Ошибка terminate called after throwing an instance of 'std::invalid_argument'

При сравнении отрицательных чисел ошибка в консоли terminate called after throwing an instance of 'std::invalid_argument'

Вот код программы:

#include <iostream>
#include <vector>
#include <string>
#include <stdio.h>
#include <math.h>

using namespace std;


class Fraction
{
public:
    Fraction(int numerator, int denominator);
    int get_whole();
    string toString();
    double toDouble();
    Fraction plus(Fraction that);
    Fraction minus(Fraction that);
    Fraction times(Fraction that);
    Fraction by(Fraction that);
    bool isGreaterThan(Fraction that);
    bool isLessThan(Fraction that);
    bool isEqual(Fraction that);

private:
    int numerator;
    int denominator;
    int whole;
    void reduce();
};
Fraction::Fraction(int numerator, int denominator)
{
    this->numerator = numerator;
    this->denominator = denominator;
    this->whole = 0;
}
int Fraction::get_whole()
{
    return this->whole;
}
string Fraction::toString()
{
    if (this->whole == 0)
        return to_string(this->numerator) + " / " + to_string(this->denominator);
    else
        return to_string(this->whole) + " " + to_string(this->numerator) + " / " + to_string(this->denominator);
}
double Fraction::toDouble()
{
    double result = (double)this->numerator / (double)this->denominator;
    return result;
}
int gcd01(int a, int b)
{
    int nod = 1L;
    for (int i = a; i > 0; i--)
    {
        if (a % i == 0 && b % i == 0)
        {
            nod = i;
            break;
        }
    }
    return nod;
}
void Fraction::reduce()
{
    this->whole = this->numerator / this->denominator;
    if (whole != 0)
    {
        this->numerator = abs(this->numerator - (this->whole *  this->denominator));
        this->denominator = abs(this->denominator);
    }
    else
    {
        this->numerator = this->numerator - (this->whole *  this->denominator);
        this->denominator = this->denominator;
    }
    int gcd = gcd01(this->numerator, this->denominator);
    if (this->numerator % gcd == 0 && this->denominator % gcd == 0)
    {
        this->numerator /= gcd;
        this->denominator /= gcd;
    }
}
Fraction Fraction::plus(Fraction that)
{
    int den = this->denominator * that.denominator;
    int firstNum = this->numerator * that.denominator;
    int secondNum = that.numerator * this->denominator;
    int num = firstNum + secondNum;
    Fraction result(num, den);
    result.reduce();
    return result;
}
Fraction Fraction::minus(Fraction that)
{
    int den = this->denominator * that.denominator;
    int firstNum = this->numerator * that.denominator;
    int secondNum = that.numerator * this->denominator;
    int num = firstNum - secondNum;
    Fraction result(num, den);
    result.reduce();
    return result;
}
Fraction Fraction::times(Fraction that)
{
    int num = this->numerator * that.numerator;
    int den = this->denominator * that.denominator;
    Fraction result(num, den);
    result.reduce();
    return result;
}
Fraction Fraction::by(Fraction that)
{
    int num = this->numerator * that.denominator;
    int den = this->denominator * that.numerator;
    Fraction result(num, den);
    result.reduce();
    return result;
}
bool Fraction::isGreaterThan(Fraction that)
{
    int den = this->denominator * that.denominator;
    int firstNum = (den / this->denominator) * this->numerator;
    int secondNum = (den / that.denominator) * that.numerator;
    this->reduce();
    that.reduce();
    if (firstNum > secondNum)
        return true;
    else
        return false;
}
bool Fraction::isLessThan(Fraction that)
{
    int den = this->denominator * that.denominator;
    int firstNum = (den / this->denominator) * this->numerator;
    int secondNum = (den / that.denominator) * that.numerator;
    this->reduce();
    that.reduce();
    if (firstNum < secondNum)
        return true;
    else
        return false;
}
bool Fraction::isEqual(Fraction that)
{
    int den = this->denominator * that.denominator;
    int firstNum = (den / this->denominator) * this->numerator;
    int secondNum = (den / that.denominator) * that.numerator;
    this->reduce();
    that.reduce();
    if (firstNum == secondNum)
        return true;
    else
        return false;
}
vector<string> split(string line, char c)
{
    vector<string> result;
    string tmpstr = "";
    for (int i = 0; i < line.length(); i++)
    {
        if (line[i] != c)
            tmpstr += line[i];
        else
        {
            result.push_back(tmpstr);
            tmpstr = "";
        }
    }

    if (line[line.length() - 1] != ' ')
        result.push_back(tmpstr);

    return result;
}
int main()
{
    int num1, den1, num2, den2;
    string numm_1, demm_1, numm_2, demm_2;
    string input1, input2 = "";
    cout << "Enter the first fraction: ";
    getline(cin, input1);
    cout << "Enter the second fraction: ";
    getline(cin, input2);
    vector<string> out1 = split(input1, ' ');
    vector<string> out2 = split(input2, ' ');
    numm_1 = input1.substr(0,1);
    demm_1 = input1.substr(2,3);
    num1 = stoi(numm_1);
    den1 = stoi(demm_1);
    numm_2 = input2.substr(0,1);
    demm_2 = input2.substr(2,3);
    num2 = stoi(numm_2);
    den2 = stoi(demm_2);
    Fraction fract1(num1, den1);
    Fraction fract2(num2, den2);
    Fraction plus = fract1.plus(fract2);
    cout << fract1.toString() << " + " << fract2.toString() << " = " << plus.toString() << "\n";
    Fraction minus = fract1.minus(fract2);
    cout << fract1.toString() << " - " << fract2.toString() << " = " << minus.toString() << "\n";
    Fraction times = fract1.times(fract2);
    cout << fract1.toString() << " * " << fract2.toString() << " = " << times.toString() << "\n";
    Fraction by = fract1.by(fract2);
    cout << fract1.toString() << " / " << fract2.toString() << " = " << by.toString() << "\n";
    return 0;
}