[jcelerier]

> Turns out for this structure hash quality just doesn't matter very much. The identity function is all you need for mediocre results, and a really great hash produces only slightly improved results with this data structure, yet running the hash costs performance so why bother ?

Then it's the good choice - I'm pretty confident the only reason std::hash is explicitly in the standard library is because it's the only way they could find to enable people to specialize hash functions for their own custom types - checking when it was introduced (C++ TR1: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n18...), std::hash is in a subsection of the "unordered containers" section and is really shown as an implementation detail of those (that has to be user-customizable).

If you use another hash table implementation, likely you're also going to choose a hash function that works well with it - I use Ankerl's in my own code for instance which 1/ comes with a modern hash implementation, wyhash, and 2/ obviously I specialize for the really specific cases I need performance for with the known statistical properties of my inputs. But I wouldn't want to pay for this cost for std::unordered_map