89 lines
7.9 KiB
HTML
89 lines
7.9 KiB
HTML
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<title>std::function 的推导指引</title>
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<h1 id="firstHeading" class="firstHeading"><code>std::function</code> 的推导指引</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-utility.html">utility</a>‎ | <a href="cpp-utility-functional.html">functional</a>‎ | <a href="cpp-utility-functional-function.html">function</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-functional.html"><functional></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> R, <span class="kw1">class</span>... <span class="me1">ArgTypes</span><span class="sy1">></span><br />
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function<span class="br0">(</span>R<span class="br0">(</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">(</span>ArgTypes...<span class="br0">)</span><span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> function<span class="sy1"><</span>R<span class="br0">(</span>ArgTypes...<span class="br0">)</span><span class="sy1">></span><span class="sy4">;</span></span></div>
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</td>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 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="sy1">></span><br />
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function<span class="br0">(</span>F<span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> function<span class="sy1"><</span><span class="coMULTI">/*see below*/</span><span class="sy1">></span><span class="sy4">;</span></span></div>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<div class="t-li1"><span class="t-li">1)</span> 为 <span class="t-lc"><a href="cpp-utility-functional-function.html">std::function</a></span> 提供此<a href="cpp-language-class_template_argument_deduction.html">推导指引</a>以允许从函数推导。</div>
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<div class="t-li1"><span class="t-li">2)</span> 此重载仅若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">&</span>F<span class="sy4">::</span><span class="me2">operator</span><span class="br0">(</span><span class="br0">)</span></span></span> 在作为不求值运算数时为良构且 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="sy3">&</span>F<span class="sy4">::</span><span class="me2">operator</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span></span></span> 拥有形式 <span class="t-c"><span class="mw-geshi cpp source-cpp">R<span class="br0">(</span>G<span class="sy4">::</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">(</span>A...<span class="br0">)</span></span></span> （可选地有 cv 限定，可选地有 noexcept ， 可选地有左值引用限定）而 <code>G</code> 为类类型才参与重载决议。推导类型为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-functional-function.html"><span class="kw1059">std::<span class="me2">function</span></span></a><span class="sy1"><</span>R<span class="br0">(</span>A...<span class="br0">)</span><span class="sy1">></span></span></span> 。</div>
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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>这些推导指引所推导的类型可能在之后的标准版本中更改（特别是若在之后的版本中添加 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">noexcept</span></span></span> 支持到 <span class="t-lc"><a href="cpp-utility-functional-function.html">std::function</a></span> 则这可能发生）。</p>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <functional></span>
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<span class="kw4">int</span> func<span class="br0">(</span><span class="kw4">double</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
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<a href="cpp-utility-functional-function.html"><span class="kw1059">std::<span class="me2">function</span></span></a> f<span class="br0">{</span>func<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 指引 #1 推导 function<int(double)></span>
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<span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">5</span><span class="sy4">;</span>
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<a href="cpp-utility-functional-function.html"><span class="kw1059">std::<span class="me2">function</span></span></a> g <span class="sy1">=</span> <span class="br0">[</span><span class="sy3">&</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">double</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> i<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 指引 #2 推导 function<int(double)></span>
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<span class="br0">}</span></pre></div>
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<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
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<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。</p>
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<table class="dsctable" style="font-size:0.8em">
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<tr>
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<th>DR</th>
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<th>应用于</th>
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<th>出版时的行为</th>
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<th>正确行为</th>
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<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue3238">LWG 3238</a></td>
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<td>C++17</td>
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<td><span class="t-v">(2)</span> 的行为不明确</td>
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<td>使之明确</td>
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