VDM-10 Language Manual

Chapter 14. Top-level Specification (VDM++ and VDM-RT)
Functions, operations, instance variables and constants 2 in a class may be either class attributes
or instance attributes. This is indicated by the keyword static: if the declaration is preceded by
the keyword static then it represents a class attribute, otherwise it denotes an instance attribute.
Other class components are by default always either class attributes or instance attributes as
follows:
• Type definitions are always class attributes.
• Thread definitions are always instance attributes.
thread(s).
Thus, each active object has its own
• Synchronization definitions are always instance attributes. Thus, each object has its own
“history” when it has been created.
In addition, the interface or accessibility of a class member may be explicitly defined using an
access specifier: one of public, private or protected. The meaning of these specifiers is:
public: Any class may use such members
protected: Only subclasses of the current class may use such members
private: No other class may use such members - they may only be used in the class in which
they are specified.
The default access to any class member is private. That is, if no access specifier is given for a
member it is private.
This is summarized in table 14.1. A few provisos apply here:
• Granting access to instance variables (i.e. through a public or protected access specifier)
gives both read and write access to these instance variables.
• Public instance variables may be read (but not written) using the dot (for object instance
variables) or back-quote (for class instance variables) notation e.g. a public instance variable
v of an object o may be accessed as o.v.
• Access specifiers may only be used with type, value, function, operation and instance variable
definitions; they cannot be used with thread or synchronization definitions.
• It is not possible to convert a class attribute into an instance attribute, or vice-versa.
• For inherited classes, the interface to the subclass is the same as the interface to its superclasses
extended with the new definitions within the subclass.
• Access to an inherited member cannot be made more restrictive e.g. a public instance variable
in a superclass cannot be redeclared as a private instance variable in a subclass.
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