Protocol conformance in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of protocol conformance with default implementation

What is the output of this Swift code when calling dog.speak()?

Swift

protocol Animal {
    func speak() -> String
}

extension Animal {
    func speak() -> String {
        return "Some sound"
    }
}

struct Dog: Animal {
    func speak() -> String {
        return "Woof!"
    }
}

let dog = Dog()
print(dog.speak())

A "Dog speaks"

B "Some sound"

C Compilation error: Missing method implementation

D "Woof!"

💡 Hint

Check if the struct provides its own implementation of the protocol method.

The struct Dog provides its own implementation of speak(), so that version is called instead of the default from the protocol extension.

❓ Predict Output

intermediate

Protocol conformance with optional method

What will be printed when running this Swift code?

Swift

@objc protocol Greeter {
    @objc optional func greet() -> String
}

class Person: Greeter {
}

let p = Person()
print(p.greet?() ?? "No greeting")

A Runtime error: Method not found

B "Optional greeting"

C "No greeting"

D Compilation error: Optional protocol methods require @objc

💡 Hint

Check if the class implements the optional method or not.

Person does not implement greet(), so calling greet?() returns nil, triggering the nil-coalescing operator to print "No greeting".

🔧 Debug

advanced

Fix the protocol conformance error

This code does not compile. What is the cause of the error?

Swift

protocol Vehicle {
    var speed: Int { get set }
    func description() -> String
}

struct Car: Vehicle {
    var speed: Int
    func description() -> String {
        return "Car speed is \(speed)"
    }
}

let myCar = Car(speed: 100)
print(myCar.description())

A The protocol Vehicle must declare speed as 'let' instead of 'var'.

B The struct Car must mark speed as 'var' to conform to the protocol's get set requirement.

C The struct Car must mark speed as 'let' to conform to the protocol.

D The description() method must be marked as 'mutating' in Car.

💡 Hint

Check the property requirements in the protocol and how Car declares speed.

The protocol requires speed to be gettable and settable ({ get set }). Car declares speed as 'let', which only provides { get }, causing a conformance error.

❓ Predict Output

advanced

Protocol conformance with associated types

What is the output of this Swift code?

Swift

protocol Container {
    associatedtype Item
    var items: [Item] { get set }
    func count() -> Int
}

struct IntContainer: Container {
    var items: [Int]
    func count() -> Int {
        items.count
    }
}

let container = IntContainer(items: [1, 2, 3, 4])
print(container.count())

A 4

B Compilation error: Missing associatedtype implementation

C 0

D Runtime error

💡 Hint

Check how many items are in the array and what count() returns.

IntContainer conforms to Container with Item as Int. The count() method returns the number of items in the array, which is 4.

❓ Predict Output

expert

Protocol conformance and method dispatch

What is the output of this Swift code?

Swift

protocol Speaker {
    func speak() -> String
}

struct Cat: Speaker {
    func speak() -> String {
        "Meow"
    }
}

func makeSpeak(_ speaker: Speaker) {
    print(speaker.speak())
}

let cat = Cat()
makeSpeak(cat)

A "Meow"

B "speak()"

C Compilation error: Protocol method not implemented

D Runtime error: Protocol witness table missing

💡 Hint

Check if Cat implements the required method and how protocol methods are called.

Cat implements speak(), so when passed as Speaker, calling speak() prints "Meow".