[commonlisp94]

> something about within an allocation and one past the end?

Yeah, I forgot about that. So I agree there is some subtly which is likely to catch beginners.

Your example could safely be:

   if (std::distance(begin, end) > 5) 

Another approach I would recommend is to write a `guarded_advance` which takes an integer and the end pointer.

Also note that the situation you are describing is still a little unusual because the baseline assumption is it takes linear time to advance an iterator by more than 1 increment.

> but neither will valid increment between two integers. No need for iterators in that case either.

The purpose of an iterator is to abstract data structure access. The coordinate inside a complex data structure may not be representable by an integer.