[Boost-users] [MPL] Fold a vector of pairs

Subject: [Boost-users] [MPL] Fold a vector of pairs
From: Peter Nyssen (peter_at_[hidden])
Date: 2012-03-14 02:13:47

Hi all,

I'm trying to understand how to use mpl::fold to transform a vector like 
this:
vector<
     pair<tag1, A>
     pair<tag2, B>
     pair<tag1, C>
 >

into the following:
pair<
     vector<A,C>
     vector<B>
 >

Following test program fails to compile and I don't understand why.
If you could explain me what I'm missing here, you will gain my eternal 
respect.

Kind regards,
Peter

//--------------------------------------------

#include <boost/mpl/if.hpp>
#include <boost/mpl/int.hpp>
#include <boost/mpl/fold.hpp>
#include <boost/mpl/pair.hpp>
#include <boost/mpl/push_back.hpp>
#include <boost/mpl/vector.hpp>

namespace mpl = boost::mpl;

struct tag1 {};
struct tag2 {};

template <typename T>
struct is_tag1 : boost::false_type {};

template <>
struct is_tag1<tag1> : boost::true_type {};

int main()
{
     typedef mpl::vector<
         mpl::pair<tag1, mpl::int_<1> >,
         mpl::pair<tag2, mpl::int_<2> >,
         mpl::pair<tag1, mpl::int_<3> >
 > initial_vector;

     typedef typename mpl::fold<
         initial_vector,
         mpl::pair<mpl::vector<>, mpl::vector<> >,
         mpl::if_<
             is_tag1<mpl::first<mpl::_2>::type >,
             mpl::push_back<
                 mpl::first<mpl::_1>::type,
                 mpl::second<mpl::_2>::type
 >,
             mpl::push_back<
                 mpl::second<mpl::_1>::type,
                 mpl::second<mpl::_2>::type
 >
 >
 >::type sorted_pair;

     return 0;
}

//--------------------------------------------

*$* g++ -Wall -I ../boost/ -o test test.cpp
In file included from test.cpp:5:0:
../boost/boost/mpl/pair.hpp: In instantiation of 
'boost::mpl::first<mpl_::arg<2> >':
test.cpp:32:40:   instantiated from here
../boost/boost/mpl/pair.hpp:43:31: fout: no type named 'first' in 
'struct mpl_::arg<2>'
test.cpp: In functie 'int main()':
test.cpp:32:47: fout: template argument 1 is invalid
In file included from test.cpp:5:0:
../boost/boost/mpl/pair.hpp: At global scope:
../boost/boost/mpl/pair.hpp: In instantiation of 
'boost::mpl::first<mpl_::arg<1> >':
test.cpp:34:36:   instantiated from here
../boost/boost/mpl/pair.hpp:43:31: fout: no type named 'first' in 
'struct mpl_::arg<1>'
../boost/boost/mpl/pair.hpp: In instantiation of 
'boost::mpl::second<mpl_::arg<2> >':
test.cpp:35:37:   instantiated from here
../boost/boost/mpl/pair.hpp:56:32: fout: no type named 'second' in 
'struct mpl_::arg<2>'
test.cpp: In functie 'int main()':
test.cpp:36:13: fout: template argument 1 is invalid
test.cpp:36:13: fout: template argument 2 is invalid
In file included from test.cpp:5:0:
../boost/boost/mpl/pair.hpp: At global scope:
../boost/boost/mpl/pair.hpp: In instantiation of 
'boost::mpl::second<mpl_::arg<1> >':
test.cpp:38:37:   instantiated from here
../boost/boost/mpl/pair.hpp:56:32: fout: no type named 'second' in 
'struct mpl_::arg<1>'
test.cpp: In functie 'int main()':
test.cpp:40:13: fout: template argument 1 is invalid
test.cpp:40:13: fout: template argument 2 is invalid
test.cpp:41:9: fout: template argument 1 is invalid
test.cpp:41:9: fout: template argument 2 is invalid
test.cpp:41:9: fout: template argument 3 is invalid
test.cpp:42:5: fout: template argument 3 is invalid
test.cpp:42:24: fout: invalid type in declaration before ';' token
**