160 lines
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160 lines
12 KiB
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<title>std::make_from_tuple</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_make_from_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_from_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></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> T, <span class="kw1">class</span> Tuple<span class="sy1">></span><br />
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<span class="kw4">constexpr</span> T make_from_tuple<span class="br0">(</span>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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<p>构造 <code>T</code> 类型对象，以元组 <code>t</code> 的元素为构造函数的参数。</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>t</td>
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<td>-</td>
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<td>元组，其元素被用作 <code>T</code> 构造函数的参数</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>被构造的 <code>T</code> 对象。</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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<p>因为<a href="cpp-language-copy_elision.html">受保证的复制消除</a>，不需要 <code>T</code> 为可移动。</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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<table class="eq-fun-cpp-table">
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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="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> T make_from_tuple_impl<span class="br0">(</span> 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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static_assert<span class="br0">(</span><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span>T,
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decltype<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-declval.html"><span class="kw1119">std::<span class="me2">declval</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="br0">)</span>...<span class="sy1">></span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> T<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> T, <span class="kw1">class</span> Tuple<span class="sy1">></span>
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<span class="kw4">constexpr</span> T make_from_tuple<span class="br0">(</span> 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">make_from_tuple_impl</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>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="kw1">struct</span> Foo <span class="br0">{</span>
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Foo<span class="br0">(</span><span class="kw4">int</span> first, <span class="kw4">float</span> second, <span class="kw4">int</span> third<span class="br0">)</span> <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> first <span class="sy1"><<</span> <span class="st0">", "</span> <span class="sy1"><<</span> second <span class="sy1"><<</span> <span class="st0">", "</span> <span class="sy1"><<</span> third <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>
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<span class="br0">}</span><span class="sy4">;</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="kw4">auto</span> tuple <span class="sy1">=</span> <a href="cpp-utility-tuple-make_tuple.html"><span class="kw1111">std::<span class="me2">make_tuple</span></span></a><span class="br0">(</span><span class="nu0">42</span>, <span class="nu17">3.14f</span>, <span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">make_from_tuple</span><span class="sy1"><</span>Foo<span class="sy1">></span><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>tuple<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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</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">42, 3.14, 0</pre></div>
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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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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue3528">LWG 3528</a></td>
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<td>C++17</td>
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<td>在 1-元组的情况下允许含有 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">reinterpret_cast</span></span></span> 等的转型</td>
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<td>不允许</td>
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</tr>
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</table>
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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-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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<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>
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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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</table>
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</div>
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<div class="visualClear"></div>
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