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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::variant&lt;Types...&gt;::</span>valueless_by_exception</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><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> <span class="kw4">bool</span> valueless_by_exception<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</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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<p>&#24403;&#19988;&#20165;&#24403; <code>variant</code> &#20445;&#26377;&#20540;&#26102;&#36820;&#22238; <code>false</code> &#12290;</p>
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">&#27880;&#35299;</span></h3>
<p><code>variant</code> &#21487;&#20197;&#22312;&#19979;&#21015;&#24773;&#24418;&#20013;&#21464;&#24471;&#26080;&#20540;&#65306;</p>
<ul>
<li>&#65288;&#20445;&#35777;&#65289;&#22312;<a href="cpp-utility-variant-operator=.html">&#31227;&#21160;&#36171;&#20540;</a>&#20013;&#65292;&#31227;&#21160;&#21021;&#22987;&#21270;&#25152;&#21547;&#20540;&#26102;&#25243;&#20986;&#24322;&#24120;</li>
<li>&#65288;&#21487;&#36873;&#65289;&#22312;<a href="cpp-utility-variant-operator=.html">&#22797;&#21046;&#36171;&#20540;</a>&#20013;&#65292;&#22797;&#21046;&#21021;&#22987;&#21270;&#25152;&#21547;&#20540;&#26102;&#25243;&#20986;&#24322;&#24120;</li>
<li>&#65288;&#21487;&#36873;&#65289;&#22312;&#31867;&#22411;&#26356;&#25913;<a href="cpp-utility-variant-operator=.html">&#36171;&#20540;</a>&#26399;&#38388;&#65292;&#21021;&#22987;&#21270;&#25152;&#21547;&#20540;&#26102;&#25243;&#20986;&#24322;&#24120;</li>
<li>&#65288;&#21487;&#36873;&#65289;&#22312;&#31867;&#22411;&#26356;&#25913; <a href="cpp-utility-variant-emplace.html"><code>emplace</code></a> &#26399;&#38388;&#65292;&#21021;&#22987;&#21270;&#25152;&#21547;&#20540;&#26102;&#25243;&#20986;&#24322;&#24120;</li>
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<p>&#22240;&#20026; <code>variant</code> &#20915;&#19981;&#23481;&#35768;&#20998;&#37197;&#21160;&#24577;&#20869;&#23384;&#65292;&#25925;&#22312;&#36825;&#20123;&#24773;&#20917;&#19979;&#19981;&#21487;&#33021;&#20445;&#30041;&#20808;&#21069;&#30340;&#20540;&#12290;</p>
<p>&#36825;&#36824;&#24212;&#29992;&#21040;&#38750;&#31867;&#31867;&#22411;&#30340; <code>variant</code> &#65306;</p>
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<pre class="de1"><span class="kw1">struct</span> S <span class="br0">{</span>
operator <span class="kw4">int</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">throw</span> <span class="nu0">42</span><span class="sy4">;</span> <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</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">float</span>, <span class="kw4">int</span><span class="sy1">&gt;</span> v<span class="br0">{</span><span class="nu0">12</span>.<span class="me1">f</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// OK</span>
v.<span class="me1">emplace</span><span class="sy1">&lt;</span><span class="nu0">1</span><span class="sy1">&gt;</span><span class="br0">(</span>S<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// v &#21487;&#33021;&#20026;&#26080;&#20540;</span></pre></div>
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<p>&#22240;&#24322;&#24120;&#26080;&#20540;&#30340; <code>variant</code> &#34987;&#24403;&#20570;&#22312;&#38750;&#27861;&#24577;&#65306; <a href="cpp-utility-variant-index.html"><code>index</code></a> &#36820;&#22238; <a href="cpp-utility-variant-variant_npos.html"><code>variant_npos</code></a> &#65292; <a href="cpp-utility-variant-get.html"><code>get</code></a> &#19982; <a href="cpp-utility-variant-visit.html"><code>visit</code></a> &#25243;&#20986; <a href="cpp-utility-variant-bad_variant_access.html"><code>bad_variant_access</code></a> &#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;cassert&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;stdexcept&gt;</span>
<span class="co2">#include &lt;string&gt;</span>
<span class="co2">#include &lt;variant&gt;</span>
<span class="kw1">struct</span> Demo <span class="br0">{</span>
Demo<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
Demo<span class="br0">(</span><span class="kw4">const</span> Demo<span class="sy3">&amp;</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">throw</span> <a href="cpp-error-domain_error.html"><span class="kw766">std::<span class="me2">domain_error</span></span></a><span class="br0">(</span><span class="st0">&quot;copy ctor&quot;</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
Demo<span class="sy3">&amp;</span> operator<span class="sy1">=</span> <span class="br0">(</span><span class="kw4">const</span> Demo<span class="sy3">&amp;</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="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1">&lt;</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a>, Demo<span class="sy1">&gt;</span> var<span class="br0">{</span><span class="st0">&quot;str&quot;</span><span class="br0">}</span><span class="sy4">;</span>
<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span>var.<span class="me1">index</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">get</span><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">==</span> <span class="st0">&quot;str&quot;</span><span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span>var.<span class="me1">valueless_by_exception</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw2">false</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">try</span> <span class="br0">{</span>
var <span class="sy1">=</span> Demo<span class="br0">{</span><span class="nu0">666</span><span class="br0">}</span><span class="sy4">;</span>
<span class="br0">}</span> <span class="kw1">catch</span> <span class="br0">(</span><span class="kw4">const</span> <a href="cpp-error-domain_error.html"><span class="kw766">std::<span class="me2">domain_error</span></span></a><span class="sy3">&amp;</span> ex<span class="br0">)</span> <span class="br0">{</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;1) Exception: &quot;</span> <span class="sy1">&lt;&lt;</span> ex.<span class="me1">what</span><span class="br0">(</span><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="br0">}</span>
<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span>var.<span class="me1">index</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <a href="cpp-utility-variant-variant_npos.html"><span class="kw1090">std::<span class="me2">variant_npos</span></span></a><span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span>var.<span class="me1">valueless_by_exception</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw2">true</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co1">// &#29616;&#22312; var &ldquo;&#26080;&#20540;&rdquo;&#65292;&#27492;&#20026; var &#30340;&#21021;&#22987;&#21270;&#36807;&#31243;&#20013;&#24341;&#21457;&#30340;&#24322;&#24120;&#23548;&#33268;&#30340;&#38750;&#27861;&#29366;&#24577;&#12290;</span>
<span class="kw1">try</span> <span class="br0">{</span>
std<span class="sy4">::</span><span class="me2">get</span><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> <span class="kw1">catch</span> <span class="br0">(</span><span class="kw4">const</span> <a href="cpp-utility-variant-bad_variant_access.html"><span class="kw1089">std::<span class="me2">bad_variant_access</span></span></a><span class="sy3">&amp;</span> ex<span class="br0">)</span> <span class="br0">{</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;2) Exception: &quot;</span> <span class="sy1">&lt;&lt;</span> ex.<span class="me1">what</span><span class="br0">(</span><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="br0">}</span>
var <span class="sy1">=</span> <span class="st0">&quot;str2&quot;</span><span class="sy4">;</span>
<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span>var.<span class="me1">index</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">get</span><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">==</span> <span class="st0">&quot;str2&quot;</span><span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span>var.<span class="me1">valueless_by_exception</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw2">false</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div>
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<p>&#21487;&#33021;&#30340;&#36755;&#20986;&#65306;</p>
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<div class="text source-text">
<pre class="de1">1) Exception: copy ctor
2) Exception: std::get: variant is valueless</pre></div>
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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-get.html"><span class="t-lines"><span>std::get<span class="t-dsc-small">(std::variant)</span></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>&#20197;&#32473;&#23450;&#32034;&#24341;&#25110;&#31867;&#22411;&#65288;&#33509;&#31867;&#22411;&#21807;&#19968;&#65289;&#35835;&#21462; <code>variant</code> &#30340;&#20540;&#65292;&#38169;&#35823;&#26102;&#25243;&#20986;&#24322;&#24120;<br />
<span class="t-mark">(&#20989;&#25968;&#27169;&#26495;)</span></td>
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<div class="t-dsc-member-div">
<div><a href="cpp-utility-variant-index.html"><span class="t-lines"><span>index</span></span></a></div>
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<td>&#36820;&#22238; <code>variant</code> &#25152;&#20445;&#26377;&#21487;&#36873;&#39033;&#30340;&#38646;&#22522;&#19979;&#26631;<br />
<span class="t-mark">(&#20844;&#24320;&#25104;&#21592;&#20989;&#25968;)</span></td>
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<div class="t-dsc-member-div">
<div><a href="cpp-utility-variant-bad_variant_access.html"><span class="t-lines"><span>bad_variant_access</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>
<td>&#38750;&#27861;&#22320;&#35775;&#38382; <code>variant</code> &#30340;&#20540;&#26102;&#25243;&#20986;&#30340;&#24322;&#24120;<br />
<span class="t-mark">(&#31867;)</span></td>
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