[boost] [proto]context interface allowing "packaged" node args?

From: Larry Evans (cppljevans_at_[hidden])
Date: 2007-06-01 07:35:43

Hi Eric,

I need a context with something like the following methods:

     template< typename Tag, typename Left, typename Right >
     result_type operator()(expr<Tag, Left, Right> const &expr) const
     {
         std::cout<<std::setw(indent)
           <<""<<"binary_expr:"<<expr.get_instance()<<"\n";
         indent+=2;
         proto::eval(left, *this);
         proto::eval(right, *this);
         indent-=2;
     }

where the expression tree node argument remains "packaged":

   (expr<Tag, Left, Right> const &expr)

in contrast to the unpacked version:

   (Tag, Left const &left, Right const &right)

shown in, for example:

http://boost-sandbox.sourceforge.net/libs/proto/doc/html/boost_proto/user_s_guide/calculator.html

IOW, the calculator.html page suggests an interface like:

     template< typename Tag, typename Left, typename Right >
     result_type operator()(Tag, Left const &left, Right const &right) const
     {
         std::cout<<std::setw(indent)<<""<<"binary_expr\n";
         indent+=2;
         proto::eval(left, *this);
         proto::eval(right, *this);
         indent-=2;
     }

where there's no access to the "packaged" extended node,
expr<Tag,Left,Right>, where the extension is, for example:

struct xmpl_domain;

struct xmpl_delta
{
     unsigned const my_instance;
     static unsigned our_instances;
     xmpl_delta(void)
     : my_instance(++our_instances)
     {}
     xmpl_delta(xmpl_delta const&)
     : my_instance(++our_instances)
     {}
     void operator=(xmpl_delta const&)
     {}
     unsigned get_instance(void)const
     {
         return my_instance;
     }
};
unsigned xmpl_delta::our_instances=0;

template<typename Expr>
struct xmpl_expression
   : proto::extends<Expr, xmpl_expression<Expr>, xmpl_domain>
   , xmpl_delta
{ ... };

I've looked at the formated output of the preprocessed context.hpp;
however, I'm still having difficulty figuring out if this
"packaged" method interfaces is possible.

Any help is appreciated.

TIA.

-regards,
Larry