VLSI DV Interview Puzzles · All levels

super Method Call in Override Chain

A derived override calls `super.report()`. Predict final string and explain call chain.

Puzzle

Difficulty: Hard · Puzzle 6 of 6 · Topic: Virtual Method Puzzles

A derived override calls `super.report()`. Predict final string and explain call chain.

Code

systemverilog

class base_r;
  virtual function string report();
    return "B";
  endfunction
endclass

class mid_r extends base_r;
  function string report();
    return {"M-", super.report()};
  endfunction
endclass

class ext_r extends mid_r;
  function string report();
    return {"E-", super.report()};
  endfunction
endclass

initial begin
  base_r h = new ext_r();
  $display("%s", h.report());
end

Hint

`super` is lexical (next class in inheritance chain), not dynamic back to most-derived.

Step-by-step solution

diagram

1) Dynamic dispatch picks `ext_r::report` for `h.report()`.
2) `ext_r` calls `super.report()` => `mid_r::report`.
3) `mid_r` calls its `super.report()` => `base_r::report`.
4) Concatenation yields `E-M-B`.

Answer

Answer: Final output is `E-M-B`; dynamic entry is most-derived, then explicit `super` walks up one lexical level each call.

Why candidates get it wrong

Some candidates think `super` re-dispatches dynamically and can recurse to most-derived.

Interviewer follow-up

How would behavior change if `mid_r::report` were declared non-virtual?

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