On Mon, Jun 2, 2008 at 7:07 PM, Steven Watanabe <watanabesj@gmail.com> wrote:

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

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