Which Method Runs Through a Base Handle?

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

WhyWhat IfProve ItBreak ItCompareScale It

1. Which Method Runs Through a Base Handle?
2. ComparePure Virtual Methods vs Empty Virtual MethodsL1 Junior

   Compare a pure virtual method in a virtual class with a virtual method that has an empty body.

   Reveal the Model Answer

   A pure virtual method has no body. Its class must be declared virtual, which means abstract: nobody can construct it. A subclass that does not implement the method cannot be constructed either. An empty virtual body compiles everywhere and silently does nothing when a subclass forgets the override. Use pure virtual for methods every subclass must provide, and an empty body for optional hooks.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 A pure virtual method has no body. Key point 2 Its class must be declared virtual, which means abstract: nobody can construct it. Key point 3 A subclass that does not implement the method cannot be constructed either. Key point 4 An empty virtual body compiles everywhere and silently does nothing when a subclass forgets the override. Key point 5 Use pure virtual for methods every subclass must provide, and an empty body for optional hooks.

   Misconception

   "An abstract class cannot have any implemented methods."

   A virtual class may mix implemented methods with pure virtual ones; only construction is forbidden.

   Source IEEE 1800-2017 §8.21

   • Lesson 4.4Virtual Methods and Dynamic Dispatch
3. What IfConstructors Along the Class ChainL1 Junior

   Asked if they mention constructors

   How do constructors work along txn and rd_txn extends txn? What happens if rd_txn::new never calls super.new?

   Reveal the Model Answer

   Constructing a rd_txn runs the base constructor first, through super.new(...), which must be the first statement. If you leave it out, the compiler inserts super.new() with no arguments. That fails to compile when the base constructor has a required argument, and silently uses the defaults when the argument has one.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 Constructing a rd_txn runs the base constructor first, through super.new(...), which must be the first statement. Key point 2 If you leave it out, the compiler inserts super.new() with no arguments. Key point 3 That fails to compile when the base constructor has a required argument, and silently uses the defaults when the argument has one.

   Half-Truth

   "Forgetting super.new is a compile error, so it cannot get past."

   Only when the base argument has no default. With a default, the implicit call compiles and the base fields keep their defaults.

   Source IEEE 1800-2017 §8.17

   • Lesson 4.3Inheritance and super.new
   • Problem 4.3.1Derived Read Transaction
4. Break ItThe Transaction Whose Id Was Always ZeroL1 JuniorL2 Mid-Level

   Every rd_txn in the log has id=0, although the test constructs them with new(7, 'h40). The constructor sets addr correctly. What is missing?

   class txn;
     int id;
     function new(int id = 0); this.id = id; endfunction
   endclass
   class rd_txn extends txn;
     int addr;
     function new(int id, int addr);
       this.addr = addr;
     endfunction
   endclass

   Reveal the Model Answer

   The derived constructor never calls super.new(id), so the compiler inserted super.new(), which used the default id = 0. The id argument of rd_txn::new is just a local name that nothing reads. Add super.new(id); as the first statement.

   Read the diagram as text

   • Test: e = new(5, 'h80, 2)
   • err_txn::new: derived constructor
   • axi_txn::new: base constructor
   • The new object: id, addr, err_beat

   • Test → err_txn::new: new(...)
   • err_txn::new → axi_txn::new: super.new(...)
   • axi_txn::new → The new object: id, addr
   • err_txn::new → The new object: err_beat

   Each class sets what it declares. The derived class reaches the base part only through super.new.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 The derived constructor never calls super.new(id), so the compiler inserted super.new(), which used the default id = 0. Key point 2 The id argument of rd_txn::new is just a local name that nothing reads. Key point 3 Add super.new(id); as the first statement.

   Misconception

   "The base constructor sees the derived constructor's id argument because the names match."

   Arguments are local to their method; matching names connect nothing. The twin prints id=0 until super.new(id) is added.

   Run It

   A derived constructor that sets its own field but never calls super.new.

   RUNVerilator · simulator service

   Edit the code, then run it

   OutputWaves

   RunReset codeExpected: id=0 addr=00000040 FAIL: without super.new(id), an implicit super.new() ran with the default id 0

   See the Fix

   class txn;
     int id;
     function new(int id = 0); this.id = id; endfunction
   endclass
   class rd_txn extends txn;
     int addr;
     function new(int id, int addr);
       super.new(id);             // the fix: pass id to the base constructor
       this.addr = addr;
     endfunction
   endclass
   module tb;
     rd_txn r;
     initial begin
       r = new(7, 'h40);
       $display("id=%0d addr=%h", r.id, r.addr);
       if (r.id == 7) $display("PASS: the base constructor received the id");
       else $display("FAIL: without super.new(id), an implicit super.new() ran with the default id 0");
     end
   endmodule
   

   Prints: id=7 addr=00000040 PASS: the base constructor received the id

   • Lesson 4.3Inheritance and super.new
5. WhyWhy Methods Are Not Virtual by DefaultL1 JuniorL2 Mid-Level

   If virtual methods are so useful, why doesn't SystemVerilog make every method virtual?

   Reveal the Model Answer

   A non-virtual method is a promise from the base class. Callers that use a base handle always get that exact behaviour, whatever a subclass does. It also lets the compiler call the method directly instead of looking it up per object. The base-class author decides which behaviour is meant to be replaced, and marks only those methods virtual.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 A non-virtual method is a promise from the base class. Key point 2 Callers that use a base handle always get that exact behaviour, whatever a subclass does. Key point 3 It also lets the compiler call the method directly instead of looking it up per object. Key point 4 The base-class author decides which behaviour is meant to be replaced, and marks only those methods virtual.

   Misconception

   "Non-virtual methods cannot be overridden."

   They can be redefined in a subclass, and calls through a derived handle see the new version. Only base-handle calls ignore it.

   Source IEEE 1800-2017 §8.14

   • Lesson 4.4Virtual Methods and Dynamic Dispatch
6. Scale ItSwapping Transaction Types Without Editing the DriverL2 Mid-LevelL3 Senior

   A test wants error-injecting transactions on one agent only, without editing the driver or the generator. How do you design for that?

   Reveal the Model Answer

   The generator creates every transaction in one place that a test can override, such as a factory. The driver only calls virtual methods on base handles. The test extends the transaction, overrides the virtual hooks that inject the error, and registers the subclass for that one agent. UVM's factory with an instance override is this pattern packaged, which is the main reason to use it at scale.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 The generator creates every transaction in one place that a test can override, such as a factory. Key point 2 The driver only calls virtual methods on base handles. Key point 3 The test extends the transaction, overrides the virtual hooks that inject the error, and registers the subclass for that one agent. Key point 4 UVM's factory with an instance override is this pattern packaged, which is the main reason to use it at scale.

   What Interviewers Listen For (Senior)

   A good senior answer says the one place that constructs objects must be open to override. It states the price: every method a test may change must be virtual from the start.

   Wrong Altitude

   "Add an inject_error flag to the base transaction and test it in the driver."

   Each new error kind then edits the base class and the driver, the two files shared by every test. Overrides keep test-specific behaviour in the test.

   • Lesson 4.4Virtual Methods and Dynamic Dispatch
   • Lesson 4.7Problem: A Transaction Factory
   • Problem 4.7.1Factory by Opcode
   • Problem 4.7.2Validated Factory Arguments
7. Prove ItSolve 4.7.1: Factory by OpcodeSimulator-graded

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

   Open Problem 4.7.1