Currently unordered_ use doubly-linked lists and provide iterators that can be decremented.

It is sufficient to provide forward iterators and use single linked list. This will save some memory and help to address #1036 to some extent. Also it will address DevCom-10035118

From #1036 (comment):

unordered_meow really shouldn't be backed by a doubly-linked list; it is required to be forward but not bidi, so it doesn't need sentinel nodes. We should be able to arrange its representation (including the bucket "map") to avoid allocating memory.

vNext note: Resolving this issue will require breaking binary compatibility. We won't be able to accept pull requests for this issue until the vNext branch is available. See #169 for more information.