Re: [Boost-users] [multi_index].[composite_key] Index creation at run-time.

From: Joaquin M Lopez Munoz (joaquin_at_[hidden])
Date: 2006-06-07 18:00:05

Prashant Thakre <prashant.thakre <at> gmail.com> writes:

> 
> Hi,
>  Guess, I should have given a code snippet :(
> 
>  >But, how are you planning to actually implement this?
>  >Certainly you can't do the following:
>  struct element
>  {
>   public:
>    boost::tuple<t0,t1,...tn> tp;
>   // ...
>  };
> 
>  As per your first suggestion:
>  >1. If the maxium number of keys of the composite_key is bounded
>  >then you can use that maximum and later on, at run-time, take
>  >advantage of only the first N key slots.
>   struct element
>   {
>    //Assumption: no more than 10 elements, a reasonable limit I guess.
>    //
>    boost::tuple<t0,t1,...t9> tp;
>    const std::string & tuple_element(const int ind) const {
>                  return tp.get<ind>;
>         }
>    //...
>   };

Well, this can't work as is, cause tp.get<ind> requires
that ind be a compile-time constant, which is not (it is
an argument to tuple_element). Actually, I think it
would be hard (though not impossible) to treat tuples,
whose dimension is compile-time, as run-time sized objects.
Let's turn to the second approach:

>  As per this suggestion:
>  >2. You can have std::vector<> as the key, the comparison semantics
>  >of this container in some ways resembles that of an infinite-length
>  >composite key.
>   struct element
>   {
>    std::vector<const std::string > tp;
>    const std::string & tuple_element(const int ind) const {
>                  return tp[ind];
>          }
>    //...
>   };

OK, here, instead of a composite_key instantiation,
you can take advantage directly of std::vector comparison
semantics (you don't really need tuple_element):

typedef multi_index_container<
  element,
  indexed_by<
    hashed_unique<
      member<element,std::vector<const std::string>,&element::tp>
    >
    // other indices, maybe
  >
> element_collection;

Now, you can use n-sized vectors everywhere, where n is
determined at run-time. Does this suit your needs?

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo