Introduction

EnTT is a header-only, tiny and easy to use entity-component system (and much more) written in modern C++.
The entity-component-system (also known as ECS) is an architectural pattern used mostly in game development.

Design decisions

A bitset-free entity-component system

EnTT offers a bitset-free entity-component system that doesn't require users to specify the set of components neither at compile-time nor at runtime.
This is why users can instantiate the core class simply like:

entt::registry registry;

In place of its more annoying and error-prone counterpart:

entt::registry<comp_0, comp_1, ..., comp_n> registry;

Furthermore, it isn't necessary to announce the existence of a component type. When the time comes, just use it and that's all.

Pay per use

EnTT is entirely designed around the principle that users have to pay only for what they want.

When it comes to using an entity-component system, the tradeoff is usually between performance and memory usage. The faster it is, the more memory it uses. Even worse, some approaches tend to heavily affect other functionalities like the construction and destruction of components to favor iterations, even when it isn't strictly required. In fact, slightly worse performance along non-critical paths are the right price to pay to reduce memory usage and have overall better perfomance sometimes and I've always wondered why this kind of tools do not leave me the choice.
EnTT follows a completely different approach. It gets the best out from the basic data structures and gives users the possibility to pay more for higher performance where needed.

So far, this choice has proven to be a good one and I really hope it can be for many others besides me.

All or nothing

EnTT is such that at every moment a pair (T *, size) is available to directly access all the instances of a given component type T.
This was a guideline and a design decision that influenced many choices, for better and for worse. I cannot say whether it will be useful or not to the reader, but it's worth to mention it since it's one of the corner stones of this library.

Many of the tools described below give the possibility to get this information and have been designed around this need.
The rest is experimentation and the desire to invent something new, hoping to have succeeded.

Stateless systems

EnTT is designed so that it can work with stateless systems. In other words, all systems can be free functions and there is no need to define them as classes (although nothing prevents users from doing so).
This is possible because the main class with which the users will work provides all what is needed to act as the sole source of truth of an application.

Vademecum

The registry to store, the views and the groups to iterate. That's all.

An entity (the E of an ECS) is an opaque identifier that users should use as-is. Inspecting an identifier isn't recommended since its format can change in future and a registry has all the functionalities to query them out-of-the-box. The type entt::entity implements the concept of entity identifier.
Components (the C of an ECS) must be both move constructible and move assignable. They are list initialized by using the parameters provided to construct the component itself. No need to register components or their types neither with the registry nor with the entity-component system at all.
Systems (the S of an ECS) can be plain functions, functors, lambdas and so on. It's not required to announce them in any case and have no requirements.

The following sections explain in short how to use the entity-component system, the core part of the whole library.
The project is composed of many other classes in addition to those describe below. For more details, please refer to the inline documentation.

Pools

In EnTT, pools of components are made available through a specialized version of a sparse set.

Each pool contains all the instances of a single component, as well as all the entities to which it's assigned. Sparse arrays are also paged to avoid wasting memory in some cases while packed arrays are not for obvious reasons.
Pools also make available at any time a pointer to the packed lists of entities and components they contain, in addition to the number of elements in use. For this reason, pools can rearrange their items in order to keep the internal arrays tightly packed and maximize performance.

At the moment, it's possible to specialize pools within certain limits, although a more flexible and user-friendly model is under development.

The Registry, the Entity and the Component

A registry can store and manage entities, as well as create views and groups to iterate the underlying data structures.
The class template basic_registry lets users decide what's the preferred type to represent an entity. Because std::uint32_t is large enough for almost all the cases, there exists also the enum class entt::entity that wraps it and the alias entt::registry for entt::basic_registry<entt::entity>.

Entities are represented by entity identifiers. An entity identifier carries information about the entity itself and its version.
User defined identifiers can be introduced by means of enum classes and custom types for which a specialization of entt_traits exists. For this purpose, entt_traits is also defined as a sfinae-friendly class template. In theory, integral types can also be used as entity identifiers, even though this may break in future and isn't recommended in general.

A registry is used both to construct and to destroy entities:

// constructs a naked entity with no components and returns its identifier
auto entity = registry.create();
 
// destroys an entity and all its components
registry.destroy(entity);

The create member function accepts also a hint and has an overload that gets two iterators and can be used to generate multiple entities at once efficiently. Similarly, the destroy member function works also with a range of entities:

// destroys all the entities in a range
auto view = registry.view<a_component, another_component>();
registry.destroy(view.begin(), view.end());

When an entity is destroyed, the registry can freely reuse it internally with a slightly different identifier. In particular, the version of an entity is increased after destruction (unless the overload that forces a version is used instead of the default one).
Users can probe an identifier to know the information it carries:

// returns true if the entity is still valid, false otherwise
bool b = registry.valid(entity);
 
// gets the version contained in the entity identifier
auto version = registry.version(entity);
 
// gets the actual version for the given entity
auto curr = registry.current(entity);

Components can be assigned to or removed from entities at any time. As for the entities, the registry offers a set of functions to use to work with components.

The emplace member function template creates, initializes and assigns to an entity the given component. It accepts a variable number of arguments to use to construct the component itself if present:

registry.emplace<position>(entity, 0., 0.);
 
// ...
 
auto &vel = registry.emplace<velocity>(entity);
vel.dx = 0.;
vel.dy = 0.;

The default storage detects aggregate types internally and exploits aggregate initialization when possible.
Therefore, it's not strictly necessary to define a constructor for each type, in accordance with the rules of the language.

On the other hand, insert works with ranges and can be used to:

Assign the same component to all entities at once when a type is specified as a template parameter or an instance is passed as an argument:

// default initialized type assigned by copy to all entities
registry.insert<position>(first, last);
 
// user-defined instance assigned by copy to all entities
registry.insert(from, to, position{0., 0.});

Assign a set of components to the entities when a range is provided (the length of the range of components must be the same of that of entities):

// first and last specify the range of entities, instances points to the first element of the range of components

registry.insert<position>(first, last, instances);

If an entity already has the given component, the replace and patch member function templates can be used to update it:

// replaces the component in-place
registry.patch<position>(entity, [](auto &pos) { pos.x = pos.y = 0.; });
 
// constructs a new instance from a list of arguments and replaces the component
registry.replace<position>(entity, 0., 0.);

When it's unknown whether an entity already owns an instance of a component, emplace_or_replace is the function to use instead:

registry.emplace_or_replace<position>(entity, 0., 0.);

This is a slightly faster alternative for the following snippet:

if(registry.has<velocity>(entity)) {
    registry.replace<velocity>(entity, 0., 0.);
} else {
    registry.emplace<velocity>(entity, 0., 0.);
}

The has and any member functions may also be useful if in doubt about whether or not an entity has all the components in a set or any of them:

// true if entity has all the given components
bool all = registry.has<position, velocity>(entity);
 
// true if entity has at least one of the given components
bool any = registry.any<position, velocity>(entity);

If the goal is to delete a component from an entity that owns it, the remove member function template is the way to go:

registry.remove<position>(entity);

When in doubt whether the entity owns the component, use the remove_if_exists member function instead. It behaves similarly to remove but it discards the component if and only if it exists, otherwise it returns safely to the caller:

registry.remove_if_exists<position>(entity);

The clear member function works similarly and can be used to either:

Remove all instances of the given components from the entities that own them:

registry.clear<position>();

Or destroy all entities in a registry at once:

registry.clear();

Finally, references to components can be retrieved simply as:

const auto &cregistry = registry;
 
// const and non-const reference
const auto &crenderable = cregistry.get<renderable>(entity);
auto &renderable = registry.get<renderable>(entity);
 
// const and non-const references
const auto [cpos, cvel] = cregistry.get<position, velocity>(entity);
auto [pos, vel] = registry.get<position, velocity>(entity);

The get member function template gives direct access to the component of an entity stored in the underlying data structures of the registry. There exists also an alternative member function named try_get that returns a pointer to the component owned by an entity if any, a null pointer otherwise.

Observe changes

Because of how the registry works internally, it stores a bunch of signal handlers for each pool in order to notify some of its data structures on the construction and destruction of components or when an instance of a component is explicitly replaced by the user.
These signal handlers are also exposed and made available to users. These are the basic bricks to build fancy things like dependencies and reactive systems.

To get a sink to be used to connect and disconnect listeners so as to be notified on the creation of a component, use the on_construct member function:

// connects a free function
registry.on_construct<position>().connect<&my_free_function>();
 
// connects a member function
registry.on_construct<position>().connect<&my_class::member>(instance);
 
// disconnects a free function
registry.on_construct<position>().disconnect<&my_free_function>();
 
// disconnects a member function
registry.on_construct<position>().disconnect<&my_class::member>(instance);

To be notified when a component is destroyed, use the on_destroy member function instead. Finally, the on_update member function will return a sink to which to connect listeners to observe changes.
In the last case, given the way C++ works, it's also necessary to use specific member functions to allow the signal to be triggered. In particular, listeners attached to on_update will only be invoked following a call to replace or patch.

The function type of a listener should be equivalent to the following:

void(entt::registry &, entt::entity);

In all cases, listeners are provided with the registry that triggered the notification and the involved entity.

Note also that:

Listeners for the construction signals are invoked after components have been assigned to entities.
Listeners designed to observe changes are invoked after components have been updated.
Listeners for the destruction signals are invoked before components have been removed from entities.

There are also some limitations on what a listener can and cannot do:

Connecting and disconnecting other functions from within the body of a listener should be avoided. It can lead to undefined behavior in some cases.
Removing the component from within the body of a listener that observes the construction or update of instances of a given type isn't allowed.
Assigning and removing components from within the body of a listener that observes the destruction of instances of a given type should be avoided. It can lead to undefined behavior in some cases. This type of listeners is intended to provide users with an easy way to perform cleanup and nothing more.

To a certain extent, these limitations don't apply. However, it's risky to try to force them and users should respect the limitations unless they know exactly what they are doing.

Events and therefore listeners must not be used as replacements for systems. They shouldn't contain much logic and interactions with a registry should be kept to a minimum. Moreover, the greater the number of listeners, the greater the performance hit when components are created or destroyed.

Please, refer to the documentation of the signal class to know all the features it offers.
There are many useful but less known functionalities that aren't described here, such as the connection objects or the possibility to attach listeners with a list of parameters that is shorter than that of the signal itself.

They call me Reactive System

Signals are the basic tools to construct reactive systems, even if they aren't enough on their own. EnTT tries to take another step in that direction with the observer class template.
In order to explain what reactive systems are, this is a slightly revised quote from the documentation of the library that first introduced this tool, Entitas:

>Imagine you have 100 fighting units on the battlefield but only 10 of them >changed their positions. Instead of using a normal system and updating all 100 >entities depending on the position, you can use a reactive system which will >only update the 10 changed units. So efficient.

In EnTT, this means to iterating over a reduced set of entities and components with respect to what would otherwise be returned from a view or a group.
On these words, however, the similarities with the proposal of Entitas also end. The rules of the language and the design of the library obviously impose and allow different things.

An observer is initialized with an instance of a registry and a set of rules that describes what are the entities to intercept. As an example:

entt::observer observer{registry, entt::collector.update<sprite>()};

The class is default constructible and can be reconfigured at any time by means of the connect member function. Moreover, instances can be disconnected from the underlying registries through the disconnect member function.
The observer offers also what is needed to query the internal state and to know if it's empty or how many entities it contains. Moreover, it can return a raw pointer to the list of entities it contains.

However, the most important features of this class are that:

It's iterable and therefore users can easily walk through the list of entities by means of a range-for loop or the each member function.
It's clearable and therefore users can consume the entities and literally reset the observer after each iteration.

These aspects make the observer an incredibly powerful tool to know at any time what are the entities that matched the given rules since the last time one asked:

for(const auto entity: observer) {
    // ...
}
 
observer.clear();

The snippet above is equivalent to the following:

observer.each([](const auto entity) {
    // ...
});

At least as long as the observer isn't const. This means that the non-const overload of each does also reset the underlying data structure before to return to the caller, while the const overload does not for obvious reasons.

The collector is an utility aimed to generate a list of matchers (the actual rules) to use with an observer instead.
There are two types of matchers:

Observing matcher: an observer will return at least all the living entities for which one or more of the given components have been updated and not yet destroyed.

entt::collector.update<sprite>();

Updated in this case means that all listeners attached to on_update are invoked. In order for this to happen, specific functions such as patch must be used. Refer to the specific documentation for more details.

Grouping matcher: an observer will return at least all the living entities that would have entered the given group if it existed and that would have not yet left it.

entt::collector.group<position, velocity>(entt::exclude<destroyed>);

A grouping matcher supports also exclusion lists as well as single components.

Roughly speaking, an observing matcher intercepts the entities for which the given components are updated while a grouping matcher tracks the entities that have assigned the given components since the last time one asked.
If an entity already has all the components except one and the missing type is assigned to it, the entity is intercepted by a grouping matcher.

In addition, a matcher can be filtered with a where clause:

entt::collector.update<sprite>().where<position>(entt::exclude<velocity>);

This clause introduces a way to intercept entities if and only if they are already part of a hypothetical group. If they are not, they aren't returned by the observer, no matter if they matched the given rule.
In the example above, whenever the component sprite of an entity is updated, the observer probes the entity itself to verify that it has at least position and has not velocity before to store it aside. If one of the two conditions of the filter isn't respected, the entity is discared, no matter what.

A where clause accepts a theoretically unlimited number of types as well as multiple elements in the exclusion list. Moreover, every matcher can have its own clause and multiple clauses for the same matcher are combined in a single one.

Sorting: is it possible?

Sorting entities and components is possible with EnTT. In particular, it's feasible with an in-place algorithm that doesn't require memory allocations nor anything else and is therefore particularly convenient.
With this in mind, there are two functions that respond to slightly different needs:

Components can be sorted either directly:

registry.sort<renderable>([](const auto &lhs, const auto &rhs) {
    return lhs.z < rhs.z;
});

Or by accessing their entities:

registry.sort<renderable>([](const entt::entity lhs, const entt::entity rhs) {
    return entt::registry::entity(lhs) < entt::registry::entity(rhs);
});

There exists also the possibility to use a custom sort function object for when the usage pattern is known. As an example, in case of an almost sorted pool, quick sort could be much slower than insertion sort.

Components can be sorted according to the order imposed by another component:

registry.sort<movement, physics>();

In this case, instances of movement are arranged in memory so that cache misses are minimized when the two components are iterated together.

As a side note, the use of groups limits the possibility of sorting pools of components. Refer to the specific documentation for more details.

Helpers

The so called helpers are small classes and functions mainly designed to offer built-in support for the most basic functionalities.
The list of helpers will grow longer as time passes and new ideas come out.

Null entity

In EnTT, the entt::null variable models the concept of null entity.
The library guarantees that the following expression always returns false:

registry.valid(entt::null);

A registry rejects the null entity in all cases because it isn't considered valid. It also means that the null entity cannot own components.
The type of the null entity is internal and should not be used for any purpose other than defining the null entity itself. However, there exist implicit conversions from the null entity to identifiers of any allowed type:

entt::entity null = entt::null;

Similarly, the null entity can be compared to any other identifier:

const auto entity = registry.create();

const bool null = (entity == entt::null);

Be aware that entt::null and entity 0 aren't the same thing. Likewise, a zero initialized entity isn't the same as entt::null. Therefore, although entt::entity{} is in some sense an alias for entity 0, none of them can be used to create a null entity.

To entity

Sometimes it's useful to get the entity from a component instance.
This is what the entt::to_entity helper does. It accepts a registry and an instance of a component and returns the entity associated with the latter:

const auto entity = entt::to_entity(registry, position);

This utility doesn't perform any check on the validity of the component. Therefore, trying to take the entity of an invalid element or of an instance that isn't associated with the given registry can result in undefined behavior.

Dependencies

The registry class is designed to be able to create short circuits between its functions. This simplifies the definition of dependencies between different operations.
For example, the following adds (or replaces) the component a_type whenever my_type is assigned to an entity:

registry.on_construct<my_type>().connect<&entt::registry::emplace_or_replace<a_type>>();

Similarly, the code shown below removes a_type from an entity whenever my_type is assigned to it:

registry.on_construct<my_type>().connect<&entt::registry::remove<a_type>>();

A dependency can also be easily broken as follows:

registry.on_construct<my_type>().disconnect<&entt::registry::emplace_or_replace<a_type>>();

There are many other types of dependencies. In general, most of the functions that accept an entity as the first argument are good candidates for this purpose.

Invoke

Sometimes it's useful to be able to directly invoke a member function of a component as a callback. It's already possible in practice but requires users to extend their classes and this may not always be possible.
The invoke helper allows to propagate the signal in these cases:

registry.on_construct<clazz>().connect<entt::invoke<&clazz::func>>();

All it does is pick up the right component for the received entity and invoke the requested method, passing on the arguments if necessary.

Handle

A handle is a thin wrapper around an entity and a registry. It provides the same functions that the registry offers for working with components, such as emplace, get, patch, remove and so on. The difference being that the entity is implicitly passed to the registry.
It's default constructible as an invalid handle that contains a null registry and a null entity. When it contains a null registry, calling functions that delegate execution to the registry will cause an undefined behavior, so it's recommended to check the validity of the handle with implicit cast to bool when in doubt.
A handle is also non-owning, meaning that it can be freely copied and moved around without affecting its entity (in fact, handles happen to be trivially copyable). An implication of this is that mutability becomes part of the type.

There are two aliases that use entt::entity as their default entity: entt::handle and entt::const_handle.
Users can also easily create their own aliases for custom identifiers as:

using my_handle = entt::basic_handle<my_identifier>;

using my_const_handle = entt::basic_handle<const my_identifier>;

Handles are also implicitly convertible to const handles out of the box but not the other way around.
A handle stores a non-const pointer to a registry and therefore it can do all the things that can be done with a non-const registry. On the other hand, a const handles store const pointers to registries and offer a restricted set of functionalities.

This class is intended to simplify function signatures. In case of functions that take a registry and an entity and do most of their work on that entity, users might want to consider using handles, either const or non-const.

Context variables

It is often convenient to assign context variables to a registry, so as to make it the only source of truth of an application.
This is possible by means of a member function named set to use to create a context variable from a given type. Either ctx or try_ctx can be used to retrieve the newly created instance, while unset is meant to clear the variable if needed.

Example of use:

// creates a new context variable initialized with the given values
registry.set<my_type>(42, 'c');
 
// gets the context variable
const auto &var = registry.ctx<my_type>();
 
// if in doubts, probe the registry to avoid assertions in case of errors
if(auto *ptr = registry.try_ctx<my_type>(); ptr) {
    // uses the context variable associated with the registry, if any
}
 
// unsets the context variable
registry.unset<my_type>();

The type of a context variable must be such that it's default constructible and can be moved. The set member function either creates a new instance of the context variable or overwrites an already existing one if any. The try_ctx member function returns a pointer to the context variable if it exists, otherwise it returns a null pointer.

Organizer

The organizer class template offers minimal support (but sufficient in many cases) for creating an execution graph from functions and their requirements on resources.
The resulting tasks aren't executed in any case. This isn't the goal of this tool. Instead, they are returned to the user in the form of a graph that allows for safe execution.

The functions are added in order of execution to the organizer. Free functions and member functions are supported as template parameters, however there is also the possibility to pass pointers to free functions or decayed lambdas as parameters to the emplace member function:

entt::organizer organizer;
 
// adds a free function to the organizer
organizer.emplace<&free_function>();
 
// adds a member function and an instance on which to invoke it to the organizer
clazz instance;
organizer.emplace<&clazz::member_function>(&instance);
 
// adds a decayed lambda directly
organizer.emplace(+[](const void *, entt::registry &) { /* ... */ });

As for free functions and member functions, these are the parameters that can be presented by their function types and that will be correctly handled:

A possibly constant reference to a registry. The one passed to the task when it's run will also be passed to the function as-is.
An entt::view with any possible combination of types. It will be created from the registry passed to the task and supplied directly to the function.
A possibly constant reference to any type T. It will be interpreted as context variable, which will be created within the registry and passed to the function.

The function type for free functions and decayed lambdas passed as parameters to emplace is void(const void *, entt::registry &) instead. The registry is the same as provided to the task. The first parameter is an optional pointer to user defined data to provide upon registration:

clazz instance;

organizer.emplace(+[](const void *, entt::registry &) { /* ... */ }, &instance);

In all cases, it's also possible to associate a name with the task when creating it. For example:

organizer.emplace<&free_function>("func");

When a function of any type is registered with the organizer, everything it accesses is considered a resource (views are unpacked and their types are treated as resources). The constness of the type also dictates its access mode (RO/RW). In turn, this affects the resulting graph, since it influences the possibility of launching tasks in parallel.
As for the registry, if a function doesn't explicitly request it or requires a constant reference to it, it's considered a read-only access. Otherwise, it's considered as read-write access. All functions will still have the registry among their resources.

When registering a function, users can also require resources that aren't in the list of parameters of the function itself. These are declared as template parameters:

organizer.emplace<&free_function, position, velocity>("func");

Similarly, users can override the access mode of a type again via template parameters:

organizer.emplace<&free_function, const renderable>("func");

In this case, even if renderable appears among the parameters of the function as not constant, it will be treated as constant as regards the generation of the task graph.

To generate the task graph, the organizer offers the graph member function:

std::vector<entt::organizer::vertex> graph = organizer.graph();

The graph is returned in the form of an adjacency list. Each vertex offers the following features:

ro_count and rw_count: they return the number of resources accessed in read-only or read-write mode.
ro_dependency and rw_dependency: useful for retrieving the type info objects associated with the parameters of the underlying function.
top_level: indicates whether a node is a top level one, that is, it has no entering edges.
info: returns the type info object associated with the underlying function.
name: returns the name associated with the given vertex if any, a null pointer otherwise.
callback: a pointer to the function to execute and whose function type is void(const void *, entt::registry &).
data: optional data to provide to the callback.
children: the vertices reachable from the given node, in the form of indices within the adjacency list.

Since the creation of pools and resources within the registry isn't necessarily thread safe, each vertex also offers a prepare function which can be called to setup a registry for execution with the created graph:

auto graph = organizer.graph();
entt::registry registry;
 
for(auto &&node: graph) {
    node.prepare(registry);
}

The actual scheduling of the tasks is the responsibility of the user, who can use the preferred tool.

Meet the runtime

EnTT takes full advantage of what the language offers at compile-time.
However, by combining these feature with a tool for static polymorphism, it's also possible to have opaque proxies to work with type-less pools at runtime.

These objects are returned by the storage member function, which accepts a type_info object as an argument rather than a compile-time type (the same returned by the visit member function):

auto storage = registry.storage(info);

By default and to stay true with the philosophy of the library, the API of a proxy is minimal and doesn't allow users to do much.
However, it's also completely customizable in a generic way and with the possibility of defining specific behaviors for given types.

This section won't go into detail on how to define a poly storage to get all the possible functionalities out of it. EnTT already contains enough snippets to get inspiration from, both in the test suite and in the example folder.
In short, users will have to define their own concepts (see the entt::poly documentation for this) and register them via the poly_storage_traits class template, which has been designed as sfinae-friendly for the purpose.

Once the concept that a poly storage must adhere to has been properly defined, copying an entity will be as easy as:

registry.visit(entity, [&](const auto info) {
    auto storage = registry.storage(info);
    storage->emplace(registry, other, storage->get(entity));
});

Where other is the entity to which the elements should be replicated.
Similarly, copying entire pools between different registries can look like this:

registry.visit([&](const auto info) {
    auto storage = registry.storage(info);
    other.storage(info)->insert(other, storage->data(), storage->raw(), storage->size());
});

Where this time other represents the destination registry.

So, all in all, EnTT shifts the complexity to the one-time definition of a concept that reflects the user's needs, and then leaves room for ease of use within the codebase.
The possibility of extreme customization is the icing on the cake in this sense, allowing users to design this tool around their own requirements.

Snapshot: complete vs continuous

The registry class offers basic support to serialization.
It doesn't convert components to bytes directly, there wasn't the need of another tool for serialization out there. Instead, it accepts an opaque object with a suitable interface (namely an archive) to serialize its internal data structures and restore them later. The way types and instances are converted to a bunch of bytes is completely in charge to the archive and thus to final users.

The goal of the serialization part is to allow users to make both a dump of the entire registry or a narrower snapshot, that is to select only the components in which they are interested.
Intuitively, the use cases are different. As an example, the first approach is suitable for local save/restore functionalities while the latter is suitable for creating client-server applications and for transferring somehow parts of the representation side to side.

To take a snapshot of a registry, use the snapshot class:

output_archive output;
 
entt::snapshot{registry}
    .entities(output)
    .component<a_component, another_component>(output);

It isn't necessary to invoke all functions each and every time. What functions to use in which case mostly depends on the goal and there is not a golden rule for that.

The entities member function makes the snapshot serialize all entities (both those still alive and those destroyed) along with their versions.
On the other hand, the component member function is a function template the aim of which is to store aside components. The presence of a template parameter list is a consequence of a couple of design choices from the past and in the present:

First of all, there is no reason to force a user to serialize all the components at once and most of the times it isn't desiderable. As an example, in case the stuff for the HUD in a game is put into the registry for some reasons, its components can be freely discarded during a serialization step because probably the software already knows how to reconstruct them correctly.
Furthermore, the registry makes heavy use of type-erasure techniques internally and doesn't know at any time what component types it contains. Therefore being explicit at the call site is mandatory.

There exists also another version of the component member function that accepts a range of entities to serialize. This version is a bit slower than the other one, mainly because it iterates the range of entities more than once for internal purposes. However, it can be used to filter out those entities that shouldn't be serialized for some reasons.
As an example:

const auto view = registry.view<serialize>();
output_archive output;
 
entt::snapshot{registry}.component<a_component, another_component>(output, view.cbegin(), view.cend());

Note that component stores items along with entities. It means that it works properly without a call to the entities member function.

Once a snapshot is created, there exist mainly two ways to load it: as a whole and in a kind of continuous mode.
The following sections describe both loaders and archives in details.

Snapshot loader

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.
To use it, just pass to the constructor a valid registry:

input_archive input;
 
entt::snapshot_loader{registry}
    .entities(input)
    .component<a_component, another_component>(input)
    .orphans();

It isn't necessary to invoke all functions each and every time. What functions to use in which case mostly depends on the goal and there is not a golden rule for that. For obvious reasons, what is important is that the data are restored in exactly the same order in which they were serialized.

The entities member function restores the sets of entities and the versions that they originally had at the source.

The component member function restores all and only the components specified and assigns them to the right entities. Note that the template parameter list must be exactly the same used during the serialization.

The orphans member function literally destroys those entities that have no components attached. It's usually useless if the snapshot is a full dump of the source. However, in case all the entities are serialized but only few components are saved, it could happen that some of the entities have no components once restored. The best the users can do to deal with them is to destroy those entities and thus update their versions.

Continuous loader

A continuous loader is designed to load data from a source registry to a (possibly) non-empty destination. The loader can accommodate in a registry more than one snapshot in a sort of continuous loading that updates the destination one step at a time.
Identifiers that entities originally had are not transferred to the target. Instead, the loader maps remote identifiers to local ones while restoring a snapshot. Because of that, this kind of loader offers a way to update automatically identifiers that are part of components (as an example, as data members or gathered in a container).
Another difference with the snapshot loader is that the continuous loader has an internal state that must persist over time. Therefore, there is no reason to limit its lifetime to that of a temporary object.

Example of use:

entt::continuous_loader loader{registry};
input_archive input;
 
loader.entities(input)
    .component<a_component, another_component, dirty_component>(input, &dirty_component::parent, &dirty_component::child)
    .orphans()
    .shrink();

It isn't necessary to invoke all functions each and every time. What functions to use in which case mostly depends on the goal and there is not a golden rule for that. For obvious reasons, what is important is that the data are restored in exactly the same order in which they were serialized.

The entities member function restores groups of entities and maps each entity to a local counterpart when required. In other terms, for each remote entity identifier not yet registered by the loader, it creates a local identifier so that it can keep the local entity in sync with the remote one.

The component member function restores all and only the components specified and assigns them to the right entities.
In case the component contains entities itself (either as data members of type entt::entity or as containers of entities), the loader can update them automatically. To do that, it's enough to specify the data members to update as shown in the example.

The orphans member function literally destroys those entities that have no components after a restore. It has exactly the same purpose described in the previous section and works the same way.

Finally, shrink helps to purge local entities that no longer have a remote conterpart. Users should invoke this member function after restoring each snapshot, unless they know exactly what they are doing.

One example to rule them all

EnTT comes with some examples (actually some tests) that show how to integrate a well known library for serialization as an archive. It uses Cereal C++ under the hood, mainly because I wanted to learn how it works at the time I was writing the code.

The code is not production-ready and it isn't neither the only nor (probably) the best way to do it. However, feel free to use it at your own risk.

The basic idea is to store everything in a group of queues in memory, then bring everything back to the registry with different loaders.

Views and Groups

First of all, it's worth answering a question: why views and groups?
Briefly, they're a good tool to enforce single responsibility. A system that has access to a registry can create and destroy entities, as well as assign and remove components. On the other side, a system that has access to a view or a group can only iterate, read and update entities and components.
It is a subtle difference that can help designing a better software sometimes.

Views

A view behaves differently if it's constructed for a single component or if it has been created to iterate multiple components. Even the API is slightly different in the two cases.

Single component views are specialized in order to give a boost in terms of performance in all the situations. This kind of views can access the underlying data structures directly and avoid superfluous checks. There is nothing as fast as a single component view. In fact, they walk through a packed array of components and return them one at a time.
Single component views offer a bunch of functionalities to get the number of entities they are going to return and a raw access to the entity list as well as to the component list. It's also possible to ask a view if it contains a given entity.
Refer to the inline documentation for all the details.

Multi component views iterate entities that have at least all the given components in their bags. During construction, these views look at the number of entities available for each component and pick up a reference to the smallest set of candidates in order to speed up iterations.
They offer fewer functionalities than single component views. In particular, a multi component view exposes utility functions to get the estimated number of entities it is going to return and to know if it contains a given entity.
Refer to the inline documentation for all the details.

There is no need to store views aside for they are extremely cheap to construct, even though valid views can be copied without problems and reused freely.
Views also return newly created and correctly initialized iterators whenever begin or end are invoked.

Views share the way they are created by means of a registry:

// single component view
auto single = registry.view<position>();
 
// multi component view
auto multi = registry.view<position, velocity>();

Filtering entities by components is also supported:

auto view = registry.view<position, velocity>(entt::exclude<renderable>);

To iterate a view, either use it in a range-for loop:

auto view = registry.view<position, velocity, renderable>();
 
for(auto entity: view) {
    // a component at a time ...
    auto &position = view.get<position>(entity);
    auto &velocity = view.get<velocity>(entity);
 
    // ... multiple components ...
    auto [pos, vel] = view.get<position, velocity>(entity);
 
    // ... all components at once
    auto [pos, vel, rend] = view.get(entity);
 
    // ...
}

Or rely on the each member functions to iterate both entities and components at once:

// through a callback
registry.view<position, velocity>().each([](auto entity, auto &pos, auto &vel) {
    // ...
});
 
// using an input iterator
for(auto &&[entity, pos, vel]: registry.view<position, velocity>().each()) {
    // ...
}

Note that entities can also be excluded from the parameter list when received through a callback and this can improve even further the performance during iterations.
Since they aren't explicitly instantiated, empty components aren't returned in any case.

As a side note, in the case of single component views, get accepts but doesn't strictly require a template parameter, since the type is implicitly defined. However, when the type isn't specified, for consistency with the multi component view, the instance will be returned using a tuple:

auto view = registry.view<const renderable>();
 
for(auto entity: view) {
    auto [renderable] = view.get(entity);
    // ...
}

Note: prefer the get member function of a view instead of that of a registry during iterations to get the types iterated by the view itself.

View pack

The view pack allows users to combine multiple views into a single view-like iterable object, while also giving them full control over which view should lead the iteration.
This object returns all and only the entities present in all views. Its intended primary use is for custom storage and views, but it can also be very convenient in everyday use.

The creation of a view pack tries to mimic C++20 ranges:

auto view = registry.view<position>();
auto other = registry.view<velocity>();
 
auto pack = view | other;

The return type is a specialization of the class template entt::view_pack. This is nothing more than a view-like iterable object that combines two or more views into a single instance.
The first view used to create a pack will also be the same that will lead the iteration.

A view pack offers functionalities similar to those of a multi component view, especially with regard to the possibilities of iteration. In particular, it only returns entities if iterated directly:

for(auto entt: pack) {
    // ...
}

On the other hand, both the (optional) entity and the components are returned when the each member function is used, be it with callback or to get an extended iterable object:

// with a callback
pack.each([](const auto entt, auto &pos, auto &vel) { /* ... */ });
 
// with an extended iterable object
for(auto [entt, pos, vel]: pack.each()) {
    // ...
}

Furthermore, the constness of the types returned by a view pack is directly inherited by the views that compose it:

(registry.view<position>() | registry.view<const velocity>()).each([](auto &pos, const auto &vel) {
    // ...
});

Read also the dedicated section to know how a view pack is involved in the creation and use of custom storage and pools.

Runtime views

Runtime views iterate entities that have at least all the given components in their bags. During construction, these views look at the number of entities available for each component and pick up a reference to the smallest set of candidates in order to speed up iterations.
They offer more or less the same functionalities of a multi component view. However, they don't expose a get member function and users should refer to the registry that generated the view to access components. In particular, a runtime view exposes utility functions to get the estimated number of entities it is going to return and to know whether it's empty or not. It's also possible to ask a runtime view if it contains a given entity.
Refer to the inline documentation for all the details.

Runtime views are pretty cheap to construct and should not be stored aside in any case. They should be used immediately after creation and then they should be thrown away. The reasons for this go far beyond the scope of this document.
To iterate a runtime view, either use it in a range-for loop:

entt::id_type types[] = { entt::type_hash<position>::value(), entt::type_hash<velocity>::value() };
auto view = registry.runtime_view(std::cbegin(types), std::cend(types));
 
for(auto entity: view) {
    // ...
}

Or rely on the each member function to iterate entities:

entt::id_type types[] = { entt::type_hash<position>::value(), entt::type_hash<velocity>::value() };
 
registry.runtime_view(std::cbegin(types), std::cend(types)).each([](auto entity) {
    // ...
});

Performance are exactly the same in both cases.
Filtering entities by components is also supported for this kind of views:

entt::id_type components[] = { entt::type_hash<position>::value() };
entt::id_type filter[] = { entt::type_hash<velocity>::value() };
auto view = registry.runtime_view(std::cbegin(components), std::cend(components), std::cbegin(filter), std::cend(filter));

Note: runtime views are meant for all those cases where users don't know at compile-time what components to use to iterate entities. If possible, don't use runtime views as their performance are inferior to those of the other views.

Groups

Groups are meant to iterate multiple components at once and to offer a faster alternative to multi component views.
Groups overcome the performance of the other tools available but require to get the ownership of components and this sets some constraints on pools. On the other side, groups aren't an automatism that increases memory consumption, affects functionalities and tries to optimize iterations for all the possible combinations of components. Users can decide when to pay for groups and to what extent.
The most interesting aspect of groups is that they fit usage patterns. Other solutions around usually try to optimize everything, because it is known that somewhere within the everything there are also our usage patterns. However this has a cost that isn't negligible, both in terms of performance and memory usage. Ironically, users pay the price also for things they don't want and this isn't something I like much. Even worse, one cannot easily disable such a behavior. Groups work differently instead and are designed to optimize only the real use cases when users find they need to.
Another nice-to-have feature of groups is that they have no impact on memory consumption, put aside full non-owning groups that are pretty rare and should be avoided as long as possible.

All groups affect to an extent the creation and destruction of their components. This is due to the fact that they must observe changes in the pools of interest and arrange data correctly when needed for the types they own.
That being said, the way groups operate is beyond the scope of this document. However, it's unlikely that users will be able to appreciate the impact of groups on the other functionalities of a registry.

Groups offer a bunch of functionalities to get the number of entities they are going to return and a raw access to the entity list as well as to the component list for owned components. It's also possible to ask a group if it contains a given entity.
Refer to the inline documentation for all the details.

There is no need to store groups aside for they are extremely cheap to construct, even though valid groups can be copied without problems and reused freely.
A group performs an initialization step the very first time it's requested and this could be quite costly. To avoid it, consider creating the group when no components have been assigned yet. If the registry is empty, preparation is extremely fast. Groups also return newly created and correctly initialized iterators whenever begin or end are invoked.

To iterate groups, either use them in a range-for loop:

auto group = registry.group<position>(entt::get<velocity, renderable>);
 
for(auto entity: group) {
    // a component at a time ...
    auto &position = group.get<position>(entity);
    auto &velocity = group.get<velocity>(entity);
 
    // ... multiple components ...
    auto [pos, vel] = group.get<position, velocity>(entity);
 
    // ... all components at once
    auto [pos, vel, rend] = group.get(entity);
 
    // ...
}

Or rely on the each member functions to iterate both entities and components at once:

// through a callback
registry.group<position>(entt::get<velocity>).each([](auto entity, auto &pos, auto &vel) {
    // ...
});
 
// using an input iterator
for(auto &&[entity, pos, vel]: registry.group<position>(entt::get<velocity>).each()) {
    // ...
}

Note that entities can also be excluded from the parameter list when received through a callback and this can improve even further the performance during iterations.
Since they aren't explicitly instantiated, empty components aren't returned in any case.

Note: prefer the get member function of a group instead of that of a registry during iterations to get the types iterated by the group itself.

Full-owning groups

A full-owning group is the fastest tool an user can expect to use to iterate multiple components at once. It iterates all the components directly, no indirection required. This type of groups performs more or less as if users are accessing sequentially a bunch of packed arrays of components all sorted identically, with no jumps nor branches.

A full-owning group is created as:

auto group = registry.group<position, velocity>();

Filtering entities by components is also supported:

auto group = registry.group<position, velocity>(entt::exclude<renderable>);

Once created, the group gets the ownership of all the components specified in the template parameter list and arranges their pools as needed.

Sorting owned components is no longer allowed once the group has been created. However, full-owning groups can be sorted by means of their sort member functions. Sorting a full-owning group affects all its instances.

Partial-owning groups

A partial-owning group works similarly to a full-owning group for the components it owns, but relies on indirection to get components owned by other groups. This isn't as fast as a full-owning group, but it's already much faster than views when there are only one or two free components to retrieve (the most common cases likely). In the worst case, it's not slower than views anyway.

A partial-owning group is created as:

auto group = registry.group<position>(entt::get<velocity>);

Filtering entities by components is also supported:

auto group = registry.group<position>(entt::get<velocity>, entt::exclude<renderable>);

Once created, the group gets the ownership of all the components specified in the template parameter list and arranges their pools as needed. The ownership of the types provided via entt::get doesn't pass to the group instead.

Sorting owned components is no longer allowed once the group has been created. However, partial-owning groups can be sorted by means of their sort member functions. Sorting a partial-owning group affects all its instances.

Non-owning groups

Non-owning groups are usually fast enough, for sure faster than views and well suited for most of the cases. However, they require custom data structures to work properly and they increase memory consumption. As a rule of thumb, users should avoid using non-owning groups, if possible.

A non-owning group is created as:

auto group = registry.group<>(entt::get<position, velocity>);

Filtering entities by components is also supported:

auto group = registry.group<>(entt::get<position, velocity>, entt::exclude<renderable>);

The group doesn't receive the ownership of any type of component in this case. This type of groups is therefore the least performing in general, but also the only one that can be used in any situation to slightly improve performance.

Non-owning groups can be sorted by means of their sort member functions. Sorting a non-owning group affects all its instances.

Nested groups

A type of component cannot be owned by two or more conflicting groups such as:

registry.group<transform, sprite>().
registry.group<transform, rotation>().

However, the same type can be owned by groups belonging to the same family, also called nested groups, such as:

registry.group<sprite, transform>().
registry.group<sprite, transform, rotation>().

Fortunately, these are also very common cases if not the most common ones.
It allows to increase performance on a greater number of component combinations.

Two nested groups are such that they own at least one componet type and the list of component types involved by one of them is contained entirely in that of the other. More specifically, this applies independently to all component lists used to define a group.
Therefore, the rules for defining whether two or more groups are nested can be summarized as:

One of the groups involves one or more additional component types with respect to the other, whether they are owned, observed or excluded.
The list of component types owned by the most restrictive group is the same or contains entirely that of the others. This also applies to the list of observed and excluded components.

It means that nested groups extend their parents by adding more conditions in the form of new components.

As mentioned, the components don't necessarily have to be all owned so that two groups can be considered nested. The following definitions are fully valid:

registry.group<sprite>(entt::get<renderable>).
registry.group<sprite, transform>(entt::get<renderable>).
registry.group<sprite, transform>(entt::get<renderable, rotation>).

Exclusion lists also play their part in this respect. When it comes to defining nested groups, an excluded component type T is treated as being an observed type not_T. Therefore, consider these two definitions:

registry.group<sprite, transform>().
registry.group<sprite, transform>(entt::exclude<rotation>).

They are treated as if users were defining the following groups:

group<sprite, transform>().
group<sprite, transform>(entt::get<not_rotation>).

Where not_rotation is an empty tag present only when rotation is not.

Because of this, to define a new group that is more restrictive than an existing one, it's enough to take the list of component types of the latter and extend it by adding new component types either owned, observed or excluded, without any precautions depending on the case.
The opposite is also true. To define a larger group, it will be enough to take an existing one and remove constraints from it, in whatever form they are expressed.
Note that the greater the number of component types involved by a group, the more restrictive it is.

Despite the extreme flexibility of nested groups which allow to independently use component types either owned, observed or excluded, the real strength of this tool lies in the possibility of defining a greater number of groups that own the same components, thus offering the best performance in more cases.
In fact, given a list of component types involved by a group, the greater the number of those owned, the greater the performance of the group itself.

As a side note, it's no longer possible to sort all groups when defining nested ones. This is because the most restrictive group shares its elements with the less restrictive ones and ordering the latter would invalidate the former.
However, given a family of nested groups, it's still possible to sort the most restrictive of them. To prevent users from having to remember which of their groups is the most restrictive, the registry class offers the sortable member function to know if a group can be sorted or not.

Invalid views and groups

Views and groups as returned by a registry are generally valid. However, there are some exceptions where an invalid object might be returned.
In these cases, they should be renewed as soon as possible. In fact, an invalid view or group contains a broken reference to one or more pools and this will never be fixed. The view or the group will continue to return no data, even if the pool for the pending reference is created in the registry in the meantime.

There is only one case in which an invalid object can be returned, that is when the view or the group is created from a constant reference to a registry in which the required pools haven't yet been created.
Pools are typically created whenever any method is used on a non-const registry. This also means that creating views and groups from a non-const registry can never result in an invalid object.

It's also perfectly fine to use an invalid view or group, to invoke each on them or to iterate them like any other object. The only difference from a valid view or group is that the invalid ones will always appear as empty.
In general, when views and groups are created on the fly and used at the same time, then discarded immediately afterwards, it doesn't matter whether or not they may be invalid. Therefore, this remains the recommended approach.

To know if a view or a group is properly initialized, both can be converted to bool explicitly and used in a guard.

Types: const, non-const and all in between

The registry class offers two overloads when it comes to constructing views and groups: a const version and a non-const one. The former accepts only const types as template parameters, the latter accepts both const and non-const types instead.
It means that views and groups can be constructed from a const registry and they propagate the constness of the registry to the types involved. As an example:

entt::view<const position, const velocity> view = std::as_const(registry).view<const position, const velocity>();

Consider the following definition for a non-const view instead:

entt::view<position, const velocity> view = registry.view<position, const velocity>();

In the example above, view can be used to access either read-only or writable position components while velocity components are read-only in all cases.
Similarly, these statements are all valid:

position &pos = view.get<position>(entity);
const position &cpos = view.get<const position>(entity);
const velocity &cpos = view.get<const velocity>(entity);
std::tuple<position &, const velocity &> tup = view.get<position, const velocity>(entity);
std::tuple<const position &, const velocity &> ctup = view.get<const position, const velocity>(entity);

It's not possible to get non-const references to velocity components from the same view instead and these will result in compilation errors:

velocity &cpos = view.get<velocity>(entity);
std::tuple<position &, velocity &> tup = view.get<position, velocity>(entity);
std::tuple<const position &, velocity &> ctup = view.get<const position, velocity>(entity);

The each member functions also propagates constness to its return values:

view.each([](auto entity, position &pos, const velocity &vel) {
    // ...
});

A caller can still refer to the position components through a const reference because of the rules of the language that fortunately already allow it.

The same concepts apply to groups as well.

Give me everything

Views and groups are narrow windows on the entire list of entities. They work by filtering entities according to their components.
In some cases there may be the need to iterate all the entities still in use regardless of their components. The registry offers a specific member function to do that:

registry.each([](auto entity) {
    // ...
});

It returns to the caller all the entities that are still in use.
As a rule of thumb, consider using a view or a group if the goal is to iterate entities that have a determinate set of components. These tools are usually much faster than combining this function with a bunch of custom tests.
In all the other cases, this is the way to go.

There exists also another member function to use to retrieve orphans. An orphan is an entity that is still in use and has no assigned components.
The signature of the function is the same of each:

registry.orphans([](auto entity) {
    // ...
});

To test the orphanity of a single entity, use the member function orphan instead. It accepts a valid entity identifer as an argument and returns true in case the entity is an orphan, false otherwise.

In general, all these functions can result in poor performance.
each is fairly slow because of some checks it performs on each and every entity. For similar reasons, orphans can be even slower. Both functions should not be used frequently to avoid the risk of a performance hit.

What is allowed and what is not

Most of the ECS available out there don't allow to create and destroy entities and components during iterations.
EnTT partially solves the problem with a few limitations:

Creating entities and components is allowed during iterations in most cases.
Deleting the current entity or removing its components is allowed during iterations. For all the other entities, destroying them or removing their components isn't allowed and can result in undefined behavior.

In these cases, iterators aren't invalidated. To be clear, it doesn't mean that also references will continue to be valid.
Consider the following example:

registry.view<position>([&](const auto entity, auto &pos) {
    registry.emplace<position>(registry.create(), 0., 0.);
    pos.x = 0.; // warning: dangling pointer
});

The each member function won't break (because iterators aren't invalidated) but there are no guarantees on references. Use a common range-for loop and get components directly from the view or move the creation of components at the end of the function to avoid dangling pointers.

Iterators are invalidated instead and the behavior is undefined if an entity is modified or destroyed and it's not the one currently returned by the iterator nor a newly created one.
To work around it, possible approaches are:

Store aside the entities and the components to be removed and perform the operations at the end of the iteration.
Mark entities and components with a proper tag component that indicates they must be purged, then perform a second iteration to clean them up one by one.

A notable side effect of this feature is that the number of required allocations is further reduced in most of the cases.

More performance, more constraints

Groups are a (much) faster alternative to views. However, the higher the performance, the greater the constraints on what is allowed and what is not.
In particular, groups add in some rare cases a limitation on the creation of components during iterations. It happens in quite particular cases. Given the nature and the scope of the groups, it isn't something in which it will happen to come across probably, but it's good to know it anyway.

First of all, it must be said that creating components while iterating a group isn't a problem at all and can be done freely as it happens with the views. The same applies to the destruction of components and entities, for which the rules mentioned above apply.

The additional limitation pops out instead when a given component that is owned by a group is iterated outside of it. In this case, adding components that are part of the group itself may invalidate the iterators. There are no further limitations to the destruction of components and entities.
Fortunately, this isn't always true. In fact, it almost never is and this happens only under certain conditions. In particular:

Iterating a type of component that is part of a group with a single component view and adding to an entity all the components required to get it into the group may invalidate the iterators.
Iterating a type of component that is part of a group with a multi component view and adding to an entity all the components required to get it into the group can invalidate the iterators, unless users specify another type of component to use to induce the order of iteration of the view (in this case, the former is treated as a free type and isn't affected by the limitation).

In other words, the limitation doesn't exist as long as a type is treated as a free type (as an example with multi component views and partial- or non-owning groups) or iterated with its own group, but it can occur if the type is used as a main type to rule on an iteration.
This happens because groups own the pools of their components and organize the data internally to maximize performance. Because of that, full consistency for owned components is guaranteed only when they are iterated as part of their groups or as free types with multi component views and groups in general.

Empty type optimization

An empty type T is such that std::is_empty_v<T> returns true. They are also the same types for which empty base optimization (EBO) is possibile.
EnTT handles these types in a special way, optimizing both in terms of performance and memory usage. However, this also has consequences that are worth mentioning.

When an empty type is detected, it's not instantiated in any case. Therefore, only the entities to which it's assigned are made available.
There doesn't exist a way to iterate empty types. Views and groups will never return instances of empty types (for example, during a call to each) and some functions such as try_get or the raw access to the list of components aren't available for them. Finally, the sort functionality accepts only callbacks that require to return entities rather than components:

registry.sort<empty_type>([](const entt::entity lhs, const entt::entity rhs) {
    return entt::registry::entity(lhs) < entt::registry::entity(rhs);
});

On the other hand, iterations are faster because only the entities to which the type is assigned are considered. Moreover, less memory is used, mainly because there doesn't exist any instance of the component, no matter how many entities it is assigned to.

More in general, none of the features offered by the library is affected, but for the ones that require to return actual instances.
This optimization can be disabled by defining the ENTT_NO_ETO macro. In this case, empty types will be treated like all other types, no matter what.

Multithreading

In general, the entire registry isn't thread safe as it is. Thread safety isn't something that users should want out of the box for several reasons. Just to mention one of them: performance.
Views, groups and consequently the approach adopted by EnTT are the great exception to the rule. It's true that views, groups and iterators in general aren't thread safe by themselves. Because of this users shouldn't try to iterate a set of components and modify the same set concurrently. However:

As long as a thread iterates the entities that have the component X or assign and removes that component from a set of entities, another thread can safely do the same with components Y and Z and everything will work like a charm. As a trivial example, users can freely execute the rendering system and iterate the renderable entities while updating a physic component concurrently on a separate thread.
Similarly, a single set of components can be iterated by multiple threads as long as the components are neither assigned nor removed in the meantime. In other words, a hypothetical movement system can start multiple threads, each of which will access the components that carry information about velocity and position for its entities.

This kind of entity-component systems can be used in single threaded applications as well as along with async stuff or multiple threads. Moreover, typical thread based models for ECS don't require a fully thread safe registry to work. Actually, users can reach the goal with the registry as it is while working with most of the common models.

Because of the few reasons mentioned above and many others not mentioned, users are completely responsible for synchronization whether required. On the other hand, they could get away with it without having to resort to particular expedients.

Finally, EnTT can be configured via a few compile-time definitions to make some of its parts implicitly thread-safe, roughly speaking only the ones that really make sense and can't be turned around.
In particular, when multiple instances of objects referencing the type index generator (such as the registry class) are used in different threads, then it might be useful to define ENTT_USE_ATOMIC.
See the relevant documentation for more information.

Iterators

A special mention is needed for the iterators returned by views and groups. Most of the times they meet the requirements of random access iterators, in all cases they meet at least the requirements of bidirectional iterators.
In other terms, they are suitable for use with the parallel algorithms of the standard library. If it's not clear, this is a great thing.

As an example, this kind of iterators can be used in combination with std::for_each and std::execution::par to parallelize the visit and therefore the update of the components returned by a view or a group, as long as the constraints previously discussed are respected:

auto view = registry.view<position, const velocity>();
 
std::for_each(std::execution::par_unseq, view.begin(), view.end(), [&view](auto entity) {
    // ...
});

This can increase the throughput considerably, even without resorting to who knows what artifacts that are difficult to maintain over time.

Unfortunately, because of the limitations of the current revision of the standard, the parallel std::for_each accepts only forward iterators. This means that the default iterators provided by the library cannot return proxy objects as references and must return actual reference types instead.
This may change in the future and the iterators will almost certainly return both the entities and a list of references to their components by default sooner or later. Multi-pass guarantee won't break in any case and the performance should even benefit from it further.

Beyond this document

There are many other features and functions not listed in this document.
EnTT and in particular its ECS part is in continuous development and some things could be forgotten, others could have been omitted on purpose to reduce the size of this file. Unfortunately, some parts may even be outdated and still to be updated.

For further information, it's recommended to refer to the documentation included in the code itself or join the official channels to ask a question.

Introduction

Design decisions

A bitset-free entity-component system

Pay per use

All or nothing

Stateless systems

Vademecum

Pools

The Registry, the Entity and the Component

Observe changes

They call me Reactive System

Sorting: is it possible?

Helpers

Null entity

To entity

Dependencies

Invoke

Handle

Context variables

Organizer

Meet the runtime

Snapshot: complete vs continuous

Snapshot loader

Continuous loader

Archives

One example to rule them all

Views and Groups

Views

View pack

Runtime views

Groups

Full-owning groups

Partial-owning groups

Non-owning groups

Nested groups

Invalid views and groups

Types: const, non-const and all in between

Give me everything

What is allowed and what is not

More performance, more constraints

Empty type optimization

Multithreading

Iterators

Beyond this document