Re: [Boost-users] [MultiIndex] is it possible to create multi-key key-extractors?

Subject: Re: [Boost-users] [MultiIndex] is it possible to create multi-key key-extractors?
From: Mostafa (mostafa_working_away_at_[hidden])
Date: 2010-08-08 22:28:55

On Sun, 08 Aug 2010 17:29:49 -0700, Steven Watanabe <watanabesj_at_[hidden]>  
wrote:

> AMDG
>
> Mostafa wrote:
>> For the following struct:
>>
>> struct Flower
>> {
>>   int type;
>>   string name;
>>   std::list<string> aliases;
>> };
>>
>> I want to create a multiindexed container that indexes Flower objects  
>> by type, name, and aliases, where aliases is some iteratable collection  
>> of strings.  I just don't know if it's possible to create a multi-key  
>> key-extractor for aliases.  So far I've come up with the following:
>
> I assume that you want to be able to look up
> the Flower from a string, which should be
> one of the aliases?  You can't do this with
> multi-index.

Yes.

> A single object corresponds to a single key.

That's pretty much what I figured from the documentation, I was just  
hoping for some kind of workaround that I might have missed.

Well, here goes a feature request ... (A rough outline.)

I propose adding a key-extractorS concept to the multi-index library.  
Something along the lines of the existing key-extractor concept, modified  
such that

key_extractorS::result_type is a Boost.Range of keys

and providing adaptor classes for creating such a Range from  
multi_index::value_type member variables, methods ..., (basically a  
key-extractorS counterpart for the equivalent existing key-extractors,  
where approriate of course).

The motivation for this request is that a multi-index element can be  
indexed by a collection of values associated with that element.

As an example: (rehashing from previous posts)

struct Flower
{
   int getType() const;
   string name;
   std::list<string> aliases;
};

struct Nursery
{
   std::list<Flower *> flowers;
};

1) a multi-index of Flower objects each indexed by an alias from  
Flower::aliases,
2.1) a multi-index of Nursery objects each indexed by an alias from  
Nursery::flowers.aliases,
2.2) and and also indexed by Nursery::flowers.getType()

Does this make sense? Should this be a new post, and if so should it be  
posted in the dev's group?

Thanks,

-Mostafa