updated version

first draft - to be reviewed

.gitignore

@@ -1,2 +1 @@
-# QtCreator
 *.user

.travis.yml

@@ -11,6 +11,13 @@ matrix:
         sources: ['ubuntu-toolchain-r-test']
         packages: ['g++-6']
     env: COMPILER=g++-6
+  - os: linux
+    compiler: gcc
+    addons:
+      apt:
+        sources: ['ubuntu-toolchain-r-test']
+        packages: ['g++-7']
+    env: COMPILER=g++-7
   - os: linux
     compiler: clang
     addons:
@@ -18,23 +25,34 @@ matrix:
         sources: ['ubuntu-toolchain-r-test', 'llvm-toolchain-trusty-4.0']
         packages: ['clang-4.0', 'libstdc++-4.9-dev']
     env: COMPILER=clang++-4.0
+  - os: linux
+    compiler: clang
+    addons:
+      apt:
+        sources: ['ubuntu-toolchain-r-test', 'llvm-toolchain-trusty-5.0']
+        packages: ['clang-5.0', 'libstdc++-4.9-dev']
+    env: COMPILER=clang++-5.0
   - os: osx
     osx_image: xcode8.3
     compiler: clang
     env: COMPILER=clang++
+  - os: osx
+    osx_image: xcode9.1
+    compiler: clang
+    env: COMPILER=clang++
   - os: linux
     compiler: gcc
     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
 
 notifications:
   email:
@@ -51,5 +69,5 @@ install:
 
 script:
 - mkdir -p build && cd build
-- cmake -DCMAKE_BUILD_TYPE=Release .. && make -j4
+- cmake .. && make -j4
 - CTEST_OUTPUT_ON_FAILURE=1 make test

AUTHORS

@@ -5,3 +5,5 @@ Michele Caini aka skypjack
 # Contributors
 
 Paolo Monteverde aka morbo84
+David Nerjes aka DavidHamburg
+Indi Kernick aka Kerndog73

CMakeLists.txt

@@ -16,7 +16,7 @@ endif()
 # Project configuration
 #
 
-project(entt VERSION 1.0.0)
+project(entt VERSION 2.6.1)
 
 if(NOT CMAKE_BUILD_TYPE)
     set(CMAKE_BUILD_TYPE Debug)
@@ -29,47 +29,69 @@ 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) 2018 ${PROJECT_AUTHOR} <${PROJECT_AUTHOR_EMAIL}>")
 message("*")
 
 #
-# Compile stuff
+# Compiler stuff
 #
 
 set(CMAKE_CXX_STANDARD 14)
+set(CMAKE_CXX_EXTENSIONS OFF)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
-set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -Wl,--no-undefined")
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pedantic -Wall -Wconversion")
-set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -O0 -g -DDEBUG")
-set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -O3 -DRELEASE")
+
+if(NOT MSVC)
+    include(CheckCXXSourceCompiles)
+
+    set(OLD_CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -stdlib=libc++")
+
+    check_cxx_source_compiles("
+        #include<type_traits>
+        int main() { return std::is_same<int, int>::value ? 0 : 1; }
+    " HAS_LIBCPP)
+
+    if(NOT HAS_LIBCPP)
+        set(CMAKE_CXX_FLAGS "${OLD_CMAKE_CXX_FLAGS}")
+    endif()
+
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pedantic -Wall")
+    set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -DRELEASE")
+    set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -O0 -g -DDEBUG")
+    set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -Wl,--no-undefined")
+
+    if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+        # it seems that -O3 ruins the performance when using clang ...
+        set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -O2")
+    else()
+        # ... on the other side, GCC is incredibly comfortable with it.
+        set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -O3")
+    endif()
+endif()
 
 #
-# CMake configuration
+# Include EnTT
 #
 
-set(PROJECT_CMAKE_IN ${entt_SOURCE_DIR}/cmake/in)
-set(PROJECT_DEPS_DIR ${entt_SOURCE_DIR}/deps)
-set(PROJECT_SRC_DIR ${entt_SOURCE_DIR}/src)
-
-set(PROJECT_RUNTIME_OUTPUT_DIRECTORY bin)
+include_directories(${entt_SOURCE_DIR}/src)
 
 #
-# Enable test support using ctest-like interface
+# Tests
 #
 
 option(BUILD_TESTING "Enable testing with ctest." ON)
 
-#
-# build testing stuff if required
-#
-
 if(BUILD_TESTING)
     set(THREADS_PREFER_PTHREAD_FLAG ON)
     find_package(Threads REQUIRED)
 
+    option(BUILD_BENCHMARK "Build benchmark." OFF)
+    option(BUILD_MOD "Build mod example." OFF)
+    option(BUILD_SNAPSHOT "Build snapshot example." OFF)
+
     # 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)
+    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)
@@ -78,3 +100,29 @@ if(BUILD_TESTING)
     enable_testing()
     add_subdirectory(test)
 endif()
+
+#
+# Documentation
+#
+
+find_package(Doxygen 1.8)
+
+if(DOXYGEN_FOUND)
+    add_subdirectory(docs)
+endif()
+
+#
+# AOB
+#
+
+add_custom_target(
+    entt_aob
+    SOURCES
+        appveyor.yml
+        AUTHORS
+        CONTRIBUTING
+        LICENSE
+        README.md
+        TODO
+        .travis.yml
+)

CONTRIBUTING

@@ -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 informaion about the system on which you are experiencing it and point out
+  the version of `EnTT` you are using (tag or commit).
+
+* If you found a bug and you wrote a patch to fix it, open a new
+  [pull request](https://github.com/skypjack/entt/pulls) with your code.
+  **Please**, add some tests to avoid regressions in future if possible, it
+  would be really appreciated. Note that the `EnTT` framework has a
+  [coverage at 100%](https://coveralls.io/github/skypjack/entt?branch=master)
+  (at least it was at 100% at the time I wrote this file) and this is the reason
+  for which you can be confident with using it in a production environment.
+
+* If you want to propose a new feature and you know how to code it, **please**
+  do not issue directly a pull request. Before to do it,
+  [create a new issue](https://github.com/skypjack/entt/issues/new) to discuss
+  your proposal. Other users could be interested in your idea and the discussion
+  that will follow can refine it and therefore give us a better solution.
+
+* If you want to request a new feature, I'm available for hiring. Take a look at
+  [my profile](https://github.com/skypjack) and feel free to write me.

LICENSE

@@ -1,6 +1,6 @@
 The MIT License (MIT)
 
-Copyright (c) 2017 Michele Caini
+Copyright (c) 2018 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

TODO

@@ -0,0 +1,13 @@
+* custom allocators and EnTT allocator-aware in general (long term feature, I don't actually need it at the moment) - see #22
+* scene management (I prefer the concept of spaces, that is a kind of scene anyway)
+* review doc: separate it in multiple md/dox files, reduce the readme to a minimum and provide users with links to the online documentation on gh-pages
+* debugging tools (#60): the issue online already contains interesting tips on this, look at it
+* dynamic view, useful for runtime ecs, can be filled with the desired pool at runtime and are not constrained to a compile-time list of components
+* define basic reactive systems (track entities to which component is attached, track entities from which component is removed, and so on)
+* define systems as composable mixins (initializazion, reactive, update, whatever) with flexible auto-detected arguments (registry, views, etc)
+* create dedicated flat map based on types implementation (sort of "type map") for types to use within the registry and so on...
+* ease the assignment of tags as string (use a template class with a non-type template parameter behind the scene)
+* improve CMake interface, see mail from Malte
+* is registry/utility.hpp really required?
+* "singleton mode" for tags (see #66)
+* AOB

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 +13,8 @@ configuration:
   - Release
 
 before_build:
-  - deps.bat
   - cd %BUILD_DIR%
-  - cmake .. -G"%CMAKE_GENERATOR_NAME%"
+  - cmake .. -DCMAKE_CXX_FLAGS=/D_SILENCE_TR1_NAMESPACE_DEPRECATION_WARNING -G"Visual Studio 15 2017"
 
 build:
   parallel: true

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/googletest.in

@@ -6,7 +6,7 @@ include(ExternalProject)
 ExternalProject_Add(
     googletest
     GIT_REPOSITORY https://github.com/google/googletest.git
-    GIT_TAG master
+    GIT_TAG release-1.8.0
     DOWNLOAD_DIR ${GOOGLETEST_DEPS_DIR}
     TMP_DIR ${GOOGLETEST_DEPS_DIR}/tmp
     STAMP_DIR ${GOOGLETEST_DEPS_DIR}/stamp

docs/CMakeLists.txt

@@ -0,0 +1,24 @@
+#
+# Doxygen configuration (documentation)
+#
+
+set(TARGET_DOCS docs)
+
+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(
+    ${TARGET_DOCS}
+    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}/
+)

docs/LICENSE

@@ -0,0 +1,395 @@
+Attribution 4.0 International
+
+=======================================================================
+
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+For the avoidance of doubt, this Section 4 supplements and does not
+replace Your obligations under this Public License where the Licensed
+Rights include other Copyright and Similar Rights.
+
+
+Section 5 -- Disclaimer of Warranties and Limitation of Liability.
+
+  a. UNLESS OTHERWISE SEPARATELY UNDERTAKEN BY THE LICENSOR, TO THE
+     EXTENT POSSIBLE, THE LICENSOR OFFERS THE LICENSED MATERIAL AS-IS
+     AND AS-AVAILABLE, AND MAKES NO REPRESENTATIONS OR WARRANTIES OF
+     ANY KIND CONCERNING THE LICENSED MATERIAL, WHETHER EXPRESS,
+     IMPLIED, STATUTORY, OR OTHER. THIS INCLUDES, WITHOUT LIMITATION,
+     WARRANTIES OF TITLE, MERCHANTABILITY, FITNESS FOR A PARTICULAR
+     PURPOSE, NON-INFRINGEMENT, ABSENCE OF LATENT OR OTHER DEFECTS,
+     ACCURACY, OR THE PRESENCE OR ABSENCE OF ERRORS, WHETHER OR NOT
+     KNOWN OR DISCOVERABLE. WHERE DISCLAIMERS OF WARRANTIES ARE NOT
+     ALLOWED IN FULL OR IN PART, THIS DISCLAIMER MAY NOT APPLY TO YOU.
+
+  b. TO THE EXTENT POSSIBLE, IN NO EVENT WILL THE LICENSOR BE LIABLE
+     TO YOU ON ANY LEGAL THEORY (INCLUDING, WITHOUT LIMITATION,
+     NEGLIGENCE) OR OTHERWISE FOR ANY DIRECT, SPECIAL, INDIRECT,
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+     COSTS, EXPENSES, OR DAMAGES ARISING OUT OF THIS PUBLIC LICENSE OR
+     USE OF THE LICENSED MATERIAL, EVEN IF THE LICENSOR HAS BEEN
+     ADVISED OF THE POSSIBILITY OF SUCH LOSSES, COSTS, EXPENSES, OR
+     DAMAGES. WHERE A LIMITATION OF LIABILITY IS NOT ALLOWED IN FULL OR
+     IN PART, THIS LIMITATION MAY NOT APPLY TO YOU.
+
+  c. The disclaimer of warranties and limitation of liability provided
+     above shall be interpreted in a manner that, to the extent
+     possible, most closely approximates an absolute disclaimer and
+     waiver of all liability.
+
+
+Section 6 -- Term and Termination.
+
+  a. This Public License applies for the term of the Copyright and
+     Similar Rights licensed here. However, if You fail to comply with
+     this Public License, then Your rights under this Public License
+     terminate automatically.
+
+  b. Where Your right to use the Licensed Material has terminated under
+     Section 6(a), it reinstates:
+
+       1. automatically as of the date the violation is cured, provided
+          it is cured within 30 days of Your discovery of the
+          violation; or
+
+       2. upon express reinstatement by the Licensor.
+
+     For the avoidance of doubt, this Section 6(b) does not affect any
+     right the Licensor may have to seek remedies for Your violations
+     of this Public License.
+
+  c. For the avoidance of doubt, the Licensor may also offer the
+     Licensed Material under separate terms or conditions or stop
+     distributing the Licensed Material at any time; however, doing so
+     will not terminate this Public License.
+
+  d. Sections 1, 5, 6, 7, and 8 survive termination of this Public
+     License.
+
+
+Section 7 -- Other Terms and Conditions.
+
+  a. The Licensor shall not be bound by any additional or different
+     terms or conditions communicated by You unless expressly agreed.
+
+  b. Any arrangements, understandings, or agreements regarding the
+     Licensed Material not stated herein are separate from and
+     independent of the terms and conditions of this Public License.
+
+
+Section 8 -- Interpretation.
+
+  a. For the avoidance of doubt, this Public License does not, and
+     shall not be interpreted to, reduce, limit, restrict, or impose
+     conditions on any use of the Licensed Material that could lawfully
+     be made without permission under this Public License.
+
+  b. To the extent possible, if any provision of this Public License is
+     deemed unenforceable, it shall be automatically reformed to the
+     minimum extent necessary to make it enforceable. If the provision
+     cannot be reformed, it shall be severed from this Public License
+     without affecting the enforceability of the remaining terms and
+     conditions.
+
+  c. No term or condition of this Public License will be waived and no
+     failure to comply consented to unless expressly agreed to by the
+     Licensor.
+
+  d. Nothing in this Public License constitutes or may be interpreted
+     as a limitation upon, or waiver of, any privileges and immunities
+     that apply to the Licensor or You, including from the legal
+     processes of any jurisdiction or authority.
+
+
+=======================================================================
+
+Creative Commons is not a party to its public
+licenses. Notwithstanding, Creative Commons may elect to apply one of
+its public licenses to material it publishes and in those instances
+will be considered the “Licensor.” The text of the Creative Commons
+public licenses is dedicated to the public domain under the CC0 Public
+Domain Dedication. Except for the limited purpose of indicating that
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+public licenses.
+
+Creative Commons may be contacted at creativecommons.org.

docs/extra.dox

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

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 +1,10 @@
+#ifndef ENTT_CONFIG_CONFIG_H
+#define ENTT_CONFIG_CONFIG_H
+
+
+#ifndef ENTT_NOEXCEPT
+#define ENTT_NOEXCEPT noexcept
+#endif
+
+
+#endif // ENTT_CONFIG_CONFIG_H

src/entt/core/algorithm.hpp

@@ -0,0 +1,76 @@
+#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 StdSort {
+    /**
+     * @brief Sorts the element in a range.
+     *
+     * Sorts the element 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 {
+        std::sort(std::move(first), std::move(last), std::move(compare));
+    }
+};
+
+
+/*! @brief Function object for performing insertion sort. */
+struct InsertionSort {
+    /**
+     * @brief Sorts the element in a range.
+     *
+     * Sorts the element 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 {
+        auto it = first + 1;
+
+        while(it != last) {
+            auto value = *it;
+            auto pre = it;
+
+            while(pre != first && compare(value, *(pre-1))) {
+                *pre = *(pre-1);
+                --pre;
+            }
+
+            *pre = value;
+            ++it;
+        }
+    }
+};
+
+
+}
+
+
+#endif // ENTT_CORE_ALGORITHM_HPP

src/entt/core/family.hpp

@@ -0,0 +1,53 @@
+#ifndef ENTT_CORE_FAMILY_HPP
+#define ENTT_CORE_FAMILY_HPP
+
+
+#include <type_traits>
+#include <cstddef>
+#include <atomic>
+#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 {
+    static std::atomic<std::size_t> identifier;
+
+    template<typename...>
+    static std::size_t family() ENTT_NOEXCEPT {
+        static const std::size_t value = identifier.fetch_add(1);
+        return value;
+    }
+
+public:
+    /*! @brief Unsigned integer type. */
+    using family_type = std::size_t;
+
+    /**
+     * @brief Returns an unique identifier for the given type.
+     * @return Statically generated unique identifier for the given type.
+     */
+    template<typename... Type>
+    inline static family_type type() ENTT_NOEXCEPT {
+        return family<std::decay_t<Type>...>();
+    }
+};
+
+
+template<typename... Types>
+std::atomic<std::size_t> Family<Types...>::identifier{};
+
+
+}
+
+
+#endif // ENTT_CORE_FAMILY_HPP

src/entt/core/hashed_string.hpp

@@ -0,0 +1,111 @@
+#ifndef ENTT_CORE_HASHED_STRING_HPP
+#define ENTT_CORE_HASHED_STRING_HPP
+
+
+#include <cstddef>
+#include <cstdint>
+#include "../config/config.h"
+
+
+namespace entt {
+
+
+/**
+ * @brief Zero overhead resource 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 HashedString final {
+    struct ConstCharWrapper final {
+        // non-explicit constructor on purpose
+        constexpr ConstCharWrapper(const char *str) ENTT_NOEXCEPT: str{str} {}
+        const char *str;
+    };
+
+    static constexpr std::uint64_t offset = 14695981039346656037ull;
+    static constexpr std::uint64_t prime = 1099511628211ull;
+
+    // Fowler–Noll–Vo hash function v. 1a - the good
+    static constexpr std::uint64_t helper(std::uint64_t partial, const char *str) ENTT_NOEXCEPT {
+        return str[0] == 0 ? partial : helper((partial^str[0])*prime, str+1);
+    }
+
+public:
+    /*! @brief Unsigned integer type. */
+    using hash_type = std::uint64_t;
+
+    /**
+     * @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}
+     * HashedString sh{"my.png"};
+     * @endcode
+     *
+     * @tparam N Number of characters of the identifier.
+     * @param str Human-readable identifer.
+     */
+    template <std::size_t N>
+    constexpr HashedString(const char (&str)[N]) ENTT_NOEXCEPT
+        : hash{helper(offset, str)}, str{str}
+    {}
+
+    /**
+     * @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 HashedString(ConstCharWrapper wrapper) ENTT_NOEXCEPT
+        : hash{helper(offset, wrapper.str)}, str{wrapper.str}
+    {}
+
+    /**
+     * @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; }
+
+    /**
+     * @brief Returns the numeric representation of a hashed string.
+     * @return The numeric representation of the instance.
+     */
+    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 HashedString &other) const ENTT_NOEXCEPT {
+        return hash == other.hash;
+    }
+
+private:
+    const hash_type hash;
+    const char *str;
+};
+
+
+/**
+ * @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 HashedString &lhs, const HashedString &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+}
+
+
+#endif // ENTT_CORE_HASHED_STRING_HPP

src/entt/core/ident.hpp

@@ -0,0 +1,109 @@
+#ifndef ENTT_CORE_IDENT_HPP
+#define ENTT_CORE_IDENT_HPP
+
+
+#include <type_traits>
+#include <cstddef>
+#include <utility>
+#include "../config/config.h"
+
+
+namespace entt {
+
+
+namespace internal {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+template<typename... Types>
+struct Identifier final: Identifier<Types>... {
+    using identifier_type = std::size_t;
+
+    template<std::size_t... Indexes>
+    constexpr Identifier(std::index_sequence<Indexes...>) ENTT_NOEXCEPT
+        : Identifier<Types>{std::index_sequence<Indexes>{}}...
+    {}
+
+    template<typename Type>
+    constexpr std::size_t get() const ENTT_NOEXCEPT {
+        return Identifier<std::decay_t<Type>>::get();
+    }
+};
+
+
+template<typename Type>
+struct Identifier<Type> {
+    using identifier_type = std::size_t;
+
+    template<std::size_t Index>
+    constexpr Identifier(std::index_sequence<Index>) ENTT_NOEXCEPT
+        : index{Index}
+    {}
+
+    constexpr std::size_t get() const ENTT_NOEXCEPT {
+        return index;
+    }
+
+private:
+    const std::size_t index;
+};
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond TURN_OFF_DOXYGEN
+ */
+
+
+}
+
+
+/**
+ * @brief Types identifers.
+ *
+ * Variable template used to generate identifiers at compile-time for the given
+ * types. Use the `constexpr` `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}
+ * constexpr auto identifiers = entt::ident<AType, AnotherType>;
+ *
+ * switch(aTypeIdentifier) {
+ * case identifers.get<AType>():
+ *     // ...
+ *     break;
+ * case identifers.get<AnotherType>():
+ *     // ...
+ *     break;
+ * default:
+ *     // ...
+ * }
+ * @endcode
+ *
+ * @note
+ * In case of single type list, `get` isn't a member function template:
+ * @code{.cpp}
+ * func(std::integral_constant<
+ *     entt::ident<AType>::identifier_type,
+ *     entt::ident<AType>::get()
+ * >{});
+ * @endcode
+ *
+ * @tparam Types List of types for which to generate identifiers.
+ */
+template<typename... Types>
+constexpr auto ident = internal::Identifier<std::decay_t<Types>...>{std::make_index_sequence<sizeof...(Types)>{}};
+
+
+}
+
+
+#endif // ENTT_CORE_IDENT_HPP

src/entt/entity/actor.hpp

@@ -0,0 +1,211 @@
+#ifndef ENTT_ENTITY_ACTOR_HPP
+#define ENTT_ENTITY_ACTOR_HPP
+
+
+#include <cassert>
+#include <utility>
+#include "../config/config.h"
+#include "registry.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 Actor {
+    /*! @brief Type of registry used internally. */
+    using registry_type = Registry<Entity>;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+
+    /**
+     * @brief Constructs an actor by using the given registry.
+     * @param reg An entity-component system properly initialized.
+     */
+    Actor(Registry<Entity> &reg)
+        : reg{reg}, entt{reg.create()}
+    {}
+
+    /*! @brief Default destructor. */
+    virtual ~Actor() {
+        reg.destroy(entt);
+    }
+
+    /*! @brief Default copy constructor. */
+    Actor(const Actor &) = default;
+    /*! @brief Default move constructor. */
+    Actor(Actor &&) = default;
+
+    /*! @brief Default copy assignment operator. @return This actor. */
+    Actor & operator=(const Actor &) = default;
+    /*! @brief Default move assignment operator. @return This actor. */
+    Actor & operator=(Actor &&) = default;
+
+    /**
+     * @brief Assigns the given tag to an actor.
+     *
+     * A new instance of the given tag is created and initialized with the
+     * arguments provided (the tag must have a proper constructor or be of
+     * aggregate type). Then the tag is removed from its previous owner (if any)
+     * and assigned to the actor.
+     *
+     * @tparam Tag Type of the tag to create.
+     * @tparam Args Types of arguments to use to construct the tag.
+     * @param args Parameters to use to initialize the tag.
+     * @return A reference to the newly created tag.
+     */
+    template<typename Tag, typename... Args>
+    Tag & assign(tag_t, Args &&... args) {
+        return (reg.template remove<Tag>(), reg.template assign<Tag>(tag_t{}, entt, std::forward<Args>(args)...));
+    }
+
+    /**
+     * @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>
+    Component & assign(Args &&... args) {
+        return reg.template accommodate<Component>(entt, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Removes the given tag from an actor.
+     * @tparam Tag Type of the tag to remove.
+     */
+    template<typename Tag>
+    void remove(tag_t) {
+        assert(has<Tag>(tag_t{}));
+        reg.template remove<Tag>();
+    }
+
+    /**
+     * @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 owns the given tag.
+     * @tparam Tag Type of the tag for which to perform the check.
+     * @return True if the actor owns the tag, false otherwise.
+     */
+    template<typename Tag>
+    bool has(tag_t) const ENTT_NOEXCEPT {
+        return (reg.template has<Tag>() && (reg.template attachee<Tag>() == 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 a reference to the given tag for an actor.
+     * @tparam Tag Type of the tag to get.
+     * @return A reference to the instance of the tag owned by the actor.
+     */
+    template<typename Tag>
+    const Tag & get(tag_t) const ENTT_NOEXCEPT {
+        assert(has<Tag>(tag_t{}));
+        return reg.template get<Tag>();
+    }
+
+    /**
+     * @brief Returns a reference to the given tag for an actor.
+     * @tparam Tag Type of the tag to get.
+     * @return A reference to the instance of the tag owned by the actor.
+     */
+    template<typename Tag>
+    inline Tag & get(tag_t) ENTT_NOEXCEPT {
+        return const_cast<Tag &>(const_cast<const Actor *>(this)->get<Tag>(tag_t{}));
+    }
+
+    /**
+     * @brief Returns a reference to the given component for an actor.
+     * @tparam Component Type of the component to get.
+     * @return A reference to the instance of the component owned by the actor.
+     */
+    template<typename Component>
+    const Component & get() const ENTT_NOEXCEPT {
+        return reg.template get<Component>(entt);
+    }
+
+    /**
+     * @brief Returns a reference to the given component for an actor.
+     * @tparam Component Type of the component to get.
+     * @return A reference to the instance of the component owned by the actor.
+     */
+    template<typename Component>
+    inline Component & get() ENTT_NOEXCEPT {
+        return const_cast<Component &>(const_cast<const Actor *>(this)->get<Component>());
+    }
+
+    /**
+     * @brief Returns a reference to the underlying registry.
+     * @return A reference to the underlying registry.
+     */
+    const registry_type & registry() const ENTT_NOEXCEPT {
+        return reg;
+    }
+
+    /**
+     * @brief Returns a reference to the underlying registry.
+     * @return A reference to the underlying registry.
+     */
+    inline registry_type & registry() ENTT_NOEXCEPT {
+        return const_cast<registry_type &>(const_cast<const Actor *>(this)->registry());
+    }
+
+    /**
+     * @brief Returns the entity associated with an actor.
+     * @return The entity associated with the actor.
+     */
+    entity_type entity() const ENTT_NOEXCEPT {
+        return entt;
+    }
+
+private:
+    registry_type &reg;
+    Entity entt;
+};
+
+
+/**
+ * @brief Default actor class.
+ *
+ * The default actor is the best choice for almost all the applications.<br/>
+ * Users should have a really good reason to choose something different.
+ */
+using DefaultActor = Actor<DefaultRegistry::entity_type>;
+
+
+}
+
+
+#endif // ENTT_ENTITY_ACTOR_HPP

src/entt/entity/entt_traits.hpp

@@ -0,0 +1,96 @@
+#ifndef ENTT_ENTITY_ENTT_TRAITS_HPP
+#define ENTT_ENTITY_ENTT_TRAITS_HPP
+
+
+#include <cstdint>
+
+
+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 Mask to use to get the entity number out of an identifier. */
+    static constexpr auto entity_mask = 0xFFF;
+    /*! @brief Mask to use to get the version out of an identifier. */
+    static constexpr auto 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:
+ *
+ * * 24 bits for the entity number (suitable for almost all the games).
+ * * 8 bit for the version (resets in [0-255]).
+ */
+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 Mask to use to get the entity number out of an identifier. */
+    static constexpr auto entity_mask = 0xFFFFF;
+    /*! @brief Mask to use to get the version out of an identifier. */
+    static constexpr auto 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:
+ *
+ * * 40 bits for the entity number (an indecently large number).
+ * * 24 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 Mask to use to get the entity number out of an identifier. */
+    static constexpr auto entity_mask = 0xFFFFFFFFFF;
+    /*! @brief Mask to use to get the version out of an identifier. */
+    static constexpr auto version_mask = 0xFFFFFF;
+    /*! @brief Extent of the entity number within an identifier. */
+    static constexpr auto entity_shift = 40;
+};
+
+
+}
+
+
+#endif // ENTT_ENTITY_ENTT_TRAITS_HPP

src/entt/entity/helper.hpp

@@ -0,0 +1,84 @@
+#ifndef ENTT_ENTITY_HELPER_HPP
+#define ENTT_ENTITY_HELPER_HPP
+
+
+#include "../signal/sigh.hpp"
+#include "registry.hpp"
+#include "utility.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @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 Types of components to assign to an entity if triggered.
+ * @param registry A valid reference to a registry.
+ * @param entity A valid entity identifier.
+ */
+template<typename Entity, typename... Component>
+void dependency(Registry<Entity> &registry, const Entity entity) {
+    using accumulator_type = int[];
+    accumulator_type accumulator = { ((registry.template has<Component>(entity) ? void() : (registry.template assign<Component>(entity), void())), 0)... };
+    (void)accumulator;
+}
+
+
+/**
+ * @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 `AType` and `AnotherType` whenever `MyType` is
+ * assigned to an entity:
+ * @code{.cpp}
+ * entt::DefaultRegistry registry;
+ * entt::dependency<AType, AnotherType>(registry.construction<MyType>());
+ * @endcode
+ *
+ * @tparam Dependency Types of components to assign to an entity if triggered.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @param sink A sink object properly initialized.
+ */
+template<typename... Dependency, typename Entity>
+void dependency(Sink<void(Registry<Entity> &, const Entity)> sink) {
+    sink.template connect<dependency<Entity, 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 `MyType` and the
+ * components `AType` and `AnotherType`:
+ * @code{.cpp}
+ * entt::DefaultRegistry registry;
+ * entt::dependency<AType, AnotherType>(entt::break_t{}, registry.construction<MyType>());
+ * @endcode
+ *
+ * @tparam Dependency Types of components used to create the dependency.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @param sink A sink object properly initialized.
+ */
+template<typename... Dependency, typename Entity>
+void dependency(break_t, Sink<void(Registry<Entity> &, const Entity)> sink) {
+    sink.template disconnect<dependency<Entity, Dependency...>>();
+}
+
+
+}
+
+
+#endif // ENTT_ENTITY_HELPER_HPP

src/entt/entity/prototype.hpp

@@ -0,0 +1,452 @@
+#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"
+
+
+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 Prototype final {
+    using fn_type = void(*)(const Prototype &, Registry<Entity> &, const Entity);
+    using component_type = typename Registry<Entity>::component_type;
+
+    template<typename Component>
+    struct Wrapper { Component component; };
+
+    struct Handler {
+        fn_type accommodate;
+        fn_type assign;
+    };
+
+public:
+    /*! @brief Registry type. */
+    using registry_type = 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 registry A valid reference to a registry.
+     */
+    Prototype(Registry<Entity> &registry)
+        : registry{registry},
+          entity{registry.create()}
+    {}
+
+    /**
+     * @brief Releases all its resources.
+     */
+    ~Prototype() {
+        registry.destroy(entity);
+    }
+
+    /**
+     * @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) {
+        fn_type accommodate = [](const Prototype &prototype, Registry<Entity> &other, const Entity dst) {
+            const auto &wrapper = prototype.registry.template get<Wrapper<Component>>(prototype.entity);
+            other.template accommodate<Component>(dst, wrapper.component);
+        };
+
+        fn_type assign = [](const Prototype &prototype, Registry<Entity> &other, const Entity dst) {
+            if(!other.template has<Component>(dst)) {
+                const auto &wrapper = prototype.registry.template get<Wrapper<Component>>(prototype.entity);
+                other.template accommodate<Component>(dst, wrapper.component);
+            }
+        };
+
+        handlers[registry.template type<Component>()] = Handler{accommodate, assign};
+        auto &wrapper = registry.template accommodate<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 {
+        registry.template reset<Wrapper<Component>>(entity);
+        handlers.erase(registry.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 registry.template has<Wrapper<Component>...>(entity);
+    }
+
+    /**
+     * @brief Returns a reference to the given component.
+     *
+     * @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 Type of component to get.
+     * @return A reference to the component owned by the prototype.
+     */
+    template<typename Component>
+    const Component & get() const ENTT_NOEXCEPT {
+        return registry.template get<Wrapper<Component>>(entity).component;
+    }
+
+    /**
+     * @brief Returns a reference to the given component.
+     *
+     * @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 Type of component to get.
+     * @return A reference to the component owned by the prototype.
+     */
+    template<typename Component>
+    inline Component & get() ENTT_NOEXCEPT {
+        return const_cast<Component &>(const_cast<const Prototype *>(this)->get<Component>());
+    }
+
+    /**
+     * @brief Returns a reference to the given components.
+     *
+     * @warning
+     * Attempting to get components from a prototype that doesn't own them
+     * results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * prototype doesn't own instances of the given components.
+     *
+     * @tparam Component Type of components to get.
+     * @return References to the components owned by the prototype.
+     */
+    template<typename... Component>
+    inline std::enable_if_t<(sizeof...(Component) > 1), std::tuple<const Component &...>>
+    get() const ENTT_NOEXCEPT {
+        return std::tuple<const Component &...>{get<Component>()...};
+    }
+
+    /**
+     * @brief Returns a reference to the given components.
+     *
+     * @warning
+     * Attempting to get components from a prototype that doesn't own them
+     * results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * prototype doesn't own instances of the given components.
+     *
+     * @tparam Component Type of components to get.
+     * @return References to the components owned by the prototype.
+     */
+    template<typename... Component>
+    inline std::enable_if_t<(sizeof...(Component) > 1), std::tuple<Component &...>>
+    get() ENTT_NOEXCEPT {
+        return std::tuple<Component &...>{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 entity = other.create();
+        assign(other, entity);
+        return entity;
+    }
+
+    /**
+     * @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.
+     */
+    entity_type create() const {
+        const auto entity = registry.create();
+        assign(registry, entity);
+        return entity;
+    }
+
+    /**
+     * @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.
+     */
+    void assign(const entity_type dst) const {
+        for(auto &handler: handlers) {
+            handler.second.assign(*this, registry, 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 accommodate(registry_type &other, const entity_type dst) const {
+        for(auto &handler: handlers) {
+            handler.second.accommodate(*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.
+     */
+    void accommodate(const entity_type dst) const {
+        for(auto &handler: handlers) {
+            handler.second.accommodate(*this, registry, 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(registry, 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(registry);
+    }
+
+private:
+    std::unordered_map<component_type, Handler> handlers;
+    Registry<Entity> &registry;
+    entity_type entity;
+};
+
+
+/**
+ * @brief Default prototype
+ *
+ * The default prototype is the best choice for almost all the
+ * applications.<br/>
+ * Users should have a really good reason to choose something different.
+ */
+using DefaultPrototype = Prototype<std::uint32_t>;
+
+
+}
+
+
+#endif // ENTT_ENTITY_PROTOTYPE_HPP

src/entt/entity/snapshot.hpp

@@ -0,0 +1,720 @@
+#ifndef ENTT_ENTITY_SNAPSHOT_HPP
+#define ENTT_ENTITY_SNAPSHOT_HPP
+
+
+#include <array>
+#include <cstddef>
+#include <utility>
+#include <cassert>
+#include <iterator>
+#include <type_traits>
+#include <unordered_map>
+#include "../config/config.h"
+#include "entt_traits.hpp"
+#include "utility.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Forward declaration of the registry class.
+ */
+template<typename>
+class Registry;
+
+
+/**
+ * @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 and tags 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 Snapshot final {
+    /*! @brief A registry is allowed to create snapshots. */
+    friend class Registry<Entity>;
+
+    using follow_fn_type = Entity(*)(const Registry<Entity> &, const Entity);
+
+    Snapshot(const Registry<Entity> &registry, Entity seed, follow_fn_type follow) ENTT_NOEXCEPT
+        : registry{registry},
+          seed{seed},
+          follow{follow}
+    {}
+
+    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 entity = *(first++);
+
+            if(registry.template has<Component>(entity)) {
+                archive(entity, registry.template get<Component>(entity));
+            }
+        }
+    }
+
+    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 entity = *(begin++);
+            using accumulator_type = std::size_t[];
+            accumulator_type accumulator = { (registry.template has<Component>(entity) ? ++size[Indexes] : size[Indexes])... };
+            (void)accumulator;
+        }
+
+        using accumulator_type = int[];
+        accumulator_type accumulator = { (get<Component>(archive, size[Indexes], first, last), 0)... };
+        (void)accumulator;
+    }
+
+public:
+    /*! @brief Copying a snapshot isn't allowed. */
+    Snapshot(const Snapshot &) = delete;
+    /*! @brief Default move constructor. */
+    Snapshot(Snapshot &&) = default;
+
+    /*! @brief Copying a snapshot isn't allowed. @return This snapshot. */
+    Snapshot & operator=(const Snapshot &) = delete;
+    /*! @brief Default move assignment operator. @return This snapshot. */
+    Snapshot & operator=(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 Snapshot & entities(Archive &archive) const {
+        archive(static_cast<Entity>(registry.size()));
+        registry.each([&archive](const auto entity) { archive(entity); });
+        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 Snapshot & destroyed(Archive &archive) const {
+        auto size = registry.capacity() - registry.size();
+        archive(static_cast<Entity>(size));
+        auto curr = seed;
+
+        for(; size; --size) {
+            archive(curr);
+            curr = follow(registry, curr);
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Puts aside the given component.
+     *
+     * Each instance is serialized together with the entity to which it belongs.
+     * Entities are serialized along with their versions.
+     *
+     * @tparam Component Type of component 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 Snapshot & component(Archive &archive) const {
+        const auto sz = registry.template size<Component>();
+        const auto *entities = registry.template data<Component>();
+
+        archive(static_cast<Entity>(sz));
+
+        for(std::remove_const_t<decltype(sz)> i{}; i < sz; ++i) {
+            const auto entity = entities[i];
+            archive(entity, registry.template get<Component>(entity));
+        };
+
+        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>
+    std::enable_if_t<(sizeof...(Component) > 1), const Snapshot &>
+    component(Archive &archive) const {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { 0, (component<Component>(archive), 0)... };
+        (void)accumulator;
+        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 Snapshot & component(Archive &archive, It first, It last) const {
+        component<Component...>(archive, first, last, std::make_index_sequence<sizeof...(Component)>{});
+        return *this;
+    }
+
+    /**
+     * @brief Puts aside the given tag.
+     *
+     * Each instance is serialized together with the entity to which it belongs.
+     * Entities are serialized along with their versions.
+     *
+     * @tparam Tag Type of tag 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 Tag, typename Archive>
+    const Snapshot & tag(Archive &archive) const {
+        const bool has = registry.template has<Tag>();
+
+        // numerical length is forced for tags to facilitate loading
+        archive(has ? Entity(1): Entity{});
+
+        if(has) {
+            archive(registry.template attachee<Tag>(), registry.template get<Tag>());
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Puts aside the given tags.
+     *
+     * Each instance is serialized together with the entity to which it belongs.
+     * Entities are serialized along with their versions.
+     *
+     * @tparam Tag Types of tags 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... Tag, typename Archive>
+    std::enable_if_t<(sizeof...(Tag) > 1), const Snapshot &>
+    tag(Archive &archive) const {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { 0, (tag<Tag>(archive), 0)... };
+        (void)accumulator;
+        return *this;
+    }
+
+private:
+    const Registry<Entity> &registry;
+    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 SnapshotLoader final {
+    /*! @brief A registry is allowed to create snapshot loaders. */
+    friend class Registry<Entity>;
+
+    using assure_fn_type = void(*)(Registry<Entity> &, const Entity, const bool);
+
+    SnapshotLoader(Registry<Entity> &registry, assure_fn_type assure_fn) ENTT_NOEXCEPT
+        : registry{registry},
+          assure_fn{assure_fn}
+    {
+        // restore a snapshot as a whole requires a clean registry
+        assert(!registry.capacity());
+    }
+
+    template<typename Archive>
+    void assure(Archive &archive, bool destroyed) const {
+        Entity length{};
+        archive(length);
+
+        while(length--) {
+            Entity entity{};
+            archive(entity);
+            assure_fn(registry, entity, destroyed);
+        }
+    }
+
+    template<typename Type, typename Archive, typename... Args>
+    void assign(Archive &archive, Args... args) const {
+        Entity length{};
+        archive(length);
+
+        while(length--) {
+            Entity entity{};
+            Type instance{};
+            archive(entity, instance);
+            static constexpr auto destroyed = false;
+            assure_fn(registry, entity, destroyed);
+            registry.template assign<Type>(args..., entity, static_cast<const Type &>(instance));
+        }
+    }
+
+public:
+    /*! @brief Copying a snapshot loader isn't allowed. */
+    SnapshotLoader(const SnapshotLoader &) = delete;
+    /*! @brief Default move constructor. */
+    SnapshotLoader(SnapshotLoader &&) = default;
+
+    /*! @brief Copying a snapshot loader isn't allowed. @return This loader. */
+    SnapshotLoader & operator=(const SnapshotLoader &) = delete;
+    /*! @brief Default move assignment operator. @return This loader. */
+    SnapshotLoader & operator=(SnapshotLoader &&) = 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 SnapshotLoader & 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 SnapshotLoader & 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 SnapshotLoader & component(Archive &archive) const {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { 0, (assign<Component>(archive), 0)... };
+        (void)accumulator;
+        return *this;
+    }
+
+    /**
+     * @brief Restores tags 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 tag is assigned
+     * doesn't exist yet, the loader will take care to create it with the
+     * version it originally had.
+     *
+     * @tparam Tag Types of tags 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... Tag, typename Archive>
+    const SnapshotLoader & tag(Archive &archive) const {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { 0, (assign<Tag>(archive, tag_t{}), 0)... };
+        (void)accumulator;
+        return *this;
+    }
+
+    /**
+     * @brief Destroys those entities that have neither components nor tags.
+     *
+     * In case all the entities were serialized but only part of the components
+     * and tags was saved, it could happen that some of the entities have
+     * neither components nor tags once restored.<br/>
+     * This functions helps to identify and destroy those entities.
+     *
+     * @return A valid loader to continue restoring data.
+     */
+    const SnapshotLoader & orphans() const {
+        registry.orphans([this](const auto entity) {
+            registry.destroy(entity);
+        });
+
+        return *this;
+    }
+
+private:
+    Registry<Entity> &registry;
+    assure_fn_type assure_fn;
+};
+
+
+/**
+ * @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 ContinuousLoader final {
+    using traits_type = entt_traits<Entity>;
+
+    void destroy(Entity entity) {
+        const auto it = remloc.find(entity);
+
+        if(it == remloc.cend()) {
+            const auto local = registry.create();
+            remloc.emplace(entity, std::make_pair(local, true));
+            registry.destroy(local);
+        }
+    }
+
+    void restore(Entity entity) {
+        const auto it = remloc.find(entity);
+
+        if(it == remloc.cend()) {
+            const auto local = registry.create();
+            remloc.emplace(entity, std::make_pair(local, true));
+        } else {
+            remloc[entity].first =
+                    registry.valid(remloc[entity].first)
+                    ? remloc[entity].first
+                    : registry.create();
+
+            // set the dirty flag
+            remloc[entity].second = true;
+        }
+    }
+
+    template<typename Type, typename Member>
+    std::enable_if_t<std::is_same<Member, Entity>::value>
+    update(Type &instance, Member Type:: *member) {
+        instance.*member = map(instance.*member);
+    }
+
+    template<typename Type, typename Member>
+    std::enable_if_t<std::is_same<typename std::iterator_traits<typename Member::iterator>::value_type, Entity>::value>
+    update(Type &instance, Member Type:: *member) {
+        for(auto &entity: instance.*member) {
+            entity = map(entity);
+        }
+    }
+
+    template<typename Other, typename Type, typename Member>
+    std::enable_if_t<!std::is_same<Other, Type>::value>
+    update(Other &, Member Type:: *) {}
+
+    template<typename Archive>
+    void assure(Archive &archive, void(ContinuousLoader:: *member)(Entity)) {
+        Entity length{};
+        archive(length);
+
+        while(length--) {
+            Entity entity{};
+            archive(entity);
+            (this->*member)(entity);
+        }
+    }
+
+    template<typename Component>
+    void reset() {
+        for(auto &&ref: remloc) {
+            const auto local = ref.second.first;
+
+            if(registry.valid(local)) {
+                registry.template reset<Component>(local);
+            }
+        }
+    }
+
+    template<typename Other, typename Archive, typename Func, typename... Type, typename... Member>
+    void assign(Archive &archive, Func func, Member Type:: *... member) {
+        Entity length{};
+        archive(length);
+
+        while(length--) {
+            Entity entity{};
+            Other instance{};
+
+            archive(entity, instance);
+            restore(entity);
+
+            using accumulator_type = int[];
+            accumulator_type accumulator = { 0, (update(instance, member), 0)... };
+            (void)accumulator;
+
+            func(map(entity), instance);
+        }
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+
+    /**
+     * @brief Constructs a loader that is bound to a given registry.
+     * @param registry A valid reference to a registry.
+     */
+    ContinuousLoader(Registry<entity_type> &registry) ENTT_NOEXCEPT
+        : registry{registry}
+    {}
+
+    /*! @brief Copying a snapshot loader isn't allowed. */
+    ContinuousLoader(const ContinuousLoader &) = delete;
+    /*! @brief Default move constructor. */
+    ContinuousLoader(ContinuousLoader &&) = default;
+
+    /*! @brief Copying a snapshot loader isn't allowed. @return This loader. */
+    ContinuousLoader & operator=(const ContinuousLoader &) = delete;
+    /*! @brief Default move assignment operator. @return This loader. */
+    ContinuousLoader & operator=(ContinuousLoader &&) = 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>
+    ContinuousLoader & entities(Archive &archive) {
+        assure(archive, &ContinuousLoader::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>
+    ContinuousLoader & destroyed(Archive &archive) {
+        assure(archive, &ContinuousLoader::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>
+    ContinuousLoader & component(Archive &archive, Member Type:: *... member) {
+        auto apply = [this](const auto entity, const auto &component) {
+            registry.template accommodate<std::decay_t<decltype(component)>>(entity, component);
+        };
+
+        using accumulator_type = int[];
+        accumulator_type accumulator = { 0, (reset<Component>(), assign<Component>(archive, apply, member...), 0)... };
+        (void)accumulator;
+        return *this;
+    }
+
+    /**
+     * @brief Restores tags 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 tag 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 Tag Type of tag 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... Tag, typename Archive, typename... Type, typename... Member>
+    ContinuousLoader & tag(Archive &archive, Member Type:: *... member) {
+        auto apply = [this](const auto entity, const auto &tag) {
+            registry.template assign<std::decay_t<decltype(tag)>>(tag_t{}, entity, tag);
+        };
+
+        using accumulator_type = int[];
+        accumulator_type accumulator = { 0, (registry.template remove<Tag>(), assign<Tag>(archive, apply, member...), 0)... };
+        (void)accumulator;
+        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.
+     */
+    ContinuousLoader & 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(registry.valid(local)) {
+                    registry.destroy(local);
+                }
+
+                it = remloc.erase(it);
+            }
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Destroys those entities that have neither components nor tags.
+     *
+     * In case all the entities were serialized but only part of the components
+     * and tags was saved, it could happen that some of the entities have
+     * neither components nor tags once restored.<br/>
+     * This functions helps to identify and destroy those entities.
+     *
+     * @return A non-const reference to this loader.
+     */
+    ContinuousLoader & orphans() {
+        registry.orphans([this](const auto entity) {
+            registry.destroy(entity);
+        });
+
+        return *this;
+    }
+
+    /**
+     * @brief Tests if a loader knows about a given entity.
+     * @param entity An entity identifier.
+     * @return True if `entity` is managed by the loader, false otherwise.
+     */
+    bool has(entity_type entity) const ENTT_NOEXCEPT {
+        return (remloc.find(entity) != remloc.cend());
+    }
+
+    /**
+     * @brief Returns the identifier to which an entity refers.
+     *
+     * @warning
+     * Attempting to use an entity that isn't managed by the loader results in
+     * undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * loader doesn't knows about the entity.
+     *
+     * @param entity An entity identifier.
+     * @return The identifier to which `entity` refers in the target registry.
+     */
+    entity_type map(entity_type entity) const ENTT_NOEXCEPT {
+        assert(has(entity));
+        return remloc.find(entity)->second.first;
+    }
+
+private:
+    std::unordered_map<Entity, std::pair<Entity, bool>> remloc;
+    Registry<Entity> &registry;
+};
+
+
+}
+
+
+#endif // ENTT_ENTITY_SNAPSHOT_HPP

src/entt/entity/sparse_set.hpp

@@ -0,0 +1,970 @@
+#ifndef ENTT_ENTITY_SPARSE_SET_HPP
+#define ENTT_ENTITY_SPARSE_SET_HPP
+
+
+#include <algorithm>
+#include <iterator>
+#include <numeric>
+#include <utility>
+#include <vector>
+#include <cstddef>
+#include <cassert>
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/algorithm.hpp"
+#include "entt_traits.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Sparse set.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error, but for a few reasonable cases.
+ */
+template<typename...>
+class SparseSet;
+
+
+/**
+ * @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. View and PersistentView are 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 SparseSet<Entity> {
+    using traits_type = entt_traits<Entity>;
+
+    struct Iterator final {
+        using difference_type = std::size_t;
+        using value_type = Entity;
+        using pointer = const value_type *;
+        using reference = value_type;
+        using iterator_category = std::input_iterator_tag;
+
+        Iterator(pointer direct, std::size_t pos)
+            : direct{direct}, pos{pos}
+        {}
+
+        Iterator & operator++() ENTT_NOEXCEPT {
+            return --pos, *this;
+        }
+
+        Iterator operator++(int) ENTT_NOEXCEPT {
+            Iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        Iterator & operator+=(const difference_type value) ENTT_NOEXCEPT {
+            pos -= value;
+            return *this;
+        }
+
+        Iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
+            return Iterator{direct, pos-value};
+        }
+
+        bool operator==(const Iterator &other) const ENTT_NOEXCEPT {
+            return other.pos == pos;
+        }
+
+        inline bool operator!=(const Iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+        reference operator*() const ENTT_NOEXCEPT {
+            return direct[pos-1];
+        }
+
+    private:
+        pointer direct;
+        std::size_t pos;
+    };
+
+    static constexpr auto pending = ~typename traits_type::entity_type{};
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Entity dependent position type. */
+    using pos_type = entity_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Input iterator type. */
+    using iterator_type = Iterator;
+    /*! @brief Constant input iterator type. */
+    using const_iterator_type = Iterator;
+
+    /*! @brief Default constructor. */
+    SparseSet() ENTT_NOEXCEPT = default;
+
+    /*! @brief Default destructor. */
+    virtual ~SparseSet() ENTT_NOEXCEPT = default;
+
+    /*! @brief Copying a sparse set isn't allowed. */
+    SparseSet(const SparseSet &) = delete;
+    /*! @brief Default move constructor. */
+    SparseSet(SparseSet &&) = default;
+
+    /*! @brief Copying a sparse set isn't allowed. @return This sparse set. */
+    SparseSet & operator=(const SparseSet &) = delete;
+    /*! @brief Default move assignment operator. @return This sparse set. */
+    SparseSet & operator=(SparseSet &&) = default;
+
+    /**
+     * @brief Increases the capacity of a sparse set.
+     *
+     * If the new capacity is greater than the current capacity, new storage is
+     * allocated, otherwise the method does nothing.
+     *
+     * @param cap Desired capacity.
+     */
+    void reserve(const size_type cap) {
+        direct.reserve(cap);
+    }
+
+    /**
+     * @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();
+    }
+
+    /**
+     * @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.
+     */
+    const_iterator_type cbegin() const ENTT_NOEXCEPT {
+        return const_iterator_type{direct.data(), direct.size()};
+    }
+
+    /**
+     * @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.
+     */
+    inline const_iterator_type begin() const ENTT_NOEXCEPT {
+        return cbegin();
+    }
+
+    /**
+     * @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.
+     */
+    inline iterator_type begin() ENTT_NOEXCEPT {
+        return cbegin();
+    }
+
+    /**
+     * @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.
+     */
+    const_iterator_type cend() const ENTT_NOEXCEPT {
+        return const_iterator_type{direct.data(), {}};
+    }
+
+    /**
+     * @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.
+     */
+    inline const_iterator_type end() const ENTT_NOEXCEPT {
+        return cend();
+    }
+
+    /**
+     * @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.
+     */
+    inline iterator_type end() ENTT_NOEXCEPT {
+        return cend();
+    }
+
+    /**
+     * @brief Checks if a sparse set contains an entity.
+     * @param entity A valid entity identifier.
+     * @return True if the sparse set contains the entity, false otherwise.
+     */
+    bool has(const entity_type entity) const ENTT_NOEXCEPT {
+        const auto pos = size_type(entity & traits_type::entity_mask);
+        // testing against pending permits to avoid accessing the direct vector
+        return (pos < reverse.size()) && (reverse[pos] != pending);
+    }
+
+    /**
+     * @brief Checks if a sparse set contains an entity (unsafe).
+     *
+     * Alternative version of `has`. It accesses the underlying data structures
+     * without bounds checking and thus it's both unsafe and risky to use.<br/>
+     * You should not invoke directly this function unless you know exactly what
+     * you are doing. Prefer the `has` member function instead.
+     *
+     * @warning
+     * Attempting to use an entity that doesn't belong to the sparse set can
+     * result in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode in case of
+     * bounds violation.
+     *
+     * @param entity A valid entity identifier.
+     * @return True if the sparse set contains the entity, false otherwise.
+     */
+    bool fast(const entity_type entity) const ENTT_NOEXCEPT {
+        const auto pos = size_type(entity & traits_type::entity_mask);
+        assert(pos < reverse.size());
+        // testing against pending permits to avoid accessing the direct vector
+        return (reverse[pos] != pending);
+    }
+
+    /**
+     * @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 entity A valid entity identifier.
+     * @return The position of the entity in the sparse set.
+     */
+    pos_type get(const entity_type entity) const ENTT_NOEXCEPT {
+        assert(has(entity));
+        return reverse[entity & traits_type::entity_mask];
+    }
+
+    /**
+     * @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 entity A valid entity identifier.
+     */
+    void construct(const entity_type entity) {
+        assert(!has(entity));
+        const auto pos = size_type(entity & traits_type::entity_mask);
+
+        if(!(pos < reverse.size())) {
+            const auto value = pending;
+            reverse.resize(pos+1, value);
+        }
+
+        reverse[pos] = pos_type(direct.size());
+        direct.push_back(entity);
+    }
+
+    /**
+     * @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 entity A valid entity identifier.
+     */
+    virtual void destroy(const entity_type entity) {
+        assert(has(entity));
+        const auto back = direct.back();
+        auto &candidate = reverse[entity & traits_type::entity_mask];
+        // swapping isn't required here, we are getting rid of the last element
+        reverse[back & traits_type::entity_mask] = candidate;
+        direct[candidate] = back;
+        candidate = pending;
+        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 pos_type lhs, const pos_type rhs) ENTT_NOEXCEPT {
+        assert(lhs < direct.size());
+        assert(rhs < direct.size());
+        auto &src = direct[lhs];
+        auto &dst = direct[rhs];
+        std::swap(reverse[src & traits_type::entity_mask], reverse[dst & traits_type::entity_mask]);
+        std::swap(src, dst);
+    }
+
+    /**
+     * @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.
+     */
+    void respect(const SparseSet<Entity> &other) ENTT_NOEXCEPT {
+        auto from = other.cbegin();
+        auto to = other.cend();
+
+        pos_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<pos_type> reverse;
+    std::vector<entity_type> direct;
+};
+
+
+/**
+ * @brief Extended sparse set implementation.
+ *
+ * This specialization of a sparse set associates an object to an entity. The
+ * main purpose of this class is to use 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.
+ *
+ * @sa SparseSet<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>
+class SparseSet<Entity, Type>: public SparseSet<Entity> {
+    using underlying_type = SparseSet<Entity>;
+
+    template<bool Const>
+    struct Iterator final {
+        using difference_type = std::size_t;
+        using value_type = std::conditional_t<Const, const Type, Type>;
+        using pointer = value_type *;
+        using reference = value_type &;
+        using iterator_category = std::input_iterator_tag;
+
+        Iterator(pointer instances, std::size_t pos)
+            : instances{instances}, pos{pos}
+        {}
+
+        Iterator & operator++() ENTT_NOEXCEPT {
+            return --pos, *this;
+        }
+
+        Iterator operator++(int) ENTT_NOEXCEPT {
+            Iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        Iterator & operator+=(const difference_type value) ENTT_NOEXCEPT {
+            pos -= value;
+            return *this;
+        }
+
+        Iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
+            return Iterator{instances, pos-value};
+        }
+
+        bool operator==(const Iterator &other) const ENTT_NOEXCEPT {
+            return other.pos == pos;
+        }
+
+        inline bool operator!=(const Iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+        reference operator*() const ENTT_NOEXCEPT {
+            return instances[pos-1];
+        }
+
+        pointer operator->() const ENTT_NOEXCEPT {
+            return (instances+pos-1);
+        }
+
+    private:
+        pointer instances;
+        std::size_t pos;
+    };
+
+public:
+    /*! @brief Type of the objects associated to the entities. */
+    using object_type = Type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename underlying_type::entity_type;
+    /*! @brief Entity dependent position type. */
+    using pos_type = typename underlying_type::pos_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = typename underlying_type::size_type;
+    /*! @brief Input iterator type. */
+    using iterator_type = Iterator<false>;
+    /*! @brief Constant input iterator type. */
+    using const_iterator_type = Iterator<true>;
+
+    /*! @brief Default constructor. */
+    SparseSet() ENTT_NOEXCEPT = default;
+
+    /*! @brief Copying a sparse set isn't allowed. */
+    SparseSet(const SparseSet &) = delete;
+    /*! @brief Default move constructor. */
+    SparseSet(SparseSet &&) = default;
+
+    /*! @brief Copying a sparse set isn't allowed. @return This sparse set. */
+    SparseSet & operator=(const SparseSet &) = delete;
+    /*! @brief Default move assignment operator. @return This sparse set. */
+    SparseSet & operator=(SparseSet &&) = default;
+
+    /**
+     * @brief Increases the capacity of a sparse set.
+     *
+     * If the new capacity is greater than the current capacity, new storage is
+     * allocated, otherwise the method does nothing.
+     *
+     * @param cap Desired capacity.
+     */
+    void reserve(const size_type cap) {
+        underlying_type::reserve(cap);
+        instances.reserve(cap);
+    }
+
+    /**
+     * @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 sparse set in the expected order.
+     *
+     * @return A pointer to the array of objects.
+     */
+    const object_type * raw() const ENTT_NOEXCEPT {
+        return instances.data();
+    }
+
+    /**
+     * @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 sparse set in the expected order.
+     *
+     * @return A pointer to the array of objects.
+     */
+    object_type * raw() ENTT_NOEXCEPT {
+        return instances.data();
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * The returned iterator points to the first instance of the given type. 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 either `sort`
+     * or `respect`.
+     *
+     * @return An iterator to the first instance of the given type.
+     */
+    const_iterator_type cbegin() const ENTT_NOEXCEPT {
+        return const_iterator_type{instances.data(), instances.size()};
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * The returned iterator points to the first instance of the given type. 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 either `sort`
+     * or `respect`.
+     *
+     * @return An iterator to the first instance of the given type.
+     */
+    inline const_iterator_type begin() const ENTT_NOEXCEPT {
+        return cbegin();
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * The returned iterator points to the first instance of the given type. 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 either `sort`
+     * or `respect`.
+     *
+     * @return An iterator to the first instance of the given type.
+     */
+    iterator_type begin() ENTT_NOEXCEPT {
+        return iterator_type{instances.data(), instances.size()};
+    }
+
+    /**
+     * @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.data(), {}};
+    }
+
+    /**
+     * @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.
+     */
+    inline const_iterator_type end() const ENTT_NOEXCEPT {
+        return cend();
+    }
+
+    /**
+     * @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 end() ENTT_NOEXCEPT {
+        return iterator_type{instances.data(), {}};
+    }
+
+    /**
+     * @brief Returns the object associated to an entity.
+     *
+     * @warning
+     * Attempting to use 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 entity A valid entity identifier.
+     * @return The object associated to the entity.
+     */
+    const object_type & get(const entity_type entity) const ENTT_NOEXCEPT {
+        return instances[underlying_type::get(entity)];
+    }
+
+    /**
+     * @brief Returns the object associated to an entity.
+     *
+     * @warning
+     * Attempting to use 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 entity A valid entity identifier.
+     * @return The object associated to the entity.
+     */
+    inline object_type & get(const entity_type entity) ENTT_NOEXCEPT {
+        return const_cast<object_type &>(const_cast<const SparseSet *>(this)->get(entity));
+    }
+
+    /**
+     * @brief Assigns an entity to a sparse set and constructs its object.
+     *
+     * @note
+     * _Sfinae'd_ function.<br/>
+     * This version is used for types that can be constructed in place directly.
+     * It doesn't work well with aggregates because of the placement new usually
+     * performed under the hood during an _emplace back_.
+     *
+     * @warning
+     * Attempting to use 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 Args Types of arguments to use to construct the object.
+     * @param entity A valid entity identifier.
+     * @param args Parameters to use to construct an object for the entity.
+     * @return The object associated to the entity.
+     */
+    template<typename... Args>
+    std::enable_if_t<std::is_constructible<Type, Args...>::value, object_type &>
+    construct(const entity_type entity, Args &&... args) {
+        underlying_type::construct(entity);
+        instances.emplace_back(std::forward<Args>(args)...);
+        return instances.back();
+    }
+
+    /**
+     * @brief Assigns an entity to a sparse set and constructs its object.
+     *
+     * @note
+     * _Sfinae'd_ function.<br/>
+     * Fallback for aggregates and types in general 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 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 Args Types of arguments to use to construct the object.
+     * @param entity A valid entity identifier.
+     * @param args Parameters to use to construct an object for the entity.
+     * @return The object associated to the entity.
+     */
+    template<typename... Args>
+    std::enable_if_t<!std::is_constructible<Type, Args...>::value, object_type &>
+    construct(const entity_type entity, Args &&... args) {
+        underlying_type::construct(entity);
+        instances.emplace_back(Type{std::forward<Args>(args)...});
+        return instances.back();
+    }
+
+    /**
+     * @brief Removes an entity from a sparse set and destroies its object.
+     *
+     * @warning
+     * Attempting to use 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 entity A valid entity identifier.
+     */
+    void destroy(const entity_type entity) override {
+        // swapping isn't required here, we are getting rid of the last element
+        // however, we must protect ourselves from self assignments (see #37)
+        auto tmp = std::move(instances.back());
+        instances[underlying_type::get(entity)] = std::move(tmp);
+        instances.pop_back();
+        underlying_type::destroy(entity);
+    }
+
+    /**
+     * @brief Sort components according to the given comparison function.
+     *
+     * Sort the elements so that iterating the sparse set with a couple of
+     * iterators returns them in the expected order. See `begin` and `end` for
+     * more details.
+     *
+     * The comparison function object must return `true` if the first element
+     * is _less_ than the second one, `false` otherwise. The signature of the
+     * comparison function should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * bool(const 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 funtion 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.
+     * @param compare A valid comparison function object.
+     * @param sort A valid sort function object.
+     */
+    template<typename Compare, typename Sort = StdSort>
+    void sort(Compare compare, Sort sort = Sort{}) {
+        std::vector<pos_type> copy(instances.size());
+        std::iota(copy.begin(), copy.end(), 0);
+
+        sort(copy.begin(), copy.end(), [this, compare = std::move(compare)](const auto lhs, const auto rhs) {
+            return compare(const_cast<const object_type &>(instances[rhs]), const_cast<const object_type &>(instances[lhs]));
+        });
+
+        for(pos_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 components according to the order of the entities 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.
+     * Components 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 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 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 SparseSet<Entity> &other) ENTT_NOEXCEPT {
+        auto from = other.cbegin();
+        auto to = other.cend();
+
+        pos_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 sparse set.
+     */
+    void reset() override {
+        underlying_type::reset();
+        instances.clear();
+    }
+
+private:
+    std::vector<object_type> instances;
+};
+
+
+}
+
+
+#endif // ENTT_ENTITY_SPARSE_SET_HPP

src/entt/entity/utility.hpp

@@ -0,0 +1,27 @@
+#ifndef ENTT_ENTITY_UTILITY_HPP
+#define ENTT_ENTITY_UTILITY_HPP
+
+
+namespace entt {
+
+
+/*! @brief Tag class type used to disambiguate overloads. */
+struct tag_t final {};
+
+
+/*! @brief Persistent view type used to disambiguate overloads. */
+struct persistent_t final {};
+
+
+/*! @brief Raw view type used to disambiguate overloads. */
+struct raw_t final {};
+
+
+/*! @brief Break type used to disambiguate overloads. */
+struct break_t final {};
+
+
+}
+
+
+#endif // ENTT_ENTITY_UTILITY_HPP

src/entt/entt.hpp

@@ -0,0 +1,23 @@
+#include "core/algorithm.hpp"
+#include "core/family.hpp"
+#include "core/hashed_string.hpp"
+#include "core/ident.hpp"
+#include "entity/actor.hpp"
+#include "entity/entt_traits.hpp"
+#include "entity/helper.hpp"
+#include "entity/prototype.hpp"
+#include "entity/registry.hpp"
+#include "entity/snapshot.hpp"
+#include "entity/sparse_set.hpp"
+#include "entity/utility.hpp"
+#include "entity/view.hpp"
+#include "locator/locator.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/locator/locator.hpp

@@ -0,0 +1,116 @@
+#ifndef ENTT_LOCATOR_LOCATOR_HPP
+#define ENTT_LOCATOR_LOCATOR_HPP
+
+
+#include <memory>
+#include <utility>
+#include <cassert>
+#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 ServiceLocator final {
+    /*! @brief Type of service offered. */
+    using service_type = Service;
+
+    /*! @brief Default constructor, deleted on purpose. */
+    ServiceLocator() = delete;
+    /*! @brief Default destructor, deleted on purpose. */
+    ~ServiceLocator() = 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) {
+        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:
+    static std::shared_ptr<Service> service;
+};
+
+
+template<typename Service>
+std::shared_ptr<Service> ServiceLocator<Service>::service{};
+
+
+}
+
+
+#endif // ENTT_LOCATOR_LOCATOR_HPP

src/entt/process/process.hpp

@@ -0,0 +1,339 @@
+#ifndef ENTT_PROCESS_PROCESS_HPP
+#define ENTT_PROCESS_PROCESS_HPP
+
+
+#include <type_traits>
+#include <functional>
+#include <utility>
+#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(void *);
+ *   @endcode
+ *
+ *   It's invoked at the first tick, immediately before an update. The `void *`
+ *   parameter is an opaque pointer to user data (if any) forwarded directly to
+ *   the process during an update.
+ *
+ * * @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 state>
+    using tag = std::integral_constant<State, state>;
+
+    template<typename Target = Derived>
+    auto tick(int, tag<State::UNINITIALIZED>, void *data)
+    -> decltype(std::declval<Target>().init(data)) {
+        static_cast<Target *>(this)->init(data);
+    }
+
+    template<typename Target = Derived>
+    auto tick(int, tag<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, tag<State::SUCCEEDED>)
+    -> decltype(std::declval<Target>().succeeded()) {
+        static_cast<Target *>(this)->succeeded();
+    }
+
+    template<typename Target = Derived>
+    auto tick(int, tag<State::FAILED>)
+    -> decltype(std::declval<Target>().failed()) {
+        static_cast<Target *>(this)->failed();
+    }
+
+    template<typename Target = Derived>
+    auto tick(int, tag<State::ABORTED>)
+    -> decltype(std::declval<Target>().aborted()) {
+        static_cast<Target *>(this)->aborted();
+    }
+
+    template<State S, typename... Args>
+    void tick(char, tag<S>, 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<Process, Derived>::value, "!");
+    }
+
+    /**
+     * @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, tag<State::UNINITIALIZED>{}, data);
+            current = State::RUNNING;
+            // no break on purpose, tasks are executed immediately
+        case State::RUNNING:
+            tick(0, tag<State::RUNNING>{}, delta, data);
+        default:
+            // suppress warnings
+            break;
+        }
+
+        // if it's dead, it must be notified and removed immediately
+        switch(current) {
+        case State::SUCCEEDED:
+            tick(0, tag<State::SUCCEEDED>{});
+            current = State::FINISHED;
+            break;
+        case State::FAILED:
+            tick(0, tag<State::FAILED>{});
+            current = State::FINISHED;
+            stopped = true;
+            break;
+        case State::ABORTED:
+            tick(0, tag<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 ProcessAdaptor: Process<ProcessAdaptor<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>
+    ProcessAdaptor(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 +1,321 @@
+#ifndef ENTT_PROCESS_SCHEDULER_HPP
+#define ENTT_PROCESS_SCHEDULER_HPP
+
+
+#include <vector>
+#include <memory>
+#include <utility>
+#include <iterator>
+#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<MyProcess>(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 final {
+    template<typename T>
+    struct type_t { using type = T; };
+
+    struct ProcessHandler final {
+        using instance_type = std::unique_ptr<void, void(*)(void *)>;
+        using update_type = bool(*)(ProcessHandler &, Delta, void *);
+        using abort_type = void(*)(ProcessHandler &, bool);
+        using next_type = std::unique_ptr<ProcessHandler>;
+
+        instance_type instance;
+        update_type update;
+        abort_type abort;
+        next_type next;
+    };
+
+    template<typename Lambda>
+    struct Then final: Lambda {
+        Then(Lambda &&lambda, ProcessHandler *handler)
+            : Lambda{std::forward<Lambda>(lambda)}, handler{handler}
+        {}
+
+        template<typename Proc, typename... Args>
+        decltype(auto) then(Args &&... args) && {
+            static_assert(std::is_base_of<Process<Proc, Delta>, Proc>::value, "!");
+            handler = Lambda::operator()(handler, type_t<Proc>{}, std::forward<Args>(args)...);
+            return std::move(*this);
+        }
+
+        template<typename Func>
+        decltype(auto) then(Func &&func) && {
+            using Proc = ProcessAdaptor<std::decay_t<Func>, Delta>;
+            return std::move(*this).template then<Proc>(std::forward<Func>(func));
+        }
+
+    private:
+        ProcessHandler *handler;
+    };
+
+    template<typename Proc>
+    static bool update(ProcessHandler &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);
+                dead = handler.update(handler, delta, data);
+            } else {
+                handler.instance.reset();
+            }
+        }
+
+        return dead;
+    }
+
+    template<typename Proc>
+    static void abort(ProcessHandler &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);
+    }
+
+    auto then(ProcessHandler *handler) {
+        auto lambda = [](ProcessHandler *handler, auto next, auto... args) {
+            using Proc = typename decltype(next)::type;
+
+            if(handler) {
+                auto proc = typename ProcessHandler::instance_type{new Proc{std::forward<decltype(args)>(args)...}, &Scheduler::deleter<Proc>};
+                handler->next.reset(new ProcessHandler{std::move(proc), &Scheduler::update<Proc>, &Scheduler::abort<Proc>, nullptr});
+                handler = handler->next.get();
+            }
+
+            return handler;
+        };
+
+        return Then<decltype(lambda)>{std::move(lambda), handler};
+    }
+
+public:
+    /*! @brief Unsigned integer type. */
+    using size_type = typename std::vector<ProcessHandler>::size_type;
+
+    /*! @brief Default constructor. */
+    Scheduler() ENTT_NOEXCEPT = default;
+
+    /*! @brief Copying a scheduler isn't allowed. */
+    Scheduler(const Scheduler &) = delete;
+    /*! @brief Default move constructor. */
+    Scheduler(Scheduler &&) = default;
+
+    /*! @brief Copying a scheduler isn't allowed. @return This scheduler. */
+    Scheduler & operator=(const Scheduler &) = delete;
+    /*! @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<MyProcess>(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<MyOtherProcess>();
+     * @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<Process<Proc, Delta>, Proc>::value, "!");
+
+        auto proc = typename ProcessHandler::instance_type{new Proc{std::forward<Args>(args)...}, &Scheduler::deleter<Proc>};
+        ProcessHandler handler{std::move(proc), &Scheduler::update<Proc>, &Scheduler::abort<Proc>, nullptr};
+        handlers.push_back(std::move(handler));
+
+        return then(&handlers.back());
+    }
+
+    /**
+     * @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, auto succeed, auto fail);
+     * @endcode
+     *
+     * Where:
+     *
+     * * `delta` is the elapsed time.
+     * * `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<MyProcess>(arguments...);
+     * @endcode
+     *
+     * @sa ProcessAdaptor
+     *
+     * @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 = ProcessAdaptor<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<ProcessHandler> handlers{};
+};
+
+
+}
+
+
+#endif // ENTT_PROCESS_SCHEDULER_HPP

src/entt/resource/cache.hpp

@@ -0,0 +1,201 @@
+#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"
+
+
+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 ResourceCache {
+    using container_type = std::unordered_map<HashedString::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 = HashedString;
+
+    /*! @brief Default constructor. */
+    ResourceCache() = default;
+
+    /*! @brief Copying a cache isn't allowed. */
+    ResourceCache(const ResourceCache &) ENTT_NOEXCEPT = delete;
+    /*! @brief Default move constructor. */
+    ResourceCache(ResourceCache &&) ENTT_NOEXCEPT = default;
+
+    /*! @brief Copying a cache isn't allowed. @return This cache. */
+    ResourceCache & operator=(const ResourceCache &) ENTT_NOEXCEPT = delete;
+    /*! @brief Default move assignment operator. @return This cache. */
+    ResourceCache & operator=(ResourceCache &&) ENTT_NOEXCEPT = 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.
+     *
+     * @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 True if the resource is ready to use, false otherwise.
+     */
+    template<typename Loader, typename... Args>
+    bool load(const resource_type id, Args &&... args) {
+        static_assert(std::is_base_of<ResourceLoader<Loader, Resource>, Loader>::value, "!");
+
+        bool loaded = true;
+
+        if(resources.find(id) == resources.cend()) {
+            std::shared_ptr<Resource> resource = Loader{}.get(std::forward<Args>(args)...);
+            loaded = (static_cast<bool>(resource) ? (resources[id] = std::move(resource), loaded) : false);
+        }
+
+        return loaded;
+    }
+
+    /**
+     * @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.
+     *
+     * @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 True if the resource is ready to use, false otherwise.
+     */
+    template<typename Loader, typename... Args>
+    bool 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>
+    ResourceHandle<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 ResourceHandle
+     *
+     * @param id Unique resource identifier.
+     * @return A handle for the given resource.
+     */
+    ResourceHandle<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 {
+        auto it = resources.find(id);
+
+        if(it != resources.end()) {
+            resources.erase(it);
+        }
+    }
+
+private:
+    container_type resources;
+};
+
+
+}
+
+
+#endif // ENTT_RESOURCE_CACHE_HPP

src/entt/resource/handle.hpp

@@ -0,0 +1,116 @@
+#ifndef ENTT_RESOURCE_HANDLE_HPP
+#define ENTT_RESOURCE_HANDLE_HPP
+
+
+#include <memory>
+#include <utility>
+#include <cassert>
+#include "../config/config.h"
+
+
+namespace entt {
+
+
+template<typename Resource>
+class ResourceCache;
+
+
+/**
+ * @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 ResourceHandle final {
+    /*! @brief Resource handles are friends of their caches. */
+    friend class ResourceCache<Resource>;
+
+    ResourceHandle(std::shared_ptr<Resource> res) ENTT_NOEXCEPT
+        : resource{std::move(res)}
+    {}
+
+public:
+    /*! @brief Default copy constructor. */
+    ResourceHandle(const ResourceHandle &) ENTT_NOEXCEPT = default;
+    /*! @brief Default move constructor. */
+    ResourceHandle(ResourceHandle &&) ENTT_NOEXCEPT = default;
+
+    /*! @brief Default copy assignment operator. @return This handle. */
+    ResourceHandle & operator=(const ResourceHandle &) ENTT_NOEXCEPT = default;
+    /*! @brief Default move assignment operator. @return This handle. */
+    ResourceHandle & operator=(ResourceHandle &&) 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 {
+        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 {
+        assert(static_cast<bool>(resource));
+        return resource.get();
+    }
+
+    /**
+     * @brief Returns 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 +1,62 @@
+#ifndef ENTT_RESOURCE_LOADER_HPP
+#define ENTT_RESOURCE_LOADER_HPP
+
+
+#include <memory>
+
+
+namespace entt {
+
+
+template<typename Resource>
+class ResourceCache;
+
+
+/**
+ * @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 MyResource {};
+ *
+ * struct MyLoader: entt::ResourceLoader<MyLoader, MyResource> {
+ *     std::shared_ptr<MyResource> load(int) const {
+ *         // use the integer value somehow
+ *         return std::make_shared<MyResource>();
+ *     }
+ * };
+ * @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 ResourceLoader {
+    /*! @brief Resource loaders are friends of their caches. */
+    friend class ResourceCache<Resource>;
+
+    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 +1,138 @@
+#ifndef ENTT_SIGNAL_DELEGATE_HPP
+#define ENTT_SIGNAL_DELEGATE_HPP
+
+
+#include <utility>
+#include "../config/config.h"
+
+
+namespace entt {
+
+
+/**
+ * @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 send around functions and member functions.
+ *
+ * Unmanaged delegate for function pointers and member functions. 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 and member functions 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...)> final {
+    using proto_type = Ret(*)(void *, Args...);
+    using stub_type = std::pair<void *, proto_type>;
+
+    static Ret fallback(void *, Args...) ENTT_NOEXCEPT { return {}; }
+
+    template<Ret(*Function)(Args...)>
+    static Ret proto(void *, Args... args) {
+        return (Function)(args...);
+    }
+
+    template<typename Class, Ret(Class:: *Member)(Args...)>
+    static Ret proto(void *instance, Args... args) {
+        return (static_cast<Class *>(instance)->*Member)(args...);
+    }
+
+public:
+    /*! @brief Default constructor. */
+    Delegate() ENTT_NOEXCEPT
+        : stub{std::make_pair(nullptr, &fallback)}
+    {}
+
+    /**
+     * @brief Binds a free function to a delegate.
+     * @tparam Function A valid free function pointer.
+     */
+    template<Ret(*Function)(Args...)>
+    void connect() ENTT_NOEXCEPT {
+        stub = std::make_pair(nullptr, &proto<Function>);
+    }
+
+    /**
+     * @brief Connects a member function for a given instance to a delegate.
+     *
+     * The delegate isn't responsible for the connected object. Users must
+     * guarantee that the lifetime of the instance overcomes the one of the
+     * delegate.
+     *
+     * @tparam Class Type of class to which the member function belongs.
+     * @tparam Member Member function to connect to the delegate.
+     * @param instance A valid instance of type pointer to `Class`.
+     */
+    template<typename Class, Ret(Class:: *Member)(Args...)>
+    void connect(Class *instance) ENTT_NOEXCEPT {
+        stub = std::make_pair(instance, &proto<Class, Member>);
+    }
+
+    /**
+     * @brief Resets a delegate.
+     *
+     * After a reset, a delegate can be safely invoked with no effect.
+     */
+    void reset() ENTT_NOEXCEPT {
+        stub = std::make_pair(nullptr, &fallback);
+    }
+
+    /**
+     * @brief Triggers a delegate.
+     * @param args Arguments to use to invoke the underlying function.
+     * @return The value returned by the underlying function.
+     */
+    Ret operator()(Args... args) const {
+        return stub.second(stub.first, args...);
+    }
+
+    /**
+     * @brief Checks if the contents of the two delegates are different.
+     *
+     * Two delegates are identical if they contain the same listener.
+     *
+     * @param other Delegate with which to compare.
+     * @return True if the two delegates are identical, false otherwise.
+     */
+    bool operator==(const Delegate<Ret(Args...)> &other) const ENTT_NOEXCEPT {
+        return stub.first == other.stub.first && stub.second == other.stub.second;
+    }
+
+private:
+    stub_type stub;
+};
+
+
+/**
+ * @brief Checks if the contents of the two delegates are different.
+ *
+ * Two delegates are identical if they contain the same listener.
+ *
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ * @param lhs A valid delegate object.
+ * @param rhs A valid delegate object.
+ * @return True if the two delegates are different, false otherwise.
+ */
+template<typename Ret, typename... Args>
+bool operator!=(const Delegate<Ret(Args...)> &lhs, const Delegate<Ret(Args...)> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+}
+
+
+#endif // ENTT_SIGNAL_DELEGATE_HPP

src/entt/signal/dispatcher.hpp

@@ -0,0 +1,188 @@
+#ifndef ENTT_SIGNAL_DISPATCHER_HPP
+#define ENTT_SIGNAL_DISPATCHER_HPP
+
+
+#include <vector>
+#include <memory>
+#include <utility>
+#include <cstdint>
+#include <algorithm>
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/family.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 must have the following function type:
+ * @code{.cpp}
+ * void(const Event &)
+ * @endcode
+ *
+ * Member functions named `receive` are automatically detected and registered or
+ * unregistered by the dispatcher. 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 final {
+    using event_family = Family<struct InternalDispatcherEventFamily>;
+
+    template<typename Class, typename Event>
+    using instance_type = typename SigH<void(const Event &)>::template instance_type<Class>;
+
+    struct BaseSignalWrapper {
+        virtual ~BaseSignalWrapper() = default;
+        virtual void publish() = 0;
+    };
+
+    template<typename Event>
+    struct SignalWrapper final: BaseSignalWrapper {
+        using sink_type = typename SigH<void(const Event &)>::sink_type;
+
+        void publish() override {
+            const auto &curr = current++;
+            current %= std::extent<decltype(events)>::value;
+            std::for_each(events[curr].cbegin(), events[curr].cend(), [this](const auto &event) { signal.publish(event); });
+            events[curr].clear();
+        }
+
+        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].push_back({ std::forward<Args>(args)... });
+        }
+
+    private:
+        SigH<void(const Event &)> signal{};
+        std::vector<Event> events[2];
+        int current{};
+    };
+
+    template<typename Event>
+    SignalWrapper<Event> & wrapper() {
+        const auto type = event_family::type<Event>();
+
+        if(!(type < wrappers.size())) {
+            wrappers.resize(type + 1);
+        }
+
+        if(!wrappers[type]) {
+            wrappers[type] = std::make_unique<SignalWrapper<Event>>();
+        }
+
+        return static_cast<SignalWrapper<Event> &>(*wrappers[type]);
+    }
+
+public:
+    /*! @brief Type of sink for the given event. */
+    template<typename Event>
+    using sink_type = typename SignalWrapper<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 SigH::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 wrapper<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) {
+        wrapper<Event>().trigger(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 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 enqueue(Args &&... args) {
+        wrapper<Event>().enqueue(std::forward<Args>(args)...);
+    }
+
+    /**
+     * @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() {
+        wrapper<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 &wrapper = wrappers[pos-1];
+
+            if(wrapper) {
+                wrapper->publish();
+            }
+        }
+    }
+
+private:
+    std::vector<std::unique_ptr<BaseSignalWrapper>> wrappers;
+};
+
+
+}
+
+
+#endif // ENTT_SIGNAL_DISPATCHER_HPP

src/entt/signal/emitter.hpp

@@ -0,0 +1,336 @@
+#ifndef ENTT_SIGNAL_EMITTER_HPP
+#define ENTT_SIGNAL_EMITTER_HPP
+
+
+#include <type_traits>
+#include <functional>
+#include <algorithm>
+#include <utility>
+#include <cstdint>
+#include <memory>
+#include <vector>
+#include <list>
+#include "../config/config.h"
+#include "../core/family.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:
+ *
+ * ```cpp
+ * struct MyEmitter: Emitter<MyEmitter> {
+ *     // ...
+ * }
+ * ```
+ *
+ * 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 InternalEmitterHandlerFamily>;
+
+    struct BaseHandler {
+        virtual ~BaseHandler() = default;
+        virtual bool empty() const ENTT_NOEXCEPT = 0;
+        virtual void clear() ENTT_NOEXCEPT = 0;
+    };
+
+    template<typename Event>
+    struct Handler final: BaseHandler {
+        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(onceL.cbegin(), onceL.cend(), pred) &&
+                    std::all_of(onL.cbegin(), onL.cend(), pred);
+        }
+
+        void clear() ENTT_NOEXCEPT override {
+            if(publishing) {
+                auto func = [](auto &&element) { element.first = true; };
+                std::for_each(onceL.begin(), onceL.end(), func);
+                std::for_each(onL.begin(), onL.end(), func);
+            } else {
+                onceL.clear();
+                onL.clear();
+            }
+        }
+
+        inline connection_type once(listener_type listener) {
+            return onceL.emplace(onceL.cend(), false, std::move(listener));
+        }
+
+        inline connection_type on(listener_type listener) {
+            return onL.emplace(onL.cend(), false, std::move(listener));
+        }
+
+        void erase(connection_type conn) ENTT_NOEXCEPT {
+            conn->first = true;
+
+            if(!publishing) {
+                auto pred = [](auto &&element) { return element.first; };
+                onceL.remove_if(pred);
+                onL.remove_if(pred);
+            }
+        }
+
+        void publish(const Event &event, Derived &ref) {
+            container_type currentL;
+            onceL.swap(currentL);
+
+            auto func = [&event, &ref](auto &&element) {
+                return element.first ? void() : element.second(event, ref);
+            };
+
+            publishing = true;
+
+            std::for_each(onL.rbegin(), onL.rend(), func);
+            std::for_each(currentL.rbegin(), currentL.rend(), func);
+
+            publishing = false;
+
+            onL.remove_if([](auto &&element) { return element.first; });
+        }
+
+    private:
+        bool publishing{false};
+        container_type onceL{};
+        container_type onL{};
+    };
+
+    template<typename Event>
+    Handler<Event> & handler() ENTT_NOEXCEPT {
+        const std::size_t family = handler_family::type<Event>();
+
+        if(!(family < handlers.size())) {
+            handlers.resize(family+1);
+        }
+
+        if(!handlers[family]) {
+            handlers[family] = std::make_unique<Handler<Event>>();
+        }
+
+        return static_cast<Handler<Event> &>(*handlers[family]);
+    }
+
+public:
+    /** @brief Type of listeners accepted for the given event. */
+    template<typename Event>
+    using Listener = typename 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 final: private 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 Handler<Event>::connection_type conn)
+            : Handler<Event>::connection_type{std::move(conn)}
+        {}
+
+        /*! @brief Default copy constructor. */
+        Connection(const Connection &) = default;
+        /*! @brief Default move constructor. */
+        Connection(Connection &&) = default;
+
+        /**
+         * @brief Default copy assignment operator.
+         * @return This connection.
+         */
+        Connection & operator=(const Connection &) = default;
+
+        /**
+         * @brief Default move assignment operator.
+         * @return This connection.
+         */
+        Connection & operator=(Connection &&) = default;
+    };
+
+    /*! @brief Default constructor. */
+    Emitter() ENTT_NOEXCEPT = default;
+
+    /*! @brief Default destructor. */
+    virtual ~Emitter() ENTT_NOEXCEPT {
+        static_assert(std::is_base_of<Emitter<Derived>, Derived>::value, "!");
+    }
+
+    /*! @brief Copying an emitter isn't allowed. */
+    Emitter(const Emitter &) = delete;
+    /*! @brief Default move constructor. */
+    Emitter(Emitter &&) = default;
+
+    /*! @brief Copying an emitter isn't allowed. @return This emitter. */
+    Emitter & operator=(const Emitter &) = delete;
+    /*! @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) {
+        handler<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 listener The listener to register.
+     * @return Connection object that can be used to disconnect the listener.
+     */
+    template<typename Event>
+    Connection<Event> on(Listener<Event> listener) {
+        return handler<Event>().on(std::move(listener));
+    }
+
+    /**
+     * @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 listener The listener to register.
+     * @return Connection object that can be used to disconnect the listener.
+     */
+    template<typename Event>
+    Connection<Event> once(Listener<Event> listener) {
+        return handler<Event>().once(std::move(listener));
+    }
+
+    /**
+     * @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 {
+        handler<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 {
+        handler<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 &&handler) {
+            return handler ? 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 {
+        const std::size_t family = handler_family::type<Event>();
+
+        return (!(family < handlers.size()) ||
+                !handlers[family] ||
+                static_cast<Handler<Event> &>(*handlers[family]).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 &&handler) {
+            return !handler || handler->empty();
+        });
+    }
+
+private:
+    std::vector<std::unique_ptr<BaseHandler>> handlers{};
+};
+
+
+}
+
+
+#endif // ENTT_SIGNAL_EMITTER_HPP

src/entt/signal/sigh.hpp

@@ -0,0 +1,382 @@
+#ifndef ENTT_SIGNAL_SIGH_HPP
+#define ENTT_SIGNAL_SIGH_HPP
+
+
+#include <algorithm>
+#include <utility>
+#include <vector>
+#include "../config/config.h"
+
+
+namespace entt {
+
+
+namespace internal {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+template<typename, typename>
+struct Invoker;
+
+
+template<typename Ret, typename... Args, typename Collector>
+struct Invoker<Ret(Args...), Collector> {
+    using proto_type = Ret(*)(void *, Args...);
+    using call_type = std::pair<void *, proto_type>;
+
+    virtual ~Invoker() = default;
+
+    bool invoke(Collector &collector, proto_type proto, void *instance, Args... args) const {
+        return collector(proto(instance, args...));
+    }
+};
+
+
+template<typename... Args, typename Collector>
+struct Invoker<void(Args...), Collector> {
+    using proto_type = void(*)(void *, Args...);
+    using call_type = std::pair<void *, proto_type>;
+
+    virtual ~Invoker() = default;
+
+    bool invoke(Collector &, proto_type proto, void *instance, Args... args) const {
+        return (proto(instance, args...), true);
+    }
+};
+
+
+template<typename Ret>
+struct NullCollector final {
+    using result_type = Ret;
+    bool operator()(result_type) const ENTT_NOEXCEPT { return true; }
+};
+
+
+template<>
+struct NullCollector<void> final {
+    using result_type = void;
+    bool operator()() const ENTT_NOEXCEPT { return true; }
+};
+
+
+template<typename>
+struct DefaultCollector;
+
+
+template<typename Ret, typename... Args>
+struct DefaultCollector<Ret(Args...)> final {
+    using collector_type = NullCollector<Ret>;
+};
+
+
+template<typename Function>
+using DefaultCollectorType = typename DefaultCollector<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::DefaultCollectorType<Function>>
+class 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...)> final {
+    /*! @brief A signal is allowed to create sinks. */
+    template<typename, typename>
+    friend class SigH;
+
+    using proto_type = Ret(*)(void *, Args...);
+    using call_type = std::pair<void *, proto_type>;
+
+    template<Ret(*Function)(Args...)>
+    static Ret proto(void *, Args... args) {
+        return (Function)(args...);
+    }
+
+    template<typename Class, Ret(Class:: *Member)(Args... args)>
+    static Ret proto(void *instance, Args... args) {
+        return (static_cast<Class *>(instance)->*Member)(args...);
+    }
+
+    Sink(std::vector<call_type> &calls)
+        : calls{calls}
+    {}
+
+public:
+    /**
+     * @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<Ret(*Function)(Args...)>
+    void connect() {
+        disconnect<Function>();
+        calls.emplace_back(nullptr, &proto<Function>);
+    }
+
+    /**
+     * @brief Connects a member function for a given instance to a signal.
+     *
+     * The signal isn't responsible for the connected object. Users must
+     * guarantee that the lifetime of the instance overcomes the one of the
+     * signal. On the other side, the signal handler performs checks to
+     * avoid multiple connections for the same member function of a given
+     * instance.
+     *
+     * @tparam Class Type of class to which the member function belongs.
+     * @tparam Member Member function to connect to the signal.
+     * @param instance A valid instance of type pointer to `Class`.
+     */
+    template <typename Class, Ret(Class:: *Member)(Args...) = &Class::receive>
+    void connect(Class *instance) {
+        disconnect<Class, Member>(instance);
+        calls.emplace_back(instance, &proto<Class, Member>);
+    }
+
+    /**
+     * @brief Disconnects a free function from a signal.
+     * @tparam Function A valid free function pointer.
+     */
+    template<Ret(*Function)(Args...)>
+    void disconnect() {
+        call_type target{nullptr, &proto<Function>};
+        calls.erase(std::remove(calls.begin(), calls.end(), std::move(target)), calls.end());
+    }
+
+    /**
+     * @brief Disconnects the given member function from a signal.
+     * @tparam Class Type of class to which the member function belongs.
+     * @tparam Member Member function to connect to the signal.
+     * @param instance A valid instance of type pointer to `Class`.
+     */
+    template<typename Class, Ret(Class:: *Member)(Args...)>
+    void disconnect(Class *instance) {
+        call_type target{instance, &proto<Class, Member>};
+        calls.erase(std::remove(calls.begin(), calls.end(), std::move(target)), calls.end());
+    }
+
+    /**
+     * @brief Removes all existing connections for the given instance.
+     * @tparam Class Type of class to which the member function belongs.
+     * @param instance A valid instance of type pointer to `Class`.
+     */
+    template<typename Class>
+    void disconnect(Class *instance) {
+        auto func = [instance](const call_type &call) { return call.first == instance; };
+        calls.erase(std::remove_if(calls.begin(), calls.end(), std::move(func)), calls.end());
+    }
+
+    /**
+     * @brief Disconnects all the listeners from a signal.
+     */
+    void disconnect() {
+        calls.clear();
+    }
+
+private:
+    std::vector<call_type> &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>
+class SigH<Ret(Args...), Collector> final: private internal::Invoker<Ret(Args...), Collector> {
+    using call_type = typename internal::Invoker<Ret(Args...), Collector>::call_type;
+
+public:
+    /*! @brief Unsigned integer type. */
+    using size_type = typename std::vector<call_type>::size_type;
+    /*! @brief Collector type. */
+    using collector_type = Collector;
+    /*! @brief Sink type. */
+    using sink_type = Sink<Ret(Args...)>;
+
+    /**
+     * @brief Instance type when it comes to connecting member functions.
+     * @tparam Class Type of class to which the member function belongs.
+     */
+    template<typename Class>
+    using instance_type = Class *;
+
+    /**
+     * @brief Number of listeners connected to the signal.
+     * @return Number of listeners currently connected.
+     */
+    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() {
+        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.second(call.first, 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.second, call.first, 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);
+    }
+
+    /**
+     * @brief Checks if the contents of the two signals are identical.
+     *
+     * Two signals are identical if they have the same size and the same
+     * listeners registered exactly in the same order.
+     *
+     * @param other Signal with which to compare.
+     * @return True if the two signals are identical, false otherwise.
+     */
+    bool operator==(const SigH &other) const ENTT_NOEXCEPT {
+        return std::equal(calls.cbegin(), calls.cend(), other.calls.cbegin(), other.calls.cend());
+    }
+
+private:
+    std::vector<call_type> calls;
+};
+
+
+/**
+ * @brief Checks if the contents of the two signals are different.
+ *
+ * Two signals are identical if they have the same size and the same
+ * listeners registered exactly in the same order.
+ *
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ * @param lhs A valid signal object.
+ * @param rhs A valid signal object.
+ * @return True if the two signals are different, false otherwise.
+ */
+template<typename Ret, typename... Args>
+bool operator!=(const SigH<Ret(Args...)> &lhs, const SigH<Ret(Args...)> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+}
+
+
+#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 +2,80 @@
 # Tests configuration
 #
 
-set(COMMON_LINK_LIBS gtest_main Threads::Threads)
+add_library(odr OBJECT odr.cpp)
 
-# List of available targets
+macro(ADD_ENTT_TEST TEST_NAME TEST_SOURCE)
+    add_executable(${TEST_NAME} $<TARGET_OBJECTS:odr> ${TEST_SOURCE})
+    target_link_libraries(${TEST_NAME} PRIVATE gtest_main Threads::Threads)
+    add_test(NAME ${TEST_NAME} COMMAND ${TEST_NAME})
+endmacro()
 
-set(TARGET_ENTT entt)
-set(TARGET_BENCHMARK benchmark)
+# Test benchmark
 
-# Test TARGET_ENTT
+if(BUILD_BENCHMARK)
+    ADD_ENTT_TEST(benchmark benchmark/benchmark.cpp)
+endif()
 
-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 mod
 
-# Test TARGET_BENCHMARK
+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)
 
-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()
+    set(MOD_TEST_SOURCE ${DUKTAPE_SRC_DIR}/duktape.c mod/mod.cpp)
+    ADD_ENTT_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)
+
+    ADD_ENTT_TEST(cereal snapshot/snapshot.cpp)
+    target_include_directories(cereal PRIVATE ${CEREAL_SRC_DIR})
+endif()
+
+# Test core
+
+ADD_ENTT_TEST(algorithm entt/core/algorithm.cpp)
+ADD_ENTT_TEST(family entt/core/family.cpp)
+ADD_ENTT_TEST(hashed_string entt/core/hashed_string.cpp)
+ADD_ENTT_TEST(ident entt/core/ident.cpp)
+
+# Test entity
+
+ADD_ENTT_TEST(actor entt/entity/actor.cpp)
+ADD_ENTT_TEST(helper entt/entity/helper.cpp)
+ADD_ENTT_TEST(prototype entt/entity/prototype.cpp)
+ADD_ENTT_TEST(registry entt/entity/registry.cpp)
+ADD_ENTT_TEST(snapshot entt/entity/snapshot.cpp)
+ADD_ENTT_TEST(sparse_set entt/entity/sparse_set.cpp)
+ADD_ENTT_TEST(view entt/entity/view.cpp)
+
+# Test locator
+
+ADD_ENTT_TEST(locator entt/locator/locator.cpp)
+
+# Test process
+
+ADD_ENTT_TEST(process entt/process/process.cpp)
+ADD_ENTT_TEST(scheduler entt/process/scheduler.cpp)
+
+# Test resource
+
+ADD_ENTT_TEST(resource entt/resource/resource.cpp)
+
+# Test signal
+
+ADD_ENTT_TEST(delegate entt/signal/delegate.cpp)
+ADD_ENTT_TEST(dispatcher entt/signal/dispatcher.cpp)
+ADD_ENTT_TEST(emitter entt/signal/emitter.cpp)
+ADD_ENTT_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/benchmark/benchmark.cpp

@@ -0,0 +1,602 @@
+#include <iostream>
+#include <cstddef>
+#include <cstdint>
+#include <chrono>
+#include <gtest/gtest.h>
+#include <entt/entity/registry.hpp>
+
+struct Position {
+    std::uint64_t x;
+    std::uint64_t y;
+};
+
+struct Velocity {
+    std::uint64_t x;
+    std::uint64_t y;
+};
+
+template<std::size_t>
+struct Comp { int x; };
+
+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(Benchmark, Construct) {
+    entt::DefaultRegistry registry;
+
+    std::cout << "Constructing 1000000 entities" << std::endl;
+
+    Timer timer;
+
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
+        registry.create();
+    }
+
+    timer.elapsed();
+}
+
+TEST(Benchmark, Destroy) {
+    entt::DefaultRegistry registry;
+
+    std::cout << "Destroying 1000000 entities" << std::endl;
+
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
+        registry.create();
+    }
+
+    Timer timer;
+
+    registry.each([&registry](auto entity) {
+        registry.destroy(entity);
+    });
+
+    timer.elapsed();
+}
+
+TEST(Benchmark, IterateCreateDeleteSingleComponent) {
+    entt::DefaultRegistry registry;
+
+    std::cout << "Looping 10000 times creating and deleting a random number of entities" << std::endl;
+
+    Timer timer;
+
+    auto view = registry.view<Position>();
+
+    for(int i = 0; i < 10000; i++) {
+        for(int j = 0; j < 10000; j++) {
+            const auto entity = registry.create();
+            registry.assign<Position>(entity);
+        }
+
+        for(auto entity: view) {
+            if(rand() % 2 == 0) {
+                registry.destroy(entity);
+            }
+        }
+    }
+
+    timer.elapsed();
+}
+
+TEST(Benchmark, IterateSingleComponent1M) {
+    entt::DefaultRegistry registry;
+
+    std::cout << "Iterating over 1000000 entities, one component" << std::endl;
+
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
+        const auto entity = registry.create();
+        registry.assign<Position>(entity);
+    }
+
+    auto test = [&registry](auto func) {
+        Timer timer;
+        registry.view<Position>().each(func);
+        timer.elapsed();
+    };
+
+    test([](auto, const auto &) {});
+    test([](auto, auto &... comp) {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { (comp.x = {}, 0)... };
+        (void)accumulator;
+    });
+}
+
+TEST(Benchmark, IterateSingleComponentRaw1M) {
+    entt::DefaultRegistry registry;
+
+    std::cout << "Iterating over 1000000 entities, one component, raw view" << std::endl;
+
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
+        const auto entity = registry.create();
+        registry.assign<Position>(entity);
+    }
+
+    auto test = [&registry](auto func) {
+        Timer timer;
+        registry.view<Position>(entt::raw_t{}).each(func);
+        timer.elapsed();
+    };
+
+    test([](const auto &) {});
+    test([](auto &... comp) {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { (comp.x = {}, 0)... };
+        (void)accumulator;
+    });
+}
+
+TEST(Benchmark, IterateTwoComponents1M) {
+    entt::DefaultRegistry registry;
+
+    std::cout << "Iterating over 1000000 entities, two components" << std::endl;
+
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
+        const auto entity = registry.create();
+        registry.assign<Position>(entity);
+        registry.assign<Velocity>(entity);
+    }
+
+    auto test = [&registry](auto func) {
+        Timer timer;
+        registry.view<Position, Velocity>().each(func);
+        timer.elapsed();
+    };
+
+    test([](auto, const auto &...) {});
+    test([](auto, auto &... comp) {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { (comp.x = {}, 0)... };
+        (void)accumulator;
+    });
+}
+
+TEST(Benchmark, IterateTwoComponents1MHalf) {
+    entt::DefaultRegistry registry;
+
+    std::cout << "Iterating over 1000000 entities, two components, half of the entities have all the components" << std::endl;
+
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
+        const auto entity = registry.create();
+        registry.assign<Velocity>(entity);
+
+        if(i % 2) {
+            registry.assign<Position>(entity);
+        }
+    }
+
+    auto test = [&registry](auto func) {
+        Timer timer;
+        registry.view<Position, Velocity>().each(func);
+        timer.elapsed();
+    };
+
+    test([](auto, const auto &...) {});
+    test([](auto, auto &... comp) {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { (comp.x = {}, 0)... };
+        (void)accumulator;
+    });
+}
+
+TEST(Benchmark, IterateTwoComponents1MOne) {
+    entt::DefaultRegistry registry;
+
+    std::cout << "Iterating over 1000000 entities, two components, only one entity has all the components" << std::endl;
+
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
+        const auto entity = registry.create();
+        registry.assign<Velocity>(entity);
+
+        if(i == 5000000L) {
+            registry.assign<Position>(entity);
+        }
+    }
+
+    auto test = [&registry](auto func) {
+        Timer timer;
+        registry.view<Position, Velocity>().each(func);
+        timer.elapsed();
+    };
+
+    test([](auto, const auto &...) {});
+    test([](auto, auto &... comp) {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { (comp.x = {}, 0)... };
+        (void)accumulator;
+    });
+}
+
+TEST(Benchmark, IterateTwoComponentsPersistent1M) {
+    entt::DefaultRegistry registry;
+    registry.prepare<Position, Velocity>();
+
+    std::cout << "Iterating over 1000000 entities, two components, persistent view" << std::endl;
+
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
+        const auto entity = registry.create();
+        registry.assign<Position>(entity);
+        registry.assign<Velocity>(entity);
+    }
+
+    auto test = [&registry](auto func) {
+        Timer timer;
+        registry.view<Position, Velocity>(entt::persistent_t{}).each(func);
+        timer.elapsed();
+    };
+
+    test([](auto, const auto &...) {});
+    test([](auto, auto &... comp) {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { (comp.x = {}, 0)... };
+        (void)accumulator;
+    });
+}
+
+TEST(Benchmark, IterateFiveComponents1M) {
+    entt::DefaultRegistry registry;
+
+    std::cout << "Iterating over 1000000 entities, five components" << std::endl;
+
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
+        const auto entity = registry.create();
+        registry.assign<Position>(entity);
+        registry.assign<Velocity>(entity);
+        registry.assign<Comp<1>>(entity);
+        registry.assign<Comp<2>>(entity);
+        registry.assign<Comp<3>>(entity);
+    }
+
+    auto test = [&registry](auto func) {
+        Timer timer;
+        registry.view<Position, Velocity, Comp<1>, Comp<2>, Comp<3>>().each(func);
+        timer.elapsed();
+    };
+
+    test([](auto, const auto &...) {});
+    test([](auto, auto &... comp) {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { (comp.x = {}, 0)... };
+        (void)accumulator;
+    });
+}
+
+TEST(Benchmark, IterateFiveComponents1MHalf) {
+    entt::DefaultRegistry registry;
+
+    std::cout << "Iterating over 1000000 entities, five components, half of the entities have all the components" << std::endl;
+
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
+        const auto entity = registry.create();
+        registry.assign<Velocity>(entity);
+        registry.assign<Comp<1>>(entity);
+        registry.assign<Comp<2>>(entity);
+        registry.assign<Comp<3>>(entity);
+
+        if(i % 2) {
+            registry.assign<Position>(entity);
+        }
+    }
+
+    auto test = [&registry](auto func) {
+        Timer timer;
+        registry.view<Position, Velocity, Comp<1>, Comp<2>, Comp<3>>().each(func);
+        timer.elapsed();
+    };
+
+    test([](auto, const auto &...) {});
+    test([](auto, auto &... comp) {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { (comp.x = {}, 0)... };
+        (void)accumulator;
+    });
+}
+
+TEST(Benchmark, IterateFiveComponents1MOne) {
+    entt::DefaultRegistry registry;
+
+    std::cout << "Iterating over 1000000 entities, five components, only one entity has all the components" << std::endl;
+
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
+        const auto entity = registry.create();
+        registry.assign<Velocity>(entity);
+        registry.assign<Comp<1>>(entity);
+        registry.assign<Comp<2>>(entity);
+        registry.assign<Comp<3>>(entity);
+
+        if(i == 5000000L) {
+            registry.assign<Position>(entity);
+        }
+    }
+
+    auto test = [&registry](auto func) {
+        Timer timer;
+        registry.view<Position, Velocity, Comp<1>, Comp<2>, Comp<3>>().each(func);
+        timer.elapsed();
+    };
+
+    test([](auto, const auto &...) {});
+    test([](auto, auto &... comp) {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { (comp.x = {}, 0)... };
+        (void)accumulator;
+    });
+}
+
+TEST(Benchmark, IterateFiveComponentsPersistent1M) {
+    entt::DefaultRegistry registry;
+    registry.prepare<Position, Velocity, Comp<1>, Comp<2>, Comp<3>>();
+
+    std::cout << "Iterating over 1000000 entities, five components, persistent view" << std::endl;
+
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
+        const auto entity = registry.create();
+        registry.assign<Position>(entity);
+        registry.assign<Velocity>(entity);
+        registry.assign<Comp<1>>(entity);
+        registry.assign<Comp<2>>(entity);
+        registry.assign<Comp<3>>(entity);
+    }
+
+    auto test = [&registry](auto func) {
+        Timer timer;
+        registry.view<Position, Velocity, Comp<1>, Comp<2>, Comp<3>>(entt::persistent_t{}).each(func);
+        timer.elapsed();
+    };
+
+    test([](auto, const auto &...) {});
+    test([](auto, auto &... comp) {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { (comp.x = {}, 0)... };
+        (void)accumulator;
+    });
+}
+
+TEST(Benchmark, IterateTenComponents1M) {
+    entt::DefaultRegistry registry;
+
+    std::cout << "Iterating over 1000000 entities, ten components" << std::endl;
+
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
+        const auto entity = registry.create();
+        registry.assign<Position>(entity);
+        registry.assign<Velocity>(entity);
+        registry.assign<Comp<1>>(entity);
+        registry.assign<Comp<2>>(entity);
+        registry.assign<Comp<3>>(entity);
+        registry.assign<Comp<4>>(entity);
+        registry.assign<Comp<5>>(entity);
+        registry.assign<Comp<6>>(entity);
+        registry.assign<Comp<7>>(entity);
+        registry.assign<Comp<8>>(entity);
+    }
+
+    auto test = [&registry](auto func) {
+        Timer timer;
+        registry.view<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>().each(func);
+        timer.elapsed();
+    };
+
+    test([](auto, const auto &...) {});
+    test([](auto, auto &... comp) {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { (comp.x = {}, 0)... };
+        (void)accumulator;
+    });
+}
+
+TEST(Benchmark, IterateTenComponents1MHalf) {
+    entt::DefaultRegistry registry;
+
+    std::cout << "Iterating over 1000000 entities, ten components, half of the entities have all the components" << std::endl;
+
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
+        const auto entity = registry.create();
+        registry.assign<Velocity>(entity);
+        registry.assign<Comp<1>>(entity);
+        registry.assign<Comp<2>>(entity);
+        registry.assign<Comp<3>>(entity);
+        registry.assign<Comp<4>>(entity);
+        registry.assign<Comp<5>>(entity);
+        registry.assign<Comp<6>>(entity);
+        registry.assign<Comp<7>>(entity);
+        registry.assign<Comp<8>>(entity);
+
+        if(i % 2) {
+            registry.assign<Position>(entity);
+        }
+    }
+
+    auto test = [&registry](auto func) {
+        Timer timer;
+        registry.view<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>().each(func);
+        timer.elapsed();
+    };
+
+    test([](auto, auto &...) {});
+    test([](auto, auto &... comp) {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { (comp.x = {}, 0)... };
+        (void)accumulator;
+    });
+}
+
+TEST(Benchmark, IterateTenComponents1MOne) {
+    entt::DefaultRegistry registry;
+
+    std::cout << "Iterating over 1000000 entities, ten components, only one entity has all the components" << std::endl;
+
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
+        const auto entity = registry.create();
+        registry.assign<Velocity>(entity);
+        registry.assign<Comp<1>>(entity);
+        registry.assign<Comp<2>>(entity);
+        registry.assign<Comp<3>>(entity);
+        registry.assign<Comp<4>>(entity);
+        registry.assign<Comp<5>>(entity);
+        registry.assign<Comp<6>>(entity);
+        registry.assign<Comp<7>>(entity);
+        registry.assign<Comp<8>>(entity);
+
+        if(i == 5000000L) {
+            registry.assign<Position>(entity);
+        }
+    }
+
+    auto test = [&registry](auto func) {
+        Timer timer;
+        registry.view<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>().each(func);
+        timer.elapsed();
+    };
+
+    test([](auto, const auto &...) {});
+    test([](auto, auto &... comp) {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { (comp.x = {}, 0)... };
+        (void)accumulator;
+    });
+}
+
+TEST(Benchmark, IterateTenComponentsPersistent1M) {
+    entt::DefaultRegistry registry;
+    registry.prepare<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
+
+    std::cout << "Iterating over 1000000 entities, ten components, persistent view" << std::endl;
+
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
+        const auto entity = registry.create();
+        registry.assign<Position>(entity);
+        registry.assign<Velocity>(entity);
+        registry.assign<Comp<1>>(entity);
+        registry.assign<Comp<2>>(entity);
+        registry.assign<Comp<3>>(entity);
+        registry.assign<Comp<4>>(entity);
+        registry.assign<Comp<5>>(entity);
+        registry.assign<Comp<6>>(entity);
+        registry.assign<Comp<7>>(entity);
+        registry.assign<Comp<8>>(entity);
+    }
+
+    auto test = [&registry](auto func) {
+        Timer timer;
+        registry.view<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>(entt::persistent_t{}).each(func);
+        timer.elapsed();
+    };
+
+    test([](auto, const auto &...) {});
+    test([](auto, auto &... comp) {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { (comp.x = {}, 0)... };
+        (void)accumulator;
+    });
+}
+
+TEST(Benchmark, SortSingle) {
+    entt::DefaultRegistry registry;
+
+    std::cout << "Sort 150000 entities, one component" << std::endl;
+
+    for(std::uint64_t i = 0; i < 150000L; i++) {
+        const auto entity = registry.create();
+        registry.assign<Position>(entity, i, i);
+    }
+
+    Timer timer;
+
+    registry.sort<Position>([](const auto &lhs, const auto &rhs) {
+        return lhs.x < rhs.x && lhs.y < rhs.y;
+    });
+
+    timer.elapsed();
+}
+
+TEST(Benchmark, SortMulti) {
+    entt::DefaultRegistry registry;
+
+    std::cout << "Sort 150000 entities, two components" << std::endl;
+
+    for(std::uint64_t i = 0; i < 150000L; i++) {
+        const auto entity = registry.create();
+        registry.assign<Position>(entity, i, i);
+        registry.assign<Velocity>(entity, i, i);
+    }
+
+    registry.sort<Position>([](const auto &lhs, const auto &rhs) {
+        return lhs.x < rhs.x && lhs.y < rhs.y;
+    });
+
+    Timer timer;
+
+    registry.sort<Velocity, Position>();
+
+    timer.elapsed();
+}
+
+TEST(Benchmark, AlmostSortedStdSort) {
+    entt::DefaultRegistry registry;
+    entt::DefaultRegistry::entity_type entities[3];
+
+    std::cout << "Sort 150000 entities, almost sorted, std::sort" << std::endl;
+
+    for(std::uint64_t i = 0; i < 150000L; i++) {
+        const auto entity = registry.create();
+        registry.assign<Position>(entity, i, i);
+
+        if(!(i % 50000)) {
+            entities[i / 50000] = entity;
+        }
+    }
+
+    for(std::uint64_t i = 0; i < 3; ++i) {
+        registry.destroy(entities[i]);
+        const auto entity = registry.create();
+        registry.assign<Position>(entity, 50000 * i, 50000 * i);
+    }
+
+    Timer timer;
+
+    registry.sort<Position>([](const auto &lhs, const auto &rhs) {
+        return lhs.x > rhs.x && lhs.y > rhs.y;
+    });
+
+    timer.elapsed();
+}
+
+TEST(Benchmark, AlmostSortedInsertionSort) {
+    entt::DefaultRegistry registry;
+    entt::DefaultRegistry::entity_type entities[3];
+
+    std::cout << "Sort 150000 entities, almost sorted, insertion sort" << std::endl;
+
+    for(std::uint64_t i = 0; i < 150000L; i++) {
+        const auto entity = registry.create();
+        registry.assign<Position>(entity, i, i);
+
+        if(!(i % 50000)) {
+            entities[i / 50000] = entity;
+        }
+    }
+
+    for(std::uint64_t i = 0; i < 3; ++i) {
+        registry.destroy(entities[i]);
+        const auto entity = registry.create();
+        registry.assign<Position>(entity, 50000 * i, 50000 * i);
+    }
+
+    Timer timer;
+
+    registry.sort<Position>([](const auto &lhs, const auto &rhs) {
+        return lhs.x > rhs.x && lhs.y > rhs.y;
+    }, entt::InsertionSort{});
+
+    timer.elapsed();
+}

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 +1,26 @@
+#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::StdSort sort;
+
+    sort(arr.begin(), arr.end());
+
+    for(auto i = 0; i < 4; ++i) {
+        ASSERT_LT(arr[i], arr[i+1]);
+    }
+}
+
+TEST(Algorithm, InsertionSort) {
+    std::array<int, 5> arr{{4, 1, 3, 2, 0}};
+    entt::InsertionSort sort;
+
+    sort(arr.begin(), arr.end());
+
+    for(auto i = 0; i < 4; ++i) {
+        ASSERT_LT(arr[i], arr[i+1]);
+    }
+}

test/entt/core/family.cpp

@@ -0,0 +1,22 @@
+#include <gtest/gtest.h>
+#include <entt/core/family.hpp>
+
+using my_family = entt::Family<struct MyFamily>;
+using your_family = entt::Family<struct YourFamily>;
+
+TEST(Family, Functionalities) {
+    auto myFamilyType = my_family::type<struct MyFamilyType>();
+    auto mySameFamilyType = my_family::type<struct MyFamilyType>();
+    auto myOtherFamilyType = my_family::type<struct MyOtherFamilyType>();
+    auto yourFamilyType = your_family::type<struct YourFamilyType>();
+
+    ASSERT_EQ(myFamilyType, mySameFamilyType);
+    ASSERT_NE(myFamilyType, myOtherFamilyType);
+    ASSERT_EQ(myFamilyType, yourFamilyType);
+}
+
+TEST(Family, Uniqueness) {
+    ASSERT_EQ(my_family::type<int>(), my_family::type<int &>());
+    ASSERT_EQ(my_family::type<int>(), my_family::type<int &&>());
+    ASSERT_EQ(my_family::type<int>(), my_family::type<const int &>());
+}

test/entt/core/hashed_string.cpp

@@ -0,0 +1,49 @@
+#include <cstddef>
+#include <gtest/gtest.h>
+#include <entt/core/hashed_string.hpp>
+
+static constexpr bool ptr(const char *str) {
+    using hash_type = entt::HashedString::hash_type;
+
+    return (static_cast<hash_type>(entt::HashedString{str}) == entt::HashedString{str}
+            && static_cast<const char *>(entt::HashedString{str}) == str
+            && entt::HashedString{str} == entt::HashedString{str}
+            && !(entt::HashedString{str} != entt::HashedString{str}));
+}
+
+template<std::size_t N>
+static constexpr bool ref(const char (&str)[N]) {
+    using hash_type = entt::HashedString::hash_type;
+
+    return (static_cast<hash_type>(entt::HashedString{str}) == entt::HashedString{str}
+            && static_cast<const char *>(entt::HashedString{str}) == str
+            && entt::HashedString{str} == entt::HashedString{str}
+            && !(entt::HashedString{str} != entt::HashedString{str}));
+}
+
+TEST(HashedString, Constexprness) {
+    // how would you test a constexpr otherwise?
+    static_assert(ptr("foo"), "!");
+    static_assert(ref("bar"), "!");
+    ASSERT_TRUE(true);
+}
+
+TEST(HashedString, Functionalities) {
+    using hash_type = entt::HashedString::hash_type;
+
+    const char *bar = "bar";
+
+    auto fooHs = entt::HashedString{"foo"};
+    auto barHs = entt::HashedString{bar};
+
+    ASSERT_NE(static_cast<hash_type>(fooHs), static_cast<hash_type>(barHs));
+    ASSERT_EQ(static_cast<const char *>(fooHs), "foo");
+    ASSERT_EQ(static_cast<const char *>(barHs), bar);
+
+    ASSERT_TRUE(fooHs == fooHs);
+    ASSERT_TRUE(fooHs != barHs);
+
+    entt::HashedString hs{"foobar"};
+
+    ASSERT_EQ(static_cast<hash_type>(hs), 0x85944171f73967e8);
+}

test/entt/core/ident.cpp

@@ -0,0 +1,33 @@
+#include <type_traits>
+#include <gtest/gtest.h>
+#include <entt/core/ident.hpp>
+
+struct AType {};
+struct AnotherType {};
+
+TEST(Identifier, Uniqueness) {
+    constexpr auto ID = entt::ident<AType, AnotherType>;
+    constexpr AType anInstance;
+    constexpr AnotherType anotherInstance;
+
+    ASSERT_NE(ID.get<AType>(), ID.get<AnotherType>());
+    ASSERT_EQ(ID.get<AType>(), ID.get<decltype(anInstance)>());
+    ASSERT_NE(ID.get<AType>(), ID.get<decltype(anotherInstance)>());
+    ASSERT_EQ(ID.get<AType>(), ID.get<AType>());
+    ASSERT_EQ(ID.get<AnotherType>(), ID.get<AnotherType>());
+
+    // test uses in constant expressions
+    switch(ID.get<AnotherType>()) {
+    case ID.get<AType>():
+        FAIL();
+        break;
+    case ID.get<AnotherType>():
+        SUCCEED();
+    }
+}
+
+TEST(Identifier, SingleType) {
+    constexpr auto ID = entt::ident<AType>;
+    std::integral_constant<decltype(ID)::identifier_type, ID.get()> ic;
+    (void)ic;
+}

test/entt/entity/actor.cpp

@@ -0,0 +1,76 @@
+#include <functional>
+#include <gtest/gtest.h>
+#include <entt/entity/actor.hpp>
+#include <entt/entity/registry.hpp>
+
+struct ActorComponent final {};
+struct ActorTag final {};
+
+TEST(Actor, Component) {
+    entt::DefaultRegistry registry;
+    entt::DefaultActor actor{registry};
+    const auto &cactor = actor;
+
+    ASSERT_EQ(&registry, &actor.registry());
+    ASSERT_EQ(&registry, &cactor.registry());
+    ASSERT_TRUE(registry.empty<ActorComponent>());
+    ASSERT_FALSE(registry.empty());
+    ASSERT_FALSE(actor.has<ActorComponent>());
+
+    const auto &component = actor.assign<ActorComponent>();
+
+    ASSERT_EQ(&component, &actor.get<ActorComponent>());
+    ASSERT_EQ(&component, &cactor.get<ActorComponent>());
+    ASSERT_FALSE(registry.empty<ActorComponent>());
+    ASSERT_FALSE(registry.empty());
+    ASSERT_TRUE(actor.has<ActorComponent>());
+
+    actor.remove<ActorComponent>();
+
+    ASSERT_TRUE(registry.empty<ActorComponent>());
+    ASSERT_FALSE(registry.empty());
+    ASSERT_FALSE(actor.has<ActorComponent>());
+}
+
+TEST(Actor, Tag) {
+    entt::DefaultRegistry registry;
+    entt::DefaultActor actor{registry};
+    const auto &cactor = actor;
+
+    ASSERT_EQ(&registry, &actor.registry());
+    ASSERT_EQ(&registry, &cactor.registry());
+    ASSERT_FALSE(registry.has<ActorTag>());
+    ASSERT_FALSE(actor.has<ActorTag>(entt::tag_t{}));
+
+    const auto &tag = actor.assign<ActorTag>(entt::tag_t{});
+
+    ASSERT_EQ(&tag, &actor.get<ActorTag>(entt::tag_t{}));
+    ASSERT_EQ(&tag, &cactor.get<ActorTag>(entt::tag_t{}));
+    ASSERT_TRUE(registry.has<ActorTag>());
+    ASSERT_FALSE(registry.empty());
+    ASSERT_TRUE(actor.has<ActorTag>(entt::tag_t{}));
+
+    actor.remove<ActorTag>(entt::tag_t{});
+
+    ASSERT_FALSE(registry.has<ActorTag>());
+    ASSERT_FALSE(registry.empty());
+    ASSERT_FALSE(actor.has<ActorTag>(entt::tag_t{}));
+}
+
+TEST(Actor, EntityLifetime) {
+    entt::DefaultRegistry registry;
+    auto *actor = new entt::DefaultActor{registry};
+    actor->assign<ActorComponent>();
+
+    ASSERT_FALSE(registry.empty<ActorComponent>());
+    ASSERT_FALSE(registry.empty());
+
+    registry.each([actor](const auto entity) {
+        ASSERT_EQ(actor->entity(), entity);
+    });
+
+    delete actor;
+
+    ASSERT_TRUE(registry.empty<ActorComponent>());
+    ASSERT_TRUE(registry.empty());
+}

test/entt/entity/helper.cpp

@@ -0,0 +1,49 @@
+#include <gtest/gtest.h>
+#include <entt/entity/helper.hpp>
+#include <entt/entity/registry.hpp>
+
+TEST(Dependency, Functionalities) {
+    entt::DefaultRegistry registry;
+    const auto entity = registry.create();
+    entt::dependency<double, float>(registry.construction<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::dependency<double, float>(entt::break_t{}, registry.construction<int>());
+    registry.assign<int>(entity);
+
+    ASSERT_FALSE(registry.has<double>(entity));
+    ASSERT_FALSE(registry.has<float>(entity));
+}

test/entt/entity/prototype.cpp

@@ -0,0 +1,135 @@
+#include <gtest/gtest.h>
+#include <entt/entity/prototype.hpp>
+#include <entt/entity/registry.hpp>
+
+TEST(Prototype, SameRegistry) {
+    entt::DefaultRegistry registry;
+    entt::DefaultPrototype prototype{registry};
+    const auto &cprototype = prototype;
+
+    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(cprototype.get<char>(), 'c');
+    ASSERT_EQ(std::get<0>(prototype.get<int, char>()), 3);
+    ASSERT_EQ(std::get<1>(cprototype.get<int, char>()), 'c');
+
+    const auto e0 = prototype();
+
+    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.accommodate(e1);
+
+    ASSERT_EQ(registry.get<char>(e1), 'c');
+}
+
+TEST(Prototype, OtherRegistry) {
+    entt::DefaultRegistry registry;
+    entt::DefaultRegistry repository;
+    entt::DefaultPrototype prototype{repository};
+    const auto &cprototype = prototype;
+
+    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(cprototype.get<char>(), 'c');
+    ASSERT_EQ(std::get<0>(prototype.get<int, char>()), 3);
+    ASSERT_EQ(std::get<1>(cprototype.get<int, char>()), 'c');
+
+    const auto e0 = prototype(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.accommodate(registry, e1);
+
+    ASSERT_EQ(registry.get<char>(e1), 'c');
+}
+
+TEST(Prototype, RAII) {
+    entt::DefaultRegistry registry;
+
+    {
+        entt::DefaultPrototype prototype{registry};
+        prototype.set<int>(0);
+
+        ASSERT_FALSE(registry.empty());
+    }
+
+    ASSERT_TRUE(registry.empty());
+}

test/entt/entity/registry.cpp

@@ -0,0 +1,743 @@
+#include <unordered_map>
+#include <unordered_set>
+#include <functional>
+#include <type_traits>
+#include <gtest/gtest.h>
+#include <entt/entity/entt_traits.hpp>
+#include <entt/entity/registry.hpp>
+
+struct Listener {
+    template<typename Component>
+    void incrComponent(entt::DefaultRegistry &registry, entt::DefaultRegistry::entity_type entity) {
+        ASSERT_TRUE(registry.valid(entity));
+        ASSERT_TRUE(registry.has<Component>(entity));
+        last = entity;
+        ++counter;
+    }
+
+    template<typename Tag>
+    void incrTag(entt::DefaultRegistry &registry, entt::DefaultRegistry::entity_type entity) {
+        ASSERT_TRUE(registry.valid(entity));
+        ASSERT_TRUE(registry.has<Tag>());
+        ASSERT_EQ(registry.attachee<Tag>(), entity);
+        last = entity;
+        ++counter;
+    }
+
+    template<typename Component>
+    void decrComponent(entt::DefaultRegistry &registry, entt::DefaultRegistry::entity_type entity) {
+        ASSERT_TRUE(registry.valid(entity));
+        ASSERT_TRUE(registry.has<Component>(entity));
+        last = entity;
+        --counter;
+    }
+
+    template<typename Tag>
+    void decrTag(entt::DefaultRegistry &registry, entt::DefaultRegistry::entity_type entity) {
+        ASSERT_TRUE(registry.valid(entity));
+        ASSERT_TRUE(registry.has<Tag>());
+        ASSERT_EQ(registry.attachee<Tag>(), entity);
+        last = entity;
+        --counter;
+    }
+
+    entt::DefaultRegistry::entity_type last;
+    int counter{0};
+};
+
+TEST(DefaultRegistry, Types) {
+    entt::DefaultRegistry registry;
+
+    ASSERT_EQ(registry.type<int>(entt::tag_t{}), registry.type<int>(entt::tag_t{}));
+    ASSERT_EQ(registry.type<int>(), registry.type<int>());
+
+    ASSERT_NE(registry.type<int>(entt::tag_t{}), registry.type<double>(entt::tag_t{}));
+    ASSERT_NE(registry.type<int>(), registry.type<double>(entt::tag_t{}));
+}
+
+TEST(DefaultRegistry, Functionalities) {
+    entt::DefaultRegistry registry;
+
+    ASSERT_EQ(registry.size(), entt::DefaultRegistry::size_type{0});
+    ASSERT_NO_THROW(registry.reserve(42));
+    ASSERT_NO_THROW(registry.reserve<int>(8));
+    ASSERT_NO_THROW(registry.reserve<char>(8));
+    ASSERT_TRUE(registry.empty());
+
+    ASSERT_EQ(registry.capacity(), entt::DefaultRegistry::size_type{0});
+    ASSERT_EQ(registry.size<int>(), entt::DefaultRegistry::size_type{0});
+    ASSERT_EQ(registry.size<char>(), entt::DefaultRegistry::size_type{0});
+    ASSERT_TRUE(registry.empty<int>());
+    ASSERT_TRUE(registry.empty<char>());
+
+    const auto e0 = registry.create();
+    const auto e1 = registry.create();
+
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    ASSERT_TRUE(registry.has<>(e0));
+    ASSERT_TRUE(registry.has<>(e1));
+
+    ASSERT_EQ(registry.capacity(), entt::DefaultRegistry::size_type{2});
+    ASSERT_EQ(registry.size<int>(), entt::DefaultRegistry::size_type{1});
+    ASSERT_EQ(registry.size<char>(), entt::DefaultRegistry::size_type{1});
+    ASSERT_FALSE(registry.empty<int>());
+    ASSERT_FALSE(registry.empty<char>());
+
+    ASSERT_NE(e0, e1);
+
+    ASSERT_FALSE(registry.has<int>(e0));
+    ASSERT_TRUE(registry.has<int>(e1));
+    ASSERT_FALSE(registry.has<char>(e0));
+    ASSERT_TRUE(registry.has<char>(e1));
+    ASSERT_FALSE((registry.has<int, char>(e0)));
+    ASSERT_TRUE((registry.has<int, char>(e1)));
+
+    ASSERT_EQ(registry.assign<int>(e0, 42), 42);
+    ASSERT_EQ(registry.assign<char>(e0, 'c'), 'c');
+    ASSERT_NO_THROW(registry.remove<int>(e1));
+    ASSERT_NO_THROW(registry.remove<char>(e1));
+
+    ASSERT_TRUE(registry.has<int>(e0));
+    ASSERT_FALSE(registry.has<int>(e1));
+    ASSERT_TRUE(registry.has<char>(e0));
+    ASSERT_FALSE(registry.has<char>(e1));
+    ASSERT_TRUE((registry.has<int, char>(e0)));
+    ASSERT_FALSE((registry.has<int, char>(e1)));
+
+    const auto e2 = registry.create();
+
+    registry.accommodate<int>(e2, registry.get<int>(e0));
+    registry.accommodate<char>(e2, registry.get<char>(e0));
+
+    ASSERT_TRUE(registry.has<int>(e2));
+    ASSERT_TRUE(registry.has<char>(e2));
+    ASSERT_EQ(registry.get<int>(e0), 42);
+    ASSERT_EQ(registry.get<char>(e0), 'c');
+
+    ASSERT_EQ(std::get<0>(registry.get<int, char>(e0)), 42);
+    ASSERT_EQ(std::get<1>(static_cast<const entt::DefaultRegistry &>(registry).get<int, char>(e0)), 'c');
+
+    ASSERT_EQ(registry.get<int>(e0), registry.get<int>(e2));
+    ASSERT_EQ(registry.get<char>(e0), registry.get<char>(e2));
+    ASSERT_NE(&registry.get<int>(e0), &registry.get<int>(e2));
+    ASSERT_NE(&registry.get<char>(e0), &registry.get<char>(e2));
+
+    ASSERT_NO_THROW(registry.replace<int>(e0, 0));
+    ASSERT_EQ(registry.get<int>(e0), 0);
+
+    ASSERT_NO_THROW(registry.accommodate<int>(e0, 1));
+    ASSERT_NO_THROW(registry.accommodate<int>(e1, 1));
+    ASSERT_EQ(static_cast<const entt::DefaultRegistry &>(registry).get<int>(e0), 1);
+    ASSERT_EQ(static_cast<const entt::DefaultRegistry &>(registry).get<int>(e1), 1);
+
+    ASSERT_EQ(registry.size(), entt::DefaultRegistry::size_type{3});
+    ASSERT_FALSE(registry.empty());
+
+    ASSERT_EQ(registry.version(e2), entt::DefaultRegistry::version_type{0});
+    ASSERT_EQ(registry.current(e2), entt::DefaultRegistry::version_type{0});
+    ASSERT_EQ(registry.capacity(), entt::DefaultRegistry::size_type{3});
+    ASSERT_NO_THROW(registry.destroy(e2));
+    ASSERT_EQ(registry.capacity(), entt::DefaultRegistry::size_type{3});
+    ASSERT_EQ(registry.version(e2), entt::DefaultRegistry::version_type{0});
+    ASSERT_EQ(registry.current(e2), entt::DefaultRegistry::version_type{1});
+
+    ASSERT_TRUE(registry.valid(e0));
+    ASSERT_TRUE(registry.fast(e0));
+    ASSERT_TRUE(registry.valid(e1));
+    ASSERT_TRUE(registry.fast(e1));
+    ASSERT_FALSE(registry.valid(e2));
+    ASSERT_FALSE(registry.fast(e2));
+
+    ASSERT_EQ(registry.size(), entt::DefaultRegistry::size_type{2});
+    ASSERT_FALSE(registry.empty());
+
+    ASSERT_NO_THROW(registry.reset());
+
+    ASSERT_EQ(registry.size(), entt::DefaultRegistry::size_type{0});
+    ASSERT_TRUE(registry.empty());
+
+    const auto e3 = registry.create();
+
+    registry.assign<int>(e3);
+    registry.assign<char>(e3);
+
+    ASSERT_EQ(registry.size<int>(), entt::DefaultRegistry::size_type{1});
+    ASSERT_EQ(registry.size<char>(), entt::DefaultRegistry::size_type{1});
+    ASSERT_FALSE(registry.empty<int>());
+    ASSERT_FALSE(registry.empty<char>());
+
+    ASSERT_NO_THROW(registry.reset<int>());
+
+    ASSERT_EQ(registry.size<int>(), entt::DefaultRegistry::size_type{0});
+    ASSERT_EQ(registry.size<char>(), entt::DefaultRegistry::size_type{1});
+    ASSERT_TRUE(registry.empty<int>());
+    ASSERT_FALSE(registry.empty<char>());
+
+    ASSERT_NO_THROW(registry.reset());
+
+    ASSERT_EQ(registry.size<int>(), entt::DefaultRegistry::size_type{0});
+    ASSERT_EQ(registry.size<char>(), entt::DefaultRegistry::size_type{0});
+    ASSERT_TRUE(registry.empty<int>());
+    ASSERT_TRUE(registry.empty<char>());
+
+    const auto e4 = registry.create();
+    const auto e5 = registry.create();
+
+    registry.assign<int>(e4);
+
+    ASSERT_NO_THROW(registry.reset<int>(e4));
+    ASSERT_NO_THROW(registry.reset<int>(e5));
+
+    ASSERT_EQ(registry.size<int>(), entt::DefaultRegistry::size_type{0});
+    ASSERT_EQ(registry.size<char>(), entt::DefaultRegistry::size_type{0});
+    ASSERT_TRUE(registry.empty<int>());
+}
+
+TEST(DefaultRegistry, RawData) {
+    entt::DefaultRegistry registry;
+    const entt::DefaultRegistry &cregistry = registry;
+    const auto entity = registry.create();
+
+    ASSERT_EQ(registry.raw<int>(), nullptr);
+    ASSERT_EQ(cregistry.raw<int>(), nullptr);
+    ASSERT_EQ(cregistry.data<int>(), nullptr);
+
+    registry.assign<int>(entity, 42);
+
+    ASSERT_NE(registry.raw<int>(), nullptr);
+    ASSERT_NE(cregistry.raw<int>(), nullptr);
+    ASSERT_NE(cregistry.data<int>(), nullptr);
+
+    ASSERT_EQ(*registry.raw<int>(), 42);
+    ASSERT_EQ(*cregistry.raw<int>(), 42);
+    ASSERT_EQ(*cregistry.data<int>(), entity);
+}
+
+TEST(DefaultRegistry, CreateDestroyCornerCase) {
+    entt::DefaultRegistry registry;
+
+    const auto e0 = registry.create();
+    const auto e1 = registry.create();
+
+    registry.destroy(e0);
+    registry.destroy(e1);
+
+    registry.each([](auto) { FAIL(); });
+
+    ASSERT_EQ(registry.current(e0), entt::DefaultRegistry::version_type{1});
+    ASSERT_EQ(registry.current(e1), entt::DefaultRegistry::version_type{1});
+}
+
+TEST(DefaultRegistry, VersionOverflow) {
+    entt::DefaultRegistry registry;
+
+    const auto entity = registry.create();
+    registry.destroy(entity);
+
+    ASSERT_EQ(registry.version(entity), entt::DefaultRegistry::version_type{});
+
+    for(auto i = entt::entt_traits<entt::DefaultRegistry::entity_type>::version_mask; i; --i) {
+        ASSERT_NE(registry.current(entity), registry.version(entity));
+        registry.destroy(registry.create());
+    }
+
+    ASSERT_EQ(registry.current(entity), registry.version(entity));
+}
+
+TEST(DefaultRegistry, Each) {
+    entt::DefaultRegistry registry;
+    entt::DefaultRegistry::size_type tot;
+    entt::DefaultRegistry::size_type match;
+
+    registry.create();
+    registry.assign<int>(registry.create());
+    registry.create();
+    registry.assign<int>(registry.create());
+    registry.create();
+
+    tot = 0u;
+    match = 0u;
+
+    registry.each([&](auto entity) {
+        if(registry.has<int>(entity)) { ++match; }
+        registry.create();
+        ++tot;
+    });
+
+    ASSERT_EQ(tot, 5u);
+    ASSERT_EQ(match, 2u);
+
+    tot = 0u;
+    match = 0u;
+
+    registry.each([&](auto entity) {
+        if(registry.has<int>(entity)) {
+            registry.destroy(entity);
+            ++match;
+        }
+
+        ++tot;
+    });
+
+    ASSERT_EQ(tot, 10u);
+    ASSERT_EQ(match, 2u);
+
+    tot = 0u;
+    match = 0u;
+
+    registry.each([&](auto entity) {
+        if(registry.has<int>(entity)) { ++match; }
+        registry.destroy(entity);
+        ++tot;
+    });
+
+    ASSERT_EQ(tot, 8u);
+    ASSERT_EQ(match, 0u);
+
+    registry.each([&](auto) { FAIL(); });
+}
+
+TEST(DefaultRegistry, Orphans) {
+    entt::DefaultRegistry registry;
+    entt::DefaultRegistry::size_type tot{};
+
+    registry.assign<int>(registry.create());
+    registry.create();
+    registry.assign<int>(registry.create());
+    registry.create();
+    registry.assign<double>(entt::tag_t{}, registry.create());
+
+    registry.orphans([&](auto) { ++tot; });
+    ASSERT_EQ(tot, 2u);
+    tot = 0u;
+
+    registry.each([&](auto entity) { registry.reset<int>(entity); });
+    registry.orphans([&](auto) { ++tot; });
+    ASSERT_EQ(tot, 4u);
+    registry.reset();
+    tot = 0u;
+
+    registry.orphans([&](auto) { ++tot; });
+    ASSERT_EQ(tot, 0u);
+}
+
+TEST(DefaultRegistry, CreateDestroyEntities) {
+    entt::DefaultRegistry registry;
+    entt::DefaultRegistry::entity_type pre{}, post{};
+
+    for(int i = 0; i < 10; ++i) {
+        const auto entity = registry.create();
+        registry.assign<double>(entity);
+    }
+
+    registry.reset();
+
+    for(int i = 0; i < 7; ++i) {
+        const auto entity = registry.create();
+        registry.assign<int>(entity);
+        if(i == 3) { pre = entity; }
+    }
+
+    registry.reset();
+
+    for(int i = 0; i < 5; ++i) {
+        const auto entity = registry.create();
+        if(i == 3) { post = entity; }
+    }
+
+    ASSERT_FALSE(registry.valid(pre));
+    ASSERT_TRUE(registry.valid(post));
+    ASSERT_NE(registry.version(pre), registry.version(post));
+    ASSERT_EQ(registry.version(pre) + 1, registry.version(post));
+    ASSERT_EQ(registry.current(pre), registry.current(post));
+}
+
+TEST(DefaultRegistry, AttachSetRemoveTags) {
+    entt::DefaultRegistry registry;
+    const auto &cregistry = registry;
+
+    ASSERT_FALSE(registry.has<int>());
+
+    const auto entity = registry.create();
+    registry.assign<int>(entt::tag_t{}, entity, 42);
+
+    ASSERT_TRUE(registry.has<int>());
+    ASSERT_TRUE(registry.has<int>(entt::tag_t{}, entity));
+    ASSERT_EQ(registry.get<int>(), 42);
+    ASSERT_EQ(cregistry.get<int>(), 42);
+    ASSERT_EQ(registry.attachee<int>(), entity);
+
+    registry.replace<int>(entt::tag_t{}, 3);
+
+    ASSERT_TRUE(registry.has<int>());
+    ASSERT_TRUE(registry.has<int>(entt::tag_t{}, entity));
+    ASSERT_EQ(registry.get<int>(), 3);
+    ASSERT_EQ(cregistry.get<int>(), 3);
+    ASSERT_EQ(registry.attachee<int>(), entity);
+
+    const auto other = registry.create();
+    registry.move<int>(other);
+
+    ASSERT_TRUE(registry.has<int>());
+    ASSERT_FALSE(registry.has<int>(entt::tag_t{}, entity));
+    ASSERT_TRUE(registry.has<int>(entt::tag_t{}, other));
+    ASSERT_EQ(registry.get<int>(), 3);
+    ASSERT_EQ(cregistry.get<int>(), 3);
+    ASSERT_EQ(registry.attachee<int>(), other);
+
+    registry.remove<int>();
+
+    ASSERT_FALSE(registry.has<int>());
+    ASSERT_FALSE(registry.has<int>(entt::tag_t{}, entity));
+    ASSERT_FALSE(registry.has<int>(entt::tag_t{}, other));
+
+    registry.assign<int>(entt::tag_t{}, entity, 42);
+    registry.destroy(entity);
+
+    ASSERT_FALSE(registry.has<int>());
+    ASSERT_FALSE(registry.has<int>(entt::tag_t{}, entity));
+    ASSERT_FALSE(registry.has<int>(entt::tag_t{}, other));
+}
+
+TEST(DefaultRegistry, StandardView) {
+    entt::DefaultRegistry registry;
+    auto mview = registry.view<int, char>();
+    auto iview = registry.view<int>();
+    auto cview = registry.view<char>();
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0, 0);
+    registry.assign<char>(e0, 'c');
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1, 0);
+
+    const auto e2 = registry.create();
+    registry.assign<int>(e2, 0);
+    registry.assign<char>(e2, 'c');
+
+    ASSERT_EQ(iview.size(), decltype(iview)::size_type{3});
+    ASSERT_EQ(cview.size(), decltype(cview)::size_type{2});
+
+    decltype(mview)::size_type cnt{0};
+    mview.each([&cnt](auto...) { ++cnt; });
+
+    ASSERT_EQ(cnt, decltype(mview)::size_type{2});
+}
+
+TEST(DefaultRegistry, PersistentView) {
+    entt::DefaultRegistry registry;
+    auto view = registry.view<int, char>(entt::persistent_t{});
+
+    ASSERT_TRUE((registry.contains<int, char>()));
+    ASSERT_FALSE((registry.contains<int, double>()));
+
+    registry.prepare<int, double>();
+
+    ASSERT_TRUE((registry.contains<int, double>()));
+
+    registry.discard<int, double>();
+
+    ASSERT_FALSE((registry.contains<int, double>()));
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0, 0);
+    registry.assign<char>(e0, 'c');
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1, 0);
+
+    const auto e2 = registry.create();
+    registry.assign<int>(e2, 0);
+    registry.assign<char>(e2, 'c');
+
+    decltype(view)::size_type cnt{0};
+    view.each([&cnt](auto...) { ++cnt; });
+
+    ASSERT_EQ(cnt, decltype(view)::size_type{2});
+}
+
+TEST(DefaultRegistry, RawView) {
+    entt::DefaultRegistry registry;
+    auto view = registry.view<int>(entt::raw_t{});
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0, 0);
+    registry.assign<char>(e0, 'c');
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1, 0);
+    registry.assign<char>(e1, 'c');
+
+    decltype(view)::size_type cnt{0};
+    view.each([&cnt](auto &...) { ++cnt; });
+
+    ASSERT_EQ(cnt, decltype(view)::size_type{2});
+}
+
+TEST(DefaultRegistry, CleanStandardViewAfterReset) {
+    entt::DefaultRegistry registry;
+    auto view = registry.view<int>();
+    registry.assign<int>(registry.create(), 0);
+
+    ASSERT_EQ(view.size(), entt::DefaultRegistry::size_type{1});
+
+    registry.reset();
+
+    ASSERT_EQ(view.size(), entt::DefaultRegistry::size_type{0});
+}
+
+TEST(DefaultRegistry, CleanPersistentViewAfterReset) {
+    entt::DefaultRegistry registry;
+    auto view = registry.view<int, char>(entt::persistent_t{});
+
+    const auto entity = registry.create();
+    registry.assign<int>(entity, 0);
+    registry.assign<char>(entity, 'c');
+
+    ASSERT_EQ(view.size(), entt::DefaultRegistry::size_type{1});
+
+    registry.reset();
+
+    ASSERT_EQ(view.size(), entt::DefaultRegistry::size_type{0});
+}
+
+TEST(DefaultRegistry, CleanRawViewAfterReset) {
+    entt::DefaultRegistry registry;
+    auto view = registry.view<int>(entt::raw_t{});
+    registry.assign<int>(registry.create(), 0);
+
+    ASSERT_EQ(view.size(), entt::DefaultRegistry::size_type{1});
+
+    registry.reset();
+
+    ASSERT_EQ(view.size(), entt::DefaultRegistry::size_type{0});
+}
+
+TEST(DefaultRegistry, CleanTagsAfterReset) {
+    entt::DefaultRegistry registry;
+    const auto entity = registry.create();
+    registry.assign<int>(entt::tag_t{}, entity);
+
+    ASSERT_TRUE(registry.has<int>());
+
+    registry.reset();
+
+    ASSERT_FALSE(registry.has<int>());
+}
+
+TEST(DefaultRegistry, SortSingle) {
+    entt::DefaultRegistry registry;
+
+    int val = 0;
+
+    registry.assign<int>(registry.create(), val++);
+    registry.assign<int>(registry.create(), val++);
+    registry.assign<int>(registry.create(), val++);
+
+    for(auto entity: registry.view<int>()) {
+        ASSERT_EQ(registry.get<int>(entity), --val);
+    }
+
+    registry.sort<int>(std::less<int>{});
+
+    for(auto entity: registry.view<int>()) {
+        ASSERT_EQ(registry.get<int>(entity), val++);
+    }
+}
+
+TEST(DefaultRegistry, SortMulti) {
+    entt::DefaultRegistry registry;
+
+    unsigned int uval = 0u;
+    int ival = 0;
+
+    for(auto i = 0; i < 3; ++i) {
+        const auto entity = registry.create();
+        registry.assign<unsigned int>(entity, uval++);
+        registry.assign<int>(entity, ival++);
+    }
+
+    for(auto entity: registry.view<unsigned int>()) {
+        ASSERT_EQ(registry.get<unsigned int>(entity), --uval);
+    }
+
+    for(auto entity: registry.view<int>()) {
+        ASSERT_EQ(registry.get<int>(entity), --ival);
+    }
+
+    registry.sort<unsigned int>(std::less<unsigned int>{});
+    registry.sort<int, unsigned int>();
+
+    for(auto entity: registry.view<unsigned int>()) {
+        ASSERT_EQ(registry.get<unsigned int>(entity), uval++);
+    }
+
+    for(auto entity: registry.view<int>()) {
+        ASSERT_EQ(registry.get<int>(entity), ival++);
+    }
+}
+
+TEST(DefaultRegistry, ComponentsWithTypesFromStandardTemplateLibrary) {
+    // see #37 - the test shouldn't crash, that's all
+    entt::DefaultRegistry registry;
+    const auto entity = registry.create();
+    registry.assign<std::unordered_set<int>>(entity).insert(42);
+    registry.destroy(entity);
+}
+
+TEST(DefaultRegistry, ConstructWithComponents) {
+    // it should compile, that's all
+    entt::DefaultRegistry registry;
+    const auto value = 0;
+    registry.assign<int>(registry.create(), value);
+}
+
+TEST(DefaultRegistry, MergeTwoRegistries) {
+    using entity_type = entt::DefaultRegistry::entity_type;
+
+    entt::DefaultRegistry src;
+    entt::DefaultRegistry dst;
+
+    std::unordered_map<entity_type, entity_type> ref;
+
+    auto merge = [&ref](const auto &view, auto &dst) {
+        view.each([&](auto entity, const auto &component) {
+            if(ref.find(entity) == ref.cend()) {
+                const auto other = dst.create();
+                dst.template assign<std::decay_t<decltype(component)>>(other, component);
+                ref.emplace(entity, other);
+            } else {
+                using component_type = std::decay_t<decltype(component)>;
+                dst.template assign<component_type>(ref[entity], component);
+            }
+        });
+    };
+
+    auto e0 = src.create();
+    src.assign<int>(e0);
+    src.assign<float>(e0);
+    src.assign<double>(e0);
+
+    auto e1 = src.create();
+    src.assign<char>(e1);
+    src.assign<float>(e1);
+    src.assign<int>(e1);
+
+    auto e2 = dst.create();
+    dst.assign<int>(e2);
+    dst.assign<char>(e2);
+    dst.assign<double>(e2);
+
+    auto e3 = dst.create();
+    dst.assign<float>(e3);
+    dst.assign<int>(e3);
+
+    auto eq = [](auto begin, auto end) { ASSERT_EQ(begin, end); };
+    auto ne = [](auto begin, auto end) { ASSERT_NE(begin, end); };
+
+    eq(dst.view<int, float, double>().begin(), dst.view<int, float, double>().end());
+    eq(dst.view<char, float, int>().begin(), dst.view<char, float, int>().end());
+
+    merge(src.view<int>(), dst);
+    merge(src.view<char>(), dst);
+    merge(src.view<double>(), dst);
+    merge(src.view<float>(), dst);
+
+    ne(dst.view<int, float, double>().begin(), dst.view<int, float, double>().end());
+    ne(dst.view<char, float, int>().begin(), dst.view<char, float, int>().end());
+}
+
+TEST(DefaultRegistry, ComponentSignals) {
+    entt::DefaultRegistry registry;
+    Listener listener;
+
+    registry.construction<int>().connect<Listener, &Listener::incrComponent<int>>(&listener);
+    registry.destruction<int>().connect<Listener, &Listener::decrComponent<int>>(&listener);
+
+    auto e0 = registry.create();
+    auto e1 = registry.create();
+
+    registry.assign<int>(e0);
+    registry.assign<int>(e1);
+
+    ASSERT_EQ(listener.counter, 2);
+    ASSERT_EQ(listener.last, e1);
+
+    registry.remove<int>(e0);
+
+    ASSERT_EQ(listener.counter, 1);
+    ASSERT_EQ(listener.last, e0);
+
+    registry.destruction<int>().disconnect<Listener, &Listener::decrComponent<int>>(&listener);
+    registry.remove<int>(e1);
+
+    ASSERT_EQ(listener.counter, 1);
+    ASSERT_EQ(listener.last, e0);
+
+    registry.construction<int>().disconnect<Listener, &Listener::incrComponent<int>>(&listener);
+    registry.assign<int>(e1);
+
+    ASSERT_EQ(listener.counter, 1);
+    ASSERT_EQ(listener.last, e0);
+
+    registry.construction<int>().connect<Listener, &Listener::incrComponent<int>>(&listener);
+    registry.destruction<int>().connect<Listener, &Listener::decrComponent<int>>(&listener);
+    registry.assign<int>(e0);
+    registry.reset<int>(e1);
+
+    ASSERT_EQ(listener.counter, 1);
+    ASSERT_EQ(listener.last, e1);
+
+    registry.reset<int>();
+
+    ASSERT_EQ(listener.counter, 0);
+    ASSERT_EQ(listener.last, e0);
+
+    registry.assign<int>(e0);
+    registry.assign<int>(e1);
+    registry.destroy(e1);
+
+    ASSERT_EQ(listener.counter, 1);
+    ASSERT_EQ(listener.last, e1);
+}
+
+TEST(DefaultRegistry, TagSignals) {
+    entt::DefaultRegistry registry;
+    Listener listener;
+
+    registry.construction<int>(entt::tag_t{}).connect<Listener, &Listener::incrTag<int>>(&listener);
+    registry.destruction<int>(entt::tag_t{}).connect<Listener, &Listener::decrTag<int>>(&listener);
+
+    auto e0 = registry.create();
+    registry.assign<int>(entt::tag_t{}, e0);
+
+    ASSERT_EQ(listener.counter, 1);
+    ASSERT_EQ(listener.last, e0);
+
+    auto e1 = registry.create();
+    registry.move<int>(e1);
+    registry.remove<int>();
+
+    ASSERT_EQ(listener.counter, 0);
+    ASSERT_EQ(listener.last, e1);
+
+    registry.construction<int>(entt::tag_t{}).disconnect<Listener, &Listener::incrTag<int>>(&listener);
+    registry.destruction<int>(entt::tag_t{}).disconnect<Listener, &Listener::decrTag<int>>(&listener);
+    registry.assign<int>(entt::tag_t{}, e0);
+    registry.remove<int>();
+
+    ASSERT_EQ(listener.counter, 0);
+    ASSERT_EQ(listener.last, e1);
+
+    registry.construction<int>(entt::tag_t{}).connect<Listener, &Listener::incrTag<int>>(&listener);
+    registry.destruction<int>(entt::tag_t{}).connect<Listener, &Listener::decrTag<int>>(&listener);
+
+    registry.assign<int>(entt::tag_t{}, e0);
+    registry.destroy(e0);
+
+    ASSERT_EQ(listener.counter, 0);
+    ASSERT_EQ(listener.last, e0);
+}

test/entt/entity/snapshot.cpp

@@ -0,0 +1,575 @@
+#include <tuple>
+#include <queue>
+#include <vector>
+#include <gtest/gtest.h>
+#include <entt/entity/registry.hpp>
+
+template<typename Storage>
+struct OutputArchive {
+    OutputArchive(Storage &storage)
+        : storage{storage}
+    {}
+
+    template<typename... Value>
+    void operator()(const Value &... value) {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { (std::get<std::queue<Value>>(storage).push(value), 0)... };
+        (void)accumulator;
+    }
+
+private:
+    Storage &storage;
+};
+
+template<typename Storage>
+struct InputArchive {
+    InputArchive(Storage &storage)
+        : storage{storage}
+    {}
+
+    template<typename... Value>
+    void operator()(Value &... value) {
+        auto assign = [this](auto &value) {
+            auto &queue = std::get<std::queue<std::decay_t<decltype(value)>>>(storage);
+            value = queue.front();
+            queue.pop();
+        };
+
+        using accumulator_type = int[];
+        accumulator_type accumulator = { (assign(value), 0)... };
+        (void)accumulator;
+    }
+
+private:
+    Storage &storage;
+};
+
+struct AComponent {};
+
+struct AnotherComponent {
+    int key;
+    int value;
+};
+
+struct WhatAComponent {
+    entt::DefaultRegistry::entity_type bar;
+    std::vector<entt::DefaultRegistry::entity_type> quux;
+};
+
+TEST(Snapshot, Dump) {
+    entt::DefaultRegistry 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.assign<float>(entt::tag_t{}, e3, .3f);
+
+    const auto e4 = registry.create();
+    registry.assign<AComponent>(entt::tag_t{}, e4);
+
+    registry.destroy(e1);
+    auto v1 = registry.current(e1);
+
+    using storage_type = std::tuple<
+        std::queue<entt::DefaultRegistry::entity_type>,
+        std::queue<int>,
+        std::queue<char>,
+        std::queue<double>,
+        std::queue<float>,
+        std::queue<bool>,
+        std::queue<AComponent>,
+        std::queue<AnotherComponent>,
+        std::queue<WhatAComponent>
+    >;
+
+    storage_type storage;
+    OutputArchive<storage_type> output{storage};
+    InputArchive<storage_type> input{storage};
+
+    registry.snapshot()
+            .entities(output)
+            .destroyed(output)
+            .component<int, char, AnotherComponent, double>(output)
+            .tag<float, bool, AComponent>(output);
+
+    registry.reset();
+
+    ASSERT_FALSE(registry.valid(e0));
+    ASSERT_FALSE(registry.valid(e1));
+    ASSERT_FALSE(registry.valid(e2));
+    ASSERT_FALSE(registry.valid(e3));
+    ASSERT_FALSE(registry.valid(e4));
+
+    registry.restore()
+            .entities(input)
+            .destroyed(input)
+            .component<int, char, AnotherComponent, double>(input)
+            .tag<float, bool, AComponent>(input)
+            .orphans();
+
+    ASSERT_TRUE(registry.valid(e0));
+    ASSERT_FALSE(registry.valid(e1));
+    ASSERT_TRUE(registry.valid(e2));
+    ASSERT_TRUE(registry.valid(e3));
+    ASSERT_TRUE(registry.valid(e4));
+
+    ASSERT_FALSE(registry.orphan(e0));
+    ASSERT_FALSE(registry.orphan(e2));
+    ASSERT_FALSE(registry.orphan(e3));
+    ASSERT_FALSE(registry.orphan(e4));
+
+    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<float>());
+    ASSERT_EQ(registry.attachee<float>(), e3);
+    ASSERT_EQ(registry.get<float>(), .3f);
+
+    ASSERT_TRUE(registry.has<AComponent>());
+    ASSERT_EQ(registry.attachee<AComponent>(), e4);
+
+    ASSERT_TRUE(registry.empty<AnotherComponent>());
+    ASSERT_FALSE(registry.has<long int>());
+}
+
+TEST(Snapshot, Partial) {
+    entt::DefaultRegistry 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.assign<float>(entt::tag_t{}, e3, .3f);
+
+    const auto e4 = registry.create();
+    registry.assign<AComponent>(entt::tag_t{}, e4);
+
+    registry.destroy(e1);
+    auto v1 = registry.current(e1);
+
+    using storage_type = std::tuple<
+        std::queue<entt::DefaultRegistry::entity_type>,
+        std::queue<int>,
+        std::queue<char>,
+        std::queue<double>,
+        std::queue<float>,
+        std::queue<bool>,
+        std::queue<AComponent>,
+        std::queue<WhatAComponent>
+    >;
+
+    storage_type storage;
+    OutputArchive<storage_type> output{storage};
+    InputArchive<storage_type> input{storage};
+
+    registry.snapshot()
+            .entities(output)
+            .destroyed(output)
+            .component<char, int>(output)
+            .tag<bool, float>(output);
+
+    registry.reset();
+
+    ASSERT_FALSE(registry.valid(e0));
+    ASSERT_FALSE(registry.valid(e1));
+    ASSERT_FALSE(registry.valid(e2));
+    ASSERT_FALSE(registry.valid(e3));
+    ASSERT_FALSE(registry.valid(e4));
+
+    registry.restore()
+            .entities(input)
+            .destroyed(input)
+            .component<char, int>(input)
+            .tag<bool, float>(input);
+
+    ASSERT_TRUE(registry.valid(e0));
+    ASSERT_FALSE(registry.valid(e1));
+    ASSERT_TRUE(registry.valid(e2));
+    ASSERT_TRUE(registry.valid(e3));
+    ASSERT_TRUE(registry.valid(e4));
+
+    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');
+    ASSERT_TRUE(registry.orphan(e4));
+
+    ASSERT_TRUE(registry.has<float>());
+    ASSERT_EQ(registry.attachee<float>(), e3);
+    ASSERT_EQ(registry.get<float>(), .3f);
+    ASSERT_FALSE(registry.has<long int>());
+
+    registry.snapshot()
+            .tag<float>(output)
+            .destroyed(output)
+            .entities(output);
+
+    registry.reset();
+
+    ASSERT_FALSE(registry.valid(e0));
+    ASSERT_FALSE(registry.valid(e1));
+    ASSERT_FALSE(registry.valid(e2));
+    ASSERT_FALSE(registry.valid(e3));
+    ASSERT_FALSE(registry.valid(e4));
+
+    registry.restore()
+            .tag<float>(input)
+            .destroyed(input)
+            .entities(input)
+            .orphans();
+
+    ASSERT_FALSE(registry.valid(e0));
+    ASSERT_FALSE(registry.valid(e1));
+    ASSERT_FALSE(registry.valid(e2));
+    ASSERT_TRUE(registry.valid(e3));
+    ASSERT_FALSE(registry.valid(e4));
+}
+
+TEST(Snapshot, Iterator) {
+    entt::DefaultRegistry registry;
+
+    for(auto i = 0; i < 50; ++i) {
+        const auto entity = registry.create();
+        registry.assign<AnotherComponent>(entity, i, i);
+
+        if(i % 2) {
+            registry.assign<AComponent>(entity);
+        }
+    }
+
+    using storage_type = std::tuple<
+        std::queue<entt::DefaultRegistry::entity_type>,
+        std::queue<AnotherComponent>
+    >;
+
+    storage_type storage;
+    OutputArchive<storage_type> output{storage};
+    InputArchive<storage_type> input{storage};
+
+    const auto view = registry.view<AComponent>();
+    const auto size = view.size();
+
+    registry.snapshot().component<AnotherComponent>(output, view.cbegin(), view.cend());
+    registry.reset();
+    registry.restore().component<AnotherComponent>(input);
+
+    ASSERT_EQ(registry.view<AnotherComponent>().size(), size);
+
+    registry.view<AnotherComponent>().each([](const auto entity, auto &&...) {
+        ASSERT_TRUE(entity % 2);
+    });
+}
+
+TEST(Snapshot, Continuous) {
+    using entity_type = entt::DefaultRegistry::entity_type;
+
+    entt::DefaultRegistry src;
+    entt::DefaultRegistry dst;
+
+    entt::ContinuousLoader<entity_type> loader{dst};
+
+    std::vector<entity_type> entities;
+    entity_type entity;
+
+    using storage_type = std::tuple<
+        std::queue<entity_type>,
+        std::queue<AComponent>,
+        std::queue<AnotherComponent>,
+        std::queue<WhatAComponent>,
+        std::queue<double>
+    >;
+
+    storage_type storage;
+    OutputArchive<storage_type> output{storage};
+    InputArchive<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<AComponent>(entity);
+        src.assign<AnotherComponent>(entity, i, i);
+
+        if(i % 2) {
+            src.assign<WhatAComponent>(entity, entity);
+        } else if(i == 2) {
+            src.assign<double>(entt::tag_t{}, entity, .3);
+        }
+    }
+
+    src.view<WhatAComponent>().each([&entities](auto, auto &whatAComponent) {
+        whatAComponent.quux.insert(whatAComponent.quux.begin(), entities.begin(), entities.end());
+    });
+
+    entity = dst.create();
+    dst.assign<AComponent>(entity);
+    dst.assign<AnotherComponent>(entity, -1, -1);
+
+    src.snapshot()
+            .entities(output)
+            .destroyed(output)
+            .component<AComponent, AnotherComponent, WhatAComponent>(output)
+            .tag<double>(output);
+
+    loader.entities(input)
+            .destroyed(input)
+            .component<AComponent, AnotherComponent, WhatAComponent>(input, &WhatAComponent::bar, &WhatAComponent::quux)
+            .tag<double>(input)
+            .orphans();
+
+    decltype(dst.size()) aComponentCnt{};
+    decltype(dst.size()) anotherComponentCnt{};
+    decltype(dst.size()) whatAComponentCnt{};
+
+    dst.each([&dst, &aComponentCnt](auto entity) {
+        ASSERT_TRUE(dst.has<AComponent>(entity));
+        ++aComponentCnt;
+    });
+
+    dst.view<AnotherComponent>().each([&anotherComponentCnt](auto, const auto &component) {
+        ASSERT_EQ(component.value, component.key < 0 ? -1 : component.key);
+        ++anotherComponentCnt;
+    });
+
+    dst.view<WhatAComponent>().each([&dst, &whatAComponentCnt](auto entity, const auto &component) {
+        ASSERT_EQ(entity, component.bar);
+
+        for(auto entity: component.quux) {
+            ASSERT_TRUE(dst.valid(entity));
+        }
+
+        ++whatAComponentCnt;
+    });
+
+    ASSERT_TRUE(dst.has<double>());
+    ASSERT_EQ(dst.get<double>(), .3);
+
+    src.view<AnotherComponent>().each([](auto, auto &component) {
+        component.value = 2 * component.key;
+    });
+
+    auto size = dst.size();
+
+    src.snapshot()
+            .entities(output)
+            .destroyed(output)
+            .component<AComponent, WhatAComponent, AnotherComponent>(output)
+            .tag<double>(output);
+
+    loader.entities(input)
+            .destroyed(input)
+            .component<AComponent, WhatAComponent, AnotherComponent>(input, &WhatAComponent::bar, &WhatAComponent::quux)
+            .tag<double>(input)
+            .orphans();
+
+    ASSERT_EQ(size, dst.size());
+
+    ASSERT_EQ(dst.size<AComponent>(), aComponentCnt);
+    ASSERT_EQ(dst.size<AnotherComponent>(), anotherComponentCnt);
+    ASSERT_EQ(dst.size<WhatAComponent>(), whatAComponentCnt);
+    ASSERT_TRUE(dst.has<double>());
+
+    dst.view<AnotherComponent>().each([](auto, auto &component) {
+        ASSERT_EQ(component.value, component.key < 0 ? -1 : (2 * component.key));
+    });
+
+    entity = src.create();
+
+    src.view<WhatAComponent>().each([entity](auto, auto &component) {
+        component.bar = entity;
+    });
+
+    src.snapshot()
+            .entities(output)
+            .destroyed(output)
+            .component<WhatAComponent, AComponent, AnotherComponent>(output)
+            .tag<double>(output);
+
+    loader.entities(input)
+            .destroyed(input)
+            .component<WhatAComponent, AComponent, AnotherComponent>(input, &WhatAComponent::bar, &WhatAComponent::quux)
+            .tag<double>(input)
+            .orphans();
+
+    dst.view<WhatAComponent>().each([&loader, entity](auto, auto &component) {
+        ASSERT_EQ(component.bar, loader.map(entity));
+    });
+
+    entities.clear();
+    for(auto entity: src.view<AComponent>()) {
+        entities.push_back(entity);
+    }
+
+    src.destroy(entity);
+    loader.shrink();
+
+    src.snapshot()
+            .entities(output)
+            .destroyed(output)
+            .component<AComponent, AnotherComponent, WhatAComponent>(output)
+            .tag<double>(output);
+
+    loader.entities(input)
+            .destroyed(input)
+            .component<AComponent, AnotherComponent, WhatAComponent>(input, &WhatAComponent::bar, &WhatAComponent::quux)
+            .tag<double>(input)
+            .orphans()
+            .shrink();
+
+    dst.view<WhatAComponent>().each([&dst](auto, auto &component) {
+        ASSERT_FALSE(dst.valid(component.bar));
+    });
+
+    ASSERT_FALSE(loader.has(entity));
+
+    entity = src.create();
+
+    src.view<WhatAComponent>().each([entity](auto, auto &component) {
+        component.bar = entity;
+    });
+
+    dst.reset<AComponent>();
+    aComponentCnt = src.size<AComponent>();
+
+    src.snapshot()
+            .entities(output)
+            .destroyed(output)
+            .component<AComponent, WhatAComponent, AnotherComponent>(output)
+            .tag<double>(output);
+
+    loader.entities(input)
+            .destroyed(input)
+            .component<AComponent, WhatAComponent, AnotherComponent>(input, &WhatAComponent::bar, &WhatAComponent::quux)
+            .tag<double>(input)
+            .orphans();
+
+    ASSERT_EQ(dst.size<AComponent>(), aComponentCnt);
+    ASSERT_TRUE(dst.has<double>());
+
+    src.reset<AComponent>();
+    src.remove<double>();
+    aComponentCnt = {};
+
+    src.snapshot()
+            .entities(output)
+            .destroyed(output)
+            .component<WhatAComponent, AComponent, AnotherComponent>(output)
+            .tag<double>(output);
+
+    loader.entities(input)
+            .destroyed(input)
+            .component<WhatAComponent, AComponent, AnotherComponent>(input, &WhatAComponent::bar, &WhatAComponent::quux)
+            .tag<double>(input)
+            .orphans();
+
+    ASSERT_EQ(dst.size<AComponent>(), aComponentCnt);
+    ASSERT_FALSE(dst.has<double>());
+}
+
+TEST(Snapshot, ContinuousMoreOnShrink) {
+    using entity_type = entt::DefaultRegistry::entity_type;
+
+    entt::DefaultRegistry src;
+    entt::DefaultRegistry dst;
+
+    entt::ContinuousLoader<entity_type> loader{dst};
+
+    using storage_type = std::tuple<
+        std::queue<entity_type>,
+        std::queue<AComponent>
+    >;
+
+    storage_type storage;
+    OutputArchive<storage_type> output{storage};
+    InputArchive<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) {
+    using entity_type = entt::DefaultRegistry::entity_type;
+
+    entt::DefaultRegistry src;
+    entt::DefaultRegistry dst;
+
+    entt::ContinuousLoader<entity_type> loader{dst};
+
+    using storage_type = std::tuple<
+        std::queue<entity_type>,
+        std::queue<WhatAComponent>
+    >;
+
+    storage_type storage;
+    OutputArchive<storage_type> output{storage};
+    InputArchive<storage_type> input{storage};
+
+    src.create();
+    src.create();
+
+    src.reset();
+
+    auto parent = src.create();
+    auto child = src.create();
+
+    src.assign<WhatAComponent>(entt::tag_t{}, child, parent).quux.push_back(parent);
+    src.assign<WhatAComponent>(child, child).quux.push_back(child);
+
+    src.snapshot().entities(output)
+            .component<WhatAComponent>(output)
+            .tag<WhatAComponent>(output);
+
+    loader.entities(input)
+            .component<WhatAComponent>(input, &WhatAComponent::bar, &WhatAComponent::quux)
+            .tag<WhatAComponent>(input, &WhatAComponent::bar, &WhatAComponent::quux);
+
+    ASSERT_FALSE(dst.valid(parent));
+    ASSERT_FALSE(dst.valid(child));
+
+    ASSERT_TRUE(dst.has<WhatAComponent>());
+    ASSERT_EQ(dst.attachee<WhatAComponent>(), loader.map(child));
+    ASSERT_EQ(dst.get<WhatAComponent>().bar, loader.map(parent));
+    ASSERT_EQ(dst.get<WhatAComponent>().quux[0], loader.map(parent));
+    ASSERT_TRUE(dst.has<WhatAComponent>(loader.map(child)));
+
+    const auto &component = dst.get<WhatAComponent>(loader.map(child));
+
+    ASSERT_EQ(component.bar, loader.map(child));
+    ASSERT_EQ(component.quux[0], loader.map(child));
+}

test/entt/entity/sparse_set.cpp

@@ -0,0 +1,703 @@
+#include <unordered_set>
+#include <gtest/gtest.h>
+#include <entt/entity/sparse_set.hpp>
+
+TEST(SparseSetNoType, Functionalities) {
+    entt::SparseSet<unsigned int> set;
+    const auto &cset = set;
+
+    ASSERT_NO_THROW(set.reserve(42));
+    ASSERT_TRUE(set.empty());
+    ASSERT_EQ(set.size(), 0u);
+    ASSERT_EQ(cset.begin(), cset.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(cset.begin(), cset.end());
+    ASSERT_NE(set.begin(), set.end());
+    ASSERT_FALSE(set.has(0));
+    ASSERT_TRUE(set.has(42));
+    ASSERT_TRUE(set.fast(42));
+    ASSERT_EQ(set.get(42), 0u);
+
+    set.destroy(42);
+
+    ASSERT_TRUE(set.empty());
+    ASSERT_EQ(set.size(), 0u);
+    ASSERT_EQ(cset.begin(), cset.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);
+
+    set.reset();
+
+    ASSERT_TRUE(set.empty());
+    ASSERT_EQ(set.size(), 0u);
+    ASSERT_EQ(cset.begin(), cset.end());
+    ASSERT_EQ(set.begin(), set.end());
+    ASSERT_FALSE(set.has(0));
+    ASSERT_FALSE(set.has(42));
+
+    (void)entt::SparseSet<unsigned int>{std::move(set)};
+    entt::SparseSet<unsigned int> other;
+    other = std::move(set);
+}
+
+TEST(SparseSetNoType, DataBeginEnd) {
+    entt::SparseSet<unsigned int> 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);
+
+    auto it = set.begin();
+
+    ASSERT_EQ(*it, 42u);
+    ASSERT_EQ(*(it+1), 12u);
+    ASSERT_EQ(*(it+2), 3u);
+    ASSERT_EQ(it += 3, set.end());
+
+    auto begin = set.begin();
+    auto end = set.end();
+
+    ASSERT_EQ(*(begin++), 42u);
+    ASSERT_EQ(*(begin++), 12u);
+    ASSERT_EQ(*(begin++), 3u);
+
+    ASSERT_EQ(begin, end);
+
+    auto cbegin = set.cbegin();
+    auto cend = set.cend();
+
+    ASSERT_NE(cbegin, cend);
+    ASSERT_EQ(cbegin+3, cend);
+    ASSERT_NE(cbegin, cend);
+    ASSERT_EQ(cbegin += 3, cend);
+    ASSERT_EQ(cbegin, cend);
+}
+
+TEST(SparseSetNoType, RespectDisjoint) {
+    entt::SparseSet<unsigned int> lhs;
+    entt::SparseSet<unsigned int> rhs;
+    const auto &clhs = lhs;
+
+    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(clhs.get(3), 0u);
+    ASSERT_EQ(clhs.get(12), 1u);
+    ASSERT_EQ(clhs.get(42), 2u);
+}
+
+TEST(SparseSetNoType, RespectOverlap) {
+    entt::SparseSet<unsigned int> lhs;
+    entt::SparseSet<unsigned int> rhs;
+    const auto &clhs = lhs;
+
+    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(clhs.get(3), 0u);
+    ASSERT_EQ(clhs.get(12), 2u);
+    ASSERT_EQ(clhs.get(42), 1u);
+}
+
+TEST(SparseSetNoType, RespectOrdered) {
+    entt::SparseSet<unsigned int> lhs;
+    entt::SparseSet<unsigned int> 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(SparseSetNoType, RespectReverse) {
+    entt::SparseSet<unsigned int> lhs;
+    entt::SparseSet<unsigned int> 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(SparseSetNoType, RespectUnordered) {
+    entt::SparseSet<unsigned int> lhs;
+    entt::SparseSet<unsigned int> 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(SparseSetWithType, Functionalities) {
+    entt::SparseSet<unsigned int, int> set;
+    const auto &cset = set;
+
+    ASSERT_NO_THROW(set.reserve(42));
+    ASSERT_TRUE(set.empty());
+    ASSERT_EQ(set.size(), 0u);
+    ASSERT_EQ(cset.begin(), cset.end());
+    ASSERT_EQ(set.begin(), set.end());
+    ASSERT_FALSE(set.has(0));
+    ASSERT_FALSE(set.has(42));
+
+    set.construct(42, 3);
+
+    ASSERT_FALSE(set.empty());
+    ASSERT_EQ(set.size(), 1u);
+    ASSERT_NE(cset.begin(), cset.end());
+    ASSERT_NE(set.begin(), set.end());
+    ASSERT_FALSE(set.has(0));
+    ASSERT_TRUE(set.has(42));
+    ASSERT_TRUE(set.fast(42));
+    ASSERT_EQ(set.get(42), 3);
+
+    set.destroy(42);
+
+    ASSERT_TRUE(set.empty());
+    ASSERT_EQ(set.size(), 0u);
+    ASSERT_EQ(cset.begin(), cset.end());
+    ASSERT_EQ(set.begin(), set.end());
+    ASSERT_FALSE(set.has(0));
+    ASSERT_FALSE(set.has(42));
+
+    set.construct(42, 12);
+
+    ASSERT_EQ(set.get(42), 12);
+
+    set.reset();
+
+    ASSERT_TRUE(set.empty());
+    ASSERT_EQ(set.size(), 0u);
+    ASSERT_EQ(cset.begin(), cset.end());
+    ASSERT_EQ(set.begin(), set.end());
+    ASSERT_FALSE(set.has(0));
+    ASSERT_FALSE(set.has(42));
+
+    (void)entt::SparseSet<unsigned int>{std::move(set)};
+    entt::SparseSet<unsigned int> other;
+    other = std::move(set);
+}
+
+TEST(SparseSetWithType, AggregatesMustWork) {
+    struct AggregateType { int value; };
+    // the goal of this test is to enforce the requirements for aggregate types
+    entt::SparseSet<unsigned int, AggregateType>{}.construct(0, 42);
+}
+
+TEST(SparseSetWithType, TypesFromStandardTemplateLibraryMustWork) {
+    // see #37 - this test shouldn't crash, that's all
+    entt::SparseSet<unsigned int, std::unordered_set<int>> set;
+    set.construct(0).insert(42);
+    set.destroy(0);
+}
+
+TEST(SparseSetWithType, RawBeginEnd) {
+    entt::SparseSet<unsigned int, int> set;
+
+    set.construct(3, 3);
+    set.construct(12, 6);
+    set.construct(42, 9);
+
+    ASSERT_EQ(set.get(3), 3);
+    ASSERT_EQ(set.get(12), 6);
+    ASSERT_EQ(set.get(42), 9);
+
+    ASSERT_EQ(*(set.raw() + 0u), 3);
+    ASSERT_EQ(*(set.raw() + 1u), 6);
+    ASSERT_EQ(*(set.raw() + 2u), 9);
+
+    auto begin = set.begin();
+    auto end = set.end();
+
+    ASSERT_EQ(*(begin++), 9);
+    ASSERT_EQ(*(begin++), 6);
+    ASSERT_EQ(*(begin++), 3);
+    ASSERT_EQ(begin, end);
+
+    auto cbegin = set.cbegin();
+    auto cend = set.cend();
+
+    ASSERT_NE(cbegin, cend);
+    ASSERT_EQ(cbegin+3, cend);
+    ASSERT_NE(cbegin, cend);
+    ASSERT_EQ(cbegin += 3, cend);
+    ASSERT_EQ(cbegin, cend);
+}
+
+TEST(SparseSetWithType, SortOrdered) {
+    entt::SparseSet<unsigned int, int> set;
+
+    set.construct(12, 12);
+    set.construct(42, 9);
+    set.construct(7, 6);
+    set.construct(3, 3);
+    set.construct(9, 1);
+
+    ASSERT_EQ(set.get(12), 12);
+    ASSERT_EQ(set.get(42), 9);
+    ASSERT_EQ(set.get(7), 6);
+    ASSERT_EQ(set.get(3), 3);
+    ASSERT_EQ(set.get(9), 1);
+
+    set.sort([](auto lhs, auto rhs) {
+        return lhs < rhs;
+    });
+
+    ASSERT_EQ(*(set.raw() + 0u), 12);
+    ASSERT_EQ(*(set.raw() + 1u), 9);
+    ASSERT_EQ(*(set.raw() + 2u), 6);
+    ASSERT_EQ(*(set.raw() + 3u), 3);
+    ASSERT_EQ(*(set.raw() + 4u), 1);
+
+    auto begin = set.begin();
+    auto end = set.end();
+
+    ASSERT_EQ(*(begin++), 1);
+    ASSERT_EQ(*(begin++), 3);
+    ASSERT_EQ(*(begin++), 6);
+    ASSERT_EQ(*(begin++), 9);
+    ASSERT_EQ(*(begin++), 12);
+    ASSERT_EQ(begin, end);
+}
+
+TEST(SparseSetWithType, SortReverse) {
+    entt::SparseSet<unsigned int, int> set;
+
+    set.construct(12, 1);
+    set.construct(42, 3);
+    set.construct(7, 6);
+    set.construct(3, 9);
+    set.construct(9, 12);
+
+    ASSERT_EQ(set.get(12), 1);
+    ASSERT_EQ(set.get(42), 3);
+    ASSERT_EQ(set.get(7), 6);
+    ASSERT_EQ(set.get(3), 9);
+    ASSERT_EQ(set.get(9), 12);
+
+    set.sort([](auto lhs, auto rhs) {
+        return lhs < rhs;
+    });
+
+    ASSERT_EQ(*(set.raw() + 0u), 12);
+    ASSERT_EQ(*(set.raw() + 1u), 9);
+    ASSERT_EQ(*(set.raw() + 2u), 6);
+    ASSERT_EQ(*(set.raw() + 3u), 3);
+    ASSERT_EQ(*(set.raw() + 4u), 1);
+
+    auto begin = set.begin();
+    auto end = set.end();
+
+    ASSERT_EQ(*(begin++), 1);
+    ASSERT_EQ(*(begin++), 3);
+    ASSERT_EQ(*(begin++), 6);
+    ASSERT_EQ(*(begin++), 9);
+    ASSERT_EQ(*(begin++), 12);
+    ASSERT_EQ(begin, end);
+}
+
+TEST(SparseSetWithType, SortUnordered) {
+    entt::SparseSet<unsigned int, int> set;
+
+    set.construct(12, 6);
+    set.construct(42, 3);
+    set.construct(7, 1);
+    set.construct(3, 9);
+    set.construct(9, 12);
+
+    ASSERT_EQ(set.get(12), 6);
+    ASSERT_EQ(set.get(42), 3);
+    ASSERT_EQ(set.get(7), 1);
+    ASSERT_EQ(set.get(3), 9);
+    ASSERT_EQ(set.get(9), 12);
+
+    set.sort([](auto lhs, auto rhs) {
+        return lhs < rhs;
+    });
+
+    ASSERT_EQ(*(set.raw() + 0u), 12);
+    ASSERT_EQ(*(set.raw() + 1u), 9);
+    ASSERT_EQ(*(set.raw() + 2u), 6);
+    ASSERT_EQ(*(set.raw() + 3u), 3);
+    ASSERT_EQ(*(set.raw() + 4u), 1);
+
+    auto begin = set.begin();
+    auto end = set.end();
+
+    ASSERT_EQ(*(begin++), 1);
+    ASSERT_EQ(*(begin++), 3);
+    ASSERT_EQ(*(begin++), 6);
+    ASSERT_EQ(*(begin++), 9);
+    ASSERT_EQ(*(begin++), 12);
+    ASSERT_EQ(begin, end);
+}
+
+TEST(SparseSetWithType, RespectDisjoint) {
+    entt::SparseSet<unsigned int, int> lhs;
+    entt::SparseSet<unsigned int, int> rhs;
+    const auto &clhs = lhs;
+
+    lhs.construct(3, 3);
+    lhs.construct(12, 6);
+    lhs.construct(42, 9);
+
+    ASSERT_EQ(clhs.get(3), 3);
+    ASSERT_EQ(clhs.get(12), 6);
+    ASSERT_EQ(clhs.get(42), 9);
+
+    lhs.respect(rhs);
+
+    ASSERT_EQ(*(clhs.raw() + 0u), 3);
+    ASSERT_EQ(*(clhs.raw() + 1u), 6);
+    ASSERT_EQ(*(clhs.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(SparseSetWithType, RespectOverlap) {
+    entt::SparseSet<unsigned int, int> lhs;
+    entt::SparseSet<unsigned int, int> rhs;
+    const auto &clhs = lhs;
+
+    lhs.construct(3, 3);
+    lhs.construct(12, 6);
+    lhs.construct(42, 9);
+    rhs.construct(12, 6);
+
+    ASSERT_EQ(clhs.get(3), 3);
+    ASSERT_EQ(clhs.get(12), 6);
+    ASSERT_EQ(clhs.get(42), 9);
+    ASSERT_EQ(rhs.get(12), 6);
+
+    lhs.respect(rhs);
+
+    ASSERT_EQ(*(clhs.raw() + 0u), 3);
+    ASSERT_EQ(*(clhs.raw() + 1u), 9);
+    ASSERT_EQ(*(clhs.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(SparseSetWithType, RespectOrdered) {
+    entt::SparseSet<unsigned int, int> lhs;
+    entt::SparseSet<unsigned int, 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(SparseSetWithType, RespectReverse) {
+    entt::SparseSet<unsigned int, int> lhs;
+    entt::SparseSet<unsigned int, 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(SparseSetWithType, RespectUnordered) {
+    entt::SparseSet<unsigned int, int> lhs;
+    entt::SparseSet<unsigned int, 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(SparseSetWithType, ReferencesGuaranteed) {
+    struct InternalType { int value; };
+
+    entt::SparseSet<unsigned int, InternalType> 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(SparseSetWithType, MoveOnlyComponent) {
+    struct MoveOnlyComponent {
+        MoveOnlyComponent() = default;
+        ~MoveOnlyComponent() = default;
+        MoveOnlyComponent(const MoveOnlyComponent &) = delete;
+        MoveOnlyComponent(MoveOnlyComponent &&) = default;
+        MoveOnlyComponent & operator=(const MoveOnlyComponent &) = delete;
+        MoveOnlyComponent & operator=(MoveOnlyComponent &&) = default;
+    };
+
+    // it's purpose is to ensure that move only components are always accepted
+    entt::SparseSet<unsigned int, MoveOnlyComponent> set;
+    (void)set;
+}

test/entt/entity/view.cpp

@@ -0,0 +1,625 @@
+#include <gtest/gtest.h>
+#include <entt/entity/registry.hpp>
+#include <entt/entity/view.hpp>
+
+TEST(View, SingleComponent) {
+    entt::DefaultRegistry registry;
+    auto view = registry.view<char>();
+
+    const auto e0 = registry.create();
+    const auto e1 = registry.create();
+
+    ASSERT_TRUE(view.empty());
+
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    ASSERT_NO_THROW(registry.view<char>().begin()++);
+    ASSERT_NO_THROW(++registry.view<char>().begin());
+
+    ASSERT_NE(view.begin(), view.end());
+    ASSERT_EQ(view.size(), typename decltype(view)::size_type{1});
+    ASSERT_FALSE(view.empty());
+
+    registry.assign<char>(e0);
+
+    ASSERT_EQ(view.size(), typename decltype(view)::size_type{2});
+
+    view.get(e0) = '1';
+    view.get(e1) = '2';
+
+    for(auto entity: view) {
+        const auto &cview = static_cast<const decltype(view) &>(view);
+        ASSERT_TRUE(cview.get(entity) == '1' || cview.get(entity) == '2');
+    }
+
+    ASSERT_EQ(*(view.data() + 0), e1);
+    ASSERT_EQ(*(view.data() + 1), e0);
+
+    ASSERT_EQ(*(view.raw() + 0), '2');
+    ASSERT_EQ(*(static_cast<const decltype(view) &>(view).raw() + 1), '1');
+
+    registry.remove<char>(e0);
+    registry.remove<char>(e1);
+
+    ASSERT_EQ(view.begin(), view.end());
+    ASSERT_TRUE(view.empty());
+}
+
+TEST(View, SingleComponentBeginEnd) {
+    entt::DefaultRegistry registry;
+    auto view = registry.view<int>();
+    const auto &cview = view;
+
+    for(auto i = 0; i < 3; ++i) {
+        registry.assign<int>(registry.create());
+    }
+
+    auto test = [](auto begin, auto end) {
+        ASSERT_NE(begin, end);
+        ASSERT_NE(++begin, end);
+        ASSERT_NE(begin++, end);
+        ASSERT_EQ(begin+1, end);
+        ASSERT_NE(begin, end);
+        ASSERT_EQ((begin += 1), end);
+        ASSERT_EQ(begin, end);
+    };
+
+    test(view.begin(), view.end());
+    test(cview.begin(), cview.end());
+    test(view.cbegin(), view.cend());
+}
+
+TEST(View, SingleComponentContains) {
+    entt::DefaultRegistry registry;
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+
+    registry.destroy(e0);
+
+    auto view = registry.view<int>();
+
+    ASSERT_FALSE(view.contains(e0));
+    ASSERT_TRUE(view.contains(e1));
+}
+
+TEST(View, SingleComponentEmpty) {
+    entt::DefaultRegistry registry;
+
+    const auto e0 = registry.create();
+    registry.assign<char>(e0);
+    registry.assign<double>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<char>(e1);
+
+    auto view = registry.view<int>();
+
+    ASSERT_EQ(view.size(), entt::DefaultRegistry::size_type{0});
+
+    for(auto entity: view) {
+        (void)entity;
+        FAIL();
+    }
+}
+
+TEST(View, SingleComponentEach) {
+    entt::DefaultRegistry registry;
+
+    registry.assign<int>(registry.create());
+    registry.assign<int>(registry.create());
+
+    auto view = registry.view<int>();
+    const auto &cview = static_cast<const decltype(view) &>(view);
+    std::size_t cnt = 0;
+
+    view.each([&cnt](auto, int &) { ++cnt; });
+
+    ASSERT_EQ(cnt, std::size_t{2});
+
+    cview.each([&cnt](auto, const int &) { --cnt; });
+
+    ASSERT_EQ(cnt, std::size_t{0});
+}
+
+TEST(View, MultipleComponent) {
+    entt::DefaultRegistry registry;
+    auto view = registry.view<int, char>();
+
+    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.view<char>().begin();
+
+    ASSERT_EQ(*it, e1);
+    ASSERT_EQ(*(it+1), e0);
+    ASSERT_EQ(it += 2, registry.view<char>().end());
+
+    ASSERT_NO_THROW((registry.view<int, char>().begin()++));
+    ASSERT_NO_THROW((++registry.view<int, char>().begin()));
+
+    ASSERT_NE(view.begin(), view.end());
+    ASSERT_EQ(view.begin()+1, 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) {
+        const auto &cview = static_cast<const decltype(view) &>(view);
+        ASSERT_EQ(std::get<0>(cview.get<int, char>(entity)), 42);
+        ASSERT_EQ(std::get<1>(view.get<int, char>(entity)), '2');
+        ASSERT_EQ(cview.get<char>(entity), '2');
+    }
+
+    registry.remove<char>(e0);
+    registry.remove<char>(e1);
+}
+
+TEST(View, MultipleComponentBeginEnd) {
+    entt::DefaultRegistry registry;
+    auto view = registry.view<int, char>();
+    const auto &cview = view;
+
+    for(auto i = 0; i < 3; ++i) {
+        const auto entity = registry.create();
+        registry.assign<int>(entity);
+        registry.assign<char>(entity);
+    }
+
+    auto test = [](auto begin, auto end) {
+        ASSERT_NE(begin, end);
+        ASSERT_NE(++begin, end);
+        ASSERT_NE(begin++, end);
+        ASSERT_EQ(begin+1, end);
+        ASSERT_NE(begin, end);
+        ASSERT_EQ((begin += 1), end);
+        ASSERT_EQ(begin, end);
+    };
+
+    test(cview.begin(), cview.end());
+    test(view.begin(), view.end());
+    test(view.cbegin(), view.cend());
+}
+
+TEST(View, MultipleComponentContains) {
+    entt::DefaultRegistry registry;
+
+    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);
+
+    auto view = registry.view<int, char>();
+
+    ASSERT_FALSE(view.contains(e0));
+    ASSERT_TRUE(view.contains(e1));
+}
+
+TEST(View, MultipleComponentEmpty) {
+    entt::DefaultRegistry 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);
+
+    auto view = registry.view<char, int, float>();
+
+    for(auto entity: view) {
+        (void)entity;
+        FAIL();
+    }
+}
+
+TEST(View, MultipleComponentEach) {
+    entt::DefaultRegistry registry;
+
+    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 view = registry.view<int, char>();
+    const auto &cview = static_cast<const decltype(view) &>(view);
+    std::size_t cnt = 0;
+
+    view.each([&cnt](auto, int &, char &) { ++cnt; });
+
+    ASSERT_EQ(cnt, std::size_t{2});
+
+    cview.each([&cnt](auto, const int &, const char &) { --cnt; });
+
+    ASSERT_EQ(cnt, std::size_t{0});
+}
+
+TEST(View, MultipleComponentEachWithHoles) {
+    entt::DefaultRegistry registry;
+
+    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);
+
+    auto view = registry.view<char, int>();
+
+    view.each([e0](auto entity, const char &c, const int &i) {
+        if(e0 == entity) {
+            ASSERT_EQ(c, '0');
+            ASSERT_EQ(i, 0);
+        } else {
+            FAIL();
+        }
+    });
+}
+
+TEST(PersistentView, Prepare) {
+    entt::DefaultRegistry registry;
+    registry.prepare<int, char>();
+    auto view = registry.view<int, char>(entt::persistent_t{});
+
+    ASSERT_TRUE(view.empty());
+
+    const auto e0 = registry.create();
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    ASSERT_FALSE(view.empty());
+    ASSERT_NO_THROW((registry.view<int, char>(entt::persistent_t{}).begin()++));
+    ASSERT_NO_THROW((++registry.view<int, char>(entt::persistent_t{}).begin()));
+
+    ASSERT_NE(view.begin(), view.end());
+    ASSERT_EQ(view.size(), typename decltype(view)::size_type{1});
+
+    registry.assign<int>(e0);
+
+    ASSERT_EQ(view.size(), typename decltype(view)::size_type{2});
+
+    registry.remove<int>(e0);
+
+    ASSERT_EQ(view.size(), typename decltype(view)::size_type{1});
+
+    registry.get<char>(e0) = '1';
+    registry.get<char>(e1) = '2';
+    registry.get<int>(e1) = 42;
+
+    for(auto entity: view) {
+        const auto &cview = static_cast<const decltype(view) &>(view);
+        ASSERT_EQ(std::get<0>(cview.get<int, char>(entity)), 42);
+        ASSERT_EQ(std::get<1>(view.get<int, char>(entity)), '2');
+        ASSERT_EQ(cview.get<char>(entity), '2');
+    }
+
+    ASSERT_EQ(*(view.data() + 0), e1);
+
+    registry.remove<char>(e0);
+    registry.remove<char>(e1);
+
+    ASSERT_EQ(view.begin(), view.end());
+    ASSERT_TRUE(view.empty());
+}
+
+TEST(PersistentView, NoPrepare) {
+    entt::DefaultRegistry registry;
+    auto view = registry.view<int, char>(entt::persistent_t{});
+
+    ASSERT_TRUE(view.empty());
+
+    const auto e0 = registry.create();
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    ASSERT_FALSE(view.empty());
+    ASSERT_NO_THROW((registry.view<int, char>(entt::persistent_t{}).begin()++));
+    ASSERT_NO_THROW((++registry.view<int, char>(entt::persistent_t{}).begin()));
+
+    ASSERT_NE(view.begin(), view.end());
+    ASSERT_EQ(view.size(), typename decltype(view)::size_type{1});
+
+    registry.assign<int>(e0);
+
+    ASSERT_EQ(view.size(), typename decltype(view)::size_type{2});
+
+    registry.remove<int>(e0);
+
+    ASSERT_EQ(view.size(), typename decltype(view)::size_type{1});
+
+    registry.get<char>(e0) = '1';
+    registry.get<char>(e1) = '2';
+    registry.get<int>(e1) = 42;
+
+    for(auto entity: view) {
+        const auto &cview = static_cast<const decltype(view) &>(view);
+        ASSERT_EQ(std::get<0>(cview.get<int, char>(entity)), 42);
+        ASSERT_EQ(std::get<1>(view.get<int, char>(entity)), '2');
+        ASSERT_EQ(cview.get<char>(entity), '2');
+    }
+
+    ASSERT_EQ(*(view.data() + 0), e1);
+
+    registry.remove<char>(e0);
+    registry.remove<char>(e1);
+
+    ASSERT_EQ(view.begin(), view.end());
+    ASSERT_TRUE(view.empty());
+}
+
+TEST(PersistentView, BeginEnd) {
+    entt::DefaultRegistry registry;
+    auto view = registry.view<int, char>(entt::persistent_t{});
+    const auto &cview = view;
+
+    for(auto i = 0; i < 3; ++i) {
+        const auto entity = registry.create();
+        registry.assign<int>(entity);
+        registry.assign<char>(entity);
+    }
+
+    auto test = [](auto begin, auto end) {
+        ASSERT_NE(begin, end);
+        ASSERT_NE(++begin, end);
+        ASSERT_NE(begin++, end);
+        ASSERT_EQ(begin+1, end);
+        ASSERT_NE(begin, end);
+        ASSERT_EQ((begin += 1), end);
+        ASSERT_EQ(begin, end);
+    };
+
+    test(cview.begin(), cview.end());
+    test(view.begin(), view.end());
+    test(view.cbegin(), view.cend());
+}
+
+TEST(PersistentView, Contains) {
+    entt::DefaultRegistry registry;
+
+    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);
+
+    auto view = registry.view<int, char>(entt::persistent_t{});
+
+    ASSERT_FALSE(view.contains(e0));
+    ASSERT_TRUE(view.contains(e1));
+}
+
+TEST(PersistentView, Empty) {
+    entt::DefaultRegistry 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.view<char, int, float>(entt::persistent_t{})) {
+        (void)entity;
+        FAIL();
+    }
+
+    for(auto entity: registry.view<double, char, int, float>(entt::persistent_t{})) {
+        (void)entity;
+        FAIL();
+    }
+}
+
+TEST(PersistentView, Each) {
+    entt::DefaultRegistry registry;
+    registry.prepare<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 view = registry.view<int, char>(entt::persistent_t{});
+    const auto &cview = static_cast<const decltype(view) &>(view);
+    std::size_t cnt = 0;
+
+    view.each([&cnt](auto, int &, char &) { ++cnt; });
+
+    ASSERT_EQ(cnt, std::size_t{2});
+
+    cview.each([&cnt](auto, const int &, const char &) { --cnt; });
+
+    ASSERT_EQ(cnt, std::size_t{0});
+}
+
+TEST(PersistentView, Sort) {
+    entt::DefaultRegistry registry;
+    registry.prepare<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++);
+
+    auto view = registry.view<int, unsigned int>(entt::persistent_t{});
+
+    for(auto entity: view) {
+        ASSERT_EQ(view.get<unsigned int>(entity), --uval);
+        ASSERT_EQ(view.get<int>(entity), --ival);
+    }
+
+    registry.sort<unsigned int>(std::less<unsigned int>{});
+    view.sort<unsigned int>();
+
+    for(auto entity: view) {
+        ASSERT_EQ(view.get<unsigned int>(entity), uval++);
+        ASSERT_EQ(view.get<int>(entity), ival++);
+    }
+}
+
+TEST(RawView, Functionalities) {
+    entt::DefaultRegistry registry;
+    auto view = registry.view<char>(entt::raw_t{});
+
+    ASSERT_TRUE(view.empty());
+
+    const auto e0 = registry.create();
+    const auto e1 = registry.create();
+
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    ASSERT_FALSE(view.empty());
+    ASSERT_NO_THROW(registry.view<char>(entt::raw_t{}).begin()++);
+    ASSERT_NO_THROW(++registry.view<char>(entt::raw_t{}).begin());
+
+    ASSERT_NE(view.begin(), view.end());
+    ASSERT_EQ(view.size(), typename decltype(view)::size_type{1});
+
+    registry.assign<char>(e0);
+
+    ASSERT_EQ(view.size(), typename decltype(view)::size_type{2});
+
+    registry.get<char>(e0) = '1';
+    registry.get<char>(e1) = '2';
+
+    for(auto &&component: view) {
+        ASSERT_TRUE(component == '1' || component == '2');
+    }
+
+    ASSERT_EQ(*(view.data() + 0), e1);
+    ASSERT_EQ(*(view.data() + 1), e0);
+
+    ASSERT_EQ(*(view.raw() + 0), '2');
+    ASSERT_EQ(*(static_cast<const decltype(view) &>(view).raw() + 1), '1');
+
+    for(auto &&component: view) {
+        // verifies that iterators return references to components
+        component = '0';
+    }
+
+    for(auto &&component: view) {
+        ASSERT_TRUE(component == '0');
+    }
+
+    registry.remove<char>(e0);
+    registry.remove<char>(e1);
+
+    ASSERT_EQ(view.begin(), view.end());
+    ASSERT_TRUE(view.empty());
+}
+
+TEST(RawView, BeginEnd) {
+    entt::DefaultRegistry registry;
+    auto view = registry.view<int>(entt::raw_t{});
+    const auto &cview = view;
+
+    for(auto i = 0; i < 3; ++i) {
+        registry.assign<int>(registry.create());
+    }
+
+    auto test = [](auto begin, auto end) {
+        ASSERT_NE(begin, end);
+        ASSERT_NE(++begin, end);
+        ASSERT_NE(begin++, end);
+        ASSERT_EQ(begin+1, end);
+        ASSERT_NE(begin, end);
+        ASSERT_EQ((begin += 1), end);
+        ASSERT_EQ(begin, end);
+    };
+
+    test(cview.begin(), cview.end());
+    test(view.begin(), view.end());
+    test(view.cbegin(), view.cend());
+}
+
+TEST(RawView, Empty) {
+    entt::DefaultRegistry registry;
+
+    const auto e0 = registry.create();
+    registry.assign<char>(e0);
+    registry.assign<double>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<char>(e1);
+
+    auto view = registry.view<int>(entt::raw_t{});
+
+    ASSERT_EQ(view.size(), entt::DefaultRegistry::size_type{0});
+
+    for(auto &&component: view) {
+        (void)component;
+        FAIL();
+    }
+}
+
+TEST(RawView, Each) {
+    entt::DefaultRegistry registry;
+
+    registry.assign<int>(registry.create(), 1);
+    registry.assign<int>(registry.create(), 3);
+
+    auto view = registry.view<int>(entt::raw_t{});
+    const auto &cview = static_cast<const decltype(view) &>(view);
+    std::size_t cnt = 0;
+
+    view.each([&cnt](int &v) { cnt += (v % 2); });
+
+    ASSERT_EQ(cnt, std::size_t{2});
+
+    cview.each([&cnt](const int &v) { cnt -= (v % 2); });
+
+    ASSERT_EQ(cnt, std::size_t{0});
+}

test/entt/locator/locator.cpp

@@ -0,0 +1,49 @@
+#include <gtest/gtest.h>
+#include <entt/locator/locator.hpp>
+
+struct AService {};
+
+struct AnotherService {
+    virtual void f(bool) = 0;
+    bool check{false};
+};
+
+struct DerivedService: AnotherService {
+    DerivedService(int): AnotherService{} {}
+    void f(bool b) override { check = b; }
+};
+
+TEST(ServiceLocator, Functionalities) {
+    using entt::ServiceLocator;
+
+    ASSERT_TRUE(ServiceLocator<AService>::empty());
+    ASSERT_TRUE(ServiceLocator<AnotherService>::empty());
+
+    ServiceLocator<AService>::set();
+
+    ASSERT_FALSE(ServiceLocator<AService>::empty());
+    ASSERT_TRUE(ServiceLocator<AnotherService>::empty());
+
+    ServiceLocator<AService>::reset();
+
+    ASSERT_TRUE(ServiceLocator<AService>::empty());
+    ASSERT_TRUE(ServiceLocator<AnotherService>::empty());
+
+    ServiceLocator<AService>::set(std::make_shared<AService>());
+
+    ASSERT_FALSE(ServiceLocator<AService>::empty());
+    ASSERT_TRUE(ServiceLocator<AnotherService>::empty());
+
+    ServiceLocator<AnotherService>::set<DerivedService>(42);
+
+    ASSERT_FALSE(ServiceLocator<AService>::empty());
+    ASSERT_FALSE(ServiceLocator<AnotherService>::empty());
+
+    ServiceLocator<AnotherService>::get().lock()->f(!ServiceLocator<AnotherService>::get().lock()->check);
+
+    ASSERT_TRUE(ServiceLocator<AnotherService>::get().lock()->check);
+
+    ServiceLocator<AnotherService>::ref().f(!ServiceLocator<AnotherService>::get().lock()->check);
+
+    ASSERT_FALSE(ServiceLocator<AnotherService>::get().lock()->check);
+}

test/entt/process/process.cpp

@@ -0,0 +1,206 @@
+#include <gtest/gtest.h>
+#include <cstdint>
+#include <entt/process/process.hpp>
+
+struct FakeProcess: entt::Process<FakeProcess, int> {
+    using process_type = entt::Process<FakeProcess, int>;
+
+    void succeed() noexcept { process_type::succeed(); }
+    void fail() noexcept { process_type::fail(); }
+    void pause() noexcept { process_type::pause(); }
+    void unpause() noexcept { process_type::unpause(); }
+
+    void init(void *) { initInvoked = true; }
+    void succeeded() { succeededInvoked = true; }
+    void failed() { failedInvoked = true; }
+    void aborted() { abortedInvoked = true; }
+
+    void update(delta_type, void *data) {
+        if(data) {
+            (*static_cast<int *>(data))++;
+        }
+
+        updateInvoked = true;
+    }
+
+    bool initInvoked{false};
+    bool updateInvoked{false};
+    bool succeededInvoked{false};
+    bool failedInvoked{false};
+    bool abortedInvoked{false};
+};
+
+TEST(Process, Basics) {
+    FakeProcess process;
+
+    ASSERT_FALSE(process.alive());
+    ASSERT_FALSE(process.dead());
+    ASSERT_FALSE(process.paused());
+
+    process.succeed();
+    process.fail();
+    process.abort();
+    process.pause();
+    process.unpause();
+
+    ASSERT_FALSE(process.alive());
+    ASSERT_FALSE(process.dead());
+    ASSERT_FALSE(process.paused());
+
+    process.tick(0);
+
+    ASSERT_TRUE(process.alive());
+    ASSERT_FALSE(process.dead());
+    ASSERT_FALSE(process.paused());
+
+    process.pause();
+
+    ASSERT_TRUE(process.alive());
+    ASSERT_FALSE(process.dead());
+    ASSERT_TRUE(process.paused());
+
+    process.unpause();
+
+    ASSERT_TRUE(process.alive());
+    ASSERT_FALSE(process.dead());
+    ASSERT_FALSE(process.paused());
+}
+
+TEST(Process, Succeeded) {
+    FakeProcess process;
+
+    process.tick(0);
+    process.succeed();
+    process.tick(0);
+
+    ASSERT_FALSE(process.alive());
+    ASSERT_TRUE(process.dead());
+    ASSERT_FALSE(process.paused());
+
+    ASSERT_TRUE(process.initInvoked);
+    ASSERT_TRUE(process.updateInvoked);
+    ASSERT_TRUE(process.succeededInvoked);
+    ASSERT_FALSE(process.failedInvoked);
+    ASSERT_FALSE(process.abortedInvoked);
+}
+
+TEST(Process, Fail) {
+    FakeProcess process;
+
+    process.tick(0);
+    process.fail();
+    process.tick(0);
+
+    ASSERT_FALSE(process.alive());
+    ASSERT_TRUE(process.dead());
+    ASSERT_FALSE(process.paused());
+
+    ASSERT_TRUE(process.initInvoked);
+    ASSERT_TRUE(process.updateInvoked);
+    ASSERT_FALSE(process.succeededInvoked);
+    ASSERT_TRUE(process.failedInvoked);
+    ASSERT_FALSE(process.abortedInvoked);
+}
+
+TEST(Process, Data) {
+    FakeProcess process;
+    int value = 0;
+
+    process.tick(0, &value);
+    process.succeed();
+    process.tick(0, &value);
+
+    ASSERT_FALSE(process.alive());
+    ASSERT_TRUE(process.dead());
+    ASSERT_FALSE(process.paused());
+
+    ASSERT_EQ(value, 1);
+    ASSERT_TRUE(process.initInvoked);
+    ASSERT_TRUE(process.updateInvoked);
+    ASSERT_TRUE(process.succeededInvoked);
+    ASSERT_FALSE(process.failedInvoked);
+    ASSERT_FALSE(process.abortedInvoked);
+}
+
+TEST(Process, AbortNextTick) {
+    FakeProcess process;
+
+    process.tick(0);
+    process.abort();
+    process.tick(0);
+
+    ASSERT_FALSE(process.alive());
+    ASSERT_TRUE(process.dead());
+    ASSERT_FALSE(process.paused());
+
+    ASSERT_TRUE(process.initInvoked);
+    ASSERT_TRUE(process.updateInvoked);
+    ASSERT_FALSE(process.succeededInvoked);
+    ASSERT_FALSE(process.failedInvoked);
+    ASSERT_TRUE(process.abortedInvoked);
+}
+
+TEST(Process, AbortImmediately) {
+    FakeProcess process;
+
+    process.tick(0);
+    process.abort(true);
+
+    ASSERT_FALSE(process.alive());
+    ASSERT_TRUE(process.dead());
+    ASSERT_FALSE(process.paused());
+
+    ASSERT_TRUE(process.initInvoked);
+    ASSERT_TRUE(process.updateInvoked);
+    ASSERT_FALSE(process.succeededInvoked);
+    ASSERT_FALSE(process.failedInvoked);
+    ASSERT_TRUE(process.abortedInvoked);
+}
+
+TEST(ProcessAdaptor, Resolved) {
+    bool updated = false;
+    auto lambda = [&updated](std::uint64_t, void *, auto resolve, auto) {
+        ASSERT_FALSE(updated);
+        updated = true;
+        resolve();
+    };
+
+    auto process = entt::ProcessAdaptor<decltype(lambda), std::uint64_t>{lambda};
+
+    process.tick(0);
+
+    ASSERT_TRUE(process.dead());
+    ASSERT_TRUE(updated);
+}
+
+TEST(ProcessAdaptor, Rejected) {
+    bool updated = false;
+    auto lambda = [&updated](std::uint64_t, void *, auto, auto rejected) {
+        ASSERT_FALSE(updated);
+        updated = true;
+        rejected();
+    };
+
+    auto process = entt::ProcessAdaptor<decltype(lambda), std::uint64_t>{lambda};
+
+    process.tick(0);
+
+    ASSERT_TRUE(process.rejected());
+    ASSERT_TRUE(updated);
+}
+
+TEST(ProcessAdaptor, Data) {
+    int value = 0;
+
+    auto lambda = [](std::uint64_t, void *data, auto resolve, auto) {
+        *static_cast<int *>(data) = 42;
+        resolve();
+    };
+
+    auto process = entt::ProcessAdaptor<decltype(lambda), std::uint64_t>{lambda};
+
+    process.tick(0, &value);
+
+    ASSERT_TRUE(process.dead());
+    ASSERT_EQ(value, 42);
+}

test/entt/process/scheduler.cpp

@@ -0,0 +1,113 @@
+#include <functional>
+#include <gtest/gtest.h>
+#include <entt/process/scheduler.hpp>
+#include <entt/process/process.hpp>
+
+struct FooProcess: entt::Process<FooProcess, int> {
+    FooProcess(std::function<void()> onUpdate, std::function<void()> onAborted)
+        : onUpdate{onUpdate}, onAborted{onAborted}
+    {}
+
+    void update(delta_type, void *) { onUpdate(); }
+    void aborted() { onAborted(); }
+
+    std::function<void()> onUpdate;
+    std::function<void()> onAborted;
+};
+
+struct SucceededProcess: entt::Process<SucceededProcess, int> {
+    void update(delta_type, void *) {
+        ASSERT_FALSE(updated);
+        updated = true;
+        ++invoked;
+        succeed();
+    }
+
+    static unsigned int invoked;
+    bool updated = false;
+};
+
+unsigned int SucceededProcess::invoked = 0;
+
+struct FailedProcess: entt::Process<FailedProcess, int> {
+    void update(delta_type, void *) {
+        ASSERT_FALSE(updated);
+        updated = true;
+        fail();
+    }
+
+    bool updated = false;
+};
+
+TEST(Scheduler, Functionalities) {
+    entt::Scheduler<int> scheduler{};
+
+    bool updated = false;
+    bool aborted = false;
+
+    ASSERT_EQ(scheduler.size(), entt::Scheduler<int>::size_type{});
+    ASSERT_TRUE(scheduler.empty());
+
+    scheduler.attach<FooProcess>(
+                [&updated](){ updated = true; },
+                [&aborted](){ aborted = true; }
+    );
+
+    ASSERT_NE(scheduler.size(), entt::Scheduler<int>::size_type{});
+    ASSERT_FALSE(scheduler.empty());
+
+    scheduler.update(0);
+    scheduler.abort(true);
+
+    ASSERT_TRUE(updated);
+    ASSERT_TRUE(aborted);
+
+    ASSERT_NE(scheduler.size(), entt::Scheduler<int>::size_type{});
+    ASSERT_FALSE(scheduler.empty());
+
+    scheduler.clear();
+
+    ASSERT_EQ(scheduler.size(), entt::Scheduler<int>::size_type{});
+    ASSERT_TRUE(scheduler.empty());
+}
+
+TEST(Scheduler, Then) {
+    entt::Scheduler<int> scheduler;
+
+    scheduler.attach<SucceededProcess>()
+            .then<SucceededProcess>()
+            .then<FailedProcess>()
+            .then<SucceededProcess>();
+
+    for(auto i = 0; i < 8; ++i) {
+        scheduler.update(0);
+    }
+
+    ASSERT_EQ(SucceededProcess::invoked, 2u);
+}
+
+TEST(Scheduler, Functor) {
+    entt::Scheduler<int> scheduler;
+
+    bool firstFunctor = false;
+    bool secondFunctor = false;
+
+    scheduler.attach([&firstFunctor](auto, void *, auto resolve, auto){
+        ASSERT_FALSE(firstFunctor);
+        firstFunctor = true;
+        resolve();
+    }).then([&secondFunctor](auto, void *, auto, auto reject){
+        ASSERT_FALSE(secondFunctor);
+        secondFunctor = true;
+        reject();
+    }).then([](auto...){
+        FAIL();
+    });
+
+    for(auto i = 0; i < 8; ++i) {
+        scheduler.update(0);
+    }
+
+    ASSERT_TRUE(firstFunctor);
+    ASSERT_TRUE(secondFunctor);
+}

test/entt/resource/resource.cpp

@@ -0,0 +1,85 @@
+#include <gtest/gtest.h>
+#include <entt/resource/cache.hpp>
+
+struct Resource { const int value; };
+
+struct Loader: entt::ResourceLoader<Loader, Resource> {
+    std::shared_ptr<Resource> load(int value) const {
+        return std::shared_ptr<Resource>(new Resource{ value });
+    }
+};
+
+struct BrokenLoader: entt::ResourceLoader<BrokenLoader, Resource> {
+    std::shared_ptr<Resource> load(int) const {
+        return nullptr;
+    }
+};
+
+TEST(ResourceCache, Functionalities) {
+    entt::ResourceCache<Resource> cache;
+
+    constexpr auto hs1 = entt::HashedString{"res1"};
+    constexpr auto hs2 = entt::HashedString{"res2"};
+
+    ASSERT_EQ(cache.size(), entt::ResourceCache<Resource>::size_type{});
+    ASSERT_TRUE(cache.empty());
+    ASSERT_FALSE(cache.contains(hs1));
+    ASSERT_FALSE(cache.contains(hs2));
+
+    ASSERT_FALSE(cache.load<BrokenLoader>(hs1, 42));
+    ASSERT_FALSE(cache.reload<BrokenLoader>(hs1, 42));
+
+    ASSERT_EQ(cache.size(), entt::ResourceCache<Resource>::size_type{});
+    ASSERT_TRUE(cache.empty());
+    ASSERT_FALSE(cache.contains(hs1));
+    ASSERT_FALSE(cache.contains(hs2));
+
+    ASSERT_TRUE(cache.load<Loader>(hs1, 42));
+    ASSERT_TRUE(cache.reload<Loader>(hs1, 42));
+
+    ASSERT_NE(cache.size(), entt::ResourceCache<Resource>::size_type{});
+    ASSERT_FALSE(cache.empty());
+    ASSERT_TRUE(cache.contains(hs1));
+    ASSERT_FALSE(cache.contains(hs2));
+    ASSERT_EQ((*cache.handle(hs1)).value, 42);
+
+    ASSERT_TRUE(cache.load<Loader>(hs2, 42));
+
+    ASSERT_NE(cache.size(), entt::ResourceCache<Resource>::size_type{});
+    ASSERT_FALSE(cache.empty());
+    ASSERT_TRUE(cache.contains(hs1));
+    ASSERT_TRUE(cache.contains(hs2));
+    ASSERT_EQ((*cache.handle(hs1)).value, 42);
+    ASSERT_EQ(cache.handle(hs2)->value, 42);
+
+    ASSERT_NO_THROW(cache.discard(hs1));
+
+    ASSERT_FALSE(cache.contains(hs1));
+    ASSERT_TRUE(cache.contains(hs2));
+    ASSERT_EQ(cache.handle(hs2)->value, 42);
+
+    ASSERT_TRUE(cache.load<Loader>(hs1, 42));
+    ASSERT_NO_THROW(cache.clear());
+
+    ASSERT_EQ(cache.size(), entt::ResourceCache<Resource>::size_type{});
+    ASSERT_TRUE(cache.empty());
+    ASSERT_FALSE(cache.contains(hs1));
+    ASSERT_FALSE(cache.contains(hs2));
+
+    ASSERT_TRUE(cache.load<Loader>(hs1, 42));
+
+    ASSERT_NE(cache.size(), entt::ResourceCache<Resource>::size_type{});
+    ASSERT_FALSE(cache.empty());
+    ASSERT_TRUE(cache.handle(hs1));
+    ASSERT_FALSE(cache.handle(hs2));
+
+    ASSERT_TRUE(cache.handle(hs1));
+    ASSERT_EQ(&cache.handle(hs1).get(), &static_cast<const Resource &>(cache.handle(hs1)));
+    ASSERT_NO_THROW(cache.clear());
+
+    ASSERT_EQ(cache.size(), entt::ResourceCache<Resource>::size_type{});
+    ASSERT_TRUE(cache.empty());
+
+    ASSERT_TRUE(cache.temp<Loader>(42));
+    ASSERT_TRUE(cache.empty());
+}

test/entt/signal/delegate.cpp

@@ -0,0 +1,45 @@
+#include <gtest/gtest.h>
+#include <entt/signal/delegate.hpp>
+
+int delegateFunction(int i) {
+    return i*i;
+}
+
+struct DelegateFunctor {
+    int operator()(int i) {
+        return i+i;
+    }
+};
+
+TEST(Delegate, Functionalities) {
+    entt::Delegate<int(int)> ffdel;
+    entt::Delegate<int(int)> mfdel;
+    DelegateFunctor functor;
+
+    ASSERT_EQ(ffdel(42), int{});
+    ASSERT_EQ(mfdel(42), int{});
+
+    ffdel.connect<&delegateFunction>();
+    mfdel.connect<DelegateFunctor, &DelegateFunctor::operator()>(&functor);
+
+    ASSERT_EQ(ffdel(3), 9);
+    ASSERT_EQ(mfdel(3), 6);
+
+    ffdel.reset();
+    mfdel.reset();
+
+    ASSERT_EQ(ffdel(42), int{});
+    ASSERT_EQ(mfdel(42), int{});
+}
+
+TEST(Delegate, Comparison) {
+    entt::Delegate<int(int)> delegate;
+    entt::Delegate<int(int)> def;
+    delegate.connect<&delegateFunction>();
+
+    ASSERT_EQ(def, entt::Delegate<int(int)>{});
+    ASSERT_NE(def, delegate);
+
+    ASSERT_TRUE(def == entt::Delegate<int(int)>{});
+    ASSERT_TRUE (def != delegate);
+}

test/entt/signal/dispatcher.cpp

@@ -0,0 +1,40 @@
+#include <memory>
+#include <gtest/gtest.h>
+#include <entt/signal/dispatcher.hpp>
+
+struct AnEvent {};
+struct AnotherEvent {};
+
+struct Receiver {
+    void receive(const AnEvent &) { ++cnt; }
+    void reset() { cnt = 0; }
+    int cnt{0};
+};
+
+TEST(Dispatcher, Functionalities) {
+    entt::Dispatcher dispatcher;
+    Receiver receiver;
+
+    dispatcher.template sink<AnEvent>().connect(&receiver);
+    dispatcher.template trigger<AnEvent>();
+    dispatcher.template enqueue<AnEvent>();
+    dispatcher.template enqueue<AnotherEvent>();
+    dispatcher.update<AnotherEvent>();
+
+    ASSERT_EQ(receiver.cnt, 1);
+
+    dispatcher.update<AnEvent>();
+    dispatcher.template trigger<AnEvent>();
+
+    ASSERT_EQ(receiver.cnt, 3);
+
+    receiver.reset();
+
+    dispatcher.template sink<AnEvent>().disconnect(&receiver);
+    dispatcher.template trigger<AnEvent>();
+    dispatcher.template enqueue<AnEvent>();
+    dispatcher.update();
+    dispatcher.template trigger<AnEvent>();
+
+    ASSERT_EQ(receiver.cnt, 0);
+}

test/entt/signal/emitter.cpp

@@ -0,0 +1,117 @@
+#include <gtest/gtest.h>
+#include <entt/signal/emitter.hpp>
+
+struct TestEmitter: entt::Emitter<TestEmitter> {};
+
+struct FooEvent { int i; char c; };
+struct BarEvent {};
+
+TEST(Emitter, Clear) {
+    TestEmitter emitter;
+
+    ASSERT_TRUE(emitter.empty());
+
+    emitter.on<FooEvent>([](const auto &, const auto &){});
+
+    ASSERT_FALSE(emitter.empty());
+    ASSERT_FALSE(emitter.empty<FooEvent>());
+    ASSERT_TRUE(emitter.empty<BarEvent>());
+
+    emitter.clear<BarEvent>();
+
+    ASSERT_FALSE(emitter.empty());
+    ASSERT_FALSE(emitter.empty<FooEvent>());
+    ASSERT_TRUE(emitter.empty<BarEvent>());
+
+    emitter.clear<FooEvent>();
+
+    ASSERT_TRUE(emitter.empty());
+    ASSERT_TRUE(emitter.empty<FooEvent>());
+    ASSERT_TRUE(emitter.empty<BarEvent>());
+
+    emitter.on<FooEvent>([](const auto &, const auto &){});
+    emitter.on<BarEvent>([](const auto &, const auto &){});
+
+    ASSERT_FALSE(emitter.empty());
+    ASSERT_FALSE(emitter.empty<FooEvent>());
+    ASSERT_FALSE(emitter.empty<BarEvent>());
+
+    emitter.clear();
+
+    ASSERT_TRUE(emitter.empty());
+    ASSERT_TRUE(emitter.empty<FooEvent>());
+    ASSERT_TRUE(emitter.empty<BarEvent>());
+}
+
+TEST(Emitter, ClearPublishing) {
+    TestEmitter emitter;
+    bool invoked = false;
+
+    ASSERT_TRUE(emitter.empty());
+
+    emitter.on<BarEvent>([&invoked](const auto &, auto &em){
+        invoked = true;
+        em.clear();
+    });
+
+    emitter.publish<BarEvent>();
+
+    ASSERT_TRUE(emitter.empty());
+    ASSERT_TRUE(invoked);
+}
+
+TEST(Emitter, On) {
+    TestEmitter emitter;
+
+    emitter.on<FooEvent>([](const auto &, const auto &){});
+
+    ASSERT_FALSE(emitter.empty());
+    ASSERT_FALSE(emitter.empty<FooEvent>());
+
+    emitter.publish<FooEvent>(0, 'c');
+
+    ASSERT_FALSE(emitter.empty());
+    ASSERT_FALSE(emitter.empty<FooEvent>());
+}
+
+TEST(Emitter, Once) {
+    TestEmitter emitter;
+
+    emitter.once<BarEvent>([](const auto &, const auto &){});
+
+    ASSERT_FALSE(emitter.empty());
+    ASSERT_FALSE(emitter.empty<BarEvent>());
+
+    emitter.publish<BarEvent>();
+
+    ASSERT_TRUE(emitter.empty());
+    ASSERT_TRUE(emitter.empty<BarEvent>());
+}
+
+TEST(Emitter, OnceAndErase) {
+    TestEmitter emitter;
+
+    auto conn = emitter.once<FooEvent>([](const auto &, const auto &){});
+
+    ASSERT_FALSE(emitter.empty());
+    ASSERT_FALSE(emitter.empty<FooEvent>());
+
+    emitter.erase(conn);
+
+    ASSERT_TRUE(emitter.empty());
+    ASSERT_TRUE(emitter.empty<FooEvent>());
+}
+
+TEST(Emitter, OnAndErase) {
+    TestEmitter emitter;
+
+    auto conn = emitter.on<BarEvent>([](const auto &, const auto &){});
+
+    ASSERT_FALSE(emitter.empty());
+    ASSERT_FALSE(emitter.empty<BarEvent>());
+
+    emitter.erase(conn);
+
+    ASSERT_TRUE(emitter.empty());
+    ASSERT_TRUE(emitter.empty<BarEvent>());
+}

test/entt/signal/sigh.cpp

@@ -0,0 +1,222 @@
+#include <utility>
+#include <vector>
+#include <gtest/gtest.h>
+#include <entt/signal/sigh.hpp>
+
+struct SigHListener {
+    static void f(int &v) { v = 42; }
+
+    bool g(int) { k = !k; return true; }
+    bool h(int) { return k; }
+
+    void i() {}
+    void l() {}
+
+    bool k{false};
+};
+
+template<typename Ret>
+struct TestCollectAll {
+    std::vector<Ret> vec{};
+    static int f() { return 42; }
+    static int g() { return 42; }
+    bool operator()(Ret r) noexcept {
+        vec.push_back(r);
+        return true;
+    }
+};
+
+template<>
+struct TestCollectAll<void> {
+    std::vector<int> vec{};
+    static void h() {}
+    bool operator()() noexcept {
+        return true;
+    }
+};
+
+template<typename Ret>
+struct TestCollectFirst {
+    std::vector<Ret> vec{};
+    static int f() { return 42; }
+    bool operator()(Ret r) noexcept {
+        vec.push_back(r);
+        return false;
+    }
+};
+
+TEST(SigH, Lifetime) {
+    using signal = entt::SigH<void(void)>;
+
+    ASSERT_NO_THROW(signal{});
+
+    signal src{}, other{};
+
+    ASSERT_NO_THROW(signal{src});
+    ASSERT_NO_THROW(signal{std::move(other)});
+    ASSERT_NO_THROW(src = other);
+    ASSERT_NO_THROW(src = std::move(other));
+
+    ASSERT_NO_THROW(delete new signal{});
+}
+
+TEST(SigH, Comparison) {
+    entt::SigH<void()> sig1;
+    entt::SigH<void()> sig2;
+
+    SigHListener s1;
+    SigHListener s2;
+
+    sig1.sink().connect<SigHListener, &SigHListener::i>(&s1);
+    sig2.sink().connect<SigHListener, &SigHListener::i>(&s2);
+
+    ASSERT_FALSE(sig1 == sig2);
+    ASSERT_TRUE(sig1 != sig2);
+
+    sig1.sink().disconnect<SigHListener, &SigHListener::i>(&s1);
+    sig2.sink().disconnect<SigHListener, &SigHListener::i>(&s2);
+
+    sig1.sink().connect<SigHListener, &SigHListener::i>(&s1);
+    sig2.sink().connect<SigHListener, &SigHListener::l>(&s1);
+
+    ASSERT_FALSE(sig1 == sig2);
+    ASSERT_TRUE(sig1 != sig2);
+
+    sig1.sink().disconnect<SigHListener, &SigHListener::i>(&s1);
+    sig2.sink().disconnect<SigHListener, &SigHListener::l>(&s1);
+
+    ASSERT_TRUE(sig1 == sig2);
+    ASSERT_FALSE(sig1 != sig2);
+
+    sig1.sink().connect<SigHListener, &SigHListener::i>(&s1);
+    sig1.sink().connect<SigHListener, &SigHListener::l>(&s1);
+    sig2.sink().connect<SigHListener, &SigHListener::i>(&s1);
+    sig2.sink().connect<SigHListener, &SigHListener::l>(&s1);
+
+    ASSERT_TRUE(sig1 == sig2);
+
+    sig1.sink().disconnect<SigHListener, &SigHListener::i>(&s1);
+    sig1.sink().disconnect<SigHListener, &SigHListener::l>(&s1);
+    sig2.sink().disconnect<SigHListener, &SigHListener::i>(&s1);
+    sig2.sink().disconnect<SigHListener, &SigHListener::l>(&s1);
+
+    sig1.sink().connect<SigHListener, &SigHListener::i>(&s1);
+    sig1.sink().connect<SigHListener, &SigHListener::l>(&s1);
+    sig2.sink().connect<SigHListener, &SigHListener::l>(&s1);
+    sig2.sink().connect<SigHListener, &SigHListener::i>(&s1);
+
+    ASSERT_FALSE(sig1 == sig2);
+}
+
+TEST(SigH, Clear) {
+    entt::SigH<void(int &)> sigh;
+    sigh.sink().connect<&SigHListener::f>();
+
+    ASSERT_FALSE(sigh.empty());
+
+    sigh.sink().disconnect();
+
+    ASSERT_TRUE(sigh.empty());
+}
+
+TEST(SigH, Swap) {
+    entt::SigH<void(int &)> sigh1;
+    entt::SigH<void(int &)> sigh2;
+
+    sigh1.sink().connect<&SigHListener::f>();
+
+    ASSERT_FALSE(sigh1.empty());
+    ASSERT_TRUE(sigh2.empty());
+
+    std::swap(sigh1, sigh2);
+
+    ASSERT_TRUE(sigh1.empty());
+    ASSERT_FALSE(sigh2.empty());
+}
+
+TEST(SigH, Functions) {
+    entt::SigH<void(int &)> sigh;
+    int v = 0;
+
+    sigh.sink().connect<&SigHListener::f>();
+    sigh.publish(v);
+
+    ASSERT_FALSE(sigh.empty());
+    ASSERT_EQ((entt::SigH<bool(int)>::size_type)1, sigh.size());
+    ASSERT_EQ(42, v);
+
+    v = 0;
+    sigh.sink().disconnect<&SigHListener::f>();
+    sigh.publish(v);
+
+    ASSERT_TRUE(sigh.empty());
+    ASSERT_EQ((entt::SigH<bool(int)>::size_type)0, sigh.size());
+    ASSERT_EQ(0, v);
+
+    sigh.sink().connect<&SigHListener::f>();
+}
+
+TEST(SigH, Members) {
+    SigHListener s;
+    SigHListener *ptr = &s;
+    entt::SigH<bool(int)> sigh;
+
+    sigh.sink().connect<SigHListener, &SigHListener::g>(ptr);
+    sigh.publish(42);
+
+    ASSERT_TRUE(s.k);
+    ASSERT_FALSE(sigh.empty());
+    ASSERT_EQ((entt::SigH<bool(int)>::size_type)1, sigh.size());
+
+    sigh.sink().disconnect<SigHListener, &SigHListener::g>(ptr);
+    sigh.publish(42);
+
+    ASSERT_TRUE(s.k);
+    ASSERT_TRUE(sigh.empty());
+    ASSERT_EQ((entt::SigH<bool(int)>::size_type)0, sigh.size());
+
+    sigh.sink().connect<SigHListener, &SigHListener::g>(ptr);
+    sigh.sink().connect<SigHListener, &SigHListener::h>(ptr);
+
+    ASSERT_FALSE(sigh.empty());
+    ASSERT_EQ((entt::SigH<bool(int)>::size_type)2, sigh.size());
+
+    sigh.sink().disconnect(ptr);
+
+    ASSERT_TRUE(sigh.empty());
+    ASSERT_EQ((entt::SigH<bool(int)>::size_type)0, sigh.size());
+}
+
+TEST(SigH, Collector) {
+    entt::SigH<void(), TestCollectAll<void>> sigh_void;
+
+    sigh_void.sink().connect<&TestCollectAll<void>::h>();
+    auto collector_void = sigh_void.collect();
+
+    ASSERT_FALSE(sigh_void.empty());
+    ASSERT_TRUE(collector_void.vec.empty());
+
+    entt::SigH<int(), TestCollectAll<int>> sigh_all;
+
+    sigh_all.sink().connect<&TestCollectAll<int>::f>();
+    sigh_all.sink().connect<&TestCollectAll<int>::f>();
+    sigh_all.sink().connect<&TestCollectAll<int>::g>();
+    auto collector_all = sigh_all.collect();
+
+    ASSERT_FALSE(sigh_all.empty());
+    ASSERT_FALSE(collector_all.vec.empty());
+    ASSERT_EQ((std::vector<int>::size_type)2, collector_all.vec.size());
+    ASSERT_EQ(42, collector_all.vec[0]);
+    ASSERT_EQ(42, collector_all.vec[1]);
+
+    entt::SigH<int(), TestCollectFirst<int>> sigh_first;
+
+    sigh_first.sink().connect<&TestCollectFirst<int>::f>();
+    sigh_first.sink().connect<&TestCollectFirst<int>::f>();
+    auto collector_first = sigh_first.collect();
+
+    ASSERT_FALSE(sigh_first.empty());
+    ASSERT_FALSE(collector_first.vec.empty());
+    ASSERT_EQ((std::vector<int>::size_type)1, collector_first.vec.size());
+    ASSERT_EQ(42, collector_first.vec[0]);
+}

test/mod/mod.cpp

@@ -0,0 +1,429 @@
+#include <gtest/gtest.h>
+#include <cassert>
+#include <map>
+#include <string>
+#include <duktape.h>
+#include <entt/entity/registry.hpp>
+
+template<typename Type>
+struct tag { using type = Type; };
+
+struct Position {
+    double x;
+    double y;
+};
+
+struct Renderable {};
+
+struct DuktapeRuntime {
+    std::map<duk_uint_t, std::string> components;
+};
+
+template<typename Comp>
+duk_ret_t set(duk_context *ctx, entt::DefaultRegistry &registry) {
+    const auto entity = duk_require_uint(ctx, 0);
+    registry.accommodate<Comp>(entity);
+    return 0;
+}
+
+template<>
+duk_ret_t set<Position>(duk_context *ctx, entt::DefaultRegistry &registry) {
+    const auto entity = duk_require_uint(ctx, 0);
+    const auto x = duk_require_number(ctx, 2);
+    const auto y = duk_require_number(ctx, 3);
+    registry.accommodate<Position>(entity, x, y);
+    return 0;
+}
+
+template<>
+duk_ret_t set<DuktapeRuntime>(duk_context *ctx, entt::DefaultRegistry &registry) {
+    const auto entity = duk_require_uint(ctx, 0);
+    const auto type = duk_require_uint(ctx, 1);
+
+    duk_dup(ctx, 2);
+
+    if(!registry.has<DuktapeRuntime>(entity)) {
+        registry.assign<DuktapeRuntime>(entity).components[type] = duk_json_encode(ctx, -1);
+    } else {
+        registry.get<DuktapeRuntime>(entity).components[type] = duk_json_encode(ctx, -1);
+    }
+
+    duk_pop(ctx);
+
+    return 0;
+}
+
+template<typename Comp>
+duk_ret_t unset(duk_context *ctx, entt::DefaultRegistry &registry) {
+    const auto entity = duk_require_uint(ctx, 0);
+    registry.remove<Comp>(entity);
+    return 0;
+}
+
+template<>
+duk_ret_t unset<DuktapeRuntime>(duk_context *ctx, entt::DefaultRegistry &registry) {
+    const auto entity = duk_require_uint(ctx, 0);
+    const auto type = duk_require_uint(ctx, 1);
+
+    auto &components = registry.get<DuktapeRuntime>(entity).components;
+    assert(components.find(type) != components.cend());
+    components.erase(type);
+
+    if(components.empty()) {
+        registry.remove<DuktapeRuntime>(entity);
+    }
+
+    return 0;
+}
+
+template<typename Comp>
+duk_ret_t has(duk_context *ctx, entt::DefaultRegistry &registry) {
+    const auto entity = duk_require_uint(ctx, 0);
+    duk_push_boolean(ctx, registry.has<Comp>(entity));
+    return 1;
+}
+
+template<>
+duk_ret_t has<DuktapeRuntime>(duk_context *ctx, entt::DefaultRegistry &registry) {
+    const auto entity = duk_require_uint(ctx, 0);
+    duk_push_boolean(ctx, registry.has<DuktapeRuntime>(entity));
+
+    if(registry.has<DuktapeRuntime>(entity)) {
+        const auto type = duk_require_uint(ctx, 1);
+        const auto &components = registry.get<DuktapeRuntime>(entity).components;
+        duk_push_boolean(ctx, components.find(type) != components.cend());
+    } else {
+        duk_push_false(ctx);
+    }
+
+    return 1;
+}
+
+template<typename Comp>
+duk_ret_t get(duk_context *ctx, entt::DefaultRegistry &registry) {
+    assert(registry.has<Comp>(duk_require_uint(ctx, 0)));
+    duk_push_object(ctx);
+    return 1;
+}
+
+template<>
+duk_ret_t get<Position>(duk_context *ctx, entt::DefaultRegistry &registry) {
+    const auto entity = duk_require_uint(ctx, 0);
+    const auto &position = registry.get<Position>(entity);
+
+    const auto idx = duk_push_object(ctx);
+
+    duk_push_string(ctx, "x");
+    duk_push_number(ctx, position.x);
+    duk_def_prop(ctx, idx, DUK_DEFPROP_HAVE_VALUE);
+
+    duk_push_string(ctx, "y");
+    duk_push_number(ctx, position.y);
+    duk_def_prop(ctx, idx, DUK_DEFPROP_HAVE_VALUE);
+
+    return 1;
+}
+
+template<>
+duk_ret_t get<DuktapeRuntime>(duk_context *ctx, entt::DefaultRegistry &registry) {
+    const auto entity = duk_require_uint(ctx, 0);
+    const auto type = duk_require_uint(ctx, 1);
+
+    auto &runtime = registry.get<DuktapeRuntime>(entity);
+    assert(runtime.components.find(type) != runtime.components.cend());
+
+    duk_push_string(ctx, runtime.components[type].c_str());
+    duk_json_decode(ctx, -1);
+
+    return 1;
+}
+
+class DuktapeRegistry {
+    // I'm pretty sure I won't have more than 99 components in the example
+    static constexpr entt::DefaultRegistry::component_type udef = 100;
+
+    struct Func {
+        using func_type = duk_ret_t(*)(duk_context *, entt::DefaultRegistry &);
+        using test_type = bool(entt::DefaultRegistry:: *)(entt::DefaultRegistry::entity_type) const;
+
+        func_type set;
+        func_type unset;
+        func_type has;
+        func_type get;
+        test_type test;
+    };
+
+    template<typename... Comp>
+    void reg() {
+        using accumulator_type = int[];
+        accumulator_type acc = { (func[registry.type<Comp>()] = {
+                                     &::set<Comp>,
+                                     &::unset<Comp>,
+                                     &::has<Comp>,
+                                     &::get<Comp>,
+                                     &entt::DefaultRegistry::has<Comp>
+                                 }, 0)... };
+        (void)acc;
+    }
+
+    static DuktapeRegistry & instance(duk_context *ctx) {
+        duk_push_this(ctx);
+
+        duk_push_string(ctx, DUK_HIDDEN_SYMBOL("dreg"));
+        duk_get_prop(ctx, -2);
+        auto &dreg = *static_cast<DuktapeRegistry *>(duk_require_pointer(ctx, -1));
+        duk_pop_2(ctx);
+
+        return dreg;
+    }
+
+    template<Func::func_type Func::*Op>
+    static duk_ret_t invoke(duk_context *ctx) {
+        auto &dreg = instance(ctx);
+        auto &func = dreg.func;
+        auto &registry = dreg.registry;
+        auto type = duk_require_uint(ctx, 1);
+
+        if(type >= udef) {
+            type = registry.type<DuktapeRuntime>();
+        }
+
+        assert(func.find(type) != func.cend());
+
+        return (func[type].*Op)(ctx, registry);
+    }
+
+public:
+    DuktapeRegistry(entt::DefaultRegistry &registry)
+        : registry{registry}
+    {
+        reg<Position, Renderable, DuktapeRuntime>();
+    }
+
+    static duk_ret_t identifier(duk_context *ctx) {
+        static auto next = udef;
+        duk_push_uint(ctx, next++);
+        return 1;
+    }
+
+    static duk_ret_t create(duk_context *ctx) {
+        auto &dreg = instance(ctx);
+        duk_push_uint(ctx, dreg.registry.create());
+        return 1;
+    }
+
+    static duk_ret_t set(duk_context *ctx) {
+        return invoke<&Func::set>(ctx);
+    }
+
+    static duk_ret_t unset(duk_context *ctx) {
+        return invoke<&Func::unset>(ctx);
+    }
+
+    static duk_ret_t has(duk_context *ctx) {
+        return invoke<&Func::has>(ctx);
+    }
+
+    static duk_ret_t get(duk_context *ctx) {
+        return invoke<&Func::get>(ctx);
+    }
+
+    static duk_ret_t entities(duk_context *ctx) {
+        const duk_idx_t nargs = duk_get_top(ctx);
+        auto &dreg = instance(ctx);
+        duk_uarridx_t pos = 0;
+
+        duk_push_array(ctx);
+
+        dreg.registry.each([ctx, nargs, &pos, &dreg](auto entity) {
+            auto &registry = dreg.registry;
+            auto &func = dreg.func;
+            bool match = true;
+
+            for (duk_idx_t arg = 0; match && arg < nargs; arg++) {
+                auto type = duk_require_uint(ctx, arg);
+
+                if(type < udef) {
+                    assert(func.find(type) != func.cend());
+                    match = (registry.*func[type].test)(entity);
+                } else {
+                    const auto ctype = registry.type<DuktapeRuntime>();
+                    assert(func.find(ctype) != func.cend());
+                    match = (registry.*func[ctype].test)(entity);
+
+                    if(match) {
+                        auto &components = registry.get<DuktapeRuntime>(entity).components;
+                        match = (components.find(type) != components.cend());
+                    }
+                }
+            }
+
+            if(match) {
+                duk_push_uint(ctx, entity);
+                duk_put_prop_index(ctx, -2, pos++);
+            }
+        });
+
+        return 1;
+    }
+
+private:
+    std::map<duk_uint_t, Func> func;
+    entt::DefaultRegistry &registry;
+};
+
+const duk_function_list_entry js_DuktapeRegistry_methods[] = {
+    { "identifier", &DuktapeRegistry::identifier, 0 },
+    { "create", &DuktapeRegistry::create, 0 },
+    { "set", &DuktapeRegistry::set, DUK_VARARGS },
+    { "unset", &DuktapeRegistry::unset, 2 },
+    { "has", &DuktapeRegistry::has, 2 },
+    { "get", &DuktapeRegistry::get, 2 },
+    { "entities", &DuktapeRegistry::entities, DUK_VARARGS },
+    { nullptr, nullptr, 0 }
+};
+
+void exportTypes(duk_context *ctx, entt::DefaultRegistry &registry) {
+    auto exportType = [](auto *ctx, auto &registry, auto idx, auto type, const auto *name) {
+        duk_push_string(ctx, name);
+        duk_push_uint(ctx, registry.template type<typename decltype(type)::type>());
+        duk_def_prop(ctx, idx, DUK_DEFPROP_HAVE_VALUE | DUK_DEFPROP_CLEAR_WRITABLE);
+    };
+
+    auto idx = duk_push_object(ctx);
+
+    exportType(ctx, registry, idx, tag<Position>{}, "POSITION");
+    exportType(ctx, registry, idx, tag<Renderable>{}, "RENDERABLE");
+
+    duk_put_global_string(ctx, "Types");
+}
+
+void exportDuktapeRegistry(duk_context *ctx, DuktapeRegistry &dreg) {
+    auto idx = duk_push_object(ctx);
+
+    duk_push_string(ctx, DUK_HIDDEN_SYMBOL("dreg"));
+    duk_push_pointer(ctx, &dreg);
+    duk_put_prop(ctx, idx);
+
+    duk_put_function_list(ctx, idx, js_DuktapeRegistry_methods);
+    duk_put_global_string(ctx, "Registry");
+}
+
+TEST(Mod, Duktape) {
+    entt::DefaultRegistry registry;
+    DuktapeRegistry dreg{registry};
+    duk_context *ctx = duk_create_heap_default();
+
+    if(!ctx) {
+        FAIL();
+    }
+
+    exportTypes(ctx, registry);
+    exportDuktapeRegistry(ctx, dreg);
+
+    const char *s0 = ""
+            "Types[\"PLAYING_CHARACTER\"] = Registry.identifier();"
+            "Types[\"VELOCITY\"] = Registry.identifier();"
+            "";
+
+    if(duk_peval_string(ctx, s0)) {
+        FAIL();
+    }
+
+    const auto e0 = registry.create();
+    registry.assign<Position>(e0, 0., 0.);
+    registry.assign<Renderable>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<Position>(e1, 0., 0.);
+
+    const char *s1 = ""
+            "Registry.entities(Types.POSITION, Types.RENDERABLE).forEach(function(entity) {"
+                "Registry.set(entity, Types.POSITION, 100., 100.);"
+            "});"
+            "var entity = Registry.create();"
+            "Registry.set(entity, Types.POSITION, 100., 100.);"
+            "Registry.set(entity, Types.RENDERABLE);"
+            "";
+
+    if(duk_peval_string(ctx, s1)) {
+        FAIL();
+    }
+
+    ASSERT_EQ(registry.view<DuktapeRuntime>().size(), 0u);
+    ASSERT_EQ(registry.view<Position>().size(), 3u);
+    ASSERT_EQ(registry.view<Renderable>().size(), 2u);
+
+    registry.view<Position>().each([&registry](auto entity, const auto &position) {
+        ASSERT_FALSE(registry.has<DuktapeRuntime>(entity));
+
+        if(registry.has<Renderable>(entity)) {
+            ASSERT_EQ(position.x, 100.);
+            ASSERT_EQ(position.y, 100.);
+        } else {
+            ASSERT_EQ(position.x, 0.);
+            ASSERT_EQ(position.y, 0.);
+        }
+    });
+
+    const char *s2 = ""
+            "Registry.entities(Types.POSITION).forEach(function(entity) {"
+                "if(!Registry.has(entity, Types.RENDERABLE)) {"
+                    "Registry.set(entity, Types.VELOCITY, { \"dx\": -100., \"dy\": -100. });"
+                    "Registry.set(entity, Types.PLAYING_CHARACTER, {});"
+                "}"
+            "});"
+            "";
+
+    if(duk_peval_string(ctx, s2)) {
+        FAIL();
+    }
+
+    ASSERT_EQ(registry.view<DuktapeRuntime>().size(), 1u);
+    ASSERT_EQ(registry.view<Position>().size(), 3u);
+    ASSERT_EQ(registry.view<Renderable>().size(), 2u);
+
+    registry.view<DuktapeRuntime>().each([](auto, const DuktapeRuntime &runtime) {
+        ASSERT_EQ(runtime.components.size(), 2u);
+    });
+
+    const char *s3 = ""
+            "Registry.entities(Types.POSITION, Types.RENDERABLE, Types.VELOCITY, Types.PLAYING_CHARACTER).forEach(function(entity) {"
+                "var velocity = Registry.get(entity, Types.VELOCITY);"
+                "Registry.set(entity, Types.POSITION, velocity.dx, velocity.dy)"
+            "});"
+            "";
+
+    if(duk_peval_string(ctx, s3)) {
+        FAIL();
+    }
+
+    ASSERT_EQ(registry.view<DuktapeRuntime>().size(), 1u);
+    ASSERT_EQ(registry.view<Position>().size(), 3u);
+    ASSERT_EQ(registry.view<Renderable>().size(), 2u);
+
+    registry.view<Position, Renderable, DuktapeRuntime>().each([](auto, const Position &position, const auto &...) {
+        ASSERT_EQ(position.x, -100.);
+        ASSERT_EQ(position.y, -100.);
+    });
+
+    const char *s4 = ""
+            "Registry.entities(Types.VELOCITY, Types.PLAYING_CHARACTER).forEach(function(entity) {"
+                "Registry.unset(entity, Types.VELOCITY);"
+                "Registry.unset(entity, Types.PLAYING_CHARACTER);"
+            "});"
+            "Registry.entities(Types.POSITION).forEach(function(entity) {"
+                "Registry.unset(entity, Types.POSITION);"
+            "});"
+            "";
+
+    if(duk_peval_string(ctx, s4)) {
+        FAIL();
+    }
+
+    ASSERT_EQ(registry.view<DuktapeRuntime>().size(), 0u);
+    ASSERT_EQ(registry.view<Position>().size(), 0u);
+    ASSERT_EQ(registry.view<Renderable>().size(), 2u);
+
+    duk_destroy_heap(ctx);
+}

test/odr.cpp

@@ -0,0 +1 @@
+#include <entt/entt.hpp>

test/registry.cpp

@@ -1,233 +0,0 @@
-#include <gtest/gtest.h>
-#include <registry.hpp>
-
-TEST(DefaultRegistry, Functionalities) {
-    using registry_type = entt::DefaultRegistry<int, char>;
-
-    registry_type registry;
-
-    ASSERT_EQ(registry.size(), registry_type::size_type{0});
-    ASSERT_EQ(registry.capacity(), registry_type::size_type{0});
-    ASSERT_TRUE(registry.empty());
-
-    ASSERT_TRUE(registry.empty<int>());
-    ASSERT_TRUE(registry.empty<char>());
-
-    registry_type::entity_type e1 = registry.create();
-    registry_type::entity_type e2 = registry.create<int, char>();
-
-    ASSERT_FALSE(registry.empty<int>());
-    ASSERT_FALSE(registry.empty<char>());
-
-    ASSERT_NE(e1, e2);
-
-    ASSERT_FALSE(registry.has<int>(e1));
-    ASSERT_TRUE(registry.has<int>(e2));
-    ASSERT_FALSE(registry.has<char>(e1));
-    ASSERT_TRUE(registry.has<char>(e2));
-    ASSERT_TRUE((registry.has<int, char>(e2)));
-    ASSERT_FALSE((registry.has<int, char>(e1)));
-
-    ASSERT_EQ(registry.assign<int>(e1, 42), 42);
-    ASSERT_EQ(registry.assign<char>(e1, 'c'), 'c');
-    ASSERT_NO_THROW(registry.remove<int>(e2));
-    ASSERT_NO_THROW(registry.remove<char>(e2));
-
-    ASSERT_TRUE(registry.has<int>(e1));
-    ASSERT_FALSE(registry.has<int>(e2));
-    ASSERT_TRUE(registry.has<char>(e1));
-    ASSERT_FALSE(registry.has<char>(e2));
-    ASSERT_TRUE((registry.has<int, char>(e1)));
-    ASSERT_FALSE((registry.has<int, char>(e2)));
-
-    registry_type::entity_type e3 = registry.clone(e1);
-
-    ASSERT_TRUE(registry.has<int>(e3));
-    ASSERT_TRUE(registry.has<char>(e3));
-    ASSERT_EQ(registry.get<int>(e1), 42);
-    ASSERT_EQ(registry.get<char>(e1), 'c');
-    ASSERT_EQ(registry.get<int>(e1), registry.get<int>(e3));
-    ASSERT_EQ(registry.get<char>(e1), registry.get<char>(e3));
-    ASSERT_NE(&registry.get<int>(e1), &registry.get<int>(e3));
-    ASSERT_NE(&registry.get<char>(e1), &registry.get<char>(e3));
-
-    ASSERT_NO_THROW(registry.copy(e2, e1));
-    ASSERT_TRUE(registry.has<int>(e2));
-    ASSERT_TRUE(registry.has<char>(e2));
-    ASSERT_EQ(registry.get<int>(e1), 42);
-    ASSERT_EQ(registry.get<char>(e1), 'c');
-    ASSERT_EQ(registry.get<int>(e1), registry.get<int>(e2));
-    ASSERT_EQ(registry.get<char>(e1), registry.get<char>(e2));
-    ASSERT_NE(&registry.get<int>(e1), &registry.get<int>(e2));
-    ASSERT_NE(&registry.get<char>(e1), &registry.get<char>(e2));
-
-    ASSERT_NO_THROW(registry.replace<int>(e1, 0));
-    ASSERT_EQ(registry.get<int>(e1), 0);
-    ASSERT_NO_THROW(registry.copy<int>(e2, e1));
-    ASSERT_EQ(registry.get<int>(e2), 0);
-    ASSERT_NE(&registry.get<int>(e1), &registry.get<int>(e2));
-
-    ASSERT_NO_THROW(registry.remove<int>(e2));
-    ASSERT_NO_THROW(registry.accomodate<int>(e1, 1));
-    ASSERT_NO_THROW(registry.accomodate<int>(e2, 1));
-    ASSERT_EQ(static_cast<const registry_type &>(registry).get<int>(e1), 1);
-    ASSERT_EQ(static_cast<const registry_type &>(registry).get<int>(e2), 1);
-
-    ASSERT_EQ(registry.size(), registry_type::size_type{3});
-    ASSERT_EQ(registry.capacity(), registry_type::size_type{3});
-    ASSERT_FALSE(registry.empty());
-
-    ASSERT_NO_THROW(registry.destroy(e3));
-
-    ASSERT_TRUE(registry.valid(e1));
-    ASSERT_TRUE(registry.valid(e2));
-    ASSERT_FALSE(registry.valid(e3));
-
-    ASSERT_EQ(registry.size(), registry_type::size_type{2});
-    ASSERT_EQ(registry.capacity(), registry_type::size_type{3});
-    ASSERT_FALSE(registry.empty());
-
-    ASSERT_NO_THROW(registry.reset());
-
-    ASSERT_EQ(registry.size(), registry_type::size_type{0});
-    ASSERT_EQ(registry.capacity(), registry_type::size_type{0});
-    ASSERT_TRUE(registry.empty());
-
-    registry.create<int, char>();
-
-    ASSERT_FALSE(registry.empty<int>());
-    ASSERT_FALSE(registry.empty<char>());
-
-    ASSERT_NO_THROW(registry.reset<int>());
-
-    ASSERT_TRUE(registry.empty<int>());
-    ASSERT_FALSE(registry.empty<char>());
-
-    ASSERT_NO_THROW(registry.reset());
-
-    ASSERT_TRUE(registry.empty<int>());
-    ASSERT_TRUE(registry.empty<char>());
-
-    e1 = registry.create<int>();
-    e2 = registry.create();
-
-    ASSERT_NO_THROW(registry.reset<int>(e1));
-    ASSERT_NO_THROW(registry.reset<int>(e2));
-    ASSERT_TRUE(registry.empty<int>());
-}
-
-TEST(DefaultRegistry, Copy) {
-    using registry_type = entt::DefaultRegistry<int, char, double>;
-
-    registry_type registry;
-
-    registry_type::entity_type e1 = registry.create<int, char>();
-    registry_type::entity_type e2 = registry.create<int, double>();
-
-    ASSERT_TRUE(registry.has<int>(e1));
-    ASSERT_TRUE(registry.has<char>(e1));
-    ASSERT_FALSE(registry.has<double>(e1));
-
-    ASSERT_TRUE(registry.has<int>(e2));
-    ASSERT_FALSE(registry.has<char>(e2));
-    ASSERT_TRUE(registry.has<double>(e2));
-
-    ASSERT_NO_THROW(registry.copy(e2, e1));
-
-    ASSERT_TRUE(registry.has<int>(e1));
-    ASSERT_TRUE(registry.has<char>(e1));
-    ASSERT_FALSE(registry.has<double>(e1));
-
-    ASSERT_TRUE(registry.has<int>(e2));
-    ASSERT_TRUE(registry.has<char>(e2));
-    ASSERT_FALSE(registry.has<double>(e2));
-
-    ASSERT_FALSE(registry.empty<int>());
-    ASSERT_FALSE(registry.empty<char>());
-    ASSERT_TRUE(registry.empty<double>());
-
-    registry.reset();
-}
-
-TEST(DefaultRegistry, ViewSingleComponent) {
-    using registry_type = entt::DefaultRegistry<int, char>;
-
-    registry_type registry;
-
-    registry_type::entity_type e1 = registry.create();
-    registry_type::entity_type e2 = registry.create<int, char>();
-
-    ASSERT_NO_THROW(registry.view<char>().begin()++);
-    ASSERT_NO_THROW(++registry.view<char>().begin());
-
-    auto view = registry.view<char>();
-
-    ASSERT_NE(view.begin(), view.end());
-    ASSERT_EQ(view.size(), typename registry_type::view_type<char>::size_type{1});
-
-    registry.assign<char>(e1);
-
-    ASSERT_EQ(view.size(), typename registry_type::view_type<char>::size_type{2});
-
-    registry.remove<char>(e1);
-    registry.remove<char>(e2);
-
-    ASSERT_EQ(view.begin(), view.end());
-    ASSERT_NO_THROW(registry.reset());
-}
-
-TEST(DefaultRegistry, ViewMultipleComponent) {
-    using registry_type = entt::DefaultRegistry<int, char>;
-
-    registry_type registry;
-
-    registry_type::entity_type e1 = registry.create<char>();
-    registry_type::entity_type e2 = registry.create<int, char>();
-
-    ASSERT_NO_THROW((registry.view<int, char>().begin()++));
-    ASSERT_NO_THROW((++registry.view<int, char>().begin()));
-
-    auto view = registry.view<int, char>();
-
-    ASSERT_NE(view.begin(), view.end());
-
-    registry.remove<char>(e1);
-    registry.remove<char>(e2);
-    view.reset();
-
-    ASSERT_EQ(view.begin(), view.end());
-    ASSERT_NO_THROW(registry.reset());
-}
-
-TEST(DefaultRegistry, EmptyViewSingleComponent) {
-    using registry_type = entt::DefaultRegistry<char, int, double>;
-
-    registry_type registry;
-
-    registry.create<char, double>();
-    registry.create<char>();
-
-    auto view = registry.view<int>();
-
-    ASSERT_EQ(view.size(), registry_type::size_type{0});
-
-    registry.reset();
-}
-
-TEST(DefaultRegistry, EmptyViewMultipleComponent) {
-    using registry_type = entt::DefaultRegistry<char, int, float, double>;
-
-    registry_type registry;
-
-    registry.create<double, int, float>();
-    registry.create<char, float>();
-
-    auto view = registry.view<char, int, float>();
-
-    for(auto entity: view) {
-        (void)entity;
-        FAIL();
-    }
-
-    registry.reset();
-}

test/snapshot/snapshot.cpp

@@ -0,0 +1,181 @@
+#include <gtest/gtest.h>
+#include <sstream>
+#include <vector>
+#include <cereal/archives/json.hpp>
+#include <entt/entity/registry.hpp>
+
+struct Position {
+    float x;
+    float y;
+};
+
+struct Timer {
+    int duration;
+    int elapsed{0};
+};
+
+struct Relationship {
+    entt::DefaultRegistry::entity_type parent;
+};
+
+template<typename Archive>
+void serialize(Archive &archive, Position &position) {
+  archive(position.x, position.y);
+}
+
+template<typename Archive>
+void serialize(Archive &archive, Timer &timer) {
+  archive(timer.duration);
+}
+
+template<typename Archive>
+void serialize(Archive &archive, Relationship &relationship) {
+  archive(relationship.parent);
+}
+
+TEST(Snapshot, Full) {
+    std::stringstream storage;
+
+    entt::DefaultRegistry source;
+    entt::DefaultRegistry destination;
+
+    auto e0 = source.create();
+    source.assign<Position>(e0, 16.f, 16.f);
+
+    source.destroy(source.create());
+
+    auto e1 = source.create();
+    source.assign<Position>(e1, .8f, .0f);
+    source.assign<Relationship>(e1, e0);
+
+    auto e2 = source.create();
+
+    auto e3 = source.create();
+    source.assign<Timer>(e3, 1000, 100);
+
+    source.destroy(e2);
+    auto v2 = source.current(e2);
+
+    {
+        // output finishes flushing its contents when it goes out of scope
+        cereal::JSONOutputArchive output{storage};
+        source.snapshot().entities(output).destroyed(output)
+                .component<Position, Timer, Relationship>(output);
+    }
+
+    cereal::JSONInputArchive input{storage};
+    destination.restore().entities(input).destroyed(input)
+            .component<Position, Timer, Relationship>(input);
+
+    ASSERT_TRUE(destination.valid(e0));
+    ASSERT_TRUE(destination.has<Position>(e0));
+    ASSERT_EQ(destination.get<Position>(e0).x, 16.f);
+    ASSERT_EQ(destination.get<Position>(e0).y, 16.f);
+
+    ASSERT_TRUE(destination.valid(e1));
+    ASSERT_TRUE(destination.has<Position>(e1));
+    ASSERT_EQ(destination.get<Position>(e1).x, .8f);
+    ASSERT_EQ(destination.get<Position>(e1).y, .0f);
+    ASSERT_TRUE(destination.has<Relationship>(e1));
+    ASSERT_EQ(destination.get<Relationship>(e1).parent, e0);
+
+    ASSERT_FALSE(destination.valid(e2));
+    ASSERT_EQ(destination.current(e2), v2);
+
+    ASSERT_TRUE(destination.valid(e3));
+    ASSERT_TRUE(destination.has<Timer>(e3));
+    ASSERT_EQ(destination.get<Timer>(e3).duration, 1000);
+    ASSERT_EQ(destination.get<Timer>(e3).elapsed, 0);
+}
+
+TEST(Snapshot, Continuous) {
+    std::stringstream storage;
+
+    entt::DefaultRegistry source;
+    entt::DefaultRegistry destination;
+
+    std::vector<entt::DefaultRegistry::entity_type> entities;
+    for(auto i = 0; i < 10; ++i) {
+        entities.push_back(source.create());
+    }
+
+    for(auto entity: entities) {
+        source.destroy(entity);
+    }
+
+    auto e0 = source.create();
+    source.assign<Position>(e0, 0.f, 0.f);
+    source.assign<Relationship>(e0, e0);
+
+    auto e1 = source.create();
+    source.assign<Position>(e1, 1.f, 1.f);
+    source.assign<Relationship>(e1, e0);
+
+    auto e2 = source.create();
+    source.assign<Position>(e2, .2f, .2f);
+    source.assign<Relationship>(e2, e0);
+
+    auto e3 = source.create();
+    source.assign<Timer>(e3, 1000, 1000);
+    source.assign<Relationship>(e3, e2);
+
+    {
+        // output finishes flushing its contents when it goes out of scope
+        cereal::JSONOutputArchive output{storage};
+        source.snapshot().entities(output).component<Position, Relationship, Timer>(output);
+    }
+
+    cereal::JSONInputArchive input{storage};
+    entt::ContinuousLoader<entt::DefaultRegistry::entity_type> loader{destination};
+    loader.entities(input)
+            .component<Position, Relationship>(input, &Relationship::parent)
+            .component<Timer>(input);
+
+    ASSERT_FALSE(destination.valid(e0));
+    ASSERT_TRUE(loader.has(e0));
+
+    auto l0 = loader.map(e0);
+
+    ASSERT_TRUE(destination.valid(l0));
+    ASSERT_TRUE(destination.has<Position>(l0));
+    ASSERT_EQ(destination.get<Position>(l0).x, 0.f);
+    ASSERT_EQ(destination.get<Position>(l0).y, 0.f);
+    ASSERT_TRUE(destination.has<Relationship>(l0));
+    ASSERT_EQ(destination.get<Relationship>(l0).parent, l0);
+
+    ASSERT_FALSE(destination.valid(e1));
+    ASSERT_TRUE(loader.has(e1));
+
+    auto l1 = loader.map(e1);
+
+    ASSERT_TRUE(destination.valid(l1));
+    ASSERT_TRUE(destination.has<Position>(l1));
+    ASSERT_EQ(destination.get<Position>(l1).x, 1.f);
+    ASSERT_EQ(destination.get<Position>(l1).y, 1.f);
+    ASSERT_TRUE(destination.has<Relationship>(l1));
+    ASSERT_EQ(destination.get<Relationship>(l1).parent, l0);
+
+    ASSERT_FALSE(destination.valid(e2));
+    ASSERT_TRUE(loader.has(e2));
+
+    auto l2 = loader.map(e2);
+
+    ASSERT_TRUE(destination.valid(l2));
+    ASSERT_TRUE(destination.has<Position>(l2));
+    ASSERT_EQ(destination.get<Position>(l2).x, .2f);
+    ASSERT_EQ(destination.get<Position>(l2).y, .2f);
+    ASSERT_TRUE(destination.has<Relationship>(l2));
+    ASSERT_EQ(destination.get<Relationship>(l2).parent, l0);
+
+    ASSERT_FALSE(destination.valid(e3));
+    ASSERT_TRUE(loader.has(e3));
+
+    auto l3 = loader.map(e3);
+
+    ASSERT_TRUE(destination.valid(l3));
+    ASSERT_TRUE(destination.has<Timer>(l3));
+    ASSERT_EQ(destination.get<Timer>(l3).duration, 1000);
+    ASSERT_EQ(destination.get<Timer>(l3).elapsed, 0);
+    ASSERT_TRUE(destination.has<Relationship>(l3));
+    ASSERT_EQ(destination.get<Relationship>(l3).parent, l2);
+}