Crash Course: core functionalities

Introduction
Compile-time identifiers
Runtime identifiers
Hashed strings
- Conflicts
Monostate

Introduction

EnTT comes with a bunch of core functionalities mostly used by the other parts of the library itself.
Hardly users will include these features in their code, but it's worth describing what EnTT offers so as not to reinvent the wheel in case of need.

Compile-time identifiers

Sometimes it's useful to be able to give unique identifiers to types at compile-time.
There are plenty of different solutions out there and I could have used one of them. However, I decided to spend my time to define a compact and versatile tool that fully embraces what the modern C++ has to offer.

The result of my efforts is the identifier class template:

#include <ident.hpp>

// defines the identifiers for the given types
using id = entt::identifier<a_type, another_type>;

// ...

switch(a_type_identifier) {
case id::type<a_type>:
    // ...
    break;
case id::type<another_type>:
    // ...
    break;
default:
    // ...
}

This is all what the class template has to offer: a type inline variable that contains a numerical identifier for the given type. It can be used in any context where constant expressions are required.

As long as the list remains unchanged, identifiers are also guaranteed to be the same for every run. In case they have been used in a production environment and a type has to be removed, one can just use a placeholder to left the other identifiers unchanged:

template<typename> struct ignore_type {};

using id = entt::identifier<
    a_type_still_valid,
    ignore_type<a_type_no_longer_valid>,
    another_type_still_valid
>;

A bit ugly to see, but it works at least.

Runtime identifiers

Sometimes it's useful to be able to give unique identifiers to types at runtime.
There are plenty of different solutions out there and I could have used one of them. In fact, I adapted the most common one to my requirements and used it extensively within the entire library.

It's the family class. Here is an example of use directly from the entity-component system:

using component_family = entt::family<struct internal_registry_component_family>;

// ...

template<typename Component>
component_type component() const noexcept {
    return component_family::type<Component>;
}

This is all what a family has to offer: a type inline variable that contains a numerical identifier for the given type.

Please, note that identifiers aren't guaranteed to be the same for every run. Indeed it mostly depends on the flow of execution.

Hashed strings

A hashed string is a zero overhead unique identifier. Users can use human-readable identifiers in the codebase while using their numeric counterparts at runtime, thus without affecting performance.
The class has an implicit constexpr constructor that chews a bunch of characters. Once created, all what one can do with it is getting back the original string or converting it into a number.
The good part is that a hashed string can be used wherever a constant expression is required and no string-to-number conversion will take place at runtime if used carefully.

Example of use:

auto load(entt::hashed_string::hash_type resource) {
    // uses the numeric representation of the resource to load and return it
}

auto resource = load(entt::hashed_string{"gui/background"});

There is also a user defined literal dedicated to hashed strings to make them more user-friendly:

constexpr auto str = "text"_hs;

Conflicts

The hashed string class uses internally FNV-1a to compute the numeric counterpart of a string. Because of the pigeonhole principle, conflicts are possible. This is a fact.
There is no silver bullet to solve the problem of conflicts when dealing with hashing functions. In this case, the best solution seemed to be to give up. That's all.
After all, human-readable unique identifiers aren't something strictly defined and over which users have not the control. Choosing a slightly different identifier is probably the best solution to make the conflict disappear in this case.

Monostate

The monostate pattern is often presented as an alternative to a singleton based configuration system. This is exactly its purpose in EnTT. Moreover, this implementation is thread safe by design (hopefully).
Keys are represented by hashed strings, values are basic types like ints or bools. Values of different types can be associated to each key, even more than one at a time. Because of this, users must pay attention to use the same type both during an assignment and when they try to read back their data. Otherwise, they will probably incur in unexpected results.