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

On Thu, Mar 15, 2012 at 3:29 AM, Peter Nyssen 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::secondmpl::_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 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< Attribute >, AttrTraits::value>::type, &Attribute >::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 >, // int, // &Attribute >::value

...

,

mpl::push_back< mpl::_1, bmi::ordered_non_unique< bmi::member< Attribute >, AttrTraits::value>::type,

Ditto.

&Attribute >::value // Attribute >, // int, // &Attribute >::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