Polymorphism in grandchild class

SystemVerilog
1

The way I though polymorphism works was that when we call a method the simulator checks the class to see the function declaration. If the method is virtual the handler is checked to see which class type it holds (parent or extended) and execute the method inside that class type. If the method isn’t virtual it executes it without checking the class type.


class a;
  virtual function display;
    $display("a");
  endfunction
endclass
class b extends a;
  function display;
    $display("b");
  endfunction
endclass
class c extends b;
  function display;
    $display("c");
  endfunction
endclass
module tb;
  b hb;
  c hc;
  initial begin 
    hc = new();
    hb = hc;
    hb.display();
  end
endmodule

The code above display “c” which doesn’t make sense to me since display is not virtual in class b so I expect it to display “b”. So what is wrong exactly in my understanding?

And if this is how it works, I don’t see the need to declare functions as virtual in extended classes so why should or shouldn’t we do so?

2

In reply to M.Younis:

See query on polymorphism | Verification Academy

Also see OOPS terminologies in SV | Verification Academy

3

In reply to M.Younis:

you need to remember: once virtual always virtual.

If you define some function virtual in any place of hierarchy, than it will be always viritual in other parts of hierarchy. No matter if we write virtual word for child functions.

4

In this example… can “c” grandchild directly access “a” method without parent “b” ? since super.display() will display parent “b” ?

I would like to get from class “c” method display only “a” and “c”, not “b”. How do we get that?

class a;
   virtual function display;
     $display("a");
   endfunction
 endclass
 class b extends a;
   function display;
     $display("b");
   endfunction
 endclass
 class c extends b;
   function display;
     $display("c");
   endfunction
 endclass
5 
 class parent_class;
  
   //function display();
   virtual function display();
      $display("Im Parent \n");
   endfunction
 endclass
 
 class child_class extends parent_class;
  
   virtual function display_c();
     super.display();
 
     $display("Im Child\n");
   endfunction
   
 endclass
 
 class grand_child_class extends child_class;
  
   function display_c(); 
     super.display();
     $display("Im Grand Child\n");
   endfunction
   
 endclass
 
 module inheritence;
   initial begin
     parent_class p=new();
     child_class c=new();
     grand_child_class g=new();
     c = g;
     g.display_c();
   end
 endmodule
6