139 lines
10 KiB
HTML
139 lines
10 KiB
HTML
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<title>std::tuple_element<std::array></title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>tuple_element<span style="font-size:0.7em; line-height:130%"><std::array></span></h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-container.html">container</a>‎ | <a href="cpp-container-array.html">array</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-array.html"><array></a></code></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> I, <span class="kw1">class</span> T, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> N <span class="sy1">></span><br />
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<span class="kw1">struct</span> tuple_element<span class="sy1"><</span>I, <a href="cpp-container-array.html"><span class="kw1268">std::<span class="me2">array</span></span></a><span class="sy1"><</span>T, N<span class="sy1">></span> <span class="sy1">></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-cxx11">(C++11 起)</span></td>
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<p>使用类 tuple 接口，提供 array 元素类型的编译时带下标访问</p>
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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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<td>成员类型</td>
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<td>定义</td>
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<tr class="t-dsc">
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<td>type</td>
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<td>array 的元素类型</td>
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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><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> I, <span class="kw1">class</span> T<span class="sy1">></span>
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<span class="kw1">struct</span> tuple_element<span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> I, <span class="kw1">class</span> T, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> N<span class="sy1">></span>
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<span class="kw1">struct</span> tuple_element<span class="sy1"><</span>I, <a href="cpp-container-array.html"><span class="kw1268">std::<span class="me2">array</span></span></a><span class="sy1"><</span>T,N<span class="sy1">></span> <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="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</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 <array></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <tuple></span>
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<span class="co2">#include <type_traits></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">// 定义 array 并获取位于位置 0 的元素类型</span>
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<a href="cpp-container-array.html"><span class="kw1268">std::<span class="me2">array</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, <span class="nu0">10</span><span class="sy1">></span> data <span class="br0">{</span><span class="nu0">0</span>, <span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">3</span>, <span class="nu0">4</span>, <span class="nu0">5</span>, <span class="nu0">6</span>, <span class="nu0">7</span>, <span class="nu0">8</span>, <span class="nu0">9</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">using</span> T <span class="sy1">=</span> <a href="cpp-utility-tuple-tuple_element.html"><span class="kw1099">std::<span class="me2">tuple_element</span></span></a><span class="sy1"><</span><span class="nu0">0</span>, decltype<span class="br0">(</span>data<span class="br0">)</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span><span class="sy4">;</span> <span class="co1">// int</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <a href="cpp-io-manip-boolalpha.html"><span class="kw1764">std::<span class="me2">boolalpha</span></span></a><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <a href="cpp-types-is_same.html"><span class="kw588">std::<span class="me2">is_same</span></span></a><span class="sy1"><</span>T, <span class="kw4">int</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="kw4">const</span> <span class="kw4">auto</span> const_data <span class="sy1">=</span> data<span class="sy4">;</span>
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<span class="kw1">using</span> CT <span class="sy1">=</span> <a href="cpp-utility-tuple-tuple_element.html"><span class="kw1099">std::<span class="me2">tuple_element</span></span></a><span class="sy1"><</span><span class="nu0">0</span>, decltype<span class="br0">(</span>const_data<span class="br0">)</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span><span class="sy4">;</span> <span class="co1">// const int</span>
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<span class="co1">// tuple_element 的结果取决于仿 tuple 类型的 cv 限定</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <a href="cpp-types-is_same.html"><span class="kw588">std::<span class="me2">is_same</span></span></a><span class="sy1"><</span>T, CT<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span> <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="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <a href="cpp-types-is_same.html"><span class="kw588">std::<span class="me2">is_same</span></span></a><span class="sy1"><</span>CT, <span class="kw4">const</span> <span class="kw4">int</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span> <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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<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">true
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false
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true</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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<td><a href="cpp-language-structured_binding.html">结构化绑定</a> <span class="t-mark-rev t-since-cxx17">(C++17)</span></td>
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<td>绑定指定的名字到初始化器的子对象或元组元素</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_element.html"><span class="t-lines"><span>std::tuple_element<span class="t-dsc-small"><std::tuple></span></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>获得指定元素的类型<br />
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<span class="t-mark">(类模板特化)</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-pair-tuple_element.html"><span class="t-lines"><span>std::tuple_element<span class="t-dsc-small"><std::pair></span></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>获得 <code>pair</code> 中元素的类型<br />
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<span class="t-mark">(类模板特化)</span></td>
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</tr>
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</div>
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<div class="visualClear"></div>
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</div>
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