swift中关于！的解包操做

// 声明可选型arr
var arr: CFArray?

// 指定arr1为CFArray类型
//let arr1 = arr as CFArray
// 该类型为可选型,但取值时必定不为空
//let arr1 = arr as CFArray!
//let arr2: CFArray! = arr

// 强制arr3类型为CFArray
//let arr3 = arr as! CFArray
// 强制arr4类型为CFArray
//let arr4: CFArray = arr
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