Protocol conformance via extension in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of protocol conformance via extension

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

Swift

protocol Greetable {
    func greet() -> String
}

struct Person {}

extension Person: Greetable {
    func greet() -> String {
        return "Hello from extension!"
    }
}

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

A Hello from extension!

B Hello from Person struct!

C Compilation error: 'Person' does not conform to protocol 'Greetable'

D Runtime error: method not found

💡 Hint

Think about how Swift allows adding protocol conformance in extensions.

In Swift, you can make a type conform to a protocol by implementing the required methods inside an extension. Here, Person conforms to Greetable via the extension, so calling greet() returns the string defined there.

❓ Predict Output

intermediate

Protocol method call with extension conformance

What will be printed when running this Swift code?

Swift

protocol Describable {
    func describe() -> String
}

struct Car {}

extension Car: Describable {
    func describe() -> String {
        "A fast car"
    }
}

func printDescription(_ item: Describable) {
    print(item.describe())
}

let myCar = Car()
printDescription(myCar)

A Compilation error: Cannot convert value of type 'Car' to expected argument type 'Describable'

B Empty string

C Runtime error: method describe() not found

D A fast car

💡 Hint

Check how the extension adds protocol conformance to Car.

The extension makes Car conform to Describable by implementing describe(). Passing myCar to printDescription works because Car conforms to Describable. The output is the string returned by describe().

🔧 Debug

advanced

Why does this protocol conformance fail?

Given this Swift code, why does it fail to compile?

Swift

protocol Runnable {
    func run() -> String
}

struct Animal {}

extension Animal: Runnable {}

extension Animal {
    func run() -> String {
        "Running fast"
    }
}

let a = Animal()
print(a.run())

A No error, prints Running fast

B Compilation error because protocol conformance requires the method inside the extension that declares conformance

C Compilation error because Animal already has run() method in extension

D Runtime error because method run() is not implemented

💡 Hint

Check where the protocol method is implemented relative to the conformance declaration.

In Swift, when you declare protocol conformance in an extension, the required methods must be implemented in the same extension. Here, run() is implemented in a separate extension, so the compiler does not see it as fulfilling the protocol requirement, causing a compilation error.

📝 Syntax

advanced

Identify the syntax error in protocol conformance via extension

Which option contains the syntax error preventing this Swift code from compiling?

Swift

protocol Flyable {
    func fly() -> String
}

struct Bird {}

extension Bird: Flyable {
    func fly() -> String {
        return "Flying high"
    }
}

let b = Bird()
print(b.fly())

A extension Bird: Flyable { func fly() -> String { return "Flying high" }

B extension Bird: Flyable { func fly() -> String { "Flying high" } }

C extension Bird: Flyable { func fly() -> String { return "Flying high" } }

D extension Bird: Flyable { func fly() -> String { print("Flying high") } }

💡 Hint

Look for missing braces or incomplete blocks.

Option A is missing the closing brace for the extension, causing a syntax error. Option A returns void from a function expecting String, causing a type error. Options A and B are valid syntax; B uses implicit return which is allowed.

🚀 Application

expert

How many methods must be implemented for protocol conformance via extension?

Given this protocol and struct, how many methods must be implemented inside the extension to make Robot conform to Worker?

Swift

protocol Worker {
    func start() -> String
    func stop() -> String
}

struct Robot {}

// Extension to make Robot conform to Worker goes here

A No methods, just declare conformance in extension

B One method: either start() or stop()

C Two methods: start() and stop()

D Three methods: start(), stop(), and reset()

💡 Hint

Protocols require all their methods to be implemented to conform.

The protocol Worker requires two methods: start() and stop(). To conform, the extension must implement both methods. Declaring conformance without implementing methods causes a compile error.