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| <h3 class="section">7.8 Namespace Association</h3> |
| |
| <p><strong>Caution:</strong> The semantics of this extension are not fully |
| defined. Users should refrain from using this extension as its |
| semantics may change subtly over time. It is possible that 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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