Initializers and designated init in Swift - Step-by-Step Execution

Concept Flow - Initializers and designated init

Start: Create instance

↓

Call designated init

↓

Initialize all properties

↓

Call superclass init if subclass

↓

Instance ready to use

↓

End

The flow shows how a designated initializer sets up all properties and calls superclass initializers if needed, preparing the instance for use.

Execution Sample

Swift

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

let p = Person(name: "Anna")

This code creates a Person instance by calling its designated initializer to set the name property.

Execution Table

Step	Action	Property 'name' Value	Instance State
1	Call Person.init(name: "Anna")	uninitialized	Instance created, properties uninitialized
2	Assign self.name = "Anna"	Anna	Property 'name' initialized
3	Initializer completes	Anna	Instance ready to use
4	Return instance to caller	Anna	Instance assigned to variable p

💡 Initialization completes after all properties are set and initializer returns

Variable Tracker

Variable	Start	After Step 2	Final
self.name	uninitialized	Anna	Anna
p	nil	nil	Person instance with name 'Anna'

Key Moments - 2 Insights

Why must all properties be initialized before the initializer ends?

Because Swift requires all stored properties to have a value before the instance is used, as shown in step 3 of the execution_table where 'name' is set before completion.

What is a designated initializer?

It is the main initializer that fully initializes all properties and calls superclass initializers if needed, as shown in step 1 where Person.init is called.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'self.name' after step 1?

A empty string

B "Anna"

C uninitialized

D nil

💡 Hint

Check the 'Property 'name' Value' column at step 1 in execution_table

At which step does the instance become ready to use?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Instance State' column in execution_table for when initialization completes

If the property 'name' was not assigned, what would happen?

A The initializer would complete successfully

B The compiler would give an error

C The instance would have a default name

D The instance would be nil

💡 Hint

Swift requires all properties to be initialized before initializer ends, see key_moments explanation

Concept Snapshot

class MyClass {
  var prop: Type
  init(prop: Type) {
    self.prop = prop  // Designated init sets all properties
  }
}
- Designated init fully initializes all stored properties
- Must call superclass init if subclass
- Instance ready after init completes

Full Transcript

This visual trace shows how Swift's designated initializer works. When creating an instance, the designated initializer is called first. It sets all stored properties, like 'name' in the example. Only after all properties are initialized does the initializer complete and the instance becomes ready to use. This ensures safe and predictable object creation. If any property is not set, Swift will give a compile-time error. The flow also includes calling superclass initializers if the class inherits from another. This step-by-step trace helps beginners see how initialization happens internally.

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