Classes are reference types in Swift - Mini Project: Build & Apply

Classes are reference types

📖 Scenario: Imagine you have a simple app that keeps track of a book's details. You want to understand how changing one book's information can affect other references to the same book.

🎯 Goal: You will create a Book class, make two variables point to the same book, change the title through one variable, and see how it affects the other variable. This will show that classes are reference types.

📋 What You'll Learn

Create a class called Book with a title property of type String

Create a variable firstBook that is an instance of Book with the title "Swift Programming"

Create a variable secondBook and assign it to firstBook

Change the title of secondBook to "Advanced Swift"

Print the title of firstBook to show it has changed

💡 Why This Matters

🌍 Real World

Understanding reference types is important when working with objects in apps, so you know how data changes affect your program.

💼 Career

Many programming jobs require knowledge of how classes and objects work, especially in Swift for iOS app development.

1

Create the Book class

Create a class called Book with a title property of type String. Initialize title using an initializer.

Swift

// Create the Book class with a title property
// Your code here

Hint

Use class Book {} and add a var title: String inside. Add an init method to set the title.

2

Create firstBook instance

Create a variable called firstBook and assign it an instance of Book with the title "Swift Programming".

Swift

class Book {
    var title: String
    
    init(title: String) {
        self.title = title
    }
}

// Create firstBook instance with title "Swift Programming"
// Your code here

Hint

Use var firstBook = Book(title: "Swift Programming") to create the instance.

3

Assign secondBook to firstBook and change title

Create a variable called secondBook and assign it to firstBook. Then change the title of secondBook to "Advanced Swift".

Swift

class Book {
    var title: String
    
    init(title: String) {
        self.title = title
    }
}

var firstBook = Book(title: "Swift Programming")

// Assign secondBook to firstBook
// Change secondBook's title to "Advanced Swift"
// Your code here

Hint

Assign secondBook = firstBook and then set secondBook.title = "Advanced Swift".

4

Print the title of firstBook

Print the title of firstBook using print to show it has changed to "Advanced Swift".

Swift

class Book {
    var title: String
    
    init(title: String) {
        self.title = title
    }
}

var firstBook = Book(title: "Swift Programming")
var secondBook = firstBook
secondBook.title = "Advanced Swift"

// Print the title of firstBook
// Your code here

Hint

Use print(firstBook.title) to see the changed title.

Practice

1. What happens when you assign an instance of a class to another variable in Swift?

easy

A. A new copy of the object is created.

B. Both variables refer to the same object in memory.

C. The original object is deleted.

D. The variables become constants automatically.

Solution

1. Step 1: Understand class assignment behavior

   In Swift, classes are reference types, so assigning one instance to another variable copies the reference, not the object.

2. Step 2: Effect of assignment on variables

   Both variables point to the same memory location, so changes via one variable affect the other.

3. Final Answer:

   Both variables refer to the same object in memory. -> Option B

4. Quick Check:

   Classes are reference types = A [OK]

Hint: Class assignment copies reference, not object [OK]

Common Mistakes:

• Thinking a new object is created on assignment
• Confusing classes with structs (value types)
• Assuming variables become constants automatically

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

easy

A. class Dog: String { var name }

B. struct Dog { var name: String }

C. class Dog { var name: String }

D. func Dog() { var name: String }

Solution

1. Step 1: Identify correct class syntax

   In Swift, a class is declared using the keyword 'class' followed by the class name and curly braces containing properties.

2. Step 2: Check each option

   class Dog { var name: String } correctly declares a class with a property. struct Dog { var name: String } declares a struct, not a class. class Dog: String { var name } has invalid syntax with inheritance from String and missing property type. func Dog() { var name: String } uses 'func' which is for functions, not classes.

3. Final Answer:

   class Dog { var name: String } -> Option C

4. Quick Check:

   Correct class syntax = A [OK]

Hint: Use 'class' keyword and braces for class declaration [OK]

Common Mistakes:

• Using 'struct' instead of 'class'
• Incorrect inheritance syntax
• Using 'func' instead of 'class'

3. What will be printed by the following Swift code?

class Counter {
  var count = 0
}

let a = Counter()
let b = a
b.count = 5
print(a.count)

medium

A. 0

B. Compilation error

C. nil

D. 5

Solution

1. Step 1: Understand reference assignment

   Variable 'b' is assigned to 'a', so both refer to the same Counter instance.

2. Step 2: Effect of changing 'b.count'

   Changing 'b.count' to 5 changes the shared object's property, so 'a.count' also becomes 5.

3. Final Answer:

   5 -> Option D

4. Quick Check:

   Class references share data = 5 [OK]

Hint: Changing one reference changes all references [OK]

Common Mistakes:

• Assuming 'a.count' stays 0 after changing 'b.count'
• Expecting a compilation error
• Confusing with value type behavior

4. Identify the error in this Swift code snippet:

class Person {
  var name: String
}

let p1 = Person()
p1.name = "Alice"

medium

A. Class Person cannot be instantiated with 'let'.

B. Cannot assign value to 'name' because it is a constant.

C. Missing initializer for 'name' property in class Person.

D. No error; code runs fine.

Solution

1. Step 1: Check class property initialization

   The class Person has a non-optional property 'name' without an initial value or initializer.

2. Step 2: Understand Swift's requirement for property initialization

   Swift requires all non-optional properties to be initialized before use, either with a default value or in an initializer.

3. Final Answer:

   Missing initializer for 'name' property in class Person. -> Option A

4. Quick Check:

   Non-optional properties need initialization = C [OK]

Hint: Non-optional properties must be initialized [OK]

Common Mistakes:

• Thinking 'let' prevents property assignment
• Assuming classes can't be instantiated with 'let'
• Ignoring property initialization rules

5. You want to create a shared settings object that multiple parts of your app can modify and see updates immediately. Which approach best uses Swift classes to achieve this?

hard

A. Create a single instance of a Settings class and pass its reference around.

B. Create multiple instances of a Settings struct and copy them as needed.

C. Use global variables instead of classes for shared data.

D. Use a function that returns a new Settings object every time.

Solution

1. Step 1: Understand shared mutable state with classes

   Classes are reference types, so a single instance shared by multiple parts allows changes to be seen everywhere.

2. Step 2: Evaluate options for shared settings

   Create a single instance of a Settings class and pass its reference around. uses one shared instance, which fits the requirement. Create multiple instances of a Settings struct and copy them as needed. uses structs (value types), so copies won't reflect changes. Use global variables instead of classes for shared data. uses globals, which is not recommended for maintainability. Use a function that returns a new Settings object every time. creates new objects each time, so no shared state.

3. Final Answer:

   Create a single instance of a Settings class and pass its reference around. -> Option A

4. Quick Check:

   Shared mutable object = B [OK]

Hint: Use one shared class instance for shared mutable data [OK]

Common Mistakes:

• Using structs which copy data instead of sharing
• Relying on global variables for sharing
• Creating new instances instead of sharing one