Specification of Reactive Hardware/Software Systems - Electronic ...

4.14 Attributes and Variables 109
4.14.2.1 Instance Variables
POOSL offers two sorts of variables: instance variables and local variables. Instance
variables are specified in class definitions. At run-time, every instance (object) has
a collection of variables as specified in its class. These variables can take on values
thereby determining the state (of the variables) of the object. Attributes from the Object
Class Model become instance variables in class definitions. Instance variables serve also
to store references to other objects. The instance variables form the data contents of the
object. These contents live in principle as long as the object itself, but may be modified
during the lifetime of the object.
4.14.2.2 Local Variables
Besides data that is stored as instance variables, there is temporary data that is stored
in local variables in methods. Local variables originate from the chosen algorithm or
expression that ’implements’ a method in POOSL. A different implementation may
require different types of local variables. Therefore local variables do not play any role
during object modelling.
4.14.2.3 Data Objects
Classes describe data objects or process objects in POOSL. Both can have instance variables
and local variables. These variables refer to data objects. Data objects are created
from their classes and get instance variables, as specified in their classes. These variables
store references to other data objects. Initially they are all set to nil. Nil is a data object
that can be considered to belong to no class at all. User defined data objects may have a
user defined service initialise to give their instance variables an initial value, other than
nil.
References can also arise very implicitly from data object creation. A data (or process)
object can for instance contract out some calculation or action. The object therefore
creates a temporary data object of an appropriate class, and both can be considered to
have a temporary link to each other for passing parameters and results. These identities
have a very temporary and implicitly created knowledge of each others identifiers.
There is no need for explicit references. After the completion of service, say m, the
lifetime of the created object ends. An example is given by the following expression:
x : new(C) m
This expression leads to the following behaviour. An object of class C is created, receives
a message m, performs the corresponding method and returns the object that results from
the method evaluation. The final result is that x refers to the resulting object.
4.14.2.4 Process Objects
Process objects in POOSL are quite different from data objects. Data objects are sequential
entities. Process objects offer concurrency. The internal behaviour of process objects is