Inheritance & Polymorphism

Overview

Inheritance lets a class extend another, reusing its properties and methods while adding or overriding behavior. Polymorphism is the payoff: code written against a base-class handle works unchanged when handed a derived object — provided the methods involved are virtual. This pair of features is what makes testbench reuse possible: a generic driver drives any transaction derived from the base, a scoreboard compares any derived type through virtual compare(), and tests swap in error-injecting transactions without touching the environment. It is also the densest interview territory in SystemVerilog — dispatch rules, casting, and construction order all hide subtle traps.

Sub-topics

1. extends & super — derived classes, overriding, base bus_txn → axi_txn example.

2. Virtual Methods & Dynamic Dispatch — declared-type vs object-type resolution, dispatch-table mental model.

3. Upcasting, Downcasting & $cast — implicit upcast, runtime-checked downcast, failure handling.

4. Abstract Classes & Interface Classes — virtual class, pure virtual methods, implements contracts.

5. Polymorphic Containers — base-handle queues holding mixed derived objects, factory creation.

6. Inheritance Interview Traps — five classic puzzles with worked answers.

INHERITANCE & POLYMORPHISM — topic map

            virtual class txn_base          ← 4. abstract contract
                 │ extends
            ┌────┴─────┐
        bus_txn      irq_txn
            │ extends
         axi_txn                            ← 1. extends & super

  base handle ──► derived object
       │
       ├─ b.calc()  virtual? object type wins   ← 2. dynamic dispatch
       ├─ $cast(d, b)  checked downcast          ← 3. casting
       ├─ txn_base q[$]  mixed types in one queue← 5. containers
       └─ traps: non-virtual / hidden props /
                 ctor order / sibling cast       ← 6. interview traps

Key takeaways

• extends reuses and specializes; virtual methods make base-handle code dispatch to derived behavior.

• Upcasts are implicit and safe; downcasts need $cast and a runtime check.

• Polymorphic containers plus virtual methods are the backbone of every reusable verification environment.