superbuild/modules/entt/docs/md/entity.md

Crash Course: entity-component system

Table of Contents

• Introduction
• Design decisions
  • A bitset-free entity-component system
  • Pay per use
  • All or nothing
  • Stateless systems
• Vademecum
• The Registry, the Entity and the Component
  • Observe changes
  • Runtime components
    • A journey through a plugin
  • Sorting: is it possible?
  • Snapshot: complete vs continuous
    • Snapshot loader
    • Continuous loader
    • Archives
    • One example to rule them all
  • Prototype
  • Helpers
    • Dependency function
    • Tags
  • Null entity
  • Context variables
• Views and Groups
  • Views
  • Runtime views
  • Groups
    • Full-owning groups
    • Partial-owning groups
    • Non-owning 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

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 is a bitset-free entity-component system that doesn't require users to specify the component set at compile-time.
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;

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.
The disadvantage of this approach is that users need to know the systems they are working on and the tools they are using. Otherwise, the risk to ruin the performance along critical paths is high.

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, because it's one of the corner stones of this library.

Many of the tools described below, from the registry to the views and up to the groups give the possibility to get this information and have been designed around this need, which was and remains one of my main requirements during the development.
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.

To be honest, this point became part of the design principles at a later date, but has also become one of the cornerstones of the library to date, as stateless systems are widely used and appreciated in general.

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 just use as-is and store around if needed. Do not try to inspect an entity identifier, its format can change in future and a registry offers all the functionalities to query them out-of-the-box. The underlying type of an entity (either std::uint16_t, std::uint32_t or std::uint64_t) can be specified when defining a registry (actually entt::registry is nothing more than an alias for entt::basic_registry<entt::entity> and entt::entity is an alias for std::uint32_t).
Components (the C of an ECS) should be plain old data structures or more complex and movable data structures with a proper constructor. Actually, the sole requirement of a component type is that it 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) are just plain functions, functors, lambdas or whatever users want. They can accept a registry, a view or a group of any type and use them the way they prefer. No need to register systems or their types neither with the registry nor with the entity-component system at all.

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

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 alias entt::entity for it, as well as the alias entt::registry for entt::basic_registry<entt::entity>.

Entities are represented by entity identifiers. An entity identifier is an opaque type that users should not inspect or modify in any way. It carries information about the entity itself and its version.

A registry can be 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);

There exists also an overload of the create and destroy member functions that accepts two iterators, that is a range to assign or to destroy. It can be used to create or destroy multiple entities at once:

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

In both cases, the create member function accepts also a list of default constructible types of components to assign to the entities before to return. It's a faster alternative to the creation and subsequent assignment of components in separate steps.

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 each and every time it's discarded.
In case entity identifiers are stored around, the registry offers all the functionalities required to test them and to get out of them the information they carry:

// 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 with a few calls to member functions of the registry. As for the entities, the registry offers also a set of functionalities users can use to work with the components.

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

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

// ...

auto &velocity = registry.assign<velocity>(entity);
vel.dx = 0.;
vel.dy = 0.;

If an entity already has the given component, the replace member function template can be used to replace it:

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

// ...

auto &velocity = registry.replace<velocity>(entity);
vel.dx = 0.;
vel.dy = 0.;

In case users want to assign a component to an entity, but it's unknown whether the entity already has it or not, assign_or_replace does the work in a single call (there is a performance penalty to pay for this mainly due to the fact that it has to check if the entity already has the given component or not):

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

// ...

auto &velocity = registry.assign_or_replace<velocity>(entity);
vel.dx = 0.;
vel.dy = 0.;

Note that assign_or_replace is a slightly faster alternative for the following if/else statement and nothing more:

if(registry.has<comp>(entity)) {
    registry.replace<comp>(entity, arg1, argN);
} else {
    registry.assign<comp>(entity, arg1, argN);
}

As already shown, if in doubt about whether or not an entity has one or more components, the has member function template may be useful:

bool b = registry.has<position, velocity>(entity);

On the other side, if the goal is to delete a single component, the remove member function template is the way to go when it's certain that the entity owns a copy of the component:

registry.remove<position>(entity);

Otherwise consider to use the reset member function. It behaves similarly to remove but with a strictly defined behavior (and a performance penalty is the price to pay for this). In particular it removes the component if and only if it exists, otherwise it returns safely to the caller:

registry.reset<position>(entity);

There exist also two other versions of the reset member function:

• If no entity is passed to it, reset will remove the given component from each entity that has it:

  registry.reset<position>();
  

• If neither the entity nor the component are specified, all the entities still in use and their components are destroyed:

  registry.reset();
  

Finally, references to components can be retrieved simply by doing this:

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 components are destroyed, use the on_destroy member function instead. Finally, the on_replace member function will return a sink to which to connect listeners to observe changes on components.

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

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

Where Component is intuitively the type of component of interest. In other words, a listener is provided with the registry that triggered the notification and the entity affected by the change, in addition to the newly created instance.
The sink returned by the on_replace member function accepts listeners the signature of which is the same of that of the construction signal. The one of the destruction signal is also similar, except for the Component parameter:

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

This is mainly due to performance reasons. While the component is made available after the construction, it is not when destroyed. Because of that, there are no reasons to get it from the underlying storage unless the user requires so. In this case, the registry is made available for the purpose.

Note also that:

• Listeners for the construction signal are invoked after components have been assigned to entities.
• Listeners designed to observe changes are invoked before components have been replaced and therefore before newly created instances have been assigned to entities.
• Listeners for the destruction signal are invoked before components have been removed from entities.
• The order of invocation of the listeners isn't guaranteed in any case.

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

• Connecting and disconnecting other functions from within the body of a listener should be avoided. It can lead to undefined behavior in some cases.
• 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. Subtle bugs are the price to pay in case of errors otherwise.

In general, events and therefore listeners must not be used as replacements for systems. They should not contain much logic and interactions with a registry should be kept to a minimum, if possible. Note also that the greater the number of listeners, the greater the performance hit when components are created or destroyed.

Runtime components

Defining components at runtime is useful to support plugin systems and mods in general. However, it seems impossible with a tool designed around a bunch of templates. Indeed it's not that difficult.
Of course, some features cannot be easily exported into a runtime environment. As an example, sorting a group of components defined at runtime isn't for free if compared to most of the other operations. However, the basic functionalities of an entity-component system such as EnTT fit the problem perfectly and can also be used to manage runtime components if required.
All that is necessary to do it is to know the identifiers of the components. An identifier is nothing more than a number or similar that can be used at runtime to work with the type system.

In EnTT, identifiers are easily accessible:

entt::registry registry;

// component identifier
auto type = registry.type<position>();

Once the identifiers are made available, almost everything becomes pretty simple.

A journey through a plugin

EnTT comes with an example (actually a test) that shows how to integrate compile-time and runtime components in a stack based JavaScript environment. It uses Duktape 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 overall performance can be highly improved. However, I sacrificed optimizations in favor of a more readable piece of code. I hope I succeeded.
Note also that this isn't neither the only nor (probably) the best way to do it. In fact, the right way depends on the scripting language and the problem one is facing in general.
That being said, feel free to use it at your own risk.

The basic idea is that of creating a compile-time component aimed to map all the runtime components assigned to an entity.
Identifiers come in use to address the right function from a map when invoked from the runtime environment and to filter entities when iterating.
With a bit of gymnastic, one can narrow views and improve the performance to some extent but it was not the goal of the example.

Sorting: is it possible?

It goes without saying that sorting entities and components is possible with EnTT.
In fact, 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, as long as it adheres to the requirements described in the inline documentation.
  This is possible mainly because users can get much more with a custom sort function object if the usage pattern is known. As an example, in case of an almost sorted pool, quick sort could be much, 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.

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 the registry, use the snapshot member function. It returns a temporary object properly initialized to save the whole registry or parts of it.

Example of use:

output_archive output;

registry.snapshot()
    .entities(output)
    .destroyed(output)
    .component<a_component, another_component>(output);

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

The entities member function asks the registry to serialize all the entities that are still in use along with their versions. On the other side, the destroyed member function tells to the registry to serialize the entities that have been destroyed and are no longer in use.
These two functions can be used to save and restore the whole set of entities with the versions they had during serialization.

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 the HUD correctly from scratch.

• Furthermore, the registry makes heavy use of type-erasure techniques internally and doesn't know at any time what types of components it contains. Therefore being explicit at the call point 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;

registry.snapshot().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 do that, the registry offers a member function named loader that returns a temporary object properly initialized to restore a snapshot.

Example of use:

input_archive input;

registry.loader()
    .entities(input)
    .destroyed(input)
    .component<a_component, another_component>(input)
    .orphans();

It isn't necessary to invoke all these functions each and every time. What functions to use in which case mostly depends on the goal and there is not a golden rule to do 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 and destroyed member functions restore the sets of entities and the versions that the entities 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 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 does not need to work with the private data structures of a registry. Furthermore, it has an internal state that must persist over time. Therefore, there is no reason to create it by means of a registry, or to limit its lifetime to that of a temporary object.

Example of use:

entt::continuous_loader<entt::entity> loader{registry};
input_archive input;

loader.entities(input)
    .destroyed(input)
    .component<a_component, another_component, dirty_component>(input, &dirty_component::parent, &dirty_component::child)
    .orphans()
    .shrink();

It isn't necessary to invoke all these functions each and every time. What functions to use in which case mostly depends on the goal and there is not a golden rule to do 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 and destroyed member functions restore groups of entities and map each entity to a local counterpart when required. In other terms, for each remote entity identifier not yet registered by the loader, the latter 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.

Archives

Archives must publicly expose a predefined set of member functions. The API is straightforward and consists only of a group of function call operators that are invoked by the snapshot class and the loaders.

In particular:

• An output archive, the one used when creating a snapshot, must expose a function call operator with the following signature to store entities:

  void operator()(entt::entity);
  

  Where entt::entity is the type of the entities used by the registry. Note that all the member functions of the snapshot class make also an initial call to this endpoint to save the size of the set they are going to store.
  In addition, an archive must accept a pair of entity and component for each type to be serialized. Therefore, given a type T, the archive must contain a function call operator with the following signature:

  void operator()(entt::entity, const T &);
  

  The output archive can freely decide how to serialize the data. The register is not affected at all by the decision.

• An input archive, the one used when restoring a snapshot, must expose a function call operator with the following signature to load entities:

  void operator()(entt::entity &);
  

  Where entt::entity is the type of the entities used by the registry. Each time the function is invoked, the archive must read the next element from the underlying storage and copy it in the given variable. Note that all the member functions of a loader class make also an initial call to this endpoint to read the size of the set they are going to load.
  In addition, the archive must accept a pair of entity and component for each type to be restored. Therefore, given a type T, the archive must contain a function call operator with the following signature:

  void operator()(entt::entity &, T &);
  

  Every time such an operator is invoked, the archive must read the next elements from the underlying storage and copy them in the given variables.

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.

Prototype

A prototype defines a type of an application in terms of its parts. They can be used to assign components to entities of a registry at once.
Roughly speaking, in most cases prototypes can be considered just as templates to use to initialize entities according to concepts. In fact, users can create how many prototypes they want, each one initialized differently from the others.

The following is an example of use of a prototype:

entt::registry registry;
entt::prototype prototype{registry};

prototype.set<position>(100.f, 100.f);
prototype.set<velocity>(0.f, 0.f);

// ...

const auto entity = prototype();

To assign and remove components from a prototype, it offers two dedicated member functions named set and unset. The has member function can be used to know if a given prototype contains one or more components and the get member function can be used to retrieve the components.

Creating an entity from a prototype is straightforward:

• To create a new entity from scratch and assign it a prototype, this is the way to go:

  const auto entity = prototype();
  

  It is equivalent to the following invokation:

  const auto entity = prototype.create();
  

• In case we want to initialize an already existing entity, we can provide the operator() directly with the entity identifier:

  prototype(entity);
  

  It is equivalent to the following invokation:

  prototype.assign(entity);
  

  Note that existing components aren't overwritten in this case. Only those components that the entity doesn't own yet are copied over. All the other components remain unchanged.

• Finally, to assign or replace all the components for an entity, thus overwriting existing ones:

  prototype.assign_or_replace(entity);
  

In the examples above, the prototype uses its underlying registry to create entities and components both for its purposes and when it's cloned. To use a different repository to clone a prototype, all the member functions accept also a reference to a valid registry as a first argument.

Prototypes are a very useful tool that can save a lot of typing sometimes. Furthermore, the codebase may be easier to maintain, since updating a prototype is much less error prone than jumping around in the codebase to update all the snippets copied and pasted around to initialize entities and components.

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.

Dependency function

A dependency function is a predefined listener, actually a function template to use to automatically assign components to an entity when a type has a dependency on some other types.
The following adds components a_type and another_type whenever my_type is assigned to an entity:

entt::connnect<a_type, another_type>(registry.on_construct<my_type>());

A component is assigned to an entity and thus default initialized only in case the entity itself hasn't it yet. It means that already existent components won't be overriden.
A dependency can easily be broken by means of the following function template:

entt::disconnect<a_type, another_type>(registry.on_construct<my_type>());

Tags

There's nothing magical about the way tags can be assigned to entities while avoiding a performance hit at runtime. Nonetheless, the syntax can be annoying and that's why a more user-friendly shortcut is provided to do it.
This shortcut is the alias template entt::tag.

If used in combination with hashed strings, it helps to use tags where types would be required otherwise. As an example:

registry.assign<entt::tag<"enemy"_hs>>(entity);

Null entity

In EnTT, there exists a sort of null entity made available to users that is accessible via the entt::null variable.
The library guarantees that the following expression always returns false:

registry.valid(entt::null);

In other terms, a registry will reject the null entity in all cases because it isn't considered valid. It means that the null entity cannot own components for obvious reasons.
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);

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. Later on, either ctx or try_ctx can be used to retrieve the newly created instance and unset is there to literally reset it 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. This fits well with the if statement with initializer.

Views and Groups

First of all, it is worth answering an obvious question: why views and groups?
Briefly, they are 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 entities and their components, then read or update the data members of the latter.
It is a subtle difference that can help designing a better software sometimes.

More in details, views are a non-intrusive tool to access entities and components without affecting other functionalities or increasing the memory consumption. On the other side, groups are an intrusive tool that allows to reach higher performance along critical paths but has also a price to pay for that.

There are mainly two kinds of views: compile-time (also known as view) and runtime (also known as runtime_view).
The former require that users indicate at compile-time what are the components involved and can make several optimizations because of that. The latter can be constructed at runtime instead and are a bit slower to iterate entities and components.
In both cases, creating and destroying a view isn't expensive at all because views don't have any type of initialization. Moreover, views don't affect any other functionality of the registry and keep memory usage at a minimum.

Groups come in three different flavors: full-owning groups, partial-owning groups and non-owning groups. The main difference between them is in terms of performance.
Groups can literally own one or more types of components. It means that they will be allowed to rearrange pools so as to speed up iterations. Roughly speaking: the more components a group owns, the faster it is to iterate them. On the other side, a given component can belong only to one group, so users have to define groups carefully to get the best out of them.

Continue reading for more details or refer to the inline documentation.

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 their companion views for single component. In particular, a multi component 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 view if it contains a given entity.
Refer to the inline documentation for all the details.

There is no need to store views around for they are extremely cheap to construct, even though they 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>();

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

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

for(auto entity: view) {
    // a component at a time ...
    auto &position = view.get<position>(entity);
    auto &velocity = view.get<velocity>(entity);

    // ... or multiple components at once
    auto &[pos, vel] = view.get<position, velocity>(entity);

    // ...
}

Or rely on the each member function to iterate entities and get all their components at once:

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

The each member function is highly optimized. Unless users want to iterate only entities or get only some of the components, this should be the preferred approach. Note that the entity can also be excluded from the parameter list if not required, but this won't improve performance for multi component views.
There exists also an alternative version of each named less that works exactly as its counterpart but for the fact that it doesn't return empty components to the caller.

As a side note, when using a single component view, the most common error is to invoke get with the type of the component as a template parameter. This is probably due to the fact that it's required for multi component views:

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

for(auto entity: view) {
    const auto &vel = view.get<const velocity>(entity);
    // ...
}

However, in case of a single component view, get doesn't accept a template parameter, since it's implicitly defined by the view itself:

auto view = registry.view<const renderable>();

for(auto entity: view) {
    const auto &renderable = view.get(entity);
    // ...
}

Note: prefer the get member function of a view instead of the get member function template of a registry during iterations, if possible. However, keep in mind that it works only with the components of the view itself.

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 view are extremely cheap to construct and should not be stored around 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:

using component_type = typename decltype(registry)::component_type;
component_type types[] = { registry.type<position>(), registry.type<velocity>() };

auto view = registry.runtime_view(std::cbegin(types), std::cend(types));

for(auto entity: view) {
    // a component at a time ...
    auto &position = registry.get<position>(entity);
    auto &velocity = registry.get<velocity>(entity);

    // ... or multiple components at once
    auto &[pos, vel] = registry.get<position, velocity>(entity);

    // ...
}

Or rely on the each member function to iterate entities:

using component_type = typename decltype(registry)::component_type;
component_type types[] = { registry.type<position>(), registry.type<velocity>() };

auto view = registry.runtime_view(std::cbegin(types), std::cend(types)).each([](auto entity) {
    // ...
});

Performance are exactly the same in both cases.

Note: runtime views are meant for all those cases where users don't know at compile-time what components to use to iterate entities. This is particularly well suited to plugin systems and mods in general. Where possible, don't use runtime views, as their performance are slightly inferior to those of the other views.

Groups

Groups are meant to iterate multiple components at once and offer a faster alternative to views. Roughly speaking, they just play in another league when compared to 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 other functionalities of the 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 around for they are extremely cheap to construct, even though they 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>);

for(auto entity: group) {
    // a component at a time ...
    auto &position = group.get<position>(entity);
    auto &velocity = group.get<velocity>(entity);

    // ... or multiple components at once
    auto &[pos, vel] = group.get<position, velocity>(entity);

    // ...
}

Or rely on the each member function to iterate entities and get all their components at once:

registry.group<position>(entt::get<velocity>).each([](auto entity, auto &pos, auto &vel) {
    // ...
});

The each member function is highly optimized. Unless users want to iterate only entities, this should be the preferred approach. Note that the entity can also be excluded from the parameter list if not required and it can improve even further the performance during iterations.

Note: prefer the get member function of a group instead of the get member function template of a registry during iterations, if possible. However, keep in mind that it works only with the components of the group itself.

Let's go a bit deeper into the different types of groups made available by this library to know how they are constructed and what are the differences between them.

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.

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 so as to iterate all of them as fast as possible.

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, if required. Sorting a full-owning group affects all the instance of the same group (it means that users don't have to call sort on each instance to sort all of them because they share the underlying data structure).

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 so as to iterate all of them as fast as possible. 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, if required. Sorting a partial-owning group affects all the instance of the same group (it means that users don't have to call sort on each instance to sort all of them because they share the underlying data structure).

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 improve a performance where necessary.

Non-owning groups can be sorted by means of their sort member functions, if required. Sorting a non-owning group affects all the instance of the same group (it means that users don't have to call sort on each instance to sort all of them because they share the set of entities).

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 both const and non-const types as template parameters, the latter accepts only const types instead.
It means that views and groups can be constructed also 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.
In other terms, 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);

Similarly, the each member functions will propagate constness to the type of the components returned during iterations:

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

Obviously, 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 by means of the given function.
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 almost all 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.assign<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. All the iterators as well as the get member functions of the registry, the views and the groups will return temporary objects. Similarly, some functions such as try_get or the raw access to the list of components aren't available for this kind of types.
On the other hand, iterations are faster because only the entities to which the type is assigned are considered. Moreover, less memory is used, since there doesn't exist any instance of the component, no matter how many entities it is assigned to.

For similar reasons, wherever a function type of a listener accepts a component, it cannot be caught by a non-const reference. Capture it by copy or by const reference instead.

More in general, none of the features offered by the library is affected, but for the ones that require to return actual instances.

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.

Iterators

A special mention is needed for the iterators returned by the views and the groups. Most of the time they meet the requirements of random access iterators, in all cases they meet at least the requirements of forward 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 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 sooner or later. Multi-pass guarantee won't break in any case and the performance should even benefit from it further.