74 lines
5.6 KiB
HTML
74 lines
5.6 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>C++ 具名要求：可移动赋值 (MoveAssignable) (C++11 起)</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_named_req_MoveAssignable 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"><span style="font-size:0.7em; line-height:130%">C++ 具名要求：</span><span style="font-style:italic">可移动赋值</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(MoveAssignable)</span> <span class="t-mark-rev t-since-cxx11">(C++11 起)</span></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-named_req.html">named req</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>指定该类型的实例可以从<a href="cpp-language-value_category.html">右值</a>实参赋值。</p>
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<h3><span class="mw-headline" id=".E8.A6.81.E6.B1.82">要求</span></h3>
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<p>若满足下列条件，则类型 <code>T</code> 满足<span style="font-style:italic">可移动赋值</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(MoveAssignable)</span> ：</p>
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<p>给定</p>
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<ul>
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<li><code>T</code> 类型的可修改<a href="cpp-language-value_category.html">左值表达式</a> <code>t</code></li>
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<li><code>T</code> 类型的<a href="cpp-language-value_category.html">右值</a>表达式 <code>rv</code></li>
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</ul>
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<p>下列表达式必须合法并拥有其指定的效果</p>
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<table class="wikitable">
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<tr>
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<th>表达式</th>
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<th>返回类型</th>
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<th>返回值</th>
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<th>后条件</th>
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</tr>
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<tr>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">t <span class="sy1">=</span> rv</span></span></td>
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<td><code>T&</code></td>
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<td><code>t</code></td>
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<td>若 <code>t</code> 与 <code>rv</code> 不指代同一对象，则 <code>t</code> 的值等价于 <code>rv</code> 在赋值前的值。
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<p><code>rv</code> 的新值未指定</p>
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</td>
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</tr>
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</table>
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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>类型不必为满足此类型要求而实现<a href="cpp-language-move_assignment.html">移动赋值运算符</a>：按值或按 <code>const Type&</code> 接收其参数的<a href="cpp-language-copy_assignment.html">复制赋值运算符</a>会绑定到右值实参。</p>
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<p>若<span style="font-style:italic">可移动赋值</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(MoveAssignable)</span> 类实现了移动赋值运算符，则它亦可实现<a href="cpp-utility-move.html">移动语义</a>，以获得“<code>rv</code> 在赋值后的值未指定”这一事实的优势。</p>
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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-types-is_move_assignable.html"><span class="t-lines"><span>is_move_assignable</span><span>is_trivially_move_assignable</span><span>is_nothrow_move_assignable</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>检查类型是否有拥有移动赋值运算符<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-concepts-assignable_from.html"><span class="t-lines"><span>assignable_from</span></span></a></div>
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<div><span class="t-lines"><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>指定一个类型能从另一类型赋值<br />
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<span class="t-mark">(概念)</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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