Re: [boost] [range] How to extend Boost.Range?

From: Rob Stewart (stewart_at_[hidden])
Date: 2005-09-16 08:03:37

I've been thinking about this issue and wonder if the following
wouldn't solve the problem neatly for both conforming and
non-conforming compilers:

   // whatever/foo.h
   namespace whatever
   {
      class foo
      {
      ...
      };

      // for conforming compilers
      unspecified begin(foo const &) { ... };
      ...
   }

   // for non-conforming compilers
   namespace boost { namespace range {
      using whatever::begin;
   } }

   // boost/range/begin.hpp
   namespace boost
   {
      inline template <T>
      unspecified begin(T const & range_i)
      {
         using boost::range::begin;
         return begin(range_i);
      }
   }

If the author of foo has no other need for begin(), then
whatever/foo.h could be done like this instead:

   // whatever/foo.h
   namespace whatever
   {
      class foo
      {
      ...
      };
   }

   namespace boost { namespace range {
      unspecified begin(foo const &) { ... };
   } }

If whatever::foo belongs to a user of Boost.Range that only uses
conforming compilers, then foo.h only needs to be:

   // whatever/foo.h
   namespace whatever
   {
      class foo
      {
      ...
      };

      unspecified begin(foo const &) { ... };
      ...
   }

If whatever::foo provides get_begin() instead, for whatever
reason, then foo.h becomes:

   // whatever/foo.h
   namespace whatever
   {
      class foo
      {
      ...
      };

      unspecified get_begin(foo const &) { ... };
      ...
   }

   namespace boost { namespace range {
      inline unspecified begin(foo const & foo_i)
      {
         return get_begin(foo_i);
      }
   } }

This approach provides a number of benefits:

- we can use the most obvious name, boost::range::begin, for the
  point of customization
- benefits from ADL when supported
- Types in namespace std or built-in types (pointers) can be
  retrofitted in boost::range and will work like everything else
  (though you must include the appropriate header to get the
  required boost::range::begin() overload)
- flexibility for those wishing to support Boost.Range
- non-conformance code can be excised easily later

Did I miss something?

-- 
Rob Stewart                           stewart_at_[hidden]
Software Engineer                     http://www.sig.com
Susquehanna International Group, LLP  using std::disclaimer;