VLSI DV Interview Puzzles · All levels

Dynamic Array Resize with Class Handles

Explain why changing `da[0].v` also changes `db[0].v` after resize-copy.

Puzzle

Difficulty: Hard · Puzzle 4 of 6 · Topic: Queue and Array Puzzles

Explain why changing `da[0].v` also changes `db[0].v` after resize-copy.

Code

systemverilog

class item;
  int v;
  function new(int v); this.v = v; endfunction
endclass

module p4;
  item da[];
  item db[];
  initial begin
    da = new[2];
    da[0] = new(10);
    da[1] = new(20);
    db = new[3](da);
    db[2] = new(30);
    da[0].v = 99;
    $display("da0=%0d db0=%0d db2=%0d", da[0].v, db[0].v, db[2].v);
  end
endmodule

Hint

`new[N](old)` copies elements, but if element type is class handle, copied value is the handle.

Step-by-step solution

diagram

1) `db = new[3](da)` copies first two elements from `da` into `db`.
2) Those elements are class handles, so db[0] and da[0] reference the same object.
3) Updating `da[0].v` updates that shared object, visible through `db[0].v`.
4) `db[2]` is independently constructed and remains 30.

Answer

Answer: da0=99, db0=99, db2=30

Why candidates get it wrong

Candidates conflate array copy (by value) with deep copy of objects stored in the array.

Interviewer follow-up

What code pattern gives deep-copy semantics for arrays of class handles?

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