如何访问STL字符串类中的成员变量?

I'm working on one of the programming challenges in the book Starting Out With C++ Early Objects 7th Edition and one of the assignments asks to create a class which is derived from the STL string class. I'm posting the question for the purpose of understanding what I am allowed to do and how I am supposed to implement the solution so that no one offers more advanced suggestions.I'm working on one of the programming challenge