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Properly casting structs


I'm trying to cast my child struct to my base struct, but without any luck.

I declared this child struct, which inherits from my base struct.

 struct FItemWeaponInfo : public FItemBaseInfo

Then I tried to add a FItemWeaponInfo struct to an array of FItemBaseInfo(the variable called Inventory).

 FItemWeaponInfo WeaponTest;

But I can't seem to get it to work, I tried casting, but I couldn't make that work either.

Any ideas?

Product Version: UE 4.12 Preview
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asked May 08 '16 at 09:52 PM in C++ Programming

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I have tried to reproduce your problem and i don't have any problem when i add my FItemWeaponInfo to my array of ItemWeaponInfo.

.h file

 #pragma once
 #include "GameFramework/Actor.h"
 #include "TestStruct.generated.h"
 USTRUCT(BlueprintType, Blueprintable)
 struct FItemBaseInfo {
     UPROPERTY(Category = "Item Base Info", BlueprintReadWrite, EditAnywhere)
     int32 Id;
     UPROPERTY(Category = "Item Base Info", BlueprintReadWrite, EditAnywhere)
     FName Title;
 USTRUCT(BlueprintType, Blueprintable)
 struct FItemWeaponInfo : public FItemBaseInfo {
     UPROPERTY(Category = "Item Weapon Info", BlueprintReadWrite, EditAnywhere)
     bool Enable;
 class PROTOTYPE_4_12_API ATestStruct : public AActor
     UPROPERTY(Category = "Items", BlueprintReadWrite, EditAnywhere)
     TArray<FItemBaseInfo> Items;
     // Called when the game starts or when spawned
     virtual void BeginPlay() override;
     UFUNCTION(Category = "Items", BlueprintCallable)
     void AddToArray();

.cpp file

 #include "Prototype_4_12.h"
 #include "TestStruct.h"
 // Called when the game starts or when spawned
 void ATestStruct::BeginPlay()
 void ATestStruct::AddToArray()
     //Create items
     FItemBaseInfo ItemBase;
     ItemBase.Title = "Title1";
     FItemWeaponInfo ItemWeapon;
     ItemWeapon.Title = "Title2";
     //add to array

Result :

alt text

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answered May 09 '16 at 12:01 PM

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avatar image Wrekk May 09 '16 at 12:55 PM

Huh, you're right. I had a chain of inheritance, like weapon inherits from stats, stats inherits from base. But I had forgotten to actually inherit from the base on the stat info. My bad.

Thank you, though. Probably would've taken me a while to figure it out. :)

avatar image Wrekk May 09 '16 at 06:00 PM

I have another problem though. When I cast it from FItemBaseInfo to FItemWeaponInfo from the Inventory array, the variables contained doesn't seem to work.

I tried this

 FItemWeaponInfo WeaponTest = FItemWeaponInfo();
 WeaponTest.WeaponDamage = 125.f;
 GEngine->AddOnScreenDebugMessage(-1, 2.f, FColor::Red, FString::SanitizeFloat(WeaponTest.WeaponDamage));

 for (int i = 0; i < Inventory.Num(); i++)
     FItemWeaponInfo* WeaponInfoTest = static_cast<FItemWeaponInfo*>(&Inventory[i]);
         GEngine->AddOnScreenDebugMessage(-1, 2.f, FColor::Red, FString::SanitizeFloat(WeaponInfoTest->WeaponDamage));

But it didn't print the value I set it to.

EDIT: The first debug message printed the correct value, but the second one(in the for loop) printed -431602080, which I'm guessing is just because it isn't set to a value (which it should be).

avatar image ellocator Jan 28 '19 at 03:12 PM

The static_cast will always return a pointer, even if it fails to cast. It just gives you a garbage-memory pointer struct which passes the "if"-check unfortunately.

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