[String！]和[String]之间有什么区别！

Question

I'm quite new to Swift and want to know the difference between [String!] and [String]! . Both are non optional? but [String]! indicates nil when uninitialized and [String!] does not?

Answer 1

[String!] is an array of implicitly unwrapped String s

[String]! is an implicitly unwrapped array of String s

That means that the first one can contain nil values but cannot itself be nil . The second one could itself be nil but its contents are always non- nil .

Answer 2

[String!]

This is an array of Implicitly Unwrapped Strings .

It means that in each element of this array you could find a String or nil .

let list0: [String!] = ["Hello", nil, "world", nil]

[String]!

This is an Implicitly Unwrapped Array that contains String .

It means that in a variable declared like this you can find nil or an array where every element is a valid String .

let list1: [String]! = nil
let list2: [String]! = ["Hello", "world"]

Answer 3

[String!] - this is a non optional array of optional Strings .
[String]! - this is a optional array of non optional Strings .