116 lines
13 KiB
HTML
116 lines
13 KiB
HTML
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<title>std::variant<Types...>::swap</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_variant_swap 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::variant<Types...>::</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-variant.html">variant</a></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> swap<span class="br0">(</span> variant<span class="sy3">&</span> rhs <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><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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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> <span class="kw4">void</span> swap<span class="br0">(</span> variant<span class="sy3">&</span> rhs <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><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<p>交换二个 <code>variant</code> 对象。</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>rhs</code> 均<a href="cpp-utility-variant-valueless_by_exception.html">因异常无值</a>，则不做任何事；</li>
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<li>否则，若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 与 <code>rhs</code> 保有同一可选项，则调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">swap<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span>i<span class="sy1">></span><span class="br0">(</span><span class="sy2">*</span>this<span class="br0">)</span>, std<span class="sy4">:</span>get<span class="sy1"><</span>i<span class="sy1">></span><span class="br0">(</span>rhs<span class="br0">)</span><span class="br0">)</span></span></span> ，其中 <code>i</code> 等于 <code>index()</code> 。若抛出异常，则值的状态依赖于被调用交换函数的异常安全；</li>
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<li>否则，交换 <code>rhs</code> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 的值。若抛出异常，则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 与 <code>rhs</code> 的状态依赖于 <code>variant</code> 移动构造函数的异常安全。</li>
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</ul>
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<p>除非对于所有 <code>Types...</code> 中的 <code>T_i</code> ， <code>T_i</code> 左值<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> 且 <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_i<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，否则行为未定义。</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>rhs</td>
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<td>-</td>
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<td>要交换的 <code>variant</code> 对象</td>
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</tr>
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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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<p>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">this<span class="sy2">-</span><span class="sy1">></span>index<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> rhs.<span class="me1">index</span><span class="br0">(</span><span class="br0">)</span></span></span> ，则可能抛出任何 <span class="t-c"><span class="mw-geshi cpp source-cpp">swap<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span>i<span class="sy1">></span><span class="br0">(</span><span class="sy2">*</span>this<span class="br0">)</span>, std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span>i<span class="sy1">></span><span class="br0">(</span>rhs<span class="br0">)</span><span class="br0">)</span></span></span> 所抛的异常，其中 <code>i</code> 等于 <span class="t-c"><span class="mw-geshi cpp source-cpp">index<span class="br0">(</span><span class="br0">)</span></span></span> 。</p>
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<p>否则，可能抛出任何当前 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">rhs</span></span> 所保有的可选项的移动构造函数所抛的异常。</p>
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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><span class="br0">(</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>Types<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>Types<span class="sy1">></span><span class="br0">)</span> <span class="sy3">&&</span> ...<span class="br0">)</span><span class="br0">)</span></span></span></div>
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</div>
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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="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <variant></span>
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<span class="co2">#include <string></span>
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<span class="co2">#include <iostream></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-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, <a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy1">></span> v1<span class="br0">{</span><span class="nu0">2</span><span class="br0">}</span>, v2<span class="br0">{</span><span class="st0">"abc"</span><span class="br0">}</span><span class="sy4">;</span>
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<a href="cpp-utility-variant-visit.html"><span class="kw1086">std::<span class="me2">visit</span></span></a><span class="br0">(</span><span class="br0">[</span><span class="br0">]</span> <span class="br0">(</span><span class="kw4">auto</span><span class="sy3">&&</span> x<span class="br0">)</span> <span class="br0">{</span> <a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> x <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span> <span class="br0">}</span>, v1<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-utility-variant-visit.html"><span class="kw1086">std::<span class="me2">visit</span></span></a><span class="br0">(</span><span class="br0">[</span><span class="br0">]</span> <span class="br0">(</span><span class="kw4">auto</span><span class="sy3">&&</span> x<span class="br0">)</span> <span class="br0">{</span> <a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> x <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="br0">}</span>, v2<span class="br0">)</span><span class="sy4">;</span>
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v1.<span class="me1">swap</span><span class="br0">(</span>v2<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-utility-variant-visit.html"><span class="kw1086">std::<span class="me2">visit</span></span></a><span class="br0">(</span><span class="br0">[</span><span class="br0">]</span> <span class="br0">(</span><span class="kw4">auto</span><span class="sy3">&&</span> x<span class="br0">)</span> <span class="br0">{</span> <a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> x <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span> <span class="br0">}</span>, v1<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-utility-variant-visit.html"><span class="kw1086">std::<span class="me2">visit</span></span></a><span class="br0">(</span><span class="br0">[</span><span class="br0">]</span> <span class="br0">(</span><span class="kw4">auto</span><span class="sy3">&&</span> x<span class="br0">)</span> <span class="br0">{</span> <a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> x <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="br0">}</span>, v2<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">2 abc
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abc 2</pre></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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<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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</table>
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</div>
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<div class="visualClear"></div>
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