/** Engine API **/
class IEvent
{
public:
  virtual const EventTypeID GetEventTypeID() const = 0;
};

template <typename T>
class Event : public IEvent
{
public:
  static const EventTypeID EVENT_TYPE_ID;

  virtual const EventTypeID GetEventTypeID() const override
  {
    return EVENT_TYPE_ID;
  }
};

/** Client **/

class KeydownEvent : public Event<KeydownEvent>
{
  ...
};

class Secure
{	
	int data;
	
public:
	static std::shared_ptr<Secure> CreateSecureObj(int data);
	class Key
	{
		friend std::shared_ptr<Secure> Secure::CreateSecureObj(int data);
		Key() {};
	};
	
	explicit Secure(const Key& key,  int data) : data{data} {};
};

std::shared_ptr<Secure> Secure::CreateSecureObj(int data)
{
	return std::make_shared<Secure>(Secure::Key(), data);
}

auto pSecure1 = Secure::CreateSecureObj(10); // OK
auto pSecure2 = std::make_shared<Secure>(Secure::Key(), 10); // Error
auto pSecure3 = new Secure(Secure::Key(), 10) // Error