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

The traditional copy-ctor and friends take a const& parameter for reasons specified above. However, you should also look up move-semantics and r-value references (to be part of C++0x, if all goes well) to see why and when you will use copy-ctors without a const& parameter. Another place to look at is the implementation of smart pointers such as auto_ptr (which have transfer of ownership semantics) where non-const parameters are useful.

Answer 2

It can also come handy if you want to copy an object you only have a const reference to for example

...
const Vector& getPosition();
...

Vector* v = new Vector(getPosition());

If it wasn't for Vector(const Vector& other) that example would create a syntax error.