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<title>std::monostate</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_variant_monostate 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>monostate</h1>
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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">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 起)</span></td>
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<p>有意为行为良好的 <span class="t-lc"><a href="cpp-utility-variant.html">std::variant</a></span> 中空可选项所用的单位类型。具体而言，非可默认构造的 <code>variant</code> 可以列 <code>std::monostate</code> 为其首个可选项：这使得 <code>variant</code> 自身可默认构造。</p>
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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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<div><span class="t-lines"><span>(构造函数)</span></span></div>
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<div><span class="t-lines"><span><span class="t-mark">(隐式声明)</span></span></span></div>
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<td>平凡隐式默认/复制/移动构造函数<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 t-dsc-member-nobold-div">
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<div><span class="t-lines"><span>(析构函数)</span></span></div>
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<div><span class="t-lines"><span><span class="t-mark">(隐式声明)</span></span></span></div>
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<td>平凡隐式析构函数<br />
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<span class="t-mark">(公开成员函数)</span></td>
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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">(隐式声明)</span></span></span></div>
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<td>平凡隐式复制/移动赋值<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=".E9.9D.9E.E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">非成员函数</span></h3>
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<div class="t-member">
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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==, !=, <, <=, >, >=, <=><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 起)</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>
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<p><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">false</span><span class="sy4">;</span> <span class="br0">}</span><br />
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<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">false</span><span class="sy4">;</span> <span class="br0">}</span><br />
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<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><br /></p>
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<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></div>
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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-rev t-until-cxx20">(C++20 前)</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"><=></span><span class="br0">(</span>monostate, monostate<span class="br0">)</span> <span class="kw1">noexcept</span><br />
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<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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<p>所有 <code>std::monostate</code> 实例比较相等。</p>
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<p><code><</code>、 <code><=</code>、 <code>></code>、 <code>>=</code> 及 <code>!=</code> 运算符分别从 <code>operator<=></code> 与 <code>operator==</code> <a href="cpp-language-operators.html#.E5.85.B3.E7.B3.BB.E8.BF.90.E7.AE.97.E7.AC.A6">合成</a>。</p>
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<h3><span class="mw-headline" id=".E8.BE.85.E5.8A.A9.E7.B1.BB">辅助类</span></h3>
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<div class="t-member">
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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><std::monostate></small></span></h2>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1"><></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"><</span>monostate<span class="sy1">></span><span class="sy4">;</span></span></div>
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<p>为 <code>std::monostate</code> 特化 <span class="t-lc"><a href="cpp-utility-hash.html">std::hash</a></span> 算法</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="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <variant></span>
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<span class="co2">#include <iostream></span>
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<span class="kw1">struct</span> S
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<span class="br0">{</span>
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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>
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<span class="kw4">int</span> i<span class="sy4">;</span>
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<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>
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<span class="co1">// 若无 monostate 类型则此声明将失败。</span>
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<span class="co1">// 这是因为 S 不可默认构造。</span>
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<a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">monostate</span>, S<span class="sy1">></span> var<span class="sy4">;</span>
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<span class="co1">// var.index() 现为 0 ——首个元素</span>
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<span class="co1">// std::get<S> 将抛异常！我等需要赋一个值</span>
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var <span class="sy1">=</span> <span class="nu0">12</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span>S<span class="sy1">></span><span class="br0">(</span>var<span class="br0">)</span>.<span class="me1">i</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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<p>输出：</p>
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<pre class="de1">12</pre></div>
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<p><br /></p>
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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><a href="cpp-utility-variant-variant.html"><span class="t-lines"><span>(构造函数)</span></span></a></div>
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</div>
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</td>
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<td>构造 <code>variant</code> 对象<br />
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<span class="t-mark">(公开成员函数)</span></td>
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<div class="visualClear"></div>
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