dear all: i'm a game developer, and my projects used a lot of multi_index(thanks, it's a great library). one of main problems of game development is to rank players according to they level. usually we use boost::interprocess::map for the task. but more, players should know which rank they are and who is at rank 10000 (for example). these requirements lead to two new methods for map: iterator find_by_rank(int idx) int rank(iterator i) i implemented an argumented rbtree data structure according to "Algorithms and Data Structures " (see the attachment rank_tree.hpp), and latter i find coutertree which is in boost review schedule. but IMHO, the best way is to integrate it to multi_index, and if anyone interests, i can do this by some efforts. yours sincerely