144 lines
9.1 KiB
HTML
144 lines
9.1 KiB
HTML
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<title>std::forward_as_tuple</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_tuple_forward_as_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>forward_as_tuple</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>‎ | <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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<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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tuple<span class="sy1"><</span>Types<span class="sy3">&&</span>...<span class="sy1">></span> forward_as_tuple<span class="br0">(</span> Types<span class="sy3">&&</span>... <span class="me1">args</span> <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></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>... <span class="me1">Types</span> <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> tuple<span class="sy1"><</span>Types<span class="sy3">&&</span>...<span class="sy1">></span> forward_as_tuple<span class="br0">(</span> Types<span class="sy3">&&</span>... <span class="me1">args</span> <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></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>构造到 <code>args</code> 中参数的，适于转发成函数参数的引用的 tuple 。该 tuple 在以右值为参数时拥有右值引用数据成员，否则拥有左值引用数据成员。</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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<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>如同以 <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>Types<span class="sy3">&&</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></span> 创建的 <span class="t-lc"><a href="cpp-utility-tuple.html">std::tuple</a></span> 对象。</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>若参数是临时量，则 <code>forward_as_tuple</code> 不延续其生存期；必须在完整表达式结尾前使用它们。</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-live-link"></div>
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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 <map></span>
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<span class="co2">#include <tuple></span>
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<span class="co2">#include <string></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-map.html"><span class="kw1280">std::<span class="me2">map</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, <a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="sy1">></span> m<span class="sy4">;</span>
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m.<span class="me1">emplace</span><span class="br0">(</span><a href="cpp-utility-piecewise_construct.html"><span class="kw1106">std::<span class="me2">piecewise_construct</span></span></a>,
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std<span class="sy4">::</span><span class="me2">forward_as_tuple</span><span class="br0">(</span><span class="nu0">10</span><span class="br0">)</span>,
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std<span class="sy4">::</span><span class="me2">forward_as_tuple</span><span class="br0">(</span><span class="nu0">20</span>, <span class="st0">'a'</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">"m[10] = "</span> <span class="sy1"><<</span> m<span class="br0">[</span><span class="nu0">10</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">// 下面是错误：它产生保有二个悬垂引用的 std::tuple<int&&, char&&></span>
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<span class="co1">//</span>
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<span class="co1">// auto t = std::forward_as_tuple(20, 'a');</span>
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<span class="co1">// m.emplace(std::piecewise_construct, std::forward_as_tuple(10), t);</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">m[10] = aaaaaaaaaaaaaaaaaaaa</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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<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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</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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<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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</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 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 class="t-dsc">
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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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</table>
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</div>
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<div class="visualClear"></div>
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