Extending built-in types in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean to extend a built-in type in Swift?

Extending a built-in type means adding new features like methods, computed properties, or initializers to existing types such as Int, String, or Array without changing their original code.

beginner

How do you declare an extension for a built-in type in Swift?

Use the keyword extension followed by the type name, then add new methods or properties inside curly braces. For example:

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

intermediate

Can extensions add stored properties to built-in types in Swift?

No, extensions cannot add stored properties. They can only add computed properties, methods, initializers, and nested types.

beginner

Why might you want to extend a built-in type?

To add useful functionality that fits your app's needs without subclassing or rewriting the original type. For example, adding a method to format a number or check a string's content.

intermediate

Example: What does this Swift extension do?<br><pre>extension String {<br>  var isPalindrome: Bool {<br>    let cleaned = self.lowercased().filter { $0.isLetter }<br>    return cleaned == String(cleaned.reversed())<br>  }<br>}</pre>

It adds a computed property isPalindrome to String that returns true if the string reads the same forwards and backwards, ignoring case and non-letter characters.

Which keyword is used to add new functionality to a built-in type in Swift?

A override

B inheritance

C extension

D protocol

The extension keyword is used to add new methods or properties to existing types.

Can you add stored properties to a built-in type using an extension?

A Yes, always

B No, never

C Only if the type is a class

D Only if the property is static

Extensions cannot add stored properties to any type, only computed properties or methods.

What is the value of 5.squared() if extension Int { func squared() -> Int { self * self } } is defined?

A 25

B 10

C 5

D Error

The method returns the number multiplied by itself, so 5 * 5 = 25.

Which of these can you add to a built-in type using an extension?

A Change stored property values

B Stored properties

C Change existing method implementations

D Computed properties

Extensions can add computed properties but cannot override or change existing methods or stored properties.

Why are extensions useful in Swift?

A They let you add functionality without subclassing

B They replace the original type's code

C They allow modifying private properties

D They make types mutable

Extensions let you add new features to existing types without subclassing or changing original code.

Explain how to extend a built-in type in Swift and what you can add with an extension.

Think about adding new abilities to a type without changing its original code.

Describe a real-life example where extending a built-in type would be helpful.

Imagine you want to add a shortcut to check or transform data on a common type.

Practice

1. What is the main purpose of extending built-in types in Swift?

easy

A. To change the stored properties of existing types

B. To delete existing methods from built-in types

C. To add new methods or computed properties to existing types

D. To create subclasses of built-in types

Solution

1. Step 1: Understand what extensions do

   Extensions allow adding new methods or computed properties to existing types without subclassing.

2. Step 2: Identify what extensions cannot do

   Extensions cannot add stored properties or delete existing methods.

3. Final Answer:

   To add new methods or computed properties to existing types -> Option C

4. Quick Check:

   Extensions add methods/properties = B [OK]

Hint: Remember: extensions add features, not stored data [OK]

Common Mistakes:

• Thinking extensions can add stored properties
• Confusing extensions with subclassing
• Believing extensions can remove methods

2. Which of the following is the correct syntax to extend the built-in Int type with a method isEven() that returns a Bool?

easy

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

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

C. extension Int { func isEven { return self % 2 == 0 } }

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

Solution

1. Step 1: Identify the correct keyword for extension

   Swift uses the keyword extension to add methods to existing types.

2. Step 2: Check method syntax

   The method must have parentheses and a return type: func isEven() -> Bool. The body uses self % 2 == 0.

3. Final Answer:

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

4. Quick Check:

   Correct extension syntax = D [OK]

Hint: Use 'extension' keyword and full method signature [OK]

Common Mistakes:

• Using 'extend' instead of 'extension'
• Omitting parentheses in method declaration
• Missing 'func' keyword before method name

3. Consider this Swift code:

extension String {
    func reversedString() -> String {
        return String(self.reversed())
    }
}

let word = "hello"
print(word.reversedString())

What is the output when this code runs?

medium

A. Error: reversed() not found

B. hello

C. nil

D. olleh

Solution

1. Step 1: Understand the extension method

   The method reversedString() returns a new String made by reversing the characters of the original string.

2. Step 2: Apply method to 'hello'

   Reversing "hello" results in "olleh".

3. Final Answer:

   olleh -> Option D

4. Quick Check:

   String reversed = olleh [OK]

Hint: Remember reversed() returns characters reversed [OK]

Common Mistakes:

• Expecting original string output
• Thinking reversed() returns an error
• Confusing reversed() with reverse()

4. What is wrong with this Swift extension code?

extension Double {
    var squared: Double {
        return self * self
    }

    var storedValue: Double = 0.0
}

medium

A. Extensions cannot add stored properties like 'storedValue'

B. Computed properties must use 'func' keyword

C. The 'squared' property must be a method, not a property

D. The extension must be declared as 'class extension'

Solution

1. Step 1: Review extension property rules

   Extensions can add computed properties but cannot add stored properties.

2. Step 2: Identify the stored property

   The line var storedValue: Double = 0.0 tries to add a stored property, which is not allowed.

3. Final Answer:

   Extensions cannot add stored properties like 'storedValue' -> Option A

4. Quick Check:

   No stored properties in extensions = A [OK]

Hint: Remember: only computed properties allowed in extensions [OK]

Common Mistakes:

• Trying to add stored properties in extensions
• Confusing computed properties with stored properties
• Thinking extensions require 'class' keyword

5. You want to extend the built-in Array type in Swift to add a method secondElement() that returns the second element if it exists, or nil otherwise. Which of the following extensions correctly implements this?

hard

A. extension Array {
func secondElement() -> Element {
return self[1]
}
}

B. extension Array {
func secondElement() -> Element? {
return self.count > 1 ? self[1] : nil
}
}

C. extension Array {
var secondElement: Element? {
if self.count >= 2 { return self[2] } else { return nil }
}
}

D. extension Array {
func secondElement() -> Element? {
if self.count > 0 { return self[1] } else { return nil }
}
}

Solution

1. Step 1: Check method signature and return type

   The method should return an optional Element (Element?) because the second element might not exist.

2. Step 2: Verify index and condition

   Arrays are zero-indexed, so the second element is at index 1. The condition should check if count > 1 to safely access index 1.

3. Step 3: Compare options

   The correct one uses ternary operator: return self.count > 1 ? self[1] : nil. The non-optional return crashes on short arrays. The computed property uses index 2 (third element). The if-else checks count > 0, allowing index 1 access when count == 1, causing runtime crash.

4. Final Answer:

   extension Array { func secondElement() -> Element? { return self.count > 1 ? self[1] : nil } } -> Option B

5. Quick Check:

   Safe optional return with index 1 = A [OK]

Hint: Check array count > 1 and return optional Element [OK]

Common Mistakes:

• Accessing wrong index (like 2 instead of 1)
• Returning non-optional Element causing crash
• Using computed property but with wrong index