Structs vs Classes, properties & inits

Overview

Value vs reference

struct Point { var x = 0, y = 0 }
class CPoint { var x = 0, y = 0 }

var a = Point(x: 1, y: 1)
var b = a          // copy
b.x = 9
print("struct a.x =", a.x, "b.x =", b.x) // 1, 9

let c = CPoint(); c.x = 1
let d = c          // same reference
d.x = 9
print("class c.x =", c.x, "d.x =", d.x) // 9, 9

Properties

struct Rectangle {
    var width: Double      // stored
    var height: Double     // stored
    var area: Double {     // computed
        return width * height
    }
}
let r = Rectangle(width: 3, height: 4)
print(r.area)  // 12

Observers

struct Counter {
    var value: Int = 0 {
        willSet { print("will set to", newValue) }
        didSet  { print("did set from", oldValue) }
    }
}
var cnt = Counter()
cnt.value = 5

Initializers

struct User {
    var name: String
    // memberwise init is synthesized: User(name:)
}
class Person {
    var name: String
    init(name: String) {    // designated
        self.name = name
    }
    convenience init() {    // convenience
        self.init(name: "Guest")
    }
}
let u = User(name: "Ada")
let p = Person()
print(u.name, p.name)

deinit

class Temp {
    let id: Int
    init(id: Int) { self.id = id }
    deinit { print("Temp \\(id) deinit") }
}
do {
    let t = Temp(id: 1)
    print("using", t.id)
} // t goes out of scope and deinit may run

Struct vs Class Q

Recap