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<h1 class="title">Bidirectional Traversal Concept</h1>
<div class="document" id="bidirectional-traversal-concept">
<p>A class or built-in type <tt class="literal"><span class="pre">X</span></tt> models the <em>Bidirectional Traversal</em>
concept if, in addition to <tt class="literal"><span class="pre">X</span></tt> meeting the requirements of Forward
Traversal Iterator, the following expressions are valid and respect
the stated semantics.</p>
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<tr><th colspan="3">Bidirectional Traversal Iterator Requirements (in addition to Forward Traversal
Iterator)</th>
</tr>
<tr><th>Expression</th>
<th>Return Type</th>
<th>Assertion/Semantics /
Pre-/Post-condition</th>
</tr>
</thead>
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<tr><td><tt class="literal"><span class="pre">--r</span></tt></td>
<td><tt class="literal"><span class="pre">X&</span></tt></td>
<td>pre: there exists
<tt class="literal"><span class="pre">s</span></tt> such that <tt class="literal"><span class="pre">r</span>
<span class="pre">==</span> <span class="pre">++s</span></tt>. post:
<tt class="literal"><span class="pre">s</span></tt> is
dereferenceable.
<tt class="literal"><span class="pre">--(++r)</span> <span class="pre">==</span> <span class="pre">r</span></tt>.
<tt class="literal"><span class="pre">--r</span> <span class="pre">==</span> <span class="pre">--s</span></tt>
implies <tt class="literal"><span class="pre">r</span> <span class="pre">==</span>
<span class="pre">s</span></tt>. <tt class="literal"><span class="pre">&r</span> <span class="pre">==</span> <span class="pre">&--r</span></tt>.</td>
</tr>
<tr><td><tt class="literal"><span class="pre">r--</span></tt></td>
<td>convertible to <tt class="literal"><span class="pre">const</span> <span class="pre">X&</span></tt></td>
<td><pre class="first last literal-block">
{
X tmp = r;
--r;
return tmp;
}
</pre>
</td>
</tr>
<tr><td><tt class="literal"><span class="pre">iterator_traversal<X>::type</span></tt></td>
<td>Convertible to
<tt class="literal"><span class="pre">bidirectional_traversal_tag</span></tt></td>
<td> </td>
</tr>
</tbody>
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