Перегрузка операторов. Урок 3

//Перегрузка операторов. Урок 3
#include <iostream>
#include <numeric>
#include <vector>
#include <iostream>

using namespace std;

class Rational {
public:
    Rational() = default;

    Rational(int numerator)
        : numerator_(numerator)
        , denominator_(1)
    {
    }

    Rational(int numerator, int denominator)
        : numerator_(numerator)
        , denominator_(denominator)
    {
        Normalize();
    }

    int Numerator() const {
        return numerator_;
    }

    int Denominator() const {
        return denominator_;
    }

private:
    void Normalize() {
        if (denominator_ < 0) {
            numerator_ = -numerator_;
            denominator_ = -denominator_;
        }
        int n = gcd(numerator_, denominator_);
        numerator_ /= n;
        denominator_ /= n;
    }

    int numerator_ = 0;
    int denominator_ = 1;
};

ostream& operator<<(ostream& output, Rational rational) {
    output << rational.Numerator() << '/' << rational.Denominator();
    return output;
}

istream& operator>>(istream& input, Rational& rational) {
    int numerator, denominator;
    char slash;
    input >> numerator >> slash >> denominator;
    rational = Rational{numerator, denominator};
    return input;
}


Rational operator+(Rational left, Rational right) {
    const int numerator = left.Numerator() * right.Denominator()
        + right.Numerator() * left.Denominator();
    const int denominator = left.Denominator() * right.Denominator();

    return { numerator, denominator };
}

Rational operator-(Rational left, Rational right) {
    const int numerator = left.Numerator() * right.Denominator()
        - right.Numerator() * left.Denominator();
    const int denominator = left.Denominator() * right.Denominator();

    return { numerator, denominator };
}

Rational operator-(Rational value) {
    return { -value.Numerator(), value.Denominator()};
}

Rational operator+(Rational value) {
    return value;
}