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In SOM, the IBM System Object Model, a class is a run-time object that defines the behavior of its instances by creating an instance method table. Because classes are objects, their behavior is defined by other classes (called metaclasses). For example, a “Before/After Metaclass” can be used to define the implementation of classes that, by suitable construction of their instance method tables, arrange for each invocation of a method to be preceded by execution of a “before method” and followed by execution of an “after method”. This paper introduces and solves the problem of composing different Before/After Metaclasses in the context of SOM. An enabling element in the solution is SOM's concept of derived metaclasses, i.e., at run-time a SOM system derives the appropriate metaclass of a class based on the classes of its parents and an optional metaclass constraint.

The panel will discuss emerging “industrial” solutions that help programmers to develop distributed and client/server applications based on objects supporting so-called openness and heterogeneity.