[boost] [thread] thread_group::size complexity

Hi, The current implementation could have O(n) complexity due to the use of std::list. Even if we would have thread_group with no too mant threads, it sounds extrange to me to have a function size without constant complexity. As your implementation do not needs splice, what do you think about having a thread_group::size with constant complexity by adding an internal counter for example? Regards, Vicente