Swift

Optionals

• ? makes absence explicit in the type — String? vs String.
• if let / guard let unwrap safely. guard exits scope immediately — prefer it for preconditions.
• ?? provides a default for nil. Never force-unwrap (!) in production unless the nil case is a programmer error that should crash loudly.

guard let email = user.email else { return }
let display = nickname ?? username

Value vs reference types

• Default to struct — value semantics, copy-on-write, no shared-mutation bugs.
• Use class only when identity matters (reference equality, inheritance), or when Objective-C interop requires it.
• Use actor to encapsulate mutable state that must be safe to access from multiple async tasks.