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Returning an interface from C++?

In my current project, I'm converting some blueprints into C++. One of these blueprints is able to search through all actors within the current scene, and return the interface for the first valid object which utilizes said interface:

FindMarionettistInScene

As you can see, returning an interface works just fine in blueprints. In C++, however, I've tried several different ways to return an interface, and can't get any of them to compile. Here's some of the following setups (and their corresponding errors) that I've tried:


  • ScriptInterface.h(144): error C2664: 'void FScriptInterface::SetObject(UObject )' : cannot convert argument 1 from 'IMarionettistInterface ' to 'UObject ' MarionettistUtilities.h(8) : see reference to function template instantiation 'InterfaceType &TScriptInterface::operator =(UObjectType )' being compiled with...

  • ScriptInterface.h(146): error C2440: 'initializing' : cannot convert from 'UMarionettistInterface ' to 'IMarionettistInterface ' MarionettistUtilities.h(8) : see reference to function template instantiation 'InterfaceType &TScriptInterface::operator =(UObjectType *)' being compiled with...

  • MarionettistUtilities.h(8): error C2664: 'void UMarionettistUtilities::FindMarionettistInScene(TEnumAsByte,bool &,IMarionettistInterface )' : cannot convert argument 3 from 'TScriptInterface' to 'IMarionettistInterface

  • MarionettistUtilities.h(8): error C2664: 'void UMarionettistUtilities::FindMarionettistInScene(TEnumAsByte,bool &,UMarionettistInterface )' : cannot convert argument 3 from 'TScriptInterface' to 'UMarionettistInterface '

  • void FindMarionettistInScene(TEnumAsByte MarionettistName, bool &Error, const IMarionettistInterface &Interface);

    MarionettistUtilities.h(12) : Missing '*' in Expected a pointer type


... Anyhow, I'm sure you get the idea. I'm a bit stumped how this is actually done.

Oh, and in case you're interested, line #8 of MarionettistUtilities.h is GENERATED_UCLASS_BODY(), and line #12 is the above function I'm trying to compile (currently, it's the only one in the class).

So what is the correct way to convert this blueprint into C++?

Product Version: UE 4.8
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asked Sep 18 '15 at 04:24 AM in C++ Programming

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MichaelWion
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2 answers: sort voted first

I found that the following definition compiles:

 static void FindMarionettistInScene(TEnumAsByte<EMarionettist> MarionettistName, bool &Error, TScriptInterface<IMarionettistInterface> &Interface);
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answered Sep 20 '15 at 09:17 AM

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MichaelWion
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Hi MichaelWion,

Unfortunately I can't se your image but You can do two things to workaround this:

1) You can add method GetObject to You interface and simply return UObject* from it for every class that will implement this interface:

 virtual UObject* IMarionettistInterface::GetObject() { return nullptr; }
 
 class Marionettist : public AActor, public IMarionettistInterface
 {   
     virtual UObject* GetObject() override {return this;}
 }

This way You will get UObject* so compiler won't complain.

2) You can return UObject* in the method instead of IMarionettistInterface* so there is no problem with conversion.

 UObject* FindMarionettistInScene(TEnumAsByte MarionettistName, bool &Error);

You should remember that IInterface is just a pure C++ class so it's not convertible to UObject* in any way.

Here is a link to the documentation where You can check what are benefits of using UInterfaces.

Interfaces

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answered Sep 18 '15 at 01:13 PM

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Mateusz.Wiech
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avatar image MichaelWion Sep 20 '15 at 09:18 AM

Thanks, see my answer to see how I solved it without a workaround

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