How can I pull variadic template arguments off from the tail instead of the head?

Question

For silly reasons I\'ll not go into here, I need the commented out line to work and the line above it it to not work:

template
         


        

Answer 1

I've made my code work a little bit like lists in Haskell - because, well, TMP is purely functional language inside C++.

add_to_pack is equivalent to Haskell's list constructor (:). drop_from_end is implemented as (in Haskell notation) \x list -> take (length list - x) list, where take n just takes first n elements of the list.

I suppose you could use std::tuple directly instead of pack, but I liked this solution better, because it doesn't misuse tuple as template parameter pack holder. :)

Here's the code:

#include 
#include  // for std::conditional


template 
struct pack
{ };


template 
struct add_to_pack;

template 
struct add_to_pack>
{
  typedef pack type;
};


template 
struct convert_to_tuple;

template 
struct convert_to_tuple>
{
  typedef std::tuple type;
};


template 
struct take;

template 
struct take
{
  typedef pack<> type;
};

template 
struct take
{
  typedef
    typename std::conditional<
      (N > 0),
      typename add_to_pack<
        Head,
        typename take<
          N - 1,
          Tail...
        >::type
      >::type,
      pack<>
    >::type type;
};  


template 
struct drop_from_end
{
  // Add these asserts if needed.
  //static_assert(N >= 0,
  //  "Cannot drop negative number of elements!");

  //static_assert(N <= static_cast(sizeof...(A)),
  //  "Cannot drop more elements than size of pack!")

  typedef
    typename convert_to_tuple<
      typename take<
        static_cast(sizeof...(A)) - N,
        A...
      >::type
    >::type type;
};


int main()
{
  drop_from_end<2, const char*, double, int, int>::type b{"pi", 3.1415};
}

And here's the code at work: via ideone.com.

---

The take struct is more or less equivalent to following Haskell code:

take n []     = []
take n (x:xs)
  | n > 0     = x : take (n - 1) xs
  | otherwise = []