C++11 std::async

//g++ -std=c++11 -pthread -g std_future.cpp -o main
// async example
#include <iostream>       // std::cout
#include <future>         // std::async, std::future

// a non-optimized way of checking for prime numbers:
bool is_prime (long int x) {
  std::cout << "Calculating " << x << ". Please, wait...\n";
  for (int i=2; i<x; ++i) if (x%i==0) return false;
  return true;
}

int main ()
{
  long int x3=2147483647;
  long int x2=623222313;
  long int x1=444444443;
  // call is_prime(313222313) asynchronously:
  std::cout << "Start Checking whether " << x1 << " is prime.\n";
  std::future<bool> fut1 = std::async (std::launch::async, is_prime,x1);
  
  std::cout << "Start Checking whether " << x2 << " is prime.\n";
  std::future<bool> fut2 = std::async (std::launch::async, is_prime,x2);
  
  std::cout << "Start Checking whether " << x3 << " is prime.\n";
  std::future<bool> fut3 = std::async (std::launch::async, is_prime,x3);


  std::cout << x1 << (fut1.get() ? " is prime\n" : " is not prime\n");

  std::cout << x2 << (fut2.get() ? " is prime\n" : " is not prime\n");
  
  std::cout << x3 << (fut3.get() ? " is prime\n" : " is not prime\n");
  
  return 0;
}