190 lines
14 KiB
HTML
190 lines
14 KiB
HTML
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<title>std::barrier</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_thread_barrier 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>barrier</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-barrier.html"><barrier></a></code></div>
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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> CompletionFunction <span class="sy1">=</span> <span class="coMULTI">/* see below */</span><span class="sy1">></span><br />
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<span class="kw1">class</span> barrier<span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<p>类模板 <code>std::barrier</code> 提供允许至多为期待数量的线程阻塞直至期待数量的线程到达该屏障。不同于 <a href="cpp-thread-latch.html"><tt>std::latch</tt></a> ，屏障可重用：一旦到达的线程从屏障阶段的同步点除阻，则可重用同一屏障。</p>
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<p>屏障对象的生存期由屏障阶段的序列组成。每个阶段定义一个<i>阶段同步点</i>。在阶段中到达屏障的线程能通过调用 <a href="cpp-thread-barrier-wait.html"><code>wait</code></a> 在阶段同步点上阻塞，而且将保持阻塞直至运行阶段完成步骤。</p>
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<p><i>屏障阶段</i>由以下步骤组成：</p>
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<ol>
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<li>每次调用 <a href="cpp-thread-barrier-arrive.html"><code>arrive</code></a> 或 <a href="cpp-thread-barrier-arrive_and_drop.html"><code>arrive_and_drop</code></a> 减少<i>期待计数</i>。</li>
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<li>期待计数抵达零时，运行<i>阶段完成步骤</i>。完成步骤调用完成函数对象，并除阻所有在阶段同步点上阻塞的线程。完成步骤的结束<a href="cpp-atomic-memory_order.html#.E5.BC.BA.E5.85.88.E5.8F.91.E7.94.9F.E4.BA.8E">强先发生于</a>所有从完成步骤所除阻的调用的返回。
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<ul>
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<li>对于特化 <code>std::barrier<></code> （使用默认模板实参），完成步骤作为对 <code>arrive</code> 或 <code>arrive_and_drop</code> 的导致期待计数抵达零的调用的一部分运行。</li>
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<li>对于其他特化，完成步骤在该阶段期间到达屏障的线程之一上运行。而若在完成步骤中调用屏障对象的 <code>wait</code> 以外的成员函数，则行为未定义。</li>
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</ul>
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</li>
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<li>完成步骤结束时，重置期待计数为构造中指定的值，可能为 <code>arrive_and_drop</code> 调用所调整，并开始下一阶段。</li>
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</ol>
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<p>同时调用 <code>barrier</code> 的成员函数，除了析构函数，不引入数据竞争。</p>
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<h3><span class="mw-headline" id=".E6.A8.A1.E6.9D.BF.E5.BD.A2.E5.8F.82">模板形参</span></h3>
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<td>CompletionFunction</td>
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<td>-</td>
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<td>函数对象类型</td>
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<tr class="t-par-req">
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<td colspan="3">-
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<code>CompletionFunction</code> 必须满足<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> 和 <a href="cpp-named_req-Destructible.html"><span style="font-style:italic">可析构</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Destructible)</span></a> 的要求。<span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">is_nothrow_invocable_v</span><span class="sy1"><</span>CompletionFunction<span class="sy3">&</span><span class="sy1">></span></span></span> 必须为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 。</td>
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<p><code>CompletionFunction</code> 的默认模板实参是未指定的函数对象类型，额外满足<a href="cpp-named_req-DefaultConstructible.html"><span style="font-style:italic">可默认构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(DefaultConstructible)</span></a> 的要求。以无参数调用其左值无效果。</p>
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<p>每个屏障对象表现为如同它保有一个 <code>CompletionFunction</code> 类型的非静态数据成员 <code>completion_</code> ，并在每个阶段完成步骤通过 <span class="t-c"><span class="mw-geshi cpp source-cpp">completion_<span class="br0">(</span><span class="br0">)</span></span></span> 调用它。</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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<td>名称</td>
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<td>定义</td>
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<td><code>arrival_token</code></td>
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<td>未指定的对象类型，符合<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-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> 及<a href="cpp-named_req-Destructible.html"><span style="font-style:italic">可析构</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Destructible)</span></a> 的要求</td>
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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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<div class="t-dsc-member-div t-dsc-member-nobold-div">
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<div><a href="cpp-thread-barrier-barrier.html"><span class="t-lines"><span>(构造函数)</span></span></a></div>
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</td>
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<td>构造 <code>barrier</code><br />
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<span class="t-mark">(公开成员函数)</span></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-barrier-~barrier.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>销毁 <code>barrier</code><br />
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<span class="t-mark">(公开成员函数)</span></td>
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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><code>barrier</code> 不可赋值<br />
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<span class="t-mark">(公开成员函数)</span></td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-thread-barrier-arrive.html"><span class="t-lines"><span>arrive</span></span></a></div>
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</div>
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</td>
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<td>到达屏障并减少期待计数<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><a href="cpp-thread-barrier-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>在阶段同步点阻塞，直至运行其阶段完成步骤<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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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-thread-barrier-arrive_and_wait.html"><span class="t-lines"><span>arrive_and_wait</span></span></a></div>
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</div>
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</td>
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<td>到达屏障并把期待计数减少一，然后阻塞直至当前阶段完成<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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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-thread-barrier-arrive_and_drop.html"><span class="t-lines"><span>arrive_and_drop</span></span></a></div>
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</div>
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</td>
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<td>将后继阶段的初始期待计数和当前阶段的期待计数均减少一<br />
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<span class="t-mark">(公开成员函数)</span></td>
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<td colspan="2">
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<h5><span class="mw-headline" id=".E5.B8.B8.E9.87.8F">常量</span></h5>
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<tr class="t-dsc">
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<div class="t-dsc-member-div">
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<div><a href="cpp-thread-barrier-max.html"><span class="t-lines"><span>max</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-cmark">[静态]</span></span></span></div>
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</td>
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<td>实现所支持的期待计数的最大值<br />
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<span class="t-mark">(公开静态成员函数)</span></td>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style="">
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<td class="mbox-text" style="">本节未完成<br />
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原因：暂无示例</td>
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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 class="t-dsc-member-div">
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<div><a href="cpp-thread-latch.html"><span class="t-lines"><span>latch</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>单次使用的线程屏障<br />
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<span class="t-mark">(类)</span></td>
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<div class="visualClear"></div>
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