203 lines
15 KiB
HTML
203 lines
15 KiB
HTML
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<title>std::ranges::prev</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_iterator_ranges_prev 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::ranges::</span>prev</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>调用签名</div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> std<span class="sy4">::</span><span class="me2">bidirectional_iterator</span> I <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> I prev<span class="br0">(</span> I i <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> std<span class="sy4">::</span><span class="me2">bidirectional_iterator</span> I <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> I prev<span class="br0">(</span> I i, 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="sy4">;</span></span></div>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> std<span class="sy4">::</span><span class="me2">bidirectional_iterator</span> I <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> I prev<span class="br0">(</span> I i, std<span class="sy4">::</span><span class="me2">iter_difference_t</span><span class="sy1"><</span>I<span class="sy1">></span> n, I bound <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(3)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<td></td>
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<td></td>
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</table>
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<p>返回迭代器 <code>i</code> 的第 <i>n</i> 个前驱。</p>
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<p>此页面上描述的仿函数实体是 <i>niebloid</i> ，即：</p>
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<ul>
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<li>调用它们中的任何一者时不可指定显式模板实参。</li>
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<li>它们无一对<a href="cpp-language-adl.html">实参依赖查找</a>可见。</li>
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<li>在<a href="cpp-language-unqualified_lookup.html">通常无限定查找</a>寻找函数调用运算符左侧的名称找到它们之一时，它禁止<a href="cpp-language-adl.html">实参依赖查找</a>。</li>
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</ul>
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<p>实际上，它们能以函数对象，或以某些特殊编译器扩展实现。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<td>i</td>
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<td>-</td>
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<td>迭代器</td>
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<tr class="t-par">
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<td>n</td>
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<td>-</td>
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<td>要减少 <code>i</code> 的次数</td>
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<td>bound</td>
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<td>-</td>
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<td>代表 <code>i</code> 所指向的范围的起始的迭代器</td>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<div class="t-li1"><span class="t-li">1)</span> <code>i</code> 的前驱</div>
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<div class="t-li1"><span class="t-li">2)</span> 迭代器 <code>i</code> 的第 <i>n</i> 个前驱</div>
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<div class="t-li1"><span class="t-li">3)</span> 迭代器 <code>i</code> 的第 <i>n</i> 个前驱，或首个比较等于 <code>bound</code> 的迭代器，取决于何者先来到。</div>
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<h3><span class="mw-headline" id=".E5.A4.8D.E6.9D.82.E5.BA.A6">复杂度</span></h3>
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<div class="t-li1"><span class="t-li">1)</span> 常数</div>
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<div class="t-li1"><span class="t-li">2,3)</span> 若 <code>I</code> 实现 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">random_access_iterator</span><span class="sy1"><</span>I<span class="sy1">></span></span></span> 则为常数；否则为线性。</div>
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<h3><span class="mw-headline" id=".E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0">可能的实现</span></h3>
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<table class="eq-fun-cpp-table">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="kw1">struct</span> prev_fn <span class="br0">{</span>
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<span class="kw1">template</span><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">bidirectional_iterator</span> I<span class="sy1">></span>
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<span class="kw4">constexpr</span> I operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>I i<span class="br0">)</span> <span class="kw4">const</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> <span class="sy2">--</span>i<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">template</span><span class="sy1"><</span> std<span class="sy4">::</span><span class="me2">bidirectional_iterator</span> I <span class="sy1">></span>
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<span class="kw4">constexpr</span> I operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>I i, 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="kw4">const</span>
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<span class="br0">{</span>
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ranges<span class="sy4">::</span><span class="me2">advance</span><span class="br0">(</span>i, <span class="sy2">-</span>n<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> i<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">template</span><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">bidirectional_iterator</span> I<span class="sy1">></span>
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<span class="kw4">constexpr</span> I operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>I i, std<span class="sy4">::</span><span class="me2">iter_difference_t</span><span class="sy1"><</span>I<span class="sy1">></span> n, I bound<span class="br0">)</span> <span class="kw4">const</span>
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<span class="br0">{</span>
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ranges<span class="sy4">::</span><span class="me2">advance</span><span class="br0">(</span>i, <span class="sy2">-</span>n, bound<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> i<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">inline</span> <span class="kw4">constexpr</span> <span class="kw4">auto</span> prev <span class="sy1">=</span> prev_fn<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></pre></div>
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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>尽管对于容器，表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">--</span>r.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span></span></span> 经常能编译，但不保证如此： <span class="t-c"><span class="mw-geshi cpp source-cpp">r.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span></span></span> 是右值表达式，而无迭代器要求指定右值的自减保证可用。尤其是迭代器实现为指针或其 <code>operator--</code> 为左值引用限定时， <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">--</span>r.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span></span></span> 不能编译，而 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">prev</span><span class="br0">(</span>r.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span></span></span> 能。</p>
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<p>不实现 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">common_range</span></span></span> 的范围加剧了这一情况。例如对于某些底层类型， <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">transform_view</span><span class="sy4">::</span><span class="me2">end</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">transform_view</span><span class="sy4">::</span><span class="me2">begin</span></span></span> 拥有不同的返回类型，故 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">--</span>r.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span></span></span> 将不能编译。此非 <code>ranges::prev</code> 所能帮助，但有变通方法。</p>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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<div class="t-example-live-link"></div>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <iostream></span>
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<span class="co2">#include <iterator></span>
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<span class="co2">#include <vector></span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> v<span class="br0">{</span> <span class="nu0">3</span>, <span class="nu0">1</span>, <span class="nu0">4</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">auto</span> pv <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">prev</span><span class="br0">(</span>v.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, <span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="sy2">*</span>pv <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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pv <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">prev</span><span class="br0">(</span>pv, <span class="nu0">42</span>, v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="sy2">*</span>pv <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>输出：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="text source-text">
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<pre class="de1">1
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3</pre></div>
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</div>
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</div>
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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-ranges-next.html"><span class="t-lines"><span>ranges::next</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>自增迭代器给定的距离或到边界<br />
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<span class="t-mark">(niebloid)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-iterator-ranges-advance.html"><span class="t-lines"><span>ranges::advance</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>令迭代器前进给定的距离或到给定的边界<br />
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<span class="t-mark">(niebloid)</span></td>
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</tr>
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<tr class="t-dsc">
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<div class="t-dsc-member-div">
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<div><a href="cpp-iterator-prev.html"><span class="t-lines"><span>prev</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>令迭代器自减<br />
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<span class="t-mark">(函数模板)</span></td>
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</tr>
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</table>
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</div>
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<div class="visualClear"></div>
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</div>
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</body>
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