155 lines
12 KiB
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155 lines
12 KiB
HTML
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<title>std::move_if_noexcept</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_move_if_noexcept 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_if_noexcept</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>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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<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 /></span>
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<p><span class="kw1">typename</span> <a href="cpp-types-conditional.html"><span class="kw633">std::<span class="me2">conditional</span></span></a><span class="sy1"><</span> <br />
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<span class="sy3">!</span><a href="cpp-types-is_move_constructible.html"><span class="kw542">std::<span class="me2">is_nothrow_move_constructible</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span> <span class="sy3">&&</span> <a href="cpp-types-is_copy_constructible.html"><span class="kw532">std::<span class="me2">is_copy_constructible</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span>,<br />
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<span class="kw4">const</span> T<span class="sy3">&</span>,<br />
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T<span class="sy3">&&</span><br /></p>
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<span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span> move_if_noexcept<span class="br0">(</span>T<span class="sy3">&</span> x<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></div>
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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 /></span>
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<p><span class="kw4">constexpr</span> <span class="kw1">typename</span> <a href="cpp-types-conditional.html"><span class="kw633">std::<span class="me2">conditional</span></span></a><span class="sy1"><</span> <br />
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<span class="sy3">!</span><a href="cpp-types-is_move_constructible.html"><span class="kw542">std::<span class="me2">is_nothrow_move_constructible</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span> <span class="sy3">&&</span> <a href="cpp-types-is_copy_constructible.html"><span class="kw532">std::<span class="me2">is_copy_constructible</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span>,<br />
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<span class="kw4">const</span> T<span class="sy3">&</span>,<br />
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T<span class="sy3">&&</span><br /></p>
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<span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span> move_if_noexcept<span class="br0">(</span>T<span class="sy3">&</span> x<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></div>
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<td><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></td>
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<p>若参数的移动构造函数不抛异常，则 <code>move_if_noexcept</code> 获得到参数的右值引用，否则获得左值引用。它典型地用于组合移动语义和强异常保证。</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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<td>x</td>
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<td>-</td>
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<td>要移动或复制的对象</td>
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</tr>
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</table>
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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>根据异常保证为 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>x<span class="br0">)</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp">x</span></span> 。</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
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<p>它可为如 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">vector</span><span class="sy4">::</span><span class="me2">resize</span></span></span> 这些必须分配新存储然后从旧存储移动或复制元素到新存储的函数所用。若在此操作中发生异常，则 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">vector</span><span class="sy4">::</span><span class="me2">resize</span></span></span> 撤销任何它做到此点的操作，这仅若使用 <code>std::move_if_noexcept</code> 确定使用移动构造还是复制构造才可行。（除非复制构造不可用，该情况下还是会使用移动构造，且可能抛弃强异常保证）</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 <iostream></span>
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<span class="co2">#include <utility></span>
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<span class="kw1">struct</span> Bad
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<span class="br0">{</span>
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Bad<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
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Bad<span class="br0">(</span>Bad<span class="sy3">&&</span><span class="br0">)</span> <span class="co1">// 可能抛出</span>
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<span class="br0">{</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Throwing move constructor called<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">}</span>
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Bad<span class="br0">(</span><span class="kw4">const</span> Bad<span class="sy3">&</span><span class="br0">)</span> <span class="co1">// 亦可能抛出</span>
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<span class="br0">{</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Throwing copy constructor called<span class="es1">\n</span>"</span><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> Good
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<span class="br0">{</span>
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Good<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
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Good<span class="br0">(</span>Good<span class="sy3">&&</span><span class="br0">)</span> <span class="kw1">noexcept</span> <span class="co1">// 将不抛出</span>
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<span class="br0">{</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Non-throwing move constructor called<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">}</span>
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Good<span class="br0">(</span><span class="kw4">const</span> Good<span class="sy3">&</span><span class="br0">)</span> <span class="kw1">noexcept</span> <span class="co1">// 将不抛出</span>
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<span class="br0">{</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Non-throwing copy constructor called<span class="es1">\n</span>"</span><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="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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Good g<span class="sy4">;</span>
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Bad b<span class="sy4">;</span>
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Good g2 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">move_if_noexcept</span><span class="br0">(</span>g<span class="br0">)</span><span class="sy4">;</span>
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Bad b2 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">move_if_noexcept</span><span class="br0">(</span>b<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">Non-throwing move constructor called
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Throwing copy constructor called</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E5.A4.8D.E6.9D.82.E5.BA.A6">复杂度</span></h3>
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<p>常数</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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<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.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>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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</table>
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</div>
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<div class="visualClear"></div>
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</div>
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</body>
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