C ++我减少分数的函数不会减少负数

Question

我创建了一个C ++程序，让用户输入两个分数，然后减少分数，添加分数，减去它们，乘以它们，然后将它们分开，然后显示结果。

当我输入负数时，我遇到了减少分数的问题。该程序完美无缺地使用正数，但是当我输入负数时，它不会简化。相反，黑屏告诉我这个：

Enter a value for the numerator: -3
Enter a value for the denominator: 2
Enter a value for the numerator: 5
Enter a value for the denominator: 2
The fractions reduced are -3/2 and 5/2
When added it gives 1
When subtracted it gives -16/4` **Does not reduce**

When multiplied it gives -15/4
When divided it gives -6/10 ` **Does not reduce**

They are not equal
The built in fixed 2 argument constructor gives the fraction 3/5
The default constructor gives the fraction 1/2
Would you like to go again? (Y/N):

这是功能：

//Function that simplifies fraction.
rational rational::reduce()
{
    //Set the greatest common factor to the denominator * numerator
    int GCF = denom * numer;
    //Create a loop that decreases the GCF by 1 each time.
    for (GCF; GCF > 1; GCF--)
    {
        //If the numerator and the denominator both have a remainder of zero when dividing by the GCF
        if ((numer % GCF == 0) && (denom % GCF == 0))
        {
            numer /= GCF;
            denom /= GCF;
        }
    }
    rational answer;
    //Store the numerator and denominator into a rational variable
    answer.numer = numer;
    answer.denom = denom;
    //Return the answer
    return answer;
}

对于我做错的任何帮助都表示赞赏，我是初学者，只是我们。