78 lines
7.5 KiB
HTML
78 lines
7.5 KiB
HTML
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<title>std::invocable, std::regular_invocable</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>invocable, <span style="font-size:0.7em; line-height:130%">std::</span>regular_invocable</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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<td></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> F, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">></span><br /></span>
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<p>concept invocable <span class="sy1">=</span><br />
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requires<span class="br0">(</span>F<span class="sy3">&&</span> f, Args<span class="sy3">&&</span>... <span class="me1">args</span><span class="br0">)</span> <span class="br0">{</span><br />
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<a href="cpp-utility-functional-invoke.html"><span class="kw1058">std::<span class="me2">invoke</span></span></a><span class="br0">(</span><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>F<span class="sy1">></span><span class="br0">(</span>f<span class="br0">)</span>, <a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>Args<span class="sy1">></span><span class="br0">(</span>args<span class="br0">)</span>...<span class="br0">)</span><span class="sy4">;</span><br />
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<span class="coMULTI">/* 不要求保持相等性 */</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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<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 />
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concept regular_invocable <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">invocable</span><span class="sy1"><</span>F, Args...<span class="sy1">></span><span class="sy4">;</span></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><code>invocable</code> 概念指定能用函数模板 <span class="t-lc"><a href="cpp-utility-functional-invoke.html">std::invoke</a></span> 以参数集 <code>Args...</code> 调用可调用类型 <code>F</code> 。</p>
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<p><code>regular_invocable</code> 概念对 <code>invocable</code> 概念增加 <code>invoke</code> 必须保持相等性且不修改可调用对象或参数。</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.E6.84.8F">注意</span></h3>
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<p><code>invocable</code> 与 <code>regular_invocable</code> 间的区别是纯语义上的。</p>
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<p>随机数生成器可满足 <code>invocable</code> 但不能满足 <code>regular_invocable</code> （除了<a rel="nofollow" class="external text" href="https://xkcd.com/221/">搞笑</a><a rel="nofollow" class="external text" href="http://dilbert.com/strip/2001-10-25">的</a>）。</p>
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