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

From: Eric Niebler (eric_at_[hidden])
Date: 2007-08-27 11:54:16

(Sorry for the delay, I'm just back from a week-long vacation.)

Maurizio Vitale wrote:
> On Jun 1, 2007, at 10:36 AM, Eric Niebler wrote:
>> 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>
>>
>>
 > Eric, just for my understanding, your suggestion is also the only way
 > to proceed when the result type depends on Expr, right?

Not so. If your context type inherits from callable_context, you can 
define a nested result class template for calculating the return type. 
In this way, the return type can be made to depend on the expression type.

HTH,

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