236 lines
36 KiB
HTML
236 lines
36 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>移动构造函数</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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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_move_constructor 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">移动构造函数</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> 的移动构造函数是非模板<a href="cpp-language-initializer_list.html" class="mw-redirect">构造函数</a>，它的首个形参是 <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>，且没有其他形参，或剩余形参均有默认值。</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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<td colspan="10" class="t-sdsc-sep"></td>
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<tr class="t-sdsc">
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<td><span class="t-spar">类名</span> ( <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 class="t-sdsc">
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<td><span class="t-spar">类名</span> ( <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 class="t-sdsc">
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<td><span class="t-spar">类名</span> ( <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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<p>其中 <span class="t-spar">类名</span> 必须指名当前类（或类模板的当前实例化），或在命名空间作用域或友元声明中声明时，必须是有限定的类名。</p>
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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> 移动构造函数的典型声明。</div>
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<div class="t-li1"><span class="t-li">2)</span> 强制编译器生成移动构造函数。</div>
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<div class="t-li1"><span class="t-li">3)</span> 避免隐式生成移动构造函数。</div>
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<p>当从同类型的<span class="t-rev-inl t-until-cxx17"><span><a href="cpp-language-value_category.html#.E5.8F.B3.E5.80.BC">右值</a>（亡值或纯右值）</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>亡值</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span><a href="cpp-language-initialization.html">初始化</a>（<a href="cpp-language-direct_initialization.html">直接初始化</a>或<a href="cpp-language-copy_initialization.html">复制初始化</a>）对象时，会调用移动构造函数，情况包括：</p>
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<ul>
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<li>初始化：<span class="t-c"><span class="mw-geshi cpp source-cpp">T a <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>b<span class="br0">)</span><span class="sy4">;</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp">T a<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>b<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span></span></span>，其中 <code>b</code> 的类型是 <code>T</code> ；</li>
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<li>函数实参传递：<span class="t-c"><span class="mw-geshi cpp source-cpp">f<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>a<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span></span></span>，其中 <code>a</code> 的类型是 <code>T</code> 且 <code>f</code> 是 <span class="t-c"><span class="mw-geshi cpp source-cpp">Ret f<span class="br0">(</span>T t<span class="br0">)</span></span></span> ；</li>
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<li>函数返回：在像 <span class="t-c"><span class="mw-geshi cpp source-cpp">T f<span class="br0">(</span><span class="br0">)</span></span></span> 这样的函数中的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> a<span class="sy4">;</span></span></span>，其中 <code>a</code> 的类型是 <code>T</code>，且 <code>T</code> 有移动构造函数。</li>
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</ul>
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<p>当初始化器是纯右值时，<span class="t-rev-inl t-until-cxx17"><span>通常会优化掉</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>始终不会进行</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>对移动构造函数的调用，见<a href="cpp-language-copy_elision.html">复制消除</a>。</p>
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<p>典型的移动构造函数“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，并使其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> 移动可以导致实参被置为空。但是不应该依赖此类行为。对于某些类型，例如 <span class="t-lc"><a href="cpp-memory-unique_ptr.html">std::unique_ptr</a></span>，移动后的状态是完全指定的。</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.E6.9E.84.E9.80.A0.E5.87.BD.E6.95.B0">隐式声明的移动构造函数</span></h3>
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<p>如果不对类类型（<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构造函数，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<a href="cpp-language-copy_constructor.html">复制构造函数</a>；</li>
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<li>没有用户声明的<a href="cpp-language-copy_assignment.html">复制赋值运算符</a>；</li>
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<li>没有用户声明的<a href="cpp-language-move_assignment.html">移动赋值运算符</a>；</li>
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<li>没有用户声明的<a href="cpp-language-destructor.html">析构函数</a>；</li>
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</ul>
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<p>那么编译器将声明一个移动构造函数作为这个类的非 <a href="cpp-language-explicit.html">explicit</a> 的 <code>inline public</code> 成员，签名为 <code>T::T(T&&)</code>。</p>
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<p>一个类可以拥有多个移动构造函数，例如 <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="sy4">::</span><span class="me2">T</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="sy4">::</span><span class="me2">T</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构造函数时，用户仍然可以通过关键词 <code>default</code> 强制编译器生成隐式声明的移动构造函数。</p>
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<p>隐式声明（或在其首个声明被预置）的移动构造函数具有<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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<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.E6.9E.84.E9.80.A0.E5.87.BD.E6.95.B0">弃置的隐式声明的移动构造函数</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构造函数被定义为<i>弃置的</i>：</p>
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<ul>
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<li><code>T</code> 拥有无法移动（拥有被弃置、不可访问或有歧义的移动构造函数）的非静态数据成员；</li>
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<li><code>T</code> 拥有无法移动（拥有被弃置、不可访问或有歧义的移动构造函数）的直接或虚基类；</li>
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<li><code>T</code> 拥有带被弃置或不可访问的析构函数的直接或虚基类；</li>
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<li><code>T</code> 是联合式的类，且拥有带非平凡移动构造函数的变体成员。</li>
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</ul>
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<p><a href="cpp-language-overload_resolution.html">重载决议</a>忽略被弃置的预置移动构造函数（否则它会阻止从右值复制初始化）。</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.E6.9E.84.E9.80.A0.E5.87.BD.E6.95.B0">平凡的移动构造函数</span></h3>
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<p>如果满足下列所有条件，类 <code>T</code> 移动构造函数是平凡的：</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构造函数都是平凡的；</li>
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<li>为 <code>T</code> 的每个类类型（或类类型数组）的非静态成员选择的移动构造函数都是平凡的。</li>
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</ul>
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<p>平凡的移动构造函数是与平凡的复制构造函数实施相同动作的构造函数，即它如同用 <span class="t-lc"><a href="cpp-string-byte-memmove.html">std::memmove</a></span> 来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.E6.9E.84.E9.80.A0.E5.87.BD.E6.95.B0">合格的移动构造函数</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>没有被弃置的移动构造函数是合格的。</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构造函数是合格的：</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的移动构造函数。</li>
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</ul>
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</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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</tr>
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</table>
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<p>合格的移动构造函数的平凡性确定该类是否为<a href="cpp-language-lifetime.html#.E9.9A.90.E5.BC.8F.E7.94.9F.E5.AD.98.E6.9C.9F.E7.B1.BB.E5.9E.8B">隐式生存期类型</a>，以及该类是否为<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.E6.9E.84.E9.80.A0.E5.87.BD.E6.95.B0">隐式定义的移动构造函数</span></h3>
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<p>如果隐式声明的移动构造函数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>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>时，它会被编译器定义（生成并编译函数体）。对于联合体类型，隐式定义的移动构造函数（如同以 <span class="t-lc"><a href="cpp-string-byte-memmove.html">std::memmove</a></span>）复制它的对象表示。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>），该构造函数用以<a href="cpp-language-value_category.html#.E4.BA.A1.E5.80.BC">亡值</a>实参执行的直接初始化，按照初始化顺序，对对象的各基类和非静态成员进行完整的逐对象移动。如果它满足对于 <a href="cpp-language-constexpr.html">constexpr 构造函数</a>的要求，那么生成的移动构造函数也是 <code>constexpr</code> 的。</p>
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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>为使<a href="cpp-language-exceptions.html#.E5.BC.82.E5.B8.B8.E5.AE.89.E5.85.A8">强异常保证</a>可行，用户定义的移动构造函数不应该抛出异常。例如，<span class="t-lc"><a href="cpp-container-vector.html">std::vector</a></span> 在需要重新放置元素时，基于 <span class="t-lc"><a href="cpp-utility-move_if_noexcept.html">std::move_if_noexcept</a></span> 在移动和复制之间选择。</p>
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<p>如果同时提供了复制和移动构造函数而没有其他可行的构造函数，那么重载决议在实参是相同类型的<a href="cpp-language-value_category.html"><i>右值</i></a>（如 <span class="t-lc">std::move</span> 的结果的<a href="cpp-language-value_category.html"><i>亡值</i></a>，或如无名临时量的<a href="cpp-language-value_category.html"><i>纯右值</i></a>）时选择移动构造函数，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/运算符）时选择复制构造函数。如果只提供复制构造函数，那么重载决议对于所有实参类别都会选择它（只要它接收到 const 的引用，因为右值能绑定到 const 引用），这使得复制可以作为在移动不可用时的后备。</p>
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<p>当接收右值引用为其形参时，构造函数被称作“移动构造函数”。它没有义务移动任何内容，不要求类拥有要被移动的资源，而且在受允许（但可能没意义）的以 const 右值引用（const T&&）为形参的情况中，‘移动构造函数’可能无法移动资源。</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 <iomanip></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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<span class="kw4">int</span> k<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>, k<span class="br0">(</span><span class="sy2">-</span><span class="nu0">1</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>, k<span class="br0">(</span>o.<span class="me1">k</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="kw1">noexcept</span> <span class="sy4">:</span>
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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="co1">// 类类型成员的显式移动</span>
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k<span class="br0">(</span><a href="cpp-utility-exchange.html"><span class="kw1118">std::<span class="me2">exchange</span></span></a><span class="br0">(</span>o.<span class="me1">k</span>, <span class="nu0">0</span><span class="br0">)</span><span class="br0">)</span> <span class="co1">// 非类类型成员的显式移动</span>
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<span class="br0">{</span> <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>
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<span class="br0">{</span>
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<span class="kw1">return</span> a<span class="sy4">;</span>
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<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">// 隐式移动构造函数 B::(B&&)</span>
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<span class="co1">// 调用 A 的移动构造函数</span>
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<span class="co1">// 调用 s2 的移动构造函数</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构造函数 C::(C&&)</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">// 析构函数阻止隐式移动构造函数 D::(D&&)</span>
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D<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">// 强制生成移动构造函数</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-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 A<span class="es1">\n</span>"</span><span class="sy4">;</span>
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A 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">// 按值返回时，从函数形参移动构造其目标</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，a1.s = "</span> <span class="sy1"><<</span> <a href="cpp-io-manip-quoted.html"><span class="kw1802">std::<span class="me2">quoted</span></span></a><span class="br0">(</span>a1.<span class="me1">s</span><span class="br0">)</span>
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<span class="sy1"><<</span> <span class="st0">" a1.k = "</span> <span class="sy1"><<</span> a1.<span class="me1">k</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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A 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">// 从亡值移动构造</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，a1.s = "</span> <span class="sy1"><<</span> <a href="cpp-io-manip-quoted.html"><span class="kw1802">std::<span class="me2">quoted</span></span></a><span class="br0">(</span>a1.<span class="me1">s</span><span class="br0">)</span>
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<span class="sy1"><<</span> <span class="st0">" a1.k = "</span> <span class="sy1"><<</span> a1.<span class="me1">k</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
|
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"尝试移动 B<span class="es1">\n</span>"</span><span class="sy4">;</span>
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B b1<span class="sy4">;</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">"移动前，b1.s = "</span> <span class="sy1"><<</span> <a href="cpp-io-manip-quoted.html"><span class="kw1802">std::<span class="me2">quoted</span></span></a><span class="br0">(</span>b1.<span class="me1">s</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">"<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
B 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">// 调用隐式移动构造函数</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> <span class="sy1"><<</span> <a href="cpp-io-manip-quoted.html"><span class="kw1802">std::<span class="me2">quoted</span></span></a><span class="br0">(</span>b1.<span class="me1">s</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">"<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
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|
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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 C<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
C c1<span class="sy4">;</span>
|
|
C 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">// 调用复制构造函数</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">"尝试移动 D<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
D d1<span class="sy4">;</span>
|
|
D 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>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
<p>输出：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="text source-text">
|
|
<pre class="de1">尝试移动 A
|
|
移动前，a1.s = "测试" a1.k = -1
|
|
移动后，a1.s = "" a1.k = 0
|
|
尝试移动 B
|
|
移动前，b1.s = "测试"
|
|
移动后，b1.s = ""
|
|
尝试移动 C
|
|
移动失败！
|
|
尝试移动 D</pre></div>
|
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</div>
|
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</div>
|
|
<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
|
|
<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>
|
|
<table class="dsctable" style="font-size:0.8em">
|
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<tr>
|
|
<th>DR</th>
|
|
<th>应用于</th>
|
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<th>出版时的行为</th>
|
|
<th>正确行为</th>
|
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</tr>
|
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<tr>
|
|
<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>会调用非平凡复制构造函数的预置移动构造函数被弃置；<br />
|
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被弃置的预置移动构造函数仍参与重载决议</td>
|
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<td>允许调用这种复制构造函数；<br />
|
|
使重载决议将其忽略</td>
|
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</tr>
|
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<tr>
|
|
<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>
|
|
<td>volatile 子对象使移动构造函数非平凡（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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</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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