Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

thread_pool::join

Joins the threads.

void join();

This function blocks until the threads in the pool have completed. If stop() is not called prior to join(), the join() call will wait until the pool has no more outstanding work.