Cannot Forward Declare for a TSubClassOf or for a UPROPERTY() Member. How Can I Have Co-Dependency?

Development Programming & Scripting C++
1

So in cases like this, I would normally forward declare the classes I’ll be referencing in a struct.h file, and then include struct.h with the classes that use it, then avoid Infinite Recursion with the .cpp file.

However, I’ve hit 2 major problems.

  1. TSubclassOf<template> cannot accept forward declarations.
  2. UPROPERTY() cannot accept forward declarations of structs (won’t recognize it as a USTRUCT())

The won’t as far as I know that is. If there is a way to make either of these accept the forward declaration, then problem solved.

[HR][/HR]
[SUP]struct.h[/SUP]


USTRUCT(BlueprintType)
struct FItemStruct
{
    GENERATED_BODY()

    UPROPERTY(BlueprintReadWrite, Category = "Inventory")
        TSubclassOf<AItem> myItem; //ERROR IF: I forward declare AItem

};

[SUP]item.h[/SUP]


struct FItemStruct;

UCLASS()
class MYPROJECT2_API AItem : public AActor
{
    GENERATED_BODY()

public:    
    AItem();


protected:
    //UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Item")
        FItemStruct *itemDetails; //Cannot make UPROPERTY() from a forward declared struct

};

[HR][/HR]
I absolutely must have the TSubclassOf<template> (unless there is an exact equivalent) in the struct, and it would be ideal to have itemDetails be a UPROPERTY() for the designers to edit the information cleanly.

Is there a way to make either of these accept the forward declaration? If not, is there a way to get the same desired behavior?

2

In case like that, because we have to deal with different compilers for multiple platforms, I create AItem_base to use it on both the Struct and as a base class for AItem.
Later I add templated auto casting routines for algorithms that must be members of the AItem class.

3

I’ve run into this quite often.

Although it seems counter-intuitive, I’ve found the forward declaration often works when including the SubclassOf.h header. Still not certain as to why.



#include "Templates/SubclassOf.h"

class USomeType;

You may know already, but if not - you can only forward-declare pointers to types.

4
  • Make sure you’ve included CoreMinimal.h.
  • Forward declarations in TSubclassOf works completely fine as long as you do not initialize it in the header.
  • You can’t make a UPROPERTY to a struct pointer, and you can’t forward declare a struct property (as it would not know the size).
5

I have CoreMinimal.h included. You cannot create a pointer for TSubclassOf, so how are you getting it to completely work?


class AItem;

USTRUCT(BlueprintType)
struct FItemStruct
{
    GENERATED_BODY()

    UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Inventory")
        TSubclassOf<AItem> myItem; //Error, does not compile.   Cannot make it pointer either, error for being a pointer
};

[HR][/HR]

Omgosh duh… I forgot I could do that, Its been way too long since I’ve had to. Thanks! (This is my first time on these forums, idk how to, if I even can, accept your response as the answer)
[HR][/HR]

Thank you, that’s very good to know! I’ll certainly keep that in mind for future reference.

6

Hello, can you explain me what exactly you are saying? I am stuck in similar problem now. I want 2 Pawns to assess each other, call functions on eachother, gives me ability to choose another pawn in details panel etc. Can you please help me?

7

Just ran into another iteration of this:
TSubclassOf works fine in arrays and variables but when being used in a TMap as the key, VS compiler throws an error.