<html lang="en"> <head> <title>Namespace Association - Using the GNU Compiler Collection (GCC)</title> <meta http-equiv="Content-Type" content="text/html"> <meta name="description" content="Using the GNU Compiler Collection (GCC)"> <meta name="generator" content="makeinfo 4.13"> <link title="Top" rel="start" href="index.html#Top"> <link rel="up" href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" title="C++ Extensions"> <link rel="prev" href="Function-Multiversioning.html#Function-Multiversioning" title="Function Multiversioning"> <link rel="next" href="Type-Traits.html#Type-Traits" title="Type Traits"> <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> <!-- Copyright (C) 1988-2015 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being ``Funding Free Software'', the Front-Cover Texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the section entitled ``GNU Free Documentation License''. (a) The FSF's Front-Cover Text is: A GNU Manual (b) The FSF's Back-Cover Text is: You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.--> <meta http-equiv="Content-Style-Type" content="text/css"> <style type="text/css"><!-- pre.display { font-family:inherit } pre.format { font-family:inherit } pre.smalldisplay { font-family:inherit; font-size:smaller } pre.smallformat { font-family:inherit; font-size:smaller } pre.smallexample { font-size:smaller } pre.smalllisp { font-size:smaller } span.sc { font-variant:small-caps } span.roman { font-family:serif; font-weight:normal; } span.sansserif { font-family:sans-serif; font-weight:normal; } --></style> </head> <body> <div class="node"> <a name="Namespace-Association"></a> <p> Next: <a rel="next" accesskey="n" href="Type-Traits.html#Type-Traits">Type Traits</a>, Previous: <a rel="previous" accesskey="p" href="Function-Multiversioning.html#Function-Multiversioning">Function Multiversioning</a>, Up: <a rel="up" accesskey="u" href="C_002b_002b-Extensions.html#C_002b_002b-Extensions">C++ Extensions</a> <hr> </div> <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> </body></html>