Protocol inheritance in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of protocol inheritance with default implementation

What is the output of this Swift code using protocol inheritance and default implementations?

Swift

protocol A {
    func greet() -> String
}

protocol B: A {
    func greet() -> String
}

extension A {
    func greet() -> String {
        return "Hello from A"
    }
}

extension B {
    func greet() -> String {
        return "Hello from B"
    }
}

struct MyStruct: B {}

let obj = MyStruct()
print(obj.greet())

A Compilation error: Ambiguous method

B Hello from A

C Hello from B

D Runtime error

💡 Hint

Remember that protocol inheritance allows overriding default implementations in child protocols.

The struct MyStruct conforms to protocol B, which inherits from A. Both A and B provide default implementations of greet(). The implementation in B overrides the one in A, so calling greet() on MyStruct uses B's version.

❓ Predict Output

intermediate

Value of property from multiple inherited protocols

What is the value of the property 'description' when accessed from an instance of struct C?

Swift

protocol P1 {
    var description: String { get }
}

protocol P2: P1 {
    var description: String { get }
}

extension P1 {
    var description: String { "P1" }
}

extension P2 {
    var description: String { "P2" }
}

struct C: P2 {}

let c = C()
print(c.description)

A P2

B Compilation error: Protocol requirement not satisfied

C Runtime error

D P1

💡 Hint

Check which protocol's extension provides the property implementation that is used.

Since P2 inherits from P1 and provides its own default implementation of description, the struct C uses P2's version.

🔧 Debug

advanced

Identify the error in protocol inheritance with conflicting requirements

What error does this Swift code produce?

Swift

protocol Alpha {
    func doSomething() -> Int
}

protocol Beta {
    func doSomething() -> String
}

protocol Gamma: Alpha, Beta {}

struct Test: Gamma {
    func doSomething() -> Int {
        return 42
    }
}

A Type 'Test' does not conform to protocol 'Beta'

B Type 'Test' does not conform to protocol 'Alpha'

C No error, compiles successfully

D Ambiguous use of 'doSomething()'

💡 Hint

Check the return types required by each protocol and what the struct implements.

Protocol Beta requires doSomething() returning String, but Test only implements the version returning Int, so it does not conform to Beta.

📝 Syntax

advanced

Correct syntax for protocol inheritance declaration

Which option shows the correct syntax to declare a protocol 'C' that inherits from protocols 'A' and 'B'?

A protocol C inherits A, B {}

B protocol C: A, B {}

C protocol C extends A, B {}

D protocol C < A, B > {}

💡 Hint

Swift uses a colon ':' to declare protocol inheritance.

In Swift, protocol inheritance is declared using a colon followed by a comma-separated list of protocols.

🚀 Application

expert

Determine the number of methods required by a conforming type

Given these protocols and extensions, how many methods must a struct implement to conform to protocol Z?

Swift

protocol X {
    func method1()
}

protocol Y: X {
    func method2()
}

protocol Z: Y {
    func method3()
}

extension X {
    func method1() { print("X method1") }
}

extension Y {
    func method2() { print("Y method2") }
}

// No extension for Z

struct MyStruct: Z {
    func method3() { print("Z method3") }
}

A 0

B 2

C 3

D 1

💡 Hint

Check which methods have default implementations and which do not.

method1 and method2 have default implementations in protocol extensions X and Y. Only method3 lacks a default and must be implemented.