SystemVerilog Datatype: Class (Part 2)

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Inheritance

It is quite common for a class definition to contain a property that is another class type. For example, we can define a new class Trapezoid that has two class properties - one is an object of type Triangle and another a structure type point.

 
class Trapezoid; 
   // class properties
   Triangle t1; 
   point p4;
   ... // rest of the definition 
endclass

While the above works, there is a better way of handling this, which makes accessing t1 from a variable of type Trapezoid much easier. To see how to do this, note that, Trapezoid is a specialization of Triangle, i.e., Trapezoid contains all that Triangle does, and then some more. Or, to put it differently, Trapezoid 'extends' the class Triangle and 'inherits' its (i.e., Triangle's) members. Triangle is said to be the parent (or, base) class of Trapezoid and Trapezoid a subclass (or, derived class) Triangle. This is how it looks in formal syntax:

class Trapezoid extends Triangle; 
   point p4;
   ... // rest of the definition
endclass