Re: [Boost-users] [multi index].[composite key] Index creation at run-time.

From: Prashant Thakre (prashant.thakre_at_[hidden])
Date: 2006-06-08 14:46:30

One final suggestion :) regarding the choice of index_container in
terms of performance,
memory requirements, serialization support and insert/lookup
opreations considering that std::vector<const std::string> will be the
basis for this container :
1.Composite keys and hashed indices.
2.Hashed indices.

Thanks a lot for clarifying all the doubts.
-- 
regards,
Prashant Thakre
----- Original Message -----
From: "Joaquín Mª López Muñoz" <joaquin_at_[hidden]>
Newsgroups: gmane.comp.lib.boost.user
Sent: Thursday, June 08, 2006 6:51 PM
Subject: Re: [multi index].[composite key] Index creation at run-time.
Prashant Thakre ha escrito:
> Hi,
>
> > OK, here, instead of a composite_key instantiation,
> > you can take advantage directly of std::vector comparison
> > semantics (you don't really need tuple_element):
> >
> > typedef multi_index_container<
> >   element,
> >   indexed_by<
> >     hashed_unique<
> >       member<element,std::vector<const std::string>,&element::tp>
> >     >
> >     // other indices, maybe
> >   >
> > > element_collection;
> >
>
>  Is this equivalent to having a member std::string tp which is constructed at
> run-time from all the elements of std::vector< const std::string > ?
Depends on how you construct that string: if you just concatenate
the vector elements, then no, it's not equivalent. For instance, the following
two vectors
{"mast","err","apparent"}
{"master","rap","parent"}
both map to "masterrapparent".
> Also, how does the hash function generate the key ?
Boost.Hash combines the vector element's hash values as described at
http://boost.org/doc/html/id2490467.html
B.MI also lets you specify your own hashing mechanism if you need it:
http://boost.org/libs/multi_index/doc/advanced_topics.html#hash_spec
Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo