Forward Declaration of class in C++.

a.cpp

#include "a.h"
#include "b.h"

std::string A::name() {
  return "class A";
}

void A::check(B* b) {
    std::cout << "From class A: object is from ";
    b->name();
}

a.h

#include <iostream>
#ifndef a_h
#define a_h
#include "b.h"

class B;

class A {
    public:
        A()= default;
        std::string name();
        void check(B* b);
};
#endif // end a_h

b.cpp

#include "b.h"
#include "a.h"

void B::check(A& a) {
    std::string name =  a.name();
    std::cout << "From class B: object is from " << name << std::endl;
}
B::~B() {
    std::cout << "B object destroyed\n";
}
void B::name() {
  std::cout <<  "class B\n";
}

b.h

#ifndef b_h
#define b_h
#include <iostream>
#include "a.h"

class A;

class B {
    public:
        void name();
        void check(A& a);
        ~B();
};
#endif // end b_h

main.cpp

// we can include both 'a.h' and 'b.h' or either one
#include <iostream>
#include "b.h"
#include "a.h"

using namespace std;

int main() {
    cout<<"Forward Declaration in C++" << endl;
    B *b = new B;
    A a;
    b->check(a);
    a.check(b);
    delete b; // without this the object b is not destroyed
    return 0;
}