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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>variant_alternative, <span style="font-size:0.7em; line-height:130%">std::</span>variant_alternative_t</h1>
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<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-variant.html">variant</a></span></div>
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<div>&#23450;&#20041;&#20110;&#22836;&#25991;&#20214; <code><a href="cpp-header-variant.html">&lt;variant&gt;</a></code></div>
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<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> T<span class="sy1">&gt;</span><br />
<span class="kw1">struct</span> variant_alternative<span class="sy4">;</span> <span class="coMULTI">/* undefined */</span></span></div>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 &#36215;)</span></td>
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<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>... <span class="me1">Types</span><span class="sy1">&gt;</span><br />
<span class="kw1">struct</span> variant_alternative<span class="sy1">&lt;</span>I, variant<span class="sy1">&lt;</span>Types...<span class="sy1">&gt;&gt;</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 &#36215;)</span></td>
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<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> T<span class="sy1">&gt;</span> <span class="kw1">class</span> variant_alternative<span class="sy1">&lt;</span>I, <span class="kw4">const</span> T<span class="sy1">&gt;</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 &#36215;)</span></td>
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<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> T<span class="sy1">&gt;</span><br /></span>
<p><span class="kw1">class</span> variant_alternative<span class="sy1">&lt;</span>I, <span class="kw4">volatile</span> T<span class="sy1">&gt;</span><span class="sy4">;</span><br />
<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> T<span class="sy1">&gt;</span><br /></p>
<span class="kw1">class</span> variant_alternative<span class="sy1">&lt;</span>I, <span class="kw4">const</span> <span class="kw4">volatile</span> T<span class="sy1">&gt;</span><span class="sy4">;</span></div>
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<td>(3)</td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 &#36215;)</span><br />
<span class="t-mark">(C++20 &#20013;&#24323;&#29992;)</span></td>
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<p>&#25552;&#20379;&#23545;&#21487;&#26377; cv &#38480;&#23450; <code>variant</code> &#30340;&#31867;&#22411;&#32534;&#35793;&#26102;&#19979;&#26631;&#35775;&#38382;&#65292;&#23558; <code>variant</code> &#30340; cv &#38480;&#23450;&#19982;&#21487;&#36873;&#39033;&#30340; cv &#38480;&#23450;&#65288;&#33509;&#23384;&#22312;&#65289;&#30456;&#32467;&#21512;&#12290;</p>
<p>&#27491;&#24335;&#32780;&#35328;&#65292;</p>
<div class="t-li1"><span class="t-li">2)</span> &#28385;&#36275;&#25317;&#26377;&#25104;&#21592; typedef <code>type</code> &#31561;&#20110;&#24207;&#21495;&#20026; <code>I</code> &#30340;&#21487;&#36873;&#39033;&#31867;&#22411;&#30340;<a href="cpp-named_req-TransformationTrait.html"><span style="font-style:italic">&#21464;&#25442;&#29305;&#24449;</span> <span style="font-family: Georgia, &#39;DejaVu Serif&#39;, serif; font-style:italic">(TransformationTrait)</span></a> &#35201;&#27714;</div>
<div class="t-li1"><span class="t-li">3)</span> &#28385;&#36275;&#25317;&#26377;&#25104;&#21592; typedef <code>type</code> &#20998;&#21035;&#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-add_cv.html"><span class="kw604">std::<span class="me2">add_const_t</span></span></a><span class="sy1">&lt;</span>std<span class="sy4">::</span><span class="me2">variant_alternative_t</span><span class="sy1">&lt;</span>I,T<span class="sy1">&gt;&gt;</span></span></span> &#12289; <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-add_cv.html"><span class="kw605">std::<span class="me2">add_volatile_t</span></span></a><span class="sy1">&lt;</span>std<span class="sy4">::</span><span class="me2">variant_alternative_t</span><span class="sy1">&lt;</span>I,T<span class="sy1">&gt;&gt;</span></span></span> &#21644; <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-add_cv.html"><span class="kw603">std::<span class="me2">add_cv_t</span></span></a><span class="sy1">&lt;</span>std<span class="sy4">::</span><span class="me2">variant_alternative_t</span><span class="sy1">&lt;</span>I,T<span class="sy1">&gt;&gt;</span></span></span> &#30340;<a href="cpp-named_req-TransformationTrait.html"><span style="font-style:italic">&#21464;&#25442;&#29305;&#24449;</span> <span style="font-family: Georgia, &#39;DejaVu Serif&#39;, serif; font-style:italic">(TransformationTrait)</span></a> &#35201;&#27714;</div>
<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E7.B1.BB.E5.9E.8B">&#25104;&#21592;&#31867;&#22411;</span></h3>
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<td>&#25104;&#21592;&#31867;&#22411;</td>
<td>&#23450;&#20041;</td>
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<td>type</td>
<td><code>variant</code> &#30340;&#31532; <code>I</code> &#30340;&#21487;&#36873;&#39033;&#30340;&#31867;&#22411;&#65292;&#20854;&#20013; <code>I</code> &#24517;&#39035;&#22312; <code>[0, sizeof...(Types))</code> &#20013;&#65292;&#21542;&#21017;&#31243;&#24207;&#20026;&#35884;&#26500;</td>
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<h3><span class="mw-headline" id=".E8.BE.85.E5.8A.A9.E5.88.AB.E5.90.8D.E6.A8.A1.E6.9D.BF">&#36741;&#21161;&#21035;&#21517;&#27169;&#26495;</span></h3>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;</span>size_t I, <span class="kw1">class</span> T<span class="sy1">&gt;</span><br />
<span class="kw1">using</span> variant_alternative_t <span class="sy1">=</span> <span class="kw1">typename</span> variant_alternative<span class="sy1">&lt;</span>I, T<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">type</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 &#36215;)</span></td>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">&#31034;&#20363;</span></h3>
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<pre class="de1"><span class="co2">#include &lt;variant&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
<span class="kw1">using</span> my_variant <span class="sy1">=</span> <a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span>, <span class="kw4">float</span><span class="sy1">&gt;</span><span class="sy4">;</span>
static_assert<span class="br0">(</span><a href="cpp-types-is_same.html"><span class="kw589">std::<span class="me2">is_same_v</span></span></a>
<span class="sy1">&lt;</span><span class="kw4">int</span>, std<span class="sy4">::</span><span class="me2">variant_alternative_t</span><span class="sy1">&lt;</span><span class="nu0">0</span>, my_variant<span class="sy1">&gt;&gt;</span><span class="br0">)</span><span class="sy4">;</span>
static_assert<span class="br0">(</span><a href="cpp-types-is_same.html"><span class="kw589">std::<span class="me2">is_same_v</span></span></a>
<span class="sy1">&lt;</span><span class="kw4">float</span>, std<span class="sy4">::</span><span class="me2">variant_alternative_t</span><span class="sy1">&lt;</span><span class="nu0">1</span>, my_variant<span class="sy1">&gt;&gt;</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co1">// variant &#31867;&#22411;&#19978;&#30340; cv &#38480;&#23450;&#20256;&#25773;&#32473;&#25552;&#21462;&#20986;&#30340;&#21487;&#36873;&#39033;&#31867;&#22411;&#12290;</span>
static_assert<span class="br0">(</span><a href="cpp-types-is_same.html"><span class="kw589">std::<span class="me2">is_same_v</span></span></a>
<span class="sy1">&lt;</span><span class="kw4">const</span> <span class="kw4">int</span>, std<span class="sy4">::</span><span class="me2">variant_alternative_t</span><span class="sy1">&lt;</span><span class="nu0">0</span>, <span class="kw4">const</span> my_variant<span class="sy1">&gt;&gt;</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<a href="cpp-io-cout.html"><span class="kw1756">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;All static assertions passed<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div>
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<p>&#36755;&#20986;&#65306;</p>
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<pre class="de1">All static assertions passed.</pre></div>
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<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>
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<th>DR</th>
<th>&#24212;&#29992;&#20110;</th>
<th>&#20986;&#29256;&#26102;&#30340;&#34892;&#20026;</th>
<th>&#27491;&#30830;&#34892;&#20026;</th>
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<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue2974">LWG 2974</a></td>
<td>C++17</td>
<td>&#36234;&#30028;&#19979;&#26631;&#23548;&#33268;&#26410;&#23450;&#20041;&#34892;&#20026;</td>
<td>&#20351;&#20043;&#20026;&#35884;&#26500;</td>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">&#21442;&#38405;</span></h3>
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<div><a href="cpp-utility-variant-variant_size.html"><span class="t-lines"><span>variant_size</span><span>variant_size_v</span></span></a></div>
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
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<td>&#22312;&#32534;&#35793;&#26102;&#33719;&#24471; <code>variant</code> &#21487;&#36873;&#39033;&#21015;&#34920;&#30340;&#22823;&#23567;<br />
<span class="t-mark">(&#31867;&#27169;&#26495;)</span> <span class="t-mark">(&#21464;&#37327;&#27169;&#26495;)</span></td>
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<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>
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<td>&#33719;&#24471;&#25351;&#23450;&#20803;&#32032;&#30340;&#31867;&#22411;<br />
<span class="t-mark">(&#31867;&#27169;&#26495;&#29305;&#21270;)</span></td>
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