Difficulty: Hard · Puzzle 4 of 6 · Topic: Handle and Copy Puzzles
This `do_copy` tries to deep-copy nested policy object, but simulation crashes with null-handle access. Identify root cause and fix.
class qos extends uvm_object;
`uvm_object_utils(qos)
int prio;
function new(string name="qos"); super.new(name); endfunction
endclass
class pkt extends uvm_sequence_item;
`uvm_object_utils(pkt)
qos policy;
function new(string name="pkt");
super.new(name);
endfunction
function void do_copy(uvm_object rhs);
pkt r;
if (!$cast(r, rhs)) return;
policy.copy(r.policy);
endfunction
endclassBefore calling `policy.copy(...)`, what must be true about destination handle allocation?
1) `policy` is never constructed in `new`, so destination handle remains null.
2) `policy.copy(r.policy)` dereferences null and triggers runtime fatal/null-handle error.
3) Allocate destination before copy (`if (policy == null) policy = qos::type_id::create(...)`).
4) Also guard source null case and set destination null accordingly.Answer: Crash occurs because `policy` is null in destination; deep-copy code dereferences an unallocated nested handle.
Engineers remember to copy nested fields but forget destination object ownership/allocation lifecycle.
How would you implement symmetric null-handling so copy preserves null vs non-null state exactly?