[boost] Re: [range] Collection Concept

From: David Abrahams (dave_at_[hidden])
Date: 2004-09-02 19:23:22

"Thorsten Ottosen" <nesotto_at_[hidden]> writes:

> "Thorsten Ottosen" <nesotto_at_[hidden]> wrote in message
> news:ch7fj0$vbj$1_at_sea.gmane.org...
> | "David Abrahams" <dave_at_[hidden]> wrote in message
> | news:un008zsze.fsf_at_boost-consulting.com...
>
> | | > | Sounds like Collection is a refinement of Range.
> | | > well, I dropped the external/internal concept definitions in the
> | | > range docs, so currently ranges are defined by free-standing
> | | > functions and metafunctions and not members.
> | | If you have generalized free-standing functions/metafunctions that
> | | work for STL collections, they should work for the Collection
> | | concept too.
> | yes, of course. but that doesn't make a collection a refinement of a
> | range.
>
> Let me explain a little better this tricky situation. Since the Range
> concepts are described in terms of free-standing functions, we kind of
> loose the ability to say a class is a model of a Range concept if that
> class only has member functions in its interface.
>
> So since Collection has col.begin() and col.end() and not begin( col )
> and end( col ) it is not per se a model of any Range concept.
>
> Boost.Range then comes with an implementation of various Range
> concepts for *any* class that has begin() and end() members.  So when
> this header with this implementation is included, all Collections
> model certain Range concepts; otherwise they do not.
>
> And since Ranges are complete decoupled from the class/member function
> idea, it does not make sense for classes to refine these concepts.
>
> hope it helps

What we need is somthing like this:

namespace boost { namespace range {

namespace detail
{
  template <class T>
  typename range_iterator<T>::type begin(T& x)
  {
     return x.begin()
  }
  
  template <class T>
  typename range_iterator<T>::type adl_begin(T& x)
  {
      return begin(x); // will use ADL if a begin is defined
  }
}

template <class T>
typename range_iterator<T>::type begin(T& x)
{
  return detail::adl_begin(x)
}

}}

Then you change the Range concept to require that
boost::range::begin(col) works.  Of course, Collection will only be a
refinement if there is no incorrect begin hanging around for that
type... so maybe this is fruitless.

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com