71 lines
6.3 KiB
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71 lines
6.3 KiB
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<title>std::packaged_task 的推导指引</title>
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<h1 id="firstHeading" class="firstHeading"><code>std::packaged_task</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-thread.html">thread</a>‎ | <a href="cpp-thread-packaged_task.html">packaged task</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-future.html"><future></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">Args</span><span class="sy1">></span><br />
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packaged_task<span class="br0">(</span>R<span class="br0">(</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">(</span>Args...<span class="br0">)</span><span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> packaged_task<span class="sy1"><</span>R<span class="br0">(</span>Args...<span class="br0">)</span><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-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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packaged_task<span class="br0">(</span>F<span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> packaged_task<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-thread-packaged_task.html">std::packaged_task</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-thread-packaged_task.html"><span class="kw2183">std::<span class="me2">packaged_task</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=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <future></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-thread-packaged_task.html"><span class="kw2183">std::<span class="me2">packaged_task</span></span></a> f<span class="br0">{</span>func<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 推导出 packaged_task<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-thread-packaged_task.html"><span class="kw2183">std::<span class="me2">packaged_task</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">// 推导出 packaged_task<int(double)></span>
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<span class="br0">}</span></pre></div>
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