C++ UInterface cast failing and other problems

Development Programming & Scripting C++
1

This is the guide I’ve followed: Interfaces | Unreal Engine Documentation
Being updated to Ue 4.14 I suppose it’s better to follow than 2 years old tutorials.

My Interface


// Fill out your copyright notice in the Description page of Project Settings.

#pragma once

#include "CoreMinimal.h"
#include "InteractiveInterface.generated.h"

// This class does not need to be modified.
UINTERFACE(Blueprintable)
class UInteractiveInterface : public UInterface
{
	GENERATED_BODY()
};

/**
* Tutorial: https://docs.unrealengine.com/latest/INT/Programming/UnrealArchitecture/Reference/Interfaces/
*/
class MID_API IInteractiveInterface
{
	GENERATED_BODY()

		// Add interface functions to this class. This is the class that will be inherited to implement this interface.
public:
	UFUNCTION(BlueprintNativeEvent, Category = Gameplay)
		void Execute(UObject* caller, UActorComponent* component = nullptr, int interactionType = 0);

	virtual void Execute_Implementation(UObject* caller, UActorComponent* component = nullptr, int interactionType = 0);
};

Where I use it:


AActor* result = RayCastForObject();

	if (result != nullptr)
	{
		
		if (result->GetClass()->ImplementsInterface(UInteractiveInterface::StaticClass()) )
		{
			IInteractiveInterface* interf = Cast<IInteractiveInterface>(result);
			interf->Execute(this);
			return true;
		}
	}
	return false;

First Issue: I cannot a BlueprintActor that implements the interface.

Second Issue: if I use the C++ Actor class that implements the interface or its derived blueprint the cast works but I cannot call the function Execute(). I can call Execute_Execute() but it doesn’t use default parameters.

2

Don’t call the function Execute, because that will make life difficult.

The first parameter of Execute is the object you’re calling the function on. In your case:



interf->Execute_MyFunction(interf, this);
3

Casting won’t work when the interface is added at the blueprint level. It can’t, because the object doesn’t actually derive from IWhateverInterface at the C++ level at all.
The most generic way that works with objects implementing the interface in both C++ and Blueprint, is to call the execute function statically. No need for an interface pointer, just take an object, check it implements the interface, then do:


IWhateverInterface::Execute_SomeFunction(Object, Parameters...);

And yeah, it probably won’t duplicate the default arguments. Not really a big deal, default arguments are mostly evil anyway.

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