El 03/11/2022 a las 8:06, Pavel Vazharov escribió:
On Wed, Nov 2, 2022 at 10:05 PM Joaquin M López Muñoz <joaquinlopezmunoz@gmail.com> wrote:
Insert+modify works fine as far as I can see: [...]
Your particular problem hints at the more general issue that Boost.MultiIndex indices do not have something like try_emplace, which is the semantics you're after. The reason why this function is not available is that we have set-like indices, not map-like indices. [...]
Hi,
Thanks for the response.
Yes, you are right, I'm looking for an API with semantics like try_emplace and now I understand the complications for it for the case of boost multi_index.
I'm currently using emplace + modify API. My initial question was not very clear about this. However, my understanding is that the modify call will try to reposition the node because it can't be sure if the user callback hasn't changed something in the entry which would require repositioning of the node in any of the indices. Is my understanding of the modify API correct? As I'm using hashed + sequenced indices, I know that setting only the mapped_type in the modify call won't cause any repositioning in the both of the above indices. Thus I was trying to avoid the additional work which happens internally in the modify call.
modify repositions the element after the modifier is called, but *only* if necessary: in the interest of performance, a pre-check is done for each index, and if the element remains in place no further work is done. This in-place check is faster than actually re-positioning the element: https://github.com/boostorg/multi_index/blob/develop/include/boost/multi_ind... To sum up, there's some overhead even if your modifier does not touch keys, but not has high as actually reinserting the element.
BTW, thanks for working on boost::unordered_flat_map because it'll allow us to remove 3rd party dependency from our projects as we are currently using https://greg7mdp.github.io/parallel-hashmap/.
Glad to hear about that! It'd be great if you could download the upcoming beta and provide early feedback before Boost 1.81 ships. Joaquín M López Muñoz