257 lines
20 KiB
HTML
257 lines
20 KiB
HTML
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<title>std::condition_variable</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_thread_condition_variable 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>condition_variable</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>定义于头文件 <code><a href="cpp-header-condition_variable.html"><condition_variable></a></code></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">class</span> condition_variable<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><code>condition_variable</code> 类是同步原语，能用于阻塞一个线程，或同时阻塞多个线程，直至另一线程修改共享变量（<i>条件</i>）并通知 <code>condition_variable</code> 。</p>
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<p>有意修改变量的线程必须</p>
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<ol>
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<li>获得 <code>std::mutex</code> （常通过 <span class="t-lc"><a href="cpp-thread-lock_guard.html">std::lock_guard</a></span> ）</li>
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<li>在保有锁时进行修改</li>
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<li>在 <code>std::condition_variable</code> 上执行 <span class="t-lc"><a href="cpp-thread-condition_variable-notify_one.html">notify_one</a></span> 或 <span class="t-lc"><a href="cpp-thread-condition_variable-notify_all.html">notify_all</a></span> （不需要为通知保有锁）</li>
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</ol>
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<p>即使共享变量是原子的，也必须在互斥下修改它，以正确地发布修改到等待的线程。</p>
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<p>任何有意在 <code>std::condition_variable</code> 上等待的线程必须</p>
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<ol>
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<li>在与用于保护共享变量者相同的互斥上获得 <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></li>
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<li>执行下列之一：</li>
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</ol>
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<dl>
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<dd>
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<ol>
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<li>检查条件，是否为已更新或提醒它的情况</li>
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<li>执行 <span class="t-lc"><a href="cpp-thread-condition_variable-wait.html">wait</a></span> 、 <span class="t-lc"><a href="cpp-thread-condition_variable-wait_for.html">wait_for</a></span> 或 <span class="t-lc"><a href="cpp-thread-condition_variable-wait_until.html">wait_until</a></span> ，等待操作自动释放互斥，并悬挂线程的执行。</li>
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<li>condition_variable 被通知时，时限消失或<a href="https://en.wikipedia.org/wiki/Spurious_wakeup" class="extiw" title="enwiki:Spurious wakeup">虚假唤醒</a>发生，线程被唤醒，且自动重获得互斥。之后线程应检查条件，若唤醒是虚假的，则继续等待。</li>
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</ol>
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<dl>
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<dd>或者</dd>
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</dl>
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<ol>
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<li>使用 <span class="t-lc"><a href="cpp-thread-condition_variable-wait.html">wait</a></span> 、 <span class="t-lc"><a href="cpp-thread-condition_variable-wait_for.html">wait_for</a></span> 及 <span class="t-lc"><a href="cpp-thread-condition_variable-wait_until.html">wait_until</a></span> 的有谓词重载，它们包揽以上三个步骤</li>
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</ol>
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</dd>
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</dl>
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<p><code>std::condition_variable</code> 只可与 <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> 一同使用；此限制在一些平台上允许最大效率。 <span class="t-lc"><a href="cpp-thread-condition_variable_any.html">std::condition_variable_any</a></span> 提供可与任何<a href="cpp-named_req-BasicLockable.html"><span style="font-style:italic">基本可锁定</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(BasicLockable)</span></a> 对象，例如 <span class="t-lc"><a href="cpp-thread-shared_lock.html">std::shared_lock</a></span> 一同使用的条件变量。</p>
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<p>condition_variable 容许 <span class="t-lc"><a href="cpp-thread-condition_variable-wait.html">wait</a></span> 、 <span class="t-lc"><a href="cpp-thread-condition_variable-wait_for.html">wait_for</a></span> 、 <span class="t-lc"><a href="cpp-thread-condition_variable-wait_until.html">wait_until</a></span> 、 <span class="t-lc"><a href="cpp-thread-condition_variable-notify_one.html">notify_one</a></span> 及 <span class="t-lc"><a href="cpp-thread-condition_variable-notify_all.html">notify_all</a></span> 成员函数的同时调用。</p>
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<p>类 <code>std::condition_variable</code> 是<a href="cpp-named_req-StandardLayoutType.html"><span style="font-style:italic">标准布局类型</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(StandardLayoutType)</span></a> 。它非<a href="cpp-named_req-CopyConstructible.html"><span style="font-style:italic">可复制构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyConstructible)</span></a> 、<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> 、<a href="cpp-named_req-CopyAssignable.html"><span style="font-style:italic">可复制赋值</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyAssignable)</span></a> 或<a href="cpp-named_req-MoveAssignable.html"><span style="font-style:italic">可移动赋值</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(MoveAssignable)</span></a> 。</p>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E7.B1.BB.E5.9E.8B">成员类型</span></h3>
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<table class="t-dsc-begin">
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<td>成员类型</td>
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<td>定义</td>
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<tr class="t-dsc">
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<td><code>native_handle_type</code></td>
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<td><i>实现定义</i></td>
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</table>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">成员函数</span></h3>
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<table class="t-dsc-begin">
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<div class="t-dsc-member-div t-dsc-member-nobold-div">
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<div><a href="cpp-thread-condition_variable-condition_variable.html"><span class="t-lines"><span>(构造函数)</span></span></a></div>
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</div>
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</td>
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<td>构造对象<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 t-dsc-member-nobold-div">
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<div><a href="cpp-thread-condition_variable-~condition_variable.html"><span class="t-lines"><span>(析构函数)</span></span></a></div>
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</div>
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</td>
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<td>析构对象<br />
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<span class="t-mark">(公开成员函数)</span></td>
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<tr class="t-dsc">
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<div class="t-dsc-member-div">
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<div><span class="t-lines"><span>operator=</span></span></div>
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<div><span class="t-lines"><span><span class="t-cmark">[被删除]</span></span></span></div>
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</div>
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</td>
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<td>不可复制赋值<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>
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<td colspan="2">
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<h5><span class="mw-headline" id=".E9.80.9A.E7.9F.A5">通知</span></h5>
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</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-notify_one.html"><span class="t-lines"><span>notify_one</span></span></a></div>
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</div>
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</td>
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<td>通知一个等待的线程<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</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-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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</tr>
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<tr>
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<td colspan="2">
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<h5><span class="mw-headline" id=".E7.AD.89.E5.BE.85">等待</span></h5>
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</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.html"><span class="t-lines"><span>wait</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<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_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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<tr>
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<td colspan="2">
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<h5><span class="mw-headline" id=".E5.8E.9F.E7.94.9F.E5.8F.A5.E6.9F.84">原生句柄</span></h5>
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</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-native_handle.html"><span class="t-lines"><span>native_handle</span></span></a></div>
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</div>
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</td>
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<td>返回原生句柄<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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<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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<p>与 <span class="t-lc"><a href="cpp-thread-mutex.html">std::mutex</a></span> 组合使用 <code>condition_variable</code> ，以促进线程间交流。</p>
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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 <string></span>
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<span class="co2">#include <thread></span>
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<span class="co2">#include <mutex></span>
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<span class="co2">#include <condition_variable></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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std<span class="sy4">::</span><span class="me2">condition_variable</span> cv<span class="sy4">;</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> data<span class="sy4">;</span>
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<span class="kw4">bool</span> ready <span class="sy1">=</span> <span class="kw2">false</span><span class="sy4">;</span>
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<span class="kw4">bool</span> processed <span class="sy1">=</span> <span class="kw2">false</span><span class="sy4">;</span>
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<span class="kw4">void</span> worker_thread<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="co1">// 等待直至 main() 发送数据</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>m<span class="br0">)</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> ready<span class="sy4">;</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 等待后，我们占有锁。</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">"Worker thread is processing data<span class="es1">\n</span>"</span><span class="sy4">;</span>
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data <span class="sy2">+</span><span class="sy1">=</span> <span class="st0">" after processing"</span><span class="sy4">;</span>
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<span class="co1">// 发送数据回 main()</span>
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processed <span class="sy1">=</span> <span class="kw2">true</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">"Worker thread signals data processing completed<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="co1">// 通知前完成手动解锁，以避免等待线程才被唤醒就阻塞（细节见 notify_one ）</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>
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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> worker<span class="br0">(</span>worker_thread<span class="br0">)</span><span class="sy4">;</span>
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data <span class="sy1">=</span> <span class="st0">"Example data"</span><span class="sy4">;</span>
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<span class="co1">// 发送数据到 worker 线程</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>m<span class="br0">)</span><span class="sy4">;</span>
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ready <span class="sy1">=</span> <span class="kw2">true</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">"main() signals data ready for processing<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_one</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 等候 worker</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>m<span class="br0">)</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> processed<span class="sy4">;</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-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Back in main(), data = "</span> <span class="sy1"><<</span> data <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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worker.<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">main() signals data ready for processing
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Worker thread is processing data
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Worker thread signals data processing completed
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Back in main(), data = Example data after processing</pre></div>
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</div>
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</div>
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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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