review: iterators + fixed bug on raw views

Add dispatcher.hpp to entt.hpp

.gitignore

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

CMakeLists.txt

@@ -16,7 +16,7 @@ endif()
 # Project configuration
 #
 
-project(entt VERSION 2.4.0)
+project(entt VERSION 2.5.0)
 
 if(NOT CMAKE_BUILD_TYPE)
     set(CMAKE_BUILD_TYPE Debug)
@@ -29,7 +29,7 @@ set(PROJECT_AUTHOR_EMAIL "michele.caini@gmail.com")
 
 message("*")
 message("* ${PROJECT_NAME} v${PROJECT_VERSION} (${CMAKE_BUILD_TYPE})")
-message("* Copyright (c) 2017 ${PROJECT_AUTHOR} <${PROJECT_AUTHOR_EMAIL}>")
+message("* Copyright (c) 2018 ${PROJECT_AUTHOR} <${PROJECT_AUTHOR_EMAIL}>")
 message("*")
 
 #
@@ -37,13 +37,28 @@ message("*")
 #
 
 set(CMAKE_CXX_STANDARD 14)
+set(CMAKE_CXX_EXTENSIONS OFF)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
 if(NOT MSVC)
-    set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -Wl,--no-undefined")
+    include(CheckCXXSourceCompiles)
+
+    set(OLD_CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -stdlib=libc++")
+
+    check_cxx_source_compiles("
+        #include<type_traits>
+        int main() { return std::is_same<int, int>::value ? 0 : 1; }
+    " HAS_LIBCPP)
+
+    if(NOT HAS_LIBCPP)
+        set(CMAKE_CXX_FLAGS "${OLD_CMAKE_CXX_FLAGS}")
+    endif()
+
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pedantic -Wall")
     set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -DRELEASE")
     set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -O0 -g -DDEBUG")
+    set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -Wl,--no-undefined")
 
     if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
         # it seems that -O3 ruins the performance when using clang ...
@@ -72,6 +87,7 @@ if(BUILD_TESTING)
 
     option(BUILD_BENCHMARK "Build benchmark." OFF)
     option(BUILD_MOD "Build mod example." OFF)
+    option(BUILD_SNAPSHOT "Build snapshot example." OFF)
 
     # gtest, gtest_main, gmock and gmock_main targets are available from now on
     set(GOOGLETEST_DEPS_DIR ${entt_SOURCE_DIR}/deps/googletest)
@@ -106,5 +122,6 @@ add_custom_target(
         AUTHORS
         LICENSE
         README.md
+        TODO
         .travis.yml
 )

LICENSE

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

TODO

@@ -0,0 +1,9 @@
+* custom allocators and EnTT allocator-aware in general (long term feature, I don't actually need it at the moment) - see #22
+* to analyze, long term feature: systems organizer based on dependency graphs for implicit parallelism (I don't want to think anymore in future :-))
+* scene management (I prefer the concept of spaces, that is a kind of scene anyway)
+* blueprint registry - kind of factory to create entitites template for initialization (get rid of the extra versions of Registry::create)
+* 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
+* signals on component creation/destruction: crtp + internal detection, probably it works - test it!!
+* define a macro for the noexcept policy, so as to provide users with an easy way to disable exception handling
+* AOB

cmake/in/cereal.in

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

cmake/in/duktape.in

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

src/entt/core/family.hpp

@@ -4,6 +4,7 @@
 
 #include<type_traits>
 #include<cstddef>
+#include<atomic>
 
 
 namespace entt {
@@ -18,14 +19,11 @@ namespace entt {
  */
 template<typename...>
 class Family {
-    static std::size_t identifier() noexcept {
-        static std::size_t value = 0;
-        return value++;
-    }
+    static std::atomic<std::size_t> identifier;
 
     template<typename...>
     static std::size_t family() noexcept {
-        static const std::size_t value = identifier();
+        static const std::size_t value = identifier.fetch_add(1);
         return value;
     }
 
@@ -38,12 +36,16 @@ public:
      * @return Statically generated unique identifier for the given type.
      */
     template<typename... Type>
-    static family_type type() noexcept {
+    inline static family_type type() noexcept {
         return family<std::decay_t<Type>...>();
     }
 };
 
 
+template<typename... Types>
+std::atomic<std::size_t> Family<Types...>::identifier{};
+
+
 }
 
 

src/entt/core/hashed_string.hpp

@@ -2,6 +2,7 @@
 #define ENTT_CORE_HASHED_STRING_HPP
 
 
+#include <cstddef>
 #include <cstdint>
 
 

src/entt/entity/actor.hpp

@@ -22,6 +22,8 @@ template<typename Entity, typename Delta>
 struct Actor {
     /*! @brief Type of registry used internally. */
     using registry_type = Registry<Entity>;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
     /*! @brief Type used to provide elapsed time. */
     using delta_type = Delta;
 
@@ -63,8 +65,8 @@ struct Actor {
      * @return A reference to the newly created component.
      */
     template<typename Component, typename... Args>
-    Component & set(Args&&... args) {
-        return reg.template accomodate<Component>(entity, std::forward<Args>(args)...);
+    Component & set(Args &&... args) {
+        return reg.template accommodate<Component>(entity, std::forward<Args>(args)...);
     }
 
     /**
@@ -143,7 +145,7 @@ private:
  * @tparam Delta Type to use to provide elapsed time.
  */
 template<typename Delta>
-using DefaultActor = Actor<std::uint32_t, Delta>;
+using DefaultActor = Actor<DefaultRegistry::entity_type, Delta>;
 
 
 }

src/entt/entity/entt_traits.hpp

@@ -1,5 +1,5 @@
-#ifndef ENTT_ENTITY_ENTT_HPP
-#define ENTT_ENTITY_ENTT_HPP
+#ifndef ENTT_ENTITY_ENTT_TRAITS_HPP
+#define ENTT_ENTITY_ENTT_TRAITS_HPP
 
 
 #include <cstdint>
@@ -58,11 +58,11 @@ struct entt_traits<std::uint32_t> {
     using version_type = std::uint16_t;
 
     /*! @brief Mask to use to get the entity number out of an identifier. */
-    static constexpr auto entity_mask = 0xFFFFFF;
+    static constexpr auto entity_mask = 0xFFFFF;
     /*! @brief Mask to use to get the version out of an identifier. */
-    static constexpr auto version_mask = 0xFF;
+    static constexpr auto version_mask = 0xFFF;
     /*! @brief Extent of the entity number within an identifier. */
-    static constexpr auto entity_shift = 24;
+    static constexpr auto entity_shift = 20;
 };
 
 
@@ -93,4 +93,4 @@ struct entt_traits<std::uint64_t> {
 }
 
 
-#endif // ENTT_ENTITY_ENTT_HPP
+#endif // ENTT_ENTITY_ENTT_TRAITS_HPP

src/entt/entity/registry.hpp

@@ -2,6 +2,7 @@
 #define ENTT_ENTITY_REGISTRY_HPP
 
 
+#include <tuple>
 #include <vector>
 #include <memory>
 #include <utility>
@@ -9,9 +10,11 @@
 #include <cstdint>
 #include <cassert>
 #include <algorithm>
+#include <type_traits>
 #include "../core/family.hpp"
+#include "entt_traits.hpp"
+#include "snapshot.hpp"
 #include "sparse_set.hpp"
-#include "traits.hpp"
 #include "view.hpp"
 
 
@@ -55,7 +58,7 @@ class Registry {
         using test_fn_type = bool(Registry::*)(Entity) const;
 
         template<typename... Args>
-        Component & construct(Registry &registry, Entity entity, Args&&... args) {
+        Component & construct(Registry &registry, Entity entity, Args &&... args) {
             auto &component = SparseSet<Entity, Component>::construct(entity, std::forward<Args>(args)...);
 
             for(auto &&listener: listeners) {
@@ -111,7 +114,7 @@ class Registry {
     }
 
     template<typename Component>
-    Pool<Component> & ensure() {
+    Pool<Component> & assure() {
         const auto ctype = component_family::type<Component>();
 
         if(!(ctype < pools.size())) {
@@ -136,7 +139,6 @@ class Registry {
 
         if(!handlers[vtype]) {
             using accumulator_type = int[];
-
             auto set = std::make_unique<SparseSet<Entity>>();
 
             for(auto entity: view<Component...>()) {
@@ -144,7 +146,7 @@ class Registry {
             }
 
             accumulator_type accumulator = {
-                (ensure<Component>().append(set.get(), &Registry::has<Component...>), 0)...
+                (assure<Component>().append(set.get(), &Registry::has<Component...>), 0)...
             };
 
             handlers[vtype] = std::move(set);
@@ -228,7 +230,7 @@ public:
      * @return Number of entities still in use.
      */
     size_type size() const noexcept {
-        return entities.size() - available.size();
+        return entities.size() - available;
     }
 
     /**
@@ -242,7 +244,7 @@ public:
      */
     template<typename Component>
     void reserve(size_type cap) {
-        ensure<Component>().reserve(cap);
+        assure<Component>().reserve(cap);
     }
 
     /**
@@ -255,7 +257,6 @@ public:
      */
     void reserve(size_type cap) {
         entities.reserve(cap);
-        available.reserve(cap);
     }
 
     /**
@@ -282,19 +283,41 @@ public:
      * @return True if at least an entity is still in use, false otherwise.
      */
     bool empty() const noexcept {
-        return entities.size() == available.size();
+        return entities.size() == available;
     }
 
     /**
-     * @brief Verifies if an entity identifier still refers to a valid entity.
+     * @brief Checks if an entity identifier refers to a valid entity.
      * @param entity An entity identifier, either valid or not.
-     * @return True if the identifier is still valid, false otherwise.
+     * @return True if the identifier is valid, false otherwise.
      */
     bool valid(entity_type entity) const noexcept {
-        using promotion_type = std::conditional_t<sizeof(size_type) >= sizeof(entity_type), size_type, entity_type>;
-        // explicit promotion to avoid warnings with std::uint16_t
-        const entity_type entt = promotion_type{entity} & traits_type::entity_mask;
-        return (entt < entities.size() && entities[entt] == entity);
+        const auto pos = size_type(entity & traits_type::entity_mask);
+        return (pos < entities.size() && entities[pos] == entity);
+    }
+
+    /**
+     * @brief Checks if an entity identifier refers to a valid entity.
+     *
+     * Alternative version of `valid`. It accesses the internal data structures
+     * without bounds checking and thus it's both unsafe and risky to use.<br/>
+     * You should not invoke directly this function unless you know exactly what
+     * you are doing. Prefer the `valid` member function instead.
+     *
+     * @warning
+     * Attempting to use an entity that doesn't belong to the registry can
+     * result in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode in case of
+     * bounds violation.
+     *
+     * @param entity A valid entity identifier.
+     * @return True if the identifier is valid, false otherwise.
+     */
+    bool fast(entity_type entity) const noexcept {
+        const auto pos = size_type(entity & traits_type::entity_mask);
+        assert(pos < entities.size());
+        // the in-use control bit permits to avoid accessing the direct vector
+        return (entities[pos] == entity);
     }
 
     /**
@@ -324,11 +347,9 @@ public:
      * @return Actual version for the given entity identifier.
      */
     version_type current(entity_type entity) const noexcept {
-        using promotion_type = std::conditional_t<sizeof(size_type) >= sizeof(entity_type), size_type, entity_type>;
-        // explicit promotion to avoid warnings with std::uint16_t
-        const auto entt = promotion_type{entity} & traits_type::entity_mask;
-        assert(entt < entities.size());
-        return version_type((entities[entt] >> traits_type::entity_shift) & traits_type::version_mask);
+        const auto pos = size_type(entity & traits_type::entity_mask);
+        assert(pos < entities.size());
+        return version_type((entities[pos] >> traits_type::entity_shift) & traits_type::version_mask);
     }
 
     /**
@@ -351,10 +372,10 @@ public:
      * @return A valid entity identifier.
      */
     template<typename... Component>
-    entity_type create(Component&&... components) noexcept {
+    entity_type create(Component &&... components) noexcept {
         using accumulator_type = int[];
         const auto entity = create();
-        accumulator_type accumulator = { 0, (ensure<Component>().construct(*this, entity, std::forward<Component>(components)), 0)... };
+        accumulator_type accumulator = { 0, (assure<std::decay_t<Component>>().construct(*this, entity, std::forward<Component>(components)), 0)... };
         (void)accumulator;
         return entity;
     }
@@ -381,7 +402,7 @@ public:
     entity_type create() noexcept {
         using accumulator_type = int[];
         const auto entity = create();
-        accumulator_type accumulator = { 0, (ensure<Component>().construct(*this, entity), 0)... };
+        accumulator_type accumulator = { 0, (assure<Component>().construct(*this, entity), 0)... };
         (void)accumulator;
         return entity;
     }
@@ -406,14 +427,18 @@ public:
     entity_type create() noexcept {
         entity_type entity;
 
-        if(available.empty()) {
+        if(available) {
+            const auto entt = next;
+            const auto version = entities[entt] & (~traits_type::entity_mask);
+
+            entity = entt | version;
+            next = entities[entt] & traits_type::entity_mask;
+            entities[entt] = entity;
+            --available;
+        } else {
             entity = entity_type(entities.size());
             assert(entity < traits_type::entity_mask);
-            assert((entity >> traits_type::entity_shift) == entity_type{});
             entities.push_back(entity);
-        } else {
-            entity = available.back();
-            available.pop_back();
         }
 
         return entity;
@@ -437,11 +462,12 @@ public:
     void destroy(entity_type entity) {
         assert(valid(entity));
         const auto entt = entity & traits_type::entity_mask;
-        const auto version = version_type{1} + ((entity >> traits_type::entity_shift) & traits_type::version_mask);
-        const auto next = entt | (version << traits_type::entity_shift);
+        const auto version = (((entity >> traits_type::entity_shift) + 1) & traits_type::version_mask) << traits_type::entity_shift;
+        const auto node = (available ? next : ((entt + 1) & traits_type::entity_mask)) | version;
 
-        entities[entt] = next;
-        available.push_back(next);
+        entities[entt] = node;
+        next = entt;
+        ++available;
 
         for(auto &&cpool: pools) {
             if(cpool && cpool->has(entity)) {
@@ -451,7 +477,7 @@ public:
     }
 
     /**
-     * @brief Attaches a tag to an entity.
+     * @brief Attaches the given tag to an entity.
      *
      * Usually, pools of components allocate enough memory to store a bunch of
      * elements even if only one of them is used. On the other hand, there are
@@ -472,7 +498,7 @@ public:
      * @return A reference to the newly created tag.
      */
     template<typename Tag, typename... Args>
-    Tag & attach(entity_type entity, Args&&... args) {
+    Tag & attach(entity_type entity, Args &&... args) {
         assert(valid(entity));
         assert(!has<Tag>());
         const auto ttype = tag_family::type<Tag>();
@@ -481,13 +507,13 @@ public:
             tags.resize(ttype + 1);
         }
 
-        tags[ttype].reset(new Attaching<Tag>{entity, { std::forward<Args>(args)... }});
+        tags[ttype].reset(new Attaching<Tag>{entity, Tag{std::forward<Args>(args)...}});
 
         return static_cast<Attaching<Tag> *>(tags[ttype].get())->tag;
     }
 
     /**
-     * @brief Removes a tag from its owner, if any.
+     * @brief Removes the given tag from its owner, if any.
      * @tparam Tag Type of tag to remove.
      */
     template<typename Tag>
@@ -498,7 +524,7 @@ public:
     }
 
     /**
-     * @brief Checks if a tag has an owner.
+     * @brief Checks if the given tag has an owner.
      * @tparam Tag Type of tag for which to perform the check.
      * @return True if the tag already has an owner, false otherwise.
      */
@@ -513,7 +539,7 @@ public:
     }
 
     /**
-     * @brief Returns a reference to a tag.
+     * @brief Returns a reference to the given tag.
      *
      * @warning
      * Attempting to get a tag that hasn't an owner results in undefined
@@ -531,7 +557,7 @@ public:
     }
 
     /**
-     * @brief Returns a reference to a tag.
+     * @brief Returns a reference to the given tag.
      *
      * @warning
      * Attempting to get a tag that hasn't an owner results in undefined
@@ -548,7 +574,56 @@ public:
     }
 
     /**
-     * @brief Gets the owner of a tag, if any.
+     * @brief Replaces the given tag.
+     *
+     * 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).
+     *
+     * @warning
+     * Attempting to replace a tag that hasn't an owner results in undefined
+     * behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * tag hasn't been previously attached to an entity.
+     *
+     * @tparam Tag Type of tag to replace.
+     * @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 tag.
+     */
+    template<typename Tag, typename... Args>
+    Tag & set(Args &&... args) {
+        return get<Tag>() = Tag{std::forward<Args>(args)...};
+    }
+
+    /**
+     * @brief Changes the owner of the given tag.
+     *
+     * The ownership of the tag is transferred from one entity to another.
+     *
+     * @warning
+     * Attempting to use an invalid entity or to transfer the ownership of a tag
+     * that hasn't an owner results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode in case of
+     * invalid entity or if the tag hasn't been previously attached to an
+     * entity.
+     *
+     * @tparam Tag Type of tag of which to transfer the ownership.
+     * @param entity A valid entity identifier.
+     * @return A valid entity identifier.
+     */
+    template<typename Tag>
+    entity_type move(entity_type entity) {
+        assert(valid(entity));
+        assert(has<Tag>());
+        const auto ttype = tag_family::type<Tag>();
+        const auto owner = tags[ttype]->entity;
+        tags[ttype]->entity = entity;
+        return owner;
+    }
+
+    /**
+     * @brief Gets the owner of the given tag, if any.
      *
      * @warning
      * Attempting to get the owner of a tag that hasn't been previously attached
@@ -586,9 +661,9 @@ public:
      * @return A reference to the newly created component.
      */
     template<typename Component, typename... Args>
-    Component & assign(entity_type entity, Args&&... args) {
+    Component & assign(entity_type entity, Args &&... args) {
         assert(valid(entity));
-        return ensure<Component>().construct(*this, entity, std::forward<Args>(args)...);
+        return assure<Component>().construct(*this, entity, std::forward<Args>(args)...);
     }
 
     /**
@@ -644,7 +719,7 @@ public:
      *
      * @tparam Component Type of component to get.
      * @param entity A valid entity identifier.
-     * @return A reference to the instance of the component owned by the entity.
+     * @return A reference to the component owned by the entity.
      */
     template<typename Component>
     const Component & get(entity_type entity) const noexcept {
@@ -664,13 +739,53 @@ public:
      *
      * @tparam Component Type of component to get.
      * @param entity A valid entity identifier.
-     * @return A reference to the instance of the component owned by the entity.
+     * @return A reference to the component owned by the entity.
      */
     template<typename Component>
     Component & get(entity_type entity) noexcept {
         return const_cast<Component &>(const_cast<const Registry *>(this)->get<Component>(entity));
     }
 
+    /**
+     * @brief Returns a reference to the given components for an entity.
+     *
+     * @warning
+     * Attempting to use an invalid entity or to get components from an entity
+     * that doesn't own them results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode in case of
+     * invalid entity or if the entity doesn't own instances of the given
+     * components.
+     *
+     * @tparam Component Type of components to get.
+     * @param entity A valid entity identifier.
+     * @return References to the components owned by the entity.
+     */
+    template<typename... Component>
+    std::enable_if_t<(sizeof...(Component) > 1), std::tuple<const Component &...>>
+    get(entity_type entity) const noexcept {
+        return std::tuple<const Component &...>{get<Component>(entity)...};
+    }
+
+    /**
+     * @brief Returns a reference to the given components for an entity.
+     *
+     * @warning
+     * Attempting to use an invalid entity or to get components from an entity
+     * that doesn't own them results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode in case of
+     * invalid entity or if the entity doesn't own instances of the given
+     * components.
+     *
+     * @tparam Component Type of components to get.
+     * @param entity A valid entity identifier.
+     * @return References to the components owned by the entity.
+     */
+    template<typename... Component>
+    std::enable_if_t<(sizeof...(Component) > 1), std::tuple<Component &...>>
+    get(entity_type entity) noexcept {
+        return std::tuple<Component &...>{get<Component>(entity)...};
+    }
+
     /**
      * @brief Replaces the given component for an entity.
      *
@@ -692,9 +807,8 @@ public:
      * @return A reference to the newly created component.
      */
     template<typename Component, typename... Args>
-    Component & replace(entity_type entity, Args&&... args) {
-        assert(valid(entity));
-        return (pool<Component>().get(entity) = Component{std::forward<Args>(args)...});
+    Component & replace(entity_type entity, Args &&... args) {
+        return (get<Component>(entity) = Component{std::forward<Args>(args)...});
     }
 
     /**
@@ -710,7 +824,7 @@ public:
      * }
      * @endcode
      *
-     * Prefer this function anyway because it has slighlty better
+     * Prefer this function anyway because it has slightly better
      * performance.
      *
      * @warning
@@ -725,9 +839,9 @@ public:
      * @return A reference to the newly created component.
      */
     template<typename Component, typename... Args>
-    Component & accomodate(entity_type entity, Args&&... args) {
+    Component & accommodate(entity_type entity, Args &&... args) {
         assert(valid(entity));
-        auto &cpool = ensure<Component>();
+        auto &cpool = assure<Component>();
 
         return (cpool.has(entity)
                 ? (cpool.get(entity) = Component{std::forward<Args>(args)...})
@@ -737,7 +851,7 @@ public:
     /**
      * @brief Sorts the pool of entities for the given component.
      *
-     * The order of the elements in a pool is highly affected by assignements
+     * The order of the elements in a pool is highly affected by assignments
      * of components to entities and deletions. Components are arranged to
      * maximize the performance during iterations and users should not make any
      * assumption on the order.<br/>
@@ -759,13 +873,13 @@ public:
      */
     template<typename Component, typename Compare>
     void sort(Compare compare) {
-        ensure<Component>().sort(std::move(compare));
+        assure<Component>().sort(std::move(compare));
     }
 
     /**
      * @brief Sorts two pools of components in the same way.
      *
-     * The order of the elements in a pool is highly affected by assignements
+     * The order of the elements in a pool is highly affected by assignments
      * of components to entities and deletions. Components are arranged to
      * maximize the performance during iterations and users should not make any
      * assumption on the order.
@@ -783,9 +897,9 @@ public:
      * the following rules:
      *
      * * All the entities in `A` that are also in `B` are returned first
-     * according to the order they have in `B`.
+     *   according to the order they have in `B`.
      * * All the entities in `A` that are not in `B` are returned in no
-     * particular order after all the other entities.
+     *   particular order after all the other entities.
      *
      * Any subsequent change to `B` won't affect the order in `A`.
      *
@@ -794,7 +908,7 @@ public:
      */
     template<typename To, typename From>
     void sort() {
-        ensure<To>().respect(ensure<From>());
+        assure<To>().respect(assure<From>());
     }
 
     /**
@@ -837,11 +951,11 @@ public:
         if(managed<Component>()) {
             auto &cpool = pool<Component>();
 
-            for(auto entity: entities) {
+            each([&cpool](auto entity) {
                 if(cpool.has(entity)) {
                     cpool.destroy(entity);
                 }
-            }
+            });
         }
     }
 
@@ -849,61 +963,106 @@ public:
      * @brief Resets a whole registry.
      *
      * Destroys all the entities. After a call to `reset`, all the entities
-     * previously created are recycled with a new version number. In case entity
+     * still in use are recycled with a new version number. In case entity
      * identifers are stored around, the `current` member function can be used
      * to know if they are still valid.
      */
     void reset() {
-        available.clear();
-
-        for(auto &&entity: entities) {
-            const auto version = version_type{1} + ((entity >> traits_type::entity_shift) & traits_type::version_mask);
-            entity = (entity & traits_type::entity_mask) | (version << traits_type::entity_shift);
-            available.push_back(entity);
-        }
-
-        for(auto &&handler: handlers) {
-            if(handler) {
-                handler->reset();
-            }
-        }
-
-        for(auto &&pool: pools) {
-            if(pool) {
-                pool->reset();
-            }
-        }
-
-        for(auto &&tag: tags) {
-            tag.reset();
-        }
+        each([this](auto entity) {
+            destroy(entity);
+        });
     }
 
     /**
-     * @brief Iterate entities and applies them the given function object.
+     * @brief Iterates all the entities that are still in use.
      *
-     * The function object is invoked for each entity, no matter if it's in use
-     * or not.<br/>
+     * The function object is invoked for each entity that is still in use.<br/>
      * The signature of the function should be equivalent to the following:
      *
      * @code{.cpp}
      * void(entity_type);
      * @endcode
      *
+     * This function is fairly slow and should not be used frequently.<br/>
      * Consider using a view if the goal is to iterate entities that have a
      * determinate set of components. A view is usually faster than combining
-     * this function with a bunch of custom tests.
+     * this function with a bunch of custom tests.<br/>
+     * On the other side, this function can be used to iterate all the entities
+     * that are in use, regardless of their components.
      *
      * @tparam Func Type of the function object to invoke.
      * @param func A valid function object.
      */
     template<typename Func>
     void each(Func func) const {
-        for(auto pos = entities.size(); pos > size_type{0}; --pos) {
-            func(entities[pos-1]);
+        if(available) {
+            for(auto pos = entities.size(); pos; --pos) {
+                const entity_type curr = pos - 1;
+                const auto entity = entities[curr];
+                const auto entt = entity & traits_type::entity_mask;
+
+                if(curr == entt) {
+                    func(entity);
+                }
+            }
+        } else {
+            for(auto pos = entities.size(); pos; --pos) {
+                func(entities[pos-1]);
+            }
         }
     }
 
+    /**
+     * @brief Checks if an entity is an orphan.
+     *
+     * An orphan is an entity that has neither assigned components nor
+     * tags.
+     *
+     * @param entity A valid entity identifier.
+     * @return True if the entity is an orphan, false otherwise.
+     */
+    bool orphan(entity_type entity) const {
+        assert(valid(entity));
+        bool orphan = true;
+
+        for(std::size_t i = 0; i < pools.size() && orphan; ++i) {
+            const auto &pool = pools[i];
+            orphan = !(pool && pool->has(entity));
+        }
+
+        for(std::size_t i = 0; i < tags.size() && orphan; ++i) {
+            const auto &tag = tags[i];
+            orphan = !(tag && (tag->entity == entity));
+        }
+
+        return orphan;
+    }
+
+    /**
+     * @brief Iterates orphans and applies them the given function object.
+     *
+     * The function object is invoked for each entity that is still in use and
+     * has neither assigned components nor tags.<br/>
+     * The signature of the function should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * void(entity_type);
+     * @endcode
+     *
+     * This function can be very slow and should not be used frequently.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void orphans(Func func) const {
+        each([func = std::move(func), this](auto entity) {
+            if(orphan(entity)) {
+                func(entity);
+            }
+        });
+    }
+
     /**
      * @brief Returns a standard view for the given components.
      *
@@ -915,13 +1074,13 @@ public:
      * As a rule of thumb, storing a view should never be an option.
      *
      * Standard views do their best to iterate the smallest set of candidate
-     * entites. In particular:
+     * entities. In particular:
      *
      * * Single component views are incredibly fast and iterate a packed array
-     * of entities, all of which has the given component.
+     *   of entities, all of which has the given component.
      * * Multi component views look at the number of entities available for each
-     * component and pick up a reference to the smallest set of candidates to
-     * test for the given components.
+     *   component and pick up a reference to the smallest set of candidates to
+     *   test for the given components.
      *
      * @note
      * Multi component views are pretty fast. However their performance tend to
@@ -932,13 +1091,14 @@ public:
      * @see View
      * @see View<Entity, Component>
      * @see PersistentView
+     * @see RawView
      *
      * @tparam Component Type of components used to construct the view.
      * @return A newly created standard view.
      */
     template<typename... Component>
     View<Entity, Component...> view() {
-        return View<Entity, Component...>{ensure<Component>()...};
+        return View<Entity, Component...>{assure<Component>()...};
     }
 
     /**
@@ -953,7 +1113,7 @@ public:
      * requested.<br/>
      * To avoid costly operations, internal data structures for persistent views
      * can be prepared with this function. Just use the same set of components
-     * that would have been used otherwise to contruct the view.
+     * that would have been used otherwise to construct the view.
      *
      * @tparam Component Types of components used to prepare the view.
      */
@@ -1012,16 +1172,16 @@ public:
      * initialization.<br/>
      * As a rule of thumb, storing a view should never be an option.
      *
-     * Persistent views are the right choice to iterate entites when the number
+     * Persistent views are the right choice to iterate entities when the number
      * of components grows up and the most of the entities have all the given
      * components.<br/>
      * However they have also drawbacks:
      *
      * * Each kind of persistent view requires a dedicated data structure that
-     * is allocated within the registry and it increases memory pressure.
+     *   is allocated within the registry and it increases memory pressure.
      * * Internal data structures used to construct persistent views must be
-     * kept updated and it affects slightly construction and destruction of
-     * entities and components.
+     *   kept updated and it affects slightly construction and destruction of
+     *   entities and components.
      *
      * That being said, persistent views are an incredibly powerful tool if used
      * with care and offer a boost of performance undoubtedly.
@@ -1035,6 +1195,7 @@ public:
      * @see View
      * @see View<Entity, Component>
      * @see PersistentView
+     * @see RawView
      *
      * @tparam Component Types of components used to construct the view.
      * @return A newly created persistent view.
@@ -1045,12 +1206,112 @@ public:
         return PersistentView<Entity, Component...>{handler<Component...>(), pool<Component>()...};
     }
 
+    /**
+     * @brief Returns a raw view for the given component.
+     *
+     * This kind of views are created on the fly and share with the registry its
+     * internal data structures.<br/>
+     * Feel free to discard a view after the use. Creating and destroying a view
+     * is an incredibly cheap operation because they do not require any type of
+     * initialization.<br/>
+     * As a rule of thumb, storing a view should never be an option.
+     *
+     * Raw views are incredibly fast and must be considered the best tool to
+     * iterate components whenever knowing the entities to which they belong
+     * isn't required.
+     *
+     * @see View
+     * @see View<Entity, Component>
+     * @see PersistentView
+     * @see RawView
+     *
+     * @tparam Component Type of component used to construct the view.
+     * @return A newly created raw view.
+     */
+    template<typename Component>
+    RawView<Entity, Component> raw() {
+        return RawView<Entity, Component>{assure<Component>()};
+    }
+
+    /**
+     * @brief Returns a temporary object to use to create snapshots.
+     *
+     * A snapshot is either a full or a partial dump of a registry.<br/>
+     * It can be used to save and restore its internal state or to keep two or
+     * more instances of this class in sync, as an example in a client-server
+     * architecture.
+     *
+     * @return A not movable and not copyable object to use to take snasphosts.
+     */
+    Snapshot<Entity> snapshot() const {
+        using follow_fn_type = entity_type(*)(const Registry &, entity_type);
+        using raw_fn_type = const entity_type *(*)(const Registry &, component_type);
+        const entity_type seed = available ? (next | (entities[next] & ~traits_type::entity_mask)) : next;
+
+        follow_fn_type follow = [](const Registry &registry, entity_type entity) -> entity_type {
+            const auto &entities = registry.entities;
+            const auto entt = entity & traits_type::entity_mask;
+            const auto next = entities[entt] & traits_type::entity_mask;
+            return (next | (entities[next] & ~traits_type::entity_mask));
+        };
+
+        raw_fn_type raw = [](const Registry &registry, component_type component) -> const entity_type * {
+            const auto &pools = registry.pools;
+            return (component < pools.size() && pools[component]) ? pools[component]->data() : nullptr;
+        };
+
+        return { *this, seed, available, follow, raw };
+    }
+
+    /**
+     * @brief Returns a temporary object to use to load snapshots.
+     *
+     * A snapshot is either a full or a partial dump of a registry.<br/>
+     * It can be used to save and restore its internal state or to keep two or
+     * more instances of this class in sync, as an example in a client-server
+     * architecture.
+     *
+     * @warning
+     * The loader returned by this function requires that the registry be empty.
+     * In case it isn't, all the data will be automatically deleted before to
+     * return.
+     *
+     * @return A not movable and not copyable object to use to load snasphosts.
+     */
+    SnapshotLoader<Entity> restore() {
+        using assure_fn_type = void(*)(Registry &, entity_type, bool);
+
+        assure_fn_type assure = [](Registry &registry, entity_type entity, bool destroyed) {
+            using promotion_type = std::conditional_t<sizeof(size_type) >= sizeof(entity_type), size_type, entity_type>;
+            // explicit promotion to avoid warnings with std::uint16_t
+            const auto entt = promotion_type{entity} & traits_type::entity_mask;
+            auto &entities = registry.entities;
+
+            if(!(entt < entities.size())) {
+                auto curr = entities.size();
+                entities.resize(entt + 1);
+                std::iota(entities.data() + curr, entities.data() + entt, entity_type(curr));
+            }
+
+            entities[entt] = entity;
+
+            if(destroyed) {
+                registry.destroy(entity);
+                const auto version = (entity & (~traits_type::entity_mask));
+                entities[entt] = ((entities[entt] & traits_type::entity_mask) | version);
+            }
+        };
+
+        return { (*this = {}), assure };
+    }
+
 private:
     std::vector<std::unique_ptr<SparseSet<Entity>>> handlers;
     std::vector<std::unique_ptr<SparseSet<Entity>>> pools;
     std::vector<std::unique_ptr<Attachee>> tags;
-    std::vector<entity_type> available;
     std::vector<entity_type> entities;
+    size_type available{};
+    entity_type next{};
 };
 
 

src/entt/entity/snapshot.hpp

@@ -0,0 +1,713 @@
+#ifndef ENTT_ENTITY_SNAPSHOT_HPP
+#define ENTT_ENTITY_SNAPSHOT_HPP
+
+
+#include <unordered_map>
+#include <algorithm>
+#include <cstddef>
+#include <utility>
+#include <cassert>
+#include <iterator>
+#include <type_traits>
+#include "entt_traits.hpp"
+
+
+namespace entt {
+
+
+/**
+ * @brief Forward declaration of the registry class.
+ */
+template<typename>
+class Registry;
+
+
+/**
+ * @brief Utility class to create snapshots from a registry.
+ *
+ * A _snapshot_ can be either a dump of the entire registry or a narrower
+ * selection of components and tags of interest.<br/>
+ * This type can be used in both cases if provided with a correctly configured
+ * output archive.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+class Snapshot final {
+    /*! @brief A registry is allowed to create snapshots. */
+    friend class Registry<Entity>;
+
+    using follow_fn_type = Entity(*)(const Registry<Entity> &, Entity);
+    using raw_fn_type = const Entity *(*)(const Registry<Entity> &, typename Registry<Entity>::component_type);
+
+    Snapshot(const Registry<Entity> &registry, Entity seed, std::size_t size, follow_fn_type follow, raw_fn_type raw) noexcept
+        : registry{registry},
+          seed{seed},
+          size{size},
+          follow{follow},
+          raw{raw}
+    {}
+
+    Snapshot(const Snapshot &) = default;
+    Snapshot(Snapshot &&) = default;
+
+    Snapshot & operator=(const Snapshot &) = default;
+    Snapshot & operator=(Snapshot &&) = default;
+
+    template<typename Component, typename Archive>
+    void get(Archive &archive, const Registry<Entity> &registry) {
+        const auto component = registry.template component<Component>();
+        const auto sz = registry.template size<Component>();
+        const auto *entities = raw(registry, component);
+
+        archive(static_cast<Entity>(sz));
+
+        for(std::remove_const_t<decltype(sz)> i{}; i < sz; ++i) {
+            const auto entity = entities[i];
+            archive(entity);
+            archive(registry.template get<Component>(entity));
+        };
+    }
+
+    template<typename Tag, typename Archive>
+    void get(Archive &archive) {
+        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>());
+            archive(registry.template get<Tag>());
+        }
+    }
+
+public:
+    /**
+     * @brief Puts aside all the entities that are still in use.
+     *
+     * Entities are serialized along with their versions. Destroyed entities are
+     * not taken in consideration by this function.
+     *
+     * @tparam Archive Type of output archive.
+     * @param archive A valid reference to an output archive.
+     * @return An object of this type to continue creating the snapshot.
+     */
+    template<typename Archive>
+    Snapshot entities(Archive &archive) && {
+        archive(static_cast<Entity>(registry.size()));
+        registry.each([&archive, this](auto entity) { archive(entity); });
+        return *this;
+    }
+
+    /**
+     * @brief Puts aside destroyed entities.
+     *
+     * Entities are serialized along with their versions. Entities that are
+     * still in use are not taken in consideration by this function.
+     *
+     * @tparam Archive Type of output archive.
+     * @param archive A valid reference to an output archive.
+     * @return An object of this type to continue creating the snapshot.
+     */
+    template<typename Archive>
+    Snapshot destroyed(Archive &archive) && {
+        archive(static_cast<Entity>(size));
+
+        if(size) {
+            auto curr = seed;
+            archive(curr);
+
+            for(auto i = size - 1; i; --i) {
+                curr = follow(registry, curr);
+                archive(curr);
+            }
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Puts aside the given components.
+     *
+     * Each component is serialized together with the entity to which it
+     * belongs. Entities are serialized along with their versions.
+     *
+     * @tparam Component Types of components to serialize.
+     * @tparam Archive Type of output archive.
+     * @param archive A valid reference to an output archive.
+     * @return An object of this type to continue creating the snapshot.
+     */
+    template<typename... Component, typename Archive>
+    Snapshot component(Archive &archive) && {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { 0, (get<Component>(archive, registry), 0)... };
+        (void)accumulator;
+        return *this;
+    }
+
+    /**
+     * @brief Puts aside the given tags.
+     *
+     * Each tag 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>
+    Snapshot tag(Archive &archive) && {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { 0, (get<Tag>(archive), 0)... };
+        (void)accumulator;
+        return *this;
+    }
+
+private:
+    const Registry<Entity> &registry;
+    const Entity seed;
+    const std::size_t size;
+    follow_fn_type follow;
+    raw_fn_type raw;
+};
+
+
+/**
+ * @brief Utility class to restore a snapshot as a whole.
+ *
+ * A snapshot loader requires that the destination registry be empty and loads
+ * all the data at once while keeping intact the identifiers that the entities
+ * originally had.<br/>
+ * An example of use is the implementation of a save/restore utility.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+class SnapshotLoader final {
+    /*! @brief A registry is allowed to create snapshot loaders. */
+    friend class Registry<Entity>;
+
+    using assure_fn_type = void(*)(Registry<Entity> &, Entity, bool);
+
+    SnapshotLoader(Registry<Entity> &registry, assure_fn_type assure_fn) noexcept
+        : registry{registry},
+          assure_fn{assure_fn}
+    {
+        // restore a snapshot as a whole requires a clean registry
+        assert(!registry.capacity());
+    }
+
+    SnapshotLoader(const SnapshotLoader &) = default;
+    SnapshotLoader(SnapshotLoader &&) = default;
+
+    SnapshotLoader & operator=(const SnapshotLoader &) = default;
+    SnapshotLoader & operator=(SnapshotLoader &&) = default;
+
+    template<typename Archive, typename Func>
+    void each(Archive &archive, Func func) {
+        Entity length{};
+        archive(length);
+
+        while(length) {
+            Entity entity{};
+            archive(entity);
+            func(entity);
+            --length;
+        }
+    }
+
+    template<typename Component, typename Archive>
+    void assign(Archive &archive) {
+        each(archive, [&archive, this](auto entity) {
+            static constexpr auto destroyed = false;
+            assure_fn(registry, entity, destroyed);
+            archive(registry.template assign<Component>(entity));
+        });
+    }
+
+    template<typename Tag, typename Archive>
+    void attach(Archive &archive) {
+        each(archive, [&archive, this](auto entity) {
+            static constexpr auto destroyed = false;
+            assure_fn(registry, entity, destroyed);
+            archive(registry.template attach<Tag>(entity));
+        });
+    }
+
+public:
+    /**
+     * @brief Restores entities that were in use during serialization.
+     *
+     * This function restores the entities that were in use during serialization
+     * and gives them the versions they originally had.
+     *
+     * @tparam Archive Type of input archive.
+     * @param archive A valid reference to an input archive.
+     * @return A valid loader to continue restoring data.
+     */
+    template<typename Archive>
+    SnapshotLoader entities(Archive &archive) && {
+        each(archive, [this](auto entity) {
+            static constexpr auto destroyed = false;
+            assure_fn(registry, entity, destroyed);
+        });
+
+        return *this;
+    }
+
+    /**
+     * @brief Restores entities that were destroyed during serialization.
+     *
+     * This function restores the entities that were destroyed during
+     * serialization and gives them the versions they originally had.
+     *
+     * @tparam Archive Type of input archive.
+     * @param archive A valid reference to an input archive.
+     * @return A valid loader to continue restoring data.
+     */
+    template<typename Archive>
+    SnapshotLoader destroyed(Archive &archive) && {
+        each(archive, [this](auto entity) {
+            static constexpr auto destroyed = true;
+            assure_fn(registry, entity, destroyed);
+        });
+
+        return *this;
+    }
+
+    /**
+     * @brief Restores components and assigns them to the right entities.
+     *
+     * The template parameter list must be exactly the same used during
+     * serialization. In the event that the entity to which the component is
+     * assigned doesn't exist yet, the loader will take care to create it with
+     * the version it originally had.
+     *
+     * @tparam Component Types of components to restore.
+     * @tparam Archive Type of input archive.
+     * @param archive A valid reference to an input archive.
+     * @return A valid loader to continue restoring data.
+     */
+    template<typename... Component, typename Archive>
+    SnapshotLoader component(Archive &archive) && {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { 0, (assign<Component>(archive), 0)... };
+        (void)accumulator;
+        return *this;
+    }
+
+    /**
+     * @brief Restores tags and assigns them to the right entities.
+     *
+     * The template parameter list must be exactly the same used during
+     * serialization. In the event that the entity to which the tag is assigned
+     * doesn't exist yet, the loader will take care to create it with the
+     * version it originally had.
+     *
+     * @tparam Tag Types of tags to restore.
+     * @tparam Archive Type of input archive.
+     * @param archive A valid reference to an input archive.
+     * @return A valid loader to continue restoring data.
+     */
+    template<typename... Tag, typename Archive>
+    SnapshotLoader tag(Archive &archive) && {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { 0, (attach<Tag>(archive), 0)... };
+        (void)accumulator;
+        return *this;
+    }
+
+
+    /**
+     * @brief Destroys those entities that have neither components nor tags.
+     *
+     * In case all the entities were serialized but only part of the components
+     * and tags was saved, it could happen that some of the entities have
+     * neither components nor tags once restored.<br/>
+     * This functions helps to identify and destroy those entities.
+     *
+     * @return A valid loader to continue restoring data.
+     */
+    SnapshotLoader orphans() && {
+        registry.orphans([this](auto entity) {
+            registry.destroy(entity);
+        });
+
+        return *this;
+    }
+
+private:
+    Registry<Entity> &registry;
+    assure_fn_type assure_fn;
+};
+
+
+/**
+ * @brief Utility class for _continuous loading_.
+ *
+ * A _continuous loader_ is designed to load data from a source registry to a
+ * (possibly) non-empty destination. The loader can accomodate in a registry
+ * more than one snapshot in a sort of _continuous loading_ that updates the
+ * destination one step at a time.<br/>
+ * Identifiers that entities originally had are not transferred to the target.
+ * Instead, the loader maps remote identifiers to local ones while restoring a
+ * snapshot.<br/>
+ * An example of use is the implementation of a client-server applications with
+ * the requirement of transferring somehow parts of the representation side to
+ * side.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+class ContinuousLoader final {
+    using traits_type = entt_traits<Entity>;
+
+    Entity destroy(Entity entity) {
+        const auto it = remloc.find(entity);
+
+        if(it == remloc.cend()) {
+            const auto local = registry.create();
+            remloc.emplace(entity, std::make_pair(local, true));
+            registry.destroy(local);
+        }
+
+        return remloc[entity].first;
+    }
+
+    Entity restore(Entity entity) {
+        const auto it = remloc.find(entity);
+
+        if(it == remloc.cend()) {
+            const auto local = registry.create();
+            remloc.emplace(entity, std::make_pair(local, true));
+        } else {
+            remloc[entity].first =
+                    registry.valid(remloc[entity].first)
+                    ? remloc[entity].first
+                    : registry.create();
+
+            // set the dirty flag
+            remloc[entity].second = true;
+        }
+
+        return remloc[entity].first;
+    }
+
+    template<typename Instance, typename Type>
+    std::enable_if_t<std::is_same<Type, Entity>::value>
+    update(Instance &instance, Type Instance::*member) {
+        instance.*member = map(instance.*member);
+    }
+
+    template<typename Instance, typename Type>
+    std::enable_if_t<std::is_same<typename std::iterator_traits<typename Type::iterator>::value_type, Entity>::value>
+    update(Instance &instance, Type Instance::*member) {
+        for(auto &entity: (instance.*member)) {
+            entity = map(entity);
+        }
+    }
+
+    template<typename Archive, typename Func>
+    void each(Archive &archive, Func func) {
+        Entity length{};
+        archive(length);
+
+        while(length) {
+            Entity entity{};
+            archive(entity);
+            func(entity);
+            --length;
+        }
+    }
+
+    template<typename Component>
+    void reset() {
+        for(auto &&ref: remloc) {
+            const auto local = ref.second.first;
+
+            if(registry.valid(local)) {
+                registry.template reset<Component>(local);
+            }
+        }
+    }
+
+    template<typename Component, typename Archive>
+    void assign(Archive &archive) {
+        reset<Component>();
+
+        each(archive, [&archive, this](auto entity) {
+            entity = restore(entity);
+            archive(registry.template accommodate<Component>(entity));
+        });
+    }
+
+    template<typename Component, typename Archive, typename... Type>
+    void assign(Archive &archive, Type Component::*... member) {
+        reset<Component>();
+
+        each(archive, [&archive, member..., this](auto entity) {
+            entity = restore(entity);
+            auto &component = registry.template accommodate<Component>(entity);
+            archive(component);
+
+            using accumulator_type = int[];
+            accumulator_type accumulator = { 0, (update(component, member), 0)... };
+            (void)accumulator;
+        });
+    }
+
+    template<typename Tag, typename Archive>
+    void attach(Archive &archive) {
+        registry.template remove<Tag>();
+
+        each(archive, [&archive, this](auto entity) {
+            entity = restore(entity);
+            archive(registry.template attach<Tag>(entity));
+        });
+    }
+
+    template<typename Tag, typename Archive, typename... Type>
+    void attach(Archive &archive, Type Tag::*... member) {
+        registry.template remove<Tag>();
+
+        each(archive, [&archive, member..., this](auto entity) {
+            entity = restore(entity);
+            auto &tag = registry.template attach<Tag>(entity);
+            archive(tag);
+
+            using accumulator_type = int[];
+            accumulator_type accumulator = { 0, (update(tag, member), 0)... };
+            (void)accumulator;
+        });
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+
+    /**
+     * @brief Constructs a loader that is bound to a given registry.
+     * @param registry A valid reference to a registry.
+     */
+    ContinuousLoader(Registry<entity_type> &registry) noexcept
+        : registry{registry}
+    {}
+
+    /*! @brief Default copy constructor. */
+    ContinuousLoader(const ContinuousLoader &) = default;
+    /*! @brief Default move constructor. */
+    ContinuousLoader(ContinuousLoader &&) = default;
+
+    /*! @brief Default copy assignment operator. @return This loader. */
+    ContinuousLoader & operator=(const ContinuousLoader &) = default;
+    /*! @brief Default move assignment operator. @return This loader. */
+    ContinuousLoader & operator=(ContinuousLoader &&) = default;
+
+    /**
+     * @brief Restores entities that were in use during serialization.
+     *
+     * This function restores the entities that were in use during serialization
+     * and creates local counterparts for them if required.
+     *
+     * @tparam Archive Type of input archive.
+     * @param archive A valid reference to an input archive.
+     * @return A non-const reference to this loader.
+     */
+    template<typename Archive>
+    ContinuousLoader & entities(Archive &archive) {
+        each(archive, [this](auto entity) { restore(entity); });
+        return *this;
+    }
+
+    /**
+     * @brief Restores entities that were destroyed during serialization.
+     *
+     * This function restores the entities that were destroyed during
+     * serialization and creates local counterparts for them if required.
+     *
+     * @tparam Archive Type of input archive.
+     * @param archive A valid reference to an input archive.
+     * @return A non-const reference to this loader.
+     */
+    template<typename Archive>
+    ContinuousLoader & destroyed(Archive &archive) {
+        each(archive, [this](auto entity) { destroy(entity); });
+        return *this;
+    }
+
+    /**
+     * @brief Restores components and assigns them to the right entities.
+     *
+     * The template parameter list must be exactly the same used during
+     * serialization. In the event that the entity to which the component is
+     * assigned doesn't exist yet, the loader will take care to create a local
+     * counterpart for it.
+     *
+     * @tparam Component Types of components to restore.
+     * @tparam Archive Type of input archive.
+     * @param archive A valid reference to an input archive.
+     * @return A non-const reference to this loader.
+     */
+    template<typename... Component, typename Archive>
+    ContinuousLoader & component(Archive &archive) {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { 0, (assign<Component>(archive), 0)... };
+        (void)accumulator;
+        return *this;
+    }
+
+    /**
+     * @brief Restores components and assigns them to the right entities.
+     *
+     * The template parameter list must be exactly the same used during
+     * serialization. In the event that the entity to which the component is
+     * assigned doesn't exist yet, the loader will take care to create a local
+     * counterpart for it.<br/>
+     * Members can be either data members of type entity_type or containers of
+     * entities. In both cases, the loader will visit them and update the
+     * entities by replacing each one with its local counterpart.
+     *
+     * @tparam Component Type of component to restore.
+     * @tparam Archive Type of input archive.
+     * @tparam Type Types of members to update with their local counterparts.
+     * @param archive A valid reference to an input archive.
+     * @param member Members to update with their local counterparts.
+     * @return A non-const reference to this loader.
+     */
+    template<typename Component, typename Archive, typename... Type>
+    ContinuousLoader & component(Archive &archive, Type Component::*... member) {
+        assign(archive, member...);
+        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.
+     *
+     * @tparam Tag Types of tags to restore.
+     * @tparam Archive Type of input archive.
+     * @param archive A valid reference to an input archive.
+     * @return A non-const reference to this loader.
+     */
+    template<typename... Tag, typename Archive>
+    ContinuousLoader & tag(Archive &archive) {
+        using accumulator_type = int[];
+        accumulator_type accumulator = { 0, (attach<Tag>(archive), 0)... };
+        (void)accumulator;
+        return *this;
+    }
+
+    /**
+     * @brief Restores tags and assigns them to the right entities.
+     *
+     * The template parameter list must be exactly the same used during
+     * serialization. In the event that the entity to which the tag is assigned
+     * doesn't exist yet, the loader will take care to create 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 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>
+    ContinuousLoader & tag(Archive &archive, Type Tag::*... member) {
+        attach<Tag>(archive, member...);
+        return *this;
+    }
+
+    /**
+     * @brief Helps to purge entities that no longer have a conterpart.
+     *
+     * Users should invoke this member function after restoring each snapshot,
+     * unless they know exactly what they are doing.
+     *
+     * @return A non-const reference to this loader.
+     */
+    ContinuousLoader & shrink() {
+        auto it = remloc.begin();
+
+        while(it != remloc.cend()) {
+            const auto local = it->second.first;
+            bool &dirty = it->second.second;
+
+            if(dirty) {
+                dirty = false;
+                ++it;
+            } else {
+                if(registry.valid(local)) {
+                    registry.destroy(local);
+                }
+
+                it = remloc.erase(it);
+            }
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Destroys those entities that have neither components nor tags.
+     *
+     * In case all the entities were serialized but only part of the components
+     * and tags was saved, it could happen that some of the entities have
+     * neither components nor tags once restored.<br/>
+     * This functions helps to identify and destroy those entities.
+     *
+     * @return A non-const reference to this loader.
+     */
+    ContinuousLoader & orphans() {
+        registry.orphans([this](auto entity) {
+            registry.destroy(entity);
+        });
+
+        return *this;
+    }
+
+    /**
+     * @brief Tests if a loader knows about a given entity.
+     * @param entity An entity identifier.
+     * @return True if `entity` is managed by the loader, false otherwise.
+     */
+    bool has(entity_type entity) {
+        return !(remloc.find(entity) == remloc.cend());
+    }
+
+    /**
+     * @brief Returns the identifier to which an entity refers.
+     *
+     * @warning
+     * Attempting to use an entity that isn't managed by the loader results in
+     * undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * loader doesn't knows about the entity.
+     *
+     * @param entity An entity identifier.
+     * @return The identifier to which `entity` refers in the target registry.
+     */
+    entity_type map(entity_type entity) {
+        assert(has(entity));
+        return remloc[entity].first;
+    }
+
+private:
+    std::unordered_map<Entity, std::pair<Entity, bool>> remloc;
+    Registry<Entity> &registry;
+};
+
+
+}
+
+
+#endif // ENTT_ENTITY_SNAPSHOT_HPP

src/entt/entity/sparse_set.hpp

@@ -3,13 +3,14 @@
 
 
 #include <algorithm>
+#include <iterator>
 #include <numeric>
 #include <utility>
 #include <vector>
 #include <cstddef>
 #include <cassert>
 #include <type_traits>
-#include "traits.hpp"
+#include "entt_traits.hpp"
 
 
 namespace entt {
@@ -56,10 +57,14 @@ template<typename Entity>
 class SparseSet<Entity> {
     using traits_type = entt_traits<Entity>;
 
-    struct Iterator {
+    struct Iterator final {
+        using difference_type = std::size_t;
         using value_type = Entity;
+        using pointer = value_type *;
+        using reference = value_type;
+        using iterator_category = std::input_iterator_tag;
 
-        Iterator(const std::vector<Entity> *direct, std::size_t pos)
+        Iterator(const std::vector<value_type> &direct, std::size_t pos)
             : direct{direct}, pos{pos}
         {}
 
@@ -72,20 +77,29 @@ class SparseSet<Entity> {
             return ++(*this), orig;
         }
 
+        Iterator & operator+=(difference_type value) noexcept {
+            pos -= value;
+            return *this;
+        }
+
+        Iterator operator+(difference_type value) noexcept {
+            return Iterator{direct, pos-value};
+        }
+
         bool operator==(const Iterator &other) const noexcept {
-            return other.pos == pos && other.direct == direct;
+            return other.pos == pos;
         }
 
         bool operator!=(const Iterator &other) const noexcept {
             return !(*this == other);
         }
 
-        value_type operator*() const noexcept {
-            return (*direct)[pos-1];
+        reference operator*() const noexcept {
+            return direct[pos-1];
         }
 
     private:
-        const std::vector<Entity> *direct;
+        const std::vector<value_type> &direct;
         std::size_t pos;
     };
 
@@ -126,10 +140,23 @@ public:
      * @param cap Desired capacity.
      */
     void reserve(size_type cap) {
-        reverse.reserve(cap);
         direct.reserve(cap);
     }
 
+    /**
+     * @brief Returns the extent of a sparse set.
+     *
+     * The extent of a sparse set is also the size of the internal sparse array.
+     * There is no guarantee that the internal packed array has the same size.
+     * Usually the size of the internal sparse array is equal or greater than
+     * the one of the internal packed array.
+     *
+     * @return Extent of the sparse set.
+     */
+    size_type extent() const noexcept {
+        return reverse.size();
+    }
+
     /**
      * @brief Returns the number of elements in a sparse set.
      *
@@ -159,7 +186,7 @@ public:
      * always a valid range, even if the container is empty.
      *
      * @note
-     * There are no guarantees on the order, even though `sort` has been
+     * There are no guarantees on the order, even though `respect` has been
      * previously invoked. Internal data structures arrange elements to maximize
      * performance. Accessing them directly gives a performance boost but less
      * guarantees. Use `begin` and `end` if you want to iterate the sparse set
@@ -174,34 +201,34 @@ public:
     /**
      * @brief Returns an iterator to the beginning.
      *
-     * The returned iterator points to the first element of the internal packed
+     * The returned iterator points to the first entity of the internal packed
      * array. If the sparse set is empty, the returned iterator will be equal to
      * `end()`.
      *
      * @note
-     * Input iterators stay true to the order imposed by a call to `sort`.
+     * Input iterators stay true to the order imposed by a call to `respect`.
      *
-     * @return An iterator to the first element of the internal packed array.
+     * @return An iterator to the first entity of the internal packed array.
      */
     iterator_type begin() const noexcept {
-        return Iterator{&direct, direct.size()};
+        return Iterator{direct, direct.size()};
     }
 
     /**
      * @brief Returns an iterator to the end.
      *
-     * The returned iterator points to the element following the last element in
+     * The returned iterator points to the element following the last entity in
      * the internal packed array. Attempting to dereference the returned
      * iterator results in undefined behavior.
      *
      * @note
-     * Input iterators stay true to the order imposed by a call to `sort`.
+     * Input iterators stay true to the order imposed by a call to `respect`.
      *
-     * @return An iterator to the element following the last element of the
+     * @return An iterator to the element following the last entity of the
      * internal packed array.
      */
     iterator_type end() const noexcept {
-        return Iterator{&direct, 0};
+        return Iterator{direct, 0};
     }
 
     /**
@@ -210,11 +237,33 @@ public:
      * @return True if the sparse set contains the entity, false otherwise.
      */
     bool has(entity_type entity) const noexcept {
-        using promotion_type = std::conditional_t<sizeof(size_type) >= sizeof(entity_type), size_type, entity_type>;
-        // explicit promotion to avoid warnings with std::uint16_t
-        const auto entt = promotion_type{entity} & traits_type::entity_mask;
+        const auto pos = size_type(entity & traits_type::entity_mask);
         // the in-use control bit permits to avoid accessing the direct vector
-        return (entt < reverse.size()) && (reverse[entt] & in_use);
+        return (pos < reverse.size()) && (reverse[pos] & in_use);
+    }
+
+    /**
+     * @brief Checks if a sparse set contains an entity (unsafe).
+     *
+     * Alternative version of `has`. It accesses the underlying data structures
+     * without bounds checking and thus it's both unsafe and risky to use.<br/>
+     * You should not invoke directly this function unless you know exactly what
+     * you are doing. Prefer the `has` member function instead.
+     *
+     * @warning
+     * Attempting to use an entity that doesn't belong to the sparse set can
+     * result in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode in case of
+     * bounds violation.
+     *
+     * @param entity A valid entity identifier.
+     * @return True if the sparse set contains the entity, false otherwise.
+     */
+    bool fast(entity_type entity) const noexcept {
+        const auto pos = size_type(entity & traits_type::entity_mask);
+        assert(pos < reverse.size());
+        // the in-use control bit permits to avoid accessing the direct vector
+        return (reverse[pos] & in_use);
     }
 
     /**
@@ -233,7 +282,7 @@ public:
         assert(has(entity));
         const auto entt = entity & traits_type::entity_mask;
         // we must get rid of the in-use bit for it's not part of the position
-        return reverse[entt] & ~in_use;
+        return reverse[entt] & traits_type::entity_mask;
     }
 
     /**
@@ -249,17 +298,15 @@ public:
      */
     void construct(entity_type entity) {
         assert(!has(entity));
-        using promotion_type = std::conditional_t<sizeof(size_type) >= sizeof(entity_type), size_type, entity_type>;
-        // explicit promotion to avoid warnings with std::uint16_t
-        const auto entt = promotion_type{entity} & traits_type::entity_mask;
+        const auto pos = size_type(entity & traits_type::entity_mask);
 
-        if(!(entt < reverse.size())) {
-            reverse.resize(entt+1, pos_type{});
+        if(!(pos < reverse.size())) {
+            reverse.resize(pos+1, pos_type{});
         }
 
         // we exploit the fact that pos_type is equal to entity_type and pos has
         // traits_type::version_mask bits unused we can use to mark it as in-use
-        reverse[entt] = pos_type(direct.size()) | in_use;
+        reverse[pos] = pos_type(direct.size()) | in_use;
         direct.emplace_back(entity);
     }
 
@@ -278,10 +325,9 @@ public:
         assert(has(entity));
         const auto entt = entity & traits_type::entity_mask;
         const auto back = direct.back() & traits_type::entity_mask;
-        const auto pos = reverse[entt] & ~in_use;
-        // the order matters: if back and entt are the same (for the sparse set
-        // has size 1), switching the two lines below doesn't work as expected
-        reverse[back] = pos | in_use;
+        // we must get rid of the in-use bit for it's not part of the position
+        const auto pos = reverse[entt] & traits_type::entity_mask;
+        reverse[back] = reverse[entt];
         reverse[entt] = pos;
         // swapping isn't required here, we are getting rid of the last element
         direct[pos] = direct.back();
@@ -322,22 +368,22 @@ public:
      * sets by using one of them as a master and the other one as a slave.
      *
      * Iterating the sparse set with a couple of iterators returns elements in
-     * the expected order after a call to `sort`. See `begin` and `end` for more
-     * details.
+     * the expected order after a call to `respect`. See `begin` and `end` for
+     * more details.
      *
      * @note
      * Attempting to iterate elements using the raw pointer returned by `data`
-     * gives no guarantees on the order, even though `sort` has been invoked.
+     * gives no guarantees on the order, even though `respect` has been invoked.
      *
      * @param other The sparse sets that imposes the order of the entities.
      */
-    virtual void respect(const SparseSet<Entity> &other) noexcept {
+    void respect(const SparseSet<Entity> &other) noexcept {
         auto from = other.begin();
         auto to = other.end();
 
         pos_type pos = direct.size() - 1;
 
-        while(pos > 0 && from != to) {
+        while(pos && from != to) {
             if(has(*from)) {
                 if(*from != direct[pos]) {
                     swap(pos, get(*from));
@@ -390,6 +436,56 @@ template<typename Entity, typename Type>
 class SparseSet<Entity, Type>: public SparseSet<Entity> {
     using underlying_type = SparseSet<Entity>;
 
+    struct Iterator final {
+        using difference_type = std::size_t;
+        using value_type = Type;
+        using pointer = value_type *;
+        using reference = value_type &;
+        using iterator_category = std::input_iterator_tag;
+
+        Iterator(std::vector<value_type> &instances, std::size_t pos)
+            : instances{instances}, pos{pos}
+        {}
+
+        Iterator & operator++() noexcept {
+            return --pos, *this;
+        }
+
+        Iterator operator++(int) noexcept {
+            Iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        Iterator & operator+=(difference_type value) noexcept {
+            pos -= value;
+            return *this;
+        }
+
+        Iterator operator+(difference_type value) noexcept {
+            return Iterator{instances, pos-value};
+        }
+
+        bool operator==(const Iterator &other) const noexcept {
+            return other.pos == pos;
+        }
+
+        bool operator!=(const Iterator &other) const noexcept {
+            return !(*this == other);
+        }
+
+        reference operator*() noexcept {
+            return instances[pos-1];
+        }
+
+        pointer operator->() noexcept {
+            return &instances.data()[pos-1];
+        }
+
+    private:
+        std::vector<value_type> &instances;
+        std::size_t pos;
+    };
+
 public:
     /*! @brief Type of the objects associated to the entities. */
     using object_type = Type;
@@ -400,7 +496,7 @@ public:
     /*! @brief Unsigned integer type. */
     using size_type = typename underlying_type::size_type;
     /*! @brief Input iterator type. */
-    using iterator_type = typename underlying_type::iterator_type;
+    using iterator_type = Iterator;
 
     /*! @brief Default constructor. */
     SparseSet() noexcept = default;
@@ -435,11 +531,11 @@ public:
      * always a valid range, even if the container is empty.
      *
      * @note
-     * There are no guarantees on the order, even though `sort` has been
-     * previously invoked. Internal data structures arrange elements to maximize
-     * performance. Accessing them directly gives a performance boost but less
-     * guarantees. Use `begin` and `end` if you want to iterate the sparse set
-     * in the expected order.
+     * There are no guarantees on the order, even though either `sort` or
+     * `respect` has been previously invoked. Internal data structures arrange
+     * elements to maximize performance. Accessing them directly gives a
+     * performance boost but less guarantees. Use `begin` and `end` if you want
+     * to iterate the sparse set in the expected order.
      *
      * @return A pointer to the array of objects.
      */
@@ -454,11 +550,11 @@ public:
      * always a valid range, even if the container is empty.
      *
      * @note
-     * There are no guarantees on the order, even though `sort` has been
-     * previously invoked. Internal data structures arrange elements to maximize
-     * performance. Accessing them directly gives a performance boost but less
-     * guarantees. Use `begin` and `end` if you want to iterate the sparse set
-     * in the expected order.
+     * There are no guarantees on the order, even though either `sort` or
+     * `respect` has been previously invoked. Internal data structures arrange
+     * elements to maximize performance. Accessing them directly gives a
+     * performance boost but less guarantees. Use `begin` and `end` if you want
+     * to iterate the sparse set in the expected order.
      *
      * @return A pointer to the array of objects.
      */
@@ -466,6 +562,40 @@ public:
         return instances.data();
     }
 
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * The returned iterator points to the first instance of the given type. If
+     * the sparse set is empty, the returned iterator will be equal to `end()`.
+     *
+     * @note
+     * Input iterators stay true to the order imposed by a call to either `sort`
+     * or `respect`.
+     *
+     * @return An iterator to the first instance of the given type.
+     */
+    iterator_type begin() noexcept {
+        return Iterator{instances, instances.size()};
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     *
+     * The returned iterator points to the element following the last instance
+     * of the given type. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @note
+     * Input iterators stay true to the order imposed by a call to either `sort`
+     * or `respect`.
+     *
+     * @return An iterator to the element following the last instance of the
+     * given type.
+     */
+    iterator_type end() noexcept {
+        return Iterator{instances, 0};
+    }
+
     /**
      * @brief Returns the object associated to an entity.
      *
@@ -501,6 +631,12 @@ public:
     /**
      * @brief Assigns an entity to a sparse set and constructs its object.
      *
+     * @note
+     * _Sfinae'd_ function.<br/>
+     * This version is used for types that can be constructed in place directly.
+     * It doesn't work well with aggregates because of the placement new usually
+     * performed under the hood during an _emplace back_.
+     *
      * @warning
      * Attempting to use an entity that already belongs to the sparse set
      * results in undefined behavior.<br/>
@@ -513,10 +649,38 @@ public:
      * @return The object associated to the entity.
      */
     template<typename... Args>
-    object_type & construct(entity_type entity, Args&&... args) {
+    std::enable_if_t<std::is_constructible<Type, Args...>::value, object_type &>
+    construct(entity_type entity, Args &&... args) {
         underlying_type::construct(entity);
-        // emplace_back doesn't work well with PODs because of its placement new
-        instances.push_back({ std::forward<Args>(args)... });
+        instances.emplace_back(std::forward<Args>(args)...);
+        return instances.back();
+    }
+
+    /**
+     * @brief Assigns an entity to a sparse set and constructs its object.
+     *
+     * @note
+     * _Sfinae'd_ function.<br/>
+     * Fallback for aggregates and types in general that do not work well with a
+     * placement new as performed usually under the hood during an
+     * _emplace back_.
+     *
+     * @warning
+     * Attempting to use an entity that already belongs to the sparse set
+     * results in undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * sparse set already contains the given entity.
+     *
+     * @tparam Args Types of arguments to use to construct the object.
+     * @param entity A valid entity identifier.
+     * @param args Parameters to use to construct an object for the entity.
+     * @return The object associated to the entity.
+     */
+    template<typename... Args>
+    std::enable_if_t<!std::is_constructible<Type, Args...>::value, object_type &>
+    construct(entity_type entity, Args &&... args) {
+        underlying_type::construct(entity);
+        instances.emplace_back(Type{std::forward<Args>(args)...});
         return instances.back();
     }
 
@@ -533,7 +697,9 @@ public:
      */
     void destroy(entity_type entity) override {
         // swapping isn't required here, we are getting rid of the last element
-        instances[underlying_type::get(entity)] = std::move(instances.back());
+        // however, we must protect ourselves from self assignments (see #37)
+        auto tmp = std::move(instances.back());
+        instances[underlying_type::get(entity)] = std::move(tmp);
         instances.pop_back();
         underlying_type::destroy(entity);
     }
@@ -554,8 +720,9 @@ public:
      * @endcode
      *
      * @note
-     * Attempting to iterate elements using the raw pointer returned by `data`
-     * gives no guarantees on the order, even though `sort` has been invoked.
+     * Attempting to iterate elements using a raw pointer returned by a call to
+     * either `data` or `raw` gives no guarantees on the order, even though
+     * `sort` has been invoked.
      *
      * @tparam Compare Type of comparison function object.
      * @param compare A valid comparison function object.
@@ -574,8 +741,8 @@ public:
             auto next = copy[curr];
 
             while(curr != next) {
-                auto lhs = copy[curr];
-                auto rhs = copy[next];
+                const auto lhs = copy[curr];
+                const auto rhs = copy[next];
                 std::swap(instances[lhs], instances[rhs]);
                 underlying_type::swap(lhs, rhs);
                 copy[curr] = curr;
@@ -598,26 +765,29 @@ public:
      * sets by using one of them as a master and the other one as a slave.
      *
      * Iterating the sparse set with a couple of iterators returns elements in
-     * the expected order after a call to `sort`. See `begin` and `end` for more
-     * details.
+     * the expected order after a call to `respect`. See `begin` and `end` for
+     * more details.
      *
      * @note
-     * Attempting to iterate elements using the raw pointer returned by `data`
-     * gives no guarantees on the order, even though `sort` has been invoked.
+     * Attempting to iterate elements using a raw pointer returned by a call to
+     * either `data` or `raw` gives no guarantees on the order, even though
+     * `respect` has been invoked.
      *
      * @param other The sparse sets that imposes the order of the entities.
      */
-    void respect(const SparseSet<Entity> &other) noexcept override {
+    void respect(const SparseSet<Entity> &other) noexcept {
         auto from = other.begin();
         auto to = other.end();
 
         pos_type pos = underlying_type::size() - 1;
-        const auto *direct = underlying_type::data();
+        const auto *local = underlying_type::data();
 
-        while(pos > 0 && from != to) {
-            if(underlying_type::has(*from)) {
-                if(*from != *(direct + pos)) {
-                    auto candidate = underlying_type::get(*from);
+        while(pos && from != to) {
+            const auto curr = *from;
+
+            if(underlying_type::has(curr)) {
+                if(curr != *(local + pos)) {
+                    auto candidate = underlying_type::get(curr);
                     std::swap(instances[pos], instances[candidate]);
                     underlying_type::swap(pos, candidate);
                 }

src/entt/entity/view.hpp

@@ -2,21 +2,32 @@
 #define ENTT_ENTITY_VIEW_HPP
 
 
+#include <cassert>
+#include <array>
 #include <tuple>
 #include <utility>
 #include <algorithm>
+#include <type_traits>
+#include "entt_traits.hpp"
 #include "sparse_set.hpp"
 
 
 namespace entt {
 
 
+/**
+ * @brief Forward declaration of the registry class.
+ */
+template<typename>
+class Registry;
+
+
 /**
  * @brief Persistent view.
  *
  * A persistent view returns all the entities and only the entities that have
  * at least the given components. Moreover, it's guaranteed that the entity list
- * is thightly packed in memory for fast iterations.<br/>
+ * is tightly packed in memory for fast iterations.<br/>
  * In general, persistent views don't stay true to the order of any set of
  * components unless users explicitly sort them.
  *
@@ -26,10 +37,11 @@ namespace entt {
  *
  * * 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).
+ *   given components is removed from the entity to which the iterator points).
  *
- * In all the other cases, modify the pools of the given components somehow
- * invalidates all the iterators and using them results in undefined behavior.
+ * 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 with the Registry
@@ -46,6 +58,7 @@ namespace entt {
  *
  * @sa View
  * @sa View<Entity, Component>
+ * @sa RawView
  *
  * @tparam Entity A valid entity type (see entt_traits for more details).
  * @tparam Component Types of components iterated by the view.
@@ -54,35 +67,27 @@ template<typename Entity, typename... Component>
 class PersistentView final {
     static_assert(sizeof...(Component) > 1, "!");
 
+    /*! @brief A registry is allowed to create views. */
+    friend class Registry<Entity>;
+
     template<typename Comp>
     using pool_type = SparseSet<Entity, Comp>;
 
     using view_type = SparseSet<Entity>;
+    using pattern_type = std::tuple<pool_type<Component> &...>;
+
+    PersistentView(view_type &view, pool_type<Component> &... pools) noexcept
+        : view{view}, pools{pools...}
+    {}
 
 public:
-    /*! Input iterator type. */
+    /*! @brief Input iterator type. */
     using iterator_type = typename view_type::iterator_type;
     /*! @brief Underlying entity identifier. */
     using entity_type = typename view_type::entity_type;
     /*! @brief Unsigned integer type. */
     using size_type = typename view_type::size_type;
 
-    /**
-     * @brief Constructs a persistent view around a dedicated pool of entities.
-     *
-     * A persistent view is created out of:
-     *
-     * * A dedicated pool of entities that is shared between all the persistent
-     * views of the same type.
-     * * A bunch of pools of components to which to refer to get instances.
-     *
-     * @param view Shared reference to a dedicated pool of entities.
-     * @param pools References to pools of components.
-     */
-    PersistentView(view_type &view, pool_type<Component>&... pools) noexcept
-        : view{view}, pools{pools...}
-    {}
-
     /**
      * @brief Returns the number of entities that have the given components.
      * @return Number of entities that have the given components.
@@ -144,6 +149,15 @@ public:
         return view.end();
     }
 
+    /**
+     * @brief Checks if a view contains an entity.
+     * @param entity A valid entity identifier.
+     * @return True if the view contains the given entity, false otherwise.
+     */
+    bool contains(entity_type entity) const noexcept {
+        return view.has(entity) && (view.data()[view.get(entity)] == entity);
+    }
+
     /**
      * @brief Returns the component assigned to the given entity.
      *
@@ -154,32 +168,33 @@ public:
      * Attempting to use an invalid component type results in a compilation
      * error. Attempting to use an entity that doesn't belong to the view
      * results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if
-     * the view doesn't contain the given entity.
+     * An assertion will abort the execution at runtime in debug mode if the
+     * view doesn't contain the given entity.
      *
-     * @tparam Comp Type of the component to get.
+     * @tparam Comp Type of component to get.
      * @param entity A valid entity identifier.
      * @return The component assigned to the entity.
      */
     template<typename Comp>
     const Comp & get(entity_type entity) const noexcept {
+        assert(contains(entity));
         return std::get<pool_type<Comp> &>(pools).get(entity);
     }
 
     /**
      * @brief Returns the component assigned to the given entity.
      *
-     * Prefer this function instead of `Registry::get` during iterations.
-     * It has far better performance than its companion function.
+     * Prefer this function instead of `Registry::get` during iterations. It has
+     * far better performance than its companion function.
      *
      * @warning
      * Attempting to use an invalid component type results in a compilation
      * error. Attempting to use an entity that doesn't belong to the view
      * results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if
-     * the view doesn't contain the given entity.
+     * An assertion will abort the execution at runtime in debug mode if the
+     * view doesn't contain the given entity.
      *
-     * @tparam Comp Type of the component to get.
+     * @tparam Comp Type of component to get.
      * @param entity A valid entity identifier.
      * @return The component assigned to the entity.
      */
@@ -189,7 +204,56 @@ public:
     }
 
     /**
-     * @brief Iterate the entities and applies them the given function object.
+     * @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 invalid component types results in a compilation error.
+     * Attempting to use an entity that doesn't belong to the view results in
+     * undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * view doesn't contain the given entity.
+     *
+     * @tparam Comp Types of the components to get.
+     * @param entity A valid entity identifier.
+     * @return The components assigned to the entity.
+     */
+    template<typename... Comp>
+    std::enable_if_t<(sizeof...(Comp) > 1), std::tuple<const Comp &...>>
+    get(entity_type entity) const noexcept {
+        assert(contains(entity));
+        return std::tuple<const Comp &...>{get<Comp>(entity)...};
+    }
+
+    /**
+     * @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 invalid component types results in a compilation error.
+     * Attempting to use an entity that doesn't belong to the view results in
+     * undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * view doesn't contain the given entity.
+     *
+     * @tparam Comp Types of the components to get.
+     * @param entity A valid entity identifier.
+     * @return The components assigned to the entity.
+     */
+    template<typename... Comp>
+    std::enable_if_t<(sizeof...(Comp) > 1), std::tuple<Comp &...>>
+    get(entity_type entity) noexcept {
+        assert(contains(entity));
+        return std::tuple<Comp &...>{get<Comp>(entity)...};
+    }
+
+    /**
+     * @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 const references to all the components of the
@@ -205,13 +269,14 @@ public:
      */
     template<typename Func>
     void each(Func func) const {
-        for(auto entity: *this) {
+        for(auto entity: view) {
             func(entity, get<Component>(entity)...);
         }
     }
 
     /**
-     * @brief Iterate the entities and applies them the given function object.
+     * @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 the components of the
@@ -246,7 +311,7 @@ public:
      * can quickly ruin the order imposed to the pool of entities shared between
      * the persistent views.
      *
-     * @tparam Comp Type of the component to use to impose the order.
+     * @tparam Comp Type of component to use to impose the order.
      */
     template<typename Comp>
     void sort() {
@@ -255,7 +320,7 @@ public:
 
 private:
     view_type &view;
-    std::tuple<pool_type<Component> &...> pools;
+    const pattern_type pools;
 };
 
 
@@ -277,10 +342,11 @@ private:
  *
  * * 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).
+ *   given components is removed from the entity to which the iterator points).
  *
- * In all the other cases, modify the pools of the given components somehow
- * invalidates all the iterators and using them results in undefined behavior.
+ * 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 with the Registry
@@ -293,6 +359,7 @@ private:
  *
  * @sa View<Entity, Component>
  * @sa PersistentView
+ * @sa RawView
  *
  * @tparam Entity A valid entity type (see entt_traits for more details).
  * @tparam Component Types of components iterated by the view.
@@ -301,28 +368,45 @@ template<typename Entity, typename... Component>
 class View final {
     static_assert(sizeof...(Component) > 1, "!");
 
+    /*! @brief A registry is allowed to create views. */
+    friend class Registry<Entity>;
+
     template<typename Comp>
     using pool_type = SparseSet<Entity, Comp>;
 
-    using base_pool_type = SparseSet<Entity>;
-    using underlying_iterator_type = typename base_pool_type::iterator_type;
-    using repo_type = std::tuple<pool_type<Component> &...>;
+    using view_type = SparseSet<Entity>;
+    using underlying_iterator_type = typename view_type::iterator_type;
+    using unchecked_type = std::array<const view_type *, (sizeof...(Component) - 1)>;
+    using pattern_type = std::tuple<pool_type<Component> &...>;
+    using traits_type = entt_traits<Entity>;
 
     class Iterator {
+        using size_type = typename view_type::size_type;
+
         inline bool valid() const noexcept {
-            using accumulator_type = bool[];
-            auto entity = *begin;
-            bool all = true;
-            accumulator_type accumulator =  { all, (all = all && std::get<pool_type<Component> &>(pools).has(entity))... };
-            (void)accumulator;
-            return all;
+            const auto entity = *begin;
+            const auto sz = size_type(entity & traits_type::entity_mask);
+            auto pos = unchecked.size();
+
+            if(sz < extent) {
+                for(; pos && unchecked[pos-1]->fast(entity); --pos);
+            }
+
+            return !pos;
         }
 
     public:
-        using value_type = typename base_pool_type::entity_type;
+        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 = typename underlying_iterator_type::iterator_category;
 
-        Iterator(const repo_type &pools, underlying_iterator_type begin, underlying_iterator_type end) noexcept
-            : pools{pools}, begin{begin}, end{end}
+        Iterator(unchecked_type unchecked, size_type extent, underlying_iterator_type begin, underlying_iterator_type end) noexcept
+            : unchecked{unchecked},
+              extent{extent},
+              begin{begin},
+              end{end}
         {
             if(begin != end && !valid()) {
                 ++(*this);
@@ -330,9 +414,7 @@ class View final {
         }
 
         Iterator & operator++() noexcept {
-            ++begin;
-            while(begin != end && !valid()) { ++begin; }
-            return *this;
+            return (++begin != end && !valid()) ? ++(*this) : *this;
         }
 
         Iterator operator++(int) noexcept {
@@ -340,6 +422,14 @@ class View final {
             return ++(*this), orig;
         }
 
+        Iterator & operator+=(difference_type value) noexcept {
+            return ((begin += value) != end && !valid()) ? ++(*this) : *this;
+        }
+
+        Iterator operator+(difference_type value) noexcept {
+            return Iterator{unchecked, extent, begin+value, end};
+        }
+
         bool operator==(const Iterator &other) const noexcept {
             return other.begin == begin;
         }
@@ -353,29 +443,34 @@ class View final {
         }
 
     private:
-        const repo_type &pools;
+        const unchecked_type unchecked;
+        const size_type extent;
         underlying_iterator_type begin;
         underlying_iterator_type end;
     };
 
-public:
-    /*! Input iterator type. */
-    using iterator_type = Iterator;
-    /*! @brief Underlying entity identifier. */
-    using entity_type = typename base_pool_type::entity_type;
-    /*! @brief Unsigned integer type. */
-    using size_type = typename base_pool_type::size_type;
-
-    /**
-     * @brief Constructs a view out of a bunch of pools of components.
-     * @param pools References to pools of components.
-     */
-    View(pool_type<Component>&... pools) noexcept
-        : pools{pools...}, view{nullptr}
+    View(pool_type<Component> &... pools) noexcept
+        : pools{pools...}, view{nullptr}, unchecked{}
     {
         reset();
     }
 
+public:
+    /*! @brief Input iterator type. */
+    using iterator_type = Iterator;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename view_type::entity_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = typename view_type::size_type;
+
+    /**
+     * @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 noexcept {
+        return view->size();
+    }
+
     /**
      * @brief Returns an iterator to the first entity that has the given
      * components.
@@ -391,7 +486,8 @@ public:
      * @return An iterator to the first entity that has the given components.
      */
     iterator_type begin() const noexcept {
-        return Iterator{pools, view->begin(), view->end()};
+        const auto extent = std::min({ std::get<pool_type<Component> &>(pools).extent()... });
+        return Iterator{unchecked, extent, view->begin(), view->end()};
     }
 
     /**
@@ -410,28 +506,47 @@ public:
      * given components.
      */
     iterator_type end() const noexcept {
-        return Iterator{pools, view->end(), view->end()};
+        const auto extent = std::min({ std::get<pool_type<Component> &>(pools).extent()... });
+        return Iterator{unchecked, extent, view->end(), view->end()};
+    }
+
+    /**
+     * @brief Checks if a view contains an entity.
+     * @param entity A valid entity identifier.
+     * @return True if the view contains the given entity, false otherwise.
+     */
+    bool contains(entity_type entity) const noexcept {
+        const auto extent = std::min({ std::get<pool_type<Component> &>(pools).extent()... });
+        const auto sz = size_type(entity & traits_type::entity_mask);
+        auto pos = unchecked.size();
+
+        if(sz < extent && view->has(entity) && (view->data()[view->get(entity)] == entity)) {
+            for(; pos && unchecked[pos-1]->fast(entity); --pos);
+        }
+
+        return !pos;
     }
 
     /**
      * @brief Returns the component assigned to the given entity.
      *
-     * Prefer this function instead of `Registry::get` during iterations.
-     * It has far better performance than its companion function.
+     * Prefer this function instead of `Registry::get` during iterations. It has
+     * far better performance than its companion function.
      *
      * @warning
      * Attempting to use an invalid component type results in a compilation
      * error. Attempting to use an entity that doesn't belong to the view
      * results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if
-     * the view doesn't contain the given entity.
+     * An assertion will abort the execution at runtime in debug mode if the
+     * view doesn't contain the given entity.
      *
-     * @tparam Comp Type of the component to get.
+     * @tparam Comp Type of component to get.
      * @param entity A valid entity identifier.
      * @return The component assigned to the entity.
      */
     template<typename Comp>
     const Comp & get(entity_type entity) const noexcept {
+        assert(contains(entity));
         return std::get<pool_type<Comp> &>(pools).get(entity);
     }
 
@@ -445,10 +560,10 @@ public:
      * Attempting to use an invalid component type results in a compilation
      * error. Attempting to use an entity that doesn't belong to the view
      * results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if
-     * the view doesn't contain the given entity.
+     * An assertion will abort the execution at runtime in debug mode if the
+     * view doesn't contain the given entity.
      *
-     * @tparam Comp Type of the component to get.
+     * @tparam Comp Type of component to get.
      * @param entity A valid entity identifier.
      * @return The component assigned to the entity.
      */
@@ -458,7 +573,56 @@ public:
     }
 
     /**
-     * @brief Iterate the entities and applies them the given function object.
+     * @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 invalid component types results in a compilation error.
+     * Attempting to use an entity that doesn't belong to the view results in
+     * undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * view doesn't contain the given entity.
+     *
+     * @tparam Comp Types of the components to get.
+     * @param entity A valid entity identifier.
+     * @return The components assigned to the entity.
+     */
+    template<typename... Comp>
+    std::enable_if_t<(sizeof...(Comp) > 1), std::tuple<const Comp &...>>
+    get(entity_type entity) const noexcept {
+        assert(contains(entity));
+        return std::tuple<const Comp &...>{get<Comp>(entity)...};
+    }
+
+    /**
+     * @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 invalid component types results in a compilation error.
+     * Attempting to use an entity that doesn't belong to the view results in
+     * undefined behavior.<br/>
+     * An assertion will abort the execution at runtime in debug mode if the
+     * view doesn't contain the given entity.
+     *
+     * @tparam Comp Types of the components to get.
+     * @param entity A valid entity identifier.
+     * @return The components assigned to the entity.
+     */
+    template<typename... Comp>
+    std::enable_if_t<(sizeof...(Comp) > 1), std::tuple<Comp &...>>
+    get(entity_type entity) noexcept {
+        assert(contains(entity));
+        return std::tuple<Comp &...>{get<Comp>(entity)...};
+    }
+
+    /**
+     * @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 const references to all the components of the
@@ -474,13 +638,26 @@ public:
      */
     template<typename Func>
     void each(Func func) const {
-        for(auto entity: *this) {
-            func(entity, get<Component>(entity)...);
+        const auto extent = std::min({ std::get<pool_type<Component> &>(pools).extent()... });
+
+        for(auto entity: *view) {
+            const auto sz = size_type(entity & traits_type::entity_mask);
+
+            if(sz < extent) {
+                auto pos = unchecked.size();
+
+                for(; pos && unchecked[pos-1]->fast(entity); --pos);
+
+                if(!pos) {
+                    func(entity, get<Component>(entity)...);
+                }
+            }
         }
     }
 
     /**
-     * @brief Iterate the entities and applies them the given function object.
+     * @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 the components of the
@@ -513,15 +690,28 @@ public:
      */
     void reset() {
         using accumulator_type = size_type[];
-        auto probe = [this](auto sz, auto &pool) { return pool.size() < sz ? (view = &pool, pool.size()) : sz; };
         size_type sz = std::max({ std::get<pool_type<Component> &>(pools).size()... }) + std::size_t{1};
-        accumulator_type accumulator = { sz, (sz = probe(sz, std::get<pool_type<Component> &>(pools)))... };
-        (void)accumulator;
+        size_type pos{};
+
+        auto probe = [this](auto sz, const auto &pool) {
+            return pool.size() < sz ? (view = &pool, pool.size()) : sz;
+        };
+
+        auto filter = [this](auto pos, const auto &pool) {
+            return (view != &pool) ? (unchecked[pos++] = &pool, pos) : pos;
+        };
+
+        accumulator_type probing = { (sz = probe(sz, std::get<pool_type<Component> &>(pools)))... };
+        accumulator_type filtering = { (pos = filter(pos, std::get<pool_type<Component> &>(pools)))... };
+
+        (void)filtering;
+        (void)probing;
     }
 
 private:
-    repo_type pools;
-    base_pool_type *view;
+    const pattern_type pools;
+    const view_type *view;
+    unchecked_type unchecked;
 };
 
 
@@ -530,7 +720,7 @@ private:
  *
  * Single component views are specialized in order to get a boost in terms of
  * performance. This kind of views can access the underlying data structure
- * directly and avoid superflous checks.<br/>
+ * directly and avoid superfluous checks.<br/>
  * Order of elements during iterations are highly dependent on the order of the
  * underlying data structure. See SparseSet and its specializations for more
  * details.
@@ -539,11 +729,11 @@ private:
  *
  * 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).
+ * * New instances of the given component are created and assigned to entities.
+ * * The entity currently pointed is modified (as an example, the given
+ *   component is removed from the entity to which the iterator points).
  *
- * In all the other cases, modify the pools of the given components somehow
+ * In all the other cases, modifying the pool of the given component in any way
  * invalidates all the iterators and using them results in undefined behavior.
  *
  * @note
@@ -557,32 +747,33 @@ private:
  *
  * @sa View
  * @sa PersistentView
+ * @sa RawView
  *
  * @tparam Entity A valid entity type (see entt_traits for more details).
- * @tparam Component Type of the component iterated by the view.
+ * @tparam Component Type of component iterated by the view.
  */
 template<typename Entity, typename Component>
 class View<Entity, Component> final {
+    /*! @brief A registry is allowed to create views. */
+    friend class Registry<Entity>;
+
+    using view_type = SparseSet<Entity>;
     using pool_type = SparseSet<Entity, Component>;
 
+    View(pool_type &pool) noexcept
+        : pool{pool}
+    {}
+
 public:
-    /*! Input iterator type. */
-    using iterator_type = typename pool_type::iterator_type;
+    /*! @brief Input iterator type. */
+    using iterator_type = typename view_type::iterator_type;
     /*! @brief Underlying entity identifier. */
     using entity_type = typename pool_type::entity_type;
     /*! @brief Unsigned integer type. */
     using size_type = typename pool_type::size_type;
-    /*! Type of the component iterated by the view. */
+    /*! @brief Type of component iterated by the view. */
     using raw_type = typename pool_type::object_type;
 
-    /**
-     * @brief Constructs a view out of a pool of components.
-     * @param pool A reference to a pool of components.
-     */
-    View(pool_type &pool) noexcept
-        : pool{pool}
-    {}
-
     /**
      * @brief Returns the number of entities that have the given component.
      * @return Number of entities that have the given component.
@@ -654,7 +845,7 @@ public:
      * @return An iterator to the first entity that has the given component.
      */
     iterator_type begin() const noexcept {
-        return pool.begin();
+        return pool.view_type::begin();
     }
 
     /**
@@ -673,7 +864,16 @@ public:
      * given component.
      */
     iterator_type end() const noexcept {
-        return pool.end();
+        return pool.view_type::end();
+    }
+
+    /**
+     * @brief Checks if a view contains an entity.
+     * @param entity A valid entity identifier.
+     * @return True if the view contains the given entity, false otherwise.
+     */
+    bool contains(entity_type entity) const noexcept {
+        return pool.has(entity) && (pool.data()[pool.view_type::get(entity)] == entity);
     }
 
     /**
@@ -692,6 +892,7 @@ public:
      * @return The component assigned to the entity.
      */
     const Component & get(entity_type entity) const noexcept {
+        assert(contains(entity));
         return pool.get(entity);
     }
 
@@ -715,7 +916,8 @@ public:
     }
 
     /**
-     * @brief Iterate the entities and applies them the given function object.
+     * @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 const reference to the component of the view.<br/>
@@ -730,13 +932,16 @@ public:
      */
     template<typename Func>
     void each(Func func) const {
-        for(auto entity: *this) {
+        const view_type &view = pool;
+
+        for(auto entity: view) {
             func(entity, get(entity));
         }
     }
 
     /**
-     * @brief Iterate the entities and applies them the given function object.
+     * @brief Iterates entities and components and applies the given function
+     * object to them.
      *
      * The function object is invoked for each entity. It is provided with the
      * entity itself and a reference to the component of the view.<br/>
@@ -761,6 +966,162 @@ private:
 };
 
 
+/**
+ * @brief Raw view.
+ *
+ * Raw views are meant to easily iterate components without having to resort to
+ * using any other member function, so as to further increase the performance.
+ * Whenever knowing the entity to which a component belongs isn't required, this
+ * should be the preferred tool.<br/>
+ * Order of elements during iterations are highly dependent on the order of the
+ * underlying data structure. See SparseSet and its specializations for more
+ * details.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New instances of the given component are created and assigned to entities.
+ * * The entity to which the component belongs is modified (as an example, the
+ *   given component is destroyed).
+ *
+ * In all the other cases, modifying the pool of the given component in any way
+ * invalidates all the iterators and using them results in undefined behavior.
+ *
+ * @note
+ * Views share a reference to the underlying data structure with the Registry
+ * that generated them. Therefore any change to the entities and to the
+ * components made by means of the registry are immediately reflected by views.
+ *
+ * @warning
+ * Lifetime of a view must overcome the one of the registry that generated it.
+ * In any other case, attempting to use a view results in undefined behavior.
+ *
+ * @sa View
+ * @sa View<Entity, Component>
+ * @sa PersistentView
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Component Type of component iterated by the view.
+ */
+template<typename Entity, typename Component>
+class RawView final {
+    /*! @brief A registry is allowed to create views. */
+    friend class Registry<Entity>;
+
+    using view_type = SparseSet<Entity>;
+    using pool_type = SparseSet<Entity, Component>;
+
+    RawView(pool_type &pool) noexcept
+        : pool{pool}
+    {}
+
+public:
+    /*! @brief Input iterator type. */
+    using iterator_type = typename pool_type::iterator_type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename pool_type::entity_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = typename pool_type::size_type;
+    /*! @brief Type of component iterated by the view. */
+    using raw_type = typename pool_type::object_type;
+
+    /**
+     * @brief Returns the number of instances of the given type.
+     * @return Number of instances of the given component.
+     */
+    size_type size() const noexcept {
+        return pool.size();
+    }
+
+    /**
+     * @brief Direct access to the list of components.
+     *
+     * The returned pointer is such that range `[raw(), raw() + size()]` is
+     * always a valid range, even if the container is empty.
+     *
+     * @note
+     * There are no guarantees on the order of the components. Use `begin` and
+     * `end` if you want to iterate the view in the expected order.
+     *
+     * @return A pointer to the array of components.
+     */
+    raw_type * raw() noexcept {
+        return pool.raw();
+    }
+
+    /**
+     * @brief Direct access to the list of components.
+     *
+     * The returned pointer is such that range `[raw(), raw() + size()]` is
+     * always a valid range, even if the container is empty.
+     *
+     * @note
+     * There are no guarantees on the order of the components. Use `begin` and
+     * `end` if you want to iterate the view in the expected order.
+     *
+     * @return A pointer to the array of components.
+     */
+    const raw_type * raw() const noexcept {
+        return pool.raw();
+    }
+
+    /**
+     * @brief Direct access to the list of entities.
+     *
+     * The returned pointer is such that range `[data(), data() + size()]` is
+     * always a valid range, even if the container is empty.
+     *
+     * @note
+     * There are no guarantees on the order of the entities. Use `begin` and
+     * `end` if you want to iterate the view in the expected order.
+     *
+     * @return A pointer to the array of entities.
+     */
+    const entity_type * data() const noexcept {
+        return pool.data();
+    }
+
+    /**
+     * @brief Returns an iterator to the first instance of the given type.
+     *
+     * The returned iterator points to the first instance of the given type. 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 instance of the given type.
+     */
+    iterator_type begin() const noexcept {
+        return pool.begin();
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last instance of the given
+     * type.
+     *
+     * 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 to the underlying data
+     * structures.
+     *
+     * @return An iterator to the element following the last instance of the
+     * given type.
+     */
+    iterator_type end() const noexcept {
+        return pool.end();
+    }
+
+private:
+    pool_type &pool;
+};
+
+
 }
 
 

src/entt/entt.hpp

@@ -2,9 +2,10 @@
 #include "core/hashed_string.hpp"
 #include "core/ident.hpp"
 #include "entity/actor.hpp"
+#include "entity/entt_traits.hpp"
 #include "entity/registry.hpp"
+#include "entity/snapshot.hpp"
 #include "entity/sparse_set.hpp"
-#include "entity/traits.hpp"
 #include "entity/view.hpp"
 #include "locator/locator.hpp"
 #include "process/process.hpp"
@@ -14,6 +15,7 @@
 #include "resource/loader.hpp"
 #include "signal/bus.hpp"
 #include "signal/delegate.hpp"
+#include "signal/dispatcher.hpp"
 #include "signal/emitter.hpp"
 #include "signal/sigh.hpp"
 #include "signal/signal.hpp"

src/entt/locator/locator.hpp

@@ -14,7 +14,7 @@ namespace entt {
  * @brief Service locator, nothing more.
  *
  * A service locator can be used to do what it promises: locate services.<br/>
- * Usually service locators are tighly bound to the services they expose and
+ * Usually service locators are tightly bound to the services they expose and
  * thus it's hard to define a general purpose class to do that. This template
  * based implementation tries to fill the gap and to get rid of the burden of
  * defining a different specific locator for each application.
@@ -78,7 +78,7 @@ struct ServiceLocator final {
      * @param args Parameters to use to construct the service.
      */
     template<typename Impl = Service, typename... Args>
-    inline static void set(Args&&... args) {
+    inline static void set(Args &&... args) {
         service = std::make_shared<Impl>(std::forward<Args>(args)...);
     }
 

src/entt/process/process.hpp

@@ -19,33 +19,42 @@ namespace entt {
  * required:
  *
  * * @code{.cpp}
- *   void update(Delta);
+ *   void update(Delta, void *);
  *   @endcode
+ *
  *   It's invoked once per tick until a process is explicitly aborted or it
  *   terminates either with or without errors. Even though it's not mandatory to
  *   declare this member function, as a rule of thumb each process should at
- *   least define it to work properly.
+ *   least define it to work properly. The `void *` parameter is an opaque
+ *   pointer to user data (if any) forwarded directly to the process during an
+ *   update.
  *
  * * @code{.cpp}
- *   void init();
+ *   void init(void *);
  *   @endcode
- *   It's invoked at the first tick, immediately before an update.
+ *
+ *   It's invoked at the first tick, immediately before an update. The `void *`
+ *   parameter is an opaque pointer to user data (if any) forwarded directly to
+ *   the process during an update.
  *
  * * @code{.cpp}
  *   void succeeded();
  *   @endcode
+ *
  *   It's invoked in case of success, immediately after an update and during the
  *   same tick.
  *
  * * @code{.cpp}
  *   void failed();
  *   @endcode
+ *
  *   It's invoked in case of errors, immediately after an update and during the
  *   same tick.
  *
  * * @code{.cpp}
  *   void aborted();
  *   @endcode
+ *
  *   It's invoked only if a process is explicitly aborted. There is no guarantee
  *   that it executes in the same tick, this depends solely on whether the
  *   process is aborted immediately or not.
@@ -105,7 +114,7 @@ class Process {
     }
 
     template<State S, typename... Args>
-    void tick(char, tag<S>, Args&&...) {}
+    void tick(char, tag<S>, Args &&...) {}
 
 protected:
     /**
@@ -161,7 +170,7 @@ public:
     using delta_type = Delta;
 
     /*! @brief Default destructor. */
-    ~Process() noexcept {
+    virtual ~Process() noexcept {
         static_assert(std::is_base_of<Process, Derived>::value, "!");
     }
 
@@ -273,12 +282,13 @@ private:
  * following:
  *
  * @code{.cpp}
- * void(Delta delta, auto succeed, auto fail);
+ * void(Delta delta, void *data, auto succeed, auto fail);
  * @endcode
  *
  * Where:
  *
  * * `delta` is the elapsed time.
+ * * `data` is an opaque pointer to user data if any, `nullptr` otherwise.
  * * `succeed` is a function to call when a process terminates with success.
  * * `fail` is a function to call when a process terminates with errors.
  *
@@ -307,7 +317,7 @@ struct ProcessAdaptor: Process<ProcessAdaptor<Func, Delta>, Delta>, private Func
      * @param args Parameters to use to initialize the actual process.
      */
     template<typename... Args>
-    ProcessAdaptor(Args&&... args)
+    ProcessAdaptor(Args &&... args)
         : Func{std::forward<Args>(args)...}
     {}
 
@@ -317,7 +327,7 @@ struct ProcessAdaptor: Process<ProcessAdaptor<Func, Delta>, Delta>, private Func
      * @param data Optional data.
      */
     void update(Delta delta, void *data) {
-        Func::operator()(delta, data, [this](){ this->succeed(); }, [this](){ this->fail(); });
+        Func::operator()(delta, data, [this]() { this->succeed(); }, [this]() { this->fail(); });
     }
 };
 

src/entt/process/scheduler.hpp

@@ -28,7 +28,7 @@ namespace entt {
  * Example of use (pseudocode):
  *
  * @code{.cpp}
- * scheduler.attach([](auto delta, auto succeed, auto fail) {
+ * scheduler.attach([](auto delta, void *, auto succeed, auto fail) {
  *     // code
  * }).then<MyProcess>(arguments...);
  * @endcode
@@ -64,7 +64,7 @@ class Scheduler final {
         {}
 
         template<typename Proc, typename... Args>
-        decltype(auto) then(Args&&... args) && {
+        decltype(auto) then(Args &&... args) && {
             static_assert(std::is_base_of<Process<Proc, Delta>, Proc>::value, "!");
             handler = Lambda::operator()(handler, tag<Proc>{}, std::forward<Args>(args)...);
             return std::move(*this);
@@ -181,7 +181,7 @@ public:
      * // schedules a task in the form of a process class
      * scheduler.attach<MyProcess>(arguments...)
      * // appends a child in the form of a lambda function
-     * .then([](auto delta, auto succeed, auto fail) {
+     * .then([](auto delta, void *, auto succeed, auto fail) {
      *     // code
      * })
      * // appends a child in the form of another process class
@@ -194,7 +194,7 @@ public:
      * @return An opaque object to use to concatenate processes.
      */
     template<typename Proc, typename... Args>
-    auto attach(Args&&... 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>};
@@ -237,11 +237,11 @@ public:
      *
      * @code{.cpp}
      * // schedules a task in the form of a lambda function
-     * scheduler.attach([](auto delta, auto succeed, auto fail) {
+     * scheduler.attach([](auto delta, void *, auto succeed, auto fail) {
      *     // code
      * })
      * // appends a child in the form of another lambda function
-     * .then([](auto delta, auto succeed, auto fail) {
+     * .then([](auto delta, void *, auto succeed, auto fail) {
      *     // code
      * })
      * // appends a child in the form of a process class
@@ -274,7 +274,7 @@ public:
     void update(Delta delta, void *data = nullptr) {
         bool clean = false;
 
-        for(auto pos = handlers.size(); pos > 0; --pos) {
+        for(auto pos = handlers.size(); pos; --pos) {
             auto &handler = handlers[pos-1];
             const bool dead = handler.update(handler, delta, data);
             clean = clean || dead;

src/entt/resource/cache.hpp

@@ -92,7 +92,7 @@ public:
      * @return True if the resource is ready to use, false otherwise.
      */
     template<typename Loader, typename... Args>
-    bool load(resource_type id, Args&&... args) {
+    bool load(resource_type id, Args &&... args) {
         static_assert(std::is_base_of<ResourceLoader<Loader, Resource>, Loader>::value, "!");
 
         bool loaded = true;
@@ -125,8 +125,8 @@ public:
      * @return True if the resource is ready to use, false otherwise.
      */
     template<typename Loader, typename... Args>
-    void reload(resource_type id, Args&&... args) {
-        return (discard(id), load(id, std::forward<Args>(args)...));
+    bool reload(resource_type id, Args &&... args) {
+        return (discard(id), load<Loader>(id, std::forward<Args>(args)...));
     }
 
     /**
@@ -142,7 +142,7 @@ public:
      * @return A handle for the given resource.
      */
     template<typename Loader, typename... Args>
-    ResourceHandle<Resource> temp(Args&&... args) const {
+    ResourceHandle<Resource> temp(Args &&... args) const {
         return { Loader{}.get(std::forward<Args>(args)...) };
     }
 

src/entt/resource/loader.hpp

@@ -17,7 +17,7 @@ class ResourceCache;
  *
  * Resource loaders must inherit from this class and stay true to the CRTP
  * idiom. Moreover, a resource loader must expose a public, const member
- * function named `load` that accepts a variable number of arguments and return
+ * function named `load` that accepts a variable number of arguments and returns
  * a shared pointer to the resource just created.<br/>
  * As an example:
  *
@@ -50,7 +50,7 @@ class ResourceLoader {
     friend class ResourceCache<Resource>;
 
     template<typename... Args>
-    std::shared_ptr<Resource> get(Args&&... args) const {
+    std::shared_ptr<Resource> get(Args &&... args) const {
         return static_cast<const Loader *>(this)->load(std::forward<Args>(args)...);
     }
 };

src/entt/signal/bus.hpp

@@ -142,7 +142,7 @@ public:
      * @param args Arguments to use to construct the event.
      */
     template<typename Type, typename... Args>
-    void publish(Args&&... args) {
+    void publish(Args &&... args) {
         Bus<Sig, Type>::publish(std::forward<Args>(args)...);
     }
 };
@@ -263,7 +263,7 @@ public:
      * @param args Arguments to use to construct the event.
      */
     template<typename... Args>
-    void publish(Args&&... args) {
+    void publish(Args &&... args) {
         signal.publish({ std::forward<Args>(args)... });
     }
 

src/entt/signal/dispatcher.hpp

@@ -59,12 +59,12 @@ class Dispatcher final {
         }
 
         template<typename... Args>
-        inline void trigger(Args&&... args) {
+        inline void trigger(Args &&... args) {
             signal.publish({ std::forward<Args>(args)... });
         }
 
         template<typename... Args>
-        inline void enqueue(std::size_t current, Args&&... args) {
+        inline void enqueue(std::size_t current, Args &&... args) {
             events[current].push_back({ std::forward<Args>(args)... });
         }
 
@@ -127,8 +127,9 @@ public:
      * automatically detected and unregistered if available.
      *
      * @warning
-     * Disonnecting a listener during an update may lead to unexpected behavior.
-     * Unregister listeners before or after invoking the update if possible.
+     * Disconnecting a listener during an update may lead to unexpected
+     * behavior. Unregister listeners before or after invoking the update if
+     * possible.
      *
      * @tparam Event Type of event from which to disconnect the function.
      * @tparam Class Type of class to which the member function belongs.
@@ -151,7 +152,7 @@ public:
      * @param args Arguments to use to construct the event.
      */
     template<typename Event, typename... Args>
-    void trigger(Args&&... args) {
+    void trigger(Args &&... args) {
         wrapper<Event>().trigger(std::forward<Args>(args)...);
     }
 
@@ -166,7 +167,7 @@ public:
      * @param args Arguments to use to construct the event.
      */
     template<typename Event, typename... Args>
-    void enqueue(Args&&... args) {
+    void enqueue(Args &&... args) {
         wrapper<Event>().enqueue(buffer(mode), std::forward<Args>(args)...);
     }
 
@@ -181,7 +182,9 @@ public:
         const auto buf = buffer(mode);
         mode = !mode;
 
-        for(auto &&wrapper: wrappers) {
+        for(auto pos = wrappers.size(); pos; --pos) {
+            auto &wrapper = wrappers[pos-1];
+
             if(wrapper) {
                 wrapper->publish(buf);
             }

src/entt/signal/emitter.hpp

@@ -52,7 +52,7 @@ class Emitter {
         using connection_type = typename container_type::iterator;
 
         bool empty() const noexcept override {
-            auto pred = [](auto &&element){ return element.first; };
+            auto pred = [](auto &&element) { return element.first; };
 
             return std::all_of(onceL.cbegin(), onceL.cend(), pred) &&
                     std::all_of(onL.cbegin(), onL.cend(), pred);
@@ -60,7 +60,7 @@ class Emitter {
 
         void clear() noexcept override {
             if(publishing) {
-                auto func = [](auto &&element){ element.first = true; };
+                auto func = [](auto &&element) { element.first = true; };
                 std::for_each(onceL.begin(), onceL.end(), func);
                 std::for_each(onL.begin(), onL.end(), func);
             } else {
@@ -81,7 +81,7 @@ class Emitter {
             conn->first = true;
 
             if(!publishing) {
-                auto pred = [](auto &&element){ return element.first; };
+                auto pred = [](auto &&element) { return element.first; };
                 onceL.remove_if(pred);
                 onL.remove_if(pred);
             }
@@ -102,7 +102,7 @@ class Emitter {
 
             publishing = false;
 
-            onL.remove_if([](auto &&element){ return element.first; });
+            onL.remove_if([](auto &&element) { return element.first; });
         }
 
     private:
@@ -215,7 +215,7 @@ public:
      * @param args Parameters to use to initialize the event.
      */
     template<typename Event, typename... Args>
-    void publish(Args&&... args) {
+    void publish(Args &&... args) {
         handler<Event>().publish({ std::forward<Args>(args)... }, *static_cast<Derived *>(this));
     }
 
@@ -287,7 +287,7 @@ public:
      * @brief Disconnects all the listeners for the given event type.
      *
      * All the connections previously returned for the given event are
-     * invalidated. Using them results in undefined behaviour.
+     * invalidated. Using them results in undefined behavior.
      *
      * @tparam Event Type of event to reset.
      */
@@ -300,11 +300,11 @@ public:
      * @brief Disconnects all the listeners.
      *
      * All the connections previously returned are invalidated. Using them
-     * results in undefined behaviour.
+     * results in undefined behavior.
      */
     void clear() noexcept {
         std::for_each(handlers.begin(), handlers.end(),
-                      [](auto &&handler){ if(handler) { handler->clear(); } });
+                      [](auto &&handler) { if(handler) { handler->clear(); } });
     }
 
     /**
@@ -327,7 +327,7 @@ public:
      */
     bool empty() const noexcept {
         return std::all_of(handlers.cbegin(), handlers.cend(),
-                           [](auto &&handler){ return !handler || handler->empty(); });
+                           [](auto &&handler) { return !handler || handler->empty(); });
     }
 
 private:

src/entt/signal/sigh.hpp

@@ -24,21 +24,25 @@ struct Invoker<Ret(Args...), Collector> {
 
     virtual ~Invoker() = default;
 
-    template<typename SFINAE = Ret>
-    typename std::enable_if_t<std::is_void<SFINAE>::value, bool>
-    invoke(Collector &, proto_type proto, void *instance, Args... args) {
-        proto(instance, args...);
-        return true;
-    }
-
-    template<typename SFINAE = Ret>
-    typename std::enable_if_t<!std::is_void<SFINAE>::value, bool>
-    invoke(Collector &collector, proto_type proto, void *instance, Args... args) {
+    bool invoke(Collector &collector, proto_type proto, void *instance, Args... args) {
         return collector(proto(instance, args...));
     }
 };
 
 
+template<typename... Args, typename Collector>
+struct Invoker<void(Args...), Collector> {
+    using proto_type = void(*)(void *, Args...);
+    using call_type = std::pair<void *, proto_type>;
+
+    virtual ~Invoker() = default;
+
+    bool invoke(Collector &, proto_type proto, void *instance, Args... args) {
+        return (proto(instance, args...), true);
+    }
+};
+
+
 template<typename Ret>
 struct NullCollector final {
     using result_type = Ret;
@@ -100,7 +104,7 @@ class SigH;
  *
  * * `Param` is a type to which `Ret` can be converted.
  * * The return type is true if the handler must stop collecting data, false
- *   otherwise.
+ * otherwise.
  *
  * @tparam Ret Return type of a function type.
  * @tparam Args Types of arguments of a function type.
@@ -229,7 +233,8 @@ public:
      * @param args Arguments to use to invoke listeners.
      */
     void publish(Args... args) {
-        for(auto &&call: calls) {
+        for(auto pos = calls.size(); pos; --pos) {
+            auto &call = calls[pos-1];
             call.second(call.first, args...);
         }
     }
@@ -242,7 +247,9 @@ public:
     collector_type collect(Args... args) {
         collector_type collector;
 
-        for(auto &&call: calls) {
+        for(auto pos = calls.size(); pos; --pos) {
+            auto &call = calls[pos-1];
+
             if(!this->invoke(collector, call.second, call.first, args...)) {
                 break;
             }

src/entt/signal/signal.hpp

@@ -167,11 +167,17 @@ public:
      * @param args Arguments to use to invoke listeners.
      */
     void publish(Args... args) {
-        for(auto it = calls.rbegin(), end = calls.rend(); it != end; it++) {
-            if(!(it->second)(it->first, args...)) {
-                calls.erase(std::next(it).base());
+        std::vector<call_type> next;
+
+        for(auto pos = calls.size(); pos; --pos) {
+            auto &call = calls[pos-1];
+
+            if((call.second)(call.first, args...)) {
+                next.push_back(call);
             }
         }
+
+        calls.swap(next);
     }
 
     /**

test/CMakeLists.txt

@@ -19,16 +19,42 @@ endif()
 # Test mod
 
 if(BUILD_MOD)
+    set(DUKTAPE_DEPS_DIR ${entt_SOURCE_DIR}/deps/duktape)
+    configure_file(${entt_SOURCE_DIR}/cmake/in/duktape.in ${DUKTAPE_DEPS_DIR}/CMakeLists.txt)
+    execute_process(COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" . WORKING_DIRECTORY ${DUKTAPE_DEPS_DIR})
+    execute_process(COMMAND ${CMAKE_COMMAND} --build . WORKING_DIRECTORY ${DUKTAPE_DEPS_DIR})
+    set(DUKTAPE_SRC_DIR ${DUKTAPE_DEPS_DIR}/src/src)
+
     add_executable(
         mod
         $<TARGET_OBJECTS:odr>
-        mod/duktape.c
+        ${DUKTAPE_SRC_DIR}/duktape.c
         mod/mod.cpp
     )
+    target_include_directories(mod PRIVATE ${DUKTAPE_SRC_DIR})
     target_link_libraries(mod PRIVATE gtest_main Threads::Threads m)
     add_test(NAME mod COMMAND mod)
 endif()
 
+# Test snapshot
+
+if(BUILD_SNAPSHOT)
+    set(CEREAL_DEPS_DIR ${entt_SOURCE_DIR}/deps/cereal)
+    configure_file(${entt_SOURCE_DIR}/cmake/in/cereal.in ${CEREAL_DEPS_DIR}/CMakeLists.txt)
+    execute_process(COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" . WORKING_DIRECTORY ${CEREAL_DEPS_DIR})
+    execute_process(COMMAND ${CMAKE_COMMAND} --build . WORKING_DIRECTORY ${CEREAL_DEPS_DIR})
+    set(CEREAL_SRC_DIR ${CEREAL_DEPS_DIR}/src/include)
+
+    add_executable(
+        snapshot
+        $<TARGET_OBJECTS:odr>
+        snapshot/snapshot.cpp
+    )
+    target_include_directories(snapshot PRIVATE ${CEREAL_SRC_DIR})
+    target_link_libraries(snapshot PRIVATE gtest_main Threads::Threads)
+    add_test(NAME snapshot COMMAND snapshot)
+endif()
+
 # Test core
 
 add_executable(
@@ -48,6 +74,7 @@ add_executable(
     $<TARGET_OBJECTS:odr>
     entt/entity/actor.cpp
     entt/entity/registry.cpp
+    entt/entity/snapshot.cpp
     entt/entity/sparse_set.cpp
     entt/entity/view.cpp
 )

test/benchmark/benchmark.cpp

@@ -1,9 +1,8 @@
-#include <gtest/gtest.h>
 #include <iostream>
 #include <cstddef>
 #include <cstdint>
 #include <chrono>
-#include <vector>
+#include <gtest/gtest.h>
 #include <entt/entity/registry.hpp>
 
 struct Position {
@@ -17,7 +16,7 @@ struct Velocity {
 };
 
 template<std::size_t>
-struct Comp {};
+struct Comp { int x; };
 
 struct Timer final {
     Timer(): start{std::chrono::system_clock::now()} {}
@@ -34,11 +33,11 @@ private:
 TEST(Benchmark, Construct) {
     entt::DefaultRegistry registry;
 
-    std::cout << "Constructing 10000000 entities" << std::endl;
+    std::cout << "Constructing 1000000 entities" << std::endl;
 
     Timer timer;
 
-    for(std::uint64_t i = 0; i < 10000000L; i++) {
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
         registry.create();
     }
 
@@ -47,19 +46,18 @@ TEST(Benchmark, Construct) {
 
 TEST(Benchmark, Destroy) {
     entt::DefaultRegistry registry;
-    std::vector<entt::DefaultRegistry::entity_type> entities{};
 
-    std::cout << "Destroying 10000000 entities" << std::endl;
+    std::cout << "Destroying 1000000 entities" << std::endl;
 
-    for(std::uint64_t i = 0; i < 10000000L; i++) {
-        entities.push_back(registry.create());
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
+        registry.create();
     }
 
     Timer timer;
 
-    for(auto entity: entities) {
+    registry.each([&registry](auto entity) {
         registry.destroy(entity);
-    }
+    });
 
     timer.elapsed();
 }
@@ -79,21 +77,24 @@ TEST(Benchmark, IterateCreateDeleteSingleComponent) {
         }
 
         for(auto entity: view) {
+            const auto &position = view.get(entity);
+            (void)position;
+
             if(rand() % 2 == 0) {
                 registry.destroy(entity);
             }
-        }
+        };
     }
 
     timer.elapsed();
 }
 
-TEST(Benchmark, IterateSingleComponent10M) {
+TEST(Benchmark, IterateSingleComponent1M) {
     entt::DefaultRegistry registry;
 
-    std::cout << "Iterating over 10000000 entities, one component" << std::endl;
+    std::cout << "Iterating over 1000000 entities, one component" << std::endl;
 
-    for(std::uint64_t i = 0; i < 10000000L; i++) {
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
         registry.create<Position>();
     }
 
@@ -102,12 +103,28 @@ TEST(Benchmark, IterateSingleComponent10M) {
     timer.elapsed();
 }
 
-TEST(Benchmark, IterateTwoComponents10M) {
+TEST(Benchmark, IterateSingleComponentRaw1M) {
     entt::DefaultRegistry registry;
 
-    std::cout << "Iterating over 10000000 entities, two components" << std::endl;
+    std::cout << "Iterating over 1000000 entities, one component, raw view" << std::endl;
 
-    for(std::uint64_t i = 0; i < 10000000L; i++) {
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
+        registry.create<Position>();
+    }
+
+    Timer timer;
+    for(auto &&component: registry.raw<Position>()) {
+        (void)component;
+    }
+    timer.elapsed();
+}
+
+TEST(Benchmark, IterateTwoComponents1M) {
+    entt::DefaultRegistry registry;
+
+    std::cout << "Iterating over 1000000 entities, two components" << std::endl;
+
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
         registry.create<Position, Velocity>();
     }
 
@@ -116,12 +133,12 @@ TEST(Benchmark, IterateTwoComponents10M) {
     timer.elapsed();
 }
 
-TEST(Benchmark, IterateTwoComponents10MHalf) {
+TEST(Benchmark, IterateTwoComponents1MHalf) {
     entt::DefaultRegistry registry;
 
-    std::cout << "Iterating over 10000000 entities, two components, half of the entities have all the components" << std::endl;
+    std::cout << "Iterating over 1000000 entities, two components, half of the entities have all the components" << std::endl;
 
-    for(std::uint64_t i = 0; i < 10000000L; i++) {
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
         auto entity = registry.create<Velocity>();
         if(i % 2) { registry.assign<Position>(entity); }
     }
@@ -131,12 +148,12 @@ TEST(Benchmark, IterateTwoComponents10MHalf) {
     timer.elapsed();
 }
 
-TEST(Benchmark, IterateTwoComponents10MOne) {
+TEST(Benchmark, IterateTwoComponents1MOne) {
     entt::DefaultRegistry registry;
 
-    std::cout << "Iterating over 10000000 entities, two components, only one entity has all the components" << std::endl;
+    std::cout << "Iterating over 1000000 entities, two components, only one entity has all the components" << std::endl;
 
-    for(std::uint64_t i = 0; i < 10000000L; i++) {
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
         auto entity = registry.create<Velocity>();
         if(i == 5000000L) { registry.assign<Position>(entity); }
     }
@@ -146,13 +163,13 @@ TEST(Benchmark, IterateTwoComponents10MOne) {
     timer.elapsed();
 }
 
-TEST(Benchmark, IterateTwoComponentsPersistent10M) {
+TEST(Benchmark, IterateTwoComponentsPersistent1M) {
     entt::DefaultRegistry registry;
     registry.prepare<Position, Velocity>();
 
-    std::cout << "Iterating over 10000000 entities, two components, persistent view" << std::endl;
+    std::cout << "Iterating over 1000000 entities, two components, persistent view" << std::endl;
 
-    for(std::uint64_t i = 0; i < 10000000L; i++) {
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
         registry.create<Position, Velocity>();
     }
 
@@ -161,44 +178,12 @@ TEST(Benchmark, IterateTwoComponentsPersistent10M) {
     timer.elapsed();
 }
 
-TEST(Benchmark, IterateTwoComponentsPersistent10MHalf) {
-    entt::DefaultRegistry registry;
-    registry.prepare<Position, Velocity>();
-
-    std::cout << "Iterating over 10000000 entities, two components, persistent view, half of the entities have all the components" << std::endl;
-
-    for(std::uint64_t i = 0; i < 10000000L; i++) {
-        auto entity = registry.create<Velocity>();
-        if(i % 2) { registry.assign<Position>(entity); }
-    }
-
-    Timer timer;
-    registry.persistent<Position, Velocity>().each([](auto, auto &...) {});
-    timer.elapsed();
-}
-
-TEST(Benchmark, IterateTwoComponentsPersistent10MOne) {
-    entt::DefaultRegistry registry;
-    registry.prepare<Position, Velocity>();
-
-    std::cout << "Iterating over 10000000 entities, two components, persistent view, only one entity has all the components" << std::endl;
-
-    for(std::uint64_t i = 0; i < 10000000L; i++) {
-        auto entity = registry.create<Velocity>();
-        if(i == 5000000L) { registry.assign<Position>(entity); }
-    }
-
-    Timer timer;
-    registry.persistent<Position, Velocity>().each([](auto, auto &...) {});
-    timer.elapsed();
-}
-
-TEST(Benchmark, IterateFiveComponents10M) {
+TEST(Benchmark, IterateFiveComponents1M) {
     entt::DefaultRegistry registry;
 
-    std::cout << "Iterating over 10000000 entities, five components" << std::endl;
+    std::cout << "Iterating over 1000000 entities, five components" << std::endl;
 
-    for(std::uint64_t i = 0; i < 10000000L; i++) {
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
         registry.create<Position, Velocity, Comp<1>, Comp<2>, Comp<3>>();
     }
 
@@ -207,57 +192,43 @@ TEST(Benchmark, IterateFiveComponents10M) {
     timer.elapsed();
 }
 
-TEST(Benchmark, IterateTenComponents10M) {
+TEST(Benchmark, IterateFiveComponents1MHalf) {
     entt::DefaultRegistry registry;
 
-    std::cout << "Iterating over 10000000 entities, ten components" << std::endl;
+    std::cout << "Iterating over 1000000 entities, five components, half of the entities have all the components" << std::endl;
 
-    for(std::uint64_t i = 0; i < 10000000L; i++) {
-        registry.create<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
-    }
-
-    Timer timer;
-    registry.view<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>().each([](auto, auto &...) {});
-    timer.elapsed();
-}
-
-TEST(Benchmark, IterateTenComponents10MHalf) {
-    entt::DefaultRegistry registry;
-
-    std::cout << "Iterating over 10000000 entities, ten components, half of the entities have all the components" << std::endl;
-
-    for(std::uint64_t i = 0; i < 10000000L; i++) {
-        auto entity = registry.create<Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
+        auto entity = registry.create<Velocity, Comp<1>, Comp<2>, Comp<3>>();
         if(i % 2) { registry.assign<Position>(entity); }
     }
 
     Timer timer;
-    registry.view<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>().each([](auto, auto &...) {});
+    registry.view<Position, Velocity, Comp<1>, Comp<2>, Comp<3>>().each([](auto, auto &...) {});
     timer.elapsed();
 }
 
-TEST(Benchmark, IterateTenComponents10MOne) {
+TEST(Benchmark, IterateFiveComponents1MOne) {
     entt::DefaultRegistry registry;
 
-    std::cout << "Iterating over 10000000 entities, ten components, only one entity has all the components" << std::endl;
+    std::cout << "Iterating over 1000000 entities, five components, only one entity has all the components" << std::endl;
 
-    for(std::uint64_t i = 0; i < 10000000L; i++) {
-        auto entity = registry.create<Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
+        auto entity = registry.create<Velocity, Comp<1>, Comp<2>, Comp<3>>();
         if(i == 5000000L) { registry.assign<Position>(entity); }
     }
 
     Timer timer;
-    registry.view<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>().each([](auto, auto &...) {});
+    registry.view<Position, Velocity, Comp<1>, Comp<2>, Comp<3>>().each([](auto, auto &...) {});
     timer.elapsed();
 }
 
-TEST(Benchmark, IterateFiveComponentsPersistent10M) {
+TEST(Benchmark, IterateFiveComponentsPersistent1M) {
     entt::DefaultRegistry registry;
     registry.prepare<Position, Velocity, Comp<1>, Comp<2>, Comp<3>>();
 
-    std::cout << "Iterating over 10000000 entities, five components, persistent view" << std::endl;
+    std::cout << "Iterating over 1000000 entities, five components, persistent view" << std::endl;
 
-    for(std::uint64_t i = 0; i < 10000000L; i++) {
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
         registry.create<Position, Velocity, Comp<1>, Comp<2>, Comp<3>>();
     }
 
@@ -266,46 +237,58 @@ TEST(Benchmark, IterateFiveComponentsPersistent10M) {
     timer.elapsed();
 }
 
-TEST(Benchmark, IterateTenComponentsPersistent10M) {
+TEST(Benchmark, IterateTenComponents1M) {
     entt::DefaultRegistry registry;
-    registry.prepare<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
 
-    std::cout << "Iterating over 10000000 entities, ten components, persistent view" << std::endl;
+    std::cout << "Iterating over 1000000 entities, ten components" << std::endl;
 
-    for(std::uint64_t i = 0; i < 10000000L; i++) {
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
         registry.create<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
     }
 
     Timer timer;
-    registry.persistent<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>().each([](auto, auto &...) {});
+    registry.view<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>().each([](auto, auto &...) {});
     timer.elapsed();
 }
 
-TEST(Benchmark, IterateTenComponentsPersistent10MHalf) {
+TEST(Benchmark, IterateTenComponents1MHalf) {
     entt::DefaultRegistry registry;
-    registry.prepare<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
 
-    std::cout << "Iterating over 10000000 entities, ten components, persistent view, half of the entities have all the components" << std::endl;
+    std::cout << "Iterating over 1000000 entities, ten components, half of the entities have all the components" << std::endl;
 
-    for(std::uint64_t i = 0; i < 10000000L; i++) {
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
         auto entity = registry.create<Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
         if(i % 2) { registry.assign<Position>(entity); }
     }
 
     Timer timer;
-    registry.persistent<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>().each([](auto, auto &...) {});
+    registry.view<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>().each([](auto, auto &...) {});
     timer.elapsed();
 }
 
-TEST(Benchmark, IterateTenComponentsPersistent10MOne) {
+TEST(Benchmark, IterateTenComponents1MOne) {
+    entt::DefaultRegistry registry;
+
+    std::cout << "Iterating over 1000000 entities, ten components, only one entity has all the components" << std::endl;
+
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
+        auto entity = registry.create<Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
+        if(i == 5000000L) { registry.assign<Position>(entity); }
+    }
+
+    Timer timer;
+    registry.view<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>().each([](auto, auto &...) {});
+    timer.elapsed();
+}
+
+TEST(Benchmark, IterateTenComponentsPersistent1M) {
     entt::DefaultRegistry registry;
     registry.prepare<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
 
-    std::cout << "Iterating over 10000000 entities, ten components, persistent view, only one entity has all the components" << std::endl;
+    std::cout << "Iterating over 1000000 entities, ten components, persistent view" << std::endl;
 
-    for(std::uint64_t i = 0; i < 10000000L; i++) {
-        auto entity = registry.create<Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
-        if(i == 5000000L) { registry.assign<Position>(entity); }
+    for(std::uint64_t i = 0; i < 1000000L; i++) {
+        registry.create<Position, Velocity, Comp<1>, Comp<2>, Comp<3>, Comp<4>, Comp<5>, Comp<6>, Comp<7>, Comp<8>>();
     }
 
     Timer timer;
@@ -315,13 +298,11 @@ TEST(Benchmark, IterateTenComponentsPersistent10MOne) {
 
 TEST(Benchmark, SortSingle) {
     entt::DefaultRegistry registry;
-    std::vector<entt::DefaultRegistry::entity_type> entities{};
 
     std::cout << "Sort 150000 entities, one component" << std::endl;
 
     for(std::uint64_t i = 0; i < 150000L; i++) {
-        auto entity = registry.create<Position>({ i, i });
-        entities.push_back(entity);
+        registry.create<Position>({ i, i });
     }
 
     Timer timer;
@@ -335,13 +316,11 @@ TEST(Benchmark, SortSingle) {
 
 TEST(Benchmark, SortMulti) {
     entt::DefaultRegistry registry;
-    std::vector<entt::DefaultRegistry::entity_type> entities{};
 
     std::cout << "Sort 150000 entities, two components" << std::endl;
 
     for(std::uint64_t i = 0; i < 150000L; i++) {
-        auto entity = registry.create<Position, Velocity>({ i, i }, { i, i });
-        entities.push_back(entity);
+        registry.create<Position, Velocity>({ i, i }, { i, i });
     }
 
     registry.sort<Position>([](const auto &lhs, const auto &rhs) {

test/entt/core/hashed_string.cpp

@@ -1,7 +1,18 @@
+#include <cstddef>
 #include <gtest/gtest.h>
 #include <entt/core/hashed_string.hpp>
 
-constexpr bool check(const char *str) {
+constexpr bool ptr(const char *str) {
+    using hash_type = entt::HashedString::hash_type;
+
+    return (static_cast<hash_type>(entt::HashedString{str}) == entt::HashedString{str}
+            && static_cast<const char *>(entt::HashedString{str}) == str
+            && entt::HashedString{str} == entt::HashedString{str}
+            && !(entt::HashedString{str} != entt::HashedString{str}));
+}
+
+template<std::size_t N>
+constexpr bool ref(const char (&str)[N]) {
     using hash_type = entt::HashedString::hash_type;
 
     return (static_cast<hash_type>(entt::HashedString{str}) == entt::HashedString{str}
@@ -11,8 +22,9 @@ constexpr bool check(const char *str) {
 }
 
 TEST(HashedString, Constexprness) {
-    // how would you test a constepxr otherwise?
-    static_assert(check("foobar"), "!");
+    // how would you test a constexpr otherwise?
+    static_assert(ptr("foo"), "!");
+    static_assert(ref("bar"), "!");
     ASSERT_TRUE(true);
 }
 
@@ -29,7 +41,7 @@ TEST(HashedString, Functionalities) {
     ASSERT_EQ(static_cast<const char *>(barHs), bar);
 
     ASSERT_TRUE(fooHs == fooHs);
-    ASSERT_FALSE(fooHs == barHs);
+    ASSERT_TRUE(fooHs != barHs);
 
     entt::HashedString hs{"foobar"};
 

test/entt/entity/registry.cpp

@@ -1,5 +1,9 @@
+#include <unordered_map>
+#include <unordered_set>
 #include <functional>
+#include <type_traits>
 #include <gtest/gtest.h>
+#include <entt/entity/entt_traits.hpp>
 #include <entt/entity/registry.hpp>
 
 TEST(DefaultRegistry, Functionalities) {
@@ -7,6 +11,8 @@ TEST(DefaultRegistry, Functionalities) {
 
     ASSERT_EQ(registry.size(), entt::DefaultRegistry::size_type{0});
     ASSERT_NO_THROW(registry.reserve(42));
+    ASSERT_NO_THROW(registry.reserve<int>(8));
+    ASSERT_NO_THROW(registry.reserve<char>(8));
     ASSERT_TRUE(registry.empty());
 
     ASSERT_EQ(registry.capacity(), entt::DefaultRegistry::size_type{0});
@@ -15,11 +21,11 @@ TEST(DefaultRegistry, Functionalities) {
     ASSERT_TRUE(registry.empty<int>());
     ASSERT_TRUE(registry.empty<char>());
 
-    auto e1 = registry.create();
-    auto e2 = registry.create<int, char>();
+    auto e0 = registry.create();
+    auto e1 = registry.create<int, char>();
 
+    ASSERT_TRUE(registry.has<>(e0));
     ASSERT_TRUE(registry.has<>(e1));
-    ASSERT_TRUE(registry.has<>(e2));
 
     ASSERT_EQ(registry.capacity(), entt::DefaultRegistry::size_type{2});
     ASSERT_EQ(registry.size<int>(), entt::DefaultRegistry::size_type{1});
@@ -27,63 +33,70 @@ TEST(DefaultRegistry, Functionalities) {
     ASSERT_FALSE(registry.empty<int>());
     ASSERT_FALSE(registry.empty<char>());
 
-    ASSERT_NE(e1, e2);
-
-    ASSERT_FALSE(registry.has<int>(e1));
-    ASSERT_TRUE(registry.has<int>(e2));
-    ASSERT_FALSE(registry.has<char>(e1));
-    ASSERT_TRUE(registry.has<char>(e2));
-    ASSERT_FALSE((registry.has<int, char>(e1)));
-    ASSERT_TRUE((registry.has<int, char>(e2)));
-
-    ASSERT_EQ(registry.assign<int>(e1, 42), 42);
-    ASSERT_EQ(registry.assign<char>(e1, 'c'), 'c');
-    ASSERT_NO_THROW(registry.remove<int>(e2));
-    ASSERT_NO_THROW(registry.remove<char>(e2));
+    ASSERT_NE(e0, e1);
 
+    ASSERT_FALSE(registry.has<int>(e0));
     ASSERT_TRUE(registry.has<int>(e1));
-    ASSERT_FALSE(registry.has<int>(e2));
+    ASSERT_FALSE(registry.has<char>(e0));
     ASSERT_TRUE(registry.has<char>(e1));
-    ASSERT_FALSE(registry.has<char>(e2));
+    ASSERT_FALSE((registry.has<int, char>(e0)));
     ASSERT_TRUE((registry.has<int, char>(e1)));
-    ASSERT_FALSE((registry.has<int, char>(e2)));
 
-    auto e3 = registry.create();
+    ASSERT_EQ(registry.assign<int>(e0, 42), 42);
+    ASSERT_EQ(registry.assign<char>(e0, 'c'), 'c');
+    ASSERT_NO_THROW(registry.remove<int>(e1));
+    ASSERT_NO_THROW(registry.remove<char>(e1));
 
-    registry.accomodate<int>(e3, registry.get<int>(e1));
-    registry.accomodate<char>(e3, registry.get<char>(e1));
+    ASSERT_TRUE(registry.has<int>(e0));
+    ASSERT_FALSE(registry.has<int>(e1));
+    ASSERT_TRUE(registry.has<char>(e0));
+    ASSERT_FALSE(registry.has<char>(e1));
+    ASSERT_TRUE((registry.has<int, char>(e0)));
+    ASSERT_FALSE((registry.has<int, char>(e1)));
 
-    ASSERT_TRUE(registry.has<int>(e3));
-    ASSERT_TRUE(registry.has<char>(e3));
-    ASSERT_EQ(registry.get<int>(e1), 42);
-    ASSERT_EQ(registry.get<char>(e1), 'c');
-    ASSERT_EQ(registry.get<int>(e1), registry.get<int>(e3));
-    ASSERT_EQ(registry.get<char>(e1), registry.get<char>(e3));
-    ASSERT_NE(&registry.get<int>(e1), &registry.get<int>(e3));
-    ASSERT_NE(&registry.get<char>(e1), &registry.get<char>(e3));
+    auto e2 = registry.create();
 
-    ASSERT_NO_THROW(registry.replace<int>(e1, 0));
-    ASSERT_EQ(registry.get<int>(e1), 0);
+    registry.accommodate<int>(e2, registry.get<int>(e0));
+    registry.accommodate<char>(e2, registry.get<char>(e0));
 
-    ASSERT_NO_THROW(registry.accomodate<int>(e1, 1));
-    ASSERT_NO_THROW(registry.accomodate<int>(e2, 1));
+    ASSERT_TRUE(registry.has<int>(e2));
+    ASSERT_TRUE(registry.has<char>(e2));
+    ASSERT_EQ(registry.get<int>(e0), 42);
+    ASSERT_EQ(registry.get<char>(e0), 'c');
+
+    ASSERT_EQ(std::get<0>(registry.get<int, char>(e0)), 42);
+    ASSERT_EQ(std::get<1>(static_cast<const entt::DefaultRegistry &>(registry).get<int, char>(e0)), 'c');
+
+    ASSERT_EQ(registry.get<int>(e0), registry.get<int>(e2));
+    ASSERT_EQ(registry.get<char>(e0), registry.get<char>(e2));
+    ASSERT_NE(&registry.get<int>(e0), &registry.get<int>(e2));
+    ASSERT_NE(&registry.get<char>(e0), &registry.get<char>(e2));
+
+    ASSERT_NO_THROW(registry.replace<int>(e0, 0));
+    ASSERT_EQ(registry.get<int>(e0), 0);
+
+    ASSERT_NO_THROW(registry.accommodate<int>(e0, 1));
+    ASSERT_NO_THROW(registry.accommodate<int>(e1, 1));
+    ASSERT_EQ(static_cast<const entt::DefaultRegistry &>(registry).get<int>(e0), 1);
     ASSERT_EQ(static_cast<const entt::DefaultRegistry &>(registry).get<int>(e1), 1);
-    ASSERT_EQ(static_cast<const entt::DefaultRegistry &>(registry).get<int>(e2), 1);
 
     ASSERT_EQ(registry.size(), entt::DefaultRegistry::size_type{3});
     ASSERT_FALSE(registry.empty());
 
-    ASSERT_EQ(registry.version(e3), entt::DefaultRegistry::version_type{0});
-    ASSERT_EQ(registry.current(e3), entt::DefaultRegistry::version_type{0});
+    ASSERT_EQ(registry.version(e2), entt::DefaultRegistry::version_type{0});
+    ASSERT_EQ(registry.current(e2), entt::DefaultRegistry::version_type{0});
     ASSERT_EQ(registry.capacity(), entt::DefaultRegistry::size_type{3});
-    ASSERT_NO_THROW(registry.destroy(e3));
+    ASSERT_NO_THROW(registry.destroy(e2));
     ASSERT_EQ(registry.capacity(), entt::DefaultRegistry::size_type{3});
-    ASSERT_EQ(registry.version(e3), entt::DefaultRegistry::version_type{0});
-    ASSERT_EQ(registry.current(e3), entt::DefaultRegistry::version_type{1});
+    ASSERT_EQ(registry.version(e2), entt::DefaultRegistry::version_type{0});
+    ASSERT_EQ(registry.current(e2), entt::DefaultRegistry::version_type{1});
 
+    ASSERT_TRUE(registry.valid(e0));
+    ASSERT_TRUE(registry.fast(e0));
     ASSERT_TRUE(registry.valid(e1));
-    ASSERT_TRUE(registry.valid(e2));
-    ASSERT_FALSE(registry.valid(e3));
+    ASSERT_TRUE(registry.fast(e1));
+    ASSERT_FALSE(registry.valid(e2));
+    ASSERT_FALSE(registry.fast(e2));
 
     ASSERT_EQ(registry.size(), entt::DefaultRegistry::size_type{2});
     ASSERT_FALSE(registry.empty());
@@ -114,24 +127,58 @@ TEST(DefaultRegistry, Functionalities) {
     ASSERT_TRUE(registry.empty<int>());
     ASSERT_TRUE(registry.empty<char>());
 
-    e1 = registry.create<int>();
-    e2 = registry.create();
+    e0 = registry.create<int>();
+    e1 = registry.create();
 
+    ASSERT_NO_THROW(registry.reset<int>(e0));
     ASSERT_NO_THROW(registry.reset<int>(e1));
-    ASSERT_NO_THROW(registry.reset<int>(e2));
 
     ASSERT_EQ(registry.size<int>(), entt::DefaultRegistry::size_type{0});
     ASSERT_EQ(registry.size<char>(), entt::DefaultRegistry::size_type{0});
     ASSERT_TRUE(registry.empty<int>());
 }
 
+TEST(DefaultRegistry, CreateDestroyCornerCase) {
+    entt::DefaultRegistry registry;
+
+    auto e0 = registry.create();
+    auto e1 = registry.create();
+
+    registry.destroy(e0);
+    registry.destroy(e1);
+
+    registry.each([](auto) { FAIL(); });
+
+    ASSERT_EQ(registry.current(e0), entt::DefaultRegistry::version_type{1});
+    ASSERT_EQ(registry.current(e1), entt::DefaultRegistry::version_type{1});
+}
+
+TEST(DefaultRegistry, VersionOverflow) {
+    entt::DefaultRegistry registry;
+
+    auto entity = registry.create();
+    registry.destroy(entity);
+
+    ASSERT_EQ(registry.version(entity), entt::DefaultRegistry::version_type{});
+
+    for(auto i = entt::entt_traits<entt::DefaultRegistry::entity_type>::version_mask; i; --i) {
+        ASSERT_NE(registry.current(entity), registry.version(entity));
+        registry.destroy(registry.create());
+    }
+
+    ASSERT_EQ(registry.current(entity), registry.version(entity));
+}
+
 TEST(DefaultRegistry, Each) {
     entt::DefaultRegistry registry;
     entt::DefaultRegistry::size_type tot;
     entt::DefaultRegistry::size_type match;
 
+    registry.create();
     registry.create<int>();
+    registry.create();
     registry.create<int>();
+    registry.create();
 
     tot = 0u;
     match = 0u;
@@ -142,7 +189,22 @@ TEST(DefaultRegistry, Each) {
         ++tot;
     });
 
-    ASSERT_EQ(tot, 2u);
+    ASSERT_EQ(tot, 5u);
+    ASSERT_EQ(match, 2u);
+
+    tot = 0u;
+    match = 0u;
+
+    registry.each([&](auto entity) {
+        if(registry.has<int>(entity)) {
+            registry.destroy(entity);
+            ++match;
+        }
+
+        ++tot;
+    });
+
+    ASSERT_EQ(tot, 10u);
     ASSERT_EQ(match, 2u);
 
     tot = 0u;
@@ -154,21 +216,35 @@ TEST(DefaultRegistry, Each) {
         ++tot;
     });
 
-    ASSERT_EQ(tot, 4u);
-    ASSERT_EQ(match, 2u);
-
-    tot = 0u;
-    match = 0u;
-
-    registry.each([&](auto entity) {
-        if(registry.has<int>(entity)) { ++match; }
-        ++tot;
-    });
-
-    ASSERT_EQ(tot, 4u);
+    ASSERT_EQ(tot, 8u);
     ASSERT_EQ(match, 0u);
+
+    registry.each([&](auto) { FAIL(); });
 }
 
+TEST(DefaultRegistry, Orphans) {
+    entt::DefaultRegistry registry;
+    entt::DefaultRegistry::size_type tot{};
+
+    registry.create<int>();
+    registry.create();
+    registry.create<int>();
+    registry.create();
+    registry.attach<double>(registry.create());
+
+    registry.orphans([&](auto) { ++tot; });
+    ASSERT_EQ(tot, 2u);
+    tot = 0u;
+
+    registry.each([&](auto entity) { registry.reset<int>(entity); });
+    registry.orphans([&](auto) { ++tot; });
+    ASSERT_EQ(tot, 4u);
+    registry.reset();
+    tot = 0u;
+
+    registry.orphans([&](auto) { ++tot; });
+    ASSERT_EQ(tot, 0u);
+}
 
 TEST(DefaultRegistry, Types) {
     entt::DefaultRegistry registry;
@@ -182,18 +258,34 @@ TEST(DefaultRegistry, Types) {
 
 TEST(DefaultRegistry, CreateDestroyEntities) {
     entt::DefaultRegistry registry;
+    entt::DefaultRegistry::entity_type pre{}, post{};
 
-    auto pre = registry.create<double>();
-    registry.destroy(pre);
-    auto post = registry.create<double>();
+    for(int i = 0; i < 10; ++i) {
+        registry.create<double>();
+    }
+
+    registry.reset();
+
+    for(int i = 0; i < 7; ++i) {
+        auto entity = registry.create<int>();
+        if(i == 3) { pre = entity; }
+    }
+
+    registry.reset();
+
+    for(int i = 0; i < 5; ++i) {
+        auto entity = registry.create();
+        if(i == 3) { post = entity; }
+    }
 
     ASSERT_FALSE(registry.valid(pre));
     ASSERT_TRUE(registry.valid(post));
     ASSERT_NE(registry.version(pre), registry.version(post));
+    ASSERT_EQ(registry.version(pre) + 1, registry.version(post));
     ASSERT_EQ(registry.current(pre), registry.current(post));
 }
 
-TEST(DefaultRegistry, AttachRemoveTags) {
+TEST(DefaultRegistry, AttachSetRemoveTags) {
     entt::DefaultRegistry registry;
     const auto &cregistry = registry;
 
@@ -207,6 +299,21 @@ TEST(DefaultRegistry, AttachRemoveTags) {
     ASSERT_EQ(cregistry.get<int>(), 42);
     ASSERT_EQ(registry.attachee<int>(), entity);
 
+    registry.set<int>(3);
+
+    ASSERT_TRUE(registry.has<int>());
+    ASSERT_EQ(registry.get<int>(), 3);
+    ASSERT_EQ(cregistry.get<int>(), 3);
+    ASSERT_EQ(registry.attachee<int>(), entity);
+
+    auto other = registry.create();
+    registry.move<int>(other);
+
+    ASSERT_TRUE(registry.has<int>());
+    ASSERT_EQ(registry.get<int>(), 3);
+    ASSERT_EQ(cregistry.get<int>(), 3);
+    ASSERT_EQ(registry.attachee<int>(), other);
+
     registry.remove<int>();
 
     ASSERT_FALSE(registry.has<int>());
@@ -346,3 +453,58 @@ TEST(DefaultRegistry, SortMulti) {
         ASSERT_EQ(registry.get<int>(entity), ival++);
     }
 }
+
+TEST(DefaultRegistry, ComponentsWithTypesFromStandardTemplateLibrary) {
+    // see #37 - the test shouldn't crash, that's all
+    entt::DefaultRegistry registry;
+    auto entity = registry.create();
+    registry.assign<std::unordered_set<int>>(entity).insert(42);
+    registry.destroy(entity);
+}
+
+TEST(DefaultRegistry, ConstructWithComponents) {
+    // it should compile, that's all
+    entt::DefaultRegistry registry;
+    const auto value = 0;
+    registry.create(value);
+}
+
+TEST(DefaultRegistry, MergeTwoRegistries) {
+    using entity_type = entt::DefaultRegistry::entity_type;
+
+    entt::DefaultRegistry src;
+    entt::DefaultRegistry dst;
+
+    std::unordered_map<entity_type, entity_type> ref;
+
+    auto merge = [&ref](const auto &view, auto &dst) {
+        view.each([&](auto entity, const auto &component) {
+            if(ref.find(entity) == ref.cend()) {
+                ref.emplace(entity, dst.create(component));
+            } else {
+                using component_type = std::decay_t<decltype(component)>;
+                dst.template assign<component_type>(ref[entity], component);
+            }
+        });
+    };
+
+    src.create<int, float, double>();
+    src.create<char, float, int>();
+
+    dst.create<int, char, double>();
+    dst.create<float, int>();
+
+    auto eq = [](auto begin, auto end) { ASSERT_EQ(begin, end); };
+    auto ne = [](auto begin, auto end) { ASSERT_NE(begin, end); };
+
+    eq(dst.view<int, float, double>().begin(), dst.view<int, float, double>().end());
+    eq(dst.view<char, float, int>().begin(), dst.view<char, float, int>().end());
+
+    merge(src.view<int>(), dst);
+    merge(src.view<char>(), dst);
+    merge(src.view<double>(), dst);
+    merge(src.view<float>(), dst);
+
+    ne(dst.view<int, float, double>().begin(), dst.view<int, float, double>().end());
+    ne(dst.view<char, float, int>().begin(), dst.view<char, float, int>().end());
+}

test/entt/entity/snapshot.cpp

@@ -0,0 +1,489 @@
+#include <tuple>
+#include <queue>
+#include <vector>
+#include <gtest/gtest.h>
+#include <entt/entity/registry.hpp>
+
+template<typename Storage>
+struct OutputArchive {
+    OutputArchive(Storage &storage)
+        : storage{storage}
+    {}
+
+    template<typename Value>
+    void operator()(const Value &value) {
+        std::get<std::queue<Value>>(storage).push(value);
+    }
+
+private:
+    Storage &storage;
+};
+
+template<typename Storage>
+struct InputArchive {
+    InputArchive(Storage &storage)
+        : storage{storage}
+    {}
+
+    template<typename Value>
+    void operator()(Value &value) {
+        auto &queue = std::get<std::queue<Value>>(storage);
+        value = queue.front();
+        queue.pop();
+    }
+
+private:
+    Storage &storage;
+};
+
+struct AComponent {};
+
+struct AnotherComponent {
+    int key;
+    int value;
+};
+
+struct Foo {
+    entt::DefaultRegistry::entity_type bar;
+    std::vector<entt::DefaultRegistry::entity_type> quux;
+};
+
+TEST(Snapshot, Dump) {
+    entt::DefaultRegistry registry;
+
+    auto e0 = registry.create();
+    registry.assign<int>(e0, 42);
+    registry.assign<char>(e0, 'c');
+    registry.assign<double>(e0, .1);
+
+    auto e1 = registry.create();
+
+    auto e2 = registry.create();
+    registry.assign<int>(e2, 3);
+
+    auto e3 = registry.create();
+    registry.assign<char>(e3, '0');
+    registry.attach<float>(e3, .3f);
+
+    auto e4 = registry.create();
+    registry.attach<AComponent>(e4);
+
+    registry.destroy(e1);
+    auto v1 = registry.current(e1);
+
+    using storage_type = std::tuple<
+        std::queue<entt::DefaultRegistry::entity_type>,
+        std::queue<int>,
+        std::queue<char>,
+        std::queue<double>,
+        std::queue<float>,
+        std::queue<bool>,
+        std::queue<AComponent>,
+        std::queue<AnotherComponent>,
+        std::queue<Foo>
+    >;
+
+    storage_type storage;
+    OutputArchive<storage_type> output{storage};
+    InputArchive<storage_type> input{storage};
+
+    registry.snapshot()
+            .entities(output)
+            .destroyed(output)
+            .component<int, char, AnotherComponent, double>(output)
+            .tag<float, bool, AComponent>(output);
+
+    registry.reset();
+
+    ASSERT_FALSE(registry.valid(e0));
+    ASSERT_FALSE(registry.valid(e1));
+    ASSERT_FALSE(registry.valid(e2));
+    ASSERT_FALSE(registry.valid(e3));
+    ASSERT_FALSE(registry.valid(e4));
+
+    registry.restore()
+            .entities(input)
+            .destroyed(input)
+            .component<int, char, AnotherComponent, double>(input)
+            .tag<float, bool, AComponent>(input)
+            .orphans();
+
+    ASSERT_TRUE(registry.valid(e0));
+    ASSERT_FALSE(registry.valid(e1));
+    ASSERT_TRUE(registry.valid(e2));
+    ASSERT_TRUE(registry.valid(e3));
+    ASSERT_TRUE(registry.valid(e4));
+
+    ASSERT_FALSE(registry.orphan(e0));
+    ASSERT_FALSE(registry.orphan(e2));
+    ASSERT_FALSE(registry.orphan(e3));
+    ASSERT_FALSE(registry.orphan(e4));
+
+    ASSERT_EQ(registry.get<int>(e0), 42);
+    ASSERT_EQ(registry.get<char>(e0), 'c');
+    ASSERT_EQ(registry.get<double>(e0), .1);
+    ASSERT_EQ(registry.current(e1), v1);
+    ASSERT_EQ(registry.get<int>(e2), 3);
+    ASSERT_EQ(registry.get<char>(e3), '0');
+
+    ASSERT_TRUE(registry.has<float>());
+    ASSERT_EQ(registry.attachee<float>(), e3);
+    ASSERT_EQ(registry.get<float>(), .3f);
+
+    ASSERT_TRUE(registry.has<AComponent>());
+    ASSERT_EQ(registry.attachee<AComponent>(), e4);
+
+    ASSERT_TRUE(registry.empty<AnotherComponent>());
+    ASSERT_FALSE(registry.has<long int>());
+}
+
+TEST(Snapshot, Partial) {
+    entt::DefaultRegistry registry;
+
+    auto e0 = registry.create();
+    registry.assign<int>(e0, 42);
+    registry.assign<char>(e0, 'c');
+    registry.assign<double>(e0, .1);
+
+    auto e1 = registry.create();
+
+    auto e2 = registry.create();
+    registry.assign<int>(e2, 3);
+
+    auto e3 = registry.create();
+    registry.assign<char>(e3, '0');
+    registry.attach<float>(e3, .3f);
+
+    auto e4 = registry.create();
+    registry.attach<AComponent>(e4);
+
+    registry.destroy(e1);
+    auto v1 = registry.current(e1);
+
+    using storage_type = std::tuple<
+        std::queue<entt::DefaultRegistry::entity_type>,
+        std::queue<int>,
+        std::queue<char>,
+        std::queue<double>,
+        std::queue<float>,
+        std::queue<bool>,
+        std::queue<AComponent>,
+        std::queue<Foo>
+    >;
+
+    storage_type storage;
+    OutputArchive<storage_type> output{storage};
+    InputArchive<storage_type> input{storage};
+
+    registry.snapshot()
+            .entities(output)
+            .destroyed(output)
+            .component<char, int>(output)
+            .tag<bool, float>(output);
+
+    registry.reset();
+
+    ASSERT_FALSE(registry.valid(e0));
+    ASSERT_FALSE(registry.valid(e1));
+    ASSERT_FALSE(registry.valid(e2));
+    ASSERT_FALSE(registry.valid(e3));
+    ASSERT_FALSE(registry.valid(e4));
+
+    registry.restore()
+            .entities(input)
+            .destroyed(input)
+            .component<char, int>(input)
+            .tag<bool, float>(input);
+
+    ASSERT_TRUE(registry.valid(e0));
+    ASSERT_FALSE(registry.valid(e1));
+    ASSERT_TRUE(registry.valid(e2));
+    ASSERT_TRUE(registry.valid(e3));
+    ASSERT_TRUE(registry.valid(e4));
+
+    ASSERT_EQ(registry.get<int>(e0), 42);
+    ASSERT_EQ(registry.get<char>(e0), 'c');
+    ASSERT_FALSE(registry.has<double>(e0));
+    ASSERT_EQ(registry.current(e1), v1);
+    ASSERT_EQ(registry.get<int>(e2), 3);
+    ASSERT_EQ(registry.get<char>(e3), '0');
+    ASSERT_TRUE(registry.orphan(e4));
+
+    ASSERT_TRUE(registry.has<float>());
+    ASSERT_EQ(registry.attachee<float>(), e3);
+    ASSERT_EQ(registry.get<float>(), .3f);
+    ASSERT_FALSE(registry.has<long int>());
+
+    registry.snapshot()
+            .tag<float>(output)
+            .destroyed(output)
+            .entities(output);
+
+    registry.reset();
+
+    ASSERT_FALSE(registry.valid(e0));
+    ASSERT_FALSE(registry.valid(e1));
+    ASSERT_FALSE(registry.valid(e2));
+    ASSERT_FALSE(registry.valid(e3));
+    ASSERT_FALSE(registry.valid(e4));
+
+    registry.restore()
+            .tag<float>(input)
+            .destroyed(input)
+            .entities(input)
+            .orphans();
+
+    ASSERT_FALSE(registry.valid(e0));
+    ASSERT_FALSE(registry.valid(e1));
+    ASSERT_FALSE(registry.valid(e2));
+    ASSERT_TRUE(registry.valid(e3));
+    ASSERT_FALSE(registry.valid(e4));
+}
+
+TEST(Snapshot, Continuous) {
+    using entity_type = entt::DefaultRegistry::entity_type;
+
+    entt::DefaultRegistry src;
+    entt::DefaultRegistry dst;
+
+    entt::ContinuousLoader<entity_type> loader{dst};
+
+    std::vector<entity_type> entities;
+    entity_type entity;
+
+    using storage_type = std::tuple<
+        std::queue<entity_type>,
+        std::queue<AComponent>,
+        std::queue<AnotherComponent>,
+        std::queue<Foo>,
+        std::queue<double>
+    >;
+
+    storage_type storage;
+    OutputArchive<storage_type> output{storage};
+    InputArchive<storage_type> input{storage};
+
+    for(int i = 0; i < 10; ++i) {
+        src.create();
+    }
+
+    src.each([&src](auto entity) {
+        src.destroy(entity);
+    });
+
+    for(int i = 0; i < 5; ++i) {
+        entity = src.create();
+        entities.push_back(entity);
+
+        src.assign<AComponent>(entity);
+        src.assign<AnotherComponent>(entity, i, i);
+
+        if(i % 2) {
+            src.assign<Foo>(entity, entity);
+        } else if(i == 2) {
+            src.attach<double>(entity, .3);
+        }
+    }
+
+    src.view<Foo>().each([&entities](auto, auto &foo) {
+        foo.quux.insert(foo.quux.begin(), entities.begin(), entities.end());
+    });
+
+    entity = dst.create();
+    dst.assign<AComponent>(entity);
+    dst.assign<AnotherComponent>(entity, -1, -1);
+
+    src.snapshot()
+            .entities(output)
+            .destroyed(output)
+            .component<AComponent, AnotherComponent, Foo>(output)
+            .tag<double>(output);
+
+    loader.entities(input)
+            .destroyed(input)
+            .component<AComponent, AnotherComponent>(input)
+            .component<Foo>(input, &Foo::bar, &Foo::quux)
+            .tag<double>(input)
+            .orphans();
+
+    decltype(dst.size()) aComponentCnt{};
+    decltype(dst.size()) anotherComponentCnt{};
+    decltype(dst.size()) fooCnt{};
+
+    dst.each([&dst, &aComponentCnt](auto entity) {
+        ASSERT_TRUE(dst.has<AComponent>(entity));
+        ++aComponentCnt;
+    });
+
+    dst.view<AnotherComponent>().each([&anotherComponentCnt](auto, const auto &component) {
+        ASSERT_EQ(component.value, component.key < 0 ? -1 : component.key);
+        ++anotherComponentCnt;
+    });
+
+    dst.view<Foo>().each([&dst, &fooCnt](auto entity, const auto &component) {
+        ASSERT_EQ(entity, component.bar);
+
+        for(auto entity: component.quux) {
+            ASSERT_TRUE(dst.valid(entity));
+        }
+
+        ++fooCnt;
+    });
+
+    ASSERT_TRUE(dst.has<double>());
+    ASSERT_EQ(dst.get<double>(), .3);
+
+    src.view<AnotherComponent>().each([](auto, auto &component) {
+        component.value = 2 * component.key;
+    });
+
+    auto size = dst.size();
+
+    src.snapshot()
+            .entities(output)
+            .destroyed(output)
+            .component<AComponent, AnotherComponent, Foo>(output)
+            .tag<double>(output);
+
+    loader.entities(input)
+            .destroyed(input)
+            .component<AComponent, AnotherComponent>(input)
+            .component<Foo>(input, &Foo::bar, &Foo::quux)
+            .tag<double>(input)
+            .orphans();
+
+    ASSERT_EQ(size, dst.size());
+
+    ASSERT_EQ(dst.size<AComponent>(), aComponentCnt);
+    ASSERT_EQ(dst.size<AnotherComponent>(), anotherComponentCnt);
+    ASSERT_EQ(dst.size<Foo>(), fooCnt);
+    ASSERT_TRUE(dst.has<double>());
+
+    dst.view<AnotherComponent>().each([](auto, auto &component) {
+        ASSERT_EQ(component.value, component.key < 0 ? -1 : (2 * component.key));
+    });
+
+    entity = src.create();
+
+    src.view<Foo>().each([entity](auto, auto &component) {
+        component.bar = entity;
+    });
+
+    src.snapshot()
+            .entities(output)
+            .destroyed(output)
+            .component<AComponent, AnotherComponent, Foo>(output)
+            .tag<double>(output);
+
+    loader.entities(input)
+            .destroyed(input)
+            .component<AComponent, AnotherComponent>(input)
+            .component<Foo>(input, &Foo::bar, &Foo::quux)
+            .tag<double>(input)
+            .orphans();
+
+    dst.view<Foo>().each([&loader, entity](auto, auto &component) {
+        ASSERT_EQ(component.bar, loader.map(entity));
+    });
+
+    entities.clear();
+    for(auto entity: src.view<AComponent>()) {
+        entities.push_back(entity);
+    }
+
+    src.destroy(entity);
+    loader.shrink();
+
+    src.snapshot()
+            .entities(output)
+            .destroyed(output)
+            .component<AComponent, AnotherComponent, Foo>(output)
+            .tag<double>(output);
+
+    loader.entities(input)
+            .destroyed(input)
+            .component<AComponent, AnotherComponent>(input)
+            .component<Foo>(input, &Foo::bar, &Foo::quux)
+            .tag<double>(input)
+            .orphans()
+            .shrink();
+
+    dst.view<Foo>().each([&dst, &loader, entity](auto, auto &component) {
+        ASSERT_FALSE(dst.valid(component.bar));
+    });
+
+    ASSERT_FALSE(loader.has(entity));
+
+    entity = src.create();
+
+    src.view<Foo>().each([entity](auto, auto &component) {
+        component.bar = entity;
+    });
+
+    dst.reset<AComponent>();
+    aComponentCnt = src.size<AComponent>();
+
+    src.snapshot()
+            .entities(output)
+            .destroyed(output)
+            .component<AComponent, AnotherComponent, Foo>(output)
+            .tag<double>(output);
+
+    loader.entities(input)
+            .destroyed(input)
+            .component<AComponent, AnotherComponent>(input)
+            .component<Foo>(input, &Foo::bar, &Foo::quux)
+            .tag<double>(input)
+            .orphans();
+
+    ASSERT_EQ(dst.size<AComponent>(), aComponentCnt);
+    ASSERT_TRUE(dst.has<double>());
+
+    src.reset<AComponent>();
+    src.remove<double>();
+    aComponentCnt = {};
+
+    src.snapshot()
+            .entities(output)
+            .destroyed(output)
+            .component<AComponent, AnotherComponent, Foo>(output)
+            .tag<double>(output);
+
+    loader.entities(input)
+            .destroyed(input)
+            .component<AComponent, AnotherComponent>(input)
+            .component<Foo>(input, &Foo::bar, &Foo::quux)
+            .tag<double>(input)
+            .orphans();
+
+    ASSERT_EQ(dst.size<AComponent>(), aComponentCnt);
+    ASSERT_FALSE(dst.has<double>());
+}
+
+TEST(Snapshot, ContinuousMoreOnShrink) {
+    using entity_type = entt::DefaultRegistry::entity_type;
+
+    entt::DefaultRegistry src;
+    entt::DefaultRegistry dst;
+
+    entt::ContinuousLoader<entity_type> loader{dst};
+
+    using storage_type = std::tuple<
+        std::queue<entity_type>,
+        std::queue<AComponent>
+    >;
+
+    storage_type storage;
+    OutputArchive<storage_type> output{storage};
+    InputArchive<storage_type> input{storage};
+
+    auto entity = src.create();
+    src.snapshot().entities(output);
+    loader.entities(input).shrink();
+
+    ASSERT_TRUE(dst.valid(entity));
+
+    loader.shrink();
+
+    ASSERT_FALSE(dst.valid(entity));
+}

test/entt/entity/sparse_set.cpp

@@ -1,3 +1,4 @@
+#include <unordered_set>
 #include <gtest/gtest.h>
 #include <entt/entity/sparse_set.hpp>
 
@@ -20,6 +21,7 @@ TEST(SparseSetNoType, Functionalities) {
     ASSERT_NE(set.begin(), set.end());
     ASSERT_FALSE(set.has(0));
     ASSERT_TRUE(set.has(42));
+    ASSERT_TRUE(set.fast(42));
     ASSERT_EQ(set.get(42), 0u);
 
     set.destroy(42);
@@ -62,19 +64,183 @@ TEST(SparseSetNoType, DataBeginEnd) {
     ASSERT_EQ(*(set.data() + 1u), 12u);
     ASSERT_EQ(*(set.data() + 2u), 42u);
 
+    auto it = set.begin();
+
+    ASSERT_EQ(*it, 42u);
+    ASSERT_EQ(*(it+1), 12u);
+    ASSERT_EQ(*(it+2), 3u);
+    ASSERT_EQ(it += 3, set.end());
+
     auto begin = set.begin();
     auto end = set.end();
 
     ASSERT_EQ(*(begin++), 42u);
     ASSERT_EQ(*(begin++), 12u);
     ASSERT_EQ(*(begin++), 3u);
+
     ASSERT_EQ(begin, end);
 }
 
-TEST(SparseSetWithType, AggregatesMustWork) {
-    struct AggregateType { int value; };
-    // the goal of this test is to enforce the requirements for aggregate types
-    entt::SparseSet<unsigned int, AggregateType>{}.construct(0, 42);
+TEST(SparseSetNoType, RespectDisjoint) {
+    entt::SparseSet<unsigned int> lhs;
+    entt::SparseSet<unsigned int> rhs;
+    const auto &clhs = lhs;
+
+    lhs.construct(3);
+    lhs.construct(12);
+    lhs.construct(42);
+
+    ASSERT_EQ(lhs.get(3), 0u);
+    ASSERT_EQ(lhs.get(12), 1u);
+    ASSERT_EQ(lhs.get(42), 2u);
+
+    lhs.respect(rhs);
+
+    ASSERT_EQ(clhs.get(3), 0u);
+    ASSERT_EQ(clhs.get(12), 1u);
+    ASSERT_EQ(clhs.get(42), 2u);
+}
+
+TEST(SparseSetNoType, RespectOverlap) {
+    entt::SparseSet<unsigned int> lhs;
+    entt::SparseSet<unsigned int> rhs;
+    const auto &clhs = lhs;
+
+    lhs.construct(3);
+    lhs.construct(12);
+    lhs.construct(42);
+
+    rhs.construct(12);
+
+    ASSERT_EQ(lhs.get(3), 0u);
+    ASSERT_EQ(lhs.get(12), 1u);
+    ASSERT_EQ(lhs.get(42), 2u);
+
+    lhs.respect(rhs);
+
+    ASSERT_EQ(clhs.get(3), 0u);
+    ASSERT_EQ(clhs.get(12), 2u);
+    ASSERT_EQ(clhs.get(42), 1u);
+}
+
+TEST(SparseSetNoType, RespectOrdered) {
+    entt::SparseSet<unsigned int> lhs;
+    entt::SparseSet<unsigned int> rhs;
+
+    lhs.construct(1);
+    lhs.construct(2);
+    lhs.construct(3);
+    lhs.construct(4);
+    lhs.construct(5);
+
+    ASSERT_EQ(lhs.get(1), 0u);
+    ASSERT_EQ(lhs.get(2), 1u);
+    ASSERT_EQ(lhs.get(3), 2u);
+    ASSERT_EQ(lhs.get(4), 3u);
+    ASSERT_EQ(lhs.get(5), 4u);
+
+    rhs.construct(6);
+    rhs.construct(1);
+    rhs.construct(2);
+    rhs.construct(3);
+    rhs.construct(4);
+    rhs.construct(5);
+
+    ASSERT_EQ(rhs.get(6), 0u);
+    ASSERT_EQ(rhs.get(1), 1u);
+    ASSERT_EQ(rhs.get(2), 2u);
+    ASSERT_EQ(rhs.get(3), 3u);
+    ASSERT_EQ(rhs.get(4), 4u);
+    ASSERT_EQ(rhs.get(5), 5u);
+
+    rhs.respect(lhs);
+
+    ASSERT_EQ(rhs.get(6), 0u);
+    ASSERT_EQ(rhs.get(1), 1u);
+    ASSERT_EQ(rhs.get(2), 2u);
+    ASSERT_EQ(rhs.get(3), 3u);
+    ASSERT_EQ(rhs.get(4), 4u);
+    ASSERT_EQ(rhs.get(5), 5u);
+}
+
+TEST(SparseSetNoType, RespectReverse) {
+    entt::SparseSet<unsigned int> lhs;
+    entt::SparseSet<unsigned int> rhs;
+
+    lhs.construct(1);
+    lhs.construct(2);
+    lhs.construct(3);
+    lhs.construct(4);
+    lhs.construct(5);
+
+    ASSERT_EQ(lhs.get(1), 0u);
+    ASSERT_EQ(lhs.get(2), 1u);
+    ASSERT_EQ(lhs.get(3), 2u);
+    ASSERT_EQ(lhs.get(4), 3u);
+    ASSERT_EQ(lhs.get(5), 4u);
+
+    rhs.construct(5);
+    rhs.construct(4);
+    rhs.construct(3);
+    rhs.construct(2);
+    rhs.construct(1);
+    rhs.construct(6);
+
+    ASSERT_EQ(rhs.get(5), 0u);
+    ASSERT_EQ(rhs.get(4), 1u);
+    ASSERT_EQ(rhs.get(3), 2u);
+    ASSERT_EQ(rhs.get(2), 3u);
+    ASSERT_EQ(rhs.get(1), 4u);
+    ASSERT_EQ(rhs.get(6), 5u);
+
+    rhs.respect(lhs);
+
+    ASSERT_EQ(rhs.get(6), 0u);
+    ASSERT_EQ(rhs.get(1), 1u);
+    ASSERT_EQ(rhs.get(2), 2u);
+    ASSERT_EQ(rhs.get(3), 3u);
+    ASSERT_EQ(rhs.get(4), 4u);
+    ASSERT_EQ(rhs.get(5), 5u);
+}
+
+TEST(SparseSetNoType, RespectUnordered) {
+    entt::SparseSet<unsigned int> lhs;
+    entt::SparseSet<unsigned int> rhs;
+
+    lhs.construct(1);
+    lhs.construct(2);
+    lhs.construct(3);
+    lhs.construct(4);
+    lhs.construct(5);
+
+    ASSERT_EQ(lhs.get(1), 0u);
+    ASSERT_EQ(lhs.get(2), 1u);
+    ASSERT_EQ(lhs.get(3), 2u);
+    ASSERT_EQ(lhs.get(4), 3u);
+    ASSERT_EQ(lhs.get(5), 4u);
+
+    rhs.construct(3);
+    rhs.construct(2);
+    rhs.construct(6);
+    rhs.construct(1);
+    rhs.construct(4);
+    rhs.construct(5);
+
+    ASSERT_EQ(rhs.get(3), 0u);
+    ASSERT_EQ(rhs.get(2), 1u);
+    ASSERT_EQ(rhs.get(6), 2u);
+    ASSERT_EQ(rhs.get(1), 3u);
+    ASSERT_EQ(rhs.get(4), 4u);
+    ASSERT_EQ(rhs.get(5), 5u);
+
+    rhs.respect(lhs);
+
+    ASSERT_EQ(rhs.get(6), 0u);
+    ASSERT_EQ(rhs.get(1), 1u);
+    ASSERT_EQ(rhs.get(2), 2u);
+    ASSERT_EQ(rhs.get(3), 3u);
+    ASSERT_EQ(rhs.get(4), 4u);
+    ASSERT_EQ(rhs.get(5), 5u);
 }
 
 TEST(SparseSetWithType, Functionalities) {
@@ -95,6 +261,7 @@ TEST(SparseSetWithType, Functionalities) {
     ASSERT_NE(set.begin(), set.end());
     ASSERT_FALSE(set.has(0));
     ASSERT_TRUE(set.has(42));
+    ASSERT_TRUE(set.fast(42));
     ASSERT_EQ(set.get(42), 3);
 
     set.destroy(42);
@@ -122,6 +289,19 @@ TEST(SparseSetWithType, Functionalities) {
     other = std::move(set);
 }
 
+TEST(SparseSetWithType, AggregatesMustWork) {
+    struct AggregateType { int value; };
+    // the goal of this test is to enforce the requirements for aggregate types
+    entt::SparseSet<unsigned int, AggregateType>{}.construct(0, 42);
+}
+
+TEST(SparseSetWithType, TypesFromStandardTemplateLibraryMustWork) {
+    // see #37 - this test shouldn't crash, that's all
+    entt::SparseSet<unsigned int, std::unordered_set<int>> set;
+    set.construct(0).insert(42);
+    set.destroy(0);
+}
+
 TEST(SparseSetWithType, RawBeginEnd) {
     entt::SparseSet<unsigned int, int> set;
 
@@ -140,9 +320,9 @@ TEST(SparseSetWithType, RawBeginEnd) {
     auto begin = set.begin();
     auto end = set.end();
 
-    ASSERT_EQ(set.get(*(begin++)), 9);
-    ASSERT_EQ(set.get(*(begin++)), 6);
-    ASSERT_EQ(set.get(*(begin++)), 3);
+    ASSERT_EQ(*(begin++), 9);
+    ASSERT_EQ(*(begin++), 6);
+    ASSERT_EQ(*(begin++), 3);
     ASSERT_EQ(begin, end);
 }
 
@@ -174,11 +354,11 @@ TEST(SparseSetWithType, SortOrdered) {
     auto begin = set.begin();
     auto end = set.end();
 
-    ASSERT_EQ(set.get(*(begin++)), 1);
-    ASSERT_EQ(set.get(*(begin++)), 3);
-    ASSERT_EQ(set.get(*(begin++)), 6);
-    ASSERT_EQ(set.get(*(begin++)), 9);
-    ASSERT_EQ(set.get(*(begin++)), 12);
+    ASSERT_EQ(*(begin++), 1);
+    ASSERT_EQ(*(begin++), 3);
+    ASSERT_EQ(*(begin++), 6);
+    ASSERT_EQ(*(begin++), 9);
+    ASSERT_EQ(*(begin++), 12);
     ASSERT_EQ(begin, end);
 }
 
@@ -210,11 +390,11 @@ TEST(SparseSetWithType, SortReverse) {
     auto begin = set.begin();
     auto end = set.end();
 
-    ASSERT_EQ(set.get(*(begin++)), 1);
-    ASSERT_EQ(set.get(*(begin++)), 3);
-    ASSERT_EQ(set.get(*(begin++)), 6);
-    ASSERT_EQ(set.get(*(begin++)), 9);
-    ASSERT_EQ(set.get(*(begin++)), 12);
+    ASSERT_EQ(*(begin++), 1);
+    ASSERT_EQ(*(begin++), 3);
+    ASSERT_EQ(*(begin++), 6);
+    ASSERT_EQ(*(begin++), 9);
+    ASSERT_EQ(*(begin++), 12);
     ASSERT_EQ(begin, end);
 }
 
@@ -246,11 +426,11 @@ TEST(SparseSetWithType, SortUnordered) {
     auto begin = set.begin();
     auto end = set.end();
 
-    ASSERT_EQ(set.get(*(begin++)), 1);
-    ASSERT_EQ(set.get(*(begin++)), 3);
-    ASSERT_EQ(set.get(*(begin++)), 6);
-    ASSERT_EQ(set.get(*(begin++)), 9);
-    ASSERT_EQ(set.get(*(begin++)), 12);
+    ASSERT_EQ(*(begin++), 1);
+    ASSERT_EQ(*(begin++), 3);
+    ASSERT_EQ(*(begin++), 6);
+    ASSERT_EQ(*(begin++), 9);
+    ASSERT_EQ(*(begin++), 12);
     ASSERT_EQ(begin, end);
 }
 
@@ -263,9 +443,9 @@ TEST(SparseSetWithType, RespectDisjoint) {
     lhs.construct(12, 6);
     lhs.construct(42, 9);
 
-    ASSERT_EQ(lhs.get(3), 3);
-    ASSERT_EQ(lhs.get(12), 6);
-    ASSERT_EQ(lhs.get(42), 9);
+    ASSERT_EQ(clhs.get(3), 3);
+    ASSERT_EQ(clhs.get(12), 6);
+    ASSERT_EQ(clhs.get(42), 9);
 
     lhs.respect(rhs);
 
@@ -273,12 +453,12 @@ TEST(SparseSetWithType, RespectDisjoint) {
     ASSERT_EQ(*(clhs.raw() + 1u), 6);
     ASSERT_EQ(*(clhs.raw() + 2u), 9);
 
-    auto begin = clhs.begin();
-    auto end = clhs.end();
+    auto begin = lhs.begin();
+    auto end = lhs.end();
 
-    ASSERT_EQ(clhs.get(*(begin++)), 9);
-    ASSERT_EQ(clhs.get(*(begin++)), 6);
-    ASSERT_EQ(clhs.get(*(begin++)), 3);
+    ASSERT_EQ(*(begin++), 9);
+    ASSERT_EQ(*(begin++), 6);
+    ASSERT_EQ(*(begin++), 3);
     ASSERT_EQ(begin, end);
 }
 
@@ -292,9 +472,9 @@ TEST(SparseSetWithType, RespectOverlap) {
     lhs.construct(42, 9);
     rhs.construct(12, 6);
 
-    ASSERT_EQ(lhs.get(3), 3);
-    ASSERT_EQ(lhs.get(12), 6);
-    ASSERT_EQ(lhs.get(42), 9);
+    ASSERT_EQ(clhs.get(3), 3);
+    ASSERT_EQ(clhs.get(12), 6);
+    ASSERT_EQ(clhs.get(42), 9);
     ASSERT_EQ(rhs.get(12), 6);
 
     lhs.respect(rhs);
@@ -303,12 +483,12 @@ TEST(SparseSetWithType, RespectOverlap) {
     ASSERT_EQ(*(clhs.raw() + 1u), 9);
     ASSERT_EQ(*(clhs.raw() + 2u), 6);
 
-    auto begin = clhs.begin();
-    auto end = clhs.end();
+    auto begin = lhs.begin();
+    auto end = lhs.end();
 
-    ASSERT_EQ(clhs.get(*(begin++)), 6);
-    ASSERT_EQ(clhs.get(*(begin++)), 9);
-    ASSERT_EQ(clhs.get(*(begin++)), 3);
+    ASSERT_EQ(*(begin++), 6);
+    ASSERT_EQ(*(begin++), 9);
+    ASSERT_EQ(*(begin++), 3);
     ASSERT_EQ(begin, end);
 }
 
@@ -449,3 +629,33 @@ TEST(SparseSetWithType, RespectUnordered) {
     ASSERT_EQ(*(rhs.data() + 4u), 4u);
     ASSERT_EQ(*(rhs.data() + 5u), 5u);
 }
+
+TEST(SparseSetWithType, ReferencesGuaranteed) {
+    struct Type { int value; };
+
+    entt::SparseSet<unsigned int, Type> set;
+
+    set.construct(0, 0);
+    set.construct(1, 1);
+
+    ASSERT_EQ(set.get(0).value, 0);
+    ASSERT_EQ(set.get(1).value, 1);
+
+    for(auto &&type: set) {
+        if(type.value) {
+            type.value = 42;
+        }
+    }
+
+    ASSERT_EQ(set.get(0).value, 0);
+    ASSERT_EQ(set.get(1).value, 42);
+
+    auto begin = set.begin();
+
+    while(begin != set.end()) {
+        (begin++)->value = 3;
+    }
+
+    ASSERT_EQ(set.get(0).value, 3);
+    ASSERT_EQ(set.get(1).value, 3);
+}

test/entt/entity/view.cpp

@@ -5,8 +5,8 @@
 TEST(View, SingleComponent) {
     entt::DefaultRegistry registry;
 
-    auto e1 = registry.create();
-    auto e2 = registry.create<int, char>();
+    auto e0 = registry.create();
+    auto e1 = registry.create<int, char>();
 
     ASSERT_NO_THROW(registry.view<char>().begin()++);
     ASSERT_NO_THROW(++registry.view<char>().begin());
@@ -16,30 +16,44 @@ TEST(View, SingleComponent) {
     ASSERT_NE(view.begin(), view.end());
     ASSERT_EQ(view.size(), typename decltype(view)::size_type{1});
 
-    registry.assign<char>(e1);
+    registry.assign<char>(e0);
 
     ASSERT_EQ(view.size(), typename decltype(view)::size_type{2});
 
-    view.get(e1) = '1';
-    view.get(e2) = '2';
+    view.get(e0) = '1';
+    view.get(e1) = '2';
 
     for(auto entity: view) {
         const auto &cview = static_cast<const decltype(view) &>(view);
         ASSERT_TRUE(cview.get(entity) == '1' || cview.get(entity) == '2');
     }
 
-    ASSERT_EQ(*(view.data() + 0), e2);
-    ASSERT_EQ(*(view.data() + 1), e1);
+    ASSERT_EQ(*(view.data() + 0), e1);
+    ASSERT_EQ(*(view.data() + 1), e0);
 
     ASSERT_EQ(*(view.raw() + 0), '2');
     ASSERT_EQ(*(static_cast<const decltype(view) &>(view).raw() + 1), '1');
 
+    registry.remove<char>(e0);
     registry.remove<char>(e1);
-    registry.remove<char>(e2);
 
     ASSERT_EQ(view.begin(), view.end());
 }
 
+TEST(View, SingleComponentContains) {
+    entt::DefaultRegistry registry;
+
+    auto e0 = registry.create<int>();
+    auto e1 = registry.create<int>();
+
+    registry.destroy(e0);
+
+    auto view = registry.view<int>();
+
+    ASSERT_FALSE(view.contains(e0));
+    ASSERT_TRUE(view.contains(e1));
+}
+
 TEST(View, SingleComponentEmpty) {
     entt::DefaultRegistry registry;
 
@@ -78,8 +92,14 @@ TEST(View, SingleComponentEach) {
 TEST(View, MultipleComponent) {
     entt::DefaultRegistry registry;
 
-    auto e1 = registry.create<char>();
-    auto e2 = registry.create<int, char>();
+    auto e0 = registry.create<char>();
+    auto e1 = registry.create<int, char>();
+
+    auto it = registry.view<char>().begin();
+
+    ASSERT_EQ(*it, e1);
+    ASSERT_EQ(*(it+1), e0);
+    ASSERT_EQ(it += 2, registry.view<char>().end());
 
     ASSERT_NO_THROW((registry.view<int, char>().begin()++));
     ASSERT_NO_THROW((++registry.view<int, char>().begin()));
@@ -87,22 +107,41 @@ TEST(View, MultipleComponent) {
     auto view = registry.view<int, char>();
 
     ASSERT_NE(view.begin(), view.end());
+    ASSERT_EQ(view.begin()+1, view.end());
+    ASSERT_EQ(view.size(), decltype(view.size()){1});
 
-    view.get<char>(e1) = '1';
-    view.get<char>(e2) = '2';
+    registry.get<char>(e0) = '1';
+    registry.get<char>(e1) = '2';
+    registry.get<int>(e1) = 42;
 
     for(auto entity: view) {
         const auto &cview = static_cast<const decltype(view) &>(view);
-        ASSERT_TRUE(cview.get<char>(entity) == '2');
+        ASSERT_EQ(std::get<0>(cview.get<int, char>(entity)), 42);
+        ASSERT_EQ(std::get<1>(view.get<int, char>(entity)), '2');
+        ASSERT_EQ(cview.get<char>(entity), '2');
     }
 
+    registry.remove<char>(e0);
     registry.remove<char>(e1);
-    registry.remove<char>(e2);
     view.reset();
 
     ASSERT_EQ(view.begin(), view.end());
 }
 
+TEST(View, MultipleComponentContains) {
+    entt::DefaultRegistry registry;
+
+    auto e0 = registry.create<int, char>();
+    auto e1 = registry.create<int, char>();
+
+    registry.destroy(e0);
+
+    auto view = registry.view<int, char>();
+
+    ASSERT_FALSE(view.contains(e0));
+    ASSERT_TRUE(view.contains(e1));
+}
+
 TEST(View, MultipleComponentEmpty) {
     entt::DefaultRegistry registry;
 
@@ -140,8 +179,8 @@ TEST(PersistentView, Prepare) {
     entt::DefaultRegistry registry;
     registry.prepare<int, char>();
 
-    auto e1 = registry.create<char>();
-    auto e2 = registry.create<int, char>();
+    auto e0 = registry.create<char>();
+    auto e1 = registry.create<int, char>();
 
     ASSERT_NO_THROW((registry.persistent<int, char>().begin()++));
     ASSERT_NO_THROW((++registry.persistent<int, char>().begin()));
@@ -151,26 +190,29 @@ TEST(PersistentView, Prepare) {
     ASSERT_NE(view.begin(), view.end());
     ASSERT_EQ(view.size(), typename decltype(view)::size_type{1});
 
-    registry.assign<int>(e1);
+    registry.assign<int>(e0);
 
     ASSERT_EQ(view.size(), typename decltype(view)::size_type{2});
 
-    registry.remove<int>(e1);
+    registry.remove<int>(e0);
 
     ASSERT_EQ(view.size(), typename decltype(view)::size_type{1});
 
-    view.get<char>(e1) = '1';
-    view.get<char>(e2) = '2';
+    registry.get<char>(e0) = '1';
+    registry.get<char>(e1) = '2';
+    registry.get<int>(e1) = 42;
 
     for(auto entity: view) {
         const auto &cview = static_cast<const decltype(view) &>(view);
-        ASSERT_TRUE(cview.get<char>(entity) == '2');
+        ASSERT_EQ(std::get<0>(cview.get<int, char>(entity)), 42);
+        ASSERT_EQ(std::get<1>(view.get<int, char>(entity)), '2');
+        ASSERT_EQ(cview.get<char>(entity), '2');
     }
 
-    ASSERT_EQ(*(view.data() + 0), e2);
+    ASSERT_EQ(*(view.data() + 0), e1);
 
+    registry.remove<char>(e0);
     registry.remove<char>(e1);
-    registry.remove<char>(e2);
 
     ASSERT_EQ(view.begin(), view.end());
 }
@@ -178,8 +220,8 @@ TEST(PersistentView, Prepare) {
 TEST(PersistentView, NoPrepare) {
     entt::DefaultRegistry registry;
 
-    auto e1 = registry.create<char>();
-    auto e2 = registry.create<int, char>();
+    auto e0 = registry.create<char>();
+    auto e1 = registry.create<int, char>();
 
     ASSERT_NO_THROW((registry.persistent<int, char>().begin()++));
     ASSERT_NO_THROW((++registry.persistent<int, char>().begin()));
@@ -189,30 +231,47 @@ TEST(PersistentView, NoPrepare) {
     ASSERT_NE(view.begin(), view.end());
     ASSERT_EQ(view.size(), typename decltype(view)::size_type{1});
 
-    registry.assign<int>(e1);
+    registry.assign<int>(e0);
 
     ASSERT_EQ(view.size(), typename decltype(view)::size_type{2});
 
-    registry.remove<int>(e1);
+    registry.remove<int>(e0);
 
     ASSERT_EQ(view.size(), typename decltype(view)::size_type{1});
 
-    view.get<char>(e1) = '1';
-    view.get<char>(e2) = '2';
+    registry.get<char>(e0) = '1';
+    registry.get<char>(e1) = '2';
+    registry.get<int>(e1) = 42;
 
     for(auto entity: view) {
         const auto &cview = static_cast<const decltype(view) &>(view);
-        ASSERT_TRUE(cview.get<char>(entity) == '2');
+        ASSERT_EQ(std::get<0>(cview.get<int, char>(entity)), 42);
+        ASSERT_EQ(std::get<1>(view.get<int, char>(entity)), '2');
+        ASSERT_EQ(cview.get<char>(entity), '2');
     }
 
-    ASSERT_EQ(*(view.data() + 0), e2);
+    ASSERT_EQ(*(view.data() + 0), e1);
 
+    registry.remove<char>(e0);
     registry.remove<char>(e1);
-    registry.remove<char>(e2);
 
     ASSERT_EQ(view.begin(), view.end());
 }
 
+TEST(PersistentView, Contains) {
+    entt::DefaultRegistry registry;
+
+    auto e0 = registry.create<int, char>();
+    auto e1 = registry.create<int, char>();
+
+    registry.destroy(e0);
+
+    auto view = registry.persistent<int, char>();
+
+    ASSERT_FALSE(view.contains(e0));
+    ASSERT_TRUE(view.contains(e1));
+}
+
 TEST(PersistentView, Empty) {
     entt::DefaultRegistry registry;
 
@@ -254,20 +313,20 @@ TEST(PersistentView, Sort) {
     entt::DefaultRegistry registry;
     registry.prepare<int, unsigned int>();
 
+    auto e0 = registry.create();
     auto e1 = registry.create();
     auto e2 = registry.create();
-    auto e3 = 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<unsigned int>(e3, uval++);
 
+    registry.assign<int>(e0, ival++);
     registry.assign<int>(e1, ival++);
     registry.assign<int>(e2, ival++);
-    registry.assign<int>(e3, ival++);
 
     auto view = registry.persistent<int, unsigned int>();
 
@@ -284,3 +343,65 @@ TEST(PersistentView, Sort) {
         ASSERT_EQ(view.get<int>(entity), ival++);
     }
 }
+
+TEST(RawView, Functionalities) {
+    entt::DefaultRegistry registry;
+
+    auto e0 = registry.create();
+    auto e1 = registry.create<int, char>();
+
+    ASSERT_NO_THROW(registry.raw<char>().begin()++);
+    ASSERT_NO_THROW(++registry.raw<char>().begin());
+
+    auto view = registry.raw<char>();
+
+    ASSERT_NE(view.begin(), view.end());
+    ASSERT_EQ(view.size(), typename decltype(view)::size_type{1});
+
+    registry.assign<char>(e0);
+
+    ASSERT_EQ(view.size(), typename decltype(view)::size_type{2});
+
+    registry.get<char>(e0) = '1';
+    registry.get<char>(e1) = '2';
+
+    for(auto &&component: view) {
+        ASSERT_TRUE(component == '1' || component == '2');
+    }
+
+    ASSERT_EQ(*(view.data() + 0), e1);
+    ASSERT_EQ(*(view.data() + 1), e0);
+
+    ASSERT_EQ(*(view.raw() + 0), '2');
+    ASSERT_EQ(*(static_cast<const decltype(view) &>(view).raw() + 1), '1');
+
+    for(auto &&component: view) {
+        // verifies that iterators return references to components
+        component = '0';
+    }
+
+    for(auto &&component: view) {
+        ASSERT_TRUE(component == '0');
+    }
+
+    registry.remove<char>(e0);
+    registry.remove<char>(e1);
+
+    ASSERT_EQ(view.begin(), view.end());
+}
+
+TEST(RawView, Empty) {
+    entt::DefaultRegistry registry;
+
+    registry.create<char, double>();
+    registry.create<char>();
+
+    auto view = registry.raw<int>();
+
+    ASSERT_EQ(view.size(), entt::DefaultRegistry::size_type{0});
+
+    for(auto &&component: view) {
+        (void)component;
+        FAIL();
+    }
+}

test/entt/resource/resource.cpp

@@ -27,6 +27,7 @@ TEST(ResourceCache, Functionalities) {
     ASSERT_FALSE(cache.contains(hs2));
 
     ASSERT_FALSE(cache.load<BrokenLoader>(hs1, 42));
+    ASSERT_FALSE(cache.reload<BrokenLoader>(hs1, 42));
 
     ASSERT_EQ(cache.size(), entt::ResourceCache<Resource>::size_type{});
     ASSERT_TRUE(cache.empty());
@@ -34,6 +35,7 @@ TEST(ResourceCache, Functionalities) {
     ASSERT_FALSE(cache.contains(hs2));
 
     ASSERT_TRUE(cache.load<Loader>(hs1, 42));
+    ASSERT_TRUE(cache.reload<Loader>(hs1, 42));
 
     ASSERT_NE(cache.size(), entt::ResourceCache<Resource>::size_type{});
     ASSERT_FALSE(cache.empty());

test/entt/signal/sigh.cpp

@@ -86,6 +86,21 @@ TEST(SigH, Clear) {
     ASSERT_TRUE(sigh.empty());
 }
 
+TEST(SigH, Swap) {
+    entt::SigH<void(int &)> sigh1;
+    entt::SigH<void(int &)> sigh2;
+
+    sigh1.connect<&S::f>();
+
+    ASSERT_FALSE(sigh1.empty());
+    ASSERT_TRUE(sigh2.empty());
+
+    std::swap(sigh1, sigh2);
+
+    ASSERT_TRUE(sigh1.empty());
+    ASSERT_FALSE(sigh2.empty());
+}
+
 TEST(SigH, Functions) {
     entt::SigH<void(int &)> sigh;
     int v = 0;

test/entt/signal/signal.cpp

@@ -91,6 +91,21 @@ TEST(Signal, Clear) {
     ASSERT_TRUE(signal.empty());
 }
 
+TEST(Signal, Swap) {
+    entt::Signal<void(const int &)> sig1;
+    entt::Signal<void(const int &)> sig2;
+
+    sig1.connect<&S::f>();
+
+    ASSERT_FALSE(sig1.empty());
+    ASSERT_TRUE(sig2.empty());
+
+    std::swap(sig1, sig2);
+
+    ASSERT_TRUE(sig1.empty());
+    ASSERT_FALSE(sig2.empty());
+}
+
 TEST(Signal, Functions) {
     entt::Signal<void(const int &)> signal;
     auto val = S::i + 1;

test/mod/mod.cpp

@@ -2,8 +2,8 @@
 #include <cassert>
 #include <map>
 #include <string>
+#include <duktape.h>
 #include <entt/entity/registry.hpp>
-#include "duktape.h"
 
 template<typename Type>
 struct tag { using type = Type; };
@@ -22,7 +22,7 @@ struct DuktapeRuntime {
 template<typename Comp>
 duk_ret_t set(duk_context *ctx, entt::DefaultRegistry &registry) {
     const auto entity = duk_require_uint(ctx, 0);
-    registry.accomodate<Comp>(entity);
+    registry.accommodate<Comp>(entity);
     return 0;
 }
 
@@ -31,7 +31,7 @@ duk_ret_t set<Position>(duk_context *ctx, entt::DefaultRegistry &registry) {
     const auto entity = duk_require_uint(ctx, 0);
     const auto x = duk_require_number(ctx, 2);
     const auto y = duk_require_number(ctx, 3);
-    registry.accomodate<Position>(entity, x, y);
+    registry.accommodate<Position>(entity, x, y);
     return 0;
 }
 

test/snapshot/snapshot.cpp

@@ -0,0 +1,182 @@
+#include <gtest/gtest.h>
+#include <sstream>
+#include <vector>
+#include <cereal/archives/json.hpp>
+#include <entt/entity/registry.hpp>
+
+struct Position {
+    float x;
+    float y;
+};
+
+struct Timer {
+    int duration;
+    int elapsed{0};
+};
+
+struct Relationship {
+    entt::DefaultRegistry::entity_type parent;
+};
+
+template<class Archive>
+void serialize(Archive &archive, Position &position) {
+  archive(position.x, position.y);
+}
+
+template<class Archive>
+void serialize(Archive &archive, Timer &timer) {
+  archive(timer.duration);
+}
+
+template<class Archive>
+void serialize(Archive &archive, Relationship &relationship) {
+  archive(relationship.parent);
+}
+
+TEST(Snapshot, Full) {
+    std::stringstream storage;
+
+    entt::DefaultRegistry source;
+    entt::DefaultRegistry destination;
+
+    auto e0 = source.create();
+    source.assign<Position>(e0, 16.f, 16.f);
+
+    source.destroy(source.create());
+
+    auto e1 = source.create();
+    source.assign<Position>(e1, .8f, .0f);
+    source.assign<Relationship>(e1, e0);
+
+    auto e2 = source.create();
+
+    auto e3 = source.create();
+    source.assign<Timer>(e3, 1000, 100);
+
+    source.destroy(e2);
+    auto v2 = source.current(e2);
+
+    {
+        // output finishes flushing its contents when it goes out of scope
+        cereal::JSONOutputArchive output{storage};
+        source.snapshot().entities(output).destroyed(output)
+                .component<Position, Timer, Relationship>(output);
+    }
+
+    cereal::JSONInputArchive input{storage};
+    destination.restore().entities(input).destroyed(input)
+            .component<Position, Timer, Relationship>(input);
+
+    ASSERT_TRUE(destination.valid(e0));
+    ASSERT_TRUE(destination.has<Position>(e0));
+    ASSERT_EQ(destination.get<Position>(e0).x, 16.f);
+    ASSERT_EQ(destination.get<Position>(e0).y, 16.f);
+
+    ASSERT_TRUE(destination.valid(e1));
+    ASSERT_TRUE(destination.has<Position>(e1));
+    ASSERT_EQ(destination.get<Position>(e1).x, .8f);
+    ASSERT_EQ(destination.get<Position>(e1).y, .0f);
+    ASSERT_TRUE(destination.has<Relationship>(e1));
+    ASSERT_EQ(destination.get<Relationship>(e1).parent, e0);
+
+    ASSERT_FALSE(destination.valid(e2));
+    ASSERT_EQ(destination.current(e2), v2);
+
+    ASSERT_TRUE(destination.valid(e3));
+    ASSERT_TRUE(destination.has<Timer>(e3));
+    ASSERT_EQ(destination.get<Timer>(e3).duration, 1000);
+    ASSERT_EQ(destination.get<Timer>(e3).elapsed, 0);
+}
+
+TEST(Snapshot, Continuous) {
+    std::stringstream storage;
+
+    entt::DefaultRegistry source;
+    entt::DefaultRegistry destination;
+
+    std::vector<entt::DefaultRegistry::entity_type> entities;
+    for(auto i = 0; i < 10; ++i) {
+        entities.push_back(source.create());
+    }
+
+    for(auto entity: entities) {
+        source.destroy(entity);
+    }
+
+    auto e0 = source.create();
+    source.assign<Position>(e0, 0.f, 0.f);
+    source.assign<Relationship>(e0, e0);
+
+    auto e1 = source.create();
+    source.assign<Position>(e1, 1.f, 1.f);
+    source.assign<Relationship>(e1, e0);
+
+    auto e2 = source.create();
+    source.assign<Position>(e2, .2f, .2f);
+    source.assign<Relationship>(e2, e0);
+
+    auto e3 = source.create();
+    source.assign<Timer>(e3, 1000, 1000);
+    source.assign<Relationship>(e3, e2);
+
+    {
+        // output finishes flushing its contents when it goes out of scope
+        cereal::JSONOutputArchive output{storage};
+        source.snapshot().entities(output).component<Position, Relationship, Timer>(output);
+    }
+
+    cereal::JSONInputArchive input{storage};
+    entt::ContinuousLoader<entt::DefaultRegistry::entity_type> loader{destination};
+    loader.entities(input)
+            .component<Position>(input)
+            .component<Relationship>(input, &Relationship::parent)
+            .component<Timer>(input);
+
+    ASSERT_FALSE(destination.valid(e0));
+    ASSERT_TRUE(loader.has(e0));
+
+    auto l0 = loader.map(e0);
+
+    ASSERT_TRUE(destination.valid(l0));
+    ASSERT_TRUE(destination.has<Position>(l0));
+    ASSERT_EQ(destination.get<Position>(l0).x, 0.f);
+    ASSERT_EQ(destination.get<Position>(l0).y, 0.f);
+    ASSERT_TRUE(destination.has<Relationship>(l0));
+    ASSERT_EQ(destination.get<Relationship>(l0).parent, l0);
+
+    ASSERT_FALSE(destination.valid(e1));
+    ASSERT_TRUE(loader.has(e1));
+
+    auto l1 = loader.map(e1);
+
+    ASSERT_TRUE(destination.valid(l1));
+    ASSERT_TRUE(destination.has<Position>(l1));
+    ASSERT_EQ(destination.get<Position>(l1).x, 1.f);
+    ASSERT_EQ(destination.get<Position>(l1).y, 1.f);
+    ASSERT_TRUE(destination.has<Relationship>(l1));
+    ASSERT_EQ(destination.get<Relationship>(l1).parent, l0);
+
+    ASSERT_FALSE(destination.valid(e2));
+    ASSERT_TRUE(loader.has(e2));
+
+    auto l2 = loader.map(e2);
+
+    ASSERT_TRUE(destination.valid(l2));
+    ASSERT_TRUE(destination.has<Position>(l2));
+    ASSERT_EQ(destination.get<Position>(l2).x, .2f);
+    ASSERT_EQ(destination.get<Position>(l2).y, .2f);
+    ASSERT_TRUE(destination.has<Relationship>(l2));
+    ASSERT_EQ(destination.get<Relationship>(l2).parent, l0);
+
+    ASSERT_FALSE(destination.valid(e3));
+    ASSERT_TRUE(loader.has(e3));
+
+    auto l3 = loader.map(e3);
+
+    ASSERT_TRUE(destination.valid(l3));
+    ASSERT_TRUE(destination.has<Timer>(l3));
+    ASSERT_EQ(destination.get<Timer>(l3).duration, 1000);
+    ASSERT_EQ(destination.get<Timer>(l3).elapsed, 0);
+    ASSERT_TRUE(destination.has<Relationship>(l3));
+    ASSERT_EQ(destination.get<Relationship>(l3).parent, l2);
+}