Re: [hash2] Difference from paper: HashAlgorithm interface vs. function object

From: Peter Dimov (pdimov_at_[hidden])
Date: 2024-12-12 13:22:00

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.