Using the GNU Compiler Collection (GCC): Namespace Association

Next: Type Traits, Previous: Function Multiversioning, Up: C++ Extensions [Contents][Index]

### 7.9 Namespace Association

Caution: The semantics of this extension are equivalent to C++ 2011 inline namespaces. Users should use inline namespaces instead as this extension will be removed in future versions of G++.

A using-directive with `__attribute ((strong))` is stronger than a normal using-directive in two ways:

- Templates from the used namespace can be specialized and explicitly instantiated as though they were members of the using namespace.
- The using namespace is considered an associated namespace of all templates in the used namespace for purposes of argument-dependent name lookup.

The used namespace must be nested within the using namespace so that normal unqualified lookup works properly.

This is useful for composing a namespace transparently from implementation namespaces. For example:

    namespace std {
      namespace debug {
        template <class T> struct A { };
      }
      using namespace debug __attribute ((__strong__));
      template <> struct A<int> { };   // ok to specialize

      template <class T> void f (A<T>);
    }

    int main()
    {
      f (std::A<float>());             // lookup finds std::f
      f (std::A<int>());
    }