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