Re: [Boost-users] Question about boost::multi_index emplace operations

3 Nov 2022

      El 03/11/2022 a las 8:06, Pavel Vazharov escribió:
...
On Wed, Nov 2, 2022 at 10:05 PM Joaquin M López Muñoz 
 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