A Copy That Shares Its Payload

Where the interviewer goes next. Open a node to practise that follow-up.

WhyWhat IfProve ItBreak ItCompare

1. A Copy That Shares Its Payload
2. WhyTwo Handles, One ObjectL0 FresherL1 Junior

   Why is a copy needed at all? What does this print?

   class item; int v; endclass
   module tb;
     item h1, h2;
     initial begin
       h1 = new; h1.v = 1;
       h2 = h1;                    // copies the handle
       h2.v = 2;
       $display("h1.v=%0d", h1.v);
     end
   endmodule
   

   Reveal the Model Answer

   It prints h1.v=2. A class variable holds a handle, a reference to an object, and h2 = h1 copies only the handle. Both names now refer to the one object new created, so a write through either is seen through both.

   RUNVerilator · simulator service

   Edit the code, then run it

   OutputWaves

   RunReset codeExpected: h1.v=2

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 It prints h1.v=2. Key point 2 A class variable holds a handle, a reference to an object, and h2 = h1 copies only the handle. Key point 3 Both names now refer to the one object new created, so a write through either is seen through both.

   Misconception

   "Assigning one class variable to another copies the object, like a struct."

   Structs are values; classes are references. Only new creates an object, and assignment never calls it.

   Source IEEE 1800-2017 §8.4

   • Lesson 4.1Handles, Objects and Constructors
   • Problem 4.1.1Construct a Valid Packet
3. What IfA Generator That Reused One ObjectL1 Junior

   Asked if they write a generator

   A generator creates one txn before its loop, sets a new address each pass and puts it in a mailbox. The driver receives three transactions, all with the last address. Why?

   gen: put addr=100
   gen: put addr=200
   gen: put addr=300
   drv: got addr=300
   drv: got addr=300
   drv: got addr=300

   Reveal the Model Answer

   A mailbox stores what it is given, and for a class that is a handle. All three messages refer to the same object, which holds the address written last. Construct a new object inside the loop for every transaction.

   Read the diagram as text

   • Generator: uses handle tr
   • exp_q: a queue of handles
   • One object: built before the loop
   • Four objects: one new() per pass

   • Generator → One object: tr.id = i
   • Generator → exp_q: push_back(tr)
   • exp_q → One object: refers to
   • Generator → Four objects: tr = new(i)
   • exp_q → Four objects: refers to

   A push stores a handle. Only new makes a second object.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 A mailbox stores what it is given, and for a class that is a handle. Key point 2 All three messages refer to the same object, which holds the address written last. Key point 3 Construct a new object inside the loop for every transaction.

   Misconception

   "put() stores a copy of the object."

   Nothing in the mailbox calls new or copy(). The twin reads back three entries that all show the last address.

   Run It

   One object, three puts, three gets.

   RUNVerilator · simulator service

   Edit the code, then run it

   OutputWaves

   RunReset codeExpected: 300 300 300 FAIL: the mailbox holds three handles to one object, so every message shows the last address

   See the Fix

   class txn; int addr; endclass
   module tb;
     mailbox #(txn) mb = new;
     txn t, got;
     int first;
     initial begin
       for (int i = 0; i < 3; i++) begin
         t = new;                // the fix: a new object for every transaction
         t.addr = 'h100 * (i + 1);
         mb.put(t);
       end
       for (int i = 0; i < 3; i++) begin
         mb.get(got);
         if (i == 0) first = got.addr;
         $write("%h ", got.addr[11:0]);
       end
       $display("");
       if (first == 'h100) $display("PASS: each message refers to its own object");
       else $display("FAIL: the mailbox holds three handles to one object, so every message shows the last address");
     end
   endmodule
   

   Prints: 100 200 300 PASS: each message refers to its own object

   • Lesson 3.5Mailboxes and Backpressure
   • Lesson 4.1Handles, Objects and Constructors
   • Problem 3.5.1Exact-Count Consumer
4. Break ItThe Copy That Forgot the Subclass FieldsL2 Mid-Level

   A crc_pkt extends pkt and adds a crc field, but does not override the virtual copy_from(pkt rhs). The scoreboard copies packets through pkt handles. What breaks?

   Reveal the Model Answer

   The call is virtual, but crc_pkt has no override, so the base body runs and copies only id. The copy looks right in every base field while crc keeps whatever it held before. Every class that adds data must override the copy method. It calls super for the inherited fields and casts rhs to reach its own.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 The call is virtual, but crc_pkt has no override, so the base body runs and copies only id. Key point 2 The copy looks right in every base field while crc keeps whatever it held before. Key point 3 Every class that adds data must override the copy method. Key point 4 It calls super for the inherited fields and casts rhs to reach its own.

   Misconception

   "copy_from() is virtual, so it copies whatever the object really is."

   Virtual only chooses which body runs. Without an override the base body runs, and it knows nothing about crc.

   Run It

   Two crc_pkt objects copied through a pkt handle with only the base copy_from().

   RUNVerilator · simulator service

   Edit the code, then run it

   OutputWaves

   RunReset codeExpected: dst.id=4 dst.crc=0 FAIL: the inherited copy_from() copied only the base fields, so crc was lost

   See the Fix

   class pkt;
     int id;
     virtual function void copy_from(pkt rhs);
       id = rhs.id;
     endfunction
   endclass
   class crc_pkt extends pkt;
     int crc;
     virtual function void copy_from(pkt rhs);
       crc_pkt r;
       super.copy_from(rhs);       // the fix: copy the base fields, then this class's own
       if ($cast(r, rhs)) crc = r.crc;
     endfunction
   endclass
   module tb;
     crc_pkt src, dst;
     pkt h;
     initial begin
       src = new; src.id = 4; src.crc = 'h5a;
       dst = new;
       h = dst;
       h.copy_from(src);           // the scoreboard copies through base handles
       $display("dst.id=%0d dst.crc=%0h", dst.id, dst.crc);
       if (dst.crc == 'h5a) $display("PASS: the derived copy_from() copied crc too");
       else $display("FAIL: the inherited copy_from() copied only the base fields, so crc was lost");
     end
   endmodule
   

   Prints: dst.id=4 dst.crc=5a PASS: the derived copy_from() copied crc too

   • Lesson 4.2Shallow Copies and Deep Copies
   • Lesson 4.4Virtual Methods and Dynamic Dispatch
5. CompareFilling an Object vs Cloning OneL2 Mid-Level

   Compare a copy(rhs) method that fills an existing object with a clone() method that returns a new one.

   Reveal the Model Answer

   copy(rhs) writes into an object the caller already owns. Each class copies its own fields and calls super.copy(rhs) for the rest. It needs a $cast of rhs to reach the derived fields. clone() constructs the right derived type, usually by calling copy() on a fresh object, and returns it through a base handle. Methodologies such as UVM provide both on the same pattern.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 copy(rhs) writes into an object the caller already owns. Key point 2 Each class copies its own fields and calls super.copy(rhs) for the rest. Key point 3 It needs a $cast of rhs to reach the derived fields. Key point 4 clone() constructs the right derived type, usually by calling copy() on a fresh object, and returns it through a base handle. Key point 5 Methodologies such as UVM provide both on the same pattern.

   Half-Truth

   "clone() can be written once in the base class."

   Only the derived class can construct an object of its own type. So each class must override clone(), or a macro must generate it.

   • Lesson 4.2Shallow Copies and Deep Copies
   • Lesson 4.5Checked Casts and Type Safety
6. Prove ItSolve 4.2.6: The Copy That Shares Its PayloadSimulator-graded

   Show it, don't just say it: this Silicon Lab problem is graded by the simulator.

   Open Problem 4.2.6