Protocol conformance in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of protocol conformance with default implementation

What is the output of this Swift code when calling dog.speak()?

Swift

protocol Animal {
    func speak() -> String
}

extension Animal {
    func speak() -> String {
        return "Some sound"
    }
}

struct Dog: Animal {
    func speak() -> String {
        return "Woof!"
    }
}

let dog = Dog()
print(dog.speak())

A "Dog speaks"

B "Some sound"

C Compilation error: Missing method implementation

D "Woof!"

💡 Hint

Check if the struct provides its own implementation of the protocol method.

The struct Dog provides its own implementation of speak(), so that version is called instead of the default from the protocol extension.

❓ Predict Output

intermediate

Protocol conformance with optional method

What will be printed when running this Swift code?

Swift

@objc protocol Greeter {
    @objc optional func greet() -> String
}

class Person: Greeter {
}

let p = Person()
print(p.greet?() ?? "No greeting")

A Runtime error: Method not found

B "Optional greeting"

C "No greeting"

D Compilation error: Optional protocol methods require @objc

💡 Hint

Check if the class implements the optional method or not.

Person does not implement greet(), so calling greet?() returns nil, triggering the nil-coalescing operator to print "No greeting".

🔧 Debug

advanced

Fix the protocol conformance error

This code does not compile. What is the cause of the error?

Swift

protocol Vehicle {
    var speed: Int { get set }
    func description() -> String
}

struct Car: Vehicle {
    var speed: Int
    func description() -> String {
        return "Car speed is \(speed)"
    }
}

let myCar = Car(speed: 100)
print(myCar.description())

A The protocol Vehicle must declare speed as 'let' instead of 'var'.

B The struct Car must mark speed as 'var' to conform to the protocol's get set requirement.

C The struct Car must mark speed as 'let' to conform to the protocol.

D The description() method must be marked as 'mutating' in Car.

💡 Hint

Check the property requirements in the protocol and how Car declares speed.

The protocol requires speed to be gettable and settable ({ get set }). Car declares speed as 'let', which only provides { get }, causing a conformance error.

❓ Predict Output

advanced

Protocol conformance with associated types

What is the output of this Swift code?

Swift

protocol Container {
    associatedtype Item
    var items: [Item] { get set }
    func count() -> Int
}

struct IntContainer: Container {
    var items: [Int]
    func count() -> Int {
        items.count
    }
}

let container = IntContainer(items: [1, 2, 3, 4])
print(container.count())

A 4

B Compilation error: Missing associatedtype implementation

C 0

D Runtime error

💡 Hint

Check how many items are in the array and what count() returns.

IntContainer conforms to Container with Item as Int. The count() method returns the number of items in the array, which is 4.

❓ Predict Output

expert

Protocol conformance and method dispatch

What is the output of this Swift code?

Swift

protocol Speaker {
    func speak() -> String
}

struct Cat: Speaker {
    func speak() -> String {
        "Meow"
    }
}

func makeSpeak(_ speaker: Speaker) {
    print(speaker.speak())
}

let cat = Cat()
makeSpeak(cat)

A "Meow"

B "speak()"

C Compilation error: Protocol method not implemented

D Runtime error: Protocol witness table missing

💡 Hint

Check if Cat implements the required method and how protocol methods are called.

Cat implements speak(), so when passed as Speaker, calling speak() prints "Meow".

Practice

1. What does it mean for a Swift type to conform to a protocol?

easy

A. It can only be a class, not a struct or enum.

B. It inherits all properties and methods from the protocol automatically.

C. It implements all required methods and properties defined by the protocol.

D. It must override all methods from the protocol.

Solution

1. Step 1: Understand protocol requirements

   A protocol defines methods and properties that a type must implement to conform.

2. Step 2: Check what conformance means

   Conforming means the type provides implementations for all required members.

3. Final Answer:

   It implements all required methods and properties defined by the protocol. -> Option C

4. Quick Check:

   Protocol conformance = Implement required members [OK]

Hint: Conformance means implementing all required protocol parts [OK]

Common Mistakes:

• Thinking conformance means inheritance
• Believing only classes can conform
• Assuming methods must be overridden

2. Which of the following is the correct syntax to declare that a struct conforms to a protocol named Drawable?

easy

A. struct Shape inherits Drawable { }

B. struct Shape : Drawable { }

C. struct Shape implements Drawable { }

D. struct Shape extends Drawable { }

Solution

1. Step 1: Recall Swift syntax for protocol conformance

   Swift uses a colon (:) after the type name to declare protocol conformance.

2. Step 2: Match syntax options

   Only struct Shape : Drawable { } uses the correct colon syntax for protocol conformance.

3. Final Answer:

   struct Shape : Drawable { } -> Option B

4. Quick Check:

   Protocol conformance syntax uses ':' [OK]

Hint: Use ':' after type name to conform to protocol [OK]

Common Mistakes:

• Using 'implements' or 'inherits' keywords
• Confusing inheritance with protocol conformance
• Using 'extends' which is not Swift syntax

3. What will be the output of this Swift code?

protocol Greetable {
    func greet() -> String
}

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

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

medium

A. Hello!

B. Error: Missing greet() implementation

C. nil

D. Hello! Person

Solution

1. Step 1: Check protocol and struct conformance

   The protocol requires a greet() method returning String. Person implements greet() returning "Hello!".

2. Step 2: Analyze print output

   Calling p.greet() returns "Hello!", so print outputs that string.

3. Final Answer:

   Hello! -> Option A

4. Quick Check:

   Implemented greet() returns "Hello!" [OK]

Hint: Implemented method returns string printed directly [OK]

Common Mistakes:

• Assuming missing implementation error
• Expecting default protocol method output
• Confusing print output with type description

4. Identify the error in this Swift code that tries to conform to the Equatable protocol:

struct Point: Equatable {
    var x: Int
    var y: Int
}

medium

A. Missing implementation of == operator function

B. Struct cannot conform to Equatable

C. Properties must be constants (let) to conform

D. No error, code compiles fine

Solution

1. Step 1: Understand Equatable conformance rules

   Swift automatically synthesizes == for structs with Equatable properties.

2. Step 2: Check struct properties and conformance

   Both x and y are Int (Equatable), so compiler provides == automatically.

3. Final Answer:

   No error, code compiles fine -> Option D

4. Quick Check:

   Automatic Equatable synthesis = No error [OK]

Hint: Swift auto-synthesizes Equatable for simple structs [OK]

Common Mistakes:

• Thinking manual == is always needed
• Believing structs can't conform
• Assuming properties must be let

5. Given this protocol and struct, how can you modify Vehicle to conform to CustomStringConvertible and print a description?

protocol CustomStringConvertible {
    var description: String { get }
}

struct Vehicle {
    var make: String
    var model: String
}

hard

A. Add extension Vehicle: CustomStringConvertible { var description: String { "\(make) \(model)" } }

B. Add func description() -> String { return "\(make) \(model)" } inside Vehicle

C. Add var description: String = "\(make) \(model)" inside Vehicle

D. Add override var description: String { "\(make) \(model)" } inside Vehicle

Solution

1. Step 1: Understand protocol property requirement

   CustomStringConvertible requires a read-only computed property 'description' of type String.

2. Step 2: Implement description correctly

   Implementing description as a computed property in an extension is the correct way.

3. Final Answer:

   Add extension Vehicle: CustomStringConvertible { var description: String { "\(make) \(model)" } } -> Option A

4. Quick Check:

   Computed property in extension = Correct conformance [OK]

Hint: Use computed property in extension for protocol conformance [OK]

Common Mistakes:

• Trying to add description as a method
• Using stored property instead of computed
• Using override keyword in struct