353 lines
24 KiB
HTML
353 lines
24 KiB
HTML
<!DOCTYPE html>
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>std::variant</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_variant skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>variant</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></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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<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 />
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<span class="kw1">class</span> variant<span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<p>类模板 <code>std::variant</code> 表示一个类型安全的<a href="cpp-language-union.html">联合体</a>。 <code>std::variant</code> 的一个实例在任意时刻要么保有其一个可选类型之一的值，要么在错误情况下无值（此状态难以达成，见 <a href="cpp-utility-variant-valueless_by_exception.html"><tt>valueless_by_exception</tt></a> ）。</p>
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<p>与联合体在<a href="cpp-language-aggregate_initialization.html">聚合初始化</a>中的行为一致, 若 variant 保有某个对象类型 <code>T</code> 的值，则直接于 <code>variant</code> 的对象表示中分配 <code>T</code> 的对象表示。不允许 <code>variant</code> 分配额外的（动态）内存。</p>
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<p><code>variant</code> 不容许保有引用、数组，或类型 <code>void</code> 。空 <code>variant</code> 亦为病式（可用 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">variant</span><span class="sy1"><</span><a href="cpp-utility-variant-monostate.html"><span class="kw1094">std::<span class="me2">monostate</span></span></a><span class="sy1">></span></span></span> 代替）。</p>
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<p><code>variant</code> 容许保有同一类型多于一次，而且可保有同一类型的不同 cv 限定版本。</p>
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<p>同联合体，默认构造的 <code>variant</code> 保有其首个选项的值，除非该选项不是可默认构造的（该情况下 variant 亦非可默认构造：能用辅助类 <a href="cpp-utility-variant-monostate.html"><tt>std::monostate</tt></a> 使这种 <code>variant</code> 可默认构造）。</p>
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<h3><span class="mw-headline" id=".E6.A8.A1.E6.9D.BF.E5.BD.A2.E5.8F.82">模板形参</span></h3>
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<table class="t-par-begin">
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<tr class="t-par">
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<td>Types</td>
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<td>-</td>
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<td>可存储于此 <code>variant</code> 中的类型。所有类型必须满足<a href="cpp-named_req-Destructible.html"><span style="font-style:italic">可析构</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Destructible)</span></a> 要求（特别是不允许数组类型和非对象类型）。</td>
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</table>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">成员函数</span></h3>
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<div class="t-dsc-member-div t-dsc-member-nobold-div">
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<div><a href="cpp-utility-variant-variant.html"><span class="t-lines"><span>(构造函数)</span></span></a></div>
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</div>
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</td>
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<td>构造 <code>variant</code> 对象<br />
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<span class="t-mark">(公开成员函数)</span></td>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div t-dsc-member-nobold-div">
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<div><a href="cpp-utility-variant-~variant.html"><span class="t-lines"><span>(析构函数)</span></span></a></div>
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</div>
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</td>
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<td>析构 <code>variant</code> 同其所含的值<br />
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<span class="t-mark">(公开成员函数)</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-utility-variant-operator=.html"><span class="t-lines"><span>operator=</span></span></a></div>
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</div>
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</td>
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<td>赋值 <code>variant</code><br />
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<span class="t-mark">(公开成员函数)</span></td>
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<tr>
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<td colspan="2">
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<h5><span class="mw-headline" id=".E8.A7.82.E5.AF.9F.E5.99.A8">观察器</span></h5>
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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-utility-variant-index.html"><span class="t-lines"><span>index</span></span></a></div>
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</div>
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</td>
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<td>返回 <code>variant</code> 所保有可选项的零基下标<br />
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<span class="t-mark">(公开成员函数)</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-utility-variant-valueless_by_exception.html"><span class="t-lines"><span>valueless_by_exception</span></span></a></div>
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</div>
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</td>
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<td>检查 <code>variant</code> 是否在非法状态<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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<tr>
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<td colspan="2">
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<h5><span class="mw-headline" id=".E4.BF.AE.E6.94.B9.E5.99.A8">修改器</span></h5>
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</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-utility-variant-emplace.html"><span class="t-lines"><span>emplace</span></span></a></div>
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</div>
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</td>
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<td>原位构造 <code>variant</code> 中的值<br />
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<span class="t-mark">(公开成员函数)</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-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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</tr>
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</table>
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<h3><span class="mw-headline" id=".E9.9D.9E.E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">非成员函数</span></h3>
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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-visit.html"><span class="t-lines"><span>visit</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>以一或多个 <code>variant</code> 所保有的各实参调用所提供的函数对象<br />
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<span class="t-mark">(函数模板)</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-utility-variant-holds_alternative.html"><span class="t-lines"><span>holds_alternative</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>检查某个 <code>variant</code> 是否当前持有某个给定类型<br />
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<span class="t-mark">(函数模板)</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-utility-variant-get.html"><span class="t-lines"><span>std::get<span class="t-dsc-small">(std::variant)</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>以给定索引或类型（若类型唯一）读取 <code>variant</code> 的值，错误时抛出异常<br />
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<span class="t-mark">(函数模板)</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-utility-variant-get_if.html"><span class="t-lines"><span>get_if</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>以给定索引或类型（若其唯一），获得指向被指向的 variant 的值的指针，错误时返回空指针<br />
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<span class="t-mark">(函数模板)</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-utility-variant-operator_cmp.html"><span class="t-lines"><span>operator==</span><span>operator!=</span><span>operator<</span><span>operator<=</span><span>operator></span><span>operator>=</span><span>operator<=></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><span class="t-mark-rev t-since-cxx17">(C++17)</span></span><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span><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>以所含值比较 <code>variant</code> 对象<br />
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<span class="t-mark">(函数模板)</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-utility-variant-swap2.html"><span class="t-lines"><span>std::swap<span class="t-dsc-small">(std::variant)</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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<h3><span class="mw-headline" id=".E8.BE.85.E5.8A.A9.E7.B1.BB">辅助类</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-variant-monostate.html"><span class="t-lines"><span>monostate</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>用作非可默认构造类型的 <code>variant</code> 的首个可选项的占位符类型<br />
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<span class="t-mark">(类)</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-utility-variant-bad_variant_access.html"><span class="t-lines"><span>bad_variant_access</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>非法地访问 <code>variant</code> 的值时抛出的异常<br />
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<span class="t-mark">(类)</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-utility-variant-variant_size.html"><span class="t-lines"><span>variant_size</span><span>variant_size_v</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>在编译时获得 <code>variant</code> 可选项列表的大小<br />
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<span class="t-mark">(类模板)</span> <span class="t-mark">(变量模板)</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-utility-variant-variant_alternative.html"><span class="t-lines"><span>variant_alternative</span><span>variant_alternative_t</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>在编译时获得以其下标指定的可选项的类型<br />
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<span class="t-mark">(类模板)</span> <span class="t-mark">(别名模板)</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-utility-variant-hash.html"><span class="t-lines"><span>std::hash<span class="t-dsc-small"><std::variant></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-utility-hash.html">std::hash</a></span> 算法<br />
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<span class="t-mark">(类模板特化)</span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E8.BE.85.E5.8A.A9.E5.AF.B9.E8.B1.A1">辅助对象</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-variant-variant_npos.html"><span class="t-lines"><span>variant_npos</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>非法状态的 <code>variant</code> 的下标<br />
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<span class="t-mark">(常量)</span></td>
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</tr>
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</table>
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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 <variant></span>
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<span class="co2">#include <string></span>
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<span class="co2">#include <cassert></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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std<span class="sy4">::</span><span class="me2">variant</span><span class="sy1"><</span><span class="kw4">int</span>, <span class="kw4">float</span><span class="sy1">></span> v, w<span class="sy4">;</span>
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v <span class="sy1">=</span> <span class="nu0">12</span><span class="sy4">;</span> <span class="co1">// v 含 int</span>
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<span class="kw4">int</span> i <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span><span class="br0">(</span>v<span class="br0">)</span><span class="sy4">;</span>
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w <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span><span class="br0">(</span>v<span class="br0">)</span><span class="sy4">;</span>
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w <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span><span class="nu0">0</span><span class="sy1">></span><span class="br0">(</span>v<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 与前一行效果相同</span>
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w <span class="sy1">=</span> v<span class="sy4">;</span> <span class="co1">// 与前一行效果相同</span>
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<span class="co1">// std::get<double>(v); // 错误： [int, float] 中无 double</span>
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<span class="co1">// std::get<3>(v); // 错误：合法下标值为 0 与 1</span>
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<span class="kw1">try</span> <span class="br0">{</span>
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std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span><span class="kw4">float</span><span class="sy1">></span><span class="br0">(</span>w<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// w 含 int 而非 float ：将抛出</span>
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<span class="br0">}</span>
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<span class="kw1">catch</span> <span class="br0">(</span><span class="kw4">const</span> <a href="cpp-utility-variant-bad_variant_access.html"><span class="kw1088">std::<span class="me2">bad_variant_access</span></span></a><span class="sy3">&</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
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<span class="kw1">using</span> <span class="kw1">namespace</span> std<span class="sy4">::</span><span class="me2">literals</span><span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">variant</span><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="sy1">></span> x<span class="br0">(</span><span class="st0">"abc"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 转换构造函数在无歧义时起作用</span>
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x <span class="sy1">=</span> <span class="st0">"def"</span><span class="sy4">;</span> <span class="co1">// 转换赋值在无歧义时亦起作用</span>
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std<span class="sy4">::</span><span class="me2">variant</span><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a>, <span class="kw4">void</span> <span class="kw4">const</span><span class="sy2">*</span><span class="sy1">></span> y<span class="br0">(</span><span class="st0">"abc"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 传递 char const * 时转换成 void const *</span>
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<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span><a href="cpp-utility-variant-holds_alternative.html"><span class="kw1086">std::<span class="me2">holds_alternative</span></span></a><span class="sy1"><</span><span class="kw4">void</span> <span class="kw4">const</span><span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span>y<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 成功</span>
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y <span class="sy1">=</span> <span class="st0">"xyz"</span>s<span class="sy4">;</span>
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<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span><a href="cpp-utility-variant-holds_alternative.html"><span class="kw1086">std::<span class="me2">holds_alternative</span></span></a><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="sy1">></span><span class="br0">(</span>y<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 成功</span>
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<span class="br0">}</span></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://cplusplus.github.io/LWG/issue2901">LWG 2901</a></td>
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<td>C++17</td>
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<td>提供 <span class="t-lc"><a href="cpp-memory-uses_allocator.html">std::uses_allocator</a></span> 的特化，但 <code>variant</code> 不能正确支持分配器</td>
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<td>移除特化</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-in_place.html"><span class="t-lines"><span>in_place</span> <span>in_place_type</span> <span>in_place_index</span> <span>in_place_t</span> <span>in_place_type_t</span> <span>in_place_index_t</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>原位构造标签<br />
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<span class="t-mark">(类模板)</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-utility-optional.html"><span class="t-lines"><span>optional</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>可能或可能不保有一个对象的包装器<br />
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<span class="t-mark">(类模板)</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-utility-any.html"><span class="t-lines"><span>any</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>可保有任何<a href="cpp-named_req-CopyConstructible.html"><span style="font-style:italic">可复制构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyConstructible)</span></a> 类型的实例的对象。<br />
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<span class="t-mark">(类)</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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</div>
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</body>
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</html>
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