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97 lines
3.8 KiB
HTML
97 lines
3.8 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Namespace Association - Using the GNU Compiler Collection (GCC)</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="Using the GNU Compiler Collection (GCC)">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" title="C++ Extensions">
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<link rel="prev" href="Function-Multiversioning.html#Function-Multiversioning" title="Function Multiversioning">
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<link rel="next" href="Type-Traits.html#Type-Traits" title="Type Traits">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Funding Free Software'', the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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``GNU Free Documentation License''.
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development.-->
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<style type="text/css"><!--
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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Namespace-Association"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Type-Traits.html#Type-Traits">Type Traits</a>,
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Previous: <a rel="previous" accesskey="p" href="Function-Multiversioning.html#Function-Multiversioning">Function Multiversioning</a>,
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Up: <a rel="up" accesskey="u" href="C_002b_002b-Extensions.html#C_002b_002b-Extensions">C++ Extensions</a>
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<hr>
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</div>
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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>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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<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>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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</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>This is useful for composing a namespace transparently from
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implementation namespaces. For example:
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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>
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</body></html>
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