Re: [Boost-users] [multi_index] modify_range?

From: Jeff Flinn (TriumphSprint2000_at_[hidden])
Date: 2006-03-16 17:09:34

Joaquín Mª López Muñoz wrote:
> Jeff Flinn ha escrito:
>
>> JOAQUIN LOPEZ MU?Z wrote:
>>
>>> There is a problem, though: consider the following:
>>>
>>> // buggy call ro modify_range
>>> modify_range(m,m.begin(),m.end(),adder(2));
>>>
>>> What's the problem with this? Well, after *increasing*
>>> the value of an element, this is repositioned
>>> further to the end of the container, and consequentely
>>> modify_range will visit it again and engage in a
>>> neverending loop (modulo overflows.)
>>
>> Would modifying a non_key(or non_derived_key dependency) ever cause
>> repositioning?
>
> I'm not sure if you mean this, but modifying a part of an object from
> which no
>
> key extractor depends results in no repositioning. For instance
>
> struct element{
>  int x,y,z
> };
>
> typedef multi_index_container<
>   element,
>   indexed_by<
>     ordered_unique<member<element,int,&element::x>,
>     ordered_non_unique<member<element,int,&element::y>,
>   >
>> multi_index_t;
>
> modifying element::z in the example above does not cause repositioning
> ever. Is this what you were referring to?

Yes, that's what I was referring to. So in the above case, there's no need 
for the 'stable' version.

>>>   // calling modify_unstable_range is now OK
>>>   modify_unstable_range(m,m.begin(),m.end(),adder(2));
>>
>> Doesn't this actually provide modify_key_range functionality?
>
> No, to provide such functionality just replace the occurences
> of .modify(...) with .modify_key(...);

So wouldn't the only use of modify_unstable_range be for modifying keys and 
indirect members? So only a single modify_unstable_range calling .modify_key 
would suffice? Just thinking out loud here, until I have access to the docs.

Thanks for your library and your help. :)

Jeff