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110 lines
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<title>std::bidirectional_iterator</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_iterator_bidirectional_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>bidirectional_iterator</h1>
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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>定义于头文件 <code><a href="cpp-header-iterator.html"><iterator></a></code></div>
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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 bidirectional_iterator <span class="sy1">=</span><br />
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std<span class="sy4">::</span><span class="me2">forward_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="kw655">std::<span class="me2">bidirectional_iterator_tag</span></span></a><span class="sy1">></span> <span class="sy3">&&</span><br />
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requires<span class="br0">(</span>I i<span class="br0">)</span> <span class="br0">{</span><br />
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<span class="br0">{</span> <span class="sy2">--</span>i <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">></span> std<span class="sy4">::</span><span class="me2">same_as</span><span class="sy1"><</span>I<span class="sy3">&</span><span class="sy1">></span><span class="sy4">;</span><br />
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<span class="br0">{</span> i<span class="sy2">--</span> <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">></span> std<span class="sy4">::</span><span class="me2">same_as</span><span class="sy1"><</span>I<span class="sy1">></span><span class="sy4">;</span><br /></p>
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<span class="br0">}</span><span class="sy4">;</span></div>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<p>概念 <code>bidirectional_iterator</code> 细化 <a href="cpp-iterator-forward_iterator.html"><code>forward_iterator</code></a> ，添加后移迭代器的能力。</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>称双向迭代器 <code>r</code> <i>可自减</i>，若且唯若存在某个 <code>s</code> 使得 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">++</span>s <span class="sy1">==</span> r</span></span> 。</p>
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<p><code>bidirectional_iterator<I></code> 得到实现，仅若其所蕴含的概念均被实现，且给定二个 <code>I</code> 类型对象 <code>a</code> 与 <code>b</code> ：</p>
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<ul>
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<li>若 <code>a</code> 可自减，则 <code>a</code> 在表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">--</span>a</span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">a<span class="sy2">--</span></span></span> 的定义域中。</li>
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<li>前自减生成指代运算数的左值： <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-addressof.html"><span class="kw757">std::<span class="me2">addressof</span></span></a><span class="br0">(</span><span class="sy2">--</span>a<span class="br0">)</span> <span class="sy1">==</span> <a href="cpp-memory-addressof.html"><span class="kw757">std::<span class="me2">addressof</span></span></a><span class="br0">(</span>a<span class="br0">)</span></span></span> ；</li>
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<li>后自减生成运算数先前的值：若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>a <span class="sy1">==</span> b<span class="br0">)</span></span></span> ，则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>a<span class="sy2">--</span> <span class="sy1">==</span> b<span class="br0">)</span></span></span> 。</li>
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<li>后自减和前自减在运算数上进行相同操作：若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>a <span class="sy1">==</span> b<span class="br0">)</span></span></span> ，则求值 <code>a--</code> 与 <code>--b</code> 两者后， <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>a <span class="sy1">==</span> b<span class="br0">)</span></span></span> 仍然成立。</li>
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<li>自增和自减互逆：</li>
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</ul>
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<dd>
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<li>若 <code>a</code> 可自增且 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>a <span class="sy1">==</span> b<span class="br0">)</span></span></span> ，则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span><span class="sy2">--</span><span class="br0">(</span><span class="sy2">++</span>a<span class="br0">)</span> <span class="sy1">==</span> b<span class="br0">)</span></span></span> 。</li>
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<li>若 <code>a</code> 可自减且 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>a <span class="sy1">==</span> b<span class="br0">)</span></span></span> ，则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span><span class="sy2">--</span><span class="br0">(</span><span class="sy2">++</span>a<span class="br0">)</span> <span class="sy1">==</span> b<span class="br0">)</span></span></span> 。</li>
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<h3><span class="mw-headline" id=".E7.9B.B8.E7.AD.89.E6.80.A7.E4.BF.9D.E6.8C.81">相等性保持</span></h3>
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<p>若表达式对给定的相等输入产生相等输出，则它<i>保持相等性</i>。</p>
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<li>表达式的输入由其操作数组成。</li>
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<li>表达式的输出由其结果和表达式所修改的所有操作数（若存在）组成。</li>
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</ul>
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<p>在标准概念的规范中，操作数定义为仅包含下列内容的最大子表达式：</p>
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<ul>
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<li><a href="cpp-language-expressions.html#.E5.88.9D.E7.AD.89.E8.A1.A8.E8.BE.BE.E5.BC.8F">标识表达式</a>，及</li>
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<li>对 <a href="cpp-utility-move.html"><tt>std::move</tt></a> 、 <span class="t-lc"><a href="cpp-utility-forward.html">std::forward</a></span> 与 <span class="t-lc"><a href="cpp-utility-declval.html">std::declval</a></span> 的调用。</li>
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<p>每个操作数的 cv 限定与值类别，是通过假设每个模板类型形参代表一个 cv 无限定的非数组对象类型确定的。</p>
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<p>进一步要求每个要求保持相等性的表达式都<i>稳定</i>：这种表达式带相同输入对象的二次求值必须拥有相等的输出，而无任何对这些输入对象的显式中间修改。</p>
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<p>除非另外提醒，每个用于 <i>requires 表达式</i>中的表达式都要求保持相等性且稳定，而表达式的求值必须只修改其非常操作数。必须不修改常操作数。</p>
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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-BidirectionalIterator.html"><span style="font-style:italic">老式双向迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyBidirectionalIterator)</span></a> 要求， <code>bidirectional_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-forward_iterator.html"><span class="t-lines"><span>forward_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 <a href="cpp-iterator-input_iterator.html"><code>input_iterator</code></a> 为向前迭代器，支持相等比较与多趟操作<br />
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<span class="t-mark">(概念)</span></td>
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