Re: Question about boost::multi_index emplace operations

From: Joaquin M López Muñoz (joaquinlopezmunoz_at_[hidden])
Date: 2022-11-03 08:26:15

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_at_[hidden]> wrote:
>
>     Insert+modify works fine as far as I can see:
>     Â [...]
>
>     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_index/hashed_index.hpp#L1578-L1589

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