What does an exclamation mark in a property in Swift language? [duplicate]

It's a variable of type "implicitly-unwrapped optional String". Essentially, every access of implicitStr is treated as if it were written implicitStr! (thus unwrapping the value).

This, of course, will cause a crash if the value is nil. You can still test the implicit optional via if implicitStr != nil, or use it in optional chaining as var foo = implicitStr?.uppercaseString. So you can still use it just as safely as a normal optional; it's just biased toward the case where the value is not nil.

Implicitly-unwrapped optionals are quite useful in cases where the value may not be present at initialization, but are set early and unlikely to become nil again. (For example, a variable you set in -awakeFromNib might reasonably be an implicitly-unwrapped optional.)

Further, since Objective-C methods can return both nil and object types, their return values cannot be modeled as non-optional. To avoid requiring liberal use of forced unwrapping whenever dealing with Cocoa APIs, though, the parameters and return types of Cocoa APIs are usually represented as implicitly-unwrapped optionals.