Protocols

Code

import Foundation

protocol Drawable {
    func draw() -> String
    var area: Double { get }
}

// Conforming type
struct Circle: Drawable {
    let radius: Double
    var area: Double { Double.pi * radius * radius }
    func draw() -> String { "Circle r=\(radius)" }
}

struct Square: Drawable {
    let side: Double
    var area: Double { side * side }
    func draw() -> String { "Square s=\(side)" }
}

// Protocol as existential type
func describe(_ shapes: [any Drawable]) {
    for s in shapes {
        print("\(s.draw()) area=\(s.area)")
    }
}

describe([Circle(radius: 2), Square(side: 3)])

// Protocol extension with default behavior
extension Drawable {
    func describe() -> String { "\(draw()) area=\(area)" }
}

let c = Circle(radius: 1)
print(c.describe())

// Protocol with associated type
protocol Container {
    associatedtype Item
    var count: Int { get }
    mutating func append(_ item: Item)
    subscript(i: Int) -> Item { get }
}

Explanation

Protocols declare a contract of properties and methods that conforming types must implement. Existential types like any Drawable let you store mixed conforming types in a collection. Protocol extensions provide default implementations, giving Swift a form of trait-based composition without inheritance.

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