VDM-10 Language Manual

Chapter 10
Instance Variables (VDM++ and VDM-RT)
Both an object instantiated from a class description and the class itself can have an internal state,
also called the instance variables of the object or class. In the case of objects, we also refer to this
state as the global state of the object.
Syntax: instance variable definitions = ‘instance’, ‘variables’,
[ instance variable definition,
{ ‘;’, instance variable definition } ] ;
instance variable definition = access assignment definition
| invariant definition ;
access assignment definition = ([ access ], [ ‘static’ ]) | ([ ‘static’ ], [ access ]),
assignment definition ;
assignment definition = identifier, ‘:’, type, [ ‘:=’, expression ] ;
invariant definition = ‘inv’, expression ;
Semantics: The section describing the internal state is preceded by the keyword instance
variables. A list of instance variable definitions and/or invariant definitions follows.
Each instance variable definition consists of an instance variable name with its corresponding
type indication and may also include an initial value and access and static specifiers.
Details of the access and static specifiers can be found in section 14.4.
It is possible to restrict the values of the instance variables by means of invariant definitions.
Each invariant definition, involving one or more instance variables, may be defined over
the values of the instance variables of objects of a class. All instance variables in the class
including those inherited from superclasses are visible in the invariant expression. Each invariant
definition must be a boolean expression that limits the values of the instance variables
to those where the expression is true. All invariant expressions must be true during the entire
lifetime of each object of the class.
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