C / C++ / MFC

//ClassA.h
#pragma once;

#include <iostream>
using namespace std;

#include "ClassB.h"
class ClassB;

class ClassA {
public:
	ClassA () {
		this->b = new ClassB(this);
	}

	void printer() {
		cout << "Hello world from A!\n";
	}

private:
	ClassB* b;

};

//ClassB.h
#pragma once;

#include <iostream>
using namespace std;

#include "ClassA.h"
class ClassA;

class ClassB {
public:
	ClassB(ClassA *a) {
		this->ca = a;
		cout << "Constructor B \n\n";
		ca->printer();
	}

private:
	ClassA *ca;
};

#include <windows.h>

#include <iostream>
using namespace std;

#include "ClassA.h"
#include "ClassB.h"

void main() {
	ClassA a;
	Sleep(10000);
}