Protocol extensions with default implementations in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a protocol named Printable.

Swift

protocol [1] {
    func printDescription()
}

Drag options to blanks, or click blank then click option'

ADescribable

BPrintableProtocol

CPrintableExtension

DPrintable

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different protocol name than 'Printable'.

Forgetting the 'protocol' keyword.

The protocol is named Printable to match the requirement.

2fill in blank

medium

Complete the code to add a default implementation for printDescription() in a protocol extension.

Swift

extension Printable {
    func [1]() {
        print("Default description")
    }
}

Drag options to blanks, or click blank then click option'

AshowDescription

Bdescribe

CprintDescription

Ddisplay

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different method name than the protocol requires.

Omitting the method implementation.

The method name must match the protocol requirement: printDescription().

3fill in blank

hard

Fix the error in the struct that conforms to Printable but does not implement printDescription().

Swift

struct Document: Printable {
    var title: String

    func [1]() {
        print("Document title: \(title)")
    }
}

Drag options to blanks, or click blank then click option'

AprintDescription

Bdescribe

CshowTitle

DdisplayTitle

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different method name than the protocol requires.

Not implementing the required method or relying on a wrong name.

The struct must implement printDescription() or use the default implementation.

4fill in blank

hard

Fill both blanks to create a protocol extension that adds a new method and a default implementation.

Swift

extension Printable {
    func [1]() {
        print("Extra info")
    }

    func [2]() {
        print("Default description")
    }
}

Drag options to blanks, or click blank then click option'

AextraInfo

BprintDescription

CshowInfo

Ddescribe

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing method names or using names not matching the protocol.

Not providing default implementation for the protocol method.

The new method is named extraInfo() and the default implementation for the protocol method is printDescription().

5fill in blank

hard

Fill all three blanks to create a struct that uses the default implementation and calls the extra method.

Swift

struct Report: Printable {
    var name: String

    func [1]() {
        print("Report name: \(name)")
    }

    func show() {
        [2]()
        [3]()
    }
}

Drag options to blanks, or click blank then click option'

AprintDescription

BextraInfo

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong method names in the struct.

Not calling the methods correctly inside 'show()'.

The struct overrides printDescription() and calls it and extraInfo() inside show().

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