declaring function member as friend

dfmf.cpp

#include <iostream>

class B; /// forward declaration to know the types in function members in Class A

class A
{
public:
    void modify (B &,const int &);
    int &display (B &);

};

class B
{
public:
    friend void A::modify (B &,const int &); /// declares function member A::modify a friend
    friend int &A::display (B &); /// the same

private:
    int data_no = 0;
};

/// declaring this after the declaration of class b to know the compiler what type can be accessed

void A::modify(B &b,const int &i)
{
    b.data_no = i;
}

int &A::display (B &b)
{
    return b.data_no;
}


int main ()
{
    A a;
    B b;

    a.modify(b, 22);
    std::cout << a.display(b) << std::endl;

    return 0;
}