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

Subject: Re: [boost] [interprocess][multi_index] emplace() is not sufficient for maps
From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2009-08-05 03:52:37

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));

Best,

Ion