| <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="C_002b_002b-Attributes.html#C_002b_002b-Attributes" title="C++ Attributes"> |
| <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, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, |
| 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, |
| 2008 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 1.2 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> |
| <link rel="stylesheet" type="text/css" href="../cs.css"> |
| </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="C_002b_002b-Attributes.html#C_002b_002b-Attributes">C++ Attributes</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">7.8 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> |
| </body></html> |
| |
