SystemVerilog Datatype: Class (Part 2)
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Inheritance and Scope
Note that, Trapezoid is indeed an extension of the class
Triangle and every Trapezoid object is a perfectly
legal Triangle object. For example, a handle of a Trapezoid
object can be assigned to a Triangle type variable.
Trapezoid tpzd = new;
Triangle t = tpzd;
This gives rise to an interesting situation. Consider a scenario where
both Trapezoid and Triangle contain a property (or a method)
of the same name, but of different definitions. A common occurence of
this situation is the method new(). Which definition will t
in the above code inherit? Is it the definition of the property/method that
Trapezoid has or will it be from Triangle?
In such a scenario, SystemVerilog rules dictate that an object will inherit
the definition of its own type. So in this example, t will inherit the
property/method definition of Triangle (and not of Trapezoid).
Before we finish on the topic of parent and subclasses, one convenient tip
for writing code: you can refer to a variable in the parent class using the
keyword super. So, from within Trapezoid, it is always
possible to read the value of, say, p1.xy.x of Triangle as
super.p1.xy.x.
[Next time: Constructors simplify the use of an object handle
and helps us to understand how to use multiple of them together. The
concept of subclasses is a powerful one. Using these two concepts
 particularly the notion of subclasses  next time we will see how
SystemVerilog can help you achieve data encapsulation and the tricky
business of polymorphism.]
Previous: Inheritance 
Next: Data encapsulation
