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

4.4 Data Objects and Process Objects 67
This problem is not studied in this thesis and requires further research.
4.3.4.5 Inheritance Anomaly
Another problem for the introduction of implementation inheritance is the so called
inheritance anomaly. The inheritance anomaly is a conflict between inheritance and
concurrency. This problem is described by S. Matsuoka and A. Yonezawa [MY93], and is
recognised as a serious problem for combing inheritance and concurrency. The problem
raises with the inheritance of synchronisation constraints in concurrent object-oriented
languages. Synchronisation constraints are used to control the acceptance of messages.
Concurrent objects typically need such power because they operate concurrently in
contrast to objects in a sequential language that operate alternately. We offer concurrency.
Further research is necessary for a balanced addition of implementation inheritance to
concurrency.
4.3.5 Class or Object
Another design decision for a method is whether a class is an object or a separate
phenomenon. We decided to define classes as a separate concept. Therefore we have
no problem with respect to encapsulation. An object encapsulates its contents after
instantiation. A class is used as a template for instantiation, which lets its objects share
their behaviour specification. In contrast, a class that is an object must again be an
instance of some class. This latter class is a class of classes, a so-called metaclass. For
example, Smalltalk [GR89] incorporates this conceptual approach. If a class is really an
object, its contents should be encapsulated. Its contents can only be obtained via the
message interface. However, a class must have greater rights than objects in accessing
attributes (internal variables). The conflict with encapsulation is described by Wegner
in [Weg90]. When attributes are completely encapsulated there is no sharing possible
between objects. This is orthogonal to the decision to share (inherit) the attributes of a
direct superclass.
4.4 Data Objects and Process Objects
4.4.1 Introduction
A very introductory description of objects has been presented in Section 4.2. For a good
understanding of objects we have to cover many aspects and properties. The many
conceptual choices made in the construction of object-oriented languages and methods
determine the precise meaning of objects in such a language or method. In depth study
reveals that there are almost as many different concepts of object as there are languages
and methods.
We choose to define two sorts of objects in our method: Data objects and Process objects.
These sorts have a different semantics and purpose.