What does an exclamation mark mean in the Swift language?

Question

The Swift Programming Language guide has the following example:

class Person {
    let name: String
    init(name: String) { self.name = name }
    var apar         


        

Answer 1

Simple the Optional variable allows nil to be stored.

var str : String? = nil

str = "Data"

To convert Optional to the Specific DataType, We unwrap the variable using the keyword "!"

func get(message : String){
   return
}

get(message : str!)  // Unwapped to pass as String