Difference between optional values in swift?

Question

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?

Answer 1

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

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1. 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"
   }
   

2. 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"
   

3. 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