[Boost-users] [Unordered] [Multi-Index] Set lookup from something else than key_type

Subject: [Boost-users] [Unordered] [Multi-Index] Set lookup from something else than key_type
From: Dominique Devienne (ddevienne_at_[hidden])
Date: 2009-09-01 12:24:14

We currently have a unordered_map< Uid, shared_ptr<Identity> >,
where Identity already contains the Uid.

So I thought I could avoid the duplicated Uid by using an
unordered_set< shared_ptr<Identity>, MyHash, MyPred >
to hash/compare by the internal Uid, but unlike a Multi-Index hashed_index [1],
unordered doesn't seem to provide:

    "lookup operations accepting search keys different from the
key_type of the index,
    which is a specially useful facility when key_type object are
expensive to create."

Any reason why unordered cannot allow similar "compatible key" lookups?

What kind of overhead is there (memory use, runtime), if any, in using
Multi-Index as
opposed to unordered_set in this scenario?

Using Multi-Index yields a more complex type and slower compiles
it seems due to its meta-programming uses I guess.

Thanks for any insight. --DD

[1] http://www.boost.org/doc/libs/1_40_0/libs/multi_index/doc/tutorial/basics.html#special_lookup