#include <mutex>
#include <stdexcept>


void do()
{

  static std::mutex mutex;

  std::lock_guard<std::mutex> lock(mutex);

  std::ofstream file("file.txt");
  if (!file.is_open())
    throw std::runtime_error("Can't open file!");

  file << "Add some text!";

} // if exception is thrown, we skip the writing to file and the scope closes, releasing all objects

// State Pattern

class Context {
  public:
  void request() {}
};

class State {
  public:
  virtual void handle() {}
};

class RealState : public State
{
  void handle() override { (void)(0); }
};

class GreatState : public State
{
  void handle() override { (void)(0); }
};

// Visitor
class CarElement {};
class CarElementVisitor {
  public:

};


// Double-checked locking with singleton
class Singleton {
  public:
  static Singleton* instance();

  private:
  Singleton() = default;

  static std::atomic<Singleton*> s_instance;
  static std::mutex s_mutex;

};

Singleton* Singleton::GetInstance()
{
  Singleton* p = s_instance.load(std::memory_order_acquire);
  if (!p)
  {
    std::lock_guard<std::mutex> lock(s_mutex);
    p = s_instance.load(std::memory_order_relaxed);
    if (!p)
    {
      p = new Singleton();
      s_instance.store(p, std::memory_order_acquire);
    }
  }
  return p;
}