Re: [Boost-users] [boost-users][multi-index] hashed identity: lookup by hash value

From: JOAQUIN M. LOPEZ MUÑOZ (joaquin_at_[hidden])
Date: 2008-07-13 15:57:25

________________________________________
De: boost-users-bounces_at_[hidden] [boost-users-bounces_at_[hidden]] En nombre de Igor R [boost.lists_at_[hidden]]
Enviado el: domingo, 13 de julio de 2008 21:29
Para: boost-users_at_[hidden]
Asunto: Re: [Boost-users] [boost-users][multi-index] hashed identity: lookup by hash value

[...]
> What's not working is the second approach - with the hash_value
> template function as a the extractor of an additional key (it's not
> compiling).

It's weird, cause it ought to work. For instance, the following does it
nicely both in GCC 3.4.4 and MSVC++ 8.0:

  #include <boost/multi_index/global_fun.hpp>
  #include <cstddef>
  #include <iostream>

  struct Element{};

  template<typename T>
  std::size_t my_hash_value(const T&){return 0;}

  namespace mi=boost::multi_index;

  typedef mi::global_fun<const Element&,std::size_t,&my_hash_value > key_extractor;

  int main()
  {
    key_extractor k;
    Element       e;
    std::cout<<k(e)<<std::endl;
  }

What error messages are you getting? You can make your compiler's life
easier by being a little more explicit when passing my_hash_value (though it's
not obligatory to do so):

  typedef mi::global_fun<const Element&,std::size_t,&my_hash_value<Element> > key_extractor;

If you provide me with a little more info about the errors you're getting maybe
I can be more helpful.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo