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

From: Maurizio Vitale (maurizio.vitale_at_[hidden])
Date: 2007-08-20 10:39:30

Eric, just for my understanding, your suggestion is also the only way  
to proceed when the result type depends on Expr, right?
Thanks,

        Maurizio

On Jun 1, 2007, at 10:36 AM, Eric Niebler wrote:

> 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
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