According to http://boost.org/libs/iterator/doc/iterator_facade.html#minus """ template <class Dr1, class V1, class TC1, class R1, class D1, class Dr2, class V2, class TC2, class R2, class D2> typename enable_if_interoperable<Dr1,Dr2,difference>::type operator -(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs, iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs); Return Type: if is_convertible<Dr2,Dr1>::value then difference shall be iterator_traits<Dr1>::difference_type. Otherwise difference shall be iterator_traits<Dr2>::difference_type Returns: if is_convertible<Dr2,Dr1>::value then -((Dr1 const&)lhs).distance_to((Dr2 const&)rhs). Otherwise, ((Dr2 const&)rhs).distance_to((Dr1 const&)lhs). """ Is the preference for the former case over the latter intentional? This caused some surprise when I tried to re-implement my cycle_iterator_adapator (a kind of circular buffer adaptor). std::copy (b, a, ...) will do: template<typename _RandomAccessIter, typename _OutputIter> inline _OutputIter __copy(_RandomAccessIter __first, _RandomAccessIter __last, _OutputIter __result, random_access_iterator_tag) { typedef typename iterator_traits<_RandomAccessIter>::difference_type _Distance; for (_Distance __n = __last - __first; __n > 0; --__n) { *__result = *__first; ++__first; ++__result; } return __result; } As I implemented cycle_iterator_adaptor distance_to, it was expecting that given iterators [a,b], you probably want a positive distance from a to b, but the way iterator_adaptors is implemented, it wants a negative distance. Not hard to workaround, but it seemed odd that the default case was inverted, and I wondered if that was really intentional.