[Question] C++ Interfaces?

Ive been coding a USE system for my game, and im trying to make a interface to use with multiple inheritance and casting.
Ive tried the UINTERFACE macro(like UCLASS), but is giving me a lot of problems and i cant find proper explanation on how to set it up correctly in the documentation.

Dear Victor,

You are in luck!

I wrote an entire tutorial on using UE4 C++ Interfaces!

UE4 C++ Interfaces are awesome!

What I display below is my own research, without any assistance from Epic, so I can’t proclaim it is optimal but it works great for me :slight_smile:

#My tutorial on UE4 C++ Interfaces

http://forums.epicgames.com/threads/972861-20-TUTORIALS-C-for-UE4-gt-gt-New-How-to-Tick-Mesh-Anims-Even-When-Not-Rendered?p=31703539&viewfull=1#post31703539

Here’s a code sample from the tutorial:

#.h

// Copyright 1998-2013 Epic Games, Inc. All Rights Reserved.

#pragma once

#include "ToStringInterface.generated.h"

/** Class needed to support InterfaceCast(Object) */
UINTERFACE(MinimalAPI)
class UToStringInterface : public UInterface
{
	GENERATED_UINTERFACE_BODY()
};

class IToStringInterface
{
	GENERATED_IINTERFACE_BODY()
	
	virtual FString ToString();
};

#.cpp

// Copyright 1998-2013 Epic Games, Inc. All Rights Reserved.

#include "VictoryGame.h"

//////////////////////////////////////////////////////////////////////////
// ToStringInterface

UToStringInterface::UToStringInterface(const class FPostConstructInitializeProperties& PCIP)
	: Super(PCIP)
{
	
}

//This is required for compiling, would also let you know if somehow you called
//the base event/function rather than the over-rided version
FString IToStringInterface::ToString()
{
	return "IToStringInterface::ToString()";
}

#Class using Interface Via Multiple Inheritance

UCLASS(placeable)
class AFlower : public ASkeletalMeshActor,  public IToStringInterface
{
	GENERATED_UCLASS_BODY()
	
	FLinearColor FlowerColor;
	
	FVector FlowerScale3D;
	
	void OpenBlossom();
	void CloseUpForTheNight();

//IToStringInterface
public:
	
	virtual FString ToString() OVERRIDE;
	
};

#Flower.cpp

//other flower code

//IToStringInterface
FString AFlower::ToString()
{
	return "Flower!";
}

#Interface Cast
//Check Flower
TheInterface = InterfaceCast(MyFlower);

if(TheInterface)
{
	ClientMessage("Flower Uses Interface");
	ClientMessage(TheInterface->ToString());
}

Thanks a lot, Rama, this was just what i needed.

Just thought I’d add a few points.

1 - If you make the interface methods ‘pure virtual’ like so:

virtual FString ToString() = 0;

You won’t need to supply an implementation, and it will throw a compile error when you don’t implement it in a deriving class. This is much safer than providing a default implementation that logs a run-time message.


2 - If you want to make your methods BlueprintCallable UFUNCTION’s, you need to mark the UINTERFACE like so:

UINTERFACE(meta=(CannotImplementInterfaceInBlueprint))

3 - Also, your interface types cannot be UPROPERTY’s, because the system has no way to serialize a reference to the type (they don’t derive UObject). This greatly reduces their usefulness for anything other than cast-checking or method parameters.


Edit: Just on that last point, there is an engine type (TInterfaceType<> I think?) that can allow you to pass interface types as UFUNCTION parameters. I haven’t experimented with it much, so I don’t know its limitations, but it may even let you serialize one as a UPROPERTY somehow? I don’t know, but if you really need that functionality, you could investigate it.

"1 - If you make the interface methods ‘pure virtual’ like so:

virtual FString ToString() = 0;
You won’t need to supply an implementation, and it will throw a compile error when you don’t implement it in a deriving class. This is much safer than providing a default implementation that logs a run-time message."

Wow this is a great tip! Thanks Andrew!

Rama

Finally managed to do the interface properly, my USE system works flawless now. Ill go pack it a bit better and release the snippets in the forum

Glad you got it all figured out Victor!

:slight_smile:

Rama

Did you create the interface file from the editor or manually through the VS project? I tried doing from the editor but I can’t find the “Interface” class, should I use “Object” ?

Never mind, I created them from the editor and changed them in VS. Thanks everyone.

Hello @jdavid82. It is good to see that you appear to have solved the issue you were experiencing. I will be closing this post due to its age. If you need any additional help with this, please feel free to post a new question.

Have a great day.