From: cp now (cppnow_at_[hidden])
Date: 2008-01-29 09:00:46
On Jan 22, 2008 5:36 AM, Steven Watanabe <watanabesj_at_[hidden]> wrote:
> 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
>
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>
Thanks a ton. Once I saw it the apply made much more sense.
-v