[boost] [lambda] extension question

From: Brock Peabody (brock.peabody_at_[hidden])
Date: 2005-07-18 10:19:00

Hi,

I'm trying to learn how to make my own lambda compatible function
objects and I'm hoping that someone can nudge me in the right direction.
My first try is called tuple_element.   It takes an index as a template
parameter and a tuple as an argument, returning the element at the given
index:

      //should print "x"
   cout << tuple_element<1>(_1)(tuple<int,std::string>(2, "x"));

I've got everything working except for the sig template.  tuple_element
looks like (mostly copied from lambda source code):

        template <int Index> struct tuple_element_action {};
        
        template <int Index, typename Arg>
        inline const
        lambda_functor<
                lambda_functor_base<
                        tuple_element_action<Index>,
                        tuple<lambda_functor<Arg> >
		>
	>
        tuple_element(const lambda_functor<Arg>& a) { 

                return 
                        lambda_functor_base<
                                tuple_element_action<Index>, 
                                tuple<lambda_functor<Arg> >
			> 
                        ( tuple<lambda_functor<Arg> >(a));
        }

and the action:

        template<typename Args, int Index>
        class 
        lambda_functor_base<tuple_element_action<Index>, Args> {
        public:

                Args args;

                template <class T> struct sig : tuples::element<Index,T>
{};

                explicit lambda_functor_base(const Args& a) : args(a) {}

                template<class RET, CALL_TEMPLATE_ARGS>
                RET call(CALL_FORMAL_ARGS) const {

                        return detail::select(get<0>(args),
CALL_ACTUAL_ARGS).get<Index>();
                }
        };

When I try to compile my example in VC 7.1 I get an error that seems to
indicate that the type being passed to the sig template is
boost::tuples::null_type.  Everything works if I hardwire it like:

   template <class T> struct sig { typedef std::string type; };

Any ideas?

Thanks,
Brock