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

From: Eric Niebler (eric_at_[hidden])
Date: 2007-06-01 10:36:29

Larry Evans wrote:
> 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)

Sure. Only context types derived from callable_context get unpacked 
nodes like that. If you don't inherit from callable_context, you'll get 
the whole node without any modification. The interface is a bit 
different, though. You'll need something like:

struct my_context
{
     template<typename Expr>
     struct eval
     {
         typedef ... result_type;
         result_type
         operator()(Expr &expr, my_context &ctx) const
         {
             return ...;
         }
     };
};

If you want to handle binary nodes specially, you would define the eval 
member template as:

     template<typename Expr, long Arity = Expr::arity::value>
     struct eval

and then partially specialize for binary nodes:

     template<typename Expr>
     struct eval<Expr, 2>

HTH,

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com