Scripts and Programming

I really need help with this programming challenge that I have to do. I have no idea what to do, if someone could get me started on the right path, I would greatly appreciate it.

Here are my instructions:

Write a class EncryptableString that is derived from the STL string class. The EncryptableString class adds a member function

void encrypt( )

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 cbb. Assume that all characters that are part of an EncryptableString 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.

I don’t know what the STL string class is, nor do I know how to encrypt a string by modifying it via ASCII characters.