Question

My own reasoning for it is that there is no random access and there is no way to know the bounds. But then why do we have std::advance? [EDIT] And come to think of it, why is there no std::deadvance (for lack of a better word)?

Answer 1

You are right; the requirements for operator+/- is that it be an O(1) operation, which cannot be met by bidirectional iterators. std::advance has no such speed requirement (but will use it when available, e.g., for random access iterators).

Note that boost has boost::next and boost::prior implementations for iterators; I’m not sure what their status is on standardization but if it’s in boost it’s solid.