Thanks David, it works beautifully. Here is my code including conversion to fusion list plus printing of the foreign key fields. #include "boost/mpl/vector.hpp" #include <boost/mpl/find_if.hpp> #include <boost/mpl/at.hpp> #include <boost/mpl/filter_view.hpp> #include <boost/fusion/sequence.hpp> #include <boost/fusion/algorithm.hpp> using namespace std; using namespace boost; namespace fields { struct id { void print() { cout << "id" << endl; } }; struct name { void print() { cout << "name" << endl;} }; struct address { void print() { cout << "address" << endl; }}; struct birthday { void print() { cout << "birthday" << endl; }}; } namespace tags { struct primary {}; struct foreign {}; struct no_key {}; } typedef mpl::vector<fields::id , tags::primary> id_field_t; typedef mpl::vector<fields::name , tags::foreign> name_field_t; typedef mpl::vector<fields::address , tags::foreign> address_field_t; typedef mpl::vector<fields::birthday, tags::no_key> birthday_field_t; typedef mpl::vector< id_field_t , name_field_t , address_field_t , birthday_field_t > table; // find all foreign keys fields typedef mpl::filter_view<table, is_same<mpl::back<mpl::_>, tags::foreign> >::type iter_2; typedef fusion::result_of::as_list<iter_2>::type foreign_keys_t; struct print { template <typename T> void operator()( T const& vec ) const { fusion::at<mpl::int_<0> >(vec).print(); } }; int _tmain(int argc, _TCHAR* argv[]) { BOOST_STATIC_ASSERT(( is_same< mpl::deref<iter>::type, id_field_t

::value ));

foreign_keys_t foreign_keys; for_each( foreign_keys, do_something() ); return 0; }

...

// find all foreign keys fields typedef mpl::filter_view<table, is_same<mpl::back<mpl::_>, tags::foreign> >::type iter_2; ^^^^^^ not needed

Why are you calling this type sequence "iter_2"? It's not an iterator, is it?

Sorry this was my error. But how can I use the filter_view result to do something useful? For what I understand the filter_view results in a lazy forward sequence and though, why not use it with the transform1 algorithm. The following code displays what I trying to do. #include "boost/mpl/vector.hpp" #include "boost/mpl/list.hpp" #include <boost/mpl/find_if.hpp> #include <boost/mpl/at.hpp> #include <boost/mpl/filter_view.hpp> #include <boost/mpl/transform.hpp> using namespace std; using namespace boost; namespace fields { struct id { void print() { cout << "id" << endl; } }; struct name { void print() { cout << "name" << endl;} }; struct address { void print() { cout << "address" << endl; }}; struct birthday { void print() { cout << "birthday" << endl; }}; } namespace tags { struct primary {}; struct foreign {}; struct no_key {}; } typedef mpl::vector<fields::id , tags::primary> id_field_t; typedef mpl::vector<fields::name , tags::foreign> name_field_t; typedef mpl::vector<fields::address , tags::foreign> address_field_t; typedef mpl::vector<fields::birthday, tags::no_key> birthday_field_t; typedef mpl::vector< id_field_t , name_field_t , address_field_t , birthday_field_t > table; // finds primary key field typedef mpl::find_if< table, is_same< mpl::at< mpl::_1, mpl::int_<1>

, tags::primary > >::type primary_key_t;

template <typename T> struct add_vector { typedef mpl::at_c< T, 0 >::type field_t; typedef std::vector<field_t> type; }; // find all foreign keys fields typedef mpl::filter_view<table, is_same<mpl::back<mpl::_>, tags::foreign> > foreign_keys_view_t; typedef mpl::transform1< foreign_keys_view_t, add_vector<mpl::_1>

::type foreign_key_vectors_t;

int _tmain(int argc, _TCHAR* argv[]) { return 0; } Regards, Christian

Sorry to intervene this discussion and may be put into other direction... Looks like you are going to emulate a db-table-like view? If so why don't you like the idea of using Boost.MultiIndex lib? It is extremely powerful (also offers the tag dispatching techniques), already debugged, easy to use and very flexible...

I'm always open for new ideas. I have never dealt with boost::MultiIndex. So I'm not aware of the advantages in using it for my project. I will take a look at it. Is there any sample code that does something similar? Regards, Christian