klarkxy/hustoj
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity
Files
master
hustoj/web/cppreference-zh/chmhelp/cpp-utility-pair-tuple_element.html
klarkxy 196e706fe4 上传cppreference
2025-08-01 12:57:54 +08:00

191 lines
14 KiB
HTML
Raw Permalink Blame History

<!DOCTYPE html>
<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
<head><meta http-equiv="x-ua-compatible" content="ie=edge">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>std::tuple_element&lt;std::pair&gt;</title>
<link rel="stylesheet" href="ext.css" />
<link rel="stylesheet" href="site_modules.css" />
</head>
<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_pair_tuple_element skin-cppreference2 action-view cpp-navbar">
<div id="cpp-content-base">
<div id="content"><a id="top"></a>
<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>tuple_element<small>&lt;std::pair&gt;</small></h1>
<div id="bodyContent">
<div id="contentSub"><span class="subpages">&lt; <a href="cpp.html">cpp</a>&lrm; | <a href="cpp-utility.html">utility</a>&lrm; | <a href="cpp-utility-pair.html">pair</a></span></div>
<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
<table class="t-dcl-begin">
<tbody>
<tr class="t-dsc-header">
<td>
<div>&#23450;&#20041;&#20110;&#22836;&#25991;&#20214; <code><a href="cpp-header-utility.html">&lt;utility&gt;</a></code></div>
</td>
<td></td>
<td></td>
</tr>
<tr class="t-dcl t-since-cxx11">
<td>
<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</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> T1, <span class="kw1">class</span> T2 <span class="sy1">&gt;</span><br />
<span class="kw1">struct</span> tuple_element<span class="sy1">&lt;</span>I, <a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1">&lt;</span>T1, T2<span class="sy1">&gt;&gt;</span><span class="sy4">;</span></span></div>
</td>
<td class="t-dcl-nopad"></td>
<td><span class="t-mark-rev t-since-cxx11">(C++11 &#36215;)</span></td>
</tr>
<tr class="t-dcl-sep">
<td></td>
<td></td>
<td></td>
</tr>
</tbody>
</table>
<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-tuple-tuple_element.html"><span class="kw1099">std::<span class="me2">tuple_element</span></span></a></span></span> &#23545; pair &#30340;&#37096;&#20998;&#29305;&#21270;&#20351;&#29992;&#31867; tuple &#35821;&#27861;&#65292;&#25552;&#20379; pair &#20803;&#32032;&#31867;&#22411;&#30340;&#32534;&#35793;&#26102;&#35775;&#38382;&#12290;&#33509; <code>I &gt;= 2</code> &#21017;&#31243;&#24207;&#20026;&#30149;&#24335;&#12290;</p>
<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E7.B1.BB.E5.9E.8B">&#25104;&#21592;&#31867;&#22411;</span></h3>
<table class="t-dsc-begin">
<tr class="t-dsc-hitem">
<td>&#25104;&#21592;&#31867;&#22411;</td>
<td>&#23450;&#20041;</td>
</tr>
<tr class="t-dsc">
<td><code>type</code></td>
<td><code>I == 0</code> &#21017;&#20026; <code>T1</code><br />
<code>I == 1</code> &#21017;&#20026; <code>T2</code></td>
</tr>
</table>
<h3><span class="mw-headline" id=".E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0">&#21487;&#33021;&#30340;&#23454;&#29616;</span></h3>
<table class="eq-fun-cpp-table">
<tr>
<td>
<div dir="ltr" class="mw-geshi" style="text-align: left;">
<div class="cpp source-cpp">
<pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> I, <span class="kw1">typename</span> T<span class="sy1">&gt;</span>
<span class="kw1">struct</span> tuple_element<span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> I, <span class="kw1">typename</span> T1, <span class="kw1">typename</span> T2<span class="sy1">&gt;</span>
<span class="kw1">struct</span> tuple_element<span class="sy1">&lt;</span>I, <a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1">&lt;</span>T1,T2<span class="sy1">&gt;</span> <span class="sy1">&gt;</span>
<span class="br0">{</span>
static_assert<span class="br0">(</span>I <span class="sy1">&lt;</span> <span class="nu0">2</span>, <span class="st0">&quot;std::pair has only 2 elements!&quot;</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T1, <span class="kw1">typename</span> T2<span class="sy1">&gt;</span>
<span class="kw1">struct</span> tuple_element<span class="sy1">&lt;</span><span class="nu0">0</span>, <a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1">&lt;</span>T1,T2<span class="sy1">&gt;</span> <span class="sy1">&gt;</span>
<span class="br0">{</span>
<span class="kw1">using</span> type <span class="sy1">=</span> T1<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T1, <span class="kw1">typename</span> T2<span class="sy1">&gt;</span>
<span class="kw1">struct</span> tuple_element<span class="sy1">&lt;</span><span class="nu0">1</span>, <a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1">&lt;</span>T1,T2<span class="sy1">&gt;</span> <span class="sy1">&gt;</span>
<span class="br0">{</span>
<span class="kw1">using</span> type <span class="sy1">=</span> T2<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div>
</div>
</td>
</tr>
</table>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">&#31034;&#20363;</span></h3>
<div class="t-example">
<div class="t-example-live-link"></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;">
<div class="cpp source-cpp">
<pre class="de1"><span class="co2">#include &lt;tuple&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;string&gt;</span>
<span class="kw1">namespace</span> detail <span class="br0">{</span>
<span class="kw1">template</span> <span class="sy1">&lt;</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a><span class="sy1">&gt;</span>
<span class="kw1">struct</span> index_tag <span class="br0">{</span> <span class="kw1">explicit</span> <span class="kw4">constexpr</span> index_tag<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">&gt;</span>
<span class="kw4">constexpr</span> T get_val_dispatch<span class="br0">(</span><a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1">&lt;</span>T, U<span class="sy1">&gt;</span> <span class="kw4">const</span><span class="sy3">&amp;</span> pair, index_tag<span class="sy1">&lt;</span><span class="nu0">0</span><span class="sy1">&gt;</span><span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">return</span> pair.<span class="me1">first</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">&gt;</span>
<span class="kw4">constexpr</span> U get_val_dispatch<span class="br0">(</span><a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1">&lt;</span>T, U<span class="sy1">&gt;</span> <span class="kw4">const</span><span class="sy3">&amp;</span> pair, index_tag<span class="sy1">&lt;</span><span class="nu0">1</span><span class="sy1">&gt;</span><span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">return</span> pair.<span class="me1">second</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="br0">}</span> <span class="co1">// namespace detail</span>
<span class="kw1">template</span> <span class="sy1">&lt;</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> N, <span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">&gt;</span>
<span class="kw4">auto</span> <span class="kw4">constexpr</span> get_val<span class="br0">(</span><a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1">&lt;</span>T, U<span class="sy1">&gt;</span> <span class="kw4">const</span><span class="sy3">&amp;</span> pair<span class="br0">)</span>
<span class="sy2">-</span><span class="sy1">&gt;</span> <span class="kw1">typename</span> <a href="cpp-utility-tuple-tuple_element.html"><span class="kw1099">std::<span class="me2">tuple_element</span></span></a><span class="sy1">&lt;</span>N, <a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1">&lt;</span>T, U<span class="sy1">&gt;&gt;</span><span class="sy4">::</span><span class="me2">type</span>
<span class="br0">{</span>
<span class="kw1">return</span> detail<span class="sy4">::</span><span class="me2">get_val_dispatch</span><span class="br0">(</span>pair, detail<span class="sy4">::</span><span class="me2">index_tag</span><span class="sy1">&lt;</span>N<span class="sy1">&gt;</span><span class="br0">{</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<span class="kw4">auto</span> var <span class="sy1">=</span> <a href="cpp-utility-pair-make_pair.html"><span class="kw1097">std::<span class="me2">make_pair</span></span></a><span class="br0">(</span><span class="nu0">1</span>, <a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="br0">{</span><span class="st0">&quot;one&quot;</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> get_val<span class="sy1">&lt;</span><span class="nu0">0</span><span class="sy1">&gt;</span><span class="br0">(</span>var<span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot; = &quot;</span> <span class="sy1">&lt;&lt;</span> get_val<span class="sy1">&lt;</span><span class="nu0">1</span><span class="sy1">&gt;</span><span class="br0">(</span>var<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div>
</div>
<p>&#36755;&#20986;&#65306;</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;">
<div class="text source-text">
<pre class="de1">1 = one</pre></div>
</div>
</div>
<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">&#32570;&#38519;&#25253;&#21578;</span></h3>
<p>&#19979;&#21015;&#26356;&#25913;&#34892;&#20026;&#30340;&#32570;&#38519;&#25253;&#21578;&#36861;&#28335;&#22320;&#24212;&#29992;&#20110;&#20197;&#21069;&#20986;&#29256;&#30340; C++ &#26631;&#20934;&#12290;</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th>DR</th>
<th>&#24212;&#29992;&#20110;</th>
<th>&#20986;&#29256;&#26102;&#30340;&#34892;&#20026;</th>
<th>&#27491;&#30830;&#34892;&#20026;</th>
</tr>
<tr>
<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue2974">LWG 2974</a></td>
<td>C++11</td>
<td>&#36234;&#30028;&#19979;&#26631;&#20351;&#29992;&#19981;&#23450;&#20041;&#30340;&#21021;&#31561;&#27169;&#26495;</td>
<td>&#20351;&#20043;&#20026;&#38750;&#33391;&#26500;&#65288;&#30828;&#38169;&#35823;&#65289;</td>
</tr>
</table>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">&#21442;&#38405;</span></h3>
<table class="t-dsc-begin">
<tr class="t-dsc">
<td><a href="cpp-language-structured_binding.html">&#32467;&#26500;&#21270;&#32465;&#23450;</a> <span class="t-mark-rev t-since-cxx17">(C++17)</span></td>
<td>&#32465;&#23450;&#25351;&#23450;&#30340;&#21517;&#23383;&#21040;&#21021;&#22987;&#21270;&#22120;&#30340;&#23376;&#23545;&#35937;&#25110;&#20803;&#32452;&#20803;&#32032;</td>
</tr>
<tr class="t-dsc">
<td>
<div class="t-dsc-member-div">
<div><a href="cpp-utility-tuple-tuple_element.html"><span class="t-lines"><span>std::tuple_element<span class="t-dsc-small">&lt;std::tuple&gt;</span></span></span></a></div>
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
</div>
</td>
<td>&#33719;&#24471;&#25351;&#23450;&#20803;&#32032;&#30340;&#31867;&#22411;<br />
<span class="t-mark">(&#31867;&#27169;&#26495;&#29305;&#21270;)</span></td>
</tr>
<tr class="t-dsc">
<td>
<div class="t-dsc-member-div">
<div><a href="cpp-container-array-tuple_element.html"><span class="t-lines"><span>std::tuple_element<span class="t-dsc-small">&lt;std::array&gt;</span></span></span></a></div>
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
</div>
</td>
<td>&#33719;&#24471; <code>array</code> &#20803;&#32032;&#30340;&#31867;&#22411;<br />
<span class="t-mark">(&#31867;&#27169;&#26495;&#29305;&#21270;)</span></td>
</tr>
<tr class="t-dsc">
<td>
<div class="t-dsc-member-div">
<div><a href="cpp-utility-pair-tuple_size.html"><span class="t-lines"><span>std::tuple_size<span class="t-dsc-small">&lt;std::pair&gt;</span></span></span></a></div>
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
</div>
</td>
<td>&#33719;&#24471; <code>pair</code> &#30340;&#22823;&#23567;<br />
<span class="t-mark">(&#31867;&#27169;&#26495;&#29305;&#21270;)</span></td>
</tr>
</table>
</div>
<div class="visualClear"></div>
</div>
</div>
</div>
</body>
</html>
Reference in New Issue View Git Blame Copy Permalink