Question

The code below, which is giving a correct output of the addition, changes x value of the first object b after doing it.

class numbers{
public:
    int x;
    numbers(int i1){
        x = i1;
    }
    numbers operator+ (numbers num){
        x = x + num.x;
        return(x);
    }
};

int main(){
    numbers a (2);
    numbers b (3);
    numbers c (5);
    numbers d (7);
    cout << a.x << b.x << c.x << d.x << endl; // returns 2357
    numbers final (100); //this value won't be shown
    final = a+b+c+d; 
    cout << a.x << b.x << c.x << d.x << endl; // returns 5357
    cout << final.x; //returns 17 (2+3+5+7)
    system("pause");
}

The question is, how does this addition class works exactly? I mean, why is x from object a modified?
I though only x from final object would be modified.

Thanks 🙂

Answer 1

The call to operator+ is just like any other member function call. a + b translates to a.operator+(b). So the line x = x + num.x; in this case is actually assigning to a.x. To achieve what you want you need to instead populate a new numbers with the new value i.e.

numbers operator+ (numbers num) const {
    return numbers(x + num.x)
}

Also note the const – which would have given you a compile error when you made that mistake.