Re: [Boost-users] a tuple from a pair?

5 Oct 2011

      On 10/6/2011 3:07 AM, Krzysztof Żelechowski wrote:
...
Krzysztof Żelechowski wrote:
...
How come a tuple, being a generalisation of a pair, cannot be constructed
from one?
I mean something along the lines
make_tuple (pair const &p) { return make_tuple (p.first, p_second); }
Would be good to have.
It turns out that TR1 supports constructing a tuple directly from a pair, 
so Boost should catch up.
Boost, is at the forefront of C++ dev and in fact spearheaded the
development of many std libraries, including tuple. FYI, there's
a more powerful tuples library called boost.fusion that includes a
(simple) TR1 tuple implementation.

Here's how you do it in fusion:

  #include <boost/fusion/tuple.hpp>
  #include <boost/fusion/adapted/std_pair.hpp>
  #include <iostream>

  int main()
  {
      using boost::fusion::tuple;
      using std::pair;

      pair<int, int> p(1, 2);

      tuple<int, int> t = p;
      std::cout << t << std::endl;

      return 0;
  }

But fusion gives you more than that and beyond TR1. It's the
unification of all kinds of "tuples" you can imagine. For example:

  #include <boost/fusion/container/vector.hpp>
  #include <boost/fusion/sequence/io.hpp>
  #include <boost/fusion/adapted/boost_array.hpp>
  #include <boost/array.hpp>
  #include <iostream>

  int main()
  {
      // fusion vector is TR1 tuples in steroids
      using boost::fusion::vector;
      using boost::array;

      array<int, 2> p = { 1, 2 };

      vector<int, int> t = p;
      std::cout << t << std::endl;

      return 0;
  }

Or how about:

  #include <boost/fusion/container/vector.hpp>
  #include <boost/fusion/sequence/io.hpp>
  #include <boost/fusion/adapted/struct.hpp>
  #include <iostream>

  // user defined struct
  struct my_struct
  {
      int a;
      int b;
  };

  BOOST_FUSION_ADAPT_STRUCT(my_struct,
      (int, a)
      (int, b)
  );

  int main()
  {
      // fusion vector is TR1 tuples in steroids
      using boost::fusion::vector;

      my_struct p = { 1, 2 };

      vector<int, int> t = p;
      std::cout << t << std::endl;

      return 0;
  }

See http://tinyurl.com/3tx6kys for more.

Regards,
-- 
Joel de Guzman
http://www.boostpro.com
http://boost-spirit.com