updated single include file

Small typo fix in wiki

.github/FUNDING.yml

@@ -0,0 +1,12 @@
+# These are supported funding model platforms
+
+github: skypjack
+patreon:
+open_collective:
+ko_fi:
+tidelift:
+community_bridge:
+liberapay:
+issuehunt:
+otechie:
+custom: https://www.paypal.me/skypjack

.github/workflows/build.yml

@@ -0,0 +1,79 @@
+name: build
+
+on: [push, pull_request]
+
+jobs:
+
+  linux:
+    timeout-minutes: 10
+
+    strategy:
+      matrix:
+        compiler: [g++, clang++]
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Compile tests
+      working-directory: build
+      env:
+        CXX: ${{ matrix.compiler }}
+      run: |
+        cmake -DBUILD_LIB=ON -DBUILD_EXAMPLE=ON ..
+        make -j4
+    - name: Run tests
+      working-directory: build
+      env:
+        CTEST_OUTPUT_ON_FAILURE: 1
+      run: ctest --timeout 5 -C Debug -j4
+
+  windows:
+    timeout-minutes: 10
+
+    strategy:
+      matrix:
+        os: [windows-latest, windows-2016]
+        toolset: [clang-cl, default, v141]
+        include:
+          - toolset: clang-cl
+            toolset_option: -T"ClangCl"
+          - toolset: v141
+            toolset_option: -T"v141"
+        exclude:
+          - os: windows-2016
+            toolset: clang-cl
+          - os: windows-2016
+            toolset: v141
+
+    runs-on: ${{ matrix.os }}
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Compile tests
+      working-directory: build
+      run: |
+        cmake -DBUILD_LIB=ON -DBUILD_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
+
+  macos:
+    timeout-minutes: 10
+    runs-on: macOS-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Compile tests
+      working-directory: build
+      run: |
+        cmake -DBUILD_LIB=ON -DBUILD_EXAMPLE=ON ..
+        make -j4
+    - name: Run tests
+      working-directory: build
+      env:
+        CTEST_OUTPUT_ON_FAILURE: 1
+      run: ctest --timeout 5 -C Debug -j4

.github/workflows/coverage.yml

@@ -0,0 +1,33 @@
+name: coverage
+
+on: [push, pull_request]
+
+jobs:
+
+  codecov:
+    timeout-minutes: 10
+    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_LIB=ON -DBUILD_EXAMPLE=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 .

.github/workflows/deploy.yml

@@ -0,0 +1,40 @@
+name: deploy
+
+on:
+  release:
+    types: published
+
+jobs:
+
+  homebrew-entt:
+    timeout-minutes: 5
+    runs-on: ubuntu-latest
+
+    env:
+      GH_REPO: homebrew-entt
+      FORMULA: entt.rb
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Clone repository
+      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
+    - name: Prepare formula
+      working-directory: build
+      run: |
+        cd $GH_REPO
+        curl "https://github.com/${{ github.repository }}/archive/${{ github.ref }}.tar.gz" --location --fail --silent --show-error --output archive.tar.gz
+        sed -i -e '/url/s/".*"/"'$(echo "https://github.com/${{ github.repository }}/archive/${{ github.ref }}.tar.gz" | sed -e 's/[\/&]/\\&/g')'"/' $FORMULA
+        sed -i -e '/sha256/s/".*"/"'$(openssl sha256 archive.tar.gz | cut -d " " -f 2)'"/' $FORMULA
+    - name: Update remote
+      working-directory: build
+      run: |
+        cd $GH_REPO
+        git config --local user.email "action@github.com"
+        git config --local user.name "$GITHUB_ACTOR"
+        git add $FORMULA
+        git commit -m "Update to ${{ github.ref }}"
+        git push origin master

.gitignore

@@ -1,2 +1,12 @@
-# QtCreator
+# Conan
+conan/test_package/build
+
+# IDEs
 *.user
+.idea
+.vscode
+.vs
+CMakeSettings.json
+
+# Bazel
+/bazel-*

.travis.yml

@@ -1,55 +0,0 @@
-language: cpp
-dist: trusty
-sudo: false
-
-matrix:
-  include:
-  - os: linux
-    compiler: gcc
-    addons:
-      apt:
-        sources: ['ubuntu-toolchain-r-test']
-        packages: ['g++-6']
-    env: COMPILER=g++-6
-  - os: linux
-    compiler: clang
-    addons:
-      apt:
-        sources: ['ubuntu-toolchain-r-test', 'llvm-toolchain-trusty-4.0']
-        packages: ['clang-4.0', 'libstdc++-4.9-dev']
-    env: COMPILER=clang++-4.0
-  - os: osx
-    osx_image: xcode8.3
-    compiler: clang
-    env: COMPILER=clang++
-  - os: linux
-    compiler: gcc
-    addons:
-      apt:
-        sources: ['ubuntu-toolchain-r-test']
-        packages: ['g++-6']
-    env:
-      - COMPILER=g++-6
-      - CXXFLAGS="-O0 --coverage -fno-inline -fno-inline-small-functions -fno-default-inline"
-    before_script:
-      - pip install --user cpp-coveralls
-    after_success:
-      - coveralls --gcov gcov-6 --gcov-options '\-lp' --root ${TRAVIS_BUILD_DIR} --build-root ${TRAVIS_BUILD_DIR}/build --extension cpp --extension hpp --exclude deps --include src
-
-notifications:
-  email:
-    on_success: never
-    on_failure: always
-
-install:
-- echo ${PATH}
-- cmake --version
-- export CXX=${COMPILER}
-- echo ${CXX}
-- ${CXX} --version
-- ${CXX} -v
-
-script:
-- mkdir -p build && cd build
-- cmake -DCMAKE_BUILD_TYPE=Release .. && make -j4
-- CTEST_OUTPUT_ON_FAILURE=1 make test

AUTHORS

@@ -1,7 +1,49 @@
 # Author
 
-Michele Caini aka skypjack
+skypjack
 
 # Contributors
 
-Paolo Monteverde aka morbo84
+alexames
+BenediktConze
+bjadamson
+ceeac
+ColinH
+corystegel
+Croydon
+cugone
+dbacchet
+dBagrat
+djarek
+DonKult
+drglove
+eliasdaler
+erez-o
+eugeneko
+gale83
+ghost
+grdowns
+Green-Sky
+Innokentiy-Alaytsev
+Kerndog73
+Lawrencemm
+markand
+mhammerc
+Milerius
+morbo84
+m-waka
+NixAJ
+Oortonaut
+Paolo-Oliverio
+pgruenbacher
+prowolf
+stefanofiorentino
+suVrik
+szunhammer
+The5-1
+vblanco20-1
+willtunnels
+WizardIke
+w1th0utnam3
+xissburg
+zaucy

BUILD.bazel

@@ -0,0 +1,15 @@
+_msvc_copts = ["/std:c++17"]	
+_gcc_copts = ["-std=c++17"]
+
+cc_library(
+    name = "entt",
+    visibility = ["//visibility:public"],
+    strip_include_prefix = "src",
+    hdrs = glob(["src/**/*.h", "src/**/*.hpp"]),
+    copts = select({
+      "@bazel_tools//src/conditions:windows": _msvc_copts,
+      "@bazel_tools//src/conditions:windows_msvc": _msvc_copts,
+      "@bazel_tools//src/conditions:windows_msys": _msvc_copts,
+      "//conditions:default": _gcc_copts,
+    }),
+)

CMakeLists.txt

@@ -2,7 +2,7 @@
 # EnTT
 #
 
-cmake_minimum_required(VERSION 3.2)
+cmake_minimum_required(VERSION 3.12.4)
 
 #
 # Building in-tree is not allowed (we take care of your craziness).
@@ -12,69 +12,183 @@ if(${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_BINARY_DIR})
     message(FATAL_ERROR "Prevented in-tree built. Please create a build directory outside of the source code and call cmake from there. Thank you.")
 endif()
 
+#
+# Read project version
+#
+
+set(ENTT_VERSION_REGEX "#define ENTT_VERSION_.*[ \t]+(.+)")
+file(STRINGS "${CMAKE_CURRENT_SOURCE_DIR}/src/entt/config/version.h" ENTT_VERSION REGEX ${ENTT_VERSION_REGEX})
+list(TRANSFORM ENTT_VERSION REPLACE ${ENTT_VERSION_REGEX} "\\1")
+string(JOIN "." ENTT_VERSION ${ENTT_VERSION})
+
 #
 # Project configuration
 #
 
-project(entt VERSION 1.0.0)
+project(
+    EnTT
+    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
+)
 
 if(NOT CMAKE_BUILD_TYPE)
     set(CMAKE_BUILD_TYPE Debug)
 endif()
 
-set(SETTINGS_ORGANIZATION "Michele Caini")
-set(SETTINGS_APPLICATION ${PROJECT_NAME})
-set(PROJECT_AUTHOR "Michele Caini")
-set(PROJECT_AUTHOR_EMAIL "michele.caini@gmail.com")
+message(VERBOSE "*")
+message(VERBOSE "* ${PROJECT_NAME} v${PROJECT_VERSION} (${CMAKE_BUILD_TYPE})")
+message(VERBOSE "* Copyright (c) 2017-2020 Michele Caini <michele.caini@gmail.com>")
+message(VERBOSE "*")
 
-message("*")
-message("* ${PROJECT_NAME} v${PROJECT_VERSION} (${CMAKE_BUILD_TYPE})")
-message("* Copyright (c) 2017 ${PROJECT_AUTHOR} <${PROJECT_AUTHOR_EMAIL}>")
-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)
 
 #
-# Compile stuff
+# Compiler stuff
 #
 
-set(CMAKE_CXX_STANDARD 14)
-set(CMAKE_CXX_STANDARD_REQUIRED ON)
-set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -Wl,--no-undefined")
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pedantic -Wall -Wconversion")
-set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -O0 -g -DDEBUG")
-set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -O3 -DRELEASE")
+if(NOT WIN32 AND USE_LIBCPP)
+    include(CheckCXXSourceCompiles)
+    include(CMakePushCheckState)
+
+    cmake_push_check_state()
+
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -stdlib=libc++")
+
+    check_cxx_source_compiles("
+        #include<type_traits>
+        int main() { return std::is_same_v<int, char>; }
+    " HAS_LIBCPP)
+
+    if(NOT HAS_LIBCPP)
+        message(VERBOSE "The option USE_LIBCPP is set (by default) but libc++ is not available. The flag will not be added to the target.")
+    endif()
+
+    cmake_pop_check_state()
+endif()
 
 #
-# CMake configuration
+# Add EnTT target
 #
 
-set(PROJECT_CMAKE_IN ${entt_SOURCE_DIR}/cmake/in)
-set(PROJECT_DEPS_DIR ${entt_SOURCE_DIR}/deps)
-set(PROJECT_SRC_DIR ${entt_SOURCE_DIR}/src)
+include(GNUInstallDirs)
 
-set(PROJECT_RUNTIME_OUTPUT_DIRECTORY bin)
+add_library(EnTT INTERFACE)
+add_library(EnTT::EnTT ALIAS EnTT)
+
+target_include_directories(
+    EnTT
+    INTERFACE
+        $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src>
+        $<INSTALL_INTERFACE:${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>)
+endif()
+
+if(HAS_LIBCPP)
+    target_compile_options(EnTT BEFORE INTERFACE -stdlib=libc++)
+endif()
+
+target_compile_features(EnTT INTERFACE cxx_std_17)
 
 #
-# Enable test support using ctest-like interface
+# Install EnTT
 #
 
-option(BUILD_TESTING "Enable testing with ctest." ON)
+include(CMakePackageConfigHelpers)
+
+install(
+    TARGETS EnTT
+    EXPORT EnTTTargets
+    ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
+)
+
+write_basic_package_version_file(
+    EnTTConfigVersion.cmake
+    VERSION ${PROJECT_VERSION}
+    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 ${CMAKE_INSTALL_LIBDIR}/EnTT/cmake
+    NAMESPACE EnTT::
+)
+
+install(
+    FILES
+        ${PROJECT_BINARY_DIR}/EnTTConfig.cmake
+        ${PROJECT_BINARY_DIR}/EnTTConfigVersion.cmake
+    DESTINATION ${CMAKE_INSTALL_LIBDIR}/EnTT/cmake
+)
+
+install(DIRECTORY src/ DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
+
+export(PACKAGE EnTT)
 
 #
-# build testing stuff if required
+# Tests
 #
 
+include(CTest)
+
 if(BUILD_TESTING)
-    set(THREADS_PREFER_PTHREAD_FLAG ON)
-    find_package(Threads REQUIRED)
-
-    # gtest, gtest_main, gmock and gmock_main targets are available from now on
-    set(GOOGLETEST_DEPS_DIR ${PROJECT_DEPS_DIR}/googletest)
-    configure_file(${PROJECT_CMAKE_IN}/googletest.in ${GOOGLETEST_DEPS_DIR}/CMakeLists.txt)
-    execute_process(COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" . WORKING_DIRECTORY ${GOOGLETEST_DEPS_DIR})
-    execute_process(COMMAND ${CMAKE_COMMAND} --build . WORKING_DIRECTORY ${GOOGLETEST_DEPS_DIR})
-    set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
-    add_subdirectory(${GOOGLETEST_DEPS_DIR}/src ${GOOGLETEST_DEPS_DIR}/build)
+    option(FIND_GTEST_PACKAGE "Enable finding gtest package." OFF)
+    option(BUILD_BENCHMARK "Build benchmark." OFF)
+    option(BUILD_EXAMPLE "Build examples." OFF)
+    option(BUILD_LIB "Build lib tests." OFF)
+    option(BUILD_SNAPSHOT "Build snapshot test with Cereal." OFF)
 
     enable_testing()
     add_subdirectory(test)
 endif()
+
+#
+# Documentation
+#
+
+option(BUILD_DOCS "Enable building with documentation." OFF)
+
+if(BUILD_DOCS)
+    find_package(Doxygen 1.8)
+
+    if(DOXYGEN_FOUND)
+        add_subdirectory(docs)
+    endif()
+endif()
+
+#
+# AOB
+#
+
+add_custom_target(
+    aob
+    SOURCES
+        .github/workflows/build.yml
+        .github/workflows/coverage.yml
+        .github/workflows/deploy.yml
+        .github/FUNDING.yml
+        AUTHORS
+        CONTRIBUTING.md
+        LICENSE
+        README.md
+        TODO
+)

CONTRIBUTING.md

@@ -0,0 +1,43 @@
+# Contributing
+
+First of all, thank you very much for taking the time to contribute to the
+`EnTT` framework.<br/>
+How to do it mostly depends on the type of contribution:
+
+* If you have a question, **please** ensure there isn't already an answer for
+  you by searching on GitHub under
+  [issues](https://github.com/skypjack/entt/issues). Do not forget to search
+  also through the closed ones. If you are unable to find a proper answer, feel
+  free to [open a new issue](https://github.com/skypjack/entt/issues/new).
+  Usually, questions are marked as such and closed in a few days.
+
+* If you want to fix a typo in the inline documentation or in the README file,
+  if you want to add some new sections or if you want to help me with the
+  language by reviewing what I wrote so far (I'm not a native speaker after
+  all), **please** open a new
+  [pull request](https://github.com/skypjack/entt/pulls) with your changes.
+
+* If you found a bug, **please** ensure there isn't already an answer for you by
+  searching on GitHub under [issues](https://github.com/skypjack/entt/issues).
+  If you are unable to find an open issue addressing the problem, feel free to
+  [open a new one](https://github.com/skypjack/entt/issues/new). **Please**, do
+  not forget to carefully describe how to reproduce the problem, then add all
+  the information about the system on which you are experiencing it and point
+  out the version of `EnTT` you are using (tag or commit).
+
+* If you found a bug and you wrote a patch to fix it, open a new
+  [pull request](https://github.com/skypjack/entt/pulls) with your code.
+  **Please**, add some tests to avoid regressions in future if possible, it
+  would be really appreciated. Note that the `EnTT` framework has a
+  [coverage at 100%](https://coveralls.io/github/skypjack/entt?branch=master)
+  (at least it was at 100% at the time I wrote this file) and this is the reason
+  for which you can be confident with using it in a production environment.
+
+* If you want to propose a new feature and you know how to code it, **please**
+  do not issue directly a pull request. Before to do it,
+  [create a new issue](https://github.com/skypjack/entt/issues/new) to discuss
+  your proposal. Other users could be interested in your idea and the discussion
+  that will follow can refine it and therefore give us a better solution.
+
+* If you want to request a new feature, I'm available for hiring. Take a look at
+  [my profile](https://github.com/skypjack) and feel free to write me.

LICENSE

@@ -1,6 +1,6 @@
 The MIT License (MIT)
 
-Copyright (c) 2017 Michele Caini
+Copyright (c) 2017-2020 Michele Caini
 
 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

@@ -1,349 +1,426 @@
-# EnTT - Entity-Component System in modern C++
+![EnTT: Gaming meets modern C++](https://user-images.githubusercontent.com/1812216/42513718-ee6e98d0-8457-11e8-9baf-8d83f61a3097.png)
 
-[![Build Status](https://travis-ci.org/skypjack/entt.svg?branch=master)](https://travis-ci.org/skypjack/uvw)
-[![Build status](https://ci.appveyor.com/api/projects/status/rvhaabjmghg715ck?svg=true)](https://ci.appveyor.com/project/skypjack/entt)
-[![Coverage Status](https://coveralls.io/repos/github/skypjack/entt/badge.svg?branch=master)](https://coveralls.io/github/skypjack/entt?branch=master)
+<!--
+@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/cOUcm1)
+[![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)
+
+`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/>
+[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/>
+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:
+
+---
+
+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) or the
+[discord server](https://discord.gg/5BjPWBd) and meet other users like you. The
+more we are, the better for everyone.
+
+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.
+
+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.
+
+# Table of Contents
+
+* [Introduction](#introduction)
+  * [Code Example](#code-example)
+  * [Motivation](#motivation)
+  * [Performance](#performance)
+* [Build Instructions](#build-instructions)
+  * [Requirements](#requirements)
+  * [Library](#library)
+  * [Documentation](#documentation)
+  * [Tests](#tests)
+* [Packaging Tools](#packaging-tools)
+* [EnTT in Action](#entt-in-action)
+* [Contributors](#contributors)
+* [License](#license)
+* [Support](#support)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
 
 # Introduction
 
-`EnTT` is a header-only, tiny and easy to use Entity-Component System in modern C++.<br/>
-ECS is an architectural pattern used mostly in game development. For further details:
+The entity-component-system (also known as _ECS_) is an architectural pattern
+used mostly in game development. For further details:
 
 * [Entity Systems Wiki](http://entity-systems.wikidot.com/)
 * [Evolve Your Hierarchy](http://cowboyprogramming.com/2007/01/05/evolve-your-heirachy/)
 * [ECS on Wikipedia](https://en.wikipedia.org/wiki/Entity%E2%80%93component%E2%80%93system)
 
+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).
+* 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.
+* 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 smallest and most basic implementation of a **service locator** ever seen.
+* A built-in, non-intrusive and macro-free runtime **reflection system**.
+* 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.
+* A general purpose **event emitter** as a CRTP idiom based class template.
+* And **much more**! Check out the
+  [**wiki**](https://github.com/skypjack/entt/wiki).
+
+Consider this list a work in progress as well as the project. The whole API is
+fully documented in-code for those who are brave enough to read it.
+
+It is also known that `EnTT` (version 3) 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 <iostream>
-#include <registry.hpp>
+#include <entt/entt.hpp>
+#include <cstdint>
 
-struct Position {
+struct position {
     float x;
     float y;
 };
 
-struct Velocity {
+struct velocity {
     float dx;
     float dy;
 };
 
-using ECS = entt::DefaultRegistry<Position, Velocity>;
+void update(entt::registry &registry) {
+    auto view = registry.view<position, velocity>();
+
+    for(auto entity: view) {
+        // gets only the components that are going to be used ...
+
+        auto &vel = view.get<velocity>(entity);
+
+        vel.dx = 0.;
+        vel.dy = 0.;
+
+        // ...
+    }
+}
+
+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;
+
+        // ...
+    });
+}
 
 int main() {
-    ECS ecs;
+    entt::registry registry;
+    std::uint64_t dt = 16;
 
     for(auto i = 0; i < 10; ++i) {
-        auto entity = ecs.create();
-        ecs.assign<Position>(entity, i * 1.f, i * 1.f);
-        if(i % 2 == 0) { ecs.assign<Velocity>(entity, i * .1f, i * .1f); }
+        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); }
     }
 
-    std::cout << "single component view" << std::endl;
+    update(dt, registry);
+    update(registry);
 
-    for(auto entity: ecs.view<Position>()) {
-        auto &position = ecs.get<Position>(entity);
-        std::cout << position.x << "," << position.y << std::endl;
-    }
-
-    std::cout << "multi component view" << std::endl;
-
-    for(auto entity: ecs.view<Position, Velocity>()) {
-        auto &position = ecs.get<Position>(entity);
-        auto &velocity = ecs.get<Velocity>(entity);
-        std::cout << position.x << "," << position.y << " - " << velocity.dx << "," << velocity.dy << std::endl;
-        if(entity % 4) { ecs.remove<Velocity>(entity); }
-        else { ecs.destroy(entity); }
-    }
-
-    std::cout << "single component view" << std::endl;
-
-    for(auto entity: ecs.view<Position>()) {
-        auto &position = ecs.get<Position>(entity);
-        std::cout << position.x << "," << position.y << std::endl;
-    }
-
-    std::cout << "multi component view" << std::endl;
-
-    for(auto entity: ecs.view<Position, Velocity>()) {
-        auto &position = ecs.get<Position>(entity);
-        auto &velocity = ecs.get<Velocity>(entity);
-        std::cout << position.x << "," << position.y << " - " << velocity.dx << "," << velocity.dy << std::endl;
-        if(entity % 4) { ecs.remove<Velocity>(entity); }
-        else { ecs.destroy(entity); }
-    }
-
-    ecs.reset();
+    // ...
 }
 ```
 
 ## Motivation
 
-I started using another well known Entity-Component System named [`EntityX`](https://github.com/alecthomas/entityx).<br/>
-While I was playing with it, I found that I didn't like that much the way it manages its memory.
-Moreover, I was pretty sure that one could achieve better performance with a slightly modified pool under the hood.<br/>
-That's also the reason for which the interface is quite similar to the one of `EntityX`, so that `EnTT` can be used as a
-drop-in replacement for it with a minimal effort.
+I started developing `EnTT` for the _wrong_ reason: my goal was to design an
+entity-component system to beat another well known open source solution both in
+terms of performance and possibly memory usage.<br/>
+In the end, I did it, but it wasn't very satisfying. Actually it wasn't
+satisfying at all. The fastest and nothing more, fairly little indeed. When I
+realized it, I tried hard to keep intact the great performance of `EnTT` and to
+add all the features I wanted to see in *my own library* at the same time.
 
-### Performance
+Nowadays, `EnTT` is finally what I was looking for: still faster than its
+_competitors_, lower memory usage in the average case, a really good API and an
+amazing set of features. And even more, of course.
 
-As it stands right now, `EnTT` is just fast enough for my requirements if compared to my first choice (that was already
-amazingly fast).
-These are the results of the twos when compiled with GCC 6.3:
+## Performance
 
-| Benchmark | EntityX (experimental/compile_time) | EnTT |
-|-----------|-------------|-------------|
-| Creating 10M entities | 0.187042s | **0.0928331s** |
-| Destroying 10M entities | 0.0735151s | **0.060166s** |
-| Iterating over 10M entities, unpacking one component | 0.00784801s | **1.02e-07s** |
-| Iterating over 10M entities, unpacking two components | 0.00865273s | **0.00326714s** |
-| Iterating over 10M entities, unpacking five components | 0.0122006s | **0.00323354s** |
-| Iterating over 10M entities, unpacking ten components | 0.0100089s | **0.00323615s** |
-| Iterating over 50M entities, unpacking one component | 0.0394404s | **1.14e-07s** |
-| Iterating over 50M entities, unpacking two components | 0.0400407s | **0.0179783s** |
+The proposed entity-component system is incredibly fast to iterate entities and
+components, this is a fact. Some compilers make a lot of optimizations because
+of how `EnTT` works, some others aren't that good. In general, if we consider
+real world cases, `EnTT` is somewhere between a bit and much faster than many of
+the other solutions around, although I couldn't check them all for obvious
+reasons.
 
-These are the results of the twos when compiled with Clang 3.8.1:
+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.
 
-| Benchmark | EntityX (experimental/compile_time) | EnTT |
-|-----------|-------------|-------------|
-| Creating 10M entities | 0.268049s | **0.0899998s** |
-| Destroying 10M entities | **0.0713912s** | 0.078663s |
-| Iterating over 10M entities, unpacking one component | 0.00863192s | **3.05e-07s** |
-| Iterating over 10M entities, unpacking two components | 0.00780158s | **2.5434e-05s** |
-| Iterating over 10M entities, unpacking five components | 0.00829669s | **2.5497e-05s** |
-| Iterating over 10M entities, unpacking ten components | 0.00789789s | **2.5563e-05s** |
-| Iterating over 50M entities, unpacking one component | 0.0423244s | **1.94e-07s** |
-| Iterating over 50M entities, unpacking two components | 0.0435464s | **0.00012661s** |
+Honestly I got tired of updating the README file whenever there is an
+improvement.<br/>
+There are already a lot of projects out there that use `EnTT` as a basis for
+comparison (this should already tell you a lot). Many of these benchmarks are
+completely wrong, many others are simply incomplete, good at omitting some
+information and using the wrong function to compare a given feature. Certainly
+there are also good ones but they age quickly if nobody updates them, especially
+when the library they are dealing with is actively developed.
 
-I don't know what Clang does to squeeze out of `EnTT` the performance above, but I'd say that it does it incredibly well.
+The choice to use `EnTT` should be based on its carefully designed API, its
+set of features and the general performance, **not** because some single
+benchmark shows it to be the fastest tool available.
 
-See [benchmark.cpp](https://github.com/skypjack/entt/blob/master/test/benchmark.cpp) for further details.<br/>
-Of course, I'll try to get out of it more features and better performance anyway in the future, mainly for fun.
-If you want to contribute and have any suggestion, feel free to make a PR or open an issue to discuss them.
-
-### Benchmarks / Comparisons
-
-`EnTT` includes its own benchmarks, mostly similar to the ones of `EntityX` so as to compare them.<br/>
-On Github you can find also a [benchmark suite](https://github.com/abeimler/ecs_benchmark) testing `EntityX` (both the official version and the compile-time one), `Anax` and `Artemis C++` with up to 10M entities.
+In the future I'll likely try to get even better performance while still adding
+new features, mainly for fun.<br/>
+If you want to contribute and/or have suggestions, feel free to make a PR or
+open an issue to discuss your idea.
 
 # Build Instructions
 
 ## Requirements
 
-To be able to use `EnTT`, users must provide a full-featured compiler that supports at least C++14.<br/>
-CMake version 3.4 or later is mandatory to compile the tests, you don't have to install it otherwise.
+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
 
-`EnTT` is a header-only library. This means that including the `registry.hpp` header is enough to use it.<br/>
-It's a matter of adding the following line at the top of a file:
+`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
+interested only in the entity-component system, consider to include the sole
+`entity/registry.hpp` header instead.<br/>
+It's a matter of adding the following line to the top of a file:
 
 ```cpp
-#include <registry.hpp>
+#include <entt/entt.hpp>
 ```
 
-Then pass the proper `-I` argument to the compiler to add the `src` directory to the include paths.<br/>
+Use the line below to include only the entity-component system instead:
+
+```cpp
+#include <entt/entity/registry.hpp>
+```
+
+Then pass the proper `-I` argument to the compiler to add the `src` directory to
+the include paths.
 
 ## Documentation
 
-### API Reference
+The documentation is based on [doxygen](http://www.doxygen.nl/).
+To build it:
 
-`EnTT` contains three main actors: the *registry*, the *view* and the *pool*.<br/>
-Unless you have specific requirements of memory management, the default registry (that used the pool provided with
-`EnTT`) should be good enough for any use. Customization is an option anyway, so that you can use your own pool as
-long as it offers the expected interface.
+    $ cd build
+    $ cmake .. -DBUILD_DOCS=ON
+    $ make
 
-#### The Registry
+The API reference will be created in HTML format within the directory
+`build/docs/html`. To navigate it with your favorite browser:
 
-There are three options to instantiate your own registry:
+    $ cd build
+    $ your_favorite_browser docs/html/index.html
 
-* By using the default one:
-
-    ```cpp
-    auto registry = entt::DefaultRegistry<Components...>{args...};
-    ```
-
-  That is, you must provide the whole list of components to be registered with the default registry.
-
-* By using the standard one:
-
-    ```cpp
-    auto registry = entt::StandardRegistry<std::uint16_t, Components...>{args...};
-    ```
-
-  That is, you must provide the whole list of components to be registered with the default registry **and** the desired type for the entities. Note that the default type is `std::uint32_t`, that is larger enough for almost all the games but also too big for the most of the games.
-
-* By using your own pool:
-
-    ```cpp
-    auto registry = entt::Registry<DesiredEntityType, YourOwnPool<Components...>>{args...};
-    ```
-
-  Note that the registry expects a class template where the template parameters are the components to be managed.
-
-In both cases, `args...` parameters are forwarded to the underlying pool during the construction.<br/>
-There are no requirements for the components but to be moveable, therefore POD types are just fine.
-
-Once you have created a registry, the followings are the exposed member functions:
-
-* `size`: returns the number of entities still alive.
-* `capacity`: returns the maximum number of entities created till now.
-* `valid`: returns true if the entity is still in use, false otherwise.
-* `empty<Component>`: returns `true` if at least an instance of `Component` exists, `false` otherwise.
-* `empty`: returns `true` if all the entities have been destroyed, `false` otherwise.
-* `create<Components...>`: creates a new entity and assigns it the given components, then returns the entity.
-* `create`: creates a new entity and returns it, no components assigned.
-* `destroy`: destroys the entity and all its components.
-* `assign<Component>(entity, args...)`: assigns the given component to the entity and uses `args...` to initialize it.
-* `remove<Component>(entity)`: removes the given component from the entity.
-* `has<Components...>(entity)`: returns `true` if the entity has the given components, `false` otherwise.
-* `get<Component>(entity)`: returns a reference to the given component for the entity (undefined behaviour if the entity has not the component).
-* `replace<Component>(entity, args...)`: replaces the given component for the entity, using `args...` to create the new component.
-* `accomodate<Component>(entity, args...)`: replaces the given component for the entity if it exists, otherwise assigns it to the entity and uses `args...` to initialize it.
-* `clone(entity)`: clones an entity and all its components, then returns the new entity identifier.
-* `copy<Component>(to, from)`: copies a component from an entity to another one (both the entities must already have been assigned the component, undefined behaviour otherwise).
-* `copy(to, from)`: copies all the components and their contents from an entity to another one (comoonents are created or destroyed if needed).
-* `reset<Component>(entity)`: removes the given component from the entity if assigned.
-* `reset<Component>()`: destroys all the instances of `Component`.
-* `reset()`: resets the pool and destroys all the entities and their components.
-* `view<Components...>()`: gets a view of the entities that have the given components (see below for further details).
-
-Note that entities are numbers and nothing more. They are not classes and they have no member functions at all.
-
-#### The View
-
-There are three different kinds of view, each one with a slighlty different interface:
-
-* The _single component view_.
-* The _multi component view_.
-
-All of them are iterable. In other terms they have `begin` and `end` member functions that are suitable for a range-based for loop:
-
-```cpp
-auto view = registry.view<Position, Velocity>();
-
-for(auto entity: view) {
-    // do whatever you want with your entities
-}
-```
-
-Iterators are extremely poor, they are meant exclusively to be used to iterate over a set of entities.<br/>
-Guaranteed exposed member functions are:
-
-* `operator++()`
-* `operator++(int)`
-* `operator==()`
-* `operator!=()`
-* `operator*()`
-
-The single component view has an additional member function:
-
-* `size()`: returns the exact number of expected entities.
-
-The multi component view has an additional member function:
-
-* `reset()`: reorganizes internal data so as to further create optimized iterators (use it whenever the data within the registry are known to be changed).
-
-All the views can be used more than once. They return newly created and correctly initialized iterators whenever
-`begin` or `end` is invoked. Anyway views and iterators are tiny objects and the time to construct them can be safely ignored.
-I'd suggest not to store them anywhere and to invoke the `Registry::view` member function at each iteration to get a properly
-initialized view over which to iterate.
-
-**Note**: If underlying sets are modified, iterators are invalidated and using them is undefined behaviour.<br/>
-Do not try to assign or remove components on which you are iterating to entities. There are no guarantees.
-
-**Note**: Iterators aren't thread safe. Do no try to iterate over a set of components and modify them concurrently.
-That being said, as long as a thread iterates over the entities that have the component `X` or assign and removes
-that component from a set of entities and another thread does something similar with components `Y` and `Z`, it shouldn't be a
-problem at all.<br/>
-As an example, that means that users can freely run the rendering system over the renderable entities and update the physics
-concurrently on a separate thread if needed.
-
-#### The Pool
-
-Custom pools for a given component can be defined as a specialization of the class template `ComponentPool`.<br/>
-In particular:
-
-```cpp
-template<>
-struct ComponentPool<Entity, MyComponent> final {
-    // ...
-};
-```
-
-Where `Entity` is the desired type for the entities, `MyComponent` the type of the component to be stored.
-
-A custom pool should expose at least the following member functions:
-
-* `bool empty() const noexcept;`
-* `size_type capacity() const noexcept;`
-* `size_type size() const noexcept;`
-* `iterator_type begin() noexcept;`
-* `const_iterator_type begin() const noexcept;`
-* `iterator_type end() noexcept;`
-* `const_iterator_type end() const noexcept;`
-* `bool has(entity_type entity) const noexcept;`
-* `const component_type & get(entity_type entity) const noexcept;`
-* `component_type & get(entity_type entity) noexcept;`
-* `template<typename... Args> component_type & construct(entity_type entity, Args... args);`
-* `void destroy(entity_type entity);`
-* `void reset();`
-
-This is a fast and easy way to define a custom pool specialization for a given component (as an example, if the
-component `X` requires to be ordered internally somehow during construction or destruction operations) and to use the
-default pool for all the other components.<br/>
-It's a mattrer of including the given specialization along with the registry, so that it can find it during the instantiation.<br/>
-In this case, users are not required to use the more explicit `Registry` class. Instead, they can still use `entt::DefaultRegistry`.
-
-In cases when the per-component pools are not good enough, the registry can be initialized with a custom pool.<br/>
-In other terms, `entt::Registry` has a template template parameter that can be used to provide both the pool and the list of
-components:
-
-```cpp
-auto registry = entt::Registry<Entity, MyCustomPool<Component1, Component2>>{};
-```
-
-Even though the underlying pool doesn't store the components separately, the registry must know them to be able to do
-specific actions (like `destroy` or `copy`). That's why they must be explicitly specified.<br/>
-A generic pool should expose at least the following memeber functions:
-
-* `template<typename Component> bool empty() const noexcept;`
-* `template<typename Component> size_type capacity() const noexcept;`
-* `template<typename Component> size_type size() const noexcept;`
-* `template<typename Component> iterator_type begin() noexcept;`
-* `template<typename Component> const_iterator_type begin() const noexcept;`
-* `template<typename Component> iterator_type end() noexcept;`
-* `template<typename Component> const_iterator_type end() const noexcept;`
-* `template<typename Component> bool has(entity_type entity) const noexcept;`
-* `template<typename Component> const Comp & get(entity_type entity) const noexcept;`
-* `template<typename Component> Comp & get(entity_type entity) noexcept;`
-* `template<typename Component, typename... Args> Comp & construct(entity_type entity, Args... args);`
-* `template<typename Component> void destroy(entity_type entity);`
-* `template<typename Component> void reset();`
-* `void reset();`
-
-Good luck. If you come out with a more performant components pool, do not forget to make a PR so that I can add it to
-the list of available ones. I would be glad to have such a contribution to the project!!
+<!--
+@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/>
-Run the script `deps.sh` to download it. It is good practice to do it every time one pull the project.
+`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`.
 
-Then, to build the tests:
+To build the most basic set of tests:
 
 * `$ cd build`
-* `$ cmake ..`
+* `$ cmake -DBUILD_TESTING=ON ..`
 * `$ make`
 * `$ make test`
 
+Note that benchmarks are not part of this set.
+
+# Packaging Tools
+
+`EnTT` is available for some of the most known packaging tools. In particular:
+
+* [`Conan`](https://github.com/conan-io/conan-center-index), the C/C++ Package
+  Manager for Developers.
+
+* [`vcpkg`](https://github.com/Microsoft/vcpkg), Microsoft VC++ Packaging
+  Tool.<br/>
+  You can download and install `EnTT` in just a few simple steps:
+
+  ```
+  $ git clone https://github.com/Microsoft/vcpkg.git
+  $ cd vcpkg
+  $ ./bootstrap-vcpkg.sh
+  $ ./vcpkg integrate install
+  $ vcpkg install entt
+  ```
+
+  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
+  [create an issue or pull request](https://github.com/Microsoft/vcpkg) on the
+  `vcpkg` repository.
+
+* [`Homebrew`](https://github.com/skypjack/homebrew-entt), the missing package
+  manager for macOS.<br/>
+  Available as a homebrew formula. Just type the following to install it:
+
+  ```
+  brew install skypjack/entt/entt
+  ```
+
+* [`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.
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# EnTT in Action
+
+`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.
+
+[Here](https://github.com/skypjack/entt/wiki/EnTT-in-Action) you can find an
+incomplete list of games, applications and articles that can be used as a
+reference.
+
+If you know of other resources out there that are about `EnTT`, feel free to
+open an issue or a PR and I'll be glad to add them to the list.
+
 # Contributors
 
-If you want to contribute, please send patches as pull requests against the branch master.<br/>
-Check the [contributors list](https://github.com/skypjack/entt/blob/master/AUTHORS) to see who has partecipated so far.
+`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.
+
+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
+[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.
+
+If you decide to participate, please see the guidelines for
+[contributing](CONTRIBUTING.md) before to create issues or pull
+requests.<br/>
+Take also a look at the
+[contributors list](https://github.com/skypjack/entt/blob/master/AUTHORS) to
+know who has participated so far.
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
 
 # License
 
-Code and documentation Copyright (c) 2017 Michele Caini.<br/>
-Code released under [the MIT license](https://github.com/skypjack/entt/blob/master/LICENSE).
+Code and documentation Copyright (c) 2017-2020 Michele Caini.<br/>
+Logo Copyright (c) 2018-2020 Richard Caseres.
+
+Code released under
+[the MIT license](https://github.com/skypjack/entt/blob/master/LICENSE).
+Documentation released under
+[CC BY 4.0](https://creativecommons.org/licenses/by/4.0/).<br/>
+Logo 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

@@ -0,0 +1,27 @@
+* 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)
+* allow to replace std:: with custom implementations
+* add examples (and credits) from @alanjfs :)
+* static reflection, hint: template<> meta_type_t<Type>: meta_descriptor<name, func..., props..., etc...> (see #342)
+* update documentation for meta, it contains less than half of the actual feature
+
+WIP:
+* pagination doesn't work nicely across boundaries probably, give it a look. RO operations are fine, adding components maybe not.
+* make it easier to hook into the type system and describe how to do that to eg auto-generate meta types on first use
+* add observer functions aside observer class
+* introduce the component iterators for non-contiguous collections of entities (multi component views, observers, user defined collections)
+* snapshot: support for range-based archives
+* update snapshot documentation to describe alternatives
+* custom pools.
+* the Perfect Model.
+* page size 0 -> page less mode
+* add ::reach and rev iterators to proxy objects for faster and unsafe iterations
+* add example: 64 bit ids with 32 bits reserved for users' purposes
+* composable views and "faster views", deprecate non-owning groups
+* offset instead of pages in the sparse set? top level mask for sparse sets?
+* add meta dynamic cast (search base for T in parent, we have the meta type already)
+* make meta base/conv node work with storage/any
+* deprecate/remove meta_base, meta_conv, ...

WORKSPACE

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

appveyor.yml

@@ -1,23 +0,0 @@
-# can use variables like {build} and {branch}
-version: 1.0.{build}
-
-image: Visual Studio 2015
-
-environment:
-  BUILD_DIR: "%APPVEYOR_BUILD_FOLDER%\\build"
-
-platform:
-  - Win32
-
-configuration:
-  - Release
-
-before_build:
-  - deps.bat
-  - cd %BUILD_DIR%
-  - cmake .. -G"%CMAKE_GENERATOR_NAME%"
-
-build:
-  parallel: true
-  project: build/entt.sln
-  verbosity: minimal

cmake/in/EnTTConfig.cmake.in

@@ -0,0 +1,5 @@
+@PACKAGE_INIT@
+
+set(EnTT_VERSION "@PROJECT_VERSION@")
+include("${CMAKE_CURRENT_LIST_DIR}/EnTTTargets.cmake")
+check_required_components("@PROJECT_NAME@")

cmake/in/googletest.in

@@ -1,19 +0,0 @@
-project(googletest-download NONE)
-cmake_minimum_required(VERSION 3.2)
-
-include(ExternalProject)
-
-ExternalProject_Add(
-    googletest
-    GIT_REPOSITORY https://github.com/google/googletest.git
-    GIT_TAG master
-    DOWNLOAD_DIR ${GOOGLETEST_DEPS_DIR}
-    TMP_DIR ${GOOGLETEST_DEPS_DIR}/tmp
-    STAMP_DIR ${GOOGLETEST_DEPS_DIR}/stamp
-    SOURCE_DIR ${GOOGLETEST_DEPS_DIR}/src
-    BINARY_DIR ${GOOGLETEST_DEPS_DIR}/build
-    CONFIGURE_COMMAND ""
-    BUILD_COMMAND ""
-    INSTALL_COMMAND ""
-    TEST_COMMAND ""
-)

conan/build.py

@@ -0,0 +1,37 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+from cpt.packager import ConanMultiPackager
+import os
+
+if __name__ == "__main__":
+    username = os.getenv("GITHUB_ACTOR")
+    tag_version = os.getenv("GITHUB_REF")
+    tag_package = os.getenv("GITHUB_REPOSITORY")
+    login_username = os.getenv("CONAN_LOGIN_USERNAME")
+    package_version = tag_version.replace("refs/tags/v", "")
+    package_name = tag_package.replace("skypjack/", "")
+    reference = "{}/{}".format(package_name, package_version)
+    channel = os.getenv("CONAN_CHANNEL", "stable")
+    upload = os.getenv("CONAN_UPLOAD")
+    stable_branch_pattern = os.getenv("CONAN_STABLE_BRANCH_PATTERN", r"v\d+\.\d+\.\d+.*")
+    test_folder = os.getenv("CPT_TEST_FOLDER", os.path.join("conan", "test_package"))
+    upload_only_when_stable = os.getenv("CONAN_UPLOAD_ONLY_WHEN_STABLE", True)
+    disable_shared = os.getenv("CONAN_DISABLE_SHARED_BUILD", "False")
+
+    builder = ConanMultiPackager(username=username,
+                                 reference=reference,
+                                 channel=channel,
+                                 login_username=login_username,
+                                 upload=upload,
+                                 stable_branch_pattern=stable_branch_pattern,
+                                 upload_only_when_stable=upload_only_when_stable,
+                                 test_folder=test_folder)
+    builder.add()
+
+    filtered_builds = []
+    for settings, options, env_vars, build_requires, reference in builder.items:
+        if disable_shared == "False" or not options["{}:shared".format(package_name)]:
+             filtered_builds.append([settings, options, env_vars, build_requires])
+    builder.builds = filtered_builds
+
+    builder.run()

conan/ci/build.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+set -e
+set -x
+
+conan user
+python conan/build.py

conan/ci/install.sh

@@ -0,0 +1,6 @@
+#!/bin/bash
+
+set -e
+set -x
+
+pip install -U conan_package_tools conan

conan/test_package/CMakeLists.txt

@@ -0,0 +1,13 @@
+cmake_minimum_required(VERSION 3.7.2)
+project(test_package)
+
+set(CMAKE_VERBOSE_MAKEFILE TRUE)
+
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
+include(${CMAKE_BINARY_DIR}/conanbuildinfo.cmake)
+conan_basic_setup()
+
+add_executable(${PROJECT_NAME} test_package.cpp)
+target_link_libraries(${PROJECT_NAME} ${CONAN_LIBS})

conan/test_package/conanfile.py

@@ -0,0 +1,19 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+
+from conans import ConanFile, CMake
+import os
+
+
+class TestPackageConan(ConanFile):
+    settings = "os", "compiler", "build_type", "arch"
+    generators = "cmake"
+
+    def build(self):
+        cmake = CMake(self)
+        cmake.configure()
+        cmake.build()
+
+    def test(self):
+        bin_path = os.path.join("bin", "test_package")
+        self.run(bin_path, run_environment=True)

conan/test_package/test_package.cpp

@@ -0,0 +1,56 @@
+#include <entt/entt.hpp>
+#include <cstdint>
+
+struct position {
+    float x;
+    float y;
+};
+
+struct velocity {
+    float dx;
+    float dy;
+};
+
+void update(entt::registry &registry) {
+    auto view = registry.view<position, velocity>();
+
+    for(auto entity: view) {
+        // gets only the components that are going to be used ...
+
+        auto &vel = view.get<velocity>(entity);
+
+        vel.dx = 0.;
+        vel.dy = 0.;
+
+        // ...
+    }
+}
+
+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;
+
+        // ...
+    });
+}
+
+int main() {
+    entt::registry registry;
+    std::uint64_t dt = 16;
+
+    for(auto i = 0; i < 10; ++i) {
+        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);
+
+    // ...
+
+    return EXIT_SUCCESS;
+}

conanfile.py

@@ -0,0 +1,27 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+from conans import ConanFile
+
+
+class EnttConan(ConanFile):
+    name = "entt"
+    description = "Gaming meets modern C++ - a fast and reliable entity-component system (ECS) and much more "
+    topics = ("conan," "entt", "gaming", "entity", "ecs")
+    url = "https://github.com/skypjack/entt"
+    homepage = url
+    author = "Michele Caini <michele.caini@gmail.com>"
+    license = "MIT"
+    exports = ["LICENSE"]
+    exports_sources = ["src/*"]
+    no_copy_source = True
+
+    def package(self):
+        self.copy(pattern="LICENSE", dst="licenses")
+        self.copy(pattern="*", dst="include", src="src", keep_path=True)
+
+    def package_info(self):
+        if not self.in_local_cache:
+            self.cpp_info.includedirs = ["src"]
+
+    def package_id(self):
+        self.info.header_only()

deps/.gitignore

@@ -1,2 +0,0 @@
-*
-!.gitignore

docs/CMakeLists.txt

@@ -0,0 +1,37 @@
+#
+# Doxygen configuration (documentation)
+#
+
+set(DOXY_DEPS_DIRECTORY ${EnTT_SOURCE_DIR}/deps)
+set(DOXY_SOURCE_DIRECTORY ${EnTT_SOURCE_DIR}/src)
+set(DOXY_DOCS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})
+set(DOXY_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
+
+configure_file(doxy.in doxy.cfg @ONLY)
+
+add_custom_target(
+    docs ALL
+    COMMAND ${DOXYGEN_EXECUTABLE} ${CMAKE_CURRENT_BINARY_DIR}/doxy.cfg
+    WORKING_DIRECTORY ${EnTT_SOURCE_DIR}
+    VERBATIM
+    SOURCES
+        dox/extra.dox
+        md/config.md
+        md/core.md
+        md/entity.md
+        md/faq.md
+        md/lib.md
+        md/links.md
+        md/locator.md
+        md/meta.md
+        md/process.md
+        md/references.md
+        md/resource.md
+        md/signal.md
+        doxy.in
+)
+
+install(
+    DIRECTORY ${DOXY_OUTPUT_DIRECTORY}/html
+    DESTINATION share/${PROJECT_NAME}-${PROJECT_VERSION}/
+)

docs/dox/extra.dox

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

docs/md/config.md

@@ -0,0 +1,109 @@
+# Crash Course: configuration
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [Definitions](#definitions)
+  * [ENTT_STANDALONE](#entt_standalone)
+  * [ENTT_NOEXCEPT](#entt_noexcept)
+  * [ENTT_HS_SUFFIX and ENTT_HWS_SUFFIX](#entt_hs_suffix_and_entt_hws_suffix)
+  * [ENTT_USE_ATOMIC](#entt_use_atomic)
+  * [ENTT_ID_TYPE](#entt_id_type)
+  * [ENTT_PAGE_SIZE](#entt_page_size)
+  * [ENTT_ASSERT](#entt_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_STANDALONE
+
+`EnTT` is designed in such a way that it works (almost) everywhere out of the
+box. However, this is the result of many refinements over time and a compromise
+regarding some optimizations.<br/>
+It's worth noting that users can get a small performance boost by passing this
+definition to the compiler when the library is used in a standalone application.
+
+## ENTT_NOEXCEPT
+
+The purpose of this parameter is to suppress the use of `noexcept` by this
+library.<br/>
+To do this, simply define the variable without assigning any value to it.
+
+## ENTT_HS_SUFFIX and ENTT_HWS_SUFFIX
+
+The `hashed_string` class introduces the `_hs` and `_hws` suffixes to accompany
+its user defined literals.<br/>
+In the case of conflicts or even just to change these suffixes, it's possible to
+do so by associating new ones with these definitions.
+
+## 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_PAGE_SIZE
+
+As is known, the ECS module of `EnTT` is based on _sparse sets_. What is less
+known perhaps is that these are paged to reduce memory consumption in some
+corner cases.<br/>
+The default size of a page is 32kB 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 `assert`s for the purpose of
+detecting errors in debug builds. However, these assertions may in turn affect
+performance to an extent.<br/>
+This option is meant to disable all controls.
+
+## 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
+
+After many adventures, `EnTT` finally works fine across boundaries.<br/>
+To do this, the library mixes some non-standard language features with others
+that are perfectly compliant.<br/>
+This definition will prevent the library from using non-standard techniques,
+that is, functionalities that aren't fully compliant with the standard C++.

docs/md/core.md

@@ -0,0 +1,471 @@
+# Crash Course: core functionalities
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [Unique sequential identifiers](#unique-sequential-identifiers)
+  * [Compile-time generator](#compile-time-generator)
+  * [Runtime generator](#runtime-generator)
+* [Hashed strings](#hashed-strings)
+  * [Wide characters](wide-characters)
+  * [Conflicts](#conflicts)
+* [Monostate](#monostate)
+* [Type support](#type-support)
+  * [Type info](#type-info)
+    * [Almost unique identifiers](#almost-unique-identifiers)
+  * [Type index](#type-index)
+  * [Type traits](#type-traits)
+    * [Member class type](#member-class-type)
+    * [Integral constant](#integral-constant)
+    * [Tag](#tag)
+* [Utilities](#utilities)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+`EnTT` comes with a bunch of core functionalities mostly used by the other parts
+of the library itself.<br/>
+Hardly users will include these features in their code, but it's worth
+describing what `EnTT` offers so as not to reinvent the wheel in case of need.
+
+# Unique sequential identifiers
+
+Sometimes it's useful to be able to give unique, sequential numeric identifiers
+to types either at compile-time or runtime.<br/>
+There are plenty of different solutions for this out there and I could have used
+one of them. However, I decided to spend my time to define a couple of tools
+that fully embraces what the modern C++ has to offer.
+
+## Compile-time generator
+
+To generate sequential numeric identifiers at compile-time, `EnTT` offers the
+`identifier` class template:
+
+```cpp
+// defines the identifiers for the given types
+using id = entt::identifier<a_type, another_type>;
+
+// ...
+
+switch(a_type_identifier) {
+case id::type<a_type>:
+    // ...
+    break;
+case id::type<another_type>:
+    // ...
+    break;
+default:
+    // ...
+}
+```
+
+This is all what this class template has to offer: a `type` inline variable that
+contains a numeric identifier for the given type. It can be used in any context
+where constant expressions are required.
+
+As long as the list remains unchanged, identifiers are also guaranteed to be
+stable across different runs. In case they have been used in a production
+environment and a type has to be removed, one can just use a placeholder to left
+the other identifiers unchanged:
+
+```cpp
+template<typename> struct ignore_type {};
+
+using id = entt::identifier<
+    a_type_still_valid,
+    ignore_type<a_type_no_longer_valid>,
+    another_type_still_valid
+>;
+```
+
+Perhaps a bit ugly to see in a codebase but it gets the job done at least.
+
+## Runtime generator
+
+To generate sequential numeric identifiers at runtime, `EnTT` offers the
+`family` class template:
+
+```cpp
+// defines a custom generator
+using id = entt::family<struct my_tag>;
+
+// ...
+
+const auto a_type_id = id::type<a_type>;
+const auto another_type_id = id::type<another_type>;
+```
+
+This is all what a _family_ has to offer: a `type` inline variable that contains
+a numeric identifier for the given type.<br/>
+The generator is customizable, so as to get different _sequences_ for different
+purposes if needed.
+
+Please, note that identifiers aren't guaranteed to be stable across different
+runs. Indeed it mostly depends on the flow of execution.
+
+# Hashed strings
+
+A hashed string is a zero overhead unique identifier. Users can use
+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/>
+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.
+
+Example of use:
+
+```cpp
+auto load(entt::hashed_string::hash_type resource) {
+    // uses the numeric representation of the resource to load and return it
+}
+
+auto resource = load(entt::hashed_string{"gui/background"});
+```
+
+There is also a _user defined literal_ dedicated to hashed strings to make them
+more user-friendly:
+
+```cpp
+constexpr auto str = "text"_hs;
+```
+
+## Wide characters
+
+The hashed string has a design that is close to that of an `std::basic_string`.
+It means that `hashed_string` is nothing more than an alias for
+`basic_hashed_string<char>`. For those who want to use the C++ type for wide
+character representation, there exists also the alias `hashed_wstring` for
+`basic_hashed_string<wchar_t>`.<br/>
+In this case, the user defined literal to use to create hashed strings on the
+fly is `_hws`:
+
+```cpp
+constexpr auto str = "text"_hws;
+```
+
+Note that the hash type of the `hashed_wstring` is the same of its counterpart.
+
+## Conflicts
+
+The hashed string class uses internally FNV-1a to compute the numeric
+counterpart of a string. Because of the _pigeonhole principle_, conflicts are
+possible. This is a fact.<br/>
+There is no silver bullet to solve the problem of conflicts when dealing with
+hashing functions. In this case, the best solution seemed to be to give up.
+That's all.<br/>
+After all, human-readable unique identifiers aren't something strictly defined
+and over which users have not the control. Choosing a slightly different
+identifier is probably the best solution to make the conflict disappear in this
+case.
+
+# Monostate
+
+The monostate pattern is often presented as an alternative to a singleton based
+configuration system. This is exactly its purpose in `EnTT`. Moreover, this
+implementation is thread safe by design (hopefully).<br/>
+Keys are represented by hashed strings, values are basic types like `int`s or
+`bool`s. Values of different types can be associated to each key, even more than
+one at a time. Because of this, users must pay attention to use the same type
+both during an assignment and when they try to read back their data. Otherwise,
+they will probably incur in unexpected results.
+
+Example of use:
+
+```cpp
+entt::monostate<entt::hashed_string{"mykey"}>{} = true;
+entt::monostate<"mykey"_hs>{} = 42;
+
+// ...
+
+const bool b = entt::monostate<"mykey"_hs>{};
+const int i = entt::monostate<entt::hashed_string{"mykey"}>{};
+```
+
+# Type support
+
+`EnTT` provides some basic information about types of all kinds.<br/>
+It also offers additional features that are not yet available in the standard
+library or that will never be.
+
+## Type info
+
+This class template 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.
+
+Currently, there are a couple of information available:
+
+* The numeric identifier associated with a given type:
+
+  ```cpp
+  auto id = entt::type_info<my_type>::id();
+  ```
+
+  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 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.
+
+* The _name_ associated with a given type:
+
+  ```cpp
+  auto name = entt::type_info<my_type>::name();
+  ```
+
+  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. As for the numeric identifier, 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.
+
+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:
+
+```cpp
+template<typename Type>
+struct entt::type_info<Type, std::enable_if_t<has_custom_data_v<Type>>> {
+    static constexpr entt::id_type id() ENTT_NOEXCEPT {
+        return Type::custom_id();
+    }
+
+    static constexpr std::string_view name() ENTT_NOEXCEPT {
+        return Type::custom_name();
+    }
+};
+```
+
+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 in many cases.<br/>
+Please refer to the dedicated section for more details.
+
+### Almost unique identifiers
+
+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/>
+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/>
+If the types have the same name and belong to the same namespace then their
+identifiers _could_ be identical (they won't necessarily be the same though).
+
+Fortunately, there are several easy ways to deal with this:
+
+* The most trivial one is to define the `ENTT_STANDARD_CPP` macro. Runtime
+  identifiers don't suffer from the same problem in fact. However, this solution
+  doesn't work well with a plugin system, where the libraries aren't linked.
+
+* Another possibility is to specialize the `type_info` 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.
+
+* A fully customized identifier generation policy (based for example on enum
+  classes or preprocessing steps) may represent yet another option.
+
+These are just some examples of possible approaches to the problem but there are
+many others. As already mentioned above, since users have full control over
+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.
+
+## Type index
+
+Types in `EnTT` are assigned also unique, sequential _indexes_ generated at
+runtime:
+
+```cpp
+auto index = entt::type_index<my_type>::value();
+```
+
+This value may differ from the numeric identifier of a type and 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.
+
+## Type traits
+
+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
+
+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
+when members are passed as template arguments.<br/>
+The purpose of this utility is to extract the class type in a few lines of code:
+
+```cpp
+template<typename Member>
+using clazz = entt::member_class_t<Member>;
+```
+
+### Integral constant
+
+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.emplace<entt::tag<"enemy"_hs>>(entity);
+```
+
+However, this isn't the only permitted use. Literally any value convertible to
+`id_type` is a good candidate, such as the named constants of an unscoped enum.
+
+# 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

@@ -0,0 +1,218 @@
+# Frequently Asked Questions
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [FAQ](#faq)
+  * [Why is my debug build on Windows so slow?](#why-is-my-debug-build-on-windows-so-slow)
+  * [How can I represent hierarchies with my components?](#how-can-i-represent-hierarchies-with-my-components)
+  * [Custom entity identifiers: yay or nay?](#custom-entity-identifiers-yay-or-nay)
+  * [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
+-->
+
+# Introduction
+
+This is a constantly updated section where I'll try to put the answers to the
+most frequently asked questions.<br/>
+If you don't find your answer here, there are two cases: nobody has done it yet
+or this section needs updating. In both cases, try to
+[open a new issue](https://github.com/skypjack/entt/issues/new) or enter the
+[gitter channel](https://gitter.im/skypjack/entt) and ask your question.
+Probably someone already has an answer for you and we can then integrate this
+part of the documentation.
+
+# FAQ
+
+## Why is my debug build on Windows so slow?
+
+`EnTT` is an experimental project that I also use to keep me up-to-date with the
+latest revision of the language and the standard library. For this reason, it's
+likely that some classes you're working with are using standard containers under
+the hood.<br/>
+Unfortunately, it's known that the standard containers aren't particularly
+performing in debugging (the reasons for this go beyond this document) and are
+even less so on Windows apparently. Fortunately this can also be mitigated a
+lot, achieving good results in many cases.
+
+First of all, there are two things to do in a Windows project:
+
+* Disable the [`/JMC`](https://docs.microsoft.com/cpp/build/reference/jmc)
+  option (_Just My Code_ debugging), available starting in Visual Studio 2017
+  version 15.8.
+
+* Set the [`_ITERATOR_DEBUG_LEVEL`](https://docs.microsoft.com/cpp/standard-library/iterator-debug-level)
+  macro to 0. This will disable checked iterators and iterator debugging.
+
+Moreover, the macro `ENTT_ASSERT` should be redefined to disable internal checks
+made by `EnTT` in debug:
+
+```cpp
+#define ENTT_ASSERT(...) ((void)0)
+```
+
+These asserts are introduced to help the users but they require to access to the
+underlying containers and therefore risk ruining the performance in some cases.
+
+With these changes, debug performance should increase enough for most cases. If
+you want something more, you can can also switch to an optimization level `O0`
+or preferably `O1`.
+
+## How can I represent hierarchies with my components?
+
+This is one of the first questions that anyone makes when starting to work with
+the entity-component-system architectural pattern.<br/>
+There are several approaches to the problem and what’s the best one depends
+mainly on the real problem one is facing. In all cases, how to do it doesn't
+strictly depend on the library in use, but the latter can certainly allow or
+not different techniques depending on how the data are laid out.
+
+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.
+
+## Custom entity identifiers: yay or nay?
+
+Custom entity identifiers are definitely a good idea in two cases at least:
+
+* If `std::uint32_t` is too large or isn't large enough for your purposes, since
+  this is the underlying type of `entt::entity`.
+* If you want to avoid conflicts when using multiple registries.
+
+Identifiers can be defined through enum classes and custom types for which a
+specialization of `entt_traits` exists. For this purpose, `entt_traits` is also
+defined as a _sfinae-friendly_ class template.<br/>
+In fact, this is a definition equivalent to that of `entt::entity`:
+
+```cpp
+enum class entity: std::uint32_t {};
+```
+
+In theory, integral types can also be used as entity identifiers, even though
+this may break in future and isn't recommended in general.
+
+## Warning C4307: integral constant overflow
+
+According to [this](https://github.com/skypjack/entt/issues/121) issue, using a
+hashed string under VS could generate a warning.<br/>
+First of all, I want to reassure you: it's expected and harmless. However, it
+can be annoying.
+
+To suppress it and if you don't want to suppress all the other warnings as well,
+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}
+#else
+#define HS(str) entt::hashed_string{str}
+#endif
+```
+
+With an example of use included:
+
+```cpp
+constexpr auto identifier = HS("my/resource/identifier");
+```
+
+Thanks to [huwpascoe](https://github.com/huwpascoe) for the courtesy.
+
+## Warning C4003: the min, the max and the macro
+
+On Windows, a header file defines two macros `min` and `max` which may result in
+conflicts with their counterparts in the standard library and therefore in
+errors during compilation.
+
+It's a pretty big problem but fortunately it's not a problem of `EnTT` and there
+is a fairly simple solution to it.<br/>
+It consists in defining the `NOMINMAX` macro before to include any other header
+so as to get rid of the extra definitions:
+
+```cpp
+#define NOMINMAX
+```
+
+Please refer to [this](https://github.com/skypjack/entt/issues/96) issue for
+more details.
+
+## The standard and the non-copyable types
+
+`EnTT` uses internally the trait `std::is_copy_constructible_v` to check if a
+component is actually copyable. This trait doesn't check if an object can
+actually be copied but only verifies if there is a copy constructor
+available.<br/>
+This can lead to surprising results due to some idiosyncrasies of the standard
+mainly related to the need to guarantee backward compatibility.
+
+For example, `std::vector` defines a copy constructor no matter if its value
+type is copyable or not. As a result, `std::is_copy_constructible_v` is true
+for the following specialization:
+
+```cpp
+struct type {
+    std::vector<std::unique_ptr<action>> vec;
+};
+```
+
+When trying to assign an instance of this type to an entity in the ECS part,
+this may trigger a compilation error because we cannot really make a copy of
+it.<br/>
+As a workaround, users can mark the type explicitly as non-copyable:
+
+```cpp
+struct type {
+    type(const type &) = delete;
+    type & operator=(const type &) = delete;
+
+    std::vector<std::unique_ptr<action>> vec;
+};
+```
+
+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

@@ -0,0 +1,136 @@
+# Push EnTT across boundaries
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Working across boundaries](#working-across-boundaries)
+  * [The EnTT way](#the-entt-way)
+  * [Meta context](#meta-context)
+  * [Memory management](#memory-management)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# 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 with 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.
+
+## The EnTT way
+
+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
+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_index` class template makes all
+types _indexable_ instead, so as to speed up the lookup.
+
+In general, these classes don't arouse much interest. The only exceptions are:
+
+* When a conflict between identifiers occurs (definitely uncommon though) or
+  when the default solution proposed by `EnTT` isn't suitable for the user's
+  purposes.<br/>
+  The section dedicated to `type_info` contains all the details to get around
+  the problem in a concise and elegant way. Please refer to the specific
+  documentation.
+
+* When working with linked libraries that also export all required symbols.<br/>
+  Compile definitions `ENTT_API_EXPORT` and `ENTT_API_IMPORT` should be passed
+  respectively where there is a need to import or export the symbols defined by
+  `EnTT`, so as to make everything work nicely across boundaries.
+
+* When working with plugins or shared libraries that don't export any symbol. In
+  this case, `type_index` confuses the other classes by giving potentially wrong
+  information to them.<br/>
+  To avoid problems, it's required to provide a custom generator or to suppress
+  the index generation as a whole:
+
+  ```cpp
+  template<typename Type>
+  struct entt::type_index<Type> {};
+  ```
+
+  All classes that use `type_index` perform also a check on the possibility of
+  creating indexes for types. If it's not a viable solution, they fallback on
+  the type id provided by `type_info`. The latter makes everything stable across
+  boundaries.<br/>
+  This is why suppressing the generation of the indexes solves the problem. In
+  case it's still necessary to associate sequential indexes with types, users
+  can refer to the `family` class, although knowing that these will not be
+  stable across boundaries.
+
+For anyone who needs more details, the test suite contains multiple examples
+covering the most common cases.<br/>
+It goes without saying that it's impossible to cover all the 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
+_shared_ to allow the use of meta types across boundaries.
+
+Sharing a context is trivial though. First of all, the local one must be
+acquired in the main space:
+
+```cpp
+entt::meta_ctx ctx{};
+```
+
+Then, it must passed to the receiving space that will set it as its global
+context, thus releasing the local one that remains available but is no longer
+referred to by the runtime reflection system:
+
+```cpp
+entt::meta_ctx::bind(ctx);
+```
+
+From now on, both spaces will refer to the same context and on it will be
+attached the new visible meta types, no matter where they are created.<br/>
+A context can also be reset and then associated again locally as:
+
+```cpp
+entt::meta_ctx::bind{entt::meta_ctx{});
+```
+
+This is allowed because local and global contexts are separated. Therefore, it's
+always possible to make the local context the current one again.
+
+Before to release a context, all locally registered types should be reset to
+avoid dangling references. Otherwise, if a type is accessed from another space
+by name, there could be an attempt to address its parts that are no longer
+available.
+
+## Memory Management
+
+There is another subtle problem due to memory management that can lead to
+headaches.<br/>
+It can occur where there are pools of objects (such as components or events)
+dynamically created on demand. This is usually not a problem when working with
+linked libraries that rely on the same dynamic runtime. However, it can occur in
+the case of plugins or statically linked runtimes.
+
+As an example, imagine creating an instance of `registry` in the main executable
+and sharing it with a plugin. If the latter starts working with a component that
+is unknown to the former, a dedicated pool is created within the registry on
+first use.<br/>
+As one can guess, this pool is instantiated on a different side of the boundary
+from the `registry`. Therefore, the instance is now managing memory from
+different spaces and this can quickly lead to crashes if not properly addressed.
+
+To overcome the risk, it's recommended to use well-defined interfaces that make
+fundamental types pass through the boundaries, isolating the instances of the
+`EnTT` classes from time to time and as appropriate.<br/>
+Refer to the test suite for some examples, read the documentation available
+online about this type of issues or consult someone who has already had such
+experiences to avoid problems.

docs/md/links.md

@@ -0,0 +1,159 @@
+# EnTT in Action
+
+`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.
+
+Below an incomplete list of games, applications and articles that can be used as
+a reference. Where I put the word _apparently_ means that the use of `EnTT` is
+documented but the authors didn't make explicit announcements or contacted me
+directly.
+
+I hope this list can grow much more in the future:
+
+* Games:
+  * [Minecraft](https://minecraft.net/en-us/attribution/) by
+    [Mojang](https://mojang.com/): of course, **that** Minecraft, see the
+    open source attributions page for more details.
+  * [Land of the Rair](https://github.com/LandOfTheRair/core2): the new backend
+    of [a retro-style MUD](https://rair.land/) for the new age.
+  * [Openblack](https://github.com/openblack/openblack): open source
+    reimplementation of the game _Black & White_ (2001).
+  * [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
+    to make Pacman with `EnTT`.
+  * [Wacman](https://github.com/carlfindahl/wacman): a pacman clone with OpenGL.
+  * [Classic Tower Defence](https://github.com/kerndog73/Classic-Tower-Defence):
+    a tiny little tower defence game featuring a homemade font.
+    [Check it out](https://indi-kernick.itch.io/classic-tower-defence).
+  * [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`.
+  * [Randballs](https://github.com/gale93/randballs): simple `SFML` and `EnTT`
+    playground.
+  * [EnTT Tower Defense](https://github.com/Daivuk/tddod): a data oriented tower
+    defense example.
+  * [EnTT Breakout](https://github.com/vblanco20-1/entt-breakout): simple
+    example of a breakout game, using `SDL` and `EnTT`.
+  * [Arcade puzzle game with EnTT](https://github.com/MasonRG/ArcadePuzzleGame):
+    arcade puzzle game made in C++ using the `SDL2` and `EnTT` libraries.
+  * [Snake with EnTT](https://github.com/MasonRG/SnakeGame): simple snake game
+    made in C++ with the `SDL2` and `EnTT` libraries.
+  * [Mirrors lasers and robots](https://github.com/guillaume-haerinck/imac-tower-defense):
+    a small tower defense game based on mirror orientation.
+  * [PopHead](https://github.com/SPC-Some-Polish-Coders/PopHead/): 2D, Zombie,
+    RPG game made from scratch in C++.
+  * [Robotligan](https://github.com/Trisslotten/robotligan): multiplayer
+    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.
+
+* Engines and the like:
+  * [Fling Engine](https://github.com/flingengine/FlingEngine): a Vulkan game
+    engine with a focus on data oriented design.
+  * [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
+    [demo code](https://github.com/Green-Sky/imgui_entt_entity_editor_demo)).
+  * [SgOgl](https://github.com/stwe/SgOgl): a game engine library for OpenGL
+    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.
+  * [LM-Engine](https://github.com/Lawrencemm/LM-Engine): the Vim of game
+    engines.
+  * [TiltedOnline](https://github.com/tiltedphoques/TiltedOnline) by
+    [Tilted Phoques](https://github.com/tiltedphoques): Skyrim Together, a mod
+    intended to sync the player and the world state accross multiple clients.
+
+* 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.
+  * [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
+    battle built on `UE4`.
+  * [Experimenting with ECS in UE4](http://victor.madtriangles.com/code%20experiment/2018/03/25/post-ue4-ecs-battle.html):
+    interesting article about `UE4` and `EnTT`.
+  * [Implementing ECS architecture in UE4](https://forums.unrealengine.com/development-discussion/c-gameplay-programming/1449913-implementing-ecs-architecture-in-ue4-giant-space-battle):
+    giant space battle.
+  * [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 (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)).
+  * [Game Engine series](https://www.youtube.com/c/TheChernoProject/videos) by
+    [The Cherno](https://github.com/TheCherno):
+    - [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).
+  * [Ability Creator](https://www.erichildebrand.net/blog/ability-creator-project-retrospect):
+    project retrospect by [Eric Hildebrand](https://www.erichildebrand.net/).
+
+* Any Other Business:
+  * [ArcGIS Runtime SDKs](https://developers.arcgis.com/arcgis-runtime/) by
+    [Esri](https://www.esri.com/): they use `EnTT` for the internal ECS and the
+    cross platform C++ rendering engine. The SDKs are utilized by a lot of
+    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).
+  * [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.
+  * [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`).
+  * [Godot meets EnTT](https://github.com/portaloffreedom/godot_entt_example/):
+    a simple example on how to use `EnTT` within
+    [`Godot`](https://godotengine.org/).
+  * [Godot and GameNetworkingSockets meet EnTT](https://github.com/portaloffreedom/godot_entt_net_example):
+    a simple example on how to use `EnTT` and
+    [`GameNetworkingSockets`](https://github.com/ValveSoftware/GameNetworkingSockets)
+    within [`Godot`](https://godotengine.org/).
+  * [MatchOneEntt](https://github.com/mhaemmerle/MatchOneEntt): port of
+    [Match One](https://github.com/sschmid/Match-One) for `Entitas-CSharp`.
+  * GitHub contains also
+    [many other examples](https://github.com/search?o=desc&q=%22skypjack%2Fentt%22&s=indexed&type=Code)
+    of use of `EnTT` from which to take inspiration if interested.
+
+If you know of other resources out there that are about `EnTT`, feel free to
+open an issue or a PR and I'll be glad to add them to this page.

docs/md/locator.md

@@ -0,0 +1,75 @@
+# Crash Course: service locator
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [Service locator](#service-locator)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+Usually service locators are tightly bound to the services they expose and it's
+hard to define a general purpose solution. This template based implementation
+tries to fill the gap and to get rid of the burden of defining a different
+specific locator for each application.<br/>
+This class is tiny, partially unsafe and thus risky to use. Moreover it doesn't
+fit probably most of the scenarios in which a service locator is required. Look
+at it as a small tool that can sometimes be useful if users know how to handle
+it.
+
+# Service locator
+
+The API is straightforward. The basic idea is that services are implemented by
+means of interfaces and rely on polymorphism.<br/>
+The locator is instantiated with the base type of the service if any and a
+concrete implementation is provided along with all the parameters required to
+initialize it. As an example:
+
+```cpp
+// the service has no base type, a locator is used to treat it as a kind of singleton
+entt::service_locator<my_service>::set(params...);
+
+// sets up an opaque service
+entt::service_locator<audio_interface>::set<audio_implementation>(params...);
+
+// resets (destroys) the service
+entt::service_locator<audio_interface>::reset();
+```
+
+The locator can also be queried to know if an active service is currently set
+and to retrieve it if necessary (either as a pointer or as a reference):
+
+```cpp
+// no service currently set
+auto empty = entt::service_locator<audio_interface>::empty();
+
+// gets a (possibly empty) shared pointer to the service ...
+std::shared_ptr<audio_interface> ptr = entt::service_locator<audio_interface>::get();
+
+// ... or a reference, but it's undefined behaviour if the service isn't set yet
+audio_interface &ref = entt::service_locator<audio_interface>::ref();
+```
+
+A common use is to wrap the different locators in a container class, creating
+aliases for the various services:
+
+```cpp
+struct locator {
+    using camera = entt::service_locator<camera_interface>;
+    using audio = entt::service_locator<audio_interface>;
+    // ...
+};
+
+// ...
+
+void init() {
+    locator::camera::set<camera_null>();
+    locator::audio::set<audio_implementation>(params...);
+    // ...
+}
+```

docs/md/meta.md

@@ -0,0 +1,884 @@
+# Crash Course: runtime reflection system
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [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)
+  * [Enjoy the runtime](#enjoy-the-runtime)
+  * [Container support](#container-support)
+  * [Pointer-like types](#pointer-like-types)
+  * [Policies: the more, the less](#policies-the-more-the-less)
+  * [Named constants and enums](#named-constants-and-enums)
+  * [Properties and meta objects](#properties-and-meta-objects)
+  * [Unregister types](#unregister-types)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# 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
+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/>
+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 the opposite.
+
+# Names and identifiers
+
+The meta system doesn't force users to rely on the tools provided by the library
+when it comes to working with names and identifiers. It does this by offering an
+API that works with opaque identifiers that may or may not be generated by means
+of a hashed string.<br/>
+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.
+
+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
+follows:
+
+```cpp
+auto factory = entt::meta<my_type>().type("reflected_type"_hs);
+```
+
+For what it's worth, this is likely completely equivalent to:
+
+```cpp
+auto factory = entt::meta<my_type>().type(42);
+```
+
+Obviously, human-readable identifiers are more convenient to use and highly
+recommended.
+
+# Reflection in a nutshell
+
+Reflection always starts from real types (users cannot reflect imaginary types
+and it would not make much sense, we wouldn't be talking about reflection
+anymore).<br/>
+To create a meta node, the library provides the `meta` function that accepts a
+type to reflect as a template parameter:
+
+```cpp
+auto factory = entt::meta<my_type>();
+```
+
+This isn't enough to _export_ the given type and make it visible though.<br/>
+The returned value is a factory object to use to continue building the meta
+type. In order to make the type _visible_, users can assign it an identifier:
+
+```cpp
+auto factory = entt::meta<my_type>().type("reflected_type"_hs);
+```
+
+Or use the default one, that is, the built-in identifier for the given type:
+
+```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 returns the factory itself or
+a decorated version of it. This object can be used to add the following:
+
+* _Constructors_. Actual constructors can be assigned to a reflected type by
+  specifying their list of arguments. Free functions (namely, factories) can be
+  used as well, as long as the return type is the expected one. From a client's
+  point of view, nothing changes if a constructor is a free function or an
+  actual constructor.<br/>
+  Use the `ctor` member function for this purpose:
+
+  ```cpp
+  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/>
+  Use the `dtor` member function for this purpose:
+
+  ```cpp
+  entt::meta<my_type>().dtor<&destroy>();
+  ```
+
+  A function should neither delete nor explicitly invoke the destructor of a
+  given instance.
+
+* _Data members_. Both real data members of the underlying type and static and
+  global variables, as well as constants of any kind, can be attached to a meta
+  type. From 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/>
+  Use the `data` member function for this purpose:
+
+  ```cpp
+  entt::meta<my_type>()
+      .data<&my_type::static_variable>("static"_hs)
+      .data<&my_type::data_member>("member"_hs)
+      .data<&global_variable>("global"_hs);
+  ```
+
+  This function requires as an argument the identifier to give to the meta data
+  once created. Users can then access meta data at runtime by searching for them
+  by _name_.<br/>
+  Data members can also be defined by means of a _setter_ and _getter_. 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);
+  ```
+
+  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/>
+  Use the `func` member function for this purpose:
+
+  ```cpp
+  entt::meta<my_type>()
+      .func<&my_type::static_function>("static"_hs)
+      .func<&my_type::member_function>("member"_hs)
+      .func<&free_function>("free"_hs);
+  ```
+
+  This 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_.
+
+* _Base classes_. A base class is such that the underlying type is actually
+  derived from it. In this case, the reflection system tracks the relationship
+  and allows for implicit casts at runtime when required.<br/>
+  Use the `base` member function for this purpose:
+
+  ```cpp
+  entt::meta<derived_type>().base<base_type>();
+  ```
+
+  From now on, wherever a `base_type` is required, an instance of `derived_type`
+  will also be accepted.
+
+* _Conversion functions_. Actual types can be converted, this is a fact. Just
+  think of the relationship between a `double` and an `int` to see it. Similar
+  to bases, conversion functions allow users to define conversions that will be
+  implicitly performed by the reflection system when required.<br/>
+  Use the `conv` member function for this purpose:
+
+  ```cpp
+  entt::meta<double>().conv<int>();
+  ```
+
+That's all, everything users need to create meta types and enjoy the reflection
+system. At first glance it may not seem that much, but users usually learn to
+appreciate it over time.<br/>
+Also, do not forget what these few lines hide under the hood: a built-in,
+non-intrusive and macro-free system for reflection in C++. Features that are
+definitely worth the price, at least for me.
+
+## Any as in any type
+
+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 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:
+
+```cpp
+// a container for an int
+entt::meta_any any{0};
+
+// an empty container
+entt::meta_any empty{};
+```
+
+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 non-const 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, it's possible to create non-owning copies of `meta_any` from existing
+ones:
+
+```cpp
+// aliasing constructor
+entt::meta_any ref = any.ref();
+```
+
+In this case, it doesn't matter if the starting 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/>
+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 also has 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.
+
+## 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.
+
+To search for a reflected type there are a few options:
+
+```cpp
+// direct access to a reflected type
+auto by_type = entt::resolve<my_type>();
+
+// lookup of a reflected type by identifier
+auto by_id = entt::resolve_id("reflected_type"_hs);
+
+// lookup of a reflected type by type id
+auto by_type_id = entt::resolve_type(entt::type_info<my_type>::id());
+```
+
+There exits also an overload of the `resolve` function to use to iterate all the
+reflected types at once as well as a `resolve_if` function to use to perform
+more refined searches when needed:
+
+```cpp
+resolve([](auto type) {
+    // ...
+});
+
+auto by_lookup = resolve_if([](auto type) { return type.is_floating_point(); });
+```
+
+In all cases, the returned value is an instance of `meta_type`. This kind of
+objects offer an API to know their _runtime identifiers_, to iterate all the
+meta objects associated with them and even to build instances of the underlying
+type.<br/>
+Refer to the inline documentation for all the details.
+
+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
+  auto data = entt::resolve<my_type>().data("member"_hs);
+  ```
+
+  The returned type is `meta_data` and may be invalid if there is no meta data
+  object associated with the given identifier.<br/>
+  A meta data object offers an API to query the underlying type (for example, to
+  know if it's a const or a static one), to get the meta type of the variable
+  and to set or get the contained value.
+
+* _Meta functions_. They are accessed by _name_:
+
+  ```cpp
+  auto func = entt::resolve<my_type>().func("member"_hs);
+  ```
+
+  The returned type is `meta_func` and may be invalid if there is no meta
+  function object associated with the given identifier.<br/>
+  A meta function object offers an API to query the underlying type (for
+  example, to know if it's a const or a static function), to know the number of
+  arguments, the meta return type and the meta types of the parameters. In
+  addition, a meta function object can be used to invoke the underlying function
+  and then get the return value in the form of a `meta_any` object.
+
+* _Meta bases_. They are accessed through the _name_ of the base types:
+
+  ```cpp
+  auto base = entt::resolve<derived_type>().base("base"_hs);
+  ```
+
+  The returned type is `meta_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.
+
+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:
+
+```cpp
+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:
+
+```cpp
+entt::resolve<my_type>().data([](auto data) {
+    // ...
+});
+```
+
+A meta type can be used to `construct` actual instances of the underlying
+type.<br/>
+In particular, the `construct` member function accepts a variable number of
+arguments and searches for a match. It then returns a `meta_any` object that may
+or may not be initialized, depending on whether a suitable constructor has been
+found or not.
+
+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.
+
+Meta types and meta objects in general contain much more than what is said: a
+plethora of functions in addition to those listed whose purposes and uses go
+unfortunately beyond the scope of this document.<br/>
+I invite anyone interested in the subject to look at the code, experiment and
+read the inline documentation to get the best out of this powerful tool.
+
+## Container support
+
+The meta module supports containers of all types out of the box.<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 by the meta module, 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{std::ref(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.handle(), 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 deliberately minimal but theoretically 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`, 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::uniqe_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 containers 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_meta_pointer_like()) {
+    // meta type equivalent to that of int
+    if(entt::meta_any ref = *any; ref) {
+        // ...
+    }
+}
+```
+
+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 returned
+object is valid. For example, invalid instances are returned when the wrapped
+object hasn'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.
+
+## Policies: the more, the less
+
+Policies are a kind of compile-time directives that can be used when recording
+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
+returned wrapped in a `meta_any` object which, by default, makes a copy of it.
+For large objects or if the caller wants to access the original instance, this
+behavior isn't desirable. Policies are there to offer a solution to this and
+other problems.
+
+There are a few alternatives available at the moment:
+
+* The _as-is_ policy, associated with the type `entt::as_is_t`.<br/>
+  This is the default policy. In general, it should never be used explicitly,
+  since it's implicitly selected if no other policy is specified.<br/>
+  In this case, the return values of the functions as well as the properties
+  exposed as data members are always returned by copy in a dedicated wrapper and
+  therefore associated with their original meta types.
+
+* 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:
+
+  ```cpp
+  entt::meta<my_type>().func<&my_type::member_function, entt::as_void_t>("member"_hs);
+  ```
+
+* The _as-ref_ policy, associated with the type `entt::as_ref_t`.<br/>
+  It allows to build wrappers that act as references to unmanaged objects.
+  Modifying the object contained in the wrapper for which the _reference_ 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).
+
+  As an example of use:
+
+  ```cpp
+  entt::meta<my_type>().data<&my_type::data_member, entt::as_ref_t>("member"_hs);
+  ```
+
+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.
+
+## Named constants and enums
+
+A special mention should be made for constant values and enums. It wouldn't be
+necessary, but it will help distracted readers.
+
+As mentioned, the `data` member function can be used to reflect constants of any
+type among the other things.<br/>
+This allows users to create meta types for enums that will work exactly like any
+other meta type built from a class. Similarly, arithmetic types can be enriched
+with constants of special meaning where required.<br/>
+Personally, I find it very useful not to export what is the difference between
+enums and classes in C++ directly in the space of the reflected types.
+
+All the values thus exported will appear to users as if they were constant data
+members of the reflected types.
+
+Exporting constant values or elements from an enum is as simple as ever:
+
+```cpp
+entt::meta<my_enum>()
+        .data<my_enum::a_value>("a_value"_hs)
+        .data<my_enum::another_value>("another_value"_hs);
+
+entt::meta<int>().data<2048>("max_int"_hs);
+```
+
+It goes without saying that accessing them is trivial as well. It's a matter of
+doing the following, as with any other data member of a meta type:
+
+```cpp
+auto value = entt::resolve<my_enum>().data("a_value"_hs).get({}).cast<my_enum>();
+auto max = entt::resolve<int>().data("max_int"_hs).get({}).cast<int>();
+```
+
+As a side note, remember that all this happens behind the scenes without any
+allocation because of the small object optimization performed by the `meta_any`
+class.
+
+## Properties and meta objects
+
+Sometimes (for example, when it comes to creating an editor) it might be useful
+to attach properties to the meta objects created. Fortunately, this is possible
+for most of them.<br/>
+For the meta objects that support properties, the member functions of the
+factory used for registering them will return a decorated version of the factory
+itself. The latter can be used to attach properties to the last created meta
+object.<br/>
+Apparently, it's more difficult to say than to do:
+
+```cpp
+entt::meta<my_type>().type("reflected_type"_hs).prop("tooltip"_hs, "message");
+```
+
+Properties are always in the key/value form. There are no restrictions on the
+type of the key or value, as long as they are copy constructible objects.<br/>
+Multiple formats are supported when it comes to defining a property:
+
+* Properties as key/value pairs:
+
+  ```cpp
+  entt::meta<my_type>().type("reflected_type"_hs).prop("tooltip"_hs, "message");
+  ```
+
+* Properties as `std::pair`s:
+
+  ```cpp
+  entt::meta<my_type>().type("reflected_type"_hs).prop(std::make_pair("tooltip"_hs, "message"));
+  ```
+
+* Key only properties:
+
+  ```cpp
+  entt::meta<my_type>().type("reflected_type"_hs).prop(my_enum::key_only);
+  ```
+
+* Properties as `std::tuple`s:
+
+  ```cpp
+  entt::meta<my_type>().type("reflected_type"_hs).prop(std::make_tuple(std::make_pair("tooltip"_hs, "message"), my_enum::key_only));
+  ```
+
+  A tuple contains one or more properties. All of them are treated individually.
+
+* Annotations:
+
+  ```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
+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) {
+    // ...
+});
+
+// search for a given property by name
+auto prop = entt::resolve<my_type>().prop("tooltip"_hs);
+```
+
+Meta properties are objects having a fairly poor interface, all in all. They
+only provide the `key` and the `value` member functions to be used to retrieve
+the key and the value contained in the form of `meta_any` objects, respectively.
+
+## Unregister types
+
+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
+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/>
+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::resolve<my_type>().reset();
+```
+
+The type can be re-registered later with a completely different name and form.

docs/md/process.md

@@ -0,0 +1,212 @@
+# Crash Course: cooperative scheduler
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [The process](#the-process)
+  * [Adaptor](#adaptor)
+* [The scheduler](#the-scheduler)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+Sometimes processes are a useful tool to work around the strict definition of a
+system and introduce logic in a different way, usually without resorting to the
+introduction of other components.
+
+`EnTT` offers a minimal support to this paradigm by introducing a few classes
+that users can use to define and execute cooperative processes.
+
+# The process
+
+A typical process must inherit from the `process` class template that stays true
+to the CRTP idiom. Moreover, derived classes must specify what's the intended
+type for elapsed times.
+
+A 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):
+
+* `void update(Delta, void *);`
+
+  It's invoked once per tick until a process is explicitly aborted or it
+  terminates either with or without errors. Even though it's not mandatory to
+  declare this member function, as a rule of thumb each process should at
+  least define it to work properly. The `void *` parameter is an opaque pointer
+  to user data (if any) forwarded directly to the process during an update.
+
+* `void init();`
+
+  It's invoked when the process joins the running queue of a scheduler. This
+  happens as soon as it's attached to the scheduler if the process is a top
+  level one, otherwise when it replaces its parent if the process is a
+  continuation.
+
+* `void succeeded();`
+
+  It's invoked in case of success, immediately after an update and during the
+  same tick.
+
+* `void failed();`
+
+  It's invoked in case of errors, immediately after an update and during the
+  same tick.
+
+* `void aborted();`
+
+  It's invoked only if a process is explicitly aborted. There is no guarantee
+  that it executes in the same tick, this depends solely on whether the
+  process is aborted immediately or not.
+
+Derived classes can also change the internal state of a process by invoking
+`succeed` and `fail`, as well as `pause` and `unpause` the process itself. All
+these are protected member functions made available to be able to manage the
+life cycle of a process from a derived class.
+
+Here is a minimal example for the sake of curiosity:
+
+```cpp
+struct my_process: entt::process<my_process, std::uint32_t> {
+    using delta_type = std::uint32_t;
+
+    my_process(delta_type delay)
+        : remaining{delay}
+    {}
+
+    void update(delta_type delta, void *) {
+        remaining -= std::min(remaining, delta);
+
+        // ...
+
+        if(!remaining) {
+            succeed();
+        }
+    }
+
+private:
+    delta_type remaining;
+};
+```
+
+## Adaptor
+
+Lambdas and functors can't be used directly with a scheduler for they are not
+properly defined processes with managed life cycles.<br/>
+This class helps in filling the gap and turning lambdas and functors into
+full featured processes usable by a scheduler.
+
+The function call operator has a signature similar to the one of the `update`
+function of a process but for the fact that it receives two extra arguments to
+call whenever a process is terminated with success or with an error:
+
+```cpp
+void(Delta delta, void *data, auto succeed, auto fail);
+```
+
+Parameters have the following meaning:
+
+* `delta` is the elapsed time.
+* `data` is an opaque pointer to user data if any, `nullptr` otherwise.
+* `succeed` is a function to call when a process terminates with success.
+* `fail` is a function to call when a process terminates with errors.
+
+Both `succeed` and `fail` accept no parameters at all.
+
+Note that usually users shouldn't worry about creating adaptors at all. A
+scheduler creates them internally each and every time a lambda or a functor is
+used as a process.
+
+# The scheduler
+
+A cooperative scheduler runs different processes and helps managing their life
+cycles.
+
+Each process is invoked once per tick. If it terminates, it's removed
+automatically from the scheduler and it's never invoked again. Otherwise it's
+a good candidate to run one more time the next tick.<br/>
+A process can also have a child. In this case, the parent process is replaced
+with its child when it terminates and only if it returns with success. In case
+of errors, both the parent process and its child are discarded. This way, it's
+easy to create chain of processes to run sequentially.
+
+Using a scheduler is straightforward. To create it, users must provide only the
+type for the elapsed times and no arguments at all:
+
+```cpp
+entt::scheduler<std::uint32_t> scheduler;
+```
+
+It has member functions to query its internal data structures, like `empty` or
+`size`, as well as a `clear` utility to reset it to a clean state:
+
+```cpp
+// checks if there are processes still running
+const auto empty = scheduler.empty();
+
+// gets the number of processes still running
+entt::scheduler<std::uint32_t>::size_type size = scheduler.size();
+
+// resets the scheduler to its initial state and discards all the processes
+scheduler.clear();
+```
+
+To attach a process to a scheduler there are mainly two ways:
+
+* If the process inherits from the `process` class template, it's enough to
+  indicate its type and submit all the parameters required to construct it to
+  the `attach` member function:
+
+  ```cpp
+  scheduler.attach<my_process>(1000u);
+  ```
+
+* Otherwise, in case of a lambda or a functor, it's enough to provide an
+  instance of the class to the `attach` member function:
+
+  ```cpp
+  scheduler.attach([](auto...){ /* ... */ });
+  ```
+
+In both cases, the return value is an opaque object that offers a `then` member
+function to use to create chains of processes to run sequentially.<br/>
+As a minimal example of use:
+
+```cpp
+// schedules a task in the form of a lambda function
+scheduler.attach([](auto delta, void *, auto succeed, auto fail) {
+    // ...
+})
+// appends a child in the form of another lambda function
+.then([](auto delta, void *, auto succeed, auto fail) {
+    // ...
+})
+// appends a child in the form of a process class
+.then<my_process>(1000u);
+```
+
+To update a scheduler and therefore all its processes, the `update` member
+function is the way to go:
+
+```cpp
+// updates all the processes, no user data are provided
+scheduler.update(delta);
+
+// updates all the processes and provides them with custom data
+scheduler.update(delta, &data);
+```
+
+In addition to these functions, the scheduler offers an `abort` member function
+that can be used to discard all the running processes at once:
+
+```cpp
+// aborts all the processes abruptly ...
+scheduler.abort(true);
+
+// ... or gracefully during the next tick
+scheduler.abort();
+```

docs/md/references.md

@@ -0,0 +1,46 @@
+# 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:
+  * [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.
+
+* Go:
+  * [gecs](https://github.com/tutumagi/gecs): a sparse sets based ECS inspired 
+    by `EnTT`.
+
+* Javascript:
+  * [ecsy](https://github.com/MozillaReality/ecsy): I haven't had the time to
+    investigate the underlying design of `ecsy` but it looks cool anyway.
+
+* 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.
+
+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

@@ -0,0 +1,231 @@
+# Crash Course: resource management
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [The resource, the loader and the cache](#the-resource-the-loader-and-the-cache)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+Resource management is usually one of the most critical part of a software like
+a game. Solutions are often tuned to the particular application. There exist
+several approaches and all of them are perfectly fine as long as they fit the
+requirements of the piece of software in which they are used.<br/>
+Examples are loading everything on start, loading on request, predictive
+loading, and so on.
+
+`EnTT` doesn't pretend to offer a _one-fits-all_ solution for the different
+cases. Instead, it offers a minimal and perhaps trivial cache that can be useful
+most of the time during prototyping and sometimes even in a production
+environment.<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.
+
+# The resource, the loader and the cache
+
+There are three main actors in the model: the resource, the loader and the
+cache.
+
+The _resource_ is whatever users want it to be. An image, a video, an audio,
+whatever. There are no limits.<br/>
+As a minimal example:
+
+```cpp
+struct my_resource { const int value; };
+```
+
+A _loader_ is a class the aim of which is to load a specific resource. It has to
+inherit directly from a dedicated base class as in the following example:
+
+```cpp
+struct my_loader final: entt::resource_loader<my_loader, my_resource> {
+    // ...
+};
+```
+
+Where `my_resource` is the type of resources it creates.<br/>
+A resource loader must also expose a public const member function named `load`
+that accepts a variable number of arguments and returns a shared pointer to a
+resource.<br/>
+As an example:
+
+```cpp
+struct my_loader: entt::resource_loader<my_loader, my_resource> {
+    std::shared_ptr<my_resource> load(int value) const {
+        // ...
+        return std::shared_ptr<my_resource>(new my_resource{ value });
+    }
+};
+```
+
+In general, resource loaders should not have a state or retain data of any type.
+They should let the cache manage their resources instead.<br/>
+As a side note, base class and CRTP idiom aren't strictly required with the
+current implementation. One could argue that a cache can easily work with
+loaders of any type. However, future changes won't be breaking ones by forcing
+the use of a base class today and that's why the model is already in its place.
+
+Finally, a cache is a specialization of a class template tailored to a specific
+resource:
+
+```cpp
+using my_cache = entt::resource_cache<my_resource>;
+
+// ...
+
+my_cache cache{};
+```
+
+The idea is to create different caches for different types of resources and to
+manage each one independently in the most appropriate way.<br/>
+As a (very) trivial example, audio tracks can survive in most of the scenes of
+an application while meshes can be associated with a single scene and then
+discarded when users leave it.
+
+A cache offers a set of basic functionalities to query its internal state and to
+_organize_ it:
+
+```cpp
+// gets the number of resources managed by a cache
+const auto size = cache.size();
+
+// checks if a cache contains at least a valid resource
+const auto empty = cache.empty();
+
+// clears a cache and discards its content
+cache.clear();
+```
+
+Besides these member functions, a cache contains what is needed to load, use and
+discard resources of the given type.<br/>
+Before exploring this part of the interface, it makes sense to mention how
+resources are identified. They have type `id_type` and therefore they can be
+created explicitly as in the following example:
+
+```cpp
+constexpr auto identifier = "my/resource/identifier"_hs;
+// this is equivalent to the following
+constexpr entt::id_type hs = entt::hashed_string{"my/resource/identifier"};
+```
+
+The class `hashed_string` is described in a dedicated section, so I won't go in
+details here.
+
+Resources are loaded and thus stored in a cache through the `load` member
+function. It accepts the loader to use as a template parameter, the resource
+identifier and the parameters used to construct the resource as arguments:
+
+```cpp
+// uses the identifier declared above
+cache.load<my_loader>(identifier, 0);
+
+// uses a const char * directly as an identifier
+cache.load<my_loader>("another/identifier"_hs, 42);
+```
+
+The function returns a handle to the resource, whether it already exists or is
+loaded. In case the loader returns an invalid pointer, the handle is invalid as
+well and therefore it can be easily used with an `if` statement:
+
+```cpp
+if(entt::resource_handle handle = cache.load<my_loader>("another/identifier"_hs, 42); handle) {
+    // ...
+}
+```
+
+Before trying to load a resource, the `contains` member function can be used to
+know if a cache already contains a specific resource:
+
+```cpp
+auto exists = cache.contains("my/identifier"_hs);
+```
+
+There exists also a member function to use to force a reload of an already
+existing resource if needed:
+
+```cpp
+auto handle = cache.reload<my_loader>("another/identifier"_hs, 42);
+```
+
+As above, the function returns a handle to the resource that is invalid in case
+of errors. The `reload` member function is a kind of alias of the following
+snippet:
+
+```cpp
+cache.discard(identifier);
+cache.load<my_loader>(identifier, 42);
+```
+
+Where the `discard` member function is used to get rid of a resource if loaded.
+In case the cache doesn't contain a resource for the given identifier, `discard`
+does nothing and returns immediately.
+
+So far, so good. Resources are finally loaded and stored within the cache.<br/>
+They are returned to users in the form of handles. To get one of them later on:
+
+```cpp
+auto handle = cache.handle("my/identifier"_hs);
+```
+
+The idea behind a handle is the same of the flyweight pattern. In other terms,
+resources aren't copied around. Instead, instances are shared between handles.
+Users of a resource own a handle that guarantees that a resource isn't destroyed
+until all the handles are destroyed, even if the resource itself is removed from
+the cache.<br/>
+Handles are tiny objects both movable and copyable. They return the contained
+resource as a (possibly const) reference on request:
+
+* By means of the `get` member function:
+
+  ```cpp
+  auto &resource = handle.get();
+  ```
+
+* Using the proper cast operator:
+
+  ```cpp
+  auto &resource = handle;
+  ```
+
+* Through the dereference operator:
+
+  ```cpp
+  auto &resource = *handle;
+  ```
+
+The resource can also be accessed directly using the arrow operator if required:
+
+```cpp
+auto value = handle->value;
+```
+
+To test if a handle is still valid, the cast operator to `bool` allows users to
+use it in a guard:
+
+```cpp
+if(handle) {
+    // ...
+}
+```
+
+Finally, in case there is the need to load a resource and thus to get a handle
+without storing the resource itself in the cache, users can rely on the `temp`
+member function template.<br/>
+The declaration is similar to that of `load`, a (possibly invalid) handle for
+the resource is returned also in this case:
+
+```cpp
+if(auto handle = cache.temp<my_loader>(42); handle) {
+    // ...
+}
+```
+
+Do not forget to test the handle for validity. Otherwise, getting a reference to
+the resource it points may result in undefined behavior.

docs/md/signal.md

@@ -0,0 +1,581 @@
+# Crash Course: events, signals and everything in between
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [Delegate](#delegate)
+  * [Runtime arguments](#runtime-arguments)
+  * [Lambda support](#lambda-support)
+* [Signals](#signals)
+* [Event dispatcher](#event-dispatcher)
+* [Event emitter](#event-emitter)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+Signals are usually a core part of games and software architectures in
+general.<br/>
+Roughly speaking, they help to decouple the various parts of a system while
+allowing them to communicate with each other somehow.
+
+The so called _modern C++_ comes with a tool that can be useful in these terms,
+the `std::function`. As an example, it can be used to create delegates.<br/>
+However, there is no guarantee that an `std::function` does not perform
+allocations under the hood and this could be problematic sometimes. Furthermore,
+it solves a problem but may not adapt well to other requirements that may arise
+from time to time.
+
+In case that the flexibility and power of an `std::function` isn't required or
+if the price to pay for them is too high,` EnTT` offers a complete set of
+lightweight classes to solve the same and many other problems.
+
+# Delegate
+
+A delegate can be used as a general purpose invoker with no memory overhead for
+free functions and members provided along with an instance on which to invoke
+them.<br/>
+It doesn't claim to be a drop-in replacement for an `std::function`, so don't
+expect to use it whenever an `std::function` fits well. That said, it's most
+likely even a better fit than an `std::function` in a lot of cases, so expect to
+use it quite a lot anyway.
+
+The interface is trivial. It offers a default constructor to create empty
+delegates:
+
+```cpp
+entt::delegate<int(int)> delegate{};
+```
+
+All what is needed to create an instance is to specify the type of the function
+the delegate will _contain_, that is the signature of the free function or the
+member one wants to assign to it.
+
+Attempting to use an empty delegate by invoking its function call operator
+results in undefined behavior or most likely a crash. Before to use a delegate,
+it must be initialized.<br/>
+There exists a bunch of overloads of the `connect` member function to do that.
+As an example of use:
+
+```cpp
+int f(int i) { return i; }
+
+struct my_struct {
+    int f(const int &i) const { return i; }
+};
+
+// bind a free function to the delegate
+delegate.connect<&f>();
+
+// bind a member function to the delegate
+my_struct instance;
+delegate.connect<&my_struct::f>(instance);
+```
+
+The delegate class accepts also data members, if needed. In this case, the
+function type of the delegate is such that the parameter list is empty and the
+value of the data member is at least convertible to the return type.
+
+Free functions having type equivalent to `void(T &, args...)` are accepted as
+well. The first argument `T &` is considered a payload and the function will
+receive it back every time it's invoked. In other terms, this works just fine
+with the above definition:
+
+```cpp
+void g(const char &c, int i) { /* ... */ }
+const char c = 'c';
+
+delegate.connect<&g>(c);
+delegate(42);
+```
+
+The function `g` will be invoked with a reference to `c` and `42`. However, the
+function type of the delegate is still `void(int)`. This is also the signature
+of its function call operator.
+
+Another interesting aspect of the delegate class is that it accepts also
+functions with a list of parameters that is shorter than that of the function
+type used to specialize the delegate itself.<br/>
+The following code is therefore perfectly valid:
+
+```cpp
+void g() { /* ... */ }
+delegate.connect<&g>();
+delegate(42);
+```
+
+Where the function type of the delegate is `void(int)` as above. It goes without
+saying that the extra arguments are silently discarded internally.<br/>
+This is a nice-to-have feature in a lot of cases, as an example when the
+`delegate` class is used as a building block of a signal-slot system.
+
+To create and initialize a delegate at once, there are a few specialized
+constructors. Because of the rules of the language, the listener is provided by
+means of the `entt::connect_arg` variable template:
+
+```cpp
+entt::delegate<int(int)> func{entt::connect_arg<&f>};
+```
+
+Aside `connect`, a `disconnect` counterpart isn't provided. Instead, there
+exists a `reset` member function to use to clear a delegate.<br/>
+To know if a delegate is empty, it can be used explicitly in every conditional
+statement:
+
+```cpp
+if(delegate) {
+    // ...
+}
+```
+
+Finally, to invoke a delegate, the function call operator is the way to go as
+already shown in the examples above:
+
+```cpp
+auto ret = delegate(42);
+```
+
+In all cases, the listeners don't have to strictly follow the signature of the
+delegate. As long as a listener can be invoked with the given arguments to yield
+a result that is convertible to the given result type, everything works just
+fine.
+
+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:
+
+```
+entt::delegate<void(my_struct &, int)> delegate;
+delegate.connect<&my_struct::f>();
+
+my_struct instance;
+delegate(instance, 42);
+```
+
+In this case, it's not possible to deduce the function type since the first
+argument doesn't necessarily have to be a reference (for example, it can be a
+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
+to members. Listeners can be any kind of objects and users are in charge of
+connecting and disconnecting them from a signal to avoid crashes due to
+different lifetimes. On the other side, performance shouldn't be affected that
+much by the presence of such a signal handler.<br/>
+Signals make use of delegates internally and therefore they undergo the same
+rules and offer similar functionalities. It may be a good idea to consult the
+documentation of the `delegate` class for further information.
+
+A signal handler can be used as a private data member without exposing any
+_publish_ functionality to the clients of a class. The basic idea is to impose a
+clear separation between the signal itself and the `sink` class, that is a tool
+to be used to connect and disconnect listeners on the fly.
+
+The API of a signal handler is straightforward. If a collector is supplied to
+the signal when something is published, all the values returned by the listeners
+can be literally _collected_ and used later by the caller. Otherwise, the class
+works just like a plain signal that emits events from time to time.<br/>
+To create instances of signal handlers it is sufficient to provide the type of
+function to which they refer:
+
+```cpp
+entt::sigh<void(int, char)> signal;
+```
+
+Signals offer all the basic functionalities required to know how many listeners
+they contain (`size`) or if they contain at least a listener (`empty`), as well
+as a function to use to swap handlers (`swap`).
+
+Besides them, there are member functions to use both to connect and disconnect
+listeners in all their forms by means of a sink:
+
+```cpp
+void foo(int, char) { /* ... */ }
+
+struct listener {
+    void bar(const int &, char) { /* ... */ }
+};
+
+// ...
+
+entt::sink sink{signal};
+listener instance;
+
+sink.connect<&foo>();
+sink.connect<&listener::bar>(instance);
+
+// ...
+
+// disconnects a free function
+sink.disconnect<&foo>();
+
+// disconnect a member function of an instance
+sink.disconnect<&listener::bar>(instance);
+
+// disconnect all member functions of an instance, if any
+sink.disconnect(instance);
+
+// discards all listeners at once
+sink.disconnect();
+```
+
+As shown above, the listeners don't have to strictly follow the signature of the
+signal. As long as a listener can be invoked with the given arguments to yield a
+result that is convertible to the given return type, everything works just
+fine.<br/>
+It's also possible to connect a listener before other listeners already
+contained by the signal. The `before` function returns a `sink` object correctly
+initialized for the purpose that can be used to connect one or more listeners in
+order and in the desired position:
+
+```cpp
+sink.before<&foo>().connect<&listener::bar>(instance);
+```
+
+In all cases, the `connect` member function returns by default a `connection`
+object to be used as an alternative to break a connection by means of its
+`release` member function. A `scoped_connection` can also be created from a
+connection. In this case, the link is broken automatically as soon as the object
+goes out of scope.
+
+Once listeners are attached (or even if there are no listeners at all), events
+and data in general can be published through a signal by means of the `publish`
+member function:
+
+```cpp
+signal.publish(42, 'c');
+```
+
+To collect data, the `collect` member function should be used instead. Below is
+a minimal example to show how to use it:
+
+```cpp
+int f() { return 0; }
+int g() { return 1; }
+
+// ...
+
+entt::sigh<int()> signal;
+entt::sink sink{signal};
+
+sink.connect<&f>();
+sink.connect<&g>();
+
+std::vector<int> vec{};
+signal.collect([&vec](int value) { vec.push_back(value); });
+
+assert(vec[0] == 0);
+assert(vec[1] == 1);
+```
+
+A collector must expose a function operator that accepts as an argument a type
+to which the return type of the listeners can be converted. Moreover, it can
+optionally return a boolean value that is true to stop collecting data, false
+otherwise. This way one can avoid calling all the listeners in case it isn't
+necessary.<br/>
+Functors can also be used in place of a lambda. Since the collector is copied
+when invoking the `collect` member function, `std::ref` is the way to go in this
+case:
+
+```cpp
+struct my_collector {
+    std::vector<int> vec{};
+
+    bool operator()(int v) noexcept {
+        vec.push_back(v);
+        return true;
+    }
+};
+
+// ...
+
+my_collector collector;
+signal.collect(std::ref(collector));
+```
+
+# Event dispatcher
+
+The event dispatcher class is designed so as to be used in a loop. It allows
+users both to trigger immediate events or to queue events to be published all
+together once per tick.<br/>
+This class shares part of its API with the one of the signal handler, but it
+doesn't require that all the types of events are specified when declared:
+
+```cpp
+// define a general purpose dispatcher
+entt::dispatcher dispatcher{};
+```
+
+In order to register an instance of a class to a dispatcher, its type must
+expose one or more member functions the arguments of which are such that `E &`
+can be converted to them for each type of event `E`, no matter what the return
+value is.<br/>
+The name of the member function aimed to receive the event must be provided to
+the `connect` member function of the sink in charge for the specific event:
+
+```cpp
+struct an_event { int value; };
+struct another_event {};
+
+struct listener {
+    void receive(const an_event &) { /* ... */ }
+    void method(const another_event &) { /* ... */ }
+};
+
+// ...
+
+listener listener;
+dispatcher.sink<an_event>().connect<&listener::receive>(listener);
+dispatcher.sink<another_event>().connect<&listener::method>(listener);
+```
+
+The `disconnect` member function follows the same pattern and can be used to
+remove one listener at a time or all of them at once:
+
+```cpp
+dispatcher.sink<an_event>().disconnect<&listener::receive>(listener);
+dispatcher.sink<another_event>().disconnect(listener);
+```
+
+The `trigger` member function serves the purpose of sending an immediate event
+to all the listeners registered so far. It offers a convenient approach that
+relieves users from having to create the event itself. Instead, it's enough to
+specify the type of event and provide all the parameters required to construct
+it.<br/>
+As an example:
+
+```cpp
+dispatcher.trigger<an_event>(42);
+dispatcher.trigger<another_event>();
+```
+
+Listeners are invoked immediately, order of execution isn't guaranteed. This
+method can be used to push around urgent messages like an _is terminating_
+notification on a mobile app.
+
+On the other hand, the `enqueue` member function queues messages together and
+allows to maintain control over the moment they are sent to listeners. The
+signature of this method is more or less the same of `trigger`:
+
+```cpp
+dispatcher.enqueue<an_event>(42);
+dispatcher.enqueue<another_event>();
+```
+
+Events are stored aside until the `update` member function is invoked, then all
+the messages that are still pending are sent to the listeners at once:
+
+```cpp
+// emits all the events of the given type at once
+dispatcher.update<an_event>();
+
+// emits all the events queued so far at once
+dispatcher.update();
+```
+
+This way users can embed the dispatcher in a loop and literally dispatch events
+once per tick to their systems.
+
+# Event emitter
+
+A general purpose event emitter thought mainly for those cases where it comes to
+working with asynchronous stuff.<br/>
+Originally designed to fit the requirements of
+[`uvw`](https://github.com/skypjack/uvw) (a wrapper for `libuv` written in
+modern C++), it was adapted later to be included in this library.
+
+To create a custom emitter type, derived classes must inherit directly from the
+base class as:
+
+```cpp
+struct my_emitter: emitter<my_emitter> {
+    // ...
+}
+```
+
+The full list of accepted types of events isn't required. Handlers are created
+internally on the fly and thus each type of event is accepted by default.
+
+Whenever an event is published, an emitter provides the listeners with a
+reference to itself along with a reference to the event. Therefore listeners
+have an handy way to work with it without incurring in the need of capturing a
+reference to the emitter itself.<br/>
+In addition, an opaque object is returned each time a connection is established
+between an emitter and a listener, allowing the caller to disconnect them at a
+later time.<br/>
+The opaque object used to handle connections is both movable and copyable. On
+the other side, an event emitter is movable but not copyable by default.
+
+To create new instances of an emitter, no arguments are required:
+
+```cpp
+my_emitter emitter{};
+```
+
+Listeners must be movable and callable objects (free functions, lambdas,
+functors, `std::function`s, whatever) whose function type is compatible with:
+
+```cpp
+void(Event &, my_emitter &)
+```
+
+Where `Event` is the type of event they want to listen.<br/>
+There are two ways to attach a listener to an event emitter that differ
+slightly from each other:
+
+* To register a long-lived listener, use the `on` member function. It is meant
+  to register a listener designed to be invoked more than once for the given
+  event type.<br/>
+  As an example:
+
+  ```cpp
+  auto conn = emitter.on<my_event>([](const my_event &event, my_emitter &emitter) {
+      // ...
+  });
+  ```
+
+  The connection object can be freely discarded. Otherwise, it can be used later
+  to disconnect the listener if required.
+
+* To register a short-lived listener, use the `once` member function. It is
+  meant to register a listener designed to be invoked only once for the given
+  event type. The listener is automatically disconnected after the first
+  invocation.<br/>
+  As an example:
+
+  ```cpp
+  auto conn = emitter.once<my_event>([](const my_event &event, my_emitter &emitter) {
+      // ...
+  });
+  ```
+
+  The connection object can be freely discarded. Otherwise, it can be used later
+  to disconnect the listener if required.
+
+In both cases, the connection object can be used with the `erase` member
+function:
+
+```cpp
+emitter.erase(conn);
+```
+
+There are also two member functions to use either to disconnect all the
+listeners for a given type of event or to clear the emitter:
+
+```cpp
+// removes all the listener for the specific event
+emitter.clear<my_event>();
+
+// removes all the listeners registered so far
+emitter.clear();
+```
+
+To send an event to all the listeners that are interested in it, the `publish`
+member function offers a convenient approach that relieves users from having to
+create the event:
+
+```cpp
+struct my_event { int i; };
+
+// ...
+
+emitter.publish<my_event>(42);
+```
+
+Finally, the `empty` member function tests if there exists at least either a
+listener registered with the event emitter or to a given type of event:
+
+```cpp
+bool empty;
+
+// checks if there is any listener registered for the specific event
+empty = emitter.empty<my_event>();
+
+// checks it there are listeners registered with the event emitter
+empty = emitter.empty();
+```
+
+In general, the event emitter is a handy tool when the derived classes _wrap_
+asynchronous operations, because it introduces a _nice-to-have_ model based on
+events and listeners that kindly hides the complexity behind the scenes. However
+it is not limited to such uses.

scripts/amalgamate.py

@@ -0,0 +1,299 @@
+#!/usr/bin/env python
+# coding=utf-8
+
+# amalgamate.py - Amalgamate C source and header files.
+# Copyright (c) 2012, Erik Edlund <erik.edlund@32767.se>
+# 
+# Redistribution and use in source and binary forms, with or without modification,
+# are permitted provided that the following conditions are met:
+# 
+#  * Redistributions of source code must retain the above copyright notice,
+#  this list of conditions and the following disclaimer.
+# 
+#  * Redistributions in binary form must reproduce the above copyright notice,
+#  this list of conditions and the following disclaimer in the documentation
+#  and/or other materials provided with the distribution.
+# 
+#  * Neither the name of Erik Edlund, nor the names of its contributors may
+#  be used to endorse or promote products derived from this software without
+#  specific prior written permission.
+# 
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+# WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+# DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
+# ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+# (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+# LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
+# ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+# SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+
+import argparse
+import datetime
+import json
+import os
+import re
+
+
+class Amalgamation(object):
+
+    # Prepends self.source_path to file_path if needed.
+    def actual_path(self, file_path):
+        if not os.path.isabs(file_path):
+            file_path = os.path.join(self.source_path, file_path)
+        return file_path
+
+    # Search included file_path in self.include_paths and
+    # in source_dir if specified.
+    def find_included_file(self, file_path, source_dir):
+        search_dirs = self.include_paths[:]
+        if source_dir:
+            search_dirs.insert(0, source_dir)
+
+        for search_dir in search_dirs:
+            search_path = os.path.join(search_dir, file_path)
+            if os.path.isfile(self.actual_path(search_path)):
+                return search_path
+        return None
+
+    def __init__(self, args):
+        with open(args.config, 'r') as f:
+            config = json.loads(f.read())
+            for key in config:
+                setattr(self, key, config[key])
+
+            self.verbose = args.verbose == "yes"
+            self.prologue = args.prologue
+            self.source_path = args.source_path
+            self.included_files = []
+
+    # Generate the amalgamation and write it to the target file.
+    def generate(self):
+        amalgamation = ""
+
+        if self.prologue:
+            with open(self.prologue, 'r') as f:
+                amalgamation += datetime.datetime.now().strftime(f.read())
+
+        if self.verbose:
+            print("Config:")
+            print(" target        = {0}".format(self.target))
+            print(" working_dir   = {0}".format(os.getcwd()))
+            print(" include_paths = {0}".format(self.include_paths))
+        print("Creating amalgamation:")
+        for file_path in self.sources:
+            # Do not check the include paths while processing the source
+            # list, all given source paths must be correct.
+            # actual_path = self.actual_path(file_path)
+            print(" - processing \"{0}\"".format(file_path))
+            t = TranslationUnit(file_path, self, True)
+            amalgamation += t.content
+
+        with open(self.target, 'w') as f:
+            f.write(amalgamation)
+
+        print("...done!\n")
+        if self.verbose:
+            print("Files processed: {0}".format(self.sources))
+            print("Files included: {0}".format(self.included_files))
+        print("")
+
+
+def _is_within(match, matches):
+    for m in matches:
+        if match.start() > m.start() and \
+                match.end() < m.end():
+            return True
+    return False
+
+
+class TranslationUnit(object):
+    # // C++ comment.
+    cpp_comment_pattern = re.compile(r"//.*?\n")
+
+    # /* C comment. */
+    c_comment_pattern = re.compile(r"/\*.*?\*/", re.S)
+
+    # "complex \"stri\\\ng\" value".
+    string_pattern = re.compile("[^']" r'".*?(?<=[^\\])"', re.S)
+
+    # Handle simple include directives. Support for advanced
+    # directives where macros and defines needs to expanded is
+    # not a concern right now.
+    include_pattern = re.compile(
+        r'#\s*include\s+(<|")(?P<path>.*?)("|>)', re.S)
+
+    # #pragma once
+    pragma_once_pattern = re.compile(r'#\s*pragma\s+once', re.S)
+
+    # Search for pattern in self.content, add the match to
+    # contexts if found and update the index accordingly.
+    def _search_content(self, index, pattern, contexts):
+        match = pattern.search(self.content, index)
+        if match:
+            contexts.append(match)
+            return match.end()
+        return index + 2
+
+    # Return all the skippable contexts, i.e., comments and strings
+    def _find_skippable_contexts(self):
+        # Find contexts in the content in which a found include
+        # directive should not be processed.
+        skippable_contexts = []
+
+        # Walk through the content char by char, and try to grab
+        # skippable contexts using regular expressions when found.
+        i = 1
+        content_len = len(self.content)
+        while i < content_len:
+            j = i - 1
+            current = self.content[i]
+            previous = self.content[j]
+
+            if current == '"':
+                # String value.
+                i = self._search_content(j, self.string_pattern,
+                                         skippable_contexts)
+            elif current == '*' and previous == '/':
+                # C style comment.
+                i = self._search_content(j, self.c_comment_pattern,
+                                         skippable_contexts)
+            elif current == '/' and previous == '/':
+                # C++ style comment.
+                i = self._search_content(j, self.cpp_comment_pattern,
+                                         skippable_contexts)
+            else:
+                # Skip to the next char.
+                i += 1
+
+        return skippable_contexts
+
+    # Returns True if the match is within list of other matches
+
+    # Removes pragma once from content
+    def _process_pragma_once(self):
+        content_len = len(self.content)
+        if content_len < len("#include <x>"):
+            return 0
+
+        # Find contexts in the content in which a found include
+        # directive should not be processed.
+        skippable_contexts = self._find_skippable_contexts()
+
+        pragmas = []
+        pragma_once_match = self.pragma_once_pattern.search(self.content)
+        while pragma_once_match:
+            if not _is_within(pragma_once_match, skippable_contexts):
+                pragmas.append(pragma_once_match)
+
+            pragma_once_match = self.pragma_once_pattern.search(self.content,
+                                                                pragma_once_match.end())
+
+        # Handle all collected pragma once directives.
+        prev_end = 0
+        tmp_content = ''
+        for pragma_match in pragmas:
+            tmp_content += self.content[prev_end:pragma_match.start()]
+            prev_end = pragma_match.end()
+        tmp_content += self.content[prev_end:]
+        self.content = tmp_content
+
+    # Include all trivial #include directives into self.content.
+    def _process_includes(self):
+        content_len = len(self.content)
+        if content_len < len("#include <x>"):
+            return 0
+
+        # Find contexts in the content in which a found include
+        # directive should not be processed.
+        skippable_contexts = self._find_skippable_contexts()
+
+        # Search for include directives in the content, collect those
+        # which should be included into the content.
+        includes = []
+        include_match = self.include_pattern.search(self.content)
+        while include_match:
+            if not _is_within(include_match, skippable_contexts):
+                include_path = include_match.group("path")
+                search_same_dir = include_match.group(1) == '"'
+                found_included_path = self.amalgamation.find_included_file(
+                    include_path, self.file_dir if search_same_dir else None)
+                if found_included_path:
+                    includes.append((include_match, found_included_path))
+
+            include_match = self.include_pattern.search(self.content,
+                                                        include_match.end())
+
+        # Handle all collected include directives.
+        prev_end = 0
+        tmp_content = ''
+        for include in includes:
+            include_match, found_included_path = include
+            tmp_content += self.content[prev_end:include_match.start()]
+            tmp_content += "// {0}\n".format(include_match.group(0))
+            if found_included_path not in self.amalgamation.included_files:
+                t = TranslationUnit(found_included_path, self.amalgamation, False)
+                tmp_content += t.content
+            prev_end = include_match.end()
+        tmp_content += self.content[prev_end:]
+        self.content = tmp_content
+
+        return len(includes)
+
+    # Make all content processing
+    def _process(self):
+        if not self.is_root:
+            self._process_pragma_once()
+        self._process_includes()
+
+    def __init__(self, file_path, amalgamation, is_root):
+        self.file_path = file_path
+        self.file_dir = os.path.dirname(file_path)
+        self.amalgamation = amalgamation
+        self.is_root = is_root
+
+        self.amalgamation.included_files.append(self.file_path)
+
+        actual_path = self.amalgamation.actual_path(file_path)
+        if not os.path.isfile(actual_path):
+            raise IOError("File not found: \"{0}\"".format(file_path))
+        with open(actual_path, 'r') as f:
+            self.content = f.read()
+            self._process()
+
+
+def main():
+    description = "Amalgamate C source and header files."
+    usage = " ".join([
+        "amalgamate.py",
+        "[-v]",
+        "-c path/to/config.json",
+        "-s path/to/source/dir",
+        "[-p path/to/prologue.(c|h)]"
+    ])
+    argsparser = argparse.ArgumentParser(
+        description=description, usage=usage)
+
+    argsparser.add_argument("-v", "--verbose", dest="verbose",
+                            choices=["yes", "no"], metavar="", help="be verbose")
+
+    argsparser.add_argument("-c", "--config", dest="config",
+                            required=True, metavar="", help="path to a JSON config file")
+
+    argsparser.add_argument("-s", "--source", dest="source_path",
+                            required=True, metavar="", help="source code path")
+
+    argsparser.add_argument("-p", "--prologue", dest="prologue",
+                            required=False, metavar="", help="path to a C prologue file")
+
+    amalgamation = Amalgamation(argsparser.parse_args())
+    amalgamation.generate()
+
+
+if __name__ == "__main__":
+    main()

scripts/config.json

@@ -0,0 +1,8 @@
+{
+	"project": "entt",
+	"target": "single_include/entt/entt.hpp",
+	"sources": [
+		"src/entt/entt.hpp"
+	],
+	"include_paths": ["src"]
+}

scripts/update_homebrew.sh

@@ -0,0 +1,60 @@
+#!/bin/sh
+
+# only argument should be the version to upgrade to
+if [ $# != 1 ]
+then
+  echo "Expected a version tag like v2.7.1"
+  exit 1
+fi
+
+VERSION="$1"
+URL="https://github.com/skypjack/entt/archive/$VERSION.tar.gz"
+FORMULA="entt.rb"
+
+echo "Updating homebrew package to $VERSION"
+
+echo "Cloning..."
+git clone https://github.com/skypjack/homebrew-entt.git
+if [ $? != 0 ]
+then
+  exit 1
+fi
+cd homebrew-entt
+
+# download the repo at the version
+# exit with error messages if curl fails
+echo "Curling..."
+curl "$URL" --location --fail --silent --show-error --output archive.tar.gz
+if [ $? != 0 ]
+then
+  exit 1
+fi
+
+# compute sha256 hash
+echo "Hashing..."
+HASH="$(openssl sha256 archive.tar.gz | cut -d " " -f 2)"
+
+# delete the archive
+rm archive.tar.gz
+
+echo "Sedding..."
+
+# change the url in the formula file
+# the slashes in the URL must be escaped
+ESCAPED_URL="$(echo "$URL" | sed -e 's/[\/&]/\\&/g')"
+sed -i -e '/url/s/".*"/"'$ESCAPED_URL'"/' $FORMULA
+
+# change the hash in the formula file
+sed -i -e '/sha256/s/".*"/"'$HASH'"/' $FORMULA
+
+# delete temporary file created by sed
+rm -rf "$FORMULA-e"
+
+# update remote repo
+echo "Gitting..."
+git add entt.rb
+git commit -m "Update to $VERSION"
+git push origin master
+
+# out of homebrew-entt dir
+cd ..

src/component_pool.hpp

@@ -1,235 +0,0 @@
-#ifndef ENTT_COMPONENT_POOL_HPP
-#define ENTT_COMPONENT_POOL_HPP
-
-
-#include <utility>
-#include <vector>
-#include <cassert>
-
-
-namespace entt {
-
-
-template<typename, typename, typename...>
-class ComponentPool;
-
-
-template<typename Entity, typename Component>
-class ComponentPool<Entity, Component> {
-public:
-    using component_type = Component;
-    using entity_type = Entity;
-    using pos_type = entity_type;
-    using size_type = typename std::vector<component_type>::size_type;
-    using iterator_type = typename std::vector<entity_type>::iterator;
-    using const_iterator_type = typename std::vector<entity_type>::const_iterator;
-
-private:
-    inline bool valid(entity_type entity) const noexcept {
-        return entity < reverse.size() && reverse[entity] < direct.size() && direct[reverse[entity]] == entity;
-    }
-
-public:
-    explicit ComponentPool(size_type dim = 4098) noexcept {
-        assert(!(dim < 0));
-        data.reserve(dim);
-    }
-
-    ComponentPool(ComponentPool &&) = default;
-
-    ~ComponentPool() noexcept {
-        assert(empty());
-    }
-
-    ComponentPool & operator=(ComponentPool &&) = default;
-
-    bool empty() const noexcept {
-        return data.empty();
-    }
-
-    size_type capacity() const noexcept {
-        return data.capacity();
-    }
-
-    size_type size() const noexcept {
-        return data.size();
-    }
-
-    iterator_type begin() noexcept {
-        return direct.begin();
-    }
-
-    const_iterator_type cbegin() const noexcept {
-        return direct.cbegin();
-    }
-
-    iterator_type end() noexcept {
-        return direct.end();
-    }
-
-    const_iterator_type cend() const noexcept {
-        return direct.cend();
-    }
-
-    bool has(entity_type entity) const noexcept {
-        return valid(entity);
-    }
-
-    const component_type & get(entity_type entity) const noexcept {
-        assert(valid(entity));
-        return data[reverse[entity]];
-    }
-
-    component_type & get(entity_type entity) noexcept {
-        return const_cast<component_type &>(const_cast<const ComponentPool *>(this)->get(entity));
-    }
-
-    template<typename... Args>
-    component_type & construct(entity_type entity, Args... args) {
-        assert(!valid(entity));
-
-        if(!(entity < reverse.size())) {
-            reverse.resize(entity+1);
-        }
-
-        reverse[entity] = pos_type(direct.size());
-        direct.emplace_back(entity);
-        data.push_back({ args... });
-
-        return data.back();
-    }
-
-    void destroy(entity_type entity) {
-        assert(valid(entity));
-
-        auto last = direct.size() - 1;
-
-        reverse[direct[last]] = reverse[entity];
-        direct[reverse[entity]] = direct[last];
-        data[reverse[entity]] = std::move(data[last]);
-
-        direct.pop_back();
-        data.pop_back();
-    }
-
-    void reset() {
-        data.clear();
-        reverse.resize(0);
-        direct.clear();
-    }
-
-private:
-    std::vector<component_type> data;
-    std::vector<pos_type> reverse;
-    std::vector<entity_type> direct;
-};
-
-
-template<typename Entity, typename Component, typename... Components>
-class ComponentPool
-        : ComponentPool<Entity, Component>, ComponentPool<Entity, Components>...
-{
-    template<typename Comp>
-    using Pool = ComponentPool<Entity, Comp>;
-
-public:
-    using entity_type = typename Pool<Component>::entity_type;
-    using pos_type = typename Pool<Component>::pos_type;
-    using size_type = typename Pool<Component>::size_type;
-    using iterator_type = typename Pool<Component>::iterator_type;
-    using const_iterator_type = typename Pool<Component>::const_iterator_type;
-
-    explicit ComponentPool(size_type dim = 4098) noexcept
-#ifdef _MSC_VER
-        : ComponentPool<Entity, Component>{dim}, ComponentPool<Entity, Components>{dim}...
-#else
-        : Pool<Component>{dim}, Pool<Components>{dim}...
-#endif
-    {
-        assert(!(dim < 0));
-    }
-
-    ComponentPool(const ComponentPool &) = delete;
-    ComponentPool(ComponentPool &&) = delete;
-
-    ComponentPool & operator=(const ComponentPool &) = delete;
-    ComponentPool & operator=(ComponentPool &&) = delete;
-
-    template<typename Comp>
-    bool empty() const noexcept {
-        return Pool<Comp>::empty();
-    }
-
-    template<typename Comp>
-    size_type capacity() const noexcept {
-        return Pool<Comp>::capacity();
-    }
-
-    template<typename Comp>
-    size_type size() const noexcept {
-        return Pool<Comp>::size();
-    }
-
-    template<typename Comp>
-    iterator_type begin() noexcept {
-        return Pool<Comp>::begin();
-    }
-
-    template<typename Comp>
-    const_iterator_type cbegin() const noexcept {
-        return Pool<Comp>::cbegin();
-    }
-
-    template<typename Comp>
-    iterator_type end() noexcept {
-        return Pool<Comp>::end();
-    }
-
-    template<typename Comp>
-    const_iterator_type cend() const noexcept {
-        return Pool<Comp>::cend();
-    }
-
-    template<typename Comp>
-    bool has(entity_type entity) const noexcept {
-        return Pool<Comp>::has(entity);
-    }
-
-    template<typename Comp>
-    const Comp & get(entity_type entity) const noexcept {
-        return Pool<Comp>::get(entity);
-    }
-
-    template<typename Comp>
-    Comp & get(entity_type entity) noexcept {
-        return const_cast<Comp &>(const_cast<const ComponentPool *>(this)->get<Comp>(entity));
-    }
-
-    template<typename Comp, typename... Args>
-    Comp & construct(entity_type entity, Args... args) {
-        return Pool<Comp>::construct(entity, args...);
-    }
-
-    template<typename Comp>
-    void destroy(entity_type entity) {
-        Pool<Comp>::destroy(entity);
-    }
-
-    template<typename Comp>
-    void reset() {
-        Pool<Comp>::reset();
-    }
-
-    void reset() {
-        using accumulator_type = int[];
-        Pool<Component>::reset();
-        accumulator_type accumulator = { (Pool<Components>::reset(), 0)... };
-        (void)accumulator;
-    }
-};
-
-
-}
-
-
-#endif // ENTT_COMPONENT_POOL_HPP

src/entt/config/config.h

@@ -0,0 +1,80 @@
+#ifndef ENTT_CONFIG_CONFIG_H
+#define ENTT_CONFIG_CONFIG_H
+
+
+#ifndef ENTT_NOEXCEPT
+#   define ENTT_NOEXCEPT noexcept
+#endif
+
+
+#ifndef ENTT_HS_SUFFIX
+#   define ENTT_HS_SUFFIX _hs
+#endif
+
+
+#ifndef ENTT_HWS_SUFFIX
+#   define ENTT_HWS_SUFFIX _hws
+#endif
+
+
+#ifndef ENTT_USE_ATOMIC
+#   define ENTT_MAYBE_ATOMIC(Type) Type
+#else
+#   include <atomic>
+#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#endif
+
+
+#ifndef ENTT_ID_TYPE
+#   include <cstdint>
+#   define ENTT_ID_TYPE std::uint32_t
+#endif
+
+
+#ifndef ENTT_PAGE_SIZE
+#   define ENTT_PAGE_SIZE 32768
+#endif
+
+
+#ifndef ENTT_ASSERT
+#   include <cassert>
+#   define ENTT_ASSERT(condition) assert(condition)
+#endif
+
+
+#ifndef ENTT_NO_ETO
+#   include <type_traits>
+#   define ENTT_IS_EMPTY(Type) std::is_empty<Type>
+#else
+#   include <type_traits>
+#   define ENTT_IS_EMPTY(Type) std::false_type
+#endif
+
+
+#ifndef ENTT_STANDARD_CPP
+#   if defined __clang__ || (defined __GNUC__ && __GNUC__ > 8)
+#       define ENTT_PRETTY_FUNCTION_CONSTEXPR
+#       define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '='
+#       define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#   elif 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_CONSTEXPR
+#       define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#       define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#   endif
+#endif
+
+
+#ifndef ENTT_STANDALONE
+#   define ENTT_FAST_PATH(...) false
+#else
+#   define ENTT_FAST_PATH(Cond) Cond
+#endif
+
+
+#endif

src/entt/config/version.h

@@ -0,0 +1,10 @@
+#ifndef ENTT_CONFIG_VERSION_H
+#define ENTT_CONFIG_VERSION_H
+
+
+#define ENTT_VERSION_MAJOR 3
+#define ENTT_VERSION_MINOR 5
+#define ENTT_VERSION_PATCH 0
+
+
+#endif

src/entt/core/algorithm.hpp

@@ -0,0 +1,144 @@
+#ifndef ENTT_CORE_ALGORITHM_HPP
+#define ENTT_CORE_ALGORITHM_HPP
+
+
+#include <vector>
+#include <utility>
+#include <iterator>
+#include <algorithm>
+#include <functional>
+#include "utility.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Function object to wrap `std::sort` in a class type.
+ *
+ * Unfortunately, `std::sort` cannot be passed as template argument to a class
+ * template or a function template.<br/>
+ * This class fills the gap by wrapping some flavors of `std::sort` in a
+ * function object.
+ */
+struct std_sort {
+    /**
+     * @brief Sorts the elements in a range.
+     *
+     * Sorts the elements in a range using the given binary comparison function.
+     *
+     * @tparam It Type of random access iterator.
+     * @tparam Compare Type of comparison function object.
+     * @tparam Args Types of arguments to forward to the sort function.
+     * @param first An iterator to the first element of the range to sort.
+     * @param last An iterator past the last element of the range to sort.
+     * @param compare A valid comparison function object.
+     * @param args Arguments to forward to the sort function, if any.
+     */
+    template<typename It, typename Compare = std::less<>, typename... Args>
+    void operator()(It first, It last, Compare compare = Compare{}, Args &&... args) const {
+        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 {
+    /**
+     * @brief Sorts the elements in a range.
+     *
+     * Sorts the elements in a range using the given binary comparison function.
+     *
+     * @tparam It Type of random access iterator.
+     * @tparam Compare Type of comparison function object.
+     * @param first An iterator to the first element of the range to sort.
+     * @param last An iterator past the last element of the range to sort.
+     * @param compare A valid comparison function object.
+     */
+    template<typename It, typename Compare = std::less<>>
+    void operator()(It first, It last, Compare compare = Compare{}) const {
+        if(first < last) {
+            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));
+                }
+
+                *pre = std::move(value);
+            }
+        }
+    }
+};
+
+
+/**
+ * @brief Function object for performing LSD radix sort.
+ * @tparam Bit Number of bits processed per pass.
+ * @tparam N Maximum number of bits to sort.
+ */
+template<std::size_t Bit, std::size_t N>
+struct radix_sort {
+    static_assert((N % Bit) == 0, "The maximum number of bits to sort must be a multiple of the number of bits processed per pass");
+
+    /**
+     * @brief Sorts the elements in a range.
+     *
+     * Sorts the elements in a range using the given _getter_ to access the
+     * actual data to be sorted.
+     *
+     * This implementation is inspired by the online book
+     * [Physically Based Rendering](http://www.pbr-book.org/3ed-2018/Primitives_and_Intersection_Acceleration/Bounding_Volume_Hierarchies.html#RadixSort).
+     *
+     * @tparam It Type of random access iterator.
+     * @tparam Getter Type of _getter_ function object.
+     * @param first An iterator to the first element of the range to sort.
+     * @param last An iterator past the last element of the range to sort.
+     * @param getter A valid _getter_ function object.
+     */
+    template<typename It, typename Getter = identity>
+    void operator()(It first, It last, Getter getter = Getter{}) const {
+        if(first < last) {
+            static constexpr auto mask = (1 << Bit) - 1;
+            static constexpr auto buckets = 1 << Bit;
+            static constexpr auto passes = N / Bit;
+
+            using value_type = typename std::iterator_traits<It>::value_type;
+            std::vector<value_type> aux(std::distance(first, last));
+
+            auto part = [getter = std::move(getter)](auto from, auto to, auto out, auto start) {
+                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];
+                });
+
+                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++);
+                });
+
+                std::for_each(from, to, [&getter, &out, &index, start](value_type &item) {
+                    out[index[(getter(item) >> start) & mask]++] = std::move(item);
+                });
+            };
+
+            for(std::size_t pass = 0; pass < (passes & ~1); pass += 2) {
+                part(first, last, aux.begin(), pass * Bit);
+                part(aux.begin(), aux.end(), first, (pass + 1) * Bit);
+            }
+
+            if constexpr(passes & 1) {
+                part(first, last, aux.begin(), (passes - 1) * Bit);
+                std::move(aux.begin(), aux.end(), first);
+            }
+        }
+    }
+};
+
+
+}
+
+
+#endif

src/entt/core/attribute.h

@@ -0,0 +1,33 @@
+#ifndef ENTT_CORE_ATTRIBUTE_H
+#define ENTT_CORE_ATTRIBUTE_H
+
+
+#ifndef ENTT_EXPORT
+#   if defined _WIN32 || defined __CYGWIN__ || defined _MSC_VER
+#       define ENTT_EXPORT __declspec(dllexport)
+#       define ENTT_IMPORT __declspec(dllimport)
+#       define ENTT_HIDDEN
+#   elif defined __GNUC__ && __GNUC__ >= 4
+#       define ENTT_EXPORT __attribute__((visibility("default")))
+#       define ENTT_IMPORT __attribute__((visibility("default")))
+#       define ENTT_HIDDEN __attribute__((visibility("hidden")))
+#   else /* Unsupported compiler */
+#       define ENTT_EXPORT
+#       define ENTT_IMPORT
+#       define ENTT_HIDDEN
+#   endif
+#endif
+
+
+#ifndef ENTT_API
+#   if defined ENTT_API_EXPORT
+#       define ENTT_API ENTT_EXPORT
+#   elif defined ENTT_API_IMPORT
+#       define ENTT_API ENTT_IMPORT
+#   else /* No API */
+#       define ENTT_API
+#   endif
+#endif
+
+
+#endif

src/entt/core/family.hpp

@@ -0,0 +1,37 @@
+#ifndef ENTT_CORE_FAMILY_HPP
+#define ENTT_CORE_FAMILY_HPP
+
+
+#include "../config/config.h"
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Dynamic identifier generator.
+ *
+ * Utility class template that can be used to assign unique identifiers to types
+ * at runtime. Use different specializations to create separate sets of
+ * identifiers.
+ */
+template<typename...>
+class family {
+    inline static ENTT_MAYBE_ATOMIC(id_type) identifier{};
+
+public:
+    /*! @brief Unsigned integer type. */
+    using family_type = id_type;
+
+    /*! @brief Statically generated unique identifier for the given type. */
+    template<typename... Type>
+    // at the time I'm writing, clang crashes during compilation if auto is used instead of family_type
+    inline static const family_type type = identifier++;
+};
+
+
+}
+
+
+#endif

src/entt/core/fwd.hpp

@@ -0,0 +1,18 @@
+#ifndef ENTT_CORE_FWD_HPP
+#define ENTT_CORE_FWD_HPP
+
+
+#include "../config/config.h"
+
+
+namespace entt {
+
+
+/*! @brief Alias declaration for type identifiers. */
+using id_type = ENTT_ID_TYPE;
+
+
+}
+
+
+#endif

src/entt/core/hashed_string.hpp

@@ -0,0 +1,263 @@
+#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;
+    static constexpr std::uint32_t offset = 2166136261;
+    static constexpr std::uint32_t prime = 16777619;
+};
+
+
+template<>
+struct fnv1a_traits<std::uint64_t> {
+    using type = std::uint64_t;
+    static constexpr std::uint64_t offset = 14695981039346656037ull;
+    static constexpr std::uint64_t prime = 1099511628211ull;
+};
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Zero overhead unique identifier.
+ *
+ * A hashed string is a compile-time tool that allows users to use
+ * human-readable identifers in the codebase while using their numeric
+ * counterparts at runtime.<br/>
+ * Because of that, a hashed string can also be used in constant expressions if
+ * required.
+ *
+ * @tparam Char Character type.
+ */
+template<typename Char>
+class basic_hashed_string {
+    using traits_type = internal::fnv1a_traits<id_type>;
+
+    struct const_wrapper {
+        // non-explicit constructor on purpose
+        constexpr const_wrapper(const Char *curr) ENTT_NOEXCEPT: str{curr} {}
+        const Char *str;
+    };
+
+    // Fowler–Noll–Vo hash function v. 1a - the good
+    [[nodiscard]] static constexpr id_type helper(const Char *curr) ENTT_NOEXCEPT {
+        auto value = traits_type::offset;
+
+        while(*curr != 0) {
+            value = (value ^ static_cast<traits_type::type>(*(curr++))) * traits_type::prime;
+        }
+
+        return value;
+    }
+
+public:
+    /*! @brief Character type. */
+    using value_type = Char;
+    /*! @brief Unsigned integer type. */
+    using hash_type = id_type;
+
+    /**
+     * @brief Returns directly the numeric representation of a string.
+     *
+     * Forcing template resolution avoids implicit conversions. An
+     * human-readable identifier can be anything but a plain, old bunch of
+     * characters.<br/>
+     * Example of use:
+     * @code{.cpp}
+     * const auto value = basic_hashed_string<char>::to_value("my.png");
+     * @endcode
+     *
+     * @tparam N Number of characters of the identifier.
+     * @param str Human-readable identifer.
+     * @return The numeric representation of the string.
+     */
+    template<std::size_t N>
+    [[nodiscard]] static constexpr hash_type value(const value_type (&str)[N]) ENTT_NOEXCEPT {
+        return helper(str);
+    }
+
+    /**
+     * @brief Returns directly the numeric representation of a string.
+     * @param wrapper Helps achieving the purpose by relying on overloading.
+     * @return The numeric representation of the string.
+     */
+    [[nodiscard]] static hash_type value(const_wrapper wrapper) ENTT_NOEXCEPT {
+        return helper(wrapper.str);
+    }
+
+    /**
+     * @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 hash_type value(const value_type *str, std::size_t size) ENTT_NOEXCEPT {
+        id_type partial{traits_type::offset};
+        while(size--) { partial = (partial^(str++)[0])*traits_type::prime; }
+        return partial;
+    }
+
+    /*! @brief Constructs an empty hashed string. */
+    constexpr basic_hashed_string() ENTT_NOEXCEPT
+        : str{nullptr}, hash{}
+    {}
+
+    /**
+     * @brief Constructs a hashed string from an array of const characters.
+     *
+     * Forcing template resolution avoids implicit conversions. An
+     * human-readable identifier can be anything but a plain, old bunch of
+     * characters.<br/>
+     * Example of use:
+     * @code{.cpp}
+     * basic_hashed_string<char> hs{"my.png"};
+     * @endcode
+     *
+     * @tparam N Number of characters of the identifier.
+     * @param curr Human-readable identifer.
+     */
+    template<std::size_t N>
+    constexpr basic_hashed_string(const value_type (&curr)[N]) ENTT_NOEXCEPT
+        : str{curr}, hash{helper(curr)}
+    {}
+
+    /**
+     * @brief Explicit constructor on purpose to avoid constructing a hashed
+     * string directly from a `const value_type *`.
+     * @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)}
+    {}
+
+    /**
+     * @brief Returns the human-readable representation of a hashed string.
+     * @return The string used to initialize the instance.
+     */
+    [[nodiscard]] constexpr const value_type * data() const ENTT_NOEXCEPT {
+        return str;
+    }
+
+    /**
+     * @brief Returns the numeric representation of a hashed string.
+     * @return The numeric representation of the instance.
+     */
+    [[nodiscard]] constexpr hash_type value() const ENTT_NOEXCEPT {
+        return hash;
+    }
+
+    /*! @copydoc 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.
+     */
+    [[nodiscard]] constexpr operator hash_type() const ENTT_NOEXCEPT { return value(); }
+
+    /**
+     * @brief Compares two hashed strings.
+     * @param other Hashed string with which to compare.
+     * @return True if the two hashed strings are identical, false otherwise.
+     */
+    [[nodiscard]] constexpr bool operator==(const basic_hashed_string &other) const ENTT_NOEXCEPT {
+        return hash == other.hash;
+    }
+
+private:
+    const value_type *str;
+    hash_type hash;
+};
+
+
+/**
+ * @brief Deduction guide.
+ *
+ * It allows to deduce the character type of the hashed string directly from a
+ * human-readable identifer provided to the constructor.
+ *
+ * @tparam Char Character type.
+ * @tparam N Number of characters of the identifier.
+ * @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>;
+
+
+/**
+ * @brief Compares two hashed strings.
+ * @tparam Char Character type.
+ * @param lhs A valid hashed string.
+ * @param rhs A valid hashed string.
+ * @return True if the two hashed strings are identical, false otherwise.
+ */
+template<typename Char>
+[[nodiscard]] constexpr bool operator!=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) 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>;
+
+
+}
+
+
+/**
+ * @brief User defined literal for hashed strings.
+ * @param str The literal without its suffix.
+ * @return A properly initialized hashed string.
+ */
+[[nodiscard]] constexpr entt::hashed_string operator"" ENTT_HS_SUFFIX(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.
+ */
+[[nodiscard]] constexpr entt::hashed_wstring operator"" ENTT_HWS_SUFFIX(const wchar_t *str, std::size_t) ENTT_NOEXCEPT {
+    return entt::hashed_wstring{str};
+}
+
+
+#endif

src/entt/core/ident.hpp

@@ -0,0 +1,66 @@
+#ifndef ENTT_CORE_IDENT_HPP
+#define ENTT_CORE_IDENT_HPP
+
+
+#include <tuple>
+#include <cstddef>
+#include <utility>
+#include <type_traits>
+#include "../config/config.h"
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Types identifiers.
+ *
+ * Variable template used to generate identifiers at compile-time for the given
+ * types. Use the `get` member function to know what's the identifier associated
+ * to the specific type.
+ *
+ * @note
+ * Identifiers are constant expression and can be used in any context where such
+ * an expression is required. As an example:
+ * @code{.cpp}
+ * using id = entt::identifier<a_type, another_type>;
+ *
+ * switch(a_type_identifier) {
+ * case id::type<a_type>:
+ *     // ...
+ *     break;
+ * case id::type<another_type>:
+ *     // ...
+ *     break;
+ * default:
+ *     // ...
+ * }
+ * @endcode
+ *
+ * @tparam Types List of types for which to generate identifiers.
+ */
+template<typename... Types>
+class identifier {
+    using tuple_type = std::tuple<std::decay_t<Types>...>;
+
+    template<typename Type, std::size_t... Indexes>
+    [[nodiscard]] static constexpr id_type get(std::index_sequence<Indexes...>) {
+        static_assert(std::disjunction_v<std::is_same<Type, Types>...>, "Invalid type");
+        return (0 + ... + (std::is_same_v<Type, std::tuple_element_t<Indexes, tuple_type>> ? id_type(Indexes) : id_type{}));
+    }
+
+public:
+    /*! @brief Unsigned integer 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...>{});
+};
+
+
+}
+
+
+#endif

src/entt/core/monostate.hpp

@@ -0,0 +1,62 @@
+#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.
+ *
+ * A minimal, yet complete configuration system built on top of the monostate
+ * pattern. Thread safe by design, it works only with basic types like `int`s or
+ * `bool`s.<br/>
+ * Multiple types and therefore more than one value can be associated with a
+ * single key. Because of this, users must pay attention to use the same type
+ * both during an assignment and when they try to read back their data.
+ * Otherwise, they can incur in unexpected results.
+ */
+template<id_type>
+struct monostate {
+    /**
+     * @brief Assigns a value of a specific type to a given key.
+     * @tparam Type Type of the value to assign.
+     * @param val User data to assign to the given key.
+     */
+    template<typename Type>
+    void operator=(Type val) const ENTT_NOEXCEPT {
+        value<Type> = val;
+    }
+
+    /**
+     * @brief Gets a value of a specific type for a given key.
+     * @tparam Type Type of the value to get.
+     * @return Stored value, if any.
+     */
+    template<typename Type>
+    operator Type() const ENTT_NOEXCEPT {
+        return value<Type>;
+    }
+
+private:
+    template<typename Type>
+    inline static ENTT_MAYBE_ATOMIC(Type) value{};
+};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Value Value used to differentiate between different variables.
+ */
+template<id_type Value>
+inline monostate<Value> monostate_v = {};
+
+
+}
+
+
+#endif

src/entt/core/type_info.hpp

@@ -0,0 +1,144 @@
+#ifndef ENTT_CORE_TYPE_INFO_HPP
+#define ENTT_CORE_TYPE_INFO_HPP
+
+
+#include <string_view>
+#include "../config/config.h"
+#include "../core/attribute.h"
+#include "hashed_string.hpp"
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+struct ENTT_API type_index {
+    [[nodiscard]] static id_type next() ENTT_NOEXCEPT {
+        static ENTT_MAYBE_ATOMIC(id_type) value{};
+        return value++;
+    }
+};
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto 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
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Type index.
+ * @tparam Type Type for which to generate a sequential identifier.
+ */
+template<typename Type, typename = void>
+struct ENTT_API type_index {
+    /**
+     * @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;
+    }
+};
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * indexable, false otherwise.
+ * @tparam Type Potentially indexable type.
+ */
+template<typename, typename = void>
+struct has_type_index: std::false_type {};
+
+
+/*! @brief has_type_index */
+template<typename Type>
+struct has_type_index<Type, std::void_t<decltype(type_index<Type>::value())>>: std::true_type {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type Potentially indexable type.
+ */
+template<typename Type>
+inline constexpr bool has_type_index_v = has_type_index<Type>::value;
+
+
+/**
+ * @brief Type info.
+ * @tparam Type Type for which to generate information.
+ */
+template<typename Type, typename = void>
+struct type_info {
+    /**
+     * @brief Returns the numeric representation of a given type.
+     * @return The numeric representation of the given type.
+     */
+#if defined ENTT_PRETTY_FUNCTION_CONSTEXPR
+    [[nodiscard]] static constexpr id_type id() ENTT_NOEXCEPT {
+        constexpr auto value = hashed_string::value(ENTT_PRETTY_FUNCTION);
+        return value;
+    }
+#elif defined ENTT_PRETTY_FUNCTION
+    [[nodiscard]] static id_type id() ENTT_NOEXCEPT {
+        static const auto value = hashed_string::value(ENTT_PRETTY_FUNCTION);
+        return value;
+    }
+#else
+    [[nodiscard]] static id_type id() ENTT_NOEXCEPT {
+        return type_index<Type>::value();
+    }
+#endif
+
+    /**
+     * @brief Returns the name of a given type.
+     * @return The name of the given type.
+     */
+#if defined ENTT_PRETTY_FUNCTION_CONSTEXPR
+    [[nodiscard]] static constexpr std::string_view name() ENTT_NOEXCEPT {
+        constexpr auto value = internal::type_name<Type>();
+        return value;
+    }
+#elif defined ENTT_PRETTY_FUNCTION
+    [[nodiscard]] static std::string_view name() ENTT_NOEXCEPT {
+        static const auto value = internal::type_name<Type>();
+        return value;
+    }
+#else
+    [[nodiscard]] static constexpr std::string_view name() ENTT_NOEXCEPT {
+        return internal::type_name<Type>();
+    }
+#endif
+};
+
+
+}
+
+
+#endif

src/entt/core/type_traits.hpp

@@ -0,0 +1,262 @@
+#ifndef ENTT_CORE_TYPE_TRAITS_HPP
+#define ENTT_CORE_TYPE_TRAITS_HPP
+
+
+#include <cstddef>
+#include <utility>
+#include <type_traits>
+#include "../config/config.h"
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Using declaration to be used to _repeat_ the same type a number of
+ * times equal to the size of a given parameter pack.
+ * @tparam Type A type to repeat.
+ */
+template<typename Type, typename>
+using unpack_as_t = Type;
+
+
+/**
+ * @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_v = 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 ease 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 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 */
+{};
+
+
+/*! @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>
+inline 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;
+
+
+/**
+ * @brief Compile-time number of elements in a type list.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list_size<type_list<Type...>>
+        : std::integral_constant<std::size_t, sizeof...(Type)>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam List Type list.
+ */
+template<class List>
+inline constexpr auto type_list_size_v = type_list_size<List>::value;
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_cat;
+
+
+/*! @brief Concatenates multiple type lists. */
+template<>
+struct type_list_cat<> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<>;
+};
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @tparam List Other type lists, if any.
+ */
+template<typename... Type, typename... Other, typename... List>
+struct type_list_cat<type_list<Type...>, type_list<Other...>, List...> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = typename type_list_cat<type_list<Type..., Other...>, List...>::type;
+};
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list_cat<type_list<Type...>> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<Type...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists to concatenate.
+ */
+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.
+ * @tparam Other The other types provided by the given type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_unique<type_list<Type, Other...>> {
+    /*! @brief A type list without duplicate types. */
+    using type = std::conditional_t<
+        std::disjunction_v<std::is_same<Type, Other>...>,
+        typename type_list_unique<type_list<Other...>>::type,
+        type_list_cat_t<type_list<Type>, typename type_list_unique<type_list<Other...>>::type>
+    >;
+};
+
+
+/*! @brief Removes duplicates types from a type list. */
+template<>
+struct type_list_unique<type_list<>> {
+    /*! @brief A type list without duplicate types. */
+    using type = type_list<>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type A 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 given type is
+ * equality comparable, false otherwise.
+ * @tparam Type Potentially equality comparable type.
+ */
+template<typename Type, typename = std::void_t<>>
+struct is_equality_comparable: std::false_type {};
+
+
+/*! @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
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type Potentially equality comparable type.
+ */
+template<class Type>
+inline constexpr auto is_equality_comparable_v = is_equality_comparable<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is empty
+ * and the empty type optimization is enabled, false otherwise.
+ * @tparam Type Potential empty type.
+ */
+template<typename Type, typename = void>
+struct is_eto_eligible
+    : ENTT_IS_EMPTY(Type)
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type Potential empty type.
+ */
+template<typename Type>
+inline constexpr auto is_eto_eligible_v = is_eto_eligible<Type>::value;
+
+
+/**
+ * @brief Extracts the class of a non-static member object or function.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+class member_class {
+    static_assert(std::is_member_pointer_v<Member>, "Invalid pointer type to non-static member object or function");
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...));
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...) const);
+
+    template<typename Class, typename Type>
+    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.
+ */
+template<typename Member>
+using member_class_t = typename member_class<Member>::type;
+
+
+}
+
+
+#endif

src/entt/core/utility.hpp

@@ -0,0 +1,105 @@
+#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 Returns its argument unchanged.
+     * @tparam Type Type of the argument.
+     * @param value The actual argument.
+     * @return The submitted value as-is.
+     */
+    template<class Type>
+    [[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.
+ * @tparam Class Type of class to which the member belongs.
+ * @param member A valid pointer to a member.
+ * @return Pointer to the member.
+ */
+template<typename Type, typename Class>
+[[nodiscard]] constexpr auto overload(Type Class:: *member) ENTT_NOEXCEPT { return member; }
+
+
+/**
+ * @brief Constant utility to disambiguate overloaded functions.
+ * @tparam Func Function type of the desired overload.
+ * @param func A valid pointer to a function.
+ * @return Pointer to the function.
+ */
+template<typename Func>
+[[nodiscard]] constexpr auto overload(Func *func) ENTT_NOEXCEPT { return func; }
+
+
+/**
+ * @brief Helper type for visitors.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+struct overloaded: Func... {
+    using Func::operator()...;
+};
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+overloaded(Func...) -> overloaded<Func...>;
+
+
+/**
+ * @brief Basic implementation of a y-combinator.
+ * @tparam Func Type of a potentially recursive function.
+ */
+template<class Func>
+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)}
+    {}
+
+    /**
+     * @brief Invokes a y-combinator and therefore its underlying function.
+     * @tparam Args Types of arguments to use to invoke the underlying function.
+     * @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 {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+    /*! @copydoc operator()() */
+    template <class... Args>
+    decltype(auto) operator()(Args &&... args) {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+private:
+    Func func;
+};
+
+
+}
+
+
+#endif

src/entt/entity/actor.hpp

@@ -0,0 +1,206 @@
+#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 [[deprecated("Consider using the handle class instead")]] 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{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 emplace_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>
+    [[nodiscard]] 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>
+    [[nodiscard]] decltype(auto) get() const {
+        return std::as_const(*reg).template get<Component...>(entt);
+    }
+
+    /*! @copydoc get */
+    template<typename... Component>
+    [[nodiscard]] 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>
+    [[nodiscard]] auto try_get() const {
+        return std::as_const(*reg).template try_get<Component...>(entt);
+    }
+
+    /*! @copydoc try_get */
+    template<typename... Component>
+    [[nodiscard]] 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.
+     */
+    [[nodiscard]] const registry_type & backend() const ENTT_NOEXCEPT {
+        return *reg;
+    }
+
+    /*! @copydoc backend */
+    [[nodiscard]] 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.
+     */
+    [[nodiscard]] 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.
+     */
+    [[nodiscard]] explicit operator bool() const {
+        return reg && reg->valid(entt);
+    }
+
+private:
+    entity_type entt;
+    registry_type *reg;
+};
+
+
+}
+
+
+#endif

src/entt/entity/entity.hpp

@@ -0,0 +1,221 @@
+#ifndef ENTT_ENTITY_ENTITY_HPP
+#define ENTT_ENTITY_ENTITY_HPP
+
+
+#include <cstddef>
+#include <cstdint>
+#include <type_traits>
+#include "../config/config.h"
+
+
+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, typename = void>
+struct entt_traits;
+
+
+/**
+ * @brief Entity traits for enumeration types.
+ * @tparam Type The type to check.
+ */
+template<typename Type>
+struct entt_traits<Type, std::enable_if_t<std::is_enum_v<Type>>>
+        : entt_traits<std::underlying_type_t<Type>>
+{};
+
+
+/**
+ * @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::int16_t;
+
+    /*! @brief Mask to use to get the entity number out of an identifier. */
+    static constexpr entity_type entity_mask = 0xFFF;
+    /*! @brief Mask to use to get the version out of an identifier. */
+    static constexpr entity_type version_mask = 0xF;
+    /*! @brief Extent of the entity number within an identifier. */
+    static constexpr std::size_t entity_shift = 12u;
+};
+
+
+/**
+ * @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::int32_t;
+
+    /*! @brief Mask to use to get the entity number out of an identifier. */
+    static constexpr entity_type entity_mask = 0xFFFFF;
+    /*! @brief Mask to use to get the version out of an identifier. */
+    static constexpr entity_type version_mask = 0xFFF;
+    /*! @brief Extent of the entity number within an identifier. */
+    static constexpr std::size_t entity_shift = 20u;
+};
+
+
+/**
+ * @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 entity_type entity_mask = 0xFFFFFFFF;
+    /*! @brief Mask to use to get the version out of an identifier. */
+    static constexpr entity_type version_mask = 0xFFFFFFFF;
+    /*! @brief Extent of the entity number within an identifier. */
+    static constexpr std::size_t entity_shift = 32u;
+};
+
+
+/**
+ * @brief Converts an entity type to its underlying type.
+ * @tparam Entity The value type.
+ * @param entity The value to convert.
+ * @return The integral representation of the given value.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr auto to_integral(const Entity entity) ENTT_NOEXCEPT {
+    return static_cast<typename entt_traits<Entity>::entity_type>(entity);
+}
+
+
+/*! @brief Null object for all entity identifiers.  */
+struct null_t {
+    /**
+     * @brief Converts the null object to identifiers of any type.
+     * @tparam Entity Type of entity identifier.
+     * @return The null representation for the given identifier.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr operator Entity() const ENTT_NOEXCEPT {
+        return Entity{entt_traits<Entity>::entity_mask};
+    }
+
+    /**
+     * @brief Compares two null objects.
+     * @return True in all cases.
+     */
+    [[nodiscard]] constexpr bool operator==(null_t) const ENTT_NOEXCEPT {
+        return true;
+    }
+
+    /**
+     * @brief Compares two null objects.
+     * @return False in all cases.
+     */
+    [[nodiscard]] constexpr bool operator!=(null_t) const ENTT_NOEXCEPT {
+        return false;
+    }
+
+    /**
+     * @brief Compares a null object and an entity identifier of any type.
+     * @tparam Entity Type of entity identifier.
+     * @param entity Entity identifier with which to compare.
+     * @return False if the two elements differ, true otherwise.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr bool operator==(const Entity entity) const ENTT_NOEXCEPT {
+        return (to_integral(entity) & entt_traits<Entity>::entity_mask) == to_integral(static_cast<Entity>(*this));
+    }
+
+    /**
+     * @brief Compares a null object and an entity identifier of any type.
+     * @tparam Entity Type of entity identifier.
+     * @param entity Entity identifier with which to compare.
+     * @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 null object and an entity identifier of any type.
+ * @tparam Entity Type of entity identifier.
+ * @param entity 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>
+[[nodiscard]] constexpr bool operator==(const Entity entity, null_t other) ENTT_NOEXCEPT {
+    return other.operator==(entity);
+}
+
+
+/**
+ * @brief Compares a null object and an entity identifier of any type.
+ * @tparam Entity Type of entity identifier.
+ * @param entity Entity identifier with which to compare.
+ * @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, null_t other) ENTT_NOEXCEPT {
+    return !(other == entity);
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @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.
+ */
+inline constexpr null_t null{};
+
+
+}
+
+
+#endif

src/entt/entity/fwd.hpp

@@ -0,0 +1,110 @@
+#ifndef ENTT_ENTITY_FWD_HPP
+#define ENTT_ENTITY_FWD_HPP
+
+
+#include "../core/fwd.hpp"
+
+
+namespace entt {
+
+
+template <typename>
+class basic_registry;
+
+
+template<typename...>
+class basic_view;
+
+
+template<typename>
+class basic_runtime_view;
+
+
+template<typename...>
+class basic_group;
+
+
+template<typename>
+class basic_observer;
+
+
+template <typename>
+struct basic_actor;
+
+
+template<typename>
+struct basic_handle;
+
+
+template<typename>
+class basic_snapshot;
+
+
+template<typename>
+class basic_snapshot_loader;
+
+
+template<typename>
+class basic_continuous_loader;
+
+
+/*! @brief Default entity identifier. */
+enum class entity: id_type {};
+
+
+/*! @brief Alias declaration for the most common use case. */
+using registry = basic_registry<entity>;
+
+
+/*! @brief Alias declaration for the most common use case. */
+using observer = basic_observer<entity>;
+
+
+/*! @brief Alias declaration for the most common use case. */
+using actor [[deprecated("Consider using the handle class instead")]] = basic_actor<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. */
+using snapshot = basic_snapshot<entity>;
+
+
+/*! @brief Alias declaration for the most common use case. */
+using snapshot_loader = basic_snapshot_loader<entity>;
+
+
+/*! @brief Alias declaration for the most common use case. */
+using continuous_loader = basic_continuous_loader<entity>;
+
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Types Types of components iterated by the view.
+ */
+template<typename... Types>
+using view = basic_view<entity, Types...>;
+
+
+/*! @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.
+ */
+template<typename... Types>
+using group = basic_group<entity, Types...>;
+
+
+}
+
+
+#endif

src/entt/entity/handle.hpp

@@ -0,0 +1,280 @@
+#ifndef ENTT_ENTITY_HANDLE_HPP
+#define ENTT_ENTITY_HANDLE_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).
+ */
+template<typename Entity>
+struct basic_handle {
+    /*! @brief Underlying entity identifier. */
+    using entity_type = std::remove_const_t<Entity>;
+
+    /*! @brief Type of registry accepted by the handle. */
+    using registry_type = std::conditional_t<
+        std::is_const_v<Entity>,
+        const basic_registry<entity_type>,
+        basic_registry<entity_type>
+    >;
+
+    /**
+     * @brief Constructs a handle from a given registry and entity.
+     * @param ref An instance of the registry class.
+     * @param value An entity identifier.
+     */
+    basic_handle(registry_type &ref, entity_type value = null) ENTT_NOEXCEPT
+        : reg{&ref}, entt{value}
+    {}
+
+    /**
+     * @brief Assigns an entity to a handle.
+     * @param value An entity identifier.
+     * @return This handle.
+     */
+    basic_handle & operator=(const entity_type value) ENTT_NOEXCEPT {
+        entt = value;
+        return *this;
+    }
+
+    /**
+     * @brief Assigns the null object to a handle.
+     * @return This handle.
+     */
+    basic_handle & operator=(null_t) ENTT_NOEXCEPT {
+        return (*this = static_cast<entity_type>(null));
+    }
+
+    /**
+     * @brief Constructs a const handle from a non-const one.
+     * @return A const handle referring to the same entity.
+     */
+    [[nodiscard]] operator basic_handle<const entity_type>() const ENTT_NOEXCEPT {
+        return {*reg, entt};
+    }
+
+    /**
+     * @brief Converts a handle to its underlying entity.
+     * @return An entity identifier.
+     */
+    [[nodiscard]] operator entity_type() const ENTT_NOEXCEPT {
+        return entity();
+    }
+
+    /**
+     * @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]] explicit operator bool() const {
+        return reg->valid(entt);
+    }
+
+    /**
+     * @brief Returns a reference to the underlying registry.
+     * @return A reference to the underlying registry.
+     */
+    [[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 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 {
+        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 {
+        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 {
+        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 {
+        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.
+     */
+    template<typename... Components>
+    void remove() const {
+        reg->template remove<Components...>(entt);
+    }
+
+    /**
+     * @brief Removes the given components from a handle.
+     * @sa basic_registry::remove_if_exists
+     * @tparam Component Types of components to remove.
+     * @return The number of components actually removed.
+     */
+    template<typename... Components>
+    decltype(auto) remove_if_exists() const {
+        return reg->template remove_if_exists<Components...>(entt);
+    }
+
+    /**
+     * @brief Removes all the components from a handle and makes it orphaned.
+     * @sa basic_registry::remove_all
+     */
+    void remove_all() const {
+        reg->remove_all(entt);
+    }
+
+    /**
+     * @brief Checks if a handle has all the given components.
+     * @sa basic_registry::has
+     * @tparam Component Components for which to perform the check.
+     * @return True if the handle has all the components, false otherwise.
+     */
+    template<typename... Components>
+    [[nodiscard]] decltype(auto) has() const {
+        return reg->template has<Components...>(entt);
+    }
+
+    /**
+     * @brief Checks if a handle has at least one of the given components.
+     * @sa basic_registry::any
+     * @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... Components>
+    [[nodiscard]] decltype(auto) any() const {
+        return reg->template any<Components...>(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... Components>
+    [[nodiscard]] decltype(auto) get() const {
+        return reg->template get<Components...>(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 {
+        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... Components>
+    [[nodiscard]] decltype(auto) try_get() const {
+        return reg->template try_get<Components...>(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 types for its components.
+     * @sa basic_registry::visit
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void visit(Func &&func) const {
+        reg->visit(entt, std::forward<Func>(func));
+    }
+
+private:
+    registry_type *reg;
+    entity_type entt;
+};
+
+
+/**
+ * @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>;
+
+
+}
+
+
+#endif

src/entt/entity/helper.hpp

@@ -0,0 +1,149 @@
+#ifndef ENTT_ENTITY_HELPER_HPP
+#define ENTT_ENTITY_HELPER_HPP
+
+
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/type_traits.hpp"
+#include "../signal/delegate.hpp"
+#include "registry.hpp"
+#include "fwd.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>
+struct as_view {
+    /*! @brief Type of registry to convert. */
+    using registry_type = std::conditional_t<Const, const basic_registry<Entity>, basic_registry<Entity>>;
+
+    /**
+     * @brief Constructs a converter for a given registry.
+     * @param source A valid reference to a registry.
+     */
+    as_view(registry_type &source) ENTT_NOEXCEPT: reg{source} {}
+
+    /**
+     * @brief Conversion function from a registry to a view.
+     * @tparam Exclude Types of components used to filter the view.
+     * @tparam Component Type of components used to construct the view.
+     * @return A newly created view.
+     */
+    template<typename Exclude, typename... Component>
+    operator basic_view<Entity, Exclude, Component...>() const {
+        return reg.template view<Component...>(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_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>;
+
+
+/**
+ * @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>
+struct as_group {
+    /*! @brief Type of registry to convert. */
+    using registry_type = std::conditional_t<Const, const basic_registry<Entity>, basic_registry<Entity>>;
+
+    /**
+     * @brief Constructs a converter for a given registry.
+     * @param source A valid reference to a registry.
+     */
+    as_group(registry_type &source) ENTT_NOEXCEPT: reg{source} {}
+
+    /**
+     * @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 Owned Types of components owned by the group.
+     * @return A newly created group.
+     */
+    template<typename Exclude, typename Get, typename... Owned>
+    operator basic_group<Entity, Exclude, Get, Owned...>() const {
+        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>;
+
+
+/*! @copydoc as_group */
+template<typename Entity>
+as_group(const basic_registry<Entity> &) ENTT_NOEXCEPT -> as_group<true, 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.
+ * @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 component 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 &component) {
+    return *(reg.template data<Component>() + (&component - reg.template raw<Component>()));
+}
+
+
+}
+
+
+#endif

src/entt/entity/observer.hpp

@@ -0,0 +1,441 @@
+#ifndef ENTT_ENTITY_OBSERVER_HPP
+#define ENTT_ENTITY_OBSERVER_HPP
+
+
+#include <limits>
+#include <cstddef>
+#include <cstdint>
+#include <utility>
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/type_traits.hpp"
+#include "registry.hpp"
+#include "storage.hpp"
+#include "utility.hpp"
+#include "entity.hpp"
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/*! @brief Grouping matcher. */
+template<typename...>
+struct matcher {};
+
+
+/**
+ * @brief Collector.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error, but for a few reasonable cases.
+ */
+template<typename...>
+struct basic_collector;
+
+
+/**
+ * @brief Collector.
+ *
+ * A collector contains a set of rules (literally, matchers) to use to track
+ * entities.<br/>
+ * Its main purpose is to generate a descriptor that allows an observer to know
+ * how to connect to a registry.
+ */
+template<>
+struct basic_collector<> {
+    /**
+     * @brief Adds a grouping matcher to the collector.
+     * @tparam AllOf Types of components tracked by the matcher.
+     * @tparam NoneOf Types of components used to filter out entities.
+     * @return The updated collector.
+     */
+    template<typename... AllOf, typename... NoneOf>
+    static constexpr auto group(exclude_t<NoneOf...> = {}) ENTT_NOEXCEPT {
+        return basic_collector<matcher<type_list<>, type_list<>, type_list<NoneOf...>, AllOf...>>{};
+    }
+
+    /**
+     * @brief Adds an observing matcher to the collector.
+     * @tparam AnyOf Type of component for which changes should be detected.
+     * @return The updated collector.
+     */
+    template<typename AnyOf>
+    static constexpr auto update() ENTT_NOEXCEPT {
+        return basic_collector<matcher<type_list<>, type_list<>, AnyOf>>{};
+    }
+};
+
+/**
+ * @brief Collector.
+ * @copydetails basic_collector<>
+ * @tparam Reject Untracked types used to filter out entities.
+ * @tparam Require Untracked types required by the matcher.
+ * @tparam Rule Specific details of the current matcher.
+ * @tparam Other Other matchers.
+ */
+template<typename... Reject, typename... Require, typename... Rule, typename... Other>
+struct basic_collector<matcher<type_list<Reject...>, type_list<Require...>, Rule...>, Other...> {
+    /*! @brief Current matcher. */
+    using current_type = matcher<type_list<Reject...>, type_list<Require...>, Rule...>;
+
+    /**
+     * @brief Adds a grouping matcher to the collector.
+     * @tparam AllOf Types of components tracked by the matcher.
+     * @tparam NoneOf Types of components used to filter out entities.
+     * @return The updated collector.
+     */
+    template<typename... AllOf, typename... NoneOf>
+    static constexpr auto group(exclude_t<NoneOf...> = {}) ENTT_NOEXCEPT {
+        return basic_collector<matcher<type_list<>, type_list<>, type_list<NoneOf...>, AllOf...>, current_type, Other...>{};
+    }
+
+    /**
+     * @brief Adds an observing matcher to the collector.
+     * @tparam AnyOf Type of component for which changes should be detected.
+     * @return The updated collector.
+     */
+    template<typename AnyOf>
+    static constexpr auto update() ENTT_NOEXCEPT {
+        return basic_collector<matcher<type_list<>, type_list<>, AnyOf>, current_type, Other...>{};
+    }
+
+    /**
+     * @brief Updates the filter of the last added matcher.
+     * @tparam AllOf Types of components required by the matcher.
+     * @tparam NoneOf Types of components used to filter out entities.
+     * @return The updated collector.
+     */
+    template<typename... AllOf, typename... NoneOf>
+    static constexpr auto where(exclude_t<NoneOf...> = {}) ENTT_NOEXCEPT {
+        using extended_type = matcher<type_list<Reject..., NoneOf...>, type_list<Require..., AllOf...>, Rule...>;
+        return basic_collector<extended_type, Other...>{};
+    }
+};
+
+
+/*! @brief Variable template used to ease the definition of collectors. */
+inline constexpr basic_collector<> collector{};
+
+
+/**
+ * @brief Observer.
+ *
+ * An observer returns all the entities and only the entities that fit the
+ * requirements of at least one matcher. Moreover, it's guaranteed that the
+ * entity list is tightly packed in memory for fast iterations.<br/>
+ * In general, observers don't stay true to the order of any set of components.
+ *
+ * Observers work mainly with two types of matchers, provided through a
+ * collector:
+ *
+ * * Observing matcher: an observer will return at least all the living entities
+ *   for which one or more of the given 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.
+ *
+ * If an entity respects the requirements of multiple matchers, it will be
+ * returned once and only once by the observer in any case.
+ *
+ * Matchers support also filtering by means of a _where_ clause that accepts
+ * both a list of types and an exclusion list.<br/>
+ * Whenever a matcher finds that an entity matches its requirements, the
+ * condition of the filter is verified before to register the entity itself.
+ * Moreover, a registered entity isn't returned by the observer if the condition
+ * set by the filter is broken in the meantime.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New instances of the given components are created and assigned to entities.
+ * * The entity currently pointed is modified (as an example, if one of the
+ *   given components is removed from the entity to which the iterator points).
+ * * The entity currently pointed is destroyed.
+ *
+ * In all the other cases, modifying the pools of the given components in any
+ * way invalidates all the iterators and using them results in undefined
+ * behavior.
+ *
+ * @warning
+ * Lifetime of an observer doesn't necessarily have to overcome that of the
+ * registry to which it is connected. However, the observer must be disconnected
+ * from the registry before being destroyed to avoid crashes due to dangling
+ * pointers.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+class basic_observer {
+    using payload_type = std::uint32_t;
+
+    template<typename>
+    struct matcher_handler;
+
+    template<typename... Reject, typename... Require, typename AnyOf>
+    struct matcher_handler<matcher<type_list<Reject...>, type_list<Require...>, AnyOf>> {
+        template<std::size_t Index>
+        static void maybe_valid_if(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.emplace(entt);
+                }
+
+                obs.view.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.erase(entt);
+            }
+        }
+
+        template<std::size_t Index>
+        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_update<AnyOf>().template connect<&maybe_valid_if<Index>>(obs);
+            reg.template on_destroy<AnyOf>().template connect<&discard_if<Index>>(obs);
+        }
+
+        static void disconnect(basic_observer &obs, basic_registry<Entity> &reg) {
+            (reg.template on_destroy<Require>().disconnect(obs), ...);
+            (reg.template on_construct<Reject>().disconnect(obs), ...);
+            reg.template on_update<AnyOf>().disconnect(obs);
+            reg.template on_destroy<AnyOf>().disconnect(obs);
+        }
+    };
+
+    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.emplace(entt);
+                }
+
+                obs.view.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.erase(entt);
+            }
+        }
+
+        template<std::size_t Index>
+        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_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<AllOf>().template connect<&discard_if<Index>>(obs), ...);
+            (reg.template on_construct<NoneOf>().template connect<&discard_if<Index>>(obs), ...);
+        }
+
+        static void disconnect(basic_observer &obs, basic_registry<Entity> &reg) {
+            (reg.template on_destroy<Require>().disconnect(obs), ...);
+            (reg.template on_construct<Reject>().disconnect(obs), ...);
+            (reg.template on_construct<AllOf>().disconnect(obs), ...);
+            (reg.template on_destroy<NoneOf>().disconnect(obs), ...);
+            (reg.template on_destroy<AllOf>().disconnect(obs), ...);
+            (reg.template on_construct<NoneOf>().disconnect(obs), ...);
+        }
+    };
+
+    template<typename... Matcher>
+    static void disconnect(basic_observer &obs, basic_registry<Entity> &reg) {
+        (matcher_handler<Matcher>::disconnect(obs, reg), ...);
+    }
+
+    template<typename... Matcher, std::size_t... Index>
+    void connect(basic_registry<Entity> &reg, std::index_sequence<Index...>) {
+        static_assert(sizeof...(Matcher) < std::numeric_limits<payload_type>::digits, "Too many matchers");
+        (matcher_handler<Matcher>::template connect<Index>(*this, reg), ...);
+        release = &basic_observer::disconnect<Matcher...>;
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Random access iterator type. */
+    using iterator = typename sparse_set<Entity>::iterator;
+
+    /*! @brief Default constructor. */
+    basic_observer()
+        : target{}, release{}, view{}
+    {}
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    basic_observer(const basic_observer &) = delete;
+    /*! @brief Default move constructor, deleted on purpose. */
+    basic_observer(basic_observer &&) = delete;
+
+    /**
+     * @brief Creates an observer and connects it to a given registry.
+     * @tparam Matcher Types of matchers to use to initialize the observer.
+     * @param reg A valid reference to a registry.
+     */
+    template<typename... Matcher>
+    basic_observer(basic_registry<entity_type> &reg, basic_collector<Matcher...>)
+        : target{&reg},
+          release{},
+          view{}
+    {
+        connect<Matcher...>(reg, std::index_sequence_for<Matcher...>{});
+    }
+
+    /*! @brief Default destructor. */
+    ~basic_observer() = default;
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This observer.
+     */
+    basic_observer & operator=(const basic_observer &) = delete;
+
+    /**
+     * @brief Default move assignment operator, deleted on purpose.
+     * @return This observer.
+     */
+    basic_observer & operator=(basic_observer &&) = delete;
+
+    /**
+     * @brief Connects an observer to a given registry.
+     * @tparam Matcher Types of matchers to use to initialize the observer.
+     * @param reg A valid reference to a registry.
+     */
+    template<typename... Matcher>
+    void connect(basic_registry<entity_type> &reg, basic_collector<Matcher...>) {
+        disconnect();
+        connect<Matcher...>(reg, std::index_sequence_for<Matcher...>{});
+        target = &reg;
+        view.clear();
+    }
+
+    /*! @brief Disconnects an observer from the registry it keeps track of. */
+    void disconnect() {
+        if(release) {
+            release(*this, *target);
+            release = nullptr;
+        }
+    }
+
+    /**
+     * @brief Returns the number of elements in an observer.
+     * @return Number of elements.
+     */
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return view.size();
+    }
+
+    /**
+     * @brief Checks whether an observer is empty.
+     * @return True if the observer is empty, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return view.empty();
+    }
+
+    /**
+     * @brief Direct access to the list of entities of the observer.
+     *
+     * The returned pointer is such that range `[data(), data() + size()]` is
+     * always a valid range, even if the container is empty.
+     *
+     * @note
+     * Entities are in the reverse order as returned by the `begin`/`end`
+     * iterators.
+     *
+     * @return A pointer to the array of entities.
+     */
+    [[nodiscard]] const entity_type * data() const ENTT_NOEXCEPT {
+        return view.data();
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the observer.
+     *
+     * The returned iterator points to the first entity of the observer. If the
+     * container is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first entity of the observer.
+     */
+    [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+        return view.sparse_set<entity_type>::begin();
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the observer.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the observer. Attempting to dereference the returned iterator results in
+     * undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the
+     * observer.
+     */
+    [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+        return view.sparse_set<entity_type>::end();
+    }
+
+    /*! @brief Clears the underlying container. */
+    void clear() ENTT_NOEXCEPT {
+        view.clear();
+    }
+
+    /**
+     * @brief Iterates entities and applies the given function object to them.
+     *
+     * The function object is invoked for each entity.<br/>
+     * The signature of the function must be equivalent to the following form:
+     *
+     * @code{.cpp}
+     * void(const entity_type);
+     * @endcode
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        for(const auto entity: *this) {
+            func(entity);
+        }
+    }
+
+    /**
+     * @brief Iterates entities and applies the given function object to them,
+     * then clears the observer.
+     *
+     * @sa each
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) {
+        std::as_const(*this).each(std::move(func));
+        clear();
+    }
+
+private:
+    basic_registry<entity_type> *target;
+    void(* release)(basic_observer &, basic_registry<entity_type> &);
+    storage<entity_type, payload_type> view;
+};
+
+
+}
+
+
+#endif

src/entt/entity/pool.hpp

@@ -0,0 +1,53 @@
+#ifndef ENTT_ENTITY_POOL_HPP
+#define ENTT_ENTITY_POOL_HPP
+
+
+#include <type_traits>
+#include "storage.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Applies component-to-pool conversion and defines the resulting type as
+ * the member typedef type.
+ *
+ * Formally:
+ *
+ * * If the component type is a non-const one, the member typedef type is the
+ *   declared storage type.
+ * * If the component type is a const one, the member typedef type is the
+ *   declared storage type, except it has a const-qualifier added.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Type Type of objects assigned to the entities.
+ */
+template<typename Entity, typename Type, typename = void>
+struct pool {
+    /*! @brief Resulting type after component-to-pool conversion. */
+    using type = storage<Entity, Type>;
+};
+
+
+/*! @copydoc pool */
+template<typename Entity, typename Type>
+struct pool<Entity, const Type> {
+    /*! @brief Resulting type after component-to-pool conversion. */
+    using type = std::add_const_t<typename pool<Entity, std::remove_const_t<Type>>::type>;
+};
+
+
+/**
+ * @brief Alias declaration to use to make component-to-pool conversions.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Type Type of objects assigned to the entities.
+ */
+template<typename Entity, typename Type>
+using pool_t = typename pool<Entity, Type>::type;
+
+
+}
+
+
+#endif

src/entt/entity/runtime_view.hpp

@@ -0,0 +1,250 @@
+#ifndef ENTT_ENTITY_RUNTIME_VIEW_HPP
+#define ENTT_ENTITY_RUNTIME_VIEW_HPP
+
+
+#include <iterator>
+#include <vector>
+#include <utility>
+#include <algorithm>
+#include <type_traits>
+#include "../config/config.h"
+#include "sparse_set.hpp"
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Runtime view.
+ *
+ * Runtime views iterate over those entities that have at least all the given
+ * components in their bags. During initialization, a runtime view looks at the
+ * number of entities available for each component and picks up a reference to
+ * the smallest set of candidate entities in order to get a performance boost
+ * when iterate.<br/>
+ * Order of elements during iterations are highly dependent on the order of the
+ * underlying data structures. See sparse_set and its specializations for more
+ * details.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New instances of the given components are created and assigned to entities.
+ * * The entity currently pointed is modified (as an example, if one of the
+ *   given components is removed from the entity to which the iterator points).
+ * * The entity currently pointed is destroyed.
+ *
+ * In all the other cases, modifying the pools of the given components in any
+ * way invalidates all the iterators and using them results in undefined
+ * behavior.
+ *
+ * @note
+ * Views share references to the underlying data structures of the registry that
+ * generated them. Therefore any change to the entities and to the components
+ * made by means of the registry are immediately reflected by the views, unless
+ * a pool was missing when the view was built (in this case, the view won't
+ * have a valid reference and won't be updated accordingly).
+ *
+ * @warning
+ * Lifetime of a view must not overcome that of the registry that generated it.
+ * In any other case, attempting to use a view results in undefined behavior.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+class basic_runtime_view {
+    /*! @brief A registry is allowed to create views. */
+    friend class basic_registry<Entity>;
+
+    using underlying_iterator = typename sparse_set<Entity>::iterator;
+
+    class view_iterator final {
+        friend class basic_runtime_view<Entity>;
+
+        view_iterator(const std::vector<const sparse_set<Entity> *> &cpools, const std::vector<const sparse_set<Entity> *> &ignore, underlying_iterator curr) ENTT_NOEXCEPT
+            : pools{&cpools},
+              filter{&ignore},
+              it{curr}
+        {
+            if(it != (*pools)[0]->end() && !valid()) {
+                ++(*this);
+            }
+        }
+
+        [[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 difference_type = typename underlying_iterator::difference_type;
+        using value_type = typename underlying_iterator::value_type;
+        using pointer = typename underlying_iterator::pointer;
+        using reference = typename underlying_iterator::reference;
+        using iterator_category = std::bidirectional_iterator_tag;
+
+        view_iterator() ENTT_NOEXCEPT = default;
+
+        view_iterator & operator++() {
+            while(++it != (*pools)[0]->end() && !valid());
+            return *this;
+        }
+
+        view_iterator operator++(int) {
+            view_iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        view_iterator & operator--() ENTT_NOEXCEPT {
+            while(--it != (*pools)[0]->begin() && !valid());
+            return *this;
+        }
+
+        view_iterator operator--(int) ENTT_NOEXCEPT {
+            view_iterator orig = *this;
+            return operator--(), orig;
+        }
+
+        [[nodiscard]] bool operator==(const view_iterator &other) const ENTT_NOEXCEPT {
+            return other.it == it;
+        }
+
+        [[nodiscard]] bool operator!=(const view_iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+        [[nodiscard]] pointer operator->() const {
+            return it.operator->();
+        }
+
+        [[nodiscard]] reference operator*() const {
+            return *operator->();
+        }
+
+    private:
+        const std::vector<const sparse_set<Entity> *> *pools;
+        const std::vector<const sparse_set<Entity> *> *filter;
+        underlying_iterator it;
+    };
+
+    basic_runtime_view(std::vector<const sparse_set<Entity> *> cpools, std::vector<const sparse_set<Entity> *> epools) ENTT_NOEXCEPT
+        : pools{std::move(cpools)},
+          filter{std::move(epools)}
+    {
+        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());
+    }
+
+    [[nodiscard]] bool valid() const {
+        return !pools.empty() && pools.front();
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Bidirectional iterator type. */
+    using iterator = view_iterator;
+
+    /**
+     * @brief Estimates the number of entities that have the given components.
+     * @return Estimated number of entities that have the given components.
+     */
+    [[nodiscard]] size_type size() const {
+        return valid() ? pools.front()->size() : size_type{};
+    }
+
+    /**
+     * @brief Checks if the view is definitely empty.
+     * @return True if the view is definitely empty, false otherwise.
+     */
+    [[nodiscard]] bool empty() const {
+        return !valid() || pools.front()->empty();
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity that has the given
+     * components.
+     *
+     * The returned iterator points to the first entity that has the given
+     * components. If the view is empty, the returned iterator will be equal to
+     * `end()`.
+     *
+     * @note
+     * Iterators stay true to the order imposed to the underlying data
+     * structures.
+     *
+     * @return An iterator to the first entity that has the given components.
+     */
+    [[nodiscard]] iterator begin() const {
+        return valid() ? iterator{pools, filter, pools[0]->begin()} : iterator{};
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity that has the
+     * given components.
+     *
+     * The returned iterator points to the entity following the last entity that
+     * has the given components. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @note
+     * 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.
+     */
+    [[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.
+     * @return True if the view contains the given entity, false otherwise.
+     */
+    [[nodiscard]] bool contains(const entity_type entt) const {
+        return valid() && std::all_of(pools.cbegin(), pools.cend(), [entt](const auto *curr) { return curr->contains(entt); })
+                && std::none_of(filter.cbegin(), filter.cend(), [entt](const auto *curr) { return curr && curr->contains(entt); });
+    }
+
+    /**
+     * @brief Iterates entities and applies the given function object to them.
+     *
+     * The function object is invoked for each entity. It is provided only with
+     * the entity itself. To get the components, users can use the registry with
+     * which the view was built.<br/>
+     * The signature of the function should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * void(const entity_type);
+     * @endcode
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        for(const auto entity: *this) {
+            func(entity);
+        }
+    }
+
+private:
+    std::vector<const sparse_set<Entity> *> pools;
+    std::vector<const sparse_set<Entity> *> filter;
+};
+
+
+}
+
+
+#endif

src/entt/entity/snapshot.hpp

@@ -0,0 +1,565 @@
+#ifndef ENTT_ENTITY_SNAPSHOT_HPP
+#define ENTT_ENTITY_SNAPSHOT_HPP
+
+
+#include <array>
+#include <vector>
+#include <cstddef>
+#include <utility>
+#include <iterator>
+#include <type_traits>
+#include <unordered_map>
+#include "../config/config.h"
+#include "entity.hpp"
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Utility class to create snapshots from a registry.
+ *
+ * A _snapshot_ can be either a dump of the entire registry or a narrower
+ * selection of components of interest.<br/>
+ * This type can be used in both cases if provided with a correctly configured
+ * output archive.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+class basic_snapshot {
+    /*! @brief A registry is allowed to create snapshots. */
+    friend class basic_registry<Entity>;
+
+    using traits_type = 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));
+
+        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));
+                }
+            }
+        }
+    }
+
+    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{};
+        auto begin = first;
+
+        while(begin != last) {
+            const auto entt = *(begin++);
+            ((reg->template has<Component>(entt) ? ++size[Indexes] : size[Indexes]), ...);
+        }
+
+        (get<Component>(archive, size[Indexes], 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;
+
+    /**
+     * @brief Puts aside all the entities from the underlying registry.
+     *
+     * Entities are serialized along with their versions. Destroyed entities are
+     * taken in consideration as well by this function.
+     *
+     * @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 {
+        const auto sz = reg->size();
+        auto first = reg->data();
+        const auto last = first + sz;
+
+        archive(typename traits_type::entity_type(sz));
+
+        while(first != last) {
+            archive(*(first++));
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Puts aside the given components.
+     *
+     * Each instance is serialized together with the entity to which it belongs.
+     * Entities are serialized along with their versions.
+     *
+     * @tparam Component Types of components to serialize.
+     * @tparam Archive Type of output archive.
+     * @param archive A valid reference to an output archive.
+     * @return An object of this type to continue creating the snapshot.
+     */
+    template<typename... Component, typename Archive>
+    const basic_snapshot & component(Archive &archive) const {
+        (component<Component>(archive, reg->template data<Component>(), reg->template data<Component>() + reg->template size<Component>()), ...);
+        return *this;
+    }
+
+    /**
+     * @brief Puts aside the given components for the entities in a range.
+     *
+     * Each instance is serialized together with the entity to which it belongs.
+     * Entities are serialized along with their versions.
+     *
+     * @tparam Component Types of components to serialize.
+     * @tparam Archive Type of output archive.
+     * @tparam It Type of input iterator.
+     * @param archive A valid reference to an output archive.
+     * @param first An iterator to the first element of the range to serialize.
+     * @param last An iterator past the last element of the range to serialize.
+     * @return An object of this type to continue creating the snapshot.
+     */
+    template<typename... Component, typename Archive, typename It>
+    const basic_snapshot & component(Archive &archive, It first, It last) const {
+        component<Component...>(archive, first, last, std::index_sequence_for<Component...>{});
+        return *this;
+    }
+
+private:
+    const basic_registry<entity_type> *reg;
+};
+
+
+/**
+ * @brief Utility class to restore a snapshot as a whole.
+ *
+ * A snapshot loader requires that the destination registry be empty and loads
+ * all the data at once while keeping intact the identifiers that the entities
+ * originally had.<br/>
+ * An example of use is the implementation of a save/restore utility.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+class basic_snapshot_loader {
+    /*! @brief A registry is allowed to create snapshot loaders. */
+    friend class basic_registry<Entity>;
+
+    using traits_type = entt_traits<Entity>;
+
+    template<typename Type, typename Archive>
+    void assign(Archive &archive) const {
+        typename traits_type::entity_type length{};
+        archive(length);
+
+        entity_type entt{};
+
+        if constexpr(std::is_empty_v<Type>) {
+            while(length--) {
+                archive(entt);
+                const auto entity = reg->valid(entt) ? entt : reg->create(entt);
+                ENTT_ASSERT(entity == entt);
+                reg->template emplace<Type>(entity);
+            }
+        } else {
+            Type instance{};
+
+            while(length--) {
+                archive(entt, instance);
+                const auto entity = reg->valid(entt) ? entt : reg->create(entt);
+                ENTT_ASSERT(entity == entt);
+                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());
+    }
+
+    /*! @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;
+
+    /**
+     * @brief Restores entities that were in use during serialization.
+     *
+     * This function restores the entities that were in use during serialization
+     * and gives them the versions they originally had.
+     *
+     * @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 & entities(Archive &archive) const {
+        typename traits_type::entity_type length{};
+
+        archive(length);
+        std::vector<entity_type> all(length);
+
+        for(decltype(length) pos{}; pos < length; ++pos) {
+            archive(all[pos]);
+        }
+
+        reg->assign(all.cbegin(), all.cend());
+
+        return *this;
+    }
+
+    /**
+     * @brief Restores components and assigns them to the right entities.
+     *
+     * The template parameter list must be exactly the same used during
+     * serialization. In the event that the entity to which the component is
+     * assigned doesn't exist yet, the loader will take care to create it with
+     * the version it originally had.
+     *
+     * @tparam Component Types of components to restore.
+     * @tparam Archive Type of input archive.
+     * @param archive A valid reference to an input archive.
+     * @return A valid loader to continue restoring data.
+     */
+    template<typename... Component, typename Archive>
+    const basic_snapshot_loader & component(Archive &archive) const {
+        (assign<Component>(archive), ...);
+        return *this;
+    }
+
+    /**
+     * @brief Destroys those entities that have no components.
+     *
+     * In case all the entities were serialized but only part of the components
+     * was saved, it could happen that some of the entities have no components
+     * once restored.<br/>
+     * This functions helps to identify and destroy those entities.
+     *
+     * @return A valid loader to continue restoring data.
+     */
+    const basic_snapshot_loader & orphans() const {
+        reg->orphans([this](const auto entt) {
+            reg->destroy(entt);
+        });
+
+        return *this;
+    }
+
+private:
+    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 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.
+ * Instead, the loader maps remote identifiers to local ones while restoring a
+ * snapshot.<br/>
+ * An example of use is the implementation of a client-server applications with
+ * the requirement of transferring somehow parts of the representation side to
+ * side.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+class basic_continuous_loader {
+    using traits_type = entt_traits<Entity>;
+
+    void destroy(Entity entt) {
+        const auto it = remloc.find(entt);
+
+        if(it == remloc.cend()) {
+            const auto local = reg->create();
+            remloc.emplace(entt, std::make_pair(local, true));
+            reg->destroy(local);
+        }
+    }
+
+    void restore(Entity entt) {
+        const auto it = remloc.find(entt);
+
+        if(it == remloc.cend()) {
+            const auto local = reg->create();
+            remloc.emplace(entt, std::make_pair(local, true));
+        } else {
+            remloc[entt].first = reg->valid(remloc[entt].first) ? remloc[entt].first : reg->create();
+            // set the dirty flag
+            remloc[entt].second = true;
+        }
+    }
+
+    template<typename Container>
+    auto update(int, Container &container)
+    -> decltype(typename Container::mapped_type{}, void()) {
+        // map like container
+        Container other;
+
+        for(auto &&pair: container) {
+            using first_type = std::remove_const_t<typename std::decay_t<decltype(pair)>::first_type>;
+            using second_type = typename std::decay_t<decltype(pair)>::second_type;
+
+            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_type>) {
+                other.emplace(map(pair.first), std::move(pair.second));
+            } else {
+                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));
+            }
+        }
+
+        std::swap(container, other);
+    }
+
+    template<typename Container>
+    auto update(char, Container &container)
+    -> decltype(typename Container::value_type{}, void()) {
+        // vector like container
+        static_assert(std::is_same_v<typename Container::value_type, entity_type>, "Invalid value type");
+
+        for(auto &&entt: container) {
+            entt = map(entt);
+        }
+    }
+
+    template<typename Other, typename Type, typename Member>
+    void update([[maybe_unused]] Other &instance, [[maybe_unused]] Member Type:: *member) {
+        if constexpr(!std::is_same_v<Other, Type>) {
+            return;
+        } else if constexpr(std::is_same_v<Member, entity_type>) {
+            instance.*member = map(instance.*member);
+        } else {
+            // maybe a container? let's try...
+            update(0, instance.*member);
+        }
+    }
+
+    template<typename Component>
+    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);
+            }
+        }
+    }
+
+    template<typename Other, typename Archive, typename... Type, typename... Member>
+    void assign(Archive &archive, [[maybe_unused]] Member Type:: *... member) {
+        typename traits_type::entity_type length{};
+        archive(length);
+
+        entity_type entt{};
+
+        if constexpr(std::is_empty_v<Other>) {
+            while(length--) {
+                archive(entt);
+                restore(entt);
+                reg->template emplace_or_replace<Other>(map(entt));
+            }
+        } else {
+            Other instance{};
+
+            while(length--) {
+                archive(entt, instance);
+                (update(instance, member), ...);
+                restore(entt);
+                reg->template emplace_or_replace<Other>(map(entt), std::move(instance));
+            }
+        }
+    }
+
+public:
+    /*! @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_continuous_loader(basic_registry<entity_type> &source) ENTT_NOEXCEPT
+        : 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;
+
+    /**
+     * @brief Restores entities that were in use during serialization.
+     *
+     * This function restores the entities that were in use 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 & entities(Archive &archive) {
+        typename traits_type::entity_type length{};
+        entity_type entt{};
+
+        archive(length);
+
+        for(decltype(length) pos{}; pos < length; ++pos) {
+            archive(entt);
+
+            if(const auto entity = (to_integral(entt) & traits_type::entity_mask); entity == pos) {
+                restore(entt);
+            } else {
+                destroy(entt);
+            }
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Restores components and assigns them to the right entities.
+     *
+     * The template parameter list must be exactly the same used during
+     * serialization. In the event that the entity to which the component is
+     * assigned doesn't exist yet, the loader will take care to create a local
+     * counterpart for it.<br/>
+     * Members can be either data members of type entity_type or containers of
+     * entities. In both cases, the loader will visit them and update the
+     * entities by replacing each one with its local counterpart.
+     *
+     * @tparam Component Type of component to restore.
+     * @tparam Archive Type of input archive.
+     * @tparam Type Types of components to update with local counterparts.
+     * @tparam Member Types of members to update with their local counterparts.
+     * @param archive A valid reference to an input archive.
+     * @param member Members to update with their local counterparts.
+     * @return A non-const reference to this loader.
+     */
+    template<typename... Component, typename Archive, typename... Type, typename... Member>
+    basic_continuous_loader & component(Archive &archive, Member Type:: *... member) {
+        (remove_if_exists<Component>(), ...);
+        (assign<Component>(archive, member...), ...);
+        return *this;
+    }
+
+    /**
+     * @brief Helps to purge entities that no longer have a conterpart.
+     *
+     * Users should invoke this member function after restoring each snapshot,
+     * unless they know exactly what they are doing.
+     *
+     * @return A non-const reference to this loader.
+     */
+    basic_continuous_loader & shrink() {
+        auto it = remloc.begin();
+
+        while(it != remloc.cend()) {
+            const auto local = it->second.first;
+            bool &dirty = it->second.second;
+
+            if(dirty) {
+                dirty = false;
+                ++it;
+            } else {
+                if(reg->valid(local)) {
+                    reg->destroy(local);
+                }
+
+                it = remloc.erase(it);
+            }
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Destroys those entities that have no components.
+     *
+     * In case all the entities were serialized but only part of the components
+     * was saved, it could happen that some of the entities have no components
+     * once restored.<br/>
+     * This functions helps to identify and destroy those entities.
+     *
+     * @return A non-const reference to this loader.
+     */
+    basic_continuous_loader & orphans() {
+        reg->orphans([this](const auto entt) {
+            reg->destroy(entt);
+        });
+
+        return *this;
+    }
+
+    /**
+     * @brief Tests if a loader knows about a given entity.
+     * @param entt An entity identifier.
+     * @return True if `entity` is managed by the loader, false otherwise.
+     */
+    [[nodiscard]] bool contains(entity_type entt) const ENTT_NOEXCEPT {
+        return (remloc.find(entt) != remloc.cend());
+    }
+
+    /**
+     * @brief Returns the identifier to which an entity refers.
+     * @param entt An entity identifier.
+     * @return The local identifier if any, the null entity otherwise.
+     */
+    [[nodiscard]] entity_type map(entity_type entt) const ENTT_NOEXCEPT {
+        const auto it = remloc.find(entt);
+        entity_type other = null;
+
+        if(it != remloc.cend()) {
+            other = it->second.first;
+        }
+
+        return other;
+    }
+
+private:
+    std::unordered_map<entity_type, std::pair<entity_type, bool>> remloc;
+    basic_registry<entity_type> *reg;
+};
+
+
+}
+
+
+#endif

src/entt/entity/sparse_set.hpp

@@ -0,0 +1,616 @@
+#ifndef ENTT_ENTITY_SPARSE_SET_HPP
+#define ENTT_ENTITY_SPARSE_SET_HPP
+
+
+#include <iterator>
+#include <utility>
+#include <vector>
+#include <memory>
+#include <cstddef>
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/algorithm.hpp"
+#include "entity.hpp"
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Basic sparse set implementation.
+ *
+ * Sparse set or packed array or whatever is the name users give it.<br/>
+ * Two arrays: an _external_ one and an _internal_ one; a _sparse_ one and a
+ * _packed_ one; one used for direct access through contiguous memory, the other
+ * one used to get the data through an extra level of indirection.<br/>
+ * This is largely used by the registry to offer users the fastest access ever
+ * to the components. Views and groups in general are almost entirely designed
+ * around sparse sets.
+ *
+ * This type of data structure is widely documented in the literature and on the
+ * web. This is nothing more than a customized implementation suitable for the
+ * purpose of the framework.
+ *
+ * @note
+ * Internal data structures arrange elements to maximize performance. There are
+ * no guarantees that entities are returned in the insertion order when iterate
+ * a sparse set. Do not make assumption on the order in any case.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+class sparse_set {
+    static_assert(ENTT_PAGE_SIZE && ((ENTT_PAGE_SIZE & (ENTT_PAGE_SIZE - 1)) == 0), "ENTT_PAGE_SIZE must be a power of two");
+    static constexpr auto entt_per_page = ENTT_PAGE_SIZE / sizeof(Entity);
+
+    using traits_type = entt_traits<Entity>;
+    using page_type = std::unique_ptr<Entity[]>;
+
+    class sparse_set_iterator final {
+        friend class sparse_set<Entity>;
+
+        using packed_type = std::vector<Entity>;
+        using index_type = typename traits_type::difference_type;
+
+        sparse_set_iterator(const packed_type &ref, const index_type idx) ENTT_NOEXCEPT
+            : packed{&ref}, index{idx}
+        {}
+
+    public:
+        using difference_type = index_type;
+        using value_type = Entity;
+        using pointer = const value_type *;
+        using reference = const value_type &;
+        using iterator_category = std::random_access_iterator_tag;
+
+        sparse_set_iterator() ENTT_NOEXCEPT = default;
+
+        sparse_set_iterator & operator++() ENTT_NOEXCEPT {
+            return --index, *this;
+        }
+
+        sparse_set_iterator operator++(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        sparse_set_iterator & operator--() ENTT_NOEXCEPT {
+            return ++index, *this;
+        }
+
+        sparse_set_iterator operator--(int) ENTT_NOEXCEPT {
+            sparse_set_iterator orig = *this;
+            return operator--(), orig;
+        }
+
+        sparse_set_iterator & operator+=(const difference_type value) ENTT_NOEXCEPT {
+            index -= value;
+            return *this;
+        }
+
+        sparse_set_iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
+            sparse_set_iterator copy = *this;
+            return (copy += value);
+        }
+
+        sparse_set_iterator & operator-=(const difference_type value) ENTT_NOEXCEPT {
+            return (*this += -value);
+        }
+
+        sparse_set_iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
+            return (*this + -value);
+        }
+
+        difference_type operator-(const sparse_set_iterator &other) const ENTT_NOEXCEPT {
+            return other.index - index;
+        }
+
+        [[nodiscard]] reference operator[](const difference_type value) const {
+            const auto pos = size_type(index-value-1u);
+            return (*packed)[pos];
+        }
+
+        [[nodiscard]] bool operator==(const sparse_set_iterator &other) const ENTT_NOEXCEPT {
+            return other.index == index;
+        }
+
+        [[nodiscard]] bool operator!=(const sparse_set_iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+        [[nodiscard]] bool operator<(const sparse_set_iterator &other) const ENTT_NOEXCEPT {
+            return index > other.index;
+        }
+
+        [[nodiscard]] bool operator>(const sparse_set_iterator &other) const ENTT_NOEXCEPT {
+            return index < other.index;
+        }
+
+        [[nodiscard]] bool operator<=(const sparse_set_iterator &other) const ENTT_NOEXCEPT {
+            return !(*this > other);
+        }
+
+        [[nodiscard]] bool operator>=(const sparse_set_iterator &other) const ENTT_NOEXCEPT {
+            return !(*this < other);
+        }
+
+        [[nodiscard]] pointer operator->() const {
+            const auto pos = size_type(index-1u);
+            return &(*packed)[pos];
+        }
+
+        [[nodiscard]] reference operator*() const {
+            return *operator->();
+        }
+
+    private:
+        const packed_type *packed;
+        index_type index;
+    };
+
+    [[nodiscard]] auto page(const Entity entt) const ENTT_NOEXCEPT {
+        return size_type{(to_integral(entt) & traits_type::entity_mask) / entt_per_page};
+    }
+
+    [[nodiscard]] auto offset(const Entity entt) const ENTT_NOEXCEPT {
+        return size_type{to_integral(entt) & (entt_per_page - 1)};
+    }
+
+    [[nodiscard]] page_type & assure(const std::size_t pos) {
+        if(!(pos < sparse.size())) {
+            sparse.resize(pos+1);
+        }
+
+        if(!sparse[pos]) {
+            sparse[pos].reset(new entity_type[entt_per_page]);
+            // null is safe in all cases for our purposes
+            for(auto *first = sparse[pos].get(), *last = first + entt_per_page; first != last; ++first) {
+                *first = null;
+            }
+        }
+
+        return sparse[pos];
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Random access iterator type. */
+    using iterator = sparse_set_iterator;
+    /*! @brief Reverse iterator type. */
+    using reverse_iterator = const entity_type *;
+
+    /*! @brief Default constructor. */
+    sparse_set() = default;
+
+    /*! @brief Default move constructor. */
+    sparse_set(sparse_set &&) = default;
+
+    /*! @brief Default destructor. */
+    virtual ~sparse_set() = default;
+
+    /*! @brief Default move assignment operator. @return This sparse set. */
+    sparse_set & operator=(sparse_set &&) = default;
+
+    /**
+     * @brief Increases the capacity of a sparse set.
+     *
+     * If the new capacity is greater than the current capacity, new storage is
+     * allocated, otherwise the method does nothing.
+     *
+     * @param cap Desired capacity.
+     */
+    void reserve(const size_type cap) {
+        packed.reserve(cap);
+    }
+
+    /**
+     * @brief Returns the number of elements that a sparse set has currently
+     * allocated space for.
+     * @return Capacity of the sparse set.
+     */
+    [[nodiscard]] size_type capacity() const ENTT_NOEXCEPT {
+        return packed.capacity();
+    }
+
+    /*! @brief Requests the removal of unused capacity. */
+    void shrink_to_fit() {
+        // conservative approach
+        if(packed.empty()) {
+            sparse.clear();
+        }
+
+        sparse.shrink_to_fit();
+        packed.shrink_to_fit();
+    }
+
+    /**
+     * @brief Returns the extent of a sparse set.
+     *
+     * The extent of a sparse set is also the size of the internal sparse array.
+     * There is no guarantee that the internal packed array has the same size.
+     * Usually the size of the internal sparse array is equal or greater than
+     * the one of the internal packed array.
+     *
+     * @return Extent of the sparse set.
+     */
+    [[nodiscard]] size_type extent() const ENTT_NOEXCEPT {
+        return sparse.size() * entt_per_page;
+    }
+
+    /**
+     * @brief Returns the number of elements in a sparse set.
+     *
+     * The number of elements is also the size of the internal packed array.
+     * There is no guarantee that the internal sparse array has the same size.
+     * Usually the size of the internal sparse array is equal or greater than
+     * the one of the internal packed array.
+     *
+     * @return Number of elements.
+     */
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return packed.size();
+    }
+
+    /**
+     * @brief Checks whether a sparse set is empty.
+     * @return True if the sparse set is empty, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return packed.empty();
+    }
+
+    /**
+     * @brief Direct access to the internal packed array.
+     *
+     * The returned pointer is such that range `[data(), data() + size()]` is
+     * always a valid range, even if the container is empty.
+     *
+     * @note
+     * Entities are in the reverse order as returned by the `begin`/`end`
+     * iterators.
+     *
+     * @return A pointer to the internal packed array.
+     */
+    [[nodiscard]] const entity_type * data() const ENTT_NOEXCEPT {
+        return packed.data();
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * The returned iterator points to the first entity of the internal packed
+     * array. If the sparse set is empty, the returned iterator will be equal to
+     * `end()`.
+     *
+     * @return An iterator to the first entity of the internal packed array.
+     */
+    [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+        const typename traits_type::difference_type pos = packed.size();
+        return iterator{packed, pos};
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     *
+     * The returned iterator points to the element following the last entity in
+     * the internal packed array. Attempting to dereference the returned
+     * iterator results in undefined behavior.
+     *
+     * @return An iterator to the element following the last entity of the
+     * internal packed array.
+     */
+    [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+        return iterator{packed, {}};
+    }
+
+    /**
+     * @brief Returns a reverse iterator to the beginning.
+     *
+     * The returned iterator points to the first entity of the reversed internal
+     * packed array. If the sparse set is empty, the returned iterator will be
+     * equal to `rend()`.
+     *
+     * @return An iterator to the first entity of the reversed internal packed
+     * array.
+     */
+    [[nodiscard]] reverse_iterator rbegin() const ENTT_NOEXCEPT {
+        return packed.data();
+    }
+
+    /**
+     * @brief Returns a reverse iterator to the end.
+     *
+     * The returned iterator points to the element following the last entity in
+     * the reversed internal packed array. Attempting to dereference the
+     * returned iterator results in undefined behavior.
+     *
+     * @return An iterator to the element following the last entity of the
+     * reversed internal packed array.
+     */
+    [[nodiscard]] reverse_iterator rend() const ENTT_NOEXCEPT {
+        return rbegin() + packed.size();
+    }
+
+    /**
+     * @brief Finds an entity.
+     * @param entt A valid entity identifier.
+     * @return An iterator to the given entity if it's found, past the end
+     * iterator otherwise.
+     */
+    [[nodiscard]] iterator find(const entity_type entt) const {
+        return contains(entt) ? --(end() - index(entt)) : end();
+    }
+
+    /**
+     * @brief Checks if a sparse set contains an entity.
+     * @param entt A valid entity identifier.
+     * @return True if the sparse set contains the entity, false otherwise.
+     */
+    [[nodiscard]] bool contains(const entity_type entt) const {
+        const auto curr = page(entt);
+        // testing against null permits to avoid accessing the packed array
+        return (curr < sparse.size() && sparse[curr] && sparse[curr][offset(entt)] != null);
+    }
+
+    /**
+     * @brief Returns the position of an entity in a sparse set.
+     *
+     * @warning
+     * Attempting to get the position of an entity that doesn't belong to the
+     * sparse set results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * sparse set doesn't contain the given entity.
+     *
+     * @param entt A valid entity identifier.
+     * @return The position of the entity in the sparse set.
+     */
+    [[nodiscard]] size_type index(const entity_type entt) const {
+        ENTT_ASSERT(contains(entt));
+        return size_type{to_integral(sparse[page(entt)][offset(entt)])};
+    }
+
+    /**
+     * @brief Assigns an entity to a sparse set.
+     *
+     * @warning
+     * Attempting to assign an entity that already belongs to the sparse set
+     * results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * sparse set already contains the given entity.
+     *
+     * @param entt A valid entity identifier.
+     */
+    void emplace(const entity_type entt) {
+        ENTT_ASSERT(!contains(entt));
+        assure(page(entt))[offset(entt)] = entity_type(static_cast<typename traits_type::entity_type>(packed.size()));
+        packed.push_back(entt);
+    }
+
+    /**
+     * @brief Assigns one or more entities to a sparse set.
+     *
+     * @warning
+     * Attempting to assign an entity that already belongs to the sparse set
+     * results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * sparse set already contains the given entity.
+     *
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     */
+    template<typename It>
+    void insert(It first, It last) {
+        auto next = static_cast<typename traits_type::entity_type>(packed.size());
+        packed.insert(packed.end(), first, last);
+
+        while(first != last) {
+            const auto entt = *(first++);
+            ENTT_ASSERT(!contains(entt));
+            assure(page(entt))[offset(entt)] = entity_type(next++);
+        }
+    }
+
+    /**
+     * @brief Removes an entity from a sparse set.
+     *
+     * @warning
+     * Attempting to remove an entity that doesn't belong to the sparse set
+     * results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * sparse set doesn't contain the given entity.
+     *
+     * @param entt A valid entity identifier.
+     */
+    void erase(const entity_type entt) {
+        ENTT_ASSERT(contains(entt));
+        const auto curr = page(entt);
+        const auto pos = offset(entt);
+        packed[size_type{to_integral(sparse[curr][pos])}] = packed.back();
+        sparse[page(packed.back())][offset(packed.back())] = sparse[curr][pos];
+        sparse[curr][pos] = null;
+        packed.pop_back();
+    }
+
+    /**
+     * @brief Swaps two entities in the internal packed array.
+     *
+     * For what it's worth, this function affects both the internal sparse array
+     * and the internal packed array. Users should not care of that anyway.
+     *
+     * @warning
+     * Attempting to swap entities that don't belong to the sparse set results
+     * in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * sparse set doesn't contain the given entities.
+     *
+     * @param lhs A valid entity identifier.
+     * @param rhs A valid entity identifier.
+     */
+    virtual void swap(const entity_type lhs, const entity_type rhs) {
+        auto &from = sparse[page(lhs)][offset(lhs)];
+        auto &to = sparse[page(rhs)][offset(rhs)];
+        std::swap(packed[size_type{to_integral(from)}], packed[size_type{to_integral(to)}]);
+        std::swap(from, to);
+    }
+
+    /**
+     * @brief Sort elements according to the given comparison function.
+     *
+     * Sort the elements so that iterating the range with a couple of iterators
+     * returns them in the expected order. See `begin` and `end` for more
+     * details.
+     *
+     * The comparison function object must return `true` if the first element
+     * is _less_ than the second one, `false` otherwise. The signature of the
+     * comparison function should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * bool(const Entity, const Entity);
+     * @endcode
+     *
+     * Moreover, the comparison function object shall induce a
+     * _strict weak ordering_ on the values.
+     *
+     * The sort function oject must offer a member function template
+     * `operator()` that accepts three arguments:
+     *
+     * * An iterator to the first element of the range to sort.
+     * * An iterator past the last element of the range to sort.
+     * * A comparison function to use to compare the elements.
+     *
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param first An iterator to the first element of the range to sort.
+     * @param last An iterator past the last element of the range to sort.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename Compare, typename Sort = std_sort, typename... Args>
+    void sort(iterator first, iterator last, Compare compare, Sort algo = Sort{}, Args &&... args) {
+        ENTT_ASSERT(!(last < first));
+        ENTT_ASSERT(!(last > end()));
+
+        const auto length = std::distance(first, last);
+        const auto skip = std::distance(last, end());
+        const auto to = packed.rend() - skip;
+        const auto from = to - length;
+
+        algo(from, to, std::move(compare), std::forward<Args>(args)...);
+
+        for(size_type pos = skip, end = skip+length; pos < end; ++pos) {
+            sparse[page(packed[pos])][offset(packed[pos])] = entity_type(static_cast<typename traits_type::entity_type>(pos));
+        }
+    }
+
+    /**
+     * @brief Sort elements according to the given comparison function.
+     *
+     * @sa sort
+     *
+     * This function is a slightly slower version of `sort` that invokes the
+     * caller to indicate which entities are swapped.<br/>
+     * It's recommended when the caller wants to sort its own data structures to
+     * align them with the order induced in the sparse set.
+     *
+     * The signature of the callback should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * bool(const Entity, const Entity);
+     * @endcode
+     *
+     * @tparam Apply Type of function object to invoke to notify the caller.
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param first An iterator to the first element of the range to sort.
+     * @param last An iterator past the last element of the range to sort.
+     * @param apply A valid function object to use as a callback.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename Apply, typename Compare, typename Sort = std_sort, typename... Args>
+    void arrange(iterator first, iterator last, Apply apply, Compare compare, Sort algo = Sort{}, Args &&... args) {
+        ENTT_ASSERT(!(last < first));
+        ENTT_ASSERT(!(last > end()));
+
+        const auto length = std::distance(first, last);
+        const auto skip = std::distance(last, end());
+        const auto to = packed.rend() - skip;
+        const auto from = to - length;
+
+        algo(from, to, std::move(compare), std::forward<Args>(args)...);
+
+        for(size_type pos = skip, end = skip+length; pos < end; ++pos) {
+            auto curr = pos;
+            auto next = index(packed[curr]);
+
+            while(curr != next) {
+                apply(packed[curr], packed[next]);
+                sparse[page(packed[curr])][offset(packed[curr])] = entity_type(static_cast<typename traits_type::entity_type>(curr));
+
+                curr = next;
+                next = index(packed[curr]);
+            }
+        }
+    }
+
+    /**
+     * @brief Sort entities according to their order in another sparse set.
+     *
+     * Entities that are part of both the sparse sets are ordered internally
+     * according to the order they have in `other`. All the other entities goes
+     * to the end of the list and there are no guarantees on their order.<br/>
+     * In other terms, this function can be used to impose the same order on two
+     * sets by using one of them as a master and the other one as a slave.
+     *
+     * Iterating the sparse set with a couple of iterators returns elements in
+     * the expected order after a call to `respect`. See `begin` and `end` for
+     * more details.
+     *
+     * @param other The sparse sets that imposes the order of the entities.
+     */
+    void respect(const sparse_set &other) {
+        const auto to = other.end();
+        auto from = other.begin();
+
+        size_type pos = packed.size() - 1;
+
+        while(pos && from != to) {
+            if(contains(*from)) {
+                if(*from != packed[pos]) {
+                    swap(packed[pos], *from);
+                }
+
+                --pos;
+            }
+
+            ++from;
+        }
+    }
+
+    /**
+     * @brief Clears a sparse set.
+     */
+    void clear() ENTT_NOEXCEPT {
+        sparse.clear();
+        packed.clear();
+    }
+
+private:
+    std::vector<page_type> sparse;
+    std::vector<entity_type> packed;
+};
+
+
+}
+
+
+#endif

src/entt/entity/storage.hpp

@@ -0,0 +1,578 @@
+#ifndef ENTT_ENTITY_STORAGE_HPP
+#define ENTT_ENTITY_STORAGE_HPP
+
+
+#include <algorithm>
+#include <iterator>
+#include <utility>
+#include <vector>
+#include <cstddef>
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/algorithm.hpp"
+#include "../core/type_traits.hpp"
+#include "entity.hpp"
+#include "sparse_set.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Basic storage implementation.
+ *
+ * This class is a refinement of a sparse set that associates an object to an
+ * entity. The main purpose of this class is to extend sparse sets to store
+ * components in a registry. It guarantees fast access both to the elements and
+ * to the entities.
+ *
+ * @note
+ * Entities and objects have the same order. It's guaranteed both in case of raw
+ * access (either to entities or objects) and when using random or input access
+ * iterators.
+ *
+ * @note
+ * Internal data structures arrange elements to maximize performance. There are
+ * no guarantees that objects are returned in the insertion order when iterate
+ * a storage. Do not make assumption on the order in any case.
+ *
+ * @warning
+ * Empty types aren't explicitly instantiated. Therefore, many of the functions
+ * normally available for non-empty types will not be available for empty ones.
+ *
+ * @sa sparse_set<Entity>
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Type Type of objects assigned to the entities.
+ */
+template<typename Entity, typename Type, typename = std::void_t<>>
+class storage: public sparse_set<Entity> {
+    static_assert(std::is_move_constructible_v<Type> && std::is_move_assignable_v<Type>, "The managed type must be at least move constructible and assignable");
+
+    using underlying_type = sparse_set<Entity>;
+    using traits_type = entt_traits<Entity>;
+
+    template<bool Const>
+    class storage_iterator final {
+        friend class storage<Entity, Type>;
+
+        using instance_type = std::conditional_t<Const, const std::vector<Type>, std::vector<Type>>;
+        using index_type = typename traits_type::difference_type;
+
+        storage_iterator(instance_type &ref, const index_type idx) ENTT_NOEXCEPT
+            : instances{&ref}, index{idx}
+        {}
+
+    public:
+        using difference_type = index_type;
+        using value_type = Type;
+        using pointer = std::conditional_t<Const, const value_type *, value_type *>;
+        using reference = std::conditional_t<Const, const value_type &, value_type &>;
+        using iterator_category = std::random_access_iterator_tag;
+
+        storage_iterator() ENTT_NOEXCEPT = default;
+
+        storage_iterator & operator++() ENTT_NOEXCEPT {
+            return --index, *this;
+        }
+
+        storage_iterator operator++(int) ENTT_NOEXCEPT {
+            storage_iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        storage_iterator & operator--() ENTT_NOEXCEPT {
+            return ++index, *this;
+        }
+
+        storage_iterator operator--(int) ENTT_NOEXCEPT {
+            storage_iterator orig = *this;
+            return operator--(), orig;
+        }
+
+        storage_iterator & operator+=(const difference_type value) ENTT_NOEXCEPT {
+            index -= value;
+            return *this;
+        }
+
+        storage_iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
+            storage_iterator copy = *this;
+            return (copy += value);
+        }
+
+        storage_iterator & operator-=(const difference_type value) ENTT_NOEXCEPT {
+            return (*this += -value);
+        }
+
+        storage_iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
+            return (*this + -value);
+        }
+
+        difference_type operator-(const storage_iterator &other) const ENTT_NOEXCEPT {
+            return other.index - index;
+        }
+
+        [[nodiscard]] reference operator[](const difference_type value) const ENTT_NOEXCEPT {
+            const auto pos = size_type(index-value-1);
+            return (*instances)[pos];
+        }
+
+        [[nodiscard]] bool operator==(const storage_iterator &other) const ENTT_NOEXCEPT {
+            return other.index == index;
+        }
+
+        [[nodiscard]] bool operator!=(const storage_iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+        [[nodiscard]] bool operator<(const storage_iterator &other) const ENTT_NOEXCEPT {
+            return index > other.index;
+        }
+
+        [[nodiscard]] bool operator>(const storage_iterator &other) const ENTT_NOEXCEPT {
+            return index < other.index;
+        }
+
+        [[nodiscard]] bool operator<=(const storage_iterator &other) const ENTT_NOEXCEPT {
+            return !(*this > other);
+        }
+
+        [[nodiscard]] bool operator>=(const storage_iterator &other) const ENTT_NOEXCEPT {
+            return !(*this < other);
+        }
+
+        [[nodiscard]] pointer operator->() const ENTT_NOEXCEPT {
+            const auto pos = size_type(index-1u);
+            return &(*instances)[pos];
+        }
+
+        [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
+            return *operator->();
+        }
+
+    private:
+        instance_type *instances;
+        index_type index;
+    };
+
+public:
+    /*! @brief Type of the objects associated with the entities. */
+    using object_type = Type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Random access iterator type. */
+    using iterator = storage_iterator<false>;
+    /*! @brief Constant random access iterator type. */
+    using const_iterator = storage_iterator<true>;
+    /*! @brief Reverse iterator type. */
+    using reverse_iterator = Type *;
+    /*! @brief Constant reverse iterator type. */
+    using const_reverse_iterator = const Type *;
+
+
+    /**
+     * @brief Increases the capacity of a storage.
+     *
+     * If the new capacity is greater than the current capacity, new storage is
+     * allocated, otherwise the method does nothing.
+     *
+     * @param cap Desired capacity.
+     */
+    void reserve(const size_type cap) {
+        underlying_type::reserve(cap);
+        instances.reserve(cap);
+    }
+
+    /*! @brief Requests the removal of unused capacity. */
+    void shrink_to_fit() {
+        underlying_type::shrink_to_fit();
+        instances.shrink_to_fit();
+    }
+
+    /**
+     * @brief Direct access to the array of objects.
+     *
+     * The returned pointer is such that range `[raw(), raw() + size()]` is
+     * always a valid range, even if the container is empty.
+     *
+     * @note
+     * Objects are in the reverse order as returned by the `begin`/`end`
+     * iterators.
+     *
+     * @return A pointer to the array of objects.
+     */
+    [[nodiscard]] const object_type * raw() const ENTT_NOEXCEPT {
+        return instances.data();
+    }
+
+    /*! @copydoc raw */
+    [[nodiscard]] object_type * raw() ENTT_NOEXCEPT {
+        return const_cast<object_type *>(std::as_const(*this).raw());
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * The returned iterator points to the first instance of the internal array.
+     * If the storage 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 {
+        const typename traits_type::difference_type pos = underlying_type::size();
+        return const_iterator{instances, pos};
+    }
+
+    /*! @copydoc cbegin */
+    [[nodiscard]] const_iterator begin() const ENTT_NOEXCEPT {
+        return cbegin();
+    }
+
+    /*! @copydoc begin */
+    [[nodiscard]] iterator begin() ENTT_NOEXCEPT {
+        const typename traits_type::difference_type pos = underlying_type::size();
+        return iterator{instances, pos};
+    }
+
+    /**
+     * @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 const_iterator{instances, {}};
+    }
+
+    /*! @copydoc cend */
+    [[nodiscard]] const_iterator end() const ENTT_NOEXCEPT {
+        return cend();
+    }
+
+    /*! @copydoc end */
+    [[nodiscard]] iterator end() ENTT_NOEXCEPT {
+        return iterator{instances, {}};
+    }
+
+    /**
+     * @brief Returns a reverse iterator to the beginning.
+     *
+     * The returned iterator points to the first instance of the reversed
+     * internal array. If the storage is empty, the returned iterator will be
+     * equal to `rend()`.
+     *
+     * @return An iterator to the first instance of the reversed internal array.
+     */
+    [[nodiscard]] const_reverse_iterator crbegin() const ENTT_NOEXCEPT {
+        return instances.data();
+    }
+
+    /*! @copydoc crbegin */
+    [[nodiscard]] const_reverse_iterator rbegin() const ENTT_NOEXCEPT {
+        return crbegin();
+    }
+
+    /*! @copydoc rbegin */
+    [[nodiscard]] reverse_iterator rbegin() ENTT_NOEXCEPT {
+        return instances.data();
+    }
+
+    /**
+     * @brief Returns a reverse iterator to the end.
+     *
+     * The returned iterator points to the element following the last instance
+     * of the reversed internal array. Attempting to dereference the returned
+     * iterator results in undefined behavior.
+     *
+     * @return An iterator to the element following the last instance of the
+     * reversed internal array.
+     */
+    [[nodiscard]] const_reverse_iterator crend() const ENTT_NOEXCEPT {
+        return crbegin() + instances.size();
+    }
+
+    /*! @copydoc crend */
+    [[nodiscard]] const_reverse_iterator rend() const ENTT_NOEXCEPT {
+        return crend();
+    }
+
+    /*! @copydoc rend */
+    [[nodiscard]] reverse_iterator rend() ENTT_NOEXCEPT {
+        return rbegin() + instances.size();
+    }
+
+    /**
+     * @brief Returns the object associated with an entity.
+     *
+     * @warning
+     * Attempting to use an entity that doesn't belong to the storage results in
+     * undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * storage doesn't contain the given entity.
+     *
+     * @param entt A valid entity identifier.
+     * @return The object associated with the entity.
+     */
+    [[nodiscard]] const object_type & get(const entity_type entt) const {
+        return instances[underlying_type::index(entt)];
+    }
+
+    /*! @copydoc get */
+    [[nodiscard]] object_type & get(const entity_type entt) {
+        return const_cast<object_type &>(std::as_const(*this).get(entt));
+    }
+
+    /**
+     * @brief Returns a pointer to the object associated with an entity, if any.
+     * @param entt A valid entity identifier.
+     * @return The object associated with the entity, if any.
+     */
+    [[nodiscard]] const object_type * try_get(const entity_type entt) const {
+        return underlying_type::contains(entt) ? (instances.data() + underlying_type::index(entt)) : nullptr;
+    }
+
+    /*! @copydoc try_get */
+    [[nodiscard]] object_type * try_get(const entity_type entt) {
+        return const_cast<object_type *>(std::as_const(*this).try_get(entt));
+    }
+
+    /**
+     * @brief Assigns an entity to a storage and constructs its object.
+     *
+     * This version accept both types that can be constructed in place directly
+     * and types like aggregates that do not work well with a placement new as
+     * performed usually under the hood during an _emplace back_.
+     *
+     * @warning
+     * Attempting to use an entity that already belongs to the storage results
+     * in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * storage already contains the given entity.
+     *
+     * @tparam Args Types of arguments to use to construct the object.
+     * @param entt A valid entity identifier.
+     * @param args Parameters to use to construct an object for the entity.
+     */
+    template<typename... Args>
+    void emplace(const entity_type entt, Args &&... args) {
+        if constexpr(std::is_aggregate_v<object_type>) {
+            instances.push_back(Type{std::forward<Args>(args)...});
+        } else {
+            instances.emplace_back(std::forward<Args>(args)...);
+        }
+
+        // entity goes after component in case constructor throws
+        underlying_type::emplace(entt);
+    }
+
+    /**
+     * @brief Assigns one or more entities to a storage and constructs their
+     * objects from a given instance.
+     *
+     * @warning
+     * Attempting to assign an entity that already belongs to the storage
+     * results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * storage already contains the given entity.
+     *
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param value An instance of the object to construct.
+     */
+    template<typename It>
+    void insert(It first, It last, const object_type &value = {}) {
+        instances.insert(instances.end(), std::distance(first, last), value);
+        // entities go after components in case constructors throw
+        underlying_type::insert(first, last);
+    }
+
+    /**
+     * @brief Assigns one or more entities to a storage and constructs their
+     * objects from a given range.
+     *
+     * @sa construct
+     *
+     * @tparam EIt Type of input iterator.
+     * @tparam CIt Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param from An iterator to the first element of the range of objects.
+     * @param to An iterator past the last element of the range of objects.
+     */
+    template<typename EIt, typename CIt>
+    void insert(EIt first, EIt last, CIt from, CIt to) {
+        instances.insert(instances.end(), from, to);
+        // entities go after components in case constructors throw
+        underlying_type::insert(first, last);
+    }
+
+    /**
+     * @brief Removes an entity from a storage and destroys its object.
+     *
+     * @warning
+     * Attempting to use an entity that doesn't belong to the storage results in
+     * undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * storage doesn't contain the given entity.
+     *
+     * @param entt A valid entity identifier.
+     */
+    void erase(const entity_type entt) {
+        auto other = std::move(instances.back());
+        instances[underlying_type::index(entt)] = std::move(other);
+        instances.pop_back();
+        underlying_type::erase(entt);
+    }
+
+    /**
+     * @brief Swaps entities and objects in the internal packed arrays.
+     *
+     * @warning
+     * Attempting to swap entities that don't belong to the sparse set results
+     * in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * sparse set doesn't contain the given entities.
+     *
+     * @param lhs A valid entity identifier.
+     * @param rhs A valid entity identifier.
+     */
+    void swap(const entity_type lhs, const entity_type rhs) override {
+        std::swap(instances[underlying_type::index(lhs)], instances[underlying_type::index(rhs)]);
+        underlying_type::swap(lhs, rhs);
+    }
+
+    /**
+     * @brief Sort elements according to the given comparison function.
+     *
+     * Sort the elements so that iterating the range with a couple of iterators
+     * returns them in the expected order. See `begin` and `end` for more
+     * details.
+     *
+     * The comparison function object must return `true` if the first element
+     * is _less_ than the second one, `false` otherwise. The signature of the
+     * comparison function should be equivalent to one of the following:
+     *
+     * @code{.cpp}
+     * bool(const Entity, const Entity);
+     * bool(const Type &, const Type &);
+     * @endcode
+     *
+     * Moreover, the comparison function object shall induce a
+     * _strict weak ordering_ on the values.
+     *
+     * The sort function oject must offer a member function template
+     * `operator()` that accepts three arguments:
+     *
+     * * An iterator to the first element of the range to sort.
+     * * An iterator past the last element of the range to sort.
+     * * A comparison function to use to compare the elements.
+     *
+     * @warning
+     * Empty types are never instantiated. Therefore, only comparison function
+     * objects that require to return entities rather than components are
+     * accepted.
+     *
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param first An iterator to the first element of the range to sort.
+     * @param last An iterator past the last element of the range to sort.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename Compare, typename Sort = std_sort, typename... Args>
+    void sort(iterator first, iterator last, Compare compare, Sort algo = Sort{}, Args &&... args) {
+        ENTT_ASSERT(!(last < first));
+        ENTT_ASSERT(!(last > end()));
+
+        const auto from = underlying_type::begin() + std::distance(begin(), first);
+        const auto to = from + std::distance(first, last);
+
+        const auto apply = [this](const auto lhs, const auto rhs) {
+            std::swap(instances[underlying_type::index(lhs)], instances[underlying_type::index(rhs)]);
+        };
+
+        if constexpr(std::is_invocable_v<Compare, const object_type &, const object_type &>) {
+            underlying_type::arrange(from, to, std::move(apply), [this, compare = std::move(compare)](const auto lhs, const auto rhs) {
+                return compare(std::as_const(instances[underlying_type::index(lhs)]), std::as_const(instances[underlying_type::index(rhs)]));
+            }, std::move(algo), std::forward<Args>(args)...);
+        } else {
+            underlying_type::arrange(from, to, std::move(apply), std::move(compare), std::move(algo), std::forward<Args>(args)...);
+        }
+    }
+
+    /*! @brief Clears a storage. */
+    void clear() {
+        underlying_type::clear();
+        instances.clear();
+    }
+
+private:
+    std::vector<object_type> instances;
+};
+
+
+/*! @copydoc storage */
+template<typename Entity, typename Type>
+class storage<Entity, Type, std::enable_if_t<is_eto_eligible_v<Type>>>: public sparse_set<Entity> {
+    using underlying_type = sparse_set<Entity>;
+
+public:
+    /*! @brief Type of the objects associated with the entities. */
+    using object_type = Type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+
+    /**
+     * @brief Assigns an entity to a storage and constructs its object.
+     *
+     * @warning
+     * Attempting to use an entity that already belongs to the storage results
+     * in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * storage already contains the given entity.
+     *
+     * @tparam Args Types of arguments to use to construct the object.
+     * @param entt A valid entity identifier.
+     * @param args Parameters to use to construct an object for the entity.
+     */
+    template<typename... Args>
+    void emplace(const entity_type entt, Args &&... args) {
+        [[maybe_unused]] object_type instance{std::forward<Args>(args)...};
+        underlying_type::emplace(entt);
+    }
+
+    /**
+     * @brief Assigns one or more entities to a storage.
+     *
+     * @warning
+     * Attempting to assign an entity that already belongs to the storage
+     * results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * storage already contains the given entity.
+     *
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     */
+    template<typename It>
+    void insert(It first, It last, const object_type & = {}) {
+        underlying_type::insert(first, last);
+    }
+};
+
+
+}
+
+
+#endif

src/entt/entity/utility.hpp

@@ -0,0 +1,46 @@
+#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{};
+
+
+}
+
+
+#endif

src/entt/entt.hpp

@@ -0,0 +1,44 @@
+#include "core/algorithm.hpp"
+#include "core/attribute.h"
+#include "core/family.hpp"
+#include "core/hashed_string.hpp"
+#include "core/ident.hpp"
+#include "core/monostate.hpp"
+#include "core/type_info.hpp"
+#include "core/type_traits.hpp"
+#include "core/utility.hpp"
+#include "entity/actor.hpp"
+#include "entity/entity.hpp"
+#include "entity/group.hpp"
+#include "entity/handle.hpp"
+#include "entity/helper.hpp"
+#include "entity/observer.hpp"
+#include "entity/pool.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/container.hpp"
+#include "meta/ctx.hpp"
+#include "meta/factory.hpp"
+#include "meta/internal.hpp"
+#include "meta/meta.hpp"
+#include "meta/pointer.hpp"
+#include "meta/policy.hpp"
+#include "meta/range.hpp"
+#include "meta/resolve.hpp"
+#include "meta/type_traits.hpp"
+#include "platform/android-ndk-r17.hpp"
+#include "process/process.hpp"
+#include "process/scheduler.hpp"
+#include "resource/cache.hpp"
+#include "resource/handle.hpp"
+#include "resource/loader.hpp"
+#include "signal/delegate.hpp"
+#include "signal/dispatcher.hpp"
+#include "signal/emitter.hpp"
+#include "signal/sigh.hpp"

src/entt/fwd.hpp

@@ -0,0 +1,4 @@
+#include "core/fwd.hpp"
+#include "entity/fwd.hpp"
+#include "resource/fwd.hpp"
+#include "signal/fwd.hpp"

src/entt/locator/locator.hpp

@@ -0,0 +1,111 @@
+#ifndef ENTT_LOCATOR_LOCATOR_HPP
+#define ENTT_LOCATOR_LOCATOR_HPP
+
+
+#include <memory>
+#include <utility>
+#include "../config/config.h"
+
+
+namespace entt {
+
+
+/**
+ * @brief Service locator, nothing more.
+ *
+ * A service locator can be used to do what it promises: locate services.<br/>
+ * Usually service locators are tightly bound to the services they expose and
+ * thus it's hard to define a general purpose class to do that. This template
+ * based implementation tries to fill the gap and to get rid of the burden of
+ * defining a different specific locator for each application.
+ *
+ * @tparam Service Type of service managed by the locator.
+ */
+template<typename Service>
+struct service_locator {
+    /*! @brief Type of service offered. */
+    using service_type = Service;
+
+    /*! @brief Default constructor, deleted on purpose. */
+    service_locator() = delete;
+    /*! @brief Default destructor, deleted on purpose. */
+    ~service_locator() = delete;
+
+    /**
+     * @brief Tests if a valid service implementation is set.
+     * @return True if the service is set, false otherwise.
+     */
+    [[nodiscard]] static bool empty() ENTT_NOEXCEPT {
+        return !static_cast<bool>(service);
+    }
+
+    /**
+     * @brief Returns a weak pointer to a service implementation, if any.
+     *
+     * Clients of a service shouldn't retain references to it. The recommended
+     * way is to retrieve the service implementation currently set each and
+     * every time the need of using it arises. Otherwise users can incur in
+     * unexpected behaviors.
+     *
+     * @return A reference to the service implementation currently set, if any.
+     */
+    [[nodiscard]] static std::weak_ptr<Service> get() ENTT_NOEXCEPT {
+        return service;
+    }
+
+    /**
+     * @brief Returns a weak reference to a service implementation, if any.
+     *
+     * Clients of a service shouldn't retain references to it. The recommended
+     * way is to retrieve the service implementation currently set each and
+     * every time the need of using it arises. Otherwise users can incur in
+     * unexpected behaviors.
+     *
+     * @warning
+     * In case no service implementation has been set, a call to this function
+     * results in undefined behavior.
+     *
+     * @return A reference to the service implementation currently set, if any.
+     */
+    [[nodiscard]] static Service & ref() ENTT_NOEXCEPT {
+        return *service;
+    }
+
+    /**
+     * @brief Sets or replaces a service.
+     * @tparam Impl Type of the new service to use.
+     * @tparam Args Types of arguments to use to construct the service.
+     * @param args Parameters to use to construct the service.
+     */
+    template<typename Impl = Service, typename... Args>
+    static void set(Args &&... args) {
+        service = std::make_shared<Impl>(std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Sets or replaces a service.
+     * @param ptr Service to use to replace the current one.
+     */
+    static void set(std::shared_ptr<Service> ptr) {
+        ENTT_ASSERT(static_cast<bool>(ptr));
+        service = std::move(ptr);
+    }
+
+    /**
+     * @brief Resets a service.
+     *
+     * The service is no longer valid after a reset.
+     */
+    static void reset() {
+        service.reset();
+    }
+
+private:
+    inline static std::shared_ptr<Service> service = nullptr;
+};
+
+
+}
+
+
+#endif

src/entt/meta/container.hpp

@@ -0,0 +1,387 @@
+#ifndef ENTT_META_CONTAINER_HPP
+#define ENTT_META_CONTAINER_HPP
+
+
+#include <array>
+#include <map>
+#include <set>
+#include <unordered_map>
+#include <unordered_set>
+#include <utility>
+#include <vector>
+#include "../config/config.h"
+#include "type_traits.hpp"
+
+
+namespace entt {
+
+
+namespace internal {
+
+
+template<typename Container, template<typename> class... Trait>
+struct container_traits: public Trait<Container>... {};
+
+
+/**
+ * @brief Basic STL-compatible container traits
+ * @tparam Container The type of the container.
+ */
+template<typename Container>
+struct basic_container {
+    /*! @brief Iterator type of the container. */
+    using iterator = typename Container::iterator;
+    /*! @brief Unsigned integer type. */
+    using size_type = typename Container::size_type;
+    /*! @brief Value type of the container. */
+    using value_type = typename Container::value_type;
+
+    /**
+     * @brief Returns the size of the given container.
+     * @param cont The container for which to return the size.
+     * @return The size of the given container.
+     */
+    [[nodiscard]] static size_type size(const Container &cont) ENTT_NOEXCEPT {
+        return cont.size();
+    }
+
+    /**
+     * @brief Returns an iterator to the first element of the given container.
+     * @param cont The container for which to return the iterator.
+     * @return An iterator to the first element of the given container.
+     */
+    [[nodiscard]] static iterator begin(Container &cont) {
+        return cont.begin();
+    }
+
+    /**
+     * @brief Returns an iterator past the last element of the given container.
+     * @param cont The container for which to return the iterator.
+     * @return An iterator past the last element of the given container.
+     */
+    [[nodiscard]] static iterator end(Container &cont) {
+        return cont.end();
+    }
+};
+
+
+/**
+ * @brief Basic STL-compatible associative container traits
+ * @tparam Container The type of the container.
+ */
+template<typename Container>
+struct basic_associative_container {
+    /*! @brief Key type of the sequence container. */
+    using key_type = typename Container::key_type;
+
+    /**
+     * @brief Returns an iterator to the element with key equivalent to the given
+     * one, if any.
+     * @param cont The container in which to search for the element.
+     * @param key The key of the element to search.
+     * @return An iterator to the element with the given key, if any.
+     */
+    [[nodiscard]] static typename Container::iterator find(Container &cont, const key_type &key) {
+        return cont.find(key);
+    }
+};
+
+
+/**
+ * @brief Basic STL-compatible dynamic container traits
+ * @tparam Container The type of the container.
+ */
+template<typename Container>
+struct basic_dynamic_container {
+    /**
+     * @brief Clears the content of the given container.
+     * @param cont The container for which to clear the content.
+     * @return True in case of success, false otherwise.
+     */
+    [[nodiscard]] static bool clear(Container &cont) {
+        return cont.clear(), true;
+    }
+};
+
+
+/**
+ * @brief Basic STL-compatible dynamic associative container traits
+ * @tparam Container The type of the container.
+ */
+template<typename Container>
+struct basic_dynamic_associative_container {
+    /**
+     * @brief Removes the specified element from the given container.
+     * @param cont The container from which to remove the element.
+     * @param key The element to remove.
+     * @return A bool denoting whether the removal took place.
+     */
+    [[nodiscard]] static bool erase(Container &cont, const typename Container::key_type &key) {
+        const auto sz = cont.size();
+        return cont.erase(key) != sz;
+    }
+};
+
+
+/**
+ * @brief Basic STL-compatible sequence container traits
+ * @tparam Container The type of the container.
+ */
+template<typename Container>
+struct basic_sequence_container {
+    /**
+     * @brief Returns a reference to the element at the specified location of the
+     * given container (no bounds checking is performed).
+     * @param cont The container from which to get the element.
+     * @param pos The position of the element to return.
+     * @return A reference to the requested element.
+     */
+    [[nodiscard]] static typename Container::value_type & get(Container &cont, typename Container::size_type pos) {
+        return cont[pos];
+    }
+};
+
+
+/**
+ * @brief STL-compatible dynamic associative key-only container traits
+ * @tparam Container The type of the container.
+ */
+template<typename Container>
+struct dynamic_associative_key_only_container {
+    /**
+     * @brief Inserts an element into the given container.
+     * @param cont The container in which to insert the element.
+     * @param key The element to insert.
+     * @return A bool denoting whether the insertion took place.
+     */
+    [[nodiscard]] static bool insert(Container &cont, const typename Container::key_type &key) {
+        return cont.insert(key).second;
+    }
+};
+
+
+/**
+ * @brief STL-compatible dynamic key-value associative container traits
+ * @tparam Container The type of the container.
+ */
+template<typename Container>
+struct dynamic_associative_key_value_container {
+    /**
+     * @brief Inserts an element (a key/value pair) into the given container.
+     * @param cont The container in which to insert the element.
+     * @param key The key of the element to insert.
+     * @param value The value of the element to insert.
+     * @return A bool denoting whether the insertion took place.
+     */
+    [[nodiscard]] static bool insert(Container &cont, const typename Container::key_type &key, const typename Container::mapped_type &value) {
+        return cont.insert(std::make_pair(key, value)).second;
+    }
+};
+
+
+/**
+ * @brief STL-compatible dynamic sequence container traits
+ * @tparam Container The type of the container.
+ */
+template<typename Container>
+struct dynamic_sequence_container {
+    /**
+     * @brief Resizes the given container to contain the given number of elements.
+     * @param cont The container to resize.
+     * @param sz The new size of the container.
+     * @return True in case of success, false otherwise.
+     */
+    [[nodiscard]] static bool resize(Container &cont, typename Container::size_type sz) {
+        return (cont.resize(sz), true);
+    }
+
+    /**
+     * @brief Inserts an element at the specified location of the given container.
+     * @param cont The container into which to insert the element.
+     * @param it Iterator before which the element will be inserted.
+     * @param value Element value to insert.
+     * @return A pair consisting of an iterator to the inserted element (in case
+     * of success) and a bool denoting whether the insertion took place.
+     */
+    [[nodiscard]] static std::pair<typename Container::iterator, bool> insert(Container &cont, typename Container::iterator it, const typename Container::value_type &value) {
+        return { cont.insert(it, value), true };
+    }
+
+    /**
+     * @brief Removes the element at the specified location from the given container.
+     * @param cont The container from which to remove the element.
+     * @param it Iterator to the element to remove.
+     * @return A pair consisting of an iterator following the last removed
+     * element (in case of success) and a bool denoting whether the insertion
+     * took place.
+     */
+    [[nodiscard]] static std::pair<typename Container::iterator, bool> erase(Container &cont, typename Container::iterator it) {
+        return { cont.erase(it), true };
+    }
+};
+
+
+/**
+ * @brief STL-compatible fixed sequence container traits
+ * @tparam Container The type of the container.
+ */
+template<typename Container>
+struct fixed_sequence_container {
+    /**
+     * @brief Does nothing.
+     * @return False to indicate failure in all cases.
+     */
+    [[nodiscard]] static bool resize(const Container &, typename Container::size_type) {
+        return false;
+    }
+
+    /**
+     * @brief Does nothing.
+     * @return False to indicate failure in all cases.
+     */
+    [[nodiscard]] static bool clear(const Container &) {
+        return false;
+    }
+
+    /**
+     * @brief Does nothing.
+     * @return A pair consisting of an invalid iterator and a false value to
+     * indicate failure in all cases.
+     */
+    [[nodiscard]] static std::pair<typename Container::iterator, bool> insert(const Container &, typename Container::iterator, const typename Container::value_type &) {
+        return { {}, false };
+    }
+
+    /**
+     * @brief Does nothing.
+     * @return A pair consisting of an invalid iterator and a false value to
+     * indicate failure in all cases.
+     */
+    [[nodiscard]] static std::pair<typename Container::iterator, bool> erase(const Container &, typename Container::iterator) {
+        return { {}, false };
+    }
+};
+
+
+}
+
+
+/**
+ * @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::container_traits<
+            std::vector<Type, Args...>,
+            internal::basic_container,
+            internal::basic_dynamic_container,
+            internal::basic_sequence_container,
+            internal::dynamic_sequence_container
+        >
+{};
+
+
+/**
+ * @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::container_traits<
+            std::array<Type, N>,
+            internal::basic_container,
+            internal::basic_sequence_container,
+            internal::fixed_sequence_container
+        >
+{};
+
+
+/**
+ * @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::container_traits<
+            std::map<Key, Value, Args...>,
+            internal::basic_container,
+            internal::basic_associative_container,
+            internal::basic_dynamic_container,
+            internal::basic_dynamic_associative_container,
+            internal::dynamic_associative_key_value_container
+        >
+{
+    /*! @brief Mapped type of the sequence container. */
+    using mapped_type = typename std::map<Key, Value, Args...>::mapped_type;
+};
+
+
+/**
+ * @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::container_traits<
+            std::unordered_map<Key, Value, Args...>,
+            internal::basic_container,
+            internal::basic_associative_container,
+            internal::basic_dynamic_container,
+            internal::basic_dynamic_associative_container,
+            internal::dynamic_associative_key_value_container
+        >
+{
+    /*! @brief Mapped type of the sequence container. */
+    using mapped_type = typename std::unordered_map<Key, Value, Args...>::mapped_type;
+};
+
+
+/**
+ * @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::container_traits<
+            std::set<Key, Args...>,
+            internal::basic_container,
+            internal::basic_associative_container,
+            internal::basic_dynamic_container,
+            internal::basic_dynamic_associative_container,
+            internal::dynamic_associative_key_only_container
+        >
+{};
+
+
+/**
+ * @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::container_traits<
+            std::unordered_set<Key, Args...>,
+            internal::basic_container,
+            internal::basic_associative_container,
+            internal::basic_dynamic_container,
+            internal::basic_dynamic_associative_container,
+            internal::dynamic_associative_key_only_container
+        >
+{};
+
+
+}
+
+
+#endif

src/entt/meta/ctx.hpp

@@ -0,0 +1,68 @@
+#ifndef ENTT_META_CTX_HPP
+#define ENTT_META_CTX_HPP
+
+
+#include "../core/attribute.h"
+#include "../config/config.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;
+    }
+};
+
+
+}
+
+
+/**
+ * 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()};
+};
+
+
+}
+
+
+#endif

src/entt/meta/factory.hpp

@@ -0,0 +1,721 @@
+#ifndef ENTT_META_FACTORY_HPP
+#define ENTT_META_FACTORY_HPP
+
+
+#include <array>
+#include <cstddef>
+#include <functional>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include "../config/config.h"
+#include "../core/fwd.hpp"
+#include "../core/type_info.hpp"
+#include "../core/type_traits.hpp"
+#include "internal.hpp"
+#include "meta.hpp"
+#include "policy.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename, bool = false>
+struct meta_function_helper;
+
+
+template<typename Ret, typename... Args, bool Const>
+struct meta_function_helper<Ret(Args...), Const> {
+    using return_type = std::remove_cv_t<std::remove_reference_t<Ret>>;
+    using args_type = std::tuple<std::remove_cv_t<std::remove_reference_t<Args>>...>;
+
+    static constexpr auto is_const = Const;
+
+    [[nodiscard]] static auto arg(typename internal::meta_func_node::size_type index) ENTT_NOEXCEPT {
+        return std::array<meta_type_node *, sizeof...(Args)>{{meta_info<Args>::resolve()...}}[index];
+    }
+};
+
+
+template<typename Ret, typename... Args, typename Class>
+constexpr meta_function_helper<Ret(Args...), true>
+to_meta_function_helper(Ret(Class:: *)(Args...) const);
+
+
+template<typename Ret, typename... Args, typename Class>
+constexpr meta_function_helper<Ret(Args...)>
+to_meta_function_helper(Ret(Class:: *)(Args...));
+
+
+template<typename Ret, typename... Args>
+constexpr meta_function_helper<Ret(Args...)>
+to_meta_function_helper(Ret(*)(Args...));
+
+
+constexpr void to_meta_function_helper(...);
+
+
+template<typename Candidate>
+using meta_function_helper_t = decltype(to_meta_function_helper(std::declval<Candidate>()));
+
+
+template<typename Type, typename... Args, std::size_t... Indexes>
+[[nodiscard]] meta_any construct(meta_any * const args, std::index_sequence<Indexes...>) {
+    [[maybe_unused]] auto direct = std::make_tuple((args+Indexes)->try_cast<Args>()...);
+    return ((std::get<Indexes>(direct) || (args+Indexes)->convert<Args>()) && ...)
+            ? Type{(std::get<Indexes>(direct) ? *std::get<Indexes>(direct) : (args+Indexes)->cast<Args>())...}
+            : meta_any{};
+}
+
+
+template<typename Type, auto Data>
+[[nodiscard]] bool setter([[maybe_unused]] meta_handle instance, [[maybe_unused]] meta_any value) {
+    bool accepted = false;
+
+    if constexpr(std::is_function_v<std::remove_reference_t<std::remove_pointer_t<decltype(Data)>>> || std::is_member_function_pointer_v<decltype(Data)>) {
+        using helper_type = meta_function_helper_t<decltype(Data)>;
+        using data_type = std::tuple_element_t<!std::is_member_function_pointer_v<decltype(Data)>, typename helper_type::args_type>;
+
+        if(auto * const clazz = instance->try_cast<Type>(); clazz) {
+            if(auto * const direct = value.try_cast<data_type>(); direct || value.convert<data_type>()) {
+                std::invoke(Data, *clazz, direct ? *direct : value.cast<data_type>());
+                accepted = true;
+            }
+        }
+    } else if constexpr(std::is_member_object_pointer_v<decltype(Data)>) {
+        using data_type = std::remove_cv_t<std::remove_reference_t<decltype(std::declval<Type>().*Data)>>;
+
+        if constexpr(!std::is_array_v<data_type>) {
+            if(auto * const clazz = instance->try_cast<Type>(); clazz) {
+                if(auto * const direct = value.try_cast<data_type>(); direct || value.convert<data_type>()) {
+                    std::invoke(Data, clazz) = (direct ? *direct : value.cast<data_type>());
+                    accepted = true;
+                }
+            }
+        }
+    } else {
+        using data_type = std::remove_cv_t<std::remove_reference_t<decltype(*Data)>>;
+
+        if constexpr(!std::is_array_v<data_type>) {
+            if(auto * const direct = value.try_cast<data_type>(); direct || value.convert<data_type>()) {
+                *Data = (direct ? *direct : value.cast<data_type>());
+                accepted = true;
+            }
+        }
+    }
+
+    return accepted;
+}
+
+
+template<typename Type, auto Data, typename Policy>
+[[nodiscard]] meta_any getter([[maybe_unused]] meta_handle instance) {
+    [[maybe_unused]] auto dispatch = [](auto &&value) {
+        if constexpr(std::is_same_v<Policy, as_void_t>) {
+            return meta_any{std::in_place_type<void>, std::forward<decltype(value)>(value)};
+        } else if constexpr(std::is_same_v<Policy, as_ref_t>) {
+            return meta_any{std::ref(std::forward<decltype(value)>(value))};
+        } else {
+            static_assert(std::is_same_v<Policy, as_is_t>, "Policy not supported");
+            return meta_any{std::forward<decltype(value)>(value)};
+        }
+    };
+
+    if constexpr(std::is_function_v<std::remove_reference_t<std::remove_pointer_t<decltype(Data)>>> || std::is_member_function_pointer_v<decltype(Data)>) {
+        auto * const clazz = instance->try_cast<Type>();
+        return clazz ? dispatch(std::invoke(Data, *clazz)) : meta_any{};
+    } else if constexpr(std::is_member_object_pointer_v<decltype(Data)>) {
+        if constexpr(std::is_array_v<std::remove_cv_t<std::remove_reference_t<decltype(std::declval<Type>().*Data)>>>) {
+            return meta_any{};
+        } else {
+            auto * const clazz = instance->try_cast<Type>();
+            return clazz ? dispatch(std::invoke(Data, clazz)) : meta_any{};
+        }
+    } else if constexpr(std::is_pointer_v<std::decay_t<decltype(Data)>>) {
+        if constexpr(std::is_array_v<std::remove_pointer_t<decltype(Data)>>) {
+            return meta_any{};
+        } else {
+            return dispatch(*Data);
+        }
+    } else {
+        return dispatch(Data);
+    }
+}
+
+
+template<typename Type, auto Candidate, typename Policy, std::size_t... Indexes>
+[[nodiscard]] meta_any invoke([[maybe_unused]] meta_handle instance, meta_any *args, std::index_sequence<Indexes...>) {
+    using helper_type = meta_function_helper_t<decltype(Candidate)>;
+
+    auto dispatch = [](auto *... params) {
+        if constexpr(std::is_void_v<typename helper_type::return_type> || std::is_same_v<Policy, as_void_t>) {
+            std::invoke(Candidate, *params...);
+            return meta_any{std::in_place_type<void>};
+        } else if constexpr(std::is_same_v<Policy, as_ref_t>) {
+            return meta_any{std::ref(std::invoke(Candidate, *params...))};
+        } else {
+            static_assert(std::is_same_v<Policy, as_is_t>, "Policy not supported");
+            return meta_any{std::invoke(Candidate, *params...)};
+        }
+    };
+
+    [[maybe_unused]] const auto direct = std::make_tuple([](meta_any *any, auto *value) {
+        using arg_type = std::remove_reference_t<decltype(*value)>;
+
+        if(!value && any->convert<arg_type>()) {
+            value = any->try_cast<arg_type>();
+        }
+
+        return value;
+    }(args+Indexes, (args+Indexes)->try_cast<std::tuple_element_t<Indexes, typename helper_type::args_type>>())...);
+
+    if constexpr(std::is_function_v<std::remove_reference_t<std::remove_pointer_t<decltype(Candidate)>>>) {
+        return (std::get<Indexes>(direct) && ...) ? dispatch(std::get<Indexes>(direct)...) : meta_any{};
+    } else {
+        auto * const clazz = instance->try_cast<Type>();
+        return (clazz && (std::get<Indexes>(direct) && ...)) ? dispatch(clazz, std::get<Indexes>(direct)...) : meta_any{};
+    }
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Meta factory to be used for reflection purposes.
+ *
+ * The meta factory is an utility class used to reflect types, data members and
+ * functions of all sorts. This class ensures that the underlying web of types
+ * is built correctly and performs some checks in debug mode to ensure that
+ * there are no subtle errors at runtime.
+ */
+template<typename...>
+class meta_factory;
+
+
+/**
+ * @brief Extended meta factory to be used for reflection purposes.
+ * @tparam Type Reflected type for which the factory was created.
+ * @tparam Spec Property specialization pack used to disambiguate overloads.
+ */
+template<typename Type, typename... Spec>
+class meta_factory<Type, Spec...>: public meta_factory<Type> {
+    [[nodiscard]] bool exists(const meta_any &key, const internal::meta_prop_node *node) ENTT_NOEXCEPT {
+        return node && (node->key() == key || exists(key, node->next));
+    }
+
+    template<std::size_t Step = 0, std::size_t... Index, typename... Property, typename... Other>
+    void unpack(std::index_sequence<Index...>, std::tuple<Property...> property, Other &&... other) {
+        unroll<Step>(choice<3>, std::move(std::get<Index>(property))..., std::forward<Other>(other)...);
+    }
+
+    template<std::size_t Step = 0, typename... Property, typename... Other>
+    void unroll(choice_t<3>, std::tuple<Property...> property, Other &&... other) {
+        unpack<Step>(std::index_sequence_for<Property...>{}, std::move(property), std::forward<Other>(other)...);
+    }
+
+    template<std::size_t Step = 0, typename... Property, typename... Other>
+    void unroll(choice_t<2>, std::pair<Property...> property, Other &&... other) {
+        assign<Step>(std::move(property.first), std::move(property.second));
+        unroll<Step+1>(choice<3>, std::forward<Other>(other)...);
+    }
+
+    template<std::size_t Step = 0, typename Property, typename... Other>
+    std::enable_if_t<!std::is_invocable_v<Property>>
+    unroll(choice_t<1>, Property &&property, Other &&... other) {
+        assign<Step>(std::forward<Property>(property));
+        unroll<Step+1>(choice<3>, std::forward<Other>(other)...);
+    }
+
+    template<std::size_t Step = 0, typename Func, typename... Other>
+    void unroll(choice_t<0>, Func &&invocable, Other &&... other) {
+        unroll<Step>(choice<3>, std::forward<Func>(invocable)(), std::forward<Other>(other)...);
+    }
+
+    template<std::size_t>
+    void unroll(choice_t<0>) {}
+
+    template<std::size_t = 0, typename Key, typename... Value>
+    void assign(Key &&key, Value &&... value) {
+        static const auto property{std::make_tuple(std::forward<Key>(key), std::forward<Value>(value)...)};
+
+        static internal::meta_prop_node node{
+            nullptr,
+            []() -> meta_any {
+                return std::get<0>(property);
+            },
+            []() -> meta_any {
+                if constexpr(sizeof...(Value) == 0) {
+                    return {};
+                } else {
+                    return std::get<1>(property);
+                }
+            }
+        };
+
+        ENTT_ASSERT(!exists(node.key(), *curr));
+        node.next = *curr;
+        *curr = &node;
+    }
+
+public:
+    /**
+     * @brief Constructs an extended factory from a given node.
+     * @param target The underlying node to which to assign the properties.
+     */
+    meta_factory(internal::meta_prop_node **target) ENTT_NOEXCEPT
+        : curr{target}
+    {}
+
+    /**
+     * @brief Assigns a property to the last meta object created.
+     *
+     * Both the key and the value (if any) must be at least copy constructible.
+     *
+     * @tparam PropertyOrKey Type of the property or property key.
+     * @tparam Value Optional type of the property value.
+     * @param property_or_key Property or property key.
+     * @param value Optional property value.
+     * @return A meta factory for the parent type.
+     */
+    template<typename PropertyOrKey, typename... Value>
+    auto prop(PropertyOrKey &&property_or_key, Value &&... value) && {
+        if constexpr(sizeof...(Value) == 0) {
+            unroll(choice<3>, std::forward<PropertyOrKey>(property_or_key));
+        } else {
+            assign(std::forward<PropertyOrKey>(property_or_key), std::forward<Value>(value)...);
+        }
+
+        return meta_factory<Type, Spec..., PropertyOrKey, Value...>{curr};
+    }
+
+    /**
+     * @brief Assigns properties to the last meta object created.
+     *
+     * Both the keys and the values (if any) must be at least copy
+     * constructible.
+     *
+     * @tparam Property Types of the properties.
+     * @param property Properties to assign to the last meta object created.
+     * @return A meta factory for the parent type.
+     */
+    template <typename... Property>
+    auto props(Property... property) && {
+        unroll(choice<3>, std::forward<Property>(property)...);
+        return meta_factory<Type, Spec..., Property...>{curr};
+    }
+
+private:
+    internal::meta_prop_node **curr;
+};
+
+
+/**
+ * @brief Basic meta factory to be used for reflection purposes.
+ * @tparam Type Reflected type for which the factory was created.
+ */
+template<typename Type>
+class meta_factory<Type> {
+    template<typename Node>
+    bool exists(const Node *candidate, const Node *node) ENTT_NOEXCEPT {
+        return node && (node == candidate || exists(candidate, node->next));
+    }
+
+    template<typename Node>
+    bool exists(const id_type id, const Node *node) ENTT_NOEXCEPT {
+        return node && (node->id == id || exists(id, node->next));
+    }
+
+public:
+    /**
+     * @brief Makes a meta type _searchable_.
+     * @param id Optional unique identifier.
+     * @return An extended meta factory for the given type.
+     */
+    auto type(const id_type id = type_info<Type>::id()) {
+        auto * const node = internal::meta_info<Type>::resolve();
+
+        ENTT_ASSERT(!exists(id, *internal::meta_context::global()));
+        ENTT_ASSERT(!exists(node, *internal::meta_context::global()));
+        node->id = id;
+        node->next = *internal::meta_context::global();
+        *internal::meta_context::global() = node;
+
+        return meta_factory<Type, Type>{&node->prop};
+    }
+
+    /**
+     * @brief Assigns a meta base to a meta type.
+     *
+     * A reflected base class must be a real base class of the reflected type.
+     *
+     * @tparam Base Type of the base class to assign to the meta type.
+     * @return A meta factory for the parent type.
+     */
+    template<typename Base>
+    auto base() ENTT_NOEXCEPT {
+        static_assert(std::is_base_of_v<Base, Type>, "Invalid base type");
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_base_node node{
+            type,
+            nullptr,
+            &internal::meta_info<Base>::resolve,
+            [](const void *instance) ENTT_NOEXCEPT -> const void * {
+                return static_cast<const Base *>(static_cast<const Type *>(instance));
+            }
+        };
+
+        ENTT_ASSERT(!exists(&node, type->base));
+        node.next = type->base;
+        type->base = &node;
+
+        return meta_factory<Type>{};
+    }
+
+    /**
+     * @brief Assigns a meta conversion function to a meta type.
+     *
+     * The given type must be such that an instance of the reflected type can be
+     * converted to it.
+     *
+     * @tparam To Type of the conversion function to assign to the meta type.
+     * @return A meta factory for the parent type.
+     */
+    template<typename To>
+    auto conv() ENTT_NOEXCEPT {
+        static_assert(std::is_convertible_v<Type, To>, "Could not convert to the required type");
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_conv_node node{
+            type,
+            nullptr,
+            &internal::meta_info<To>::resolve,
+            [](const void *instance) -> meta_any {
+                return static_cast<To>(*static_cast<const Type *>(instance));
+            }
+        };
+
+        ENTT_ASSERT(!exists(&node, type->conv));
+        node.next = type->conv;
+        type->conv = &node;
+
+        return meta_factory<Type>{};
+    }
+
+    /**
+     * @brief Assigns a meta conversion function to a meta type.
+     *
+     * Conversion functions can be either free functions or member
+     * functions.<br/>
+     * In case of free functions, they must accept a const reference to an
+     * instance of the parent type as an argument. In case of member functions,
+     * they should have no arguments at all.
+     *
+     * @tparam Candidate The actual function to use for the conversion.
+     * @return A meta factory for the parent type.
+     */
+    template<auto Candidate>
+    auto conv() ENTT_NOEXCEPT {
+        using conv_type = std::invoke_result_t<decltype(Candidate), Type &>;
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_conv_node node{
+            type,
+            nullptr,
+            &internal::meta_info<conv_type>::resolve,
+            [](const void *instance) -> meta_any {
+                return std::invoke(Candidate, *static_cast<const Type *>(instance));
+            }
+        };
+
+        ENTT_ASSERT(!exists(&node, type->conv));
+        node.next = type->conv;
+        type->conv = &node;
+
+        return meta_factory<Type>{};
+    }
+
+    /**
+     * @brief Assigns a meta constructor to a meta type.
+     *
+     * Free functions can be assigned to meta types in the role of constructors.
+     * All that is required is that they return an instance of the underlying
+     * type.<br/>
+     * From a client's point of view, nothing changes if a constructor of a meta
+     * type is a built-in one or a free function.
+     *
+     * @tparam Func The actual function to use as a constructor.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @return An extended meta factory for the parent type.
+     */
+    template<auto Func, typename Policy = as_is_t>
+    auto ctor() ENTT_NOEXCEPT {
+        using helper_type = internal::meta_function_helper_t<decltype(Func)>;
+        static_assert(std::is_same_v<typename helper_type::return_type, Type>, "The function doesn't return an object of the required type");
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_ctor_node node{
+            type,
+            nullptr,
+            nullptr,
+            std::tuple_size_v<typename helper_type::args_type>,
+            &helper_type::arg,
+            [](meta_any * const any) {
+                return internal::invoke<Type, Func, Policy>({}, any, std::make_index_sequence<std::tuple_size_v<typename helper_type::args_type>>{});
+            }
+        };
+
+        ENTT_ASSERT(!exists(&node, type->ctor));
+        node.next = type->ctor;
+        type->ctor = &node;
+
+        return meta_factory<Type, std::integral_constant<decltype(Func), Func>>{&node.prop};
+    }
+
+    /**
+     * @brief Assigns a meta constructor to a meta type.
+     *
+     * A meta constructor is uniquely identified by the types of its arguments
+     * and is such that there exists an actual constructor of the underlying
+     * type that can be invoked with parameters whose types are those given.
+     *
+     * @tparam Args Types of arguments to use to construct an instance.
+     * @return An extended meta factory for the parent type.
+     */
+    template<typename... Args>
+    auto ctor() ENTT_NOEXCEPT {
+        using helper_type = internal::meta_function_helper_t<Type(*)(Args...)>;
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_ctor_node node{
+            type,
+            nullptr,
+            nullptr,
+            std::tuple_size_v<typename helper_type::args_type>,
+            &helper_type::arg,
+            [](meta_any * const any) {
+                return internal::construct<Type, std::remove_cv_t<std::remove_reference_t<Args>>...>(any, std::make_index_sequence<std::tuple_size_v<typename helper_type::args_type>>{});
+            }
+        };
+
+        ENTT_ASSERT(!exists(&node, type->ctor));
+        node.next = type->ctor;
+        type->ctor = &node;
+
+        return meta_factory<Type, Type(Args...)>{&node.prop};
+    }
+
+    /**
+     * @brief Assigns a meta destructor to a meta type.
+     *
+     * Free functions can be assigned to meta types in the role of destructors.
+     * The signature of the function should identical to the following:
+     *
+     * @code{.cpp}
+     * void(Type &);
+     * @endcode
+     *
+     * The purpose is to give users the ability to free up resources that
+     * require special treatment before an object is actually destroyed.
+     *
+     * @tparam Func The actual function to use as a destructor.
+     * @return A meta factory for the parent type.
+     */
+    template<auto Func>
+    auto dtor() ENTT_NOEXCEPT {
+        static_assert(std::is_invocable_v<decltype(Func), Type &>, "The function doesn't accept an object of the type provided");
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        ENTT_ASSERT(!type->dtor);
+
+        type->dtor = [](void *instance) {
+            if(instance) {
+                std::invoke(Func, *static_cast<Type *>(instance));
+            }
+        };
+
+        return meta_factory<Type>{};
+    }
+
+    /**
+     * @brief Assigns a meta data to a meta type.
+     *
+     * Both data members and static and global variables, as well as constants
+     * of any kind, can be assigned to a meta type.<br/>
+     * From a client's point of view, all the variables associated with the
+     * reflected object will appear as if they were part of the type itself.
+     *
+     * @tparam Data The actual variable to attach to the meta type.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @param id Unique identifier.
+     * @return An extended meta factory for the parent type.
+     */
+    template<auto Data, typename Policy = as_is_t>
+    auto data(const id_type id) ENTT_NOEXCEPT {
+        if constexpr(std::is_member_object_pointer_v<decltype(Data)>) {
+            return data<Data, Data, Policy>(id);
+        } else {
+            using data_type = std::remove_pointer_t<std::decay_t<decltype(Data)>>;
+            auto * const type = internal::meta_info<Type>::resolve();
+
+            static internal::meta_data_node node{
+                {},
+                type,
+                nullptr,
+                nullptr,
+                true,
+                &internal::meta_info<data_type>::resolve,
+                []() -> std::remove_const_t<decltype(internal::meta_data_node::set)> {
+                    if constexpr(std::is_same_v<Type, data_type> || std::is_const_v<data_type>) {
+                        return nullptr;
+                    } else {
+                        return &internal::setter<Type, Data>;
+                    }
+                }(),
+                &internal::getter<Type, Data, Policy>
+            };
+
+            ENTT_ASSERT(!exists(id, type->data));
+            ENTT_ASSERT(!exists(&node, type->data));
+            node.id = id;
+            node.next = type->data;
+            type->data = &node;
+
+            return meta_factory<Type, std::integral_constant<decltype(Data), Data>>{&node.prop};
+        }
+    }
+
+    /**
+     * @brief Assigns a meta data to a meta type by means of its setter and
+     * getter.
+     *
+     * Setters and getters can be either free functions, member functions or a
+     * mix of them.<br/>
+     * In case of free functions, setters and getters must accept a reference to
+     * an instance of the parent type as their first argument. A setter has then
+     * an extra argument of a type convertible to that of the parameter to
+     * set.<br/>
+     * In case of member functions, getters have no arguments at all, while
+     * setters has an argument of a type convertible to that of the parameter to
+     * set.
+     *
+     * @tparam Setter The actual function to use as a setter.
+     * @tparam Getter The actual function to use as a getter.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @param id Unique identifier.
+     * @return An extended meta factory for the parent type.
+     */
+    template<auto Setter, auto Getter, typename Policy = as_is_t>
+    auto data(const id_type id) ENTT_NOEXCEPT {
+        using underlying_type = std::remove_reference_t<std::invoke_result_t<decltype(Getter), Type &>>;
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_data_node node{
+            {},
+            type,
+            nullptr,
+            nullptr,
+            false,
+            &internal::meta_info<underlying_type>::resolve,
+            []() -> std::remove_const_t<decltype(internal::meta_data_node::set)> {
+                if constexpr(std::is_same_v<decltype(Setter), std::nullptr_t> || (std::is_member_object_pointer_v<decltype(Setter)> && std::is_const_v<underlying_type>)) {
+                    return nullptr;
+                } else {
+                    return &internal::setter<Type, Setter>;
+                }
+            }(),
+            &internal::getter<Type, Getter, Policy>
+        };
+
+        ENTT_ASSERT(!exists(id, type->data));
+        ENTT_ASSERT(!exists(&node, type->data));
+        node.id = id;
+        node.next = type->data;
+        type->data = &node;
+
+        return meta_factory<Type, std::integral_constant<decltype(Setter), Setter>, std::integral_constant<decltype(Getter), Getter>>{&node.prop};
+    }
+
+    /**
+     * @brief Assigns a meta funcion to a meta type.
+     *
+     * Both member functions and free functions can be assigned to a meta
+     * type.<br/>
+     * From a client's point of view, all the functions associated with the
+     * reflected object will appear as if they were part of the type itself.
+     *
+     * @tparam Candidate The actual function to attach to the meta type.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @param id Unique identifier.
+     * @return An extended meta factory for the parent type.
+     */
+    template<auto Candidate, typename Policy = as_is_t>
+    auto func(const id_type id) ENTT_NOEXCEPT {
+        using helper_type = internal::meta_function_helper_t<decltype(Candidate)>;
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_func_node node{
+            {},
+            type,
+            nullptr,
+            nullptr,
+            std::tuple_size_v<typename helper_type::args_type>,
+            helper_type::is_const,
+            !std::is_member_function_pointer_v<decltype(Candidate)>,
+            &internal::meta_info<std::conditional_t<std::is_same_v<Policy, as_void_t>, void, typename helper_type::return_type>>::resolve,
+            &helper_type::arg,
+            [](meta_handle instance, meta_any *args) {
+                return internal::invoke<Type, Candidate, Policy>(*instance, args, std::make_index_sequence<std::tuple_size_v<typename helper_type::args_type>>{});
+            }
+        };
+
+        ENTT_ASSERT(!exists(id, type->func));
+        ENTT_ASSERT(!exists(&node, type->func));
+        node.id = id;
+        node.next = type->func;
+        type->func = &node;
+
+        return meta_factory<Type, std::integral_constant<decltype(Candidate), Candidate>>{&node.prop};
+    }
+};
+
+
+/**
+ * @brief Utility function to use for reflection.
+ *
+ * This is the point from which everything starts.<br/>
+ * By invoking this function with a type that is not yet reflected, a meta type
+ * is created to which it will be possible to attach meta objects through a
+ * dedicated factory.
+ *
+ * @tparam Type Type to reflect.
+ * @return A meta factory for the given type.
+ */
+template<typename Type>
+[[nodiscard]] auto meta() ENTT_NOEXCEPT {
+    auto * const node = internal::meta_info<Type>::resolve();
+    // extended meta factory to allow assigning properties to opaque meta types
+    return meta_factory<Type, Type>{&node->prop};
+}
+
+
+}
+
+
+#endif

src/entt/meta/internal.hpp

@@ -0,0 +1,455 @@
+#ifndef ENTT_META_INTERNAL_HPP
+#define ENTT_META_INTERNAL_HPP
+
+
+#include <cstddef>
+#include <functional>
+#include <iterator>
+#include <type_traits>
+#include <utility>
+#include "../core/attribute.h"
+#include "../config/config.h"
+#include "../core/fwd.hpp"
+#include "../core/type_info.hpp"
+#include "../core/type_traits.hpp"
+#include "type_traits.hpp"
+
+
+namespace entt {
+
+
+class meta_any;
+struct meta_handle;
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+class meta_storage {
+    using storage_type = std::aligned_storage_t<sizeof(void *), alignof(void *)>;
+    using copy_fn_type = void(meta_storage &, const meta_storage &);
+    using steal_fn_type = void(meta_storage &, meta_storage &);
+    using destroy_fn_type = void(meta_storage &);
+
+    template<typename Type, typename = std::void_t<>>
+    struct type_traits {
+        template<typename... Args>
+        static void instance(meta_storage &buffer, Args &&... args) {
+            buffer.instance = new Type{std::forward<Args>(args)...};
+            new (&buffer.storage) Type *{static_cast<Type *>(buffer.instance)};
+        }
+
+        static void destroy(meta_storage &buffer) {
+            delete static_cast<Type *>(buffer.instance);
+        }
+
+        static void copy(meta_storage &to, const meta_storage &from) {
+            to.instance = new Type{*static_cast<const Type *>(from.instance)};
+            new (&to.storage) Type *{static_cast<Type *>(to.instance)};
+        }
+
+        static void steal(meta_storage &to, meta_storage &from) {
+            new (&to.storage) Type *{static_cast<Type *>(from.instance)};
+            to.instance = from.instance;
+        }
+    };
+
+    template<typename Type>
+    struct type_traits<Type, std::enable_if_t<sizeof(Type) <= sizeof(void *) && std::is_nothrow_move_constructible_v<Type>>> {
+        template<typename... Args>
+        static void instance(meta_storage &buffer, Args &&... args) {
+            buffer.instance = new (&buffer.storage) Type{std::forward<Args>(args)...};
+        }
+
+        static void destroy(meta_storage &buffer) {
+            static_cast<Type *>(buffer.instance)->~Type();
+        }
+
+        static void copy(meta_storage &to, const meta_storage &from) {
+            to.instance = new (&to.storage) Type{*static_cast<const Type *>(from.instance)};
+        }
+
+        static void steal(meta_storage &to, meta_storage &from) {
+            to.instance = new (&to.storage) Type{std::move(*static_cast<Type *>(from.instance))};
+            destroy(from);
+        }
+    };
+
+public:
+    /*! @brief Default constructor. */
+    meta_storage() ENTT_NOEXCEPT
+        : storage{},
+          instance{},
+          destroy_fn{},
+          copy_fn{},
+          steal_fn{}
+    {}
+
+    template<typename Type, typename... Args>
+    explicit meta_storage(std::in_place_type_t<Type>, [[maybe_unused]] Args &&... args)
+        : meta_storage{}
+    {
+        if constexpr(!std::is_void_v<Type>) {
+            type_traits<Type>::instance(*this, std::forward<Args>(args)...);
+            destroy_fn = &type_traits<Type>::destroy;
+            copy_fn = &type_traits<Type>::copy;
+            steal_fn = &type_traits<Type>::steal;
+        }
+    }
+
+    template<typename Type>
+    meta_storage(std::reference_wrapper<Type> value)
+        : meta_storage{}
+    {
+        instance = &value.get();
+    }
+
+    template<typename Type, typename = std::enable_if_t<!std::is_same_v<std::remove_cv_t<std::remove_reference_t<Type>>, meta_storage>>>
+    meta_storage(Type &&value)
+        : meta_storage{std::in_place_type<std::remove_cv_t<std::remove_reference_t<Type>>>, std::forward<Type>(value)}
+    {}
+
+    meta_storage(const meta_storage &other)
+        : meta_storage{}
+    {
+        (other.copy_fn ? other.copy_fn : [](auto &to, const auto &from) { to.instance = from.instance; })(*this, other);
+        destroy_fn = other.destroy_fn;
+        copy_fn = other.copy_fn;
+        steal_fn = other.steal_fn;
+    }
+
+    meta_storage(meta_storage &&other)
+        : meta_storage{}
+    {
+        swap(*this, other);
+    }
+
+    ~meta_storage() {
+        if(destroy_fn) {
+            destroy_fn(*this);
+        }
+    }
+
+    meta_storage & operator=(meta_storage other) {
+        swap(other, *this);
+        return *this;
+    }
+
+    [[nodiscard]] const void * data() const ENTT_NOEXCEPT {
+        return instance;
+    }
+
+    [[nodiscard]] void * data() ENTT_NOEXCEPT {
+        return const_cast<void *>(std::as_const(*this).data());
+    }
+
+    template<typename Type, typename... Args>
+    void emplace(Args &&... args) {
+        *this = meta_storage{std::in_place_type<Type>, std::forward<Args>(args)...};
+    }
+
+    [[nodiscard]] meta_storage ref() const ENTT_NOEXCEPT {
+        meta_storage other{};
+        other.instance = instance;
+        return other;
+    }
+
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return !(instance == nullptr);
+    }
+
+    friend void swap(meta_storage &lhs, meta_storage &rhs) {
+        using std::swap;
+
+        if(lhs.steal_fn && rhs.steal_fn) {
+            meta_storage buffer{};
+            lhs.steal_fn(buffer, lhs);
+            rhs.steal_fn(lhs, rhs);
+            lhs.steal_fn(rhs, buffer);
+        } else if(lhs.steal_fn) {
+            lhs.steal_fn(rhs, lhs);
+        } else if(rhs.steal_fn) {
+            rhs.steal_fn(lhs, rhs);
+        } else {
+            swap(lhs.instance, rhs.instance);
+        }
+
+        swap(lhs.destroy_fn, rhs.destroy_fn);
+        swap(lhs.copy_fn, rhs.copy_fn);
+        swap(lhs.steal_fn, rhs.steal_fn);
+    }
+
+private:
+    storage_type storage;
+    void *instance;
+    destroy_fn_type *destroy_fn;
+    copy_fn_type *copy_fn;
+    steal_fn_type *steal_fn;
+};
+
+
+struct meta_type_node;
+
+
+struct meta_prop_node {
+    meta_prop_node * next;
+    meta_any(* const key)();
+    meta_any(* const value)();
+};
+
+
+struct meta_base_node {
+    meta_type_node * const parent;
+    meta_base_node * next;
+    meta_type_node *(* const type)() ENTT_NOEXCEPT;
+    const void *(* const cast)(const void *) ENTT_NOEXCEPT;
+};
+
+
+struct meta_conv_node {
+    meta_type_node * const parent;
+    meta_conv_node * next;
+    meta_type_node *(* const type)() ENTT_NOEXCEPT;
+    meta_any(* const conv)(const void *);
+};
+
+
+struct meta_ctor_node {
+    using size_type = std::size_t;
+    meta_type_node * const parent;
+    meta_ctor_node * next;
+    meta_prop_node * prop;
+    const size_type size;
+    meta_type_node *(* const arg)(size_type) ENTT_NOEXCEPT;
+    meta_any(* const invoke)(meta_any * const);
+};
+
+
+struct meta_data_node {
+    id_type id;
+    meta_type_node * const parent;
+    meta_data_node * next;
+    meta_prop_node * prop;
+    const bool is_static;
+    meta_type_node *(* const type)() ENTT_NOEXCEPT;
+    bool(* const set)(meta_handle, meta_any);
+    meta_any(* const get)(meta_handle);
+};
+
+
+struct meta_func_node {
+    using size_type = std::size_t;
+    id_type id;
+    meta_type_node * const parent;
+    meta_func_node * next;
+    meta_prop_node * prop;
+    const size_type size;
+    const bool is_const;
+    const bool is_static;
+    meta_type_node *(* const ret)() ENTT_NOEXCEPT;
+    meta_type_node *(* const arg)(size_type) ENTT_NOEXCEPT;
+    meta_any(* const invoke)(meta_handle, meta_any *);
+};
+
+
+struct meta_type_node {
+    using size_type = std::size_t;
+    const id_type type_id;
+    id_type id;
+    meta_type_node * next;
+    meta_prop_node * prop;
+    const bool is_void;
+    const bool is_integral;
+    const bool is_floating_point;
+    const bool is_array;
+    const bool is_enum;
+    const bool is_union;
+    const bool is_class;
+    const bool is_pointer;
+    const bool is_function_pointer;
+    const bool is_member_object_pointer;
+    const bool is_member_function_pointer;
+    const bool is_pointer_like;
+    const bool is_sequence_container;
+    const bool is_associative_container;
+    const size_type rank;
+    size_type(* const extent)(size_type);
+    bool(* const compare)(const void *, const void *);
+    meta_type_node *(* const remove_pointer)() ENTT_NOEXCEPT;
+    meta_type_node *(* const remove_extent)() ENTT_NOEXCEPT;
+    meta_base_node *base{nullptr};
+    meta_conv_node *conv{nullptr};
+    meta_ctor_node *ctor{nullptr};
+    meta_data_node *data{nullptr};
+    meta_func_node *func{nullptr};
+    void(* dtor)(void *){nullptr};
+};
+
+
+template<typename Node>
+class meta_range {
+    struct range_iterator {
+        using difference_type = std::ptrdiff_t;
+        using value_type = Node;
+        using pointer = value_type *;
+        using reference = value_type &;
+        using iterator_category = std::forward_iterator_tag;
+
+        range_iterator() ENTT_NOEXCEPT = default;
+
+        range_iterator(Node *head) ENTT_NOEXCEPT
+            : node{head}
+        {}
+
+        range_iterator & operator++() ENTT_NOEXCEPT {
+            return node = node->next, *this;
+        }
+
+        range_iterator operator++(int) ENTT_NOEXCEPT {
+            range_iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        [[nodiscard]] bool operator==(const range_iterator &other) const ENTT_NOEXCEPT {
+            return other.node == node;
+        }
+
+        [[nodiscard]] bool operator!=(const range_iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+        [[nodiscard]] pointer operator->() const ENTT_NOEXCEPT {
+            return node;
+        }
+
+        [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
+            return *operator->();
+        }
+
+    private:
+        Node *node{nullptr};
+    };
+
+public:
+    using iterator = range_iterator;
+
+    meta_range() ENTT_NOEXCEPT = default;
+
+    meta_range(Node *head)
+        : node{head}
+    {}
+
+    [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+        return iterator{node};
+    }
+
+    [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+        return iterator{};
+    }
+
+private:
+    Node *node{nullptr};
+};
+
+
+template<auto Member, typename Op>
+auto find_if(const Op &op, const meta_type_node *node)
+-> std::decay_t<decltype(node->*Member)> {
+    std::decay_t<decltype(node->*Member)> ret = nullptr;
+
+    for(auto &&curr: meta_range{node->*Member}) {
+        if(op(&curr)) {
+            ret = &curr;
+            break;
+        }
+    }
+
+    if(!ret) {
+        for(auto &&curr: meta_range{node->base}) {
+            if(ret = find_if<Member>(op, curr.type()); ret) {
+                break;
+            }
+        }
+    }
+
+    return ret;
+}
+
+
+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 bool compare(const void *lhs, const void *rhs) {
+        if constexpr(!std::is_function_v<Type> && is_equality_comparable_v<Type>) {
+            return *static_cast<const Type *>(lhs) == *static_cast<const Type *>(rhs);
+        } else {
+            return lhs == rhs;
+        }
+    }
+
+    template<std::size_t... Index>
+    [[nodiscard]] static auto extent(meta_type_node::size_type dim, std::index_sequence<Index...>) {
+        meta_type_node::size_type ext{};
+        ((ext = (dim == Index ? std::extent_v<Type, Index> : ext)), ...);
+        return ext;
+    }
+
+public:
+    [[nodiscard]] static meta_type_node * resolve() ENTT_NOEXCEPT {
+        static meta_type_node node{
+            type_info<Type>::id(),
+            {},
+            nullptr,
+            nullptr,
+            std::is_void_v<Type>,
+            std::is_integral_v<Type>,
+            std::is_floating_point_v<Type>,
+            std::is_array_v<Type>,
+            std::is_enum_v<Type>,
+            std::is_union_v<Type>,
+            std::is_class_v<Type>,
+            std::is_pointer_v<Type>,
+            std::is_pointer_v<Type> && std::is_function_v<std::remove_pointer_t<Type>>,
+            std::is_member_object_pointer_v<Type>,
+            std::is_member_function_pointer_v<Type>,
+            is_meta_pointer_like_v<Type>,
+            has_meta_sequence_container_traits_v<Type>,
+            has_meta_associative_container_traits_v<Type>,
+            std::rank_v<Type>,
+            [](meta_type_node::size_type dim) {
+                return extent(dim, std::make_index_sequence<std::rank_v<Type>>{});
+            },
+            &compare, // workaround for an issue with VS2017
+            &meta_node<std::remove_const_t<std::remove_pointer_t<Type>>>::resolve,
+            &meta_node<std::remove_const_t<std::remove_extent_t<Type>>>::resolve
+        };
+
+        return &node;
+    }
+};
+
+
+template<typename... Type>
+struct meta_info: meta_node<std::remove_cv_t<std::remove_reference_t<Type>>...> {};
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+}
+
+
+#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 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
+{};
+
+
+}
+
+
+#endif

src/entt/meta/policy.hpp

@@ -0,0 +1,27 @@
+#ifndef ENTT_META_POLICY_HPP
+#define ENTT_META_POLICY_HPP
+
+
+namespace entt {
+
+
+/*! @brief Empty class type used to request the _as ref_ policy. */
+struct as_ref_t {};
+
+
+/*! @brief Disambiguation tag. */
+inline constexpr as_ref_t as_ref;
+
+
+/*! @brief Empty class type used to request the _as-is_ policy. */
+struct as_is_t {};
+
+
+/*! @brief Empty class type used to request the _as void_ policy. */
+struct as_void_t {};
+
+
+}
+
+
+#endif

src/entt/meta/range.hpp

@@ -0,0 +1,98 @@
+#ifndef ENTT_META_RANGE_HPP
+#define ENTT_META_RANGE_HPP
+
+
+#include "internal.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Iterable range to use to iterate all types of meta objects.
+ * @tparam Type Type of meta objects iterated.
+ */
+template<typename Type>
+class meta_range {
+    struct range_iterator {
+        using difference_type = std::ptrdiff_t;
+        using value_type = Type;
+        using pointer = void;
+        using reference = value_type;
+        using iterator_category = std::input_iterator_tag;
+        using node_type = typename Type::node_type;
+
+        range_iterator() ENTT_NOEXCEPT = default;
+
+        range_iterator(node_type *head) ENTT_NOEXCEPT
+            : it{head}
+        {}
+
+        range_iterator & operator++() ENTT_NOEXCEPT {
+            return ++it, *this;
+        }
+
+        range_iterator operator++(int) ENTT_NOEXCEPT {
+            range_iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
+            return it.operator->();
+        }
+
+        [[nodiscard]] bool operator==(const range_iterator &other) const ENTT_NOEXCEPT {
+            return other.it == it;
+        }
+
+        [[nodiscard]] bool operator!=(const range_iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+    private:
+        typename internal::meta_range<node_type>::iterator it{};
+    };
+
+public:
+    /*! @brief Node type. */
+    using node_type = typename Type::node_type;
+    /*! @brief Input iterator type. */
+    using iterator = range_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]] iterator begin() const ENTT_NOEXCEPT {
+        return iterator{node};
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     * @return An iterator to the element following the last meta object of the
+     * range.
+     */
+    [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+        return iterator{};
+    }
+
+private:
+    node_type *node{nullptr};
+};
+
+
+}
+
+
+#endif

src/entt/meta/resolve.hpp

@@ -0,0 +1,59 @@
+#ifndef ENTT_META_RESOLVE_HPP
+#define ENTT_META_RESOLVE_HPP
+
+
+#include <algorithm>
+#include "ctx.hpp"
+#include "meta.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_info<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_id(const id_type id) ENTT_NOEXCEPT {
+    internal::meta_range range{*internal::meta_context::global()};
+    return std::find_if(range.begin(), range.end(), [id](const auto &curr) { return curr.id == id; }).operator->();
+}
+
+
+/**
+ * @brief Returns the meta type associated with a given type id, if any.
+ * @param id Unique identifier.
+ * @return The meta type associated with the given type id, if any.
+ */
+[[nodiscard]] inline meta_type resolve_type(const id_type id) ENTT_NOEXCEPT {
+    internal::meta_range range{*internal::meta_context::global()};
+    return std::find_if(range.begin(), range.end(), [id](const auto &curr) { return curr.type_id == id; }).operator->();
+}
+
+
+}
+
+
+#endif

src/entt/meta/type_traits.hpp

@@ -0,0 +1,119 @@
+#ifndef ENTT_META_TYPE_TRAITS_HPP
+#define ENTT_META_TYPE_TRAITS_HPP
+
+
+#include <type_traits>
+
+
+namespace entt {
+
+
+/**
+ * @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 support for meta
+ * sequence containers is enabled for the given type, false otherwise.
+ * @tparam Type Potentially sequence container type.
+ */
+template<typename Type, typename = void>
+struct has_meta_sequence_container_traits: std::false_type {};
+
+
+/*! @copydoc has_meta_sequence_container_traits */
+template<typename Type>
+struct has_meta_sequence_container_traits<Type, std::void_t<typename meta_sequence_container_traits<Type>::value_type>>
+        : std::true_type
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type Potentially sequence container type.
+ */
+template<typename Type>
+inline constexpr auto has_meta_sequence_container_traits_v = has_meta_sequence_container_traits<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if support for meta
+ * associative containers is enabled for the given type, false otherwise.
+ * @tparam Type Potentially associative container type.
+ */
+template<typename, typename = void>
+struct has_meta_associative_container_traits: std::false_type {};
+
+
+/*! @copydoc has_meta_associative_container_traits */
+template<typename Type>
+struct has_meta_associative_container_traits<Type, std::void_t<typename meta_associative_container_traits<Type>::key_type>>
+        : std::true_type
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type Potentially associative container type.
+ */
+template<typename Type>
+inline constexpr auto has_meta_associative_container_traits_v = has_meta_associative_container_traits<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a meta associative
+ * container claims to wrap a key-only type, false otherwise.
+ * @tparam Type Potentially key-only meta associative container type.
+ */
+template<typename, typename = void>
+struct is_key_only_meta_associative_container: std::true_type {};
+
+
+/*! @copydoc is_key_only_meta_associative_container */
+template<typename Type>
+struct is_key_only_meta_associative_container<Type, std::void_t<typename meta_associative_container_traits<Type>::mapped_type>>
+        : std::false_type
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type Potentially key-only meta associative container type.
+ */
+template<typename Type>
+inline constexpr auto is_key_only_meta_associative_container_v = is_key_only_meta_associative_container<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is a
+ * pointer-like type from the point of view of the meta system, false otherwise.
+ * @tparam Type Potentially pointer-like type.
+ */
+template<typename>
+struct is_meta_pointer_like: std::false_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;
+
+
+}
+
+
+#endif

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

@@ -0,0 +1,86 @@
+#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(...);
+
+
+}
+
+
+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;
+
+
+}
+
+
+#endif
+#endif
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+#endif

src/entt/process/process.hpp

@@ -0,0 +1,337 @@
+#ifndef ENTT_PROCESS_PROCESS_HPP
+#define ENTT_PROCESS_PROCESS_HPP
+
+
+#include <utility>
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/type_traits.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Base class for processes.
+ *
+ * This class stays true to the CRTP idiom. Derived classes must specify what's
+ * the intended type for elapsed times.<br/>
+ * A process should expose publicly the following member functions whether
+ * required:
+ *
+ * * @code{.cpp}
+ *   void update(Delta, void *);
+ *   @endcode
+ *
+ *   It's invoked once per tick until a process is explicitly aborted or it
+ *   terminates either with or without errors. Even though it's not mandatory to
+ *   declare this member function, as a rule of thumb each process should at
+ *   least define it to work properly. The `void *` parameter is an opaque
+ *   pointer to user data (if any) forwarded directly to the process during an
+ *   update.
+ *
+ * * @code{.cpp}
+ *   void init();
+ *   @endcode
+ *
+ *   It's invoked when the process joins the running queue of a scheduler. This
+ *   happens as soon as it's attached to the scheduler if the process is a top
+ *   level one, otherwise when it replaces its parent if the process is a
+ *   continuation.
+ *
+ * * @code{.cpp}
+ *   void succeeded();
+ *   @endcode
+ *
+ *   It's invoked in case of success, immediately after an update and during the
+ *   same tick.
+ *
+ * * @code{.cpp}
+ *   void failed();
+ *   @endcode
+ *
+ *   It's invoked in case of errors, immediately after an update and during the
+ *   same tick.
+ *
+ * * @code{.cpp}
+ *   void aborted();
+ *   @endcode
+ *
+ *   It's invoked only if a process is explicitly aborted. There is no guarantee
+ *   that it executes in the same tick, this depends solely on whether the
+ *   process is aborted immediately or not.
+ *
+ * Derived classes can change the internal state of a process by invoking the
+ * `succeed` and `fail` protected member functions and even pause or unpause the
+ * process itself.
+ *
+ * @sa scheduler
+ *
+ * @tparam Derived Actual type of process that extends the class template.
+ * @tparam Delta Type to use to provide elapsed time.
+ */
+template<typename Derived, typename Delta>
+class process {
+    enum class state: unsigned int {
+        UNINITIALIZED = 0,
+        RUNNING,
+        PAUSED,
+        SUCCEEDED,
+        FAILED,
+        ABORTED,
+        FINISHED
+    };
+
+    template<typename Target = Derived>
+    auto next(integral_constant<state::UNINITIALIZED>)
+    -> decltype(std::declval<Target>().init(), void()) {
+        static_cast<Target *>(this)->init();
+    }
+
+    template<typename Target = Derived>
+    auto next(integral_constant<state::RUNNING>, Delta delta, void *data)
+    -> decltype(std::declval<Target>().update(delta, data), void()) {
+        static_cast<Target *>(this)->update(delta, data);
+    }
+
+    template<typename Target = Derived>
+    auto next(integral_constant<state::SUCCEEDED>)
+    -> decltype(std::declval<Target>().succeeded(), void()) {
+        static_cast<Target *>(this)->succeeded();
+    }
+
+    template<typename Target = Derived>
+    auto next(integral_constant<state::FAILED>)
+    -> decltype(std::declval<Target>().failed(), void()) {
+        static_cast<Target *>(this)->failed();
+    }
+
+    template<typename Target = Derived>
+    auto next(integral_constant<state::ABORTED>)
+    -> decltype(std::declval<Target>().aborted(), void()) {
+        static_cast<Target *>(this)->aborted();
+    }
+
+    void next(...) const ENTT_NOEXCEPT {}
+
+protected:
+    /**
+     * @brief Terminates a process with success if it's still alive.
+     *
+     * The function is idempotent and it does nothing if the process isn't
+     * alive.
+     */
+    void succeed() ENTT_NOEXCEPT {
+        if(alive()) {
+            current = state::SUCCEEDED;
+        }
+    }
+
+    /**
+     * @brief Terminates a process with errors if it's still alive.
+     *
+     * The function is idempotent and it does nothing if the process isn't
+     * alive.
+     */
+    void fail() ENTT_NOEXCEPT {
+        if(alive()) {
+            current = state::FAILED;
+        }
+    }
+
+    /**
+     * @brief Stops a process if it's in a running state.
+     *
+     * The function is idempotent and it does nothing if the process isn't
+     * running.
+     */
+    void pause() ENTT_NOEXCEPT {
+        if(current == state::RUNNING) {
+            current = state::PAUSED;
+        }
+    }
+
+    /**
+     * @brief Restarts a process if it's paused.
+     *
+     * The function is idempotent and it does nothing if the process isn't
+     * paused.
+     */
+    void unpause() ENTT_NOEXCEPT {
+        if(current  == state::PAUSED) {
+            current  = state::RUNNING;
+        }
+    }
+
+public:
+    /*! @brief Type used to provide elapsed time. */
+    using delta_type = Delta;
+
+    /*! @brief Default destructor. */
+    virtual ~process() {
+        static_assert(std::is_base_of_v<process, Derived>, "Incorrect use of the class template");
+    }
+
+    /**
+     * @brief Aborts a process if it's still alive.
+     *
+     * The function is idempotent and it does nothing if the process isn't
+     * alive.
+     *
+     * @param immediately Requests an immediate operation.
+     */
+    void abort(const bool immediately = false) {
+        if(alive()) {
+            current = state::ABORTED;
+
+            if(immediately) {
+                tick({});
+            }
+        }
+    }
+
+    /**
+     * @brief Returns true if a process is either running or paused.
+     * @return True if the process is still alive, false otherwise.
+     */
+    [[nodiscard]] bool alive() const ENTT_NOEXCEPT {
+        return current == state::RUNNING || current == state::PAUSED;
+    }
+
+    /**
+     * @brief Returns true if a process is already terminated.
+     * @return True if the process is terminated, false otherwise.
+     */
+    [[nodiscard]] bool dead() const ENTT_NOEXCEPT {
+        return current == state::FINISHED;
+    }
+
+    /**
+     * @brief Returns true if a process is currently paused.
+     * @return True if the process is paused, false otherwise.
+     */
+    [[nodiscard]] bool paused() const ENTT_NOEXCEPT {
+        return current == state::PAUSED;
+    }
+
+    /**
+     * @brief Returns true if a process terminated with errors.
+     * @return True if the process terminated with errors, false otherwise.
+     */
+    [[nodiscard]] bool rejected() const ENTT_NOEXCEPT {
+        return stopped;
+    }
+
+    /**
+     * @brief Updates a process and its internal state if required.
+     * @param delta Elapsed time.
+     * @param data Optional data.
+     */
+    void tick(const Delta delta, void *data = nullptr) {
+        switch (current) {
+        case state::UNINITIALIZED:
+            next(integral_constant<state::UNINITIALIZED>{});
+            current = state::RUNNING;
+            break;
+        case state::RUNNING:
+            next(integral_constant<state::RUNNING>{}, delta, data);
+            break;
+        default:
+            // suppress warnings
+            break;
+        }
+
+        // if it's dead, it must be notified and removed immediately
+        switch(current) {
+        case state::SUCCEEDED:
+            next(integral_constant<state::SUCCEEDED>{});
+            current = state::FINISHED;
+            break;
+        case state::FAILED:
+            next(integral_constant<state::FAILED>{});
+            current = state::FINISHED;
+            stopped = true;
+            break;
+        case state::ABORTED:
+            next(integral_constant<state::ABORTED>{});
+            current = state::FINISHED;
+            stopped = true;
+            break;
+        default:
+            // suppress warnings
+            break;
+        }
+    }
+
+private:
+    state current{state::UNINITIALIZED};
+    bool stopped{false};
+};
+
+
+/**
+ * @brief Adaptor for lambdas and functors to turn them into processes.
+ *
+ * Lambdas and functors can't be used directly with a scheduler for they are not
+ * properly defined processes with managed life cycles.<br/>
+ * This class helps in filling the gap and turning lambdas and functors into
+ * full featured processes usable by a scheduler.
+ *
+ * The signature of the function call operator should be equivalent to the
+ * following:
+ *
+ * @code{.cpp}
+ * void(Delta delta, void *data, auto succeed, auto fail);
+ * @endcode
+ *
+ * Where:
+ *
+ * * `delta` is the elapsed time.
+ * * `data` is an opaque pointer to user data if any, `nullptr` otherwise.
+ * * `succeed` is a function to call when a process terminates with success.
+ * * `fail` is a function to call when a process terminates with errors.
+ *
+ * The signature of the function call operator of both `succeed` and `fail`
+ * is equivalent to the following:
+ *
+ * @code{.cpp}
+ * void();
+ * @endcode
+ *
+ * Usually users shouldn't worry about creating adaptors. A scheduler will
+ * create them internally each and avery time a lambda or a functor is used as
+ * a process.
+ *
+ * @sa process
+ * @sa scheduler
+ *
+ * @tparam Func Actual type of process.
+ * @tparam Delta Type to use to provide elapsed time.
+ */
+template<typename Func, typename Delta>
+struct process_adaptor: process<process_adaptor<Func, Delta>, Delta>, private Func {
+    /**
+     * @brief Constructs a process adaptor from a lambda or a functor.
+     * @tparam Args Types of arguments to use to initialize the actual process.
+     * @param args Parameters to use to initialize the actual process.
+     */
+    template<typename... Args>
+    process_adaptor(Args &&... args)
+        : Func{std::forward<Args>(args)...}
+    {}
+
+    /**
+     * @brief Updates a process and its internal state if required.
+     * @param delta Elapsed time.
+     * @param data Optional data.
+     */
+    void update(const Delta delta, void *data) {
+        Func::operator()(delta, data, [this]() { this->succeed(); }, [this]() { this->fail(); });
+    }
+};
+
+
+}
+
+
+#endif

src/entt/process/scheduler.hpp

@@ -0,0 +1,300 @@
+#ifndef ENTT_PROCESS_SCHEDULER_HPP
+#define ENTT_PROCESS_SCHEDULER_HPP
+
+
+#include <vector>
+#include <memory>
+#include <utility>
+#include <algorithm>
+#include <type_traits>
+#include "../config/config.h"
+#include "process.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Cooperative scheduler for processes.
+ *
+ * A cooperative scheduler runs processes and helps managing their life cycles.
+ *
+ * Each process is invoked once per tick. If a process terminates, it's
+ * removed automatically from the scheduler and it's never invoked again.<br/>
+ * A process can also have a child. In this case, the process is replaced with
+ * its child when it terminates if it returns with success. In case of errors,
+ * both the process and its child are discarded.
+ *
+ * Example of use (pseudocode):
+ *
+ * @code{.cpp}
+ * scheduler.attach([](auto delta, void *, auto succeed, auto fail) {
+ *     // code
+ * }).then<my_process>(arguments...);
+ * @endcode
+ *
+ * In order to invoke all scheduled processes, call the `update` member function
+ * passing it the elapsed time to forward to the tasks.
+ *
+ * @sa process
+ *
+ * @tparam Delta Type to use to provide elapsed time.
+ */
+template<typename Delta>
+class scheduler {
+    struct process_handler {
+        using instance_type = std::unique_ptr<void, void(*)(void *)>;
+        using update_fn_type = bool(process_handler &, Delta, void *);
+        using abort_fn_type = void(process_handler &, bool);
+        using next_type = std::unique_ptr<process_handler>;
+
+        instance_type instance;
+        update_fn_type *update;
+        abort_fn_type *abort;
+        next_type next;
+    };
+
+    struct continuation {
+        continuation(process_handler *ref)
+            : handler{ref}
+        {
+            ENTT_ASSERT(handler);
+        }
+
+        template<typename Proc, typename... Args>
+        continuation then(Args &&... args) {
+            static_assert(std::is_base_of_v<process<Proc, Delta>, Proc>, "Invalid process type");
+            auto proc = typename process_handler::instance_type{new Proc{std::forward<Args>(args)...}, &scheduler::deleter<Proc>};
+            handler->next.reset(new process_handler{std::move(proc), &scheduler::update<Proc>, &scheduler::abort<Proc>, nullptr});
+            handler = handler->next.get();
+            return *this;
+        }
+
+        template<typename Func>
+        continuation then(Func &&func) {
+            return then<process_adaptor<std::decay_t<Func>, Delta>>(std::forward<Func>(func));
+        }
+
+    private:
+        process_handler *handler;
+    };
+
+    template<typename Proc>
+    [[nodiscard]] static bool update(process_handler &handler, const Delta delta, void *data) {
+        auto *process = static_cast<Proc *>(handler.instance.get());
+        process->tick(delta, data);
+
+        auto dead = process->dead();
+
+        if(dead) {
+            if(handler.next && !process->rejected()) {
+                handler = std::move(*handler.next);
+                // forces the process to exit the uninitialized state
+                dead = handler.update(handler, {}, nullptr);
+            } else {
+                handler.instance.reset();
+            }
+        }
+
+        return dead;
+    }
+
+    template<typename Proc>
+    static void abort(process_handler &handler, const bool immediately) {
+        static_cast<Proc *>(handler.instance.get())->abort(immediately);
+    }
+
+    template<typename Proc>
+    static void deleter(void *proc) {
+        delete static_cast<Proc *>(proc);
+    }
+
+public:
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+
+    /*! @brief Default constructor. */
+    scheduler() = default;
+
+    /*! @brief Default move constructor. */
+    scheduler(scheduler &&) = default;
+
+    /*! @brief Default move assignment operator. @return This scheduler. */
+    scheduler & operator=(scheduler &&) = default;
+
+    /**
+     * @brief Number of processes currently scheduled.
+     * @return Number of processes currently scheduled.
+     */
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return handlers.size();
+    }
+
+    /**
+     * @brief Returns true if at least a process is currently scheduled.
+     * @return True if there are scheduled processes, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return handlers.empty();
+    }
+
+    /**
+     * @brief Discards all scheduled processes.
+     *
+     * Processes aren't aborted. They are discarded along with their children
+     * and never executed again.
+     */
+    void clear() {
+        handlers.clear();
+    }
+
+    /**
+     * @brief Schedules a process for the next tick.
+     *
+     * Returned value is an opaque object that can be used to attach a child to
+     * the given process. The child is automatically scheduled when the process
+     * terminates and only if the process returns with success.
+     *
+     * Example of use (pseudocode):
+     *
+     * @code{.cpp}
+     * // schedules a task in the form of a process class
+     * scheduler.attach<my_process>(arguments...)
+     * // appends a child in the form of a lambda function
+     * .then([](auto delta, void *, auto succeed, auto fail) {
+     *     // code
+     * })
+     * // appends a child in the form of another process class
+     * .then<my_other_process>();
+     * @endcode
+     *
+     * @tparam Proc Type of process to schedule.
+     * @tparam Args Types of arguments to use to initialize the process.
+     * @param args Parameters to use to initialize the process.
+     * @return An opaque object to use to concatenate processes.
+     */
+    template<typename Proc, typename... Args>
+    auto attach(Args &&... args) {
+        static_assert(std::is_base_of_v<process<Proc, Delta>, Proc>, "Invalid process type");
+        auto proc = typename process_handler::instance_type{new Proc{std::forward<Args>(args)...}, &scheduler::deleter<Proc>};
+        process_handler handler{std::move(proc), &scheduler::update<Proc>, &scheduler::abort<Proc>, nullptr};
+        // forces the process to exit the uninitialized state
+        handler.update(handler, {}, nullptr);
+        return continuation{&handlers.emplace_back(std::move(handler))};
+    }
+
+    /**
+     * @brief Schedules a process for the next tick.
+     *
+     * A process can be either a lambda or a functor. The scheduler wraps both
+     * of them in a process adaptor internally.<br/>
+     * The signature of the function call operator should be equivalent to the
+     * following:
+     *
+     * @code{.cpp}
+     * void(Delta delta, void *data, auto succeed, auto fail);
+     * @endcode
+     *
+     * Where:
+     *
+     * * `delta` is the elapsed time.
+     * * `data` is an opaque pointer to user data if any, `nullptr` otherwise.
+     * * `succeed` is a function to call when a process terminates with success.
+     * * `fail` is a function to call when a process terminates with errors.
+     *
+     * The signature of the function call operator of both `succeed` and `fail`
+     * is equivalent to the following:
+     *
+     * @code{.cpp}
+     * void();
+     * @endcode
+     *
+     * Returned value is an opaque object that can be used to attach a child to
+     * the given process. The child is automatically scheduled when the process
+     * terminates and only if the process returns with success.
+     *
+     * Example of use (pseudocode):
+     *
+     * @code{.cpp}
+     * // schedules a task in the form of a lambda function
+     * scheduler.attach([](auto delta, void *, auto succeed, auto fail) {
+     *     // code
+     * })
+     * // appends a child in the form of another lambda function
+     * .then([](auto delta, void *, auto succeed, auto fail) {
+     *     // code
+     * })
+     * // appends a child in the form of a process class
+     * .then<my_process>(arguments...);
+     * @endcode
+     *
+     * @sa process_adaptor
+     *
+     * @tparam Func Type of process to schedule.
+     * @param func Either a lambda or a functor to use as a process.
+     * @return An opaque object to use to concatenate processes.
+     */
+    template<typename Func>
+    auto attach(Func &&func) {
+        using Proc = process_adaptor<std::decay_t<Func>, Delta>;
+        return attach<Proc>(std::forward<Func>(func));
+    }
+
+    /**
+     * @brief Updates all scheduled processes.
+     *
+     * All scheduled processes are executed in no specific order.<br/>
+     * If a process terminates with success, it's replaced with its child, if
+     * any. Otherwise, if a process terminates with an error, it's removed along
+     * with its child.
+     *
+     * @param delta Elapsed time.
+     * @param data Optional data.
+     */
+    void update(const Delta delta, void *data = nullptr) {
+        bool clean = false;
+
+        for(auto pos = handlers.size(); pos; --pos) {
+            auto &handler = handlers[pos-1];
+            const bool dead = handler.update(handler, delta, data);
+            clean = clean || dead;
+        }
+
+        if(clean) {
+            handlers.erase(std::remove_if(handlers.begin(), handlers.end(), [](auto &handler) {
+                return !handler.instance;
+            }), handlers.end());
+        }
+    }
+
+    /**
+     * @brief Aborts all scheduled processes.
+     *
+     * Unless an immediate operation is requested, the abort is scheduled for
+     * the next tick. Processes won't be executed anymore in any case.<br/>
+     * Once a process is fully aborted and thus finished, it's discarded along
+     * with its child, if any.
+     *
+     * @param immediately Requests an immediate operation.
+     */
+    void abort(const bool immediately = false) {
+        decltype(handlers) exec;
+        exec.swap(handlers);
+
+        for(auto &&handler: exec) {
+            handler.abort(handler, immediately);
+        }
+
+        std::move(handlers.begin(), handlers.end(), std::back_inserter(exec));
+        handlers.swap(exec);
+    }
+
+private:
+    std::vector<process_handler> handlers{};
+};
+
+
+}
+
+
+#endif

src/entt/resource/cache.hpp

@@ -0,0 +1,239 @@
+#ifndef ENTT_RESOURCE_CACHE_HPP
+#define ENTT_RESOURCE_CACHE_HPP
+
+
+#include <memory>
+#include <type_traits>
+#include <unordered_map>
+#include <utility>
+#include "../config/config.h"
+#include "../core/fwd.hpp"
+#include "handle.hpp"
+#include "loader.hpp"
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Simple cache for resources of a given type.
+ *
+ * Minimal implementation of a cache for resources of a given type. It doesn't
+ * offer much functionalities but it's suitable for small or medium sized
+ * applications and can be freely inherited to add targeted functionalities for
+ * large sized applications.
+ *
+ * @tparam Resource Type of resources managed by a cache.
+ */
+template<typename Resource>
+struct resource_cache {
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Type of resources managed by a cache. */
+    using resource_type = Resource;
+
+    /*! @brief Default constructor. */
+    resource_cache() = default;
+
+    /*! @brief Default move constructor. */
+    resource_cache(resource_cache &&) = default;
+
+    /*! @brief Default move assignment operator. @return This cache. */
+    resource_cache & operator=(resource_cache &&) = default;
+
+    /**
+     * @brief Number of resources managed by a cache.
+     * @return Number of resources currently stored.
+     */
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return resources.size();
+    }
+
+    /**
+     * @brief Returns true if a cache contains no resources, false otherwise.
+     * @return True if the cache contains no resources, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return resources.empty();
+    }
+
+    /**
+     * @brief Clears a cache and discards all its resources.
+     *
+     * Handles are not invalidated and the memory used by a resource isn't
+     * freed as long as at least a handle keeps the resource itself alive.
+     */
+    void clear() ENTT_NOEXCEPT {
+        resources.clear();
+    }
+
+    /**
+     * @brief Loads the resource that corresponds to a given identifier.
+     *
+     * In case an identifier isn't already present in the cache, it loads its
+     * resource and stores it aside for future uses. Arguments are forwarded
+     * directly to the loader in order to construct properly the requested
+     * resource.
+     *
+     * @note
+     * If the identifier is already present in the cache, this function does
+     * nothing and the arguments are simply discarded.
+     *
+     * @warning
+     * If the resource cannot be loaded correctly, the returned handle will be
+     * invalid and any use of it will result in undefined behavior.
+     *
+     * @tparam Loader Type of loader to use to load the resource if required.
+     * @tparam Args Types of arguments to use to load the resource if required.
+     * @param id Unique resource identifier.
+     * @param args Arguments to use to load the resource if required.
+     * @return A handle for the given resource.
+     */
+    template<typename Loader, typename... Args>
+    resource_handle<Resource> load(const id_type id, Args &&... args) {
+        static_assert(std::is_base_of_v<resource_loader<Loader, Resource>, Loader>, "Invalid loader type");
+        resource_handle<Resource> resource{};
+
+        if(auto it = resources.find(id); it == resources.cend()) {
+            if(auto instance = Loader{}.get(std::forward<Args>(args)...); instance) {
+                resources[id] = instance;
+                resource = std::move(instance);
+            }
+        } else {
+            resource = it->second;
+        }
+
+        return resource;
+    }
+
+    /**
+     * @brief Reloads a resource or loads it for the first time if not present.
+     *
+     * Equivalent to the following snippet (pseudocode):
+     *
+     * @code{.cpp}
+     * cache.discard(id);
+     * cache.load(id, args...);
+     * @endcode
+     *
+     * Arguments are forwarded directly to the loader in order to construct
+     * properly the requested resource.
+     *
+     * @warning
+     * If the resource cannot be loaded correctly, the returned handle will be
+     * invalid and any use of it will result in undefined behavior.
+     *
+     * @tparam Loader Type of loader to use to load the resource.
+     * @tparam Args Types of arguments to use to load the resource.
+     * @param id Unique resource identifier.
+     * @param args Arguments to use to load the resource.
+     * @return A handle for the given resource.
+     */
+    template<typename Loader, typename... Args>
+    resource_handle<Resource> reload(const id_type id, Args &&... args) {
+        return (discard(id), load<Loader>(id, std::forward<Args>(args)...));
+    }
+
+    /**
+     * @brief Creates a temporary handle for a resource.
+     *
+     * Arguments are forwarded directly to the loader in order to construct
+     * properly the requested resource. The handle isn't stored aside and the
+     * cache isn't in charge of the lifetime of the resource itself.
+     *
+     * @tparam Loader Type of loader to use to load the resource.
+     * @tparam Args Types of arguments to use to load the resource.
+     * @param args Arguments to use to load the resource.
+     * @return A handle for the given resource.
+     */
+    template<typename Loader, typename... Args>
+    [[nodiscard]] resource_handle<Resource> temp(Args &&... args) const {
+        return { Loader{}.get(std::forward<Args>(args)...) };
+    }
+
+    /**
+     * @brief Creates a handle for a given resource identifier.
+     *
+     * A resource handle can be in a either valid or invalid state. In other
+     * terms, a resource handle is properly initialized with a resource if the
+     * cache contains the resource itself. Otherwise the returned handle is
+     * uninitialized and accessing it results in undefined behavior.
+     *
+     * @sa resource_handle
+     *
+     * @param id Unique resource identifier.
+     * @return A handle for the given resource.
+     */
+    [[nodiscard]] resource_handle<Resource> handle(const id_type id) const {
+        auto it = resources.find(id);
+        return { it == resources.end() ? nullptr : it->second };
+    }
+
+    /**
+     * @brief Checks if a cache contains a given identifier.
+     * @param id Unique resource identifier.
+     * @return True if the cache contains the resource, false otherwise.
+     */
+    [[nodiscard]] bool contains(const id_type id) const {
+        return (resources.find(id) != resources.cend());
+    }
+
+    /**
+     * @brief Discards the resource that corresponds to a given identifier.
+     *
+     * Handles are not invalidated and the memory used by the resource isn't
+     * freed as long as at least a handle keeps the resource itself alive.
+     *
+     * @param id Unique resource identifier.
+     */
+    void discard(const id_type id) {
+        if(auto it = resources.find(id); it != resources.end()) {
+            resources.erase(it);
+        }
+    }
+
+    /**
+     * @brief Iterates all resources.
+     *
+     * The function object is invoked for each element. It is provided with
+     * either the resource identifier, the resource handle or both of them.<br/>
+     * The signature of the function must be equivalent to one of the following
+     * forms:
+     *
+     * @code{.cpp}
+     * void(const entt::id_type);
+     * void(entt::resource_handle<Resource>);
+     * void(const entt::id_type, entt::resource_handle<Resource>);
+     * @endcode
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template <typename Func>
+    void each(Func func) const {
+        auto begin = resources.begin();
+        auto end = resources.end();
+
+        while(begin != end) {
+            auto curr = begin++;
+
+            if constexpr(std::is_invocable_v<Func, id_type>) {
+                func(curr->first);
+            } else if constexpr(std::is_invocable_v<Func, resource_handle<Resource>>) {
+                func(resource_handle{ curr->second });
+            } else {
+                func(curr->first, resource_handle{ curr->second });
+            }
+        }
+    }
+
+private:
+    std::unordered_map<id_type, std::shared_ptr<Resource>> resources;
+};
+
+
+}
+
+
+#endif

src/entt/resource/fwd.hpp

@@ -0,0 +1,23 @@
+#ifndef ENTT_RESOURCE_FWD_HPP
+#define ENTT_RESOURCE_FWD_HPP
+
+
+namespace entt {
+
+
+template<typename>
+struct resource_cache;
+
+
+template<typename>
+class resource_handle;
+
+
+template<typename, typename>
+class resource_loader;
+
+
+}
+
+
+#endif

src/entt/resource/handle.hpp

@@ -0,0 +1,116 @@
+#ifndef ENTT_RESOURCE_HANDLE_HPP
+#define ENTT_RESOURCE_HANDLE_HPP
+
+
+#include <memory>
+#include <utility>
+#include "../config/config.h"
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Shared resource handle.
+ *
+ * A shared resource handle is a small class that wraps a resource and keeps it
+ * alive even if it's deleted from the cache. It can be either copied or
+ * moved. A handle shares a reference to the same resource with all the other
+ * handles constructed for the same identifier.<br/>
+ * As a rule of thumb, resources should never be copied nor moved. Handles are
+ * the way to go to keep references to them.
+ *
+ * @tparam Resource Type of resource managed by a handle.
+ */
+template<typename Resource>
+class resource_handle {
+    /*! @brief Resource handles are friends of their caches. */
+    friend struct resource_cache<Resource>;
+
+    resource_handle(std::shared_ptr<Resource> res) ENTT_NOEXCEPT
+        : resource{std::move(res)}
+    {}
+
+public:
+    /*! @brief Default constructor. */
+    resource_handle() ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Gets a reference to the managed resource.
+     *
+     * @warning
+     * The behavior is undefined if the handle doesn't contain a resource.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * handle is empty.
+     *
+     * @return A reference to the managed resource.
+     */
+    [[nodiscard]] const Resource & get() const ENTT_NOEXCEPT {
+        ENTT_ASSERT(static_cast<bool>(resource));
+        return *resource;
+    }
+
+    /*! @copydoc get */
+    [[nodiscard]] Resource & get() ENTT_NOEXCEPT {
+        return const_cast<Resource &>(std::as_const(*this).get());
+    }
+
+    /*! @copydoc get */
+    [[nodiscard]] operator const Resource & () const ENTT_NOEXCEPT {
+        return get();
+    }
+
+    /*! @copydoc get */
+    [[nodiscard]] operator Resource & () ENTT_NOEXCEPT {
+        return get();
+    }
+
+    /*! @copydoc get */
+    [[nodiscard]] const Resource & operator *() const ENTT_NOEXCEPT {
+        return get();
+    }
+
+    /*! @copydoc get */
+    [[nodiscard]] Resource & operator *() ENTT_NOEXCEPT {
+        return get();
+    }
+
+    /**
+     * @brief Gets a pointer to the managed resource.
+     *
+     * @warning
+     * The behavior is undefined if the handle doesn't contain a resource.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * handle is empty.
+     *
+     * @return A pointer to the managed resource or `nullptr` if the handle
+     * contains no resource at all.
+     */
+    [[nodiscard]] const Resource * operator->() const ENTT_NOEXCEPT {
+        ENTT_ASSERT(static_cast<bool>(resource));
+        return resource.get();
+    }
+
+    /*! @copydoc operator-> */
+    [[nodiscard]] Resource * operator->() ENTT_NOEXCEPT {
+        return const_cast<Resource *>(std::as_const(*this).operator->());
+    }
+
+    /**
+     * @brief Returns true if a handle contains a resource, false otherwise.
+     * @return True if the handle contains a resource, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return static_cast<bool>(resource);
+    }
+
+private:
+    std::shared_ptr<Resource> resource;
+};
+
+
+}
+
+
+#endif

src/entt/resource/loader.hpp

@@ -0,0 +1,65 @@
+#ifndef ENTT_RESOURCE_LOADER_HPP
+#define ENTT_RESOURCE_LOADER_HPP
+
+
+#include <memory>
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Base class for resource loaders.
+ *
+ * Resource loaders must inherit from this class and stay true to the CRTP
+ * idiom. Moreover, a resource loader must expose a public, const member
+ * function named `load` that accepts a variable number of arguments and returns
+ * a shared pointer to the resource just created.<br/>
+ * As an example:
+ *
+ * @code{.cpp}
+ * struct my_resource {};
+ *
+ * struct my_loader: entt::resource_loader<my_loader, my_resource> {
+ *     std::shared_ptr<my_resource> load(int) const {
+ *         // use the integer value somehow
+ *         return std::make_shared<my_resource>();
+ *     }
+ * };
+ * @endcode
+ *
+ * In general, resource loaders should not have a state or retain data of any
+ * type. They should let the cache manage their resources instead.
+ *
+ * @note
+ * Base class and CRTP idiom aren't strictly required with the current
+ * implementation. One could argue that a cache can easily work with loaders of
+ * any type. However, future changes won't be breaking ones by forcing the use
+ * of a base class today and that's why the model is already in its place.
+ *
+ * @tparam Loader Type of the derived class.
+ * @tparam Resource Type of resource for which to use the loader.
+ */
+template<typename Loader, typename Resource>
+class resource_loader {
+    /*! @brief Resource loaders are friends of their caches. */
+    friend struct resource_cache<Resource>;
+
+    /**
+     * @brief Loads the resource and returns it.
+     * @tparam Args Types of arguments for the loader.
+     * @param args Arguments for the loader.
+     * @return The resource just loaded or an empty pointer in case of errors.
+     */
+    template<typename... Args>
+    [[nodiscard]] std::shared_ptr<Resource> get(Args &&... args) const {
+        return static_cast<const Loader *>(this)->load(std::forward<Args>(args)...);
+    }
+};
+
+
+}
+
+
+#endif

src/entt/signal/delegate.hpp

@@ -0,0 +1,362 @@
+#ifndef ENTT_SIGNAL_DELEGATE_HPP
+#define ENTT_SIGNAL_DELEGATE_HPP
+
+
+#include <tuple>
+#include <cstddef>
+#include <utility>
+#include <functional>
+#include <type_traits>
+#include "../config/config.h"
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename Ret, typename... Args>
+auto function_pointer(Ret(*)(Args...)) -> Ret(*)(Args...);
+
+
+template<typename Ret, typename Type, typename... Args, typename Other>
+auto function_pointer(Ret(*)(Type, Args...), Other &&) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Ret, typename... Args, typename... Other>
+auto function_pointer(Ret(Class:: *)(Args...), Other &&...) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Ret, typename... Args, typename... Other>
+auto function_pointer(Ret(Class:: *)(Args...) const, Other &&...) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Type, typename... Other>
+auto function_pointer(Type Class:: *, Other &&...) -> Type(*)();
+
+
+template<typename... Type>
+using function_pointer_t = decltype(internal::function_pointer(std::declval<Type>()...));
+
+
+template<typename... Class, typename Ret, typename... Args>
+[[nodiscard]] constexpr auto index_sequence_for(Ret(*)(Args...)) {
+    return std::index_sequence_for<Class..., Args...>{};
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/*! @brief Used to wrap a function or a member of a specified type. */
+template<auto>
+struct connect_arg_t {};
+
+
+/*! @brief Constant of type connect_arg_t used to disambiguate calls. */
+template<auto Func>
+inline constexpr connect_arg_t<Func> connect_arg{};
+
+
+/**
+ * @brief Basic delegate implementation.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error unless the template parameter is a function type.
+ */
+template<typename>
+class delegate;
+
+
+/**
+ * @brief Utility class to use to send around functions and members.
+ *
+ * Unmanaged delegate for function pointers and members. Users of this class are
+ * in charge of disconnecting instances before deleting them.
+ *
+ * A delegate can be used as a general purpose invoker without memory overhead
+ * for free functions possibly with payloads and bound or unbound members.
+ *
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ */
+template<typename Ret, typename... Args>
+class delegate<Ret(Args...)> {
+    template<auto Candidate, std::size_t... Index>
+    [[nodiscard]] auto wrap(std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        return [](const void *, Args... args) -> Ret {
+            [[maybe_unused]] const auto arguments = std::forward_as_tuple(std::forward<Args>(args)...);
+            return Ret(std::invoke(Candidate, std::forward<std::tuple_element_t<Index, std::tuple<Args...>>>(std::get<Index>(arguments))...));
+        };
+    }
+
+    template<auto Candidate, typename Type, std::size_t... Index>
+    [[nodiscard]] auto wrap(Type &, std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        return [](const void *payload, Args... args) -> Ret {
+            [[maybe_unused]] const auto arguments = std::forward_as_tuple(std::forward<Args>(args)...);
+            Type *curr = static_cast<Type *>(const_cast<std::conditional_t<std::is_const_v<Type>, const void *, void *>>(payload));
+            return Ret(std::invoke(Candidate, *curr, std::forward<std::tuple_element_t<Index, std::tuple<Args...>>>(std::get<Index>(arguments))...));
+        };
+    }
+
+    template<auto Candidate, typename Type, std::size_t... Index>
+    [[nodiscard]] auto wrap(Type *, std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        return [](const void *payload, Args... args) -> Ret {
+            [[maybe_unused]] const auto arguments = std::forward_as_tuple(std::forward<Args>(args)...);
+            Type *curr = static_cast<Type *>(const_cast<std::conditional_t<std::is_const_v<Type>, const void *, void *>>(payload));
+            return Ret(std::invoke(Candidate, curr, std::forward<std::tuple_element_t<Index, std::tuple<Args...>>>(std::get<Index>(arguments))...));
+        };
+    }
+
+public:
+    /*! @brief Function type of the contained target. */
+    using function_type = Ret(const void *, Args...);
+    /*! @brief Function type of the delegate. */
+    using type = Ret(Args...);
+    /*! @brief Return type of the delegate. */
+    using result_type = Ret;
+
+    /*! @brief Default constructor. */
+    delegate() ENTT_NOEXCEPT
+        : fn{nullptr}, data{nullptr}
+    {}
+
+    /**
+     * @brief Constructs a delegate and connects a free function or an unbound
+     * member.
+     * @tparam Candidate Function or member to connect to the delegate.
+     */
+    template<auto Candidate>
+    delegate(connect_arg_t<Candidate>) ENTT_NOEXCEPT {
+        connect<Candidate>();
+    }
+
+    /**
+     * @brief Constructs a delegate and connects a free function with payload or
+     * a bound member.
+     * @tparam Candidate Function or member to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    delegate(connect_arg_t<Candidate>, Type &&value_or_instance) ENTT_NOEXCEPT {
+        connect<Candidate>(std::forward<Type>(value_or_instance));
+    }
+
+    /**
+     * @brief Constructs a delegate and connects an user defined function with
+     * optional payload.
+     * @param function Function to connect to the delegate.
+     * @param payload User defined arbitrary data.
+     */
+    delegate(function_type *function, const void *payload = nullptr) ENTT_NOEXCEPT {
+        connect(function, payload);
+    }
+
+    /**
+     * @brief Connects a free function or an unbound member to a delegate.
+     * @tparam Candidate Function or member to connect to the delegate.
+     */
+    template<auto Candidate>
+    void connect() ENTT_NOEXCEPT {
+        data = nullptr;
+
+        if constexpr(std::is_invocable_r_v<Ret, decltype(Candidate), Args...>) {
+            fn = [](const void *, Args... args) -> Ret {
+                return Ret(std::invoke(Candidate, std::forward<Args>(args)...));
+            };
+        } else if constexpr(std::is_member_pointer_v<decltype(Candidate)>) {
+            fn = wrap<Candidate>(internal::index_sequence_for<std::tuple_element_t<0, std::tuple<Args...>>>(internal::function_pointer_t<decltype(Candidate)>{}));
+        } else {
+            fn = wrap<Candidate>(internal::index_sequence_for(internal::function_pointer_t<decltype(Candidate)>{}));
+        }
+    }
+
+    /**
+     * @brief Connects a free function with payload or a bound member to a
+     * delegate.
+     *
+     * The delegate isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of the instance overcomes
+     * the one of the delegate.<br/>
+     * When used to connect a free function with payload, its signature must be
+     * such that the instance is the first argument before the ones used to
+     * define the delegate itself.
+     *
+     * @tparam Candidate Function or member to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid reference that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void connect(Type &value_or_instance) ENTT_NOEXCEPT {
+        data = &value_or_instance;
+
+        if constexpr(std::is_invocable_r_v<Ret, decltype(Candidate), Type &, Args...>) {
+            fn = [](const void *payload, Args... args) -> Ret {
+                Type *curr = static_cast<Type *>(const_cast<std::conditional_t<std::is_const_v<Type>, const void *, void *>>(payload));
+                return Ret(std::invoke(Candidate, *curr, std::forward<Args>(args)...));
+            };
+        } else {
+            fn = wrap<Candidate>(value_or_instance, internal::index_sequence_for(internal::function_pointer_t<decltype(Candidate), Type>{}));
+        }
+    }
+
+    /**
+     * @brief Connects a free function with payload or a bound member to a
+     * delegate.
+     *
+     * @sa connect(Type &)
+     *
+     * @tparam Candidate Function or member to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid pointer that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void connect(Type *value_or_instance) ENTT_NOEXCEPT {
+        data = value_or_instance;
+
+        if constexpr(std::is_invocable_r_v<Ret, decltype(Candidate), Type *, Args...>) {
+            fn = [](const void *payload, Args... args) -> Ret {
+                Type *curr = static_cast<Type *>(const_cast<std::conditional_t<std::is_const_v<Type>, const void *, void *>>(payload));
+                return Ret(std::invoke(Candidate, curr, std::forward<Args>(args)...));
+            };
+        } else {
+            fn = wrap<Candidate>(value_or_instance, internal::index_sequence_for(internal::function_pointer_t<decltype(Candidate), Type>{}));
+        }
+    }
+
+    /**
+     * @brief Connects an user defined function with optional payload to a
+     * delegate.
+     *
+     * The delegate isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of an instance overcomes
+     * the one of the delegate.<br/>
+     * The payload is returned as the first argument to the target function in
+     * all cases.
+     *
+     * @param function Function to connect to the delegate.
+     * @param payload User defined arbitrary data.
+     */
+    void connect(function_type *function, const void *payload = nullptr) ENTT_NOEXCEPT {
+        fn = function;
+        data = payload;
+    }
+
+    /**
+     * @brief Resets a delegate.
+     *
+     * After a reset, a delegate cannot be invoked anymore.
+     */
+    void reset() ENTT_NOEXCEPT {
+        fn = nullptr;
+        data = nullptr;
+    }
+
+    /**
+     * @brief Returns the instance or the payload linked to a delegate, if any.
+     * @return An opaque pointer to the underlying data.
+     */
+    [[nodiscard]] const void * instance() const ENTT_NOEXCEPT {
+        return data;
+    }
+
+    /**
+     * @brief Triggers a delegate.
+     *
+     * The delegate invokes the underlying function and returns the result.
+     *
+     * @warning
+     * Attempting to trigger an invalid delegate results in undefined
+     * behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * delegate has not yet been set.
+     *
+     * @param args Arguments to use to invoke the underlying function.
+     * @return The value returned by the underlying function.
+     */
+    Ret operator()(Args... args) const {
+        ENTT_ASSERT(fn);
+        return fn(data, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Checks whether a delegate actually stores a listener.
+     * @return False if the delegate is empty, true otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        // no need to test also data
+        return !(fn == nullptr);
+    }
+
+    /**
+     * @brief Compares the contents of two delegates.
+     * @param other Delegate with which to compare.
+     * @return False if the two contents differ, true otherwise.
+     */
+    [[nodiscard]] bool operator==(const delegate<Ret(Args...)> &other) const ENTT_NOEXCEPT {
+        return fn == other.fn && data == other.data;
+    }
+
+private:
+    function_type *fn;
+    const void *data;
+};
+
+
+/**
+ * @brief Compares the contents of two delegates.
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ * @param lhs A valid delegate object.
+ * @param rhs A valid delegate object.
+ * @return True if the two contents differ, false otherwise.
+ */
+template<typename Ret, typename... Args>
+[[nodiscard]] bool operator!=(const delegate<Ret(Args...)> &lhs, const delegate<Ret(Args...)> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Candidate Function or member to connect to the delegate.
+ */
+template<auto Candidate>
+delegate(connect_arg_t<Candidate>) ENTT_NOEXCEPT
+-> delegate<std::remove_pointer_t<internal::function_pointer_t<decltype(Candidate)>>>;
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Candidate Function or member to connect to the delegate.
+ * @tparam Type Type of class or type of payload.
+ */
+template<auto Candidate, typename Type>
+delegate(connect_arg_t<Candidate>, Type &&) ENTT_NOEXCEPT
+-> delegate<std::remove_pointer_t<internal::function_pointer_t<decltype(Candidate), Type>>>;
+
+
+/*! @brief Deduction guide. */
+template<typename Ret, typename... Args>
+delegate(Ret(*)(const void *, Args...), const void * = nullptr) ENTT_NOEXCEPT
+-> delegate<Ret(Args...)>;
+
+
+}
+
+
+#endif

src/entt/signal/dispatcher.hpp

@@ -0,0 +1,278 @@
+#ifndef ENTT_SIGNAL_DISPATCHER_HPP
+#define ENTT_SIGNAL_DISPATCHER_HPP
+
+
+#include <cstddef>
+#include <memory>
+#include <type_traits>
+#include <utility>
+#include <vector>
+#include "../config/config.h"
+#include "../core/fwd.hpp"
+#include "../core/type_info.hpp"
+#include "sigh.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Basic dispatcher implementation.
+ *
+ * A dispatcher can be used either to trigger an immediate event or to enqueue
+ * events to be published all together once per tick.<br/>
+ * Listeners are provided in the form of member functions. For each event of
+ * type `Event`, listeners are such that they can be invoked with an argument of
+ * type `Event &`, no matter what the return type is.
+ *
+ * The dispatcher creates instances of the `sigh` class internally. Refer to the
+ * documentation of the latter for more details.
+ */
+class dispatcher {
+    struct basic_pool {
+        virtual ~basic_pool() = default;
+        virtual void publish() = 0;
+        virtual void disconnect(void *) = 0;
+        virtual void clear() ENTT_NOEXCEPT = 0;
+        [[nodiscard]] virtual id_type type_id() const ENTT_NOEXCEPT = 0;
+    };
+
+    template<typename Event>
+    struct pool_handler final: basic_pool {
+        using signal_type = sigh<void(Event &)>;
+        using sink_type = typename signal_type::sink_type;
+
+        void publish() override {
+            const auto length = events.size();
+
+            for(std::size_t pos{}; pos < length; ++pos) {
+                signal.publish(events[pos]);
+            }
+
+            events.erase(events.cbegin(), events.cbegin()+length);
+        }
+
+        void disconnect(void *instance) override {
+            sink().disconnect(instance);
+        }
+
+        void clear() ENTT_NOEXCEPT override {
+            events.clear();
+        }
+
+        [[nodiscard]] sink_type sink() ENTT_NOEXCEPT {
+            return entt::sink{signal};
+        }
+
+        template<typename... Args>
+        void trigger(Args &&... args) {
+            Event instance{std::forward<Args>(args)...};
+            signal.publish(instance);
+        }
+
+        template<typename... Args>
+        void enqueue(Args &&... args) {
+            if constexpr(std::is_aggregate_v<Event>) {
+                events.push_back(Event{std::forward<Args>(args)...});
+            } else {
+                events.emplace_back(std::forward<Args>(args)...);
+            }
+        }
+
+        [[nodiscard]] id_type type_id() const ENTT_NOEXCEPT override {
+            return type_info<Event>::id();
+        }
+
+    private:
+        signal_type signal{};
+        std::vector<Event> events;
+    };
+
+    template<typename Event>
+    [[nodiscard]] pool_handler<Event> & assure() {
+        static_assert(std::is_same_v<Event, std::decay_t<Event>>, "Invalid event type");
+
+        if constexpr(ENTT_FAST_PATH(has_type_index_v<Event>)) {
+            const auto index = type_index<Event>::value();
+
+            if(!(index < pools.size())) {
+                pools.resize(index+1u);
+            }
+
+            if(!pools[index]) {
+                pools[index].reset(new pool_handler<Event>{});
+            }
+
+            return static_cast<pool_handler<Event> &>(*pools[index]);
+        } else {
+            auto it = std::find_if(pools.begin(), pools.end(), [id = type_info<Event>::id()](const auto &cpool) { return id == cpool->type_id(); });
+            return static_cast<pool_handler<Event> &>(it == pools.cend() ? *pools.emplace_back(new pool_handler<Event>{}) : **it);
+        }
+    }
+
+public:
+    /**
+     * @brief Returns a sink object for the given event.
+     *
+     * A sink is an opaque object used to connect listeners to events.
+     *
+     * The function type for a listener is _compatible_ with:
+     * @code{.cpp}
+     * void(Event &);
+     * @endcode
+     *
+     * The order of invocation of the listeners isn't guaranteed.
+     *
+     * @sa sink
+     *
+     * @tparam Event Type of event of which to get the sink.
+     * @return A temporary sink object.
+     */
+    template<typename Event>
+    [[nodiscard]] auto sink() {
+        return assure<Event>().sink();
+    }
+
+    /**
+     * @brief Triggers an immediate event of the given type.
+     *
+     * All the listeners registered for the given type are immediately notified.
+     * The event is discarded after the execution.
+     *
+     * @tparam Event Type of event to trigger.
+     * @tparam Args Types of arguments to use to construct the event.
+     * @param args Arguments to use to construct the event.
+     */
+    template<typename Event, typename... Args>
+    void trigger(Args &&... args) {
+        assure<Event>().trigger(std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Triggers an immediate event of the given type.
+     *
+     * All the listeners registered for the given type are immediately notified.
+     * The event is discarded after the execution.
+     *
+     * @tparam Event Type of event to trigger.
+     * @param event An instance of the given type of event.
+     */
+    template<typename Event>
+    void trigger(Event &&event) {
+        assure<std::decay_t<Event>>().trigger(std::forward<Event>(event));
+    }
+
+    /**
+     * @brief Enqueues an event of the given type.
+     *
+     * An event of the given type is queued. No listener is invoked. Use the
+     * `update` member function to notify listeners when ready.
+     *
+     * @tparam Event Type of event to enqueue.
+     * @tparam Args Types of arguments to use to construct the event.
+     * @param args Arguments to use to construct the event.
+     */
+    template<typename Event, typename... Args>
+    void enqueue(Args &&... args) {
+        assure<Event>().enqueue(std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Enqueues an event of the given type.
+     *
+     * An event of the given type is queued. No listener is invoked. Use the
+     * `update` member function to notify listeners when ready.
+     *
+     * @tparam Event Type of event to enqueue.
+     * @param event An instance of the given type of event.
+     */
+    template<typename Event>
+    void enqueue(Event &&event) {
+        assure<std::decay_t<Event>>().enqueue(std::forward<Event>(event));
+    }
+
+    /**
+     * @brief Utility function to disconnect everything related to a given value
+     * or instance from a dispatcher.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<typename Type>
+    void disconnect(Type &value_or_instance) {
+        disconnect(&value_or_instance);
+    }
+
+    /**
+     * @brief Utility function to disconnect everything related to a given value
+     * or instance from a dispatcher.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<typename Type>
+    void disconnect(Type *value_or_instance) {
+        for(auto &&cpool: pools) {
+            if(cpool) {
+                cpool->disconnect(value_or_instance);
+            }
+        }
+    }
+
+    /**
+     * @brief Discards all the events queued so far.
+     *
+     * If no types are provided, the dispatcher will clear all the existing
+     * pools.
+     *
+     * @tparam Event Type of events to discard.
+     */
+    template<typename... Event>
+    void clear() {
+        if constexpr(sizeof...(Event) == 0) {
+            for(auto &&cpool: pools) {
+                if(cpool) {
+                    cpool->clear();
+                }
+            }
+        } else {
+            (assure<Event>().clear(), ...);
+        }
+    }
+
+    /**
+     * @brief Delivers all the pending events of the given type.
+     *
+     * This method is blocking and it doesn't return until all the events are
+     * delivered to the registered listeners. It's responsibility of the users
+     * to reduce at a minimum the time spent in the bodies of the listeners.
+     *
+     * @tparam Event Type of events to send.
+     */
+    template<typename Event>
+    void update() {
+        assure<Event>().publish();
+    }
+
+    /**
+     * @brief Delivers all the pending events.
+     *
+     * This method is blocking and it doesn't return until all the events are
+     * delivered to the registered listeners. It's responsibility of the users
+     * to reduce at a minimum the time spent in the bodies of the listeners.
+     */
+    void update() const {
+        for(auto pos = pools.size(); pos; --pos) {
+            if(auto &&cpool = pools[pos-1]; cpool) {
+                cpool->publish();
+            }
+        }
+    }
+
+private:
+    std::vector<std::unique_ptr<basic_pool>> pools;
+};
+
+
+}
+
+
+#endif

src/entt/signal/emitter.hpp

@@ -0,0 +1,335 @@
+#ifndef ENTT_SIGNAL_EMITTER_HPP
+#define ENTT_SIGNAL_EMITTER_HPP
+
+
+#include <algorithm>
+#include <functional>
+#include <iterator>
+#include <list>
+#include <memory>
+#include <type_traits>
+#include <utility>
+#include <vector>
+#include "../config/config.h"
+#include "../core/fwd.hpp"
+#include "../core/type_info.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief General purpose event emitter.
+ *
+ * The emitter class template follows the CRTP idiom. To create a custom emitter
+ * type, derived classes must inherit directly from the base class as:
+ *
+ * @code{.cpp}
+ * struct my_emitter: emitter<my_emitter> {
+ *     // ...
+ * }
+ * @endcode
+ *
+ * Pools for the type of events are created internally on the fly. It's not
+ * required to specify in advance the full list of accepted types.<br/>
+ * Moreover, whenever an event is published, an emitter provides the listeners
+ * with a reference to itself along with a reference to the event. Therefore
+ * listeners have an handy way to work with it without incurring in the need of 
+ * capturing a reference to the emitter.
+ *
+ * @tparam Derived Actual type of emitter that extends the class template.
+ */
+template<typename Derived>
+class emitter {
+    struct basic_pool {
+        virtual ~basic_pool() = default;
+        virtual bool empty() const ENTT_NOEXCEPT = 0;
+        virtual void clear() ENTT_NOEXCEPT = 0;
+        virtual id_type type_id() const ENTT_NOEXCEPT = 0;
+    };
+
+    template<typename Event>
+    struct pool_handler final: basic_pool {
+        using listener_type = std::function<void(Event &, Derived &)>;
+        using element_type = std::pair<bool, listener_type>;
+        using container_type = std::list<element_type>;
+        using connection_type = typename container_type::iterator;
+
+        [[nodiscard]] bool empty() const ENTT_NOEXCEPT override {
+            auto pred = [](auto &&element) { return element.first; };
+
+            return std::all_of(once_list.cbegin(), once_list.cend(), pred) &&
+                    std::all_of(on_list.cbegin(), on_list.cend(), pred);
+        }
+
+        void clear() ENTT_NOEXCEPT override {
+            if(publishing) {
+                for(auto &&element: once_list) {
+                    element.first = true;
+                }
+
+                for(auto &&element: on_list) {
+                    element.first = true;
+                }
+            } else {
+                once_list.clear();
+                on_list.clear();
+            }
+        }
+
+        connection_type once(listener_type listener) {
+            return once_list.emplace(once_list.cend(), false, std::move(listener));
+        }
+
+        connection_type on(listener_type listener) {
+            return on_list.emplace(on_list.cend(), false, std::move(listener));
+        }
+
+        void erase(connection_type conn) {
+            conn->first = true;
+
+            if(!publishing) {
+                auto pred = [](auto &&element) { return element.first; };
+                once_list.remove_if(pred);
+                on_list.remove_if(pred);
+            }
+        }
+
+        void publish(Event &event, Derived &ref) {
+            container_type swap_list;
+            once_list.swap(swap_list);
+
+            publishing = true;
+
+            for(auto &&element: on_list) {
+                element.first ? void() : element.second(event, ref);
+            }
+
+            for(auto &&element: swap_list) {
+                element.first ? void() : element.second(event, ref);
+            }
+
+            publishing = false;
+
+            on_list.remove_if([](auto &&element) { return element.first; });
+        }
+
+        [[nodiscard]] id_type type_id() const ENTT_NOEXCEPT override {
+            return type_info<Event>::id();
+        }
+
+    private:
+        bool publishing{false};
+        container_type once_list{};
+        container_type on_list{};
+    };
+
+    template<typename Event>
+    [[nodiscard]] const pool_handler<Event> & assure() const {
+        static_assert(std::is_same_v<Event, std::decay_t<Event>>, "Invalid event type");
+
+        if constexpr(ENTT_FAST_PATH(has_type_index_v<Event>)) {
+            const auto index = type_index<Event>::value();
+
+            if(!(index < pools.size())) {
+                pools.resize(index+1u);
+            }
+
+            if(!pools[index]) {
+                pools[index].reset(new pool_handler<Event>{});
+            }
+
+            return static_cast<pool_handler<Event> &>(*pools[index]);
+        } else {
+            auto it = std::find_if(pools.begin(), pools.end(), [id = type_info<Event>::id()](const auto &cpool) { return id == cpool->type_id(); });
+            return static_cast<pool_handler<Event> &>(it == pools.cend() ? *pools.emplace_back(new pool_handler<Event>{}) : **it);
+        }
+    }
+
+    template<typename Event>
+    [[nodiscard]] pool_handler<Event> & assure() {
+        return const_cast<pool_handler<Event> &>(std::as_const(*this).template assure<Event>());
+    }
+
+public:
+    /** @brief Type of listeners accepted for the given event. */
+    template<typename Event>
+    using listener = typename pool_handler<Event>::listener_type;
+
+    /**
+     * @brief Generic connection type for events.
+     *
+     * Type of the connection object returned by the event emitter whenever a
+     * listener for the given type is registered.<br/>
+     * It can be used to break connections still in use.
+     *
+     * @tparam Event Type of event for which the connection is created.
+     */
+    template<typename Event>
+    struct connection: private pool_handler<Event>::connection_type {
+        /** @brief Event emitters are friend classes of connections. */
+        friend class emitter;
+
+        /*! @brief Default constructor. */
+        connection() = default;
+
+        /**
+         * @brief Creates a connection that wraps its underlying instance.
+         * @param conn A connection object to wrap.
+         */
+        connection(typename pool_handler<Event>::connection_type conn)
+            : pool_handler<Event>::connection_type{std::move(conn)}
+        {}
+    };
+
+    /*! @brief Default constructor. */
+    emitter() = default;
+
+    /*! @brief Default destructor. */
+    virtual ~emitter() {
+        static_assert(std::is_base_of_v<emitter<Derived>, Derived>, "Incorrect use of the class template");
+    }
+
+    /*! @brief Default move constructor. */
+    emitter(emitter &&) = default;
+
+    /*! @brief Default move assignment operator. @return This emitter. */
+    emitter & operator=(emitter &&) = default;
+
+    /**
+     * @brief Emits the given event.
+     *
+     * All the listeners registered for the specific event type are invoked with
+     * the given event. The event type must either have a proper constructor for
+     * the arguments provided or be an aggregate type.
+     *
+     * @tparam Event Type of event to publish.
+     * @tparam Args Types of arguments to use to construct the event.
+     * @param args Parameters to use to initialize the event.
+     */
+    template<typename Event, typename... Args>
+    void publish(Args &&... args) {
+        Event instance{std::forward<Args>(args)...};
+        assure<Event>().publish(instance, *static_cast<Derived *>(this));
+    }
+
+    /**
+     * @brief Registers a long-lived listener with the event emitter.
+     *
+     * This method can be used to register a listener designed to be invoked
+     * more than once for the given event type.<br/>
+     * The connection returned by the method can be freely discarded. It's meant
+     * to be used later to disconnect the listener if required.
+     *
+     * The listener is as a callable object that can be moved and the type of
+     * which is _compatible_ with `void(Event &, Derived &)`.
+     *
+     * @note
+     * Whenever an event is emitted, the emitter provides the listener with a
+     * reference to the derived class. Listeners don't have to capture those
+     * instances for later uses.
+     *
+     * @tparam Event Type of event to which to connect the listener.
+     * @param instance The listener to register.
+     * @return Connection object that can be used to disconnect the listener.
+     */
+    template<typename Event>
+    connection<Event> on(listener<Event> instance) {
+        return assure<Event>().on(std::move(instance));
+    }
+
+    /**
+     * @brief Registers a short-lived listener with the event emitter.
+     *
+     * This method can be used to register a listener designed to be invoked
+     * only once for the given event type.<br/>
+     * The connection returned by the method can be freely discarded. It's meant
+     * to be used later to disconnect the listener if required.
+     *
+     * The listener is as a callable object that can be moved and the type of
+     * which is _compatible_ with `void(Event &, Derived &)`.
+     *
+     * @note
+     * Whenever an event is emitted, the emitter provides the listener with a
+     * reference to the derived class. Listeners don't have to capture those
+     * instances for later uses.
+     *
+     * @tparam Event Type of event to which to connect the listener.
+     * @param instance The listener to register.
+     * @return Connection object that can be used to disconnect the listener.
+     */
+    template<typename Event>
+    connection<Event> once(listener<Event> instance) {
+        return assure<Event>().once(std::move(instance));
+    }
+
+    /**
+     * @brief Disconnects a listener from the event emitter.
+     *
+     * Do not use twice the same connection to disconnect a listener, it results
+     * in undefined behavior. Once used, discard the connection object.
+     *
+     * @tparam Event Type of event of the connection.
+     * @param conn A valid connection.
+     */
+    template<typename Event>
+    void erase(connection<Event> conn) {
+        assure<Event>().erase(std::move(conn));
+    }
+
+    /**
+     * @brief Disconnects all the listeners for the given event type.
+     *
+     * All the connections previously returned for the given event are
+     * invalidated. Using them results in undefined behavior.
+     *
+     * @tparam Event Type of event to reset.
+     */
+    template<typename Event>
+    void clear() {
+        assure<Event>().clear();
+    }
+
+    /**
+     * @brief Disconnects all the listeners.
+     *
+     * All the connections previously returned are invalidated. Using them
+     * results in undefined behavior.
+     */
+    void clear() ENTT_NOEXCEPT {
+        for(auto &&cpool: pools) {
+            if(cpool) {
+                cpool->clear();
+            }
+        }
+    }
+
+    /**
+     * @brief Checks if there are listeners registered for the specific event.
+     * @tparam Event Type of event to test.
+     * @return True if there are no listeners registered, false otherwise.
+     */
+    template<typename Event>
+    [[nodiscard]] bool empty() const {
+        return assure<Event>().empty();
+    }
+
+    /**
+     * @brief Checks if there are listeners registered with the event emitter.
+     * @return True if there are no listeners registered, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return std::all_of(pools.cbegin(), pools.cend(), [](auto &&cpool) {
+            return !cpool || cpool->empty();
+        });
+    }
+
+private:
+    mutable std::vector<std::unique_ptr<basic_pool>> pools{};
+};
+
+
+}
+
+
+#endif

src/entt/signal/fwd.hpp

@@ -0,0 +1,36 @@
+#ifndef ENTT_SIGNAL_FWD_HPP
+#define ENTT_SIGNAL_FWD_HPP
+
+
+namespace entt {
+
+
+template<typename>
+class delegate;
+
+
+class dispatcher;
+
+
+template<typename>
+class emitter;
+
+
+class connection;
+
+
+struct scoped_connection;
+
+
+template<typename>
+class sink;
+
+
+template<typename>
+class sigh;
+
+
+}
+
+
+#endif

src/entt/signal/sigh.hpp

@@ -0,0 +1,519 @@
+#ifndef ENTT_SIGNAL_SIGH_HPP
+#define ENTT_SIGNAL_SIGH_HPP
+
+
+#include <vector>
+#include <utility>
+#include <iterator>
+#include <algorithm>
+#include <functional>
+#include <type_traits>
+#include "../config/config.h"
+#include "delegate.hpp"
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Sink class.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error unless the template parameter is a function type.
+ *
+ * @tparam Function A valid function type.
+ */
+template<typename Function>
+class sink;
+
+
+/**
+ * @brief Unmanaged signal handler.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error unless the template parameter is a function type.
+ *
+ * @tparam Function A valid function type.
+ */
+template<typename Function>
+class sigh;
+
+
+/**
+ * @brief Unmanaged signal handler.
+ *
+ * It works directly with references to classes and pointers to member functions
+ * as well as pointers to free functions. Users of this class are in charge of
+ * disconnecting instances before deleting them.
+ *
+ * This class serves mainly two purposes:
+ *
+ * * Creating signals to use later to notify a bunch of listeners.
+ * * Collecting results from a set of functions like in a voting system.
+ *
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ */
+template<typename Ret, typename... Args>
+class sigh<Ret(Args...)> {
+    /*! @brief A sink is allowed to modify a signal. */
+    friend class sink<Ret(Args...)>;
+
+public:
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Sink type. */
+    using sink_type = sink<Ret(Args...)>;
+
+    /**
+     * @brief Instance type when it comes to connecting member functions.
+     * @tparam Class Type of class to which the member function belongs.
+     */
+    template<typename Class>
+    using instance_type = Class *;
+
+    /**
+     * @brief Number of listeners connected to the signal.
+     * @return Number of listeners currently connected.
+     */
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return calls.size();
+    }
+
+    /**
+     * @brief Returns false if at least a listener is connected to the signal.
+     * @return True if the signal has no listeners connected, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return calls.empty();
+    }
+
+    /**
+     * @brief Triggers a signal.
+     *
+     * All the listeners are notified. Order isn't guaranteed.
+     *
+     * @param args Arguments to use to invoke listeners.
+     */
+    void publish(Args... args) const {
+        for(auto &&call: std::as_const(calls)) {
+            call(args...);
+        }
+    }
+
+    /**
+     * @brief Collects return values from the listeners.
+     *
+     * The collector must expose a call operator with the following properties:
+     *
+     * * The return type is either `void` or such that it's convertible to
+     *   `bool`. In the second case, a true value will stop the iteration.
+     * * The list of parameters is empty if `Ret` is `void`, otherwise it
+     *   contains a single element such that `Ret` is convertible to it.
+     *
+     * @tparam Func Type of collector to use, if any.
+     * @param func A valid function object.
+     * @param args Arguments to use to invoke listeners.
+     */
+    template<typename Func>
+    void collect(Func func, Args... args) const {
+        for(auto &&call: calls) {
+            if constexpr(std::is_void_v<Ret>) {
+                if constexpr(std::is_invocable_r_v<bool, Func>) {
+                    call(args...);
+                    if(func()) { break; }
+                } else {
+                    call(args...);
+                    func();
+                }
+            } else {
+                if constexpr(std::is_invocable_r_v<bool, Func, Ret>) {
+                    if(func(call(args...))) { break; }
+                } else {
+                    func(call(args...));
+                }
+            }
+        }
+    }
+
+private:
+    std::vector<delegate<Ret(Args...)>> calls;
+};
+
+
+/**
+ * @brief Connection class.
+ *
+ * Opaque object the aim of which is to allow users to release an already
+ * estabilished connection without having to keep a reference to the signal or
+ * the sink that generated it.
+ */
+class connection {
+    /*! @brief A sink is allowed to create connection objects. */
+    template<typename>
+    friend class sink;
+
+    connection(delegate<void(void *)> fn, void *ref)
+        : disconnect{fn}, signal{ref}
+    {}
+
+public:
+    /*! @brief Default constructor. */
+    connection() = default;
+
+    /**
+     * @brief Checks whether a connection is properly initialized.
+     * @return True if the connection is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return static_cast<bool>(disconnect);
+    }
+
+    /*! @brief Breaks the connection. */
+    void release() {
+        if(disconnect) {
+            disconnect(signal);
+            disconnect.reset();
+        }
+    }
+
+private:
+    delegate<void(void *)> disconnect;
+    void *signal{};
+};
+
+
+/**
+ * @brief Scoped connection class.
+ *
+ * Opaque object the aim of which is to allow users to release an already
+ * estabilished connection without having to keep a reference to the signal or
+ * the sink that generated it.<br/>
+ * A scoped connection automatically breaks the link between the two objects
+ * when it goes out of scope.
+ */
+struct scoped_connection {
+    /*! @brief Default constructor. */
+    scoped_connection() = default;
+
+    /**
+     * @brief Constructs a scoped connection from a basic connection.
+     * @param other A valid connection object.
+     */
+    scoped_connection(const connection &other)
+        : conn{other}
+    {}
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    scoped_connection(const scoped_connection &) = delete;
+
+    /*! @brief Automatically breaks the link on destruction. */
+    ~scoped_connection() {
+        conn.release();
+    }
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This scoped connection.
+     */
+    scoped_connection & operator=(const scoped_connection &) = delete;
+
+    /**
+     * @brief Acquires a connection.
+     * @param other The connection object to acquire.
+     * @return This scoped connection.
+     */
+    scoped_connection & operator=(connection other) {
+        conn = std::move(other);
+        return *this;
+    }
+
+    /**
+     * @brief Checks whether a scoped connection is properly initialized.
+     * @return True if the connection is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return static_cast<bool>(conn);
+    }
+
+    /*! @brief Breaks the connection. */
+    void release() {
+        conn.release();
+    }
+
+private:
+    connection conn;
+};
+
+
+/**
+ * @brief Sink class.
+ *
+ * A sink is used to connect listeners to signals and to disconnect them.<br/>
+ * The function type for a listener is the one of the signal to which it
+ * belongs.
+ *
+ * The clear separation between a signal and a sink permits to store the former
+ * as private data member without exposing the publish functionality to the
+ * users of the class.
+ * 
+ * @warning
+ * Lifetime of a sink must not overcome that of the signal to which it refers.
+ * In any other case, attempting to use a sink results in undefined behavior.
+ *
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ */
+template<typename Ret, typename... Args>
+class sink<Ret(Args...)> {
+    using signal_type = sigh<Ret(Args...)>;
+    using difference_type = typename std::iterator_traits<typename decltype(signal_type::calls)::iterator>::difference_type;
+
+    template<auto Candidate, typename Type>
+    static void release(Type value_or_instance, void *signal) {
+        sink{*static_cast<signal_type *>(signal)}.disconnect<Candidate>(value_or_instance);
+    }
+
+    template<auto Candidate>
+    static void release(void *signal) {
+        sink{*static_cast<signal_type *>(signal)}.disconnect<Candidate>();
+    }
+
+public:
+    /**
+     * @brief Constructs a sink that is allowed to modify a given signal.
+     * @param ref A valid reference to a signal object.
+     */
+    sink(sigh<Ret(Args...)> &ref) ENTT_NOEXCEPT
+        : offset{},
+          signal{&ref}
+    {}
+
+    /**
+     * @brief Returns false if at least a listener is connected to the sink.
+     * @return True if the sink has no listeners connected, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return signal->calls.empty();
+    }
+
+    /**
+     * @brief Returns a sink that connects before a given free function or an
+     * unbound member.
+     * @tparam Function A valid free function pointer.
+     * @return A properly initialized sink object.
+     */
+    template<auto Function>
+    [[nodiscard]] sink before() {
+        delegate<Ret(Args...)> call{};
+        call.template connect<Function>();
+
+        const auto &calls = signal->calls;
+        const auto it = std::find(calls.cbegin(), calls.cend(), std::move(call));
+
+        sink other{*this};
+        other.offset = std::distance(it, calls.cend());
+        return other;
+    }
+
+    /**
+     * @brief Returns a sink that connects before a free function with payload
+     * or a bound member.
+     * @tparam Candidate Member or free function to look for.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     * @return A properly initialized sink object.
+     */
+    template<auto Candidate, typename Type>
+    [[nodiscard]] sink before(Type &&value_or_instance) {
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>(value_or_instance);
+
+        const auto &calls = signal->calls;
+        const auto it = std::find(calls.cbegin(), calls.cend(), std::move(call));
+
+        sink other{*this};
+        other.offset = std::distance(it, calls.cend());
+        return other;
+    }
+
+    /**
+     * @brief Returns a sink that connects before a given instance or specific
+     * payload.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     * @return A properly initialized sink object.
+     */
+    template<typename Type>
+    [[nodiscard]] sink before(Type &value_or_instance) {
+        return before(&value_or_instance);
+    }
+
+    /**
+     * @brief Returns a sink that connects before a given instance or specific
+     * payload.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid pointer that fits the purpose.
+     * @return A properly initialized sink object.
+     */
+    template<typename Type>
+    [[nodiscard]] sink before(Type *value_or_instance) {
+        sink other{*this};
+
+        if(value_or_instance) {
+            const auto &calls = signal->calls;
+            const auto it = std::find_if(calls.cbegin(), calls.cend(), [value_or_instance](const auto &delegate) {
+                return delegate.instance() == value_or_instance;
+            });
+
+            other.offset = std::distance(it, calls.cend());
+        }
+
+        return other;
+    }
+
+    /**
+     * @brief Returns a sink that connects before anything else.
+     * @return A properly initialized sink object.
+     */
+    [[nodiscard]] sink before() {
+        sink other{*this};
+        other.offset = signal->calls.size();
+        return other;
+    }
+
+    /**
+     * @brief Connects a free function or an unbound member to a signal.
+     *
+     * The signal handler performs checks to avoid multiple connections for the
+     * same function.
+     *
+     * @tparam Candidate Function or member to connect to the signal.
+     * @return A properly initialized connection object.
+     */
+    template<auto Candidate>
+    connection connect() {
+        disconnect<Candidate>();
+
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>();
+        signal->calls.insert(signal->calls.end() - offset, std::move(call));
+
+        delegate<void(void *)> conn{};
+        conn.template connect<&release<Candidate>>();
+        return { std::move(conn), signal };
+    }
+
+    /**
+     * @brief Connects a free function with payload or a bound member to a
+     * signal.
+     *
+     * The signal isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of the instance overcomes
+     * the one of the signal. On the other side, the signal handler performs
+     * checks to avoid multiple connections for the same function.<br/>
+     * When used to connect a free function with payload, its signature must be
+     * such that the instance is the first argument before the ones used to
+     * define the signal itself.
+     *
+     * @tparam Candidate Function or member to connect to the signal.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     * @return A properly initialized connection object.
+     */
+    template<auto Candidate, typename Type>
+    connection connect(Type &&value_or_instance) {
+        disconnect<Candidate>(value_or_instance);
+
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>(value_or_instance);
+        signal->calls.insert(signal->calls.end() - offset, std::move(call));
+
+        delegate<void(void *)> conn{};
+        conn.template connect<&release<Candidate, Type>>(value_or_instance);
+        return { std::move(conn), signal };
+    }
+
+    /**
+     * @brief Disconnects a free function or an unbound member from a signal.
+     * @tparam Candidate Function or member to disconnect from the signal.
+     */
+    template<auto Candidate>
+    void disconnect() {
+        auto &calls = signal->calls;
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>();
+        calls.erase(std::remove(calls.begin(), calls.end(), std::move(call)), calls.end());
+    }
+
+    /**
+     * @brief Disconnects a free function with payload or a bound member from a
+     * signal.
+     * @tparam Candidate Function or member to disconnect from the signal.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void disconnect(Type &&value_or_instance) {
+        auto &calls = signal->calls;
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>(value_or_instance);
+        calls.erase(std::remove(calls.begin(), calls.end(), std::move(call)), calls.end());
+    }
+
+    /**
+     * @brief Disconnects free functions with payload or bound members from a
+     * signal.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<typename Type>
+    void disconnect(Type &value_or_instance) {
+        disconnect(&value_or_instance);
+    }
+
+    /**
+     * @brief Disconnects free functions with payload or bound members from a
+     * signal.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<typename Type>
+    void disconnect(Type *value_or_instance) {
+        if(value_or_instance) {
+            auto &calls = signal->calls;
+            calls.erase(std::remove_if(calls.begin(), calls.end(), [value_or_instance](const auto &delegate) {
+                return delegate.instance() == value_or_instance;
+            }), calls.end());
+        }
+    }
+
+    /*! @brief Disconnects all the listeners from a signal. */
+    void disconnect() {
+        signal->calls.clear();
+    }
+
+private:
+    difference_type offset;
+    signal_type *signal;
+};
+
+
+/**
+ * @brief Deduction guide.
+ *
+ * It allows to deduce the function type of a sink directly from the signal it
+ * refers to.
+ *
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ */
+template<typename Ret, typename... Args>
+sink(sigh<Ret(Args...)> &) ENTT_NOEXCEPT -> sink<Ret(Args...)>;
+
+
+}
+
+
+#endif

src/ident.hpp

@@ -1,43 +0,0 @@
-#ifndef ENTT_IDENT_HPP
-#define ENTT_IDENT_HPP
-
-
-#include<type_traits>
-#include<cstddef>
-#include<utility>
-
-
-namespace entt {
-
-
-namespace details {
-
-
-template<typename Type>
-struct Wrapper {
-    using type = Type;
-    constexpr Wrapper(std::size_t index): index{index} {}
-    const std::size_t index;
-};
-
-template<typename... Types>
-struct Identifier final: Wrapper<Types>... {
-    template<std::size_t... Indexes>
-    constexpr Identifier(std::index_sequence<Indexes...>): Wrapper<Types>{Indexes}... {}
-
-    template<typename Type>
-    constexpr std::size_t get() const { return Wrapper<std::decay_t<Type>>::index; }
-};
-
-
-}
-
-
-template<typename... Types>
-constexpr auto ident = details::Identifier<std::decay_t<Types>...>{std::make_index_sequence<sizeof...(Types)>{}};
-
-
-}
-
-
-#endif // ENTT_IDENT_HPP

src/registry.hpp

@@ -1,414 +0,0 @@
-#ifndef ENTT_REGISTRY_HPP
-#define ENTT_REGISTRY_HPP
-
-
-#include <vector>
-#include <bitset>
-#include <utility>
-#include <cstddef>
-#include <iterator>
-#include <cassert>
-#include <type_traits>
-#include "component_pool.hpp"
-#include "ident.hpp"
-
-
-namespace entt {
-
-
-template<typename...>
-class View;
-
-
-template<template<typename...> class Pool, typename Entity, typename... Components, typename Type, typename... Types>
-class View<Pool<Entity, Components...>, Type, Types...> final {
-    using pool_type = Pool<Entity, Components...>;
-    using entity_type = typename pool_type::entity_type;
-    using mask_type = std::bitset<sizeof...(Components)+1>;
-    using underlying_iterator_type = typename pool_type::const_iterator_type;
-
-    class ViewIterator;
-
-public:
-    using iterator_type = ViewIterator;
-    using const_iterator_type = iterator_type;
-    using size_type = typename pool_type::size_type;
-
-private:
-    class ViewIterator {
-        inline bool valid() const noexcept {
-            return ((mask[*begin] & bitmask) == bitmask);
-        }
-
-    public:
-        using value_type = entity_type;
-        using difference_type = std::ptrdiff_t;
-        using reference = entity_type &;
-        using pointer = entity_type *;
-        using iterator_category = std::input_iterator_tag;
-
-        ViewIterator(underlying_iterator_type begin, underlying_iterator_type end, const mask_type &bitmask, const mask_type *mask) noexcept
-            : begin{begin}, end{end}, bitmask{bitmask}, mask{mask}
-        {
-            if(begin != end && !valid()) {
-                ++(*this);
-            }
-        }
-
-        ViewIterator & operator++() noexcept {
-            ++begin;
-            while(begin != end && !valid()) { ++begin; }
-            return *this;
-        }
-
-        ViewIterator operator++(int) noexcept {
-            ViewIterator orig = *this;
-            return ++(*this), orig;
-        }
-
-        bool operator==(const ViewIterator &other) const noexcept {
-            return other.begin == begin;
-        }
-
-        bool operator!=(const ViewIterator &other) const noexcept {
-            return !(*this == other);
-        }
-
-        value_type operator*() const noexcept {
-            return *begin;
-        }
-
-    private:
-        underlying_iterator_type begin;
-        underlying_iterator_type end;
-        const mask_type bitmask;
-        const mask_type *mask;
-    };
-
-    template<typename Comp>
-    void prefer(size_type &size) noexcept {
-        auto sz = pool.template size<Comp>();
-
-        if(sz < size) {
-            from = pool.template begin<Type>();
-            to = pool.template end<Type>();
-            size = sz;
-        }
-    }
-
-public:
-    explicit View(pool_type &pool, const mask_type *mask) noexcept
-        : from{pool.template begin<Type>()},
-          to{pool.template end<Type>()},
-          pool{pool},
-          mask{mask}
-    {
-        using accumulator_type = int[];
-        size_type size = pool.template size<Type>();
-        bitmask.set(ident<Components...>.template get<Type>());
-        accumulator_type types = { 0, (bitmask.set(ident<Components...>.template get<Types>()), 0)... };
-        accumulator_type pref = { 0, (prefer<Types>(size), 0)... };
-        (void)types, (void)pref;
-    }
-
-    const_iterator_type begin() const noexcept {
-        return ViewIterator{from, to, bitmask, mask};
-    }
-
-    iterator_type begin() noexcept {
-        return const_cast<const View *>(this)->begin();
-    }
-
-    const_iterator_type end() const noexcept {
-        return ViewIterator{to, to, bitmask, mask};
-    }
-
-    iterator_type end() noexcept {
-        return const_cast<const View *>(this)->end();
-    }
-
-    void reset() noexcept {
-        using accumulator_type = int[];
-        from = pool.template begin<Type>();
-        to = pool.template end<Type>();
-        size_type size = pool.template size<Type>();
-        accumulator_type accumulator = { 0, (prefer<Types>(size), 0)... };
-        (void)accumulator;
-    }
-
-private:
-    underlying_iterator_type from;
-    underlying_iterator_type to;
-    pool_type &pool;
-    const mask_type *mask;
-    mask_type bitmask;
-};
-
-
-template<template<typename...> class Pool, typename Entity, typename... Components, typename Type>
-class View<Pool<Entity, Components...>, Type> final {
-    using pool_type = Pool<Entity, Components...>;
-
-public:
-    using size_type = typename pool_type::size_type;
-    using iterator_type = typename pool_type::const_iterator_type;
-    using const_iterator_type = iterator_type;
-
-    explicit View(pool_type &pool) noexcept
-        : pool{pool}
-    {}
-
-    const_iterator_type cbegin() const noexcept {
-        return pool.template cbegin<Type>();
-    }
-
-    iterator_type begin() noexcept {
-        return pool.template begin<Type>();
-    }
-
-    const_iterator_type cend() const noexcept {
-        return pool.template cend<Type>();
-    }
-
-    iterator_type end() noexcept {
-        return pool.template end<Type>();
-    }
-
-    size_type size() const noexcept {
-        return pool.template size<Type>();
-    }
-
-private:
-    pool_type &pool;
-};
-
-
-template<typename>
-class Registry;
-
-
-template<template<typename...> class Pool, typename Entity, typename... Components>
-class Registry<Pool<Entity, Components...>> {
-    static_assert(sizeof...(Components) > 1, "!");
-
-    using pool_type = Pool<Entity, Components...>;
-    using mask_type = std::bitset<sizeof...(Components)+1>;
-
-    static constexpr auto validity_bit = sizeof...(Components);
-
-public:
-    using entity_type = typename pool_type::entity_type;
-    using size_type = typename std::vector<mask_type>::size_type;
-
-private:
-    template<typename Comp>
-    void clone(entity_type to, entity_type from) {
-        if(entities[from].test(ident<Components...>.template get<Comp>())) {
-            assign<Comp>(to, pool.template get<Comp>(from));
-        }
-    }
-
-    template<typename Comp>
-    void sync(entity_type to, entity_type from) {
-        bool src = entities[from].test(ident<Components...>.template get<Comp>());
-        bool dst = entities[to].test(ident<Components...>.template get<Comp>());
-
-        if(src && dst) {
-            copy<Comp>(to, from);
-        } else if(src) {
-            clone<Comp>(to, from);
-        } else if(dst) {
-            remove<Comp>(to);
-        }
-    }
-
-public:
-    template<typename... Comp>
-    using view_type = View<pool_type, Comp...>;
-
-    template<typename... Args>
-    Registry(Args&&... args)
-        : pool{std::forward<Args>(args)...}
-    {}
-
-    Registry(const Registry &) = delete;
-    Registry(Registry &&) = delete;
-
-    Registry & operator=(const Registry &) = delete;
-    Registry & operator=(Registry &&) = delete;
-
-    size_type size() const noexcept {
-        return entities.size() - available.size();
-    }
-
-    size_type capacity() const noexcept {
-        return entities.size();
-    }
-
-    template<typename Comp>
-    bool empty() const noexcept {
-        return pool.template empty<Comp>();
-    }
-
-    bool empty() const noexcept {
-        return entities.empty();
-    }
-
-    bool valid(entity_type entity) const noexcept {
-        return (entity < entities.size() && entities[entity].test(validity_bit));
-    }
-
-    template<typename... Comp>
-    entity_type create() noexcept {
-        using accumulator_type = int[];
-        auto entity = create();
-        accumulator_type accumulator = { 0, (assign<Comp>(entity), 0)... };
-        (void)accumulator;
-        return entity;
-    }
-
-    entity_type create() noexcept {
-        entity_type entity;
-
-        if(available.empty()) {
-            entity = entity_type(entities.size());
-            entities.emplace_back();
-        } else {
-            entity = available.back();
-            available.pop_back();
-        }
-
-        entities[entity].set(validity_bit);
-
-        return entity;
-    }
-
-    void destroy(entity_type entity) {
-        assert(valid(entity));
-        using accumulator_type = int[];
-        accumulator_type accumulator = { 0, (reset<Components>(entity), 0)... };
-        available.push_back(entity);
-        entities[entity].reset();
-        (void)accumulator;
-    }
-
-    template<typename Comp, typename... Args>
-    Comp & assign(entity_type entity, Args... args) {
-        assert(valid(entity));
-        entities[entity].set(ident<Components...>.template get<Comp>());
-        return pool.template construct<Comp>(entity, args...);
-    }
-
-    template<typename Comp>
-    void remove(entity_type entity) {
-        assert(valid(entity));
-        entities[entity].reset(ident<Components...>.template get<Comp>());
-        pool.template destroy<Comp>(entity);
-    }
-
-    template<typename... Comp>
-    bool has(entity_type entity) const noexcept {
-        assert(valid(entity));
-        using accumulator_type = bool[];
-        bool all = true;
-        auto &mask = entities[entity];
-        accumulator_type accumulator = { true, (all = all && mask.test(ident<Components...>.template get<Comp>()))... };
-        (void)accumulator;
-        return all;
-    }
-
-    template<typename Comp>
-    const Comp & get(entity_type entity) const noexcept {
-        return pool.template get<Comp>(entity);
-    }
-
-    template<typename Comp>
-    Comp & get(entity_type entity) noexcept {
-        return pool.template get<Comp>(entity);
-    }
-
-    template<typename Comp, typename... Args>
-    Comp & replace(entity_type entity, Args... args) {
-        return (pool.template get<Comp>(entity) = Comp{args...});
-    }
-
-    template<typename Comp, typename... Args>
-    Comp & accomodate(entity_type entity, Args... args) {
-        assert(valid(entity));
-
-        return (entities[entity].test(ident<Components...>.template get<Comp>())
-                ? this->template replace<Comp>(entity, std::forward<Args>(args)...)
-                : this->template assign<Comp>(entity, std::forward<Args>(args)...));
-    }
-
-    entity_type clone(entity_type from) {
-        assert(valid(from));
-        using accumulator_type = int[];
-        auto to = create();
-        accumulator_type accumulator = { 0, (clone<Components>(to, from), 0)... };
-        (void)accumulator;
-        return to;
-    }
-
-    template<typename Comp>
-    Comp & copy(entity_type to, entity_type from) {
-        return (pool.template get<Comp>(to) = pool.template get<Comp>(from));
-    }
-
-    void copy(entity_type to, entity_type from) {
-        using accumulator_type = int[];
-        accumulator_type accumulator = { 0, (sync<Components>(to, from), 0)... };
-        (void)accumulator;
-    }
-
-    template<typename Comp>
-    void reset(entity_type entity) {
-        assert(valid(entity));
-
-        if(entities[entity].test(ident<Components...>.template get<Comp>())) {
-            remove<Comp>(entity);
-        }
-    }
-
-    template<typename Comp>
-    void reset() {
-        for(entity_type entity = 0, last = entity_type(entities.size()); entity < last; ++entity) {
-            if(entities[entity].test(ident<Components...>.template get<Comp>())) {
-                remove<Comp>(entity);
-            }
-        }
-    }
-
-    void reset() {
-        entities.clear();
-        available.clear();
-        pool.reset();
-    }
-
-    template<typename... Comp>
-    std::enable_if_t<(sizeof...(Comp) == 1), view_type<Comp...>>
-    view() noexcept { return view_type<Comp...>{pool}; }
-
-    template<typename... Comp>
-    std::enable_if_t<(sizeof...(Comp) > 1), view_type<Comp...>>
-    view() noexcept { return view_type<Comp...>{pool, entities.data()}; }
-
-private:
-    std::vector<mask_type> entities;
-    std::vector<entity_type> available;
-    pool_type pool;
-};
-
-
-template<typename Entity, typename... Components>
-using StandardRegistry = Registry<ComponentPool<Entity, Components...>>;
-
-
-template<typename... Components>
-using DefaultRegistry = Registry<ComponentPool<std::uint32_t, Components...>>;
-
-
-}
-
-
-#endif // ENTT_REGISTRY_HPP

test/CMakeLists.txt

@@ -2,25 +2,209 @@
 # Tests configuration
 #
 
-set(COMMON_LINK_LIBS gtest_main Threads::Threads)
+include(FetchContent)
 
-# List of available targets
+set(THREADS_PREFER_PTHREAD_FLAG ON)
+find_package(Threads REQUIRED)
 
-set(TARGET_ENTT entt)
-set(TARGET_BENCHMARK benchmark)
+if(FIND_GTEST_PACKAGE)
+    find_package(GTest REQUIRED)
+else()
+    FetchContent_Declare(
+        googletest
+        GIT_REPOSITORY https://github.com/google/googletest.git
+        GIT_TAG master
+        GIT_SHALLOW 1
+    )
 
-# Test TARGET_ENTT
+    FetchContent_GetProperties(googletest)
 
-add_executable(${TARGET_ENTT} component_pool.cpp registry.cpp)
-target_include_directories(${TARGET_ENTT} PRIVATE ${PROJECT_SRC_DIR})
-target_link_libraries(${TARGET_ENTT} PRIVATE ${COMMON_LINK_LIBS})
-add_test(NAME ${TARGET_ENTT} COMMAND ${TARGET_ENTT})
+    if(NOT googletest_POPULATED)
+        FetchContent_Populate(googletest)
+        set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
+        add_subdirectory(${googletest_SOURCE_DIR} ${googletest_BINARY_DIR})
+    endif()
 
-# Test TARGET_BENCHMARK
+    add_library(GTest::Main ALIAS gtest_main)
 
-IF(CMAKE_BUILD_TYPE MATCHES Release)
-    add_executable(${TARGET_BENCHMARK} benchmark.cpp)
-    target_include_directories(${TARGET_BENCHMARK} PRIVATE ${PROJECT_SRC_DIR})
-    target_link_libraries(${TARGET_BENCHMARK} PRIVATE ${COMMON_LINK_LIBS})
-    add_test(NAME ${TARGET_BENCHMARK} COMMAND ${TARGET_BENCHMARK})
-ENDIF()
+    target_compile_features(gtest PUBLIC cxx_std_17)
+    target_compile_features(gtest_main PUBLIC cxx_std_17)
+    target_compile_features(gmock PUBLIC cxx_std_17)
+    target_compile_features(gmock_main PUBLIC cxx_std_17)
+endif()
+
+include_directories($<TARGET_PROPERTY:EnTT,INTERFACE_INCLUDE_DIRECTORIES>)
+add_compile_options($<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_OPTIONS>)
+
+function(SETUP_TARGET TARGET_NAME)
+    set_target_properties(${TARGET_NAME} PROPERTIES CXX_EXTENSIONS OFF)
+    target_link_libraries(${TARGET_NAME} PRIVATE EnTT)
+
+    target_compile_options(
+        ${TARGET_NAME}
+        PRIVATE
+            $<$<NOT:$<PLATFORM_ID:Windows>>:-pedantic -fvisibility=hidden -Wall -Wshadow -Wno-deprecated-declarations>
+            $<$<PLATFORM_ID:Windows>:/EHsc /W1 /wd4996 /w14800>
+    )
+
+    target_compile_options(
+        ${TARGET_NAME}
+        PRIVATE
+            $<$<AND:$<CONFIG:Debug>,$<NOT:$<PLATFORM_ID:Windows>>>:-O0 -g>
+            $<$<AND:$<CONFIG:Release>,$<NOT:$<PLATFORM_ID:Windows>>>:-O2>
+            $<$<AND:$<CONFIG:Debug>,$<PLATFORM_ID:Windows>>:/Od>
+            $<$<AND:$<CONFIG:Release>,$<PLATFORM_ID:Windows>>:/O2>
+    )
+
+    target_compile_definitions(${TARGET_NAME} PRIVATE ENTT_STANDALONE ${ARGN})
+endfunction()
+
+add_library(odr OBJECT odr.cpp)
+SETUP_TARGET(odr)
+
+function(SETUP_BASIC_TEST TEST_NAME TEST_SOURCES)
+    add_executable(${TEST_NAME} $<TARGET_OBJECTS:odr> ${TEST_SOURCES})
+    target_link_libraries(${TEST_NAME} PRIVATE GTest::Main Threads::Threads)
+    SETUP_TARGET(${TEST_NAME} ${ARGN})
+    add_test(NAME ${TEST_NAME} COMMAND ${TEST_NAME})
+endfunction()
+
+function(SETUP_LIB_TEST TEST_NAME)
+    add_library(_${TEST_NAME} SHARED $<TARGET_OBJECTS:odr> lib/${TEST_NAME}/lib.cpp)
+    SETUP_TARGET(_${TEST_NAME} ENTT_API_EXPORT)
+    SETUP_BASIC_TEST(lib_${TEST_NAME} lib/${TEST_NAME}/main.cpp ENTT_API_IMPORT)
+    target_link_libraries(lib_${TEST_NAME} PRIVATE _${TEST_NAME})
+endfunction()
+
+function(SETUP_PLUGIN_TEST TEST_NAME)
+    add_library(_${TEST_NAME} MODULE $<TARGET_OBJECTS:odr> lib/${TEST_NAME}/plugin.cpp)
+    SETUP_TARGET(_${TEST_NAME} NOMINMAX ${ARGVN})
+    SETUP_BASIC_TEST(lib_${TEST_NAME} lib/${TEST_NAME}/main.cpp NOMINMAX PLUGIN="$<TARGET_FILE:_${TEST_NAME}>" ${ARGVN})
+    target_include_directories(_${TEST_NAME} PRIVATE ${cr_INCLUDE_DIR})
+    target_include_directories(lib_${TEST_NAME} PRIVATE ${cr_INCLUDE_DIR})
+    target_link_libraries(lib_${TEST_NAME} PRIVATE ${CMAKE_DL_LIBS})
+endfunction()
+
+# Test benchmark
+
+if(BUILD_BENCHMARK)
+    SETUP_BASIC_TEST(benchmark benchmark/benchmark.cpp)
+endif()
+
+# Test example
+
+if(BUILD_EXAMPLE)
+    SETUP_BASIC_TEST(custom_identifier example/custom_identifier.cpp)
+endif()
+
+# Test lib
+
+if(BUILD_LIB)
+    FetchContent_Declare(
+        cr
+        GIT_REPOSITORY https://github.com/fungos/cr.git
+        GIT_TAG master
+        GIT_SHALLOW 1
+    )
+
+    FetchContent_GetProperties(cr)
+
+    if(NOT cr_POPULATED)
+        FetchContent_Populate(cr)
+        set(cr_INCLUDE_DIR ${cr_SOURCE_DIR})
+    endif()
+
+    SETUP_LIB_TEST(dispatcher)
+    SETUP_LIB_TEST(emitter)
+    SETUP_LIB_TEST(meta)
+    SETUP_LIB_TEST(registry)
+
+    SETUP_PLUGIN_TEST(dispatcher_plugin)
+    SETUP_PLUGIN_TEST(emitter_plugin)
+    SETUP_PLUGIN_TEST(meta_plugin)
+    SETUP_PLUGIN_TEST(registry_plugin)
+
+    SETUP_PLUGIN_TEST(meta_plugin_std ENTT_STANDARD_CPP)
+endif()
+
+# Test snapshot
+
+if(BUILD_SNAPSHOT)
+    FetchContent_Declare(
+        cereal
+        GIT_REPOSITORY https://github.com/USCiLab/cereal.git
+        GIT_TAG v1.2.2
+        GIT_SHALLOW 1
+    )
+
+    FetchContent_GetProperties(cereal)
+
+    if(NOT cereal_POPULATED)
+        FetchContent_Populate(cereal)
+        set(cereal_INCLUDE_DIR ${cereal_SOURCE_DIR}/include)
+    endif()
+
+    SETUP_BASIC_TEST(cereal snapshot/snapshot.cpp)
+    target_include_directories(cereal PRIVATE ${cereal_INCLUDE_DIR})
+endif()
+
+# Test core
+
+SETUP_BASIC_TEST(algorithm entt/core/algorithm.cpp)
+SETUP_BASIC_TEST(family entt/core/family.cpp)
+SETUP_BASIC_TEST(hashed_string entt/core/hashed_string.cpp)
+SETUP_BASIC_TEST(ident entt/core/ident.cpp)
+SETUP_BASIC_TEST(monostate entt/core/monostate.cpp)
+SETUP_BASIC_TEST(type_info entt/core/type_info.cpp)
+SETUP_BASIC_TEST(type_traits entt/core/type_traits.cpp)
+SETUP_BASIC_TEST(utility entt/core/utility.cpp)
+
+# Test entity
+
+SETUP_BASIC_TEST(actor entt/entity/actor.cpp)
+SETUP_BASIC_TEST(entity entt/entity/entity.cpp)
+SETUP_BASIC_TEST(group entt/entity/group.cpp)
+SETUP_BASIC_TEST(handle entt/entity/handle.cpp)
+SETUP_BASIC_TEST(helper entt/entity/helper.cpp)
+SETUP_BASIC_TEST(observer entt/entity/observer.cpp)
+SETUP_BASIC_TEST(registry entt/entity/registry.cpp)
+SETUP_BASIC_TEST(registry_no_eto entt/entity/registry_no_eto.cpp ENTT_NO_ETO)
+SETUP_BASIC_TEST(runtime_view entt/entity/runtime_view.cpp)
+SETUP_BASIC_TEST(snapshot entt/entity/snapshot.cpp)
+SETUP_BASIC_TEST(sparse_set entt/entity/sparse_set.cpp)
+SETUP_BASIC_TEST(storage entt/entity/storage.cpp)
+SETUP_BASIC_TEST(view entt/entity/view.cpp)
+
+# Test locator
+
+SETUP_BASIC_TEST(locator entt/locator/locator.cpp)
+
+# Test meta
+
+SETUP_BASIC_TEST(meta_any entt/meta/meta_any.cpp)
+SETUP_BASIC_TEST(meta_base entt/meta/meta_base.cpp)
+SETUP_BASIC_TEST(meta_container entt/meta/meta_container.cpp)
+SETUP_BASIC_TEST(meta_conv entt/meta/meta_conv.cpp)
+SETUP_BASIC_TEST(meta_ctor entt/meta/meta_ctor.cpp)
+SETUP_BASIC_TEST(meta_data entt/meta/meta_data.cpp)
+SETUP_BASIC_TEST(meta_func entt/meta/meta_func.cpp)
+SETUP_BASIC_TEST(meta_pointer entt/meta/meta_pointer.cpp)
+SETUP_BASIC_TEST(meta_prop entt/meta/meta_prop.cpp)
+SETUP_BASIC_TEST(meta_range entt/meta/meta_range.cpp)
+SETUP_BASIC_TEST(meta_type entt/meta/meta_type.cpp)
+
+# Test process
+
+SETUP_BASIC_TEST(process entt/process/process.cpp)
+SETUP_BASIC_TEST(scheduler entt/process/scheduler.cpp)
+
+# Test resource
+
+SETUP_BASIC_TEST(resource entt/resource/resource.cpp)
+
+# Test signal
+
+SETUP_BASIC_TEST(delegate entt/signal/delegate.cpp)
+SETUP_BASIC_TEST(dispatcher entt/signal/dispatcher.cpp)
+SETUP_BASIC_TEST(emitter entt/signal/emitter.cpp)
+SETUP_BASIC_TEST(sigh entt/signal/sigh.cpp)