http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58153
Bug ID: 58153 Summary: unordered_multimap::erase(iterator) is not constant-time when many entries have the same key Product: gcc Version: 4.8.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: libstdc++ Assignee: unassigned at gcc dot gnu.org Reporter: temporal at gmail dot com It appears that if an unordered_multimap has k entries with the same key, then erase(iter) for any of those entries is O(k) rather than constant-time. The problem is that _Hashtable::erase() searches through all nodes in the bucket looking for the one previous to the one being removed. This is reasonable for unordered_map, where buckets are expected to have no more than a couple entries. But it is surprising for unordered_multimap, whose whole purpose is to support multiple entries with the same key (and therefore the same bucket). I do not know exactly what the standard requires here, but all of the references I can find claim that erase(iter) should be average-time O(1), and none of them suggest that having a large number of entries with the same key should cause trouble. FWIW, it looks like libc++ has the same behavior. Maybe my expectations were wrong, and unordered_multimap was never meant to contain more than a couple entries per key?