Re: [boost] hash_combine vs hash_value

Subject: Re: [boost] hash_combine vs hash_value
From: Michael Goldshteyn (mgoldshteyn_at_[hidden])
Date: 2011-03-25 11:32:47

The thing that bothers me about the use "hash_combine" for even the first 
hashed value of a bunch of values is the fact that:

size_t myHashVal=0;

boost::hash_combine(myHashVal,firstVal);
...

becomes

size_t seed=0;
seed ^= hash_value(myFirstHashVal) + 0x9e3779b9; // + (seed << 6) + (seed >> 
2) does nothing for seed==0
...

The xor'ing of the first value doesn't bother me, but this displacement by 
0x9e3779b9 of the first value does, since for 32-bit size_t values, we've 
just lost a bunch of info. It seems like the formula will result in less of 
the first value being applied to the overall hash, due to the "potential" 
unsigned 32-bit overflow caused by the addition. Maybe I'm misinterpreting 
the effect. If so, please correct me.

Thanks,

Michael Goldshteyn