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

Here are some examples:

var name:String = "Hello World"
var word:String?

Where word is an optional value. means it may or may not contain a value.

word = name 

Here name has a value so we can assign it

var cow:String = nil
var dog:String!

Where dog is forcefully unwrapped means it must contain a value

dog = cow

The application will crash because we are assign nil to unwrapped