How to make boost::thread_group execute a fixed number of parallel threads

Question

This is the code to create a thread_group and execute all threads in parallel:

boost::thread_group group;
for (int i = 0; i < 15; ++i)
    group.create_thread(aFunctionToExecute);
group.join_all();

This code will execute all threads at once. What I want to do is to execute them all but 4 maximum in parallel. When on is terminated, another one is executed until there are no more to execute.

Answer 1

Another, more efficient solution would be to have each thread callback to the primary thread when they are finished, and the handler on the primary thread could launch a new thread each time. This prevents the repetitive calls to timed_join, as the primary thread won't do anything until the callback is triggered.