Why inheritance is class-only in Swift - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a class that inherits from another class.

Swift

class Dog: [1] {
    func bark() {
        print("Woof!")
    }
}

Drag options to blanks, or click blank then click option'

Aenum

Bstruct

Cprotocol

DAnimal

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to inherit from a struct or protocol.

In Swift, a class inherits from another class by specifying the parent class name after a colon.

2fill in blank

medium

Complete the code to override a method in a subclass.

Swift

class Animal {
    func sound() {
        print("Some sound")
    }
}

class Cat: Animal {
    override func [1]() {
        print("Meow")
    }
}

Drag options to blanks, or click blank then click option'

Abark

Bmeow

Csound

Dnoise

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different method name that does not exist in the parent class.

The subclass overrides the method from the parent class by using the same method name.

3fill in blank

hard

Fix the error in the code by choosing the correct type that supports inheritance.

Swift

[1] Vehicle {
    var wheels: Int
    init(wheels: Int) {
        self.wheels = wheels
    }
}

class Car: Vehicle {
    var brand: String
    init(brand: String, wheels: Int) {
        self.brand = brand
        super.init(wheels: wheels)
    }
}

Drag options to blanks, or click blank then click option'

Aclass

Bstruct

CVehicle

Dprotocol

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to inherit from a struct or protocol.

Only classes can be inherited from in Swift. Structs cannot be used as a superclass.

4fill in blank

hard

Fill both blanks to create a subclass that calls the superclass initializer.

Swift

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

class Student: [1] {
    var id: Int
    init(name: String, id: Int) {
        self.id = id
        [2].init(name: name)
    }
}

Drag options to blanks, or click blank then click option'

APerson

Bsuper

Cstruct

Dself

Attempts:

3 left

💡 Hint

Common Mistakes

Using self.init instead of super.init to call the superclass initializer.

The subclass Student inherits from Person and calls the superclass initializer using super.init.

5fill in blank

hard

Fill all three blanks to define a class-only protocol and a class that conforms to it.

Swift

protocol [1]: [2] {
    func play()
}

class Guitarist: [3] {
    func play() {
        print("Playing guitar")
    }
}

Drag options to blanks, or click blank then click option'

APlayable

BAnyObject

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to restrict protocols to structs or enums.

Protocols can be restricted to classes by inheriting from AnyObject. The class then conforms to the protocol.

Practice

1. Why is inheritance only allowed for classes in Swift?

easy

A. Because only classes can have properties

B. Because structs and enums are faster to create

C. Because classes are reference types and share data by reference

D. Because structs and enums cannot have methods

Solution

1. Step 1: Understand type behavior in Swift

   Classes are reference types, meaning instances share the same data in memory.

2. Step 2: Compare with structs and enums

   Structs and enums are value types, so they copy data instead of sharing it.

3. Final Answer:

   Because classes are reference types and share data by reference -> Option C

4. Quick Check:

   Inheritance = Reference types only [OK]

Hint: Inheritance needs shared data, so only classes can do it [OK]

Common Mistakes:

• Thinking structs can inherit like classes
• Believing enums can have inheritance
• Confusing methods with inheritance

2. Which Swift declaration correctly shows inheritance allowed only for classes?

easy

A. class MyClass: ParentClass {}

B. struct MyStruct: ParentClass {}

C. enum MyEnum: ParentClass {}

D. protocol MyProtocol: ParentClass {}

Solution

1. Step 1: Identify inheritance syntax in Swift

   Only classes can inherit from other classes using the colon syntax.

2. Step 2: Check each declaration

   Structs, enums, and protocols cannot inherit from classes.

3. Final Answer:

   class MyClass: ParentClass {} -> Option A

4. Quick Check:

   Inheritance syntax = class only [OK]

Hint: Only classes use colon for inheritance from classes [OK]

Common Mistakes:

• Trying to inherit a class in a struct
• Using enum to inherit a class
• Confusing protocol inheritance with class inheritance

3. What will happen if you try this Swift code?

struct ParentStruct {}
struct ChildStruct: ParentStruct {}

medium

A. ChildStruct inherits ParentStruct successfully

B. Compile-time error: structs cannot inherit

C. Runtime error when creating ChildStruct

D. ChildStruct copies ParentStruct properties automatically

Solution

1. Step 1: Analyze struct inheritance rules

   Structs in Swift cannot inherit from other structs or classes.

2. Step 2: Predict compiler behavior

   The compiler will produce an error because inheritance is not allowed for structs.

3. Final Answer:

   Compile-time error: structs cannot inherit -> Option B

4. Quick Check:

   Struct inheritance = error [OK]

Hint: Structs cannot inherit; expect compile error [OK]

Common Mistakes:

• Assuming structs inherit like classes
• Expecting runtime error instead of compile error
• Thinking structs copy properties via inheritance

4. Find the error in this Swift code:

enum ParentEnum {}
class ChildClass: ParentEnum {}

medium

A. Enums cannot be inherited by classes

B. Enums cannot have child classes

C. Classes cannot inherit from enums

D. All of the above

Solution

1. Step 1: Check inheritance rules for enums and classes

   Enums are value types and cannot be inherited or used as base classes.

2. Step 2: Understand class inheritance restrictions

   Classes can only inherit from other classes, not enums.

3. Step 3: Combine observations

   All statements about enums and classes here are correct.

4. Final Answer:

   All of the above -> Option D

5. Quick Check:

   Enum inheritance = disallowed [OK]

Hint: Classes inherit classes only; enums can't be base classes [OK]

Common Mistakes:

• Thinking enums can be base classes
• Confusing enum cases with inheritance
• Ignoring that classes can't inherit enums

5. You want to share behavior between a struct and a class in Swift. Given inheritance is class-only, what is the best approach?

hard

A. Use a protocol to define shared behavior

B. Convert the struct to a class to inherit

C. Make the struct inherit from the class

D. Use enum inheritance to share code

Solution

1. Step 1: Recall inheritance limits

   Inheritance works only for classes, so structs cannot inherit from classes.

2. Step 2: Identify alternative sharing method

   Protocols allow both structs and classes to share behavior by conforming to them.

3. Step 3: Evaluate options

   Using protocols is the best way to share behavior without inheritance.

4. Final Answer:

   Use a protocol to define shared behavior -> Option A

5. Quick Check:

   Protocols share behavior across types [OK]

Hint: Use protocols to share behavior with structs and classes [OK]

Common Mistakes:

• Trying to inherit struct from class
• Converting structs unnecessarily to classes
• Thinking enums can inherit