VLSI DV Interview Puzzles · All levels

String Substr with Runtime Class Cast

Predict all outputs from string methods and both `$cast` attempts.

Puzzle

Difficulty: Easy · Puzzle 1 of 6 · Topic: String and Casting Puzzles

Predict all outputs from string methods and both `$cast` attempts.

Code

systemverilog

class base_pkt; endclass
class good_pkt extends base_pkt;
  int id = 42;
endclass
class bad_pkt extends base_pkt; endclass

module p1;
  string s = "dv puzzle";
  base_pkt b1, b2;
  good_pkt g;
  initial begin
    b1 = new good_pkt();
    b2 = new bad_pkt();

    $display("substr=%s", s.substr(3,8));
    $display("toupper=%s", s.toupper());

    if ($cast(g, b1)) $display("cast1 ok id=%0d", g.id);
    else              $display("cast1 fail");

    if ($cast(g, b2)) $display("cast2 ok id=%0d", g.id);
    else              $display("cast2 fail");
  end
endmodule

Hint

`substr(i,j)` uses inclusive indices. `$cast` checks runtime object type, not just declared handle type.

Step-by-step solution

diagram

1) `substr(3,8)` on `"dv puzzle"` returns `"puzzle"` (index 3 through 8 inclusive).
2) `toupper()` returns `"DV PUZZLE"`.
3) `b1` points to `good_pkt`, so `$cast(g,b1)` succeeds and prints id 42.
4) `b2` points to `bad_pkt`, so cast to `good_pkt` fails.

Answer

Answer: substr=puzzle, toupper=DV PUZZLE, cast1 ok id=42, cast2 fail

Why candidates get it wrong

Candidates confuse static type compatibility with dynamic runtime-type compatibility.

Interviewer follow-up

What happens to `g` after the failed second cast in this code?

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