August 2021
Intermediate to advanced
450 pages
5h 35m
Chinese
Content preview from C++语言导学（原书第2版）
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15.7.2 packaged_task
我们应该如何向一个需要结果的任务引入future呢？又如何向一个生成结果的线程引入对应的promise呢？标准库提供了packaged_task类型去简化将任务连接到future和promise的设置。一个packaged_task提供了一层包装代码，实现将某个任务的返回值或异常放入一个promise中（就像15.7.1节中代码所做的）。如果你通过调用get_future（）来获取结果，packaged_task会返回给你对应其promise的future。例如，我们可以将两个任务连接起来，它们分别使用标准库accumulate（）（参见14.3节）算法将一个vector<double>中的一半元素累加起来：
packaged_task模板接受模板参数表示任务的类型（本例中为Task_type，double（double*，double*，double）的别名），并接受构造函数参数作为任务（本例中为accum）。move（）操作是必需的，因为packaged_task不能被拷贝。原因在于packaged_task是一种资源句柄：它拥有一个promise且（间接）负责其任务所拥有的资源。
请注意这段代码没有显式使用锁：通过使用packaged_task，我们可以集中精力于要完成的任务，而不必操心用来管理它们通信的机制。两个任务运行于两个独立的线程上，因此可以并行执行。 ...
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