很多人在刚上手swift时对于Optional中 ‘!’ 和 ‘?’ 的使用十分不理解，接下来我会谈一谈自己对于这两个符号的使用方式的理解。

先来说说Optional的概念，以方便之后的理解。来看一下下面的代码：

var a : String = nil              // 编译错误，String类型不能为nil
var b : String = “Hello!”

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Swift 中的普通类型不再能设为nil。那如何表示这个值不存在呢？所以引进了Optional的概念：代表 nil 或某个具体的值。例如：

var c : String? = nil       
var d : String? = “Hello!” 

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String? 就是一个Optional，它既能够被具体类型赋值, 也可以赋值为nil。通过 String 和 Optional的比较，发现Optional 就相当于把具体类型和 nil 打包捆绑在了一起，转变成了一种新的类型。

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