[swift-users] Collection's "past the end" endIndex

[Tim Vermeulen]

Why is endIndex one greater than the last valid subscript argument, rather than simply the last valid subscript argument? I’m sure there are great reasons for this, but I just can’t think of them myself.

I’m implementing a binary tree and I want to implement BidirectionalCollection. Each index contains a (private) reference to the corresponding node in the tree. However, endIndex shouldn’t point to anything, and it makes things very complicated if I want a constant time implementation of index(before:). How are we supposed to deal with endIndex if the index is something more complex than a simple integer?