Re: [boost] [review] hash functions

From: Peter Dimov (pdimov_at_[hidden])
Date: 2005-03-13 10:50:56

Dave Harris wrote:
> In-Reply-To: <007e01c52597$b62c1980$6601a8c0_at_pdimov>
> pdimov_at_[hidden] (Peter Dimov) wrote (abridged):
>> pair<int, int>, int[2], struct { int a; int b; } are different
>> representations of the same thing; it is not strictly necessary for
>> them to use the same algorithm, but it's desirable and will
>> eliminate a source of confusion and mistakes.
>
> Really? What kind of mistakes?
>
> In my experience, if you want two different things to hash the same,
> you need to write their hash functions with that in mind. It is a special
> case which justifies not using the default hash function.

Maybe, but this is not an argument for making the hash values different.

>>     size_t hash_range( size_t seed, It first, It last );
>
> I am also tempted by:
>
>     size_t hash_range( It first, It last, size_t seed=0 );
>
> perhaps replacing the 0 with some other default value. So people can
> ignore it if they don't need it.
>
> But doesn't this mean that hash_value( pair<int,int> ) also needs a
> seed argument, if you want it to match hash_range( int[2] ) ?

No. The seed version of hash_range is used when you want to compute the hash 
function of a sequence, but you don't have access to the whole sequence at 
once. It's goal is not to provide a way to change the initial seed, but to 
make it possible to continue from an intermediate seed.