Question

I’m trying to help a coworker get something compiled – essentially, he was trying to reduce dependencies for a small executable we need to make from a larger software system.

I’m not sure I can fully explain the problem as I don’t completely understand it… but I’m going to show what’s going on here:

Library A:  File: A.h

namespace CF {
    typedef sometype B;
};

Library C:   File C.h

//Forward declare Class
class CF::B;

Class D {
    public:
        B* UseB();
};

Library C:   File C.cpp

#include "C.h"
#include "A.h"
using CF::B;

B* D::UseB()
{
    return new B;
}

Sorry, I know this looks a little crazy but I have tried to simplify it from the set of files that we’re actually dealing with.

We’re typically getting either a multiple definition error on CF::B, or when we play with the code and change it around, sometimes in the CPP file it just doesn’t recognize the type of CF::B.

I guess my first question is… can I forward declare the typedef like we’ve tried, or is there some other way to deal with the fact that B is a typedef in CF namespace, and we don’t want it to be directly included in the C.h file?

Answer 1

A forward declaration would be more like

namespace CF { class B; }

The compiler cannot make anything out of CF::B unless it already knows CF to be a namespace.

You also cannot forward declare a typedef, because the compiler must know if B is a class or a built in type. Some built in types have special rules, like char* or void*.