Compatibility with std::hash

[JVApen]

boost::hash_combine is very useful, however, it does require to implement a hash function twice in order to be used. std::hash and boost::hash_value.

On stackoverflow, several 'workarounds' exist for making this easier, all assume you are able to encapsulate the includes to hash.hpp in your own wrapping header.

Would it be possible to allow hash_combine ... to fall back to std::hash when no hash_value is available?

https://stackoverflow.com/a/51915825/2466431

[pdimov]

That's an interesting idea that can allow us to eliminate the various hash_value overloads for standard types, and is apparently also what Abseil does by default. But it's not quite as trivial as in the SO answer, because today, it's typical for a user-defined type to declare both hash_value (for boost::hash) and specialize std::hash. Something like this, for example:

struct uuid
{
    unsigned char data[ 16 ];

    inline friend std::size_t hash_value( uuid const& u ) noexcept
    {
        return boost::hash_value( u.data );
    }

    inline friend bool operator==( uuid const& u1, uuid const& u2 ) noexcept
    {
        return std::equal( u1.data + 0, u1.data + 16, u2.data );
    }
};

namespace std
{

template<> struct hash< ::uuid >
{
    std::size_t operator()( uuid const& u ) const noexcept
    {
        return hash_value( u );
    }
};

} // namespace std

And in that case, if there's an overload for boost::hash_value for all std-hashable types, we're disturbingly close to infinite recursion. It doesn't seem to happen here, but I'm not particularly comfortable with this arrangement.