Protocol extensions for shared behavior in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of protocol extension method call

What is the output of this Swift code using protocol extensions?

Swift

protocol Greetable {
    func greet() -> String
}

extension Greetable {
    func greet() -> String {
        return "Hello from protocol extension"
    }
}

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

struct Robot: Greetable {}

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

A Compilation error: Robot does not implement greet()

B

Hello from Person
Hello from protocol extension

C

Hello from protocol extension
Hello from protocol extension

D

Hello from Person
Hello from Person

💡 Hint

Remember that structs can override protocol extension methods by implementing them directly.

Person implements greet(), so its method is called. Robot does not implement greet(), so it uses the protocol extension's default implementation.

🧠 Conceptual

intermediate

Protocol extension default implementation usage

Which statement about protocol extensions providing default implementations is true?

A Default implementations are ignored if the protocol is used as a type.

B Default implementations override any implementation in the conforming type.

C Protocol extensions cannot provide default implementations for protocol methods.

D Default implementations in protocol extensions are used only if the conforming type does not provide its own implementation.

💡 Hint

Think about how Swift chooses which method to call when both protocol extension and conforming type have the same method.

If a conforming type implements a method, that implementation is used. Otherwise, the protocol extension's default is used.

🔧 Debug

advanced

Why does this protocol extension method not get called?

Given this code, why does the protocol extension method not get called when using a protocol-typed variable?

Swift

protocol Drawable {
    func draw()
}

extension Drawable {
    func draw() {
        print("Drawing from extension")
    }
}

struct Circle: Drawable {
    func draw() {
        print("Drawing Circle")
    }
}

let shape: Drawable = Circle()
shape.draw()

A The Circle's draw() method is called because the variable is of protocol type and Circle implements draw().

B The protocol extension's draw() method is called because the variable is of protocol type.

C Compilation error because protocol extension methods cannot be called on protocol-typed variables.

D Runtime error because draw() is ambiguous.

💡 Hint

Consider how Swift dispatches protocol methods when called on a variable typed as the protocol.

When a protocol method is called on a variable typed as the protocol, Swift uses dynamic dispatch and calls the conforming type's implementation if it exists.

📝 Syntax

advanced

Which option causes a syntax error in protocol extension?

Which of the following protocol extension code snippets will cause a syntax error?

A

extension Sequence {
    func firstElement() -> Element? {
        return self.first
    }
}

B

extension Collection {
    func isNotEmpty() -> Bool {
        return !self.isEmpty
    }
}

C

extension Array {
    func reversedArray() -> Array {
        return self.reversed()
    }
}

D

extension Dictionary {
    func keysArray() -> [Key] {
        return Array(self.keys)
    }
}

💡 Hint

Check the return type and method used in the extension carefully.

The reversed() method returns a ReversedCollection>, not an Array. Returning it as Array without conversion causes a type mismatch error.

🚀 Application

expert

How many items are in the resulting dictionary?

Consider this Swift code using protocol extensions and conforming types. How many key-value pairs does the dictionary contain after execution?

Swift

protocol Identifiable {
    var id: String { get }
}

extension Identifiable {
    func info() -> String {
        return "ID: \(id)"
    }
}

struct User: Identifiable {
    var id: String
}

struct Admin: Identifiable {
    var id: String
    func info() -> String {
        return "Admin ID: \(id)"
    }
}

let users: [Identifiable] = [User(id: "u1"), Admin(id: "a1"), User(id: "u2")]

var dict = [String: String]()
for user in users {
    dict[user.id] = user.info()
}

print(dict.count)

A 3

B 2

C Compilation error

D 1

💡 Hint

Each user has a unique id used as dictionary key.

The array has three Identifiable items with unique ids. The loop adds all three to the dictionary, so count is 3.

Practice

1. What is the main purpose of using protocol extensions in Swift?

easy

A. To store data inside protocols

B. To add shared behavior to multiple types without repeating code

C. To create new classes that inherit from protocols

D. To prevent types from conforming to protocols

Solution

1. Step 1: Understand protocol extensions

   Protocol extensions allow adding default method or property implementations to protocols.

2. Step 2: Identify the benefit

   This lets many types share behavior without rewriting the same code.

3. Final Answer:

   To add shared behavior to multiple types without repeating code -> Option B

4. Quick Check:

   Protocol extensions = shared behavior [OK]

Hint: Protocol extensions add default behavior to many types [OK]

Common Mistakes:

• Thinking protocols can store data
• Confusing protocol extensions with inheritance
• Believing protocol extensions prevent conformance

2. Which of the following is the correct syntax to add a default method greet() to a protocol Greeter using an extension?

easy

A. extension Greeter: greet() { print("Hello") }

B. protocol Greeter { extension func greet() { print("Hello") } }

C. extension Greeter { func greet() { print("Hello") } }

D. protocol Greeter { func greet() { print("Hello") } }

Solution

1. Step 1: Recall protocol extension syntax

   To add default behavior, use extension ProtocolName { func method() { ... } }.

2. Step 2: Check each option

   extension Greeter { func greet() { print("Hello") } } correctly uses extension Greeter { func greet() { ... } }. Others misuse protocol or extension syntax.

3. Final Answer:

   extension Greeter { func greet() { print("Hello") } } -> Option C

4. Quick Check:

   Extension syntax = extension ProtocolName { func ... } [OK]

Hint: Use 'extension ProtocolName { func ... }' for default methods [OK]

Common Mistakes:

• Putting method inside protocol body with implementation
• Using 'extension' keyword inside protocol declaration
• Trying to add method with colon syntax

3. Consider this Swift code:

protocol Vehicle {
    func start()
}

extension Vehicle {
    func start() {
        print("Starting vehicle")
    }
}

struct Car: Vehicle {}

let myCar = Car()
myCar.start()

What will be printed when this code runs?

medium

A. Starting vehicle

B. No output

C. Compile error: missing start() implementation

D. Runtime error

Solution

1. Step 1: Understand protocol and extension

   The protocol Vehicle requires a start() method. The extension provides a default implementation.

2. Step 2: Check struct Car conformance

   Car conforms to Vehicle but does not implement start(), so it uses the default from the extension.

3. Final Answer:

   Starting vehicle -> Option A

4. Quick Check:

   Default method runs if not overridden [OK]

Hint: Default extension methods run if type doesn't override [OK]

Common Mistakes:

• Expecting compile error for missing method
• Thinking no output will print
• Confusing runtime error with missing implementation

4. This code tries to add a default method to a protocol but causes an error:

protocol Speaker {
    func speak()
}

extension Speaker {
    func speak() {
        print("Speaking")
    }
}

struct Person: Speaker {
    func speak() {
        print("Hello")
    }
}

let p = Person()
p.speak()

What is the error and how to fix it?

medium

A. No error; output is 'Hello'

B. Error: protocol method cannot have default implementation; remove extension

C. Error: struct must mark speak() as 'override'; add 'override' keyword

D. Error: protocol must declare speak() as optional

Solution

1. Step 1: Analyze protocol and extension

   Protocol Speaker requires speak(). Extension provides default implementation.

2. Step 2: Check struct Person implementation

   Person implements speak(), overriding the default. This is allowed and correct.

3. Final Answer:

   No error; output is 'Hello' -> Option A

4. Quick Check:

   Override default method by implementing in type [OK]

Hint: Implementing method in type overrides default extension [OK]

Common Mistakes:

• Thinking default methods cause errors
• Trying to use 'override' keyword in struct
• Believing protocol methods must be optional

5. You want to add a shared method describe() to multiple unrelated types Cat and Dog without repeating code. Which approach using protocol extensions is best?

hard

A. Add a global function describe() that takes Any type

B. Add describe() method separately in Cat and Dog structs

C. Use class inheritance from a base class with describe() method

D. Create a protocol Animal with describe() and add default implementation in extension; make Cat and Dog conform

Solution

1. Step 1: Identify shared behavior need

   Both Cat and Dog need the same describe() method without repeating code.

2. Step 2: Use protocol with extension

   Define protocol Animal with describe(), then provide default describe() in protocol extension. Make Cat and Dog conform to Animal.

3. Final Answer:

   Create a protocol Animal with describe() and add default implementation in extension; make Cat and Dog conform -> Option D

4. Quick Check:

   Protocol extension = shared behavior for unrelated types [OK]

Hint: Use protocol + extension for shared methods across types [OK]

Common Mistakes:

• Repeating code in each type
• Using class inheritance when types are unrelated
• Using global functions losing type safety