How to create a UINTERFACE

Development Programming & Scripting UI
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1

Creating a trivial interface seen by the Unreal Editor is literally driving me crazy…

I googled a lot and found nothing useful for 4.7.
Can anyone tell me how the hell do I create an interface? It’s a foundamental thing in programming, it’s not acceptable to renounce using it!

2

I already read this tutorial and it’s extremely easy as you say. The problem is that from the editor I cannot create a new class inheriting from UInterface so the .generated.h defining the UINTERFACE() macro is not created and if I extend simply UObject in order to edit it later the .generated.h does not contain the macro, instead has #undef UINTERFACE

THAT is the real problem, setting up the two classes needed is pretty straightforward

3

Yeah, creating Interface is not as easy as it should (meaning, it should be simpler) but it’s still pretty easy :

Here is how I implemented one of mine :

UINTERFACE(MinimalAPI)
class UMyCheckpoint : public UInterface
{
	GENERATED_UINTERFACE_BODY()
};

class MYGAME_API IMyCheckpoint
{
	GENERATED_IINTERFACE_BODY()

	virtual void SaveCheckpoint(int8 aCheckpointID) = 0;
	virtual void LoadCheckpoint(int8 aCheckpointID) = 0;
};

There is some things to know about IInterface and UInterface, I think Rama explain it best here : A new, community-hosted Unreal Engine Wiki - Announcements - Unreal Engine Forums

Hope this helps!

Mick

1 Like
4

Ok, don’t worry, it’s pretty easy :slight_smile: Yes the tutorial do not mention how, but it automatically generates once you compile. I was asking myself the same question when first created it. Here is my COMPLETE .h :
// Fill out your copyright notice in the Description page of Project Settings.

#pragma once

#include "JotunnCheckpoint.generated.h"


UINTERFACE(MinimalAPI)
class UJotunnCheckpoint : public UInterface
{
	GENERATED_UINTERFACE_BODY()
};

class MYGAME_API IJotunnCheckpoint
{
	GENERATED_IINTERFACE_BODY()

	virtual void SaveCheckpoint(int8 aCheckpointID) = 0;
	virtual void LoadCheckpoint(int8 aCheckpointID) = 0;
};

And my complete .cpp:

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

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

#include "Jotunn.h"
#include "JotunnCheckpoint.h"

UJotunnCheckpoint::UJotunnCheckpoint(const class FObjectInitializer& ObjectInitializer)
: Super(ObjectInitializer)
{

}

Just create a new class using UObject, then replace for UInterface.

Replace the MYGAME_API for yours (if your project is name MyGreatGame, it should be MYGREATGAME_API )

After that compile your game in Visual Studio and UBT will create the .generated.h for you.

5

Well you sure helped, I would never imagine that the code would be generated at compile time!! Thank you very much!

Unfortunately I encountered other errors… If I don’t ask too much can you please give me a hint?

here is the .h

#pragma once

#include "GenericSwitch.generated.h"

UINTERFACE(MinimalAPI)
class UNIDUNGEON_API UGenericSwitch : public UInterface
{
	GENERATED_UINTERFACE_BODY()
};

class UNIDUNGEON_API IGenericSwitch
{
	GENERATED_IINTERFACE_BODY()
public:
	/*UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = Switch)
		int32 statesCount = 2;
	UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = Switch)
		int32 currentState = 0;

	UFUNCTION(BlueprintCallable, Category = Switch)
		virtual void Increment();
	UFUNCTION(BlueprintCallable, Category = Switch)
		virtual void Decrement();*/

	virtual int32 GetStatesCount() = 0;
	virtual int32 GetCurrentState() = 0;
};

here are the compiler errors:

Error 1 error C2487: ‘GetPrivateStaticClass’ : member of dll interface class may not be declared with dll interface d:\unrealengine\unidungeon\source\unidungeon\GenericSwitch.h 13 1 UniDungeon
Error 3 error C2487: ‘GetPrivateStaticClass’ : member of dll interface class may not be declared with dll interface D:\UnrealEngine\UniDungeon\Source\UniDungeon\GenericSwitch.h 13 1 UniDungeon
Error 2 error C2487: ‘UGenericSwitch::{ctor}’ : member of dll interface class may not be declared with dll interface d:\unrealengine\unidungeon\source\unidungeon\GenericSwitch.h 13 1 UniDungeon
Error 4 error C2487: ‘UGenericSwitch::{ctor}’ : member of dll interface class may not be declared with dll interface D:\UnrealEngine\UniDungeon\Source\UniDungeon\GenericSwitch.h 13 1 UniDungeon

6

Great job!!
I’m now seing further errors because my project’s generated cpp file was declaring a constructor for my interface (I initially created the class as a Ubject, i guess that’s the reason).

Thank you very much for your help!!!

7

Ha, nice one, I had my project with my working interface in 4.7.1, since you had this issue I went ahead and tried to do the same in 4.7.5, and it did the same error as you! After some messing around, I found that the “MinimalAPI” in UINTERFACE was the issue. Looks like they changed that somewhere along the way.

So just use UINTERFACE() instead of UINTERFACE(MinimalAPI)

1 Like
8

You can accept my answer than :slight_smile: Have a nice day!