GNU R object-oriented programming with or without references

 Methods and classes for object-oriented programming in R with or
 without references. Large effort has been made on making definition of
 methods as simple as possible with a minimum of maintenance for package
 developers. The package has been developed since 2001 and is now
 considered very stable. This is a cross-platform package implemented in
 pure R that defines standard S3 classes without any tricks.