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<h1 class="title">Bidirectional Traversal Concept</h1>

<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
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<p>A class or built-in type <tt class="docutils literal"><span class="pre">X</span></tt> models the <em>Bidirectional Traversal</em>
concept if, in addition to <tt class="docutils 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>
<table border="1" class="docutils">
<colgroup>
<col width="38%" />
<col width="37%" />
<col width="25%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head" colspan="3">Bidirectional Traversal Iterator Requirements (in addition to Forward Traversal
Iterator)</th>
</tr>
<tr><th class="head">Expression</th>
<th class="head">Return Type</th>
<th class="head">Assertion/Semantics /
Pre-/Post-condition</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="docutils literal"><span class="pre">--r</span></tt></td>
<td><tt class="docutils literal"><span class="pre">X&amp;</span></tt></td>
<td>pre: there exists
<tt class="docutils literal"><span class="pre">s</span></tt> such that <tt class="docutils literal"><span class="pre">r</span>
<span class="pre">==</span> <span class="pre">++s</span></tt>.  post:
<tt class="docutils literal"><span class="pre">s</span></tt> is
dereferenceable.
<tt class="docutils literal"><span class="pre">--(++r)</span> <span class="pre">==</span> <span class="pre">r</span></tt>.
<tt class="docutils literal"><span class="pre">--r</span> <span class="pre">==</span> <span class="pre">--s</span></tt>
implies <tt class="docutils literal"><span class="pre">r</span> <span class="pre">==</span>
<span class="pre">s</span></tt>. <tt class="docutils literal"><span class="pre">&amp;r</span> <span class="pre">==</span> <span class="pre">&amp;--r</span></tt>.</td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">r--</span></tt></td>
<td>convertible to <tt class="docutils literal"><span class="pre">const</span> <span class="pre">X&amp;</span></tt></td>
<td><pre class="first last literal-block">
{
  X tmp = r;
  --r;
  return tmp;
}
</pre>
</td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">iterator_traversal&lt;X&gt;::type</span></tt></td>
<td>Convertible to
<tt class="docutils literal"><span class="pre">bidirectional_traversal_tag</span></tt></td>
<td>&nbsp;</td>
</tr>
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