Re: [Boost-users] MPL: Instantiating a template across permutations of types

From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2008-01-21 19:06:29

AMDG

cp now wrote:
> Hello.
>
> I am trying to instantiate a template taking two parameters across a 
> series of types contained in mpl::vector's, and place the resulting 
> types in an mpl::vector.
>
> template <class A, class B> class T {};
> typedef mpl::vector<int, double, char> Atypes;
> typedef mpl::vector<double, std::string> Btypes;
>
> template <template <class, class> class TEMPLATE,
>    class ATYPE,
>    class BTYPES,
>    class CLASSES>
> struct unary_fold_inner : mpl::fold<
>    BTYPES,
>    CLASSES,
>    mpl::push_back<mpl::_1, TEMPLATE<ATYPE, mpl::_2> >
> > {};
>
> template <template <class, class> class TEMPLATE,
>    class ATYPES,
>    class BTYPES>
> struct unary_fold : mpl::fold<
>    ATYPES,
>    mpl::vector<>,
>    typename detail::unary_fold_inner<TEMPLATE, mpl::_2, BTYPES, 
> mpl::_1>::type
> > {};

Problem #1.  This causes the metafunction unary_inner_fold to be 
evaluated immediately.
Don't put ::type here.  You want unary_inner_fold to be evaluated by 
mpl::fold.

Problem #2
MPL Lambda can't handle template template arguments.  Use a metafunction 
class instead.

>
> typedef unary_fold<
>    T,
>    Atypes,
>    Btypes
> >::type CLASSES;

The following gives the correct answers for me:

#include <string>

#include <boost/mpl/vector.hpp>
#include <boost/mpl/fold.hpp>
#include <boost/mpl/push_back.hpp>
#include <boost/mpl/apply.hpp>
#include <boost/mpl/quote.hpp>

namespace mpl = boost::mpl;

template <class A, class B> class T {};
typedef mpl::vector<int, double, char> Atypes;
typedef mpl::vector<double, std::string> Btypes;

template <class TEMPLATE,
   class ATYPE,
   class BTYPES,
   class CLASSES>
struct unary_fold_inner : mpl::fold<
   BTYPES,
   CLASSES,
   mpl::push_back<mpl::_1, mpl::apply2<TEMPLATE, ATYPE, mpl::_2> >
 > {};

template <class TEMPLATE,
   class ATYPES,
   class BTYPES>
struct unary_fold : mpl::fold<
   ATYPES,
   mpl::vector<>,
   typename unary_fold_inner<TEMPLATE, mpl::_2, BTYPES, mpl::_1>
 > {};

//typedef  CLASSES;

template<class Message>
struct print {};

typedef print<
    unary_fold<
        mpl::quote2<T>,
       Atypes,
       Btypes
    >::type
 >::type CLASSES;

In Christ,
Steven Watanabe