174 lines
18 KiB
HTML
174 lines
18 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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<title>std::move</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_move skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>move</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-utility.html"><utility></a></code></div>
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<td></td>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> T <span class="sy1">></span><br />
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<span class="kw1">typename</span> <a href="cpp-types-remove_reference.html"><span class="kw606">std::<span class="me2">remove_reference</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span><span class="sy3">&&</span> move<span class="br0">(</span> T<span class="sy3">&&</span> t <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span><br />
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<span class="t-mark-rev t-until-cxx14">(C++14 前)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> T <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> <a href="cpp-types-remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&&</span> move<span class="br0">(</span> T<span class="sy3">&&</span> t <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></td>
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<p><code>std::move</code> 用于<i>指示</i>对象 <code>t</code> 可以“被移动”，即允许从 <code>t</code> 到另一对象的有效率的资源传递。</p>
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<p>特别是， <code>std::move</code> 生成标识其参数 <code>t</code> 的<a href="cpp-language-value_category.html">亡值表达式</a>。它准确地等价于到右值引用类型的 static_cast 。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<tr class="t-par">
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<td>t</td>
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<td>-</td>
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<td>要被移动的对象</td>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">static_cast</span><span class="sy1"><</span><span class="kw1">typename</span> <a href="cpp-types-remove_reference.html"><span class="kw606">std::<span class="me2">remove_reference</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span><span class="sy3">&&</span><span class="sy1">></span><span class="br0">(</span>t<span class="br0">)</span></span></span></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>以<a href="cpp-language-value_category.html">右值</a>参数（如临时对象的<a href="cpp-language-value_category.html">纯右值</a>或如 <code>std::move</code> 所产生者的<a href="cpp-language-value_category.html">亡值</a>之一）调用函数时，<a href="cpp-language-overload_resolution.html">重载决议</a>选择接受右值引用参数的版本（包含<a href="cpp-language-move_constructor.html">移动构造函数</a>、<a href="cpp-language-move_assignment.html">移动赋值运算符</a>及常规成员函数，如 <span class="t-lc"><a href="cpp-container-vector-push_back.html">std::vector::push_back</a></span> ）。若参数标识一个占有资源的对象，则这些重载拥有<i>移动</i>参数所保有的任何资源的选择，但不强求如此。例如，链表的移动构造函数可以复制指向表头的指针，并将 <span class="t-c"><span class="mw-geshi cpp source-cpp">nullptr</span></span> 存储到参数中，而非分配并复制逐个结点。</p>
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<p><a href="cpp-language-reference.html">右值引用</a>变量的名称是<a href="cpp-language-value_category.html">左值</a>，而若要绑定到接受右值引用参数的重载，就必须转换到<a href="cpp-language-value_category.html">亡值</a>，此乃<a href="cpp-language-move_constructor.html">移动构造函数</a>与<a href="cpp-language-move_assignment.html">移动赋值运算符</a>典型地使用 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">move</span></span></span> 的原因：</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="co1">// 简单的移动构造函数</span>
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A<span class="br0">(</span>A<span class="sy3">&&</span> arg<span class="br0">)</span> <span class="sy4">:</span> member<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>arg.<span class="me1">member</span><span class="br0">)</span><span class="br0">)</span> <span class="co1">// 表达式 "arg.member" 为左值</span>
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<span class="br0">{</span><span class="br0">}</span>
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<span class="co1">// 简单的移动赋值运算符</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> <span class="br0">{</span>
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member <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>other.<span class="me1">member</span><span class="br0">)</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></pre></div>
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</div>
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<p>一个例外是当函数参数类型是到模板形参的右值引用（“转发引用”或“通用引用”）时，该情况下转而使用 <span class="t-lc"><a href="cpp-utility-forward.html">std::forward</a></span> 。</p>
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<p>除非另外指定，否则所有已被移动的标准库对象被置于合法但未指定的状态。即只有无前提的函数，例如赋值运算符，才能安全地在对象被移动后使用：</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"><a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy1">></span> v<span class="sy4">;</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> str <span class="sy1">=</span> <span class="st0">"example"</span><span class="sy4">;</span>
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v.<span class="me1">push_back</span><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>str<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// str 现在合法但未指定</span>
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str.<span class="me1">back</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 若 size() == 0 则为未定义行为： back() 拥有前提 !empty()</span>
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str.<span class="me1">clear</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK ， clear() 无前提</span></pre></div>
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</div>
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<p>而且，以亡值参数调用的标准库函数可以假设该参数是到对象的唯一引用；若它从左值带 <code>std::move</code> 构造，则不进行别名检查。然而标准库类型的自移动赋值保证将对象置于合法（但未指定的）状态：</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"><a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> v <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">2</span>, <span class="nu0">3</span>, <span class="nu0">3</span><span class="br0">}</span><span class="sy4">;</span>
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v <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>v<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// v 的值未指定</span></pre></div>
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</div>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="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 <iostream></span>
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<span class="co2">#include <utility></span>
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<span class="co2">#include <vector></span>
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<span class="co2">#include <string></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-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> str <span class="sy1">=</span> <span class="st0">"Hello"</span><span class="sy4">;</span>
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<a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy1">></span> v<span class="sy4">;</span>
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<span class="co1">// 使用 push_back(const T&) 重载，</span>
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<span class="co1">// 表示我们将带来复制 str 的成本</span>
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v.<span class="me1">push_back</span><span class="br0">(</span>str<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">"After copy, str is <span class="es1">\"</span>"</span> <span class="sy1"><<</span> str <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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<span class="co1">// 使用右值引用 push_back(T&&) 重载，</span>
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<span class="co1">// 表示不复制字符串；而是</span>
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<span class="co1">// str 的内容被移动进 vector</span>
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<span class="co1">// 这个开销比较低，但也意味着 str 现在可能为空。</span>
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v.<span class="me1">push_back</span><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>str<span class="br0">)</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">"After move, str is <span class="es1">\"</span>"</span> <span class="sy1"><<</span> str <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">"The contents of the vector are <span class="es1">\"</span>"</span> <span class="sy1"><<</span> v<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span>
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<span class="sy1"><<</span> <span class="st0">"<span class="es1">\"</span>, <span class="es1">\"</span>"</span> <span class="sy1"><<</span> v<span class="br0">[</span><span class="nu0">1</span><span class="br0">]</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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<span class="br0">}</span></pre></div>
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</div>
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<p>可能的输出：</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">After copy, str is "Hello"
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After move, str is ""
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The contents of the vector are "Hello", "Hello"</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<table class="t-dsc-begin">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-forward.html"><span class="t-lines"><span>forward</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></div>
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</div>
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</td>
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<td>转发一个函数实参<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-move_if_noexcept.html"><span class="t-lines"><span>move_if_noexcept</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></div>
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</div>
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</td>
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<td>若移动构造函数不抛出则获得右值引用<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-algorithm-move.html"><span class="t-lines"><span>move</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></div>
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</div>
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</td>
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<td>将某一范围的元素移动到一个新的位置<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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</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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