65 lines
6.0 KiB
HTML
65 lines
6.0 KiB
HTML
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<title>C++ 具名要求：可相等比较 (EqualityComparable)</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_named_req_EqualityComparable skin-cppreference2 action-view cpp-navbar">
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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">可相等比较</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(EqualityComparable)</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">可相等比较</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(EqualityComparable)</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且拥有其指定的效果</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>返回类型</th>
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<th>要求</th>
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</tr>
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<tr>
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<td><code><b>a == b</b></code></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>建立一种<a rel="nofollow" class="external text" href="https://en.wikipedia.org/wiki/Equivalence_relation">等价关系</a>，即满足下列性质：
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<ul>
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<li>对于 <code>a</code> 的所有值，<span class="t-c"><span class="mw-geshi cpp source-cpp">a <span class="sy1">==</span> a</span></span> 产生 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</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">b <span class="sy1">==</span> a</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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</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">用户定义的 operator==</a>。</p>
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<p>对于既为<span style="font-style:italic">可相等比较</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(EqualityComparable)</span> 亦为<a href="cpp-named_req-LessThanComparable.html"><span style="font-style:italic">可小于比较</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LessThanComparable)</span></a> 的类型， C++ 标准库对<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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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-concepts-equality_comparable.html"><span class="t-lines"><span>equality_comparable</span><span>equality_comparable_with</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>指定运算符 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">==</span></span></span> 为等价关系<br />
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<span class="t-mark">(概念)</span></td>
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