[boost] [repost][multi_index][hash][rfc] Hashed indices preview + Boost.Hash

From: Joaquín Mª López Muñoz (joaquin_at_[hidden])
Date: 2005-02-16 08:20:22

Allow me to repost this message from a couple of days ago, since
nobody responded :( In another situation, I could proceed according
to my own judgement, but point (6), in particular,  necessitates a
public discussion, IMHO. Thank you,

Hello all,

You can find a preview version of Boost.MultiIndex including hashed
indices plus Daniel James' Boost.Hash library at the Sandbox Files
area:

multi_index_140205_plus_hash_part_1.zip
multi_index_140205_plus_hash_part_2.zip

(split in two for maximum size reasons.)

There are a number of things we'd like to ask you feedback about:

1. Hashed indices are modeled according to the latest TR1 draft, with
some significant diversions:
1.1. erase() does not throw ever, even it the user-provided Hash object
throws. This is imposed by the general framework of Boost.MultiIndex.
This goodie does not come for free: if the Hash object throws, erase()
decays to a linear search. Not that there's any alternative, but I'd
like
to point it out.
1.2. insert() has strong exception safety (TR1 only requires basic.)
Again, the way Boost.MultiIndex is designed forces this.
1.3. hash indices iterators are bidirectional.
1.4. Usual implementations use one pointer per bucket, whereas
Boost.MultiIndex has two pointers per bucket. This addresses potential
performance problems of insert() and erase() under low load conditions,
as discussed in
http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2005/n1756.pdf
issue 6.14. Is this convenient?
2. The material is complete except for a section in the tutorial and
possibly
one more example (example 8 already exercises hashed indices.) Tests
fully sport hashed indices. The reference is also updated.
3. All in all, I'd greatly appreciate your comments on this:
suitability,
documentation, performance, etc. I plan to commit this to the CVS
if nobody objects in a reasonable amount of time (a couple of weeks.)
I'm open to all kind of suggestions --I'm eager to recieve them, as it
happens :)

4. As for Boost.Hash, this library by Daniel James' implements the
ideas for std::tr1::hash (albeit in boost namespace) proposed by
Peter Dimov in the aforementioned paper, issue 6.18. The idea
is to provide a general purpose hash<> utility for use by
  * Boost.MultiIndex
  * TR1-based unordered containers (Daniel is working on this.)
  * Eventually, wrapping by Boost.TR1.
5. The library comes fully documented and tested, and in our view
well prepared for uploading to the CVS.
6. Yet, this hasn't undergone any formal review, so some of you might
(with reason) object to its being commited to the CVS. From our point
of view, we have three valid alternatives:
  * Boost members agree to have it in CVS without more ado.
  * As this is used by Boost.MultiIndex, Boost.Hash is suitable for
  fasttrack review.
  * This is untolerable and the library should be push_back()'ed to
  the review queue. Meanwhile, Boost.Hash should live as an impl
  detail of Boost.MultiIndex.

We hope you find the material useful and look forward to knowing
your opinions about the issues brought forward.

Best,

Joaquín M López Muñoz
Daniel James