function member as friend

fmf.cpp

#include <iostream>

class Person;

class option
{
public:

    typedef int number;

    option () = default;
    option (std::string &, std::string &, number &, number &);

    void modify (const std::string &,const std::string &, Person &);
    void modify (std::string &, std::string &, number &, number&, Person &);


};

class Person
{
    public:
        friend void display (Person &, std::ostream &);
        typedef int number;

        Person () = default;
        Person (const std::string &n, const std::string &a, const number &ag, const number &id) :
                name (n), address (a), age (ag), idno (id) { }

        Person (const std::string &n, const std::string &a) :
                name (n), address (a) { }

        friend void option::modify (const std::string &,const std::string &, Person &);
        friend void option::modify (std::string &, std::string &, number &, number &, Person &);

    private:
        std::string name, address;
        number age = 0, idno = 0;

};

void option::modify (const std::string &n,const std::string &a, Person &p1)
{
    p1.name = n;
    p1.address = a;
}

void option::modify (std::string &n, std::string &a, number &ag, number &id, Person &p1)
{
    p1.name = n;
    p1.address = a;
    p1.age = ag;
    p1.idno = id;

}

void display (Person &p1, std::ostream & os)
{
    os << std::endl
       << p1.name << std::endl
       << p1.address << std::endl
       << p1.age << std::endl
       << p1.idno << std::endl;
}

int main ()
{
    Person p1("kryssel", "las pinas city", 22, 23);

    display (p1, std::cout);

    option op1;

    op1.modify ("kryssel tillada", "las pinas city", p1);

    display (p1, std::cout);

    return 0;
}