1<?xml version="1.0" encoding="utf-8" ?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> 4<head> 5<meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> 6<meta name="generator" content="Docutils 0.7: http://docutils.sourceforge.net/" /> 7<title>The MPL Reference Manual: Metafunction</title> 8<link rel="stylesheet" href="../style.css" type="text/css" /> 9</head> 10<body class="docframe refmanual"> 11<table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./metafunctions-concepts.html" class="navigation-link">Prev</a> <a href="./metafunction-class.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group">Back <a href="./metafunction-class.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./metafunctions-concepts.html" class="navigation-link">Up</a> <a href="../refmanual.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./refmanual_toc.html" class="navigation-link">Full TOC</a></span></td> 12<td class="header-group page-location"><a href="../refmanual.html" class="navigation-link">Front Page</a> / <a href="./metafunctions.html" class="navigation-link">Metafunctions</a> / <a href="./metafunctions-concepts.html" class="navigation-link">Concepts</a> / <a href="./metafunction.html" class="navigation-link">Metafunction</a></td> 13</tr></table><div class="header-separator"></div> 14<div class="section" id="metafunction"> 15<h1><a class="toc-backref" href="./metafunctions-concepts.html#id1512">Metafunction</a></h1> 16<div class="section" id="id822"> 17<h3><a class="subsection-title" href="#description" name="description">Description</a></h3> 18<p id="nullary-metafunction">A <em>metafunction</em> is a class or a class template that represents a 19function invocable at compile-time. An non-nullary metafunction is 20invoked by instantiating the class template with particular 21template parameters (metafunction arguments); the result of the 22metafunction application is accessible through the instantiation's 23nested <tt class="literal"><span class="pre">type</span></tt> typedef. All metafunction's arguments must be types 24(i.e. only <em>type template parameters</em> are allowed). A metafunction 25can have a variable number of parameters. A <em>nullary metafunction</em> is 26represented as a (template) class with a nested <tt class="literal"><span class="pre">type</span></tt> typename 27member.</p> 28</div> 29<div class="section" id="id823"> 30<h3><a class="subsection-title" href="#expression-requirements" name="expression-requirements">Expression requirements</a></h3> 31<p>In the following table and subsequent specifications, <tt class="literal"><span class="pre">f</span></tt> is a <a class="reference internal" href="./metafunction.html">Metafunction</a>.</p> 32<table border="1" class="docutils table"> 33<colgroup> 34<col width="38%" /> 35<col width="28%" /> 36<col width="33%" /> 37</colgroup> 38<thead valign="bottom"> 39<tr><th class="head">Expression</th> 40<th class="head">Type</th> 41<th class="head">Complexity</th> 42</tr> 43</thead> 44<tbody valign="top"> 45<tr><td><tt class="literal"><span class="pre">f::type</span></tt></td> 46<td>Any type</td> 47<td>Unspecified.</td> 48</tr> 49<tr><td><tt class="literal"><span class="pre">f<>::type</span></tt></td> 50<td>Any type</td> 51<td>Unspecified.</td> 52</tr> 53<tr><td><tt class="literal"><span class="pre">f<a1,..,an>::type</span></tt></td> 54<td>Any type</td> 55<td>Unspecified.</td> 56</tr> 57</tbody> 58</table> 59</div> 60<div class="section" id="id824"> 61<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3> 62<pre class="literal-block"> 63typedef f::type x; 64</pre> 65<table class="docutils field-list" frame="void" rules="none"> 66<col class="field-name" /> 67<col class="field-body" /> 68<tbody valign="top"> 69<tr class="field"><th class="field-name">Precondition:</th><td class="field-body"><tt class="literal"><span class="pre">f</span></tt> is a nullary <a class="reference internal" href="./metafunction.html">Metafunction</a>; <tt class="literal"><span class="pre">f::type</span></tt> is a <em>type-name</em>.</td> 70</tr> 71<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><tt class="literal"><span class="pre">x</span></tt> is the result of the metafunction invocation.</td> 72</tr> 73</tbody> 74</table> 75<!-- ................................................................................... --> 76<pre class="literal-block"> 77typedef f<>::type x; 78</pre> 79<table class="docutils field-list" frame="void" rules="none"> 80<col class="field-name" /> 81<col class="field-body" /> 82<tbody valign="top"> 83<tr class="field"><th class="field-name">Precondition:</th><td class="field-body"><tt class="literal"><span class="pre">f</span></tt> is a nullary <a class="reference internal" href="./metafunction.html">Metafunction</a>; <tt class="literal"><span class="pre">f<>::type</span></tt> is a <em>type-name</em>.</td> 84</tr> 85<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><tt class="literal"><span class="pre">x</span></tt> is the result of the metafunction invocation.</td> 86</tr> 87</tbody> 88</table> 89<!-- ................................................................................... --> 90<pre class="literal-block"> 91typedef f<a1,<em>...</em> a<em>n</em>>::type x; 92</pre> 93<table class="docutils field-list" frame="void" rules="none"> 94<col class="field-name" /> 95<col class="field-body" /> 96<tbody valign="top"> 97<tr class="field"><th class="field-name">Precondition:</th><td class="field-body"><tt class="literal"><span class="pre">f</span></tt> is an <em>n</em>-ary <a class="reference internal" href="./metafunction.html">Metafunction</a>; <tt class="literal"><span class="pre">a1</span></tt>,... <tt class="literal"><span class="pre">an</span></tt> are types; 98<tt class="literal"><span class="pre">f<a1,...an>::type</span></tt> is a <em>type-name</em>.</td> 99</tr> 100<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><tt class="literal"><span class="pre">x</span></tt> is the result of the metafunction invocation 101with the actual arguments <tt class="literal"><span class="pre">a1</span></tt>,... <tt class="literal"><span class="pre">an</span></tt>.</td> 102</tr> 103</tbody> 104</table> 105</div> 106<div class="section" id="id825"> 107<h3><a class="subsection-title" href="#models" name="models">Models</a></h3> 108<ul class="simple"> 109<li><a class="reference internal" href="./identity.html">identity</a></li> 110<li><a class="reference internal" href="./plus.html">plus</a></li> 111<li><a class="reference internal" href="./begin.html">begin</a></li> 112<li><a class="reference internal" href="./insert.html">insert</a></li> 113<li><a class="reference internal" href="./fold.html">fold</a></li> 114</ul> 115</div> 116<div class="section" id="id826"> 117<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3> 118<p><a class="reference internal" href="./metafunctions.html">Metafunctions</a>, <a class="reference internal" href="./metafunction-class.html">Metafunction Class</a>, <a class="reference internal" href="./lambda-expression.html">Lambda Expression</a>, <a class="reference internal" href="./invocation.html">invocation</a>, <a class="reference internal" href="./apply.html">apply</a>, <a class="reference internal" href="./lambda.html">lambda</a>, <a class="reference internal" href="./bind.html">bind</a></p> 119<!-- Metafunctions/Concepts//Metafunction Class |20 --> 120</div> 121</div> 122 123<div class="footer-separator"></div> 124<table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./metafunctions-concepts.html" class="navigation-link">Prev</a> <a href="./metafunction-class.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group">Back <a href="./metafunction-class.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./metafunctions-concepts.html" class="navigation-link">Up</a> <a href="../refmanual.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./refmanual_toc.html" class="navigation-link">Full TOC</a></span></td> 125<td><div class="copyright-footer"><div class="copyright">Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams</div> 126Distributed under the Boost Software License, Version 1.0. (See accompanying 127file LICENSE_1_0.txt or copy at <a class="reference external" href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)</div></td></tr></table></body> 128</html> 129