Question

I have a situation as such

A.h

#ifndef _CLASSA
#define _CLASSA
class B;

class A {
    virtual void addTo(B*) {}
};

#endif

B.h

#ifndef _CLASSB
#define _CLASSB

#include "A.h"

class B : public A {
    B();
    void addTo(B* b) {
         // blah blah blah
    }
};

#endif

B.cpp

#include "B.h"

B::B() : A() {}

main.cpp

#include "B.h"

int main() {
    A* b = new B();
    B* d = new B();
    b->addTo(d);
}

The project won’t compile. If I forward declare B in the A header, the compiler complains about the expectation of a class in B.h. If I include the B.h header in A.h, the compiler can’t resolve the base class. Is this possible?

Answer 1

Yes, include A.h in B.h and forward declare B in A.h.

If I forward declare B in the A header, the compiler complains about the expectation of a class in B.h

The way you have the code now it should work. But I suspect you’re actually using B inside the method addTo, in which case a forward declaration is not enough. You need to separate the implementation to an implementation file and include B there.

EDIT: As DeadMg pointed out, class B : class A { isn’t valid syntax, you probably want class B : A or class B : public A.