circular includes

A.cpp

#include "A.hpp"
#include "B.hpp"

void A::f(B* b)
{
  b->g(this);
}

A.hpp

#pragma once

struct B;

struct A
{
  void f(B* b);
};

B.cpp

#include "B.hpp"
#include "A.hpp"

void B::g(A* a)
{
  a->f(this);
}

B.hpp

#pragma once

struct A;

struct B
{
  void g(A* a);
};