First Interlude Inadequacy of Simple Encapsulation

A little experience with the use of abstract data types shows that it often isn’t entirely possible to hide the implementation of a type. A common example arises in the case where a data structure is used to contain a collection of individual values and the program needs to iterate systematically through these values. Unless the programming language provides a way to access these values through the abstract data type, the application program will have to make explicit references to the data structure, thereby compromising any attempt to suppress and localize details of the implementation.