#include <iostream>
#include <vector>
#include <mutex>
#include <condition_variable>
#include <thread>

#define NUM_THREADS 4

std::condition_variable cond;
std::mutex mx;
int started = 0;

void
do_thread ()
{
  std::unique_lock<std::mutex> lock(mx);
  std::cout << "Start thread " << started << std::endl;
  if(++started >= NUM_THREADS)
    cond.notify_all();
  else
    cond.wait(lock);
}

int
main ()
{
  std::vector<std::thread> vec;
  for (int i = 0; i < NUM_THREADS; ++i)
    vec.emplace_back(&do_thread);
  for (int i = 0; i < NUM_THREADS; ++i)
    vec[i].join();
  vec.clear();
  return 0;
}