Overloading + correctly in c++ - c++

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);
}
};

Answer 2

There are at least two problems in your code. First, you've declared operator+ to be a friend, so it is a free function, with no this. Regretfully, from the code, I'm not able to figure out what you're trying to do (overload unary + or overload binary +), so it's difficult to say more. Second, you're trying to return the results of new, which is a pointer, not an object type. operator+ should always return an object type, so you don't want the new.

Answer 3

Assuming that your operator+ is a binary operator that takes two Linias as input and returns a Linea having added coefficients, you may want to define a free function operator+ overload, with proper const correctness of the input Linias, like this:
Linia operator+(const Linia& lhs, const Linia& rhs)
{
return Linia
(
lhs.a + rhs.a,
lhs.b + rhs.b,
lhs.c + rhs.c
);
}

Answer 4

Since you declared it as a friend, your addition operator is not a member function, hence it must take two parameters, and has no access to this:
friend Linia operator+ (const Linia& lhs, const Linia& rhs)
{
return Linia(lhs.a, lhs.b, lhs.c + rhs/lhs.b);
}
This follows the logic in your code but will only work if there is an operator double operator/(const Linia& lhs, double). Something that intuitively would make sense would be
friend Linia operator+ (const Linia& lhs, const Linia& rhs)
{
return Linia(lhs.a+rhs.a, lhs.b+rhs.b, lhs.c + rhs.c);
}
Finally, you could avoid the friend declaration completely and just declare the operator as a standard non-member function. It doesn't access any private or protected data, so it doesn't need to be a friend.