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<h3 class="section">7.9 Namespace Association</h3>
<p><strong>Caution:</strong> 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++.
<p>A using-directive with <code>__attribute ((strong))</code> is stronger
than a normal using-directive in two ways:
<ul>
<li>Templates from the used namespace can be specialized and explicitly
instantiated as though they were members of the using namespace.
<li>The using namespace is considered an associated namespace of all
templates in the used namespace for purposes of argument-dependent
name lookup.
</ul>
<p>The used namespace must be nested within the using namespace so that
normal unqualified lookup works properly.
<p>This is useful for composing a namespace transparently from
implementation namespaces. For example:
<pre class="smallexample"> namespace std {
namespace debug {
template <class T> struct A { };
}
using namespace debug __attribute ((__strong__));
template <> struct A<int> { }; // <span class="roman">OK to specialize</span>
template <class T> void f (A<T>);
}
int main()
{
f (std::A<float>()); // <span class="roman">lookup finds</span> std::f
f (std::A<int>());
}
</pre>
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