Hi all, this is a follow-up of my question of yesterday (Fold a vector of pairs). The example here is a lot more elaborate and more like the final code I'd like to achieve. I still get errors when compiling the code below related the use of mpl::second<mpl::_2>. I'm trying to generate a multi-index container type that will store objects generated using inherit_linearly. The objects contain a number of attributes (all int in this example) based on a typelist of attribute tags. I want to generate this container by providing the object type and a index_list as shown here: http://www.boost.org/doc/libs/1_49_0/libs/multi_index/doc/tutorial/technique... I'm trying to compose the index_list using some kind of MPL tag dispatch based on the unique & non_unique tags to generate unique & non_unique ordered indices for the container. It's clear to me that the errors have to do with the MPL placeholder expressions used in the 3 types passed to the multi_index::member template class. This is confirmed because if I replace these 3 template parameters by the ones which are comment out just below the program compiles fine. From my previous question I get that the placeholder expressions need to be evaluated lazily (so no ::type or ::value - obviously the code below is thus incorrect). I've been reading section 3.3 of the MPL book on 'Handling Placeholders' a couple of times and from this it seems I need 3 metafunctions that call apply<> on the passed placeholder expressions and which would return each of the types to be passed to multi_index::member<>? I've been trying different approaches but none have been successful... I'm still not grasping how to handle these placeholder expressions... I hope the great minds on this list can bring some light in my clouded mind... Thanks & kind regards, Peter //------------------------------------------------- #include <boost/mpl/at.hpp> #include <boost/mpl/if.hpp> #include <boost/mpl/inherit.hpp> #include <boost/mpl/inherit_linearly.hpp> #include <boost/mpl/int.hpp> #include <boost/mpl/fold.hpp> #include <boost/mpl/pair.hpp> #include <boost/mpl/push_back.hpp> #include <boost/mpl/vector.hpp> #include <boost/multi_index_container.hpp> #include <boost/multi_index/indexed_by.hpp> #include <boost/multi_index/member.hpp> #include <boost/multi_index/ordered_index.hpp> namespace mpl = boost::mpl; namespace bmi = boost::multi_index; enum attributeTags { ATTR_1, ATTR_2, ATTR_3 }; template <int attrId> struct AttrTraits { typedef int type; }; template <typename AttrT> struct Attribute { typedef AttrTraits<AttrT::value> ATraits; typename ATraits::type value; }; template <typename AttrList> struct Object : public boost::mpl::inherit_linearly< AttrList, boost::mpl::inherit<boost::mpl::_1, Attribute<boost::mpl::_2> >

::type {};

struct unique {}; struct non_unique {}; template <typename T> struct is_unique : boost::false_type {}; template <> struct is_unique<unique> : boost::true_type {}; template<typename ObjectT, typename IndicesMap> struct ObjectContainer { typedef typename mpl::fold< IndicesMap, mpl::vector<>, mpl::if_< is_unique<mpl::first<mpl::_2> >, mpl::push_back< mpl::_1, bmi::ordered_unique< bmi::member< Attribute<boost::mpl::second<boost::mpl::_2> >, AttrTraits<boost::mpl::second<boost::mpl::_2>::value>::type, &Attribute<boost::mpl::second<boost::mpl::_2> >::value // Attribute<boost::mpl::int_<0> >, // int, // &Attribute<boost::mpl::int_<0> >::value

,

mpl::push_back< mpl::_1, bmi::ordered_non_unique< bmi::member< Attribute<boost::mpl::second<boost::mpl::_2> >, AttrTraits<boost::mpl::second<boost::mpl::_2>::value>::type, &Attribute<boost::mpl::second<boost::mpl::_2> >::value // Attribute<boost::mpl::int_<1> >, // int, // &Attribute<boost::mpl::int_<1> >::value

::type IndexList;

typedef boost::multi_index_container<ObjectT, IndexList> Container_t; Container_t Container; }; int main() { typedef boost::mpl::vector< boost::mpl::int_<ATTR_1>, boost::mpl::int_<ATTR_2>, boost::mpl::int_<ATTR_3>

AttributeList_1;

typedef Object<AttributeList_1> Object_1; typedef mpl::vector< mpl::pair<unique, mpl::int_<ATTR_1> >, mpl::pair<non_unique, mpl::int_<ATTR_2> >

Object_1_Indicex;

ObjectContainer<Object_1, Object_1_Indicex> Object1Container; return 0; } //-------------------------------------------------