ProtocolDispatch

Protocol Dispatch

Difficulty: medium

Problem Statement

Consider the following type hierarchy

protocol P {
    func a() -> String
}

extension P {
    func a() -> String { return "P.a" }
    func b() -> String { return "P.b" }
}

struct S: P {
    func a() -> String { return "S.a" }
    func b() -> String { return "S.b" }
}

class C: P {
    init() {}
    func a() -> String { return "C.a" }
    func b() -> String { return "C.b" }
}

What is the result of running the following functions?

func challenge1() -> (a: String, b: String) {
    let obj = S()
    return (a: obj.a(), b: obj.b())
}

func challenge2() -> (a: String, b: String) {
    let obj = C()
    return (a: obj.a(), b: obj.b())
}

func challenge3() -> (a: String, b: String) {
    let obj: P = S()
    return (a: obj.a(), b: obj.b())
}

func challenge4() -> (a: String, b: String) {
    let obj: P = C()
    return (a: obj.a(), b: obj.b())
}

func challenge5() -> (a: String, b: String) {
    func challenge<T: P>(_ obj: T) -> (a: String, b: String) {
        return (a: obj.a(), b: obj.b())
    }
    return challenge(S())
}

func challenge6() -> (a: String, b: String) {
    func challenge<T: P>(_ obj: T) -> (a: String, b: String) {
        return (a: obj.a(), b: obj.b())
    }
    return challenge(C())
}

func challenge7() -> (a: String, b: String) {
    func challenge<T: C>(_ obj: T) -> (a: String, b: String) {
        return (a: obj.a(), b: obj.b())
    }
    return challenge(C())
}

For each challengeX function write a solutionX function returning the result you expect.

func solution1() -> (a: String, b: String) {
    // ?
}

solution1().a == challenge1().a // true
solution1().b == challenge1().b // true

Further Instructions

To start working on this challenge open Challenge.swift and uncomment the code skeletons.

To run unit tests that validate your code, uncomment the body of the test methods in ChallengeTests.swift and hit CMD + U in Xcode. On Linux you can run the tests by executing swift test in the package directory.

To view the solutions open Solutions.swift.