[Boost-users] [MultiIndex] Prevent automatic re-indexing after "modify"?

Subject: [Boost-users] [MultiIndex] Prevent automatic re-indexing after "modify"?
From: Deniz Bahadir (deniz.bahadir_at_[hidden])
Date: 2016-12-14 08:06:05

Hi guys,
Hi Joaquín,

I have a question concerning the automatic re-indexing of 
Boost.MultiIndex indexes.

Is it somehow possible to modify a member of the element-type that is 
used as key for (another) index without triggering automatic 
re-indexing? (Of course, I want to be able to trigger manual re-indexing 
later.)
Or is it at least possible to adjust the code so that this can be achieved?

To understand my question, here is my use-case and why I need it.

I have a boost::multi_index_container with multiple indexes (one 
hashed_unique and four hashed_non_unique).
I am accessing a range of elements through a hashed_non_unique index 
'A'. Then I am iterating through these elements and want to modify the 
value of a member 'M' of each element. However, that member is used when 
calculating the key for another hashed_non_unique index 'B'.
Therefore, I must use the "modify" function of index 'A'.

The problem is now that it takes forever to modify the entire range. (We 
are talking about 100 thousands if not millions of entries that need to 
be modified.)
Probably because re-indexing is triggered after each call to "modify". 
However, in my case it should be perfectly fine to postpone the 
re-indexing after I modified the entire range, because I am not going to 
access any element through index 'B' until I finished modifying the 
entire range.

As a workaround I removed index 'B' from the container and declared the 
element-member 'M' as mutable. This allows me to directly change the 
value of 'M'.
The speedup is astonishing. It now finishes almost in no time.

But this workaround is not really feasible as it requires reimplementing 
index 'B' by somehow tracking the information outside of the 
multi_index_container.

Therefore, my question really is:

Is it possible to provide an "unsafe_modify" function, which does the 
same as "modify" but does not trigger automatic re-indexing, and an 
"refresh_index" function, which (manually) triggers the re-indexing?

Thanks,
Deniz