Re: [boost] [Range] breaking update pending

From: Thorsten Ottosen (tottosen_at_[hidden])
Date: 2006-02-05 18:43:59

Shunsuke Sogame wrote:
> Thorsten Ottosen wrote:
>>
>>How will a different mechanism save you from provding specializations
>>of alle the types you want to support?
> 
> 
> I tried to write Boost.Range implementation of ATL/WTL collections.
> But I gave up. There are 13 classes.

Is that without counting typdefs like CString etc?

That is a lot.

> They form a simple hierarchy, but we cannot take advantage of it.
> Partial class template specialization ignores base/derived relationship.

right.

> And "primary" range_detail::boost_range_begin(C&) eats all the derived classes.

yes, a wierd feature of the language that we have to lieve with.

> Thus, I have to write 13 implementations.

maybe the easiest would be to write a function, say, make_mfc_range(),
that returns an iterator_range<...> of some sort. Then on all range 
boundaries you just call make_mfc_range(mfc_rng).

(well,not great, but at least something)

> Now I see ADL customization way is blob.
> Type itself has another job, that is, implementation selector.
> Left the job to tag...
> 
> For example, std::pair range implementation could be simplified to something like:
> 
> 
>    namespace customization {
> 
> 
>      namespace std_pair_detail {
> 
>        struct tag
>        { };
> 
>      } // namespace std_pair_detail
> 
> 
>      template< class T >
>      struct tag< std::pair<T, T> > :
>        boost::mpl::identity<std_pair_detail::tag>
>      { };
> 
> 
>      template< class PairT >
>      struct metafunctions<PairT, std_pair_detail::tag>
>      {
>        typedef typename PairT::first_type iterator;
>        typedef iterator const_iterator;
>      };
> 
> 
>      template< >
>      struct functions<std_pair_detail::tag>
>      {
>        template< class PairT >
>        typename boost::range_iterator<PairT>::type
>        begin(PairT& p)
>        {
>          return p.first;
>        }
> 
>        template< class PairT >
>        typename boost::range_iterator<PairT>::type
>        end(PairT& p)
>        {
>          return p.second;
>        }
>      };
> 
>    }
> 
> 
> Now 'PairT' is almost concept.
> A type derived from std::pair<> specializes only tag<>.

that is clever.

> Moreover, 'const' overloads disappear.

also clever.

> (though such overloads also are possible.)
> Of course, default implementation can call 'boost_range_begin'.
> Compatibility holds.
> 
> I said range metafunctions were "public virtual interfaces".
> IMHO, next version should make them non-virtual.

I don't plan to add such customization, sorry. But Iød be happy to
include your support for mfc with the library.

Maybe you can get some help from the preprocessor?

-Thorsten