friend7.c

this is a gcc file, you can download it and learn its usetility, for further detail please look at t

// Origin: Martin v. L鰓is  <[e-mail]>
// Test that a friend declaration with an explicit :: finds the right fn.
// Build don't link:

namespace M {
class S; 
}
void foo(M::S *);

namespace M {
class S {
  friend void (::foo)(S *);
  void Fn(); 
  static S s;
};
} 

void (::foo)(M::S *ptr) {
  M::S::s.Fn();
  ptr->Fn();
}