build system: suppress a few other warnings here and there

very useful in CI/CD pipelines

.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 +1,75 @@
+name: build
+
+on: [push, pull_request]
+
+jobs:
+
+  linux:
+    timeout-minutes: 10
+
+    strategy:
+      matrix:
+        compiler: [g++, clang++]
+
+    runs-on: ubuntu-latest
+
+    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: 10
+
+    strategy:
+      matrix:
+        os: [windows-latest, windows-2016]
+        toolset: [clang-cl, default]
+        include:
+          - toolset: clang-cl
+            toolset_option: -T"ClangCl"
+        exclude:
+          - os: windows-2016
+            toolset: clang-cl
+
+    runs-on: ${{ matrix.os }}
+
+    steps:
+    - uses: actions/checkout@v1
+    - name: Compile tests
+      working-directory: build
+      run: |
+        cmake -DBUILD_TESTING=ON -DBUILD_LIB=ON ${{ matrix.toolset_option }} ..
+        cmake --build . -j 4
+    - name: Run tests
+      working-directory: build
+      env:
+        CTEST_OUTPUT_ON_FAILURE: 1
+      run: ctest --timeout 5 -C Debug -j4
+
+  macos:
+    timeout-minutes: 10
+    runs-on: macOS-latest
+
+    steps:
+    - uses: actions/checkout@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: 10
+    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/

.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 +1,44 @@
 # 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
+stefanofiorentino
+suVrik
+szunhammer
+The5-1
+vblanco20-1
+willtunnels
+WizardIke
+w1th0utnam3
+xissburg
+zaucy

CMakeLists.txt

@@ -2,7 +2,7 @@
 # EnTT
 #
 
-cmake_minimum_required(VERSION 3.2)
+cmake_minimum_required(VERSION 3.12.4)
 
 #
 # Building in-tree is not allowed (we take care of your craziness).
@@ -12,75 +12,164 @@ if(${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_BINARY_DIR})
     message(FATAL_ERROR "Prevented in-tree built. Please create a build directory outside of the source code and call cmake from there. Thank you.")
 endif()
 
+#
+# Read project version
+#
+
+set(ENTT_VERSION_REGEX "#define ENTT_VERSION_.*[ \t]+(.+)")
+file(STRINGS "${CMAKE_CURRENT_SOURCE_DIR}/src/entt/config/version.h" ENTT_VERSION REGEX ${ENTT_VERSION_REGEX})
+list(TRANSFORM ENTT_VERSION REPLACE ${ENTT_VERSION_REGEX} "\\1")
+string(JOIN "." ENTT_VERSION ${ENTT_VERSION})
+
 #
 # Project configuration
 #
 
-project(entt VERSION 2.2.0)
+project(
+    EnTT
+    VERSION ${ENTT_VERSION}
+    DESCRIPTION "Gaming meets modern C++ - a fast and reliable entity-component system (ECS) and much more"
+    HOMEPAGE_URL "https://github.com/skypjack/entt"
+    LANGUAGES CXX
+)
 
 if(NOT CMAKE_BUILD_TYPE)
     set(CMAKE_BUILD_TYPE Debug)
 endif()
 
-set(SETTINGS_ORGANIZATION "Michele Caini")
-set(SETTINGS_APPLICATION ${PROJECT_NAME})
-set(PROJECT_AUTHOR "Michele Caini")
-set(PROJECT_AUTHOR_EMAIL "michele.caini@gmail.com")
-
 message("*")
 message("* ${PROJECT_NAME} v${PROJECT_VERSION} (${CMAKE_BUILD_TYPE})")
-message("* Copyright (c) 2017 ${PROJECT_AUTHOR} <${PROJECT_AUTHOR_EMAIL}>")
+message("* Copyright (c) 2017-2020 Michele Caini <michele.caini@gmail.com>")
 message("*")
 
+option(USE_LIBCPP "Use libc++ by adding -stdlib=libc++ flag if availbale." ON)
+option(USE_ASAN "Use address sanitizer by adding -fsanitize=address -fno-omit-frame-pointer flags" OFF)
+
 #
 # 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)
+include(GNUInstallDirs)
 
-set(PROJECT_RUNTIME_OUTPUT_DIRECTORY bin)
+add_library(EnTT INTERFACE)
+add_library(EnTT::EnTT ALIAS EnTT)
+
+target_include_directories(
+    EnTT
+    INTERFACE
+        $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/src>
+        $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
+)
+
+target_compile_definitions(
+    EnTT
+    INTERFACE
+        $<$<AND:$<CONFIG:Debug>,$<NOT:$<CXX_COMPILER_ID:MSVC>>>:DEBUG>
+        $<$<AND:$<CONFIG:Release>,$<NOT:$<CXX_COMPILER_ID:MSVC>>>:RELEASE>
+)
+
+if(USE_ASAN)
+    target_compile_options(EnTT INTERFACE $<$<CONFIG:Debug>:-fsanitize=address -fno-omit-frame-pointer>)
+    target_link_libraries(EnTT INTERFACE $<$<CONFIG:Debug>:-fsanitize=address -fno-omit-frame-pointer>)
+endif()
+
+if(HAS_LIBCPP)
+    target_compile_options(EnTT BEFORE INTERFACE -stdlib=libc++)
+endif()
+
+target_compile_features(EnTT INTERFACE cxx_std_17)
+
+#
+# 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)
+    option(BUILD_BENCHMARK "Build benchmark." OFF)
+    option(BUILD_LIB "Build lib example." OFF)
+    option(BUILD_SNAPSHOT "Build snapshot example." OFF)
 
     enable_testing()
     add_subdirectory(test)
@@ -90,8 +179,30 @@ 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
+#
+
+set(
+    AOB_SOURCES
+    .github/workflows/build.yml
+    .github/workflows/coverage.yml
+    .github/FUNDING.yml
+    AUTHORS
+    CONTRIBUTING.md
+    LICENSE
+    README.md
+    TODO
+)
+
+add_custom_target(aob SOURCES ${AOB_SOURCES})

CONTRIBUTING.md

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

LICENSE

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

TODO

@@ -0,0 +1,28 @@
+* long term feature: shared_ptr less locator and resource cache
+* custom allocators and EnTT allocator-aware in general (long term feature, I don't actually need it at the moment) - see #22
+* debugging tools (#60): the issue online already contains interesting tips on this, look at it
+* work stealing job system (see #100) + mt scheduler based on const awareness for types
+* meta: sort of meta view based on meta stuff to iterate entities, void * and meta info objects (remove runtime views, welcome reflection)
+* add opaque input iterators to views and groups that return tuples <entity, T &...> (proxy), multi-pass guaranteed
+* allow to replace std:: with custom implementations
+* custom (decoupled) pools ==> N-buffering, shared components, multi-model, hibitsets, and so on
+* add examples (and credits) from @alanjfs :)
+* static reflection, hint: template<> meta_type_t<Type>: meta_descriptor<name, func..., props..., etc...> (see #342)
+* observer: user defined filters (eg .replace<T, &function> or .group<T, U, &func>)
+* use underlying_type as entity type within pools and registry? it would make different registries work together flawlessy
+* can we write a bool conv func for entt::entity that silently compares it to null?
+* reset... reset everywhere...
+* is it possible to make 0 the entity null?
+* document undocumented parts (entt::overload and a few others)
+* any-of rule for views/groups (eg entity has A and any of B/C/D)
+  - get -> all, exclude -> none
+* review prepare after clone and the others have been removed
+* remove copy-ctor/op from sparse set and storage (it doesn't make much sense)
+
+* WIP:
+ - deprecate snapshot, loader, ...
+ - provide documentation to describe alternatives
+
+* WIP: snapshot rework/deprecation
+ - remove snapshot/loader from registry, make them external (faster) tools
+ - deprecate snapshot classes, update documentation to describe alternatives

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

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/googletest.in

@@ -1,19 +0,0 @@
-project(googletest-download NONE)
-cmake_minimum_required(VERSION 3.2)
-
-include(ExternalProject)
-
-ExternalProject_Add(
-    googletest
-    GIT_REPOSITORY https://github.com/google/googletest.git
-    GIT_TAG release-1.8.0
-    DOWNLOAD_DIR ${GOOGLETEST_DEPS_DIR}
-    TMP_DIR ${GOOGLETEST_DEPS_DIR}/tmp
-    STAMP_DIR ${GOOGLETEST_DEPS_DIR}/stamp
-    SOURCE_DIR ${GOOGLETEST_DEPS_DIR}/src
-    BINARY_DIR ${GOOGLETEST_DEPS_DIR}/build
-    CONFIGURE_COMMAND ""
-    BUILD_COMMAND ""
-    INSTALL_COMMAND ""
-    TEST_COMMAND ""
-)

conan/build.py

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

conan/ci/build.sh

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

conan/ci/install.sh

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

conan/test_package/CMakeLists.txt

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

conan/test_package/conanfile.py

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

conan/test_package/test_package.cpp

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

deps/.gitignore

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

docs/CMakeLists.txt

@@ -2,23 +2,35 @@
 # Doxygen configuration (documentation)
 #
 
-set(TARGET_DOCS docs)
-
-set(DOXY_IN_FILE doxy.in)
-
-set(DOXY_SOURCE_DIRECTORY ${PROJECT_SRC_DIR})
+set(DOXY_DEPS_DIRECTORY ${EnTT_SOURCE_DIR}/deps)
+set(DOXY_SOURCE_DIRECTORY ${EnTT_SOURCE_DIR}/src)
 set(DOXY_DOCS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})
 set(DOXY_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
-set(DOXY_CFG_FILE doxy.cfg)
 
-configure_file(${DOXY_IN_FILE} ${DOXY_CFG_FILE} @ONLY)
+configure_file(doxy.in doxy.cfg @ONLY)
+
+set(
+    DOC_SOURCES
+    dox/extra.dox
+    md/core.md
+    md/entity.md
+    md/faq.md
+    md/lib.md
+    md/links.md
+    md/locator.md
+    md/meta.md
+    md/process.md
+    md/resource.md
+    md/signal.md
+    doxy.in
+)
 
 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 ${DOC_SOURCES}
 )
 
 install(

docs/LICENSE

@@ -1,395 +0,0 @@
-Attribution 4.0 International
-
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-
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docs/doxy.in

@@ -1,4 +1,4 @@
-# Doxyfile 1.8.13
+# Doxyfile 1.8.16
 
 # This file describes the settings to be used by the documentation system
 # doxygen (www.doxygen.org) for a project.
@@ -17,11 +17,11 @@
 # Project related configuration options
 #---------------------------------------------------------------------------
 
-# This tag specifies the encoding used for all characters in the config file
-# that follow. The default is UTF-8 which is also the encoding used for all text
-# before the first occurrence of this tag. Doxygen uses libiconv (or the iconv
-# built into libc) for the transcoding. See http://www.gnu.org/software/libiconv
-# for the list of possible encodings.
+# This tag specifies the encoding used for all characters in the configuration
+# file that follow. The default is UTF-8 which is also the encoding used for all
+# text before the first occurrence of this tag. Doxygen uses libiconv (or the
+# iconv built into libc) for the transcoding. See
+# https://www.gnu.org/software/libiconv/ for the list of possible encodings.
 # The default value is: UTF-8.
 
 DOXYFILE_ENCODING      = UTF-8
@@ -93,6 +93,14 @@ ALLOW_UNICODE_NAMES    = NO
 
 OUTPUT_LANGUAGE        = English
 
+# The OUTPUT_TEXT_DIRECTION tag is used to specify the direction in which all
+# documentation generated by doxygen is written. Doxygen will use this
+# information to generate all generated output in the proper direction.
+# Possible values are: None, LTR, RTL and Context.
+# The default value is: None.
+
+OUTPUT_TEXT_DIRECTION  = None
+
 # If the BRIEF_MEMBER_DESC tag is set to YES, doxygen will include brief member
 # descriptions after the members that are listed in the file and class
 # documentation (similar to Javadoc). Set to NO to disable this.
@@ -179,6 +187,16 @@ SHORT_NAMES            = NO
 
 JAVADOC_AUTOBRIEF      = NO
 
+# If the JAVADOC_BANNER tag is set to YES then doxygen will interpret a line
+# such as
+# /***************
+# as being the beginning of a Javadoc-style comment "banner". If set to NO, the
+# Javadoc-style will behave just like regular comments and it will not be
+# interpreted by doxygen.
+# The default value is: NO.
+
+JAVADOC_BANNER         = NO
+
 # If the QT_AUTOBRIEF tag is set to YES then doxygen will interpret the first
 # line (until the first dot) of a Qt-style comment as the brief description. If
 # set to NO, the Qt-style will behave just like regular Qt-style comments (thus
@@ -226,7 +244,12 @@ TAB_SIZE               = 4
 # will allow you to put the command \sideeffect (or @sideeffect) in the
 # documentation, which will result in a user-defined paragraph with heading
 # "Side Effects:". You can put \n's in the value part of an alias to insert
-# newlines.
+# newlines (in the resulting output). You can put ^^ in the value part of an
+# alias to insert a newline as if a physical newline was in the original file.
+# When you need a literal { or } or , in the value part of an alias you have to
+# escape them by means of a backslash (\), this can lead to conflicts with the
+# commands \{ and \} for these it is advised to use the version @{ and @} or use
+# a double escape (\\{ and \\})
 
 ALIASES                =
 
@@ -264,17 +287,26 @@ OPTIMIZE_FOR_FORTRAN   = NO
 
 OPTIMIZE_OUTPUT_VHDL   = NO
 
+# Set the OPTIMIZE_OUTPUT_SLICE tag to YES if your project consists of Slice
+# sources only. Doxygen will then generate output that is more tailored for that
+# language. For instance, namespaces will be presented as modules, types will be
+# separated into more groups, etc.
+# The default value is: NO.
+
+OPTIMIZE_OUTPUT_SLICE  = NO
+
 # Doxygen selects the parser to use depending on the extension of the files it
 # parses. With this tag you can assign which parser to use for a given
 # extension. Doxygen has a built-in mapping, but you can override or extend it
 # using this tag. The format is ext=language, where ext is a file extension, and
 # language is one of the parsers supported by doxygen: IDL, Java, Javascript,
-# C#, C, C++, D, PHP, Objective-C, Python, Fortran (fixed format Fortran:
-# FortranFixed, free formatted Fortran: FortranFree, unknown formatted Fortran:
-# Fortran. In the later case the parser tries to guess whether the code is fixed
-# or free formatted code, this is the default for Fortran type files), VHDL. For
-# instance to make doxygen treat .inc files as Fortran files (default is PHP),
-# and .f files as C (default is Fortran), use: inc=Fortran f=C.
+# Csharp (C#), C, C++, D, PHP, md (Markdown), Objective-C, Python, Slice,
+# Fortran (fixed format Fortran: FortranFixed, free formatted Fortran:
+# FortranFree, unknown formatted Fortran: Fortran. In the later case the parser
+# tries to guess whether the code is fixed or free formatted code, this is the
+# default for Fortran type files), VHDL, tcl. For instance to make doxygen treat
+# .inc files as Fortran files (default is PHP), and .f files as C (default is
+# Fortran), use: inc=Fortran f=C.
 #
 # Note: For files without extension you can use no_extension as a placeholder.
 #
@@ -285,7 +317,7 @@ EXTENSION_MAPPING      =
 
 # If the MARKDOWN_SUPPORT tag is enabled then doxygen pre-processes all comments
 # according to the Markdown format, which allows for more readable
-# documentation. See http://daringfireball.net/projects/markdown/ for details.
+# documentation. See https://daringfireball.net/projects/markdown/ for details.
 # The output of markdown processing is further processed by doxygen, so you can
 # mix doxygen, HTML, and XML commands with Markdown formatting. Disable only in
 # case of backward compatibilities issues.
@@ -297,10 +329,10 @@ MARKDOWN_SUPPORT       = YES
 # to that level are automatically included in the table of contents, even if
 # they do not have an id attribute.
 # Note: This feature currently applies only to Markdown headings.
-# Minimum value: 0, maximum value: 99, default value: 0.
+# Minimum value: 0, maximum value: 99, default value: 5.
 # This tag requires that the tag MARKDOWN_SUPPORT is set to YES.
 
-TOC_INCLUDE_HEADINGS   = 4
+TOC_INCLUDE_HEADINGS   = 5
 
 # When enabled doxygen tries to link words that correspond to documented
 # classes, or namespaces to their corresponding documentation. Such a link can
@@ -327,7 +359,7 @@ BUILTIN_STL_SUPPORT    = NO
 CPP_CLI_SUPPORT        = NO
 
 # Set the SIP_SUPPORT tag to YES if your project consists of sip (see:
-# http://www.riverbankcomputing.co.uk/software/sip/intro) sources only. Doxygen
+# https://www.riverbankcomputing.com/software/sip/intro) sources only. Doxygen
 # will parse them like normal C++ but will assume all classes use public instead
 # of private inheritance when no explicit protection keyword is present.
 # The default value is: NO.
@@ -433,6 +465,12 @@ EXTRACT_ALL            = NO
 
 EXTRACT_PRIVATE        = NO
 
+# If the EXTRACT_PRIV_VIRTUAL tag is set to YES, documented private virtual
+# methods of a class will be included in the documentation.
+# The default value is: NO.
+
+EXTRACT_PRIV_VIRTUAL   = NO
+
 # If the EXTRACT_PACKAGE tag is set to YES, all members with package or internal
 # scope will be included in the documentation.
 # The default value is: NO.
@@ -511,7 +549,7 @@ INTERNAL_DOCS          = NO
 # names in lower-case letters. If set to YES, upper-case letters are also
 # allowed. This is useful if you have classes or files whose names only differ
 # in case and if your file system supports case sensitive file names. Windows
-# and Mac users are advised to set this option to NO.
+# (including Cygwin) ands Mac users are advised to set this option to NO.
 # The default value is: system dependent.
 
 CASE_SENSE_NAMES       = YES
@@ -698,7 +736,7 @@ LAYOUT_FILE            =
 # The CITE_BIB_FILES tag can be used to specify one or more bib files containing
 # the reference definitions. This must be a list of .bib files. The .bib
 # extension is automatically appended if omitted. This requires the bibtex tool
-# to be installed. See also http://en.wikipedia.org/wiki/BibTeX for more info.
+# to be installed. See also https://en.wikipedia.org/wiki/BibTeX for more info.
 # For LaTeX the style of the bibliography can be controlled using
 # LATEX_BIB_STYLE. To use this feature you need bibtex and perl available in the
 # search path. See also \cite for info how to create references.
@@ -743,7 +781,8 @@ WARN_IF_DOC_ERROR      = YES
 # This WARN_NO_PARAMDOC option can be enabled to get warnings for functions that
 # are documented, but have no documentation for their parameters or return
 # value. If set to NO, doxygen will only warn about wrong or incomplete
-# parameter documentation, but not about the absence of documentation.
+# parameter documentation, but not about the absence of documentation. If
+# EXTRACT_ALL is set to YES then this flag will automatically be disabled.
 # The default value is: NO.
 
 WARN_NO_PARAMDOC       = YES
@@ -780,12 +819,14 @@ 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
 # libiconv (or the iconv built into libc) for the transcoding. See the libiconv
-# documentation (see: http://www.gnu.org/software/libiconv) for the list of
+# documentation (see: https://www.gnu.org/software/libiconv/) for the list of
 # possible encodings.
 # The default value is: UTF-8.
 
@@ -803,11 +844,9 @@ INPUT_ENCODING         = UTF-8
 # *.c++, *.java, *.ii, *.ixx, *.ipp, *.i++, *.inl, *.idl, *.ddl, *.odl, *.h,
 # *.hh, *.hxx, *.hpp, *.h++, *.cs, *.d, *.php, *.php4, *.php5, *.phtml, *.inc,
 # *.m, *.markdown, *.md, *.mm, *.dox, *.py, *.pyw, *.f90, *.f95, *.f03, *.f08,
-# *.f, *.for, *.tcl, *.vhd, *.vhdl, *.ucf and *.qsf.
+# *.f, *.for, *.tcl, *.vhd, *.vhdl, *.ucf, *.qsf and *.ice.
 
-FILE_PATTERNS          = *.cpp \
-                         *.c++ \
-                         *.h \
+FILE_PATTERNS          = *.h \
                          *.hpp \
                          *.md \
                          *.dox
@@ -825,7 +864,7 @@ RECURSIVE              = YES
 # Note that relative paths are relative to the directory from which doxygen is
 # run.
 
-EXCLUDE                =
+EXCLUDE                = @DOXY_DEPS_DIRECTORY@
 
 # The EXCLUDE_SYMLINKS tag can be used to select whether or not files or
 # directories that are symbolic links (a Unix file system feature) are excluded
@@ -963,7 +1002,7 @@ INLINE_SOURCES         = NO
 STRIP_CODE_COMMENTS    = YES
 
 # If the REFERENCED_BY_RELATION tag is set to YES then for each documented
-# function all documented functions referencing it will be listed.
+# entity all documented functions referencing it will be listed.
 # The default value is: NO.
 
 REFERENCED_BY_RELATION = NO
@@ -995,12 +1034,12 @@ SOURCE_TOOLTIPS        = YES
 # If the USE_HTAGS tag is set to YES then the references to source code will
 # point to the HTML generated by the htags(1) tool instead of doxygen built-in
 # source browser. The htags tool is part of GNU's global source tagging system
-# (see http://www.gnu.org/software/global/global.html). You will need version
+# (see https://www.gnu.org/software/global/global.html). You will need version
 # 4.8.6 or higher.
 #
 # To use it do the following:
 # - Install the latest version of global
-# - Enable SOURCE_BROWSER and USE_HTAGS in the config file
+# - Enable SOURCE_BROWSER and USE_HTAGS in the configuration file
 # - Make sure the INPUT points to the root of the source tree
 # - Run doxygen as normal
 #
@@ -1028,7 +1067,7 @@ VERBATIM_HEADERS       = YES
 # rich C++ code for which doxygen's built-in parser lacks the necessary type
 # information.
 # Note: The availability of this option depends on whether or not doxygen was
-# generated with the -Duse-libclang=ON option for CMake.
+# generated with the -Duse_libclang=ON option for CMake.
 # The default value is: NO.
 
 CLANG_ASSISTED_PARSING = NO
@@ -1041,6 +1080,16 @@ CLANG_ASSISTED_PARSING = NO
 
 CLANG_OPTIONS          =
 
+# If clang assisted parsing is enabled you can provide the clang parser with the
+# path to the compilation database (see:
+# http://clang.llvm.org/docs/HowToSetupToolingForLLVM.html) used when the files
+# were built. This is equivalent to specifying the "-p" option to a clang tool,
+# such as clang-check. These options will then be passed to the parser.
+# Note: The availability of this option depends on whether or not doxygen was
+# generated with the -Duse_libclang=ON option for CMake.
+
+CLANG_DATABASE_PATH    =
+
 #---------------------------------------------------------------------------
 # Configuration options related to the alphabetical class index
 #---------------------------------------------------------------------------
@@ -1159,7 +1208,7 @@ HTML_EXTRA_FILES       =
 # The HTML_COLORSTYLE_HUE tag controls the color of the HTML output. Doxygen
 # will adjust the colors in the style sheet and background images according to
 # this color. Hue is specified as an angle on a colorwheel, see
-# http://en.wikipedia.org/wiki/Hue for more information. For instance the value
+# https://en.wikipedia.org/wiki/Hue for more information. For instance the value
 # 0 represents red, 60 is yellow, 120 is green, 180 is cyan, 240 is blue, 300
 # purple, and 360 is red again.
 # Minimum value: 0, maximum value: 359, default value: 220.
@@ -1195,6 +1244,17 @@ HTML_COLORSTYLE_GAMMA  = 80
 
 HTML_TIMESTAMP         = NO
 
+# If the HTML_DYNAMIC_MENUS tag is set to YES then the generated HTML
+# documentation will contain a main index with vertical navigation menus that
+# are dynamically created via Javascript. If disabled, the navigation index will
+# consists of multiple levels of tabs that are statically embedded in every HTML
+# page. Disable this option to support browsers that do not have Javascript,
+# like the Qt help browser.
+# The default value is: YES.
+# This tag requires that the tag GENERATE_HTML is set to YES.
+
+HTML_DYNAMIC_MENUS     = YES
+
 # If the HTML_DYNAMIC_SECTIONS tag is set to YES then the generated HTML
 # documentation will contain sections that can be hidden and shown after the
 # page has loaded.
@@ -1218,13 +1278,13 @@ HTML_INDEX_NUM_ENTRIES = 100
 
 # If the GENERATE_DOCSET tag is set to YES, additional index files will be
 # generated that can be used as input for Apple's Xcode 3 integrated development
-# environment (see: http://developer.apple.com/tools/xcode/), introduced with
-# OSX 10.5 (Leopard). To create a documentation set, doxygen will generate a
+# environment (see: https://developer.apple.com/xcode/), introduced with OSX
+# 10.5 (Leopard). To create a documentation set, doxygen will generate a
 # Makefile in the HTML output directory. Running make will produce the docset in
 # that directory and running make install will install the docset in
 # ~/Library/Developer/Shared/Documentation/DocSets so that Xcode will find it at
-# startup. See http://developer.apple.com/tools/creatingdocsetswithdoxygen.html
-# for more information.
+# startup. See https://developer.apple.com/library/archive/featuredarticles/Doxy
+# genXcode/_index.html for more information.
 # The default value is: NO.
 # This tag requires that the tag GENERATE_HTML is set to YES.
 
@@ -1263,7 +1323,7 @@ DOCSET_PUBLISHER_NAME  = Publisher
 # If the GENERATE_HTMLHELP tag is set to YES then doxygen generates three
 # additional HTML index files: index.hhp, index.hhc, and index.hhk. The
 # index.hhp is a project file that can be read by Microsoft's HTML Help Workshop
-# (see: http://www.microsoft.com/en-us/download/details.aspx?id=21138) on
+# (see: https://www.microsoft.com/en-us/download/details.aspx?id=21138) on
 # Windows.
 #
 # The HTML Help Workshop contains a compiler that can convert all HTML output
@@ -1339,7 +1399,7 @@ QCH_FILE               =
 
 # The QHP_NAMESPACE tag specifies the namespace to use when generating Qt Help
 # Project output. For more information please see Qt Help Project / Namespace
-# (see: http://qt-project.org/doc/qt-4.8/qthelpproject.html#namespace).
+# (see: https://doc.qt.io/archives/qt-4.8/qthelpproject.html#namespace).
 # The default value is: org.doxygen.Project.
 # This tag requires that the tag GENERATE_QHP is set to YES.
 
@@ -1347,7 +1407,7 @@ QHP_NAMESPACE          = org.doxygen.Project
 
 # The QHP_VIRTUAL_FOLDER tag specifies the namespace to use when generating Qt
 # Help Project output. For more information please see Qt Help Project / Virtual
-# Folders (see: http://qt-project.org/doc/qt-4.8/qthelpproject.html#virtual-
+# Folders (see: https://doc.qt.io/archives/qt-4.8/qthelpproject.html#virtual-
 # folders).
 # The default value is: doc.
 # This tag requires that the tag GENERATE_QHP is set to YES.
@@ -1356,7 +1416,7 @@ QHP_VIRTUAL_FOLDER     = doc
 
 # If the QHP_CUST_FILTER_NAME tag is set, it specifies the name of a custom
 # filter to add. For more information please see Qt Help Project / Custom
-# Filters (see: http://qt-project.org/doc/qt-4.8/qthelpproject.html#custom-
+# Filters (see: https://doc.qt.io/archives/qt-4.8/qthelpproject.html#custom-
 # filters).
 # This tag requires that the tag GENERATE_QHP is set to YES.
 
@@ -1364,7 +1424,7 @@ QHP_CUST_FILTER_NAME   =
 
 # The QHP_CUST_FILTER_ATTRS tag specifies the list of the attributes of the
 # custom filter to add. For more information please see Qt Help Project / Custom
-# Filters (see: http://qt-project.org/doc/qt-4.8/qthelpproject.html#custom-
+# Filters (see: https://doc.qt.io/archives/qt-4.8/qthelpproject.html#custom-
 # filters).
 # This tag requires that the tag GENERATE_QHP is set to YES.
 
@@ -1372,7 +1432,7 @@ QHP_CUST_FILTER_ATTRS  =
 
 # The QHP_SECT_FILTER_ATTRS tag specifies the list of the attributes this
 # project's filter section matches. Qt Help Project / Filter Attributes (see:
-# http://qt-project.org/doc/qt-4.8/qthelpproject.html#filter-attributes).
+# https://doc.qt.io/archives/qt-4.8/qthelpproject.html#filter-attributes).
 # This tag requires that the tag GENERATE_QHP is set to YES.
 
 QHP_SECT_FILTER_ATTRS  =
@@ -1465,7 +1525,7 @@ EXT_LINKS_IN_WINDOW    = NO
 
 FORMULA_FONTSIZE       = 10
 
-# Use the FORMULA_TRANPARENT tag to determine whether or not the images
+# Use the FORMULA_TRANSPARENT tag to determine whether or not the images
 # generated for formulas are transparent PNGs. Transparent PNGs are not
 # supported properly for IE 6.0, but are supported on all modern browsers.
 #
@@ -1477,7 +1537,7 @@ FORMULA_FONTSIZE       = 10
 FORMULA_TRANSPARENT    = YES
 
 # Enable the USE_MATHJAX option to render LaTeX formulas using MathJax (see
-# http://www.mathjax.org) which uses client side Javascript for the rendering
+# https://www.mathjax.org) which uses client side Javascript for the rendering
 # instead of using pre-rendered bitmaps. Use this if you do not have LaTeX
 # installed or if you want to formulas look prettier in the HTML output. When
 # enabled you may also need to install MathJax separately and configure the path
@@ -1504,11 +1564,11 @@ MATHJAX_FORMAT         = HTML-CSS
 # MATHJAX_RELPATH should be ../mathjax. The default value points to the MathJax
 # Content Delivery Network so you can quickly see the result without installing
 # MathJax. However, it is strongly recommended to install a local copy of
-# MathJax from http://www.mathjax.org before deployment.
-# The default value is: http://cdn.mathjax.org/mathjax/latest.
+# MathJax from https://www.mathjax.org before deployment.
+# The default value is: https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.5/.
 # This tag requires that the tag USE_MATHJAX is set to YES.
 
-MATHJAX_RELPATH        = http://cdn.mathjax.org/mathjax/latest
+MATHJAX_RELPATH        = https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.5/
 
 # The MATHJAX_EXTENSIONS tag can be used to specify one or more MathJax
 # extension names that should be enabled during MathJax rendering. For example
@@ -1566,7 +1626,7 @@ SERVER_BASED_SEARCH    = NO
 #
 # Doxygen ships with an example indexer (doxyindexer) and search engine
 # (doxysearch.cgi) which are based on the open source search engine library
-# Xapian (see: http://xapian.org/).
+# Xapian (see: https://xapian.org/).
 #
 # See the section "External Indexing and Searching" for details.
 # The default value is: NO.
@@ -1579,7 +1639,7 @@ EXTERNAL_SEARCH        = NO
 #
 # Doxygen ships with an example indexer (doxyindexer) and search engine
 # (doxysearch.cgi) which are based on the open source search engine library
-# Xapian (see: http://xapian.org/). See the section "External Indexing and
+# Xapian (see: https://xapian.org/). See the section "External Indexing and
 # Searching" for details.
 # This tag requires that the tag SEARCHENGINE is set to YES.
 
@@ -1631,21 +1691,35 @@ LATEX_OUTPUT           = latex
 # The LATEX_CMD_NAME tag can be used to specify the LaTeX command name to be
 # invoked.
 #
-# Note that when enabling USE_PDFLATEX this option is only used for generating
-# bitmaps for formulas in the HTML output, but not in the Makefile that is
-# written to the output directory.
-# The default file is: latex.
+# Note that when not enabling USE_PDFLATEX the default is latex when enabling
+# USE_PDFLATEX the default is pdflatex and when in the later case latex is
+# chosen this is overwritten by pdflatex. For specific output languages the
+# default can have been set differently, this depends on the implementation of
+# the output language.
 # This tag requires that the tag GENERATE_LATEX is set to YES.
 
-LATEX_CMD_NAME         = latex
+LATEX_CMD_NAME         =
 
 # The MAKEINDEX_CMD_NAME tag can be used to specify the command name to generate
 # index for LaTeX.
+# Note: This tag is used in the Makefile / make.bat.
+# See also: LATEX_MAKEINDEX_CMD for the part in the generated output file
+# (.tex).
 # The default file is: makeindex.
 # This tag requires that the tag GENERATE_LATEX is set to YES.
 
 MAKEINDEX_CMD_NAME     = makeindex
 
+# The LATEX_MAKEINDEX_CMD tag can be used to specify the command name to
+# generate index for LaTeX. In case there is no backslash (\) as first character
+# it will be automatically added in the LaTeX code.
+# Note: This tag is used in the generated output file (.tex).
+# See also: MAKEINDEX_CMD_NAME for the part in the Makefile / make.bat.
+# The default value is: makeindex.
+# This tag requires that the tag GENERATE_LATEX is set to YES.
+
+LATEX_MAKEINDEX_CMD    = makeindex
+
 # If the COMPACT_LATEX tag is set to YES, doxygen generates more compact LaTeX
 # documents. This may be useful for small projects and may help to save some
 # trees in general.
@@ -1766,7 +1840,7 @@ LATEX_SOURCE_CODE      = NO
 
 # The LATEX_BIB_STYLE tag can be used to specify the style to use for the
 # bibliography, e.g. plainnat, or ieeetr. See
-# http://en.wikipedia.org/wiki/BibTeX and \cite for more info.
+# https://en.wikipedia.org/wiki/BibTeX and \cite for more info.
 # The default value is: plain.
 # This tag requires that the tag GENERATE_LATEX is set to YES.
 
@@ -1780,6 +1854,14 @@ LATEX_BIB_STYLE        = plain
 
 LATEX_TIMESTAMP        = NO
 
+# The LATEX_EMOJI_DIRECTORY tag is used to specify the (relative or absolute)
+# path from which the emoji images will be read. If a relative path is entered,
+# it will be relative to the LATEX_OUTPUT directory. If left blank the
+# LATEX_OUTPUT directory will be used.
+# This tag requires that the tag GENERATE_LATEX is set to YES.
+
+LATEX_EMOJI_DIRECTORY  =
+
 #---------------------------------------------------------------------------
 # Configuration options related to the RTF output
 #---------------------------------------------------------------------------
@@ -1819,9 +1901,9 @@ COMPACT_RTF            = NO
 
 RTF_HYPERLINKS         = NO
 
-# Load stylesheet definitions from file. Syntax is similar to doxygen's config
-# file, i.e. a series of assignments. You only have to provide replacements,
-# missing definitions are set to their default value.
+# Load stylesheet definitions from file. Syntax is similar to doxygen's
+# configuration file, i.e. a series of assignments. You only have to provide
+# replacements, missing definitions are set to their default value.
 #
 # See also section "Doxygen usage" for information on how to generate the
 # default style sheet that doxygen normally uses.
@@ -1830,8 +1912,8 @@ RTF_HYPERLINKS         = NO
 RTF_STYLESHEET_FILE    =
 
 # Set optional variables used in the generation of an RTF document. Syntax is
-# similar to doxygen's config file. A template extensions file can be generated
-# using doxygen -e rtf extensionFile.
+# similar to doxygen's configuration file. A template extensions file can be
+# generated using doxygen -e rtf extensionFile.
 # This tag requires that the tag GENERATE_RTF is set to YES.
 
 RTF_EXTENSIONS_FILE    =
@@ -1917,6 +1999,13 @@ XML_OUTPUT             = xml
 
 XML_PROGRAMLISTING     = YES
 
+# If the XML_NS_MEMB_FILE_SCOPE tag is set to YES, doxygen will include
+# namespace members in file scope as well, matching the HTML output.
+# The default value is: NO.
+# This tag requires that the tag GENERATE_XML is set to YES.
+
+XML_NS_MEMB_FILE_SCOPE = NO
+
 #---------------------------------------------------------------------------
 # Configuration options related to the DOCBOOK output
 #---------------------------------------------------------------------------
@@ -1949,9 +2038,9 @@ DOCBOOK_PROGRAMLISTING = NO
 #---------------------------------------------------------------------------
 
 # If the GENERATE_AUTOGEN_DEF tag is set to YES, doxygen will generate an
-# AutoGen Definitions (see http://autogen.sf.net) file that captures the
-# structure of the code including all documentation. Note that this feature is
-# still experimental and incomplete at the moment.
+# AutoGen Definitions (see http://autogen.sourceforge.net/) file that captures
+# the structure of the code including all documentation. Note that this feature
+# is still experimental and incomplete at the moment.
 # The default value is: NO.
 
 GENERATE_AUTOGEN_DEF   = NO
@@ -2118,12 +2207,6 @@ EXTERNAL_GROUPS        = YES
 
 EXTERNAL_PAGES         = YES
 
-# The PERL_PATH should be the absolute path and name of the perl script
-# interpreter (i.e. the result of 'which perl').
-# The default file (with absolute path) is: /usr/bin/perl.
-
-PERL_PATH              = /usr/bin/perl
-
 #---------------------------------------------------------------------------
 # Configuration options related to the dot tool
 #---------------------------------------------------------------------------
@@ -2137,15 +2220,6 @@ PERL_PATH              = /usr/bin/perl
 
 CLASS_DIAGRAMS         = YES
 
-# You can define message sequence charts within doxygen comments using the \msc
-# command. Doxygen will then run the mscgen tool (see:
-# http://www.mcternan.me.uk/mscgen/)) to produce the chart and insert it in the
-# documentation. The MSCGEN_PATH tag allows you to specify the directory where
-# the mscgen tool resides. If left empty the tool is assumed to be found in the
-# default search path.
-
-MSCGEN_PATH            =
-
 # You can include diagrams made with dia in doxygen documentation. Doxygen will
 # then run dia to produce the diagram and insert it in the documentation. The
 # DIA_PATH tag allows you to specify the directory where the dia binary resides.

docs/md/core.md

@@ -0,0 +1,296 @@
+# 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)
+* [Type info](#type-info)
+  * [Almost unique identifiers](#almost-unique-identifiers)
+* [Traits](#traits)
+  * [Member class type](#member-class-type)
+  * [Tags](#tags)
+<!--
+@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"}>{};
+```
+
+# Type info
+
+The `type_info` class template is meant to provide some basic information about
+types of all kinds.<br/>
+Currently, the only information available is the numeric identifier associated
+with a given type:
+
+```cpp
+auto id = entt::type_info<my_type>::id();
+```
+
+In general, the `id` function is also `constexpr` but this isn't guaranteed for
+all compilers and platforms (although it's valid with the most well-known and
+popular compilers).<br/>
+This function **can** use non-standard features of the language for its own
+purposes. This allows it to provide compile-time identifiers that remain stable
+between different runs. However, it's possible to force the use of standard
+features only by defining the macro `ENTT_STANDARD_CPP`. In this case, there is
+no guarantee that the identifiers are stable across executions though. Moreover,
+identifiers are generated at runtime and are no longer a compile-time thing.
+
+An external type system can also be used if needed. In fact, `type_info` can be
+specialized by type and is also _sfinae-friendly_ in order to allow more refined
+specializations such as:
+
+```cpp
+template<typename Type>
+struct entt::type_info<Type, std::void_d<decltype(Type::custom_id())>> {
+    static constexpr ENTT_ID_TYPE id() ENTT_NOEXCEPT {
+        return Type::custom_id();
+    }
+};
+```
+
+Note that this class template and its specializations are widely used within
+`EnTT`. It also plays a very important role in making `EnTT` work transparently
+across boundaries.<br/>
+Please refer to the dedicated section for more details.
+
+## Almost unique identifiers
+
+Since the default non-standard, compile-time implementation makes use of hashed
+strings, it may happen that two types are assigned the same numeric
+identifier.<br/>
+In fact, although this is quite rare, it's not entirely excluded.
+
+Another case where two types are assigned the same identifier is when classes
+from different contexts (for example two or more libraries loaded at runtime)
+have the same fully qualified name.<br/>
+Since the default model is based on the name of the classes, if the types belong
+to the same namespace then their identifiers _could_ be identical (they won't
+necessarily be the same though).
+
+Fortunately, there are several easy ways to deal with this:
+
+* The most trivial one is to define the `ENTT_STANDARD_CPP` macro. Note that
+  runtime identifiers don't suffer from the sam problem. However, this solution
+  doesn't work well with a plugin system, where the libraries aren't linked.
+
+* Another possibility is to specialize the `type_info` class for one of the
+  conflicting types, in order to assign it a custom identifier. This is probably
+  the easiest solution that also preserves the feature of the tool.
+
+* A fully customized identifier generation policy (based for example on enum
+  classes or preprocessing steps) may represent yet another option.
+
+These are just some examples of possible approaches to the problem but there are
+many others. As already mentioned above, since users have full control over
+their types, this problem is in any case easy to solve and should not worry too
+much.<br/>
+In all likelihood, it will never happen to run into a conflict anyway.
+
+# Traits
+
+This section contains a handful of utilities and traits not present in the
+standard template library but which can be useful in everyday life.
+
+## Member class type
+
+The `auto` template parameter introduced with C++17 made it possible to simplify
+many class templates and template functions but also made the class type opaque
+when members are passed as template arguments.<br/>
+The purpose of this utility is to extract the class type in a few lines of code:
+
+```cpp
+template<typename Member>
+using clazz = entt::member_class_t<Member>;
+```
+
+## Tags
+
+Since in `EnTT` the type identified by `ENTT_ID_TYPE` is very important and
+widely used, there is a more user-friendly shortcut for the creation of integral
+constants based on it.<br/>
+This shortcut is the alias template `entt::tag`.
+
+If used in combination with hashed strings, it helps to use human-readable names
+where types would be required otherwise. As an example:
+
+```cpp
+registry.assign<entt::tag<"enemy"_hs>>(entity);
+```
+
+However, this isn't the only permitted use. Literally any value convertible to
+`ENTT_ID_TYPE` is a good candidate, such as the named constants of an unscoped
+enum.

docs/md/faq.md

@@ -0,0 +1,200 @@
+# 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_ASSERT` should be redefined to disable internal checks
+made by `EnTT` in debug:
+
+```cpp
+#define ENTT_ASSERT(...) ((void)0)
+```
+
+These asserts are introduced to help the users but they require to access to the
+underlying containers and therefore risk ruining the performance in some cases.
+
+With these changes, debug performance should increase enough for most cases. If
+you want something more, you can can also switch to an optimization level `O0`
+or preferably `O1`.
+
+## How can I represent hierarchies with my components?
+
+This is one of the first questions that anyone makes when starting to work with
+the entity-component-system architectural pattern.<br/>
+There are several approaches to the problem and what’s the best one depends
+mainly on the real problem one is facing. In all cases, how to do it doesn't
+strictly depend on the library in use, but the latter can certainly allow or
+not different techniques depending on how the data are laid out.
+
+I tried to describe some of the techniques that fit well with the model of
+`EnTT`. [Here](https://skypjack.github.io/2019-06-25-ecs-baf-part-4/) is the
+first post of a series that tries to explore the problem. More will probably
+come in future.
+
+Long story short, you can always define a tree where the nodes expose implicit
+lists of children by means of the following type:
+
+```cpp
+struct relationship {
+    std::size_t children{};
+    entt::entity first{entt::null};
+    entt::entity prev{entt::null};
+    entt::entity next{entt::null};
+    entt::entity parent{entt::null};
+    // ... other data members ...
+};
+```
+
+The sort functionalities of `EnTT`, the groups and all the other features of the
+library can help then to get the best in terms of data locality and therefore
+performance from this component.
+
+## Custom entity identifiers: yay or nay?
+
+Custom entity identifiers are definitely a good idea in two cases at least:
+
+* If `std::uint32_t` 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(suppress:4307)) entt::hashed_string{str}\
+#else
+#define HS(str) entt::hashed_string{str}
+#endif
+```
+
+With an example of use included:
+
+```cpp
+constexpr auto identifier = HS("my/resource/identifier");
+```
+
+Thanks to [huwpascoe](https://github.com/huwpascoe) for the courtesy.
+
+## Warning C4003: the min, the max and the macro
+
+On Windows, a header file defines two macros `min` and `max` which may result in
+conflicts with their counterparts in the standard library and therefore in
+errors during compilation.
+
+It's a pretty big problem but fortunately it's not a problem of `EnTT` and there
+is a fairly simple solution to it.<br/>
+It consists in defining the `NOMINMAX` macro before to include any other header
+so as to get rid of the extra definitions:
+
+```cpp
+#define NOMINMAX
+```
+
+Please refer to [this](https://github.com/skypjack/entt/issues/96) issue for
+more details.
+
+## The standard and the non-copyable types
+
+`EnTT` uses internally the trait `std::is_copy_constructible_v` to check if a
+component is actually copyable. This trait doesn't check if an object can
+actually be copied but only verifies if there is a copy constructor
+available.<br/>
+This can lead to surprising results due to some idiosyncrasies of the standard
+mainly related to the need to guarantee backward compatibility.
+
+For example, `std::vector` defines a copy constructor no matter if its value
+type is copyable or not. As a result, `std::is_copy_constructible_v` is true
+for the following specialization:
+
+```cpp
+struct type {
+    std::vector<std::unique_ptr<action>> vec;
+};
+```
+
+When trying to assign an instance of this type to an entity in the ECS part,
+this may trigger a compilation error because we cannot really make a copy of
+it.<br/>
+As a workaround, users can mark the type explicitly as non-copyable:
+
+```cpp
+struct type {
+    type(const type &) = delete;
+    type & operator=(const type &) = delete;
+
+    std::vector<std::unique_ptr<action>> vec;
+};
+```
+
+Unfortunately, this will also disable aggregate initialization.

docs/md/lib.md

@@ -0,0 +1,103 @@
+# Push EnTT across boundaries
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [The EnTT way](#the-entt-way)
+* [Meta context](#meta-context)
+* [Memory management](#memory-management)
+<!--
+@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 was mainly due to a custom utility used to assign unique, sequential
+identifiers to different types.<br/>
+Fortunately, nowadays using `EnTT` across boundaries is straightforward. In
+fact, everything just works transparently in almost all cases. There are only a
+few exceptions, easy to deal with anyway.
+
+# The EnTT way
+
+Many classes in `EnTT` make extensive use of type erasure for their purposes.
+This isn't a problem in itself (in fact, it's the basis of an API so convenient
+to use). However, a way is needed to recognize the objects whose type has been
+erased on the other side of a boundary.<br/>
+The `type_info` class template is how identifiers are generated and thus made
+available to the rest of the library.
+
+In general, this class doesn't arouse much interest. The only exception is in
+case of conflicts between identifiers (definitely uncommon though) or where the
+default solution proposed by `EnTT` isn't suitable for the user's purposes.<br/>
+The section dedicated to this core class contains all the details to get around
+the problem in a concise and elegant way. Please refer to the specific
+documentation.
+
+# Meta context
+
+The runtime reflection system deserves a special mention when it comes to using
+it across boundaries.<br/>
+Since it's linked to a static context to which the visible components are
+attached and different contexts don't relate to each other, they must be
+_shared_ to allow the use of meta types across boundaries.
+
+Sharing a context is trivial though. First of all, the local one must be
+acquired in the main space:
+
+```cpp
+entt::meta_ctx ctx{};
+```
+
+Then, it must passed to the receiving space that will set it as its global
+context, thus releasing the local one that remains available but is no longer
+referred to by the runtime reflection system:
+
+```cpp
+entt::meta_ctx::bind(ctx);
+```
+
+From now on, both spaces will refer to the same context and on it will be
+attached the new visible meta types, no matter where they are created.<br/>
+A context can also be reset and then associated again locally as:
+
+```cpp
+entt::meta_ctx::bind{entt::meta_ctx{});
+```
+
+This is allowed because local and global contexts are separated. Therefore, it's
+always possible to make the local context the current one again.
+
+Before to release a context, all locally registered types should be reset to
+avoid dangling references. Otherwise, if a type is accessed from another space
+by name, there could be an attempt to address its parts that are no longer
+available.
+
+# Memory Management
+
+There is another subtle problem due to memory management that can lead to
+headaches.<br/>
+It can occur where there are pools of objects (such as components or events)
+dynamically created on demand. This is usually not a problem when working with
+linked libraries that rely on the same dynamic runtime. However, it can occur in
+the case of plugins or statically linked runtimes.
+
+As an example, imagine creating an instance of `registry` in the main executable
+and sharing it with a plugin. If the latter starts working with a component that
+is unknown to the former, a dedicated pool is created within the registry on
+first use.<br/>
+As one can guess, this pool is instantiated on a different side of the boundary
+from the `registry`. Therefore, the instance is now managing memory from
+different spaces and this can quickly lead to crashes if not properly addressed.
+
+To overcome the risk, it's recommended to use well-defined interfaces that make
+fundamental types pass through the boundaries, isolating the instances of the
+`EnTT` classes from time to time and as appropriate.<br/>
+Refer to the test suite for some examples, read the documentation available
+online about this type of issues or consult someone who has already had such
+experiences to avoid problems.

docs/md/links.md

@@ -0,0 +1,127 @@
+# EnTT in Action
+
+`EnTT` is widely used in private and commercial applications. I cannot even
+mention most of them because of some signatures I put on some documents time
+ago. Fortunately, there are also people who took the time to implement open
+source projects based on `EnTT` and did not hold back when it came to
+documenting them.
+
+Below an incomplete list of games, applications and articles that can be used as
+a reference. Where I put the word _apparently_ means that the use of `EnTT` is
+documented but the authors didn't make explicit announcements or contacted me
+directly.
+
+I hope this list can grow much more in the future:
+
+* Games:
+  * [Minecraft](https://minecraft.net/en-us/attribution/) by
+    [Mojang](https://mojang.com/): of course, **that** Minecraft, see the
+    open source attributions page for more details.
+  * [Land of the Rair](https://github.com/LandOfTheRair/core2): the new backend
+    of [a retro-style MUD](https://rair.land/) for the new age.
+  * [Openblack](https://github.com/openblack/openblack): open source
+    reimplementation of the game _Black & White_ (2001).
+  * [Face Smash](https://play.google.com/store/apps/details?id=com.gamee.facesmash):
+    a game to play with your face.
+  * [EnTT Pacman](https://github.com/Kerndog73/EnTT-Pacman): an example of how
+    to make Pacman with `EnTT`.
+  * [Wacman](https://github.com/carlfindahl/wacman): a pacman clone with OpenGL.
+  * [Classic Tower Defence](https://github.com/kerndog73/Classic-Tower-Defence):
+    a tiny little tower defence game featuring a homemade font.
+    [Check it out](https://indi-kernick.itch.io/classic-tower-defence).
+  * [The Machine](https://github.com/Kerndog73/The-Machine): a box pushing
+    puzzler with logic gates and other cool stuff.
+    [Check it out](https://indi-kernick.itch.io/the-machine-web-version).
+  * [EnttPong](https://github.com/reworks/EnttPong): an example of how to make
+    Pong with `EnTT`.
+  * [Randballs](https://github.com/gale93/randballs): simple `SFML` and `EnTT`
+    playground.
+  * [EnTT Tower Defense](https://github.com/Daivuk/tddod): a data oriented tower
+    defense example.
+  * [EnTT Breakout](https://github.com/vblanco20-1/entt-breakout): simple
+    example of a breakout game, using `SDL` and `EnTT`.
+  * [Arcade puzzle game with EnTT](https://github.com/MasonRG/ArcadePuzzleGame):
+    arcade puzzle game made in C++ using the `SDL2` and `EnTT` libraries.
+  * [Snake with EnTT](https://github.com/MasonRG/SnakeGame): simple snake game
+    made in C++ with the `SDL2` and `EnTT` libraries.
+  * [Mirrors lasers and robots](https://github.com/guillaume-haerinck/imac-tower-defense):
+    a small tower defense game based on mirror orientation.
+  * [PopHead](https://github.com/SPC-Some-Polish-Coders/PopHead/): 2D, Zombie,
+    RPG game made from scratch in C++.
+  * [Robotligan](https://github.com/Trisslotten/robotligan): multiplayer
+    football game.
+  * [DungeonSlayer](https://github.com/alohaeee/DungeonSlayer): 2D game made
+    from scratch in C++.
+
+* Engines and the like:
+  * [Fling Engine](https://github.com/flingengine/FlingEngine): a Vulkan game
+    engine with a focus on data oriented design.
+  * [Apparently](https://teamwisp.github.io/credits/)
+    [Wisp](https://teamwisp.github.io/product/) by
+    [Team Wisp](https://teamwisp.github.io/): an advanced real-time ray tracing
+    renderer built for the demands of video game artists.
+  * [starlight](https://github.com/DomRe/starlight): game programming framework
+    using `Allegro`, `Lua` and modern C++.
+  * [Apparently](https://github.com/JosiahWI/qub3d-libdeps)
+    [Qub3d](https://qub3d.org/): because blocks should be open source.
+  * [shiva](https://github.com/Milerius/shiva): modern C++ engine with
+    modularity.
+  * [NovusCore](https://github.com/novuscore/NovusCore): a modern take on World
+    of Warcraft emulation.
+  * [ImGui/EnTT editor](https://github.com/Green-Sky/imgui_entt_entity_editor):
+    a drop-in, single-file entity editor for `EnTT` that uses `ImGui` as
+    graphical backend (with
+    [demo code](https://github.com/Green-Sky/imgui_entt_entity_editor_demo)).
+  * [SgOgl](https://github.com/stwe/SgOgl): a game engine library for OpenGL
+    developed for educational purposes.
+  * [Lumos](https://github.com/jmorton06/Lumos): game engine written in C++
+    using OpenGL and Vulkan.
+
+* 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).
+  * [Sequentity](https://github.com/alanjfs/sequentity): A MIDI-like
+    sequencer/tracker for C++ and `ImGui` (with `Magnum` and `EnTT`).
+  * [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 +1,591 @@
+# 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 users to rely on the tools provided by the library
+when it comes to working with names and identifiers. It does this by offering an
+API that works with opaque identifiers that may or may not be generated by means
+of a hashed string.<br/>
+This means that users can assign any type of identifier to the meta objects, as
+long as they are numeric. It doesn't matter if they are generated at runtime, at
+compile-time or with custom functions.
+
+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.<br/>
+Similarly, to create a copy that works as a light reference for the managed
+object, it's possible to dereference a given `meta_any`:
+
+```cpp
+entt::meta_any ref = *any;
+```
+
+It doesn't matter if the starting container actually holds an object or acts as
+a reference for unmanaged elements, the new instance thus created won't create
+copies and will only serve as a reference for the original item.<br/>
+It means that, starting from the example above, both `ref` and` any` will point
+to the same object, whether it's initially contained in `any` or already an
+unmanaged one. This is particularly useful for passing instances of `meta_any`
+belonging to the external context by reference to a function or a constructor
+rather than making copies of them.
+
+The `meta_any` class has also a `type` member function that returns the meta
+type of the contained value, if any. The member functions `try_cast`, `cast` and
+`convert` are then used to know if the underlying object has a given type as a
+base or if it can be converted implicitly to it.
+
+## 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 instances of the underlying
+type.<br/>
+Refer to the inline documentation for all the details.
+
+The meta objects that compose a meta type are accessed in the following ways:
+
+* _Meta constructors_. They are accessed by types of arguments:
+
+  ```cpp
+  auto ctor = entt::resolve<my_type>().ctor<int, char>();
+  ```
+
+  The returned type is `meta_ctor` and may be invalid if there is no constructor
+  that accepts the supplied arguments or at least some types from which they are
+  derived or to which they can be converted.<br/>
+  A meta constructor offers an API to know the number of its arguments and their
+  expected meta types. Furthermor, it's possible to invoke it and therefore to
+  construct new instances of the underlying type.
+
+* _Meta data_. They are accessed by _name_:
+
+  ```cpp
+  auto data = entt::resolve<my_type>().data("member"_hs);
+  ```
+
+  The returned type is `meta_data` and may be invalid if there is no meta data
+  object associated with the given identifier.<br/>
+  A meta data object offers an API to query the underlying type (for example, to
+  know if it's a const or a static one), to get the meta type of the variable
+  and to set or get the contained value.
+
+* _Meta functions_. They are accessed by _name_:
+
+  ```cpp
+  auto func = entt::resolve<my_type>().func("member"_hs);
+  ```
+
+  The returned type is `meta_func` and may be invalid if there is no meta
+  function object associated with the given identifier.<br/>
+  A meta function object offers an API to query the underlying type (for
+  example, to know if it's a const or a static function), to know the number of
+  arguments, the meta return type and the meta types of the parameters. In
+  addition, a meta function object can be used to invoke the underlying function
+  and then get the return value in the form of a `meta_any` object.
+
+* _Meta bases_. They are accessed through the _name_ of the base types:
+
+  ```cpp
+  auto base = entt::resolve<derived_type>().base("base"_hs);
+  ```
+
+  The returned type is `meta_base` and may be invalid if there is no meta base
+  object associated with the given identifier.<br/>
+  Meta bases aren't meant to be used directly, even though they are freely
+  accessible. They expose only a few methods to use to know the meta type of the
+  base class and to convert a raw pointer between types.
+
+* _Meta conversion functions_. They are accessed by type:
+
+  ```cpp
+  auto conv = entt::resolve<double>().conv<int>();
+  ```
+
+  The returned type is `meta_conv` and may be invalid if there is no meta
+  conversion function associated with the given type.<br/>
+  The meta conversion functions are as thin as the meta bases and with a very
+  similar interface. The sole difference is that they return a newly created
+  instance wrapped in a `meta_any` object when they convert between different
+  types.
+
+All the objects thus obtained as well as the meta types can be explicitly
+converted to a boolean value to check if they are valid:
+
+```cpp
+if(auto func = entt::resolve<my_type>().func("member"_hs); func) {
+    // ...
+}
+```
+
+Furthermore, all meta objects can be iterated through an overload that accepts a
+callback through which to return them. As an example:
+
+```cpp
+entt::resolve<my_type>().data([](auto data) {
+    // ...
+});
+```
+
+A meta type can be used to `construct` actual instances of the underlying
+type.<br/>
+In particular, the `construct` member function accepts a variable number of
+arguments and searches for a match. It then returns a `meta_any` object that may
+or may not be initialized, depending on whether a suitable constructor has been
+found or not.
+
+There is no object that wraps the destructor of a meta type nor a `destroy`
+member function in its API. The reason is quickly explained: destructors are
+invoked implicitly by `meta_any` behind the scenes and users have not to deal
+with them explicitly. Furthermore, they have no name, cannot be searched and
+wouldn't have member functions to expose anyway.<br/>
+Therefore, exposing destructors would be pointless and would add nothing to the
+library itself.
+
+Meta types and meta objects in general contain much more than what is said: a
+plethora of functions in addition to those listed whose purposes and uses go
+unfortunately beyond the scope of this document.<br/>
+I invite anyone interested in the subject to look at the code, experiment and
+read the inline documentation to get the best out of this powerful tool.
+
+## 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 that generated
+  them. Modifying the object contained in the wrapper for which the _aliasing_
+  was requested will make it possible to directly modify the instance used to
+  initialize the wrapper itself.<br/>
+  This policy works with constructors (for example, when objects are taken from
+  an external container rather than created on demand), data members and
+  functions in general (as long as their return types are lvalue references).
+
+  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 +1,212 @@
+# Crash Course: cooperative scheduler
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [The process](#the-process)
+  * [Adaptor](#adaptor)
+* [The scheduler](#the-scheduler)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+Sometimes processes are a useful tool to work around the strict definition of a
+system and introduce logic in a different way, usually without resorting to the
+introduction of other components.
+
+`EnTT` offers a minimal support to this paradigm by introducing a few classes
+that users can use to define and execute cooperative processes.
+
+# The process
+
+A typical process must inherit from the `process` class template that stays true
+to the CRTP idiom. Moreover, derived classes must specify what's the intended
+type for elapsed times.
+
+A process should expose publicly the following member functions whether
+required (note that it isn't required to define a function unless the derived
+class wants to _override_ the default behavior):
+
+* `void update(Delta, void *);`
+
+  It's invoked once per tick until a process is explicitly aborted or it
+  terminates either with or without errors. Even though it's not mandatory to
+  declare this member function, as a rule of thumb each process should at
+  least define it to work properly. The `void *` parameter is an opaque pointer
+  to user data (if any) forwarded directly to the process during an update.
+
+* `void init();`
+
+  It's invoked when the process joins the running queue of a scheduler. This
+  happens as soon as it's attached to the scheduler if the process is a top
+  level one, otherwise when it replaces its parent if the process is a
+  continuation.
+
+* `void succeeded();`
+
+  It's invoked in case of success, immediately after an update and during the
+  same tick.
+
+* `void failed();`
+
+  It's invoked in case of errors, immediately after an update and during the
+  same tick.
+
+* `void aborted();`
+
+  It's invoked only if a process is explicitly aborted. There is no guarantee
+  that it executes in the same tick, this depends solely on whether the
+  process is aborted immediately or not.
+
+Derived classes can also change the internal state of a process by invoking
+`succeed` and `fail`, as well as `pause` and `unpause` the process itself. All
+these are protected member functions made available to be able to manage the
+life cycle of a process from a derived class.
+
+Here is a minimal example for the sake of curiosity:
+
+```cpp
+struct my_process: entt::process<my_process, std::uint32_t> {
+    using delta_type = std::uint32_t;
+
+    my_process(delta_type delay)
+        : remaining{delay}
+    {}
+
+    void update(delta_type delta, void *) {
+        remaining -= std::min(remaining, delta);
+
+        // ...
+
+        if(!remaining) {
+            succeed();
+        }
+    }
+
+private:
+    delta_type remaining;
+};
+```
+
+## Adaptor
+
+Lambdas and functors can't be used directly with a scheduler for they are not
+properly defined processes with managed life cycles.<br/>
+This class helps in filling the gap and turning lambdas and functors into
+full featured processes usable by a scheduler.
+
+The function call operator has a signature similar to the one of the `update`
+function of a process but for the fact that it receives two extra arguments to
+call whenever a process is terminated with success or with an error:
+
+```cpp
+void(Delta delta, void *data, auto succeed, auto fail);
+```
+
+Parameters have the following meaning:
+
+* `delta` is the elapsed time.
+* `data` is an opaque pointer to user data if any, `nullptr` otherwise.
+* `succeed` is a function to call when a process terminates with success.
+* `fail` is a function to call when a process terminates with errors.
+
+Both `succeed` and `fail` accept no parameters at all.
+
+Note that usually users shouldn't worry about creating adaptors at all. A
+scheduler creates them internally each and every time a lambda or a functor is
+used as a process.
+
+# The scheduler
+
+A cooperative scheduler runs different processes and helps managing their life
+cycles.
+
+Each process is invoked once per tick. If it terminates, it's removed
+automatically from the scheduler and it's never invoked again. Otherwise it's
+a good candidate to run one more time the next tick.<br/>
+A process can also have a child. In this case, the parent process is replaced
+with its child when it terminates and only if it returns with success. In case
+of errors, both the parent process and its child are discarded. This way, it's
+easy to create chain of processes to run sequentially.
+
+Using a scheduler is straightforward. To create it, users must provide only the
+type for the elapsed times and no arguments at all:
+
+```cpp
+entt::scheduler<std::uint32_t> scheduler;
+```
+
+It has member functions to query its internal data structures, like `empty` or
+`size`, as well as a `clear` utility to reset it to a clean state:
+
+```cpp
+// checks if there are processes still running
+const auto empty = scheduler.empty();
+
+// gets the number of processes still running
+entt::scheduler<std::uint32_t>::size_type size = scheduler.size();
+
+// resets the scheduler to its initial state and discards all the processes
+scheduler.clear();
+```
+
+To attach a process to a scheduler there are mainly two ways:
+
+* If the process inherits from the `process` class template, it's enough to
+  indicate its type and submit all the parameters required to construct it to
+  the `attach` member function:
+
+  ```cpp
+  scheduler.attach<my_process>(1000u);
+  ```
+
+* Otherwise, in case of a lambda or a functor, it's enough to provide an
+  instance of the class to the `attach` member function:
+
+  ```cpp
+  scheduler.attach([](auto...){ /* ... */ });
+  ```
+
+In both cases, the return value is an opaque object that offers a `then` member
+function to use to create chains of processes to run sequentially.<br/>
+As a minimal example of use:
+
+```cpp
+// schedules a task in the form of a lambda function
+scheduler.attach([](auto delta, void *, auto succeed, auto fail) {
+    // ...
+})
+// appends a child in the form of another lambda function
+.then([](auto delta, void *, auto succeed, auto fail) {
+    // ...
+})
+// appends a child in the form of a process class
+.then<my_process>(1000u);
+```
+
+To update a scheduler and therefore all its processes, the `update` member
+function is the way to go:
+
+```cpp
+// updates all the processes, no user data are provided
+scheduler.update(delta);
+
+// updates all the processes and provides them with custom data
+scheduler.update(delta, &data);
+```
+
+In addition to these functions, the scheduler offers an `abort` member function
+that can be used to discard all the running processes at once:
+
+```cpp
+// aborts all the processes abruptly ...
+scheduler.abort(true);
+
+// ... or gracefully during the next tick
+scheduler.abort();
+```

docs/md/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 +1,506 @@
+# 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 doesn't claim to be a drop-in replacement for an `std::function`, so don't
+expect to use it whenever an `std::function` fits well. That said, it's most
+likely even a better fit than an `std::function` in a lot of cases, so expect to
+use it quite a lot anyway.
+
+The interface is trivial. It offers a default constructor to create empty
+delegates:
+
+```cpp
+entt::delegate<int(int)> delegate{};
+```
+
+All what is needed to create an instance is to specify the type of the function
+the delegate will _contain_, that is the signature of the free function or the
+member one wants to assign to it.
+
+Attempting to use an empty delegate by invoking its function call operator
+results in undefined behavior or most likely a crash. Before to use a delegate,
+it must be initialized.<br/>
+There exists a bunch of overloads of the `connect` member function to do that.
+As an example of use:
+
+```cpp
+int f(int i) { return i; }
+
+struct my_struct {
+    int f(const int &i) { return i }
+};
+
+// bind a free function to the delegate
+delegate.connect<&f>();
+
+// bind a member function to the delegate
+my_struct instance;
+delegate.connect<&my_struct::f>(instance);
+```
+
+The delegate class accepts also data members, if needed. In this case, the
+function type of the delegate is such that the parameter list is empty and the
+value of the data member is at least convertible to the return type.
+
+Free functions having type equivalent to `void(T &, args...)` are accepted as
+well. The first argument `T &` is considered a payload and the function will
+receive it back every time it's invoked. In other terms, this works just fine
+with the above definition:
+
+```cpp
+void g(const char &c, int i) { /* ... */ }
+const char c = 'c';
+
+delegate.connect<&g>(c);
+delegate(42);
+```
+
+The function `g` will be invoked with a reference to `c` and `42`. However, the
+function type of the delegate is still `void(int)`. This is also the signature
+of its function call operator.
+
+Another interesting aspect of the delegate class is that it accepts also
+functions with a list of parameters that is shorter than that of the function
+type used to specialize the delegate itself.<br/>
+The following code is therefore perfectly valid:
+
+```cpp
+void g() { /* ... */ }
+delegate.connect<&g>();
+delegate(42);
+```
+
+Where the function type of the delegate is `void(int)` as above. It goes without
+saying that the extra arguments are silently discarded internally.<br/>
+This is a nice-to-have feature in a lot of cases, as an example when the
+`delegate` class is used as a building block of a signal-slot system.
+
+To create and initialize a delegate at once, there are a few specialized
+constructors. Because of the rules of the language, the listener is provided by
+means of the `entt::connect_arg` variable template:
+
+```cpp
+entt::delegate<int(int)> func{entt::connect_arg<&f>};
+```
+
+Aside `connect`, a `disconnect` counterpart isn't provided. Instead, there
+exists a `reset` member function to use to clear a delegate.<br/>
+To know if a delegate is empty, it can be used explicitly in every conditional
+statement:
+
+```cpp
+if(delegate) {
+    // ...
+}
+```
+
+Finally, to invoke a delegate, the function call operator is the way to go as
+already shown in the examples above:
+
+```cpp
+auto ret = delegate(42);
+```
+
+In all cases, the listeners don't have to strictly follow the signature of the
+delegate. As long as a listener can be invoked with the given arguments to yield
+a result that is convertible to the given result type, everything works just
+fine.
+
+As a side note, note that members of a class may or may not be associated with
+instances. If they are not, the first argument of the function type must be that
+of the class on which the members operate and an instance of this class must
+obviously be passed when invoking the delegate:
+
+```
+entt::delegate<void(my_struct &, int)> delegate;
+delegate.connect<&my_struct::f>();
+
+my_struct instance;
+delegate(instance, 42);
+```
+
+In this case, it's not possible to deduce the function type since the first
+argument doesn't necessarily have to be a reference (for example, it can be a
+pointer, as well as a const reference).<br/>
+Therefore, the function type must be declared explicitly for unbound members.
+
+# Signals
+
+Signal handlers work with references to classes, function pointers and pointers
+to members. Listeners can be any kind of objects and users are in charge of
+connecting and disconnecting them from a signal to avoid crashes due to
+different lifetimes. On the other side, performance shouldn't be affected that
+much by the presence of such a signal handler.<br/>
+Signals make use of delegates internally and therefore they undergo the same
+rules and offer similar functionalities. It may be a good idea to consult the
+documentation of the `delegate` class for further information.
+
+A signal handler can be used as a private data member without exposing any
+_publish_ functionality to the clients of a class. The basic idea is to impose a
+clear separation between the signal itself and the `sink` class, that is a tool
+to be used to connect and disconnect listeners on the fly.
+
+The API of a signal handler is straightforward. If a collector is supplied to
+the signal when something is published, all the values returned by the listeners
+can be literally _collected_ and used later by the caller. Otherwise, the class
+works just like a plain signal that emits events from time to time.<br/>
+To create instances of signal handlers it is sufficient to provide the type of
+function to which they refer:
+
+```cpp
+entt::sigh<void(int, char)> signal;
+```
+
+Signals offer all the basic functionalities required to know how many listeners
+they contain (`size`) or if they contain at least a listener (`empty`), as well
+as a function to use to swap handlers (`swap`).
+
+Besides them, there are member functions to use both to connect and disconnect
+listeners in all their forms by means of a sink:
+
+```cpp
+void foo(int, char) { /* ... */ }
+
+struct listener {
+    void bar(const int &, char) { /* ... */ }
+};
+
+// ...
+
+entt::sink sink{signal};
+listener instance;
+
+sink.connect<&foo>();
+sink.connect<&listener::bar>(instance);
+
+// ...
+
+// disconnects a free function
+sink.disconnect<&foo>();
+
+// disconnect a member function of an instance
+sink.disconnect<&listener::bar>(instance);
+
+// disconnect all member functions of an instance, if any
+sink.disconnect(instance);
+
+// discards all listeners at once
+sink.disconnect();
+```
+
+As shown above, the listeners don't have to strictly follow the signature of the
+signal. As long as a listener can be invoked with the given arguments to yield a
+result that is convertible to the given return type, everything works just
+fine.<br/>
+It's also possible to connect a listener before other listeners already
+contained by the signal. The `before` function returns a `sink` object correctly
+initialized for the purpose that can be used to connect one or more listeners in
+order and in the desired position:
+
+```cpp
+sink.before<&foo>().connect<&listener::bar>(instance);
+```
+
+In all cases, the `connect` member function returns by default a `connection`
+object to be used as an alternative to break a connection by means of its
+`release` member function. A `scoped_connection` can also be created from a
+connection. In this case, the link is broken automatically as soon as the object
+goes out of scope.
+
+Once listeners are attached (or even if there are no listeners at all), events
+and data in general can be published through a signal by means of the `publish`
+member function:
+
+```cpp
+signal.publish(42, 'c');
+```
+
+To collect data, the `collect` member function should be used instead. Below is
+a minimal example to show how to use it:
+
+```cpp
+int f() { return 0; }
+int g() { return 1; }
+
+// ...
+
+entt::sigh<int()> signal;
+entt::sink sink{signal};
+
+sink.connect<&f>();
+sink.connect<&g>();
+
+std::vector<int> vec{};
+signal.collect([&vec](int value) { vec.push_back(value); });
+
+assert(vec[0] == 0);
+assert(vec[1] == 1);
+```
+
+A collector must expose a function operator that accepts as an argument a type
+to which the return type of the listeners can be converted. Moreover, it can
+optionally return a boolean value that is true to stop collecting data, false
+otherwise. This way one can avoid calling all the listeners in case it isn't
+necessary.<br/>
+Functors can also be used in place of a lambda. Since the collector is copied
+when invoking the `collect` member function, `std::ref` is the way to go in this
+case:
+
+```cpp
+struct my_collector {
+    std::vector<int> vec{};
+
+    bool operator()(int v) noexcept {
+        vec.push_back(v);
+        return true;
+    }
+};
+
+// ...
+
+my_collector collector;
+signal.collect(std::ref(collector));
+```
+
+# Event dispatcher
+
+The event dispatcher class is designed so as to be used in a loop. It allows
+users both to trigger immediate events or to queue events to be published all
+together once per tick.<br/>
+This class shares part of its API with the one of the signal handler, but it
+doesn't require that all the types of events are specified when declared:
+
+```cpp
+// define a general purpose dispatcher
+entt::dispatcher dispatcher{};
+```
+
+In order to register an instance of a class to a dispatcher, its type must
+expose one or more member functions the arguments of which are such that
+`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 +1,67 @@
+#ifndef ENTT_CONFIG_CONFIG_H
+#define ENTT_CONFIG_CONFIG_H
+
+
+#ifndef ENTT_NOEXCEPT
+#   define ENTT_NOEXCEPT noexcept
+#endif
+
+
+#ifndef ENTT_HS_SUFFIX
+#   define ENTT_HS_SUFFIX _hs
+#endif
+
+
+#ifndef ENTT_HWS_SUFFIX
+#   define ENTT_HWS_SUFFIX _hws
+#endif
+
+
+#ifndef ENTT_USE_ATOMIC
+#   define ENTT_MAYBE_ATOMIC(Type) Type
+#else
+#   include <atomic>
+#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#endif
+
+
+#ifndef ENTT_ID_TYPE
+#   include <cstdint>
+#   define ENTT_ID_TYPE std::uint32_t
+#endif
+
+
+#ifndef ENTT_PAGE_SIZE
+#   define ENTT_PAGE_SIZE 32768
+#endif
+
+
+#ifndef ENTT_ASSERT
+#   include <cassert>
+#   define ENTT_ASSERT(condition) assert(condition)
+#endif
+
+
+#ifndef ENTT_DISABLE_ETO
+#   include <type_traits>
+#   define ENTT_ENABLE_ETO(Type) std::is_empty_v<Type>
+#else
+#   // sfinae-friendly definition
+#   define ENTT_ENABLE_ETO(Type) (false && std::is_empty_v<Type>)
+#endif
+
+
+#ifndef ENTT_STANDARD_CPP
+#   if defined _MSC_VER
+#      define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#      define ENTT_PRETTY_FUNCTION_CONSTEXPR ENTT_PRETTY_FUNCTION
+#   elif defined __clang__ || (defined __GNUC__ && __GNUC__ > 8)
+#      define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#      define ENTT_PRETTY_FUNCTION_CONSTEXPR ENTT_PRETTY_FUNCTION
+#   elif defined __GNUC__
+#      define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#   endif
+#endif
+
+
+#endif

src/entt/config/version.h

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

src/entt/core/algorithm.hpp

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

src/entt/core/attribute.h

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

src/entt/core/family.hpp

@@ -2,8 +2,7 @@
 #define ENTT_CORE_FAMILY_HPP
 
 
-#include<type_traits>
-#include<cstddef>
+#include "../config/config.h"
 
 
 namespace entt {
@@ -17,34 +16,21 @@ 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++;
 };
 
 
 }
 
 
-#endif // ENTT_CORE_FAMILY_HPP
+#endif

src/entt/core/hashed_string.hpp

@@ -2,108 +2,261 @@
 #define ENTT_CORE_HASHED_STRING_HPP
 
 
+#include <cstddef>
 #include <cstdint>
+#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> {
+    using type = std::uint32_t;
+    static constexpr std::uint32_t offset = 2166136261;
+    static constexpr std::uint32_t prime = 16777619;
+};
+
+
+template<>
+struct fnv1a_traits<std::uint64_t> {
+    using type = std::uint64_t;
+    static constexpr std::uint64_t offset = 14695981039346656037ull;
+    static constexpr std::uint64_t prime = 1099511628211ull;
+};
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond 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(const Char *curr) ENTT_NOEXCEPT {
+        auto value = traits_type::offset;
+
+        while(*curr != 0) {
+            value = (value ^ static_cast<traits_type::type>(*(curr++))) * traits_type::prime;
+        }
+
+        return value;
     }
 
 public:
+    /*! @brief Character type. */
+    using value_type = Char;
     /*! @brief Unsigned integer type. */
-    using hash_type = 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 value(const value_type (&str)[N]) ENTT_NOEXCEPT {
+        return helper(str);
+    }
+
+    /**
+     * @brief Returns directly the numeric representation of a string.
+     * @param wrapper Helps achieving the purpose by relying on overloading.
+     * @return The numeric representation of the string.
+     */
+    static hash_type value(const_wrapper wrapper) ENTT_NOEXCEPT {
+        return helper(wrapper.str);
+    }
+
+    /**
+     * @brief Returns directly the numeric representation of a string view.
+     * @param str Human-readable identifer.
+     * @param size Length of the string to hash.
+     * @return The numeric representation of the string.
+     */
+    static hash_type value(const value_type *str, std::size_t size) ENTT_NOEXCEPT {
+        ENTT_ID_TYPE partial{traits_type::offset};
+        while(size--) { partial = (partial^(str++)[0])*traits_type::prime; }
+        return partial;
+    }
+
+    /*! @brief Constructs an empty hashed string. */
+    constexpr basic_hashed_string() ENTT_NOEXCEPT
+        : str{nullptr}, hash{}
+    {}
+
+    /**
+     * @brief Constructs a hashed string from an array of const characters.
+     *
+     * Forcing template resolution avoids implicit conversions. An
+     * human-readable identifier can be anything but a plain, old bunch of
+     * characters.<br/>
+     * Example of use:
+     * @code{.cpp}
+     * basic_hashed_string<char> hs{"my.png"};
+     * @endcode
+     *
+     * @tparam N Number of characters of the identifier.
+     * @param curr Human-readable identifer.
+     */
+    template<std::size_t N>
+    constexpr basic_hashed_string(const value_type (&curr)[N]) ENTT_NOEXCEPT
+        : str{curr}, hash{helper(curr)}
     {}
 
     /**
      * @brief Explicit constructor on purpose to avoid constructing a hashed
-     * string directly from a `const 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(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;
+    }
+
+    /*! @copydoc data */
+    constexpr operator const value_type *() const ENTT_NOEXCEPT { return data(); }
+
+    /**
+     * @brief Returns the numeric representation of a hashed string.
+     * @return The numeric representation of the instance.
+     */
+    constexpr operator hash_type() const ENTT_NOEXCEPT { return value(); }
 
     /**
      * @brief Compares two hashed strings.
      * @param other Hashed string with which to compare.
      * @return True if the two hashed strings are identical, false otherwise.
      */
-    constexpr bool operator==(const 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>;
+
+
 }
 
 
-#endif // ENTT_CORE_HASHED_STRING_HPP
+/**
+ * @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};
+}
+
+
+#endif

src/entt/core/ident.hpp

@@ -2,72 +2,34 @@
 #define ENTT_CORE_IDENT_HPP
 
 
-#include<type_traits>
-#include<cstddef>
-#include<utility>
+#include <tuple>
+#include <cstddef>
+#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:
@@ -75,22 +37,29 @@ 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...>) {
+        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...>{});
+};
 
 
 }
 
 
-#endif // ENTT_CORE_IDENT_HPP
+#endif

src/entt/core/monostate.hpp

@@ -0,0 +1,61 @@
+#ifndef ENTT_CORE_MONOSTATE_HPP
+#define ENTT_CORE_MONOSTATE_HPP
+
+
+#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

src/entt/core/type_info.hpp

@@ -0,0 +1,80 @@
+#ifndef ENTT_CORE_TYPE_INFO_HPP
+#define ENTT_CORE_TYPE_INFO_HPP
+
+
+#include "../config/config.h"
+#include "../core/attribute.h"
+#include "hashed_string.hpp"
+
+
+#ifndef ENTT_PRETTY_FUNCTION
+#   define ENTT_TYPE_ID_API ENTT_API
+#else
+#   define ENTT_TYPE_ID_API
+#endif
+
+
+namespace entt {
+
+
+#ifndef ENTT_PRETTY_FUNCTION
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+struct ENTT_API type_id_generator {
+    static ENTT_ID_TYPE next() ENTT_NOEXCEPT {
+        static ENTT_ID_TYPE value{};
+        return value++;
+    }
+};
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond TURN_OFF_DOXYGEN
+ */
+#endif
+
+
+/**
+ * @brief Types identifiers.
+ * @tparam Type Type for which to generate an identifier.
+ */
+template<typename Type, typename = void>
+struct ENTT_TYPE_ID_API type_info {
+    /**
+     * @brief Returns the numeric representation of a given type.
+     * @return The numeric representation of the given type.
+     */
+#if defined ENTT_PRETTY_FUNCTION_CONSTEXPR
+    static constexpr ENTT_ID_TYPE id() ENTT_NOEXCEPT {
+        constexpr auto value = entt::hashed_string::value(ENTT_PRETTY_FUNCTION_CONSTEXPR);
+        return value;
+    }
+#elif defined ENTT_PRETTY_FUNCTION
+    static ENTT_ID_TYPE id() ENTT_NOEXCEPT {
+        static const auto value = entt::hashed_string::value(ENTT_PRETTY_FUNCTION);
+        return value;
+    }
+#else
+    static ENTT_ID_TYPE id() ENTT_NOEXCEPT {
+        static const ENTT_ID_TYPE value = internal::type_id_generator::next();
+        return value;
+    }
+#endif
+};
+
+
+}
+
+
+#endif

src/entt/core/type_traits.hpp

@@ -0,0 +1,229 @@
+#ifndef ENTT_CORE_TYPE_TRAITS_HPP
+#define ENTT_CORE_TYPE_TRAITS_HPP
+
+
+#include <cstddef>
+#include <utility>
+#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 Extracts the class of a non-static member object or function.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+class member_class {
+    static_assert(std::is_member_pointer_v<Member>);
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...));
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...) const);
+
+    template<typename Class, typename Type>
+    static Class * clazz(Type Class:: *);
+
+public:
+    /*! @brief The class of the given non-static member object or function. */
+    using type = std::remove_pointer_t<decltype(clazz(std::declval<Member>()))>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+using member_class_t = typename member_class<Member>::type;
+
+
+/**
+ * @brief Alias template to ease the creation of named values.
+ * @tparam Value A constant value at least convertible to `ENTT_ID_TYPE`.
+ */
+template<ENTT_ID_TYPE Value>
+using tag = std::integral_constant<ENTT_ID_TYPE, Value>;
+
+
+}
+
+
+/**
+ * @brief Defines an enum class to use for opaque identifiers and a dedicate
+ * `to_integer` function to convert the identifiers to their underlying type.
+ * @param clazz The name to use for the enum class.
+ * @param type The underlying type for the enum class.
+ */
+#define ENTT_OPAQUE_TYPE(clazz, type)\
+    enum class clazz: type {};\
+    constexpr auto to_integral(const clazz id) ENTT_NOEXCEPT {\
+        return static_cast<std::underlying_type_t<clazz>>(id);\
+    }\
+    static_assert(true)
+
+
+#endif

src/entt/core/utility.hpp

@@ -0,0 +1,105 @@
+#ifndef ENTT_CORE_UTILITY_HPP
+#define ENTT_CORE_UTILITY_HPP
+
+
+#include <utility>
+#include "../config/config.h"
+
+
+namespace entt {
+
+
+/*! @brief Identity function object (waiting for C++20). */
+struct identity {
+    /**
+     * @brief Returns its argument unchanged.
+     * @tparam Type Type of the argument.
+     * @param value The actual argument.
+     * @return The submitted value as-is.
+     */
+    template<class Type>
+    constexpr Type && operator()(Type &&value) const ENTT_NOEXCEPT {
+        return std::forward<Type>(value);
+    }
+};
+
+
+/**
+ * @brief Constant utility to disambiguate overloaded members of a class.
+ * @tparam Type Type of the desired overload.
+ * @tparam Class Type of class to which the member belongs.
+ * @param member A valid pointer to a member.
+ * @return Pointer to the member.
+ */
+template<typename Type, typename Class>
+constexpr auto overload(Type Class:: *member) ENTT_NOEXCEPT { return member; }
+
+
+/**
+ * @brief Constant utility to disambiguate overloaded functions.
+ * @tparam Func Function type of the desired overload.
+ * @param func A valid pointer to a function.
+ * @return Pointer to the function.
+ */
+template<typename Func>
+constexpr auto overload(Func *func) ENTT_NOEXCEPT { return func; }
+
+
+/**
+ * @brief Helper type for visitors.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+struct overloaded: Func... {
+    using Func::operator()...;
+};
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+overloaded(Func...) -> overloaded<Func...>;
+
+
+/**
+ * @brief Basic implementation of a y-combinator.
+ * @tparam Func Type of a potentially recursive function.
+ */
+template<class Func>
+struct y_combinator {
+    /**
+     * @brief Constructs a y-combinator from a given function.
+     * @param recursive A potentially recursive function.
+     */
+    y_combinator(Func recursive):
+        func{std::move(recursive)}
+    {}
+
+    /**
+     * @brief Invokes a y-combinator and therefore its underlying function.
+     * @tparam Args Types of arguments to use to invoke the underlying function.
+     * @param args Parameters to use to invoke the underlying function.
+     * @return Return value of the underlying function, if any.
+     */
+    template <class... Args>
+    decltype(auto) operator()(Args &&... args) const {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+    /*! @copydoc operator()() */
+    template <class... Args>
+    decltype(auto) operator()(Args &&... args) {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+private:
+    Func func;
+};
+
+
+}
+
+
+#endif

src/entt/entity/actor.hpp

@@ -3,50 +3,97 @@
 
 
 #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 Move constructor.
+     *
+     * After actor move construction, instances that have been moved from are
+     * placed in a valid but unspecified state. It's highly discouraged to
+     * continue using them.
+     *
+     * @param other The instance to move from.
+     */
+    basic_actor(basic_actor &&other) ENTT_NOEXCEPT
+        : entt{other.entt}, reg{other.reg}
+    {
+        other.entt = null;
     }
 
-    /*! @brief Default copy constructor. */
-    Actor(const Actor &) = default;
-    /*! @brief Default move constructor. */
-    Actor(Actor &&) = default;
+    /**
+     * @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 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 Constructs an actor from a given entity.
+     * @param entity A valid entity identifier.
+     * @param ref An instance of the registry class.
+     */
+    explicit basic_actor(entity_type entity, registry_type &ref) ENTT_NOEXCEPT
+        : entt{entity}, reg{&ref}
+    {
+        ENTT_ASSERT(ref.valid(entity));
+    }
+
+    /*! @brief Default destructor. */
+    virtual ~basic_actor() {
+        if(*this) {
+            reg->destroy(entt);
+        }
+    }
+
+    /**
+     * @brief Move assignment operator.
+     *
+     * After actor move assignment, instances that have been moved from are
+     * placed in a valid but unspecified state. It's highly discouraged to
+     * continue using them.
+     *
+     * @param other The instance to move from.
+     * @return This actor.
+     */
+    basic_actor & operator=(basic_actor &&other) ENTT_NOEXCEPT {
+        if(this != &other) {
+            auto tmp{std::move(other)};
+            std::swap(reg, tmp.reg);
+            std::swap(entt, tmp.entt);
+        }
+
+        return *this;
+    }
 
     /**
      * @brief Assigns the given component to an actor.
@@ -63,8 +110,8 @@ 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)...);
     }
 
     /**
@@ -72,81 +119,88 @@ 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 {
+        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 {
+        return std::as_const(*reg).template get<Component...>(entt);
+    }
+
+    /*! @copydoc get */
+    template<typename... Component>
+    decltype(auto) get() {
+        return reg->template get<Component...>(entt);
     }
 
     /**
-     * @brief Returns 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 {
+        return std::as_const(*reg).template try_get<Component...>(entt);
+    }
+
+    /*! @copydoc try_get */
+    template<typename... Component>
+    auto try_get() {
+        return reg->template try_get<Component...>(entt);
     }
 
     /**
      * @brief Returns a reference to the underlying registry.
-     * @return A reference to the underlying registry
+     * @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 {
+        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>;
-
-
 }
 
 
-#endif // ENTT_ENTITY_ACTOR_HPP
+#endif

src/entt/entity/entity.hpp

@@ -0,0 +1,175 @@
+#ifndef ENTT_ENTITY_ENTITY_HPP
+#define ENTT_ENTITY_ENTITY_HPP
+
+
+#include <cstdint>
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/type_traits.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Entity traits.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error unless the template parameter is an accepted entity type.
+ */
+template<typename>
+struct entt_traits;
+
+
+/**
+ * @brief Entity traits for a 16 bits entity identifier.
+ *
+ * A 16 bits entity identifier guarantees:
+ *
+ * * 12 bits for the entity number (up to 4k entities).
+ * * 4 bit for the version (resets in [0-15]).
+ */
+template<>
+struct entt_traits<std::uint16_t> {
+    /*! @brief Underlying entity type. */
+    using entity_type = std::uint16_t;
+    /*! @brief Underlying version type. */
+    using version_type = std::uint8_t;
+    /*! @brief Difference type. */
+    using difference_type = std::int32_t;
+
+    /*! @brief Mask to use to get the entity number out of an identifier. */
+    static constexpr std::uint16_t entity_mask = 0xFFF;
+    /*! @brief Mask to use to get the version out of an identifier. */
+    static constexpr std::uint16_t version_mask = 0xF;
+    /*! @brief Extent of the entity number within an identifier. */
+    static constexpr auto entity_shift = 12;
+};
+
+
+/**
+ * @brief Entity traits for a 32 bits entity identifier.
+ *
+ * A 32 bits entity identifier guarantees:
+ *
+ * * 20 bits for the entity number (suitable for almost all the games).
+ * * 12 bit for the version (resets in [0-4095]).
+ */
+template<>
+struct entt_traits<std::uint32_t> {
+    /*! @brief Underlying entity type. */
+    using entity_type = std::uint32_t;
+    /*! @brief Underlying version type. */
+    using version_type = std::uint16_t;
+    /*! @brief Difference type. */
+    using difference_type = std::int64_t;
+
+    /*! @brief Mask to use to get the entity number out of an identifier. */
+    static constexpr std::uint32_t entity_mask = 0xFFFFF;
+    /*! @brief Mask to use to get the version out of an identifier. */
+    static constexpr std::uint32_t version_mask = 0xFFF;
+    /*! @brief Extent of the entity number within an identifier. */
+    static constexpr auto entity_shift = 20;
+};
+
+
+/**
+ * @brief Entity traits for a 64 bits entity identifier.
+ *
+ * A 64 bits entity identifier guarantees:
+ *
+ * * 32 bits for the entity number (an indecently large number).
+ * * 32 bit for the version (an indecently large number).
+ */
+template<>
+struct entt_traits<std::uint64_t> {
+    /*! @brief Underlying entity type. */
+    using entity_type = std::uint64_t;
+    /*! @brief Underlying version type. */
+    using version_type = std::uint32_t;
+    /*! @brief Difference type. */
+    using difference_type = std::int64_t;
+
+    /*! @brief Mask to use to get the entity number out of an identifier. */
+    static constexpr std::uint64_t entity_mask = 0xFFFFFFFF;
+    /*! @brief Mask to use to get the version out of an identifier. */
+    static constexpr std::uint64_t version_mask = 0xFFFFFFFF;
+    /*! @brief Extent of the entity number within an identifier. */
+    static constexpr auto entity_shift = 32;
+};
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+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_integral(entity) & traits_type<Entity>::entity_mask) == to_integral(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

src/entt/entity/fwd.hpp

@@ -0,0 +1,122 @@
+#ifndef ENTT_ENTITY_FWD_HPP
+#define ENTT_ENTITY_FWD_HPP
+
+
+#include "../config/config.h"
+#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{};
+
+
+/*! @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. */
+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

src/entt/entity/group.hpp

@@ -0,0 +1,890 @@
+#ifndef ENTT_ENTITY_GROUP_HPP
+#define ENTT_ENTITY_GROUP_HPP
+
+
+#include <tuple>
+#include <utility>
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/type_traits.hpp"
+#include "sparse_set.hpp"
+#include "storage.hpp"
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief 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 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.
+ *
+ * @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 other cases, modifying the pools iterated by the group 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 instances 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_ entities and components).
+ *
+ * @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 a group or some pools are empty.
+     * @tparam Component Types of components in which one is interested.
+     * @return True if the group or the pools 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 Returns the first entity that has the given components, if any.
+     * @return The first entity that has the given components if one exists, the
+     * null entity otherwise.
+     */
+    entity_type front() const {
+        const auto it = begin();
+        return it != end() ? *it : null;
+    }
+
+    /**
+     * @brief Returns the last entity that has the given components, if any.
+     * @return The last entity that has the given components if one exists, the
+     * null entity otherwise.
+     */
+    entity_type back() const {
+        const auto it = std::make_reverse_iterator(end());
+        return it != std::make_reverse_iterator(begin()) ? *it : null;
+    }
+
+    /**
+     * @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 {
+        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 {
+        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 {
+        return handler->has(entt);
+    }
+
+    /**
+     * @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 counterpart.
+     *
+     * @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_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 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 instances 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 other cases, modifying the pools iterated by the group 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 a group or some pools are empty.
+     * @tparam Component Types of components in which one is interested.
+     * @return True if the group or the pools 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 Returns the first entity that has the given components, if any.
+     * @return The first entity that has the given components if one exists, the
+     * null entity otherwise.
+     */
+    entity_type front() const {
+        const auto it = begin();
+        return it != end() ? *it : null;
+    }
+
+    /**
+     * @brief Returns the last entity that has the given components, if any.
+     * @return The last entity that has the given components if one exists, the
+     * null entity otherwise.
+     */
+    entity_type back() const {
+        const auto it = std::make_reverse_iterator(end());
+        return it != std::make_reverse_iterator(begin()) ? *it : null;
+    }
+
+    /**
+     * @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 {
+        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 {
+        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 {
+        return std::get<0>(pools)->has(entt) && (std::get<0>(pools)->index(entt) < (*length));
+    }
+
+    /**
+     * @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 counterpart.
+     *
+     * @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_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

src/entt/entity/helper.hpp

@@ -0,0 +1,117 @@
+#ifndef ENTT_ENTITY_HELPER_HPP
+#define ENTT_ENTITY_HELPER_HPP
+
+
+#include <type_traits>
+#include "../core/type_traits.hpp"
+#include "../config/config.h"
+#include "registry.hpp"
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Converts a registry to a view.
+ * @tparam Const Constness of the accepted registry.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<bool Const, typename Entity>
+struct as_view {
+    /*! @brief Type of registry to convert. */
+    using registry_type = std::conditional_t<Const, const 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>;
+
+
+}
+
+
+#endif

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 <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 basic_registry<Entity> &reg, const Entity entt) {
+            if(reg.template has<Require...>(entt) && !(reg.template has<Reject>(entt) || ...)) {
+                if(auto *comp = obs.view.try_get(entt); !comp) {
+                    obs.view.construct(entt);
+                }
+
+                obs.view.get(entt) |= (1 << Index);
+            }
+        }
+
+        template<std::size_t Index>
+        static void discard_if(basic_observer &obs, const basic_registry<Entity> &, const Entity entt) {
+            if(auto *value = obs.view.try_get(entt); value && !(*value &= (~(1 << Index)))) {
+                obs.view.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 basic_registry<Entity> &reg, const Entity entt) {
+            if(reg.template has<AllOf..., Require...>(entt) && !(reg.template has<NoneOf>(entt) || ...) && !(reg.template has<Reject>(entt) || ...)) {
+                if(auto *comp = obs.view.try_get(entt); !comp) {
+                    obs.view.construct(entt);
+                }
+
+                obs.view.get(entt) |= (1 << Index);
+            }
+        }
+
+        template<std::size_t Index>
+        static void discard_if(basic_observer &obs, const basic_registry<Entity> &, const Entity entt) {
+            if(auto *value = obs.view.try_get(entt); value && !(*value &= (~(1 << Index)))) {
+                obs.view.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()
+        : target{}, release{}, view{}
+    {}
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    basic_observer(const basic_observer &) = delete;
+    /*! @brief Default move constructor, deleted on purpose. */
+    basic_observer(basic_observer &&) = delete;
+
+    /**
+     * @brief Creates an observer and connects it to a given registry.
+     * @tparam Matcher Types of matchers to use to initialize the observer.
+     * @param reg A valid reference to a registry.
+     */
+    template<typename... Matcher>
+    basic_observer(basic_registry<entity_type> &reg, basic_collector<Matcher...>)
+        : target{&reg},
+          release{},
+          view{}
+    {
+        connect<Matcher...>(reg, std::index_sequence_for<Matcher...>{});
+    }
+
+    /*! @brief Default destructor. */
+    ~basic_observer() = default;
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This observer.
+     */
+    basic_observer & operator=(const basic_observer &) = delete;
+
+    /**
+     * @brief Default move assignment operator, deleted on purpose.
+     * @return This observer.
+     */
+    basic_observer & operator=(basic_observer &&) = delete;
+
+    /**
+     * @brief Connects an observer to a given registry.
+     * @tparam Matcher Types of matchers to use to initialize the observer.
+     * @param reg A valid reference to a registry.
+     */
+    template<typename... Matcher>
+    void connect(basic_registry<entity_type> &reg, basic_collector<Matcher...>) {
+        disconnect();
+        connect<Matcher...>(reg, std::index_sequence_for<Matcher...>{});
+        target = &reg;
+        view.clear();
+    }
+
+    /*! @brief Disconnects an observer from the registry it keeps track of. */
+    void disconnect() {
+        if(release) {
+            release(*this, *target);
+            release = nullptr;
+        }
+    }
+
+    /**
+     * @brief Returns the number of elements in an observer.
+     * @return Number of elements.
+     */
+    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 Clears the underlying container. */
+    void clear() ENTT_NOEXCEPT {
+        view.clear();
+    }
+
+    /**
+     * @brief Iterates entities and applies the given function object to them.
+     *
+     * The function object is invoked for each entity.<br/>
+     * The signature of the function must be equivalent to the following form:
+     *
+     * @code{.cpp}
+     * void(const entity_type);
+     * @endcode
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        static_assert(std::is_invocable_v<Func, entity_type>);
+
+        for(const auto entity: *this) {
+            func(entity);
+        }
+    }
+
+    /**
+     * @brief Iterates entities and applies the given function object to them,
+     * then clears the observer.
+     *
+     * @sa each
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) {
+        std::as_const(*this).each(std::move(func));
+        clear();
+    }
+
+private:
+    basic_registry<entity_type> *target;
+    void(* release)(basic_observer &, basic_registry<entity_type> &);
+    storage<entity_type, payload_type> view;
+};
+
+
+}
+
+
+#endif

src/entt/entity/runtime_view.hpp

@@ -0,0 +1,263 @@
+#ifndef ENTT_ENTITY_RUNTIME_VIEW_HPP
+#define ENTT_ENTITY_RUNTIME_VIEW_HPP
+
+
+#include <iterator>
+#include <vector>
+#include <utility>
+#include <algorithm>
+#include <type_traits>
+#include "../config/config.h"
+#include "sparse_set.hpp"
+#include "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>;
+
+        using direct_type = std::vector<const sparse_set<Entity> *>;
+
+        iterator(const direct_type *all, underlying_iterator_type curr) ENTT_NOEXCEPT
+            : pools{all},
+              it{curr}
+        {
+            if(it != (*pools)[0]->end() && !valid()) {
+                ++(*this);
+            }
+        }
+
+        bool valid() const {
+            return std::all_of(pools->begin()++, pools->end(), [entt = *it](const auto *curr) {
+                return curr->has(entt);
+            });
+        }
+
+    public:
+        using difference_type = typename underlying_iterator_type::difference_type;
+        using value_type = typename underlying_iterator_type::value_type;
+        using pointer = typename underlying_iterator_type::pointer;
+        using reference = typename underlying_iterator_type::reference;
+        using iterator_category = std::bidirectional_iterator_tag;
+
+        iterator() ENTT_NOEXCEPT = default;
+
+        iterator & operator++() {
+            while(++it != (*pools)[0]->end() && !valid());
+            return *this;
+        }
+
+        iterator operator++(int) {
+            iterator orig = *this;
+            return operator++(), orig;
+        }
+
+        iterator & operator--() ENTT_NOEXCEPT {
+            while(--it != (*pools)[0]->begin() && !valid());
+            return *this;
+        }
+
+        iterator operator--(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return operator--(), orig;
+        }
+
+        bool operator==(const iterator &other) const ENTT_NOEXCEPT {
+            return other.it == it;
+        }
+
+        bool operator!=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+        pointer operator->() const {
+            return it.operator->();
+        }
+
+        reference operator*() const {
+            return *operator->();
+        }
+
+    private:
+        const direct_type *pools;
+        underlying_iterator_type it;
+    };
+
+    basic_runtime_view(std::vector<const sparse_set<Entity> *> others) ENTT_NOEXCEPT
+        : pools{std::move(others)}
+    {
+        const auto it = std::min_element(pools.begin(), pools.end(), [](const auto *lhs, const auto *rhs) {
+            return (!lhs && rhs) || (lhs && rhs && lhs->size() < rhs->size());
+        });
+
+        // brings the best candidate (if any) on front of the vector
+        std::rotate(pools.begin(), it, pools.end());
+    }
+
+    bool valid() const {
+        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 {
+        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 {
+        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 {
+        iterator_type it{};
+
+        if(valid()) {
+            it = { &pools, pools[0]->begin() };
+        }
+
+        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 {
+        iterator_type it{};
+
+        if(valid()) {
+            it = { &pools, pools[0]->end() };
+        }
+
+        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 {
+        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 {
+        for(const auto entity: *this) {
+            func(entity);
+        }
+    }
+
+private:
+    std::vector<const sparse_set<Entity> *> pools;
+};
+
+
+}
+
+
+#endif

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([[maybe_unused]] Other &instance, [[maybe_unused]] Member Type:: *member) {
+        if constexpr(!std::is_same_v<Other, Type>) {
+            return;
+        } else if constexpr(std::is_same_v<Member, Entity>) {
+            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 remove_if_exists<Component>(local);
+            }
+        }
+    }
+
+    template<typename Other, typename Archive, typename... Type, typename... Member>
+    void assign(Archive &archive, [[maybe_unused]] Member Type:: *... member) {
+        typename traits_type::entity_type length{};
+        archive(length);
+
+        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

src/entt/entity/sparse_set.hpp

@@ -3,26 +3,21 @@
 
 
 #include <algorithm>
+#include <iterator>
 #include <utility>
 #include <vector>
+#include <memory>
 #include <cstddef>
-#include <cassert>
-#include "traits.hpp"
+#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 +25,9 @@ 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 +46,229 @@ 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 = std::vector<Entity>;
+        using index_type = typename traits_type::difference_type;
+
+        iterator(const 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;
-            return ++(*this), orig;
+        iterator operator++(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return operator++(), 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 operator--(), 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 {
+            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 {
+            const auto pos = size_type(index-1);
+            return &(*direct)[pos];
+        }
+
+        reference operator*() const {
+            return *operator->();
         }
 
     private:
-        const std::vector<Entity> *direct;
-        std::size_t pos;
+        const direct_type *direct;
+        index_type index;
     };
 
-    static constexpr Entity in_use = 1 << traits_type::entity_shift;
+    auto page(const Entity entt) const ENTT_NOEXCEPT {
+        return std::size_t{(to_integral(entt) & traits_type::entity_mask) / entt_per_page};
+    }
+
+    auto offset(const Entity entt) const ENTT_NOEXCEPT {
+        return std::size_t{to_integral(entt) & (entt_per_page - 1)};
+    }
+
+    Entity * assure(const std::size_t pos) {
+        if(!(pos < reverse.size())) {
+            reverse.resize(pos+1);
+        }
+
+        if(!reverse[pos]) {
+            reverse[pos] = std::make_unique<entity_type[]>(entt_per_page);
+            // null is safe in all cases for our purposes
+            std::fill_n(reverse[pos].get(), entt_per_page, null);
+        }
+
+        return reverse[pos].get();
+    }
 
 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]) {
+                std::copy_n(other.reverse[pos].get(), entt_per_page, assure(pos));
+            }
+        }
+    }
+
+    /*! @brief Default move constructor. */
+    sparse_set(sparse_set &&) = default;
 
     /*! @brief Default destructor. */
-    virtual ~SparseSet() noexcept = default;
+    virtual ~sparse_set() = 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 +280,7 @@ public:
      *
      * @return Number of elements.
      */
-    size_type size() const noexcept {
+    size_type size() const ENTT_NOEXCEPT {
         return direct.size();
     }
 
@@ -133,7 +288,7 @@ 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 +299,7 @@ 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 +307,65 @@ 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 {
+        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 {
+        const auto curr = page(entt);
+        // testing against null permits to avoid accessing the direct vector
+        return (curr < reverse.size() && reverse[curr] && reverse[curr][offset(entt)] != null);
     }
 
     /**
@@ -209,14 +377,12 @@ 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_ASSERT(has(entt));
+        return size_type(reverse[page(entt)][offset(entt)]);
     }
 
     /**
@@ -228,20 +394,35 @@ 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));
+        assure(page(entt))[offset(entt)] = 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 input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     */
+    template<typename It>
+    void construct(It first, It last) {
+        std::for_each(first, last, [this, next = direct.size()](const auto entt) mutable {
+            ENTT_ASSERT(!has(entt));
+            assure(page(entt))[offset(entt)] = 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 +434,20 @@ 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));
+        const auto curr = page(entt);
+        const auto pos = offset(entt);
+        direct[size_type(reverse[curr][pos])] = entity_type(direct.back());
+        reverse[page(direct.back())][offset(direct.back())] = reverse[curr][pos];
+        reverse[curr][pos] = 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.
@@ -284,302 +461,176 @@ 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) {
+        auto &from = reverse[page(lhs)][offset(lhs)];
+        auto &to = reverse[page(rhs)][offset(rhs)];
+        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) {
+            reverse[page(direct[pos])][offset(direct[pos])] = 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]);
+                reverse[page(direct[curr])][offset(direct[curr])] = 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) {
+        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.
+     * @brief Clears a sparse set.
      */
-    virtual void reset() {
+    void clear() ENTT_NOEXCEPT {
         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;
-};
-
-
 }
 
 
-#endif // ENTT_ENTITY_SPARSE_SET_HPP
+#endif

src/entt/entity/storage.hpp

@@ -0,0 +1,718 @@
+#ifndef ENTT_ENTITY_STORAGE_HPP
+#define ENTT_ENTITY_STORAGE_HPP
+
+
+#include <algorithm>
+#include <iterator>
+#include <utility>
+#include <vector>
+#include <cstddef>
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/algorithm.hpp"
+#include "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 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 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 operator++(), orig;
+        }
+
+        iterator & operator--() ENTT_NOEXCEPT {
+            return ++index, *this;
+        }
+
+        iterator operator--(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return operator--(), 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 {
+        return instances[underlying_type::index(entt)];
+    }
+
+    /*! @copydoc get */
+    object_type & get(const entity_type entt) {
+        return const_cast<object_type &>(std::as_const(*this).get(entt));
+    }
+
+    /**
+     * @brief Returns a pointer to the object associated with an entity, if any.
+     * @param entt A valid entity identifier.
+     * @return The object associated with the entity, if any.
+     */
+    const object_type * try_get(const entity_type entt) const {
+        return underlying_type::has(entt) ? (instances.data() + underlying_type::index(entt)) : nullptr;
+    }
+
+    /*! @copydoc try_get */
+    object_type * try_get(const entity_type entt) {
+        return const_cast<object_type *>(std::as_const(*this).try_get(entt));
+    }
+
+    /**
+     * @brief Assigns an entity to a storage and constructs its object.
+     *
+     * This version accept both types that can be constructed in place directly
+     * and types like aggregates that do not work well with a placement new as
+     * performed usually under the hood during an _emplace back_.
+     *
+     * @warning
+     * Attempting to use an entity that already belongs to the storage results
+     * in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * storage already contains the given entity.
+     *
+     * @tparam Args Types of arguments to use to construct the object.
+     * @param entt A valid entity identifier.
+     * @param args Parameters to use to construct an object for the entity.
+     */
+    template<typename... Args>
+    void construct(const entity_type entt, Args &&... args) {
+        if constexpr(std::is_aggregate_v<object_type>) {
+            instances.push_back(Type{std::forward<Args>(args)...});
+        } else {
+            instances.emplace_back(std::forward<Args>(args)...);
+        }
+
+        // entity goes after component in case constructor throws
+        underlying_type::construct(entt);
+    }
+
+    /**
+     * @brief Assigns one or more entities to a storage and constructs their
+     * objects with from a given instance.
+     *
+     * @warning
+     * Attempting to assign an entity that already belongs to the storage
+     * results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * storage already contains the given entity.
+     *
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param value An instance of the object to construct.
+     */
+    template<typename It>
+    std::enable_if_t<std::is_same_v<typename std::iterator_traits<It>::value_type, entity_type>, void>
+    construct(It first, It last, const object_type &value = {}) {
+        instances.insert(instances.end(), std::distance(first, last), value);
+        // entities go after components in case constructors throw
+        underlying_type::construct(first, last);
+    }
+
+    /**
+     * @brief Assigns one or more entities to a storage and constructs their
+     * objects from a given range.
+     *
+     * @sa construct
+     *
+     * @tparam EIt Type of input iterator.
+     * @tparam CIt Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param value An iterator to the first element of the range of objects.
+     */
+    template<typename EIt, typename CIt>
+    std::enable_if_t<std::is_same_v<typename std::iterator_traits<EIt>::value_type, entity_type>, void>
+    construct(EIt first, EIt last, CIt value) {
+        instances.insert(instances.end(), value, value + std::distance(first, last));
+        // entities go after components in case constructors throw
+        underlying_type::construct(first, last);
+    }
+
+    /**
+     * @brief Removes an entity from a storage and destroys its object.
+     *
+     * @warning
+     * Attempting to use an entity that doesn't belong to the storage results in
+     * undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * storage doesn't contain the given entity.
+     *
+     * @param entt A valid entity identifier.
+     */
+    void 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) 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 Clears a storage. */
+    void clear() {
+        underlying_type::clear();
+        instances.clear();
+    }
+
+private:
+    std::vector<object_type> instances;
+};
+
+
+/*! @copydoc storage */
+template<typename Entity, typename Type>
+class storage<Entity, Type, std::enable_if_t<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 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 operator++(), orig;
+        }
+
+        iterator & operator--() ENTT_NOEXCEPT {
+            return ++index, *this;
+        }
+
+        iterator operator--(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return operator--(), 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_ASSERT(underlying_type::has(entt));
+        return {};
+    }
+
+    /**
+     * @brief Assigns an entity to a storage and constructs its object.
+     *
+     * @warning
+     * Attempting to use an entity that already belongs to the storage results
+     * in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * storage already contains the given entity.
+     *
+     * @tparam Args Types of arguments to use to construct the object.
+     * @param entt A valid entity identifier.
+     */
+    template<typename... Args>
+    void construct(const entity_type entt, Args &&...) {
+        underlying_type::construct(entt);
+    }
+
+    /**
+     * @brief Assigns one or more entities to a storage.
+     *
+     * @warning
+     * Attempting to assign an entity that already belongs to the storage
+     * results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * storage already contains the given entity.
+     *
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     */
+    template<typename It>
+    std::enable_if_t<std::is_same_v<typename std::iterator_traits<It>::value_type, entity_type>, void>
+    construct(It first, It last, const object_type & = {}) {
+        underlying_type::construct(first, last);
+    }
+
+    /*! @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)...);
+    }
+};
+
+
+}
+
+
+#endif

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/entt.hpp

@@ -1,19 +1,33 @@
+#include "core/algorithm.hpp"
+#include "core/attribute.h"
 #include "core/family.hpp"
 #include "core/hashed_string.hpp"
 #include "core/ident.hpp"
+#include "core/monostate.hpp"
+#include "core/type_info.hpp"
+#include "core/type_traits.hpp"
+#include "core/utility.hpp"
 #include "entity/actor.hpp"
+#include "entity/entity.hpp"
+#include "entity/group.hpp"
+#include "entity/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/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);
     }
 
@@ -96,20 +96,16 @@ 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{};
-
-
 }
 
 
-#endif // ENTT_LOCATOR_LOCATOR_HPP
+#endif

src/entt/meta/factory.hpp

@@ -0,0 +1,880 @@
+#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 "../core/utility.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_any instance, [[maybe_unused]] meta_any index, [[maybe_unused]] meta_any value) {
+    bool accepted = false;
+
+    if constexpr(!Const) {
+        if constexpr(std::is_function_v<std::remove_reference_t<std::remove_pointer_t<decltype(Data)>>> || std::is_member_function_pointer_v<decltype(Data)>) {
+            using helper_type = meta_function_helper_t<decltype(Data)>;
+            using data_type = std::tuple_element_t<!std::is_member_function_pointer_v<decltype(Data)>, typename helper_type::args_type>;
+            static_assert(std::is_invocable_v<decltype(Data), Type &, data_type>);
+            auto * const clazz = instance.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 = instance.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_any instance, [[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>, std::forward<decltype(value)>(value)};
+        } 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_reference_t<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 = instance.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 = instance.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_any instance, meta_any *args, std::index_sequence<Indexes...>) {
+    using helper_type = meta_function_helper_t<decltype(Candidate)>;
+
+    auto dispatch = [](auto *... params) {
+        if constexpr(std::is_void_v<typename helper_type::return_type> || std::is_same_v<Policy, as_void_t>) {
+            std::invoke(Candidate, *params...);
+            return meta_any{std::in_place_type<void>};
+        } else if constexpr(std::is_same_v<Policy, as_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 *value) {
+        using arg_type = std::remove_reference_t<decltype(*value)>;
+
+        if(!value && any->convert<arg_type>()) {
+            value = any->try_cast<arg_type>();
+        }
+
+        return value;
+    }(args+Indexes, (args+Indexes)->try_cast<std::tuple_element_t<Indexes, typename helper_type::args_type>>())...);
+
+    if constexpr(std::is_function_v<std::remove_reference_t<std::remove_pointer_t<decltype(Candidate)>>>) {
+        return (std::get<Indexes>(direct) && ...) ? dispatch(std::get<Indexes>(direct)...) : meta_any{};
+    } else {
+        auto * const clazz = instance.try_cast<Type>();
+        return (clazz && (std::get<Indexes>(direct) && ...)) ? dispatch(clazz, std::get<Indexes>(direct)...) : meta_any{};
+    }
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond TURN_OFF_DOXYGEN
+ */
+
+
+/**
+ * @brief Meta factory to be used for reflection purposes.
+ *
+ * The meta factory is an utility class used to reflect types, data members and
+ * functions of all sorts. This class ensures that the underlying web of types
+ * is built correctly and performs some checks in debug mode to ensure that
+ * there are no subtle errors at runtime.
+ */
+template<typename...>
+class meta_factory;
+
+
+/**
+ * @brief Extended meta factory to be used for reflection purposes.
+ * @tparam Type Reflected type for which the factory was created.
+ * @tparam Spec Property specialization pack used to disambiguate overloads.
+ */
+template<typename Type, typename... Spec>
+class meta_factory<Type, Spec...>: public meta_factory<Type> {
+    bool exists(const meta_any &key, const internal::meta_prop_node *node) ENTT_NOEXCEPT {
+        return node && (node->key() == key || exists(key, node->next));
+    }
+
+    template<std::size_t Step = 0, std::size_t... Index, typename... Property, typename... Other>
+    void unpack(std::index_sequence<Index...>, std::tuple<Property...> property, Other &&... other) {
+        unroll<Step>(choice<3>, std::move(std::get<Index>(property))..., std::forward<Other>(other)...);
+    }
+
+    template<std::size_t Step = 0, typename... Property, typename... Other>
+    void unroll(choice_t<3>, std::tuple<Property...> property, Other &&... other) {
+        unpack<Step>(std::index_sequence_for<Property...>{}, std::move(property), std::forward<Other>(other)...);
+    }
+
+    template<std::size_t Step = 0, typename... Property, typename... Other>
+    void unroll(choice_t<2>, std::pair<Property...> property, Other &&... other) {
+        assign<Step>(std::move(property.first), std::move(property.second));
+        unroll<Step+1>(choice<3>, std::forward<Other>(other)...);
+    }
+
+    template<std::size_t Step = 0, typename Property, typename... Other>
+    std::enable_if_t<!std::is_invocable_v<Property>>
+    unroll(choice_t<1>, Property &&property, Other &&... other) {
+        assign<Step>(std::forward<Property>(property));
+        unroll<Step+1>(choice<3>, std::forward<Other>(other)...);
+    }
+
+    template<std::size_t Step = 0, typename Func, typename... Other>
+    void unroll(choice_t<0>, Func &&invocable, Other &&... other) {
+        unroll<Step>(choice<3>, std::forward<Func>(invocable)(), std::forward<Other>(other)...);
+    }
+
+    template<std::size_t>
+    void unroll(choice_t<0>) {}
+
+    template<std::size_t = 0, typename Key, typename... Value>
+    void assign(Key &&key, Value &&... value) {
+        static const auto property{std::make_tuple(std::forward<Key>(key), std::forward<Value>(value)...)};
+
+        static internal::meta_prop_node node{
+            nullptr,
+            []() -> meta_any {
+                return std::get<0>(property);
+            },
+            []() -> meta_any {
+                if constexpr(sizeof...(Value) == 0) {
+                    return {};
+                } else {
+                    return std::get<1>(property);
+                }
+            }
+        };
+
+        ENTT_ASSERT(!exists(node.key(), *curr));
+        node.next = *curr;
+        *curr = &node;
+    }
+
+public:
+    /**
+     * @brief Constructs an extended factory from a given node.
+     * @param target The underlying node to which to assign the properties.
+     */
+    meta_factory(entt::internal::meta_prop_node **target) ENTT_NOEXCEPT
+        : curr{target}
+    {}
+
+    /**
+     * @brief Assigns a property to the last meta object created.
+     *
+     * Both the key and the value (if any) must be at least copy constructible.
+     *
+     * @tparam PropertyOrKey Type of the property or property key.
+     * @tparam Value Optional type of the property value.
+     * @param property_or_key Property or property key.
+     * @param value Optional property value.
+     * @return A meta factory for the parent type.
+     */
+    template<typename PropertyOrKey, typename... Value>
+    auto prop(PropertyOrKey &&property_or_key, Value &&... value) && {
+        if constexpr(sizeof...(Value) == 0) {
+            unroll(choice<3>, std::forward<PropertyOrKey>(property_or_key));
+        } else {
+            assign(std::forward<PropertyOrKey>(property_or_key), std::forward<Value>(value)...);
+        }
+
+        return meta_factory<Type, Spec..., PropertyOrKey, Value...>{curr};
+    }
+
+    /**
+     * @brief Assigns properties to the last meta object created.
+     *
+     * Both the keys and the values (if any) must be at least copy
+     * constructible.
+     *
+     * @tparam Property Types of the properties.
+     * @param property Properties to assign to the last meta object created.
+     * @return A meta factory for the parent type.
+     */
+    template <typename... Property>
+    auto props(Property... property) && {
+        unroll(choice<3>, std::forward<Property>(property)...);
+        return meta_factory<Type, Spec..., Property...>{curr};
+    }
+
+private:
+    entt::internal::meta_prop_node **curr;
+};
+
+
+/**
+ * @brief Basic meta factory to be used for reflection purposes.
+ * @tparam Type Reflected type for which the factory was created.
+ */
+template<typename Type>
+class meta_factory<Type> {
+    template<typename Node>
+    bool exists(const Node *candidate, const Node *node) ENTT_NOEXCEPT {
+        return node && (node == candidate || exists(candidate, node->next));
+    }
+
+    template<typename Node>
+    bool exists(const ENTT_ID_TYPE alias, const Node *node) ENTT_NOEXCEPT {
+        return node && (node->alias == alias || exists(alias, node->next));
+    }
+
+public:
+    /**
+     * @brief Extends a meta type by assigning it an alias.
+     * @param value Unique identifier.
+     * @return An extended meta factory for the given type.
+     */
+    auto alias(const ENTT_ID_TYPE value) ENTT_NOEXCEPT {
+        auto * const node = internal::meta_info<Type>::resolve();
+
+        ENTT_ASSERT(!exists(value, *internal::meta_info<>::global));
+        ENTT_ASSERT(!exists(node, *internal::meta_info<>::global));
+        node->alias = value;
+        node->next = *internal::meta_info<>::global;
+        *internal::meta_info<>::global = node;
+
+        return meta_factory<Type, Type>{&node->prop};
+    }
+
+    /*! @copydoc alias */
+    [[deprecated("Use ::alias instead")]]
+    auto type(const ENTT_ID_TYPE value) ENTT_NOEXCEPT {
+        return alias(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(!exists(&node, type->base));
+        node.next = type->base;
+        type->base = &node;
+
+        return meta_factory<Type>{};
+    }
+
+    /**
+     * @brief Assigns a meta conversion function to a meta type.
+     *
+     * The given type must be such that an instance of the reflected type can be
+     * converted to it.
+     *
+     * @tparam To Type of the conversion function to assign to the meta type.
+     * @return A meta factory for the parent type.
+     */
+    template<typename To>
+    auto conv() ENTT_NOEXCEPT {
+        static_assert(std::is_convertible_v<Type, To>);
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_conv_node node{
+            type,
+            nullptr,
+            &internal::meta_info<To>::resolve,
+            [](const void *instance) -> meta_any {
+                return static_cast<To>(*static_cast<const Type *>(instance));
+            }
+        };
+
+        ENTT_ASSERT(!exists(&node, type->conv));
+        node.next = type->conv;
+        type->conv = &node;
+
+        return meta_factory<Type>{};
+    }
+
+    /**
+     * @brief Assigns a meta conversion function to a meta type.
+     *
+     * Conversion functions can be either free functions or member
+     * functions.<br/>
+     * In case of free functions, they must accept a const reference to an
+     * instance of the parent type as an argument. In case of member functions,
+     * they should have no arguments at all.
+     *
+     * @tparam Candidate The actual function to use for the conversion.
+     * @return A meta factory for the parent type.
+     */
+    template<auto Candidate>
+    auto conv() ENTT_NOEXCEPT {
+        using conv_type = std::invoke_result_t<decltype(Candidate), Type &>;
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_conv_node node{
+            type,
+            nullptr,
+            &internal::meta_info<conv_type>::resolve,
+            [](const void *instance) -> meta_any {
+                return std::invoke(Candidate, *static_cast<const Type *>(instance));
+            }
+        };
+
+        ENTT_ASSERT(!exists(&node, type->conv));
+        node.next = type->conv;
+        type->conv = &node;
+
+        return meta_factory<Type>{};
+    }
+
+    /**
+     * @brief Assigns a meta constructor to a meta type.
+     *
+     * Free functions can be assigned to meta types in the role of constructors.
+     * All that is required is that they return an instance of the underlying
+     * type.<br/>
+     * From a client's point of view, nothing changes if a constructor of a meta
+     * type is a built-in one or a free function.
+     *
+     * @tparam Func The actual function to use as a constructor.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @return An extended meta factory for the parent type.
+     */
+    template<auto Func, typename Policy = as_is_t>
+    auto ctor() ENTT_NOEXCEPT {
+        using helper_type = internal::meta_function_helper_t<decltype(Func)>;
+        static_assert(std::is_same_v<typename helper_type::return_type, Type>);
+        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(!exists(&node, type->ctor));
+        node.next = type->ctor;
+        type->ctor = &node;
+
+        return meta_factory<Type, std::integral_constant<decltype(Func), Func>>{&node.prop};
+    }
+
+    /**
+     * @brief Assigns a meta constructor to a meta type.
+     *
+     * A meta constructor is uniquely identified by the types of its arguments
+     * and is such that there exists an actual constructor of the underlying
+     * type that can be invoked with parameters whose types are those given.
+     *
+     * @tparam Args Types of arguments to use to construct an instance.
+     * @return An extended meta factory for the parent type.
+     */
+    template<typename... Args>
+    auto ctor() ENTT_NOEXCEPT {
+        using helper_type = internal::meta_function_helper_t<Type(*)(Args...)>;
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_ctor_node node{
+            type,
+            nullptr,
+            nullptr,
+            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(!exists(&node, type->ctor));
+        node.next = type->ctor;
+        type->ctor = &node;
+
+        return meta_factory<Type, Type(Args...)>{&node.prop};
+    }
+
+    /**
+     * @brief Assigns a meta destructor to a meta type.
+     *
+     * Free functions can be assigned to meta types in the role of destructors.
+     * The signature of the function should identical to the following:
+     *
+     * @code{.cpp}
+     * void(Type &);
+     * @endcode
+     *
+     * The purpose is to give users the ability to free up resources that
+     * require special treatment before an object is actually destroyed.
+     *
+     * @tparam Func The actual function to use as a destructor.
+     * @return A meta factory for the parent type.
+     */
+    template<auto Func>
+    auto dtor() ENTT_NOEXCEPT {
+        static_assert(std::is_invocable_v<decltype(Func), Type &>);
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_dtor_node node{
+            type,
+            [](void *instance) {
+                if(instance) {
+                    std::invoke(Func, *static_cast<Type *>(instance));
+                }
+            }
+        };
+
+        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 alias 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 alias) 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_any, meta_any, meta_any) { return false; },
+                [](meta_any, 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(!exists(alias, type->data));
+        ENTT_ASSERT(!exists(curr, type->data));
+        curr->alias = alias;
+        curr->next = type->data;
+        type->data = curr;
+
+        return 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 alias 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 alias) 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(!exists(alias, type->data));
+        ENTT_ASSERT(!exists(&node, type->data));
+        node.alias = alias;
+        node.next = type->data;
+        type->data = &node;
+
+        return meta_factory<Type, std::integral_constant<decltype(Setter), Setter>, std::integral_constant<decltype(Getter), Getter>>{&node.prop};
+    }
+
+    /**
+     * @brief Assigns a meta funcion to a meta type.
+     *
+     * Both member functions and free functions can be assigned to a meta
+     * type.<br/>
+     * From a client's point of view, all the functions associated with the
+     * reflected object will appear as if they were part of the type itself.
+     *
+     * @tparam Candidate The actual function to attach to the meta type.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @param alias 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 alias) 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_any instance, meta_any *args) {
+                return internal::invoke<Type, Candidate, Policy>(std::move(instance), args, helper_type::index_sequence);
+            }
+        };
+
+        ENTT_ASSERT(!exists(alias, type->func));
+        ENTT_ASSERT(!exists(&node, type->func));
+        node.alias = alias;
+        node.next = type->func;
+        type->func = &node;
+
+        return 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.
+     *
+     * @return An extended meta factory for the given type.
+     */
+    auto reset() ENTT_NOEXCEPT {
+        auto * const node = internal::meta_info<Type>::resolve();
+        auto **it = internal::meta_info<>::global;
+
+        while(*it && *it != node) {
+            it = &(*it)->next;
+        }
+
+        if(*it) {
+            *it = (*it)->next;
+        }
+
+        const auto unregister_all = y_combinator{
+            [](auto &&self, auto **curr, auto... member) {
+                while(*curr) {
+                    auto *prev = *curr;
+                    (self(&(prev->*member)), ...);
+                    *curr = prev->next;
+                    prev->next = nullptr;
+                }
+            }
+        };
+
+        unregister_all(&node->prop);
+        unregister_all(&node->base);
+        unregister_all(&node->conv);
+        unregister_all(&node->ctor, &internal::meta_ctor_node::prop);
+        unregister_all(&node->data, &internal::meta_data_node::prop);
+        unregister_all(&node->func, &internal::meta_func_node::prop);
+
+        node->alias = {};
+        node->next = nullptr;
+        node->dtor = nullptr;
+
+        return meta_factory<Type, Type>{&node->prop};
+    }
+};
+
+
+/**
+ * @brief Utility function to use for reflection.
+ *
+ * This is the point from which everything starts.<br/>
+ * By invoking this function with a type that is not yet reflected, a meta type
+ * is created to which it will be possible to attach meta objects through a
+ * dedicated factory.
+ *
+ * @tparam Type Type to reflect.
+ * @return An meta factory for the given type.
+ */
+template<typename Type>
+inline meta_factory<Type> meta() ENTT_NOEXCEPT {
+    auto * const node = internal::meta_info<Type>::resolve();
+    // extended meta factory to allow assigning properties to opaque meta types
+    return meta_factory<Type, Type>{&node->prop};
+}
+
+
+/**
+ * @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 alias.
+ * @param alias Unique identifier.
+ * @return The meta type associated with the given alias, if any.
+ */
+inline meta_type resolve(const ENTT_ID_TYPE alias) ENTT_NOEXCEPT {
+    return internal::find_if([alias](const auto *curr) {
+        return curr->alias == alias;
+    }, *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) {
+    internal::visit<meta_type>(std::move(op), *internal::meta_info<>::global);
+}
+
+
+}
+
+
+#endif

src/entt/meta/policy.hpp

@@ -0,0 +1,27 @@
+#ifndef ENTT_META_POLICY_HPP
+#define ENTT_META_POLICY_HPP
+
+
+namespace entt {
+
+
+/*! @brief Empty class type used to request the _as 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

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() {
+        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) {
         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(); });
     }
 };
 
@@ -332,4 +337,4 @@ struct ProcessAdaptor: Process<ProcessAdaptor<Func, Delta>, Delta>, private Func
 }
 
 
-#endif // ENTT_PROCESS_PROCESS_HPP
+#endif

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() = 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());
         }
@@ -296,24 +277,24 @@ public:
      *
      * @param immediately Requests an immediate operation.
      */
-    void abort(bool immediately = false) {
+    void abort(const bool immediately = false) {
         decltype(handlers) exec;
         exec.swap(handlers);
 
-        std::for_each(exec.begin(), exec.end(), [immediately](auto &handler) {
+        for(auto &&handler: exec) {
             handler.abort(handler, immediately);
-        });
+        }
 
         std::move(handlers.begin(), handlers.end(), std::back_inserter(exec));
         handlers.swap(exec);
     }
 
 private:
-    std::vector<ProcessHandler> handlers{};
+    std::vector<process_handler> handlers{};
 };
 
 
 }
 
 
-#endif // ENTT_PROCESS_SCHEDULER_HPP
+#endif

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;
     }
 
     /**
@@ -118,66 +121,120 @@ 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 {
+        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) {
+        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;
 };
 
 
 }
 
 
-#endif // ENTT_RESOURCE_CACHE_HPP
+#endif

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

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/>
@@ -94,15 +79,23 @@ 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); }
+    explicit operator bool() const ENTT_NOEXCEPT {
+        return static_cast<bool>(resource);
+    }
 
 private:
     std::shared_ptr<Resource> resource;
@@ -112,4 +105,4 @@ private:
 }
 
 
-#endif // ENTT_RESOURCE_HANDLE_HPP
+#endif

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)...);
     }
 };
@@ -59,4 +62,4 @@ class ResourceLoader {
 }
 
 
-#endif // ENTT_RESOURCE_LOADER_HPP
+#endif

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 +2,75 @@
 #define ENTT_SIGNAL_DELEGATE_HPP
 
 
+#include <tuple>
+#include <cstddef>
 #include <utility>
+#include <functional>
+#include <type_traits>
+#include "../config/config.h"
 
 
 namespace entt {
 
 
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename Ret, typename... Args>
+auto function_pointer(Ret(*)(Args...)) -> Ret(*)(Args...);
+
+
+template<typename Ret, typename Type, typename... Args, typename Other>
+auto function_pointer(Ret(*)(Type, Args...), Other &&) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Ret, typename... Args, typename... Other>
+auto function_pointer(Ret(Class:: *)(Args...), Other &&...) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Ret, typename... Args, typename... Other>
+auto function_pointer(Ret(Class:: *)(Args...) const, Other &&...) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Type, typename... Other>
+auto function_pointer(Type Class:: *, Other &&...) -> Type(*)();
+
+
+template<typename... Type>
+using function_pointer_t = decltype(internal::function_pointer(std::declval<Type>()...));
+
+
+template<typename... Class, typename Ret, typename... Args>
+constexpr auto index_sequence_for(Ret(*)(Args...)) {
+    return std::index_sequence_for<Class..., 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.
  *
@@ -15,123 +78,253 @@ 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.
+ * A delegate can be used as a general purpose invoker without memory overhead
+ * for free functions possibly with payloads and bound or unbound members.
  *
  * @tparam Ret Return type of a function type.
  * @tparam Args Types of arguments of a function type.
  */
 template<typename Ret, typename... Args>
-class Delegate<Ret(Args...)> 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 *, Args...);
 
-    static Ret fallback(void *, Args...) noexcept { return {}; }
-
-    template<Ret(*Function)(Args...)>
-    static Ret proto(void *, Args... args) {
-        return (Function)(args...);
+    template<auto Candidate, std::size_t... Index>
+    auto wrap(std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        return [](const void *, Args... args) -> Ret {
+            const auto arguments = std::forward_as_tuple(std::forward<Args>(args)...);
+            return Ret(std::invoke(Candidate, std::forward<std::tuple_element_t<Index, std::tuple<Args...>>>(std::get<Index>(arguments))...));
+        };
     }
 
-    template<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>
+    auto wrap(Type &, std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        return [](const void *payload, Args... args) -> Ret {
+            const auto arguments = std::forward_as_tuple(std::forward<Args>(args)...);
+            Type *curr = static_cast<Type *>(const_cast<std::conditional_t<std::is_const_v<Type>, const void *, void *>>(payload));
+            return Ret(std::invoke(Candidate, *curr, std::forward<std::tuple_element_t<Index, std::tuple<Args...>>>(std::get<Index>(arguments))...));
+        };
+    }
+
+    template<auto Candidate, typename Type, std::size_t... Index>
+    auto wrap(Type *, std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        return [](const void *payload, Args... args) -> Ret {
+            const auto arguments = std::forward_as_tuple(std::forward<Args>(args)...);
+            Type *curr = static_cast<Type *>(const_cast<std::conditional_t<std::is_const_v<Type>, const void *, void *>>(payload));
+            return Ret(std::invoke(Candidate, curr, std::forward<std::tuple_element_t<Index, std::tuple<Args...>>>(std::get<Index>(arguments))...));
+        };
     }
 
 public:
+    /*! @brief Function type of the 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.
-     * @tparam Function A valid free function pointer.
+     * @brief Constructs a delegate and connects a free function or an unbound
+     * member.
+     * @tparam Candidate Function or member to connect to the delegate.
      */
-    template<Ret(*Function)(Args...)>
-    void connect() noexcept {
-        stub = std::make_pair(nullptr, &proto<Function>);
+    template<auto Candidate>
+    delegate(connect_arg_t<Candidate>) ENTT_NOEXCEPT
+        : delegate{}
+    {
+        connect<Candidate>();
     }
 
     /**
-     * @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
+     * @brief Constructs a delegate and connects a free function with payload or
+     * a bound member.
+     * @tparam Candidate Function or member to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    delegate(connect_arg_t<Candidate>, Type &&value_or_instance) ENTT_NOEXCEPT
+        : delegate{}
+    {
+        connect<Candidate>(std::forward<Type>(value_or_instance));
+    }
+
+    /**
+     * @brief Connects a free function or an unbound member to a delegate.
+     * @tparam Candidate Function or member to connect to the delegate.
+     */
+    template<auto Candidate>
+    void connect() ENTT_NOEXCEPT {
+        data = nullptr;
+
+        if constexpr(std::is_invocable_r_v<Ret, decltype(Candidate), Args...>) {
+            fn = [](const void *, Args... args) -> Ret {
+                return Ret(std::invoke(Candidate, std::forward<Args>(args)...));
+            };
+        } else if constexpr(std::is_member_pointer_v<decltype(Candidate)>) {
+            fn = wrap<Candidate>(internal::index_sequence_for<std::tuple_element_t<0, std::tuple<Args...>>>(internal::function_pointer_t<decltype(Candidate)>{}));
+        } else {
+            fn = wrap<Candidate>(internal::index_sequence_for(internal::function_pointer_t<decltype(Candidate)>{}));
+        }
+    }
+
+    /**
+     * @brief Connects a free function with payload or a bound member to a
      * delegate.
      *
-     * @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`.
+     * The delegate isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of the instance overcomes
+     * the one  of the delegate.<br/>
+     * When used to connect a free function with payload, its signature must be
+     * such that the instance is the first argument before the ones used to
+     * define the delegate itself.
+     *
+     * @tparam Candidate Function or member to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid reference that fits the purpose.
      */
-    template<typename Class, Ret(Class::*Member)(Args...)>
-    void connect(Class *instance) noexcept {
-        stub = std::make_pair(instance, &proto<Class, Member>);
+    template<auto Candidate, typename Type>
+    void connect(Type &value_or_instance) ENTT_NOEXCEPT {
+        data = &value_or_instance;
+
+        if constexpr(std::is_invocable_r_v<Ret, decltype(Candidate), Type &, Args...>) {
+            fn = [](const void *payload, Args... args) -> Ret {
+                Type *curr = static_cast<Type *>(const_cast<std::conditional_t<std::is_const_v<Type>, const void *, void *>>(payload));
+                return Ret(std::invoke(Candidate, *curr, std::forward<Args>(args)...));
+            };
+        } else {
+            fn = wrap<Candidate>(value_or_instance, internal::index_sequence_for(internal::function_pointer_t<decltype(Candidate), Type>{}));
+        }
+    }
+
+    /**
+     * @brief Connects a free function with payload or a bound member to a
+     * delegate.
+     *
+     * @sa connect(Type &)
+     *
+     * @tparam Candidate Function or member to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid pointer that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void connect(Type *value_or_instance) ENTT_NOEXCEPT {
+        data = value_or_instance;
+
+        if constexpr(std::is_invocable_r_v<Ret, decltype(Candidate), Type *, Args...>) {
+            fn = [](const void *payload, Args... args) -> Ret {
+                Type *curr = static_cast<Type *>(const_cast<std::conditional_t<std::is_const_v<Type>, const void *, void *>>(payload));
+                return Ret(std::invoke(Candidate, curr, std::forward<Args>(args)...));
+            };
+        } else {
+            fn = wrap<Candidate>(value_or_instance, internal::index_sequence_for(internal::function_pointer_t<decltype(Candidate), Type>{}));
+        }
     }
 
     /**
      * @brief 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<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 == nullptr);
+    }
+
+    /**
+     * @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.
+ * @tparam Candidate Function or member to connect to the delegate.
+ */
+template<auto Candidate>
+delegate(connect_arg_t<Candidate>) ENTT_NOEXCEPT
+-> delegate<std::remove_pointer_t<internal::function_pointer_t<decltype(Candidate)>>>;
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Candidate Function or member to connect to the delegate.
+ * @tparam Type Type of class or type of payload.
+ */
+template<auto Candidate, typename Type>
+delegate(connect_arg_t<Candidate>, Type &&) ENTT_NOEXCEPT
+-> delegate<std::remove_pointer_t<internal::function_pointer_t<decltype(Candidate), Type>>>;
+
+
 }
 
 
-#endif // ENTT_SIGNAL_DELEGATE_HPP
+#endif

src/entt/signal/dispatcher.hpp

@@ -4,10 +4,11 @@
 
 #include <vector>
 #include <memory>
+#include <cstddef>
 #include <utility>
-#include <cstdint>
-#include "../core/family.hpp"
-#include "signal.hpp"
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/type_info.hpp"
 #include "sigh.hpp"
 
 
@@ -20,124 +21,100 @@ 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>;
-
-    template<typename Class, typename Event>
-    using instance_type = typename Sig<void(const Event &)>::template instance_type<Class>;
-
-    struct BaseSignalWrapper {
-        virtual ~BaseSignalWrapper() = default;
-        virtual void publish(std::size_t) = 0;
+class dispatcher {
+    struct basic_pool {
+        virtual ~basic_pool() = default;
+        virtual void publish() = 0;
+        virtual void clear() ENTT_NOEXCEPT = 0;
+        virtual ENTT_ID_TYPE type_id() const ENTT_NOEXCEPT = 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 pool_handler: basic_pool {
+        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() ENTT_NOEXCEPT 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) {
-            signal.publish({ std::forward<Args>(args)... });
+        void trigger(Args &&... args) {
+            signal.publish(Event{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)...);
+        }
+
+        ENTT_ID_TYPE type_id() const ENTT_NOEXCEPT override {
+            return type_info<Event>::id();
         }
 
     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;
-    }
-
     template<typename Event>
-    SignalWrapper<Event> & wrapper() {
-        auto type = event_family::type<Event>();
+    pool_handler<Event> & assure() {
+        static_assert(std::is_same_v<Event, std::decay_t<Event>>);
+        static std::size_t index{pools.size()};
 
-        if(!(type < wrappers.size())) {
-            wrappers.resize(type + 1);
+        if(const auto length = pools.size(); !(index < length) || pools[index]->type_id() != type_info<Event>::id()) {
+            for(index = {}; index < length && pools[index]->type_id() != type_info<Event>::id(); ++index);
+
+            if(index == pools.size()) {
+                pools.emplace_back(new pool_handler<Event>{});
+            }
         }
 
-        if(!wrappers[type]) {
-            wrappers[type] = std::make_unique<SignalWrapper<Event>>();
-        }
-
-        return static_cast<SignalWrapper<Event> &>(*wrappers[type]);
+        return static_cast<pool_handler<Event> &>(*pools[index]);
     }
 
 public:
-    /*! @brief Default constructor. */
-    Dispatcher() noexcept
-        : wrappers{}, mode{false}
-    {}
-
     /**
-     * @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>
+    auto sink() {
+        return assure<Event>().sink();
     }
 
     /**
@@ -151,8 +128,22 @@ 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));
     }
 
     /**
@@ -161,62 +152,81 @@ 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 clear() {
+        if constexpr(sizeof...(Event) == 0) {
+            for(auto &&cpool: pools) {
+                cpool->clear();
+            }
+        } else {
+            (assure<Event>().clear(), ...);
+        }
+    }
+
+    /**
+     * @brief Delivers all the pending events of the given type.
+     *
+     * This method is blocking and it doesn't return until all the events are
+     * delivered to the registered listeners. It's responsibility of the users
+     * to reduce at a minimum the time spent in the bodies of the listeners.
+     *
+     * @tparam Event Type of events to send.
+     */
+    template<typename Event>
+    void update() {
+        assure<Event>().publish();
     }
 
     /**
      * @brief Delivers all the pending events.
      *
      * This method is blocking and it doesn't return until all the events are
-     * delivered to the registered listeners. It's 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 = pools.size(); pos; --pos) {
+            pools[pos-1]->publish();
         }
     }
 
 private:
-    std::vector<std::unique_ptr<BaseSignalWrapper>> wrappers;
-    bool mode;
+    std::vector<std::unique_ptr<basic_pool>> pools;
 };
 
 
-/**
- * @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>;
-
-
 }
 
 
-#endif // ENTT_SIGNAL_DISPATCHER_HPP
+#endif

src/entt/signal/emitter.hpp

@@ -6,10 +6,12 @@
 #include <functional>
 #include <algorithm>
 #include <utility>
-#include <cstdint>
 #include <memory>
 #include <vector>
 #include <list>
+#include <iterator>
+#include "../config/config.h"
+#include "../core/type_info.hpp"
 
 
 namespace entt {
@@ -21,13 +23,13 @@ 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
+ * Pools for the type of events are created internally on the fly. It's not
  * required to specify in advance the full list of accepted types.<br/>
  * Moreover, whenever an event is published, an emitter provides the listeners
  * with a reference to itself along with a const reference to the event.
@@ -37,110 +39,115 @@ 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 {
+    struct basic_pool {
+        virtual ~basic_pool() = default;
+        virtual bool empty() const ENTT_NOEXCEPT = 0;
+        virtual void clear() ENTT_NOEXCEPT = 0;
+        virtual ENTT_ID_TYPE type_id() const ENTT_NOEXCEPT = 0;
     };
 
     template<typename Event>
-    struct Handler final: BaseHandler {
+    struct pool_handler: basic_pool {
         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);
+                for(auto &&element: once_list) {
+                    element.first = true;
+                }
+
+                for(auto &&element: on_list) {
+                    element.first = true;
+                }
             } 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) {
             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);
-
-            auto func = [&event, &ref](auto &&element) {
-                return element.first ? void() : element.second(event, ref);
-            };
+            container_type swap_list;
+            once_list.swap(swap_list);
 
             publishing = true;
 
-            std::for_each(onL.rbegin(), onL.rend(), func);
-            std::for_each(currentL.rbegin(), currentL.rend(), func);
+            for(auto &&element: on_list) {
+                element.first ? void() : element.second(event, ref);
+            }
+
+            for(auto &&element: swap_list) {
+                element.first ? void() : element.second(event, ref);
+            }
 
             publishing = false;
 
-            onL.remove_if([](auto &&element){ return element.first; });
+            on_list.remove_if([](auto &&element) { return element.first; });
+        }
+
+        ENTT_ID_TYPE type_id() const ENTT_NOEXCEPT override {
+            return type_info<Event>::id();
         }
 
     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++;
-    }
+    template<typename Event>
+    const pool_handler<Event> & assure() const {
+        static_assert(std::is_same_v<Event, std::decay_t<Event>>);
+        static std::size_t index{pools.size()};
 
-    template<typename>
-    static std::size_t type() noexcept {
-        static std::size_t value = next();
-        return value;
+        if(const auto length = pools.size(); !(index < length) || pools[index]->type_id() != type_info<Event>::id()) {
+            for(index = {}; index < length && pools[index]->type_id() != type_info<Event>::id(); ++index);
+
+            if(index == pools.size()) {
+                pools.emplace_back(new pool_handler<Event>{});
+            }
+        }
+
+        return static_cast<pool_handler<Event> &>(*pools[index]);
     }
 
     template<typename Event>
-    Handler<Event> & handler() noexcept {
-        std::size_t family = type<Event>();
-
-        if(!(family < handlers.size())) {
-            handlers.resize(family+1);
-        }
-
-        if(!handlers[family]) {
-            handlers[family] = std::make_unique<Handler<Event>>();
-        }
-
-        return static_cast<Handler<Event> &>(*handlers[family]);
+    pool_handler<Event> & assure() {
+        return const_cast<pool_handler<Event> &>(std::as_const(*this).template assure<Event>());
     }
 
 public:
-    /** @brief Type of listeners accepted for the given 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 pool_handler<Event>::listener_type;
 
     /**
      * @brief Generic connection type for events.
@@ -152,56 +159,35 @@ 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 pool_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() = 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 pool_handler<Event>::connection_type conn)
+            : pool_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() = default;
 
     /*! @brief Default destructor. */
-    virtual ~Emitter() noexcept {
-        static_assert(std::is_base_of<Emitter<Derived>, Derived>::value, "!");
+    virtual ~emitter() {
+        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 +201,8 @@ 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(Event{std::forward<Args>(args)...}, *static_cast<Derived *>(this));
     }
 
     /**
@@ -236,12 +222,12 @@ 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 +247,12 @@ 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 +265,33 @@ 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) {
+        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() {
+        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 {
+        for(auto &&cpool: pools) {
+            cpool->clear();
+        }
     }
 
     /**
@@ -313,29 +300,26 @@ 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 {
+        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(pools.cbegin(), pools.cend(), [](auto &&cpool) {
+            return cpool->empty();
+        });
     }
 
 private:
-    std::vector<std::unique_ptr<BaseHandler>> handlers{};
+    mutable std::vector<std::unique_ptr<basic_pool>> pools{};
 };
 
 
 }
 
 
-#endif // ENTT_SIGNAL_EMITTER_HPP
+#endif

src/entt/signal/fwd.hpp

@@ -0,0 +1,37 @@
+#ifndef ENTT_SIGNAL_FWD_HPP
+#define ENTT_SIGNAL_FWD_HPP
+
+
+namespace entt {
+
+
+/*! @class delegate */
+template<typename>
+class delegate;
+
+/*! @class dispatcher */
+class dispatcher;
+
+/*! @class emitter */
+template<typename>
+class emitter;
+
+/*! @class connection */
+class connection;
+
+/*! @class scoped_connection */
+struct scoped_connection;
+
+/*! @class sink */
+template<typename>
+class sink;
+
+/*! @class sigh */
+template<typename>
+class sigh;
+
+
+}
+
+
+#endif

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.
      *
@@ -228,76 +96,420 @@ 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 {
+        for(auto &&call: std::as_const(calls)) {
+            call(args...);
         }
     }
 
     /**
      * @brief Collects return values from the listeners.
+     *
+     * The collector must expose a call operator with the following properties:
+     *
+     * * The return type is either `void` or such that it's convertible to
+     *   `bool`. In the second case, a true value will stop the iteration.
+     * * The list of parameters is empty if `Ret` is `void`, otherwise it
+     *   contains a single element such that `Ret` is convertible to it.
+     *
+     * @tparam Func Type of collector to use, if any.
+     * @param func A valid function object.
      * @param args Arguments to use to invoke listeners.
-     * @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 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 {
+    /*! @brief Default constructor. */
+    scoped_connection() = default;
+
+    /**
+     * @brief Constructs a scoped connection from a basic connection.
+     * @param other A valid connection object.
+     */
+    scoped_connection(const connection &other)
+        : conn{other}
+    {}
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    scoped_connection(const scoped_connection &) = delete;
+
+    /*! @brief Automatically breaks the link on destruction. */
+    ~scoped_connection() {
+        conn.release();
+    }
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This scoped connection.
+     */
+    scoped_connection & operator=(const scoped_connection &) = delete;
+
+    /**
+     * @brief Acquires a connection.
+     * @param other The connection object to acquire.
+     * @return This scoped connection.
+     */
+    scoped_connection & operator=(connection other) {
+        conn = std::move(other);
+        return *this;
+    }
+
+    /**
+     * @brief Checks whether a scoped connection is properly initialized.
+     * @return True if the connection is properly initialized, false otherwise.
+     */
+    explicit operator bool() const ENTT_NOEXCEPT {
+        return static_cast<bool>(conn);
+    }
+
+    /*! @brief Breaks the connection. */
+    void release() {
+        conn.release();
+    }
+
+private:
+    connection conn;
+};
+
+
+/**
+ * @brief Sink class.
+ *
+ * A sink is used to connect listeners to signals and to disconnect them.<br/>
+ * The function type for a listener is the one of the signal to which it
+ * belongs.
+ *
+ * The clear separation between a signal and a sink permits to store the former
+ * as private data member without exposing the publish functionality to the
+ * users of the class.
  *
  * @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 Candidate>
+    static void release(void *signal) {
+        sink{*static_cast<signal_type *>(signal)}.disconnect<Candidate>();
+    }
+
+public:
+    /**
+     * @brief Constructs a sink that is allowed to modify a given signal.
+     * @param ref A valid reference to a signal object.
+     */
+    sink(sigh<Ret(Args...)> &ref) ENTT_NOEXCEPT
+        : offset{},
+          signal{&ref}
+    {}
+
+    /**
+     * @brief Returns false if at least a listener is connected to the sink.
+     * @return True if the sink has no listeners connected, false otherwise.
+     */
+    bool empty() const ENTT_NOEXCEPT {
+        return signal->calls.empty();
+    }
+
+    /**
+     * @brief Returns a sink that connects before a given free function or an
+     * unbound member.
+     * @tparam Function A valid free function pointer.
+     * @return A properly initialized sink object.
+     */
+    template<auto Function>
+    sink before() {
+        delegate<Ret(Args...)> call{};
+        call.template connect<Function>();
+
+        const auto &calls = signal->calls;
+        const auto it = std::find(calls.cbegin(), calls.cend(), std::move(call));
+
+        sink other{*this};
+        other.offset = std::distance(it, calls.cend());
+        return other;
+    }
+
+    /**
+     * @brief Returns a sink that connects before a free function with payload
+     * or a bound member.
+     * @tparam Candidate Member or free function to look for.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     * @return A properly initialized sink object.
+     */
+    template<auto Candidate, typename Type>
+    sink before(Type &&value_or_instance) {
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>(std::forward<Type>(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 or an unbound member to a signal.
+     *
+     * The signal handler performs checks to avoid multiple connections for the
+     * same function.
+     *
+     * @tparam Candidate Function or member to connect to the signal.
+     * @return A properly initialized connection object.
+     */
+    template<auto Candidate>
+    connection connect() {
+        disconnect<Candidate>();
+
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>();
+        signal->calls.insert(signal->calls.end() - offset, std::move(call));
+
+        delegate<void(void *)> conn{};
+        conn.template connect<&release<Candidate>>();
+        return { std::move(conn), signal };
+    }
+
+    /**
+     * @brief Connects a free function with payload or a bound member to a
+     * signal.
+     *
+     * The signal isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of the instance overcomes
+     * the one  of the 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 Function or member to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     * @return A properly initialized connection object.
+     */
+    template<auto Candidate, typename Type>
+    connection connect(Type &&value_or_instance) {
+        disconnect<Candidate>(value_or_instance);
+
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>(value_or_instance);
+        signal->calls.insert(signal->calls.end() - offset, std::move(call));
+
+        delegate<void(void *)> conn{};
+        conn.template connect<&release<Candidate, Type>>(value_or_instance);
+        return { std::move(conn), signal };
+    }
+
+    /**
+     * @brief Disconnects a free function or an unbound member from a signal.
+     * @tparam Candidate Function or member to disconnect from the delegate.
+     */
+    template<auto Candidate>
+    void disconnect() {
+        auto &calls = signal->calls;
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>();
+        calls.erase(std::remove(calls.begin(), calls.end(), std::move(call)), calls.end());
+    }
+
+    /**
+     * @brief Disconnects a free function with payload or a bound member from a
+     * signal.
+     * @tparam Candidate Function or member to disconnect from the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void disconnect(Type &&value_or_instance) {
+        auto &calls = signal->calls;
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>(std::forward<Type>(value_or_instance));
+        calls.erase(std::remove(calls.begin(), calls.end(), std::move(call)), calls.end());
+    }
+
+    /**
+     * @brief Disconnects free functions with payload or bound members from a
+     * signal.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<typename Type>
+    void disconnect(Type &value_or_instance) {
+        disconnect(&value_or_instance);
+    }
+
+    /**
+     * @brief Disconnects free functions with payload or bound members from a
+     * signal.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<typename Type>
+    void disconnect(Type *value_or_instance) {
+        if(value_or_instance) {
+            auto &calls = signal->calls;
+            calls.erase(std::remove_if(calls.begin(), calls.end(), [value_or_instance](const auto &delegate) {
+                return delegate.instance() == value_or_instance;
+            }), calls.end());
+        }
+    }
+
+    /*! @brief Disconnects all the listeners from a signal. */
+    void disconnect() {
+        signal->calls.clear();
+    }
+
+private:
+    difference_type offset;
+    signal_type *signal;
+};
+
+
+/**
+ * @brief Deduction guide.
+ *
+ * It allows to deduce the function type of a sink directly from the signal it
+ * refers to.
+ *
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ */
+template<typename Ret, typename... Args>
+sink(sigh<Ret(Args...)> &) ENTT_NOEXCEPT -> sink<Ret(Args...)>;
 
 
 }
 
 
-#endif // ENTT_SIGNAL_SIGH_HPP
+#endif

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 +2,201 @@
 # Tests configuration
 #
 
-include_directories(${PROJECT_SRC_DIR})
+include(FetchContent)
+
+set(THREADS_PREFER_PTHREAD_FLAG ON)
+find_package(Threads REQUIRED)
+
+if(FIND_GTEST_PACKAGE)
+    find_package(GTest REQUIRED)
+else()
+    FetchContent_Declare(
+        googletest
+        GIT_REPOSITORY https://github.com/google/googletest.git
+        GIT_TAG master
+        GIT_SHALLOW 1
+    )
+
+    FetchContent_GetProperties(googletest)
+
+    if(NOT googletest_POPULATED)
+        FetchContent_Populate(googletest)
+        set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
+        add_subdirectory(${googletest_SOURCE_DIR} ${googletest_BINARY_DIR})
+    endif()
+
+    add_library(GTest::Main ALIAS gtest_main)
+
+    target_compile_features(gtest PUBLIC cxx_std_17)
+    target_compile_features(gtest_main PUBLIC cxx_std_17)
+    target_compile_features(gmock PUBLIC cxx_std_17)
+    target_compile_features(gmock_main PUBLIC cxx_std_17)
+endif()
+
+include_directories($<TARGET_PROPERTY:EnTT,INTERFACE_INCLUDE_DIRECTORIES>)
+add_compile_options($<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_OPTIONS>)
+
+function(SETUP_TARGET TARGET_NAME)
+    set_target_properties(${TARGET_NAME} PROPERTIES CXX_EXTENSIONS OFF)
+    target_link_libraries(${TARGET_NAME} PRIVATE EnTT)
+
+    target_compile_options(
+        ${TARGET_NAME}
+        PRIVATE
+            $<$<NOT:$<PLATFORM_ID:Windows>>:-pedantic -fvisibility=hidden -Wall -Wshadow -Wno-deprecated-declarations>
+            $<$<PLATFORM_ID:Windows>:/EHsc /W1 /wd4996 /w14800>
+    )
+
+    target_compile_options(
+        ${TARGET_NAME}
+        PRIVATE
+            $<$<AND:$<CONFIG:Debug>,$<NOT:$<PLATFORM_ID:Windows>>>:-O0 -g>
+            $<$<AND:$<CONFIG:Release>,$<NOT:$<PLATFORM_ID:Windows>>>:-O2>
+            $<$<AND:$<CONFIG:Debug>,$<PLATFORM_ID:Windows>>:/Od>
+            $<$<AND:$<CONFIG:Release>,$<PLATFORM_ID:Windows>>:/O2>
+    )
+endfunction()
 
 add_library(odr OBJECT odr.cpp)
+SETUP_TARGET(odr)
+
+function(SETUP_BASIC_TEST TEST_NAME TEST_SOURCES)
+    add_executable(${TEST_NAME} $<TARGET_OBJECTS:odr> ${TEST_SOURCES})
+    target_link_libraries(${TEST_NAME} PRIVATE GTest::Main Threads::Threads)
+    SETUP_TARGET(${TEST_NAME})
+    add_test(NAME ${TEST_NAME} COMMAND ${TEST_NAME})
+endfunction()
+
+function(SETUP_LIB_TEST TEST_NAME)
+    add_library(_${TEST_NAME} SHARED lib/${TEST_NAME}/lib.cpp)
+    SETUP_TARGET(_${TEST_NAME})
+    SETUP_BASIC_TEST(lib_${TEST_NAME} lib/${TEST_NAME}/main.cpp)
+    target_compile_definitions(_${TEST_NAME} PRIVATE ENTT_API_EXPORT ${ARGV1})
+    target_compile_definitions(lib_${TEST_NAME} PRIVATE ENTT_API_IMPORT ${ARGV1})
+    target_link_libraries(lib_${TEST_NAME} PRIVATE _${TEST_NAME})
+endfunction()
+
+function(SETUP_PLUGIN_TEST TEST_NAME)
+    add_library(_${TEST_NAME} MODULE lib/${TEST_NAME}/plugin.cpp)
+    SETUP_TARGET(_${TEST_NAME})
+    SETUP_BASIC_TEST(lib_${TEST_NAME} lib/${TEST_NAME}/main.cpp)
+    target_include_directories(_${TEST_NAME} PRIVATE ${cr_INCLUDE_DIR})
+    target_include_directories(lib_${TEST_NAME} PRIVATE ${cr_INCLUDE_DIR})
+    target_compile_definitions(lib_${TEST_NAME} PRIVATE NOMINMAX PLUGIN="$<TARGET_FILE:_${TEST_NAME}>" ${ARGV1})
+    target_compile_definitions(_${TEST_NAME} PRIVATE NOMINMAX ${ARGV1})
+    target_link_libraries(lib_${TEST_NAME} PRIVATE ${CMAKE_DL_LIBS})
+endfunction()
 
 # Test benchmark
 
-if(CMAKE_BUILD_TYPE MATCHES Release)
-    add_executable(
-        benchmark
-        $<TARGET_OBJECTS:odr>
-        entt/entity/benchmark.cpp
+if(BUILD_BENCHMARK)
+    SETUP_BASIC_TEST(benchmark benchmark/benchmark.cpp)
+endif()
+
+# Test lib
+
+if(BUILD_LIB)
+    FetchContent_Declare(
+        cr
+        GIT_REPOSITORY https://github.com/fungos/cr.git
+        GIT_TAG master
+        GIT_SHALLOW 1
     )
-    target_link_libraries(benchmark PRIVATE gtest_main Threads::Threads)
-    add_test(NAME benchmark COMMAND benchmark)
+
+    FetchContent_GetProperties(cr)
+
+    if(NOT cr_POPULATED)
+        FetchContent_Populate(cr)
+        set(cr_INCLUDE_DIR ${cr_SOURCE_DIR})
+    endif()
+
+    SETUP_LIB_TEST(dispatcher)
+    SETUP_LIB_TEST(emitter)
+    SETUP_LIB_TEST(meta)
+    SETUP_LIB_TEST(registry)
+
+    SETUP_LIB_TEST(dispatcher_std ENTT_STANDARD_CPP)
+    SETUP_LIB_TEST(emitter_std ENTT_STANDARD_CPP)
+    SETUP_LIB_TEST(meta_std ENTT_STANDARD_CPP)
+    SETUP_LIB_TEST(registry_std ENTT_STANDARD_CPP)
+
+    SETUP_PLUGIN_TEST(dispatcher_plugin)
+    SETUP_PLUGIN_TEST(emitter_plugin)
+    SETUP_PLUGIN_TEST(meta_plugin)
+    SETUP_PLUGIN_TEST(registry_plugin)
+
+    SETUP_PLUGIN_TEST(dispatcher_plugin_std ENTT_STANDARD_CPP)
+    SETUP_PLUGIN_TEST(emitter_plugin_std ENTT_STANDARD_CPP)
+    SETUP_PLUGIN_TEST(meta_plugin_std ENTT_STANDARD_CPP)
+    SETUP_PLUGIN_TEST(registry_plugin_std ENTT_STANDARD_CPP)
+endif()
+
+# Test snapshot
+
+if(BUILD_SNAPSHOT)
+    FetchContent_Declare(
+        cereal
+        GIT_REPOSITORY https://github.com/USCiLab/cereal.git
+        GIT_TAG v1.2.2
+        GIT_SHALLOW 1
+    )
+
+    FetchContent_GetProperties(cereal)
+
+    if(NOT cereal_POPULATED)
+        FetchContent_Populate(cereal)
+        set(cereal_INCLUDE_DIR ${cereal_SOURCE_DIR}/include)
+    endif()
+
+    SETUP_BASIC_TEST(cereal snapshot/snapshot.cpp)
+    target_include_directories(cereal PRIVATE ${cereal_INCLUDE_DIR})
 endif()
 
 # Test core
 
-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_BASIC_TEST(algorithm entt/core/algorithm.cpp)
+SETUP_BASIC_TEST(family entt/core/family.cpp)
+SETUP_BASIC_TEST(hashed_string entt/core/hashed_string.cpp)
+SETUP_BASIC_TEST(ident entt/core/ident.cpp)
+SETUP_BASIC_TEST(monostate entt/core/monostate.cpp)
+SETUP_BASIC_TEST(type_info entt/core/type_info.cpp)
+SETUP_BASIC_TEST(type_traits entt/core/type_traits.cpp)
+SETUP_BASIC_TEST(utility entt/core/utility.cpp)
 
 # Test entity
 
-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_BASIC_TEST(actor entt/entity/actor.cpp)
+SETUP_BASIC_TEST(entity entt/entity/entity.cpp)
+SETUP_BASIC_TEST(group entt/entity/group.cpp)
+SETUP_BASIC_TEST(helper entt/entity/helper.cpp)
+SETUP_BASIC_TEST(observer entt/entity/observer.cpp)
+SETUP_BASIC_TEST(registry entt/entity/registry.cpp)
+SETUP_BASIC_TEST(runtime_view entt/entity/runtime_view.cpp)
+SETUP_BASIC_TEST(snapshot entt/entity/snapshot.cpp)
+SETUP_BASIC_TEST(sparse_set entt/entity/sparse_set.cpp)
+SETUP_BASIC_TEST(storage entt/entity/storage.cpp)
+SETUP_BASIC_TEST(view entt/entity/view.cpp)
 
 # Test locator
 
-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_BASIC_TEST(locator entt/locator/locator.cpp)
+
+# Test meta
+
+SETUP_BASIC_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_BASIC_TEST(process entt/process/process.cpp)
+SETUP_BASIC_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_BASIC_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_BASIC_TEST(delegate entt/signal/delegate.cpp)
+SETUP_BASIC_TEST(dispatcher entt/signal/dispatcher.cpp)
+SETUP_BASIC_TEST(emitter entt/signal/emitter.cpp)
+SETUP_BASIC_TEST(sigh entt/signal/sigh.cpp)

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::value(foobar), 0xbf9cf968);
+    // how would you test a constexpr otherwise?
+    (void)std::integral_constant<hash_type, entt::hashed_string::value("quux")>{};
+}
+
+TEST(HashedString, StringView) {
+    std::string str{"__foobar__"};
+    std::string_view view{str.data()+2, 6};
+    ASSERT_EQ(entt::hashed_string::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::value(foobar), 0xbf9cf968);
+    // how would you test a constexpr otherwise?
+    (void)std::integral_constant<hash_type, entt::hashed_wstring::value(L"quux")>{};
+}
+
+TEST(HashedWString, StringView) {
+    std::wstring str{L"__foobar__"};
+    std::wstring_view view{str.data()+2, 6};
+    ASSERT_EQ(entt::hashed_wstring::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;
 }

test/entt/core/monostate.cpp

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

test/entt/core/type_info.cpp

@@ -0,0 +1,9 @@
+#include <gtest/gtest.h>
+#include <entt/core/hashed_string.hpp>
+#include <entt/core/type_info.hpp>
+
+TEST(TypeId, Functionalities) {
+    ASSERT_NE(entt::type_info<int>::id(), entt::type_info<const int>::id());
+    ASSERT_NE(entt::type_info<int>::id(), entt::type_info<char>::id());
+    ASSERT_EQ(entt::type_info<int>::id(), entt::type_info<int>::id());
+}

test/entt/core/type_traits.cpp

@@ -0,0 +1,43 @@
+#include <gtest/gtest.h>
+#include <entt/config/config.h>
+#include <entt/core/hashed_string.hpp>
+#include <entt/core/type_traits.hpp>
+
+TEST(Choice, Functionalities) {
+    ASSERT_TRUE((std::is_base_of_v<entt::choice_t<0>, entt::choice_t<1>>));
+    ASSERT_FALSE((std::is_base_of_v<entt::choice_t<1>, entt::choice_t<0>>));
+}
+
+TEST(TypeList, Functionalities) {
+    using type = entt::type_list<int, char>;
+    using other = entt::type_list<double>;
+
+    ASSERT_EQ(entt::type_list_size_v<type>, 2u);
+    ASSERT_EQ(entt::type_list_size_v<other>, 1u);
+    ASSERT_TRUE((std::is_same_v<entt::type_list_cat_t<type, other, type, other>, entt::type_list<int, char, double, int, char, double>>));
+    ASSERT_TRUE((std::is_same_v<entt::type_list_cat_t<type, other>, entt::type_list<int, char, double>>));
+    ASSERT_TRUE((std::is_same_v<entt::type_list_cat_t<type, type>, entt::type_list<int, char, int, char>>));
+    ASSERT_TRUE((std::is_same_v<entt::type_list_unique_t<entt::type_list_cat_t<type, type>>, entt::type_list<int, char>>));
+}
+
+TEST(IsEqualityComparable, Functionalities) {
+    ASSERT_TRUE(entt::is_equality_comparable_v<int>);
+    ASSERT_FALSE(entt::is_equality_comparable_v<void>);
+}
+
+TEST(MemberClass, Functionalities) {
+    struct clazz {
+        char foo(int) { return {}; }
+        int bar(double, float) const { return {}; }
+        bool quux;
+    };
+
+    ASSERT_TRUE((std::is_same_v<clazz, entt::member_class_t<decltype(&clazz::foo)>>));
+    ASSERT_TRUE((std::is_same_v<clazz, entt::member_class_t<decltype(&clazz::bar)>>));
+    ASSERT_TRUE((std::is_same_v<clazz, entt::member_class_t<decltype(&clazz::quux)>>));
+}
+
+TEST(Tag, Functionalities) {
+    ASSERT_EQ(entt::tag<"foobar"_hs>::value, entt::hashed_string::value("foobar"));
+    ASSERT_TRUE((std::is_same_v<typename entt::tag<"foobar"_hs>::value_type, ENTT_ID_TYPE>));
+}

test/entt/core/utility.cpp

@@ -0,0 +1,60 @@
+#include <utility>
+#include <gtest/gtest.h>
+#include <entt/core/utility.hpp>
+
+struct Functions {
+    static void foo(int) {}
+    static void foo() {}
+
+    void bar(int) {}
+    void bar() {}
+};
+
+TEST(Utility, Identity) {
+    entt::identity identity;
+    int value = 42;
+
+    ASSERT_EQ(identity(value), value);
+    ASSERT_EQ(&identity(value), &value);
+}
+
+TEST(Utility, Overload) {
+    ASSERT_EQ(entt::overload<void(int)>(&Functions::foo), static_cast<void(*)(int)>(&Functions::foo));
+    ASSERT_EQ(entt::overload<void()>(&Functions::foo), static_cast<void(*)()>(&Functions::foo));
+
+    ASSERT_EQ(entt::overload<void(int)>(&Functions::bar), static_cast<void(Functions:: *)(int)>(&Functions::bar));
+    ASSERT_EQ(entt::overload<void()>(&Functions::bar), static_cast<void(Functions:: *)()>(&Functions::bar));
+
+    Functions instance;
+
+    ASSERT_NO_THROW(entt::overload<void(int)>(&Functions::foo)(0));
+    ASSERT_NO_THROW(entt::overload<void()>(&Functions::foo)());
+
+    ASSERT_NO_THROW((instance.*entt::overload<void(int)>(&Functions::bar))(0));
+    ASSERT_NO_THROW((instance.*entt::overload<void()>(&Functions::bar))());
+}
+
+TEST(Utility, Overloaded) {
+    int iv = 0;
+    char cv = '\0';
+
+    entt::overloaded func{
+        [&iv](int value) { iv = value; },
+        [&cv](char value) { cv = value; }
+    };
+
+    func(42);
+    func('c');
+
+    ASSERT_EQ(iv, 42);
+    ASSERT_EQ(cv, 'c');
+}
+
+TEST(Utility, YCombinator) {
+    entt::y_combinator gauss([](auto &&self, unsigned int value) -> unsigned int {
+        return value ? (value + self(value-1u)) : 0;
+    });
+
+    ASSERT_EQ(gauss(3u), 3u*4u/2u);
+    ASSERT_EQ(std::as_const(gauss)(7u), 7u*8u/2u);
+}

test/entt/entity/actor.cpp

@@ -3,55 +3,90 @@
 #include <entt/entity/actor.hpp>
 #include <entt/entity/registry.hpp>
 
-struct TestActor: entt::DefaultActor<unsigned int> {
-    using entt::DefaultActor<unsigned int>::DefaultActor;
-    void update(unsigned int) {}
-};
+TEST(Actor, Component) {
+    entt::registry registry;
+    entt::actor actor{registry};
 
-struct Position final {};
-struct Velocity final {};
-
-TEST(Actor, Functionalities) {
-    entt::DefaultRegistry registry;
-    TestActor *actor = new TestActor{registry};
-    const auto &cactor = *actor;
-
-    ASSERT_EQ(&registry, &actor->registry());
-    ASSERT_EQ(&registry, &cactor.registry());
-    ASSERT_TRUE(registry.empty<Position>());
-    ASSERT_TRUE(registry.empty<Velocity>());
+    ASSERT_EQ(&registry, &actor.backend());
+    ASSERT_EQ(&registry, &std::as_const(actor).backend());
+    ASSERT_TRUE(registry.empty<int>());
     ASSERT_FALSE(registry.empty());
-    ASSERT_FALSE(actor->has<Position>());
-    ASSERT_FALSE(actor->has<Velocity>());
+    ASSERT_FALSE(actor.has<int>());
 
-    const auto &position = actor->set<Position>();
+    const auto &cint = actor.assign<int>();
+    const auto &cchar = actor.assign<char>();
 
-    ASSERT_EQ(&position, &actor->get<Position>());
-    ASSERT_EQ(&position, &cactor.get<Position>());
-    ASSERT_FALSE(registry.empty<Position>());
-    ASSERT_TRUE(registry.empty<Velocity>());
+    ASSERT_EQ(&cint, &actor.get<int>());
+    ASSERT_EQ(&cchar, &std::as_const(actor).get<char>());
+    ASSERT_EQ(&cint, &std::get<0>(actor.get<int, char>()));
+    ASSERT_EQ(&cchar, &std::get<1>(actor.get<int, char>()));
+    ASSERT_EQ(&cint, std::get<0>(actor.try_get<int, char, double>()));
+    ASSERT_EQ(&cchar, std::get<1>(actor.try_get<int, char, double>()));
+    ASSERT_EQ(nullptr, std::get<2>(actor.try_get<int, char, double>()));
+    ASSERT_EQ(nullptr, actor.try_get<double>());
+    ASSERT_EQ(&cchar, actor.try_get<char>());
+    ASSERT_EQ(&cint, actor.try_get<int>());
+
+    ASSERT_FALSE(registry.empty<int>());
     ASSERT_FALSE(registry.empty());
-    ASSERT_TRUE(actor->has<Position>());
-    ASSERT_FALSE(actor->has<Velocity>());
+    ASSERT_TRUE((actor.has<int, char>()));
+    ASSERT_FALSE(actor.has<double>());
 
-    actor->unset<Position>();
+    actor.remove<int>();
 
-    ASSERT_TRUE(registry.empty<Position>());
-    ASSERT_TRUE(registry.empty<Velocity>());
+    ASSERT_TRUE(registry.empty<int>());
     ASSERT_FALSE(registry.empty());
-    ASSERT_FALSE(actor->has<Position>());
-    ASSERT_FALSE(actor->has<Velocity>());
+    ASSERT_FALSE(actor.has<int>());
+}
 
-    actor->set<Position>();
-    actor->set<Velocity>();
+TEST(Actor, FromEntity) {
+    entt::registry registry;
+    const auto entity = registry.create();
 
+    registry.assign<int>(entity, 42);
+    registry.assign<char>(entity, 'c');
+
+    entt::actor actor{entity, registry};
+
+    ASSERT_TRUE(actor);
+    ASSERT_EQ(entity, actor.entity());
+    ASSERT_TRUE((actor.has<int, char>()));
+    ASSERT_EQ(actor.get<int>(), 42);
+    ASSERT_EQ(actor.get<char>(), 'c');
+}
+
+TEST(Actor, EntityLifetime) {
+    entt::registry registry;
+    entt::actor actor{};
+
+    ASSERT_FALSE(actor);
+
+    actor = entt::actor{registry};
+    actor.assign<int>();
+
+    ASSERT_TRUE(actor);
+    ASSERT_FALSE(registry.empty<int>());
     ASSERT_FALSE(registry.empty());
-    ASSERT_FALSE(registry.empty<Position>());
-    ASSERT_FALSE(registry.empty<Velocity>());
+
+    registry.destroy(actor.entity());
+
+    ASSERT_FALSE(actor);
+}
+
+TEST(Actor, ActorLifetime) {
+    entt::registry registry;
+    auto *actor = new entt::actor{registry};
+    actor->assign<int>();
+
+    ASSERT_FALSE(registry.empty<int>());
+    ASSERT_FALSE(registry.empty());
+
+    registry.each([actor](const auto entity) {
+        ASSERT_EQ(actor->entity(), entity);
+    });
 
     delete actor;
 
+    ASSERT_TRUE(registry.empty<int>());
     ASSERT_TRUE(registry.empty());
-    ASSERT_TRUE(registry.empty<Position>());
-    ASSERT_TRUE(registry.empty<Velocity>());
 }

test/entt/entity/benchmark.cpp

@@ -1,355 +0,0 @@
-#include <gtest/gtest.h>
-#include <iostream>
-#include <cstddef>
-#include <chrono>
-#include <vector>
-#include <entt/entity/registry.hpp>
-
-struct Position {
-    uint64_t x;
-    uint64_t y;
-};
-
-struct Velocity {
-    uint64_t x;
-    uint64_t y;
-};
-
-template<std::size_t>
-struct Comp {};
-
-struct Timer final {
-    Timer(): start{std::chrono::system_clock::now()} {}
-
-    void elapsed() {
-        auto now = std::chrono::system_clock::now();
-        std::cout << std::chrono::duration<double>(now - start).count() << " seconds" << std::endl;
-    }
-
-private:
-    std::chrono::time_point<std::chrono::system_clock> start;
-};
-
-TEST(Benchmark, Construct) {
-    entt::DefaultRegistry registry;
-
-    std::cout << "Constructing 10000000 entities" << std::endl;
-
-    Timer timer;
-
-    for(uint64_t i = 0; i < 10000000L; i++) {
-        registry.create();
-    }
-
-    timer.elapsed();
-}
-
-TEST(Benchmark, Destroy) {
-    entt::DefaultRegistry registry;
-    std::vector<entt::DefaultRegistry::entity_type> entities{};
-
-    std::cout << "Destroying 10000000 entities" << std::endl;
-
-    for(uint64_t i = 0; i < 10000000L; i++) {
-        entities.push_back(registry.create());
-    }
-
-    Timer timer;
-
-    for(auto entity: entities) {
-        registry.destroy(entity);
-    }
-
-    timer.elapsed();
-}
-
-TEST(Benchmark, IterateCreateDeleteSingleComponent) {
-    entt::DefaultRegistry registry;
-
-    std::cout << "Looping 10000 times creating and deleting a random number of entities" << std::endl;
-
-    Timer timer;
-
-    auto view = registry.view<Position>();
-
-    for(int i = 0; i < 10000; i++) {
-        for(int j = 0; j < 10000; j++) {
-            registry.create<Position>();
-        }
-
-        for(auto entity: view) {
-            if(rand() % 2 == 0) {
-                registry.destroy(entity);
-            }
-        }
-    }
-
-    timer.elapsed();
-}
-
-TEST(Benchmark, IterateSingleComponent10M) {
-    entt::DefaultRegistry registry;
-
-    std::cout << "Iterating over 10000000 entities, one component" << std::endl;
-
-    for(uint64_t i = 0; i < 10000000L; i++) {
-        registry.create<Position>();
-    }
-
-    Timer timer;
-    registry.view<Position>().each([](auto, auto &) {});
-    timer.elapsed();
-}
-
-TEST(Benchmark, IterateTwoComponents10M) {
-    entt::DefaultRegistry registry;
-
-    std::cout << "Iterating over 10000000 entities, two components" << std::endl;
-
-    for(uint64_t i = 0; i < 10000000L; i++) {
-        registry.create<Position, Velocity>();
-    }
-
-    Timer timer;
-    registry.view<Position, Velocity>().each([](auto, auto &...) {});
-    timer.elapsed();
-}
-
-TEST(Benchmark, IterateTwoComponents10MHalf) {
-    entt::DefaultRegistry registry;
-
-    std::cout << "Iterating over 10000000 entities, two components, half of the entities have all the components" << std::endl;
-
-    for(uint64_t i = 0; i < 10000000L; i++) {
-        auto entity = registry.create<Velocity>();
-        if(i % 2) { registry.assign<Position>(entity); }
-    }
-
-    Timer timer;
-    registry.view<Position, Velocity>().each([](auto, auto &...) {});
-    timer.elapsed();
-}
-
-TEST(Benchmark, IterateTwoComponents10MOne) {
-    entt::DefaultRegistry registry;
-
-    std::cout << "Iterating over 10000000 entities, two components, only one entity has all the components" << std::endl;
-
-    for(uint64_t i = 0; i < 10000000L; i++) {
-        auto entity = registry.create<Velocity>();
-        if(i == 5000000L) { registry.assign<Position>(entity); }
-    }
-
-    Timer timer;
-    registry.view<Position, Velocity>().each([](auto, auto &...) {});
-    timer.elapsed();
-}
-
-TEST(Benchmark, IterateTwoComponentsPersistent10M) {
-    entt::DefaultRegistry registry;
-    registry.prepare<Position, Velocity>();
-
-    std::cout << "Iterating over 10000000 entities, two components, persistent view" << std::endl;
-
-    for(uint64_t i = 0; i < 10000000L; i++) {
-        registry.create<Position, Velocity>();
-    }
-
-    Timer timer;
-    registry.persistent<Position, Velocity>().each([](auto, auto &...) {});
-    timer.elapsed();
-}
-
-TEST(Benchmark, IterateTwoComponentsPersistent10MHalf) {
-    entt::DefaultRegistry registry;
-    registry.prepare<Position, Velocity>();
-
-    std::cout << "Iterating over 10000000 entities, two components, persistent view, half of the entities have all the components" << std::endl;
-
-    for(uint64_t i = 0; i < 10000000L; i++) {
-        auto entity = registry.create<Velocity>();
-        if(i % 2) { registry.assign<Position>(entity); }
-    }
-
-    Timer timer;
-    registry.persistent<Position, Velocity>().each([](auto, auto &...) {});
-    timer.elapsed();
-}
-
-TEST(Benchmark, IterateTwoComponentsPersistent10MOne) {
-    entt::DefaultRegistry registry;
-    registry.prepare<Position, Velocity>();
-
-    std::cout << "Iterating over 10000000 entities, two components, persistent view, only one entity has all the components" << std::endl;
-
-    for(uint64_t i = 0; i < 10000000L; i++) {
-        auto entity = registry.create<Velocity>();
-        if(i == 5000000L) { registry.assign<Position>(entity); }
-    }
-
-    Timer timer;
-    registry.persistent<Position, Velocity>().each([](auto, auto &...) {});
-    timer.elapsed();
-}
-
-TEST(Benchmark, IterateFiveComponents10M) {
-    entt::DefaultRegistry registry;
-
-    std::cout << "Iterating over 10000000 entities, five components" << std::endl;
-
-    for(uint64_t i = 0; i < 10000000L; i++) {
-        registry.create<Position, Velocity, Comp<1>, Comp<2>, Comp<3>>();
-    }
-
-    Timer timer;
-    registry.view<Position, Velocity, Comp<1>, Comp<2>, Comp<3>>().each([](auto, auto &...) {});
-    timer.elapsed();
-}
-
-TEST(Benchmark, IterateTenComponents10M) {
-    entt::DefaultRegistry registry;
-
-    std::cout << "Iterating over 10000000 entities, ten components" << std::endl;
-
-    for(uint64_t i = 0; i < 10000000L; i++) {
-        registry.create<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
-    }
-
-    Timer timer;
-    registry.view<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>().each([](auto, auto &...) {});
-    timer.elapsed();
-}
-
-TEST(Benchmark, IterateTenComponents10MHalf) {
-    entt::DefaultRegistry registry;
-
-    std::cout << "Iterating over 10000000 entities, ten components, half of the entities have all the components" << std::endl;
-
-    for(uint64_t i = 0; i < 10000000L; i++) {
-        auto entity = registry.create<Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
-        if(i % 2) { registry.assign<Position>(entity); }
-    }
-
-    Timer timer;
-    registry.view<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>().each([](auto, auto &...) {});
-    timer.elapsed();
-}
-
-TEST(Benchmark, IterateTenComponents10MOne) {
-    entt::DefaultRegistry registry;
-
-    std::cout << "Iterating over 10000000 entities, ten components, only one entity has all the components" << std::endl;
-
-    for(uint64_t i = 0; i < 10000000L; i++) {
-        auto entity = registry.create<Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
-        if(i == 5000000L) { registry.assign<Position>(entity); }
-    }
-
-    Timer timer;
-    registry.view<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>().each([](auto, auto &...) {});
-    timer.elapsed();
-}
-
-TEST(Benchmark, IterateFiveComponentsPersistent10M) {
-    entt::DefaultRegistry registry;
-    registry.prepare<Position, Velocity, Comp<1>, Comp<2>, Comp<3>>();
-
-    std::cout << "Iterating over 10000000 entities, five components, persistent view" << std::endl;
-
-    for(uint64_t i = 0; i < 10000000L; i++) {
-        registry.create<Position, Velocity, Comp<1>, Comp<2>, Comp<3>>();
-    }
-
-    Timer timer;
-    registry.persistent<Position, Velocity, Comp<1>, Comp<2>, Comp<3>>().each([](auto, auto &...) {});
-    timer.elapsed();
-}
-
-TEST(Benchmark, IterateTenComponentsPersistent10M) {
-    entt::DefaultRegistry registry;
-    registry.prepare<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
-
-    std::cout << "Iterating over 10000000 entities, ten components, persistent view" << std::endl;
-
-    for(uint64_t i = 0; i < 10000000L; i++) {
-        registry.create<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
-    }
-
-    Timer timer;
-    registry.persistent<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>().each([](auto, auto &...) {});
-    timer.elapsed();
-}
-
-TEST(Benchmark, IterateTenComponentsPersistent10MHalf) {
-    entt::DefaultRegistry registry;
-    registry.prepare<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
-
-    std::cout << "Iterating over 10000000 entities, ten components, persistent view, half of the entities have all the components" << std::endl;
-
-    for(uint64_t i = 0; i < 10000000L; i++) {
-        auto entity = registry.create<Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
-        if(i % 2) { registry.assign<Position>(entity); }
-    }
-
-    Timer timer;
-    registry.persistent<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>().each([](auto, auto &...) {});
-    timer.elapsed();
-}
-
-TEST(Benchmark, IterateTenComponentsPersistent10MOne) {
-    entt::DefaultRegistry registry;
-    registry.prepare<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
-
-    std::cout << "Iterating over 10000000 entities, ten components, persistent view, only one entity has all the components" << std::endl;
-
-    for(uint64_t i = 0; i < 10000000L; i++) {
-        auto entity = registry.create<Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
-        if(i == 5000000L) { registry.assign<Position>(entity); }
-    }
-
-    Timer timer;
-    registry.persistent<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>().each([](auto, auto &...) {});
-    timer.elapsed();
-}
-
-TEST(Benchmark, SortSingle) {
-    entt::DefaultRegistry registry;
-    std::vector<entt::DefaultRegistry::entity_type> entities{};
-
-    std::cout << "Sort 150000 entities, one component" << std::endl;
-
-    for(uint64_t i = 0; i < 150000L; i++) {
-        auto entity = registry.create<Position>({ i, i });
-        entities.push_back(entity);
-    }
-
-    Timer timer;
-
-    registry.sort<Position>([](const auto &lhs, const auto &rhs) {
-        return lhs.x < rhs.x && lhs.y < rhs.y;
-    });
-
-    timer.elapsed();
-}
-
-TEST(Benchmark, SortMulti) {
-    entt::DefaultRegistry registry;
-    std::vector<entt::DefaultRegistry::entity_type> entities{};
-
-    std::cout << "Sort 150000 entities, two components" << std::endl;
-
-    for(uint64_t i = 0; i < 150000L; i++) {
-        auto entity = registry.create<Position, Velocity>({ i, i }, { i, i });
-        entities.push_back(entity);
-    }
-
-    registry.sort<Position>([](const auto &lhs, const auto &rhs) {
-        return lhs.x < rhs.x && lhs.y < rhs.y;
-    });
-
-    Timer timer;
-
-    registry.sort<Velocity, Position>();
-
-    timer.elapsed();
-}

test/entt/entity/entity.cpp

@@ -0,0 +1,29 @@
+#include <functional>
+#include <type_traits>
+#include <gtest/gtest.h>
+#include <entt/entity/entity.hpp>
+#include <entt/entity/registry.hpp>
+
+TEST(Entity, Null) {
+    using traits_type = entt::entt_traits<std::underlying_type_t<entt::entity>>;
+
+    entt::registry registry{};
+    const auto entity = registry.create();
+
+    registry.assign<int>(entity, 42);
+
+    ASSERT_FALSE(entt::entity{} == entt::null);
+    ASSERT_TRUE(entt::entity{traits_type::entity_mask} == entt::null);
+    ASSERT_TRUE(entt::entity{~typename traits_type::entity_type{}} == entt::null);
+
+    ASSERT_TRUE(entt::null == entt::null);
+    ASSERT_FALSE(entt::null != entt::null);
+
+    ASSERT_FALSE(entity == entt::null);
+    ASSERT_FALSE(entt::null == entity);
+
+    ASSERT_TRUE(entity != entt::null);
+    ASSERT_TRUE(entt::null != entity);
+
+    ASSERT_FALSE(registry.valid(entt::null));
+}

test/entt/entity/helper.cpp

@@ -0,0 +1,24 @@
+#include <gtest/gtest.h>
+#include <entt/core/hashed_string.hpp>
+#include <entt/entity/helper.hpp>
+#include <entt/entity/registry.hpp>
+#include <entt/core/type_traits.hpp>
+
+TEST(Helper, AsView) {
+    entt::registry registry;
+    const entt::registry cregistry;
+
+    ([](entt::view<entt::exclude_t<>, int>) {})(entt::as_view{registry});
+    ([](entt::view<entt::exclude_t<int>, char, double>) {})(entt::as_view{registry});
+    ([](entt::view<entt::exclude_t<int>, const char, double>) {})(entt::as_view{registry});
+    ([](entt::view<entt::exclude_t<int>, const char, const double>) {})(entt::as_view{registry});
+}
+
+TEST(Helper, AsGroup) {
+    entt::registry registry;
+    const entt::registry cregistry;
+
+    ([](entt::group<entt::exclude_t<int>, entt::get_t<char>, double>) {})(entt::as_group{registry});
+    ([](entt::group<entt::exclude_t<int>, entt::get_t<const char>, double>) {})(entt::as_group{registry});
+    ([](entt::group<entt::exclude_t<int>, entt::get_t<const char>, const double>) {})(entt::as_group{registry});
+}

test/entt/entity/observer.cpp

@@ -0,0 +1,342 @@
+#include <tuple>
+#include <cstddef>
+#include <type_traits>
+#include <gtest/gtest.h>
+#include <entt/entity/observer.hpp>
+#include <entt/entity/registry.hpp>
+
+TEST(Observer, Functionalities) {
+    entt::registry registry;
+    entt::observer observer{registry, entt::collector.group<int>()};
+
+    ASSERT_EQ(observer.size(), 0u);
+    ASSERT_TRUE(observer.empty());
+    ASSERT_EQ(observer.data(), nullptr);
+    ASSERT_EQ(observer.begin(), observer.end());
+
+    const auto entity = registry.create();
+    registry.assign<int>(entity);
+
+    ASSERT_EQ(observer.size(), 1u);
+    ASSERT_FALSE(observer.empty());
+    ASSERT_NE(observer.data(), nullptr);
+    ASSERT_EQ(*observer.data(), entity);
+    ASSERT_NE(observer.begin(), observer.end());
+    ASSERT_EQ(++observer.begin(), observer.end());
+    ASSERT_EQ(*observer.begin(), entity);
+
+    observer.clear();
+
+    ASSERT_EQ(observer.size(), 0u);
+    ASSERT_TRUE(observer.empty());
+
+    observer.disconnect();
+    registry.remove<int>(entity);
+    registry.assign<int>(entity);
+
+    ASSERT_EQ(observer.size(), 0u);
+    ASSERT_TRUE(observer.empty());
+}
+
+TEST(Observer, AllOf) {
+    constexpr auto collector =  entt::collector
+            .group<int, char>(entt::exclude<float>)
+            .group<int, double>();
+
+    entt::registry registry;
+    entt::observer observer{registry, collector};
+    const auto entity = registry.create();
+
+    ASSERT_TRUE(observer.empty());
+
+    registry.assign<int>(entity);
+    registry.assign<char>(entity);
+
+    ASSERT_EQ(observer.size(), 1u);
+    ASSERT_FALSE(observer.empty());
+    ASSERT_EQ(*observer.data(), entity);
+
+    registry.assign<double>(entity);
+
+    ASSERT_FALSE(observer.empty());
+
+    registry.remove<int>(entity);
+
+    ASSERT_TRUE(observer.empty());
+
+    registry.assign<float>(entity);
+    registry.assign<int>(entity);
+
+    ASSERT_FALSE(observer.empty());
+
+    registry.remove<double>(entity);
+
+    ASSERT_TRUE(observer.empty());
+
+    registry.assign<double>(entity);
+    observer.clear();
+
+    ASSERT_TRUE(observer.empty());
+
+    observer.disconnect();
+    registry.assign_or_replace<int>(entity);
+    registry.assign_or_replace<char>(entity);
+    registry.remove_if_exists<float>(entity);
+
+    ASSERT_TRUE(observer.empty());
+}
+
+TEST(Observer, AllOfFiltered) {
+    constexpr auto collector =  entt::collector
+            .group<int>().where<char>(entt::exclude<double>);
+
+    entt::registry registry;
+    entt::observer observer{registry, collector};
+    const auto entity = registry.create();
+
+    ASSERT_TRUE(observer.empty());
+
+    registry.assign<int>(entity);
+
+    ASSERT_EQ(observer.size(), 0u);
+    ASSERT_TRUE(observer.empty());
+    ASSERT_EQ(observer.data(), nullptr);
+
+    registry.remove<int>(entity);
+    registry.assign<char>(entity);
+    registry.assign<double>(entity);
+    registry.assign<int>(entity);
+
+    ASSERT_TRUE(observer.empty());
+
+    registry.remove<int>(entity);
+    registry.remove<double>(entity);
+    registry.assign<int>(entity);
+
+    ASSERT_EQ(observer.size(), 1u);
+    ASSERT_FALSE(observer.empty());
+    ASSERT_EQ(*observer.data(), entity);
+
+    registry.assign<double>(entity);
+
+    ASSERT_TRUE(observer.empty());
+
+    registry.remove<double>(entity);
+
+    ASSERT_TRUE(observer.empty());
+
+    observer.disconnect();
+    registry.remove<int>(entity);
+    registry.assign<int>(entity);
+
+    ASSERT_TRUE(observer.empty());
+}
+
+TEST(Observer, Observe) {
+    entt::registry registry;
+    entt::observer observer{registry, entt::collector.replace<int>().replace<char>()};
+    const auto entity = registry.create();
+
+    ASSERT_TRUE(observer.empty());
+
+    registry.assign<int>(entity);
+    registry.assign<char>(entity);
+
+    ASSERT_TRUE(observer.empty());
+
+    registry.assign_or_replace<int>(entity);
+
+    ASSERT_EQ(observer.size(), 1u);
+    ASSERT_FALSE(observer.empty());
+    ASSERT_EQ(*observer.data(), entity);
+
+    observer.clear();
+    registry.replace<char>(entity);
+
+    ASSERT_FALSE(observer.empty());
+
+    observer.clear();
+
+    ASSERT_TRUE(observer.empty());
+
+    observer.disconnect();
+    registry.assign_or_replace<int>(entity);
+    registry.assign_or_replace<char>(entity);
+
+    ASSERT_TRUE(observer.empty());
+}
+
+TEST(Observer, ObserveFiltered) {
+    constexpr auto collector =  entt::collector
+            .replace<int>().where<char>(entt::exclude<double>);
+
+    entt::registry registry;
+    entt::observer observer{registry, collector};
+    const auto entity = registry.create();
+
+    ASSERT_TRUE(observer.empty());
+
+    registry.assign<int>(entity);
+    registry.replace<int>(entity);
+
+    ASSERT_EQ(observer.size(), 0u);
+    ASSERT_TRUE(observer.empty());
+    ASSERT_EQ(observer.data(), nullptr);
+
+    registry.assign<char>(entity);
+    registry.assign<double>(entity);
+    registry.replace<int>(entity);
+
+    ASSERT_TRUE(observer.empty());
+
+    registry.remove<double>(entity);
+    registry.replace<int>(entity);
+
+    ASSERT_EQ(observer.size(), 1u);
+    ASSERT_FALSE(observer.empty());
+    ASSERT_EQ(*observer.data(), entity);
+
+    registry.assign<double>(entity);
+
+    ASSERT_TRUE(observer.empty());
+
+    registry.remove<double>(entity);
+
+    ASSERT_TRUE(observer.empty());
+
+    observer.disconnect();
+    registry.replace<int>(entity);
+
+    ASSERT_TRUE(observer.empty());
+}
+
+TEST(Observer, AllOfObserve) {
+    entt::registry registry;
+    entt::observer observer{};
+    const auto entity = registry.create();
+
+    observer.connect(registry, entt::collector.group<int>().replace<char>());
+
+    ASSERT_TRUE(observer.empty());
+
+    registry.assign<int>(entity);
+    registry.assign<char>(entity);
+    registry.replace<char>(entity);
+    registry.remove<int>(entity);
+
+    ASSERT_EQ(observer.size(), 1u);
+    ASSERT_FALSE(observer.empty());
+    ASSERT_EQ(*observer.data(), entity);
+
+    registry.remove<char>(entity);
+    registry.assign<char>(entity);
+
+    ASSERT_TRUE(observer.empty());
+
+    registry.replace<char>(entity);
+    observer.clear();
+
+    ASSERT_TRUE(observer.empty());
+
+    observer.disconnect();
+    registry.assign_or_replace<int>(entity);
+    registry.assign_or_replace<char>(entity);
+
+    ASSERT_TRUE(observer.empty());
+}
+
+TEST(Observer, CrossRulesCornerCase) {
+    entt::registry registry;
+    entt::observer observer{registry, entt::collector.group<int>().group<char>()};
+    const auto entity = registry.create();
+
+    registry.assign<int>(entity);
+    observer.clear();
+
+    ASSERT_TRUE(observer.empty());
+
+    registry.assign<char>(entity);
+    registry.remove<int>(entity);
+
+    ASSERT_FALSE(observer.empty());
+}
+
+TEST(Observer, Each) {
+    entt::registry registry;
+    entt::observer observer{registry, entt::collector.group<int>()};
+    const auto entity = registry.create();
+    registry.assign<int>(entity);
+
+    ASSERT_FALSE(observer.empty());
+    ASSERT_EQ(observer.size(), 1u);
+
+    std::as_const(observer).each([entity](const auto entt) {
+        ASSERT_EQ(entity, entt);
+    });
+
+    ASSERT_FALSE(observer.empty());
+    ASSERT_EQ(observer.size(), 1u);
+
+    observer.each([entity](const auto entt) {
+        ASSERT_EQ(entity, entt);
+    });
+
+    ASSERT_TRUE(observer.empty());
+    ASSERT_EQ(observer.size(), 0u);
+}
+
+TEST(Observer, MultipleFilters) {
+    constexpr auto collector =  entt::collector
+            .replace<int>().where<char>()
+            .replace<double>().where<float>();
+
+    entt::registry registry;
+    entt::observer observer{registry, collector};
+    const auto entity = registry.create();
+
+    ASSERT_TRUE(observer.empty());
+
+    registry.assign_or_replace<int>(entity);
+    registry.assign<char>(entity);
+
+    ASSERT_TRUE(observer.empty());
+
+    registry.assign_or_replace<int>(entity);
+
+    ASSERT_EQ(observer.size(), 1u);
+    ASSERT_FALSE(observer.empty());
+    ASSERT_EQ(*observer.data(), entity);
+
+    observer.clear();
+    registry.assign<double>(entity);
+
+    ASSERT_TRUE(observer.empty());
+
+    registry.assign_or_replace<double>(entity);
+    registry.assign<float>(entity);
+
+    ASSERT_TRUE(observer.empty());
+
+    registry.assign_or_replace<double>(entity);
+
+    ASSERT_EQ(observer.size(), 1u);
+    ASSERT_FALSE(observer.empty());
+    ASSERT_EQ(*observer.data(), entity);
+
+    registry.remove<float>(entity);
+
+    ASSERT_TRUE(observer.empty());
+
+    registry.assign_or_replace<int>(entity);
+
+    ASSERT_EQ(observer.size(), 1u);
+    ASSERT_FALSE(observer.empty());
+    ASSERT_EQ(*observer.data(), entity);
+
+    observer.clear();
+    observer.disconnect();
+
+    registry.assign_or_replace<int>(entity);
+
+    ASSERT_TRUE(observer.empty());
+}