1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Function templates next() and prior()</title> 5<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../index.html" title="Chapter 1. Boost.Iterator"> 8<link rel="up" href="../algorithms.html" title="Algorithms"> 9<link rel="prev" href="distance.html" title="Function template distance()"> 10<link rel="next" href="../upgrading.html" title="Upgrading from the old Boost Iterator Adaptor Library"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="distance.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../algorithms.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../upgrading.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h3 class="title"> 27<a name="iterator.algorithms.next_prior"></a><a class="link" href="next_prior.html" title="Function templates next() and prior()">Function templates <code class="computeroutput"><span class="identifier">next</span><span class="special">()</span></code> and 28 <code class="computeroutput"><span class="identifier">prior</span><span class="special">()</span></code></a> 29</h3></div></div></div> 30<p> 31 Certain data types, such as the C++ Standard Library's forward and bidirectional 32 iterators, do not provide addition and subtraction via <code class="computeroutput"><span class="keyword">operator</span><span class="special">+()</span></code> or <code class="computeroutput"><span class="keyword">operator</span><span class="special">-()</span></code>. This means that non-modifying computation 33 of the next or prior value requires a temporary, even though <code class="computeroutput"><span class="keyword">operator</span><span class="special">++()</span></code> 34 or <code class="computeroutput"><span class="keyword">operator</span><span class="special">--()</span></code> 35 is provided. It also means that writing code like <code class="computeroutput"><span class="identifier">itr</span><span class="special">+</span><span class="number">1</span></code> inside a 36 template restricts the iterator category to random access iterators. 37 </p> 38<p> 39 The <code class="computeroutput"><span class="identifier">next</span><span class="special">()</span></code> 40 and <code class="computeroutput"><span class="identifier">prior</span><span class="special">()</span></code> 41 functions defined in <code class="computeroutput"><span class="identifier">boost</span><span class="special">/</span><span class="identifier">next_prior</span><span class="special">.</span><span class="identifier">hpp</span></code> provide 42 a simple way around these problems. 43 </p> 44<h5> 45<a name="iterator.algorithms.next_prior.h0"></a> 46 <span class="phrase"><a name="iterator.algorithms.next_prior.synopsis"></a></span><a class="link" href="next_prior.html#iterator.algorithms.next_prior.synopsis">Synopsis</a> 47 </h5> 48<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> 49<span class="identifier">T</span> <span class="identifier">next</span><span class="special">(</span><span class="identifier">T</span> <span class="identifier">x</span><span class="special">)</span> 50<span class="special">{</span> 51 <span class="keyword">return</span> <span class="special">++</span><span class="identifier">x</span><span class="special">;</span> 52<span class="special">}</span> 53 54<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Distance</span><span class="special">></span> 55<span class="identifier">T</span> <span class="identifier">next</span><span class="special">(</span><span class="identifier">T</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">Distance</span> <span class="identifier">n</span><span class="special">)</span> 56<span class="special">{</span> 57 <span class="identifier">std</span><span class="special">::</span><span class="identifier">advance</span><span class="special">(</span><span class="identifier">x</span><span class="special">,</span> <span class="identifier">n</span><span class="special">);</span> 58 <span class="keyword">return</span> <span class="identifier">x</span><span class="special">;</span> 59<span class="special">}</span> 60 61<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> 62<span class="identifier">T</span> <span class="identifier">prior</span><span class="special">(</span><span class="identifier">T</span> <span class="identifier">x</span><span class="special">)</span> 63<span class="special">{</span> 64 <span class="keyword">return</span> <span class="special">--</span><span class="identifier">x</span><span class="special">;</span> 65<span class="special">}</span> 66 67<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Distance</span><span class="special">></span> 68<span class="identifier">T</span> <span class="identifier">prior</span><span class="special">(</span><span class="identifier">T</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">Distance</span> <span class="identifier">n</span><span class="special">)</span> 69<span class="special">{</span> 70 <span class="identifier">std</span><span class="special">::</span><span class="identifier">advance</span><span class="special">(</span><span class="identifier">x</span><span class="special">,</span> <span class="special">-</span><span class="identifier">n</span><span class="special">);</span> 71 <span class="keyword">return</span> <span class="identifier">x</span><span class="special">;</span> 72<span class="special">}</span> 73</pre> 74<div class="note"><table border="0" summary="Note"> 75<tr> 76<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td> 77<th align="left">Note</th> 78</tr> 79<tr><td align="left" valign="top"><p> 80 Function implementations above are given for exposition only. The actual 81 implementation has the same effect for iterators, but has different properties, 82 as documented later. 83 </p></td></tr> 84</table></div> 85<h5> 86<a name="iterator.algorithms.next_prior.h1"></a> 87 <span class="phrase"><a name="iterator.algorithms.next_prior.usage"></a></span><a class="link" href="next_prior.html#iterator.algorithms.next_prior.usage">Usage</a> 88 </h5> 89<p> 90 Usage is simple: 91 </p> 92<pre class="programlisting"><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">iterator</span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">get_some_iterator</span><span class="special">();</span> 93<span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">iterator</span> <span class="identifier">prev</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">prior</span><span class="special">(</span><span class="identifier">p</span><span class="special">);</span> 94<span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">iterator</span> <span class="identifier">next</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">next</span><span class="special">(</span><span class="identifier">prev</span><span class="special">,</span> <span class="number">2</span><span class="special">);</span> 95</pre> 96<p> 97 The distance from the given iterator should be supplied as an absolute value. 98 For example, the iterator four iterators prior to the given iterator <code class="computeroutput"><span class="identifier">p</span></code> may be obtained by <code class="computeroutput"><span class="identifier">prior</span><span class="special">(</span><span class="identifier">p</span><span class="special">,</span> 99 <span class="number">4</span><span class="special">)</span></code>. 100 </p> 101<p> 102 With C++11, the Standard Library provides <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">next</span><span class="special">()</span></code> and <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">prev</span><span class="special">()</span></code> function templates, which serve the same 103 purpose. However, there are advantages to <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">next</span><span class="special">()</span></code> and <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">prior</span><span class="special">()</span></code>. 104 </p> 105<p> 106 First, <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">next</span><span class="special">()</span></code> 107 and <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">prior</span><span class="special">()</span></code> 108 are compatible not only with iterators but with any type that provides arithmetic 109 operators <code class="computeroutput"><span class="keyword">operator</span><span class="special">++()</span></code>, 110 <code class="computeroutput"><span class="keyword">operator</span><span class="special">--()</span></code>, 111 <code class="computeroutput"><span class="keyword">operator</span><span class="special">+()</span></code>, 112 <code class="computeroutput"><span class="keyword">operator</span><span class="special">-()</span></code>, 113 <code class="computeroutput"><span class="keyword">operator</span><span class="special">+=()</span></code> 114 or <code class="computeroutput"><span class="keyword">operator</span><span class="special">-=()</span></code>. 115 For example, this is possible: 116 </p> 117<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span> 118<span class="keyword">int</span> <span class="identifier">y</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">next</span><span class="special">(</span><span class="identifier">x</span><span class="special">,</span> <span class="number">5</span><span class="special">);</span> 119<span class="identifier">assert</span><span class="special">(</span><span class="identifier">y</span> <span class="special">==</span> <span class="number">15</span><span class="special">);</span> 120</pre> 121<p> 122 Second, <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">next</span><span class="special">()</span></code> 123 and <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">prior</span><span class="special">()</span></code> 124 use <a class="link" href="../concepts/traversal.html" title="Traversal">traversal categories</a> 125 to select the most efficient implementation. For some kinds of iterators, 126 such as <a class="link" href="../specialized/transform.html" title="Transform Iterator">transform iterators</a>, 127 the standard iterator category does not reflect the traversal category correctly 128 and therefore <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">next</span><span class="special">()</span></code> 129 and <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">prev</span><span class="special">()</span></code> 130 will fall back to linear complexity. 131 </p> 132<h5> 133<a name="iterator.algorithms.next_prior.h2"></a> 134 <span class="phrase"><a name="iterator.algorithms.next_prior.acknowledgements"></a></span><a class="link" href="next_prior.html#iterator.algorithms.next_prior.acknowledgements">Acknowledgements</a> 135 </h5> 136<p> 137 Contributed by <a href="http://www.boost.org/people/dave_abrahams.htm" target="_top">Dave 138 Abrahams</a>. Two-argument versions by Daniel Walker. 139 </p> 140</div> 141<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 142<td align="left"></td> 143<td align="right"><div class="copyright-footer">Copyright © 2003, 2005 David Abrahams Jeremy Siek Thomas 144 Witt<p> 145 Distributed under the Boost Software License, Version 1.0. (See accompanying 146 file LICENSE_1_0.txt or copy at <ulink url="http://www.boost.org/LICENSE_1_0.txt"> 147 http://www.boost.org/LICENSE_1_0.txt </ulink>) 148 </p> 149</div></td> 150</tr></table> 151<hr> 152<div class="spirit-nav"> 153<a accesskey="p" href="distance.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../algorithms.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../upgrading.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 154</div> 155</body> 156</html> 157
