Re: [boost] Re: [review] hash functions

From: Peter Dimov (pdimov_at_[hidden])
Date: 2005-03-16 13:09:33

Dave Harris wrote:
> In-Reply-To: <01c301c52989$d50a2f40$6801a8c0_at_pdimov>
> pdimov_at_[hidden] (Peter Dimov) wrote (abridged):
>> I think that it is better to fix the zero trap in hash_combine,
>> because this does not depend on users remembering to initialize the
>> seed to default_seed, instead of zero.
>
> Did you see my suggestion of changing the primitive hash_value
> functions instead? That also does not depend on users remembering to
> initialise the seed.
>
>    size_t hash_value( int v ) {
>        return size_t(v) + default_seed;
>    }
>
> And similarly for long, etc. Is there a reason to reject this?

I wonder what do we gain from this. From the point of view of hash_combine 
the effect is the same, and we now rely on the user overloads of hash_value 
to not produce a zero.

>> 2. Add a hash_range overload that takes a seed.
>>
>> 2a. void hash_range( size_t & seed, It first, It last );
>> 2b. size_t hash_range( size_t seed, It first, It last );
>> 2c. size_t hash_range( It first, It last, size_t seed );
>>
>> which are listed in order of my preference.
>
> So we're talking about a family of 3 functions:
>
> 2a:
>    void hash_combine( size_t &seed, T value );
>    void hash_range( size_t &seed, It first, It last );
>    size_t hash_range( It first, It last );
>
> 2b:
>    void hash_combine( size_t &seed, T value );
>    size_t hash_range( size_t seed, It first, It last );
>    size_t hash_range( It first, It last );
>
> 2c:
>    void hash_combine( size_t &seed, T value );
>    size_t hash_range( It first, It last, size_t seed=0 );
>
> I think 2b is very error-prone, because of the lack of consistency
> between how hash_combine and the seeded version of hash_range return
> their results. 2a is less error-prone, but it seems needlessly confusing 
> to
> have the two versions of hash_range return their results in different
> ways.

This reflects their intended use. The two argument overload is used when one 
has a whole range and wants its hash value, as in the hash_value overload 
for std::vector, for example.

The three argument overload is used when one has an intermediate seed which 
should accumulate the next part of the input.