swift - Why Implicitly unwrapped optional was not assigned? -

in following code, why 5 not assigned "somevar" ?

class viewcontroller: uiviewcontroller {      var somevar : int?      override func viewdidload() {         super.viewdidload()         somevar! = int(5) // why 5 not assigned somevar here     } } 

background:

somevar declared optional variable means if variable nil command using variable ignored.

example:

class viewcontroller: uiviewcontroller {      var somevar : int?      override func viewdidload() {         super.viewdidload()         somevar? = 5 // command ignored bcz somevar nil     } } 

to forcefully execute command on our own risk use "implicitly unwrapped optional" sure command executed, in case following line executed

somevar! = 5  

fatal error: unexpectedly found nil while unwrapping optional value

when line executed, why "5" not assigned "somevar" instead fatal error occurs?

class viewcontroller: uiviewcontroller {      var somevar : int?      override func viewdidload() {         super.viewdidload()         somevar! = 5     } } 

when something! (emphasis on ! mark), "force reading" (force unwrapping) optional.

that above code tries read something before assigning new value.
since something nil, code explodes.

to illustrate:

var somevar: int?  print(somevar!) // code explodes! 💥  print(somevar) // output "nil"  somevar = 5  print(somevar!) // output "5"  print(somevar) // output "optional(5)" 

as @leodabus stated, covered in apple's awesome swift book.
(btw book! 📘❤️)