struct SBox { var n: Int }
class CBox { var n: Int; init(_ n: Int) { self.n = n } }
var s1 = SBox(n: 1); var s2 = s1
s2.n = 99            // s1 is untouched: a struct is copied
let c1 = CBox(1); let c2 = c1
c2.n = 99            // c1 sees it: a class is shared
print(s1.n, c1.n)

struct copies, class shares