Type casting with as, as?, as! in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift code using 'as?'?

Consider the following Swift code snippet. What will be printed?

Swift

class Animal {}
class Dog: Animal {}

let animal: Animal = Dog()
if let dog = animal as? Dog {
    print("Dog cast succeeded")
} else {
    print("Dog cast failed")
}

A Dog cast succeeded

B Dog cast failed

C Compile-time error

D Runtime crash

💡 Hint

Remember that 'as?' tries to cast safely and returns nil if it fails.

The variable animal actually holds a Dog instance. Using as? safely casts it to Dog, so the cast succeeds and prints "Dog cast succeeded".

❓ Predict Output

intermediate

What happens when using 'as!' to cast an incompatible type?

What is the result of running this Swift code?

Swift

class Animal {}
class Cat: Animal {}
class Dog: Animal {}

let animal: Animal = Cat()
let dog = animal as! Dog
print("Cast succeeded")

A Runtime crash with a fatal error

B Prints "Cast succeeded"

C Compile-time error

D Prints nothing

💡 Hint

Think about what happens when you force cast to a wrong type.

The variable animal holds a Cat instance. Forcing a cast to Dog with as! causes a runtime crash because the types are incompatible.

🧠 Conceptual

advanced

Which statement about 'as' casting is true?

Choose the correct statement about the as keyword in Swift.

A <code>as</code> can only be used to cast between unrelated types.

B <code>as</code> always performs a runtime check and returns an optional.

C <code>as</code> is used for upcasting and bridging between types without runtime checks.

D <code>as</code> is used to force cast and crashes if the cast fails.

💡 Hint

Think about when as can be used without optional or force casting.

as is used for upcasting (casting to a superclass or protocol) or bridging (e.g., between Swift and Objective-C types) and does not perform runtime checks. It never returns an optional or crashes.

❓ Predict Output

advanced

What is the output of this code using 'as?' with an array?

What will this Swift code print?

Swift

let mixedArray: [Any] = ["Hello", 42, 3.14]
if let stringArray = mixedArray as? [String] {
    print("String array: \(stringArray)")
} else {
    print("Cast to [String] failed")
}

A String array: ["Hello", "42", "3.14"]

B Cast to [String] failed

C Compile-time error

D String array: ["Hello"]

💡 Hint

Remember that as? requires all elements to be of the target type.

The array contains different types: String, Int, and Double. Casting the whole array to [String] fails because not all elements are strings, so the else branch runs.

🔧 Debug

expert

Why does this forced cast crash at runtime?

Examine the code below. Why does it crash at runtime?

Swift

class Vehicle {}
class Car: Vehicle {}
class Bike: Vehicle {}

let vehicle: Vehicle = Vehicle()
let car = vehicle as! Car
print("Car cast succeeded")

A Because <code>Car</code> is not a subclass of <code>Vehicle</code>.

B Because <code>as!</code> cannot be used with classes.

C Because <code>vehicle</code> is nil and cannot be cast.

D Because <code>vehicle</code> is not an instance of <code>Car</code>, so forced cast fails.

💡 Hint

Check the actual type of the instance stored in vehicle.

The variable vehicle holds an instance of Vehicle itself, not a subclass Car. Forced casting to Car fails at runtime causing a crash.