forward declarations

Question

Why do I need to use forward declarations for a class when I am already including the relevant header file? It has fixed my problem but confused me!

essentially clas

Answer 1

If you have a circular declaration dependence, you'll probably have something like this:

• A.h defined class A and starts with #include "B.h"

• B.h defined class B and starts with #include "A.h"

Now, your header files will have include guards, so they're only included once. So in the context of B.h, there's only a single inclusion of A.h, so you'll get something that's effectively like this:

class A { B * bp; };  // from `#include "A.h"`

class B { A * ap; };

Now you're stuck, because the first line is incorrect without having the declaration of B. You fix this by adding a forward declaration to A.h:

// A.h
#ifndef H_A
#define H_A

class B;
class A { B * bp; };

#endif