Protocol extensions with default implementations in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of protocol extension default method call

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

Swift

protocol Greeter {
    func greet()
}

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

struct Person: Greeter {}

let person = Person()
person.greet()

A Hello from protocol extension

B Compile-time error: Missing implementation of greet()

C Runtime error: Method greet() not found

D No output

💡 Hint

Protocol extensions provide default implementations if the conforming type does not implement the method.

Since Person does not implement greet(), the default implementation in the protocol extension is called, printing the message.

❓ Predict Output

intermediate

Overriding default implementation in protocol extension

What will be printed when greet() is called on dog?

Swift

protocol Greeter {
    func greet()
}

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

struct Dog: Greeter {
    func greet() {
        print("Woof! Hello from Dog")
    }
}

let dog = Dog()
dog.greet()

A Hello from protocol extension

B Compile-time error: Duplicate method implementation

C Woof! Hello from Dog

D No output

💡 Hint

If the conforming type implements the method, it overrides the default from the extension.

Dog provides its own greet() method, so calling greet() on dog uses Dog's implementation.

🔧 Debug

advanced

Why does this code print the protocol extension method instead of the struct's method?

Given the code below, why does calling greeter.greet() print "Hello from protocol extension" instead of "Hi from Cat"?

Swift

protocol Greeter {
}

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

struct Cat: Greeter {
    func greet() {
        print("Hi from Cat")
    }
}

let cat = Cat()
let greeter: Greeter = cat
greeter.greet()

A Because Cat does not implement greet(), the default method is called.

B Because the method is called on a protocol-typed variable, the protocol extension method is used instead of the struct's method.

C Because greet() is a static method, the protocol extension method is always called.

D Because the protocol requires @objc, the method dispatch fails to call Cat's method.

💡 Hint

Consider how Swift dispatches methods when called on protocol-typed variables.

When calling a method on a variable typed as the protocol, Swift uses static dispatch for protocol extension methods, so the default implementation is called instead of the struct's override.

📝 Syntax

advanced

Which option correctly adds a default implementation to a protocol method with parameters?

Given the protocol below, which option correctly provides a default implementation for describe(age:) in a protocol extension?

Swift

protocol Describable {
    func describe(age: Int)
}

A

extension Describable {
    func describe(age: Int) {
        print("Age is \(age)")
    }
}

B

extension Describable {
    func describe() {
        print("Age is unknown")
    }
}

C

extension Describable {
    func describe(age) {
        print("Age is \(age)")
    }
}

D

extension Describable {
    func describe(age: String) {
        print("Age is \(age)")
    }
}

💡 Hint

The method signature in the extension must exactly match the protocol's method signature.

Option A matches the protocol method signature exactly, providing a valid default implementation. Option A has no parameters, C misses the parameter type, and A changes the parameter type, all causing errors or mismatches.

🚀 Application

expert

Using protocol extensions to add default behavior with constraints

Consider the protocol Summable below. Which option correctly uses a protocol extension with a constraint to provide a default implementation of sum() only for arrays of Int?

Swift

protocol Summable {
    associatedtype Element
    func sum() -> Element
}

A

extension Summable {
    func sum() -> Element {
        return 0
    }
}

B

extension Summable where Element == Int {
    func sum() -> Int {
        return self.reduce(0, +)
    }
}

C

extension Summable where Element: Numeric {
    func sum() -> Element {
        return (self as! [Element]).reduce(0, +)
    }
}

D

extension Summable where Element == Int {
    func sum() -> Int {
        return (self as! [Int]).reduce(0, +)
    }
}

💡 Hint

You must constrain the extension to Element == Int and cast self to [Int] to use reduce.

Option D correctly constrains the extension to Element == Int and safely casts self to [Int] to call reduce. Option D provides a generic implementation returning 0, which may not be correct. Option D uses Element: Numeric but casts self to [Element] without guaranteeing self is an array. Option D tries to call reduce directly on self, which is not guaranteed to be a collection.

Practice

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

easy

A. To allow protocols to store properties

B. To create new classes that inherit from protocols

C. To enforce that all conforming types must implement every method

D. To provide default method implementations that all conforming types can use

Solution

1. Step 1: Understand protocol extensions

   Protocol extensions allow adding methods or properties with default behavior to protocols.

2. Step 2: Identify the purpose of default implementations

   Default implementations let conforming types use common code without writing it themselves.

3. Final Answer:

   To provide default method implementations that all conforming types can use -> Option D

4. Quick Check:

   Default implementations = shared code for all [OK]

Hint: Default implementations give shared code to all conforming types [OK]

Common Mistakes:

• Thinking protocols can store properties
• Confusing protocol inheritance with extensions
• Believing all methods must be implemented manually

2. Which of the following is the correct syntax to add a default implementation for a protocol method in Swift?

easy

A. extension MyProtocol { func greet() { print(\"Hello\") } }

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

C. func greet() in MyProtocol { print(\"Hello\") }

D. MyProtocol { func greet() { print(\"Hello\") } }"

Solution

1. Step 1: Recall syntax for protocol extensions

   Default implementations are added inside an extension of the protocol.

2. Step 2: Check each option

   extension MyProtocol { func greet() { print(\"Hello\") } } correctly uses extension MyProtocol { func greet() { ... } }. Others misuse keywords or structure.

3. Final Answer:

   extension MyProtocol { func greet() { print("Hello") } } -> Option A

4. Quick Check:

   Use extension to add defaults [OK]

Hint: Use 'extension ProtocolName' to add default methods [OK]

Common Mistakes:

• Trying to put method bodies inside protocol declaration
• Using 'extension' inside protocol block
• Omitting the 'extension' keyword

3. Consider the code below:

protocol Speaker {
    func speak()
}

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

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

struct Robot: Speaker {}

let p = Person()
p.speak()

let r = Robot()
r.speak()

What is the output when this code runs?

medium

A. Person speaking
Robot speaking

B. Speaking from extension
Speaking from extension

C. Person speaking
Speaking from extension

D. Compilation error

Solution

1. Step 1: Understand method resolution for protocol extensions

   If a conforming type provides its own method, it overrides the default from the extension.

2. Step 2: Analyze the structs

   Person implements speak() so it prints "Person speaking". Robot does not implement it, so uses default "Speaking from extension".

3. Final Answer:

   Person speaking Speaking from extension -> Option C

4. Quick Check:

   Own method overrides default extension [OK]

Hint: Own method overrides default extension method [OK]

Common Mistakes:

• Assuming default is always called
• Expecting Robot to cause error
• Confusing protocol declaration with implementation

4. The following code tries to add a default implementation for describe() in a protocol extension, but it causes a compile error. What is the mistake?

protocol Describable {
    func describe()
}

extension Describable {
    func describe
    {
        print("Default description")
    }
}

medium

A. Protocol cannot have methods without bodies

B. Missing parentheses after method name in extension

C. Extension cannot provide method implementations

D. Missing 'func' keyword in extension

Solution

1. Step 1: Check method syntax in extension

   Method definitions require parentheses even if no parameters are present.

2. Step 2: Identify the error

   The extension method func describe is missing parentheses () after the name.

3. Final Answer:

   Missing parentheses after method name in extension -> Option B

4. Quick Check:

   Method syntax needs () even if empty [OK]

Hint: Always include () after method name in definitions [OK]

Common Mistakes:

• Omitting parentheses in method declaration
• Thinking protocol methods must have bodies
• Believing extensions can't implement methods

5. You want to create a protocol Identifiable with a property id and a default method identify() that prints the id. How would you write the protocol and extension so that conforming types can use the default identify() without extra code?

hard

A. protocol Identifiable { var id: String { get } }\nextension Identifiable { func identify() { print(\"ID: \(id)\") } }

B. protocol Identifiable { func identify() }\nextension Identifiable { var id: String { return \"default\" } }

C. protocol Identifiable { var id: String }\nextension Identifiable { func identify() { print(id) } }

D. protocol Identifiable { var id: String { get set } }\nextension Identifiable { func identify() { print(\"ID: \(id)\") } }"

Solution

1. Step 1: Define protocol with read-only property

   Use var id: String { get } to require an id property that conforming types must provide.

2. Step 2: Provide default identify() method in extension

   Extension adds identify() that prints the id using string interpolation.

3. Final Answer:

   protocol Identifiable { var id: String { get } }\nextension Identifiable { func identify() { print("ID: \(id)") } } -> Option A

4. Quick Check:

   Protocol requires id; extension adds identify() [OK]

Hint: Require property with { get } and add method in extension [OK]

Common Mistakes:

• Using var id: String without { get } or { get set }
• Trying to provide stored properties in protocol extension
• Defining identify() in protocol but not implementing in extension