An interesting polymorphism behavior/mis-behavior?

SystemVerilog
,
6

In reply to dave_59:

Could someone help explain this behavior difference? Shouldn’t functions that are virtual be always virtual? Does it matter where it is made virtual-parent/child class.

Variant 1.

module test;
   class A;
      virtual function void showMsg(int val);
         $display("This is A");
      endfunction
   endclass
   class B extends A;
       function void showMsg(int val);
         $display("This is B, val = %0d", val);
      endfunction
   endclass
   class C extends B;
      function void showMsg(int val);
         $display("This is C, val=%0d", val+1);
      endfunction
   endclass
   initial begin
      A a;
      B b;
      C c;
      b = new();
      a = b;
     a.showMsg(1);
      c = new();
      b = c;
     b.showMsg(7); 
   end
endmodule

Result-
This is B, val = 1
This is C, val=8

Variant 2:

module test;
   class A;
       function void showMsg(int val);
         $display("This is A");
      endfunction
   endclass
   class B extends A;
       virtual function void showMsg(int val);
         $display("This is B, val = %0d", val);
      endfunction
   endclass
   class C extends B;
      function void showMsg(int val);
         $display("This is C, val=%0d", val+1);
      endfunction
   endclass
   initial begin
      A a;
      B b;
      C c;
      b = new();
      a = b;
     a.showMsg(1);
      c = new();
      b = c;
     b.showMsg(7); 
   end
endmodule

Result -
This is A
This is C, val=8

The only difference between the two is the place where virtual keyword is used. Rest of the method prototype being exactly the same.