什么先运行：分区器或组合器？(What runs first: the partitioner or the combiner?)

 std::condition_variable不指定在调用notify_one时哪个等待线程被唤醒。 因此，您应该编写不关心哪个线程被唤醒的代码。 标准模式是无论哪个线程被唤醒，该线程都应该完成需要完成的工作。 
 
 如果您要求线程按特定顺序唤醒，请使用其他机制。 例如，您可以为每个线程分别设置std::condition_variable ，然后在需要工具时将线程放入队列中。 当一个线程递交工具时，它可以发信号通知与队列前面的线程相对应的条件变量。 那个线程会被唤醒，其他线程将保持睡眠状态（模仿虚假唤醒）。 

std::condition_variable does not specify which waiting thread is woken when you call notify_one. You should therefore write code that doesn't care which thread is woken. The standard pattern is that whichever thread is woken, that thread should do the work that needs to be done.
 
If you require that the threads are woken in a specific order, then use a different mechanism. You could, for example, have a separate std::condition_variable for each thread, and then put the threads in a queue when they need tools. As a thread hands in the tools, it could then signal the condition variable corresponding to the thread at the front of the queue. That thread will then be woken, and the others will remain sleeping (modulo spurious wake-ups).