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

182 Modelling of Concurrent Reactive Behaviour
defined identifiers in contrast to data object identifiers that are not user defined names
(see Paragraph 6.3.9.3).
A dynamic link exists when the object that plays the role of receiver is an acquaintance
of the object that plays the role of sender. The relation enables the possibility to use
the identifier as destination specification. Having an acquaintance does not prescribe
to use the identifier of the acquaintance for all communication with this acquaintance.
A dynamic link does also not prescribe a dynamic usage of the link. If the identifier is
never passed to another object the property of being dynamic is not exploited. Notice
that a receiver can also have (or obtain) the name of the sender. This means that there is
another dynamic link in the opposite direction.
In Section 4.8 about properties of composites we defined visibility and dependency.
These concepts are related to the concept of dynamic link. A dynamic link is a sufficient
condition for visibility. An object that is dependent on another object must have a link to
it. This link can be static or dynamic. Having a link does not imply dependency. After
all, having a link does not specify the need for a service.
6.3.9.3 Dynamic Links with Data Objects
Data objects cannot have a static link, because they neither have channels nor can they
execute message-send and message-receive statements. In the previous paragraph we
discussed dynamic links between process objects. These links are based on identifiers
that are user defined. In contrast, data objects have dynamic links that are created
automatically when data objects are created.
Data objects are instantiated from a class, by the execution of a new statement by another
object. This other object can be either a process object or a data object. After creation this
other object has a dynamic link to the new data object. This link is created by using an
automatically generated identifier that must be stored in a variable. This variable holds
a reference to the newly created object. This makes the changing of communication
links with data objects very flexible. After all, references can be passed as easy as values
between variables.
A process object has usually a collection of data objects and hides those objects in its
abstraction boundary. Data objects cannot be shared among process objects. There
cannot be a dynamic link between two data objects in different process objects. If
different processes must perform subsequent operations on a data object, this data
object must be passed. This is why data objects are also called travelling objects (see
Subsection 4.4.5). The transport of a data object is simulated by the creation of a deep
copy. So instead of passing a reference to a data object from the sending process to the
receiving process, a new reference to a new (deep) copy of the data object is created at
the receiver side.