<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <meta name="generator" content="Docutils 0.7: http://docutils.sourceforge.net/" /> <title>The MPL Reference Manual: front</title> <link rel="stylesheet" href="../style.css" type="text/css" /> </head> <body class="docframe refmanual"> <table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./erase-key.html" class="navigation-link">Prev</a> <a href="./has-key.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./erase-key.html" class="navigation-link">Back</a> <a href="./has-key.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./intrinsic-metafunctions.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="./sequences.html" class="navigation-link">Sequences</a> / <a href="./intrinsic-metafunctions.html" class="navigation-link">Intrinsic Metafunctions</a> / <a href="./front.html" class="navigation-link">front</a></td> </tr></table><div class="header-separator"></div> <div class="section" id="front"> <h1><a class="toc-backref" href="./intrinsic-metafunctions.html#id1434">front</a></h1> <div class="section" id="id256"> <h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3> <pre class="literal-block"> template< typename Sequence > struct <a href="./front.html" class="identifier">front</a> { typedef <em>unspecified</em> type; }; </pre> </div> <div class="section" id="id257"> <h3><a class="subsection-title" href="#description" name="description">Description</a></h3> <p>Returns the first element in the sequence.</p> </div> <div class="section" id="id258"> <h3><a class="subsection-title" href="#header" name="header">Header</a></h3> <pre class="literal-block"> #include <<a href="../../../../boost/mpl/front.hpp" class="header">boost/mpl/front.hpp</a>> </pre> </div> <div class="section" id="id259"> <h3><a class="subsection-title" href="#model-of" name="model-of">Model of</a></h3> <p><a class="reference internal" href="./tag-dispatched-metafunction.html">Tag Dispatched Metafunction</a></p> </div> <div class="section" id="id260"> <h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3> <table border="1" class="docutils table"> <colgroup> <col width="18%" /> <col width="27%" /> <col width="55%" /> </colgroup> <thead valign="bottom"> <tr><th class="head">Parameter</th> <th class="head">Requirement</th> <th class="head">Description</th> </tr> </thead> <tbody valign="top"> <tr><td><tt class="literal"><span class="pre">Sequence</span></tt></td> <td><a class="reference internal" href="./forward-sequence.html">Forward Sequence</a></td> <td>A sequence to be examined.</td> </tr> </tbody> </table> </div> <div class="section" id="id261"> <h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3> <p>For any <a class="reference internal" href="./forward-sequence.html">Forward Sequence</a> <tt class="literal"><span class="pre">s</span></tt>:</p> <pre class="literal-block"> typedef <a href="./front.html" class="identifier">front</a><s>::type t; </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">Return type:</th><td class="field-body"><p class="first">A type.</p> </td> </tr> <tr class="field"><th class="field-name">Precondition:</th><td class="field-body"><p class="first"><tt class="literal"><span class="pre"><a href="./empty.html" class="identifier">empty</a><s>::value</span> <span class="pre">==</span> <span class="pre">false</span></tt>.</p> </td> </tr> <tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">Equivalent to</p> <pre class="last literal-block"> typedef <a href="./deref.html" class="identifier">deref</a>< <a href="./begin.html" class="identifier">begin</a><s>::type >::type t; </pre> </td> </tr> </tbody> </table> </div> <div class="section" id="id262"> <h3><a class="subsection-title" href="#complexity" name="complexity">Complexity</a></h3> <p>Amortized constant time.</p> </div> <div class="section" id="id263"> <h3><a class="subsection-title" href="#example" name="example">Example</a></h3> <pre class="literal-block"> typedef <a href="./list.html" class="identifier">list</a><long>::type types1; typedef <a href="./list.html" class="identifier">list</a><int,long>::type types2; typedef <a href="./list.html" class="identifier">list</a><char,int,long>::type types3; <a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same< <a href="./front.html" class="identifier">front</a><types1>::type, long > )); <a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same< <a href="./front.html" class="identifier">front</a><types2>::type, int> )); <a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same< <a href="./front.html" class="identifier">front</a><types3>::type, char> )); </pre> </div> <div class="section" id="id264"> <h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3> <p><a class="reference internal" href="./forward-sequence.html">Forward Sequence</a>, <a class="reference internal" href="./back.html">back</a>, <a class="reference internal" href="./push-front.html">push_front</a>, <a class="reference internal" href="./begin.html">begin</a>, <a class="reference internal" href="./deref.html">deref</a>, <a class="reference internal" href="./at.html">at</a></p> <!-- Sequences/Intrinsic Metafunctions//has_key --> </div> </div> <div class="footer-separator"></div> <table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./erase-key.html" class="navigation-link">Prev</a> <a href="./has-key.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./erase-key.html" class="navigation-link">Back</a> <a href="./has-key.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./intrinsic-metafunctions.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><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. 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