284 lines
36 KiB
HTML
284 lines
36 KiB
HTML
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<title>std::async</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_thread_async skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>async</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-thread.html">thread</a></span></div>
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<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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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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<td rowspan="142">(1)</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> Function, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">></span><br /></span>
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<p><a href="cpp-thread-future.html"><span class="kw2180">std::<span class="me2">future</span></span></a><span class="sy1"><</span><a href="cpp-types-result_of.html"><span class="kw640">std::<span class="me2">result_of_t</span></span></a><span class="sy1"><</span><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>Function<span class="sy1">></span><span class="br0">(</span><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>Args<span class="sy1">></span>...<span class="br0">)</span><span class="sy1">>></span><br /></p>
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async<span class="br0">(</span> Function<span class="sy3">&&</span> f, Args<span class="sy3">&&</span>... <span class="me1">args</span> <span class="br0">)</span><span class="sy4">;</span></div>
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<td class="t-dcl-nopad"></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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<tr class="t-dcl t-since-cxx17 t-until-cxx20">
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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> Function, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">></span><br /></span>
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<p><a href="cpp-thread-future.html"><span class="kw2180">std::<span class="me2">future</span></span></a><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">invoke_result_t</span><span class="sy1"><</span><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>Function<span class="sy1">></span>,<br />
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<a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>Args<span class="sy1">></span>...<span class="sy1">>></span><br /></p>
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async<span class="br0">(</span> Function<span class="sy3">&&</span> f, Args<span class="sy3">&&</span>... <span class="me1">args</span> <span class="br0">)</span><span class="sy4">;</span></div>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span><br />
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<span class="t-mark-rev t-until-cxx20">(C++20 前)</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> Function, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">></span><br /></span>
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<p><span class="br0">[</span><span class="br0">[</span>nodiscard<span class="br0">]</span><span class="br0">]</span><br />
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<a href="cpp-thread-future.html"><span class="kw2180">std::<span class="me2">future</span></span></a><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">invoke_result_t</span><span class="sy1"><</span><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>Function<span class="sy1">></span>,<br />
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<a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>Args<span class="sy1">></span>...<span class="sy1">>></span><br /></p>
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async<span class="br0">(</span> Function<span class="sy3">&&</span> f, Args<span class="sy3">&&</span>... <span class="me1">args</span> <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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<td rowspan="142">(2)</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> Function, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">></span><br /></span>
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<p><a href="cpp-thread-future.html"><span class="kw2180">std::<span class="me2">future</span></span></a><span class="sy1"><</span><a href="cpp-types-result_of.html"><span class="kw640">std::<span class="me2">result_of_t</span></span></a><span class="sy1"><</span><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>Function<span class="sy1">></span><span class="br0">(</span><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>Args<span class="sy1">></span>...<span class="br0">)</span><span class="sy1">>></span><br /></p>
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async<span class="br0">(</span> <a href="cpp-thread-launch.html"><span class="kw2184">std::<span class="me2">launch</span></span></a> policy, Function<span class="sy3">&&</span> f, Args<span class="sy3">&&</span>... <span class="me1">args</span> <span class="br0">)</span><span class="sy4">;</span></div>
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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"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> Function, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">></span><br /></span>
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<p><a href="cpp-thread-future.html"><span class="kw2180">std::<span class="me2">future</span></span></a><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">invoke_result_t</span><span class="sy1"><</span><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>Function<span class="sy1">></span>,<br />
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<a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>Args<span class="sy1">></span>...<span class="sy1">>></span><br /></p>
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async<span class="br0">(</span> <a href="cpp-thread-launch.html"><span class="kw2184">std::<span class="me2">launch</span></span></a> policy, Function<span class="sy3">&&</span> f, Args<span class="sy3">&&</span>... <span class="me1">args</span> <span class="br0">)</span><span class="sy4">;</span></div>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span><br />
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<span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td>
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<tr class="t-dcl t-since-cxx20">
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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> Function, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">></span><br /></span>
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<p><span class="br0">[</span><span class="br0">[</span>nodiscard<span class="br0">]</span><span class="br0">]</span><br />
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<a href="cpp-thread-future.html"><span class="kw2180">std::<span class="me2">future</span></span></a><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">invoke_result_t</span><span class="sy1"><</span><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>Function<span class="sy1">></span>,<br />
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<a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>Args<span class="sy1">></span>...<span class="sy1">>></span><br /></p>
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async<span class="br0">(</span> <a href="cpp-thread-launch.html"><span class="kw2184">std::<span class="me2">launch</span></span></a> policy, Function<span class="sy3">&&</span> f, Args<span class="sy3">&&</span>... <span class="me1">args</span> <span class="br0">)</span><span class="sy4">;</span></div>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<p>函数模板 <code>async</code> 异步地运行函数 <code>f</code> (潜在地在可能是线程池一部分的分离线程中),并返回最终将保有该函数调用结果的 <span class="t-lc"><a href="cpp-thread-future.html">std::future</a></span> 。</p>
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<div class="t-li1"><span class="t-li">1)</span> 表现如同以 <span class="t-c"><span class="mw-geshi cpp source-cpp">policy</span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-thread-launch.html"><span class="kw2185">std::<span class="me2">launch</span><span class="sy4">::</span><span class="me2">async</span></span></a> <span class="sy3">|</span> <a href="cpp-thread-launch.html"><span class="kw2186">std::<span class="me2">launch</span><span class="sy4">::</span><span class="me2">deferred</span></span></a></span></span> 调用 <span class="t-v">(2)</span> 。换言之, <code>f</code> 可能执行于另一线程,或者它可能在查询产生的 <span class="t-lc"><a href="cpp-thread-future.html">std::future</a></span> 的值时同步运行。</div>
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<div class="t-li1"><span class="t-li">2)</span> 按照特定的执行策略 <code>policy</code> ,以参数 <code>args</code> 调用函数 <code>f</code> :
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<li>若设置 <i>async</i> 标志(即 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>policy <span class="sy3">&</span> <a href="cpp-thread-launch.html"><span class="kw2185">std::<span class="me2">launch</span><span class="sy4">::</span><span class="me2">async</span></span></a><span class="br0">)</span> <span class="sy3">!</span><span class="sy1">=</span> <span class="nu0">0</span></span></span> ),则 <code>async</code> 在新的执行线程(初始化所有线程局域对象后)执行可调用对象 <code>f</code> ,如同产出 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-thread-thread.html"><span class="kw2146">std::<span class="me2">thread</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></span> ,除了若 <code>f</code> 返回值或抛出异常,则于可通过 <code>async</code> 返回给调用方的 <span class="t-lc"><a href="cpp-thread-future.html">std::future</a></span> 访问的共享状态存储结果。</li>
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<li>若设置 <i>deferred</i> 标志(即 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>policy <span class="sy3">&</span> <a href="cpp-thread-launch.html"><span class="kw2186">std::<span class="me2">launch</span><span class="sy4">::</span><span class="me2">deferred</span></span></a><span class="br0">)</span> <span class="sy3">!</span><span class="sy1">=</span> <span class="nu0">0</span></span></span> ),则 <code>async</code> 以同 <span class="t-lc"><a href="cpp-thread-thread.html">std::thread</a></span> 构造函数的方式转换 <code>f</code> 与 <code>args...</code> ,但不产出新的执行线程。而是进行<i>惰性求值</i>:在 <code>async</code> 所返回的 <span class="t-lc"><a href="cpp-thread-future.html">std::future</a></span> 上首次调用非定时等待函数,将导致在当前线程(不必是最初调用 <code>std::async</code> 的线程)中,以 <code>args...</code> (作为右值传递)的副本调用 <code>f</code> (亦作为右值)的副本。将结果或异常置于关联到该 future 的共享状态,然后才令它就绪。对同一 <span class="t-lc"><a href="cpp-thread-future.html">std::future</a></span> 的所有后续访问都会立即返回结果。</li>
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<li>若 <code>policy</code> 中设置了 <span class="t-lc"><a href="cpp-thread-launch.html">std::launch::async</a></span> 和 <span class="t-lc"><a href="cpp-thread-launch.html">std::launch::deferred</a></span> 两个标志,则进行异步执行还是惰性求值取决于实现。</li>
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<li>若 <code>policy</code> 中未设置 <span class="t-lc"><a href="cpp-thread-launch.html">std::launch::async</a></span> 或 <span class="t-lc"><a href="cpp-thread-launch.html">std::launch::deferred</a></span> 或任何实现定义策略标志,则行为未定义。</li>
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</ul>
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</dd>
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<p>任何情况下,对 <code>std::async</code> 的调用<i>同步于</i>(定义于 <span class="t-lc"><a href="cpp-atomic-memory_order.html">std::memory_order</a></span> )对 <code>f</code> 的调用,且 <code>f</code> 的完成<i>先序于</i>令共享状态就绪。若选择 <code>async</code> 策略,则关联线程的完成<i>同步于</i>首个等待于共享状态上的函数的成功返回,或最后一个释放共享状态的函数的返回,两者的先到来者。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>Function<span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span></span></span> 或 <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>Args<span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span></span></span> 中的每个类型不能从其对应的实参构造,则程序为谬构。</p>
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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>要调用的<a href="cpp-named_req-Callable.html"><span style="font-style:italic">可调用</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Callable)</span></a> 对象</td>
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<tr class="t-par">
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<td>args...</td>
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<td>-</td>
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<td>传递给 <code>f</code> 的参数</td>
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</tr>
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<tr class="t-par">
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<td>policy</td>
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<td>-</td>
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<td>位掩码值,每个单独位控制允许的执行方法
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<table class="t-dsc-begin">
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<tr class="t-dsc-hitem">
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<td>位</td>
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<td>解释</td>
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<tr class="t-dsc">
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<td><span class="t-lc"><a href="cpp-thread-launch.html">std::launch::async</a></span></td>
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<td>启用异步求值</td>
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</tr>
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<tr class="t-dsc">
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<td><span class="t-lc"><a href="cpp-thread-launch.html">std::launch::deferred</a></span></td>
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<td>启用惰性求值</td>
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</tr>
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</table>
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<p><br /></p>
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</td>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<p>指代此次调用 <code>std::async</code> 所创建的共享状态的 <span class="t-lc"><a href="cpp-thread-future.html">std::future</a></span> 。</p>
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<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<p>若运行策略等于 <span class="t-lc"><a href="cpp-thread-launch.html">std::launch::async</a></span> 且实现无法开始新线程(该情况下,若运行策略为 <code>async|deferred</code> 或设置了额外位,则它将回退到 deferred 或实现定义的策略),则抛出以 <span class="t-lc"><a href="cpp-error-errc.html">std::errc::resource_unavailable_try_again</a></span> 为错误条件的 <span class="t-lc"><a href="cpp-error-system_error.html">std::system_error</a></span> ,或者若无法分配内部数据结构所用的内存,则为 <span class="t-lc"><a href="cpp-memory-new-bad_alloc.html">std::bad_alloc</a></span> 。</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>实现可以通过在默认运行策略中启用额外(实现定义的)位,扩展 <span class="t-lc"><strong class="selflink">std::async</strong></span> 第一重载的行为。</p>
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<p>实现定义的运行策略的例子是同步策略(在 async 调用内立即执行)和任务策略(类似 async ,但不清理线程局域对象)。</p>
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<p>若从 <code>std::async</code> 获得的 <code>std::future</code> 未被移动或绑定到引用,则在完整表达式结尾, <span class="t-lc"><a href="cpp-thread-future.html">std::future</a></span> 的析构函数将阻塞直至异步计算完成,实质上令如下代码同步:</p>
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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">std<span class="sy4">::</span><span class="me2">async</span><span class="br0">(</span><a href="cpp-thread-launch.html"><span class="kw2185">std::<span class="me2">launch</span><span class="sy4">::</span><span class="me2">async</span></span></a>, <span class="br0">[</span><span class="br0">]</span><span class="br0">{</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 临时量的析构函数等待 f()</span>
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std<span class="sy4">::</span><span class="me2">async</span><span class="br0">(</span><a href="cpp-thread-launch.html"><span class="kw2185">std::<span class="me2">launch</span><span class="sy4">::</span><span class="me2">async</span></span></a>, <span class="br0">[</span><span class="br0">]</span><span class="br0">{</span> g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// f() 完成前不开始</span></pre></div>
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</div>
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<p>(注意,以调用 std::async 以外的方式获得的 std::future 的析构函数决不阻塞)</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 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 <iostream></span>
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<span class="co2">#include <vector></span>
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<span class="co2">#include <algorithm></span>
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<span class="co2">#include <numeric></span>
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<span class="co2">#include <future></span>
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<span class="co2">#include <string></span>
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<span class="co2">#include <mutex></span>
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<a href="cpp-thread-mutex.html"><span class="kw2154">std::<span class="me2">mutex</span></span></a> m<span class="sy4">;</span>
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<span class="kw1">struct</span> X <span class="br0">{</span>
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<span class="kw4">void</span> foo<span class="br0">(</span><span class="kw4">int</span> i, <span class="kw4">const</span> <a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy3">&</span> str<span class="br0">)</span> <span class="br0">{</span>
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<a href="cpp-thread-lock_guard.html"><span class="kw2166">std::<span class="me2">lock_guard</span></span></a><span class="sy1"><</span><a href="cpp-thread-mutex.html"><span class="kw2154">std::<span class="me2">mutex</span></span></a><span class="sy1">></span> lk<span class="br0">(</span>m<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> str <span class="sy1"><<</span> <span class="st0">' '</span> <span class="sy1"><<</span> i <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">void</span> bar<span class="br0">(</span><span class="kw4">const</span> <a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy3">&</span> str<span class="br0">)</span> <span class="br0">{</span>
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<a href="cpp-thread-lock_guard.html"><span class="kw2166">std::<span class="me2">lock_guard</span></span></a><span class="sy1"><</span><a href="cpp-thread-mutex.html"><span class="kw2154">std::<span class="me2">mutex</span></span></a><span class="sy1">></span> lk<span class="br0">(</span>m<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> str <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">int</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span> <span class="br0">{</span>
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<a href="cpp-thread-lock_guard.html"><span class="kw2166">std::<span class="me2">lock_guard</span></span></a><span class="sy1"><</span><a href="cpp-thread-mutex.html"><span class="kw2154">std::<span class="me2">mutex</span></span></a><span class="sy1">></span> lk<span class="br0">(</span>m<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> i <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="kw1">return</span> i <span class="sy2">+</span> <span class="nu0">10</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">typename</span> RandomIt<span class="sy1">></span>
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<span class="kw4">int</span> parallel_sum<span class="br0">(</span>RandomIt beg, RandomIt end<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw4">auto</span> len <span class="sy1">=</span> end <span class="sy2">-</span> beg<span class="sy4">;</span>
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<span class="kw1">if</span> <span class="br0">(</span>len <span class="sy1"><</span> <span class="nu0">1000</span><span class="br0">)</span>
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<span class="kw1">return</span> <a href="cpp-algorithm-accumulate.html"><span class="kw1651">std::<span class="me2">accumulate</span></span></a><span class="br0">(</span>beg, end, <span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span>
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RandomIt mid <span class="sy1">=</span> beg <span class="sy2">+</span> len<span class="sy2">/</span><span class="nu0">2</span><span class="sy4">;</span>
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<span class="kw4">auto</span> handle <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">async</span><span class="br0">(</span><a href="cpp-thread-launch.html"><span class="kw2185">std::<span class="me2">launch</span><span class="sy4">::</span><span class="me2">async</span></span></a>,
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parallel_sum<span class="sy1"><</span>RandomIt<span class="sy1">></span>, mid, end<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">int</span> sum <span class="sy1">=</span> parallel_sum<span class="br0">(</span>beg, mid<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> sum <span class="sy2">+</span> handle.<span class="me1">get</span><span class="br0">(</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-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> v<span class="br0">(</span><span class="nu0">10000</span>, <span class="nu0">1</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">"The sum is "</span> <span class="sy1"><<</span> parallel_sum<span class="br0">(</span>v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, v.<span class="me1">end</span><span class="br0">(</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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X x<span class="sy4">;</span>
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<span class="co1">// 以默认策略调用 x.foo(42, "Hello") :</span>
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<span class="co1">// 可能同时打印 "Hello 42" 或延迟执行</span>
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<span class="kw4">auto</span> a1 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">async</span><span class="br0">(</span><span class="sy3">&</span>X<span class="sy4">::</span><span class="me2">foo</span>, <span class="sy3">&</span>x, <span class="nu0">42</span>, <span class="st0">"Hello"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 以 deferred 策略调用 x.bar("world!")</span>
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<span class="co1">// 调用 a2.get() 或 a2.wait() 时打印 "world!"</span>
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<span class="kw4">auto</span> a2 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">async</span><span class="br0">(</span><a href="cpp-thread-launch.html"><span class="kw2186">std::<span class="me2">launch</span><span class="sy4">::</span><span class="me2">deferred</span></span></a>, <span class="sy3">&</span>X<span class="sy4">::</span><span class="me2">bar</span>, x, <span class="st0">"world!"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 以 async 策略调用 X()(43) :</span>
|
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<span class="co1">// 同时打印 "43"</span>
|
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<span class="kw4">auto</span> a3 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">async</span><span class="br0">(</span><a href="cpp-thread-launch.html"><span class="kw2185">std::<span class="me2">launch</span><span class="sy4">::</span><span class="me2">async</span></span></a>, X<span class="br0">(</span><span class="br0">)</span>, <span class="nu0">43</span><span class="br0">)</span><span class="sy4">;</span>
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a2.<span class="me1">wait</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 打印 "world!"</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> a3.<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> <span class="co1">// 打印 "53"</span>
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<span class="br0">}</span> <span class="co1">// 若 a1 在此点未完成,则 a1 的析构函数在此打印 "Hello 42"</span></pre></div>
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</div>
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<p>可能的输出:</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">The sum is 10000
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43
|
|
world!
|
|
53
|
|
Hello 42</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/issue2021">LWG 2021</a></td>
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<td>C++11</td>
|
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<td>返回类型不正确, deferred 的情况下参数的值类别不明</td>
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<td>更正返回类型,明确使用右值</td>
|
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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/issue2120">LWG 2120</a></td>
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<td>C++11</td>
|
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<td>未设置标准或实现定义的策略的情况下行为不明</td>
|
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<td>行为未定义</td>
|
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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/issue3476">LWG 3476</a></td>
|
|
<td>C++11</td>
|
|
<td>要求 <code>Function</code> 与 <code>Args...</code> 为<a href="cpp-named_req-MoveConstructible.html"><span style="font-style:italic">可移动构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(MoveConstructible)</span></a> 而无指定的额外移动构造</td>
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<td>移除要求</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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</div>
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</div>
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</div>
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</body>
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</html>
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