[Boost-users] Selecting one element per fusion sequence using mpl::int_s

Subject: [Boost-users] Selecting one element per fusion sequence using mpl::int_s
From: gsermaid (linuxfever_at_[hidden])
Date: 2014-06-28 20:38:15

Dear all,

I am trying to create a fusion sequence whose elements are non-const references to the elements of given set of sequences. In particular, for a given sequence of integral constants, say vector< int_<2>, int_<1> >, and two fusion sequences, say X = vector< X0, X1, X2, X3 > and Y = vector< Y0, Y1, Y2 >, I would like to create the following sequence

R = vector< X2&, Y1& >, such that at<0>(R) == at<2>(X) and at<1>(R) == at<1>(Y).

I am using fusion::transform to achieve the above but the transform function takes the sequences as const references so I end up with R = vector<const X2&, const Y1&>, which is not what I want. Below is my code; any ideas on how to get a sequence of non-const references would be greatly appreciated. Thanks for reading!

#include <boost/fusion/include/vector.hpp>
#include <boost/fusion/include/transform.hpp>
#include <boost/fusion/sequence/io/out.hpp>
#include <boost/fusion/include/at.hpp>
#include <boost/mpl/assert.hpp>
#include <boost/type_traits.hpp>

#include <iostream>

using namespace boost::fusion;
using boost::mpl::int_;

using my_ints = vector< int_<2>, int_<1> >;
using my_x = vector<int, double, unsigned, float>;
using my_y = vector<unsigned, float, long>;
using joined = vector<my_x, my_y>;


struct my_functor {
  
  template <class I, class Seq>
  typename result_of::at<Seq, I>::type operator()(I& i, Seq& seq) const {
    return boost::fusion::at<I>(seq);
  }
};

int main() {
  
  my_ints ints;
  my_x x(-2, 3.5, 15, -1.3);
  my_y y(4, 10.1, 100);
  joined jv(x, y);

  auto tv = transform(ints, jv, my_functor());
  std::cout << tv << std::endl; // prints (15 10.1)
  
  // these two fail to compile because the first argument is const-qualified
  BOOST_MPL_ASSERT(( boost::is_same< decltype(at_c<0>(tv)), unsigned& > ));
  BOOST_MPL_ASSERT(( boost::is_same< decltype(at_c<1>(tv)), float& > ));
}