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

3 Jun 2008

      Hello Steven,

thank you very much for your nice and clean solution.
May I ask you to post the variadic version as it is, even if it doesn't
compile. I'll try to find how to make it work. It will give me a good
starting point to learn more about these techniques.

Istvan

On Mon, Jun 2, 2008 at 7:07 PM, Steven Watanabe 
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 
#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
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