c++ - Combination of types using boost::mpl -

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i have list of types, want construct list of combinations 2 elements. example:

namespace mpl = boost::mpl; typedef mpl::vector<int, long> typelist; // mpl magic... // wanted list equivalent to: typedef mpl::vector<pair<int, int>, pair<int, long>,                     pair<long, int>, pair<long, long> > combinations;

here, pair<t1,t2> std::pair<t1,t2>, or mpl::vector<t1,t2>. how this? interested in removing duplicates when consider pair<t1, t2> == pair<t2, t1>.
thanks.

the list of combinations of single type int list of types mpl::vector<int, long> can computed invoking mpl::fold:

typedef fold<     mpl::vector<int, long>, vector<>,      push_back<mpl::_1, std::pair<int, mpl::_2> >  >::type list_of_pairs;

now, if wrap separate meta-function , invoke types of initial typelist get:

typedef mpl::vector<int, long> typelist;  template <typename t, typename result> struct list_of_pairs   : mpl::fold<typelist, result,          mpl::push_back<mpl::_1, std::pair<t, mpl::_2> > >  {};  typedef mpl::fold<     typelist, mpl::vector<>, mpl::lambda<list_of_pairs<mpl::_2, mpl::_1> > >::type result_type;  boost_mpl_assert(     mpl::equal<result_type,          mpl::vector4<             std::pair<int, int>, std::pair<int,long>,             std::pair<long,int>, std::pair<long,long>          > >::value);

edit: answering second question:

making result containing unique elements (in sense mentioned) bit more involved. first need define meta function comparing 2 elements , returning mpl::true_/mpl::false_:

template <typename p1, typename p2> struct pairs_are_equal   : mpl::or_<         mpl::and_<             is_same<typename p1::first_type, typename p2::first_type>,             is_same<typename p1::second_type, typename p2::second_type> >,         mpl::and_<             is_same<typename p1::first_type, typename p2::second_type>,              is_same<typename p1::second_type, typename p2::first_type> > > {};

then need define meta-function tries find given element in given list:

template <typename list, typename t> struct list_doesnt_have_element   : is_same<         typename mpl::find_if<list, pairs_are_equal<mpl::_1, t> >::type,          typename mpl::end<list>::type> {};

now, can utilized build new list, making sure no duplicates inserted:

typedef mpl::fold<     result_type, mpl::vector<>,     mpl::if_<         mpl::lambda<list_doesnt_have_element<mpl::_1, mpl::_2> >,          mpl::push_back<mpl::_1, mpl::_2>, mpl::_1>  >::type unique_result_type;

all top of head, may need tweaking here or there. idea should correct.

edit: minor corrections outlined @rafak


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