Concept in object-oriented design. Creation of object classes by creating a new classes whose instance state contains references to one or more objects of different classes, and then forwards method invocations on itself to these wrapped objects. This is sometimes also called wrapping, or overloading.
See also: Snit's Not Incr Tcl.
I am currently working on xoins that is planned to do the same with XOTcl instead of Itcl.