Having trouble forward declaring a struct

Development Programming & Scripting
1

I’m forward declaring my struct here in VoxelizedActor.h:

#pragma once

#include "GameFramework/Actor.h"
#include "TheGrid.h"
#include "Grabbable.h"
#include "VoxelizedActor.generated.h"

struct FAxisOrientation;
...

UCLASS()
class VRGAME_API AVoxelizedActor : public AGrabbable
{
	GENERATED_BODY()
	
...
private:
	UPROPERTY()
	FAxisOrientation gridOrientation;
};

And I define it at the top of TheGrid.h:

#pragma once

#include "GameFramework/Actor.h"
#include "VoxelizedActor.h"
#include "TheGrid.generated.h"

USTRUCT(BlueprintType)
struct FAxisOrientation {
    GENERATED_BODY()
 public:
    UPROPERTY(EditAnywhere, BlueprintReadWrite, Category="AxisAlignedOrientation")
    uint8 x;  // axis * 90 = degree

    UPROPERTY(EditAnywhere, BlueprintReadWrite, Category="AxisAlignedOrientation")
    uint8 y;

    UPROPERTY(EditAnywhere, BlueprintReadWrite, Category="AxisAlignedOrientation")
    uint8 z;

    FAxisOrientation() : x(0), y(0), z(0) {}
    FAxisOrientation(uint8 X, uint8 Y, uint8 Z) : x(X), y(Y), z(Z) {}
};
...

But I keep getting the error: “VoxelizedActor.h(71): Unrecognized type ‘FAxisOrientation’ - type must be UCLASS, USTRUCT, or ENUM”

As far as I can tell, this is a circular dependency issue, but I’m not sure what I’m doing wrong. Can anyone help?

2

The reason it fails is because when declaring a field, the compiler must know the size of that type. While the compiler doesn’t know the size of FAxisOrientation, you could declare a field of type FAxisOrientation * instead.

If I were you, I would ditch the forward declaration and reverse the direction of includes: i.e. remove #include VoxelizedActor.h from your struct header and add #include TheGrid.h to the top of your class header.

3

Whoops - it seems you already #include TheGrid.h in your class header. But because you have circular includes, this include doesn’t actually do anything at the moment.

4

Hey Soup,

You have 2 issues:
The circular Includes.
And the incomplete type

Circular Includes appear when you have 2 classes:
A and B

//A.h

#include "B.h"

//B.h

#include "A.h"

This means that A.h includes itself which includes itself…and so on. The same goes with B

Incomplete Types:
If you use Member which are defined as Values (not Pointers), you have to include these because they have to have access to the default Constructor.
FAxisOrientation Orientation; //Calls Default Constructor.

If you want to have a member or type FAxisOrientation it has to be a pointer OR you have to #include the .h file which defines this struct.

Greetings

5

Thanks! This helped a lot. I switched the forward declaration around, and now everything works. And I got rid of the extra include - I hadn’t realized I only needed one when doing forward declarations.