198 lines
16 KiB
HTML
198 lines
16 KiB
HTML
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<title>std::variant_size, std::variant_size_v</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_variant_variant_size 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>variant_size, <span style="font-size:0.7em; line-height:130%">std::</span>variant_size_v</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-variant.html">variant</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-variant.html"><variant></a></code></div>
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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> variant_size<span class="sy4">;</span> <span class="coMULTI">/* undefined */</span></span></div>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</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 /></span>
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<p><span class="kw1">struct</span> variant_size<span class="sy1"><</span><a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1"><</span>Types...<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>, sizeof...<span class="br0">(</span>Types<span class="br0">)</span><span class="sy1">></span> <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span></div>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</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> T<span class="sy1">></span> <span class="kw1">class</span> variant_size<span class="sy1"><</span><span class="kw4">const</span> T<span class="sy1">></span><span class="sy4">;</span></span></div>
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<td>(3)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</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> T<span class="sy1">></span> <span class="kw1">class</span> variant_size<span class="sy1"><</span><span class="kw4">volatile</span> T<span class="sy1">></span><span class="sy4">;</span><br />
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span> <span class="kw1">class</span> variant_size<span class="sy1"><</span><span class="kw4">const</span> <span class="kw4">volatile</span> T<span class="sy1">></span><span class="sy4">;</span></span></div>
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<td>(3)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span><br />
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<span class="t-mark">(C++20 中弃用)</span></td>
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<p>提供作为编译时常量表达式，对可有 cv 限定的 <code>variant</code> 中可选项数量的访问。</p>
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<p>正式地，</p>
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<div class="t-li1"><span class="t-li">2)</span> 满足基特征为 <code>std::integral_constant<std::size_t, sizeof...(Types)></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> 要求</div>
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<div class="t-li1"><span class="t-li">3)</span> 满足基特征为 <code>std::integral_constant<std::size_t, variant_size<T>></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> 要求</div>
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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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<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> variant_size_v <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">variant_size</span><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 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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<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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<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><code>sizeof...(Types)</code><br />
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<span class="t-mark">(公开静态成员常量)</span></td>
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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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<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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<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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<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>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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<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>类型</td>
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<td>定义</td>
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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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<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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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p>所有 <code>std::variant_size</code> 的特化都满足以某些 <code>N</code> 的 <code>std::integral_constant<std::size_t, N></code> 为<i>基特征</i>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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<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 <any></span>
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<span class="co2">#include <cstdio></span>
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<span class="co2">#include <variant></span>
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static_assert<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">variant_size_v</span><span class="sy1"><</span><a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1"><>></span> <span class="sy1">==</span> <span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">variant_size_v</span><span class="sy1"><</span><a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">>></span> <span class="sy1">==</span> <span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">variant_size_v</span><span class="sy1"><</span><a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, <span class="kw4">int</span><span class="sy1">>></span> <span class="sy1">==</span> <span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">variant_size_v</span><span class="sy1"><</span><a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, <span class="kw4">int</span>, <span class="kw4">int</span><span class="sy1">>></span> <span class="sy1">==</span> <span class="nu0">3</span><span class="br0">)</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">variant_size_v</span><span class="sy1"><</span><a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, <span class="kw4">float</span>, <span class="kw4">double</span><span class="sy1">>></span> <span class="sy1">==</span> <span class="nu0">3</span><span class="br0">)</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">variant_size_v</span><span class="sy1"><</span><a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1"><</span><a href="cpp-utility-variant-monostate.html"><span class="kw1093">std::<span class="me2">monostate</span></span></a>, <span class="kw4">void</span><span class="sy1">>></span> <span class="sy1">==</span> <span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">variant_size_v</span><span class="sy1"><</span><a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1"><</span><span class="kw4">const</span> <span class="kw4">int</span>, <span class="kw4">const</span> <span class="kw4">float</span><span class="sy1">>></span> <span class="sy1">==</span> <span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">variant_size_v</span><span class="sy1"><</span><a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1"><</span><a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1"><</span><a href="cpp-utility-any.html"><span class="kw1071">std::<span class="me2">any</span></span></a><span class="sy1">>>></span> <span class="sy1">==</span> <span class="nu0">1</span><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> <span class="br0">{</span> <a href="cpp-io-c-puts.html"><span class="kw1828">std::<span class="me2">puts</span></span></a><span class="br0">(</span><span class="st0">"All static assertions passed."</span><span class="br0">)</span><span class="sy4">;</span> <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">All static assertions passed.</pre></div>
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</div>
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</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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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-variant-variant_alternative.html"><span class="t-lines"><span>variant_alternative</span><span>variant_alternative_t</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>在编译时获得以其下标指定的可选项的类型<br />
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<span class="t-mark">(类模板)</span> <span class="t-mark">(别名模板)</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-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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</table>
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</div>
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<div class="visualClear"></div>
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</div>
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</body>
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