template<typename date_type> 
class year_based_generator {
public:
  // types
  typedef date_type::calendar_type calendar_type;
  typedef calendar_type::year_type year_type;    

  // construct/copy/destruct
  year_based_generator();
  ~year_based_generator();

  // public member functions
  virtual date_type get_date(year_type) const;
  virtual std::string to_string() const;
};