9. Non-member binary operator syntax

• 
  // rational.h
  #include <iostream>
  using namespace std;
  class Rational {
      friend ostream& operator<<( ostream& os, Rational const& rat );
      //...
  };
  
  // rational.cpp
  ostream& operator<<( ostream& os, Rational const& rat )
  {
      os << "(" << rat.m_n << "/" << rat.m_d << ")";
      return os;
  }
  
  // main.cpp
  #include <iostream>
  #include "rational.h"
  using namespace std;
  int main( )
  {
      Rational r1;
      cout << r1 << endl;
      return 0;
  }