65 lines
7.1 KiB
HTML
65 lines
7.1 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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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>C++ 具名要求：可小于比较 (LessThanComparable)</title>
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<link rel="stylesheet" href="ext.css" />
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<link rel="stylesheet" href="site_modules.css" />
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_named_req_LessThanComparable skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<div id="content"><a id="top"></a>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">C++ 具名要求：</span><span style="font-style:italic">可小于比较</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LessThanComparable)</span></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-named_req.html">named req</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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<p>类型必须能使用 < 运算符且结果应当具有标准语义。</p>
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<h3><span class="mw-headline" id=".E8.A6.81.E6.B1.82">要求</span></h3>
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<p>以下情况下，类型 <code>T</code> 满足<span style="font-style:italic">可小于比较</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LessThanComparable)</span> ：</p>
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<p>给定</p>
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<ul>
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<li><code>T</code> 或 <code>const T</code> 类型的表达式 <code>a</code>、<code>b</code> 与 <code>c</code></li>
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</ul>
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<p>下列表达式必须合法并拥有其指定的效果</p>
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<table class="wikitable">
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<tr>
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<th>表达式</th>
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<th>返回值</th>
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<th>要求</th>
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</tr>
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<tr>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">a <span class="sy1"><</span> b</span></span></td>
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<td><a href="cpp-language-implicit_conversion.html">可隐式转换</a>为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span></td>
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<td>建立<a href="https://en.wikipedia.org/wiki/Strict_weak_ordering" class="extiw" title="enwiki:Strict weak ordering">严格弱序</a>关系，即具有下列属性：
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<ul>
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<li>对于所有 <code>a</code>，<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">!</span><span class="br0">(</span>a <span class="sy1"><</span> a<span class="br0">)</span></span></span></li>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">a <span class="sy1"><</span> b</span></span> 则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">!</span><span class="br0">(</span>b <span class="sy1"><</span> a<span class="br0">)</span></span></span></li>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">a <span class="sy1"><</span> b</span></span> 且 <span class="t-c"><span class="mw-geshi cpp source-cpp">b <span class="sy1"><</span> c</span></span> 则 <span class="t-c"><span class="mw-geshi cpp source-cpp">a <span class="sy1"><</span> c</span></span></li>
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<li>定义 <span class="t-c"><span class="mw-geshi cpp source-cpp">equiv<span class="br0">(</span>a, b<span class="br0">)</span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">!</span><span class="br0">(</span>a <span class="sy1"><</span> b<span class="br0">)</span> <span class="sy3">&&</span> <span class="sy3">!</span><span class="br0">(</span>b <span class="sy1"><</span> a<span class="br0">)</span></span></span>，若 <span class="t-c"><span class="mw-geshi cpp source-cpp">equiv<span class="br0">(</span>a, b<span class="br0">)</span></span></span> 且 <span class="t-c"><span class="mw-geshi cpp source-cpp">equiv<span class="br0">(</span>b, c<span class="br0">)</span></span></span>，则 <span class="t-c"><span class="mw-geshi cpp source-cpp">equiv<span class="br0">(</span>a, c<span class="br0">)</span></span></span></li>
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</ul>
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</td>
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</tr>
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</table>
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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>为满足此要求，没有内建<a href="cpp-language-operator_comparison.html">比较运算符</a>的类型必须提供<a href="cpp-language-operators.html#.E5.85.B3.E7.B3.BB.E8.BF.90.E7.AE.97.E7.AC.A6">用户定义的 operator<</a>。</p>
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<p>对于既<a href="cpp-named_req-EqualityComparable.html"><span style="font-style:italic">可相等比较</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(EqualityComparable)</span></a> 又<span style="font-style:italic">可小于比较</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LessThanComparable)</span> 的类型，C++ 标准库在<i>相等</i>（即表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">a <span class="sy1">==</span> b</span></span> 的值）和<i>等价</i>（即表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">!</span><span class="br0">(</span>a <span class="sy1"><</span> b<span class="br0">)</span> <span class="sy3">&&</span> <span class="sy3">!</span><span class="br0">(</span>b <span class="sy1"><</span> a<span class="br0">)</span></span></span> 的值）间做出区别。</p>
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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-named-req-div">
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<div><a href="cpp-named_req-Compare.html"><span class="t-lines"><span>Compare</span><span><i>比较</i></span></span></a></div>
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</div>
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</td>
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<td>建立顺序关系的<a href="cpp-named_req-BinaryPredicate.html"><span style="font-style:italic">二元谓词</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(BinaryPredicate)</span></a><br />
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<span class="t-mark">(具名要求)</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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</div>
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</div>
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</body>
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</html>
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