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

Subject: Re: [boost] [interprocess][multi_index] emplace() is not sufficient for maps
From: Thorsten Ottosen (thorsten.ottosen_at_[hidden])
Date: 2009-08-20 09:27:29

Ion Gaztañaga skrev:
> Thorsten Ottosen escribió:
>> 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) );
> 
> Is it too expensive to use move insertion from a type with non-const key?
> 
> //note that key is non-const!
> std::pair<key_type, value_type> v(/**/);
> 
> m.insert(boost::move(v));

Yes, because the mapped object is not fast to move. The way I code 
around it currently is to allocate all state in an object on the heap. I 
then do

   PriorityMap::iterator i = open_.emplace( &nodes_.back() ); 

   i->second.swap( newData );

since swap is now efficient. But it is still not optimally efficient.

-Thorsten