130 lines
8.6 KiB
HTML
130 lines
8.6 KiB
HTML
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<title>std::reference_wrapper<T>::reference_wrapper</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_functional_reference_wrapper_reference_wrapper 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::reference_wrapper<T>::</span>reference_wrapper</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-utility.html">utility</a>‎ | <a href="cpp-utility-functional.html">functional</a>‎ | <a href="cpp-utility-functional-reference_wrapper.html">reference wrapper</a></span></div>
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<table class="t-dcl-begin">
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<td rowspan="142">(1)</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> U <span class="sy1">></span><br />
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reference_wrapper<span class="br0">(</span> U<span class="sy3">&&</span> x <span class="br0">)</span> <span class="kw1">noexcept</span><span class="br0">(</span><span class="coMULTI">/*see below*/</span><span class="br0">)</span> <span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-until-cxx20">(C++20 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> U <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> reference_wrapper<span class="br0">(</span> U<span class="sy3">&&</span> x <span class="br0">)</span> <span class="kw1">noexcept</span><span class="br0">(</span><span class="coMULTI">/*see below*/</span><span class="br0">)</span> <span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<td rowspan="142">(2)</td>
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<div><span class="mw-geshi cpp source-cpp">reference_wrapper<span class="br0">(</span> <span class="kw4">const</span> reference_wrapper<span class="sy3">&</span> other <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-until-cxx20">(C++20 前)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> reference_wrapper<span class="br0">(</span> <span class="kw4">const</span> reference_wrapper<span class="sy3">&</span> other <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-cxx20">(C++20 起)</span></td>
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<p>构造新的引用包装。</p>
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<div class="t-li1"><span class="t-li">1)</span> 如同用 <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="sy3">&</span> t <span class="sy1">=</span> <a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="br0">(</span>x<span class="br0">)</span><span class="sy4">;</span></span></span> 转换 <code>x</code> 为 <code>T&</code> ，然后存储到 <code>t</code> 的引用。此重载仅若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">typename</span> <a href="cpp-types-decay.html"><span class="kw628">std::<span class="me2">decay</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span></span></span> 与 <code>reference_wrapper</code> 不是同一类型且表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">FUN<span class="br0">(</span><a href="cpp-utility-declval.html"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span></span></span> 为良式才参与重载决议，其中 <code>FUN</code> 指名虚构函数集
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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="kw4">void</span> FUN<span class="br0">(</span>T<span class="sy3">&</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span>
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<span class="kw4">void</span> FUN<span class="br0">(</span>T<span class="sy3">&&</span><span class="br0">)</span> <span class="sy1">=</span> delete<span class="sy4">;</span></pre></div>
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<div class="t-li1"><span class="t-li">2)</span> 复制构造函数。存储到 <code>other.get()</code> 的引用。</div>
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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>要包装的对象</td>
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</tr>
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<tr class="t-par">
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<td>other</td>
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<td>-</td>
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<td>另一 reference_wrapper</td>
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<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<div class="t-li1"><span class="t-li">1)</span>
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<div class="t-noexcept-full"><a href="cpp-language-noexcept_spec.html"><tt>noexcept</tt></a> 说明：
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<div class="t-noexcept-box"><span class="t-cc"><span class="mw-geshi cpp source-cpp"><span class="kw1">noexcept</span><span class="br0">(</span><span class="kw1">noexcept</span><span class="br0">(</span>FUN<span class="br0">(</span><a href="cpp-utility-declval.html"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span></span></span></div>
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</div>
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，其中 <code>FUN</code> 是以上描述中的虚构函数集。</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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<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style="">
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<td class="mbox-empty-cell"></td>
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<td class="mbox-text" style="">本节未完成<br />
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原因：暂无示例</td>
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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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<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue2993">LWG 2993</a></td>
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<td>C++11</td>
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<td>被删除的 <code>reference_wrapper(T&&)</code> 构造函数在某些情况下干涉重载决议</td>
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<td>替换成构造函数模板</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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