189 lines
16 KiB
HTML
189 lines
16 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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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>std::apply</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_apply 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>apply</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-tuple.html"><tuple></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> F, <span class="kw1">class</span> Tuple<span class="sy1">></span><br />
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<span class="kw4">constexpr</span> decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span> apply<span class="br0">(</span>F<span class="sy3">&&</span> f, Tuple<span class="sy3">&&</span> t<span class="br0">)</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-cxx17">(C++17 起)</span></td>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<td></td>
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<p>以参数的元组调用<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> 对象 <code>f</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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<tr class="t-par">
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<td>f</td>
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<td>-</td>
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<td>要调用的<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> 对象</td>
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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>以其元素为 <code>f</code> 的参数的元组</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><code>f</code> 所返回的值。</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>元组不必是 <span class="t-lc"><a href="cpp-utility-tuple.html">std::tuple</a></span> ，可以为任何支持 <span class="t-lc">std::get</span> 和 <span class="t-lc">std::tuple_size</span> 的类型所替代；特别是可以用 <span class="t-lc"><a href="cpp-container-array.html">std::array</a></span> 和 <span class="t-lc"><a href="cpp-utility-pair.html">std::pair</a></span> 。</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">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> F, <span class="kw1">class</span> Tuple, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a>... <span class="me1">I</span><span class="sy1">></span>
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<span class="kw4">constexpr</span> decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span> apply_impl<span class="br0">(</span>F<span class="sy3">&&</span> f, Tuple<span class="sy3">&&</span> t, <a href="cpp-utility-integer_sequence.html"><span class="kw1107">std::<span class="me2">index_sequence</span></span></a><span class="sy1"><</span>I...<span class="sy1">></span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="co1">// 此实现从 C++20 起合法（经由 P1065R2 ）</span>
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<span class="co1">// C++17 中，实际上在此需要 std::invoke 的 constexpr 对应</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><a href="cpp-utility-forward.html"><span class="kw1116">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>F<span class="sy1">></span><span class="br0">(</span>f<span class="br0">)</span>, std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span>I<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>Tuple<span class="sy1">></span><span class="br0">(</span>t<span class="br0">)</span><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="br0">}</span> <span class="co1">// namespace detail</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> F, <span class="kw1">class</span> Tuple<span class="sy1">></span>
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<span class="kw4">constexpr</span> decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span> apply<span class="br0">(</span>F<span class="sy3">&&</span> f, Tuple<span class="sy3">&&</span> t<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> detail<span class="sy4">::</span><span class="me2">apply_impl</span><span class="br0">(</span>
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<a href="cpp-utility-forward.html"><span class="kw1116">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>F<span class="sy1">></span><span class="br0">(</span>f<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>Tuple<span class="sy1">></span><span class="br0">(</span>t<span class="br0">)</span>,
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<a href="cpp-utility-integer_sequence.html"><span class="kw1109">std::<span class="me2">make_index_sequence</span></span></a><span class="sy1"><</span><a href="cpp-utility-tuple-tuple_size.html"><span class="kw1102">std::<span class="me2">tuple_size_v</span></span></a><span class="sy1"><</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>Tuple<span class="sy1">>>></span><span class="br0">{</span><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 <utility></span>
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<span class="kw4">int</span> add<span class="br0">(</span><span class="kw4">int</span> first, <span class="kw4">int</span> second<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> first <span class="sy2">+</span> second<span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
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T add_generic<span class="br0">(</span>T first, T second<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> first <span class="sy2">+</span> second<span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw4">auto</span> add_lambda <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">auto</span> first, <span class="kw4">auto</span> second<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> first <span class="sy2">+</span> second<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span>... <span class="me1">Ts</span><span class="sy1">></span>
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<a href="cpp-io-basic_ostream.html"><span class="kw1728">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> operator<span class="sy1"><<</span><span class="br0">(</span><a href="cpp-io-basic_ostream.html"><span class="kw1728">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> os, <a href="cpp-utility-tuple.html"><span class="kw1098">std::<span class="me2">tuple</span></span></a><span class="sy1"><</span>Ts...<span class="sy1">></span> <span class="kw4">const</span><span class="sy3">&</span> theTuple<span class="br0">)</span>
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<span class="br0">{</span>
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std<span class="sy4">::</span><span class="me2">apply</span>
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<span class="br0">(</span>
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<span class="br0">[</span><span class="sy3">&</span>os<span class="br0">]</span><span class="br0">(</span>Ts <span class="kw4">const</span><span class="sy3">&</span>... <span class="me1">tupleArgs</span><span class="br0">)</span>
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<span class="br0">{</span>
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os <span class="sy1"><<</span> <span class="st0">'['</span><span class="sy4">;</span>
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<a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> n<span class="br0">{</span><span class="nu0">0</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="br0">(</span><span class="br0">(</span>os <span class="sy1"><<</span> tupleArgs <span class="sy1"><<</span> <span class="br0">(</span><span class="sy2">++</span>n <span class="sy3">!</span><span class="sy1">=</span> sizeof...<span class="br0">(</span>Ts<span class="br0">)</span> <span class="sy4">?</span> <span class="st0">", "</span> <span class="sy4">:</span> <span class="st0">""</span><span class="br0">)</span><span class="br0">)</span>, ...<span class="br0">)</span><span class="sy4">;</span>
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os <span class="sy1"><<</span> <span class="st0">']'</span><span class="sy4">;</span>
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<span class="br0">}</span>, theTuple
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<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> os<span class="sy4">;</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">// OK</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> std<span class="sy4">::</span><span class="me2">apply</span><span class="br0">(</span>add, <a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="br0">(</span><span class="nu0">1</span>, <span class="nu0">2</span><span class="br0">)</span><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">// 错误：无法推导函数类型</span>
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<span class="co1">// std::cout << std::apply(add_generic, std::make_pair(2.0f, 3.0f)) << '\n'; </span>
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<span class="co1">// OK</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> std<span class="sy4">::</span><span class="me2">apply</span><span class="br0">(</span>add_lambda, <a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="br0">(</span><span class="nu17">2.0f</span>, <span class="nu17">3.0f</span><span class="br0">)</span><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">// 进阶示例</span>
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<a href="cpp-utility-tuple.html"><span class="kw1098">std::<span class="me2">tuple</span></span></a> myTuple<span class="br0">(</span><span class="nu0">25</span>, <span class="st0">"Hello"</span>, <span class="nu17">9.31f</span>, <span class="st0">'c'</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> myTuple <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">3
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5
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[25, Hello, 9.31, c]</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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<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-make_tuple.html"><span class="t-lines"><span>make_tuple</span></span></a></div>
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</div>
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</td>
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<td>创建一个 <code>tuple</code> 对象，其类型根据各实参类型定义<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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<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>
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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-make_from_tuple.html"><span class="t-lines"><span>make_from_tuple</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>以一个实参元组构造对象<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-functional-invoke.html"><span class="t-lines"><span>invoke</span><span>invoke_r</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><span class="t-mark-rev t-since-cxx23">(C++23)</span></span></span></div>
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</div>
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</td>
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<td>以给定实参<span class="t-rev-inl t-since-cxx23"><span>和可能指定的返回类型</span> <span><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></span></span>调用任意<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> 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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</body>
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