128 lines
12 KiB
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128 lines
12 KiB
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<title>std::to_underlying</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_to_underlying 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>to_underlying</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-utility.html">utility</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-utility.html"><utility></a></code></div>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> Enum <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> <a href="cpp-types-underlying_type.html"><span class="kw638">std::<span class="me2">underlying_type_t</span></span></a><span class="sy1"><</span>Enum<span class="sy1">></span> to_underlying<span class="br0">(</span> Enum e <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></td>
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<p>转换枚举到其底层类型。等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> <span class="kw1">static_cast</span><span class="sy1"><</span><a href="cpp-types-underlying_type.html"><span class="kw638">std::<span class="me2">underlying_type_t</span></span></a><span class="sy1"><</span>Enum<span class="sy1">>></span><span class="br0">(</span>e<span class="br0">)</span><span class="sy4">;</span></span></span> 。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<td>e</td>
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<td>-</td>
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<td>要转换的枚举值</td>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<p><code>Enum</code> 的底层类型的整数值，从 <code>e</code> 转换得到。</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p><code>std::to_underlying</code> 能用于避免将枚举类型转换成其底层类型以外的整数类型。</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 <cstdint></span>
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<span class="co2">#include <iomanip></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <type_traits></span>
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<span class="co2">#include <utility></span>
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<span class="kw1">namespace</span> cxx <span class="br0">{</span>
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<span class="co2">#if defined(__cpp_lib_to_underlying)</span>
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<span class="kw1">using</span> std<span class="sy4">::</span><span class="me2">to_underlying</span><span class="sy4">;</span>
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<span class="co2">#else</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> Enum<span class="sy1">></span>
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<span class="kw4">constexpr</span> <a href="cpp-types-underlying_type.html"><span class="kw638">std::<span class="me2">underlying_type_t</span></span></a><span class="sy1"><</span>Enum<span class="sy1">></span>
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to_underlying<span class="br0">(</span>Enum e<span class="br0">)</span> <span class="kw1">noexcept</span> <span class="br0">{</span>
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<span class="kw1">return</span> <span class="kw1">static_cast</span><span class="sy1"><</span><a href="cpp-types-underlying_type.html"><span class="kw638">std::<span class="me2">underlying_type_t</span></span></a><span class="sy1"><</span>Enum<span class="sy1">>></span><span class="br0">(</span>e<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="co2">#endif</span>
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<span class="br0">}</span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw2">enum</span> <span class="kw1">class</span> E1 <span class="sy4">:</span> <span class="kw4">char</span> <span class="br0">{</span> e <span class="br0">}</span><span class="sy4">;</span>
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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"><</span><span class="kw4">char</span>, decltype<span class="br0">(</span>cxx<span class="sy4">::</span><span class="me2">to_underlying</span><span class="br0">(</span>E1<span class="sy4">::</span><span class="me2">e</span><span class="br0">)</span><span class="br0">)</span><span class="sy1">></span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw2">enum</span> <span class="kw1">struct</span> E2 <span class="sy4">:</span> <span class="kw4">long</span> <span class="br0">{</span> e <span class="br0">}</span><span class="sy4">;</span>
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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"><</span><span class="kw4">long</span>, decltype<span class="br0">(</span>cxx<span class="sy4">::</span><span class="me2">to_underlying</span><span class="br0">(</span>E2<span class="sy4">::</span><span class="me2">e</span><span class="br0">)</span><span class="br0">)</span><span class="sy1">></span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw2">enum</span> E3 <span class="sy4">:</span> <span class="kw4">unsigned</span> <span class="br0">{</span> e <span class="br0">}</span><span class="sy4">;</span>
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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"><</span><span class="kw4">unsigned</span>, decltype<span class="br0">(</span>cxx<span class="sy4">::</span><span class="me2">to_underlying</span><span class="br0">(</span>e<span class="br0">)</span><span class="br0">)</span><span class="sy1">></span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw2">enum</span> <span class="kw1">class</span> ColorMask <span class="sy4">:</span> <a href="cpp-types-integer.html"><span class="kw122">std::<span class="me2">uint32_t</span></span></a> <span class="br0">{</span>
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red <span class="sy1">=</span> <span class="nu12">0xFF</span>, green <span class="sy1">=</span> <span class="br0">(</span>red <span class="sy1"><<</span> <span class="nu0">8</span><span class="br0">)</span>, blue <span class="sy1">=</span> <span class="br0">(</span>green <span class="sy1"><<</span> <span class="nu0">8</span><span class="br0">)</span>, alpha <span class="sy1">=</span> <span class="br0">(</span>blue <span class="sy1"><<</span> <span class="nu0">8</span><span class="br0">)</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <a href="cpp-io-manip-hex.html"><span class="kw1782">std::<span class="me2">hex</span></span></a> <span class="sy1"><<</span> <a href="cpp-io-manip-uppercase.html"><span class="kw1774">std::<span class="me2">uppercase</span></span></a> <span class="sy1"><<</span> <a href="cpp-io-manip-setfill.html"><span class="kw1795">std::<span class="me2">setfill</span></span></a><span class="br0">(</span><span class="st0">'0'</span><span class="br0">)</span>
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<span class="sy1"><<</span> <a href="cpp-io-manip-setw.html"><span class="kw1797">std::<span class="me2">setw</span></span></a><span class="br0">(</span><span class="nu0">8</span><span class="br0">)</span> <span class="sy1"><<</span> cxx<span class="sy4">::</span><span class="me2">to_underlying</span><span class="br0">(</span>ColorMask<span class="sy4">::</span><span class="me2">red</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <a href="cpp-io-manip-setw.html"><span class="kw1797">std::<span class="me2">setw</span></span></a><span class="br0">(</span><span class="nu0">8</span><span class="br0">)</span> <span class="sy1"><<</span> cxx<span class="sy4">::</span><span class="me2">to_underlying</span><span class="br0">(</span>ColorMask<span class="sy4">::</span><span class="me2">green</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <a href="cpp-io-manip-setw.html"><span class="kw1797">std::<span class="me2">setw</span></span></a><span class="br0">(</span><span class="nu0">8</span><span class="br0">)</span> <span class="sy1"><<</span> cxx<span class="sy4">::</span><span class="me2">to_underlying</span><span class="br0">(</span>ColorMask<span class="sy4">::</span><span class="me2">blue</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <a href="cpp-io-manip-setw.html"><span class="kw1797">std::<span class="me2">setw</span></span></a><span class="br0">(</span><span class="nu0">8</span><span class="br0">)</span> <span class="sy1"><<</span> cxx<span class="sy4">::</span><span class="me2">to_underlying</span><span class="br0">(</span>ColorMask<span class="sy4">::</span><span class="me2">alpha</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="co1">// std::underlying_type_t<ColorMask> x = ColorMask::alpha; // 错误：无已知转换</span>
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<span class="br0">[</span><span class="br0">[</span>maybe_unused<span class="br0">]</span><span class="br0">]</span>
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<a href="cpp-types-underlying_type.html"><span class="kw638">std::<span class="me2">underlying_type_t</span></span></a><span class="sy1"><</span>ColorMask<span class="sy1">></span> y <span class="sy1">=</span> cxx<span class="sy4">::</span><span class="me2">to_underlying</span><span class="br0">(</span>ColorMask<span class="sy4">::</span><span class="me2">alpha</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK</span>
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<span class="br0">}</span></pre></div>
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<p>输出：</p>
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<div class="text source-text">
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<pre class="de1">000000FF
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0000FF00
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00FF0000
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FF000000</pre></div>
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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 class="t-dsc-member-div">
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<div><a href="cpp-types-underlying_type.html"><span class="t-lines"><span>underlying_type</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>获取给定枚举类型的底层整数类型<br />
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<span class="t-mark">(类模板)</span></td>
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