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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::condition_variable::</span>wait</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.html">condition variable</a></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> wait<span class="br0">(</span> <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><span class="sy3">&</span> lock <span class="br0">)</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> Predicate <span class="sy1">></span><br />
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<span class="kw4">void</span> wait<span class="br0">(</span> <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><span class="sy3">&</span> lock, Predicate pred <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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<p><code>wait</code> 导致当前线程阻塞直至条件变量被通知，或虚假唤醒发生，可选地循环直至满足某谓词。</p>
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<div class="t-li1"><span class="t-li">1)</span> 原子地解锁 <code>lock</code> ，阻塞当前执行线程，并将它添加到于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 上等待的线程列表。线程将在执行 <span class="t-lc"><a href="cpp-thread-condition_variable-notify_all.html">notify_all()</a></span> 或 <span class="t-lc"><a href="cpp-thread-condition_variable-notify_one.html">notify_one()</a></span> 时被解除阻塞。解阻塞时，无关乎原因， <code>lock</code> 再次锁定且 <code>wait</code> 退出。</div>
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<div class="t-li1"><span class="t-li">2)</span> 等价于
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<pre class="de1"><span class="kw1">while</span> <span class="br0">(</span><span class="sy3">!</span>pred<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span>
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wait<span class="br0">(</span>lock<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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此重载可用于在等待特定条件成为 <code>true</code> 时忽略虚假唤醒。注意进入此方法前，必须得到 <code>lock</code> ， <code>wait(lock)</code> 退出后也会重获得它，即能以 <code>lock</code> 为对 <code>pred()</code> 访问的保障。</div>
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<p><br />
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若这些函数不能满足后置条件（ <span class="t-c"><span class="mw-geshi cpp source-cpp">lock.<span class="me1">owns_lock</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw2">true</span></span></span> 且调用方线程锁定 <span class="t-c"><span class="mw-geshi cpp source-cpp">lock.<span class="me1">mutex</span><span class="br0">(</span><span class="br0">)</span></span></span> ），则调用 <span class="t-lc"><a href="cpp-error-terminate.html">std::terminate</a></span> 。例如，这可能在重锁定互斥抛异常的情况下发生。</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>lock</td>
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<td>-</td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><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></span></span> 类型对象，必须为当前线程所锁定</td>
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<tr class="t-par">
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<td>pred</td>
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<td>-</td>
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<td>等待是否应该持续则返回 ​<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 的谓词。<br />
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<p>谓词函数的签名应等价于如下者：</p>
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<p><span class="t-cc"><span class="mw-geshi cpp source-cpp"> <span class="kw4">bool</span> pred<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></span>​</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=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<div class="t-li1"><span class="t-li">1)</span> 不抛出</div>
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<div class="t-li1"><span class="t-li">2)</span> 同 <span class="t-v">(1)</span> 但亦会传播 <code>pred</code> 所抛的异常</div>
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<p><br /></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>若当前线程未锁定 <a href="cpp-thread-unique_lock-mutex.html"><span class="t-c"><span class="mw-geshi cpp source-cpp">lock.<span class="me1">mutex</span><span class="br0">(</span><span class="br0">)</span></span></span></a> ，则调用此函数是未定义行为。</p>
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<p>若 <a href="cpp-thread-unique_lock-mutex.html"><span class="t-c"><span class="mw-geshi cpp source-cpp">lock.<span class="me1">mutex</span><span class="br0">(</span><span class="br0">)</span></span></span></a> 与所有其他当前等待在同一条件变量上的线程所用的互斥不相同，则调用此函数是未定义行为。</p>
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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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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-template-editlink" style=""></div>
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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 <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.html"><span class="kw2171">std::<span class="me2">condition_variable</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> <span class="co1">// 此互斥用于三个目的：</span>
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<span class="co1">// 1) 同步到 i 的访问</span>
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<span class="co1">// 2) 同步到 std::cerr 的访问</span>
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<span class="co1">// 3) 为条件变量 cv</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">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-cerr.html"><span class="kw1760">std::<span class="me2">cerr</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-cerr.html"><span class="kw1760">std::<span class="me2">cerr</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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<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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<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>cv_m<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cerr.html"><span class="kw1760">std::<span class="me2">cerr</span></span></a> <span class="sy1"><<</span> <span class="st0">"Notifying...<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">}</span>
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cv.<span class="me1">notify_all</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</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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<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>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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<a href="cpp-io-cerr.html"><span class="kw1760">std::<span class="me2">cerr</span></span></a> <span class="sy1"><<</span> <span class="st0">"Notifying again...<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">}</span>
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cv.<span class="me1">notify_all</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-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>waits<span class="br0">)</span>, t3<span class="br0">(</span>waits<span class="br0">)</span>, t4<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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t3.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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t4.<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>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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Waiting...
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Waiting...
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Notifying...
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Notifying again...
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...finished waiting. i == 1
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...finished waiting. i == 1
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...finished waiting. i == 1</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/issue2135">LWG 2135</a></td>
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<td>C++11</td>
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<td><code>wait</code> 在解锁/重锁失败时抛异常</td>
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<td>调用 <span class="t-lc"><a href="cpp-error-terminate.html">std::terminate</a></span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<table class="t-dsc-begin">
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-thread-condition_variable-wait_for.html"><span class="t-lines"><span>wait_for</span></span></a></div>
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</div>
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</td>
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<td>阻塞当前线程，直到条件变量被唤醒，或到指定时限时长后<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-thread-condition_variable-wait_until.html"><span class="t-lines"><span>wait_until</span></span></a></div>
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</div>
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</td>
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<td>阻塞当前线程，直到条件变量被唤醒，或直到抵达指定时间点<br />
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<span class="t-mark">(公开成员函数)</span></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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