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: begin</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="./back.html" class="navigation-link">Prev</a> <a href="./clear.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./back.html" class="navigation-link">Back</a> <a href="./clear.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> 12<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="./begin.html" class="navigation-link">begin</a></td> 13</tr></table><div class="header-separator"></div> 14<div class="section" id="begin"> 15<h1><a class="toc-backref" href="./intrinsic-metafunctions.html#id1428">begin</a></h1> 16<div class="section" id="id202"> 17<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3> 18<pre class="literal-block"> 19template< 20 typename X 21 > 22struct <a href="./begin.html" class="identifier">begin</a> 23{ 24 typedef <em>unspecified</em> type; 25}; 26</pre> 27</div> 28<div class="section" id="id203"> 29<h3><a class="subsection-title" href="#description" name="description">Description</a></h3> 30<p>Returns an iterator that points to the first element of the sequence. If 31the argument is not a <a class="reference internal" href="./forward-sequence.html">Forward Sequence</a>, returns <a class="reference internal" href="./void.html">void_</a>.</p> 32</div> 33<div class="section" id="id204"> 34<h3><a class="subsection-title" href="#header" name="header">Header</a></h3> 35<pre class="literal-block"> 36#include <<a href="../../../../boost/mpl/begin_end.hpp" class="header">boost/mpl/begin_end.hpp</a>> 37</pre> 38</div> 39<div class="section" id="id205"> 40<h3><a class="subsection-title" href="#model-of" name="model-of">Model of</a></h3> 41<p><a class="reference internal" href="./tag-dispatched-metafunction.html">Tag Dispatched Metafunction</a></p> 42</div> 43<div class="section" id="id206"> 44<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3> 45<table border="1" class="docutils table"> 46<colgroup> 47<col width="18%" /> 48<col width="22%" /> 49<col width="60%" /> 50</colgroup> 51<thead valign="bottom"> 52<tr><th class="head">Parameter</th> 53<th class="head">Requirement</th> 54<th class="head">Description</th> 55</tr> 56</thead> 57<tbody valign="top"> 58<tr><td><tt class="literal"><span class="pre">X</span></tt></td> 59<td>Any type</td> 60<td>A type whose begin iterator, if any, will be 61returned.</td> 62</tr> 63</tbody> 64</table> 65</div> 66<div class="section" id="id207"> 67<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3> 68<p>For any arbitrary type <tt class="literal"><span class="pre">x</span></tt>:</p> 69<pre class="literal-block"> 70typedef <a href="./begin.html" class="identifier">begin</a><x>::type first; 71</pre> 72<table class="docutils field-list" frame="void" rules="none"> 73<col class="field-name" /> 74<col class="field-body" /> 75<tbody valign="top"> 76<tr class="field"><th class="field-name">Return type:</th><td class="field-body"><a class="reference internal" href="./forward-iterator.html">Forward Iterator</a> or <a class="reference internal" href="./void.html">void_</a>.</td> 77</tr> 78<tr class="field"><th class="field-name">Semantics:</th><td class="field-body">If <tt class="literal"><span class="pre">x</span></tt> is a <a class="reference internal" href="./forward-sequence.html">Forward Sequence</a>, <tt class="literal"><span class="pre">first</span></tt> is an iterator pointing to the 79first element of <tt class="literal"><span class="pre">s</span></tt>; otherwise <tt class="literal"><span class="pre">first</span></tt> is <a class="reference internal" href="./void.html">void_</a>.</td> 80</tr> 81<tr class="field"><th class="field-name">Postcondition:</th><td class="field-body">If <tt class="literal"><span class="pre">first</span></tt> is an iterator, it is either dereferenceable or past-the-end; it 82is past-the-end if and only if <tt class="literal"><span class="pre"><a href="./size.html" class="identifier">size</a><x>::value</span> <span class="pre">==</span> <span class="pre">0</span></tt>.</td> 83</tr> 84</tbody> 85</table> 86</div> 87<div class="section" id="id208"> 88<h3><a class="subsection-title" href="#complexity" name="complexity">Complexity</a></h3> 89<p>Amortized constant time.</p> 90</div> 91<div class="section" id="id209"> 92<h3><a class="subsection-title" href="#example" name="example">Example</a></h3> 93<pre class="literal-block"> 94typedef <a href="./vector.html" class="identifier">vector</a>< unsigned char,unsigned short, 95 unsigned int,unsigned long > unsigned_types; 96 97typedef <a href="./begin.html" class="identifier">begin</a><unsigned_types>::type iter; 98<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same< <a href="./deref.html" class="identifier">deref</a><iter>::type, unsigned char > )); 99 100<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same< <a href="./begin.html" class="identifier">begin</a><int>::type, <a href="./void.html" class="identifier">void_</a> > )); 101</pre> 102</div> 103<div class="section" id="id210"> 104<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3> 105<p><a class="reference internal" href="./iterators.html">Iterators</a>, <a class="reference internal" href="./forward-sequence.html">Forward Sequence</a>, <a class="reference internal" href="./end.html">end</a>, <a class="reference internal" href="./size.html">size</a>, <a class="reference internal" href="./empty.html">empty</a></p> 106<!-- Sequences/Intrinsic Metafunctions//clear --> 107</div> 108</div> 109 110<div class="footer-separator"></div> 111<table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./back.html" class="navigation-link">Prev</a> <a href="./clear.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./back.html" class="navigation-link">Back</a> <a href="./clear.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> 112<td><div class="copyright-footer"><div class="copyright">Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams</div> 113Distributed under the Boost Software License, Version 1.0. (See accompanying 114file 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> 115</html> 116