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 09:21:10

Joaquin M Lopez Munoz skrev:

>>
>> 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?
> 
> Why can't you just do the following?
> 
> map::mapped_type x=...;
> map::key_type    k=modify(x);
> m.emplace(boost::move(k),boost::move(x));
> 

I could, but moving the mapped object is just as expensive as 
copy-construction, and it is exactly this I want to avoid.

-Thorsten