On Tue, Mar 13, 2012 at 11:13 PM, Peter Nyssen <peter@dynsws.com> wrote:
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;
Try removing all the ::type's in the fold operation and rely on lazy evaluation. mpl:first< mpl::_1 >::type is equivalent to mpl::_1::first, which of course doesn't exist and isn't what you want; but mpl::first< mpl::_1 > is a placeholder expression equivalent to the mapping ( T0, ... ) -> T0::first, which looks like what you want.
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>’
:: cough ::
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
See if that helps. (Too lazy to try it on my own.)
- Jeff