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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.


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