Re: tuples in C++11

From:

Casey Carter <casey_at_carter_dot_net@nospam.invalid>

Newsgroups:

comp.lang.c++

Date:

Tue, 14 Aug 2012 16:47:01 -0500

Message-ID:

<k0eh0l$lur$1@dont-email.me>

On 2012-08-14 13:17, Single Stage to Orbit wrote:

Is it even possible to have something like this:

for (auto& i : tuples)
{
         for (unsigned j = 0; j < 3; ++j)
         {
                 std::cout << i.get<j>();
                 if (j < 3)
                         std::cout << " ";
         }

         std::cout << '\n';
}

It's possible if you convert the iteration into recursion, and implement
the recursion with some nasty template meta-programming:

#include <iostream>
#include <tuple>
#include <vector>

template <std::size_t Index, std::size_t Max, typename Tuple>
struct dump_helper
{
   static void dump(std::ostream& os, const Tuple& t)
   {
     if (Index > 0)
       os << ' ';
     os << std::get<Index>(t);
     dump_helper<Index+1,Max,Tuple>::dump(os, t);
   }
};

template <std::size_t Max, typename Tuple>
struct dump_helper<Max,Max,Tuple>
{
   static void dump(std::ostream&, const Tuple&)
   {}
};

template <typename... Args>
inline void dump(std::ostream& os, const std::tuple<Args...>& t)
{
   typedef std::tuple<Args...> Tuple;
   dump_helper<0,sizeof...(Args),Tuple>::dump(os, t);
}

int main()
{
   typedef std::tuple<int, int, int> tuple3;
   std::vector<tuple3> tuples;

   tuples.push_back(tuple3(1, 2, 3));
   tuples.push_back(tuple3(7, 8, 9));
   tuples.push_back(tuple3(4, 5, 6));

   for (const auto& i : tuples)
   {
     dump(std::cout, i);
     std::cout << '\n';
   }

   return 0;
}