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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 &copy; 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:&nbsp;<a name="Bound%20member%20functions">Bound member functions</a>,Next:&nbsp;<a rel="next" accesskey="n" href="C---Attributes.html#C++%20Attributes">C++ Attributes</a>,Previous:&nbsp;<a rel="previous" accesskey="p" href="Template-Instantiation.html#Template%20Instantiation">Template Instantiation</a>,Up:&nbsp;<a rel="up" accesskey="u" href="C---Extensions.html#C++%20Extensions">C++ Extensions</a><hr><br></div><h3 class="section">Extracting the function pointer from a bound pointer to member function</h3><p>In C++, pointer to member functions (PMFs) are implemented using a widepointer of sorts to handle all the possible call mechanisms; the PMFneeds to store information about how to adjust the <code>this</code> pointer,and if the function pointed to is virtual, where to find the vtable, andwhere in the vtable to look for the member function.  If you are usingPMFs in an inner loop, you should really reconsider that decision.  Ifthat is not an option, you can extract the pointer to the function thatwould be called for a given object/PMF pair and call it directly insidethe inner loop, to save a bit of time.   <p>Note that you will still be paying the penalty for the call through afunction pointer; on most modern architectures, such a call defeats thebranch prediction features of the CPU.  This is also true of normalvirtual function calls.   <p>The syntax for this extension is<pre class="smallexample">     extern A a;     extern int (A::*fp)();     typedef int (*fptr)(A *);          fptr p = (fptr)(a.*fp);     </pre>   <p>For PMF constants (i.e. expressions of the form <code>&amp;Klasse::Member</code>),no object is needed to obtain the address of the function.  They can beconverted to function pointers directly:<pre class="smallexample">     fptr p1 = (fptr)(&amp;A::foo);     </pre>   <p>You must specify <code>-Wno-pmf-conversions</code> to use this extension.   </body></html>
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