ref – http://stackoverflow.com/questions/24083842/what-is-the-difference-between-string-and-string-two-ways-of-creating-an-opti

Use ? if the value can become nil in the future, so that you test for this. variables with ? will degrade gracefully if nil.

Use ! if it really shouldn’t become nil in the future, but it needs to be nil initially. If the variable is nil and you use exclamation, it will crash.