VLSI DV Interview Puzzles · All levels

Non-Virtual vs Virtual Through Base Handle

A framework stores derived objects in base handles. Predict output and explain why only one method dispatches to derived.

Puzzle

Difficulty: Easy · Puzzle 1 of 6 · Topic: Virtual Method Puzzles

A framework stores derived objects in base handles. Predict output and explain why only one method dispatches to derived.

Code

systemverilog

class checker_base;
  function string mode();
    return "BASE";
  endfunction
  virtual function string detail();
    return "base-detail";
  endfunction
endclass

class checker_ext extends checker_base;
  function string mode();
    return "EXT";
  endfunction
  function string detail();
    return "ext-detail";
  endfunction
endclass

initial begin
  checker_base c;
  c = new checker_ext();
  $display("mode=%s detail=%s", c.mode(), c.detail());
end

Hint

Dispatch behavior is determined by virtual-ness in the base declaration.

Step-by-step solution

diagram

1) `mode()` is non-virtual in base, so call binds statically to `checker_base::mode`.
2) `detail()` is virtual in base, so call dispatches dynamically to `checker_ext::detail`.
3) Output becomes `mode=BASE detail=ext-detail`.

Answer

Answer: Output is `mode=BASE detail=ext-detail`; non-virtual methods bind to static handle type while virtual methods use runtime object type.

Why candidates get it wrong

Declaring method in derived does not create polymorphism unless base declaration is virtual.

Interviewer follow-up

If `mode` is made virtual only in derived, does behavior change through `checker_base` handle?

Navigate