Re: [Boost-users] [mpl][fusion] Metafunction for extracting a type from a map

From: Christian Henning (chhenning_at_[hidden])
Date: 2007-03-22 21:18:17

Thanks, I tried it but seems like my compiler (MSVC 7.1) doesn't eat
it. The error messages are:

c:\boost\boost\mpl\aux_\preprocessed\plain\apply_wrap.hpp(38) : error
C2903: 'apply' : symbol is neither a class template nor a function
template
        c:\boost\boost\mpl\aux_\preprocessed\plain\apply.hpp(43) : see
reference to class template instantiation
'boost::mpl::apply_wrap1<F,T1>' being compiled
        with
        [
            F=boost::mpl::lambda<boost::is_same<boost::mpl::at_c<boost::mpl::_1,1>,int>>::type,
            T1=boost::mpl::deref<boost::mpl::aux::iter_fold_if_null_step<boost::mpl::iter_fold_if<mat,void,boost::mpl::arg<1>,boost::mpl::protect<boost::mpl::aux::find_if_pred<boost::is_same<boost::mpl::at_c<boost::mpl::_1,1>,int>>>>::first_,void>::iterator>::type
        ]
        c:\boost\boost\mpl\aux_\iter_apply.hpp(28) : see reference to
class template instantiation 'boost::mpl::apply1<F,T1>' being compiled
        with
        [
            F=boost::is_same<boost::mpl::at_c<boost::mpl::_1,1>,int>,
            T1=boost::mpl::deref<boost::mpl::aux::iter_fold_if_null_step<boost::mpl::iter_fold_if<mat,void,boost::mpl::arg<1>,boost::mpl::protect<boost::mpl::aux::find_if_pred<boost::is_same<boost::mpl::at_c<boost::mpl::_1,1>,int>>>>::first_,void>::iterator>::type
        ]
        c:\boost\boost\mpl\not.hpp(41) : see reference to class
template instantiation 'boost::mpl::aux::iter_apply1<F,Iterator>'
being compiled
        with
        [
            F=boost::is_same<boost::mpl::at_c<boost::mpl::_1,1>,int>,
            Iterator=boost::mpl::aux::iter_fold_if_null_step<boost::mpl::iter_fold_if<mat,void,boost::mpl::arg<1>,boost::mpl::protect<boost::mpl::aux::find_if_pred<boost::is_same<boost::mpl::at_c<boost::mpl::_1,1>,int>>>>::first_,void>::iterator
        ]
        c:\boost\boost\mpl\aux_\preprocessed\plain\and.hpp(25) : see
reference to class template instantiation 'boost::mpl::not_<T>' being
compiled
        with
        [
            T=boost::mpl::aux::iter_apply1<boost::is_same<boost::mpl::at_c<boost::mpl::_1,1>,int>,boost::mpl::aux::iter_fold_if_null_step<boost::mpl::iter_fold_if<mat,void,boost::mpl::arg<1>,boost::mpl::protect<boost::mpl::aux::find_if_pred<boost::is_same<boost::mpl::at_c<boost::mpl::_1,1>,int>>>>::first_,void>::iterator>
        ]
        c:\boost\boost\mpl\aux_\preprocessed\plain\and.hpp(55) : see
reference to class template instantiation
'boost::mpl::aux::and_impl<C_,T1,T2,T3,T4>' being compiled
        with
        [
            C_=true,
            T1=boost::mpl::apply1<boost::mpl::protect<boost::mpl::aux::find_if_pred<boost::is_same<boost::mpl::at_c<boost::mpl::_1,1>,int>>>,boost::mpl::aux::iter_fold_if_null_step<boost::mpl::iter_fold_if<mat,void,boost::mpl::arg<1>,boost::mpl::protect<boost::mpl::aux::find_if_pred<boost::is_same<boost::mpl::at_c<boost::mpl::_1,1>,int>>>>::first_,void>::iterator>,
            T2=boost::mpl::true_,
            T3=boost::mpl::true_,
            T4=boost::mpl::true_
        ]
[snip]

The code looks like this:

#include "boost/mpl/vector.hpp"
#include <boost/mpl/find_if.hpp>
#include <boost/mpl/at.hpp>

using namespace std;
using namespace boost;

typedef mpl::vector<char, char> vec_1;
typedef mpl::vector<char, int> vec_2;

typedef mpl::vector<vec_1, vec_2> mat;

//typedef mpl::find_if<mat, is_same<mpl::_1, int> >::type iter;

typedef mpl::find_if<mat, is_same<mpl::at_c<mpl::_1,1>, int> >::type iter;

int _tmain(int argc, _TCHAR* argv[])
{
   BOOST_STATIC_ASSERT(( is_same< mpl::deref<iter>::type, vec_2 >::value ));

        return 0;
}

Regards,
Christian

On 3/22/07, Chris Weed <chrisweed_at_[hidden]> wrote:
> On 3/22/07, Christian Henning <chhenning_at_[hidden]> wrote:
> > Thanks Chris for the quick response. I was trying find_if but I don't
> > think it provides the whole solution. Below is the minimal example,
> > that doesn't compile since the static assertion fires.
> >
> > #include "boost/mpl/vector.hpp"
> > #include <boost/mpl/find_if.hpp>
> >
> > using namespace std;
> > using namespace boost;
> >
> > typedef mpl::vector<char, char> vec_1;
> > typedef mpl::vector<char, int> vec_2;
> >
> > typedef mpl::vector<vec_1, vec_2> mat;
> >
>
> I would think you need to use mpl::at_c here. I haven't test this, but
> how about:
>
> typedef mpl::find_if<mat, is_same<mpl::at_c<mpl::_1,1>, int> >::type iter;
>
> >
> > int _tmain(int argc, _TCHAR* argv[])
> > {
> >    BOOST_STATIC_ASSERT(( is_same< mpl::deref<iter>::type, vec_2 >::value ));
> >
> >         return 0;
> > }
> >
> > Do you know what I'm missing here?
> >
> > Christian
> > _______________________________________________
> > Boost-users mailing list
> > Boost-users_at_[hidden]
> > http://lists.boost.org/mailman/listinfo.cgi/boost-users
> >
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