Circular includes and forward declarations

[Rossman231]

Hi, so I'm having a bit of an issue, and I know it has to do with the fact that I have circular includes, but it seems somewhat necessary. Here's essentially what I have:

// Base.h
#pragma once

#include "SpecificChild.h"

class Base {
SpecificChild* mySpecificChild;
};

// SpecificChild.h
#pragma once

#include "Base.h"

class SpecificChild : public Base {
};

The issue is that when Base.cpp gets compiled, it includes Base.h. Base.h in turn includes SpecificChild.h. SpecificChild.h tries to include Base.h but sees nothing because of the #pragma once (essentially an #ifdef _BASE_H ... #endif for those not familiar with MSVC). I have had this issue before when I was dealing with two different, unrelated classes, and that was fixed by putting a simple forward declaration in each header file. However, putting "class Base;" above SpecificChild's declaration (and putting "class SpecificChild;" above Base's declaration just in case), still does not help. I get the following error:
"'Base' : base class undefined"

Obviously, and easy and cheap solution to this would be just to change the reference to SpecificChild in Base to just a Base, but I would like to use stuff specific to SpecificChild, and it seems pointless to cast it every time I want to do so.

Is there some specific way of forwardly declaring a class that you intend to use as a base? Thanks.

[treuss]

The problem is not the includes but the logic of your class struction.

SpecificChild is derived from Base, so a SpecificChild object is also a Base object.

Now a Base object includes a SpecificChild object, which means each Base object includes a Base object which includes a Base object which includes ....

How would such an object look like? Infinitely large? Or infinitely small as it does not contain anything other than infinitely small objects?

PS: Congratulations!!! First post and correct usage of code tags ! Haven't seen that for quite some time.

[Rossman231]

Ok, well I didn't actually compile the thing given, it's really a pointer. I'll fix it in the original post. That fixes the whole issue with a Base being in a Base (which of course is a huge issue).

[treuss]

So now you can replace your include

#include "SpecificChild.h"

with a forward declaration

class SpecificChild;

Problem solved.

[Rossman231]

Alright that seems to work, thank you!