64 lines
5.3 KiB
HTML
64 lines
5.3 KiB
HTML
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<title>std::predicate</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_concepts_predicate 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>predicate</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> F, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">></span><br /></span>
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<p>concept predicate <span class="sy1">=</span><br />
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std<span class="sy4">::</span><span class="me2">regular_invocable</span><span class="sy1"><</span>F, Args...<span class="sy1">></span> <span class="sy3">&&</span><br /></p>
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boolean<span class="sy2">-</span>testable<span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">invoke_result_t</span><span class="sy1"><</span>F, Args...<span class="sy1">>></span><span class="sy4">;</span></div>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<p>概念 <code>predicate<F, Args...></code> 指定 <code>F</code> 是谓词，接受类型和值类别由 <code>Args...</code> 编码的参数，即能以这些参数调用它并产生 <a href="cpp-concepts-boolean-testable.html"><code>boolean-testable</code></a> 结果。</p>
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<p>注意 <a href="cpp-concepts-invocable.html"><code>regular_invocable</code></a> 要求调用既不修改可调用对象亦不修改参数，并保持相等性。</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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