124 lines
5.4 KiB
HTML
124 lines
5.4 KiB
HTML
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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) 1988-2016 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 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Using the GNU Compiler Collection (GCC): Bound member functions</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC): Bound member functions">
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<meta name="keywords" content="Using the GNU Compiler Collection (GCC): Bound member functions">
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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="C_002b_002b-Attributes.html#C_002b_002b-Attributes" rel="next" title="C++ Attributes">
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<link href="Template-Instantiation.html#Template-Instantiation" rel="prev" title="Template Instantiation">
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<style type="text/css">
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</style>
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Bound-member-functions"></a>
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<div class="header">
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<p>
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Next: <a href="C_002b_002b-Attributes.html#C_002b_002b-Attributes" accesskey="n" rel="next">C++ Attributes</a>, Previous: <a href="Template-Instantiation.html#Template-Instantiation" accesskey="p" rel="prev">Template Instantiation</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="Extracting-the-Function-Pointer-from-a-Bound-Pointer-to-Member-Function"></a>
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<h3 class="section">7.6 Extracting the Function Pointer from a Bound Pointer to Member Function</h3>
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<a name="index-pmf"></a>
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<a name="index-pointer-to-member-function"></a>
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<a name="index-bound-pointer-to-member-function"></a>
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<p>In C++, pointer to member functions (PMFs) are implemented using a wide
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pointer of sorts to handle all the possible call mechanisms; the PMF
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needs to store information about how to adjust the ‘<samp>this</samp>’ pointer,
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and if the function pointed to is virtual, where to find the vtable, and
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where in the vtable to look for the member function. If you are using
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PMFs in an inner loop, you should really reconsider that decision. If
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that is not an option, you can extract the pointer to the function that
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would be called for a given object/PMF pair and call it directly inside
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the inner loop, to save a bit of time.
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</p>
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<p>Note that you still pay the penalty for the call through a
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function pointer; on most modern architectures, such a call defeats the
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branch prediction features of the CPU. This is also true of normal
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virtual function calls.
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</p>
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<p>The syntax for this extension is
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</p>
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<div class="smallexample">
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<pre class="smallexample">extern A a;
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extern int (A::*fp)();
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typedef int (*fptr)(A *);
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fptr p = (fptr)(a.*fp);
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</pre></div>
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<p>For PMF constants (i.e. expressions of the form ‘<samp>&Klasse::Member</samp>’),
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no object is needed to obtain the address of the function. They can be
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converted to function pointers directly:
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</p>
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<div class="smallexample">
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<pre class="smallexample">fptr p1 = (fptr)(&A::foo);
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</pre></div>
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<a name="index-Wno_002dpmf_002dconversions-1"></a>
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<p>You must specify <samp>-Wno-pmf-conversions</samp> to use this extension.
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</p>
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</body>
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</html>
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