264 lines
26 KiB
HTML
264 lines
26 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::reference_wrapper</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_functional_reference_wrapper skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</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></span></div>
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<div>定义于头文件 <code><a href="cpp-header-functional.html"><functional></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 />
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<span class="kw1">class</span> reference_wrapper<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></td>
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<td></td>
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<p><code>std::reference_wrapper</code> 是包装引用于可复制、可赋值对象的类模板。它常用作将引用存储入无法正常保有引用的标准容器（类似 <span class="t-lc"><a href="cpp-container-vector.html">std::vector</a></span> ）的机制。</p>
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<p>特别是， <code>std::reference_wrapper</code> 是围绕到类型 <code>T</code> 的对象引用或函数引用的<a href="cpp-named_req-CopyConstructible.html"><span style="font-style:italic">可复制构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyConstructible)</span></a> 且<a href="cpp-named_req-CopyAssignable.html"><span style="font-style:italic">可复制赋值</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyAssignable)</span></a> 的包装器。 <code>std::reference_wrapper</code> 的实例是对象（它们可被复制或存储于容器），但它们能隐式转换成 <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="sy3">&</span></span></span> ，故能以之为以引用接收底层类型的函数的参数。</p>
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<p>若存储的引用<a href="cpp-named_req-Callable.html"><span style="font-style:italic">可调用</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Callable)</span></a> ，则可以相同参数调用 <code>std::reference_wrapper</code> 。</p>
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<p>辅助函数 <span class="t-lc"><a href="cpp-utility-functional-ref.html">std::ref</a></span> 与 <span class="t-lc"><a href="cpp-utility-functional-ref.html">std::cref</a></span> 常用于生成 <code>std::reference_wrapper</code> 对象。</p>
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<p><code>std::reference_wrapper</code> 亦用于按引用传递对象给 <span class="t-lc"><a href="cpp-utility-functional-bind.html">std::bind</a></span> 或 <span class="t-lc"><a href="cpp-thread-thread.html">std::thread</a></span> 的构造函数。</p>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx17">
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<p>保证 <code>std::reference_wrapper</code> 为<a href="cpp-named_req-TriviallyCopyable.html"><span style="font-style:italic">可平凡复制</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(TriviallyCopyable)</span></a> 。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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</table>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx20">
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<p><code>T</code> 可为不完整类型。</p>
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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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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E7.B1.BB.E5.9E.8B">成员类型</span></h3>
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<table class="t-dsc-begin">
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<tr class="t-dsc-hitem">
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<td>类型</td>
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<td>定义</td>
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<tr class="t-dsc">
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<td><code>type</code></td>
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<td><code>T</code></td>
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</tr>
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<tr class="t-dsc">
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<td><code>result_type</code><span class="t-mark-rev t-deprecated-cxx17">(C++17 中弃用)</span><span class="t-mark-rev t-until-cxx20">(C++20 中移除)</span></td>
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<td>若 <code>T</code> 为函数则为 <code>T</code> 的返回类型。否则不定义。</td>
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</tr>
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<tr class="t-dsc">
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<td><code>argument_type</code><span class="t-mark-rev t-deprecated-cxx17">(C++17 中弃用)</span><span class="t-mark-rev t-until-cxx20">(C++20 中移除)</span></td>
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<td>1) 若 <code>T</code> 为接受一个 <code>A1</code> 类型参数的函数或指向函数指针，则 <code>argument_type</code> 为 <code>A1</code> 。<br />
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<p>2) 若 <code>T</code> 是不接收参数的指向类 <code>T0</code> 成员函数的指针，则 <code>argument_type</code> 为 <code>T0*</code> ，可能有 cv 限定<br />
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3) 若 <code>T</code> 是拥有成员类型 <code>T::argument_type</code> 的类类型，则 <code>argument_type</code> 为其别名</p>
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</td>
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</tr>
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<tr class="t-dsc">
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<td><code>first_argument_type</code><span class="t-mark-rev t-deprecated-cxx17">(C++17 中弃用)</span><span class="t-mark-rev t-until-cxx20">(C++20 中移除)</span></td>
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<td>1) 若 <code>T</code> 是接收二个 <code>A1</code> 与 <code>A2</code> 类型参数的函数或指向函数指针，则 <code>first_argument_type</code> 为 <code>A1</code> 。<br />
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<p>2) 若 <code>T</code> 是接收一个参数的指向 <code>T0</code> 成员函数指针，则 <code>first_argument_type</code> 为 <code>T0*</code> ，可能有 cv 限定<br />
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3) 若 <code>T</code> 是拥有成员类型 <code>T::first_argument_type</code> 的类类型，则 <code>first_argument_type</code> 为其别名</p>
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</td>
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</tr>
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<tr class="t-dsc">
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<td><code>second_argument_type</code><span class="t-mark-rev t-deprecated-cxx17">(C++17 中弃用)</span><span class="t-mark-rev t-until-cxx20">(C++20 中移除)</span></td>
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<td>1) 若 <code>T</code> 是接收二个 <code>A1</code> 与 <code>A2</code> 类型参数的函数或指向函数指针，则 <code>second_argument_type</code> 为 <code>A2</code> 。<br />
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<p>2) 若 <code>T</code> 是接收一个 <code>A1</code> 类型参数的指向 <code>T0</code> 成员函数指针，则 <code>second_argument_type</code> 为 <code>A1</code> ，可有 cv 限定<br />
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3) 若 <code>T</code> 是拥有成员类型 <code>T::second_argument_type</code> 的类类型，则 <code>second_argument_type</code> 为其别名</p>
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</td>
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</table>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">成员函数</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 t-dsc-member-nobold-div">
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<div><a href="cpp-utility-functional-reference_wrapper-reference_wrapper.html"><span class="t-lines"><span>(构造函数)</span></span></a></div>
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</div>
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</td>
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<td>存储引用于新的 <span class="t-lc"><strong class="selflink">std::reference_wrapper</strong></span> 对象<br />
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<span class="t-mark">(公开成员函数)</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-functional-reference_wrapper-operator=.html"><span class="t-lines"><span>operator=</span></span></a></div>
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</div>
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</td>
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<td>重绑定 <span class="t-lc"><strong class="selflink">std::reference_wrapper</strong></span><br />
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<span class="t-mark">(公开成员函数)</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-functional-reference_wrapper-get.html"><span class="t-lines"><span>get</span><span>operator T&</span></span></a></div>
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</div>
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</td>
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<td>访问存储的引用<br />
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<span class="t-mark">(公开成员函数)</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-functional-reference_wrapper-operator().html"><span class="t-lines"><span>operator()</span></span></a></div>
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</div>
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</td>
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<td>调用其所存储的函数<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E6.8E.A8.E5.AF.BC.E6.8C.87.E5.BC.95.28C.2B.2B17_.E8.B5.B7.29"><a href="cpp-utility-functional-reference_wrapper-deduction_guides.html">推导指引</a><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></h3>
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<h3><span class="mw-headline" id=".E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0">可能的实现</span></h3>
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<table class="eq-fun-cpp-table">
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<tr>
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<td>
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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="kw1">namespace</span> detail <span class="br0">{</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span> T<span class="sy3">&</span> FUN<span class="br0">(</span>T<span class="sy3">&</span> t<span class="br0">)</span> <span class="kw1">noexcept</span> <span class="br0">{</span> <span class="kw1">return</span> t<span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span> <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>
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<span class="br0">}</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
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<span class="kw1">class</span> reference_wrapper <span class="br0">{</span>
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<span class="kw1">public</span><span class="sy4">:</span>
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<span class="co1">// 类型</span>
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<span class="kw1">using</span> type <span class="sy1">=</span> T<span class="sy4">;</span>
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<span class="co1">// 构造/复制/销毁</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> U, <span class="kw1">class</span> <span class="sy1">=</span> decltype<span class="br0">(</span>
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detail<span class="sy4">::</span><span class="me2">FUN</span><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span><a href="cpp-utility-declval.html"><span class="kw1119">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>,
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<a href="cpp-types-enable_if.html"><span class="kw631">std::<span class="me2">enable_if_t</span></span></a><span class="sy1"><</span><span class="sy3">!</span><a href="cpp-types-is_same.html"><span class="kw589">std::<span class="me2">is_same_v</span></span></a><span class="sy1"><</span>reference_wrapper, std<span class="sy4">::</span><span class="me2">remove_cvref_t</span><span class="sy1"><</span>U<span class="sy1">>>></span><span class="br0">(</span><span class="br0">)</span>
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<span class="br0">)</span><span class="sy1">></span>
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<span class="kw4">constexpr</span> reference_wrapper<span class="br0">(</span>U<span class="sy3">&&</span> u<span class="br0">)</span> <span class="kw1">noexcept</span><span class="br0">(</span><span class="kw1">noexcept</span><span class="br0">(</span>detail<span class="sy4">::</span><span class="me2">FUN</span><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span><a href="cpp-utility-forward.html"><span class="kw1116">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="br0">(</span>u<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span>
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<span class="sy4">:</span> _ptr<span class="br0">(</span><a href="cpp-memory-addressof.html"><span class="kw757">std::<span class="me2">addressof</span></span></a><span class="br0">(</span>detail<span class="sy4">::</span><span class="me2">FUN</span><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span><a href="cpp-utility-forward.html"><span class="kw1116">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="br0">(</span>u<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
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reference_wrapper<span class="br0">(</span><span class="kw4">const</span> reference_wrapper<span class="sy3">&</span><span class="br0">)</span> <span class="kw1">noexcept</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span>
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<span class="co1">// 赋值</span>
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reference_wrapper<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span><span class="kw4">const</span> reference_wrapper<span class="sy3">&</span> x<span class="br0">)</span> <span class="kw1">noexcept</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span>
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<span class="co1">// 访问</span>
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<span class="kw4">constexpr</span> operator T<span class="sy3">&</span> <span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="sy2">*</span>_ptr<span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw4">constexpr</span> T<span class="sy3">&</span> get<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="sy2">*</span>_ptr<span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span>... <span class="me1">ArgTypes</span> <span class="sy1">></span>
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<span class="kw4">constexpr</span> std<span class="sy4">::</span><span class="me2">invoke_result_t</span><span class="sy1"><</span>T<span class="sy3">&</span>, ArgTypes...<span class="sy1">></span>
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operator<span class="br0">(</span><span class="br0">)</span> <span class="br0">(</span> ArgTypes<span class="sy3">&&</span>... <span class="me1">args</span> <span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span>
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<span class="kw1">return</span> <a href="cpp-utility-functional-invoke.html"><span class="kw1058">std::<span class="me2">invoke</span></span></a><span class="br0">(</span>get<span class="br0">(</span><span class="br0">)</span>, <a href="cpp-utility-forward.html"><span class="kw1116">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>ArgTypes<span class="sy1">></span><span class="br0">(</span>args<span class="br0">)</span>...<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">private</span><span class="sy4">:</span>
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T<span class="sy2">*</span> _ptr<span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="co1">// 推导指引</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
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reference_wrapper<span class="br0">(</span>T<span class="sy3">&</span><span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> reference_wrapper<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">;</span></pre></div>
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</div>
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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=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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<p>演示将 reference_wrapper 作为引用的容器使用，这令用多重下标访问同一容器可行</p>
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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 <algorithm></span>
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<span class="co2">#include <list></span>
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<span class="co2">#include <vector></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <numeric></span>
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<span class="co2">#include <random></span>
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<span class="co2">#include <functional></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-container-list.html"><span class="kw1274">std::<span class="me2">list</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> l<span class="br0">(</span><span class="nu0">10</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-algorithm-iota.html"><span class="kw1654">std::<span class="me2">iota</span></span></a><span class="br0">(</span>l.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, l.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, <span class="sy2">-</span><span class="nu0">4</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-container-vector.html"><span class="kw1268">std::<span class="me2">vector</span></span></a><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">reference_wrapper</span><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">>></span> v<span class="br0">(</span>l.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, l.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 不能在 list 上用 shuffle （要求随机访问），但能在 vector 上使用它</span>
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<a href="cpp-algorithm-random_shuffle.html"><span class="kw1606">std::<span class="me2">shuffle</span></span></a><span class="br0">(</span>v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, v.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, <a href="cpp-numeric-random-mersenne_twister_engine.html"><span class="kw1488">std::<span class="me2">mt19937</span></span></a><span class="br0">{</span><a href="cpp-numeric-random-random_device.html"><span class="kw1485">std::<span class="me2">random_device</span></span></a><span class="br0">{</span><span class="br0">}</span><span class="br0">(</span><span class="br0">)</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</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">"Contents of the list: "</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> n <span class="sy4">:</span> l<span class="br0">)</span> <a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> n <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span> <a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <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="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Contents of the list, as seen through a shuffled vector: "</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> i <span class="sy4">:</span> v<span class="br0">)</span> <a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> i <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span> <a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <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="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Doubling the values in the initial list...<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span><span class="sy3">&</span> i <span class="sy4">:</span> l<span class="br0">)</span> <span class="br0">{</span>
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i <span class="sy2">*</span><span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</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">"Contents of the list, as seen through a shuffled vector: "</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> i <span class="sy4">:</span> v<span class="br0">)</span> <a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> i <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span> <a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">'<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">Contents of the list: -4 -3 -2 -1 0 1 2 3 4 5
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Contents of the list, as seen through a shuffled vector: -1 2 -2 1 5 0 3 -3 -4 4
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Doubling the values in the initial list...
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Contents of the list, as seen through a shuffled vector: -2 4 -4 2 10 0 6 -6 -8 8</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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<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-functional-ref.html"><span class="t-lines"><span>ref</span><span>cref</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></div>
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</div>
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</td>
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<td>创建具有从其实参推导的类型的 <span class="t-lc"><strong class="selflink">std::reference_wrapper</strong></span><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-functional-bind.html"><span class="t-lines"><span>bind</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>绑定一或多个实参到函数对象<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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</html>
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