Why classes exist alongside structs in Swift - See It in Action

Why Classes Exist Alongside Structs in Swift

📖 Scenario: Imagine you are building a simple app to manage information about books and their authors. You want to understand when to use structs and when to use classes in Swift.

🎯 Goal: You will create a struct and a class to represent books and authors, then see how they behave differently when copied or changed.

📋 What You'll Learn

Create a struct called Book with properties title and pages

Create a class called Author with properties name and booksWritten

Show how changing a copy of a Book struct does not affect the original

Show how changing a copy of an Author class affects the original

Print the results to observe the differences

💡 Why This Matters

🌍 Real World

Understanding when to use structs or classes helps you write safer and more efficient Swift code, especially in apps where data sharing and copying matter.

💼 Career

Swift developers often decide between structs and classes to optimize app performance and behavior, making this knowledge essential for iOS development jobs.

1

Create the Book struct

Create a struct called Book with two properties: title of type String and pages of type Int. Then create a variable firstBook of type Book with title "Swift Basics" and pages 200.

Swift

# Your code here

Hint

Use struct keyword to define Book. Properties are declared with var.

2

Create the Author class

Create a class called Author with two properties: name of type String and booksWritten of type Int. Then create a variable firstAuthor of type Author with name "Alice" and booksWritten 3.

Swift

struct Book {
    var title: String
    var pages: Int
}

var firstBook = Book(title: "Swift Basics", pages: 200)

# Your code here

Hint

Use class keyword and an initializer init to set properties.

3

Copy and change the Book struct

Create a new variable secondBook and set it equal to firstBook. Then change secondBook.pages to 250.

Swift

struct Book {
    var title: String
    var pages: Int
}

var firstBook = Book(title: "Swift Basics", pages: 200)

class Author {
    var name: String
    var booksWritten: Int
    
    init(name: String, booksWritten: Int) {
        self.name = name
        self.booksWritten = booksWritten
    }
}

var firstAuthor = Author(name: "Alice", booksWritten: 3)

# Your code here

Hint

Assign firstBook to secondBook and then change secondBook.pages.

4

Copy and change the Author class and print results

Create a new variable secondAuthor and set it equal to firstAuthor. Then change secondAuthor.booksWritten to 5. Finally, print firstBook.pages, secondBook.pages, firstAuthor.booksWritten, and secondAuthor.booksWritten each on its own line.

Swift

struct Book {
    var title: String
    var pages: Int
}

var firstBook = Book(title: "Swift Basics", pages: 200)

class Author {
    var name: String
    var booksWritten: Int
    
    init(name: String, booksWritten: Int) {
        self.name = name
        self.booksWritten = booksWritten
    }
}

var firstAuthor = Author(name: "Alice", booksWritten: 3)

var secondBook = firstBook
secondBook.pages = 250

# Your code here

Hint

Assign firstAuthor to secondAuthor, change secondAuthor.booksWritten, then print all values.

Practice

1. Why do classes exist alongside structs in Swift?

easy

A. Because classes cannot store data, only methods

B. Because structs are slower than classes in all cases

C. Because classes support shared references and inheritance, while structs do not

D. Because structs allow inheritance but classes do not

Solution

1. Step 1: Understand the difference between classes and structs

   Structs are value types, meaning each copy is independent. Classes are reference types, meaning multiple variables can share the same instance.

2. Step 2: Identify why classes are needed

   Classes allow shared data through references and support inheritance, which structs do not.

3. Final Answer:

   Because classes support shared references and inheritance, while structs do not -> Option C

4. Quick Check:

   Classes = shared references + inheritance [OK]

Hint: Classes share data and support inheritance; structs do not [OK]

Common Mistakes:

• Thinking structs support inheritance
• Believing classes cannot store data
• Assuming structs are always slower

2. Which of the following is the correct way to declare a class in Swift?

easy

A. struct MyClass { var name: String }

B. class MyClass { var name: String }

C. func MyClass() { var name: String }

D. let MyClass = class { var name: String }

Solution

1. Step 1: Recall Swift syntax for class declaration

   Classes in Swift are declared using the keyword class followed by the class name and braces.

2. Step 2: Check each option

   class MyClass { var name: String } correctly uses class MyClass { var name: String }. struct MyClass { var name: String } uses struct, not class. func MyClass() { var name: String } uses func, which is incorrect. let MyClass = class { var name: String } uses invalid syntax.

3. Final Answer:

   class MyClass { var name: String } -> Option B

4. Quick Check:

   Class declaration uses 'class' keyword [OK]

Hint: Use 'class' keyword to declare a class in Swift [OK]

Common Mistakes:

• Using 'struct' instead of 'class'
• Trying to declare class inside a function
• Incorrect syntax with 'let' or 'func'

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

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

var p1 = Person(name: "Alice")
var p2 = p1
p2.name = "Bob"
print(p1.name)

medium

A. Bob

B. Alice

C. Compilation error

D. nil

Solution

1. Step 1: Understand class reference behavior

   Classes are reference types, so p1 and p2 point to the same object.

2. Step 2: Analyze the code changes

   Changing p2.name to "Bob" changes the shared object. Printing p1.name shows "Bob".

3. Final Answer:

   Bob -> Option A

4. Quick Check:

   Class variables share references, so name = Bob [OK]

Hint: Classes share instances; changing one changes all references [OK]

Common Mistakes:

• Thinking p1 and p2 are independent copies
• Expecting 'Alice' as output
• Confusing class with struct behavior

4. Identify the error in this Swift code that tries to use inheritance with a struct:

struct Vehicle {
  var speed: Int
}

struct Car: Vehicle {
  var brand: String
}

medium

A. Cannot declare variables inside structs

B. Missing initializer in Car struct

C. Syntax error: colon should be replaced with comma

D. Structs cannot inherit from other structs or classes

Solution

1. Step 1: Understand inheritance rules in Swift

   In Swift, only classes support inheritance. Structs cannot inherit from other structs or classes.

2. Step 2: Analyze the code

   The code tries to make Car inherit from Vehicle, both structs, which is invalid.

3. Final Answer:

   Structs cannot inherit from other structs or classes -> Option D

4. Quick Check:

   Inheritance = classes only, structs no [OK]

Hint: Only classes support inheritance, not structs [OK]

Common Mistakes:

• Thinking structs can inherit
• Confusing initializer errors with inheritance
• Misreading colon syntax as error

5. You want to create a data model where multiple parts of your app share and modify the same user profile data. Which Swift type should you choose and why?

hard

A. Use a class because it allows shared references and identity

B. Use a class because structs cannot store properties

C. Use a struct because it supports inheritance for flexibility

D. Use a struct because it copies data safely between parts

Solution

1. Step 1: Identify the need for shared data

   The requirement is that multiple parts share and modify the same user profile data, meaning shared references are needed.

2. Step 2: Choose the correct Swift type

   Classes are reference types that allow shared references and identity, so changes in one place reflect everywhere. Structs copy data, so changes do not propagate.

3. Final Answer:

   Use a class because it allows shared references and identity -> Option A

4. Quick Check:

   Shared data needs class reference type [OK]

Hint: Shared mutable data needs classes, not structs [OK]

Common Mistakes:

• Choosing structs for shared mutable data
• Thinking structs support inheritance
• Believing classes cannot store properties