1<html lang="en"> 2<head> 3<title>Bound member functions - 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="Template-Instantiation.html#Template-Instantiation" title="Template Instantiation"> 10<link rel="next" href="C_002b_002b-Attributes.html#C_002b_002b-Attributes" title="C++ Attributes"> 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="Bound-member-functions"></a> 51<p> 52Next: <a rel="next" accesskey="n" href="C_002b_002b-Attributes.html#C_002b_002b-Attributes">C++ Attributes</a>, 53Previous: <a rel="previous" accesskey="p" href="Template-Instantiation.html#Template-Instantiation">Template Instantiation</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">7.6 Extracting the function pointer from a bound pointer to member function</h3> 59 60<p><a name="index-pmf-3281"></a><a name="index-pointer-to-member-function-3282"></a><a name="index-bound-pointer-to-member-function-3283"></a> 61In C++, pointer to member functions (PMFs) are implemented using a wide 62pointer of sorts to handle all the possible call mechanisms; the PMF 63needs to store information about how to adjust the ‘<samp><span class="samp">this</span></samp>’ pointer, 64and if the function pointed to is virtual, where to find the vtable, and 65where in the vtable to look for the member function. If you are using 66PMFs in an inner loop, you should really reconsider that decision. If 67that is not an option, you can extract the pointer to the function that 68would be called for a given object/PMF pair and call it directly inside 69the inner loop, to save a bit of time. 70 71 <p>Note that you will still be paying the penalty for the call through a 72function pointer; on most modern architectures, such a call defeats the 73branch prediction features of the CPU. This is also true of normal 74virtual function calls. 75 76 <p>The syntax for this extension is 77 78<pre class="smallexample"> extern A a; 79 extern int (A::*fp)(); 80 typedef int (*fptr)(A *); 81 82 fptr p = (fptr)(a.*fp); 83</pre> 84 <p>For PMF constants (i.e. expressions of the form ‘<samp><span class="samp">&Klasse::Member</span></samp>’), 85no object is needed to obtain the address of the function. They can be 86converted to function pointers directly: 87 88<pre class="smallexample"> fptr p1 = (fptr)(&A::foo); 89</pre> 90 <p><a name="index-Wno_002dpmf_002dconversions-3284"></a>You must specify <samp><span class="option">-Wno-pmf-conversions</span></samp> to use this extension. 91 92 </body></html> 93 94