On Jan 22, 2008 5:36 AM, Steven Watanabe <watanabesj@gmail.com> 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