VDM-10 Language Manual

VDM-10 Language Manual
14.3 Inheritance
The concept of inheritance is essential to object orientation. When one defines a class as a subclass
of an already existing class the definition of the subclass introduces an extended class, which is
composed of the definitions of the superclass together with the definitions of the newly defined
subclass.
Through inheritance, a subclass inherits from the superclass:
• Its instance variables. This also includes all invariants and their restrictions on the allowed
modifications of the state.
• Its operation and function definitions.
• Its value and type definitions.
• Its synchronization definitions as described in section 15.2.
A name conflict occurs when two constructs of the same kind and with the same name are
inherited from different superclasses. Name conflicts must be explicitly resolved through name
qualification, i.e. prefixing the construct with the name of the superclass and a ‘-sign (back-quote)
(see also section 20).
Example: In the first example, we see that inheritance can be exploited to allow a class definition
to be used as an abstract interface which subclasses must implement:
✞
class Sort
instance variables
protected data : seq of int
operations
initial_data : seq of int ==> ()
initial_data (l) ==
data := l;
sort_ascending : () ==> ()
sort_ascending () == is subclass responsibility;
end Sort
class SelectionSort is subclass of Sort
functions
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