Cross dependencies without forward declaring all used functions?

Question

I have class A (in A.h) which depends on class B in (B.h) and vice versa. Forward declaring the used functions works, but this means I have to update everywhere where I forw

Answer 1

The best way is to have a forward declaration header:

a.fwd.h

#pragma once
class A;

a.h

#pragma once
#include "a.fwd.h"
#include "b.fwd.h"
class A
{
    A(B*);
};

etc.

This way, each class provides its own forward declarations - localised alongside the header where it belongs - checked for consistency with the real declarations and definitions by including the forward declaration header in the header, and the header in the implementation.

Answer 2

If you only need to work with pointers or references to a class at the declaration level, you can do it like this:

A.h

class B; // forward class declaration

class A {
    A(B &);
};

B.h

class A;

class B {
    B(A &);
};

B.cpp

#include "B.h"
#include "A.h" // now we get the full declaration of A

B::B(A &a) {
    a.foo(5);
}

Mutual dependencies like this are tough to deal with but sometimes unavoidable.

If A and B depend on the implementations of each other, then you've got a system design problem that you need to resolve before proceeding further.