Hidden friend idiom

hidden_friend_idiom.cpp

namespace N {
  class MyType {
    Mytype& operator+=(const MyType&);
    void print(std::ostream&);
    void swap(MyType&) noexcept;
    
    friend MyType operator+(MyType a, const MyType& b) {
      a += b;
      return a;
    }
    
    friend std::ostream& operator<<(std::ostream& os, const MyType& b) {
      b.print(os);
      return os;
    }
    
    friend void swap(MyType& a, MyType& b) noexcept {
      a.swap(b);
    }
  };
} //namespace N

void test(N::MyType& t) {
  t + t;          // considers N::operator+
  std::cout << t; // considers N::operator<<
  swap(t, t);     // considers N::swap
}