Classes

Classes provide reference semantics, inheritance, and deinitializers—use when instances must share identity (view controllers, network clients) or you need subclassing.

Class basics

class Counter {
    var count = 0
    func increment() { count += 1 }
}

let a = Counter()
let b = a
b.increment()
print(a.count)  // 1 — shared instance

Inheritance

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

Swift classes support single inheritance; protocols fill multiple-behavior needs.

Important interview questions and answers

1. Q: When use class over struct?
   A: Shared mutable state, Objective-C interop, inheritance, or ARC-managed resources tied to instance lifetime.
2. Q: final keyword?
   A: Prevents subclassing or overriding—default for performance and API stability in many frameworks.

Self-check

1. Do two variables referencing one class instance share mutations?
2. How many superclasses can a Swift class have?

Tip: Use classes when instances must share identity—view controllers, delegates, network clients.