mutating कीवर्ड

struct Rectangle {
    var width = 0
    var height = 0
    func area() -> Int {
        return width * height
    }
}
let r = Rectangle(width: 3, height: 4)
print(r.area())

enum Light {
    case on, off
    mutating func toggle() {
        self = (self == .on) ? .off : .on
    }
}
var l = Light.off
l.toggle()
print(l)

struct Counter {
    var value = 0
    mutating func bump() { value += 1 }
}
var c = Counter()
c.bump()
// let d = Counter(); d.bump() // error
print(c.value)

struct Player {
    var x = 0
    var y = 0
    mutating func move(dx: Int, dy: Int) {
        x += dx
        y += dy
    }
}
var p = Player()
p.move(dx: 3, dy: 5)
print(p.x, p.y)

struct Stack {
    var items: [Int] = []
    mutating func pop() -> Int? {
        return items.popLast()
    }
}
var s = Stack(items: [1, 2, 3])
print(s.pop() ?? -1)
print(s.items)

var nums = [1, 2]
nums.append(3)
print(nums)

struct Wallet {
    var coins = 0
    var label = "empty"
    mutating func add(_ n: Int) {
        coins += n
        label = coins > 0 ? "has coins" : "empty"
    }
}
var w = Wallet()
w.add(5)
print(w.coins, w.label)

struct Accumulator {
    var total = 0
    mutating func process() {
        total += 1
        total *= 10
    }
}
var a = Accumulator()
a.process()
print(a.total)

class Box {
    var value = 0
    func bump() { value += 1 }
}
let box = Box()
box.bump()
print(box.value)

struct C { var v = 0; mutating func inc() { v += 1 } }
var c = C()
c.inc()
print(c.v)