182 lines
18 KiB
HTML
182 lines
18 KiB
HTML
<!DOCTYPE html>
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<title>std::compare_weak_order_fallback</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_compare_compare_weak_order_fallback skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>compare_weak_order_fallback</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 id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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<td>
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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">inline</span> <span class="kw1">namespace</span> <span class="coMULTI">/* unspecified */</span> <span class="br0">{</span><br /></span>
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<p> <span class="kw1">inline</span> <span class="kw4">constexpr</span> <span class="coMULTI">/* unspecified */</span><br />
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compare_weak_order_fallback <span class="sy1">=</span> <span class="coMULTI">/* unspecified */</span><span class="sy4">;</span><br /></p>
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<span class="br0">}</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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<div>调用签名</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> T, <span class="kw1">class</span> U <span class="sy1">></span><br /></span>
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<p> requires <span class="coMULTI">/* see below */</span><br />
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<span class="kw4">constexpr</span> std<span class="sy4">::</span><span class="me2">weak_ordering</span><br /></p>
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compare_weak_order_fallback<span class="br0">(</span>T<span class="sy3">&&</span> t, U<span class="sy3">&&</span> u<span class="br0">)</span> <span class="kw1">noexcept</span><span class="br0">(</span><span class="coMULTI">/* see below */</span><span class="br0">)</span><span class="sy4">;</span></div>
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<p>进行 <code>t</code> 与 <code>u</code> 上的三路比较并产生 <span class="t-lc">std::weak_ordering</span> 类型结果，即使运算符 <code><=></code> 不可用。</p>
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<p>设 <code>t</code> 与 <code>u</code> 为表达式而 <code>T</code> 与 <code>U</code> 分别代表 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="br0">(</span>t<span class="br0">)</span><span class="br0">)</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="br0">(</span>u<span class="br0">)</span><span class="br0">)</span></span></span> ， <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">compare_weak_order_fallback</span><span class="br0">(</span>t, u<span class="br0">)</span></span></span> 表达式等价于：</p>
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<ul>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_same.html"><span class="kw589">std::<span class="me2">is_same_v</span></span></a><span class="sy1"><</span><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>T<span class="sy1">></span>, <a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>U<span class="sy1">>></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ：
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<ul>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">weak_order</span><span class="br0">(</span>t, u<span class="br0">)</span></span></span> ，若它为良构的表达式；</li>
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<li>否则，为</li>
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</ul>
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</li>
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</ul>
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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">t <span class="sy1">==</span> u <span class="sy4">?</span> std<span class="sy4">::</span><span class="me2">weak_ordering</span><span class="sy4">::</span><span class="me2">equivalent</span> <span class="sy4">:</span>
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t <span class="sy1"><</span> u <span class="sy4">?</span> std<span class="sy4">::</span><span class="me2">weak_ordering</span><span class="sy4">::</span><span class="me2">less</span> <span class="sy4">:</span>
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std<span class="sy4">::</span><span class="me2">weak_ordering</span><span class="sy4">::</span><span class="me2">greater</span></pre></div>
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<dd>
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<dd>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">t <span class="sy1">==</span> u</span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">t <span class="sy1"><</span> u</span></span> 均为良构且可转换到 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span> ，除了只求值 <code>t</code> 与 <code>u</code> 一次。</dd>
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</dl>
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</dd>
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</dl>
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<ul>
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<li>所有其他情况下 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">compare_weak_order_fallback</span><span class="br0">(</span>t, u<span class="br0">)</span></span></span> 为非良构，这能在出现于模板实例化的立即语境时导致<a href="cpp-language-sfinae.html">替换失败</a>。</li>
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</ul>
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<h3><span class="mw-headline" id=".E8.A1.A8.E8.BE.BE.E5.BC.8F.E7.AD.89.E4.BB.B7">表达式等价</span></h3>
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<p>表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> <i>表达式等价</i>于表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">f</span></span> ，若 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">f</span></span> 拥有相同效果，均为<a href="cpp-language-noexcept_spec.html">潜在抛出</a>或均非潜在抛出（即 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">noexcept</span><span class="br0">(</span>e<span class="br0">)</span> <span class="sy1">==</span> <span class="kw1">noexcept</span><span class="br0">(</span>f<span class="br0">)</span></span></span> ），且均为<a href="cpp-language-constant_expression.html">常量子表达式</a>或均非常量子表达式。</p>
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<h3><span class="mw-headline" id=".E5.AE.9A.E5.88.B6.E7.82.B9.E5.AF.B9.E8.B1.A1">定制点对象</span></h3>
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<p>名字 <code>std::compare_weak_order_fallback</code> 代表一个<i>定制点对象</i>，它是<a href="cpp-named_req-LiteralType.html">字面</a> <a href="cpp-concepts-semiregular.html"><code>semiregular</code></a> 类类型的 const <a href="cpp-named_req-FunctionObject.html">函数对象</a>。为说明目的，以 <code><i>__compare_weak_order_fallback_fn</i></code> 表示其类型的 cv 无限定版本。</p>
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<p><code><i>__compare_weak_order_fallback_fn</i></code> 的所有实例均相等。在相同参数上调用类型 <code><i>__compare_weak_order_fallback_fn</i></code> 的不同实例的效果是等价的，无关乎指代该实例的表达式是左值还是右值，以及是否为 const 限定（然而不要求 volatile 限定的实例可调用）。从而能自由地复制 <code>std::compare_weak_order_fallback</code> 并且能彼此替代地使用其副本。</p>
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<p>给定类型集合 <code>Args...</code> ，若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-declval.html"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1"><</span>Args<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span>...</span></span> 满足上面对于 <code>std::compare_weak_order_fallback</code> 的参数要求，则 <code><i>__compare_weak_order_fallback_fn</i></code> 实现 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">invocable</span><span class="sy1"><</span>__compare_weak_order_fallback_fn, Args...<span class="sy1">></span></span></span>、 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">invocable</span><span class="sy1"><</span><span class="kw4">const</span> __compare_weak_order_fallback_fn, Args...<span class="sy1">></span></span></span>、 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">invocable</span><span class="sy1"><</span>__compare_weak_order_fallback_fn<span class="sy3">&</span>, Args...<span class="sy1">></span></span></span> 及 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">invocable</span><span class="sy1"><</span><span class="kw4">const</span> __compare_weak_order_fallback_fn<span class="sy3">&</span>, Args...<span class="sy1">></span></span></span> 。否则， <code><i>__compare_weak_order_fallback_fn</i></code> 的函数调用运算符不参与重载决议。</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="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 <iostream></span>
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<span class="co2">#include <compare></span>
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<span class="co1">// 不支持 <=></span>
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<span class="kw1">struct</span> Rational_1 <span class="br0">{</span>
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<span class="kw4">int</span> num<span class="sy4">;</span>
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<span class="kw4">int</span> den<span class="sy4">;</span> <span class="co1">// > 0</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">inline</span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1"><</span><span class="br0">(</span>Rational_1 lhs, Rational_1 rhs<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> lhs.<span class="me1">num</span> <span class="sy2">*</span> rhs.<span class="me1">den</span> <span class="sy1"><</span> rhs.<span class="me1">num</span> <span class="sy2">*</span> lhs.<span class="me1">den</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">inline</span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1">==</span><span class="br0">(</span>Rational_1 lhs, Rational_1 rhs<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> lhs.<span class="me1">num</span> <span class="sy2">*</span> rhs.<span class="me1">den</span> <span class="sy1">==</span> rhs.<span class="me1">num</span> <span class="sy2">*</span> lhs.<span class="me1">den</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="co1">// 支持 <=></span>
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<span class="kw1">struct</span> Rational_2 <span class="br0">{</span>
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<span class="kw4">int</span> num<span class="sy4">;</span>
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<span class="kw4">int</span> den<span class="sy4">;</span> <span class="co1">// > 0</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">inline</span> <span class="kw4">constexpr</span> std<span class="sy4">::</span><span class="me2">weak_ordering</span> operator<span class="sy1"><=></span><span class="br0">(</span>Rational_2 lhs, Rational_2 rhs<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> lhs.<span class="me1">num</span> <span class="sy2">*</span> rhs.<span class="me1">den</span> <span class="sy1"><=></span> rhs.<span class="me1">num</span> <span class="sy2">*</span> lhs.<span class="me1">den</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">void</span> print<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">weak_ordering</span> value<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">if</span> <span class="br0">(</span>value <span class="sy1">==</span> <span class="nu0">0</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">"equal<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="br0">(</span>value <span class="sy1"><</span> <span class="nu0">0</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">"less<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">"greater<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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Rational_1 a<span class="br0">{</span><span class="nu0">1</span>, <span class="nu0">2</span><span class="br0">}</span><span class="sy4">;</span>
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Rational_1 b<span class="br0">{</span><span class="nu0">3</span>, <span class="nu0">4</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="co1">// print(a <=> b); // 不起效</span>
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print<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">compare_weak_order_fallback</span><span class="br0">(</span>a, b<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 起效，默认到 < 与 ==</span>
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Rational_2 c<span class="br0">{</span><span class="nu0">6</span>, <span class="nu0">5</span><span class="br0">}</span><span class="sy4">;</span>
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Rational_2 d<span class="br0">{</span><span class="nu0">8</span>, <span class="nu0">7</span><span class="br0">}</span><span class="sy4">;</span>
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print<span class="br0">(</span>c <span class="sy1"><=></span> d<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 起效</span>
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print<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">compare_weak_order_fallback</span><span class="br0">(</span>c, d<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 起效</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">less
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greater
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greater</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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<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_order.html"><span class="t-lines"><span>weak_order</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>进行三路比较并产生 <code>std::weak_ordering</code> 类型结果<br />
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<span class="t-mark">(定制点对象)</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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