159 lines
14 KiB
HTML
159 lines
14 KiB
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::ranges::less</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_functional_ranges_less 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::ranges::</span>less</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-functional.html">functional</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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<div>定义于头文件 <code><a href="cpp-header-functional.html"><functional></a></code></div>
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</td>
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<td></td>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">struct</span> less<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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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<td></td>
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<p>进行比较的函数对象。从实参推导函数调用运算符的形参类型（但非返回类型）。</p>
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<h3><span class="mw-headline" id=".E5.AE.9E.E7.8E.B0.E5.AE.9A.E4.B9.89.E7.9A.84.E6.8C.87.E9.92.88.E4.B8.8A.E7.9A.84.E4.B8.A5.E6.A0.BC.E5.85.A8.E5.BA.8F">实现定义的指针上的严格全序</span></h3>
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<p>若参数间的 <code><</code> 运算符调用指针的内建比较运算符，则函数调用运算符产生实现定义的指针上的严格全序，即使内建的 <code><</code> 运算符不如此。</p>
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<p>实现定义的严格全序与比较运算符（ <code><=></code>、 <code><</code>、 <code>></code>、 <code><=</code> 及 <code>>=</code> ）所施加的偏序一致，并在下列标准函数对象间一致：</p>
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<ul>
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<li><span class="t-lc"><a href="cpp-utility-functional-less.html">std::less</a></span>、 <span class="t-lc"><a href="cpp-utility-functional-greater.html">std::greater</a></span>、 <span class="t-lc"><a href="cpp-utility-functional-less_equal.html">std::less_equal</a></span> 及 <span class="t-lc"><a href="cpp-utility-functional-greater_equal.html">std::greater_equal</a></span> ，当模板实参为指针类型或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> 时</li>
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</ul>
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<ul>
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<li><a href="cpp-utility-functional-ranges-equal_to.html"><tt>std::ranges::equal_to</tt></a>、 <a href="cpp-utility-functional-ranges-not_equal_to.html"><tt>std::ranges::not_equal_to</tt></a>、 <strong class="selflink"><tt>std::ranges::less</tt></strong>、 <a href="cpp-utility-functional-ranges-greater.html"><tt>std::ranges::greater</tt></a>、 <a href="cpp-utility-functional-ranges-less_equal.html"><tt>std::ranges::less_equal</tt></a>、 <a href="cpp-utility-functional-ranges-greater_equal.html"><tt>std::ranges::greater_equal</tt></a> 及 <span class="t-lc">std::compare_three_way</span></li>
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</ul>
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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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<table class="t-dsc-begin">
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<td>成员类型</td>
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<td>定义</td>
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<td><code>is_transparent</code></td>
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<td>/* 未指定 */</td>
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</table>
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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 class="t-dsc-member-div">
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<div><span class="t-lines"><span>operator()</span></span></div>
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</td>
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<td>检查第一参数是否<i>小于</i>第二参数<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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<div class="t-member">
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<h2><span class="mw-headline" id="std::ranges::less::operator.28.29"><span style="font-size:0.7em; line-height:130%">std::ranges::less::</span>operator()</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">class</span> T, <span class="kw1">class</span> U <span class="sy1">></span><br /></span>
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<p> requires std<span class="sy4">::</span><span class="me2">totally_ordered_with</span><span class="sy1"><</span>T, U<span class="sy1">></span> <span class="co1">// 有不同的语义要求</span><br /></p>
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<span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>T<span class="sy3">&&</span> t, U<span class="sy3">&&</span> u<span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span></div>
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<td class="t-dcl-nopad"></td>
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<p>比较 <code>t</code> 与 <code>u</code> ，等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> <a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span>t<span class="br0">)</span> <span class="sy1"><</span> <a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="br0">(</span>u<span class="br0">)</span><span class="sy4">;</span></span></span> ，除非该表达式解析成调用比较指针的内建 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy1"><</span></span></span> 。</p>
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<p>当函数调用不调用内建的比较指针的运算符时，若 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">totally_ordered_with</span><span class="sy1"><</span>T, U<span class="sy1">></span></span></span> 未得到满足则行为未定义。</p>
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<p>调用可能会使用比较 <code>P</code> 类型指针的内建运算符时，转而以如下方式确定结果：</p>
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<ul>
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<li>若在 <code>P</code> 类型的所有指针值上的实现定义严格全序中，（可能为转换后的）第一参数的值前趋（可能为转换后的）第二参数的值，则返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 。此严格全序与内建运算符 <code><</code> 、 <code>></code> 、 <code><=</code> 及 <code>>=</code> 所施加的偏序一致。</li>
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<li>否则返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。</li>
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</ul>
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<p>行为未定义，除非从 <code>T</code> 与 <code>U</code> 到 <code>P</code> 的转换序列均保持相等性（见下方）。</p>
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<h3><span class="mw-headline" id=".E7.9B.B8.E7.AD.89.E6.80.A7.E4.BF.9D.E6.8C.81">相等性保持</span></h3>
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<p>若表达式对给定的相等输入产生相等输出，则它<i>保持相等性</i>。</p>
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<ul>
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<li>表达式的输入由其操作数组成。</li>
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<li>表达式的输出由其结果和表达式所修改的所有操作数（若存在）组成。</li>
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</ul>
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<p>在标准概念的规范中，操作数定义为仅包含下列内容的最大子表达式：</p>
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<ul>
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<li><a href="cpp-language-expressions.html#.E5.88.9D.E7.AD.89.E8.A1.A8.E8.BE.BE.E5.BC.8F">标识表达式</a>，及</li>
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<li>对 <a href="cpp-utility-move.html"><tt>std::move</tt></a> 、 <span class="t-lc"><a href="cpp-utility-forward.html">std::forward</a></span> 与 <span class="t-lc"><a href="cpp-utility-declval.html">std::declval</a></span> 的调用。</li>
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</ul>
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<p>每个操作数的 cv 限定与值类别，是通过假设每个模板类型形参代表一个 cv 无限定的非数组对象类型确定的。</p>
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<p>进一步要求每个要求保持相等性的表达式都<i>稳定</i>：这种表达式带相同输入对象的二次求值必须拥有相等的输出，而无任何对这些输入对象的显式中间修改。</p>
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</div>
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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>不同于 <span class="t-lc"><a href="cpp-utility-functional-less.html">std::less</a></span> ， <code>std::ranges::less</code> 要求全部六个比较运算符 <code><</code> 、 <code><=</code> 、 <code>></code> 、 <code>>=</code> 、 <code>==</code> 及 <code>!=</code> 合法（经由 <a href="cpp-concepts-totally_ordered.html"><code>totally_ordered_with</code></a> 约束）。</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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<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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</tr>
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</table>
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<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
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<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。</p>
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<table class="dsctable" style="font-size:0.8em">
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<tr>
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<th>DR</th>
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<th>应用于</th>
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<th>出版时的行为</th>
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<th>正确行为</th>
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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue3530">LWG 3530</a></td>
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<td>C++20</td>
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<td>在比较指针时放松了语法检查</td>
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<td>仅放松语义要求</td>
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</tr>
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</table>
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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-utility-functional-less.html"><span class="t-lines"><span>less</span></span></a></div>
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</div>
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</td>
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<td>实现 <span class="t-c"><span class="mw-geshi cpp source-cpp">x <span class="sy1"><</span> y</span></span> 的函数对象<br />
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<span class="t-mark">(类模板)</span></td>
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</table>
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</div>
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<div class="visualClear"></div>
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