Classes BSONObjBuilder and BSONArrayBuilder both derive from BSONBuilderBase, which is an abstract base class that makes virtual several key methods of BSONObjBuilder (and BSONArrayBuilder).

However, nowhere in the codebase, other than as a base class for BSONObjBuilder and BSONArrayBuilder, is this base class actually used. BSONObjBuilder and BSONArrayBuilder are the only subclasses, and no other references to the base type are found.