Conditional conformance in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of conditional conformance with Equatable

What is the output of this Swift code using conditional conformance for Equatable?

Swift

struct Box<T> {
    let value: T
}
extension Box: Equatable where T: Equatable {
    static func == (lhs: Box<T>, rhs: Box<T>) -> Bool {
        return lhs.value == rhs.value
    }
}

let box1 = Box(value: 5)
let box2 = Box(value: 5)
let box3 = Box(value: 10)

print(box1 == box2)
print(box1 == box3)

A false\nfalse

B true\nfalse

C true\ntrue

D Compilation error

💡 Hint

Think about how conditional conformance allows Box to be Equatable only if T is Equatable.

The Box struct conforms to Equatable only when its generic type T conforms to Equatable. Since Int is Equatable, Box is Equatable. box1 and box2 have the same value 5, so they are equal. box1 and box3 have different values, so they are not equal.

❓ Predict Output

intermediate

Output of conditional conformance with CustomStringConvertible

What will this Swift code print when using conditional conformance for CustomStringConvertible?

Swift

struct Wrapper<T> {
    let item: T
}
extension Wrapper: CustomStringConvertible where T: CustomStringConvertible {
    var description: String {
        return "Wrapper contains: \(item.description)"
    }
}

struct Person: CustomStringConvertible {
    let name: String
    var description: String { name }
}

let p = Person(name: "Alice")
let w = Wrapper(item: p)
print(w.description)

A Wrapper contains: Alice

B Wrapper contains: Person

C Compilation error

D Wrapper contains: Optional("Alice")

💡 Hint

Check how the description property is used in the extension.

The Wrapper struct conforms to CustomStringConvertible only if T does. Person conforms and returns its name as description. So Wrapper's description returns "Wrapper contains: Alice".

❓ Predict Output

advanced

Output of conditional conformance with Codable

What is the output of this Swift code that uses conditional conformance for Codable?

Swift

struct Container<T> {
    let content: T
}
extension Container: Codable where T: Codable {}

struct Item: Codable {
    let id: Int
    let name: String
}

let item = Item(id: 1, name: "Book")
let container = Container(content: item)

let encoder = JSONEncoder()
if let jsonData = try? encoder.encode(container),
   let jsonString = String(data: jsonData, encoding: .utf8) {
    print(jsonString)
} else {
    print("Encoding failed")
}

A Encoding failed

B {"id":1,"name":"Book"}

C {"content":{"id":1,"name":"Book"}}

D Compilation error

💡 Hint

Look at how Container encodes its content property.

Container conforms to Codable only if T does. Item conforms to Codable, so Container can be encoded. The JSON string represents the container with a content key holding the item data.

❓ Predict Output

advanced

Output of conditional conformance with Sequence and Collection

What will this Swift code print when using conditional conformance for Collection?

Swift

struct MyArray<T> {
    var elements: [T]
}
extension MyArray: Collection where T: Equatable {
    typealias Index = Int
    var startIndex: Int { elements.startIndex }
    var endIndex: Int { elements.endIndex }
    subscript(position: Int) -> T { elements[position] }
    func index(after i: Int) -> Int { elements.index(after: i) }
}

let arr = MyArray(elements: [1, 2, 3])
print(arr.contains(2))

A true

B Runtime error

C Compilation error

D false

💡 Hint

Check how Collection conformance enables contains method.

MyArray conforms to Collection only if T is Equatable. Since Int is Equatable, MyArray is a Collection and has contains method. The array contains 2, so it prints true.

❓ Predict Output

expert

Output of nested conditional conformance with Optional and Equatable

What is the output of this Swift code using nested conditional conformance with Optional and Equatable?

Swift

extension Optional: Equatable where Wrapped: Equatable {
    public static func == (lhs: Optional<Wrapped>, rhs: Optional<Wrapped>) -> Bool {
        switch (lhs, rhs) {
        case (.some(let l), .some(let r)):
            return l == r
        case (.none, .none):
            return true
        default:
            return false
        }
    }
}

let a: Int? = 5
let b: Int? = 5
let c: Int? = nil

print(a == b)
print(a == c)
print(c == nil)

A Compilation error

B false\nfalse\nfalse

C true\ntrue\nfalse

D true\nfalse\ntrue

💡 Hint

Consider how Optional Equatable compares values and nil.

Optional conforms to Equatable only if Wrapped does. Int is Equatable, so Optional is Equatable. Two optionals with same value are equal, optional with value and nil are not equal, and nil equals nil.