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

72 Abstraction of a Problem Domain
C. Object C returns object B as result. So the expression is evaluated now to simply:
B ¡ B
This means that message == B is sent to B. In this message == is the message name,
which corresponds to an operation in the receiver. The message name is followed by
parameter B. So, B is requested to check via its primitive method == whether parameter
B is a reference to himself. The result is an object of class boolean, in this case the object
true.
Besides the equality symbol ’==’ there is a symbol ’=’. The usual meaning is that it
is an equivalence operation. This operation does not have any predefined semantics. It
depends on a class definition what it means. We recommend to use it for an equivalence
operation, and to define a boolean result for it. This matches the intuitive notion that A
= B results in true if A and B have a particular resemblance of properties.
4.4.2.6 Deep copy
The main purpose of data objects is to model dynamic entities, that are transformed in
the system and transported through the system. A complex (data) structure is usually
represented by a complex structure of data objects that are linked together by references.
There is usually one data object that represents the structure, and that is the entry point
for messages to the structure. When passing such an object, it must be determined what
passing means. This can mean passing the individual object or the complete structure
that is connected to the object by its references. In order to distinguish these situations
we use the concepts of ’shallow copy’, and of ’deep copy’. A shallow copy copies only
the representative object (top level object). Objects that are linked to this object by
references are not copied. A deep copy creates a copy of a complete structure which is the
representative object, and all objects (indirectly) linked to it. This concept will appear to
be important for the understanding of the concept of travelling objects see 4.4.5.
4.4.3 Process Objects
4.4.3.1 Definition
Data objects offer computational power and dynamic properties comparable with other
sequential object-oriented languages. Process objects offer the power to express concurrent
independent system parts connected in a static structure. Process objects solely
communicate via channels. They are connected in a static interconnect topology of
channels.
Process objects are instantiated from Process Classes. Process Classes can be visualised
in Object Class Diagrams, together with Data Classes. The symbol has been shown in
Figure 4.2. The class sort field must be filled with the P of Process class.
Behaviour of a process object is specified in its Class definition. Such a class definition
is a formal behaviour description in the form of a POOSL Process Class. Process classes