14.
Classes
Written by Eli Ganim
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Structures introduced you to named types. In this chapter, you’ll get acquainted with classes, which are much like structures — they are named types with properties and methods.
You’ll learn classes are reference types, as opposed to value types, and have substantially different capabilities and benefits than their structure counterparts. While you’ll often use structures in your apps to represent values, you’ll generally use classes to represent objects.
What does values vs. objects mean, though?
Creating Classes
Consider the following class definition in Swift:
class Person {
var firstName: String
var lastName: String
init(firstName: String, lastName: String) {
self.firstName = firstName
self.lastName = lastName
}
var fullName: String {
"\(firstName) \(lastName)"
}
}
let john = Person(firstName: "Johnny", lastName: "Appleseed")
That’s simple enough! It may surprise you that the definition is almost identical to its struct counterpart. The keyword class is followed by the name of the class, and everything in the curly braces is a member of that class.
But you can also see some differences between a class and a struct: The class above defines an initializer that sets both firstName and lastName to initial values. Unlike a struct, a class doesn’t provide a memberwise initializer automatically — which means you must provide it yourself if you need it. If you forget to provide an initializer, the Swift compiler will flag that as an error:
Default initialization aside, the initialization rules for classes and structs are very similar. Class initializers are functions marked init, and all stored properties must be assigned initial values before the end of init.
There is much more to class initialization, but you’ll have to wait until Chapter 15, “Advanced Classes”, which will introduce the concept of inheritance and its effect on initialization rules. This chapter will stick with basic class initializers so that you can get comfortable with classes in Swift.
Reference Types
In Swift, an instance of a structure is an immutable value, whereas an instance of a class is a mutable object. Classes are reference types, so a variable of a class type doesn’t store an actual instance — it stores a reference to a location in memory that stores the instance.
class SimplePerson {
let name: String
init(name: String) {
self.name = name
}
}
var var1 = SimplePerson(name: "John")
var var2 = var1
struct SimplePerson {
let name: String
}
The Heap vs. the Stack
When you create a reference type using a class, the system often stores the actual instance in a region of memory known as the heap that has a dynamic lifetime. Instances of value types typically reside in a region of memory called the stack that lives only as long as the current scope.
Working with References
In Chapter 11, “Structures”, you saw the copy semantics involved when working with structures and other value types. Here’s a little reminder, using the Location and DeliveryArea structures from that chapter:
struct Location {
let x: Int
let y: Int
}
struct DeliveryArea {
var range: Double
let center: Location
}
var area1 = DeliveryArea(range: 2.5,
center: Location(x: 2, y: 4))
var area2 = area1
print(area1.range) // 2.5
print(area2.range) // 2.5
area1.range = 4
print(area1.range) // 4.0
print(area2.range) // 2.5
var homeOwner = john // "Johnny Appleseed"
john.firstName = "John" // John wants to use his short name!
john.firstName // "John"
homeOwner.firstName // "John"
Mini-Exercise
Change the value of lastName on homeOwner, then try reading fullName on both john and homeOwner. What do you observe?
Object Identity
In the previous code sample, it’s easy to see that john and homeOwner are pointing to the same object. The code is short, and both references are named variables. What if you want to see if the value behind a variable is John?
john === homeOwner // true
let imposterJohn = Person(firstName: "Johnny",
lastName: "Appleseed")
john === homeOwner // true
john === imposterJohn // false
imposterJohn === homeOwner // false
// Assignment of existing variables changes the instances the variables reference.
homeOwner = imposterJohn
john === homeOwner // false
homeOwner = john
john === homeOwner // true
// Create fake, imposter Johns. Use === to see if any of these imposters are our real John.
var imposters = (0...100).map { _ in
Person(firstName: "John", lastName: "Appleseed")
}
// Equality (==) is not effective when John cannot be identified by his name alone
imposters.contains {
$0.firstName == john.firstName && $0.lastName == john.lastName
} // true
// Check to ensure the real John is not found among the imposters.
imposters.contains {
$0 === john
} // false
// Now hide the "real" John somewhere among the imposters.
imposters.insert(john, at: Int.random(in: 0..<100))
// John can now be found among the imposters.
imposters.contains {
$0 === john
} // true
// Since `Person` is a reference type, you can use === to grab the real John out of the list of imposters and modify the value.
// The original `john` variable will print the new last name!
if let indexOfJohn = imposters.firstIndex(where:
{ $0 === john }) {
imposters[indexOfJohn].lastName = "Bananapeel"
}
john.fullName // John Bananapeel
Mini-Exercise
Write a function memberOf(person: Person, group: [Person]) -> Bool that will return true if person can be found inside group and false if it can not.
Methods and Mutability
As you’ve read before, instances of classes are mutable objects, whereas instances of structures are immutable values. The following example illustrates this difference:
struct Grade {
let letter: String
let points: Double
let credits: Double
}
class Student {
var firstName: String
var lastName: String
var grades: [Grade] = []
init(firstName: String, lastName: String) {
self.firstName = firstName
self.lastName = lastName
}
func recordGrade(_ grade: Grade) {
grades.append(grade)
}
}
let jane = Student(firstName: "Jane", lastName: "Appleseed")
var history = Grade(letter: "B", points: 9.0, credits: 3.0)
var math = Grade(letter: "A", points: 16.0, credits: 4.0)
jane.recordGrade(history)
jane.recordGrade(math)
Mutability and Constants
The previous example may have had you wondering how you could modify jane even though it was a constant. After all, when you define a constant, it doesn’t change. If you recall the discussion of value types vs. reference types, it’s important to remember that, with reference types, the value is a reference.
var jane = Student(firstName: "Jane", lastName: "Appleseed")
jane = Student(firstName: "John", lastName: "Appleseed")
Mini-Exercise
Add a computed property to Student that returns the student’s Grade Point Average or GPA. A GPA is defined as the number of points earned divided by the number of credits taken. For the example above, Jane earned (9 + 16 = 25) points while taking (3 + 4 = 7) credits, making her GPA (25 / 7 = 3.57).
Understanding State and Side Effects
Since the very nature of classes is that they are both referenced and mutable, programmers have many possibilities and many concerns. Remember: If you update a class instance with a new value, every reference to that instance will also see the new value.
var credits = 0.0
func recordGrade(_ grade: Grade) {
grades.append(grade)
credits += grade.credits
}
jane.credits // 7
// The teacher made a mistake; math has 5 credits
math = Grade(letter: "A", points: 20.0, credits: 5.0)
jane.recordGrade(math)
jane.credits // 12, not 8!
Extending a Class Using an Extension
As you saw with structs, classes can be re-opened using the extension keyword to add methods and computed properties. Add a fullName computed property to Student:
extension Student {
var fullName: String {
"\(firstName) \(lastName)"
}
}
When to Use a Class Versus a Struct
You may wonder when to use a class vs. a struct. Here are some general guidelines.
Values vs. Objects
While there are no hard-and-fast rules, you should consider value versus reference semantics and use structures as values and classes as objects with identity.
Speed
Speed considerations are a thing, as structs rely on the faster stack while classes rely on the slower heap. If you’ll have many more instances (hundreds and greater), or if these instances will only exist in memory for a short time — lean towards using a struct. If your instance will have a longer lifecycle in memory, or if you’ll create relatively few instances, then class instances on the heap shouldn’t create too much overhead.
Minimalist Approach
Another approach is to use only what you need. Use structures if your data will never change or you need a simple data store. If you need to update your data and it contains logic to update its state, then use a class. Often, it’s best to begin with a struct. If you need the behavior of a class sometime later, you can convert the struct to a class.
Structures vs. Classes Recap
Structures
- Useful for representing values.
- Implicit copying of values.
- Becomes completely immutable when declared with
let. - Fast memory allocation (stack).
Classes
- Useful for representing objects with an identity.
- Implicit sharing of objects.
- Internals can remain mutable even when declared with
let. - Slower memory allocation (heap).
Challenges
Before moving on, here are some challenges to test your knowledge of classes. It’s best to try and solve them yourself, but solutions are available if you get stuck. These came with the download or are available at the printed book’s source code link listed in the introduction.
Challenge 1: Movie Lists
Imagine you’re writing a movie-viewing app in Swift. Users can create lists of movies and share those lists with other users. Create a User and a List class that uses reference semantics to help maintain lists between users.
Challenge 2: T-shirt Store
Your challenge here is to build a set of entities to support a T-shirt store. Decide if each entity should be a class or a struct and why.
Key Points
- Like structures, classes are a named type that can have properties and methods.
- Classes use references that are shared on assignment.
- Class instances are called objects.
- Objects are mutable.
- Mutability introduces state, which adds complexity when managing your objects.
- Use classes when you want reference semantics; structures for value semantics.