301 lines
38 KiB
HTML
301 lines
38 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>移动赋值运算符</title>
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<link rel="stylesheet" href="ext.css" />
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<link rel="stylesheet" href="site_modules.css" />
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</head>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_move_assignment 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">移动赋值运算符</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-language.html">language</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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<p>类 <code>T</code> 的移动赋值运算符是名为 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy1">=</span></span></span>的非模板<a href="cpp-language-member_functions.html">非静态成员函数</a>，它接受恰好一个 <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="sy3">&&</span></span></span>、<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> T<span class="sy3">&&</span></span></span>、<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">volatile</span> T<span class="sy3">&&</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <span class="kw4">volatile</span> T<span class="sy3">&&</span></span></span> 类型的形参。</p>
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<h3><span class="mw-headline" id=".E8.AF.AD.E6.B3.95">语法</span></h3>
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<table class="t-sdsc-begin">
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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</tr>
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<tr class="t-sdsc">
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<td><span class="t-spar">类名</span> <code><b>&</b></code> <span class="t-spar">类名</span> <code><b>:: operator=</b></code> ( <span class="t-spar">类名</span> <code>&&</code> )</td>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</tr>
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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</tr>
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<tr class="t-sdsc">
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<td><span class="t-spar">类名</span> <code><b>&</b></code> <span class="t-spar">类名</span> <code><b>:: operator=</b></code> ( <span class="t-spar">类名</span> <code>&&</code> ) = <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">default</span><span class="sy4">;</span></span></span></td>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</tr>
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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</tr>
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<tr class="t-sdsc">
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<td><span class="t-spar">类名</span> <code><b>&</b></code> <span class="t-spar">类名</span> <code><b>:: operator=</b></code> ( <span class="t-spar">类名</span> <code>&&</code> ) = <span class="t-c"><span class="mw-geshi cpp source-cpp">delete<span class="sy4">;</span></span></span></td>
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<td>(3)</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</tr>
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
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<div class="t-li1"><span class="t-li">1)</span> 移动赋值运算符的典型声明。</div>
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<div class="t-li1"><span class="t-li">2)</span> 强制编译器生成移动赋值运算符。</div>
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<div class="t-li1"><span class="t-li">3)</span> 避免隐式移动赋值。</div>
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<p>每当<a href="cpp-language-overload_resolution.html">重载决议</a>选择移动赋值运算符时，它都会被调用，例如当对象出现在赋值表达式左侧，而其右侧是同类型或可隐式转换的类型的右值时。</p>
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<p>典型的移动赋值运算符“窃取”实参曾保有的资源（例如指向动态分配对象的指针，文件描述符，TCP socket，输入输出流，运行的线程，等等），而非复制它们，并使得实参遗留在某个合法但不确定的状态。例如，从 <span class="t-lc"><a href="cpp-string-basic_string.html">std::string</a></span> 或从 <span class="t-lc"><a href="cpp-container-vector.html">std::vector</a></span> 移动赋值可能导致实参被置空。然而这并不保证会发生。移动赋值与普通赋值相比，其定义较为宽松而非更严格；在完成时，普通赋值必须留下数据的两份副本，而移动赋值只要求留下一份。</p>
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<h3><span class="mw-headline" id=".E9.9A.90.E5.BC.8F.E5.A3.B0.E6.98.8E.E7.9A.84.E7.A7.BB.E5.8A.A8.E8.B5.8B.E5.80.BC.E8.BF.90.E7.AE.97.E7.AC.A6">隐式声明的移动赋值运算符</span></h3>
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<p>如果没有对类类型（<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">struct</span></span></span>、<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">class</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">union</span></span></span>）提供任何用户定义的移动赋值运算符，且满足下列所有条件：</p>
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<ul>
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<li>没有用户声明的复制构造函数；</li>
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<li>没有用户声明的移动构造函数；</li>
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<li>没有用户声明的复制赋值运算符；</li>
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<li>没有用户声明的析构函数，</li>
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</ul>
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<p>那么编译器将声明一个，作为类的 <code>inline public</code> 成员，并拥有签名 <code>T& T::operator=(T&&)</code>。</p>
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<p>类可以拥有多个移动赋值运算符，如 <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="sy3">&</span> T<span class="sy4">::</span><span class="me2">operator</span><span class="sy1">=</span><span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&&</span><span class="br0">)</span></span></span> 和 <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="sy3">&</span> T<span class="sy4">::</span><span class="me2">operator</span><span class="sy1">=</span><span class="br0">(</span>T<span class="sy3">&&</span><span class="br0">)</span></span></span>。当存在用户定义的移动赋值运算符时，用户仍然可以通过关键词 <code>default</code> 强迫编译器生成隐式声明的移动赋值运算符。</p>
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<p>隐式声明（或在其首个声明被预置）的移动赋值运算符具有<span class="t-rev-inl t-until-cxx17"><span><a href="cpp-language-except_spec.html">动态异常说明</a></span> <span><span class="t-mark-rev t-until-cxx17">(C++17 前)</span></span></span><span class="t-rev-inl t-since-cxx17"><span><a href="cpp-language-noexcept_spec.html">noexcept 说明</a></span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>中所描述的异常说明。</p>
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<p>因为每个类总是会声明赋值运算符（移动或复制），所以基类的赋值运算符始终被隐藏。当使用 using 声明从基类带入赋值运算符，且它的实参类型与派生类的隐式赋值运算符的实参类型相同时，该 using 声明也会被隐式声明隐藏。</p>
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<h3><span class="mw-headline" id=".E5.BC.83.E7.BD.AE.E7.9A.84.E9.9A.90.E5.BC.8F.E5.A3.B0.E6.98.8E.E7.9A.84.E7.A7.BB.E5.8A.A8.E8.B5.8B.E5.80.BC.E8.BF.90.E7.AE.97.E7.AC.A6">弃置的隐式声明的移动赋值运算符</span></h3>
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<p>如果满足下列任一条件，那么类 <code>T</code> 的隐式声明或预置的移动赋值运算符被定义为<i>弃置的</i>：</p>
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<ul>
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<li><code>T</code> 拥有 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> 限定的非静态数据成员。</li>
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<li><code>T</code> 拥有引用类型的非静态数据成员。</li>
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<li><code>T</code> 拥有无法移动赋值（拥有被弃置、不可访问或有歧义的移动赋值运算符）的非静态数据成员。</li>
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<li><code>T</code> 拥有无法移动赋值（拥有被弃置、不可访问或有歧义的移动赋值运算符）的直接基类或虚基类。</li>
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</ul>
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<p><a href="cpp-language-overload_resolution.html">重载决议</a>忽略被弃置的隐式声明的移动赋值运算符。</p>
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<h3><span class="mw-headline" id=".E5.B9.B3.E5.87.A1.E7.9A.84.E7.A7.BB.E5.8A.A8.E8.B5.8B.E5.80.BC.E8.BF.90.E7.AE.97.E7.AC.A6">平凡的移动赋值运算符</span></h3>
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<p>如果满足下列所有条件，那么类 <code>T</code> 的移动赋值运算符是平凡的：</p>
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<ul>
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<li>它不是用户提供的（即它是隐式定义或预置的）；</li>
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<li><code>T</code> 没有虚成员函数；</li>
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<li><code>T</code> 没有虚基类；</li>
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<li>为 <code>T</code> 的每个直接基类选择的移动赋值运算符都是平凡的；</li>
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<li>为 <code>T</code> 的每个类类型（或类类型的数组）的非静态数据成员选择的移动赋值运算符都是平凡的；</li>
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</ul>
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<p>平凡移动赋值运算符实施与平凡复制赋值运算符相同的动作，即如同以 <span class="t-lc"><a href="cpp-string-byte-memmove.html">std::memmove</a></span> 进行对象表示的复制。所有与 C 兼容的数据类型（POD 类型）都可以平凡移动。</p>
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<h3><span class="mw-headline" id=".E5.90.88.E6.A0.BC.E7.9A.84.E7.A7.BB.E5.8A.A8.E8.B5.8B.E5.80.BC.E8.BF.90.E7.AE.97.E7.AC.A6">合格的移动赋值运算符</span></h3>
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<table class="t-rev-begin">
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<tr class="t-rev t-until-cxx20">
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<td>
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<p>没有被弃置的移动赋值运算符是合格的。</p>
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</td>
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<td><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td>
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</tr>
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<tr class="t-rev t-since-cxx20">
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<td>
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<p>满足下列所有条件的移动赋值运算符是合格的：</p>
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<ul>
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<li>它没有被弃置，且</li>
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<li>满足它的所有<a href="cpp-language-constraints.html">关联约束</a>（如果存在），且</li>
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<li>没有比它<a href="cpp-language-constraints.html#.E7.BA.A6.E6.9D.9F.E7.9A.84.E9.83.A8.E5.88.86.E6.8E.92.E5.BA.8F">更受约束</a>且拥有相同的第一形参类型和相同的 cv 或引用限定符（如果存在）的移动赋值运算符。</li>
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</ul>
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</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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</table>
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<p>合格移动赋值运算符的平凡性确定该类是否为该类是否为<a href="cpp-named_req-TriviallyCopyable.html">可平凡复制类型</a>。</p>
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<h3><span class="mw-headline" id=".E9.9A.90.E5.BC.8F.E5.AE.9A.E4.B9.89.E7.9A.84.E7.A7.BB.E5.8A.A8.E8.B5.8B.E5.80.BC.E8.BF.90.E7.AE.97.E7.AC.A6">隐式定义的移动赋值运算符</span></h3>
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<p>如果隐式声明的移动赋值运算符既没有被弃置也不平凡，那么当它被 <a href="cpp-language-definition.html#ODR_.E5.BC.8F.E4.BD.BF.E7.94.A8">ODR 式使用</a><span class="t-rev-inl t-since-cxx14"><span>或用于<a href="cpp-language-constant_expression.html#.E5.B8.B8.E9.87.8F.E6.B1.82.E5.80.BC.E6.89.80.E9.9C.80.E8.A6.81.E7.9A.84.E5.87.BD.E6.95.B0.E4.B8.8E.E5.8F.98.E9.87.8F">常量求值</a></span> <span><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></span></span>时，它会被编译器定义（即生成并编译函数体）。</p>
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<p>对于联合体类型，隐式定义的移动赋值运算符（如用 <span class="t-lc"><a href="cpp-string-byte-memmove.html">std::memmove</a></span>）复制其对象表示。</p>
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<p>对于非联合类类型（<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">class</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">struct</span></span></span>），移动赋值运算符按照声明顺序对对象的各直接基类和直接非静态成员进行完整的逐成员移动赋值，其中对标量用内建运算符，对数组用逐元素移动赋值，而对类类型用移动赋值运算符（非虚调用）。</p>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx14">
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<td>
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<p>如果满足下列所有条件，那么类 <code>T</code> 的隐式定义的复制赋值运算符是 <a href="cpp-language-constexpr.html"><code>constexpr</code></a> 的：</p>
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<ul>
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<li><code>T</code> 是<a href="cpp-named_req-LiteralType.html">字面类型</a>，且</li>
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<li>移动每个直接基类子对象时选中的赋值运算符都是 constexpr 函数，且</li>
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<li>移动 <code>T</code> 的每个类（或其数组）类型的数据成员时选中的赋值运算符都是 constexpr 函数。</li>
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</ul>
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</td>
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<td><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></td>
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</tr>
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</table>
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<p>与复制赋值一样，隐式定义的移动赋值运算符是否会多次对在继承网格中可通过多于一条路径访问的虚基类子对象赋值是未指明的：</p>
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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="kw1">struct</span> V
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<span class="br0">{</span>
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V<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span>V<span class="sy3">&&</span> other<span class="br0">)</span> <span class="br0">{</span>
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<span class="co1">// 这可能会被调用一或两次</span>
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<span class="co1">// 如果调用两次，那么 'other' 是刚被移动的 V 子对象</span>
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<span class="kw1">return</span> <span class="sy2">*</span>this<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">struct</span> A <span class="sy4">:</span> <span class="kw1">virtual</span> V <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// operator= 调用 V::operator=</span>
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<span class="kw1">struct</span> B <span class="sy4">:</span> <span class="kw1">virtual</span> V <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// operator= 调用 V::operator=</span>
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<span class="kw1">struct</span> C <span class="sy4">:</span> B, A <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// operator= 调用 B::operator=，然后调用 A::operator=</span>
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<span class="co1">// 但可能只调用一次 V::operator=</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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C c1, c2<span class="sy4">;</span>
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c2 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>c1<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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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p>如果复制和移动赋值运算符都有提供，那么重载决议会在实参是<a href="cpp-language-value_category.html"><i>右值</i></a>（例如无名临时量的<a href="cpp-language-value_category.html"><i>纯右值</i></a> 或 <span class="t-lc">std::move</span> 的结果的<a href="cpp-language-value_category.html"><i>亡值</i></a>）时选择移动赋值，而在实参是<a href="cpp-language-value_category.html"><i>左值</i></a>（具名对象或返回左值引用的函数或运算符）时选择复制赋值。如果只提供了复制赋值，那么重载决议对于所有值类别都会选择它（只要它按值或按到 const 的引用接收其实参），从而当移动赋值不可用时，复制赋值将会成为它的后备。</p>
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<p>隐式定义的移动赋值运算符是否会多次对在继承网格中可通过多于一条路径访问的虚基类子对象赋值是未指明的（同样适用于<a href="cpp-language-copy_assignment.html">复制赋值</a>）。</p>
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<p>有关用户定义的移动赋值运算符应当有哪些行为，见<a href="cpp-language-operators.html#.E8.B5.8B.E5.80.BC.E8.BF.90.E7.AE.97.E7.AC.A6">赋值运算符重载</a>。</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 <string></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <utility></span>
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<span class="kw1">struct</span> A
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<span class="br0">{</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> s<span class="sy4">;</span>
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A<span class="br0">(</span><span class="br0">)</span> <span class="sy4">:</span> s<span class="br0">(</span><span class="st0">"测试"</span><span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span>
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A<span class="br0">(</span><span class="kw4">const</span> A<span class="sy3">&</span> o<span class="br0">)</span> <span class="sy4">:</span> s<span class="br0">(</span>o.<span class="me1">s</span><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> <span class="st0">"移动失败！<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
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A<span class="br0">(</span>A<span class="sy3">&&</span> o<span class="br0">)</span> <span class="sy4">:</span> s<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>o.<span class="me1">s</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span>
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A<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span><span class="kw4">const</span> A<span class="sy3">&</span> other<span class="br0">)</span>
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<span class="br0">{</span>
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s <span class="sy1">=</span> other.<span class="me1">s</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">"复制赋值<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="kw1">return</span> <span class="sy2">*</span>this<span class="sy4">;</span>
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<span class="br0">}</span>
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A<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span>A<span class="sy3">&&</span> other<span class="br0">)</span>
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<span class="br0">{</span>
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s <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>other.<span class="me1">s</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">"移动赋值<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="kw1">return</span> <span class="sy2">*</span>this<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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A f<span class="br0">(</span>A a<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> a<span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw1">struct</span> B <span class="sy4">:</span> A
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<span class="br0">{</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> s2<span class="sy4">;</span>
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<span class="kw4">int</span> n<span class="sy4">;</span>
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<span class="co1">// 隐式移动赋值运算符 B& B::operator=(B&&)</span>
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<span class="co1">// 调用 A 的移动赋值运算符</span>
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<span class="co1">// 调用 s2 的移动赋值运算符</span>
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<span class="co1">// 并进行 n 的逐位复制</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">struct</span> C <span class="sy4">:</span> B
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<span class="br0">{</span>
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~C<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span> <span class="co1">// 析构函数阻止隐式移动赋值</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">struct</span> D <span class="sy4">:</span> B
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<span class="br0">{</span>
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D<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span>
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~D<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span> <span class="co1">// 析构函数本会阻止隐式移动赋值</span>
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D<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span>D<span class="sy3">&&</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span> <span class="co1">// 无论如何都强制移动赋值</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 a1, a2<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">"尝试从右值临时量移动赋值 A<span class="es1">\n</span>"</span><span class="sy4">;</span>
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a1 <span class="sy1">=</span> f<span class="br0">(</span>A<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 从右值临时量移动赋值</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">"尝试从亡值移动赋值 A<span class="es1">\n</span>"</span><span class="sy4">;</span>
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a2 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>a1<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 从亡值移动赋值</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">"尝试移动赋值 B<span class="es1">\n</span>"</span><span class="sy4">;</span>
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B b1, b2<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">"移动前，b1.s = <span class="es1">\"</span>"</span> <span class="sy1"><<</span> b1.<span class="me1">s</span> <span class="sy1"><<</span> <span class="st0">"<span class="es1">\"</span><span class="es1">\n</span>"</span><span class="sy4">;</span>
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b2 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>b1<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 调用隐式移动赋值</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">"移动后，b1.s = <span class="es1">\"</span>"</span> <span class="sy1"><<</span> b1.<span class="me1">s</span> <span class="sy1"><<</span> <span class="st0">"<span class="es1">\"</span><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">"尝试移动赋值 C<span class="es1">\n</span>"</span><span class="sy4">;</span>
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C c1, c2<span class="sy4">;</span>
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c2 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>c1<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 调用复制赋值运算符</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">"尝试移动赋值 D<span class="es1">\n</span>"</span><span class="sy4">;</span>
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D d1, d2<span class="sy4">;</span>
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d2 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>d1<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">尝试从右值临时量移动赋值 A
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移动赋值
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尝试从亡值移动赋值 A
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移动赋值
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尝试移动赋值 B
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移动前，b1.s = "测试"
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移动赋值
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移动后，b1.s = ""
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尝试移动赋值 C
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复制赋值
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尝试移动赋值 D
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移动赋值</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.cmeerw.net/cwg/issue1402">CWG 1402</a></td>
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<td>C++11</td>
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<td>会调用非平凡复制赋值运算符的预置移动赋值运算符被弃置；<br />
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被弃置的预置移动赋值运算符仍参与重载决议</td>
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<td>允许调用这种复制赋值运算符；<br />
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使重载决议将其忽略</td>
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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://wg21.cmeerw.net/cwg/issue1806">CWG 1806</a></td>
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<td>C++11</td>
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<td>涉及虚基类的预置移动赋值运算符的规定缺失</td>
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<td>已添加</td>
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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://wg21.cmeerw.net/cwg/issue2094">CWG 2094</a></td>
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<td>C++11</td>
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<td>volatile 子对象使预置的移动赋值运算符非平凡（CWG 496）</td>
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<td>平凡性不受影响</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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<ul>
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<li><a href="cpp-language-constructor.html">构造函数</a></li>
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<li><a href="cpp-language-converting_constructor.html">转换构造函数</a></li>
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<li><a href="cpp-language-copy_assignment.html">复制赋值</a></li>
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<li><a href="cpp-language-copy_constructor.html">复制构造函数</a></li>
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<li><a href="cpp-language-default_constructor.html">默认构造函数</a></li>
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<li><a href="cpp-language-destructor.html">析构函数</a></li>
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<li><a href="cpp-language-initialization.html">初始化</a>
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<ul>
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<li><a href="cpp-language-aggregate_initialization.html">聚合初始化</a></li>
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<li><a href="cpp-language-constant_initialization.html">常量初始化</a></li>
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<li><a href="cpp-language-copy_initialization.html">复制初始化</a></li>
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<li><a href="cpp-language-default_initialization.html">默认初始化</a></li>
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<li><a href="cpp-language-direct_initialization.html">直接初始化</a></li>
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<li><a href="cpp-language-list_initialization.html">列表初始化</a></li>
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<li><a href="cpp-language-reference_initialization.html">引用初始化</a></li>
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<li><a href="cpp-language-value_initialization.html">值初始化</a></li>
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<li><a href="cpp-language-zero_initialization.html">零初始化</a></li>
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</ul>
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</li>
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<li><a href="cpp-language-move_constructor.html">移动构造函数</a></li>
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</ul>
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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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