Question

I’ve been reading some on forward declarations, including in this forum. They all say that it saves us from including the header file, However the following code generates an error:

#ifndef CLASSA_H_
#define CLASSA_H_

class B;

class A {

public:
    A();
    ~A();
    int getCount();
private:
    static int _count;
    int _num;
    B _b1;   //ERROR

};

compiler says:
A.h:23: error: field ‘_b1’ has incomplete type

I noticed that if i make _b1 of type B* the problem is solved.

So is forward declaration good only for pointer types?
If i want A to hold B object i have to #inlcude "B.h" ?

thanks!

Answer 1

The compiler has to know the exact definition of class B to determine at least what size to give to class A. If you use a pointer, it knows its size.

Note that circular dependencies are not possible. If you want

class A { B b; };
class B { A a; };

then A and B must have infinite size…