Is "x? .Y" in Swift the same as "x?" and then ".y"?

Yes opt?.isEmpty

different from String?

. It's called optional chaining . From a quick programming guide:

You specify an optional string by putting a question mark (?) After the optional value on which you want to call a property, method, or index, if the optional parameter is nonzero. This is very similar to placing an exclamation mark (!) After an optional value to force the expansion of its value. The main difference is that the extra chaining fails gracefully when the optional parameter is zero, whereas the forced unwrapping triggers a runtime error when the option is zero.

This creates an optional as you think var opt: String?

Now

 opt?.isEmpty //it will not crash if opt is nil 

      

        
        
        
      

    

matches the exception

opt!.isEmpty //it will crash if opt is nil 

      

        
        
        
      

    

if opt!

- nil

, than will not crash at runtime. The optional chaining is used to call long sequences of optional chaining without calling. Each additional chain returns an optional ie opt? returns the optional expands it, and if nil than not calling isEmpty else, calling isEmpty and reutns value. Also

  (opt?).isEmpty 

      

        
        
        
      

    

When you write the instruction above, it is optional (optional) and it cannot be unpacked due to curly braces. This error displays

$T2?? does not have a member named `isEmpty`

      

        
        
        
      

    

to expand it

(opt?)!.isEmpty

      

        
        
        
      

    

it will return false

Edit: More ...

   var o1 = opt?
   var o2 = ((opt?)?)

      

        
        
        
      

    

does nothing on its own, it just assigns a value o1

, o2

ie String ?. to expand opt

and o1

, o2

they are both optional and need one statement !

to expand it.

Also, please don't understand between String?

and opt?

they are both different, when? used after some type in a declaration that it makes optional, and when ?

used after a variable opt

it is used to unwrap in an optional chaining and return an optional return value

Additional materials:

try this to clarify

(((opt?)?)!).isEmpty     //it will unwrap with single !
((((opt?)?)!)?).isEmpty   //compiler will show suggestion to remove ?

      

        
        
        
      

    

the below statement makes optional optional

   var opt:String??? = "Optional"

      

        
        
        
      

    

to deploy

   opt!!!.isEmpty

      

        
        
        
      

    

EDIT2

opt?

always returns optional, but if opt

defined as String! it implicit optional

and opt?

will return optional(explicit)

. But if it opt

does opt?

n't necessarily do anything

From the Quick Programming Guide

In other words:

If the type you are trying to retrieve is not optional, it will become optional because of the optional chaining.
If the type you are trying to retrieve is already optional, it will not become more optional because of the chaining.

      

        
        
        
      

    

Thus:

If you try to retrieve an Int value through optional chaining, an Int? is always returned, no matter how many levels of chaining are used.
Similarly, if you try to retrieve an Int? value through optional chaining, an Int? is always returned, no matter how many levels of chaining are used.