Memberwise initializer in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a memberwise initializer in Swift?

A memberwise initializer is an automatically created initializer for structs in Swift. It lets you create an instance by providing values for each property directly.

beginner

Does Swift automatically create a memberwise initializer for classes?

No, Swift does not automatically create a memberwise initializer for classes. It only does this for structs.

beginner

Given this struct:

struct Car {
  var make: String
  var year: Int
}

How do you create a new Car instance using the memberwise initializer?

You create it by passing values for each property like this:<br><pre>let myCar = Car(make: "Toyota", year: 2020)</pre>

intermediate

What happens if you add a custom initializer to a struct in Swift?

If you add a custom initializer, Swift does not generate the memberwise initializer automatically anymore. You have to write it yourself if you want it.

beginner

Why is the memberwise initializer useful?

It saves time by letting you create instances quickly without writing your own initializer. It also makes your code cleaner and easier to read.

Which Swift type automatically gets a memberwise initializer?

A Enum

B Class

C Protocol

D Struct

Only structs get an automatic memberwise initializer in Swift.

What do you need to provide when using a memberwise initializer?

A A closure

B Only some properties

C Values for all properties

D No values, it uses defaults

You must provide values for all stored properties when using the memberwise initializer.

If you add a custom initializer to a struct, what happens to the memberwise initializer?

A It is removed

B It stays available

C It becomes private

D It throws an error

Adding a custom initializer removes the automatic memberwise initializer.

Can classes in Swift use memberwise initializers automatically?

A No

B Yes

C Only if they have no methods

D Only if they inherit from NSObject

Classes do not get automatic memberwise initializers in Swift.

What is the main benefit of a memberwise initializer?

A Automatically generate methods

B Quickly create instances with property values

C Manage memory automatically

D Enforce access control

Memberwise initializers let you quickly create instances by providing values for all properties.

Explain what a memberwise initializer is and how it helps when creating struct instances in Swift.

Think about how you create a struct instance without writing your own initializer.

Describe what happens to the memberwise initializer when you add a custom initializer to a Swift struct.

Consider how Swift handles initializers when you add your own.

Practice

1. What does a memberwise initializer in Swift do for a struct?

easy

A. It creates a function to print all properties.

B. It deletes all properties from the struct.

C. It automatically creates an initializer that sets all properties at once.

D. It converts the struct into a class.

Solution

1. Step 1: Understand memberwise initializer purpose

   A memberwise initializer lets you create an instance by setting all properties in one call.

2. Step 2: Recall Swift default behavior for structs

   Swift automatically provides this initializer for structs without custom initializers.

3. Final Answer:

   It automatically creates an initializer that sets all properties at once. -> Option C

4. Quick Check:

   Memberwise initializer = sets all properties at once [OK]

Hint: Memberwise initializer sets all struct properties at once [OK]

Common Mistakes:

• Thinking it prints properties
• Confusing with class initializers
• Assuming it deletes properties

2. Which of the following is the correct way to use a memberwise initializer for this struct?

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

easy

A. let myBook = Book(title -> "Swift Guide", pages -> 300)

B. let myBook = Book("Swift Guide", 300)

C. let myBook = Book(title = "Swift Guide", pages = 300)

D. let myBook = Book(title: "Swift Guide", pages: 300)

Solution

1. Step 1: Recall memberwise initializer syntax

   It uses parameter labels matching property names: title: and pages:.

2. Step 2: Check each option

   let myBook = Book(title: "Swift Guide", pages: 300) uses correct syntax with labels and values. Others use wrong syntax or missing labels.

3. Final Answer:

   let myBook = Book(title: "Swift Guide", pages: 300) -> Option D

4. Quick Check:

   Memberwise initializer syntax = labels and values [OK]

Hint: Use property names as labels in initializer call [OK]

Common Mistakes:

• Omitting labels in initializer
• Using = instead of : in parameters
• Using arrows -> instead of colons

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

struct Point {
  var x: Int
  var y: Int
}

let p = Point(x: 5, y: 10)
print(p.x + p.y)

medium

A. 510

B. 15

C. x: 5, y: 10

D. Error: No memberwise initializer

Solution

1. Step 1: Understand memberwise initializer usage

   The struct Point has two Int properties and uses the memberwise initializer to set x=5 and y=10.

2. Step 2: Calculate the print output

   p.x + p.y = 5 + 10 = 15, so print outputs 15.

3. Final Answer:

   15 -> Option B

4. Quick Check:

   5 + 10 = 15 [OK]

Hint: Add property values after initializing with memberwise initializer [OK]

Common Mistakes:

• Concatenating numbers as strings
• Expecting a print of property names
• Assuming no initializer exists

4. Identify the error in this code using a memberwise initializer:

struct Car {
  var brand: String
  var year: Int
}

let myCar = Car(brand: "Toyota")

medium

A. Missing value for 'year' property in initializer

B. Incorrect property name 'brand'

C. Cannot create instance of struct

D. Syntax error in struct declaration

Solution

1. Step 1: Check memberwise initializer requirements

   All properties must be initialized. 'year' is missing in the initializer call.

2. Step 2: Identify error cause

   Since 'year' is not given a value, the code will cause a compile error.

3. Final Answer:

   Missing value for 'year' property in initializer -> Option A

4. Quick Check:

   All properties must be set in memberwise initializer [OK]

Hint: Provide all properties when using memberwise initializer [OK]

Common Mistakes:

• Forgetting to initialize all properties
• Assuming default values without setting
• Misnaming properties in initializer

5. Given this struct with a default property value:

struct User {
  var name: String
  var age: Int = 18
}

Which initializer call is valid and why?

hard

A. let u = User(name: "Anna") // age uses default value

B. let u = User(name: "Anna", age: 20) // must always provide age

C. let u = User() // no parameters allowed

D. let u = User(age: 18) // name can be omitted

Solution

1. Step 1: Understand default property values in structs

   Properties with default values can be omitted in memberwise initializer calls.

2. Step 2: Analyze each initializer call

   let u = User(name: "Anna") // age uses default value provides 'name' only; 'age' uses default 18, which is valid. Others omit required 'name' or incorrectly require all parameters.

3. Final Answer:

   let u = User(name: "Anna") // age uses default value -> Option A

4. Quick Check:

   Default property values allow skipping parameters [OK]

Hint: Skip properties with default values in initializer call [OK]

Common Mistakes:

• Trying to omit non-default properties
• Assuming all properties must be provided
• Calling initializer with no parameters when required