293 lines
19 KiB
HTML
293 lines
19 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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<title>std::tuple_size</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_tuple_size skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<div id="content"><a id="top"></a>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>tuple_size</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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<table class="t-dcl-begin">
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<tr class="t-dsc-header">
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<td>
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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>
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<td></td>
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<td></td>
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</tr>
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<tr class="t-dsc-header">
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<td>
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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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</td>
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<td></td>
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<td></td>
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</tr>
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<tr class="t-dsc-header">
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<td>
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<div>定义于头文件 <code><a href="cpp-header-utility.html"><utility></a></code></div>
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</td>
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<td></td>
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<td></td>
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</tr>
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<tr class="t-dsc-header">
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<td>
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<div>定义于头文件 <code><a href="cpp-header-ranges.html"><ranges></a></code></div>
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</td>
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<td></td>
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<td>
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<div><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></div>
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</td>
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</tr>
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<tr class="t-dcl t-since-cxx11">
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<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="sy1">></span><br />
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<span class="kw1">struct</span> tuple_size<span class="sy4">;</span> <span class="co1">// 不定义</span></span></div>
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</td>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx11">
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<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="sy1">></span><br /></span>
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<p><span class="kw1">struct</span> tuple_size<span class="sy1"><</span> <span class="kw4">const</span> T <span class="sy1">></span><br /></p>
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<span class="sy4">:</span> <a href="cpp-types-integral_constant.html"><span class="kw647">std::<span class="me2">integral_constant</span></span></a><span class="sy1"><</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a>, <a href="cpp-utility-tuple-tuple_size.html"><span class="kw1101">std::<span class="me2">tuple_size</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span><span class="sy1">></span> <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span></div>
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</td>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx11">
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<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="sy1">></span><br /></span>
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<p><span class="kw1">struct</span> tuple_size<span class="sy1"><</span> <span class="kw4">volatile</span> T <span class="sy1">></span><br /></p>
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<span class="sy4">:</span> <a href="cpp-types-integral_constant.html"><span class="kw647">std::<span class="me2">integral_constant</span></span></a><span class="sy1"><</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a>, <a href="cpp-utility-tuple-tuple_size.html"><span class="kw1101">std::<span class="me2">tuple_size</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span><span class="sy1">></span> <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span></div>
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</td>
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<td>(3)</td>
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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">(C++20 中弃用)</span></td>
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<tr class="t-dcl t-since-cxx11">
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<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="sy1">></span><br /></span>
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<p><span class="kw1">struct</span> tuple_size<span class="sy1"><</span> <span class="kw4">const</span> <span class="kw4">volatile</span> T <span class="sy1">></span><br /></p>
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<span class="sy4">:</span> <a href="cpp-types-integral_constant.html"><span class="kw647">std::<span class="me2">integral_constant</span></span></a><span class="sy1"><</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a>, <a href="cpp-utility-tuple-tuple_size.html"><span class="kw1101">std::<span class="me2">tuple_size</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span><span class="sy1">></span> <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span></div>
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</td>
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<td>(4)</td>
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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">(C++20 中弃用)</span></td>
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</tr>
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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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</tr>
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</tbody>
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</table>
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<p>提供作为常量表达式的元组式类型中元素数的访问。</p>
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<div class="t-li1"><span class="t-li">1)</span> 不定义主模板。要求显式（全）或部分特化以令类型为元组式。</div>
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<div class="t-li1"><span class="t-li">2-4)</span> 对 cv 限定类型的特化默认复用对应 cv 无限定版本的 <code>value</code> 。</div>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx17">
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<td>
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<p><code>std::tuple_size</code> 与核心语言交互：它能在元组式情况中提供<a href="cpp-language-structured_binding.html">结构化绑定</a>支持。</p>
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<p><span class="t-v">(2-4)</span> 为 SFINAE 友好：若在作为不求值操作数时 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-tuple-tuple_size.html"><span class="kw1101">std::<span class="me2">tuple_size</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span></span></span> 为非良构，则它们不提供成员 <code>value</code> 。如同在与 <code>tuple_size</code> 和 <code>T</code> 无关的场景进行访问检查。仅考虑表达式的立即语境的合法性。这允许</p>
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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 <utility></span>
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<span class="kw1">struct</span> X <span class="br0">{</span> <span class="kw4">int</span> a, b<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">const</span> <span class="kw4">auto</span> <span class="br0">[</span>x, y<span class="br0">]</span> <span class="sy1">=</span> X<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 结构化绑定声明首先尝试 tuple_size<const X> ，而它尝试</span>
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<span class="co1">// tuple_size<X>::value ，然后遇到软错误，绑定到公开数据成员</span></pre></div>
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</div>
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</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E7.89.B9.E5.8C.96">特化</span></h3>
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<p>标准库对标准库类型提供下列特化：</p>
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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-tuple_size.html"><span class="t-lines"><span>std::tuple_size<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>在编译时获得 <code>tuple</code> 的大小<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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<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_size.html"><span class="t-lines"><span>std::tuple_size<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> 的大小<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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<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-container-array-tuple_size.html"><span class="t-lines"><span>std::tuple_size<span class="t-dsc-small"><std::array></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>array</code> 的大小<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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<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-ranges-subrange-tuple_size.html"><span class="t-lines"><span>std::tuple_size<span class="t-dsc-small"><std::ranges::subrange></span></span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>获得 <span class="t-lc">std::ranges::subrange</span> 的组分数量<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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</table>
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<p><code>std::tuple_size</code> 的所有特化满足<i>基特征</i>为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-integral_constant.html"><span class="kw647">std::<span class="me2">integral_constant</span></span></a><span class="sy1"><</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a>, N<span class="sy1">></span></span></span> 某个 <code>N</code> 的<a href="cpp-named_req-UnaryTypeTrait.html"><span style="font-style:italic">一元类型特征</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(UnaryTypeTrait)</span></a> 。</p>
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<p>用户可以对程序定义类型特化 <code>std::tuple_size</code> 以令它们为元组式。程序定义特化必须满足上述要求。</p>
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<p>通常只需要定制对 cv 无限定类型的特化。</p>
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<h3><span class="mw-headline" id=".E8.BE.85.E5.8A.A9.E5.8F.98.E9.87.8F.E6.A8.A1.E6.9D.BF">辅助变量模板</span></h3>
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<table class="t-dcl-begin">
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<tbody>
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<tr class="t-dsc-header">
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<td>
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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>
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<td></td>
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<td></td>
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</tr>
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<tr class="t-dcl t-since-cxx17">
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<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="sy1">></span><br />
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<span class="kw1">inline</span> <span class="kw4">constexpr</span> <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> tuple_size_v <span class="sy1">=</span> tuple_size<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span><span class="sy4">;</span></span></div>
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</td>
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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>
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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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</tr>
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</tbody>
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</table>
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<div class="t-inherited">
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<h2><span class="mw-headline" id=".E7.BB.A7.E6.89.BF.E8.87.AA_std::integral_constant">继承自 <a href="cpp-types-integral_constant.html"><span style="font-size:0.7em; line-height:130%">std::</span>integral_constant</a></span></h2>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.B8.B8.E9.87.8F">成员常量</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><span class="t-lines"><span>value</span></span></div>
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<div><span class="t-lines"><span><span class="t-cmark">[静态]</span></span></span></div>
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</div>
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</td>
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<td>对于标准特化，为元组式类型 <code>T</code> 的元素数<br />
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<span class="t-mark">(公开静态成员常量)</span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">成员函数</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><span class="t-lines"><span>operator std::size_t</span></span></div>
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</div>
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</td>
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<td>转换对象为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a></span></span> ，返回 <code>value</code><br />
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<span class="t-mark">(公开成员函数)</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><span class="t-lines"><span>operator()</span></span></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx14">(C++14)</span></span></span></div>
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</div>
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</td>
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<td>返回 <code>value</code><br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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</table>
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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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<table class="t-dsc-begin">
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<tr class="t-dsc-hitem">
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<td>类型</td>
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<td>定义</td>
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</tr>
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<tr class="t-dsc">
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<td><code>value_type</code></td>
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<td><code>std::size_t</code></td>
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</tr>
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<tr class="t-dsc">
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<td><code>type</code></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-integral_constant.html"><span class="kw647">std::<span class="me2">integral_constant</span></span></a><span class="sy1"><</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a>, value<span class="sy1">></span></span></span></td>
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</tr>
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</table>
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</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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<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style="">
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<tr>
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<td class="mbox-empty-cell"></td>
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<td class="mbox-text" style="">本节未完成<br />
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原因：暂无示例</td>
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</tr>
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</table>
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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/issue2212">LWG 2212</a></td>
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<td>C++11</td>
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<td>未在某些头文件中要求对 cv 类型的特化，这导致歧义</td>
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<td>已要求</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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<tr class="t-dsc">
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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>
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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_element.html"><span class="t-lines"><span>tuple_element</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>获得元组式类型的元素类型<br />
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<span class="t-mark">(类模板)</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-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>
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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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</div>
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</div>
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</body>
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</html>
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