The problem is, there's no copying going on in the constructor. It creates a new, empty hash. That's not so useful.

The only other reason something like this would be useful is if you want to create an object of the same type without knowing what that type is. I can't think of when that would be useful, and there are other ways to solve this, but I'd allow it in this case.

Otherwise, it adds nothing. (Some people would argue that a copy constructor should have a different name, like "clone" or such. I lean toward the idea that methods should each do one thing well.)