1<html lang="en"> 2<head> 3<title>Namespace Association - Using the GNU Compiler Collection (GCC)</title> 4<meta http-equiv="Content-Type" content="text/html"> 5<meta name="description" content="Using the GNU Compiler Collection (GCC)"> 6<meta name="generator" content="makeinfo 4.13"> 7<link title="Top" rel="start" href="index.html#Top"> 8<link rel="up" href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" title="C++ Extensions"> 9<link rel="prev" href="C_002b_002b-Attributes.html#C_002b_002b-Attributes" title="C++ Attributes"> 10<link rel="next" href="Type-Traits.html#Type-Traits" title="Type Traits"> 11<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 12<!-- 13Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 141998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 152010 Free Software Foundation, Inc. 16 17Permission is granted to copy, distribute and/or modify this document 18under the terms of the GNU Free Documentation License, Version 1.3 or 19any later version published by the Free Software Foundation; with the 20Invariant Sections being ``Funding Free Software'', the Front-Cover 21Texts being (a) (see below), and with the Back-Cover Texts being (b) 22(see below). A copy of the license is included in the section entitled 23``GNU Free Documentation License''. 24 25(a) The FSF's Front-Cover Text is: 26 27 A GNU Manual 28 29(b) The FSF's Back-Cover Text is: 30 31 You have freedom to copy and modify this GNU Manual, like GNU 32 software. Copies published by the Free Software Foundation raise 33 funds for GNU development.--> 34<meta http-equiv="Content-Style-Type" content="text/css"> 35<style type="text/css"><!-- 36 pre.display { font-family:inherit } 37 pre.format { font-family:inherit } 38 pre.smalldisplay { font-family:inherit; font-size:smaller } 39 pre.smallformat { font-family:inherit; font-size:smaller } 40 pre.smallexample { font-size:smaller } 41 pre.smalllisp { font-size:smaller } 42 span.sc { font-variant:small-caps } 43 span.roman { font-family:serif; font-weight:normal; } 44 span.sansserif { font-family:sans-serif; font-weight:normal; } 45--></style> 46<link rel="stylesheet" type="text/css" href="../cs.css"> 47</head> 48<body> 49<div class="node"> 50<a name="Namespace-Association"></a> 51<p> 52Next: <a rel="next" accesskey="n" href="Type-Traits.html#Type-Traits">Type Traits</a>, 53Previous: <a rel="previous" accesskey="p" href="C_002b_002b-Attributes.html#C_002b_002b-Attributes">C++ Attributes</a>, 54Up: <a rel="up" accesskey="u" href="C_002b_002b-Extensions.html#C_002b_002b-Extensions">C++ Extensions</a> 55<hr> 56</div> 57 58<h3 class="section">7.8 Namespace Association</h3> 59 60<p><strong>Caution:</strong> The semantics of this extension are not fully 61defined. Users should refrain from using this extension as its 62semantics may change subtly over time. It is possible that this 63extension will be removed in future versions of G++. 64 65 <p>A using-directive with <code>__attribute ((strong))</code> is stronger 66than a normal using-directive in two ways: 67 68 <ul> 69<li>Templates from the used namespace can be specialized and explicitly 70instantiated as though they were members of the using namespace. 71 72 <li>The using namespace is considered an associated namespace of all 73templates in the used namespace for purposes of argument-dependent 74name lookup. 75</ul> 76 77 <p>The used namespace must be nested within the using namespace so that 78normal unqualified lookup works properly. 79 80 <p>This is useful for composing a namespace transparently from 81implementation namespaces. For example: 82 83<pre class="smallexample"> namespace std { 84 namespace debug { 85 template <class T> struct A { }; 86 } 87 using namespace debug __attribute ((__strong__)); 88 template <> struct A<int> { }; // <span class="roman">ok to specialize</span> 89 90 template <class T> void f (A<T>); 91 } 92 93 int main() 94 { 95 f (std::A<float>()); // <span class="roman">lookup finds</span> std::f 96 f (std::A<int>()); 97 } 98</pre> 99 </body></html> 100 101