using9.c

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

// Build don't link:
// 
// Copyright (C) 2000 Free Software Foundation, Inc.
// Contributed by Nathan Sidwell 26 Feb 2001 <[e-mail]>

// Bug 75. using declarations cannot introduce functions which ambiguate
// those in the current namespace, BUT here we're reaccessing the current
// namespace -- the function is not being 'introduced'.

extern int a();
struct x {};

using ::x;
using ::a;

extern "C" void foo ();

namespace {
  extern "C" int foo ();
  using ::foo; // ERROR - already in use
}