работа с классом

#include <iostream>
 
int nod(int a, int b) {
  if (a < 0) a = -a;
  if (b < 0) b = -b;  
  while (a != 0 && b != 0)
    if (a >= b)
      a %= b;
    else
      b %= a;
  return a + b;
}
 
class Rational {
 public:
  Rational() : numerator_(0), denominator_(1) {}
  Rational(int number) : numerator_(number), denominator_(1) {}
  Rational(int numerator, int denominator)
    : numerator_(numerator), denominator_(denominator) {
    optimize_();
  }
  int getNumerator() const { return numerator_; }
  int getDenominator() const { return denominator_; }
 private:
  void optimize_() {
    int nod_ = nod(numerator_, denominator_);
    numerator_ /= nod_;
    denominator_ /= nod_;
    if (denominator_ < 0) {
      numerator_ = -numerator_;
      denominator_ = -denominator_;
    }
  }
  int numerator_;
  int denominator_;
};
 
std::ostream &operator<<(std::ostream &stream, const Rational &number) {
  return stream << number.getNumerator() << "/" << number.getDenominator();
}
 
Rational operator+(const Rational &a, const Rational &b) {
  return Rational(a.getNumerator() * b.getDenominator() +
    b.getNumerator() * a.getDenominator(),
    a.getDenominator() * b.getDenominator());
    
}
 
Rational operator-(const Rational &a, const Rational &b) {
  return Rational(a.getNumerator() * b.getDenominator() -
    b.getNumerator() * a.getDenominator(),
    a.getDenominator() * b.getDenominator());
}
 
Rational operator*(const Rational &a, const Rational &b) {
  return Rational(a.getNumerator() * b.getNumerator(),
    a.getDenominator() * b.getDenominator());
}
 
Rational operator/(const Rational &a, const Rational &b) {
  return Rational(a.getNumerator() * b.getDenominator(),
    a.getDenominator() * b.getNumerator());
}
 
int main(int argc, char *argv[]) {
  Rational a(10, 15);
  Rational b(6, 21);
  Rational c(7);
  std::cout << "a = " << a << ", b = " << b << std::endl;
  std::cout << "a + b = " << (a + b) << std::endl;
  std::cout << "a - b = " << (a - b) << std::endl;
  std::cout << "a * b = " << (a * b) << std::endl;
  std::cout << "a / b = " << (a / b) << std::endl;
  std::cout << "c = " << c << std::endl;
  std::cout << "c - 3 = " << (c - 3) << std::endl;
  std::cout << "c + 3 = " << (c + 3) << std::endl;
  std::cout << "c / 3 = " << (c / 3) << std::endl;
  std::cout << "1 - c / 3 = " << (1 - c / 3) << std::endl;
  return 0;
}