may be unique_ptr? or it is shared somewhere?

ReadIterator can be unique_ptr. But for WriteIterator, I don't know if it should be set to unique_ptr, because it may be copied with the FileSegment, do we need to invalidate the iterator before copying?

because it may be copied with the FileSegment, do we need to invalidate the iterator before copying?

but file segment cannot be copied

It looks like this, I tried to modify it, but sometimes we need to use a temporary pointer to keep some code readable, such as the following cases, so I think it may be better to keep it as it is, what do you think?

auto priority_iter = record->second;
priority_iter->use(cache_lock);
result_state = priority_iter->hits() >= enable_cache_hits_threshold ? FileSegment::State::EMPTY : FileSegment::State::SKIP_CACHE;

but looks like here auto & is possible, then unique_ptr will be ok

I tried to fix it, but I always get an error when compiling involves unordered_map. Maybe there is something wrong with my understanding of uniqueu_ptr :(

may be not use a pointer at all - just an object? (and in FileSegmentCell put std::optional to iterator).

Sounds feasible, do you mean all ReadIterator and WriteIterator as objects? In this case, there will be more revisions, but I may not have much time to make revisions recently (there is a lot of work to deal with in reality), and I will reconsider the relevant implementation when I have time :).

do you mean all ReadIterator and WriteIterator as objects

yes

but I may not have much time to make revisions recently (there is a lot of work to deal with in reality), and I will reconsider the relevant implementation when I have time

ok, let's leave it as it is for now