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: Bidirectional Iterator</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="./forward-iterator.html" class="navigation-link">Prev</a> <a href="./random-access-iterator.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./forward-iterator.html" class="navigation-link">Back</a> <a href="./random-access-iterator.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./iterators-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> 12<td class="header-group page-location"><a href="../refmanual.html" class="navigation-link">Front Page</a> / <a href="./iterators.html" class="navigation-link">Iterators</a> / <a href="./iterators-concepts.html" class="navigation-link">Concepts</a> / <a href="./bidirectional-iterator.html" class="navigation-link">Bidirectional Iterator</a></td> 13</tr></table><div class="header-separator"></div> 14<div class="section" id="bidirectional-iterator"> 15<h1><a class="toc-backref" href="./iterators-concepts.html#id1451">Bidirectional Iterator</a></h1> 16<div class="section" id="id390"> 17<h3><a class="subsection-title" href="#description" name="description">Description</a></h3> 18<p>A <a class="reference internal" href="./bidirectional-iterator.html">Bidirectional Iterator</a> is a <a class="reference internal" href="./forward-iterator.html">Forward Iterator</a> that provides a way to 19obtain an iterator to the previous element in a sequence.</p> 20</div> 21<div class="section" id="id391"> 22<h3><a class="subsection-title" href="#refinement-of" name="refinement-of">Refinement of</a></h3> 23<p><a class="reference internal" href="./forward-iterator.html">Forward Iterator</a></p> 24</div> 25<div class="section" id="id392"> 26<h3><a class="subsection-title" href="#definitions" name="definitions">Definitions</a></h3> 27<ul class="simple"> 28<li>a bidirectional iterator <tt class="literal"><span class="pre">i</span></tt> is <cite>decrementable</cite> if there is a "previous" 29iterator, that is, if <tt class="literal"><span class="pre"><a href="./prior.html" class="identifier">prior</a><i>::type</span></tt> expression is well-defined; 30iterators pointing to the first element of the sequence are not 31decrementable.</li> 32</ul> 33</div> 34<div class="section" id="id393"> 35<h3><a class="subsection-title" href="#expression-requirements" name="expression-requirements">Expression requirements</a></h3> 36<p>In addition to the requirements defined in <a class="reference internal" href="./forward-iterator.html">Forward Iterator</a>, 37the following requirements must be met.</p> 38<table border="1" class="docutils table"> 39<colgroup> 40<col width="25%" /> 41<col width="46%" /> 42<col width="29%" /> 43</colgroup> 44<thead valign="bottom"> 45<tr><th class="head">Expression</th> 46<th class="head">Type</th> 47<th class="head">Complexity</th> 48</tr> 49</thead> 50<tbody valign="top"> 51<tr><td><tt class="literal"><span class="pre"><a href="./next.html" class="identifier">next</a><i>::type</span></tt></td> 52<td><a class="reference internal" href="./bidirectional-iterator.html">Bidirectional Iterator</a></td> 53<td>Amortized constant time</td> 54</tr> 55<tr><td><tt class="literal"><span class="pre"><a href="./prior.html" class="identifier">prior</a><i>::type</span></tt></td> 56<td><a class="reference internal" href="./bidirectional-iterator.html">Bidirectional Iterator</a></td> 57<td>Amortized constant time</td> 58</tr> 59<tr><td><tt class="literal"><span class="pre">i::category</span></tt></td> 60<td><a class="reference internal" href="./integral-constant.html">Integral Constant</a>, convertible 61to <tt class="literal"><span class="pre">bidirectional_iterator_tag</span></tt></td> 62<td>Constant time</td> 63</tr> 64</tbody> 65</table> 66</div> 67<div class="section" id="id394"> 68<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3> 69<pre class="literal-block"> 70typedef <a href="./prior.html" class="identifier">prior</a><i>::type j; 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">Precondition:</th><td class="field-body"><tt class="literal"><span class="pre">i</span></tt> is decrementable</td> 77</tr> 78<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><tt class="literal"><span class="pre">j</span></tt> is an iterator pointing to the previous element of the 79sequence</td> 80</tr> 81<tr class="field"><th class="field-name">Postcondition:</th><td class="field-body"><tt class="literal"><span class="pre">j</span></tt> is dereferenceable and incrementable</td> 82</tr> 83</tbody> 84</table> 85</div> 86<div class="section" id="id395"> 87<h3><a class="subsection-title" href="#invariants" name="invariants">Invariants</a></h3> 88<p>For any bidirectional iterators <tt class="literal"><span class="pre">i</span></tt> and <tt class="literal"><span class="pre">j</span></tt> the following invariants 89always hold:</p> 90<ul class="simple"> 91<li>If <tt class="literal"><span class="pre">i</span></tt> is incrementable, then <tt class="literal"><span class="pre"><a href="./prior.html" class="identifier">prior</a><</span> <span class="pre"><a href="./next.html" class="identifier">next</a><i>::type</span> <span class="pre">>::type</span></tt> is a null 92operation; similarly, if <tt class="literal"><span class="pre">i</span></tt> is decrementable, <tt class="literal"><span class="pre"><a href="./next.html" class="identifier">next</a><</span> <span class="pre"><a href="./prior.html" class="identifier">prior</a><i>::type</span> <span class="pre">>::type</span></tt> 93is a null operation.</li> 94</ul> 95</div> 96<div class="section" id="id396"> 97<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3> 98<p><a class="reference internal" href="./iterators.html">Iterators</a>, <a class="reference internal" href="./forward-iterator.html">Forward Iterator</a>, <a class="reference internal" href="./random-access-iterator.html">Random Access Iterator</a>, <a class="reference internal" href="./bidirectional-sequence.html">Bidirectional Sequence</a>, <a class="reference internal" href="./prior.html">prior</a></p> 99<!-- Iterators/Concepts//Random Access Iterator |30 --> 100</div> 101</div> 102 103<div class="footer-separator"></div> 104<table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./forward-iterator.html" class="navigation-link">Prev</a> <a href="./random-access-iterator.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./forward-iterator.html" class="navigation-link">Back</a> <a href="./random-access-iterator.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./iterators-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> 105<td><div class="copyright-footer"><div class="copyright">Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams</div> 106Distributed under the Boost Software License, Version 1.0. (See accompanying 107file 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> 108</html> 109