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

From: Prashant Thakre (prashant.thakre_at_[hidden])
Date: 2006-06-07 07:08:05

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>;
        }
   //...
  };

 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];
         }
   //...
  };

 template < std::size_t hash_table_size>
 struct element_hash
  {
         int operator() ( const std::string & hash_string ) const
         {
            std::string::const_iterator hash_iter = hash_string.begin();
             register int hash_value = 0;
              //...
             return hash_value;
         }
 };

  typedef multi_index_container<
   element,
   indexed_by<
     hashed_unique < // indexed by elements of tuple/vector
       composite_key<
         element,
           const_mem_fun< element, const std::string &,
&element::tuple_element(0) >
       >, // end of composite key def
       composite_key_hash <
         element_hash< 8191 >
       > // end of key hash
     > // end of hash
   > // end of inedexed_by
>
> > element_collection;

 regards,
 Prashant Thakre

> ----- Mensaje original -----
> De: Prashant Thakre <prashant.thakre_at_[hidden]>
> Fecha: Martes, Junio 6, 2006 9:06 pm
> Asunto: Re: [Boost-users] [multi_index].[composite_key] Index creation
> atrun-time.
>
>
> > Hi,
> > Thanks for the reply.
>
> You're welcome. I'm afraid I'll have to ask
> for some more info before I can try to answer
> your original question, though :(
>
>
> > This is what I am trying to achieve:
> > a) Each entry in the container will have tuple containing n-elements
> > tuple < t0, t1 ..., tn >, where n is known at run-time. This entry
> > will also have other elements.
>
>
> But, how are you planning to actually implement this?
> Certainly you can't do the following:
>
>
> class element
> {
> public:
>   boost::tuple<t0,t1,...tn> tp;
>   ...
> };
>
>
> because you don't know n at compile time. This question
> is important, since how you actually implement the
> run-time knowledge of n will surely determine the way
> you can plug a composite_key into the scheme. If you
> can provide some actual code, no matter how sketchy,
> showing how you implement your element type, so much
> the better.
>
>
> Looking fwd to your feedback,
>
>
> Joaquín M López Muñoz
> Telefónica, Investigación y Desarrollo