Re: STL Map uses hashing?

From:

=?UTF-8?B?RXJpayBXaWtzdHLDtm0=?= <Erik-wikstrom@telia.com>

Newsgroups:

comp.lang.c++

Date:

Sat, 31 May 2008 16:15:34 GMT

Message-ID:

<GQe0k.7819$R_4.6299@newsb.telia.net>

On 2008-05-31 17:50, Juha Nieminen wrote:

James Kanze wrote:

I don't know about the "typically faster". Their average access
time is faster IF the hash function is good. Typically, I find
that it's not always that good (although the situation is
improving).

Question: Will the new C++ standard require for the libraries to
provide high-quality default hashing functions for internal types (and
perhaps some other standard types such as std::string), or will the user
be forced to always provide one himself?

Creating good-quality hashing functions are not a trivial task at all
(and the subject of continuing extensive research). One cannot expect
the average user to invent a good-quality function without extensive
knowledge and experience on the subject.

Currently the parametrised hash struct (which is used by the unordered
containers) is required to be instantiable for integers, float, double,
pointers, and string-types. It says nothing about the quality of the
implementation.

--
Erik Wikstr??m