[Boost-users] a few more questions

Subject: [Boost-users] a few more questions
From: e r (erwann.rogard_at_[hidden])
Date: 2008-09-10 20:42:03

David Abrahams wrote:
> on Mon Sep 08 2008, e r <erwann.rogard-AT-gmail.com> wrote:
> 
>> hello,
>>
>> i have something like this:
>>
>> *hpp :
>>
>> namespace tag{
>>     template<typename Id>
>>     struct sequence{
>>         static ::boost::parameter::keyword<
>>             a_<Id> >& a;
>>         static ::boost::parameter::keyword<
>>             b_<Id> >& b;
>>
>>         static ::boost::parameter::keyword<
>>             arg_method_<Id> >& arg_method;
>>     };
>> }
>>
>> namespace sequence_arg{
>>     struct method{};
>>     struct a:public method{};
>>     struct b:public method{};
>> }
>>
>> class sequence{
>>   public:
>>         template<typename Id,typename ArgPack>
>>         sequence(const Id& id,const ArgPack&args){
>> fill_elements(id,args,args[tag::sequence<Id>::arg_method]);
>> }
>>
>>  private:
>>         template<typename Id,typename ArgPack>
>>         void fill_elements(
>>             const Id& id,
>>             const ArgPack& args,
>>             sequence_arg::a method
>>         );
>>         template<typename Id,typename ArgPack>
>>         void fill_elements(
>>             const Id& id,
>>             const ArgPack& args,
>>             sequence_arg::b method
>>         );
>> };
>>
>> *.cpp :
>>
>> sequence seq(
>>   id(),
>>   (
>>   tag::sequence::a = a_value,
>>   tag::sequence::arg_method = sequence_arg::a()
>>   )
>> );

oops, I made a mistake, it should be

sequence seq(
  id(),//id instance of Id
  (
  tag::sequence<Id>::a = a_value,
  tag::sequence<Id>::arg_method = sequence_arg::a()
  )
);

The reason I use Id is for example to discriminate between different
objects of the same type within a host object.

>>
>> So far so good except there is redundancy of information about which
>> method to use, so I'm looking for a way to be able to call say
>>
>> sequence seq(
>>   id(),
>>   (
>>   tag::sequence::a = a_value
>>   )
>> )
>>
>> Any suggestion?
> 
> Use the macros -- they automatically generate the SFINAE you need so
> that the right overload will be chosen -- and add type requirements.
> See
> http://www.boost.org/doc/libs/1_36_0/libs/parameter/doc/html/index.html#signature-matching-and-overloading
> and
> http://www.boost.org/doc/libs/1_36_0/libs/parameter/doc/html/index.html#adding-type-requirements.
> 
>   BOOST_PARAMETER_FUNCTION(
>      (void), fill_elements, (tag),
>      (required (id,*) (method, a))
>      (optional ... ))
>      {
>         ...
>      }
>
>   BOOST_PARAMETER_FUNCTION(
>      (void), fill_elements, (tag),
>      (required (id,*) (method, b))
>      (optional ... ))
>      {
>         ...
>      }
>
> HTH,
>

Much appreciated, Thanks!

I think I follow you, but to keep faithful to my example you probably meant:

BOOST_PARAMETER_CONST_MEMBER_FUNCTION(
   (void), fill_elements, (tag),
   (required (id,*) (a, *))
   (optional ... ))
   {
      // say x = sqrt(a)
   }
BOOST_PARAMETER_CONST_MEMBER_FUNCTION(
   (void), fill_elements, (tag),
   (required (id,*) (b, *))
   (optional ... ))
   {
      //say x = b^2
   }

Am I correct?

I still have a problem, because as I corrected above, my tag is indexed
by Id.

I guess I should then have something like this?

class sequence{
   public:
         template<typename Id,typename ArgPack>
         sequence(const Id& id,const ArgPack&args){
 fill_elements(id,args,find_method<Id,ArgPack>::type());
 }
...
};

where find_method<Id,ArgPack> looks for which of sequence<Id>::a and
sequence<Id>::b is present in ArgPack, and returns type sequence_arg::a
or sequence_arg::b accordingly.

I've looked into arglist.hpp, which seems to be what ArgPacks are made
of, but I'd need a little guidance in getting started to write an
appropriate MPL algorithm.

Thanks again!