From: Rob Stewart (stewart_at_[hidden])
Date: 2005-09-20 16:29:51
From: David Abrahams <dave_at_[hidden]>
> Rob Stewart <stewart_at_[hidden]> writes:
> > 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);
> > }
> > }
> Careful. Many of the problems you can run into with
> dispatching onlyshow up in the case where the definition of
> begin you want to beselected *follows* the template that uses
> it.
Right. That was the case you'd been discussing and I missed its
application here. However, if the author of whatever::foo
defines, as a matter of convention or coercion,* begin() in
namespace boost::range in the same header as foo, and doesn't
invoke it before it's definition, then the problem order won't
occur.
> > Did I miss something?
> Two-phase name lookup.
> 14.6.4 Dependent name resolution [temp.dep.res]
LOL It seems you like quoting that now that you've found it! I
do know about two-phase lookup, but I don't have to deal with it
often so I don't always recognize the trouble it can cause a
solution like this.
Nevertheless, won't the problem only occur for non-conforming
compilers and only when the definition of
boost::range::begin(foot const &) is in a separate header
(provided the author/maintainer of foo follows the prescribed
course when it's in the same header)?
> Unless you arrange for boost::range to be an associated namespace offoo, you're setting up dangerous #include order dependencies.
Of course.
* It would be documented as the only correct thing to do were
this an accepted workaround for non-conforming compilers.
-- Rob Stewart stewart_at_[hidden] Software Engineer http://www.sig.com Susquehanna International Group, LLP using std::disclaimer;