[4.6.1] Returning reference to structure gives error

Development Programming & Scripting Blueprint
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1

I am trying to return a reference to a structure which can be called by blueprints. I can get it to compile in c++, but in blueprints, I get an error “Error The direction of Return Value doesn’t match the direction of parameter ReturnValue”.

Here’s my definition:

USTRUCT( BlueprintType )
struct FRaptorActivateTask
{
...
};


UFUNCTION( BlueprintCallable, Category = RaptorInteractive )
const struct FRaptorActivateTask& GetInteractTask( ERaptorInteractType::Type InteractTaskType ) const;

I only get the blueprint error when the node is fully connected to a exec chain.

I noticed a similar definition in UPrimitiveComponent:

USTRUCT(BlueprintType)
struct FWalkableSlopeOverride
{
...
};

UFUNCTION(BlueprintCallable, Category="Physics")
const struct FWalkableSlopeOverride& GetWalkableSlopeOverride() const;

This gives the exact same blueprint compile error.

So, either my class and UPrimitiveComponent are doing it wrong, or the compiler is doing something wrong.

Please advise.

2

Hi ,

Thank you for pointing this out to us. I ran some tests and was able to see the results that you described. I have entered a report to have this investigated further (UE-8693).

3

This bug is also present in 4.7 preview.

29161-struct_bug.png
29161-struct_bug.png1086×789 270 KB

4

Ok, found my mistake.

Of course you can’t set any value of the struct if it’s const. Just remove the const keyword, and it should work!

5

Yes! That was it. I glossed over my “const” as well. Making non-const removed the error. I guess what would be helpful is to change the error to something like “can’t set value on const reference”.

6

hey mate, i asked a similar question. This is what happens when you try to return a const reference via “normal” c++ way.

You can return a reference via the C way using it as an OutParam.

here is the link to my question.

however i havnt figured how to return it as const

7

This still does not explain to me why it is impossible to return such a value as const. If you don’t want to alter the returned value, it should be const

8

Hi Wallenstein,

Blueprints don’t actually know what const means, so if this is only being used in Blueprints, the const keyword doesn’t help much. What I did find that worked in this case is instead of using const struct FTestStruct& AMyActor::GetTestStruct() const, use const struct FTestStruct AMyActor::GetTestStruct() const instead. Leaving the & out seems to allow this to work as expected.

9

While this may work, it shouldn’t be this way. You should be able to return a const reference from a function. If the blueprint system then chooses to copy this into a new non-const/ref structure, so be it, the inefficiencies of BP climb.

The problem lies here:

		const bool bOutParam = (Property->HasAnyPropertyFlags(CPF_OutParm | CPF_ReturnParm) && !(Property->HasAnyPropertyFlags(CPF_ReferenceParm)));

		if ( ((SourcePin->Direction == EGPD_Input) && bOutParam) || ((SourcePin->Direction == EGPD_Output) && !bOutParam))
		{
			MessageLog.Error(*FString::Printf(*LOCTEXT("DirectionMismatchParameter_Error", "The direction of @@ doesn't match the direction of parameter %s").ToString(), *Property->GetName()), SourcePin);
		}

An an out param (i.e. return value) cannot be a reference, const or not, which is entirely silly.

10

Hi TTaM,

You are correct that you will receive an error message when trying to have a reference as a return value (as in the original post, for example). This is unfortunately a current limitation of Blueprints.

However, you are still able to pass parameters to a BlueprintCallable function as references, and those should function normally. There is one thing to keep in mind, however. If you write your function declaration like you normally would in C++, the parameter will appear on the node as an output pin instead of as an input pin. For example, the following code will give you a Return Float output pin:

UFUNCTION(BlueprintCallable, Category = TestCat)
void TestFunc(float& ReturnFloat);

However, if you add UPARAM(ref) before the parameter, it will instead appear as an input pin:

UFUNCTION(BlueprintCallable, Category = TestCat)
void TestFunc(UPARAM(ref) float& ReturnFloat);

You can also make the parameters that are references const if you wish.