Re: [boost] [Range] breaking update pending

From: Shunsuke Sogame (mb2act_at_[hidden])
Date: 2006-02-05 03:34:37

Thorsten Ottosen wrote:
> Shunsuke Sogame wrote:
> 
>>Thorsten Ottosen wrote:
>>
> 
> 
>>>>>Can you elaborate on what exactly you would like to see?
> 
> 
>>>>Moreover, the different version of ATL has different template parameters.
>>>
>>>
>>>if ATL actually uses a namespace, you could add a unconstrained overload
>>>of range_begin/range_end.
>>
>>
>>ATL/WTL has also many containers that can conform to Boost.Range.
>>I request tag-dispatching!
> 
> 
> 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.
They form a simple hierarchy, but we cannot take advantage of it.
Partial class template specialization ignores base/derived relationship.
And "primary" range_detail::boost_range_begin(C&) eats all the derived classes.
Thus, I have to write 13 implementations.

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<>.

Moreover, 'const' overloads disappear.
(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.
By doing this, you can contrive any nice customization way anytime.

-- 
Regards,
Shunsuke Sogame