VDM-10 Language Manual

Chapter 12. Operation Definitions
here. However there is a conceptual distinction between them which dictates which should
be used and when. The pre-condition specifies what callers to the operation must ensure
for correct behaviour; the exception clauses indicate that the operation being specified takes
responsibility for error handling when an exception condition is satisfied. Hence normally
exception clauses and pre-conditions do not overlap.
The next VDM-SL example of an operation uses the following state definition:
✞
✡✝
state qsys of
q : Queue
end
The next VDM++/VDM-RT example of an operation uses the following instance variable
definition:
✞
✡✝
instance variables
q : Queue
✆
✆
This example shows how exceptions with an implicit definition can be used:
✞
✡✝
DEQUEUE() e: [Elem]
ext wr q : Queue
post q˜ = [e] ˆ q
errs QUEUE_EMPTY: q = [] -> q = q˜ and e = nil
✆
This is a dequeue operation which uses a global variable q of type Queue to get an element
e of type Elem out of the queue. The exceptional case here is that the queue in which the
exception clause specifies how the operation should behave is empty.
Note that the VDM-SL interpreter creates a function here:
✞
✡✝
post_DEQUEUE: [Elem] * qsys * qsys +> bool
✆
12.1 Constructors (VDM++ and VDM-RT)
Constructors are operations which have the same name as the class in which they are defined and
which create new instances of that class. Their return type must therefore be the same class name,
and if a return value is specified this should be self though this can optionally be omitted.
Multiple constructors can be defined in a single class using operation overloading as described
in section 14.2.
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