Create my own cart class named cart

[eluser]ipsod[/eluser]
Just go with the standard overwriting of CI classes. When you define a class using your prefix (usually MY_), it allows you to call it by saying $CI->cart->(etc.), effectively replacing CI's cart class.

Not sure why you'd care what the text file and class are titled when you can call it this way, unless it's some sort of severe OCD, in which case, seek help elsewhere.