swift中關於！的解包操做

// 聲明可選型arr
var arr: CFArray?

// 指定arr1爲CFArray類型
//let arr1 = arr as CFArray
// 該類型爲可選型,但取值時必定不爲空
//let arr1 = arr as CFArray!
//let arr2: CFArray! = arr

// 強制arr3類型爲CFArray
//let arr3 = arr as! CFArray
// 強制arr4類型爲CFArray
//let arr4: CFArray = arr
複製代碼