Curiously Recurring Template Pattern

crtpExample.cpp

/*
 * Introduction to Curiously Recurring Template Pattern
 *
 * Ref: https://theolizer.com/cpp-school2/cpp-school2-19/
 */
#include <iostream>
#include <iomanip>
#include <cstdint>
#include <tuple>

#define isDebug() 0

#if isDebug()
// This is a big picture.
template <class tDerived>
class CrtpBase {};

class CrtpDerived : CrtpBase<CrtpDerived> {};
#endif


template <class tDerived>
struct CompareBase {
  // based on operator<
  bool operator>(tDerived const& iRhs) const {
    return (iRhs < static_cast<tDerived const&>(*this));
  }

  bool operator==(tDerived const& iRhs) const {
    return !(static_cast<tDerived const&>(*this) < iRhs) && !(iRhs < static_cast<tDerived const&>(*this));
  }

  bool operator !=(tDerived const& iRhs) const {
    return (static_cast<tDerived const &>(*this) < iRhs) || (iRhs < static_cast<const &>(*this));
  }

  bool operator<=(tDerived const& iRhs) const {
    return !(iRhs < static_cast<tDerived const&>(*this));
  }

  bool operator>=(tDerived const& iRhs) const {
    return !(static_cast<tDerived const&>(*this) < iRhs);
  }
};


// struct Date {
//   uint16_t mYear;
//   uint8_t mMonth;
//   uint8_t mDay;
//
//   Date(uint16_t iYear, uint8_t iMonth, uint8_t iDay)
//     : mYear(iYear), mMonth(iMonth), mDay(iDay) {}
//
//   bool operator<(Date const& iRhs) const {
//     return std::tie(mYear, mMonth, mDay) < std::tie(iRhs.mYear, iRhs.mMonth, iRhs.mDay);
//   }
//
//   // Deduce from operator<
//   bool operator>(Date const& iRhs) const { return iRhs < *this; }
//   bool operator==(Date const& iRhs) const { return !(*this < iRhs) && !(iRhs < *this); }
//   bool operator!=(Date const& iRhs) const { return (*this < iRhs) || (iRhs < *this); }
//   bool operator<=(Date const& iRhs) const { return !(iRhs < *this); }
//   bool operator>=(Date const& iRhs) const { return !(*this < iRhs); }
// };
struct Date : public CompareBase<Date> {
  uint16_t mYear;
  uint8_t mMonth;
  uint8_t mDay;
  Date(uint16_t iYear, uint8_t iMonth, uint8_t iDay)
    : mYear(iYear), mMonth(iMonth), mDay(iDay) {}

  bool operator<(Date const& iRhs) const {
    return  std::tie(mYear, mMonth, mDay) < std::tie(iRhs.mYear, iRhs.mMonth, iRhs.mDay);
  }
};

struct SizeString : public std::string, public CompareBase<SizeString> {
  using std::string::string;

  bool operator<(SizeString const& iRhs) const {
    return size() < iRhs.size();
  }
};

// After the above definitions, we wanted to define order btwn strings by their length.
// struct SizeString : public std::string {
//   using std::string::string;
//
//   bool operator<(SizeString const& iRhs) const {
//     return size() < iRhs.size();
//   }

  /* This repetitive definition is tedious and easy to cause mistakes.
   * Here, we can utilize CRTP.
   * bool operator>(SizeString const& iRhs) const { return  iRhs < *this; }
   * bool operator==(SizeString const& iRhs) const { return  !(*this < iRhs) && !(iRhs < *this); }
   * bool operator!=(SizeString const& iRhs) const { return  (*this < iRhs) || (iRhs < *this); }
   * bool operator<=(SizeString const& iRhs) const { return  !(*this < iRhs); }
   * bool operator>=(SizeString const& iRhs) const { return  !(iRhs < *this); }
   */
// };

int main() {
  Date aToday(2019, 7, 18);
  Date aTomorrow(2019, 7, 19);

  std::cout << std::boolalpha;
  std::cout << "aToday < aTomorrow = " << (aToday < aTomorrow) << std::endl;
}