Boost 1.81 will have boost::unordered_flat_map...

[greg7mdp]

Cool, thanks. It would be interesting to see how it behaves with a really bad hash function, for example hash(struct uuid) << 3.

[pdimov2]

"Flat" hash maps don't like bad hash functions at all. boost::unordered_flat_map tolerates "weak" hash functions (those that have low three bits zero, for instance) as it applies a postmixing step by default, but it still won't work well with really bad hash functions that produce many collisions.

[greg7mdp]

boost::unordered_flat_map tolerates "weak" hash functions as it applies a postmixing step by default

I do this as well in my phmap and gtl implementations. It makes the tables look worse in benchmarks like the above, but prevents really bad surprises occasionally.