Classes, Reference Semantics & Inheritance

In Plain English

When you need shared mutable state or single-inheritance hierarchies, use Classes. Master reference semantics (multiple variables pointing to the same instance), custom initializers, subclassing with extends, super.init calls, and deinitializers (deinit).

The Mental Model

"A class instance is like a Google Doc: when you share the link with 5 team members, everyone edits the exact same live document. Changes made by any member are instantly visible to everyone."

Deep Dive: How It Works

Reference Semantics: Assigning a class instance to a new variable creates another reference to the exact same object in memory.

Identity Operator (===): Checks whether two variables point to the exact same memory address (unlike == which checks value equality).

Inheritance: Subclasses inherit methods and properties from a superclass (class Manager: Employee).

Deinitializer (deinit): Executes automatically when an instance reference count drops to zero and memory is deallocated.

Syntax Blueprint

class Animal {
    func speak() { print("...") }
}
class Dog: Animal {
    override func speak() { print("Woof!") }
}

Inherit with : BaseClass and override methods with the override keyword.

Core Rules to Remember

Reference Types: Variables hold pointers to memory allocated on the heap.

Required Initializers: Subclasses must call super.init() after initializing their own stored properties.

Final Classes: Prefix with final class to disallow subclassing and enable static method dispatch optimization.

Common Beginner Traps & How to Fix Them

Mistake:Forgetting the override keyword when redefining a superclass method.

Why it happens: Swift compiler requires explicit override to prevent accidental method shadowing.

How to fix: Add the override keyword before func.

Pro Tip: Use final on classes whenever inheritance is not explicitly planned to improve compiler optimization.

Live Interactive Example

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Class Reference Semantics and Inheritance

Swift 5.10 / LLVM

Output Console

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How it works: Mutating v2.speed updated v1 because both reference the exact same heap instance.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Create an Account Class Hierarchy

Create a class Account with var balance: Double and an init(balance: Double).
Create a subclass PremiumAccount: Account with a method addBonus(amount: Double) that adds amount to balance.
In top-level code, create let acc = PremiumAccount(balance: 100.0), call acc.addBonus(amount: 25.0), and print "Balance: $\(acc.balance)".

Sandbox Output

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Deep Dive: How It Works

Reference Semantics: Assigning a class instance to a new variable creates another reference to the exact same object in memory.

Identity Operator (===): Checks whether two variables point to the exact same memory address (unlike == which checks value equality).

Inheritance: Subclasses inherit methods and properties from a superclass (class Manager: Employee).

Deinitializer (deinit): Executes automatically when an instance reference count drops to zero and memory is deallocated.