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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="id-1.3.49.3.8.3"></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="id-1.3.49.3.8.4"></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="id-1.3.49.3.8.5"></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="id-1.3.49.3.8.6"></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="id-1.3.49.3.8.7"></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="id-1.3.49.3.8.8"></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="id-1.3.49.3.8.9"></a><h2>Complexity</h2>
<p>
All iterator operations must take amortized constant time.
</p>
</div>
<div class="refsect1">
<a name="id-1.3.49.3.8.10"></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="id-1.3.49.3.8.11"></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="id-1.3.49.3.8.12"></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
of Notre Dame and the Pervasive Technology Labs at Indiana
University. For technical information contact Andrew Lumsdaine
at the Pervasive Technology Labs at Indiana University. For
administrative and license questions contact the Advanced
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Indianapolis, Indiana 46202, phone 317-278-4100, fax
317-274-5902.</p>
<p>Some concepts based on versions from the MTL draft manual
and Boost Graph and Property Map documentation, the SGI Standard
Template Library documentation and the Hewlett-Packard STL,
under the following license:
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