Introduction

This is a constantly updated section where I'm trying to put the answers to the most frequently asked questions.
If you don't find your answer here, there are two cases: nobody has done it yet or this section needs updating. In both cases, you can open a new issue or enter either the gitter channel or the discord server to ask for help.
Probably someone already has an answer for you and we can then integrate this part of the documentation.

FAQ

Why is my debug build on Windows so slow?

EnTT is an experimental project that I also use to keep me up-to-date with the latest revision of the language and the standard library. For this reason, it's likely that some classes you're working with are using standard containers under the hood.
Unfortunately, it's known that the standard containers aren't particularly performing in debugging (the reasons for this go beyond this document) and are even less so on Windows apparently. Fortunately this can also be mitigated a lot, achieving good results in many cases.

First of all, there are two things to do in a Windows project:

Disable the /JMC option (Just My Code debugging), available starting with Visual Studio 2017 version 15.8.
Set the _ITERATOR_DEBUG_LEVEL macro to 0. This will disable checked iterators and iterator debugging.

Moreover, set the ENTT_DISABLE_ASSERT variable or redefine the ENTT_ASSERT macro to disable internal debug checks in EnTT:

#define ENTT_ASSERT(...) ((void)0)

These asserts are introduced to help the users but they require to access to the underlying containers and therefore risk ruining the performance in some cases.

With these changes, debug performance should increase enough in most cases. If you want something more, you can also switch to an optimization level O0 or preferably O1.

How can I represent hierarchies with my components?

This is one of the first questions that anyone makes when starting to work with the entity-component-system architectural pattern.
There are several approaches to the problem and the best one depends mainly on the real problem one is facing. In all cases, how to do it doesn't strictly depend on the library in use, but the latter certainly allows or not different techniques depending on how the data are laid out.

I tried to describe some of the approaches that fit well with the model of EnTT. This is the first post of a series that tries to explore the problem. More will probably come in future.
In addition, EnTT also offers the possibility to create stable storage types and therefore have pointer stability for one, all or some components. This is by far the most convenient solution when it comes to creating hierarchies and whatnot. See the documentation for the ECS part of the library and in particular what concerns the component_traits class for further details.

Custom entity identifiers: yay or nay?

Custom entity identifiers are definitely a good idea in two cases at least:

If std::uint32_t isn't large enough for your purposes, since this is the underlying type of entt::entity.
If you want to avoid conflicts when using multiple registries.

Identifiers can be defined through enum classes and class types that define an entity_type member of type std::uint32_t or std::uint64_t.
In fact, this is a definition equivalent to that of entt::entity:

enum class entity: std::uint32_t {};

There is no limit to the number of identifiers that can be defined.

Warning C4003: the min, the max and the macro

On Windows, a header file defines two macros min and max which may result in conflicts with their counterparts in the standard library and therefore in errors during compilation.

It's a pretty big problem but fortunately it's not a problem of EnTT and there is a fairly simple solution to it.
It consists in defining the NOMINMAX macro before including any other header so as to get rid of the extra definitions:

#define NOMINMAX

Please refer to this issue for more details.

The standard and the non-copyable types

EnTT uses internally the trait std::is_copy_constructible_v to check if a component is actually copyable. However, this trait doesn't really check whether a type is actually copyable. Instead, it just checks that a suitable copy constructor and copy operator exist.
This can lead to surprising results due to some idiosyncrasies of the standard.

For example, std::vector defines a copy constructor that is conditionally enabled depending on whether the value type is copyable or not. As a result, std::is_copy_constructible_v returns true for the following specialization:

struct type {
    std::vector<std::unique_ptr<action>> vec;
};

However, the copy constructor is effectively disabled upon specialization. Therefore, trying to assign an instance of this type to an entity may trigger a compilation error.
As a workaround, users can mark the type explicitly as non-copyable. This also suppresses the implicit generation of the move constructor and operator, which will therefore have to be defaulted accordingly:

struct type {
    type(const type &) = delete;
    type(type &&) = default;
 
    type & operator=(const type &) = delete;
    type & operator=(type &&) = default;
 
    std::vector<std::unique_ptr<action>> vec;
};

Note that aggregate initialization is also disabled as a consequence.
Fortunately, this type of trick is quite rare. The bad news is that there is no way to deal with it at the library level, this being due to the design of the language. On the other hand, the fact that the language itself also offers a way to mitigate the problem makes it manageable.

Which functions trigger which signals

Storage classes offer three signals that are emitted following specific operations. Maybe not everyone knows what these operations are, though.
If this isn't clear, below you can find a vademecum for this purpose:

on_created is invoked when a component is first added (neither modified nor replaced) to an entity.
on_update is called whenever an existing component is modified or replaced.
on_destroyed is called when a component is explicitly or implicitly removed from an entity.

Among the most controversial functions can be found emplace_or_replace and destroy. However, following the above rules, it's quite simple to know what will happen.
In the first case, on_created is invoked if the entity has not the component, otherwise the latter is replaced and therefore on_update is triggered. As for the second case, components are removed from their entities and thus freed when they are recycled. It means that on_destroyed is triggered for every component owned by the entity that is destroyed.

Duplicate storage for the same component

It's rare but you can see double sometimes, especially when it comes to storage. This can be caused by a conflict in the hash assigned to the various component types (one of a kind) or by bugs in your compiler (more common apparently).
Regardless of the cause, EnTT offers a customization point that also serves as a solution in this case:

template<>
struct entt::type_hash<Type> final {
    [[nodiscard]] static constexpr id_type value() noexcept {
        return hashed_string::value("Type");
    }
 
    [[nodiscard]] constexpr operator id_type() const noexcept {
        return value();
    }
};

Specializing type_hash directly bypasses the default implementation offered by EnTT, thus avoiding any possible conflicts or compiler bugs.

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