OCL2_ — section 7.4.6 (Re-typing or casting) on p.13

In section 7.4.6 (Re-typing or casting) on p.13, it is stated that "An object can only be re-typed to one of its subtypes". In other words, in the expression object.oclAsType(A), A must be a subtype of the type of the object. However, in section 7.5.8 (Accessing overridden properties of supertypes) on p.20 an example is given of object.oclAsType(A), where A is a supertype of object. This contradicts the assertion made earlier. In section 11.2.4 there are no constraints given on oclAsType that would rule out the usage shown in the second paragraph. However, in the semantics section on p.A-25 it is only stated that for oclAsType, the the target type must be a subtype of the source type or vice versa, supporting again the usage in the second paragraph. Judging from these, I assume that the restriction given on p.13 is incorrect.

Type-casting is more general than the set of known subtypes and supertypes. If the OCL type system can be considered as open (meaning that OCL expressions may be applied to objects of types not known in the context in which it was defined), then it must be permitted to attempt a cast to any other type. UML implements multiple inheritance, so that given any two types, it is always a possibility that an object could be classified by both, even if they do not apparently have any relationship. This is particularly important for constraints defined in type libraries.

Because OCL provides type tests oclIsKindOf() and oclIsTypeOf(), it is reasonable that failure to cast result in invalid to indicate an error condition. Note that a type-testing expression such as self.oclAsType(SomeType).oclIsInvalid() is correct, though it may be considered as poor style.

The effect of casting is only a parse-time re-typing to provide visibility of features not defined for the original type, and a run-time assertion of the required type of an object. It cannot change the type of an object or coerce an object to an instance of a different type, nor can it provide access to hidden or overidden features of a supertype. For this, a new syntax is required that statically indicates the definition of a feature by the classifier that defines it.