doc

only one entity has all the components

.gitignore

@@ -1,2 +1,5 @@
-# QtCreator
 *.user
+conan/test_package/build
+
+# IDEs
+.idea

.travis.yml

@@ -2,6 +2,38 @@ language: cpp
 dist: trusty
 sudo: false
 
+env:
+  global:
+    - CONAN_USERNAME="skypjack"
+    - CONAN_PACKAGE_NAME="entt"
+    - CONAN_HEADER_ONLY="True"
+    - NON_CONAN_DEPLOYMENT="True"
+
+conan-buildsteps: &conan-buildsteps
+  before_install:
+    # use this step if you desire to manipulate CONAN variables programmatically
+    - NON_CONAN_DEPLOYMENT="False"
+  install:
+    - chmod +x ./conan/ci/install.sh
+    - ./conan/ci/install.sh
+  script:
+    - chmod +x ./conan/ci/build.sh
+    - ./conan/ci/build.sh
+  # the following are dummies to overwrite default build steps
+  before_script:
+    - true
+  after_success:
+    - true
+  if: tag IS present
+conan-linux: &conan-linux
+  os: linux
+  sudo: required
+  language: python
+  python: "3.6"
+  services:
+    - docker
+  <<: *conan-buildsteps
+
 matrix:
   include:
   - os: linux
@@ -9,17 +41,17 @@ matrix:
     addons:
       apt:
         sources: ['ubuntu-toolchain-r-test']
-        packages: ['g++-6']
-    env: COMPILER=g++-6
+        packages: ['g++-7']
+    env: COMPILER=g++-7
   - 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
+        sources: ['ubuntu-toolchain-r-test', 'llvm-toolchain-trusty-6.0']
+        packages: ['clang-6.0', 'g++-7']
+    env: COMPILER=clang++-6.0
   - os: osx
-    osx_image: xcode8.3
+    osx_image: xcode10
     compiler: clang
     env: COMPILER=clang++
   - os: linux
@@ -27,14 +59,17 @@ matrix:
     addons:
       apt:
         sources: ['ubuntu-toolchain-r-test']
-        packages: ['g++-6']
+        packages: ['g++-7']
     env:
-      - COMPILER=g++-6
+      - COMPILER=g++-7
       - 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
+      - coveralls --gcov gcov-7 --gcov-options '\-lp' --root ${TRAVIS_BUILD_DIR} --build-root ${TRAVIS_BUILD_DIR}/build --extension cpp --extension hpp --exclude deps --include src
+  # Conan testing and uploading
+  - <<: *conan-linux
+    env: CONAN_GCC_VERSIONS=8 CONAN_DOCKER_IMAGE=conanio/gcc8
 
 notifications:
   email:
@@ -51,5 +86,12 @@ install:
 
 script:
 - mkdir -p build && cd build
-- cmake -DCMAKE_BUILD_TYPE=Release .. && make -j4
-- CTEST_OUTPUT_ON_FAILURE=1 make test
+- cmake -DBUILD_TESTING=ON -DCMAKE_BUILD_TYPE=Release .. && make -j4
+- CTEST_OUTPUT_ON_FAILURE=1 ctest -j4
+
+deploy:
+  provider: script
+  script: scripts/update_packages.sh $TRAVIS_TAG
+  on:
+    tags: true
+    condition: “$NON_CONAN_DEPLOYMENT = “True”

AUTHORS

@@ -1,7 +1,37 @@
 # Author
 
-Michele Caini aka skypjack
+skypjack
 
 # Contributors
 
-Paolo Monteverde aka morbo84
+BenediktConze
+bjadamson
+ceeac
+ColinH
+corystegel
+Croydon
+dbacchet
+dBagrat
+djarek
+DonKult
+drglove
+eugeneko
+gale83
+ghost
+Green-Sky
+Milerius
+morbo84
+m-waka
+Kerndog73
+Paolo-Oliverio
+pgruenbacher
+prowolf
+vblanco20-1
+willtunnels
+WizardIke
+w1th0utnam3
+
+# Special thanks for patrons
+
+Arn
+tpisto

CMakeLists.txt

@@ -2,7 +2,7 @@
 # EnTT
 #
 
-cmake_minimum_required(VERSION 3.2)
+cmake_minimum_required(VERSION 3.7.2)
 
 #
 # Building in-tree is not allowed (we take care of your craziness).
@@ -16,7 +16,9 @@ endif()
 # Project configuration
 #
 
-project(entt VERSION 1.0.0)
+project(EnTT VERSION 3.0.0)
+
+include(GNUInstallDirs)
 
 if(NOT CMAKE_BUILD_TYPE)
     set(CMAKE_BUILD_TYPE Debug)
@@ -29,52 +31,195 @@ set(PROJECT_AUTHOR_EMAIL "michele.caini@gmail.com")
 
 message("*")
 message("* ${PROJECT_NAME} v${PROJECT_VERSION} (${CMAKE_BUILD_TYPE})")
-message("* Copyright (c) 2017 ${PROJECT_AUTHOR} <${PROJECT_AUTHOR_EMAIL}>")
+message("* Copyright (c) 2017-2019 ${PROJECT_AUTHOR} <${PROJECT_AUTHOR_EMAIL}>")
 message("*")
 
-#
-# Compile 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")
+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)
+option(USE_COMPILE_OPTIONS "Use compile options from EnTT." ON)
 
 #
-# CMake configuration
+# Compiler stuff
 #
 
-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)
+if(NOT MSVC AND USE_LIBCPP)
+    include(CheckCXXSourceCompiles)
+    include(CMakePushCheckState)
 
-set(PROJECT_RUNTIME_OUTPUT_DIRECTORY bin)
+    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, int> ? 0 : 1; }
+    " HAS_LIBCPP)
+
+    if(NOT HAS_LIBCPP)
+        message(WARNING "The option USE_LIBCPP is set (by default) but libc++ is not available. The flag will not be added to the target.")
+    endif()
+
+    cmake_pop_check_state()
+endif()
 
 #
-# Enable test support using ctest-like interface
+# Add EnTT target
 #
 
-option(BUILD_TESTING "Enable testing with ctest." ON)
+add_library(EnTT INTERFACE)
+
+configure_file(${EnTT_SOURCE_DIR}/cmake/in/version.h.in ${EnTT_SOURCE_DIR}/src/entt/config/version.h @ONLY)
+
+target_include_directories(
+    EnTT INTERFACE
+    $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/src>
+    $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
+)
+
+target_compile_definitions(
+    EnTT
+    INTERFACE $<$<AND:$<CONFIG:Debug>,$<NOT:$<CXX_COMPILER_ID:MSVC>>>:DEBUG>
+    INTERFACE $<$<AND:$<CONFIG:Release>,$<NOT:$<CXX_COMPILER_ID:MSVC>>>:RELEASE>
+)
+
+if(USE_ASAN)
+    target_compile_options(EnTT INTERFACE $<$<AND:$<CONFIG:Debug>,$<NOT:$<CXX_COMPILER_ID:MSVC>>>:-fsanitize=address -fno-omit-frame-pointer>)
+    target_link_libraries(EnTT INTERFACE $<$<AND:$<CONFIG:Debug>,$<NOT:$<CXX_COMPILER_ID:MSVC>>>:-fsanitize=address -fno-omit-frame-pointer>)
+endif()
+
+if(USE_COMPILE_OPTIONS)
+    target_compile_options(
+        EnTT
+        INTERFACE $<$<AND:$<CONFIG:Debug>,$<NOT:$<CXX_COMPILER_ID:MSVC>>>:-O0 -g>
+        # it seems that -O3 ruins a bit the performance when using clang ...
+        INTERFACE $<$<AND:$<CONFIG:Release>,$<CXX_COMPILER_ID:Clang>>:-O2>
+        # ... on the other side, GCC is incredibly comfortable with it.
+        INTERFACE $<$<AND:$<CONFIG:Release>,$<CXX_COMPILER_ID:GNU>>:-O3>
+    )
+endif()
+
+if(HAS_LIBCPP)
+    target_compile_options(EnTT BEFORE INTERFACE -stdlib=libc++)
+endif()
+
+target_compile_features(EnTT INTERFACE cxx_std_17)
 
 #
-# build testing stuff if required
+# Install EnTT
 #
 
+if(${CMAKE_SYSTEM_NAME} STREQUAL "Windows")
+    set(CUSTOM_INSTALL_CONFIGDIR cmake)
+else()
+    set(CUSTOM_INSTALL_CONFIGDIR ${CMAKE_INSTALL_LIBDIR}/cmake/entt)
+endif()
+
+install(DIRECTORY src/ DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
+install(TARGETS EnTT EXPORT EnTTTargets)
+
+export(EXPORT EnTTTargets FILE ${EnTT_BINARY_DIR}/EnTTTargets.cmake)
+
+install(
+    EXPORT EnTTTargets
+    FILE EnTTTargets.cmake
+    DESTINATION ${CUSTOM_INSTALL_CONFIGDIR}
+)
+
+#
+# Build tree package config file
+#
+
+configure_file(cmake/in/EnTTBuildConfig.cmake.in EnTTConfig.cmake @ONLY)
+
+include(CMakePackageConfigHelpers)
+
+#
+# Install tree package config file
+#
+
+configure_package_config_file(
+    cmake/in/EnTTConfig.cmake.in
+    ${CUSTOM_INSTALL_CONFIGDIR}/EnTTConfig.cmake
+    INSTALL_DESTINATION ${CUSTOM_INSTALL_CONFIGDIR}
+    PATH_VARS CMAKE_INSTALL_INCLUDEDIR
+)
+
+write_basic_package_version_file(
+    ${EnTT_BINARY_DIR}/EnTTConfigVersion.cmake
+    VERSION ${PROJECT_VERSION}
+    COMPATIBILITY AnyNewerVersion
+)
+
+install(
+    FILES
+        ${EnTT_BINARY_DIR}/${CUSTOM_INSTALL_CONFIGDIR}/EnTTConfig.cmake
+        ${EnTT_BINARY_DIR}/EnTTConfigVersion.cmake
+    DESTINATION ${CUSTOM_INSTALL_CONFIGDIR}
+)
+
+export(PACKAGE EnTT)
+
+#
+# Tests
+#
+
+option(BUILD_TESTING "Enable testing with ctest." OFF)
+
 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)
+
+    if(FIND_GTEST_PACKAGE)
+        find_package(GTest REQUIRED)
+    else()
+        # gtest, gtest_main, gmock and gmock_main targets are available from now on
+        set(GOOGLETEST_DEPS_DIR ${EnTT_SOURCE_DIR}/deps/googletest)
+        configure_file(${EnTT_SOURCE_DIR}/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)
+        target_compile_features(gmock_main PRIVATE $<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_FEATURES>)
+        target_compile_features(gmock PRIVATE $<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_FEATURES>)
+        add_library(GTest::Main ALIAS gtest_main)
+    endif()
+
+    option(BUILD_BENCHMARK "Build benchmark." OFF)
+    option(BUILD_LIB "Build lib example." OFF)
+    option(BUILD_MOD "Build mod example." OFF)
+    option(BUILD_SNAPSHOT "Build snapshot example." 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(
+    entt_aob
+    SOURCES
+        appveyor.yml
+        AUTHORS
+        LICENSE
+        README.md
+        TODO
+        .travis.yml
+)

LICENSE

@@ -1,6 +1,6 @@
 The MIT License (MIT)
 
-Copyright (c) 2017 Michele Caini
+Copyright (c) 2017-2019 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 +1,408 @@
-# 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)
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+[![GitHub version](https://badge.fury.io/gh/skypjack%2Fentt.svg)](http://badge.fury.io/gh/skypjack%2Fentt)
+[![LoC](https://tokei.rs/b1/github/skypjack/entt)](https://github.com/skypjack/entt)
+[![Build Status](https://travis-ci.org/skypjack/entt.svg?branch=master)](https://travis-ci.org/skypjack/entt)
 [![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)
+[![Gitter chat](https://badges.gitter.im/skypjack/entt.png)](https://gitter.im/skypjack/entt)
+[![Donate](https://img.shields.io/badge/Donate-PayPal-green.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, the
+[**ArcGIS Runtime SDKs**](https://developers.arcgis.com/arcgis-runtime/) by Esri
+and [**The Forge**](https://github.com/ConfettiFX/The-Forge) by Confetti. Read
+on to find out what it can offer you.
+
+---
+
+Do you want to **keep up with changes** or do you have a **question** that
+doesn't require you to open an issue?<br/>
+Join the [gitter channel](https://gitter.im/skypjack/entt) and meet other users
+like you. The more we are, the better for everyone.
+
+If you use `EnTT` and you want to say thanks or support the project, please
+**consider becoming a patron**:
+
+[![Patreon](https://c5.patreon.com/external/logo/become_a_patron_button.png)](https://www.patreon.com/bePatron?c=1772573)
+
+# 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)
+  * [Patreon](#patreon)
+  * [Donation](#donation)
+  * [Hire me](#hire-me)
+<!--
+@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 identifiers**.
+* 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 support
+  the users and avoid reinventing the wheel (dependencies, snapshot, actor class
+  for those who aren't confident with the architecture 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.
+
+Currently, `EnTT` is tested on Linux, Microsoft Windows and OSX. It has proven
+to work also on both Android and iOS.<br/>
+Most likely it won't be problematic on other systems as well, but it hasn't been
+sufficiently tested so far.
+
 ## 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.assign<position>(entity, i * 1.f, i * 1.f);
+        if(i % 2 == 0) { registry.assign<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 |
+As it stands right now, `EnTT` is just fast enough for my requirements when
+compared to my first choice (it was already amazingly fast actually).<br/>
+Below is a comparison between the two (both of them compiled with GCC 7.3.0 on a
+Dell XPS 13 from mid 2014):
+
+| Benchmark | EntityX (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** |
+| Create 1M entities | 0.0147s | **0.0046s** |
+| Destroy 1M entities | 0.0053s | **0.0045s** |
+| 1M entities, one component | 0.0012s | **1.9e-07s** |
+| 1M entities, two components | 0.0012s | **3.8e-07s** |
+| 1M entities, two components<br/>Half of the entities have all the components | 0.0009s | **3.8e-07s** |
+| 1M entities, two components<br/>One of the entities has all the components | 0.0008s | **1.0e-06s** |
+| 1M entities, five components | 0.0010s | **7.0e-07s** |
+| 1M entities, ten components | 0.0011s | **1.2e-06s** |
+| 1M entities, ten components<br/>Half of the entities have all the components | 0.0010s | **1.2e-06s** |
+| 1M entities, ten components<br/>One of the entities has all the components | 0.0008s | **1.2e-06s** |
+| Sort 150k entities, one component<br/>Arrays are in reverse order | - | **0.0036s** |
+| Sort 150k entities, enforce permutation<br/>Arrays are in reverse order | - | **0.0005s** |
+| Sort 150k entities, one component<br/>Arrays are almost sorted, std::sort | - | **0.0035s** |
+| Sort 150k entities, one component<br/>Arrays are almost sorted, insertion sort | - | **0.0007s** |
 
-These are the results of the twos when compiled with Clang 3.8.1:
+Note: The default version of `EntityX` (`master` branch) wasn't added to the
+comparison because it's already much slower than its compile-time counterpart.
 
-| 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** |
+Pretty interesting results, aren't them? In fact, these benchmarks are the ones
+used by `EntityX` to show _how fast it is_. To be honest, they aren't so good
+and these results shouldn't be taken too seriously (indeed they are completely
+unrealistic).<br/>
+The proposed entity-component system is incredibly fast to iterate entities,
+this is a fact. The compiler can make a lot of optimizations because of how
+`EnTT` works, even more when components aren't used at all. This is exactly the
+case for these benchmarks. On the other hand, if we consider real world cases,
+`EnTT` is somewhere between a bit and much faster than the other solutions
+around when users also access the components and not just the entities, although
+it isn't as fast as reported by these benchmarks.<br/>
+This is why they are completely wrong and cannot be used to evaluate any of the
+entity-component-system libraries out there.
 
-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 3.2 or later.
+* Doxygen version 1.8 or later.
+
+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
+-->
+It's also available [online](https://skypjack.github.io/entt/) for the latest
+version.<br/>
+Finally, 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://bintray.com/skypjack/conan/entt%3Askypjack/_latestVersion),
+  the C/C++ Package Manager for Developers.
+* [`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
+  ```
+* [`vcpkg`](https://github.com/Microsoft/vcpkg/tree/master/ports/entt),
+  Microsoft VC++ Packaging Tool.
+
+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](docs/md/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) 2017-2019 Michele Caini.<br/>
+Logo Copyright (c) 2018-2019 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
+
+## Patreon
+
+Become a [patron](https://www.patreon.com/bePatron?c=1772573) and get access to
+extra content, help me spend more time on the projects you love and create new
+ones for you. Your support will help me to continue the work done so far and
+make it more professional and feature-rich every day.<br/>
+It takes very little to
+[become a patron](https://www.patreon.com/bePatron?c=1772573) and thus help the
+software you use every day. Don't miss the chance.
+
+## Donation
+
+Developing and maintaining `EnTT` takes some time and lots of coffee. I'd like
+to add more and more functionalities in future and turn it in a full-featured
+solution.<br/>
+If you want to support this project, you can offer me an espresso. I'm from
+Italy, we're used to turning the best coffee ever in code. If you find that
+it's not enough, feel free to support me the way you prefer.<br/>
+Take a look at the donation button at the top of the page for more details or
+just click [here](https://www.paypal.me/skypjack).
+
+## Hire me
+
+If you start using `EnTT` and need help, if you want a new feature and want me
+to give it the highest priority, if you have any other reason to contact me:
+do not hesitate. I'm available for hiring.<br/>
+Feel free to take a look at my [profile](https://github.com/skypjack) and
+contact me by mail.
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->

TODO

@@ -0,0 +1,28 @@
+* long term feature: templated generic vm
+* long term feature: shared_ptr less locator
+* long term feature: shared_ptr less 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
+* runner proposal: https://en.wikipedia.org/wiki/Fork%E2%80%93join_model https://slide-rs.github.io/specs/03_dispatcher.html
+* work stealing job system (see #100)
+* meta: sort of meta view based on meta stuff to iterate entities, void * and meta info objects
+* allow for built-in parallel each if possible
+* allow to replace std:: with custom implementations
+* remove runtime views, welcome reflection and what about snapshot?
+* empty components model allows for shared components and prefabs unity-like
+  - each with entity return the shared component multiple times, one per entity that refers to it
+  - each components only return actual component, so shared components are returned only once
+* types defined at runtime that refer to the same compile-time type (but to different pools) are possible, the library is almost there
+* add take functionality, eg registry.take(entity, other); where it takes the entity and all its components from registry and move them to other
+* add opaque input iterators to views and groups that return tuples <entity, T &...> (proxy), multi-pass guaranteed
+* add fast lane for raw iterations, extend mt doc to describe allowed add/remove with pre-allocations on fast lanes
+* review sparse set to allow customization (mix pack in the spec, base is position only)
+  - non-owning groups can iterate pages and skip empty ones, this should mitigate the lack of the packed array
+* review 64 bit id: user defined area + dedicated member on the registry to set it
+* early out in views using bitmasks with bloom filter like access based on modulus
+  - standard each, use bitmask to speed up the whole thing and avoid accessing the pools to test for the page
+  - iterator based each with a couple of iterators passed from outside (use bitmask + has)
+* stable component handle that isn't affected by reallocations
+* multi component registry::remove and some others?
+* move CONTRIBUTING.md within doc (for GitHub)
+* reactive systems

appveyor.yml

@@ -1,7 +1,7 @@
 # can use variables like {build} and {branch}
 version: 1.0.{build}
 
-image: Visual Studio 2015
+image: Visual Studio 2017
 
 environment:
   BUILD_DIR: "%APPVEYOR_BUILD_FOLDER%\\build"
@@ -13,9 +13,11 @@ configuration:
   - Release
 
 before_build:
-  - deps.bat
   - cd %BUILD_DIR%
-  - cmake .. -G"%CMAKE_GENERATOR_NAME%"
+  - cmake .. -DBUILD_TESTING=ON -DBUILD_LIB=ON -DCMAKE_CXX_FLAGS=/W1 -G"Visual Studio 15 2017"
+
+after_build:
+  - ctest -C Release -j4
 
 build:
   parallel: true

cmake/in/EnTTBuildConfig.cmake.in

@@ -0,0 +1,6 @@
+set(ENTT_VERSION "@PROJECT_VERSION@")
+set(ENTT_INCLUDE_DIRS "@CMAKE_CURRENT_SOURCE_DIR@/src")
+
+if(NOT CMAKE_VERSION VERSION_LESS "3.0")
+    include("${CMAKE_CURRENT_LIST_DIR}/EnTTTargets.cmake")
+endif()

cmake/in/EnTTConfig.cmake.in

@@ -0,0 +1,11 @@
+set(ENTT_VERSION "@PROJECT_VERSION@")
+
+@PACKAGE_INIT@
+
+set_and_check(ENTT_INCLUDE_DIRS "@PACKAGE_CMAKE_INSTALL_INCLUDEDIR@")
+
+if(NOT CMAKE_VERSION VERSION_LESS "3.0")
+    include("${CMAKE_CURRENT_LIST_DIR}/EnTTTargets.cmake")
+endif()
+
+check_required_components("@PROJECT_NAME@")

cmake/in/cereal.in

@@ -0,0 +1,19 @@
+project(cereal-download NONE)
+cmake_minimum_required(VERSION 3.2)
+
+include(ExternalProject)
+
+ExternalProject_Add(
+    cereal
+    GIT_REPOSITORY https://github.com/USCiLab/cereal.git
+    GIT_TAG v1.2.2
+    DOWNLOAD_DIR ${CEREAL_DEPS_DIR}
+    TMP_DIR ${CEREAL_DEPS_DIR}/tmp
+    STAMP_DIR ${CEREAL_DEPS_DIR}/stamp
+    SOURCE_DIR ${CEREAL_DEPS_DIR}/src
+    BINARY_DIR ${CEREAL_DEPS_DIR}/build
+    CONFIGURE_COMMAND ""
+    BUILD_COMMAND ""
+    INSTALL_COMMAND ""
+    TEST_COMMAND ""
+)

cmake/in/duktape.in

@@ -0,0 +1,19 @@
+project(duktape-download NONE)
+cmake_minimum_required(VERSION 3.2)
+
+include(ExternalProject)
+
+ExternalProject_Add(
+    duktape
+    GIT_REPOSITORY https://github.com/svaarala/duktape-releases.git
+    GIT_TAG v2.2.0
+    DOWNLOAD_DIR ${DUKTAPE_DEPS_DIR}
+    TMP_DIR ${DUKTAPE_DEPS_DIR}/tmp
+    STAMP_DIR ${DUKTAPE_DEPS_DIR}/stamp
+    SOURCE_DIR ${DUKTAPE_DEPS_DIR}/src
+    BINARY_DIR ${DUKTAPE_DEPS_DIR}/build
+    CONFIGURE_COMMAND ""
+    BUILD_COMMAND ""
+    INSTALL_COMMAND ""
+    TEST_COMMAND ""
+)

cmake/in/version.h.in

@@ -0,0 +1,11 @@
+#ifndef ENTT_CONFIG_VERSION_H
+#define ENTT_CONFIG_VERSION_H
+
+
+#define ENTT_VERSION "@PROJECT_VERSION@"
+#define ENTT_VERSION_MAJOR @PROJECT_VERSION_MAJOR@
+#define ENTT_VERSION_MINOR @PROJECT_VERSION_MINOR@
+#define ENTT_VERSION_PATCH @PROJECT_VERSION_PATCH@
+
+
+#endif // ENTT_CONFIG_VERSION_H

conan/build.py

@@ -0,0 +1,45 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+from cpt.packager import ConanMultiPackager
+import os
+
+if __name__ == "__main__":
+    login_username = os.getenv("CONAN_LOGIN_USERNAME")
+    username = os.getenv("CONAN_USERNAME")
+    tag_version = os.getenv("CONAN_PACKAGE_VERSION", os.getenv("TRAVIS_TAG"))
+    package_version = tag_version.replace("v", "")
+    package_name_unset = "SET-CONAN_PACKAGE_NAME-OR-CONAN_REFERENCE"
+    package_name = os.getenv("CONAN_PACKAGE_NAME", package_name_unset)
+    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)
+    header_only = os.getenv("CONAN_HEADER_ONLY", False)
+    pure_c = os.getenv("CONAN_PURE_C", False)
+
+    disable_shared = os.getenv("CONAN_DISABLE_SHARED_BUILD", "False")
+    if disable_shared == "True" and package_name == package_name_unset:
+        raise Exception("CONAN_DISABLE_SHARED_BUILD: True is only supported when you define CONAN_PACKAGE_NAME")
+
+    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)
+    if header_only == "False":
+        builder.add_common_builds(pure_c=pure_c)
+    else:
+        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.assign<position>(entity, i * 1.f, i * 1.f);
+        if(i % 2 == 0) { registry.assign<velocity>(entity, i * .1f, i * .1f); }
+    }
+
+    update(dt, registry);
+    update(registry);
+
+    // ...
+
+    return EXIT_SUCCESS;
+}

conanfile.py

@@ -0,0 +1,23 @@
+#!/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_id(self):
+        self.info.header_only()

docs/CMakeLists.txt

@@ -0,0 +1,39 @@
+#
+# Doxygen configuration (documentation)
+#
+
+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 doxy.in
+)
+
+install(
+    DIRECTORY ${DOXY_OUTPUT_DIRECTORY}/html
+    DESTINATION share/${PROJECT_NAME}-${PROJECT_VERSION}/
+)
+
+add_custom_target(
+    docs_aob
+    SOURCES
+        md/CONTRIBUTING.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/resource.md
+        md/signal.md
+        dox/extra.dox
+)

docs/dox/extra.dox

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

docs/md/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.

docs/md/core.md

@@ -0,0 +1,167 @@
+# Crash Course: core functionalities
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [Compile-time identifiers](#compile-time-identifiers)
+* [Runtime identifiers](#runtime-identifiers)
+* [Hashed strings](#hashed-strings)
+  * [Conflicts](#conflicts)
+* [Monostate](#monostate)
+<!--
+@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.
+
+# Compile-time identifiers
+
+Sometimes it's useful to be able to give unique identifiers to types at
+compile-time.<br/>
+There are plenty of different solutions out there and I could have used one of
+them. However, I decided to spend my time to define a compact and versatile tool
+that fully embraces what the modern C++ has to offer.
+
+The _result of my efforts_ is the `identifier` class template:
+
+```cpp
+#include <ident.hpp>
+
+// 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 the class template has to offer: a `type` inline variable that
+contains a numerical identifier for the given type. It can be used in any
+context where constant expressions are required.
+
+As long as the list remains unchanged, identifiers are also guaranteed to be the
+same for every run. In case they have been used in a production environment and
+a type has to be removed, one can just use a placeholder to left the other
+identifiers unchanged:
+
+```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
+>;
+```
+
+A bit ugly to see, but it works at least.
+
+# Runtime identifiers
+
+Sometimes it's useful to be able to give unique identifiers to types at
+runtime.<br/>
+There are plenty of different solutions out there and I could have used one of
+them. In fact, I adapted the most common one to my requirements and used it
+extensively within the entire library.
+
+It's the `family` class. Here is an example of use directly from the
+entity-component system:
+
+```cpp
+using component_family = entt::family<struct internal_registry_component_family>;
+
+// ...
+
+template<typename Component>
+component_type component() const noexcept {
+    return component_family::type<Component>;
+}
+```
+
+This is all what a _family_ has to offer: a `type` inline variable that contains
+a numerical identifier for the given type.
+
+Please, note that identifiers aren't guaranteed to be the same for every run.
+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;
+```
+
+## 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"}>{};
+```

docs/md/faq.md

@@ -0,0 +1,55 @@
+# 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)
+<!--
+@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_DISABLE_ASSERT` should be defined to disable internal
+checks made by `EnTT` in debug. These are asserts introduced to help the users,
+but 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`.

docs/md/lib.md

@@ -0,0 +1,155 @@
+# Push EnTT across boundaries
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [Named types and traits class](#named-types-and-traits-class)
+  * [Do not mix types](#do-not-mix-types)
+* [Macros, macros everywhere](#macros-macros-everywhere)
+  * [Conflicts](#conflicts)
+* [Allocations: the dark side of the force](#allocations-the-dark-side-of-the-force)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+`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 is due mainly to a custom utility used to assign unique, sequential
+identifiers to different types. Unfortunately, this tool is used by several core
+classes (the `registry` among the others) that are thus almost unusable across
+boundaries.<br/>
+The reasons for that are beyond the purposes of this document. However, the good
+news is that `EnTT` also offers now a way to overcome this limit and to push
+things across boundaries without problems when needed.
+
+# Named types and traits class
+
+To allow a type to work properly across boundaries when used by a class that
+requires to assign unique identifiers to types, users must specialize a class
+template to literally give a compile-time name to the type itself.<br/>
+The name of the class template is `name_type_traits` and the specialization must
+be such that it exposes a static constexpr data member named `value` having type
+either `ENTT_ID_TYPE` or `entt::hashed_string::hash_type`. Its value is the user
+defined unique identifier assigned to the specific type.<br/>
+Identifiers are not to be sequentially generated in this case.
+
+As an example:
+
+```cpp
+struct my_type { /* ... */ };
+
+template<>
+struct entt::named_type_traits<my_type> {
+    static constexpr auto value = "my_type"_hs;
+};
+```
+
+Because of the rules of the language, the specialization must reside in the
+global namespace or in the `entt` namespace. There is no way to change this rule
+unfortunately, because it doesn't depend on the library itself.
+
+The good aspect of this approach is that it's not intrusive at all. The other
+way around was in fact forcing users to inherit all their classes from a common
+base. Something to avoid, at least from my point of view.<br/>
+However, despite the fact that it's not intrusive, it would be great if it was
+also easier to use and a bit less error-prone. This is why a bunch of macros
+exist to ease defining named types.
+
+## Do not mix types
+
+Someone might think that this trick is valid only for the types to push across
+boundaries. This isn't how things work. In fact, the problem is more complex
+than that.<br/>
+As a rule of thumb, users should never mix named and non-named types. Whenever
+a type is given a name, all the types must be given a name. As an example,
+consider the `registry` class template: in case it is pushed across boundaries,
+all the types of components should be assigned a name to avoid subtle bugs.
+
+Indeed, this constraint can be relaxed in many cases. However, it is difficult
+to define a general rule to follow that is not the most stringent, unless users
+know exactly what they are doing. Therefore, I won't elaborate on giving further
+details on the topic.
+
+# Macros, macros everywhere
+
+The library comes with a set of predefined macros to use to declare named types
+or export already existing ones. In particular:
+
+* `ENTT_NAMED_TYPE` can be used to assign a name to already existing types. This
+  macro must be used in the global namespace even when the types to be named are
+  not.
+
+  ```cpp
+  ENTT_NAMED_TYPE(my_type)
+  ENTT_NAMED_TYPE(ns::another_type)
+  ```
+
+* `ENTT_NAMED_STRUCT` can be used to define and export a struct at the same
+  time. It accepts also an optional namespace in which to define the given type.
+  This macro must be used in the global namespace.
+
+  ```cpp
+  ENTT_NAMED_STRUCT(my_type, { /* struct definition */})
+  ENTT_NAMED_STRUCT(ns, another_type, { /* struct definition */})
+  ```
+
+* `ENTT_NAMED_CLASS` can be used to define and export a class at the same
+  time. It accepts also an optional namespace in which to define the given type.
+  This macro must be used in the global namespace.
+
+  ```cpp
+  ENTT_NAMED_CLASS(my_type, { /* class definition */})
+  ENTT_NAMED_CLASS(ns, another_type, { /* class definition */})
+  ```
+
+Nested namespaces are supported out of the box as well in all cases. As an
+example:
+
+```cpp
+ENTT_NAMED_STRUCT(nested::ns, my_type, { /* struct definition */})
+```
+
+These macros can be used to avoid specializing the `named_type_traits` class
+template. In all cases, the name of the class is used also as a seed to generate
+the compile-time unique identifier.
+
+## Conflicts
+
+When using macros, unique identifiers are 32/64 bit integers generated by
+hashing strings during compilation. Therefore, conflicts are rare but still
+possible. In case of conflicts, everything simply will get broken at runtime and
+the strangest things will probably take place.<br/>
+Unfortunately, there is no safe way to prevent it. If this happens, it will be
+enough to give a different value to one of the conflicting types to solve the
+problem. To do this, users can either assign a different name to the class or
+directly define a specialization for the `named_type_traits` class template.
+
+# Allocations: the dark side of the force
+
+As long as `EnTT` won't support custom allocators, another problem with
+allocations will remain alive instead. This is in fact easily solved, or at
+least it is if one knows it.
+
+To allow users to add types dynamically, the library makes extensive use of type
+erasure techniques and dynamic allocations for pools (whether they are for
+components, events or anything else). The problem occurs when, for example, a
+registry is created on one side of a boundary and a pool is dynamically created
+on the other side. In the best case, everything will crash at the exit, while at
+worst it will do so at runtime.<br/>
+To avoid problems, the pools must be generated from the same side of the
+boundary where the object that owns them is also created. As an example, when
+the registry is created in the main executable and used across boundaries for a
+given type of component, the pool for that type must be created before passing
+around the registry itself. To do this is fortunately quite easy, since it is
+sufficient to invoke any of the methods that involve the given type (continuing
+the example with the registry, a call to `reserve` or `size` is more than
+enough).
+
+Maybe one day some dedicated methods will be added that do nothing but create a
+pool for a given type. Until now it has been preferred to keep the API cleaner
+as they are not strictly necessary.

docs/md/links.md

@@ -0,0 +1,93 @@
+# 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.
+  * [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`.
+  * [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`.
+
+* Engines/Frameworks:
+  * [The Forge](https://github.com/ConfettiFX/The-Forge) by
+    [Confett](http://www.confettispecialfx.com/): a cross-platform rendering
+    framework.
+  * [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.
+
+* Emulators:
+  * [NovusCore](https://github.com/novuscore/NovusCore): A modern take on World
+    of Warcraft emulation.
+
+* Articles 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`.
+
+* Any Other Business:
+  * The [ArcGIS Runtime SDKs](https://developers.arcgis.com/arcgis-runtime/)
+    by [Esri](https://www.esri.com/): they use `EnTT` for the internal ECS and
+    the cross platform C++ rendering engine. The SDKs are utilized by a lot of
+    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).
+  * [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.
+  * [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 +1,414 @@
+# Crash Course: reflection system
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [Reflection in a nutshell](#reflection-in-a-nutshell)
+* [Any as in any type](#any-as-in-any-type)
+* [Enjoy the runtime](#enjoy-the-runtime)
+* [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 vice versa.
+
+# 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 _reflect_ a type, the library provides the `reflect` function:
+
+```cpp
+auto factory = entt::reflect<my_type>("reflected_type");
+```
+
+It accepts the type to reflect as a template parameter and an optional name as
+an argument. Names are important because users can retrieve meta types at
+runtime by searching for them by name. However, there are cases in which users
+can be interested in adding features to a reflected type so that the reflection
+system can use it correctly under the hood, but they don't want to allow
+searching the type by name.<br/>
+In both cases, the returned value is a factory object to use to continue
+building the meta type.
+
+A factory is such that all its member functions returns the factory itself.
+It can be used to extend the reflected type and 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::reflect<my_type>("reflected").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::reflect<my_type>("reflected").dtor<&destroy>();
+  ```
+
+* _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::reflect<my_type>("reflected")
+      .data<&my_type::static_variable>("static")
+      .data<&my_type::data_member>("member")
+      .data<&global_variable>("global");
+  ```
+
+  This function requires as an argument the name to give to the meta data once
+  created. Users can then access meta data at runtime by searching for them by
+  name.<br/>
+  Data members can be set also by means of a couple of functions, namely a
+  setter and a getter. Setters and getters can be either free functions, member
+  functions or mixed ones, as long as they respect the required signatures.<br/>
+  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::reflect<my_type>("reflected")
+      .func<&my_type::static_function>("static")
+      .func<&my_type::member_function>("member")
+      .func<&free_function>("free");
+  ```
+
+  This function requires as an argument the name 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::reflect<derived_type>("derived").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::reflect<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.
+
+A meta any object provides 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 though the reflection system, allocations are at zero.
+
+A meta any object can be created by any other object or as an empty container
+to initialize later:
+
+```cpp
+// a meta any object that contains an int
+entt::meta_any any{0};
+
+// an empty meta any object
+entt::meta_any empty{};
+```
+
+It can be constructed or assigned by copy and move and it takes the burden of
+destroying the contained object when required.<br/>
+A meta any object has a `type` member function that returns the meta type of the
+contained value, if any. The member functions `can_cast` and `can_convert` are
+used to know if the underlying object has a given type as a base or if it can be
+converted implicitly to it. Similarly, `cast` and `convert` do what they promise
+and return the expected value.
+
+# 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 two options: by type or by name. In
+both cases, the search can be done by means of the `resolve` function:
+
+```cpp
+// search for a reflected type by type
+auto by_type = entt::resolve<my_type>();
+
+// search for a reflected type by name
+auto by_name = entt::resolve("reflected_type");
+```
+
+There exits also a third overload of the `resolve` function to use to iterate
+all the reflected types at once:
+
+```cpp
+resolve([](auto type) {
+    // ...
+});
+```
+
+In all cases, the returned value is an instance of `meta_type`. This type of
+objects offer an API to know the _runtime name_ of the type, to iterate all the
+meta objects associated with them and even to build or destroy 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 arguments, the expected
+  meta types and to invoke it, therefore to construct a new instance of the
+  underlying type.
+
+* _Meta destructor_. It's returned by a dedicated function:
+
+  ```cpp
+  auto dtor = entt::resolve<my_type>().dtor();
+  ```
+
+  The returned type is `meta_dtor` and may be invalid if there is no custom
+  destructor set for the given meta type.<br/>
+  All what a meta destructor has to offer is a way to invoke it on a given
+  instance. Be aware that the result may not be what is expected.
+
+* _Meta data_. They are accessed by name:
+
+  ```cpp
+  auto data = entt::resolve<my_type>().data("member");
+  ```
+
+  The returned type is `meta_data` and may be invalid if there is no meta data
+  object associated with the given name.<br/>
+  A meta data object offers an API to query the underlying type (ie 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");
+  ```
+
+  The returned type is `meta_func` and may be invalid if there is no meta
+  function object associated with the given name.<br/>
+  A meta function object offers an API to query the underlying type (ie 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 meta any object.
+
+
+* _Meta bases_. They are accessed through the name of the base types:
+
+  ```cpp
+  auto base = entt::resolve<derived_type>().base("base");
+  ```
+
+  The returned type is `meta_base` and may be invalid if there is no meta base
+  object associated with the given name.<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
+auto func = entt::resolve<my_type>().func("member");
+
+if(func) {
+    // ...
+}
+```
+
+Furthermore, all meta objects with the exception of meta destructors 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 also be used to `construct` or `destroy` 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 returns a `meta_any` object that may or
+may not be initialized, depending on whether a suitable constructor has been
+found or not. On the other side, the `destroy` member function accepts instances
+of `meta_any` as well as actual objects by reference and invokes the registered
+destructor if any or a default one.<br/>
+Be aware that the result of a call to `destroy` may not be what is expected.
+
+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 official documentation to get the best out of this powerful tool.
+
+# 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::reflect<my_enum>()
+        .data<my_enum::a_value>("a_value")
+        .data<my_enum::another_value>("another_value");
+
+entt::reflect<int>().data<2048>("max_int");
+```
+
+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").get({}).cast<my_enum>();
+auto max = entt::resolve<int>().data("max_int").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 (ie when it comes to creating an editor) it might be useful to be able
+to attach properties to the meta objects created. Fortunately, this is possible
+for most of them.<br/>
+To attach a property to a meta object, no matter what as long as it supports
+properties, it is sufficient to provide an object at the time of construction
+such that `std::get<0>` and `std::get<1>` are valid for it. In other terms, the
+properties are nothing more than key/value pairs users can put in an
+`std::pair`. As an example:
+
+```cpp
+entt::reflect<my_type>("reflected", std::make_pair("tooltip"_hs, "message"));
+```
+
+The meta objects that support properties offer then a couple of member functions
+named `prop` to iterate them at once and to search a specific property by key:
+
+```cpp
+// iterate all the 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.
+
+To unregister a type, users can use the `unregister` function from the global
+namespace:
+
+```cpp
+entt::unregister<my_type>();
+```
+
+This function returns a boolean value that is true if the type is actually
+registered with the reflection system, false otherwise.<br/>
+The type can be re-registered later with a completely different name and form.

docs/md/process.md

@@ -0,0 +1,208 @@
+# 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;
+
+    void update(delta_type delta, void *) {
+        remaining -= std::min(remaining, delta);
+
+        // ...
+
+        if(!remaining) {
+            succeed();
+        }
+    }
+
+private:
+    delta_type remaining{1000u};
+};
+```
+
+## 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>("foobar");
+  ```
+
+* 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>();
+```
+
+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/resource.md

@@ -0,0 +1,238 @@
+# 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 the 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_resource_cache = entt::resource_cache<my_resource>;
+
+// ...
+
+my_resource_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 to explore this part of the interface, it makes sense to mention how
+resources are identified. The type of the identifiers to use is defined as:
+
+```cpp
+entt::resource_cache<resource>::resource_type
+```
+
+Where `resource_type` is an alias for `entt::hashed_string::hash_type`.
+Therefore, resource identifiers are created explicitly as in the following
+example:
+
+```cpp
+constexpr auto identifier = entt::resource_cache<resource>::resource_type{"my/resource/identifier"_hs};
+// this is equivalent to the following
+constexpr auto 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(auto 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 const reference on request:
+
+* By means of the `get` member function:
+
+  ```cpp
+  const auto &resource = handle.get();
+  ```
+
+* Using the proper cast operator:
+
+  ```cpp
+  const auto &resource = handle;
+  ```
+
+* Through the dereference operator:
+
+  ```cpp
+  const 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 +1,447 @@
+# Crash Course: events, signals and everything in between
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [Delegate](#delegate)
+* [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 potential of an `std::function` are not
+required or where you are looking for something different, `EnTT` offers a full
+set of classes to solve completely different 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 does not claim to be a drop-in replacement for an `std::function`, so do not
+expect to use it whenever an `std::function` fits well. However, it can be used
+to send opaque delegates around to be used to invoke functions as needed.
+
+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 function 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) { 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.<br/>
+Functions having type equivalent to `void(T *, args...)` are accepted as well.
+In this case, `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 pointer to `c` and `42`. However, the
+function type of the delegate is still `void(int)`, mainly because this is also
+the signature of its function call operator.
+
+To create and initialize a delegate at once, there are also some 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
+usual:
+
+```cpp
+auto ret = delegate(42);
+```
+
+As shown above, listeners do not have to strictly follow the signature of the
+delegate. As long as a listener can be invoked with the given arguments to yield
+a result that is convertible to the given result type, everything works just
+fine.
+
+Probably too much small and pretty poor of functionalities, but the delegate
+class can help in a lot of cases and it has shown that it is worth keeping it
+within the library.
+
+# Signals
+
+Signal handlers work with naked pointers, function pointers and pointers to
+member functions. 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/>
+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 its _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. The most important thing is that
+it comes in two forms: with and without a collector. In case a signal is
+associated with a collector, all the values returned by the listeners can be
+literally _collected_ and used later by the caller. Otherwise it works just like
+a plain signal that emits events from time to time.<br/>
+
+**Note**: collectors are allowed only in case of function types whose the return
+type isn't `void` for obvious reasons.
+
+To create instances of signal handlers there exist mainly two ways:
+
+```cpp
+// no collector type
+entt::sigh<void(int, char)> signal;
+
+// explicit collector type
+entt::sigh<void(int, char), my_collector<bool>> collector;
+```
+
+As expected, they offer all the basic functionalities required to know how many
+listeners they contain (`size`) or if they contain at least a listener (`empty`)
+and even to swap two signal 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) { /* ... */ }
+};
+
+// ...
+
+listener instance;
+
+signal.sink().connect<&foo>();
+signal.sink().connect<&listener::bar>(&instance);
+
+// ...
+
+// disconnects a free function
+signal.sink().disconnect<&foo>();
+
+// disconnect a member function of an instance
+signal.sink().disconnect<&listener::bar>(&instance);
+
+// discards all the listeners at once
+signal.sink().disconnect();
+```
+
+As shown above, listeners do not have to strictly follow the signature of the
+signal. As long as a listener can be invoked with the given arguments to yield a
+result that is convertible to the given result type, everything works just fine.
+
+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
+struct my_collector {
+    std::vector<int> vec{};
+
+    bool operator()(int v) noexcept {
+        vec.push_back(v);
+        return true;
+    }
+};
+
+int f() { return 0; }
+int g() { return 1; }
+
+// ...
+
+entt::sigh<int(), my_collector<int>> signal;
+
+signal.sink().connect<&f>();
+signal.sink().connect<&g>();
+
+my_collector collector = signal.collect();
+
+assert(collector.vec[0] == 0);
+assert(collector.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 has to
+return a boolean value that is false to stop collecting data, true otherwise.
+This way one can avoid calling all the listeners in case it isn't necessary.
+
+# 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 that works with naked pointers
+entt::dispatcher dispatcher{};
+```
+
+In order to register an instance of a class to a dispatcher, its type must
+expose one or more member functions the arguments of which are such that
+`const E &` can be converted to them for each type of event `E`, no matter what
+the return value is.<br/>
+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
+selectively remove listeners:
+
+```cpp
+dispatcher.sink<an_event>().disconnect<&listener::receive>(&listener);
+dispatcher.sink<another_event>().disconnect<&listener::method>(&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<my_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 const reference to the event. Therefore
+listeners have an handy way to work with it without incurring in the need of
+capturing a reference to the emitter itself.<br/>
+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:
+
+```cpp
+void(const 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="$(sed -e 's/[\/&]/\\&/g' <<< "$URL")"
+sed -i -e '/url/s/".*"/"'$ESCAPED_URL'"/' $FORMULA
+
+# change the hash in the formula file
+sed -i -e '/sha256/s/".*"/"'$HASH'"/' $FORMULA
+
+# delete temporary file created by sed
+rm "$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 ..

scripts/update_packages.sh

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+scripts/update_homebrew.sh $1

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 +1,44 @@
+#ifndef ENTT_CONFIG_CONFIG_H
+#define ENTT_CONFIG_CONFIG_H
+
+
+#ifndef ENTT_NOEXCEPT
+#define ENTT_NOEXCEPT noexcept
+#endif // ENTT_NOEXCEPT
+
+
+#ifndef ENTT_HS_SUFFIX
+#define ENTT_HS_SUFFIX _hs
+#endif // ENTT_HS_SUFFIX
+
+
+#ifndef ENTT_NO_ATOMIC
+#include <atomic>
+template<typename Type>
+using maybe_atomic_t = std::atomic<Type>;
+#else // ENTT_NO_ATOMIC
+template<typename Type>
+using maybe_atomic_t = Type;
+#endif // ENTT_NO_ATOMIC
+
+
+#ifndef ENTT_ID_TYPE
+#include <cstdint>
+#define ENTT_ID_TYPE std::uint32_t
+#endif // ENTT_ID_TYPE
+
+
+#ifndef ENTT_PAGE_SIZE
+#define ENTT_PAGE_SIZE 32768
+#endif
+
+
+#ifndef ENTT_DISABLE_ASSERT
+#include <cassert>
+#define ENTT_ASSERT(condition) assert(condition)
+#else // ENTT_DISABLE_ASSERT
+#define ENTT_ASSERT(...) ((void)0)
+#endif // ENTT_DISABLE_ASSERT
+
+
+#endif // ENTT_CONFIG_CONFIG_H

src/entt/config/version.h

@@ -0,0 +1,11 @@
+#ifndef ENTT_CONFIG_VERSION_H
+#define ENTT_CONFIG_VERSION_H
+
+
+#define ENTT_VERSION "3.0.0"
+#define ENTT_VERSION_MAJOR 3
+#define ENTT_VERSION_MINOR 0
+#define ENTT_VERSION_PATCH 0
+
+
+#endif // ENTT_CONFIG_VERSION_H

src/entt/core/algorithm.hpp

@@ -0,0 +1,78 @@
+#ifndef ENTT_CORE_ALGORITHM_HPP
+#define ENTT_CORE_ALGORITHM_HPP
+
+
+#include <functional>
+#include <algorithm>
+#include <utility>
+
+
+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) {
+            auto it = first + 1;
+
+            while(it != last) {
+                auto pre = it++;
+                auto value = *pre;
+
+                while(pre-- != first && compare(value, *pre)) {
+                    *(pre+1) = *pre;
+                }
+
+                *(pre+1) = value;
+            }
+        }
+    }
+};
+
+
+}
+
+
+#endif // ENTT_CORE_ALGORITHM_HPP

src/entt/core/family.hpp

@@ -0,0 +1,40 @@
+#ifndef ENTT_CORE_FAMILY_HPP
+#define ENTT_CORE_FAMILY_HPP
+
+
+#include <type_traits>
+#include "../config/config.h"
+
+
+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 maybe_atomic_t<ENTT_ID_TYPE> identifier;
+
+    template<typename...>
+    inline static const auto inner = identifier++;
+
+public:
+    /*! @brief Unsigned integer type. */
+    using family_type = ENTT_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 in place of family_type here
+    inline static const family_type type = inner<std::decay_t<Type>...>;
+};
+
+
+}
+
+
+#endif // ENTT_CORE_FAMILY_HPP

src/entt/core/hashed_string.hpp

@@ -0,0 +1,213 @@
+#ifndef ENTT_CORE_HASHED_STRING_HPP
+#define ENTT_CORE_HASHED_STRING_HPP
+
+
+#include <cstddef>
+#include "../config/config.h"
+
+
+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> {
+    static constexpr std::uint32_t offset = 2166136261;
+    static constexpr std::uint32_t prime = 16777619;
+};
+
+
+template<>
+struct fnv1a_traits<std::uint64_t> {
+    static constexpr std::uint64_t offset = 14695981039346656037ull;
+    static constexpr std::uint64_t prime = 1099511628211ull;
+};
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond TURN_OFF_DOXYGEN
+ */
+
+
+/**
+ * @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.
+ */
+class hashed_string {
+    using traits_type = internal::fnv1a_traits<ENTT_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
+    inline static constexpr ENTT_ID_TYPE helper(ENTT_ID_TYPE partial, const char *curr) ENTT_NOEXCEPT {
+        return curr[0] == 0 ? partial : helper((partial^curr[0])*traits_type::prime, curr+1);
+    }
+
+public:
+    /*! @brief Unsigned integer type. */
+    using hash_type = ENTT_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 = hashed_string::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>
+    inline static constexpr hash_type to_value(const char (&str)[N]) ENTT_NOEXCEPT {
+        return helper(traits_type::offset, 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.
+     */
+    inline static hash_type to_value(const_wrapper wrapper) ENTT_NOEXCEPT {
+        return helper(traits_type::offset, 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.
+     */
+    inline static hash_type to_value(const char *str, std::size_t size) ENTT_NOEXCEPT {
+        ENTT_ID_TYPE partial{traits_type::offset};
+        while(size--) { partial = (partial^(str++)[0])*traits_type::prime; }
+        return partial;
+    }
+
+    /*! @brief Constructs an empty hashed string. */
+    constexpr hashed_string() ENTT_NOEXCEPT
+        : str{nullptr}, hash{}
+    {}
+
+    /**
+     * @brief Constructs a hashed string from an array of const chars.
+     *
+     * 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}
+     * hashed_string hs{"my.png"};
+     * @endcode
+     *
+     * @tparam N Number of characters of the identifier.
+     * @param curr Human-readable identifer.
+     */
+    template<std::size_t N>
+    constexpr hashed_string(const char (&curr)[N]) ENTT_NOEXCEPT
+        : str{curr}, hash{helper(traits_type::offset, curr)}
+    {}
+
+    /**
+     * @brief Explicit constructor on purpose to avoid constructing a hashed
+     * string directly from a `const char *`.
+     * @param wrapper Helps achieving the purpose by relying on overloading.
+     */
+    explicit constexpr hashed_string(const_wrapper wrapper) ENTT_NOEXCEPT
+        : str{wrapper.str}, hash{helper(traits_type::offset, wrapper.str)}
+    {}
+
+    /**
+     * @brief Returns the human-readable representation of a hashed string.
+     * @return The string used to initialize the instance.
+     */
+    constexpr const char * data() const ENTT_NOEXCEPT {
+        return str;
+    }
+
+    /**
+     * @brief Returns the numeric representation of a hashed string.
+     * @return The numeric representation of the instance.
+     */
+    constexpr hash_type value() const ENTT_NOEXCEPT {
+        return hash;
+    }
+
+    /**
+     * @brief Returns the human-readable representation of a hashed string.
+     * @return The string used to initialize the instance.
+     */
+    constexpr operator const char *() const ENTT_NOEXCEPT { return str; }
+
+    /*! @copydoc value */
+    constexpr operator hash_type() const ENTT_NOEXCEPT { return hash; }
+
+    /**
+     * @brief Compares two hashed strings.
+     * @param other Hashed string with which to compare.
+     * @return True if the two hashed strings are identical, false otherwise.
+     */
+    constexpr bool operator==(const hashed_string &other) const ENTT_NOEXCEPT {
+        return hash == other.hash;
+    }
+
+private:
+    const char *str;
+    hash_type hash;
+};
+
+
+/**
+ * @brief Compares two hashed strings.
+ * @param lhs A valid hashed string.
+ * @param rhs A valid hashed string.
+ * @return True if the two hashed strings are identical, false otherwise.
+ */
+constexpr bool operator!=(const hashed_string &lhs, const hashed_string &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+}
+
+
+/**
+ * @brief User defined literal for hashed strings.
+ * @param str The literal without its suffix.
+ * @return A properly initialized hashed string.
+ */
+constexpr entt::hashed_string operator"" ENTT_HS_SUFFIX(const char *str, std::size_t) ENTT_NOEXCEPT {
+    return entt::hashed_string{str};
+}
+
+
+#endif // ENTT_CORE_HASHED_STRING_HPP

src/entt/core/ident.hpp

@@ -0,0 +1,64 @@
+#ifndef ENTT_CORE_IDENT_HPP
+#define ENTT_CORE_IDENT_HPP
+
+
+#include <tuple>
+#include <utility>
+#include <type_traits>
+#include "../config/config.h"
+
+
+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>
+    static constexpr ENTT_ID_TYPE get(std::index_sequence<Indexes...>) ENTT_NOEXCEPT {
+        static_assert(std::disjunction_v<std::is_same<Type, Types>...>);
+        return (0 + ... + (std::is_same_v<Type, std::tuple_element_t<Indexes, tuple_type>> ? ENTT_ID_TYPE(Indexes) : ENTT_ID_TYPE{}));
+    }
+
+public:
+    /*! @brief Unsigned integer type. */
+    using identifier_type = ENTT_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::make_index_sequence<sizeof...(Types)>{});
+};
+
+
+}
+
+
+#endif // ENTT_CORE_IDENT_HPP

src/entt/core/monostate.hpp

@@ -0,0 +1,63 @@
+#ifndef ENTT_CORE_MONOSTATE_HPP
+#define ENTT_CORE_MONOSTATE_HPP
+
+
+#include <cassert>
+#include "../config/config.h"
+#include "hashed_string.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<hashed_string::hash_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 maybe_atomic_t<Type> value{};
+};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Value Value used to differentiate between different variables.
+ */
+template<hashed_string::hash_type Value>
+inline monostate<Value> monostate_v = {};
+
+
+}
+
+
+#endif // ENTT_CORE_MONOSTATE_HPP

src/entt/core/type_traits.hpp

@@ -0,0 +1,228 @@
+#ifndef ENTT_CORE_TYPE_TRAITS_HPP
+#define ENTT_CORE_TYPE_TRAITS_HPP
+
+
+#include <type_traits>
+#include "../core/hashed_string.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief A class to use to push around lists of types, nothing more.
+ * @tparam Type Types provided by the given type list.
+ */
+template<typename... Type>
+struct type_list {
+    /*! @brief Unsigned integer type. */
+    static constexpr auto size = sizeof...(Type);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_cat;
+
+
+/**
+ * @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 Traits class used mainly to push things across boundaries. */
+template<typename>
+struct named_type_traits;
+
+
+/**
+ * @brief Specialization used to get rid of constness.
+ * @tparam Type Named type.
+ */
+template<typename Type>
+struct named_type_traits<const Type>
+        : named_type_traits<Type>
+{};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type Potentially named type.
+ */
+template<typename Type>
+using named_type_traits_t = typename named_type_traits<Type>::type;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type has a
+ * name. In all other cases, `value` is false.
+ */
+template<typename, typename = std::void_t<>>
+struct is_named_type: std::false_type {};
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type has a
+ * name. In all other cases, `value` is false.
+ * @tparam Type Potentially named type.
+ */
+template<typename Type>
+struct is_named_type<Type, std::void_t<named_type_traits_t<std::decay_t<Type>>>>: std::true_type {};
+
+
+/**
+ * @brief Helper variable template.
+ *
+ * True if a given type has a name, false otherwise.
+ *
+ * @tparam Type Potentially named type.
+ */
+template<class Type>
+constexpr auto is_named_type_v = is_named_type<Type>::value;
+
+
+}
+
+
+/**
+ * @brief Utility macro to deal with an issue of MSVC.
+ *
+ * See _msvc-doesnt-expand-va-args-correctly_ on SO for all the details.
+ *
+ * @param args Argument to expand.
+ */
+#define ENTT_EXPAND(args) args
+
+
+/**
+ * @brief Makes an already existing type a named type.
+ * @param type Type to assign a name to.
+ */
+#define ENTT_NAMED_TYPE(type)\
+    template<>\
+    struct entt::named_type_traits<type>\
+        : std::integral_constant<typename entt::hashed_string::hash_type, entt::hashed_string::to_value(#type)>\
+    {\
+        static_assert(std::is_same_v<std::decay_t<type>, type>);\
+    };
+
+
+/**
+ * @brief Defines a named type (to use for structs).
+ * @param clazz Name of the type to define.
+ * @param body Body of the type to define.
+ */
+#define ENTT_NAMED_STRUCT_ONLY(clazz, body)\
+    struct clazz body;\
+    ENTT_NAMED_TYPE(clazz)
+
+
+/**
+ * @brief Defines a named type (to use for structs).
+ * @param ns Namespace where to define the named type.
+ * @param clazz Name of the type to define.
+ * @param body Body of the type to define.
+ */
+#define ENTT_NAMED_STRUCT_WITH_NAMESPACE(ns, clazz, body)\
+    namespace ns { struct clazz body; }\
+    ENTT_NAMED_TYPE(ns::clazz)
+
+
+/*! @brief Utility function to simulate macro overloading. */
+#define ENTT_NAMED_STRUCT_OVERLOAD(_1, _2, _3, FUNC, ...) FUNC
+/*! @brief Defines a named type (to use for structs). */
+#define ENTT_NAMED_STRUCT(...) ENTT_EXPAND(ENTT_NAMED_STRUCT_OVERLOAD(__VA_ARGS__, ENTT_NAMED_STRUCT_WITH_NAMESPACE, ENTT_NAMED_STRUCT_ONLY,)(__VA_ARGS__))
+
+
+/**
+ * @brief Defines a named type (to use for classes).
+ * @param clazz Name of the type to define.
+ * @param body Body of the type to define.
+ */
+#define ENTT_NAMED_CLASS_ONLY(clazz, body)\
+    class clazz body;\
+    ENTT_NAMED_TYPE(clazz)
+
+
+/**
+ * @brief Defines a named type (to use for classes).
+ * @param ns Namespace where to define the named type.
+ * @param clazz Name of the type to define.
+ * @param body Body of the type to define.
+ */
+#define ENTT_NAMED_CLASS_WITH_NAMESPACE(ns, clazz, body)\
+    namespace ns { class clazz body; }\
+    ENTT_NAMED_TYPE(ns::clazz)
+
+
+/*! @brief Utility function to simulate macro overloading. */
+#define ENTT_NAMED_CLASS_MACRO(_1, _2, _3, FUNC, ...) FUNC
+/*! @brief Defines a named type (to use for classes). */
+#define ENTT_NAMED_CLASS(...) ENTT_EXPAND(ENTT_NAMED_CLASS_MACRO(__VA_ARGS__, ENTT_NAMED_CLASS_WITH_NAMESPACE, ENTT_NAMED_CLASS_ONLY,)(__VA_ARGS__))
+
+
+#endif // ENTT_CORE_TYPE_TRAITS_HPP

src/entt/core/utility.hpp

@@ -0,0 +1,32 @@
+#ifndef ENTT_CORE_UTILITY_HPP
+#define ENTT_CORE_UTILITY_HPP
+
+
+namespace entt {
+
+
+/**
+ * @brief Constant utility to disambiguate overloaded member functions.
+ * @tparam Type Function type of the desired overload.
+ * @tparam Class Type of class to which the member functions belong.
+ * @param member A valid pointer to a member function.
+ * @return Pointer to the member function.
+ */
+template<typename Type, typename Class>
+constexpr auto overload(Type Class:: *member) { return member; }
+
+
+/**
+ * @brief Constant utility to disambiguate overloaded functions.
+ * @tparam Type Function type of the desired overload.
+ * @param func A valid pointer to a function.
+ * @return Pointer to the function.
+ */
+template<typename Type>
+constexpr auto overload(Type *func) { return func; }
+
+
+}
+
+
+#endif // ENTT_CORE_UTILITY_HPP

src/entt/entity/actor.hpp

@@ -0,0 +1,180 @@
+#ifndef ENTT_ENTITY_ACTOR_HPP
+#define ENTT_ENTITY_ACTOR_HPP
+
+
+#include <cassert>
+#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 entity-component systems.
+ *
+ * Tiny wrapper around a registry, for all those users that aren't confident
+ * with entity-component systems and prefer to iterate objects directly.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+struct basic_actor {
+    /*! @brief Type of registry used internally. */
+    using registry_type = basic_registry<Entity>;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+
+    /**
+     * @brief Constructs an actor by using the given registry.
+     * @param ref An entity-component system properly initialized.
+     */
+    basic_actor(registry_type &ref)
+        : reg{&ref}, entt{ref.create()}
+    {}
+
+    /*! @brief Default destructor. */
+    virtual ~basic_actor() {
+        reg->destroy(entt);
+    }
+
+    /**
+     * @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)
+        : reg{other.reg}, entt{other.entt}
+    {
+        other.entt = null;
+    }
+
+    /**
+     * @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) {
+        if(this != &other) {
+            auto tmp{std::move(other)};
+            std::swap(reg, tmp.reg);
+            std::swap(entt, tmp.entt);
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Assigns the given component to an actor.
+     *
+     * A new instance of the given component is created and initialized with the
+     * arguments provided (the component must have a proper constructor or be of
+     * aggregate type). Then the component is assigned to the actor.<br/>
+     * In case the actor already has a component of the given type, it's
+     * replaced with the new one.
+     *
+     * @tparam Component Type of the component to create.
+     * @tparam Args Types of arguments to use to construct the component.
+     * @param args Parameters to use to initialize the component.
+     * @return A reference to the newly created component.
+     */
+    template<typename Component, typename... Args>
+    decltype(auto) assign(Args &&... args) {
+        return reg->template assign_or_replace<Component>(entt, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Removes the given component from an actor.
+     * @tparam Component Type of the component to remove.
+     */
+    template<typename Component>
+    void remove() {
+        reg->template remove<Component>(entt);
+    }
+
+    /**
+     * @brief Checks if an actor has the given component.
+     * @tparam Component Type of the component for which to perform the check.
+     * @return True if the actor has the component, false otherwise.
+     */
+    template<typename Component>
+    bool has() const ENTT_NOEXCEPT {
+        return reg->template has<Component>(entt);
+    }
+
+    /**
+     * @brief Returns references to the given components for an actor.
+     * @tparam Component Types of components to get.
+     * @return References to the components owned by the actor.
+     */
+    template<typename... Component>
+    decltype(auto) get() const ENTT_NOEXCEPT {
+        return std::as_const(*reg).template get<Component...>(entt);
+    }
+
+    /*! @copydoc get */
+    template<typename... Component>
+    decltype(auto) get() ENTT_NOEXCEPT {
+        return reg->template get<Component...>(entt);
+    }
+
+    /**
+     * @brief Returns pointers to the given components for an actor.
+     * @tparam Component Types of components to get.
+     * @return Pointers to the components owned by the actor.
+     */
+    template<typename... Component>
+    auto try_get() const ENTT_NOEXCEPT {
+        return std::as_const(*reg).template try_get<Component...>(entt);
+    }
+
+    /*! @copydoc try_get */
+    template<typename... Component>
+    auto try_get() ENTT_NOEXCEPT {
+        return reg->template try_get<Component...>(entt);
+    }
+
+    /**
+     * @brief Returns a reference to the underlying registry.
+     * @return A reference to the underlying registry.
+     */
+    inline const registry_type & backend() const ENTT_NOEXCEPT {
+        return *reg;
+    }
+
+    /*! @copydoc backend */
+    inline 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.
+     */
+    inline entity_type entity() const ENTT_NOEXCEPT {
+        return entt;
+    }
+
+private:
+    registry_type *reg;
+    Entity entt;
+};
+
+
+}
+
+
+#endif // ENTT_ENTITY_ACTOR_HPP

src/entt/entity/entity.hpp

@@ -0,0 +1,169 @@
+#ifndef ENTT_ENTITY_ENTITY_HPP
+#define ENTT_ENTITY_ENTITY_HPP
+
+
+#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>
+struct entt_traits;
+
+
+/**
+ * @brief Entity traits for a 16 bits entity identifier.
+ *
+ * A 16 bits entity identifier guarantees:
+ *
+ * * 12 bits for the entity number (up to 4k entities).
+ * * 4 bit for the version (resets in [0-15]).
+ */
+template<>
+struct entt_traits<std::uint16_t> {
+    /*! @brief Underlying entity type. */
+    using entity_type = std::uint16_t;
+    /*! @brief Underlying version type. */
+    using version_type = std::uint8_t;
+    /*! @brief Difference type. */
+    using difference_type = std::int32_t;
+
+    /*! @brief Mask to use to get the entity number out of an identifier. */
+    static constexpr std::uint16_t entity_mask = 0xFFF;
+    /*! @brief Mask to use to get the version out of an identifier. */
+    static constexpr std::uint16_t version_mask = 0xF;
+    /*! @brief Extent of the entity number within an identifier. */
+    static constexpr auto entity_shift = 12;
+};
+
+
+/**
+ * @brief Entity traits for a 32 bits entity identifier.
+ *
+ * A 32 bits entity identifier guarantees:
+ *
+ * * 20 bits for the entity number (suitable for almost all the games).
+ * * 12 bit for the version (resets in [0-4095]).
+ */
+template<>
+struct entt_traits<std::uint32_t> {
+    /*! @brief Underlying entity type. */
+    using entity_type = std::uint32_t;
+    /*! @brief Underlying version type. */
+    using version_type = std::uint16_t;
+    /*! @brief Difference type. */
+    using difference_type = std::int64_t;
+
+    /*! @brief Mask to use to get the entity number out of an identifier. */
+    static constexpr std::uint32_t entity_mask = 0xFFFFF;
+    /*! @brief Mask to use to get the version out of an identifier. */
+    static constexpr std::uint32_t version_mask = 0xFFF;
+    /*! @brief Extent of the entity number within an identifier. */
+    static constexpr auto entity_shift = 20;
+};
+
+
+/**
+ * @brief Entity traits for a 64 bits entity identifier.
+ *
+ * A 64 bits entity identifier guarantees:
+ *
+ * * 32 bits for the entity number (an indecently large number).
+ * * 32 bit for the version (an indecently large number).
+ */
+template<>
+struct entt_traits<std::uint64_t> {
+    /*! @brief Underlying entity type. */
+    using entity_type = std::uint64_t;
+    /*! @brief Underlying version type. */
+    using version_type = std::uint32_t;
+    /*! @brief Difference type. */
+    using difference_type = std::int64_t;
+
+    /*! @brief Mask to use to get the entity number out of an identifier. */
+    static constexpr std::uint64_t entity_mask = 0xFFFFFFFF;
+    /*! @brief Mask to use to get the version out of an identifier. */
+    static constexpr std::uint64_t version_mask = 0xFFFFFFFF;
+    /*! @brief Extent of the entity number within an identifier. */
+    static constexpr auto entity_shift = 32;
+};
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+struct null {
+    template<typename Entity>
+    constexpr operator Entity() const ENTT_NOEXCEPT {
+        using traits_type = entt_traits<Entity>;
+        return traits_type::entity_mask | (traits_type::version_mask << traits_type::entity_shift);
+    }
+
+    constexpr bool operator==(null) const ENTT_NOEXCEPT {
+        return true;
+    }
+
+    constexpr bool operator!=(null) const ENTT_NOEXCEPT {
+        return false;
+    }
+
+    template<typename Entity>
+    constexpr bool operator==(const Entity entity) const ENTT_NOEXCEPT {
+        return entity == static_cast<Entity>(*this);
+    }
+
+    template<typename Entity>
+    constexpr bool operator!=(const Entity entity) const ENTT_NOEXCEPT {
+        return entity != static_cast<Entity>(*this);
+    }
+};
+
+
+template<typename Entity>
+constexpr bool operator==(const Entity entity, null other) ENTT_NOEXCEPT {
+    return other == entity;
+}
+
+
+template<typename Entity>
+constexpr bool operator!=(const Entity entity, null other) ENTT_NOEXCEPT {
+    return other != entity;
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond TURN_OFF_DOXYGEN
+ */
+
+
+/**
+ * @brief Null entity.
+ *
+ * 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.
+ */
+constexpr auto null = internal::null{};
+
+
+}
+
+
+#endif // ENTT_ENTITY_ENTITY_HPP

src/entt/entity/fwd.hpp

@@ -0,0 +1,89 @@
+#ifndef ENTT_ENTITY_FWD_HPP
+#define ENTT_ENTITY_FWD_HPP
+
+
+#include <cstdint>
+#include "../config/config.h"
+
+
+namespace entt {
+
+/*! @class basic_registry */
+template <typename>
+class basic_registry;
+
+/*! @class basic_view */
+template<typename, typename...>
+class basic_view;
+
+/*! @class basic_runtime_view */
+template<typename>
+class basic_runtime_view;
+
+/*! @class basic_group */
+template<typename...>
+class basic_group;
+
+/*! @class basic_actor */
+template <typename>
+struct basic_actor;
+
+/*! @class basic_prototype */
+template<typename>
+class basic_prototype;
+
+/*! @class basic_snapshot */
+template<typename>
+class basic_snapshot;
+
+/*! @class basic_snapshot_loader */
+template<typename>
+class basic_snapshot_loader;
+
+/*! @class basic_continuous_loader */
+template<typename>
+class basic_continuous_loader;
+
+/*! @brief Alias declaration for the most common use case. */
+using entity = std::uint32_t;
+
+/*! @brief Alias declaration for the most common use case. */
+using registry = basic_registry<entity>;
+
+/*! @brief Alias declaration for the most common use case. */
+using actor = basic_actor<entity>;
+
+/*! @brief Alias declaration for the most common use case. */
+using prototype = basic_prototype<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 Component 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 // ENTT_ENTITY_FWD_HPP

src/entt/entity/group.hpp

@@ -0,0 +1,779 @@
+#ifndef ENTT_ENTITY_GROUP_HPP
+#define ENTT_ENTITY_GROUP_HPP
+
+
+#include <tuple>
+#include <utility>
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/type_traits.hpp"
+#include "sparse_set.hpp"
+#include "storage.hpp"
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Alias for lists of observed components.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+struct get_t: type_list<Type...> {};
+
+
+/**
+ * @brief Variable template for lists of observed components.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+constexpr get_t<Type...> get{};
+
+
+/**
+ * @brief Group.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error, but for a few reasonable cases.
+ */
+template<typename...>
+class basic_group;
+
+
+/**
+ * @brief Non-owning group.
+ *
+ * A non-owning group returns all the entities and only the entities that have
+ * at least the given components. Moreover, it's guaranteed that the entity list
+ * is tightly packed in memory for fast iterations.<br/>
+ * In general, non-owning groups don't stay true to the order of any set of
+ * components unless users explicitly sort them.
+ *
+ * @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).
+ *
+ * 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
+ * Groups 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 all the
+ * groups.<br/>
+ * Moreover, sorting a non-owning group affects all the instance of the same
+ * group (it means that users don't have to call `sort` on each instance to sort
+ * all of them because they share the set of entities).
+ *
+ * @warning
+ * Lifetime of a group must overcome the one of the registry that generated it.
+ * In any other case, attempting to use a group results in undefined behavior.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Get Types of components observed by the group.
+ */
+template<typename Entity, typename... Get>
+class basic_group<Entity, get_t<Get...>> {
+    static_assert(sizeof...(Get) > 0);
+
+    /*! @brief A registry is allowed to create groups. */
+    friend class basic_registry<Entity>;
+
+    template<typename Component>
+    using pool_type = std::conditional_t<std::is_const_v<Component>, const storage<Entity, std::remove_const_t<Component>>, storage<Entity, Component>>;
+
+    // we could use pool_type<Get> *..., but vs complains about it and refuses to compile for unknown reasons (likely a bug)
+    basic_group(sparse_set<Entity> *ref, storage<Entity, std::remove_const_t<Get>> *... get) ENTT_NOEXCEPT
+        : handler{ref},
+          pools{get...}
+    {}
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename sparse_set<Entity>::entity_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = typename sparse_set<Entity>::size_type;
+    /*! @brief Input iterator type. */
+    using iterator_type = typename sparse_set<Entity>::iterator_type;
+
+    /**
+     * @brief Returns the number of existing components of the given type.
+     * @tparam Component Type of component of which to return the size.
+     * @return Number of existing components of the given type.
+     */
+    template<typename Component>
+    size_type size() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Component> *>(pools)->size();
+    }
+
+    /**
+     * @brief Returns the number of entities that have the given components.
+     * @return Number of entities that have the given components.
+     */
+    size_type size() const ENTT_NOEXCEPT {
+        return handler->size();
+    }
+
+    /**
+     * @brief Returns the number of elements that a group has currently
+     * allocated space for.
+     * @return Capacity of the group.
+     */
+    size_type capacity() const ENTT_NOEXCEPT {
+        return handler->capacity();
+    }
+
+    /*! @brief Requests the removal of unused capacity. */
+    void shrink_to_fit() {
+        handler->shrink_to_fit();
+    }
+
+    /**
+     * @brief Checks whether the pool of a given component is empty.
+     * @tparam Component Type of component in which one is interested.
+     * @return True if the pool of the given component is empty, false
+     * otherwise.
+     */
+    template<typename Component>
+    bool empty() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Component> *>(pools)->empty();
+    }
+
+    /**
+     * @brief Checks whether the group is empty.
+     * @return True if the group is empty, false otherwise.
+     */
+    bool empty() const ENTT_NOEXCEPT {
+        return handler->empty();
+    }
+
+    /**
+     * @brief Direct access to the list of components of a given pool.
+     *
+     * The returned pointer is such that range
+     * `[raw<Component>(), raw<Component>() + size<Component>()]` is always a
+     * valid range, even if the container is empty.
+     *
+     * @note
+     * There are no guarantees on the order of the components. Use `begin` and
+     * `end` if you want to iterate the group in the expected order.
+     *
+     * @tparam Component Type of component in which one is interested.
+     * @return A pointer to the array of components.
+     */
+    template<typename Component>
+    Component * raw() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Component> *>(pools)->raw();
+    }
+
+    /**
+     * @brief Direct access to the list of entities of a given pool.
+     *
+     * The returned pointer is such that range
+     * `[data<Component>(), data<Component>() + size<Component>()]` is always a
+     * valid range, even if the container is empty.
+     *
+     * @note
+     * There are no guarantees on the order of the entities. Use `begin` and
+     * `end` if you want to iterate the group in the expected order.
+     *
+     * @tparam Component Type of component in which one is interested.
+     * @return A pointer to the array of entities.
+     */
+    template<typename Component>
+    const entity_type * data() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Component> *>(pools)->data();
+    }
+
+    /**
+     * @brief Direct access to the list of entities.
+     *
+     * The returned pointer is such that range `[data(), data() + size()]` is
+     * always a valid range, even if the container is empty.
+     *
+     * @note
+     * There are no guarantees on the order of the entities. Use `begin` and
+     * `end` if you want to iterate the group in the expected order.
+     *
+     * @return A pointer to the array of entities.
+     */
+    const entity_type * data() const ENTT_NOEXCEPT {
+        return handler->data();
+    }
+
+    /**
+     * @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 group is empty, the returned iterator will be equal to
+     * `end()`.
+     *
+     * @note
+     * Input iterators stay true to the order imposed to the underlying data
+     * structures.
+     *
+     * @return An iterator to the first entity that has the given components.
+     */
+    iterator_type begin() const ENTT_NOEXCEPT {
+        return handler->begin();
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity that has the
+     * given components.
+     *
+     * The returned iterator points to the entity following the last entity that
+     * has the given components. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @note
+     * Input iterators stay true to the order imposed to the underlying data
+     * structures.
+     *
+     * @return An iterator to the entity following the last entity that has the
+     * given components.
+     */
+    iterator_type end() const ENTT_NOEXCEPT {
+        return handler->end();
+    }
+
+    /**
+     * @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.
+     */
+    iterator_type find(const entity_type entt) const ENTT_NOEXCEPT {
+        const auto it = handler->find(entt);
+        return it != end() && *it == entt ? it : end();
+    }
+
+    /**
+     * @brief Returns the identifier that occupies the given position.
+     * @param pos Position of the element to return.
+     * @return The identifier that occupies the given position.
+     */
+    entity_type operator[](const size_type pos) const ENTT_NOEXCEPT {
+        return begin()[pos];
+    }
+
+    /**
+     * @brief Checks if a group contains an entity.
+     * @param entt A valid entity identifier.
+     * @return True if the group contains the given entity, false otherwise.
+     */
+    bool contains(const entity_type entt) const ENTT_NOEXCEPT {
+        return find(entt) != end();
+    }
+
+    /**
+     * @brief Returns the components assigned to the given entity.
+     *
+     * Prefer this function instead of `registry::get` during iterations. It has
+     * far better performance than its companion function.
+     *
+     * @warning
+     * Attempting to use an invalid component type results in a compilation
+     * error. Attempting to use an entity that doesn't belong to the group
+     * results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * group doesn't contain the given entity.
+     *
+     * @tparam Component Types of components to get.
+     * @param entt A valid entity identifier.
+     * @return The components assigned to the entity.
+     */
+    template<typename... Component>
+    decltype(auto) get([[maybe_unused]] const entity_type entt) const ENTT_NOEXCEPT {
+        ENTT_ASSERT(contains(entt));
+
+        if constexpr(sizeof...(Component) == 1) {
+            return (std::get<pool_type<Component> *>(pools)->get(entt), ...);
+        } else {
+            return std::tuple<decltype(get<Component>(entt))...>{get<Component>(entt)...};
+        }
+    }
+
+    /**
+     * @brief Iterates entities and components and applies the given function
+     * object to them.
+     *
+     * The function object is invoked for each entity. It is provided with the
+     * entity itself and a set of references to all its components. The
+     * _constness_ of the components is as requested.<br/>
+     * The signature of the function must be equivalent to one of the following
+     * forms:
+     *
+     * @code{.cpp}
+     * void(const entity_type, Get &...);
+     * void(Get &...);
+     * @endcode
+     *
+     * @note
+     * Empty types aren't explicitly instantiated. Therefore, temporary objects
+     * are returned during iterations. They can be caught only by copy or with
+     * const references.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    inline void each(Func func) const {
+        for(const auto entt: *handler) {
+            if constexpr(std::is_invocable_v<Func, decltype(get<Get>({}))...>) {
+                func(std::get<pool_type<Get> *>(pools)->get(entt)...);
+            } else {
+                func(entt, std::get<pool_type<Get> *>(pools)->get(entt)...);
+            }
+        };
+    }
+
+    /**
+     * @brief Sort the shared pool of entities according to the given component.
+     *
+     * Non-owning groups of the same type share with the registry a pool of
+     * entities with  its own order that doesn't depend on the order of any pool
+     * of components. Users can order the underlying data structure so that it
+     * respects the order of the pool of the given component.
+     *
+     * @note
+     * The shared pool of entities and thus its order is affected by the changes
+     * to each and every pool that it tracks. Therefore changes to those pools
+     * can quickly ruin the order imposed to the pool of entities shared between
+     * the non-owning groups.
+     *
+     * @tparam Component Type of component to use to impose the order.
+     */
+    template<typename Component>
+    void sort() const {
+        handler->respect(*std::get<pool_type<Component> *>(pools));
+    }
+
+private:
+    sparse_set<entity_type> *handler;
+    const std::tuple<pool_type<Get> *...> pools;
+};
+
+
+/**
+ * @brief Owning group.
+ *
+ * Owning groups return all the entities and only the entities that have at
+ * least the given components. Moreover:
+ *
+ * * It's guaranteed that the entity list is tightly packed in memory for fast
+ *   iterations.
+ * * It's guaranteed that the lists of owned components are tightly packed in
+ *   memory for even faster iterations and to allow direct access.
+ * * They stay true to the order of the owned components and all the owned
+ *   components have the same order in memory.
+ *
+ * The more types of components are owned by a group, the faster it is to
+ * iterate them.
+ *
+ * @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).
+ *
+ * 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
+ * Groups 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 all the
+ * groups.
+ * Moreover, sorting an owning group affects all the instance of the same group
+ * (it means that users don't have to call `sort` on each instance to sort all
+ * of them because they share the underlying data structure).
+ *
+ * @warning
+ * Lifetime of a group must overcome the one of the registry that generated it.
+ * In any other case, attempting to use a group results in undefined behavior.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Get Types of components observed by the group.
+ * @tparam Owned Types of components owned by the group.
+ */
+template<typename Entity, typename... Get, typename... Owned>
+class basic_group<Entity, get_t<Get...>, Owned...> {
+    static_assert(sizeof...(Get) + sizeof...(Owned) > 0);
+
+    /*! @brief A registry is allowed to create groups. */
+    friend class basic_registry<Entity>;
+
+    template<typename Component>
+    using pool_type = std::conditional_t<std::is_const_v<Component>, const storage<Entity, std::remove_const_t<Component>>, storage<Entity, Component>>;
+
+    template<typename Component>
+    using component_iterator_type = decltype(std::declval<pool_type<Component>>().begin());
+
+    template<typename Component>
+    decltype(auto) from_index(const typename sparse_set<Entity>::size_type index) {
+        static_assert(!std::is_empty_v<Component>);
+
+        if constexpr(std::disjunction_v<std::is_same<Component, Owned>...>) {
+            return std::as_const(*std::get<pool_type<Component> *>(pools)).raw()[index];
+        } else {
+            return std::as_const(*std::get<pool_type<Component> *>(pools)).get(data()[index]);
+        }
+    }
+
+    template<typename Component>
+    inline auto swap(int, pool_type<Component> *cpool, const std::size_t lhs, const std::size_t rhs)
+    -> decltype(cpool->raw(), void()) {
+        std::swap(cpool->raw()[lhs], cpool->raw()[rhs]);
+        cpool->swap(lhs, rhs);
+    }
+
+    template<typename Component>
+    inline void swap(char, pool_type<Component> *cpool, const std::size_t lhs, const std::size_t rhs) {
+        cpool->swap(lhs, rhs);
+    }
+
+    // we could use pool_type<Type> *..., but vs complains about it and refuses to compile for unknown reasons (likely a bug)
+    basic_group(const typename basic_registry<Entity>::size_type *sz, storage<Entity, std::remove_const_t<Owned>> *... owned, storage<Entity, std::remove_const_t<Get>> *... get) ENTT_NOEXCEPT
+        : length{sz},
+          pools{owned..., get...}
+    {}
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename sparse_set<Entity>::entity_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = typename sparse_set<Entity>::size_type;
+    /*! @brief Input iterator type. */
+    using iterator_type = typename sparse_set<Entity>::iterator_type;
+
+    /**
+     * @brief Returns the number of existing components of the given type.
+     * @tparam Component Type of component of which to return the size.
+     * @return Number of existing components of the given type.
+     */
+    template<typename Component>
+    size_type size() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Component> *>(pools)->size();
+    }
+
+    /**
+     * @brief Returns the number of entities that have the given components.
+     * @return Number of entities that have the given components.
+     */
+    size_type size() const ENTT_NOEXCEPT {
+        return *length;
+    }
+
+    /**
+     * @brief Checks whether the pool of a given component is empty.
+     * @tparam Component Type of component in which one is interested.
+     * @return True if the pool of the given component is empty, false
+     * otherwise.
+     */
+    template<typename Component>
+    bool empty() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Component> *>(pools)->empty();
+    }
+
+    /**
+     * @brief Checks whether the group is empty.
+     * @return True if the group is empty, false otherwise.
+     */
+    bool empty() const ENTT_NOEXCEPT {
+        return !*length;
+    }
+
+    /**
+     * @brief Direct access to the list of components of a given pool.
+     *
+     * The returned pointer is such that range
+     * `[raw<Component>(), raw<Component>() + size<Component>()]` is always a
+     * valid range, even if the container is empty.<br/>
+     * Moreover, in case the group owns the given component, the range
+     * `[raw<Component>(), raw<Component>() + size()]` is such that it contains
+     * the instances that are part of the group itself.
+     *
+     * @note
+     * There are no guarantees on the order of the components. Use `begin` and
+     * `end` if you want to iterate the group in the expected order.
+     *
+     * @tparam Component Type of component in which one is interested.
+     * @return A pointer to the array of components.
+     */
+    template<typename Component>
+    Component * raw() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Component> *>(pools)->raw();
+    }
+
+    /**
+     * @brief Direct access to the list of entities of a given pool.
+     *
+     * The returned pointer is such that range
+     * `[data<Component>(), data<Component>() + size<Component>()]` is always a
+     * valid range, even if the container is empty.<br/>
+     * Moreover, in case the group owns the given component, the range
+     * `[data<Component>(), data<Component>() + size()]` is such that it
+     * contains the entities that are part of the group itself.
+     *
+     * @note
+     * There are no guarantees on the order of the entities. Use `begin` and
+     * `end` if you want to iterate the group in the expected order.
+     *
+     * @tparam Component Type of component in which one is interested.
+     * @return A pointer to the array of entities.
+     */
+    template<typename Component>
+    const entity_type * data() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Component> *>(pools)->data();
+    }
+
+    /**
+     * @brief Direct access to the list of entities.
+     *
+     * The returned pointer is such that range `[data(), data() + size()]` is
+     * always a valid range, even if the container is empty.
+     *
+     * @note
+     * There are no guarantees on the order of the entities. Use `begin` and
+     * `end` if you want to iterate the group in the expected order.
+     *
+     * @return A pointer to the array of entities.
+     */
+    const entity_type * data() const ENTT_NOEXCEPT {
+        return std::get<0>(pools)->data();
+    }
+
+    /**
+     * @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 group is empty, the returned iterator will be equal to
+     * `end()`.
+     *
+     * @note
+     * Input iterators stay true to the order imposed to the underlying data
+     * structures.
+     *
+     * @return An iterator to the first entity that has the given components.
+     */
+    iterator_type begin() const ENTT_NOEXCEPT {
+        return std::get<0>(pools)->sparse_set<entity_type>::end() - *length;
+    }
+
+    /**
+     * @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
+     * Input iterators stay true to the order imposed to the underlying data
+     * structures.
+     *
+     * @return An iterator to the entity following the last entity that has the
+     * given components.
+     */
+    iterator_type end() const ENTT_NOEXCEPT {
+        return std::get<0>(pools)->sparse_set<entity_type>::end();
+    }
+
+    /**
+     * @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.
+     */
+    iterator_type find(const entity_type entt) const ENTT_NOEXCEPT {
+        const auto it = std::get<0>(pools)->find(entt);
+        return it != end() && it >= begin() && *it == entt ? it : end();
+    }
+
+    /**
+     * @brief Returns the identifier that occupies the given position.
+     * @param pos Position of the element to return.
+     * @return The identifier that occupies the given position.
+     */
+    entity_type operator[](const size_type pos) const ENTT_NOEXCEPT {
+        return begin()[pos];
+    }
+
+    /**
+     * @brief Checks if a group contains an entity.
+     * @param entt A valid entity identifier.
+     * @return True if the group contains the given entity, false otherwise.
+     */
+    bool contains(const entity_type entt) const ENTT_NOEXCEPT {
+        return find(entt) != end();
+    }
+
+    /**
+     * @brief Returns the components assigned to the given entity.
+     *
+     * Prefer this function instead of `registry::get` during iterations. It has
+     * far better performance than its companion function.
+     *
+     * @warning
+     * Attempting to use an invalid component type results in a compilation
+     * error. Attempting to use an entity that doesn't belong to the group
+     * results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * group doesn't contain the given entity.
+     *
+     * @tparam Component Types of components to get.
+     * @param entt A valid entity identifier.
+     * @return The components assigned to the entity.
+     */
+    template<typename... Component>
+    decltype(auto) get([[maybe_unused]] const entity_type entt) const ENTT_NOEXCEPT {
+        ENTT_ASSERT(contains(entt));
+
+        if constexpr(sizeof...(Component) == 1) {
+            return (std::get<pool_type<Component> *>(pools)->get(entt), ...);
+        } else {
+            return std::tuple<decltype(get<Component>(entt))...>{get<Component>(entt)...};
+        }
+    }
+
+    /**
+     * @brief Iterates entities and components and applies the given function
+     * object to them.
+     *
+     * The function object is invoked for each entity. It is provided with the
+     * entity itself and a set of references to all its components. The
+     * _constness_ of the components is as requested.<br/>
+     * The signature of the function must be equivalent to one of the following
+     * forms:
+     *
+     * @code{.cpp}
+     * void(const entity_type, Owned &..., Get &...);
+     * void(Owned &..., Get &...);
+     * @endcode
+     *
+     * @note
+     * Empty types aren't explicitly instantiated. Therefore, temporary objects
+     * are returned during iterations. They can be caught only by copy or with
+     * const references.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    inline void each(Func func) const {
+        auto raw = std::make_tuple((std::get<pool_type<Owned> *>(pools)->end() - *length)...);
+        [[maybe_unused]] auto data = std::get<0>(pools)->sparse_set<entity_type>::end() - *length;
+
+        for(auto next = *length; next; --next) {
+            if constexpr(std::is_invocable_v<Func, decltype(get<Owned>({}))..., decltype(get<Get>({}))...>) {
+                if constexpr(sizeof...(Get) == 0) {
+                    func(*(std::get<component_iterator_type<Owned>>(raw)++)...);
+                } else {
+                    const auto entt = *(data++);
+                    func(*(std::get<component_iterator_type<Owned>>(raw)++)..., std::get<pool_type<Get> *>(pools)->get(entt)...);
+                }
+            } else {
+                const auto entt = *(data++);
+                func(entt, *(std::get<component_iterator_type<Owned>>(raw)++)..., std::get<pool_type<Get> *>(pools)->get(entt)...);
+            }
+        }
+    }
+
+    /**
+     * @brief Sort a group according to the given comparison function.
+     *
+     * Sort the group so that iterating it with a couple of iterators returns
+     * entities and components 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 Component &..., const Component &...);
+     * bool(const Entity, const Entity);
+     * @endcode
+     *
+     * Where `Component` are either owned types or not but still such that they
+     * are iterated by the group.<br/>
+     * 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.
+     *
+     * The comparison function object received by the sort function object
+     * hasn't necessarily the type of the one passed along with the other
+     * parameters to this member function.
+     *
+     * @note
+     * Attempting to iterate elements using a raw pointer returned by a call to
+     * either `data` or `raw` gives no guarantees on the order, even though
+     * `sort` has been invoked.
+     *
+     * @tparam Component Optional types of components to compare.
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @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... Component, typename Compare, typename Sort = std_sort, typename... Args>
+    void sort(Compare compare, Sort algo = Sort{}, Args &&... args) {
+        std::vector<size_type> copy(*length);
+        std::iota(copy.begin(), copy.end(), 0);
+
+        if constexpr(sizeof...(Component) == 0) {
+            algo(copy.rbegin(), copy.rend(), [compare = std::move(compare), data = data()](const auto lhs, const auto rhs) {
+                return compare(data[lhs], data[rhs]);
+            }, std::forward<Args>(args)...);
+        } else {
+            algo(copy.rbegin(), copy.rend(), [compare = std::move(compare), this](const auto lhs, const auto rhs) {
+                // useless this-> used to suppress a warning with clang
+                return compare(this->from_index<Component>(lhs)..., this->from_index<Component>(rhs)...);
+            }, std::forward<Args>(args)...);
+        }
+
+        for(size_type pos = 0, last = copy.size(); pos < last; ++pos) {
+            auto curr = pos;
+            auto next = copy[curr];
+
+            while(curr != next) {
+                const auto lhs = copy[curr];
+                const auto rhs = copy[next];
+                (swap<Owned>(0, std::get<pool_type<Owned> *>(pools), lhs, rhs), ...);
+                copy[curr] = curr;
+                curr = next;
+                next = copy[curr];
+            }
+        }
+    }
+
+private:
+    const typename basic_registry<Entity>::size_type *length;
+    const std::tuple<pool_type<Owned> *..., pool_type<Get> *...> pools;
+};
+
+
+}
+
+
+#endif // ENTT_ENTITY_GROUP_HPP

src/entt/entity/helper.hpp

@@ -0,0 +1,211 @@
+#ifndef ENTT_ENTITY_HELPER_HPP
+#define ENTT_ENTITY_HELPER_HPP
+
+
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/hashed_string.hpp"
+#include "../signal/sigh.hpp"
+#include "registry.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Converts a registry to a view.
+ * @tparam Const Constness of the accepted registry.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<bool Const, typename Entity>
+struct as_view {
+    /*! @brief Type of registry to convert. */
+    using registry_type = std::conditional_t<Const, const entt::basic_registry<Entity>, entt::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 Component Type of components used to construct the view.
+     * @return A newly created view.
+     */
+    template<typename... Component>
+    inline operator entt::basic_view<Entity, Component...>() const {
+        return reg.template view<Component...>();
+    }
+
+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 entt::basic_registry<Entity>, entt::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.
+     *
+     * @note
+     * Unfortunately, only full owning groups are supported because of an issue
+     * with msvc that doesn't manage to correctly deduce types.
+     *
+     * @tparam Owned Types of components owned by the group.
+     * @return A newly created group.
+     */
+    template<typename... Owned>
+    inline operator entt::basic_group<Entity, get_t<>, Owned...>() const {
+        return reg.template group<Owned...>();
+    }
+
+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 Dependency function prototype.
+ *
+ * A _dependency function_ is a built-in listener to use to automatically assign
+ * components to an entity when a type has a dependency on some other types.
+ *
+ * This is a prototype function to use to create dependencies.<br/>
+ * It isn't intended for direct use, although nothing forbids using it freely.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Component Type of component that triggers the dependency handler.
+ * @tparam Dependency Types of components to assign to an entity if triggered.
+ * @param reg A valid reference to a registry.
+ * @param entt A valid entity identifier.
+ */
+template<typename Entity, typename Component, typename... Dependency>
+void dependency(basic_registry<Entity> &reg, const Entity entt, const Component &) {
+    ((reg.template has<Dependency>(entt) ? void() : (reg.template assign<Dependency>(entt), void())), ...);
+}
+
+
+/**
+ * @brief Connects a dependency function to the given sink.
+ *
+ * A _dependency function_ is a built-in listener to use to automatically assign
+ * components to an entity when a type has a dependency on some other types.
+ *
+ * The following adds components `a_type` and `another_type` whenever `my_type`
+ * is assigned to an entity:
+ * @code{.cpp}
+ * entt::registry registry;
+ * entt::connect<a_type, another_type>(registry.construction<my_type>());
+ * @endcode
+ *
+ * @tparam Dependency Types of components to assign to an entity if triggered.
+ * @tparam Component Type of component that triggers the dependency handler.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @param sink A sink object properly initialized.
+ */
+template<typename... Dependency, typename Component, typename Entity>
+inline void connect(sink<void(basic_registry<Entity> &, const Entity, Component &)> sink) {
+    sink.template connect<dependency<Entity, Component, Dependency...>>();
+}
+
+
+/**
+ * @brief Disconnects a dependency function from the given sink.
+ *
+ * A _dependency function_ is a built-in listener to use to automatically assign
+ * components to an entity when a type has a dependency on some other types.
+ *
+ * The following breaks the dependency between the component `my_type` and the
+ * components `a_type` and `another_type`:
+ * @code{.cpp}
+ * entt::registry registry;
+ * entt::disconnect<a_type, another_type>(registry.construction<my_type>());
+ * @endcode
+ *
+ * @tparam Dependency Types of components used to create the dependency.
+ * @tparam Component Type of component that triggers the dependency handler.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @param sink A sink object properly initialized.
+ */
+template<typename... Dependency, typename Component, typename Entity>
+inline void disconnect(sink<void(basic_registry<Entity> &, const Entity, Component &)> sink) {
+    sink.template disconnect<dependency<Entity, Component, Dependency...>>();
+}
+
+
+/**
+ * @brief Alias template to ease the assignment of tags to entities.
+ *
+ * If used in combination with hashed strings, it simplifies the assignment of
+ * tags to entities and the use of tags in general where a type would be
+ * required otherwise.<br/>
+ * As an example and where the user defined literal for hashed strings hasn't
+ * been changed:
+ * @code{.cpp}
+ * entt::registry registry;
+ * registry.assign<entt::tag<"enemy"_hs>>(entity);
+ * @endcode
+ *
+ * @note
+ * Tags are empty components and therefore candidates for the empty component
+ * optimization.
+ *
+ * @tparam Value The numeric representation of an instance of hashed string.
+ */
+template<typename hashed_string::hash_type Value>
+using tag = std::integral_constant<typename hashed_string::hash_type, Value>;
+
+
+}
+
+
+#endif // ENTT_ENTITY_HELPER_HPP

src/entt/entity/prototype.hpp

@@ -0,0 +1,483 @@
+#ifndef ENTT_ENTITY_PROTOTYPE_HPP
+#define ENTT_ENTITY_PROTOTYPE_HPP
+
+
+#include <tuple>
+#include <utility>
+#include <cstddef>
+#include <type_traits>
+#include <unordered_map>
+#include "../config/config.h"
+#include "registry.hpp"
+#include "entity.hpp"
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Prototype container for _concepts_.
+ *
+ * A prototype is used to define a _concept_ in terms of components.<br/>
+ * Prototypes act as templates for those specific types of an application which
+ * users would otherwise define through a series of component assignments to
+ * entities. In other words, prototypes can be used to assign components to
+ * entities of a registry at once.
+ *
+ * @note
+ * Components used along with prototypes must be copy constructible. Prototypes
+ * wrap component types with custom types, so they do not interfere with other
+ * users of the registry they were built with.
+ *
+ * @warning
+ * Prototypes directly use their underlying registries to store entities and
+ * components for their purposes. Users must ensure that the lifetime of a
+ * registry and its contents exceed that of the prototypes that use it.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+class basic_prototype {
+    using basic_fn_type = void(const basic_prototype &, basic_registry<Entity> &, const Entity);
+    using component_type = typename basic_registry<Entity>::component_type;
+
+    template<typename Component>
+    struct component_wrapper { Component component; };
+
+    struct component_handler {
+        basic_fn_type *assign_or_replace;
+        basic_fn_type *assign;
+    };
+
+    void release() {
+        if(reg->valid(entity)) {
+            reg->destroy(entity);
+        }
+    }
+
+public:
+    /*! @brief Registry type. */
+    using registry_type = basic_registry<Entity>;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+
+    /**
+     * @brief Constructs a prototype that is bound to a given registry.
+     * @param ref A valid reference to a registry.
+     */
+    basic_prototype(registry_type &ref)
+        : reg{&ref},
+          entity{ref.create()}
+    {}
+
+    /**
+     * @brief Releases all its resources.
+     */
+    ~basic_prototype() {
+        release();
+    }
+
+    /**
+     * @brief Move constructor.
+     *
+     * After prototype 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_prototype(basic_prototype &&other)
+        : handlers{std::move(other.handlers)},
+          reg{other.reg},
+          entity{other.entity}
+    {
+        other.entity = null;
+    }
+
+    /**
+     * @brief Move assignment operator.
+     *
+     * After prototype 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 prototype.
+     */
+    basic_prototype & operator=(basic_prototype &&other) {
+        if(this != &other) {
+            auto tmp{std::move(other)};
+            handlers.swap(tmp.handlers);
+            std::swap(reg, tmp.reg);
+            std::swap(entity, tmp.entity);
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Assigns to or replaces the given component of a prototype.
+     * @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>
+    Component & set(Args &&... args) {
+        component_handler handler;
+
+        handler.assign_or_replace = [](const basic_prototype &proto, registry_type &other, const Entity dst) {
+            const auto &wrapper = proto.reg->template get<component_wrapper<Component>>(proto.entity);
+            other.template assign_or_replace<Component>(dst, wrapper.component);
+        };
+
+        handler.assign = [](const basic_prototype &proto, registry_type &other, const Entity dst) {
+            if(!other.template has<Component>(dst)) {
+                const auto &wrapper = proto.reg->template get<component_wrapper<Component>>(proto.entity);
+                other.template assign<Component>(dst, wrapper.component);
+            }
+        };
+
+        handlers[reg->template type<Component>()] = handler;
+        auto &wrapper = reg->template assign_or_replace<component_wrapper<Component>>(entity, Component{std::forward<Args>(args)...});
+        return wrapper.component;
+    }
+
+    /**
+     * @brief Removes the given component from a prototype.
+     * @tparam Component Type of component to remove.
+     */
+    template<typename Component>
+    void unset() ENTT_NOEXCEPT {
+        reg->template reset<component_wrapper<Component>>(entity);
+        handlers.erase(reg->template type<Component>());
+    }
+
+    /**
+     * @brief Checks if a prototype owns all the given components.
+     * @tparam Component Components for which to perform the check.
+     * @return True if the prototype owns all the components, false otherwise.
+     */
+    template<typename... Component>
+    bool has() const ENTT_NOEXCEPT {
+        return reg->template has<component_wrapper<Component>...>(entity);
+    }
+
+    /**
+     * @brief Returns references to the given components.
+     *
+     * @warning
+     * Attempting to get a component from a prototype that doesn't own it
+     * results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * prototype doesn't own an instance of the given component.
+     *
+     * @tparam Component Types of components to get.
+     * @return References to the components owned by the prototype.
+     */
+    template<typename... Component>
+    decltype(auto) get() const ENTT_NOEXCEPT {
+        if constexpr(sizeof...(Component) == 1) {
+            return (std::as_const(*reg).template get<component_wrapper<Component...>>(entity).component);
+        } else {
+            return std::tuple<std::add_const_t<Component> &...>{get<Component>()...};
+        }
+    }
+
+    /*! @copydoc get */
+    template<typename... Component>
+    inline decltype(auto) get() ENTT_NOEXCEPT {
+        if constexpr(sizeof...(Component) == 1) {
+            return (const_cast<Component &>(std::as_const(*this).template get<Component>()), ...);
+        } else {
+            return std::tuple<Component &...>{get<Component>()...};
+        }
+    }
+
+    /**
+     * @brief Returns pointers to the given components.
+     * @tparam Component Types of components to get.
+     * @return Pointers to the components owned by the prototype.
+     */
+    template<typename... Component>
+    auto try_get() const ENTT_NOEXCEPT {
+        if constexpr(sizeof...(Component) == 1) {
+            const auto *wrapper = reg->template try_get<component_wrapper<Component...>>(entity);
+            return wrapper ? &wrapper->component : nullptr;
+        } else {
+            return std::tuple<std::add_const_t<Component> *...>{try_get<Component>()...};
+        }
+    }
+
+    /*! @copydoc try_get */
+    template<typename... Component>
+    inline auto try_get() ENTT_NOEXCEPT {
+        if constexpr(sizeof...(Component) == 1) {
+            return (const_cast<Component *>(std::as_const(*this).template try_get<Component>()), ...);
+        } else {
+            return std::tuple<Component *...>{try_get<Component>()...};
+        }
+    }
+
+    /**
+     * @brief Creates a new entity using a given prototype.
+     *
+     * Utility shortcut, equivalent to the following snippet:
+     *
+     * @code{.cpp}
+     * const auto entity = registry.create();
+     * prototype(registry, entity);
+     * @endcode
+     *
+     * @note
+     * The registry may or may not be different from the one already used by
+     * the prototype. There is also an overload that directly uses the
+     * underlying registry.
+     *
+     * @param other A valid reference to a registry.
+     * @return A valid entity identifier.
+     */
+    entity_type create(registry_type &other) const {
+        const auto entt = other.create();
+        assign(other, entt);
+        return entt;
+    }
+
+    /**
+     * @brief Creates a new entity using a given prototype.
+     *
+     * Utility shortcut, equivalent to the following snippet:
+     *
+     * @code{.cpp}
+     * const auto entity = registry.create();
+     * prototype(entity);
+     * @endcode
+     *
+     * @note
+     * This overload directly uses the underlying registry as a working space.
+     * Therefore, the components of the prototype and of the entity will share
+     * the same registry.
+     *
+     * @return A valid entity identifier.
+     */
+    inline entity_type create() const {
+        return create(*reg);
+    }
+
+    /**
+     * @brief Assigns the components of a prototype to a given entity.
+     *
+     * Assigning a prototype to an entity won't overwrite existing components
+     * under any circumstances.<br/>
+     * In other words, only those components that the entity doesn't own yet are
+     * copied over. All the other components remain unchanged.
+     *
+     * @note
+     * The registry may or may not be different from the one already used by
+     * the prototype. There is also an overload that directly uses the
+     * underlying registry.
+     *
+     * @warning
+     * Attempting to use an invalid entity results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode in case of
+     * invalid entity.
+     *
+     * @param other A valid reference to a registry.
+     * @param dst A valid entity identifier.
+     */
+    void assign(registry_type &other, const entity_type dst) const {
+        for(auto &handler: handlers) {
+            handler.second.assign(*this, other, dst);
+        }
+    }
+
+    /**
+     * @brief Assigns the components of a prototype to a given entity.
+     *
+     * Assigning a prototype to an entity won't overwrite existing components
+     * under any circumstances.<br/>
+     * In other words, only those components that the entity doesn't own yet are
+     * copied over. All the other components remain unchanged.
+     *
+     * @note
+     * This overload directly uses the underlying registry as a working space.
+     * Therefore, the components of the prototype and of the entity will share
+     * the same registry.
+     *
+     * @warning
+     * Attempting to use an invalid entity results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode in case of
+     * invalid entity.
+     *
+     * @param dst A valid entity identifier.
+     */
+    inline void assign(const entity_type dst) const {
+        assign(*reg, dst);
+    }
+
+    /**
+     * @brief Assigns or replaces the components of a prototype for an entity.
+     *
+     * Existing components are overwritten, if any. All the other components
+     * will be copied over to the target entity.
+     *
+     * @note
+     * The registry may or may not be different from the one already used by
+     * the prototype. There is also an overload that directly uses the
+     * underlying registry.
+     *
+     * @warning
+     * Attempting to use an invalid entity results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode in case of
+     * invalid entity.
+     *
+     * @param other A valid reference to a registry.
+     * @param dst A valid entity identifier.
+     */
+    void assign_or_replace(registry_type &other, const entity_type dst) const {
+        for(auto &handler: handlers) {
+            handler.second.assign_or_replace(*this, other, dst);
+        }
+    }
+
+    /**
+     * @brief Assigns or replaces the components of a prototype for an entity.
+     *
+     * Existing components are overwritten, if any. All the other components
+     * will be copied over to the target entity.
+     *
+     * @note
+     * This overload directly uses the underlying registry as a working space.
+     * Therefore, the components of the prototype and of the entity will share
+     * the same registry.
+     *
+     * @warning
+     * Attempting to use an invalid entity results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode in case of
+     * invalid entity.
+     *
+     * @param dst A valid entity identifier.
+     */
+    inline void assign_or_replace(const entity_type dst) const {
+        assign_or_replace(*reg, dst);
+    }
+
+    /**
+     * @brief Assigns the components of a prototype to an entity.
+     *
+     * Assigning a prototype to an entity won't overwrite existing components
+     * under any circumstances.<br/>
+     * In other words, only the components that the entity doesn't own yet are
+     * copied over. All the other components remain unchanged.
+     *
+     * @note
+     * The registry may or may not be different from the one already used by
+     * the prototype. There is also an overload that directly uses the
+     * underlying registry.
+     *
+     * @warning
+     * Attempting to use an invalid entity results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode in case of
+     * invalid entity.
+     *
+     * @param other A valid reference to a registry.
+     * @param dst A valid entity identifier.
+     */
+    inline void operator()(registry_type &other, const entity_type dst) const ENTT_NOEXCEPT {
+        assign(other, dst);
+    }
+
+    /**
+     * @brief Assigns the components of a prototype to an entity.
+     *
+     * Assigning a prototype to an entity won't overwrite existing components
+     * under any circumstances.<br/>
+     * In other words, only the components that the entity doesn't own yet are
+     * copied over. All the other components remain unchanged.
+     *
+     * @note
+     * This overload directly uses the underlying registry as a working space.
+     * Therefore, the components of the prototype and of the entity will share
+     * the same registry.
+     *
+     * @warning
+     * Attempting to use an invalid entity results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode in case of
+     * invalid entity.
+     *
+     * @param dst A valid entity identifier.
+     */
+    inline void operator()(const entity_type dst) const ENTT_NOEXCEPT {
+        assign(*reg, dst);
+    }
+
+    /**
+     * @brief Creates a new entity using a given prototype.
+     *
+     * Utility shortcut, equivalent to the following snippet:
+     *
+     * @code{.cpp}
+     * const auto entity = registry.create();
+     * prototype(registry, entity);
+     * @endcode
+     *
+     * @note
+     * The registry may or may not be different from the one already used by
+     * the prototype. There is also an overload that directly uses the
+     * underlying registry.
+     *
+     * @param other A valid reference to a registry.
+     * @return A valid entity identifier.
+     */
+    inline entity_type operator()(registry_type &other) const ENTT_NOEXCEPT {
+        return create(other);
+    }
+
+    /**
+     * @brief Creates a new entity using a given prototype.
+     *
+     * Utility shortcut, equivalent to the following snippet:
+     *
+     * @code{.cpp}
+     * const auto entity = registry.create();
+     * prototype(entity);
+     * @endcode
+     *
+     * @note
+     * This overload directly uses the underlying registry as a working space.
+     * Therefore, the components of the prototype and of the entity will share
+     * the same registry.
+     *
+     * @return A valid entity identifier.
+     */
+    inline entity_type operator()() const ENTT_NOEXCEPT {
+        return create(*reg);
+    }
+
+    /**
+     * @brief Returns a reference to the underlying registry.
+     * @return A reference to the underlying registry.
+     */
+    inline const registry_type & backend() const ENTT_NOEXCEPT {
+        return *reg;
+    }
+
+    /*! @copydoc backend */
+    inline registry_type & backend() ENTT_NOEXCEPT {
+        return const_cast<registry_type &>(std::as_const(*this).backend());
+    }
+
+private:
+    std::unordered_map<component_type, component_handler> handlers;
+    registry_type *reg;
+    entity_type entity;
+};
+
+
+}
+
+
+#endif // ENTT_ENTITY_PROTOTYPE_HPP

src/entt/entity/runtime_view.hpp

@@ -0,0 +1,275 @@
+#ifndef ENTT_ENTITY_RUNTIME_VIEW_HPP
+#define ENTT_ENTITY_RUNTIME_VIEW_HPP
+
+
+#include <iterator>
+#include <cassert>
+#include <vector>
+#include <utility>
+#include <algorithm>
+#include "../config/config.h"
+#include "sparse_set.hpp"
+#include "entity.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).
+ *
+ * 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 overcome the one 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_type = typename sparse_set<Entity>::iterator_type;
+    using extent_type = typename sparse_set<Entity>::size_type;
+    using traits_type = entt_traits<Entity>;
+
+    class iterator {
+        friend class basic_runtime_view<Entity>;
+
+        iterator(underlying_iterator_type first, underlying_iterator_type last, const sparse_set<Entity> * const *others, const sparse_set<Entity> * const *length, extent_type ext) ENTT_NOEXCEPT
+            : begin{first},
+              end{last},
+              from{others},
+              to{length},
+              extent{ext}
+        {
+            if(begin != end && !valid()) {
+                ++(*this);
+            }
+        }
+
+        bool valid() const ENTT_NOEXCEPT {
+            const auto entt = *begin;
+            const auto sz = size_type(entt & traits_type::entity_mask);
+
+            return sz < extent && std::all_of(from, to, [entt](const auto *view) {
+                return view->has(entt);
+            });
+        }
+
+    public:
+        using difference_type = typename underlying_iterator_type::difference_type;
+        using value_type = typename underlying_iterator_type::value_type;
+        using pointer = typename underlying_iterator_type::pointer;
+        using reference = typename underlying_iterator_type::reference;
+        using iterator_category = std::forward_iterator_tag;
+
+        iterator() ENTT_NOEXCEPT = default;
+
+        iterator & operator++() ENTT_NOEXCEPT {
+            return (++begin != end && !valid()) ? ++(*this) : *this;
+        }
+
+        iterator operator++(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        bool operator==(const iterator &other) const ENTT_NOEXCEPT {
+            return other.begin == begin;
+        }
+
+        inline bool operator!=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+        pointer operator->() const ENTT_NOEXCEPT {
+            return begin.operator->();
+        }
+
+        inline reference operator*() const ENTT_NOEXCEPT {
+            return *operator->();
+        }
+
+    private:
+        underlying_iterator_type begin;
+        underlying_iterator_type end;
+        const sparse_set<Entity> * const *from;
+        const sparse_set<Entity> * const *to;
+        extent_type extent;
+    };
+
+    basic_runtime_view(std::vector<const sparse_set<Entity> *> others) ENTT_NOEXCEPT
+        : pools{std::move(others)}
+    {
+        const auto it = std::min_element(pools.begin(), pools.end(), [](const auto *lhs, const auto *rhs) {
+            return (!lhs && rhs) || (lhs && rhs && lhs->size() < rhs->size());
+        });
+
+        // brings the best candidate (if any) on front of the vector
+        std::rotate(pools.begin(), it, pools.end());
+    }
+
+    extent_type min() const ENTT_NOEXCEPT {
+        extent_type extent{};
+
+        if(valid()) {
+            const auto it = std::min_element(pools.cbegin(), pools.cend(), [](const auto *lhs, const auto *rhs) {
+                return lhs->extent() < rhs->extent();
+            });
+
+            extent = (*it)->extent();
+        }
+
+        return extent;
+    }
+
+    inline bool valid() const ENTT_NOEXCEPT {
+        return !pools.empty() && pools.front();
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename sparse_set<Entity>::entity_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = typename sparse_set<Entity>::size_type;
+    /*! @brief Input iterator type. */
+    using iterator_type = iterator;
+
+    /**
+     * @brief Estimates the number of entities that have the given components.
+     * @return Estimated number of entities that have the given components.
+     */
+    size_type size() const ENTT_NOEXCEPT {
+        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.
+     */
+    bool empty() const ENTT_NOEXCEPT {
+        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
+     * Input iterators stay true to the order imposed to the underlying data
+     * structures.
+     *
+     * @return An iterator to the first entity that has the given components.
+     */
+    iterator_type begin() const ENTT_NOEXCEPT {
+        iterator_type it{};
+
+        if(valid()) {
+            const auto &pool = *pools.front();
+            const auto * const *data = pools.data();
+            it = { pool.begin(), pool.end(), data + 1, data + pools.size(), min() };
+        }
+
+        return it;
+    }
+
+    /**
+     * @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
+     * Input iterators stay true to the order imposed to the underlying data
+     * structures.
+     *
+     * @return An iterator to the entity following the last entity that has the
+     * given components.
+     */
+    iterator_type end() const ENTT_NOEXCEPT {
+        iterator_type it{};
+
+        if(valid()) {
+            const auto &pool = *pools.front();
+            it = { pool.end(), pool.end(), nullptr, nullptr, min() };
+        }
+
+        return it;
+    }
+
+    /**
+     * @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.
+     */
+    bool contains(const entity_type entt) const ENTT_NOEXCEPT {
+        return valid() && std::all_of(pools.cbegin(), pools.cend(), [entt](const auto *view) {
+            return view->has(entt) && view->data()[view->get(entt)] == 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 {
+        std::for_each(begin(), end(), func);
+    }
+
+private:
+    std::vector<const sparse_set<Entity> *> pools;
+};
+
+
+}
+
+
+#endif // ENTT_ENTITY_RUNTIME_VIEW_HPP

src/entt/entity/snapshot.hpp

@@ -0,0 +1,589 @@
+#ifndef ENTT_ENTITY_SNAPSHOT_HPP
+#define ENTT_ENTITY_SNAPSHOT_HPP
+
+
+#include <array>
+#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 follow_fn_type = Entity(const basic_registry<Entity> &, const Entity);
+
+    basic_snapshot(const basic_registry<Entity> *source, Entity init, follow_fn_type *fn) ENTT_NOEXCEPT
+        : reg{source},
+          seed{init},
+          follow{fn}
+    {}
+
+    template<typename Component, typename Archive, typename It>
+    void get(Archive &archive, std::size_t sz, It first, It last) const {
+        archive(static_cast<Entity>(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 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 that are still in use.
+     *
+     * Entities are serialized along with their versions. Destroyed entities are
+     * not taken in consideration by this function.
+     *
+     * @tparam Archive Type of output archive.
+     * @param archive A valid reference to an output archive.
+     * @return An object of this type to continue creating the snapshot.
+     */
+    template<typename Archive>
+    const basic_snapshot & entities(Archive &archive) const {
+        archive(static_cast<Entity>(reg->alive()));
+        reg->each([&archive](const auto entt) { archive(entt); });
+        return *this;
+    }
+
+    /**
+     * @brief Puts aside destroyed entities.
+     *
+     * Entities are serialized along with their versions. Entities that are
+     * still in use are not taken in consideration by this function.
+     *
+     * @tparam Archive Type of output archive.
+     * @param archive A valid reference to an output archive.
+     * @return An object of this type to continue creating the snapshot.
+     */
+    template<typename Archive>
+    const basic_snapshot & destroyed(Archive &archive) const {
+        auto size = reg->size() - reg->alive();
+        archive(static_cast<Entity>(size));
+
+        if(size) {
+            auto curr = seed;
+            archive(curr);
+
+            for(--size; size; --size) {
+                curr = follow(*reg, curr);
+                archive(curr);
+            }
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Puts aside the given components.
+     *
+     * Each instance is serialized together with the entity to which it belongs.
+     * Entities are serialized along with their versions.
+     *
+     * @tparam Component Types of components to serialize.
+     * @tparam Archive Type of output archive.
+     * @param archive A valid reference to an output archive.
+     * @return An object of this type to continue creating the snapshot.
+     */
+    template<typename... Component, typename Archive>
+    const basic_snapshot & component(Archive &archive) const {
+        if constexpr(sizeof...(Component) == 1) {
+            const auto sz = reg->template size<Component...>();
+            const auto *entities = reg->template data<Component...>();
+
+            archive(static_cast<Entity>(sz));
+
+            for(std::remove_const_t<decltype(sz)> i{}; i < sz; ++i) {
+                const auto entt = entities[i];
+
+                if constexpr(std::is_empty_v<Component...>) {
+                    archive(entt);
+                } else {
+                    archive(entt, reg->template get<Component...>(entt));
+                }
+            };
+        } else {
+            (component<Component>(archive), ...);
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Puts aside the given components for the entities in a range.
+     *
+     * Each instance is serialized together with the entity to which it belongs.
+     * Entities are serialized along with their versions.
+     *
+     * @tparam Component Types of components to serialize.
+     * @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::make_index_sequence<sizeof...(Component)>{});
+        return *this;
+    }
+
+private:
+    const basic_registry<Entity> *reg;
+    const Entity seed;
+    follow_fn_type *follow;
+};
+
+
+/**
+ * @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 force_fn_type = void(basic_registry<Entity> &, const Entity, const bool);
+
+    basic_snapshot_loader(basic_registry<Entity> *source, force_fn_type *fn) ENTT_NOEXCEPT
+        : reg{source},
+          force{fn}
+    {
+        // to restore a snapshot as a whole requires a clean registry
+        ENTT_ASSERT(reg->empty());
+    }
+
+    template<typename Archive>
+    void assure(Archive &archive, bool destroyed) const {
+        Entity length{};
+        archive(length);
+
+        while(length--) {
+            Entity entt{};
+            archive(entt);
+            force(*reg, entt, destroyed);
+        }
+    }
+
+    template<typename Type, typename Archive, typename... Args>
+    void assign(Archive &archive, Args... args) const {
+        Entity length{};
+        archive(length);
+
+        while(length--) {
+            static constexpr auto destroyed = false;
+            Entity entt{};
+
+            if constexpr(std::is_empty_v<Type>) {
+                archive(entt);
+                force(*reg, entt, destroyed);
+                reg->template assign<Type>(args..., entt);
+            } else {
+                Type instance{};
+                archive(entt, instance);
+                force(*reg, entt, destroyed);
+                reg->template assign<Type>(args..., entt, std::as_const(instance));
+            }
+        }
+    }
+
+public:
+    /*! @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 {
+        static constexpr auto destroyed = false;
+        assure(archive, destroyed);
+        return *this;
+    }
+
+    /**
+     * @brief Restores entities that were destroyed during serialization.
+     *
+     * This function restores the entities that were destroyed during
+     * serialization and gives them the versions they originally had.
+     *
+     * @tparam Archive Type of input archive.
+     * @param archive A valid reference to an input archive.
+     * @return A valid loader to continue restoring data.
+     */
+    template<typename Archive>
+    const basic_snapshot_loader & destroyed(Archive &archive) const {
+        static constexpr auto destroyed = true;
+        assure(archive, destroyed);
+        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> *reg;
+    force_fn_type *force;
+};
+
+
+/**
+ * @brief Utility class for _continuous loading_.
+ *
+ * A _continuous loader_ is designed to load data from a source registry to a
+ * (possibly) non-empty destination. The loader can accomodate in a registry
+ * 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 Other, typename Type, typename Member>
+    void update(Other &instance, Member Type:: *member) {
+        if constexpr(!std::is_same_v<Other, Type>) {
+            return;
+        } else if constexpr(std::is_same_v<Member, Entity>) {
+            instance.*member = map(instance.*member);
+        } else {
+            // maybe a container? let's try...
+            for(auto &entt: instance.*member) {
+                entt = map(entt);
+            }
+        }
+    }
+
+    template<typename Archive>
+    void assure(Archive &archive, void(basic_continuous_loader:: *member)(Entity)) {
+        Entity length{};
+        archive(length);
+
+        while(length--) {
+            Entity entt{};
+            archive(entt);
+            (this->*member)(entt);
+        }
+    }
+
+    template<typename Component>
+    void reset() {
+        for(auto &&ref: remloc) {
+            const auto local = ref.second.first;
+
+            if(reg->valid(local)) {
+                reg->template reset<Component>(local);
+            }
+        }
+    }
+
+    template<typename Other, typename Archive, typename... Type, typename... Member>
+    void assign(Archive &archive, [[maybe_unused]] Member Type:: *... member) {
+        Entity length{};
+        archive(length);
+
+        while(length--) {
+            Entity entt{};
+
+            if constexpr(std::is_empty_v<Other>) {
+                archive(entt);
+                restore(entt);
+                reg->template assign_or_replace<Other>(map(entt));
+            } else {
+                Other instance{};
+                archive(entt, instance);
+                (update(instance, member), ...);
+                restore(entt);
+                reg->template assign_or_replace<Other>(map(entt), std::as_const(instance));
+            }
+        }
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+
+    /**
+     * @brief Constructs a loader 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) {
+        assure(archive, &basic_continuous_loader::restore);
+        return *this;
+    }
+
+    /**
+     * @brief Restores entities that were destroyed during serialization.
+     *
+     * This function restores the entities that were destroyed during
+     * serialization and creates local counterparts for them if required.
+     *
+     * @tparam Archive Type of input archive.
+     * @param archive A valid reference to an input archive.
+     * @return A non-const reference to this loader.
+     */
+    template<typename Archive>
+    basic_continuous_loader & destroyed(Archive &archive) {
+        assure(archive, &basic_continuous_loader::destroy);
+        return *this;
+    }
+
+    /**
+     * @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) {
+        (reset<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.
+     */
+    bool has(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.
+     */
+    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, std::pair<Entity, bool>> remloc;
+    basic_registry<Entity> *reg;
+};
+
+
+}
+
+
+#endif // ENTT_ENTITY_SNAPSHOT_HPP

src/entt/entity/sparse_set.hpp

@@ -0,0 +1,535 @@
+#ifndef ENTT_ENTITY_SPARSE_SET_HPP
+#define ENTT_ENTITY_SPARSE_SET_HPP
+
+
+#include <algorithm>
+#include <iterator>
+#include <utility>
+#include <vector>
+#include <memory>
+#include <cstddef>
+#include "../config/config.h"
+#include "entity.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
+ * 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.
+ *
+ * @note
+ * Internal data structures arrange elements to maximize performance. Because of
+ * that, there are no guarantees that elements have the expected order when
+ * iterate directly the internal packed array (see `data` and `size` member
+ * functions for that). Use `begin` and `end` instead.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+class sparse_set {
+    using traits_type = entt_traits<Entity>;
+
+    static_assert(ENTT_PAGE_SIZE && ((ENTT_PAGE_SIZE & (ENTT_PAGE_SIZE - 1)) == 0));
+    static constexpr auto entt_per_page = ENTT_PAGE_SIZE / sizeof(typename entt_traits<Entity>::entity_type);
+
+    class iterator {
+        friend class sparse_set<Entity>;
+
+        using direct_type = const std::vector<Entity>;
+        using index_type = typename traits_type::difference_type;
+
+        iterator(direct_type *ref, const index_type idx) ENTT_NOEXCEPT
+            : direct{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;
+
+        iterator() ENTT_NOEXCEPT = default;
+
+        iterator & operator++() ENTT_NOEXCEPT {
+            return --index, *this;
+        }
+
+        iterator operator++(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        iterator & operator--() ENTT_NOEXCEPT {
+            return ++index, *this;
+        }
+
+        iterator operator--(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return --(*this), orig;
+        }
+
+        iterator & operator+=(const difference_type value) ENTT_NOEXCEPT {
+            index -= value;
+            return *this;
+        }
+
+        iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
+            return iterator{direct, index-value};
+        }
+
+        inline iterator & operator-=(const difference_type value) ENTT_NOEXCEPT {
+            return (*this += -value);
+        }
+
+        inline iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
+            return (*this + -value);
+        }
+
+        difference_type operator-(const iterator &other) const ENTT_NOEXCEPT {
+            return other.index - index;
+        }
+
+        reference operator[](const difference_type value) const ENTT_NOEXCEPT {
+            const auto pos = size_type(index-value-1);
+            return (*direct)[pos];
+        }
+
+        bool operator==(const iterator &other) const ENTT_NOEXCEPT {
+            return other.index == index;
+        }
+
+        inline bool operator!=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+        bool operator<(const iterator &other) const ENTT_NOEXCEPT {
+            return index > other.index;
+        }
+
+        bool operator>(const iterator &other) const ENTT_NOEXCEPT {
+            return index < other.index;
+        }
+
+        inline bool operator<=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this > other);
+        }
+
+        inline bool operator>=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this < other);
+        }
+
+        pointer operator->() const ENTT_NOEXCEPT {
+            const auto pos = size_type(index-1);
+            return &(*direct)[pos];
+        }
+
+        inline reference operator*() const ENTT_NOEXCEPT {
+            return *operator->();
+        }
+
+    private:
+        direct_type *direct;
+        index_type index;
+    };
+
+    void assure(const std::size_t page) {
+        if(!(page < reverse.size())) {
+            reverse.resize(page+1);
+        }
+
+        if(!reverse[page].first) {
+            reverse[page].first = std::make_unique<entity_type[]>(entt_per_page);
+            // null is safe in all cases for our purposes
+            std::fill_n(reverse[page].first.get(), entt_per_page, null);
+        }
+    }
+
+    auto index(const Entity entt) const ENTT_NOEXCEPT {
+        const auto identifier = entt & traits_type::entity_mask;
+        const auto page = size_type(identifier / entt_per_page);
+        const auto offset = size_type(identifier & (entt_per_page - 1));
+        return std::make_pair(page, offset);
+    }
+
+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_type = iterator;
+
+    /*! @brief Default constructor. */
+    sparse_set() = default;
+
+    /**
+     * @brief Copy constructor.
+     * @param other The instance to copy from.
+     */
+    sparse_set(const sparse_set &other)
+        : reverse{},
+          direct{other.direct}
+    {
+        for(size_type i = {}, last = other.reverse.size(); i < last; ++i) {
+            if(other.reverse[i].first) {
+                assure(i);
+                std::copy_n(other.reverse[i].first.get(), entt_per_page, reverse[i].first.get());
+                reverse[i].second = other.reverse[i].second;
+            }
+        }
+    }
+
+    /*! @brief Default move constructor. */
+    sparse_set(sparse_set &&) = default;
+
+    /*! @brief Default destructor. */
+    virtual ~sparse_set() ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Copy assignment operator.
+     * @param other The instance to copy from.
+     * @return This sparse set.
+     */
+    sparse_set & operator=(const sparse_set &other) {
+        if(&other != this) {
+            auto tmp{other};
+            *this = std::move(tmp);
+        }
+
+        return *this;
+    }
+
+    /*! @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.
+     */
+    virtual void reserve(const size_type cap) {
+        direct.reserve(cap);
+    }
+
+    /**
+     * @brief Returns the number of elements that a sparse set has currently
+     * allocated space for.
+     * @return Capacity of the sparse set.
+     */
+    size_type capacity() const ENTT_NOEXCEPT {
+        return direct.capacity();
+    }
+
+    /*! @brief Requests the removal of unused capacity. */
+    virtual void shrink_to_fit() {
+        while(!reverse.empty() && !reverse.back().second) {
+            reverse.pop_back();
+        }
+
+        for(auto &&data: reverse) {
+            if(!data.second) {
+                data.first.reset();
+            }
+        }
+
+        reverse.shrink_to_fit();
+        direct.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.
+     */
+    size_type extent() const ENTT_NOEXCEPT {
+        return reverse.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.
+     */
+    size_type size() const ENTT_NOEXCEPT {
+        return direct.size();
+    }
+
+    /**
+     * @brief Checks whether a sparse set is empty.
+     * @return True if the sparse set is empty, false otherwise.
+     */
+    bool empty() const ENTT_NOEXCEPT {
+        return direct.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
+     * There are no guarantees on the order, even though `respect` has been
+     * previously invoked. Internal data structures arrange elements to maximize
+     * performance. Accessing them directly gives a performance boost but less
+     * guarantees. Use `begin` and `end` if you want to iterate the sparse set
+     * in the expected order.
+     *
+     * @return A pointer to the internal packed array.
+     */
+    const entity_type * data() const ENTT_NOEXCEPT {
+        return direct.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()`.
+     *
+     * @note
+     * Input iterators stay true to the order imposed by a call to `respect`.
+     *
+     * @return An iterator to the first entity of the internal packed array.
+     */
+    iterator_type begin() const ENTT_NOEXCEPT {
+        const typename traits_type::difference_type pos = direct.size();
+        return iterator_type{&direct, 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.
+     *
+     * @note
+     * Input iterators stay true to the order imposed by a call to `respect`.
+     *
+     * @return An iterator to the element following the last entity of the
+     * internal packed array.
+     */
+    iterator_type end() const ENTT_NOEXCEPT {
+        return iterator_type{&direct, {}};
+    }
+
+    /**
+     * @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.
+     */
+    iterator_type find(const entity_type entt) const ENTT_NOEXCEPT {
+        return has(entt) ? --(end() - get(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.
+     */
+    bool has(const entity_type entt) const ENTT_NOEXCEPT {
+        auto [page, offset] = index(entt);
+        // testing against null permits to avoid accessing the direct vector
+        return (page < reverse.size() && reverse[page].second && reverse[page].first[offset] != 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.
+     */
+    size_type get(const entity_type entt) const ENTT_NOEXCEPT {
+        ENTT_ASSERT(has(entt));
+        auto [page, offset] = index(entt);
+        return size_type(reverse[page].first[offset]);
+    }
+
+    /**
+     * @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 construct(const entity_type entt) {
+        ENTT_ASSERT(!has(entt));
+        auto [page, offset] = index(entt);
+        assure(page);
+        reverse[page].first[offset] = entity_type(direct.size());
+        reverse[page].second++;
+        direct.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 forward 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 batch(It first, It last) {
+        std::for_each(first, last, [next = entity_type(direct.size()), this](const auto entt) mutable {
+            ENTT_ASSERT(!has(entt));
+            auto [page, offset] = index(entt);
+            assure(page);
+            reverse[page].first[offset] = next++;
+            reverse[page].second++;
+        });
+
+        direct.insert(direct.end(), first, last);
+    }
+
+    /**
+     * @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.
+     */
+    virtual void destroy(const entity_type entt) {
+        ENTT_ASSERT(has(entt));
+        auto [from_page, from_offset] = index(entt);
+        auto [to_page, to_offset] = index(direct.back());
+        direct[size_type(reverse[from_page].first[from_offset])] = direct.back();
+        reverse[to_page].first[to_offset] = reverse[from_page].first[from_offset];
+        reverse[from_page].first[from_offset] = null;
+        reverse[from_page].second--;
+        direct.pop_back();
+    }
+
+    /**
+     * @brief Swaps the position of 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 position within the sparse set.
+     * @param rhs A valid position within the sparse set.
+     */
+    void swap(const size_type lhs, const size_type rhs) ENTT_NOEXCEPT {
+        ENTT_ASSERT(lhs < direct.size());
+        ENTT_ASSERT(rhs < direct.size());
+        auto [src_page, src_offset] = index(direct[lhs]);
+        auto [dst_page, dst_offset] = index(direct[rhs]);
+        std::swap(reverse[src_page].first[src_offset], reverse[dst_page].first[dst_offset]);
+        std::swap(direct[lhs], direct[rhs]);
+    }
+
+    /**
+     * @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 guarantess 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.
+     *
+     * @note
+     * Attempting to iterate elements using the raw pointer returned by `data`
+     * gives no guarantees on the order, even though `respect` has been invoked.
+     *
+     * @param other The sparse sets that imposes the order of the entities.
+     */
+    virtual void respect(const sparse_set &other) ENTT_NOEXCEPT {
+        const auto to = other.end();
+        auto from = other.begin();
+
+        size_type pos = direct.size() - 1;
+
+        while(pos && from != to) {
+            if(has(*from)) {
+                if(*from != direct[pos]) {
+                    swap(pos, get(*from));
+                }
+
+                --pos;
+            }
+
+            ++from;
+        }
+    }
+
+    /**
+     * @brief Resets a sparse set.
+     */
+    virtual void reset() {
+        reverse.clear();
+        direct.clear();
+    }
+
+private:
+    std::vector<std::pair<std::unique_ptr<entity_type[]>, size_type>> reverse;
+    std::vector<entity_type> direct;
+};
+
+
+}
+
+
+#endif // ENTT_ENTITY_SPARSE_SET_HPP

src/entt/entity/storage.hpp

@@ -0,0 +1,698 @@
+#ifndef ENTT_ENTITY_STORAGE_HPP
+#define ENTT_ENTITY_STORAGE_HPP
+
+
+#include <algorithm>
+#include <iterator>
+#include <numeric>
+#include <utility>
+#include <vector>
+#include <cstddef>
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/algorithm.hpp"
+#include "sparse_set.hpp"
+#include "entity.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 input iterators.
+ *
+ * @note
+ * Internal data structures arrange elements to maximize performance. Because of
+ * that, there are no guarantees that elements have the expected order when
+ * iterate directly the internal packed array (see `raw` and `size` member
+ * functions for that). Use `begin` and `end` instead.
+ *
+ * @warning
+ * Empty types aren't explicitly instantiated. Temporary objects are returned in
+ * place of the instances of the components and raw access isn't available for
+ * them.
+ *
+ * @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 basic_storage: public sparse_set<Entity> {
+    using underlying_type = sparse_set<Entity>;
+    using traits_type = entt_traits<Entity>;
+
+    template<bool Const>
+    class iterator {
+        friend class basic_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;
+
+        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;
+
+        iterator() ENTT_NOEXCEPT = default;
+
+        iterator & operator++() ENTT_NOEXCEPT {
+            return --index, *this;
+        }
+
+        iterator operator++(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        iterator & operator--() ENTT_NOEXCEPT {
+            return ++index, *this;
+        }
+
+        iterator operator--(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return --(*this), orig;
+        }
+
+        iterator & operator+=(const difference_type value) ENTT_NOEXCEPT {
+            index -= value;
+            return *this;
+        }
+
+        iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
+            return iterator{instances, index-value};
+        }
+
+        inline iterator & operator-=(const difference_type value) ENTT_NOEXCEPT {
+            return (*this += -value);
+        }
+
+        inline iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
+            return (*this + -value);
+        }
+
+        difference_type operator-(const iterator &other) const ENTT_NOEXCEPT {
+            return other.index - index;
+        }
+
+        reference operator[](const difference_type value) const ENTT_NOEXCEPT {
+            const auto pos = size_type(index-value-1);
+            return (*instances)[pos];
+        }
+
+        bool operator==(const iterator &other) const ENTT_NOEXCEPT {
+            return other.index == index;
+        }
+
+        inline bool operator!=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+        bool operator<(const iterator &other) const ENTT_NOEXCEPT {
+            return index > other.index;
+        }
+
+        bool operator>(const iterator &other) const ENTT_NOEXCEPT {
+            return index < other.index;
+        }
+
+        inline bool operator<=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this > other);
+        }
+
+        inline bool operator>=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this < other);
+        }
+
+        pointer operator->() const ENTT_NOEXCEPT {
+            const auto pos = size_type(index-1);
+            return &(*instances)[pos];
+        }
+
+        inline 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 = typename underlying_type::entity_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = typename underlying_type::size_type;
+    /*! @brief Random access iterator type. */
+    using iterator_type = iterator<false>;
+    /*! @brief Constant random access iterator type. */
+    using const_iterator_type = iterator<true>;
+
+    /**
+     * @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) override {
+        underlying_type::reserve(cap);
+        instances.reserve(cap);
+    }
+
+    /*! @brief Requests the removal of unused capacity. */
+    void shrink_to_fit() override {
+        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
+     * There are no guarantees on the order, even though either `sort` or
+     * `respect` has been previously invoked. Internal data structures arrange
+     * elements to maximize performance. Accessing them directly gives a
+     * performance boost but less guarantees. Use `begin` and `end` if you want
+     * to iterate the storage in the expected order.
+     *
+     * @return A pointer to the array of objects.
+     */
+    const object_type * raw() const ENTT_NOEXCEPT {
+        return instances.data();
+    }
+
+    /*! @copydoc raw */
+    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 given type. If
+     * the storage is empty, the returned iterator will be equal to `end()`.
+     *
+     * @note
+     * Input iterators stay true to the order imposed by a call to either `sort`
+     * or `respect`.
+     *
+     * @return An iterator to the first instance of the given type.
+     */
+    const_iterator_type cbegin() const ENTT_NOEXCEPT {
+        const typename traits_type::difference_type pos = underlying_type::size();
+        return const_iterator_type{&instances, pos};
+    }
+
+    /*! @copydoc cbegin */
+    inline const_iterator_type begin() const ENTT_NOEXCEPT {
+        return cbegin();
+    }
+
+    /*! @copydoc begin */
+    iterator_type begin() ENTT_NOEXCEPT {
+        const typename traits_type::difference_type pos = underlying_type::size();
+        return iterator_type{&instances, pos};
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     *
+     * The returned iterator points to the element following the last instance
+     * of the given type. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @note
+     * Input iterators stay true to the order imposed by a call to either `sort`
+     * or `respect`.
+     *
+     * @return An iterator to the element following the last instance of the
+     * given type.
+     */
+    const_iterator_type cend() const ENTT_NOEXCEPT {
+        return const_iterator_type{&instances, {}};
+    }
+
+    /*! @copydoc cend */
+    inline const_iterator_type end() const ENTT_NOEXCEPT {
+        return cend();
+    }
+
+    /*! @copydoc end */
+    iterator_type end() ENTT_NOEXCEPT {
+        return iterator_type{&instances, {}};
+    }
+
+    /**
+     * @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.
+     */
+    const object_type & get(const entity_type entt) const ENTT_NOEXCEPT {
+        return instances[underlying_type::get(entt)];
+    }
+
+    /*! @copydoc get */
+    inline object_type & get(const entity_type entt) ENTT_NOEXCEPT {
+        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.
+     */
+    const object_type * try_get(const entity_type entt) const ENTT_NOEXCEPT {
+        return underlying_type::has(entt) ? (instances.data() + underlying_type::get(entt)) : nullptr;
+    }
+
+    /*! @copydoc try_get */
+    inline object_type * try_get(const entity_type entt) ENTT_NOEXCEPT {
+        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.
+     * @return The object associated with the entity.
+     */
+    template<typename... Args>
+    object_type & construct(const entity_type entt, Args &&... args) {
+        if constexpr(std::is_aggregate_v<object_type>) {
+            instances.emplace_back(Type{std::forward<Args>(args)...});
+        } else {
+            instances.emplace_back(std::forward<Args>(args)...);
+        }
+
+        // entity goes after component in case constructor throws
+        underlying_type::construct(entt);
+        return instances.back();
+    }
+
+    /**
+     * @brief Assigns one or more entities to a storage and constructs their
+     * objects.
+     *
+     * The object type must be at least default constructible.
+     *
+     * @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 forward iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @return A pointer to the array of instances just created and sorted the
+     * same of the entities.
+     */
+    template<typename It>
+    object_type * batch(It first, It last) {
+        static_assert(std::is_default_constructible_v<object_type>);
+        const auto skip = instances.size();
+        instances.insert(instances.end(), last-first, {});
+        // entity goes after component in case constructor throws
+        underlying_type::batch(first, last);
+        return instances.data() + skip;
+    }
+
+    /**
+     * @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 destroy(const entity_type entt) override {
+        std::swap(instances[underlying_type::get(entt)], instances.back());
+        instances.pop_back();
+        underlying_type::destroy(entt);
+    }
+
+    /**
+     * @brief Sort instances according to the given comparison function.
+     *
+     * Sort the elements so that iterating the storage 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.
+     *
+     * The comparison function object received by the sort function object
+     * hasn't necessarily the type of the one passed along with the other
+     * parameters to this member function.
+     *
+     * @note
+     * Attempting to iterate elements using a raw pointer returned by a call to
+     * either `data` or `raw` gives no guarantees on the order, even though
+     * `sort` has been invoked.
+     *
+     * @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 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(Compare compare, Sort algo = Sort{}, Args &&... args) {
+        std::vector<size_type> copy(instances.size());
+        std::iota(copy.begin(), copy.end(), 0);
+
+        if constexpr(std::is_invocable_v<Compare, const object_type &, const object_type &>) {
+            static_assert(!std::is_empty_v<object_type>);
+
+            algo(copy.rbegin(), copy.rend(), [this, compare = std::move(compare)](const auto lhs, const auto rhs) {
+                return compare(std::as_const(instances[lhs]), std::as_const(instances[rhs]));
+            }, std::forward<Args>(args)...);
+        } else {
+            algo(copy.rbegin(), copy.rend(), [compare = std::move(compare), data = underlying_type::data()](const auto lhs, const auto rhs) {
+                return compare(data[lhs], data[rhs]);
+            }, std::forward<Args>(args)...);
+        }
+
+        for(size_type pos = 0, last = copy.size(); pos < last; ++pos) {
+            auto curr = pos;
+            auto next = copy[curr];
+
+            while(curr != next) {
+                const auto lhs = copy[curr];
+                const auto rhs = copy[next];
+                std::swap(instances[lhs], instances[rhs]);
+                underlying_type::swap(lhs, rhs);
+                copy[curr] = curr;
+                curr = next;
+                next = copy[curr];
+            }
+        }
+    }
+
+    /**
+     * @brief Sort instances according to the order of the entities in another
+     * sparse set.
+     *
+     * Entities that are part of both the storage 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 guarantess on their order.
+     * Instances are sorted according to the entities to which they belong.<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 storage with a couple of iterators returns elements in the
+     * expected order after a call to `respect`. See `begin` and `end` for more
+     * details.
+     *
+     * @note
+     * Attempting to iterate elements using a raw pointer returned by a call to
+     * either `data` or `raw` gives no guarantees on the order, even though
+     * `respect` has been invoked.
+     *
+     * @param other The sparse sets that imposes the order of the entities.
+     */
+    void respect(const sparse_set<Entity> &other) ENTT_NOEXCEPT override {
+        const auto to = other.end();
+        auto from = other.begin();
+
+        size_type pos = underlying_type::size() - 1;
+        const auto *local = underlying_type::data();
+
+        while(pos && from != to) {
+            const auto curr = *from;
+
+            if(underlying_type::has(curr)) {
+                if(curr != *(local + pos)) {
+                    auto candidate = underlying_type::get(curr);
+                    std::swap(instances[pos], instances[candidate]);
+                    underlying_type::swap(pos, candidate);
+                }
+
+                --pos;
+            }
+
+            ++from;
+        }
+    }
+
+    /*! @brief Resets a storage. */
+    void reset() override {
+        underlying_type::reset();
+        instances.clear();
+    }
+
+private:
+    std::vector<object_type> instances;
+};
+
+
+/*! @copydoc basic_storage */
+template<typename Entity, typename Type>
+class basic_storage<Entity, Type, std::enable_if_t<std::is_empty_v<Type>>>: public sparse_set<Entity> {
+    using underlying_type = sparse_set<Entity>;
+    using traits_type = entt_traits<Entity>;
+
+    class iterator {
+        friend class basic_storage<Entity, Type>;
+
+        using index_type = typename traits_type::difference_type;
+
+        iterator(const index_type idx) ENTT_NOEXCEPT
+            : index{idx}
+        {}
+
+    public:
+        using difference_type = index_type;
+        using value_type = Type;
+        using pointer = const value_type *;
+        using reference = value_type;
+        using iterator_category = std::input_iterator_tag;
+
+        iterator() ENTT_NOEXCEPT = default;
+
+        iterator & operator++() ENTT_NOEXCEPT {
+            return --index, *this;
+        }
+
+        iterator operator++(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        iterator & operator--() ENTT_NOEXCEPT {
+            return ++index, *this;
+        }
+
+        iterator operator--(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return --(*this), orig;
+        }
+
+        iterator & operator+=(const difference_type value) ENTT_NOEXCEPT {
+            index -= value;
+            return *this;
+        }
+
+        iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
+            return iterator{index-value};
+        }
+
+        inline iterator & operator-=(const difference_type value) ENTT_NOEXCEPT {
+            return (*this += -value);
+        }
+
+        inline iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
+            return (*this + -value);
+        }
+
+        difference_type operator-(const iterator &other) const ENTT_NOEXCEPT {
+            return other.index - index;
+        }
+
+        reference operator[](const difference_type) const ENTT_NOEXCEPT {
+            return {};
+        }
+
+        bool operator==(const iterator &other) const ENTT_NOEXCEPT {
+            return other.index == index;
+        }
+
+        inline bool operator!=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+        bool operator<(const iterator &other) const ENTT_NOEXCEPT {
+            return index > other.index;
+        }
+
+        bool operator>(const iterator &other) const ENTT_NOEXCEPT {
+            return index < other.index;
+        }
+
+        inline bool operator<=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this > other);
+        }
+
+        inline bool operator>=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this < other);
+        }
+
+        pointer operator->() const ENTT_NOEXCEPT {
+            return nullptr;
+        }
+
+        inline reference operator*() const ENTT_NOEXCEPT {
+            return {};
+        }
+
+    private:
+        index_type index;
+    };
+
+public:
+    /*! @brief Type of the objects associated with the entities. */
+    using object_type = Type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename underlying_type::entity_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = typename underlying_type::size_type;
+    /*! @brief Random access iterator type. */
+    using iterator_type = iterator;
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * The returned iterator points to the first instance of the given type. If
+     * the storage is empty, the returned iterator will be equal to `end()`.
+     *
+     * @note
+     * Input iterators stay true to the order imposed by a call to either `sort`
+     * or `respect`.
+     *
+     * @return An iterator to the first instance of the given type.
+     */
+    iterator_type cbegin() const ENTT_NOEXCEPT {
+        const typename traits_type::difference_type pos = underlying_type::size();
+        return iterator_type{pos};
+    }
+
+    /*! @copydoc cbegin */
+    inline iterator_type begin() const ENTT_NOEXCEPT {
+        return cbegin();
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     *
+     * The returned iterator points to the element following the last instance
+     * of the given type. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @note
+     * Input iterators stay true to the order imposed by a call to either `sort`
+     * or `respect`.
+     *
+     * @return An iterator to the element following the last instance of the
+     * given type.
+     */
+    iterator_type cend() const ENTT_NOEXCEPT {
+        return iterator_type{};
+    }
+
+    /*! @copydoc cend */
+    inline iterator_type end() const ENTT_NOEXCEPT {
+        return cend();
+    }
+
+    /**
+     * @brief Returns the object associated with an entity.
+     *
+     * @note
+     * Empty types aren't explicitly instantiated. Therefore, this function
+     * always returns a temporary 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.
+     * @return The object associated with the entity.
+     */
+    object_type get([[maybe_unused]] const entity_type entt) const ENTT_NOEXCEPT {
+        ENTT_ASSERT(underlying_type::has(entt));
+        return {};
+    }
+};
+
+/*! @copydoc basic_storage */
+template<typename Entity, typename Type>
+struct storage: basic_storage<Entity, Type> {};
+
+
+}
+
+
+#endif // ENTT_ENTITY_STORAGE_HPP

src/entt/entity/view.hpp

@@ -0,0 +1,662 @@
+#ifndef ENTT_ENTITY_VIEW_HPP
+#define ENTT_ENTITY_VIEW_HPP
+
+
+#include <iterator>
+#include <array>
+#include <tuple>
+#include <utility>
+#include <algorithm>
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/type_traits.hpp"
+#include "sparse_set.hpp"
+#include "storage.hpp"
+#include "entity.hpp"
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Multi component view.
+ *
+ * Multi component views iterate over those entities that have at least all the
+ * given components in their bags. During initialization, a multi component 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).
+ *
+ * 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 views.
+ *
+ * @warning
+ * Lifetime of a view must overcome the one of the registry that generated it.
+ * In any other case, attempting to use a view results in undefined behavior.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Component Types of components iterated by the view.
+ */
+template<typename Entity, typename... Component>
+class basic_view {
+    static_assert(sizeof...(Component) > 1);
+
+    /*! @brief A registry is allowed to create views. */
+    friend class basic_registry<Entity>;
+
+    template<typename Comp>
+    using pool_type = std::conditional_t<std::is_const_v<Comp>, const storage<Entity, std::remove_const_t<Comp>>, storage<Entity, Comp>>;
+
+    template<typename Comp>
+    using component_iterator_type = decltype(std::declval<pool_type<Comp>>().begin());
+
+    using underlying_iterator_type = typename sparse_set<Entity>::iterator_type;
+    using unchecked_type = std::array<const sparse_set<Entity> *, (sizeof...(Component) - 1)>;
+    using traits_type = entt_traits<Entity>;
+
+    class iterator {
+        friend class basic_view<Entity, Component...>;
+
+        using extent_type = typename sparse_set<Entity>::size_type;
+
+        iterator(unchecked_type other, underlying_iterator_type first, underlying_iterator_type last) ENTT_NOEXCEPT
+            : unchecked{other},
+              begin{first},
+              end{last},
+              extent{min(std::make_index_sequence<other.size()>{})}
+        {
+            if(begin != end && !valid()) {
+                ++(*this);
+            }
+        }
+
+        template<auto... Indexes>
+        extent_type min(std::index_sequence<Indexes...>) const ENTT_NOEXCEPT {
+            return std::min({ std::get<Indexes>(unchecked)->extent()... });
+        }
+
+        bool valid() const ENTT_NOEXCEPT {
+            const auto entt = *begin;
+            const auto sz = size_type(entt& traits_type::entity_mask);
+
+            return sz < extent && std::all_of(unchecked.cbegin(), unchecked.cend(), [entt](const sparse_set<Entity> *view) {
+                return view->has(entt);
+            });
+        }
+
+    public:
+        using difference_type = typename underlying_iterator_type::difference_type;
+        using value_type = typename underlying_iterator_type::value_type;
+        using pointer = typename underlying_iterator_type::pointer;
+        using reference = typename underlying_iterator_type::reference;
+        using iterator_category = std::forward_iterator_tag;
+
+        iterator() ENTT_NOEXCEPT = default;
+
+        iterator & operator++() ENTT_NOEXCEPT {
+            return (++begin != end && !valid()) ? ++(*this) : *this;
+        }
+
+        iterator operator++(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        bool operator==(const iterator &other) const ENTT_NOEXCEPT {
+            return other.begin == begin;
+        }
+
+        inline bool operator!=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+        pointer operator->() const ENTT_NOEXCEPT {
+            return begin.operator->();
+        }
+
+        inline reference operator*() const ENTT_NOEXCEPT {
+            return *operator->();
+        }
+
+    private:
+        unchecked_type unchecked;
+        underlying_iterator_type begin;
+        underlying_iterator_type end;
+        extent_type extent;
+    };
+
+    // we could use pool_type<Component> *..., but vs complains about it and refuses to compile for unknown reasons (likely a bug)
+    basic_view(storage<Entity, std::remove_const_t<Component>> *... ref) ENTT_NOEXCEPT
+        : pools{ref...}
+    {}
+
+    const sparse_set<Entity> * candidate() const ENTT_NOEXCEPT {
+        return std::min({ static_cast<const sparse_set<Entity> *>(std::get<pool_type<Component> *>(pools))... }, [](const auto *lhs, const auto *rhs) {
+            return lhs->size() < rhs->size();
+        });
+    }
+
+    unchecked_type unchecked(const sparse_set<Entity> *view) const ENTT_NOEXCEPT {
+        unchecked_type other{};
+        typename unchecked_type::size_type pos{};
+        ((std::get<pool_type<Component> *>(pools) == view ? nullptr : (other[pos++] = std::get<pool_type<Component> *>(pools))), ...);
+        return other;
+    }
+
+    template<typename Comp, typename Other>
+    inline decltype(auto) get([[maybe_unused]] component_iterator_type<Comp> it, [[maybe_unused]] pool_type<Other> *cpool, [[maybe_unused]] const Entity entt) const ENTT_NOEXCEPT {
+        if constexpr(std::is_same_v<Comp, Other>) {
+            return *it;
+        } else {
+            return cpool->get(entt);
+        }
+    }
+
+    template<typename Comp, typename... Other, typename Func>
+    void each(type_list<Other...>, Func func) const {
+        const auto end = std::get<pool_type<Comp> *>(pools)->sparse_set<Entity>::end();
+        auto begin = std::get<pool_type<Comp> *>(pools)->sparse_set<Entity>::begin();
+
+        std::for_each(begin, end, [raw = std::get<pool_type<Comp> *>(pools)->begin(), &func, this](const auto entity) mutable {
+            auto curr = raw++;
+
+            if((std::get<pool_type<Other> *>(pools)->has(entity) && ...)) {
+                if constexpr(std::is_invocable_v<Func, decltype(get<Component>({}))...>) {
+                    func(get<Comp, Component>(curr, std::get<pool_type<Component> *>(pools), entity)...);
+                } else {
+                    func(entity, get<Comp, Component>(curr, std::get<pool_type<Component> *>(pools), entity)...);
+                }
+            }
+        });
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename sparse_set<Entity>::entity_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = typename sparse_set<Entity>::size_type;
+    /*! @brief Input iterator type. */
+    using iterator_type = iterator;
+
+    /**
+     * @brief Returns the number of existing components of the given type.
+     * @tparam Comp Type of component of which to return the size.
+     * @return Number of existing components of the given type.
+     */
+    template<typename Comp>
+    size_type size() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Comp> *>(pools)->size();
+    }
+
+    /**
+     * @brief Estimates the number of entities that have the given components.
+     * @return Estimated number of entities that have the given components.
+     */
+    size_type size() const ENTT_NOEXCEPT {
+        return std::min({ std::get<pool_type<Component> *>(pools)->size()... });
+    }
+
+    /**
+     * @brief Checks whether the pool of a given component is empty.
+     * @tparam Comp Type of component in which one is interested.
+     * @return True if the pool of the given component is empty, false
+     * otherwise.
+     */
+    template<typename Comp>
+    bool empty() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Comp> *>(pools)->empty();
+    }
+
+    /**
+     * @brief Checks if the view is definitely empty.
+     * @return True if the view is definitely empty, false otherwise.
+     */
+    bool empty() const ENTT_NOEXCEPT {
+        return (std::get<pool_type<Component> *>(pools)->empty() || ...);
+    }
+
+    /**
+     * @brief Direct access to the list of components of a given pool.
+     *
+     * The returned pointer is such that range
+     * `[raw<Comp>(), raw<Comp>() + size<Comp>()]` is always a valid range, even
+     * if the container is empty.
+     *
+     * @note
+     * There are no guarantees on the order of the components. Use `begin` and
+     * `end` if you want to iterate the view in the expected order.
+     *
+     * @tparam Comp Type of component in which one is interested.
+     * @return A pointer to the array of components.
+     */
+    template<typename Comp>
+    Comp * raw() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Comp> *>(pools)->raw();
+    }
+
+    /**
+     * @brief Direct access to the list of entities of a given pool.
+     *
+     * The returned pointer is such that range
+     * `[data<Comp>(), data<Comp>() + size<Comp>()]` is always a valid range,
+     * even if the container is empty.
+     *
+     * @note
+     * There are no guarantees on the order of the entities. Use `begin` and
+     * `end` if you want to iterate the view in the expected order.
+     *
+     * @tparam Comp Type of component in which one is interested.
+     * @return A pointer to the array of entities.
+     */
+    template<typename Comp>
+    const entity_type * data() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Comp> *>(pools)->data();
+    }
+
+    /**
+     * @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
+     * Input iterators stay true to the order imposed to the underlying data
+     * structures.
+     *
+     * @return An iterator to the first entity that has the given components.
+     */
+    iterator_type begin() const ENTT_NOEXCEPT {
+        const auto *view = candidate();
+        return iterator_type{unchecked(view), view->begin(), view->end()};
+    }
+
+    /**
+     * @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
+     * Input iterators stay true to the order imposed to the underlying data
+     * structures.
+     *
+     * @return An iterator to the entity following the last entity that has the
+     * given components.
+     */
+    iterator_type end() const ENTT_NOEXCEPT {
+        const auto *view = candidate();
+        return iterator_type{unchecked(view), view->end(), view->end()};
+    }
+
+    /**
+     * @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.
+     */
+    iterator_type find(const entity_type entt) const ENTT_NOEXCEPT {
+        const auto *view = candidate();
+        iterator_type it{unchecked(view), view->find(entt), view->end()};
+        return (it != end() && *it == entt) ? it : end();
+    }
+
+    /**
+     * @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.
+     */
+    bool contains(const entity_type entt) const ENTT_NOEXCEPT {
+        return find(entt) != end();
+    }
+
+    /**
+     * @brief Returns the components assigned to the given entity.
+     *
+     * Prefer this function instead of `registry::get` during iterations. It has
+     * far better performance than its companion function.
+     *
+     * @warning
+     * Attempting to use an invalid component type results in a compilation
+     * error. Attempting to use an entity that doesn't belong to the view
+     * results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * view doesn't contain the given entity.
+     *
+     * @tparam Comp Types of components to get.
+     * @param entt A valid entity identifier.
+     * @return The components assigned to the entity.
+     */
+    template<typename... Comp>
+    decltype(auto) get([[maybe_unused]] const entity_type entt) const ENTT_NOEXCEPT {
+        ENTT_ASSERT(contains(entt));
+
+        if constexpr(sizeof...(Comp) == 1) {
+            return (std::get<pool_type<Comp> *>(pools)->get(entt), ...);
+        } else {
+            return std::tuple<decltype(get<Comp>(entt))...>{get<Comp>(entt)...};
+        }
+    }
+
+    /**
+     * @brief Iterates entities and components and applies the given function
+     * object to them.
+     *
+     * The function object is invoked for each entity. It is provided with the
+     * entity itself and a set of references to all its components. The
+     * _constness_ of the components is as requested.<br/>
+     * The signature of the function must be equivalent to one of the following
+     * forms:
+     *
+     * @code{.cpp}
+     * void(const entity_type, Component &...);
+     * void(Component &...);
+     * @endcode
+     *
+     * @note
+     * Empty types aren't explicitly instantiated. Therefore, temporary objects
+     * are returned during iterations. They can be caught only by copy or with
+     * const references.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    inline void each(Func func) const {
+        const auto *view = candidate();
+        ((std::get<pool_type<Component> *>(pools) == view ? each<Component>(std::move(func)) : void()), ...);
+    }
+
+    /**
+     * @brief Iterates entities and components and applies the given function
+     * object to them.
+     *
+     * The function object is invoked for each entity. It is provided with the
+     * entity itself and a set of references to all its components. The
+     * _constness_ of the components is as requested.<br/>
+     * The signature of the function must be equivalent to one of the following
+     * forms:
+     *
+     * @code{.cpp}
+     * void(const entity_type, Component &...);
+     * void(Component &...);
+     * @endcode
+     *
+     * The pool of the suggested component is used to lead the iterations. The
+     * returned entities will therefore respect the order of the pool associated
+     * with that type.<br/>
+     * It is no longer guaranteed that the performance is the best possible, but
+     * there will be greater control over the order of iteration.
+     *
+     * @note
+     * Empty types aren't explicitly instantiated. Therefore, temporary objects
+     * are returned during iterations. They can be caught only by copy or with
+     * const references.
+     *
+     * @tparam Comp Type of component to use to enforce the iteration order.
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Comp, typename Func>
+    inline void each(Func func) const {
+        using other_type = type_list_cat_t<std::conditional_t<std::is_same_v<Comp, Component>, type_list<>, type_list<Component>>...>;
+        each<Comp>(other_type{}, std::move(func));
+    }
+
+private:
+    const std::tuple<pool_type<Component> *...> pools;
+};
+
+
+/**
+ * @brief Single component view specialization.
+ *
+ * Single component views are specialized in order to get a boost in terms of
+ * performance. This kind of views can access the underlying data structure
+ * directly and avoid superfluous checks.<br/>
+ * Order of elements during iterations are highly dependent on the order of the
+ * underlying data structure. See sparse_set and its specializations for more
+ * details.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New instances of the given component are created and assigned to entities.
+ * * The entity currently pointed is modified (as an example, the given
+ *   component is removed from the entity to which the iterator points).
+ *
+ * In all the other cases, modifying the pool of the given component in any way
+ * invalidates all the iterators and using them results in undefined behavior.
+ *
+ * @note
+ * Views share a reference to the underlying data structure 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 views.
+ *
+ * @warning
+ * Lifetime of a view must overcome the one of the registry that generated it.
+ * In any other case, attempting to use a view results in undefined behavior.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Component Type of component iterated by the view.
+ */
+template<typename Entity, typename Component>
+class basic_view<Entity, Component> {
+    /*! @brief A registry is allowed to create views. */
+    friend class basic_registry<Entity>;
+
+    using pool_type = std::conditional_t<std::is_const_v<Component>, const storage<Entity, std::remove_const_t<Component>>, storage<Entity, Component>>;
+
+    basic_view(pool_type *ref) ENTT_NOEXCEPT
+        : pool{ref}
+    {}
+
+public:
+    /*! @brief Type of component iterated by the view. */
+    using raw_type = Component;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename pool_type::entity_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = typename pool_type::size_type;
+    /*! @brief Input iterator type. */
+    using iterator_type = typename sparse_set<Entity>::iterator_type;
+
+    /**
+     * @brief Returns the number of entities that have the given component.
+     * @return Number of entities that have the given component.
+     */
+    size_type size() const ENTT_NOEXCEPT {
+        return pool->size();
+    }
+
+    /**
+     * @brief Checks whether the view is empty.
+     * @return True if the view is empty, false otherwise.
+     */
+    bool empty() const ENTT_NOEXCEPT {
+        return pool->empty();
+    }
+
+    /**
+     * @brief Direct access to the list of components.
+     *
+     * The returned pointer is such that range `[raw(), raw() + size()]` is
+     * always a valid range, even if the container is empty.
+     *
+     * @note
+     * There are no guarantees on the order of the components. Use `begin` and
+     * `end` if you want to iterate the view in the expected order.
+     *
+     * @return A pointer to the array of components.
+     */
+    raw_type * raw() const ENTT_NOEXCEPT {
+        return pool->raw();
+    }
+
+    /**
+     * @brief Direct access to the list of entities.
+     *
+     * The returned pointer is such that range `[data(), data() + size()]` is
+     * always a valid range, even if the container is empty.
+     *
+     * @note
+     * There are no guarantees on the order of the entities. Use `begin` and
+     * `end` if you want to iterate the view in the expected order.
+     *
+     * @return A pointer to the array of entities.
+     */
+    const entity_type * data() const ENTT_NOEXCEPT {
+        return pool->data();
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity that has the given
+     * component.
+     *
+     * The returned iterator points to the first entity that has the given
+     * component. If the view is empty, the returned iterator will be equal to
+     * `end()`.
+     *
+     * @note
+     * Input iterators stay true to the order imposed to the underlying data
+     * structures.
+     *
+     * @return An iterator to the first entity that has the given component.
+     */
+    iterator_type begin() const ENTT_NOEXCEPT {
+        return pool->sparse_set<Entity>::begin();
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity that has the
+     * given component.
+     *
+     * The returned iterator points to the entity following the last entity that
+     * has the given component. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @note
+     * Input iterators stay true to the order imposed to the underlying data
+     * structures.
+     *
+     * @return An iterator to the entity following the last entity that has the
+     * given component.
+     */
+    iterator_type end() const ENTT_NOEXCEPT {
+        return pool->sparse_set<Entity>::end();
+    }
+
+    /**
+     * @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.
+     */
+    iterator_type find(const entity_type entt) const ENTT_NOEXCEPT {
+        const auto it = pool->find(entt);
+        return it != end() && *it == entt ? it : end();
+    }
+
+    /**
+     * @brief Returns the identifier that occupies the given position.
+     * @param pos Position of the element to return.
+     * @return The identifier that occupies the given position.
+     */
+    entity_type operator[](const size_type pos) const ENTT_NOEXCEPT {
+        return begin()[pos];
+    }
+
+    /**
+     * @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.
+     */
+    bool contains(const entity_type entt) const ENTT_NOEXCEPT {
+        return find(entt) != end();
+    }
+
+    /**
+     * @brief Returns the component assigned to the given entity.
+     *
+     * Prefer this function instead of `registry::get` during iterations. It has
+     * far better performance than its companion function.
+     *
+     * @warning
+     * Attempting to use an entity that doesn't belong to the view results in
+     * undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * view doesn't contain the given entity.
+     *
+     * @param entt A valid entity identifier.
+     * @return The component assigned to the entity.
+     */
+    decltype(auto) get(const entity_type entt) const ENTT_NOEXCEPT {
+        ENTT_ASSERT(contains(entt));
+        return pool->get(entt);
+    }
+
+    /**
+     * @brief Iterates entities and components and applies the given function
+     * object to them.
+     *
+     * The function object is invoked for each entity. It is provided with the
+     * entity itself and a reference to its component. The _constness_ of the
+     * component is as requested.<br/>
+     * The signature of the function must be equivalent to one of the following
+     * forms:
+     *
+     * @code{.cpp}
+     * void(const entity_type, Component &);
+     * void(Component &);
+     * @endcode
+     *
+     * @note
+     * Empty types aren't explicitly instantiated. Therefore, temporary objects
+     * are returned during iterations. They can be caught only by copy or with
+     * const references.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        if constexpr(std::is_invocable_v<Func, decltype(get({}))>) {
+            std::for_each(pool->begin(), pool->end(), std::move(func));
+        } else {
+            std::for_each(pool->sparse_set<Entity>::begin(), pool->sparse_set<Entity>::end(), [&func, raw = pool->begin()](const auto entt) mutable {
+                func(entt, *(raw++));
+            });
+        }
+    }
+
+private:
+    pool_type *pool;
+};
+
+
+}
+
+
+#endif // ENTT_ENTITY_VIEW_HPP

src/entt/entt.hpp

@@ -0,0 +1,30 @@
+#include "core/algorithm.hpp"
+#include "core/family.hpp"
+#include "core/hashed_string.hpp"
+#include "core/ident.hpp"
+#include "core/monostate.hpp"
+#include "core/type_traits.hpp"
+#include "core/utility.hpp"
+#include "entity/actor.hpp"
+#include "entity/entity.hpp"
+#include "entity/group.hpp"
+#include "entity/helper.hpp"
+#include "entity/prototype.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/view.hpp"
+#include "locator/locator.hpp"
+#include "meta/factory.hpp"
+#include "meta/meta.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 +1,3 @@
+#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.
+     */
+    inline 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.
+     */
+    inline 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.
+     */
+    inline 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>
+    inline 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.
+     */
+    inline 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.
+     */
+    inline static void reset() {
+        service.reset();
+    }
+
+private:
+    inline static std::shared_ptr<Service> service = nullptr;
+};
+
+
+}
+
+
+#endif // ENTT_LOCATOR_LOCATOR_HPP

src/entt/meta/factory.hpp

@@ -0,0 +1,687 @@
+#ifndef ENTT_META_FACTORY_HPP
+#define ENTT_META_FACTORY_HPP
+
+
+#include <utility>
+#include <algorithm>
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/hashed_string.hpp"
+#include "meta.hpp"
+
+
+namespace entt {
+
+
+template<typename>
+class meta_factory;
+
+
+template<typename Type, typename... Property>
+meta_factory<Type> reflect(const char *str, Property &&... property) ENTT_NOEXCEPT;
+
+
+template<typename Type>
+bool unregister() ENTT_NOEXCEPT;
+
+
+/**
+ * @brief A meta factory to be used for reflection purposes.
+ *
+ * A meta factory is an utility class used to reflect types, data 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.
+ *
+ * @tparam Type Reflected type for which the factory was created.
+ */
+template<typename Type>
+class meta_factory {
+    static_assert(std::is_object_v<Type> && !(std::is_const_v<Type> || std::is_volatile_v<Type>));
+
+    template<typename Node>
+    inline bool duplicate(const hashed_string &name, const Node *node) ENTT_NOEXCEPT {
+        return node ? node->name == name || duplicate(name, node->next) : false;
+    }
+
+    inline bool duplicate(const meta_any &key, const internal::meta_prop_node *node) ENTT_NOEXCEPT {
+        return node ? node->key() == key || duplicate(key, node->next) : false;
+    }
+
+    template<typename>
+    internal::meta_prop_node * properties() {
+        return nullptr;
+    }
+
+    template<typename Owner, typename Property, typename... Other>
+    internal::meta_prop_node * properties(Property &&property, Other &&... other) {
+        static std::remove_cv_t<std::remove_reference_t<Property>> prop{};
+
+        static internal::meta_prop_node node{
+            nullptr,
+            []() -> meta_any {
+                return std::get<0>(prop);
+            },
+            []() -> meta_any {
+                return std::get<1>(prop);
+            },
+            []() -> meta_prop {
+                return &node;
+            }
+        };
+
+        prop = std::forward<Property>(property);
+        node.next = properties<Owner>(std::forward<Other>(other)...);
+        ENTT_ASSERT(!duplicate(meta_any{std::get<0>(prop)}, node.next));
+        return &node;
+    }
+
+    template<typename... Property>
+    meta_factory type(hashed_string name, Property &&... property) ENTT_NOEXCEPT {
+        static internal::meta_type_node node{
+            {},
+            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_function_v<Type>,
+            std::is_member_object_pointer_v<Type>,
+            std::is_member_function_pointer_v<Type>,
+            std::extent_v<Type>,
+            []() -> meta_type {
+                return internal::meta_info<std::remove_pointer_t<Type>>::resolve();
+            },
+            &internal::destroy<Type>,
+            []() -> meta_type {
+                return &node;
+            }
+        };
+
+        node.name = name;
+        node.next = internal::meta_info<>::type;
+        node.prop = properties<Type>(std::forward<Property>(property)...);
+        ENTT_ASSERT(!duplicate(name, node.next));
+        ENTT_ASSERT(!internal::meta_info<Type>::type);
+        internal::meta_info<Type>::type = &node;
+        internal::meta_info<>::type = &node;
+
+        return *this;
+    }
+
+    void unregister_prop(internal::meta_prop_node **prop) {
+        while(*prop) {
+            auto *node = *prop;
+            *prop = node->next;
+            node->next = nullptr;
+        }
+    }
+
+    void unregister_dtor() {
+        if(auto node = internal::meta_info<Type>::type->dtor; node) {
+            internal::meta_info<Type>::type->dtor = nullptr;
+            *node->underlying = nullptr;
+        }
+    }
+
+    template<auto Member>
+    auto unregister_all(int)
+    -> decltype((internal::meta_info<Type>::type->*Member)->prop, void()) {
+        while(internal::meta_info<Type>::type->*Member) {
+            auto node = internal::meta_info<Type>::type->*Member;
+            internal::meta_info<Type>::type->*Member = node->next;
+            unregister_prop(&node->prop);
+            node->next = nullptr;
+            *node->underlying = nullptr;
+        }
+    }
+
+    template<auto Member>
+    void unregister_all(char) {
+        while(internal::meta_info<Type>::type->*Member) {
+            auto node = internal::meta_info<Type>::type->*Member;
+            internal::meta_info<Type>::type->*Member = node->next;
+            node->next = nullptr;
+            *node->underlying = nullptr;
+        }
+    }
+
+    bool unregister() ENTT_NOEXCEPT {
+        const auto registered = internal::meta_info<Type>::type;
+
+        if(registered) {
+            if(auto *curr = internal::meta_info<>::type; curr == internal::meta_info<Type>::type) {
+                internal::meta_info<>::type = internal::meta_info<Type>::type->next;
+            } else {
+                while(curr && curr->next != internal::meta_info<Type>::type) {
+                    curr = curr->next;
+                }
+
+                if(curr) {
+                    curr->next = internal::meta_info<Type>::type->next;
+                }
+            }
+
+            unregister_prop(&internal::meta_info<Type>::type->prop);
+            unregister_all<&internal::meta_type_node::base>(0);
+            unregister_all<&internal::meta_type_node::conv>(0);
+            unregister_all<&internal::meta_type_node::ctor>(0);
+            unregister_all<&internal::meta_type_node::data>(0);
+            unregister_all<&internal::meta_type_node::func>(0);
+            unregister_dtor();
+
+            internal::meta_info<Type>::type->name = {};
+            internal::meta_info<Type>::type->next = nullptr;
+            internal::meta_info<Type>::type = nullptr;
+        }
+
+        return registered;
+    }
+
+    meta_factory() ENTT_NOEXCEPT = default;
+
+public:
+    template<typename Other, typename... Property>
+    friend meta_factory<Other> reflect(const char *str, Property &&... property) ENTT_NOEXCEPT;
+
+    template<typename Other>
+    friend bool unregister() ENTT_NOEXCEPT;
+
+    /**
+     * @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>
+    meta_factory base() ENTT_NOEXCEPT {
+        static_assert(std::is_base_of_v<Base, Type>);
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_base_node node{
+            &internal::meta_info<Type>::template base<Base>,
+            type,
+            nullptr,
+            &internal::meta_info<Base>::resolve,
+            [](void *instance) -> void * {
+                return static_cast<Base *>(static_cast<Type *>(instance));
+            },
+            []() -> meta_base {
+                return &node;
+            }
+        };
+
+        node.next = type->base;
+        ENTT_ASSERT((!internal::meta_info<Type>::template base<Base>));
+        internal::meta_info<Type>::template base<Base> = &node;
+        type->base = &node;
+
+        return *this;
+    }
+
+    /**
+     * @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>
+    meta_factory conv() ENTT_NOEXCEPT {
+        static_assert(std::is_convertible_v<Type, To>);
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_conv_node node{
+            &internal::meta_info<Type>::template conv<To>,
+            type,
+            nullptr,
+            &internal::meta_info<To>::resolve,
+            [](void *instance) -> meta_any {
+                return static_cast<To>(*static_cast<Type *>(instance));
+            },
+            []() -> meta_conv {
+                return &node;
+            }
+        };
+
+        node.next = type->conv;
+        ENTT_ASSERT((!internal::meta_info<Type>::template conv<To>));
+        internal::meta_info<Type>::template conv<To> = &node;
+        type->conv = &node;
+
+        return *this;
+    }
+
+    /**
+     * @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 Property Types of properties to assign to the meta data.
+     * @param property Properties to assign to the meta data.
+     * @return A meta factory for the parent type.
+     */
+    template<auto Func, typename... Property>
+    meta_factory ctor(Property &&... property) ENTT_NOEXCEPT {
+        using helper_type = internal::meta_function_helper<std::integral_constant<decltype(Func), Func>>;
+        static_assert(std::is_same_v<typename helper_type::return_type, Type>);
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_ctor_node node{
+            &internal::meta_info<Type>::template ctor<typename helper_type::args_type>,
+            type,
+            nullptr,
+            nullptr,
+            helper_type::size,
+            &helper_type::arg,
+            [](meta_any * const any) {
+                return internal::invoke<Type, Func>(nullptr, any, std::make_index_sequence<helper_type::size>{});
+            },
+            []() -> meta_ctor {
+                return &node;
+            }
+        };
+
+        node.next = type->ctor;
+        node.prop = properties<typename helper_type::args_type>(std::forward<Property>(property)...);
+        ENTT_ASSERT((!internal::meta_info<Type>::template ctor<typename helper_type::args_type>));
+        internal::meta_info<Type>::template ctor<typename helper_type::args_type> = &node;
+        type->ctor = &node;
+
+        return *this;
+    }
+
+    /**
+     * @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.
+     * @tparam Property Types of properties to assign to the meta data.
+     * @param property Properties to assign to the meta data.
+     * @return A meta factory for the parent type.
+     */
+    template<typename... Args, typename... Property>
+    meta_factory ctor(Property &&... property) ENTT_NOEXCEPT {
+        using helper_type = internal::meta_function_helper<Type(Args...)>;
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_ctor_node node{
+            &internal::meta_info<Type>::template ctor<typename helper_type::args_type>,
+            type,
+            nullptr,
+            nullptr,
+            helper_type::size,
+            &helper_type::arg,
+            [](meta_any * const any) {
+                return internal::construct<Type, Args...>(any, std::make_index_sequence<helper_type::size>{});
+            },
+            []() -> meta_ctor {
+                return &node;
+            }
+        };
+
+        node.next = type->ctor;
+        node.prop = properties<typename helper_type::args_type>(std::forward<Property>(property)...);
+        ENTT_ASSERT((!internal::meta_info<Type>::template ctor<typename helper_type::args_type>));
+        internal::meta_info<Type>::template ctor<typename helper_type::args_type> = &node;
+        type->ctor = &node;
+
+        return *this;
+    }
+
+    /**
+     * @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
+     *
+     * From a client's point of view, nothing changes if the destructor of a
+     * meta type is the default one or a custom one.
+     *
+     * @tparam Func The actual function to use as a destructor.
+     * @return A meta factory for the parent type.
+     */
+    template<auto *Func>
+    meta_factory dtor() ENTT_NOEXCEPT {
+        static_assert(std::is_invocable_v<decltype(Func), Type *>);
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_dtor_node node{
+            &internal::meta_info<Type>::template dtor<Func>,
+            type,
+            [](meta_handle handle) {
+                return handle.type() == internal::meta_info<Type>::resolve()->meta()
+                        ? ((*Func)(static_cast<Type *>(handle.data())), true)
+                        : false;
+            },
+            []() -> meta_dtor {
+                return &node;
+            }
+        };
+
+        ENTT_ASSERT(!internal::meta_info<Type>::type->dtor);
+        ENTT_ASSERT((!internal::meta_info<Type>::template dtor<Func>));
+        internal::meta_info<Type>::template dtor<Func> = &node;
+        internal::meta_info<Type>::type->dtor = &node;
+
+        return *this;
+    }
+
+    /**
+     * @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 Property Types of properties to assign to the meta data.
+     * @param str The name to assign to the meta data.
+     * @param property Properties to assign to the meta data.
+     * @return A meta factory for the parent type.
+     */
+    template<auto Data, typename... Property>
+    meta_factory data(const char *str, Property &&... property) ENTT_NOEXCEPT {
+        auto * const type = internal::meta_info<Type>::resolve();
+        internal::meta_data_node *curr = nullptr;
+
+        if constexpr(std::is_same_v<Type, decltype(Data)>) {
+            static internal::meta_data_node node{
+                &internal::meta_info<Type>::template data<Data>,
+                {},
+                type,
+                nullptr,
+                nullptr,
+                true,
+                true,
+                &internal::meta_info<Type>::resolve,
+                [](meta_handle, meta_any, meta_any) { return false; },
+                [](meta_handle, meta_any) -> meta_any { return Data; },
+                []() -> meta_data {
+                    return &node;
+                }
+            };
+
+            node.prop = properties<std::integral_constant<Type, Data>>(std::forward<Property>(property)...);
+            curr = &node;
+        } else if constexpr(std::is_member_object_pointer_v<decltype(Data)>) {
+            using data_type = std::remove_reference_t<decltype(std::declval<Type>().*Data)>;
+
+            static internal::meta_data_node node{
+                &internal::meta_info<Type>::template data<Data>,
+                {},
+                type,
+                nullptr,
+                nullptr,
+                std::is_const_v<data_type>,
+                !std::is_member_object_pointer_v<decltype(Data)>,
+                &internal::meta_info<data_type>::resolve,
+                &internal::setter<std::is_const_v<data_type>, Type, Data>,
+                &internal::getter<Type, Data>,
+                []() -> meta_data {
+                    return &node;
+                }
+            };
+
+            node.prop = properties<std::integral_constant<decltype(Data), Data>>(std::forward<Property>(property)...);
+            curr = &node;
+        } else {
+            static_assert(std::is_pointer_v<decltype(Data)>);
+            using data_type = std::remove_pointer_t<decltype(Data)>;
+
+            static internal::meta_data_node node{
+                &internal::meta_info<Type>::template data<Data>,
+                {},
+                type,
+                nullptr,
+                nullptr,
+                std::is_const_v<data_type>,
+                !std::is_member_object_pointer_v<decltype(Data)>,
+                &internal::meta_info<data_type>::resolve,
+                &internal::setter<std::is_const_v<data_type>, Type, Data>,
+                &internal::getter<Type, Data>,
+                []() -> meta_data {
+                    return &node;
+                }
+            };
+
+            node.prop = properties<std::integral_constant<decltype(Data), Data>>(std::forward<Property>(property)...);
+            curr = &node;
+        }
+
+        curr->name = hashed_string{str};
+        curr->next = type->data;
+        ENTT_ASSERT(!duplicate(hashed_string{str}, curr->next));
+        ENTT_ASSERT((!internal::meta_info<Type>::template data<Data>));
+        internal::meta_info<Type>::template data<Data> = curr;
+        type->data = curr;
+
+        return *this;
+    }
+
+    /**
+     * @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 pointer 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 Property Types of properties to assign to the meta data.
+     * @param str The name to assign to the meta data.
+     * @param property Properties to assign to the meta data.
+     * @return A meta factory for the parent type.
+     */
+    template<auto Setter, auto Getter, typename... Property>
+    meta_factory data(const char *str, Property &&... property) ENTT_NOEXCEPT {
+        using owner_type = std::tuple<std::integral_constant<decltype(Setter), Setter>, std::integral_constant<decltype(Getter), Getter>>;
+        using underlying_type = std::invoke_result_t<decltype(Getter), Type *>;
+        static_assert(std::is_invocable_v<decltype(Setter), Type *, underlying_type>);
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_data_node node{
+            &internal::meta_info<Type>::template data<Setter, Getter>,
+            {},
+            type,
+            nullptr,
+            nullptr,
+            false,
+            false,
+            &internal::meta_info<underlying_type>::resolve,
+            &internal::setter<false, Type, Setter>,
+            &internal::getter<Type, Getter>,
+            []() -> meta_data {
+                return &node;
+            }
+        };
+
+        node.name = hashed_string{str};
+        node.next = type->data;
+        node.prop = properties<owner_type>(std::forward<Property>(property)...);
+        ENTT_ASSERT(!duplicate(hashed_string{str}, node.next));
+        ENTT_ASSERT((!internal::meta_info<Type>::template data<Setter, Getter>));
+        internal::meta_info<Type>::template data<Setter, Getter> = &node;
+        type->data = &node;
+
+        return *this;
+    }
+
+    /**
+     * @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 Func The actual function to attach to the meta type.
+     * @tparam Property Types of properties to assign to the meta function.
+     * @param str The name to assign to the meta function.
+     * @param property Properties to assign to the meta function.
+     * @return A meta factory for the parent type.
+     */
+    template<auto Func, typename... Property>
+    meta_factory func(const char *str, Property &&... property) ENTT_NOEXCEPT {
+        using owner_type = std::integral_constant<decltype(Func), Func>;
+        using func_type = internal::meta_function_helper<std::integral_constant<decltype(Func), Func>>;
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_func_node node{
+            &internal::meta_info<Type>::template func<Func>,
+            {},
+            type,
+            nullptr,
+            nullptr,
+            func_type::size,
+            func_type::is_const,
+            func_type::is_static,
+            &internal::meta_info<typename func_type::return_type>::resolve,
+            &func_type::arg,
+            [](meta_handle handle, meta_any *any) {
+                return internal::invoke<Type, Func>(handle, any, std::make_index_sequence<func_type::size>{});
+            },
+            []() -> meta_func {
+                return &node;
+            }
+        };
+
+        node.name = hashed_string{str};
+        node.next = type->func;
+        node.prop = properties<owner_type>(std::forward<Property>(property)...);
+        ENTT_ASSERT(!duplicate(hashed_string{str}, node.next));
+        ENTT_ASSERT((!internal::meta_info<Type>::template func<Func>));
+        internal::meta_info<Type>::template func<Func> = &node;
+        type->func = &node;
+
+        return *this;
+    }
+};
+
+
+/**
+ * @brief Basic 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 data and functions through
+ * a dedicated factory.
+ *
+ * @tparam Type Type to reflect.
+ * @tparam Property Types of properties to assign to the reflected type.
+ * @param str The name to assign to the reflected type.
+ * @param property Properties to assign to the reflected type.
+ * @return A meta factory for the given type.
+ */
+template<typename Type, typename... Property>
+inline meta_factory<Type> reflect(const char *str, Property &&... property) ENTT_NOEXCEPT {
+    return meta_factory<Type>{}.type(hashed_string{str}, std::forward<Property>(property)...);
+}
+
+
+/**
+ * @brief Basic 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 data and functions through
+ * a dedicated factory.
+ *
+ * @tparam Type Type to reflect.
+ * @return A meta factory for the given type.
+ */
+template<typename Type>
+inline meta_factory<Type> reflect() ENTT_NOEXCEPT {
+    return meta_factory<Type>{};
+}
+
+
+/**
+ * @brief Basic function to unregister a type.
+ *
+ * This function unregisters a type and all its data members, member functions
+ * and properties, as well as its constructors, destructors and conversion
+ * functions if any.<br/>
+ * Base classes aren't unregistered but the link between the two types is
+ * removed.
+ *
+ * @tparam Type Type to unregister.
+ * @return True if the type to unregister exists, false otherwise.
+ */
+template<typename Type>
+inline bool unregister() ENTT_NOEXCEPT {
+    return meta_factory<Type>().unregister();
+}
+
+
+/**
+ * @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>
+inline meta_type resolve() ENTT_NOEXCEPT {
+    return internal::meta_info<Type>::resolve()->meta();
+}
+
+
+/**
+ * @brief Returns the meta type associated with a given name.
+ * @param str The name to use to search for a meta type.
+ * @return The meta type associated with the given name, if any.
+ */
+inline meta_type resolve(const char *str) ENTT_NOEXCEPT {
+    const auto *curr = internal::find_if([name = hashed_string{str}](auto *node) {
+        return node->name == name;
+    }, internal::meta_info<>::type);
+
+    return curr ? curr->meta() : meta_type{};
+}
+
+
+/**
+ * @brief Iterates all the reflected types.
+ * @tparam Op Type of the function object to invoke.
+ * @param op A valid function object.
+ */
+template<typename Op>
+void resolve(Op op) ENTT_NOEXCEPT {
+    internal::iterate([op = std::move(op)](auto *node) {
+        op(node->meta());
+    }, internal::meta_info<>::type);
+}
+
+
+}
+
+
+#endif // ENTT_META_FACTORY_HPP

src/entt/process/process.hpp

@@ -0,0 +1,340 @@
+#ifndef ENTT_PROCESS_PROCESS_HPP
+#define ENTT_PROCESS_PROCESS_HPP
+
+
+#include <utility>
+#include <type_traits>
+#include "../config/config.h"
+
+
+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<state value>
+    using state_value_t = std::integral_constant<state, value>;
+
+    template<typename Target = Derived>
+    auto tick(int, state_value_t<state::UNINITIALIZED>)
+    -> decltype(std::declval<Target>().init()) {
+        static_cast<Target *>(this)->init();
+    }
+
+    template<typename Target = Derived>
+    auto tick(int, state_value_t<state::RUNNING>, Delta delta, void *data)
+    -> decltype(std::declval<Target>().update(delta, data)) {
+        static_cast<Target *>(this)->update(delta, data);
+    }
+
+    template<typename Target = Derived>
+    auto tick(int, state_value_t<state::SUCCEEDED>)
+    -> decltype(std::declval<Target>().succeeded()) {
+        static_cast<Target *>(this)->succeeded();
+    }
+
+    template<typename Target = Derived>
+    auto tick(int, state_value_t<state::FAILED>)
+    -> decltype(std::declval<Target>().failed()) {
+        static_cast<Target *>(this)->failed();
+    }
+
+    template<typename Target = Derived>
+    auto tick(int, state_value_t<state::ABORTED>)
+    -> decltype(std::declval<Target>().aborted()) {
+        static_cast<Target *>(this)->aborted();
+    }
+
+    template<state value, typename... Args>
+    void tick(char, state_value_t<value>, Args &&...) 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() ENTT_NOEXCEPT {
+        static_assert(std::is_base_of_v<process, Derived>);
+    }
+
+    /**
+     * @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) ENTT_NOEXCEPT {
+        if(alive()) {
+            current = state::ABORTED;
+
+            if(immediately) {
+                tick(0);
+            }
+        }
+    }
+
+    /**
+     * @brief Returns true if a process is either running or paused.
+     * @return True if the process is still alive, false otherwise.
+     */
+    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.
+     */
+    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.
+     */
+    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.
+     */
+    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:
+            tick(0, state_value_t<state::UNINITIALIZED>{});
+            current = state::RUNNING;
+            break;
+        case state::RUNNING:
+            tick(0, state_value_t<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:
+            tick(0, state_value_t<state::SUCCEEDED>{});
+            current = state::FINISHED;
+            break;
+        case state::FAILED:
+            tick(0, state_value_t<state::FAILED>{});
+            current = state::FINISHED;
+            stopped = true;
+            break;
+        case state::ABORTED:
+            tick(0, state_value_t<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 // ENTT_PROCESS_PROCESS_HPP

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>);
+            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>
+    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 = typename std::vector<process_handler>::size_type;
+
+    /*! @brief Default constructor. */
+    scheduler() ENTT_NOEXCEPT = 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.
+     */
+    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.
+     */
+    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>);
+        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);
+
+        std::for_each(exec.begin(), exec.end(), [immediately](auto &handler) {
+            handler.abort(handler, immediately);
+        });
+
+        std::move(handlers.begin(), handlers.end(), std::back_inserter(exec));
+        handlers.swap(exec);
+    }
+
+private:
+    std::vector<process_handler> handlers{};
+};
+
+
+}
+
+
+#endif // ENTT_PROCESS_SCHEDULER_HPP

src/entt/resource/cache.hpp

@@ -0,0 +1,207 @@
+#ifndef ENTT_RESOURCE_CACHE_HPP
+#define ENTT_RESOURCE_CACHE_HPP
+
+
+#include <memory>
+#include <utility>
+#include <type_traits>
+#include <unordered_map>
+#include "../config/config.h"
+#include "../core/hashed_string.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>
+class resource_cache {
+    using container_type = std::unordered_map<hashed_string::hash_type, std::shared_ptr<Resource>>;
+
+public:
+    /*! @brief Unsigned integer type. */
+    using size_type = typename container_type::size_type;
+    /*! @brief Type of resources managed by a cache. */
+    using resource_type = typename hashed_string::hash_type;
+
+    /*! @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.
+     */
+    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.
+     */
+    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 resource_type id, Args &&... args) {
+        static_assert(std::is_base_of_v<resource_loader<Loader, Resource>, Loader>);
+        resource_handle<Resource> handle{};
+
+        if(auto it = resources.find(id); it == resources.cend()) {
+            if(auto resource = Loader{}.get(std::forward<Args>(args)...); resource) {
+                resources[id] = resource;
+                handle = std::move(resource);
+            }
+        } else {
+            handle = it->second;
+        }
+
+        return handle;
+    }
+
+    /**
+     * @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 resource_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>
+    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.
+     */
+    resource_handle<Resource> handle(const resource_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.
+     */
+    bool contains(const resource_type id) const ENTT_NOEXCEPT {
+        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 resource_type id) ENTT_NOEXCEPT {
+        if(auto it = resources.find(id); it != resources.end()) {
+            resources.erase(it);
+        }
+    }
+
+private:
+    container_type resources;
+};
+
+
+}
+
+
+#endif // ENTT_RESOURCE_CACHE_HPP

src/entt/resource/fwd.hpp

@@ -0,0 +1,27 @@
+#ifndef ENTT_RESOURCE_FWD_HPP
+#define ENTT_RESOURCE_FWD_HPP
+
+
+#include "../config/config.h"
+
+
+namespace entt {
+
+
+/*! @class resource_cache */
+template<typename>
+class resource_cache;
+
+/*! @class resource_handle */
+template<typename>
+class resource_handle;
+
+/*! @class resource_loader */
+template<typename, typename>
+class resource_loader;
+
+
+}
+
+
+#endif // ENTT_RESOURCE_FWD_HPP

src/entt/resource/handle.hpp

@@ -0,0 +1,106 @@
+#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 class 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.
+     */
+    const Resource & get() const ENTT_NOEXCEPT {
+        ENTT_ASSERT(static_cast<bool>(resource));
+        return *resource;
+    }
+
+    /**
+     * @brief Casts a handle and 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.
+     */
+    inline operator const Resource &() const ENTT_NOEXCEPT { return get(); }
+
+    /**
+     * @brief Dereferences a handle to obtain 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.
+     */
+    inline const Resource & operator *() const ENTT_NOEXCEPT { return get(); }
+
+    /**
+     * @brief Gets a pointer to the managed resource from a handle.
+     *
+     * @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.
+     */
+    inline const Resource * operator->() const ENTT_NOEXCEPT {
+        ENTT_ASSERT(static_cast<bool>(resource));
+        return resource.get();
+    }
+
+    /**
+     * @brief Returns true if a handle contains a resource, false otherwise.
+     * @return True if the handle contains a resource, false otherwise.
+     */
+    explicit operator bool() const { return static_cast<bool>(resource); }
+
+private:
+    std::shared_ptr<Resource> resource;
+};
+
+
+}
+
+
+#endif // ENTT_RESOURCE_HANDLE_HPP

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 class 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>
+    std::shared_ptr<Resource> get(Args &&... args) const {
+        return static_cast<const Loader *>(this)->load(std::forward<Args>(args)...);
+    }
+};
+
+
+}
+
+
+#endif // ENTT_RESOURCE_LOADER_HPP

src/entt/signal/delegate.hpp

@@ -0,0 +1,282 @@
+#ifndef ENTT_SIGNAL_DELEGATE_HPP
+#define ENTT_SIGNAL_DELEGATE_HPP
+
+
+#include <cstring>
+#include <algorithm>
+#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 to_function_pointer(Ret(*)(Args...)) -> Ret(*)(Args...);
+
+
+template<typename Ret, typename... Args, typename Type>
+auto to_function_pointer(Ret(*)(Type *, Args...), Type *) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Ret, typename... Args>
+auto to_function_pointer(Ret(Class:: *)(Args...), Class *) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Ret, typename... Args>
+auto to_function_pointer(Ret(Class:: *)(Args...) const, Class *) -> Ret(*)(Args...);
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond TURN_OFF_DOXYGEN
+ */
+
+
+/*! @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>
+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 general purpose invoker with no memory overhead for
+ * free functions (with or without payload) and members provided along with an
+ * instance on which to invoke them.
+ *
+ * @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...)> {
+    using proto_fn_type = Ret(const void *, Args...);
+
+public:
+    /*! @brief Function type of the delegate. */
+    using function_type = Ret(Args...);
+
+    /*! @brief Default constructor. */
+    delegate() ENTT_NOEXCEPT
+        : fn{nullptr}, data{nullptr}
+    {}
+
+    /**
+     * @brief Constructs a delegate and connects a free function to it.
+     * @tparam Function A valid free function pointer.
+     */
+    template<auto Function>
+    delegate(connect_arg_t<Function>) ENTT_NOEXCEPT
+        : delegate{}
+    {
+        connect<Function>();
+    }
+
+    /**
+     * @brief Constructs a delegate and connects a member for a given instance
+     * or a free function with payload.
+     * @tparam Candidate Member or free function 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>
+    delegate(connect_arg_t<Candidate>, Type *value_or_instance) ENTT_NOEXCEPT
+        : delegate{}
+    {
+        connect<Candidate>(value_or_instance);
+    }
+
+    /**
+     * @brief Connects a free function to a delegate.
+     * @tparam Function A valid free function pointer.
+     */
+    template<auto Function>
+    void connect() ENTT_NOEXCEPT {
+        static_assert(std::is_invocable_r_v<Ret, decltype(Function), Args...>);
+        data = nullptr;
+
+        fn = [](const void *, Args... args) -> Ret {
+            // this allows void(...) to eat return values and avoid errors
+            return Ret(std::invoke(Function, args...));
+        };
+    }
+
+    /**
+     * @brief Connects a member function for a given instance or a free function
+     * with payload 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 Member or free function 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 {
+        static_assert(std::is_invocable_r_v<Ret, decltype(Candidate), Type *, Args...>);
+        data = value_or_instance;
+
+        fn = [](const void *payload, Args... args) -> Ret {
+            Type *curr = nullptr;
+
+            if constexpr(std::is_const_v<Type>) {
+                curr = static_cast<Type *>(payload);
+            } else {
+                curr = static_cast<Type *>(const_cast<void *>(payload));
+            }
+
+            // this allows void(...) to eat return values and avoid errors
+            return Ret(std::invoke(Candidate, curr, args...));
+        };
+    }
+
+    /**
+     * @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 linked to a delegate, if any.
+     * @return An opaque pointer to the instance linked to the delegate, if any.
+     */
+    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, args...);
+    }
+
+    /**
+     * @brief Checks whether a delegate actually stores a listener.
+     * @return False if the delegate is empty, true otherwise.
+     */
+    explicit operator bool() const ENTT_NOEXCEPT {
+        // no need to test also data
+        return fn;
+    }
+
+    /**
+     * @brief Checks if the connected functions differ.
+     *
+     * Instances connected to delegates are ignored by this operator. Use the
+     * `instance` member function instead.
+     *
+     * @param other Delegate with which to compare.
+     * @return False if the connected functions differ, true otherwise.
+     */
+    bool operator==(const delegate<Ret(Args...)> &other) const ENTT_NOEXCEPT {
+        return fn == other.fn;
+    }
+
+private:
+    proto_fn_type *fn;
+    const void *data;
+};
+
+
+/**
+ * @brief Checks if the connected functions differ.
+ *
+ * Instances connected to delegates are ignored by this operator. Use the
+ * `instance` member function instead.
+ *
+ * @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 connected functions differ, false otherwise.
+ */
+template<typename Ret, typename... Args>
+bool operator!=(const delegate<Ret(Args...)> &lhs, const delegate<Ret(Args...)> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Deduction guide.
+ *
+ * It allows to deduce the function type of the delegate directly from a
+ * function provided to the constructor.
+ *
+ * @tparam Function A valid free function pointer.
+ */
+template<auto Function>
+delegate(connect_arg_t<Function>) ENTT_NOEXCEPT
+-> delegate<std::remove_pointer_t<decltype(internal::to_function_pointer(Function))>>;
+
+
+/**
+ * @brief Deduction guide.
+ *
+ * It allows to deduce the function type of the delegate directly from a member
+ * or a free function with payload provided to the constructor.
+ *
+ * @tparam Candidate Member or free function 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<decltype(internal::to_function_pointer(Candidate, std::declval<Type *>()))>>;
+
+
+}
+
+
+#endif // ENTT_SIGNAL_DELEGATE_HPP

src/entt/signal/dispatcher.hpp

@@ -0,0 +1,247 @@
+#ifndef ENTT_SIGNAL_DISPATCHER_HPP
+#define ENTT_SIGNAL_DISPATCHER_HPP
+
+
+#include <vector>
+#include <memory>
+#include <utility>
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/family.hpp"
+#include "../core/type_traits.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 `const Event &`, no matter what the return type is.
+ *
+ * The type of the instances is `Class *` (a naked pointer). It means that users
+ * must guarantee that the lifetimes of the instances overcome the one of the
+ * dispatcher itself to avoid crashes.
+ */
+class dispatcher {
+    using event_family = family<struct internal_dispatcher_event_family>;
+
+    template<typename Class, typename Event>
+    using instance_type = typename sigh<void(const Event &)>::template instance_type<Class>;
+
+    struct base_wrapper {
+        virtual ~base_wrapper() = default;
+        virtual void publish() = 0;
+    };
+
+    template<typename Event>
+    struct signal_wrapper: base_wrapper {
+        using signal_type = sigh<void(const Event &)>;
+        using sink_type = typename signal_type::sink_type;
+
+        void publish() override {
+            for(const auto &event: events[current]) {
+                signal.publish(event);
+            }
+
+            events[current++].clear();
+            current %= std::extent<decltype(events)>::value;
+        }
+
+        inline sink_type sink() ENTT_NOEXCEPT {
+            return signal.sink();
+        }
+
+        template<typename... Args>
+        inline void trigger(Args &&... args) {
+            signal.publish({ std::forward<Args>(args)... });
+        }
+
+        template<typename... Args>
+        inline void enqueue(Args &&... args) {
+            events[current].emplace_back(std::forward<Args>(args)...);
+        }
+
+    private:
+        signal_type signal{};
+        std::vector<Event> events[2];
+        int current{};
+    };
+
+    struct wrapper_data {
+        std::unique_ptr<base_wrapper> wrapper;
+        ENTT_ID_TYPE runtime_type;
+    };
+
+    template<typename Event>
+    static auto type() ENTT_NOEXCEPT {
+        if constexpr(is_named_type_v<Event>) {
+            return named_type_traits<Event>::value;
+        } else {
+            return event_family::type<Event>;
+        }
+    }
+
+    template<typename Event>
+    signal_wrapper<Event> & assure() {
+        const auto wtype = type<Event>();
+        wrapper_data *wdata = nullptr;
+
+        if constexpr(is_named_type_v<Event>) {
+            const auto it = std::find_if(wrappers.begin(), wrappers.end(), [wtype](const auto &candidate) {
+                return candidate.wrapper && candidate.runtime_type == wtype;
+            });
+
+            wdata = (it == wrappers.cend() ? &wrappers.emplace_back() : &(*it));
+        } else {
+            if(!(wtype < wrappers.size())) {
+                wrappers.resize(wtype+1);
+            }
+
+            wdata = &wrappers[wtype];
+
+            if(wdata->wrapper && wdata->runtime_type != wtype) {
+                wrappers.emplace_back();
+                std::swap(wrappers[wtype], wrappers.back());
+                wdata = &wrappers[wtype];
+            }
+        }
+
+        if(!wdata->wrapper) {
+            wdata->wrapper = std::make_unique<signal_wrapper<Event>>();
+            wdata->runtime_type = wtype;
+        }
+
+        return static_cast<signal_wrapper<Event> &>(*wdata->wrapper);
+    }
+
+public:
+    /*! @brief Type of sink for the given event. */
+    template<typename Event>
+    using sink_type = typename signal_wrapper<Event>::sink_type;
+
+    /**
+     * @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:
+     * @code{.cpp}
+     * void(const 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>
+    inline sink_type<Event> sink() ENTT_NOEXCEPT {
+        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>
+    inline 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>
+    inline 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>
+    inline 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>
+    inline void enqueue(Event &&event) {
+        assure<std::decay_t<Event>>().enqueue(std::forward<Event>(event));
+    }
+
+    /**
+     * @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>
+    inline 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.
+     */
+    inline void update() const {
+        for(auto pos = wrappers.size(); pos; --pos) {
+            auto &wdata = wrappers[pos-1];
+
+            if(wdata.wrapper) {
+                wdata.wrapper->publish();
+            }
+        }
+    }
+
+private:
+    std::vector<wrapper_data> wrappers;
+};
+
+
+}
+
+
+#endif // ENTT_SIGNAL_DISPATCHER_HPP

src/entt/signal/emitter.hpp

@@ -0,0 +1,343 @@
+#ifndef ENTT_SIGNAL_EMITTER_HPP
+#define ENTT_SIGNAL_EMITTER_HPP
+
+
+#include <type_traits>
+#include <functional>
+#include <algorithm>
+#include <utility>
+#include <memory>
+#include <vector>
+#include <list>
+#include "../config/config.h"
+#include "../core/family.hpp"
+#include "../core/type_traits.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
+ *
+ * Handlers 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 const reference to the event.
+ * Therefore listeners have an handy way to work with it without incurring in
+ * the need of capturing a reference to the emitter.
+ *
+ * @tparam Derived Actual type of emitter that extends the class template.
+ */
+template<typename Derived>
+class emitter {
+    using handler_family = family<struct internal_emitter_handler_family>;
+
+    struct base_handler {
+        virtual ~base_handler() = default;
+        virtual bool empty() const ENTT_NOEXCEPT = 0;
+        virtual void clear() ENTT_NOEXCEPT = 0;
+    };
+
+    template<typename Event>
+    struct event_handler: base_handler {
+        using listener_type = std::function<void(const Event &, Derived &)>;
+        using element_type = std::pair<bool, listener_type>;
+        using container_type = std::list<element_type>;
+        using connection_type = typename container_type::iterator;
+
+        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) {
+                auto func = [](auto &&element) { element.first = true; };
+                std::for_each(once_list.begin(), once_list.end(), func);
+                std::for_each(on_list.begin(), on_list.end(), func);
+            } else {
+                once_list.clear();
+                on_list.clear();
+            }
+        }
+
+        inline connection_type once(listener_type listener) {
+            return once_list.emplace(once_list.cend(), false, std::move(listener));
+        }
+
+        inline connection_type on(listener_type listener) {
+            return on_list.emplace(on_list.cend(), false, std::move(listener));
+        }
+
+        void erase(connection_type conn) ENTT_NOEXCEPT {
+            conn->first = true;
+
+            if(!publishing) {
+                auto pred = [](auto &&element) { return element.first; };
+                once_list.remove_if(pred);
+                on_list.remove_if(pred);
+            }
+        }
+
+        void publish(const Event &event, Derived &ref) {
+            container_type swap_list;
+            once_list.swap(swap_list);
+
+            auto func = [&event, &ref](auto &&element) {
+                return element.first ? void() : element.second(event, ref);
+            };
+
+            publishing = true;
+
+            std::for_each(on_list.rbegin(), on_list.rend(), func);
+            std::for_each(swap_list.rbegin(), swap_list.rend(), func);
+
+            publishing = false;
+
+            on_list.remove_if([](auto &&element) { return element.first; });
+        }
+
+    private:
+        bool publishing{false};
+        container_type once_list{};
+        container_type on_list{};
+    };
+
+    struct handler_data {
+        std::unique_ptr<base_handler> handler;
+        ENTT_ID_TYPE runtime_type;
+    };
+
+    template<typename Event>
+    static auto type() ENTT_NOEXCEPT {
+        if constexpr(is_named_type_v<Event>) {
+            return named_type_traits<Event>::value;
+        } else {
+            return handler_family::type<Event>;
+        }
+    }
+
+    template<typename Event>
+    event_handler<Event> * assure() const ENTT_NOEXCEPT {
+        const auto htype = type<Event>();
+        handler_data *hdata = nullptr;
+
+        if constexpr(is_named_type_v<Event>) {
+            const auto it = std::find_if(handlers.begin(), handlers.end(), [htype](const auto &candidate) {
+                return candidate.handler && candidate.runtime_type == htype;
+            });
+
+            hdata = (it == handlers.cend() ? &handlers.emplace_back() : &(*it));
+        } else {
+            if(!(htype < handlers.size())) {
+                handlers.resize(htype+1);
+            }
+
+            hdata = &handlers[htype];
+
+            if(hdata->handler && hdata->runtime_type != htype) {
+                handlers.emplace_back();
+                std::swap(handlers[htype], handlers.back());
+                hdata = &handlers[htype];
+            }
+        }
+
+        if(!hdata->handler) {
+            hdata->handler = std::make_unique<event_handler<Event>>();
+            hdata->runtime_type = htype;
+        }
+
+        return static_cast<event_handler<Event> *>(hdata->handler.get());
+    }
+
+public:
+    /** @brief Type of listeners accepted for the given event. */
+    template<typename Event>
+    using listener = typename event_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 event_handler<Event>::connection_type {
+        /** @brief Event emitters are friend classes of connections. */
+        friend class emitter;
+
+        /*! @brief Default constructor. */
+        connection() ENTT_NOEXCEPT = default;
+
+        /**
+         * @brief Creates a connection that wraps its underlying instance.
+         * @param conn A connection object to wrap.
+         */
+        connection(typename event_handler<Event>::connection_type conn)
+            : event_handler<Event>::connection_type{std::move(conn)}
+        {}
+    };
+
+    /*! @brief Default constructor. */
+    emitter() ENTT_NOEXCEPT = default;
+
+    /*! @brief Default destructor. */
+    virtual ~emitter() ENTT_NOEXCEPT {
+        static_assert(std::is_base_of_v<emitter<Derived>, Derived>);
+    }
+
+    /*! @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) {
+        assure<Event>()->publish({ std::forward<Args>(args)... }, *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 `void(const 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 `void(const 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) ENTT_NOEXCEPT {
+        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() ENTT_NOEXCEPT {
+        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 {
+        std::for_each(handlers.begin(), handlers.end(), [](auto &&hdata) {
+            return hdata.handler ? hdata.handler->clear() : void();
+        });
+    }
+
+    /**
+     * @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>
+    bool empty() const ENTT_NOEXCEPT {
+        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.
+     */
+    bool empty() const ENTT_NOEXCEPT {
+        return std::all_of(handlers.cbegin(), handlers.cend(), [](auto &&hdata) {
+            return !hdata.handler || hdata.handler->empty();
+        });
+    }
+
+private:
+    mutable std::vector<handler_data> handlers{};
+};
+
+
+}
+
+
+#endif // ENTT_SIGNAL_EMITTER_HPP

src/entt/signal/fwd.hpp

@@ -0,0 +1,27 @@
+#ifndef ENTT_SIGNAL_FWD_HPP
+#define ENTT_SIGNAL_FWD_HPP
+
+
+#include "../config/config.h"
+
+
+namespace entt {
+
+
+/*! @class delegate */
+template<typename>
+class delegate;
+
+/*! @class sink */
+template<typename>
+class sink;
+
+/*! @class sigh */
+template<typename, typename>
+struct sigh;
+
+
+}
+
+
+#endif // ENTT_SIGNAL_FWD_HPP

src/entt/signal/sigh.hpp

@@ -0,0 +1,357 @@
+#ifndef ENTT_SIGNAL_SIGH_HPP
+#define ENTT_SIGNAL_SIGH_HPP
+
+
+#include <algorithm>
+#include <utility>
+#include <vector>
+#include <functional>
+#include <type_traits>
+#include "../config/config.h"
+#include "delegate.hpp"
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename, typename>
+struct invoker;
+
+
+template<typename Ret, typename... Args, typename Collector>
+struct invoker<Ret(Args...), Collector> {
+    virtual ~invoker() = default;
+
+    bool invoke(Collector &collector, const delegate<Ret(Args...)> &delegate, Args... args) const {
+        return collector(delegate(args...));
+    }
+};
+
+
+template<typename... Args, typename Collector>
+struct invoker<void(Args...), Collector> {
+    virtual ~invoker() = default;
+
+    bool invoke(Collector &, const delegate<void(Args...)> &delegate, Args... args) const {
+        return (delegate(args...), true);
+    }
+};
+
+
+template<typename Ret>
+struct null_collector {
+    using result_type = Ret;
+    bool operator()(result_type) const ENTT_NOEXCEPT { return true; }
+};
+
+
+template<>
+struct null_collector<void> {
+    using result_type = void;
+    bool operator()() const ENTT_NOEXCEPT { return true; }
+};
+
+
+template<typename>
+struct default_collector;
+
+
+template<typename Ret, typename... Args>
+struct default_collector<Ret(Args...)> {
+    using collector_type = null_collector<Ret>;
+};
+
+
+template<typename Function>
+using default_collector_type = typename default_collector<Function>::collector_type;
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond TURN_OFF_DOXYGEN
+ */
+
+
+/**
+ * @brief Sink implementation.
+ *
+ * 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 declaration.
+ *
+ * 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.
+ * @tparam Collector Type of collector to use, if any.
+ */
+template<typename Function, typename Collector = internal::default_collector_type<Function>>
+struct sigh;
+
+
+/**
+ * @brief Sink implementation.
+ *
+ * A sink is an opaque object used to connect listeners to signals.<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 a class.
+ *
+ * @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...)> {
+    /*! @brief A signal is allowed to create sinks. */
+    template<typename, typename>
+    friend struct sigh;
+
+    template<typename Type>
+    Type * payload_type(Ret(*)(Type *, Args...));
+
+    sink(std::vector<delegate<Ret(Args...)>> *ref) ENTT_NOEXCEPT
+        : calls{ref}
+    {}
+
+public:
+    /**
+     * @brief Returns false if at least a listener is connected to the sink.
+     * @return True if the sink has no listeners connected, false otherwise.
+     */
+    bool empty() const ENTT_NOEXCEPT {
+        return calls->empty();
+    }
+
+    /**
+     * @brief Connects a free function to a signal.
+     *
+     * The signal handler performs checks to avoid multiple connections for free
+     * functions.
+     *
+     * @tparam Function A valid free function pointer.
+     */
+    template<auto Function>
+    void connect() {
+        disconnect<Function>();
+        delegate<Ret(Args...)> delegate{};
+        delegate.template connect<Function>();
+        calls->emplace_back(std::move(delegate));
+    }
+
+    /**
+     * @brief Connects a member function or a free function with payload 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 delegate. 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 delegate itself.
+     *
+     * @tparam Candidate Member or free function 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) {
+        if constexpr(std::is_member_function_pointer_v<decltype(Candidate)>) {
+            disconnect<Candidate>(value_or_instance);
+        } else {
+            disconnect<Candidate>();
+        }
+
+        delegate<Ret(Args...)> delegate{};
+        delegate.template connect<Candidate>(value_or_instance);
+        calls->emplace_back(std::move(delegate));
+    }
+
+    /**
+     * @brief Disconnects a free function from a signal.
+     * @tparam Function A valid free function pointer.
+     */
+    template<auto Function>
+    void disconnect() {
+        delegate<Ret(Args...)> delegate{};
+
+        if constexpr(std::is_invocable_r_v<Ret, decltype(Function), Args...>) {
+            delegate.template connect<Function>();
+        } else {
+            decltype(payload_type(Function)) payload = nullptr;
+            delegate.template connect<Function>(payload);
+        }
+
+        calls->erase(std::remove(calls->begin(), calls->end(), std::move(delegate)), calls->end());
+    }
+
+    /**
+     * @brief Disconnects a given member function from a signal.
+     * @tparam Member Member function to disconnect from the signal.
+     * @tparam Class Type of class to which the member function belongs.
+     * @param instance A valid instance of type pointer to `Class`.
+     */
+    template<auto Member, typename Class>
+    void disconnect(Class *instance) {
+        static_assert(std::is_member_function_pointer_v<decltype(Member)>);
+        delegate<Ret(Args...)> delegate{};
+        delegate.template connect<Member>(instance);
+        calls->erase(std::remove_if(calls->begin(), calls->end(), [&delegate](const auto &other) {
+            return other == delegate && other.instance() == delegate.instance();
+        }), calls->end());
+    }
+
+    /**
+     * @brief Disconnects all the listeners from a signal.
+     */
+    void disconnect() {
+        calls->clear();
+    }
+
+private:
+    std::vector<delegate<Ret(Args...)>> *calls;
+};
+
+
+/**
+ * @brief Unmanaged signal handler definition.
+ *
+ * Unmanaged signal handler. It works directly with naked pointers 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 used later to notify a bunch of listeners.
+ * * Collecting results from a set of functions like in a voting system.
+ *
+ * The default collector does nothing. To properly collect data, define and use
+ * a class that has a call operator the signature of which is `bool(Param)` and:
+ *
+ * * `Param` is a type to which `Ret` can be converted.
+ * * The return type is true if the handler must stop collecting data, false
+ * otherwise.
+ *
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ * @tparam Collector Type of collector to use, if any.
+ */
+template<typename Ret, typename... Args, typename Collector>
+struct sigh<Ret(Args...), Collector>: private internal::invoker<Ret(Args...), Collector> {
+    /*! @brief Unsigned integer type. */
+    using size_type = typename std::vector<delegate<Ret(Args...)>>::size_type;
+    /*! @brief Collector type. */
+    using collector_type = Collector;
+    /*! @brief Sink type. */
+    using sink_type = entt::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.
+     */
+    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.
+     */
+    bool empty() const ENTT_NOEXCEPT {
+        return calls.empty();
+    }
+
+    /**
+     * @brief Returns a sink object for the given signal.
+     *
+     * A sink is an opaque object used to connect listeners to signals.<br/>
+     * The function type for a listener is the one of the signal to which it
+     * belongs. The order of invocation of the listeners isn't guaranteed.
+     *
+     * @return A temporary sink object.
+     */
+    sink_type sink() ENTT_NOEXCEPT {
+        return { &calls };
+    }
+
+    /**
+     * @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 pos = calls.size(); pos; --pos) {
+            auto &call = calls[pos-1];
+            call(args...);
+        }
+    }
+
+    /**
+     * @brief Collects return values from the listeners.
+     * @param args Arguments to use to invoke listeners.
+     * @return An instance of the collector filled with collected data.
+     */
+    collector_type collect(Args... args) const {
+        collector_type collector;
+
+        for(auto &&call: calls) {
+            if(!this->invoke(collector, call, args...)) {
+                break;
+            }
+        }
+
+        return collector;
+    }
+
+    /**
+     * @brief Swaps listeners between the two signals.
+     * @param lhs A valid signal object.
+     * @param rhs A valid signal object.
+     */
+    friend void swap(sigh &lhs, sigh &rhs) {
+        using std::swap;
+        swap(lhs.calls, rhs.calls);
+    }
+
+private:
+    std::vector<delegate<Ret(Args...)>> calls;
+};
+
+
+}
+
+
+#endif // ENTT_SIGNAL_SIGH_HPP

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 +2,128 @@
 # Tests configuration
 #
 
-set(COMMON_LINK_LIBS gtest_main Threads::Threads)
+include_directories($<TARGET_PROPERTY:EnTT,INTERFACE_INCLUDE_DIRECTORIES>)
+add_compile_options($<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_OPTIONS>)
 
-# List of available targets
+macro(SETUP_LIBRARY_TARGET LIB_TARGET)
+    set_target_properties(${LIB_TARGET} PROPERTIES CXX_EXTENSIONS OFF)
+    target_compile_definitions(${LIB_TARGET} PRIVATE $<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_DEFINITIONS>)
+    target_compile_features(${LIB_TARGET} PRIVATE $<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_FEATURES>)
+    target_compile_options(${LIB_TARGET} PRIVATE $<$<NOT:$<CXX_COMPILER_ID:MSVC>>:-pedantic -Wall>)
+    target_compile_options(${LIB_TARGET} PRIVATE $<$<CXX_COMPILER_ID:MSVC>:/EHsc>)
+endmacro()
 
-set(TARGET_ENTT entt)
-set(TARGET_BENCHMARK benchmark)
+add_library(odr OBJECT odr.cpp)
+SETUP_LIBRARY_TARGET(odr)
 
-# Test TARGET_ENTT
+macro(SETUP_AND_ADD_TEST TEST_NAME TEST_SOURCE)
+    add_executable(${TEST_NAME} $<TARGET_OBJECTS:odr> ${TEST_SOURCE})
+    set_target_properties(${TEST_NAME} PROPERTIES CXX_EXTENSIONS OFF)
+    target_link_libraries(${TEST_NAME} PRIVATE EnTT GTest::Main Threads::Threads)
+    target_compile_definitions(${TEST_NAME} PRIVATE $<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_DEFINITIONS>)
+    target_compile_features(${TEST_NAME} PRIVATE $<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_FEATURES>)
+    target_compile_options(${TEST_NAME} PRIVATE $<$<NOT:$<CXX_COMPILER_ID:MSVC>>:-pedantic -Wall>)
+    target_compile_options(${TEST_NAME} PRIVATE $<$<CXX_COMPILER_ID:MSVC>:/EHsc>)
+    add_test(NAME ${TEST_NAME} COMMAND ${TEST_NAME})
+endmacro()
 
-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})
+# Test benchmark
 
-# Test TARGET_BENCHMARK
+if(BUILD_BENCHMARK)
+    SETUP_AND_ADD_TEST(benchmark benchmark/benchmark.cpp)
+endif()
 
-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()
+# Test lib
+
+if(BUILD_LIB)
+    add_library(a_module_shared SHARED lib/a_module.cpp)
+    add_library(a_module_static STATIC lib/a_module.cpp)
+    add_library(another_module_shared SHARED lib/another_module.cpp)
+    add_library(another_module_static STATIC lib/another_module.cpp)
+
+    SETUP_LIBRARY_TARGET(a_module_shared)
+    SETUP_LIBRARY_TARGET(a_module_static)
+    SETUP_LIBRARY_TARGET(another_module_shared)
+    SETUP_LIBRARY_TARGET(another_module_static)
+
+    SETUP_AND_ADD_TEST(lib_shared lib/lib.cpp)
+    target_link_libraries(lib_shared PRIVATE a_module_shared another_module_shared)
+
+    SETUP_AND_ADD_TEST(lib_static lib/lib.cpp)
+    target_link_libraries(lib_static PRIVATE a_module_static another_module_static)
+endif()
+
+# Test mod
+
+if(BUILD_MOD)
+    set(DUKTAPE_DEPS_DIR ${EnTT_SOURCE_DIR}/deps/duktape)
+    configure_file(${EnTT_SOURCE_DIR}/cmake/in/duktape.in ${DUKTAPE_DEPS_DIR}/CMakeLists.txt)
+    execute_process(COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" . WORKING_DIRECTORY ${DUKTAPE_DEPS_DIR})
+    execute_process(COMMAND ${CMAKE_COMMAND} --build . WORKING_DIRECTORY ${DUKTAPE_DEPS_DIR})
+    set(DUKTAPE_SRC_DIR ${DUKTAPE_DEPS_DIR}/src/src)
+
+    set(MOD_TEST_SOURCE ${DUKTAPE_SRC_DIR}/duktape.c mod/mod.cpp)
+    SETUP_AND_ADD_TEST(mod "${MOD_TEST_SOURCE}")
+    target_include_directories(mod PRIVATE ${DUKTAPE_SRC_DIR})
+endif()
+
+# Test snapshot
+
+if(BUILD_SNAPSHOT)
+    set(CEREAL_DEPS_DIR ${EnTT_SOURCE_DIR}/deps/cereal)
+    configure_file(${EnTT_SOURCE_DIR}/cmake/in/cereal.in ${CEREAL_DEPS_DIR}/CMakeLists.txt)
+    execute_process(COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" . WORKING_DIRECTORY ${CEREAL_DEPS_DIR})
+    execute_process(COMMAND ${CMAKE_COMMAND} --build . WORKING_DIRECTORY ${CEREAL_DEPS_DIR})
+    set(CEREAL_SRC_DIR ${CEREAL_DEPS_DIR}/src/include)
+
+    SETUP_AND_ADD_TEST(cereal snapshot/snapshot.cpp)
+    target_include_directories(cereal PRIVATE ${CEREAL_SRC_DIR})
+endif()
+
+# Test core
+
+SETUP_AND_ADD_TEST(algorithm entt/core/algorithm.cpp)
+SETUP_AND_ADD_TEST(family entt/core/family.cpp)
+SETUP_AND_ADD_TEST(hashed_string entt/core/hashed_string.cpp)
+SETUP_AND_ADD_TEST(ident entt/core/ident.cpp)
+SETUP_AND_ADD_TEST(monostate entt/core/monostate.cpp)
+SETUP_AND_ADD_TEST(type_traits entt/core/type_traits.cpp)
+SETUP_AND_ADD_TEST(utility entt/core/utility.cpp)
+
+# Test entity
+
+SETUP_AND_ADD_TEST(actor entt/entity/actor.cpp)
+SETUP_AND_ADD_TEST(entity entt/entity/entity.cpp)
+SETUP_AND_ADD_TEST(group entt/entity/group.cpp)
+SETUP_AND_ADD_TEST(helper entt/entity/helper.cpp)
+SETUP_AND_ADD_TEST(prototype entt/entity/prototype.cpp)
+SETUP_AND_ADD_TEST(registry entt/entity/registry.cpp)
+SETUP_AND_ADD_TEST(runtime_view entt/entity/runtime_view.cpp)
+SETUP_AND_ADD_TEST(snapshot entt/entity/snapshot.cpp)
+SETUP_AND_ADD_TEST(sparse_set entt/entity/sparse_set.cpp)
+SETUP_AND_ADD_TEST(storage entt/entity/storage.cpp)
+SETUP_AND_ADD_TEST(view entt/entity/view.cpp)
+
+# Test locator
+
+SETUP_AND_ADD_TEST(locator entt/locator/locator.cpp)
+
+# Test meta
+
+SETUP_AND_ADD_TEST(meta entt/meta/meta.cpp)
+
+# Test process
+
+SETUP_AND_ADD_TEST(process entt/process/process.cpp)
+SETUP_AND_ADD_TEST(scheduler entt/process/scheduler.cpp)
+
+# Test resource
+
+SETUP_AND_ADD_TEST(resource entt/resource/resource.cpp)
+
+# Test signal
+
+SETUP_AND_ADD_TEST(delegate entt/signal/delegate.cpp)
+SETUP_AND_ADD_TEST(dispatcher entt/signal/dispatcher.cpp)
+SETUP_AND_ADD_TEST(emitter entt/signal/emitter.cpp)
+SETUP_AND_ADD_TEST(sigh entt/signal/sigh.cpp)

test/benchmark.cpp

@@ -1,409 +0,0 @@
-#include <gtest/gtest.h>
-#include <registry.hpp>
-#include <iostream>
-#include <cstddef>
-#include <chrono>
-#include <vector>
-
-struct Position {
-    uint64_t x;
-    uint64_t y;
-};
-
-struct Velocity {
-    uint64_t x;
-    uint64_t y;
-};
-
-template<std::size_t>
-struct Comp {};
-
-struct Timer final {
-    Timer(): start{std::chrono::system_clock::now()} {}
-
-    void elapsed() {
-        auto now = std::chrono::system_clock::now();
-        std::cout << std::chrono::duration<double>(now - start).count() << " seconds" << std::endl;
-    }
-
-private:
-    std::chrono::time_point<std::chrono::system_clock> start;
-};
-
-TEST(DefaultRegistry, Construct) {
-    using registry_type = entt::DefaultRegistry<Position, Velocity>;
-
-    registry_type registry;
-
-    std::cout << "Constructing 10000000 entities" << std::endl;
-
-    Timer timer;
-    for (uint64_t i = 0; i < 10000000L; i++) {
-        registry.create();
-    }
-
-    timer.elapsed();
-    registry.reset();
-}
-
-TEST(DefaultRegistry, Destroy) {
-    using registry_type = entt::DefaultRegistry<Position, Velocity>;
-
-    registry_type registry;
-    std::vector<registry_type::entity_type> entities{};
-
-    std::cout << "Destroying 10000000 entities" << std::endl;
-
-    for (uint64_t i = 0; i < 10000000L; i++) {
-        entities.push_back(registry.create());
-    }
-
-    Timer timer;
-
-    for (auto entity: entities) {
-        registry.destroy(entity);
-    }
-
-    timer.elapsed();
-}
-
-TEST(DefaultRegistry, IterateCreateDeleteSingleComponent) {
-    using registry_type = entt::DefaultRegistry<Position, Velocity>;
-
-    registry_type registry;
-
-    std::cout << "Looping 10000 times creating and deleting a random number of entities" << std::endl;
-
-    Timer timer;
-
-    for(int i = 0; i < 10000; i++) {
-        for(int j = 0; j < 10000; j++) {
-            registry.create<Position>();
-        }
-
-        auto view = registry.view<Position>();
-
-        for(auto entity: view) {
-            if(rand() % 2 == 0) {
-                registry.destroy(entity);
-            }
-        }
-    }
-
-    timer.elapsed();
-    registry.reset();
-}
-
-TEST(DefaultRegistry, IterateSingleComponent10M) {
-    using registry_type = entt::DefaultRegistry<Position, Velocity>;
-
-    registry_type registry;
-
-    std::cout << "Iterating over 10000000 entities, one component" << std::endl;
-
-    for (uint64_t i = 0; i < 10000000L; i++) {
-        registry.create<Position>();
-    }
-
-    Timer timer;
-
-    auto view = registry.view<Position>();
-
-    for(auto entity: view) {
-        auto &position = registry.get<Position>(entity);
-        (void)position;
-    }
-
-    timer.elapsed();
-    registry.reset();
-}
-
-TEST(DefaultRegistry, IterateTwoComponents10M) {
-    using registry_type = entt::DefaultRegistry<Position, Velocity>;
-
-    registry_type registry;
-
-    std::cout << "Iterating over 10000000 entities, two components" << std::endl;
-
-    for (uint64_t i = 0; i < 10000000L; i++) {
-        registry.create<Position, Velocity>();
-    }
-
-    Timer timer;
-
-    auto view = registry.view<Position, Velocity>();
-
-    for(auto entity: view) {
-        auto &position = registry.get<Position>(entity);
-        auto &velocity = registry.get<Velocity>(entity);
-        (void)position;
-        (void)velocity;
-    }
-
-    timer.elapsed();
-    registry.reset();
-}
-
-TEST(DefaultRegistry, IterateTwoComponents10MHalf) {
-    using registry_type = entt::DefaultRegistry<Position, Velocity>;
-
-    registry_type registry;
-
-    std::cout << "Iterating over 10000000 entities, two components, half of the entities have all the components" << std::endl;
-
-    for (uint64_t i = 0; i < 10000000L; i++) {
-        auto entity = registry.create<Velocity>();
-        if(i % 2) { registry.assign<Position>(entity); }
-    }
-
-    Timer timer;
-
-    auto view = registry.view<Position, Velocity>();
-
-    for(auto entity: view) {
-        auto &position = registry.get<Position>(entity);
-        auto &velocity = registry.get<Velocity>(entity);
-        (void)position;
-        (void)velocity;
-    }
-
-    timer.elapsed();
-    registry.reset();
-}
-
-TEST(DefaultRegistry, IterateTwoComponents10MOne) {
-    using registry_type = entt::DefaultRegistry<Position, Velocity>;
-
-    registry_type registry;
-
-    std::cout << "Iterating over 10000000 entities, two components, only one entity has all the components" << std::endl;
-
-    for (uint64_t i = 0; i < 10000000L; i++) {
-        auto entity = registry.create<Velocity>();
-        if(i == 5000000L) { registry.assign<Position>(entity); }
-    }
-
-    Timer timer;
-
-    auto view = registry.view<Position, Velocity>();
-
-    for(auto entity: view) {
-        auto &position = registry.get<Position>(entity);
-        auto &velocity = registry.get<Velocity>(entity);
-        (void)position;
-        (void)velocity;
-    }
-
-    timer.elapsed();
-    registry.reset();
-}
-
-TEST(DefaultRegistry, IterateSingleComponent50M) {
-    using registry_type = entt::DefaultRegistry<Position, Velocity>;
-
-    registry_type registry;
-
-    std::cout << "Iterating over 50000000 entities, one component" << std::endl;
-
-    for (uint64_t i = 0; i < 50000000L; i++) {
-        registry.create<Position>();
-    }
-
-    Timer timer;
-
-    auto view = registry.view<Position>();
-
-    for(auto entity: view) {
-        auto &position = registry.get<Position>(entity);
-        (void)position;
-    }
-
-    timer.elapsed();
-    registry.reset();
-}
-
-TEST(DefaultRegistry, IterateTwoComponents50M) {
-    using registry_type = entt::DefaultRegistry<Position, Velocity>;
-
-    registry_type registry;
-
-    std::cout << "Iterating over 50000000 entities, two components" << std::endl;
-
-    for (uint64_t i = 0; i < 50000000L; i++) {
-        registry.create<Position, Velocity>();
-    }
-
-    Timer timer;
-
-    auto view = registry.view<Position, Velocity>();
-
-    for(auto entity: view) {
-        auto &position = registry.get<Position>(entity);
-        auto &velocity = registry.get<Velocity>(entity);
-        (void)position;
-        (void)velocity;
-    }
-
-    timer.elapsed();
-    registry.reset();
-}
-
-TEST(DefaultRegistry, IterateFiveComponents10M) {
-    using registry_type = entt::DefaultRegistry<Position, Velocity, Comp<1>, Comp<2>, Comp<3>>;
-
-    registry_type registry;
-
-    std::cout << "Iterating over 10000000 entities, five components" << std::endl;
-
-    for (uint64_t i = 0; i < 10000000L; i++) {
-        registry.create<Position, Velocity, Comp<1>, Comp<2>, Comp<3>>();
-    }
-
-    Timer timer;
-
-    auto view = registry.view<Position, Velocity, Comp<1>, Comp<2>, Comp<3>>();
-
-    for(auto entity: view) {
-        auto &position = registry.get<Position>(entity);
-        auto &velocity = registry.get<Velocity>(entity);
-        auto &comp1 = registry.get<Comp<1>>(entity);
-        auto &comp2 = registry.get<Comp<2>>(entity);
-        auto &comp3 = registry.get<Comp<3>>(entity);
-        (void)position;
-        (void)velocity;
-        (void)comp1;
-        (void)comp2;
-        (void)comp3;
-    }
-
-    timer.elapsed();
-    registry.reset();
-}
-
-TEST(DefaultRegistry, IterateTenComponents10M) {
-    using registry_type = entt::DefaultRegistry<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>;
-
-    registry_type registry;
-
-    std::cout << "Iterating over 10000000 entities, ten components" << std::endl;
-
-    for (uint64_t i = 0; i < 10000000L; i++) {
-        registry.create<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
-    }
-
-    Timer timer;
-
-    auto view = registry.view<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
-
-    for(auto entity: view) {
-        auto &position = registry.get<Position>(entity);
-        auto &velocity = registry.get<Velocity>(entity);
-        auto &comp1 = registry.get<Comp<1>>(entity);
-        auto &comp2 = registry.get<Comp<2>>(entity);
-        auto &comp3 = registry.get<Comp<3>>(entity);
-        auto &comp4 = registry.get<Comp<4>>(entity);
-        auto &comp5 = registry.get<Comp<5>>(entity);
-        auto &comp6 = registry.get<Comp<6>>(entity);
-        auto &comp7 = registry.get<Comp<7>>(entity);
-        auto &comp8 = registry.get<Comp<8>>(entity);
-        (void)position;
-        (void)velocity;
-        (void)comp1;
-        (void)comp2;
-        (void)comp3;
-        (void)comp4;
-        (void)comp5;
-        (void)comp6;
-        (void)comp7;
-        (void)comp8;
-    }
-
-    timer.elapsed();
-    registry.reset();
-}
-
-TEST(DefaultRegistry, IterateTenComponents10MHalf) {
-    using registry_type = entt::DefaultRegistry<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>;
-
-    registry_type registry;
-
-    std::cout << "Iterating over 10000000 entities, ten components, half of the entities have all the components" << std::endl;
-
-    for (uint64_t i = 0; i < 10000000L; i++) {
-        auto entity = registry.create<Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
-        if(i % 2) { registry.assign<Position>(entity); }
-    }
-
-    Timer timer;
-
-    auto view = registry.view<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
-
-    for(auto entity: view) {
-        auto &position = registry.get<Position>(entity);
-        auto &velocity = registry.get<Velocity>(entity);
-        auto &comp1 = registry.get<Comp<1>>(entity);
-        auto &comp2 = registry.get<Comp<2>>(entity);
-        auto &comp3 = registry.get<Comp<3>>(entity);
-        auto &comp4 = registry.get<Comp<4>>(entity);
-        auto &comp5 = registry.get<Comp<5>>(entity);
-        auto &comp6 = registry.get<Comp<6>>(entity);
-        auto &comp7 = registry.get<Comp<7>>(entity);
-        auto &comp8 = registry.get<Comp<8>>(entity);
-        (void)position;
-        (void)velocity;
-        (void)comp1;
-        (void)comp2;
-        (void)comp3;
-        (void)comp4;
-        (void)comp5;
-        (void)comp6;
-        (void)comp7;
-        (void)comp8;
-    }
-
-    timer.elapsed();
-    registry.reset();
-}
-
-TEST(DefaultRegistry, IterateTenComponents10MOne) {
-    using registry_type = entt::DefaultRegistry<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>;
-
-    registry_type registry;
-
-    std::cout << "Iterating over 10000000 entities, ten components, only one entity has all the components" << std::endl;
-
-    for (uint64_t i = 0; i < 10000000L; i++) {
-        auto entity = registry.create<Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
-        if(i == 5000000L) { registry.assign<Position>(entity); }
-    }
-
-    Timer timer;
-
-    auto view = registry.view<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
-
-    for(auto entity: view) {
-        auto &position = registry.get<Position>(entity);
-        auto &velocity = registry.get<Velocity>(entity);
-        auto &comp1 = registry.get<Comp<1>>(entity);
-        auto &comp2 = registry.get<Comp<2>>(entity);
-        auto &comp3 = registry.get<Comp<3>>(entity);
-        auto &comp4 = registry.get<Comp<4>>(entity);
-        auto &comp5 = registry.get<Comp<5>>(entity);
-        auto &comp6 = registry.get<Comp<6>>(entity);
-        auto &comp7 = registry.get<Comp<7>>(entity);
-        auto &comp8 = registry.get<Comp<8>>(entity);
-        (void)position;
-        (void)velocity;
-        (void)comp1;
-        (void)comp2;
-        (void)comp3;
-        (void)comp4;
-        (void)comp5;
-        (void)comp6;
-        (void)comp7;
-        (void)comp8;
-    }
-
-    timer.elapsed();
-    registry.reset();
-}

test/component_pool.cpp

@@ -1,166 +0,0 @@
-#include <cstddef>
-#include <gtest/gtest.h>
-#include <component_pool.hpp>
-
-TEST(ComponentPool, Functionalities) {
-    using pool_type = entt::ComponentPool<std::uint8_t, int, double>;
-
-    pool_type pool{0};
-
-    ASSERT_TRUE(pool.empty<int>());
-    ASSERT_TRUE(pool.empty<double>());
-    ASSERT_EQ(pool.capacity<int>(), pool_type::size_type{0});
-    ASSERT_EQ(pool.capacity<double>(), pool_type::size_type{0});
-    ASSERT_EQ(pool.size<int>(), pool_type::size_type{0});
-    ASSERT_EQ(pool.size<double>(), pool_type::size_type{0});
-    ASSERT_EQ(pool.begin<int>(), pool.end<int>());
-    ASSERT_EQ(pool.begin<double>(), pool.end<double>());
-    ASSERT_FALSE(pool.has<int>(0));
-    ASSERT_FALSE(pool.has<double>(0));
-}
-
-TEST(ComponentPool, ConstructDestroy) {
-    using pool_type = entt::ComponentPool<std::uint8_t, double, int>;
-
-    pool_type pool{4};
-
-    ASSERT_EQ(pool.construct<int>(0, 42), 42);
-    ASSERT_FALSE(pool.empty<int>());
-    ASSERT_TRUE(pool.empty<double>());
-    ASSERT_EQ(pool.capacity<int>(), pool_type::size_type{4});
-    ASSERT_EQ(pool.capacity<double>(), pool_type::size_type{4});
-    ASSERT_EQ(pool.size<int>(), pool_type::size_type{1});
-    ASSERT_EQ(pool.size<double>(), pool_type::size_type{0});
-    ASSERT_TRUE(pool.has<int>(0));
-    ASSERT_FALSE(pool.has<double>(0));
-    ASSERT_FALSE(pool.has<int>(1));
-    ASSERT_FALSE(pool.has<double>(1));
-
-    ASSERT_EQ(pool.construct<int>(1), 0);
-    ASSERT_FALSE(pool.empty<int>());
-    ASSERT_TRUE(pool.empty<double>());
-    ASSERT_EQ(pool.capacity<int>(), pool_type::size_type{4});
-    ASSERT_EQ(pool.capacity<double>(), pool_type::size_type{4});
-    ASSERT_EQ(pool.size<int>(), pool_type::size_type{2});
-    ASSERT_EQ(pool.size<double>(), pool_type::size_type{0});
-    ASSERT_TRUE(pool.has<int>(0));
-    ASSERT_FALSE(pool.has<double>(0));
-    ASSERT_TRUE(pool.has<int>(1));
-    ASSERT_FALSE(pool.has<double>(1));
-    ASSERT_NE(pool.get<int>(0), pool.get<int>(1));
-    ASSERT_NE(&pool.get<int>(0), &pool.get<int>(1));
-
-    ASSERT_NO_THROW(pool.destroy<int>(0));
-    ASSERT_FALSE(pool.empty<int>());
-    ASSERT_TRUE(pool.empty<double>());
-    ASSERT_EQ(pool.capacity<int>(), pool_type::size_type{4});
-    ASSERT_EQ(pool.capacity<double>(), pool_type::size_type{4});
-    ASSERT_EQ(pool.size<int>(), pool_type::size_type{1});
-    ASSERT_EQ(pool.size<double>(), pool_type::size_type{0});
-    ASSERT_FALSE(pool.has<int>(0));
-    ASSERT_FALSE(pool.has<double>(0));
-    ASSERT_TRUE(pool.has<int>(1));
-    ASSERT_FALSE(pool.has<double>(1));
-
-    ASSERT_NO_THROW(pool.destroy<int>(1));
-    ASSERT_TRUE(pool.empty<int>());
-    ASSERT_TRUE(pool.empty<double>());
-    ASSERT_EQ(pool.capacity<int>(), pool_type::size_type{4});
-    ASSERT_EQ(pool.capacity<double>(), pool_type::size_type{4});
-    ASSERT_EQ(pool.size<int>(), pool_type::size_type{0});
-    ASSERT_EQ(pool.size<int>(), pool_type::size_type{0});
-    ASSERT_FALSE(pool.has<int>(0));
-    ASSERT_FALSE(pool.has<double>(0));
-    ASSERT_FALSE(pool.has<int>(1));
-    ASSERT_FALSE(pool.has<double>(1));
-
-    int *comp[] = {
-        &pool.construct<int>(0, 0),
-        &pool.construct<int>(1, 1),
-        nullptr,
-        &pool.construct<int>(3, 3)
-    };
-
-    ASSERT_FALSE(pool.empty<int>());
-    ASSERT_TRUE(pool.empty<double>());
-    ASSERT_EQ(pool.capacity<int>(), pool_type::size_type{4});
-    ASSERT_EQ(pool.capacity<double>(), pool_type::size_type{4});
-    ASSERT_EQ(pool.size<int>(), pool_type::size_type{3});
-    ASSERT_EQ(pool.size<double>(), pool_type::size_type{0});
-    ASSERT_TRUE(pool.has<int>(0));
-    ASSERT_FALSE(pool.has<double>(0));
-    ASSERT_TRUE(pool.has<int>(1));
-    ASSERT_FALSE(pool.has<double>(1));
-    ASSERT_FALSE(pool.has<int>(2));
-    ASSERT_FALSE(pool.has<double>(2));
-    ASSERT_TRUE(pool.has<int>(3));
-    ASSERT_FALSE(pool.has<double>(3));
-    ASSERT_EQ(&pool.get<int>(0), comp[0]);
-    ASSERT_EQ(&pool.get<int>(1), comp[1]);
-    ASSERT_EQ(&pool.get<int>(3), comp[3]);
-    ASSERT_EQ(pool.get<int>(0), 0);
-    ASSERT_EQ(pool.get<int>(1), 1);
-    ASSERT_EQ(pool.get<int>(3), 3);
-
-    ASSERT_NO_THROW(pool.destroy<int>(0));
-    ASSERT_NO_THROW(pool.destroy<int>(1));
-    ASSERT_NO_THROW(pool.destroy<int>(3));
-}
-
-TEST(ComponentPool, HasGet) {
-    using pool_type = entt::ComponentPool<std::uint8_t, int, char>;
-
-    pool_type pool;
-    const pool_type &cpool = pool;
-
-    int &comp = pool.construct<int>(0, 42);
-
-    ASSERT_EQ(pool.get<int>(0), comp);
-    ASSERT_EQ(pool.get<int>(0), 42);
-    ASSERT_TRUE(pool.has<int>(0));
-
-    ASSERT_EQ(cpool.get<int>(0), comp);
-    ASSERT_EQ(cpool.get<int>(0), 42);
-    ASSERT_TRUE(cpool.has<int>(0));
-
-    ASSERT_NO_THROW(pool.destroy<int>(0));
-}
-
-TEST(ComponentPool, BeginEndReset) {
-    using pool_type = entt::ComponentPool<std::uint8_t, int, char>;
-
-    pool_type pool{2};
-
-    ASSERT_EQ(pool.construct<int>(0, 0), 0);
-    ASSERT_EQ(pool.construct<int>(2, 2), 2);
-    ASSERT_EQ(pool.construct<int>(3, 3), 3);
-    ASSERT_EQ(pool.construct<int>(1, 1), 1);
-
-    ASSERT_EQ(pool.size<int>(), decltype(pool.size<int>()){4});
-    ASSERT_EQ(*(pool.begin<int>()+0), typename pool_type::entity_type{0});
-    ASSERT_EQ(*(pool.begin<int>()+1), typename pool_type::entity_type{2});
-    ASSERT_EQ(*(pool.begin<int>()+2), typename pool_type::entity_type{3});
-    ASSERT_EQ(*(pool.begin<int>()+3), typename pool_type::entity_type{1});
-
-    pool.destroy<int>(2);
-
-    ASSERT_EQ(pool.size<int>(), decltype(pool.size<int>()){3});
-    ASSERT_EQ(*(pool.begin<int>()+0), typename pool_type::entity_type{0});
-    ASSERT_EQ(*(pool.begin<int>()+1), typename pool_type::entity_type{1});
-    ASSERT_EQ(*(pool.begin<int>()+2), typename pool_type::entity_type{3});
-
-    ASSERT_EQ(pool.construct<char>(0, 'c'), 'c');
-
-    ASSERT_FALSE(pool.empty<int>());
-    ASSERT_FALSE(pool.empty<char>());
-
-    ASSERT_NO_THROW(pool.reset<char>());
-
-    ASSERT_FALSE(pool.empty<int>());
-    ASSERT_TRUE(pool.empty<char>());
-
-    ASSERT_NO_THROW(pool.reset());
-
-    ASSERT_TRUE(pool.empty<int>());
-    ASSERT_TRUE(pool.empty<char>());
-}

test/entt/core/algorithm.cpp

@@ -0,0 +1,33 @@
+#include <array>
+#include <gtest/gtest.h>
+#include <entt/core/algorithm.hpp>
+
+TEST(Algorithm, StdSort) {
+    // well, I'm pretty sure it works, it's std::sort!!
+    std::array<int, 5> arr{{4, 1, 3, 2, 0}};
+    entt::std_sort sort;
+
+    sort(arr.begin(), arr.end());
+
+    for(typename decltype(arr)::size_type i = 0; i < (arr.size() - 1); ++i) {
+        ASSERT_LT(arr[i], arr[i+1]);
+    }
+}
+
+TEST(Algorithm, InsertionSort) {
+    std::array<int, 5> arr{{4, 1, 3, 2, 0}};
+    entt::insertion_sort sort;
+
+    sort(arr.begin(), arr.end());
+
+    for(typename decltype(arr)::size_type i = 0; i < (arr.size() - 1); ++i) {
+        ASSERT_LT(arr[i], arr[i+1]);
+    }
+}
+
+TEST(Algorithm, InsertionSortEmptyContainer) {
+    std::vector<int> vec{};
+    entt::insertion_sort sort;
+    // this should crash with asan enabled if we break the constraint
+    sort(vec.begin(), vec.end());
+}

test/entt/core/family.cpp

@@ -0,0 +1,22 @@
+#include <gtest/gtest.h>
+#include <entt/core/family.hpp>
+
+using a_family = entt::family<struct a_family_type>;
+using another_family = entt::family<struct another_family_type>;
+
+TEST(Family, Functionalities) {
+    auto t1 = a_family::type<int>;
+    auto t2 = a_family::type<int>;
+    auto t3 = a_family::type<char>;
+    auto t4 = another_family::type<double>;
+
+    ASSERT_EQ(t1, t2);
+    ASSERT_NE(t1, t3);
+    ASSERT_EQ(t1, t4);
+}
+
+TEST(Family, Uniqueness) {
+    ASSERT_EQ(a_family::type<int>, a_family::type<int &>);
+    ASSERT_EQ(a_family::type<int>, a_family::type<int &&>);
+    ASSERT_EQ(a_family::type<int>, a_family::type<const int &>);
+}

test/entt/core/hashed_string.cpp

@@ -0,0 +1,64 @@
+#include <string>
+#include <string_view>
+#include <type_traits>
+#include <gtest/gtest.h>
+#include <entt/core/hashed_string.hpp>
+
+TEST(HashedString, Functionalities) {
+    using hash_type = entt::hashed_string::hash_type;
+
+    const char *bar = "bar";
+
+    auto foo_hs = entt::hashed_string{"foo"};
+    auto bar_hs = entt::hashed_string{bar};
+
+    ASSERT_NE(static_cast<hash_type>(foo_hs), static_cast<hash_type>(bar_hs));
+    ASSERT_STREQ(static_cast<const char *>(foo_hs), "foo");
+    ASSERT_STREQ(static_cast<const char *>(bar_hs), bar);
+    ASSERT_STREQ(foo_hs.data(), "foo");
+    ASSERT_STREQ(bar_hs.data(), bar);
+
+    ASSERT_EQ(foo_hs, foo_hs);
+    ASSERT_NE(foo_hs, bar_hs);
+
+    entt::hashed_string hs{"foobar"};
+
+    ASSERT_EQ(static_cast<hash_type>(hs), 0xbf9cf968);
+    ASSERT_EQ(hs.value(), 0xbf9cf968);
+
+    ASSERT_EQ(foo_hs, "foo"_hs);
+    ASSERT_NE(bar_hs, "foo"_hs);
+}
+
+TEST(HashedString, Empty) {
+    using hash_type = entt::hashed_string::hash_type;
+
+    entt::hashed_string hs{};
+
+    ASSERT_EQ(static_cast<hash_type>(hs), hash_type{});
+    ASSERT_EQ(static_cast<const char *>(hs), nullptr);
+}
+
+TEST(HashedString, Constexprness) {
+    using hash_type = entt::hashed_string::hash_type;
+    // how would you test a constexpr otherwise?
+    (void)std::integral_constant<hash_type, entt::hashed_string{"quux"}>{};
+    (void)std::integral_constant<hash_type, "quux"_hs>{};
+    ASSERT_TRUE(true);
+}
+
+TEST(HashedString, ToValue) {
+    using hash_type = entt::hashed_string::hash_type;
+
+    const char *foobar = "foobar";
+
+    ASSERT_EQ(entt::hashed_string::to_value(foobar), 0xbf9cf968);
+    // how would you test a constexpr otherwise?
+    (void)std::integral_constant<hash_type, entt::hashed_string::to_value("quux")>{};
+}
+
+TEST(HashedString, StringView) {
+    std::string str{"__foobar__"};
+    std::string_view view{str.data()+2, 6};
+    ASSERT_EQ(entt::hashed_string::to_value(view.data(), view.size()), 0xbf9cf968);
+}

test/entt/core/ident.cpp

@@ -0,0 +1,32 @@
+#include <type_traits>
+#include <gtest/gtest.h>
+#include <entt/core/ident.hpp>
+
+struct a_type {};
+struct another_type {};
+
+TEST(Identifier, Uniqueness) {
+    using id = entt::identifier<a_type, another_type>;
+    constexpr a_type an_instance;
+    constexpr another_type another_instance;
+
+    ASSERT_NE(id::type<a_type>, id::type<another_type>);
+    ASSERT_EQ(id::type<a_type>, id::type<decltype(an_instance)>);
+    ASSERT_NE(id::type<a_type>, id::type<decltype(another_instance)>);
+    ASSERT_EQ(id::type<a_type>, id::type<a_type>);
+    ASSERT_EQ(id::type<another_type>, id::type<another_type>);
+
+    // test uses in constant expressions
+    switch(id::type<another_type>) {
+    case id::type<a_type>:
+        FAIL();
+    case id::type<another_type>:
+        SUCCEED();
+    }
+}
+
+TEST(Identifier, SingleType) {
+    using id = entt::identifier<a_type>;
+    std::integral_constant<id::identifier_type, id::type<a_type>> ic;
+    (void)ic;
+}

test/entt/core/monostate.cpp

@@ -0,0 +1,20 @@
+#include <gtest/gtest.h>
+#include <entt/core/hashed_string.hpp>
+#include <entt/core/monostate.hpp>
+
+TEST(Monostate, Functionalities) {
+    const bool b_pre = entt::monostate<entt::hashed_string{"foobar"}>{};
+    const int i_pre = entt::monostate<"foobar"_hs>{};
+
+    ASSERT_FALSE(b_pre);
+    ASSERT_EQ(i_pre, int{});
+
+    entt::monostate<"foobar"_hs>{} = true;
+    entt::monostate_v<"foobar"_hs> = 42;
+
+    const bool &b_post = entt::monostate<"foobar"_hs>{};
+    const int &i_post = entt::monostate_v<entt::hashed_string{"foobar"}>;
+
+    ASSERT_TRUE(b_post);
+    ASSERT_EQ(i_post, 42);
+}

test/entt/core/type_traits.cpp

@@ -0,0 +1,14 @@
+#include <gtest/gtest.h>
+#include <entt/core/type_traits.hpp>
+
+TEST(TypeList, Functionalities) {
+    using type = entt::type_list<int, char>;
+    using other = entt::type_list<double>;
+
+    ASSERT_EQ((type::size), (decltype(type::size){2}));
+    ASSERT_EQ((other::size), (decltype(other::size){1}));
+    ASSERT_TRUE((std::is_same_v<entt::type_list_cat_t<type, other, type, other>, entt::type_list<int, char, double, int, char, double>>));
+    ASSERT_TRUE((std::is_same_v<entt::type_list_cat_t<type, other>, entt::type_list<int, char, double>>));
+    ASSERT_TRUE((std::is_same_v<entt::type_list_cat_t<type, type>, entt::type_list<int, char, int, char>>));
+    ASSERT_TRUE((std::is_same_v<entt::type_list_unique_t<entt::type_list_cat_t<type, type>>, entt::type_list<int, char>>));
+}

test/entt/core/utility.cpp

@@ -0,0 +1,19 @@
+#include <gtest/gtest.h>
+#include <entt/core/utility.hpp>
+
+struct Functions {
+    static void foo(int) {}
+    static void foo() {}
+
+    void bar(int) {}
+    void bar() {}
+};
+
+
+TEST(Utility, Overload) {
+    ASSERT_EQ(entt::overload<void(int)>(&Functions::foo), static_cast<void(*)(int)>(&Functions::foo));
+    ASSERT_EQ(entt::overload<void()>(&Functions::foo), static_cast<void(*)()>(&Functions::foo));
+
+    ASSERT_EQ(entt::overload<void(int)>(&Functions::bar), static_cast<void(Functions:: *)(int)>(&Functions::bar));
+    ASSERT_EQ(entt::overload<void()>(&Functions::bar), static_cast<void(Functions:: *)()>(&Functions::bar));
+}

test/entt/entity/actor.cpp

@@ -0,0 +1,59 @@
+#include <functional>
+#include <gtest/gtest.h>
+#include <entt/entity/actor.hpp>
+#include <entt/entity/registry.hpp>
+
+TEST(Actor, Component) {
+    entt::registry registry;
+    entt::actor actor{registry};
+
+    ASSERT_EQ(&registry, &actor.backend());
+    ASSERT_EQ(&registry, &std::as_const(actor).backend());
+    ASSERT_TRUE(registry.empty<int>());
+    ASSERT_FALSE(registry.empty());
+    ASSERT_FALSE(actor.has<int>());
+
+    const auto &cint = actor.assign<int>();
+    const auto &cchar = actor.assign<char>();
+
+    ASSERT_EQ(&cint, &actor.get<int>());
+    ASSERT_EQ(&cchar, &std::as_const(actor).get<char>());
+    ASSERT_EQ(&cint, &std::get<0>(actor.get<int, char>()));
+    ASSERT_EQ(&cchar, &std::get<1>(actor.get<int, char>()));
+    ASSERT_EQ(&cint, std::get<0>(actor.try_get<int, char, double>()));
+    ASSERT_EQ(&cchar, std::get<1>(actor.try_get<int, char, double>()));
+    ASSERT_EQ(nullptr, std::get<2>(actor.try_get<int, char, double>()));
+    ASSERT_EQ(nullptr, actor.try_get<double>());
+    ASSERT_EQ(&cchar, actor.try_get<char>());
+    ASSERT_EQ(&cint, actor.try_get<int>());
+
+    ASSERT_FALSE(registry.empty<int>());
+    ASSERT_FALSE(registry.empty());
+    ASSERT_TRUE(actor.has<int>());
+    ASSERT_TRUE(actor.has<char>());
+    ASSERT_FALSE(actor.has<double>());
+
+    actor.remove<int>();
+
+    ASSERT_TRUE(registry.empty<int>());
+    ASSERT_FALSE(registry.empty());
+    ASSERT_FALSE(actor.has<int>());
+}
+
+TEST(Actor, EntityLifetime) {
+    entt::registry registry;
+    auto *actor = new entt::actor{registry};
+    actor->assign<int>();
+
+    ASSERT_FALSE(registry.empty<int>());
+    ASSERT_FALSE(registry.empty());
+
+    registry.each([actor](const auto entity) {
+        ASSERT_EQ(actor->entity(), entity);
+    });
+
+    delete actor;
+
+    ASSERT_TRUE(registry.empty<int>());
+    ASSERT_TRUE(registry.empty());
+}

test/entt/entity/entity.cpp

@@ -0,0 +1,24 @@
+#include <functional>
+#include <gtest/gtest.h>
+#include <entt/entity/entity.hpp>
+#include <entt/entity/registry.hpp>
+
+TEST(Traits, Null) {
+    entt::registry registry{};
+
+    const auto entity = registry.create();
+    registry.assign<int>(entity, 42);
+
+    ASSERT_TRUE(~entt::entity{} == entt::null);
+
+    ASSERT_TRUE(entt::null == entt::null);
+    ASSERT_FALSE(entt::null != entt::null);
+
+    ASSERT_FALSE(entity == entt::null);
+    ASSERT_FALSE(entt::null == entity);
+
+    ASSERT_TRUE(entity != entt::null);
+    ASSERT_TRUE(entt::null != entity);
+
+    ASSERT_FALSE(registry.valid(entt::null));
+}

test/entt/entity/group.cpp

@@ -0,0 +1,892 @@
+#include <utility>
+#include <iterator>
+#include <algorithm>
+#include <gtest/gtest.h>
+#include <entt/entity/registry.hpp>
+#include <entt/entity/group.hpp>
+
+struct empty_type {};
+struct boxed_int { int value; };
+
+TEST(NonOwningGroup, Functionalities) {
+    entt::registry registry;
+    auto group = registry.group<>(entt::get<int, char>);
+    auto cgroup = std::as_const(registry).group<>(entt::get<const int, const char>);
+
+    ASSERT_TRUE(group.empty());
+    ASSERT_TRUE(group.empty<int>());
+    ASSERT_TRUE(cgroup.empty<const char>());
+
+    const auto e0 = registry.create();
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    ASSERT_FALSE(group.empty());
+    ASSERT_FALSE(group.empty<int>());
+    ASSERT_FALSE(cgroup.empty<const char>());
+    ASSERT_NO_THROW((group.begin()++));
+    ASSERT_NO_THROW((++cgroup.begin()));
+
+    ASSERT_NE(group.begin(), group.end());
+    ASSERT_NE(cgroup.begin(), cgroup.end());
+    ASSERT_EQ(group.size(), typename decltype(group)::size_type{1});
+    ASSERT_EQ(group.size<int>(), typename decltype(group)::size_type{1});
+    ASSERT_EQ(cgroup.size<const char>(), typename decltype(group)::size_type{2});
+
+    registry.assign<int>(e0);
+
+    ASSERT_EQ(group.size(), typename decltype(group)::size_type{2});
+    ASSERT_EQ(group.size<int>(), typename decltype(group)::size_type{2});
+    ASSERT_EQ(cgroup.size<const char>(), typename decltype(group)::size_type{2});
+
+    registry.remove<int>(e0);
+
+    ASSERT_EQ(group.size(), typename decltype(group)::size_type{1});
+    ASSERT_EQ(group.size<int>(), typename decltype(group)::size_type{1});
+    ASSERT_EQ(cgroup.size<const char>(), typename decltype(group)::size_type{2});
+
+    registry.get<char>(e0) = '1';
+    registry.get<char>(e1) = '2';
+    registry.get<int>(e1) = 42;
+
+    for(auto entity: group) {
+        ASSERT_EQ(std::get<0>(cgroup.get<const int, const char>(entity)), 42);
+        ASSERT_EQ(std::get<1>(group.get<int, char>(entity)), '2');
+        ASSERT_EQ(cgroup.get<const char>(entity), '2');
+    }
+
+    ASSERT_EQ(*(group.data() + 0), e1);
+
+    ASSERT_EQ(*(group.data<int>() + 0), e1);
+    ASSERT_EQ(*(group.data<char>() + 0), e0);
+    ASSERT_EQ(*(cgroup.data<const char>() + 1), e1);
+
+    ASSERT_EQ(*(group.raw<int>() + 0), 42);
+    ASSERT_EQ(*(group.raw<char>() + 0), '1');
+    ASSERT_EQ(*(cgroup.raw<const char>() + 1), '2');
+
+    registry.remove<char>(e0);
+    registry.remove<char>(e1);
+
+    ASSERT_EQ(group.begin(), group.end());
+    ASSERT_EQ(cgroup.begin(), cgroup.end());
+    ASSERT_TRUE(group.empty());
+
+    ASSERT_TRUE(group.capacity());
+
+    group.shrink_to_fit();
+
+    ASSERT_FALSE(group.capacity());
+}
+
+TEST(NonOwningGroup, ElementAccess) {
+    entt::registry registry;
+    auto group = registry.group<>(entt::get<int, char>);
+    auto cgroup = std::as_const(registry).group<>(entt::get<const int, const char>);
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0);
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    for(typename decltype(group)::size_type i{}; i < group.size(); ++i) {
+        ASSERT_EQ(group[i], i ? e0 : e1);
+        ASSERT_EQ(cgroup[i], i ? e0 : e1);
+    }
+}
+
+TEST(NonOwningGroup, Contains) {
+    entt::registry registry;
+    auto group = registry.group<>(entt::get<int, char>);
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0);
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    registry.destroy(e0);
+
+    ASSERT_FALSE(group.contains(e0));
+    ASSERT_TRUE(group.contains(e1));
+}
+
+TEST(NonOwningGroup, Empty) {
+    entt::registry registry;
+
+    const auto e0 = registry.create();
+    registry.assign<double>(e0);
+    registry.assign<int>(e0);
+    registry.assign<float>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<char>(e1);
+    registry.assign<float>(e1);
+
+    for(auto entity: registry.group<>(entt::get<char, int, float>)) {
+        (void)entity;
+        FAIL();
+    }
+
+    for(auto entity: registry.group<>(entt::get<double, char, int, float>)) {
+        (void)entity;
+        FAIL();
+    }
+}
+
+TEST(NonOwningGroup, Each) {
+    entt::registry registry;
+    auto group = registry.group<>(entt::get<int, char>);
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0);
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    auto cgroup = std::as_const(registry).group<>(entt::get<const int, const char>);
+    std::size_t cnt = 0;
+
+    group.each([&cnt](auto, int &, char &) { ++cnt; });
+    group.each([&cnt](int &, char &) { ++cnt; });
+
+    ASSERT_EQ(cnt, std::size_t{4});
+
+    cgroup.each([&cnt](auto, const int &, const char &) { --cnt; });
+    cgroup.each([&cnt](const int &, const char &) { --cnt; });
+
+    ASSERT_EQ(cnt, std::size_t{0});
+}
+
+TEST(NonOwningGroup, Sort) {
+    entt::registry registry;
+    auto group = registry.group<>(entt::get<const int, unsigned int>);
+
+    const auto e0 = registry.create();
+    const auto e1 = registry.create();
+    const auto e2 = registry.create();
+
+    auto uval = 0u;
+    auto ival = 0;
+
+    registry.assign<unsigned int>(e0, uval++);
+    registry.assign<unsigned int>(e1, uval++);
+    registry.assign<unsigned int>(e2, uval++);
+
+    registry.assign<int>(e0, ival++);
+    registry.assign<int>(e1, ival++);
+    registry.assign<int>(e2, ival++);
+
+    for(auto entity: group) {
+        ASSERT_EQ(group.get<unsigned int>(entity), --uval);
+        ASSERT_EQ(group.get<const int>(entity), --ival);
+    }
+
+    registry.sort<unsigned int>(std::less<unsigned int>{});
+    group.sort<unsigned int>();
+
+    for(auto entity: group) {
+        ASSERT_EQ(group.get<unsigned int>(entity), uval++);
+        ASSERT_EQ(group.get<const int>(entity), ival++);
+    }
+}
+
+TEST(NonOwningGroup, IndexRebuiltOnDestroy) {
+    entt::registry registry;
+    auto group = registry.group<>(entt::get<int, unsigned int>);
+
+    const auto e0 = registry.create();
+    const auto e1 = registry.create();
+
+    registry.assign<unsigned int>(e0, 0u);
+    registry.assign<unsigned int>(e1, 1u);
+
+    registry.assign<int>(e0, 0);
+    registry.assign<int>(e1, 1);
+
+    registry.destroy(e0);
+    registry.assign<int>(registry.create(), 42);
+
+    ASSERT_EQ(group.size(), typename decltype(group)::size_type{1});
+    ASSERT_EQ(group[{}], e1);
+    ASSERT_EQ(group.get<int>(e1), 1);
+    ASSERT_EQ(group.get<unsigned int>(e1), 1u);
+
+    group.each([e1](auto entity, auto ivalue, auto uivalue) {
+        ASSERT_EQ(entity, e1);
+        ASSERT_EQ(ivalue, 1);
+        ASSERT_EQ(uivalue, 1u);
+    });
+}
+
+TEST(NonOwningGroup, ConstNonConstAndAllInBetween) {
+    entt::registry registry;
+    auto group = registry.group<>(entt::get<int, const char>);
+
+    ASSERT_EQ(group.size(), decltype(group.size()){0});
+
+    const auto entity = registry.create();
+    registry.assign<int>(entity, 0);
+    registry.assign<char>(entity, 'c');
+
+    ASSERT_EQ(group.size(), decltype(group.size()){1});
+
+    ASSERT_TRUE((std::is_same_v<decltype(group.get<int>(0)), int &>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.get<const char>(0)), const char &>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.get<int, const char>(0)), std::tuple<int &, const char &>>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.raw<const char>()), const char *>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.raw<int>()), int *>));
+
+    group.each([](auto, auto &&i, auto &&c) {
+        ASSERT_TRUE((std::is_same_v<decltype(i), int &>));
+        ASSERT_TRUE((std::is_same_v<decltype(c), const char &>));
+    });
+}
+
+TEST(NonOwningGroup, Find) {
+    entt::registry registry;
+    auto group = registry.group<>(entt::get<int, const char>);
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0);
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    const auto e2 = registry.create();
+    registry.assign<int>(e2);
+    registry.assign<char>(e2);
+
+    const auto e3 = registry.create();
+    registry.assign<int>(e3);
+    registry.assign<char>(e3);
+
+    registry.remove<int>(e1);
+
+    ASSERT_NE(group.find(e0), group.end());
+    ASSERT_EQ(group.find(e1), group.end());
+    ASSERT_NE(group.find(e2), group.end());
+    ASSERT_NE(group.find(e3), group.end());
+
+    auto it = group.find(e2);
+
+    ASSERT_EQ(*it, e2);
+    ASSERT_EQ(*(++it), e3);
+    ASSERT_EQ(*(++it), e0);
+    ASSERT_EQ(++it, group.end());
+    ASSERT_EQ(++group.find(e0), group.end());
+
+    const auto e4 = registry.create();
+    registry.destroy(e4);
+    const auto e5 = registry.create();
+    registry.assign<int>(e5);
+    registry.assign<char>(e5);
+
+    ASSERT_NE(group.find(e5), group.end());
+    ASSERT_EQ(group.find(e4), group.end());
+}
+
+TEST(NonOwningGroup, ExcludedComponents) {
+    entt::registry registry;
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0, 0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1, 1);
+    registry.assign<char>(e1);
+
+    const auto group = registry.group<>(entt::get<int>, entt::exclude<char>);
+
+    const auto e2 = registry.create();
+    registry.assign<int>(e2, 2);
+
+    const auto e3 = registry.create();
+    registry.assign<int>(e3, 3);
+    registry.assign<char>(e3);
+
+    for(const auto entity: group) {
+        if(entity == e0) {
+            ASSERT_EQ(group.get<int>(e0), 0);
+        } else if(entity == e2) {
+            ASSERT_EQ(group.get<int>(e2), 2);
+        } else {
+            FAIL();
+        }
+    }
+
+    registry.assign<char>(e0);
+    registry.assign<char>(e2);
+
+    ASSERT_TRUE(group.empty());
+
+    registry.remove<char>(e1);
+    registry.remove<char>(e3);
+
+    for(const auto entity: group) {
+        if(entity == e1) {
+            ASSERT_EQ(group.get<int>(e1), 1);
+        } else if(entity == e3) {
+            ASSERT_EQ(group.get<int>(e3), 3);
+        } else {
+            FAIL();
+        }
+    }
+}
+
+TEST(NonOwningGroup, EmptyAndNonEmptyTypes) {
+    entt::registry registry;
+    const auto group = registry.group<>(entt::get<int, empty_type>);
+
+    const auto e0 = registry.create();
+    registry.assign<empty_type>(e0);
+    registry.assign<int>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<empty_type>(e1);
+    registry.assign<int>(e1);
+
+    registry.assign<int>(registry.create());
+
+    for(const auto entity: group) {
+        ASSERT_TRUE(entity == e0 || entity == e1);
+    }
+
+    group.each([e0, e1](const auto entity, const int &, empty_type) {
+        ASSERT_TRUE(entity == e0 || entity == e1);
+    });
+
+    ASSERT_EQ(group.size(), typename decltype(group)::size_type{2});
+}
+
+TEST(NonOwningGroup, TrackEntitiesOnComponentDestruction) {
+    entt::registry registry;
+    const auto group = registry.group<>(entt::get<int>, entt::exclude<char>);
+    const auto cgroup = std::as_const(registry).group<>(entt::get<const int>, entt::exclude<char>);
+
+    const auto entity = registry.create();
+    registry.assign<int>(entity);
+    registry.assign<char>(entity);
+
+    ASSERT_TRUE(group.empty());
+    ASSERT_TRUE(cgroup.empty());
+
+    registry.remove<char>(entity);
+
+    ASSERT_FALSE(group.empty());
+    ASSERT_FALSE(cgroup.empty());
+}
+
+TEST(OwningGroup, Functionalities) {
+    entt::registry registry;
+    auto group = registry.group<int>(entt::get<char>);
+    auto cgroup = std::as_const(registry).group<const int>(entt::get<const char>);
+
+    ASSERT_TRUE(group.empty());
+    ASSERT_TRUE(group.empty<int>());
+    ASSERT_TRUE(cgroup.empty<const char>());
+
+    const auto e0 = registry.create();
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    ASSERT_FALSE(group.empty());
+    ASSERT_FALSE(group.empty<int>());
+    ASSERT_FALSE(cgroup.empty<const char>());
+    ASSERT_NO_THROW((group.begin()++));
+    ASSERT_NO_THROW((++cgroup.begin()));
+
+    ASSERT_NE(group.begin(), group.end());
+    ASSERT_NE(cgroup.begin(), cgroup.end());
+    ASSERT_EQ(group.size(), typename decltype(group)::size_type{1});
+    ASSERT_EQ(group.size<int>(), typename decltype(group)::size_type{1});
+    ASSERT_EQ(cgroup.size<const char>(), typename decltype(group)::size_type{2});
+
+    registry.assign<int>(e0);
+
+    ASSERT_EQ(group.size(), typename decltype(group)::size_type{2});
+    ASSERT_EQ(group.size<int>(), typename decltype(group)::size_type{2});
+    ASSERT_EQ(cgroup.size<const char>(), typename decltype(group)::size_type{2});
+
+    registry.remove<int>(e0);
+
+    ASSERT_EQ(group.size(), typename decltype(group)::size_type{1});
+    ASSERT_EQ(group.size<int>(), typename decltype(group)::size_type{1});
+    ASSERT_EQ(cgroup.size<const char>(), typename decltype(group)::size_type{2});
+
+    registry.get<char>(e0) = '1';
+    registry.get<char>(e1) = '2';
+    registry.get<int>(e1) = 42;
+
+    ASSERT_EQ(*(cgroup.raw<const int>() + 0), 42);
+    ASSERT_EQ(*(group.raw<int>() + 0), 42);
+
+    for(auto entity: group) {
+        ASSERT_EQ(std::get<0>(cgroup.get<const int, const char>(entity)), 42);
+        ASSERT_EQ(std::get<1>(group.get<int, char>(entity)), '2');
+        ASSERT_EQ(cgroup.get<const char>(entity), '2');
+    }
+
+    ASSERT_EQ(*(group.data() + 0), e1);
+
+    ASSERT_EQ(*(group.data<int>() + 0), e1);
+    ASSERT_EQ(*(group.data<char>() + 0), e0);
+    ASSERT_EQ(*(cgroup.data<const char>() + 1), e1);
+
+    ASSERT_EQ(*(group.raw<int>() + 0), 42);
+    ASSERT_EQ(*(group.raw<char>() + 0), '1');
+    ASSERT_EQ(*(cgroup.raw<const char>() + 1), '2');
+
+    registry.remove<char>(e0);
+    registry.remove<char>(e1);
+
+    ASSERT_EQ(group.begin(), group.end());
+    ASSERT_EQ(cgroup.begin(), cgroup.end());
+    ASSERT_TRUE(group.empty());
+}
+
+TEST(OwningGroup, ElementAccess) {
+    entt::registry registry;
+    auto group = registry.group<int>(entt::get<char>);
+    auto cgroup = std::as_const(registry).group<const int>(entt::get<const char>);
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0);
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    for(typename decltype(group)::size_type i{}; i < group.size(); ++i) {
+        ASSERT_EQ(group[i], i ? e0 : e1);
+        ASSERT_EQ(cgroup[i], i ? e0 : e1);
+    }
+}
+
+TEST(OwningGroup, Contains) {
+    entt::registry registry;
+    auto group = registry.group<int>(entt::get<char>);
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0);
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    registry.destroy(e0);
+
+    ASSERT_FALSE(group.contains(e0));
+    ASSERT_TRUE(group.contains(e1));
+}
+
+TEST(OwningGroup, Empty) {
+    entt::registry registry;
+
+    const auto e0 = registry.create();
+    registry.assign<double>(e0);
+    registry.assign<int>(e0);
+    registry.assign<float>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<char>(e1);
+    registry.assign<float>(e1);
+
+    for(auto entity: registry.group<char, int>(entt::get<float>)) {
+        (void)entity;
+        FAIL();
+    }
+
+    for(auto entity: registry.group<double, float>(entt::get<char, int>)) {
+        (void)entity;
+        FAIL();
+    }
+}
+
+TEST(OwningGroup, Each) {
+    entt::registry registry;
+    auto group = registry.group<int>(entt::get<char>);
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0);
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    auto cgroup = std::as_const(registry).group<const int>(entt::get<const char>);
+    std::size_t cnt = 0;
+
+    group.each([&cnt](auto, int &, char &) { ++cnt; });
+    group.each([&cnt](int &, char &) { ++cnt; });
+
+    ASSERT_EQ(cnt, std::size_t{4});
+
+    cgroup.each([&cnt](auto, const int &, const char &) { --cnt; });
+    cgroup.each([&cnt](const int &, const char &) { --cnt; });
+
+    ASSERT_EQ(cnt, std::size_t{0});
+}
+
+TEST(OwningGroup, SortOrdered) {
+    entt::registry registry;
+    auto group = registry.group<boxed_int, char>();
+
+    entt::entity entities[5] = {
+        registry.create(),
+        registry.create(),
+        registry.create(),
+        registry.create(),
+        registry.create()
+    };
+
+    registry.assign<boxed_int>(entities[0], 12);
+    registry.assign<char>(entities[0], 'a');
+
+    registry.assign<boxed_int>(entities[1], 9);
+    registry.assign<char>(entities[1], 'b');
+
+    registry.assign<boxed_int>(entities[2], 6);
+    registry.assign<char>(entities[2], 'c');
+
+    registry.assign<boxed_int>(entities[3], 1);
+    registry.assign<boxed_int>(entities[4], 2);
+
+    group.sort([&group](const auto lhs, const auto rhs) {
+        return group.get<boxed_int>(lhs).value < group.get<boxed_int>(rhs).value;
+    });
+
+    ASSERT_EQ(*(group.data() + 0u), entities[0]);
+    ASSERT_EQ(*(group.data() + 1u), entities[1]);
+    ASSERT_EQ(*(group.data() + 2u), entities[2]);
+    ASSERT_EQ(*(group.data() + 3u), entities[3]);
+    ASSERT_EQ(*(group.data() + 4u), entities[4]);
+
+    ASSERT_EQ((group.raw<boxed_int>() + 0u)->value, 12);
+    ASSERT_EQ((group.raw<boxed_int>() + 1u)->value, 9);
+    ASSERT_EQ((group.raw<boxed_int>() + 2u)->value, 6);
+    ASSERT_EQ((group.raw<boxed_int>() + 3u)->value, 1);
+    ASSERT_EQ((group.raw<boxed_int>() + 4u)->value, 2);
+
+    ASSERT_EQ(*(group.raw<char>() + 0u), 'a');
+    ASSERT_EQ(*(group.raw<char>() + 1u), 'b');
+    ASSERT_EQ(*(group.raw<char>() + 2u), 'c');
+}
+
+TEST(OwningGroup, SortReverse) {
+    entt::registry registry;
+    auto group = registry.group<boxed_int, char>();
+
+    entt::entity entities[5] = {
+        registry.create(),
+        registry.create(),
+        registry.create(),
+        registry.create(),
+        registry.create()
+    };
+
+    registry.assign<boxed_int>(entities[0], 6);
+    registry.assign<char>(entities[0], 'a');
+
+    registry.assign<boxed_int>(entities[1], 9);
+    registry.assign<char>(entities[1], 'b');
+
+    registry.assign<boxed_int>(entities[2], 12);
+    registry.assign<char>(entities[2], 'c');
+
+    registry.assign<boxed_int>(entities[3], 1);
+    registry.assign<boxed_int>(entities[4], 2);
+
+    group.sort<boxed_int>([](const auto &lhs, const auto &rhs) {
+        return lhs.value < rhs.value;
+    });
+
+    ASSERT_EQ(*(group.data() + 0u), entities[2]);
+    ASSERT_EQ(*(group.data() + 1u), entities[1]);
+    ASSERT_EQ(*(group.data() + 2u), entities[0]);
+    ASSERT_EQ(*(group.data() + 3u), entities[3]);
+    ASSERT_EQ(*(group.data() + 4u), entities[4]);
+
+    ASSERT_EQ((group.raw<boxed_int>() + 0u)->value, 12);
+    ASSERT_EQ((group.raw<boxed_int>() + 1u)->value, 9);
+    ASSERT_EQ((group.raw<boxed_int>() + 2u)->value, 6);
+    ASSERT_EQ((group.raw<boxed_int>() + 3u)->value, 1);
+    ASSERT_EQ((group.raw<boxed_int>() + 4u)->value, 2);
+
+    ASSERT_EQ(*(group.raw<char>() + 0u), 'c');
+    ASSERT_EQ(*(group.raw<char>() + 1u), 'b');
+    ASSERT_EQ(*(group.raw<char>() + 2u), 'a');
+}
+
+TEST(OwningGroup, SortUnordered) {
+    entt::registry registry;
+    auto group = registry.group<boxed_int>(entt::get<char>);
+
+    entt::entity entities[7] = {
+        registry.create(),
+        registry.create(),
+        registry.create(),
+        registry.create(),
+        registry.create(),
+        registry.create(),
+        registry.create()
+    };
+
+    registry.assign<boxed_int>(entities[0], 6);
+    registry.assign<char>(entities[0], 'c');
+
+    registry.assign<boxed_int>(entities[1], 3);
+    registry.assign<char>(entities[1], 'b');
+
+    registry.assign<boxed_int>(entities[2], 1);
+    registry.assign<char>(entities[2], 'a');
+
+    registry.assign<boxed_int>(entities[3], 9);
+    registry.assign<char>(entities[3], 'd');
+
+    registry.assign<boxed_int>(entities[4], 12);
+    registry.assign<char>(entities[4], 'e');
+
+    registry.assign<boxed_int>(entities[5], 4);
+    registry.assign<boxed_int>(entities[6], 5);
+
+    group.sort<char>([](const auto lhs, const auto rhs) {
+        return lhs < rhs;
+    });
+
+    ASSERT_EQ(*(group.data() + 0u), entities[4]);
+    ASSERT_EQ(*(group.data() + 1u), entities[3]);
+    ASSERT_EQ(*(group.data() + 2u), entities[0]);
+    ASSERT_EQ(*(group.data() + 3u), entities[1]);
+    ASSERT_EQ(*(group.data() + 4u), entities[2]);
+    ASSERT_EQ(*(group.data() + 5u), entities[5]);
+    ASSERT_EQ(*(group.data() + 6u), entities[6]);
+
+    ASSERT_EQ((group.raw<boxed_int>() + 0u)->value, 12);
+    ASSERT_EQ((group.raw<boxed_int>() + 1u)->value, 9);
+    ASSERT_EQ((group.raw<boxed_int>() + 2u)->value, 6);
+    ASSERT_EQ((group.raw<boxed_int>() + 3u)->value, 3);
+    ASSERT_EQ((group.raw<boxed_int>() + 4u)->value, 1);
+    ASSERT_EQ((group.raw<boxed_int>() + 5u)->value, 4);
+    ASSERT_EQ((group.raw<boxed_int>() + 6u)->value, 5);
+
+    ASSERT_EQ(*(group.raw<char>() + 0u), 'c');
+    ASSERT_EQ(*(group.raw<char>() + 1u), 'b');
+    ASSERT_EQ(*(group.raw<char>() + 2u), 'a');
+    ASSERT_EQ(*(group.raw<char>() + 3u), 'd');
+    ASSERT_EQ(*(group.raw<char>() + 4u), 'e');
+}
+
+TEST(OwningGroup, IndexRebuiltOnDestroy) {
+    entt::registry registry;
+    auto group = registry.group<int>(entt::get<unsigned int>);
+
+    const auto e0 = registry.create();
+    const auto e1 = registry.create();
+
+    registry.assign<unsigned int>(e0, 0u);
+    registry.assign<unsigned int>(e1, 1u);
+
+    registry.assign<int>(e0, 0);
+    registry.assign<int>(e1, 1);
+
+    registry.destroy(e0);
+    registry.assign<int>(registry.create(), 42);
+
+    ASSERT_EQ(group.size(), typename decltype(group)::size_type{1});
+    ASSERT_EQ(group[{}], e1);
+    ASSERT_EQ(group.get<int>(e1), 1);
+    ASSERT_EQ(group.get<unsigned int>(e1), 1u);
+
+    group.each([e1](auto entity, auto ivalue, auto uivalue) {
+        ASSERT_EQ(entity, e1);
+        ASSERT_EQ(ivalue, 1);
+        ASSERT_EQ(uivalue, 1u);
+    });
+}
+
+TEST(OwningGroup, ConstNonConstAndAllInBetween) {
+    entt::registry registry;
+    auto group = registry.group<int, const char>(entt::get<double, const float>);
+
+    ASSERT_EQ(group.size(), decltype(group.size()){0});
+
+    const auto entity = registry.create();
+    registry.assign<int>(entity, 0);
+    registry.assign<char>(entity, 'c');
+    registry.assign<double>(entity, 0.);
+    registry.assign<float>(entity, 0.f);
+
+    ASSERT_EQ(group.size(), decltype(group.size()){1});
+
+    ASSERT_TRUE((std::is_same_v<decltype(group.get<int>(0)), int &>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.get<const char>(0)), const char &>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.get<double>(0)), double &>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.get<const float>(0)), const float &>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.get<int, const char, double, const float>(0)), std::tuple<int &, const char &, double &, const float &>>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.raw<const float>()), const float *>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.raw<double>()), double *>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.raw<const char>()), const char *>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.raw<int>()), int *>));
+
+    group.each([](auto, auto &&i, auto &&c, auto &&d, auto &&f) {
+        ASSERT_TRUE((std::is_same_v<decltype(i), int &>));
+        ASSERT_TRUE((std::is_same_v<decltype(c), const char &>));
+        ASSERT_TRUE((std::is_same_v<decltype(d), double &>));
+        ASSERT_TRUE((std::is_same_v<decltype(f), const float &>));
+    });
+}
+
+TEST(OwningGroup, Find) {
+    entt::registry registry;
+    auto group = registry.group<int>(entt::get<const char>);
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0);
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    const auto e2 = registry.create();
+    registry.assign<int>(e2);
+    registry.assign<char>(e2);
+
+    const auto e3 = registry.create();
+    registry.assign<int>(e3);
+    registry.assign<char>(e3);
+
+    registry.remove<int>(e1);
+
+    ASSERT_NE(group.find(e0), group.end());
+    ASSERT_EQ(group.find(e1), group.end());
+    ASSERT_NE(group.find(e2), group.end());
+    ASSERT_NE(group.find(e3), group.end());
+
+    auto it = group.find(e2);
+
+    ASSERT_EQ(*it, e2);
+    ASSERT_EQ(*(++it), e3);
+    ASSERT_EQ(*(++it), e0);
+    ASSERT_EQ(++it, group.end());
+    ASSERT_EQ(++group.find(e0), group.end());
+
+    const auto e4 = registry.create();
+    registry.destroy(e4);
+    const auto e5 = registry.create();
+    registry.assign<int>(e5);
+    registry.assign<char>(e5);
+
+    ASSERT_NE(group.find(e5), group.end());
+    ASSERT_EQ(group.find(e4), group.end());
+}
+
+TEST(OwningGroup, ExcludedComponents) {
+    entt::registry registry;
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0, 0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1, 1);
+    registry.assign<char>(e1);
+
+    const auto group = registry.group<int>(entt::exclude<char, double>);
+
+    const auto e2 = registry.create();
+    registry.assign<int>(e2, 2);
+
+    const auto e3 = registry.create();
+    registry.assign<int>(e3, 3);
+    registry.assign<double>(e3);
+
+    for(const auto entity: group) {
+        if(entity == e0) {
+            ASSERT_EQ(group.get<int>(e0), 0);
+        } else if(entity == e2) {
+            ASSERT_EQ(group.get<int>(e2), 2);
+        } else {
+            FAIL();
+        }
+    }
+
+    registry.assign<char>(e0);
+    registry.assign<double>(e2);
+
+    ASSERT_TRUE(group.empty());
+
+    registry.remove<char>(e1);
+    registry.remove<double>(e3);
+
+    for(const auto entity: group) {
+        if(entity == e1) {
+            ASSERT_EQ(group.get<int>(e1), 1);
+        } else if(entity == e3) {
+            ASSERT_EQ(group.get<int>(e3), 3);
+        } else {
+            FAIL();
+        }
+    }
+}
+
+TEST(OwningGroup, EmptyAndNonEmptyTypes) {
+    entt::registry registry;
+    const auto group = registry.group<empty_type>(entt::get<int>);
+
+    const auto e0 = registry.create();
+    registry.assign<empty_type>(e0);
+    registry.assign<int>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<empty_type>(e1);
+    registry.assign<int>(e1);
+
+    registry.assign<int>(registry.create());
+
+    for(const auto entity: group) {
+        ASSERT_TRUE(entity == e0 || entity == e1);
+    }
+
+    group.each([e0, e1](const auto entity, empty_type, const int &) {
+        ASSERT_TRUE(entity == e0 || entity == e1);
+    });
+
+    ASSERT_EQ(group.size(), typename decltype(group)::size_type{2});
+}
+
+TEST(OwningGroup, TrackEntitiesOnComponentDestruction) {
+    entt::registry registry;
+    const auto group = registry.group<int>(entt::exclude<char>);
+    const auto cgroup = std::as_const(registry).group<const int>(entt::exclude<char>);
+
+    const auto entity = registry.create();
+    registry.assign<int>(entity);
+    registry.assign<char>(entity);
+
+    ASSERT_TRUE(group.empty());
+    ASSERT_TRUE(cgroup.empty());
+
+    registry.remove<char>(entity);
+
+    ASSERT_FALSE(group.empty());
+    ASSERT_FALSE(cgroup.empty());
+}

test/entt/entity/helper.cpp

@@ -0,0 +1,106 @@
+#include <gtest/gtest.h>
+#include <entt/core/hashed_string.hpp>
+#include <entt/entity/helper.hpp>
+#include <entt/entity/registry.hpp>
+#include <entt/core/type_traits.hpp>
+
+TEST(Helper, AsView) {
+    entt::registry registry;
+    const entt::registry cregistry;
+
+    ([](entt::view<int, char>) {})(entt::as_view{registry});
+    ([](entt::view<const int, char>) {})(entt::as_view{registry});
+    ([](entt::view<const double>) {})(entt::as_view{cregistry});
+}
+
+TEST(Helper, AsGroup) {
+    entt::registry registry;
+    const entt::registry cregistry;
+
+    ([](entt::group<entt::get_t<>, double, float>) {})(entt::as_group{registry});
+    ([](entt::group<entt::get_t<>, const double, float>) {})(entt::as_group{registry});
+    ([](entt::group<entt::get_t<>, const double, const float>) {})(entt::as_group{cregistry});
+}
+
+TEST(Helper, Dependency) {
+    entt::registry registry;
+    const auto entity = registry.create();
+    entt::connect<double, float>(registry.on_construct<int>());
+
+    ASSERT_FALSE(registry.has<double>(entity));
+    ASSERT_FALSE(registry.has<float>(entity));
+
+    registry.assign<char>(entity);
+
+    ASSERT_FALSE(registry.has<double>(entity));
+    ASSERT_FALSE(registry.has<float>(entity));
+
+    registry.assign<int>(entity);
+
+    ASSERT_TRUE(registry.has<double>(entity));
+    ASSERT_TRUE(registry.has<float>(entity));
+    ASSERT_EQ(registry.get<double>(entity), .0);
+    ASSERT_EQ(registry.get<float>(entity), .0f);
+
+    registry.get<double>(entity) = .3;
+    registry.get<float>(entity) = .1f;
+    registry.remove<int>(entity);
+    registry.assign<int>(entity);
+
+    ASSERT_EQ(registry.get<double>(entity), .3);
+    ASSERT_EQ(registry.get<float>(entity), .1f);
+
+    registry.remove<int>(entity);
+    registry.remove<float>(entity);
+    registry.assign<int>(entity);
+
+    ASSERT_TRUE(registry.has<float>(entity));
+    ASSERT_EQ(registry.get<double>(entity), .3);
+    ASSERT_EQ(registry.get<float>(entity), .0f);
+
+    registry.remove<int>(entity);
+    registry.remove<double>(entity);
+    registry.remove<float>(entity);
+    entt::disconnect<double, float>(registry.on_construct<int>());
+    registry.assign<int>(entity);
+
+    ASSERT_FALSE(registry.has<double>(entity));
+    ASSERT_FALSE(registry.has<float>(entity));
+}
+
+TEST(Dependency, MultipleListenersOnTheSameType) {
+    entt::registry registry;
+    entt::connect<double>(registry.on_construct<int>());
+    entt::connect<char>(registry.on_construct<int>());
+
+    const auto entity = registry.create();
+    registry.assign<int>(entity);
+
+    ASSERT_TRUE(registry.has<double>(entity));
+    ASSERT_TRUE(registry.has<char>(entity));
+}
+
+TEST(Helper, Tag) {
+    entt::registry registry;
+    const auto entity = registry.create();
+    registry.assign<entt::tag<"foobar"_hs>>(entity);
+    registry.assign<int>(entity, 42);
+    int counter{};
+
+    ASSERT_FALSE(registry.has<entt::tag<"barfoo"_hs>>(entity));
+    ASSERT_TRUE(registry.has<entt::tag<"foobar"_hs>>(entity));
+
+    for(auto entt: registry.view<int, entt::tag<"foobar"_hs>>()) {
+        (void)entt;
+        ++counter;
+    }
+
+    ASSERT_NE(counter, 0);
+
+    for(auto entt: registry.view<entt::tag<"foobar"_hs>>()) {
+        (void)entt;
+        --counter;
+    }
+
+    ASSERT_EQ(counter, 0);
+}

test/entt/entity/prototype.cpp

@@ -0,0 +1,161 @@
+#include <gtest/gtest.h>
+#include <entt/entity/prototype.hpp>
+#include <entt/entity/registry.hpp>
+
+TEST(Prototype, SameRegistry) {
+    entt::registry registry;
+    entt::prototype prototype{registry};
+
+    ASSERT_EQ(&registry, &prototype.backend());
+    ASSERT_EQ(&registry, &std::as_const(prototype).backend());
+    ASSERT_FALSE(registry.empty());
+    ASSERT_FALSE((prototype.has<int, char>()));
+
+    ASSERT_EQ(prototype.set<int>(2), 2);
+    ASSERT_EQ(prototype.set<int>(3), 3);
+    ASSERT_EQ(prototype.set<char>('c'), 'c');
+
+    ASSERT_EQ(prototype.get<int>(), 3);
+    ASSERT_EQ(std::as_const(prototype).get<char>(), 'c');
+    ASSERT_EQ(std::get<0>(prototype.get<int, char>()), 3);
+    ASSERT_EQ(std::get<1>(std::as_const(prototype).get<int, char>()), 'c');
+
+    ASSERT_NE(prototype.try_get<int>(), nullptr);
+    ASSERT_NE(prototype.try_get<char>(), nullptr);
+    ASSERT_EQ(prototype.try_get<double>(), nullptr);
+    ASSERT_EQ(*prototype.try_get<int>(), 3);
+    ASSERT_EQ(*std::as_const(prototype).try_get<char>(), 'c');
+    ASSERT_EQ(*std::get<0>(prototype.try_get<int, char, double>()), 3);
+    ASSERT_EQ(*std::get<1>(std::as_const(prototype).try_get<int, char, double>()), 'c');
+
+    const auto e0 = prototype.create();
+
+    ASSERT_TRUE((prototype.has<int, char>()));
+    ASSERT_FALSE(registry.orphan(e0));
+
+    const auto e1 = prototype();
+    prototype(e0);
+
+    ASSERT_FALSE(registry.orphan(e0));
+    ASSERT_FALSE(registry.orphan(e1));
+
+    ASSERT_TRUE((registry.has<int, char>(e0)));
+    ASSERT_TRUE((registry.has<int, char>(e1)));
+
+    registry.remove<int>(e0);
+    registry.remove<int>(e1);
+    prototype.unset<int>();
+
+    ASSERT_FALSE((prototype.has<int, char>()));
+    ASSERT_FALSE((prototype.has<int>()));
+    ASSERT_TRUE((prototype.has<char>()));
+
+    prototype(e0);
+    prototype(e1);
+
+    ASSERT_FALSE(registry.has<int>(e0));
+    ASSERT_FALSE(registry.has<int>(e1));
+
+    ASSERT_EQ(registry.get<char>(e0), 'c');
+    ASSERT_EQ(registry.get<char>(e1), 'c');
+
+    registry.get<char>(e0) = '*';
+    prototype.assign(e0);
+
+    ASSERT_EQ(registry.get<char>(e0), '*');
+
+    registry.get<char>(e1) = '*';
+    prototype.assign_or_replace(e1);
+
+    ASSERT_EQ(registry.get<char>(e1), 'c');
+}
+
+TEST(Prototype, OtherRegistry) {
+    entt::registry registry;
+    entt::registry repository;
+    entt::prototype prototype{repository};
+
+    ASSERT_TRUE(registry.empty());
+    ASSERT_FALSE((prototype.has<int, char>()));
+
+    ASSERT_EQ(prototype.set<int>(2), 2);
+    ASSERT_EQ(prototype.set<int>(3), 3);
+    ASSERT_EQ(prototype.set<char>('c'), 'c');
+
+    ASSERT_EQ(prototype.get<int>(), 3);
+    ASSERT_EQ(std::as_const(prototype).get<char>(), 'c');
+    ASSERT_EQ(std::get<0>(prototype.get<int, char>()), 3);
+    ASSERT_EQ(std::get<1>(std::as_const(prototype).get<int, char>()), 'c');
+
+    const auto e0 = prototype.create(registry);
+
+    ASSERT_TRUE((prototype.has<int, char>()));
+    ASSERT_FALSE(registry.orphan(e0));
+
+    const auto e1 = prototype(registry);
+    prototype(registry, e0);
+
+    ASSERT_FALSE(registry.orphan(e0));
+    ASSERT_FALSE(registry.orphan(e1));
+
+    ASSERT_TRUE((registry.has<int, char>(e0)));
+    ASSERT_TRUE((registry.has<int, char>(e1)));
+
+    registry.remove<int>(e0);
+    registry.remove<int>(e1);
+    prototype.unset<int>();
+
+    ASSERT_FALSE((prototype.has<int, char>()));
+    ASSERT_FALSE((prototype.has<int>()));
+    ASSERT_TRUE((prototype.has<char>()));
+
+    prototype(registry, e0);
+    prototype(registry, e1);
+
+    ASSERT_FALSE(registry.has<int>(e0));
+    ASSERT_FALSE(registry.has<int>(e1));
+
+    ASSERT_EQ(registry.get<char>(e0), 'c');
+    ASSERT_EQ(registry.get<char>(e1), 'c');
+
+    registry.get<char>(e0) = '*';
+    prototype.assign(registry, e0);
+
+    ASSERT_EQ(registry.get<char>(e0), '*');
+
+    registry.get<char>(e1) = '*';
+    prototype.assign_or_replace(registry, e1);
+
+    ASSERT_EQ(registry.get<char>(e1), 'c');
+}
+
+TEST(Prototype, RAII) {
+    entt::registry registry;
+
+    {
+        entt::prototype prototype{registry};
+        prototype.set<int>(0);
+
+        ASSERT_FALSE(registry.empty());
+    }
+
+    ASSERT_TRUE(registry.empty());
+}
+
+TEST(Prototype, MoveConstructionAssignment) {
+    entt::registry registry;
+
+    entt::prototype prototype{registry};
+    prototype.set<int>(0);
+    auto other{std::move(prototype)};
+    const auto e0 = other();
+
+    ASSERT_EQ(registry.size(), entt::registry::size_type{2});
+    ASSERT_TRUE(registry.has<int>(e0));
+
+    prototype = std::move(other);
+    const auto e1 = prototype();
+
+    ASSERT_EQ(registry.size(), entt::registry::size_type{3});
+    ASSERT_TRUE(registry.has<int>(e1));
+}

test/entt/entity/runtime_view.cpp

@@ -0,0 +1,208 @@
+#include <iterator>
+#include <gtest/gtest.h>
+#include <entt/entity/registry.hpp>
+#include <entt/entity/runtime_view.hpp>
+
+TEST(RuntimeView, Functionalities) {
+    entt::registry registry;
+    using component_type = typename decltype(registry)::component_type;
+
+    // forces the creation of the pools
+    registry.reserve<int>(0);
+    registry.reserve<char>(0);
+
+    component_type types[] = { registry.type<int>(), registry.type<char>() };
+    auto view = registry.runtime_view(std::begin(types), std::end(types));
+
+    ASSERT_TRUE(view.empty());
+
+    const auto e0 = registry.create();
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+
+    ASSERT_FALSE(view.empty());
+
+    registry.assign<char>(e1);
+
+    auto it = registry.runtime_view(std::begin(types), std::end(types)).begin();
+
+    ASSERT_EQ(*it, e1);
+    ASSERT_EQ(++it, (registry.runtime_view(std::begin(types), std::end(types)).end()));
+
+    ASSERT_NO_THROW((registry.runtime_view(std::begin(types), std::end(types)).begin()++));
+    ASSERT_NO_THROW((++registry.runtime_view(std::begin(types), std::end(types)).begin()));
+
+    ASSERT_NE(view.begin(), view.end());
+    ASSERT_EQ(view.size(), decltype(view.size()){1});
+
+    registry.get<char>(e0) = '1';
+    registry.get<char>(e1) = '2';
+    registry.get<int>(e1) = 42;
+
+    for(auto entity: view) {
+        ASSERT_EQ(registry.get<int>(entity), 42);
+        ASSERT_EQ(registry.get<char>(entity), '2');
+    }
+}
+
+TEST(RuntimeView, Iterator) {
+    entt::registry registry;
+    using component_type = typename decltype(registry)::component_type;
+
+    const auto entity = registry.create();
+    registry.assign<int>(entity);
+    registry.assign<char>(entity);
+
+    component_type types[] = { registry.type<int>(), registry.type<char>() };
+    auto view = registry.runtime_view(std::begin(types), std::end(types));
+    using iterator_type = typename decltype(view)::iterator_type;
+
+    iterator_type end{view.begin()};
+    iterator_type begin{};
+    begin = view.end();
+    std::swap(begin, end);
+
+    ASSERT_EQ(begin, view.begin());
+    ASSERT_EQ(end, view.end());
+    ASSERT_NE(begin, end);
+
+    ASSERT_EQ(view.begin()++, view.begin());
+    ASSERT_EQ(++view.begin(), view.end());
+}
+
+TEST(RuntimeView, Contains) {
+    entt::registry registry;
+    using component_type = typename decltype(registry)::component_type;
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0);
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    registry.destroy(e0);
+
+    component_type types[] = { registry.type<int>(), registry.type<char>() };
+    auto view = registry.runtime_view(std::begin(types), std::end(types));
+
+    ASSERT_FALSE(view.contains(e0));
+    ASSERT_TRUE(view.contains(e1));
+}
+
+TEST(RuntimeView, Empty) {
+    entt::registry registry;
+    using component_type = typename decltype(registry)::component_type;
+
+    const auto e0 = registry.create();
+    registry.assign<double>(e0);
+    registry.assign<int>(e0);
+    registry.assign<float>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<char>(e1);
+    registry.assign<float>(e1);
+
+    component_type types[] = { registry.type<char>(), registry.type<int>(), registry.type<float>() };
+    auto view = registry.runtime_view(std::begin(types), std::end(types));
+
+    for(auto entity: view) {
+        (void)entity;
+        FAIL();
+    }
+}
+
+TEST(RuntimeView, Each) {
+    entt::registry registry;
+    using component_type = typename decltype(registry)::component_type;
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0);
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    component_type types[] = { registry.type<int>(), registry.type<char>() };
+    auto view = registry.runtime_view(std::begin(types), std::end(types));
+    std::size_t cnt = 0;
+
+    view.each([&cnt](auto) { ++cnt; });
+
+    ASSERT_EQ(cnt, std::size_t{2});
+}
+
+TEST(RuntimeView, EachWithHoles) {
+    entt::registry registry;
+    using component_type = typename decltype(registry)::component_type;
+
+    const auto e0 = registry.create();
+    const auto e1 = registry.create();
+    const auto e2 = registry.create();
+
+    registry.assign<char>(e0, '0');
+    registry.assign<char>(e1, '1');
+
+    registry.assign<int>(e0, 0);
+    registry.assign<int>(e2, 2);
+
+    component_type types[] = { registry.type<int>(), registry.type<char>() };
+    auto view = registry.runtime_view(std::begin(types), std::end(types));
+
+    view.each([e0](auto entity) {
+        ASSERT_EQ(e0, entity);
+    });
+}
+
+TEST(RuntimeView, MissingPool) {
+    entt::registry registry;
+    using component_type = typename decltype(registry)::component_type;
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0);
+
+    component_type types[] = { registry.type<int>(), registry.type<char>() };
+    auto view = registry.runtime_view(std::begin(types), std::end(types));
+
+    ASSERT_TRUE(view.empty());
+    ASSERT_EQ(view.size(), decltype(view.size()){0});
+
+    registry.assign<char>(e0);
+
+    ASSERT_TRUE(view.empty());
+    ASSERT_EQ(view.size(), decltype(view.size()){0});
+    ASSERT_FALSE(view.contains(e0));
+
+    view.each([](auto) { FAIL(); });
+
+    for(auto entity: view) {
+        (void)entity;
+        FAIL();
+    }
+}
+
+TEST(RuntimeView, EmptyRange) {
+    entt::registry registry;
+    using component_type = typename decltype(registry)::component_type;
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0);
+
+    const component_type *ptr = nullptr;
+    auto view = registry.runtime_view(ptr, ptr);
+
+    ASSERT_TRUE(view.empty());
+    ASSERT_EQ(view.size(), decltype(view.size()){0});
+    ASSERT_FALSE(view.contains(e0));
+
+    view.each([](auto) { FAIL(); });
+
+    for(auto entity: view) {
+        (void)entity;
+        FAIL();
+    }
+}

test/entt/entity/snapshot.cpp

@@ -0,0 +1,495 @@
+#include <tuple>
+#include <queue>
+#include <vector>
+#include <gtest/gtest.h>
+#include <entt/entity/registry.hpp>
+#include <entt/entity/entity.hpp>
+
+template<typename Storage>
+struct output_archive {
+    output_archive(Storage &instance)
+        : storage{instance}
+    {}
+
+    template<typename... Value>
+    void operator()(const Value &... value) {
+        (std::get<std::queue<Value>>(storage).push(value), ...);
+    }
+
+private:
+    Storage &storage;
+};
+
+template<typename Storage>
+struct input_archive {
+    input_archive(Storage &instance)
+        : storage{instance}
+    {}
+
+    template<typename... Value>
+    void operator()(Value &... value) {
+        auto assign = [this](auto &val) {
+            auto &queue = std::get<std::queue<std::decay_t<decltype(val)>>>(storage);
+            val = queue.front();
+            queue.pop();
+        };
+
+        (assign(value), ...);
+    }
+
+private:
+    Storage &storage;
+};
+
+struct a_component {};
+
+struct another_component {
+    int key;
+    int value;
+};
+
+struct what_a_component {
+    entt::entity bar;
+    std::vector<entt::entity> quux;
+};
+
+TEST(Snapshot, Dump) {
+    entt::registry registry;
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0, 42);
+    registry.assign<char>(e0, 'c');
+    registry.assign<double>(e0, .1);
+
+    const auto e1 = registry.create();
+
+    const auto e2 = registry.create();
+    registry.assign<int>(e2, 3);
+
+    const auto e3 = registry.create();
+    registry.assign<a_component>(e3);
+    registry.assign<char>(e3, '0');
+
+    registry.destroy(e1);
+    auto v1 = registry.current(e1);
+
+    using storage_type = std::tuple<
+        std::queue<entt::entity>,
+        std::queue<int>,
+        std::queue<char>,
+        std::queue<double>,
+        std::queue<a_component>,
+        std::queue<another_component>
+    >;
+
+    storage_type storage;
+    output_archive<storage_type> output{storage};
+    input_archive<storage_type> input{storage};
+
+    registry.snapshot()
+            .entities(output)
+            .destroyed(output)
+            .component<int, char, double, a_component, another_component>(output);
+
+    registry.reset();
+
+    ASSERT_FALSE(registry.valid(e0));
+    ASSERT_FALSE(registry.valid(e1));
+    ASSERT_FALSE(registry.valid(e2));
+    ASSERT_FALSE(registry.valid(e3));
+
+    registry.loader()
+            .entities(input)
+            .destroyed(input)
+            .component<int, char, double, a_component, another_component>(input)
+            .orphans();
+
+    ASSERT_TRUE(registry.valid(e0));
+    ASSERT_FALSE(registry.valid(e1));
+    ASSERT_TRUE(registry.valid(e2));
+    ASSERT_TRUE(registry.valid(e3));
+
+    ASSERT_FALSE(registry.orphan(e0));
+    ASSERT_FALSE(registry.orphan(e2));
+    ASSERT_FALSE(registry.orphan(e3));
+
+    ASSERT_EQ(registry.get<int>(e0), 42);
+    ASSERT_EQ(registry.get<char>(e0), 'c');
+    ASSERT_EQ(registry.get<double>(e0), .1);
+    ASSERT_EQ(registry.current(e1), v1);
+    ASSERT_EQ(registry.get<int>(e2), 3);
+    ASSERT_EQ(registry.get<char>(e3), '0');
+    ASSERT_TRUE(registry.has<a_component>(e3));
+
+    ASSERT_TRUE(registry.empty<another_component>());
+}
+
+TEST(Snapshot, Partial) {
+    entt::registry registry;
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0, 42);
+    registry.assign<char>(e0, 'c');
+    registry.assign<double>(e0, .1);
+
+    const auto e1 = registry.create();
+
+    const auto e2 = registry.create();
+    registry.assign<int>(e2, 3);
+
+    const auto e3 = registry.create();
+    registry.assign<char>(e3, '0');
+
+    registry.destroy(e1);
+    auto v1 = registry.current(e1);
+
+    using storage_type = std::tuple<
+        std::queue<entt::entity>,
+        std::queue<int>,
+        std::queue<char>,
+        std::queue<double>
+    >;
+
+    storage_type storage;
+    output_archive<storage_type> output{storage};
+    input_archive<storage_type> input{storage};
+
+    registry.snapshot()
+            .entities(output)
+            .destroyed(output)
+            .component<char, int>(output);
+
+    registry.reset();
+
+    ASSERT_FALSE(registry.valid(e0));
+    ASSERT_FALSE(registry.valid(e1));
+    ASSERT_FALSE(registry.valid(e2));
+    ASSERT_FALSE(registry.valid(e3));
+
+    registry.loader()
+            .entities(input)
+            .destroyed(input)
+            .component<char, int>(input);
+
+    ASSERT_TRUE(registry.valid(e0));
+    ASSERT_FALSE(registry.valid(e1));
+    ASSERT_TRUE(registry.valid(e2));
+    ASSERT_TRUE(registry.valid(e3));
+
+    ASSERT_EQ(registry.get<int>(e0), 42);
+    ASSERT_EQ(registry.get<char>(e0), 'c');
+    ASSERT_FALSE(registry.has<double>(e0));
+    ASSERT_EQ(registry.current(e1), v1);
+    ASSERT_EQ(registry.get<int>(e2), 3);
+    ASSERT_EQ(registry.get<char>(e3), '0');
+
+    registry.snapshot()
+            .entities(output)
+            .destroyed(output);
+
+    registry.reset();
+
+    ASSERT_FALSE(registry.valid(e0));
+    ASSERT_FALSE(registry.valid(e1));
+    ASSERT_FALSE(registry.valid(e2));
+    ASSERT_FALSE(registry.valid(e3));
+
+    registry.loader()
+            .entities(input)
+            .destroyed(input)
+            .orphans();
+
+    ASSERT_FALSE(registry.valid(e0));
+    ASSERT_FALSE(registry.valid(e1));
+    ASSERT_FALSE(registry.valid(e2));
+    ASSERT_FALSE(registry.valid(e3));
+}
+
+TEST(Snapshot, Iterator) {
+    entt::registry registry;
+
+    for(auto i = 0; i < 50; ++i) {
+        const auto entity = registry.create();
+        registry.assign<another_component>(entity, i, i);
+
+        if(i % 2) {
+            registry.assign<a_component>(entity);
+        }
+    }
+
+    using storage_type = std::tuple<
+        std::queue<entt::entity>,
+        std::queue<another_component>
+    >;
+
+    storage_type storage;
+    output_archive<storage_type> output{storage};
+    input_archive<storage_type> input{storage};
+
+    const auto view = registry.view<a_component>();
+    const auto size = view.size();
+
+    registry.snapshot().component<another_component>(output, view.begin(), view.end());
+    registry.reset();
+    registry.loader().component<another_component>(input);
+
+    ASSERT_EQ(registry.view<another_component>().size(), size);
+
+    registry.view<another_component>().each([](const auto entity, const auto &) {
+        ASSERT_TRUE(entity % 2);
+    });
+}
+
+TEST(Snapshot, Continuous) {
+    entt::registry src;
+    entt::registry dst;
+
+    entt::continuous_loader loader{dst};
+
+    std::vector<entt::entity> entities;
+    entt::entity entity;
+
+    using storage_type = std::tuple<
+        std::queue<entt::entity>,
+        std::queue<another_component>,
+        std::queue<what_a_component>,
+        std::queue<double>
+    >;
+
+    storage_type storage;
+    output_archive<storage_type> output{storage};
+    input_archive<storage_type> input{storage};
+
+    for(int i = 0; i < 10; ++i) {
+        src.create();
+    }
+
+    src.reset();
+
+    for(int i = 0; i < 5; ++i) {
+        entity = src.create();
+        entities.push_back(entity);
+
+        src.assign<a_component>(entity);
+        src.assign<another_component>(entity, i, i);
+
+        if(i % 2) {
+            src.assign<what_a_component>(entity, entity);
+        }
+    }
+
+    src.view<what_a_component>().each([&entities](auto, auto &what_a_component) {
+        what_a_component.quux.insert(what_a_component.quux.begin(), entities.begin(), entities.end());
+    });
+
+    entity = dst.create();
+    dst.assign<a_component>(entity);
+    dst.assign<another_component>(entity, -1, -1);
+
+    src.snapshot()
+            .entities(output)
+            .destroyed(output)
+            .component<a_component, another_component, what_a_component>(output);
+
+    loader.entities(input)
+            .destroyed(input)
+            .component<a_component, another_component, what_a_component>(input, &what_a_component::bar, &what_a_component::quux)
+            .orphans();
+
+    decltype(dst.size()) a_component_cnt{};
+    decltype(dst.size()) another_component_cnt{};
+    decltype(dst.size()) what_a_component_cnt{};
+
+    dst.each([&dst, &a_component_cnt](auto entt) {
+        ASSERT_TRUE(dst.has<a_component>(entt));
+        ++a_component_cnt;
+    });
+
+    dst.view<another_component>().each([&another_component_cnt](auto, const auto &component) {
+        ASSERT_EQ(component.value, component.key < 0 ? -1 : component.key);
+        ++another_component_cnt;
+    });
+
+    dst.view<what_a_component>().each([&dst, &what_a_component_cnt](auto entt, const auto &component) {
+        ASSERT_EQ(entt, component.bar);
+
+        for(auto child: component.quux) {
+            ASSERT_TRUE(dst.valid(child));
+        }
+
+        ++what_a_component_cnt;
+    });
+
+    src.view<another_component>().each([](auto, auto &component) {
+        component.value = 2 * component.key;
+    });
+
+    auto size = dst.size();
+
+    src.snapshot()
+            .entities(output)
+            .destroyed(output)
+            .component<a_component, what_a_component, another_component>(output);
+
+    loader.entities(input)
+            .destroyed(input)
+            .component<a_component, what_a_component, another_component>(input, &what_a_component::bar, &what_a_component::quux)
+            .orphans();
+
+    ASSERT_EQ(size, dst.size());
+
+    ASSERT_EQ(dst.size<a_component>(), a_component_cnt);
+    ASSERT_EQ(dst.size<another_component>(), another_component_cnt);
+    ASSERT_EQ(dst.size<what_a_component>(), what_a_component_cnt);
+
+    dst.view<another_component>().each([](auto, auto &component) {
+        ASSERT_EQ(component.value, component.key < 0 ? -1 : (2 * component.key));
+    });
+
+    entity = src.create();
+
+    src.view<what_a_component>().each([entity](auto, auto &component) {
+        component.bar = entity;
+    });
+
+    src.snapshot()
+            .entities(output)
+            .destroyed(output)
+            .component<what_a_component, a_component, another_component>(output);
+
+    loader.entities(input)
+            .destroyed(input)
+            .component<what_a_component, a_component, another_component>(input, &what_a_component::bar, &what_a_component::quux)
+            .orphans();
+
+    dst.view<what_a_component>().each([&loader, entity](auto, auto &component) {
+        ASSERT_EQ(component.bar, loader.map(entity));
+    });
+
+    entities.clear();
+    for(auto entt: src.view<a_component>()) {
+        entities.push_back(entt);
+    }
+
+    src.destroy(entity);
+    loader.shrink();
+
+    src.snapshot()
+            .entities(output)
+            .destroyed(output)
+            .component<a_component, another_component, what_a_component>(output);
+
+    loader.entities(input)
+            .destroyed(input)
+            .component<a_component, another_component, what_a_component>(input, &what_a_component::bar, &what_a_component::quux)
+            .orphans()
+            .shrink();
+
+    dst.view<what_a_component>().each([&dst](auto, auto &component) {
+        ASSERT_FALSE(dst.valid(component.bar));
+    });
+
+    ASSERT_FALSE(loader.has(entity));
+
+    entity = src.create();
+
+    src.view<what_a_component>().each([entity](auto, auto &component) {
+        component.bar = entity;
+    });
+
+    dst.reset<a_component>();
+    a_component_cnt = src.size<a_component>();
+
+    src.snapshot()
+            .entities(output)
+            .destroyed(output)
+            .component<a_component, what_a_component, another_component>(output);
+
+    loader.entities(input)
+            .destroyed(input)
+            .component<a_component, what_a_component, another_component>(input, &what_a_component::bar, &what_a_component::quux)
+            .orphans();
+
+    ASSERT_EQ(dst.size<a_component>(), a_component_cnt);
+
+    src.reset<a_component>();
+    a_component_cnt = {};
+
+    src.snapshot()
+            .entities(output)
+            .destroyed(output)
+            .component<what_a_component, a_component, another_component>(output);
+
+    loader.entities(input)
+            .destroyed(input)
+            .component<what_a_component, a_component, another_component>(input, &what_a_component::bar, &what_a_component::quux)
+            .orphans();
+
+    ASSERT_EQ(dst.size<a_component>(), a_component_cnt);
+}
+
+TEST(Snapshot, MoreOnShrink) {
+    entt::registry src;
+    entt::registry dst;
+
+    entt::continuous_loader loader{dst};
+
+    using storage_type = std::tuple<std::queue<entt::entity>>;
+
+    storage_type storage;
+    output_archive<storage_type> output{storage};
+    input_archive<storage_type> input{storage};
+
+    auto entity = src.create();
+    src.snapshot().entities(output);
+    loader.entities(input).shrink();
+
+    ASSERT_TRUE(dst.valid(entity));
+
+    loader.shrink();
+
+    ASSERT_FALSE(dst.valid(entity));
+}
+
+TEST(Snapshot, SyncDataMembers) {
+    entt::registry src;
+    entt::registry dst;
+
+    entt::continuous_loader loader{dst};
+
+    using storage_type = std::tuple<
+        std::queue<entt::entity>,
+        std::queue<what_a_component>
+    >;
+
+    storage_type storage;
+    output_archive<storage_type> output{storage};
+    input_archive<storage_type> input{storage};
+
+    src.create();
+    src.create();
+
+    src.reset();
+
+    auto parent = src.create();
+    auto child = src.create();
+
+    src.assign<what_a_component>(parent, entt::null);
+    src.assign<what_a_component>(child, parent).quux.push_back(child);
+
+    src.snapshot().entities(output).component<what_a_component>(output);
+    loader.entities(input).component<what_a_component>(input, &what_a_component::bar, &what_a_component::quux);
+
+    ASSERT_FALSE(dst.valid(parent));
+    ASSERT_FALSE(dst.valid(child));
+
+    ASSERT_TRUE(dst.has<what_a_component>(loader.map(parent)));
+    ASSERT_TRUE(dst.has<what_a_component>(loader.map(child)));
+
+    ASSERT_EQ(dst.get<what_a_component>(loader.map(parent)).bar, static_cast<entt::entity>(entt::null));
+
+    const auto &component = dst.get<what_a_component>(loader.map(child));
+
+    ASSERT_EQ(component.bar, loader.map(parent));
+    ASSERT_EQ(component.quux[0], loader.map(child));
+}

test/entt/entity/sparse_set.cpp

@@ -0,0 +1,397 @@
+#include <cstdint>
+#include <utility>
+#include <iterator>
+#include <type_traits>
+#include <gtest/gtest.h>
+#include <entt/entity/sparse_set.hpp>
+
+struct empty_type {};
+struct boxed_int { int value; };
+
+TEST(SparseSet, Functionalities) {
+    entt::sparse_set<std::uint64_t> set;
+
+    set.reserve(42);
+
+    ASSERT_EQ(set.capacity(), 42);
+    ASSERT_TRUE(set.empty());
+    ASSERT_EQ(set.size(), 0u);
+    ASSERT_EQ(std::as_const(set).begin(), std::as_const(set).end());
+    ASSERT_EQ(set.begin(), set.end());
+    ASSERT_FALSE(set.has(0));
+    ASSERT_FALSE(set.has(42));
+
+    set.construct(42);
+
+    ASSERT_EQ(set.get(42), 0u);
+
+    ASSERT_FALSE(set.empty());
+    ASSERT_EQ(set.size(), 1u);
+    ASSERT_NE(std::as_const(set).begin(), std::as_const(set).end());
+    ASSERT_NE(set.begin(), set.end());
+    ASSERT_FALSE(set.has(0));
+    ASSERT_TRUE(set.has(42));
+    ASSERT_EQ(set.get(42), 0u);
+
+    set.destroy(42);
+
+    ASSERT_TRUE(set.empty());
+    ASSERT_EQ(set.size(), 0u);
+    ASSERT_EQ(std::as_const(set).begin(), std::as_const(set).end());
+    ASSERT_EQ(set.begin(), set.end());
+    ASSERT_FALSE(set.has(0));
+    ASSERT_FALSE(set.has(42));
+
+    set.construct(42);
+
+    ASSERT_FALSE(set.empty());
+    ASSERT_EQ(set.get(42), 0u);
+
+    ASSERT_TRUE(std::is_move_constructible_v<decltype(set)>);
+    ASSERT_TRUE(std::is_move_assignable_v<decltype(set)>);
+
+    entt::sparse_set<std::uint64_t> cpy{set};
+    set = cpy;
+
+    ASSERT_FALSE(set.empty());
+    ASSERT_FALSE(cpy.empty());
+    ASSERT_EQ(set.get(42), 0u);
+    ASSERT_EQ(cpy.get(42), 0u);
+
+    entt::sparse_set<std::uint64_t> other{std::move(set)};
+
+    set = std::move(other);
+    other = std::move(set);
+
+    ASSERT_TRUE(set.empty());
+    ASSERT_FALSE(other.empty());
+    ASSERT_EQ(other.get(42), 0u);
+
+    other.reset();
+
+    ASSERT_TRUE(other.empty());
+    ASSERT_EQ(other.size(), 0u);
+    ASSERT_EQ(std::as_const(other).begin(), std::as_const(other).end());
+    ASSERT_EQ(other.begin(), other.end());
+    ASSERT_FALSE(other.has(0));
+    ASSERT_FALSE(other.has(42));
+}
+
+TEST(SparseSet, Pagination) {
+    entt::sparse_set<std::uint64_t> set;
+    constexpr auto entt_per_page = ENTT_PAGE_SIZE / sizeof(std::uint64_t);
+
+    ASSERT_EQ(set.extent(), 0);
+
+    set.construct(entt_per_page-1);
+
+    ASSERT_EQ(set.extent(), entt_per_page);
+    ASSERT_TRUE(set.has(entt_per_page-1));
+
+    set.construct(entt_per_page);
+
+    ASSERT_EQ(set.extent(), 2 * entt_per_page);
+    ASSERT_TRUE(set.has(entt_per_page-1));
+    ASSERT_TRUE(set.has(entt_per_page));
+    ASSERT_FALSE(set.has(entt_per_page+1));
+
+    set.destroy(entt_per_page-1);
+
+    ASSERT_EQ(set.extent(), 2 * entt_per_page);
+    ASSERT_FALSE(set.has(entt_per_page-1));
+    ASSERT_TRUE(set.has(entt_per_page));
+
+    set.shrink_to_fit();
+    set.destroy(entt_per_page);
+
+    ASSERT_EQ(set.extent(), 2 * entt_per_page);
+    ASSERT_FALSE(set.has(entt_per_page-1));
+    ASSERT_FALSE(set.has(entt_per_page));
+
+    set.shrink_to_fit();
+
+    ASSERT_EQ(set.extent(), 0);
+}
+
+TEST(SparseSet, BatchAdd) {
+    entt::sparse_set<std::uint64_t> set;
+    entt::sparse_set<std::uint64_t>::entity_type entities[2];
+
+    entities[0] = 3;
+    entities[1] = 42;
+
+    set.construct(12);
+    set.batch(std::begin(entities), std::end(entities));
+    set.construct(24);
+
+    ASSERT_TRUE(set.has(entities[0]));
+    ASSERT_TRUE(set.has(entities[1]));
+    ASSERT_FALSE(set.has(0));
+    ASSERT_FALSE(set.has(9));
+    ASSERT_TRUE(set.has(12));
+    ASSERT_TRUE(set.has(24));
+
+    ASSERT_FALSE(set.empty());
+    ASSERT_EQ(set.size(), 4u);
+    ASSERT_EQ(set.get(12), 0u);
+    ASSERT_EQ(set.get(entities[0]), 1u);
+    ASSERT_EQ(set.get(entities[1]), 2u);
+    ASSERT_EQ(set.get(24), 3u);
+}
+
+TEST(SparseSet, Iterator) {
+    using iterator_type = typename entt::sparse_set<std::uint64_t>::iterator_type;
+
+    entt::sparse_set<std::uint64_t> set;
+    set.construct(3);
+
+    iterator_type end{set.begin()};
+    iterator_type begin{};
+    begin = set.end();
+    std::swap(begin, end);
+
+    ASSERT_EQ(begin, set.begin());
+    ASSERT_EQ(end, set.end());
+    ASSERT_NE(begin, end);
+
+    ASSERT_EQ(begin++, set.begin());
+    ASSERT_EQ(begin--, set.end());
+
+    ASSERT_EQ(begin+1, set.end());
+    ASSERT_EQ(end-1, set.begin());
+
+    ASSERT_EQ(++begin, set.end());
+    ASSERT_EQ(--begin, set.begin());
+
+    ASSERT_EQ(begin += 1, set.end());
+    ASSERT_EQ(begin -= 1, set.begin());
+
+    ASSERT_EQ(begin + (end - begin), set.end());
+    ASSERT_EQ(begin - (begin - end), set.end());
+
+    ASSERT_EQ(end - (end - begin), set.begin());
+    ASSERT_EQ(end + (begin - end), set.begin());
+
+    ASSERT_EQ(begin[0], *set.begin());
+
+    ASSERT_LT(begin, end);
+    ASSERT_LE(begin, set.begin());
+
+    ASSERT_GT(end, begin);
+    ASSERT_GE(end, set.end());
+
+    ASSERT_EQ(*begin, 3);
+    ASSERT_EQ(*begin.operator->(), 3);
+}
+
+TEST(SparseSet, Find) {
+    entt::sparse_set<std::uint64_t> set;
+    set.construct(3);
+    set.construct(42);
+    set.construct(99);
+
+    ASSERT_NE(set.find(3), set.end());
+    ASSERT_NE(set.find(42), set.end());
+    ASSERT_NE(set.find(99), set.end());
+    ASSERT_EQ(set.find(0), set.end());
+
+    auto it = set.find(99);
+
+    ASSERT_EQ(*it, 99);
+    ASSERT_EQ(*(++it), 42);
+    ASSERT_EQ(*(++it), 3);
+    ASSERT_EQ(++it, set.end());
+    ASSERT_EQ(++set.find(3), set.end());
+}
+
+TEST(SparseSet, Data) {
+    entt::sparse_set<std::uint64_t> set;
+
+    set.construct(3);
+    set.construct(12);
+    set.construct(42);
+
+    ASSERT_EQ(set.get(3), 0u);
+    ASSERT_EQ(set.get(12), 1u);
+    ASSERT_EQ(set.get(42), 2u);
+
+    ASSERT_EQ(*(set.data() + 0u), 3u);
+    ASSERT_EQ(*(set.data() + 1u), 12u);
+    ASSERT_EQ(*(set.data() + 2u), 42u);
+}
+
+TEST(SparseSet, RespectDisjoint) {
+    entt::sparse_set<std::uint64_t> lhs;
+    entt::sparse_set<std::uint64_t> rhs;
+
+    lhs.construct(3);
+    lhs.construct(12);
+    lhs.construct(42);
+
+    ASSERT_EQ(lhs.get(3), 0u);
+    ASSERT_EQ(lhs.get(12), 1u);
+    ASSERT_EQ(lhs.get(42), 2u);
+
+    lhs.respect(rhs);
+
+    ASSERT_EQ(std::as_const(lhs).get(3), 0u);
+    ASSERT_EQ(std::as_const(lhs).get(12), 1u);
+    ASSERT_EQ(std::as_const(lhs).get(42), 2u);
+}
+
+TEST(SparseSet, RespectOverlap) {
+    entt::sparse_set<std::uint64_t> lhs;
+    entt::sparse_set<std::uint64_t> rhs;
+
+    lhs.construct(3);
+    lhs.construct(12);
+    lhs.construct(42);
+
+    rhs.construct(12);
+
+    ASSERT_EQ(lhs.get(3), 0u);
+    ASSERT_EQ(lhs.get(12), 1u);
+    ASSERT_EQ(lhs.get(42), 2u);
+
+    lhs.respect(rhs);
+
+    ASSERT_EQ(std::as_const(lhs).get(3), 0u);
+    ASSERT_EQ(std::as_const(lhs).get(12), 2u);
+    ASSERT_EQ(std::as_const(lhs).get(42), 1u);
+}
+
+TEST(SparseSet, RespectOrdered) {
+    entt::sparse_set<std::uint64_t> lhs;
+    entt::sparse_set<std::uint64_t> rhs;
+
+    lhs.construct(1);
+    lhs.construct(2);
+    lhs.construct(3);
+    lhs.construct(4);
+    lhs.construct(5);
+
+    ASSERT_EQ(lhs.get(1), 0u);
+    ASSERT_EQ(lhs.get(2), 1u);
+    ASSERT_EQ(lhs.get(3), 2u);
+    ASSERT_EQ(lhs.get(4), 3u);
+    ASSERT_EQ(lhs.get(5), 4u);
+
+    rhs.construct(6);
+    rhs.construct(1);
+    rhs.construct(2);
+    rhs.construct(3);
+    rhs.construct(4);
+    rhs.construct(5);
+
+    ASSERT_EQ(rhs.get(6), 0u);
+    ASSERT_EQ(rhs.get(1), 1u);
+    ASSERT_EQ(rhs.get(2), 2u);
+    ASSERT_EQ(rhs.get(3), 3u);
+    ASSERT_EQ(rhs.get(4), 4u);
+    ASSERT_EQ(rhs.get(5), 5u);
+
+    rhs.respect(lhs);
+
+    ASSERT_EQ(rhs.get(6), 0u);
+    ASSERT_EQ(rhs.get(1), 1u);
+    ASSERT_EQ(rhs.get(2), 2u);
+    ASSERT_EQ(rhs.get(3), 3u);
+    ASSERT_EQ(rhs.get(4), 4u);
+    ASSERT_EQ(rhs.get(5), 5u);
+}
+
+TEST(SparseSet, RespectReverse) {
+    entt::sparse_set<std::uint64_t> lhs;
+    entt::sparse_set<std::uint64_t> rhs;
+
+    lhs.construct(1);
+    lhs.construct(2);
+    lhs.construct(3);
+    lhs.construct(4);
+    lhs.construct(5);
+
+    ASSERT_EQ(lhs.get(1), 0u);
+    ASSERT_EQ(lhs.get(2), 1u);
+    ASSERT_EQ(lhs.get(3), 2u);
+    ASSERT_EQ(lhs.get(4), 3u);
+    ASSERT_EQ(lhs.get(5), 4u);
+
+    rhs.construct(5);
+    rhs.construct(4);
+    rhs.construct(3);
+    rhs.construct(2);
+    rhs.construct(1);
+    rhs.construct(6);
+
+    ASSERT_EQ(rhs.get(5), 0u);
+    ASSERT_EQ(rhs.get(4), 1u);
+    ASSERT_EQ(rhs.get(3), 2u);
+    ASSERT_EQ(rhs.get(2), 3u);
+    ASSERT_EQ(rhs.get(1), 4u);
+    ASSERT_EQ(rhs.get(6), 5u);
+
+    rhs.respect(lhs);
+
+    ASSERT_EQ(rhs.get(6), 0u);
+    ASSERT_EQ(rhs.get(1), 1u);
+    ASSERT_EQ(rhs.get(2), 2u);
+    ASSERT_EQ(rhs.get(3), 3u);
+    ASSERT_EQ(rhs.get(4), 4u);
+    ASSERT_EQ(rhs.get(5), 5u);
+}
+
+TEST(SparseSet, RespectUnordered) {
+    entt::sparse_set<std::uint64_t> lhs;
+    entt::sparse_set<std::uint64_t> rhs;
+
+    lhs.construct(1);
+    lhs.construct(2);
+    lhs.construct(3);
+    lhs.construct(4);
+    lhs.construct(5);
+
+    ASSERT_EQ(lhs.get(1), 0u);
+    ASSERT_EQ(lhs.get(2), 1u);
+    ASSERT_EQ(lhs.get(3), 2u);
+    ASSERT_EQ(lhs.get(4), 3u);
+    ASSERT_EQ(lhs.get(5), 4u);
+
+    rhs.construct(3);
+    rhs.construct(2);
+    rhs.construct(6);
+    rhs.construct(1);
+    rhs.construct(4);
+    rhs.construct(5);
+
+    ASSERT_EQ(rhs.get(3), 0u);
+    ASSERT_EQ(rhs.get(2), 1u);
+    ASSERT_EQ(rhs.get(6), 2u);
+    ASSERT_EQ(rhs.get(1), 3u);
+    ASSERT_EQ(rhs.get(4), 4u);
+    ASSERT_EQ(rhs.get(5), 5u);
+
+    rhs.respect(lhs);
+
+    ASSERT_EQ(rhs.get(6), 0u);
+    ASSERT_EQ(rhs.get(1), 1u);
+    ASSERT_EQ(rhs.get(2), 2u);
+    ASSERT_EQ(rhs.get(3), 3u);
+    ASSERT_EQ(rhs.get(4), 4u);
+    ASSERT_EQ(rhs.get(5), 5u);
+}
+
+TEST(SparseSet, CanModifyDuringIteration) {
+    entt::sparse_set<std::uint64_t> set;
+    set.construct(0);
+
+    ASSERT_EQ(set.capacity(), entt::sparse_set<std::uint64_t>::size_type{1});
+
+    const auto it = set.begin();
+    set.reserve(entt::sparse_set<std::uint64_t>::size_type{2});
+
+    ASSERT_EQ(set.capacity(), entt::sparse_set<std::uint64_t>::size_type{2});
+
+    // this should crash with asan enabled if we break the constraint
+    const auto entity = *it;
+    (void)entity;
+}

test/entt/entity/storage.cpp

@@ -0,0 +1,697 @@
+#include <memory>
+#include <utility>
+#include <iterator>
+#include <exception>
+#include <type_traits>
+#include <unordered_set>
+#include <gtest/gtest.h>
+#include <entt/entity/storage.hpp>
+
+struct empty_type {};
+struct boxed_int { int value; };
+
+struct throwing_component {
+    struct constructor_exception: std::exception {};
+
+    [[noreturn]] throwing_component() { throw constructor_exception{}; }
+
+    // necessary to avoid the short-circuit construct() logic for empty objects
+    int data;
+};
+
+TEST(Storage, Functionalities) {
+    entt::storage<std::uint64_t, int> set;
+
+    set.reserve(42);
+
+    ASSERT_EQ(set.capacity(), 42);
+    ASSERT_TRUE(set.empty());
+    ASSERT_EQ(set.size(), 0u);
+    ASSERT_EQ(std::as_const(set).begin(), std::as_const(set).end());
+    ASSERT_EQ(set.begin(), set.end());
+    ASSERT_FALSE(set.has(0));
+    ASSERT_FALSE(set.has(41));
+
+    set.construct(41, 3);
+
+    ASSERT_FALSE(set.empty());
+    ASSERT_EQ(set.size(), 1u);
+    ASSERT_NE(std::as_const(set).begin(), std::as_const(set).end());
+    ASSERT_NE(set.begin(), set.end());
+    ASSERT_FALSE(set.has(0));
+    ASSERT_TRUE(set.has(41));
+    ASSERT_EQ(set.get(41), 3);
+    ASSERT_EQ(*set.try_get(41), 3);
+    ASSERT_EQ(set.try_get(99), nullptr);
+
+    set.destroy(41);
+
+    ASSERT_TRUE(set.empty());
+    ASSERT_EQ(set.size(), 0u);
+    ASSERT_EQ(std::as_const(set).begin(), std::as_const(set).end());
+    ASSERT_EQ(set.begin(), set.end());
+    ASSERT_FALSE(set.has(0));
+    ASSERT_FALSE(set.has(41));
+
+    set.construct(41, 12);
+
+    ASSERT_EQ(set.get(41), 12);
+    ASSERT_EQ(*set.try_get(41), 12);
+    ASSERT_EQ(set.try_get(99), nullptr);
+
+    set.reset();
+
+    ASSERT_TRUE(set.empty());
+    ASSERT_EQ(set.size(), 0u);
+    ASSERT_EQ(std::as_const(set).begin(), std::as_const(set).end());
+    ASSERT_EQ(set.begin(), set.end());
+    ASSERT_FALSE(set.has(0));
+    ASSERT_FALSE(set.has(41));
+
+    ASSERT_EQ(set.capacity(), 42);
+
+    set.shrink_to_fit();
+
+    ASSERT_EQ(set.capacity(), 0);
+
+    (void)entt::storage<std::uint64_t, int>{std::move(set)};
+    entt::storage<std::uint64_t, int> other;
+    other = std::move(set);
+}
+
+TEST(Storage, EmptyType) {
+    entt::storage<std::uint64_t, empty_type> set;
+
+    set.construct(42);
+    set.construct(99);
+
+    ASSERT_TRUE(set.has(42));
+    ASSERT_TRUE(set.has(99));
+
+    auto &&component = set.get(42);
+
+    ASSERT_TRUE((std::is_same_v<decltype(component), empty_type &&>));
+}
+
+TEST(Storage, BatchAdd) {
+    entt::storage<std::uint64_t, int> set;
+    entt::storage<std::uint64_t, int>::entity_type entities[2];
+
+    entities[0] = 3;
+    entities[1] = 42;
+
+    set.reserve(4);
+    set.construct(12, 21);
+    auto *component = set.batch(std::begin(entities), std::end(entities));
+    set.construct(24, 42);
+
+    ASSERT_TRUE(set.has(entities[0]));
+    ASSERT_TRUE(set.has(entities[1]));
+    ASSERT_FALSE(set.has(0));
+    ASSERT_FALSE(set.has(9));
+    ASSERT_TRUE(set.has(12));
+    ASSERT_TRUE(set.has(24));
+
+    ASSERT_FALSE(set.empty());
+    ASSERT_EQ(set.size(), 4u);
+    ASSERT_EQ(set.get(12), 21);
+    ASSERT_EQ(set.get(entities[0]), 0);
+    ASSERT_EQ(set.get(entities[1]), 0);
+    ASSERT_EQ(set.get(24), 42);
+
+    component[0] = 1;
+    component[1] = 2;
+
+    ASSERT_EQ(set.get(entities[0]), 1);
+    ASSERT_EQ(set.get(entities[1]), 2);
+}
+
+TEST(Storage, BatchAddEmptyType) {
+    entt::storage<std::uint64_t, empty_type> set;
+    entt::storage<std::uint64_t, empty_type>::entity_type entities[2];
+
+    entities[0] = 3;
+    entities[1] = 42;
+
+    set.reserve(4);
+    set.construct(12);
+    set.batch(std::begin(entities), std::end(entities));
+    set.construct(24);
+
+    ASSERT_TRUE(set.has(entities[0]));
+    ASSERT_TRUE(set.has(entities[1]));
+    ASSERT_FALSE(set.has(0));
+    ASSERT_FALSE(set.has(9));
+    ASSERT_TRUE(set.has(12));
+    ASSERT_TRUE(set.has(24));
+
+    ASSERT_FALSE(set.empty());
+    ASSERT_EQ(set.size(), 4u);
+
+    auto &&component = set.get(entities[0]);
+
+    ASSERT_TRUE((std::is_same_v<decltype(component), empty_type &&>));
+}
+
+TEST(Storage, AggregatesMustWork) {
+    struct aggregate_type { int value; };
+    // the goal of this test is to enforce the requirements for aggregate types
+    entt::storage<std::uint64_t, aggregate_type>{}.construct(0, 42);
+}
+
+TEST(Storage, TypesFromStandardTemplateLibraryMustWork) {
+    // see #37 - this test shouldn't crash, that's all
+    entt::storage<std::uint64_t, std::unordered_set<int>> set;
+    set.construct(0).insert(42);
+    set.destroy(0);
+}
+
+TEST(Storage, Iterator) {
+    using iterator_type = typename entt::storage<std::uint64_t, boxed_int>::iterator_type;
+
+    entt::storage<std::uint64_t, boxed_int> set;
+    set.construct(3, 42);
+
+    iterator_type end{set.begin()};
+    iterator_type begin{};
+    begin = set.end();
+    std::swap(begin, end);
+
+    ASSERT_EQ(begin, set.begin());
+    ASSERT_EQ(end, set.end());
+    ASSERT_NE(begin, end);
+
+    ASSERT_EQ(begin++, set.begin());
+    ASSERT_EQ(begin--, set.end());
+
+    ASSERT_EQ(begin+1, set.end());
+    ASSERT_EQ(end-1, set.begin());
+
+    ASSERT_EQ(++begin, set.end());
+    ASSERT_EQ(--begin, set.begin());
+
+    ASSERT_EQ(begin += 1, set.end());
+    ASSERT_EQ(begin -= 1, set.begin());
+
+    ASSERT_EQ(begin + (end - begin), set.end());
+    ASSERT_EQ(begin - (begin - end), set.end());
+
+    ASSERT_EQ(end - (end - begin), set.begin());
+    ASSERT_EQ(end + (begin - end), set.begin());
+
+    ASSERT_EQ(begin[0].value, set.begin()->value);
+
+    ASSERT_LT(begin, end);
+    ASSERT_LE(begin, set.begin());
+
+    ASSERT_GT(end, begin);
+    ASSERT_GE(end, set.end());
+}
+
+TEST(Storage, ConstIterator) {
+    using iterator_type = typename entt::storage<std::uint64_t, boxed_int>::const_iterator_type;
+
+    entt::storage<std::uint64_t, boxed_int> set;
+    set.construct(3, 42);
+
+    iterator_type cend{set.cbegin()};
+    iterator_type cbegin{};
+    cbegin = set.cend();
+    std::swap(cbegin, cend);
+
+    ASSERT_EQ(cbegin, set.cbegin());
+    ASSERT_EQ(cend, set.cend());
+    ASSERT_NE(cbegin, cend);
+
+    ASSERT_EQ(cbegin++, set.cbegin());
+    ASSERT_EQ(cbegin--, set.cend());
+
+    ASSERT_EQ(cbegin+1, set.cend());
+    ASSERT_EQ(cend-1, set.cbegin());
+
+    ASSERT_EQ(++cbegin, set.cend());
+    ASSERT_EQ(--cbegin, set.cbegin());
+
+    ASSERT_EQ(cbegin += 1, set.cend());
+    ASSERT_EQ(cbegin -= 1, set.cbegin());
+
+    ASSERT_EQ(cbegin + (cend - cbegin), set.cend());
+    ASSERT_EQ(cbegin - (cbegin - cend), set.cend());
+
+    ASSERT_EQ(cend - (cend - cbegin), set.cbegin());
+    ASSERT_EQ(cend + (cbegin - cend), set.cbegin());
+
+    ASSERT_EQ(cbegin[0].value, set.cbegin()->value);
+
+    ASSERT_LT(cbegin, cend);
+    ASSERT_LE(cbegin, set.cbegin());
+
+    ASSERT_GT(cend, cbegin);
+    ASSERT_GE(cend, set.cend());
+}
+
+TEST(Storage, IteratorEmptyType) {
+    using iterator_type = typename entt::storage<std::uint64_t, empty_type>::iterator_type;
+    entt::storage<std::uint64_t, empty_type> set;
+    set.construct(3);
+
+    iterator_type end{set.begin()};
+    iterator_type begin{};
+    begin = set.end();
+    std::swap(begin, end);
+
+    ASSERT_EQ(begin, set.begin());
+    ASSERT_EQ(end, set.end());
+    ASSERT_NE(begin, end);
+
+    ASSERT_EQ(begin++, set.begin());
+    ASSERT_EQ(begin--, set.end());
+
+    ASSERT_EQ(begin+1, set.end());
+    ASSERT_EQ(end-1, set.begin());
+
+    ASSERT_EQ(++begin, set.end());
+    ASSERT_EQ(--begin, set.begin());
+
+    ASSERT_EQ(begin += 1, set.end());
+    ASSERT_EQ(begin -= 1, set.begin());
+
+    ASSERT_EQ(begin + (end - begin), set.end());
+    ASSERT_EQ(begin - (begin - end), set.end());
+
+    ASSERT_EQ(end - (end - begin), set.begin());
+    ASSERT_EQ(end + (begin - end), set.begin());
+
+    ASSERT_EQ(set.begin().operator->(), nullptr);
+
+    ASSERT_LT(begin, end);
+    ASSERT_LE(begin, set.begin());
+
+    ASSERT_GT(end, begin);
+    ASSERT_GE(end, set.end());
+
+    set.construct(33);
+    auto &&component = *begin;
+
+    ASSERT_TRUE((std::is_same_v<decltype(component), empty_type &&>));
+}
+
+TEST(Storage, Raw) {
+    entt::storage<std::uint64_t, int> set;
+
+    set.construct(3, 3);
+    set.construct(12, 6);
+    set.construct(42, 9);
+
+    ASSERT_EQ(set.get(3), 3);
+    ASSERT_EQ(std::as_const(set).get(12), 6);
+    ASSERT_EQ(set.get(42), 9);
+
+    ASSERT_EQ(*(set.raw() + 0u), 3);
+    ASSERT_EQ(*(std::as_const(set).raw() + 1u), 6);
+    ASSERT_EQ(*(set.raw() + 2u), 9);
+}
+
+TEST(Storage, SortOrdered) {
+    entt::storage<std::uint64_t, boxed_int> set;
+
+    set.construct(12, boxed_int{12});
+    set.construct(42, boxed_int{9});
+    set.construct(7, boxed_int{6});
+    set.construct(3, boxed_int{3});
+    set.construct(9, boxed_int{1});
+
+    ASSERT_EQ(set.get(12).value, 12);
+    ASSERT_EQ(set.get(42).value, 9);
+    ASSERT_EQ(set.get(7).value, 6);
+    ASSERT_EQ(set.get(3).value, 3);
+    ASSERT_EQ(set.get(9).value, 1);
+
+    set.sort([](auto lhs, auto rhs) {
+        return lhs.value < rhs.value;
+    });
+
+    ASSERT_EQ((set.raw() + 0u)->value, 12);
+    ASSERT_EQ((set.raw() + 1u)->value, 9);
+    ASSERT_EQ((set.raw() + 2u)->value, 6);
+    ASSERT_EQ((set.raw() + 3u)->value, 3);
+    ASSERT_EQ((set.raw() + 4u)->value, 1);
+
+    auto begin = set.begin();
+    auto end = set.end();
+
+    ASSERT_EQ((begin++)->value, 1);
+    ASSERT_EQ((begin++)->value, 3);
+    ASSERT_EQ((begin++)->value, 6);
+    ASSERT_EQ((begin++)->value, 9);
+    ASSERT_EQ((begin++)->value, 12);
+    ASSERT_EQ(begin, end);
+}
+
+TEST(Storage, SortReverse) {
+    entt::storage<std::uint64_t, boxed_int> set;
+
+    set.construct(12, boxed_int{1});
+    set.construct(42, boxed_int{3});
+    set.construct(7, boxed_int{6});
+    set.construct(3, boxed_int{9});
+    set.construct(9, boxed_int{12});
+
+    ASSERT_EQ(set.get(12).value, 1);
+    ASSERT_EQ(set.get(42).value, 3);
+    ASSERT_EQ(set.get(7).value, 6);
+    ASSERT_EQ(set.get(3).value, 9);
+    ASSERT_EQ(set.get(9).value, 12);
+
+    set.sort([&set](std::uint64_t lhs, std::uint64_t rhs) {
+        return set.get(lhs).value < set.get(rhs).value;
+    });
+
+    ASSERT_EQ((set.raw() + 0u)->value, 12);
+    ASSERT_EQ((set.raw() + 1u)->value, 9);
+    ASSERT_EQ((set.raw() + 2u)->value, 6);
+    ASSERT_EQ((set.raw() + 3u)->value, 3);
+    ASSERT_EQ((set.raw() + 4u)->value, 1);
+
+    auto begin = set.begin();
+    auto end = set.end();
+
+    ASSERT_EQ((begin++)->value, 1);
+    ASSERT_EQ((begin++)->value, 3);
+    ASSERT_EQ((begin++)->value, 6);
+    ASSERT_EQ((begin++)->value, 9);
+    ASSERT_EQ((begin++)->value, 12);
+    ASSERT_EQ(begin, end);
+}
+
+TEST(Storage, SortUnordered) {
+    entt::storage<std::uint64_t, boxed_int> set;
+
+    set.construct(12, boxed_int{6});
+    set.construct(42, boxed_int{3});
+    set.construct(7, boxed_int{1});
+    set.construct(3, boxed_int{9});
+    set.construct(9, boxed_int{12});
+
+    ASSERT_EQ(set.get(12).value, 6);
+    ASSERT_EQ(set.get(42).value, 3);
+    ASSERT_EQ(set.get(7).value, 1);
+    ASSERT_EQ(set.get(3).value, 9);
+    ASSERT_EQ(set.get(9).value, 12);
+
+    set.sort([](auto lhs, auto rhs) {
+        return lhs.value < rhs.value;
+    });
+
+    ASSERT_EQ((set.raw() + 0u)->value, 12);
+    ASSERT_EQ((set.raw() + 1u)->value, 9);
+    ASSERT_EQ((set.raw() + 2u)->value, 6);
+    ASSERT_EQ((set.raw() + 3u)->value, 3);
+    ASSERT_EQ((set.raw() + 4u)->value, 1);
+
+    auto begin = set.begin();
+    auto end = set.end();
+
+    ASSERT_EQ((begin++)->value, 1);
+    ASSERT_EQ((begin++)->value, 3);
+    ASSERT_EQ((begin++)->value, 6);
+    ASSERT_EQ((begin++)->value, 9);
+    ASSERT_EQ((begin++)->value, 12);
+    ASSERT_EQ(begin, end);
+}
+
+TEST(Storage, RespectDisjoint) {
+    entt::storage<std::uint64_t, int> lhs;
+    entt::storage<std::uint64_t, int> rhs;
+
+    lhs.construct(3, 3);
+    lhs.construct(12, 6);
+    lhs.construct(42, 9);
+
+    ASSERT_EQ(std::as_const(lhs).get(3), 3);
+    ASSERT_EQ(std::as_const(lhs).get(12), 6);
+    ASSERT_EQ(std::as_const(lhs).get(42), 9);
+
+    lhs.respect(rhs);
+
+    ASSERT_EQ(*(std::as_const(lhs).raw() + 0u), 3);
+    ASSERT_EQ(*(std::as_const(lhs).raw() + 1u), 6);
+    ASSERT_EQ(*(std::as_const(lhs).raw() + 2u), 9);
+
+    auto begin = lhs.begin();
+    auto end = lhs.end();
+
+    ASSERT_EQ(*(begin++), 9);
+    ASSERT_EQ(*(begin++), 6);
+    ASSERT_EQ(*(begin++), 3);
+    ASSERT_EQ(begin, end);
+}
+
+TEST(Storage, RespectOverlap) {
+    entt::storage<std::uint64_t, int> lhs;
+    entt::storage<std::uint64_t, int> rhs;
+
+    lhs.construct(3, 3);
+    lhs.construct(12, 6);
+    lhs.construct(42, 9);
+    rhs.construct(12, 6);
+
+    ASSERT_EQ(std::as_const(lhs).get(3), 3);
+    ASSERT_EQ(std::as_const(lhs).get(12), 6);
+    ASSERT_EQ(std::as_const(lhs).get(42), 9);
+    ASSERT_EQ(rhs.get(12), 6);
+
+    lhs.respect(rhs);
+
+    ASSERT_EQ(*(std::as_const(lhs).raw() + 0u), 3);
+    ASSERT_EQ(*(std::as_const(lhs).raw() + 1u), 9);
+    ASSERT_EQ(*(std::as_const(lhs).raw() + 2u), 6);
+
+    auto begin = lhs.begin();
+    auto end = lhs.end();
+
+    ASSERT_EQ(*(begin++), 6);
+    ASSERT_EQ(*(begin++), 9);
+    ASSERT_EQ(*(begin++), 3);
+    ASSERT_EQ(begin, end);
+}
+
+TEST(Storage, RespectOrdered) {
+    entt::storage<std::uint64_t, int> lhs;
+    entt::storage<std::uint64_t, int> rhs;
+
+    lhs.construct(1, 0);
+    lhs.construct(2, 0);
+    lhs.construct(3, 0);
+    lhs.construct(4, 0);
+    lhs.construct(5, 0);
+
+    ASSERT_EQ(lhs.get(1), 0);
+    ASSERT_EQ(lhs.get(2), 0);
+    ASSERT_EQ(lhs.get(3), 0);
+    ASSERT_EQ(lhs.get(4), 0);
+    ASSERT_EQ(lhs.get(5), 0);
+
+    rhs.construct(6, 0);
+    rhs.construct(1, 0);
+    rhs.construct(2, 0);
+    rhs.construct(3, 0);
+    rhs.construct(4, 0);
+    rhs.construct(5, 0);
+
+    ASSERT_EQ(rhs.get(6), 0);
+    ASSERT_EQ(rhs.get(1), 0);
+    ASSERT_EQ(rhs.get(2), 0);
+    ASSERT_EQ(rhs.get(3), 0);
+    ASSERT_EQ(rhs.get(4), 0);
+    ASSERT_EQ(rhs.get(5), 0);
+
+    rhs.respect(lhs);
+
+    ASSERT_EQ(*(lhs.data() + 0u), 1u);
+    ASSERT_EQ(*(lhs.data() + 1u), 2u);
+    ASSERT_EQ(*(lhs.data() + 2u), 3u);
+    ASSERT_EQ(*(lhs.data() + 3u), 4u);
+    ASSERT_EQ(*(lhs.data() + 4u), 5u);
+
+    ASSERT_EQ(*(rhs.data() + 0u), 6u);
+    ASSERT_EQ(*(rhs.data() + 1u), 1u);
+    ASSERT_EQ(*(rhs.data() + 2u), 2u);
+    ASSERT_EQ(*(rhs.data() + 3u), 3u);
+    ASSERT_EQ(*(rhs.data() + 4u), 4u);
+    ASSERT_EQ(*(rhs.data() + 5u), 5u);
+}
+
+TEST(Storage, RespectReverse) {
+    entt::storage<std::uint64_t, int> lhs;
+    entt::storage<std::uint64_t, int> rhs;
+
+    lhs.construct(1, 0);
+    lhs.construct(2, 0);
+    lhs.construct(3, 0);
+    lhs.construct(4, 0);
+    lhs.construct(5, 0);
+
+    ASSERT_EQ(lhs.get(1), 0);
+    ASSERT_EQ(lhs.get(2), 0);
+    ASSERT_EQ(lhs.get(3), 0);
+    ASSERT_EQ(lhs.get(4), 0);
+    ASSERT_EQ(lhs.get(5), 0);
+
+    rhs.construct(5, 0);
+    rhs.construct(4, 0);
+    rhs.construct(3, 0);
+    rhs.construct(2, 0);
+    rhs.construct(1, 0);
+    rhs.construct(6, 0);
+
+    ASSERT_EQ(rhs.get(5), 0);
+    ASSERT_EQ(rhs.get(4), 0);
+    ASSERT_EQ(rhs.get(3), 0);
+    ASSERT_EQ(rhs.get(2), 0);
+    ASSERT_EQ(rhs.get(1), 0);
+    ASSERT_EQ(rhs.get(6), 0);
+
+    rhs.respect(lhs);
+
+    ASSERT_EQ(*(lhs.data() + 0u), 1u);
+    ASSERT_EQ(*(lhs.data() + 1u), 2u);
+    ASSERT_EQ(*(lhs.data() + 2u), 3u);
+    ASSERT_EQ(*(lhs.data() + 3u), 4u);
+    ASSERT_EQ(*(lhs.data() + 4u), 5u);
+
+    ASSERT_EQ(*(rhs.data() + 0u), 6u);
+    ASSERT_EQ(*(rhs.data() + 1u), 1u);
+    ASSERT_EQ(*(rhs.data() + 2u), 2u);
+    ASSERT_EQ(*(rhs.data() + 3u), 3u);
+    ASSERT_EQ(*(rhs.data() + 4u), 4u);
+    ASSERT_EQ(*(rhs.data() + 5u), 5u);
+}
+
+TEST(Storage, RespectUnordered) {
+    entt::storage<std::uint64_t, int> lhs;
+    entt::storage<std::uint64_t, int> rhs;
+
+    lhs.construct(1, 0);
+    lhs.construct(2, 0);
+    lhs.construct(3, 0);
+    lhs.construct(4, 0);
+    lhs.construct(5, 0);
+
+    ASSERT_EQ(lhs.get(1), 0);
+    ASSERT_EQ(lhs.get(2), 0);
+    ASSERT_EQ(lhs.get(3), 0);
+    ASSERT_EQ(lhs.get(4), 0);
+    ASSERT_EQ(lhs.get(5), 0);
+
+    rhs.construct(3, 0);
+    rhs.construct(2, 0);
+    rhs.construct(6, 0);
+    rhs.construct(1, 0);
+    rhs.construct(4, 0);
+    rhs.construct(5, 0);
+
+    ASSERT_EQ(rhs.get(3), 0);
+    ASSERT_EQ(rhs.get(2), 0);
+    ASSERT_EQ(rhs.get(6), 0);
+    ASSERT_EQ(rhs.get(1), 0);
+    ASSERT_EQ(rhs.get(4), 0);
+    ASSERT_EQ(rhs.get(5), 0);
+
+    rhs.respect(lhs);
+
+    ASSERT_EQ(*(lhs.data() + 0u), 1u);
+    ASSERT_EQ(*(lhs.data() + 1u), 2u);
+    ASSERT_EQ(*(lhs.data() + 2u), 3u);
+    ASSERT_EQ(*(lhs.data() + 3u), 4u);
+    ASSERT_EQ(*(lhs.data() + 4u), 5u);
+
+    ASSERT_EQ(*(rhs.data() + 0u), 6u);
+    ASSERT_EQ(*(rhs.data() + 1u), 1u);
+    ASSERT_EQ(*(rhs.data() + 2u), 2u);
+    ASSERT_EQ(*(rhs.data() + 3u), 3u);
+    ASSERT_EQ(*(rhs.data() + 4u), 4u);
+    ASSERT_EQ(*(rhs.data() + 5u), 5u);
+}
+
+TEST(Storage, RespectOverlapEmptyType) {
+    entt::storage<std::uint64_t, empty_type> lhs;
+    entt::storage<std::uint64_t, empty_type> rhs;
+
+    lhs.construct(3);
+    lhs.construct(12);
+    lhs.construct(42);
+
+    rhs.construct(12);
+
+    ASSERT_EQ(lhs.sparse_set<std::uint64_t>::get(3), 0u);
+    ASSERT_EQ(lhs.sparse_set<std::uint64_t>::get(12), 1u);
+    ASSERT_EQ(lhs.sparse_set<std::uint64_t>::get(42), 2u);
+
+    lhs.respect(rhs);
+
+    ASSERT_EQ(std::as_const(lhs).sparse_set<std::uint64_t>::get(3), 0u);
+    ASSERT_EQ(std::as_const(lhs).sparse_set<std::uint64_t>::get(12), 2u);
+    ASSERT_EQ(std::as_const(lhs).sparse_set<std::uint64_t>::get(42), 1u);
+}
+
+TEST(Storage, CanModifyDuringIteration) {
+    entt::storage<std::uint64_t, int> set;
+    set.construct(0, 42);
+
+    ASSERT_EQ(set.capacity(), (entt::storage<std::uint64_t, int>::size_type{1}));
+
+    const auto it = set.cbegin();
+    set.reserve(entt::storage<std::uint64_t, int>::size_type{2});
+
+    ASSERT_EQ(set.capacity(), (entt::storage<std::uint64_t, int>::size_type{2}));
+
+    // this should crash with asan enabled if we break the constraint
+    const auto entity = *it;
+    (void)entity;
+}
+
+TEST(Storage, ReferencesGuaranteed) {
+    entt::storage<std::uint64_t, boxed_int> set;
+
+    set.construct(0, 0);
+    set.construct(1, 1);
+
+    ASSERT_EQ(set.get(0).value, 0);
+    ASSERT_EQ(set.get(1).value, 1);
+
+    for(auto &&type: set) {
+        if(type.value) {
+            type.value = 42;
+        }
+    }
+
+    ASSERT_EQ(set.get(0).value, 0);
+    ASSERT_EQ(set.get(1).value, 42);
+
+    auto begin = set.begin();
+
+    while(begin != set.end()) {
+        (begin++)->value = 3;
+    }
+
+    ASSERT_EQ(set.get(0).value, 3);
+    ASSERT_EQ(set.get(1).value, 3);
+}
+
+TEST(Storage, MoveOnlyComponent) {
+    // the purpose is to ensure that move only components are always accepted
+    entt::storage<std::uint64_t, std::unique_ptr<int>> set;
+    (void)set;
+}
+
+TEST(Storage, ConstructorExceptionDoesNotAddToSet) {
+    entt::storage<std::uint64_t, throwing_component> set;
+
+    try {
+        set.construct(0);
+        FAIL() << "Expected constructor_exception to be thrown";
+    } catch (const throwing_component::constructor_exception &) {
+        ASSERT_TRUE(set.empty());
+    }
+}