Not necessarily specific to Folly, but I've wondered why vector classes don't have a "short vector optimization" like string classes do. That is, why don't they store 24 bytes or so in place? Is it because the required iterators would take-up too much space anyway?
folly::small_vector does just that (and it lets you use one bit for a mutex, too! -- we have a lot of memory-constrained apps so we had to design data structures for them).
