117 lines
21 KiB
HTML
117 lines
21 KiB
HTML
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<title>std::random_access_iterator</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_iterator_random_access_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>random_access_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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<td></td>
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<td></td>
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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 random_access_iterator <span class="sy1">=</span><br />
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std<span class="sy4">::</span><span class="me2">bidirectional_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="kw656">std::<span class="me2">random_access_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">totally_ordered</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">sized_sentinel_for</span><span class="sy1"><</span>I, I<span class="sy1">></span> <span class="sy3">&&</span><br />
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requires<span class="br0">(</span>I i, <span class="kw4">const</span> I j, <span class="kw4">const</span> std<span class="sy4">::</span><span class="me2">iter_difference_t</span><span class="sy1"><</span>I<span class="sy1">></span> n<span class="br0">)</span> <span class="br0">{</span><br />
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<span class="br0">{</span> i <span class="sy2">+</span><span class="sy1">=</span> n <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> j <span class="sy2">+</span> n <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 />
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<span class="br0">{</span> n <span class="sy2">+</span> j <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 />
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<span class="br0">{</span> i <span class="sy2">-</span><span class="sy1">=</span> n <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> j <span class="sy2">-</span> n <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 />
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<span class="br0">{</span> j<span class="br0">[</span>n<span class="br0">]</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>std<span class="sy4">::</span><span class="me2">iter_reference_t</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 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>概念 <code>random_access_iterator</code> 细化 <a href="cpp-iterator-bidirectional_iterator.html"><code>bidirectional_iterator</code></a> ，添加用 <code>+=</code> 、 <code>-=</code> 、 <code>+</code> 和 <code>-</code> 运算符的常数时间前进、用 <code>-</code> 的常数时间距离计算，及带下标的数组记法支持。</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>令 <code>a</code> 与 <code>b</code> 为 <code>I</code> 类型的合法迭代器，使得从 <code>a</code> 可抵达 <code>b</code> ，并令 <code>n</code> 为等于 <span class="t-c"><span class="mw-geshi cpp source-cpp">b <span class="sy2">-</span> a</span></span> 的 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">iter_difference_t</span><span class="sy1"><</span>I<span class="sy1">></span></span></span> 类型值。 <code>random_access_iterator<I></code> 仅若其所蕴含的概念均被实现并且符合下列条件才得到实现：</p>
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<ul>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>a <span class="sy2">+</span><span class="sy1">=</span> n<span class="br0">)</span></span></span> 等于 <span class="t-c"><span class="mw-geshi cpp source-cpp">b</span></span> 。</li>
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<li><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>a <span class="sy2">+</span><span class="sy1">=</span> n<span class="br0">)</span></span></span> 等于 <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>a<span class="br0">)</span></span></span> 。</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>a <span class="sy2">+</span> n<span class="br0">)</span></span></span> 等于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>a <span class="sy2">+</span><span class="sy1">=</span> n<span class="br0">)</span></span></span> 。</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>a <span class="sy2">+</span> n<span class="br0">)</span></span></span> 等于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>n <span class="sy2">+</span> a<span class="br0">)</span></span></span> 。</li>
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<li>对于任何二个正整数 <code>x</code> 与 <code>y</code> ，若 <span class="t-c"><span class="mw-geshi cpp source-cpp">a <span class="sy2">+</span> <span class="br0">(</span>x <span class="sy2">+</span> y<span class="br0">)</span></span></span> 合法，则 <span class="t-c"><span class="mw-geshi cpp source-cpp">a <span class="sy2">+</span> <span class="br0">(</span>x <span class="sy2">+</span> y<span class="br0">)</span></span></span> 等于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>a <span class="sy2">+</span> x<span class="br0">)</span> <span class="sy2">+</span> y</span></span> 。</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">a <span class="sy2">+</span> <span class="nu0">0</span></span></span> 等于 <span class="t-c"><span class="mw-geshi cpp source-cpp">a</span></span> 。</li>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>a <span class="sy2">+</span> <span class="br0">(</span>n <span class="sy2">-</span> <span class="nu0">1</span><span class="br0">)</span><span class="br0">)</span></span></span> 合法，则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">--</span>b</span></span> 等于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>a <span class="sy2">+</span> <span class="br0">(</span>n <span class="sy2">-</span> <span class="nu0">1</span><span class="br0">)</span><span class="br0">)</span></span></span> 。</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>b <span class="sy2">+</span><span class="sy1">=</span> <span class="sy2">-</span>n<span class="br0">)</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>b <span class="sy2">-</span><span class="sy1">=</span> n<span class="br0">)</span></span></span> 都等于 <span class="t-c"><span class="mw-geshi cpp source-cpp">a</span></span> 。</li>
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<li><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>b <span class="sy2">-</span><span class="sy1">=</span> n<span class="br0">)</span></span></span> 等于 <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>b<span class="br0">)</span></span></span> 。</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>b <span class="sy2">-</span> n<span class="br0">)</span></span></span> 等于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>b <span class="sy2">-</span><span class="sy1">=</span> n<span class="br0">)</span></span></span> 。</li>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">b</span></span> 可解引用，则 <span class="t-c"><span class="mw-geshi cpp source-cpp">a<span class="br0">[</span>n<span class="br0">]</span></span></span> 合法并等于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>b</span></span> 。</li>
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<li><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> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 。</li>
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<li>每个要求的操作均拥有常数时间复杂度。</li>
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</ul>
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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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<ul>
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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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</ul>
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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=".E9.9A.90.E5.BC.8F.E8.A1.A8.E8.BE.BE.E5.BC.8F.E5.8F.98.E7.A7.8D">隐式表达式变种</span></h3>
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<p>使用不修改某 const 左值运算数的表达式的 <i>requires 表达式</i>亦隐式要求该表达式的额外变种对给定运算数接受非 const 左值或（可为 const 的）右值，除非以有区别的语义显式要求这种表达式变种。这些<i>隐式表达式变种</i>必须符合与声明的表达式的相同的语义。不指定实现以何种程度校验变种的语法。</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-RandomAccessIterator.html"><span style="font-style:italic">老式随机访问迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyRandomAccessIterator)</span></a> 要求， <code>random_access_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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<table class="t-dsc-begin">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-iterator-bidirectional_iterator.html"><span class="t-lines"><span>bidirectional_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>
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<td>指定 <a href="cpp-iterator-forward_iterator.html"><code>forward_iterator</code></a> 为双向迭代器，支持向后移动<br />
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<span class="t-mark">(概念)</span></td>
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</table>
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</div>
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<div class="visualClear"></div>
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</body>
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