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) );
> Why can't you just do the following?
> map::mapped_type x=...;
> map::key_type k=modify(x);
I could, but moving the mapped object is just as expensive as
copy-construction, and it is exactly this I want to avoid.
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