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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:

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>());