| <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
| <html> |
| <!-- Copyright (C) 1988-2015 Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.3 or |
| any later version published by the Free Software Foundation; with the |
| Invariant Sections being "Funding Free Software", the Front-Cover |
| Texts being (a) (see below), and with the Back-Cover Texts being (b) |
| (see below). A copy of the license is included in the section entitled |
| "GNU Free Documentation License". |
| |
| (a) The FSF's Front-Cover Text is: |
| |
| A GNU Manual |
| |
| (b) The FSF's Back-Cover Text is: |
| |
| 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. --> |
| <!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ --> |
| <head> |
| <title>Using the GNU Compiler Collection (GCC): Bound member functions</title> |
| |
| <meta name="description" content="Using the GNU Compiler Collection (GCC): Bound member functions"> |
| <meta name="keywords" content="Using the GNU Compiler Collection (GCC): Bound member functions"> |
| <meta name="resource-type" content="document"> |
| <meta name="distribution" content="global"> |
| <meta name="Generator" content="makeinfo"> |
| <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> |
| <link href="index.html#Top" rel="start" title="Top"> |
| <link href="Option-Index.html#Option-Index" rel="index" title="Option Index"> |
| <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> |
| <link href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" rel="up" title="C++ Extensions"> |
| <link href="C_002b_002b-Attributes.html#C_002b_002b-Attributes" rel="next" title="C++ Attributes"> |
| <link href="Template-Instantiation.html#Template-Instantiation" rel="prev" title="Template Instantiation"> |
| <style type="text/css"> |
| <!-- |
| a.summary-letter {text-decoration: none} |
| blockquote.smallquotation {font-size: smaller} |
| div.display {margin-left: 3.2em} |
| div.example {margin-left: 3.2em} |
| div.indentedblock {margin-left: 3.2em} |
| div.lisp {margin-left: 3.2em} |
| div.smalldisplay {margin-left: 3.2em} |
| div.smallexample {margin-left: 3.2em} |
| div.smallindentedblock {margin-left: 3.2em; font-size: smaller} |
| div.smalllisp {margin-left: 3.2em} |
| kbd {font-style:oblique} |
| pre.display {font-family: inherit} |
| pre.format {font-family: inherit} |
| pre.menu-comment {font-family: serif} |
| pre.menu-preformatted {font-family: serif} |
| pre.smalldisplay {font-family: inherit; font-size: smaller} |
| pre.smallexample {font-size: smaller} |
| pre.smallformat {font-family: inherit; font-size: smaller} |
| pre.smalllisp {font-size: smaller} |
| span.nocodebreak {white-space:nowrap} |
| span.nolinebreak {white-space:nowrap} |
| span.roman {font-family:serif; font-weight:normal} |
| span.sansserif {font-family:sans-serif; font-weight:normal} |
| ul.no-bullet {list-style: none} |
| --> |
| </style> |
| |
| |
| </head> |
| |
| <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> |
| <a name="Bound-member-functions"></a> |
| <div class="header"> |
| <p> |
| 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> |
| </div> |
| <hr> |
| <a name="Extracting-the-function-pointer-from-a-bound-pointer-to-member-function"></a> |
| <h3 class="section">7.6 Extracting the function pointer from a bound pointer to member function</h3> |
| <a name="index-pmf"></a> |
| <a name="index-pointer-to-member-function"></a> |
| <a name="index-bound-pointer-to-member-function"></a> |
| |
| <p>In C++, pointer to member functions (PMFs) are implemented using a wide |
| pointer of sorts to handle all the possible call mechanisms; the PMF |
| needs to store information about how to adjust the ‘<samp>this</samp>’ pointer, |
| and if the function pointed to is virtual, where to find the vtable, and |
| where in the vtable to look for the member function. If you are using |
| PMFs in an inner loop, you should really reconsider that decision. If |
| that is not an option, you can extract the pointer to the function that |
| would be called for a given object/PMF pair and call it directly inside |
| the inner loop, to save a bit of time. |
| </p> |
| <p>Note that you still pay the penalty for the call through a |
| function pointer; on most modern architectures, such a call defeats the |
| branch prediction features of the CPU. This is also true of normal |
| virtual function calls. |
| </p> |
| <p>The syntax for this extension is |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">extern A a; |
| extern int (A::*fp)(); |
| typedef int (*fptr)(A *); |
| |
| fptr p = (fptr)(a.*fp); |
| </pre></div> |
| |
| <p>For PMF constants (i.e. expressions of the form ‘<samp>&Klasse::Member</samp>’), |
| no object is needed to obtain the address of the function. They can be |
| converted to function pointers directly: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">fptr p1 = (fptr)(&A::foo); |
| </pre></div> |
| |
| <a name="index-Wno_002dpmf_002dconversions-1"></a> |
| <p>You must specify <samp>-Wno-pmf-conversions</samp> to use this extension. |
| </p> |
| |
| |
| |
| </body> |
| </html> |
