161 lines
14 KiB
HTML
161 lines
14 KiB
HTML
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<title>std::optional<T>::swap</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_optional_swap skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<div id="content"><a id="top"></a>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::optional<T>::</span>swap</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-utility.html">utility</a>‎ | <a href="cpp-utility-optional.html">optional</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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<tbody></tbody>
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<tr class="t-dcl t-since-cxx17 t-until-cxx20">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> swap<span class="br0">(</span> optional<span class="sy3">&</span> other <span class="br0">)</span> <span class="kw1">noexcept</span><span class="br0">(</span><span class="coMULTI">/* see below */</span><span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span><br />
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<span class="t-mark-rev t-until-cxx20">(C++20 前)</span></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="kw4">constexpr</span> <span class="kw4">void</span> swap<span class="br0">(</span> optional<span class="sy3">&</span> other <span class="br0">)</span> <span class="kw1">noexcept</span><span class="br0">(</span><span class="coMULTI">/* see below */</span><span class="br0">)</span><span class="sy4">;</span></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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</tr>
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</tbody>
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<tbody>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<td></td>
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</tbody>
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</table>
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<p>与 <code>other</code> 交换内容。</p>
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<ul>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 和 <code>other</code> 均不含值，则函数无效果。</li>
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</ul>
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<ul>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 与 <code>other</code> 仅有一个含值（称此对象为 <code>in</code> ，另一者为 <code>un</code> ），则从 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span><span class="sy2">*</span>in<span class="br0">)</span></span></span> <a href="cpp-language-direct_initialization.html">直接初始化</a> <code>un</code> 所含值，随后如同通过 <span class="t-c"><span class="mw-geshi cpp source-cpp">in<span class="sy2">-</span><span class="sy1">></span>T<span class="sy4">::</span>~T<span class="br0">(</span><span class="br0">)</span></span></span> 析构 <code>in</code> 所含值。此调用后， <code>in</code> 不含值； <code>un</code> 含值。</li>
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</ul>
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<ul>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 与 <code>other</code> 均含值，则通过调用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-algorithm-swap.html"><span class="kw1599">std::<span class="me2">swap</span></span></a><span class="br0">(</span><span class="sy2">**</span>this, <span class="sy2">*</span>other<span class="br0">)</span></span></span> 交换所含值。 <code>T</code> 左值必须满足<a href="cpp-named_req-Swappable.html"><span style="font-style:italic">可交换</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Swappable)</span></a> 。</li>
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</ul>
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<p>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_move_constructible.html"><span class="kw539">std::<span class="me2">is_move_constructible_v</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 则程序非良构。</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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<tr class="t-par">
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<td>other</td>
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<td>-</td>
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<td>要交换内容的 <code>optional</code> 对象</td>
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</tr>
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</table>
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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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<p>（无）</p>
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<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<div class="t-noexcept-full"><a href="cpp-language-noexcept_spec.html"><tt>noexcept</tt></a> 说明：
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<div class="t-noexcept-box"><span class="t-cc"><span class="mw-geshi cpp source-cpp"><span class="kw1">noexcept</span><span class="br0">(</span><a href="cpp-types-is_move_constructible.html"><span class="kw543">std::<span class="me2">is_nothrow_move_constructible_v</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> <span class="sy3">&&</span><br />
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<a href="cpp-types-is_swappable.html"><span class="kw577">std::<span class="me2">is_nothrow_swappable_v</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">)</span></span></span></div>
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</div>
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<p>在抛异常的情况下， <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 和 <code>other</code> 所含值的状态由 <code>T</code> 的 <code>swap</code> 或 <code>T</code> 的移动构造函数的异常安全保证确定，取决于所调用者。对于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 和 <code>other</code> ，若对象含值，则令它继续含值，反之亦然。</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 <string></span>
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<span class="co2">#include <optional></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-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy1">></span> opt1<span class="br0">(</span><span class="st0">"First example text"</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy1">></span> opt2<span class="br0">(</span><span class="st0">"2nd text"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw2">enum</span> Swap <span class="br0">{</span> Before, After <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">auto</span> print_opts <span class="sy1">=</span> <span class="br0">[</span><span class="sy3">&</span><span class="br0">]</span><span class="br0">(</span>Swap e<span class="br0">)</span> <span class="br0">{</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="br0">(</span>e <span class="sy1">==</span> Before <span class="sy4">?</span> <span class="st0">"Before swap:<span class="es1">\n</span>"</span> <span class="sy4">:</span> <span class="st0">"After swap:<span class="es1">\n</span>"</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="st0">"opt1 contains '"</span> <span class="sy1"><<</span> opt1.<span class="me1">value_or</span><span class="br0">(</span><span class="st0">""</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">"'<span class="es1">\n</span>"</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="st0">"opt2 contains '"</span> <span class="sy1"><<</span> opt2.<span class="me1">value_or</span><span class="br0">(</span><span class="st0">""</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">"'<span class="es1">\n</span>"</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="br0">(</span>e <span class="sy1">==</span> Before <span class="sy4">?</span> <span class="st0">"---SWAP---<span class="es1">\n</span>"</span><span class="sy4">:</span> <span class="st0">"<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span>
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print_opts<span class="br0">(</span>Before<span class="br0">)</span><span class="sy4">;</span>
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opt1.<span class="me1">swap</span><span class="br0">(</span>opt2<span class="br0">)</span><span class="sy4">;</span>
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print_opts<span class="br0">(</span>After<span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 在仅一者含值时交换</span>
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opt1 <span class="sy1">=</span> <span class="st0">"Lorem ipsum dolor sit amet, consectetur tincidunt."</span><span class="sy4">;</span>
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opt2.<span class="me1">reset</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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print_opts<span class="br0">(</span>Before<span class="br0">)</span><span class="sy4">;</span>
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opt1.<span class="me1">swap</span><span class="br0">(</span>opt2<span class="br0">)</span><span class="sy4">;</span>
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print_opts<span class="br0">(</span>After<span class="br0">)</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">Before swap:
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opt1 contains 'First example text'
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opt2 contains '2nd text'
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---SWAP---
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After swap:
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opt1 contains '2nd text'
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opt2 contains 'First example text'
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Before swap:
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opt1 contains 'Lorem ipsum dolor sit amet, consectetur tincidunt.'
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opt2 contains ''
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---SWAP---
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After swap:
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opt1 contains ''
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opt2 contains 'Lorem ipsum dolor sit amet, consectetur tincidunt.'</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
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<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。</p>
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<table class="dsctable" style="font-size:0.8em">
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<tr>
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<th>DR</th>
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<th>应用于</th>
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<th>出版时的行为</th>
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<th>正确行为</th>
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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://wg21.link/P2231R1">P2231R1</a></td>
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<td>C++20</td>
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<td><code>swap</code> 不是 constexpr 而要求的操作在 C++20 中能为 constexpr</td>
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<td>使之为 constexpr</td>
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</tr>
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</table>
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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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<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-utility-optional-swap2.html"><span class="t-lines"><span>std::swap<span class="t-dsc-small">(std::optional)</span></span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
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</div>
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</td>
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<td>特化 <span class="t-lc"><a href="cpp-algorithm-swap.html">std::swap</a></span> 算法<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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</div>
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</body>
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