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186 lines
9.3 KiB
HTML
186 lines
9.3 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2018 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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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>C++ Interface (Using the GNU Compiler Collection (GCC))</title>
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<meta name="description" content="C++ Interface (Using the GNU Compiler Collection (GCC))">
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<meta name="keywords" content="C++ Interface (Using the GNU Compiler Collection (GCC))">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" rel="up" title="C++ Extensions">
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<link href="Template-Instantiation.html#Template-Instantiation" rel="next" title="Template Instantiation">
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<link href="Vague-Linkage.html#Vague-Linkage" rel="prev" title="Vague Linkage">
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<style type="text/css">
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<!--
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a.summary-letter {text-decoration: none}
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blockquote.indentedblock {margin-right: 0em}
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blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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div.lisp {margin-left: 3.2em}
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div.smalldisplay {margin-left: 3.2em}
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div.smallexample {margin-left: 3.2em}
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div.smalllisp {margin-left: 3.2em}
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kbd {font-style: oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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pre.menu-comment {font-family: serif}
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pre.menu-preformatted {font-family: serif}
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pre.smalldisplay {font-family: inherit; font-size: smaller}
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pre.smallexample {font-size: smaller}
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pre.smallformat {font-family: inherit; font-size: smaller}
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pre.smalllisp {font-size: smaller}
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span.nolinebreak {white-space: nowrap}
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span.roman {font-family: initial; font-weight: normal}
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span.sansserif {font-family: sans-serif; font-weight: normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="C_002b_002b-Interface"></a>
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<div class="header">
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<p>
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Next: <a href="Template-Instantiation.html#Template-Instantiation" accesskey="n" rel="next">Template Instantiation</a>, Previous: <a href="Vague-Linkage.html#Vague-Linkage" accesskey="p" rel="prev">Vague Linkage</a>, Up: <a href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" accesskey="u" rel="up">C++ Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="C_002b_002b-Interface-and-Implementation-Pragmas"></a>
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<h3 class="section">7.4 C++ Interface and Implementation Pragmas</h3>
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<a name="index-interface-and-implementation-headers_002c-C_002b_002b"></a>
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<a name="index-C_002b_002b-interface-and-implementation-headers"></a>
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<a name="index-pragmas_002c-interface-and-implementation"></a>
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<p><code>#pragma interface</code> and <code>#pragma implementation</code> provide the
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user with a way of explicitly directing the compiler to emit entities
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with vague linkage (and debugging information) in a particular
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translation unit.
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</p>
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<p><em>Note:</em> These <code>#pragma</code>s have been superceded as of GCC 2.7.2
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by COMDAT support and the “key method” heuristic
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mentioned in <a href="Vague-Linkage.html#Vague-Linkage">Vague Linkage</a>. Using them can actually cause your
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program to grow due to unnecessary out-of-line copies of inline
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functions.
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</p>
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<dl compact="compact">
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<dt><code>#pragma interface</code></dt>
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<dt><code>#pragma interface "<var>subdir</var>/<var>objects</var>.h"</code></dt>
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<dd><a name="index-_0023pragma-interface"></a>
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<p>Use this directive in <em>header files</em> that define object classes, to save
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space in most of the object files that use those classes. Normally,
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local copies of certain information (backup copies of inline member
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functions, debugging information, and the internal tables that implement
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virtual functions) must be kept in each object file that includes class
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definitions. You can use this pragma to avoid such duplication. When a
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header file containing ‘<samp>#pragma interface</samp>’ is included in a
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compilation, this auxiliary information is not generated (unless
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the main input source file itself uses ‘<samp>#pragma implementation</samp>’).
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Instead, the object files contain references to be resolved at link
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time.
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</p>
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<p>The second form of this directive is useful for the case where you have
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multiple headers with the same name in different directories. If you
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use this form, you must specify the same string to ‘<samp>#pragma
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implementation</samp>’.
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</p>
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</dd>
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<dt><code>#pragma implementation</code></dt>
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<dt><code>#pragma implementation "<var>objects</var>.h"</code></dt>
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<dd><a name="index-_0023pragma-implementation"></a>
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<p>Use this pragma in a <em>main input file</em>, when you want full output from
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included header files to be generated (and made globally visible). The
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included header file, in turn, should use ‘<samp>#pragma interface</samp>’.
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Backup copies of inline member functions, debugging information, and the
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internal tables used to implement virtual functions are all generated in
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implementation files.
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</p>
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<a name="index-implied-_0023pragma-implementation"></a>
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<a name="index-_0023pragma-implementation_002c-implied"></a>
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<a name="index-naming-convention_002c-implementation-headers"></a>
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<p>If you use ‘<samp>#pragma implementation</samp>’ with no argument, it applies to
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an include file with the same basename<a name="DOCF4" href="#FOOT4"><sup>4</sup></a> as your source
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file. For example, in <samp>allclass.cc</samp>, giving just
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‘<samp>#pragma implementation</samp>’
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by itself is equivalent to ‘<samp>#pragma implementation "allclass.h"</samp>’.
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</p>
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<p>Use the string argument if you want a single implementation file to
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include code from multiple header files. (You must also use
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‘<samp>#include</samp>’ to include the header file; ‘<samp>#pragma
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implementation</samp>’ only specifies how to use the file—it doesn’t actually
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include it.)
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</p>
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<p>There is no way to split up the contents of a single header file into
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multiple implementation files.
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</p></dd>
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</dl>
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<a name="index-inlining-and-C_002b_002b-pragmas"></a>
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<a name="index-C_002b_002b-pragmas_002c-effect-on-inlining"></a>
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<a name="index-pragmas-in-C_002b_002b_002c-effect-on-inlining"></a>
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<p>‘<samp>#pragma implementation</samp>’ and ‘<samp>#pragma interface</samp>’ also have an
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effect on function inlining.
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</p>
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<p>If you define a class in a header file marked with ‘<samp>#pragma
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interface</samp>’, the effect on an inline function defined in that class is
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similar to an explicit <code>extern</code> declaration—the compiler emits
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no code at all to define an independent version of the function. Its
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definition is used only for inlining with its callers.
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</p>
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<a name="index-fno_002dimplement_002dinlines-1"></a>
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<p>Conversely, when you include the same header file in a main source file
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that declares it as ‘<samp>#pragma implementation</samp>’, the compiler emits
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code for the function itself; this defines a version of the function
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that can be found via pointers (or by callers compiled without
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inlining). If all calls to the function can be inlined, you can avoid
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emitting the function by compiling with <samp>-fno-implement-inlines</samp>.
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If any calls are not inlined, you will get linker errors.
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</p>
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<div class="footnote">
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<hr>
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<h4 class="footnotes-heading">Footnotes</h4>
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<h3><a name="FOOT4" href="#DOCF4">(4)</a></h3>
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<p>A file’s <em>basename</em>
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is the name stripped of all leading path information and of trailing
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suffixes, such as ‘<samp>.h</samp>’ or ‘<samp>.C</samp>’ or ‘<samp>.cc</samp>’.</p>
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</div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Template-Instantiation.html#Template-Instantiation" accesskey="n" rel="next">Template Instantiation</a>, Previous: <a href="Vague-Linkage.html#Vague-Linkage" accesskey="p" rel="prev">Vague Linkage</a>, Up: <a href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" accesskey="u" rel="up">C++ Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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