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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> |
| <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> |
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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 --> |
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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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