Re: [boost] [functional/hash] integer hash function

Subject: Re: [boost] [functional/hash] integer hash function
From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2011-10-04 18:19:25

On 10/04/2011 10:03 PM, Tim Blechmann wrote:
> i was quite amazed that the default hash function for integers seems to be the
> identity function:
>
> from boost/functional/hash/hash.hpp:
>      inline std::size_t hash_value(int v)
>      {
>          return static_cast<std::size_t>(v);
>      }
>
> while this is probably the most efficient hash function, it is also the worst
> possible. is this done intentionally?

It's only bad if you know your possible values are not equally 
distributed over the whole range of "int".

Which is not something a default hashing function can know.