Question

I’m using learning the basic of boost.thread. So far, I can create each thread one by one manually to let them run at the same time. However, when creating by loop, it runs sequentially not concurrency anymore.

#include <iostream>
#include <boost/thread.hpp>
void workerFunc()
{
  boost::posix_time::seconds workTime(3);
  std::cout << "Worker: Running" << '\n';
  boost::this_thread::sleep(workTime);
  std::cout<< "Worker: Finished" << '\n';
}

int main()
{
  std::cout << "main: startup" << '\n';
  boost::thread workerThread(workerFunc);
  std::cout << "main: waiting for thread" << '\n';
  //these are ok
  boost::thread t(workerFunc), t2(workerFunc), t3(workerFunc), t4(workerFunc);     
  t.join();
  t2.join();
  t3.join();
  t4.join();
  //these are not
  for (int i = 0; i < 2; ++i)
    {
      boost::thread z(workerFunc);
      z.join();
    }
  std::cout << "main:done" << '\n';
  return 0;
}

Answer 1

  for (int i = 0; i < 2; ++i)
    {
      boost::thread z(workerFunc);
      z.join();
    }

You are starting your thread and then immediately waiting for it to complete!

EDIT

One of several alternative hacks besides thread groups.

  std::vector<boost::thread *> z;

  for (int i = 0; i < 2; ++i)
    z.push_back(new boost::thread(workerFunc));

  for (int i = 0; i < 2; ++i)
  {
      z[i]->join();
      delete z[i];
  }