C++ - Should I make `operator+` const? And does it return a reference?

Question

When a class overloads operator+, should it be declared const since it does not do any assignment on the object? Also, I know that operator= and operator+= return a reference because an assignment is made. But, what about operator+? When I implement it should I make a copy of the current object, add the given object to that, and return that value?

Here is what I have:

class Point
{
public:
    int x, int y;

    Point& operator += (const Point& other) {
        X += other.x;
        Y += other.y;
        return *this;
    }

    // The above seems pretty straightforward to me, but what about this?:
    Point operator + (const Point& other) const { // Should this be const?
        Point copy;
        copy.x = x + other.x;
        copy.y = y + other.y;
        return copy;
    }
};

Is this a correct implementation of the operator+? Or is there something I am overlooking that could cause trouble or unwanted/undefined behavior?

Answer 1

Better than that, you should make it a free function:

Point operator+( Point lhs, const Point& rhs ) { // lhs is a copy
    lhs += rhs;
    return lhs;
}

But yes, if you leave it as a member function it should be const as it does not modify the left hand side object.

Regarding whether to return a reference or a copy, the advice for operator overloading is do as fundamental types do (i.e. do as ints do). In this case, addition for two integers returns a separate integer that is not a reference to neither one of the inputs.