Answer: Protocol extensions let you add default implementations to protocol methods, so conforming types get behavior for free without inheriting from a base class.

Code Example:

protocol Greetable {
    var name: String { get }
    func greet() -> String
}

extension Greetable {
    func greet() -> String {
        return "Hello, \(name)!"
    }
}

struct User: Greetable {
    var name: String
    // greet() is provided for free by the extension
}

let user = User(name: "Mia")
print(user.greet()) // "Hello, Mia!"

Key Points:

• Enables composition over inheritance
• Multiple protocols can be adopted (vs single class inheritance)
• Constrained extensions let you add methods only for specific types