Subsystem Access Snippets

WARNING

This is a work in progress! I'm trying, okay? I have a full time job and it's NOT THIS. So if things are incomplete... uhhhh. I'LL FIGURE IT OUT (or bug me about it ok?)

The following is a toolbox of snippets useful for accessing different subsystems in unreal engine. For each of these following subsystem access snippets, I'm going to try to provide

1. A concept to enforce inputs related to retrieving the related subsystem
2. A templated function to accept the concept backed input
3. Some examples of usage!

TIP

If your unreal engine version isn't running C++ 20 (some versions of 5.X run C++ 17) then you won't be able to implement concepts (that's fine, just be careful!)

World Subsystem

Templated Function

The regular templated function simply assumes you're really only using this with UObjects, as it

template <typename WorldAwareType = UObject>
static ThisClass* Get(WorldAwareType* InWorldAwareContext)
{
    UWorld* CurrentWorld = InWorldAwareContext->GetWorld();

    if(!IsValid(CurrentWorld))
    {
        return nullptr;
    }

    return CurrentWorld->GetSubsystem<ThisClass>();
}

Concept-backed Templated Function

The concept backed version uses a concept to assert that whatever is passed in to this function is "world aware". We could write this as being derived from UObject (since all UObject types have a GetWorld), but we're going broader to say you could also pass in a struct or any type that implements a GetWorld() function.

You'll also note that the concept enforces IsValid since the template function calls it. So we just need to be sure we enforce at compile time that our type is going to not explode on calling those particular functions.

template <typename T>
concept CIsWorldAware = requires() 
{
    /** Implements a GetWorld Function that returns a UWorld */
    { T->GetWorld() } -> CSameAs<UWorld>

    /** Implements an IsValid Function that returns bool */
    { IsValid(T) } -> CSameAs<bool>
}

template <CIsWorldAware WorldAwareType>
static ThisClass* Get(WorldAwareType* InWorldAwareContext)
{
    UWorld* CurrentWorld = InWorldAwareContext->GetWorld();

    if(!IsValid(CurrentWorld))
    {
        return nullptr;
    }

    return CurrentWorld->GetSubsystem<ThisClass>();
}

Game Instance Subsystem

Local Player Subsystem