#include <iostream>
#include <memory>

namespace base {

class Clock {
 public:
  virtual int Now() = 0;
  virtual ~Clock() {}
};

class DefaultClock : public Clock {
 public:
  DefaultClock() {
    std::cout << "Constructed DefaultClock" << std::endl;
  }

  virtual ~DefaultClock() {
    std::cout << "Destroyed DefaultClock" << std::endl;
  }

  int Now() override {
    static int now = 42;
    return now++;
  };
};

class MockClock : public Clock {
 public:
  MockClock(int fixed) : fixed_(fixed), now_(fixed) {
    std::cout << "Constructed MockClock" << fixed_ << std::endl;
  }

  virtual ~MockClock() {
    std::cout << "Destroyed MockClock" << fixed_ << std::endl;
  }

  int Now() override { return now_; }

  void SetNow(int now) { now_ = now; }
 private:
  int fixed_;
  int now_;
};

template <typename T, typename O = std::nullptr_t>
class NoDestructor {
 public:
  // Not constexpr; just write static constexpr T x = ...; if the value should
  // be a constexpr.
  template <typename... Args>
  explicit NoDestructor(Args&&... args) {
    new (storage_) T(std::forward<Args>(args)...);
  }

  // Allows copy and move construction of the contained type, to allow
  // construction from an initializer list, e.g. for std::vector.
  explicit NoDestructor(const T& x) { new (storage_) T(x); }
  explicit NoDestructor(T&& x) { new (storage_) T(std::move(x)); }

  NoDestructor(const NoDestructor&) = delete;
  NoDestructor& operator=(const NoDestructor&) = delete;

  ~NoDestructor() = default;

  const T& operator*() const { return *get(); }
  T& operator*() { return *get(); }

  const T* operator->() const { return get(); }
  T* operator->() { return get(); }

  const T* get() const { return reinterpret_cast<const T*>(storage_); }
  T* get() { return reinterpret_cast<T*>(storage_); }

 private:
  alignas(T) char storage_[sizeof(T)];
};

}

// Example class allowing to inject custom Clock.
class Example {
 public:
   static void PrintNow() {
     std::cout << GetClock().Now() << std::endl;
   }

   static void SetClockForTesting(std::unique_ptr<base::Clock>& clock) {
     GetClockPtr() = std::move(clock);
   }

 private:
  static std::unique_ptr<base::Clock>& GetClockPtr() {
    static base::NoDestructor<std::unique_ptr<base::Clock>> clock([] {
      return std::make_unique<base::DefaultClock>();
    }());
    return *clock;
  }

  static base::Clock& GetClock() {
    return *GetClockPtr().get();
  }
};

// Test code.
void test() {
    std::unique_ptr<base::Clock> mock_clock =
      std::make_unique<base::MockClock>(11);
    auto* raw_mock = static_cast<base::MockClock*>(mock_clock.get());
    Example::SetClockForTesting(mock_clock);
    Example::PrintNow();
    raw_mock->SetNow(22);
}


int main() {
  std::cout << "Start: nothing should be constructed before" << std::endl;
  std::cout << "===========================================" << std::endl;
  Example::PrintNow();
  Example::PrintNow();
  test();
  Example::PrintNow();
  std::unique_ptr<base::Clock> mock_clock =
    std::make_unique<base::MockClock>(33);
  Example::SetClockForTesting(mock_clock);
  Example::PrintNow();
  std::cout << "======================================" << std::endl;
  std::cout << "End: nothing should be destroyed after" << std::endl;
}