In Chapter 47, you learned about the definition of two classes, header and link, that enable you to construct lists of railroad_car objects. So that you did not have to deal with too much complexity at once, all member variables and member functions in those two classes are in the public portion of the class definitions, where they are too freely accessible.

Of course, add, advance, access, endp, reset and the header constructor are used in the train program and therefore should be in the public portion of the header class. In this chapter, you learn how to make all other member variables and member functions inaccessible, except through those six functions, which constitute the public interface. Along the way, you learn about friends and why they are needed.