“undefined reference to” operator overloading in C++

Question

I want to create class Q(rational numbers), which have the following functions:

1)print(): printing rational number in form :numerator/denominator

2)cancel(): performs cancelation, if it's possible: (3/9 = 1/3)

3)value(): value of the rational number as double type;

4)Operator overloading with addition, subtraction, division, multiplication.

Here is my code:

#include <iostream>
#include <vector>
#include <string>
#include <stdexcept>
using namespace std;


class Q{
private:
    int num;
    int denom;

public:
    Q();
    Q(int num,int denom);
    void print();
    Q cancel();
    void value();
    friend Q operator*(Q lhs,Q rhs);
    friend Q operator/(Q lhs,Q rhs);
    friend Q operator+(Q lhs,Q rhs);
    friend Q operator-(Q lhs,Q rhs);
};

Q::Q(int numerator,int denominator){
    num = numerator;
    denom = denominator;
}


void Q::print(){
    cout << to_string(num) + "/" + to_string(denom);
}

Q Q::cancel(){
    vector<int> canc_val = {2,3,5,7};
    while(true){
        if(num % canc_val[0] == 0 && denom % canc_val[0] == 0){
            num /= canc_val[0];
            denom /= canc_val[0];
        } else if(num % canc_val[1] == 0 && denom % canc_val[1] == 0){
            num /= canc_val[1];
            denom /= canc_val[1];
        } else if(num % canc_val[2] == 0 && denom % canc_val[2] == 0){
            num /= canc_val[2];
            denom /= canc_val[2];
        }else if(num % canc_val[3] == 0 && denom % canc_val[3] == 0){
            num /= canc_val[3];
            denom /= canc_val[3];
        }else{
            break;
        }
    }
    return Q(num,denom);
}

void Q::value(){
    double res;
    res = num/denom;
    cout << res;
}

Q operator*(Q lhs,Q rhs){
    Q temp;
    temp.num = lhs.num * rhs.num;
    temp.denom = lhs.denom * rhs.denom;
    return temp;
}

Q operator/(Q lhs,Q rhs){
    Q temp;
    temp.num = lhs.num * rhs.denom;
    temp.denom = lhs.denom * rhs.num;
    return temp;
}

Q operator+(Q lhs,Q rhs){
    Q temp;
    temp.num = lhs.num * rhs.denom + rhs.num*lhs.denom;
    temp.denom = lhs.denom * rhs.denom;
    return temp;
}

Q operator-(Q lhs,Q rhs){
    Q temp;
    temp.num = lhs.num * rhs.denom - rhs.num*lhs.denom;
    temp.denom = lhs.denom * rhs.denom;
    return temp;
}

int main(){
    Q number_1(3,7);
    Q number_2(3,9);
    Q number_3(15,125);

    number_1.print();
    cout << "\n";
    number_2.print();
    number_2.cancel();
    cout << "\n";
    number_2.print();
    cout << "\n";
    number_3.value();
    cout << "\n";
    number_3.cancel();
    cout << "\n";
    number_3.print();

    Q number_4 = number_1 + number_2;
    number_4.print();
    number_4.cancel();
    cout << "\n";
    number_4.print();

    Q number_5 = number_2 - number_3;
    number_5.print();
    number_5.cancel();
    cout << "\n";
    number_5.print();


    Q number_6 = number_1 * number_1;
    number_6.print();
    number_6.cancel();
    cout << "\n";
    number_6.print();

    Q number_7 = number_2 / number_3;
    number_7.print();
    number_7.cancel();
    cout << "\n";
    number_7.print();

}

But compiler throws me an error:

/tmp/cce4zPM4.o: In function `operator*(Q&, Q&)':
:(.text+0x170): undefined reference to `Q::Q()'
/tmp/cce4zPM4.o: In function `operator/(Q&, Q&)':
:(.text+0x1b0): undefined reference to `Q::Q()'
/tmp/cce4zPM4.o: In function `operator+(Q&, Q&)':
:(.text+0x1f0): undefined reference to `Q::Q()'
/tmp/cce4zPM4.o: In function `operator-(Q&, Q&)':
:(.text+0x230): undefined reference to `Q::Q()'
collect2: error: ld returned 1 exit status

I have no idea how to fix it. Can you help me ?

Answer 1

There is no definition for Q::Q() , only a declaration, but you must provide a definition, like this:

Q::Q() :
    num(0),
    denom(1)
{
}

Don't just define it without an initialisation list, because this would mean that your data members have indeterminate values, eventually causing undefined behaviour.

Answer 2

This happens because you never implemented Q::Q() constructor.

Fortunately, you can rewrite your code to avoid using this constructor by calling Q::Q(int, int) instead.

Here is operator+ implementation example:

Q operator+(const Q& lhs, const Q& rhs) {
    return Q(lhs.num * rhs.denom + rhs.num*lhs.denom, lhs.denom * rhs.denom);
}

Note the use of const Q& instead of Q to avoid copying objects.

Consider removing Q::Q() constructor, because it is not clear what number it should represent. Perhaps you could also alter the two-argument constructor to default denominator to 1 :

Q(int num, int denom = 1);

This would let you create integers without specifying a denominator.

You should also consider normalizing your number in the constructor by dividing out GCD(num, denom) from both sides. This would make it easier to compare rational numbers in code.