Protocols

Protocols define requirements that types adopt—Swift’s primary abstraction for polymorphism, comparable to interfaces in Java, Kotlin, and C#, but with extension default implementations.

Defining and conforming

protocol Describable {
    var label: String { get }
    func describe() -> String
}

struct User: Describable {
    let label: String
    func describe() -> String { "User: \(label)" }
}

Protocol-oriented programming

Extend protocols with default implementations; compose small protocols instead of deep class hierarchies—core Swift design philosophy.

Important interview questions and answers

1. Q: Protocol vs abstract class?
   A: Protocols work with structs, enums, and classes; support retroactive modeling via extensions; no stored state in the protocol itself.
2. Q: Any vs protocol type?
   A: Existential any Protocol (Swift 5.7+) boxes heterogeneous conformers; generics preserve concrete types at compile time.

Self-check

1. Can structs conform to protocols?
2. What keyword marks protocol adoption?

Tip: Compose small protocols—Swift favors protocol-oriented design over deep inheritance.