C++ Fraction class does not work on negatives - c++

Question

I am trying to create a fraction class that can do operations on fractions just like the way we did it by hand in elementary school. It works fine with positive numbers but I have tried implementing it with negative numbers and I get breakpoint error. It would be great if someone can tell me what is wrong with it.
#include <iostream>
#include <cmath>
using namespace std;
class fraction
{
private:
long int n;
long int d;
long int gcd();
public:
fraction(long int, long int);
long int num(); //returns num
long int denom(); //returns denom
void print(); //print fraction
void reduce(); //reduce fraction to lowest terms
friend double convert(fraction); //convert function to double
friend fraction operator+ (fraction, fraction);//add two fractions, answer in reduced form
friend fraction operator- (fraction, fraction);//subtract two fractions, reduced
};
long int fraction::gcd()
{
long int divisor = 0;
for (long int i = 1; (i <= n && i <= d) ; i++)
{
if ((n % i == 0) && (d % i == 0))
{
divisor=i;
}
}
return divisor;
}
fraction::fraction(long int x, long int y) //constructor
{
n=x;
d=y;
if ((x <= 0) && (y <= 0))
{
n = -x;
d = -y;
}
}
void fraction::reduce() //change value of n and d
{
long int num = n/gcd();
long int denom = d/gcd();
n = num;
d = denom;
}
long int fraction::num()
{
return n;
}
long int fraction::denom()
{
return d;
}
void fraction::print()
{
cout << n << "/" << d;
}
fraction operator- (fraction x, fraction y)
{
x.reduce();
y.reduce();
fraction temp;
temp.n = (x.n)*(y.d) - y.n*(x.d);
temp.d = (x.d)*(y.d);
temp.reduce();
return temp;
}
int main()
{
fraction f(-5,100), g(-1,-2);
(f-g).print; //returns error!
return 0;
}

Answer 1

The error is caused by the fact that print() is a non-const member function.
When you use:
(f-g).print();
The function is called an a temporary object, which is good for calling const member functions, not non-const member functions.
You can resolve this problem by:
Changing print() to a const member function.
By assigning f-g to an object and calling print on that object.
fraction res = f-g;
res.print();
I would recommend using the first method.
Update
You have a problem in gcd when either n or d is negative. Change it to:
long int fraction::gcd()
{
long int divisor = 1;
// Deal with only positive numbers when computing the gcd.
long int tempN = n < 0 ? -n : n;
long int tempD = d < 0 ? -d : d;
for (long int i = 1; (i <= tempN && i <= tempD) ; i++)
{
if ((tempN % i == 0) && (tempD % i == 0))
{
divisor=i;
}
}
return divisor;
}