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4<title>Concept BidirectionalIterator</title>
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25<div class="refentry" lang="en">
26<a name="BidirectionalIterator"></a><div class="titlepage"></div>
27<div class="refnamediv">
28<h2><span class="refentrytitle">Concept BidirectionalIterator</span></h2>
29<p>BidirectionalIterator</p>
30</div>
31<div class="refsect1" lang="en">
32<a name="id2577363"></a><h2>Description</h2>
33<p>A bidirectional iterator is an iterator that can read through a sequence
34  of values.  It can move in either direction through the sequence, and can
35  be either mutable (data pointed to by it can be changed) or not mutable.</p>
36<p>An iterator represents a position in a sequence.  Therefore, the
37  iterator can point into the sequence (returning a value when dereferenced
38  and being incrementable), or be off-the-end (and not dereferenceable or
39  incrementable).</p>
40</div>
41<div class="refsect1" lang="en">
42<a name="id2577379"></a><h2>Refinement of</h2>
43<div class="itemizedlist"><ul type="disc"><li><p><a href="ForwardIterator.html" title="Concept ForwardIterator">ForwardIterator</a></p></li></ul></div>
44</div>
45<div class="refsect1" lang="en">
46<a name="id2577391"></a><h2>Associated types</h2>
47<div class="itemizedlist"><ul type="disc">
48<li>
49<p><span class="bold"><strong>value_type</strong></span></p>
50<pre class="literallayout">std::iterator_traits&lt;Iter&gt;::value_type</pre>
51<p>The value type of the iterator</p>
52</li>
53<li>
54<p><span class="bold"><strong>category</strong></span></p>
55<pre class="literallayout">std::iterator_traits&lt;Iter&gt;::iterator_category</pre>
56<p>The category of the iterator</p>
57</li>
58</ul></div>
59</div>
60<div class="refsect1" lang="en">
61<a name="id2577432"></a><h2>Notation</h2>
62<div class="variablelist"><dl>
63<dt><span class="term">Iter</span></dt>
64<dd>A type playing the role of iterator-type in the <a href="BidirectionalIterator.html" title="Concept BidirectionalIterator">BidirectionalIterator</a> concept.</dd>
65<dt>
66<span class="term"><code class="varname">i</code>, </span><span class="term"><code class="varname">j</code></span>
67</dt>
68<dd>Objects of type Iter</dd>
69<dt><span class="term"><code class="varname">x</code></span></dt>
70<dd>Object of type value_type</dd>
71</dl></div>
72</div>
73<div class="refsect1" lang="en">
74<a name="id2577471"></a><h2>Type expressions</h2>
75<div class="variablelist"><dl>
76<dt><span class="term">Category tag</span></dt>
77<dd><p><span class="type">category</span> must be
78                  derived from <span class="type">std::bidirectional_iterator_tag</span>.
79                </p></dd>
80</dl></div>
81</div>
82<div class="refsect1" lang="en">
83<a name="id2577490"></a><h2>Valid expressions</h2>
84<div class="informaltable"><table class="table">
85<colgroup>
86<col>
87<col>
88<col>
89<col>
90<col>
91<col>
92</colgroup>
93<thead><tr>
94<th>Name</th>
95<th>Expression</th>
96<th>Type</th>
97<th>Precondition</th>
98<th>Semantics</th>
99<th>Postcondition</th>
100</tr></thead>
101<tbody>
102<tr>
103<td><p>Predecrement</p></td>
104<td><p>--i</p></td>
105<td><p><span class="type">Iter &amp;</span></p></td>
106<td><p><code class="computeroutput">i</code> is incrementable (not
107    off-the-end) and some dereferenceable iterator <code class="computeroutput">j</code> exists
108    such that <code class="computeroutput">i == ++j</code></p></td>
109<td> </td>
110<td> </td>
111</tr>
112<tr>
113<td><p>Postdecrement</p></td>
114<td><p>i--</p></td>
115<td><p><span class="type">Iter</span></p></td>
116<td><p>Same as for predecrement</p></td>
117<td><p>Equivalent to <code class="computeroutput">{Iter j = i; --i; return j;}</code></p></td>
118<td><p><code class="computeroutput">i</code> is dereferenceable or
119    off-the-end</p></td>
120</tr>
121</tbody>
122</table></div>
123</div>
124<div class="refsect1" lang="en">
125<a name="id2577583"></a><h2>Complexity</h2>
126<p>
127  All iterator operations must take amortized constant time.
128  </p>
129</div>
130<div class="refsect1" lang="en">
131<a name="id2577590"></a><h2>Invariants</h2>
132<div class="variablelist"><dl>
133<dt><span class="term">Predecrement must return object</span></dt>
134<dd><p><code class="computeroutput">&amp;i = &amp;(--i)</code></p></dd>
135<dt><span class="term">Unique path through sequence</span></dt>
136<dd><p><code class="computeroutput">i == j</code> implies <code class="computeroutput">--i == --j</code></p></dd>
137<dt><span class="term">Increment and decrement are inverses</span></dt>
138<dd><p><code class="computeroutput">++i; --i;</code> and <code class="computeroutput">--i; ++i;</code> must end up with the
139  value of <code class="computeroutput">i</code> unmodified, if <code class="computeroutput">i</code> both of the
140  operations in the pair are valid.
141  </p></dd>
142</dl></div>
143</div>
144<div class="refsect1" lang="en">
145<a name="id2577653"></a><h2>Models</h2>
146<div class="itemizedlist"><ul type="disc">
147<li><span class="simplelist"><span class="type">T *</span></span></li>
148<li><span class="simplelist"><span class="type">std::list&lt;T&gt;::iterator</span></span></li>
149</ul></div>
150</div>
151<div class="refsect1" lang="en">
152<a name="id2577674"></a><h2>See also</h2>
153<div class="itemizedlist"><ul type="disc"><li><p><a href="RandomAccessIterator.html" title="Concept RandomAccessIterator">RandomAccessIterator</a></p></li></ul></div>
154</div>
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158<td align="right"><small>Copyright © 2001, 2002 Indiana University<br>Copyright © 2000, 2001 University of Notre Dame du Lac<br>Copyright © 2000 Jeremy Siek, Lie-Quan Lee, Andrew Lumsdaine<br>Copyright © 1996-1999 Silicon Graphics Computer Systems, Inc.<br>Copyright © 1994 Hewlett-Packard Company</small></td>
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