Struct is reset when passed through interface?

Development Programming & Scripting C++
1

I’m trying to pass a struct through an interface, but all it’s data gets reset. Not sure if I’m doing something wrong here.

//The Struct


USTRUCT(BlueprintType)
struct FDamageParams
{
    GENERATED_BODY()
public:
    float **PrevHealth**;
   // other params
};

//ControllerInterface.h


UFUNCTION(BlueprintCallable, BlueprintNativeEvent) void Damage(FDamageParams **params**);

//Calling the interface on the controller when pawn damage is taken


**params.PrevHealth** = CurHealth;
CurHealth -= 5;

IControllerInterface::Execute_Damage(GetController(), **params**);

//Receiving the damage on the controller


void AHeroController::Damage_Implementation(FDamageParams **params**) {
***// params.PrevHealth, and all values inside params are reset to 0!***
    HealthBar_ins->SetScalarParameterValue("Prev", **params.PrevHealth**);
    HealthBar_ins->SetScalarParameterValue("Cur", Pawn->CurHealth);
}

The struct works fine to be passed all around, but the moment it passes through an interface function it gets completely reset.
How can I solve this?

Thanks!

2

You are passing the FDamageParams variable by value. When you pass by value, the compiler makes a copy of the variable with default data.

In this case, you want to pass by reference:



void AHeroController::Damage_Implementation(FDamageParams &params);

I strongly suggest you read up on the difference between passing by value and passing by reference.

3

That’s not how passing by value works anywhere. It should copy the struct with the values that are set into. It doesn’t (or shouldn’t) copy “with default data”.

4

I figured out the problem:


USTRUCT(BlueprintType)
 struct FDamageParams {    
GENERATED_BODY()
public:    
**UPROPERTY() float PrevHealth;**    
// other params
};

Making it a UPROPERTY (and restarting unreal) allowed the property to persist through the interface.

But thanks for the info guys, I didn’t even think about passing as a reference, that would have most likely worked as well!
Might actually go back someday and change many things into references where relevant.

Edit: Turns out I added UPROPERTY on a different value, NOT on PrevHealth, even though it randomly started working the next day. Just Unreal being Unreal I guess.

5

You are completely right. I misspoke! Apologies. I should have just suggested passing by reference.

Thanks for keeping me honest!