Difference between optional values in swift?

问题

What is the difference between:

var title:String? = "Title" //1
var title:String! = "Title" //2
var title:String = "Title" //3

What am I saying if I were to set title in each way and am I forced to unwrap each variable in a different way?

回答1:

Think about ? and ! like a box that might have a value or not.

I recommend this article.

Optional box that might have value or might not, and that optional box is not unwrapped.

var title:String? = "Title" //second and third picture

You use unwrapped value like that:

if let title = title {
    //do sth with title, here is the same like let title: String = "Title"
}

Optional box that might have a value or might not, and that optional box is actually unwrapped. If there is a value and you access that value, that is ok (second image, just replace ? with !), but if there is no value, then app crash (third image, just replace ? with !)
```
var title:String! = "Title"
```
That variable have a value for sure, and you cannot assign to this value nil (because it is not optional). Optional means that there is a value or there is no value (nil):
```
var title:String = "Title" //first picture
```

回答2:

`var title:String? = "Title"`

title currently has a value of Title, but in the future it could possibly be nil. I will need to unwrap it using optional binding:

if let unwrappedTitle = title {
   println(unwrappedTitle)
}

Or by forcing the unwrap with the ! character

let unwrappedTitle = title!

The above will crash if title is nil

`var title:String! = "Title"`

title currently has a value of "Title". It could possibly be nil, but I know that it never will be when I am using it. You don't need to unwrap this with optional binding or by forcing the unwrap with the ! character.

Your program will crash if this value is ever accessed while nil, but the compiler will let you set this value to nil.

`var title:String = "Title"`

title currently has a value of "Title". This may change, but the variable title will always have some string value. I don't need to check for nil with optional binding or by forcing an unwrap. The compiler will not let you build if you try to set title to nil.

回答3:

var title:String? = "Title" //1 Nil Possible-Force Unwrap needed-Nil checking when use
var title1:String! = "Title"//2 Nil Possible-Force Unwrap not needed-Nil cheking when Use
var title2:String = "Title" //3 Nil Not possible as its initailize when declared-No force unwrap needed-No Nil Cheking is needed.

来源：https://stackoverflow.com/questions/29054096/difference-between-optional-values-in-swift

标签

swift

optional-values