The correspondence between records in a relational database and C++ objects is not as close as one would hope. (For example, C++ concepts like pointer, member function, and derived class don't have obvious RDBMS counterparts.) The rows of a table must still be converted into objects, and this can be a lot of work for the programmer and the CPU. In fact, it has been estimated that as much as 30% of programming effort and code space is devoted to converting data from database or file formats into and out of program-internal formats such as objects [ATK]. The gap between database formats and program-internal formats is called impedance mismatch.
In contrast to relational databases, object databases are collections of objects organized into classes that are related by association and specialization. But what type of objects? C++ objects? Java objects? Smalltalk objects? If an object database contains C++ objects, then impedance mismatch is eliminated between the database and C++ client programs, but not between the database and Java or Smalltalk programs. To resolve this problem a consortium of companies has formed the Object Database Management Group[1] (ODMG) to define standards for object oriented database management systems (OODBMS). Version 2 of the standard (ODMG 2.0) appeared in 1997.
ODMG 2.0 includes specifications of an object model[2]
(i.e., language-independent definitions of object oriented concepts such as
object, class, inheritance, attribute, method, etc.); OQL, the Object Query
Language (a language with SQL-like syntax for searching object oriented
databases); and language bindings for C++, Java, and Smalltalk. GemStone,
Using an object database[3], a C++ (or Java or Smalltalk) program may refer to objects without worrying if they are in main memory or secondary memory. If the requested object is in secondary memory, an object fault occurs, and the OODBMS transparently locates the requested object using the object's unique object identifier number (OID), translates the object into a C++ (or Java or Smalltalk) object, then loads the object into main memory. If the program updates an object (and commits to the update), the procedure is reversed: the ODBMS translates the C++ (or Java or Smalltalk) object into an ODMG object, then writes the translated object back to the database.[4]
[1] [WWW 12]
[2] The ODMG object model extends the OMG object model. In addition to the CORBA standard, OMG is sponsoring UML as a standard modeling language, which can be used to describe their object model.
[3] At the moment, there aren't that many OODBMSs around. Most companies already have large relational databases, and converting them to object databases would be too expensive and too risky.
[4] See [ROG] for examples of using ODMG with C++.