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From: "temporal at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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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
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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?