/*
 * ChangeLog:
 *   2014-05-21.0829 by LJEvans
 *     downloaded from attachment to:
 *
From: Roland Bock <rbock@eudoxos.de>
Newsgroups: gmane.comp.lib.boost.devel
Subject: Re: [GSoC][MPL11] Post C++Now update
Date: Wed, 21 May 2014 00:00:03 +0200
Lines: 137
Approved: news@gmane.org
Message-ID: <537BD063.4070102@eudoxos.de>
 *
 *   2014-05-21.1136 by LJEvans
 *     Reformatted and added many comments
 *     to clarify how it works.
 */
#include <utility>
#include <iostream>

namespace detail
{
  struct any 
  { 
      template<typename T> 
      any(T&& t)
      /**
       * @brief
       *   Provide a conversion constructor
       *   for any type.
       */
      {
          std::cout<<"any::CTOR:t="<<t<<"\n";
      } 
  };

    template
    < typename T
    , typename U
    > 
  struct first 
  { 
      typedef T type; 
  };

    template
    < typename ...Ts
      //sizeof...(Ts)+1 is the index of the argument desired
      //in the static select method below.
    >
    struct 
  select_impl 
  {
        template<typename U, typename ...Vs>
          static 
        U&&
      select
        ( typename first<any, Ts>::type... 
          //Produces sizeof...(Ts) any formal arguments.
          //Since each any can be constructed from 
          //T&& for any T, the 1st sizeof...(Ts)
          //actual arguments to this function can
          //converted to an any.
          //This leaves the next , unconverted, actual arg 
          //in argument list as the argument desired.
        , U&& u //the desired argument [the (sizeof...(Ts)+1)-th argument].
        , Vs&&...
        ) 
      {
          return static_cast<U&&>(u);
      }
  };

    template
    < std::size_t... Idx
    , typename... Ts
    >
      static 
    auto 
  select
    ( const std::index_sequence<Idx...>&
    , Ts&&... ts
    )
    /**
     *  @brief
     *    Return (N+1)-th ts, where N=sizeof...(Idx).
     */
  {
      return select_impl
             < decltype(Idx)... 
               //converts std::size_t args to types(all of type std::size_t)
               //because called function uses detail::first metafunction
               //which requires type args.
             >::select
                ( static_cast<Ts&&>(ts)... //forward the actual arguments.
                );
  }
}//exit detail namespace

  template<std::size_t N, typename ...Ts>
  auto 
nth(Ts &&...ts)
  /**
   * @brief
   *    return (N+1)-th ts, where 0-th ts is first one.
   */
{
    return detail::select(std::make_index_sequence<N>(), static_cast<Ts&&>(ts)...);
}

int main()
{
    {
        auto x = ::nth<7>(0,"1",'2',3,"4",'5',6,"7");
        std::cout << x << std::endl;
    }
    {
        auto x = ::nth<0>(0,"1",'2',3,"4",'5',6,"7");
        std::cout << x << std::endl;
    }
    {
        auto x = ::nth<1>(0,"1",'2',3,"4",'5',6,"7");
        std::cout << x << std::endl;
    }
    return 0;
}