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: 2009-09-18 18:17:40

Thorsten Ottosen escribió:
> Thorsten Ottosen skrev:
>> 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.
> 
> I actually got into a situation where I need another member function.
> The issue arises when I use map/set as a priority_queue and want to
> limit the size of the queue. In this situation I will be erasing one 
> element and inserting one element again and again. Unless the map/set
> keeps a free-list of previously erased elements (which I don't think 
> they do normally) we will be deallocating and allocating a new node over 
> and over again.
> 
> Therefore I would like to see the follwing member:
> 
>   iterator modify_key( const_iterator element, const Key& new_key );
> 
> This function only needs to rebalance/resort the keys and can completely 
> avoid deallocation/allocation.

Interesting. What about exception safety? We can't assure strong 
guarantee. It could just erase the element if the change fails? We could 
also use the assignment operator instead of destroy+construct to reuse 
some resources.

Best,

Ion