In the C++ class library, features that implement the interface to the ODMG Object Model
are prefixed d_. Examples are d_Float, d_String, d_Short for base data types and d_List,
d_Set, and d_Bag for collection types. There is also a class d_Iterator for the Iterator class and
a class d_Extent for class extents. In addition, a template class d_Ref(T) is defined for each
class T in the database schema that can refer to both persistent and transient objects of class T.