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Subject: [Boostusers] Selecting one element per fusion sequence using mpl::int_s
From: gsermaid (linuxfever_at_[hidden])
Date: 20140628 20:38:15
Dear all,
I am trying to create a fusion sequence whose elements are nonconst 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 nonconst 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 constqualified
BOOST_MPL_ASSERT(( boost::is_same< decltype(at_c<0>(tv)), unsigned& > ));
BOOST_MPL_ASSERT(( boost::is_same< decltype(at_c<1>(tv)), float& > ));
}
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