Structs

Basics of Structs

struct Repository {
    let identifier: Int
    let name: String
    var description: String?
}

This defines a Repository struct with three stored properties, an integer identifier, a string name, and an optional string description. The identifier and name are constants, as they’ve been declared using the let-keyword. Once set during initialization, they cannot be modified. The description is a variable. Modifying it updates the value of the structure.

Structure types automatically receive a memberwise initializer if they do not define any of their own custom initializers. The structure receives a memberwise initializer even if it has stored properties that do not have default values.

Repository contains three stored properties of which only description has a default value (nil). Further it defines no initializers of its own, so it receives a memberwise initializer for free:

let newRepository = Repository(identifier: 0, name: "New Repository", description: "Brand New Repository")

Structs are value types

Unlike classes, which are passed by reference, structures are passed through copying:

first = "Hello"
second = first
first += " World!"
// first == "Hello World!"
// second == "Hello"

String is a structure, therefore it’s copied on assignment.

Structures also cannot be compared using identity operator:

window0 === window1 // works because a window is a class instance
"hello" === "hello" // error: binary operator '===' cannot be applied to two 'String' operands

Every two structure instances are deemed identical if they compare equal.

Collectively, these traits that differentiate structures from classes are what makes structures value types.

Mutating a Struct

A method of a struct that change the value of the struct itself must be prefixed with the mutating keyword

struct Counter {
    private var value = 0
    
    mutating func next() {
        value += 1
    }
}

When you can use mutating methods

The mutating methods are only available on struct values inside variables.

var counter = Counter()
counter.next()

When you can NOT use mutating methods

On the other hand, mutating methods are NOT available on struct values inside constants

let counter = Counter()
counter.next()
//  error: cannot use mutating member on immutable value: 'counter' is a 'let' constant

Structs cannot inherit

Unlike classes, structures cannot inherit:

class MyView: NSView { }  // works

struct MyInt: Int { } // error: inheritance from non-protocol type 'Int'

Structures, however, can adopt protocols:

struct Vector: Hashable { ... }  // works

Accessing members of struct

In Swift, structures use a simple “dot syntax” to access their members.

For example:

struct DeliveryRange {
  var range: Double
  let center: Location
}
let storeLocation = Location(latitude: 44.9871,
                             longitude: -93.2758)
var pizzaRange = DeliveryRange(range: 200,
                               center: storeLocation)

You can access(print) the range like this:

    print(pizzaRange.range) // 200

You can even access members of members using dot syntax:

 print(pizzaRange.center.latitude) // 44.9871

Similar to how you can read values with dot syntax, you can also assign them.

pizzaRange.range = 250