The Metaclass Framework

In SOM, classes are objects; metaclasses are classes and thus are objects, too. Also depicted are the three primitive class objects of the SOM run time: SOMClass, SOMobject, and SOMClassMgr.
Primitive objects of the SOM run time

The important point to be aware of here is that any class that is a subclass of SOMClass is a metaclass This chapter describes metaclasses that are available in SOMobjects Toolkit. There are two kinds of metaclasses:

Framework metaclasses-metaclasses for building new metaclasses and Utility metaclasses - metaclasses to help you write applications

Briefly, the SOMobjects Toolkit provides the following metaclasses of each category for use by programmers:

• Framework metaclasses:

SOMMBeforeAfter

Used to create a metaclass that has "before" and "after" methods for all methods (inherited or introduced) invoked on instances of its classes.

• Utility metaclasses:

SOMMSingleInstance

Used to create a class that may have at most one instance.

Provides tracing for every invocation of all methods on instances of its classes.

The following sections describe each metaclass more fully. ClassorganizationoftheMetaclassFramework .