[boost] Re: [review] hash functions

From: Daniel James (daniel_at_[hidden])
Date: 2005-03-15 17:03:28

Peter Dimov wrote:
> Dave Harris wrote:
> 
>>
>> But suppose I have pair< pair<int,int>, pair<int,int> > ? Shouldn't
>> that yield the same hash value as int[4] ?
> 
> 
> I don't expect a pair< pair<int, int>, pair<int, int> > to be frequently 
> refactored into an int[4]. So no, I don't think that it should 
> necessarily have the same hash value as an int[4]. Similarly, I don't 
> think that vector< vector<int> > should necessarily have the same hash 
> value as its flattened vector<int> counterpart.

They definitely should not for vector< vector<int> >. boost::hash should 
reflect std::equal_to, and std::equal_to does not flatten sequences.

And since we're treating pairs like ranges of two values, nested pairs 
should mirror nested vectors.

Of course, when using a custom equality function object which flattens 
sequences, a custom hash function with this behaviour is correct.