cow.swift

struct Cow<Food> {
    class Stomach {
        var food: Food? = nil

        func copy() -> Stomach {
            let stomach = Stomach()
            stomach.food = food
            return stomach
        }
    }

    var stomach = Stomach()

    mutating func eat(_ food: Food) {
        if !isKnownUniquelyReferenced(&stomach) {
            stomach = stomach.copy()
        }
        stomach.food = food
    }
}

class Grass {}

var grass1 = Grass()
var grass2 = Grass()

print("grass1 = 0x" + String(unsafeBitCast(grass1, to: Int.self), radix: 16))
print("grass2 = 0x" + String(unsafeBitCast(grass2, to: Int.self), radix: 16))

var cow1 = Cow<Grass>()
cow1.eat(grass1)
print("grass1 = 0x" + String(unsafeBitCast(cow1.stomach.food, to: Int.self), radix: 16))


var cow2 = cow1
print("grass1 = 0x" + String(unsafeBitCast(cow2.stomach.food, to: Int.self), radix: 16))

cow1.eat(grass2)
print("grass1 = 0x" + String(unsafeBitCast(cow2.stomach.food, to: Int.self), radix: 16))