On Thu, Mar 15, 2012 at 3:29 AM, Peter Nyssen <peter@dynsws.com> wrote:
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>.
[...] (My response is independent of your provided context.)
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'm not sure what you mean here.
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
//-------------------------------------------------
[...snip includes...]
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; };
Okay, AttrTraits is not a proper MPL-compatible member function, as it operates on non-type template parameters. This has problems below...
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
I don't know where the "&"s came from (email problems at some point), but the ::value and ::type are, as you've observed, triggering immediate evaluation, and you want lazy evaluation. But like I mentioned above, Boost.MPL can only handle metafunctions which take type template parameters, so your use of AttrTraits is DOA. I suggest either redefine AttrTraits to take a Boost.MPL integral constant or define a wrapper that has the same effect.
// 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,
Ditto.
&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; };
[...snip main...] See if the above suggestion gets you anywhere. - Jeff