boost::threadpool::pool vs.boost::thread_group

boost::thread_group is a convenience class for performing thread management operations on a collection of threads. For example, instead of having to iterate over std::vector<boost::thread>, invoking join() on each thread, the thread_group provides a convenient join_all() member function.

With boost::thread, regardless of it being managed by boost::thread_group, the lifetime of the thread is often dependent on the work in which the thread is doing. For example, if a thread is created to perform a computationally expensive calculation, then the thread can exit once the result has been calculated. If the work is short-lived, then the overhead of creating and destroying threads can affect performance.

On the other hand, a threadpool is a pattern, where a number of threads services a number of task/work. The lifetime of the thread is not directly associated with the lifetime of the task. To continue with the previous example, the application would schedule the computationally expensive calculation to run within the thread pool. The work will be queued within the threadpool, and one of the threadpool's threads will be selected to perform the work. Once the calculation has completed, the thread goes back to waiting for more work to be scheduled with the threadpool.

As shown in this threadpool example, a threadpool can be implemented with boost::thread_group to manage lifetime of threads, and boost::asio::io_service for task/work dispatching.