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122 lines
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<title>std::condition_variable_any::notify_one</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::condition_variable_any::</span>notify_one</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-condition_variable_any.html">condition variable any</a></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> notify_one<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<p>若任何线程在 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 上等待，则调用 <code>notify_one</code> 会解阻塞等待线程之一。</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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<p>（无）</p>
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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>（无）</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>notify_one()</code>/<code>notify_all()</code> 的效果与 <code>wait()</code>/<code>wait_for()</code>/<code>wait_until()</code> 的三个原子部分的每一者（解锁+等待、唤醒和锁定）以能看做原子变量<a href="cpp-atomic-memory_order.html#.E4.BF.AE.E6.94.B9.E9.A1.BA.E5.BA.8F">修改顺序</a>单独全序发生：顺序对此单独的 condition_variable 是特定的。譬如，这使得 <code>notify_one()</code> 不可能被延迟并解锁正好在进行 <code>notify_one()</code> 调用后开始等待的线程。</p>
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<p>通知线程不必保有等待线程所保有的同一互斥上的锁；实际上这么做是劣化，因为被通知线程将立即再次阻塞，等待通知线程释放锁。然而一些实现（尤其是许多 pthread 的实现）辨识此情形，在通知调用中，直接从条件变量队列转移等待线程到互斥队列，而不唤醒它，以避免此“急促并等待”场景。</p>
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<p>然而，在要求精确调度事件时，可能必须在处于锁下时通知，例如，在若满足条件则线程将退出程序，导致析构通知线程的 condition_variable 的情况下。互斥解锁之后，但在通知前的虚假唤醒可能导致通知在被销毁对象上调用。</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="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <iostream></span>
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<span class="co2">#include <condition_variable></span>
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<span class="co2">#include <thread></span>
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<span class="co2">#include <chrono></span>
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<a href="cpp-thread-condition_variable_any.html"><span class="kw2172">std::<span class="me2">condition_variable_any</span></span></a> cv<span class="sy4">;</span>
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<a href="cpp-thread-mutex.html"><span class="kw2154">std::<span class="me2">mutex</span></span></a> cv_m<span class="sy4">;</span>
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<span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
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<span class="kw4">bool</span> done <span class="sy1">=</span> <span class="kw2">false</span><span class="sy4">;</span>
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<span class="kw4">void</span> waits<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-thread-unique_lock.html"><span class="kw2167">std::<span class="me2">unique_lock</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>cv_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> <span class="st0">"Waiting... <span class="es1">\n</span>"</span><span class="sy4">;</span>
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cv.<span class="me1">wait</span><span class="br0">(</span>lk, <span class="br0">[</span><span class="br0">]</span><span class="br0">{</span><span class="kw1">return</span> i <span class="sy1">==</span> <span class="nu0">1</span><span class="sy4">;</span><span class="br0">}</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">"...finished waiting. i == 1<span class="es1">\n</span>"</span><span class="sy4">;</span>
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done <span class="sy1">=</span> <span class="kw2">true</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">void</span> signals<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-thread-sleep_for.html"><span class="kw2149">std::<span class="me2">this_thread</span><span class="sy4">::</span><span class="me2">sleep_for</span></span></a><span class="br0">(</span><a href="cpp-chrono-duration.html"><span class="kw997">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">seconds</span></span></a><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</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">"Notifying falsely...<span class="es1">\n</span>"</span><span class="sy4">;</span>
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cv.<span class="me1">notify_one</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 等待线程被通知 i == 0.</span>
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<span class="co1">// cv.wait 唤醒，检查 i ，再回到等待</span>
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<a href="cpp-thread-unique_lock.html"><span class="kw2167">std::<span class="me2">unique_lock</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>cv_m<span class="br0">)</span><span class="sy4">;</span>
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i <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
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<span class="kw1">while</span> <span class="br0">(</span><span class="sy3">!</span>done<span class="br0">)</span>
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<span class="br0">{</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">"Notifying true change...<span class="es1">\n</span>"</span><span class="sy4">;</span>
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lk.<span class="me1">unlock</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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cv.<span class="me1">notify_one</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 等待线程被通知 i == 1 ， cv.wait 返回</span>
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<a href="cpp-thread-sleep_for.html"><span class="kw2149">std::<span class="me2">this_thread</span><span class="sy4">::</span><span class="me2">sleep_for</span></span></a><span class="br0">(</span><a href="cpp-chrono-duration.html"><span class="kw997">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">seconds</span></span></a><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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lk.<span class="me1">lock</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="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-thread.html"><span class="kw2146">std::<span class="me2">thread</span></span></a> t1<span class="br0">(</span>waits<span class="br0">)</span>, t2<span class="br0">(</span>signals<span class="br0">)</span><span class="sy4">;</span>
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t1.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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t2.<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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<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">Waiting...
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Notifying falsely...
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Notifying true change...
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...finished waiting. i == 1</pre></div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<div><a href="cpp-thread-condition_variable_any-notify_all.html"><span class="t-lines"><span>notify_all</span></span></a></div>
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</div>
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</td>
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<td>通知所有等待的线程<br />
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<span class="t-mark">(公开成员函数)</span></td>
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