1<?xml version="1.0" encoding="utf-8" ?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> 4<head> 5<meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> 6<meta name="generator" content="Docutils 0.7: http://docutils.sourceforge.net/" /> 7<title>The MPL Reference Manual: Forward Iterator</title> 8<link rel="stylesheet" href="../style.css" type="text/css" /> 9</head> 10<body class="docframe refmanual"> 11<table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./iterators-concepts.html" class="navigation-link">Prev</a> <a href="./bidirectional-iterator.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group">Back <a href="./bidirectional-iterator.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./iterators-concepts.html" class="navigation-link">Up</a> <a href="../refmanual.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./refmanual_toc.html" class="navigation-link">Full TOC</a></span></td> 12<td class="header-group page-location"><a href="../refmanual.html" class="navigation-link">Front Page</a> / <a href="./iterators.html" class="navigation-link">Iterators</a> / <a href="./iterators-concepts.html" class="navigation-link">Concepts</a> / <a href="./forward-iterator.html" class="navigation-link">Forward Iterator</a></td> 13</tr></table><div class="header-separator"></div> 14<div class="section" id="forward-iterator"> 15<h1><a class="toc-backref" href="./iterators-concepts.html#id1450">Forward Iterator</a></h1> 16<div class="section" id="id384"> 17<h3><a class="subsection-title" href="#description" name="description">Description</a></h3> 18<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 19to an element of a <a class="reference internal" href="./forward-sequence.html">Forward Sequence</a>. It allows to access the element through 20a dereference operation, and provides a way to obtain an iterator to 21the next element in a sequence.</p> 22<!-- A [Forward Iterator] guarantees a linear traversal over 23the sequence. --> 24</div> 25<div class="section" id="id385"> 26<h3><a class="subsection-title" href="#definitions" name="definitions">Definitions</a></h3> 27<ul class="simple"> 28<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> 29is a well-defined expression.</li> 30<li>An iterator is <cite>past-the-end</cite> if it points beyond the last element of a 31sequence; past-the-end iterators are non-dereferenceable.</li> 32<li>An iterator <tt class="literal"><span class="pre">i</span></tt> is <cite>incrementable</cite> if there is a "next" iterator, that 33is, 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 34not incrementable.</li> 35<li>Two iterators into the same sequence are <cite>equivalent</cite> if they have the same 36type.</li> 37<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 38application 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> 39is equivalent to <tt class="literal"><span class="pre">j</span></tt>.</li> 40<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 41<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> 42<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> 43</ul> 44</div> 45<div class="section" id="id386"> 46<h3><a class="subsection-title" href="#expression-requirements" name="expression-requirements">Expression requirements</a></h3> 47<table border="1" class="docutils table"> 48<colgroup> 49<col width="25%" /> 50<col width="46%" /> 51<col width="29%" /> 52</colgroup> 53<thead valign="bottom"> 54<tr><th class="head">Expression</th> 55<th class="head">Type</th> 56<th class="head">Complexity</th> 57</tr> 58</thead> 59<tbody valign="top"> 60<tr><td><tt class="literal"><span class="pre"><a href="./deref.html" class="identifier">deref</a><i>::type</span></tt></td> 61<td>Any type</td> 62<td>Amortized constant time</td> 63</tr> 64<tr><td><tt class="literal"><span class="pre"><a href="./next.html" class="identifier">next</a><i>::type</span></tt></td> 65<td><a class="reference internal" href="./forward-iterator.html">Forward Iterator</a></td> 66<td>Amortized constant time</td> 67</tr> 68<tr><td><tt class="literal"><span class="pre">i::category</span></tt></td> 69<td><a class="reference internal" href="./integral-constant.html">Integral Constant</a>, convertible 70to <tt class="literal"><span class="pre">forward_iterator_tag</span></tt></td> 71<td>Constant time</td> 72</tr> 73</tbody> 74</table> 75</div> 76<div class="section" id="id387"> 77<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3> 78<pre class="literal-block"> 79typedef <a href="./deref.html" class="identifier">deref</a><i>::type j; 80</pre> 81<table class="docutils field-list" frame="void" rules="none"> 82<col class="field-name" /> 83<col class="field-body" /> 84<tbody valign="top"> 85<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> 86</tr> 87<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> 88</tr> 89</tbody> 90</table> 91<!-- .......................................................................... --> 92<pre class="literal-block"> 93typedef <a href="./next.html" class="identifier">next</a><i>::type j; 94</pre> 95<table class="docutils field-list" frame="void" rules="none"> 96<col class="field-name" /> 97<col class="field-body" /> 98<tbody valign="top"> 99<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> 100</tr> 101<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> 102</tr> 103<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> 104</tr> 105</tbody> 106</table> 107<!-- .......................................................................... --> 108<pre class="literal-block"> 109typedef i::category c; 110</pre> 111<table class="docutils field-list" frame="void" rules="none"> 112<col class="field-name" /> 113<col class="field-body" /> 114<tbody valign="top"> 115<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> 116</tr> 117</tbody> 118</table> 119</div> 120<div class="section" id="id388"> 121<h3><a class="subsection-title" href="#invariants" name="invariants">Invariants</a></h3> 122<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> 123<ul class="simple"> 124<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 125element.</li> 126<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 127dereferenceable as well.</li> 128<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> 129and <tt class="literal"><span class="pre"><a href="./deref.html" class="identifier">deref</a><j>::type</span></tt> are identical.</li> 130<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 131incrementable as well.</li> 132<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> 133and <tt class="literal"><span class="pre"><a href="./next.html" class="identifier">next</a><j>::type</span></tt> are equivalent.</li> 134</ul> 135</div> 136<div class="section" id="id389"> 137<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3> 138<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> 139<!-- Iterators/Concepts//Bidirectional Iterator |20 --> 140</div> 141</div> 142 143<div class="footer-separator"></div> 144<table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./iterators-concepts.html" class="navigation-link">Prev</a> <a href="./bidirectional-iterator.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group">Back <a href="./bidirectional-iterator.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./iterators-concepts.html" class="navigation-link">Up</a> <a href="../refmanual.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./refmanual_toc.html" class="navigation-link">Full TOC</a></span></td> 145<td><div class="copyright-footer"><div class="copyright">Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams</div> 146Distributed under the Boost Software License, Version 1.0. (See accompanying 147file LICENSE_1_0.txt or copy at <a class="reference external" href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)</div></td></tr></table></body> 148</html> 149