#include <iostream>
#include <thread>
#include <vector>

// Code to be run by the additional threads.
void thread_code(int id) {
  std::cout << "Hello World from thread " << id << std::endl;
}

int main(int, char *[]) {
  int constexpr NUM_THREADS = 4;

  // A vector to keep the thread objects.
  std::vector<std::thread> threads;

  // Create the threads and put their objects in-place in the vector.
  for (int i = 0; i < NUM_THREADS; ++i) {
    threads.emplace_back(thread_code, i);
  }

  // Main thread continues.
  std::cout << "Now there are " << NUM_THREADS + 1 << " threads running.\n";

  // Wait for all additional threads to finish.
  for (auto &thr : threads) {
    thr.join();
  }

  std::cout << "Now there is only one thread." << std::endl;
  return 0;
}