as?, as!はどちらもdowncast、??はnilに備える演算子、?がfalseに備える演算子であるのと似ている

implicitlyUnwrappedOptional.swift

var a: Any? = nil
a = 2
//a = 2.0                                                                       
let b = a as? Int //可能ならOptionalへ、失敗ならnil
let c = a as! Int?
//let d = a as Int? compile error!!
let d = a as? Int?
//let e = a! // forced unwrap
print(b)
print(c)
print(d)
//
print(a.dynamicType)
print(b.dynamicType)
print(c.dynamicType)

//Optional(2)
//Optional(2)
//Optional(Optional(2))
//Optional<protocol<>>
//Optional<Int>
//Optional<Int>
var p: Any? = nil
p = 4
let q = p as? Int ?? 0
print(q)
print(q.dynamicType)
p = nil
let r = p as? Int ?? 0
print(r)
print(r.dynamicType)
p = 8
let s = p as? Int
print(s)

//  4                                                                           
//  Int
//  0
//  Int
//  Optional(8)