168 lines
12 KiB
HTML
168 lines
12 KiB
HTML
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<title>std::common_comparison_category</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_compare_common_comparison_category 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>common_comparison_category</h1>
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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-compare.html"><compare></a></code></div>
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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>... <span class="me1">Ts</span><span class="sy1">></span><br /></span>
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<p><span class="kw1">struct</span> common_comparison_category <span class="br0">{</span><br />
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<span class="kw1">using</span> type <span class="sy1">=</span> <span class="coMULTI">/*see below*/</span> <span class="sy4">;</span><br /></p>
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<span class="br0">}</span><span class="sy4">;</span></div>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<p>类模板 <code>std::common_comparison_category</code> 为所有模板实参 <code>Ts...</code> 所能转换到的最强比较类别提供别名（作为成员 typedef <code>type</code> ）。</p>
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<p><br />
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详细而言， n 个类型 T0...Tn-1 的公共比较类型 U 定义如下：</p>
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<ul>
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<li>若任何 Ti 不是比较类别类型（ <a href="cpp-utility-compare-partial_ordering.html"><tt>std::partial_ordering</tt></a> 、 <a href="cpp-utility-compare-weak_ordering.html"><tt>std::weak_ordering</tt></a> 、 <a href="cpp-utility-compare-strong_ordering.html"><tt>std::strong_ordering</tt></a> ），则 U 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> 。</li>
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<li>否则，若至少一个 Ti 为 <a href="cpp-utility-compare-partial_ordering.html"><tt>std::partial_ordering</tt></a> ，则 U 为 <a href="cpp-utility-compare-partial_ordering.html"><tt>std::partial_ordering</tt></a></li>
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<li>否则，若至少一个 Ti 为 <a href="cpp-utility-compare-weak_ordering.html"><tt>std::weak_ordering</tt></a> ，则 U 为 <a href="cpp-utility-compare-weak_ordering.html"><tt>std::weak_ordering</tt></a></li>
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<li>否则（若每个 Ti 均为 <a href="cpp-utility-compare-strong_ordering.html"><tt>std::strong_ordering</tt></a> 或列表为空），则 U 为 <a href="cpp-utility-compare-strong_ordering.html"><tt>std::strong_ordering</tt></a> 。</li>
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</ul>
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<h3><span class="mw-headline" id=".E6.A8.A1.E6.9D.BF.E5.BD.A2.E5.8F.82">模板形参</span></h3>
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<td>...Ts</td>
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<td>-</td>
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<td>可为空的类型列表</td>
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<h3><span class="mw-headline" id=".E8.BE.85.E5.8A.A9.E6.A8.A1.E6.9D.BF">辅助模板</span></h3>
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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>... <span class="me1">Ts</span> <span class="sy1">></span><br />
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<span class="kw1">using</span> common_comparison_category_t <span class="sy1">=</span> <span class="kw1">typename</span> common_comparison_category<span class="sy1"><</span>Ts...<span class="sy1">></span><span class="sy4">::</span><span class="me2">type</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-cxx20">(C++20 起)</span></td>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E7.B1.BB.E5.9E.8B">成员类型</span></h3>
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<td>成员类型</td>
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<td>定义</td>
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<tr class="t-dsc">
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<td><code>type</code></td>
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<td>最强的公共比较类别（定义如上）</td>
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<h3><span class="mw-headline" id=".E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0">可能的实现</span></h3>
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<div class="cpp source-cpp">
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<pre class="de1"><span class="kw1">namespace</span> detail <span class="br0">{</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw4">unsigned</span> <span class="kw4">int</span><span class="sy1">></span>
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<span class="kw1">struct</span> common_cmpcat_base <span class="br0">{</span> <span class="kw1">using</span> type <span class="sy1">=</span> <span class="kw4">void</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><></span>
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<span class="kw1">struct</span> common_cmpcat_base<span class="sy1"><</span>0u<span class="sy1">></span> <span class="br0">{</span> <span class="kw1">using</span> type <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">strong_ordering</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><></span>
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<span class="kw1">struct</span> common_cmpcat_base<span class="sy1"><</span>2u<span class="sy1">></span> <span class="br0">{</span> <span class="kw1">using</span> type <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">partial_ordering</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><></span>
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<span class="kw1">struct</span> common_cmpcat_base<span class="sy1"><</span>4u<span class="sy1">></span> <span class="br0">{</span> <span class="kw1">using</span> type <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">weak_ordering</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><></span>
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<span class="kw1">struct</span> common_cmpcat_base<span class="sy1"><</span>6u<span class="sy1">></span> <span class="br0">{</span> <span class="kw1">using</span> type <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">partial_ordering</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="br0">}</span> <span class="co1">// namespace detail</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span>...<span class="me1">Ts</span><span class="sy1">></span>
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<span class="kw1">struct</span> common_comparison_category <span class="sy4">:</span>
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detail<span class="sy4">::</span><span class="me2">common_cmpcat_base</span><span class="sy1"><</span><span class="br0">(</span>0u <span class="sy3">|</span> ... <span class="sy3">|</span>
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<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>Ts, std<span class="sy4">::</span><span class="me2">strong_ordering</span><span class="sy1">></span> <span class="sy4">?</span> 0u <span class="sy4">:</span>
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<a href="cpp-types-is_same.html"><span class="kw589">std::<span class="me2">is_same_v</span></span></a><span class="sy1"><</span>Ts, std<span class="sy4">::</span><span class="me2">weak_ordering</span><span class="sy1">></span> <span class="sy4">?</span> 4u <span class="sy4">:</span>
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<a href="cpp-types-is_same.html"><span class="kw589">std::<span class="me2">is_same_v</span></span></a><span class="sy1"><</span>Ts, std<span class="sy4">::</span><span class="me2">partial_ordering</span><span class="sy1">></span> <span class="sy4">?</span> 2u <span class="sy4">:</span> 1u<span class="br0">)</span>
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<span class="br0">)</span><span class="sy1">></span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span></pre></div>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style="">
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<td class="mbox-empty-cell"></td>
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<td class="mbox-text" style="">本节未完成<br />
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原因：暂无示例</td>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<table class="t-dsc-begin">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-compare-strong_ordering.html"><span class="t-lines"><span>strong_ordering</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>三路比较的结果类型，支持所有 6 种运算符且可替换<br />
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<span class="t-mark">(类)</span></td>
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</tr>
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<tr class="t-dsc">
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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-compare-weak_ordering.html"><span class="t-lines"><span>weak_ordering</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>三路比较的结果类型，支持所有 6 种运算符且不可替换<br />
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<span class="t-mark">(类)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-compare-partial_ordering.html"><span class="t-lines"><span>partial_ordering</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>三路比较的结果类型，支持所有 6 种运算符，不可替换，并允许不可比较的值<br />
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<span class="t-mark">(类)</span></td>
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</tr>
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</table>
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</div>
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<div class="visualClear"></div>
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</div>
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