101 lines
10 KiB
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101 lines
10 KiB
HTML
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<title>std::forward_iterator</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_iterator_forward_iterator skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>forward_iterator</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-iterator.html">iterator</a></span></div>
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<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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<table class="t-dcl-begin">
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<div>定义于头文件 <code><a href="cpp-header-iterator.html"><iterator></a></code></div>
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<td></td>
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<tr class="t-dcl t-since-cxx20">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> I<span class="sy1">></span><br /></span>
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<p> concept forward_iterator <span class="sy1">=</span><br />
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std<span class="sy4">::</span><span class="me2">input_iterator</span><span class="sy1"><</span>I<span class="sy1">></span> <span class="sy3">&&</span><br />
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std<span class="sy4">::</span><span class="me2">derived_from</span><span class="sy1"><</span><span class="coMULTI">/*ITER_CONCEPT*/</span><span class="sy1"><</span>I<span class="sy1">></span>, <a href="cpp-iterator-iterator_tags.html"><span class="kw654">std::<span class="me2">forward_iterator_tag</span></span></a><span class="sy1">></span> <span class="sy3">&&</span><br />
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std<span class="sy4">::</span><span class="me2">incrementable</span><span class="sy1"><</span>I<span class="sy1">></span> <span class="sy3">&&</span><br /></p>
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std<span class="sy4">::</span><span class="me2">sentinel_for</span><span class="sy1"><</span>I, I<span class="sy1">></span><span class="sy4">;</span></div>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<p>此概念细化 <span class="t-lc">std::input_iterator</span> ，通过要求 <code>I</code> 亦实现 <code>std::incrementable</code> （从而使之适合多趟算法），并保证指向同一范围的二个迭代器可相互比较。</p>
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<h3><span class="mw-headline" id=".E8.BF.AD.E4.BB.A3.E5.99.A8.E6.A6.82.E5.BF.B5.E7.A1.AE.E5.AE.9A">迭代器概念确定</span></h3>
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<p>此概念的定义通过仅用于阐释的别名模板 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="coMULTI">/*ITER_CONCEPT*/</span></span></span> 说明。</p>
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<p>为确定 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="coMULTI">/*ITER_CONCEPT*/</span><span class="sy1"><</span>I<span class="sy1">></span></span></span> ，令 <span class="t-c"><span class="mw-geshi cpp source-cpp">ITER_TRAITS<span class="sy1"><</span>I<span class="sy1">></span></span></span> 若特化 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-iterator-iterator_traits.html"><span class="kw651">std::<span class="me2">iterator_traits</span></span></a><span class="sy1"><</span>I<span class="sy1">></span></span></span> 从主模板生成则指代 <span class="t-c"><span class="mw-geshi cpp source-cpp">I</span></span> ，否则指代 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-iterator-iterator_traits.html"><span class="kw651">std::<span class="me2">iterator_traits</span></span></a><span class="sy1"><</span>I<span class="sy1">></span></span></span> ：</p>
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<ul>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">ITER_TRAITS<span class="sy1"><</span>I<span class="sy1">></span><span class="sy4">::</span><span class="me2">iterator_concept</span></span></span> 合法并指名类型，则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="coMULTI">/*ITER_CONCEPT*/</span><span class="sy1"><</span>I<span class="sy1">></span></span></span> 指代该类型。</li>
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<li>否则，若 <span class="t-c"><span class="mw-geshi cpp source-cpp">ITER_TRAITS<span class="sy1"><</span>I<span class="sy1">></span><span class="sy4">::</span><span class="me2">iterator_category</span></span></span> 合法并指名类型，则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="coMULTI">/*ITER_CONCEPT*/</span><span class="sy1"><</span>I<span class="sy1">></span></span></span> 指代该类型。</li>
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<li>否则，若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-iterator-iterator_traits.html"><span class="kw651">std::<span class="me2">iterator_traits</span></span></a><span class="sy1"><</span>I<span class="sy1">></span></span></span> 从主模板生成，则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="coMULTI">/*ITER_CONCEPT*/</span><span class="sy1"><</span>I<span class="sy1">></span></span></span> 指代 <span class="t-lc"><a href="cpp-iterator-iterator_tags.html">std::random_access_iterator_tag</a></span> 。</li>
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<li>否则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="coMULTI">/*ITER_CONCEPT*/</span><span class="sy1"><</span>I<span class="sy1">></span></span></span> 不指代类型并导致替换失败。</li>
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</ul>
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<h3><span class="mw-headline" id=".E8.AF.AD.E4.B9.89.E8.A6.81.E6.B1.82">语义要求</span></h3>
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<p><code>I</code> 实现 <code>std::forward_iterator</code> 当且仅当 <code>I</code> 实现其所蕴含的所有概念，并且给定 <code>I</code> 类型对象 <code>i</code> 与 <code>j</code> ：</p>
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<ul>
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<li>若符合下列条件，则迭代器 <code>i</code> 与 <code>j</code> 间的比较拥有定义的结果：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li><code>i</code> 与 <code>j</code> 为指向同一底层序列的迭代器，或</li>
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<li><code>i</code> 与 <code>j</code> 均被值初始化，该情况下它们比较相等。</li>
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</ul>
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</dd>
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</dl>
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<ul>
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<li>从指向范围中的向前迭代器获得的指针与引用在该范围存在时保持合法。</li>
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<li>若 <code>i</code> 与 <code>j</code> 可解引用，则它们提供<i>多趟保证</i>，即：</li>
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<li><code>i == j</code> 蕴含 <code>++i == ++j</code> ，且</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span><span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span><span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">auto</span> x<span class="br0">)</span><span class="br0">{</span> <span class="sy2">++</span>x<span class="sy4">;</span> <span class="br0">}</span><span class="br0">(</span>i<span class="br0">)</span>, <span class="sy2">*</span>i<span class="br0">)</span></span></span> 等价于 <code>*i</code> 。</li>
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</ul>
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</dd>
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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p>不同于<a href="cpp-named_req-ForwardIterator.html"><span style="font-style:italic">老式向前迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyForwardIterator)</span></a> 要求， <code>forward_iterator</code> 概念不要求解引用返回左值。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<div class="t-dsc-member-div">
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<div><a href="cpp-iterator-input_iterator.html"><span class="t-lines"><span>input_iterator</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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<td>指定类型为输入迭代器，即可读取其所引用的值，且可前/后自增<br />
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<span class="t-mark">(概念)</span></td>
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