Re: [boost] Alternative solution to intermediate results problem of expression templates.

From: Joel de Guzman (joel_at_[hidden])
Date: 2005-10-23 10:15:38

Korcan Hussein wrote:
> "Larry Evans" <cppljevans_at_[hidden]> wrote in message 
> news:435B9E00.3030607_at_cox-internet.com...
> 
>>On 10/22/2005 06:58 AM, Larry Evans wrote:
>>Are you suggesting this as an alternative to Spirit subrules?
> 
> 
> Doesn't need to replace subrules but could be an alternative implementation 
> for them, if you read the part:
> 
> [qoute]
> ... It might not be apparent but behind the scenes, plain rules are actually 
> implemented using a pointer to a runtime polymorphic abstract class that 
> holds the dynamic type of the parser assigned to it. When a Spirit 
> expression is assigned to a rule, its type is encapsulated in a concrete 
> subclass of the abstract class. A virtual parse function delegates the 
> parsing to the encapsulated object.
> Rules have drawbacks though:
> 
>  It is coupled to a specific scanner type. The rule is tied to a specific 
> scanner [see The Scanner Business].
>  The rule's parse member function has a virtual function call overhead that 
> cannot be inlined.
> 
> [/quote]

That quote pertains to (type-erased rules) not subrules.
Subrules in Spirit are completely static. No type erasure.
Please read the http://www.boost.org/libs/spirit/doc/subrules.html
carefully. Read below the lines that you have quoted. Those lines
you quoted were there just to explain the problems of type-erased
rules.

I'm quite interested if you have a better alternative. For example,
how do you write the Spirit calculator using your scheme:

     rule<> group, factor, term, expression;

     group       = '(' >> expression >> ')';
     factor      = integer | group;
     term        = factor >> *(('*' >> factor) | ('/' >> factor));
     expression  = term >> *(('+' >> term) | ('-' >> term));

?

Regards,
-- 
Joel de Guzman
http://www.boost-consulting.com
http://spirit.sf.net