141 lines
11 KiB
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141 lines
11 KiB
HTML
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<title>std::swap(std::variant)</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_variant_swap2 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>swap<small>(std::variant)</small></h1>
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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>定义于头文件 <code><a href="cpp-header-variant.html"><variant></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>... <span class="me1">Types</span><span class="sy1">></span><br /></span>
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<p><span class="kw4">void</span> swap<span class="br0">(</span> <a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1"><</span>Types...<span class="sy1">></span><span class="sy3">&</span> lhs,<br /></p>
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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>Types...<span class="sy1">></span><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></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="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span>... <span class="me1">Types</span><span class="sy1">></span><br /></span>
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<p><span class="kw4">constexpr</span> <span class="kw4">void</span> swap<span class="br0">(</span> <a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1"><</span>Types...<span class="sy1">></span><span class="sy3">&</span> lhs,<br /></p>
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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>Types...<span class="sy1">></span><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></div>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<p>为 <span class="t-lc"><a href="cpp-utility-variant.html">std::variant</a></span> 重载 <span class="t-lc"><a href="cpp-algorithm-swap.html">std::swap</a></span> 算法。等效地调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">lhs.<span class="me1">swap</span><span class="br0">(</span>rhs<span class="br0">)</span></span></span> 。</p>
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<p>此重载仅若 <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_i<span class="sy1">></span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_swappable.html"><span class="kw573">std::<span class="me2">is_swappable_v</span></span></a><span class="sy1"><</span>T_i<span class="sy1">></span></span></span> 对于所有 <code>Types...</code> 中的 <code>T_i</code> 皆为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 才参与重载决议。</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>lhs, rhs</td>
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<td>-</td>
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<td>要交换值的 <code>variant</code> 对象</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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<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><span class="kw1">noexcept</span><span class="br0">(</span>lhs.<span class="me1">swap</span><span class="br0">(</span>rhs<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span></span></span></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="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">auto</span> print <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">auto</span> <span class="kw4">const</span><span class="sy3">&</span> v, <span class="kw4">char</span> term <span class="sy1">=</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="br0">)</span> <span class="br0">{</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> o<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> o<span class="sy4">;</span> <span class="br0">}</span>, v<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> term<span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</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">123</span><span class="br0">}</span>, v2<span class="br0">{</span><span class="st0">"XYZ"</span><span class="br0">}</span><span class="sy4">;</span>
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print<span class="br0">(</span>v1, <span class="st0">' '</span><span class="br0">)</span><span class="sy4">;</span>
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print<span class="br0">(</span>v2<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-algorithm-swap.html"><span class="kw1599">std::<span class="me2">swap</span></span></a><span class="br0">(</span>v1, v2<span class="br0">)</span><span class="sy4">;</span>
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print<span class="br0">(</span>v1, <span class="st0">' '</span><span class="br0">)</span><span class="sy4">;</span>
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print<span class="br0">(</span>v2<span class="br0">)</span><span class="sy4">;</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">double</span>, <a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy1">></span> v3<span class="br0">{</span><span class="nu16">3.14</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="co1">// std::swap(v1, v3); // 错误： ~ 不一致的形参包</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">123 XYZ
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XYZ 123</pre></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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<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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<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-utility-variant-swap.html"><span class="t-lines"><span>swap</span></span></a></div>
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</div>
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</td>
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<td>与另一 <code>variant</code> 交换<br />
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<span class="t-mark">(公开成员函数)</span></td>
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