Why C++ copy constructor must use const object?

Question

I understand that when we define a class copy constructor of the class is necessary as Rule of three states. I also notice that the argument of the copy constructor is usual

Answer 1

As several other answers point out, a copy constructor that modified its argument would be an unpleasant surprise. That is not, however, the only problem. Copy constructors are sometimes used with arguments that are temporaries. (Example: return from function.) And non-const references to temporaries don't fly, as explained elsewhere on SO.