Re: [boost] [review] hash functions

From: Dave Harris (brangdon_at_[hidden])
Date: 2005-03-10 06:37:12

In-Reply-To: <00cd01c524e3$bf3cb4d0$6501a8c0_at_pdimov2>
pdimov_at_[hidden] (Peter Dimov) wrote (abridged):
> A pure function would entice you to define hash_value for (x, y)
> as hash_combine(x, y). 

And that would be bad because...?

    template <typename T1, typename T2>
    size_t hash_value( const std::pair<T1,T2> &pair ) {
        return hash_combine(
              hash_value(pair.first), hash_value(pair.second) );
    }

looks good to me. 1 line of code instead of 4, no spurious variable, no 
side effects, half as many calls to hash_combine(), easier to exploit 
instruction-level parallelism. 

> We can fix this by initializing the seed in hash_range with something 
> other than zero. It isn't a problem for strings, where \0 is rare, but 
> may be a problem for arbitrary sequences. 

That would help.

How about fixing it in hash_combine instead? At the moment 
hash_combine(0,0) == 0. If you fix it in hash_range, you also need to fix 
it in every other place a seed is used, eg for pairs.

-- Dave Harris, Nottingham, UK