This info seems to be scattered across a ton of different docs pages and blog posts so I’m consolidating it here for my own sanity.

This example is an Interactable interface, something a player can “interact” with. I don’t know if you should actually include Interface in the name of your interface but I did here.

Create interface

You can do this with the usual class generator but note the following!

Important: Note the UINTERFACE(BlueprintType) and the second _Implementation function definition, as well as the U and I prefixes.

UINTERFACE(BlueprintType)
class UInteractableInterface : public UInterface
{
	GENERATED_BODY()
};

class FLAB_API IInteractableInterface
{
	GENERATED_BODY()

	UFUNCTION(BlueprintNativeEvent, BlueprintCallable)
	bool Interact(AFlabCharacter* Interactor);
	virtual bool Interact_Implementation(AFlabCharacter* Interactor);
};

Implement interface in C++

Put the I version of your interface in your class definition and make it a BlueprintType:

UCLASS(BlueprintType)
class ALootItem : public AActor, public IInteractableInterface

Override both methods in your h file:

public:
	UFUNCTION(BlueprintCallable, BlueprintNativeEvent, Category = "Loot")
	bool Interact(bool ExampleArg);
	virtual bool Interact_Implementation(bool ExampleArg) override;

Implement only the _Implementation method in your cpp file:

bool ALootItem::Interact_Implementation(bool ExampleArg)
{
	return !ExampleArg;
}

Test for & use interface functions in C++

Don’t call Interact or Execute_Interact directly, and don’t use ImplementsInterface because it doesn’t seem to work

AActor* Target = GetSomeActor();

// The Target could be some interactable object, like an ALootItem
if (Target->Implements<UInteractableInterface>())
{
	// Now we can safely use our interface function
	bool ExampleArg = false;
	bool Result = IInteractableInterface::Execute_Interact(Target, ExampleArg);
}

Use in Blueprints

Open the “Class Settings” in your Blueprint:

Under the “Interfaces” category add your interface:

The overridable methods will then be listed next to your Blueprint functions under “Interfaces”: