Re: [Boost-users] [multi_index] sorted iteration of values

Subject: Re: [Boost-users] [multi_index] sorted iteration of values
From: Joaquín Mª López Muñoz (joaquin_at_[hidden])
Date: 2013-09-04 04:12:58

El 04/09/2013 9:41, Mathieu Champlon escribió:
> On 04/09/2013 08:15, Joaquín Mª López Muñoz wrote:
>> In fact, you can get rid of index #2: when using a composite key you
>> can search by providing only the first m fields of the key, so for
>> all practical purposes index #2 is redundant with #0.
>>
>
> With #2 I can erase by doing :
>
>     units_.get< 2 >().erase( &publisher );
>
> With #0 it seems I need to do :
>
>     auto range = units_.equal_range( &publisher );
>     units_.erase( range.first, range.second );
>
> Or is there a shorter way ?

Well, erase is the only lookup function where you can't provide a
partial specification of the
composite key, so you have to do it as you write above.

> Also if my use case rarely uses #0 but uses #1 a lot (for instance #0
> is for adding/erasing
> while #1 is used to perform some operations very often), would it be
> more efficient to invert
> the indices or is it just a matter of whichever is the more convenient
> to write ?

Efficiency is not affected, it's just a matter of convenience (#index 0
can be used directly
without using get<0>().)

(In reality, efficiency can be affected, but probably in a negligible
way: when inserting an
element, Boost.MultiIndex inserts it first in index #0, then in #1,
etc., and if some index
fails (because of unicity constraints), previous insertions are undone.
So, if index A is more
likely to ban insertions than index B, it potentially pays off to have
index A at a lower
position than B.)

Joaquín M López Muñoz
Telefónica Digital

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