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From: "temporal at gmail dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug libstdc++/58153] New: unordered_multimap::erase(iterator) is not constant-time when many entries have the same key Date: Wed, 14 Aug 2013 10:11:00 -0000 [thread overview] Message-ID: <bug-58153-4@http.gcc.gnu.org/bugzilla/> (raw) 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?
next reply other threads:[~2013-08-14 10:11 UTC|newest] Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top 2013-08-14 10:11 temporal at gmail dot com [this message] 2013-08-14 10:35 ` [Bug libstdc++/58153] " redi at gcc dot gnu.org 2013-08-14 11:16 ` temporal at gmail dot com 2013-08-24 16:30 ` fdumont at gcc dot gnu.org 2013-08-24 19:16 ` temporal at gmail dot com 2013-08-26 20:06 ` fdumont at gcc dot gnu.org 2013-08-27 3:10 ` temporal at gmail dot com 2015-04-09 14:49 ` redi at gcc dot gnu.org
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