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>());
}