[boost] Re: [range] Collection Concept

From: Thorsten Ottosen (nesotto_at_[hidden])
Date: 2004-09-03 03:11:48

"David Abrahams" <dave_at_[hidden]> wrote in message news:ur7pkurnp.fsf_at_boost-consulting.com...
| "Thorsten Ottosen" <nesotto_at_[hidden]> writes:

| > 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)
| }
|
| }}

apart from the adl stuff, it already is like that. Did you mean to put in the range namespace to
avoid problems with classes in boost?

| Then you change the Range concept to require that
| boost::range::begin(col) works.

hm...yeah...one could perhaps state both syntaxes as part of the concept, eg

Valid expressions
============
begin( a )
 or
a.begin()

| Of course, Collection will only be a
| refinement if there is no incorrect begin hanging around for that
| type... so maybe this is fruitless.

well, I agree that we have not got it complete right yet.

perhaps the Range concept could specify

Primary Template Behavior
==================
return a.begin()

?

And maybe we don't have to talk about collection as a refinement; maybe a new term is in order, like "act",
sa in "all Collections may act as Ranges".

-Thorsten, the confused