71 lines
8.7 KiB
HTML
71 lines
8.7 KiB
HTML
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<h1 id="firstHeading" class="firstHeading"><i>boolean-testable</i></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><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> B<span class="sy1">></span><br /></span>
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<p>concept __boolean_testable_impl <span class="sy1">=</span> <span class="co1">// 仅用于阐释</span><br /></p>
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std<span class="sy4">::</span><span class="me2">convertible_to</span><span class="sy1"><</span>B, <span class="kw4">bool</span><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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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> B<span class="sy1">></span><br /></span>
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<p>concept boolean<span class="sy2">-</span>testable <span class="sy1">=</span> <span class="co1">// 仅用于阐释</span><br />
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__boolean_testable_impl<span class="sy1"><</span>B<span class="sy1">></span> <span class="sy3">&&</span><br />
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requires <span class="br0">(</span>B<span class="sy3">&&</span> b<span class="br0">)</span> <span class="br0">{</span><br />
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<span class="br0">{</span> <span class="sy3">!</span><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>B<span class="sy1">></span><span class="br0">(</span>b<span class="br0">)</span> <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">></span> __boolean_testable_impl<span class="sy4">;</span><br /></p>
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<span class="br0">}</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>仅用于阐释的概念 <code><i>boolean-testable</i></code> 指定可转换成 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span> 的表达式，和逻辑运算符拥有通常行为（包括短路），即使是对于二个不同的 <code><i>boolean-testable</i></code> 类型的要求。</p>
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<p>正式而言，为满足仅用于阐释的概念 <code>__boolean_testable_impl</code> ，类型必须不定义任何成员 <code>operator&&</code> 与 <code>operator||</code> ，而且无可生成的非成员 <code>operator&&</code> 及 <code>operator||</code> 可对<a href="cpp-language-adl.html">实参依赖查找</a>可见。另外给定使 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="br0">(</span>e<span class="br0">)</span><span class="br0">)</span></span></span> 为 <code>B</code> 的表达式 <code>e</code> ， <code><i>boolean-testable</i></code> 仅若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>e<span class="br0">)</span> <span class="sy1">==</span> <span class="sy3">!</span><span class="kw4">bool</span><span class="br0">(</span><span class="sy3">!</span>e<span class="br0">)</span></span></span> 才得到实现。</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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<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=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p><code><i>boolean-testable</i></code> 类型的例子包括 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span>、 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-integral_constant.html"><span class="kw649">std::<span class="me2">true_type</span></span></a></span></span>、 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-bitset.html"><span class="kw1027">std::<span class="me2">bitset</span></span></a><span class="sy1"><</span>N<span class="sy1">></span><span class="sy4">::</span><span class="me2">reference</span></span></span> 和 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span><span class="sy2">*</span></span></span> 。</p>
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