> unreal-cpp _

The subsystem collection asks each non-abstract class's CDO whether to create an instance. A subclass does not automatically replace its parent; both can exist side by side. This explains how to move the implementation into Blueprint, how to keep a C++ fallback when the child is missing, and why GetSubsystem<T>() can still find the derived instance.

Five subsystem base classes, each with a different owner. Where the engine creates and tears down each collection, how world startup relates to actor BeginPlay, and where initialization order is no longer guaranteed.

Cast<IMyInterface>(Obj) returns nullptr when an object's interface implementation exists only in Blueprint, even though ImplementsInterface() returns true for that same object. The engine skips entries marked bImplementedByK2, so casting to an interface is safe only when that interface is explicitly blocked from Blueprint implementation.