ready to cut v3.1.1

He's one of contributors to the idea of nested groups!! :)

.github/FUNDING.yml

@@ -0,0 +1,12 @@
+# These are supported funding model platforms
+
+github: # [skypjack] available only for those enrolled in the github sponsor program
+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

.gitignore

@@ -1 +1,8 @@
 *.user
+conan/test_package/build
+
+# IDEs
+.idea
+
+# Bazel
+/bazel-*

.travis.yml

@@ -2,15 +2,40 @@ language: cpp
 dist: trusty
 sudo: false
 
+env:
+  global:
+    - CONAN_USERNAME="skypjack"
+    - CONAN_PACKAGE_NAME="entt"
+    - CONAN_HEADER_ONLY="True"
+    - NON_CONAN_DEPLOYMENT="True"
+
+conan-buildsteps: &conan-buildsteps
+  before_install:
+    # use this step if you desire to manipulate CONAN variables programmatically
+    - NON_CONAN_DEPLOYMENT="False"
+  install:
+    - chmod +x ./conan/ci/install.sh
+    - ./conan/ci/install.sh
+  script:
+    - chmod +x ./conan/ci/build.sh
+    - ./conan/ci/build.sh
+  # the following are dummies to overwrite default build steps
+  before_script:
+    - true
+  after_success:
+    - true
+  if: tag IS present
+conan-linux: &conan-linux
+  os: linux
+  sudo: required
+  language: python
+  python: "3.6"
+  services:
+    - docker
+  <<: *conan-buildsteps
+
 matrix:
   include:
-  - os: linux
-    compiler: gcc
-    addons:
-      apt:
-        sources: ['ubuntu-toolchain-r-test']
-        packages: ['g++-6']
-    env: COMPILER=g++-6
   - os: linux
     compiler: gcc
     addons:
@@ -22,22 +47,11 @@ matrix:
     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
+        sources: ['ubuntu-toolchain-r-test', 'llvm-toolchain-trusty-6.0']
+        packages: ['clang-6.0', 'g++-7']
+    env: COMPILER=clang++-6.0
   - os: osx
-    osx_image: xcode8.3
-    compiler: clang
-    env: COMPILER=clang++
-  - os: osx
-    osx_image: xcode9.4
+    osx_image: xcode10
     compiler: clang
     env: COMPILER=clang++
   - os: linux
@@ -53,6 +67,9 @@ matrix:
       - 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
+  # Conan testing and uploading
+  - <<: *conan-linux
+    env: CONAN_GCC_VERSIONS=8 CONAN_DOCKER_IMAGE=conanio/gcc8
 
 notifications:
   email:
@@ -69,11 +86,12 @@ install:
 
 script:
 - mkdir -p build && cd build
-- cmake .. && make -j4
-- CTEST_OUTPUT_ON_FAILURE=1 make test
+- cmake -DBUILD_TESTING=ON -DBUILD_LIB=ON .. && make -j4
+- CTEST_OUTPUT_ON_FAILURE=1 ctest --timeout 5 -C Debug -j4
 
 deploy:
   provider: script
   script: scripts/update_packages.sh $TRAVIS_TAG
   on:
     tags: true
+    condition: “$NON_CONAN_DEPLOYMENT = “True”

AUTHORS

@@ -1,11 +1,37 @@
 # Author
 
-Michele Caini aka skypjack
+skypjack
 
 # Contributors
 
-Paolo Monteverde aka morbo84
-David Nerjes aka DavidHamburg
-Indi Kernick aka Kerndog73
-Malte Müller-Rowold aka m-waka
-Richard Caseres aka richardbmx
+BenediktConze
+bjadamson
+ceeac
+ColinH
+corystegel
+Croydon
+dbacchet
+dBagrat
+djarek
+DonKult
+drglove
+eugeneko
+gale83
+ghost
+Green-Sky
+Kerndog73
+Lawrencemm
+mhammerc
+Milerius
+morbo84
+m-waka
+NixAJ
+Paolo-Oliverio
+pgruenbacher
+prowolf
+The5-1
+vblanco20-1
+willtunnels
+WizardIke
+w1th0utnam3
+zaucy

BUILD.bazel

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

CMakeLists.txt

@@ -16,7 +16,7 @@ endif()
 # Project configuration
 #
 
-project(EnTT VERSION 2.7.2)
+project(EnTT VERSION 3.1.1)
 
 include(GNUInstallDirs)
 
@@ -31,7 +31,7 @@ set(PROJECT_AUTHOR_EMAIL "michele.caini@gmail.com")
 
 message("*")
 message("* ${PROJECT_NAME} v${PROJECT_VERSION} (${CMAKE_BUILD_TYPE})")
-message("* Copyright (c) 2017-2018 ${PROJECT_AUTHOR} <${PROJECT_AUTHOR_EMAIL}>")
+message("* Copyright (c) 2017-2019 ${PROJECT_AUTHOR} <${PROJECT_AUTHOR_EMAIL}>")
 message("*")
 
 option(USE_LIBCPP "Use libc++ by adding -stdlib=libc++ flag if availbale." ON)
@@ -52,7 +52,7 @@ if(NOT MSVC AND USE_LIBCPP)
 
     check_cxx_source_compiles("
         #include<type_traits>
-        int main() { return std::is_same<int, int>::value ? 0 : 1; }
+        int main() { return std::is_same_v<int, char>; }
     " HAS_LIBCPP)
 
     if(NOT HAS_LIBCPP)
@@ -68,6 +68,8 @@ endif()
 
 add_library(EnTT INTERFACE)
 
+configure_file(${EnTT_SOURCE_DIR}/cmake/in/version.h.in ${EnTT_SOURCE_DIR}/src/entt/config/version.h @ONLY)
+
 target_include_directories(
     EnTT INTERFACE
     $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/src>
@@ -100,7 +102,7 @@ if(HAS_LIBCPP)
     target_compile_options(EnTT BEFORE INTERFACE -stdlib=libc++)
 endif()
 
-target_compile_features(EnTT INTERFACE cxx_std_14)
+target_compile_features(EnTT INTERFACE cxx_std_17)
 
 #
 # Install EnTT
@@ -140,7 +142,6 @@ configure_package_config_file(
     ${CUSTOM_INSTALL_CONFIGDIR}/EnTTConfig.cmake
     INSTALL_DESTINATION ${CUSTOM_INSTALL_CONFIGDIR}
     PATH_VARS CMAKE_INSTALL_INCLUDEDIR
-    NO_CHECK_REQUIRED_COMPONENTS_MACRO
 )
 
 write_basic_package_version_file(
@@ -162,7 +163,7 @@ 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)
@@ -180,10 +181,13 @@ if(BUILD_TESTING)
         execute_process(COMMAND ${CMAKE_COMMAND} --build . WORKING_DIRECTORY ${GOOGLETEST_DEPS_DIR})
         set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
         add_subdirectory(${GOOGLETEST_DEPS_DIR}/src ${GOOGLETEST_DEPS_DIR}/build)
+        target_compile_features(gmock_main PRIVATE $<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_FEATURES>)
+        target_compile_features(gmock PRIVATE $<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_FEATURES>)
         add_library(GTest::Main ALIAS gtest_main)
     endif()
 
     option(BUILD_BENCHMARK "Build benchmark." OFF)
+    option(BUILD_LIB "Build lib example." OFF)
     option(BUILD_MOD "Build mod example." OFF)
     option(BUILD_SNAPSHOT "Build snapshot example." OFF)
 
@@ -214,10 +218,9 @@ add_custom_target(
     SOURCES
         appveyor.yml
         AUTHORS
+        CONTRIBUTING.md
         LICENSE
         README.md
         TODO
         .travis.yml
-        docs/CONTRIBUTING.md
-        docs/extra.dox
 )

CONTRIBUTING.md

@@ -22,8 +22,8 @@ How to do it mostly depends on the type of contribution:
   If you are unable to find an open issue addressing the problem, feel free to
   [open a new one](https://github.com/skypjack/entt/issues/new). **Please**, do
   not forget to carefully describe how to reproduce the problem, then add all
-  the informaion about the system on which you are experiencing it and point out
-  the version of `EnTT` you are using (tag or commit).
+  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.

LICENSE

@@ -1,6 +1,6 @@
 The MIT License (MIT)
 
-Copyright (c) 2017-2018 Michele Caini
+Copyright (c) 2017-2019 Michele Caini
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal

TODO

@@ -1,15 +1,33 @@
+* long term feature: templated generic vm
+* long term feature: shared_ptr less locator
+* long term feature: shared_ptr less resource cache
 * custom allocators and EnTT allocator-aware in general (long term feature, I don't actually need it at the moment) - see #22
-* scene management (I prefer the concept of spaces, that is a kind of scene anyway)
-* review doc: separate it in multiple md/dox files, reduce the readme to a minimum and provide users with links to the online documentation on gh-pages
 * debugging tools (#60): the issue online already contains interesting tips on this, look at it
-* define basic reactive systems (track entities to which component is attached, track entities from which component is removed, and so on)
-* define systems as composable mixins (initializazion, reactive, update, whatever) with flexible auto-detected arguments (registry, views, etc)
-* create dedicated flat map based on types implementation (sort of "type map") for types to use within the registry and so on...
-* registry::create with a "hint" on the entity identifier to use, it should ease combining multiple registries
-* optimize for empty components, it would be a mid improvement in terms of memory usage
-* add some lazy iterative sorters like "single bubble sort loop"
-* can we do more for shared libraries? who knows... see #144
+* runner proposal: https://en.wikipedia.org/wiki/Fork%E2%80%93join_model https://slide-rs.github.io/specs/03_dispatcher.html
 * work stealing job system (see #100)
-* reflection system (maybe)
-* C++17. That's all.
-* AOB
+  - mt scheduler based on const awareness for types
+* meta: sort of meta view based on meta stuff to iterate entities, void * and meta info objects
+* allow for built-in parallel each if possible
+* allow to replace std:: with custom implementations
+* remove runtime views, welcome reflection and what about snapshot?
+* types defined at runtime that refer to the same compile-time type (but to different pools) are possible, the library is almost there
+* add opaque input iterators to views and groups that return tuples <entity, T &...> (proxy), multi-pass guaranteed
+* add fast lane for raw iterations, extend mt doc to describe allowed add/remove with pre-allocations on fast lanes
+* registry.each<T...>(first, last) by iterators, entities/components guaranteed
+* multi component registry::remove and some others?
+* built-in support for dual (or N-) buffering
+* allow for custom stomp functions
+* deprecate/replace snapshot
+
+TODO
+* custom (decoupled) pools ==> double buffering, shared components, multi-model
+* make meta work across boundaries
+  - inline variables are fine here, only the head represents a problem
+  - we should always resolve by looking into the list of types when working across boundaries, no direct resolve
+* nested groups: AB/ABC/ABCD/... (hints: sort, check functions)
+* snapshot rework/deprecation
+  * create(hint: entity) -> force-create
+  * assign<T...>(first, last)
+* use unordered_map for named pools and context variables:
+  * use direct access (pool-like) also for context variables
+  * improves multi-stomp

WORKSPACE

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

appveyor.yml

@@ -1,20 +1,42 @@
 # can use variables like {build} and {branch}
 version: 1.0.{build}
 
-image: Visual Studio 2017
+skip_tags: true
+
+image:
+- Visual Studio 2019
+- Visual Studio 2017
 
 environment:
   BUILD_DIR: "%APPVEYOR_BUILD_FOLDER%\\build"
-
-platform:
-  - Win32
+  CTEST_OUTPUT_ON_FAILURE: 1
 
 configuration:
-  - Release
+  - Debug
+
+matrix:
+  fast_finish: true
+
+for:
+  -
+    matrix:
+      only:
+        - image: Visual Studio 2019
+    environment:
+      CMAKE_GENERATOR: "Visual Studio 16 2019"
+  -
+    matrix:
+      only:
+        - image: Visual Studio 2017
+    environment:
+      CMAKE_GENERATOR: "Visual Studio 15 2017"
 
 before_build:
   - cd %BUILD_DIR%
-  - cmake .. -DCMAKE_CXX_FLAGS=/D_SILENCE_TR1_NAMESPACE_DEPRECATION_WARNING -G"Visual Studio 15 2017"
+  - cmake .. -DBUILD_TESTING=ON -DBUILD_LIB=ON -DCMAKE_CXX_FLAGS=/W1 -G"%CMAKE_GENERATOR%"
+
+after_build:
+  - ctest --timeout 5 -C Debug -j4
 
 build:
   parallel: true

bazel/BUILD.bazel

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

bazel/copts.bzl

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

cmake/in/version.h.in

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

conan/build.py

@@ -0,0 +1,45 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+from cpt.packager import ConanMultiPackager
+import os
+
+if __name__ == "__main__":
+    login_username = os.getenv("CONAN_LOGIN_USERNAME")
+    username = os.getenv("CONAN_USERNAME")
+    tag_version = os.getenv("CONAN_PACKAGE_VERSION", os.getenv("TRAVIS_TAG"))
+    package_version = tag_version.replace("v", "")
+    package_name_unset = "SET-CONAN_PACKAGE_NAME-OR-CONAN_REFERENCE"
+    package_name = os.getenv("CONAN_PACKAGE_NAME", package_name_unset)
+    reference = "{}/{}".format(package_name, package_version)
+    channel = os.getenv("CONAN_CHANNEL", "stable")
+    upload = os.getenv("CONAN_UPLOAD")
+    stable_branch_pattern = os.getenv("CONAN_STABLE_BRANCH_PATTERN", r"v\d+\.\d+\.\d+.*")
+    test_folder = os.getenv("CPT_TEST_FOLDER", os.path.join("conan", "test_package"))
+    upload_only_when_stable = os.getenv("CONAN_UPLOAD_ONLY_WHEN_STABLE", True)
+    header_only = os.getenv("CONAN_HEADER_ONLY", False)
+    pure_c = os.getenv("CONAN_PURE_C", False)
+
+    disable_shared = os.getenv("CONAN_DISABLE_SHARED_BUILD", "False")
+    if disable_shared == "True" and package_name == package_name_unset:
+        raise Exception("CONAN_DISABLE_SHARED_BUILD: True is only supported when you define CONAN_PACKAGE_NAME")
+
+    builder = ConanMultiPackager(username=username,
+                                 reference=reference,
+                                 channel=channel,
+                                 login_username=login_username,
+                                 upload=upload,
+                                 stable_branch_pattern=stable_branch_pattern,
+                                 upload_only_when_stable=upload_only_when_stable,
+                                 test_folder=test_folder)
+    if header_only == "False":
+        builder.add_common_builds(pure_c=pure_c)
+    else:
+        builder.add()
+
+    filtered_builds = []
+    for settings, options, env_vars, build_requires, reference in builder.items:
+        if disable_shared == "False" or not options["{}:shared".format(package_name)]:
+             filtered_builds.append([settings, options, env_vars, build_requires])
+    builder.builds = filtered_builds
+
+    builder.run()

conan/ci/build.sh

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

conan/ci/install.sh

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

conan/test_package/CMakeLists.txt

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

conan/test_package/conanfile.py

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

conan/test_package/test_package.cpp

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

conanfile.py

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

docs/CMakeLists.txt

@@ -20,3 +20,19 @@ install(
     DIRECTORY ${DOXY_OUTPUT_DIRECTORY}/html
     DESTINATION share/${PROJECT_NAME}-${PROJECT_VERSION}/
 )
+
+add_custom_target(
+    docs_aob
+    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
+)

docs/doxy.in

@@ -780,7 +780,9 @@ WARN_LOGFILE           =
 # spaces. See also FILE_PATTERNS and EXTENSION_MAPPING
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-INPUT                  = @DOXY_SOURCE_DIRECTORY@ @DOXY_DOCS_DIRECTORY@ @PROJECT_SOURCE_DIR@/README.md
+INPUT                  = @DOXY_SOURCE_DIRECTORY@ \
+			 @DOXY_DOCS_DIRECTORY@ \
+			 @PROJECT_SOURCE_DIR@/README.md
 
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@@ -805,9 +807,7 @@ INPUT_ENCODING         = UTF-8
 # *.m, *.markdown, *.md, *.mm, *.dox, *.py, *.pyw, *.f90, *.f95, *.f03, *.f08,
 # *.f, *.for, *.tcl, *.vhd, *.vhdl, *.ucf and *.qsf.
 
-FILE_PATTERNS          = *.cpp \
-                         *.c++ \
-                         *.h \
+FILE_PATTERNS          = *.h \
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                          *.md \
                          *.dox

docs/md/core.md

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

docs/md/faq.md

@@ -0,0 +1,163 @@
+# 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)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+This is a constantly updated section where I'll try to put the answers to the
+most frequently asked questions.<br/>
+If you don't find your answer here, there are two cases: nobody has done it yet
+or this section needs updating. In both cases, try to
+[open a new issue](https://github.com/skypjack/entt/issues/new) or enter the
+[gitter channel](https://gitter.im/skypjack/entt) and ask your question.
+Probably someone already has an answer for you and we can then integrate this
+part of the documentation.
+
+# FAQ
+
+## Why is my debug build on Windows so slow?
+
+`EnTT` is an experimental project that I also use to keep me up-to-date with the
+latest revision of the language and the standard library. For this reason, it's
+likely that some classes you're working with are using standard containers under
+the hood.<br/>
+Unfortunately, it's known that the standard containers aren't particularly
+performing in debugging (the reasons for this go beyond this document) and are
+even less so on Windows apparently. Fortunately this can also be mitigated a
+lot, achieving good results in many cases.
+
+First of all, there are two things to do in a Windows project:
+
+* Disable the [`/JMC`](https://docs.microsoft.com/cpp/build/reference/jmc)
+  option (_Just My Code_ debugging), available starting in Visual Studio 2017
+  version 15.8.
+
+* Set the [`_ITERATOR_DEBUG_LEVEL`](https://docs.microsoft.com/cpp/standard-library/iterator-debug-level)
+  macro to 0. This will disable checked iterators and iterator debugging.
+
+Moreover, the macro `ENTT_DISABLE_ASSERT` should be defined to disable internal
+checks made by `EnTT` in debug. These are asserts introduced to help the users,
+but require to access to the underlying containers and therefore risk ruining
+the performance in some cases.
+
+With these changes, debug performance should increase enough for most cases. If
+you want something more, you can can also switch to an optimization level `O0`
+or preferably `O1`.
+
+## How can I represent hierarchies with my components?
+
+This is one of the first questions that anyone makes when starting to work with
+the entity-component-system architectural pattern.<br/>
+There are several approaches to the problem and what’s the best one depends
+mainly on the real problem one is facing. In all cases, how to do it doesn't
+strictly depend on the library in use, but the latter can certainly allow or
+not different techniques depending on how the data are laid out.
+
+I tried to describe some of the techniques that fit well with the model of
+`EnTT`. [Here](https://skypjack.github.io/2019-06-25-ecs-baf-part-4/) is the
+first post of a series that tries to explore the problem. More will probably
+come in future.
+
+Long story short, you can always define a tree where the nodes expose implicit
+lists of children by means of the following type:
+
+```cpp
+struct relationship {
+    std::size_t children{};
+    entt::entity first{entt::null};
+    entt::entity prev{entt::null};
+    entt::entity next{entt::null};
+    entt::entity parent{entt::null};
+    // ... other data members ...
+};
+```
+
+The sort functionalities of `EnTT`, the groups and all the other features of the
+library can help then to get the best in terms of data locality and therefore
+performance from this component.
+
+## Custom entity identifiers: yay or nay?
+
+Custom entity identifiers are definitely a good idea in two cases at least:
+
+* If `std::uint32_t` isn't large enough as an underlying type.
+* If you want to avoid conflicts when using multiple registries.
+
+These identifiers are nothing more than enum classes with some salt.<br/>
+To simplify the creation of new identifiers, `EnTT` provides the macro
+`ENTT_OPAQUE_TYPE` that accepts two arguments:
+
+* The name you want to give to the new identifier (watch out for namespaces).
+* The underlying type to use (either `std::uint16_t`, `std::uint32_t`
+  or `std::uint64_t`).
+
+In fact, this is the definition of `entt::entity`:
+
+```cpp
+ENTT_OPAQUE_TYPE(entity, std::uint32_t)
+```
+
+The use of this macro is highly recommended, so as not to run into problems if
+the requirements for the identifiers should change in the future.
+
+## Warning C4307: integral constant overflow
+
+According to [this](https://github.com/skypjack/entt/issues/121) issue, using a
+hashed string under VS could generate a warning.<br/>
+First of all, I want to reassure you: it's expected and harmless. However, it
+can be annoying.
+
+To suppress it and if you don't want to suppress all the other warnings as well,
+here is a workaround in the form of a macro:
+
+```cpp
+#if defined(_MSC_VER)
+    #define HS(str)\
+        __pragma(warning(push))\
+        __pragma(warning(disable:4307))\
+        entt::hashed_string{str}\
+        __pragma(warning(pop))
+#else
+    #define HS(str) entt::hashed_string{str}
+#endif
+```
+
+With an example of use included:
+
+```cpp
+constexpr auto identifier = HS("my/resource/identifier");
+```
+
+Thanks to [huwpascoe](https://github.com/huwpascoe) for the courtesy.
+
+## Warning C4003: the min, the max and the macro
+
+On Windows, a header file defines two macros `min` and `max` which may result in
+conflicts with their counterparts in the standard library and therefore in
+errors during compilation.
+
+It's a pretty big problem but fortunately it's not a problem of `EnTT` and there
+is a fairly simple solution to it.<br/>
+It consists in defining the `NOMINMAX` macro before to include any other header
+so as to get rid of the extra definitions:
+
+```cpp
+#define NOMINMAX
+```
+
+Please refer to [this](https://github.com/skypjack/entt/issues/96) issue for
+more details.

docs/md/lib.md

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

docs/md/links.md

@@ -0,0 +1,100 @@
+# EnTT in Action
+
+`EnTT` is widely used in private and commercial applications. I cannot even
+mention most of them because of some signatures I put on some documents time
+ago. Fortunately, there are also people who took the time to implement open
+source projects based on `EnTT` and did not hold back when it came to
+documenting them.
+
+Below an incomplete list of games, applications and articles that can be used as
+a reference. Where I put the word _apparently_ means that the use of `EnTT` is
+documented but the authors didn't make explicit announcements or contacted me
+directly.
+
+I hope this list can grow much more in the future:
+
+* Games:
+  * [Minecraft](https://minecraft.net/en-us/attribution/) by
+    [Mojang](https://mojang.com/): of course, **that** Minecraft, see the
+    open source attributions page for more details.
+  * [Face Smash](https://play.google.com/store/apps/details?id=com.gamee.facesmash):
+    a game to play with your face.
+  * [EnTT Pacman](https://github.com/Kerndog73/EnTT-Pacman): an example of how
+    to make Pacman with `EnTT`.
+  * [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.
+
+* Engines and the like:
+  * [Apparently](https://teamwisp.github.io/credits/)
+    [Wisp](https://teamwisp.github.io/product/) by
+    [Team Wisp](https://teamwisp.github.io/): an advanced real-time ray tracing
+    renderer built for the demands of video game artists.
+  * [starlight](https://github.com/DomRe/starlight): game programming framework
+    using `Allegro`, `Lua` and modern C++.
+  * [Apparently](https://github.com/JosiahWI/qub3d-libdeps)
+    [Qub3d](https://qub3d.org/): because blocks should be open source.
+  * [shiva](https://github.com/Milerius/shiva): modern C++ engine with
+    modularity.
+  * [NovusCore](https://github.com/novuscore/NovusCore): A modern take on World
+    of Warcraft emulation.
+  * [ImGui/EnTT editor](https://github.com/Green-Sky/imgui_entt_entity_editor):
+    A drop-in, single-file entity editor for `EnTT` that uses `ImGui` as
+    graphical backend (with
+    [demo code](https://github.com/Green-Sky/imgui_entt_entity_editor_demo)).
+
+* Articles and blog posts
+  * [Some posts](https://skypjack.github.io/tags/#entt) on my personal
+    [blog](https://skypjack.github.io/) are about `EnTT`, for those who want to
+    know **more** on this project.
+  * [Space Battle: Huge edition](http://victor.madtriangles.com/code%20experiment/2018/06/11/post-ecs-battle-huge.html):
+    huge space battle built entirely from scratch.
+  * [Space Battle](https://github.com/vblanco20-1/ECS_SpaceBattle): huge space
+    battle built on `UE4`.
+  * [Experimenting with ECS in UE4](http://victor.madtriangles.com/code%20experiment/2018/03/25/post-ue4-ecs-battle.html):
+    interesting article about `UE4` and `EnTT`.
+  * [Implementing ECS architecture in UE4](https://forums.unrealengine.com/development-discussion/c-gameplay-programming/1449913-implementing-ecs-architecture-in-ue4-giant-space-battle):
+    giant space battle.
+  * [Conan Adventures (SFML and EnTT in C++)](https://leinnan.github.io/blog/conan-adventuressfml-and-entt-in-c.html):
+    create projects in modern C++ using `SFML`, `EnTT`, `Conan` and `CMake`.
+
+* Any Other Business:
+  * The [ArcGIS Runtime SDKs](https://developers.arcgis.com/arcgis-runtime/)
+    by [Esri](https://www.esri.com/): they use `EnTT` for the internal ECS and
+    the cross platform C++ rendering engine. The SDKs are utilized by a lot of
+    enterprise custom apps, as well as by Esri for its own public applications
+    such as
+    [Explorer](https://play.google.com/store/apps/details?id=com.esri.explorer),
+    [Collector](https://play.google.com/store/apps/details?id=com.esri.arcgis.collector)
+    and
+    [Navigator](https://play.google.com/store/apps/details?id=com.esri.navigator).
+  * [Apparently](https://www.linkedin.com/in/skypjack/)
+    [NIO](https://www.nio.io/): there was a collaboration to make some changes
+    to `EnTT`, at the time used for internal projects.
+  * [Apparently](https://www.linkedin.com/jobs/view/architekt-c%2B%2B-at-tieto-1219512333/)
+    [Tieto](https://www.tieto.com/): they published a job post where `EnTT` was
+    listed on their software stack.
+  * [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 +1,511 @@
+# Crash Course: runtime reflection system
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [Names and identifiers](#names-and-identifiers)
+* [Reflection in a nutshell](#reflection-in-a-nutshell)
+  * [Any as in any type](#any-as-in-any-type)
+  * [Enjoy the runtime](#enjoy-the-runtime)
+  * [Policies: the more, the less](#policies-the-more-the-less)
+  * [Named constants and enums](#named-constants-and-enums)
+  * [Properties and meta objects](#properties-and-meta-objects)
+  * [Unregister types](#unregister-types)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+Reflection (or rather, its lack) is a trending topic in the C++ world and, in
+the specific case of `EnTT`, a tool that can unlock a lot of other features. I
+looked for a third-party library that met my needs on the subject, but I always
+came across some details that I didn't like: macros, being intrusive, too many
+allocations. In one word: unsatisfactory.<br/>
+I finally decided to write a built-in, non-intrusive and macro-free runtime
+reflection system for `EnTT`. Maybe I didn't do better than others or maybe yes,
+time will tell me, but at least I can model this tool around the library to
+which it belongs and not vice versa.
+
+# Names and identifiers
+
+The meta system doesn't force the user to use the tools provided by the library
+when it comes to working with names and identifiers. It does this by offering an
+API that works with opaque identifiers that may or may not be generated by means
+of a hashed string.<br/>
+This means that users can assign any type of identifier to the meta objects, as
+long as they are numeric. It doesn't matter if they are generated at runtime, at
+compile-time or with custom functions.
+
+However, the examples in the following sections are all based on the
+`hashed_string` class as provided by this library. Therefore, where an
+identifier is required, it's likely that a user defined literal is used as
+follows:
+
+```cpp
+auto factory = entt::reflect<my_type>("reflected_type"_hs);
+```
+
+For what it's worth, this is likely completely equivalent to:
+
+```cpp
+auto factory = entt::reflect<my_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 _reflect_ a type, the library provides the `reflect` function:
+
+```cpp
+auto factory = entt::reflect<my_type>("reflected_type"_hs);
+```
+
+It accepts the type to reflect as a template parameter and an optional
+identifier as an argument. Identifiers are important because users can retrieve
+meta types at runtime by searching for them by _name_. However, there are cases
+in which users can be interested in adding features to a reflected type so that
+the reflection system can use it correctly under the hood, but they don't want
+to allow searching the type by _name_.<br/>
+In both cases, the returned value is a factory object to use to continue
+building the meta type.
+
+A factory is such that all its member functions returns the factory itself.
+It can be used to extend the reflected type and add the following:
+
+* _Constructors_. Actual constructors can be assigned to a reflected type by
+  specifying their list of arguments. Free functions (namely, factories) can be
+  used as well, as long as the return type is the expected one. From a client's
+  point of view, nothing changes if a constructor is a free function or an
+  actual constructor.<br/>
+  Use the `ctor` member function for this purpose:
+
+  ```cpp
+  entt::reflect<my_type>("reflected"_hs).ctor<int, char>().ctor<&factory>();
+  ```
+
+* _Destructors_. Free functions can be set as destructors of reflected types.
+  The purpose is to give users the ability to free up resources that require
+  special treatment before an object is actually destroyed.<br/>
+  Use the `dtor` member function for this purpose:
+
+  ```cpp
+  entt::reflect<my_type>("reflected"_hs).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::reflect<my_type>("reflected"_hs)
+      .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::reflect<my_type>("reflected"_hs)
+      .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::reflect<derived_type>("derived"_hs).base<base_type>();
+  ```
+
+  From now on, wherever a `base_type` is required, an instance of `derived_type`
+  will also be accepted.
+
+* _Conversion functions_. Actual types can be converted, this is a fact. Just
+  think of the relationship between a `double` and an `int` to see it. Similar
+  to bases, conversion functions allow users to define conversions that will be
+  implicitly performed by the reflection system when required.<br/>
+  Use the `conv` member function for this purpose:
+
+  ```cpp
+  entt::reflect<double>().conv<int>();
+  ```
+
+That's all, everything users need to create meta types and enjoy the reflection
+system. At first glance it may not seem that much, but users usually learn to
+appreciate it over time.<br/>
+Also, do not forget what these few lines hide under the hood: a built-in,
+non-intrusive and macro-free system for reflection in C++. Features that are
+definitely worth the price, at least for me.
+
+## Any as in any type
+
+The reflection system comes with its own meta any type. It may seem redundant
+since C++17 introduced `std::any`, but it is not.<br/>
+In fact, the _type_ returned by an `std::any` is a const reference to an
+`std::type_info`, an implementation defined class that's not something everyone
+wants to see in a software. Furthermore, the class `std::type_info` suffers from
+some design flaws and there is even no way to _convert_ an `std::type_info` into
+a meta type, thus linking the two worlds.
+
+A meta any object provides an API similar to that of its most famous counterpart
+and serves the same purpose of being an opaque container for any type of
+value.<br/>
+It minimizes the allocations required, which are almost absent thanks to _SBO_
+techniques. In fact, unless users deal with _fat types_ and create instances of
+them though the reflection system, allocations are at zero.
+
+A meta any object can be created by any other object or as an empty container
+to initialize later:
+
+```cpp
+// a meta any object that contains an int
+entt::meta_any any{0};
+
+// an empty meta any object
+entt::meta_any empty{};
+```
+
+It takes the burden of destroying the contained instance when required.<br/>
+Moreover, it can be used as an opaque container for unmanaged objects if needed:
+
+```cpp
+int value;
+entt::meta_any any{entt::as_alias, value};
+```
+
+In this case, the contained instance is never destroyed and users must ensure
+that the lifetime of the object exceeds that of the container.
+
+A meta any object has a `type` member function that returns the meta type of the
+contained value, if any. The member functions `try_cast`, `cast` and `convert`
+are 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 type of
+objects offer an API to know the _runtime identifier_ of the type, to iterate
+all the meta objects associated with them and even to build or destroy instances
+of the underlying type.<br/>
+Refer to the inline documentation for all the details.
+
+The meta objects that compose a meta type are accessed in the following ways:
+
+* _Meta constructors_. They are accessed by types of arguments:
+
+  ```cpp
+  auto ctor = entt::resolve<my_type>().ctor<int, char>();
+  ```
+
+  The returned type is `meta_ctor` and may be invalid if there is no constructor
+  that accepts the supplied arguments or at least some types from which they are
+  derived or to which they can be converted.<br/>
+  A meta constructor offers an API to know the number of arguments, the expected
+  meta types and to invoke it, therefore to construct a new instance of the
+  underlying type.
+
+* _Meta destructor_. It's returned by a dedicated function:
+
+  ```cpp
+  auto dtor = entt::resolve<my_type>().dtor();
+  ```
+
+  The returned type is `meta_dtor` and may be invalid if there is no custom
+  destructor set for the given meta type.<br/>
+  All what a meta destructor has to offer is a way to invoke it on a given
+  instance. Be aware that the result may not be what is expected.
+
+* _Meta data_. They are accessed by _name_:
+
+  ```cpp
+  auto data = entt::resolve<my_type>().data("member"_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 (ie to know if
+  it's a const or a static one), to get the meta type of the variable and to set
+  or get the contained value.
+
+* _Meta functions_. They are accessed by _name_:
+
+  ```cpp
+  auto func = entt::resolve<my_type>().func("member"_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 (ie to know
+  if it's a const or a static function), to know the number of arguments, the
+  meta return type and the meta types of the parameters. In addition, a meta
+  function object can be used to invoke the underlying function and then get the
+  return value in the form of meta any object.
+
+* _Meta bases_. They are accessed through the _name_ of the base types:
+
+  ```cpp
+  auto base = entt::resolve<derived_type>().base("base"_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
+auto func = entt::resolve<my_type>().func("member"_hs);
+
+if(func) {
+    // ...
+}
+```
+
+Furthermore, all meta objects with the exception of meta destructors can be
+iterated through an overload that accepts a callback through which to return
+them. As an example:
+
+```cpp
+entt::resolve<my_type>().data([](auto data) {
+    // ...
+});
+```
+
+A meta type can also be used to `construct` or `destroy` actual instances of the
+underlying type.<br/>
+In particular, the `construct` member function accepts a variable number of
+arguments and searches for a match. It returns a `meta_any` object that may or
+may not be initialized, depending on whether a suitable constructor has been
+found or not. On the other side, the `destroy` member function accepts instances
+of `meta_any` as well as actual objects by reference and invokes the registered
+destructor if any.<br/>
+Be aware that the result of a call to `destroy` may not be what is expected. The
+purpose is to give users the ability to free up resources that require special
+treatment and **not** to actually destroy instances.
+
+Meta types and meta objects in general contain much more than what is said: a
+plethora of functions in addition to those listed whose purposes and uses go
+unfortunately beyond the scope of this document.<br/>
+I invite anyone interested in the subject to look at the code, experiment and
+read the official 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::reflect<my_type>("reflected"_hs)
+      .func<&my_type::member_function, entt::as_void_t>("member"_hs);
+  ```
+
+* The _as-alias_ policy, associated with the type `entt::as_alias_t`.<br/>
+  It allows to build wrappers that act as aliases for the objects used to
+  initialize them. Modifying the object contained in the wrapper for which the
+  _aliasing_ was requested will make it possible to directly modify the instance
+  used to initialize the wrapper itself.<br/>
+  This policy works with constructors (for example, when objects are taken from
+  an external container rather than created on demand), data members and
+  functions in general (as long as their return types are lvalue references).
+
+  As an example of use:
+
+  ```cpp
+  entt::reflect<my_type>("reflected"_hs)
+      .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::reflect<my_enum>()
+        .data<my_enum::a_value>("a_value"_hs)
+        .data<my_enum::another_value>("another_value"_hs);
+
+entt::reflect<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 be able to attach properties to the meta objects created. Fortunately, this
+is possible for most of them.<br/>
+To attach a property to a meta object, no matter what as long as it supports
+properties, it is sufficient to provide an object at the time of construction
+such that `std::get<0>` and `std::get<1>` are valid for it. In other terms, the
+properties are nothing more than key/value pairs users can put in an
+`std::pair`. As an example:
+
+```cpp
+entt::reflect<my_type>("reflected"_hs, std::make_pair("tooltip"_hs, "message"));
+```
+
+The meta objects that support properties offer then a couple of member functions
+named `prop` to iterate them at once and to search a specific property by key:
+
+```cpp
+// iterate all the properties of a meta type
+entt::resolve<my_type>().prop([](auto prop) {
+    // ...
+});
+
+// search for a given property by name
+auto prop = entt::resolve<my_type>().prop("tooltip"_hs);
+```
+
+Meta properties are objects having a fairly poor interface, all in all. They
+only provide the `key` and the `value` member functions to be used to retrieve
+the key and the value contained in the form of meta any objects, respectively.
+
+## Unregister types
+
+A type registered with the reflection system can also be unregistered. This
+means unregistering all its data members, member functions, conversion functions
+and so on. However, the base classes won't be unregistered, since they don't
+necessarily depend on it. Similarly, implicitly generated types (as an example,
+the meta types implicitly generated for function parameters when needed) won't
+be unregistered.
+
+To unregister a type, users can use the `unregister` function from the global
+namespace:
+
+```cpp
+entt::unregister<my_type>();
+```
+
+This function returns a boolean value that is true if the type is actually
+registered with the reflection system, false otherwise.<br/>
+The type can be re-registered later with a completely different name and form.

docs/md/process.md

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

docs/md/resource.md

@@ -0,0 +1,238 @@
+# Crash Course: resource management
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [The resource, the loader and the cache](#the-resource-the-loader-and-the-cache)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+Resource management is usually one of the most critical part of a software like
+a game. Solutions are often tuned to the particular application. There exist
+several approaches and all of them are perfectly fine as long as they fit the
+requirements of the piece of software in which they are used.<br/>
+Examples are loading everything on start, loading on request, predictive
+loading, and so on.
+
+`EnTT` doesn't pretend to offer a _one-fits-all_ solution for the different
+cases. Instead, it offers a minimal and perhaps trivial cache that can be useful
+most of the time during prototyping and sometimes even in a production
+environment.<br/>
+For those interested in the subject, the plan is to improve it considerably over
+time in terms of performance, memory usage and functionalities. Hoping to make
+it, of course, one step at a time.
+
+# The resource, the loader and the cache
+
+There are three main actors in the model: the resource, the loader and the
+cache.
+
+The _resource_ is whatever users want it to be. An image, a video, an audio,
+whatever. There are no limits.<br/>
+As a minimal example:
+
+```cpp
+struct my_resource { const int value; };
+```
+
+A _loader_ is a class the aim of which is to load a specific resource. It has to
+inherit directly from the dedicated base class as in the following example:
+
+```cpp
+struct my_loader final: entt::resource_loader<my_loader, my_resource> {
+    // ...
+};
+```
+
+Where `my_resource` is the type of resources it creates.<br/>
+A resource loader must also expose a public const member function named `load`
+that accepts a variable number of arguments and returns a shared pointer to a
+resource.<br/>
+As an example:
+
+```cpp
+struct my_loader: entt::resource_loader<my_loader, my_resource> {
+    std::shared_ptr<my_resource> load(int value) const {
+        // ...
+        return std::shared_ptr<my_resource>(new my_resource{ value });
+    }
+};
+```
+
+In general, resource loaders should not have a state or retain data of any type.
+They should let the cache manage their resources instead.<br/>
+As a side note, base class and CRTP idiom aren't strictly required with the
+current implementation. One could argue that a cache can easily work with
+loaders of any type. However, future changes won't be breaking ones by forcing
+the use of a base class today and that's why the model is already in its place.
+
+Finally, a cache is a specialization of a class template tailored to a specific
+resource:
+
+```cpp
+using my_resource_cache = entt::resource_cache<my_resource>;
+
+// ...
+
+my_resource_cache cache{};
+```
+
+The idea is to create different caches for different types of resources and to
+manage each one independently in the most appropriate way.<br/>
+As a (very) trivial example, audio tracks can survive in most of the scenes of
+an application while meshes can be associated with a single scene and then
+discarded when users leave it.
+
+A cache offers a set of basic functionalities to query its internal state and to
+_organize_ it:
+
+```cpp
+// gets the number of resources managed by a cache
+const auto size = cache.size();
+
+// checks if a cache contains at least a valid resource
+const auto empty = cache.empty();
+
+// clears a cache and discards its content
+cache.clear();
+```
+
+Besides these member functions, a cache contains what is needed to load, use and
+discard resources of the given type.<br/>
+Before to explore this part of the interface, it makes sense to mention how
+resources are identified. The type of the identifiers to use is defined as:
+
+```cpp
+entt::resource_cache<resource>::resource_type
+```
+
+Where `resource_type` is an alias for `entt::hashed_string::hash_type`.
+Therefore, resource identifiers are created explicitly as in the following
+example:
+
+```cpp
+constexpr auto identifier = entt::resource_cache<resource>::resource_type{"my/resource/identifier"_hs};
+// this is equivalent to the following
+constexpr auto hs = entt::hashed_string{"my/resource/identifier"};
+```
+
+The class `hashed_string` is described in a dedicated section, so I won't go in
+details here.
+
+Resources are loaded and thus stored in a cache through the `load` member
+function. It accepts the loader to use as a template parameter, the resource
+identifier and the parameters used to construct the resource as arguments:
+
+```cpp
+// uses the identifier declared above
+cache.load<my_loader>(identifier, 0);
+
+// uses a const char * directly as an identifier
+cache.load<my_loader>("another/identifier"_hs, 42);
+```
+
+The function returns a handle to the resource, whether it already exists or is
+loaded. In case the loader returns an invalid pointer, the handle is invalid as
+well and therefore it can be easily used with an `if` statement:
+
+```cpp
+if(auto handle = cache.load<my_loader>("another/identifier"_hs, 42); handle) {
+    // ...
+}
+```
+
+Before trying to load a resource, the `contains` member function can be used to
+know if a cache already contains a specific resource:
+
+```cpp
+auto exists = cache.contains("my/identifier"_hs);
+```
+
+There exists also a member function to use to force a reload of an already
+existing resource if needed:
+
+```cpp
+auto handle = cache.reload<my_loader>("another/identifier"_hs, 42);
+```
+
+As above, the function returns a handle to the resource that is invalid in case
+of errors. The `reload` member function is a kind of alias of the following
+snippet:
+
+```cpp
+cache.discard(identifier);
+cache.load<my_loader>(identifier, 42);
+```
+
+Where the `discard` member function is used to get rid of a resource if loaded.
+In case the cache doesn't contain a resource for the given identifier, `discard`
+does nothing and returns immediately.
+
+So far, so good. Resources are finally loaded and stored within the cache.<br/>
+They are returned to users in the form of handles. To get one of them later on:
+
+```cpp
+auto handle = cache.handle("my/identifier"_hs);
+```
+
+The idea behind a handle is the same of the flyweight pattern. In other terms,
+resources aren't copied around. Instead, instances are shared between handles.
+Users of a resource own a handle that guarantees that a resource isn't destroyed
+until all the handles are destroyed, even if the resource itself is removed from
+the cache.<br/>
+Handles are tiny objects both movable and copyable. They return the contained
+resource as a const reference on request:
+
+* By means of the `get` member function:
+
+  ```cpp
+  const auto &resource = handle.get();
+  ```
+
+* Using the proper cast operator:
+
+  ```cpp
+  const auto &resource = handle;
+  ```
+
+* Through the dereference operator:
+
+  ```cpp
+  const auto &resource = *handle;
+  ```
+
+The resource can also be accessed directly using the arrow operator if required:
+
+```cpp
+auto value = handle->value;
+```
+
+To test if a handle is still valid, the cast operator to `bool` allows users to
+use it in a guard:
+
+```cpp
+if(handle) {
+    // ...
+}
+```
+
+Finally, in case there is the need to load a resource and thus to get a handle
+without storing the resource itself in the cache, users can rely on the `temp`
+member function template.<br/>
+The declaration is similar to that of `load`, a (possibly invalid) handle for
+the resource is returned also in this case:
+
+```cpp
+if(auto handle = cache.temp<my_loader>(42); handle) {
+    // ...
+}
+```
+
+Do not forget to test the handle for validity. Otherwise, getting a reference to
+the resource it points may result in undefined behavior.

docs/md/signal.md

@@ -0,0 +1,479 @@
+# Crash Course: events, signals and everything in between
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [Delegate](#delegate)
+* [Signals](#signals)
+* [Event dispatcher](#event-dispatcher)
+* [Event emitter](#event-emitter)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+Signals are usually a core part of games and software architectures in
+general.<br/>
+Roughly speaking, they help to decouple the various parts of a system while
+allowing them to communicate with each other somehow.
+
+The so called _modern C++_ comes with a tool that can be useful in these terms,
+the `std::function`. As an example, it can be used to create delegates.<br/>
+However, there is no guarantee that an `std::function` does not perform
+allocations under the hood and this could be problematic sometimes. Furthermore,
+it solves a problem but may not adapt well to other requirements that may arise
+from time to time.
+
+In case that the flexibility and power of an `std::function` isn't required or
+if the price to pay for them is too high,` EnTT` offers a complete set of
+lightweight classes to solve the same and many other problems.
+
+# Delegate
+
+A delegate can be used as a general purpose invoker with no memory overhead for
+free functions and members provided along with an instance on which to invoke
+them.<br/>
+It does not claim to be a drop-in replacement for an `std::function`, so do not
+expect to use it whenever an `std::function` fits well. That said, it's most
+likely even a better fit than an `std::function` in a lot of cases, so expect to
+use it quite a lot anyway.
+
+The interface is trivial. It offers a default constructor to create empty
+delegates:
+
+```cpp
+entt::delegate<int(int)> delegate{};
+```
+
+All what is needed to create an instance is to specify the type of the function
+the delegate will _contain_, that is the signature of the free function or the
+member one wants to assign to it.
+
+Attempting to use an empty delegate by invoking its function call operator
+results in undefined behavior or most likely a crash. Before to use a delegate,
+it must be initialized.<br/>
+There exists a bunch of overloads of the `connect` member function to do that.
+As an example of use:
+
+```cpp
+int f(int i) { return i; }
+
+struct my_struct {
+    int f(const int &i) { return i }
+};
+
+// bind a free function to the delegate
+delegate.connect<&f>();
+
+// bind a member function to the delegate
+my_struct instance;
+delegate.connect<&my_struct::f>(instance);
+```
+
+The delegate class accepts also data members, if needed. In this case, the
+function type of the delegate is such that the parameter list is empty and the
+value of the data member is at least convertible to the return type.
+
+Free functions having type equivalent to `void(T &, args...)` are accepted as
+well. In this case, `T &` is considered a payload and the function will receive
+it back every time it's invoked. In other terms, this works just fine with the
+above definition:
+
+```cpp
+void g(const char &c, int i) { /* ... */ }
+const char c = 'c';
+
+delegate.connect<&g>(c);
+delegate(42);
+```
+
+The function `g` will be invoked with a reference to `c` and `42`. However, the
+function type of the delegate is still `void(int)`. This is also the signature
+of its function call operator.
+
+Another interesting aspect of the delegate class is that it accepts also
+functions with a list of parameters that is shorter than that of the function
+type used to specialize the delegate itself.<br/>
+The following code is therefore perfectly valid:
+
+```cpp
+void g() { /* ... */ }
+delegate.connect<&g>();
+delegate(42);
+```
+
+Where the function type of the delegate is `void(int)` as above. It goes without
+saying that the extra arguments are silently discarded internally.<br/>v
+This is a nice-to-have feature in a lot of cases, as an example when the
+`delegate` class is used as a building block of a signal-slot system.
+
+To create and initialize a delegate at once, there are a few specialized
+constructors. Because of the rules of the language, the listener is provided by
+means of the `entt::connect_arg` variable template:
+
+```cpp
+entt::delegate<int(int)> func{entt::connect_arg<&f>};
+```
+
+Aside `connect`, a `disconnect` counterpart isn't provided. Instead, there
+exists a `reset` member function to use to clear a delegate.<br/>
+To know if a delegate is empty, it can be used explicitly in every conditional
+statement:
+
+```cpp
+if(delegate) {
+    // ...
+}
+```
+
+Finally, to invoke a delegate, the function call operator is the way to go as
+already shown in the examples above:
+
+```cpp
+auto ret = delegate(42);
+```
+
+In all cases, the listeners don't have to strictly follow the signature of the
+delegate. As long as a listener can be invoked with the given arguments to yield
+a result that is convertible to the given result type, everything works just
+fine.
+
+# Signals
+
+Signal handlers work with references to classes, function pointers and pointers
+to members. Listeners can be any kind of objects and users are in charge of
+connecting and disconnecting them from a signal to avoid crashes due to
+different lifetimes. On the other side, performance shouldn't be affected that
+much by the presence of such a signal handler.<br/>
+Signals make use of delegates internally and therefore they undergo the same
+rules and offer similar functionalities. It may be a good idea to consult the
+documentation of the `delegate` class for further information.
+
+A signal handler can be used as a private data member without exposing any
+_publish_ functionality to the clients of a class. The basic idea is to impose a
+clear separation between the signal itself and the `sink` class, that is a tool
+to be used to connect and disconnect listeners on the fly.
+
+The API of a signal handler is straightforward. The most important thing is that
+it comes in two forms: with and without a collector. In case a signal is
+provided with a collector, all the values returned by the listeners can be
+literally _collected_ and used later by the caller. Otherwise it works just like
+a plain signal that emits events from time to time.<br/>
+To create instances of signal handlers it is sufficient to provide the type of
+function to which they refer:
+
+```cpp
+entt::sigh<void(int, char)> signal;
+```
+
+Signals offer all the basic functionalities required to know how many listeners
+they contain (`size`) or if they contain at least a listener (`empty`), as well
+as a function to use to swap handlers (`swap`).
+
+Besides them, there are member functions to use both to connect and disconnect
+listeners in all their forms by means of a sink:
+
+```cpp
+void foo(int, char) { /* ... */ }
+
+struct listener {
+    void bar(const int &, char) { /* ... */ }
+};
+
+// ...
+
+entt::sink sink{signal};
+listener instance;
+
+sink.connect<&foo>();
+sink.connect<&listener::bar>(instance);
+
+// ...
+
+// disconnects a free function
+sink.disconnect<&foo>();
+
+// disconnect a member function of an instance
+sink.disconnect<&listener::bar>(instance);
+
+// disconnect all the member functions of an instance, if any
+sink.disconnect(instance);
+
+// discards all the listeners at once
+sink.disconnect();
+```
+
+As shown above, the listeners don't have to strictly follow the signature of the
+signal. As long as a listener can be invoked with the given arguments to yield a
+result that is convertible to the given return type, everything works just
+fine.<br/>
+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"]
+}

src/entt/config/config.h

@@ -12,5 +12,36 @@
 #endif // ENTT_HS_SUFFIX
 
 
+#ifndef ENTT_HWS_SUFFIX
+#define ENTT_HWS_SUFFIX _hws
+#endif // ENTT_HWS_SUFFIX
+
+
+#ifndef ENTT_NO_ATOMIC
+#include <atomic>
+#define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#else // ENTT_NO_ATOMIC
+#define ENTT_MAYBE_ATOMIC(Type) Type
+#endif // ENTT_NO_ATOMIC
+
+
+#ifndef ENTT_ID_TYPE
+#include <cstdint>
+#define ENTT_ID_TYPE std::uint32_t
+#endif // ENTT_ID_TYPE
+
+
+#ifndef ENTT_PAGE_SIZE
+#define ENTT_PAGE_SIZE 32768
+#endif // ENTT_PAGE_SIZE
+
+
+#ifndef ENTT_DISABLE_ASSERT
+#include <cassert>
+#define ENTT_ASSERT(condition) assert(condition)
+#else // ENTT_DISABLE_ASSERT
+#define ENTT_ASSERT(...) ((void)0)
+#endif // ENTT_DISABLE_ASSERT
+
 
 #endif // ENTT_CONFIG_CONFIG_H

src/entt/config/version.h

@@ -0,0 +1,11 @@
+#ifndef ENTT_CONFIG_VERSION_H
+#define ENTT_CONFIG_VERSION_H
+
+
+#define ENTT_VERSION "3.1.1"
+#define ENTT_VERSION_MAJOR 3
+#define ENTT_VERSION_MINOR 1
+#define ENTT_VERSION_PATCH 1
+
+
+#endif // ENTT_CONFIG_VERSION_H

src/entt/core/algorithm.hpp

@@ -2,9 +2,11 @@
 #define ENTT_CORE_ALGORITHM_HPP
 
 
-#include <functional>
-#include <algorithm>
 #include <utility>
+#include <iterator>
+#include <algorithm>
+#include <functional>
+#include "utility.hpp"
 
 
 namespace entt {
@@ -18,7 +20,7 @@ namespace entt {
  * This class fills the gap by wrapping some flavors of `std::sort` in a
  * function object.
  */
-struct StdSort final {
+struct std_sort {
     /**
      * @brief Sorts the elements in a range.
      *
@@ -40,7 +42,7 @@ struct StdSort final {
 
 
 /*! @brief Function object for performing insertion sort. */
-struct InsertionSort final {
+struct insertion_sort {
     /**
      * @brief Sorts the elements in a range.
      *
@@ -54,51 +56,81 @@ struct InsertionSort final {
      */
     template<typename It, typename Compare = std::less<>>
     void operator()(It first, It last, Compare compare = Compare{}) const {
-        auto it = first + 1;
+        if(first < last) {
+            for(auto it = first+1; it < last; ++it) {
+                auto value = std::move(*it);
+                auto pre = it;
 
-        while(it != last) {
-            auto value = *it;
-            auto pre = it;
+                for(; pre > first && compare(value, *(pre-1)); --pre) {
+                    *pre = std::move(*(pre-1));
+                }
 
-            while(pre != first && compare(value, *(pre-1))) {
-                *pre = *(pre-1);
-                --pre;
+                *pre = std::move(value);
             }
-
-            *pre = value;
-            ++it;
         }
     }
 };
 
 
-/*! @brief Function object for performing bubble sort (single iteration). */
-struct OneShotBubbleSort final {
+/**
+ * @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 Tries to sort the elements in a range.
+     * @brief Sorts the elements in a range.
      *
-     * Performs a single iteration to sort the elements in a range using the
-     * given binary comparison function. The range may not be completely sorted
-     * after running this function.
+     * 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 Compare Type of comparison function object.
+     * @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 compare A valid comparison function object.
+     * @param getter A valid _getter_ function object.
      */
-    template<typename It, typename Compare = std::less<>>
-    void operator()(It first, It last, Compare compare = Compare{}) const {
-        if(first != last) {
-            auto it = first++;
+    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;
 
-            while(first != last) {
-                if(compare(*first, *it)) {
-                    using std::swap;
-                    std::swap(*first, *it);
-                }
+            using value_type = typename std::iterator_traits<It>::value_type;
+            std::vector<value_type> aux(std::distance(first, last));
 
-                it = first++;
+            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);
             }
         }
     }

src/entt/core/family.hpp

@@ -3,8 +3,6 @@
 
 
 #include <type_traits>
-#include <cstddef>
-#include <atomic>
 #include "../config/config.h"
 
 
@@ -19,34 +17,24 @@ namespace entt {
  * identifiers.
  */
 template<typename...>
-class Family {
-    static std::atomic<std::size_t> identifier;
+class family {
+    inline static ENTT_MAYBE_ATOMIC(ENTT_ID_TYPE) identifier;
 
     template<typename...>
-    static std::size_t family() ENTT_NOEXCEPT {
-        static const std::size_t value = identifier.fetch_add(1);
-        return value;
-    }
+    // clang (since version 9) started to complain if auto is used instead of ENTT_ID_TYPE
+    inline static const ENTT_ID_TYPE inner = 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>
-    inline static family_type type() ENTT_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 = inner<std::decay_t<Type>...>;
 };
 
 
-template<typename... Types>
-std::atomic<std::size_t> Family<Types...>::identifier{};
-
-
 }
 
 

src/entt/core/hashed_string.hpp

@@ -3,7 +3,6 @@
 
 
 #include <cstddef>
-#include <cstdint>
 #include "../config/config.h"
 
 
@@ -11,72 +10,170 @@ namespace entt {
 
 
 /**
- * @brief Zero overhead resource identifier.
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename>
+struct fnv1a_traits;
+
+
+template<>
+struct fnv1a_traits<std::uint32_t> {
+    static constexpr std::uint32_t offset = 2166136261;
+    static constexpr std::uint32_t prime = 16777619;
+};
+
+
+template<>
+struct fnv1a_traits<std::uint64_t> {
+    static constexpr std::uint64_t offset = 14695981039346656037ull;
+    static constexpr std::uint64_t prime = 1099511628211ull;
+};
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond TURN_OFF_DOXYGEN
+ */
+
+
+/**
+ * @brief Zero overhead unique identifier.
  *
  * A hashed string is a compile-time tool that allows users to use
  * human-readable identifers in the codebase while using their numeric
  * counterparts at runtime.<br/>
  * Because of that, a hashed string can also be used in constant expressions if
  * required.
+ *
+ * @tparam Char Character type.
  */
-class HashedString final {
-    struct ConstCharWrapper final {
+template<typename Char>
+class basic_hashed_string {
+    using traits_type = internal::fnv1a_traits<ENTT_ID_TYPE>;
+
+    struct const_wrapper {
         // non-explicit constructor on purpose
-        constexpr ConstCharWrapper(const char *str) ENTT_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 = 14695981039346656037ull;
-    static constexpr std::uint64_t prime = 1099511628211ull;
-
     // Fowler–Noll–Vo hash function v. 1a - the good
-    static constexpr std::uint64_t helper(std::uint64_t partial, const char *str) ENTT_NOEXCEPT {
-        return str[0] == 0 ? partial : helper((partial^str[0])*prime, str+1);
+    static constexpr ENTT_ID_TYPE helper(ENTT_ID_TYPE partial, const Char *curr) ENTT_NOEXCEPT {
+        return curr[0] == 0 ? partial : helper((partial^curr[0])*traits_type::prime, curr+1);
     }
 
 public:
+    /*! @brief Character type. */
+    using value_type = Char;
     /*! @brief Unsigned integer type. */
-    using hash_type = std::uint64_t;
+    using hash_type = ENTT_ID_TYPE;
 
     /**
-     * @brief Constructs a hashed string from an array of const chars.
+     * @brief Returns directly the numeric representation of a string.
      *
      * Forcing template resolution avoids implicit conversions. An
      * human-readable identifier can be anything but a plain, old bunch of
      * characters.<br/>
      * Example of use:
      * @code{.cpp}
-     * HashedString sh{"my.png"};
+     * const auto value = basic_hashed_string<char>::to_value("my.png");
      * @endcode
      *
      * @tparam N Number of characters of the identifier.
      * @param str Human-readable identifer.
+     * @return The numeric representation of the string.
      */
-    template <std::size_t N>
-    constexpr HashedString(const char (&str)[N]) ENTT_NOEXCEPT
-        : hash{helper(offset, str)}, str{str}
+    template<std::size_t N>
+    static constexpr hash_type to_value(const value_type (&str)[N]) ENTT_NOEXCEPT {
+        return helper(traits_type::offset, str);
+    }
+
+    /**
+     * @brief Returns directly the numeric representation of a string.
+     * @param wrapper Helps achieving the purpose by relying on overloading.
+     * @return The numeric representation of the string.
+     */
+    static hash_type to_value(const_wrapper wrapper) ENTT_NOEXCEPT {
+        return helper(traits_type::offset, wrapper.str);
+    }
+
+    /**
+     * @brief Returns directly the numeric representation of a string view.
+     * @param str Human-readable identifer.
+     * @param size Length of the string to hash.
+     * @return The numeric representation of the string.
+     */
+    static hash_type to_value(const value_type *str, std::size_t size) ENTT_NOEXCEPT {
+        ENTT_ID_TYPE partial{traits_type::offset};
+        while(size--) { partial = (partial^(str++)[0])*traits_type::prime; }
+        return partial;
+    }
+
+    /*! @brief Constructs an empty hashed string. */
+    constexpr basic_hashed_string() ENTT_NOEXCEPT
+        : str{nullptr}, hash{}
+    {}
+
+    /**
+     * @brief Constructs a hashed string from an array of const characters.
+     *
+     * Forcing template resolution avoids implicit conversions. An
+     * human-readable identifier can be anything but a plain, old bunch of
+     * characters.<br/>
+     * Example of use:
+     * @code{.cpp}
+     * basic_hashed_string<char> hs{"my.png"};
+     * @endcode
+     *
+     * @tparam N Number of characters of the identifier.
+     * @param curr Human-readable identifer.
+     */
+    template<std::size_t N>
+    constexpr basic_hashed_string(const value_type (&curr)[N]) ENTT_NOEXCEPT
+        : str{curr}, hash{helper(traits_type::offset, curr)}
     {}
 
     /**
      * @brief Explicit constructor on purpose to avoid constructing a hashed
-     * string directly from a `const char *`.
-     *
+     * string directly from a `const value_type *`.
      * @param wrapper Helps achieving the purpose by relying on overloading.
      */
-    explicit constexpr HashedString(ConstCharWrapper wrapper) ENTT_NOEXCEPT
-        : hash{helper(offset, wrapper.str)}, str{wrapper.str}
+    explicit constexpr basic_hashed_string(const_wrapper wrapper) ENTT_NOEXCEPT
+        : str{wrapper.str}, hash{helper(traits_type::offset, wrapper.str)}
     {}
 
     /**
      * @brief Returns the human-readable representation of a hashed string.
      * @return The string used to initialize the instance.
      */
-    constexpr operator const char *() const ENTT_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 hash_type value() const ENTT_NOEXCEPT {
+        return hash;
+    }
+
+    /**
+     * @brief Returns the human-readable representation of a hashed string.
+     * @return The string used to initialize the instance.
+     */
+    constexpr operator const value_type *() const ENTT_NOEXCEPT { return str; }
+
+    /*! @copydoc value */
     constexpr operator hash_type() const ENTT_NOEXCEPT { return hash; }
 
     /**
@@ -84,27 +181,52 @@ public:
      * @param other Hashed string with which to compare.
      * @return True if the two hashed strings are identical, false otherwise.
      */
-    constexpr bool operator==(const HashedString &other) const ENTT_NOEXCEPT {
+    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) ENTT_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>;
+
+
 }
 
 
@@ -113,8 +235,18 @@ constexpr bool operator!=(const HashedString &lhs, const HashedString &rhs) ENTT
  * @param str The literal without its suffix.
  * @return A properly initialized hashed string.
  */
-constexpr entt::HashedString operator"" ENTT_HS_SUFFIX(const char *str, std::size_t) ENTT_NOEXCEPT {
-    return entt::HashedString{str};
+constexpr entt::hashed_string operator"" ENTT_HS_SUFFIX(const char *str, std::size_t) ENTT_NOEXCEPT {
+    return entt::hashed_string{str};
+}
+
+
+/**
+ * @brief User defined literal for hashed wstrings.
+ * @param str The literal without its suffix.
+ * @return A properly initialized hashed wstring.
+ */
+constexpr entt::hashed_wstring operator"" ENTT_HWS_SUFFIX(const wchar_t *str, std::size_t) ENTT_NOEXCEPT {
+    return entt::hashed_wstring{str};
 }
 
 

src/entt/core/ident.hpp

@@ -2,44 +2,15 @@
 #define ENTT_CORE_IDENT_HPP
 
 
-#include <type_traits>
-#include <cstddef>
-#include <utility>
 #include <tuple>
+#include <utility>
+#include <type_traits>
 #include "../config/config.h"
 
 
 namespace entt {
 
 
-namespace internal {
-
-
-/**
- * @cond TURN_OFF_DOXYGEN
- * Internal details not to be documented.
- */
-
-
-template<typename...>
-struct IsPartOf;
-
-template<typename Type, typename Current, typename... Other>
-struct IsPartOf<Type, Current, Other...>: std::conditional_t<std::is_same<Type, Current>::value, std::true_type, IsPartOf<Type, Other...>> {};
-
-template<typename Type>
-struct IsPartOf<Type>: std::false_type {};
-
-
-/**
- * Internal details not to be documented.
- * @endcond TURN_OFF_DOXYGEN
- */
-
-
-}
-
-
 /**
  * @brief Types identifiers.
  *
@@ -51,13 +22,13 @@ struct IsPartOf<Type>: std::false_type {};
  * Identifiers are constant expression and can be used in any context where such
  * an expression is required. As an example:
  * @code{.cpp}
- * using ID = entt::Identifier<AType, AnotherType>;
+ * using id = entt::identifier<a_type, another_type>;
  *
- * switch(aTypeIdentifier) {
- * case ID::get<AType>():
+ * switch(a_type_identifier) {
+ * case id::type<a_type>:
  *     // ...
  *     break;
- * case ID::get<AnotherType>():
+ * case id::type<another_type>:
  *     // ...
  *     break;
  * default:
@@ -68,33 +39,22 @@ struct IsPartOf<Type>: std::false_type {};
  * @tparam Types List of types for which to generate identifiers.
  */
 template<typename... Types>
-class Identifier final {
+class identifier {
     using tuple_type = std::tuple<std::decay_t<Types>...>;
 
     template<typename Type, std::size_t... Indexes>
-    static constexpr std::size_t get(std::index_sequence<Indexes...>) ENTT_NOEXCEPT {
-        static_assert(internal::IsPartOf<Type, Types...>::value, "!");
-
-        std::size_t max{};
-        using accumulator_type = std::size_t[];
-        accumulator_type accumulator = { (max = std::is_same<Type, std::tuple_element_t<Indexes, tuple_type>>::value ? Indexes : max)... };
-        (void)accumulator;
-        return max;
+    static constexpr ENTT_ID_TYPE get(std::index_sequence<Indexes...>) ENTT_NOEXCEPT {
+        static_assert(std::disjunction_v<std::is_same<Type, Types>...>);
+        return (0 + ... + (std::is_same_v<Type, std::tuple_element_t<Indexes, tuple_type>> ? ENTT_ID_TYPE(Indexes) : ENTT_ID_TYPE{}));
     }
 
 public:
     /*! @brief Unsigned integer type. */
-    using identifier_type = std::size_t;
+    using identifier_type = ENTT_ID_TYPE;
 
-    /**
-     * @brief Returns the identifier associated with a given type.
-     * @tparam Type of which to return the identifier.
-     * @return The identifier associated with the given type.
-     */
+    /*! @brief Statically generated unique identifier for the given type. */
     template<typename Type>
-    static constexpr identifier_type get() ENTT_NOEXCEPT {
-        return get<std::decay_t<Type>>(std::make_index_sequence<sizeof...(Types)>{});
-    }
+    static constexpr identifier_type type = get<std::decay_t<Type>>(std::make_index_sequence<sizeof...(Types)>{});
 };
 
 

src/entt/core/monostate.hpp

@@ -2,10 +2,8 @@
 #define ENTT_CORE_MONOSTATE_HPP
 
 
-#include <atomic>
 #include <cassert>
-#include "family.hpp"
-#include "hashed_string.hpp"
+#include "../config/config.h"
 
 
 namespace entt {
@@ -22,8 +20,8 @@ namespace entt {
  * both during an assignment and when they try to read back their data.
  * Otherwise, they can incur in unexpected results.
  */
-template<HashedString::hash_type>
-struct Monostate {
+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.
@@ -31,7 +29,7 @@ struct Monostate {
      */
     template<typename Type>
     void operator=(Type val) const ENTT_NOEXCEPT {
-        Monostate::value<Type> = val;
+        value<Type> = val;
     }
 
     /**
@@ -41,18 +39,21 @@ struct Monostate {
      */
     template<typename Type>
     operator Type() const ENTT_NOEXCEPT {
-        return Monostate::value<Type>;
+        return value<Type>;
     }
 
 private:
     template<typename Type>
-    static std::atomic<Type> value;
+    inline static ENTT_MAYBE_ATOMIC(Type) value{};
 };
 
 
-template<HashedString::hash_type ID>
-template<typename Type>
-std::atomic<Type> Monostate<ID>::value{};
+/**
+ * @brief Helper variable template.
+ * @tparam Value Value used to differentiate between different variables.
+ */
+template<ENTT_ID_TYPE Value>
+inline monostate<Value> monostate_v = {};
 
 
 }

src/entt/core/type_traits.hpp

@@ -0,0 +1,277 @@
+#ifndef ENTT_CORE_TYPE_TRAITS_HPP
+#define ENTT_CORE_TYPE_TRAITS_HPP
+
+
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/hashed_string.hpp"
+
+
+namespace entt {
+
+
+/*! @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 Traits class used mainly to push things across boundaries. */
+template<typename>
+struct named_type_traits;
+
+
+/**
+ * @brief Specialization used to get rid of constness.
+ * @tparam Type Named type.
+ */
+template<typename Type>
+struct named_type_traits<const Type>
+        : named_type_traits<Type>
+{};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type Potentially named type.
+ */
+template<typename Type>
+using named_type_traits_t = typename named_type_traits<Type>::type;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type has a
+ * name. In all other cases, `value` is false.
+ */
+template<typename, typename = std::void_t<>>
+struct is_named_type: std::false_type {};
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type has a
+ * name. In all other cases, `value` is false.
+ * @tparam Type Potentially named type.
+ */
+template<typename Type>
+struct is_named_type<Type, std::void_t<named_type_traits_t<std::decay_t<Type>>>>: std::true_type {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type Potentially named type.
+ */
+template<class Type>
+constexpr auto is_named_type_v = is_named_type<Type>::value;
+
+
+/**
+ * @brief Defines an enum class to use for opaque identifiers and a dedicate
+ * `to_integer` function to convert the identifiers to their underlying type.
+ * @param clazz The name to use for the enum class.
+ * @param type The underlying type for the enum class.
+ */
+#define ENTT_OPAQUE_TYPE(clazz, type)\
+    enum class clazz: type {};\
+    constexpr auto to_integer(const clazz id) ENTT_NOEXCEPT {\
+        return std::underlying_type_t<clazz>(id);\
+    }
+
+
+}
+
+
+/**
+ * @brief Utility macro to deal with an issue of MSVC.
+ *
+ * See _msvc-doesnt-expand-va-args-correctly_ on SO for all the details.
+ *
+ * @param args Argument to expand.
+ */
+#define ENTT_EXPAND(args) args
+
+
+/**
+ * @brief Makes an already existing type a named type.
+ *
+ * The current definition contains a workaround for Clang 6 because it fails to
+ * deduce correctly the type to use to specialize the class template.<br/>
+ * With a compiler that fully supports C++17 and works fine with deduction
+ * guides, the following should be fine instead:
+ *
+ * @code{.cpp}
+ * std::integral_constant<ENTT_ID_TYPE, entt::basic_hashed_string{#type}>
+ * @endcode
+ *
+ * In order to support even sligthly older compilers, I prefer to stick to the
+ * implementation below.
+ *
+ * @param type Type to assign a name to.
+ */
+#define ENTT_NAMED_TYPE(type)\
+    template<>\
+    struct entt::named_type_traits<type>\
+        : std::integral_constant<ENTT_ID_TYPE, entt::basic_hashed_string<std::remove_cv_t<std::remove_pointer_t<std::decay_t<decltype(#type)>>>>{#type}>\
+    {\
+        static_assert(std::is_same_v<std::decay_t<type>, type>);\
+    };
+
+
+/**
+ * @brief Defines a named type (to use for structs).
+ * @param clazz Name of the type to define.
+ * @param body Body of the type to define.
+ */
+#define ENTT_NAMED_STRUCT_ONLY(clazz, body)\
+    struct clazz body;\
+    ENTT_NAMED_TYPE(clazz)
+
+
+/**
+ * @brief Defines a named type (to use for structs).
+ * @param ns Namespace where to define the named type.
+ * @param clazz Name of the type to define.
+ * @param body Body of the type to define.
+ */
+#define ENTT_NAMED_STRUCT_WITH_NAMESPACE(ns, clazz, body)\
+    namespace ns { struct clazz body; }\
+    ENTT_NAMED_TYPE(ns::clazz)
+
+
+/*! @brief Utility function to simulate macro overloading. */
+#define ENTT_NAMED_STRUCT_OVERLOAD(_1, _2, _3, FUNC, ...) FUNC
+/*! @brief Defines a named type (to use for structs). */
+#define ENTT_NAMED_STRUCT(...) ENTT_EXPAND(ENTT_NAMED_STRUCT_OVERLOAD(__VA_ARGS__, ENTT_NAMED_STRUCT_WITH_NAMESPACE, ENTT_NAMED_STRUCT_ONLY,)(__VA_ARGS__))
+
+
+/**
+ * @brief Defines a named type (to use for classes).
+ * @param clazz Name of the type to define.
+ * @param body Body of the type to define.
+ */
+#define ENTT_NAMED_CLASS_ONLY(clazz, body)\
+    class clazz body;\
+    ENTT_NAMED_TYPE(clazz)
+
+
+/**
+ * @brief Defines a named type (to use for classes).
+ * @param ns Namespace where to define the named type.
+ * @param clazz Name of the type to define.
+ * @param body Body of the type to define.
+ */
+#define ENTT_NAMED_CLASS_WITH_NAMESPACE(ns, clazz, body)\
+    namespace ns { class clazz body; }\
+    ENTT_NAMED_TYPE(ns::clazz)
+
+
+/*! @brief Utility function to simulate macro overloading. */
+#define ENTT_NAMED_CLASS_MACRO(_1, _2, _3, FUNC, ...) FUNC
+/*! @brief Defines a named type (to use for classes). */
+#define ENTT_NAMED_CLASS(...) ENTT_EXPAND(ENTT_NAMED_CLASS_MACRO(__VA_ARGS__, ENTT_NAMED_CLASS_WITH_NAMESPACE, ENTT_NAMED_CLASS_ONLY,)(__VA_ARGS__))
+
+
+#endif // ENTT_CORE_TYPE_TRAITS_HPP

src/entt/core/utility.hpp

@@ -0,0 +1,50 @@
+#ifndef ENTT_CORE_UTILITY_HPP
+#define ENTT_CORE_UTILITY_HPP
+
+
+#include "../config/config.h"
+
+
+namespace entt {
+
+
+/*! @brief Identity function object (waiting for C++20). */
+struct identity {
+    /**
+     * @brief Returns its argument unchanged.
+     * @tparam Type Type of the argument.
+     * @param value The actual argument.
+     * @return The submitted value as-is.
+     */
+    template<class Type>
+    constexpr Type && operator()(Type &&value) const ENTT_NOEXCEPT {
+        return std::forward<Type>(value);
+    }
+};
+
+
+/**
+ * @brief Constant utility to disambiguate overloaded member functions.
+ * @tparam Type Function type of the desired overload.
+ * @tparam Class Type of class to which the member functions belong.
+ * @param member A valid pointer to a member function.
+ * @return Pointer to the member function.
+ */
+template<typename Type, typename Class>
+constexpr auto overload(Type Class:: *member) ENTT_NOEXCEPT { return member; }
+
+
+/**
+ * @brief Constant utility to disambiguate overloaded functions.
+ * @tparam Type Function type of the desired overload.
+ * @param func A valid pointer to a function.
+ * @return Pointer to the function.
+ */
+template<typename Type>
+constexpr auto overload(Type *func) ENTT_NOEXCEPT { return func; }
+
+
+}
+
+
+#endif // ENTT_CORE_UTILITY_HPP

src/entt/entity/actor.hpp

@@ -4,67 +4,96 @@
 
 #include <cassert>
 #include <utility>
+#include <type_traits>
 #include "../config/config.h"
 #include "registry.hpp"
+#include "entity.hpp"
+#include "fwd.hpp"
 
 
 namespace entt {
 
 
 /**
- * @brief Dedicated to those who aren't confident with entity-component systems.
+ * @brief Dedicated to those who aren't confident with the
+ * entity-component-system architecture.
  *
  * Tiny wrapper around a registry, for all those users that aren't confident
- * with entity-component systems and prefer to iterate objects directly.
+ * with entity-component-system architecture and prefer to iterate objects
+ * directly.
  *
  * @tparam Entity A valid entity type (see entt_traits for more details).
  */
 template<typename Entity>
-struct Actor {
+struct basic_actor {
     /*! @brief Type of registry used internally. */
-    using registry_type = Registry<Entity>;
+    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}, entt{reg.create()}
+    basic_actor() ENTT_NOEXCEPT
+        : entt{entt::null}, reg{nullptr}
     {}
 
-    /*! @brief Default destructor. */
-    virtual ~Actor() {
-        reg.destroy(entt);
-    }
-
-    /*! @brief Default copy constructor. */
-    Actor(const Actor &) = default;
-    /*! @brief Default move constructor. */
-    Actor(Actor &&) = default;
-
-    /*! @brief Default copy assignment operator. @return This actor. */
-    Actor & operator=(const Actor &) = default;
-    /*! @brief Default move assignment operator. @return This actor. */
-    Actor & operator=(Actor &&) = default;
+    /**
+     * @brief 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 Assigns the given tag to an actor.
-     *
-     * A new instance of the given tag is created and initialized with the
-     * arguments provided (the tag must have a proper constructor or be of
-     * aggregate type). Then the tag is removed from its previous owner (if any)
-     * and assigned to the actor.
-     *
-     * @tparam Tag Type of the tag to create.
-     * @tparam Args Types of arguments to use to construct the tag.
-     * @param args Parameters to use to initialize the tag.
-     * @return A reference to the newly created tag.
+     * @brief Constructs an actor from a given entity.
+     * @param entity A valid entity identifier.
+     * @param ref An instance of the registry class.
      */
-    template<typename Tag, typename... Args>
-    Tag & assign(tag_t, Args &&... args) {
-        return (reg.template remove<Tag>(), reg.template assign<Tag>(tag_t{}, entt, std::forward<Args>(args)...));
+    explicit basic_actor(entity_type entity, registry_type &ref)
+        : entt{entity}, reg{&ref}
+    {
+        ENTT_ASSERT(ref.valid(entity));
+    }
+
+    /*! @brief Default destructor. */
+    virtual ~basic_actor() {
+        if(*this) {
+            reg->destroy(entt);
+        }
+    }
+
+    /**
+     * @brief Move constructor.
+     *
+     * After actor move construction, instances that have been moved from are
+     * placed in a valid but unspecified state. It's highly discouraged to
+     * continue using them.
+     *
+     * @param other The instance to move from.
+     */
+    basic_actor(basic_actor &&other)
+        : entt{other.entt}, reg{other.reg}
+    {
+        other.entt = null;
+    }
+
+    /**
+     * @brief Move assignment operator.
+     *
+     * After actor move assignment, instances that have been moved from are
+     * placed in a valid but unspecified state. It's highly discouraged to
+     * continue using them.
+     *
+     * @param other The instance to move from.
+     * @return This actor.
+     */
+    basic_actor & operator=(basic_actor &&other) {
+        if(this != &other) {
+            auto tmp{std::move(other)};
+            std::swap(reg, tmp.reg);
+            std::swap(entt, tmp.entt);
+        }
+
+        return *this;
     }
 
     /**
@@ -82,18 +111,8 @@ struct Actor {
      * @return A reference to the newly created component.
      */
     template<typename Component, typename... Args>
-    Component & assign(Args &&... args) {
-        return reg.template accommodate<Component>(entt, std::forward<Args>(args)...);
-    }
-
-    /**
-     * @brief Removes the given tag from an actor.
-     * @tparam Tag Type of the tag to remove.
-     */
-    template<typename Tag>
-    void remove(tag_t) {
-        assert(has<Tag>(tag_t{}));
-        reg.template remove<Tag>();
+    decltype(auto) assign(Args &&... args) {
+        return reg->template assign_or_replace<Component>(entt, std::forward<Args>(args)...);
     }
 
     /**
@@ -102,109 +121,86 @@ struct Actor {
      */
     template<typename Component>
     void remove() {
-        reg.template remove<Component>(entt);
+        reg->template remove<Component>(entt);
     }
 
     /**
-     * @brief Checks if an actor owns the given tag.
-     * @tparam Tag Type of the tag for which to perform the check.
-     * @return True if the actor owns the tag, false otherwise.
+     * @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 Tag>
-    bool has(tag_t) const ENTT_NOEXCEPT {
-        return (reg.template has<Tag>() && (reg.template attachee<Tag>() == entt));
-    }
-
-    /**
-     * @brief Checks if an actor has the given component.
-     * @tparam Component Type of the component for which to perform the check.
-     * @return True if the actor has the component, false otherwise.
-     */
-    template<typename Component>
+    template<typename... Component>
     bool has() const ENTT_NOEXCEPT {
-        return reg.template has<Component>(entt);
+        return (reg->template has<Component>(entt) && ...);
     }
 
     /**
-     * @brief Returns a reference to the given tag for an actor.
-     * @tparam Tag Type of the tag to get.
-     * @return A reference to the instance of the tag owned by the actor.
+     * @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 Tag>
-    const Tag & get(tag_t) const ENTT_NOEXCEPT {
-        assert(has<Tag>(tag_t{}));
-        return reg.template get<Tag>();
+    template<typename... Component>
+    decltype(auto) get() const ENTT_NOEXCEPT {
+        return std::as_const(*reg).template get<Component...>(entt);
+    }
+
+    /*! @copydoc get */
+    template<typename... Component>
+    decltype(auto) get() ENTT_NOEXCEPT {
+        return reg->template get<Component...>(entt);
     }
 
     /**
-     * @brief Returns a reference to the given tag for an actor.
-     * @tparam Tag Type of the tag to get.
-     * @return A reference to the instance of the tag owned by the actor.
+     * @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 Tag>
-    inline Tag & get(tag_t) ENTT_NOEXCEPT {
-        return const_cast<Tag &>(const_cast<const Actor *>(this)->get<Tag>(tag_t{}));
+    template<typename... Component>
+    auto try_get() const ENTT_NOEXCEPT {
+        return std::as_const(*reg).template try_get<Component...>(entt);
     }
 
-    /**
-     * @brief Returns a reference to the given component for an actor.
-     * @tparam Component Type of the component to get.
-     * @return A reference to the instance of the component owned by the actor.
-     */
-    template<typename Component>
-    const Component & get() const ENTT_NOEXCEPT {
-        return reg.template get<Component>(entt);
-    }
-
-    /**
-     * @brief Returns a reference to the given component for an actor.
-     * @tparam Component Type of the component to get.
-     * @return A reference to the instance of the component owned by the actor.
-     */
-    template<typename Component>
-    inline Component & get() ENTT_NOEXCEPT {
-        return const_cast<Component &>(const_cast<const Actor *>(this)->get<Component>());
+    /*! @copydoc try_get */
+    template<typename... Component>
+    auto try_get() ENTT_NOEXCEPT {
+        return reg->template try_get<Component...>(entt);
     }
 
     /**
      * @brief Returns a reference to the underlying registry.
      * @return A reference to the underlying registry.
      */
-    inline const registry_type & registry() const ENTT_NOEXCEPT {
-        return reg;
+    const registry_type & backend() const ENTT_NOEXCEPT {
+        return *reg;
     }
 
-    /**
-     * @brief Returns a reference to the underlying registry.
-     * @return A reference to the underlying registry.
-     */
-    inline registry_type & registry() ENTT_NOEXCEPT {
-        return const_cast<registry_type &>(const_cast<const Actor *>(this)->registry());
+    /*! @copydoc backend */
+    registry_type & backend() ENTT_NOEXCEPT {
+        return const_cast<registry_type &>(std::as_const(*this).backend());
     }
 
     /**
      * @brief Returns the entity associated with an actor.
      * @return The entity associated with the actor.
      */
-    inline entity_type entity() const ENTT_NOEXCEPT {
+    entity_type entity() const ENTT_NOEXCEPT {
         return entt;
     }
 
+    /**
+     * @brief Checks if an actor refers to a valid entity or not.
+     * @return True if the actor refers to a valid entity, false otherwise.
+     */
+    explicit operator bool() const ENTT_NOEXCEPT {
+        return reg && reg->valid(entt);
+    }
+
 private:
-    registry_type &reg;
-    Entity entt;
+    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.
- */
-using DefaultActor = Actor<DefaultRegistry::entity_type>;
-
-
 }
 
 

src/entt/entity/entity.hpp

@@ -2,14 +2,100 @@
 #define ENTT_ENTITY_ENTITY_HPP
 
 
+#include <cstdint>
+#include <type_traits>
 #include "../config/config.h"
-#include "entt_traits.hpp"
 
 
 namespace entt {
 
 
-namespace internal {
+/**
+ * @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;
+};
 
 
 /**
@@ -18,47 +104,51 @@ namespace internal {
  */
 
 
-template<typename Entity>
-static constexpr auto null = ~typename entt_traits<Entity>::entity_type{};
+namespace internal {
 
 
-struct Null {
-    explicit constexpr Null() = default;
+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 null<Entity>;
+        return Entity{traits_type<Entity>::entity_mask};
     }
 
-    constexpr bool operator==(Null) const ENTT_NOEXCEPT {
+    constexpr bool operator==(null) const ENTT_NOEXCEPT {
         return true;
     }
 
-    constexpr bool operator!=(Null) const ENTT_NOEXCEPT {
+    constexpr bool operator!=(null) const ENTT_NOEXCEPT {
         return false;
     }
 
     template<typename Entity>
     constexpr bool operator==(const Entity entity) const ENTT_NOEXCEPT {
-        return entity == null<Entity>;
+        return (to_integer(entity) & traits_type<Entity>::entity_mask) == to_integer(static_cast<Entity>(*this));
     }
 
     template<typename Entity>
     constexpr bool operator!=(const Entity entity) const ENTT_NOEXCEPT {
-        return entity != null<Entity>;
+        return !(entity == *this);
     }
 };
 
 
 template<typename Entity>
-constexpr bool operator==(const Entity entity, Null null) ENTT_NOEXCEPT {
-    return null == entity;
+constexpr bool operator==(const Entity entity, null other) ENTT_NOEXCEPT {
+    return other == entity;
 }
 
 
 template<typename Entity>
-constexpr bool operator!=(const Entity entity, Null null) ENTT_NOEXCEPT {
-    return null != entity;
+constexpr bool operator!=(const Entity entity, null other) ENTT_NOEXCEPT {
+    return other != entity;
+}
+
+
 }
 
 
@@ -68,17 +158,14 @@ constexpr bool operator!=(const Entity entity, Null null) ENTT_NOEXCEPT {
  */
 
 
-}
-
-
 /**
- * @brief Null entity.
+ * @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{};
+constexpr auto null = internal::null{};
 
 
 }

src/entt/entity/entt_traits.hpp

@@ -1,102 +0,0 @@
-#ifndef ENTT_ENTITY_ENTT_TRAITS_HPP
-#define ENTT_ENTITY_ENTT_TRAITS_HPP
-
-
-#include <cstdint>
-
-
-namespace entt {
-
-
-/**
- * @brief Entity traits.
- *
- * Primary template isn't defined on purpose. All the specializations give a
- * compile-time error unless the template parameter is an accepted entity type.
- */
-template<typename>
-struct entt_traits;
-
-
-/**
- * @brief Entity traits for a 16 bits entity identifier.
- *
- * A 16 bits entity identifier guarantees:
- *
- * * 12 bits for the entity number (up to 4k entities).
- * * 4 bit for the version (resets in [0-15]).
- */
-template<>
-struct entt_traits<std::uint16_t> {
-    /*! @brief Underlying entity type. */
-    using entity_type = std::uint16_t;
-    /*! @brief Underlying version type. */
-    using version_type = std::uint8_t;
-    /*! @brief Difference type. */
-    using difference_type = std::int32_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:
- *
- * * 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 auto entity_mask = 0xFFFFF;
-    /*! @brief Mask to use to get the version out of an identifier. */
-    static constexpr auto version_mask = 0xFFF;
-    /*! @brief Extent of the entity number within an identifier. */
-    static constexpr auto entity_shift = 20;
-};
-
-
-/**
- * @brief Entity traits for a 64 bits entity identifier.
- *
- * A 64 bits entity identifier guarantees:
- *
- * * 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 auto entity_mask = 0xFFFFFFFF;
-    /*! @brief Mask to use to get the version out of an identifier. */
-    static constexpr auto version_mask = 0xFFFFFFFF;
-    /*! @brief Extent of the entity number within an identifier. */
-    static constexpr auto entity_shift = 32;
-};
-
-
-}
-
-
-#endif // ENTT_ENTITY_ENTT_TRAITS_HPP

src/entt/entity/fwd.hpp

@@ -0,0 +1,100 @@
+#ifndef ENTT_ENTITY_FWD_HPP
+#define ENTT_ENTITY_FWD_HPP
+
+
+#include <cstdint>
+#include "../config/config.h"
+#include "../core/type_traits.hpp"
+
+
+namespace entt {
+
+/*! @class basic_registry */
+template <typename>
+class basic_registry;
+
+/*! @class basic_view */
+template<typename, typename...>
+class basic_view;
+
+/*! @class basic_runtime_view */
+template<typename>
+class basic_runtime_view;
+
+/*! @class basic_group */
+template<typename...>
+class basic_group;
+
+/*! @class basic_observer */
+template<typename>
+class basic_observer;
+
+/*! @class basic_actor */
+template <typename>
+struct basic_actor;
+
+/*! @class basic_prototype */
+template<typename>
+class basic_prototype;
+
+/*! @class basic_snapshot */
+template<typename>
+class basic_snapshot;
+
+/*! @class basic_snapshot_loader */
+template<typename>
+class basic_snapshot_loader;
+
+/*! @class basic_continuous_loader */
+template<typename>
+class basic_continuous_loader;
+
+/*! @brief Alias declaration for the most common use case. */
+ENTT_OPAQUE_TYPE(entity, ENTT_ID_TYPE)
+
+/*! @brief Alias declaration for the most common use case. */
+ENTT_OPAQUE_TYPE(component, ENTT_ID_TYPE)
+
+/*! @brief Alias declaration for the most common use case. */
+using registry = basic_registry<entity>;
+
+/*! @brief Alias declaration for the most common use case. */
+using observer = basic_observer<entity>;
+
+/*! @brief Alias declaration for the most common use case. */
+using actor = basic_actor<entity>;
+
+/*! @brief Alias declaration for the most common use case. */
+using prototype = basic_prototype<entity>;
+
+/*! @brief Alias declaration for the most common use case. */
+using snapshot = basic_snapshot<entity>;
+
+/*! @brief Alias declaration for the most common use case. */
+using snapshot_loader = basic_snapshot_loader<entity>;
+
+/*! @brief Alias declaration for the most common use case. */
+using continuous_loader = basic_continuous_loader<entity>;
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Component Types of components iterated by the view.
+ */
+template<typename... Types>
+using view = basic_view<entity, Types...>;
+
+/*! @brief Alias declaration for the most common use case. */
+using runtime_view = basic_runtime_view<entity>;
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Types Types of components iterated by the group.
+ */
+template<typename... Types>
+using group = basic_group<entity, Types...>;
+
+
+}
+
+
+#endif // ENTT_ENTITY_FWD_HPP

src/entt/entity/group.hpp

@@ -0,0 +1,857 @@
+#ifndef ENTT_ENTITY_GROUP_HPP
+#define ENTT_ENTITY_GROUP_HPP
+
+
+#include <tuple>
+#include <utility>
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/type_traits.hpp"
+#include "sparse_set.hpp"
+#include "storage.hpp"
+#include "utility.hpp"
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Group.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error, but for a few reasonable cases.
+ */
+template<typename...>
+class basic_group;
+
+
+/**
+ * @brief Non-owning group.
+ *
+ * A non-owning group returns all the entities and only the entities that have
+ * at least the given components. Moreover, it's guaranteed that the entity list
+ * is tightly packed in memory for fast iterations.<br/>
+ * In general, non-owning groups don't stay true to the order of any set of
+ * components unless users explicitly sort them.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New instances of the given components are created and assigned to entities.
+ * * The entity currently pointed is modified (as an example, if one of the
+ *   given components is removed from the entity to which the iterator points).
+ * * The entity currently pointed is destroyed.
+ *
+ * In all the other cases, modifying the pools of the given components in any
+ * way invalidates all the iterators and using them results in undefined
+ * behavior.
+ *
+ * @note
+ * Groups share references to the underlying data structures of the registry
+ * that generated them. Therefore any change to the entities and to the
+ * components made by means of the registry are immediately reflected by all the
+ * groups.<br/>
+ * Moreover, sorting a non-owning group affects all the instance of the same
+ * group (it means that users don't have to call `sort` on each instance to sort
+ * all of them because they share the set of entities).
+ *
+ * @warning
+ * Lifetime of a group must overcome the one of the registry that generated it.
+ * In any other case, attempting to use a group results in undefined behavior.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Exclude Types of components used to filter the group.
+ * @tparam Get Type of component observed by the group.
+ * @tparam Other Other types of components observed by the group.
+ */
+template<typename Entity, typename... Exclude, typename Get, typename... Other>
+class basic_group<Entity, exclude_t<Exclude...>, get_t<Get, Other...>> {
+    /*! @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>> *get, storage<Entity, std::remove_const_t<Other>> *... other) ENTT_NOEXCEPT
+        : handler{ref},
+          pools{get, other...}
+    {}
+
+    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 = typename sparse_set<Entity>::entity_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = typename sparse_set<Entity>::size_type;
+    /*! @brief Input iterator type. */
+    using iterator_type = typename sparse_set<Entity>::iterator_type;
+
+    /**
+     * @brief Returns the number of existing components of the given type.
+     * @tparam Component Type of component of which to return the size.
+     * @return Number of existing components of the given type.
+     */
+    template<typename Component>
+    size_type size() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Component> *>(pools)->size();
+    }
+
+    /**
+     * @brief Returns the number of entities that have the given components.
+     * @return Number of entities that have the given components.
+     */
+    size_type size() const ENTT_NOEXCEPT {
+        return handler->size();
+    }
+
+    /**
+     * @brief Returns the number of elements that a group has currently
+     * allocated space for.
+     * @return Capacity of the group.
+     */
+    size_type capacity() const ENTT_NOEXCEPT {
+        return handler->capacity();
+    }
+
+    /*! @brief Requests the removal of unused capacity. */
+    void shrink_to_fit() {
+        handler->shrink_to_fit();
+    }
+
+    /**
+     * @brief Checks whether the pool of a given component is empty.
+     * @tparam Component Type of component in which one is interested.
+     * @return True if the pool of the given component is empty, false
+     * otherwise.
+     */
+    template<typename Component>
+    bool empty() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Component> *>(pools)->empty();
+    }
+
+    /**
+     * @brief Checks whether the group is empty.
+     * @return True if the group is empty, false otherwise.
+     */
+    bool empty() const ENTT_NOEXCEPT {
+        return handler->empty();
+    }
+
+    /**
+     * @brief Direct access to the list of components of a given pool.
+     *
+     * The returned pointer is such that range
+     * `[raw<Component>(), raw<Component>() + size<Component>()]` is always a
+     * valid range, even if the container is empty.
+     *
+     * @note
+     * There are no guarantees on the order of the components. Use `begin` and
+     * `end` if you want to iterate the group in the expected order.
+     *
+     * @tparam Component Type of component in which one is interested.
+     * @return A pointer to the array of components.
+     */
+    template<typename Component>
+    Component * raw() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Component> *>(pools)->raw();
+    }
+
+    /**
+     * @brief Direct access to the list of entities of a given pool.
+     *
+     * The returned pointer is such that range
+     * `[data<Component>(), data<Component>() + size<Component>()]` is always a
+     * valid range, even if the container is empty.
+     *
+     * @note
+     * There are no guarantees on the order of the entities. Use `begin` and
+     * `end` if you want to iterate the group in the expected order.
+     *
+     * @tparam Component Type of component in which one is interested.
+     * @return A pointer to the array of entities.
+     */
+    template<typename Component>
+    const entity_type * data() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Component> *>(pools)->data();
+    }
+
+    /**
+     * @brief Direct access to the list of entities.
+     *
+     * The returned pointer is such that range `[data(), data() + size()]` is
+     * always a valid range, even if the container is empty.
+     *
+     * @note
+     * There are no guarantees on the order of the entities. Use `begin` and
+     * `end` if you want to iterate the group in the expected order.
+     *
+     * @return A pointer to the array of entities.
+     */
+    const entity_type * data() const ENTT_NOEXCEPT {
+        return handler->data();
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity that has the given
+     * components.
+     *
+     * The returned iterator points to the first entity that has the given
+     * components. If the group is empty, the returned iterator will be equal to
+     * `end()`.
+     *
+     * @note
+     * Input iterators stay true to the order imposed to the underlying data
+     * structures.
+     *
+     * @return An iterator to the first entity that has the given components.
+     */
+    iterator_type begin() const ENTT_NOEXCEPT {
+        return handler->begin();
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity that has the
+     * given components.
+     *
+     * The returned iterator points to the entity following the last entity that
+     * has the given components. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @note
+     * Input iterators stay true to the order imposed to the underlying data
+     * structures.
+     *
+     * @return An iterator to the entity following the last entity that has the
+     * given components.
+     */
+    iterator_type end() const ENTT_NOEXCEPT {
+        return handler->end();
+    }
+
+    /**
+     * @brief Finds an entity.
+     * @param entt A valid entity identifier.
+     * @return An iterator to the given entity if it's found, past the end
+     * iterator otherwise.
+     */
+    iterator_type find(const entity_type entt) const ENTT_NOEXCEPT {
+        const auto it = handler->find(entt);
+        return it != end() && *it == entt ? it : end();
+    }
+
+    /**
+     * @brief Returns the identifier that occupies the given position.
+     * @param pos Position of the element to return.
+     * @return The identifier that occupies the given position.
+     */
+    entity_type operator[](const size_type pos) const ENTT_NOEXCEPT {
+        return begin()[pos];
+    }
+
+    /**
+     * @brief Checks if a group contains an entity.
+     * @param entt A valid entity identifier.
+     * @return True if the group contains the given entity, false otherwise.
+     */
+    bool contains(const entity_type entt) const ENTT_NOEXCEPT {
+        return find(entt) != end();
+    }
+
+    /**
+     * @brief Returns the components assigned to the given entity.
+     *
+     * Prefer this function instead of `registry::get` during iterations. It has
+     * far better performance than its companion function.
+     *
+     * @warning
+     * Attempting to use an invalid component type results in a compilation
+     * error. Attempting to use an entity that doesn't belong to the group
+     * results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * group doesn't contain the given entity.
+     *
+     * @tparam Component Types of components to get.
+     * @param entt A valid entity identifier.
+     * @return The components assigned to the entity.
+     */
+    template<typename... Component>
+    decltype(auto) get([[maybe_unused]] const entity_type entt) const ENTT_NOEXCEPT {
+        ENTT_ASSERT(contains(entt));
+
+        if constexpr(sizeof...(Component) == 1) {
+            return (std::get<pool_type<Component> *>(pools)->get(entt), ...);
+        } else {
+            return std::tuple<decltype(get<Component>(entt))...>{get<Component>(entt)...};
+        }
+    }
+
+    /**
+     * @brief Iterates entities and components and applies the given function
+     * object to them.
+     *
+     * The function object is invoked for each entity. It is provided with the
+     * entity itself and a set of references to all its components. The
+     * _constness_ of the components is as requested.<br/>
+     * The signature of the function must be equivalent to one of the following
+     * forms:
+     *
+     * @code{.cpp}
+     * void(const entity_type, Get &, Other &...);
+     * void(Get &, Other &...);
+     * @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, Other...>{});
+    }
+
+    /**
+     * @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 = std::conditional_t<std::is_empty_v<Get>, type_list<>, type_list<Get>>;
+        using other_type_list = type_list_cat_t<std::conditional_t<std::is_empty_v<Other>, type_list<>, type_list<Other>>...>;
+        traverse(std::move(func), type_list_cat_t<get_type_list, other_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.
+     *
+     * The comparison function object received by the sort function object
+     * hasn't necessarily the type of the one passed along with the other
+     * parameters to this member function.
+     *
+     * @note
+     * Attempting to iterate elements using a raw pointer returned by a call to
+     * either `data` or `raw` gives no guarantees on the order, even though
+     * `sort` has been invoked.
+     *
+     * @tparam Component Optional types of components to compare.
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename... Component, typename Compare, typename Sort = std_sort, typename... Args>
+    void sort(Compare compare, Sort algo = Sort{}, Args &&... args) {
+        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 {
+            handler->sort(handler->begin(), handler->end(), [this, compare = std::move(compare)](const entity_type lhs, const entity_type rhs) {
+                // useless this-> used to suppress a warning with clang
+                return compare(this->get<Component...>(lhs), this->get<Component...>(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> *, pool_type<Other> *...> pools;
+};
+
+
+/**
+ * @brief Owning group.
+ *
+ * Owning groups return all the entities and only the entities that have at
+ * least the given components. Moreover:
+ *
+ * * It's guaranteed that the entity list is tightly packed in memory for fast
+ *   iterations.
+ * * It's guaranteed that the lists of owned components are tightly packed in
+ *   memory for even faster iterations and to allow direct access.
+ * * They stay true to the order of the owned components and all the owned
+ *   components have the same order in memory.
+ *
+ * The more types of components are owned by a group, the faster it is to
+ * iterate them.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New instances of the given components are created and assigned to entities.
+ * * The entity currently pointed is modified (as an example, if one of the
+ *   given components is removed from the entity to which the iterator points).
+ * * The entity currently pointed is destroyed.
+ *
+ * In all the other cases, modifying the pools of the given components in any
+ * way invalidates all the iterators and using them results in undefined
+ * behavior.
+ *
+ * @note
+ * Groups share references to the underlying data structures of the registry
+ * that generated them. Therefore any change to the entities and to the
+ * components made by means of the registry are immediately reflected by all the
+ * groups.
+ * Moreover, sorting an owning group affects all the instance of the same group
+ * (it means that users don't have to call `sort` on each instance to sort all
+ * of them because they share the underlying data structure).
+ *
+ * @warning
+ * Lifetime of a group must overcome the one of the registry that generated it.
+ * In any other case, attempting to use a group results in undefined behavior.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Exclude Types of components used to filter the group.
+ * @tparam Get Types of components observed by the group.
+ * @tparam Owned Type of component owned by the group.
+ * @tparam Other Other types of components owned by the group.
+ */
+template<typename Entity, typename... Exclude, typename... Get, typename Owned, typename... Other>
+class basic_group<Entity, exclude_t<Exclude...>, get_t<Get...>, Owned, Other...> {
+    /*! @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 typename basic_registry<Entity>::size_type *sz, storage<Entity, std::remove_const_t<Owned>> *owned, storage<Entity, std::remove_const_t<Other>> *... other, storage<Entity, std::remove_const_t<Get>> *... get) ENTT_NOEXCEPT
+        : length{sz},
+          pools{owned, other..., get...}
+    {}
+
+    template<typename Func, typename... Strong, typename... Weak>
+    void traverse(Func func, type_list<Strong...>, type_list<Weak...>) const {
+        [[maybe_unused]] auto raw = std::make_tuple((std::get<pool_type<Strong> *>(pools)->end() - *length)...);
+        [[maybe_unused]] auto data = std::get<pool_type<Owned> *>(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>>(raw)++)...);
+                } else {
+                    const auto entt = *(data++);
+                    func(*(std::get<component_iterator_type<Strong>>(raw)++)..., std::get<pool_type<Weak> *>(pools)->get(entt)...);
+                }
+            } else {
+                const auto entt = *(data++);
+                func(entt, *(std::get<component_iterator_type<Strong>>(raw)++)..., std::get<pool_type<Weak> *>(pools)->get(entt)...);
+            }
+        }
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename sparse_set<Entity>::entity_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = typename sparse_set<Entity>::size_type;
+    /*! @brief Input iterator type. */
+    using iterator_type = typename sparse_set<Entity>::iterator_type;
+
+    /**
+     * @brief Returns the number of existing components of the given type.
+     * @tparam Component Type of component of which to return the size.
+     * @return Number of existing components of the given type.
+     */
+    template<typename Component>
+    size_type size() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Component> *>(pools)->size();
+    }
+
+    /**
+     * @brief Returns the number of entities that have the given components.
+     * @return Number of entities that have the given components.
+     */
+    size_type size() const ENTT_NOEXCEPT {
+        return *length;
+    }
+
+    /**
+     * @brief Checks whether the pool of a given component is empty.
+     * @tparam Component Type of component in which one is interested.
+     * @return True if the pool of the given component is empty, false
+     * otherwise.
+     */
+    template<typename Component>
+    bool empty() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Component> *>(pools)->empty();
+    }
+
+    /**
+     * @brief Checks whether the group is empty.
+     * @return True if the group is empty, false otherwise.
+     */
+    bool empty() const ENTT_NOEXCEPT {
+        return !*length;
+    }
+
+    /**
+     * @brief Direct access to the list of components of a given pool.
+     *
+     * The returned pointer is such that range
+     * `[raw<Component>(), raw<Component>() + size<Component>()]` is always a
+     * valid range, even if the container is empty.<br/>
+     * Moreover, in case the group owns the given component, the range
+     * `[raw<Component>(), raw<Component>() + size()]` is such that it contains
+     * the instances that are part of the group itself.
+     *
+     * @note
+     * There are no guarantees on the order of the components. Use `begin` and
+     * `end` if you want to iterate the group in the expected order.
+     *
+     * @tparam Component Type of component in which one is interested.
+     * @return A pointer to the array of components.
+     */
+    template<typename Component>
+    Component * raw() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Component> *>(pools)->raw();
+    }
+
+    /**
+     * @brief Direct access to the list of entities of a given pool.
+     *
+     * The returned pointer is such that range
+     * `[data<Component>(), data<Component>() + size<Component>()]` is always a
+     * valid range, even if the container is empty.<br/>
+     * Moreover, in case the group owns the given component, the range
+     * `[data<Component>(), data<Component>() + size()]` is such that it
+     * contains the entities that are part of the group itself.
+     *
+     * @note
+     * There are no guarantees on the order of the entities. Use `begin` and
+     * `end` if you want to iterate the group in the expected order.
+     *
+     * @tparam Component Type of component in which one is interested.
+     * @return A pointer to the array of entities.
+     */
+    template<typename Component>
+    const entity_type * data() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Component> *>(pools)->data();
+    }
+
+    /**
+     * @brief Direct access to the list of entities.
+     *
+     * The returned pointer is such that range `[data(), data() + size()]` is
+     * always a valid range, even if the container is empty.
+     *
+     * @note
+     * There are no guarantees on the order of the entities. Use `begin` and
+     * `end` if you want to iterate the group in the expected order.
+     *
+     * @return A pointer to the array of entities.
+     */
+    const entity_type * data() const ENTT_NOEXCEPT {
+        return std::get<pool_type<Owned> *>(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<pool_type<Owned> *>(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<pool_type<Owned> *>(pools)->sparse_set<entity_type>::end();
+    }
+
+    /**
+     * @brief Finds an entity.
+     * @param entt A valid entity identifier.
+     * @return An iterator to the given entity if it's found, past the end
+     * iterator otherwise.
+     */
+    iterator_type find(const entity_type entt) const ENTT_NOEXCEPT {
+        const auto it = std::get<pool_type<Owned> *>(pools)->find(entt);
+        return it != end() && it >= begin() && *it == entt ? it : end();
+    }
+
+    /**
+     * @brief Returns the identifier that occupies the given position.
+     * @param pos Position of the element to return.
+     * @return The identifier that occupies the given position.
+     */
+    entity_type operator[](const size_type pos) const ENTT_NOEXCEPT {
+        return begin()[pos];
+    }
+
+    /**
+     * @brief Checks if a group contains an entity.
+     * @param entt A valid entity identifier.
+     * @return True if the group contains the given entity, false otherwise.
+     */
+    bool contains(const entity_type entt) const ENTT_NOEXCEPT {
+        return find(entt) != end();
+    }
+
+    /**
+     * @brief Returns the components assigned to the given entity.
+     *
+     * Prefer this function instead of `registry::get` during iterations. It has
+     * far better performance than its companion function.
+     *
+     * @warning
+     * Attempting to use an invalid component type results in a compilation
+     * error. Attempting to use an entity that doesn't belong to the group
+     * results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * group doesn't contain the given entity.
+     *
+     * @tparam Component Types of components to get.
+     * @param entt A valid entity identifier.
+     * @return The components assigned to the entity.
+     */
+    template<typename... Component>
+    decltype(auto) get([[maybe_unused]] const entity_type entt) const ENTT_NOEXCEPT {
+        ENTT_ASSERT(contains(entt));
+
+        if constexpr(sizeof...(Component) == 1) {
+            return (std::get<pool_type<Component> *>(pools)->get(entt), ...);
+        } else {
+            return std::tuple<decltype(get<Component>(entt))...>{get<Component>(entt)...};
+        }
+    }
+
+    /**
+     * @brief Iterates entities and components and applies the given function
+     * object to them.
+     *
+     * The function object is invoked for each entity. It is provided with the
+     * entity itself and a set of references to all its components. The
+     * _constness_ of the components is as requested.<br/>
+     * The signature of the function must be equivalent to one of the following
+     * forms:
+     *
+     * @code{.cpp}
+     * void(const entity_type, Owned &, Other &..., Get &...);
+     * void(Owned &, Other &..., 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, Other...>{}, 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 = std::conditional_t<std::is_empty_v<Owned>, type_list<>, type_list<Owned>>;
+        using other_type_list = type_list_cat_t<std::conditional_t<std::is_empty_v<Other>, type_list<>, type_list<Other>>...>;
+        using get_type_list = type_list_cat_t<std::conditional_t<std::is_empty_v<Get>, type_list<>, type_list<Get>>...>;
+        traverse(std::move(func), type_list_cat_t<owned_type_list, other_type_list>{}, 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 either owned types or not but still such that they
+     * are iterated by the group.<br/>
+     * Moreover, the comparison function object shall induce a
+     * _strict weak ordering_ on the values.
+     *
+     * The sort function oject must offer a member function template
+     * `operator()` that accepts three arguments:
+     *
+     * * An iterator to the first element of the range to sort.
+     * * An iterator past the last element of the range to sort.
+     * * A comparison function to use to compare the elements.
+     *
+     * The comparison function object received by the sort function object
+     * hasn't necessarily the type of the one passed along with the other
+     * parameters to this member function.
+     *
+     * @note
+     * Attempting to iterate elements using a raw pointer returned by a call to
+     * either `data` or `raw` gives no guarantees on the order, even though
+     * `sort` has been invoked.
+     *
+     * @tparam Component Optional types of components to compare.
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename... Component, typename Compare, typename Sort = std_sort, typename... Args>
+    void sort(Compare compare, Sort algo = Sort{}, Args &&... args) {
+        auto *cpool = std::get<pool_type<Owned> *>(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 {
+            cpool->sort(cpool->end()-*length, cpool->end(), [this, compare = std::move(compare)](const entity_type lhs, const entity_type rhs) {
+                // useless this-> used to suppress a warning with clang
+                return compare(this->get<Component...>(lhs), this->get<Component...>(rhs));
+            }, std::move(algo), std::forward<Args>(args)...);
+        }
+
+        for(auto next = *length; next; --next) {
+            const auto pos = next - 1;
+            const auto entt = cpool->data()[pos];
+            (std::get<pool_type<Other> *>(pools)->swap(pos, std::get<pool_type<Other> *>(pools)->index(entt)), ...);
+        }
+    }
+
+private:
+    const typename basic_registry<Entity>::size_type *length;
+    const std::tuple<pool_type<Owned> *, pool_type<Other> *..., pool_type<Get> *...> pools;
+};
+
+
+}
+
+
+#endif // ENTT_ENTITY_GROUP_HPP

src/entt/entity/helper.hpp

@@ -3,100 +3,133 @@
 
 
 #include <type_traits>
-#include "../core/hashed_string.hpp"
+#include "../config/config.h"
 #include "../signal/sigh.hpp"
 #include "registry.hpp"
-#include "utility.hpp"
 
 
 namespace entt {
 
 
 /**
- * @brief Dependency function prototype.
- *
- * A _dependency function_ is a built-in listener to use to automatically assign
- * components to an entity when a type has a dependency on some other types.
- *
- * This is a prototype function to use to create dependencies.<br/>
- * It isn't intended for direct use, although nothing forbids using it freely.
- *
+ * @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).
- * @tparam Component Types of components to assign to an entity if triggered.
- * @param registry A valid reference to a registry.
- * @param entity A valid entity identifier.
  */
-template<typename Entity, typename... Component>
-void dependency(Registry<Entity> &registry, const Entity entity) {
-    using accumulator_type = int[];
-    accumulator_type accumulator = { ((registry.template has<Component>(entity) ? void() : (registry.template assign<Component>(entity), void())), 0)... };
-    (void)accumulator;
-}
+template<bool Const, typename Entity>
+struct as_view {
+    /*! @brief Type of registry to convert. */
+    using registry_type = std::conditional_t<Const, const entt::basic_registry<Entity>, entt::basic_registry<Entity>>;
+
+    /**
+     * @brief Constructs a converter for a given registry.
+     * @param source A valid reference to a registry.
+     */
+    as_view(registry_type &source) ENTT_NOEXCEPT: reg{source} {}
+
+    /**
+     * @brief Conversion function from a registry to a view.
+     * @tparam Component Type of components used to construct the view.
+     * @return A newly created view.
+     */
+    template<typename... Component>
+    operator entt::basic_view<Entity, Component...>() const {
+        return reg.template view<Component...>();
+    }
+
+private:
+    registry_type &reg;
+};
 
 
 /**
- * @brief Connects a dependency function to the given sink.
+ * @brief Deduction guide.
  *
- * A _dependency function_ is a built-in listener to use to automatically assign
- * components to an entity when a type has a dependency on some other types.
+ * It allows to deduce the constness of a registry directly from the instance
+ * provided to the constructor.
  *
- * The following adds components `AType` and `AnotherType` whenever `MyType` is
- * assigned to an entity:
- * @code{.cpp}
- * entt::DefaultRegistry registry;
- * entt::dependency<AType, AnotherType>(registry.construction<MyType>());
- * @endcode
- *
- * @tparam Dependency Types of components to assign to an entity if triggered.
  * @tparam Entity A valid entity type (see entt_traits for more details).
- * @param sink A sink object properly initialized.
  */
-template<typename... Dependency, typename Entity>
-void dependency(Sink<void(Registry<Entity> &, const Entity)> sink) {
-    sink.template connect<dependency<Entity, Dependency...>>();
-}
+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 Disconnects a dependency function from the given sink.
- *
- * A _dependency function_ is a built-in listener to use to automatically assign
- * components to an entity when a type has a dependency on some other types.
- *
- * The following breaks the dependency between the component `MyType` and the
- * components `AType` and `AnotherType`:
- * @code{.cpp}
- * entt::DefaultRegistry registry;
- * entt::dependency<AType, AnotherType>(entt::break_t{}, registry.construction<MyType>());
- * @endcode
- *
- * @tparam Dependency Types of components used to create the dependency.
+ * @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).
- * @param sink A sink object properly initialized.
  */
-template<typename... Dependency, typename Entity>
-void dependency(break_t, Sink<void(Registry<Entity> &, const Entity)> sink) {
-    sink.template disconnect<dependency<Entity, Dependency...>>();
-}
+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 Alias template to ease the assignment of labels to entities.
+ * @brief Deduction guide.
+ *
+ * It allows to deduce the constness of a registry directly from the instance
+ * provided to the constructor.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+as_group(basic_registry<Entity> &) ENTT_NOEXCEPT -> as_group<false, Entity>;
+
+
+/*! @copydoc as_group */
+template<typename Entity>
+as_group(const basic_registry<Entity> &) ENTT_NOEXCEPT -> as_group<true, Entity>;
+
+
+/**
+ * @brief Alias template to ease the assignment of tags to entities.
  *
  * If used in combination with hashed strings, it simplifies the assignment of
- * labels to entities and the use of labels in general where a type would be
+ * tags to entities and the use of tags in general where a type would be
  * required otherwise.<br/>
  * As an example and where the user defined literal for hashed strings hasn't
  * been changed:
  * @code{.cpp}
- * entt::DefaultRegistry registry;
- * registry.assign<entt::label<"enemy"_hs>>(entity);
+ * entt::registry registry;
+ * registry.assign<entt::tag<"enemy"_hs>>(entity);
  * @endcode
  *
+ * @note
+ * Tags are empty components and therefore candidates for the empty component
+ * optimization.
+ *
  * @tparam Value The numeric representation of an instance of hashed string.
  */
-template<typename HashedString::hash_type Value>
-using label = std::integral_constant<typename HashedString::hash_type, Value>;
+template<ENTT_ID_TYPE Value>
+using tag = std::integral_constant<ENTT_ID_TYPE, Value>;
 
 
 }

src/entt/entity/observer.hpp

@@ -0,0 +1,438 @@
+#ifndef ENTT_ENTITY_OBSERVER_HPP
+#define ENTT_ENTITY_OBSERVER_HPP
+
+
+#include <limits>
+#include <cstddef>
+#include <cstdint>
+#include <utility>
+#include <algorithm>
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/type_traits.hpp"
+#include "registry.hpp"
+#include "storage.hpp"
+#include "entity.hpp"
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/*! @brief Grouping matcher. */
+template<typename...>
+struct matcher {};
+
+
+/**
+ * @brief Collector.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error, but for a few reasonable cases.
+ */
+template<typename...>
+struct basic_collector;
+
+
+/**
+ * @brief Collector.
+ *
+ * A collector contains a set of rules (literally, matchers) to use to track
+ * entities.<br/>
+ * Its main purpose is to generate a descriptor that allows an observer to know
+ * how to connect to a registry.
+ */
+template<>
+struct basic_collector<> {
+    /**
+     * @brief Adds a grouping matcher to the collector.
+     * @tparam AllOf Types of components tracked by the matcher.
+     * @tparam NoneOf Types of components used to filter out entities.
+     * @return The updated collector.
+     */
+    template<typename... AllOf, typename... NoneOf>
+    static constexpr auto group(exclude_t<NoneOf...> = {}) ENTT_NOEXCEPT {
+        return basic_collector<matcher<matcher<type_list<>, type_list<>>, type_list<NoneOf...>, type_list<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<matcher<type_list<>, type_list<>>, AnyOf>>{};
+    }
+};
+
+/**
+ * @brief Collector.
+ * @copydetails basic_collector<>
+ * @tparam AnyOf Types of components for which changes should be detected.
+ * @tparam Matcher Types of grouping matchers.
+ */
+template<typename... Reject, typename... Require, typename... Rule, typename... Other>
+struct basic_collector<matcher<matcher<type_list<Reject...>, type_list<Require...>>, Rule...>, Other...> {
+    /**
+     * @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 {
+        using first = matcher<matcher<type_list<Reject...>, type_list<Require...>>, Rule...>;
+        return basic_collector<first, Other..., matcher<matcher<type_list<>, type_list<>>, type_list<NoneOf...>, type_list<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 {
+        using first = matcher<matcher<type_list<Reject...>, type_list<Require...>>, Rule...>;
+        return basic_collector<first, Other..., matcher<matcher<type_list<>, type_list<>>, AnyOf>>{};
+    }
+
+    /**
+     * @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 {
+        return basic_collector<matcher<matcher<type_list<Reject..., NoneOf...>, type_list<Require..., AllOf...>>, Rule...>, 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<matcher<type_list<Reject...>, type_list<Require...>>, AnyOf>> {
+        template<std::size_t Index>
+        static void maybe_valid_if(basic_observer &obs, const Entity entt, const basic_registry<Entity> &reg) {
+            if(reg.template has<Require...>(entt) && !(reg.template has<Reject>(entt) || ...)) {
+                auto *comp = obs.view.try_get(entt);
+                (comp ? *comp : obs.view.construct(entt)) |= (1 << Index);
+            }
+        }
+
+        template<std::size_t Index>
+        static void discard_if(basic_observer &obs, const Entity entt) {
+            if(auto *value = obs.view.try_get(entt); value && !(*value &= (~(1 << Index)))) {
+                obs.view.destroy(entt);
+            }
+        }
+
+        template<std::size_t Index>
+        static void connect(basic_observer &obs, basic_registry<Entity> &reg) {
+            (reg.template on_destroy<Require>().template connect<&discard_if<Index>>(obs), ...);
+            (reg.template on_construct<Reject>().template connect<&discard_if<Index>>(obs), ...);
+            reg.template on_replace<AnyOf>().template connect<&maybe_valid_if<Index>>(obs);
+            reg.template on_destroy<AnyOf>().template connect<&discard_if<Index>>(obs);
+        }
+
+        static void disconnect(basic_observer &obs, basic_registry<Entity> &reg) {
+            (reg.template on_destroy<Require>().disconnect(obs), ...);
+            (reg.template on_construct<Reject>().disconnect(obs), ...);
+            reg.template on_replace<AnyOf>().disconnect(obs);
+            reg.template on_destroy<AnyOf>().disconnect(obs);
+        }
+    };
+
+    template<typename... Reject, typename... Require, typename... NoneOf, typename... AllOf>
+    struct matcher_handler<matcher<matcher<type_list<Reject...>, type_list<Require...>>, type_list<NoneOf...>, type_list<AllOf...>>> {
+        template<std::size_t Index>
+        static void maybe_valid_if(basic_observer &obs, const Entity entt, const basic_registry<Entity> &reg) {
+            if(reg.template has<AllOf...>(entt) && !(reg.template has<NoneOf>(entt) || ...)
+                    && reg.template has<Require...>(entt) && !(reg.template has<Reject>(entt) || ...))
+            {
+                auto *comp = obs.view.try_get(entt);
+                (comp ? *comp : obs.view.construct(entt)) |= (1 << Index);
+            }
+        }
+
+        template<std::size_t Index>
+        static void discard_if(basic_observer &obs, const Entity entt) {
+            if(auto *value = obs.view.try_get(entt); value && !(*value &= (~(1 << Index)))) {
+                obs.view.destroy(entt);
+            }
+        }
+
+        template<std::size_t Index>
+        static void connect(basic_observer &obs, basic_registry<Entity> &reg) {
+            (reg.template on_destroy<Require>().template connect<&discard_if<Index>>(obs), ...);
+            (reg.template on_construct<Reject>().template connect<&discard_if<Index>>(obs), ...);
+            (reg.template on_construct<AllOf>().template connect<&maybe_valid_if<Index>>(obs), ...);
+            (reg.template on_destroy<NoneOf>().template connect<&maybe_valid_if<Index>>(obs), ...);
+            (reg.template on_destroy<AllOf>().template connect<&discard_if<Index>>(obs), ...);
+            (reg.template on_construct<NoneOf>().template connect<&discard_if<Index>>(obs), ...);
+        }
+
+        static void disconnect(basic_observer &obs, basic_registry<Entity> &reg) {
+            (reg.template on_destroy<Require>().disconnect(obs), ...);
+            (reg.template on_construct<Reject>().disconnect(obs), ...);
+            (reg.template on_construct<AllOf>().disconnect(obs), ...);
+            (reg.template on_destroy<NoneOf>().disconnect(obs), ...);
+            (reg.template on_destroy<AllOf>().disconnect(obs), ...);
+            (reg.template on_construct<NoneOf>().disconnect(obs), ...);
+        }
+    };
+
+    template<typename... Matcher>
+    static void disconnect(basic_observer &obs, basic_registry<Entity> &reg) {
+        (matcher_handler<Matcher>::disconnect(obs, reg), ...);
+    }
+
+    template<typename... Matcher, std::size_t... Index>
+    void connect(basic_registry<Entity> &reg, std::index_sequence<Index...>) {
+        static_assert(sizeof...(Matcher) < std::numeric_limits<payload_type>::digits);
+        (matcher_handler<Matcher>::template connect<Index>(*this, reg), ...);
+        release = &basic_observer::disconnect<Matcher...>;
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Input iterator type. */
+    using iterator_type = typename sparse_set<Entity>::iterator_type;
+
+    /*! @brief Default constructor. */
+    basic_observer() ENTT_NOEXCEPT
+        : target{}, release{}, view{}
+    {}
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    basic_observer(const basic_observer &) = delete;
+    /*! @brief Default move constructor, deleted on purpose. */
+    basic_observer(basic_observer &&) = delete;
+
+    /**
+     * @brief Creates an observer and connects it to a given registry.
+     * @tparam Matcher Types of matchers to use to initialize the observer.
+     * @param reg A valid reference to a registry.
+     */
+    template<typename... Matcher>
+    basic_observer(basic_registry<entity_type> &reg, basic_collector<Matcher...>) ENTT_NOEXCEPT
+        : target{&reg},
+          release{},
+          view{}
+    {
+        connect<Matcher...>(reg, std::make_index_sequence<sizeof...(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::make_index_sequence<sizeof...(Matcher)>{});
+        target = &reg;
+        view.reset();
+    }
+
+    /*! @brief Disconnects an observer from the registry it keeps track of. */
+    void disconnect() {
+        if(release) {
+            release(*this, *target);
+            release = nullptr;
+        }
+    }
+
+    /**
+     * @brief Returns the number of elements in an observer.
+     * @return Number of elements.
+     */
+    size_type size() const ENTT_NOEXCEPT {
+        return view.size();
+    }
+
+    /**
+     * @brief Checks whether an observer is empty.
+     * @return True if the observer is empty, false otherwise.
+     */
+    bool empty() const ENTT_NOEXCEPT {
+        return view.empty();
+    }
+
+    /**
+     * @brief Direct access to the list of entities of the observer.
+     *
+     * The returned pointer is such that range `[data(), data() + size()]` is
+     * always a valid range, even if the container is empty.
+     *
+     * @note
+     * There are no guarantees on the order of the entities. Use `begin` and
+     * `end` if you want to iterate the observer in the expected order.
+     *
+     * @return A pointer to the array of entities.
+     */
+    const entity_type * data() const ENTT_NOEXCEPT {
+        return view.data();
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the observer.
+     *
+     * The returned iterator points to the first entity of the observer. If the
+     * container is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first entity of the observer.
+     */
+    iterator_type begin() const ENTT_NOEXCEPT {
+        return view.sparse_set<entity_type>::begin();
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the observer.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the observer. Attempting to dereference the returned iterator results in
+     * undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the
+     * observer.
+     */
+    iterator_type end() const ENTT_NOEXCEPT {
+        return view.sparse_set<entity_type>::end();
+    }
+
+    /*! @brief Resets the underlying container. */
+    void clear() {
+        view.reset();
+    }
+
+    /**
+     * @brief Iterates entities and applies the given function object to them,
+     * then clears the observer.
+     *
+     * The function object is invoked for each entity.<br/>
+     * The signature of the function must be equivalent to the following form:
+     *
+     * @code{.cpp}
+     * void(const entity_type);
+     * @endcode
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        static_assert(std::is_invocable_v<Func, entity_type>);
+        std::for_each(begin(), end(), std::move(func));
+    }
+
+    /**
+     * @brief Iterates entities and applies the given function object to them,
+     * then clears the observer.
+     *
+     * The function object is invoked for each entity.<br/>
+     * The signature of the function must be equivalent to the following form:
+     *
+     * @code{.cpp}
+     * void(const entity_type);
+     * @endcode
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) {
+        std::as_const(*this).each(std::move(func));
+        clear();
+    }
+
+private:
+    basic_registry<entity_type> *target;
+    void(* release)(basic_observer &, basic_registry<entity_type> &);
+    storage<entity_type, payload_type> view;
+};
+
+
+}
+
+
+#endif // ENTT_ENTITY_OBSERVER_HPP

src/entt/entity/prototype.hpp

@@ -1,496 +0,0 @@
-#ifndef ENTT_ENTITY_PROTOTYPE_HPP
-#define ENTT_ENTITY_PROTOTYPE_HPP
-
-
-#include <tuple>
-#include <utility>
-#include <cstddef>
-#include <type_traits>
-#include <unordered_map>
-#include "../config/config.h"
-#include "registry.hpp"
-
-
-namespace entt {
-
-
-/**
- * @brief Prototype container for _concepts_.
- *
- * A prototype is used to define a _concept_ in terms of components.<br/>
- * Prototypes act as templates for those specific types of an application which
- * users would otherwise define through a series of component assignments to
- * entities. In other words, prototypes can be used to assign components to
- * entities of a registry at once.
- *
- * @note
- * Components used along with prototypes must be copy constructible. Prototypes
- * wrap component types with custom types, so they do not interfere with other
- * users of the registry they were built with.
- *
- * @warning
- * Prototypes directly use their underlying registries to store entities and
- * components for their purposes. Users must ensure that the lifetime of a
- * registry and its contents exceed that of the prototypes that use it.
- *
- * @tparam Entity A valid entity type (see entt_traits for more details).
- */
-template<typename Entity>
-class Prototype final {
-    using basic_fn_type = void(const Prototype &, Registry<Entity> &, const Entity);
-    using component_type = typename Registry<Entity>::component_type;
-
-    template<typename Component>
-    struct Wrapper { Component component; };
-
-    struct Handler {
-        basic_fn_type *accommodate;
-        basic_fn_type *assign;
-    };
-
-    void release() {
-        if(registry->valid(entity)) {
-            registry->destroy(entity);
-        }
-    }
-
-public:
-    /*! @brief Registry type. */
-    using registry_type = Registry<Entity>;
-    /*! @brief Underlying entity identifier. */
-    using entity_type = Entity;
-    /*! @brief Unsigned integer type. */
-    using size_type = std::size_t;
-
-    /**
-     * @brief Constructs a prototype that is bound to a given registry.
-     * @param registry A valid reference to a registry.
-     */
-    Prototype(Registry<Entity> &registry)
-        : registry{&registry},
-          entity{registry.create()}
-    {}
-
-    /**
-     * @brief Releases all its resources.
-     */
-    ~Prototype() {
-        release();
-    }
-
-    /*! @brief Copying a prototype isn't allowed. */
-    Prototype(const Prototype &) = delete;
-
-    /**
-     * @brief Move constructor.
-     *
-     * After prototype move construction, instances that have been moved from
-     * are placed in a valid but unspecified state. It's highly discouraged to
-     * continue using them.
-     *
-     * @param other The instance to move from.
-     */
-    Prototype(Prototype &&other)
-        : handlers{std::move(other.handlers)},
-          registry{other.registry},
-          entity{other.entity}
-    {
-        other.entity = ~entity_type{};
-    }
-
-    /*! @brief Copying a prototype isn't allowed. @return This Prototype. */
-    Prototype & operator=(const Prototype &) = delete;
-
-    /**
-     * @brief Move assignment operator.
-     *
-     * After prototype move assignment, instances that have been moved from are
-     * placed in a valid but unspecified state. It's highly discouraged to
-     * continue using them.
-     *
-     * @param other The instance to move from.
-     * @return This Prototype.
-     */
-    Prototype & operator=(Prototype &&other) {
-        if(this != &other) {
-            auto tmp{std::move(other)};
-            handlers.swap(tmp.handlers);
-            std::swap(registry, tmp.registry);
-            std::swap(entity, tmp.entity);
-        }
-
-        return *this;
-    }
-
-    /**
-     * @brief Assigns to or replaces the given component of a prototype.
-     * @tparam Component Type of component to assign or replace.
-     * @tparam Args Types of arguments to use to construct the component.
-     * @param args Parameters to use to initialize the component.
-     * @return A reference to the newly created component.
-     */
-    template<typename Component, typename... Args>
-    Component & set(Args &&... args) {
-        basic_fn_type *accommodate = [](const Prototype &prototype, Registry<Entity> &other, const Entity dst) {
-            const auto &wrapper = prototype.registry->template get<Wrapper<Component>>(prototype.entity);
-            other.template accommodate<Component>(dst, wrapper.component);
-        };
-
-        basic_fn_type *assign = [](const Prototype &prototype, Registry<Entity> &other, const Entity dst) {
-            if(!other.template has<Component>(dst)) {
-                const auto &wrapper = prototype.registry->template get<Wrapper<Component>>(prototype.entity);
-                other.template accommodate<Component>(dst, wrapper.component);
-            }
-        };
-
-        handlers[registry->template type<Component>()] = Handler{accommodate, assign};
-        auto &wrapper = registry->template accommodate<Wrapper<Component>>(entity, Component{std::forward<Args>(args)...});
-        return wrapper.component;
-    }
-
-    /**
-     * @brief Removes the given component from a prototype.
-     * @tparam Component Type of component to remove.
-     */
-    template<typename Component>
-    void unset() ENTT_NOEXCEPT {
-        registry->template reset<Wrapper<Component>>(entity);
-        handlers.erase(registry->template type<Component>());
-    }
-
-    /**
-     * @brief Checks if a prototype owns all the given components.
-     * @tparam Component Components for which to perform the check.
-     * @return True if the prototype owns all the components, false otherwise.
-     */
-    template<typename... Component>
-    bool has() const ENTT_NOEXCEPT {
-        return registry->template has<Wrapper<Component>...>(entity);
-    }
-
-    /**
-     * @brief Returns a reference to the given component.
-     *
-     * @warning
-     * Attempting to get a component from a prototype that doesn't own it
-     * results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * prototype doesn't own an instance of the given component.
-     *
-     * @tparam Component Type of component to get.
-     * @return A reference to the component owned by the prototype.
-     */
-    template<typename Component>
-    const Component & get() const ENTT_NOEXCEPT {
-        return registry->template get<Wrapper<Component>>(entity).component;
-    }
-
-    /**
-     * @brief Returns a reference to the given component.
-     *
-     * @warning
-     * Attempting to get a component from a prototype that doesn't own it
-     * results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * prototype doesn't own an instance of the given component.
-     *
-     * @tparam Component Type of component to get.
-     * @return A reference to the component owned by the prototype.
-     */
-    template<typename Component>
-    inline Component & get() ENTT_NOEXCEPT {
-        return const_cast<Component &>(const_cast<const Prototype *>(this)->get<Component>());
-    }
-
-    /**
-     * @brief Returns a reference to the given components.
-     *
-     * @warning
-     * Attempting to get components from a prototype that doesn't own them
-     * results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * prototype doesn't own instances of the given components.
-     *
-     * @tparam Component Type of components to get.
-     * @return References to the components owned by the prototype.
-     */
-    template<typename... Component>
-    inline std::enable_if_t<(sizeof...(Component) > 1), std::tuple<const Component &...>>
-    get() const ENTT_NOEXCEPT {
-        return std::tuple<const Component &...>{get<Component>()...};
-    }
-
-    /**
-     * @brief Returns a reference to the given components.
-     *
-     * @warning
-     * Attempting to get components from a prototype that doesn't own them
-     * results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * prototype doesn't own instances of the given components.
-     *
-     * @tparam Component Type of components to get.
-     * @return References to the components owned by the prototype.
-     */
-    template<typename... Component>
-    inline std::enable_if_t<(sizeof...(Component) > 1), std::tuple<Component &...>>
-    get() ENTT_NOEXCEPT {
-        return std::tuple<Component &...>{get<Component>()...};
-    }
-
-    /**
-     * @brief Creates a new entity using a given prototype.
-     *
-     * Utility shortcut, equivalent to the following snippet:
-     *
-     * @code{.cpp}
-     * const auto entity = registry.create();
-     * prototype(registry, entity);
-     * @endcode
-     *
-     * @note
-     * The registry may or may not be different from the one already used by
-     * the prototype. There is also an overload that directly uses the
-     * underlying registry.
-     *
-     * @param other A valid reference to a registry.
-     * @return A valid entity identifier.
-     */
-    entity_type create(registry_type &other) const {
-        const auto entity = other.create();
-        assign(other, entity);
-        return entity;
-    }
-
-    /**
-     * @brief Creates a new entity using a given prototype.
-     *
-     * Utility shortcut, equivalent to the following snippet:
-     *
-     * @code{.cpp}
-     * const auto entity = registry.create();
-     * prototype(entity);
-     * @endcode
-     *
-     * @note
-     * This overload directly uses the underlying registry as a working space.
-     * Therefore, the components of the prototype and of the entity will share
-     * the same registry.
-     *
-     * @return A valid entity identifier.
-     */
-    inline entity_type create() const {
-        return create(*registry);
-    }
-
-    /**
-     * @brief Assigns the components of a prototype to a given entity.
-     *
-     * Assigning a prototype to an entity won't overwrite existing components
-     * under any circumstances.<br/>
-     * In other words, only those components that the entity doesn't own yet are
-     * copied over. All the other components remain unchanged.
-     *
-     * @note
-     * The registry may or may not be different from the one already used by
-     * the prototype. There is also an overload that directly uses the
-     * underlying registry.
-     *
-     * @warning
-     * Attempting to use an invalid entity results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode in case of
-     * invalid entity.
-     *
-     * @param other A valid reference to a registry.
-     * @param dst A valid entity identifier.
-     */
-    void assign(registry_type &other, const entity_type dst) const {
-        for(auto &handler: handlers) {
-            handler.second.assign(*this, other, dst);
-        }
-    }
-
-    /**
-     * @brief Assigns the components of a prototype to a given entity.
-     *
-     * Assigning a prototype to an entity won't overwrite existing components
-     * under any circumstances.<br/>
-     * In other words, only those components that the entity doesn't own yet are
-     * copied over. All the other components remain unchanged.
-     *
-     * @note
-     * This overload directly uses the underlying registry as a working space.
-     * Therefore, the components of the prototype and of the entity will share
-     * the same registry.
-     *
-     * @warning
-     * Attempting to use an invalid entity results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode in case of
-     * invalid entity.
-     *
-     * @param dst A valid entity identifier.
-     */
-    inline void assign(const entity_type dst) const {
-        assign(*registry, dst);
-    }
-
-    /**
-     * @brief Assigns or replaces the components of a prototype for an entity.
-     *
-     * Existing components are overwritten, if any. All the other components
-     * will be copied over to the target entity.
-     *
-     * @note
-     * The registry may or may not be different from the one already used by
-     * the prototype. There is also an overload that directly uses the
-     * underlying registry.
-     *
-     * @warning
-     * Attempting to use an invalid entity results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode in case of
-     * invalid entity.
-     *
-     * @param other A valid reference to a registry.
-     * @param dst A valid entity identifier.
-     */
-    void accommodate(registry_type &other, const entity_type dst) const {
-        for(auto &handler: handlers) {
-            handler.second.accommodate(*this, other, dst);
-        }
-    }
-
-    /**
-     * @brief Assigns or replaces the components of a prototype for an entity.
-     *
-     * Existing components are overwritten, if any. All the other components
-     * will be copied over to the target entity.
-     *
-     * @note
-     * This overload directly uses the underlying registry as a working space.
-     * Therefore, the components of the prototype and of the entity will share
-     * the same registry.
-     *
-     * @warning
-     * Attempting to use an invalid entity results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode in case of
-     * invalid entity.
-     *
-     * @param dst A valid entity identifier.
-     */
-    inline void accommodate(const entity_type dst) const {
-        accommodate(*registry, dst);
-    }
-
-    /**
-     * @brief Assigns the components of a prototype to an entity.
-     *
-     * Assigning a prototype to an entity won't overwrite existing components
-     * under any circumstances.<br/>
-     * In other words, only the components that the entity doesn't own yet are
-     * copied over. All the other components remain unchanged.
-     *
-     * @note
-     * The registry may or may not be different from the one already used by
-     * the prototype. There is also an overload that directly uses the
-     * underlying registry.
-     *
-     * @warning
-     * Attempting to use an invalid entity results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode in case of
-     * invalid entity.
-     *
-     * @param other A valid reference to a registry.
-     * @param dst A valid entity identifier.
-     */
-    inline void operator()(registry_type &other, const entity_type dst) const ENTT_NOEXCEPT {
-        assign(other, dst);
-    }
-
-    /**
-     * @brief Assigns the components of a prototype to an entity.
-     *
-     * Assigning a prototype to an entity won't overwrite existing components
-     * under any circumstances.<br/>
-     * In other words, only the components that the entity doesn't own yet are
-     * copied over. All the other components remain unchanged.
-     *
-     * @note
-     * This overload directly uses the underlying registry as a working space.
-     * Therefore, the components of the prototype and of the entity will share
-     * the same registry.
-     *
-     * @warning
-     * Attempting to use an invalid entity results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode in case of
-     * invalid entity.
-     *
-     * @param dst A valid entity identifier.
-     */
-    inline void operator()(const entity_type dst) const ENTT_NOEXCEPT {
-        assign(*registry, dst);
-    }
-
-    /**
-     * @brief Creates a new entity using a given prototype.
-     *
-     * Utility shortcut, equivalent to the following snippet:
-     *
-     * @code{.cpp}
-     * const auto entity = registry.create();
-     * prototype(registry, entity);
-     * @endcode
-     *
-     * @note
-     * The registry may or may not be different from the one already used by
-     * the prototype. There is also an overload that directly uses the
-     * underlying registry.
-     *
-     * @param other A valid reference to a registry.
-     * @return A valid entity identifier.
-     */
-    inline entity_type operator()(registry_type &other) const ENTT_NOEXCEPT {
-        return create(other);
-    }
-
-    /**
-     * @brief Creates a new entity using a given prototype.
-     *
-     * Utility shortcut, equivalent to the following snippet:
-     *
-     * @code{.cpp}
-     * const auto entity = registry.create();
-     * prototype(entity);
-     * @endcode
-     *
-     * @note
-     * This overload directly uses the underlying registry as a working space.
-     * Therefore, the components of the prototype and of the entity will share
-     * the same registry.
-     *
-     * @return A valid entity identifier.
-     */
-    inline entity_type operator()() const ENTT_NOEXCEPT {
-        return create(*registry);
-    }
-
-private:
-    std::unordered_map<component_type, Handler> handlers;
-    Registry<Entity> *registry;
-    entity_type entity;
-};
-
-
-/**
- * @brief Default prototype
- *
- * The default prototype is the best choice for almost all the
- * applications.<br/>
- * Users should have a really good reason to choose something different.
- */
-using DefaultPrototype = Prototype<DefaultRegistry::entity_type>;
-
-
-}
-
-
-#endif // ENTT_ENTITY_PROTOTYPE_HPP

src/entt/entity/runtime_view.hpp

@@ -0,0 +1,257 @@
+#ifndef ENTT_ENTITY_RUNTIME_VIEW_HPP
+#define ENTT_ENTITY_RUNTIME_VIEW_HPP
+
+
+#include <iterator>
+#include <cassert>
+#include <vector>
+#include <utility>
+#include <algorithm>
+#include <type_traits>
+#include "../config/config.h"
+#include "sparse_set.hpp"
+#include "entity.hpp"
+#include "fwd.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Runtime view.
+ *
+ * Runtime views iterate over those entities that have at least all the given
+ * components in their bags. During initialization, a runtime view looks at the
+ * number of entities available for each component and picks up a reference to
+ * the smallest set of candidate entities in order to get a performance boost
+ * when iterate.<br/>
+ * Order of elements during iterations are highly dependent on the order of the
+ * underlying data structures. See sparse_set and its specializations for more
+ * details.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New instances of the given components are created and assigned to entities.
+ * * The entity currently pointed is modified (as an example, if one of the
+ *   given components is removed from the entity to which the iterator points).
+ * * The entity currently pointed is destroyed.
+ *
+ * In all the other cases, modifying the pools of the given components in any
+ * way invalidates all the iterators and using them results in undefined
+ * behavior.
+ *
+ * @note
+ * Views share references to the underlying data structures of the registry that
+ * generated them. Therefore any change to the entities and to the components
+ * made by means of the registry are immediately reflected by the views, unless
+ * a pool was missing when the view was built (in this case, the view won't
+ * have a valid reference and won't be updated accordingly).
+ *
+ * @warning
+ * Lifetime of a view must overcome the one of the registry that generated it.
+ * In any other case, attempting to use a view results in undefined behavior.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+class basic_runtime_view {
+    /*! @brief A registry is allowed to create views. */
+    friend class basic_registry<Entity>;
+
+    using underlying_iterator_type = typename sparse_set<Entity>::iterator_type;
+    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
+
+    class iterator {
+        friend class basic_runtime_view<Entity>;
+
+        iterator(underlying_iterator_type first, underlying_iterator_type last, const sparse_set<Entity> * const *others, const sparse_set<Entity> * const *length) ENTT_NOEXCEPT
+            : begin{first},
+              end{last},
+              from{others},
+              to{length}
+        {
+            if(begin != end && !valid()) {
+                ++(*this);
+            }
+        }
+
+        bool valid() const ENTT_NOEXCEPT {
+            return std::all_of(from, to, [entt = *begin](const auto *view) {
+                return view->has(entt);
+            });
+        }
+
+    public:
+        using difference_type = typename underlying_iterator_type::difference_type;
+        using value_type = typename underlying_iterator_type::value_type;
+        using pointer = typename underlying_iterator_type::pointer;
+        using reference = typename underlying_iterator_type::reference;
+        using iterator_category = std::forward_iterator_tag;
+
+        iterator() ENTT_NOEXCEPT = default;
+
+        iterator & operator++() ENTT_NOEXCEPT {
+            return (++begin != end && !valid()) ? ++(*this) : *this;
+        }
+
+        iterator operator++(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        bool operator==(const iterator &other) const ENTT_NOEXCEPT {
+            return other.begin == begin;
+        }
+
+        bool operator!=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+        pointer operator->() const ENTT_NOEXCEPT {
+            return begin.operator->();
+        }
+
+        reference operator*() const ENTT_NOEXCEPT {
+            return *operator->();
+        }
+
+    private:
+        underlying_iterator_type begin;
+        underlying_iterator_type end;
+        const sparse_set<Entity> * const *from;
+        const sparse_set<Entity> * const *to;
+    };
+
+    basic_runtime_view(std::vector<const sparse_set<Entity> *> others) ENTT_NOEXCEPT
+        : pools{std::move(others)}
+    {
+        const auto it = std::min_element(pools.begin(), pools.end(), [](const auto *lhs, const auto *rhs) {
+            return (!lhs && rhs) || (lhs && rhs && lhs->size() < rhs->size());
+        });
+
+        // brings the best candidate (if any) on front of the vector
+        std::rotate(pools.begin(), it, pools.end());
+    }
+
+    bool valid() const ENTT_NOEXCEPT {
+        return !pools.empty() && pools.front();
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Input iterator type. */
+    using iterator_type = iterator;
+
+    /**
+     * @brief Estimates the number of entities that have the given components.
+     * @return Estimated number of entities that have the given components.
+     */
+    size_type size() const ENTT_NOEXCEPT {
+        return valid() ? pools.front()->size() : size_type{};
+    }
+
+    /**
+     * @brief Checks if the view is definitely empty.
+     * @return True if the view is definitely empty, false otherwise.
+     */
+    bool empty() const ENTT_NOEXCEPT {
+        return !valid() || pools.front()->empty();
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity that has the given
+     * components.
+     *
+     * The returned iterator points to the first entity that has the given
+     * components. If the view is empty, the returned iterator will be equal to
+     * `end()`.
+     *
+     * @note
+     * Input iterators stay true to the order imposed to the underlying data
+     * structures.
+     *
+     * @return An iterator to the first entity that has the given components.
+     */
+    iterator_type begin() const ENTT_NOEXCEPT {
+        iterator_type it{};
+
+        if(valid()) {
+            const auto &pool = *pools.front();
+            const auto * const *data = pools.data();
+            it = { pool.begin(), pool.end(), data + 1, data + pools.size() };
+        }
+
+        return it;
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity that has the
+     * given components.
+     *
+     * The returned iterator points to the entity following the last entity that
+     * has the given components. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @note
+     * Input iterators stay true to the order imposed to the underlying data
+     * structures.
+     *
+     * @return An iterator to the entity following the last entity that has the
+     * given components.
+     */
+    iterator_type end() const ENTT_NOEXCEPT {
+        iterator_type it{};
+
+        if(valid()) {
+            const auto &pool = *pools.front();
+            it = { pool.end(), pool.end(), nullptr, nullptr };
+        }
+
+        return it;
+    }
+
+    /**
+     * @brief Checks if a view contains an entity.
+     * @param entt A valid entity identifier.
+     * @return True if the view contains the given entity, false otherwise.
+     */
+    bool contains(const entity_type entt) const ENTT_NOEXCEPT {
+        return valid() && std::all_of(pools.cbegin(), pools.cend(), [entt](const auto *view) {
+            return view->find(entt) != view->end();
+        });
+    }
+
+    /**
+     * @brief Iterates entities and applies the given function object to them.
+     *
+     * The function object is invoked for each entity. It is provided only with
+     * the entity itself. To get the components, users can use the registry with
+     * which the view was built.<br/>
+     * The signature of the function should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * void(const entity_type);
+     * @endcode
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        std::for_each(begin(), end(), func);
+    }
+
+private:
+    std::vector<const sparse_set<Entity> *> pools;
+};
+
+
+}
+
+
+#endif // ENTT_ENTITY_RUNTIME_VIEW_HPP

src/entt/entity/snapshot.hpp

@@ -5,57 +5,54 @@
 #include <array>
 #include <cstddef>
 #include <utility>
-#include <cassert>
 #include <iterator>
 #include <type_traits>
 #include <unordered_map>
 #include "../config/config.h"
-#include "entt_traits.hpp"
-#include "utility.hpp"
+#include "entity.hpp"
+#include "fwd.hpp"
 
 
 namespace entt {
 
 
-/**
- * @brief Forward declaration of the registry class.
- */
-template<typename>
-class Registry;
-
-
 /**
  * @brief Utility class to create snapshots from a registry.
  *
  * A _snapshot_ can be either a dump of the entire registry or a narrower
- * selection of components and tags of interest.<br/>
+ * 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 Snapshot final {
+class basic_snapshot {
     /*! @brief A registry is allowed to create snapshots. */
-    friend class Registry<Entity>;
+    friend class basic_registry<Entity>;
 
-    using follow_fn_type = Entity(const Registry<Entity> &, const Entity);
+    using follow_fn_type = Entity(const basic_registry<Entity> &, const Entity);
+    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
 
-    Snapshot(const Registry<Entity> &registry, Entity seed, follow_fn_type *follow) ENTT_NOEXCEPT
-        : registry{registry},
-          seed{seed},
-          follow{follow}
+    basic_snapshot(const basic_registry<Entity> *source, Entity init, follow_fn_type *fn) ENTT_NOEXCEPT
+        : reg{source},
+          seed{init},
+          follow{fn}
     {}
 
     template<typename Component, typename Archive, typename It>
     void get(Archive &archive, std::size_t sz, It first, It last) const {
-        archive(static_cast<Entity>(sz));
+        archive(typename traits_type::entity_type(sz));
 
         while(first != last) {
-            const auto entity = *(first++);
+            const auto entt = *(first++);
 
-            if(registry.template has<Component>(entity)) {
-                archive(entity, registry.template get<Component>(entity));
+            if(reg->template has<Component>(entt)) {
+                if constexpr(std::is_empty_v<Component>) {
+                    archive(entt);
+                } else {
+                    archive(entt, reg->template get<Component>(entt));
+                }
             }
         }
     }
@@ -66,27 +63,19 @@ class Snapshot final {
         auto begin = first;
 
         while(begin != last) {
-            const auto entity = *(begin++);
-            using accumulator_type = std::size_t[];
-            accumulator_type accumulator = { (registry.template has<Component>(entity) ? ++size[Indexes] : size[Indexes])... };
-            (void)accumulator;
+            const auto entt = *(begin++);
+            ((reg->template has<Component>(entt) ? ++size[Indexes] : size[Indexes]), ...);
         }
 
-        using accumulator_type = int[];
-        accumulator_type accumulator = { (get<Component>(archive, size[Indexes], first, last), 0)... };
-        (void)accumulator;
+        (get<Component>(archive, size[Indexes], first, last), ...);
     }
 
 public:
-    /*! @brief Copying a snapshot isn't allowed. */
-    Snapshot(const Snapshot &) = delete;
     /*! @brief Default move constructor. */
-    Snapshot(Snapshot &&) = default;
+    basic_snapshot(basic_snapshot &&) = default;
 
-    /*! @brief Copying a snapshot isn't allowed. @return This snapshot. */
-    Snapshot & operator=(const Snapshot &) = delete;
     /*! @brief Default move assignment operator. @return This snapshot. */
-    Snapshot & operator=(Snapshot &&) = default;
+    basic_snapshot & operator=(basic_snapshot &&) = default;
 
     /**
      * @brief Puts aside all the entities that are still in use.
@@ -99,9 +88,9 @@ public:
      * @return An object of this type to continue creating the snapshot.
      */
     template<typename Archive>
-    const Snapshot & entities(Archive &archive) const {
-        archive(static_cast<Entity>(registry.alive()));
-        registry.each([&archive](const auto entity) { archive(entity); });
+    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;
     }
 
@@ -116,45 +105,23 @@ public:
      * @return An object of this type to continue creating the snapshot.
      */
     template<typename Archive>
-    const Snapshot & destroyed(Archive &archive) const {
-        auto size = registry.size() - registry.alive();
-        archive(static_cast<Entity>(size));
-        auto curr = seed;
+    const basic_snapshot & destroyed(Archive &archive) const {
+        auto size = reg->size() - reg->alive();
+        archive(typename traits_type::entity_type(size));
 
-        for(; size; --size) {
+        if(size) {
+            auto curr = seed;
             archive(curr);
-            curr = follow(registry, curr);
+
+            for(--size; size; --size) {
+                curr = follow(*reg, curr);
+                archive(curr);
+            }
         }
 
         return *this;
     }
 
-    /**
-     * @brief Puts aside the given component.
-     *
-     * Each instance is serialized together with the entity to which it belongs.
-     * Entities are serialized along with their versions.
-     *
-     * @tparam Component Type of component to serialize.
-     * @tparam Archive Type of output archive.
-     * @param archive A valid reference to an output archive.
-     * @return An object of this type to continue creating the snapshot.
-     */
-    template<typename Component, typename Archive>
-    const Snapshot & component(Archive &archive) const {
-        const auto sz = registry.template size<Component>();
-        const auto *entities = registry.template data<Component>();
-
-        archive(static_cast<Entity>(sz));
-
-        for(std::remove_const_t<decltype(sz)> i{}; i < sz; ++i) {
-            const auto entity = entities[i];
-            archive(entity, registry.template get<Component>(entity));
-        };
-
-        return *this;
-    }
-
     /**
      * @brief Puts aside the given components.
      *
@@ -167,11 +134,26 @@ public:
      * @return An object of this type to continue creating the snapshot.
      */
     template<typename... Component, typename Archive>
-    std::enable_if_t<(sizeof...(Component) > 1), const Snapshot &>
-    component(Archive &archive) const {
-        using accumulator_type = int[];
-        accumulator_type accumulator = { 0, (component<Component>(archive), 0)... };
-        (void)accumulator;
+    const basic_snapshot & component(Archive &archive) const {
+        if constexpr(sizeof...(Component) == 1) {
+            const auto sz = reg->template size<Component...>();
+            const auto *entities = reg->template data<Component...>();
+
+            archive(typename traits_type::entity_type(sz));
+
+            for(std::remove_const_t<decltype(sz)> pos{}; pos < sz; ++pos) {
+                const auto entt = entities[pos];
+
+                if constexpr(std::is_empty_v<Component...>) {
+                    archive(entt);
+                } else {
+                    archive(entt, reg->template get<Component...>(entt));
+                }
+            };
+        } else {
+            (component<Component>(archive), ...);
+        }
+
         return *this;
     }
 
@@ -190,58 +172,13 @@ public:
      * @return An object of this type to continue creating the snapshot.
      */
     template<typename... Component, typename Archive, typename It>
-    const Snapshot & component(Archive &archive, It first, It last) const {
+    const basic_snapshot & component(Archive &archive, It first, It last) const {
         component<Component...>(archive, first, last, std::make_index_sequence<sizeof...(Component)>{});
         return *this;
     }
 
-    /**
-     * @brief Puts aside the given tag.
-     *
-     * Each instance is serialized together with the entity to which it belongs.
-     * Entities are serialized along with their versions.
-     *
-     * @tparam Tag Type of tag to serialize.
-     * @tparam Archive Type of output archive.
-     * @param archive A valid reference to an output archive.
-     * @return An object of this type to continue creating the snapshot.
-     */
-    template<typename Tag, typename Archive>
-    const Snapshot & tag(Archive &archive) const {
-        const bool has = registry.template has<Tag>();
-
-        // numerical length is forced for tags to facilitate loading
-        archive(has ? Entity(1): Entity{});
-
-        if(has) {
-            archive(registry.template attachee<Tag>(), registry.template get<Tag>());
-        }
-
-        return *this;
-    }
-
-    /**
-     * @brief Puts aside the given tags.
-     *
-     * Each instance is serialized together with the entity to which it belongs.
-     * Entities are serialized along with their versions.
-     *
-     * @tparam Tag Types of tags to serialize.
-     * @tparam Archive Type of output archive.
-     * @param archive A valid reference to an output archive.
-     * @return An object of this type to continue creating the snapshot.
-     */
-    template<typename... Tag, typename Archive>
-    std::enable_if_t<(sizeof...(Tag) > 1), const Snapshot &>
-    tag(Archive &archive) const {
-        using accumulator_type = int[];
-        accumulator_type accumulator = { 0, (tag<Tag>(archive), 0)... };
-        (void)accumulator;
-        return *this;
-    }
-
 private:
-    const Registry<Entity> &registry;
+    const basic_registry<Entity> *reg;
     const Entity seed;
     follow_fn_type *follow;
 };
@@ -258,57 +195,61 @@ private:
  * @tparam Entity A valid entity type (see entt_traits for more details).
  */
 template<typename Entity>
-class SnapshotLoader final {
+class basic_snapshot_loader {
     /*! @brief A registry is allowed to create snapshot loaders. */
-    friend class Registry<Entity>;
+    friend class basic_registry<Entity>;
 
-    using assure_fn_type = void(Registry<Entity> &, const Entity, const bool);
+    using force_fn_type = void(basic_registry<Entity> &, const Entity, const bool);
+    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
 
-    SnapshotLoader(Registry<Entity> &registry, assure_fn_type *assure_fn) ENTT_NOEXCEPT
-        : registry{registry},
-          assure_fn{assure_fn}
+    basic_snapshot_loader(basic_registry<Entity> *source, force_fn_type *fn) ENTT_NOEXCEPT
+        : reg{source},
+          force{fn}
     {
-        // restore a snapshot as a whole requires a clean registry
-        assert(!registry.capacity());
+        // to restore a snapshot as a whole requires a clean registry
+        ENTT_ASSERT(reg->empty());
     }
 
     template<typename Archive>
-    void assure(Archive &archive, bool destroyed) const {
-        Entity length{};
+    void assure(Archive &archive, bool discard) const {
+        typename traits_type::entity_type length{};
         archive(length);
 
         while(length--) {
-            Entity entity{};
-            archive(entity);
-            assure_fn(registry, entity, destroyed);
+            Entity entt{};
+            archive(entt);
+            force(*reg, entt, discard);
         }
     }
 
     template<typename Type, typename Archive, typename... Args>
     void assign(Archive &archive, Args... args) const {
-        Entity length{};
+        typename traits_type::entity_type length{};
         archive(length);
 
         while(length--) {
-            Entity entity{};
-            Type instance{};
-            archive(entity, instance);
-            static constexpr auto destroyed = false;
-            assure_fn(registry, entity, destroyed);
-            registry.template assign<Type>(args..., entity, static_cast<const Type &>(instance));
+            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 Copying a snapshot loader isn't allowed. */
-    SnapshotLoader(const SnapshotLoader &) = delete;
     /*! @brief Default move constructor. */
-    SnapshotLoader(SnapshotLoader &&) = default;
+    basic_snapshot_loader(basic_snapshot_loader &&) = default;
 
-    /*! @brief Copying a snapshot loader isn't allowed. @return This loader. */
-    SnapshotLoader & operator=(const SnapshotLoader &) = delete;
     /*! @brief Default move assignment operator. @return This loader. */
-    SnapshotLoader & operator=(SnapshotLoader &&) = default;
+    basic_snapshot_loader & operator=(basic_snapshot_loader &&) = default;
 
     /**
      * @brief Restores entities that were in use during serialization.
@@ -321,9 +262,9 @@ public:
      * @return A valid loader to continue restoring data.
      */
     template<typename Archive>
-    const SnapshotLoader & entities(Archive &archive) const {
-        static constexpr auto destroyed = false;
-        assure(archive, destroyed);
+    const basic_snapshot_loader & entities(Archive &archive) const {
+        static constexpr auto discard = false;
+        assure(archive, discard);
         return *this;
     }
 
@@ -338,9 +279,9 @@ public:
      * @return A valid loader to continue restoring data.
      */
     template<typename Archive>
-    const SnapshotLoader & destroyed(Archive &archive) const {
-        static constexpr auto destroyed = true;
-        assure(archive, destroyed);
+    const basic_snapshot_loader & destroyed(Archive &archive) const {
+        static constexpr auto discard = true;
+        assure(archive, discard);
         return *this;
     }
 
@@ -358,55 +299,32 @@ public:
      * @return A valid loader to continue restoring data.
      */
     template<typename... Component, typename Archive>
-    const SnapshotLoader & component(Archive &archive) const {
-        using accumulator_type = int[];
-        accumulator_type accumulator = { 0, (assign<Component>(archive), 0)... };
-        (void)accumulator;
+    const basic_snapshot_loader & component(Archive &archive) const {
+        (assign<Component>(archive), ...);
         return *this;
     }
 
     /**
-     * @brief Restores tags and assigns them to the right entities.
-     *
-     * The template parameter list must be exactly the same used during
-     * serialization. In the event that the entity to which the tag is assigned
-     * doesn't exist yet, the loader will take care to create it with the
-     * version it originally had.
-     *
-     * @tparam Tag Types of tags to restore.
-     * @tparam Archive Type of input archive.
-     * @param archive A valid reference to an input archive.
-     * @return A valid loader to continue restoring data.
-     */
-    template<typename... Tag, typename Archive>
-    const SnapshotLoader & tag(Archive &archive) const {
-        using accumulator_type = int[];
-        accumulator_type accumulator = { 0, (assign<Tag>(archive, tag_t{}), 0)... };
-        (void)accumulator;
-        return *this;
-    }
-
-    /**
-     * @brief Destroys those entities that have neither components nor tags.
+     * @brief Destroys those entities that have no components.
      *
      * In case all the entities were serialized but only part of the components
-     * and tags was saved, it could happen that some of the entities have
-     * neither components nor tags once restored.<br/>
+     * 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 SnapshotLoader & orphans() const {
-        registry.orphans([this](const auto entity) {
-            registry.destroy(entity);
+    const basic_snapshot_loader & orphans() const {
+        reg->orphans([this](const auto entt) {
+            reg->destroy(entt);
         });
 
         return *this;
     }
 
 private:
-    Registry<Entity> &registry;
-    assure_fn_type *assure_fn;
+    basic_registry<Entity> *reg;
+    force_fn_type *force;
 };
 
 
@@ -427,63 +345,55 @@ private:
  * @tparam Entity A valid entity type (see entt_traits for more details).
  */
 template<typename Entity>
-class ContinuousLoader final {
-    using traits_type = entt_traits<Entity>;
+class basic_continuous_loader {
+    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
 
-    void destroy(Entity entity) {
-        const auto it = remloc.find(entity);
+    void destroy(Entity entt) {
+        const auto it = remloc.find(entt);
 
         if(it == remloc.cend()) {
-            const auto local = registry.create();
-            remloc.emplace(entity, std::make_pair(local, true));
-            registry.destroy(local);
+            const auto local = reg->create();
+            remloc.emplace(entt, std::make_pair(local, true));
+            reg->destroy(local);
         }
     }
 
-    void restore(Entity entity) {
-        const auto it = remloc.find(entity);
+    void restore(Entity entt) {
+        const auto it = remloc.find(entt);
 
         if(it == remloc.cend()) {
-            const auto local = registry.create();
-            remloc.emplace(entity, std::make_pair(local, true));
+            const auto local = reg->create();
+            remloc.emplace(entt, std::make_pair(local, true));
         } else {
-            remloc[entity].first =
-                    registry.valid(remloc[entity].first)
-                    ? remloc[entity].first
-                    : registry.create();
-
+            remloc[entt].first = reg->valid(remloc[entt].first) ? remloc[entt].first : reg->create();
             // set the dirty flag
-            remloc[entity].second = true;
-        }
-    }
-
-    template<typename Type, typename Member>
-    std::enable_if_t<std::is_same<Member, Entity>::value>
-    update(Type &instance, Member Type:: *member) {
-        instance.*member = map(instance.*member);
-    }
-
-    template<typename Type, typename Member>
-    std::enable_if_t<std::is_same<typename std::iterator_traits<typename Member::iterator>::value_type, Entity>::value>
-    update(Type &instance, Member Type:: *member) {
-        for(auto &entity: instance.*member) {
-            entity = map(entity);
+            remloc[entt].second = true;
         }
     }
 
     template<typename Other, typename Type, typename Member>
-    std::enable_if_t<!std::is_same<Other, Type>::value>
-    update(Other &, Member Type:: *) {}
+    void update(Other &instance, Member Type:: *member) {
+        if constexpr(!std::is_same_v<Other, Type>) {
+            return;
+        } else if constexpr(std::is_same_v<Member, Entity>) {
+            instance.*member = map(instance.*member);
+        } else {
+            // maybe a container? let's try...
+            for(auto &entt: instance.*member) {
+                entt = map(entt);
+            }
+        }
+    }
 
     template<typename Archive>
-    void assure(Archive &archive, void(ContinuousLoader:: *member)(Entity)) {
-        Entity length{};
+    void assure(Archive &archive, void(basic_continuous_loader:: *member)(Entity)) {
+        typename traits_type::entity_type length{};
         archive(length);
 
         while(length--) {
-            Entity entity{};
-            archive(entity);
-            (this->*member)(entity);
+            Entity entt{};
+            archive(entt);
+            (this->*member)(entt);
         }
     }
 
@@ -492,29 +402,31 @@ class ContinuousLoader final {
         for(auto &&ref: remloc) {
             const auto local = ref.second.first;
 
-            if(registry.valid(local)) {
-                registry.template reset<Component>(local);
+            if(reg->valid(local)) {
+                reg->template reset<Component>(local);
             }
         }
     }
 
-    template<typename Other, typename Archive, typename Func, typename... Type, typename... Member>
-    void assign(Archive &archive, Func func, Member Type:: *... member) {
-        Entity length{};
+    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 entity{};
-            Other instance{};
+            Entity entt{};
 
-            archive(entity, instance);
-            restore(entity);
-
-            using accumulator_type = int[];
-            accumulator_type accumulator = { 0, (update(instance, member), 0)... };
-            (void)accumulator;
-
-            func(map(entity), instance);
+            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));
+            }
         }
     }
 
@@ -524,21 +436,17 @@ public:
 
     /**
      * @brief Constructs a loader that is bound to a given registry.
-     * @param registry A valid reference to a registry.
+     * @param source A valid reference to a registry.
      */
-    ContinuousLoader(Registry<entity_type> &registry) ENTT_NOEXCEPT
-        : registry{registry}
+    basic_continuous_loader(basic_registry<entity_type> &source) ENTT_NOEXCEPT
+        : reg{&source}
     {}
 
-    /*! @brief Copying a snapshot loader isn't allowed. */
-    ContinuousLoader(const ContinuousLoader &) = delete;
     /*! @brief Default move constructor. */
-    ContinuousLoader(ContinuousLoader &&) = default;
+    basic_continuous_loader(basic_continuous_loader &&) = default;
 
-    /*! @brief Copying a snapshot loader isn't allowed. @return This loader. */
-    ContinuousLoader & operator=(const ContinuousLoader &) = delete;
     /*! @brief Default move assignment operator. @return This loader. */
-    ContinuousLoader & operator=(ContinuousLoader &&) = default;
+    basic_continuous_loader & operator=(basic_continuous_loader &&) = default;
 
     /**
      * @brief Restores entities that were in use during serialization.
@@ -551,8 +459,8 @@ public:
      * @return A non-const reference to this loader.
      */
     template<typename Archive>
-    ContinuousLoader & entities(Archive &archive) {
-        assure(archive, &ContinuousLoader::restore);
+    basic_continuous_loader & entities(Archive &archive) {
+        assure(archive, &basic_continuous_loader::restore);
         return *this;
     }
 
@@ -567,8 +475,8 @@ public:
      * @return A non-const reference to this loader.
      */
     template<typename Archive>
-    ContinuousLoader & destroyed(Archive &archive) {
-        assure(archive, &ContinuousLoader::destroy);
+    basic_continuous_loader & destroyed(Archive &archive) {
+        assure(archive, &basic_continuous_loader::destroy);
         return *this;
     }
 
@@ -592,45 +500,9 @@ public:
      * @return A non-const reference to this loader.
      */
     template<typename... Component, typename Archive, typename... Type, typename... Member>
-    ContinuousLoader & component(Archive &archive, Member Type:: *... member) {
-        auto apply = [this](const auto entity, const auto &component) {
-            registry.template accommodate<std::decay_t<decltype(component)>>(entity, component);
-        };
-
-        using accumulator_type = int[];
-        accumulator_type accumulator = { 0, (reset<Component>(), assign<Component>(archive, apply, member...), 0)... };
-        (void)accumulator;
-        return *this;
-    }
-
-    /**
-     * @brief Restores tags and assigns them to the right entities.
-     *
-     * The template parameter list must be exactly the same used during
-     * serialization. In the event that the entity to which the tag is assigned
-     * doesn't exist yet, the loader will take care to create a local
-     * counterpart for it.<br/>
-     * Members can be either data members of type entity_type or containers of
-     * entities. In both cases, the loader will visit them and update the
-     * entities by replacing each one with its local counterpart.
-     *
-     * @tparam Tag Type of tag to restore.
-     * @tparam Archive Type of input archive.
-     * @tparam Type Types of components to update with local counterparts.
-     * @tparam Member Types of members to update with their local counterparts.
-     * @param archive A valid reference to an input archive.
-     * @param member Members to update with their local counterparts.
-     * @return A non-const reference to this loader.
-     */
-    template<typename... Tag, typename Archive, typename... Type, typename... Member>
-    ContinuousLoader & tag(Archive &archive, Member Type:: *... member) {
-        auto apply = [this](const auto entity, const auto &tag) {
-            registry.template assign<std::decay_t<decltype(tag)>>(tag_t{}, entity, tag);
-        };
-
-        using accumulator_type = int[];
-        accumulator_type accumulator = { 0, (registry.template remove<Tag>(), assign<Tag>(archive, apply, member...), 0)... };
-        (void)accumulator;
+    basic_continuous_loader & component(Archive &archive, Member Type:: *... member) {
+        (reset<Component>(), ...);
+        (assign<Component>(archive, member...), ...);
         return *this;
     }
 
@@ -642,7 +514,7 @@ public:
      *
      * @return A non-const reference to this loader.
      */
-    ContinuousLoader & shrink() {
+    basic_continuous_loader & shrink() {
         auto it = remloc.begin();
 
         while(it != remloc.cend()) {
@@ -653,8 +525,8 @@ public:
                 dirty = false;
                 ++it;
             } else {
-                if(registry.valid(local)) {
-                    registry.destroy(local);
+                if(reg->valid(local)) {
+                    reg->destroy(local);
                 }
 
                 it = remloc.erase(it);
@@ -665,18 +537,18 @@ public:
     }
 
     /**
-     * @brief Destroys those entities that have neither components nor tags.
+     * @brief Destroys those entities that have no components.
      *
      * In case all the entities were serialized but only part of the components
-     * and tags was saved, it could happen that some of the entities have
-     * neither components nor tags once restored.<br/>
+     * 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.
      */
-    ContinuousLoader & orphans() {
-        registry.orphans([this](const auto entity) {
-            registry.destroy(entity);
+    basic_continuous_loader & orphans() {
+        reg->orphans([this](const auto entt) {
+            reg->destroy(entt);
         });
 
         return *this;
@@ -684,33 +556,32 @@ public:
 
     /**
      * @brief Tests if a loader knows about a given entity.
-     * @param entity An entity identifier.
+     * @param entt An entity identifier.
      * @return True if `entity` is managed by the loader, false otherwise.
      */
-    bool has(entity_type entity) const ENTT_NOEXCEPT {
-        return (remloc.find(entity) != remloc.cend());
+    bool has(entity_type entt) const ENTT_NOEXCEPT {
+        return (remloc.find(entt) != remloc.cend());
     }
 
     /**
      * @brief Returns the identifier to which an entity refers.
-     *
-     * @warning
-     * Attempting to use an entity that isn't managed by the loader results in
-     * undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * loader doesn't knows about the entity.
-     *
-     * @param entity An entity identifier.
-     * @return The identifier to which `entity` refers in the target registry.
+     * @param entt An entity identifier.
+     * @return The local identifier if any, the null entity otherwise.
      */
-    entity_type map(entity_type entity) const ENTT_NOEXCEPT {
-        assert(has(entity));
-        return remloc.find(entity)->second.first;
+    entity_type map(entity_type entt) const ENTT_NOEXCEPT {
+        const auto it = remloc.find(entt);
+        entity_type other = null;
+
+        if(it != remloc.cend()) {
+            other = it->second.first;
+        }
+
+        return other;
     }
 
 private:
-    std::unordered_map<Entity, std::pair<Entity, bool>> remloc;
-    Registry<Entity> &registry;
+    std::unordered_map<entity_type, std::pair<entity_type, bool>> remloc;
+    basic_registry<entity_type> *reg;
 };
 
 

src/entt/entity/storage.hpp

@@ -0,0 +1,704 @@
+#ifndef ENTT_ENTITY_STORAGE_HPP
+#define ENTT_ENTITY_STORAGE_HPP
+
+
+#include <algorithm>
+#include <iterator>
+#include <numeric>
+#include <utility>
+#include <vector>
+#include <cstddef>
+#include <type_traits>
+#include "../config/config.h"
+#include "../core/algorithm.hpp"
+#include "sparse_set.hpp"
+#include "entity.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Basic storage implementation.
+ *
+ * This class is a refinement of a sparse set that associates an object to an
+ * entity. The main purpose of this class is to extend sparse sets to store
+ * components in a registry. It guarantees fast access both to the elements and
+ * to the entities.
+ *
+ * @note
+ * Entities and objects have the same order. It's guaranteed both in case of raw
+ * access (either to entities or objects) and when using random or input access
+ * iterators.
+ *
+ * @note
+ * Internal data structures arrange elements to maximize performance. Because of
+ * that, there are no guarantees that elements have the expected order when
+ * iterate directly the internal packed array (see `raw` and `size` member
+ * functions for that). Use `begin` and `end` instead.
+ *
+ * @warning
+ * Empty types aren't explicitly instantiated. Temporary objects are returned in
+ * place of the instances of the components and raw access isn't available for
+ * them.
+ *
+ * @sa sparse_set<Entity>
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Type Type of objects assigned to the entities.
+ */
+template<typename Entity, typename Type, typename = std::void_t<>>
+class basic_storage: public sparse_set<Entity> {
+    using underlying_type = sparse_set<Entity>;
+    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
+
+    template<bool Const>
+    class iterator {
+        friend class basic_storage<Entity, Type>;
+
+        using instance_type = std::conditional_t<Const, const std::vector<Type>, std::vector<Type>>;
+        using index_type = typename traits_type::difference_type;
+
+        iterator(instance_type *ref, const index_type idx) ENTT_NOEXCEPT
+            : instances{ref}, index{idx}
+        {}
+
+    public:
+        using difference_type = index_type;
+        using value_type = Type;
+        using pointer = std::conditional_t<Const, const value_type *, value_type *>;
+        using reference = std::conditional_t<Const, const value_type &, value_type &>;
+        using iterator_category = std::random_access_iterator_tag;
+
+        iterator() ENTT_NOEXCEPT = default;
+
+        iterator & operator++() ENTT_NOEXCEPT {
+            return --index, *this;
+        }
+
+        iterator operator++(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        iterator & operator--() ENTT_NOEXCEPT {
+            return ++index, *this;
+        }
+
+        iterator operator--(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return --(*this), orig;
+        }
+
+        iterator & operator+=(const difference_type value) ENTT_NOEXCEPT {
+            index -= value;
+            return *this;
+        }
+
+        iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
+            return iterator{instances, index-value};
+        }
+
+        iterator & operator-=(const difference_type value) ENTT_NOEXCEPT {
+            return (*this += -value);
+        }
+
+        iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
+            return (*this + -value);
+        }
+
+        difference_type operator-(const iterator &other) const ENTT_NOEXCEPT {
+            return other.index - index;
+        }
+
+        reference operator[](const difference_type value) const ENTT_NOEXCEPT {
+            const auto pos = size_type(index-value-1);
+            return (*instances)[pos];
+        }
+
+        bool operator==(const iterator &other) const ENTT_NOEXCEPT {
+            return other.index == index;
+        }
+
+        bool operator!=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+        bool operator<(const iterator &other) const ENTT_NOEXCEPT {
+            return index > other.index;
+        }
+
+        bool operator>(const iterator &other) const ENTT_NOEXCEPT {
+            return index < other.index;
+        }
+
+        bool operator<=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this > other);
+        }
+
+        bool operator>=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this < other);
+        }
+
+        pointer operator->() const ENTT_NOEXCEPT {
+            const auto pos = size_type(index-1);
+            return &(*instances)[pos];
+        }
+
+        reference operator*() const ENTT_NOEXCEPT {
+            return *operator->();
+        }
+
+    private:
+        instance_type *instances;
+        index_type index;
+    };
+
+public:
+    /*! @brief Type of the objects associated with the entities. */
+    using object_type = Type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Random access iterator type. */
+    using iterator_type = iterator<false>;
+    /*! @brief Constant random access iterator type. */
+    using const_iterator_type = iterator<true>;
+
+    /**
+     * @brief Increases the capacity of a storage.
+     *
+     * If the new capacity is greater than the current capacity, new storage is
+     * allocated, otherwise the method does nothing.
+     *
+     * @param cap Desired capacity.
+     */
+    void reserve(const size_type cap) {
+        underlying_type::reserve(cap);
+        instances.reserve(cap);
+    }
+
+    /*! @brief Requests the removal of unused capacity. */
+    void shrink_to_fit() {
+        underlying_type::shrink_to_fit();
+        instances.shrink_to_fit();
+    }
+
+    /**
+     * @brief Direct access to the array of objects.
+     *
+     * The returned pointer is such that range `[raw(), raw() + size()]` is
+     * always a valid range, even if the container is empty.
+     *
+     * @note
+     * There are no guarantees on the order, even though either `sort` or
+     * `respect` has been previously invoked. Internal data structures arrange
+     * elements to maximize performance. Accessing them directly gives a
+     * performance boost but less guarantees. Use `begin` and `end` if you want
+     * to iterate the storage in the expected order.
+     *
+     * @return A pointer to the array of objects.
+     */
+    const object_type * raw() const ENTT_NOEXCEPT {
+        return instances.data();
+    }
+
+    /*! @copydoc raw */
+    object_type * raw() ENTT_NOEXCEPT {
+        return const_cast<object_type *>(std::as_const(*this).raw());
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * The returned iterator points to the first instance of the given type. If
+     * the storage is empty, the returned iterator will be equal to `end()`.
+     *
+     * @note
+     * Random access iterators stay true to the order imposed by a call to
+     * either `sort` or `respect`.
+     *
+     * @return An iterator to the first instance of the given type.
+     */
+    const_iterator_type cbegin() const ENTT_NOEXCEPT {
+        const typename traits_type::difference_type pos = underlying_type::size();
+        return const_iterator_type{&instances, pos};
+    }
+
+    /*! @copydoc cbegin */
+    const_iterator_type begin() const ENTT_NOEXCEPT {
+        return cbegin();
+    }
+
+    /*! @copydoc begin */
+    iterator_type begin() ENTT_NOEXCEPT {
+        const typename traits_type::difference_type pos = underlying_type::size();
+        return iterator_type{&instances, pos};
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     *
+     * The returned iterator points to the element following the last instance
+     * of the given type. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @note
+     * Random access iterators stay true to the order imposed by a call to
+     * either `sort` or `respect`.
+     *
+     * @return An iterator to the element following the last instance of the
+     * given type.
+     */
+    const_iterator_type cend() const ENTT_NOEXCEPT {
+        return const_iterator_type{&instances, {}};
+    }
+
+    /*! @copydoc cend */
+    const_iterator_type end() const ENTT_NOEXCEPT {
+        return cend();
+    }
+
+    /*! @copydoc end */
+    iterator_type end() ENTT_NOEXCEPT {
+        return iterator_type{&instances, {}};
+    }
+
+    /**
+     * @brief Returns the object associated with an entity.
+     *
+     * @warning
+     * Attempting to use an entity that doesn't belong to the storage results in
+     * undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * storage doesn't contain the given entity.
+     *
+     * @param entt A valid entity identifier.
+     * @return The object associated with the entity.
+     */
+    const object_type & get(const entity_type entt) const ENTT_NOEXCEPT {
+        return instances[underlying_type::index(entt)];
+    }
+
+    /*! @copydoc get */
+    object_type & get(const entity_type entt) ENTT_NOEXCEPT {
+        return const_cast<object_type &>(std::as_const(*this).get(entt));
+    }
+
+    /**
+     * @brief Returns a pointer to the object associated with an entity, if any.
+     * @param entt A valid entity identifier.
+     * @return The object associated with the entity, if any.
+     */
+    const object_type * try_get(const entity_type entt) const ENTT_NOEXCEPT {
+        return underlying_type::has(entt) ? (instances.data() + underlying_type::index(entt)) : nullptr;
+    }
+
+    /*! @copydoc try_get */
+    object_type * try_get(const entity_type entt) ENTT_NOEXCEPT {
+        return const_cast<object_type *>(std::as_const(*this).try_get(entt));
+    }
+
+    /**
+     * @brief Assigns an entity to a storage and constructs its object.
+     *
+     * This version accept both types that can be constructed in place directly
+     * and types like aggregates that do not work well with a placement new as
+     * performed usually under the hood during an _emplace back_.
+     *
+     * @warning
+     * Attempting to use an entity that already belongs to the storage results
+     * in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * storage already contains the given entity.
+     *
+     * @tparam Args Types of arguments to use to construct the object.
+     * @param entt A valid entity identifier.
+     * @param args Parameters to use to construct an object for the entity.
+     * @return The object associated with the entity.
+     */
+    template<typename... Args>
+    object_type & construct(const entity_type entt, Args &&... args) {
+        if constexpr(std::is_aggregate_v<object_type>) {
+            instances.emplace_back(Type{std::forward<Args>(args)...});
+        } else {
+            instances.emplace_back(std::forward<Args>(args)...);
+        }
+
+        // entity goes after component in case constructor throws
+        underlying_type::construct(entt);
+        return instances.back();
+    }
+
+    /**
+     * @brief Assigns one or more entities to a storage and default constructs
+     * their objects.
+     *
+     * The object type must be at least move and default insertable.
+     *
+     * @warning
+     * Attempting to assign an entity that already belongs to the storage
+     * results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * storage already contains the given entity.
+     *
+     * @tparam It Type of forward iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @return An iterator to the list of instances just created and sorted the
+     * same of the entities.
+     */
+    template<typename It>
+    iterator_type batch(It first, It last) {
+        instances.resize(instances.size() + std::distance(first, last));
+        // entity goes after component in case constructor throws
+        underlying_type::batch(first, last);
+        return begin();
+    }
+
+    /**
+     * @brief Assigns one or more entities to a storage and copy constructs
+     * their objects.
+     *
+     * The object type must be at least move and copy insertable.
+     *
+     * @sa batch
+     *
+     * @tparam It Type of forward iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param value The value to initialize the new objects with.
+     * @return An iterator to the list of instances just created and sorted the
+     * same of the entities.
+     */
+    template<typename It>
+    iterator_type batch(It first, It last, const object_type &value) {
+        instances.resize(instances.size() + std::distance(first, last), value);
+        // entity goes after component in case constructor throws
+        underlying_type::batch(first, last);
+        return begin();
+    }
+
+    /**
+     * @brief Removes an entity from a storage and destroys its object.
+     *
+     * @warning
+     * Attempting to use an entity that doesn't belong to the storage results in
+     * undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * storage doesn't contain the given entity.
+     *
+     * @param entt A valid entity identifier.
+     */
+    void destroy(const entity_type entt) {
+        auto other = std::move(instances.back());
+        instances[underlying_type::index(entt)] = std::move(other);
+        instances.pop_back();
+        underlying_type::destroy(entt);
+    }
+
+    /**
+     * @brief Swaps entities and objects in the internal packed arrays.
+     *
+     * @warning
+     * Attempting to swap entities that don't belong to the sparse set results
+     * in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * sparse set doesn't contain the given entities.
+     *
+     * @param lhs A valid position within the sparse set.
+     * @param rhs A valid position within the sparse set.
+     */
+    void swap(const size_type lhs, const size_type rhs) ENTT_NOEXCEPT override {
+        ENTT_ASSERT(lhs < instances.size());
+        ENTT_ASSERT(rhs < instances.size());
+        std::swap(instances[lhs], instances[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.
+     *
+     * The comparison function object received by the sort function object
+     * hasn't necessarily the type of the one passed along with the other
+     * parameters to this member function.
+     *
+     * @note
+     * Attempting to iterate elements using a raw pointer returned by a call to
+     * either `data` or `raw` gives no guarantees on the order, even though
+     * `sort` has been invoked.
+     *
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param 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(!(first > last));
+
+        const auto from = underlying_type::begin() + std::distance(begin(), first);
+        const auto to = from + std::distance(first, last);
+
+        if constexpr(std::is_invocable_v<Compare, const object_type &, const object_type &>) {
+            static_assert(!std::is_empty_v<object_type>);
+
+            underlying_type::sort(from, to, [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::sort(from, to, std::move(compare), std::move(algo), std::forward<Args>(args)...);
+        }
+    }
+
+    /*! @brief Resets a storage. */
+    void reset() {
+        underlying_type::reset();
+        instances.clear();
+    }
+
+private:
+    std::vector<object_type> instances;
+};
+
+
+/*! @copydoc basic_storage */
+template<typename Entity, typename Type>
+class basic_storage<Entity, Type, std::enable_if_t<std::is_empty_v<Type>>>: public sparse_set<Entity> {
+    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
+    using underlying_type = sparse_set<Entity>;
+
+    class iterator {
+        friend class basic_storage<Entity, Type>;
+
+        using index_type = typename traits_type::difference_type;
+
+        iterator(const index_type idx) ENTT_NOEXCEPT
+            : index{idx}
+        {}
+
+    public:
+        using difference_type = index_type;
+        using value_type = Type;
+        using pointer = const value_type *;
+        using reference = value_type;
+        using iterator_category = std::input_iterator_tag;
+
+        iterator() ENTT_NOEXCEPT = default;
+
+        iterator & operator++() ENTT_NOEXCEPT {
+            return --index, *this;
+        }
+
+        iterator operator++(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        iterator & operator--() ENTT_NOEXCEPT {
+            return ++index, *this;
+        }
+
+        iterator operator--(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return --(*this), orig;
+        }
+
+        iterator & operator+=(const difference_type value) ENTT_NOEXCEPT {
+            index -= value;
+            return *this;
+        }
+
+        iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
+            return iterator{index-value};
+        }
+
+        iterator & operator-=(const difference_type value) ENTT_NOEXCEPT {
+            return (*this += -value);
+        }
+
+        iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
+            return (*this + -value);
+        }
+
+        difference_type operator-(const iterator &other) const ENTT_NOEXCEPT {
+            return other.index - index;
+        }
+
+        reference operator[](const difference_type) const ENTT_NOEXCEPT {
+            return {};
+        }
+
+        bool operator==(const iterator &other) const ENTT_NOEXCEPT {
+            return other.index == index;
+        }
+
+        bool operator!=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+        bool operator<(const iterator &other) const ENTT_NOEXCEPT {
+            return index > other.index;
+        }
+
+        bool operator>(const iterator &other) const ENTT_NOEXCEPT {
+            return index < other.index;
+        }
+
+        bool operator<=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this > other);
+        }
+
+        bool operator>=(const iterator &other) const ENTT_NOEXCEPT {
+            return !(*this < other);
+        }
+
+        pointer operator->() const ENTT_NOEXCEPT {
+            return nullptr;
+        }
+
+        reference operator*() const ENTT_NOEXCEPT {
+            return {};
+        }
+
+    private:
+        index_type index;
+    };
+
+public:
+    /*! @brief Type of the objects associated with the entities. */
+    using object_type = Type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Random access iterator type. */
+    using iterator_type = iterator;
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * The returned iterator points to the first instance of the given type. If
+     * the storage is empty, the returned iterator will be equal to `end()`.
+     *
+     * @note
+     * Input iterators stay true to the order imposed by a call to either `sort`
+     * or `respect`.
+     *
+     * @return An iterator to the first instance of the given type.
+     */
+    iterator_type cbegin() const ENTT_NOEXCEPT {
+        const typename traits_type::difference_type pos = underlying_type::size();
+        return iterator_type{pos};
+    }
+
+    /*! @copydoc cbegin */
+    iterator_type begin() const ENTT_NOEXCEPT {
+        return cbegin();
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     *
+     * The returned iterator points to the element following the last instance
+     * of the given type. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @note
+     * Input iterators stay true to the order imposed by a call to either `sort`
+     * or `respect`.
+     *
+     * @return An iterator to the element following the last instance of the
+     * given type.
+     */
+    iterator_type cend() const ENTT_NOEXCEPT {
+        return iterator_type{};
+    }
+
+    /*! @copydoc cend */
+    iterator_type end() const ENTT_NOEXCEPT {
+        return cend();
+    }
+
+    /**
+     * @brief Returns the object associated with an entity.
+     *
+     * @note
+     * Empty types aren't explicitly instantiated. Therefore, this function
+     * always returns a temporary object.
+     *
+     * @warning
+     * Attempting to use an entity that doesn't belong to the storage results in
+     * undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * storage doesn't contain the given entity.
+     *
+     * @param entt A valid entity identifier.
+     * @return The object associated with the entity.
+     */
+    object_type get([[maybe_unused]] const entity_type entt) const ENTT_NOEXCEPT {
+        ENTT_ASSERT(underlying_type::has(entt));
+        return {};
+    }
+
+    /**
+     * @brief Assigns one or more entities to a storage.
+     *
+     * The object type must be at least default constructible.
+     *
+     * @warning
+     * Attempting to assign an entity that already belongs to the storage
+     * results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * storage already contains the given entity.
+     *
+     * @tparam It Type of forward iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @return An iterator to the list of instances just created and sorted the
+     * same of the entities.
+     */
+    template<typename It>
+    iterator_type batch(It first, It last, const object_type & = {}) {
+        underlying_type::batch(first, last);
+        return begin();
+    }
+};
+
+/*! @copydoc basic_storage */
+template<typename Entity, typename Type>
+struct storage: basic_storage<Entity, Type> {};
+
+
+}
+
+
+#endif // ENTT_ENTITY_STORAGE_HPP

src/entt/entity/utility.hpp

@@ -2,23 +2,42 @@
 #define ENTT_ENTITY_UTILITY_HPP
 
 
+#include "../core/type_traits.hpp"
+
+
 namespace entt {
 
 
-/*! @brief Tag class type used to disambiguate overloads. */
-struct tag_t final {};
+/**
+ * @brief Alias for exclusion lists.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+struct exclude_t: type_list<Type...> {};
 
 
-/*! @brief Persistent view type used to disambiguate overloads. */
-struct persistent_t final {};
+/**
+ * @brief Variable template for exclusion lists.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+constexpr exclude_t<Type...> exclude{};
 
 
-/*! @brief Raw view type used to disambiguate overloads. */
-struct raw_t final {};
+/**
+ * @brief Alias for lists of observed components.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+struct get_t: type_list<Type...>{};
 
 
-/*! @brief Break type used to disambiguate overloads. */
-struct break_t final {};
+/**
+ * @brief Variable template for lists of observed components.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+constexpr get_t<Type...> get{};
 
 
 }

src/entt/entt.hpp

@@ -3,17 +3,24 @@
 #include "core/hashed_string.hpp"
 #include "core/ident.hpp"
 #include "core/monostate.hpp"
+#include "core/type_traits.hpp"
+#include "core/utility.hpp"
 #include "entity/actor.hpp"
 #include "entity/entity.hpp"
-#include "entity/entt_traits.hpp"
+#include "entity/group.hpp"
 #include "entity/helper.hpp"
-#include "entity/prototype.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/storage.hpp"
 #include "entity/utility.hpp"
 #include "entity/view.hpp"
 #include "locator/locator.hpp"
+#include "meta/factory.hpp"
+#include "meta/meta.hpp"
+#include "meta/policy.hpp"
 #include "process/process.hpp"
 #include "process/scheduler.hpp"
 #include "resource/cache.hpp"

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 +4,6 @@
 
 #include <memory>
 #include <utility>
-#include <cassert>
 #include "../config/config.h"
 
 
@@ -23,20 +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() ENTT_NOEXCEPT {
+    static bool empty() ENTT_NOEXCEPT {
         return !static_cast<bool>(service);
     }
 
@@ -50,7 +49,7 @@ struct ServiceLocator final {
      *
      * @return A reference to the service implementation currently set, if any.
      */
-    inline static std::weak_ptr<Service> get() ENTT_NOEXCEPT {
+    static std::weak_ptr<Service> get() ENTT_NOEXCEPT {
         return service;
     }
 
@@ -68,7 +67,7 @@ struct ServiceLocator final {
      *
      * @return A reference to the service implementation currently set, if any.
      */
-    inline static Service & ref() ENTT_NOEXCEPT {
+    static Service & ref() ENTT_NOEXCEPT {
         return *service;
     }
 
@@ -79,7 +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)...);
     }
 
@@ -87,8 +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);
     }
 
@@ -97,19 +96,15 @@ struct ServiceLocator final {
      *
      * The service is no longer valid after a reset.
      */
-    inline static void reset() {
+    static void reset() {
         service.reset();
     }
 
 private:
-    static std::shared_ptr<Service> service;
+    inline static std::shared_ptr<Service> service = nullptr;
 };
 
 
-template<typename Service>
-std::shared_ptr<Service> ServiceLocator<Service>::service{};
-
-
 }
 
 

src/entt/meta/factory.hpp

@@ -0,0 +1,928 @@
+#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 "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 auto size = sizeof...(Args);
+    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...));
+
+
+template<typename Candidate>
+using meta_function_helper_t = decltype(to_meta_function_helper(std::declval<Candidate>()));
+
+
+template<typename Type, typename... Args, std::size_t... Indexes>
+meta_any construct(meta_any * const args, std::index_sequence<Indexes...>) {
+    [[maybe_unused]] auto direct = std::make_tuple((args+Indexes)->try_cast<Args>()...);
+    meta_any any{};
+
+    if(((std::get<Indexes>(direct) || (args+Indexes)->convert<Args>()) && ...)) {
+        any = Type{(std::get<Indexes>(direct) ? *std::get<Indexes>(direct) : (args+Indexes)->cast<Args>())...};
+    }
+
+    return any;
+}
+
+
+template<bool Const, typename Type, auto Data>
+bool setter([[maybe_unused]] meta_handle handle, [[maybe_unused]] meta_any index, [[maybe_unused]] meta_any value) {
+    bool accepted = false;
+
+    if constexpr(!Const) {
+        if constexpr(std::is_function_v<std::remove_pointer_t<decltype(Data)>> || std::is_member_function_pointer_v<decltype(Data)>) {
+            using helper_type = meta_function_helper_t<decltype(Data)>;
+            using data_type = std::tuple_element_t<!std::is_member_function_pointer_v<decltype(Data)>, typename helper_type::args_type>;
+            static_assert(std::is_invocable_v<decltype(Data), Type &, data_type>);
+            auto *direct = value.try_cast<data_type>();
+            auto *clazz = handle.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 *clazz = handle.data<Type>();
+
+            if constexpr(std::is_array_v<data_type>) {
+                using underlying_type = std::remove_extent_t<data_type>;
+                auto *direct = value.try_cast<underlying_type>();
+                auto *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 *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 *direct = value.try_cast<underlying_type>();
+                auto *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 *direct = value.try_cast<data_type>();
+
+                if(direct || value.convert<data_type>()) {
+                    *Data = (direct ? *direct : value.cast<data_type>());
+                    accepted = true;
+                }
+            }
+        }
+    }
+
+    return accepted;
+}
+
+
+template<typename Type, auto Data, typename Policy>
+meta_any getter([[maybe_unused]] meta_handle handle, [[maybe_unused]] meta_any index) {
+    auto dispatch = [](auto &&value) {
+        if constexpr(std::is_same_v<Policy, as_void_t>) {
+            return meta_any{std::in_place_type<void>};
+        } else if constexpr(std::is_same_v<Policy, as_alias_t>) {
+            return meta_any{as_alias, std::forward<decltype(value)>(value)};
+        } else {
+            static_assert(std::is_same_v<Policy, as_is_t>);
+            return meta_any{std::forward<decltype(value)>(value)};
+        }
+    };
+
+    if constexpr(std::is_function_v<std::remove_pointer_t<decltype(Data)>> || std::is_member_function_pointer_v<decltype(Data)>) {
+        static_assert(std::is_invocable_v<decltype(Data), Type &>);
+        auto *clazz = handle.data<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 *clazz = handle.data<Type>();
+
+        if constexpr(std::is_array_v<data_type>) {
+            auto *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 *idx = index.try_cast<std::size_t>();
+            return idx ? dispatch((*Data)[*idx]) : meta_any{};
+        } else {
+            return dispatch(*Data);
+        }
+    }
+}
+
+
+template<typename Type, auto Candidate, typename Policy, std::size_t... Indexes>
+meta_any invoke([[maybe_unused]] meta_handle handle, meta_any *args, std::index_sequence<Indexes...>) {
+    using helper_type = meta_function_helper_t<decltype(Candidate)>;
+
+    auto dispatch = [](auto *... args) {
+        if constexpr(std::is_void_v<typename helper_type::return_type> || std::is_same_v<Policy, as_void_t>) {
+            std::invoke(Candidate, *args...);
+            return meta_any{std::in_place_type<void>};
+        } else if constexpr(std::is_same_v<Policy, as_alias_t>) {
+            return meta_any{as_alias, std::invoke(Candidate, *args...)};
+        } else {
+            static_assert(std::is_same_v<Policy, as_is_t>);
+            return meta_any{std::invoke(Candidate, *args...)};
+        }
+    };
+
+    [[maybe_unused]] const auto direct = std::make_tuple([](meta_any *any, auto *instance) {
+        using arg_type = std::remove_reference_t<decltype(*instance)>;
+
+        if(!instance && any->convert<arg_type>()) {
+            instance = any->try_cast<arg_type>();
+        }
+
+        return instance;
+    }(args+Indexes, (args+Indexes)->try_cast<std::tuple_element_t<Indexes, typename helper_type::args_type>>())...);
+
+    if constexpr(std::is_function_v<std::remove_pointer_t<decltype(Candidate)>>) {
+        return (std::get<Indexes>(direct) && ...) ? dispatch(std::get<Indexes>(direct)...) : meta_any{};
+    } else {
+        auto *clazz = handle.data<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 A meta factory to be used for reflection purposes.
+ *
+ * A meta factory is an utility class used to reflect types, data and functions
+ * of all sorts. This class ensures that the underlying web of types is built
+ * correctly and performs some checks in debug mode to ensure that there are no
+ * subtle errors at runtime.
+ *
+ * @tparam Type Reflected type for which the factory was created.
+ */
+template<typename Type>
+class meta_factory {
+    static_assert(std::is_same_v<Type, std::decay_t<Type>>);
+
+    template<typename Node>
+    bool duplicate(const ENTT_ID_TYPE identifier, const Node *node) ENTT_NOEXCEPT {
+        return node ? node->identifier == identifier || duplicate(identifier, node->next) : false;
+    }
+
+    bool duplicate(const meta_any &key, const internal::meta_prop_node *node) ENTT_NOEXCEPT {
+        return node ? node->key() == key || duplicate(key, node->next) : false;
+    }
+
+    template<typename>
+    internal::meta_prop_node * properties() {
+        return nullptr;
+    }
+
+    template<typename Owner, typename Property, typename... Other>
+    internal::meta_prop_node * properties(Property &&property, Other &&... other) {
+        static std::remove_cv_t<std::remove_reference_t<Property>> prop{};
+
+        static internal::meta_prop_node node{
+            nullptr,
+            []() -> meta_any {
+                return std::as_const(std::get<0>(prop));
+            },
+            []() -> meta_any {
+                return std::as_const(std::get<1>(prop));
+            },
+            []() ENTT_NOEXCEPT -> meta_prop {
+                return &node;
+            }
+        };
+
+        prop = std::forward<Property>(property);
+        node.next = properties<Owner>(std::forward<Other>(other)...);
+        ENTT_ASSERT(!duplicate(meta_any{std::get<0>(prop)}, node.next));
+        return &node;
+    }
+
+    void unregister_prop(internal::meta_prop_node **prop) {
+        while(*prop) {
+            auto *node = *prop;
+            *prop = node->next;
+            node->next = nullptr;
+        }
+    }
+
+    void unregister_dtor() {
+        if(auto node = internal::meta_info<Type>::type->dtor; node) {
+            internal::meta_info<Type>::type->dtor = nullptr;
+            *node->underlying = nullptr;
+        }
+    }
+
+    template<auto Member>
+    auto unregister_all(int)
+    -> decltype((internal::meta_info<Type>::type->*Member)->prop, void()) {
+        while(internal::meta_info<Type>::type->*Member) {
+            auto node = internal::meta_info<Type>::type->*Member;
+            internal::meta_info<Type>::type->*Member = node->next;
+            unregister_prop(&node->prop);
+            node->next = nullptr;
+            *node->underlying = nullptr;
+        }
+    }
+
+    template<auto Member>
+    void unregister_all(char) {
+        while(internal::meta_info<Type>::type->*Member) {
+            auto node = internal::meta_info<Type>::type->*Member;
+            internal::meta_info<Type>::type->*Member = node->next;
+            node->next = nullptr;
+            *node->underlying = nullptr;
+        }
+    }
+
+public:
+    /*! @brief Default constructor. */
+    meta_factory() ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Extends a meta type by assigning it an identifier and properties.
+     * @tparam Property Types of properties to assign to the meta type.
+     * @param identifier Unique identifier.
+     * @param property Properties to assign to the meta type.
+     * @return A meta factory for the parent type.
+     */
+    template<typename... Property>
+    meta_factory type(const ENTT_ID_TYPE identifier, Property &&... property) ENTT_NOEXCEPT {
+        ENTT_ASSERT(!internal::meta_info<Type>::type);
+        auto *node = internal::meta_info<Type>::resolve();
+        node->identifier = identifier;
+        node->next = internal::meta_info<>::type;
+        node->prop = properties<Type>(std::forward<Property>(property)...);
+        ENTT_ASSERT(!duplicate(identifier, node->next));
+        internal::meta_info<Type>::type = node;
+        internal::meta_info<>::type = node;
+
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a meta base to a meta type.
+     *
+     * A reflected base class must be a real base class of the reflected type.
+     *
+     * @tparam Base Type of the base class to assign to the meta type.
+     * @return A meta factory for the parent type.
+     */
+    template<typename Base>
+    meta_factory base() ENTT_NOEXCEPT {
+        static_assert(std::is_base_of_v<Base, Type>);
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_base_node node{
+            &internal::meta_info<Type>::template base<Base>,
+            type,
+            nullptr,
+            &internal::meta_info<Base>::resolve,
+            [](void *instance) ENTT_NOEXCEPT -> void * {
+                return static_cast<Base *>(static_cast<Type *>(instance));
+            },
+            []() ENTT_NOEXCEPT -> meta_base {
+                return &node;
+            }
+        };
+
+        node.next = type->base;
+        ENTT_ASSERT((!internal::meta_info<Type>::template base<Base>));
+        internal::meta_info<Type>::template base<Base> = &node;
+        type->base = &node;
+
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a meta conversion function to a meta type.
+     *
+     * The given type must be such that an instance of the reflected type can be
+     * converted to it.
+     *
+     * @tparam To Type of the conversion function to assign to the meta type.
+     * @return A meta factory for the parent type.
+     */
+    template<typename To>
+    meta_factory conv() ENTT_NOEXCEPT {
+        static_assert(std::is_convertible_v<Type, To>);
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_conv_node node{
+            &internal::meta_info<Type>::template conv<To>,
+            type,
+            nullptr,
+            &internal::meta_info<To>::resolve,
+            [](const void *instance) -> meta_any {
+                return static_cast<To>(*static_cast<const Type *>(instance));
+            },
+            []() ENTT_NOEXCEPT -> meta_conv {
+                return &node;
+            }
+        };
+
+        node.next = type->conv;
+        ENTT_ASSERT((!internal::meta_info<Type>::template conv<To>));
+        internal::meta_info<Type>::template conv<To> = &node;
+        type->conv = &node;
+
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a meta 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>
+    meta_factory 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{
+            &internal::meta_info<Type>::template conv<conv_type>,
+            type,
+            nullptr,
+            &internal::meta_info<conv_type>::resolve,
+            [](const void *instance) -> meta_any {
+                return std::invoke(Candidate, *static_cast<const Type *>(instance));
+            },
+            []() ENTT_NOEXCEPT -> meta_conv {
+                return &node;
+            }
+        };
+
+        node.next = type->conv;
+        ENTT_ASSERT((!internal::meta_info<Type>::template conv<conv_type>));
+        internal::meta_info<Type>::template conv<conv_type> = &node;
+        type->conv = &node;
+
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a meta constructor to a meta type.
+     *
+     * Free functions can be assigned to meta types in the role of constructors.
+     * All that is required is that they return an instance of the underlying
+     * type.<br/>
+     * From a client's point of view, nothing changes if a constructor of a meta
+     * type is a built-in one or a free function.
+     *
+     * @tparam Func The actual function to use as a constructor.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @tparam Property Types of properties to assign to the meta data.
+     * @param property Properties to assign to the meta data.
+     * @return A meta factory for the parent type.
+     */
+    template<auto Func, typename Policy = as_is_t, typename... Property>
+    meta_factory ctor(Property &&... property) 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{
+            &internal::meta_info<Type>::template ctor<typename helper_type::args_type>,
+            type,
+            nullptr,
+            nullptr,
+            helper_type::size,
+            &helper_type::arg,
+            [](meta_any * const any) {
+                return internal::invoke<Type, Func, Policy>({}, any, std::make_index_sequence<helper_type::size>{});
+            },
+            []() ENTT_NOEXCEPT -> meta_ctor {
+                return &node;
+            }
+        };
+
+        node.next = type->ctor;
+        node.prop = properties<typename helper_type::args_type>(std::forward<Property>(property)...);
+        ENTT_ASSERT((!internal::meta_info<Type>::template ctor<typename helper_type::args_type>));
+        internal::meta_info<Type>::template ctor<typename helper_type::args_type> = &node;
+        type->ctor = &node;
+
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a meta constructor to a meta type.
+     *
+     * A meta constructor is uniquely identified by the types of its arguments
+     * and is such that there exists an actual constructor of the underlying
+     * type that can be invoked with parameters whose types are those given.
+     *
+     * @tparam Args Types of arguments to use to construct an instance.
+     * @tparam Property Types of properties to assign to the meta data.
+     * @param property Properties to assign to the meta data.
+     * @return A meta factory for the parent type.
+     */
+    template<typename... Args, typename... Property>
+    meta_factory ctor(Property &&... property) ENTT_NOEXCEPT {
+        using helper_type = internal::meta_function_helper_t<Type(*)(Args...)>;
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_ctor_node node{
+            &internal::meta_info<Type>::template ctor<typename helper_type::args_type>,
+            type,
+            nullptr,
+            nullptr,
+            helper_type::size,
+            &helper_type::arg,
+            [](meta_any * const any) {
+                return internal::construct<Type, std::remove_cv_t<std::remove_reference_t<Args>>...>(any, std::make_index_sequence<helper_type::size>{});
+            },
+            []() ENTT_NOEXCEPT -> meta_ctor {
+                return &node;
+            }
+        };
+
+        node.next = type->ctor;
+        node.prop = properties<typename helper_type::args_type>(std::forward<Property>(property)...);
+        ENTT_ASSERT((!internal::meta_info<Type>::template ctor<typename helper_type::args_type>));
+        internal::meta_info<Type>::template ctor<typename helper_type::args_type> = &node;
+        type->ctor = &node;
+
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a meta destructor to a meta type.
+     *
+     * Free functions can be assigned to meta types in the role of destructors.
+     * The signature of the function should identical to the following:
+     *
+     * @code{.cpp}
+     * void(Type &);
+     * @endcode
+     *
+     * 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>
+    meta_factory dtor() ENTT_NOEXCEPT {
+        static_assert(std::is_invocable_v<decltype(Func), Type &>);
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_dtor_node node{
+            &internal::meta_info<Type>::template dtor<Func>,
+            type,
+            [](meta_handle handle) {
+                return handle.type() == internal::meta_info<Type>::resolve()->meta()
+                        ? (std::invoke(Func, *handle.data<Type>()), true)
+                        : false;
+            },
+            []() ENTT_NOEXCEPT -> meta_dtor {
+                return &node;
+            }
+        };
+
+        ENTT_ASSERT(!internal::meta_info<Type>::type->dtor);
+        ENTT_ASSERT((!internal::meta_info<Type>::template dtor<Func>));
+        internal::meta_info<Type>::template dtor<Func> = &node;
+        internal::meta_info<Type>::type->dtor = &node;
+
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a meta data to a meta type.
+     *
+     * Both data members and static and global variables, as well as constants
+     * of any kind, can be assigned to a meta type.<br/>
+     * From a client's point of view, all the variables associated with the
+     * reflected object will appear as if they were part of the type itself.
+     *
+     * @tparam Data The actual variable to attach to the meta type.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @tparam Property Types of properties to assign to the meta data.
+     * @param identifier Unique identifier.
+     * @param property Properties to assign to the meta data.
+     * @return A meta factory for the parent type.
+     */
+    template<auto Data, typename Policy = as_is_t, typename... Property>
+    meta_factory data(const ENTT_ID_TYPE identifier, Property &&... property) ENTT_NOEXCEPT {
+        auto * const type = internal::meta_info<Type>::resolve();
+        internal::meta_data_node *curr = nullptr;
+
+        if constexpr(std::is_same_v<Type, decltype(Data)>) {
+            static_assert(std::is_same_v<Policy, as_is_t>);
+
+            static internal::meta_data_node node{
+                &internal::meta_info<Type>::template data<Data>,
+                {},
+                type,
+                nullptr,
+                nullptr,
+                true,
+                true,
+                &internal::meta_info<Type>::resolve,
+                [](meta_handle, meta_any, meta_any) { return false; },
+                [](meta_handle, meta_any) -> meta_any { return Data; },
+                []() ENTT_NOEXCEPT -> meta_data {
+                    return &node;
+                }
+            };
+
+            node.prop = properties<std::integral_constant<Type, Data>>(std::forward<Property>(property)...);
+            curr = &node;
+        } else if constexpr(std::is_member_object_pointer_v<decltype(Data)>) {
+            using data_type = std::remove_reference_t<decltype(std::declval<Type>().*Data)>;
+
+            static internal::meta_data_node node{
+                &internal::meta_info<Type>::template data<Data>,
+                {},
+                type,
+                nullptr,
+                nullptr,
+                std::is_const_v<data_type>,
+                !std::is_member_object_pointer_v<decltype(Data)>,
+                &internal::meta_info<data_type>::resolve,
+                &internal::setter<std::is_const_v<data_type>, Type, Data>,
+                &internal::getter<Type, Data, Policy>,
+                []() ENTT_NOEXCEPT -> meta_data {
+                    return &node;
+                }
+            };
+
+            node.prop = properties<std::integral_constant<decltype(Data), Data>>(std::forward<Property>(property)...);
+            curr = &node;
+        } else {
+            static_assert(std::is_pointer_v<std::decay_t<decltype(Data)>>);
+            using data_type = std::remove_pointer_t<std::decay_t<decltype(Data)>>;
+
+            static internal::meta_data_node node{
+                &internal::meta_info<Type>::template data<Data>,
+                {},
+                type,
+                nullptr,
+                nullptr,
+                std::is_const_v<data_type>,
+                !std::is_member_object_pointer_v<decltype(Data)>,
+                &internal::meta_info<data_type>::resolve,
+                &internal::setter<std::is_const_v<data_type>, Type, Data>,
+                &internal::getter<Type, Data, Policy>,
+                []() ENTT_NOEXCEPT -> meta_data {
+                    return &node;
+                }
+            };
+
+            node.prop = properties<std::integral_constant<decltype(Data), Data>>(std::forward<Property>(property)...);
+            curr = &node;
+        }
+
+        curr->identifier = identifier;
+        curr->next = type->data;
+        ENTT_ASSERT(!duplicate(identifier, curr->next));
+        ENTT_ASSERT((!internal::meta_info<Type>::template data<Data>));
+        internal::meta_info<Type>::template data<Data> = curr;
+        type->data = curr;
+
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a meta data to a meta type by means of its setter and
+     * getter.
+     *
+     * Setters and getters can be either free functions, member functions or a
+     * mix of them.<br/>
+     * In case of free functions, setters and getters must accept a 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).
+     * @tparam Property Types of properties to assign to the meta data.
+     * @param identifier Unique identifier.
+     * @param property Properties to assign to the meta data.
+     * @return A meta factory for the parent type.
+     */
+    template<auto Setter, auto Getter, typename Policy = as_is_t, typename... Property>
+    meta_factory data(const ENTT_ID_TYPE identifier, Property &&... property) ENTT_NOEXCEPT {
+        using owner_type = std::tuple<std::integral_constant<decltype(Setter), Setter>, std::integral_constant<decltype(Getter), Getter>>;
+        using underlying_type = std::invoke_result_t<decltype(Getter), Type &>;
+        static_assert(std::is_invocable_v<decltype(Setter), Type &, underlying_type>);
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_data_node node{
+            &internal::meta_info<Type>::template data<Setter, Getter>,
+            {},
+            type,
+            nullptr,
+            nullptr,
+            false,
+            false,
+            &internal::meta_info<underlying_type>::resolve,
+            &internal::setter<false, Type, Setter>,
+            &internal::getter<Type, Getter, Policy>,
+            []() ENTT_NOEXCEPT -> meta_data {
+                return &node;
+            }
+        };
+
+        node.identifier = identifier;
+        node.next = type->data;
+        node.prop = properties<owner_type>(std::forward<Property>(property)...);
+        ENTT_ASSERT(!duplicate(identifier, node.next));
+        ENTT_ASSERT((!internal::meta_info<Type>::template data<Setter, Getter>));
+        internal::meta_info<Type>::template data<Setter, Getter> = &node;
+        type->data = &node;
+
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a meta funcion to a meta type.
+     *
+     * Both member functions and free functions can be assigned to a meta
+     * type.<br/>
+     * From a client's point of view, all the functions associated with the
+     * reflected object will appear as if they were part of the type itself.
+     *
+     * @tparam Candidate The actual function to attach to the meta type.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @tparam Property Types of properties to assign to the meta function.
+     * @param identifier Unique identifier.
+     * @param property Properties to assign to the meta function.
+     * @return A meta factory for the parent type.
+     */
+    template<auto Candidate, typename Policy = as_is_t, typename... Property>
+    meta_factory func(const ENTT_ID_TYPE identifier, Property &&... property) ENTT_NOEXCEPT {
+        using owner_type = std::integral_constant<decltype(Candidate), Candidate>;
+        using helper_type = internal::meta_function_helper_t<decltype(Candidate)>;
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_func_node node{
+            &internal::meta_info<Type>::template func<Candidate>,
+            {},
+            type,
+            nullptr,
+            nullptr,
+            helper_type::size,
+            helper_type::is_const,
+            !std::is_member_function_pointer_v<decltype(Candidate)>,
+            &internal::meta_info<std::conditional_t<std::is_same_v<Policy, as_void_t>, void, typename helper_type::return_type>>::resolve,
+            &helper_type::arg,
+            [](meta_handle handle, meta_any *any) {
+                return internal::invoke<Type, Candidate, Policy>(handle, any, std::make_index_sequence<helper_type::size>{});
+            },
+            []() ENTT_NOEXCEPT -> meta_func {
+                return &node;
+            }
+        };
+
+        node.identifier = identifier;
+        node.next = type->func;
+        node.prop = properties<owner_type>(std::forward<Property>(property)...);
+        ENTT_ASSERT(!duplicate(identifier, node.next));
+        ENTT_ASSERT((!internal::meta_info<Type>::template func<Candidate>));
+        internal::meta_info<Type>::template func<Candidate> = &node;
+        type->func = &node;
+
+        return *this;
+    }
+
+    /**
+     * @brief Unregisters a meta type and all its parts.
+     *
+     * This function unregisters 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 unregistered but the link between the two types is
+     * removed.
+     *
+     * @return True if the meta type exists, false otherwise.
+     */
+    bool unregister() ENTT_NOEXCEPT {
+        const auto registered = internal::meta_info<Type>::type;
+
+        if(registered) {
+            if(auto *curr = internal::meta_info<>::type; curr == internal::meta_info<Type>::type) {
+                internal::meta_info<>::type = internal::meta_info<Type>::type->next;
+            } else {
+                while(curr && curr->next != internal::meta_info<Type>::type) {
+                    curr = curr->next;
+                }
+
+                if(curr) {
+                    curr->next = internal::meta_info<Type>::type->next;
+                }
+            }
+
+            unregister_prop(&internal::meta_info<Type>::type->prop);
+            unregister_all<&internal::meta_type_node::base>(0);
+            unregister_all<&internal::meta_type_node::conv>(0);
+            unregister_all<&internal::meta_type_node::ctor>(0);
+            unregister_all<&internal::meta_type_node::data>(0);
+            unregister_all<&internal::meta_type_node::func>(0);
+            unregister_dtor();
+
+            internal::meta_info<Type>::type->identifier = {};
+            internal::meta_info<Type>::type->next = nullptr;
+            internal::meta_info<Type>::type = nullptr;
+        }
+
+        return registered;
+    }
+};
+
+
+/**
+ * @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 data and functions through
+ * a dedicated factory.
+ *
+ * @tparam Type Type to reflect.
+ * @tparam Property Types of properties to assign to the reflected type.
+ * @param identifier Unique identifier.
+ * @param property Properties to assign to the reflected type.
+ * @return A meta factory for the given type.
+ */
+template<typename Type, typename... Property>
+inline meta_factory<Type> reflect(const ENTT_ID_TYPE identifier, Property &&... property) ENTT_NOEXCEPT {
+    return meta_factory<Type>{}.type(identifier, std::forward<Property>(property)...);
+}
+
+
+/**
+ * @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 data and functions through
+ * a dedicated factory.
+ *
+ * @tparam Type Type to reflect.
+ * @return A meta factory for the given type.
+ */
+template<typename Type>
+inline meta_factory<Type> reflect() ENTT_NOEXCEPT {
+    return meta_factory<Type>{};
+}
+
+
+/**
+ * @brief Utility function to unregister a type.
+ *
+ * This function unregisters a type and all its data members, member functions
+ * and properties, as well as its constructors, destructors and conversion
+ * functions if any.<br/>
+ * Base classes aren't unregistered but the link between the two types is
+ * removed.
+ *
+ * @tparam Type Type to unregister.
+ * @return True if the type to unregister exists, false otherwise.
+ */
+template<typename Type>
+inline bool unregister() ENTT_NOEXCEPT {
+    return meta_factory<Type>{}.unregister();
+}
+
+
+/**
+ * @brief Returns the meta type associated with a given type.
+ * @tparam Type Type to use to search for a meta type.
+ * @return The meta type associated with the given type, if any.
+ */
+template<typename Type>
+inline meta_type resolve() ENTT_NOEXCEPT {
+    return internal::meta_info<Type>::resolve()->meta();
+}
+
+
+/**
+ * @brief Returns the meta type associated with a given identifier.
+ * @param identifier Unique identifier.
+ * @return The meta type associated with the given identifier, if any.
+ */
+inline meta_type resolve(const ENTT_ID_TYPE identifier) ENTT_NOEXCEPT {
+    const auto *curr = internal::find_if([identifier](auto *node) {
+        return node->identifier == identifier;
+    }, internal::meta_info<>::type);
+
+    return curr ? curr->meta() : meta_type{};
+}
+
+
+/**
+ * @brief Iterates all the reflected types.
+ * @tparam Op Type of the function object to invoke.
+ * @param op A valid function object.
+ */
+template<typename Op>
+inline std::enable_if_t<std::is_invocable_v<Op, meta_type>, void>
+resolve(Op op) ENTT_NOEXCEPT {
+    internal::iterate([op = std::move(op)](auto *node) {
+        op(node->meta());
+    }, internal::meta_info<>::type);
+}
+
+
+}
+
+
+#endif // ENTT_META_FACTORY_HPP

src/entt/meta/policy.hpp

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

src/entt/process/process.hpp

@@ -2,9 +2,8 @@
 #define ENTT_PROCESS_PROCESS_HPP
 
 
-#include <type_traits>
-#include <functional>
 #include <utility>
+#include <type_traits>
 #include "../config/config.h"
 
 
@@ -31,12 +30,13 @@ namespace entt {
  *   update.
  *
  * * @code{.cpp}
- *   void init(void *);
+ *   void init();
  *   @endcode
  *
- *   It's invoked at the first tick, immediately before an update. The `void *`
- *   parameter is an opaque pointer to user data (if any) forwarded directly to
- *   the process during an update.
+ *   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();
@@ -64,14 +64,14 @@ namespace entt {
  * `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 {
-    enum class State: unsigned int {
+class process {
+    enum class state: unsigned int {
         UNINITIALIZED = 0,
         RUNNING,
         PAUSED,
@@ -81,41 +81,41 @@ class Process {
         FINISHED
     };
 
-    template<State state>
-    using tag = std::integral_constant<State, state>;
+    template<state value>
+    using state_value_t = std::integral_constant<state, value>;
 
     template<typename Target = Derived>
-    auto tick(int, tag<State::UNINITIALIZED>, void *data)
-    -> decltype(std::declval<Target>().init(data)) {
-        static_cast<Target *>(this)->init(data);
+    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, void *data)
+    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 &&...) const ENTT_NOEXCEPT {}
+    template<state value, typename... Args>
+    void tick(char, state_value_t<value>, Args &&...) const ENTT_NOEXCEPT {}
 
 protected:
     /**
@@ -126,7 +126,7 @@ protected:
      */
     void succeed() ENTT_NOEXCEPT {
         if(alive()) {
-            current = State::SUCCEEDED;
+            current = state::SUCCEEDED;
         }
     }
 
@@ -138,7 +138,7 @@ protected:
      */
     void fail() ENTT_NOEXCEPT {
         if(alive()) {
-            current = State::FAILED;
+            current = state::FAILED;
         }
     }
 
@@ -149,8 +149,8 @@ protected:
      * running.
      */
     void pause() ENTT_NOEXCEPT {
-        if(current == State::RUNNING) {
-            current = State::PAUSED;
+        if(current == state::RUNNING) {
+            current = state::PAUSED;
         }
     }
 
@@ -161,8 +161,8 @@ protected:
      * paused.
      */
     void unpause() ENTT_NOEXCEPT {
-        if(current  == State::PAUSED) {
-            current  = State::RUNNING;
+        if(current  == state::PAUSED) {
+            current  = state::RUNNING;
         }
     }
 
@@ -171,8 +171,8 @@ public:
     using delta_type = Delta;
 
     /*! @brief Default destructor. */
-    virtual ~Process() ENTT_NOEXCEPT {
-        static_assert(std::is_base_of<Process, Derived>::value, "!");
+    virtual ~process() ENTT_NOEXCEPT {
+        static_assert(std::is_base_of_v<process, Derived>);
     }
 
     /**
@@ -185,7 +185,7 @@ public:
      */
     void abort(const bool immediately = false) ENTT_NOEXCEPT {
         if(alive()) {
-            current = State::ABORTED;
+            current = state::ABORTED;
 
             if(immediately) {
                 tick(0);
@@ -198,7 +198,7 @@ public:
      * @return True if the process is still alive, false otherwise.
      */
     bool alive() const ENTT_NOEXCEPT {
-        return current == State::RUNNING || current == State::PAUSED;
+        return current == state::RUNNING || current == state::PAUSED;
     }
 
     /**
@@ -206,7 +206,7 @@ public:
      * @return True if the process is terminated, false otherwise.
      */
     bool dead() const ENTT_NOEXCEPT {
-        return current == State::FINISHED;
+        return current == state::FINISHED;
     }
 
     /**
@@ -214,7 +214,7 @@ public:
      * @return True if the process is paused, false otherwise.
      */
     bool paused() const ENTT_NOEXCEPT {
-        return current == State::PAUSED;
+        return current == state::PAUSED;
     }
 
     /**
@@ -232,12 +232,13 @@ public:
      */
     void tick(const Delta delta, void *data = nullptr) {
         switch (current) {
-        case State::UNINITIALIZED:
-            tick(0, tag<State::UNINITIALIZED>{}, data);
-            current = State::RUNNING;
-            // no break on purpose, tasks are executed immediately
-        case State::RUNNING:
-            tick(0, tag<State::RUNNING>{}, delta, data);
+        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;
@@ -245,18 +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:
@@ -266,7 +267,7 @@ public:
     }
 
 private:
-    State current{State::UNINITIALIZED};
+    state current{state::UNINITIALIZED};
     bool stopped{false};
 };
 
@@ -304,21 +305,21 @@ 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)...}
     {}
 

src/entt/process/scheduler.hpp

@@ -30,23 +30,23 @@ namespace entt {
  * @code{.cpp}
  * 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 {
-    struct ProcessHandler final {
+class scheduler {
+    struct process_handler {
         using instance_type = std::unique_ptr<void, void(*)(void *)>;
-        using update_fn_type = bool(ProcessHandler &, Delta, void *);
-        using abort_fn_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_fn_type *update;
@@ -54,30 +54,33 @@ class Scheduler final {
         next_type next;
     };
 
-    struct Then final {
-        Then(ProcessHandler *handler)
-            : 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 = Scheduler::then<Proc>(handler, 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, const Delta delta, void *data) {
+    static bool update(process_handler &handler, const Delta delta, void *data) {
         auto *process = static_cast<Proc *>(handler.instance.get());
         process->tick(delta, data);
 
@@ -86,7 +89,8 @@ class Scheduler final {
         if(dead) {
             if(handler.next && !process->rejected()) {
                 handler = std::move(*handler.next);
-                dead = handler.update(handler, delta, data);
+                // forces the process to exit the uninitialized state
+                dead = handler.update(handler, {}, nullptr);
             } else {
                 handler.instance.reset();
             }
@@ -96,7 +100,7 @@ class Scheduler final {
     }
 
     template<typename Proc>
-    static void abort(ProcessHandler &handler, const bool immediately) {
+    static void abort(process_handler &handler, const bool immediately) {
         static_cast<Proc *>(handler.instance.get())->abort(immediately);
     }
 
@@ -105,33 +109,18 @@ class Scheduler final {
         delete static_cast<Proc *>(proc);
     }
 
-    template<typename Proc, typename... Args>
-    static auto then(ProcessHandler *handler, Args &&... args) {
-        if(handler) {
-            auto proc = typename ProcessHandler::instance_type{new Proc{std::forward<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;
-    }
-
 public:
     /*! @brief Unsigned integer type. */
-    using size_type = typename std::vector<ProcessHandler>::size_type;
+    using size_type = typename std::vector<process_handler>::size_type;
 
     /*! @brief Default constructor. */
-    Scheduler() ENTT_NOEXCEPT = default;
+    scheduler() ENTT_NOEXCEPT = default;
 
-    /*! @brief Copying a scheduler isn't allowed. */
-    Scheduler(const Scheduler &) = delete;
     /*! @brief Default move constructor. */
-    Scheduler(Scheduler &&) = default;
+    scheduler(scheduler &&) = default;
 
-    /*! @brief Copying a scheduler isn't allowed. @return This scheduler. */
-    Scheduler & operator=(const Scheduler &) = delete;
     /*! @brief Default move assignment operator. @return This scheduler. */
-    Scheduler & operator=(Scheduler &&) = default;
+    scheduler & operator=(scheduler &&) = default;
 
     /**
      * @brief Number of processes currently scheduled.
@@ -170,13 +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, 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.
@@ -186,13 +175,12 @@ public:
      */
     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()};
+        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))};
     }
 
     /**
@@ -204,12 +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.
      *
@@ -236,10 +225,10 @@ public:
      *     // 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.
@@ -247,7 +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));
     }
 
@@ -301,7 +290,7 @@ public:
     }
 
 private:
-    std::vector<ProcessHandler> handlers{};
+    std::vector<process_handler> handlers{};
 };
 
 

src/entt/resource/cache.hpp

@@ -7,9 +7,9 @@
 #include <type_traits>
 #include <unordered_map>
 #include "../config/config.h"
-#include "../core/hashed_string.hpp"
 #include "handle.hpp"
 #include "loader.hpp"
+#include "fwd.hpp"
 
 
 namespace entt {
@@ -26,27 +26,23 @@ 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>>;
+class resource_cache {
+    using container_type = std::unordered_map<ENTT_ID_TYPE, std::shared_ptr<Resource>>;
 
 public:
     /*! @brief Unsigned integer type. */
     using size_type = typename container_type::size_type;
     /*! @brief Type of resources managed by a cache. */
-    using resource_type = HashedString;
+    using resource_type = ENTT_ID_TYPE;
 
     /*! @brief Default constructor. */
-    ResourceCache() = default;
+    resource_cache() = default;
 
-    /*! @brief Copying a cache isn't allowed. */
-    ResourceCache(const ResourceCache &) ENTT_NOEXCEPT = delete;
     /*! @brief Default move constructor. */
-    ResourceCache(ResourceCache &&) ENTT_NOEXCEPT = default;
+    resource_cache(resource_cache &&) = default;
 
-    /*! @brief Copying a cache isn't allowed. @return This cache. */
-    ResourceCache & operator=(const ResourceCache &) ENTT_NOEXCEPT = delete;
     /*! @brief Default move assignment operator. @return This cache. */
-    ResourceCache & operator=(ResourceCache &&) ENTT_NOEXCEPT = default;
+    resource_cache & operator=(resource_cache &&) = default;
 
     /**
      * @brief Number of resources managed by a cache.
@@ -86,24 +82,31 @@ 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(const resource_type id, Args &&... args) {
-        static_assert(std::is_base_of<ResourceLoader<Loader, Resource>, Loader>::value, "!");
+    resource_handle<Resource> load(const resource_type id, Args &&... args) {
+        static_assert(std::is_base_of_v<resource_loader<Loader, Resource>, Loader>);
+        resource_handle<Resource> handle{};
 
-        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 resource = Loader{}.get(std::forward<Args>(args)...); resource) {
+                resources[id] = resource;
+                handle = std::move(resource);
+            }
+        } else {
+            handle = it->second;
         }
 
-        return loaded;
+        return handle;
     }
 
     /**
@@ -119,14 +122,18 @@ 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>
-    bool reload(const resource_type id, Args &&... args) {
+    resource_handle<Resource> reload(const resource_type id, Args &&... args) {
         return (discard(id), load<Loader>(id, std::forward<Args>(args)...));
     }
 
@@ -143,7 +150,7 @@ public:
      * @return A handle for the given resource.
      */
     template<typename Loader, typename... Args>
-    ResourceHandle<Resource> temp(Args &&... args) const {
+    resource_handle<Resource> temp(Args &&... args) const {
         return { Loader{}.get(std::forward<Args>(args)...) };
     }
 
@@ -155,12 +162,12 @@ public:
      * cache contains the resource itself. Otherwise the returned handle is
      * uninitialized and accessing it results in undefined behavior.
      *
-     * @sa ResourceHandle
+     * @sa resource_handle
      *
      * @param id Unique resource identifier.
      * @return A handle for the given resource.
      */
-    ResourceHandle<Resource> handle(const resource_type id) const {
+    resource_handle<Resource> handle(const resource_type id) const {
         auto it = resources.find(id);
         return { it == resources.end() ? nullptr : it->second };
     }
@@ -183,13 +190,46 @@ public:
      * @param id Unique resource identifier.
      */
     void discard(const resource_type id) ENTT_NOEXCEPT {
-        auto it = resources.find(id);
-
-        if(it != resources.end()) {
+        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 resource_type);
+     * void(resource_handle<Resource>);
+     * void(const resource_type, resource_handle<Resource>);
+     * @endcode
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template <typename Func>
+    void each(Func func) const {
+        auto begin = resources.begin();
+        auto end = resources.end();
+
+        while(begin != end) {
+            auto curr = begin++;
+
+            if constexpr(std::is_invocable_v<Func, resource_type>) {
+                func(curr->first);
+            } else if constexpr(std::is_invocable_v<Func, resource_handle<Resource>>) {
+                func(resource_handle{ curr->second });
+            } else {
+                func(curr->first, resource_handle{ curr->second });
+            }
+        }
+    }
+
 private:
     container_type resources;
 };

src/entt/resource/fwd.hpp

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

src/entt/resource/handle.hpp

@@ -4,17 +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.
  *
@@ -28,24 +24,17 @@ class ResourceCache;
  * @tparam Resource Type of resource managed by a handle.
  */
 template<typename Resource>
-class ResourceHandle final {
+class resource_handle {
     /*! @brief Resource handles are friends of their caches. */
-    friend class ResourceCache<Resource>;
+    friend class resource_cache<Resource>;
 
-    ResourceHandle(std::shared_ptr<Resource> res) ENTT_NOEXCEPT
+    resource_handle(std::shared_ptr<Resource> res) ENTT_NOEXCEPT
         : resource{std::move(res)}
     {}
 
 public:
-    /*! @brief Default copy constructor. */
-    ResourceHandle(const ResourceHandle &) ENTT_NOEXCEPT = default;
-    /*! @brief Default move constructor. */
-    ResourceHandle(ResourceHandle &&) ENTT_NOEXCEPT = default;
-
-    /*! @brief Default copy assignment operator. @return This handle. */
-    ResourceHandle & operator=(const ResourceHandle &) ENTT_NOEXCEPT = default;
-    /*! @brief Default move assignment operator. @return This handle. */
-    ResourceHandle & operator=(ResourceHandle &&) ENTT_NOEXCEPT = default;
+    /*! @brief Default constructor. */
+    resource_handle() ENTT_NOEXCEPT = default;
 
     /**
      * @brief Gets a reference to the managed resource.
@@ -58,34 +47,29 @@ public:
      * @return A reference to the managed resource.
      */
     const Resource & get() const ENTT_NOEXCEPT {
-        assert(static_cast<bool>(resource));
+        ENTT_ASSERT(static_cast<bool>(resource));
         return *resource;
     }
 
-    /**
-     * @brief Casts a handle and gets a reference to the managed resource.
-     *
-     * @warning
-     * The behavior is undefined if the handle doesn't contain a resource.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * handle is empty.
-     */
-    inline operator const Resource &() const ENTT_NOEXCEPT { return get(); }
+    /*! @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 ENTT_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/>
@@ -95,13 +79,19 @@ public:
      * @return A pointer to the managed resource or `nullptr` if the handle
      * contains no resource at all.
      */
-    inline const Resource * operator ->() const ENTT_NOEXCEPT {
-        assert(static_cast<bool>(resource));
+    const Resource * operator->() const ENTT_NOEXCEPT {
+        ENTT_ASSERT(static_cast<bool>(resource));
         return resource.get();
     }
 
+    /*! @copydoc operator-> */
+    Resource * operator->() ENTT_NOEXCEPT {
+        return const_cast<Resource *>(std::as_const(*this).operator->());
+    }
+
     /**
-     * @brief Returns true if the handle contains a resource, false otherwise.
+     * @brief Returns true if a handle contains a resource, false otherwise.
+     * @return True if the handle contains a resource, false otherwise.
      */
     explicit operator bool() const { return static_cast<bool>(resource); }
 

src/entt/resource/loader.hpp

@@ -3,15 +3,12 @@
 
 
 #include <memory>
+#include "fwd.hpp"
 
 
 namespace entt {
 
 
-template<typename Resource>
-class ResourceCache;
-
-
 /**
  * @brief Base class for resource loaders.
  *
@@ -22,12 +19,12 @@ class ResourceCache;
  * 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::resource_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
@@ -45,10 +42,16 @@ class ResourceCache;
  * @tparam Resource Type of resource for which to use the loader.
  */
 template<typename Loader, typename Resource>
-class ResourceLoader {
+class resource_loader {
     /*! @brief Resource loaders are friends of their caches. */
-    friend class ResourceCache<Resource>;
+    friend class resource_cache<Resource>;
 
+    /**
+     * @brief Loads the resource and returns it.
+     * @tparam Args Types of arguments for the loader.
+     * @param args Arguments for the loader.
+     * @return The resource just loaded or an empty pointer in case of errors.
+     */
     template<typename... Args>
     std::shared_ptr<Resource> get(Args &&... args) const {
         return static_cast<const Loader *>(this)->load(std::forward<Args>(args)...);

src/entt/signal/delegate.hpp

@@ -2,13 +2,80 @@
 #define ENTT_SIGNAL_DELEGATE_HPP
 
 
+#include <tuple>
+#include <cstring>
 #include <utility>
+#include <algorithm>
+#include <functional>
+#include <type_traits>
 #include "../config/config.h"
 
 
 namespace entt {
 
 
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename Ret, typename... Args>
+auto to_function_pointer(Ret(*)(Args...)) -> Ret(*)(Args...);
+
+
+template<typename Ret, typename... Args, typename Type, typename Payload, typename = std::enable_if_t<std::is_convertible_v<Payload &, Type &>>>
+auto to_function_pointer(Ret(*)(Type &, Args...), Payload &) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Ret, typename... Args>
+auto to_function_pointer(Ret(Class:: *)(Args...), const Class &) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Ret, typename... Args>
+auto to_function_pointer(Ret(Class:: *)(Args...) const, const Class &) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Type>
+auto to_function_pointer(Type Class:: *, const Class &) -> Type(*)();
+
+
+template<typename>
+struct function_extent;
+
+
+template<typename Ret, typename... Args>
+struct function_extent<Ret(*)(Args...)> {
+    static constexpr auto value = sizeof...(Args);
+};
+
+
+template<typename Func>
+constexpr auto function_extent_v = function_extent<Func>::value;
+
+
+}
+
+
+/**
+ * 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.
  *
@@ -16,129 +83,224 @@ namespace entt {
  * compile-time error unless the template parameter is a function type.
  */
 template<typename>
-class Delegate;
+class delegate;
 
 
 /**
- * @brief Utility class to send around functions and member functions.
+ * @brief Utility class to use to send around functions and members.
  *
- * Unmanaged delegate for function pointers and member functions. Users of this
- * class are in charge of disconnecting instances before deleting them.
+ * Unmanaged delegate for function pointers and members. Users of this class are
+ * in charge of disconnecting instances before deleting them.
  *
  * A delegate can be used as general purpose invoker with no memory overhead for
- * free functions and member functions provided along with an instance on which
- * to invoke them.
+ * free functions (with or without payload) and members provided along with an
+ * instance on which to invoke them.
  *
  * @tparam Ret Return type of a function type.
  * @tparam Args Types of arguments of a function type.
  */
 template<typename Ret, typename... Args>
-class Delegate<Ret(Args...)> final {
-    using proto_fn_type = Ret(void *, Args...);
-    using stub_type = std::pair<void *, proto_fn_type *>;
+class delegate<Ret(Args...)> {
+    using proto_fn_type = Ret(const void *, std::tuple<Args &&...>);
 
-    template<Ret(*Function)(Args...)>
-    static Ret proto(void *, Args... args) {
-        return (Function)(args...);
+    template<auto Function, std::size_t... Index>
+    void connect(std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        static_assert(std::is_invocable_r_v<Ret, decltype(Function), std::tuple_element_t<Index, std::tuple<Args...>>...>);
+        data = nullptr;
+
+        fn = [](const void *, std::tuple<Args &&...> args) -> Ret {
+            // Ret(...) makes void(...) eat the return values to avoid errors
+            return Ret(std::invoke(Function, std::forward<std::tuple_element_t<Index, std::tuple<Args...>>>(std::get<Index>(args))...));
+        };
     }
 
-    template<typename Class, Ret(Class:: *Member)(Args...)>
-    static Ret proto(void *instance, Args... args) {
-        return (static_cast<Class *>(instance)->*Member)(args...);
+    template<auto Candidate, typename Type, std::size_t... Index>
+    void connect(Type &value_or_instance, std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        static_assert(std::is_invocable_r_v<Ret, decltype(Candidate), Type &, std::tuple_element_t<Index, std::tuple<Args...>>...>);
+        data = &value_or_instance;
+
+        fn = [](const void *payload, std::tuple<Args &&...> args) -> Ret {
+            Type *curr = nullptr;
+
+            if constexpr(std::is_const_v<Type>) {
+                curr = static_cast<Type *>(payload);
+            } else {
+                curr = static_cast<Type *>(const_cast<void *>(payload));
+            }
+
+            // Ret(...) makes void(...) eat the return values to avoid errors
+            return Ret(std::invoke(Candidate, *curr, std::forward<std::tuple_element_t<Index, std::tuple<Args...>>>(std::get<Index>(args))...));
+        };
     }
 
 public:
+    /*! @brief Function type of the delegate. */
+    using function_type = Ret(Args...);
+
     /*! @brief Default constructor. */
-    Delegate() ENTT_NOEXCEPT
-        : stub{}
+    delegate() ENTT_NOEXCEPT
+        : fn{nullptr}, data{nullptr}
     {}
 
     /**
-     * @brief Checks whether a delegate actually stores a listener.
-     * @return True if the delegate is empty, false otherwise.
-     */
-    bool empty() const ENTT_NOEXCEPT {
-        // no need to test also stub.first
-        return !stub.second;
-    }
-
-    /**
-     * @brief Binds a free function to a delegate.
+     * @brief Constructs a delegate and connects a free function to it.
      * @tparam Function A valid free function pointer.
      */
-    template<Ret(*Function)(Args...)>
-    void connect() ENTT_NOEXCEPT {
-        stub = std::make_pair(nullptr, &proto<Function>);
+    template<auto Function>
+    delegate(connect_arg_t<Function>) ENTT_NOEXCEPT
+        : delegate{}
+    {
+        connect<Function>();
     }
 
     /**
-     * @brief Connects a member function for a given instance to a delegate.
-     *
-     * The delegate isn't responsible for the connected object. Users must
-     * guarantee that the lifetime of the instance overcomes the one of the
-     * delegate.
-     *
-     * @tparam Class Type of class to which the member function belongs.
-     * @tparam Member Member function to connect to the delegate.
-     * @param instance A valid instance of type pointer to `Class`.
+     * @brief Constructs a delegate and connects a member for a given instance
+     * or a free function with payload.
+     * @tparam Candidate Member or free function to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid reference that fits the purpose.
      */
-    template<typename Class, Ret(Class:: *Member)(Args...)>
-    void connect(Class *instance) ENTT_NOEXCEPT {
-        stub = std::make_pair(instance, &proto<Class, Member>);
+    template<auto Candidate, typename Type>
+    delegate(connect_arg_t<Candidate>, Type &value_or_instance) ENTT_NOEXCEPT
+        : delegate{}
+    {
+        connect<Candidate>(value_or_instance);
+    }
+
+    /**
+     * @brief Connects a free function to a delegate.
+     * @tparam Function A valid free function pointer.
+     */
+    template<auto Function>
+    void connect() ENTT_NOEXCEPT {
+        constexpr auto extent = internal::function_extent_v<decltype(internal::to_function_pointer(std::declval<decltype(Function)>()))>;
+        connect<Function>(std::make_index_sequence<extent>{});
+    }
+
+    /**
+     * @brief Connects a member function for a given instance or a free function
+     * with payload to a delegate.
+     *
+     * The delegate isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of the instance overcomes
+     * the one  of the delegate.<br/>
+     * When used to connect a free function with payload, its signature must be
+     * such that the instance is the first argument before the ones used to
+     * define the delegate itself.
+     *
+     * @tparam Candidate Member or free function to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid reference that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void connect(Type &value_or_instance) ENTT_NOEXCEPT {
+        constexpr auto extent = internal::function_extent_v<decltype(internal::to_function_pointer(std::declval<decltype(Candidate)>(), value_or_instance))>;
+        connect<Candidate>(value_or_instance, std::make_index_sequence<extent>{});
     }
 
     /**
      * @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() ENTT_NOEXCEPT {
-        stub.second = nullptr;
+        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) const {
-        return stub.second(stub.first, args...);
+        ENTT_ASSERT(fn);
+        return fn(data, std::forward_as_tuple(std::forward<Args>(args)...));
     }
 
     /**
-     * @brief Checks if the contents of the two delegates are different.
-     *
-     * Two delegates are identical if they contain the same listener.
-     *
-     * @param other Delegate with which to compare.
-     * @return True if the two delegates are identical, false otherwise.
+     * @brief Checks whether a delegate actually stores a listener.
+     * @return False if the delegate is empty, true otherwise.
      */
-    bool operator==(const Delegate<Ret(Args...)> &other) const ENTT_NOEXCEPT {
-        return stub.first == other.stub.first && stub.second == other.stub.second;
+    explicit operator bool() const ENTT_NOEXCEPT {
+        // no need to test also data
+        return fn;
+    }
+
+    /**
+     * @brief Compares the contents of two delegates.
+     * @param other Delegate with which to compare.
+     * @return False if the two contents differ, true otherwise.
+     */
+    bool operator==(const delegate<Ret(Args...)> &other) const ENTT_NOEXCEPT {
+        return fn == other.fn && data == other.data;
     }
 
 private:
-    stub_type stub;
+    proto_fn_type *fn;
+    const void *data;
 };
 
 
 /**
- * @brief Checks if the contents of the two delegates are different.
- *
- * Two delegates are identical if they contain the same listener.
- *
+ * @brief Compares the contents of two delegates.
  * @tparam Ret Return type of a function type.
  * @tparam Args Types of arguments of a function type.
  * @param lhs A valid delegate object.
  * @param rhs A valid delegate object.
- * @return True if the two delegates are different, false otherwise.
+ * @return True if the two contents differ, false otherwise.
  */
 template<typename Ret, typename... Args>
-bool operator!=(const Delegate<Ret(Args...)> &lhs, const Delegate<Ret(Args...)> &rhs) ENTT_NOEXCEPT {
+bool operator!=(const delegate<Ret(Args...)> &lhs, const delegate<Ret(Args...)> &rhs) ENTT_NOEXCEPT {
     return !(lhs == rhs);
 }
 
 
+/**
+ * @brief Deduction guide.
+ *
+ * It allows to deduce the function type of the delegate directly from a
+ * function provided to the constructor.
+ *
+ * @tparam Function A valid free function pointer.
+ */
+template<auto Function>
+delegate(connect_arg_t<Function>) ENTT_NOEXCEPT
+-> delegate<std::remove_pointer_t<decltype(internal::to_function_pointer(Function))>>;
+
+
+/**
+ * @brief Deduction guide.
+ *
+ * It allows to deduce the function type of the delegate directly from a member
+ * or a free function with payload provided to the constructor.
+ *
+ * @param value_or_instance A valid reference that fits the purpose.
+ * @tparam Candidate Member or free function to connect to the delegate.
+ * @tparam Type Type of class or type of payload.
+ */
+template<auto Candidate, typename Type>
+delegate(connect_arg_t<Candidate>, Type &value_or_instance) ENTT_NOEXCEPT
+-> delegate<std::remove_pointer_t<decltype(internal::to_function_pointer(Candidate, value_or_instance))>>;
+
+
 }
 
 

src/entt/signal/dispatcher.hpp

@@ -5,11 +5,10 @@
 #include <vector>
 #include <memory>
 #include <utility>
-#include <cstdint>
-#include <algorithm>
 #include <type_traits>
 #include "../config/config.h"
 #include "../core/family.hpp"
+#include "../core/type_traits.hpp"
 #include "sigh.hpp"
 
 
@@ -22,77 +21,109 @@ 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 function type:
- * @code{.cpp}
- * void(const Event &)
- * @endcode
+ * type `Event`, listeners are such that they can be invoked with an argument of
+ * type `const Event &`, no matter what the return type is.
  *
- * Member functions named `receive` are automatically detected and registered or
- * unregistered by the dispatcher. The type of the instances is `Class *` (a
- * naked pointer). It means that users must guarantee that the lifetimes of the
- * instances overcome the one of the dispatcher itself to avoid crashes.
+ * 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.
  */
-class Dispatcher final {
-    using event_family = Family<struct InternalDispatcherEventFamily>;
+class dispatcher {
+    using event_family = family<struct internal_dispatcher_event_family>;
 
     template<typename Class, typename Event>
-    using instance_type = typename SigH<void(const Event &)>::template instance_type<Class>;
+    using instance_type = typename sigh<void(const Event &)>::template instance_type<Class>;
 
-    struct BaseSignalWrapper {
-        virtual ~BaseSignalWrapper() = default;
+    struct base_wrapper {
+        virtual ~base_wrapper() = default;
         virtual void publish() = 0;
     };
 
     template<typename Event>
-    struct SignalWrapper final: BaseSignalWrapper {
-        using sink_type = typename SigH<void(const Event &)>::sink_type;
+    struct signal_wrapper: base_wrapper {
+        using signal_type = sigh<void(const Event &)>;
+        using sink_type = typename signal_type::sink_type;
 
         void publish() override {
-            const auto &curr = current++;
+            for(const auto &event: events[current]) {
+                signal.publish(event);
+            }
+
+            events[current++].clear();
             current %= std::extent<decltype(events)>::value;
-            std::for_each(events[curr].cbegin(), events[curr].cend(), [this](const auto &event) { signal.publish(event); });
-            events[curr].clear();
         }
 
-        inline sink_type sink() ENTT_NOEXCEPT {
-            return signal.sink();
+        sink_type sink() ENTT_NOEXCEPT {
+            return entt::sink{signal};
         }
 
         template<typename... Args>
-        inline void trigger(Args &&... args) {
+        void trigger(Args &&... args) {
             signal.publish({ std::forward<Args>(args)... });
         }
 
         template<typename... Args>
-        inline void enqueue(Args &&... args) {
-            events[current].push_back({ std::forward<Args>(args)... });
+        void enqueue(Args &&... args) {
+            events[current].emplace_back(std::forward<Args>(args)...);
         }
 
     private:
-        SigH<void(const Event &)> signal{};
+        signal_type signal{};
         std::vector<Event> events[2];
         int current{};
     };
 
+    struct wrapper_data {
+        std::unique_ptr<base_wrapper> wrapper;
+        ENTT_ID_TYPE runtime_type;
+    };
+
     template<typename Event>
-    SignalWrapper<Event> & wrapper() {
-        const auto type = event_family::type<Event>();
+    static auto type() ENTT_NOEXCEPT {
+        if constexpr(is_named_type_v<Event>) {
+            return named_type_traits<Event>::value;
+        } else {
+            return event_family::type<Event>;
+        }
+    }
 
-        if(!(type < wrappers.size())) {
-            wrappers.resize(type + 1);
+    template<typename Event>
+    signal_wrapper<Event> & assure() {
+        const auto wtype = type<Event>();
+        wrapper_data *wdata = nullptr;
+
+        if constexpr(is_named_type_v<Event>) {
+            const auto it = std::find_if(wrappers.begin(), wrappers.end(), [wtype](const auto &candidate) {
+                return candidate.wrapper && candidate.runtime_type == wtype;
+            });
+
+            wdata = (it == wrappers.cend() ? &wrappers.emplace_back() : &(*it));
+        } else {
+            if(!(wtype < wrappers.size())) {
+                wrappers.resize(wtype+1);
+            }
+
+            wdata = &wrappers[wtype];
+
+            if(wdata->wrapper && wdata->runtime_type != wtype) {
+                wrappers.emplace_back();
+                std::swap(wrappers[wtype], wrappers.back());
+                wdata = &wrappers[wtype];
+            }
         }
 
-        if(!wrappers[type]) {
-            wrappers[type] = std::make_unique<SignalWrapper<Event>>();
+        if(!wdata->wrapper) {
+            wdata->wrapper = std::make_unique<signal_wrapper<Event>>();
+            wdata->runtime_type = wtype;
         }
 
-        return static_cast<SignalWrapper<Event> &>(*wrappers[type]);
+        return static_cast<signal_wrapper<Event> &>(*wdata->wrapper);
     }
 
 public:
     /*! @brief Type of sink for the given event. */
     template<typename Event>
-    using sink_type = typename SignalWrapper<Event>::sink_type;
+    using sink_type = typename signal_wrapper<Event>::sink_type;
 
     /**
      * @brief Returns a sink object for the given event.
@@ -101,19 +132,19 @@ public:
      *
      * The function type for a listener is:
      * @code{.cpp}
-     * void(const Event &)
+     * void(const Event &);
      * @endcode
      *
      * The order of invocation of the listeners isn't guaranteed.
      *
-     * @sa SigH::Sink
+     * @sa sink
      *
      * @tparam Event Type of event of which to get the sink.
      * @return A temporary sink object.
      */
     template<typename Event>
-    inline sink_type<Event> sink() ENTT_NOEXCEPT {
-        return wrapper<Event>().sink();
+    sink_type<Event> sink() ENTT_NOEXCEPT {
+        return assure<Event>().sink();
     }
 
     /**
@@ -127,8 +158,22 @@ public:
      * @param args Arguments to use to construct the event.
      */
     template<typename Event, typename... Args>
-    inline void trigger(Args &&... args) {
-        wrapper<Event>().trigger(std::forward<Args>(args)...);
+    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));
     }
 
     /**
@@ -137,13 +182,27 @@ 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>
-    inline void enqueue(Args &&... args) {
-        wrapper<Event>().enqueue(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));
     }
 
     /**
@@ -156,8 +215,8 @@ public:
      * @tparam Event Type of events to send.
      */
     template<typename Event>
-    inline void update() {
-        wrapper<Event>().publish();
+    void update() {
+        assure<Event>().publish();
     }
 
     /**
@@ -167,18 +226,18 @@ public:
      * delivered to the registered listeners. It's responsibility of the users
      * to reduce at a minimum the time spent in the bodies of the listeners.
      */
-    inline void update() const {
+    void update() const {
         for(auto pos = wrappers.size(); pos; --pos) {
-            auto &wrapper = wrappers[pos-1];
+            auto &wdata = wrappers[pos-1];
 
-            if(wrapper) {
-                wrapper->publish();
+            if(wdata.wrapper) {
+                wdata.wrapper->publish();
             }
         }
     }
 
 private:
-    std::vector<std::unique_ptr<BaseSignalWrapper>> wrappers;
+    std::vector<wrapper_data> wrappers;
 };
 
 

src/entt/signal/emitter.hpp

@@ -6,12 +6,12 @@
 #include <functional>
 #include <algorithm>
 #include <utility>
-#include <cstdint>
 #include <memory>
 #include <vector>
 #include <list>
 #include "../config/config.h"
 #include "../core/family.hpp"
+#include "../core/type_traits.hpp"
 
 
 namespace entt {
@@ -23,11 +23,11 @@ namespace entt {
  * The emitter class template follows the CRTP idiom. To create a custom emitter
  * type, derived classes must inherit directly from the base class as:
  *
- * ```cpp
- * struct MyEmitter: Emitter<MyEmitter> {
+ * @code{.cpp}
+ * struct my_emitter: emitter<my_emitter> {
  *     // ...
  * }
- * ```
+ * @endcode
  *
  * Handlers for the type of events are created internally on the fly. It's not
  * required to specify in advance the full list of accepted types.<br/>
@@ -39,17 +39,17 @@ namespace entt {
  * @tparam Derived Actual type of emitter that extends the class template.
  */
 template<typename Derived>
-class Emitter {
-    using handler_family = Family<struct InternalEmitterHandlerFamily>;
+class emitter {
+    using handler_family = family<struct internal_emitter_handler_family>;
 
-    struct BaseHandler {
-        virtual ~BaseHandler() = default;
+    struct base_handler {
+        virtual ~base_handler() = default;
         virtual bool empty() const ENTT_NOEXCEPT = 0;
         virtual void clear() ENTT_NOEXCEPT = 0;
     };
 
     template<typename Event>
-    struct Handler final: BaseHandler {
+    struct event_handler: base_handler {
         using listener_type = std::function<void(const Event &, Derived &)>;
         using element_type = std::pair<bool, listener_type>;
         using container_type = std::list<element_type>;
@@ -58,27 +58,27 @@ class Emitter {
         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() 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);
+                std::for_each(once_list.begin(), once_list.end(), func);
+                std::for_each(on_list.begin(), on_list.end(), func);
             } else {
-                onceL.clear();
-                onL.clear();
+                once_list.clear();
+                on_list.clear();
             }
         }
 
-        inline connection_type once(listener_type listener) {
-            return onceL.emplace(onceL.cend(), false, std::move(listener));
+        connection_type once(listener_type listener) {
+            return once_list.emplace(once_list.cend(), false, std::move(listener));
         }
 
-        inline connection_type on(listener_type listener) {
-            return onL.emplace(onL.cend(), false, std::move(listener));
+        connection_type on(listener_type listener) {
+            return on_list.emplace(on_list.cend(), false, std::move(listener));
         }
 
         void erase(connection_type conn) ENTT_NOEXCEPT {
@@ -86,14 +86,14 @@ class Emitter {
 
             if(!publishing) {
                 auto pred = [](auto &&element) { return element.first; };
-                onceL.remove_if(pred);
-                onL.remove_if(pred);
+                once_list.remove_if(pred);
+                on_list.remove_if(pred);
             }
         }
 
         void publish(const Event &event, Derived &ref) {
-            container_type currentL;
-            onceL.swap(currentL);
+            container_type swap_list;
+            once_list.swap(swap_list);
 
             auto func = [&event, &ref](auto &&element) {
                 return element.first ? void() : element.second(event, ref);
@@ -101,39 +101,71 @@ class Emitter {
 
             publishing = true;
 
-            std::for_each(onL.rbegin(), onL.rend(), func);
-            std::for_each(currentL.rbegin(), currentL.rend(), func);
+            std::for_each(on_list.rbegin(), on_list.rend(), func);
+            std::for_each(swap_list.rbegin(), swap_list.rend(), func);
 
             publishing = false;
 
-            onL.remove_if([](auto &&element) { return element.first; });
+            on_list.remove_if([](auto &&element) { return element.first; });
         }
 
     private:
         bool publishing{false};
-        container_type onceL{};
-        container_type onL{};
+        container_type once_list{};
+        container_type on_list{};
+    };
+
+    struct handler_data {
+        std::unique_ptr<base_handler> handler;
+        ENTT_ID_TYPE runtime_type;
     };
 
     template<typename Event>
-    Handler<Event> & handler() ENTT_NOEXCEPT {
-        const std::size_t family = handler_family::type<Event>();
+    static auto type() ENTT_NOEXCEPT {
+        if constexpr(is_named_type_v<Event>) {
+            return named_type_traits<Event>::value;
+        } else {
+            return handler_family::type<Event>;
+        }
+    }
 
-        if(!(family < handlers.size())) {
-            handlers.resize(family+1);
+    template<typename Event>
+    event_handler<Event> * assure() const ENTT_NOEXCEPT {
+        const auto htype = type<Event>();
+        handler_data *hdata = nullptr;
+
+        if constexpr(is_named_type_v<Event>) {
+            const auto it = std::find_if(handlers.begin(), handlers.end(), [htype](const auto &candidate) {
+                return candidate.handler && candidate.runtime_type == htype;
+            });
+
+            hdata = (it == handlers.cend() ? &handlers.emplace_back() : &(*it));
+        } else {
+            if(!(htype < handlers.size())) {
+                handlers.resize(htype+1);
+            }
+
+            hdata = &handlers[htype];
+
+            if(hdata->handler && hdata->runtime_type != htype) {
+                handlers.emplace_back();
+                std::swap(handlers[htype], handlers.back());
+                hdata = &handlers[htype];
+            }
         }
 
-        if(!handlers[family]) {
-            handlers[family] = std::make_unique<Handler<Event>>();
+        if(!hdata->handler) {
+            hdata->handler = std::make_unique<event_handler<Event>>();
+            hdata->runtime_type = htype;
         }
 
-        return static_cast<Handler<Event> &>(*handlers[family]);
+        return static_cast<event_handler<Event> *>(hdata->handler.get());
     }
 
 public:
     /** @brief Type of listeners accepted for the given event. */
     template<typename Event>
-    using Listener = typename Handler<Event>::listener_type;
+    using listener = typename event_handler<Event>::listener_type;
 
     /**
      * @brief Generic connection type for events.
@@ -145,56 +177,35 @@ public:
      * @tparam Event Type of event for which the connection is created.
      */
     template<typename Event>
-    struct Connection final: private Handler<Event>::connection_type {
+    struct connection: private event_handler<Event>::connection_type {
         /** @brief Event emitters are friend classes of connections. */
-        friend class Emitter;
+        friend class emitter;
 
         /*! @brief Default constructor. */
-        Connection() ENTT_NOEXCEPT = default;
+        connection() ENTT_NOEXCEPT = default;
 
         /**
          * @brief Creates a connection that wraps its underlying instance.
          * @param conn A connection object to wrap.
          */
-        Connection(typename Handler<Event>::connection_type conn)
-            : Handler<Event>::connection_type{std::move(conn)}
+        connection(typename event_handler<Event>::connection_type conn)
+            : event_handler<Event>::connection_type{std::move(conn)}
         {}
-
-        /*! @brief Default copy constructor. */
-        Connection(const Connection &) = default;
-        /*! @brief Default move constructor. */
-        Connection(Connection &&) = default;
-
-        /**
-         * @brief Default copy assignment operator.
-         * @return This connection.
-         */
-        Connection & operator=(const Connection &) = default;
-
-        /**
-         * @brief Default move assignment operator.
-         * @return This connection.
-         */
-        Connection & operator=(Connection &&) = default;
     };
 
     /*! @brief Default constructor. */
-    Emitter() ENTT_NOEXCEPT = default;
+    emitter() ENTT_NOEXCEPT = default;
 
     /*! @brief Default destructor. */
-    virtual ~Emitter() ENTT_NOEXCEPT {
-        static_assert(std::is_base_of<Emitter<Derived>, Derived>::value, "!");
+    virtual ~emitter() ENTT_NOEXCEPT {
+        static_assert(std::is_base_of_v<emitter<Derived>, Derived>);
     }
 
-    /*! @brief Copying an emitter isn't allowed. */
-    Emitter(const Emitter &) = delete;
     /*! @brief Default move constructor. */
-    Emitter(Emitter &&) = default;
+    emitter(emitter &&) = default;
 
-    /*! @brief Copying an emitter isn't allowed. @return This emitter. */
-    Emitter & operator=(const Emitter &) = delete;
     /*! @brief Default move assignment operator. @return This emitter. */
-    Emitter & operator=(Emitter &&) = default;
+    emitter & operator=(emitter &&) = default;
 
     /**
      * @brief Emits the given event.
@@ -209,7 +220,7 @@ public:
      */
     template<typename Event, typename... Args>
     void publish(Args &&... args) {
-        handler<Event>().publish({ std::forward<Args>(args)... }, *static_cast<Derived *>(this));
+        assure<Event>()->publish({ std::forward<Args>(args)... }, *static_cast<Derived *>(this));
     }
 
     /**
@@ -229,12 +240,12 @@ public:
      * instances for later uses.
      *
      * @tparam Event Type of event to which to connect the listener.
-     * @param listener The listener to register.
+     * @param instance The listener to register.
      * @return Connection object that can be used to disconnect the listener.
      */
     template<typename Event>
-    Connection<Event> on(Listener<Event> listener) {
-        return handler<Event>().on(std::move(listener));
+    connection<Event> on(listener<Event> instance) {
+        return assure<Event>()->on(std::move(instance));
     }
 
     /**
@@ -254,12 +265,12 @@ public:
      * instances for later uses.
      *
      * @tparam Event Type of event to which to connect the listener.
-     * @param listener The listener to register.
+     * @param instance The listener to register.
      * @return Connection object that can be used to disconnect the listener.
      */
     template<typename Event>
-    Connection<Event> once(Listener<Event> listener) {
-        return handler<Event>().once(std::move(listener));
+    connection<Event> once(listener<Event> instance) {
+        return assure<Event>()->once(std::move(instance));
     }
 
     /**
@@ -272,8 +283,8 @@ public:
      * @param conn A valid connection.
      */
     template<typename Event>
-    void erase(Connection<Event> conn) ENTT_NOEXCEPT {
-        handler<Event>().erase(std::move(conn));
+    void erase(connection<Event> conn) ENTT_NOEXCEPT {
+        assure<Event>()->erase(std::move(conn));
     }
 
     /**
@@ -286,7 +297,7 @@ public:
      */
     template<typename Event>
     void clear() ENTT_NOEXCEPT {
-        handler<Event>().clear();
+        assure<Event>()->clear();
     }
 
     /**
@@ -296,8 +307,8 @@ public:
      * results in undefined behavior.
      */
     void clear() ENTT_NOEXCEPT {
-        std::for_each(handlers.begin(), handlers.end(), [](auto &&handler) {
-            return handler ? handler->clear() : void();
+        std::for_each(handlers.begin(), handlers.end(), [](auto &&hdata) {
+            return hdata.handler ? hdata.handler->clear() : void();
         });
     }
 
@@ -308,11 +319,7 @@ public:
      */
     template<typename Event>
     bool empty() const ENTT_NOEXCEPT {
-        const std::size_t family = handler_family::type<Event>();
-
-        return (!(family < handlers.size()) ||
-                !handlers[family] ||
-                static_cast<Handler<Event> &>(*handlers[family]).empty());
+        return assure<Event>()->empty();
     }
 
     /**
@@ -320,13 +327,13 @@ public:
      * @return True if there are no listeners registered, false otherwise.
      */
     bool empty() const ENTT_NOEXCEPT {
-        return std::all_of(handlers.cbegin(), handlers.cend(), [](auto &&handler) {
-            return !handler || handler->empty();
+        return std::all_of(handlers.cbegin(), handlers.cend(), [](auto &&hdata) {
+            return !hdata.handler || hdata.handler->empty();
         });
     }
 
 private:
-    std::vector<std::unique_ptr<BaseHandler>> handlers{};
+    mutable std::vector<handler_data> handlers{};
 };
 
 

src/entt/signal/fwd.hpp

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

src/entt/signal/sigh.hpp

@@ -5,90 +5,18 @@
 #include <algorithm>
 #include <utility>
 #include <vector>
+#include <functional>
+#include <type_traits>
 #include "../config/config.h"
+#include "delegate.hpp"
+#include "fwd.hpp"
 
 
 namespace entt {
 
 
-namespace internal {
-
-
 /**
- * @cond TURN_OFF_DOXYGEN
- * Internal details not to be documented.
- */
-
-
-template<typename, typename>
-struct Invoker;
-
-
-template<typename Ret, typename... Args, typename Collector>
-struct Invoker<Ret(Args...), Collector> {
-    using proto_fn_type = Ret(void *, Args...);
-    using call_type = std::pair<void *, proto_fn_type *>;
-
-    virtual ~Invoker() = default;
-
-    bool invoke(Collector &collector, proto_fn_type *proto, void *instance, Args... args) const {
-        return collector(proto(instance, args...));
-    }
-};
-
-
-template<typename... Args, typename Collector>
-struct Invoker<void(Args...), Collector> {
-    using proto_fn_type = void(void *, Args...);
-    using call_type = std::pair<void *, proto_fn_type *>;
-
-    virtual ~Invoker() = default;
-
-    bool invoke(Collector &, proto_fn_type *proto, void *instance, Args... args) const {
-        return (proto(instance, args...), true);
-    }
-};
-
-
-template<typename Ret>
-struct NullCollector final {
-    using result_type = Ret;
-    bool operator()(result_type) const ENTT_NOEXCEPT { return true; }
-};
-
-
-template<>
-struct NullCollector<void> final {
-    using result_type = void;
-    bool operator()() const ENTT_NOEXCEPT { return true; }
-};
-
-
-template<typename>
-struct DefaultCollector;
-
-
-template<typename Ret, typename... Args>
-struct DefaultCollector<Ret(Args...)> final {
-    using collector_type = NullCollector<Ret>;
-};
-
-
-template<typename Function>
-using DefaultCollectorType = typename DefaultCollector<Function>::collector_type;
-
-
-/**
- * Internal details not to be documented.
- * @endcond TURN_OFF_DOXYGEN
- */
-
-
-}
-
-
-/**
- * @brief Sink implementation.
+ * @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.
@@ -96,172 +24,46 @@ using DefaultCollectorType = typename DefaultCollector<Function>::collector_type
  * @tparam Function A valid function type.
  */
 template<typename Function>
-class Sink;
+class sink;
 
 
 /**
- * @brief Unmanaged signal handler declaration.
+ * @brief Unmanaged signal handler.
  *
  * Primary template isn't defined on purpose. All the specializations give a
  * compile-time error unless the template parameter is a function type.
  *
  * @tparam Function A valid function type.
- * @tparam Collector Type of collector to use, if any.
  */
-template<typename Function, typename Collector = internal::DefaultCollectorType<Function>>
-class SigH;
+template<typename Function>
+class sigh;
 
 
 /**
- * @brief Sink implementation.
+ * @brief Unmanaged signal handler.
  *
- * A sink is an opaque object used to connect listeners to signals.<br/>
- * The function type for a listener is the one of the signal to which it
- * belongs.
- *
- * The clear separation between a signal and a sink permits to store the
- * former as private data member without exposing the publish functionality
- * to the users of a class.
- *
- * @tparam Ret Return type of a function type.
- * @tparam Args Types of arguments of a function type.
- */
-template<typename Ret, typename... Args>
-class Sink<Ret(Args...)> final {
-    /*! @brief A signal is allowed to create sinks. */
-    template<typename, typename>
-    friend class SigH;
-
-    using proto_fn_type = Ret(void *, Args...);
-    using call_type = std::pair<void *, proto_fn_type *>;
-
-    template<Ret(*Function)(Args...)>
-    static Ret proto(void *, Args... args) {
-        return (Function)(args...);
-    }
-
-    template<typename Class, Ret(Class:: *Member)(Args... args)>
-    static Ret proto(void *instance, Args... args) {
-        return (static_cast<Class *>(instance)->*Member)(args...);
-    }
-
-    Sink(std::vector<call_type> &calls) ENTT_NOEXCEPT
-        : calls{calls}
-    {}
-
-public:
-    /**
-     * @brief Connects a free function to a signal.
-     *
-     * The signal handler performs checks to avoid multiple connections for
-     * free functions.
-     *
-     * @tparam Function A valid free function pointer.
-     */
-    template<Ret(*Function)(Args...)>
-    void connect() {
-        disconnect<Function>();
-        calls.emplace_back(nullptr, &proto<Function>);
-    }
-
-    /**
-     * @brief Connects a member function for a given instance to a signal.
-     *
-     * The signal isn't responsible for the connected object. Users must
-     * guarantee that the lifetime of the instance overcomes the one of the
-     * signal. On the other side, the signal handler performs checks to
-     * avoid multiple connections for the same member function of a given
-     * instance.
-     *
-     * @tparam Class Type of class to which the member function belongs.
-     * @tparam Member Member function to connect to the signal.
-     * @param instance A valid instance of type pointer to `Class`.
-     */
-    template <typename Class, Ret(Class:: *Member)(Args...) = &Class::receive>
-    void connect(Class *instance) {
-        disconnect<Class, Member>(instance);
-        calls.emplace_back(instance, &proto<Class, Member>);
-    }
-
-    /**
-     * @brief Disconnects a free function from a signal.
-     * @tparam Function A valid free function pointer.
-     */
-    template<Ret(*Function)(Args...)>
-    void disconnect() {
-        call_type target{nullptr, &proto<Function>};
-        calls.erase(std::remove(calls.begin(), calls.end(), std::move(target)), calls.end());
-    }
-
-    /**
-     * @brief Disconnects the given member function from a signal.
-     * @tparam Class Type of class to which the member function belongs.
-     * @tparam Member Member function to connect to the signal.
-     * @param instance A valid instance of type pointer to `Class`.
-     */
-    template<typename Class, Ret(Class:: *Member)(Args...)>
-    void disconnect(Class *instance) {
-        call_type target{instance, &proto<Class, Member>};
-        calls.erase(std::remove(calls.begin(), calls.end(), std::move(target)), calls.end());
-    }
-
-    /**
-     * @brief Removes all existing connections for the given instance.
-     * @tparam Class Type of class to which the member function belongs.
-     * @param instance A valid instance of type pointer to `Class`.
-     */
-    template<typename Class>
-    void disconnect(Class *instance) {
-        auto func = [instance](const call_type &call) { return call.first == instance; };
-        calls.erase(std::remove_if(calls.begin(), calls.end(), std::move(func)), calls.end());
-    }
-
-    /**
-     * @brief Disconnects all the listeners from a signal.
-     */
-    void disconnect() {
-        calls.clear();
-    }
-
-private:
-    std::vector<call_type> &calls;
-};
-
-
-/**
- * @brief Unmanaged signal handler definition.
- *
- * Unmanaged signal handler. It works directly with naked pointers to classes
- * and pointers to member functions as well as pointers to free functions. Users
- * of this class are in charge of disconnecting instances before deleting them.
+ * 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 internal::Invoker<Ret(Args...), Collector> {
-    using call_type = typename internal::Invoker<Ret(Args...), Collector>::call_type;
+template<typename Ret, typename... Args>
+class sigh<Ret(Args...)> {
+    /*! @brief A sink is allowed to modify a signal. */
+    friend class sink<Ret(Args...)>;
 
 public:
     /*! @brief Unsigned integer type. */
-    using size_type = typename std::vector<call_type>::size_type;
-    /*! @brief Collector type. */
-    using collector_type = Collector;
+    using size_type = typename std::vector<delegate<Ret(Args...)>>::size_type;
     /*! @brief Sink type. */
-    using sink_type = Sink<Ret(Args...)>;
+    using sink_type = entt::sink<Ret(Args...)>;
 
     /**
      * @brief Instance type when it comes to connecting member functions.
@@ -286,19 +88,6 @@ public:
         return calls.empty();
     }
 
-    /**
-     * @brief Returns a sink object for the given signal.
-     *
-     * A sink is an opaque object used to connect listeners to signals.<br/>
-     * The function type for a listener is the one of the signal to which it
-     * belongs. The order of invocation of the listeners isn't guaranteed.
-     *
-     * @return A temporary sink object.
-     */
-    sink_type sink() ENTT_NOEXCEPT {
-        return { calls };
-    }
-
     /**
      * @brief Triggers a signal.
      *
@@ -308,72 +97,346 @@ public:
      */
     void publish(Args... args) const {
         for(auto pos = calls.size(); pos; --pos) {
-            auto &call = calls[pos-1];
-            call.second(call.first, args...);
+            calls[pos-1](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) const {
-        collector_type collector;
+    template<typename Func>
+    void collect(Func func, Args... args) const {
+        bool stop = false;
 
-        for(auto &&call: calls) {
-            if(!this->invoke(collector, call.second, call.first, args...)) {
-                break;
+        for(auto pos = calls.size(); pos && !stop; --pos) {
+            if constexpr(std::is_void_v<Ret>) {
+                if constexpr(std::is_invocable_r_v<bool, Func>) {
+                    calls[pos-1](args...);
+                    stop = func();
+                } else {
+                    calls[pos-1](args...);
+                    func();
+                }
+            } else {
+                if constexpr(std::is_invocable_r_v<bool, Func, Ret>) {
+                    stop = func(calls[pos-1](args...));
+                } else {
+                    func(calls[pos-1](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 ENTT_NOEXCEPT {
-        return std::equal(calls.cbegin(), calls.cend(), other.calls.cbegin(), other.calls.cend());
     }
 
 private:
-    std::vector<call_type> calls;
+    std::vector<delegate<Ret(Args...)>> calls;
 };
 
 
 /**
- * @brief Checks if the contents of the two signals are different.
+ * @brief Connection class.
  *
- * Two signals are identical if they have the same size and the same
- * listeners registered exactly in the same order.
+ * Opaque object the aim of which is to allow users to release an already
+ * estabilished connection without having to keep a reference to the signal or
+ * the sink that generated it.
+ */
+class connection {
+    /*! @brief A sink is allowed to create connection objects. */
+    template<typename>
+    friend class sink;
+
+    connection(delegate<void(void *)> fn, void *ref)
+        : disconnect{fn}, signal{ref}
+    {}
+
+public:
+    /*! @brief Default constructor. */
+    connection() = default;
+
+    /*! @brief Default copy constructor. */
+    connection(const connection &) = default;
+
+    /**
+     * @brief Default move constructor.
+     * @param other The instance to move from.
+     */
+    connection(connection &&other)
+        : connection{}
+    {
+        std::swap(disconnect, other.disconnect);
+        std::swap(signal, other.signal);
+    }
+
+    /*! @brief Default copy assignment operator. @return This connection. */
+    connection & operator=(const connection &) = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @param other The instance to move from.
+     * @return This connection.
+     */
+    connection & operator=(connection &&other) {
+        if(this != &other) {
+            auto tmp{std::move(other)};
+            disconnect = tmp.disconnect;
+            signal = tmp.signal;
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Checks whether a connection is properly initialized.
+     * @return True if the connection is properly initialized, false otherwise.
+     */
+    explicit operator bool() const ENTT_NOEXCEPT {
+        return static_cast<bool>(disconnect);
+    }
+
+    /*! @brief Breaks the connection. */
+    void release() {
+        if(disconnect) {
+            disconnect(signal);
+            disconnect.reset();
+        }
+    }
+
+private:
+    delegate<void(void *)> disconnect;
+    void *signal{};
+};
+
+
+/**
+ * @brief Scoped connection class.
+ *
+ * Opaque object the aim of which is to allow users to release an already
+ * estabilished connection without having to keep a reference to the signal or
+ * the sink that generated it.<br/>
+ * A scoped connection automatically breaks the link between the two objects
+ * when it goes out of scope.
+ */
+struct scoped_connection: private connection {
+    /*! @brief Default constructor. */
+    scoped_connection() = default;
+
+    /**
+     * @brief Constructs a scoped connection from a basic connection.
+     * @param conn A valid connection object.
+     */
+    scoped_connection(const connection &conn)
+        : connection{conn}
+    {}
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    scoped_connection(const scoped_connection &) = delete;
+
+    /*! @brief Default move constructor. */
+    scoped_connection(scoped_connection &&) = default;
+
+    /*! @brief Automatically breaks the link on destruction. */
+    ~scoped_connection() {
+        connection::release();
+    }
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This scoped connection.
+     */
+    scoped_connection & operator=(const scoped_connection &) = delete;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This scoped connection.
+     */
+    scoped_connection & operator=(scoped_connection &&) = default;
+
+    /**
+     * @brief Copies a connection.
+     * @param other The connection object to copy.
+     * @return This scoped connection.
+     */
+    scoped_connection & operator=(const connection &other) {
+        static_cast<connection &>(*this) = other;
+        return *this;
+    }
+
+    /**
+     * @brief Moves a connection.
+     * @param other The connection object to move.
+     * @return This scoped connection.
+     */
+    scoped_connection & operator=(connection &&other) {
+        static_cast<connection &>(*this) = std::move(other);
+        return *this;
+    }
+
+    using connection::operator bool;
+    using connection::release;
+};
+
+
+/**
+ * @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) ENTT_NOEXCEPT {
-    return !(lhs == rhs);
-}
+class sink<Ret(Args...)> {
+    using signal_type = sigh<Ret(Args...)>;
+
+    template<auto Candidate, typename Type>
+    static void release(Type &value_or_instance, void *signal) {
+        sink{*static_cast<signal_type *>(signal)}.disconnect<Candidate>(value_or_instance);
+    }
+
+    template<auto Function>
+    static void release(void *signal) {
+        sink{*static_cast<signal_type *>(signal)}.disconnect<Function>();
+    }
+
+public:
+    /**
+     * @brief Constructs a sink that is allowed to modify a given signal.
+     * @param ref A valid reference to a signal object.
+     */
+    sink(sigh<Ret(Args...)> &ref) ENTT_NOEXCEPT
+        : 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 Connects a free function to a signal.
+     *
+     * The signal handler performs checks to avoid multiple connections for free
+     * functions.
+     *
+     * @tparam Function A valid free function pointer.
+     * @return A properly initialized connection object.
+     */
+    template<auto Function>
+    connection connect() {
+        disconnect<Function>();
+        delegate<void(void *)> conn{};
+        conn.template connect<&release<Function>>();
+        signal->calls.emplace_back(delegate<Ret(Args...)>{connect_arg<Function>});
+        return { std::move(conn), signal };
+    }
+
+    /**
+     * @brief Connects a member function or a free function with payload to a
+     * signal.
+     *
+     * The signal isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of the instance overcomes
+     * the one  of the delegate. On the other side, the signal handler performs
+     * checks to avoid multiple connections for the same function.<br/>
+     * When used to connect a free function with payload, its signature must be
+     * such that the instance is the first argument before the ones used to
+     * define the delegate itself.
+     *
+     * @tparam Candidate Member or free function to connect to the signal.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid reference that fits the purpose.
+     * @return A properly initialized connection object.
+     */
+    template<auto Candidate, typename Type>
+    connection connect(Type &value_or_instance) {
+        disconnect<Candidate>(value_or_instance);
+        delegate<void(void *)> conn{};
+        conn.template connect<&sink::release<Candidate, Type>>(value_or_instance);
+        signal->calls.emplace_back(delegate<Ret(Args...)>{connect_arg<Candidate>, value_or_instance});
+        return { std::move(conn), signal };
+    }
+
+    /**
+     * @brief Disconnects a free function from a signal.
+     * @tparam Function A valid free function pointer.
+     */
+    template<auto Function>
+    void disconnect() {
+        auto &calls = signal->calls;
+        delegate<Ret(Args...)> delegate{};
+        delegate.template connect<Function>();
+        calls.erase(std::remove(calls.begin(), calls.end(), delegate), calls.end());
+    }
+
+    /**
+     * @brief Disconnects a member function or a free function with payload from
+     * a signal.
+     * @tparam Candidate Member or free function to disconnect from the signal.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid reference that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void disconnect(Type &value_or_instance) {
+        auto &calls = signal->calls;
+        delegate<Ret(Args...)> delegate{};
+        delegate.template connect<Candidate>(value_or_instance);
+        calls.erase(std::remove(calls.begin(), calls.end(), delegate), calls.end());
+    }
+
+    /**
+     * @brief Disconnects member functions or free functions based on an
+     * instance or specific payload.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid reference that fits the purpose.
+     */
+    template<typename Type>
+    void disconnect(const Type &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:
+    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...)>;
 
 
 }

test/CMakeLists.txt

@@ -5,11 +5,16 @@
 include_directories($<TARGET_PROPERTY:EnTT,INTERFACE_INCLUDE_DIRECTORIES>)
 add_compile_options($<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_OPTIONS>)
 
+macro(SETUP_LIBRARY_TARGET LIB_TARGET)
+    set_target_properties(${LIB_TARGET} PROPERTIES CXX_EXTENSIONS OFF)
+    target_compile_definitions(${LIB_TARGET} PRIVATE $<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_DEFINITIONS>)
+    target_compile_features(${LIB_TARGET} PRIVATE $<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_FEATURES>)
+    target_compile_options(${LIB_TARGET} PRIVATE $<$<NOT:$<CXX_COMPILER_ID:MSVC>>:-pedantic -Wall>)
+    target_compile_options(${LIB_TARGET} PRIVATE $<$<CXX_COMPILER_ID:MSVC>:/EHsc>)
+endmacro()
+
 add_library(odr OBJECT odr.cpp)
-set_target_properties(odr PROPERTIES CXX_EXTENSIONS OFF)
-target_compile_definitions(odr PRIVATE $<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_DEFINITIONS>)
-target_compile_features(odr PRIVATE $<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_FEATURES>)
-target_compile_options(odr PRIVATE $<$<NOT:$<CXX_COMPILER_ID:MSVC>>:-pedantic -Wall>)
+SETUP_LIBRARY_TARGET(odr)
 
 macro(SETUP_AND_ADD_TEST TEST_NAME TEST_SOURCE)
     add_executable(${TEST_NAME} $<TARGET_OBJECTS:odr> ${TEST_SOURCE})
@@ -18,6 +23,7 @@ macro(SETUP_AND_ADD_TEST TEST_NAME TEST_SOURCE)
     target_compile_definitions(${TEST_NAME} PRIVATE $<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_DEFINITIONS>)
     target_compile_features(${TEST_NAME} PRIVATE $<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_FEATURES>)
     target_compile_options(${TEST_NAME} PRIVATE $<$<NOT:$<CXX_COMPILER_ID:MSVC>>:-pedantic -Wall>)
+    target_compile_options(${TEST_NAME} PRIVATE $<$<CXX_COMPILER_ID:MSVC>:/EHsc>)
     add_test(NAME ${TEST_NAME} COMMAND ${TEST_NAME})
 endmacro()
 
@@ -27,6 +33,26 @@ if(BUILD_BENCHMARK)
     SETUP_AND_ADD_TEST(benchmark benchmark/benchmark.cpp)
 endif()
 
+# Test lib
+
+if(BUILD_LIB)
+    add_library(a_module_shared SHARED lib/a_module.cpp)
+    add_library(a_module_static STATIC lib/a_module.cpp)
+    add_library(another_module_shared SHARED lib/another_module.cpp)
+    add_library(another_module_static STATIC lib/another_module.cpp)
+
+    SETUP_LIBRARY_TARGET(a_module_shared)
+    SETUP_LIBRARY_TARGET(a_module_static)
+    SETUP_LIBRARY_TARGET(another_module_shared)
+    SETUP_LIBRARY_TARGET(another_module_static)
+
+    SETUP_AND_ADD_TEST(lib_shared lib/lib.cpp)
+    target_link_libraries(lib_shared PRIVATE a_module_shared another_module_shared)
+
+    SETUP_AND_ADD_TEST(lib_static lib/lib.cpp)
+    target_link_libraries(lib_static PRIVATE a_module_static another_module_static)
+endif()
+
 # Test mod
 
 if(BUILD_MOD)
@@ -61,22 +87,31 @@ SETUP_AND_ADD_TEST(family entt/core/family.cpp)
 SETUP_AND_ADD_TEST(hashed_string entt/core/hashed_string.cpp)
 SETUP_AND_ADD_TEST(ident entt/core/ident.cpp)
 SETUP_AND_ADD_TEST(monostate entt/core/monostate.cpp)
+SETUP_AND_ADD_TEST(type_traits entt/core/type_traits.cpp)
+SETUP_AND_ADD_TEST(utility entt/core/utility.cpp)
 
 # Test entity
 
 SETUP_AND_ADD_TEST(actor entt/entity/actor.cpp)
 SETUP_AND_ADD_TEST(entity entt/entity/entity.cpp)
+SETUP_AND_ADD_TEST(group entt/entity/group.cpp)
 SETUP_AND_ADD_TEST(helper entt/entity/helper.cpp)
-SETUP_AND_ADD_TEST(prototype entt/entity/prototype.cpp)
+SETUP_AND_ADD_TEST(observer entt/entity/observer.cpp)
 SETUP_AND_ADD_TEST(registry entt/entity/registry.cpp)
+SETUP_AND_ADD_TEST(runtime_view entt/entity/runtime_view.cpp)
 SETUP_AND_ADD_TEST(snapshot entt/entity/snapshot.cpp)
 SETUP_AND_ADD_TEST(sparse_set entt/entity/sparse_set.cpp)
+SETUP_AND_ADD_TEST(storage entt/entity/storage.cpp)
 SETUP_AND_ADD_TEST(view entt/entity/view.cpp)
 
 # Test locator
 
 SETUP_AND_ADD_TEST(locator entt/locator/locator.cpp)
 
+# Test meta
+
+SETUP_AND_ADD_TEST(meta entt/meta/meta.cpp)
+
 # Test process
 
 SETUP_AND_ADD_TEST(process entt/process/process.cpp)

test/benchmark/BUILD.bazel

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

test/entt/core/BUILD.bazel

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

test/entt/core/algorithm.cpp

@@ -2,10 +2,14 @@
 #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::StdSort sort;
+    entt::std_sort sort;
 
     sort(arr.begin(), arr.end());
 
@@ -14,9 +18,23 @@ TEST(Algorithm, StdSort) {
     }
 }
 
+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::InsertionSort sort;
+    entt::insertion_sort sort;
 
     sort(arr.begin(), arr.end());
 
@@ -25,16 +43,55 @@ TEST(Algorithm, InsertionSort) {
     }
 }
 
-TEST(Algorithm, OneShotBubbleSort) {
-    std::array<int, 5> arr{{4, 1, 3, 2, 0}};
-    entt::OneShotBubbleSort sort;
+TEST(Algorithm, InsertionSortBoxedInt) {
+    std::array<boxed_int, 6> arr{{{4}, {1}, {3}, {2}, {0}, {6}}};
+    entt::insertion_sort sort;
 
-    sort(arr.begin(), arr.end());
-    sort(arr.begin(), arr.end());
-    sort(arr.begin(), arr.end());
-    sort(arr.begin(), arr.end());
+    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_EQ(a_family::type<int>, a_family::type<int &>);
+    ASSERT_EQ(a_family::type<int>, a_family::type<int &&>);
+    ASSERT_EQ(a_family::type<int>, a_family::type<const int &>);
 }

test/entt/core/hashed_string.cpp

@@ -1,34 +1,128 @@
+#include <string>
+#include <string_view>
 #include <type_traits>
 #include <gtest/gtest.h>
 #include <entt/core/hashed_string.hpp>
 
 TEST(HashedString, Functionalities) {
-    using hash_type = entt::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_EQ(fooHs, fooHs);
-    ASSERT_NE(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(fooHs, "foo"_hs);
-    ASSERT_NE(barHs, "foo"_hs);
+    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::HashedString::hash_type;
+    using hash_type = entt::hashed_string::hash_type;
     // how would you test a constexpr otherwise?
-    (void)std::integral_constant<hash_type, entt::HashedString{"quux"}>{};
+    (void)std::integral_constant<hash_type, entt::hashed_string{"quux"}>{};
     (void)std::integral_constant<hash_type, "quux"_hs>{};
     ASSERT_TRUE(true);
 }
+
+TEST(HashedString, ToValue) {
+    using hash_type = entt::hashed_string::hash_type;
+
+    const char *foobar = "foobar";
+
+    ASSERT_EQ(entt::hashed_string::to_value(foobar), 0xbf9cf968);
+    // how would you test a constexpr otherwise?
+    (void)std::integral_constant<hash_type, entt::hashed_string::to_value("quux")>{};
+}
+
+TEST(HashedString, StringView) {
+    std::string str{"__foobar__"};
+    std::string_view view{str.data()+2, 6};
+    ASSERT_EQ(entt::hashed_string::to_value(view.data(), view.size()), 0xbf9cf968);
+}
+
+TEST(HashedWString, Functionalities) {
+    using hash_type = entt::hashed_wstring::hash_type;
+
+    const wchar_t *bar = L"bar";
+
+    auto foo_hws = entt::hashed_wstring{L"foo"};
+    auto bar_hws = entt::hashed_wstring{bar};
+
+    ASSERT_NE(static_cast<hash_type>(foo_hws), static_cast<hash_type>(bar_hws));
+    ASSERT_STREQ(static_cast<const wchar_t *>(foo_hws), L"foo");
+    ASSERT_STREQ(static_cast<const wchar_t *>(bar_hws), bar);
+    ASSERT_STREQ(foo_hws.data(), L"foo");
+    ASSERT_STREQ(bar_hws.data(), bar);
+
+    ASSERT_EQ(foo_hws, foo_hws);
+    ASSERT_NE(foo_hws, bar_hws);
+
+    entt::hashed_wstring hws{L"foobar"};
+
+    ASSERT_EQ(static_cast<hash_type>(hws), 0xbf9cf968);
+    ASSERT_EQ(hws.value(), 0xbf9cf968);
+
+    ASSERT_EQ(foo_hws, L"foo"_hws);
+    ASSERT_NE(bar_hws, L"foo"_hws);
+}
+
+TEST(HashedWString, Empty) {
+    using hash_type = entt::hashed_wstring::hash_type;
+
+    entt::hashed_wstring hws{};
+
+    ASSERT_EQ(static_cast<hash_type>(hws), hash_type{});
+    ASSERT_EQ(static_cast<const wchar_t *>(hws), nullptr);
+}
+
+TEST(HashedWString, Constexprness) {
+    using hash_type = entt::hashed_wstring::hash_type;
+    // how would you test a constexpr otherwise?
+    (void)std::integral_constant<hash_type, entt::hashed_wstring{L"quux"}>{};
+    (void)std::integral_constant<hash_type, L"quux"_hws>{};
+    ASSERT_TRUE(true);
+}
+
+TEST(HashedWString, ToValue) {
+    using hash_type = entt::hashed_wstring::hash_type;
+
+    const wchar_t *foobar = L"foobar";
+
+    ASSERT_EQ(entt::hashed_wstring::to_value(foobar), 0xbf9cf968);
+    // how would you test a constexpr otherwise?
+    (void)std::integral_constant<hash_type, entt::hashed_wstring::to_value(L"quux")>{};
+}
+
+TEST(HashedWString, StringView) {
+    std::wstring str{L"__foobar__"};
+    std::wstring_view view{str.data()+2, 6};
+    ASSERT_EQ(entt::hashed_wstring::to_value(view.data(), view.size()), 0xbf9cf968);
+}
+
+TEST(BasicHashedString, DeductionGuide) {
+    static_assert(std::is_same_v<decltype(entt::basic_hashed_string{"foo"}), entt::hashed_string>);
+    static_assert(std::is_same_v<decltype(entt::basic_hashed_string{L"foo"}), entt::hashed_wstring>);
+}

test/entt/core/ident.cpp

@@ -2,31 +2,31 @@
 #include <gtest/gtest.h>
 #include <entt/core/ident.hpp>
 
-struct AType {};
-struct AnotherType {};
+struct a_type {};
+struct another_type {};
 
 TEST(Identifier, Uniqueness) {
-    using ID = entt::Identifier<AType, AnotherType>;
-    constexpr AType anInstance;
-    constexpr AnotherType anotherInstance;
+    using id = entt::identifier<a_type, another_type>;
+    constexpr a_type an_instance;
+    constexpr another_type another_instance;
 
-    ASSERT_NE(ID::get<AType>(), ID::get<AnotherType>());
-    ASSERT_EQ(ID::get<AType>(), ID::get<decltype(anInstance)>());
-    ASSERT_NE(ID::get<AType>(), ID::get<decltype(anotherInstance)>());
-    ASSERT_EQ(ID::get<AType>(), ID::get<AType>());
-    ASSERT_EQ(ID::get<AnotherType>(), ID::get<AnotherType>());
+    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<AnotherType>()) {
-    case ID::get<AType>():
+    switch(id::type<another_type>) {
+    case id::type<a_type>:
         FAIL();
-    case ID::get<AnotherType>():
+    case id::type<another_type>:
         SUCCEED();
     }
 }
 
 TEST(Identifier, SingleType) {
-    using ID = entt::Identifier<AType>;
-    std::integral_constant<ID::identifier_type, ID::get<AType>()> 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

@@ -3,18 +3,18 @@
 #include <entt/core/monostate.hpp>
 
 TEST(Monostate, Functionalities) {
-    const bool bPre = entt::Monostate<entt::HashedString{"foobar"}>{};
-    const int iPre = entt::Monostate<"foobar"_hs>{};
+    const bool b_pre = entt::monostate<entt::hashed_string{"foobar"}>{};
+    const int i_pre = entt::monostate<"foobar"_hs>{};
 
-    ASSERT_FALSE(bPre);
-    ASSERT_EQ(iPre, int{});
+    ASSERT_FALSE(b_pre);
+    ASSERT_EQ(i_pre, int{});
 
-    entt::Monostate<"foobar"_hs>{} = true;
-    entt::Monostate<"foobar"_hs>{} = 42;
+    entt::monostate<"foobar"_hs>{} = true;
+    entt::monostate_v<"foobar"_hs> = 42;
 
-    const bool &bPost = entt::Monostate<"foobar"_hs>{};
-    const int &iPost = entt::Monostate<entt::HashedString{"foobar"}>{};
+    const bool &b_post = entt::monostate<"foobar"_hs>{};
+    const int &i_post = entt::monostate_v<entt::hashed_string{"foobar"}>;
 
-    ASSERT_TRUE(bPost);
-    ASSERT_EQ(iPost, 42);
+    ASSERT_TRUE(b_post);
+    ASSERT_EQ(i_post, 42);
 }

test/entt/core/type_traits.cpp

@@ -0,0 +1,14 @@
+#include <gtest/gtest.h>
+#include <entt/core/type_traits.hpp>
+
+TEST(TypeList, Functionalities) {
+    using type = entt::type_list<int, char>;
+    using other = entt::type_list<double>;
+
+    ASSERT_EQ(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/entt/core/utility.cpp

@@ -0,0 +1,26 @@
+#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));
+}

test/entt/entity/BUILD.bazel

@@ -0,0 +1,121 @@
+load("//bazel:copts.bzl", "entt_copts")
+
+cc_test(
+    name = "actor",
+    copts = entt_copts,
+    srcs = ["actor.cpp"],
+    deps = [
+        "//:entt",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "entity",
+    copts = entt_copts,
+    srcs = ["entity.cpp"],
+    deps = [
+        "//:entt",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "group",
+    copts = entt_copts,
+    srcs = ["group.cpp"],
+    deps = [
+        "//:entt",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "helper",
+    copts = entt_copts,
+    srcs = ["helper.cpp"],
+    deps = [
+        "//:entt",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "observer",
+    copts = entt_copts,
+    srcs = ["observer.cpp"],
+    deps = [
+        "//:entt",
+        "@com_google_googletest//:gtest_main",  
+    ],
+)
+
+cc_test(
+    name = "prototype",
+    copts = entt_copts,
+    srcs = ["prototype.cpp"],
+    deps = [
+        "//:entt",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "registry",
+    copts = entt_copts,
+    srcs = ["registry.cpp"],
+    deps = [
+        "//:entt",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "runtime_view",
+    copts = entt_copts,
+    srcs = ["runtime_view.cpp"],
+    deps = [
+        "//:entt",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "snapshot",
+    copts = entt_copts,
+    srcs = ["snapshot.cpp"],
+    deps = [
+        "//:entt",
+        "@com_google_googletest//:gtest_main",  
+    ],
+)
+
+cc_test(
+    name = "sparse_set",
+    copts = entt_copts,
+    srcs = ["sparse_set.cpp"],
+    deps = [
+        "//:entt",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "storage",
+    copts = entt_copts,
+    srcs = ["storage.cpp"],
+    deps = [
+        "//:entt",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "view",
+    copts = entt_copts,
+    srcs = ["view.cpp"],
+    deps = [
+        "//:entt",
+        "@com_google_googletest//:gtest_main",
+    ],
+)

test/entt/entity/actor.cpp

@@ -3,66 +3,82 @@
 #include <entt/entity/actor.hpp>
 #include <entt/entity/registry.hpp>
 
-struct ActorComponent final {};
-struct ActorTag final {};
-
 TEST(Actor, Component) {
-    entt::DefaultRegistry registry;
-    entt::DefaultActor actor{registry};
-    const auto &cactor = actor;
+    entt::registry registry;
+    entt::actor actor{registry};
 
-    ASSERT_EQ(&registry, &actor.registry());
-    ASSERT_EQ(&registry, &cactor.registry());
-    ASSERT_TRUE(registry.empty<ActorComponent>());
+    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<ActorComponent>());
+    ASSERT_FALSE(actor.has<int>());
 
-    const auto &component = actor.assign<ActorComponent>();
+    const auto &cint = actor.assign<int>();
+    const auto &cchar = actor.assign<char>();
 
-    ASSERT_EQ(&component, &actor.get<ActorComponent>());
-    ASSERT_EQ(&component, &cactor.get<ActorComponent>());
-    ASSERT_FALSE(registry.empty<ActorComponent>());
+    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<ActorComponent>());
+    ASSERT_TRUE((actor.has<int, char>()));
+    ASSERT_FALSE(actor.has<double>());
 
-    actor.remove<ActorComponent>();
+    actor.remove<int>();
 
-    ASSERT_TRUE(registry.empty<ActorComponent>());
+    ASSERT_TRUE(registry.empty<int>());
     ASSERT_FALSE(registry.empty());
-    ASSERT_FALSE(actor.has<ActorComponent>());
+    ASSERT_FALSE(actor.has<int>());
 }
 
-TEST(Actor, Tag) {
-    entt::DefaultRegistry registry;
-    entt::DefaultActor actor{registry};
-    const auto &cactor = actor;
+TEST(Actor, FromEntity) {
+    entt::registry registry;
+    const auto entity = registry.create();
 
-    ASSERT_EQ(&registry, &actor.registry());
-    ASSERT_EQ(&registry, &cactor.registry());
-    ASSERT_FALSE(registry.has<ActorTag>());
-    ASSERT_FALSE(actor.has<ActorTag>(entt::tag_t{}));
+    registry.assign<int>(entity, 42);
+    registry.assign<char>(entity, 'c');
 
-    const auto &tag = actor.assign<ActorTag>(entt::tag_t{});
+    entt::actor actor{entity, registry};
 
-    ASSERT_EQ(&tag, &actor.get<ActorTag>(entt::tag_t{}));
-    ASSERT_EQ(&tag, &cactor.get<ActorTag>(entt::tag_t{}));
-    ASSERT_TRUE(registry.has<ActorTag>());
-    ASSERT_FALSE(registry.empty());
-    ASSERT_TRUE(actor.has<ActorTag>(entt::tag_t{}));
-
-    actor.remove<ActorTag>(entt::tag_t{});
-
-    ASSERT_FALSE(registry.has<ActorTag>());
-    ASSERT_FALSE(registry.empty());
-    ASSERT_FALSE(actor.has<ActorTag>(entt::tag_t{}));
+    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::DefaultRegistry registry;
-    auto *actor = new entt::DefaultActor{registry};
-    actor->assign<ActorComponent>();
+    entt::registry registry;
+    entt::actor actor{};
 
-    ASSERT_FALSE(registry.empty<ActorComponent>());
+    ASSERT_FALSE(actor);
+
+    actor = entt::actor{registry};
+    actor.assign<int>();
+
+    ASSERT_TRUE(actor);
+    ASSERT_FALSE(registry.empty<int>());
+    ASSERT_FALSE(registry.empty());
+
+    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) {
@@ -71,6 +87,6 @@ TEST(Actor, EntityLifetime) {
 
     delete actor;
 
-    ASSERT_TRUE(registry.empty<ActorComponent>());
+    ASSERT_TRUE(registry.empty<int>());
     ASSERT_TRUE(registry.empty());
 }

test/entt/entity/entity.cpp

@@ -1,18 +1,19 @@
 #include <functional>
+#include <type_traits>
 #include <gtest/gtest.h>
 #include <entt/entity/entity.hpp>
 #include <entt/entity/registry.hpp>
 
-template<bool>
-struct S {};
-
-TEST(Traits, Null) {
-    entt::DefaultRegistry registry{};
+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_TRUE(~typename entt::DefaultRegistry::entity_type{} == 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);

test/entt/entity/group.cpp

@@ -0,0 +1,985 @@
+#include <utility>
+#include <iterator>
+#include <algorithm>
+#include <type_traits>
+#include <gtest/gtest.h>
+#include <entt/entity/helper.hpp>
+#include <entt/entity/registry.hpp>
+#include <entt/entity/group.hpp>
+
+struct empty_type {};
+struct boxed_int { int value; };
+
+TEST(NonOwningGroup, Functionalities) {
+    entt::registry registry;
+    auto group = registry.group(entt::get<int, char>);
+    auto cgroup = std::as_const(registry).group(entt::get<const int, const char>);
+
+    ASSERT_TRUE(group.empty());
+    ASSERT_TRUE(group.empty<int>());
+    ASSERT_TRUE(cgroup.empty<const char>());
+
+    const auto e0 = registry.create();
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    ASSERT_FALSE(group.empty());
+    ASSERT_FALSE(group.empty<int>());
+    ASSERT_FALSE(cgroup.empty<const char>());
+    ASSERT_NO_THROW((group.begin()++));
+    ASSERT_NO_THROW((++cgroup.begin()));
+
+    ASSERT_NE(group.begin(), group.end());
+    ASSERT_NE(cgroup.begin(), cgroup.end());
+    ASSERT_EQ(group.size(), typename decltype(group)::size_type{1});
+    ASSERT_EQ(group.size<int>(), typename decltype(group)::size_type{1});
+    ASSERT_EQ(cgroup.size<const char>(), typename decltype(group)::size_type{2});
+
+    registry.assign<int>(e0);
+
+    ASSERT_EQ(group.size(), typename decltype(group)::size_type{2});
+    ASSERT_EQ(group.size<int>(), typename decltype(group)::size_type{2});
+    ASSERT_EQ(cgroup.size<const char>(), typename decltype(group)::size_type{2});
+
+    registry.remove<int>(e0);
+
+    ASSERT_EQ(group.size(), typename decltype(group)::size_type{1});
+    ASSERT_EQ(group.size<int>(), typename decltype(group)::size_type{1});
+    ASSERT_EQ(cgroup.size<const char>(), typename decltype(group)::size_type{2});
+
+    registry.get<char>(e0) = '1';
+    registry.get<char>(e1) = '2';
+    registry.get<int>(e1) = 42;
+
+    for(auto entity: group) {
+        ASSERT_EQ(std::get<0>(cgroup.get<const int, const char>(entity)), 42);
+        ASSERT_EQ(std::get<1>(group.get<int, char>(entity)), '2');
+        ASSERT_EQ(cgroup.get<const char>(entity), '2');
+    }
+
+    ASSERT_EQ(*(group.data() + 0), e1);
+
+    ASSERT_EQ(*(group.data<int>() + 0), e1);
+    ASSERT_EQ(*(group.data<char>() + 0), e0);
+    ASSERT_EQ(*(cgroup.data<const char>() + 1), e1);
+
+    ASSERT_EQ(*(group.raw<int>() + 0), 42);
+    ASSERT_EQ(*(group.raw<char>() + 0), '1');
+    ASSERT_EQ(*(cgroup.raw<const char>() + 1), '2');
+
+    registry.remove<char>(e0);
+    registry.remove<char>(e1);
+
+    ASSERT_EQ(group.begin(), group.end());
+    ASSERT_EQ(cgroup.begin(), cgroup.end());
+    ASSERT_TRUE(group.empty());
+
+    ASSERT_TRUE(group.capacity());
+
+    group.shrink_to_fit();
+
+    ASSERT_FALSE(group.capacity());
+}
+
+TEST(NonOwningGroup, ElementAccess) {
+    entt::registry registry;
+    auto group = registry.group(entt::get<int, char>);
+    auto cgroup = std::as_const(registry).group(entt::get<const int, const char>);
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0);
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    for(typename decltype(group)::size_type i{}; i < group.size(); ++i) {
+        ASSERT_EQ(group[i], i ? e0 : e1);
+        ASSERT_EQ(cgroup[i], i ? e0 : e1);
+    }
+}
+
+TEST(NonOwningGroup, Contains) {
+    entt::registry registry;
+    auto group = registry.group(entt::get<int, char>);
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0);
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    registry.destroy(e0);
+
+    ASSERT_FALSE(group.contains(e0));
+    ASSERT_TRUE(group.contains(e1));
+}
+
+TEST(NonOwningGroup, Empty) {
+    entt::registry registry;
+
+    const auto e0 = registry.create();
+    registry.assign<double>(e0);
+    registry.assign<int>(e0);
+    registry.assign<float>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<char>(e1);
+    registry.assign<float>(e1);
+
+    ASSERT_TRUE(registry.group(entt::get<char, int, float>).empty());
+    ASSERT_TRUE(registry.group(entt::get<double, char, int, float>).empty());
+}
+
+TEST(NonOwningGroup, Each) {
+    entt::registry registry;
+    auto group = registry.group(entt::get<int, char>);
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0);
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    auto cgroup = std::as_const(registry).group(entt::get<const int, const char>);
+    std::size_t cnt = 0;
+
+    group.each([&cnt](auto, int &, char &) { ++cnt; });
+    group.each([&cnt](int &, char &) { ++cnt; });
+
+    ASSERT_EQ(cnt, std::size_t{4});
+
+    cgroup.each([&cnt](auto, const int &, const char &) { --cnt; });
+    cgroup.each([&cnt](const int &, const char &) { --cnt; });
+
+    ASSERT_EQ(cnt, std::size_t{0});
+}
+
+TEST(NonOwningGroup, Sort) {
+    entt::registry registry;
+    auto group = registry.group(entt::get<const int, unsigned int>);
+
+    const auto e0 = registry.create();
+    const auto e1 = registry.create();
+    const auto e2 = registry.create();
+
+    registry.assign<unsigned int>(e0, 0u);
+    registry.assign<unsigned int>(e1, 1u);
+    registry.assign<unsigned int>(e2, 2u);
+
+    registry.assign<int>(e0, 0);
+    registry.assign<int>(e1, 1);
+    registry.assign<int>(e2, 2);
+
+    ASSERT_EQ(*(group.raw<unsigned int>() + 0u), 0u);
+    ASSERT_EQ(*(group.raw<unsigned int>() + 1u), 1u);
+    ASSERT_EQ(*(group.raw<unsigned int>() + 2u), 2u);
+
+    ASSERT_EQ(*(group.raw<const int>() + 0u), 0);
+    ASSERT_EQ(*(group.raw<const int>() + 1u), 1);
+    ASSERT_EQ(*(group.raw<const int>() + 2u), 2);
+
+    ASSERT_EQ(*(group.data() + 0u), e0);
+    ASSERT_EQ(*(group.data() + 1u), e1);
+    ASSERT_EQ(*(group.data() + 2u), e2);
+
+    group.sort([](const entt::entity lhs, const entt::entity rhs) {
+        return std::underlying_type_t<entt::entity>(lhs) < std::underlying_type_t<entt::entity>(rhs);
+    });
+
+    ASSERT_EQ(*(group.raw<unsigned int>() + 0u), 0u);
+    ASSERT_EQ(*(group.raw<unsigned int>() + 1u), 1u);
+    ASSERT_EQ(*(group.raw<unsigned int>() + 2u), 2u);
+
+    ASSERT_EQ(*(group.raw<const int>() + 0u), 0);
+    ASSERT_EQ(*(group.raw<const int>() + 1u), 1);
+    ASSERT_EQ(*(group.raw<const int>() + 2u), 2);
+
+    ASSERT_EQ(*(group.data() + 0u), e2);
+    ASSERT_EQ(*(group.data() + 1u), e1);
+    ASSERT_EQ(*(group.data() + 2u), e0);
+
+    group.sort<const int>([](const int lhs, const int rhs) {
+        return lhs > rhs;
+    });
+
+    ASSERT_EQ(*(group.data() + 0u), e0);
+    ASSERT_EQ(*(group.data() + 1u), e1);
+    ASSERT_EQ(*(group.data() + 2u), e2);
+}
+
+TEST(NonOwningGroup, SortAsAPool) {
+    entt::registry registry;
+    auto group = registry.group(entt::get<const int, unsigned int>);
+
+    const auto e0 = registry.create();
+    const auto e1 = registry.create();
+    const auto e2 = registry.create();
+
+    auto uval = 0u;
+    auto ival = 0;
+
+    registry.assign<unsigned int>(e0, uval++);
+    registry.assign<unsigned int>(e1, uval++);
+    registry.assign<unsigned int>(e2, uval++);
+
+    registry.assign<int>(e0, ival++);
+    registry.assign<int>(e1, ival++);
+    registry.assign<int>(e2, ival++);
+
+    for(auto entity: group) {
+        ASSERT_EQ(group.get<unsigned int>(entity), --uval);
+        ASSERT_EQ(group.get<const int>(entity), --ival);
+    }
+
+    registry.sort<unsigned int>(std::less<unsigned int>{});
+    group.sort<unsigned int>();
+
+    for(auto entity: group) {
+        ASSERT_EQ(group.get<unsigned int>(entity), uval++);
+        ASSERT_EQ(group.get<const int>(entity), ival++);
+    }
+}
+
+TEST(NonOwningGroup, IndexRebuiltOnDestroy) {
+    entt::registry registry;
+    auto group = registry.group(entt::get<int, unsigned int>);
+
+    const auto e0 = registry.create();
+    const auto e1 = registry.create();
+
+    registry.assign<unsigned int>(e0, 0u);
+    registry.assign<unsigned int>(e1, 1u);
+
+    registry.assign<int>(e0, 0);
+    registry.assign<int>(e1, 1);
+
+    registry.destroy(e0);
+    registry.assign<int>(registry.create(), 42);
+
+    ASSERT_EQ(group.size(), typename decltype(group)::size_type{1});
+    ASSERT_EQ(group[{}], e1);
+    ASSERT_EQ(group.get<int>(e1), 1);
+    ASSERT_EQ(group.get<unsigned int>(e1), 1u);
+
+    group.each([e1](auto entity, auto ivalue, auto uivalue) {
+        ASSERT_EQ(entity, e1);
+        ASSERT_EQ(ivalue, 1);
+        ASSERT_EQ(uivalue, 1u);
+    });
+}
+
+TEST(NonOwningGroup, ConstNonConstAndAllInBetween) {
+    entt::registry registry;
+    auto group = registry.group(entt::get<int, const char, std::true_type>);
+
+    ASSERT_EQ(group.size(), decltype(group.size()){0});
+
+    const auto entity = registry.create();
+    registry.assign<int>(entity, 0);
+    registry.assign<char>(entity, 'c');
+    registry.assign<std::true_type>(entity);
+
+    ASSERT_EQ(group.size(), decltype(group.size()){1});
+
+    ASSERT_TRUE((std::is_same_v<decltype(group.get<int>({})), int &>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.get<const char>({})), const char &>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.get<std::true_type>({})), std::true_type>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.get<int, const char, std::true_type>({})), std::tuple<int &, const char &, std::true_type>>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.raw<const char>()), const char *>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.raw<int>()), int *>));
+
+    group.each([](auto &&i, auto &&c, auto &&e) {
+        ASSERT_TRUE((std::is_same_v<decltype(i), int &>));
+        ASSERT_TRUE((std::is_same_v<decltype(c), const char &>));
+        ASSERT_TRUE((std::is_same_v<decltype(e), std::true_type &&>));
+    });
+}
+
+TEST(NonOwningGroup, Find) {
+    entt::registry registry;
+    auto group = registry.group(entt::get<int, const char>);
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0);
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    const auto e2 = registry.create();
+    registry.assign<int>(e2);
+    registry.assign<char>(e2);
+
+    const auto e3 = registry.create();
+    registry.assign<int>(e3);
+    registry.assign<char>(e3);
+
+    registry.remove<int>(e1);
+
+    ASSERT_NE(group.find(e0), group.end());
+    ASSERT_EQ(group.find(e1), group.end());
+    ASSERT_NE(group.find(e2), group.end());
+    ASSERT_NE(group.find(e3), group.end());
+
+    auto it = group.find(e2);
+
+    ASSERT_EQ(*it, e2);
+    ASSERT_EQ(*(++it), e3);
+    ASSERT_EQ(*(++it), e0);
+    ASSERT_EQ(++it, group.end());
+    ASSERT_EQ(++group.find(e0), group.end());
+
+    const auto e4 = registry.create();
+    registry.destroy(e4);
+    const auto e5 = registry.create();
+    registry.assign<int>(e5);
+    registry.assign<char>(e5);
+
+    ASSERT_NE(group.find(e5), group.end());
+    ASSERT_EQ(group.find(e4), group.end());
+}
+
+TEST(NonOwningGroup, ExcludedComponents) {
+    entt::registry registry;
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0, 0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1, 1);
+    registry.assign<char>(e1);
+
+    const auto group = registry.group(entt::get<int>, entt::exclude<char>);
+
+    const auto e2 = registry.create();
+    registry.assign<int>(e2, 2);
+
+    const auto e3 = registry.create();
+    registry.assign<int>(e3, 3);
+    registry.assign<char>(e3);
+
+    for(const auto entity: group) {
+        if(entity == e0) {
+            ASSERT_EQ(group.get<int>(e0), 0);
+        } else if(entity == e2) {
+            ASSERT_EQ(group.get<int>(e2), 2);
+        } else {
+            FAIL();
+        }
+    }
+
+    registry.assign<char>(e0);
+    registry.assign<char>(e2);
+
+    ASSERT_TRUE(group.empty());
+
+    registry.remove<char>(e1);
+    registry.remove<char>(e3);
+
+    for(const auto entity: group) {
+        if(entity == e1) {
+            ASSERT_EQ(group.get<int>(e1), 1);
+        } else if(entity == e3) {
+            ASSERT_EQ(group.get<int>(e3), 3);
+        } else {
+            FAIL();
+        }
+    }
+}
+
+TEST(NonOwningGroup, EmptyAndNonEmptyTypes) {
+    entt::registry registry;
+    const auto group = registry.group(entt::get<int, empty_type>);
+
+    const auto e0 = registry.create();
+    registry.assign<empty_type>(e0);
+    registry.assign<int>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<empty_type>(e1);
+    registry.assign<int>(e1);
+
+    registry.assign<int>(registry.create());
+
+    for(const auto entity: group) {
+        ASSERT_TRUE(entity == e0 || entity == e1);
+    }
+
+    group.each([e0, e1](const auto entity, const int &, empty_type) {
+        ASSERT_TRUE(entity == e0 || entity == e1);
+    });
+
+    ASSERT_EQ(group.size(), typename decltype(group)::size_type{2});
+}
+
+TEST(NonOwningGroup, TrackEntitiesOnComponentDestruction) {
+    entt::registry registry;
+    const auto group = registry.group(entt::get<int>, entt::exclude<char>);
+    const auto cgroup = std::as_const(registry).group(entt::get<const int>, entt::exclude<char>);
+
+    const auto entity = registry.create();
+    registry.assign<int>(entity);
+    registry.assign<char>(entity);
+
+    ASSERT_TRUE(group.empty());
+    ASSERT_TRUE(cgroup.empty());
+
+    registry.remove<char>(entity);
+
+    ASSERT_FALSE(group.empty());
+    ASSERT_FALSE(cgroup.empty());
+}
+
+TEST(NonOwningGroup, Less) {
+    entt::registry registry;
+    const auto entity = std::get<0>(registry.create<int, entt::tag<"empty"_hs>>());
+    registry.create<char>();
+
+    registry.group(entt::get<int, char, entt::tag<"empty"_hs>>).less([entity](const auto entt, int, char) {
+        ASSERT_EQ(entity, entt);
+    });
+
+    registry.group(entt::get<int, entt::tag<"empty"_hs>, char>).less([check = true](int, char) mutable {
+        ASSERT_TRUE(check);
+        check = false;
+    });
+
+    registry.group(entt::get<entt::tag<"empty"_hs>, int, char>).less([entity](const auto entt, int, char) {
+        ASSERT_EQ(entity, entt);
+    });
+
+    registry.group(entt::get<int, char, double>).less([entity](const auto entt, int, char, double) {
+        ASSERT_EQ(entity, entt);
+    });
+}
+
+TEST(OwningGroup, Functionalities) {
+    entt::registry registry;
+    auto group = registry.group<int>(entt::get<char>);
+    auto cgroup = std::as_const(registry).group<const int>(entt::get<const char>);
+
+    ASSERT_TRUE(group.empty());
+    ASSERT_TRUE(group.empty<int>());
+    ASSERT_TRUE(cgroup.empty<const char>());
+
+    const auto e0 = registry.create();
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    ASSERT_FALSE(group.empty());
+    ASSERT_FALSE(group.empty<int>());
+    ASSERT_FALSE(cgroup.empty<const char>());
+    ASSERT_NO_THROW((group.begin()++));
+    ASSERT_NO_THROW((++cgroup.begin()));
+
+    ASSERT_NE(group.begin(), group.end());
+    ASSERT_NE(cgroup.begin(), cgroup.end());
+    ASSERT_EQ(group.size(), typename decltype(group)::size_type{1});
+    ASSERT_EQ(group.size<int>(), typename decltype(group)::size_type{1});
+    ASSERT_EQ(cgroup.size<const char>(), typename decltype(group)::size_type{2});
+
+    registry.assign<int>(e0);
+
+    ASSERT_EQ(group.size(), typename decltype(group)::size_type{2});
+    ASSERT_EQ(group.size<int>(), typename decltype(group)::size_type{2});
+    ASSERT_EQ(cgroup.size<const char>(), typename decltype(group)::size_type{2});
+
+    registry.remove<int>(e0);
+
+    ASSERT_EQ(group.size(), typename decltype(group)::size_type{1});
+    ASSERT_EQ(group.size<int>(), typename decltype(group)::size_type{1});
+    ASSERT_EQ(cgroup.size<const char>(), typename decltype(group)::size_type{2});
+
+    registry.get<char>(e0) = '1';
+    registry.get<char>(e1) = '2';
+    registry.get<int>(e1) = 42;
+
+    ASSERT_EQ(*(cgroup.raw<const int>() + 0), 42);
+    ASSERT_EQ(*(group.raw<int>() + 0), 42);
+
+    for(auto entity: group) {
+        ASSERT_EQ(std::get<0>(cgroup.get<const int, const char>(entity)), 42);
+        ASSERT_EQ(std::get<1>(group.get<int, char>(entity)), '2');
+        ASSERT_EQ(cgroup.get<const char>(entity), '2');
+    }
+
+    ASSERT_EQ(*(group.data() + 0), e1);
+
+    ASSERT_EQ(*(group.data<int>() + 0), e1);
+    ASSERT_EQ(*(group.data<char>() + 0), e0);
+    ASSERT_EQ(*(cgroup.data<const char>() + 1), e1);
+
+    ASSERT_EQ(*(group.raw<int>() + 0), 42);
+    ASSERT_EQ(*(group.raw<char>() + 0), '1');
+    ASSERT_EQ(*(cgroup.raw<const char>() + 1), '2');
+
+    registry.remove<char>(e0);
+    registry.remove<char>(e1);
+
+    ASSERT_EQ(group.begin(), group.end());
+    ASSERT_EQ(cgroup.begin(), cgroup.end());
+    ASSERT_TRUE(group.empty());
+}
+
+TEST(OwningGroup, ElementAccess) {
+    entt::registry registry;
+    auto group = registry.group<int>(entt::get<char>);
+    auto cgroup = std::as_const(registry).group<const int>(entt::get<const char>);
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0);
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    for(typename decltype(group)::size_type i{}; i < group.size(); ++i) {
+        ASSERT_EQ(group[i], i ? e0 : e1);
+        ASSERT_EQ(cgroup[i], i ? e0 : e1);
+    }
+}
+
+TEST(OwningGroup, Contains) {
+    entt::registry registry;
+    auto group = registry.group<int>(entt::get<char>);
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0);
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    registry.destroy(e0);
+
+    ASSERT_FALSE(group.contains(e0));
+    ASSERT_TRUE(group.contains(e1));
+}
+
+TEST(OwningGroup, Empty) {
+    entt::registry registry;
+
+    const auto e0 = registry.create();
+    registry.assign<double>(e0);
+    registry.assign<int>(e0);
+    registry.assign<float>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<char>(e1);
+    registry.assign<float>(e1);
+
+    ASSERT_TRUE((registry.group<char, int>(entt::get<float>).empty()));
+    ASSERT_TRUE((registry.group<double, float>(entt::get<char, int>).empty()));
+}
+
+TEST(OwningGroup, Each) {
+    entt::registry registry;
+    auto group = registry.group<int>(entt::get<char>);
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0);
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    auto cgroup = std::as_const(registry).group<const int>(entt::get<const char>);
+    std::size_t cnt = 0;
+
+    group.each([&cnt](auto, int &, char &) { ++cnt; });
+    group.each([&cnt](int &, char &) { ++cnt; });
+
+    ASSERT_EQ(cnt, std::size_t{4});
+
+    cgroup.each([&cnt](auto, const int &, const char &) { --cnt; });
+    cgroup.each([&cnt](const int &, const char &) { --cnt; });
+
+    ASSERT_EQ(cnt, std::size_t{0});
+}
+
+TEST(OwningGroup, SortOrdered) {
+    entt::registry registry;
+    auto group = registry.group<boxed_int, char>();
+
+    entt::entity entities[5] = {
+        registry.create(),
+        registry.create(),
+        registry.create(),
+        registry.create(),
+        registry.create()
+    };
+
+    registry.assign<boxed_int>(entities[0], 12);
+    registry.assign<char>(entities[0], 'a');
+
+    registry.assign<boxed_int>(entities[1], 9);
+    registry.assign<char>(entities[1], 'b');
+
+    registry.assign<boxed_int>(entities[2], 6);
+    registry.assign<char>(entities[2], 'c');
+
+    registry.assign<boxed_int>(entities[3], 1);
+    registry.assign<boxed_int>(entities[4], 2);
+
+    group.sort([&group](const entt::entity lhs, const entt::entity rhs) {
+        return group.get<boxed_int>(lhs).value < group.get<boxed_int>(rhs).value;
+    });
+
+    ASSERT_EQ(*(group.data() + 0u), entities[0]);
+    ASSERT_EQ(*(group.data() + 1u), entities[1]);
+    ASSERT_EQ(*(group.data() + 2u), entities[2]);
+    ASSERT_EQ(*(group.data() + 3u), entities[3]);
+    ASSERT_EQ(*(group.data() + 4u), entities[4]);
+
+    ASSERT_EQ((group.raw<boxed_int>() + 0u)->value, 12);
+    ASSERT_EQ((group.raw<boxed_int>() + 1u)->value, 9);
+    ASSERT_EQ((group.raw<boxed_int>() + 2u)->value, 6);
+    ASSERT_EQ((group.raw<boxed_int>() + 3u)->value, 1);
+    ASSERT_EQ((group.raw<boxed_int>() + 4u)->value, 2);
+
+    ASSERT_EQ(*(group.raw<char>() + 0u), 'a');
+    ASSERT_EQ(*(group.raw<char>() + 1u), 'b');
+    ASSERT_EQ(*(group.raw<char>() + 2u), 'c');
+}
+
+TEST(OwningGroup, SortReverse) {
+    entt::registry registry;
+    auto group = registry.group<boxed_int, char>();
+
+    entt::entity entities[5] = {
+        registry.create(),
+        registry.create(),
+        registry.create(),
+        registry.create(),
+        registry.create()
+    };
+
+    registry.assign<boxed_int>(entities[0], 6);
+    registry.assign<char>(entities[0], 'a');
+
+    registry.assign<boxed_int>(entities[1], 9);
+    registry.assign<char>(entities[1], 'b');
+
+    registry.assign<boxed_int>(entities[2], 12);
+    registry.assign<char>(entities[2], 'c');
+
+    registry.assign<boxed_int>(entities[3], 1);
+    registry.assign<boxed_int>(entities[4], 2);
+
+    group.sort<boxed_int>([](const auto &lhs, const auto &rhs) {
+        return lhs.value < rhs.value;
+    });
+
+    ASSERT_EQ(*(group.data() + 0u), entities[2]);
+    ASSERT_EQ(*(group.data() + 1u), entities[1]);
+    ASSERT_EQ(*(group.data() + 2u), entities[0]);
+    ASSERT_EQ(*(group.data() + 3u), entities[3]);
+    ASSERT_EQ(*(group.data() + 4u), entities[4]);
+
+    ASSERT_EQ((group.raw<boxed_int>() + 0u)->value, 12);
+    ASSERT_EQ((group.raw<boxed_int>() + 1u)->value, 9);
+    ASSERT_EQ((group.raw<boxed_int>() + 2u)->value, 6);
+    ASSERT_EQ((group.raw<boxed_int>() + 3u)->value, 1);
+    ASSERT_EQ((group.raw<boxed_int>() + 4u)->value, 2);
+
+    ASSERT_EQ(*(group.raw<char>() + 0u), 'c');
+    ASSERT_EQ(*(group.raw<char>() + 1u), 'b');
+    ASSERT_EQ(*(group.raw<char>() + 2u), 'a');
+}
+
+TEST(OwningGroup, SortUnordered) {
+    entt::registry registry;
+    auto group = registry.group<boxed_int>(entt::get<char>);
+
+    entt::entity entities[7] = {
+        registry.create(),
+        registry.create(),
+        registry.create(),
+        registry.create(),
+        registry.create(),
+        registry.create(),
+        registry.create()
+    };
+
+    registry.assign<boxed_int>(entities[0], 6);
+    registry.assign<char>(entities[0], 'c');
+
+    registry.assign<boxed_int>(entities[1], 3);
+    registry.assign<char>(entities[1], 'b');
+
+    registry.assign<boxed_int>(entities[2], 1);
+    registry.assign<char>(entities[2], 'a');
+
+    registry.assign<boxed_int>(entities[3], 9);
+    registry.assign<char>(entities[3], 'd');
+
+    registry.assign<boxed_int>(entities[4], 12);
+    registry.assign<char>(entities[4], 'e');
+
+    registry.assign<boxed_int>(entities[5], 4);
+    registry.assign<boxed_int>(entities[6], 5);
+
+    group.sort<char>([](const auto lhs, const auto rhs) {
+        return lhs < rhs;
+    });
+
+    ASSERT_EQ(*(group.data() + 0u), entities[4]);
+    ASSERT_EQ(*(group.data() + 1u), entities[3]);
+    ASSERT_EQ(*(group.data() + 2u), entities[0]);
+    ASSERT_EQ(*(group.data() + 3u), entities[1]);
+    ASSERT_EQ(*(group.data() + 4u), entities[2]);
+    ASSERT_EQ(*(group.data() + 5u), entities[5]);
+    ASSERT_EQ(*(group.data() + 6u), entities[6]);
+
+    ASSERT_EQ((group.raw<boxed_int>() + 0u)->value, 12);
+    ASSERT_EQ((group.raw<boxed_int>() + 1u)->value, 9);
+    ASSERT_EQ((group.raw<boxed_int>() + 2u)->value, 6);
+    ASSERT_EQ((group.raw<boxed_int>() + 3u)->value, 3);
+    ASSERT_EQ((group.raw<boxed_int>() + 4u)->value, 1);
+    ASSERT_EQ((group.raw<boxed_int>() + 5u)->value, 4);
+    ASSERT_EQ((group.raw<boxed_int>() + 6u)->value, 5);
+
+    ASSERT_EQ(*(group.raw<char>() + 0u), 'c');
+    ASSERT_EQ(*(group.raw<char>() + 1u), 'b');
+    ASSERT_EQ(*(group.raw<char>() + 2u), 'a');
+    ASSERT_EQ(*(group.raw<char>() + 3u), 'd');
+    ASSERT_EQ(*(group.raw<char>() + 4u), 'e');
+}
+
+TEST(OwningGroup, IndexRebuiltOnDestroy) {
+    entt::registry registry;
+    auto group = registry.group<int>(entt::get<unsigned int>);
+
+    const auto e0 = registry.create();
+    const auto e1 = registry.create();
+
+    registry.assign<unsigned int>(e0, 0u);
+    registry.assign<unsigned int>(e1, 1u);
+
+    registry.assign<int>(e0, 0);
+    registry.assign<int>(e1, 1);
+
+    registry.destroy(e0);
+    registry.assign<int>(registry.create(), 42);
+
+    ASSERT_EQ(group.size(), typename decltype(group)::size_type{1});
+    ASSERT_EQ(group[{}], e1);
+    ASSERT_EQ(group.get<int>(e1), 1);
+    ASSERT_EQ(group.get<unsigned int>(e1), 1u);
+
+    group.each([e1](auto entity, auto ivalue, auto uivalue) {
+        ASSERT_EQ(entity, e1);
+        ASSERT_EQ(ivalue, 1);
+        ASSERT_EQ(uivalue, 1u);
+    });
+}
+
+TEST(OwningGroup, ConstNonConstAndAllInBetween) {
+    entt::registry registry;
+    auto group = registry.group<int, const char>(entt::get<double, const float, std::true_type>);
+
+    ASSERT_EQ(group.size(), decltype(group.size()){0});
+
+    const auto entity = registry.create();
+    registry.assign<int>(entity, 0);
+    registry.assign<char>(entity, 'c');
+    registry.assign<double>(entity, 0.);
+    registry.assign<float>(entity, 0.f);
+    registry.assign<std::true_type>(entity);
+
+    ASSERT_EQ(group.size(), decltype(group.size()){1});
+
+    ASSERT_TRUE((std::is_same_v<decltype(group.get<int>({})), int &>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.get<const char>({})), const char &>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.get<double>({})), double &>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.get<const float>({})), const float &>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.get<std::true_type>({})), std::true_type>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.get<int, const char, double, const float, std::true_type>({})), std::tuple<int &, const char &, double &, const float &, std::true_type>>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.raw<const float>()), const float *>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.raw<double>()), double *>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.raw<const char>()), const char *>));
+    ASSERT_TRUE((std::is_same_v<decltype(group.raw<int>()), int *>));
+
+    group.each([](auto &&i, auto &&c, auto &&d, auto &&f, auto &&e) {
+        ASSERT_TRUE((std::is_same_v<decltype(i), int &>));
+        ASSERT_TRUE((std::is_same_v<decltype(c), const char &>));
+        ASSERT_TRUE((std::is_same_v<decltype(d), double &>));
+        ASSERT_TRUE((std::is_same_v<decltype(f), const float &>));
+        ASSERT_TRUE((std::is_same_v<decltype(e), std::true_type &&>));
+    });
+}
+
+TEST(OwningGroup, Find) {
+    entt::registry registry;
+    auto group = registry.group<int>(entt::get<const char>);
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0);
+    registry.assign<char>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1);
+    registry.assign<char>(e1);
+
+    const auto e2 = registry.create();
+    registry.assign<int>(e2);
+    registry.assign<char>(e2);
+
+    const auto e3 = registry.create();
+    registry.assign<int>(e3);
+    registry.assign<char>(e3);
+
+    registry.remove<int>(e1);
+
+    ASSERT_NE(group.find(e0), group.end());
+    ASSERT_EQ(group.find(e1), group.end());
+    ASSERT_NE(group.find(e2), group.end());
+    ASSERT_NE(group.find(e3), group.end());
+
+    auto it = group.find(e2);
+
+    ASSERT_EQ(*it, e2);
+    ASSERT_EQ(*(++it), e3);
+    ASSERT_EQ(*(++it), e0);
+    ASSERT_EQ(++it, group.end());
+    ASSERT_EQ(++group.find(e0), group.end());
+
+    const auto e4 = registry.create();
+    registry.destroy(e4);
+    const auto e5 = registry.create();
+    registry.assign<int>(e5);
+    registry.assign<char>(e5);
+
+    ASSERT_NE(group.find(e5), group.end());
+    ASSERT_EQ(group.find(e4), group.end());
+}
+
+TEST(OwningGroup, ExcludedComponents) {
+    entt::registry registry;
+
+    const auto e0 = registry.create();
+    registry.assign<int>(e0, 0);
+
+    const auto e1 = registry.create();
+    registry.assign<int>(e1, 1);
+    registry.assign<char>(e1);
+
+    const auto group = registry.group<int>(entt::exclude<char, double>);
+
+    const auto e2 = registry.create();
+    registry.assign<int>(e2, 2);
+
+    const auto e3 = registry.create();
+    registry.assign<int>(e3, 3);
+    registry.assign<double>(e3);
+
+    for(const auto entity: group) {
+        if(entity == e0) {
+            ASSERT_EQ(group.get<int>(e0), 0);
+        } else if(entity == e2) {
+            ASSERT_EQ(group.get<int>(e2), 2);
+        } else {
+            FAIL();
+        }
+    }
+
+    registry.assign<char>(e0);
+    registry.assign<double>(e2);
+
+    ASSERT_TRUE(group.empty());
+
+    registry.remove<char>(e1);
+    registry.remove<double>(e3);
+
+    for(const auto entity: group) {
+        if(entity == e1) {
+            ASSERT_EQ(group.get<int>(e1), 1);
+        } else if(entity == e3) {
+            ASSERT_EQ(group.get<int>(e3), 3);
+        } else {
+            FAIL();
+        }
+    }
+}
+
+TEST(OwningGroup, EmptyAndNonEmptyTypes) {
+    entt::registry registry;
+    const auto group = registry.group<empty_type>(entt::get<int>);
+
+    const auto e0 = registry.create();
+    registry.assign<empty_type>(e0);
+    registry.assign<int>(e0);
+
+    const auto e1 = registry.create();
+    registry.assign<empty_type>(e1);
+    registry.assign<int>(e1);
+
+    registry.assign<int>(registry.create());
+
+    for(const auto entity: group) {
+        ASSERT_TRUE(entity == e0 || entity == e1);
+    }
+
+    group.each([e0, e1](const auto entity, empty_type, const int &) {
+        ASSERT_TRUE(entity == e0 || entity == e1);
+    });
+
+    ASSERT_EQ(group.size(), typename decltype(group)::size_type{2});
+}
+
+TEST(OwningGroup, TrackEntitiesOnComponentDestruction) {
+    entt::registry registry;
+    const auto group = registry.group<int>(entt::exclude<char>);
+    const auto cgroup = std::as_const(registry).group<const int>(entt::exclude<char>);
+
+    const auto entity = registry.create();
+    registry.assign<int>(entity);
+    registry.assign<char>(entity);
+
+    ASSERT_TRUE(group.empty());
+    ASSERT_TRUE(cgroup.empty());
+
+    registry.remove<char>(entity);
+
+    ASSERT_FALSE(group.empty());
+    ASSERT_FALSE(cgroup.empty());
+}
+
+TEST(OwningGroup, Less) {
+    entt::registry registry;
+    const auto entity = std::get<0>(registry.create<int, entt::tag<"empty"_hs>>());
+    registry.create<char>();
+
+    registry.group<int>(entt::get<char, entt::tag<"empty"_hs>>).less([entity](const auto entt, int, char) {
+        ASSERT_EQ(entity, entt);
+    });
+
+    registry.group<char>(entt::get<entt::tag<"empty"_hs>, int>).less([check = true](int, char) mutable {
+        ASSERT_TRUE(check);
+        check = false;
+    });
+
+    registry.group<entt::tag<"empty"_hs>>(entt::get<int, char>).less([entity](const auto entt, int, char) {
+        ASSERT_EQ(entity, entt);
+    });
+
+    registry.group<double>(entt::get<int, char>).less([entity](const auto entt, double, int, char) {
+        ASSERT_EQ(entity, entt);
+    });
+}

test/entt/entity/helper.cpp

@@ -2,72 +2,46 @@
 #include <entt/core/hashed_string.hpp>
 #include <entt/entity/helper.hpp>
 #include <entt/entity/registry.hpp>
+#include <entt/core/type_traits.hpp>
 
-TEST(Helper, Dependency) {
-    entt::DefaultRegistry registry;
-    const auto entity = registry.create();
-    entt::dependency<double, float>(registry.construction<int>());
+TEST(Helper, AsView) {
+    entt::registry registry;
+    const entt::registry cregistry;
 
-    ASSERT_FALSE(registry.has<double>(entity));
-    ASSERT_FALSE(registry.has<float>(entity));
-
-    registry.assign<char>(entity);
-
-    ASSERT_FALSE(registry.has<double>(entity));
-    ASSERT_FALSE(registry.has<float>(entity));
-
-    registry.assign<int>(entity);
-
-    ASSERT_TRUE(registry.has<double>(entity));
-    ASSERT_TRUE(registry.has<float>(entity));
-    ASSERT_EQ(registry.get<double>(entity), .0);
-    ASSERT_EQ(registry.get<float>(entity), .0f);
-
-    registry.get<double>(entity) = .3;
-    registry.get<float>(entity) = .1f;
-    registry.remove<int>(entity);
-    registry.assign<int>(entity);
-
-    ASSERT_EQ(registry.get<double>(entity), .3);
-    ASSERT_EQ(registry.get<float>(entity), .1f);
-
-    registry.remove<int>(entity);
-    registry.remove<float>(entity);
-    registry.assign<int>(entity);
-
-    ASSERT_TRUE(registry.has<float>(entity));
-    ASSERT_EQ(registry.get<double>(entity), .3);
-    ASSERT_EQ(registry.get<float>(entity), .0f);
-
-    registry.remove<int>(entity);
-    registry.remove<double>(entity);
-    registry.remove<float>(entity);
-    entt::dependency<double, float>(entt::break_t{}, registry.construction<int>());
-    registry.assign<int>(entity);
-
-    ASSERT_FALSE(registry.has<double>(entity));
-    ASSERT_FALSE(registry.has<float>(entity));
+    ([](entt::view<int, char>) {})(entt::as_view{registry});
+    ([](entt::view<const int, char>) {})(entt::as_view{registry});
+    ([](entt::view<const double>) {})(entt::as_view{cregistry});
 }
 
-TEST(Helper, Label) {
-    entt::DefaultRegistry 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<const int>, entt::get_t<char>, double>) {})(entt::as_group{registry});
+    ([](entt::group<entt::exclude_t<const int>, entt::get_t<const char>, double>) {})(entt::as_group{registry});
+    ([](entt::group<entt::exclude_t<const int>, entt::get_t<const char>, const double>) {})(entt::as_group{registry});
+}
+
+TEST(Helper, Tag) {
+    entt::registry registry;
     const auto entity = registry.create();
-    registry.assign<entt::label<"foobar"_hs>>(entity);
+    registry.assign<entt::tag<"foobar"_hs>>(entity);
     registry.assign<int>(entity, 42);
     int counter{};
 
-    ASSERT_FALSE(registry.has<entt::label<"barfoo"_hs>>(entity));
-    ASSERT_TRUE(registry.has<entt::label<"foobar"_hs>>(entity));
+    ASSERT_FALSE(registry.has<entt::tag<"barfoo"_hs>>(entity));
+    ASSERT_TRUE(registry.has<entt::tag<"foobar"_hs>>(entity));
 
-    for(auto entity: registry.view<int, entt::label<"foobar"_hs>>()) {
-        (void)entity;
+    for(auto entt: registry.view<int, entt::tag<"foobar"_hs>>()) {
+        (void)entt;
         ++counter;
     }
 
     ASSERT_NE(counter, 0);
 
-    for(auto entity: registry.view<entt::label<"foobar"_hs>>()) {
-        (void)entity;
+    for(auto entt: registry.view<entt::tag<"foobar"_hs>>()) {
+        (void)entt;
         --counter;
     }
 

test/entt/entity/observer.cpp

@@ -0,0 +1,283 @@
+#include <tuple>
+#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(), entt::observer::size_type{});
+    ASSERT_TRUE(observer.empty());
+    ASSERT_EQ(observer.data(), nullptr);
+    ASSERT_EQ(observer.begin(), observer.end());
+
+    const auto entity = std::get<0>(registry.create<int>());
+
+    ASSERT_EQ(observer.size(), entt::observer::size_type{1});
+    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(), entt::observer::size_type{});
+    ASSERT_TRUE(observer.empty());
+
+    observer.disconnect();
+    registry.remove<int>(entity);
+    registry.assign<int>(entity);
+
+    ASSERT_EQ(observer.size(), entt::observer::size_type{});
+    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(), entt::observer::size_type{1});
+    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.reset<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(), entt::observer::size_type{});
+    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(), entt::observer::size_type{1});
+    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(), entt::observer::size_type{1});
+    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(), entt::observer::size_type{});
+    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(), entt::observer::size_type{1});
+    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(), entt::observer::size_type{1});
+    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 = std::get<0>(registry.create<int>());
+
+    ASSERT_FALSE(observer.empty());
+    ASSERT_EQ(observer.size(), entt::observer::size_type{1});
+
+    std::as_const(observer).each([entity](const auto entt) {
+        ASSERT_EQ(entity, entt);
+    });
+
+    ASSERT_FALSE(observer.empty());
+    ASSERT_EQ(observer.size(), entt::observer::size_type{1});
+
+    observer.each([entity](const auto entt) {
+        ASSERT_EQ(entity, entt);
+    });
+
+    ASSERT_TRUE(observer.empty());
+    ASSERT_EQ(observer.size(), entt::observer::size_type{});
+}

test/entt/entity/prototype.cpp

@@ -1,153 +0,0 @@
-#include <gtest/gtest.h>
-#include <entt/entity/prototype.hpp>
-#include <entt/entity/registry.hpp>
-
-TEST(Prototype, SameRegistry) {
-    entt::DefaultRegistry registry;
-    entt::DefaultPrototype prototype{registry};
-    const auto &cprototype = prototype;
-
-    ASSERT_FALSE(registry.empty());
-    ASSERT_FALSE((prototype.has<int, char>()));
-
-    ASSERT_EQ(prototype.set<int>(2), 2);
-    ASSERT_EQ(prototype.set<int>(3), 3);
-    ASSERT_EQ(prototype.set<char>('c'), 'c');
-
-    ASSERT_EQ(prototype.get<int>(), 3);
-    ASSERT_EQ(cprototype.get<char>(), 'c');
-    ASSERT_EQ(std::get<0>(prototype.get<int, char>()), 3);
-    ASSERT_EQ(std::get<1>(cprototype.get<int, char>()), 'c');
-
-    const auto e0 = prototype.create();
-
-    ASSERT_TRUE((prototype.has<int, char>()));
-    ASSERT_FALSE(registry.orphan(e0));
-
-    const auto e1 = prototype();
-    prototype(e0);
-
-    ASSERT_FALSE(registry.orphan(e0));
-    ASSERT_FALSE(registry.orphan(e1));
-
-    ASSERT_TRUE((registry.has<int, char>(e0)));
-    ASSERT_TRUE((registry.has<int, char>(e1)));
-
-    registry.remove<int>(e0);
-    registry.remove<int>(e1);
-    prototype.unset<int>();
-
-    ASSERT_FALSE((prototype.has<int, char>()));
-    ASSERT_FALSE((prototype.has<int>()));
-    ASSERT_TRUE((prototype.has<char>()));
-
-    prototype(e0);
-    prototype(e1);
-
-    ASSERT_FALSE(registry.has<int>(e0));
-    ASSERT_FALSE(registry.has<int>(e1));
-
-    ASSERT_EQ(registry.get<char>(e0), 'c');
-    ASSERT_EQ(registry.get<char>(e1), 'c');
-
-    registry.get<char>(e0) = '*';
-    prototype.assign(e0);
-
-    ASSERT_EQ(registry.get<char>(e0), '*');
-
-    registry.get<char>(e1) = '*';
-    prototype.accommodate(e1);
-
-    ASSERT_EQ(registry.get<char>(e1), 'c');
-}
-
-TEST(Prototype, OtherRegistry) {
-    entt::DefaultRegistry registry;
-    entt::DefaultRegistry repository;
-    entt::DefaultPrototype prototype{repository};
-    const auto &cprototype = prototype;
-
-    ASSERT_TRUE(registry.empty());
-    ASSERT_FALSE((prototype.has<int, char>()));
-
-    ASSERT_EQ(prototype.set<int>(2), 2);
-    ASSERT_EQ(prototype.set<int>(3), 3);
-    ASSERT_EQ(prototype.set<char>('c'), 'c');
-
-    ASSERT_EQ(prototype.get<int>(), 3);
-    ASSERT_EQ(cprototype.get<char>(), 'c');
-    ASSERT_EQ(std::get<0>(prototype.get<int, char>()), 3);
-    ASSERT_EQ(std::get<1>(cprototype.get<int, char>()), 'c');
-
-    const auto e0 = prototype.create(registry);
-
-    ASSERT_TRUE((prototype.has<int, char>()));
-    ASSERT_FALSE(registry.orphan(e0));
-
-    const auto e1 = prototype(registry);
-    prototype(registry, e0);
-
-    ASSERT_FALSE(registry.orphan(e0));
-    ASSERT_FALSE(registry.orphan(e1));
-
-    ASSERT_TRUE((registry.has<int, char>(e0)));
-    ASSERT_TRUE((registry.has<int, char>(e1)));
-
-    registry.remove<int>(e0);
-    registry.remove<int>(e1);
-    prototype.unset<int>();
-
-    ASSERT_FALSE((prototype.has<int, char>()));
-    ASSERT_FALSE((prototype.has<int>()));
-    ASSERT_TRUE((prototype.has<char>()));
-
-    prototype(registry, e0);
-    prototype(registry, e1);
-
-    ASSERT_FALSE(registry.has<int>(e0));
-    ASSERT_FALSE(registry.has<int>(e1));
-
-    ASSERT_EQ(registry.get<char>(e0), 'c');
-    ASSERT_EQ(registry.get<char>(e1), 'c');
-
-    registry.get<char>(e0) = '*';
-    prototype.assign(registry, e0);
-
-    ASSERT_EQ(registry.get<char>(e0), '*');
-
-    registry.get<char>(e1) = '*';
-    prototype.accommodate(registry, e1);
-
-    ASSERT_EQ(registry.get<char>(e1), 'c');
-}
-
-TEST(Prototype, RAII) {
-    entt::DefaultRegistry registry;
-
-    {
-        entt::DefaultPrototype prototype{registry};
-        prototype.set<int>(0);
-
-        ASSERT_FALSE(registry.empty());
-    }
-
-    ASSERT_TRUE(registry.empty());
-}
-
-TEST(Prototype, MoveConstructionAssignment) {
-    entt::DefaultRegistry registry;
-
-    entt::DefaultPrototype prototype{registry};
-    prototype.set<int>(0);
-    auto other{std::move(prototype)};
-    const auto e0 = other();
-
-    ASSERT_EQ(registry.size(), entt::DefaultRegistry::size_type{2});
-    ASSERT_TRUE(registry.has<int>(e0));
-
-    prototype = std::move(other);
-    const auto e1 = prototype();
-
-    ASSERT_EQ(registry.size(), entt::DefaultRegistry::size_type{3});
-    ASSERT_TRUE(registry.has<int>(e1));
-}