Protocol declaration syntax in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple protocol conformance

What is the output of this Swift code when calling greet() on person?

Swift

protocol Greeter {
    func greet() -> String
}

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

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

A No output

B "Hello!"

C "Hi!"

D Error: Protocol method not implemented

💡 Hint

Check how the greet() method is implemented in Person.

The Person struct conforms to the Greeter protocol by implementing greet() returning "Hello!". So calling greet() prints "Hello!".

❓ Predict Output

intermediate

Protocol with property requirement output

What will be printed by this Swift code?

Swift

protocol Identifiable {
    var id: String { get }
}

struct User: Identifiable {
    var id: String
}

let user = User(id: "abc123")
print(user.id)

A nil

B Error: Missing property implementation

C "id"

D "abc123"

💡 Hint

Look at how id is defined and initialized in User.

The User struct implements the id property required by Identifiable. The instance is created with id = "abc123", so printing user.id outputs "abc123".

🔧 Debug

advanced

Identify the syntax error in protocol declaration

Which option contains the syntax error in this protocol declaration?

Swift

protocol Vehicle {
    var speed: Int
    func drive()
}

A The property <code>speed</code> must specify { get } or { get set }

B The function <code>drive()</code> must have a return type

C Protocols cannot have properties

D The protocol keyword is misspelled

💡 Hint

Check how properties are declared in protocols.

In Swift protocols, properties must specify whether they are readable ({ get }) or readable and writable ({ get set }). Missing this causes a syntax error.

🧠 Conceptual

advanced

Protocol inheritance syntax

Which option correctly declares a protocol FlyingCar that inherits from protocols Car and Flyable?

A protocol FlyingCar: Car, Flyable {}

B protocol FlyingCar inherits Car, Flyable {}

C protocol FlyingCar <- Car, Flyable {}

D protocol FlyingCar extends Car & Flyable {}

💡 Hint

Look at how Swift declares protocol inheritance using colons and commas.

Swift uses a colon after the protocol name followed by a comma-separated list of inherited protocols.

❓ Predict Output

expert

Output of protocol extension method call

What is the output of this Swift code?

Swift

protocol Speaker {
    func speak() -> String
}

extension Speaker {
    func speak() -> String {
        return "Hello from extension"
    }
}

struct Robot: Speaker {
    func speak() -> String {
        return "Hello from Robot"
    }
}

struct Alien: Speaker {}

let r = Robot()
let a = Alien()
print(r.speak())
print(a.speak())

A "Hello from Robot"\nCompilation error

B Compilation error: Alien does not implement speak()

C "Hello from Robot"\n"Hello from extension"

D "Hello from extension"\n"Hello from extension"

💡 Hint

Check how protocol extensions provide default implementations.

The Robot struct implements speak(), so its version is called. The Alien struct does not implement speak(), so it uses the default from the protocol extension.

Practice

1. What keyword is used in Swift to declare a protocol?

easy

A. interface

B. class

C. protocol

D. struct

Solution

1. Step 1: Recall Swift syntax for protocols

   In Swift, protocols are declared using the keyword protocol.

2. Step 2: Compare options with known syntax

   Options like interface are from other languages, and class or struct declare types, not protocols.

3. Final Answer:

   protocol -> Option C

4. Quick Check:

   Keyword for protocol declaration = protocol [OK]

Hint: Remember: Swift uses 'protocol' keyword for protocols [OK]

Common Mistakes:

• Confusing 'protocol' with 'interface' from other languages
• Using 'class' or 'struct' instead of 'protocol'
• Misspelling the keyword

2. Which of the following is the correct syntax to declare a protocol named Vehicle in Swift?

easy

A. protocol Vehicle {}

B. protocol Vehicle() {}

C. protocol Vehicle[] {}

D. protocol Vehicle<> {}

Solution

1. Step 1: Understand protocol declaration syntax

   Protocols are declared with the keyword protocol, followed by the name and curly braces without parentheses or brackets.

2. Step 2: Check each option

   protocol Vehicle {} uses correct syntax: protocol Vehicle {}. Options B, C, and D incorrectly add parentheses or brackets.

3. Final Answer:

   protocol Vehicle {} -> Option A

4. Quick Check:

   Protocol declaration uses curly braces only [OK]

Hint: Protocols use curly braces, no parentheses or brackets [OK]

Common Mistakes:

• Adding parentheses after protocol name
• Using square or angle brackets incorrectly
• Omitting curly braces

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

protocol Greetable {
    func greet() -> String
}

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

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

medium

A. Hello!

B. greet()

C. Error: Missing protocol method implementation

D. nil

Solution

1. Step 1: Understand protocol and struct implementation

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

2. Step 2: Analyze the print statement

   Creating an instance p of Person and calling p.greet() returns "Hello!", which is printed.

3. Final Answer:

   Hello! -> Option A

4. Quick Check:

   Implemented protocol method returns "Hello!" [OK]

Hint: Protocol methods must be implemented; output is returned string [OK]

Common Mistakes:

• Assuming protocol methods run automatically without implementation
• Expecting error due to protocol conformance
• Confusing method name with output

4. Identify the error in this protocol declaration:

protocol Drawable {
    func draw()
    var color: String
}

medium

A. func draw() must have a return type

B. Protocol properties must specify { get } or { get set }

C. Protocol name must start with uppercase letter

D. Missing parentheses after func draw

Solution

1. Step 1: Check protocol property declaration rules

   In Swift, protocol properties must declare if they are readable or writable using { get } or { get set }.

2. Step 2: Analyze the given property

   The property color: String lacks { get } or { get set }, causing a syntax error.

3. Final Answer:

   Protocol properties must specify { get } or { get set } -> Option B

4. Quick Check:

   Protocol property syntax requires access specifiers [OK]

Hint: Always add { get } or { get set } to protocol properties [OK]

Common Mistakes:

• Omitting { get } or { get set } in protocol properties
• Adding parentheses to property declarations
• Thinking protocol names must be uppercase

5. You want to declare a protocol Identifiable that requires a read-only id property of type String and a method identify() that returns a String. Which is the correct declaration?

hard

A. protocol Identifiable {
let id: String
func identify() -> String
}

B. protocol Identifiable {
var id: String
func identify()
}

C. protocol Identifiable {
var id: String { get set }
func identify() -> String
}

D. protocol Identifiable {
var id: String { get }
func identify() -> String
}

Solution

1. Step 1: Define read-only property in protocol

   A read-only property in a protocol must be declared with { get } to indicate it can be read but not written.

2. Step 2: Check method signature

   The method identify() must specify return type String as required.

3. Step 3: Evaluate options

   protocol Identifiable { var id: String { get } func identify() -> String } correctly declares var id: String { get } and func identify() -> String. protocol Identifiable { var id: String func identify() } misses { get } and return type. protocol Identifiable { let id: String func identify() -> String } uses let which is invalid in protocols. protocol Identifiable { var id: String { get set } func identify() -> String } allows writing with { get set }, not read-only.

4. Final Answer:

   protocol Identifiable { var id: String { get } func identify() -> String } -> Option D

5. Quick Check:

   Read-only property uses { get } and method return type matches [OK]

Hint: Use { get } for read-only properties in protocols [OK]

Common Mistakes:

• Using 'let' instead of 'var' in protocol
• Omitting { get } for read-only properties
• Missing return type in method declaration