Why is the copy-constructor argument const?

Question

 Vector(const Vector& other) // Copy constructor 
 {
    x = other.x;
    y = other.y;

Why is the argument a const?

Answer 1

Because you are not going to modify the argument other inside the copy ctor as it is const.

When you did x = other.x it essentially means this->x = other.x. So you are modifying only this object just by copying the values from other variable. Since the other variable is read-only here, it is passed as a const-ref.