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All objects correspond
to the building plan that defi nes the pertaining classes from Figure 3.24 .
I leave it up to the playful reader to think of more unhappy constellations.
How can such a situation be prevented? How can a modeler express which
object relationships are valid and which ones are not? Unfortunately, this is impossible
in a class diagram, because it describes types, and with them all object relationships
that are permitted in each context across the entire model. However, we
want to describe that certain relationships are invalid while others are mandatory
for an object, depending on the role it plays. This is where the composite structure
diagram comes in handy.
The diagram and the underlying model close the gap between type level
(classes) and object level. In between these two there is the role level. The engine
plays a role in the context of the car. The four wheels can play two roles: Each
can either be a front wheel or a rear wheel. The context-specifi c relationships
between the roles and sets are defi ned in a composite structure diagram.


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