Why should the copy constructor accept its parameter by reference in C++?

Question

Why must a copy constructor's parameter be passed by reference?

Answer 1

Because if it's not by reference, it's by value. To do that you make a copy, and to do that you call the copy constructor. But to do that, we need to make a new value, so we call the copy constructor, and so on...

(You would have infinite recursion because "to make a copy, you need to make a copy".)

Answer 2

Because pass-by-value would invoke the copy constructor :)