Optional是以enum和泛型为基础的高阶类型

结论：1、optionals使用时需要检查；2、可以通过！+赋值语句转化为非optionals。

Optional-Generic Enumeration

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

}

 

A type that represents either a wrapped value or nil, the absence of a value.

The types of shortForm and longForm in the following code sample are the same:

let shortForm: Int? = Int("42")

let longForm: Optional<Int> = Int("42")

Optionals

？！

You use optionals in situations where a value may be absent. An optional represents two possibilities: Either there is a value, and you can unwrap the optional to access that value, or there isn’t a value at all.

optionals indicate that a constant or variable is allowed to have “no value”. Optionals can be checked with an if statement to see if a value exists, and can be conditionally unwrapped with optional binding to access the optional’s value if it does exist.

1. let possibleString: String? = "An optional string."

1. let possibleString: String? = "An optional string."
2. let forcedString: String = possibleString! // requires an exclamation mark