#ifndef _A_h
#define _A_h

class A{
    public:
        A(int id);
    private:
        int _id;
        B _b; // HERE I GET A COMPILATION ERROR: B does not name a type
};

#endif

#include "A.h"
#include "B.h"
#include <cstdio>

A::A(int id): _id(id), _b(){
    printf("hellon the id is: %dn", _id);
}

#ifndef _B_h
#define _B_h

class B{
    public:
        B();
};
#endif

#include "B.h"
#include <cstdio>

B::B(){
    printf("this is hello from Bn");
}

class B; // forward declaration
class A {
public:
    A(int id);
private:
    int _id;
    B & _b;
};

error 'Class' does not name a type