Re: Best way to hash_combine a bool?

From: Peter Dimov (pdimov_at_[hidden])
Date: 2021-06-05 17:07:57

John Maddock wrote:
> What's the best way to combine a hash value with a bool?
> 
> Specifically, if you're generating a hash value for a class which has a one
> member a bool.  Normally I would just boost::hash_combine all the class
> members together, but doing that with a bool member leads to:
> 
> 1>d:\data\boost\boost\boost\multiprecision\detail\hash.hpp(20,88):
> warning C4805: '^': unsafe mix of type 'T' and type 'size_t' in operation
> 1>        with
> 1>        [
> 1>            T=bool
> 1>        ]
> 
> Followed by a long instantiation backtrace (this is from Boost.Multiprecision).
> 
> Since there's only one bool in this class, I could just apply a unary ~ to the
> value when the bool is true, but obviously that wouldn't work for classes with
> multiple bool's.  Should we modify hash_combine for this use case?

You're using your own (wrong) hash_combine, so "we" can't modify anything. :-)

You need to apply hash_value to the T argument to produce a size_t first,
instead of using it directly. This will also get rid of the casts.