Initializers and designated init in Swift - Mini Project: Build & Apply

Initializers and Designated Init in Swift

📖 Scenario: You are creating a simple app to manage information about books in a library. Each book has a title and a number of pages.

🎯 Goal: Build a Swift struct called Book with a designated initializer that sets the title and pages. Then create an instance and print its details.

📋 What You'll Learn

Create a struct called Book with two properties: title (String) and pages (Int).

Add a designated initializer init(title: String, pages: Int) to set these properties.

Create a variable myBook using the initializer with title "Swift Programming" and pages 350.

Print the book's title and pages in the format: "Title: Swift Programming, Pages: 350".

💡 Why This Matters

🌍 Real World

Initializers help set up objects with the right starting values, just like filling out a form before using a new tool.

💼 Career

Understanding initializers is essential for building apps that manage data cleanly and safely, a key skill for Swift developers.

1

Create the Book struct with properties

Create a struct called Book with two properties: title of type String and pages of type Int.

Swift

struct Book {
    // Your code here
}

Hint

Use struct keyword and declare two variables inside.

2

Add a designated initializer

Add a designated initializer init(title: String, pages: Int) inside the Book struct that sets the title and pages properties.

Swift

struct Book {
    var title: String
    var pages: Int

    // Your code here
}

Hint

Use init with parameters and assign them to self properties.

3

Create an instance of Book

Create a variable called myBook and initialize it using Book(title: "Swift Programming", pages: 350).

Swift

struct Book {
    var title: String
    var pages: Int

    init(title: String, pages: Int) {
        self.title = title
        self.pages = pages
    }
}

// Your code here

Hint

Use the initializer to create myBook with the exact title and pages.

4

Print the book details

Print the details of myBook in the format: "Title: Swift Programming, Pages: 350" using print and string interpolation.

Swift

struct Book {
    var title: String
    var pages: Int

    init(title: String, pages: Int) {
        self.title = title
        self.pages = pages
    }
}

var myBook = Book(title: "Swift Programming", pages: 350)

// Your code here

Hint

Use print("Title: \(myBook.title), Pages: \(myBook.pages)") to show the output.

Practice

1. What is the main purpose of a designated initializer in Swift?

easy

A. To fully initialize all properties of a class before use

B. To provide default values for optional properties only

C. To create a copy of an existing object

D. To deallocate memory when an object is no longer needed

Solution

1. Step 1: Understand what initializers do

   Initializers set starting values for new objects to ensure they are ready to use.

2. Step 2: Identify the role of designated initializers

   Designated initializers fully initialize all properties of a class, making the object safe and complete.

3. Final Answer:

   To fully initialize all properties of a class before use -> Option A

4. Quick Check:

   Designated initializer = full property initialization [OK]

Hint: Designated init always sets all properties fully [OK]

Common Mistakes:

• Confusing designated init with convenience init
• Thinking initializers copy objects
• Mixing initialization with deallocation

2. Which of the following is the correct syntax for a designated initializer in a Swift class?

easy

A. func init() { }

B. init() { }

C. constructor() { }

D. initialize() { }

Solution

1. Step 1: Recall Swift initializer syntax

   In Swift, initializers use the keyword init without a return type or func keyword.

2. Step 2: Identify the correct syntax

   The correct syntax is init() { }. Other options use incorrect keywords or syntax.

3. Final Answer:

   init() { } -> Option B

4. Quick Check:

   Swift designated init syntax = init() { } [OK]

Hint: Swift init uses just 'init', no func or constructor [OK]

Common Mistakes:

• Using 'func' keyword with init
• Using 'constructor' or 'initialize' instead of 'init'
• Adding return types to init

3. Consider this Swift class:

class Person {
  var name: String
  var age: Int

  init(name: String) {
    self.name = name
    self.age = 0
  }
}

What will be the value of age after creating let p = Person(name: "Anna")?

medium

A. Name of the person

B. nil

C. Undefined

D. 0

Solution

1. Step 1: Analyze the initializer code

   The initializer sets self.name = name and explicitly sets self.age = 0.

2. Step 2: Determine the value of age after initialization

   Since age is set to 0 inside the initializer, the new object will have age = 0.

3. Final Answer:

   0 -> Option D

4. Quick Check:

   Explicit age assignment = 0 [OK]

Hint: Check initializer assignments for property values [OK]

Common Mistakes:

• Assuming age is nil because not passed as parameter
• Thinking age is uninitialized or undefined
• Confusing name with age value

4. Identify the error in this Swift class initializer:

class Car {
  var model: String
  var year: Int

  init(model: String) {
    model = model
    year = 2020
  }
}

medium

A. Initializer missing return type

B. Class must inherit from NSObject

C. Properties are not initialized using self keyword

D. year property cannot be assigned a value

Solution

1. Step 1: Check property assignments in initializer

   The code assigns model = model and year = 2020 without self., so it assigns parameters to themselves, not properties.

2. Step 2: Understand correct property initialization

   Properties must be assigned with self.model = model and self.year = 2020 to set the instance variables.

3. Final Answer:

   Properties are not initialized using self keyword -> Option C

4. Quick Check:

   Use self.property = value to initialize [OK]

Hint: Use self. to assign properties inside init [OK]

Common Mistakes:

• Forgetting self. causes no property assignment
• Adding return type to init
• Thinking inheritance affects property assignment

5. You want to create a Swift class Book with properties title (String) and pages (Int). The designated initializer should allow setting both properties, but if pages is not provided, it defaults to 100. Which is the best way to write this initializer?

hard

A. init(title: String, pages: Int = 100) { self.title = title; self.pages = pages }

B. init(title: String) { self.title = title; self.pages = 100 }

C. init(title: String, pages: Int) { self.title = title }

D. init(title: String, pages: Int?) { self.title = title; self.pages = pages! }

Solution

1. Step 1: Understand default parameter values in Swift

   Swift allows default values in parameters, so pages: Int = 100 means pages defaults to 100 if not passed.

2. Step 2: Check initializer completeness and safety

   The initializer init(title: String, pages: Int = 100) { self.title = title; self.pages = pages } sets both properties properly and safely with a default value. The version with only a title parameter cannot set pages explicitly. The version taking both parameters but only assigning title misses the pages assignment. The version using an optional Int? for pages with force unwrap is unsafe.

3. Final Answer:

   init(title: String, pages: Int = 100) { self.title = title; self.pages = pages } -> Option A

4. Quick Check:

   Default parameter = safe default value [OK]

Hint: Use default parameter values for optional init args [OK]

Common Mistakes:

• Forgetting to assign all properties
• Using force unwrap (!) unsafely
• Not providing default value in parameter