116 lines
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116 lines
19 KiB
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<title>std::equality_comparable, std::equality_comparable_with</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>equality_comparable, <span style="font-size:0.7em; line-height:130%">std::</span>equality_comparable_with</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-concepts.html">concepts</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-concepts.html"><concepts></a></code></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="sy1">></span><br />
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concept equality_comparable <span class="sy1">=</span> __WeaklyEqualityComparableWith<span class="sy1"><</span>T, T<span class="sy1">></span><span class="sy4">;</span></span></div>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></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><br /></span>
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<p>concept equality_comparable_with <span class="sy1">=</span><br />
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std<span class="sy4">::</span><span class="me2">equality_comparable</span><span class="sy1"><</span>T<span class="sy1">></span> <span class="sy3">&&</span><br />
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std<span class="sy4">::</span><span class="me2">equality_comparable</span><span class="sy1"><</span>U<span class="sy1">></span> <span class="sy3">&&</span><br />
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std<span class="sy4">::</span><span class="me2">common_reference_with</span><span class="sy1"><</span><br />
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<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>,<br />
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<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="sy3">&&</span><br />
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std<span class="sy4">::</span><span class="me2">equality_comparable</span><span class="sy1"><</span><br />
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std<span class="sy4">::</span><span class="me2">common_reference_t</span><span class="sy1"><</span><br />
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<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>,<br />
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<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="sy3">&&</span><br /></p>
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__WeaklyEqualityComparableWith<span class="sy1"><</span>T, U<span class="sy1">></span><span class="sy4">;</span></div>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></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><br /></span>
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<p>concept __WeaklyEqualityComparableWith <span class="sy1">=</span> <span class="co1">// 仅用于解释</span><br />
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requires<span class="br0">(</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> t,<br />
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<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> u<span class="br0">)</span> <span class="br0">{</span><br />
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<span class="br0">{</span> t <span class="sy1">==</span> u <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">></span> boolean<span class="sy2">-</span>testable<span class="sy4">;</span><br />
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<span class="br0">{</span> t <span class="sy3">!</span><span class="sy1">=</span> u <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">></span> boolean<span class="sy2">-</span>testable<span class="sy4">;</span><br />
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<span class="br0">{</span> u <span class="sy1">==</span> t <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">></span> boolean<span class="sy2">-</span>testable<span class="sy4">;</span><br />
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<span class="br0">{</span> u <span class="sy3">!</span><span class="sy1">=</span> t <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">></span> boolean<span class="sy2">-</span>testable<span class="sy4">;</span><br /></p>
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<span class="br0">}</span><span class="sy4">;</span></div>
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<td>(3)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<div class="t-li1"><span class="t-li">1)</span> 概念 <code>std::equality_comparable</code> 指定比较运算符 <code>==</code> 和 <code>!=</code> 在 <code>T</code> 上反映相等性： <code>==</code> 若且唯若运算数相等才生成 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 。</div>
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<div class="t-li1"><span class="t-li">2)</span> 概念 <code>std::equality_comparable_with</code> 指定比较运算符 <code>==</code> 和 <code>!=</code> 在（可能混合的） <code>T</code> 和 <code>U</code> 运算数上生成与相等一致的结果。比较混合的运算数产生的结果等价于比较转换到其共用类型的运算数。</div>
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<div class="t-li1"><span class="t-li">3)</span> 仅用于阐释的概念 <code><i>__WeaklyEqualityComparableWith</i></code> 指定 <code>T</code> 类型对象和 <code>U</code> 类型对象能用 <code>==</code> 和 <code>!=</code> 彼此比较相等（以任一顺序），而比较结果一致。</div>
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<h3><span class="mw-headline" id=".E8.AF.AD.E4.B9.89.E8.A6.81.E6.B1.82">语义要求</span></h3>
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<p>这些概念仅若其所蕴含的概念均被实现才得到实现。</p>
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<div class="t-li1"><span class="t-li">1)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">equality_comparable</span><span class="sy1"><</span>T<span class="sy1">></span></span></span> 仅若符合下列条件才得到实现。给定 <code>T</code> 类型对象 <code>a</code> 和 <code>b</code> ， <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>a <span class="sy1">==</span> b<span class="br0">)</span></span></span> 若且唯若 <code>a</code> 和 <code>b</code> 相等才为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 。同 <span class="t-c"><span class="mw-geshi cpp source-cpp">a <span class="sy1">==</span> b</span></span> 保持相等性的要求，这隐含 <code>==</code> 为对称且传递，而且 <code>==</code> 进而对所有至少等于一个其他对象的 <code>a</code> 自反。</div>
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<div class="t-li1"><span class="t-li">2)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">equality_comparable_with</span><span class="sy1"><</span>T, U<span class="sy1">></span></span></span> 仅若符合下列条件才得到实现，给定任何 <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> 类型左值 <code>t</code> 和任何 <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> 类型左值 <code>u</code> ，并令 <code>C</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">common_reference_t</span><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="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></span> ，则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>t <span class="sy1">==</span> u<span class="br0">)</span> <span class="sy1">==</span> <span class="kw4">bool</span><span class="br0">(</span>C<span class="br0">(</span>t<span class="br0">)</span> <span class="sy1">==</span> C<span class="br0">(</span>u<span class="br0">)</span><span class="br0">)</span></span></span> 。</div>
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<div class="t-li1"><span class="t-li">3)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp">__WeaklyEqualityComparableWith<span class="sy1"><</span>T, U<span class="sy1">></span></span></span> 仅若符合下列条件才得到实现。给定
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<ul>
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<li><code>t</code> ， <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> 类型左值，和</li>
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<li><code>u</code> ， <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> 类型左值，</li>
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</ul>
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<p>下列为真：</p>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">t <span class="sy1">==</span> u</span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp">u <span class="sy1">==</span> t</span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp">t <span class="sy3">!</span><span class="sy1">=</span> u</span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp">u <span class="sy3">!</span><span class="sy1">=</span> t</span></span> 拥有相同定义域；</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>u <span class="sy1">==</span> t<span class="br0">)</span> <span class="sy1">==</span> <span class="kw4">bool</span><span class="br0">(</span>t <span class="sy1">==</span> u<span class="br0">)</span></span></span> ；</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>t <span class="sy3">!</span><span class="sy1">=</span> u<span class="br0">)</span> <span class="sy1">==</span> <span class="sy3">!</span><span class="kw4">bool</span><span class="br0">(</span>t <span class="sy1">==</span> u<span class="br0">)</span></span></span> ；且</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>u <span class="sy3">!</span><span class="sy1">=</span> t<span class="br0">)</span> <span class="sy1">==</span> <span class="kw4">bool</span><span class="br0">(</span>t <span class="sy3">!</span><span class="sy1">=</span> u<span class="br0">)</span></span></span> 。</li>
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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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<p>除非另外提醒，每个用于 <i>requires 表达式</i>中的表达式都要求保持相等性且稳定，而表达式的求值必须只修改其非常操作数。必须不修改常操作数。</p>
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<h3><span class="mw-headline" id=".E9.9A.90.E5.BC.8F.E8.A1.A8.E8.BE.BE.E5.BC.8F.E5.8F.98.E7.A7.8D">隐式表达式变种</span></h3>
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<p>使用不修改某 const 左值运算数的表达式的 <i>requires 表达式</i>亦隐式要求该表达式的额外变种对给定运算数接受非 const 左值或（可为 const 的）右值，除非以有区别的语义显式要求这种表达式变种。这些<i>隐式表达式变种</i>必须符合与声明的表达式的相同的语义。不指定实现以何种程度校验变种的语法。</p>
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