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<ol class="chapter"><li class="chapter-item expanded "><a href="../Introduction.html">Introduction</a></li><li class="chapter-item expanded "><div>Chapter 1. Deducing Types</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../1.DeducingTypes/item1.html">Item 1: Understanding template type deduction</a></li><li class="chapter-item expanded "><a href="../1.DeducingTypes/item2.html">Item 2: Understand auto type deduction</a></li><li class="chapter-item expanded "><a href="../1.DeducingTypes/item3.html">Item 3: Understand decltype</a></li><li class="chapter-item expanded "><a href="../1.DeducingTypes/item4.html">Item 4: Know how to view deduced types</a></li></ol></li><li class="chapter-item expanded "><div>Chapter 2. auto</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../2.Auto/item5.html">Item 5: Prefer auto to explicit type declarations</a></li><li class="chapter-item expanded "><a href="../2.Auto/item6.html">Item 6: Use the explicitly typed initializer idiom when auto deduces undesired types</a></li></ol></li><li class="chapter-item expanded "><div>Chapter 3. Moving to Modern C++</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item7.html">Item 7: Distinguish between () and {} when creating objects</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item8.html">Item 8: Prefer nullptr to 0 or NULL</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item9.html">Item 9: Prefer alias declarations to typedefs</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item10.html">Item 10: Prefer scoped enums to unscoped enums</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item11.html">Item 11: Prefer deleted functions to private undefined ones</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item12.html">Item 12: Declare overriding functions override</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item13.html">Item 13: Prefer const_iterators to iterators</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item14.html">Item 14: Declare functions noexcept if they won't emit exceptions</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item15.html">Item 15: Use constexpr whenever possible</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item16.html">Item 16: Make const member functions thread safe</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item17.html">Item 17: Understand special member funciton generation</a></li></ol></li><li class="chapter-item expanded "><div>Chapter 4. Smart Pointer</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../4.SmartPointers/item18.html">Item 18: Use std::unique_ptr for exclusive-ownership resource management</a></li><li class="chapter-item expanded "><a href="../4.SmartPointers/item19.html">Item 19: Use std::shared_ptr for shared-ownership resource management</a></li><li class="chapter-item expanded "><a href="../4.SmartPointers/item20.html">Item 20: Use std::weak_ptr for std::shared_ptr like pointers that can dangle</a></li><li class="chapter-item expanded "><a href="../4.SmartPointers/item21.html">Item 21: Prefer std::make_unique and std::make_shared to direct use of new</a></li><li class="chapter-item expanded "><a href="../4.SmartPointers/item22.html">Item 22: When using the Pimpl Idiom, define special member functions in the implementation file</a></li></ol></li><li class="chapter-item expanded "><div>Chapter 5. Rvalue References, Move Semantics, and Perfect Forwarding</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../5.RRefMovSemPerfForw/item23.html">Item 23: Understand std::move and std::forward</a></li><li class="chapter-item expanded "><a href="../5.RRefMovSemPerfForw/item24.html">Item 24: Distinguish universal references from rvalue references</a></li><li class="chapter-item expanded "><a href="../5.RRefMovSemPerfForw/item25.html">Item 25: Use std::move on rvalue references, std::forward on universal references</a></li><li class="chapter-item expanded "><a href="../5.RRefMovSemPerfForw/item26.html">Item 26: Avoid overloading on universal references</a></li><li class="chapter-item expanded "><a href="../5.RRefMovSemPerfForw/item27.html">Item 27: Familiarize yourself with alternatives to overaloading on univeral references</a></li><li class="chapter-item expanded "><a href="../5.RRefMovSemPerfForw/item28.html">Item 28: Understand reference collapsing</a></li><li class="chapter-item expanded "><a href="../5.RRefMovSemPerfForw/item29.html">Item 29: Assume that move operations are not present not cheap, and not used</a></li><li class="chapter-item expanded "><a href="../5.RRefMovSemPerfForw/item30.html">Item 30: Familiarize yourself with perfect forwarding failure cases</a></li></ol></li><li class="chapter-item expanded "><div>Chapter 6. Lambda Expressions</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../6.LambdaExpressions/item31.html">Item 31: Avoid default capture modes</a></li><li class="chapter-item expanded "><a href="../6.LambdaExpressions/item32.html">Item 32: Use init capture to move objects into closures</a></li><li class="chapter-item expanded "><a href="../6.LambdaExpressions/item33.html">Item 33: Use decltype on auto&¶meters to std::forward them</a></li><li class="chapter-item expanded "><a href="../6.LambdaExpressions/item34.html">Item 34: Prefer lambdas to std::bind</a></li></ol></li><li class="chapter-item expanded "><div>Chapter 7. The Concurrency API</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../7.TheConcurrencyAPI/Item35.html">Item 35: Prefer task-based programming to thread-based</a></li><li class="chapter-item expanded "><a href="../7.TheConcurrencyAPI/item36.html" class="active">Item 36: Specify std::launch::async if asynchronicity is essential</a></li><li class="chapter-item expanded "><a href="../7.TheConcurrencyAPI/item37.html">Item 37: Make std::threads unjionable on all paths</a></li><li class="chapter-item expanded "><a href="../7.TheConcurrencyAPI/item38.html">Item 38: Be aware of varying thread handle destructor behavior</a></li><li class="chapter-item expanded "><a href="../7.TheConcurrencyAPI/item39.html">Item 39: Consider void futures for one-shot event communication</a></li><li class="chapter-item expanded "><a href="../7.TheConcurrencyAPI/item40.html">Item 40: Use std::atomic for concurrency, volatile for special memory</a></li></ol></li><li class="chapter-item expanded "><div>Chapter 8. Tweaks</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../8.Tweaks/item41.html">Item 41: Consider pass by value for copyable parameters that are cheap to move and always copied</a></li><li class="chapter-item expanded "><a href="../8.Tweaks/item42.html">Item 42: Consider emplacement instead of insertion</a></li></ol></li></ol>
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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>
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<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="https://github.com/kelthuzadx/EffectiveModernCppChinese/blob/master/3.MovingToModernCpp/item10.md">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>
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<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="https://github.com/kelthuzadx/EffectiveModernCppChinese/blob/master/7.TheConcurrencyAPI/item38.md">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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<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>
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<pre><code class="language-cpp">auto fut1 = std::async(f); //使用默认启动策略运行f
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auto fut2 = std::async(std::launch::async | //使用async或者deferred运行f
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std::launch::deferred,
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f);
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</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="https://github.com/kelthuzadx/EffectiveModernCppChinese/blob/master/7.TheConcurrencyAPI/Item35.md">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>
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<pre><code class="language-cpp">auto fut = std::async(f); //使用默认启动策略运行f
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</code></pre>
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<ul>
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<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>
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<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>
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<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>
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</ul>
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<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>，TLS），不可能预测到哪个线程的变量被访问：</p>
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<pre><code class="language-cpp">auto fut = std::async(f); //f的TLS可能是为单独的线程建的，
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//也可能是为在fut上调用get或者wait的线程建的
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</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="https://github.com/kelthuzadx/EffectiveModernCppChinese/blob/master/7.TheConcurrencyAPI/Item35.md">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；参见条款34
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void f() //f休眠1秒，然后返回
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{
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std::this_thread::sleep_for(1s);
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}
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auto fut = std::async(f); //异步运行f（理论上）
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while (fut.wait_for(100ms) != //循环，直到f完成运行时停止...
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std::future_status::ready) //但是有可能永远不会发生！
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{
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…
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}
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</code></pre>
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<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>
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<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>
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<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>，所以无须怀疑，使用0调用<code>wait_for</code>：</p>
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<pre><code class="language-cpp">auto fut = std::async(f); //同上
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|
||
if (fut.wait_for(0s) == //如果task是deferred（被延迟）状态
|
||
std::future_status::deferred)
|
||
{
|
||
… //在fut上调用wait或get来异步调用f
|
||
} else { //task没有deferred（被延迟）
|
||
while (fut.wait_for(100ms) != //不可能无限循环（假设f完成）
|
||
std::future_status::ready) {
|
||
… //task没deferred（被延迟），也没ready（已准备）
|
||
//做并行工作直到已准备
|
||
}
|
||
… //fut是ready（已准备）状态
|
||
}
|
||
</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，拥有它会非常方便，而且编写起来很容易也使它看起来很棒。C++11版本如下：</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>
|
||
<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="https://github.com/kelthuzadx/EffectiveModernCppChinese/blob/master/5.RRefMovSemPerfForw/item25.md">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="https://github.com/kelthuzadx/EffectiveModernCppChinese/blob/master/3.MovingToModernCpp/item9.md">Item9</a>关于<em>type trait</em>的详细表述。）</p>
|
||
<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，如果std::async抛出异常它也会抛出
|
||
</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，隐含了任务可能不会被执行的意思，会影响调用基于超时的<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>
|
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