Re: [Boost-users] [boost-users][fusion] nested map access

From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2008-06-02 13:07:21

AMDG

Istvan Buki wrote:
> 1) Is MPL the right tool to transform a call to
>            get_value< general, age >( a_person )
>    into
>          fusion::at_key< general::age >( fusion::at_key< general >( 
> a_person ) )

No not really.

> 2) If it is, could someone provide me some help on how to achieve such 
> a result?

Here's a modified form of your example.  I couldn't get the variadic 
syntax to compile, so
there is a numeral tacked onto the end of the function name.

#include <boost/fusion/container/map.hpp>
#include <boost/fusion/container/generation/make_map.hpp>
#include <boost/fusion/sequence/intrinsic/at_key.hpp>
#include <boost/type_traits/remove_reference.hpp>
#include <string>
#include <iostream>

namespace result_of {
    template<class M, class K0>
    struct at_key1 : boost::fusion::result_of::at_key<M, K0> {};
    template<class M, class K0, class K1>
    struct at_key2 : boost::fusion::result_of::at_key<typename 
boost::remove_reference<typename boost::fusion::result_of::at_key<M, 
K0>::type>::type, K1> {};
}

template<class K0, class Map>
typename result_of::at_key1<Map, K0>::type at_key1(Map& m) {
    return(boost::fusion::at_key<K0>(m));
}

template<class K0, class K1, class Map>
typename result_of::at_key2<Map, K0, K1>::type at_key2(Map& m) {
    return(boost::fusion::at_key<K1>(boost::fusion::at_key<K0>(m)));
}

namespace fusion = boost::fusion;

namespace fields
{
  struct general
  {
    struct first_name ;
    struct last_name ;
    struct age ;
  } ;

  struct address
  {
    struct street ;
    struct postal_code ;
    struct country ;
  } ;
}

typedef fusion::map <
  fusion::pair< fields::general::first_name, std::string >,
  fusion::pair< fields::general::last_name, std::string >,
  fusion::pair< fields::general::age, int >
  > general_person ;

typedef fusion::map <
  fusion::pair< fields::address::street, std::string >,
  fusion::pair< fields::address::postal_code, std::string >,
  fusion::pair< fields::address::country, std::string >
 > address_person ;

typedef fusion::map <
  fusion::pair< fields::general, general_person >,
  fusion::pair< fields::address, address_person >
 > person ;

int
main()
{
  using namespace fields ;

  general_person gp =
    fusion::make_map< general::first_name,
    general::last_name, general::age >( "John", "Doe", 99 ) ;

  address_person ap =
    fusion::make_map< address::street,
    address::postal_code, address::country >( "Foo street 1", "12345", 
"Barland" ) ;

  person a_person =
    fusion::make_map< general, address >( gp, ap ) ;

  std::cout << "Age: "
//        << fusion::at_key< general::age >( fusion::at_key< general >( 
a_person ) )
        << at_key2< general, general::age>( a_person )
        << std::endl ;

  return 0 ;
}

In Christ,
Steven Watanabe