Convenience initializers in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a class with convenience initializer

What will be printed when the following Swift code runs?

Swift

class Vehicle {
    var wheels: Int
    init(wheels: Int) {
        self.wheels = wheels
    }
    convenience init() {
        self.init(wheels: 4)
    }
}

let car = Vehicle()
print(car.wheels)

A 4

B 0

C Compilation error

D nil

💡 Hint

Remember that the convenience initializer calls the designated initializer with 4 wheels.

The convenience initializer calls init(wheels: 4), so the wheels property is set to 4. Printing car.wheels outputs 4.

❓ Predict Output

intermediate

Value of property after using convenience initializer with parameters

What is the value of 'name' after running this Swift code?

Swift

class Person {
    var name: String
    init(name: String) {
        self.name = name
    }
    convenience init() {
        self.init(name: "Anonymous")
    }
}

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

A ""

B "Anonymous"

C nil

D Compilation error

💡 Hint

The convenience initializer sets the name to "Anonymous".

The convenience initializer calls init(name: "Anonymous"), so p.name is "Anonymous".

❓ Predict Output

advanced

Output of subclass using convenience initializer

What will be printed when this Swift code runs?

Swift

class Animal {
    var species: String
    init(species: String) {
        self.species = species
    }
    convenience init() {
        self.init(species: "Unknown")
    }
}

class Dog: Animal {
    var name: String
    init(name: String) {
        self.name = name
        super.init(species: "Dog")
    }
    convenience init() {
        self.init(name: "Buddy")
    }
}

let dog = Dog()
print(dog.name + " is a " + dog.species)

A Buddy is a Dog

B Buddy is a Unknown

C Runtime error

D Compilation error

💡 Hint

The Dog convenience initializer calls init(name: "Buddy") which sets species to "Dog".

Dog's convenience init calls init(name: "Buddy"), which sets name and calls super.init(species: "Dog"), so output is "Buddy is a Dog".

❓ Predict Output

advanced

Error type from incorrect convenience initializer

What error does this Swift code produce?

Swift

class Box {
    var size: Int
    init(size: Int) {
        self.size = size
    }
    convenience init() {
        self.size = 10
    }
}

A Error: Convenience initializer cannot have parameters

B No error, compiles fine

C Error: Missing return type

D Error: 'self' used before calling designated initializer

💡 Hint

In convenience initializers, you must call a designated initializer before setting properties.

The convenience initializer tries to set self.size before calling a designated initializer, which is not allowed and causes a compile-time error.

🧠 Conceptual

expert

Correct order of initialization in Swift convenience initializer

Which sequence correctly describes the order of steps when a convenience initializer is called in Swift?

A 1, 3, 2, 4

B 2, 1, 3, 4

C 1, 2, 3, 4

D 3, 2, 1, 4

💡 Hint

Convenience initializers must call a designated initializer before properties are set.

First, the convenience initializer is called (1), then it calls a designated initializer (2), which initializes all stored properties (3), and finally returns the fully initialized instance (4).