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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ok, I understand now. Indeed, "variadic" was misleading. Thanks for your help. On Tue, Jun 3, 2008 at 3:14 PM, Steven Watanabe wrote:

AMDG

Istvan Buki wrote:

...

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.

Just take the numbers off of the functions at_key1 and at_key2, so that they are overloaded. I suppose that variadic is not really the right term for this...

In Christ, Steven Watanabe

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