Extension syntax in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an extension in Swift?

An extension in Swift adds new functionality to an existing class, struct, enum, or protocol without modifying the original source code.

beginner

How do you declare an extension for a struct named Person?

Use the syntax:

extension Person {
    // new methods or properties
}

intermediate

Can extensions add stored properties to types in Swift?

No, extensions cannot add stored properties. They can only add computed properties, methods, initializers, and conform to protocols.

beginner

What is the benefit of using extensions in Swift?

Extensions help organize code better, add functionality to types you don’t own, and conform to protocols without subclassing.

beginner

Show an example of adding a method greet() to a struct Person using extension.

extension Person {
    func greet() {
        print("Hello!")
    }
}

Which of the following can you NOT add using a Swift extension?

A Computed properties

B Methods

C Stored properties

D Initializers

Extensions cannot add stored properties, only computed properties, methods, and initializers.

How do you start an extension declaration for a class named Car?

A extension Car { }

B class Car extension { }

C extend Car { }

D Car extension { }

The correct syntax is extension Car { }.

Can extensions in Swift add protocol conformance to existing types?

A No

B Yes

C Only for classes

D Only for structs

Extensions can add protocol conformance to any type.

Which keyword is used to declare an extension in Swift?

A modify

B extend

C add

D extension

The keyword extension is used to declare extensions.

What can extensions NOT do?

A Override existing methods

B Add computed properties

C Add new methods

D Add initializers

Extensions cannot override existing methods.

Explain what Swift extensions are and why they are useful.

Think about how you can add features to existing types without changing their original code.

Describe the syntax of declaring an extension and what you can add inside it.

Remember the basic structure and what features extensions support.

Practice

1. What is the main purpose of using extension in Swift?

easy

A. To add new functionality to an existing type without modifying its original code

B. To create a new subclass from an existing class

C. To replace the original implementation of a type

D. To delete methods from a type

Solution

1. Step 1: Understand what extensions do

   Extensions allow adding new features to existing types without changing their original source code.

2. Step 2: Compare options with this definition

   Only adding new functionality to an existing type without modifying its original code correctly describes this purpose. Replacing original implementation refers to overriding, creating subclasses is inheritance, and deleting methods is unrelated to extensions.

3. Final Answer:

   To add new functionality to an existing type without modifying its original code -> Option A

4. Quick Check:

   Extension purpose = Add features without changing original code [OK]

Hint: Extensions add features without changing original code [OK]

Common Mistakes:

• Confusing extensions with subclassing
• Thinking extensions replace original code
• Believing extensions can remove methods

2. Which of the following is the correct syntax to start an extension for the String type in Swift?

easy

A. extend String { }

B. extension String { }

C. extension String() { }

D. extension String[] { }

Solution

1. Step 1: Recall Swift extension syntax

   The correct syntax starts with the keyword extension followed by the type name without parentheses or brackets.

2. Step 2: Check each option

   extension String { } uses correct syntax. extend String { } uses invalid keyword. extension String() { } adds invalid parentheses. extension String[] { } uses invalid array notation.

3. Final Answer:

   extension String { } -> Option B

4. Quick Check:

   Extension syntax = extension TypeName { } [OK]

Hint: Use 'extension TypeName { }' without parentheses or brackets [OK]

Common Mistakes:

• Using 'extend' instead of 'extension'
• Adding parentheses after type name
• Using brackets after type name

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

extension Int {
    func squared() -> Int {
        return self * self
    }
}

let number = 4
print(number.squared())

medium

A. 16

B. 8

C. 4

D. Compile error

Solution

1. Step 1: Understand the extension method

   The extension adds a method squared() to Int that returns the number multiplied by itself.

2. Step 2: Calculate the output for number = 4

   Calling 4.squared() returns 4 * 4 = 16.

3. Final Answer:

   16 -> Option A

4. Quick Check:

   4 squared = 16 [OK]

Hint: Multiply the number by itself in the extension method [OK]

Common Mistakes:

• Confusing squared with doubling
• Expecting syntax error due to extension
• Forgetting to return the value

4. Identify the error in this Swift extension code:

extension Double {
    func half() -> Double {
        self / 2
    }
}

medium

A. Missing return keyword in the method

B. Extension cannot add methods to Double

C. Method name 'half' is invalid

D. No error, code is correct

Solution

1. Step 1: Check method syntax in extension

   In Swift, single-expression functions can omit the return keyword.

2. Step 2: Verify if extension on Double is allowed

   Extensions can add methods to Double. The method name 'half' is valid.

3. Final Answer:

   No error, code is correct -> Option D

4. Quick Check:

   Single-expression methods can omit return [OK]

Hint: Single-expression methods can omit 'return' keyword [OK]

Common Mistakes:

• Assuming 'return' is always required
• Thinking extensions can't add methods to built-in types
• Believing method names have restrictions

5. You want to add a computed property isEven to Int using an extension that returns true if the number is even, otherwise false. Which code correctly implements this?

hard

A. extension Int { var isEven: Bool { if self % 2 == 0 { return true } } }

B. extension Int { func isEven() -> Bool { return self % 2 == 0 } }

C. extension Int { var isEven: Bool { self % 2 == 0 } }

D. extension Int { var isEven: Bool { self / 2 == 0 } }

Solution

1. Step 1: Understand computed property syntax

   Computed properties use var with a getter. extension Int { var isEven: Bool { self % 2 == 0 } } uses correct concise syntax; the func version declares a method, the if version lacks an else branch causing compile error, the division version has wrong logic.

2. Step 2: Check logic correctness

   extension Int { var isEven: Bool { self % 2 == 0 } } uses concise expression self % 2 == 0 which returns true if even. extension Int { var isEven: Bool { if self % 2 == 0 { return true } } } omits else branch, causing compiler error. extension Int { var isEven: Bool { self / 2 == 0 } } incorrectly uses division instead of modulo.

3. Step 3: Verify distractors

   The method requires parentheses to call like num.isEven(). Incomplete if fails to compile. Division fails for even numbers like 2 (2/2 == 1 != 0).

4. Final Answer:

   extension Int { var isEven: Bool { self % 2 == 0 } } -> Option C

5. Quick Check:

   Computed property with modulo check = extension Int { var isEven: Bool { self % 2 == 0 } } [OK]

Hint: Use computed property with 'self % 2 == 0' for even check [OK]

Common Mistakes:

• Using division instead of modulo
• Confusing method with computed property
• Forgetting to return a Bool value