Swift：检查变量是否为nil

Question

I am currently learning Swift and I am experimenting with Core Data where I would like to save a linked list. To add an element at the end of the list I have a while-loop like this:

var curr = start
while (curr.nextElem != nil) {
    curr = curr.nextElem
}

The problem is though that curr.next != nil seems to be invalid, following error is shown: 'ListElem' is not convertible to UInt8

I noticed this error a few times before but always found a way around comparing but how can this be done in this case?

Answer 1

Your ListElem.nextElem property seems to be returning an actual ListElem , so it can never be nil. For it to be able to be nil, it has to be of optional type ( ListElem? ).

Also, try the Xcode 6.3 beta — most of the error messages where Swift 1.1 said "I dunno what you're doing, so I'll just say you can't convert it to UInt8 " have been replaced with better diagnostics in Swift 1.2.