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10<title>The MPL Reference Manual: Bidirectional Iterator</title>
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14<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>&nbsp;<a href="./random-access-iterator.html" class="navigation-link">Next</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./forward-iterator.html" class="navigation-link">Back</a>&nbsp;<a href="./random-access-iterator.html" class="navigation-link">Along</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./iterators-concepts.html" class="navigation-link">Up</a>&nbsp;<a href="../refmanual.html" class="navigation-link">Home</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./refmanual_toc.html" class="navigation-link">Full TOC</a></span></td>
15<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>
16</tr></table><div class="header-separator"></div>
17<div class="section" id="bidirectional-iterator">
18<h1><a class="toc-backref" href="./iterators-concepts.html#id379" name="bidirectional-iterator">Bidirectional Iterator</a></h1>
19<div class="section" id="iterators-bidirectional-description">
20<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
21<p>A <a class="reference" href="./bidirectional-iterator.html">Bidirectional Iterator</a> is a <a class="reference" href="./forward-iterator.html">Forward Iterator</a> that provides a way to
22obtain an iterator to the previous element in a sequence.</p>
23</div>
24<div class="section" id="bidirectional-refinement-of">
25<h3><a class="subsection-title" href="#refinement-of" name="refinement-of">Refinement of</a></h3>
26<p><a class="reference" href="./forward-iterator.html">Forward Iterator</a></p>
27</div>
28<div class="section" id="bidirectional-definitions">
29<h3><a class="subsection-title" href="#definitions" name="definitions">Definitions</a></h3>
30<ul class="simple">
31<li>a bidirectional iterator <tt class="literal"><span class="pre">i</span></tt> is <cite>decrementable</cite> if there is a &quot;previous&quot; 
32iterator, that is, if <tt class="literal"><span class="pre"><a href="./prior.html" class="identifier">prior</a>&lt;i&gt;::type</span></tt> expression is well-defined;
33iterators pointing to the first element of the sequence are not
34decrementable.</li>
35</ul>
36</div>
37<div class="section" id="iterators-bidirectional-expression-requirements">
38<h3><a class="subsection-title" href="#expression-requirements" name="expression-requirements">Expression requirements</a></h3>
39<p>In addition to the requirements defined in <a class="reference" href="./forward-iterator.html">Forward Iterator</a>,
40the following requirements must be met.</p>
41<table border="1" class="table">
42<colgroup>
43<col width="25%" />
44<col width="46%" />
45<col width="29%" />
46</colgroup>
47<thead valign="bottom">
48<tr><th>Expression</th>
49<th>Type</th>
50<th>Complexity</th>
51</tr>
52</thead>
53<tbody valign="top">
54<tr><td><tt class="literal"><span class="pre"><a href="./next.html" class="identifier">next</a>&lt;i&gt;::type</span></tt></td>
55<td><a class="reference" href="./bidirectional-iterator.html">Bidirectional Iterator</a></td>
56<td>Amortized constant time</td>
57</tr>
58<tr><td><tt class="literal"><span class="pre"><a href="./prior.html" class="identifier">prior</a>&lt;i&gt;::type</span></tt></td>
59<td><a class="reference" href="./bidirectional-iterator.html">Bidirectional Iterator</a></td>
60<td>Amortized constant time</td>
61</tr>
62<tr><td><tt class="literal"><span class="pre">i::category</span></tt></td>
63<td><a class="reference" href="./integral-constant.html">Integral Constant</a>, convertible
64to <tt class="literal"><span class="pre">bidirectional_iterator_tag</span></tt></td>
65<td>Constant time</td>
66</tr>
67</tbody>
68</table>
69</div>
70<div class="section" id="iterators-bidirectional-expression-semantics">
71<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
72<pre class="literal-block">
73typedef <a href="./prior.html" class="identifier">prior</a>&lt;i&gt;::type j;
74</pre>
75<table class="field-list" frame="void" rules="none">
76<col class="field-name" />
77<col class="field-body" />
78<tbody valign="top">
79<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>
80</tr>
81<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
82sequence</td>
83</tr>
84<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>
85</tr>
86</tbody>
87</table>
88</div>
89<div class="section" id="bidirectional-invariants">
90<h3><a class="subsection-title" href="#invariants" name="invariants">Invariants</a></h3>
91<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
92always hold:</p>
93<ul class="simple">
94<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>&lt;</span> <span class="pre"><a href="./next.html" class="identifier">next</a>&lt;i&gt;::type</span> <span class="pre">&gt;::type</span></tt> is a null
95operation; 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>&lt;</span> <span class="pre"><a href="./prior.html" class="identifier">prior</a>&lt;i&gt;::type</span> <span class="pre">&gt;::type</span></tt>
96is a null operation.</li>
97</ul>
98</div>
99<div class="section" id="iterators-bidirectional-see-also">
100<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
101<p><a class="reference" href="./iterators.html">Iterators</a>, <a class="reference" href="./forward-iterator.html">Forward Iterator</a>, <a class="reference" href="./random-access-iterator.html">Random Access Iterator</a>, <a class="reference" href="./bidirectional-sequence.html">Bidirectional Sequence</a>, <a class="refentry reference" href="./prior.html"><tt class="refentry literal"><span class="pre">prior</span></tt></a></p>
102<!-- modtime: November 15, 2004 02:44:55 +0000 -->
103<!-- Iterators/Concepts//Random Access Iterator |30 -->
104</div>
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