Protocol declaration syntax in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple protocol conformance

What is the output of this Swift code when calling greet() on person?

Swift

protocol Greeter {
    func greet() -> String
}

struct Person: Greeter {
    func greet() -> String {
        return "Hello!"
    }
}

let person = Person()
print(person.greet())

A No output

B "Hello!"

C "Hi!"

D Error: Protocol method not implemented

💡 Hint

Check how the greet() method is implemented in Person.

The Person struct conforms to the Greeter protocol by implementing greet() returning "Hello!". So calling greet() prints "Hello!".

❓ Predict Output

intermediate

Protocol with property requirement output

What will be printed by this Swift code?

Swift

protocol Identifiable {
    var id: String { get }
}

struct User: Identifiable {
    var id: String
}

let user = User(id: "abc123")
print(user.id)

A nil

B Error: Missing property implementation

C "id"

D "abc123"

💡 Hint

Look at how id is defined and initialized in User.

The User struct implements the id property required by Identifiable. The instance is created with id = "abc123", so printing user.id outputs "abc123".

🔧 Debug

advanced

Identify the syntax error in protocol declaration

Which option contains the syntax error in this protocol declaration?

Swift

protocol Vehicle {
    var speed: Int
    func drive()
}

A The property <code>speed</code> must specify { get } or { get set }

B The function <code>drive()</code> must have a return type

C Protocols cannot have properties

D The protocol keyword is misspelled

💡 Hint

Check how properties are declared in protocols.

In Swift protocols, properties must specify whether they are readable ({ get }) or readable and writable ({ get set }). Missing this causes a syntax error.

🧠 Conceptual

advanced

Protocol inheritance syntax

Which option correctly declares a protocol FlyingCar that inherits from protocols Car and Flyable?

A protocol FlyingCar: Car, Flyable {}

B protocol FlyingCar inherits Car, Flyable {}

C protocol FlyingCar <- Car, Flyable {}

D protocol FlyingCar extends Car & Flyable {}

💡 Hint

Look at how Swift declares protocol inheritance using colons and commas.

Swift uses a colon after the protocol name followed by a comma-separated list of inherited protocols.

❓ Predict Output

expert

Output of protocol extension method call

What is the output of this Swift code?

Swift

protocol Speaker {
    func speak() -> String
}

extension Speaker {
    func speak() -> String {
        return "Hello from extension"
    }
}

struct Robot: Speaker {
    func speak() -> String {
        return "Hello from Robot"
    }
}

struct Alien: Speaker {}

let r = Robot()
let a = Alien()
print(r.speak())
print(a.speak())

A "Hello from Robot"\nCompilation error

B Compilation error: Alien does not implement speak()

C "Hello from Robot"\n"Hello from extension"

D "Hello from extension"\n"Hello from extension"

💡 Hint

Check how protocol extensions provide default implementations.

The Robot struct implements speak(), so its version is called. The Alien struct does not implement speak(), so it uses the default from the protocol extension.