Write a class Encr ypt abl eStri ng that is derived from the STL string class. The Encrypt abl e string class adds a member function voi d encr ypt ( ) that encrypts the string contained in the object by replacing each letter with its successor in the ASCII ordering. For example, the string baa would be encrypted to ebb. Assume that all characters that are part of an Encrypta bl eStr i ng object are letters a, .. , z and A, .. , Z, and that the successor of z is a and the successor of Z is A. Test your class with a program that asks the user to enter strings that are then encrypted and printed