Currently, flat_multimap mirrors C++98's multimap concept. I'd like to propose an update to flat_multimap fulfil the C++11's multimap concept. The part I'm particularly interested are guarantees about preserving the insertion ordering for elements with equivalent keys. On ISO/IEC 14882-2011, 23.2.4 - associative container requirements, pages 741 and 742, you can see the effects for a_eq.emplace and a_eq.insert ends with "If a range containing elements equivalent to t exists in a_eq, t is inserted at the end of that range". I see no similar guarantees on the Boost's flat_multimap documentation[1]. This feature is useful, for instance, in HTTP messages, where field reordering is illegal[2] if the keys are equivalent. I did a few tests and it looks to me that flat_multimap already provides these guarantees, but before I go check the source and possibly provide patches, I need to know if there is interest in turning this accidental behaviour into intentional behaviour through documented guarantees. [1] http://www.boost.org/doc/libs/1_56_0/doc/html/boost/container/flat_multimap.... [2] http://tools.ietf.org/html/rfc7230#section-3.2.2 -- VinÃcius dos Santos Oliveira https://about.me/vinipsmaker