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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>monostate</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">struct</span> monostate <span class="br0">{</span> <span class="br0">}</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>&#26377;&#24847;&#20026;&#34892;&#20026;&#33391;&#22909;&#30340; <span class="t-lc"><a href="cpp-utility-variant.html">std::variant</a></span> &#20013;&#31354;&#21487;&#36873;&#39033;&#25152;&#29992;&#30340;&#21333;&#20301;&#31867;&#22411;&#12290;&#20855;&#20307;&#32780;&#35328;&#65292;&#38750;&#21487;&#40664;&#35748;&#26500;&#36896;&#30340; <code>variant</code> &#21487;&#20197;&#21015; <code>std::monostate</code> &#20026;&#20854;&#39318;&#20010;&#21487;&#36873;&#39033;&#65306;&#36825;&#20351;&#24471; <code>variant</code> &#33258;&#36523;&#21487;&#40664;&#35748;&#26500;&#36896;&#12290;</p>
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<td>&#24179;&#20961;&#38544;&#24335;&#40664;&#35748;/&#22797;&#21046;/&#31227;&#21160;&#26500;&#36896;&#20989;&#25968;<br />
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<td>&#24179;&#20961;&#38544;&#24335;&#26512;&#26500;&#20989;&#25968;<br />
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<div><span class="t-lines"><span>operator=</span></span></div>
<div><span class="t-lines"><span><span class="t-mark">(&#38544;&#24335;&#22768;&#26126;)</span></span></span></div>
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<td>&#24179;&#20961;&#38544;&#24335;&#22797;&#21046;/&#31227;&#21160;&#36171;&#20540;<br />
<span class="t-mark">(&#20844;&#24320;&#25104;&#21592;&#20989;&#25968;)</span></td>
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<h3><span class="mw-headline" id=".E9.9D.9E.E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">&#38750;&#25104;&#21592;&#20989;&#25968;</span></h3>
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<h2><span class="mw-headline" id="std::operator.3D.3D.2C_.21.3D.2C_.3C.2C_.3C.3D.2C_.3E.2C_.3E.3D.2C_.3C.3D.3E.28std::monostate.29"><span style="font-size:0.7em; line-height:130%">std::</span>operator==,&nbsp;!=, &lt;, &lt;=, &gt;, &gt;=, &lt;=&gt;<small>(std::monostate)</small></span></h2>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1">==</span><span class="br0">(</span>monostate, monostate<span class="br0">)</span> <span class="kw1">noexcept</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="kw2">true</span><span class="sy4">;</span> <span class="br0">}</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="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy3">!</span><span class="sy1">=</span><span class="br0">(</span>monostate, monostate<span class="br0">)</span> <span class="kw1">noexcept</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="kw2">false</span><span class="sy4">;</span> <span class="br0">}</span><br /></span>
<p><span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1">&lt;</span><span class="br0">(</span>monostate, monostate<span class="br0">)</span> <span class="kw1">noexcept</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="kw2">false</span><span class="sy4">;</span> <span class="br0">}</span><br />
<span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1">&gt;</span><span class="br0">(</span>monostate, monostate<span class="br0">)</span> <span class="kw1">noexcept</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="kw2">false</span><span class="sy4">;</span> <span class="br0">}</span><br />
<span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1">&lt;=</span><span class="br0">(</span>monostate, monostate<span class="br0">)</span> <span class="kw1">noexcept</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="kw2">true</span><span class="sy4">;</span> <span class="br0">}</span><br /></p>
<span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1">&gt;=</span><span class="br0">(</span>monostate, monostate<span class="br0">)</span> <span class="kw1">noexcept</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="kw2">true</span><span class="sy4">;</span> <span class="br0">}</span></div>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 &#36215;)</span><br />
<span class="t-mark-rev t-until-cxx20">(C++20 &#21069;)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> std<span class="sy4">::</span><span class="me2">strong_ordering</span> operator<span class="sy1">&lt;=&gt;</span><span class="br0">(</span>monostate, monostate<span class="br0">)</span> <span class="kw1">noexcept</span><br />
<span class="br0">{</span> <span class="kw1">return</span> std<span class="sy4">::</span><span class="me2">strong_ordering</span><span class="sy4">::</span><span class="me2">equal</span><span class="sy4">;</span> <span class="br0">}</span></span></div>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 &#36215;)</span></td>
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<p>&#25152;&#26377; <code>std::monostate</code> &#23454;&#20363;&#27604;&#36739;&#30456;&#31561;&#12290;</p>
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<p><code>&lt;</code>&#12289; <code>&lt;=</code>&#12289; <code>&gt;</code>&#12289; <code>&gt;=</code> &#21450; <code>!=</code> &#36816;&#31639;&#31526;&#20998;&#21035;&#20174; <code>operator&lt;=&gt;</code> &#19982; <code>operator==</code> <a href="cpp-language-operators.html#.E5.85.B3.E7.B3.BB.E8.BF.90.E7.AE.97.E7.AC.A6">&#21512;&#25104;</a>&#12290;</p>
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<h2><span class="mw-headline" id="std::hash.3Cstd::monostate.3E"><span style="font-size:0.7em; line-height:130%">std::</span>hash<small>&lt;std::monostate&gt;</small></span></h2>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;&gt;</span> <span class="kw1">struct</span> <a href="cpp-utility-hash.html"><span class="kw1123">std::<span class="me2">hash</span></span></a><span class="sy1">&lt;</span>monostate<span class="sy1">&gt;</span><span class="sy4">;</span></span></div>
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<p>&#20026; <code>std::monostate</code> &#29305;&#21270; <span class="t-lc"><a href="cpp-utility-hash.html">std::hash</a></span> &#31639;&#27861;</p>
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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">struct</span> S
<span class="br0">{</span>
S<span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span> <span class="sy4">:</span> i<span class="br0">(</span>i<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
<span class="kw4">int</span> i<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>
<span class="co1">// &#33509;&#26080; monostate &#31867;&#22411;&#21017;&#27492;&#22768;&#26126;&#23558;&#22833;&#36133;&#12290;</span>
<span class="co1">// &#36825;&#26159;&#22240;&#20026; S &#19981;&#21487;&#40664;&#35748;&#26500;&#36896;&#12290;</span>
<a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1">&lt;</span>std<span class="sy4">::</span><span class="me2">monostate</span>, S<span class="sy1">&gt;</span> var<span class="sy4">;</span>
<span class="co1">// var.index() &#29616;&#20026; 0 &mdash;&mdash;&#39318;&#20010;&#20803;&#32032;</span>
<span class="co1">// std::get&lt;S&gt; &#23558;&#25243;&#24322;&#24120;&#65281;&#25105;&#31561;&#38656;&#35201;&#36171;&#19968;&#20010;&#20540;</span>
var <span class="sy1">=</span> <span class="nu0">12</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> std<span class="sy4">::</span><span class="me2">get</span><span class="sy1">&lt;</span>S<span class="sy1">&gt;</span><span class="br0">(</span>var<span class="br0">)</span>.<span class="me1">i</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></pre></div>
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<p>&#36755;&#20986;&#65306;</p>
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<pre class="de1">12</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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<td>&#26500;&#36896; <code>variant</code> &#23545;&#35937;<br />
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