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

2 Jun 2008


      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 
#include 
#include 
#include 
#include <string>
#include <iostream>

namespace result_of {
    template
    struct at_key1 : boost::fusion::result_of::at_key {};
    template
    struct at_key2 : boost::fusion::result_of::at_key::type>::type, K1> {};
}

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

template
typename result_of::at_key2::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