Swift中? 、! 和 ??

enum Optional<T> : LogicValue, Reflectable {  
        case None  
        case Some(T)  
        init()  
        init(_ some: T)  
        /// Allow use in a Boolean context.  
        func getLogicValue() -> Bool  
       /// Haskell's fmap, which was mis-named  
        func map<U>(f: (T) -> U) -> U?  
        func getMirror() -> Mirror 
}

if name { 
// 有值再操作
}

var person:Person?
var str:String?
var num:NSNumber!

 let str:String! = "Hello"
        let greeting = "World!"
        if str != nil {
          let message = str + " " + greeting
          print(message)
}

let str:String? = "Hello"
        let greeting = "World!"
        if str != nil {
            let message = str! + " " + greeting
            print(message)
 }

let helloStr:String? = nil
let x = helloStr ?? "Hello World!"
print(x)