class-only protocols

.swift

import Foundation

// Define a class-only protocol conforming 'Vehicle' to 'AnyObject'.
protocol Vehicle: AnyObject {
    var numberOfWheels: Int { get }
    func drive()
}

// Define a class that adopts the protocol.
class Car: Vehicle {
    var numberOfWheels = 4
    func drive() {
        print("Driving a car with \(numberOfWheels) wheels.")
    }
}
// NOTE: If you convert 'Car' in a struct the compiler will give you an error 
// because the protocol 'Vehicle' is a class-only protocol (AnyObject).

// Create an instance of the Car class and call the methods
let myCar = Car()
myCar.drive()