c++ undefined reference in extraction operator overloading

Question

Learning c++ bit by bit in codeblocks, I encountered this error. I have tried it for hours and saw various links but its still not working,

main.cpp

#include <iostream> 
#include "Person.h"

using namespace std;

int main()
{
  foo:: Person p;
  //std:: cin >> p;
  std:: cout << p;
  return 0;
 }

Person.h

#ifndef PERSON_H
#define PERSON_H

namespace foo{
class Person
{
   public:
   int age;
   Person();
   friend std:: ostream& operator << (std::ostream&,const Person&);
 //friend std:: istream& operator << (std:: istream& in, Person& p);
   protected:
   private:

  // std::string name;
};

}
#endif // PERSON_H

Person.cpp

#include <iostream>
#include "Person.h"

using namespace foo;

Person::Person()
{
   age = 0;
  //name = "noname";
}


std:: ostream& operator <<(std:: ostream& out, Person const &p)
{
  out << "Extraction operator" << std:: endl << p.age << std:: endl;
  // out << p.name << std::endl;
  return out;
}

/*std:: istream& operator >>(std:: istream& in, Person &p)
{
  in >> p.age >> p.name;
  return in;
}*/

Q1. Why do we need to create the header file and Person.cpp in a seperate namespace? Guess: Is it because cout would simply mean the global namespace and then we again have a overloaded cout so which definition is the compiler going to call its not sure?

Q2. By creating a object p of Person class in foo namespace in main and calling std:: cout << p, what are we trying to do? (std is in std namespace and we want to call the overloaded operator)

Error:

undefined reference to foo:: operator<<(std::ostream&, foo::Person const&)') 

If we write the age in private it says that its is private but being a friend it should have access to private members. I know it's something related to namespace but I can't find out the reason.

no match for operator >> in std::cin Earlier it was giving the above and many other error so I tried using two namespaces but still its not working.

Answer 1

You defined operator<< in the global namespace, but declared it inside namespace foo .

Define it inside the namespace foo :

namespace foo
{
    // definition
}

Answer 2

I think you using wrong operator declaration

//friend std:: istream& operator << (std:: istream& in, Person& p);

instead of

friend std:: istream& operator >> (std:: istream& in, Person& p);

and also define your class methods in namespace where it were declared

#include <iostream>
#include "Person.h"

namespace foo
{

    Person::Person()
    {
       age = 0;
      //name = "noname";
    }


    std:: ostream& operator <<(std:: ostream& out, Person const &p)
    {
      out << "Extraction operator" << std:: endl << p.age << std:: endl;
      // out << p.name << std::endl;
      return out;
    }

    std:: istream& operator >>(std:: istream& in, Person &p)
    {
      in >> p.age >> p.name;
      return in;
    }

}

the code below runs perfectly on MS Visual C++

#include "stdafx.h"
#include <iostream>
#include <conio.h>
#include <string>

namespace foo{
    class Person
    {
       public:
       Person();
       friend std:: ostream& operator << (std::ostream&,const Person&);
       friend std:: istream& operator >> (std:: istream& in, Person& p);
       protected:
       private:
       int age;
       std::string name;
    };
}

namespace foo
{

    Person::Person()
    {
       age = 0;
       name = "noname";
    }


    std:: ostream& operator <<(std:: ostream& out, Person const &p)
    {
      out << "Extraction operator" << std:: endl << p.age << std:: endl;
      out << p.name << std::endl;
      return out;
    }

    std:: istream& operator >>(std:: istream& in, Person &p)
    {
      in >> p.age >> p.name;
      return in;
    }

}

int main()
{
  foo:: Person p;
  std:: cin >> p;
  std:: cout << p;
  _getch();
  return 0;
 }