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- <title>The MPL Reference Manual: Metafunction</title>
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- <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>
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- <div class="section" id="metafunction">
- <h1><a class="toc-backref" href="./metafunctions-concepts.html#id1512">Metafunction</a></h1>
- <div class="section" id="id822">
- <h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
- <p id="nullary-metafunction">A <em>metafunction</em> is a class or a class template that represents a
- function invocable at compile-time. An non-nullary metafunction is
- invoked by instantiating the class template with particular
- template parameters (metafunction arguments); the result of the
- metafunction application is accessible through the instantiation's
- nested <tt class="literal"><span class="pre">type</span></tt> typedef. All metafunction's arguments must be types
- (i.e. only <em>type template parameters</em> are allowed). A metafunction
- can have a variable number of parameters. A <em>nullary metafunction</em> is
- represented as a (template) class with a nested <tt class="literal"><span class="pre">type</span></tt> typename
- member.</p>
- </div>
- <div class="section" id="id823">
- <h3><a class="subsection-title" href="#expression-requirements" name="expression-requirements">Expression requirements</a></h3>
- <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>
- <table border="1" class="docutils table">
- <colgroup>
- <col width="38%" />
- <col width="28%" />
- <col width="33%" />
- </colgroup>
- <thead valign="bottom">
- <tr><th class="head">Expression</th>
- <th class="head">Type</th>
- <th class="head">Complexity</th>
- </tr>
- </thead>
- <tbody valign="top">
- <tr><td><tt class="literal"><span class="pre">f::type</span></tt></td>
- <td>Any type</td>
- <td>Unspecified.</td>
- </tr>
- <tr><td><tt class="literal"><span class="pre">f<>::type</span></tt></td>
- <td>Any type</td>
- <td>Unspecified.</td>
- </tr>
- <tr><td><tt class="literal"><span class="pre">f<a1,..,an>::type</span></tt></td>
- <td>Any type</td>
- <td>Unspecified.</td>
- </tr>
- </tbody>
- </table>
- </div>
- <div class="section" id="id824">
- <h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
- <pre class="literal-block">
- typedef f::type x;
- </pre>
- <table class="docutils field-list" frame="void" rules="none">
- <col class="field-name" />
- <col class="field-body" />
- <tbody valign="top">
- <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>
- </tr>
- <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>
- </tr>
- </tbody>
- </table>
- <!-- ................................................................................... -->
- <pre class="literal-block">
- typedef f<>::type x;
- </pre>
- <table class="docutils field-list" frame="void" rules="none">
- <col class="field-name" />
- <col class="field-body" />
- <tbody valign="top">
- <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>
- </tr>
- <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>
- </tr>
- </tbody>
- </table>
- <!-- ................................................................................... -->
- <pre class="literal-block">
- typedef f<a1,<em>...</em> a<em>n</em>>::type x;
- </pre>
- <table class="docutils field-list" frame="void" rules="none">
- <col class="field-name" />
- <col class="field-body" />
- <tbody valign="top">
- <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;
- <tt class="literal"><span class="pre">f<a1,...an>::type</span></tt> is a <em>type-name</em>.</td>
- </tr>
- <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
- with the actual arguments <tt class="literal"><span class="pre">a1</span></tt>,... <tt class="literal"><span class="pre">an</span></tt>.</td>
- </tr>
- </tbody>
- </table>
- </div>
- <div class="section" id="id825">
- <h3><a class="subsection-title" href="#models" name="models">Models</a></h3>
- <ul class="simple">
- <li><a class="reference internal" href="./identity.html">identity</a></li>
- <li><a class="reference internal" href="./plus.html">plus</a></li>
- <li><a class="reference internal" href="./begin.html">begin</a></li>
- <li><a class="reference internal" href="./insert.html">insert</a></li>
- <li><a class="reference internal" href="./fold.html">fold</a></li>
- </ul>
- </div>
- <div class="section" id="id826">
- <h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
- <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>
- <!-- Metafunctions/Concepts//Metafunction Class |20 -->
- </div>
- </div>
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- <td><div class="copyright-footer"><div class="copyright">Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams</div>
- Distributed under the Boost Software License, Version 1.0. (See accompanying
- file 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>
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