In-Reply-To: <00cd01c524e3$bf3cb4d0$6501a8c0@pdimov2> pdimov@mmltd.net (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