Systemverilog polymorphism feature of OOPS demonstrated with constraints

Question

I was working on checking my understanding on polymorphism with constraints. I wrote a sample code

class parent;
  rand int unsigned a;
  constraint a_c { a < 1000;}
  function print();
    $display("The randomized data is %d\n", a);
  endfunction
endclass

class child extends parent;
  constraint a_c { a > 50;}
endclass

module m;
  child c = new();

  initial begin
    c.randomize();
    c.print;
  end
endmodule

The output was

The randomized data is 2567677

What is going wrong here?

Looks like expected behavior. testbench.in/CR_08_CONSTRAINT_BLOCK.html --> Constraints in derived class with the same name in base class overrides the base class constraints just like task and functions. — dbugger
– dbugger, Commented Jul 1, 2016 at 2:47

dave_59 · Accepted Answer · 2016-07-01 04:41:07Z

6

That is a valid result that satisfies constraint a_c { a > 50;} Since you extended constraint a_c, it overrides the constraint in the base class. If you want the constraint to be additive, you need to give it a different name from the base class.

BTW, I suggest avoiding the terms parent and child when referring to OOP inheritance. Those terms imply distinct objects. Use base/super and derived/extended classes instead.

answered Jul 1, 2016 at 4:41

dave_59

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1 Comment

dbugger Over a year ago

+1 just for this..."BTW, I suggest avoiding the terms parent and child when referring to OOP inheritance. Those terms imply distinct objects."

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