UML is especially good at describing relationships between classes.
Jumping ahead a bit to Example 2.22, we will describe an important
relationship in UML.
In that example we introduce a Point class to represent a geometric point on
the screen, and a Square class to represent a geometric shape on the screen.
Point has two int data members and Square has two Point data members.
The two Point data members, because they are class objects, are regarded as subobjects of Square.
The Square object is regarded as the parent of its
Point subobjects; when a Square object is
destroyed so are its subobjects.
That makes the subobjects components of the parent
object, and the relationship is called composition.
In Figure 2.2,
the filled-in diamonds indicate that the instances of that class are composed (at least partially) of instance(s) of the
class on the other end of the relationship connector.
