#include <atomic>
#include <functional>
#include <thread>
#include <iostream>
 
int countdown(std::atomic<int>& jobs)
{
  int done = 0, nr;
  while ( (nr = int(--jobs)) >= 0) ++done;
  return done;
}
 
int main()
{
  int n = 1000000;
  std::atomic<int> jobs(n);
  auto mode = std::launch::async;
  auto cnt1 = std::async(mode, countdown, std::ref(jobs));
  auto cnt2 = std::async(mode, countdown, std::ref(jobs));
  int a = cnt1.get(), b = cnt2.get();
 
  std::cout << a << " + " << b << " = " << a+b << '\n';
  assert(a+b == n);
}