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.