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  26 <a name="BidirectionalIterator"></a><div class="titlepage"></div>
  27 <div class="refnamediv">
  28 <h2><span class="refentrytitle">Concept BidirectionalIterator</span></h2>
  29 <p>BidirectionalIterator</p>
  30 </div>
  31 <div class="refsect1">
  32 <a name="idm45853365421824"></a><h2>Description</h2>
  33 <p>A bidirectional iterator is an iterator that can read through a sequence
  34   of values.  It can move in either direction through the sequence, and can
  35   be either mutable (data pointed to by it can be changed) or not mutable.</p>
  36 <p>An iterator represents a position in a sequence.  Therefore, the
  37   iterator can point into the sequence (returning a value when dereferenced
  38   and being incrementable), or be off-the-end (and not dereferenceable or
  39   incrementable).</p>
  40 </div>
  41 <div class="refsect1">
  42 <a name="idm45853365420048"></a><h2>Refinement of</h2>
  43 <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>
  44 </div>
  45 <div class="refsect1">
  46 <a name="idm45853365418784"></a><h2>Associated types</h2>
  47 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  48 <li class="listitem">
  49 <p><span class="bold"><strong>value_type</strong></span></p>
  50 <pre class="literallayout">std::iterator_traits&lt;Iter&gt;::value_type</pre>
  51 <p>The value type of the iterator</p>
  52 </li>
  53 <li class="listitem">
  54 <p><span class="bold"><strong>category</strong></span></p>
  55 <pre class="literallayout">std::iterator_traits&lt;Iter&gt;::iterator_category</pre>
  56 <p>The category of the iterator</p>
  57 </li>
  58 </ul></div>
  59 </div>
  60 <div class="refsect1">
  61 <a name="idm45853365414640"></a><h2>Notation</h2>
  62 <div class="variablelist"><dl class="variablelist">
  63 <dt><span class="term">Iter</span></dt>
  64 <dd>A type playing the role of iterator-type in the <a class="link" href="BidirectionalIterator.html" title="Concept BidirectionalIterator">BidirectionalIterator</a> concept.</dd>
  65 <dt>
  66 <span class="term"><code class="varname">i</code>, </span><span class="term"><code class="varname">j</code></span>
  67 </dt>
  68 <dd>Objects of type Iter</dd>
  69 <dt><span class="term"><code class="varname">x</code></span></dt>
  70 <dd>Object of type value_type</dd>
  71 </dl></div>
  72 </div>
  73 <div class="refsect1">
  74 <a name="idm45853365410560"></a><h2>Type expressions</h2>
  75 <div class="variablelist"><dl class="variablelist">
  76 <dt><span class="term">Category tag</span></dt>
  77 <dd><p><span class="type">category</span> must be
  78                   derived from <span class="type">std::bidirectional_iterator_tag</span>.
  79                 </p></dd>
  80 </dl></div>
  81 </div>
  82 <div class="refsect1">
  83 <a name="idm45853365408640"></a><h2>Valid expressions</h2>
  84 <div class="informaltable"><table class="table">
  85 <colgroup>
  86 <col>
  87 <col>
  88 <col>
  89 <col>
  90 <col>
  91 <col>
  92 </colgroup>
  93 <thead><tr>
  94 <th>Name</th>
  95 <th>Expression</th>
  96 <th>Type</th>
  97 <th>Precondition</th>
  98 <th>Semantics</th>
  99 <th>Postcondition</th>
 100 </tr></thead>
 101 <tbody>
 102 <tr>
 103 <td><p>Predecrement</p></td>
 104 <td><p>--i</p></td>
 105 <td><p><span class="type">Iter &amp;</span></p></td>
 106 <td><p><code class="computeroutput">i</code> is incrementable (not
 107     off-the-end) and some dereferenceable iterator <code class="computeroutput">j</code> exists
 108     such that <code class="computeroutput">i == ++j</code></p></td>
 109 <td>&#160;</td>
 110 <td>&#160;</td>
 111 </tr>
 112 <tr>
 113 <td><p>Postdecrement</p></td>
 114 <td><p>i--</p></td>
 115 <td><p><span class="type">Iter</span></p></td>
 116 <td><p>Same as for predecrement</p></td>
 117 <td><p>Equivalent to <code class="computeroutput">{Iter j = i; --i; return j;}</code></p></td>
 118 <td><p><code class="computeroutput">i</code> is dereferenceable or
 119     off-the-end</p></td>
 120 </tr>
 121 </tbody>
 122 </table></div>
 123 </div>
 124 <div class="refsect1">
 125 <a name="idm45853365398528"></a><h2>Complexity</h2>
 126 <p>
 127   All iterator operations must take amortized constant time.
 128   </p>
 129 </div>
 130 <div class="refsect1">
 131 <a name="idm45853365397808"></a><h2>Invariants</h2>
 132 <div class="variablelist"><dl class="variablelist">
 133 <dt><span class="term">Predecrement must return object</span></dt>
 134 <dd><p>
 135   <code class="computeroutput">&amp;i = &amp;(--i)</code>
 136   </p></dd>
 137 <dt><span class="term">Unique path through sequence</span></dt>
 138 <dd><p>
 139   <code class="computeroutput">i == j</code> implies <code class="computeroutput">--i == --j</code>
 140   </p></dd>
 141 <dt><span class="term">Increment and decrement are inverses</span></dt>
 142 <dd><p>
 143   <code class="computeroutput">++i; --i;</code> and <code class="computeroutput">--i; ++i;</code> must end up with the
 144   value of <code class="computeroutput">i</code> unmodified, if <code class="computeroutput">i</code> both of the
 145   operations in the pair are valid.
 146   </p></dd>
 147 </dl></div>
 148 </div>
 149 <div class="refsect1">
 150 <a name="idm45853365390624"></a><h2>Models</h2>
 151 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 152 <li class="listitem"><span class="simplelist"><span class="type">T *</span></span></li>
 153 <li class="listitem"><span class="simplelist"><span class="type">std::list&lt;T&gt;::iterator</span></span></li>
 154 </ul></div>
 155 </div>
 156 <div class="refsect1">
 157 <a name="idm45853365388272"></a><h2>See also</h2>
 158 <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>
 159 </div>
 160 </div>
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