handle.
+ * @param rhs A valid handle.
+ * @return True if both handles refer to the same registry and the same
+ * entity, false otherwise.
+ */
+template<typename... Args, typename... Other>
+[[nodiscard]] bool operator==(const basic_handle<Args...> &lhs, const basic_handle<Other...> &rhs) ENTT_NOEXCEPT {
+    return lhs.registry() == rhs.registry() && lhs.entity() == rhs.entity();
+}
+
+/**
+ * @brief Compares two handles.
+ * @tparam Args Scope of the first handle.
+ * @tparam Other Scope of the second handle.
+ * @param lhs A valid handle.
+ * @param rhs A valid handle.
+ * @return False if both handles refer to the same registry and the same
+ * entity, true otherwise.
+ */
+template<typename... Args, typename... Other>
+[[nodiscard]] bool operator!=(const basic_handle<Args...> &lhs, const basic_handle<Other...> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+/**
+ * @brief Deduction guide.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+basic_handle(basic_registry<Entity> &, Entity) -> basic_handle<Entity>;
+
+/**
+ * @brief Deduction guide.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+basic_handle(const basic_registry<Entity> &, Entity) -> basic_handle<const Entity>;
+
+} // namespace entt
+
+#endif

src/entt/entity/helper.hpp

@@ -1,26 +1,26 @@
 #ifndef ENTT_ENTITY_HELPER_HPP
 #define ENTT_ENTITY_HELPER_HPP
 
-
 #include <type_traits>
-#include "../core/type_traits.hpp"
 #include "../config/config.h"
-#include "registry.hpp"
+#include "../core/fwd.hpp"
+#include "../core/type_traits.hpp"
+#include "../signal/delegate.hpp"
 #include "fwd.hpp"
-
+#include "registry.hpp"
 
 namespace entt {
 
-
 /**
  * @brief Converts a registry to a view.
- * @tparam Const Constness of the accepted registry.
  * @tparam Entity A valid entity type (see entt_traits for more details).
  */
-template<bool Const, typename Entity>
+template<typename Entity>
 struct as_view {
+    /*! @brief Underlying entity identifier. */
+    using entity_type = std::remove_const_t<Entity>;
     /*! @brief Type of registry to convert. */
-    using registry_type = std::conditional_t<Const, const entt::basic_registry<Entity>, entt::basic_registry<Entity>>;
+    using registry_type = constness_as_t<basic_registry<entity_type>, Entity>;
 
     /**
      * @brief Constructs a converter for a given registry.
@@ -35,7 +35,7 @@ struct as_view {
      * @return A newly created view.
      */
     template<typename Exclude, typename... Component>
-    operator entt::basic_view<Entity, Exclude, Component...>() const {
+    operator basic_view<entity_type, get_t<Component...>, Exclude>() const {
         return reg.template view<Component...>(Exclude{});
     }
 
@@ -43,33 +43,30 @@ 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.
- *
- * It allows to deduce the constness of a registry directly from the instance
- * provided to the constructor.
- *
  * @tparam Entity A valid entity type (see entt_traits for more details).
  */
 template<typename Entity>
-as_view(basic_registry<Entity> &) ENTT_NOEXCEPT -> as_view<false, Entity>;
-
-
-/*! @copydoc as_view */
-template<typename Entity>
-as_view(const basic_registry<Entity> &) ENTT_NOEXCEPT -> as_view<true, Entity>;
-
+as_view(const basic_registry<Entity> &) -> as_view<const Entity>;
 
 /**
  * @brief Converts a registry to a group.
- * @tparam Const Constness of the accepted registry.
  * @tparam Entity A valid entity type (see entt_traits for more details).
  */
-template<bool Const, typename Entity>
+template<typename Entity>
 struct as_group {
+    /*! @brief Underlying entity identifier. */
+    using entity_type = std::remove_const_t<Entity>;
     /*! @brief Type of registry to convert. */
-    using registry_type = std::conditional_t<Const, const entt::basic_registry<Entity>, entt::basic_registry<Entity>>;
+    using registry_type = constness_as_t<basic_registry<entity_type>, Entity>;
 
     /**
      * @brief Constructs a converter for a given registry.
@@ -79,39 +76,81 @@ struct as_group {
 
     /**
      * @brief Conversion function from a registry to a group.
-     * @tparam Exclude Types of components used to filter the group.
      * @tparam Get Types of components observed by the group.
+     * @tparam Exclude Types of components used to filter the group.
      * @tparam Owned Types of components owned by the group.
      * @return A newly created group.
      */
-    template<typename Exclude, typename Get, typename... Owned>
-    operator entt::basic_group<Entity, Exclude, Get, Owned...>() const {
-        return reg.template group<Owned...>(Get{}, Exclude{});
+    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{});
+        }
     }
 
 private:
     registry_type &reg;
 };
 
-
 /**
  * @brief Deduction guide.
- *
- * It allows to deduce the constness of a registry directly from the instance
- * provided to the constructor.
- *
  * @tparam Entity A valid entity type (see entt_traits for more details).
  */
 template<typename Entity>
-as_group(basic_registry<Entity> &) ENTT_NOEXCEPT -> as_group<false, Entity>;
+as_group(basic_registry<Entity> &) -> as_group<Entity>;
 
-
-/*! @copydoc as_group */
+/**
+ * @brief Deduction guide.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
 template<typename Entity>
-as_group(const basic_registry<Entity> &) ENTT_NOEXCEPT -> as_group<true, 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).
+ * @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) {
+    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;
+    func.template connect<Member>(reg.template get<member_class_t<decltype(Member)>>(entt));
+    func(reg, entt);
 }
 
+/**
+ * @brief Returns the entity associated with a given component.
+ *
+ * @warning
+ * Currently, this function only works correctly with the default pool 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.
+ * @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;
+    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) {
+            return *(it + dist);
+        }
+    }
+
+    return null;
+}
+
+} // namespace entt
 
 #endif

src/entt/entity/observer.hpp

@@ -1,28 +1,26 @@
 #ifndef ENTT_ENTITY_OBSERVER_HPP
 #define ENTT_ENTITY_OBSERVER_HPP
 
-
-#include <limits>
 #include <cstddef>
 #include <cstdint>
-#include <utility>
+#include <limits>
 #include <type_traits>
+#include <utility>
 #include "../config/config.h"
 #include "../core/type_traits.hpp"
-#include "registry.hpp"
-#include "storage.hpp"
+#include "../signal/delegate.hpp"
 #include "entity.hpp"
 #include "fwd.hpp"
-
+#include "registry.hpp"
+#include "storage.hpp"
+#include "utility.hpp"
 
 namespace entt {
 
-
 /*! @brief Grouping matcher. */
 template<typename...>
 struct matcher {};
 
-
 /**
  * @brief Collector.
  *
@@ -32,7 +30,6 @@ struct matcher {};
 template<typename...>
 struct basic_collector;
 
-
 /**
  * @brief Collector.
  *
@@ -60,7 +57,7 @@ struct basic_collector<> {
      * @return The updated collector.
      */
     template<typename AnyOf>
-    static constexpr auto replace() ENTT_NOEXCEPT {
+    static constexpr auto update() ENTT_NOEXCEPT {
         return basic_collector<matcher<type_list<>, type_list<>, AnyOf>>{};
     }
 };
@@ -95,7 +92,7 @@ struct basic_collector<matcher<type_list<Reject...>, type_list<Require...>, Rule
      * @return The updated collector.
      */
     template<typename AnyOf>
-    static constexpr auto replace() ENTT_NOEXCEPT {
+    static constexpr auto update() ENTT_NOEXCEPT {
         return basic_collector<matcher<type_list<>, type_list<>, AnyOf>, current_type, Other...>{};
     }
 
@@ -112,10 +109,8 @@ struct basic_collector<matcher<type_list<Reject...>, type_list<Require...>, Rule
     }
 };
 
-
 /*! @brief Variable template used to ease the definition of collectors. */
-constexpr basic_collector<> collector{};
-
+inline constexpr basic_collector<> collector{};
 
 /**
  * @brief Observer.
@@ -129,8 +124,8 @@ constexpr basic_collector<> collector{};
  * collector:
  *
  * * Observing matcher: an observer will return at least all the living entities
- *   for which one or more of the given components have been explicitly
- *   replaced and not yet destroyed.
+ *   for which one or more of the given components have been updated and not yet
+ *   destroyed.
  * * Grouping matcher: an observer will return at least all the living entities
  *   that would have entered the given group if it existed and that would have
  *   not yet left it.
@@ -176,20 +171,20 @@ 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, const basic_registry<Entity> &reg, const Entity entt) {
-            if(reg.template has<Require...>(entt) && !reg.template any<Reject...>(entt)) {
-                if(auto *comp = obs.view.try_get(entt); !comp) {
-                    obs.view.construct(entt);
+        static void maybe_valid_if(basic_observer &obs, basic_registry<Entity> &reg, const Entity entt) {
+            if(reg.template all_of<Require...>(entt) && !reg.template any_of<Reject...>(entt)) {
+                if(!obs.storage.contains(entt)) {
+                    obs.storage.emplace(entt);
                 }
 
-                obs.view.get(entt) |= (1 << Index);
+                obs.storage.get(entt) |= (1 << Index);
             }
         }
 
         template<std::size_t Index>
-        static void discard_if(basic_observer &obs, const basic_registry<Entity> &, const Entity entt) {
-            if(auto *value = obs.view.try_get(entt); value && !(*value &= (~(1 << Index)))) {
-                obs.view.destroy(entt);
+        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);
             }
         }
 
@@ -197,35 +192,43 @@ class basic_observer {
         static void connect(basic_observer &obs, basic_registry<Entity> &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_replace<AnyOf>().template connect<&maybe_valid_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_replace<AnyOf>().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>
-        static void maybe_valid_if(basic_observer &obs, const basic_registry<Entity> &reg, const Entity entt) {
-            if(reg.template has<AllOf..., Require...>(entt) && !reg.template any<NoneOf..., Reject...>(entt)) {
-                if(auto *comp = obs.view.try_get(entt); !comp) {
-                    obs.view.construct(entt);
+        template<std::size_t Index, typename... Ignore>
+        static void maybe_valid_if(basic_observer &obs, basic_registry<Entity> &reg, const Entity entt) {
+            auto condition = [&reg, entt]() {
+                if constexpr(sizeof...(Ignore) == 0) {
+                    return reg.template all_of<AllOf..., Require...>(entt) && !reg.template any_of<NoneOf..., Reject...>(entt);
+                } else {
+                    return reg.template all_of<AllOf..., Require...>(entt) && ((std::is_same_v<Ignore..., NoneOf> || !reg.template any_of<NoneOf>(entt)) && ...) && !reg.template any_of<Reject...>(entt);
+                }
+            };
+
+            if(condition()) {
+                if(!obs.storage.contains(entt)) {
+                    obs.storage.emplace(entt);
                 }
 
-                obs.view.get(entt) |= (1 << Index);
+                obs.storage.get(entt) |= (1 << Index);
             }
         }
 
         template<std::size_t Index>
-        static void discard_if(basic_observer &obs, const basic_registry<Entity> &, const Entity entt) {
-            if(auto *value = obs.view.try_get(entt); value && !(*value &= (~(1 << Index)))) {
-                obs.view.destroy(entt);
+        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);
             }
         }
 
@@ -234,7 +237,7 @@ class basic_observer {
             (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), ...);
-            (reg.template on_destroy<NoneOf>().template connect<&maybe_valid_if<Index>>(obs), ...);
+            (reg.template on_destroy<NoneOf>().template connect<&maybe_valid_if<Index, NoneOf>>(obs), ...);
             (reg.template on_destroy<AllOf>().template connect<&discard_if<Index>>(obs), ...);
             (reg.template on_construct<NoneOf>().template connect<&discard_if<Index>>(obs), ...);
         }
@@ -250,15 +253,15 @@ class basic_observer {
     };
 
     template<typename... Matcher>
-    static void disconnect(basic_observer &obs, basic_registry<Entity> &reg) {
+    static void disconnect(basic_registry<Entity> &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);
+        static_assert(sizeof...(Matcher) < std::numeric_limits<payload_type>::digits, "Too many matchers");
         (matcher_handler<Matcher>::template connect<Index>(*this, reg), ...);
-        release = &basic_observer::disconnect<Matcher...>;
+        release.template connect<&basic_observer::disconnect<Matcher...>>(reg);
     }
 
 public:
@@ -266,13 +269,13 @@ public:
     using entity_type = Entity;
     /*! @brief Unsigned integer type. */
     using size_type = std::size_t;
-    /*! @brief Input iterator type. */
-    using iterator_type = typename sparse_set<Entity>::iterator_type;
+    /*! @brief Random access iterator type. */
+    using iterator = typename basic_sparse_set<Entity>::iterator;
 
     /*! @brief Default constructor. */
     basic_observer()
-        : target{}, release{}, view{}
-    {}
+        : release{},
+          storage{} {}
 
     /*! @brief Default copy constructor, deleted on purpose. */
     basic_observer(const basic_observer &) = delete;
@@ -286,10 +289,7 @@ public:
      */
     template<typename... Matcher>
     basic_observer(basic_registry<entity_type> &reg, basic_collector<Matcher...>)
-        : target{&reg},
-          release{},
-          view{}
-    {
+        : basic_observer{} {
         connect<Matcher...>(reg, std::index_sequence_for<Matcher...>{});
     }
 
@@ -300,13 +300,13 @@ public:
      * @brief Default copy assignment operator, deleted on purpose.
      * @return This observer.
      */
-    basic_observer & operator=(const basic_observer &) = delete;
+    basic_observer &operator=(const basic_observer &) = delete;
 
     /**
      * @brief Default move assignment operator, deleted on purpose.
      * @return This observer.
      */
-    basic_observer & operator=(basic_observer &&) = delete;
+    basic_observer &operator=(basic_observer &&) = delete;
 
     /**
      * @brief Connects an observer to a given registry.
@@ -317,15 +317,14 @@ public:
     void connect(basic_registry<entity_type> &reg, basic_collector<Matcher...>) {
         disconnect();
         connect<Matcher...>(reg, std::index_sequence_for<Matcher...>{});
-        target = &reg;
-        view.clear();
+        storage.clear();
     }
 
     /*! @brief Disconnects an observer from the registry it keeps track of. */
     void disconnect() {
         if(release) {
-            release(*this, *target);
-            release = nullptr;
+            release(*this);
+            release.reset();
         }
     }
 
@@ -333,32 +332,32 @@ public:
      * @brief Returns the number of elements in an observer.
      * @return Number of elements.
      */
-    size_type size() const ENTT_NOEXCEPT {
-        return view.size();
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return storage.size();
     }
 
     /**
      * @brief Checks whether an observer is empty.
      * @return True if the observer is empty, false otherwise.
      */
-    bool empty() const ENTT_NOEXCEPT {
-        return view.empty();
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return storage.empty();
     }
 
     /**
      * @brief Direct access to the list of entities of the observer.
      *
-     * The returned pointer is such that range `[data(), data() + size()]` is
+     * The returned pointer is such that range `[data(), data() + size())` is
      * always a valid range, even if the container is empty.
      *
      * @note
-     * There are no guarantees on the order of the entities. Use `begin` and
-     * `end` if you want to iterate the observer in the expected order.
+     * Entities are in the reverse order as returned by the `begin`/`end`
+     * iterators.
      *
      * @return A pointer to the array of entities.
      */
-    const entity_type * data() const ENTT_NOEXCEPT {
-        return view.data();
+    [[nodiscard]] const entity_type *data() const ENTT_NOEXCEPT {
+        return storage.data();
     }
 
     /**
@@ -369,8 +368,8 @@ public:
      *
      * @return An iterator to the first entity of the observer.
      */
-    iterator_type begin() const ENTT_NOEXCEPT {
-        return view.sparse_set<entity_type>::begin();
+    [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+        return storage.basic_sparse_set<entity_type>::begin();
     }
 
     /**
@@ -383,13 +382,13 @@ public:
      * @return An iterator to the entity following the last entity of the
      * observer.
      */
-    iterator_type end() const ENTT_NOEXCEPT {
-        return view.sparse_set<entity_type>::end();
+    [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+        return storage.basic_sparse_set<entity_type>::end();
     }
 
     /*! @brief Clears the underlying container. */
     void clear() ENTT_NOEXCEPT {
-        view.clear();
+        storage.clear();
     }
 
     /**
@@ -407,8 +406,6 @@ public:
      */
     template<typename Func>
     void each(Func func) const {
-        static_assert(std::is_invocable_v<Func, entity_type>);
-
         for(const auto entity: *this) {
             func(entity);
         }
@@ -430,13 +427,10 @@ public:
     }
 
 private:
-    basic_registry<entity_type> *target;
-    void(* release)(basic_observer &, basic_registry<entity_type> &);
-    storage<entity_type, payload_type> view;
+    delegate<void(basic_observer &)> release;
+    basic_storage<entity_type, payload_type> storage;
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/entity/organizer.hpp

@@ -0,0 +1,487 @@
+#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 "fwd.hpp"
+#include "helper.hpp"
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename>
+struct is_view: std::false_type {};
+
+template<typename Entity, typename... Component, typename... Exclude>
+struct is_view<basic_view<Entity, get_t<Component...>, exclude_t<Exclude...>>>: std::true_type {};
+
+template<typename Type>
+inline constexpr bool is_view_v = is_view<Type>::value;
+
+template<typename Type, typename Override>
+struct unpack_type {
+    using ro = std::conditional_t<
+        type_list_contains_v<Override, std::add_const_t<Type>> || (std::is_const_v<Type> && !type_list_contains_v<Override, std::remove_const_t<Type>>),
+        type_list<std::remove_const_t<Type>>,
+        type_list<>>;
+
+    using rw = std::conditional_t<
+        type_list_contains_v<Override, std::remove_const_t<Type>> || (!std::is_const_v<Type> && !type_list_contains_v<Override, std::add_const_t<Type>>),
+        type_list<Type>,
+        type_list<>>;
+};
+
+template<typename Entity, typename... Override>
+struct unpack_type<basic_registry<Entity>, 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 Entity, typename... Component, typename... Exclude, typename... Override>
+struct unpack_type<basic_view<Entity, get_t<Component...>, exclude_t<Exclude...>>, type_list<Override...>> {
+    using ro = type_list_cat_t<type_list<Exclude...>, typename unpack_type<Component, type_list<Override...>>::ro...>;
+    using rw = type_list_cat_t<typename unpack_type<Component, type_list<Override...>>::rw...>;
+};
+
+template<typename Entity, typename... Component, typename... Exclude, typename... Override>
+struct unpack_type<const basic_view<Entity, get_t<Component...>, exclude_t<Exclude...>>, type_list<Override...>>
+    : unpack_type<basic_view<Entity, get_t<Component...>, exclude_t<Exclude...>>, type_list<Override...>> {};
+
+template<typename, typename>
+struct resource;
+
+template<typename... Args, typename... Req>
+struct resource<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...));
+
+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...));
+
+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...));
+
+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();
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief Utility class for creating a static task graph.
+ *
+ * This class offers minimal support (but sufficient in many cases) for creating
+ * an execution graph from functions and their requirements on resources.<br/>
+ * Note that the resulting tasks aren't executed in any case. This isn't the
+ * 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).
+ */
+template<typename Entity>
+class basic_organizer final {
+    using callback_type = void(const void *, basic_registry<Entity> &);
+    using prepare_type = void(basic_registry<Entity> &);
+    using dependency_type = std::size_t(const bool, const type_info **, const std::size_t);
+
+    struct vertex_data final {
+        std::size_t ro_count{};
+        std::size_t rw_count{};
+        const char *name{};
+        const void *payload{};
+        callback_type *callback{};
+        dependency_type *dependency;
+        prepare_type *prepare{};
+        const type_info *info{};
+    };
+
+    template<typename Type>
+    [[nodiscard]] static decltype(auto) extract(basic_registry<Entity> &reg) {
+        if constexpr(std::is_same_v<Type, basic_registry<Entity>>) {
+            return reg;
+        } else if constexpr(internal::is_view_v<Type>) {
+            return as_view{reg};
+        } else {
+            return reg.template ctx_or_set<std::remove_reference_t<Type>>();
+        }
+    }
+
+    template<typename... Args>
+    [[nodiscard]] static auto to_args(basic_registry<Entity> &reg, type_list<Args...>) {
+        return std::tuple<decltype(extract<Args>(reg))...>(extract<Args>(reg)...);
+    }
+
+    template<typename... Type>
+    static std::size_t fill_dependencies(type_list<Type...>, [[maybe_unused]] const type_info **buffer, [[maybe_unused]] const std::size_t count) {
+        if constexpr(sizeof...(Type) == 0u) {
+            return {};
+        } else {
+            const type_info *info[sizeof...(Type)]{&type_id<Type>()...};
+            const auto length = (std::min)(count, sizeof...(Type));
+            std::copy_n(info, length, buffer);
+            return length;
+        }
+    }
+
+    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;
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Raw task function type. */
+    using function_type = callback_type;
+
+    /*! @brief Vertex type of a task graph defined as an adjacency list. */
+    struct vertex {
+        /**
+         * @brief Constructs a vertex of the task graph.
+         * @param vtype True if the vertex is a top-level one, false otherwise.
+         * @param data The data associated with the vertex.
+         * @param edges The indices of the children in the adjacency list.
+         */
+        vertex(const bool vtype, vertex_data data, std::vector<std::size_t> edges)
+            : is_top_level{vtype},
+              node{std::move(data)},
+              reachable{std::move(edges)} {}
+
+        /**
+         * @brief Fills a buffer with the type info objects for the writable
+         * resources of a vertex.
+         * @param buffer A buffer pre-allocated by the user.
+         * @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 {
+            return node.dependency(false, buffer, length);
+        }
+
+        /**
+         * @brief Fills a buffer with the type info objects for the read-only
+         * resources of a vertex.
+         * @param buffer A buffer pre-allocated by the user.
+         * @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 {
+            return node.dependency(true, buffer, length);
+        }
+
+        /**
+         * @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 {
+            return node.ro_count;
+        }
+
+        /**
+         * @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 {
+            return node.rw_count;
+        }
+
+        /**
+         * @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 {
+            return is_top_level;
+        }
+
+        /**
+         * @brief Returns a type info object associated with a vertex.
+         * @return A properly initialized type info object.
+         */
+        const type_info &info() const ENTT_NOEXCEPT {
+            return *node.info;
+        }
+
+        /**
+         * @brief Returns a user defined name associated with a vertex, if any.
+         * @return The user defined name associated with the vertex, if any.
+         */
+        const char *name() const ENTT_NOEXCEPT {
+            return node.name;
+        }
+
+        /**
+         * @brief Returns the function associated with a vertex.
+         * @return The function associated with the vertex.
+         */
+        function_type *callback() const ENTT_NOEXCEPT {
+            return node.callback;
+        }
+
+        /**
+         * @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 {
+            return node.payload;
+        }
+
+        /**
+         * @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 {
+            return reachable;
+        }
+
+        /**
+         * @brief Prepares a registry and assures that all required resources
+         * are properly instantiated before using them.
+         * @param reg A valid registry.
+         */
+        void prepare(basic_registry<entity_type> &reg) const {
+            node.prepare ? node.prepare(reg) : void();
+        }
+
+    private:
+        bool is_top_level;
+        vertex_data node;
+        std::vector<std::size_t> reachable;
+    };
+
+    /**
+     * @brief Adds a free function to the task list.
+     * @tparam Candidate Function to add to the task list.
+     * @tparam Req Additional requirements and/or override resource access mode.
+     * @param name Optional name to associate with the task.
+     */
+    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>>;
+
+        callback_type *callback = +[](const void *, basic_registry<entity_type> &reg) {
+            std::apply(Candidate, to_args(reg, typename resource_type::args{}));
+        };
+
+        vertex_data vdata{
+            resource_type::ro::size,
+            resource_type::rw::size,
+            name,
+            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{})); },
+            &type_id<std::integral_constant<decltype(Candidate), Candidate>>()};
+
+        track_dependencies(vertices.size(), requires_registry, typename resource_type::ro{}, typename resource_type::rw{});
+        vertices.push_back(std::move(vdata));
+    }
+
+    /**
+     * @brief Adds a free function with payload or a member function with an
+     * instance to the task list.
+     * @tparam Candidate Function or member to add to the task list.
+     * @tparam Req Additional requirements and/or override resource access mode.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     * @param name Optional name to associate with the task.
+     */
+    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>>;
+
+        callback_type *callback = +[](const void *payload, basic_registry<entity_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{})));
+        };
+
+        vertex_data vdata{
+            resource_type::ro::size,
+            resource_type::rw::size,
+            name,
+            &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{})); },
+            &type_id<std::integral_constant<decltype(Candidate), Candidate>>()};
+
+        track_dependencies(vertices.size(), requires_registry, typename resource_type::ro{}, typename resource_type::rw{});
+        vertices.push_back(std::move(vdata));
+    }
+
+    /**
+     * @brief Adds an user defined function with optional payload to the task
+     * list.
+     * @tparam Req Additional requirements and/or override resource access mode.
+     * @param func Function to add to the task list.
+     * @param payload User defined arbitrary data.
+     * @param name Optional name to associate with the task.
+     */
+    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...>>;
+        track_dependencies(vertices.size(), true, typename resource_type::ro{}, typename resource_type::rw{});
+
+        vertex_data vdata{
+            resource_type::ro::size,
+            resource_type::rw::size,
+            name,
+            payload,
+            func,
+            +[](const bool rw, const type_info **buffer, const std::size_t length) { return rw ? fill_dependencies(typename resource_type::rw{}, buffer, length) : fill_dependencies(typename resource_type::ro{}, buffer, length); },
+            nullptr,
+            &type_id<void>()};
+
+        vertices.push_back(std::move(vdata));
+    }
+
+    /**
+     * @brief Generates a task graph for the current content.
+     * @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());
+
+        for(std::size_t col{}, length = vertices.size(); col < length; ++col) {
+            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(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));
+        }
+
+        return adjacency_list;
+    }
+
+    /*! @brief Erases all elements from a container. */
+    void clear() {
+        dependencies.clear();
+        vertices.clear();
+    }
+
+private:
+    dense_hash_map<id_type, std::vector<std::pair<std::size_t, bool>>, identity> dependencies;
+    std::vector<vertex_data> vertices;
+};
+
+} // namespace entt
+
+#endif

src/entt/entity/runtime_view.hpp

@@ -1,20 +1,99 @@
 #ifndef ENTT_ENTITY_RUNTIME_VIEW_HPP
 #define ENTT_ENTITY_RUNTIME_VIEW_HPP
 
-
-#include <iterator>
-#include <vector>
-#include <utility>
 #include <algorithm>
+#include <iterator>
 #include <type_traits>
+#include <utility>
+#include <vector>
 #include "../config/config.h"
-#include "sparse_set.hpp"
 #include "entity.hpp"
 #include "fwd.hpp"
-
+#include "sparse_set.hpp"
 
 namespace entt {
 
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename Type>
+class runtime_view_iterator final {
+    [[nodiscard]] bool valid() const {
+        return 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;
+
+    runtime_view_iterator(const std::vector<const Type *> &cpools, const std::vector<const Type *> &ignore, iterator_type curr) ENTT_NOEXCEPT
+        : pools{&cpools},
+          filter{&ignore},
+          it{curr} {
+        if(it != (*pools)[0]->end() && !valid()) {
+            ++(*this);
+        }
+    }
+
+    runtime_view_iterator &operator++() {
+        while(++it != (*pools)[0]->end() && !valid()) {}
+        return *this;
+    }
+
+    runtime_view_iterator operator++(int) {
+        runtime_view_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    runtime_view_iterator &operator--() ENTT_NOEXCEPT {
+        while(--it != (*pools)[0]->begin() && !valid()) {}
+        return *this;
+    }
+
+    runtime_view_iterator operator--(int) ENTT_NOEXCEPT {
+        runtime_view_iterator orig = *this;
+        return operator--(), orig;
+    }
+
+    [[nodiscard]] pointer operator->() const {
+        return it.operator->();
+    }
+
+    [[nodiscard]] reference operator*() const {
+        return *operator->();
+    }
+
+    [[nodiscard]] bool operator==(const runtime_view_iterator &other) const ENTT_NOEXCEPT {
+        return it == other.it;
+    }
+
+    [[nodiscard]] bool operator!=(const runtime_view_iterator &other) const ENTT_NOEXCEPT {
+        return !(*this == other);
+    }
+
+private:
+    const std::vector<const Type *> *pools;
+    const std::vector<const Type *> *filter;
+    iterator_type it;
+};
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
 
 /**
  * @brief Runtime view.
@@ -55,94 +134,10 @@ namespace entt {
  * @tparam Entity A valid entity type (see entt_traits for more details).
  */
 template<typename Entity>
-class basic_runtime_view {
-    /*! @brief A registry is allowed to create views. */
-    friend class basic_registry<Entity>;
+class basic_runtime_view final {
+    using basic_common_type = basic_sparse_set<Entity>;
 
-    using underlying_iterator_type = typename sparse_set<Entity>::iterator_type;
-
-    class iterator {
-        friend class basic_runtime_view<Entity>;
-
-        using direct_type = std::vector<const sparse_set<Entity> *>;
-
-        iterator(const direct_type *all, underlying_iterator_type curr) ENTT_NOEXCEPT
-            : pools{all},
-              it{curr}
-        {
-            if(it != (*pools)[0]->end() && !valid()) {
-                ++(*this);
-            }
-        }
-
-        bool valid() const {
-            return std::all_of(pools->begin()++, pools->end(), [entt = *it](const auto *curr) {
-                return curr->has(entt);
-            });
-        }
-
-    public:
-        using difference_type = typename underlying_iterator_type::difference_type;
-        using value_type = typename underlying_iterator_type::value_type;
-        using pointer = typename underlying_iterator_type::pointer;
-        using reference = typename underlying_iterator_type::reference;
-        using iterator_category = std::bidirectional_iterator_tag;
-
-        iterator() ENTT_NOEXCEPT = default;
-
-        iterator & operator++() {
-            while(++it != (*pools)[0]->end() && !valid());
-            return *this;
-        }
-
-        iterator operator++(int) {
-            iterator orig = *this;
-            return operator++(), orig;
-        }
-
-        iterator & operator--() ENTT_NOEXCEPT {
-            while(--it != (*pools)[0]->begin() && !valid());
-            return *this;
-        }
-
-        iterator operator--(int) ENTT_NOEXCEPT {
-            iterator orig = *this;
-            return operator--(), orig;
-        }
-
-        bool operator==(const iterator &other) const ENTT_NOEXCEPT {
-            return other.it == it;
-        }
-
-        bool operator!=(const iterator &other) const ENTT_NOEXCEPT {
-            return !(*this == other);
-        }
-
-        pointer operator->() const {
-            return it.operator->();
-        }
-
-        reference operator*() const {
-            return *operator->();
-        }
-
-    private:
-        const direct_type *pools;
-        underlying_iterator_type it;
-    };
-
-    basic_runtime_view(std::vector<const sparse_set<Entity> *> others) ENTT_NOEXCEPT
-        : pools{std::move(others)}
-    {
-        const auto it = std::min_element(pools.begin(), pools.end(), [](const auto *lhs, const auto *rhs) {
-            return (!lhs && rhs) || (lhs && rhs && lhs->size() < rhs->size());
-        });
-
-        // brings the best candidate (if any) on front of the vector
-        std::rotate(pools.begin(), it, pools.end());
-    }
-
-    bool valid() const {
+    [[nodiscard]] bool valid() const {
         return !pools.empty() && pools.front();
     }
 
@@ -151,23 +146,36 @@ public:
     using entity_type = Entity;
     /*! @brief Unsigned integer type. */
     using size_type = std::size_t;
-    /*! @brief Input iterator type. */
-    using iterator_type = iterator;
+    /*! @brief Bidirectional iterator type. */
+    using iterator = internal::runtime_view_iterator<basic_common_type>;
+
+    /*! @brief Default constructor to use to create empty, invalid views. */
+    basic_runtime_view() ENTT_NOEXCEPT
+        : pools{},
+          filter{} {}
 
     /**
-     * @brief Estimates the number of entities that have the given components.
-     * @return Estimated number of entities that have the given components.
+     * @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.
      */
-    size_type size() const {
-        return valid() ? pools.front()->size() : size_type{};
+    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());
+        });
+
+        // brings the best candidate (if any) on front of the vector
+        std::rotate(pools.begin(), candidate, pools.end());
     }
 
     /**
-     * @brief Checks if the view is definitely empty.
-     * @return True if the view is definitely empty, false otherwise.
+     * @brief Estimates the number of entities iterated by the view.
+     * @return Estimated number of entities iterated by the view.
      */
-    bool empty() const {
-        return !valid() || pools.front()->empty();
+    [[nodiscard]] size_type size_hint() const {
+        return valid() ? pools.front()->size() : size_type{};
     }
 
     /**
@@ -178,20 +186,10 @@ public:
      * components. If the view is empty, the returned iterator will be equal to
      * `end()`.
      *
-     * @note
-     * Input iterators stay true to the order imposed to the underlying data
-     * structures.
-     *
      * @return An iterator to the first entity that has the given components.
      */
-    iterator_type begin() const {
-        iterator_type it{};
-
-        if(valid()) {
-            it = { &pools, pools[0]->begin() };
-        }
-
-        return it;
+    [[nodiscard]] iterator begin() const {
+        return valid() ? iterator{pools, filter, pools[0]->begin()} : iterator{};
     }
 
     /**
@@ -202,32 +200,21 @@ public:
      * has the given components. Attempting to dereference the returned iterator
      * results in undefined behavior.
      *
-     * @note
-     * Input iterators stay true to the order imposed to the underlying data
-     * structures.
-     *
      * @return An iterator to the entity following the last entity that has the
      * given components.
      */
-    iterator_type end() const {
-        iterator_type it{};
-
-        if(valid()) {
-            it = { &pools, pools[0]->end() };
-        }
-
-        return it;
+    [[nodiscard]] iterator end() const {
+        return valid() ? iterator{pools, filter, pools[0]->end()} : iterator{};
     }
 
     /**
      * @brief Checks if a view contains an entity.
-     * @param entt A valid entity identifier.
+     * @param entt A valid identifier.
      * @return True if the view contains the given entity, false otherwise.
      */
-    bool contains(const entity_type entt) const {
-        return valid() && std::all_of(pools.cbegin(), pools.cend(), [entt](const auto *view) {
-            return view->find(entt) != view->end();
-        });
+    [[nodiscard]] bool contains(const entity_type entt) const {
+        return valid() && std::all_of(pools.cbegin(), pools.cend(), [entt](const auto *curr) { return curr->contains(entt); })
+               && std::none_of(filter.cbegin(), filter.cend(), [entt](const auto *curr) { return curr && curr->contains(entt); });
     }
 
     /**
@@ -253,11 +240,10 @@ public:
     }
 
 private:
-    std::vector<const sparse_set<Entity> *> pools;
+    std::vector<const basic_common_type *> pools;
+    std::vector<const basic_common_type *> filter;
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/entity/snapshot.hpp

@@ -1,21 +1,23 @@
 #ifndef ENTT_ENTITY_SNAPSHOT_HPP
 #define ENTT_ENTITY_SNAPSHOT_HPP
 
-
 #include <array>
 #include <cstddef>
-#include <utility>
 #include <iterator>
+#include <tuple>
 #include <type_traits>
-#include <unordered_map>
+#include <utility>
+#include <vector>
 #include "../config/config.h"
+#include "../container/dense_hash_map.hpp"
+#include "../core/type_traits.hpp"
+#include "component.hpp"
 #include "entity.hpp"
 #include "fwd.hpp"
-
+#include "registry.hpp"
 
 namespace entt {
 
-
 /**
  * @brief Utility class to create snapshots from a registry.
  *
@@ -28,95 +30,71 @@ namespace entt {
  */
 template<typename Entity>
 class basic_snapshot {
-    /*! @brief A registry is allowed to create snapshots. */
-    friend class basic_registry<Entity>;
-
-    using follow_fn_type = Entity(const basic_registry<Entity> &, const Entity);
-    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
-
-    basic_snapshot(const basic_registry<Entity> *source, Entity init, follow_fn_type *fn) ENTT_NOEXCEPT
-        : reg{source},
-          seed{init},
-          follow{fn}
-    {}
+    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 {
-        archive(typename traits_type::entity_type(sz));
+        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 has<Component>(entt)) {
-                if constexpr(std::is_empty_v<Component>) {
-                    archive(entt);
-                } else {
-                    archive(entt, reg->template get<Component>(entt));
-                }
+            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... Indexes>
-    void component(Archive &archive, It first, It last, std::index_sequence<Indexes...>) const {
-        std::array<std::size_t, sizeof...(Indexes)> size{};
+    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 has<Component>(entt) ? ++size[Indexes] : size[Indexes]), ...);
+            ((reg->template all_of<Component>(entt) ? ++size[Index] : 0u), ...);
         }
 
-        (get<Component>(archive, size[Indexes], first, last), ...);
+        (get<Component>(archive, size[Index], first, last), ...);
     }
 
 public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+
+    /**
+     * @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
+        : reg{&source} {}
+
     /*! @brief Default move constructor. */
     basic_snapshot(basic_snapshot &&) = default;
 
     /*! @brief Default move assignment operator. @return This snapshot. */
-    basic_snapshot & operator=(basic_snapshot &&) = default;
+    basic_snapshot &operator=(basic_snapshot &&) = default;
 
     /**
-     * @brief Puts aside all the entities that are still in use.
+     * @brief Puts aside all the entities from the underlying registry.
      *
      * Entities are serialized along with their versions. Destroyed entities are
-     * not taken in consideration by this function.
+     * taken in consideration as well by this function.
      *
      * @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 Archive>
-    const basic_snapshot & entities(Archive &archive) const {
-        archive(typename traits_type::entity_type(reg->alive()));
-        reg->each([&archive](const auto entt) { archive(entt); });
-        return *this;
-    }
+    const basic_snapshot &entities(Archive &archive) const {
+        const auto sz = reg->size();
 
-    /**
-     * @brief Puts aside destroyed entities.
-     *
-     * Entities are serialized along with their versions. Entities that are
-     * still in use are not taken in consideration by this function.
-     *
-     * @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 Archive>
-    const basic_snapshot & destroyed(Archive &archive) const {
-        auto size = reg->size() - reg->alive();
-        archive(typename traits_type::entity_type(size));
+        archive(typename entity_traits::entity_type(sz + 1u));
+        archive(reg->released());
 
-        if(size) {
-            auto curr = seed;
-            archive(curr);
-
-            for(--size; size; --size) {
-                curr = follow(*reg, curr);
-                archive(curr);
-            }
+        for(auto first = reg->data(), last = first + sz; first != last; ++first) {
+            archive(*first);
         }
 
         return *this;
@@ -134,9 +112,15 @@ public:
      * @return An object of this type to continue creating the snapshot.
      */
     template<typename... Component, typename Archive>
-    const basic_snapshot & component(Archive &archive) const {
-        (component<Component>(archive, reg->template data<Component>(), reg->template data<Component>() + reg->template size<Component>()), ...);
-        return *this;
+    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;
+        }
     }
 
     /**
@@ -154,18 +138,15 @@ public:
      * @return An object of this type to continue creating the snapshot.
      */
     template<typename... Component, typename Archive, typename It>
-    const basic_snapshot & component(Archive &archive, It first, It last) const {
+    const basic_snapshot &component(Archive &archive, It first, It last) const {
         component<Component...>(archive, first, last, std::index_sequence_for<Component...>{});
         return *this;
     }
 
 private:
-    const basic_registry<Entity> *reg;
-    const Entity seed;
-    follow_fn_type *follow;
+    const basic_registry<entity_type> *reg;
 };
 
-
 /**
  * @brief Utility class to restore a snapshot as a whole.
  *
@@ -178,60 +159,53 @@ private:
  */
 template<typename Entity>
 class basic_snapshot_loader {
-    /*! @brief A registry is allowed to create snapshot loaders. */
-    friend class basic_registry<Entity>;
+    using entity_traits = entt_traits<Entity>;
 
-    using force_fn_type = void(basic_registry<Entity> &, const Entity, const bool);
-    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
-
-    basic_snapshot_loader(basic_registry<Entity> *source, force_fn_type *fn) ENTT_NOEXCEPT
-        : reg{source},
-          force{fn}
-    {
-        // to restore a snapshot as a whole requires a clean registry
-        ENTT_ASSERT(reg->empty());
-    }
-
-    template<typename Archive>
-    void assure(Archive &archive, bool discard) const {
-        typename traits_type::entity_type length{};
+    template<typename Type, typename Archive>
+    void assign(Archive &archive) const {
+        typename entity_traits::entity_type length{};
         archive(length);
 
-        while(length--) {
-            Entity entt{};
-            archive(entt);
-            force(*reg, entt, discard);
-        }
-    }
+        entity_type entt{};
 
-    template<typename Type, typename Archive, typename... Args>
-    void assign(Archive &archive, Args... args) const {
-        typename traits_type::entity_type length{};
-        archive(length);
-
-        while(length--) {
-            static constexpr auto discard = false;
-            Entity entt{};
-
-            if constexpr(std::is_empty_v<Type>) {
+        if constexpr(ignore_as_empty_v<std::remove_const_t<Type>>) {
+            while(length--) {
                 archive(entt);
-                force(*reg, entt, discard);
-                reg->template assign<Type>(args..., entt);
-            } else {
-                Type 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);
+            }
+        } else {
+            Type instance{};
+
+            while(length--) {
                 archive(entt, instance);
-                force(*reg, entt, discard);
-                reg->template assign<Type>(args..., entt, std::as_const(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));
             }
         }
     }
 
 public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+
+    /**
+     * @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
+        : reg{&source} {
+        // restoring a snapshot as a whole requires a clean registry
+        ENTT_ASSERT(reg->empty(), "Registry must be empty");
+    }
+
     /*! @brief Default move constructor. */
     basic_snapshot_loader(basic_snapshot_loader &&) = default;
 
     /*! @brief Default move assignment operator. @return This loader. */
-    basic_snapshot_loader & operator=(basic_snapshot_loader &&) = default;
+    basic_snapshot_loader &operator=(basic_snapshot_loader &&) = default;
 
     /**
      * @brief Restores entities that were in use during serialization.
@@ -244,26 +218,18 @@ public:
      * @return A valid loader to continue restoring data.
      */
     template<typename Archive>
-    const basic_snapshot_loader & entities(Archive &archive) const {
-        static constexpr auto discard = false;
-        assure(archive, discard);
-        return *this;
-    }
+    const basic_snapshot_loader &entities(Archive &archive) const {
+        typename entity_traits::entity_type length{};
+
+        archive(length);
+        std::vector<entity_type> all(length);
+
+        for(std::size_t pos{}; pos < length; ++pos) {
+            archive(all[pos]);
+        }
+
+        reg->assign(++all.cbegin(), all.cend(), all[0u]);
 
-    /**
-     * @brief Restores entities that were destroyed during serialization.
-     *
-     * This function restores the entities that were destroyed during
-     * serialization and gives them the versions they originally had.
-     *
-     * @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 Archive>
-    const basic_snapshot_loader & destroyed(Archive &archive) const {
-        static constexpr auto discard = true;
-        assure(archive, discard);
         return *this;
     }
 
@@ -281,7 +247,7 @@ public:
      * @return A valid loader to continue restoring data.
      */
     template<typename... Component, typename Archive>
-    const basic_snapshot_loader & component(Archive &archive) const {
+    const basic_snapshot_loader &component(Archive &archive) const {
         (assign<Component>(archive), ...);
         return *this;
     }
@@ -296,25 +262,25 @@ public:
      *
      * @return A valid loader to continue restoring data.
      */
-    const basic_snapshot_loader & orphans() const {
-        reg->orphans([this](const auto entt) {
-            reg->destroy(entt);
+    const basic_snapshot_loader &orphans() const {
+        reg->each([this](const auto entt) {
+            if(reg->orphan(entt)) {
+                reg->release(entt);
+            }
         });
 
         return *this;
     }
 
 private:
-    basic_registry<Entity> *reg;
-    force_fn_type *force;
+    basic_registry<entity_type> *reg;
 };
 
-
 /**
  * @brief Utility class for _continuous loading_.
  *
  * A _continuous loader_ is designed to load data from a source registry to a
- * (possibly) non-empty destination. The loader can accomodate in a registry
+ * (possibly) non-empty destination. The loader can accommodate in a registry
  * more than one snapshot in a sort of _continuous loading_ that updates the
  * destination one step at a time.<br/>
  * Identifiers that entities originally had are not transferred to the target.
@@ -328,12 +294,10 @@ private:
  */
 template<typename Entity>
 class basic_continuous_loader {
-    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
+    using entity_traits = entt_traits<Entity>;
 
     void destroy(Entity entt) {
-        const auto it = remloc.find(entt);
-
-        if(it == remloc.cend()) {
+        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);
@@ -347,7 +311,10 @@ class basic_continuous_loader {
             const auto local = reg->create();
             remloc.emplace(entt, std::make_pair(local, true));
         } else {
-            remloc[entt].first = reg->valid(remloc[entt].first) ? remloc[entt].first : reg->create();
+            if(!reg->valid(remloc[entt].first)) {
+                remloc[entt].first = reg->create();
+            }
+
             // set the dirty flag
             remloc[entt].second = true;
         }
@@ -355,7 +322,7 @@ class basic_continuous_loader {
 
     template<typename Container>
     auto update(int, Container &container)
-    -> decltype(typename Container::mapped_type{}, void()) {
+        -> decltype(typename Container::mapped_type{}, void()) {
         // map like container
         Container other;
 
@@ -363,12 +330,12 @@ class basic_continuous_loader {
             using first_type = std::remove_const_t<typename std::decay_t<decltype(pair)>::first_type>;
             using second_type = typename std::decay_t<decltype(pair)>::second_type;
 
-            if constexpr(std::is_same_v<first_type, Entity> && std::is_same_v<second_type, Entity>) {
+            if constexpr(std::is_same_v<first_type, entity_type> && std::is_same_v<second_type, entity_type>) {
                 other.emplace(map(pair.first), map(pair.second));
-            } else if constexpr(std::is_same_v<first_type, Entity>) {
+            } else if constexpr(std::is_same_v<first_type, entity_type>) {
                 other.emplace(map(pair.first), std::move(pair.second));
             } else {
-                static_assert(std::is_same_v<second_type, Entity>);
+                static_assert(std::is_same_v<second_type, entity_type>, "Neither the key nor the value are of entity type");
                 other.emplace(std::move(pair.first), map(pair.second));
             }
         }
@@ -378,9 +345,9 @@ class basic_continuous_loader {
 
     template<typename Container>
     auto update(char, Container &container)
-    -> decltype(typename Container::value_type{}, void()) {
+        -> decltype(typename Container::value_type{}, void()) {
         // vector like container
-        static_assert(std::is_same_v<typename Container::value_type, Entity>);
+        static_assert(std::is_same_v<typename Container::value_type, entity_type>, "Invalid value type");
 
         for(auto &&entt: container) {
             entt = map(entt);
@@ -388,10 +355,10 @@ class basic_continuous_loader {
     }
 
     template<typename Other, typename Type, typename Member>
-    void update([[maybe_unused]] Other &instance, [[maybe_unused]] Member Type:: *member) {
+    void update([[maybe_unused]] Other &instance, [[maybe_unused]] Member Type::*member) {
         if constexpr(!std::is_same_v<Other, Type>) {
             return;
-        } else if constexpr(std::is_same_v<Member, Entity>) {
+        } else if constexpr(std::is_same_v<Member, entity_type>) {
             instance.*member = map(instance.*member);
         } else {
             // maybe a container? let's try...
@@ -399,47 +366,38 @@ class basic_continuous_loader {
         }
     }
 
-    template<typename Archive>
-    void assure(Archive &archive, void(basic_continuous_loader:: *member)(Entity)) {
-        typename traits_type::entity_type length{};
-        archive(length);
-
-        while(length--) {
-            Entity entt{};
-            archive(entt);
-            (this->*member)(entt);
-        }
-    }
-
     template<typename Component>
-    void reset() {
+    void remove_if_exists() {
         for(auto &&ref: remloc) {
             const auto local = ref.second.first;
 
             if(reg->valid(local)) {
-                reg->template remove_if_exists<Component>(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 traits_type::entity_type length{};
+    void assign(Archive &archive, [[maybe_unused]] Member Type::*...member) {
+        typename entity_traits::entity_type length{};
         archive(length);
 
-        while(length--) {
-            Entity entt{};
+        entity_type entt{};
 
-            if constexpr(std::is_empty_v<Other>) {
+        if constexpr(ignore_as_empty_v<std::remove_const_t<Other>>) {
+            while(length--) {
                 archive(entt);
                 restore(entt);
-                reg->template assign_or_replace<Other>(map(entt));
-            } else {
-                Other instance{};
+                reg->template emplace_or_replace<Other>(map(entt));
+            }
+        } else {
+            Other instance{};
+
+            while(length--) {
                 archive(entt, instance);
                 (update(instance, member), ...);
                 restore(entt);
-                reg->template assign_or_replace<Other>(map(entt), std::as_const(instance));
+                reg->template emplace_or_replace<Other>(map(entt), std::move(instance));
             }
         }
     }
@@ -449,18 +407,17 @@ public:
     using entity_type = Entity;
 
     /**
-     * @brief Constructs a loader that is bound to a given registry.
+     * @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}
-    {}
+        : reg{&source} {}
 
     /*! @brief Default move constructor. */
     basic_continuous_loader(basic_continuous_loader &&) = default;
 
     /*! @brief Default move assignment operator. @return This loader. */
-    basic_continuous_loader & operator=(basic_continuous_loader &&) = default;
+    basic_continuous_loader &operator=(basic_continuous_loader &&) = default;
 
     /**
      * @brief Restores entities that were in use during serialization.
@@ -473,24 +430,24 @@ public:
      * @return A non-const reference to this loader.
      */
     template<typename Archive>
-    basic_continuous_loader & entities(Archive &archive) {
-        assure(archive, &basic_continuous_loader::restore);
-        return *this;
-    }
+    basic_continuous_loader &entities(Archive &archive) {
+        typename entity_traits::entity_type length{};
+        entity_type entt{};
+
+        archive(length);
+        // discards the head of the list of destroyed entities
+        archive(entt);
+
+        for(std::size_t pos{}, last = length - 1u; pos < last; ++pos) {
+            archive(entt);
+
+            if(const auto entity = entity_traits::to_entity(entt); entity == pos) {
+                restore(entt);
+            } else {
+                destroy(entt);
+            }
+        }
 
-    /**
-     * @brief Restores entities that were destroyed during serialization.
-     *
-     * This function restores the entities that were destroyed during
-     * serialization and creates local counterparts for them if required.
-     *
-     * @tparam Archive Type of input archive.
-     * @param archive A valid reference to an input archive.
-     * @return A non-const reference to this loader.
-     */
-    template<typename Archive>
-    basic_continuous_loader & destroyed(Archive &archive) {
-        assure(archive, &basic_continuous_loader::destroy);
         return *this;
     }
 
@@ -514,8 +471,8 @@ public:
      * @return A non-const reference to this loader.
      */
     template<typename... Component, typename Archive, typename... Type, typename... Member>
-    basic_continuous_loader & component(Archive &archive, Member Type:: *... member) {
-        (reset<Component>(), ...);
+    basic_continuous_loader &component(Archive &archive, Member Type::*...member) {
+        (remove_if_exists<Component>(), ...);
         (assign<Component>(archive, member...), ...);
         return *this;
     }
@@ -528,7 +485,7 @@ public:
      *
      * @return A non-const reference to this loader.
      */
-    basic_continuous_loader & shrink() {
+    basic_continuous_loader &shrink() {
         auto it = remloc.begin();
 
         while(it != remloc.cend()) {
@@ -560,9 +517,11 @@ public:
      *
      * @return A non-const reference to this loader.
      */
-    basic_continuous_loader & orphans() {
-        reg->orphans([this](const auto entt) {
-            reg->destroy(entt);
+    basic_continuous_loader &orphans() {
+        reg->each([this](const auto entt) {
+            if(reg->orphan(entt)) {
+                reg->release(entt);
+            }
         });
 
         return *this;
@@ -570,19 +529,19 @@ public:
 
     /**
      * @brief Tests if a loader knows about a given entity.
-     * @param entt An entity identifier.
+     * @param entt A valid identifier.
      * @return True if `entity` is managed by the loader, false otherwise.
      */
-    bool has(entity_type entt) const ENTT_NOEXCEPT {
+    [[nodiscard]] bool contains(entity_type entt) const ENTT_NOEXCEPT {
         return (remloc.find(entt) != remloc.cend());
     }
 
     /**
      * @brief Returns the identifier to which an entity refers.
-     * @param entt An entity identifier.
+     * @param entt A valid identifier.
      * @return The local identifier if any, the null entity otherwise.
      */
-    entity_type map(entity_type entt) const ENTT_NOEXCEPT {
+    [[nodiscard]] entity_type map(entity_type entt) const ENTT_NOEXCEPT {
         const auto it = remloc.find(entt);
         entity_type other = null;
 
@@ -594,12 +553,10 @@ public:
     }
 
 private:
-    std::unordered_map<entity_type, std::pair<entity_type, bool>> remloc;
+    dense_hash_map<entity_type, std::pair<entity_type, bool>> remloc;
     basic_registry<entity_type> *reg;
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/entity/utility.hpp

@@ -0,0 +1,52 @@
+#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

@@ -1,27 +1,51 @@
+#include "config/version.h"
+#include "container/dense_hash_map.hpp"
+#include "container/dense_hash_set.hpp"
 #include "core/algorithm.hpp"
+#include "core/any.hpp"
 #include "core/attribute.h"
+#include "core/compressed_pair.hpp"
+#include "core/enum.hpp"
 #include "core/family.hpp"
 #include "core/hashed_string.hpp"
 #include "core/ident.hpp"
+#include "core/iterator.hpp"
+#include "core/memory.hpp"
 #include "core/monostate.hpp"
+#include "core/tuple.hpp"
 #include "core/type_info.hpp"
 #include "core/type_traits.hpp"
 #include "core/utility.hpp"
-#include "entity/actor.hpp"
+#include "entity/component.hpp"
 #include "entity/entity.hpp"
 #include "entity/group.hpp"
+#include "entity/handle.hpp"
 #include "entity/helper.hpp"
 #include "entity/observer.hpp"
+#include "entity/organizer.hpp"
 #include "entity/registry.hpp"
 #include "entity/runtime_view.hpp"
 #include "entity/snapshot.hpp"
 #include "entity/sparse_set.hpp"
 #include "entity/storage.hpp"
+#include "entity/utility.hpp"
 #include "entity/view.hpp"
 #include "locator/locator.hpp"
+#include "meta/adl_pointer.hpp"
+#include "meta/container.hpp"
+#include "meta/ctx.hpp"
 #include "meta/factory.hpp"
 #include "meta/meta.hpp"
+#include "meta/node.hpp"
+#include "meta/pointer.hpp"
 #include "meta/policy.hpp"
+#include "meta/range.hpp"
+#include "meta/resolve.hpp"
+#include "meta/template.hpp"
+#include "meta/type_traits.hpp"
+#include "meta/utility.hpp"
+#include "platform/android-ndk-r17.hpp"
+#include "poly/poly.hpp"
 #include "process/process.hpp"
 #include "process/scheduler.hpp"
 #include "resource/cache.hpp"

src/entt/fwd.hpp

@@ -1,3 +1,7 @@
+#include "container/fwd.hpp"
+#include "core/fwd.hpp"
 #include "entity/fwd.hpp"
+#include "meta/fwd.hpp"
+#include "poly/fwd.hpp"
 #include "resource/fwd.hpp"
 #include "signal/fwd.hpp"

src/entt/locator/locator.hpp

@@ -1,15 +1,12 @@
 #ifndef ENTT_LOCATOR_LOCATOR_HPP
 #define ENTT_LOCATOR_LOCATOR_HPP
 
-
 #include <memory>
 #include <utility>
 #include "../config/config.h"
 
-
 namespace entt {
 
-
 /**
  * @brief Service locator, nothing more.
  *
@@ -35,7 +32,7 @@ struct service_locator {
      * @brief Tests if a valid service implementation is set.
      * @return True if the service is set, false otherwise.
      */
-    static bool empty() ENTT_NOEXCEPT {
+    [[nodiscard]] static bool empty() ENTT_NOEXCEPT {
         return !static_cast<bool>(service);
     }
 
@@ -49,7 +46,7 @@ struct service_locator {
      *
      * @return A reference to the service implementation currently set, if any.
      */
-    static std::weak_ptr<Service> get() ENTT_NOEXCEPT {
+    [[nodiscard]] static std::weak_ptr<Service> get() ENTT_NOEXCEPT {
         return service;
     }
 
@@ -67,7 +64,7 @@ struct service_locator {
      *
      * @return A reference to the service implementation currently set, if any.
      */
-    static Service & ref() ENTT_NOEXCEPT {
+    [[nodiscard]] static Service &ref() ENTT_NOEXCEPT {
         return *service;
     }
 
@@ -78,7 +75,7 @@ struct service_locator {
      * @param args Parameters to use to construct the service.
      */
     template<typename Impl = Service, typename... Args>
-    static void set(Args &&... args) {
+    static void set(Args &&...args) {
         service = std::make_shared<Impl>(std::forward<Args>(args)...);
     }
 
@@ -87,7 +84,7 @@ struct service_locator {
      * @param ptr Service to use to replace the current one.
      */
     static void set(std::shared_ptr<Service> ptr) {
-        ENTT_ASSERT(static_cast<bool>(ptr));
+        ENTT_ASSERT(static_cast<bool>(ptr), "Null service not allowed");
         service = std::move(ptr);
     }
 
@@ -104,8 +101,6 @@ private:
     inline static std::shared_ptr<Service> service = nullptr;
 };
 
-
-}
-
+} // namespace entt
 
 #endif

src/entt/meta/adl_pointer.hpp

@@ -0,0 +1,35 @@
+#ifndef ENTT_META_ADL_POINTER_HPP
+#define ENTT_META_ADL_POINTER_HPP
+
+namespace entt {
+
+/**
+ * @brief ADL based lookup function for dereferencing meta pointer-like types.
+ * @tparam Type Element type.
+ * @param value A pointer-like object.
+ * @return The value returned from the dereferenced pointer.
+ */
+template<typename Type>
+decltype(auto) dereference_meta_pointer_like(const Type &value) {
+    return *value;
+}
+
+/**
+ * @brief Fake ADL based lookup function for meta pointer-like types.
+ * @tparam Type Element type.
+ */
+template<typename Type>
+struct adl_meta_pointer_like {
+    /**
+     * @brief Uses the default ADL based lookup method to resolve the call.
+     * @param value A pointer-like object.
+     * @return The value returned from the dereferenced pointer.
+     */
+    static decltype(auto) dereference(const Type &value) {
+        return dereference_meta_pointer_like(value);
+    }
+};
+
+} // namespace entt
+
+#endif

src/entt/meta/container.hpp

@@ -0,0 +1,274 @@
+#ifndef ENTT_META_CONTAINER_HPP
+#define ENTT_META_CONTAINER_HPP
+
+#include <array>
+#include <map>
+#include <set>
+#include <type_traits>
+#include <unordered_map>
+#include <unordered_set>
+#include <vector>
+#include "../container/dense_hash_map.hpp"
+#include "../container/dense_hash_set.hpp"
+#include "meta.hpp"
+#include "type_traits.hpp"
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename, typename = void>
+struct is_dynamic_sequence_container: std::false_type {};
+
+template<typename Type>
+struct is_dynamic_sequence_container<Type, std::void_t<decltype(&Type::reserve)>>: std::true_type {};
+
+template<typename, typename = void>
+struct is_key_only_meta_associative_container: std::true_type {};
+
+template<typename Type>
+struct is_key_only_meta_associative_container<Type, std::void_t<typename Type::mapped_type>>: std::false_type {};
+
+template<typename Type>
+struct basic_meta_sequence_container_traits {
+    using iterator = meta_sequence_container::iterator;
+    using size_type = std::size_t;
+
+    [[nodiscard]] static size_type size(const any &container) ENTT_NOEXCEPT {
+        return any_cast<const Type &>(container).size();
+    }
+
+    [[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 Type>
+struct basic_meta_associative_container_traits {
+    using iterator = meta_associative_container::iterator;
+    using size_type = std::size_t;
+
+    static constexpr auto key_only = is_key_only_meta_associative_container<Type>::value;
+
+    [[nodiscard]] static size_type size(const any &container) ENTT_NOEXCEPT {
+        return any_cast<const Type &>(container).size();
+    }
+
+    [[nodiscard]] static bool clear(any &container) {
+        if(auto *const cont = any_cast<Type>(&container); cont) {
+            cont->clear();
+            return true;
+        }
+
+        return false;
+    }
+
+    [[nodiscard]] static iterator 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 {};
+    }
+};
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief Meta sequence container traits for `std::vector`s of any type.
+ * @tparam Type The type of elements.
+ * @tparam Args Other arguments.
+ */
+template<typename Type, typename... Args>
+struct meta_sequence_container_traits<std::vector<Type, Args...>>
+    : internal::basic_meta_sequence_container_traits<std::vector<Type, 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.
+ */
+template<typename Type, auto N>
+struct meta_sequence_container_traits<std::array<Type, N>>
+    : internal::basic_meta_sequence_container_traits<std::array<Type, N>> {};
+
+/**
+ * @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.
+ */
+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...>> {};
+
+/**
+ * @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.
+ */
+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...>> {};
+
+/**
+ * @brief Meta associative container traits for `std::set`s of any type.
+ * @tparam Key The type of elements.
+ * @tparam Args Other arguments.
+ */
+template<typename Key, typename... Args>
+struct meta_associative_container_traits<std::set<Key, Args...>>
+    : internal::basic_meta_associative_container_traits<std::set<Key, Args...>> {};
+
+/**
+ * @brief Meta associative container traits for `std::unordered_set`s of any
+ * type.
+ * @tparam Key The type of elements.
+ * @tparam Args Other arguments.
+ */
+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...>> {};
+
+/**
+ * @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.
+ */
+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...>> {};
+
+/**
+ * @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.
+ */
+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...>> {};
+
+} // namespace entt
+
+#endif

src/entt/meta/ctx.hpp

@@ -0,0 +1,57 @@
+#ifndef ENTT_META_CTX_HPP
+#define ENTT_META_CTX_HPP
+
+#include "../config/config.h"
+#include "../core/attribute.h"
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+struct meta_type_node;
+
+struct ENTT_API meta_context {
+    // we could use the lines below but VS2017 returns with an ICE if combined with ENTT_API despite the code being valid C++
+    //     inline static meta_type_node *local = nullptr;
+    //     inline static meta_type_node **global = &local;
+
+    [[nodiscard]] static meta_type_node *&local() ENTT_NOEXCEPT {
+        static meta_type_node *chain = nullptr;
+        return chain;
+    }
+
+    [[nodiscard]] static meta_type_node **&global() ENTT_NOEXCEPT {
+        static meta_type_node **chain = &local();
+        return chain;
+    }
+};
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/*! @brief Opaque container for a meta context. */
+struct meta_ctx {
+    /**
+     * @brief Binds the meta system to a given context.
+     * @param other A valid context to which to bind.
+     */
+    static void bind(meta_ctx other) ENTT_NOEXCEPT {
+        internal::meta_context::global() = other.ctx;
+    }
+
+private:
+    internal::meta_type_node **ctx{&internal::meta_context::local()};
+};
+
+} // namespace entt
+
+#endif

src/entt/meta/fwd.hpp

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

src/entt/meta/node.hpp

@@ -0,0 +1,235 @@
+#ifndef ENTT_META_NODE_HPP
+#define ENTT_META_NODE_HPP
+
+#include <cstddef>
+#include <type_traits>
+#include <utility>
+#include "../config/config.h"
+#include "../core/attribute.h"
+#include "../core/enum.hpp"
+#include "../core/fwd.hpp"
+#include "../core/type_info.hpp"
+#include "../core/type_traits.hpp"
+#include "type_traits.hpp"
+
+namespace entt {
+
+class meta_any;
+class meta_type;
+struct meta_handle;
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+enum class meta_traits : std::uint32_t {
+    is_none = 0x0000,
+    is_const = 0x0001,
+    is_static = 0x0002,
+    is_arithmetic = 0x0004,
+    is_array = 0x0008,
+    is_enum = 0x0010,
+    is_class = 0x0020,
+    is_pointer = 0x0040,
+    is_meta_pointer_like = 0x0080,
+    is_meta_sequence_container = 0x0100,
+    is_meta_associative_container = 0x0200,
+    _entt_enum_as_bitmask
+};
+
+struct meta_type_node;
+
+struct meta_prop_node {
+    meta_prop_node *next;
+    const meta_any &id;
+    meta_any &value;
+};
+
+struct meta_base_node {
+    meta_base_node *next;
+    meta_type_node *const type;
+    meta_any (*const cast)(meta_any) ENTT_NOEXCEPT;
+};
+
+struct meta_conv_node {
+    meta_conv_node *next;
+    meta_type_node *const type;
+    meta_any (*const conv)(const meta_any &);
+};
+
+struct meta_ctor_node {
+    using size_type = std::size_t;
+    meta_ctor_node *next;
+    const size_type arity;
+    meta_type (*const arg)(const size_type) ENTT_NOEXCEPT;
+    meta_any (*const invoke)(meta_any *const);
+};
+
+struct meta_data_node {
+    using size_type = std::size_t;
+    id_type id;
+    meta_data_node *next;
+    meta_prop_node *prop;
+    const size_type arity;
+    const meta_traits traits;
+    meta_type_node *const type;
+    meta_type (*const arg)(const size_type) ENTT_NOEXCEPT;
+    bool (*const set)(meta_handle, meta_any);
+    meta_any (*const get)(meta_handle);
+};
+
+struct meta_func_node {
+    using size_type = std::size_t;
+    id_type id;
+    meta_func_node *next;
+    meta_prop_node *prop;
+    const size_type arity;
+    const meta_traits traits;
+    meta_type_node *const ret;
+    meta_type (*const arg)(const size_type) ENTT_NOEXCEPT;
+    meta_any (*const invoke)(meta_handle, meta_any *const);
+};
+
+struct meta_template_node {
+    using size_type = std::size_t;
+    const size_type arity;
+    meta_type_node *const type;
+    meta_type_node *(*const arg)(const size_type)ENTT_NOEXCEPT;
+};
+
+struct meta_type_node {
+    using size_type = std::size_t;
+    const type_info *info;
+    id_type id;
+    meta_type_node *next;
+    meta_prop_node *prop;
+    const size_type size_of;
+    const meta_traits traits;
+    meta_any (*const default_constructor)();
+    double (*const conversion_helper)(void *, const void *);
+    const meta_template_node *const templ;
+    meta_ctor_node *ctor{nullptr};
+    meta_base_node *base{nullptr};
+    meta_conv_node *conv{nullptr};
+    meta_data_node *data{nullptr};
+    meta_func_node *func{nullptr};
+    void (*dtor)(void *){nullptr};
+};
+
+template<typename... Args>
+meta_type_node *meta_arg_node(type_list<Args...>, const std::size_t index) ENTT_NOEXCEPT;
+
+template<typename Type>
+class ENTT_API meta_node {
+    static_assert(std::is_same_v<Type, std::remove_cv_t<std::remove_reference_t<Type>>>, "Invalid type");
+
+    [[nodiscard]] static auto *meta_default_constructor() ENTT_NOEXCEPT {
+        if constexpr(std::is_default_constructible_v<Type>) {
+            return +[]() { return meta_any{std::in_place_type<Type>}; };
+        } else {
+            return static_cast<std::decay_t<decltype(meta_type_node::default_constructor)>>(nullptr);
+        }
+    }
+
+    [[nodiscard]] static auto *meta_conversion_helper() ENTT_NOEXCEPT {
+        if constexpr(std::is_arithmetic_v<Type>) {
+            return +[](void *bin, const void *value) {
+                return bin ? static_cast<double>(*static_cast<Type *>(bin) = static_cast<Type>(*static_cast<const double *>(value))) : static_cast<double>(*static_cast<const Type *>(value));
+            };
+        } else if constexpr(std::is_enum_v<Type>) {
+            return +[](void *bin, const void *value) {
+                return bin ? static_cast<double>(*static_cast<Type *>(bin) = static_cast<Type>(static_cast<std::underlying_type_t<Type>>(*static_cast<const double *>(value)))) : static_cast<double>(*static_cast<const Type *>(value));
+            };
+        } else {
+            return static_cast<std::decay_t<decltype(meta_type_node::conversion_helper)>>(nullptr);
+        }
+    }
+
+    [[nodiscard]] static meta_template_node *meta_template_info() ENTT_NOEXCEPT {
+        if constexpr(is_complete_v<meta_template_traits<Type>>) {
+            static meta_template_node node{
+                meta_template_traits<Type>::args_type::size,
+                meta_node<typename meta_template_traits<Type>::class_type>::resolve(),
+                [](const std::size_t index) ENTT_NOEXCEPT { return meta_arg_node(typename meta_template_traits<Type>::args_type{}, index); }
+                // tricks clang-format
+            };
+
+            return &node;
+        } else {
+            return nullptr;
+        }
+    }
+
+public:
+    [[nodiscard]] static meta_type_node *resolve() ENTT_NOEXCEPT {
+        static meta_type_node node{
+            &type_id<Type>(),
+            {},
+            nullptr,
+            nullptr,
+            size_of_v<Type>,
+            internal::meta_traits::is_none
+                | (std::is_arithmetic_v<Type> ? internal::meta_traits::is_arithmetic : internal::meta_traits::is_none)
+                | (std::is_array_v<Type> ? internal::meta_traits::is_array : internal::meta_traits::is_none)
+                | (std::is_enum_v<Type> ? internal::meta_traits::is_enum : internal::meta_traits::is_none)
+                | (std::is_class_v<Type> ? internal::meta_traits::is_class : internal::meta_traits::is_none)
+                | (std::is_pointer_v<Type> ? internal::meta_traits::is_pointer : internal::meta_traits::is_none)
+                | (is_meta_pointer_like_v<Type> ? internal::meta_traits::is_meta_pointer_like : internal::meta_traits::is_none)
+                | (is_complete_v<meta_sequence_container_traits<Type>> ? internal::meta_traits::is_meta_sequence_container : internal::meta_traits::is_none)
+                | (is_complete_v<meta_associative_container_traits<Type>> ? internal::meta_traits::is_meta_associative_container : internal::meta_traits::is_none),
+            meta_default_constructor(),
+            meta_conversion_helper(),
+            meta_template_info()
+            // tricks clang-format
+        };
+
+        return &node;
+    }
+};
+
+template<typename... Args>
+[[nodiscard]] meta_type_node *meta_arg_node(type_list<Args...>, const std::size_t index) ENTT_NOEXCEPT {
+    meta_type_node *args[sizeof...(Args) + 1u]{nullptr, internal::meta_node<std::remove_const_t<std::remove_reference_t<Args>>>::resolve()...};
+    return args[index + 1u];
+}
+
+template<auto Member, typename Type>
+[[nodiscard]] static std::decay_t<decltype(std::declval<internal::meta_type_node>().*Member)> find_by(const Type &info_or_id, const internal::meta_type_node *node) {
+    for(auto *curr = node->*Member; curr; curr = curr->next) {
+        if constexpr(std::is_same_v<Type, type_info>) {
+            if(*curr->type->info == info_or_id) {
+                return curr;
+            }
+        } else if constexpr(std::is_same_v<decltype(curr), meta_base_node *>) {
+            if(curr->type->id == info_or_id) {
+                return curr;
+            }
+        } else {
+            if(curr->id == info_or_id) {
+                return curr;
+            }
+        }
+    }
+
+    for(auto *curr = node->base; curr; curr = curr->next) {
+        if(auto *ret = find_by<Member>(info_or_id, curr->type); ret) {
+            return ret;
+        }
+    }
+
+    return nullptr;
+}
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+} // namespace entt
+
+#endif

src/entt/meta/pointer.hpp

@@ -0,0 +1,48 @@
+#ifndef ENTT_META_POINTER_HPP
+#define ENTT_META_POINTER_HPP
+
+#include <memory>
+#include <type_traits>
+#include "type_traits.hpp"
+
+namespace entt {
+
+/**
+ * @brief Makes plain pointers pointer-like types for the meta system.
+ * @tparam Type Element type.
+ */
+template<typename Type>
+struct is_meta_pointer_like<Type *>
+    : std::true_type {};
+
+/**
+ * @brief Partial specialization used to reject pointers to arrays.
+ * @tparam Type Type of elements of the array.
+ * @tparam N Number of elements of the array.
+ */
+template<typename Type, std::size_t N>
+struct is_meta_pointer_like<Type (*)[N]>
+    : std::false_type {};
+
+/**
+ * @brief Makes `std::shared_ptr`s of any type pointer-like types for the meta
+ * system.
+ * @tparam Type Element type.
+ */
+template<typename Type>
+struct is_meta_pointer_like<std::shared_ptr<Type>>
+    : std::true_type {};
+
+/**
+ * @brief Makes `std::unique_ptr`s of any type pointer-like types for the meta
+ * system.
+ * @tparam Type Element type.
+ * @tparam Args Other arguments.
+ */
+template<typename Type, typename... Args>
+struct is_meta_pointer_like<std::unique_ptr<Type, Args...>>
+    : std::true_type {};
+
+} // namespace entt
+
+#endif

src/entt/meta/policy.hpp

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

src/entt/meta/range.hpp

@@ -0,0 +1,124 @@
+#ifndef ENTT_META_RANGE_HPP
+#define ENTT_META_RANGE_HPP
+
+#include <cstddef>
+#include <iterator>
+#include "../core/iterator.hpp"
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename Type, typename Node>
+struct meta_range_iterator {
+    using difference_type = std::ptrdiff_t;
+    using value_type = Type;
+    using pointer = input_iterator_pointer<value_type>;
+    using reference = value_type;
+    using iterator_category = std::input_iterator_tag;
+    using node_type = Node;
+
+    meta_range_iterator() ENTT_NOEXCEPT = default;
+
+    meta_range_iterator(node_type *head) ENTT_NOEXCEPT
+        : it{head} {}
+
+    meta_range_iterator &operator++() ENTT_NOEXCEPT {
+        return (it = it->next), *this;
+    }
+
+    meta_range_iterator operator++(int) ENTT_NOEXCEPT {
+        meta_range_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
+        return it;
+    }
+
+    [[nodiscard]] pointer operator->() const ENTT_NOEXCEPT {
+        return operator*();
+    }
+
+    [[nodiscard]] bool operator==(const meta_range_iterator &other) const ENTT_NOEXCEPT {
+        return it == other.it;
+    }
+
+    [[nodiscard]] bool operator!=(const meta_range_iterator &other) const ENTT_NOEXCEPT {
+        return !(*this == other);
+    }
+
+private:
+    node_type *it{};
+};
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief Iterable range to use to iterate all types of meta objects.
+ * @tparam Type Type of meta objects returned.
+ * @tparam Node Type of meta nodes iterated.
+ */
+template<typename Type, typename Node = typename Type::node_type>
+struct meta_range {
+    /*! @brief Node type. */
+    using node_type = Node;
+    /*! @brief Input iterator type. */
+    using iterator = internal::meta_range_iterator<Type, Node>;
+    /*! @brief Constant input iterator type. */
+    using const_iterator = iterator;
+
+    /*! @brief Default constructor. */
+    meta_range() ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Constructs a meta range from a given node.
+     * @param head The underlying node with which to construct the range.
+     */
+    meta_range(node_type *head)
+        : node{head} {}
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     * @return An iterator to the first meta object of the range.
+     */
+    [[nodiscard]] const_iterator cbegin() const ENTT_NOEXCEPT {
+        return iterator{node};
+    }
+
+    /*! @copydoc cbegin */
+    [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+        return cbegin();
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     * @return An iterator to the element following the last meta object of the
+     * range.
+     */
+    [[nodiscard]] const_iterator cend() const ENTT_NOEXCEPT {
+        return iterator{};
+    }
+
+    /*! @copydoc cend */
+    [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+        return cend();
+    }
+
+private:
+    node_type *node{nullptr};
+};
+
+} // namespace entt
+
+#endif

src/entt/meta/resolve.hpp

@@ -0,0 +1,63 @@
+#ifndef ENTT_META_RESOLVE_HPP
+#define ENTT_META_RESOLVE_HPP
+
+#include <algorithm>
+#include "../core/type_info.hpp"
+#include "ctx.hpp"
+#include "meta.hpp"
+#include "node.hpp"
+#include "range.hpp"
+
+namespace entt {
+
+/**
+ * @brief Returns the meta type associated with a given type.
+ * @tparam Type Type to use to search for a meta type.
+ * @return The meta type associated with the given type, if any.
+ */
+template<typename Type>
+[[nodiscard]] meta_type resolve() ENTT_NOEXCEPT {
+    return internal::meta_node<std::remove_const_t<std::remove_reference_t<Type>>>::resolve();
+}
+
+/**
+ * @brief Returns a range to use to visit all meta types.
+ * @return An iterable range to use to visit all meta types.
+ */
+[[nodiscard]] inline meta_range<meta_type> resolve() {
+    return *internal::meta_context::global();
+}
+
+/**
+ * @brief Returns the meta type associated with a given identifier, if any.
+ * @param id Unique identifier.
+ * @return The meta type associated with the given identifier, if any.
+ */
+[[nodiscard]] inline meta_type resolve(const id_type id) ENTT_NOEXCEPT {
+    for(auto &&curr: resolve()) {
+        if(curr.id() == id) {
+            return curr;
+        }
+    }
+
+    return {};
+}
+
+/**
+ * @brief Returns the meta type associated with a given type info object.
+ * @param info The type info object of the requested type.
+ * @return The meta type associated with the given type info object, if any.
+ */
+[[nodiscard]] inline meta_type resolve(const type_info &info) ENTT_NOEXCEPT {
+    for(auto &&curr: resolve()) {
+        if(curr.info() == info) {
+            return curr;
+        }
+    }
+
+    return {};
+}
+
+} // namespace entt
+
+#endif

src/entt/meta/template.hpp

@@ -0,0 +1,27 @@
+#ifndef ENTT_META_TEMPLATE_HPP
+#define ENTT_META_TEMPLATE_HPP
+
+#include "../core/type_traits.hpp"
+
+namespace entt {
+
+/*! @brief Utility class to disambiguate class templates. */
+template<template<typename...> typename>
+struct meta_class_template_tag {};
+
+/**
+ * @brief General purpose traits class for generating meta template information.
+ * @tparam Clazz Type of class template.
+ * @tparam Args Types of template arguments.
+ */
+template<template<typename...> typename Clazz, typename... Args>
+struct meta_template_traits<Clazz<Args...>> {
+    /*! @brief Wrapped class template. */
+    using class_type = meta_class_template_tag<Clazz>;
+    /*! @brief List of template arguments. */
+    using args_type = type_list<Args...>;
+};
+
+} // namespace entt
+
+#endif

src/entt/meta/type_traits.hpp

@@ -0,0 +1,55 @@
+#ifndef ENTT_META_TYPE_TRAITS_HPP
+#define ENTT_META_TYPE_TRAITS_HPP
+
+#include <type_traits>
+#include <utility>
+
+namespace entt {
+
+/**
+ * @brief Traits class template to be specialized to enable support for meta
+ * template information.
+ */
+template<typename>
+struct meta_template_traits;
+
+/**
+ * @brief Traits class template to be specialized to enable support for meta
+ * sequence containers.
+ */
+template<typename>
+struct meta_sequence_container_traits;
+
+/**
+ * @brief Traits class template to be specialized to enable support for meta
+ * associative containers.
+ */
+template<typename>
+struct meta_associative_container_traits;
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is a
+ * pointer-like type from the point of view of the meta system, false otherwise.
+ * @tparam Type Potentially pointer-like type.
+ */
+template<typename>
+struct is_meta_pointer_like: std::false_type {};
+
+/**
+ * @brief Partial specialization to ensure that const pointer-like types are
+ * also accepted.
+ * @tparam Type Potentially pointer-like type.
+ */
+template<typename Type>
+struct is_meta_pointer_like<const Type>: is_meta_pointer_like<Type> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type Potentially pointer-like type.
+ */
+template<typename Type>
+inline constexpr auto is_meta_pointer_like_v = is_meta_pointer_like<Type>::value;
+
+} // namespace entt
+
+#endif

src/entt/meta/utility.hpp

@@ -0,0 +1,396 @@
+#ifndef ENTT_META_UTILITY_HPP
+#define ENTT_META_UTILITY_HPP
+
+#include <cstddef>
+#include <functional>
+#include <type_traits>
+#include <utility>
+#include "../config/config.h"
+#include "../core/type_traits.hpp"
+#include "meta.hpp"
+#include "node.hpp"
+#include "policy.hpp"
+
+namespace entt {
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename, typename>
+struct meta_function_descriptor;
+
+/**
+ * @brief Meta function descriptor.
+ * @tparam Type Reflected type to which the meta function is associated.
+ * @tparam Ret Function return type.
+ * @tparam Class Actual owner of the member function.
+ * @tparam Args Function arguments.
+ */
+template<typename Type, typename Ret, typename Class, typename... Args>
+struct meta_function_descriptor<Type, Ret (Class::*)(Args...) const> {
+    /*! @brief Meta function return type. */
+    using return_type = Ret;
+    /*! @brief Meta function arguments. */
+    using args_type = std::conditional_t<std::is_base_of_v<Class, Type>, type_list<Args...>, type_list<const Class &, Args...>>;
+
+    /*! @brief True if the meta function is const, false otherwise. */
+    static constexpr auto is_const = true;
+    /*! @brief True if the meta function is static, false otherwise. */
+    static constexpr auto is_static = !std::is_base_of_v<Class, Type>;
+};
+
+/**
+ * @brief Meta function descriptor.
+ * @tparam Type Reflected type to which the meta function is associated.
+ * @tparam Ret Function return type.
+ * @tparam Class Actual owner of the member function.
+ * @tparam Args Function arguments.
+ */
+template<typename Type, typename Ret, typename Class, typename... Args>
+struct meta_function_descriptor<Type, Ret (Class::*)(Args...)> {
+    /*! @brief Meta function return type. */
+    using return_type = Ret;
+    /*! @brief Meta function arguments. */
+    using args_type = std::conditional_t<std::is_base_of_v<Class, Type>, type_list<Args...>, type_list<Class &, Args...>>;
+
+    /*! @brief True if the meta function is const, false otherwise. */
+    static constexpr auto is_const = false;
+    /*! @brief True if the meta function is static, false otherwise. */
+    static constexpr auto is_static = !std::is_base_of_v<Class, Type>;
+};
+
+/**
+ * @brief Meta function descriptor.
+ * @tparam Type Reflected type to which the meta data is associated.
+ * @tparam Class Actual owner of the data member.
+ * @tparam Ret Data member type.
+ */
+template<typename Type, typename Ret, typename Class>
+struct meta_function_descriptor<Type, Ret Class::*> {
+    /*! @brief Meta data return type. */
+    using return_type = Ret &;
+    /*! @brief Meta data arguments. */
+    using args_type = std::conditional_t<std::is_base_of_v<Class, Type>, type_list<>, type_list<Class &>>;
+
+    /*! @brief True if the meta data is const, false otherwise. */
+    static constexpr auto is_const = false;
+    /*! @brief True if the meta data is static, false otherwise. */
+    static constexpr auto is_static = !std::is_base_of_v<Class, Type>;
+};
+
+/**
+ * @brief Meta function descriptor.
+ * @tparam Type Reflected type to which the meta function is associated.
+ * @tparam Ret Function return type.
+ * @tparam MaybeType First function argument.
+ * @tparam Args Other function arguments.
+ */
+template<typename Type, typename Ret, typename MaybeType, typename... Args>
+struct meta_function_descriptor<Type, Ret (*)(MaybeType, Args...)> {
+    /*! @brief Meta function return type. */
+    using return_type = Ret;
+    /*! @brief Meta function arguments. */
+    using args_type = std::conditional_t<std::is_base_of_v<std::remove_const_t<std::remove_reference_t<MaybeType>>, Type>, type_list<Args...>, type_list<MaybeType, Args...>>;
+
+    /*! @brief True if the meta function is const, false otherwise. */
+    static constexpr auto is_const = std::is_base_of_v<std::remove_const_t<std::remove_reference_t<MaybeType>>, Type> && std::is_const_v<std::remove_reference_t<MaybeType>>;
+    /*! @brief True if the meta function is static, false otherwise. */
+    static constexpr auto is_static = !std::is_base_of_v<std::remove_const_t<std::remove_reference_t<MaybeType>>, Type>;
+};
+
+/**
+ * @brief Meta function descriptor.
+ * @tparam Type Reflected type to which the meta function is associated.
+ * @tparam Ret Function return type.
+ */
+template<typename Type, typename Ret>
+struct meta_function_descriptor<Type, Ret (*)()> {
+    /*! @brief Meta function return type. */
+    using return_type = Ret;
+    /*! @brief Meta function arguments. */
+    using args_type = type_list<>;
+
+    /*! @brief True if the meta function is const, false otherwise. */
+    static constexpr auto is_const = false;
+    /*! @brief True if the meta function is static, false otherwise. */
+    static constexpr auto is_static = true;
+};
+
+/**
+ * @brief Meta function helper.
+ *
+ * Converts a function type to be associated with a reflected type into its meta
+ * function descriptor.
+ *
+ * @tparam Type Reflected type to which the meta function is associated.
+ * @tparam Candidate The actual function to associate with the reflected type.
+ */
+template<typename Type, typename Candidate>
+class meta_function_helper {
+    template<typename Ret, typename... Args, typename Class>
+    static constexpr meta_function_descriptor<Type, Ret (Class::*)(Args...) const> get_rid_of_noexcept(Ret (Class::*)(Args...) const);
+
+    template<typename Ret, typename... Args, typename Class>
+    static constexpr meta_function_descriptor<Type, Ret (Class::*)(Args...)> get_rid_of_noexcept(Ret (Class::*)(Args...));
+
+    template<typename Ret, typename Class>
+    static constexpr meta_function_descriptor<Type, Ret Class::*> get_rid_of_noexcept(Ret Class::*);
+
+    template<typename Ret, typename... Args>
+    static constexpr meta_function_descriptor<Type, Ret (*)(Args...)> get_rid_of_noexcept(Ret (*)(Args...));
+
+    template<typename Class>
+    static constexpr meta_function_descriptor<Class, decltype(&Class::operator())> get_rid_of_noexcept(Class);
+
+public:
+    /*! @brief The meta function descriptor of the given function. */
+    using type = decltype(get_rid_of_noexcept(std::declval<Candidate>()));
+};
+
+/**
+ * @brief Helper type.
+ * @tparam Type Reflected type to which the meta function is associated.
+ * @tparam Candidate The actual function to associate with the reflected type.
+ */
+template<typename Type, typename Candidate>
+using meta_function_helper_t = typename meta_function_helper<Type, Candidate>::type;
+
+/**
+ * @brief Wraps a value depending on the given policy.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @tparam Type Type of value to wrap.
+ * @param value Value to wrap.
+ * @return A meta any containing the returned value, if any.
+ */
+template<typename Policy = as_is_t, typename Type>
+meta_any meta_dispatch([[maybe_unused]] Type &&value) {
+    if constexpr(std::is_same_v<Policy, as_void_t>) {
+        return meta_any{std::in_place_type<void>};
+    } else if constexpr(std::is_same_v<Policy, as_ref_t>) {
+        return meta_any{std::in_place_type<Type>, std::forward<Type>(value)};
+    } else if constexpr(std::is_same_v<Policy, as_cref_t>) {
+        static_assert(std::is_lvalue_reference_v<Type>, "Invalid type");
+        return meta_any{std::in_place_type<const std::remove_reference_t<Type> &>, std::as_const(value)};
+    } else {
+        static_assert(std::is_same_v<Policy, as_is_t>, "Policy not supported");
+        return meta_any{std::forward<Type>(value)};
+    }
+}
+
+/**
+ * @brief Returns the meta type of the i-th element of a list of arguments.
+ * @tparam Type Type list of the actual types of arguments.
+ * @return The meta type of the i-th element of the list of arguments.
+ */
+template<typename Type>
+[[nodiscard]] static meta_type meta_arg(const std::size_t index) ENTT_NOEXCEPT {
+    return internal::meta_arg_node(Type{}, index);
+}
+
+/**
+ * @brief Sets the value of a given variable.
+ * @tparam Type Reflected type to which the variable is associated.
+ * @tparam Data The actual variable to set.
+ * @param instance An opaque instance of the underlying type, if required.
+ * @param value Parameter to use to set the variable.
+ * @return True in case of success, false otherwise.
+ */
+template<typename Type, auto Data>
+[[nodiscard]] bool meta_setter([[maybe_unused]] meta_handle instance, [[maybe_unused]] meta_any value) {
+    if constexpr(!std::is_same_v<decltype(Data), Type> && !std::is_same_v<decltype(Data), std::nullptr_t>) {
+        if constexpr(std::is_member_function_pointer_v<decltype(Data)> || std::is_function_v<std::remove_reference_t<std::remove_pointer_t<decltype(Data)>>>) {
+            using descriptor = meta_function_helper_t<Type, decltype(Data)>;
+            using data_type = type_list_element_t<descriptor::is_static, typename descriptor::args_type>;
+
+            if(auto *const clazz = instance->try_cast<Type>(); clazz && value.allow_cast<data_type>()) {
+                std::invoke(Data, *clazz, value.cast<data_type>());
+                return true;
+            }
+        } else if constexpr(std::is_member_object_pointer_v<decltype(Data)>) {
+            using data_type = std::remove_reference_t<typename meta_function_helper_t<Type, decltype(Data)>::return_type>;
+
+            if constexpr(!std::is_array_v<data_type> && !std::is_const_v<data_type>) {
+                if(auto *const clazz = instance->try_cast<Type>(); clazz && value.allow_cast<data_type>()) {
+                    std::invoke(Data, *clazz) = value.cast<data_type>();
+                    return true;
+                }
+            }
+        } else {
+            using data_type = std::remove_reference_t<decltype(*Data)>;
+
+            if constexpr(!std::is_array_v<data_type> && !std::is_const_v<data_type>) {
+                if(value.allow_cast<data_type>()) {
+                    *Data = value.cast<data_type>();
+                    return true;
+                }
+            }
+        }
+    }
+
+    return false;
+}
+
+/**
+ * @brief Gets the value of a given variable.
+ * @tparam Type Reflected type to which the variable is associated.
+ * @tparam Data The actual variable to get.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @param instance An opaque instance of the underlying type, if required.
+ * @return A meta any containing the value of the underlying variable.
+ */
+template<typename Type, auto Data, typename Policy = as_is_t>
+[[nodiscard]] meta_any meta_getter([[maybe_unused]] meta_handle instance) {
+    if constexpr(std::is_member_pointer_v<decltype(Data)> || std::is_function_v<std::remove_reference_t<std::remove_pointer_t<decltype(Data)>>>) {
+        if constexpr(!std::is_array_v<std::remove_cv_t<std::remove_reference_t<std::invoke_result_t<decltype(Data), Type &>>>>) {
+            if constexpr(std::is_invocable_v<decltype(Data), Type &>) {
+                if(auto *clazz = instance->try_cast<Type>(); clazz) {
+                    return meta_dispatch<Policy>(std::invoke(Data, *clazz));
+                }
+            }
+
+            if constexpr(std::is_invocable_v<decltype(Data), const Type &>) {
+                if(auto *fallback = instance->try_cast<const Type>(); fallback) {
+                    return meta_dispatch<Policy>(std::invoke(Data, *fallback));
+                }
+            }
+        }
+
+        return meta_any{};
+    } else if constexpr(std::is_pointer_v<decltype(Data)>) {
+        if constexpr(std::is_array_v<std::remove_pointer_t<decltype(Data)>>) {
+            return meta_any{};
+        } else {
+            return meta_dispatch<Policy>(*Data);
+        }
+    } else {
+        return meta_dispatch<Policy>(Data);
+    }
+}
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename Type, typename Policy, typename Candidate, typename... Args>
+[[nodiscard]] meta_any meta_invoke_with_args(Candidate &&candidate, Args &&...args) {
+    if constexpr(std::is_same_v<std::invoke_result_t<decltype(candidate), Args...>, void>) {
+        std::invoke(candidate, args...);
+        return meta_any{std::in_place_type<void>};
+    } else {
+        return meta_dispatch<Policy>(std::invoke(candidate, args...));
+    }
+}
+
+template<typename Type, typename Policy, typename Candidate, std::size_t... Index>
+[[nodiscard]] meta_any meta_invoke([[maybe_unused]] meta_handle instance, Candidate &&candidate, [[maybe_unused]] meta_any *args, std::index_sequence<Index...>) {
+    using descriptor = meta_function_helper_t<Type, std::remove_reference_t<Candidate>>;
+
+    if constexpr(std::is_invocable_v<std::remove_reference_t<Candidate>, const Type &, type_list_element_t<Index, typename descriptor::args_type>...>) {
+        if(const auto *const clazz = instance->try_cast<const Type>(); clazz && ((args + Index)->allow_cast<type_list_element_t<Index, typename descriptor::args_type>>() && ...)) {
+            return meta_invoke_with_args<Type, Policy>(std::forward<Candidate>(candidate), *clazz, (args + Index)->cast<type_list_element_t<Index, typename descriptor::args_type>>()...);
+        }
+    } else if constexpr(std::is_invocable_v<std::remove_reference_t<Candidate>, Type &, type_list_element_t<Index, typename descriptor::args_type>...>) {
+        if(auto *const clazz = instance->try_cast<Type>(); clazz && ((args + Index)->allow_cast<type_list_element_t<Index, typename descriptor::args_type>>() && ...)) {
+            return meta_invoke_with_args<Type, Policy>(std::forward<Candidate>(candidate), *clazz, (args + Index)->cast<type_list_element_t<Index, typename descriptor::args_type>>()...);
+        }
+    } else {
+        if(((args + Index)->allow_cast<type_list_element_t<Index, typename descriptor::args_type>>() && ...)) {
+            return meta_invoke_with_args<Type, Policy>(std::forward<Candidate>(candidate), (args + Index)->cast<type_list_element_t<Index, typename descriptor::args_type>>()...);
+        }
+    }
+
+    return meta_any{};
+}
+
+template<typename Type, typename... Args, std::size_t... Index>
+[[nodiscard]] meta_any meta_construct(meta_any *const args, std::index_sequence<Index...>) {
+    if(((args + Index)->allow_cast<Args>() && ...)) {
+        return meta_any{std::in_place_type<Type>, (args + Index)->cast<Args>()...};
+    }
+
+    return meta_any{};
+}
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief Tries to _invoke_ an object given a list of erased parameters.
+ * @tparam Type Reflected type to which the object to _invoke_ is associated.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @tparam Candidate The type of the actual object to _invoke_.
+ * @param instance An opaque instance of the underlying type, if required.
+ * @param candidate The actual object to _invoke_.
+ * @param args Parameters to use to _invoke_ the object.
+ * @return A meta any containing the returned value, if any.
+ */
+template<typename Type, typename Policy = as_is_t, typename Candidate>
+[[nodiscard]] meta_any meta_invoke([[maybe_unused]] meta_handle instance, Candidate &&candidate, [[maybe_unused]] meta_any *const args) {
+    return internal::meta_invoke<Type, Policy>(std::move(instance), std::forward<Candidate>(candidate), args, std::make_index_sequence<meta_function_helper_t<Type, std::remove_reference_t<Candidate>>::args_type::size>{});
+}
+
+/**
+ * @brief Tries to invoke a function given a list of erased parameters.
+ * @tparam Type Reflected type to which the function is associated.
+ * @tparam Candidate The actual function to invoke.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @param instance An opaque instance of the underlying type, if required.
+ * @param args Parameters to use to invoke the function.
+ * @return A meta any containing the returned value, if any.
+ */
+template<typename Type, auto Candidate, typename Policy = as_is_t>
+[[nodiscard]] meta_any meta_invoke(meta_handle instance, meta_any *const args) {
+    return internal::meta_invoke<Type, Policy>(std::move(instance), Candidate, args, std::make_index_sequence<meta_function_helper_t<Type, std::remove_reference_t<decltype(Candidate)>>::args_type::size>{});
+}
+
+/**
+ * @brief Tries to construct an instance given a list of erased parameters.
+ * @tparam Type Actual type of the instance to construct.
+ * @tparam Args Types of arguments expected.
+ * @param args Parameters to use to construct the instance.
+ * @return A meta any containing the new instance, if any.
+ */
+template<typename Type, typename... Args>
+[[nodiscard]] meta_any meta_construct(meta_any *const args) {
+    return internal::meta_construct<Type, Args...>(args, std::index_sequence_for<Args...>{});
+}
+
+/**
+ * @brief Tries to construct an instance given a list of erased parameters.
+ * @tparam Type Reflected type to which the object to _invoke_ is associated.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @tparam Candidate The type of the actual object to _invoke_.
+ * @param args Parameters to use to _invoke_ the object.
+ * @param candidate The actual object to _invoke_.
+ * @return A meta any containing the returned value, if any.
+ */
+template<typename Type, typename Policy = as_is_t, typename Candidate>
+[[nodiscard]] meta_any meta_construct(Candidate &&candidate, meta_any *const args) {
+    if constexpr(meta_function_helper_t<Type, Candidate>::is_static) {
+        return internal::meta_invoke<Type, Policy>({}, std::forward<Candidate>(candidate), args, std::make_index_sequence<meta_function_helper_t<Type, std::remove_reference_t<Candidate>>::args_type::size>{});
+    } else {
+        return internal::meta_invoke<Type, Policy>(*args, std::forward<Candidate>(candidate), args + 1u, std::make_index_sequence<meta_function_helper_t<Type, std::remove_reference_t<Candidate>>::args_type::size>{});
+    }
+}
+
+/**
+ * @brief Tries to construct an instance given a list of erased parameters.
+ * @tparam Type Reflected type to which the function is associated.
+ * @tparam Candidate The actual function to invoke.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @param args Parameters to use to invoke the function.
+ * @return A meta any containing the returned value, if any.
+ */
+template<typename Type, auto Candidate, typename Policy = as_is_t>
+[[nodiscard]] meta_any meta_construct(meta_any *const args) {
+    return meta_construct<Type, Policy>(Candidate, args);
+}
+
+} // namespace entt
+
+#endif

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

@@ -0,0 +1,67 @@
+#ifndef ENTT_PLATFORM_ANDROID_NDK_R17_HPP
+#define ENTT_PLATFORM_ANDROID_NDK_R17_HPP
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+#ifdef __ANDROID__
+#    include <android/ndk-version.h>
+#    if __NDK_MAJOR__ == 17
+
+#        include <functional>
+#        include <type_traits>
+#        include <utility>
+
+namespace std {
+
+namespace internal {
+
+template<typename Func, typename... Args>
+constexpr auto is_invocable(int) -> decltype(std::invoke(std::declval<Func>(), std::declval<Args>()...), std::true_type{});
+
+template<typename, typename...>
+constexpr std::false_type is_invocable(...);
+
+template<typename Ret, typename Func, typename... Args>
+constexpr auto is_invocable_r(int)
+-> std::enable_if_t<decltype(std::is_convertible_v<decltype(std::invoke(std::declval<Func>(), std::declval<Args>()...)), Ret>, std::true_type>;
+
+
+template<typename, typename, typename...>
+constexpr std::false_type is_invocable_r(...);
+
+} // namespace internal
+
+template<typename Func, typename... Args>
+struct is_invocable: decltype(internal::is_invocable<Func, Args...>(0)) {};
+
+template<typename Func, typename... Argsv>
+inline constexpr bool is_invocable_v = std::is_invocable<Func, Args...>::value;
+
+template<typename Ret, typename Func, typename... Args>
+struct is_invocable_r: decltype(internal::is_invocable_r<Ret, Func, Args...>(0)) {};
+
+template<typename Ret, typename Func, typename... Args>
+inline constexpr bool is_invocable_r_v = std::is_invocable_r<Ret, Func, Args...>::value;
+
+template<typename Func, typename... Args>
+struct invoke_result {
+    using type = decltype(std::invoke(std::declval<Func>(), std::declval<Args>()...));
+};
+
+template<typename Func, typename... Args>
+using invoke_result_t = typename std::invoke_result<Func, Args...>::type;
+
+} // namespace std
+
+#    endif
+#endif
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+#endif

src/entt/poly/fwd.hpp

@@ -0,0 +1,20 @@
+#ifndef ENTT_POLY_FWD_HPP
+#define ENTT_POLY_FWD_HPP
+
+#include <type_traits>
+
+namespace entt {
+
+template<typename, std::size_t Len, std::size_t = alignof(typename std::aligned_storage_t<Len + !Len>)>
+class basic_poly;
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Concept Concept descriptor.
+ */
+template<typename Concept>
+using poly = basic_poly<Concept, sizeof(double[2])>;
+
+} // namespace entt
+
+#endif