Question

I have created class which represents line aX + bY = c and I wanted overload + operator to higher the line(return higher Line so I have done that below but compilator says invalid use of this

class Linia{
public:
    double a,b,c;
    Linia (double a, double b, double c){
        this->a = a;
        this->b = b;
        this->c = c;
    }
   friend Linia operator+ (double i){
   return new Linia(a, this->b, this->c + i/this->b);
}
};

I would like to return new Linia object which is has fields like shown above i is int i do not want to modify original object

Answer 1

You have some basic syntax issues.

• this is a pointer so you need to use -> to dereference it.

• I assume you mean this->c + i.c instead of this->c + i

• You don’t need to (and probably shouldn’t) have the operator be a friend.

• Operators that return a new instance (like operator+) should return by value, not allocate on the heap.

• Operators generally take parameters as const references (since you shouldn’t be modifying the operands).

I think you mean to have something like this:

class Linia{
public:
    double a,b,c;
    Linia (double a, double b, double c){
        this->a = a;
        this->b = b;
        this->c = c;
    }
    Linia operator+ (const Linia& i){
        return Linia(this->a, this->b, this->c + i.c / this->b);
    }
};

which you can clean up to be something like this:

class Linia{
public:
    double a,b,c;
    Linia (double a, double b, double c)
        : a(a), b(b), c(c)
    { }

    Linia operator+ (const Linia& i){
        return Linia(a, b, c + i.c / b);
    }
};