I have a problem in organizing concurrent thread launch. My program structure is the following:
#include <iostream>
#include <boost/thread.hpp>

using namespace std;

class Worker
{
   private:
       boost::thread m_Thread;

   public:
       Worker() { /* the thread is not-a-thread until we call start() */ }

       void start(int N) { m_Thread = boost::thread(&Worker::processQueue, this, N); }

       void join() { m_Thread.join(); }

       void processQueue(unsigned N) { /* Do some long long stuff... */ }
};

int main(int argc, char* argv[])
{
   Worker worker_1,
          worker_2,
          worker_3,
          worker_4;

   // How to start threads and join them in order
   // to make constantly two of them running?

   return 0;
}

I have different threads which have to work on completely different input and output data (non critical sections): an atomic operation per thread, each one with different time execution but everyone with an intense use of the CPU.
Let's say I have 10 operations to perform (10 threads): I would like to run concurrently only 2 threads because of resource consumption.

My problem is that when a threads ends its execution, I would like to suddenly start another thread performing operation 3, in order to have constantly 2 threads working, and so on until the end of operations.

How can I achieve this? I thought to insert my threads into a vector...but I have no idea on how start and join them in order to obtain what described above.

Can anyone post me a little example?

Thanks in advance.