[boost] Re: Possible tuple extensions

From: Larry Evans (cppljevans_at_[hidden])
Date: 2005-04-28 09:55:52

On 11/08/2001 10:18 AM, Aleksey Gurtovoy wrote:
[snip]
> And the extensions themselves are here:
> http://groups.yahoo.com/group/boost/files/tuple_ext/

The test code in:

   libs/tuple_ext/test/tuple_iterator_test.cpp

looks like the iterator, to be useful, requires some
templated function similar to the:

template<class Tuple> void foo(tuples::tuple_iterator<Tuple>) {}

in that file.  However, the template argument, Tuple, provides no
clue to the programmer as to the argument type or which element
in the tuple is being passed.  Wouldn't a variant argument be
more self-documenting?  In addition, wouldn't some sort of
visitor also be more self-documenting.  The visitor would take
the same argument as the tuple:

   visitor<tuple::type_list>

BTW, the zip file did not have tuple.hpp; hence, I'm guessing what
the appropriate nested type (type_list) for the tuple types
would be.  Then visitor could be defined as follows:

   template
   < typename HeadField
   , typename OtherFields
   >
   struct
visited_fields
   < HeadField
   , OtherFields
   >
   : public OtherFields
{
         virtual
       void
     visit_field( HeadField& a_field )
     =0
     ;
};

   struct
empty_visited_field
{
  protected:
       void
     visit_field
       ( void
       )
      /**@brief
       *  Only to avoid compile error during parse of subclass'
       *  using declaration.
       */
     ;
};

}//exit visitors_detail namespace

#include <boost/mpl/inherit_linearly.hpp>

template
   < typename type_list
   >
   struct
visitors
   : public boost::mpl::inherit_linearly
     < type_list
     , visited_fields
       <boost::mpl::arg<2>
       , boost::mpl::arg<1>
       >
     , empty_visited_field
     >::type
{
};