BDataIO is an abstract class that defines protocols for objects that can Read() and Write() data. BPositionIO, BDataIO's main subclass, declares an additional set of functions (ReadAt(), WriteAt(), Seek(), and Position()) for objects that can keep track of the current position in the I/O buffer; it implements Read() and Write() in terms of these other functions.

Neither BDataIO nor BPositionIO declares any data members, nor do they implement the functions in the protocols they declare. It's up to derived classes to implement them based on the properties of the particular kinds of data sources/repositories they represent.