updated version.h to v3.2.1

* Fix typo in comment
* Implemented a function for adding a list of properties to meta-item

.github/FUNDING.yml

@@ -0,0 +1,12 @@
+# These are supported funding model platforms
+
+github: skypjack
+patreon: skypjack
+open_collective: # Replace with a single Open Collective username
+ko_fi: # Replace with a single Ko-fi username
+tidelift: # Replace with a single Tidelift platform-name/package-name e.g., npm/babel
+community_bridge: # Replace with a single Community Bridge project-name e.g., cloud-foundry
+liberapay: # Replace with a single Liberapay username
+issuehunt: # Replace with a single IssueHunt username
+otechie: # Replace with a single Otechie username
+custom: https://www.paypal.me/skypjack

.github/workflows/build.yml

@@ -0,0 +1,74 @@
+name: build
+
+on: [push, pull_request]
+
+jobs:
+
+  linux:
+    timeout-minutes: 5
+
+    strategy:
+      matrix:
+        os: [ubuntu-18.04]
+        compiler: [g++, clang++]
+
+    runs-on: ${{ matrix.os }}
+
+    steps:
+    - uses: actions/checkout@v1
+    - name: Compile tests
+      working-directory: build
+      env:
+        CXX: ${{ matrix.compiler }}
+      run: |
+        cmake -DBUILD_TESTING=ON -DBUILD_LIB=ON ..
+        make -j4
+    - name: Run tests
+      working-directory: build
+      env:
+        CTEST_OUTPUT_ON_FAILURE: 1
+      run: ctest --timeout 5 -C Debug -j4
+
+  windows:
+    timeout-minutes: 5
+
+    strategy:
+      matrix:
+        os: [windows-2019, windows-2016]
+        include:
+          - os: windows-2019
+            generator: Visual Studio 16 2019
+          - os: windows-2016
+            generator: Visual Studio 15 2017
+
+    runs-on: ${{ matrix.os }}
+
+    steps:
+    - uses: actions/checkout@v1
+    - name: Compile tests
+      working-directory: build
+      run: |
+        cmake -DBUILD_TESTING=ON -DBUILD_LIB=ON -DCMAKE_CXX_FLAGS=/W1 -G"${{ matrix.generator }}" ..
+        cmake --build . -j 4
+    - name: Run tests
+      working-directory: build
+      env:
+        CTEST_OUTPUT_ON_FAILURE: 1
+      run: ctest --timeout 5 -C Debug -j4
+
+  macos:
+    timeout-minutes: 5
+    runs-on: macOS-10.14
+
+    steps:
+    - uses: actions/checkout@v1
+    - name: Compile tests
+      working-directory: build
+      run: |
+        cmake -DBUILD_TESTING=ON -DBUILD_LIB=ON ..
+        make -j4
+    - name: Run tests
+      working-directory: build
+      env:
+        CTEST_OUTPUT_ON_FAILURE: 1
+      run: ctest --timeout 5 -C Debug -j4

.github/workflows/coverage.yml

@@ -0,0 +1,33 @@
+name: coverage
+
+on: [push, pull_request]
+
+jobs:
+
+  codecov:
+    timeout-minutes: 30
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v1
+    - name: Compile tests
+      working-directory: build
+      env:
+        CXXFLAGS: "-O0 --coverage -fno-inline -fno-inline-small-functions -fno-default-inline"
+        CXX: g++
+      run: |
+        cmake -DBUILD_TESTING=ON -DBUILD_LIB=ON ..
+        make -j4
+    - name: Run tests
+      working-directory: build
+      env:
+        CTEST_OUTPUT_ON_FAILURE: 1
+      run: ctest --timeout 5 -C Debug -j4
+    - name: Upload coverage to Codecov
+      working-directory: build
+      env:
+        CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
+      run: |
+        wget https://codecov.io/bash -O codecov
+        chmod +x codecov
+        ./codecov -t $CODECOV_TOKEN -B $GITHUB_REF -s test/

.github/workflows/deploy.yml

@@ -0,0 +1,33 @@
+name: deploy
+
+on:
+  push:
+    tags:
+      - v*
+
+jobs:
+
+  conan:
+    timeout-minutes: 5
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: docker://conanio/gcc8
+    - uses: actions/checkout@v1
+    - name: Setup Python
+      uses: actions/setup-python@master
+      with:
+        version: 3.6
+    - name: Install
+      run: |
+        chmod +x conan/ci/install.sh
+        ./conan/ci/install.sh
+    - name: Deploy
+      env:
+        CONAN_LOGIN_USERNAME: ${{ secrets.CONAN_LOGIN_USERNAME }}
+        CONAN_PASSWORD: ${{ secrets.CONAN_PASSWORD }}
+        CONAN_UPLOAD: ${{ secrets.CONAN_UPLOAD }}
+        CONAN_GCC_VERSIONS: 8
+      run: |
+        chmod +x conan/ci/build.sh
+        ./conan/ci/build.sh

.gitignore

@@ -1,2 +1,10 @@
-# QtCreator
+# Conan
+conan/test_package/build
+
+# IDEs
 *.user
+.idea
+.vscode
+
+# Bazel
+/bazel-*

.travis.yml

@@ -1,73 +0,0 @@
-language: cpp
-dist: trusty
-sudo: false
-
-matrix:
-  include:
-  - os: linux
-    compiler: gcc
-    addons:
-      apt:
-        sources: ['ubuntu-toolchain-r-test']
-        packages: ['g++-6']
-    env: COMPILER=g++-6
-  - os: linux
-    compiler: gcc
-    addons:
-      apt:
-        sources: ['ubuntu-toolchain-r-test']
-        packages: ['g++-7']
-    env: COMPILER=g++-7
-  - os: linux
-    compiler: clang
-    addons:
-      apt:
-        sources: ['ubuntu-toolchain-r-test', 'llvm-toolchain-trusty-4.0']
-        packages: ['clang-4.0', 'libstdc++-4.9-dev']
-    env: COMPILER=clang++-4.0
-  - 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++-7']
-    env:
-      - 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-7 --gcov-options '\-lp' --root ${TRAVIS_BUILD_DIR} --build-root ${TRAVIS_BUILD_DIR}/build --extension cpp --extension hpp --exclude deps --include src
-
-notifications:
-  email:
-    on_success: never
-    on_failure: always
-
-install:
-- echo ${PATH}
-- cmake --version
-- export CXX=${COMPILER}
-- echo ${CXX}
-- ${CXX} --version
-- ${CXX} -v
-
-script:
-- mkdir -p build && cd build
-- cmake .. && make -j4
-- CTEST_OUTPUT_ON_FAILURE=1 make test

AUTHORS

@@ -1,7 +1,43 @@
 # Author
 
-Michele Caini aka skypjack
+skypjack
 
 # Contributors
 
-Paolo Monteverde aka morbo84
+BenediktConze
+bjadamson
+ceeac
+ColinH
+corystegel
+Croydon
+dbacchet
+dBagrat
+djarek
+DonKult
+drglove
+eugeneko
+gale83
+ghost
+grdowns
+Green-Sky
+Innokentiy-Alaytsev
+Kerndog73
+Lawrencemm
+markand
+mhammerc
+Milerius
+morbo84
+m-waka
+NixAJ
+Paolo-Oliverio
+pgruenbacher
+prowolf
+suVrik
+szunhammer
+The5-1
+vblanco20-1
+willtunnels
+WizardIke
+w1th0utnam3
+xissburg
+zaucy

BUILD.bazel

@@ -0,0 +1,9 @@
+load("//bazel:copts.bzl", "entt_copts")
+
+cc_library(
+    name = "entt",
+    visibility = ["//visibility:public"],
+    strip_include_prefix = "src",
+    hdrs = glob(["src/**/*.h", "src/**/*.hpp"]),
+    copts = entt_copts,
+)

CMakeLists.txt

@@ -2,7 +2,7 @@
 # EnTT
 #
 
-cmake_minimum_required(VERSION 3.2)
+cmake_minimum_required(VERSION 3.7.2)
 
 #
 # Building in-tree is not allowed (we take care of your craziness).
@@ -16,7 +16,9 @@ endif()
 # Project configuration
 #
 
-project(entt VERSION 2.2.0)
+project(EnTT VERSION 3.2.1 LANGUAGES CXX)
+
+include(GNUInstallDirs)
 
 if(NOT CMAKE_BUILD_TYPE)
     set(CMAKE_BUILD_TYPE Debug)
@@ -29,58 +31,171 @@ set(PROJECT_AUTHOR_EMAIL "michele.caini@gmail.com")
 
 message("*")
 message("* ${PROJECT_NAME} v${PROJECT_VERSION} (${CMAKE_BUILD_TYPE})")
-message("* Copyright (c) 2017 ${PROJECT_AUTHOR} <${PROJECT_AUTHOR_EMAIL}>")
+message("* Copyright (c) 2017-2019 ${PROJECT_AUTHOR} <${PROJECT_AUTHOR_EMAIL}>")
 message("*")
 
+option(USE_LIBCPP "Use libc++ by adding -stdlib=libc++ flag if availbale." ON)
+option(USE_ASAN "Use address sanitizer by adding -fsanitize=address -fno-omit-frame-pointer flags" OFF)
+option(USE_COMPILE_OPTIONS "Use compile options from EnTT." ON)
+
 #
 # Compiler stuff
 #
 
-set(CMAKE_CXX_STANDARD 14)
-set(CMAKE_CXX_STANDARD_REQUIRED ON)
+if(NOT WIN32 AND USE_LIBCPP)
+    include(CheckCXXSourceCompiles)
+    include(CMakePushCheckState)
 
-if(NOT MSVC)
-    set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -Wl,--no-undefined")
-    set(CMAKE_CXX_FLAGS "${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")
+    cmake_push_check_state()
 
-    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")
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -stdlib=libc++")
+
+    check_cxx_source_compiles("
+        #include<type_traits>
+        int main() { return std::is_same_v<int, char>; }
+    " HAS_LIBCPP)
+
+    if(NOT HAS_LIBCPP)
+        message(WARNING "The option USE_LIBCPP is set (by default) but libc++ is not available. The flag will not be added to the target.")
     endif()
+
+    cmake_pop_check_state()
 endif()
 
 #
-# CMake configuration
+# Add EnTT target
 #
 
-set(PROJECT_CMAKE_IN ${entt_SOURCE_DIR}/cmake/in)
-set(PROJECT_DEPS_DIR ${entt_SOURCE_DIR}/deps)
-set(PROJECT_SRC_DIR ${entt_SOURCE_DIR}/src)
+add_library(EnTT INTERFACE)
+add_library(EnTT::EnTT ALIAS EnTT)
 
-set(PROJECT_RUNTIME_OUTPUT_DIRECTORY bin)
+configure_file(${EnTT_SOURCE_DIR}/cmake/in/version.h.in ${EnTT_SOURCE_DIR}/src/entt/config/version.h @ONLY)
+
+target_include_directories(
+    EnTT INTERFACE
+    $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/src>
+    $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
+)
+
+target_compile_definitions(
+    EnTT
+    INTERFACE $<$<AND:$<CONFIG:Debug>,$<NOT:$<CXX_COMPILER_ID:MSVC>>>:DEBUG>
+    INTERFACE $<$<AND:$<CONFIG:Release>,$<NOT:$<CXX_COMPILER_ID:MSVC>>>:RELEASE>
+)
+
+if(USE_ASAN)
+    target_compile_options(EnTT INTERFACE $<$<AND:$<CONFIG:Debug>,$<NOT:$<PLATFORM_ID:Windows>>>:-fsanitize=address -fno-omit-frame-pointer>)
+    target_link_libraries(EnTT INTERFACE $<$<AND:$<CONFIG:Debug>,$<NOT:$<PLATFORM_ID:Windows>>>:-fsanitize=address -fno-omit-frame-pointer>)
+endif()
+
+if(USE_COMPILE_OPTIONS)
+    target_compile_options(
+        EnTT
+        INTERFACE $<$<AND:$<CONFIG:Debug>,$<NOT:$<PLATFORM_ID:Windows>>>:-O0 -g>
+        # it seems that -O3 ruins a bit the performance when using clang ...
+        INTERFACE $<$<AND:$<CONFIG:Release>,$<CXX_COMPILER_ID:Clang>,$<OR:$<PLATFORM_ID:Darwin>,$<PLATFORM_ID:Linux>>>:-O2>
+        # ... on the other side, GCC is incredibly comfortable with it.
+        INTERFACE $<$<AND:$<CONFIG:Release>,$<CXX_COMPILER_ID:GNU>>:-O3>
+    )
+endif()
+
+if(HAS_LIBCPP)
+    target_compile_options(EnTT BEFORE INTERFACE -stdlib=libc++)
+endif()
+
+target_compile_features(EnTT INTERFACE cxx_std_17)
+
+#
+# Install EnTT
+#
+
+if(${CMAKE_SYSTEM_NAME} STREQUAL "Windows")
+    set(CUSTOM_INSTALL_CONFIGDIR cmake)
+else()
+    set(CUSTOM_INSTALL_CONFIGDIR ${CMAKE_INSTALL_LIBDIR}/cmake/entt)
+endif()
+
+install(DIRECTORY src/ DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
+install(TARGETS EnTT EXPORT EnTTTargets)
+
+export(EXPORT EnTTTargets FILE ${EnTT_BINARY_DIR}/EnTTTargets.cmake)
+
+install(
+    EXPORT EnTTTargets
+    FILE EnTTTargets.cmake
+    DESTINATION ${CUSTOM_INSTALL_CONFIGDIR}
+    NAMESPACE EnTT::
+)
+
+#
+# Build tree package config file
+#
+
+configure_file(cmake/in/EnTTBuildConfig.cmake.in EnTTConfig.cmake @ONLY)
+
+include(CMakePackageConfigHelpers)
+
+#
+# Install tree package config file
+#
+
+configure_package_config_file(
+    cmake/in/EnTTConfig.cmake.in
+    ${CUSTOM_INSTALL_CONFIGDIR}/EnTTConfig.cmake
+    INSTALL_DESTINATION ${CUSTOM_INSTALL_CONFIGDIR}
+    PATH_VARS CMAKE_INSTALL_INCLUDEDIR
+)
+
+write_basic_package_version_file(
+    ${EnTT_BINARY_DIR}/EnTTConfigVersion.cmake
+    VERSION ${PROJECT_VERSION}
+    COMPATIBILITY AnyNewerVersion
+)
+
+install(
+    FILES
+        ${EnTT_BINARY_DIR}/${CUSTOM_INSTALL_CONFIGDIR}/EnTTConfig.cmake
+        ${EnTT_BINARY_DIR}/EnTTConfigVersion.cmake
+    DESTINATION ${CUSTOM_INSTALL_CONFIGDIR}
+)
+
+export(PACKAGE EnTT)
 
 #
 # Tests
 #
 
-option(BUILD_TESTING "Enable testing with ctest." ON)
+option(BUILD_TESTING "Enable testing with ctest." OFF)
 
 if(BUILD_TESTING)
     set(THREADS_PREFER_PTHREAD_FLAG ON)
     find_package(Threads REQUIRED)
 
-    # gtest, gtest_main, gmock and gmock_main targets are available from now on
-    set(GOOGLETEST_DEPS_DIR ${PROJECT_DEPS_DIR}/googletest)
-    configure_file(${PROJECT_CMAKE_IN}/googletest.in ${GOOGLETEST_DEPS_DIR}/CMakeLists.txt)
-    execute_process(COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" . WORKING_DIRECTORY ${GOOGLETEST_DEPS_DIR})
-    execute_process(COMMAND ${CMAKE_COMMAND} --build . WORKING_DIRECTORY ${GOOGLETEST_DEPS_DIR})
-    set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
-    add_subdirectory(${GOOGLETEST_DEPS_DIR}/src ${GOOGLETEST_DEPS_DIR}/build)
+    option(FIND_GTEST_PACKAGE "Enable finding gtest package." OFF)
+
+    if(FIND_GTEST_PACKAGE)
+        find_package(GTest REQUIRED)
+    else()
+        # gtest, gtest_main, gmock and gmock_main targets are available from now on
+        set(GOOGLETEST_DEPS_DIR ${EnTT_SOURCE_DIR}/deps/googletest)
+        configure_file(${EnTT_SOURCE_DIR}/cmake/in/googletest.in ${GOOGLETEST_DEPS_DIR}/CMakeLists.txt)
+        execute_process(COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" . WORKING_DIRECTORY ${GOOGLETEST_DEPS_DIR})
+        execute_process(COMMAND ${CMAKE_COMMAND} --build . WORKING_DIRECTORY ${GOOGLETEST_DEPS_DIR})
+        set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
+        add_subdirectory(${GOOGLETEST_DEPS_DIR}/src ${GOOGLETEST_DEPS_DIR}/build)
+        target_compile_features(gmock_main PRIVATE $<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_FEATURES>)
+        target_compile_features(gmock PRIVATE $<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_FEATURES>)
+        add_library(GTest::Main ALIAS gtest_main)
+    endif()
+
+    option(BUILD_BENCHMARK "Build benchmark." OFF)
+    option(BUILD_LIB "Build lib example." OFF)
+    option(BUILD_MOD "Build mod example." OFF)
+    option(BUILD_SNAPSHOT "Build snapshot example." OFF)
+
+    if(BUILD_MOD)
+        enable_language(C)
+    endif()
 
     enable_testing()
     add_subdirectory(test)
@@ -90,8 +205,30 @@ endif()
 # Documentation
 #
 
-find_package(Doxygen 1.8)
+option(BUILD_DOCS "Enable building with documentation." OFF)
 
-if(DOXYGEN_FOUND)
-    add_subdirectory(docs)
+if(BUILD_DOCS)
+    find_package(Doxygen 1.8)
+
+    if(DOXYGEN_FOUND)
+        add_subdirectory(docs)
+    endif()
 endif()
+
+#
+# AOB
+#
+
+FILE(GLOB GH_WORKFLOWS .github/workflows/*.yml)
+
+add_custom_target(
+    entt_aob
+    SOURCES
+        ${GH_WORKFLOWS}
+        .github/FUNDING.yml
+        AUTHORS
+        CONTRIBUTING.md
+        LICENSE
+        README.md
+        TODO
+)

CONTRIBUTING.md

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

LICENSE

@@ -1,6 +1,6 @@
 The MIT License (MIT)
 
-Copyright (c) 2017 Michele Caini
+Copyright (c) 2017-2019 Michele Caini
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal

TODO

@@ -0,0 +1,40 @@
+* long term feature: templated generic vm
+* long term feature: shared_ptr less locator
+* long term feature: shared_ptr less resource cache
+* custom allocators and EnTT allocator-aware in general (long term feature, I don't actually need it at the moment) - see #22
+* debugging tools (#60): the issue online already contains interesting tips on this, look at it
+* runner proposal: https://en.wikipedia.org/wiki/Fork%E2%80%93join_model https://slide-rs.github.io/specs/03_dispatcher.html
+* work stealing job system (see #100)
+  - mt scheduler based on const awareness for types
+* meta: sort of meta view based on meta stuff to iterate entities, void * and meta info objects
+* allow for built-in parallel each if possible
+* allow to replace std:: with custom implementations
+* remove runtime views, welcome reflection and what about snapshot?
+* types defined at runtime that refer to the same compile-time type (but to different pools) are possible, the library is almost there
+* add opaque input iterators to views and groups that return tuples <entity, T &...> (proxy), multi-pass guaranteed
+* add fast lane for raw iterations, extend mt doc to describe allowed add/remove with pre-allocations on fast lanes
+* registry.each<T...>(first, last) by iterators, entities/components guaranteed
+* built-in support for dual (or N-) buffering
+* allow for custom stomp functions
+* deprecate/replace snapshot
+* hibitset, views and non-owning groups
+* custom (decoupled) pools ==> double buffering, shared components, multi-model
+* snapshot rework/deprecation
+  - create(hint: entity) -> force-create
+  - assign<T...>(first, last)
+* use unordered_map for named pools and context variables:
+  - use direct access (pool-like) also for context variables
+  - allow for key/value variables where the key is an ENTT_ID_TYPE
+  - improves multi-stomp
+* multi component registry::remove and some others?
+  - auto foo(It first, It last = entity_type{null})
+* range based registry::remove and some others?
+* add examples (and credits) from @alanjfs :)
+* explore the possibility to wrap other backend with a XHandler component
+* use [[nodiscard]] consistently for safety purposes
+* static reflection, hint: template<> meta_type_t<Type>: meta_descriptor<name, func..., props..., etc...>
+* null support for entt::component
+* add ENTT_CUSTOM_NAMED_TYPE for custom names
+* Make another attempt to overcome named types
+* meta: members+class as fake functions, is it possible?
+* named types: almost-stable index optimization for direct access to pools, no more linear searches

WORKSPACE

@@ -0,0 +1,41 @@
+workspace(name = "com_github_skypjack_entt")
+
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+
+http_archive(
+    name = "com_google_googletest",
+    strip_prefix = "googletest-release-1.8.1",
+    url = "https://github.com/google/googletest/archive/release-1.8.1.tar.gz",
+    sha256 = "9bf1fe5182a604b4135edc1a425ae356c9ad15e9b23f9f12a02e80184c3a249c",
+)
+
+http_archive(
+    name = "org_duktape",
+    url = "https://duktape.org/duktape-2.4.0.tar.xz",
+    strip_prefix = "duktape-2.4.0",
+    sha256 = "86a89307d1633b5cedb2c6e56dc86e92679fc34b05be551722d8cc69ab0771fc",
+    build_file_content = """
+cc_library(
+    name = "duktape",
+    visibility = ["//visibility:public"],
+    strip_include_prefix = "src",
+    hdrs = ["src/duktape.h", "src/duk_config.h"],
+    srcs = ["src/duktape.c", "src/duktape.h", "src/duk_config.h"],
+)
+    """,
+)
+
+http_archive(
+    name = "bazelregistry_cereal",
+    strip_prefix = "cereal-8629f40d932d57c5337d4557327f6f22436211b7",
+    url = "https://github.com/bazelregistry/cereal/archive/8629f40d932d57c5337d4557327f6f22436211b7.zip",
+    sha256 = "c983a7a2e16b153c3de022a0818d2f4836e510a3fc3bea9d3703de79f58a90a6",
+)
+
+# This is for bazelregistry_cereal
+http_archive(
+    name = "bazelregistry_rapidjson",
+    strip_prefix = "rapidjson-6b980984dacf689be8f65be823203b967c69da04",
+    url = "https://github.com/bazelregistry/rapidjson/archive/6b980984dacf689be8f65be823203b967c69da04.zip",
+    sha256 = "82187ba8de53bab3b4fb9e56d0bae81a96fa27a115e5a6ce14c70f0ef9338965",
+)

appveyor.yml

@@ -1,22 +0,0 @@
-# can use variables like {build} and {branch}
-version: 1.0.{build}
-
-image: Visual Studio 2017
-
-environment:
-  BUILD_DIR: "%APPVEYOR_BUILD_FOLDER%\\build"
-
-platform:
-  - Win32
-
-configuration:
-  - Release
-
-before_build:
-  - cd %BUILD_DIR%
-  - cmake .. -G"Visual Studio 15 2017"
-
-build:
-  parallel: true
-  project: build/entt.sln
-  verbosity: minimal

bazel/BUILD.bazel

@@ -0,0 +1 @@
+exports_files(["copts.bzl"], visibility = ["//:__subpackages__"])

bazel/copts.bzl

@@ -0,0 +1,10 @@
+_msvc_copts = ["/std:c++17"]
+_gcc_copts = ["-std=c++17"]
+
+entt_copts = select({
+    # Windows
+    "@bazel_tools//src/conditions:windows": _msvc_copts,
+    "@bazel_tools//src/conditions:windows_msvc": _msvc_copts,
+    "@bazel_tools//src/conditions:windows_msys": _msvc_copts,
+    "//conditions:default": _gcc_copts,
+})

cmake/in/EnTTBuildConfig.cmake.in

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

cmake/in/EnTTConfig.cmake.in

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

cmake/in/cereal.in

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

cmake/in/duktape.in

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

cmake/in/googletest.in

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

cmake/in/version.h.in

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

conan/build.py

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

conan/ci/build.sh

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

conan/ci/install.sh

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

conan/test_package/CMakeLists.txt

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

conan/test_package/conanfile.py

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

conan/test_package/test_package.cpp

@@ -0,0 +1,56 @@
+#include <entt/entt.hpp>
+#include <cstdint>
+
+struct position {
+    float x;
+    float y;
+};
+
+struct velocity {
+    float dx;
+    float dy;
+};
+
+void update(entt::registry &registry) {
+    auto view = registry.view<position, velocity>();
+
+    for(auto entity: view) {
+        // gets only the components that are going to be used ...
+
+        auto &vel = view.get<velocity>(entity);
+
+        vel.dx = 0.;
+        vel.dy = 0.;
+
+        // ...
+    }
+}
+
+void update(std::uint64_t dt, entt::registry &registry) {
+    registry.view<position, velocity>().each([dt](auto &pos, auto &vel) {
+        // gets all the components of the view at once ...
+
+        pos.x += vel.dx * dt;
+        pos.y += vel.dy * dt;
+
+        // ...
+    });
+}
+
+int main() {
+    entt::registry registry;
+    std::uint64_t dt = 16;
+
+    for(auto i = 0; i < 10; ++i) {
+        auto entity = registry.create();
+        registry.assign<position>(entity, i * 1.f, i * 1.f);
+        if(i % 2 == 0) { registry.assign<velocity>(entity, i * .1f, i * .1f); }
+    }
+
+    update(dt, registry);
+    update(registry);
+
+    // ...
+
+    return EXIT_SUCCESS;
+}

conanfile.py

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

docs/CMakeLists.txt

@@ -2,26 +2,30 @@
 # Doxygen configuration (documentation)
 #
 
-set(TARGET_DOCS docs)
-
-set(DOXY_IN_FILE doxy.in)
-
-set(DOXY_SOURCE_DIRECTORY ${PROJECT_SRC_DIR})
+set(DOXY_SOURCE_DIRECTORY ${EnTT_SOURCE_DIR}/src)
 set(DOXY_DOCS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})
 set(DOXY_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
-set(DOXY_CFG_FILE doxy.cfg)
 
-configure_file(${DOXY_IN_FILE} ${DOXY_CFG_FILE} @ONLY)
+configure_file(doxy.in doxy.cfg @ONLY)
 
 add_custom_target(
-    ${TARGET_DOCS}
-    COMMAND ${DOXYGEN_EXECUTABLE} ${CMAKE_CURRENT_BINARY_DIR}/${DOXY_CFG_FILE}
-    WORKING_DIRECTORY ${entt_SOURCE_DIR}
+    docs ALL
+    COMMAND ${DOXYGEN_EXECUTABLE} ${CMAKE_CURRENT_BINARY_DIR}/doxy.cfg
+    WORKING_DIRECTORY ${EnTT_SOURCE_DIR}
     VERBATIM
-    SOURCES ${DOXY_IN_FILE}
+    SOURCES doxy.in
 )
 
 install(
     DIRECTORY ${DOXY_OUTPUT_DIRECTORY}/html
     DESTINATION share/${PROJECT_NAME}-${PROJECT_VERSION}/
 )
+
+FILE(GLOB MD_FILES md/*.md)
+
+add_custom_target(
+    docs_aob
+    SOURCES
+        dox/extra.dox
+        ${MD_FILES}
+)

docs/LICENSE

@@ -1,395 +0,0 @@
-Attribution 4.0 International
-
-=======================================================================
-
-Creative Commons Corporation ("Creative Commons") is not a law firm and
-does not provide legal services or legal advice. Distribution of
-Creative Commons public licenses does not create a lawyer-client or
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-terms and conditions, or any related information. Creative Commons
-disclaims all liability for damages resulting from their use to the
-fullest extent possible.
-
-Using Creative Commons Public Licenses
-
-Creative Commons public licenses provide a standard set of terms and
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-original works of authorship and other material subject to copyright
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-
-     Considerations for licensors: Our public licenses are
-     intended for use by those authorized to give the public
-     permission to use material in ways otherwise restricted by
-     copyright and certain other rights. Our licenses are
-     irrevocable. Licensors should read and understand the terms
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-     Licensors should also secure all rights necessary before
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-     licensed material, or material used under an exception or
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-	wiki.creativecommons.org/Considerations_for_licensors
-
-     Considerations for the public: By using one of our public
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-     Although not required by our licenses, you are encouraged to
-     respect those requests where reasonable. More_considerations
-     for the public: 
-	wiki.creativecommons.org/Considerations_for_licensees
-
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-
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-
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docs/doxy.in

@@ -780,7 +780,9 @@ WARN_LOGFILE           =
 # spaces. See also FILE_PATTERNS and EXTENSION_MAPPING
 # Note: If this tag is empty the current directory is searched.
 
-INPUT                  = @DOXY_SOURCE_DIRECTORY@ @DOXY_DOCS_DIRECTORY@ @PROJECT_SOURCE_DIR@/README.md
+INPUT                  = @DOXY_SOURCE_DIRECTORY@ \
+			 @DOXY_DOCS_DIRECTORY@ \
+			 @PROJECT_SOURCE_DIR@/README.md
 
 # This tag can be used to specify the character encoding of the source files
 # that doxygen parses. Internally doxygen uses the UTF-8 encoding. Doxygen uses
@@ -805,9 +807,7 @@ INPUT_ENCODING         = UTF-8
 # *.m, *.markdown, *.md, *.mm, *.dox, *.py, *.pyw, *.f90, *.f95, *.f03, *.f08,
 # *.f, *.for, *.tcl, *.vhd, *.vhdl, *.ucf and *.qsf.
 
-FILE_PATTERNS          = *.cpp \
-                         *.c++ \
-                         *.h \
+FILE_PATTERNS          = *.h \
                          *.hpp \
                          *.md \
                          *.dox

docs/md/core.md

@@ -0,0 +1,184 @@
+# Crash Course: core functionalities
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [Compile-time identifiers](#compile-time-identifiers)
+* [Runtime identifiers](#runtime-identifiers)
+* [Hashed strings](#hashed-strings)
+  * [Wide characters](wide-characters)
+  * [Conflicts](#conflicts)
+* [Monostate](#monostate)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+`EnTT` comes with a bunch of core functionalities mostly used by the other parts
+of the library itself.<br/>
+Hardly users will include these features in their code, but it's worth
+describing what `EnTT` offers so as not to reinvent the wheel in case of need.
+
+# Compile-time identifiers
+
+Sometimes it's useful to be able to give unique identifiers to types at
+compile-time.<br/>
+There are plenty of different solutions out there and I could have used one of
+them. However, I decided to spend my time to define a compact and versatile tool
+that fully embraces what the modern C++ has to offer.
+
+The _result of my efforts_ is the `identifier` class template:
+
+```cpp
+#include <ident.hpp>
+
+// defines the identifiers for the given types
+using id = entt::identifier<a_type, another_type>;
+
+// ...
+
+switch(a_type_identifier) {
+case id::type<a_type>:
+    // ...
+    break;
+case id::type<another_type>:
+    // ...
+    break;
+default:
+    // ...
+}
+```
+
+This is all what the class template has to offer: a `type` inline variable that
+contains a numerical identifier for the given type. It can be used in any
+context where constant expressions are required.
+
+As long as the list remains unchanged, identifiers are also guaranteed to be the
+same for every run. In case they have been used in a production environment and
+a type has to be removed, one can just use a placeholder to left the other
+identifiers unchanged:
+
+```cpp
+template<typename> struct ignore_type {};
+
+using id = entt::identifier<
+    a_type_still_valid,
+    ignore_type<a_type_no_longer_valid>,
+    another_type_still_valid
+>;
+```
+
+A bit ugly to see, but it works at least.
+
+# Runtime identifiers
+
+Sometimes it's useful to be able to give unique identifiers to types at
+runtime.<br/>
+There are plenty of different solutions out there and I could have used one of
+them. In fact, I adapted the most common one to my requirements and used it
+extensively within the entire library.
+
+It's the `family` class. Here is an example of use directly from the
+entity-component system:
+
+```cpp
+using component_family = entt::family<struct internal_registry_component_family>;
+
+// ...
+
+template<typename Component>
+component_type component() const noexcept {
+    return component_family::type<Component>;
+}
+```
+
+This is all what a _family_ has to offer: a `type` inline variable that contains
+a numerical identifier for the given type.
+
+Please, note that identifiers aren't guaranteed to be the same for every run.
+Indeed it mostly depends on the flow of execution.
+
+# Hashed strings
+
+A hashed string is a zero overhead unique identifier. Users can use
+human-readable identifiers in the codebase while using their numeric
+counterparts at runtime, thus without affecting performance.<br/>
+The class has an implicit `constexpr` constructor that chews a bunch of
+characters. Once created, all what one can do with it is getting back the
+original string or converting it into a number.<br/>
+The good part is that a hashed string can be used wherever a constant expression
+is required and no _string-to-number_ conversion will take place at runtime if
+used carefully.
+
+Example of use:
+
+```cpp
+auto load(entt::hashed_string::hash_type resource) {
+    // uses the numeric representation of the resource to load and return it
+}
+
+auto resource = load(entt::hashed_string{"gui/background"});
+```
+
+There is also a _user defined literal_ dedicated to hashed strings to make them
+more user-friendly:
+
+```cpp
+constexpr auto str = "text"_hs;
+```
+
+## Wide characters
+
+The hashed string has a design that is close to that of an `std::basic_string`.
+It means that `hashed_string` is nothing more than an alias for
+`basic_hashed_string<char>`. For those who want to use the C++ type for wide
+character representation, there exists also the alias `hashed_wstring` for
+`basic_hashed_string<wchar_t>`.<br/>
+In this case, the user defined literal to use to create hashed strings on the
+fly is `_hws`:
+
+```cpp
+constexpr auto str = "text"_hws;
+```
+
+Note that the hash type of the `hashed_wstring` is the same of its counterpart.
+
+## Conflicts
+
+The hashed string class uses internally FNV-1a to compute the numeric
+counterpart of a string. Because of the _pigeonhole principle_, conflicts are
+possible. This is a fact.<br/>
+There is no silver bullet to solve the problem of conflicts when dealing with
+hashing functions. In this case, the best solution seemed to be to give up.
+That's all.<br/>
+After all, human-readable unique identifiers aren't something strictly defined
+and over which users have not the control. Choosing a slightly different
+identifier is probably the best solution to make the conflict disappear in this
+case.
+
+# Monostate
+
+The monostate pattern is often presented as an alternative to a singleton based
+configuration system. This is exactly its purpose in `EnTT`. Moreover, this
+implementation is thread safe by design (hopefully).<br/>
+Keys are represented by hashed strings, values are basic types like `int`s or
+`bool`s. Values of different types can be associated to each key, even more than
+one at a time. Because of this, users must pay attention to use the same type
+both during an assignment and when they try to read back their data. Otherwise,
+they will probably incur in unexpected results.
+
+Example of use:
+
+```cpp
+entt::monostate<entt::hashed_string{"mykey"}>{} = true;
+entt::monostate<"mykey"_hs>{} = 42;
+
+// ...
+
+const bool b = entt::monostate<"mykey"_hs>{};
+const int i = entt::monostate<entt::hashed_string{"mykey"}>{};
+```

docs/md/faq.md

@@ -0,0 +1,199 @@
+# Frequently Asked Questions
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [FAQ](#faq)
+  * [Why is my debug build on Windows so slow?](#why-is-my-debug-build-on-windows-so-slow)
+  * [How can I represent hierarchies with my components?](#how-can-i-represent-hierarchies-with-my-components)
+  * [Custom entity identifiers: yay or nay?](#custom-entity-identifiers-yay-or-nay)
+  * [Warning C4307: integral constant overflow](#warning-C4307-integral-constant-overflow)
+  * [Warning C4003: the min, the max and the macro](#warning-C4003-the-min-the-max-and-the-macro)
+  * [The standard and the non-copyable types](#the-standard-and-the-non-copyable-types)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+This is a constantly updated section where I'll try to put the answers to the
+most frequently asked questions.<br/>
+If you don't find your answer here, there are two cases: nobody has done it yet
+or this section needs updating. In both cases, try to
+[open a new issue](https://github.com/skypjack/entt/issues/new) or enter the
+[gitter channel](https://gitter.im/skypjack/entt) and ask your question.
+Probably someone already has an answer for you and we can then integrate this
+part of the documentation.
+
+# FAQ
+
+## Why is my debug build on Windows so slow?
+
+`EnTT` is an experimental project that I also use to keep me up-to-date with the
+latest revision of the language and the standard library. For this reason, it's
+likely that some classes you're working with are using standard containers under
+the hood.<br/>
+Unfortunately, it's known that the standard containers aren't particularly
+performing in debugging (the reasons for this go beyond this document) and are
+even less so on Windows apparently. Fortunately this can also be mitigated a
+lot, achieving good results in many cases.
+
+First of all, there are two things to do in a Windows project:
+
+* Disable the [`/JMC`](https://docs.microsoft.com/cpp/build/reference/jmc)
+  option (_Just My Code_ debugging), available starting in Visual Studio 2017
+  version 15.8.
+
+* Set the [`_ITERATOR_DEBUG_LEVEL`](https://docs.microsoft.com/cpp/standard-library/iterator-debug-level)
+  macro to 0. This will disable checked iterators and iterator debugging.
+
+Moreover, the macro `ENTT_DISABLE_ASSERT` should be defined to disable internal
+checks made by `EnTT` in debug. These are asserts introduced to help the users,
+but require to access to the underlying containers and therefore risk ruining
+the performance in some cases.
+
+With these changes, debug performance should increase enough for most cases. If
+you want something more, you can can also switch to an optimization level `O0`
+or preferably `O1`.
+
+## How can I represent hierarchies with my components?
+
+This is one of the first questions that anyone makes when starting to work with
+the entity-component-system architectural pattern.<br/>
+There are several approaches to the problem and what’s the best one depends
+mainly on the real problem one is facing. In all cases, how to do it doesn't
+strictly depend on the library in use, but the latter can certainly allow or
+not different techniques depending on how the data are laid out.
+
+I tried to describe some of the techniques that fit well with the model of
+`EnTT`. [Here](https://skypjack.github.io/2019-06-25-ecs-baf-part-4/) is the
+first post of a series that tries to explore the problem. More will probably
+come in future.
+
+Long story short, you can always define a tree where the nodes expose implicit
+lists of children by means of the following type:
+
+```cpp
+struct relationship {
+    std::size_t children{};
+    entt::entity first{entt::null};
+    entt::entity prev{entt::null};
+    entt::entity next{entt::null};
+    entt::entity parent{entt::null};
+    // ... other data members ...
+};
+```
+
+The sort functionalities of `EnTT`, the groups and all the other features of the
+library can help then to get the best in terms of data locality and therefore
+performance from this component.
+
+## Custom entity identifiers: yay or nay?
+
+Custom entity identifiers are definitely a good idea in two cases at least:
+
+* If `std::uint32_t` isn't large enough as an underlying type.
+* If you want to avoid conflicts when using multiple registries.
+
+These identifiers are nothing more than enum classes with some salt.<br/>
+To simplify the creation of new identifiers, `EnTT` provides the macro
+`ENTT_OPAQUE_TYPE` that accepts two arguments:
+
+* The name you want to give to the new identifier (watch out for namespaces).
+* The underlying type to use (either `std::uint16_t`, `std::uint32_t`
+  or `std::uint64_t`).
+
+In fact, this is the definition of `entt::entity`:
+
+```cpp
+ENTT_OPAQUE_TYPE(entity, std::uint32_t)
+```
+
+The use of this macro is highly recommended, so as not to run into problems if
+the requirements for the identifiers should change in the future.
+
+## Warning C4307: integral constant overflow
+
+According to [this](https://github.com/skypjack/entt/issues/121) issue, using a
+hashed string under VS could generate a warning.<br/>
+First of all, I want to reassure you: it's expected and harmless. However, it
+can be annoying.
+
+To suppress it and if you don't want to suppress all the other warnings as well,
+here is a workaround in the form of a macro:
+
+```cpp
+#if defined(_MSC_VER)
+    #define HS(str)\
+        __pragma(warning(push))\
+        __pragma(warning(disable:4307))\
+        entt::hashed_string{str}\
+        __pragma(warning(pop))
+#else
+    #define HS(str) entt::hashed_string{str}
+#endif
+```
+
+With an example of use included:
+
+```cpp
+constexpr auto identifier = HS("my/resource/identifier");
+```
+
+Thanks to [huwpascoe](https://github.com/huwpascoe) for the courtesy.
+
+## Warning C4003: the min, the max and the macro
+
+On Windows, a header file defines two macros `min` and `max` which may result in
+conflicts with their counterparts in the standard library and therefore in
+errors during compilation.
+
+It's a pretty big problem but fortunately it's not a problem of `EnTT` and there
+is a fairly simple solution to it.<br/>
+It consists in defining the `NOMINMAX` macro before to include any other header
+so as to get rid of the extra definitions:
+
+```cpp
+#define NOMINMAX
+```
+
+Please refer to [this](https://github.com/skypjack/entt/issues/96) issue for
+more details.
+
+## The standard and the non-copyable types
+
+`EnTT` uses internally the trait `std::is_copy_constructible_v` to check if a
+component is actually copyable. This trait doesn't check if an object can
+actually be copied but only verifies if there is a copy constructor
+available.<br/>
+This can lead to surprising results due to some idiosyncrasies of the standard
+mainly related to the need to guarantee backward compatibility.
+
+For example, `std::vector` defines a copy constructor no matter if its value
+type is copyable or not. As a result, `std::is_copy_constructible_v` is true
+for the following specialization:
+
+```cpp
+struct type {
+    std::vector<std::unique_ptr<action>> vec;
+};
+```
+
+When trying to assign an instance of this type to an entity in the ECS part,
+this may trigger a compilation error because we cannot really make a copy of
+it.<br/>
+As a workaround, users can mark the type explicitly as non-copyable:
+
+```cpp
+struct type {
+    type(const type &) = delete;
+    type & operator=(const type &) = delete;
+
+    std::vector<std::unique_ptr<action>> vec;
+};
+```
+
+Unfortunately, this will also disable aggregate initialization.

docs/md/lib.md

@@ -0,0 +1,196 @@
+# Push EnTT across boundaries
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [Named types and traits class](#named-types-and-traits-class)
+  * [Do not mix types](#do-not-mix-types)
+* [Macros, macros everywhere](#macros-macros-everywhere)
+  * [Conflicts](#conflicts)
+* [Runtime reflection system](#runtime-reflection-system)
+* [Allocations: the dark side of the force](#allocations-the-dark-side-of-the-force)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+`EnTT` has historically had a limit when used across boundaries on Windows in
+general and on GNU/Linux when default visibility was set to _hidden_. The
+limitation is due mainly to a custom utility used to assign unique, sequential
+identifiers to different types. Unfortunately, this tool is used by several core
+classes (the `registry` among the others) that are thus almost unusable across
+boundaries.<br/>
+The reasons for that are beyond the purposes of this document. However, the good
+news is that `EnTT` also offers now a way to overcome this limit and to push
+things across boundaries without problems when needed.
+
+# Named types and traits class
+
+To allow a type to work properly across boundaries when used by a class that
+requires to assign unique identifiers to types, users must specialize a class
+template to literally give a compile-time name to the type itself.<br/>
+The name of the class template is `name_type_traits` and the specialization must
+be such that it exposes a static constexpr data member named `value` having type
+either `ENTT_ID_TYPE` or `entt::hashed_string::hash_type`. Its value is the user
+defined unique identifier assigned to the specific type.<br/>
+Identifiers are not to be sequentially generated in this case.
+
+As an example:
+
+```cpp
+struct my_type { /* ... */ };
+
+template<>
+struct entt::named_type_traits<my_type> {
+    static constexpr auto value = "my_type"_hs;
+};
+```
+
+Because of the rules of the language, the specialization must reside in the
+global namespace or in the `entt` namespace. There is no way to change this rule
+unfortunately, because it doesn't depend on the library itself.
+
+The good aspect of this approach is that it's not intrusive at all. The other
+way around was in fact forcing users to inherit all their classes from a common
+base. Something to avoid, at least from my point of view.<br/>
+However, despite the fact that it's not intrusive, it would be great if it was
+also easier to use and a bit less error-prone. This is why a bunch of macros
+exist to ease defining named types.
+
+## Do not mix types
+
+Someone might think that this trick is valid only for the types to push across
+boundaries. This isn't how things work. In fact, the problem is more complex
+than that.<br/>
+As a rule of thumb, users should never mix named and non-named types. Whenever
+a type is given a name, all the types must be given a name. As an example,
+consider the `registry` class template: in case it is pushed across boundaries,
+all the types of components should be assigned a name to avoid subtle bugs.
+
+Indeed, this constraint can be relaxed in many cases. However, it is difficult
+to define a general rule to follow that is not the most stringent, unless users
+know exactly what they are doing. Therefore, I won't elaborate on giving further
+details on the topic.
+
+# Macros, macros everywhere
+
+The library comes with a set of predefined macros to use to declare named types
+or export already existing ones. In particular:
+
+* `ENTT_NAMED_TYPE` can be used to assign a name to already existing types. This
+  macro must be used in the global namespace even when the types to be named are
+  not.
+
+  ```cpp
+  ENTT_NAMED_TYPE(my_type)
+  ENTT_NAMED_TYPE(ns::another_type)
+  ```
+
+* `ENTT_NAMED_STRUCT` can be used to define and export a struct at the same
+  time. It accepts also an optional namespace in which to define the given type.
+  This macro must be used in the global namespace.
+
+  ```cpp
+  ENTT_NAMED_STRUCT(my_type, { /* struct definition */})
+  ENTT_NAMED_STRUCT(ns, another_type, { /* struct definition */})
+  ```
+
+* `ENTT_NAMED_CLASS` can be used to define and export a class at the same
+  time. It accepts also an optional namespace in which to define the given type.
+  This macro must be used in the global namespace.
+
+  ```cpp
+  ENTT_NAMED_CLASS(my_type, { /* class definition */})
+  ENTT_NAMED_CLASS(ns, another_type, { /* class definition */})
+  ```
+
+Nested namespaces are supported out of the box as well in all cases. As an
+example:
+
+```cpp
+ENTT_NAMED_STRUCT(nested::ns, my_type, { /* struct definition */})
+```
+
+These macros can be used to avoid specializing the `named_type_traits` class
+template. In all cases, the name of the class is used also as a seed to generate
+the compile-time unique identifier.
+
+## Conflicts
+
+When using macros, unique identifiers are 32/64 bit integers generated by
+hashing strings during compilation. Therefore, conflicts are rare but still
+possible. In case of conflicts, everything simply will get broken at runtime and
+the strangest things will probably take place.<br/>
+Unfortunately, there is no safe way to prevent it. If this happens, it will be
+enough to give a different value to one of the conflicting types to solve the
+problem. To do this, users can either assign a different name to the class or
+directly define a specialization for the `named_type_traits` class template.
+
+# Runtime reflection system
+
+The runtime reflection system deserves a special mention when it comes to using
+it across boundaries.<br/>
+As in all other cases, it's necessary to give a name to the types. However, this
+time this isn't enough to get the job done.
+
+The runtime reflection system is linked to a static context to which the visible
+components are linked. A component is visible when it's given a name, so the
+named components are implicitly visible.<br/>
+Different contexts don't relate to each other. Therefore, to allow the use of
+named types across boundaries, a context must also be shared.
+
+Sharing a context is straightforward. First of all, the local one must be
+acquired:
+
+```
+entt::meta_ctx ctx{};
+```
+
+Then, it must be pushed across boundaries and the receiving space must set it as
+its global context, thus releasing the local one that remains available but is
+no longer referred to by the runtime reflection system:
+
+```
+entt::meta_ctx::bind(ctx);
+```
+
+From now on, both spaces will refer to the same context and on it will be
+associated the new visible meta types, no matter _where_ they are created.
+
+A context can also be reset and then associated again locally as:
+
+```
+entt::meta_ctx::bind{entt::meta_ctx{});
+```
+
+This is allowed because local and global contexts are separated. Therefore, it's
+always possible to make the local context the current one again.
+
+# Allocations: the dark side of the force
+
+As long as `EnTT` won't support custom allocators, another problem with
+allocations will remain alive instead. This is in fact easily solved, or at
+least it is if one knows it.
+
+To allow users to add types dynamically, the library makes extensive use of type
+erasure techniques and dynamic allocations for pools (whether they are for
+components, events or anything else). The problem occurs when, for example, a
+registry is created on one side of a boundary and a pool is dynamically created
+on the other side. In the best case, everything will crash at the exit, while at
+worst it will do so at runtime.<br/>
+To avoid problems, the pools must be generated from the same side of the
+boundary where the object that owns them is also created. As an example, when
+the registry is created in the main executable and used across boundaries for a
+given type of component, the pool for that type must be created before passing
+around the registry itself. To do this is fortunately quite easy, since it is
+sufficient to invoke any of the methods that involve the given type (continuing
+the example with the registry, a call to `reserve` or `size` is more than
+enough).
+
+Maybe one day some dedicated methods will be added that do nothing but create a
+pool for a given type. Until now it has been preferred to keep the API cleaner
+as they are not strictly necessary.

docs/md/links.md

@@ -0,0 +1,115 @@
+# EnTT in Action
+
+`EnTT` is widely used in private and commercial applications. I cannot even
+mention most of them because of some signatures I put on some documents time
+ago. Fortunately, there are also people who took the time to implement open
+source projects based on `EnTT` and did not hold back when it came to
+documenting them.
+
+Below an incomplete list of games, applications and articles that can be used as
+a reference. Where I put the word _apparently_ means that the use of `EnTT` is
+documented but the authors didn't make explicit announcements or contacted me
+directly.
+
+I hope this list can grow much more in the future:
+
+* Games:
+  * [Minecraft](https://minecraft.net/en-us/attribution/) by
+    [Mojang](https://mojang.com/): of course, **that** Minecraft, see the
+    open source attributions page for more details.
+  * [Land of the Rair](https://github.com/LandOfTheRair/core2): the new backend
+    of [a retro-style MUD](https://rair.land/) for the new age.
+  * [Openblack](https://github.com/openblack/openblack): open source
+    reimplementation of the game _Black & White_ (2001).
+  * [Face Smash](https://play.google.com/store/apps/details?id=com.gamee.facesmash):
+    a game to play with your face.
+  * [EnTT Pacman](https://github.com/Kerndog73/EnTT-Pacman): an example of how
+    to make Pacman with `EnTT`.
+  * [Wacman](https://github.com/carlfindahl/wacman): a pacman clone with OpenGL.
+  * [Classic Tower Defence](https://github.com/kerndog73/Classic-Tower-Defence):
+    a tiny little tower defence game featuring a homemade font.
+    [Check it out](https://indi-kernick.itch.io/classic-tower-defence).
+  * [The Machine](https://github.com/Kerndog73/The-Machine): a box pushing
+    puzzler with logic gates and other cool stuff.
+    [Check it out](https://indi-kernick.itch.io/the-machine-web-version).
+  * [EnttPong](https://github.com/reworks/EnttPong): an example of how to make
+    Pong with `EnTT`.
+  * [Randballs](https://github.com/gale93/randballs): simple `SFML` and `EnTT`
+    playground.
+  * [EnTT Tower Defense](https://github.com/Daivuk/tddod): a data oriented tower
+    defense example.
+  * [EnTT Breakout](https://github.com/vblanco20-1/entt-breakout): simple
+    example of a breakout game, using `SDL` and `EnTT`.
+  * [Arcade puzzle game with EnTT](https://github.com/MasonRG/ArcadePuzzleGame):
+    arcade puzzle game made in C++ using the `SDL2` and `EnTT` libraries.
+  * [Snake with EnTT](https://github.com/MasonRG/SnakeGame): simple snake game
+    made in C++ with the `SDL2` and `EnTT` libraries.
+  * [Mirrors lasers and robots](https://github.com/guillaume-haerinck/imac-tower-defense):
+    a small tower defense game based on mirror orientation.
+
+* Engines and the like:
+  * [Fling Engine](https://github.com/flingengine/FlingEngine): A Vulkan game
+    engine with a focus on data oriented design.
+  * [Apparently](https://teamwisp.github.io/credits/)
+    [Wisp](https://teamwisp.github.io/product/) by
+    [Team Wisp](https://teamwisp.github.io/): an advanced real-time ray tracing
+    renderer built for the demands of video game artists.
+  * [starlight](https://github.com/DomRe/starlight): game programming framework
+    using `Allegro`, `Lua` and modern C++.
+  * [Apparently](https://github.com/JosiahWI/qub3d-libdeps)
+    [Qub3d](https://qub3d.org/): because blocks should be open source.
+  * [shiva](https://github.com/Milerius/shiva): modern C++ engine with
+    modularity.
+  * [NovusCore](https://github.com/novuscore/NovusCore): A modern take on World
+    of Warcraft emulation.
+  * [ImGui/EnTT editor](https://github.com/Green-Sky/imgui_entt_entity_editor):
+    A drop-in, single-file entity editor for `EnTT` that uses `ImGui` as
+    graphical backend (with
+    [demo code](https://github.com/Green-Sky/imgui_entt_entity_editor_demo)).
+
+* Articles and blog posts:
+  * [Some posts](https://skypjack.github.io/tags/#entt) on my personal
+    [blog](https://skypjack.github.io/) are about `EnTT`, for those who want to
+    know **more** on this project.
+  * [Space Battle: Huge edition](http://victor.madtriangles.com/code%20experiment/2018/06/11/post-ecs-battle-huge.html):
+    huge space battle built entirely from scratch.
+  * [Space Battle](https://github.com/vblanco20-1/ECS_SpaceBattle): huge space
+    battle built on `UE4`.
+  * [Experimenting with ECS in UE4](http://victor.madtriangles.com/code%20experiment/2018/03/25/post-ue4-ecs-battle.html):
+    interesting article about `UE4` and `EnTT`.
+  * [Implementing ECS architecture in UE4](https://forums.unrealengine.com/development-discussion/c-gameplay-programming/1449913-implementing-ecs-architecture-in-ue4-giant-space-battle):
+    giant space battle.
+  * [Conan Adventures (SFML and EnTT in C++)](https://leinnan.github.io/blog/conan-adventuressfml-and-entt-in-c.html):
+    create projects in modern C++ using `SFML`, `EnTT`, `Conan` and `CMake`.
+
+* Any Other Business:
+  * The [ArcGIS Runtime SDKs](https://developers.arcgis.com/arcgis-runtime/)
+    by [Esri](https://www.esri.com/): they use `EnTT` for the internal ECS and
+    the cross platform C++ rendering engine. The SDKs are utilized by a lot of
+    enterprise custom apps, as well as by Esri for its own public applications
+    such as
+    [Explorer](https://play.google.com/store/apps/details?id=com.esri.explorer),
+    [Collector](https://play.google.com/store/apps/details?id=com.esri.arcgis.collector)
+    and
+    [Navigator](https://play.google.com/store/apps/details?id=com.esri.navigator).
+  * [Apparently](https://www.linkedin.com/in/skypjack/)
+    [NIO](https://www.nio.io/): there was a collaboration to make some changes
+    to `EnTT`, at the time used for internal projects.
+  * [Apparently](https://www.linkedin.com/jobs/view/architekt-c%2B%2B-at-tieto-1219512333/)
+    [Tieto](https://www.tieto.com/): they published a job post where `EnTT` was
+    listed on their software stack.
+  * [Godot meets EnTT](https://github.com/portaloffreedom/godot_entt_example/):
+    A simple example on how to use `EnTT` within
+    [`Godot`](https://godotengine.org/).
+  * [Godot and GameNetworkingSockets meet EnTT](https://github.com/portaloffreedom/godot_entt_net_example):
+    A simple example on how to use `EnTT` and
+    [`GameNetworkingSockets`](https://github.com/ValveSoftware/GameNetworkingSockets)
+    within [`Godot`](https://godotengine.org/).
+  * [MatchOneEntt](https://github.com/mhaemmerle/MatchOneEntt): port of
+    [Match One](https://github.com/sschmid/Match-One) for `Entitas-CSharp`.
+  * GitHub contains also
+    [many other examples](https://github.com/search?o=desc&q=%22skypjack%2Fentt%22&s=indexed&type=Code)
+    of use of `EnTT` from which to take inspiration if interested.
+
+If you know of other resources out there that are about `EnTT`, feel free to
+open an issue or a PR and I'll be glad to add them to this page.

docs/md/locator.md

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

docs/md/meta.md

@@ -0,0 +1,585 @@
+# Crash Course: runtime reflection system
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [Names and identifiers](#names-and-identifiers)
+* [Reflection in a nutshell](#reflection-in-a-nutshell)
+  * [Any as in any type](#any-as-in-any-type)
+  * [Enjoy the runtime](#enjoy-the-runtime)
+  * [Policies: the more, the less](#policies-the-more-the-less)
+  * [Named constants and enums](#named-constants-and-enums)
+  * [Properties and meta objects](#properties-and-meta-objects)
+  * [Unregister types](#unregister-types)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+Reflection (or rather, its lack) is a trending topic in the C++ world and, in
+the specific case of `EnTT`, a tool that can unlock a lot of other features. I
+looked for a third-party library that met my needs on the subject, but I always
+came across some details that I didn't like: macros, being intrusive, too many
+allocations. In one word: unsatisfactory.<br/>
+I finally decided to write a built-in, non-intrusive and macro-free runtime
+reflection system for `EnTT`. Maybe I didn't do better than others or maybe yes,
+time will tell me, but at least I can model this tool around the library to
+which it belongs and not vice versa.
+
+# Names and identifiers
+
+The meta system doesn't force the user to use the tools provided by the library
+when it comes to working with names and identifiers. It does this by offering an
+API that works with opaque identifiers that may or may not be generated by means
+of a hashed string.<br/>
+This means that users can assign any type of identifier to the meta objects, as
+long as they are numeric. It doesn't matter if they are generated at runtime, at
+compile-time or with custom functions.
+
+However, the examples in the following sections are all based on the
+`hashed_string` class as provided by this library. Therefore, where an
+identifier is required, it's likely that a user defined literal is used as
+follows:
+
+```cpp
+auto factory = entt::meta<my_type>().type("reflected_type"_hs);
+```
+
+For what it's worth, this is likely completely equivalent to:
+
+```cpp
+auto factory = entt::meta<my_type>().type(42);
+```
+
+Obviously, human-readable identifiers are more convenient to use and highly
+recommended.
+
+# Reflection in a nutshell
+
+Reflection always starts from real types (users cannot reflect imaginary types
+and it would not make much sense, we wouldn't be talking about reflection
+anymore).<br/>
+To create a meta node, the library provides the `meta` function that accepts a
+type to reflect as a template parameter:
+
+```cpp
+auto factory = entt::meta<my_type>();
+```
+
+This isn't enough to _export_ the given type and make it visible though.<br/>
+The returned value is a factory object to use to continue building the meta
+type. In order to make the type _visible_, users can assign it an identifier:
+
+```cpp
+auto factory = entt::meta<my_type>().type("reflected_type"_hs);
+```
+
+When working with named types, it isn't even necessary to specify the
+identifier. In fact, it isn't allowed and it will trigger a compilation
+error.<br/>
+Identifiers are important because users can retrieve meta types at runtime by
+searching for them by _name_ other than by type. On the other hand, there are
+cases in which users can be interested in adding features to a reflected type so
+that the reflection system can use it correctly under the hood, but they don't
+want to allow searching the type by _name_. In this case, it's sufficient not
+to invoke `type` and the type will not be searchable _by name_.
+
+A factory is such that all its member functions returns the factory itself or
+a decorated version of it. This object can be used to add the following:
+
+* _Constructors_. Actual constructors can be assigned to a reflected type by
+  specifying their list of arguments. Free functions (namely, factories) can be
+  used as well, as long as the return type is the expected one. From a client's
+  point of view, nothing changes if a constructor is a free function or an
+  actual constructor.<br/>
+  Use the `ctor` member function for this purpose:
+
+  ```cpp
+  entt::meta<my_type>().ctor<int, char>().ctor<&factory>();
+  ```
+
+* _Destructors_. Free functions can be set as destructors of reflected types.
+  The purpose is to give users the ability to free up resources that require
+  special treatment before an object is actually destroyed.<br/>
+  Use the `dtor` member function for this purpose:
+
+  ```cpp
+  entt::meta<my_type>().dtor<&destroy>();
+  ```
+
+  A function should neither delete nor explicitly invoke the destructor of a
+  given instance.
+
+* _Data members_. Both real data members of the underlying type and static and
+  global variables, as well as constants of any kind, can be attached to a meta
+  type. From a client's point of view, all the variables associated with the
+  reflected type will appear as if they were part of the type itself.<br/>
+  Use the `data` member function for this purpose:
+
+  ```cpp
+  entt::meta<my_type>()
+      .data<&my_type::static_variable>("static"_hs)
+      .data<&my_type::data_member>("member"_hs)
+      .data<&global_variable>("global"_hs);
+  ```
+
+  This function requires as an argument the identifier to give to the meta data
+  once created. Users can then access meta data at runtime by searching for them
+  by _name_.<br/>
+  Data members can be set also by means of a couple of functions, namely a
+  setter and a getter. Setters and getters can be either free functions, member
+  functions or mixed ones, as long as they respect the required signatures.<br/>
+  Refer to the inline documentation for all the details.
+
+* _Member functions_. Both real member functions of the underlying type and free
+  functions can be attached to a meta type. From a client's point of view, all
+  the functions associated with the reflected type will appear as if they were
+  part of the type itself.<br/>
+  Use the `func` member function for this purpose:
+
+  ```cpp
+  entt::meta<my_type>()
+      .func<&my_type::static_function>("static"_hs)
+      .func<&my_type::member_function>("member"_hs)
+      .func<&free_function>("free"_hs);
+  ```
+
+  This function requires as an argument the identifier to give to the meta
+  function once created. Users can then access meta functions at runtime by
+  searching for them by _name_.
+
+* _Base classes_. A base class is such that the underlying type is actually
+  derived from it. In this case, the reflection system tracks the relationship
+  and allows for implicit casts at runtime when required.<br/>
+  Use the `base` member function for this purpose:
+
+  ```cpp
+  entt::meta<derived_type>().base<base_type>();
+  ```
+
+  From now on, wherever a `base_type` is required, an instance of `derived_type`
+  will also be accepted.
+
+* _Conversion functions_. Actual types can be converted, this is a fact. Just
+  think of the relationship between a `double` and an `int` to see it. Similar
+  to bases, conversion functions allow users to define conversions that will be
+  implicitly performed by the reflection system when required.<br/>
+  Use the `conv` member function for this purpose:
+
+  ```cpp
+  entt::meta<double>().conv<int>();
+  ```
+
+That's all, everything users need to create meta types and enjoy the reflection
+system. At first glance it may not seem that much, but users usually learn to
+appreciate it over time.<br/>
+Also, do not forget what these few lines hide under the hood: a built-in,
+non-intrusive and macro-free system for reflection in C++. Features that are
+definitely worth the price, at least for me.
+
+## Any as in any type
+
+The reflection system comes with its own `meta_any` type. It may seem redundant
+since C++17 introduced `std::any`, but it is not.<br/>
+In fact, the _type_ returned by an `std::any` is a const reference to an
+`std::type_info`, an implementation defined class that's not something everyone
+wants to see in a software. Furthermore, the class `std::type_info` suffers from
+some design flaws and there is even no way to _convert_ an `std::type_info` into
+a meta type, thus linking the two worlds.
+
+The class `meta_any` offers an API similar to that of its most famous
+counterpart and serves the same purpose of being an opaque container for any
+type of value.<br/>
+It minimizes the allocations required, which are almost absent thanks to _SBO_
+techniques. In fact, unless users deal with _fat types_ and create instances of
+them through the reflection system, allocations are at zero.
+
+Creating instances of `meta_any`, whether empty or from existing objects, is
+trivial:
+
+```cpp
+// a container for an int
+entt::meta_any any{0};
+
+// an empty container
+entt::meta_any empty{};
+```
+
+The `meta_any` class takes also the burden of destroying the contained object
+when required.<br/>
+Furthermore, an instance of `meta_any` is not tied to a specific type.
+Therefore, the wrapper will be reconfigured by assigning it an object of a
+different type than the one contained, so as to be able to handle the new
+instance.
+
+A particularly interesting feature of this class is that it can also be used as
+an opaque container for unmanaged objects:
+
+```cpp
+int value;
+entt::meta_any any{std::ref(value)};
+```
+
+In other words, whenever `meta_any` intercepts a `reference_wrapper`, it acts as
+a reference to the original instance rather than making a copy of it. The
+contained object is never destroyed and users must ensure that its lifetime
+exceeds that of the container.
+
+The `meta_any` class has also a `type` member function that returns the meta
+type of the contained value, if any. The member functions `try_cast`, `cast` and
+`convert` are used to know if the underlying object has a given type as a base
+or if it can be converted implicitly to it.
+
+## Enjoy the runtime
+
+Once the web of reflected types has been constructed, it's a matter of using it
+at runtime where required.<br/>
+All this has the great merit that, unlike the vast majority of the things
+present in this library and closely linked to the compile-time, the reflection
+system stands in fact as a non-intrusive tool for the runtime.
+
+To search for a reflected type there are two options: by type or by _name_. In
+both cases, the search can be done by means of the `resolve` function:
+
+```cpp
+// search for a reflected type by type
+auto by_type = entt::resolve<my_type>();
+
+// search for a reflected type by name
+auto by_name = entt::resolve("reflected_type"_hs);
+```
+
+There exits also a third overload of the `resolve` function to use to iterate
+all the reflected types at once:
+
+```cpp
+resolve([](auto type) {
+    // ...
+});
+```
+
+In all cases, the returned value is an instance of `meta_type`. This kind of
+objects offer an API to know their _runtime identifiers_, to iterate all the
+meta objects associated with them and even to build or destroy instances of the
+underlying type.<br/>
+Refer to the inline documentation for all the details.
+
+The meta objects that compose a meta type are accessed in the following ways:
+
+* _Meta constructors_. They are accessed by types of arguments:
+
+  ```cpp
+  auto ctor = entt::resolve<my_type>().ctor<int, char>();
+  ```
+
+  The returned type is `meta_ctor` and may be invalid if there is no constructor
+  that accepts the supplied arguments or at least some types from which they are
+  derived or to which they can be converted.<br/>
+  A meta constructor offers an API to know the number of arguments, the expected
+  meta types and to invoke it, therefore to construct a new instance of the
+  underlying type.
+
+* _Meta destructor_. It's returned by a dedicated function:
+
+  ```cpp
+  auto dtor = entt::resolve<my_type>().dtor();
+  ```
+
+  The returned type is `meta_dtor` and may be invalid if there is no custom
+  destructor set for the given meta type.<br/>
+  All what a meta destructor has to offer is a way to invoke it on a given
+  instance. Be aware that the result may not be what is expected.
+
+* _Meta data_. They are accessed by _name_:
+
+  ```cpp
+  auto data = entt::resolve<my_type>().data("member"_hs);
+  ```
+
+  The returned type is `meta_data` and may be invalid if there is no meta data
+  object associated with the given identifier.<br/>
+  A meta data object offers an API to query the underlying type (for example, to
+  know if it's a const or a static one), to get the meta type of the variable
+  and to set or get the contained value.
+
+* _Meta functions_. They are accessed by _name_:
+
+  ```cpp
+  auto func = entt::resolve<my_type>().func("member"_hs);
+  ```
+
+  The returned type is `meta_func` and may be invalid if there is no meta
+  function object associated with the given identifier.<br/>
+  A meta function object offers an API to query the underlying type (for
+  example, to know if it's a const or a static function), to know the number of
+  arguments, the meta return type and the meta types of the parameters. In
+  addition, a meta function object can be used to invoke the underlying function
+  and then get the return value in the form of a `meta_any` object.
+
+* _Meta bases_. They are accessed through the _name_ of the base types:
+
+  ```cpp
+  auto base = entt::resolve<derived_type>().base("base"_hs);
+  ```
+
+  The returned type is `meta_base` and may be invalid if there is no meta base
+  object associated with the given identifier.<br/>
+  Meta bases aren't meant to be used directly, even though they are freely
+  accessible. They expose only a few methods to use to know the meta type of the
+  base class and to convert a raw pointer between types.
+
+* _Meta conversion functions_. They are accessed by type:
+
+  ```cpp
+  auto conv = entt::resolve<double>().conv<int>();
+  ```
+
+  The returned type is `meta_conv` and may be invalid if there is no meta
+  conversion function associated with the given type.<br/>
+  The meta conversion functions are as thin as the meta bases and with a very
+  similar interface. The sole difference is that they return a newly created
+  instance wrapped in a `meta_any` object when they convert between different
+  types.
+
+All the objects thus obtained as well as the meta types can be explicitly
+converted to a boolean value to check if they are valid:
+
+```cpp
+if(auto func = entt::resolve<my_type>().func("member"_hs); func) {
+    // ...
+}
+```
+
+Furthermore, all meta objects with the exception of meta destructors can be
+iterated through an overload that accepts a callback through which to return
+them. As an example:
+
+```cpp
+entt::resolve<my_type>().data([](auto data) {
+    // ...
+});
+```
+
+A meta type can also be used to `construct` or `destroy` actual instances of the
+underlying type.<br/>
+In particular, the `construct` member function accepts a variable number of
+arguments and searches for a match. It returns a `meta_any` object that may or
+may not be initialized, depending on whether a suitable constructor has been
+found or not. On the other side, the `destroy` member function accepts instances
+of `meta_any` as well as actual objects by reference and invokes the registered
+destructor, if any.<br/>
+Be aware that the result of a call to `destroy` may not be what is expected. The
+purpose is to give users the ability to free up resources that require special
+treatment and **not** to actually destroy instances.
+
+Meta types and meta objects in general contain much more than what is said: a
+plethora of functions in addition to those listed whose purposes and uses go
+unfortunately beyond the scope of this document.<br/>
+I invite anyone interested in the subject to look at the code, experiment and
+read the inline documentation to get the best out of this powerful tool.
+
+## Policies: the more, the less
+
+Policies are a kind of compile-time directives that can be used when recording
+reflection information.<br/>
+Their purpose is to require slightly different behavior than the default in some
+specific cases. For example, when reading a given data member, its value is
+returned wrapped in a `meta_any` object which, by default, makes a copy of it.
+For large objects or if the caller wants to access the original instance, this
+behavior isn't desirable. Policies are there to offer a solution to this and
+other problems.
+
+There are a few alternatives available at the moment:
+
+* The _as-is_ policy, associated with the type `entt::as_is_t`.<br/>
+  This is the default policy. In general, it should never be used explicitly,
+  since it's implicitly selected if no other policy is specified.<br/>
+  In this case, the return values of the functions as well as the properties
+  exposed as data members are always returned by copy in a dedicated wrapper and
+  therefore associated with their original meta types.
+
+* The _as-void_ policy, associated with the type `entt::as_void_t`.<br/>
+  Its purpose is to discard the return value of a meta object, whatever it is,
+  thus making it appear as if its type were `void`.<br/>
+  If the use with functions is obvious, it must be said that it's also possible
+  to use this policy with constructors and data members. In the first case, the
+  constructor will be invoked but the returned wrapper will actually be empty.
+  In the second case, instead, the property will not be accessible for reading.
+
+  As an example of use:
+
+  ```cpp
+  entt::meta<my_type>().func<&my_type::member_function, entt::as_void_t>("member"_hs);
+  ```
+
+* The _as-alias_ policy, associated with the type `entt::as_alias_t`.<br/>
+  It allows to build wrappers that act as aliases for the objects used to
+  initialize them. Modifying the object contained in the wrapper for which the
+  _aliasing_ was requested will make it possible to directly modify the instance
+  used to initialize the wrapper itself.<br/>
+  This policy works with constructors (for example, when objects are taken from
+  an external container rather than created on demand), data members and
+  functions in general (as long as their return types are lvalue references).
+
+  As an example of use:
+
+  ```cpp
+  entt::meta<my_type>().data<&my_type::data_member, entt::as_alias_t>("member"_hs);
+  ```
+
+Some uses are rather trivial, but it's useful to note that there are some less
+obvious corner cases that can in turn be solved with the use of policies.
+
+## Named constants and enums
+
+A special mention should be made for constant values and enums. It wouldn't be
+necessary, but it will help distracted readers.
+
+As mentioned, the `data` member function can be used to reflect constants of any
+type among the other things.<br/>
+This allows users to create meta types for enums that will work exactly like any
+other meta type built from a class. Similarly, arithmetic types can be enriched
+with constants of special meaning where required.<br/>
+Personally, I find it very useful not to export what is the difference between
+enums and classes in C++ directly in the space of the reflected types.
+
+All the values thus exported will appear to users as if they were constant data
+members of the reflected types.
+
+Exporting constant values or elements from an enum is as simple as ever:
+
+```cpp
+entt::meta<my_enum>()
+        .data<my_enum::a_value>("a_value"_hs)
+        .data<my_enum::another_value>("another_value"_hs);
+
+entt::meta<int>().data<2048>("max_int"_hs);
+```
+
+It goes without saying that accessing them is trivial as well. It's a matter of
+doing the following, as with any other data member of a meta type:
+
+```cpp
+auto value = entt::resolve<my_enum>().data("a_value"_hs).get({}).cast<my_enum>();
+auto max = entt::resolve<int>().data("max_int"_hs).get({}).cast<int>();
+```
+
+As a side note, remember that all this happens behind the scenes without any
+allocation because of the small object optimization performed by the `meta_any`
+class.
+
+## Properties and meta objects
+
+Sometimes (for example, when it comes to creating an editor) it might be useful
+to attach properties to the meta objects created. Fortunately, this is possible
+for most of them.<br/>
+For the meta objects that support properties, the member functions of the
+factory used for registering them will return a decorated version of the factory
+itself. The latter can be used to attach properties to the last created meta
+object.<br/>
+Apparently, it's more difficult to say than to do:
+
+```cpp
+entt::meta<my_type>().type("reflected_type"_hs).prop("tooltip"_hs, "message");
+```
+
+Properties are always in the key/value form. There are no restrictions on the
+type of the key or value, as long as they are copy constructible objects.<br/>
+Multiple formats are supported when it comes to defining a property:
+
+* Properties as key/value pairs:
+
+  ```cpp
+  entt::meta<my_type>().type("reflected_type"_hs).prop("tooltip"_hs, "message");
+  ```
+
+* Properties as `std::pair`s:
+
+  ```cpp
+  entt::meta<my_type>().type("reflected_type"_hs).prop(std::make_pair("tooltip"_hs, "message"));
+  ```
+
+* Key only properties:
+
+  ```cpp
+  entt::meta<my_type>().type("reflected_type"_hs).prop(my_enum::key_only);
+  ```
+
+* Properties as `std::tuple`s:
+
+  ```cpp
+  entt::meta<my_type>().type("reflected_type"_hs)
+          .prop(std::make_tuple(std::make_pair("tooltip"_hs, "message"), my_enum::key_only));
+  ```
+
+  A tuple contains one or more properties. All of them are treated individually.
+
+* Annotations:
+
+  ```cpp
+  entt::meta<my_type>().type("reflected_type"_hs).prop(&property_generator);
+  ```
+
+  An annotation is an invocable object that returns one or more properties. All
+  of them are treated individually.
+
+It's possible to invoke the `prop` function several times if needed, one for
+each property to associate with the last meta object created:
+
+```cpp
+entt::meta<my_type>()
+        .type("reflected_type"_hs)
+            .prop(entt::hashed_string{"Name"}, "Reflected Type")
+        .data<&my_type::data_member>("member"_hs)
+            .prop(std::make_pair("tooltip"_hs, "Member"))
+            .prop(my_enum::a_value, 42);
+```
+
+Alternatively, the `props` function is available to associate several properties
+at a time. However, in this case properties in the key/value form aren't
+allowed, since they would be interpreted as two different properties rather than
+a single one.
+
+The meta objects for which properties are supported are currently the meta
+types, meta constructors, meta data and meta functions. It's not possible to
+attach properties to other types of meta objects and the factory returned as a
+result of their construction won't allow such an operation.
+
+These types offer a couple of member functions named `prop` to iterate all
+properties at once or to search a specific property by key:
+
+```cpp
+// iterate all properties of a meta type
+entt::resolve<my_type>().prop([](auto prop) {
+    // ...
+});
+
+// search for a given property by name
+auto prop = entt::resolve<my_type>().prop("tooltip"_hs);
+```
+
+Meta properties are objects having a fairly poor interface, all in all. They
+only provide the `key` and the `value` member functions to be used to retrieve
+the key and the value contained in the form of `meta_any` objects, respectively.
+
+## Unregister types
+
+A type registered with the reflection system can also be unregistered. This
+means unregistering all its data members, member functions, conversion functions
+and so on. However, the base classes won't be unregistered, since they don't
+necessarily depend on it. Similarly, implicitly generated types (as an example,
+the meta types implicitly generated for function parameters when needed) won't
+be unregistered.<br/>
+Roughly speaking, unregistering a type means disconnecting all associated meta
+objects from it and making its identifier no longer visible. The underlying node
+will remain available though, as if it were implicitly generated:
+
+```cpp
+entt::meta<my_type>().reset();
+```
+
+The type can be re-registered later with a completely different name and form.

docs/md/process.md

@@ -0,0 +1,208 @@
+# Crash Course: cooperative scheduler
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [The process](#the-process)
+  * [Adaptor](#adaptor)
+* [The scheduler](#the-scheduler)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+Sometimes processes are a useful tool to work around the strict definition of a
+system and introduce logic in a different way, usually without resorting to the
+introduction of other components.
+
+`EnTT` offers a minimal support to this paradigm by introducing a few classes
+that users can use to define and execute cooperative processes.
+
+# The process
+
+A typical process must inherit from the `process` class template that stays true
+to the CRTP idiom. Moreover, derived classes must specify what's the intended
+type for elapsed times.
+
+A process should expose publicly the following member functions whether
+required (note that it isn't required to define a function unless the derived
+class wants to _override_ the default behavior):
+
+* `void update(Delta, void *);`
+
+  It's invoked once per tick until a process is explicitly aborted or it
+  terminates either with or without errors. Even though it's not mandatory to
+  declare this member function, as a rule of thumb each process should at
+  least define it to work properly. The `void *` parameter is an opaque pointer
+  to user data (if any) forwarded directly to the process during an update.
+
+* `void init();`
+
+  It's invoked when the process joins the running queue of a scheduler. This
+  happens as soon as it's attached to the scheduler if the process is a top
+  level one, otherwise when it replaces its parent if the process is a
+  continuation.
+
+* `void succeeded();`
+
+  It's invoked in case of success, immediately after an update and during the
+  same tick.
+
+* `void failed();`
+
+  It's invoked in case of errors, immediately after an update and during the
+  same tick.
+
+* `void aborted();`
+
+  It's invoked only if a process is explicitly aborted. There is no guarantee
+  that it executes in the same tick, this depends solely on whether the
+  process is aborted immediately or not.
+
+Derived classes can also change the internal state of a process by invoking
+`succeed` and `fail`, as well as `pause` and `unpause` the process itself. All
+these are protected member functions made available to be able to manage the
+life cycle of a process from a derived class.
+
+Here is a minimal example for the sake of curiosity:
+
+```cpp
+struct my_process: entt::process<my_process, std::uint32_t> {
+    using delta_type = std::uint32_t;
+
+    void update(delta_type delta, void *) {
+        remaining -= std::min(remaining, delta);
+
+        // ...
+
+        if(!remaining) {
+            succeed();
+        }
+    }
+
+private:
+    delta_type remaining{1000u};
+};
+```
+
+## Adaptor
+
+Lambdas and functors can't be used directly with a scheduler for they are not
+properly defined processes with managed life cycles.<br/>
+This class helps in filling the gap and turning lambdas and functors into
+full featured processes usable by a scheduler.
+
+The function call operator has a signature similar to the one of the `update`
+function of a process but for the fact that it receives two extra arguments to
+call whenever a process is terminated with success or with an error:
+
+```cpp
+void(Delta delta, void *data, auto succeed, auto fail);
+```
+
+Parameters have the following meaning:
+
+* `delta` is the elapsed time.
+* `data` is an opaque pointer to user data if any, `nullptr` otherwise.
+* `succeed` is a function to call when a process terminates with success.
+* `fail` is a function to call when a process terminates with errors.
+
+Both `succeed` and `fail` accept no parameters at all.
+
+Note that usually users shouldn't worry about creating adaptors at all. A
+scheduler creates them internally each and every time a lambda or a functor is
+used as a process.
+
+# The scheduler
+
+A cooperative scheduler runs different processes and helps managing their life
+cycles.
+
+Each process is invoked once per tick. If it terminates, it's removed
+automatically from the scheduler and it's never invoked again. Otherwise it's
+a good candidate to run one more time the next tick.<br/>
+A process can also have a child. In this case, the parent process is replaced
+with its child when it terminates and only if it returns with success. In case
+of errors, both the parent process and its child are discarded. This way, it's
+easy to create chain of processes to run sequentially.
+
+Using a scheduler is straightforward. To create it, users must provide only the
+type for the elapsed times and no arguments at all:
+
+```cpp
+entt::scheduler<std::uint32_t> scheduler;
+```
+
+It has member functions to query its internal data structures, like `empty` or
+`size`, as well as a `clear` utility to reset it to a clean state:
+
+```cpp
+// checks if there are processes still running
+const auto empty = scheduler.empty();
+
+// gets the number of processes still running
+entt::scheduler<std::uint32_t>::size_type size = scheduler.size();
+
+// resets the scheduler to its initial state and discards all the processes
+scheduler.clear();
+```
+
+To attach a process to a scheduler there are mainly two ways:
+
+* If the process inherits from the `process` class template, it's enough to
+  indicate its type and submit all the parameters required to construct it to
+  the `attach` member function:
+
+  ```cpp
+  scheduler.attach<my_process>("foobar");
+  ```
+
+* Otherwise, in case of a lambda or a functor, it's enough to provide an
+  instance of the class to the `attach` member function:
+
+  ```cpp
+  scheduler.attach([](auto...){ /* ... */ });
+  ```
+
+In both cases, the return value is an opaque object that offers a `then` member
+function to use to create chains of processes to run sequentially.<br/>
+As a minimal example of use:
+
+```cpp
+// schedules a task in the form of a lambda function
+scheduler.attach([](auto delta, void *, auto succeed, auto fail) {
+    // ...
+})
+// appends a child in the form of another lambda function
+.then([](auto delta, void *, auto succeed, auto fail) {
+    // ...
+})
+// appends a child in the form of a process class
+.then<my_process>();
+```
+
+To update a scheduler and therefore all its processes, the `update` member
+function is the way to go:
+
+```cpp
+// updates all the processes, no user data are provided
+scheduler.update(delta);
+
+// updates all the processes and provides them with custom data
+scheduler.update(delta, &data);
+```
+
+In addition to these functions, the scheduler offers an `abort` member function
+that can be used to discard all the running processes at once:
+
+```cpp
+// aborts all the processes abruptly ...
+scheduler.abort(true);
+
+// ... or gracefully during the next tick
+scheduler.abort();
+```

docs/md/resource.md

@@ -0,0 +1,237 @@
+# Crash Course: resource management
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [The resource, the loader and the cache](#the-resource-the-loader-and-the-cache)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+Resource management is usually one of the most critical part of a software like
+a game. Solutions are often tuned to the particular application. There exist
+several approaches and all of them are perfectly fine as long as they fit the
+requirements of the piece of software in which they are used.<br/>
+Examples are loading everything on start, loading on request, predictive
+loading, and so on.
+
+`EnTT` doesn't pretend to offer a _one-fits-all_ solution for the different
+cases. Instead, it offers a minimal and perhaps trivial cache that can be useful
+most of the time during prototyping and sometimes even in a production
+environment.<br/>
+For those interested in the subject, the plan is to improve it considerably over
+time in terms of performance, memory usage and functionalities. Hoping to make
+it, of course, one step at a time.
+
+# The resource, the loader and the cache
+
+There are three main actors in the model: the resource, the loader and the
+cache.
+
+The _resource_ is whatever users want it to be. An image, a video, an audio,
+whatever. There are no limits.<br/>
+As a minimal example:
+
+```cpp
+struct my_resource { const int value; };
+```
+
+A _loader_ is a class the aim of which is to load a specific resource. It has to
+inherit directly from the dedicated base class as in the following example:
+
+```cpp
+struct my_loader final: entt::loader<my_loader, my_resource> {
+    // ...
+};
+```
+
+Where `my_resource` is the type of resources it creates.<br/>
+A resource loader must also expose a public const member function named `load`
+that accepts a variable number of arguments and returns a shared pointer to a
+resource.<br/>
+As an example:
+
+```cpp
+struct my_loader: entt::loader<my_loader, my_resource> {
+    std::shared_ptr<my_resource> load(int value) const {
+        // ...
+        return std::shared_ptr<my_resource>(new my_resource{ value });
+    }
+};
+```
+
+In general, resource loaders should not have a state or retain data of any type.
+They should let the cache manage their resources instead.<br/>
+As a side note, base class and CRTP idiom aren't strictly required with the
+current implementation. One could argue that a cache can easily work with
+loaders of any type. However, future changes won't be breaking ones by forcing
+the use of a base class today and that's why the model is already in its place.
+
+Finally, a cache is a specialization of a class template tailored to a specific
+resource:
+
+```cpp
+using my_cache = entt::cache<my_resource>;
+
+// ...
+
+my_cache cache{};
+```
+
+The idea is to create different caches for different types of resources and to
+manage each one independently in the most appropriate way.<br/>
+As a (very) trivial example, audio tracks can survive in most of the scenes of
+an application while meshes can be associated with a single scene and then
+discarded when users leave it.
+
+A cache offers a set of basic functionalities to query its internal state and to
+_organize_ it:
+
+```cpp
+// gets the number of resources managed by a cache
+const auto size = cache.size();
+
+// checks if a cache contains at least a valid resource
+const auto empty = cache.empty();
+
+// clears a cache and discards its content
+cache.clear();
+```
+
+Besides these member functions, a cache contains what is needed to load, use and
+discard resources of the given type.<br/>
+Before to explore this part of the interface, it makes sense to mention how
+resources are identified. The type of the identifiers to use is defined as:
+
+```cpp
+entt::cache<resource>::id_type
+```
+
+Where `id_type` is an alias for `entt::hashed_string::hash_type`. Therefore,
+resource identifiers are created explicitly as in the following example:
+
+```cpp
+constexpr auto identifier = entt::cache<resource>::id_type{"my/resource/identifier"_hs};
+// this is equivalent to the following
+constexpr auto hs = entt::hashed_string{"my/resource/identifier"};
+```
+
+The class `hashed_string` is described in a dedicated section, so I won't go in
+details here.
+
+Resources are loaded and thus stored in a cache through the `load` member
+function. It accepts the loader to use as a template parameter, the resource
+identifier and the parameters used to construct the resource as arguments:
+
+```cpp
+// uses the identifier declared above
+cache.load<my_loader>(identifier, 0);
+
+// uses a const char * directly as an identifier
+cache.load<my_loader>("another/identifier"_hs, 42);
+```
+
+The function returns a handle to the resource, whether it already exists or is
+loaded. In case the loader returns an invalid pointer, the handle is invalid as
+well and therefore it can be easily used with an `if` statement:
+
+```cpp
+if(auto handle = cache.load<my_loader>("another/identifier"_hs, 42); handle) {
+    // ...
+}
+```
+
+Before trying to load a resource, the `contains` member function can be used to
+know if a cache already contains a specific resource:
+
+```cpp
+auto exists = cache.contains("my/identifier"_hs);
+```
+
+There exists also a member function to use to force a reload of an already
+existing resource if needed:
+
+```cpp
+auto handle = cache.reload<my_loader>("another/identifier"_hs, 42);
+```
+
+As above, the function returns a handle to the resource that is invalid in case
+of errors. The `reload` member function is a kind of alias of the following
+snippet:
+
+```cpp
+cache.discard(identifier);
+cache.load<my_loader>(identifier, 42);
+```
+
+Where the `discard` member function is used to get rid of a resource if loaded.
+In case the cache doesn't contain a resource for the given identifier, `discard`
+does nothing and returns immediately.
+
+So far, so good. Resources are finally loaded and stored within the cache.<br/>
+They are returned to users in the form of handles. To get one of them later on:
+
+```cpp
+auto handle = cache.handle("my/identifier"_hs);
+```
+
+The idea behind a handle is the same of the flyweight pattern. In other terms,
+resources aren't copied around. Instead, instances are shared between handles.
+Users of a resource own a handle that guarantees that a resource isn't destroyed
+until all the handles are destroyed, even if the resource itself is removed from
+the cache.<br/>
+Handles are tiny objects both movable and copyable. They return the contained
+resource as a const reference on request:
+
+* By means of the `get` member function:
+
+  ```cpp
+  const auto &resource = handle.get();
+  ```
+
+* Using the proper cast operator:
+
+  ```cpp
+  const auto &resource = handle;
+  ```
+
+* Through the dereference operator:
+
+  ```cpp
+  const auto &resource = *handle;
+  ```
+
+The resource can also be accessed directly using the arrow operator if required:
+
+```cpp
+auto value = handle->value;
+```
+
+To test if a handle is still valid, the cast operator to `bool` allows users to
+use it in a guard:
+
+```cpp
+if(handle) {
+    // ...
+}
+```
+
+Finally, in case there is the need to load a resource and thus to get a handle
+without storing the resource itself in the cache, users can rely on the `temp`
+member function template.<br/>
+The declaration is similar to that of `load`, a (possibly invalid) handle for
+the resource is returned also in this case:
+
+```cpp
+if(auto handle = cache.temp<my_loader>(42); handle) {
+    // ...
+}
+```
+
+Do not forget to test the handle for validity. Otherwise, getting a reference to
+the resource it points may result in undefined behavior.

docs/md/signal.md

@@ -0,0 +1,488 @@
+# Crash Course: events, signals and everything in between
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [Delegate](#delegate)
+* [Signals](#signals)
+* [Event dispatcher](#event-dispatcher)
+* [Event emitter](#event-emitter)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+Signals are usually a core part of games and software architectures in
+general.<br/>
+Roughly speaking, they help to decouple the various parts of a system while
+allowing them to communicate with each other somehow.
+
+The so called _modern C++_ comes with a tool that can be useful in these terms,
+the `std::function`. As an example, it can be used to create delegates.<br/>
+However, there is no guarantee that an `std::function` does not perform
+allocations under the hood and this could be problematic sometimes. Furthermore,
+it solves a problem but may not adapt well to other requirements that may arise
+from time to time.
+
+In case that the flexibility and power of an `std::function` isn't required or
+if the price to pay for them is too high,` EnTT` offers a complete set of
+lightweight classes to solve the same and many other problems.
+
+# Delegate
+
+A delegate can be used as a general purpose invoker with no memory overhead for
+free functions and members provided along with an instance on which to invoke
+them.<br/>
+It does not claim to be a drop-in replacement for an `std::function`, so do not
+expect to use it whenever an `std::function` fits well. That said, it's most
+likely even a better fit than an `std::function` in a lot of cases, so expect to
+use it quite a lot anyway.
+
+The interface is trivial. It offers a default constructor to create empty
+delegates:
+
+```cpp
+entt::delegate<int(int)> delegate{};
+```
+
+All what is needed to create an instance is to specify the type of the function
+the delegate will _contain_, that is the signature of the free function or the
+member one wants to assign to it.
+
+Attempting to use an empty delegate by invoking its function call operator
+results in undefined behavior or most likely a crash. Before to use a delegate,
+it must be initialized.<br/>
+There exists a bunch of overloads of the `connect` member function to do that.
+As an example of use:
+
+```cpp
+int f(int i) { return i; }
+
+struct my_struct {
+    int f(const int &i) { return i }
+};
+
+// bind a free function to the delegate
+delegate.connect<&f>();
+
+// bind a member function to the delegate
+my_struct instance;
+delegate.connect<&my_struct::f>(instance);
+```
+
+The delegate class accepts also data members, if needed. In this case, the
+function type of the delegate is such that the parameter list is empty and the
+value of the data member is at least convertible to the return type.
+
+Free functions having type equivalent to `void(T &, args...)` are accepted as
+well. In this case, `T &` is considered a payload and the function will receive
+it back every time it's invoked. In other terms, this works just fine with the
+above definition:
+
+```cpp
+void g(const char &c, int i) { /* ... */ }
+const char c = 'c';
+
+delegate.connect<&g>(c);
+delegate(42);
+```
+
+The function `g` will be invoked with a reference to `c` and `42`. However, the
+function type of the delegate is still `void(int)`. This is also the signature
+of its function call operator.
+
+Another interesting aspect of the delegate class is that it accepts also
+functions with a list of parameters that is shorter than that of the function
+type used to specialize the delegate itself.<br/>
+The following code is therefore perfectly valid:
+
+```cpp
+void g() { /* ... */ }
+delegate.connect<&g>();
+delegate(42);
+```
+
+Where the function type of the delegate is `void(int)` as above. It goes without
+saying that the extra arguments are silently discarded internally.<br/>v
+This is a nice-to-have feature in a lot of cases, as an example when the
+`delegate` class is used as a building block of a signal-slot system.
+
+To create and initialize a delegate at once, there are a few specialized
+constructors. Because of the rules of the language, the listener is provided by
+means of the `entt::connect_arg` variable template:
+
+```cpp
+entt::delegate<int(int)> func{entt::connect_arg<&f>};
+```
+
+Aside `connect`, a `disconnect` counterpart isn't provided. Instead, there
+exists a `reset` member function to use to clear a delegate.<br/>
+To know if a delegate is empty, it can be used explicitly in every conditional
+statement:
+
+```cpp
+if(delegate) {
+    // ...
+}
+```
+
+Finally, to invoke a delegate, the function call operator is the way to go as
+already shown in the examples above:
+
+```cpp
+auto ret = delegate(42);
+```
+
+In all cases, the listeners don't have to strictly follow the signature of the
+delegate. As long as a listener can be invoked with the given arguments to yield
+a result that is convertible to the given result type, everything works just
+fine.
+
+# Signals
+
+Signal handlers work with references to classes, function pointers and pointers
+to members. Listeners can be any kind of objects and users are in charge of
+connecting and disconnecting them from a signal to avoid crashes due to
+different lifetimes. On the other side, performance shouldn't be affected that
+much by the presence of such a signal handler.<br/>
+Signals make use of delegates internally and therefore they undergo the same
+rules and offer similar functionalities. It may be a good idea to consult the
+documentation of the `delegate` class for further information.
+
+A signal handler can be used as a private data member without exposing any
+_publish_ functionality to the clients of a class. The basic idea is to impose a
+clear separation between the signal itself and the `sink` class, that is a tool
+to be used to connect and disconnect listeners on the fly.
+
+The API of a signal handler is straightforward. If a collector is supplied to
+the signal when something is published, all the values returned by the listeners
+can be literally _collected_ and used later by the caller. Otherwise, the class
+works just like a plain signal that emits events from time to time.<br/>
+To create instances of signal handlers it is sufficient to provide the type of
+function to which they refer:
+
+```cpp
+entt::sigh<void(int, char)> signal;
+```
+
+Signals offer all the basic functionalities required to know how many listeners
+they contain (`size`) or if they contain at least a listener (`empty`), as well
+as a function to use to swap handlers (`swap`).
+
+Besides them, there are member functions to use both to connect and disconnect
+listeners in all their forms by means of a sink:
+
+```cpp
+void foo(int, char) { /* ... */ }
+
+struct listener {
+    void bar(const int &, char) { /* ... */ }
+};
+
+// ...
+
+entt::sink sink{signal};
+listener instance;
+
+sink.connect<&foo>();
+sink.connect<&listener::bar>(instance);
+
+// ...
+
+// disconnects a free function
+sink.disconnect<&foo>();
+
+// disconnect a member function of an instance
+sink.disconnect<&listener::bar>(instance);
+
+// disconnect all the member functions of an instance, if any
+sink.disconnect(instance);
+
+// discards all the listeners at once
+sink.disconnect();
+```
+
+As shown above, the listeners don't have to strictly follow the signature of the
+signal. As long as a listener can be invoked with the given arguments to yield a
+result that is convertible to the given return type, everything works just
+fine.<br/>
+It's also possible to connect a listener before other listeners already
+contained by the signal. The `before` function returns a `sink` object correctly
+initialized for the purpose that can be used to connect one or more listeners in
+order and in the desired position:
+
+```cpp
+sink.before<&foo>().connect<&listener::bar>(instance);
+```
+
+In all cases, the `connect` member function returns by default a `connection`
+object to be used as an alternative to break a connection by means of its
+`release` member function. A `scoped_connection` can also be created from a
+connection. In this case, the link is broken automatically as soon as the object
+goes out of scope.
+
+Once listeners are attached (or even if there are no listeners at all), events
+and data in general can be published through a signal by means of the `publish`
+member function:
+
+```cpp
+signal.publish(42, 'c');
+```
+
+To collect data, the `collect` member function should be used instead. Below is
+a minimal example to show how to use it:
+
+```cpp
+int f() { return 0; }
+int g() { return 1; }
+
+// ...
+
+entt::sigh<int()> signal;
+entt::sink sink{signal};
+
+sink.connect<&f>();
+sink.connect<&g>();
+
+std::vector<int> vec{};
+signal.collect([&vec](int value) { vec.push_back(value); });
+
+assert(vec[0] == 0);
+assert(vec[1] == 1);
+```
+
+A collector must expose a function operator that accepts as an argument a type
+to which the return type of the listeners can be converted. Moreover, it can
+optionally return a boolean value that is true to stop collecting data, false
+otherwise. This way one can avoid calling all the listeners in case it isn't
+necessary.<br/>
+Functors can also be used in place of a lambda. Since the collector is copied
+when invoking the `collect` member function, `std::ref` is the way to go in this
+case:
+
+```cpp
+struct my_collector {
+    std::vector<int> vec{};
+
+    bool operator()(int v) noexcept {
+        vec.push_back(v);
+        return true;
+    }
+};
+
+// ...
+
+my_collector collector;
+signal.collect(std::ref(collector));
+```
+
+# Event dispatcher
+
+The event dispatcher class is designed so as to be used in a loop. It allows
+users both to trigger immediate events or to queue events to be published all
+together once per tick.<br/>
+This class shares part of its API with the one of the signal handler, but it
+doesn't require that all the types of events are specified when declared:
+
+```cpp
+// define a general purpose dispatcher
+entt::dispatcher dispatcher{};
+```
+
+In order to register an instance of a class to a dispatcher, its type must
+expose one or more member functions the arguments of which are such that
+`const E &` can be converted to them for each type of event `E`, no matter what
+the return value is.<br/>
+The name of the member function aimed to receive the event must be provided to
+the `connect` member function of the sink in charge for the specific event:
+
+```cpp
+struct an_event { int value; };
+struct another_event {};
+
+struct listener {
+    void receive(const an_event &) { /* ... */ }
+    void method(const another_event &) { /* ... */ }
+};
+
+// ...
+
+listener listener;
+dispatcher.sink<an_event>().connect<&listener::receive>(listener);
+dispatcher.sink<another_event>().connect<&listener::method>(listener);
+```
+
+The `disconnect` member function follows the same pattern and can be used to
+remove one listener at a time or all of them at once:
+
+```cpp
+dispatcher.sink<an_event>().disconnect<&listener::receive>(listener);
+dispatcher.sink<another_event>().disconnect(listener);
+```
+
+The `trigger` member function serves the purpose of sending an immediate event
+to all the listeners registered so far. It offers a convenient approach that
+relieves users from having to create the event itself. Instead, it's enough to
+specify the type of event and provide all the parameters required to construct
+it.<br/>
+As an example:
+
+```cpp
+dispatcher.trigger<an_event>(42);
+dispatcher.trigger<another_event>();
+```
+
+Listeners are invoked immediately, order of execution isn't guaranteed. This
+method can be used to push around urgent messages like an _is terminating_
+notification on a mobile app.
+
+On the other hand, the `enqueue` member function queues messages together and
+allows to maintain control over the moment they are sent to listeners. The
+signature of this method is more or less the same of `trigger`:
+
+```cpp
+dispatcher.enqueue<an_event>(42);
+dispatcher.enqueue<another_event>();
+```
+
+Events are stored aside until the `update` member function is invoked, then all
+the messages that are still pending are sent to the listeners at once:
+
+```cpp
+// emits all the events of the given type at once
+dispatcher.update<my_event>();
+
+// emits all the events queued so far at once
+dispatcher.update();
+```
+
+This way users can embed the dispatcher in a loop and literally dispatch events
+once per tick to their systems.
+
+# Event emitter
+
+A general purpose event emitter thought mainly for those cases where it comes to
+working with asynchronous stuff.<br/>
+Originally designed to fit the requirements of
+[`uvw`](https://github.com/skypjack/uvw) (a wrapper for `libuv` written in
+modern C++), it was adapted later to be included in this library.
+
+To create a custom emitter type, derived classes must inherit directly from the
+base class as:
+
+```cpp
+struct my_emitter: emitter<my_emitter> {
+    // ...
+}
+```
+
+The full list of accepted types of events isn't required. Handlers are created
+internally on the fly and thus each type of event is accepted by default.
+
+Whenever an event is published, an emitter provides the listeners with a
+reference to itself along with a const reference to the event. Therefore
+listeners have an handy way to work with it without incurring in the need of
+capturing a reference to the emitter itself.<br/>
+In addition, an opaque object is returned each time a connection is established
+between an emitter and a listener, allowing the caller to disconnect them at a
+later time.<br/>
+The opaque object used to handle connections is both movable and copyable. On
+the other side, an event emitter is movable but not copyable by default.
+
+To create new instances of an emitter, no arguments are required:
+
+```cpp
+my_emitter emitter{};
+```
+
+Listeners must be movable and callable objects (free functions, lambdas,
+functors, `std::function`s, whatever) whose function type is:
+
+```cpp
+void(const Event &, my_emitter &)
+```
+
+Where `Event` is the type of event they want to listen.<br/>
+There are two ways to attach a listener to an event emitter that differ
+slightly from each other:
+
+* To register a long-lived listener, use the `on` member function. It is meant
+  to register a listener designed to be invoked more than once for the given
+  event type.<br/>
+  As an example:
+
+  ```cpp
+  auto conn = emitter.on<my_event>([](const my_event &event, my_emitter &emitter) {
+      // ...
+  });
+  ```
+
+  The connection object can be freely discarded. Otherwise, it can be used later
+  to disconnect the listener if required.
+
+* To register a short-lived listener, use the `once` member function. It is
+  meant to register a listener designed to be invoked only once for the given
+  event type. The listener is automatically disconnected after the first
+  invocation.<br/>
+  As an example:
+
+  ```cpp
+  auto conn = emitter.once<my_event>([](const my_event &event, my_emitter &emitter) {
+      // ...
+  });
+  ```
+
+  The connection object can be freely discarded. Otherwise, it can be used later
+  to disconnect the listener if required.
+
+In both cases, the connection object can be used with the `erase` member
+function:
+
+```cpp
+emitter.erase(conn);
+```
+
+There are also two member functions to use either to disconnect all the
+listeners for a given type of event or to clear the emitter:
+
+```cpp
+// removes all the listener for the specific event
+emitter.clear<my_event>();
+
+// removes all the listeners registered so far
+emitter.clear();
+```
+
+To send an event to all the listeners that are interested in it, the `publish`
+member function offers a convenient approach that relieves users from having to
+create the event:
+
+```cpp
+struct my_event { int i; };
+
+// ...
+
+emitter.publish<my_event>(42);
+```
+
+Finally, the `empty` member function tests if there exists at least either a
+listener registered with the event emitter or to a given type of event:
+
+```cpp
+bool empty;
+
+// checks if there is any listener registered for the specific event
+empty = emitter.empty<my_event>();
+
+// checks it there are listeners registered with the event emitter
+empty = emitter.empty();
+```
+
+In general, the event emitter is a handy tool when the derived classes _wrap_
+asynchronous operations, because it introduces a _nice-to-have_ model based on
+events and listeners that kindly hides the complexity behind the scenes. However
+it is not limited to such uses.

scripts/amalgamate.py

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

scripts/config.json

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

scripts/update_homebrew.sh

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

src/entt/config/config.h

@@ -0,0 +1,56 @@
+#ifndef ENTT_CONFIG_CONFIG_H
+#define ENTT_CONFIG_CONFIG_H
+
+
+#ifndef ENTT_NOEXCEPT
+#define ENTT_NOEXCEPT noexcept
+#endif // ENTT_NOEXCEPT
+
+
+#ifndef ENTT_HS_SUFFIX
+#define ENTT_HS_SUFFIX _hs
+#endif // ENTT_HS_SUFFIX
+
+
+#ifndef ENTT_HWS_SUFFIX
+#define ENTT_HWS_SUFFIX _hws
+#endif // ENTT_HWS_SUFFIX
+
+
+#ifndef ENTT_NO_ATOMIC
+#include <atomic>
+#define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#else // ENTT_NO_ATOMIC
+#define ENTT_MAYBE_ATOMIC(Type) Type
+#endif // ENTT_NO_ATOMIC
+
+
+#ifndef ENTT_DISABLE_ETO
+#include <type_traits>
+#define ENTT_ENABLE_ETO(Type) std::is_empty_v<Type>
+#else // ENTT_DISABLE_ETO
+// sfinae-friendly definition
+#define ENTT_ENABLE_ETO(Type) (false && std::is_empty_v<Type>)
+#endif // ENTT_DISABLE_ETO
+
+
+#ifndef ENTT_ID_TYPE
+#include <cstdint>
+#define ENTT_ID_TYPE std::uint32_t
+#endif // ENTT_ID_TYPE
+
+
+#ifndef ENTT_PAGE_SIZE
+#define ENTT_PAGE_SIZE 32768
+#endif // ENTT_PAGE_SIZE
+
+
+#ifndef ENTT_DISABLE_ASSERT
+#include <cassert>
+#define ENTT_ASSERT(condition) assert(condition)
+#else // ENTT_DISABLE_ASSERT
+#define ENTT_ASSERT(...) ((void)0)
+#endif // ENTT_DISABLE_ASSERT
+
+
+#endif // ENTT_CONFIG_CONFIG_H

src/entt/config/version.h

@@ -0,0 +1,11 @@
+#ifndef ENTT_CONFIG_VERSION_H
+#define ENTT_CONFIG_VERSION_H
+
+
+#define ENTT_VERSION "3.2.1"
+#define ENTT_VERSION_MAJOR 3
+#define ENTT_VERSION_MINOR 2
+#define ENTT_VERSION_PATCH 1
+
+
+#endif // ENTT_CONFIG_VERSION_H

src/entt/core/algorithm.hpp

@@ -0,0 +1,143 @@
+#ifndef ENTT_CORE_ALGORITHM_HPP
+#define ENTT_CORE_ALGORITHM_HPP
+
+
+#include <utility>
+#include <iterator>
+#include <algorithm>
+#include <functional>
+#include "utility.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Function object to wrap `std::sort` in a class type.
+ *
+ * Unfortunately, `std::sort` cannot be passed as template argument to a class
+ * template or a function template.<br/>
+ * This class fills the gap by wrapping some flavors of `std::sort` in a
+ * function object.
+ */
+struct std_sort {
+    /**
+     * @brief Sorts the elements in a range.
+     *
+     * Sorts the elements in a range using the given binary comparison function.
+     *
+     * @tparam It Type of random access iterator.
+     * @tparam Compare Type of comparison function object.
+     * @tparam Args Types of arguments to forward to the sort function.
+     * @param first An iterator to the first element of the range to sort.
+     * @param last An iterator past the last element of the range to sort.
+     * @param compare A valid comparison function object.
+     * @param args Arguments to forward to the sort function, if any.
+     */
+    template<typename It, typename Compare = std::less<>, typename... Args>
+    void operator()(It first, It last, Compare compare = Compare{}, Args &&... args) const {
+        std::sort(std::forward<Args>(args)..., std::move(first), std::move(last), std::move(compare));
+    }
+};
+
+
+/*! @brief Function object for performing insertion sort. */
+struct insertion_sort {
+    /**
+     * @brief Sorts the elements in a range.
+     *
+     * Sorts the elements in a range using the given binary comparison function.
+     *
+     * @tparam It Type of random access iterator.
+     * @tparam Compare Type of comparison function object.
+     * @param first An iterator to the first element of the range to sort.
+     * @param last An iterator past the last element of the range to sort.
+     * @param compare A valid comparison function object.
+     */
+    template<typename It, typename Compare = std::less<>>
+    void operator()(It first, It last, Compare compare = Compare{}) const {
+        if(first < last) {
+            for(auto it = first+1; it < last; ++it) {
+                auto value = std::move(*it);
+                auto pre = it;
+
+                for(; pre > first && compare(value, *(pre-1)); --pre) {
+                    *pre = std::move(*(pre-1));
+                }
+
+                *pre = std::move(value);
+            }
+        }
+    }
+};
+
+
+/**
+ * @brief Function object for performing LSD radix sort.
+ * @tparam Bit Number of bits processed per pass.
+ * @tparam N Maximum number of bits to sort.
+ */
+template<std::size_t Bit, std::size_t N>
+struct radix_sort {
+    static_assert((N % Bit) == 0);
+
+    /**
+     * @brief Sorts the elements in a range.
+     *
+     * Sorts the elements in a range using the given _getter_ to access the
+     * actual data to be sorted.
+     *
+     * This implementation is inspired by the online book
+     * [Physically Based Rendering](http://www.pbr-book.org/3ed-2018/Primitives_and_Intersection_Acceleration/Bounding_Volume_Hierarchies.html#RadixSort).
+     *
+     * @tparam It Type of random access iterator.
+     * @tparam Getter Type of _getter_ function object.
+     * @param first An iterator to the first element of the range to sort.
+     * @param last An iterator past the last element of the range to sort.
+     * @param getter A valid _getter_ function object.
+     */
+    template<typename It, typename Getter = identity>
+    void operator()(It first, It last, Getter getter = Getter{}) const {
+        if(first < last) {
+            static constexpr auto mask = (1 << Bit) - 1;
+            static constexpr auto buckets = 1 << Bit;
+            static constexpr auto passes = N / Bit;
+
+            using value_type = typename std::iterator_traits<It>::value_type;
+            std::vector<value_type> aux(std::distance(first, last));
+
+            auto part = [getter = std::move(getter)](auto from, auto to, auto out, auto start) {
+                std::size_t index[buckets]{};
+                std::size_t count[buckets]{};
+
+                std::for_each(from, to, [&getter, &count, start](const value_type &item) {
+                    ++count[(getter(item) >> start) & mask];
+                });
+
+                std::for_each(std::next(std::begin(index)), std::end(index), [index = std::begin(index), count = std::begin(count)](auto &item) mutable {
+                    item = *(index++) + *(count++);
+                });
+
+                std::for_each(from, to, [&getter, &out, &index, start](value_type &item) {
+                    out[index[(getter(item) >> start) & mask]++] = std::move(item);
+                });
+            };
+
+            for(std::size_t pass = 0; pass < (passes & ~1); pass += 2) {
+                part(first, last, aux.begin(), pass * Bit);
+                part(aux.begin(), aux.end(), first, (pass + 1) * Bit);
+            }
+
+            if constexpr(passes & 1) {
+                part(first, last, aux.begin(), (passes - 1) * Bit);
+                std::move(aux.begin(), aux.end(), first);
+            }
+        }
+    }
+};
+
+
+}
+
+
+#endif // ENTT_CORE_ALGORITHM_HPP

src/entt/core/family.hpp

@@ -2,8 +2,8 @@
 #define ENTT_CORE_FAMILY_HPP
 
 
-#include<type_traits>
-#include<cstddef>
+#include <type_traits>
+#include "../config/config.h"
 
 
 namespace entt {
@@ -17,30 +17,17 @@ namespace entt {
  * identifiers.
  */
 template<typename...>
-class Family {
-    static std::size_t identifier() noexcept {
-        static std::size_t value = 0;
-        return value++;
-    }
-
-    template<typename...>
-    static std::size_t family() noexcept {
-        static const std::size_t value = identifier();
-        return value;
-    }
+class family {
+    inline static ENTT_MAYBE_ATOMIC(ENTT_ID_TYPE) identifier{};
 
 public:
     /*! @brief Unsigned integer type. */
-    using family_type = std::size_t;
+    using family_type = ENTT_ID_TYPE;
 
-    /**
-     * @brief Returns an unique identifier for the given type.
-     * @return Statically generated unique identifier for the given type.
-     */
+    /*! @brief Statically generated unique identifier for the given type. */
     template<typename... Type>
-    static family_type type() noexcept {
-        return family<std::decay_t<Type>...>();
-    }
+    // at the time I'm writing, clang crashes during compilation if auto is used instead of family_type
+    inline static const family_type type = identifier++;
 };
 
 

src/entt/core/hashed_string.hpp

@@ -2,107 +2,251 @@
 #define ENTT_CORE_HASHED_STRING_HPP
 
 
-#include <cstdint>
+#include <cstddef>
+#include "../config/config.h"
 
 
 namespace entt {
 
 
 /**
- * @brief Zero overhead resource identifier.
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename>
+struct fnv1a_traits;
+
+
+template<>
+struct fnv1a_traits<std::uint32_t> {
+    static constexpr std::uint32_t offset = 2166136261;
+    static constexpr std::uint32_t prime = 16777619;
+};
+
+
+template<>
+struct fnv1a_traits<std::uint64_t> {
+    static constexpr std::uint64_t offset = 14695981039346656037ull;
+    static constexpr std::uint64_t prime = 1099511628211ull;
+};
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond TURN_OFF_DOXYGEN
+ */
+
+
+/**
+ * @brief Zero overhead unique identifier.
  *
  * A hashed string is a compile-time tool that allows users to use
  * human-readable identifers in the codebase while using their numeric
  * counterparts at runtime.<br/>
  * Because of that, a hashed string can also be used in constant expressions if
  * required.
+ *
+ * @tparam Char Character type.
  */
-class HashedString final {
-    struct ConstCharWrapper final {
+template<typename Char>
+class basic_hashed_string {
+    using traits_type = internal::fnv1a_traits<ENTT_ID_TYPE>;
+
+    struct const_wrapper {
         // non-explicit constructor on purpose
-        constexpr ConstCharWrapper(const char *str) noexcept: str{str} {}
-        const char *str;
+        constexpr const_wrapper(const Char *curr) ENTT_NOEXCEPT: str{curr} {}
+        const Char *str;
     };
 
-    static constexpr std::uint64_t offset = 14695981039346656037u;
-    static constexpr std::uint64_t prime = 1099511628211u;
-
     // Fowler–Noll–Vo hash function v. 1a - the good
-    static constexpr std::uint64_t helper(std::uint64_t partial, const char *str) noexcept {
-        return str[0] == 0 ? partial : helper((partial^str[0])*prime, str+1);
+    static constexpr ENTT_ID_TYPE helper(ENTT_ID_TYPE partial, const Char *curr) ENTT_NOEXCEPT {
+        return curr[0] == 0 ? partial : helper((partial^curr[0])*traits_type::prime, curr+1);
     }
 
 public:
+    /*! @brief Character type. */
+    using value_type = Char;
     /*! @brief Unsigned integer type. */
-    using hash_type = std::uint64_t;
+    using hash_type = ENTT_ID_TYPE;
 
     /**
-     * @brief Constructs a hashed string from an array of const chars.
+     * @brief Returns directly the numeric representation of a string.
      *
      * Forcing template resolution avoids implicit conversions. An
      * human-readable identifier can be anything but a plain, old bunch of
      * characters.<br/>
      * Example of use:
      * @code{.cpp}
-     * HashedString sh{"my.png"};
+     * const auto value = basic_hashed_string<char>::to_value("my.png");
      * @endcode
      *
      * @tparam N Number of characters of the identifier.
      * @param str Human-readable identifer.
+     * @return The numeric representation of the string.
      */
-    template <std::size_t N>
-    constexpr HashedString(const char (&str)[N]) noexcept
-        : hash{helper(offset, str)}, str{str}
+    template<std::size_t N>
+    static constexpr hash_type to_value(const value_type (&str)[N]) ENTT_NOEXCEPT {
+        return helper(traits_type::offset, str);
+    }
+
+    /**
+     * @brief Returns directly the numeric representation of a string.
+     * @param wrapper Helps achieving the purpose by relying on overloading.
+     * @return The numeric representation of the string.
+     */
+    static hash_type to_value(const_wrapper wrapper) ENTT_NOEXCEPT {
+        return helper(traits_type::offset, wrapper.str);
+    }
+
+    /**
+     * @brief Returns directly the numeric representation of a string view.
+     * @param str Human-readable identifer.
+     * @param size Length of the string to hash.
+     * @return The numeric representation of the string.
+     */
+    static hash_type to_value(const value_type *str, std::size_t size) ENTT_NOEXCEPT {
+        ENTT_ID_TYPE partial{traits_type::offset};
+        while(size--) { partial = (partial^(str++)[0])*traits_type::prime; }
+        return partial;
+    }
+
+    /*! @brief Constructs an empty hashed string. */
+    constexpr basic_hashed_string() ENTT_NOEXCEPT
+        : str{nullptr}, hash{}
+    {}
+
+    /**
+     * @brief Constructs a hashed string from an array of const characters.
+     *
+     * Forcing template resolution avoids implicit conversions. An
+     * human-readable identifier can be anything but a plain, old bunch of
+     * characters.<br/>
+     * Example of use:
+     * @code{.cpp}
+     * basic_hashed_string<char> hs{"my.png"};
+     * @endcode
+     *
+     * @tparam N Number of characters of the identifier.
+     * @param curr Human-readable identifer.
+     */
+    template<std::size_t N>
+    constexpr basic_hashed_string(const value_type (&curr)[N]) ENTT_NOEXCEPT
+        : str{curr}, hash{helper(traits_type::offset, curr)}
     {}
 
     /**
      * @brief Explicit constructor on purpose to avoid constructing a hashed
-     * string directly from a `const char *`.
-     *
+     * string directly from a `const value_type *`.
      * @param wrapper Helps achieving the purpose by relying on overloading.
      */
-    explicit constexpr HashedString(ConstCharWrapper wrapper) noexcept
-        : hash{helper(offset, wrapper.str)}, str{wrapper.str}
+    explicit constexpr basic_hashed_string(const_wrapper wrapper) ENTT_NOEXCEPT
+        : str{wrapper.str}, hash{helper(traits_type::offset, wrapper.str)}
     {}
 
     /**
      * @brief Returns the human-readable representation of a hashed string.
      * @return The string used to initialize the instance.
      */
-    constexpr operator const char *() const noexcept { return str; }
+    constexpr const value_type * data() const ENTT_NOEXCEPT {
+        return str;
+    }
 
     /**
      * @brief Returns the numeric representation of a hashed string.
      * @return The numeric representation of the instance.
      */
-    constexpr operator hash_type() const noexcept { return hash; }
+    constexpr hash_type value() const ENTT_NOEXCEPT {
+        return hash;
+    }
+
+    /**
+     * @brief Returns the human-readable representation of a hashed string.
+     * @return The string used to initialize the instance.
+     */
+    constexpr operator const value_type *() const ENTT_NOEXCEPT { return str; }
+
+    /*! @copydoc value */
+    constexpr operator hash_type() const ENTT_NOEXCEPT { return hash; }
 
     /**
      * @brief Compares two hashed strings.
      * @param other Hashed string with which to compare.
      * @return True if the two hashed strings are identical, false otherwise.
      */
-    constexpr bool operator==(const HashedString &other) const noexcept {
+    constexpr bool operator==(const basic_hashed_string &other) const ENTT_NOEXCEPT {
         return hash == other.hash;
     }
 
 private:
-    const hash_type hash;
-    const char *str;
+    const value_type *str;
+    hash_type hash;
 };
 
 
+/**
+ * @brief Deduction guide.
+ *
+ * It allows to deduce the character type of the hashed string directly from a
+ * human-readable identifer provided to the constructor.
+ *
+ * @tparam Char Character type.
+ * @tparam N Number of characters of the identifier.
+ * @param str Human-readable identifer.
+ */
+template<typename Char, std::size_t N>
+basic_hashed_string(const Char (&str)[N]) ENTT_NOEXCEPT
+-> basic_hashed_string<Char>;
+
+
 /**
  * @brief Compares two hashed strings.
+ * @tparam Char Character type.
  * @param lhs A valid hashed string.
  * @param rhs A valid hashed string.
  * @return True if the two hashed strings are identical, false otherwise.
  */
-constexpr bool operator!=(const HashedString &lhs, const HashedString &rhs) noexcept {
+template<typename Char>
+constexpr bool operator!=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) ENTT_NOEXCEPT {
     return !(lhs == rhs);
 }
 
 
+/*! @brief Aliases for common character types. */
+using hashed_string = basic_hashed_string<char>;
+
+
+/*! @brief Aliases for common character types. */
+using hashed_wstring = basic_hashed_string<wchar_t>;
+
+
+}
+
+
+/**
+ * @brief User defined literal for hashed strings.
+ * @param str The literal without its suffix.
+ * @return A properly initialized hashed string.
+ */
+constexpr entt::hashed_string operator"" ENTT_HS_SUFFIX(const char *str, std::size_t) ENTT_NOEXCEPT {
+    return entt::hashed_string{str};
+}
+
+
+/**
+ * @brief User defined literal for hashed wstrings.
+ * @param str The literal without its suffix.
+ * @return A properly initialized hashed wstring.
+ */
+constexpr entt::hashed_wstring operator"" ENTT_HWS_SUFFIX(const wchar_t *str, std::size_t) ENTT_NOEXCEPT {
+    return entt::hashed_wstring{str};
 }
 
 

src/entt/core/ident.hpp

@@ -2,72 +2,33 @@
 #define ENTT_CORE_IDENT_HPP
 
 
-#include<type_traits>
-#include<cstddef>
-#include<utility>
+#include <tuple>
+#include <utility>
+#include <type_traits>
+#include "../config/config.h"
 
 
 namespace entt {
 
 
-namespace {
-
-
-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...>)
-        : Identifier<Types>{std::index_sequence<Indexes>{}}...
-    {}
-
-    template<typename Type>
-    constexpr std::size_t get() const {
-        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>)
-        : index{Index}
-    {}
-
-    constexpr std::size_t get() const {
-        return index;
-    }
-
-private:
-    const std::size_t index;
-};
-
-
-}
-
-
 /**
- * @brief Types identifers.
+ * @brief Types identifiers.
  *
  * 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.
+ * types. Use the `get` member function to know what's the identifier associated
+ * to the specific type.
  *
  * @note
  * Identifiers are constant expression and can be used in any context where such
  * an expression is required. As an example:
  * @code{.cpp}
- * constexpr auto identifiers = entt::ident<AType, AnotherType>;
+ * using id = entt::identifier<a_type, another_type>;
  *
- * switch(aTypeIdentifier) {
- * case identifers.get<AType>():
+ * switch(a_type_identifier) {
+ * case id::type<a_type>:
  *     // ...
  *     break;
- * case identifers.get<AnotherType>():
+ * case id::type<another_type>:
  *     // ...
  *     break;
  * default:
@@ -75,19 +36,26 @@ private:
  * }
  * @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 = Identifier<std::decay_t<Types>...>{std::make_index_sequence<sizeof...(Types)>{}};
+class identifier {
+    using tuple_type = std::tuple<std::decay_t<Types>...>;
+
+    template<typename Type, std::size_t... Indexes>
+    static constexpr ENTT_ID_TYPE get(std::index_sequence<Indexes...>) ENTT_NOEXCEPT {
+        static_assert(std::disjunction_v<std::is_same<Type, Types>...>);
+        return (0 + ... + (std::is_same_v<Type, std::tuple_element_t<Indexes, tuple_type>> ? ENTT_ID_TYPE(Indexes) : ENTT_ID_TYPE{}));
+    }
+
+public:
+    /*! @brief Unsigned integer type. */
+    using identifier_type = ENTT_ID_TYPE;
+
+    /*! @brief Statically generated unique identifier for the given type. */
+    template<typename Type>
+    static constexpr identifier_type type = get<std::decay_t<Type>>(std::index_sequence_for<Types...>{});
+};
 
 
 }

src/entt/core/monostate.hpp

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

src/entt/core/type_traits.hpp

@@ -0,0 +1,333 @@
+#ifndef ENTT_CORE_TYPE_TRAITS_HPP
+#define ENTT_CORE_TYPE_TRAITS_HPP
+
+
+#include <cstddef>
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/hashed_string.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Utility class to disambiguate overloaded functions.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+struct choice_t
+        // Unfortunately, doxygen cannot parse such a construct.
+        /*! @cond TURN_OFF_DOXYGEN */
+        : choice_t<N-1>
+        /*! @endcond TURN_OFF_DOXYGEN */
+{};
+
+
+/*! @copybrief choice_t */
+template<>
+struct choice_t<0> {};
+
+
+/**
+ * @brief Variable template for the choice trick.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+constexpr choice_t<N> choice{};
+
+
+/*! @brief A class to use to push around lists of types, nothing more. */
+template<typename...>
+struct type_list {};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename>
+struct type_list_size;
+
+
+/**
+ * @brief Compile-time number of elements in a type list.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list_size<type_list<Type...>>
+        : std::integral_constant<std::size_t, sizeof...(Type)>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam List Type list.
+ */
+template<class List>
+constexpr auto type_list_size_v = type_list_size<List>::value;
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_cat;
+
+
+/*! @brief Concatenates multiple type lists. */
+template<>
+struct type_list_cat<> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<>;
+};
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @tparam List Other type lists, if any.
+ */
+template<typename... Type, typename... Other, typename... List>
+struct type_list_cat<type_list<Type...>, type_list<Other...>, List...> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = typename type_list_cat<type_list<Type..., Other...>, List...>::type;
+};
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list_cat<type_list<Type...>> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<Type...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists to concatenate.
+ */
+template<typename... List>
+using type_list_cat_t = typename type_list_cat<List...>::type;
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename>
+struct type_list_unique;
+
+
+/**
+ * @brief Removes duplicates types from a type list.
+ * @tparam Type One of the types provided by the given type list.
+ * @tparam Other The other types provided by the given type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_unique<type_list<Type, Other...>> {
+    /*! @brief A type list without duplicate types. */
+    using type = std::conditional_t<
+        std::disjunction_v<std::is_same<Type, Other>...>,
+        typename type_list_unique<type_list<Other...>>::type,
+        type_list_cat_t<type_list<Type>, typename type_list_unique<type_list<Other...>>::type>
+    >;
+};
+
+
+/*! @brief Removes duplicates types from a type list. */
+template<>
+struct type_list_unique<type_list<>> {
+    /*! @brief A type list without duplicate types. */
+    using type = type_list<>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type A type list.
+ */
+template<typename Type>
+using type_list_unique_t = typename type_list_unique<Type>::type;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * equality comparable, false otherwise.
+ * @tparam Type Potentially equality comparable type.
+ */
+template<typename Type, typename = std::void_t<>>
+struct is_equality_comparable: std::false_type {};
+
+
+/*! @copydoc is_equality_comparable */
+template<typename Type>
+struct is_equality_comparable<Type, std::void_t<decltype(std::declval<Type>() == std::declval<Type>())>>: std::true_type {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type Potentially equality comparable type.
+ */
+template<class Type>
+constexpr auto is_equality_comparable_v = is_equality_comparable<Type>::value;
+
+
+/*! @brief Traits class used mainly to push things across boundaries. */
+template<typename>
+struct named_type_traits;
+
+
+/**
+ * @brief Specialization used to get rid of constness.
+ * @tparam Type Named type.
+ */
+template<typename Type>
+struct named_type_traits<const Type>
+        : named_type_traits<Type>
+{};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type Potentially named type.
+ */
+template<typename Type>
+using named_type_traits_t = typename named_type_traits<Type>::type;
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type Potentially named type.
+ */
+template<class Type>
+constexpr auto named_type_traits_v = named_type_traits<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type has a
+ * name. In all other cases, `value` is false.
+ * @tparam Type Potentially named type.
+ */
+template<typename Type, typename = std::void_t<>>
+struct is_named_type: std::false_type {};
+
+
+/*! @copydoc is_named_type */
+template<typename Type>
+struct is_named_type<Type, std::void_t<named_type_traits_t<std::decay_t<Type>>>>: std::true_type {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type Potentially named type.
+ */
+template<class Type>
+constexpr auto is_named_type_v = is_named_type<Type>::value;
+
+
+/**
+ * @brief Defines an enum class to use for opaque identifiers and a dedicate
+ * `to_integer` function to convert the identifiers to their underlying type.
+ * @param clazz The name to use for the enum class.
+ * @param type The underlying type for the enum class.
+ */
+#define ENTT_OPAQUE_TYPE(clazz, type)\
+    enum class clazz: type {};\
+    constexpr auto to_integer(const clazz id) ENTT_NOEXCEPT {\
+        return std::underlying_type_t<clazz>(id);\
+    }\
+    static_assert(true)
+
+
+}
+
+
+/**
+ * @brief Utility macro to deal with an issue of MSVC.
+ *
+ * See _msvc-doesnt-expand-va-args-correctly_ on SO for all the details.
+ *
+ * @param args Argument to expand.
+ */
+#define ENTT_EXPAND(args) args
+
+
+/**
+ * @brief Makes an already existing type a named type.
+ *
+ * The current definition contains a workaround for Clang 6 because it fails to
+ * deduce correctly the type to use to specialize the class template.<br/>
+ * With a compiler that fully supports C++17 and works fine with deduction
+ * guides, the following should be fine instead:
+ *
+ * @code{.cpp}
+ * std::integral_constant<ENTT_ID_TYPE, entt::basic_hashed_string{#type}>
+ * @endcode
+ *
+ * In order to support even sligthly older compilers, I prefer to stick to the
+ * implementation below.
+ *
+ * @param type Type to assign a name to.
+ */
+#define ENTT_NAMED_TYPE(type)\
+    template<>\
+    struct entt::named_type_traits<type>\
+        : std::integral_constant<ENTT_ID_TYPE, entt::basic_hashed_string<std::remove_cv_t<std::remove_pointer_t<std::decay_t<decltype(#type)>>>>{#type}>\
+    {\
+        static_assert(std::is_same_v<std::remove_cv_t<type>, type>);\
+        static_assert(std::is_object_v<type>);\
+    }
+
+
+/**
+ * @brief Defines a named type (to use for structs).
+ * @param clazz Name of the type to define.
+ * @param body Body of the type to define.
+ */
+#define ENTT_NAMED_STRUCT_ONLY(clazz, body)\
+    struct clazz body;\
+    ENTT_NAMED_TYPE(clazz)
+
+
+/**
+ * @brief Defines a named type (to use for structs).
+ * @param ns Namespace where to define the named type.
+ * @param clazz Name of the type to define.
+ * @param body Body of the type to define.
+ */
+#define ENTT_NAMED_STRUCT_WITH_NAMESPACE(ns, clazz, body)\
+    namespace ns { struct clazz body; }\
+    ENTT_NAMED_TYPE(ns::clazz)
+
+
+/*! @brief Utility function to simulate macro overloading. */
+#define ENTT_NAMED_STRUCT_OVERLOAD(_1, _2, _3, FUNC, ...) FUNC
+/*! @brief Defines a named type (to use for structs). */
+#define ENTT_NAMED_STRUCT(...) ENTT_EXPAND(ENTT_NAMED_STRUCT_OVERLOAD(__VA_ARGS__, ENTT_NAMED_STRUCT_WITH_NAMESPACE, ENTT_NAMED_STRUCT_ONLY,)(__VA_ARGS__))
+
+
+/**
+ * @brief Defines a named type (to use for classes).
+ * @param clazz Name of the type to define.
+ * @param body Body of the type to define.
+ */
+#define ENTT_NAMED_CLASS_ONLY(clazz, body)\
+    class clazz body;\
+    ENTT_NAMED_TYPE(clazz)
+
+
+/**
+ * @brief Defines a named type (to use for classes).
+ * @param ns Namespace where to define the named type.
+ * @param clazz Name of the type to define.
+ * @param body Body of the type to define.
+ */
+#define ENTT_NAMED_CLASS_WITH_NAMESPACE(ns, clazz, body)\
+    namespace ns { class clazz body; }\
+    ENTT_NAMED_TYPE(ns::clazz)
+
+
+/*! @brief Utility function to simulate macro overloading. */
+#define ENTT_NAMED_CLASS_MACRO(_1, _2, _3, FUNC, ...) FUNC
+/*! @brief Defines a named type (to use for classes). */
+#define ENTT_NAMED_CLASS(...) ENTT_EXPAND(ENTT_NAMED_CLASS_MACRO(__VA_ARGS__, ENTT_NAMED_CLASS_WITH_NAMESPACE, ENTT_NAMED_CLASS_ONLY,)(__VA_ARGS__))
+
+
+#endif // ENTT_CORE_TYPE_TRAITS_HPP

src/entt/core/utility.hpp

@@ -0,0 +1,104 @@
+#ifndef ENTT_CORE_UTILITY_HPP
+#define ENTT_CORE_UTILITY_HPP
+
+
+#include "../config/config.h"
+
+
+namespace entt {
+
+
+/*! @brief Identity function object (waiting for C++20). */
+struct identity {
+    /**
+     * @brief Returns its argument unchanged.
+     * @tparam Type Type of the argument.
+     * @param value The actual argument.
+     * @return The submitted value as-is.
+     */
+    template<class Type>
+    constexpr Type && operator()(Type &&value) const ENTT_NOEXCEPT {
+        return std::forward<Type>(value);
+    }
+};
+
+
+/**
+ * @brief Constant utility to disambiguate overloaded member functions.
+ * @tparam Type Function type of the desired overload.
+ * @tparam Class Type of class to which the member functions belong.
+ * @param member A valid pointer to a member function.
+ * @return Pointer to the member function.
+ */
+template<typename Type, typename Class>
+constexpr auto overload(Type Class:: *member) ENTT_NOEXCEPT { return member; }
+
+
+/**
+ * @brief Constant utility to disambiguate overloaded functions.
+ * @tparam Type Function type of the desired overload.
+ * @param func A valid pointer to a function.
+ * @return Pointer to the function.
+ */
+template<typename Type>
+constexpr auto overload(Type *func) ENTT_NOEXCEPT { return func; }
+
+
+/**
+ * @brief Helper type for visitors.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+struct overloaded: Func... {
+    using Func::operator()...;
+};
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Func Types of function objects.
+ */
+template<class... Type>
+overloaded(Type...) -> overloaded<Type...>;
+
+
+/**
+ * @brief Basic implementation of a y-combinator.
+ * @tparam Func Type of a potentially recursive function.
+ */
+template<class Func>
+struct y_combinator {
+    /**
+     * @brief Constructs a y-combinator from a given function.
+     * @param recursive A potentially recursive function.
+     */
+    y_combinator(Func recursive):
+        func{std::move(recursive)}
+    {}
+
+    /**
+     * @brief Invokes a y-combinator and therefore its underlying function.
+     * @tparam Args Types of arguments to use to invoke the underlying function.
+     * @param args Parameters to use to invoke the underlying function.
+     * @return Return value of the underlying function, if any.
+     */
+    template <class... Args>
+    decltype(auto) operator()(Args &&... args) const {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+    /*! @copydoc operator()() */
+    template <class... Args>
+    decltype(auto) operator()(Args &&... args) {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+private:
+    Func func;
+};
+
+
+}
+
+
+#endif // ENTT_CORE_UTILITY_HPP

src/entt/entity/actor.hpp

@@ -2,51 +2,99 @@
 #define ENTT_ENTITY_ACTOR_HPP
 
 
+#include <cassert>
 #include <utility>
+#include <type_traits>
+#include "../config/config.h"
 #include "registry.hpp"
+#include "entity.hpp"
+#include "fwd.hpp"
 
 
 namespace entt {
 
 
 /**
- * @brief Dedicated to those who aren't confident with entity-component systems.
+ * @brief Dedicated to those who aren't confident with the
+ * entity-component-system architecture.
  *
  * Tiny wrapper around a registry, for all those users that aren't confident
- * with entity-component systems and prefer to iterate objects directly.
+ * with entity-component-system architecture and prefer to iterate objects
+ * directly.
  *
  * @tparam Entity A valid entity type (see entt_traits for more details).
- * @tparam Delta Type to use to provide elapsed time.
  */
-template<typename Entity, typename Delta>
-struct Actor {
+template<typename Entity>
+struct basic_actor {
     /*! @brief Type of registry used internally. */
-    using registry_type = Registry<Entity>;
-    /*! @brief Type used to provide elapsed time. */
-    using delta_type = Delta;
+    using registry_type = basic_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}, entity{reg.create()}
+    basic_actor() ENTT_NOEXCEPT
+        : entt{entt::null}, reg{nullptr}
     {}
 
-    /*! @brief Default destructor. */
-    virtual ~Actor() {
-        reg.destroy(entity);
+    /**
+     * @brief Constructs an actor from a given registry.
+     * @param ref An instance of the registry class.
+     */
+    explicit basic_actor(registry_type &ref)
+        : entt{ref.create()}, reg{&ref}
+    {}
+
+    /**
+     * @brief Constructs an actor from a given entity.
+     * @param entity A valid entity identifier.
+     * @param ref An instance of the registry class.
+     */
+    explicit basic_actor(entity_type entity, registry_type &ref)
+        : entt{entity}, reg{&ref}
+    {
+        ENTT_ASSERT(ref.valid(entity));
     }
 
-    /*! @brief Default copy constructor. */
-    Actor(const Actor &) = default;
-    /*! @brief Default move constructor. */
-    Actor(Actor &&) = default;
+    /*! @brief Default destructor. */
+    virtual ~basic_actor() {
+        if(*this) {
+            reg->destroy(entt);
+        }
+    }
 
-    /*! @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 Move constructor.
+     *
+     * After actor move construction, instances that have been moved from are
+     * placed in a valid but unspecified state. It's highly discouraged to
+     * continue using them.
+     *
+     * @param other The instance to move from.
+     */
+    basic_actor(basic_actor &&other)
+        : entt{other.entt}, reg{other.reg}
+    {
+        other.entt = null;
+    }
+
+    /**
+     * @brief Move assignment operator.
+     *
+     * After actor move assignment, instances that have been moved from are
+     * placed in a valid but unspecified state. It's highly discouraged to
+     * continue using them.
+     *
+     * @param other The instance to move from.
+     * @return This actor.
+     */
+    basic_actor & operator=(basic_actor &&other) {
+        if(this != &other) {
+            auto tmp{std::move(other)};
+            std::swap(reg, tmp.reg);
+            std::swap(entt, tmp.entt);
+        }
+
+        return *this;
+    }
 
     /**
      * @brief Assigns the given component to an actor.
@@ -63,8 +111,8 @@ struct Actor {
      * @return A reference to the newly created component.
      */
     template<typename Component, typename... Args>
-    Component & set(Args&&... args) {
-        return reg.template accomodate<Component>(entity, std::forward<Args>(args)...);
+    decltype(auto) assign(Args &&... args) {
+        return reg->template assign_or_replace<Component>(entt, std::forward<Args>(args)...);
     }
 
     /**
@@ -72,80 +120,87 @@ struct Actor {
      * @tparam Component Type of the component to remove.
      */
     template<typename Component>
-    void unset() {
-        reg.template remove<Component>(entity);
+    void remove() {
+        reg->template remove<Component>(entt);
     }
 
     /**
-     * @brief Checks if an actor has the given component.
-     * @tparam Component Type of the component for which to perform the check.
-     * @return True if the actor has the component, false otherwise.
+     * @brief Checks if an actor has the given components.
+     * @tparam Component Components for which to perform the check.
+     * @return True if the actor has all the components, false otherwise.
      */
-    template<typename Component>
-    bool has() const noexcept {
-        return reg.template has<Component>(entity);
+    template<typename... Component>
+    bool has() const ENTT_NOEXCEPT {
+        return (reg->template has<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 entity.
+     * @brief Returns references to the given components for an actor.
+     * @tparam Component Types of components to get.
+     * @return References to the components owned by the actor.
      */
-    template<typename Component>
-    const Component & get() const noexcept {
-        return reg.template get<Component>(entity);
+    template<typename... Component>
+    decltype(auto) get() const ENTT_NOEXCEPT {
+        return std::as_const(*reg).template get<Component...>(entt);
+    }
+
+    /*! @copydoc get */
+    template<typename... Component>
+    decltype(auto) get() ENTT_NOEXCEPT {
+        return reg->template get<Component...>(entt);
     }
 
     /**
-     * @brief Returns 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 entity.
+     * @brief Returns pointers to the given components for an actor.
+     * @tparam Component Types of components to get.
+     * @return Pointers to the components owned by the actor.
      */
-    template<typename Component>
-    Component & get() noexcept {
-        return const_cast<Component &>(const_cast<const Actor *>(this)->get<Component>());
+    template<typename... Component>
+    auto try_get() const ENTT_NOEXCEPT {
+        return std::as_const(*reg).template try_get<Component...>(entt);
+    }
+
+    /*! @copydoc try_get */
+    template<typename... Component>
+    auto try_get() ENTT_NOEXCEPT {
+        return reg->template try_get<Component...>(entt);
     }
 
     /**
      * @brief Returns a reference to the underlying registry.
-     * @return A reference to the underlying registry
+     * @return A reference to the underlying registry.
      */
-    const registry_type & registry() const noexcept {
-        return reg;
+    const registry_type & backend() const ENTT_NOEXCEPT {
+        return *reg;
+    }
+
+    /*! @copydoc backend */
+    registry_type & backend() ENTT_NOEXCEPT {
+        return const_cast<registry_type &>(std::as_const(*this).backend());
     }
 
     /**
-     * @brief Returns a reference to the underlying registry.
-     * @return A reference to the underlying registry
+     * @brief Returns the entity associated with an actor.
+     * @return The entity associated with the actor.
      */
-    registry_type & registry() noexcept {
-        return const_cast<registry_type &>(const_cast<const Actor *>(this)->registry());
+    entity_type entity() const ENTT_NOEXCEPT {
+        return entt;
     }
 
     /**
-     * @brief Updates an actor, whatever it means to update it.
-     * @param delta Elapsed time.
+     * @brief Checks if an actor refers to a valid entity or not.
+     * @return True if the actor refers to a valid entity, false otherwise.
      */
-    virtual void update(delta_type delta) = 0;
+    explicit operator bool() const ENTT_NOEXCEPT {
+        return reg && reg->valid(entt);
+    }
 
 private:
-    registry_type &reg;
-    Entity entity;
+    entity_type entt;
+    registry_type *reg;
 };
 
 
-/**
- * @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.
- *
- * @tparam Delta Type to use to provide elapsed time.
- */
-template<typename Delta>
-using DefaultActor = Actor<std::uint32_t, Delta>;
-
-
 }
 
 

src/entt/entity/entity.hpp

@@ -0,0 +1,174 @@
+#ifndef ENTT_ENTITY_ENTITY_HPP
+#define ENTT_ENTITY_ENTITY_HPP
+
+
+#include <cstdint>
+#include <type_traits>
+#include "../config/config.h"
+
+
+namespace entt {
+
+
+/**
+ * @brief Entity traits.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error unless the template parameter is an accepted entity type.
+ */
+template<typename>
+struct entt_traits;
+
+
+/**
+ * @brief Entity traits for a 16 bits entity identifier.
+ *
+ * A 16 bits entity identifier guarantees:
+ *
+ * * 12 bits for the entity number (up to 4k entities).
+ * * 4 bit for the version (resets in [0-15]).
+ */
+template<>
+struct entt_traits<std::uint16_t> {
+    /*! @brief Underlying entity type. */
+    using entity_type = std::uint16_t;
+    /*! @brief Underlying version type. */
+    using version_type = std::uint8_t;
+    /*! @brief Difference type. */
+    using difference_type = std::int32_t;
+
+    /*! @brief Mask to use to get the entity number out of an identifier. */
+    static constexpr std::uint16_t entity_mask = 0xFFF;
+    /*! @brief Mask to use to get the version out of an identifier. */
+    static constexpr std::uint16_t version_mask = 0xF;
+    /*! @brief Extent of the entity number within an identifier. */
+    static constexpr auto entity_shift = 12;
+};
+
+
+/**
+ * @brief Entity traits for a 32 bits entity identifier.
+ *
+ * A 32 bits entity identifier guarantees:
+ *
+ * * 20 bits for the entity number (suitable for almost all the games).
+ * * 12 bit for the version (resets in [0-4095]).
+ */
+template<>
+struct entt_traits<std::uint32_t> {
+    /*! @brief Underlying entity type. */
+    using entity_type = std::uint32_t;
+    /*! @brief Underlying version type. */
+    using version_type = std::uint16_t;
+    /*! @brief Difference type. */
+    using difference_type = std::int64_t;
+
+    /*! @brief Mask to use to get the entity number out of an identifier. */
+    static constexpr std::uint32_t entity_mask = 0xFFFFF;
+    /*! @brief Mask to use to get the version out of an identifier. */
+    static constexpr std::uint32_t version_mask = 0xFFF;
+    /*! @brief Extent of the entity number within an identifier. */
+    static constexpr auto entity_shift = 20;
+};
+
+
+/**
+ * @brief Entity traits for a 64 bits entity identifier.
+ *
+ * A 64 bits entity identifier guarantees:
+ *
+ * * 32 bits for the entity number (an indecently large number).
+ * * 32 bit for the version (an indecently large number).
+ */
+template<>
+struct entt_traits<std::uint64_t> {
+    /*! @brief Underlying entity type. */
+    using entity_type = std::uint64_t;
+    /*! @brief Underlying version type. */
+    using version_type = std::uint32_t;
+    /*! @brief Difference type. */
+    using difference_type = std::int64_t;
+
+    /*! @brief Mask to use to get the entity number out of an identifier. */
+    static constexpr std::uint64_t entity_mask = 0xFFFFFFFF;
+    /*! @brief Mask to use to get the version out of an identifier. */
+    static constexpr std::uint64_t version_mask = 0xFFFFFFFF;
+    /*! @brief Extent of the entity number within an identifier. */
+    static constexpr auto entity_shift = 32;
+};
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+class null {
+    template<typename Entity>
+    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
+
+public:
+    template<typename Entity>
+    constexpr operator Entity() const ENTT_NOEXCEPT {
+        return Entity{traits_type<Entity>::entity_mask};
+    }
+
+    constexpr bool operator==(null) const ENTT_NOEXCEPT {
+        return true;
+    }
+
+    constexpr bool operator!=(null) const ENTT_NOEXCEPT {
+        return false;
+    }
+
+    template<typename Entity>
+    constexpr bool operator==(const Entity entity) const ENTT_NOEXCEPT {
+        return (to_integer(entity) & traits_type<Entity>::entity_mask) == to_integer(static_cast<Entity>(*this));
+    }
+
+    template<typename Entity>
+    constexpr bool operator!=(const Entity entity) const ENTT_NOEXCEPT {
+        return !(entity == *this);
+    }
+};
+
+
+template<typename Entity>
+constexpr bool operator==(const Entity entity, null other) ENTT_NOEXCEPT {
+    return other == entity;
+}
+
+
+template<typename Entity>
+constexpr bool operator!=(const Entity entity, null other) ENTT_NOEXCEPT {
+    return other != entity;
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond TURN_OFF_DOXYGEN
+ */
+
+
+/**
+ * @brief Compile-time constant for null entities.
+ *
+ * There exist implicit conversions from this variable to entity identifiers of
+ * any allowed type. Similarly, there exist comparision operators between the
+ * null entity and any other entity identifier.
+ */
+constexpr auto null = internal::null{};
+
+
+}
+
+
+#endif // ENTT_ENTITY_ENTITY_HPP

src/entt/entity/fwd.hpp

@@ -0,0 +1,92 @@
+#ifndef ENTT_ENTITY_FWD_HPP
+#define ENTT_ENTITY_FWD_HPP
+
+
+#include "../config/config.h"
+#include "../core/type_traits.hpp"
+
+
+namespace entt {
+
+/*! @class basic_registry */
+template <typename>
+class basic_registry;
+
+/*! @class basic_view */
+template<typename...>
+class basic_view;
+
+/*! @class basic_runtime_view */
+template<typename>
+class basic_runtime_view;
+
+/*! @class basic_group */
+template<typename...>
+class basic_group;
+
+/*! @class basic_observer */
+template<typename>
+class basic_observer;
+
+/*! @struct basic_actor */
+template <typename>
+struct basic_actor;
+
+/*! @class basic_snapshot */
+template<typename>
+class basic_snapshot;
+
+/*! @class basic_snapshot_loader */
+template<typename>
+class basic_snapshot_loader;
+
+/*! @class basic_continuous_loader */
+template<typename>
+class basic_continuous_loader;
+
+/*! @brief Alias declaration for the most common use case. */
+ENTT_OPAQUE_TYPE(entity, ENTT_ID_TYPE);
+
+/*! @brief Alias declaration for the most common use case. */
+ENTT_OPAQUE_TYPE(component, ENTT_ID_TYPE);
+
+/*! @brief Alias declaration for the most common use case. */
+using registry = basic_registry<entity>;
+
+/*! @brief Alias declaration for the most common use case. */
+using observer = basic_observer<entity>;
+
+/*! @brief Alias declaration for the most common use case. */
+using actor = basic_actor<entity>;
+
+/*! @brief Alias declaration for the most common use case. */
+using snapshot = basic_snapshot<entity>;
+
+/*! @brief Alias declaration for the most common use case. */
+using snapshot_loader = basic_snapshot_loader<entity>;
+
+/*! @brief Alias declaration for the most common use case. */
+using continuous_loader = basic_continuous_loader<entity>;
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Types Types of components iterated by the view.
+ */
+template<typename... Types>
+using view = basic_view<entity, Types...>;
+
+/*! @brief Alias declaration for the most common use case. */
+using runtime_view = basic_runtime_view<entity>;
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Types Types of components iterated by the group.
+ */
+template<typename... Types>
+using group = basic_group<entity, Types...>;
+
+
+}
+
+
+#endif // ENTT_ENTITY_FWD_HPP

src/entt/entity/group.hpp

@@ -0,0 +1,859 @@
+#ifndef ENTT_ENTITY_GROUP_HPP
+#define ENTT_ENTITY_GROUP_HPP
+
+
+#include <tuple>
+#include <utility>
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/type_traits.hpp"
+#include "sparse_set.hpp"
+#include "storage.hpp"
+#include "utility.hpp"
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Group.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error, but for a few reasonable cases.
+ */
+template<typename...>
+class basic_group;
+
+
+/**
+ * @brief Non-owning group.
+ *
+ * A non-owning group returns all the entities and only the entities that have
+ * at least the given components. Moreover, it's guaranteed that the entity list
+ * is tightly packed in memory for fast iterations.<br/>
+ * In general, non-owning groups don't stay true to the order of any set of
+ * components unless users explicitly sort them.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New instances of the given components are created and assigned to entities.
+ * * The entity currently pointed is modified (as an example, if one of the
+ *   given components is removed from the entity to which the iterator points).
+ * * The entity currently pointed is destroyed.
+ *
+ * In all the other cases, modifying the pools of the given components in any
+ * way invalidates all the iterators and using them results in undefined
+ * behavior.
+ *
+ * @note
+ * Groups share references to the underlying data structures of the registry
+ * that generated them. Therefore any change to the entities and to the
+ * components made by means of the registry are immediately reflected by all the
+ * groups.<br/>
+ * Moreover, sorting a non-owning group affects all the instance of the same
+ * group (it means that users don't have to call `sort` on each instance to sort
+ * all of them because they share the set of entities).
+ *
+ * @warning
+ * Lifetime of a group must overcome the one of the registry that generated it.
+ * In any other case, attempting to use a group results in undefined behavior.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Exclude Types of components used to filter the group.
+ * @tparam Get Type of components observed by the group.
+ */
+template<typename Entity, typename... Exclude, typename... Get>
+class basic_group<Entity, exclude_t<Exclude...>, get_t<Get...>> {
+    /*! @brief A registry is allowed to create groups. */
+    friend class basic_registry<Entity>;
+
+    template<typename Component>
+    using pool_type = std::conditional_t<std::is_const_v<Component>, const storage<Entity, std::remove_const_t<Component>>, storage<Entity, Component>>;
+
+    // we could use pool_type<Type> *..., but vs complains about it and refuses to compile for unknown reasons (most likely a bug)
+    basic_group(sparse_set<Entity> *ref, storage<Entity, std::remove_const_t<Get>> *... gpool) ENTT_NOEXCEPT
+        : handler{ref},
+          pools{gpool...}
+    {}
+
+    template<typename Func, typename... Weak>
+    void traverse(Func func, type_list<Weak...>) const {
+        for(const auto entt: *handler) {
+            if constexpr(std::is_invocable_v<Func, decltype(get<Weak>({}))...>) {
+                func(std::get<pool_type<Weak> *>(pools)->get(entt)...);
+            } else {
+                func(entt, std::get<pool_type<Weak> *>(pools)->get(entt)...);
+            }
+        }
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Input iterator type. */
+    using iterator_type = typename sparse_set<Entity>::iterator_type;
+
+    /**
+     * @brief Returns the number of existing components of the given type.
+     * @tparam Component Type of component of which to return the size.
+     * @return Number of existing components of the given type.
+     */
+    template<typename Component>
+    size_type size() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Component> *>(pools)->size();
+    }
+
+    /**
+     * @brief Returns the number of entities that have the given components.
+     * @return Number of entities that have the given components.
+     */
+    size_type size() const ENTT_NOEXCEPT {
+        return handler->size();
+    }
+
+    /**
+     * @brief Returns the number of elements that a group has currently
+     * allocated space for.
+     * @return Capacity of the group.
+     */
+    size_type capacity() const ENTT_NOEXCEPT {
+        return handler->capacity();
+    }
+
+    /*! @brief Requests the removal of unused capacity. */
+    void shrink_to_fit() {
+        handler->shrink_to_fit();
+    }
+
+    /**
+     * @brief Checks whether the group or the pools of the given components are
+     * empty.
+     * @tparam Component Types of components in which one is interested.
+     * @return True if the group or the pools of the given components are empty,
+     * false otherwise.
+     */
+    template<typename... Component>
+    bool empty() const ENTT_NOEXCEPT {
+        if constexpr(sizeof...(Component) == 0) {
+            return handler->empty();
+        } else {
+            return (std::get<pool_type<Component> *>(pools)->empty() && ...);
+        }
+    }
+
+    /**
+     * @brief Direct access to the list of components of a given pool.
+     *
+     * The returned pointer is such that range
+     * `[raw<Component>(), raw<Component>() + size<Component>()]` is always a
+     * valid range, even if the container is empty.
+     *
+     * @note
+     * There are no guarantees on the order of the components. Use `begin` and
+     * `end` if you want to iterate the group in the expected order.
+     *
+     * @tparam Component Type of component in which one is interested.
+     * @return A pointer to the array of components.
+     */
+    template<typename Component>
+    Component * raw() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Component> *>(pools)->raw();
+    }
+
+    /**
+     * @brief Direct access to the list of entities of a given pool.
+     *
+     * The returned pointer is such that range
+     * `[data<Component>(), data<Component>() + size<Component>()]` is always a
+     * valid range, even if the container is empty.
+     *
+     * @note
+     * There are no guarantees on the order of the entities. Use `begin` and
+     * `end` if you want to iterate the group in the expected order.
+     *
+     * @tparam Component Type of component in which one is interested.
+     * @return A pointer to the array of entities.
+     */
+    template<typename Component>
+    const entity_type * data() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Component> *>(pools)->data();
+    }
+
+    /**
+     * @brief Direct access to the list of entities.
+     *
+     * The returned pointer is such that range `[data(), data() + size()]` is
+     * always a valid range, even if the container is empty.
+     *
+     * @note
+     * There are no guarantees on the order of the entities. Use `begin` and
+     * `end` if you want to iterate the group in the expected order.
+     *
+     * @return A pointer to the array of entities.
+     */
+    const entity_type * data() const ENTT_NOEXCEPT {
+        return handler->data();
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity that has the given
+     * components.
+     *
+     * The returned iterator points to the first entity that has the given
+     * components. If the group is empty, the returned iterator will be equal to
+     * `end()`.
+     *
+     * @note
+     * Input iterators stay true to the order imposed to the underlying data
+     * structures.
+     *
+     * @return An iterator to the first entity that has the given components.
+     */
+    iterator_type begin() const ENTT_NOEXCEPT {
+        return handler->begin();
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity that has the
+     * given components.
+     *
+     * The returned iterator points to the entity following the last entity that
+     * has the given components. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @note
+     * Input iterators stay true to the order imposed to the underlying data
+     * structures.
+     *
+     * @return An iterator to the entity following the last entity that has the
+     * given components.
+     */
+    iterator_type end() const ENTT_NOEXCEPT {
+        return handler->end();
+    }
+
+    /**
+     * @brief Finds an entity.
+     * @param entt A valid entity identifier.
+     * @return An iterator to the given entity if it's found, past the end
+     * iterator otherwise.
+     */
+    iterator_type find(const entity_type entt) const ENTT_NOEXCEPT {
+        const auto it = handler->find(entt);
+        return it != end() && *it == entt ? it : end();
+    }
+
+    /**
+     * @brief Returns the identifier that occupies the given position.
+     * @param pos Position of the element to return.
+     * @return The identifier that occupies the given position.
+     */
+    entity_type operator[](const size_type pos) const ENTT_NOEXCEPT {
+        return begin()[pos];
+    }
+
+    /**
+     * @brief Checks if a group contains an entity.
+     * @param entt A valid entity identifier.
+     * @return True if the group contains the given entity, false otherwise.
+     */
+    bool contains(const entity_type entt) const ENTT_NOEXCEPT {
+        return find(entt) != end();
+    }
+
+    /**
+     * @brief Returns the components assigned to the given entity.
+     *
+     * Prefer this function instead of `registry::get` during iterations. It has
+     * far better performance than its companion function.
+     *
+     * @warning
+     * Attempting to use an invalid component type results in a compilation
+     * error. Attempting to use an entity that doesn't belong to the group
+     * results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * group doesn't contain the given entity.
+     *
+     * @tparam Component Types of components to get.
+     * @param entt A valid entity identifier.
+     * @return The components assigned to the entity.
+     */
+    template<typename... Component>
+    decltype(auto) get([[maybe_unused]] const entity_type entt) const ENTT_NOEXCEPT {
+        ENTT_ASSERT(contains(entt));
+
+        if constexpr(sizeof...(Component) == 1) {
+            return (std::get<pool_type<Component> *>(pools)->get(entt), ...);
+        } else {
+            return std::tuple<decltype(get<Component>({}))...>{get<Component>(entt)...};
+        }
+    }
+
+    /**
+     * @brief Iterates entities and components and applies the given function
+     * object to them.
+     *
+     * The function object is invoked for each entity. It is provided with the
+     * entity itself and a set of references to all its components. The
+     * _constness_ of the components is as requested.<br/>
+     * The signature of the function must be equivalent to one of the following
+     * forms:
+     *
+     * @code{.cpp}
+     * void(const entity_type, Get &...);
+     * void(Get &...);
+     * @endcode
+     *
+     * @note
+     * Empty types aren't explicitly instantiated. Therefore, temporary objects
+     * are returned during iterations. They can be caught only by copy or with
+     * const references.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        traverse(std::move(func), type_list<Get...>{});
+    }
+
+    /**
+     * @brief Iterates entities and components and applies the given function
+     * object to them.
+     *
+     * The function object is invoked for each entity. It is provided with the
+     * entity itself and a set of references to non-empty components. The
+     * _constness_ of the components is as requested.<br/>
+     * The signature of the function must be equivalent to one of the following
+     * forms:
+     *
+     * @code{.cpp}
+     * void(const entity_type, Type &...);
+     * void(Type &...);
+     * @endcode
+     *
+     * @sa each
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void less(Func func) const {
+        using get_type_list = type_list_cat_t<std::conditional_t<ENTT_ENABLE_ETO(Get), type_list<>, type_list<Get>>...>;
+        traverse(std::move(func), get_type_list{});
+    }
+
+    /**
+     * @brief Sort a group according to the given comparison function.
+     *
+     * Sort the group so that iterating it with a couple of iterators returns
+     * entities and components in the expected order. See `begin` and `end` for
+     * more details.
+     *
+     * The comparison function object must return `true` if the first element
+     * is _less_ than the second one, `false` otherwise. The signature of the
+     * comparison function should be equivalent to one of the following:
+     *
+     * @code{.cpp}
+     * bool(std::tuple<Component &...>, std::tuple<Component &...>);
+     * bool(const Component &..., const Component &...);
+     * bool(const Entity, const Entity);
+     * @endcode
+     *
+     * Where `Component` are such that they are iterated by the group.<br/>
+     * Moreover, the comparison function object shall induce a
+     * _strict weak ordering_ on the values.
+     *
+     * The sort function oject must offer a member function template
+     * `operator()` that accepts three arguments:
+     *
+     * * An iterator to the first element of the range to sort.
+     * * An iterator past the last element of the range to sort.
+     * * A comparison function to use to compare the elements.
+     *
+     * @note
+     * Attempting to iterate elements using a raw pointer returned by a call to
+     * either `data` or `raw` gives no guarantees on the order, even though
+     * `sort` has been invoked.
+     *
+     * @tparam Component Optional types of components to compare.
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename... Component, typename Compare, typename Sort = std_sort, typename... Args>
+    void sort(Compare compare, Sort algo = Sort{}, Args &&... args) {
+        if constexpr(sizeof...(Component) == 0) {
+            static_assert(std::is_invocable_v<Compare, const entity_type, const entity_type>);
+            handler->sort(handler->begin(), handler->end(), std::move(compare), std::move(algo), std::forward<Args>(args)...);
+        }  else if constexpr(sizeof...(Component) == 1) {
+            handler->sort(handler->begin(), handler->end(), [this, compare = std::move(compare)](const entity_type lhs, const entity_type rhs) {
+                return compare((std::get<pool_type<Component> *>(pools)->get(lhs), ...), (std::get<pool_type<Component> *>(pools)->get(rhs), ...));
+            }, std::move(algo), std::forward<Args>(args)...);
+        } else {
+            handler->sort(handler->begin(), handler->end(), [this, compare = std::move(compare)](const entity_type lhs, const entity_type rhs) {
+                return compare(std::tuple<decltype(get<Component>({}))...>{std::get<pool_type<Component> *>(pools)->get(lhs)...}, std::tuple<decltype(get<Component>({}))...>{std::get<pool_type<Component> *>(pools)->get(rhs)...});
+            }, std::move(algo), std::forward<Args>(args)...);
+        }
+    }
+
+    /**
+     * @brief Sort the shared pool of entities according to the given component.
+     *
+     * Non-owning groups of the same type share with the registry a pool of
+     * entities with  its own order that doesn't depend on the order of any pool
+     * of components. Users can order the underlying data structure so that it
+     * respects the order of the pool of the given component.
+     *
+     * @note
+     * The shared pool of entities and thus its order is affected by the changes
+     * to each and every pool that it tracks. Therefore changes to those pools
+     * can quickly ruin the order imposed to the pool of entities shared between
+     * the non-owning groups.
+     *
+     * @tparam Component Type of component to use to impose the order.
+     */
+    template<typename Component>
+    void sort() const {
+        handler->respect(*std::get<pool_type<Component> *>(pools));
+    }
+
+private:
+    sparse_set<entity_type> *handler;
+    const std::tuple<pool_type<Get> *...> pools;
+};
+
+
+/**
+ * @brief Owning group.
+ *
+ * Owning groups return all the entities and only the entities that have at
+ * least the given components. Moreover:
+ *
+ * * It's guaranteed that the entity list is tightly packed in memory for fast
+ *   iterations.
+ * * It's guaranteed that the lists of owned components are tightly packed in
+ *   memory for even faster iterations and to allow direct access.
+ * * They stay true to the order of the owned components and all the owned
+ *   components have the same order in memory.
+ *
+ * The more types of components are owned by a group, the faster it is to
+ * iterate them.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New instances of the given components are created and assigned to entities.
+ * * The entity currently pointed is modified (as an example, if one of the
+ *   given components is removed from the entity to which the iterator points).
+ * * The entity currently pointed is destroyed.
+ *
+ * In all the other cases, modifying the pools of the given components in any
+ * way invalidates all the iterators and using them results in undefined
+ * behavior.
+ *
+ * @note
+ * Groups share references to the underlying data structures of the registry
+ * that generated them. Therefore any change to the entities and to the
+ * components made by means of the registry are immediately reflected by all the
+ * groups.
+ * Moreover, sorting an owning group affects all the instance of the same group
+ * (it means that users don't have to call `sort` on each instance to sort all
+ * of them because they share the underlying data structure).
+ *
+ * @warning
+ * Lifetime of a group must overcome the one of the registry that generated it.
+ * In any other case, attempting to use a group results in undefined behavior.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Exclude Types of components used to filter the group.
+ * @tparam Get Types of components observed by the group.
+ * @tparam Owned Types of components owned by the group.
+ */
+template<typename Entity, typename... Exclude, typename... Get, typename... Owned>
+class basic_group<Entity, exclude_t<Exclude...>, get_t<Get...>, Owned...> {
+    /*! @brief A registry is allowed to create groups. */
+    friend class basic_registry<Entity>;
+
+    template<typename Component>
+    using pool_type = std::conditional_t<std::is_const_v<Component>, const storage<Entity, std::remove_const_t<Component>>, storage<Entity, Component>>;
+
+    template<typename Component>
+    using component_iterator_type = decltype(std::declval<pool_type<Component>>().begin());
+
+    // we could use pool_type<Type> *..., but vs complains about it and refuses to compile for unknown reasons (most likely a bug)
+    basic_group(const std::size_t *ref, const std::size_t *extent, storage<Entity, std::remove_const_t<Owned>> *... opool, storage<Entity, std::remove_const_t<Get>> *... gpool) ENTT_NOEXCEPT
+        : pools{opool..., gpool...},
+          length{extent},
+          super{ref}
+    {}
+
+    template<typename Func, typename... Strong, typename... Weak>
+    void traverse(Func func, type_list<Strong...>, type_list<Weak...>) const {
+        [[maybe_unused]] auto it = std::make_tuple((std::get<pool_type<Strong> *>(pools)->end() - *length)...);
+        [[maybe_unused]] auto data = std::get<0>(pools)->sparse_set<entity_type>::end() - *length;
+
+        for(auto next = *length; next; --next) {
+            if constexpr(std::is_invocable_v<Func, decltype(get<Strong>({}))..., decltype(get<Weak>({}))...>) {
+                if constexpr(sizeof...(Weak) == 0) {
+                    func(*(std::get<component_iterator_type<Strong>>(it)++)...);
+                } else {
+                    const auto entt = *(data++);
+                    func(*(std::get<component_iterator_type<Strong>>(it)++)..., std::get<pool_type<Weak> *>(pools)->get(entt)...);
+                }
+            } else {
+                const auto entt = *(data++);
+                func(entt, *(std::get<component_iterator_type<Strong>>(it)++)..., std::get<pool_type<Weak> *>(pools)->get(entt)...);
+            }
+        }
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Input iterator type. */
+    using iterator_type = typename sparse_set<Entity>::iterator_type;
+
+    /**
+     * @brief Returns the number of existing components of the given type.
+     * @tparam Component Type of component of which to return the size.
+     * @return Number of existing components of the given type.
+     */
+    template<typename Component>
+    size_type size() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Component> *>(pools)->size();
+    }
+
+    /**
+     * @brief Returns the number of entities that have the given components.
+     * @return Number of entities that have the given components.
+     */
+    size_type size() const ENTT_NOEXCEPT {
+        return *length;
+    }
+
+    /**
+     * @brief Checks whether the group or the pools of the given components are
+     * empty.
+     * @tparam Component Types of components in which one is interested.
+     * @return True if the group or the pools of the given components are empty,
+     * false otherwise.
+     */
+    template<typename... Component>
+    bool empty() const ENTT_NOEXCEPT {
+        if constexpr(sizeof...(Component) == 0) {
+            return !*length;
+        } else {
+            return (std::get<pool_type<Component> *>(pools)->empty() && ...);
+        }
+    }
+
+    /**
+     * @brief Direct access to the list of components of a given pool.
+     *
+     * The returned pointer is such that range
+     * `[raw<Component>(), raw<Component>() + size<Component>()]` is always a
+     * valid range, even if the container is empty.<br/>
+     * Moreover, in case the group owns the given component, the range
+     * `[raw<Component>(), raw<Component>() + size()]` is such that it contains
+     * the instances that are part of the group itself.
+     *
+     * @note
+     * There are no guarantees on the order of the components. Use `begin` and
+     * `end` if you want to iterate the group in the expected order.
+     *
+     * @tparam Component Type of component in which one is interested.
+     * @return A pointer to the array of components.
+     */
+    template<typename Component>
+    Component * raw() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Component> *>(pools)->raw();
+    }
+
+    /**
+     * @brief Direct access to the list of entities of a given pool.
+     *
+     * The returned pointer is such that range
+     * `[data<Component>(), data<Component>() + size<Component>()]` is always a
+     * valid range, even if the container is empty.<br/>
+     * Moreover, in case the group owns the given component, the range
+     * `[data<Component>(), data<Component>() + size()]` is such that it
+     * contains the entities that are part of the group itself.
+     *
+     * @note
+     * There are no guarantees on the order of the entities. Use `begin` and
+     * `end` if you want to iterate the group in the expected order.
+     *
+     * @tparam Component Type of component in which one is interested.
+     * @return A pointer to the array of entities.
+     */
+    template<typename Component>
+    const entity_type * data() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Component> *>(pools)->data();
+    }
+
+    /**
+     * @brief Direct access to the list of entities.
+     *
+     * The returned pointer is such that range `[data(), data() + size()]` is
+     * always a valid range, even if the container is empty.
+     *
+     * @note
+     * There are no guarantees on the order of the entities. Use `begin` and
+     * `end` if you want to iterate the group in the expected order.
+     *
+     * @return A pointer to the array of entities.
+     */
+    const entity_type * data() const ENTT_NOEXCEPT {
+        return std::get<0>(pools)->data();
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity that has the given
+     * components.
+     *
+     * The returned iterator points to the first entity that has the given
+     * components. If the group is empty, the returned iterator will be equal to
+     * `end()`.
+     *
+     * @note
+     * Input iterators stay true to the order imposed to the underlying data
+     * structures.
+     *
+     * @return An iterator to the first entity that has the given components.
+     */
+    iterator_type begin() const ENTT_NOEXCEPT {
+        return std::get<0>(pools)->sparse_set<entity_type>::end() - *length;
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity that has the
+     * given components.
+     *
+     * The returned iterator points to the entity following the last entity that
+     * has the given components. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @note
+     * Input iterators stay true to the order imposed to the underlying data
+     * structures.
+     *
+     * @return An iterator to the entity following the last entity that has the
+     * given components.
+     */
+    iterator_type end() const ENTT_NOEXCEPT {
+        return std::get<0>(pools)->sparse_set<entity_type>::end();
+    }
+
+    /**
+     * @brief Finds an entity.
+     * @param entt A valid entity identifier.
+     * @return An iterator to the given entity if it's found, past the end
+     * iterator otherwise.
+     */
+    iterator_type find(const entity_type entt) const ENTT_NOEXCEPT {
+        const auto it = std::get<0>(pools)->find(entt);
+        return it != end() && it >= begin() && *it == entt ? it : end();
+    }
+
+    /**
+     * @brief Returns the identifier that occupies the given position.
+     * @param pos Position of the element to return.
+     * @return The identifier that occupies the given position.
+     */
+    entity_type operator[](const size_type pos) const ENTT_NOEXCEPT {
+        return begin()[pos];
+    }
+
+    /**
+     * @brief Checks if a group contains an entity.
+     * @param entt A valid entity identifier.
+     * @return True if the group contains the given entity, false otherwise.
+     */
+    bool contains(const entity_type entt) const ENTT_NOEXCEPT {
+        return find(entt) != end();
+    }
+
+    /**
+     * @brief Returns the components assigned to the given entity.
+     *
+     * Prefer this function instead of `registry::get` during iterations. It has
+     * far better performance than its companion function.
+     *
+     * @warning
+     * Attempting to use an invalid component type results in a compilation
+     * error. Attempting to use an entity that doesn't belong to the group
+     * results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * group doesn't contain the given entity.
+     *
+     * @tparam Component Types of components to get.
+     * @param entt A valid entity identifier.
+     * @return The components assigned to the entity.
+     */
+    template<typename... Component>
+    decltype(auto) get([[maybe_unused]] const entity_type entt) const ENTT_NOEXCEPT {
+        ENTT_ASSERT(contains(entt));
+
+        if constexpr(sizeof...(Component) == 1) {
+            return (std::get<pool_type<Component> *>(pools)->get(entt), ...);
+        } else {
+            return std::tuple<decltype(get<Component>({}))...>{get<Component>(entt)...};
+        }
+    }
+
+    /**
+     * @brief Iterates entities and components and applies the given function
+     * object to them.
+     *
+     * The function object is invoked for each entity. It is provided with the
+     * entity itself and a set of references to all its components. The
+     * _constness_ of the components is as requested.<br/>
+     * The signature of the function must be equivalent to one of the following
+     * forms:
+     *
+     * @code{.cpp}
+     * void(const entity_type, Owned &..., Get &...);
+     * void(Owned &..., Get &...);
+     * @endcode
+     *
+     * @note
+     * Empty types aren't explicitly instantiated. Therefore, temporary objects
+     * are returned during iterations. They can be caught only by copy or with
+     * const references.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        traverse(std::move(func), type_list<Owned...>{}, type_list<Get...>{});
+    }
+
+    /**
+     * @brief Iterates entities and components and applies the given function
+     * object to them.
+     *
+     * The function object is invoked for each entity. It is provided with the
+     * entity itself and a set of references to non-empty components. The
+     * _constness_ of the components is as requested.<br/>
+     * The signature of the function must be equivalent to one of the following
+     * forms:
+     *
+     * @code{.cpp}
+     * void(const entity_type, Type &...);
+     * void(Type &...);
+     * @endcode
+     *
+     * @sa each
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void less(Func func) const {
+        using owned_type_list = type_list_cat_t<std::conditional_t<ENTT_ENABLE_ETO(Owned), type_list<>, type_list<Owned>>...>;
+        using get_type_list = type_list_cat_t<std::conditional_t<ENTT_ENABLE_ETO(Get), type_list<>, type_list<Get>>...>;
+        traverse(std::move(func), owned_type_list{}, get_type_list{});
+    }
+
+    /**
+     * @brief Checks whether the group can be sorted.
+     * @return True if the group can be sorted, false otherwise.
+     */
+    bool sortable() const ENTT_NOEXCEPT {
+        constexpr auto size = sizeof...(Owned) + sizeof...(Get) + sizeof...(Exclude);
+        return *super == size;
+    }
+
+    /**
+     * @brief Sort a group according to the given comparison function.
+     *
+     * Sort the group so that iterating it with a couple of iterators returns
+     * entities and components in the expected order. See `begin` and `end` for
+     * more details.
+     *
+     * The comparison function object must return `true` if the first element
+     * is _less_ than the second one, `false` otherwise. The signature of the
+     * comparison function should be equivalent to one of the following:
+     *
+     * @code{.cpp}
+     * bool(std::tuple<Component &...>, std::tuple<Component &...>);
+     * bool(const Component &, const Component &);
+     * bool(const Entity, const Entity);
+     * @endcode
+     *
+     * Where `Component` are either owned types or not but still such that they
+     * are iterated by the group.<br/>
+     * Moreover, the comparison function object shall induce a
+     * _strict weak ordering_ on the values.
+     *
+     * The sort function oject must offer a member function template
+     * `operator()` that accepts three arguments:
+     *
+     * * An iterator to the first element of the range to sort.
+     * * An iterator past the last element of the range to sort.
+     * * A comparison function to use to compare the elements.
+     *
+     * @note
+     * Attempting to iterate elements using a raw pointer returned by a call to
+     * either `data` or `raw` gives no guarantees on the order, even though
+     * `sort` has been invoked.
+     *
+     * @tparam Component Optional types of components to compare.
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename... Component, typename Compare, typename Sort = std_sort, typename... Args>
+    void sort(Compare compare, Sort algo = Sort{}, Args &&... args) {
+        ENTT_ASSERT(sortable());
+        auto *cpool = std::get<0>(pools);
+
+        if constexpr(sizeof...(Component) == 0) {
+            static_assert(std::is_invocable_v<Compare, const entity_type, const entity_type>);
+            cpool->sort(cpool->end()-*length, cpool->end(), std::move(compare), std::move(algo), std::forward<Args>(args)...);
+        } else if constexpr(sizeof...(Component) == 1) {
+            cpool->sort(cpool->end()-*length, cpool->end(), [this, compare = std::move(compare)](const entity_type lhs, const entity_type rhs) {
+                return compare((std::get<pool_type<Component> *>(pools)->get(lhs), ...), (std::get<pool_type<Component> *>(pools)->get(rhs), ...));
+            }, std::move(algo), std::forward<Args>(args)...);
+        } else {
+            cpool->sort(cpool->end()-*length, cpool->end(), [this, compare = std::move(compare)](const entity_type lhs, const entity_type rhs) {
+                return compare(std::tuple<decltype(get<Component>({}))...>{std::get<pool_type<Component> *>(pools)->get(lhs)...}, std::tuple<decltype(get<Component>({}))...>{std::get<pool_type<Component> *>(pools)->get(rhs)...});
+            }, std::move(algo), std::forward<Args>(args)...);
+        }
+
+        [this](auto *head, auto *... other) {
+            for(auto next = *length; next; --next) {
+                const auto pos = next - 1;
+                [[maybe_unused]] const auto entt = head->data()[pos];
+                (other->swap(other->data()[pos], entt), ...);
+            }
+        }(std::get<pool_type<Owned> *>(pools)...);
+    }
+
+private:
+    const std::tuple<pool_type<Owned> *..., pool_type<Get> *...> pools;
+    const size_type *length;
+    const size_type *super;
+};
+
+
+}
+
+
+#endif // ENTT_ENTITY_GROUP_HPP

src/entt/entity/helper.hpp

@@ -0,0 +1,139 @@
+#ifndef ENTT_ENTITY_HELPER_HPP
+#define ENTT_ENTITY_HELPER_HPP
+
+
+#include <type_traits>
+#include "../config/config.h"
+#include "../signal/sigh.hpp"
+#include "registry.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Converts a registry to a view.
+ * @tparam Const Constness of the accepted registry.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<bool Const, typename Entity>
+struct as_view {
+    /*! @brief Type of registry to convert. */
+    using registry_type = std::conditional_t<Const, const entt::basic_registry<Entity>, entt::basic_registry<Entity>>;
+
+    /**
+     * @brief Constructs a converter for a given registry.
+     * @param source A valid reference to a registry.
+     */
+    as_view(registry_type &source) ENTT_NOEXCEPT: reg{source} {}
+
+    /**
+     * @brief Conversion function from a registry to a view.
+     * @tparam Exclude Types of components used to filter the view.
+     * @tparam Component Type of components used to construct the view.
+     * @return A newly created view.
+     */
+    template<typename Exclude, typename... Component>
+    operator entt::basic_view<Entity, Exclude, Component...>() const {
+        return reg.template view<Component...>(Exclude{});
+    }
+
+private:
+    registry_type &reg;
+};
+
+
+/**
+ * @brief Deduction guide.
+ *
+ * It allows to deduce the constness of a registry directly from the instance
+ * provided to the constructor.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+as_view(basic_registry<Entity> &) ENTT_NOEXCEPT -> as_view<false, Entity>;
+
+
+/*! @copydoc as_view */
+template<typename Entity>
+as_view(const basic_registry<Entity> &) ENTT_NOEXCEPT -> as_view<true, Entity>;
+
+
+/**
+ * @brief Converts a registry to a group.
+ * @tparam Const Constness of the accepted registry.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<bool Const, typename Entity>
+struct as_group {
+    /*! @brief Type of registry to convert. */
+    using registry_type = std::conditional_t<Const, const entt::basic_registry<Entity>, entt::basic_registry<Entity>>;
+
+    /**
+     * @brief Constructs a converter for a given registry.
+     * @param source A valid reference to a registry.
+     */
+    as_group(registry_type &source) ENTT_NOEXCEPT: reg{source} {}
+
+    /**
+     * @brief Conversion function from a registry to a group.
+     * @tparam Exclude Types of components used to filter the group.
+     * @tparam Get Types of components observed by the group.
+     * @tparam Owned Types of components owned by the group.
+     * @return A newly created group.
+     */
+    template<typename Exclude, typename Get, typename... Owned>
+    operator entt::basic_group<Entity, Exclude, Get, Owned...>() const {
+        return reg.template group<Owned...>(Get{}, Exclude{});
+    }
+
+private:
+    registry_type &reg;
+};
+
+
+/**
+ * @brief Deduction guide.
+ *
+ * It allows to deduce the constness of a registry directly from the instance
+ * provided to the constructor.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+as_group(basic_registry<Entity> &) ENTT_NOEXCEPT -> as_group<false, Entity>;
+
+
+/*! @copydoc as_group */
+template<typename Entity>
+as_group(const basic_registry<Entity> &) ENTT_NOEXCEPT -> as_group<true, Entity>;
+
+
+/**
+ * @brief Alias template to ease the assignment of tags to entities.
+ *
+ * If used in combination with hashed strings, it simplifies the assignment of
+ * tags to entities and the use of tags in general where a type would be
+ * required otherwise.<br/>
+ * As an example and where the user defined literal for hashed strings hasn't
+ * been changed:
+ * @code{.cpp}
+ * entt::registry registry;
+ * registry.assign<entt::tag<"enemy"_hs>>(entity);
+ * @endcode
+ *
+ * @note
+ * Tags are empty components and therefore candidates for the empty component
+ * optimization.
+ *
+ * @tparam Value The numeric representation of an instance of hashed string.
+ */
+template<ENTT_ID_TYPE Value>
+using tag = std::integral_constant<ENTT_ID_TYPE, Value>;
+
+
+}
+
+
+#endif // ENTT_ENTITY_HELPER_HPP

src/entt/entity/observer.hpp

@@ -0,0 +1,442 @@
+#ifndef ENTT_ENTITY_OBSERVER_HPP
+#define ENTT_ENTITY_OBSERVER_HPP
+
+
+#include <limits>
+#include <cstddef>
+#include <cstdint>
+#include <utility>
+#include <algorithm>
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/type_traits.hpp"
+#include "registry.hpp"
+#include "storage.hpp"
+#include "entity.hpp"
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/*! @brief Grouping matcher. */
+template<typename...>
+struct matcher {};
+
+
+/**
+ * @brief Collector.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error, but for a few reasonable cases.
+ */
+template<typename...>
+struct basic_collector;
+
+
+/**
+ * @brief Collector.
+ *
+ * A collector contains a set of rules (literally, matchers) to use to track
+ * entities.<br/>
+ * Its main purpose is to generate a descriptor that allows an observer to know
+ * how to connect to a registry.
+ */
+template<>
+struct basic_collector<> {
+    /**
+     * @brief Adds a grouping matcher to the collector.
+     * @tparam AllOf Types of components tracked by the matcher.
+     * @tparam NoneOf Types of components used to filter out entities.
+     * @return The updated collector.
+     */
+    template<typename... AllOf, typename... NoneOf>
+    static constexpr auto group(exclude_t<NoneOf...> = {}) ENTT_NOEXCEPT {
+        return basic_collector<matcher<type_list<>, type_list<>, type_list<NoneOf...>, AllOf...>>{};
+    }
+
+    /**
+     * @brief Adds an observing matcher to the collector.
+     * @tparam AnyOf Type of component for which changes should be detected.
+     * @return The updated collector.
+     */
+    template<typename AnyOf>
+    static constexpr auto replace() ENTT_NOEXCEPT {
+        return basic_collector<matcher<type_list<>, type_list<>, AnyOf>>{};
+    }
+};
+
+/**
+ * @brief Collector.
+ * @copydetails basic_collector<>
+ * @tparam Reject Untracked types used to filter out entities.
+ * @tparam Require Untracked types required by the matcher.
+ * @tparam Rule Specific details of the current matcher.
+ * @tparam Other Other matchers.
+ */
+template<typename... Reject, typename... Require, typename... Rule, typename... Other>
+struct basic_collector<matcher<type_list<Reject...>, type_list<Require...>, Rule...>, Other...> {
+    /*! @brief Current matcher. */
+    using current_type = matcher<type_list<Reject...>, type_list<Require...>, Rule...>;
+
+    /**
+     * @brief Adds a grouping matcher to the collector.
+     * @tparam AllOf Types of components tracked by the matcher.
+     * @tparam NoneOf Types of components used to filter out entities.
+     * @return The updated collector.
+     */
+    template<typename... AllOf, typename... NoneOf>
+    static constexpr auto group(exclude_t<NoneOf...> = {}) ENTT_NOEXCEPT {
+        return basic_collector<matcher<type_list<>, type_list<>, type_list<NoneOf...>, AllOf...>, current_type, Other...>{};
+    }
+
+    /**
+     * @brief Adds an observing matcher to the collector.
+     * @tparam AnyOf Type of component for which changes should be detected.
+     * @return The updated collector.
+     */
+    template<typename AnyOf>
+    static constexpr auto replace() ENTT_NOEXCEPT {
+        return basic_collector<matcher<type_list<>, type_list<>, AnyOf>, current_type, Other...>{};
+    }
+
+    /**
+     * @brief Updates the filter of the last added matcher.
+     * @tparam AllOf Types of components required by the matcher.
+     * @tparam NoneOf Types of components used to filter out entities.
+     * @return The updated collector.
+     */
+    template<typename... AllOf, typename... NoneOf>
+    static constexpr auto where(exclude_t<NoneOf...> = {}) ENTT_NOEXCEPT {
+        using extended_type = matcher<type_list<Reject..., NoneOf...>, type_list<Require..., AllOf...>, Rule...>;
+        return basic_collector<extended_type, Other...>{};
+    }
+};
+
+
+/*! @brief Variable template used to ease the definition of collectors. */
+constexpr basic_collector<> collector{};
+
+
+/**
+ * @brief Observer.
+ *
+ * An observer returns all the entities and only the entities that fit the
+ * requirements of at least one matcher. Moreover, it's guaranteed that the
+ * entity list is tightly packed in memory for fast iterations.<br/>
+ * In general, observers don't stay true to the order of any set of components.
+ *
+ * Observers work mainly with two types of matchers, provided through a
+ * collector:
+ *
+ * * Observing matcher: an observer will return at least all the living entities
+ *   for which one or more of the given components have been explicitly
+ *   replaced and not yet destroyed.
+ * * Grouping matcher: an observer will return at least all the living entities
+ *   that would have entered the given group if it existed and that would have
+ *   not yet left it.
+ *
+ * If an entity respects the requirements of multiple matchers, it will be
+ * returned once and only once by the observer in any case.
+ *
+ * Matchers support also filtering by means of a _where_ clause that accepts
+ * both a list of types and an exclusion list.<br/>
+ * Whenever a matcher finds that an entity matches its requirements, the
+ * condition of the filter is verified before to register the entity itself.
+ * Moreover, a registered entity isn't returned by the observer if the condition
+ * set by the filter is broken in the meantime.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New instances of the given components are created and assigned to entities.
+ * * The entity currently pointed is modified (as an example, if one of the
+ *   given components is removed from the entity to which the iterator points).
+ * * The entity currently pointed is destroyed.
+ *
+ * In all the other cases, modifying the pools of the given components in any
+ * way invalidates all the iterators and using them results in undefined
+ * behavior.
+ *
+ * @warning
+ * Lifetime of an observer doesn't necessarily have to overcome the one of the
+ * registry to which it is connected. However, the observer must be disconnected
+ * from the registry before being destroyed to avoid crashes due to dangling
+ * pointers.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+class basic_observer {
+    using payload_type = std::uint32_t;
+
+    template<typename>
+    struct matcher_handler;
+
+    template<typename... Reject, typename... Require, typename AnyOf>
+    struct matcher_handler<matcher<type_list<Reject...>, type_list<Require...>, AnyOf>> {
+        template<std::size_t Index>
+        static void maybe_valid_if(basic_observer &obs, const Entity entt, const basic_registry<Entity> &reg) {
+            if(reg.template has<Require...>(entt) && !(reg.template has<Reject>(entt) || ...)) {
+                auto *comp = obs.view.try_get(entt);
+                (comp ? *comp : obs.view.construct(entt)) |= (1 << Index);
+            }
+        }
+
+        template<std::size_t Index>
+        static void discard_if(basic_observer &obs, const Entity entt) {
+            if(auto *value = obs.view.try_get(entt); value && !(*value &= (~(1 << Index)))) {
+                obs.view.destroy(entt);
+            }
+        }
+
+        template<std::size_t Index>
+        static void connect(basic_observer &obs, basic_registry<Entity> &reg) {
+            (reg.template on_destroy<Require>().template connect<&discard_if<Index>>(obs), ...);
+            (reg.template on_construct<Reject>().template connect<&discard_if<Index>>(obs), ...);
+            reg.template on_replace<AnyOf>().template connect<&maybe_valid_if<Index>>(obs);
+            reg.template on_destroy<AnyOf>().template connect<&discard_if<Index>>(obs);
+        }
+
+        static void disconnect(basic_observer &obs, basic_registry<Entity> &reg) {
+            (reg.template on_destroy<Require>().disconnect(obs), ...);
+            (reg.template on_construct<Reject>().disconnect(obs), ...);
+            reg.template on_replace<AnyOf>().disconnect(obs);
+            reg.template on_destroy<AnyOf>().disconnect(obs);
+        }
+    };
+
+    template<typename... Reject, typename... Require, typename... NoneOf, typename... AllOf>
+    struct matcher_handler<matcher<type_list<Reject...>, type_list<Require...>, type_list<NoneOf...>, AllOf...>> {
+        template<std::size_t Index>
+        static void maybe_valid_if(basic_observer &obs, const Entity entt, const basic_registry<Entity> &reg) {
+            if(reg.template has<AllOf...>(entt) && !(reg.template has<NoneOf>(entt) || ...)
+                    && reg.template has<Require...>(entt) && !(reg.template has<Reject>(entt) || ...))
+            {
+                auto *comp = obs.view.try_get(entt);
+                (comp ? *comp : obs.view.construct(entt)) |= (1 << Index);
+            }
+        }
+
+        template<std::size_t Index>
+        static void discard_if(basic_observer &obs, const Entity entt) {
+            if(auto *value = obs.view.try_get(entt); value && !(*value &= (~(1 << Index)))) {
+                obs.view.destroy(entt);
+            }
+        }
+
+        template<std::size_t Index>
+        static void connect(basic_observer &obs, basic_registry<Entity> &reg) {
+            (reg.template on_destroy<Require>().template connect<&discard_if<Index>>(obs), ...);
+            (reg.template on_construct<Reject>().template connect<&discard_if<Index>>(obs), ...);
+            (reg.template on_construct<AllOf>().template connect<&maybe_valid_if<Index>>(obs), ...);
+            (reg.template on_destroy<NoneOf>().template connect<&maybe_valid_if<Index>>(obs), ...);
+            (reg.template on_destroy<AllOf>().template connect<&discard_if<Index>>(obs), ...);
+            (reg.template on_construct<NoneOf>().template connect<&discard_if<Index>>(obs), ...);
+        }
+
+        static void disconnect(basic_observer &obs, basic_registry<Entity> &reg) {
+            (reg.template on_destroy<Require>().disconnect(obs), ...);
+            (reg.template on_construct<Reject>().disconnect(obs), ...);
+            (reg.template on_construct<AllOf>().disconnect(obs), ...);
+            (reg.template on_destroy<NoneOf>().disconnect(obs), ...);
+            (reg.template on_destroy<AllOf>().disconnect(obs), ...);
+            (reg.template on_construct<NoneOf>().disconnect(obs), ...);
+        }
+    };
+
+    template<typename... Matcher>
+    static void disconnect(basic_observer &obs, basic_registry<Entity> &reg) {
+        (matcher_handler<Matcher>::disconnect(obs, reg), ...);
+    }
+
+    template<typename... Matcher, std::size_t... Index>
+    void connect(basic_registry<Entity> &reg, std::index_sequence<Index...>) {
+        static_assert(sizeof...(Matcher) < std::numeric_limits<payload_type>::digits);
+        (matcher_handler<Matcher>::template connect<Index>(*this, reg), ...);
+        release = &basic_observer::disconnect<Matcher...>;
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Input iterator type. */
+    using iterator_type = typename sparse_set<Entity>::iterator_type;
+
+    /*! @brief Default constructor. */
+    basic_observer() ENTT_NOEXCEPT
+        : target{}, release{}, view{}
+    {}
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    basic_observer(const basic_observer &) = delete;
+    /*! @brief Default move constructor, deleted on purpose. */
+    basic_observer(basic_observer &&) = delete;
+
+    /**
+     * @brief Creates an observer and connects it to a given registry.
+     * @tparam Matcher Types of matchers to use to initialize the observer.
+     * @param reg A valid reference to a registry.
+     */
+    template<typename... Matcher>
+    basic_observer(basic_registry<entity_type> &reg, basic_collector<Matcher...>) ENTT_NOEXCEPT
+        : target{&reg},
+          release{},
+          view{}
+    {
+        connect<Matcher...>(reg, std::index_sequence_for<Matcher...>{});
+    }
+
+    /*! @brief Default destructor. */
+    ~basic_observer() = default;
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This observer.
+     */
+    basic_observer & operator=(const basic_observer &) = delete;
+
+    /**
+     * @brief Default move assignment operator, deleted on purpose.
+     * @return This observer.
+     */
+    basic_observer & operator=(basic_observer &&) = delete;
+
+    /**
+     * @brief Connects an observer to a given registry.
+     * @tparam Matcher Types of matchers to use to initialize the observer.
+     * @param reg A valid reference to a registry.
+     */
+    template<typename... Matcher>
+    void connect(basic_registry<entity_type> &reg, basic_collector<Matcher...>) {
+        disconnect();
+        connect<Matcher...>(reg, std::index_sequence_for<Matcher...>{});
+        target = &reg;
+        view.reset();
+    }
+
+    /*! @brief Disconnects an observer from the registry it keeps track of. */
+    void disconnect() {
+        if(release) {
+            release(*this, *target);
+            release = nullptr;
+        }
+    }
+
+    /**
+     * @brief Returns the number of elements in an observer.
+     * @return Number of elements.
+     */
+    size_type size() const ENTT_NOEXCEPT {
+        return view.size();
+    }
+
+    /**
+     * @brief Checks whether an observer is empty.
+     * @return True if the observer is empty, false otherwise.
+     */
+    bool empty() const ENTT_NOEXCEPT {
+        return view.empty();
+    }
+
+    /**
+     * @brief Direct access to the list of entities of the observer.
+     *
+     * The returned pointer is such that range `[data(), data() + size()]` is
+     * always a valid range, even if the container is empty.
+     *
+     * @note
+     * There are no guarantees on the order of the entities. Use `begin` and
+     * `end` if you want to iterate the observer in the expected order.
+     *
+     * @return A pointer to the array of entities.
+     */
+    const entity_type * data() const ENTT_NOEXCEPT {
+        return view.data();
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the observer.
+     *
+     * The returned iterator points to the first entity of the observer. If the
+     * container is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first entity of the observer.
+     */
+    iterator_type begin() const ENTT_NOEXCEPT {
+        return view.sparse_set<entity_type>::begin();
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the observer.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the observer. Attempting to dereference the returned iterator results in
+     * undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the
+     * observer.
+     */
+    iterator_type end() const ENTT_NOEXCEPT {
+        return view.sparse_set<entity_type>::end();
+    }
+
+    /*! @brief Resets the underlying container. */
+    void clear() {
+        view.reset();
+    }
+
+    /**
+     * @brief Iterates entities and applies the given function object to them,
+     * then clears the observer.
+     *
+     * The function object is invoked for each entity.<br/>
+     * The signature of the function must be equivalent to the following form:
+     *
+     * @code{.cpp}
+     * void(const entity_type);
+     * @endcode
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        static_assert(std::is_invocable_v<Func, entity_type>);
+        std::for_each(begin(), end(), std::move(func));
+    }
+
+    /**
+     * @brief Iterates entities and applies the given function object to them,
+     * then clears the observer.
+     *
+     * The function object is invoked for each entity.<br/>
+     * The signature of the function must be equivalent to the following form:
+     *
+     * @code{.cpp}
+     * void(const entity_type);
+     * @endcode
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) {
+        std::as_const(*this).each(std::move(func));
+        clear();
+    }
+
+private:
+    basic_registry<entity_type> *target;
+    void(* release)(basic_observer &, basic_registry<entity_type> &);
+    storage<entity_type, payload_type> view;
+};
+
+
+}
+
+
+#endif // ENTT_ENTITY_OBSERVER_HPP

src/entt/entity/runtime_view.hpp

@@ -0,0 +1,256 @@
+#ifndef ENTT_ENTITY_RUNTIME_VIEW_HPP
+#define ENTT_ENTITY_RUNTIME_VIEW_HPP
+
+
+#include <iterator>
+#include <cassert>
+#include <vector>
+#include <utility>
+#include <algorithm>
+#include <type_traits>
+#include "../config/config.h"
+#include "sparse_set.hpp"
+#include "entity.hpp"
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Runtime view.
+ *
+ * Runtime views iterate over those entities that have at least all the given
+ * components in their bags. During initialization, a runtime view looks at the
+ * number of entities available for each component and picks up a reference to
+ * the smallest set of candidate entities in order to get a performance boost
+ * when iterate.<br/>
+ * Order of elements during iterations are highly dependent on the order of the
+ * underlying data structures. See sparse_set and its specializations for more
+ * details.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New instances of the given components are created and assigned to entities.
+ * * The entity currently pointed is modified (as an example, if one of the
+ *   given components is removed from the entity to which the iterator points).
+ * * The entity currently pointed is destroyed.
+ *
+ * In all the other cases, modifying the pools of the given components in any
+ * way invalidates all the iterators and using them results in undefined
+ * behavior.
+ *
+ * @note
+ * Views share references to the underlying data structures of the registry that
+ * generated them. Therefore any change to the entities and to the components
+ * made by means of the registry are immediately reflected by the views, unless
+ * a pool was missing when the view was built (in this case, the view won't
+ * have a valid reference and won't be updated accordingly).
+ *
+ * @warning
+ * Lifetime of a view must overcome the one of the registry that generated it.
+ * In any other case, attempting to use a view results in undefined behavior.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+class basic_runtime_view {
+    /*! @brief A registry is allowed to create views. */
+    friend class basic_registry<Entity>;
+
+    using underlying_iterator_type = typename sparse_set<Entity>::iterator_type;
+
+    class iterator {
+        friend class basic_runtime_view<Entity>;
+
+        iterator(underlying_iterator_type first, underlying_iterator_type last, const sparse_set<Entity> * const *others, const sparse_set<Entity> * const *length) ENTT_NOEXCEPT
+            : begin{first},
+              end{last},
+              from{others},
+              to{length}
+        {
+            if(begin != end && !valid()) {
+                ++(*this);
+            }
+        }
+
+        bool valid() const ENTT_NOEXCEPT {
+            return std::all_of(from, to, [entt = *begin](const auto *view) {
+                return view->has(entt);
+            });
+        }
+
+    public:
+        using difference_type = typename underlying_iterator_type::difference_type;
+        using value_type = typename underlying_iterator_type::value_type;
+        using pointer = typename underlying_iterator_type::pointer;
+        using reference = typename underlying_iterator_type::reference;
+        using iterator_category = std::forward_iterator_tag;
+
+        iterator() ENTT_NOEXCEPT = default;
+
+        iterator & operator++() ENTT_NOEXCEPT {
+            return (++begin != end && !valid()) ? ++(*this) : *this;
+        }
+
+        iterator operator++(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        bool operator==(const iterator &other) const ENTT_NOEXCEPT {
+            return other.begin == begin;
+        }
+
+        bool operator!=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+        pointer operator->() const ENTT_NOEXCEPT {
+            return begin.operator->();
+        }
+
+        reference operator*() const ENTT_NOEXCEPT {
+            return *operator->();
+        }
+
+    private:
+        underlying_iterator_type begin;
+        underlying_iterator_type end;
+        const sparse_set<Entity> * const *from;
+        const sparse_set<Entity> * const *to;
+    };
+
+    basic_runtime_view(std::vector<const sparse_set<Entity> *> others) ENTT_NOEXCEPT
+        : pools{std::move(others)}
+    {
+        const auto it = std::min_element(pools.begin(), pools.end(), [](const auto *lhs, const auto *rhs) {
+            return (!lhs && rhs) || (lhs && rhs && lhs->size() < rhs->size());
+        });
+
+        // brings the best candidate (if any) on front of the vector
+        std::rotate(pools.begin(), it, pools.end());
+    }
+
+    bool valid() const ENTT_NOEXCEPT {
+        return !pools.empty() && pools.front();
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Input iterator type. */
+    using iterator_type = iterator;
+
+    /**
+     * @brief Estimates the number of entities that have the given components.
+     * @return Estimated number of entities that have the given components.
+     */
+    size_type size() const ENTT_NOEXCEPT {
+        return valid() ? pools.front()->size() : size_type{};
+    }
+
+    /**
+     * @brief Checks if the view is definitely empty.
+     * @return True if the view is definitely empty, false otherwise.
+     */
+    bool empty() const ENTT_NOEXCEPT {
+        return !valid() || pools.front()->empty();
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity that has the given
+     * components.
+     *
+     * The returned iterator points to the first entity that has the given
+     * components. If the view is empty, the returned iterator will be equal to
+     * `end()`.
+     *
+     * @note
+     * Input iterators stay true to the order imposed to the underlying data
+     * structures.
+     *
+     * @return An iterator to the first entity that has the given components.
+     */
+    iterator_type begin() const ENTT_NOEXCEPT {
+        iterator_type it{};
+
+        if(valid()) {
+            const auto &pool = *pools.front();
+            const auto * const *data = pools.data();
+            it = { pool.begin(), pool.end(), data + 1, data + pools.size() };
+        }
+
+        return it;
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity that has the
+     * given components.
+     *
+     * The returned iterator points to the entity following the last entity that
+     * has the given components. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @note
+     * Input iterators stay true to the order imposed to the underlying data
+     * structures.
+     *
+     * @return An iterator to the entity following the last entity that has the
+     * given components.
+     */
+    iterator_type end() const ENTT_NOEXCEPT {
+        iterator_type it{};
+
+        if(valid()) {
+            const auto &pool = *pools.front();
+            it = { pool.end(), pool.end(), nullptr, nullptr };
+        }
+
+        return it;
+    }
+
+    /**
+     * @brief Checks if a view contains an entity.
+     * @param entt A valid entity identifier.
+     * @return True if the view contains the given entity, false otherwise.
+     */
+    bool contains(const entity_type entt) const ENTT_NOEXCEPT {
+        return valid() && std::all_of(pools.cbegin(), pools.cend(), [entt](const auto *view) {
+            return view->find(entt) != view->end();
+        });
+    }
+
+    /**
+     * @brief Iterates entities and applies the given function object to them.
+     *
+     * The function object is invoked for each entity. It is provided only with
+     * the entity itself. To get the components, users can use the registry with
+     * which the view was built.<br/>
+     * The signature of the function should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * void(const entity_type);
+     * @endcode
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        std::for_each(begin(), end(), func);
+    }
+
+private:
+    std::vector<const sparse_set<Entity> *> pools;
+};
+
+
+}
+
+
+#endif // ENTT_ENTITY_RUNTIME_VIEW_HPP

src/entt/entity/snapshot.hpp

@@ -0,0 +1,605 @@
+#ifndef ENTT_ENTITY_SNAPSHOT_HPP
+#define ENTT_ENTITY_SNAPSHOT_HPP
+
+
+#include <array>
+#include <cstddef>
+#include <utility>
+#include <iterator>
+#include <type_traits>
+#include <unordered_map>
+#include "../config/config.h"
+#include "entity.hpp"
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Utility class to create snapshots from a registry.
+ *
+ * A _snapshot_ can be either a dump of the entire registry or a narrower
+ * selection of components of interest.<br/>
+ * This type can be used in both cases if provided with a correctly configured
+ * output archive.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+class basic_snapshot {
+    /*! @brief A registry is allowed to create snapshots. */
+    friend class basic_registry<Entity>;
+
+    using follow_fn_type = Entity(const basic_registry<Entity> &, const Entity);
+    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
+
+    basic_snapshot(const basic_registry<Entity> *source, Entity init, follow_fn_type *fn) ENTT_NOEXCEPT
+        : reg{source},
+          seed{init},
+          follow{fn}
+    {}
+
+    template<typename Component, typename Archive, typename It>
+    void get(Archive &archive, std::size_t sz, It first, It last) const {
+        archive(typename traits_type::entity_type(sz));
+
+        while(first != last) {
+            const auto entt = *(first++);
+
+            if(reg->template has<Component>(entt)) {
+                if constexpr(std::is_empty_v<Component>) {
+                    archive(entt);
+                } else {
+                    archive(entt, reg->template get<Component>(entt));
+                }
+            }
+        }
+    }
+
+    template<typename... Component, typename Archive, typename It, std::size_t... Indexes>
+    void component(Archive &archive, It first, It last, std::index_sequence<Indexes...>) const {
+        std::array<std::size_t, sizeof...(Indexes)> size{};
+        auto begin = first;
+
+        while(begin != last) {
+            const auto entt = *(begin++);
+            ((reg->template has<Component>(entt) ? ++size[Indexes] : size[Indexes]), ...);
+        }
+
+        (get<Component>(archive, size[Indexes], first, last), ...);
+    }
+
+public:
+    /*! @brief Default move constructor. */
+    basic_snapshot(basic_snapshot &&) = default;
+
+    /*! @brief Default move assignment operator. @return This snapshot. */
+    basic_snapshot & operator=(basic_snapshot &&) = default;
+
+    /**
+     * @brief Puts aside all the entities that are still in use.
+     *
+     * Entities are serialized along with their versions. Destroyed entities are
+     * not taken in consideration by this function.
+     *
+     * @tparam Archive Type of output archive.
+     * @param archive A valid reference to an output archive.
+     * @return An object of this type to continue creating the snapshot.
+     */
+    template<typename Archive>
+    const basic_snapshot & entities(Archive &archive) const {
+        archive(typename traits_type::entity_type(reg->alive()));
+        reg->each([&archive](const auto entt) { archive(entt); });
+        return *this;
+    }
+
+    /**
+     * @brief Puts aside destroyed entities.
+     *
+     * Entities are serialized along with their versions. Entities that are
+     * still in use are not taken in consideration by this function.
+     *
+     * @tparam Archive Type of output archive.
+     * @param archive A valid reference to an output archive.
+     * @return An object of this type to continue creating the snapshot.
+     */
+    template<typename Archive>
+    const basic_snapshot & destroyed(Archive &archive) const {
+        auto size = reg->size() - reg->alive();
+        archive(typename traits_type::entity_type(size));
+
+        if(size) {
+            auto curr = seed;
+            archive(curr);
+
+            for(--size; size; --size) {
+                curr = follow(*reg, curr);
+                archive(curr);
+            }
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Puts aside the given components.
+     *
+     * Each instance is serialized together with the entity to which it belongs.
+     * Entities are serialized along with their versions.
+     *
+     * @tparam Component Types of components to serialize.
+     * @tparam Archive Type of output archive.
+     * @param archive A valid reference to an output archive.
+     * @return An object of this type to continue creating the snapshot.
+     */
+    template<typename... Component, typename Archive>
+    const basic_snapshot & component(Archive &archive) const {
+        (component<Component>(archive, reg->template data<Component>(), reg->template data<Component>() + reg->template size<Component>()), ...);
+        return *this;
+    }
+
+    /**
+     * @brief Puts aside the given components for the entities in a range.
+     *
+     * Each instance is serialized together with the entity to which it belongs.
+     * Entities are serialized along with their versions.
+     *
+     * @tparam Component Types of components to serialize.
+     * @tparam Archive Type of output archive.
+     * @tparam It Type of input iterator.
+     * @param archive A valid reference to an output archive.
+     * @param first An iterator to the first element of the range to serialize.
+     * @param last An iterator past the last element of the range to serialize.
+     * @return An object of this type to continue creating the snapshot.
+     */
+    template<typename... Component, typename Archive, typename It>
+    const basic_snapshot & component(Archive &archive, It first, It last) const {
+        component<Component...>(archive, first, last, std::index_sequence_for<Component...>{});
+        return *this;
+    }
+
+private:
+    const basic_registry<Entity> *reg;
+    const Entity seed;
+    follow_fn_type *follow;
+};
+
+
+/**
+ * @brief Utility class to restore a snapshot as a whole.
+ *
+ * A snapshot loader requires that the destination registry be empty and loads
+ * all the data at once while keeping intact the identifiers that the entities
+ * originally had.<br/>
+ * An example of use is the implementation of a save/restore utility.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+class basic_snapshot_loader {
+    /*! @brief A registry is allowed to create snapshot loaders. */
+    friend class basic_registry<Entity>;
+
+    using force_fn_type = void(basic_registry<Entity> &, const Entity, const bool);
+    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
+
+    basic_snapshot_loader(basic_registry<Entity> *source, force_fn_type *fn) ENTT_NOEXCEPT
+        : reg{source},
+          force{fn}
+    {
+        // to restore a snapshot as a whole requires a clean registry
+        ENTT_ASSERT(reg->empty());
+    }
+
+    template<typename Archive>
+    void assure(Archive &archive, bool discard) const {
+        typename traits_type::entity_type length{};
+        archive(length);
+
+        while(length--) {
+            Entity entt{};
+            archive(entt);
+            force(*reg, entt, discard);
+        }
+    }
+
+    template<typename Type, typename Archive, typename... Args>
+    void assign(Archive &archive, Args... args) const {
+        typename traits_type::entity_type length{};
+        archive(length);
+
+        while(length--) {
+            static constexpr auto discard = false;
+            Entity entt{};
+
+            if constexpr(std::is_empty_v<Type>) {
+                archive(entt);
+                force(*reg, entt, discard);
+                reg->template assign<Type>(args..., entt);
+            } else {
+                Type instance{};
+                archive(entt, instance);
+                force(*reg, entt, discard);
+                reg->template assign<Type>(args..., entt, std::as_const(instance));
+            }
+        }
+    }
+
+public:
+    /*! @brief Default move constructor. */
+    basic_snapshot_loader(basic_snapshot_loader &&) = default;
+
+    /*! @brief Default move assignment operator. @return This loader. */
+    basic_snapshot_loader & operator=(basic_snapshot_loader &&) = default;
+
+    /**
+     * @brief Restores entities that were in use during serialization.
+     *
+     * This function restores the entities that were in use during serialization
+     * and gives them the versions they originally had.
+     *
+     * @tparam Archive Type of input archive.
+     * @param archive A valid reference to an input archive.
+     * @return A valid loader to continue restoring data.
+     */
+    template<typename Archive>
+    const basic_snapshot_loader & entities(Archive &archive) const {
+        static constexpr auto discard = false;
+        assure(archive, discard);
+        return *this;
+    }
+
+    /**
+     * @brief Restores entities that were destroyed during serialization.
+     *
+     * This function restores the entities that were destroyed during
+     * serialization and gives them the versions they originally had.
+     *
+     * @tparam Archive Type of input archive.
+     * @param archive A valid reference to an input archive.
+     * @return A valid loader to continue restoring data.
+     */
+    template<typename Archive>
+    const basic_snapshot_loader & destroyed(Archive &archive) const {
+        static constexpr auto discard = true;
+        assure(archive, discard);
+        return *this;
+    }
+
+    /**
+     * @brief Restores components and assigns them to the right entities.
+     *
+     * The template parameter list must be exactly the same used during
+     * serialization. In the event that the entity to which the component is
+     * assigned doesn't exist yet, the loader will take care to create it with
+     * the version it originally had.
+     *
+     * @tparam Component Types of components to restore.
+     * @tparam Archive Type of input archive.
+     * @param archive A valid reference to an input archive.
+     * @return A valid loader to continue restoring data.
+     */
+    template<typename... Component, typename Archive>
+    const basic_snapshot_loader & component(Archive &archive) const {
+        (assign<Component>(archive), ...);
+        return *this;
+    }
+
+    /**
+     * @brief Destroys those entities that have no components.
+     *
+     * In case all the entities were serialized but only part of the components
+     * was saved, it could happen that some of the entities have no components
+     * once restored.<br/>
+     * This functions helps to identify and destroy those entities.
+     *
+     * @return A valid loader to continue restoring data.
+     */
+    const basic_snapshot_loader & orphans() const {
+        reg->orphans([this](const auto entt) {
+            reg->destroy(entt);
+        });
+
+        return *this;
+    }
+
+private:
+    basic_registry<Entity> *reg;
+    force_fn_type *force;
+};
+
+
+/**
+ * @brief Utility class for _continuous loading_.
+ *
+ * A _continuous loader_ is designed to load data from a source registry to a
+ * (possibly) non-empty destination. The loader can accomodate in a registry
+ * more than one snapshot in a sort of _continuous loading_ that updates the
+ * destination one step at a time.<br/>
+ * Identifiers that entities originally had are not transferred to the target.
+ * Instead, the loader maps remote identifiers to local ones while restoring a
+ * snapshot.<br/>
+ * An example of use is the implementation of a client-server applications with
+ * the requirement of transferring somehow parts of the representation side to
+ * side.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+class basic_continuous_loader {
+    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
+
+    void destroy(Entity entt) {
+        const auto it = remloc.find(entt);
+
+        if(it == remloc.cend()) {
+            const auto local = reg->create();
+            remloc.emplace(entt, std::make_pair(local, true));
+            reg->destroy(local);
+        }
+    }
+
+    void restore(Entity entt) {
+        const auto it = remloc.find(entt);
+
+        if(it == remloc.cend()) {
+            const auto local = reg->create();
+            remloc.emplace(entt, std::make_pair(local, true));
+        } else {
+            remloc[entt].first = reg->valid(remloc[entt].first) ? remloc[entt].first : reg->create();
+            // set the dirty flag
+            remloc[entt].second = true;
+        }
+    }
+
+    template<typename Container>
+    auto update(int, Container &container)
+    -> decltype(typename Container::mapped_type{}, void()) {
+        // map like container
+        Container other;
+
+        for(auto &&pair: container) {
+            using first_type = std::remove_const_t<typename std::decay_t<decltype(pair)>::first_type>;
+            using second_type = typename std::decay_t<decltype(pair)>::second_type;
+
+            if constexpr(std::is_same_v<first_type, Entity> && std::is_same_v<second_type, Entity>) {
+                other.emplace(map(pair.first), map(pair.second));
+            } else if constexpr(std::is_same_v<first_type, Entity>) {
+                other.emplace(map(pair.first), std::move(pair.second));
+            } else {
+                static_assert(std::is_same_v<second_type, Entity>);
+                other.emplace(std::move(pair.first), map(pair.second));
+            }
+        }
+
+        std::swap(container, other);
+    }
+
+    template<typename Container>
+    auto update(char, Container &container)
+    -> decltype(typename Container::value_type{}, void()) {
+        // vector like container
+        static_assert(std::is_same_v<typename Container::value_type, Entity>);
+
+        for(auto &&entt: container) {
+            entt = map(entt);
+        }
+    }
+
+    template<typename Other, typename Type, typename Member>
+    void update(Other &instance, Member Type:: *member) {
+        if constexpr(!std::is_same_v<Other, Type>) {
+            return;
+        } else if constexpr(std::is_same_v<Member, Entity>) {
+            instance.*member = map(instance.*member);
+        } else {
+            // maybe a container? let's try...
+            update(0, instance.*member);
+        }
+    }
+
+    template<typename Archive>
+    void assure(Archive &archive, void(basic_continuous_loader:: *member)(Entity)) {
+        typename traits_type::entity_type length{};
+        archive(length);
+
+        while(length--) {
+            Entity entt{};
+            archive(entt);
+            (this->*member)(entt);
+        }
+    }
+
+    template<typename Component>
+    void reset() {
+        for(auto &&ref: remloc) {
+            const auto local = ref.second.first;
+
+            if(reg->valid(local)) {
+                reg->template reset<Component>(local);
+            }
+        }
+    }
+
+    template<typename Other, typename Archive, typename... Type, typename... Member>
+    void assign(Archive &archive, [[maybe_unused]] Member Type:: *... member) {
+        typename traits_type::entity_type length{};
+        archive(length);
+
+        while(length--) {
+            Entity entt{};
+
+            if constexpr(std::is_empty_v<Other>) {
+                archive(entt);
+                restore(entt);
+                reg->template assign_or_replace<Other>(map(entt));
+            } else {
+                Other instance{};
+                archive(entt, instance);
+                (update(instance, member), ...);
+                restore(entt);
+                reg->template assign_or_replace<Other>(map(entt), std::as_const(instance));
+            }
+        }
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+
+    /**
+     * @brief Constructs a loader that is bound to a given registry.
+     * @param source A valid reference to a registry.
+     */
+    basic_continuous_loader(basic_registry<entity_type> &source) ENTT_NOEXCEPT
+        : reg{&source}
+    {}
+
+    /*! @brief Default move constructor. */
+    basic_continuous_loader(basic_continuous_loader &&) = default;
+
+    /*! @brief Default move assignment operator. @return This loader. */
+    basic_continuous_loader & operator=(basic_continuous_loader &&) = default;
+
+    /**
+     * @brief Restores entities that were in use during serialization.
+     *
+     * This function restores the entities that were in use during serialization
+     * and creates local counterparts for them if required.
+     *
+     * @tparam Archive Type of input archive.
+     * @param archive A valid reference to an input archive.
+     * @return A non-const reference to this loader.
+     */
+    template<typename Archive>
+    basic_continuous_loader & entities(Archive &archive) {
+        assure(archive, &basic_continuous_loader::restore);
+        return *this;
+    }
+
+    /**
+     * @brief Restores entities that were destroyed during serialization.
+     *
+     * This function restores the entities that were destroyed during
+     * serialization and creates local counterparts for them if required.
+     *
+     * @tparam Archive Type of input archive.
+     * @param archive A valid reference to an input archive.
+     * @return A non-const reference to this loader.
+     */
+    template<typename Archive>
+    basic_continuous_loader & destroyed(Archive &archive) {
+        assure(archive, &basic_continuous_loader::destroy);
+        return *this;
+    }
+
+    /**
+     * @brief Restores components and assigns them to the right entities.
+     *
+     * The template parameter list must be exactly the same used during
+     * serialization. In the event that the entity to which the component is
+     * assigned doesn't exist yet, the loader will take care to create a local
+     * counterpart for it.<br/>
+     * Members can be either data members of type entity_type or containers of
+     * entities. In both cases, the loader will visit them and update the
+     * entities by replacing each one with its local counterpart.
+     *
+     * @tparam Component Type of component to restore.
+     * @tparam Archive Type of input archive.
+     * @tparam Type Types of components to update with local counterparts.
+     * @tparam Member Types of members to update with their local counterparts.
+     * @param archive A valid reference to an input archive.
+     * @param member Members to update with their local counterparts.
+     * @return A non-const reference to this loader.
+     */
+    template<typename... Component, typename Archive, typename... Type, typename... Member>
+    basic_continuous_loader & component(Archive &archive, Member Type:: *... member) {
+        (reset<Component>(), ...);
+        (assign<Component>(archive, member...), ...);
+        return *this;
+    }
+
+    /**
+     * @brief Helps to purge entities that no longer have a conterpart.
+     *
+     * Users should invoke this member function after restoring each snapshot,
+     * unless they know exactly what they are doing.
+     *
+     * @return A non-const reference to this loader.
+     */
+    basic_continuous_loader & shrink() {
+        auto it = remloc.begin();
+
+        while(it != remloc.cend()) {
+            const auto local = it->second.first;
+            bool &dirty = it->second.second;
+
+            if(dirty) {
+                dirty = false;
+                ++it;
+            } else {
+                if(reg->valid(local)) {
+                    reg->destroy(local);
+                }
+
+                it = remloc.erase(it);
+            }
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Destroys those entities that have no components.
+     *
+     * In case all the entities were serialized but only part of the components
+     * was saved, it could happen that some of the entities have no components
+     * once restored.<br/>
+     * This functions helps to identify and destroy those entities.
+     *
+     * @return A non-const reference to this loader.
+     */
+    basic_continuous_loader & orphans() {
+        reg->orphans([this](const auto entt) {
+            reg->destroy(entt);
+        });
+
+        return *this;
+    }
+
+    /**
+     * @brief Tests if a loader knows about a given entity.
+     * @param entt An entity identifier.
+     * @return True if `entity` is managed by the loader, false otherwise.
+     */
+    bool has(entity_type entt) const ENTT_NOEXCEPT {
+        return (remloc.find(entt) != remloc.cend());
+    }
+
+    /**
+     * @brief Returns the identifier to which an entity refers.
+     * @param entt An entity identifier.
+     * @return The local identifier if any, the null entity otherwise.
+     */
+    entity_type map(entity_type entt) const ENTT_NOEXCEPT {
+        const auto it = remloc.find(entt);
+        entity_type other = null;
+
+        if(it != remloc.cend()) {
+            other = it->second.first;
+        }
+
+        return other;
+    }
+
+private:
+    std::unordered_map<entity_type, std::pair<entity_type, bool>> remloc;
+    basic_registry<entity_type> *reg;
+};
+
+
+}
+
+
+#endif // ENTT_ENTITY_SNAPSHOT_HPP

src/entt/entity/sparse_set.hpp

@@ -3,26 +3,22 @@
 
 
 #include <algorithm>
+#include <iterator>
 #include <utility>
 #include <vector>
+#include <memory>
 #include <cstddef>
-#include <cassert>
-#include "traits.hpp"
+#include <numeric>
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/algorithm.hpp"
+#include "entity.hpp"
+#include "fwd.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.
  *
@@ -30,9 +26,9 @@ class SparseSet;
  * 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 is largely used by the registry to offer users the fastest access ever
+ * to the components. Views and groups in general are almost entirely designed
+ * around sparse sets.
  *
  * This type of data structure is widely documented in the literature and on the
  * web. This is nothing more than a customized implementation suitable for the
@@ -51,69 +47,227 @@ class SparseSet;
  * @tparam Entity A valid entity type (see entt_traits for more details).
  */
 template<typename Entity>
-class SparseSet<Entity> {
-    using traits_type = entt_traits<Entity>;
+class sparse_set {
+    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
 
-    struct Iterator {
-        using value_type = Entity;
+    static_assert(ENTT_PAGE_SIZE && ((ENTT_PAGE_SIZE & (ENTT_PAGE_SIZE - 1)) == 0));
+    static constexpr auto entt_per_page = ENTT_PAGE_SIZE / sizeof(typename traits_type::entity_type);
 
-        Iterator(const std::vector<Entity> *direct, std::size_t pos)
-            : direct{direct}, pos{pos}
+    class iterator {
+        friend class sparse_set<Entity>;
+
+        using direct_type = const std::vector<Entity>;
+        using index_type = typename traits_type::difference_type;
+
+        iterator(direct_type *ref, const index_type idx) ENTT_NOEXCEPT
+            : direct{ref}, index{idx}
         {}
 
-        Iterator & operator++() noexcept {
-            return --pos, *this;
+    public:
+        using difference_type = index_type;
+        using value_type = Entity;
+        using pointer = const value_type *;
+        using reference = const value_type &;
+        using iterator_category = std::random_access_iterator_tag;
+
+        iterator() ENTT_NOEXCEPT = default;
+
+        iterator & operator++() ENTT_NOEXCEPT {
+            return --index, *this;
         }
 
-        Iterator operator++(int) noexcept {
-            Iterator orig = *this;
+        iterator operator++(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
             return ++(*this), orig;
         }
 
-        bool operator==(const Iterator &other) const noexcept {
-            return other.pos == pos && other.direct == direct;
+        iterator & operator--() ENTT_NOEXCEPT {
+            return ++index, *this;
         }
 
-        bool operator!=(const Iterator &other) const noexcept {
+        iterator operator--(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return --(*this), orig;
+        }
+
+        iterator & operator+=(const difference_type value) ENTT_NOEXCEPT {
+            index -= value;
+            return *this;
+        }
+
+        iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
+            return iterator{direct, index-value};
+        }
+
+        iterator & operator-=(const difference_type value) ENTT_NOEXCEPT {
+            return (*this += -value);
+        }
+
+        iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
+            return (*this + -value);
+        }
+
+        difference_type operator-(const iterator &other) const ENTT_NOEXCEPT {
+            return other.index - index;
+        }
+
+        reference operator[](const difference_type value) const ENTT_NOEXCEPT {
+            const auto pos = size_type(index-value-1);
+            return (*direct)[pos];
+        }
+
+        bool operator==(const iterator &other) const ENTT_NOEXCEPT {
+            return other.index == index;
+        }
+
+        bool operator!=(const iterator &other) const ENTT_NOEXCEPT {
             return !(*this == other);
         }
 
-        value_type operator*() const noexcept {
-            return (*direct)[pos-1];
+        bool operator<(const iterator &other) const ENTT_NOEXCEPT {
+            return index > other.index;
+        }
+
+        bool operator>(const iterator &other) const ENTT_NOEXCEPT {
+            return index < other.index;
+        }
+
+        bool operator<=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this > other);
+        }
+
+        bool operator>=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this < other);
+        }
+
+        pointer operator->() const ENTT_NOEXCEPT {
+            const auto pos = size_type(index-1);
+            return &(*direct)[pos];
+        }
+
+        reference operator*() const ENTT_NOEXCEPT {
+            return *operator->();
         }
 
     private:
-        const std::vector<Entity> *direct;
-        std::size_t pos;
+        direct_type *direct;
+        index_type index;
     };
 
-    static constexpr Entity in_use = 1 << traits_type::entity_shift;
+    void assure(const std::size_t page) {
+        if(!(page < reverse.size())) {
+            reverse.resize(page+1);
+        }
+
+        if(!reverse[page]) {
+            reverse[page] = std::make_unique<entity_type[]>(entt_per_page);
+            // null is safe in all cases for our purposes
+            std::fill_n(reverse[page].get(), entt_per_page, null);
+        }
+    }
+
+    auto map(const Entity entt) const ENTT_NOEXCEPT {
+        const auto identifier = to_integer(entt) & traits_type::entity_mask;
+        const auto page = size_type(identifier / entt_per_page);
+        const auto offset = size_type(identifier & (entt_per_page - 1));
+        return std::make_pair(page, offset);
+    }
 
 public:
     /*! @brief Underlying entity identifier. */
     using entity_type = Entity;
-    /*! @brief 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 Random access iterator type. */
+    using iterator_type = iterator;
 
     /*! @brief Default constructor. */
-    SparseSet() noexcept = default;
+    sparse_set() = default;
+
+    /**
+     * @brief Copy constructor.
+     * @param other The instance to copy from.
+     */
+    sparse_set(const sparse_set &other)
+        : reverse{},
+          direct{other.direct}
+    {
+        for(size_type pos{}, last = other.reverse.size(); pos < last; ++pos) {
+            if(other.reverse[pos]) {
+                assure(pos);
+                std::copy_n(other.reverse[pos].get(), entt_per_page, reverse[pos].get());
+            }
+        }
+    }
+
+    /*! @brief Default move constructor. */
+    sparse_set(sparse_set &&) = default;
 
     /*! @brief Default destructor. */
-    virtual ~SparseSet() noexcept = default;
+    virtual ~sparse_set() ENTT_NOEXCEPT = default;
 
-    /*! @brief Copying a sparse set isn't allowed. */
-    SparseSet(const SparseSet &) = delete;
-    /*! @brief Default move constructor. */
-    SparseSet(SparseSet &&) = default;
+    /**
+     * @brief Copy assignment operator.
+     * @param other The instance to copy from.
+     * @return This sparse set.
+     */
+    sparse_set & operator=(const sparse_set &other) {
+        if(&other != this) {
+            auto tmp{other};
+            *this = std::move(tmp);
+        }
+
+        return *this;
+    }
 
-    /*! @brief 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;
+    sparse_set & operator=(sparse_set &&) = default;
+
+    /**
+     * @brief Increases the capacity of a sparse set.
+     *
+     * If the new capacity is greater than the current capacity, new storage is
+     * allocated, otherwise the method does nothing.
+     *
+     * @param cap Desired capacity.
+     */
+    void reserve(const size_type cap) {
+        direct.reserve(cap);
+    }
+
+    /**
+     * @brief Returns the number of elements that a sparse set has currently
+     * allocated space for.
+     * @return Capacity of the sparse set.
+     */
+    size_type capacity() const ENTT_NOEXCEPT {
+        return direct.capacity();
+    }
+
+    /*! @brief Requests the removal of unused capacity. */
+    void shrink_to_fit() {
+        // conservative approach
+        if(direct.empty()) {
+            reverse.clear();
+        }
+
+        reverse.shrink_to_fit();
+        direct.shrink_to_fit();
+    }
+
+    /**
+     * @brief Returns the extent of a sparse set.
+     *
+     * The extent of a sparse set is also the size of the internal sparse array.
+     * There is no guarantee that the internal packed array has the same size.
+     * Usually the size of the internal sparse array is equal or greater than
+     * the one of the internal packed array.
+     *
+     * @return Extent of the sparse set.
+     */
+    size_type extent() const ENTT_NOEXCEPT {
+        return reverse.size() * entt_per_page;
+    }
 
     /**
      * @brief Returns the number of elements in a sparse set.
@@ -125,7 +279,7 @@ public:
      *
      * @return Number of elements.
      */
-    size_type size() const noexcept {
+    size_type size() const ENTT_NOEXCEPT {
         return direct.size();
     }
 
@@ -133,7 +287,7 @@ public:
      * @brief Checks whether a sparse set is empty.
      * @return True if the sparse set is empty, false otherwise.
      */
-    bool empty() const noexcept {
+    bool empty() const ENTT_NOEXCEPT {
         return direct.empty();
     }
 
@@ -144,7 +298,7 @@ public:
      * always a valid range, even if the container is empty.
      *
      * @note
-     * There are no guarantees on the order, even though `sort` has been
+     * 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
@@ -152,52 +306,65 @@ public:
      *
      * @return A pointer to the internal packed array.
      */
-    const entity_type * data() const noexcept {
+    const entity_type * data() const ENTT_NOEXCEPT {
         return direct.data();
     }
 
     /**
      * @brief Returns an iterator to the beginning.
      *
-     * The returned iterator points to the first element of the internal packed
+     * 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 `sort`.
+     * Random access iterators stay true to the order imposed by a call to
+     * `respect`.
      *
-     * @return An iterator to the first element of the internal packed array.
+     * @return An iterator to the first entity of the internal packed array.
      */
-    iterator_type begin() const noexcept {
-        return Iterator{&direct, direct.size()};
+    iterator_type begin() const ENTT_NOEXCEPT {
+        const typename traits_type::difference_type pos = direct.size();
+        return iterator_type{&direct, pos};
     }
 
     /**
      * @brief Returns an iterator to the end.
      *
-     * The returned iterator points to the element following the last element in
+     * 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 `sort`.
+     * Random access iterators stay true to the order imposed by a call to
+     * `respect`.
      *
-     * @return An iterator to the element following the last element of the
+     * @return An iterator to the element following the last entity of the
      * internal packed array.
      */
-    iterator_type end() const noexcept {
-        return Iterator{&direct, 0};
+    iterator_type end() const ENTT_NOEXCEPT {
+        return iterator_type{&direct, {}};
+    }
+
+    /**
+     * @brief Finds an entity.
+     * @param entt A valid entity identifier.
+     * @return An iterator to the given entity if it's found, past the end
+     * iterator otherwise.
+     */
+    iterator_type find(const entity_type entt) const ENTT_NOEXCEPT {
+        return has(entt) ? --(end() - index(entt)) : end();
     }
 
     /**
      * @brief Checks if a sparse set contains an entity.
-     * @param entity A valid entity identifier.
+     * @param entt A valid entity identifier.
      * @return True if the sparse set contains the entity, false otherwise.
      */
-    bool has(entity_type entity) const noexcept {
-        const auto entt = entity & traits_type::entity_mask;
-        // the in-use control bit permits to avoid accessing the direct vector
-        return (entt < reverse.size()) && (reverse[entt] & in_use);
+    bool has(const entity_type entt) const ENTT_NOEXCEPT {
+        auto [page, offset] = map(entt);
+        // testing against null permits to avoid accessing the direct vector
+        return (page < reverse.size() && reverse[page] && reverse[page][offset] != null);
     }
 
     /**
@@ -209,14 +376,13 @@ public:
      * 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.
+     * @param entt A valid entity identifier.
      * @return The position of the entity in the sparse set.
      */
-    pos_type get(entity_type entity) const noexcept {
-        assert(has(entity));
-        const auto entt = entity & traits_type::entity_mask;
-        // we must get rid of the in-use bit for it's not part of the position
-        return reverse[entt] & ~in_use;
+    size_type index(const entity_type entt) const ENTT_NOEXCEPT {
+        ENTT_ASSERT(has(entt));
+        auto [page, offset] = map(entt);
+        return size_type(reverse[page][offset]);
     }
 
     /**
@@ -228,20 +394,39 @@ public:
      * 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.
+     * @param entt A valid entity identifier.
      */
-    void construct(entity_type entity) {
-        assert(!has(entity));
-        const auto entt = entity & traits_type::entity_mask;
+    void construct(const entity_type entt) {
+        ENTT_ASSERT(!has(entt));
+        auto [page, offset] = map(entt);
+        assure(page);
+        reverse[page][offset] = entity_type(direct.size());
+        direct.push_back(entt);
+    }
 
-        if(!(entt < reverse.size())) {
-            reverse.resize(entt+1, pos_type{});
-        }
+    /**
+     * @brief Assigns one or more entities to a sparse set.
+     *
+     * @warning
+     * Attempting to assign an entity that already belongs to the sparse set
+     * results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * sparse set already contains the given entity.
+     *
+     * @tparam It Type of forward iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     */
+    template<typename It>
+    void batch(It first, It last) {
+        std::for_each(first, last, [this, next = direct.size()](const auto entt) mutable {
+            ENTT_ASSERT(!has(entt));
+            auto [page, offset] = map(entt);
+            assure(page);
+            reverse[page][offset] = entity_type(next++);
+        });
 
-        // we exploit the fact that pos_type is equal to entity_type and pos has
-        // traits_type::version_mask bits unused we can use to mark it as in-use
-        reverse[entt] = pos_type(direct.size()) | in_use;
-        direct.emplace_back(entity);
+        direct.insert(direct.end(), first, last);
     }
 
     /**
@@ -253,24 +438,20 @@ public:
      * 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.
+     * @param entt A valid entity identifier.
      */
-    virtual void destroy(entity_type entity) {
-        assert(has(entity));
-        const auto entt = entity & traits_type::entity_mask;
-        const auto back = direct.back() & traits_type::entity_mask;
-        const auto pos = reverse[entt] & ~in_use;
-        // the order matters: if back and entt are the same (for the sparse set
-        // has size 1), switching the two lines below doesn't work as expected
-        reverse[back] = pos | in_use;
-        reverse[entt] = pos;
-        // swapping isn't required here, we are getting rid of the last element
-        direct[pos] = direct.back();
+    void destroy(const entity_type entt) {
+        ENTT_ASSERT(has(entt));
+        auto [from_page, from_offset] = map(entt);
+        auto [to_page, to_offset] = map(direct.back());
+        direct[size_type(reverse[from_page][from_offset])] = entity_type(direct.back());
+        reverse[to_page][to_offset] = reverse[from_page][from_offset];
+        reverse[from_page][from_offset] = null;
         direct.pop_back();
     }
 
     /**
-     * @brief Swaps the position of two entities in the internal packed array.
+     * @brief Swaps two entities in the internal packed array.
      *
      * For what it's worth, this function affects both the internal sparse array
      * and the internal packed array. Users should not care of that anyway.
@@ -284,301 +465,179 @@ public:
      * @param lhs A valid entity identifier.
      * @param rhs A valid entity identifier.
      */
-    virtual void swap(entity_type lhs, entity_type rhs) {
-        assert(has(lhs));
-        assert(has(rhs));
-        auto &le = reverse[lhs & traits_type::entity_mask];
-        auto &re = reverse[rhs & traits_type::entity_mask];
-        // we must get rid of the in-use bit for it's not part of the position
-        std::swap(direct[le & ~in_use], direct[re & ~in_use]);
-        std::swap(le, re);
+    virtual void swap(const entity_type lhs, const entity_type rhs) ENTT_NOEXCEPT {
+        auto [src_page, src_offset] = map(lhs);
+        auto [dst_page, dst_offset] = map(rhs);
+        auto &from = reverse[src_page][src_offset];
+        auto &to = reverse[dst_page][dst_offset];
+        std::swap(direct[size_type(from)], direct[size_type(to)]);
+        std::swap(from, to);
     }
 
     /**
-     * @brief Sort entities according to the given comparison function.
+     * @brief Sort elements 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.
+     * Sort the elements so that iterating the range with a couple of iterators
+     * returns them in the expected order. See `begin` and `end` for more
+     * details.
+     *
+     * The comparison function object must return `true` if the first element
+     * is _less_ than the second one, `false` otherwise. The signature of the
+     * comparison function should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * bool(const Entity, const Entity);
+     * @endcode
+     *
+     * Moreover, the comparison function object shall induce a
+     * _strict weak ordering_ on the values.
+     *
+     * The sort function oject must offer a member function template
+     * `operator()` that accepts three arguments:
+     *
+     * * An iterator to the first element of the range to sort.
+     * * An iterator past the last element of the range to sort.
+     * * A comparison function to use to compare the elements.
      *
      * @note
-     * Attempting to iterate elements using the raw pointer returned by `data`
-     * gives no guarantees on the order, even though `sort` has been invoked.
+     * Attempting to iterate elements using a raw pointer returned by a call to
+     * `data` gives no guarantees on the order, even though `sort` has been
+     * invoked.
      *
-     * @tparam Compare Type of the comparison function.
-     * @param compare A comparison function whose signature shall be equivalent
-     * to: `bool(Entity, Entity)`.
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param first An iterator to the first element of the range to sort.
+     * @param last An iterator past the last element of the range to sort.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
      */
-    template<typename Compare>
-    void sort(Compare compare) {
-        std::vector<pos_type> copy{direct.cbegin(), direct.cend()};
-        std::sort(copy.begin(), copy.end(), [compare = std::move(compare)](auto... args) {
-            return !compare(args...);
-        });
+    template<typename Compare, typename Sort = std_sort, typename... Args>
+    void sort(iterator_type first, iterator_type last, Compare compare, Sort algo = Sort{}, Args &&... args) {
+        ENTT_ASSERT(!(last < first));
+        ENTT_ASSERT(!(last > end()));
 
-        for(pos_type i = 0; i < copy.size(); ++i) {
-            if(direct[i] != copy[i]) {
-                swap(direct[i], copy[i]);
+        const auto length = std::distance(first, last);
+        const auto skip = std::distance(last, end());
+        const auto to = direct.rend() - skip;
+        const auto from = to - length;
+
+        algo(from, to, std::move(compare), std::forward<Args>(args)...);
+
+        for(size_type pos = skip, end = skip+length; pos < end; ++pos) {
+            auto [page, offset] = map(direct[pos]);
+            reverse[page][offset] = entity_type(pos);
+        }
+    }
+
+    /**
+     * @brief Sort elements according to the given comparison function.
+     *
+     * @sa sort
+     *
+     * This function is a slightly slower version of `sort` that invokes the
+     * caller to indicate which entities are swapped.<br/>
+     * It's recommended when the caller wants to sort its own data structures to
+     * align them with the order induced in the sparse set.
+     *
+     * The signature of the callback should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * bool(const Entity, const Entity);
+     * @endcode
+     *
+     * @tparam Apply Type of function object to invoke to notify the caller.
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param first An iterator to the first element of the range to sort.
+     * @param last An iterator past the last element of the range to sort.
+     * @param apply A valid function object to use as a callback.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename Apply, typename Compare, typename Sort = std_sort, typename... Args>
+    void arrange(iterator_type first, iterator_type last, Apply apply, Compare compare, Sort algo = Sort{}, Args &&... args) {
+        ENTT_ASSERT(!(last < first));
+        ENTT_ASSERT(!(last > end()));
+
+        const auto length = std::distance(first, last);
+        const auto skip = std::distance(last, end());
+        const auto to = direct.rend() - skip;
+        const auto from = to - length;
+
+        algo(from, to, std::move(compare), std::forward<Args>(args)...);
+
+        for(size_type pos = skip, end = skip+length; pos < end; ++pos) {
+            auto curr = pos;
+            auto next = index(direct[curr]);
+
+            while(curr != next) {
+                apply(direct[curr], direct[next]);
+                auto [page, offset] = map(direct[curr]);
+                reverse[page][offset] = entity_type(curr);
+
+                curr = next;
+                next = index(direct[curr]);
             }
         }
     }
 
     /**
-     * @brief Sort entities according to their order in a sparse set.
+     * @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/>
+     * to the end of the list and there are no guarantees on their order.<br/>
      * In other terms, this function can be used to impose the same order on two
      * sets by using one of them as a master and the other one as a slave.
      *
      * Iterating the sparse set with a couple of iterators returns elements in
-     * the expected order after a call to `sort`. See `begin` and `end` for more
-     * details.
+     * 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 `sort` has been invoked.
+     * Attempting to iterate elements using a raw pointer returned by a call to
+     * `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) {
-        struct Bool { bool value{false}; };
-        std::vector<Bool> check(std::max(other.reverse.size(), reverse.size()));
+    void respect(const sparse_set &other) ENTT_NOEXCEPT {
+        const auto to = other.end();
+        auto from = other.begin();
 
-        for(auto entity: other.direct) {
-            check[entity & traits_type::entity_mask].value = true;
-        }
+        size_type pos = direct.size() - 1;
 
-        sort([this, &other, &check](auto lhs, auto rhs) {
-            const auto le = lhs & traits_type::entity_mask;
-            const auto re = rhs & traits_type::entity_mask;
+        while(pos && from != to) {
+            if(has(*from)) {
+                if(*from != direct[pos]) {
+                    swap(direct[pos], *from);
+                }
 
-            const bool bLhs = check[le].value;
-            const bool bRhs = check[re].value;
-            bool compare = false;
-
-            if(bLhs && bRhs) {
-                compare = other.get(rhs) < other.get(lhs);
-            } else if(!bLhs && !bRhs) {
-                compare = re < le;
-            } else {
-                compare = bLhs;
+                --pos;
             }
 
-            return compare;
-        });
+            ++from;
+        }
     }
 
     /**
      * @brief Resets a sparse set.
      */
-    virtual void reset() {
+    void reset() {
         reverse.clear();
         direct.clear();
     }
 
 private:
-    std::vector<pos_type> reverse;
+    std::vector<std::unique_ptr<entity_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>;
-
-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 = typename underlying_type::iterator_type;
-
-    /*! @brief Default constructor. */
-    SparseSet() 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 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 `sort` 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 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 `sort` 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() noexcept {
-        return 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(entity_type entity) const 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.
-     */
-    object_type & get(entity_type entity) 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.
-     *
-     * @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>
-    object_type & construct(entity_type entity, Args&&... args) {
-        underlying_type::construct(entity);
-        // emplace_back doesn't work well with PODs because of its placement new
-        instances.push_back({ 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(entity_type entity) override {
-        // swapping isn't required here, we are getting rid of the last element
-        instances[underlying_type::get(entity)] = std::move(instances.back());
-        instances.pop_back();
-        underlying_type::destroy(entity);
-    }
-
-    /**
-     * @brief Swaps two entities and their objects.
-     *
-     * @note
-     * This function doesn't swap objects between entities. It exchanges entity
-     * and object positions in the sparse set. It's used mainly for sorting.
-     *
-     * @warning
-     * Attempting to use entities that don't belong to the sparse set results
-     * in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * sparse set doesn't contain the given entities.
-     *
-     * @param lhs A valid entity identifier.
-     * @param rhs A valid entity identifier.
-     */
-    void swap(entity_type lhs, entity_type rhs) override {
-        std::swap(instances[underlying_type::get(lhs)], instances[underlying_type::get(rhs)]);
-        underlying_type::swap(lhs, rhs);
-    }
-
-    /**
-     * @brief Resets a sparse set.
-     */
-    void reset() override {
-        underlying_type::reset();
-        instances.clear();
-    }
-
-private:
-    std::vector<object_type> instances;
-};
-
-
 }
 
 

src/entt/entity/storage.hpp

@@ -0,0 +1,703 @@
+#ifndef ENTT_ENTITY_STORAGE_HPP
+#define ENTT_ENTITY_STORAGE_HPP
+
+
+#include <algorithm>
+#include <iterator>
+#include <numeric>
+#include <utility>
+#include <vector>
+#include <cstddef>
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/algorithm.hpp"
+#include "sparse_set.hpp"
+#include "entity.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Basic storage implementation.
+ *
+ * This class is a refinement of a sparse set that associates an object to an
+ * entity. The main purpose of this class is to extend sparse sets to store
+ * components in a registry. It guarantees fast access both to the elements and
+ * to the entities.
+ *
+ * @note
+ * Entities and objects have the same order. It's guaranteed both in case of raw
+ * access (either to entities or objects) and when using random or input access
+ * iterators.
+ *
+ * @note
+ * Internal data structures arrange elements to maximize performance. Because of
+ * that, there are no guarantees that elements have the expected order when
+ * iterate directly the internal packed array (see `raw` and `size` member
+ * functions for that). Use `begin` and `end` instead.
+ *
+ * @warning
+ * Empty types aren't explicitly instantiated. Temporary objects are returned in
+ * place of the instances of the components and raw access isn't available for
+ * them.
+ *
+ * @sa sparse_set<Entity>
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Type Type of objects assigned to the entities.
+ */
+template<typename Entity, typename Type, typename = std::void_t<>>
+class basic_storage: public sparse_set<Entity> {
+    using underlying_type = sparse_set<Entity>;
+    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
+
+    template<bool Const>
+    class iterator {
+        friend class basic_storage<Entity, Type>;
+
+        using instance_type = std::conditional_t<Const, const std::vector<Type>, std::vector<Type>>;
+        using index_type = typename traits_type::difference_type;
+
+        iterator(instance_type *ref, const index_type idx) ENTT_NOEXCEPT
+            : instances{ref}, index{idx}
+        {}
+
+    public:
+        using difference_type = index_type;
+        using value_type = Type;
+        using pointer = std::conditional_t<Const, const value_type *, value_type *>;
+        using reference = std::conditional_t<Const, const value_type &, value_type &>;
+        using iterator_category = std::random_access_iterator_tag;
+
+        iterator() ENTT_NOEXCEPT = default;
+
+        iterator & operator++() ENTT_NOEXCEPT {
+            return --index, *this;
+        }
+
+        iterator operator++(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        iterator & operator--() ENTT_NOEXCEPT {
+            return ++index, *this;
+        }
+
+        iterator operator--(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return --(*this), orig;
+        }
+
+        iterator & operator+=(const difference_type value) ENTT_NOEXCEPT {
+            index -= value;
+            return *this;
+        }
+
+        iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
+            return iterator{instances, index-value};
+        }
+
+        iterator & operator-=(const difference_type value) ENTT_NOEXCEPT {
+            return (*this += -value);
+        }
+
+        iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
+            return (*this + -value);
+        }
+
+        difference_type operator-(const iterator &other) const ENTT_NOEXCEPT {
+            return other.index - index;
+        }
+
+        reference operator[](const difference_type value) const ENTT_NOEXCEPT {
+            const auto pos = size_type(index-value-1);
+            return (*instances)[pos];
+        }
+
+        bool operator==(const iterator &other) const ENTT_NOEXCEPT {
+            return other.index == index;
+        }
+
+        bool operator!=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+        bool operator<(const iterator &other) const ENTT_NOEXCEPT {
+            return index > other.index;
+        }
+
+        bool operator>(const iterator &other) const ENTT_NOEXCEPT {
+            return index < other.index;
+        }
+
+        bool operator<=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this > other);
+        }
+
+        bool operator>=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this < other);
+        }
+
+        pointer operator->() const ENTT_NOEXCEPT {
+            const auto pos = size_type(index-1);
+            return &(*instances)[pos];
+        }
+
+        reference operator*() const ENTT_NOEXCEPT {
+            return *operator->();
+        }
+
+    private:
+        instance_type *instances;
+        index_type index;
+    };
+
+public:
+    /*! @brief Type of the objects associated with the entities. */
+    using object_type = Type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Random access iterator type. */
+    using iterator_type = iterator<false>;
+    /*! @brief Constant random access iterator type. */
+    using const_iterator_type = iterator<true>;
+
+    /**
+     * @brief Increases the capacity of a storage.
+     *
+     * If the new capacity is greater than the current capacity, new storage is
+     * allocated, otherwise the method does nothing.
+     *
+     * @param cap Desired capacity.
+     */
+    void reserve(const size_type cap) {
+        underlying_type::reserve(cap);
+        instances.reserve(cap);
+    }
+
+    /*! @brief Requests the removal of unused capacity. */
+    void shrink_to_fit() {
+        underlying_type::shrink_to_fit();
+        instances.shrink_to_fit();
+    }
+
+    /**
+     * @brief Direct access to the array of objects.
+     *
+     * The returned pointer is such that range `[raw(), raw() + size()]` is
+     * always a valid range, even if the container is empty.
+     *
+     * @note
+     * There are no guarantees on the order, even though either `sort` or
+     * `respect` has been previously invoked. Internal data structures arrange
+     * elements to maximize performance. Accessing them directly gives a
+     * performance boost but less guarantees. Use `begin` and `end` if you want
+     * to iterate the storage in the expected order.
+     *
+     * @return A pointer to the array of objects.
+     */
+    const object_type * raw() const ENTT_NOEXCEPT {
+        return instances.data();
+    }
+
+    /*! @copydoc raw */
+    object_type * raw() ENTT_NOEXCEPT {
+        return const_cast<object_type *>(std::as_const(*this).raw());
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * The returned iterator points to the first instance of the given type. If
+     * the storage is empty, the returned iterator will be equal to `end()`.
+     *
+     * @note
+     * Random access iterators stay true to the order imposed by a call to
+     * either `sort` or `respect`.
+     *
+     * @return An iterator to the first instance of the given type.
+     */
+    const_iterator_type cbegin() const ENTT_NOEXCEPT {
+        const typename traits_type::difference_type pos = underlying_type::size();
+        return const_iterator_type{&instances, pos};
+    }
+
+    /*! @copydoc cbegin */
+    const_iterator_type begin() const ENTT_NOEXCEPT {
+        return cbegin();
+    }
+
+    /*! @copydoc begin */
+    iterator_type begin() ENTT_NOEXCEPT {
+        const typename traits_type::difference_type pos = underlying_type::size();
+        return iterator_type{&instances, pos};
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     *
+     * The returned iterator points to the element following the last instance
+     * of the given type. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @note
+     * Random access iterators stay true to the order imposed by a call to
+     * either `sort` or `respect`.
+     *
+     * @return An iterator to the element following the last instance of the
+     * given type.
+     */
+    const_iterator_type cend() const ENTT_NOEXCEPT {
+        return const_iterator_type{&instances, {}};
+    }
+
+    /*! @copydoc cend */
+    const_iterator_type end() const ENTT_NOEXCEPT {
+        return cend();
+    }
+
+    /*! @copydoc end */
+    iterator_type end() ENTT_NOEXCEPT {
+        return iterator_type{&instances, {}};
+    }
+
+    /**
+     * @brief Returns the object associated with an entity.
+     *
+     * @warning
+     * Attempting to use an entity that doesn't belong to the storage results in
+     * undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * storage doesn't contain the given entity.
+     *
+     * @param entt A valid entity identifier.
+     * @return The object associated with the entity.
+     */
+    const object_type & get(const entity_type entt) const ENTT_NOEXCEPT {
+        return instances[underlying_type::index(entt)];
+    }
+
+    /*! @copydoc get */
+    object_type & get(const entity_type entt) ENTT_NOEXCEPT {
+        return const_cast<object_type &>(std::as_const(*this).get(entt));
+    }
+
+    /**
+     * @brief Returns a pointer to the object associated with an entity, if any.
+     * @param entt A valid entity identifier.
+     * @return The object associated with the entity, if any.
+     */
+    const object_type * try_get(const entity_type entt) const ENTT_NOEXCEPT {
+        return underlying_type::has(entt) ? (instances.data() + underlying_type::index(entt)) : nullptr;
+    }
+
+    /*! @copydoc try_get */
+    object_type * try_get(const entity_type entt) ENTT_NOEXCEPT {
+        return const_cast<object_type *>(std::as_const(*this).try_get(entt));
+    }
+
+    /**
+     * @brief Assigns an entity to a storage and constructs its object.
+     *
+     * This version accept both types that can be constructed in place directly
+     * and types like aggregates that do not work well with a placement new as
+     * performed usually under the hood during an _emplace back_.
+     *
+     * @warning
+     * Attempting to use an entity that already belongs to the storage results
+     * in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * storage already contains the given entity.
+     *
+     * @tparam Args Types of arguments to use to construct the object.
+     * @param entt A valid entity identifier.
+     * @param args Parameters to use to construct an object for the entity.
+     * @return The object associated with the entity.
+     */
+    template<typename... Args>
+    object_type & construct(const entity_type entt, Args &&... args) {
+        if constexpr(std::is_aggregate_v<object_type>) {
+            instances.emplace_back(Type{std::forward<Args>(args)...});
+        } else {
+            instances.emplace_back(std::forward<Args>(args)...);
+        }
+
+        // entity goes after component in case constructor throws
+        underlying_type::construct(entt);
+        return instances.back();
+    }
+
+    /**
+     * @brief Assigns one or more entities to a storage and default constructs
+     * or copy constructs their objects.
+     *
+     * The object type must be at least move and default insertable if no
+     * arguments are provided, move and copy insertable otherwise.
+     *
+     * @warning
+     * Attempting to assign an entity that already belongs to the storage
+     * results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * storage already contains the given entity.
+     *
+     * @tparam It Type of forward iterator.
+     * @tparam Args Types of arguments to use to construct the object.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param args Parameters to use to construct an object for the entities.
+     * @return An iterator to the list of instances just created and sorted the
+     * same of the entities.
+     */
+    template<typename It, typename... Args>
+    iterator_type batch(It first, It last, Args &&... args) {
+        if constexpr(sizeof...(Args) == 0) {
+            instances.resize(instances.size() + std::distance(first, last));
+        } else {
+            instances.resize(instances.size() + std::distance(first, last), Type{std::forward<Args>(args)...});
+        }
+
+        // entity goes after component in case constructor throws
+        underlying_type::batch(first, last);
+        return begin();
+    }
+
+    /**
+     * @brief Removes an entity from a storage and destroys its object.
+     *
+     * @warning
+     * Attempting to use an entity that doesn't belong to the storage results in
+     * undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * storage doesn't contain the given entity.
+     *
+     * @param entt A valid entity identifier.
+     */
+    void destroy(const entity_type entt) {
+        auto other = std::move(instances.back());
+        instances[underlying_type::index(entt)] = std::move(other);
+        instances.pop_back();
+        underlying_type::destroy(entt);
+    }
+
+    /**
+     * @brief Swaps entities and objects in the internal packed arrays.
+     *
+     * @warning
+     * Attempting to swap entities that don't belong to the sparse set results
+     * in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * sparse set doesn't contain the given entities.
+     *
+     * @param lhs A valid entity identifier.
+     * @param rhs A valid entity identifier.
+     */
+    void swap(const entity_type lhs, const entity_type rhs) ENTT_NOEXCEPT override {
+        std::swap(instances[underlying_type::index(lhs)], instances[underlying_type::index(rhs)]);
+        underlying_type::swap(lhs, rhs);
+    }
+
+    /**
+     * @brief Sort elements according to the given comparison function.
+     *
+     * Sort the elements so that iterating the range with a couple of iterators
+     * returns them in the expected order. See `begin` and `end` for more
+     * details.
+     *
+     * The comparison function object must return `true` if the first element
+     * is _less_ than the second one, `false` otherwise. The signature of the
+     * comparison function should be equivalent to one of the following:
+     *
+     * @code{.cpp}
+     * bool(const Entity, const Entity);
+     * bool(const Type &, const Type &);
+     * @endcode
+     *
+     * Moreover, the comparison function object shall induce a
+     * _strict weak ordering_ on the values.
+     *
+     * The sort function oject must offer a member function template
+     * `operator()` that accepts three arguments:
+     *
+     * * An iterator to the first element of the range to sort.
+     * * An iterator past the last element of the range to sort.
+     * * A comparison function to use to compare the elements.
+     *
+     * @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.
+     *
+     * @warning
+     * Empty types are never instantiated. Therefore, only comparison function
+     * objects that require to return entities rather than components are
+     * accepted.
+     *
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param first An iterator to the first element of the range to sort.
+     * @param last An iterator past the last element of the range to sort.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename Compare, typename Sort = std_sort, typename... Args>
+    void sort(iterator_type first, iterator_type last, Compare compare, Sort algo = Sort{}, Args &&... args) {
+        ENTT_ASSERT(!(last < first));
+        ENTT_ASSERT(!(last > end()));
+
+        const auto from = underlying_type::begin() + std::distance(begin(), first);
+        const auto to = from + std::distance(first, last);
+
+        const auto apply = [this](const auto lhs, const auto rhs) {
+            std::swap(instances[underlying_type::index(lhs)], instances[underlying_type::index(rhs)]);
+        };
+
+        if constexpr(std::is_invocable_v<Compare, const object_type &, const object_type &>) {
+            underlying_type::arrange(from, to, std::move(apply), [this, compare = std::move(compare)](const auto lhs, const auto rhs) {
+                return compare(std::as_const(instances[underlying_type::index(lhs)]), std::as_const(instances[underlying_type::index(rhs)]));
+            }, std::move(algo), std::forward<Args>(args)...);
+        } else {
+            underlying_type::arrange(from, to, std::move(apply), std::move(compare), std::move(algo), std::forward<Args>(args)...);
+        }
+    }
+
+    /*! @brief Resets a storage. */
+    void reset() {
+        underlying_type::reset();
+        instances.clear();
+    }
+
+private:
+    std::vector<object_type> instances;
+};
+
+
+/*! @copydoc basic_storage */
+template<typename Entity, typename Type>
+class basic_storage<Entity, Type, std::enable_if_t<ENTT_ENABLE_ETO(Type)>>: public sparse_set<Entity> {
+    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
+    using underlying_type = sparse_set<Entity>;
+
+    class iterator {
+        friend class basic_storage<Entity, Type>;
+
+        using index_type = typename traits_type::difference_type;
+
+        iterator(const index_type idx) ENTT_NOEXCEPT
+            : index{idx}
+        {}
+
+    public:
+        using difference_type = index_type;
+        using value_type = Type;
+        using pointer = const value_type *;
+        using reference = value_type;
+        using iterator_category = std::input_iterator_tag;
+
+        iterator() ENTT_NOEXCEPT = default;
+
+        iterator & operator++() ENTT_NOEXCEPT {
+            return --index, *this;
+        }
+
+        iterator operator++(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        iterator & operator--() ENTT_NOEXCEPT {
+            return ++index, *this;
+        }
+
+        iterator operator--(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return --(*this), orig;
+        }
+
+        iterator & operator+=(const difference_type value) ENTT_NOEXCEPT {
+            index -= value;
+            return *this;
+        }
+
+        iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
+            return iterator{index-value};
+        }
+
+        iterator & operator-=(const difference_type value) ENTT_NOEXCEPT {
+            return (*this += -value);
+        }
+
+        iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
+            return (*this + -value);
+        }
+
+        difference_type operator-(const iterator &other) const ENTT_NOEXCEPT {
+            return other.index - index;
+        }
+
+        reference operator[](const difference_type) const ENTT_NOEXCEPT {
+            return {};
+        }
+
+        bool operator==(const iterator &other) const ENTT_NOEXCEPT {
+            return other.index == index;
+        }
+
+        bool operator!=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+        bool operator<(const iterator &other) const ENTT_NOEXCEPT {
+            return index > other.index;
+        }
+
+        bool operator>(const iterator &other) const ENTT_NOEXCEPT {
+            return index < other.index;
+        }
+
+        bool operator<=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this > other);
+        }
+
+        bool operator>=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this < other);
+        }
+
+        pointer operator->() const ENTT_NOEXCEPT {
+            return nullptr;
+        }
+
+        reference operator*() const ENTT_NOEXCEPT {
+            return {};
+        }
+
+    private:
+        index_type index;
+    };
+
+public:
+    /*! @brief Type of the objects associated with the entities. */
+    using object_type = Type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Random access iterator type. */
+    using iterator_type = iterator;
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * The returned iterator points to the first instance of the given type. If
+     * the storage is empty, the returned iterator will be equal to `end()`.
+     *
+     * @note
+     * Input iterators stay true to the order imposed by a call to either `sort`
+     * or `respect`.
+     *
+     * @return An iterator to the first instance of the given type.
+     */
+    iterator_type cbegin() const ENTT_NOEXCEPT {
+        const typename traits_type::difference_type pos = underlying_type::size();
+        return iterator_type{pos};
+    }
+
+    /*! @copydoc cbegin */
+    iterator_type begin() const ENTT_NOEXCEPT {
+        return cbegin();
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     *
+     * The returned iterator points to the element following the last instance
+     * of the given type. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @note
+     * Input iterators stay true to the order imposed by a call to either `sort`
+     * or `respect`.
+     *
+     * @return An iterator to the element following the last instance of the
+     * given type.
+     */
+    iterator_type cend() const ENTT_NOEXCEPT {
+        return iterator_type{};
+    }
+
+    /*! @copydoc cend */
+    iterator_type end() const ENTT_NOEXCEPT {
+        return cend();
+    }
+
+    /**
+     * @brief Returns the object associated with an entity.
+     *
+     * @note
+     * Empty types aren't explicitly instantiated. Therefore, this function
+     * always returns a temporary object.
+     *
+     * @warning
+     * Attempting to use an entity that doesn't belong to the storage results in
+     * undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * storage doesn't contain the given entity.
+     *
+     * @param entt A valid entity identifier.
+     * @return The object associated with the entity.
+     */
+    object_type get([[maybe_unused]] const entity_type entt) const ENTT_NOEXCEPT {
+        ENTT_ASSERT(underlying_type::has(entt));
+        return {};
+    }
+
+    /**
+     * @brief Assigns one or more entities to a storage.
+     *
+     * The object type must be at least default constructible.
+     *
+     * @warning
+     * Attempting to assign an entity that already belongs to the storage
+     * results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * storage already contains the given entity.
+     *
+     * @tparam It Type of forward iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @return An iterator to the list of instances just created and sorted the
+     * same of the entities.
+     */
+    template<typename It>
+    iterator_type batch(It first, It last, const object_type & = {}) {
+        underlying_type::batch(first, last);
+        return begin();
+    }
+
+    /*! @copydoc storage::sort */
+    template<typename Compare, typename Sort = std_sort, typename... Args>
+    void sort(iterator_type first, iterator_type last, Compare compare, Sort algo = Sort{}, Args &&... args) {
+        ENTT_ASSERT(!(last < first));
+        ENTT_ASSERT(!(last > end()));
+
+        const auto from = underlying_type::begin() + std::distance(begin(), first);
+        const auto to = from + std::distance(first, last);
+
+        underlying_type::sort(from, to, std::move(compare), std::move(algo), std::forward<Args>(args)...);
+    }
+};
+
+/*! @copydoc basic_storage */
+template<typename Entity, typename Type>
+struct storage: basic_storage<Entity, Type> {};
+
+
+}
+
+
+#endif // ENTT_ENTITY_STORAGE_HPP

src/entt/entity/traits.hpp

@@ -1,96 +0,0 @@
-#ifndef ENTT_ENTITY_ENTT_HPP
-#define ENTT_ENTITY_ENTT_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 = 0xFFFFFF;
-    /*! @brief Mask to use to get the version out of an identifier. */
-    static constexpr auto version_mask = 0xFF;
-    /*! @brief Extent of the entity number within an identifier. */
-    static constexpr auto entity_shift = 24;
-};
-
-
-/**
- * @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_HPP

src/entt/entity/utility.hpp

@@ -0,0 +1,46 @@
+#ifndef ENTT_ENTITY_UTILITY_HPP
+#define ENTT_ENTITY_UTILITY_HPP
+
+
+#include "../core/type_traits.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Alias for exclusion lists.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+struct exclude_t: type_list<Type...> {};
+
+
+/**
+ * @brief Variable template for exclusion lists.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+constexpr exclude_t<Type...> exclude{};
+
+
+/**
+ * @brief Alias for lists of observed components.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+struct get_t: type_list<Type...>{};
+
+
+/**
+ * @brief Variable template for lists of observed components.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+constexpr get_t<Type...> get{};
+
+
+}
+
+
+#endif // ENTT_ENTITY_UTILITY_HPP

src/entt/entt.hpp

@@ -1,19 +1,32 @@
+#include "core/algorithm.hpp"
 #include "core/family.hpp"
 #include "core/hashed_string.hpp"
 #include "core/ident.hpp"
+#include "core/monostate.hpp"
+#include "core/type_traits.hpp"
+#include "core/utility.hpp"
 #include "entity/actor.hpp"
+#include "entity/entity.hpp"
+#include "entity/group.hpp"
+#include "entity/helper.hpp"
+#include "entity/observer.hpp"
 #include "entity/registry.hpp"
+#include "entity/runtime_view.hpp"
+#include "entity/snapshot.hpp"
 #include "entity/sparse_set.hpp"
-#include "entity/traits.hpp"
+#include "entity/storage.hpp"
+#include "entity/utility.hpp"
 #include "entity/view.hpp"
 #include "locator/locator.hpp"
+#include "meta/factory.hpp"
+#include "meta/meta.hpp"
+#include "meta/policy.hpp"
 #include "process/process.hpp"
 #include "process/scheduler.hpp"
 #include "resource/cache.hpp"
 #include "resource/handle.hpp"
 #include "resource/loader.hpp"
-#include "signal/bus.hpp"
 #include "signal/delegate.hpp"
+#include "signal/dispatcher.hpp"
 #include "signal/emitter.hpp"
 #include "signal/sigh.hpp"
-#include "signal/signal.hpp"

src/entt/fwd.hpp

@@ -0,0 +1,3 @@
+#include "entity/fwd.hpp"
+#include "resource/fwd.hpp"
+#include "signal/fwd.hpp"

src/entt/locator/locator.hpp

@@ -4,7 +4,7 @@
 
 #include <memory>
 #include <utility>
-#include <cassert>
+#include "../config/config.h"
 
 
 namespace entt {
@@ -14,7 +14,7 @@ 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 tighly bound to the services they expose and
+ * 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.
@@ -22,20 +22,20 @@ namespace entt {
  * @tparam Service Type of service managed by the locator.
  */
 template<typename Service>
-struct ServiceLocator final {
+struct service_locator {
     /*! @brief Type of service offered. */
     using service_type = Service;
 
     /*! @brief Default constructor, deleted on purpose. */
-    ServiceLocator() = delete;
+    service_locator() = delete;
     /*! @brief Default destructor, deleted on purpose. */
-    ~ServiceLocator() = delete;
+    ~service_locator() = delete;
 
     /**
      * @brief Tests if a valid service implementation is set.
      * @return True if the service is set, false otherwise.
      */
-    inline static bool empty() noexcept {
+    static bool empty() ENTT_NOEXCEPT {
         return !static_cast<bool>(service);
     }
 
@@ -49,7 +49,7 @@ struct ServiceLocator final {
      *
      * @return A reference to the service implementation currently set, if any.
      */
-    inline static std::weak_ptr<Service> get() noexcept {
+    static std::weak_ptr<Service> get() ENTT_NOEXCEPT {
         return service;
     }
 
@@ -67,7 +67,7 @@ struct ServiceLocator final {
      *
      * @return A reference to the service implementation currently set, if any.
      */
-    inline static Service & ref() noexcept {
+    static Service & ref() ENTT_NOEXCEPT {
         return *service;
     }
 
@@ -78,7 +78,7 @@ struct ServiceLocator final {
      * @param args Parameters to use to construct the service.
      */
     template<typename Impl = Service, typename... Args>
-    inline static void set(Args&&... args) {
+    static void set(Args &&... args) {
         service = std::make_shared<Impl>(std::forward<Args>(args)...);
     }
 
@@ -86,8 +86,8 @@ struct ServiceLocator final {
      * @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));
+    static void set(std::shared_ptr<Service> ptr) {
+        ENTT_ASSERT(static_cast<bool>(ptr));
         service = std::move(ptr);
     }
 
@@ -96,19 +96,15 @@ struct ServiceLocator final {
      *
      * The service is no longer valid after a reset.
      */
-    inline static void reset() {
+    static void reset() {
         service.reset();
     }
 
 private:
-    static std::shared_ptr<Service> service;
+    inline static std::shared_ptr<Service> service = nullptr;
 };
 
 
-template<typename Service>
-std::shared_ptr<Service> ServiceLocator<Service>::service{};
-
-
 }
 
 

src/entt/meta/factory.hpp

@@ -0,0 +1,858 @@
+#ifndef ENTT_META_FACTORY_HPP
+#define ENTT_META_FACTORY_HPP
+
+
+#include <tuple>
+#include <array>
+#include <cstddef>
+#include <utility>
+#include <functional>
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/type_traits.hpp"
+#include "policy.hpp"
+#include "meta.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename>
+struct meta_function_helper;
+
+
+template<typename Ret, typename... Args>
+struct meta_function_helper<Ret(Args...)> {
+    using return_type = std::remove_cv_t<std::remove_reference_t<Ret>>;
+    using args_type = std::tuple<std::remove_cv_t<std::remove_reference_t<Args>>...>;
+
+    static constexpr std::index_sequence_for<Args...> index_sequence{};
+    static constexpr auto is_const = false;
+
+    static auto arg(typename internal::meta_func_node::size_type index) ENTT_NOEXCEPT {
+        return std::array<meta_type_node *, sizeof...(Args)>{{meta_info<Args>::resolve()...}}[index];
+    }
+};
+
+
+template<typename Ret, typename... Args>
+struct meta_function_helper<Ret(Args...) const>: meta_function_helper<Ret(Args...)> {
+    static constexpr auto is_const = true;
+};
+
+
+template<typename Ret, typename... Args, typename Class>
+constexpr meta_function_helper<Ret(Args...)>
+to_meta_function_helper(Ret(Class:: *)(Args...));
+
+
+template<typename Ret, typename... Args, typename Class>
+constexpr meta_function_helper<Ret(Args...) const>
+to_meta_function_helper(Ret(Class:: *)(Args...) const);
+
+
+template<typename Ret, typename... Args>
+constexpr meta_function_helper<Ret(Args...)>
+to_meta_function_helper(Ret(*)(Args...));
+
+
+constexpr void to_meta_function_helper(...);
+
+
+template<typename Candidate>
+using meta_function_helper_t = decltype(to_meta_function_helper(std::declval<Candidate>()));
+
+
+template<typename Type, typename... Args, std::size_t... Indexes>
+meta_any construct(meta_any * const args, std::index_sequence<Indexes...>) {
+    [[maybe_unused]] auto direct = std::make_tuple((args+Indexes)->try_cast<Args>()...);
+    meta_any any{};
+
+    if(((std::get<Indexes>(direct) || (args+Indexes)->convert<Args>()) && ...)) {
+        any = Type{(std::get<Indexes>(direct) ? *std::get<Indexes>(direct) : (args+Indexes)->cast<Args>())...};
+    }
+
+    return any;
+}
+
+
+template<bool Const, typename Type, auto Data>
+bool setter([[maybe_unused]] meta_handle handle, [[maybe_unused]] meta_any index, [[maybe_unused]] meta_any value) {
+    bool accepted = false;
+
+    if constexpr(!Const) {
+        if constexpr(std::is_function_v<std::remove_pointer_t<decltype(Data)>> || std::is_member_function_pointer_v<decltype(Data)>) {
+            using helper_type = meta_function_helper_t<decltype(Data)>;
+            using data_type = std::tuple_element_t<!std::is_member_function_pointer_v<decltype(Data)>, typename helper_type::args_type>;
+            static_assert(std::is_invocable_v<decltype(Data), Type &, data_type>);
+            auto * const clazz = meta_any{handle}.try_cast<Type>();
+            auto * const direct = value.try_cast<data_type>();
+
+            if(clazz && (direct || value.convert<data_type>())) {
+                std::invoke(Data, *clazz, direct ? *direct : value.cast<data_type>());
+                accepted = true;
+            }
+        } else if constexpr(std::is_member_object_pointer_v<decltype(Data)>) {
+            using data_type = std::remove_cv_t<std::remove_reference_t<decltype(std::declval<Type>().*Data)>>;
+            static_assert(std::is_invocable_v<decltype(Data), Type *>);
+            auto * const clazz = meta_any{handle}.try_cast<Type>();
+
+            if constexpr(std::is_array_v<data_type>) {
+                using underlying_type = std::remove_extent_t<data_type>;
+                auto * const direct = value.try_cast<underlying_type>();
+                auto * const idx = index.try_cast<std::size_t>();
+
+                if(clazz && idx && (direct || value.convert<underlying_type>())) {
+                    std::invoke(Data, clazz)[*idx] = direct ? *direct : value.cast<underlying_type>();
+                    accepted = true;
+                }
+            } else {
+                auto * const direct = value.try_cast<data_type>();
+
+                if(clazz && (direct || value.convert<data_type>())) {
+                    std::invoke(Data, clazz) = (direct ? *direct : value.cast<data_type>());
+                    accepted = true;
+                }
+            }
+        } else {
+            static_assert(std::is_pointer_v<decltype(Data)>);
+            using data_type = std::remove_cv_t<std::remove_reference_t<decltype(*Data)>>;
+
+            if constexpr(std::is_array_v<data_type>) {
+                using underlying_type = std::remove_extent_t<data_type>;
+                auto * const direct = value.try_cast<underlying_type>();
+                auto * const idx = index.try_cast<std::size_t>();
+
+                if(idx && (direct || value.convert<underlying_type>())) {
+                    (*Data)[*idx] = (direct ? *direct : value.cast<underlying_type>());
+                    accepted = true;
+                }
+            } else {
+                auto * const direct = value.try_cast<data_type>();
+
+                if(direct || value.convert<data_type>()) {
+                    *Data = (direct ? *direct : value.cast<data_type>());
+                    accepted = true;
+                }
+            }
+        }
+    }
+
+    return accepted;
+}
+
+
+template<typename Type, auto Data, typename Policy>
+meta_any getter([[maybe_unused]] meta_handle handle, [[maybe_unused]] meta_any index) {
+    auto dispatch = [](auto &&value) {
+        if constexpr(std::is_same_v<Policy, as_void_t>) {
+            return meta_any{std::in_place_type<void>};
+        } else if constexpr(std::is_same_v<Policy, as_alias_t>) {
+            return meta_any{std::ref(std::forward<decltype(value)>(value))};
+        } else {
+            static_assert(std::is_same_v<Policy, as_is_t>);
+            return meta_any{std::forward<decltype(value)>(value)};
+        }
+    };
+
+    if constexpr(std::is_function_v<std::remove_pointer_t<decltype(Data)>> || std::is_member_function_pointer_v<decltype(Data)>) {
+        static_assert(std::is_invocable_v<decltype(Data), Type &>);
+        auto * const clazz = meta_any{handle}.try_cast<Type>();
+        return clazz ? dispatch(std::invoke(Data, *clazz)) : meta_any{};
+    } else if constexpr(std::is_member_object_pointer_v<decltype(Data)>) {
+        using data_type = std::remove_cv_t<std::remove_reference_t<decltype(std::declval<Type>().*Data)>>;
+        static_assert(std::is_invocable_v<decltype(Data), Type *>);
+        auto * const clazz = meta_any{handle}.try_cast<Type>();
+
+        if constexpr(std::is_array_v<data_type>) {
+            auto * const idx = index.try_cast<std::size_t>();
+            return (clazz && idx) ? dispatch(std::invoke(Data, clazz)[*idx]) : meta_any{};
+        } else {
+            return clazz ? dispatch(std::invoke(Data, clazz)) : meta_any{};
+        }
+    } else {
+        static_assert(std::is_pointer_v<std::decay_t<decltype(Data)>>);
+
+        if constexpr(std::is_array_v<std::remove_pointer_t<decltype(Data)>>) {
+            auto * const idx = index.try_cast<std::size_t>();
+            return idx ? dispatch((*Data)[*idx]) : meta_any{};
+        } else {
+            return dispatch(*Data);
+        }
+    }
+}
+
+
+template<typename Type, auto Candidate, typename Policy, std::size_t... Indexes>
+meta_any invoke([[maybe_unused]] meta_handle handle, meta_any *args, std::index_sequence<Indexes...>) {
+    using helper_type = meta_function_helper_t<decltype(Candidate)>;
+
+    auto dispatch = [](auto *... params) {
+        if constexpr(std::is_void_v<typename helper_type::return_type> || std::is_same_v<Policy, as_void_t>) {
+            std::invoke(Candidate, *params...);
+            return meta_any{std::in_place_type<void>};
+        } else if constexpr(std::is_same_v<Policy, as_alias_t>) {
+            return meta_any{std::ref(std::invoke(Candidate, *params...))};
+        } else {
+            static_assert(std::is_same_v<Policy, as_is_t>);
+            return meta_any{std::invoke(Candidate, *params...)};
+        }
+    };
+
+    [[maybe_unused]] const auto direct = std::make_tuple([](meta_any *any, auto *instance) {
+        using arg_type = std::remove_reference_t<decltype(*instance)>;
+
+        if(!instance && any->convert<arg_type>()) {
+            instance = any->try_cast<arg_type>();
+        }
+
+        return instance;
+    }(args+Indexes, (args+Indexes)->try_cast<std::tuple_element_t<Indexes, typename helper_type::args_type>>())...);
+
+    if constexpr(std::is_function_v<std::remove_pointer_t<decltype(Candidate)>>) {
+        return (std::get<Indexes>(direct) && ...) ? dispatch(std::get<Indexes>(direct)...) : meta_any{};
+    } else {
+        auto * const clazz = meta_any{handle}.try_cast<Type>();
+        return (clazz && (std::get<Indexes>(direct) && ...)) ? dispatch(clazz, std::get<Indexes>(direct)...) : meta_any{};
+    }
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond TURN_OFF_DOXYGEN
+ */
+
+
+template<typename, typename...>
+class extended_meta_factory;
+
+
+/**
+ * @brief A meta factory to be used for reflection purposes.
+ *
+ * A meta factory is an utility class used to reflect types, data and functions
+ * of all sorts. This class ensures that the underlying web of types is built
+ * correctly and performs some checks in debug mode to ensure that there are no
+ * subtle errors at runtime.
+ *
+ * @tparam Type Reflected type for which the factory was created.
+ */
+template<typename Type>
+class meta_factory {
+    template<typename Node>
+    bool duplicate(const Node *candidate, const Node *node) ENTT_NOEXCEPT {
+        return node && (node == candidate || duplicate(candidate, node->next));
+    }
+
+    template<typename Node>
+    bool duplicate(const ENTT_ID_TYPE identifier, const Node *node) ENTT_NOEXCEPT {
+        return node && (node->identifier == identifier || duplicate(identifier, node->next));
+    }
+
+    auto record(const ENTT_ID_TYPE identifier) ENTT_NOEXCEPT {
+        auto * const node = internal::meta_info<Type>::resolve();
+
+        ENTT_ASSERT(!duplicate(identifier, *internal::meta_info<>::global));
+        ENTT_ASSERT(!duplicate(node, *internal::meta_info<>::global));
+        node->identifier = identifier;
+        node->next = *internal::meta_info<>::global;
+        *internal::meta_info<>::global = node;
+
+        return extended_meta_factory<Type>{&node->prop};
+    }
+
+public:
+    /**
+     * @brief Extends a meta type by assigning it an identifier.
+     *
+     * This function is intended only for unnamed types.
+     *
+     * @param identifier Unique identifier.
+     * @return An extended meta factory for the parent type.
+     */
+    auto type(const ENTT_ID_TYPE identifier) ENTT_NOEXCEPT {
+        static_assert(!is_named_type_v<Type>);
+        return record(identifier);
+    }
+
+    /**
+     * @brief Extends a meta type by assigning it an identifier.
+     *
+     * This function is intended only for named types
+     *
+     * @return An extended meta factory for the parent type.
+     */
+    auto type() ENTT_NOEXCEPT {
+        static_assert(is_named_type_v<Type>);
+        return record(named_type_traits_t<Type>::value);
+    }
+
+    /**
+     * @brief Assigns a meta base to a meta type.
+     *
+     * A reflected base class must be a real base class of the reflected type.
+     *
+     * @tparam Base Type of the base class to assign to the meta type.
+     * @return A meta factory for the parent type.
+     */
+    template<typename Base>
+    auto base() ENTT_NOEXCEPT {
+        static_assert(std::is_base_of_v<Base, Type>);
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_base_node node{
+            type,
+            nullptr,
+            &internal::meta_info<Base>::resolve,
+            [](void *instance) ENTT_NOEXCEPT -> void * {
+                return static_cast<Base *>(static_cast<Type *>(instance));
+            }
+        };
+
+        ENTT_ASSERT(!duplicate(&node, type->base));
+        node.next = type->base;
+        type->base = &node;
+
+        return meta_factory<Type>{};
+    }
+
+    /**
+     * @brief Assigns a meta conversion function to a meta type.
+     *
+     * The given type must be such that an instance of the reflected type can be
+     * converted to it.
+     *
+     * @tparam To Type of the conversion function to assign to the meta type.
+     * @return A meta factory for the parent type.
+     */
+    template<typename To>
+    auto conv() ENTT_NOEXCEPT {
+        static_assert(std::is_convertible_v<Type, To>);
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_conv_node node{
+            type,
+            nullptr,
+            &internal::meta_info<To>::resolve,
+            [](const void *instance) -> meta_any {
+                return static_cast<To>(*static_cast<const Type *>(instance));
+            }
+        };
+
+        ENTT_ASSERT(!duplicate(&node, type->conv));
+        node.next = type->conv;
+        type->conv = &node;
+
+        return meta_factory<Type>{};
+    }
+
+    /**
+     * @brief Assigns a meta conversion function to a meta type.
+     *
+     * Conversion functions can be either free functions or member
+     * functions.<br/>
+     * In case of free functions, they must accept a const reference to an
+     * instance of the parent type as an argument. In case of member functions,
+     * they should have no arguments at all.
+     *
+     * @tparam Candidate The actual function to use for the conversion.
+     * @return A meta factory for the parent type.
+     */
+    template<auto Candidate>
+    auto conv() ENTT_NOEXCEPT {
+        using conv_type = std::invoke_result_t<decltype(Candidate), Type &>;
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_conv_node node{
+            type,
+            nullptr,
+            &internal::meta_info<conv_type>::resolve,
+            [](const void *instance) -> meta_any {
+                return std::invoke(Candidate, *static_cast<const Type *>(instance));
+            }
+        };
+
+        ENTT_ASSERT(!duplicate(&node, type->conv));
+        node.next = type->conv;
+        type->conv = &node;
+
+        return meta_factory<Type>{};
+    }
+
+    /**
+     * @brief Assigns a meta constructor to a meta type.
+     *
+     * Free functions can be assigned to meta types in the role of constructors.
+     * All that is required is that they return an instance of the underlying
+     * type.<br/>
+     * From a client's point of view, nothing changes if a constructor of a meta
+     * type is a built-in one or a free function.
+     *
+     * @tparam Func The actual function to use as a constructor.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @return An extended meta factory for the parent type.
+     */
+    template<auto Func, typename Policy = as_is_t>
+    auto ctor() ENTT_NOEXCEPT {
+        using helper_type = internal::meta_function_helper_t<decltype(Func)>;
+        static_assert(std::is_same_v<typename helper_type::return_type, Type>);
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_ctor_node node{
+            type,
+            nullptr,
+            nullptr,
+            helper_type::index_sequence.size(),
+            &helper_type::arg,
+            [](meta_any * const any) {
+                return internal::invoke<Type, Func, Policy>({}, any, helper_type::index_sequence);
+            }
+        };
+
+        ENTT_ASSERT(!duplicate(&node, type->ctor));
+        node.next = type->ctor;
+        type->ctor = &node;
+
+        return extended_meta_factory<Type, std::integral_constant<decltype(Func), Func>>{&node.prop};
+    }
+
+    /**
+     * @brief Assigns a meta constructor to a meta type.
+     *
+     * A meta constructor is uniquely identified by the types of its arguments
+     * and is such that there exists an actual constructor of the underlying
+     * type that can be invoked with parameters whose types are those given.
+     *
+     * @tparam Args Types of arguments to use to construct an instance.
+     * @return An extended meta factory for the parent type.
+     */
+    template<typename... Args>
+    auto ctor() ENTT_NOEXCEPT {
+        using helper_type = internal::meta_function_helper_t<Type(*)(Args...)>;
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_ctor_node node{
+            type,
+            nullptr,
+            nullptr,
+            helper_type::index_sequence.size(),
+            &helper_type::arg,
+            [](meta_any * const any) {
+                return internal::construct<Type, std::remove_cv_t<std::remove_reference_t<Args>>...>(any, helper_type::index_sequence);
+            }
+        };
+
+        ENTT_ASSERT(!duplicate(&node, type->ctor));
+        node.next = type->ctor;
+        type->ctor = &node;
+
+        return extended_meta_factory<Type, Type(Args...)>{&node.prop};
+    }
+
+    /**
+     * @brief Assigns a meta destructor to a meta type.
+     *
+     * Free functions can be assigned to meta types in the role of destructors.
+     * The signature of the function should identical to the following:
+     *
+     * @code{.cpp}
+     * void(Type &);
+     * @endcode
+     *
+     * The purpose is to give users the ability to free up resources that
+     * require special treatment before an object is actually destroyed.
+     *
+     * @tparam Func The actual function to use as a destructor.
+     * @return A meta factory for the parent type.
+     */
+    template<auto Func>
+    auto dtor() ENTT_NOEXCEPT {
+        static_assert(std::is_invocable_v<decltype(Func), Type &>);
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_dtor_node node{
+            type,
+            [](meta_handle handle) {
+                const auto valid = (handle.type() == internal::meta_info<Type>::resolve());
+
+                if(valid) {
+                    std::invoke(Func, *meta_any{handle}.try_cast<Type>());
+                }
+
+                return valid;
+            }
+        };
+
+        ENTT_ASSERT(!type->dtor);
+        type->dtor = &node;
+
+        return meta_factory<Type>{};
+    }
+
+    /**
+     * @brief Assigns a meta data to a meta type.
+     *
+     * Both data members and static and global variables, as well as constants
+     * of any kind, can be assigned to a meta type.<br/>
+     * From a client's point of view, all the variables associated with the
+     * reflected object will appear as if they were part of the type itself.
+     *
+     * @tparam Data The actual variable to attach to the meta type.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @param identifier Unique identifier.
+     * @return An extended meta factory for the parent type.
+     */
+    template<auto Data, typename Policy = as_is_t>
+    auto data(const ENTT_ID_TYPE identifier) ENTT_NOEXCEPT {
+        auto * const type = internal::meta_info<Type>::resolve();
+        internal::meta_data_node *curr = nullptr;
+
+        if constexpr(std::is_same_v<Type, decltype(Data)>) {
+            static_assert(std::is_same_v<Policy, as_is_t>);
+
+            static internal::meta_data_node node{
+                {},
+                type,
+                nullptr,
+                nullptr,
+                true,
+                true,
+                &internal::meta_info<Type>::resolve,
+                [](meta_handle, meta_any, meta_any) { return false; },
+                [](meta_handle, meta_any) -> meta_any { return Data; }
+            };
+
+            curr = &node;
+        } else if constexpr(std::is_member_object_pointer_v<decltype(Data)>) {
+            using data_type = std::remove_reference_t<decltype(std::declval<Type>().*Data)>;
+
+            static internal::meta_data_node node{
+                {},
+                type,
+                nullptr,
+                nullptr,
+                std::is_const_v<data_type>,
+                !std::is_member_object_pointer_v<decltype(Data)>,
+                &internal::meta_info<data_type>::resolve,
+                &internal::setter<std::is_const_v<data_type>, Type, Data>,
+                &internal::getter<Type, Data, Policy>
+            };
+
+            curr = &node;
+        } else {
+            static_assert(std::is_pointer_v<std::decay_t<decltype(Data)>>);
+            using data_type = std::remove_pointer_t<std::decay_t<decltype(Data)>>;
+
+            static internal::meta_data_node node{
+                {},
+                type,
+                nullptr,
+                nullptr,
+                std::is_const_v<data_type>,
+                !std::is_member_object_pointer_v<decltype(Data)>,
+                &internal::meta_info<data_type>::resolve,
+                &internal::setter<std::is_const_v<data_type>, Type, Data>,
+                &internal::getter<Type, Data, Policy>
+            };
+
+            curr = &node;
+        }
+
+        ENTT_ASSERT(!duplicate(identifier, type->data));
+        ENTT_ASSERT(!duplicate(curr, type->data));
+        curr->identifier = identifier;
+        curr->next = type->data;
+        type->data = curr;
+
+        return extended_meta_factory<Type, std::integral_constant<decltype(Data), Data>>{&curr->prop};
+    }
+
+    /**
+     * @brief Assigns a meta data to a meta type by means of its setter and
+     * getter.
+     *
+     * Setters and getters can be either free functions, member functions or a
+     * mix of them.<br/>
+     * In case of free functions, setters and getters must accept a reference to
+     * an instance of the parent type as their first argument. A setter has then
+     * an extra argument of a type convertible to that of the parameter to
+     * set.<br/>
+     * In case of member functions, getters have no arguments at all, while
+     * setters has an argument of a type convertible to that of the parameter to
+     * set.
+     *
+     * @tparam Setter The actual function to use as a setter.
+     * @tparam Getter The actual function to use as a getter.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @param identifier Unique identifier.
+     * @return An extended meta factory for the parent type.
+     */
+    template<auto Setter, auto Getter, typename Policy = as_is_t>
+    auto data(const ENTT_ID_TYPE identifier) ENTT_NOEXCEPT {
+        using underlying_type = std::invoke_result_t<decltype(Getter), Type &>;
+        static_assert(std::is_invocable_v<decltype(Setter), Type &, underlying_type>);
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_data_node node{
+            {},
+            type,
+            nullptr,
+            nullptr,
+            false,
+            false,
+            &internal::meta_info<underlying_type>::resolve,
+            &internal::setter<false, Type, Setter>,
+            &internal::getter<Type, Getter, Policy>
+        };
+
+        ENTT_ASSERT(!duplicate(identifier, type->data));
+        ENTT_ASSERT(!duplicate(&node, type->data));
+        node.identifier = identifier;
+        node.next = type->data;
+        type->data = &node;
+
+        return extended_meta_factory<Type, std::integral_constant<decltype(Setter), Setter>, std::integral_constant<decltype(Getter), Getter>>{&node.prop};
+    }
+
+    /**
+     * @brief Assigns a meta funcion to a meta type.
+     *
+     * Both member functions and free functions can be assigned to a meta
+     * type.<br/>
+     * From a client's point of view, all the functions associated with the
+     * reflected object will appear as if they were part of the type itself.
+     *
+     * @tparam Candidate The actual function to attach to the meta type.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @param identifier Unique identifier.
+     * @return An extended meta factory for the parent type.
+     */
+    template<auto Candidate, typename Policy = as_is_t>
+    auto func(const ENTT_ID_TYPE identifier) ENTT_NOEXCEPT {
+        using helper_type = internal::meta_function_helper_t<decltype(Candidate)>;
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_func_node node{
+            {},
+            type,
+            nullptr,
+            nullptr,
+            helper_type::index_sequence.size(),
+            helper_type::is_const,
+            !std::is_member_function_pointer_v<decltype(Candidate)>,
+            &internal::meta_info<std::conditional_t<std::is_same_v<Policy, as_void_t>, void, typename helper_type::return_type>>::resolve,
+            &helper_type::arg,
+            [](meta_handle handle, meta_any *any) {
+                return internal::invoke<Type, Candidate, Policy>(handle, any, helper_type::index_sequence);
+            }
+        };
+
+        ENTT_ASSERT(!duplicate(identifier, type->func));
+        ENTT_ASSERT(!duplicate(&node, type->func));
+        node.identifier = identifier;
+        node.next = type->func;
+        type->func = &node;
+
+        return extended_meta_factory<Type, std::integral_constant<decltype(Candidate), Candidate>>{&node.prop};
+    }
+
+    /**
+     * @brief Resets a meta type and all its parts.
+     *
+     * This function resets a meta type and all its data members, member
+     * functions and properties, as well as its constructors, destructors and
+     * conversion functions if any.<br/>
+     * Base classes aren't reset but the link between the two types is removed.
+     */
+    void reset() ENTT_NOEXCEPT {
+        internal::meta_info<Type>::reset();
+    }
+};
+
+
+/**
+ * @brief Extended meta factory to be used for reflection purposes.
+ * @tparam Type Reflected type for which the factory was created.
+ * @tparam Spec Property specialization pack used to disambiguate overloads.
+ */
+template<typename Type, typename... Spec>
+class extended_meta_factory: public meta_factory<Type> {
+    bool duplicate(const meta_any &key, const internal::meta_prop_node *node) ENTT_NOEXCEPT {
+        return node && (node->key() == key || duplicate(key, node->next));
+    }
+
+    template<std::size_t Step = 0, std::size_t... Index, typename... Property, typename... Other>
+    void unpack(std::index_sequence<Index...>, std::tuple<Property...> property, Other &&... other) {
+        unroll<Step>(choice<3>, std::move(std::get<Index>(property))..., std::forward<Other>(other)...);
+    }
+
+    template<std::size_t Step = 0, typename... Property, typename... Other>
+    void unroll(choice_t<3>, std::tuple<Property...> property, Other &&... other) {
+        unpack<Step>(std::index_sequence_for<Property...>{}, std::move(property), std::forward<Other>(other)...);
+    }
+
+    template<std::size_t Step = 0, typename... Property, typename... Other>
+    void unroll(choice_t<2>, std::pair<Property...> property, Other &&... other) {
+        assign<Step>(std::move(property.first), std::move(property.second));
+        unroll<Step+1>(choice<3>, std::forward<Other>(other)...);
+    }
+
+    template<std::size_t Step = 0, typename Property, typename... Other>
+    std::enable_if_t<!std::is_invocable_v<Property>>
+    unroll(choice_t<1>, Property &&property, Other &&... other) {
+        assign<Step>(std::forward<Property>(property));
+        unroll<Step+1>(choice<3>, std::forward<Other>(other)...);
+    }
+
+    template<std::size_t Step = 0, typename Func, typename... Other>
+    void unroll(choice_t<0>, Func &&invocable, Other &&... other) {
+        unroll<Step>(choice<3>, std::forward<Func>(invocable)(), std::forward<Other>(other)...);
+    }
+
+    template<std::size_t>
+    void unroll(choice_t<0>) {}
+
+    template<std::size_t = 0, typename Key, typename... Value>
+    void assign(Key &&key, Value &&... value) {
+        static auto property{std::make_tuple(std::forward<Key>(key), std::forward<Value>(value)...)};
+
+        static internal::meta_prop_node node{
+            nullptr,
+            []() -> meta_any {
+                return std::get<0>(property);
+            },
+            []() -> meta_any {
+                if constexpr(sizeof...(Value) == 0) {
+                    return {};
+                } else {
+                    return std::get<1>(property);
+                }
+            }
+        };
+
+        ENTT_ASSERT(!duplicate(node.key(), *curr));
+        node.next = *curr;
+        *curr = &node;
+    }
+
+public:
+    /**
+     * @brief Constructs an extended factory from a given node.
+     * @param target The underlying node to which to assign the properties.
+     */
+    extended_meta_factory(entt::internal::meta_prop_node **target)
+        : curr{target}
+    {}
+
+    /**
+     * @brief Assigns a property to the last meta object created.
+     *
+     * Both the key and the value (if any) must be at least copy constructible.
+     *
+     * @tparam PropertyOrKey Type of the property or property key.
+     * @tparam Value Optional type of the property value.
+     * @param property_or_key Property or property key.
+     * @param value Optional property value.
+     * @return A meta factory for the parent type.
+     */
+    template<typename PropertyOrKey, typename... Value>
+    auto prop(PropertyOrKey &&property_or_key, Value &&... value) && {
+        if constexpr(sizeof...(Value) == 0) {
+            unroll(choice<3>, std::forward<PropertyOrKey>(property_or_key));
+        } else {
+            assign(std::forward<PropertyOrKey>(property_or_key), std::forward<Value>(value)...);
+        }
+
+        return extended_meta_factory<Type, Spec..., PropertyOrKey, Value...>{curr};
+    }
+
+    /**
+     * @brief Assigns properties to the last meta object created.
+     *
+     * Both the keys and the values (if any) must be at least copy
+     * constructible.
+     *
+     * @tparam Property Types of the properties.
+     * @param property Properties to assign to the last meta object created.
+     * @return A meta factory for the parent type.
+     */
+    template <typename... Property>
+    auto props(Property... property) && {
+        unroll(choice<3>, std::forward<Property>(property)...);
+        return extended_meta_factory<Type, Spec..., Property...>{curr};
+    }
+
+private:
+    entt::internal::meta_prop_node **curr{nullptr};
+};
+
+
+/**
+ * @brief Utility function to use for reflection.
+ *
+ * This is the point from which everything starts.<br/>
+ * By invoking this function with a type that is not yet reflected, a meta type
+ * is created to which it will be possible to attach meta objects through a
+ * dedicated factory.
+ *
+ * @tparam Type Type to reflect.
+ * @return A meta factory for the given type.
+ */
+template<typename Type>
+inline meta_factory<Type> meta() ENTT_NOEXCEPT {
+    return meta_factory<Type>{};
+}
+
+
+/**
+ * @brief Returns the meta type associated with a given type.
+ * @tparam Type Type to use to search for a meta type.
+ * @return The meta type associated with the given type, if any.
+ */
+template<typename Type>
+inline meta_type resolve() ENTT_NOEXCEPT {
+    return internal::meta_info<Type>::resolve();
+}
+
+
+/**
+ * @brief Returns the meta type associated with a given identifier.
+ * @param identifier Unique identifier.
+ * @return The meta type associated with the given identifier, if any.
+ */
+inline meta_type resolve(const ENTT_ID_TYPE identifier) ENTT_NOEXCEPT {
+    return internal::find_if([identifier](auto *node) {
+        return node->identifier == identifier;
+    }, *internal::meta_info<>::global);
+}
+
+
+/**
+ * @brief Iterates all the reflected types.
+ * @tparam Op Type of the function object to invoke.
+ * @param op A valid function object.
+ */
+template<typename Op>
+inline std::enable_if_t<std::is_invocable_v<Op, meta_type>, void>
+resolve(Op op) ENTT_NOEXCEPT {
+    internal::iterate<meta_type>(std::move(op), *internal::meta_info<>::global);
+}
+
+
+}
+
+
+#endif // ENTT_META_FACTORY_HPP

src/entt/meta/policy.hpp

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

src/entt/process/process.hpp

@@ -2,36 +2,14 @@
 #define ENTT_PROCESS_PROCESS_HPP
 
 
-#include <type_traits>
-#include <functional>
 #include <utility>
+#include <type_traits>
+#include "../config/config.h"
 
 
 namespace entt {
 
 
-namespace {
-
-
-struct BaseProcess {
-    enum class State: unsigned int {
-        UNINITIALIZED = 0,
-        RUNNING,
-        PAUSED,
-        SUCCEEDED,
-        FAILED,
-        ABORTED,
-        FINISHED
-    };
-
-    template<State state>
-    using tag = std::integral_constant<State, state>;
-};
-
-
-}
-
-
 /**
  * @brief Base class for processes.
  *
@@ -41,33 +19,43 @@ struct BaseProcess {
  * required:
  *
  * * @code{.cpp}
- *   void update(Delta);
+ *   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.
+ *   least define it to work properly. The `void *` parameter is an opaque
+ *   pointer to user data (if any) forwarded directly to the process during an
+ *   update.
  *
  * * @code{.cpp}
  *   void init();
  *   @endcode
- *   It's invoked at the first tick, immediately before an update.
+ *
+ *   It's invoked when the process joins the running queue of a scheduler. This
+ *   happens as soon as it's attached to the scheduler if the process is a top
+ *   level one, otherwise when it replaces its parent if the process is a
+ *   continuation.
  *
  * * @code{.cpp}
  *   void succeeded();
  *   @endcode
+ *
  *   It's invoked in case of success, immediately after an update and during the
  *   same tick.
  *
  * * @code{.cpp}
  *   void failed();
  *   @endcode
+ *
  *   It's invoked in case of errors, immediately after an update and during the
  *   same tick.
  *
  * * @code{.cpp}
  *   void aborted();
  *   @endcode
+ *
  *   It's invoked only if a process is explicitly aborted. There is no guarantee
  *   that it executes in the same tick, this depends solely on whether the
  *   process is aborted immediately or not.
@@ -76,45 +64,58 @@ struct BaseProcess {
  * `succeed` and `fail` protected member functions and even pause or unpause the
  * process itself.
  *
- * @sa Scheduler
+ * @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: private BaseProcess {
+class process {
+    enum class state: unsigned int {
+        UNINITIALIZED = 0,
+        RUNNING,
+        PAUSED,
+        SUCCEEDED,
+        FAILED,
+        ABORTED,
+        FINISHED
+    };
+
+    template<state value>
+    using state_value_t = std::integral_constant<state, value>;
+
     template<typename Target = Derived>
-    auto tick(int, tag<State::UNINITIALIZED>)
+    auto tick(int, state_value_t<state::UNINITIALIZED>)
     -> decltype(std::declval<Target>().init()) {
         static_cast<Target *>(this)->init();
     }
 
     template<typename Target = Derived>
-    auto tick(int, tag<State::RUNNING>, Delta delta)
-    -> decltype(std::declval<Target>().update(delta)) {
-        static_cast<Target *>(this)->update(delta);
+    auto tick(int, state_value_t<state::RUNNING>, Delta delta, void *data)
+    -> decltype(std::declval<Target>().update(delta, data)) {
+        static_cast<Target *>(this)->update(delta, data);
     }
 
     template<typename Target = Derived>
-    auto tick(int, tag<State::SUCCEEDED>)
+    auto tick(int, state_value_t<state::SUCCEEDED>)
     -> decltype(std::declval<Target>().succeeded()) {
         static_cast<Target *>(this)->succeeded();
     }
 
     template<typename Target = Derived>
-    auto tick(int, tag<State::FAILED>)
+    auto tick(int, state_value_t<state::FAILED>)
     -> decltype(std::declval<Target>().failed()) {
         static_cast<Target *>(this)->failed();
     }
 
     template<typename Target = Derived>
-    auto tick(int, tag<State::ABORTED>)
+    auto tick(int, state_value_t<state::ABORTED>)
     -> decltype(std::declval<Target>().aborted()) {
         static_cast<Target *>(this)->aborted();
     }
 
-    template<State S, typename... Args>
-    void tick(char, tag<S>, Args&&...) {}
+    template<state value, typename... Args>
+    void tick(char, state_value_t<value>, Args &&...) const ENTT_NOEXCEPT {}
 
 protected:
     /**
@@ -123,9 +124,9 @@ protected:
      * The function is idempotent and it does nothing if the process isn't
      * alive.
      */
-    void succeed() noexcept {
+    void succeed() ENTT_NOEXCEPT {
         if(alive()) {
-            current = State::SUCCEEDED;
+            current = state::SUCCEEDED;
         }
     }
 
@@ -135,9 +136,9 @@ protected:
      * The function is idempotent and it does nothing if the process isn't
      * alive.
      */
-    void fail() noexcept {
+    void fail() ENTT_NOEXCEPT {
         if(alive()) {
-            current = State::FAILED;
+            current = state::FAILED;
         }
     }
 
@@ -147,9 +148,9 @@ protected:
      * The function is idempotent and it does nothing if the process isn't
      * running.
      */
-    void pause() noexcept {
-        if(current == State::RUNNING) {
-            current = State::PAUSED;
+    void pause() ENTT_NOEXCEPT {
+        if(current == state::RUNNING) {
+            current = state::PAUSED;
         }
     }
 
@@ -159,9 +160,9 @@ protected:
      * The function is idempotent and it does nothing if the process isn't
      * paused.
      */
-    void unpause() noexcept {
-        if(current  == State::PAUSED) {
-            current  = State::RUNNING;
+    void unpause() ENTT_NOEXCEPT {
+        if(current  == state::PAUSED) {
+            current  = state::RUNNING;
         }
     }
 
@@ -170,8 +171,8 @@ public:
     using delta_type = Delta;
 
     /*! @brief Default destructor. */
-    ~Process() noexcept {
-        static_assert(std::is_base_of<Process, Derived>::value, "!");
+    virtual ~process() ENTT_NOEXCEPT {
+        static_assert(std::is_base_of_v<process, Derived>);
     }
 
     /**
@@ -182,12 +183,12 @@ public:
      *
      * @param immediately Requests an immediate operation.
      */
-    void abort(bool immediately = false) noexcept {
+    void abort(const bool immediately = false) ENTT_NOEXCEPT {
         if(alive()) {
-            current = State::ABORTED;
+            current = state::ABORTED;
 
             if(immediately) {
-                tick(0);
+                tick({});
             }
         }
     }
@@ -196,46 +197,48 @@ public:
      * @brief Returns true if a process is either running or paused.
      * @return True if the process is still alive, false otherwise.
      */
-    bool alive() const noexcept {
-        return current == State::RUNNING || current == State::PAUSED;
+    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 noexcept {
-        return current == State::FINISHED;
+    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 noexcept {
-        return current == State::PAUSED;
+    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 noexcept {
+    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(Delta delta) {
+    void tick(const Delta delta, void *data = nullptr) {
         switch (current) {
-        case State::UNINITIALIZED:
-            tick(0, tag<State::UNINITIALIZED>{});
-            current = State::RUNNING;
-            // no break on purpose, tasks are executed immediately
-        case State::RUNNING:
-            tick(0, tag<State::RUNNING>{}, delta);
+        case state::UNINITIALIZED:
+            tick(0, state_value_t<state::UNINITIALIZED>{});
+            current = state::RUNNING;
+            break;
+        case state::RUNNING:
+            tick(0, state_value_t<state::RUNNING>{}, delta, data);
+            break;
         default:
             // suppress warnings
             break;
@@ -243,18 +246,18 @@ public:
 
         // if it's dead, it must be notified and removed immediately
         switch(current) {
-        case State::SUCCEEDED:
-            tick(0, tag<State::SUCCEEDED>{});
-            current = State::FINISHED;
+        case state::SUCCEEDED:
+            tick(0, state_value_t<state::SUCCEEDED>{});
+            current = state::FINISHED;
             break;
-        case State::FAILED:
-            tick(0, tag<State::FAILED>{});
-            current = State::FINISHED;
+        case state::FAILED:
+            tick(0, state_value_t<state::FAILED>{});
+            current = state::FINISHED;
             stopped = true;
             break;
-        case State::ABORTED:
-            tick(0, tag<State::ABORTED>{});
-            current = State::FINISHED;
+        case state::ABORTED:
+            tick(0, state_value_t<state::ABORTED>{});
+            current = state::FINISHED;
             stopped = true;
             break;
         default:
@@ -264,7 +267,7 @@ public:
     }
 
 private:
-    State current{State::UNINITIALIZED};
+    state current{state::UNINITIALIZED};
     bool stopped{false};
 };
 
@@ -281,12 +284,13 @@ private:
  * following:
  *
  * @code{.cpp}
- * void(Delta delta, auto succeed, auto fail);
+ * 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.
  *
@@ -301,30 +305,31 @@ private:
  * create them internally each and avery time a lambda or a functor is used as
  * a process.
  *
- * @sa Process
- * @sa Scheduler
+ * @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 {
+struct process_adaptor: process<process_adaptor<Func, Delta>, Delta>, private Func {
     /**
      * @brief Constructs a process adaptor from a lambda or a functor.
      * @tparam Args Types of arguments to use to initialize the actual process.
      * @param args Parameters to use to initialize the actual process.
      */
     template<typename... Args>
-    ProcessAdaptor(Args&&... args)
+    process_adaptor(Args &&... args)
         : Func{std::forward<Args>(args)...}
     {}
 
     /**
      * @brief Updates a process and its internal state if required.
      * @param delta Elapsed time.
+     * @param data Optional data.
      */
-    void update(Delta delta) {
-        Func::operator()(delta, [this](){ this->succeed(); }, [this](){ this->fail(); });
+    void update(const Delta delta, void *data) {
+        Func::operator()(delta, data, [this]() { this->succeed(); }, [this]() { this->fail(); });
     }
 };
 

src/entt/process/scheduler.hpp

@@ -5,9 +5,9 @@
 #include <vector>
 #include <memory>
 #include <utility>
-#include <iterator>
 #include <algorithm>
 #include <type_traits>
+#include "../config/config.h"
 #include "process.hpp"
 
 
@@ -28,69 +28,69 @@ namespace entt {
  * Example of use (pseudocode):
  *
  * @code{.cpp}
- * scheduler.attach([](auto delta, auto succeed, auto fail) {
+ * scheduler.attach([](auto delta, void *, auto succeed, auto fail) {
  *     // code
- * }).then<MyProcess>(arguments...);
+ * }).then<my_process>(arguments...);
  * @endcode
  *
  * In order to invoke all scheduled processes, call the `update` member function
  * passing it the elapsed time to forward to the tasks.
  *
- * @sa Process
+ * @sa process
  *
  * @tparam Delta Type to use to provide elapsed time.
  */
 template<typename Delta>
-class Scheduler final {
-    template<typename T>
-    struct tag { using type = T; };
-
-    struct ProcessHandler final {
+class scheduler {
+    struct process_handler {
         using instance_type = std::unique_ptr<void, void(*)(void *)>;
-        using update_type = bool(*)(ProcessHandler &, Delta);
-        using abort_type = void(*)(ProcessHandler &, bool);
-        using next_type = std::unique_ptr<ProcessHandler>;
+        using update_fn_type = bool(process_handler &, Delta, void *);
+        using abort_fn_type = void(process_handler &, bool);
+        using next_type = std::unique_ptr<process_handler>;
 
         instance_type instance;
-        update_type update;
-        abort_type abort;
+        update_fn_type *update;
+        abort_fn_type *abort;
         next_type next;
     };
 
-    template<typename Lambda>
-    struct Then final: Lambda {
-        Then(Lambda &&lambda, ProcessHandler *handler)
-            : Lambda{std::forward<Lambda>(lambda)}, handler{handler}
-        {}
+    struct continuation {
+        continuation(process_handler *ref)
+            : handler{ref}
+        {
+            ENTT_ASSERT(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, tag<Proc>{}, std::forward<Args>(args)...);
-            return std::move(*this);
+        continuation then(Args &&... args) {
+            static_assert(std::is_base_of_v<process<Proc, Delta>, Proc>);
+            auto proc = typename process_handler::instance_type{new Proc{std::forward<Args>(args)...}, &scheduler::deleter<Proc>};
+            handler->next.reset(new process_handler{std::move(proc), &scheduler::update<Proc>, &scheduler::abort<Proc>, nullptr});
+            handler = handler->next.get();
+            return *this;
         }
 
         template<typename Func>
-        decltype(auto) then(Func &&func) && {
-            using Proc = ProcessAdaptor<std::decay_t<Func>, Delta>;
-            return std::move(*this).template then<Proc>(std::forward<Func>(func));
+        continuation then(Func &&func) {
+            return then<process_adaptor<std::decay_t<Func>, Delta>>(std::forward<Func>(func));
         }
 
     private:
-        ProcessHandler *handler;
+        process_handler *handler;
     };
 
     template<typename Proc>
-    static bool update(ProcessHandler &handler, Delta delta) {
+    static bool update(process_handler &handler, const Delta delta, void *data) {
         auto *process = static_cast<Proc *>(handler.instance.get());
-        process->tick(delta);
+        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);
+                // forces the process to exit the uninitialized state
+                dead = handler.update(handler, {}, nullptr);
             } else {
                 handler.instance.reset();
             }
@@ -100,7 +100,7 @@ class Scheduler final {
     }
 
     template<typename Proc>
-    static void abort(ProcessHandler &handler, bool immediately) {
+    static void abort(process_handler &handler, const bool immediately) {
         static_cast<Proc *>(handler.instance.get())->abort(immediately);
     }
 
@@ -109,44 +109,24 @@ class Scheduler final {
         delete static_cast<Proc *>(proc);
     }
 
-    auto then(ProcessHandler *handler) {
-        auto lambda = [this](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;
+    using size_type = std::size_t;
 
     /*! @brief Default constructor. */
-    Scheduler() noexcept= default;
+    scheduler() ENTT_NOEXCEPT = default;
 
-    /*! @brief Copying a scheduler isn't allowed. */
-    Scheduler(const Scheduler &) = delete;
     /*! @brief Default move constructor. */
-    Scheduler(Scheduler &&) = default;
+    scheduler(scheduler &&) = default;
 
-    /*! @brief Copying a scheduler isn't allowed. @return This scheduler. */
-    Scheduler & operator=(const Scheduler &) = delete;
-    /*! @brief Default move assignament operator. @return This scheduler. */
-    Scheduler & operator=(Scheduler &&) = default;
+    /*! @brief Default move assignment operator. @return This scheduler. */
+    scheduler & operator=(scheduler &&) = default;
 
     /**
      * @brief Number of processes currently scheduled.
      * @return Number of processes currently scheduled.
      */
-    size_type size() const noexcept {
+    size_type size() const ENTT_NOEXCEPT {
         return handlers.size();
     }
 
@@ -154,7 +134,7 @@ public:
      * @brief Returns true if at least a process is currently scheduled.
      * @return True if there are scheduled processes, false otherwise.
      */
-    bool empty() const noexcept {
+    bool empty() const ENTT_NOEXCEPT {
         return handlers.empty();
     }
 
@@ -179,13 +159,13 @@ public:
      *
      * @code{.cpp}
      * // schedules a task in the form of a process class
-     * scheduler.attach<MyProcess>(arguments...)
+     * scheduler.attach<my_process>(arguments...)
      * // appends a child in the form of a lambda function
-     * .then([](auto delta, auto succeed, auto fail) {
+     * .then([](auto delta, void *, auto succeed, auto fail) {
      *     // code
      * })
      * // appends a child in the form of another process class
-     * .then<MyOtherProcess>();
+     * .then<my_other_process>();
      * @endcode
      *
      * @tparam Proc Type of process to schedule.
@@ -194,14 +174,13 @@ public:
      * @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());
+    auto attach(Args &&... args) {
+        static_assert(std::is_base_of_v<process<Proc, Delta>, Proc>);
+        auto proc = typename process_handler::instance_type{new Proc{std::forward<Args>(args)...}, &scheduler::deleter<Proc>};
+        process_handler handler{std::move(proc), &scheduler::update<Proc>, &scheduler::abort<Proc>, nullptr};
+        // forces the process to exit the uninitialized state
+        handler.update(handler, {}, nullptr);
+        return continuation{&handlers.emplace_back(std::move(handler))};
     }
 
     /**
@@ -213,12 +192,13 @@ public:
      * following:
      *
      * @code{.cpp}
-     * void(Delta delta, auto succeed, auto fail);
+     * 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.
      *
@@ -237,18 +217,18 @@ public:
      *
      * @code{.cpp}
      * // schedules a task in the form of a lambda function
-     * scheduler.attach([](auto delta, auto succeed, auto fail) {
+     * scheduler.attach([](auto delta, void *, auto succeed, auto fail) {
      *     // code
      * })
      * // appends a child in the form of another lambda function
-     * .then([](auto delta, auto succeed, auto fail) {
+     * .then([](auto delta, void *, auto succeed, auto fail) {
      *     // code
      * })
      * // appends a child in the form of a process class
-     * .then<MyProcess>(arguments...);
+     * .then<my_process>(arguments...);
      * @endcode
      *
-     * @sa ProcessAdaptor
+     * @sa process_adaptor
      *
      * @tparam Func Type of process to schedule.
      * @param func Either a lambda or a functor to use as a process.
@@ -256,7 +236,7 @@ public:
      */
     template<typename Func>
     auto attach(Func &&func) {
-        using Proc = ProcessAdaptor<std::decay_t<Func>, Delta>;
+        using Proc = process_adaptor<std::decay_t<Func>, Delta>;
         return attach<Proc>(std::forward<Func>(func));
     }
 
@@ -269,18 +249,19 @@ public:
      * with its child.
      *
      * @param delta Elapsed time.
+     * @param data Optional data.
      */
-    void update(Delta delta) {
+    void update(const Delta delta, void *data = nullptr) {
         bool clean = false;
 
-        for(auto i = handlers.size(); i > 0; --i) {
-            auto &handler = handlers[i-1];
-            const bool dead = handler.update(handler, delta);
+        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(), [delta](auto &handler) {
+            handlers.erase(std::remove_if(handlers.begin(), handlers.end(), [](auto &handler) {
                 return !handler.instance;
             }), handlers.end());
         }
@@ -296,7 +277,7 @@ public:
      *
      * @param immediately Requests an immediate operation.
      */
-    void abort(bool immediately = false) {
+    void abort(const bool immediately = false) {
         decltype(handlers) exec;
         exec.swap(handlers);
 
@@ -309,7 +290,7 @@ public:
     }
 
 private:
-    std::vector<ProcessHandler> handlers{};
+    std::vector<process_handler> handlers{};
 };
 
 

src/entt/resource/cache.hpp

@@ -6,9 +6,10 @@
 #include <utility>
 #include <type_traits>
 #include <unordered_map>
-#include "../core/hashed_string.hpp"
+#include "../config/config.h"
 #include "handle.hpp"
 #include "loader.hpp"
+#include "fwd.hpp"
 
 
 namespace entt {
@@ -25,33 +26,28 @@ namespace entt {
  * @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:
+struct cache {
     /*! @brief Unsigned integer type. */
-    using size_type = typename container_type::size_type;
+    using size_type = std::size_t;
     /*! @brief Type of resources managed by a cache. */
-    using resource_type = HashedString;
+    using resource_type = Resource;
+    /*! @brief Unique identifier type for resources. */
+    using id_type = ENTT_ID_TYPE;
 
     /*! @brief Default constructor. */
-    ResourceCache() = default;
+    cache() = default;
 
-    /*! @brief Copying a cache isn't allowed. */
-    ResourceCache(const ResourceCache &) noexcept = delete;
     /*! @brief Default move constructor. */
-    ResourceCache(ResourceCache &&) noexcept = default;
+    cache(cache &&) = default;
 
-    /*! @brief Copying a cache isn't allowed. @return This cache. */
-    ResourceCache & operator=(const ResourceCache &) noexcept = delete;
     /*! @brief Default move assignment operator. @return This cache. */
-    ResourceCache & operator=(ResourceCache &&) noexcept = default;
+    cache & operator=(cache &&) = default;
 
     /**
      * @brief Number of resources managed by a cache.
      * @return Number of resources currently stored.
      */
-    size_type size() const noexcept {
+    size_type size() const ENTT_NOEXCEPT {
         return resources.size();
     }
 
@@ -59,7 +55,7 @@ public:
      * @brief Returns true if a cache contains no resources, false otherwise.
      * @return True if the cache contains no resources, false otherwise.
      */
-    bool empty() const noexcept {
+    bool empty() const ENTT_NOEXCEPT {
         return resources.empty();
     }
 
@@ -69,12 +65,12 @@ public:
      * 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() noexcept {
+    void clear() ENTT_NOEXCEPT {
         resources.clear();
     }
 
     /**
-     * @brief Loads the resource that corresponds to the given identifier.
+     * @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
@@ -85,24 +81,31 @@ public:
      * If the identifier is already present in the cache, this function does
      * nothing and the arguments are simply discarded.
      *
+     * @warning
+     * If the resource cannot be loaded correctly, the returned handle will be
+     * invalid and any use of it will result in undefined behavior.
+     *
      * @tparam Loader Type of loader to use to load the resource if required.
      * @tparam Args Types of arguments to use to load the resource if required.
      * @param id Unique resource identifier.
      * @param args Arguments to use to load the resource if required.
-     * @return True if the resource is ready to use, false otherwise.
+     * @return A handle for the given resource.
      */
     template<typename Loader, typename... Args>
-    bool load(resource_type id, Args&&... args) {
-        static_assert(std::is_base_of<ResourceLoader<Loader, Resource>, Loader>::value, "!");
+    entt::handle<Resource> load(const id_type id, Args &&... args) {
+        static_assert(std::is_base_of_v<loader<Loader, Resource>, Loader>);
+        entt::handle<Resource> resource{};
 
-        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);
+        if(auto it = resources.find(id); it == resources.cend()) {
+            if(auto instance = Loader{}.get(std::forward<Args>(args)...); instance) {
+                resources[id] = instance;
+                resource = std::move(instance);
+            }
+        } else {
+            resource = it->second;
         }
 
-        return loaded;
+        return resource;
     }
 
     /**
@@ -118,62 +121,116 @@ public:
      * Arguments are forwarded directly to the loader in order to construct
      * properly the requested resource.
      *
+     * @warning
+     * If the resource cannot be loaded correctly, the returned handle will be
+     * invalid and any use of it will result in undefined behavior.
+     *
      * @tparam Loader Type of loader to use to load the resource.
      * @tparam Args Types of arguments to use to load the resource.
      * @param id Unique resource identifier.
      * @param args Arguments to use to load the resource.
-     * @return True if the resource is ready to use, false otherwise.
+     * @return A handle for the given resource.
      */
     template<typename Loader, typename... Args>
-    void reload(resource_type id, Args&&... args) {
-        return (discard(id), load(id, std::forward<Args>(args)...));
+    entt::handle<Resource> reload(const id_type id, Args &&... args) {
+        return (discard(id), load<Loader>(id, std::forward<Args>(args)...));
     }
 
     /**
-     * @brief Creates a handle for the given resource identifier.
+     * @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>
+    entt::handle<Resource> temp(Args &&... args) const {
+        return { Loader{}.get(std::forward<Args>(args)...) };
+    }
+
+    /**
+     * @brief Creates a handle for a given resource identifier.
      *
      * A resource handle can be in a either valid or invalid state. In other
      * terms, a resource handle is properly initialized with a resource if the
      * cache contains the resource itself. Otherwise the returned handle is
      * uninitialized and accessing it results in undefined behavior.
      *
-     * @sa ResourceHandle
+     * @sa handle
      *
      * @param id Unique resource identifier.
      * @return A handle for the given resource.
      */
-    ResourceHandle<Resource> handle(resource_type id) const {
+    entt::handle<Resource> handle(const id_type id) const {
         auto it = resources.find(id);
         return { it == resources.end() ? nullptr : it->second };
     }
 
     /**
-     * @brief Checks if a cache contains the given identifier.
+     * @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(resource_type id) const noexcept {
-        return !(resources.find(id) == resources.cend());
+    bool contains(const id_type id) const ENTT_NOEXCEPT {
+        return (resources.find(id) != resources.cend());
     }
 
     /**
-     * @brief Discards the resource that corresponds to the given identifier.
+     * @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(resource_type id) noexcept {
-        auto it = resources.find(id);
-
-        if(it != resources.end()) {
+    void discard(const id_type id) ENTT_NOEXCEPT {
+        if(auto it = resources.find(id); it != resources.end()) {
             resources.erase(it);
         }
     }
 
+    /**
+     * @brief Iterates all resources.
+     *
+     * The function object is invoked for each element. It is provided with
+     * either the resource identifier, the resource handle or both of them.<br/>
+     * The signature of the function must be equivalent to one of the following
+     * forms:
+     *
+     * @code{.cpp}
+     * void(const id_type);
+     * void(handle<Resource>);
+     * void(const id_type, handle<Resource>);
+     * @endcode
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template <typename Func>
+    void each(Func func) const {
+        auto begin = resources.begin();
+        auto end = resources.end();
+
+        while(begin != end) {
+            auto curr = begin++;
+
+            if constexpr(std::is_invocable_v<Func, id_type>) {
+                func(curr->first);
+            } else if constexpr(std::is_invocable_v<Func, entt::handle<Resource>>) {
+                func(entt::handle{ curr->second });
+            } else {
+                func(curr->first, entt::handle{ curr->second });
+            }
+        }
+    }
+
 private:
-    container_type resources;
+    std::unordered_map<id_type, std::shared_ptr<Resource>> resources;
 };
 
 

src/entt/resource/fwd.hpp

@@ -0,0 +1,24 @@
+#ifndef ENTT_RESOURCE_FWD_HPP
+#define ENTT_RESOURCE_FWD_HPP
+
+
+namespace entt {
+
+
+/*! @struct cache */
+template<typename>
+struct cache;
+
+/*! @class handle */
+template<typename>
+class handle;
+
+/*! @class loader */
+template<typename, typename>
+class loader;
+
+
+}
+
+
+#endif // ENTT_RESOURCE_FWD_HPP

src/entt/resource/handle.hpp

@@ -4,16 +4,13 @@
 
 #include <memory>
 #include <utility>
-#include <cassert>
+#include "../config/config.h"
+#include "fwd.hpp"
 
 
 namespace entt {
 
 
-template<typename Resource>
-class ResourceCache;
-
-
 /**
  * @brief Shared resource handle.
  *
@@ -27,24 +24,17 @@ class ResourceCache;
  * @tparam Resource Type of resource managed by a handle.
  */
 template<typename Resource>
-class ResourceHandle final {
+class handle {
     /*! @brief Resource handles are friends of their caches. */
-    friend class ResourceCache<Resource>;
+    friend struct cache<Resource>;
 
-    ResourceHandle(std::shared_ptr<Resource> res) noexcept
+    handle(std::shared_ptr<Resource> res) ENTT_NOEXCEPT
         : resource{std::move(res)}
     {}
 
 public:
-    /*! @brief Default copy constructor. */
-    ResourceHandle(const ResourceHandle &) noexcept = default;
-    /*! @brief Default move constructor. */
-    ResourceHandle(ResourceHandle &&) noexcept = default;
-
-    /*! @brief Default copy assignment operator. @return This handle. */
-    ResourceHandle & operator=(const ResourceHandle &) noexcept = default;
-    /*! @brief Default move assignment operator. @return This handle. */
-    ResourceHandle & operator=(ResourceHandle &&) noexcept = default;
+    /*! @brief Default constructor. */
+    handle() ENTT_NOEXCEPT = default;
 
     /**
      * @brief Gets a reference to the managed resource.
@@ -56,35 +46,30 @@ public:
      *
      * @return A reference to the managed resource.
      */
-    const Resource & get() const noexcept {
-        assert(static_cast<bool>(resource));
+    const Resource & get() const ENTT_NOEXCEPT {
+        ENTT_ASSERT(static_cast<bool>(resource));
         return *resource;
     }
 
-    /**
-     * @brief Casts a handle and gets a reference to the managed resource.
-     *
-     * @warning
-     * The behavior is undefined if the handle doesn't contain a resource.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * handle is empty.
-     */
-    inline operator const Resource & () const noexcept { return get(); }
+    /*! @copydoc get */
+    Resource & get() ENTT_NOEXCEPT {
+        return const_cast<Resource &>(std::as_const(*this).get());
+    }
+
+    /*! @copydoc get */
+    operator const Resource & () const ENTT_NOEXCEPT { return get(); }
+
+    /*! @copydoc get */
+    operator Resource & () ENTT_NOEXCEPT { return get(); }
+
+    /*! @copydoc get */
+    const Resource & operator *() const ENTT_NOEXCEPT { return get(); }
+
+    /*! @copydoc get */
+    Resource & operator *() 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 noexcept { return get(); }
-
-    /**
-     * @brief Gets a pointer to the managed resource from a handle .
+     * @brief Gets a pointer to the managed resource.
      *
      * @warning
      * The behavior is undefined if the handle doesn't contain a resource.<br/>
@@ -94,13 +79,19 @@ public:
      * @return A pointer to the managed resource or `nullptr` if the handle
      * contains no resource at all.
      */
-    inline const Resource * operator ->() const noexcept {
-        assert(static_cast<bool>(resource));
+    const Resource * operator->() const ENTT_NOEXCEPT {
+        ENTT_ASSERT(static_cast<bool>(resource));
         return resource.get();
     }
 
+    /*! @copydoc operator-> */
+    Resource * operator->() ENTT_NOEXCEPT {
+        return const_cast<Resource *>(std::as_const(*this).operator->());
+    }
+
     /**
-     * @brief Returns true if the handle contains a resource, false otherwise.
+     * @brief Returns true if a handle contains a resource, false otherwise.
+     * @return True if the handle contains a resource, false otherwise.
      */
     explicit operator bool() const { return static_cast<bool>(resource); }
 

src/entt/resource/loader.hpp

@@ -3,31 +3,28 @@
 
 
 #include <memory>
+#include "fwd.hpp"
 
 
 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 return
+ * 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 my_resource {};
  *
- * struct MyLoader: entt::ResourceLoader<MyLoader, MyResource> {
- *     std::shared_ptr<MyResource> load(int) const {
+ * struct my_loader: entt::loader<my_loader, my_resource> {
+ *     std::shared_ptr<my_resource> load(int) const {
  *         // use the integer value somehow
- *         return std::make_shared<MyResource>();
+ *         return std::make_shared<my_resource>();
  *     }
  * };
  * @endcode
@@ -45,12 +42,18 @@ class ResourceCache;
  * @tparam Resource Type of resource for which to use the loader.
  */
 template<typename Loader, typename Resource>
-class ResourceLoader {
+class loader {
     /*! @brief Resource loaders are friends of their caches. */
-    friend class ResourceCache<Resource>;
+    friend struct cache<Resource>;
 
+    /**
+     * @brief Loads the resource and returns it.
+     * @tparam Args Types of arguments for the loader.
+     * @param args Arguments for the loader.
+     * @return The resource just loaded or an empty pointer in case of errors.
+     */
     template<typename... Args>
-    std::shared_ptr<Resource> get(Args&&... args) const {
+    std::shared_ptr<Resource> get(Args &&... args) const {
         return static_cast<const Loader *>(this)->load(std::forward<Args>(args)...);
     }
 };

src/entt/signal/bus.hpp

@@ -1,305 +0,0 @@
-#ifndef ENTT_SIGNAL_BUS_HPP
-#define ENTT_SIGNAL_BUS_HPP
-
-
-#include <cstddef>
-#include <utility>
-#include "signal.hpp"
-#include "sigh.hpp"
-
-
-namespace entt {
-
-
-/**
- * @brief Minimal event bus.
- *
- * Primary template isn't defined on purpose. The main reason for which it
- * exists is to work around the doxygen's parsing capabilities. In fact, there
- * is no need to declare it actually.
- */
-template<template<typename...> class, typename...>
-class Bus;
-
-
-/**
- * @brief Event bus specialization for multiple types.
- *
- * The event bus is designed to allow an easy registration of specific member
- * functions to a bunch of signal handlers (either manager or unmanaged).
- * Classes must publicly expose the required member functions to allow the bus
- * to detect them for the purpose of registering and unregistering
- * instances.<br/>
- * In particular, for each event type `E`, a matching member function has the
- * following signature: `void receive(const E &)`. Events will be properly
- * redirected to all the listeners by calling the right member functions, if
- * any.
- *
- * @tparam Sig Type of signal handler to use.
- * @tparam Event The list of events managed by the bus.
- */
-template<template<typename...> class Sig, typename Event, typename... Other>
-class Bus<Sig, Event, Other...>
-        : private Bus<Sig, Event>, private Bus<Sig, Other>...
-{
-public:
-    /*! @brief Unsigned integer type. */
-    using size_type = std::size_t;
-
-    /**
-     * @brief Unregisters all the member functions of an instance.
-     *
-     * A bus is used to convey a certain set of events. This method detects
-     * and unregisters from the bus all the matching member functions of an
-     * instance.<br/>
-     * For each event type `E`, a matching member function has the following
-     * signature: `void receive(const E &)`.
-     *
-     * @tparam Instance Type of instance to unregister.
-     * @param instance A valid instance of the right type.
-     */
-    template<typename Instance>
-    void unreg(Instance instance) {
-        using accumulator_type = int[];
-        accumulator_type accumulator = {
-            (Bus<Sig, Event>::unreg(instance), 0),
-            (Bus<Sig, Other>::unreg(instance), 0)...
-        };
-        return void(accumulator);
-    }
-
-    /**
-     * @brief Registers all the member functions of an instance.
-     *
-     * A bus is used to convey a certain set of events. This method detects
-     * and registers to the bus all the matching member functions of an
-     * instance.<br/>
-     * For each event type `E`, a matching member function has the following
-     * signature: `void receive(const E &)`.
-     *
-     * @tparam Instance Type of instance to register.
-     * @param instance A valid instance of the right type.
-     */
-    template<typename Instance>
-    void reg(Instance instance) {
-        using accumulator_type = int[];
-        accumulator_type accumulator = {
-            (Bus<Sig, Event>::reg(instance), 0),
-            (Bus<Sig, Other>::reg(instance), 0)...
-        };
-        return void(accumulator);
-    }
-
-    /**
-     * @brief Number of listeners connected to the bus.
-     * @return Number of listeners currently connected.
-     */
-    size_type size() const noexcept {
-        using accumulator_type = std::size_t[];
-        std::size_t sz = Bus<Sig, Event>::size();
-        accumulator_type accumulator = { sz, (sz += Bus<Sig, Other>::size())... };
-        return void(accumulator), sz;
-    }
-
-    /**
-     * @brief Returns false if at least a listener is connected to the bus.
-     * @return True if the bus has no listeners connected, false otherwise.
-     */
-    bool empty() const noexcept {
-        using accumulator_type = bool[];
-        bool ret = Bus<Sig, Event>::empty();
-        accumulator_type accumulator = { ret, (ret = ret && Bus<Sig, Other>::empty())... };
-        return void(accumulator), ret;
-    }
-
-    /**
-     * @brief Connects a free function to the bus.
-     * @tparam Type Type of event to which to connect the function.
-     * @tparam Function A valid free function pointer.
-     */
-    template<typename Type, void(*Function)(const Type &)>
-    void connect() {
-        Bus<Sig, Type>::template connect<Function>();
-    }
-
-    /**
-     * @brief Disconnects a free function from the bus.
-     * @tparam Type Type of event from which to disconnect the function.
-     * @tparam Function A valid free function pointer.
-     */
-    template<typename Type, void(*Function)(const Type &)>
-    void disconnect() {
-        Bus<Sig, Type>::template disconnect<Function>();
-    }
-
-    /**
-     * @brief Publishes an event.
-     *
-     * All the listeners are notified. Order isn't guaranteed.
-     *
-     * @tparam Type Type of event to publish.
-     * @tparam Args Types of arguments to use to construct the event.
-     * @param args Arguments to use to construct the event.
-     */
-    template<typename Type, typename... Args>
-    void publish(Args&&... args) {
-        Bus<Sig, Type>::publish(std::forward<Args>(args)...);
-    }
-};
-
-
-/**
- * @brief Event bus specialization for a single type.
- *
- * The event bus is designed to allow an easy registration of a specific member
- * function to a signal handler (either manager or unmanaged).
- * Classes must publicly expose the required member function to allow the bus to
- * detect it for the purpose of registering and unregistering instances.<br/>
- * In particular, a matching member function has the following signature:
- * `void receive(const Event &)`. Events of the given type will be properly
- * redirected to all the listeners by calling the right member function, if any.
- *
- * @tparam Sig Type of signal handler to use.
- * @tparam Event Type of event managed by the bus.
- */
-template<template<typename...> class Sig, typename Event>
-class Bus<Sig, Event> {
-    using signal_type = Sig<void(const Event &)>;
-
-    template<typename Class>
-    using instance_type = typename signal_type::template instance_type<Class>;
-
-    template<typename Class>
-    auto disconnect(int, instance_type<Class> instance)
-    -> decltype(std::declval<Class>().receive(std::declval<Event>()), void()) {
-        signal.template disconnect<Class, &Class::receive>(std::move(instance));
-    }
-
-    template<typename Class>
-    auto connect(int, instance_type<Class> instance)
-    -> decltype(std::declval<Class>().receive(std::declval<Event>()), void()) {
-        signal.template connect<Class, &Class::receive>(std::move(instance));
-    }
-
-    template<typename Class> void disconnect(char, instance_type<Class>) {}
-    template<typename Class> void connect(char, instance_type<Class>) {}
-
-public:
-    /*! @brief Unsigned integer type. */
-    using size_type = typename signal_type::size_type;
-
-    /**
-     * @brief Unregisters member functions of instances.
-     *
-     * This method tries to detect and unregister from the bus matching member
-     * functions of instances.<br/>
-     * A matching member function has the following signature:
-     * `void receive(const Event &)`.
-     *
-     * @tparam Class Type of instance to unregister.
-     * @param instance A valid instance of the right type.
-     */
-    template<typename Class>
-    void unreg(instance_type<Class> instance) {
-        disconnect(0, std::move(instance));
-    }
-
-    /**
-     * @brief Tries to register an instance.
-     *
-     * This method tries to detect and register to the bus matching member
-     * functions of instances.<br/>
-     * A matching member function has the following signature:
-     * `void receive(const Event &)`.
-     *
-     * @tparam Class Type of instance to register.
-     * @param instance A valid instance of the right type.
-     */
-    template<typename Class>
-    void reg(instance_type<Class> instance) {
-        connect(0, std::move(instance));
-    }
-
-    /**
-     * @brief Number of listeners connected to the bus.
-     * @return Number of listeners currently connected.
-     */
-    size_type size() const noexcept {
-        return signal.size();
-    }
-
-    /**
-     * @brief Returns false if at least a listener is connected to the bus.
-     * @return True if the bus has no listeners connected, false otherwise.
-     */
-    bool empty() const noexcept {
-        return signal.empty();
-    }
-
-    /**
-     * @brief Connects a free function to the bus.
-     * @tparam Function A valid free function pointer.
-     */
-    template<void(*Function)(const Event &)>
-    void connect() {
-        signal.template connect<Function>();
-    }
-
-    /**
-     * @brief Disconnects a free function from the bus.
-     * @tparam Function A valid free function pointer.
-     */
-    template<void(*Function)(const Event &)>
-    void disconnect() {
-        signal.template disconnect<Function>();
-    }
-
-    /**
-     * @brief Publishes an event.
-     *
-     * All the listeners are notified. Order isn't guaranteed.
-     *
-     * @tparam Args Types of arguments to use to construct the event.
-     * @param args Arguments to use to construct the event.
-     */
-    template<typename... Args>
-    void publish(Args&&... args) {
-        signal.publish({ std::forward<Args>(args)... });
-    }
-
-private:
-    signal_type signal;
-};
-
-
-/**
- * @brief Managed event bus.
- *
- * A managed event bus uses the Signal class template as an underlying type. The
- * type of the instances is the one required by the signal handler:
- * `std::shared_ptr<Class>` (a shared pointer).
- *
- * @tparam Event The list of events managed by the bus.
- */
-template<typename... Event>
-using ManagedBus = Bus<Signal, Event...>;
-
-/**
- * @brief Unmanaged event bus.
- *
- * An unmanaged event bus uses the SigH class template as an underlying type.
- * The type of the instances is the one required by the signal handler:
- * `Class *` (a naked pointer).<br/>
- * When it comes to work with this kind of bus, users must guarantee that the
- * lifetimes of the instances overcome the one of the bus itself.
- *
- * @tparam Event The list of events managed by the bus.
- */
-template<typename... Event>
-using UnmanagedBus = Bus<SigH, Event...>;
-
-
-}
-
-
-#endif // ENTT_SIGNAL_BUS_HPP

src/entt/signal/delegate.hpp

@@ -2,12 +2,80 @@
 #define ENTT_SIGNAL_DELEGATE_HPP
 
 
+#include <tuple>
+#include <cstring>
 #include <utility>
+#include <algorithm>
+#include <functional>
+#include <type_traits>
+#include "../config/config.h"
 
 
 namespace entt {
 
 
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename Ret, typename... Args>
+auto to_function_pointer(Ret(*)(Args...)) -> Ret(*)(Args...);
+
+
+template<typename Ret, typename... Args, typename Type, typename Payload, typename = std::enable_if_t<std::is_convertible_v<const Payload *, const Type *>>>
+auto to_function_pointer(Ret(*)(Type &, Args...), const Payload *) -> Ret(*)(Args...);
+
+
+template<typename Ret, typename... Args, typename Type, typename Payload, typename = std::enable_if_t<std::is_convertible_v<const Payload *, const Type *>>>
+auto to_function_pointer(Ret(*)(Type *, Args...), const Payload *) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Ret, typename... Args>
+auto to_function_pointer(Ret(Class:: *)(Args...), const Class *) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Ret, typename... Args>
+auto to_function_pointer(Ret(Class:: *)(Args...) const, const Class *) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Type>
+auto to_function_pointer(Type Class:: *, const Class *) -> Type(*)();
+
+
+template<typename... Type>
+using to_function_pointer_t = decltype(internal::to_function_pointer(std::declval<Type>()...));
+
+
+template<typename Ret, typename... Args>
+constexpr auto index_sequence_for(Ret(*)(Args...)) {
+    return std::index_sequence_for<Args...>{};
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond TURN_OFF_DOXYGEN
+ */
+
+
+/*! @brief Used to wrap a function or a member of a specified type. */
+template<auto>
+struct connect_arg_t {};
+
+
+/*! @brief Constant of type connect_arg_t used to disambiguate calls. */
+template<auto Func>
+constexpr connect_arg_t<Func> connect_arg{};
+
+
 /**
  * @brief Basic delegate implementation.
  *
@@ -15,122 +83,274 @@ namespace entt {
  * compile-time error unless the template parameter is a function type.
  */
 template<typename>
-class Delegate;
+class delegate;
 
 
 /**
- * @brief Utility class to send around functions and member functions.
+ * @brief Utility class to use to send around functions and members.
  *
- * Unmanaged delegate for function pointers and member functions. Users of this
- * class are in charge of disconnecting instances before deleting them.
+ * Unmanaged delegate for function pointers and members. Users of this class are
+ * in charge of disconnecting instances before deleting them.
  *
  * A delegate can be used as general purpose invoker with no memory overhead for
- * free functions and member functions provided along with an instance on which
- * to invoke them.
+ * free functions (with or without payload) and members provided along with an
+ * instance on which to invoke them.
  *
  * @tparam Ret Return type of a function type.
  * @tparam Args Types of arguments of a function type.
  */
 template<typename Ret, typename... Args>
-class Delegate<Ret(Args...)> final {
-    using proto_type = Ret(*)(void *, Args...);
-    using stub_type = std::pair<void *, proto_type>;
+class delegate<Ret(Args...)> {
+    using proto_fn_type = Ret(const void *, std::tuple<Args &&...>);
 
-    static Ret fallback(void *, Args...) noexcept { return {}; }
+    template<auto Function, std::size_t... Index>
+    void connect(std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        static_assert(std::is_invocable_r_v<Ret, decltype(Function), std::tuple_element_t<Index, std::tuple<Args...>>...>);
+        data = nullptr;
 
-    template<Ret(*Function)(Args...)>
-    static Ret proto(void *, Args... args) {
-        return (Function)(args...);
+        fn = [](const void *, std::tuple<Args &&...> args) -> Ret {
+            // Ret(...) makes void(...) eat the return values to avoid errors
+            return Ret(std::invoke(Function, std::forward<std::tuple_element_t<Index, std::tuple<Args...>>>(std::get<Index>(args))...));
+        };
     }
 
-    template<typename Class, Ret(Class::*Member)(Args...)>
-    static Ret proto(void *instance, Args... args) {
-        return (static_cast<Class *>(instance)->*Member)(args...);
+    template<auto Candidate, typename Type, std::size_t... Index>
+    void connect(Type &value_or_instance, std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        static_assert(std::is_invocable_r_v<Ret, decltype(Candidate), Type &, std::tuple_element_t<Index, std::tuple<Args...>>...>);
+        data = &value_or_instance;
+
+        fn = [](const void *payload, std::tuple<Args &&...> args) -> Ret {
+            Type *curr = static_cast<Type *>(const_cast<std::conditional_t<std::is_const_v<Type>, const void *, void *>>(payload));
+            // Ret(...) makes void(...) eat the return values to avoid errors
+            return Ret(std::invoke(Candidate, *curr, std::forward<std::tuple_element_t<Index, std::tuple<Args...>>>(std::get<Index>(args))...));
+        };
+    }
+
+    template<auto Candidate, typename Type, std::size_t... Index>
+    void connect(Type *value_or_instance, std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        static_assert(std::is_invocable_r_v<Ret, decltype(Candidate), Type *, std::tuple_element_t<Index, std::tuple<Args...>>...>);
+        data = value_or_instance;
+
+        fn = [](const void *payload, std::tuple<Args &&...> args) -> Ret {
+            Type *curr = static_cast<Type *>(const_cast<std::conditional_t<std::is_const_v<Type>, const void *, void *>>(payload));
+            // Ret(...) makes void(...) eat the return values to avoid errors
+            return Ret(std::invoke(Candidate, curr, std::forward<std::tuple_element_t<Index, std::tuple<Args...>>>(std::get<Index>(args))...));
+        };
     }
 
 public:
+    /*! @brief Function type of the delegate. */
+    using function_type = Ret(Args...);
+
     /*! @brief Default constructor. */
-    Delegate() noexcept
-        : stub{std::make_pair(nullptr, &fallback)}
+    delegate() ENTT_NOEXCEPT
+        : fn{nullptr}, data{nullptr}
     {}
 
     /**
-     * @brief Binds a free function to a delegate.
+     * @brief Constructs a delegate and connects a free function to it.
      * @tparam Function A valid free function pointer.
      */
-    template<Ret(*Function)(Args...)>
-    void connect() noexcept {
-        stub = std::make_pair(nullptr, &proto<Function>);
+    template<auto Function>
+    delegate(connect_arg_t<Function>) ENTT_NOEXCEPT
+        : delegate{}
+    {
+        connect<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`.
+     * @brief Constructs a delegate and connects a member for a given instance
+     * or a free function with payload.
+     * @tparam Candidate Member or free function to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid reference that fits the purpose.
      */
-    template<typename Class, Ret(Class::*Member)(Args...)>
-    void connect(Class *instance) noexcept {
-        stub = std::make_pair(instance, &proto<Class, Member>);
+    template<auto Candidate, typename Type>
+    delegate(connect_arg_t<Candidate>, Type &value_or_instance) ENTT_NOEXCEPT
+        : delegate{}
+    {
+        connect<Candidate>(value_or_instance);
+    }
+
+    /**
+     * @brief Constructs a delegate and connects a member for a given instance
+     * or a free function with payload.
+     * @tparam Candidate Member or free function to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid pointer that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    delegate(connect_arg_t<Candidate>, Type *value_or_instance) ENTT_NOEXCEPT
+        : delegate{}
+    {
+        connect<Candidate>(value_or_instance);
+    }
+
+    /**
+     * @brief Connects a free function to a delegate.
+     * @tparam Function A valid free function pointer.
+     */
+    template<auto Function>
+    void connect() ENTT_NOEXCEPT {
+        connect<Function>(internal::index_sequence_for(internal::to_function_pointer_t<decltype(Function)>{}));
+    }
+
+    /**
+     * @brief Connects a member function for a given instance or a free function
+     * with payload to a delegate.
+     *
+     * The delegate isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of the instance overcomes
+     * the one  of the delegate.<br/>
+     * When used to connect a free function with payload, its signature must be
+     * such that the instance is the first argument before the ones used to
+     * define the delegate itself.
+     *
+     * @tparam Candidate Member or free function to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid reference that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void connect(Type &value_or_instance) ENTT_NOEXCEPT {
+        connect<Candidate>(value_or_instance, internal::index_sequence_for(internal::to_function_pointer_t<decltype(Candidate), Type *>{}));
+    }
+
+    /**
+     * @brief Connects a member function for a given instance or a free function
+     * with payload to a delegate.
+     *
+     * The delegate isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of the instance overcomes
+     * the one  of the delegate.<br/>
+     * When used to connect a free function with payload, its signature must be
+     * such that the instance is the first argument before the ones used to
+     * define the delegate itself.
+     *
+     * @tparam Candidate Member or free function to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid pointer that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void connect(Type *value_or_instance) ENTT_NOEXCEPT {
+        connect<Candidate>(value_or_instance, internal::index_sequence_for(internal::to_function_pointer_t<decltype(Candidate), Type *>{}));
     }
 
     /**
      * @brief Resets a delegate.
      *
-     * After a reset, a delegate can be safely invoked with no effect.
+     * After a reset, a delegate cannot be invoked anymore.
      */
-    void reset() noexcept {
-        stub = std::make_pair(nullptr, &fallback);
+    void reset() ENTT_NOEXCEPT {
+        fn = nullptr;
+        data = nullptr;
+    }
+
+    /**
+     * @brief Returns the instance or the payload linked to a delegate, if any.
+     * @return An opaque pointer to the underlying data.
+     */
+    const void * instance() const ENTT_NOEXCEPT {
+        return data;
     }
 
     /**
      * @brief Triggers a delegate.
+     *
+     * The delegate invokes the underlying function and returns the result.
+     *
+     * @warning
+     * Attempting to trigger an invalid delegate results in undefined
+     * behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * delegate has not yet been set.
+     *
      * @param args Arguments to use to invoke the underlying function.
      * @return The value returned by the underlying function.
      */
-    Ret operator()(Args... args) {
-        return stub.second(stub.first, args...);
+    Ret operator()(Args... args) const {
+        ENTT_ASSERT(fn);
+        return fn(data, std::forward_as_tuple(std::forward<Args>(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.
+     * @brief Checks whether a delegate actually stores a listener.
+     * @return False if the delegate is empty, true otherwise.
      */
-    bool operator==(const Delegate<Ret(Args...)> &other) const noexcept {
-        return stub.first == other.stub.first && stub.second == other.stub.second;
+    explicit operator bool() const ENTT_NOEXCEPT {
+        // no need to test also data
+        return fn;
+    }
+
+    /**
+     * @brief Compares the contents of two delegates.
+     * @param other Delegate with which to compare.
+     * @return False if the two contents differ, true otherwise.
+     */
+    bool operator==(const delegate<Ret(Args...)> &other) const ENTT_NOEXCEPT {
+        return fn == other.fn && data == other.data;
     }
 
 private:
-    stub_type stub;
+    proto_fn_type *fn;
+    const void *data;
 };
 
 
 /**
- * @brief Checks if the contents of the two delegates are different.
- *
- * Two delegates are identical if they contain the same listener.
- *
+ * @brief Compares the contents of two delegates.
  * @tparam Ret Return type of a function type.
  * @tparam Args Types of arguments of a function type.
  * @param lhs A valid delegate object.
  * @param rhs A valid delegate object.
- * @return True if the two delegates are different, false otherwise.
+ * @return True if the two contents differ, false otherwise.
  */
 template<typename Ret, typename... Args>
-bool operator!=(const Delegate<Ret(Args...)> &lhs, const Delegate<Ret(Args...)> &rhs) noexcept {
+bool operator!=(const delegate<Ret(Args...)> &lhs, const delegate<Ret(Args...)> &rhs) ENTT_NOEXCEPT {
     return !(lhs == rhs);
 }
 
 
+/**
+ * @brief Deduction guide.
+ *
+ * It allows to deduce the function type of the delegate directly from a
+ * function provided to the constructor.
+ *
+ * @tparam Function A valid free function pointer.
+ */
+template<auto Function>
+delegate(connect_arg_t<Function>) ENTT_NOEXCEPT
+-> delegate<std::remove_pointer_t<internal::to_function_pointer_t<decltype(Function)>>>;
+
+
+/**
+ * @brief Deduction guide.
+ *
+ * It allows to deduce the function type of the delegate directly from a member
+ * or a free function with payload provided to the constructor.
+ *
+ * @tparam Candidate Member or free function to connect to the delegate.
+ * @tparam Type Type of class or type of payload.
+ */
+template<auto Candidate, typename Type>
+delegate(connect_arg_t<Candidate>, Type &) ENTT_NOEXCEPT
+-> delegate<std::remove_pointer_t<internal::to_function_pointer_t<decltype(Candidate), Type *>>>;
+
+
+/**
+ * @brief Deduction guide.
+ *
+ * It allows to deduce the function type of the delegate directly from a member
+ * or a free function with payload provided to the constructor.
+ *
+ * @tparam Candidate Member or free function to connect to the delegate.
+ * @tparam Type Type of class or type of payload.
+ */
+template<auto Candidate, typename Type>
+delegate(connect_arg_t<Candidate>, Type *) ENTT_NOEXCEPT
+-> delegate<std::remove_pointer_t<internal::to_function_pointer_t<decltype(Candidate), Type *>>>;
+
+
 }
 
 

src/entt/signal/dispatcher.hpp

@@ -4,10 +4,13 @@
 
 #include <vector>
 #include <memory>
+#include <cstddef>
 #include <utility>
-#include <cstdint>
+#include <algorithm>
+#include <type_traits>
+#include "../config/config.h"
 #include "../core/family.hpp"
-#include "signal.hpp"
+#include "../core/type_traits.hpp"
 #include "sigh.hpp"
 
 
@@ -20,124 +23,135 @@ namespace entt {
  * 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 signature:
- * `void(const Event &)`. Member functions named `receive` are automatically
- * detected and registered or unregistered by the dispatcher.
+ * type `Event`, listeners are such that they can be invoked with an argument of
+ * type `const Event &`, no matter what the return type is.
  *
- * @tparam Sig Type of the signal handler to use.
+ * The types of the instances are `Class &`. Users must guarantee that the
+ * lifetimes of the objects overcome the one of the dispatcher itself to avoid
+ * crashes.
  */
-template<template<typename...> class Sig>
-class Dispatcher final {
-    using event_family = Family<struct InternalDispatcherEventFamily>;
+class dispatcher {
+    using event_family = family<struct internal_dispatcher_event_family>;
 
     template<typename Class, typename Event>
-    using instance_type = typename Sig<void(const Event &)>::template instance_type<Class>;
+    using instance_type = typename sigh<void(const Event &)>::template instance_type<Class>;
 
-    struct BaseSignalWrapper {
-        virtual ~BaseSignalWrapper() = default;
-        virtual void publish(std::size_t) = 0;
+    struct base_wrapper {
+        virtual ~base_wrapper() = default;
+        virtual void publish() = 0;
+        virtual void clear() = 0;
     };
 
     template<typename Event>
-    struct SignalWrapper final: BaseSignalWrapper {
-        void publish(std::size_t current) final override {
-            for(auto &&event: events[current]) {
-                signal.publish(event);
+    struct signal_wrapper: base_wrapper {
+        using signal_type = sigh<void(const Event &)>;
+        using sink_type = typename signal_type::sink_type;
+
+        void publish() override {
+            const auto length = events.size();
+
+            for(std::size_t pos{}; pos < length; ++pos) {
+                signal.publish(events[pos]);
             }
 
-            events[current].clear();
+            events.erase(events.cbegin(), events.cbegin()+length);
         }
 
-        template<typename Class, void(Class::*Member)(const Event &)>
-        inline void connect(instance_type<Class, Event> instance) noexcept {
-            signal.template connect<Class, Member>(std::move(instance));
+        void clear() override {
+            events.clear();
         }
 
-        template<typename Class, void(Class::*Member)(const Event &)>
-        inline void disconnect(instance_type<Class, Event> instance) noexcept {
-            signal.template disconnect<Class, Member>(std::move(instance));
+        sink_type sink() ENTT_NOEXCEPT {
+            return entt::sink{signal};
         }
 
         template<typename... Args>
-        inline void trigger(Args&&... args) {
+        void trigger(Args &&... args) {
             signal.publish({ std::forward<Args>(args)... });
         }
 
         template<typename... Args>
-        inline void enqueue(std::size_t current, Args&&... args) {
-            events[current].push_back({ std::forward<Args>(args)... });
+        void enqueue(Args &&... args) {
+            events.emplace_back(std::forward<Args>(args)...);
         }
 
     private:
-        Sig<void(const Event &)> signal{};
-        std::vector<Event> events[2];
+        signal_type signal{};
+        std::vector<Event> events;
     };
 
-    inline static std::size_t buffer(bool mode) {
-        return mode ? 0 : 1;
+    struct wrapper_data {
+        std::unique_ptr<base_wrapper> wrapper;
+        ENTT_ID_TYPE runtime_type;
+    };
+
+    template<typename Event>
+    static auto type() ENTT_NOEXCEPT {
+        if constexpr(is_named_type_v<Event>) {
+            return named_type_traits_v<Event>;
+        } else {
+            return event_family::type<std::decay_t<Event>>;
+        }
     }
 
     template<typename Event>
-    SignalWrapper<Event> & wrapper() {
-        auto type = event_family::type<Event>();
+    signal_wrapper<Event> & assure() {
+        const auto wtype = type<Event>();
+        wrapper_data *wdata = nullptr;
 
-        if(!(type < wrappers.size())) {
-            wrappers.resize(type + 1);
+        if constexpr(is_named_type_v<Event>) {
+            const auto it = std::find_if(wrappers.begin(), wrappers.end(), [wtype](const auto &candidate) {
+                return candidate.wrapper && candidate.runtime_type == wtype;
+            });
+
+            wdata = (it == wrappers.cend() ? &wrappers.emplace_back() : &(*it));
+        } else {
+            if(!(wtype < wrappers.size())) {
+                wrappers.resize(wtype+1);
+            }
+
+            wdata = &wrappers[wtype];
+
+            if(wdata->wrapper && wdata->runtime_type != wtype) {
+                wrappers.emplace_back();
+                std::swap(wrappers[wtype], wrappers.back());
+                wdata = &wrappers[wtype];
+            }
         }
 
-        if(!wrappers[type]) {
-            wrappers[type] = std::make_unique<SignalWrapper<Event>>();
+        if(!wdata->wrapper) {
+            wdata->wrapper = std::make_unique<signal_wrapper<Event>>();
+            wdata->runtime_type = wtype;
         }
 
-        return static_cast<SignalWrapper<Event> &>(*wrappers[type]);
+        return static_cast<signal_wrapper<Event> &>(*wdata->wrapper);
     }
 
 public:
-    /*! @brief Default constructor. */
-    Dispatcher() noexcept
-        : wrappers{}, mode{false}
-    {}
+    /*! @brief Type of sink for the given event. */
+    template<typename Event>
+    using sink_type = typename signal_wrapper<Event>::sink_type;
 
     /**
-     * @brief Registers a listener given in the form of a member function.
+     * @brief Returns a sink object for the given event.
      *
-     * A matching member function has the following signature:
-     * `void receive(const Event &)`. Member functions named `receive` are
-     * automatically detected and registered if available.
+     * A sink is an opaque object used to connect listeners to events.
      *
-     * @warning
-     * Connecting a listener during an update may lead to unexpected behavior.
-     * Register listeners before or after invoking the update if possible.
+     * The function type for a listener is:
+     * @code{.cpp}
+     * void(const Event &);
+     * @endcode
      *
-     * @tparam Event Type of event to which to connect the function.
-     * @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 the right type.
+     * The order of invocation of the listeners isn't guaranteed.
+     *
+     * @sa sink
+     *
+     * @tparam Event Type of event of which to get the sink.
+     * @return A temporary sink object.
      */
-    template<typename Event, typename Class, void(Class::*Member)(const Event &) = &Class::receive>
-    void connect(instance_type<Class, Event> instance) noexcept {
-        wrapper<Event>().template connect<Class, Member>(std::move(instance));
-    }
-
-    /**
-     * @brief Unregisters a listener given in the form of a member function.
-     *
-     * A matching member function has the following signature:
-     * `void receive(const Event &)`. Member functions named `receive` are
-     * automatically detected and unregistered if available.
-     *
-     * @warning
-     * Disonnecting a listener during an update may lead to unexpected behavior.
-     * Unregister listeners before or after invoking the update if possible.
-     *
-     * @tparam Event Type of event from which to disconnect the function.
-     * @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 the right type.
-     */
-    template<typename Event, typename Class, void(Class::*Member)(const Event &) = &Class::receive>
-    void disconnect(instance_type<Class, Event> instance) noexcept {
-        wrapper<Event>().template disconnect<Class, Member>(std::move(instance));
+    template<typename Event>
+    sink_type<Event> sink() ENTT_NOEXCEPT {
+        return assure<Event>().sink();
     }
 
     /**
@@ -151,8 +165,22 @@ public:
      * @param args Arguments to use to construct the event.
      */
     template<typename Event, typename... Args>
-    void trigger(Args&&... args) {
-        wrapper<Event>().trigger(std::forward<Args>(args)...);
+    void trigger(Args &&... args) {
+        assure<Event>().trigger(std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Triggers an immediate event of the given type.
+     *
+     * All the listeners registered for the given type are immediately notified.
+     * The event is discarded after the execution.
+     *
+     * @tparam Event Type of event to trigger.
+     * @param event An instance of the given type of event.
+     */
+    template<typename Event>
+    void trigger(Event &&event) {
+        assure<std::decay_t<Event>>().trigger(std::forward<Event>(event));
     }
 
     /**
@@ -161,61 +189,84 @@ public:
      * 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 Event Type of event to enqueue.
      * @tparam Args Types of arguments to use to construct the event.
      * @param args Arguments to use to construct the event.
      */
     template<typename Event, typename... Args>
-    void enqueue(Args&&... args) {
-        wrapper<Event>().enqueue(buffer(mode), std::forward<Args>(args)...);
+    void enqueue(Args &&... args) {
+        assure<Event>().enqueue(std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Enqueues an event of the given type.
+     *
+     * An event of the given type is queued. No listener is invoked. Use the
+     * `update` member function to notify listeners when ready.
+     *
+     * @tparam Event Type of event to enqueue.
+     * @param event An instance of the given type of event.
+     */
+    template<typename Event>
+    void enqueue(Event &&event) {
+        assure<std::decay_t<Event>>().enqueue(std::forward<Event>(event));
+    }
+
+    /**
+     * @brief Discards all the events queued so far.
+     *
+     * If no types are provided, the dispatcher will clear all the existing
+     * pools.
+     *
+     * @tparam Event Type of events to discard.
+     */
+    template<typename... Event>
+    void discard() {
+        if constexpr(sizeof...(Event) == 0) {
+            std::for_each(wrappers.begin(), wrappers.end(), [](auto &&wdata) {
+                if(wdata.wrapper) {
+                    wdata.wrapper->clear();
+                }
+            });
+        } else {
+            (assure<std::decay_t<Event>>().clear(), ...);
+        }
+    }
+
+    /**
+     * @brief Delivers all the pending events of the given type.
+     *
+     * This method is blocking and it doesn't return until all the events are
+     * delivered to the registered listeners. It's responsibility of the users
+     * to reduce at a minimum the time spent in the bodies of the listeners.
+     *
+     * @tparam Event Type of events to send.
+     */
+    template<typename Event>
+    void update() {
+        assure<Event>().publish();
     }
 
     /**
      * @brief Delivers all the pending events.
      *
      * This method is blocking and it doesn't return until all the events are
-     * delivered to the registered listeners. It's responsability of the users
+     * delivered to the registered listeners. It's responsibility of the users
      * to reduce at a minimum the time spent in the bodies of the listeners.
      */
-    void update() {
-        auto buf = buffer(mode);
-        mode = !mode;
-
-        for(auto &&wrapper: wrappers) {
-            if(wrapper) {
-                wrapper->publish(buf);
+    void update() const {
+        for(auto pos = wrappers.size(); pos; --pos) {
+            if(auto &wdata = wrappers[pos-1]; wdata.wrapper) {
+                wdata.wrapper->publish();
             }
         }
     }
 
 private:
-    std::vector<std::unique_ptr<BaseSignalWrapper>> wrappers;
-    bool mode;
+    std::vector<wrapper_data> wrappers;
 };
 
 
-/**
- * @brief Managed dispatcher.
- *
- * A managed dispatcher uses the Signal class template as an underlying type.
- * The type of the instances is the one required by the signal handler:
- * `std::shared_ptr<Class>` (a shared pointer).
- */
-using ManagedDispatcher = Dispatcher<Signal>;
-
-
-/**
- * @brief Unmanaged dispatcher.
- *
- * An unmanaged dispatcher uses the SigH class template as an underlying type.
- * The type of the instances is the one required by the signal handler:
- * `Class *` (a naked pointer).<br/>
- * When it comes to work with this kind of dispatcher, users must guarantee that
- * the lifetimes of the instances overcome the one of the dispatcher itself.
- */
-using UnmanagedDispatcher = Dispatcher<SigH>;
-
-
 }
 
 

src/entt/signal/emitter.hpp

@@ -6,10 +6,12 @@
 #include <functional>
 #include <algorithm>
 #include <utility>
-#include <cstdint>
 #include <memory>
 #include <vector>
 #include <list>
+#include "../config/config.h"
+#include "../core/family.hpp"
+#include "../core/type_traits.hpp"
 
 
 namespace entt {
@@ -21,11 +23,11 @@ namespace entt {
  * 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> {
+ * @code{.cpp}
+ * struct my_emitter: emitter<my_emitter> {
  *     // ...
  * }
- * ```
+ * @endcode
  *
  * Handlers for the type of events are created internally on the fly. It's not
  * required to specify in advance the full list of accepted types.<br/>
@@ -37,59 +39,61 @@ namespace entt {
  * @tparam Derived Actual type of emitter that extends the class template.
  */
 template<typename Derived>
-class Emitter {
-    struct BaseHandler {
-        virtual ~BaseHandler() = default;
-        virtual bool empty() const noexcept = 0;
-        virtual void clear() noexcept = 0;
+class emitter {
+    using handler_family = family<struct internal_emitter_handler_family>;
+
+    struct base_handler {
+        virtual ~base_handler() = default;
+        virtual bool empty() const ENTT_NOEXCEPT = 0;
+        virtual void clear() ENTT_NOEXCEPT = 0;
     };
 
     template<typename Event>
-    struct Handler final: BaseHandler {
+    struct event_handler: base_handler {
         using listener_type = std::function<void(const Event &, Derived &)>;
         using element_type = std::pair<bool, listener_type>;
         using container_type = std::list<element_type>;
         using connection_type = typename container_type::iterator;
 
-        bool empty() const noexcept override {
-            auto pred = [](auto &&element){ return element.first; };
+        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);
+            return std::all_of(once_list.cbegin(), once_list.cend(), pred) &&
+                    std::all_of(on_list.cbegin(), on_list.cend(), pred);
         }
 
-        void clear() noexcept override {
+        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);
+                auto func = [](auto &&element) { element.first = true; };
+                std::for_each(once_list.begin(), once_list.end(), func);
+                std::for_each(on_list.begin(), on_list.end(), func);
             } else {
-                onceL.clear();
-                onL.clear();
+                once_list.clear();
+                on_list.clear();
             }
         }
 
-        inline connection_type once(listener_type listener) {
-            return onceL.emplace(onceL.cend(), false, std::move(listener));
+        connection_type once(listener_type listener) {
+            return once_list.emplace(once_list.cend(), false, std::move(listener));
         }
 
-        inline connection_type on(listener_type listener) {
-            return onL.emplace(onL.cend(), false, std::move(listener));
+        connection_type on(listener_type listener) {
+            return on_list.emplace(on_list.cend(), false, std::move(listener));
         }
 
-        void erase(connection_type conn) noexcept {
+        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);
+                auto pred = [](auto &&element) { return element.first; };
+                once_list.remove_if(pred);
+                on_list.remove_if(pred);
             }
         }
 
         void publish(const Event &event, Derived &ref) {
-            container_type currentL;
-            onceL.swap(currentL);
+            container_type swap_list;
+            once_list.swap(swap_list);
 
             auto func = [&event, &ref](auto &&element) {
                 return element.first ? void() : element.second(event, ref);
@@ -97,50 +101,71 @@ class Emitter {
 
             publishing = true;
 
-            std::for_each(onL.rbegin(), onL.rend(), func);
-            std::for_each(currentL.rbegin(), currentL.rend(), func);
+            std::for_each(on_list.rbegin(), on_list.rend(), func);
+            std::for_each(swap_list.rbegin(), swap_list.rend(), func);
 
             publishing = false;
 
-            onL.remove_if([](auto &&element){ return element.first; });
+            on_list.remove_if([](auto &&element) { return element.first; });
         }
 
     private:
         bool publishing{false};
-        container_type onceL{};
-        container_type onL{};
+        container_type once_list{};
+        container_type on_list{};
     };
 
-    static std::size_t next() noexcept {
-        static std::size_t counter = 0;
-        return counter++;
-    }
+    struct handler_data {
+        std::unique_ptr<base_handler> handler;
+        ENTT_ID_TYPE runtime_type;
+    };
 
-    template<typename>
-    static std::size_t type() noexcept {
-        static std::size_t value = next();
-        return value;
+    template<typename Event>
+    static auto type() ENTT_NOEXCEPT {
+        if constexpr(is_named_type_v<Event>) {
+            return named_type_traits_v<Event>;
+        } else {
+            return handler_family::type<std::decay_t<Event>>;
+        }
     }
 
     template<typename Event>
-    Handler<Event> & handler() noexcept {
-        std::size_t family = type<Event>();
+    event_handler<Event> * assure() const ENTT_NOEXCEPT {
+        const auto htype = type<Event>();
+        handler_data *hdata = nullptr;
 
-        if(!(family < handlers.size())) {
-            handlers.resize(family+1);
+        if constexpr(is_named_type_v<Event>) {
+            const auto it = std::find_if(handlers.begin(), handlers.end(), [htype](const auto &candidate) {
+                return candidate.handler && candidate.runtime_type == htype;
+            });
+
+            hdata = (it == handlers.cend() ? &handlers.emplace_back() : &(*it));
+        } else {
+            if(!(htype < handlers.size())) {
+                handlers.resize(htype+1);
+            }
+
+            hdata = &handlers[htype];
+
+            if(hdata->handler && hdata->runtime_type != htype) {
+                handlers.emplace_back();
+                std::swap(handlers[htype], handlers.back());
+                hdata = &handlers[htype];
+            }
         }
 
-        if(!handlers[family]) {
-            handlers[family] = std::make_unique<Handler<Event>>();
+        if(!hdata->handler) {
+            hdata->handler = std::make_unique<event_handler<Event>>();
+            hdata->runtime_type = htype;
         }
 
-        return static_cast<Handler<Event> &>(*handlers[family]);
+        return static_cast<event_handler<Event> *>(hdata->handler.get());
     }
 
 public:
-    /** @brief Type of listeners accepted for the given type of event. */
+    /** @brief Type of listeners accepted for the given event. */
     template<typename Event>
-    using Listener = typename Handler<Event>::listener_type;
+    using listener = typename event_handler<Event>::listener_type;
 
     /**
      * @brief Generic connection type for events.
@@ -152,56 +177,35 @@ public:
      * @tparam Event Type of event for which the connection is created.
      */
     template<typename Event>
-    struct Connection final: private Handler<Event>::connection_type {
+    struct connection: private event_handler<Event>::connection_type {
         /** @brief Event emitters are friend classes of connections. */
-        friend class Emitter;
+        friend class emitter;
 
         /*! @brief Default constructor. */
-        Connection() noexcept = default;
+        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)}
+        connection(typename event_handler<Event>::connection_type conn)
+            : event_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 assignament 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() noexcept = default;
+    emitter() ENTT_NOEXCEPT = default;
 
     /*! @brief Default destructor. */
-    virtual ~Emitter() noexcept {
-        static_assert(std::is_base_of<Emitter<Derived>, Derived>::value, "!");
+    virtual ~emitter() ENTT_NOEXCEPT {
+        static_assert(std::is_base_of_v<emitter<Derived>, Derived>);
     }
 
-    /*! @brief Copying an emitter isn't allowed. */
-    Emitter(const Emitter &) = delete;
     /*! @brief Default move constructor. */
-    Emitter(Emitter &&) = default;
+    emitter(emitter &&) = default;
 
-    /*! @brief Copying an emitter isn't allowed. @return This emitter. */
-    Emitter & operator=(const Emitter &) = delete;
-    /*! @brief Default move assignament operator. @return This emitter. */
-    Emitter & operator=(Emitter &&) = default;
+    /*! @brief Default move assignment operator. @return This emitter. */
+    emitter & operator=(emitter &&) = default;
 
     /**
      * @brief Emits the given event.
@@ -215,8 +219,8 @@ public:
      * @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));
+    void publish(Args &&... args) {
+        assure<Event>()->publish({ std::forward<Args>(args)... }, *static_cast<Derived *>(this));
     }
 
     /**
@@ -236,12 +240,12 @@ public:
      * instances for later uses.
      *
      * @tparam Event Type of event to which to connect the listener.
-     * @param listener The listener to register.
+     * @param instance The listener to register.
      * @return Connection object that can be used to disconnect the listener.
      */
     template<typename Event>
-    Connection<Event> on(Listener<Event> listener) {
-        return handler<Event>().on(std::move(listener));
+    connection<Event> on(listener<Event> instance) {
+        return assure<Event>()->on(std::move(instance));
     }
 
     /**
@@ -261,12 +265,12 @@ public:
      * instances for later uses.
      *
      * @tparam Event Type of event to which to connect the listener.
-     * @param listener The listener to register.
+     * @param instance The listener to register.
      * @return Connection object that can be used to disconnect the listener.
      */
     template<typename Event>
-    Connection<Event> once(Listener<Event> listener) {
-        return handler<Event>().once(std::move(listener));
+    connection<Event> once(listener<Event> instance) {
+        return assure<Event>()->once(std::move(instance));
     }
 
     /**
@@ -279,32 +283,33 @@ public:
      * @param conn A valid connection.
      */
     template<typename Event>
-    void erase(Connection<Event> conn) noexcept {
-        handler<Event>().erase(std::move(conn));
+    void erase(connection<Event> conn) ENTT_NOEXCEPT {
+        assure<Event>()->erase(std::move(conn));
     }
 
     /**
      * @brief Disconnects all the listeners for the given event type.
      *
      * All the connections previously returned for the given event are
-     * invalidated. Using them results in undefined behaviour.
+     * invalidated. Using them results in undefined behavior.
      *
      * @tparam Event Type of event to reset.
      */
     template<typename Event>
-    void clear() noexcept {
-        handler<Event>().clear();
+    void clear() ENTT_NOEXCEPT {
+        assure<Event>()->clear();
     }
 
     /**
      * @brief Disconnects all the listeners.
      *
      * All the connections previously returned are invalidated. Using them
-     * results in undefined behaviour.
+     * results in undefined behavior.
      */
-    void clear() noexcept {
-        std::for_each(handlers.begin(), handlers.end(),
-                      [](auto &&handler){ if(handler) { handler->clear(); } });
+    void clear() ENTT_NOEXCEPT {
+        std::for_each(handlers.begin(), handlers.end(), [](auto &&hdata) {
+            return hdata.handler ? hdata.handler->clear() : void();
+        });
     }
 
     /**
@@ -313,25 +318,22 @@ public:
      * @return True if there are no listeners registered, false otherwise.
      */
     template<typename Event>
-    bool empty() const noexcept {
-        std::size_t family = type<Event>();
-
-        return (!(family < handlers.size()) ||
-                !handlers[family] ||
-                static_cast<Handler<Event> &>(*handlers[family]).empty());
+    bool empty() const ENTT_NOEXCEPT {
+        return assure<Event>()->empty();
     }
 
     /**
      * @brief Checks if there are listeners registered with the event emitter.
      * @return True if there are no listeners registered, false otherwise.
      */
-    bool empty() const noexcept {
-        return std::all_of(handlers.cbegin(), handlers.cend(),
-                           [](auto &&handler){ return !handler || handler->empty(); });
+    bool empty() const ENTT_NOEXCEPT {
+        return std::all_of(handlers.cbegin(), handlers.cend(), [](auto &&hdata) {
+            return !hdata.handler || hdata.handler->empty();
+        });
     }
 
 private:
-    std::vector<std::unique_ptr<BaseHandler>> handlers{};
+    mutable std::vector<handler_data> handlers{};
 };
 
 

src/entt/signal/fwd.hpp

@@ -0,0 +1,24 @@
+#ifndef ENTT_SIGNAL_FWD_HPP
+#define ENTT_SIGNAL_FWD_HPP
+
+
+namespace entt {
+
+
+/*! @class delegate */
+template<typename>
+class delegate;
+
+/*! @class sink */
+template<typename>
+class sink;
+
+/*! @class sigh */
+template<typename>
+class sigh;
+
+
+}
+
+
+#endif // ENTT_SIGNAL_FWD_HPP

src/entt/signal/sigh.hpp

@@ -2,129 +2,69 @@
 #define ENTT_SIGNAL_SIGH_HPP
 
 
-#include <algorithm>
-#include <utility>
 #include <vector>
+#include <utility>
+#include <iterator>
+#include <algorithm>
+#include <functional>
+#include <type_traits>
+#include "../config/config.h"
+#include "delegate.hpp"
+#include "fwd.hpp"
 
 
 namespace entt {
 
 
-namespace {
-
-
-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;
-
-    template<typename SFINAE = Ret>
-    typename std::enable_if<std::is_void<SFINAE>::value, bool>::type
-    invoke(Collector &, proto_type proto, void *instance, Args... args) {
-        proto(instance, args...);
-        return true;
-    }
-
-    template<typename SFINAE = Ret>
-    typename std::enable_if<!std::is_void<SFINAE>::value, bool>::type
-    invoke(Collector &collector, proto_type proto, void *instance, Args... args) {
-        return collector(proto(instance, args...));
-    }
-};
-
-
-template<typename Ret>
-struct NullCollector final {
-    using result_type = Ret;
-    bool operator()(result_type) const noexcept { return true; }
-};
-
-
-template<>
-struct NullCollector<void> final {
-    using result_type = void;
-    bool operator()() const 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;
-
-
-}
-
-
 /**
- * @brief Unmanaged signal handler declaration.
+ * @brief Sink class.
  *
  * Primary template isn't defined on purpose. All the specializations give a
  * compile-time error unless the template parameter is a function type.
  *
  * @tparam Function A valid function type.
- * @tparam Collector Type of collector to use, if any.
  */
-template<typename Function, typename Collector = DefaultCollectorType<Function>>
-class SigH;
+template<typename Function>
+class sink;
 
 
 /**
- * @brief Unmanaged signal handler definition.
+ * @brief Unmanaged signal handler.
  *
- * 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.
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error unless the template parameter is a function type.
+ *
+ * @tparam Function A valid function type.
+ */
+template<typename Function>
+class sigh;
+
+
+/**
+ * @brief Unmanaged signal handler.
+ *
+ * It works directly with references to classes and pointers to member functions
+ * as well as pointers to free functions. Users of this class are in charge of
+ * disconnecting instances before deleting them.
  *
  * This class serves mainly two purposes:
  *
- * * Creating signals used later to notify a bunch of listeners.
+ * * Creating signals to use 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 Invoker<Ret(Args...), Collector> {
-    using typename Invoker<Ret(Args...), Collector>::call_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...);
-    }
+template<typename Ret, typename... Args>
+class sigh<Ret(Args...)> {
+    /*! @brief A sink is allowed to modify a signal. */
+    friend class sink<Ret(Args...)>;
 
 public:
     /*! @brief Unsigned integer type. */
-    using size_type = typename std::vector<call_type>::size_type;
-    /*! @brief Collector type. */
-    using collector_type = Collector;
+    using size_type = std::size_t;
+    /*! @brief Sink type. */
+    using sink_type = entt::sink<Ret(Args...)>;
 
     /**
      * @brief Instance type when it comes to connecting member functions.
@@ -137,7 +77,7 @@ public:
      * @brief Number of listeners connected to the signal.
      * @return Number of listeners currently connected.
      */
-    size_type size() const noexcept {
+    size_type size() const ENTT_NOEXCEPT {
         return calls.size();
     }
 
@@ -145,82 +85,10 @@ public:
      * @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 noexcept {
+    bool empty() const ENTT_NOEXCEPT {
         return calls.empty();
     }
 
-    /**
-     * @brief Disconnects all the listeners from a signal.
-     */
-    void clear() noexcept {
-        calls.clear();
-    }
-
-    /**
-     * @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...)>
-    void connect(instance_type<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(instance_type<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(instance_type<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 Triggers a signal.
      *
@@ -228,73 +96,520 @@ public:
      *
      * @param args Arguments to use to invoke listeners.
      */
-    void publish(Args... args) {
-        for(auto &&call: calls) {
-            call.second(call.first, args...);
-        }
+    void publish(Args... args) const {
+        std::for_each(calls.cbegin(), calls.cend(), [&args...](auto &&call) {
+            call(args...);
+        });
     }
 
     /**
      * @brief Collects return values from the listeners.
+     *
+     * The collector must expose a call operator with the following properties:
+     *
+     * * The return type is either `void` or such that it's convertible to
+     *   `bool`. In the second case, a true value will stop the iteration.
+     * * The list of parameters is empty if `Ret` is `void`, otherwise it
+     *   contains a single element such that `Ret` is convertible to it.
+     *
+     * @tparam Func Type of collector to use, if any.
+     * @param func A valid function object.
      * @param args Arguments to use to invoke listeners.
-     * @return An instance of the collector filled with collected data.
      */
-    collector_type collect(Args... args) {
-        collector_type collector;
-
+    template<typename Func>
+    void collect(Func func, Args... args) const {
         for(auto &&call: calls) {
-            if(!this->invoke(collector, call.second, call.first, args...)) {
-                break;
+            if constexpr(std::is_void_v<Ret>) {
+                if constexpr(std::is_invocable_r_v<bool, Func>) {
+                    call(args...);
+                    if(func()) { break; }
+                } else {
+                    call(args...);
+                    func();
+                }
+            } else {
+                if constexpr(std::is_invocable_r_v<bool, Func, Ret>) {
+                    if(func(call(args...))) { break; }
+                } else {
+                    func(call(args...));
+                }
             }
         }
-
-        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 noexcept {
-        return std::equal(calls.cbegin(), calls.cend(), other.calls.cbegin(), other.calls.cend());
     }
 
 private:
-    std::vector<call_type> calls;
+    std::vector<delegate<Ret(Args...)>> calls;
 };
 
 
 /**
- * @brief Checks if the contents of the two signals are different.
+ * @brief Connection class.
  *
- * Two signals are identical if they have the same size and the same
- * listeners registered exactly in the same order.
+ * Opaque object the aim of which is to allow users to release an already
+ * estabilished connection without having to keep a reference to the signal or
+ * the sink that generated it.
+ */
+class connection {
+    /*! @brief A sink is allowed to create connection objects. */
+    template<typename>
+    friend class sink;
+
+    connection(delegate<void(void *)> fn, void *ref)
+        : disconnect{fn}, signal{ref}
+    {}
+
+public:
+    /*! @brief Default constructor. */
+    connection() = default;
+
+    /*! @brief Default copy constructor. */
+    connection(const connection &) = default;
+
+    /**
+     * @brief Default move constructor.
+     * @param other The instance to move from.
+     */
+    connection(connection &&other)
+        : connection{}
+    {
+        std::swap(disconnect, other.disconnect);
+        std::swap(signal, other.signal);
+    }
+
+    /*! @brief Default copy assignment operator. @return This connection. */
+    connection & operator=(const connection &) = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @param other The instance to move from.
+     * @return This connection.
+     */
+    connection & operator=(connection &&other) {
+        if(this != &other) {
+            auto tmp{std::move(other)};
+            disconnect = tmp.disconnect;
+            signal = tmp.signal;
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Checks whether a connection is properly initialized.
+     * @return True if the connection is properly initialized, false otherwise.
+     */
+    explicit operator bool() const ENTT_NOEXCEPT {
+        return static_cast<bool>(disconnect);
+    }
+
+    /*! @brief Breaks the connection. */
+    void release() {
+        if(disconnect) {
+            disconnect(signal);
+            disconnect.reset();
+        }
+    }
+
+private:
+    delegate<void(void *)> disconnect;
+    void *signal{};
+};
+
+
+/**
+ * @brief Scoped connection class.
+ *
+ * Opaque object the aim of which is to allow users to release an already
+ * estabilished connection without having to keep a reference to the signal or
+ * the sink that generated it.<br/>
+ * A scoped connection automatically breaks the link between the two objects
+ * when it goes out of scope.
+ */
+struct scoped_connection: private connection {
+    using connection::operator bool;
+    using connection::release;
+
+    /*! @brief Default constructor. */
+    scoped_connection() = default;
+
+    /**
+     * @brief Constructs a scoped connection from a basic connection.
+     * @param conn A valid connection object.
+     */
+    scoped_connection(const connection &conn)
+        : connection{conn}
+    {}
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    scoped_connection(const scoped_connection &) = delete;
+
+    /*! @brief Default move constructor. */
+    scoped_connection(scoped_connection &&) = default;
+
+    /*! @brief Automatically breaks the link on destruction. */
+    ~scoped_connection() {
+        connection::release();
+    }
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This scoped connection.
+     */
+    scoped_connection & operator=(const scoped_connection &) = delete;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This scoped connection.
+     */
+    scoped_connection & operator=(scoped_connection &&) = default;
+
+    /**
+     * @brief Copies a connection.
+     * @param other The connection object to copy.
+     * @return This scoped connection.
+     */
+    scoped_connection & operator=(const connection &other) {
+        static_cast<connection &>(*this) = other;
+        return *this;
+    }
+
+    /**
+     * @brief Moves a connection.
+     * @param other The connection object to move.
+     * @return This scoped connection.
+     */
+    scoped_connection & operator=(connection &&other) {
+        static_cast<connection &>(*this) = std::move(other);
+        return *this;
+    }
+};
+
+
+/**
+ * @brief Sink class.
+ *
+ * A sink is used to connect listeners to signals and to disconnect them.<br/>
+ * The function type for a listener is the one of the signal to which it
+ * belongs.
+ *
+ * The clear separation between a signal and a sink permits to store the former
+ * as private data member without exposing the publish functionality to the
+ * users of the class.
  *
  * @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) noexcept {
-    return !(lhs == rhs);
-}
+class sink<Ret(Args...)> {
+    using signal_type = sigh<Ret(Args...)>;
+    using difference_type = typename std::iterator_traits<typename decltype(signal_type::calls)::iterator>::difference_type;
+
+    template<auto Candidate, typename Type>
+    static void release(Type value_or_instance, void *signal) {
+        sink{*static_cast<signal_type *>(signal)}.disconnect<Candidate>(value_or_instance);
+    }
+
+    template<auto Function>
+    static void release(void *signal) {
+        sink{*static_cast<signal_type *>(signal)}.disconnect<Function>();
+    }
+
+public:
+    /**
+     * @brief Constructs a sink that is allowed to modify a given signal.
+     * @param ref A valid reference to a signal object.
+     */
+    sink(sigh<Ret(Args...)> &ref) ENTT_NOEXCEPT
+        : offset{},
+          signal{&ref}
+    {}
+
+    /**
+     * @brief Returns false if at least a listener is connected to the sink.
+     * @return True if the sink has no listeners connected, false otherwise.
+     */
+    bool empty() const ENTT_NOEXCEPT {
+        return signal->calls.empty();
+    }
+
+    /**
+     * @brief Returns a sink that connects before a given function.
+     * @tparam Function A valid free function pointer.
+     * @return A properly initialized sink object.
+     */
+    template<auto Function>
+    sink before() {
+        delegate<Ret(Args...)> call{};
+        call.template connect<Function>();
+
+        const auto &calls = signal->calls;
+        const auto it = std::find(calls.cbegin(), calls.cend(), std::move(call));
+
+        sink other{*this};
+        other.offset = std::distance(it, calls.cend());
+        return other;
+    }
+
+    /**
+     * @brief Returns a sink that connects before a given member function or
+     * free function with payload.
+     * @tparam Candidate Member or free function to look for.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid reference that fits the purpose.
+     * @return A properly initialized sink object.
+     */
+    template<auto Candidate, typename Type>
+    sink before(Type &value_or_instance) {
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>(value_or_instance);
+
+        const auto &calls = signal->calls;
+        const auto it = std::find(calls.cbegin(), calls.cend(), std::move(call));
+
+        sink other{*this};
+        other.offset = std::distance(it, calls.cend());
+        return other;
+    }
+
+    /**
+     * @brief Returns a sink that connects before a given member function or
+     * free function with payload.
+     * @tparam Candidate Member or free function to look for.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid pointer that fits the purpose.
+     * @return A properly initialized sink object.
+     */
+    template<auto Candidate, typename Type>
+    sink before(Type *value_or_instance) {
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>(value_or_instance);
+
+        const auto &calls = signal->calls;
+        const auto it = std::find(calls.cbegin(), calls.cend(), std::move(call));
+
+        sink other{*this};
+        other.offset = std::distance(it, calls.cend());
+        return other;
+    }
+
+    /**
+     * @brief Returns a sink that connects before a given instance or specific
+     * payload.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     * @return A properly initialized sink object.
+     */
+    template<typename Type>
+    sink before(Type &value_or_instance) {
+        return before(&value_or_instance);
+    }
+
+    /**
+     * @brief Returns a sink that connects before a given instance or specific
+     * payload.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid pointer that fits the purpose.
+     * @return A properly initialized sink object.
+     */
+    template<typename Type>
+    sink before(Type *value_or_instance) {
+        sink other{*this};
+
+        if(value_or_instance) {
+            const auto &calls = signal->calls;
+            const auto it = std::find_if(calls.cbegin(), calls.cend(), [value_or_instance](const auto &delegate) {
+                return delegate.instance() == value_or_instance;
+            });
+
+            other.offset = std::distance(it, calls.cend());
+        }
+
+        return other;
+    }
+
+    /**
+     * @brief Returns a sink that connects before anything else.
+     * @return A properly initialized sink object.
+     */
+    sink before() {
+        sink other{*this};
+        other.offset = signal->calls.size();
+        return other;
+    }
+
+    /**
+     * @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.
+     * @return A properly initialized connection object.
+     */
+    template<auto Function>
+    connection connect() {
+        disconnect<Function>();
+
+        delegate<Ret(Args...)> call{};
+        call.template connect<Function>();
+        signal->calls.insert(signal->calls.end() - offset, std::move(call));
+
+        delegate<void(void *)> conn{};
+        conn.template connect<&release<Function>>();
+        return { std::move(conn), signal };
+    }
+
+    /**
+     * @brief Connects a member function or a free function with payload to a
+     * signal.
+     *
+     * The signal isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of the instance overcomes
+     * the one  of the delegate. On the other side, the signal handler performs
+     * checks to avoid multiple connections for the same function.<br/>
+     * When used to connect a free function with payload, its signature must be
+     * such that the instance is the first argument before the ones used to
+     * define the delegate itself.
+     *
+     * @tparam Candidate Member or free function to connect to the signal.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid reference that fits the purpose.
+     * @return A properly initialized connection object.
+     */
+    template<auto Candidate, typename Type>
+    connection connect(Type &value_or_instance) {
+        disconnect<Candidate>(value_or_instance);
+
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>(value_or_instance);
+        signal->calls.insert(signal->calls.end() - offset, std::move(call));
+
+        delegate<void(void *)> conn{};
+        conn.template connect<&release<Candidate, Type &>>(value_or_instance);
+        return { std::move(conn), signal };
+    }
+
+    /**
+     * @brief Connects a member function or a free function with payload to a
+     * signal.
+     *
+     * The signal isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of the instance overcomes
+     * the one  of the delegate. On the other side, the signal handler performs
+     * checks to avoid multiple connections for the same function.<br/>
+     * When used to connect a free function with payload, its signature must be
+     * such that the instance is the first argument before the ones used to
+     * define the delegate itself.
+     *
+     * @tparam Candidate Member or free function to connect to the signal.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid pointer that fits the purpose.
+     * @return A properly initialized connection object.
+     */
+    template<auto Candidate, typename Type>
+    connection connect(Type *value_or_instance) {
+        disconnect<Candidate>(value_or_instance);
+
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>(value_or_instance);
+        signal->calls.insert(signal->calls.end() - offset, std::move(call));
+
+        delegate<void(void *)> conn{};
+        conn.template connect<&release<Candidate, Type *>>(value_or_instance);
+        return { std::move(conn), signal };
+    }
+
+    /**
+     * @brief Disconnects a free function from a signal.
+     * @tparam Function A valid free function pointer.
+     */
+    template<auto Function>
+    void disconnect() {
+        auto &calls = signal->calls;
+        delegate<Ret(Args...)> call{};
+        call.template connect<Function>();
+        calls.erase(std::remove(calls.begin(), calls.end(), std::move(call)), calls.end());
+    }
+
+    /**
+     * @brief Disconnects a member function or a free function with payload from
+     * a signal.
+     * @tparam Candidate Member or free function to disconnect from the signal.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid reference that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void disconnect(Type &value_or_instance) {
+        auto &calls = signal->calls;
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>(value_or_instance);
+        calls.erase(std::remove(calls.begin(), calls.end(), std::move(call)), calls.end());
+    }
+
+    /**
+     * @brief Disconnects a member function or a free function with payload from
+     * a signal.
+     * @tparam Candidate Member or free function to disconnect from the signal.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid pointer that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void disconnect(Type *value_or_instance) {
+        auto &calls = signal->calls;
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>(value_or_instance);
+        calls.erase(std::remove(calls.begin(), calls.end(), std::move(call)), calls.end());
+    }
+
+    /**
+     * @brief Disconnects member functions or free functions based on an
+     * instance or specific payload.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<typename Type>
+    void disconnect(Type &value_or_instance) {
+        disconnect(&value_or_instance);
+    }
+
+    /**
+     * @brief Disconnects member functions or free functions based on an
+     * instance or specific payload.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid pointer that fits the purpose.
+     */
+    template<typename Type>
+    void disconnect(Type *value_or_instance) {
+        if(value_or_instance) {
+            auto &calls = signal->calls;
+            calls.erase(std::remove_if(calls.begin(), calls.end(), [value_or_instance](const auto &delegate) {
+                return delegate.instance() == value_or_instance;
+            }), calls.end());
+        }
+    }
+
+    /*! @brief Disconnects all the listeners from a signal. */
+    void disconnect() {
+        signal->calls.clear();
+    }
+
+private:
+    difference_type offset;
+    signal_type *signal;
+};
+
+
+/**
+ * @brief Deduction guide.
+ *
+ * It allows to deduce the function type of a sink directly from the signal it
+ * refers to.
+ *
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ */
+template<typename Ret, typename... Args>
+sink(sigh<Ret(Args...)> &) ENTT_NOEXCEPT -> sink<Ret(Args...)>;
 
 
 }

src/entt/signal/signal.hpp

@@ -1,227 +0,0 @@
-#ifndef ENTT_SIGNAL_SIGNAL_HPP
-#define ENTT_SIGNAL_SIGNAL_HPP
-
-
-#include <memory>
-#include <vector>
-#include <utility>
-#include <cstdint>
-#include <iterator>
-#include <algorithm>
-
-
-namespace entt {
-
-
-/**
- * @brief Managed 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.
- */
-template<typename>
-class Signal;
-
-
-/**
- * @brief Managed signal handler definition.
- *
- * Managed signal handler. It works with weak pointers to classes and pointers
- * to member functions as well as pointers to free functions. References are
- * automatically removed when the instances to which they point are freed.
- *
- * This class can be used to create signals used later to notify a bunch of
- * listeners.
- *
- * @tparam Args Types of arguments of a function type.
- */
-template<typename... Args>
-class Signal<void(Args...)> final {
-    using proto_type = bool(*)(std::weak_ptr<void> &, Args...);
-    using call_type = std::pair<std::weak_ptr<void>, proto_type>;
-
-    template<void(*Function)(Args...)>
-    static bool proto(std::weak_ptr<void> &, Args... args) {
-        Function(args...);
-        return true;
-    }
-
-    template<typename Class, void(Class::*Member)(Args...)>
-    static bool proto(std::weak_ptr<void> &wptr, Args... args) {
-        bool ret = false;
-
-        if(!wptr.expired()) {
-            auto ptr = std::static_pointer_cast<Class>(wptr.lock());
-            (ptr.get()->*Member)(args...);
-            ret = true;
-        }
-
-        return ret;
-    }
-
-public:
-    /*! @brief Unsigned integer type. */
-    using size_type = std::size_t;
-
-    /**
-     * @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 = std::shared_ptr<Class>;
-
-    /**
-     * @brief Number of listeners connected to the signal.
-     * @return Number of listeners currently connected.
-     */
-    size_type size() const 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 noexcept {
-        return calls.empty();
-    }
-
-    /**
-     * @brief Disconnects all the listeners from a signal.
-     */
-    void clear() noexcept {
-        calls.clear();
-    }
-
-    /**
-     * @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<void(*Function)(Args...)>
-    void connect() {
-        disconnect<Function>();
-        calls.emplace_back(std::weak_ptr<void>{}, &proto<Function>);
-    }
-
-    /**
-     * @brief Connects a member function for a given instance to a signal.
-     *
-     * 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, void(Class::*Member)(Args...)>
-    void connect(instance_type<Class> instance) {
-        disconnect<Class, Member>(instance);
-        calls.emplace_back(std::move(instance), &proto<Class, Member>);
-    }
-
-    /**
-     * @brief Disconnects a free function from a signal.
-     * @tparam Function A valid free function pointer.
-     */
-    template<void(*Function)(Args...)>
-    void disconnect() {
-        calls.erase(std::remove_if(calls.begin(), calls.end(),
-            [](const call_type &call) { return call.second == &proto<Function> && !call.first.lock(); }
-        ), 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, void(Class::*Member)(Args...)>
-    void disconnect(instance_type<Class> instance) {
-        calls.erase(std::remove_if(calls.begin(), calls.end(),
-            [instance{std::move(instance)}](const call_type &call) { return call.second == &proto<Class, Member> && call.first.lock() == instance; }
-        ), 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(instance_type<Class> instance) {
-        calls.erase(std::remove_if(calls.begin(), calls.end(),
-            [instance{std::move(instance)}](const call_type &call) { return call.first.lock() == instance; }
-        ), calls.end());
-    }
-
-    /**
-     * @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) {
-        for(auto it = calls.rbegin(), end = calls.rend(); it != end; it++) {
-            if(!(it->second)(it->first, args...)) {
-                calls.erase(std::next(it).base());
-            }
-        }
-    }
-
-    /**
-     * @brief Swaps listeners between the two signals.
-     * @param lhs A valid signal object.
-     * @param rhs A valid signal object.
-     */
-    friend void swap(Signal &lhs, Signal &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 Signal &other) const noexcept {
-        return std::equal(calls.cbegin(), calls.cend(), other.calls.cbegin(), other.calls.cend(), [](const auto &lhs, const auto &rhs) {
-            return (lhs.second == rhs.second) && (lhs.first.lock() == rhs.first.lock());
-        });
-    }
-
-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 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... Args>
-bool operator!=(const Signal<void(Args...)> &lhs, const Signal<void(Args...)> &rhs) noexcept {
-    return !(lhs == rhs);
-}
-
-
-}
-
-
-#endif // ENTT_SIGNAL_SIGNAL_HPP

test/CMakeLists.txt

@@ -2,89 +2,128 @@
 # Tests configuration
 #
 
-include_directories(${PROJECT_SRC_DIR})
+include_directories($<TARGET_PROPERTY:EnTT,INTERFACE_INCLUDE_DIRECTORIES>)
+add_compile_options($<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_OPTIONS>)
+
+macro(SETUP_LIBRARY_TARGET LIB_TARGET)
+    set_target_properties(${LIB_TARGET} PROPERTIES CXX_EXTENSIONS OFF)
+    target_compile_definitions(${LIB_TARGET} PRIVATE $<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_DEFINITIONS>)
+    target_compile_features(${LIB_TARGET} PRIVATE $<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_FEATURES>)
+    target_compile_options(${LIB_TARGET} PRIVATE $<$<NOT:$<CXX_COMPILER_ID:MSVC>>:-pedantic -Wall -Wshadow>)
+    target_compile_options(${LIB_TARGET} PRIVATE $<$<CXX_COMPILER_ID:MSVC>:/EHsc /W2>)
+endmacro()
 
 add_library(odr OBJECT odr.cpp)
+SETUP_LIBRARY_TARGET(odr)
+
+macro(SETUP_AND_ADD_TEST TEST_NAME TEST_SOURCE)
+    add_executable(${TEST_NAME} $<TARGET_OBJECTS:odr> ${TEST_SOURCE})
+    set_target_properties(${TEST_NAME} PROPERTIES CXX_EXTENSIONS OFF)
+    target_link_libraries(${TEST_NAME} PRIVATE EnTT GTest::Main Threads::Threads)
+    target_compile_definitions(${TEST_NAME} PRIVATE $<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_DEFINITIONS>)
+    target_compile_features(${TEST_NAME} PRIVATE $<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_FEATURES>)
+    target_compile_options(${TEST_NAME} PRIVATE $<$<NOT:$<CXX_COMPILER_ID:MSVC>>:-pedantic -Wall -Wshadow>)
+    target_compile_options(${TEST_NAME} PRIVATE $<$<CXX_COMPILER_ID:MSVC>:/EHsc /W2>)
+    add_test(NAME ${TEST_NAME} COMMAND ${TEST_NAME})
+endmacro()
 
 # Test benchmark
 
-if(CMAKE_BUILD_TYPE MATCHES Release)
-    add_executable(
-        benchmark
-        $<TARGET_OBJECTS:odr>
-        entt/entity/benchmark.cpp
-    )
-    target_link_libraries(benchmark PRIVATE gtest_main Threads::Threads)
-    add_test(NAME benchmark COMMAND benchmark)
+if(BUILD_BENCHMARK)
+    SETUP_AND_ADD_TEST(benchmark benchmark/benchmark.cpp)
+endif()
+
+# Test lib
+
+if(BUILD_LIB)
+    add_library(a_module_shared SHARED lib/a_module.cpp)
+    add_library(a_module_static STATIC lib/a_module.cpp)
+    add_library(another_module_shared SHARED lib/another_module.cpp)
+    add_library(another_module_static STATIC lib/another_module.cpp)
+
+    SETUP_LIBRARY_TARGET(a_module_shared)
+    SETUP_LIBRARY_TARGET(a_module_static)
+    SETUP_LIBRARY_TARGET(another_module_shared)
+    SETUP_LIBRARY_TARGET(another_module_static)
+
+    SETUP_AND_ADD_TEST(lib_shared lib/lib.cpp)
+    target_link_libraries(lib_shared PRIVATE a_module_shared another_module_shared)
+
+    SETUP_AND_ADD_TEST(lib_static lib/lib.cpp)
+    target_link_libraries(lib_static PRIVATE a_module_static another_module_static)
+endif()
+
+# Test mod
+
+if(BUILD_MOD)
+    set(DUKTAPE_DEPS_DIR ${EnTT_SOURCE_DIR}/deps/duktape)
+    configure_file(${EnTT_SOURCE_DIR}/cmake/in/duktape.in ${DUKTAPE_DEPS_DIR}/CMakeLists.txt)
+    execute_process(COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" . WORKING_DIRECTORY ${DUKTAPE_DEPS_DIR})
+    execute_process(COMMAND ${CMAKE_COMMAND} --build . WORKING_DIRECTORY ${DUKTAPE_DEPS_DIR})
+    set(DUKTAPE_SRC_DIR ${DUKTAPE_DEPS_DIR}/src/src)
+
+    set(MOD_TEST_SOURCE ${DUKTAPE_SRC_DIR}/duktape.c mod/mod.cpp)
+    SETUP_AND_ADD_TEST(mod "${MOD_TEST_SOURCE}")
+    target_include_directories(mod PRIVATE ${DUKTAPE_SRC_DIR})
+endif()
+
+# Test snapshot
+
+if(BUILD_SNAPSHOT)
+    set(CEREAL_DEPS_DIR ${EnTT_SOURCE_DIR}/deps/cereal)
+    configure_file(${EnTT_SOURCE_DIR}/cmake/in/cereal.in ${CEREAL_DEPS_DIR}/CMakeLists.txt)
+    execute_process(COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" . WORKING_DIRECTORY ${CEREAL_DEPS_DIR})
+    execute_process(COMMAND ${CMAKE_COMMAND} --build . WORKING_DIRECTORY ${CEREAL_DEPS_DIR})
+    set(CEREAL_SRC_DIR ${CEREAL_DEPS_DIR}/src/include)
+
+    SETUP_AND_ADD_TEST(cereal snapshot/snapshot.cpp)
+    target_include_directories(cereal PRIVATE ${CEREAL_SRC_DIR})
 endif()
 
 # Test core
 
-add_executable(
-    core
-    $<TARGET_OBJECTS:odr>
-    entt/core/family.cpp
-    entt/core/hashed_string.cpp
-    entt/core/ident.cpp
-)
-target_link_libraries(core PRIVATE gtest_main Threads::Threads)
-add_test(NAME core COMMAND core)
+SETUP_AND_ADD_TEST(algorithm entt/core/algorithm.cpp)
+SETUP_AND_ADD_TEST(family entt/core/family.cpp)
+SETUP_AND_ADD_TEST(hashed_string entt/core/hashed_string.cpp)
+SETUP_AND_ADD_TEST(ident entt/core/ident.cpp)
+SETUP_AND_ADD_TEST(monostate entt/core/monostate.cpp)
+SETUP_AND_ADD_TEST(type_traits entt/core/type_traits.cpp)
+SETUP_AND_ADD_TEST(utility entt/core/utility.cpp)
 
 # Test entity
 
-add_executable(
-    entity
-    $<TARGET_OBJECTS:odr>
-    entt/entity/actor.cpp
-    entt/entity/registry.cpp
-    entt/entity/sparse_set.cpp
-    entt/entity/view.cpp
-)
-target_link_libraries(entity PRIVATE gtest_main Threads::Threads)
-add_test(NAME entity COMMAND entity)
+SETUP_AND_ADD_TEST(actor entt/entity/actor.cpp)
+SETUP_AND_ADD_TEST(entity entt/entity/entity.cpp)
+SETUP_AND_ADD_TEST(group entt/entity/group.cpp)
+SETUP_AND_ADD_TEST(helper entt/entity/helper.cpp)
+SETUP_AND_ADD_TEST(observer entt/entity/observer.cpp)
+SETUP_AND_ADD_TEST(registry entt/entity/registry.cpp)
+SETUP_AND_ADD_TEST(runtime_view entt/entity/runtime_view.cpp)
+SETUP_AND_ADD_TEST(snapshot entt/entity/snapshot.cpp)
+SETUP_AND_ADD_TEST(sparse_set entt/entity/sparse_set.cpp)
+SETUP_AND_ADD_TEST(storage entt/entity/storage.cpp)
+SETUP_AND_ADD_TEST(view entt/entity/view.cpp)
 
 # Test locator
 
-add_executable(
-    locator
-    $<TARGET_OBJECTS:odr>
-    entt/locator/locator.cpp
-)
-target_link_libraries(locator PRIVATE gtest_main Threads::Threads)
-add_test(NAME locator COMMAND locator)
+SETUP_AND_ADD_TEST(locator entt/locator/locator.cpp)
+
+# Test meta
+
+SETUP_AND_ADD_TEST(meta entt/meta/meta.cpp)
 
 # Test process
 
-add_executable(
-    process
-    $<TARGET_OBJECTS:odr>
-    entt/process/process.cpp
-    entt/process/scheduler.cpp
-)
-target_link_libraries(process PRIVATE gtest_main Threads::Threads)
-add_test(NAME process COMMAND process)
+SETUP_AND_ADD_TEST(process entt/process/process.cpp)
+SETUP_AND_ADD_TEST(scheduler entt/process/scheduler.cpp)
 
 # Test resource
 
-add_executable(
-    resource
-    $<TARGET_OBJECTS:odr>
-    entt/resource/resource.cpp
-)
-target_link_libraries(resource PRIVATE gtest_main Threads::Threads)
-add_test(NAME resource COMMAND resource)
+SETUP_AND_ADD_TEST(resource entt/resource/resource.cpp)
 
 # Test signal
 
-add_executable(
-    signal
-    $<TARGET_OBJECTS:odr>
-    entt/signal/bus.cpp
-    entt/signal/delegate.cpp
-    entt/signal/dispatcher.cpp
-    entt/signal/emitter.cpp
-    entt/signal/sigh.cpp
-    entt/signal/signal.cpp
-)
-target_link_libraries(signal PRIVATE gtest_main Threads::Threads)
-add_test(NAME signal COMMAND signal)
+SETUP_AND_ADD_TEST(delegate entt/signal/delegate.cpp)
+SETUP_AND_ADD_TEST(dispatcher entt/signal/dispatcher.cpp)
+SETUP_AND_ADD_TEST(emitter entt/signal/emitter.cpp)
+SETUP_AND_ADD_TEST(sigh entt/signal/sigh.cpp)

test/benchmark/BUILD.bazel

@@ -0,0 +1,11 @@
+load("//bazel:copts.bzl", "entt_copts")
+
+cc_binary(
+    name = "benchmark",
+    srcs = ["benchmark.cpp"],
+    copts = entt_copts,
+    deps = [
+        "//:entt",
+        "@com_google_googletest//:gtest_main",
+    ],
+)

test/entt/core/BUILD.bazel

@@ -0,0 +1,71 @@
+load("//bazel:copts.bzl", "entt_copts")
+
+cc_test(
+    name = "algorithm",
+    srcs = ["algorithm.cpp"],
+    copts = entt_copts,
+    deps = [
+        "//:entt",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "family",
+    srcs = ["family.cpp"],
+    copts = entt_copts,
+    deps = [
+        "//:entt",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "hashed_string",
+    srcs = ["hashed_string.cpp"],
+    copts = entt_copts,
+    deps = [
+        "//:entt",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "ident",
+    srcs = ["ident.cpp"],
+    copts = entt_copts,
+    deps = [
+        "//:entt",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "monostate",
+    srcs = ["monostate.cpp"],
+    copts = entt_copts,
+    deps = [
+        "//:entt",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "type_traits",
+    srcs = ["type_traits.cpp"],
+    copts = entt_copts,
+    deps = [
+        "//:entt",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "utility",
+    srcs = ["utility.cpp"],
+    copts = entt_copts,
+    deps = [
+        "//:entt",
+        "@com_google_googletest//:gtest_main",
+    ],
+)

test/entt/core/algorithm.cpp

@@ -0,0 +1,97 @@
+#include <array>
+#include <gtest/gtest.h>
+#include <entt/core/algorithm.hpp>
+
+struct boxed_int {
+    int value;
+};
+
+TEST(Algorithm, StdSort) {
+    // well, I'm pretty sure it works, it's std::sort!!
+    std::array<int, 5> arr{{4, 1, 3, 2, 0}};
+    entt::std_sort sort;
+
+    sort(arr.begin(), arr.end());
+
+    for(typename decltype(arr)::size_type i = 0; i < (arr.size() - 1); ++i) {
+        ASSERT_LT(arr[i], arr[i+1]);
+    }
+}
+
+TEST(Algorithm, StdSortBoxedInt) {
+    // well, I'm pretty sure it works, it's std::sort!!
+    std::array<boxed_int, 6> arr{{{4}, {1}, {3}, {2}, {0}, {6}}};
+    entt::std_sort sort;
+
+    sort(arr.begin(), arr.end(), [](const auto &lhs, const auto &rhs) {
+        return lhs.value > rhs.value;
+    });
+
+    for(typename decltype(arr)::size_type i = 0; i < (arr.size() - 1); ++i) {
+        ASSERT_GT(arr[i].value, arr[i+1].value);
+    }
+}
+
+TEST(Algorithm, InsertionSort) {
+    std::array<int, 5> arr{{4, 1, 3, 2, 0}};
+    entt::insertion_sort sort;
+
+    sort(arr.begin(), arr.end());
+
+    for(typename decltype(arr)::size_type i = 0; i < (arr.size() - 1); ++i) {
+        ASSERT_LT(arr[i], arr[i+1]);
+    }
+}
+
+TEST(Algorithm, InsertionSortBoxedInt) {
+    std::array<boxed_int, 6> arr{{{4}, {1}, {3}, {2}, {0}, {6}}};
+    entt::insertion_sort sort;
+
+    sort(arr.begin(), arr.end(), [](const auto &lhs, const auto &rhs) {
+        return lhs.value > rhs.value;
+    });
+
+    for(typename decltype(arr)::size_type i = 0; i < (arr.size() - 1); ++i) {
+        ASSERT_GT(arr[i].value, arr[i+1].value);
+    }
+}
+
+TEST(Algorithm, InsertionSortEmptyContainer) {
+    std::vector<int> vec{};
+    entt::insertion_sort sort;
+    // this should crash with asan enabled if we break the constraint
+    sort(vec.begin(), vec.end());
+}
+
+TEST(Algorithm, RadixSort) {
+    std::array<uint32_t, 5> arr{{4, 1, 3, 2, 0}};
+    entt::radix_sort<8, 32> sort;
+
+    sort(arr.begin(), arr.end(), [](const auto &value) {
+        return value;
+    });
+
+    for(typename decltype(arr)::size_type i = 0; i < (arr.size() - 1); ++i) {
+        ASSERT_LT(arr[i], arr[i+1]);
+    }
+}
+
+TEST(Algorithm, RadixSortBoxedInt) {
+    std::array<boxed_int, 6> arr{{{4}, {1}, {3}, {2}, {0}, {6}}};
+    entt::radix_sort<2, 6> sort;
+
+    sort(arr.rbegin(), arr.rend(), [](const auto &instance) {
+        return instance.value;
+    });
+
+    for(typename decltype(arr)::size_type i = 0; i < (arr.size() - 1); ++i) {
+        ASSERT_GT(arr[i].value, arr[i+1].value);
+    }
+}
+
+TEST(Algorithm, RadixSortEmptyContainer) {
+    std::vector<int> vec{};
+    entt::radix_sort<8, 32> sort;
+    // this should crash with asan enabled if we break the constraint
+    sort(vec.begin(), vec.end());
+}

test/entt/core/family.cpp

@@ -1,22 +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>;
+using a_family = entt::family<struct a_family_type>;
+using another_family = entt::family<struct another_family_type>;
 
 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>();
+    auto t1 = a_family::type<int>;
+    auto t2 = a_family::type<int>;
+    auto t3 = a_family::type<char>;
+    auto t4 = another_family::type<double>;
 
-    ASSERT_EQ(myFamilyType, mySameFamilyType);
-    ASSERT_NE(myFamilyType, myOtherFamilyType);
-    ASSERT_EQ(myFamilyType, yourFamilyType);
+    ASSERT_EQ(t1, t2);
+    ASSERT_NE(t1, t3);
+    ASSERT_EQ(t1, t4);
 }
 
 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 &>());
+    ASSERT_NE(a_family::type<int>, a_family::type<int &>);
+    ASSERT_NE(a_family::type<int>, a_family::type<int &&>);
+    ASSERT_NE(a_family::type<int>, a_family::type<const int &>);
 }

test/entt/core/hashed_string.cpp

@@ -1,37 +1,128 @@
+#include <string>
+#include <string_view>
+#include <type_traits>
 #include <gtest/gtest.h>
 #include <entt/core/hashed_string.hpp>
 
-constexpr bool check(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}));
-}
-
-TEST(HashedString, Constexprness) {
-    // how would you test a constepxr otherwise?
-    static_assert(check("foobar"), "!");
-    ASSERT_TRUE(true);
-}
-
 TEST(HashedString, Functionalities) {
-    using hash_type = entt::HashedString::hash_type;
+    using hash_type = entt::hashed_string::hash_type;
 
     const char *bar = "bar";
 
-    auto fooHs = entt::HashedString("foo");
-    auto barHs = entt::HashedString(bar);
+    auto foo_hs = entt::hashed_string{"foo"};
+    auto bar_hs = entt::hashed_string{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_NE(static_cast<hash_type>(foo_hs), static_cast<hash_type>(bar_hs));
+    ASSERT_STREQ(static_cast<const char *>(foo_hs), "foo");
+    ASSERT_STREQ(static_cast<const char *>(bar_hs), bar);
+    ASSERT_STREQ(foo_hs.data(), "foo");
+    ASSERT_STREQ(bar_hs.data(), bar);
 
-    ASSERT_TRUE(fooHs == fooHs);
-    ASSERT_FALSE(fooHs == barHs);
+    ASSERT_EQ(foo_hs, foo_hs);
+    ASSERT_NE(foo_hs, bar_hs);
 
-    entt::HashedString hs{"foobar"};
+    entt::hashed_string hs{"foobar"};
 
-    ASSERT_EQ(static_cast<hash_type>(hs), 0x85944171f73967e8);
+    ASSERT_EQ(static_cast<hash_type>(hs), 0xbf9cf968);
+    ASSERT_EQ(hs.value(), 0xbf9cf968);
+
+    ASSERT_EQ(foo_hs, "foo"_hs);
+    ASSERT_NE(bar_hs, "foo"_hs);
+}
+
+TEST(HashedString, Empty) {
+    using hash_type = entt::hashed_string::hash_type;
+
+    entt::hashed_string hs{};
+
+    ASSERT_EQ(static_cast<hash_type>(hs), hash_type{});
+    ASSERT_EQ(static_cast<const char *>(hs), nullptr);
+}
+
+TEST(HashedString, Constexprness) {
+    using hash_type = entt::hashed_string::hash_type;
+    // how would you test a constexpr otherwise?
+    (void)std::integral_constant<hash_type, entt::hashed_string{"quux"}>{};
+    (void)std::integral_constant<hash_type, "quux"_hs>{};
+    ASSERT_TRUE(true);
+}
+
+TEST(HashedString, ToValue) {
+    using hash_type = entt::hashed_string::hash_type;
+
+    const char *foobar = "foobar";
+
+    ASSERT_EQ(entt::hashed_string::to_value(foobar), 0xbf9cf968);
+    // how would you test a constexpr otherwise?
+    (void)std::integral_constant<hash_type, entt::hashed_string::to_value("quux")>{};
+}
+
+TEST(HashedString, StringView) {
+    std::string str{"__foobar__"};
+    std::string_view view{str.data()+2, 6};
+    ASSERT_EQ(entt::hashed_string::to_value(view.data(), view.size()), 0xbf9cf968);
+}
+
+TEST(HashedWString, Functionalities) {
+    using hash_type = entt::hashed_wstring::hash_type;
+
+    const wchar_t *bar = L"bar";
+
+    auto foo_hws = entt::hashed_wstring{L"foo"};
+    auto bar_hws = entt::hashed_wstring{bar};
+
+    ASSERT_NE(static_cast<hash_type>(foo_hws), static_cast<hash_type>(bar_hws));
+    ASSERT_STREQ(static_cast<const wchar_t *>(foo_hws), L"foo");
+    ASSERT_STREQ(static_cast<const wchar_t *>(bar_hws), bar);
+    ASSERT_STREQ(foo_hws.data(), L"foo");
+    ASSERT_STREQ(bar_hws.data(), bar);
+
+    ASSERT_EQ(foo_hws, foo_hws);
+    ASSERT_NE(foo_hws, bar_hws);
+
+    entt::hashed_wstring hws{L"foobar"};
+
+    ASSERT_EQ(static_cast<hash_type>(hws), 0xbf9cf968);
+    ASSERT_EQ(hws.value(), 0xbf9cf968);
+
+    ASSERT_EQ(foo_hws, L"foo"_hws);
+    ASSERT_NE(bar_hws, L"foo"_hws);
+}
+
+TEST(HashedWString, Empty) {
+    using hash_type = entt::hashed_wstring::hash_type;
+
+    entt::hashed_wstring hws{};
+
+    ASSERT_EQ(static_cast<hash_type>(hws), hash_type{});
+    ASSERT_EQ(static_cast<const wchar_t *>(hws), nullptr);
+}
+
+TEST(HashedWString, Constexprness) {
+    using hash_type = entt::hashed_wstring::hash_type;
+    // how would you test a constexpr otherwise?
+    (void)std::integral_constant<hash_type, entt::hashed_wstring{L"quux"}>{};
+    (void)std::integral_constant<hash_type, L"quux"_hws>{};
+    ASSERT_TRUE(true);
+}
+
+TEST(HashedWString, ToValue) {
+    using hash_type = entt::hashed_wstring::hash_type;
+
+    const wchar_t *foobar = L"foobar";
+
+    ASSERT_EQ(entt::hashed_wstring::to_value(foobar), 0xbf9cf968);
+    // how would you test a constexpr otherwise?
+    (void)std::integral_constant<hash_type, entt::hashed_wstring::to_value(L"quux")>{};
+}
+
+TEST(HashedWString, StringView) {
+    std::wstring str{L"__foobar__"};
+    std::wstring_view view{str.data()+2, 6};
+    ASSERT_EQ(entt::hashed_wstring::to_value(view.data(), view.size()), 0xbf9cf968);
+}
+
+TEST(BasicHashedString, DeductionGuide) {
+    static_assert(std::is_same_v<decltype(entt::basic_hashed_string{"foo"}), entt::hashed_string>);
+    static_assert(std::is_same_v<decltype(entt::basic_hashed_string{L"foo"}), entt::hashed_wstring>);
 }

test/entt/core/ident.cpp

@@ -2,32 +2,31 @@
 #include <gtest/gtest.h>
 #include <entt/core/ident.hpp>
 
-struct A {};
-struct B {};
+struct a_type {};
+struct another_type {};
 
 TEST(Identifier, Uniqueness) {
-    constexpr auto ID = entt::ident<A, B>;
-    constexpr A a;
-    constexpr B b;
+    using id = entt::identifier<a_type, another_type>;
+    constexpr a_type an_instance;
+    constexpr another_type another_instance;
 
-    ASSERT_NE(ID.get<A>(), ID.get<B>());
-    ASSERT_EQ(ID.get<A>(), ID.get<decltype(a)>());
-    ASSERT_NE(ID.get<A>(), ID.get<decltype(b)>());
-    ASSERT_EQ(ID.get<A>(), ID.get<A>());
-    ASSERT_EQ(ID.get<B>(), ID.get<B>());
+    ASSERT_NE(id::type<a_type>, id::type<another_type>);
+    ASSERT_EQ(id::type<a_type>, id::type<decltype(an_instance)>);
+    ASSERT_NE(id::type<a_type>, id::type<decltype(another_instance)>);
+    ASSERT_EQ(id::type<a_type>, id::type<a_type>);
+    ASSERT_EQ(id::type<another_type>, id::type<another_type>);
 
     // test uses in constant expressions
-    switch(ID.get<B>()) {
-    case ID.get<A>():
+    switch(id::type<another_type>) {
+    case id::type<a_type>:
         FAIL();
-        break;
-    case ID.get<B>():
+    case id::type<another_type>:
         SUCCEED();
     }
 }
 
 TEST(Identifier, SingleType) {
-    constexpr auto ID = entt::ident<A>;
-    std::integral_constant<decltype(ID)::identifier_type, ID.get()> ic;
+    using id = entt::identifier<a_type>;
+    std::integral_constant<id::identifier_type, id::type<a_type>> ic;
     (void)ic;
 }