How to solve the problem of mutual inclusion of header files in CPP ?

Question

For example :
// A.h
#ifndef A_H
#define A_H
#include "B.h"
class A {
B b;
}
#endif

//B.h
#ifndef B_H
#define B_H
#include "A.h"
class B {
A a;
}
#endif

one header requires the others, thus the compiler prints an error that one of the types A or B is undeclared.

Answer 1

I don't think this is possible. If you compile A.h, B.h will be compiled first, which has a member A and at this time it's undefined. Same thing for B.

The best you can do is to forward declare class A and B. Though you still can't make it work, this works for pointer and reference types.

You can refer to this post: https://stackoverflow.com/questions/8526819/c-header-files-including-each-other-mutually

Answer 2

and, also it's unnecessary to add two marcos since you aren't using them anywhere in your code. either, you can remove it or use #pragma once