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| <title>Random Access Traversal Concept</title> |
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| <div class="document" id="random-access-traversal-concept"> |
| <h1 class="title">Random Access Traversal Concept</h1> |
| |
| <!-- Copyright David Abrahams 2006. Distributed under the Boost --> |
| <!-- Software License, Version 1.0. (See accompanying --> |
| <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> |
| <p>A class or built-in type <tt class="docutils literal"><span class="pre">X</span></tt> models the <em>Random Access Traversal</em> |
| concept if the following expressions are valid and respect the stated |
| semantics. In the table below, <tt class="docutils literal"><span class="pre">Distance</span></tt> is |
| <tt class="docutils literal"><span class="pre">iterator_traits<X>::difference_type</span></tt> and <tt class="docutils literal"><span class="pre">n</span></tt> represents a |
| constant object of type <tt class="docutils literal"><span class="pre">Distance</span></tt>.</p> |
| <table border="1" class="docutils"> |
| <colgroup> |
| <col width="28%" /> |
| <col width="30%" /> |
| <col width="23%" /> |
| <col width="20%" /> |
| </colgroup> |
| <thead valign="bottom"> |
| <tr><th class="head" colspan="4">Random Access Traversal Iterator Requirements (in addition to Bidirectional Traversal)</th> |
| </tr> |
| <tr><th class="head">Expression</th> |
| <th class="head">Return Type</th> |
| <th class="head">Operational Semantics</th> |
| <th class="head">Assertion/ |
| Precondition</th> |
| </tr> |
| </thead> |
| <tbody valign="top"> |
| <tr><td><tt class="docutils literal"><span class="pre">r</span> <span class="pre">+=</span> <span class="pre">n</span></tt></td> |
| <td><tt class="docutils literal"><span class="pre">X&</span></tt></td> |
| <td><pre class="first last literal-block"> |
| { |
| Distance m = n; |
| if (m >= 0) |
| while (m--) |
| ++r; |
| else |
| while (m++) |
| --r; |
| return r; |
| } |
| </pre> |
| </td> |
| <td> </td> |
| </tr> |
| <tr><td><tt class="docutils literal"><span class="pre">a</span> <span class="pre">+</span> <span class="pre">n</span></tt>, <tt class="docutils literal"><span class="pre">n</span> <span class="pre">+</span> <span class="pre">a</span></tt></td> |
| <td><tt class="docutils literal"><span class="pre">X</span></tt></td> |
| <td><tt class="docutils literal"><span class="pre">{</span> <span class="pre">X</span> <span class="pre">tmp</span> <span class="pre">=</span> <span class="pre">a;</span> <span class="pre">return</span> <span class="pre">tmp</span> |
| <span class="pre">+=</span> <span class="pre">n;</span> <span class="pre">}</span></tt></td> |
| <td> </td> |
| </tr> |
| <tr><td><tt class="docutils literal"><span class="pre">r</span> <span class="pre">-=</span> <span class="pre">n</span></tt></td> |
| <td><tt class="docutils literal"><span class="pre">X&</span></tt></td> |
| <td><tt class="docutils literal"><span class="pre">return</span> <span class="pre">r</span> <span class="pre">+=</span> <span class="pre">-n</span></tt></td> |
| <td> </td> |
| </tr> |
| <tr><td><tt class="docutils literal"><span class="pre">a</span> <span class="pre">-</span> <span class="pre">n</span></tt></td> |
| <td><tt class="docutils literal"><span class="pre">X</span></tt></td> |
| <td><tt class="docutils literal"><span class="pre">{</span> <span class="pre">X</span> <span class="pre">tmp</span> <span class="pre">=</span> <span class="pre">a;</span> <span class="pre">return</span> <span class="pre">tmp</span> |
| <span class="pre">-=</span> <span class="pre">n;</span> <span class="pre">}</span></tt></td> |
| <td> </td> |
| </tr> |
| <tr><td><tt class="docutils literal"><span class="pre">b</span> <span class="pre">-</span> <span class="pre">a</span></tt></td> |
| <td><tt class="docutils literal"><span class="pre">Distance</span></tt></td> |
| <td><tt class="docutils literal"><span class="pre">a</span> <span class="pre"><</span> <span class="pre">b</span> <span class="pre">?</span> <span class="pre">distance(a,b)</span> |
| <span class="pre">:</span> <span class="pre">-distance(b,a)</span></tt></td> |
| <td>pre: there exists a |
| value <tt class="docutils literal"><span class="pre">n</span></tt> of |
| <tt class="docutils literal"><span class="pre">Distance</span></tt> such that |
| <tt class="docutils literal"><span class="pre">a</span> <span class="pre">+</span> <span class="pre">n</span> <span class="pre">==</span> <span class="pre">b</span></tt>. <tt class="docutils literal"><span class="pre">b</span> |
| <span class="pre">==</span> <span class="pre">a</span> <span class="pre">+</span> <span class="pre">(b</span> <span class="pre">-</span> <span class="pre">a)</span></tt>.</td> |
| </tr> |
| <tr><td><tt class="docutils literal"><span class="pre">a[n]</span></tt></td> |
| <td>convertible to T</td> |
| <td><tt class="docutils literal"><span class="pre">*(a</span> <span class="pre">+</span> <span class="pre">n)</span></tt></td> |
| <td>pre: a is a <em>Readable |
| Iterator</em></td> |
| </tr> |
| <tr><td><tt class="docutils literal"><span class="pre">a[n]</span> <span class="pre">=</span> <span class="pre">v</span></tt></td> |
| <td>convertible to T</td> |
| <td><tt class="docutils literal"><span class="pre">*(a</span> <span class="pre">+</span> <span class="pre">n)</span> <span class="pre">=</span> <span class="pre">v</span></tt></td> |
| <td>pre: a is a <em>Writable |
| iterator</em></td> |
| </tr> |
| <tr><td><tt class="docutils literal"><span class="pre">a</span> <span class="pre"><</span> <span class="pre">b</span></tt></td> |
| <td>convertible to <tt class="docutils literal"><span class="pre">bool</span></tt></td> |
| <td><tt class="docutils literal"><span class="pre">b</span> <span class="pre">-</span> <span class="pre">a</span> <span class="pre">></span> <span class="pre">0</span></tt></td> |
| <td><tt class="docutils literal"><span class="pre"><</span></tt> is a total |
| ordering relation</td> |
| </tr> |
| <tr><td><tt class="docutils literal"><span class="pre">a</span> <span class="pre">></span> <span class="pre">b</span></tt></td> |
| <td>convertible to <tt class="docutils literal"><span class="pre">bool</span></tt></td> |
| <td><tt class="docutils literal"><span class="pre">b</span> <span class="pre"><</span> <span class="pre">a</span></tt></td> |
| <td><tt class="docutils literal"><span class="pre">></span></tt> is a total |
| ordering relation</td> |
| </tr> |
| <tr><td><tt class="docutils literal"><span class="pre">a</span> <span class="pre">>=</span> <span class="pre">b</span></tt></td> |
| <td>convertible to <tt class="docutils literal"><span class="pre">bool</span></tt></td> |
| <td><tt class="docutils literal"><span class="pre">!(a</span> <span class="pre"><</span> <span class="pre">b)</span></tt></td> |
| <td> </td> |
| </tr> |
| <tr><td><tt class="docutils literal"><span class="pre">a</span> <span class="pre"><=</span> <span class="pre">b</span></tt></td> |
| <td>convertible to <tt class="docutils literal"><span class="pre">bool</span></tt></td> |
| <td><tt class="docutils literal"><span class="pre">!(a</span> <span class="pre">></span> <span class="pre">b)</span></tt></td> |
| <td> </td> |
| </tr> |
| <tr><td><tt class="docutils literal"><span class="pre">iterator_traversal<X>::type</span></tt></td> |
| <td>Convertible to |
| <tt class="docutils literal"><span class="pre">random_access_traversal_tag</span></tt></td> |
| <td> </td> |
| <td> </td> |
| </tr> |
| </tbody> |
| </table> |
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