<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <meta name="generator" content="Docutils 0.5: http://docutils.sourceforge.net/" /> <title>Random Access Traversal Concept</title> <link rel="stylesheet" href="../../../rst.css" type="text/css" /> </head> <body> <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> </div> <div class="footer"> <hr class="footer" /> <a class="reference external" href="RandomAccessTraversal.rst">View document source</a>. 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