# 08.Optional实质

## 底层实现

我们先看看`Swift`中`Optional`是怎么实现的把

```
public enum Optional<Wrapped> : _Reflectable, NilLiteralConvertible {
    case None
    case Some(Wrapped)
    /// Construct a `nil` instance.
    public init()
    /// Construct a non-`nil` instance that stores `some`.
    public init(_ some: Wrapped)
    /// If `self == nil`, returns `nil`.  Otherwise, returns `f(self!)`.
    @warn_unused_result
    public func map<U>(@noescape f: (Wrapped) throws -> U) rethrows -> U?
    /// Returns `nil` if `self` is `nil`, `f(self!)` otherwise.
    @warn_unused_result
    public func flatMap<U>(@noescape f: (Wrapped) throws -> U?) rethrows -> U?
    /// Create an instance initialized with `nil`.
    public init(nilLiteral: ())
}
```

很明显可以看出，它实际是一个枚举。 `.Some`的关联值就是我们解包出来的

## ? & ！

在我们对一个`Optional`的变量进行赋值的时候，我们实际上是将初始值放到`.Some`中，取值是则是从`.Some`中取出。

* `!`
  * 强制解包，告诉系统这个变量不为`nil` 且 `.Some`中一定有值
  * 如果为空则会崩溃
* `?`
  * 解包时会先判断是否为`nil`如果不为空则正常解包，否则会返回`nil`即默认初始值


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