Re: [Boost-users] [mpl] can't transform a map

Yuval Ronen writes:

I have the following code:

#include <utility> #include #include

template <typename MplPair> struct make_ptr_pair1 { typedef std::pair type; ^^^ mpl

};

template struct make_ptr_pair2 { typedef std::pair type; };

int main(int argc, char *argv[]) { namespace mpl = boost::mpl; typedef mpl::map< mpl::pair > my_map; //typedef mpl::transform< my_map, make_ptr_pair1mpl::_1 >::type ptr_pair_seq1; //typedef mpl::transform< my_map, make_ptr_pair2 >::type ptr_pair_seq2; return 0; }

If I uncomment any of the two mpl::transform lines in main(), the code doesn't compile (VC8). The point is to get a sequence of std::pairs out of the content of the mpl::map. Is there anything I'm doing wrong? Can mpl::transform be used for mpl::map?

It can be, but you need to specify the inserter manually, because the default (front_inserter, as per http://boost.org/libs/mpl/doc/refmanual/reversible-algorithm.html) is not suitable for accociated sequences: ... #include #include ... int main(int argc, char *argv[]) { typedef mpl::map< mpl::pair > my_map; typedef mpl::inserter< mpl::map0<>, mpl::insertmpl::_1,mpl::_2 > map_inserter; typedef mpl::transform< my_map, make_ptr_pair1mpl::_1, map_inserter >::type ptr_pair_seq1; // ^^^^^^^^^^^^^^ return 0; }

Maybe I can do it with fusion, but the above code seems to be something that's supposed to be working, isn't it?

Yes, ideally, your original code should just work. It's a design bug. I've added the issue to the library's TODO list (http://www.crystalclearsoftware.com/cgi-bin/boost_wiki/wiki.pl?MPL_TODO_List). Thanks for the report, -- Aleksey Gurtovoy MetaCommunications Engineering