Visual C++ :: Adding Rational Number Using Only Class Functions

I'm having trouble finishing this program. What I'm trying to do is create a class for Rational numbers. Get input from user for denominator and numerator. Store the numbers without a sign, and for the sign store it separately in char. I'm not supposed to use accessor functions. The part that I can't seem to know how to do is use the info that was stored in the addUp function. This function need to add two rational numbers. i need to make this function general enough that it can handle two fractions, or a fraction and a whole number, or two whole numbers. What I already have here is readin function which reads in the numerator and denominator. setnumerator and setdenominator to assign positive values. setsign should get the sign of the fraction. Finally addUp should addUp which I explained before. I have some ideas about writing the tests, but can't seem to know how to implement it to the program. The main program is still empty but all I'm supposed to do there is call the class functions.

Code:
#include <iostream>
using namespace std;
class Rational {
private:
int numerator, denominator;
char sign;

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