Re: [Boost-users] A forward iterator need not be default-constructible

I doubt there may be a sequence of length 0 at NULL.  How would you allocate such a sequence?

Actually, every iterator (singular, initialized, pointing to viable memory or not) defines its own empty range. That is, for every i, [i, i) is an empty range, which means that, if all you know is that you have that range, then it is not valid to increment, decrement, or dereference. You can certainly construct an empty range on nullptr.