Re: C++0x: variadic template puzzle

From:

SG <s.gesemann@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Tue, 7 Jul 2009 12:55:26 CST

Message-ID:

<d2f4423d-9a5d-4a67-892f-f871dc45148a@d4g2000yqa.googlegroups.com>

SG <s.gesem...@gmail.com> wrote:

[...]

   template<typename Func, typename... Args>
   inline void tuple_forward(Func && f, tuple<Args...> && args)
   {
      typedef invoke_helper<0,1,2,...,(sizeof...(Args)-1)> ih;
      ih::doit( forward<Func>(f) , move(args) );
      // Note: "args" is always an rvalue reference. There is
      // no reference collapsing involved.
   }

I havn't yet been able to come up with a solution for the "ih"-
typedef. Obviously the line is ill-formed.

I think I solved it. It required quite a bit of TMP noise, though:

    template< typename, typename >
      struct concat;

    template< int... I1, int... I2 >
      struct concat< invoke_helper<T1...>, invoke_helper<T2...> >
      {
        typedef invoke_helper<T1...,T2...> type;
      };

    template< int Count, int Start = 0 >
      struct make_helper
      {
        static const int Half1 = Count/2;
        static const int Half2 = Count-Half1;
        static const int Center = Start + Half1;
        typedef typename concat<
          typename make_helper<Half1,Start>::type,
          typename make_helper<Half2,Center>::type
        >::type type;
      };

    template<int Start>
      struct make_helper<0,Start>
      { typedef invoke_helper<> type; };

    template<int Start>
      struct make_helper<1,Start>
      { typedef invoke_helper<Start> type; };

    template<typename Func, typename... Args>
      inline void tuple_forward(Func && f, tuple<Args...> && args)
      {
         typedef typename make_helper<sizeof...(Args)>::type ih;
         ih::doit( forward<Func>(f) , move(args) );
      }

Cheers!
SG

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