How To Modify Blueprint Variable References In C++ Without Copying

Overview

Author ()

Dear Community,

As you know, if you pass an BP array or other variable reference to C++ by reference, you have to make it const or the variable will appear as an output, not an input!

ex:

//This parameter will show as an out parameter because it is a non-const reference
UFUNCTION(BlueprintPure, Category = "VictoryBPLibrary|Transform")
FRotator TransformVectorToActorSpaceAngle(AActor* Actor, FVector& ShouldBeInVector);

//In this case InVector will indeed show up as an input parameter because it is const
UFUNCTION(BlueprintPure, Category = "VictoryBPLibrary|Transform")
FRotator TransformVectorToActorSpaceAngle(AActor* Actor, const FVector& InVector);

Non-Const Input Variable by Reference?

Well what if you want to modify the BP array in C++ and send it back to Blueprints, as is ideal since C++ can do calculations faster than BP?

Your team gives you a BP array and wants you to sort it, or modify it, and you should be able to do so without creating a copy of the data just to send it back to blueprints!

Well UPARAM(ref) is the solution!

I have 3 real-life examples for you!

Example 1: Increment Integer by Reference

/** Easily increment an integer! */
UFUNCTION(BlueprintCallable, meta = (CompactNodeTitle = "++",Keywords = "increment integer"), Category = "VictoryBPLibrary|Math|Integer")
static void VictoryIntIncrement(UPARAM(ref) int32& Int, int32& IntOut);

//~~~

void UVictoryBPFunctionLibrary::VictoryIntIncrement(UPARAM(ref) int32& Int, int32& IntOut)
{  
    Int++;
    IntOut = Int; 
} 

Example 2: Sort Integer Array by Reference

/** Sort an integer array, smallest value will be at index 0 after sorting. Modifies the input array, no new data created.