172 lines
13 KiB
HTML
172 lines
13 KiB
HTML
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<title>std::compare_three_way_result</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_compare_compare_three_way_result 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>compare_three_way_result</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-compare.html"><compare></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> T, <span class="kw1">class</span> U <span class="sy1">=</span> T<span class="sy1">></span><br />
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<span class="kw1">struct</span> compare_three_way_result<span class="sy4">;</span></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>令 <code>t</code> 与 <code>u</code> 分别代表 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <a href="cpp-types-remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <a href="cpp-types-remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1"><</span>U<span class="sy1">></span></span></span> 类型的左值，若表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">t <span class="sy1"><=></span> u</span></span> 良构，则提供等于 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span>t <span class="sy1"><=></span> u<span class="br0">)</span></span></span> 的成员 typedef <code>type</code> ，否则无成员 <code>type</code> 。</p>
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<p>添加 <code>compare_three_way_result</code> 的特化的程序行为未定义。</p>
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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>名字</td>
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<td>定义</td>
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<td><code>type</code></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy1"><=></span></span></span> 在 <code>T</code> 与 <code>U</code> 的 const 限定左值上的结果类型</td>
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<h3><span class="mw-headline" id=".E8.BE.85.E5.8A.A9.E7.B1.BB.E5.9E.8B">辅助类型</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> T, <span class="kw1">class</span> U <span class="sy1">=</span> T<span class="sy1">></span><br />
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<span class="kw1">using</span> compare_three_way_result_t <span class="sy1">=</span> <span class="kw1">typename</span> compare_three_way_result<span class="sy1"><</span>T<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=".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="co1">// 由 Casey Carter 推荐</span>
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<span class="co1">// 参阅： https://github.com/microsoft/STL/pull/385#discussion_r357894054</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U <span class="sy1">=</span> T<span class="sy1">></span>
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<span class="kw1">using</span> compare_three_way_result_t <span class="sy1">=</span> decltype<span class="br0">(</span>
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<a href="cpp-utility-declval.html"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1"><</span><span class="kw4">const</span> <a href="cpp-types-remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span><span class="sy1">></span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><=></span>
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<a href="cpp-utility-declval.html"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1"><</span><span class="kw4">const</span> <a href="cpp-types-remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span><span class="sy1">></span><span class="br0">(</span><span class="br0">)</span>
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<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U <span class="sy1">=</span> T<span class="sy1">></span>
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<span class="kw1">struct</span> compare_three_way_result <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span>
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requires requires <span class="br0">{</span> <span class="kw1">typename</span> compare_three_way_result_t<span class="sy1"><</span>T, U<span class="sy1">></span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw1">struct</span> compare_three_way_result <span class="br0">{</span>
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<span class="kw1">using</span> type <span class="sy1">=</span> compare_three_way_result_t<span class="sy1"><</span>T, U<span class="sy1">></span><span class="sy4">;</span>
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<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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<div class="t-example">
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<div class="t-example-live-link"></div>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <compare></span>
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<span class="co2">#include <type_traits></span>
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<span class="co2">#include <iostream></span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> Ord<span class="sy1">></span>
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<span class="kw4">void</span> print_cmp_type<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">if</span> <span class="kw4">constexpr</span> <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>Ord, std<span class="sy4">::</span><span class="me2">strong_ordering</span><span class="sy1">></span><span class="br0">)</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> <span class="st0">"strong ordering<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="kw1">else</span> <span class="kw1">if</span> <span class="kw4">constexpr</span> <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>Ord, std<span class="sy4">::</span><span class="me2">weak_ordering</span><span class="sy1">></span><span class="br0">)</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> <span class="st0">"weak ordering<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="kw1">else</span> <span class="kw1">if</span> <span class="kw4">constexpr</span> <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>Ord, std<span class="sy4">::</span><span class="me2">partial_ordering</span><span class="sy1">></span><span class="br0">)</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> <span class="st0">"partial ordering<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="kw1">else</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> <span class="st0">"illegal comparison result type<span class="es1">\n</span>"</span><span class="sy4">;</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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print_cmp_type<span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">compare_three_way_result_t</span><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">>></span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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print_cmp_type<span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">compare_three_way_result_t</span><span class="sy1"><</span><span class="kw4">double</span><span class="sy1">>></span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>输出：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="text source-text">
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<pre class="de1">strong ordering
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partial ordering</pre></div>
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</div>
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</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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<table class="t-dsc-begin">
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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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<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-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-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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</table>
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</div>
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<div class="visualClear"></div>
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