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<html lang="en"><head><title>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 4.6"><!--Copyright © 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,1999, 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc. <p>Permission is granted to copy, distribute and/or modify this documentunder the terms of the GNU Free Documentation License, Version 1.2 orany later version published by the Free Software Foundation; with theInvariant Sections being "GNU General Public License" and "FundingFree Software", the Front-Cover texts being (a) (see below), and withthe Back-Cover Texts being (b) (see below). A copy of the license isincluded in the section entitled "GNU Free Documentation License". <p>(a) The FSF's Front-Cover Text is: <p>A GNU Manual <p>(b) The FSF's Back-Cover Text is: <p>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 }--></style></head><body><div class="node"><p>Node: <a name="Strong%20Using">Strong Using</a>,Next: <a rel="next" accesskey="n" href="Offsetof.html#Offsetof">Offsetof</a>,Previous: <a rel="previous" accesskey="p" href="C---Attributes.html#C++%20Attributes">C++ Attributes</a>,Up: <a rel="up" accesskey="u" href="C---Extensions.html#C++%20Extensions">C++ Extensions</a><hr><br></div><h3 class="section">Strong Using</h3><p><strong>Caution:</strong> The semantics of this extension are not fullydefined. Users should refrain from using this extension as itssemantics may change subtly over time. It is possible that thisextension wil be removed in future versions of G++. <p>A using-directive with <code>__attribute ((strong))</code> is strongerthan a normal using-directive in two ways: <ul><li>Templates from the used namespace can be specialized as though they were members of the using namespace. <li>The using namespace is considered an associated namespace of alltemplates in the used namespace for purposes of argument-dependentname lookup. </ul> <p>This is useful for composing a namespace transparently fromimplementation namespaces. For example:<pre class="smallexample"> namespace std { namespace debug { template <class T> struct A { }; } using namespace debug __attribute ((__strong__)); template <> struct A<int> { }; // ok to specialize template <class T> void f (A<T>); } int main() { f (std::A<float>()); // lookup finds std::f f (std::A<int>()); } </pre> </body></html>?? 快捷鍵說明
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