VDM-10 Language Manual

VDM-10 Language Manual
✡✝
end B
class C is subclass of A
instance variables
i: int := 3;
end C
class D is subclass of B,C
operations
GetValues: () ==> seq of int
GetValues() ==
return [
A‘i, -- equal to 1
B‘i, -- equal to 1 (A‘i)
]
end D
C‘i, -- equal to 3
j -- equal to 2 (A‘j)
✆
In the example objects of class D have 3 instance variables: A‘i, A‘j and C‘j. Note that
objects of class D will have only one copy of the instance variables defined in class A even though
this class is a common super class of both class B and C. Thus, in class D the names B‘j, C‘j,
D‘j and j are all referring to the same variable, A‘j. It should also be noticed that the variable
name i is ambiguous in class D as it refers to different variables in class B and class C.
14.4 Interface and Availability of Class Members
In VDM++ and VDM-RT definitions inside a class are distinguished between:
Class attribute: an attribute of a class for which there exists exactly one incarnation no matter
how many instances (possibly zero) of the class may eventually be created. Class attributes
in VDM++ and VDM-RT correspond to static class members in languages like
C++ and Java. Class (static) attributes can be referenced by prefixing the name of the attribute
with the name of the class followed by a ‘-sign (back-quote), so that, for example,
ClassName‘val refers to the value val defined in class ClassName.
Instance attribute: an attribute for which there exists one incarnation for each instance of the
class. Thus, an instance attribute is only available in an object and each object has its own
copy of its instance attributes. Instance (non-static) attributes can be referenced by prefixing
the name of the attribute with the name of the object followed by a dot, so that, for example,
object.op() invokes the operation op in the object denoted by object (provided that
op is visible to object).
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