[Boost-users] [MPL] Generating multi-index container indices based on MPL tag dispatching

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::secondmpl::_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 #include #include #include #include #include #include #include #include #include #include #include #include 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 AttrTraitsAttrT::value ATraits; typename ATraits::type value; }; template <typename AttrList> struct Object : public boost::mpl::inherit_linearly< AttrList, boost::mpl::inheritboost::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 struct ObjectContainer { typedef typename mpl::fold< IndicesMap, mpl::vector<>, mpl::if_< is_uniquempl::_2 >, mpl::push_back< mpl::_1, bmi::ordered_unique< bmi::member< Attributeboost::mpl::_2 >, AttrTraitsboost::mpl::_2::value>::type, &Attributeboost::mpl::_2 >::value // Attribute >, // int, // &Attribute >::value

,

mpl::push_back< mpl::_1, bmi::ordered_non_unique< bmi::member< Attributeboost::mpl::_2 >, AttrTraitsboost::mpl::_2::value>::type, &Attributeboost::mpl::_2 >::value // Attribute >, // int, // &Attribute >::value

::type IndexList;

typedef boost::multi_index_container Container_t; Container_t Container; }; int main() { typedef boost::mpl::vector< boost::mpl::int_, boost::mpl::int_, boost::mpl::int_

AttributeList_1;

typedef Object Object_1; typedef mpl::vector< mpl::pair >, mpl::pair >

Object_1_Indicex;

ObjectContainer Object1Container; return 0; } //-------------------------------------------------