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7<title>The MPL Reference Manual: Metafunction</title>
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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>&nbsp;<a href="./metafunction-class.html" class="navigation-link">Next</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group">Back&nbsp;<a href="./metafunction-class.html" class="navigation-link">Along</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./metafunctions-concepts.html" class="navigation-link">Up</a>&nbsp;<a href="../refmanual.html" class="navigation-link">Home</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</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#id438" name="metafunction">Metafunction</a></h1>
16<div class="section" id="metafunction-description">
17<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
18<a class="target" id="nullary-metafunction" name="nullary-metafunction"></a><p>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="metafunction-expression-requirements">
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" href="./metafunction.html">Metafunction</a>.</p>
32<table border="1" class="table">
33<colgroup>
34<col width="38%" />
35<col width="28%" />
36<col width="33%" />
37</colgroup>
38<thead valign="bottom">
39<tr><th>Expression</th>
40<th>Type</th>
41<th>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&lt;&gt;::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&lt;a1,..,an&gt;::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="metafunction-expression-semantics">
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="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" 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&lt;&gt;::type x;
78</pre>
79<table class="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" href="./metafunction.html">Metafunction</a>; <tt class="literal"><span class="pre">f&lt;&gt;::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&lt;a1,<em>...</em> a<em>n</em>&gt;::type x;
92</pre>
93<table class="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" 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&lt;a1,...an&gt;::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="metafunction-models">
107<h3><a class="subsection-title" href="#models" name="models">Models</a></h3>
108<ul class="simple">
109<li><a class="refentry reference" href="./identity.html"><tt class="refentry literal"><span class="pre">identity</span></tt></a></li>
110<li><a class="refentry reference" href="./plus.html"><tt class="refentry literal"><span class="pre">plus</span></tt></a></li>
111<li><a class="refentry reference" href="./begin.html"><tt class="refentry literal"><span class="pre">begin</span></tt></a></li>
112<li><a class="refentry reference" href="./insert.html"><tt class="refentry literal"><span class="pre">insert</span></tt></a></li>
113<li><a class="refentry reference" href="./fold.html"><tt class="refentry literal"><span class="pre">fold</span></tt></a></li>
114</ul>
115</div>
116<div class="section" id="metafunction-see-also">
117<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
118<p><a class="reference" href="./metafunctions.html">Metafunctions</a>, <a class="reference" href="./metafunction-class.html">Metafunction Class</a>, <a class="reference" href="./lambda-expression.html">Lambda Expression</a>, <a class="reference" href="./invocation.html">invocation</a>, <a class="refentry reference" href="./apply.html"><tt class="refentry literal"><span class="pre">apply</span></tt></a>, <a class="refentry reference" href="./lambda.html"><tt class="refentry literal"><span class="pre">lambda</span></tt></a>, <a class="refentry reference" href="./bind.html"><tt class="refentry literal"><span class="pre">bind</span></tt></a></p>
119<!-- modtime: November 10, 2004 04:37:49 +0000 -->
120<!-- Metafunctions/Concepts//Metafunction Class |20 -->
121</div>
122</div>
123
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