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<div class="refentry">
<a name="BidirectionalIterator"></a><div class="titlepage"></div>
<div class="refnamediv">
<h2><span class="refentrytitle">Concept BidirectionalIterator</span></h2>
<p>BidirectionalIterator</p>
</div>
<div class="refsect1">
<a name="idp634177344"></a><h2>Description</h2>
<p>A bidirectional iterator is an iterator that can read through a sequence
of values. It can move in either direction through the sequence, and can
be either mutable (data pointed to by it can be changed) or not mutable.</p>
<p>An iterator represents a position in a sequence. Therefore, the
iterator can point into the sequence (returning a value when dereferenced
and being incrementable), or be off-the-end (and not dereferenceable or
incrementable).</p>
</div>
<div class="refsect1">
<a name="idp634179152"></a><h2>Refinement of</h2>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p><a class="link" href="ForwardIterator.html" title="Concept ForwardIterator">ForwardIterator</a></p></li></ul></div>
</div>
<div class="refsect1">
<a name="idp634180624"></a><h2>Associated types</h2>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<p><span class="bold"><strong>value_type</strong></span></p>
<pre class="literallayout">std::iterator_traits<Iter>::value_type</pre>
<p>The value type of the iterator</p>
</li>
<li class="listitem">
<p><span class="bold"><strong>category</strong></span></p>
<pre class="literallayout">std::iterator_traits<Iter>::iterator_category</pre>
<p>The category of the iterator</p>
</li>
</ul></div>
</div>
<div class="refsect1">
<a name="idp634185216"></a><h2>Notation</h2>
<div class="variablelist"><dl class="variablelist">
<dt><span class="term">Iter</span></dt>
<dd>A type playing the role of iterator-type in the <a class="link" href="BidirectionalIterator.html" title="Concept BidirectionalIterator">BidirectionalIterator</a> concept.</dd>
<dt>
<span class="term"><code class="varname">i</code>, </span><span class="term"><code class="varname">j</code></span>
</dt>
<dd>Objects of type Iter</dd>
<dt><span class="term"><code class="varname">x</code></span></dt>
<dd>Object of type value_type</dd>
</dl></div>
</div>
<div class="refsect1">
<a name="idp634189712"></a><h2>Type expressions</h2>
<div class="variablelist"><dl class="variablelist">
<dt><span class="term">Category tag</span></dt>
<dd><p><span class="type">category</span> must be
derived from <span class="type">std::bidirectional_iterator_tag</span>.
</p></dd>
</dl></div>
</div>
<div class="refsect1">
<a name="idp634191872"></a><h2>Valid expressions</h2>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
<col>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>Name</th>
<th>Expression</th>
<th>Type</th>
<th>Precondition</th>
<th>Semantics</th>
<th>Postcondition</th>
</tr></thead>
<tbody>
<tr>
<td><p>Predecrement</p></td>
<td><p>--i</p></td>
<td><p><span class="type">Iter &</span></p></td>
<td><p><code class="computeroutput">i</code> is incrementable (not
off-the-end) and some dereferenceable iterator <code class="computeroutput">j</code> exists
such that <code class="computeroutput">i == ++j</code></p></td>
<td> </td>
<td> </td>
</tr>
<tr>
<td><p>Postdecrement</p></td>
<td><p>i--</p></td>
<td><p><span class="type">Iter</span></p></td>
<td><p>Same as for predecrement</p></td>
<td><p>Equivalent to <code class="computeroutput">{Iter j = i; --i; return j;}</code></p></td>
<td><p><code class="computeroutput">i</code> is dereferenceable or
off-the-end</p></td>
</tr>
</tbody>
</table></div>
</div>
<div class="refsect1">
<a name="idp634202832"></a><h2>Complexity</h2>
<p>
All iterator operations must take amortized constant time.
</p>
</div>
<div class="refsect1">
<a name="idp634203584"></a><h2>Invariants</h2>
<div class="variablelist"><dl class="variablelist">
<dt><span class="term">Predecrement must return object</span></dt>
<dd><p>
<code class="computeroutput">&i = &(--i)</code>
</p></dd>
<dt><span class="term">Unique path through sequence</span></dt>
<dd><p>
<code class="computeroutput">i == j</code> implies <code class="computeroutput">--i == --j</code>
</p></dd>
<dt><span class="term">Increment and decrement are inverses</span></dt>
<dd><p>
<code class="computeroutput">++i; --i;</code> and <code class="computeroutput">--i; ++i;</code> must end up with the
value of <code class="computeroutput">i</code> unmodified, if <code class="computeroutput">i</code> both of the
operations in the pair are valid.
</p></dd>
</dl></div>
</div>
<div class="refsect1">
<a name="idp634211536"></a><h2>Models</h2>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><span class="simplelist"><span class="type">T *</span></span></li>
<li class="listitem"><span class="simplelist"><span class="type">std::list<T>::iterator</span></span></li>
</ul></div>
</div>
<div class="refsect1">
<a name="idp634213984"></a><h2>See also</h2>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p><a class="link" href="RandomAccessIterator.html" title="Concept RandomAccessIterator">RandomAccessIterator</a></p></li></ul></div>
</div>
</div>
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<td align="right"><div class="copyright-footer">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<p>Distributed under the Boost Software License, Version 1.0.
(See accompanying file <code class="filename">LICENSE_1_0.txt</code> or copy at
<a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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<p>This product includes software developed at the University
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<p>Some concepts based on versions from the MTL draft manual
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Template Library documentation and the Hewlett-Packard STL,
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