Crash Course: resource management

Introduction
The resource, the loader and the cache

Introduction

Resource management is usually one of the most critical part of a software like a game. Solutions are often tuned to the particular application. There exist several approaches and all of them are perfectly fine as long as they fit the requirements of the piece of software in which they are used.
Examples are loading everything on start, loading on request, predictive loading, and so on.

EnTT doesn't pretend to offer a one-fits-all solution for the different cases.
Instead, the library offers a minimal, general purpose resource cache that might be useful in many cases.

The resource, the loader and the cache

Resource, loader and cache are the three main actors for the purpose.
The resource is an image, an audio, a video or any other type:

struct my_resource { const int value; };

The loader is a callable type the aim of which is to load a specific resource:

struct my_loader final {
    using result_type = std::shared_ptr<my_resource>;

    result_type operator()(int value) const {
        // ...
        return std::make_shared<my_resource>(value);
    }
};

Its function operator can accept any arguments and should return a value of the declared result type (std::shared_ptr<my_resource> in the example).
A loader can also overload its function call operator to make it possible to construct the same or another resource from different lists of arguments.

Finally, a cache is a specialization of a class template tailored to a specific resource and (optionally) a loader:

using my_cache = entt::resource_cache<my_resource, my_loader>;

// ...

my_cache cache{};

The cache is meant to be used to create different caches for different types of resources and to manage each one independently in the most appropriate way.
As a (very) trivial example, audio tracks can survive in most of the scenes of an application while meshes can be associated with a single scene only, then discarded when a player leaves it.

Resource handle

Resources aren't returned directly to the caller. Instead, they are wrapped in a resource handle identified by the entt::resource class template.
For those who know the flyweight design pattern already, that's exactly what it is. To all others, this is the time to brush up on some notions instead.

A shared pointer could have been used as a resource handle. In fact, the default handle mostly maps the interface of its standard counterpart and only adds a few things to it.
However, the handle in EnTT is designed as a standalone class template named resource. It boils down to the fact that specializing a class in the standard is often undefined behavior while having the ability to specialize the handle for one, more or all resource types could help over time.

Loaders

A loader is a class that is responsible for loading the resources.
By default, it's just a callable object which forwards its arguments to the resource itself. That is, a passthrough type. All the work is demanded to the constructor(s) of the resource itself.
Loaders also are fully customizable as expected.

A custom loader is a class with at least one function call operator and a member type named result_type.
The loader isn't required to return a resource handle. As long as return_type is suitable for constructing a handle, that's fine.

When using the default handle, it expects a resource type which is convertible to or suitable for constructing an std::shared_ptr<Type> (where Type is the actual resource type).
In other terms, the loader should return shared pointers to the given resource type. However, it isn't mandatory. Users can easily get around this constraint by specializing both the handle and the loader.

A cache forwards all its arguments to the loader if required. This means that loaders can also support tag dispatching to offer different loading policies:

struct my_loader {
    using result_type = std::shared_ptr<my_resource>;

    struct from_disk_tag{};
    struct from_network_tag{};

    template<typename Args>
    result_type operator()(from_disk_tag, Args&&... args) {
        // ...
        return std::make_shared<my_resource>(std::forward<Args>(args)...);
    }

    template<typename Args>
    result_type operator()(from_network_tag, Args&&... args) {
        // ...
        return std::make_shared<my_resource>(std::forward<Args>(args)...);
    }
}

This makes the whole loading logic quite flexible and easy to extend over time.

The cache class

The cache is the class that is asked to connect the dots.
It loads the resources, store them aside and returns handles as needed:

entt::resource_cache<my_resource, my_loader> cache{};

Under the hood, a cache is nothing more than a map where the key value has type entt::id_type while the mapped value is whatever type its loader returns.
For this reason, it offers most of the functionality a user would expect from a map, such as empty or size and so on. Similarly, it's an iterable type that also supports indexing by resource id:

for(entt::resource<my_resource> curr: cache) {
    // ...
}

if(entt::resource<my_resource> res = cache["resource/id"_hs]; res) {
    // ...
}

Please, refer to the inline documentation for all the details about the other functions (for example contains or erase).

Set aside the part of the API that this class shares with a map, it also adds something on top of it in order to address the most common requirements of a resource cache.
In particular, it doesn't have an emplace member function which is replaced by load and force_load instead (where the former loads a new resource only if not present while the second triggers a forced loading in any case):

auto ret = cache.load("resource/id"_hs);

// true only if the resource was not already present
const bool loaded = ret.second;

// takes the resource handle pointed to by the returned iterator
entt::resource<my_resource> res = *ret.first;

Note that the hashed string is used for convenience in the example above.
Resource identifiers are nothing more than integral values. Therefore, plain numbers as well as non-class enum value are accepted.

Moreover, it's worth mentioning that both the iterators of a cache and its indexing operators return resource handles rather than instances of the mapped type.
Since the cache has no control over the loader and a resource isn't required to also be convertible to bool, these handles can be invalid. This usually means an error in the user logic but it may also be an expected event.
It's therefore recommended to verify handles validity with a check in debug (for example, when loading) or an appropriate logic in retail.

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