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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>visit</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><span class="kw1">class</span> Visitor, <span class="kw1">class</span>... <span class="me1">Variants</span><span class="sy1">&gt;</span><br />
<span class="kw4">constexpr</span> <span class="coMULTI">/*see below*/</span> visit<span class="br0">(</span>Visitor<span class="sy3">&amp;&amp;</span> vis, Variants<span class="sy3">&amp;&amp;</span>... <span class="me1">vars</span><span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(1)</td>
<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><span class="kw1">class</span> R, <span class="kw1">class</span> Visitor, <span class="kw1">class</span>... <span class="me1">Variants</span><span class="sy1">&gt;</span><br />
<span class="kw4">constexpr</span> R visit<span class="br0">(</span>Visitor<span class="sy3">&amp;&amp;</span> vis, Variants<span class="sy3">&amp;&amp;</span>... <span class="me1">vars</span><span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(2)</td>
<td><span class="t-mark-rev t-since-cxx20">(C++20 &#36215;)</span></td>
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<p>&#24212;&#29992;&#35266;&#35272;&#22120; <code>vis</code> &#21040; <code>variant</code> &#21464;&#37327;&#32452; <code>vars</code> &#12290;</p>
<p>&#36825;&#20123;&#37325;&#36733;&#20165;&#33509; <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>Variants<span class="sy1">&gt;</span>...</span></span> &#20013;&#30340;&#27599;&#20010;&#31867;&#22411;&#22343;&#20026;&#65288;&#21487;&#20026; const &#38480;&#23450;&#30340;&#65289; <span class="t-lc"><a href="cpp-utility-variant.html">std::variant</a></span> &#30340;&#29305;&#21270;&#65292;&#25110;&#65288;&#21487;&#20026; const &#38480;&#23450;&#65289;&#30340;&#31867; <code>C</code> &#65292;&#28385;&#36275;&#24688;&#26377;&#19968;&#20010; <span class="t-lc"><a href="cpp-utility-variant.html">std::variant</a></span> &#29305;&#21270;&#26159; <code>C</code> &#30340;&#22522;&#31867;&#19988;&#23427;&#26159;&#20844;&#24320;&#19988;&#26080;&#27495;&#20041;&#30340;&#22522;&#31867;&#25165;&#21442;&#19982;&#37325;&#36733;&#20915;&#35758;&#12290;</p>
<p>&#31561;&#25928;&#20110;&#36820;&#22238;</p>
<p><span class="t-cc"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-functional-invoke.html"><span class="kw1058">std::<span class="me2">invoke</span></span></a><span class="br0">(</span><a href="cpp-utility-forward.html"><span class="kw1116">std::<span class="me2">forward</span></span></a><span class="sy1">&lt;</span>Visitor<span class="sy1">&gt;</span><span class="br0">(</span>vis<span class="br0">)</span>,<br />
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; std<span class="sy4">::</span><span class="me2">get</span><span class="sy1">&lt;</span>is<span class="sy1">&gt;</span><span class="br0">(</span><a href="cpp-utility-forward.html"><span class="kw1116">std::<span class="me2">forward</span></span></a><span class="sy1">&lt;</span>VariantBases<span class="sy1">&gt;</span><span class="br0">(</span>vars<span class="br0">)</span><span class="br0">)</span>...<span class="br0">)</span></span></span></p>
<p>&#65292;&#20854;&#20013; <code>VariantBases...</code> &#20013;&#30340;&#27599;&#20010;&#31867;&#22411;&#26159;&#30830;&#23450;&#22914;&#19978;&#30340;&#21807;&#19968; <span class="t-lc"><a href="cpp-utility-variant.html">std::variant</a></span> &#29305;&#21270;&#65292;&#38500;&#20102;&#33509;&#23545;&#24212;&#30340;&#23454;&#21442;&#25317;&#26377; const &#38480;&#23450;&#31867;&#22411;&#65292;&#20026;&#24038;&#20540;&#25110;&#21491;&#20540;&#65292;&#21017;&#20998;&#21035;&#28155;&#21152; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> &#12289; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">&amp;</span></span></span> &#25110; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">&amp;&amp;</span></span></span> &#21040;&#23427;&#65292;&#32780; <code>is...</code> &#26159; <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-forward.html"><span class="kw1116">std::<span class="me2">forward</span></span></a><span class="sy1">&lt;</span>VariantBases<span class="sy1">&gt;</span><span class="br0">(</span>vars<span class="br0">)</span>.<span class="me1">index</span><span class="br0">(</span><span class="br0">)</span>...</span></span> &#12290;</p>
<div class="t-li1"><span class="t-li">1)</span> &#22914;&#21516;&#29992; <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype</span></span> &#20174;&#36820;&#22238;&#30340;&#34920;&#36798;&#24335;&#25512;&#23548;&#36820;&#22238;&#31867;&#22411;&#12290;&#33509;&#19978;&#36848;&#35843;&#29992;&#19981;&#26159;&#23545;&#20110;&#25152;&#26377; <code>variant</code> &#30340;&#25152;&#26377;&#21487;&#36873;&#39033;&#31867;&#22411;&#22343;&#20026;&#21516;&#19968;&#31867;&#22411;&#21644;&#20540;&#31867;&#21035;&#30340;&#21512;&#27861;&#34920;&#36798;&#24335;&#65292;&#21017;&#35843;&#29992;&#20026;&#35884;&#26500;&#12290;</div>
<div class="t-li1"><span class="t-li">2)</span> &#36820;&#22238;&#31867;&#22411;&#20026; <code>R</code> &#12290;&#33509; <code>R</code> &#26159;&#65288;&#21487;&#26377; cv &#38480;&#23450;&#30340;&#65289; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> &#65292;&#21017;&#33293;&#24323; <code>invoke</code> &#34920;&#36798;&#24335;&#30340;&#32467;&#26524;&#12290;</div>
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">&#21442;&#25968;</span></h3>
<table class="t-par-begin">
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<td>vis</td>
<td>-</td>
<td>&#25509;&#21463;&#27599;&#20010; <code>variant</code> &#30340;&#27599;&#20010;&#21487;&#33021;&#21487;&#36873;&#39033;&#30340;<a href="cpp-named_req-Callable.html"><span style="font-style:italic">&#21487;&#35843;&#29992;</span> <span style="font-family: Georgia, &#39;DejaVu Serif&#39;, serif; font-style:italic">(Callable)</span></a> &#23545;&#35937;</td>
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<td>vars</td>
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<td>&#20256;&#36882;&#32473;&#35266;&#35272;&#22120;&#30340; <code>variant</code> &#21015;&#34920;</td>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">&#36820;&#22238;&#20540;</span></h3>
<div class="t-li1"><span class="t-li">1)</span> &#35266;&#35272;&#22120;&#30340;&#25152;&#36873;&#35843;&#29992;&#25152;&#36820;&#22238;&#30340;&#20540;&#65292;&#36716;&#25442;&#25104;&#25152;&#26377;&#21487;&#34892;&#30340; <code>std::invoke</code> &#34920;&#36798;&#24335;&#30340;&#20849;&#29992;&#31867;&#22411;&#12290;</div>
<div class="t-li1"><span class="t-li">2)</span> &#33509; <code>R</code> &#20026;&#65288;&#21487;&#26377; cv &#38480;&#23450;&#30340;&#65289; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> &#21017;&#20026;&#26080;&#65307;&#21542;&#21017;&#20026;&#36873;&#25321;&#30340;&#35266;&#35272;&#22120;&#35843;&#29992;&#25152;&#36820;&#22238;&#30340;&#20540;&#65292;&#20877;&#38544;&#24335;&#36716;&#25442;&#25104; <code>R</code> &#12290;</div>
<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">&#24322;&#24120;</span></h3>
<p>&#33509;&#20219;&#20309; <code>vars</code> &#20013;&#30340; <code>variant</code> &#20026;&#22240;&#24322;&#24120;&#26080;&#20540;&#65288; <a href="cpp-utility-variant-valueless_by_exception.html"><tt>valueless_by_exception</tt></a> &#65289;&#65292;&#21017;&#25243;&#20986; <span class="t-lc"><a href="cpp-utility-variant-bad_variant_access.html">std::bad_variant_access</a></span> &#12290;&#22914;&#21516;&#36890;&#36807; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span><a href="cpp-utility-forward.html"><span class="kw1116">std::<span class="me2">forward</span></span></a><span class="sy1">&lt;</span>VariantBases<span class="sy1">&gt;</span><span class="br0">(</span>vars<span class="br0">)</span>.<span class="me1">valueless_by_exception</span><span class="br0">(</span><span class="br0">)</span> <span class="sy3">||</span> ...<span class="br0">)</span></span></span> &#30830;&#23450;&#20219;&#20309; <code>variant</code> &#20026;&#22240;&#24322;&#24120;&#26080;&#20540;&#12290;</p>
<h3><span class="mw-headline" id=".E5.A4.8D.E6.9D.82.E5.BA.A6">&#22797;&#26434;&#24230;</span></h3>
<p>&#24403; <code>variant</code> &#30340;&#25968;&#37327;&#20026;&#38646;&#25110;&#19968;&#26102;&#65292;&#21487;&#35843;&#29992;&#23545;&#35937;&#30340;&#35843;&#29992;&#22312;&#24120;&#25968;&#26102;&#38388;&#20869;&#23454;&#29616;&#65292;&#21363;&#23427;&#19981;&#21462;&#20915;&#20110; <code>sizeof...(Types)</code> &#12290;</p>
<p>&#33509; <code>variant</code> &#30340;&#25968;&#37327;&#22823;&#20110; 1 &#65292;&#21017;&#21487;&#35843;&#29992;&#23545;&#35937;&#30340;&#35843;&#29992;&#27809;&#26377;&#22797;&#26434;&#24230;&#35201;&#27714;&#12290;</p>
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">&#27880;&#35299;</span></h3>
<p>&#20196; <i>n</i> &#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span><span class="nu0">1</span> <span class="sy2">*</span> ... <span class="sy2">*</span> <a href="cpp-utility-variant-variant_size.html"><span class="kw1091">std::<span class="me2">variant_size_v</span></span></a><span class="sy1">&lt;</span><a href="cpp-types-remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>VariantBases<span class="sy1">&gt;&gt;</span><span class="br0">)</span></span></span>&#65292;&#23454;&#29616;&#36890;&#24120;&#20250;&#20026;&#27599;&#20010; <code>std::visit</code> &#30340;&#29305;&#21270;&#29983;&#25104;&#31561;&#20215;&#20110;&#26377; <i>n</i> &#20010;&#20989;&#25968;&#25351;&#38024;&#30340;&#65288;&#21487;&#33021;&#22810;&#32500;&#30340;&#65289;&#25968;&#32452;&#30340;&#34920;&#65292;&#36825;&#19982;<a href="cpp-language-virtual.html">&#34394;&#20989;&#25968;</a>&#30340;&#23454;&#29616;&#31867;&#20284;&#12290;</p>
<p>&#23454;&#29616;&#20134;&#21487;&#20026; <code>std::visit</code> &#29983;&#25104;&#26377; <i>n</i> &#20010;&#20998;&#25903;&#30340; <a href="cpp-language-switch.html">switch &#35821;&#21477;</a>&#65288;&#20363;&#22914; MSVC STL &#23454;&#29616;&#22312; <i>n</i> &#19981;&#22823;&#20110; 256 &#26102;&#20351;&#29992; switch &#35821;&#21477;&#65289;&#12290;</p>
<p>&#22312;&#20856;&#22411;&#23454;&#29616;&#19978;&#65292;&#33021;&#35748;&#20026;&#35843;&#29992; <code>vis</code> &#30340;&#26102;&#38388;&#22797;&#26434;&#24230;&#31561;&#20110;&#35775;&#38382;&#65288;&#21487;&#33021;&#22810;&#32500;&#30340;&#65289;&#25968;&#32452;&#20013;&#30340;&#20803;&#32032;&#65292;&#25110;&#25191;&#34892; switch &#35821;&#21477;&#30340;&#26102;&#38388;&#22797;&#26434;&#24230;&#12290;</p>
<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;iomanip&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;string&gt;</span>
<span class="co2">#include &lt;type_traits&gt;</span>
<span class="co2">#include &lt;variant&gt;</span>
<span class="co2">#include &lt;vector&gt;</span>
<span class="co1">// &#35201;&#35266;&#35272;&#30340; variant</span>
<span class="kw1">using</span> var_t <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">long</span>, <span class="kw4">double</span>, <a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="sy1">&gt;</span><span class="sy4">;</span>
<span class="co1">// &#35266;&#35272;&#22120; #3 &#30340;&#36741;&#21161;&#24120;&#37327;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span><span class="sy1">&gt;</span> <span class="kw1">inline</span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> always_false_v <span class="sy1">=</span> <span class="kw2">false</span><span class="sy4">;</span>
<span class="co1">// &#35266;&#35272;&#22120; #4 &#30340;&#36741;&#21161;&#31867;&#22411;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>... <span class="me1">Ts</span><span class="sy1">&gt;</span> <span class="kw1">struct</span> overloaded <span class="sy4">:</span> Ts... <span class="br0">{</span> <span class="kw1">using</span> Ts<span class="sy4">::</span><span class="me2">operator</span><span class="br0">(</span><span class="br0">)</span>...<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="co1">// &#26174;&#24335;&#25512;&#23548;&#25351;&#24341;&#65288; C++20 &#36215;&#19981;&#38656;&#35201;&#65289;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>... <span class="me1">Ts</span><span class="sy1">&gt;</span> overloaded<span class="br0">(</span>Ts...<span class="br0">)</span> <span class="sy2">-</span><span class="sy1">&gt;</span> overloaded<span class="sy1">&lt;</span>Ts...<span class="sy1">&gt;</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-container-vector.html"><span class="kw1268">std::<span class="me2">vector</span></span></a><span class="sy1">&lt;</span>var_t<span class="sy1">&gt;</span> vec <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">10</span>, <span class="nu0">15l</span>, <span class="nu16">1.5</span>, <span class="st0">&quot;hello&quot;</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">for</span><span class="br0">(</span><span class="kw4">auto</span><span class="sy3">&amp;&amp;</span> v<span class="sy4">:</span> vec<span class="br0">)</span> <span class="br0">{</span>
<span class="co1">// 1. void &#35266;&#35272;&#22120;&#65292;&#20165;&#20026;&#20854;&#21103;&#25928;&#24212;&#35843;&#29992;</span>
std<span class="sy4">::</span><span class="me2">visit</span><span class="br0">(</span><span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">auto</span><span class="sy3">&amp;&amp;</span> arg<span class="br0">)</span><span class="br0">{</span><a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> arg<span class="sy4">;</span><span class="br0">}</span>, v<span class="br0">)</span><span class="sy4">;</span>
<span class="co1">// 2. &#36820;&#22238;&#20540;&#30340;&#35266;&#35272;&#22120;&#65292;&#36820;&#22238;&#21478;&#19968; variant &#30340;&#24120;&#35265;&#27169;&#24335;</span>
var_t w <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">visit</span><span class="br0">(</span><span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">auto</span><span class="sy3">&amp;&amp;</span> arg<span class="br0">)</span> <span class="sy2">-</span><span class="sy1">&gt;</span> var_t <span class="br0">{</span><span class="kw1">return</span> arg <span class="sy2">+</span> arg<span class="sy4">;</span><span class="br0">}</span>, v<span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;. After doubling, variant holds &quot;</span><span class="sy4">;</span>
<span class="co1">// 3. &#31867;&#22411;&#21305;&#37197;&#35266;&#35272;&#22120;&#65306;&#20134;&#33021;&#20026;&#24102; 4 &#20010;&#37325;&#36733;&#30340; operator() &#30340;&#31867;</span>
std<span class="sy4">::</span><span class="me2">visit</span><span class="br0">(</span><span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">auto</span><span class="sy3">&amp;&amp;</span> arg<span class="br0">)</span> <span class="br0">{</span>
<span class="kw1">using</span> T <span class="sy1">=</span> <a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1">&lt;</span>decltype<span class="br0">(</span>arg<span class="br0">)</span><span class="sy1">&gt;</span><span class="sy4">;</span>
<span class="kw1">if</span> <span class="kw4">constexpr</span> <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>T, <span class="kw4">int</span><span class="sy1">&gt;</span><span class="br0">)</span>
<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;int with value &quot;</span> <span class="sy1">&lt;&lt;</span> arg <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span><span class="sy4">;</span>
<span class="kw1">else</span> <span class="kw1">if</span> <span class="kw4">constexpr</span> <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>T, <span class="kw4">long</span><span class="sy1">&gt;</span><span class="br0">)</span>
<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;long with value &quot;</span> <span class="sy1">&lt;&lt;</span> arg <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span><span class="sy4">;</span>
<span class="kw1">else</span> <span class="kw1">if</span> <span class="kw4">constexpr</span> <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>T, <span class="kw4">double</span><span class="sy1">&gt;</span><span class="br0">)</span>
<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;double with value &quot;</span> <span class="sy1">&lt;&lt;</span> arg <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span><span class="sy4">;</span>
<span class="kw1">else</span> <span class="kw1">if</span> <span class="kw4">constexpr</span> <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>T, <a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="sy1">&gt;</span><span class="br0">)</span>
<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;std::string with value &quot;</span> <span class="sy1">&lt;&lt;</span> <a href="cpp-io-manip-quoted.html"><span class="kw1801">std::<span class="me2">quoted</span></span></a><span class="br0">(</span>arg<span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span><span class="sy4">;</span>
<span class="kw1">else</span>
static_assert<span class="br0">(</span>always_false_v<span class="sy1">&lt;</span>T<span class="sy1">&gt;</span>, <span class="st0">&quot;non-exhaustive visitor!&quot;</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>, w<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">auto</span><span class="sy3">&amp;&amp;</span> v<span class="sy4">:</span> vec<span class="br0">)</span> <span class="br0">{</span>
<span class="co1">// 4. &#21478;&#19968;&#31181;&#31867;&#22411;&#21305;&#37197;&#35266;&#35272;&#22120;&#65306;&#26377;&#19977;&#20010;&#37325;&#36733;&#30340; operator() &#30340;&#31867;</span>
<span class="co1">// &#27880;&#65306;&#27492;&#24773;&#20917;&#19979; &#39;(auto arg)&#39; &#27169;&#26495; operator() &#23558;&#32465;&#23450;&#21040; &#39;int&#39; &#19982; &#39;long&#39; &#65292;</span>
<span class="co1">// &#20294;&#33509;&#23427;&#19981;&#23384;&#22312;&#21017; &#39;(double arg)&#39; operator() *&#20134;&#23558;* &#32465;&#23450;&#21040; &#39;int&#39; &#19982; &#39;long&#39; &#65292;</span>
<span class="co1">// &#22240;&#20026;&#20004;&#32773;&#22343;&#21487;&#38544;&#24335;&#36716;&#25442;&#25104; double &#12290;&#20351;&#29992;&#27492;&#24418;&#24335;&#26102;&#24212;&#30041;&#24515;&#20197;&#27491;&#30830;&#22788;&#29702;&#38544;&#24335;&#36716;&#25442;&#12290;</span>
std<span class="sy4">::</span><span class="me2">visit</span><span class="br0">(</span>overloaded <span class="br0">{</span>
<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">auto</span> arg<span class="br0">)</span> <span class="br0">{</span> <a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> arg <span class="sy1">&lt;&lt;</span> <span class="st0">&#39; &#39;</span><span class="sy4">;</span> <span class="br0">}</span>,
<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">double</span> arg<span class="br0">)</span> <span class="br0">{</span> <a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <a href="cpp-io-manip-fixed.html"><span class="kw1783">std::<span class="me2">fixed</span></span></a> <span class="sy1">&lt;&lt;</span> arg <span class="sy1">&lt;&lt;</span> <span class="st0">&#39; &#39;</span><span class="sy4">;</span> <span class="br0">}</span>,
<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">const</span> <a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="sy3">&amp;</span> arg<span class="br0">)</span> <span class="br0">{</span> <a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <a href="cpp-io-manip-quoted.html"><span class="kw1801">std::<span class="me2">quoted</span></span></a><span class="br0">(</span>arg<span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&#39; &#39;</span><span class="sy4">;</span> <span class="br0">}</span>,
<span class="br0">}</span>, v<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="br0">}</span></pre></div>
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<p>&#36755;&#20986;&#65306;</p>
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<div class="text source-text">
<pre class="de1">10. After doubling, variant holds int with value 20
15. After doubling, variant holds long with value 30
1.5. After doubling, variant holds double with value 3
hello. After doubling, variant holds std::string with value &quot;hellohello&quot;
10 15 1.500000 &quot;hello&quot;</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>
<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/issue3052">LWG 3052</a></td>
<td>C++17</td>
<td>&#33509; <code>Variants</code> &#20013;&#30340;&#20219;&#20309;&#31867;&#22411;&#19981;&#26159; <span class="t-lc"><a href="cpp-utility-variant.html">std::variant</a></span> &#21017;&#25928;&#26524;&#26410;&#25351;&#23450;</td>
<td>&#24050;&#25351;&#23450;</td>
</tr>
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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 class="t-dsc-member-div">
<div><a href="cpp-utility-variant-swap.html"><span class="t-lines"><span>swap</span></span></a></div>
</div>
</td>
<td>&#19982;&#21478;&#19968; <code>variant</code> &#20132;&#25442;<br />
<span class="t-mark">(&#20844;&#24320;&#25104;&#21592;&#20989;&#25968;)</span></td>
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