C ++ friend of each class in boost :: mpl :: vector

Question

How can I get a class for each class in boost :: mpl :: vector? those. something that expands to:

template <typename mpl_vector> class A { friend class mpl_vector[0]; friend class mpl_vector[1]; ... friend class mpl_vector[n]; }; 

Answer 1

Doing this with the boost preprocessor, as Andres suggested, will work.

I tried, and it is not nice and will not compile efficiently. It is also limited to BOOST_MPL_LIMIT_VECTOR_SIZE. If his method works, then it might be a little cleaner.

classA.h:

 #if BOOST_PP_IS_ITERATING friend get_elem<mpl_vector, BOOST_PP_ITERATION()>::type; #else #ifndef SOME_INCLUSION_GUARD_H #define SOME_INCLUSION_GUARD_H #include <boost/preprocessor/iteration/iterate.hpp> #include <boost/mpl/vector.hpp> #include <boost/mpl/int.hpp> #include <boost/mpl/at.hpp> #include <boost/mpl/size.hpp> class Dummy {}; template <int Exists> struct get_elem_i { template <typename V, int N> struct get { //typedef Dummy type; typedef typename boost::mpl::at< V, boost::mpl::int_<N> >::type type; }; }; template <> struct get_elem_i<0> { template <typename V, int N> struct get { typedef Dummy type; }; }; template <typename V, int N> struct get_elem { typedef typename boost::mpl::size<V>::type size; typedef get_elem_i<N < size::value> elem; typedef typename elem::get<V, N>::type type; //typedef Dummy type; }; template <typename mpl_vector> class A { #define BOOST_PP_ITERATION_PARAMS_1 (3, (0, BOOST_MPL_LIMIT_VECTOR_SIZE, "classA.h")) ??=include BOOST_PP_ITERATE() private: int test_; }; #endif // SOME_INCLUSION_GUARD_H #endif 

This file includes itself, so be sure to specify the same name as the file in the BOOST_PP_ITERATION_PARAMS_1 bit.

In addition, this code will also cause the 'Dummy' class to be a friend of 'A'.

Answer 2

I think you need to use something like Boost.Preprocessor or Pump to specialize your template for MPL vector of different sizes. Or just specialize it manually.

You should specialize your template this way:

 template< typename mpl_vector, std::size_t size = boost::mpl::size< mpl_vector >::type::value > class A; template< typename mpl_vector > class A< mpl_vector, 0 > { }; template< typename mpl_vector > class A< mpl_vector, 1 > { friend class boost::mpl::at< mpl_vector, boost::mpl::int_< 0 > >::type; };