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TScriptInterface troubles

Development Discussion C++ Programming
#1

I’ve been using TScriptInterface so I can use an interface reference as a UProperty.

I can’t assign NULL without first typecasting to a known UObject class that implements the interface. It’s because things assigned to TScriptInterface must be UObjects that also implement the interface.

My code ends up looking like this:



//Casting to AWRPickup as a hack to get this to compile.  I just want to set the value to NULL.
TargetUsable = TScriptInterface<IWRUsableInterface>((AWRPickup *)NULL);

I even ran into the issue when I was trying to assign a pointer to my interface type to a TScriptInterface.

This can be solved by adding a constructor to TScriptInterface that just takes a pointer to the interface instead of a UObject pointer.



//was the result a usable object?
IWRUsableInterface * HitUsable = Cast<IWRUsableInterface>(HitResult.Actor.Get());

if (HitUsable)
{
	//Casting to AWRPickup as a hack to get this to compile.
	TargetUsable = (AWRPickup *)HitUsable;
}
#2

The first issue is because the assignment operator is templated and can’t resolve the type from NULL. Just do:


TargetUsable = TScriptInterface<IWRUsableInterface>();

Your second issue is I think a fundamental limitation with UE4 interfaces. You can’t assign an interface pointer, you have to always have a UObject pointer. Creating a new constructor as you say might appear to get around it, but I’m sure will end up breaking the code, since the TScriptInterface needs to store a valid UObject pointer along with the interface pointer.

#3

Ah of cource, use the default constructor to set it to NULL.

For the second thing, it could probably put an assert in there to make sure it’s casteable to a UObject.

Now I’m trying to figure out how to check if the interface reference is NULL in blueprint.
For now I ended up having to create a C++ Blueprint Library function that takes the interface and returns if it’s NULL or not by checking C++ side. Hopefully there’s a cleaner way.

#4

For checking in blueprint whether an interface variable is pointing to NULL, cast it to an object and check the object with NULL:
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By the way, this is how I set a TScriptInterface and how I clear it, by calling both SetInterface and SetObject. Using your variable naming:



IWRUsableInterface * HitUsable = Cast<IWRUsableInterface>(HitResult.Actor.Get());
if (HitUsable)
{
	TargetUsable.SetInterface(HitUsable);
	TargetUsable.SetObject(HitResult.Actor.Get());
}





TargetUsable.SetInterface(NULL);
TargetUsable.SetObject(NULL);

I’m not sure if its the intended way to set and clear a TScriptInterface, but since TScriptInterface basically acts as a wrapper for the same object as a UObject pointer and IYourInterface pointer, I think this is alright.

#5

How do you get that conversion node in Blueprint? For me if I try to check != it just creates the != node and fails to compile. I can’t even use a Cast to Object node since that still takes an object.

About the C++ though, that clears it up. I didn’t even see those functions because I was looking at the docs here: TScriptInterface | Unreal Engine Documentation

Didn’t think of looking at the super class functions: FScriptInterface | Unreal Engine Documentation

This makes everything better heh.

#6

To get the conversion node drag off the interface variable and start typing To Object (Interface), that is the one you need. :slight_smile:

#7

That worked.

Now I have another question. How do I make a variable with the interface type.

The interface type for me is passed in through an event. When I drag off of the input param pin there is no option to promote to variable. And when I try to create a new variable and specify its type the interface type doesn’t show up as a valid type when I try to enter its name in the search.

#8

Make sure your UInterface C++ class is a BlueprintType:



UINTERFACE(BlueprintType)
class UInteractable : public UInterface

Info: BlueprintType | Unreal Engine Documentation

#9

The docs still leave it a little unclear as to where we do and don’t reference the UInterface vs. the IInterface. Is the IInterface just there fore codegen or is it the other way around?