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Swift path · Values and types · Topic 2 of 13

Optionals and unwrapping

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In Swift a String is always a string. A value that may be missing has the type String?, which is shorthand for Optional<String>, an ordinary generic enum with two cases: .none, written nil, and .some(wrapped). Because it is a separate type, the compiler won't let you use a String? where a String is needed until you have said what happens when it is nil.

Why it matters: Swift makes 'might be missing' part of the type, and every way of getting the value out says something about what you expect. Force-unwrapping is the only one that can crash.

  1. 1An enum with two cases

    T? is Optional<T>: either .none (nil) or .some(value). A plain T can never be nil.

  2. 2Unwrap by binding

    if let and guard let give you a non-optional name; guard keeps the happy path unindented.

  3. 3Chain and default

    a?.b?.c stops at the first nil; ?? supplies a value when the left side is nil.

  4. 4! is a promise

    Force-unwrapping a nil traps the program; implicitly unwrapped T! does that implicitly.

1

How it works

The explanation, with examples you can run.

Getting the value out

struct Address { var city: String? }struct User { var name: String; var address: Address? } func greeting(for user: User?) -> String {    guard let user else { return "Hello, stranger" }        // Swift 5.7 shorthand for guard let user = user    let city = user.address?.city ?? "somewhere"             // chain, then default    return "Hello \(user.name) from \(city)"} print(greeting(for: nil))                                                // Hello, strangerprint(greeting(for: User(name: "Ada", address: Address(city: "London"))))  // Hello Ada from Londonprint(greeting(for: User(name: "Bo", address: nil)))                      // Hello Bo from somewhere if let n = Int("42") { print(n + 1) }                                   // 43
  • if let x / guard let x bind a non-optional name. Since Swift 5.7 you can leave out = x when the names match. guard must leave the scope in its else, and the bound name stays available afterwards, which keeps the main path flat.
  • Optional chaining a?.b?.c() evaluates to nil as soon as one link is nil. The result is always a single optional, however long the chain.
  • Nil-coalescing a ?? b gives the wrapped value or b. The right side is only evaluated when needed.
  • map and flatMap transform the wrapped value: Int(text).map { $0 * 2 }.
  • switch works too, with case .some(let v) or the case let v? pattern, because it is just an enum.

Force-unwrapping

value! returns the wrapped value or traps: the program stops with "Unexpectedly found nil while unwrapping an Optional value". Treat each ! as an assertion that nil is impossible here, and prefer a guard with a useful failure, or precondition, when it really is a programmer error.

Implicitly unwrapped optionals (var label: UILabel!) are optionals that unwrap themselves on use. They exist for values that can't be set in init but are always set before use, such as Interface Builder outlets. Anywhere else, a plain optional or a non-optional with a proper initialiser is safer.

Optionals and Objective-C

APIs from Objective-C come in with nullability from their annotations (nullable, nonnull, or a whole header in NS_ASSUME_NONNULL_BEGIN). Unannotated pointers are imported as implicitly unwrapped optionals, Swift's way of saying "unknown". The idea is close to Kotlin's platform types: the uncertainty sits at the border, so handle it there.

Design tips

Don't make something optional because it is convenient; make it optional because "missing" is a real state. An Optional<Bool> with three states is usually an enum waiting to be written. And Optional of a collection is rarely needed: an empty array usually says the same thing.

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Check yourself

3 questions. Get them all right to finish this topic.

  1. 1.let n = Int("42x"). What is the type and value of n?
  2. 2.What does guard let user else { return } do that if let user { } doesn't?
  3. 3.What is the type of user?.address?.city when city is a String property?

0 of 3 right so far. Get all three to finish this topic.

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