==运算符正在为用户制作的类工作而不会过载

Question

我已经创建了一个Rational类，并且仅对Rational vs Rational进行了重载==运算符，而不是Rational vs int比较，但它在某种程度上也适用于后一种情况。它为什么有效？这是代码：

bool Rational::operator==(const Rational& r) const
{
    if(denominator == r.getDenominator())
        if(numerator == r.getNumerator())
            return true;
    else
        return false;
}

int main()
{
    Rational r(16, 8); // 16/8
    cout << boolalpha << (r == 3) << '\n'; //false
    cout << boolalpha << (r == 2) << '\n'; //Why this returns "true" without knowing how to compare Rational with int?!
    return 0;
}

“r == 2”返回true而“r == 3”为false，但“== operator”仅为类与类比较重载... 这是所有的代码：

#include <iostream> 
#include <cstdlib>
using namespace std;

class Rational {
public:
    Rational( int nume, int denom );
    Rational( int nume );
    void setNumeDenom( int nume, int denom );
    bool operator==( const Rational& r ) const;

    int getNumerator() const { return numerator; }
    int getDenominator() const { return denominator; }

private:
    int numerator;
    int denominator;
};

int greatestCommonDivisor( int a, int b )
{
    if( b == 0 )
        return a;   
    return greatestCommonDivisor(b, a % b); 
}

Rational::Rational( int n, int d )
{
    setNumeDenom( n, d );
}

Rational::Rational( int n )
{
    setNumeDenom( n, 1 );
}

void Rational::setNumeDenom( int n, int d )
{
    int gcd = greatestCommonDivisor(n ,d);
    if( d != 0 && gcd > 0 )
    {
        numerator = n / gcd;
        denominator = d / gcd;
    }
    else
    {
        cout << "denominator cannot be Zero.\n";
        abort();
    }
}
bool Rational::operator==(const Rational& r) const
{
    if(denominator == r.getDenominator())
        if(numerator == r.getNumerator())
            return true;
    else
        return false;
}

int main()
{
    Rational r(16, 8);
    cout << boolalpha << (r == 2) << '\n'; //true
    cout << boolalpha << (r == 3) << '\n'; //false
    return 0;
}