125 lines
5.0 KiB
HTML
125 lines
5.0 KiB
HTML
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<head>
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<title>Using the GNU Compiler Collection (GCC): Namespace Association</title>
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" rel="up" title="C++ Extensions">
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<a name="Namespace-Association"></a>
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<p>
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Next: <a href="Type-Traits.html#Type-Traits" accesskey="n" rel="next">Type Traits</a>, Previous: <a href="Function-Multiversioning.html#Function-Multiversioning" accesskey="p" rel="prev">Function Multiversioning</a>, Up: <a href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" accesskey="u" rel="up">C++ Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
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<a name="Namespace-Association-1"></a>
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<h3 class="section">7.9 Namespace Association</h3>
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<p><strong>Caution:</strong> The semantics of this extension are equivalent
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to C++ 2011 inline namespaces. Users should use inline namespaces
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instead as this extension will be removed in future versions of G++.
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</p>
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<p>A using-directive with <code>__attribute ((strong))</code> is stronger
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than a normal using-directive in two ways:
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</p>
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<ul>
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<li> Templates from the used namespace can be specialized and explicitly
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instantiated as though they were members of the using namespace.
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</li><li> The using namespace is considered an associated namespace of all
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templates in the used namespace for purposes of argument-dependent
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name lookup.
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</li></ul>
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<p>The used namespace must be nested within the using namespace so that
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normal unqualified lookup works properly.
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</p>
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<p>This is useful for composing a namespace transparently from
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implementation namespaces. For example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">namespace std {
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namespace debug {
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template <class T> struct A { };
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}
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using namespace debug __attribute ((__strong__));
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template <> struct A<int> { }; // <span class="roman">OK to specialize</span>
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template <class T> void f (A<T>);
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}
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int main()
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{
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f (std::A<float>()); // <span class="roman">lookup finds</span> std::f
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f (std::A<int>());
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}
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</pre></div>
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