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

Its not specific to copy constructor. In any function if you are not going to modify the internal state of the object then object will be passed as const.

Vector(const Vector& other) 
{
     //Since other is const, only public data member and public methods which are `const` can be accessed.
}