[boost] Re: [range] update on ADL hooks -- new insight

From: David Abrahams (dave_at_[hidden])
Date: 2005-03-30 08:53:56

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

> Dear all,
>
> I'm going to change the ADL lookup hook of range functions.
>
> instead of the irritating
>
> void foo()
> {
>     using namespace boost;
>     begin(r);
> }
>
> we simply now say
>
> void foo()
> {
>     boost::begin(r);
> }
>
> and get full ADL lookup. This changes the extension protocol
> to overiding these three functions:
>
> range_adl_begin()
> range_adl_end()
> range_adl_size()
>
> Any comments before I commit?

[I hope we've dropped "adl_" from the name by now ;-) ]

A scheme like this one has at least one advantage that we should be
sure to take advantage of: it allows us to reduce the number of
overloads required by conforming range classes.  I've been working on
a similar scheme for an generalized Range concept ("Sequence" -- I
know it's used by the standard but how many suitable concept names
remain?), so here's some of my code:

----
namespace boost {
namespace sequence { 
// Do not use directly, except to specialize.
// Default implementation works for ordinary STL sequences.
template <class Sequence>
struct traits
{
    typedef typename Sequence::iterator begin_iterator;
    typedef typename Sequence::iterator end_iterator;
    typedef typename Sequence::const_iterator begin_const_iterator;
    typedef typename Sequence::const_iterator end_const_iterator;
};
    
template <class Sequence>
struct begin_iterator
{
    typedef typename sequence::traits<
        Sequence
    >::begin_iterator type;
};
// Note: this partial specialization is important!
template <class Sequence>
struct begin_iterator<Sequence const>
{
    typedef typename sequence::traits<
        Sequence
    >::begin_const_iterator type;
};
    
template <class Sequence>
typename begin_const_iterator<Sequence>::type
begin(Sequence const& s)
{
    return sequence_begin(s);
}
template <class Sequence>
typename begin_iterator<Sequence>::type
begin(Sequence& s)
{
    return sequence_begin(s);
}
}}
namespace my
{
  struct range;
  
  // Note: no overload for Range const&.  This is never 
  // called directly, so it always receives an lvalue.
  template <class Range>
  typename begin_iterator<Sequence>::type
  sequence_begin(Range& s)
  {
      return ... // whatever
  }
  ...
}
-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com