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: <br>

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

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.