Initializers and designated init in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an initializer in Swift?

An initializer is a special method that prepares an instance of a class, struct, or enum for use by setting initial values for its properties.

intermediate

What is a designated initializer?

A designated initializer is the primary initializer for a class that fully initializes all properties introduced by that class and calls an appropriate superclass initializer.

intermediate

How does a designated initializer differ from a convenience initializer?

A designated initializer initializes all properties and calls a superclass initializer, while a convenience initializer provides shortcuts and must call another initializer from the same class.

intermediate

Why must a designated initializer call a superclass initializer?

Because the superclass may have properties that need initialization, calling its initializer ensures the entire inheritance chain is properly set up.

beginner

What happens if you don’t provide any initializer in a Swift class?

Swift provides a default initializer if all properties have default values and no custom initializers are defined.

Which initializer must fully initialize all properties introduced by its class?

A Designated initializer

B Convenience initializer

C Default initializer

D Failable initializer

A designated initializer fully initializes all properties introduced by its class.

In Swift, a convenience initializer must call:

A A designated initializer from the same class

B A superclass initializer

C Itself recursively

D No other initializer

Convenience initializers must call another initializer from the same class, usually a designated initializer.

What is the main role of a designated initializer in a class hierarchy?

A To deinitialize instances

B To provide shortcuts for initialization

C To initialize all properties and call the superclass initializer

D To override superclass methods

Designated initializers initialize all properties and call the superclass initializer to ensure proper setup.

If a Swift class has all properties with default values and no custom initializers, what initializer is available?

A Designated initializer

B Default initializer

C Convenience initializer

D No initializer

Swift automatically provides a default initializer in this case.

Which of the following is true about designated initializers?

A They can only be used in structs

B They are optional

C They cannot call other initializers

D They must call a superclass designated initializer

Designated initializers must call a superclass designated initializer to ensure proper initialization.

Explain the difference between a designated initializer and a convenience initializer in Swift.

Think about who does the full setup and who provides easier ways to create instances.

Describe why calling a superclass initializer is important in a designated initializer.

Consider what happens if the superclass is not properly set up.

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