Why inheritance is class-only in Swift - See It in Action

Why inheritance is class-only in Swift

📖 Scenario: Imagine you are building a simple app to manage different types of vehicles. You want to share common features like speed and capacity but also have specific features for cars and bicycles.

🎯 Goal: Learn why inheritance in Swift works only with classes and not with structs or enums by creating a simple class hierarchy.

📋 What You'll Learn

Create a base class called Vehicle with properties speed and capacity

Create a subclass called Car that inherits from Vehicle and adds a property brand

Create a subclass called Bicycle that inherits from Vehicle and adds a property hasBasket

Print details of a Car and a Bicycle instance

💡 Why This Matters

🌍 Real World

Inheritance helps organize code when building apps with related objects like vehicles, animals, or UI elements.

💼 Career

Understanding class-only inheritance is important for Swift developers to design clean, reusable, and maintainable code.

1

Create the base class Vehicle

Create a class called Vehicle with two properties: speed of type Int and capacity of type Int. Initialize both properties in an initializer.

Swift

class Vehicle {
    // Your code here
}

Hint

Use class keyword to define a class. Add properties and an initializer method.

2

Create subclass Car inheriting from Vehicle

Create a class called Car that inherits from Vehicle. Add a property brand of type String. Create an initializer that takes speed, capacity, and brand and calls the superclass initializer for speed and capacity.

Swift

class Vehicle {
    var speed: Int
    var capacity: Int
    
    init(speed: Int, capacity: Int) {
        self.speed = speed
        self.capacity = capacity
    }
}

// Your code here

Hint

Use class Car: Vehicle to inherit. Call super.init to initialize the base class.

3

Create subclass Bicycle inheriting from Vehicle

Create a class called Bicycle that inherits from Vehicle. Add a property hasBasket of type Bool. Create an initializer that takes speed, capacity, and hasBasket and calls the superclass initializer for speed and capacity.

Swift

class Vehicle {
    var speed: Int
    var capacity: Int
    
    init(speed: Int, capacity: Int) {
        self.speed = speed
        self.capacity = capacity
    }
}

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

// Your code here

Hint

Similar to Car, inherit from Vehicle and call super.init.

4

Create instances and print details

Create an instance of Car named myCar with speed 120, capacity 5, and brand "Toyota". Create an instance of Bicycle named myBike with speed 25, capacity 1, and hasBasket true. Print the details of both using print statements.

Swift

class Vehicle {
    var speed: Int
    var capacity: Int
    
    init(speed: Int, capacity: Int) {
        self.speed = speed
        self.capacity = capacity
    }
}

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

class Bicycle: Vehicle {
    var hasBasket: Bool
    
    init(speed: Int, capacity: Int, hasBasket: Bool) {
        self.hasBasket = hasBasket
        super.init(speed: speed, capacity: capacity)
    }
}

// Your code here

Hint

Create instances with the init method and use print with string interpolation to show properties.

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