lock_guard.cpp

#include <iostream>
#include <mutex>
#include <thread>
int counter = 0;
std::mutex counter_mutex;
void worker() {
  std::lock_guard<std::mutex> lock(counter_mutex);
  if (counter == 1) {
    counter += 10;
  } else if (counter >= 10) {
    counter += 15;
  } else if (counter >= 50) {
    return;
  } else {
    ++counter;
  }
  std::cout << std::this_thread::get_id() << ": " << counter << '\n';
}
int main() {
  std::cout << __func__ << ": " << counter << '\n';
  std::thread t1(worker);
  std::thread t2(worker);
  t1.join();
  t2.join();
  std::cout << __func__ << ": " << counter << '\n';
}