Le vendredi 13 décembre 2024 à 19:24 +0200, Peter Dimov via Boost a écrit :
Julien Blanc wrote: ...
* remove default construction and too short key construction for algorithms where this is actually harmful (such as siphash). I don't have an issue with providing constructors with keys for algorithms that do not mandate it. But boost code should encourage correct usage of the algorithms it propose, not allow bad usage in the name of genericity. ... * makes copy construction requirement on hash algorithm a requirement only if hash_append_unordered_range is used. Hashing unordeder ranges is an open problem, some creative other solutions may be found, and i see no reasons why they would involve duplicating the algorithm state. Actually, if the hash is default constructible, then it is not needed. You need either one of default or copy constructible, not both.
I already answered this (twice) - using default constructed hash algorithm instances in hash_append_unordered_range makes collisions seed-independent.
And it seems we won't agree here. IMHO it's not worth it. The potential for misuse and its consequences far outweight the ease of implementation that it provides.
But this aside, these two points also contradict each other. The first one wants to remove default construction from SipHash; the second one wants to use default construction in hash_append_unordered_range. If both are applied, SipHash will not work in hash_append_unordered_range.
Obviously i failed at explaining my point, because i see no contradiction. My point is that for unordered range hashing, you can keep your scheme and need either: 1. use a default constructed hash 2. use a copy constructed hash from the given hash. This makes the hash less predictable. But not both. The current implementation uses #2, but it could use #1 if #2 is not provided and #1 is available (typical use case for #1 would be openssl md5, whereas typical for #2 is siphash in boost::hash2). If you have neither #1 or #2, then you cannot hash unordered containers. But you can hash everything else. And it will be fine for a lot of users. Hope i made myself a little clearer, Regards, Julien