VDM-10 Language Manual

Chapter 15. Synchronization Constraints (VDM++ and VDM-RT)
Inheritance of mutex constraints is completely analogous to the inheritance scheme for permission
predicates. Internally mutex constraints are always expanded into appropriate permission
predicates which are added to the existing permission predicates as a conjunction. This inheritance
scheme ensures that the result (the final permission predicate) is the same, regardless of whether
the mutex definitions are expanded in the base class and inherited as permission predicates or are
inherited as mutex definitions and only expanded in the derived class.
The intention for inheriting synchronization constraints in the way presented is to ensure, that
any derived class at least satisfies the constraints of the base class. In addition to that it must be
possible to strengthen the synchronization constraints. This can be necessary if the derived class
adds new operations as in the following example:
✞
class A
operations
writer: () ==> ()
writer() == is not yet specified
reader: () ==> ()
reader() == is not yet specified
sync
per reader => #active(writer) = 0;
per writer => #active(reader, writer) = 0;
end A
class B is subclass of A
operations
newWriter: () ==> ()
newWriter() == is not yet specified
sync
per reader => active(newWriter) = 0;
per writer => #active(newWriter) = 0;
per newWriter => #active(reader, writer, newWriter) = 0;
end B
✡✝
✆
Class A implements reader and writer operations with the permission predicates specifying the
multiple readers-single writer protocol. The derived class B adds newWriter. In order to ensure
deterministic behaviour B also has to add permission predicates for the inherited operations.
The actual permission predicates in the derived class are therefore:
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