`flex_vector` nothrow move constructible/assignable

[maierlars]

This is a result of https://github.com/arximboldi/immer/issues/244.

This is just an idea how one could make the move constructor and assignment operator noexcept. I used aligned_storage to statically reserve some memory for the nodes.

However, while I type this description I noticed a problem: I'm not familiar with the mechanism the frees memory. There is nothing that prevents the code from attempting to free the statically allocated buffers. A possible solution would be to keep the ref count always above 0.

[codecov-commenter]

Codecov Report

Merging #246 (b7756f5) into master (a5991ef) will increase coverage by 0.00%. The diff coverage is 96.00%.

@@           Coverage Diff           @@
##           master     #246   +/-   ##
=======================================
  Coverage   90.50%   90.50%           
=======================================
  Files         119      119           
  Lines       12109    12121   +12     
=======================================
+ Hits        10959    10970   +11     
- Misses       1150     1151    +1     

Impacted Files	Coverage Δ
immer/flex_vector.hpp	100.00% <ø> (ø)
immer/detail/rbts/node.hpp	79.32% <90.90%> (+0.06%)	:arrow_up:
immer/detail/rbts/rrbtree.hpp	88.65% <100.00%> (+0.09%)	:arrow_up:

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[maierlars]

How can we proceed with this PR? :)

[arximboldi]

I am happy to merge it. Just wasn't sure what your opinion is about the other improvements I suggested. I guess they could be tackled in a separate PR, would you like to do them? Should I just merge this branch as-is for now?

[maierlars]

If you are happy with this PR, please merge it. For my work it is not urgent to have the other data-structures noexcept move-constr, however, I totally agree, that this should be done at some point. Maybe I find time to look into this over the weekend, but if someone else feels like doing it, please go ahead.

[arximboldi]

Sounds good to me Lars, merged!

Also, quite interesting the use-case for Arango DB. Curious about how you are using the library and what benefits it is providing. Happy to follow up per e-mail some time if you'd like to keep that a bit more private: juanpe@sinusoid.es

[maierlars]

Sure, there a multiple use cases for immer at ArangoDB, however, the flex vector is used primarily for an in-memory representation of a sequence of operations (which is also dumped to disk). For each shard we keep the history of the recent operations on documents in that shard and replicated the operations from the leader to its followers. While new log entries are appended, old entries are eventually compacted (removed) and we take snapshots of ranges for iteration or network messages regularly.

At the time we initially designed the prototype it just felt natural to use an immutable data structure for that. And until today we see no performance loss. After all, we are writing a database and want to replicate data, so the whole business is clearly dominated by disk and network io times. In fact, I personally have less mental load, I can just create a copy: it's cheap, it's not modified, its' noexcept, and no thread is blocked by that.