Optionals

Making absence explicit

An optional (String?, short for Optional<String>) either contains a value or is nil. Non-optional types can never be nil, so the compiler forces you to handle the missing case before using an optional's value. Ways to unwrap: optional binding with if let name { ... } (shorthand since Swift 5.7) or if let n = name; guard let, which unwraps and exits early if nil, keeping the happy path unindented; the nil-coalescing operator ?? to supply a default; and optional chaining user?.address?.city, which returns nil if any link is nil. Force unwrapping with ! crashes if the value is nil; use it only when nil would be a programming error that should stop the app, and even then prefer a clear guard ... else { fatalError("reason") }. Many APIs return optionals: dictionary lookups, Int("42") conversions, first on collections. Optionals are really an enum with .some(value) and .none, so they work with switch and map/flatMap too.

A box that may be empty

An optional is a box: you must check whether something is inside before using it.

Unwrapping optionals safely

guard for early exit, ?? for defaults and optional chaining.

struct User { let name: String; let address: Address? }
struct Address { let city: String }

func shippingLabel(for user: User?) -> String {
    guard let user else {
        return "Guest checkout"            // early exit when nil
    }
    let city = user.address?.city ?? "City not set"
    return "\(user.name), \(city)"
}

let ages = ["asha": 24, "ravi": 31]
if let age = ages["asha"] {               // dictionary lookups return optionals
    print("Asha is \(age)")
}

let input = "42"
let quantity = Int(input) ?? 1            // conversion may fail -> default

let scores = [91, 64, 78]
let best = scores.max()                   // Int? because the array could be empty

Every ! is a possible crash

Force unwrapping is the most common cause of Swift crashes in real apps. When you write !, ask what happens if the value is nil, and handle that case explicitly instead.

त्वरित जाँच: What does `guard let user else { return }` do when user is nil?

• Crashes the program
• Assigns a default user
• Exits the current scope early via the else branch
• Continues with user as nil