Protocol composition in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of protocol composition with method calls

What is the output of the following Swift code using protocol composition?

Swift

protocol A {
    func greet() -> String
}

protocol B {
    func farewell() -> String
}

struct Person: A, B {
    func greet() -> String { "Hello" }
    func farewell() -> String { "Goodbye" }
}

func communicate(with entity: A & B) {
    print(entity.greet())
    print(entity.farewell())
}

let p = Person()
communicate(with: p)

A Goodbye

B Hello

C Hello\nGoodbye

D Compilation error

💡 Hint

Remember that protocol composition requires the instance to conform to all protocols listed.

The function communicate accepts an argument conforming to both protocols A and B. Person conforms to both, so both greet() and farewell() methods are called, printing "Hello" and "Goodbye".

❓ Predict Output

intermediate

Result of accessing properties in protocol composition

What will be printed when running this Swift code that uses protocol composition with properties?

Swift

protocol X {
    var xValue: Int { get }
}

protocol Y {
    var yValue: Int { get }
}

struct Point: X, Y {
    var xValue: Int
    var yValue: Int
}

func printSum(of point: X & Y) {
    print(point.xValue + point.yValue)
}

let pt = Point(xValue: 3, yValue: 7)
printSum(of: pt)

A Compilation error

B 37

C 0

D 10

💡 Hint

Check how the sum of the two property values is calculated.

The Point struct has xValue = 3 and yValue = 7. The function prints their sum, which is 10.

🔧 Debug

advanced

Identify the error in protocol composition usage

What error does this Swift code produce and why?

Swift

protocol P1 {
    func foo()
}

protocol P2 {
    func bar()
}

struct S: P1 {
    func foo() { print("foo") }
}

func test(param: P1 & P2) {
    param.foo()
    param.bar()
}

let s = S()
test(param: s)

A Error: 'S' does not conform to protocol 'P2'

B Runtime error: method 'bar' not found

C Compilation error: missing return in function

D No error, prints 'foo' and 'bar'

💡 Hint

Check which protocols the struct conforms to and what the function requires.

The struct S only conforms to P1, but the function test requires an argument conforming to both P1 and P2. This causes a compile-time error.

🧠 Conceptual

advanced

Understanding protocol composition with class inheritance

Given these Swift declarations, what is true about the variable 'obj' declared below?

Swift

protocol Alpha {
    func alphaMethod()
}

protocol Beta {
    func betaMethod()
}

class Base: Alpha {
    func alphaMethod() { print("Alpha") }
}

class Derived: Base, Beta {
    func betaMethod() { print("Beta") }
}

let obj: Alpha & Beta = Derived()

A 'obj' can only call betaMethod()

B 'obj' can call both alphaMethod() and betaMethod()

C 'obj' cannot call either method

D 'obj' can only call alphaMethod()

💡 Hint

Consider what protocols 'obj' is declared to conform to and the actual class instance.

The variable 'obj' is declared as conforming to both Alpha and Beta. The instance is Derived, which implements both methods, so both can be called.

❓ Predict Output

expert

Output of complex protocol composition with extensions

What is the output of this Swift code involving protocol composition and protocol extensions?

Swift

protocol First {
    func action() -> String
}

protocol Second {
    func action() -> String
}

extension First {
    func action() -> String { "First" }
}

extension Second {
    func action() -> String { "Second" }
}

struct Combo: First, Second {
    func action() -> String { "Combo" }
}

func performAction(on obj: First & Second) {
    print(obj.action())
}

let c = Combo()
performAction(on: c)

A Combo

B First

C Second

D Compilation error due to ambiguity

💡 Hint

Check which implementation of action() is called when the struct provides its own.

The struct Combo implements its own action() method, which overrides the default implementations from protocol extensions. So "Combo" is printed.