Re: [boost] [Review] Named parameters library

From: David Abrahams (dave_at_[hidden])
Date: 2005-07-10 11:30:28

David Abrahams <dave_at_[hidden]> writes:

> David Abrahams <dave <at> boost-consulting.com> writes:
>
>> Rene Rivera <grafik.list <at> redshift-software.com> writes:
>> 
>> It's an ODR thing.  If you then use the parameter names in a function
>> template that's not in an unnamed namespace but _is_ in multiple TU's,
>> you have technically violated the ODR.
>> 
>> In general, types in header files had better stay out of the unnamed
>> namespace.
>
> In fact, it looks like to be really safe from ODR violations we need
> to require the keyword objects to be declared as _references_ in the
> unnamed namespace using the unique_instance trick, per 
> http://thread.gmane.org/gmane.comp.lib.boost.devel/126455
> because a function template using named parameters and instantiated
> in different translation units might otherwise use different objects.
> It's very tempting to ignore this problem, because IMO it will *never*
> cause a problem in practice, and because it complicates the usage and
> documentation of the parameter library.  We also need someplace to
> put unique_instance.  It's too general to go in boost::parameter and
> it's not suitable for boost::detail since users have to touch it.
>
> Any helpful ideas about all this would be appreciated, especially if
> you can find standard text that makes it all go away ;-)
>
> For more context, please see my Nov 16th reply to Rene in 
> http://thread.gmane.org/gmane.comp.lib.boost.devel/113158
> GMane's posting nanny won't let me quote very much.

So far, the best I can come up with is:

  namespace 
  {
    using namespace boost::parameters

    keyword<foo_> foo& = instance();
    ...
  }

But the conversion from instance() to keyword<foo_>& doesn't port to
broken compilers (vc6), which leaves me with:

  namespace 
  {
    using namespace boost::parameters

    keyword<foo_> foo& = keyword<foo_>::get();
    ...
  }

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com