Функции и перегруженный оператор

cout << "Input x: ";
cin >> x;
cout << "Input y: ";
cin >>  y;
if (x==0 || y==0)
   cout << "nevozmozno rasschitat NOD" ;
   else if (x<0) x=abs(x);
   if (y<0) y=abs(y);
while (x!=y)
if (x>y) x-=y;
else y-=x;
cout << "NOD=" << x << '\n';

struct fraction
{
    long m;
    unsigned long n;
};
unsigned long NOD(unsigned long a, unsigned long b) // Наибольшее общее кратное
{
        while(b) b^=a^=b^=a%=b;
        return a;
}
void reduce(fraction* f)
{
    unsigned long nod = NOD(f->m, f->n);
    f->m/=nod;
    f->n/=nod;
}
fraction operator + (fraction f1, fraction f2)
{
    fraction result;
    result.n=f1.n / NOD (f1.n, f2.n) * f1.n;
    result.m=f1.m+f2.m;
    reduce(&result);
    return result;
}

#include <iostream>
#include <cstdlib>
#include <stdexcept>
 
unsigned long gcd(unsigned long a, unsigned long b)
{
    if(!b)
        return a;
    return gcd(b, a % b);
}
 
class fraction
{
 
public:
    fraction(long num, unsigned long denom);
    fraction(const fraction& rhs);
 
    fraction operator + (const fraction& rhs) const;
    friend std::ostream& operator << (std::ostream& os, const fraction& rhs);
 
private:
    void reduce();
 
    long            _numerator;
    unsigned long   _denominator;
};
 
fraction::fraction(long num, unsigned long denom)
{
    if(!denom)
        throw std::domain_error("Error dividing by zero");
 
    _numerator = num;
    _denominator = denom;
    reduce();
}
 
fraction::fraction(const fraction& rhs)
    : _numerator(rhs._numerator), _denominator(rhs._denominator)
{
}
 
fraction fraction::operator + (const fraction& rhs) const
{
    if(!_numerator)
        return rhs;
    else if(!rhs._numerator)
        return *this;
    else
        return fraction(
                   _numerator * rhs._denominator + _denominator * rhs._numerator,
                   _denominator * rhs._denominator);
}
 
std::ostream& operator << (std::ostream& os, const fraction& rhs)
{
    os << rhs._numerator << '/' << rhs._denominator;
    return os;
}
 
void fraction::reduce()
{
    if(_numerator)
    {
        unsigned long g = gcd(abs(_numerator), _denominator);
        _numerator /= g;
        _denominator /= g;
    }
}
 
fraction getFraction()
{
    long n;
    unsigned long d;
    std::cout << "Input numerator: ";
    std::cin >> n;
    std::cout << "Input denominator: ";
    std::cin >> d;
    return fraction(n, d);
}
 
int main()
{
    fraction f1 = getFraction();
    fraction f2 = getFraction();
    std::cout << "Fraction #1 = " << f1 << std::endl;
    std::cout << "Fraction #2 = " << f2 << std::endl;
    std::cout << "Sum = " << f1 + f2 << std::endl;
    return EXIT_SUCCESS;
}