Re: [Boost-users] [fusion] How to use transform Metafunction?

From: Christian Henning (chhenning_at_[hidden])
Date: 2007-03-19 10:34:12

Thanks Chris and Joel for your replies.

>
> There is an example (at least a test). You can get that here:
> libs/fusion/test/algorithm/transform/transform.cpp. The doc
> also contains an example (see the transform docs)

I was looking at this test. But it doesn't include a test of the
transform metafunction, I believe. Correct me if I'm wrong.

>
> This is one of the cases where Fusion differs from MPL. Fusion
> requires a polymorphic function object for the transform. See
> the docs on "Polymorphic Function Object".

I would suggest to add an example on the "Polymorphic Function Object"
page in the documentation. Also, how about adding an index at the end
of the documentation? I would like that a lot.

> Having said that,
> and looking at your code, it seems you do not need value
> transformation (which is what Polymorphic Function Objects
> do). All you are doing is type manipulation. I'd suggest you
> to first work on an mpl sequence. Then, when you get your
> desired MPL sequence, convert it to a fusion sequence through
> fusion::as_xxx (see conversions in the docs). fusion::as_xxx
> can take in any valid MPL sequence. Here's an example:
>
>      typedef
>          fusion::result_of::as_list<any_mpl_seq>::type
>      list_of_vectors;
>

This is what I'm doing right now:

#include <vector>

#include <boost/mpl/assert.hpp>
#include <boost/mpl/equal.hpp>
#include <boost/mpl/list.hpp>
#include <boost/mpl/transform.hpp>

#include <boost/fusion/algorithm/transformation/transform.hpp>
#include <boost/fusion/algorithm/iteration/for_each.hpp>
#include <boost/fusion/sequence/container.hpp>
#include <boost/fusion/sequence/conversion/as_list.hpp>

#include <boost/type_traits/add_pointer.hpp>

using namespace std;
using namespace boost;

template <typename T>
struct add_vector
{
   typedef std::vector<T> type;
};

struct A;
struct B;
struct C;

int main()
{
   typedef mpl::list< A, B, C > types;
   typedef mpl::transform1< types, add_vector<mpl::_1> >::type
mpl_list_of_vectors;

   typedef fusion::result_of::as_list<mpl_list_of_vectors>::type
fusion_list_of_vectors;

   return 0;
}

Unfortunately, this doesn't compile. I'm using VC 7.1.

hpp(33) : error C2504:
'boost::fusion::extension::begin_impl<Tag>::apply<Sequence>' : base
class undefined
        with
        [
            Tag=boost::fusion::traits::tag_of<boost::remove_const<mpl_list_of_vectors>::type>::type
        ]
        and
        [
            Sequence=mpl_list_of_vectors
        ]
        c:\boost\boost\fusion\sequence\intrinsic\begin.hpp(33) : see
reference to class template instantiation
'boost::fusion::extension::begin_impl<Tag>::apply<Sequence>' being
compiled
        with
        [
            Tag=boost::fusion::traits::tag_of<boost::remove_const<mpl_list_of_vectors>::type>::type,
            Sequence=mpl_list_of_vectors
        ]
        c:\boost\boost\fusion\sequence\conversion\as_list.hpp(26) :
see reference to class template instantiation
'boost::fusion::result_of::begin<Sequence>' being compiled
        with
        [
            Sequence=mpl_list_of_vectors
        ]
etc.

Is there anything wrong with my code?

Christian