Question

The library I’m using has class G and class S which inherits G.

I needed to add functionality to them, so I wrapped G and S, rather I inherited from them making Gnew and Snew respectively.

So, my inheritance is:

   
 G --> Gnew  
 |      
 v    
 S --> Snew  

But, I want to use Gnew in Snew and when I try to include the Gnew header (in the Snew implementation file) to use it … the include guards mask the definition of Gnew in Snew???

How can I use Gnew in Snew? Right now, the compiler wont even let me forward declare Gnew in the Snew definition file (which doesn’t make sense to me) unless I forward declare inside the class.

In Snew (if I forward declare before the Snew definition) I have:

...
Gnew *g;

The error is:

error: ISO C++ forbids declaration of ‘Gnew’ with no type

If I change Snew to say:

...
class Gnew *g;
Gnew *g;

The error is:

error: invalid use of undefined type ‘struct Snew::Gnew’

NOTE:
I was trying to abstract the problem, so I’m closing this and reopening a better phrasing of the question …

Answer 1

Where’s the cycle? And why would Gnew include the header of Snew?

[Edit]
OK, I think your inheritance arrows are opposite of what’s customary. But this should get you sorted out:

In Gnew.h:

#pragma once
#if !defined(Gnew_h)
#define Gnew_h

#include "G.h"

class Gnew : public virtual G
{
  // added functionality here.
};

#endif // Gnew_h

In Snew.h:

#pragma once
#if !defined(Snew_h)
#define Snew_h

#include "S.h"
#include "Gnew.h"

class Snew : public virtual Gnew, public virtual S
{
  // added functionality here.
};

#endif // Snew_h

You should not have to forward declare anything.

Note however that this only works as expected if S inherits virtually from G. If all these multiple inheritance issues are too much trouble, you should probably just adapt the library classes instead of inheriting from them.

Does this help?