[boost] [interprocess][multi_index] emplace() is not sufficient for maps

Subject: [boost] [interprocess][multi_index] emplace() is not sufficient for maps
From: Thorsten Ottosen (thorsten.ottosen_at_[hidden])
Date: 2009-07-24 09:49:02

Hi Ion and Joaquín,

This issue is related to node-based containers. I think you both have 
added emplace() to provide efficient in-place construction.

However, I often encounter a situation where emplace() is not sufficient 
for me.

The situation is characterized by the need to calculate the key after 
the mapped value (a heavy object) has been placed in the pair. To 
calculate the key the mapped value needs to be modified (the key is
calculated as a side-effect).

I therefore suggest that we add a new function:

   typedef ... map;
   map m;
   map::node_ptr p = m.get_node_ptr(<like emplace, but only for
                                    constructing the mapped value>);
   p->pair.first = modify( p->pair.second );
   m.insert( boost::move(p) );

Thoughts?

-Thorsten