Re: [Boost-users] [multiIndex] modify_key

From: Jeffrey Holle (jeffreyholle_at_[hidden])
Date: 2006-11-28 19:48:44

Thanks for this!
The boost::multiIndex container looks like exactly what my application 
needs.
Your sample program is very through, enough so I think is should be part 
of the library's examples.

Joaquín Mª López Muñoz wrote:
> Hello Jeffrey,
> 
> Jeffrey Holle ha escrito:
> 
>> I'm using boost version 1.33.1 and am contemplating using a multiIndex
>> container in my application.
>>
>> The held object will be something like:
>>
>>   class element {
>>     int x;
>>     int y;
>>     Object *pObject;
>>   };
>>
>> I need 2 ordered_non_unique indices using x and y and one ordered_unique
>> index on pObject.
>>
>> The problem is that the values of x and y will need to be modified
>> during the life of this container.  Since the documentation says these
>> indices are implemented using red-black trees, I think I'm heading for
>> trouble.
> 
> No, B.MI is designed to handle these situations :)
> 
>> However, the multiIndex container seems to have a "modify_key" method.
>> I can guarantee that the pObject value will not change, so if I obtain
>> an iterator to an element based on this index, will I be able to modify
>> either x or y and expect their related indices to still work?
> 
> B.MI provides updating mechanisms that take care of all the necessary
> index internal rearrangement resulting from modifications on the keys:
> 
>   modify()
>   modify_key()
>   replace()
> 
> In your particular case, if the iterator belongs to the pObject-index,
> modify_key is of no use, since the key here is precisely pObject, which
> you do not wish to update. You can use then modify(), with accept a
> modifying functor to be passed a reference to the whole element, or
> also replace(), wich is passed a new element object to substitute for
> the old one. In any case, the indices are internally kept in sync.
> 
> Please find attached a small snippet showing the various ways you
> can use the updating facilities of B.MI, both with user-defined
> modifying functors and by means of the Boost Lambda library. If
> you have further doubts, don't hesitate to ask.
> 
> HTH,
> 
> Joaquín M López Muñoz
> Telefónica, Investigación y Desarrollo
> 
> 
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