//形式1
cout << r.toString();

//形式2
cout << r;

<< (cout, r)
//或者
operator << (cout, r)

friend ostream& operator<<(ostream& out, const Rational rational);

ostream& operator<<(ostream& out, const Rational rational) {
    out << rational.numerator << "/" << rational.denominator;
    return out;
}

friend istream& operator>>(istream& in, Rational& rational);

istream& operator>>(istream& in, Rational& rational) {
    cout << "Enter numerator: ";
    in >> rational.numerator;

    cout << "Enter denominator: ";
    in >> rational.denominator;

    return in;
}

#include <iostream>
#include "Rational.h"

using namespace std;

int main()
{
    Rational r1, r2;

    cout << "第1个有理数" << endl;
    cin >> r1;

    cout << "第2个有理数" << endl;
    cin >> r2;

    cout << "r1 = " << r1 << "\nr2 = " << r2 << endl;

    return 0;
}