Can I hash a container?

[ianthehenry]

Is there a way to hash a container?

I can see that this listed as "future work" in #19, and I wanted to know if that was done/how to do it. I can see that std::hash is still not defined, but with my extremely limited knowledge of C++ I cannot find an alternative way to hash containers.

Failing that, is there some way to stably iterate over the elements in a container such that I can construct my own law-abiding hash value? I don't care too badly about performance, but allocating a vector or something just to sort it feels wrong.

[arximboldi]

You can define override std::hash and define it by iterating over the container, hashing every element and combining the hashes (using something like boost:::hash_combine or your own hash combination formula).

[ianthehenry]

I had sort of assumed that this wouldn't work for unordered types, because the iteration order wouldn't be well-defined, and a standard hash mixer would be sensitive to that. But experimentally, playing with immer::set, it seems like the iteration order actually is consistent, regardless of how I construct the set. Is that guaranteed to be the case, or am I just getting lucky?

[arximboldi]

That is guaranteed to be the case. The order of iteration depends only on the hashes of the elements themselves, not the order of insertion/removal.