C++ Interface.md

In Unreal Engine a c++ Interface is more than just an abstract base class. It is a member function system that ties into with the Blueprint runtime, making it possible to implement the functions declared by the C++ Interface in [[Blueprint Visual Script]].

There are two parts to a C++ Interface:

• Interface declaration.
• Interface definition.

I don't know if these are the correct words. I know what I mean.

The interface declaration is there to let the engine know that an interface with a specific name exists. The interface definition is where we list the functions that the interface contains. The functions should be decorated with the [[UFUNCTION]] macro with the

• BlueprintNativeEvent
• BlueprintCallable [[Function Specifier]]s. There may be alternatives to BlueprintNativeEvent, maybe BlueprintEvent for when the event should be implemented in Blueprint instead of C++.

The functions listed in the interface definition should not be implemented. An implementation will be provided by [[Unreal Build Tool]]. If a C++ function implementation is needed then it should be named with the _Implementation suffix. The generated interface function implementation will determine if a [[Blueprint Visual Script]] or C++ function should be called and call it.

The interface declaration class is decorated with the UINTERFACE macro and the name should start with U. The interface definition class should have the same name as the declaration but with I instead of U at the start.

MyInterface.h:

#pragme once
#include "" // TODO What should go here?
#include "MyInterface.generated.h" // TODO I assume this is needed. Check to make sure.

// Interface declaration.
UINTERFACE(MinimalAPI)
class UMyInterface : public UInterface
{
	GENERATED_BODY()
}

// Interface definition.
class MYMODULE_API IMyInterface
{
	GENERATED_BODY()

public:
	UFUNCTION(BlueprintNativeEvent, BlueprintCallable, Category = "My Interface")
	void MyFunction();
};

A class implementing the interface should inherit from the interface definition. Any C++ definition of a interface function should have _Implementation suffixed to the function name.

MyActor.h:

#pragma once
#include "Gameframework/Actor.h"
#include "MyInterface.h"

UCLASS(BlueprintType)
class MYMODULE_API AMyActor : public AActor, public IMyInterface
{
	GENERATED_BODY()
public:
	UFUNCTION(BlueprintNativeEvent, BlueprintCallable, Category = "My Interface")
	void MyFunction();
};

MyActor.cpp:

#include "MyActor.h"

void AMyActor::MyFunction_Implementation()
{
	// Your logic here.
}