#include <cstdlib>
#include <iomanip>
#include <iostream>
#include <mutex>

class Singleton {
private:
  static Singleton *singleton;
  static std::mutex mutex;

  Singleton() = default;

public:
  Singleton(const Singleton &other) = delete;
  Singleton &operator=(const Singleton &) = delete;

  static Singleton *GetInstance() {
    std::lock_guard<std::mutex> lock(mutex);

    if (singleton == nullptr) {
      singleton = new Singleton();
    }

    return singleton;
  }
};

Singleton *Singleton::singleton = nullptr;
std::mutex Singleton::mutex;

int main(int argc, char *argv[]) {

  std::cout << "Singleton" << std::endl;

  auto singleton1 = Singleton::GetInstance();
  auto singleton2 = Singleton::GetInstance();

  std::cout << std::setw(20) << "Singleton 1: " << singleton1 << std::endl;
  std::cout << std::setw(20) << "Singleton 2: " << singleton2 << std::endl;

  return EXIT_SUCCESS;
}