4.6.3 Subtyping

This standard allows subtyping of type definitions. The subtyping rules are defined in Clause 6. Subtyping of ObjectTypes and VariableTypes allows:

• Clients that only know the supertype to handle an instance of the subtype as if it were an instance of the supertype;

• instances of the supertype to be replaced by instances of the subtype;

• specialised types that inherit common characteristics of the base type.

In other words, subtypes reflect the structure defined by their supertype but may add additional characteristics. For example, a vendor may wish to extend a general “TemperatureSensor” VariableType by adding a Property providing the next maintenance interval. The vendor would do this by creating a new VariableType which is a TargetNode for a HasSubtype reference from the original VariableType and adding the new Property to it.