What does a leading :: mean in “using namespace ::X” in C++

Question

can somebody explain me the difference between the following namespace usages:

using namespace ::layer::module;

and

using namespa

Answer 1

A leading :: refers to the global namespace. Any qualified identifier starting with a :: will always refer to some identifier in the global namespace. The difference is when you have the same stuff in the global as well as in some local namespace:

namespace layer { namespace module {
    void f();
} }

namespace blah { 
  namespace layer { namespace module {
      void f();
  } }

  using namespace layer::module // note: no leading ::
                                // refers to local namespace layer
  void g() {
    f(); // calls blah::layer::module::f();
  }
}

namespace blubb {
  namespace layer { namespace module {
      void f();
  } }

  using namespace ::layer::module // note: leading ::
                                  // refers to global namespace layer
  void g() {
    f(); // calls ::layer::module::f();
  }
}