Julien Blanc wrote:
Le mercredi 11 décembre 2024 à 19:31 +0200, Peter Dimov via Boost a écrit :
You've already stated that you don't think copy construction should be required. But copy construction is required by the current implementation of hash_append_unordered_range.
The way you phrase it make it sounds like it could be implemented without requiring copy construction. Is that poor understanding from my side?
I feel like I already answered this question. The alternative mechanism given in N3980, constructing a temporary std::set from the unordered set elements, then hashing that, is wildly impractical for obvious reasons, performance and otherwise. Using default-constructed instances of the hash algorithm, instead of copies, makes the hashing seed-independent, which means that a way to engineer collisions will work regardless of the seed used. I say "by the current implementation" because I obviously can't say anything about hypothetical future and unknown to me implementations.