217: Mainline

Data abstraction and the notion of a public interface fit together as follows:

• Channeling member-variable references and assignments through access functions isolates you from the details of class implementation, thus providing a means to ensure that you can practice data abstraction.
• When you move member variables to the private part of a class definition, you force all member-variable references and assignments to go through the constructors, readers, writers, and other member functions in the public interface, thus providing a means to ensure that you benefit from your effort to practice data abstraction.