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| <title>The MPL Reference Manual: Forward Iterator</title> |
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| <div class="section" id="forward-iterator"> |
| <h1><a class="toc-backref" href="./iterators-concepts.html#id1450">Forward Iterator</a></h1> |
| <div class="section" id="id384"> |
| <h3><a class="subsection-title" href="#description" name="description">Description</a></h3> |
| <p>A <a class="reference internal" href="./forward-iterator.html">Forward Iterator</a> <tt class="literal"><span class="pre">i</span></tt> is a type that represents a positional reference |
| to an element of a <a class="reference internal" href="./forward-sequence.html">Forward Sequence</a>. It allows to access the element through |
| a dereference operation, and provides a way to obtain an iterator to |
| the next element in a sequence.</p> |
| <!-- A [Forward Iterator] guarantees a linear traversal over |
| the sequence. --> |
| </div> |
| <div class="section" id="id385"> |
| <h3><a class="subsection-title" href="#definitions" name="definitions">Definitions</a></h3> |
| <ul class="simple"> |
| <li>An iterator can be <cite>dereferenceable</cite>, meaning that <tt class="literal"><span class="pre"><a href="./deref.html" class="identifier">deref</a><i>::type</span></tt> |
| is a well-defined expression.</li> |
| <li>An iterator is <cite>past-the-end</cite> if it points beyond the last element of a |
| sequence; past-the-end iterators are non-dereferenceable.</li> |
| <li>An iterator <tt class="literal"><span class="pre">i</span></tt> is <cite>incrementable</cite> if there is a "next" iterator, that |
| is, if <tt class="literal"><span class="pre"><a href="./next.html" class="identifier">next</a><i>::type</span></tt> expression is well-defined; past-the-end iterators are |
| not incrementable.</li> |
| <li>Two iterators into the same sequence are <cite>equivalent</cite> if they have the same |
| type.</li> |
| <li>An iterator <tt class="literal"><span class="pre">j</span></tt> is <cite>reachable</cite> from an iterator <tt class="literal"><span class="pre">i</span></tt> if , after recursive |
| application of <tt class="literal"><span class="pre"><a href="./next.html" class="identifier">next</a></span></tt> metafunction to <tt class="literal"><span class="pre">i</span></tt> a finite number of times, <tt class="literal"><span class="pre">i</span></tt> |
| is equivalent to <tt class="literal"><span class="pre">j</span></tt>.</li> |
| <li>The notation [<tt class="literal"><span class="pre">i</span></tt>,<tt class="literal"><span class="pre">j</span></tt>) refers to a <cite>range</cite> of iterators beginning with |
| <tt class="literal"><span class="pre">i</span></tt> and up to but not including <tt class="literal"><span class="pre">j</span></tt>.</li> |
| <li>The range [<tt class="literal"><span class="pre">i</span></tt>,<tt class="literal"><span class="pre">j</span></tt>) is a <cite>valid range</cite> if <tt class="literal"><span class="pre">j</span></tt> is reachable from <tt class="literal"><span class="pre">i</span></tt>.</li> |
| </ul> |
| </div> |
| <div class="section" id="id386"> |
| <h3><a class="subsection-title" href="#expression-requirements" name="expression-requirements">Expression requirements</a></h3> |
| <table border="1" class="docutils table"> |
| <colgroup> |
| <col width="25%" /> |
| <col width="46%" /> |
| <col width="29%" /> |
| </colgroup> |
| <thead valign="bottom"> |
| <tr><th class="head">Expression</th> |
| <th class="head">Type</th> |
| <th class="head">Complexity</th> |
| </tr> |
| </thead> |
| <tbody valign="top"> |
| <tr><td><tt class="literal"><span class="pre"><a href="./deref.html" class="identifier">deref</a><i>::type</span></tt></td> |
| <td>Any type</td> |
| <td>Amortized constant time</td> |
| </tr> |
| <tr><td><tt class="literal"><span class="pre"><a href="./next.html" class="identifier">next</a><i>::type</span></tt></td> |
| <td><a class="reference internal" href="./forward-iterator.html">Forward Iterator</a></td> |
| <td>Amortized constant time</td> |
| </tr> |
| <tr><td><tt class="literal"><span class="pre">i::category</span></tt></td> |
| <td><a class="reference internal" href="./integral-constant.html">Integral Constant</a>, convertible |
| to <tt class="literal"><span class="pre">forward_iterator_tag</span></tt></td> |
| <td>Constant time</td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| <div class="section" id="id387"> |
| <h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3> |
| <pre class="literal-block"> |
| typedef <a href="./deref.html" class="identifier">deref</a><i>::type j; |
| </pre> |
| <table class="docutils field-list" frame="void" rules="none"> |
| <col class="field-name" /> |
| <col class="field-body" /> |
| <tbody valign="top"> |
| <tr class="field"><th class="field-name">Precondition:</th><td class="field-body"><tt class="literal"><span class="pre">i</span></tt> is dereferenceable</td> |
| </tr> |
| <tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><tt class="literal"><span class="pre">j</span></tt> is identical to the type of the pointed element</td> |
| </tr> |
| </tbody> |
| </table> |
| <!-- .......................................................................... --> |
| <pre class="literal-block"> |
| typedef <a href="./next.html" class="identifier">next</a><i>::type j; |
| </pre> |
| <table class="docutils field-list" frame="void" rules="none"> |
| <col class="field-name" /> |
| <col class="field-body" /> |
| <tbody valign="top"> |
| <tr class="field"><th class="field-name">Precondition:</th><td class="field-body"><tt class="literal"><span class="pre">i</span></tt> is incrementable</td> |
| </tr> |
| <tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><tt class="literal"><span class="pre">j</span></tt> is the next iterator in a sequence</td> |
| </tr> |
| <tr class="field"><th class="field-name">Postcondition:</th><td class="field-body"><tt class="literal"><span class="pre">j</span></tt> is dereferenceable or past-the-end</td> |
| </tr> |
| </tbody> |
| </table> |
| <!-- .......................................................................... --> |
| <pre class="literal-block"> |
| typedef i::category c; |
| </pre> |
| <table class="docutils field-list" frame="void" rules="none"> |
| <col class="field-name" /> |
| <col class="field-body" /> |
| <tbody valign="top"> |
| <tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><tt class="literal"><span class="pre">c</span></tt> is identical to the iterator's category tag</td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| <div class="section" id="id388"> |
| <h3><a class="subsection-title" href="#invariants" name="invariants">Invariants</a></h3> |
| <p>For any forward iterators <tt class="literal"><span class="pre">i</span></tt> and <tt class="literal"><span class="pre">j</span></tt> the following invariants always hold:</p> |
| <ul class="simple"> |
| <li><tt class="literal"><span class="pre">i</span></tt> and <tt class="literal"><span class="pre">j</span></tt> are equivalent if and only if they are pointing to the same |
| element.</li> |
| <li>If <tt class="literal"><span class="pre">i</span></tt> is dereferenceable, and <tt class="literal"><span class="pre">j</span></tt> is equivalent to <tt class="literal"><span class="pre">i</span></tt>, then <tt class="literal"><span class="pre">j</span></tt> is |
| dereferenceable as well.</li> |
| <li>If <tt class="literal"><span class="pre">i</span></tt> and <tt class="literal"><span class="pre">j</span></tt> are equivalent and dereferenceable, then <tt class="literal"><span class="pre"><a href="./deref.html" class="identifier">deref</a><i>::type</span></tt> |
| and <tt class="literal"><span class="pre"><a href="./deref.html" class="identifier">deref</a><j>::type</span></tt> are identical.</li> |
| <li>If <tt class="literal"><span class="pre">i</span></tt> is incrementable, and <tt class="literal"><span class="pre">j</span></tt> is equivalent to <tt class="literal"><span class="pre">i</span></tt>, then <tt class="literal"><span class="pre">j</span></tt> is |
| incrementable as well.</li> |
| <li>If <tt class="literal"><span class="pre">i</span></tt> and <tt class="literal"><span class="pre">j</span></tt> are equivalent and incrementable, then <tt class="literal"><span class="pre"><a href="./next.html" class="identifier">next</a><i>::type</span></tt> |
| and <tt class="literal"><span class="pre"><a href="./next.html" class="identifier">next</a><j>::type</span></tt> are equivalent.</li> |
| </ul> |
| </div> |
| <div class="section" id="id389"> |
| <h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3> |
| <p><a class="reference internal" href="./iterators.html">Iterators</a>, <a class="reference internal" href="./bidirectional-iterator.html">Bidirectional Iterator</a>, <a class="reference internal" href="./forward-sequence.html">Forward Sequence</a>, <a class="reference internal" href="./deref.html">deref</a>, <a class="reference internal" href="./next.html">next</a></p> |
| <!-- Iterators/Concepts//Bidirectional Iterator |20 --> |
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