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< title > Item 36:如果有异步的必要请指定std::launch::async - Effective Modern C++< / title >
2022-06-30 10:23:03 +08:00
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< ol class = "chapter" > < li class = "chapter-item expanded " > < a href = "../Introduction.html" > 简介< / a > < / li > < li class = "chapter-item expanded " > < div > 第一章 类型推导< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../1.DeducingTypes/item1.html" > Item 1:理解模板类型推导< / a > < / li > < li class = "chapter-item expanded " > < a href = "../1.DeducingTypes/item2.html" > Item 2:理解auto类型推导< / a > < / li > < li class = "chapter-item expanded " > < a href = "../1.DeducingTypes/item3.html" > Item 3:理解decltype< / a > < / li > < li class = "chapter-item expanded " > < a href = "../1.DeducingTypes/item4.html" > Item 4:学会查看类型推导结果< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < div > 第二章 auto< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../2.Auto/item5.html" > Item 5:优先考虑auto而非显式类型声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../2.Auto/item6.html" > Item 6:auto推导若非己愿， < / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < div > 第三章 移步现代C++< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item7.html" > Item 7:区别使用()和{}创建对象< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item8.html" > Item 8:优先考虑nullptr而非0和NULL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item9.html" > Item 9:优先考虑别名声明而非typedefs< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item10.html" > Item 10:优先考虑限域枚举而非未限域枚举< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item11.html" > Item 11:优先考虑使用deleted函数而非使用未定义的私有声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item12.html" > Item 12:使用override声明重载函数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item13.html" > Item 13:优先考虑const_iterator而非iterator< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item14.html" > Item 14:如果函数不抛出异常请使用noexcept< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item15.html" > Item 15:尽可能的使用constexpr< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item16.html" > Item 16:让const成员函数线程安全< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item17.html" > Item 17:理解特殊成员函数函数的生成< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < div > 第四章 智能指针< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item18.html" > Item 18:对于独占资源使用std::unique_ptr< / a > < / li > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item19.html" > Item 19:对于共享资源使用std::shared_ptr< / a > < / li > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item20.html" > Item 20:当std::shared_ptr可能悬空时使用std::weak_ptr< / a > < / li > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item21.html" > Item 21:优先考虑使用std::make_unique和std::make_shared而非new< / a > < / li > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item22.html" > Item 22:当使用Pimpl惯用法， < / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < div > 第五章 右值引用，移动语义，完美转发< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../5.RRefMovSemPerfForw/item23.html" > Item 23:理解std::move和std::forward< / a > < / li > < li class = "chapter-item expanded " > < a href = "../5.RRefMovSemPerfForw/item24.html" > Item 24:区别通用引用和右值引用< / a > < / li > < li class = "chapter-item expanded " > < a href = "../5.RRefMovSemPerfForw/item25.html" > Item 25:对于右值引用使用std::move， < / a > < / li > < li class = "chapter-item expanded " > < a href = "../5.RRefMovSemPerf
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< h2 id = "条款三十六如果有异步的必要请指定stdlaunchasync" > < a class = "header" href = "#条款三十六如果有异步的必要请指定stdlaunchasync" > 条款三十六：如果有异步的必要请指定< code > std::launch::async< / code > < / a > < / h2 >
< p > < strong > Item 36: Specify < code > std::launch::async< / code > if asynchronicity is essential.< / strong > < / p >
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< p > 当你调用< code > std::async< / code > 执行函数时（或者其他可调用对象），你通常希望异步执行函数。但是这并不一定是你要求< code > std::async< / code > 执行的操作。你事实上要求这个函数按照< code > std::async< / code > 启动策略来执行。有两种标准策略，每种都通过< code > std::launch< / code > 这个限域< code > enum< / code > 的一个枚举名表示（关于枚举的更多细节参见< a href = "../3.MovingToModernCpp/item10.html" > Item10< / a > ）。假定一个函数< code > f< / code > 传给< code > std::async< / code > 来执行：< / p >
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< ul >
< li > < strong > < code > std::launch::async< / code > 启动策略< / strong > 意味着< code > f< / code > 必须异步执行，即在不同的线程。< / li >
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< li > < strong > < code > std::launch::deferred< / code > 启动策略< / strong > 意味着< code > f< / code > 仅当在< code > std::async< / code > 返回的< em > future< / em > 上调用< code > get< / code > 或者< code > wait< / code > 时才执行。这表示< code > f< / code > < strong > 推迟< / strong > 到存在这样的调用时才执行（译者注：异步与并发是两个不同概念，这里侧重于惰性求值）。当< code > get< / code > 或< code > wait< / code > 被调用，< code > f< / code > 会同步执行，即调用方被阻塞，直到< code > f< / code > 运行结束。如果< code > get< / code > 和< code > wait< / code > 都没有被调用，< code > f< / code > 将不会被执行。（这是个简化说法。关键点不是要在其上调用< code > get< / code > 或< code > wait< / code > 的那个< em > future< / em > ，而是< em > future< / em > 引用的那个共享状态。（< a href = "../7.TheConcurrencyAPI/item38.html" > Item38< / a > 讨论了< em > future< / em > 与共享状态的关系。）因为< code > std::future< / code > 支持移动，也可以用来构造< code > std::shared_future< / code > ，并且因为< code > std::shared_future< / code > 可以被拷贝，对共享状态——对< code > f< / code > 传到的那个< code > std::async< / code > 进行调用产生的——进行引用的< em > future< / em > 对象，有可能与< code > std::async< / code > 返回的那个< em > future< / em > 对象不同。这非常绕口，所以经常回避这个事实，简称为在< code > std::async< / code > 返回的< em > future< / em > 上调用< code > get< / code > 或< code > wait< / code > 。）< / li >
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< / ul >
< p > 可能让人惊奇的是，< code > std::async< / code > 的默认启动策略——你不显式指定一个策略时它使用的那个——不是上面中任意一个。相反，是求或在一起的。下面的两种调用含义相同：< / p >
< pre > < code class = "language-cpp" > auto fut1 = std::async(f); //使用默认启动策略运行f
auto fut2 = std::async(std::launch::async | //使用async或者deferred运行f
std::launch::deferred,
f);
< / code > < / pre >
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< p > 因此默认策略允许< code > f< / code > 异步或者同步执行。如同< a href = "../7.TheConcurrencyAPI/Item35.html" > Item35< / a > 中指出，这种灵活性允许< code > std::async< / code > 和标准库的线程管理组件承担线程创建和销毁的责任，避免资源超额，以及平衡负载。这就是使用< code > std::async< / code > 并发编程如此方便的原因。< / p >
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< p > 但是，使用默认启动策略的< code > std::async< / code > 也有一些有趣的影响。给定一个线程< code > t< / code > 执行此语句：< / p >
< pre > < code class = "language-cpp" > auto fut = std::async(f); //使用默认启动策略运行f
< / code > < / pre >
< ul >
< li > < strong > 无法预测< code > f< / code > 是否会与< code > t< / code > 并发运行< / strong > ，因为< code > f< / code > 可能被安排延迟运行。< / li >
< li > < strong > 无法预测< code > f< / code > 是否会在与某线程相异的另一线程上执行，这个某线程在< code > fut< / code > 上调用< code > get< / code > 或< code > wait< / code > < / strong > 。如果对< code > fut< / code > 调用函数的线程是< code > t< / code > ，含义就是无法预测< code > f< / code > 是否在异于< code > t< / code > 的另一线程上执行。< / li >
< li > < strong > 无法预测< code > f< / code > 是否执行< / strong > ，因为不能确保在程序每条路径上，都会不会在< code > fut< / code > 上调用< code > get< / code > 或者< code > wait< / code > 。< / li >
< / ul >
< p > 默认启动策略的调度灵活性导致使用< code > thread_local< / code > 变量比较麻烦，因为这意味着如果< code > f< / code > 读写了< strong > 线程本地存储< / strong > （ < em > thread-local storage< / em > ， ） ， ： < / p >
< pre > < code class = "language-cpp" > auto fut = std::async(f); //f的TLS可能是为单独的线程建的，
//也可能是为在fut上调用get或者wait的线程建的
< / code > < / pre >
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< p > 这还会影响到基于< code > wait< / code > 的循环使用超时机制，因为在一个延时的任务（参见< a href = "../7.TheConcurrencyAPI/Item35.html" > Item35< / a > ）上调用< code > wait_for< / code > 或者< code > wait_until< / code > 会产生< code > std::launch::deferred< / code > 值。意味着，以下循环看似应该最终会终止，但可能实际上永远运行：< / p >
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< pre > < code class = "language-cpp" > using namespace std::literals; //为了使用C++14中的时间段后缀；
void f() //f休眠1秒，
{
std::this_thread::sleep_for(1s);
}
auto fut = std::async(f); //异步运行f（ ）
while (fut.wait_for(100ms) != //循环，
std::future_status::ready) //但是有可能永远不会发生！
{
…
}
< / code > < / pre >
< p > 如果< code > f< / code > 与调用< code > std::async< / code > 的线程并发运行（即，如果为< code > f< / code > 选择的启动策略是< code > std::launch::async< / code > ），这里没有问题（假定< code > f< / code > 最终会执行完毕），但是如果< code > f< / code > 是延迟执行，< code > fut.wait_for< / code > 将总是返回< code > std::future_status::deferred< / code > 。这永远不等于< code > std::future_status::ready< / code > ，循环会永远执行下去。< / p >
< p > 这种错误很容易在开发和单元测试中忽略，因为它可能在负载过高时才能显现出来。那些是使机器资源超额或者线程耗尽的条件，此时任务推迟执行才最有可能发生。毕竟，如果硬件没有资源耗尽，没有理由不安排任务并发执行。< / p >
< p > 修复也是很简单的：只需要检查与< code > std::async< / code > 对应的< code > future< / code > 是否被延迟执行即可，那样就会避免进入无限循环。不幸的是，没有直接的方法来查看< code > future< / code > 是否被延迟执行。相反，你必须调用一个超时函数——比如< code > wait_for< / code > 这种函数。在这个情况中，你不想等待任何事，只想查看返回值是否是< code > std::future_status::deferred< / code > ， ， < code > wait_for< / code > ： < / p >
< pre > < code class = "language-cpp" > auto fut = std::async(f); //同上
if (fut.wait_for(0s) == //如果task是deferred（ ）
std::future_status::deferred)
{
… //在fut上调用wait或get来异步调用f
} else { //task没有deferred（ ）
while (fut.wait_for(100ms) != //不可能无限循环（ ）
std::future_status::ready) {
… //task没deferred（ ） ， （ ）
//做并行工作直到已准备
}
… //fut是ready（ ）
}
< / code > < / pre >
< p > 这些各种考虑的结果就是，只要满足以下条件，< code > std::async< / code > 的默认启动策略就可以使用：< / p >
< ul >
< li > 任务不需要和执行< code > get< / code > 或< code > wait< / code > 的线程并行执行。< / li >
< li > 读写哪个线程的< code > thread_local< / code > 变量没什么问题。< / li >
< li > 可以保证会在< code > std::async< / code > 返回的< em > future< / em > 上调用< code > get< / code > 或< code > wait< / code > ，或者该任务可能永远不会执行也可以接受。< / li >
< li > 使用< code > wait_for< / code > 或< code > wait_until< / code > 编码时考虑到了延迟状态。< / li >
< / ul >
< p > 如果上述条件任何一个都满足不了，你可能想要保证< code > std::async< / code > 会安排任务进行真正的异步执行。进行此操作的方法是调用时，将< code > std::launch::async< / code > 作为第一个实参传递：< / p >
< pre > < code class = "language-cpp" > auto fut = std::async(std::launch::async, f); //异步启动f的执行
< / code > < / pre >
< p > 事实上，对于一个类似< code > std::async< / code > 行为的函数，但是会自动使用< code > std::launch::async< / code > 作为启动策略的工具， ， ： < / p >
< pre > < code class = "language-cpp" > template< typename F, typename... Ts>
inline
std::future< typename std::result_of< F(Ts...)> ::type>
reallyAsync(F& & f, Ts& & ... params) //返回异步调用f(params...)得来的future
{
return std::async(std::launch::async,
std::forward< F> (f),
std::forward< Ts> (params)...);
}
< / code > < / pre >
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< p > 这个函数接受一个可调用对象< code > f< / code > 和0或多个形参< code > params< / code > ，然后完美转发（参见< a href = "../5.RRefMovSemPerfForw/item25.html" > Item25< / a > ）给< code > std::async< / code > ，使用< code > std::launch::async< / code > 作为启动策略。就像< code > std::async< / code > 一样，返回< code > std::future< / code > 作为用< code > params< / code > 调用< code > f< / code > 得到的结果。确定结果的类型很容易，因为< em > type trait< / em > < code > std::result_of< / code > 可以提供给你。（参见< a href = "../3.MovingToModernCpp/item9.html" > Item9< / a > 关于< em > type trait< / em > 的详细表述。）< / p >
2022-06-30 10:23:03 +08:00
< p > < code > reallyAsync< / code > 就像< code > std::async< / code > 一样使用：< / p >
< pre > < code class = "language-cpp" > auto fut = reallyAsync(f); //异步运行f，
< / code > < / pre >
< p > 在C++14中， < code > reallyAsync< / code > 返回类型的推导能力可以简化函数的声明：< / p >
< pre > < code class = "language-cpp" > template< typename F, typename... Ts>
inline
auto // C++14
reallyAsync(F& & f, Ts& & ... params)
{
return std::async(std::launch::async,
std::forward< F> (f),
std::forward< Ts> (params)...);
}
< / code > < / pre >
< p > 这个版本清楚表明，< code > reallyAsync< / code > 除了使用< code > std::launch::async< / code > 启动策略之外什么也没有做。< / p >
< p > < strong > 请记住：< / strong > < / p >
< ul >
< li > < code > std::async< / code > 的默认启动策略是异步和同步执行兼有的。< / li >
< li > 这个灵活性导致访问< code > thread_local< / code > s的不确定性， ， < code > wait< / code > 的程序逻辑。< / li >
< li > 如果异步执行任务非常关键，则指定< code > std::launch::async< / code > 。< / li >
< / ul >
< / main >
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