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4.3.5 Determining the Class Precedence List

The **defclass** form for a *class* provides a total ordering on that *class* and its direct *superclasses*. This ordering is called the *local precedence order*. It is an ordered list of the *class* and its direct *superclasses*. The *class precedence list* for a class C is a total ordering on C and its *superclasses* that is consistent with the *local precedence orders* for each of C and its *superclasses*.

A *class* precedes its direct *superclasses*, and a direct *superclass* precedes all other direct *superclasses* specified to its right in the *superclasses* list of the **defclass** form. For every class C, define

RC={(C,C1),(C1,C2),...,(Cn-1,Cn)}

where C1,...,Cn are the direct *superclasses* of C in the order in which they are mentioned in the **defclass** form. These ordered pairs generate the total ordering on the class C and its direct *superclasses*.

Let SC be the set of C and its *superclasses*. Let R be

R=Uc<ELEMENT-OF>SCRc

.

The set R might or might not generate a partial ordering, depending on whether the Rc, c<ELEMENT-OF>SC, are consistent; it is assumed that they are consistent and that R generates a partial ordering. When the Rc are not consistent, it is said that R is inconsistent.

To compute the *class precedence list* for C, topologically sort the elements of SC with respect to the partial ordering generated by R. When the topological sort must select a *class* from a set of two or more *classes*, none of which are preceded by other *classes* with respect to R, the *class* selected is chosen deterministically, as described below.

If R is inconsistent, an error is signaled.

*Copyright 1996, The Harlequin Group Limited. All Rights Reserved.*