152 lines
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152 lines
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<title>std::packaged_task<R(Args...)>::packaged_task</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::packaged_task<R(Args...)>::</span>packaged_task</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><span class="mw-geshi cpp source-cpp">packaged_task<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</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-cxx11">(C++11 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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<span class="kw1">explicit</span> packaged_task<span class="br0">(</span> F<span class="sy3">&&</span> f <span class="br0">)</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-cxx11">(C++11 起)</span></td>
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<tr class="t-dcl t-since-cxx11 t-until-cxx17">
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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> Allocator<span class="sy1">></span><br />
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<span class="kw1">explicit</span> packaged_task<span class="br0">(</span> <a href="cpp-memory-allocator_arg_t.html"><span class="kw702">std::<span class="me2">allocator_arg_t</span></span></a>, <span class="kw4">const</span> Allocator<span class="sy3">&</span> a, F<span class="sy3">&&</span> f <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(3)</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span><br />
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<span class="t-mark-rev t-until-cxx17">(C++17 前)</span></td>
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<div><span class="mw-geshi cpp source-cpp">packaged_task<span class="br0">(</span> <span class="kw4">const</span> packaged_task<span class="sy3">&</span> <span class="br0">)</span> <span class="sy1">=</span> delete<span class="sy4">;</span></span></div>
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<td>(4)</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp">packaged_task<span class="br0">(</span> packaged_task<span class="sy3">&&</span> rhs <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td>(5)</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<td></td>
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<td></td>
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<td></td>
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<p>构造新的 <code>std::packaged_task</code> 对象。</p>
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<div class="t-li1"><span class="t-li">1)</span> 构造无任务且无共享状态的 <code>std::packaged_task</code> 对象。</div>
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<div class="t-li1"><span class="t-li">2)</span> 构造拥有共享状态和任务副本的 <code>std::packaged_task</code> 对象，以 <span class="t-c"><span class="mw-geshi cpp source-cpp"><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></span></span> 初始化副本。若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-decay.html"><span class="kw628">std::<span class="me2">decay</span></span></a><span class="sy1"><</span>F<span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span></span></span> 与 <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>ArgTypes...<span class="br0">)</span><span class="sy1">></span></span></span> 是同一类型，则此构造函数不参与重载决议。</div>
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<div class="t-li1"><span class="t-li">3)</span> 构造拥有共享状态和任务副本的 <code>std::packaged_task</code> 对象，以 <span class="t-c"><span class="mw-geshi cpp source-cpp"><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></span></span> 初始化副本。用提供的分配器分配存储任务所需的内存。 若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-decay.html"><span class="kw628">std::<span class="me2">decay</span></span></a><span class="sy1"><</span>F<span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span></span></span> 与 <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>ArgTypes...<span class="br0">)</span><span class="sy1">></span></span></span> 是同一类型，则此构造函数不参与重载决议。</div>
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<div class="t-li1"><span class="t-li">4)</span> 复制构造函数被删除， <code>std::packaged_task</code> 仅可移动。</div>
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<div class="t-li1"><span class="t-li">5)</span> 以 <code>rhs</code> 之前所占有的共享状态和任务构造 <code>std::packaged_task</code> ，令 <code>rhs</code> 留在无共享状态且拥有被移动后的任务的状态。</div>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<tr class="t-par">
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<td>f</td>
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<td>-</td>
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<td>要执行的可调用目标（函数、成员函数、 lambda 表达式、函数对象）</td>
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</tr>
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<tr class="t-par">
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<td>a</td>
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<td>-</td>
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<td>存储任务时所用的分配器</td>
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</tr>
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<tr class="t-par">
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<td>rhs</td>
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<td>-</td>
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<td>要移动的 <code>std::packaged_task</code></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<div class="t-li1"><span class="t-li">2)</span> <code>f</code> 的复制/移动构造函数所抛的任何异常，而若内存分配失败则可能为 <span class="t-lc"><a href="cpp-memory-new-bad_alloc.html">std::bad_alloc</a></span> 。</div>
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<div class="t-li1"><span class="t-li">3)</span> <code>f</code> 的复制/移动构造函数，而若内存分配失败则有分配器的 <code>allocate</code> 函数所抛的任何异常。</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="t-example">
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<div class="t-example-live-link"></div>
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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 <future></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <thread></span>
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<span class="kw4">int</span> fib<span class="br0">(</span><span class="kw4">int</span> n<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">if</span> <span class="br0">(</span>n <span class="sy1"><</span> <span class="nu0">3</span><span class="br0">)</span> <span class="kw1">return</span> <span class="nu0">1</span><span class="sy4">;</span>
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<span class="kw1">else</span> <span class="kw1">return</span> fib<span class="br0">(</span>n<span class="sy2">-</span><span class="nu0">1</span><span class="br0">)</span> <span class="sy2">+</span> fib<span class="br0">(</span>n<span class="sy2">-</span><span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<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><span class="sy1"><</span><span class="kw4">int</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy1">></span> fib_task<span class="br0">(</span><span class="sy3">&</span>fib<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"starting task<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="kw4">auto</span> result <span class="sy1">=</span> fib_task.<span class="me1">get_future</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-thread-thread.html"><span class="kw2146">std::<span class="me2">thread</span></span></a> t<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>fib_task<span class="br0">)</span>, <span class="nu0">42</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"waiting for task to finish..."</span> <span class="sy1"><<</span> <a href="cpp-io-manip-endl.html"><span class="kw1791">std::<span class="me2">endl</span></span></a><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> result.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"task complete<span class="es1">\n</span>"</span><span class="sy4">;</span>
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t.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>输出：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="text source-text">
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<pre class="de1">starting task
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waiting for task to finish...
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267914296
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task complete</pre></div>
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</div>
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</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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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue2067">LWG 2067</a></td>
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<td>C++11</td>
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<td>被删除的复制构造函数接收非 const 引用</td>
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<td>使之为 const</td>
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</tr>
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</table>
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</div>
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<div class="visualClear"></div>
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