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

The second case might be X::layer::module where using namespace X has already happened.

In the first case the prefix :: means "compiler, don't be clever, start at the global namespace".

Answer 2

There would be a difference if it was used in a context such as:

namespace layer {
    namespace module {
        int x;
    }
}

namespace nest {
    namespace layer {
        namespace module {
            int x;
        }
    }
    using namespace /*::*/layer::module;
}

With the initial :: the first x would be visible after the using directive, without it the second x inside nest::layer::module would be made visible.

Answer 3

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();
  }
}

Answer 4

It is called as Qualified name lookup in C++.

It means that the layer namespace being referred to is the one off the global namespace, rather than another nested namespace named layer.

For Standerdese fans:
$3.4.3/1

"The name of a class or namespace member can be referred to after the :: scope resolution operator (5.1) applied to a nested-name-specifier that nominates its class or namespace. During the lookup for a name preceding the :: scope resolution operator, object, function, and enumerator names are ignored. If the name found is not a class-name (clause 9) or namespace-name (7.3.1), the program is ill-formed."