What is the difference between optional and implicitly unwrapped optional?

Question

Consider these two declarations:

var implicitUnwrappedOptionalInt: Int! = nil

var optionalInt: Int? = nil

The result would be nil in both the cases. So what is the difference?

Answer 1

An implicitly unwrapped optional can be (will be) initialized nil BUT you are telling the compiler that it will be set to a value before you first use it. If you do try and use it before it is initialized like I do below, you will crash everytime.