181 lines
15 KiB
HTML
181 lines
15 KiB
HTML
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<title>std::make_tuple</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_tuple_make_tuple 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>make_tuple</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-tuple.html">tuple</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-tuple.html"><tuple></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>... <span class="me1">Types</span> <span class="sy1">></span><br />
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<a href="cpp-utility-tuple.html"><span class="kw1098">std::<span class="me2">tuple</span></span></a><span class="sy1"><</span>VTypes...<span class="sy1">></span> make_tuple<span class="br0">(</span> Types<span class="sy3">&&</span>... <span class="me1">args</span> <span class="br0">)</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">(C++14 起为 constexpr)</span></td>
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<p>创建 tuple 对象，从参数类型推导目标类型。</p>
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<p>对于每个 <code>Types...</code> 中的 <code>Ti</code> ， <code>Vtypes...</code> 中的对应类型 <code>Vi</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-decay.html"><span class="kw628">std::<span class="me2">decay</span></span></a><span class="sy1"><</span>Ti<span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span></span></span> ，除非应用 <span class="t-lc"><a href="cpp-types-decay.html">std::decay</a></span> 对某些类型 <code>X</code> 导致 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-functional-reference_wrapper.html"><span class="kw1068">std::<span class="me2">reference_wrapper</span></span></a><span class="sy1"><</span>X<span class="sy1">></span></span></span> ，该情况下推导的类型为 <code>X&</code> 。</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>args</td>
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<td>-</td>
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<td>构造 tuple 所用的零或更多参数</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>含给定值的 <span class="t-lc"><a href="cpp-utility-tuple.html">std::tuple</a></span> 对象，如同用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-tuple.html"><span class="kw1098">std::<span class="me2">tuple</span></span></a><span class="sy1"><</span>VTypes...<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>Types<span class="sy1">></span><span class="br0">(</span>t<span class="br0">)</span>...<span class="br0">)</span>.</span></span> 创建</p>
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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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<div class="cpp source-cpp">
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<pre class="de1"><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">struct</span> unwrap_refwrapper
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<span class="br0">{</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="br0">}</span><span class="sy4">;</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">struct</span> unwrap_refwrapper<span class="sy1"><</span><a href="cpp-utility-functional-reference_wrapper.html"><span class="kw1068">std::<span class="me2">reference_wrapper</span></span></a><span class="sy1"><</span>T<span class="sy1">>></span>
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<span class="br0">{</span>
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<span class="kw1">using</span> type <span class="sy1">=</span> T<span class="sy3">&</span><span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</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">using</span> unwrap_decay_t <span class="sy1">=</span> <span class="kw1">typename</span> unwrap_refwrapper<span class="sy1"><</span><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>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span><span class="sy4">;</span>
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<span class="co1">// 或使用 std::unwrap_ref_decay_t （ C++20 起）</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span>... <span class="me1">Types</span><span class="sy1">></span>
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<span class="kw4">constexpr</span> <span class="co1">// since C++14</span>
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<a href="cpp-utility-tuple.html"><span class="kw1098">std::<span class="me2">tuple</span></span></a><span class="sy1"><</span>unwrap_decay_t<span class="sy1"><</span>Types<span class="sy1">></span>...<span class="sy1">></span> make_tuple<span class="br0">(</span>Types<span class="sy3">&&</span>... <span class="me1">args</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> <a href="cpp-utility-tuple.html"><span class="kw1098">std::<span class="me2">tuple</span></span></a><span class="sy1"><</span>unwrap_decay_t<span class="sy1"><</span>Types<span class="sy1">></span>...<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>Types<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></pre></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 <tuple></span>
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<span class="co2">#include <functional></span>
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<a href="cpp-utility-tuple.html"><span class="kw1098">std::<span class="me2">tuple</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, <span class="kw4">int</span><span class="sy1">></span> f<span class="br0">(</span><span class="br0">)</span> <span class="co1">// 此函数返回多值</span>
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<span class="br0">{</span>
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<span class="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">5</span><span class="sy4">;</span>
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<span class="kw1">return</span> std<span class="sy4">::</span><span class="me2">make_tuple</span><span class="br0">(</span>x, <span class="nu0">7</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// return {x,7}; 于 C++17</span>
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<span class="br0">}</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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<span class="co1">// 异类 tuple 构造</span>
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<span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
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<span class="kw4">auto</span> t <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">make_tuple</span><span class="br0">(</span><span class="nu0">10</span>, <span class="st0">"Test"</span>, <span class="nu16">3.14</span>, <a href="cpp-utility-functional-ref.html"><span class="kw1069">std::<span class="me2">ref</span></span></a><span class="br0">(</span>n<span class="br0">)</span>, n<span class="br0">)</span><span class="sy4">;</span>
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n <span class="sy1">=</span> <span class="nu0">7</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">"The value of t is "</span> <span class="sy1"><<</span> <span class="st0">"("</span>
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<span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span><span class="nu0">0</span><span class="sy1">></span><span class="br0">(</span>t<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">", "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span><span class="nu0">1</span><span class="sy1">></span><span class="br0">(</span>t<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">", "</span>
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<span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span><span class="nu0">2</span><span class="sy1">></span><span class="br0">(</span>t<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">", "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span><span class="nu0">3</span><span class="sy1">></span><span class="br0">(</span>t<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">", "</span>
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<span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span><span class="nu0">4</span><span class="sy1">></span><span class="br0">(</span>t<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">")<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="co1">// 返回多值的函数</span>
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<span class="kw4">int</span> a, b<span class="sy4">;</span>
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<a href="cpp-utility-tuple-tie.html"><span class="kw1112">std::<span class="me2">tie</span></span></a><span class="br0">(</span>a, b<span class="br0">)</span> <span class="sy1">=</span> f<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> a <span class="sy1"><<</span> <span class="st0">" "</span> <span class="sy1"><<</span> b <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>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">The value of t is (10, Test, 3.14, 7, 1)
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5 7</pre></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-tuple-tie.html"><span class="t-lines"><span>tie</span></span></a></div>
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</div>
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</td>
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<td>创建左值引用的 <code>tuple</code>，或将 tuple 解包为独立对象<br />
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<span class="t-mark">(函数模板)</span></td>
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</tr>
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<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-tuple-forward_as_tuple.html"><span class="t-lines"><span>forward_as_tuple</span></span></a></div>
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</div>
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</td>
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<td>创建<a href="cpp-language-reference.html#.E8.BD.AC.E5.8F.91.E5.BC.95.E7.94.A8">转发引用</a>的 <code>tuple</code><br />
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<span class="t-mark">(函数模板)</span></td>
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<tr class="t-dsc">
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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-tuple-tuple_cat.html"><span class="t-lines"><span>tuple_cat</span></span></a></div>
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</div>
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</td>
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<td>通过连接任意数量的元组来创建一个<code>tuple</code><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-apply.html"><span class="t-lines"><span>apply</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
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</div>
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</td>
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<td>以一个实参的元组来调用函数<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-unwrap_reference.html"><span class="t-lines"><span>unwrap_reference</span><span>unwrap_ref_decay</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><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>获取包装于 <span class="t-lc"><a href="cpp-utility-functional-reference_wrapper.html">std::reference_wrapper</a></span> 的引用类型<br />
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<span class="t-mark">(类模板)</span></td>
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</table>
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</div>
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<div class="visualClear"></div>
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