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<title>Concept BidirectionalIterator</title>
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<div class="refentry" lang="en">
<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" lang="en">
<a name="id976672"></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" lang="en">
<a name="id976688"></a><h2>Refinement of</h2>
<div class="itemizedlist"><ul type="disc"><li><p><a href="ForwardIterator.html" title="Concept ForwardIterator">ForwardIterator</a></p></li></ul></div>
</div>
<div class="refsect1" lang="en">
<a name="id976700"></a><h2>Associated types</h2>
<div class="itemizedlist"><ul type="disc">
<li>
<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>
<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" lang="en">
<a name="id976740"></a><h2>Notation</h2>
<div class="variablelist"><dl>
<dt><span class="term">Iter</span></dt>
<dd>A type playing the role of iterator-type in the <a 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" lang="en">
<a name="id976778"></a><h2>Type expressions</h2>
<div class="variablelist"><dl>
<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" lang="en">
<a name="id976797"></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" lang="en">
<a name="id976889"></a><h2>Complexity</h2>
<p>
All iterator operations must take amortized constant time.
</p>
</div>
<div class="refsect1" lang="en">
<a name="id976896"></a><h2>Invariants</h2>
<div class="variablelist"><dl>
<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" lang="en">
<a name="id976958"></a><h2>Models</h2>
<div class="itemizedlist"><ul type="disc">
<li><span class="simplelist"><span class="type">T *</span></span></li>
<li><span class="simplelist"><span class="type">std::list<T>::iterator</span></span></li>
</ul></div>
</div>
<div class="refsect1" lang="en">
<a name="id976979"></a><h2>See also</h2>
<div class="itemizedlist"><ul type="disc"><li><p><a href="RandomAccessIterator.html" title="Concept RandomAccessIterator">RandomAccessIterator</a></p></li></ul></div>
</div>
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