Shallow Copy in Nested Payload

Puzzle

Difficulty: Easy · Puzzle 1 of 6 · Topic: Handle and Copy Puzzles

You clone a transaction before pushing into an expected queue. Predict the print and name the exact ownership bug.

Code

class payload;
  rand bit [7:0] data[];
  function new();
    data = new[2];
  endfunction
endclass

class txn extends uvm_sequence_item;
  `uvm_object_utils(txn)
  rand bit [31:0] addr;
  payload pld;

  function new(string name = "txn");
    super.new(name);
    pld = new();
  endfunction

  function void do_copy(uvm_object rhs);
    txn r;
    if (!$cast(r, rhs)) return;
    super.do_copy(rhs);
    addr = r.addr;
    pld  = r.pld;
  endfunction
endclass

initial begin
  txn a = txn::type_id::create("a");
  txn b = txn::type_id::create("b");
  a.pld.data[0] = 8'h11;
  b.copy(a);
  a.pld.data[0] = 8'hEE;
  $display("a=%0h b=%0h", a.pld.data[0], b.pld.data[0]);
end

Hint

Focus on what `pld = r.pld` copies: object contents or just the handle value.

Step-by-step solution

1) `b.copy(a)` calls `do_copy`, which assigns `b.pld` to the same handle as `a.pld`.
2) After copy, both `a.pld` and `b.pld` refer to one payload object.
3) Updating `a.pld.data[0]` updates shared storage, so `b.pld.data[0]` changes too.
4) Fix by allocating/cloning payload inside `do_copy` (`pld = payload::type_id::create(...)` or `pld = r.pld.clone()`).

Answer

Answer: It prints `a=ee b=ee`; the bug is shallow copy of nested handle `pld` inside `do_copy`.

Why candidates get it wrong

Calling `copy()` does not guarantee deep copy; UVM only performs what your override explicitly implements.

Interviewer follow-up

Show a `do_compare` snippet that can detect aliasing by checking handle identity in addition to field equality.

Related topics

• Back to Handle and Copy Puzzles

• OOP & Testbench Puzzles section