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. <3 Rama */
UFUNCTION(BlueprintCallable, meta = (Keywords = "sort integer array"), Category = "VictoryBPLibrary|Array")
static void VictorySortIntArray(UPARAM(ref) TArray<int32>& IntArray, TArray<int32>& IntArrayRef);

//~~~

void UVictoryBPFunctionLibrary::VictorySortIntArray(UPARAM(ref) TArray<int32>& IntArray, TArray<int32>& IntArrayRef)
{
	IntArray.Sort();
	IntArrayRef = IntArray;
}

Example 3: Sort Float Array by Reference

/** Sort a float array, smallest value will be at index 0 after sorting. Modifies the input array, no new data created. */
UFUNCTION(BlueprintCallable, meta = (Keywords = "sort float array"), Category = "VictoryBPLibrary|Array")
static void VictorySortFloatArray(UPARAM(ref) TArray<float>& FloatArray, TArray<float>& FloatArrayRef);

//~~~

void UVictoryBPFunctionLibrary::VictorySortFloatArray(UPARAM(ref) TArray<float>& FloatArray, TArray<float>& FloatArrayRef)
{
	FloatArray.Sort();
	FloatArrayRef = FloatArray;
}

Efficiency And Stability

Because the variables are references rather than hard data, these operations are very efficient, because no new data is being created!

The array data is never duplicated, only sorted and a reference to the array passed back out to BP!

Yay!

Conclusion

Now you know how you can perform high-speed calculations on BP variables in C++ and then send the results back to BP in the most efficient and stable way possible: by reference!

Enjoy!

Rama