Why is there no operator +/- for a bidirectional iterator?

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

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.