Re: [boost] [parameter] named template parameters

Subject: Re: [boost] [parameter] named template parameters
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2011-11-03 14:32:00

On Thu, Nov 3, 2011 at 11:05 AM, Lorenzo Caminiti <lorcaminiti_at_[hidden]>wrote:

> On Wed, Nov 2, 2011 at 6:46 PM, Jeffrey Lee Hellrung, Jr.
> <jeffrey.hellrung_at_[hidden]> wrote:
> > On Wed, Nov 2, 2011 at 2:00 PM, Lorenzo Caminiti <lorcaminiti_at_[hidden]
> >wrote:
> >
> >> Hello all (and especially Boost.Parameter's authors),
> >>
> >> The Boost.Parameter docs suggest to implement parameter-enabled class
> >> templates using a series of typedes for the argument pack and the
> >> actual argument types. I think it is best to define extra template
> >> parameters with default values to old the argument pack Args and named
> >> template parameters ClassType, etc as shown below. This allows to use
> >> the named template parameters within the class declaration for example
> >> in a base class (boost::python::class_<ClassType, ...>) instead that
> >> just in the class definition.
> >>
> >> Do you see any problem in using extra template parameter to define the
> >> argument pack and named template parameters?
> >>
> >
> > I *think* I know what you're asking, and I *think* it's fine, but I
> > basically didn't even try parsing the code below due to formatting :/
>
> OK, let me try again (I tested the formatting of the code below
> sending an email to myself and it showed up fine):
>
[...]

> template<
>      typename Arg123X0
>    , typename Arg123X1 = boost::parameter::void_
>    , typename Arg123X2 = boost::parameter::void_
>    , typename Arg123X3 = boost::parameter::void_
>    , typename Args123 = typename boost::parameter::parameters<
>        // IF(IS_EMPTY(DEFAULT(p)), required, optional)
>          boost::parameter::required<
>              tag::ClassType
>            , boost::is_class<boost::mpl::_>
>          >
>        , boost::parameter::optional<
>            // IF(IS_DEDUCED(p), deduced<tag::NAME(p)>, tag::NAME(p))
>              boost::parameter::deduced<tag::BaseList>
>            // COMMA_IF(COMPL(IS_EMPTY(REQUIRES(p)))) REQUIRES(p)
>            , boost::is_base_and_derived<detail::bases_tag, boost::mpl::_>
>          >
>        , boost::parameter::optional<
>              boost::parameter::deduced<tag::HeldType>
>            , boost::mpl::not_<
>                boost::mpl::or_<
>                      boost::is_base_and_derived<detail::bases_tag,
>                            boost::mpl::_>
>                    , boost::is_same<boost::noncopyable, boost::mpl::_>
>                >
>              >
>          >
>        , boost::parameter::optional<
>              boost::parameter::deduced<tag::Copyable>
>            , boost::is_same<boost::noncopyable, boost::mpl::_>
>          >
>      >::bind<Arg123X0, Arg123X1, Arg123X2, Arg123X3>::type
>    , typename ClassType = typename boost::parameter::value_type<
>            Args123, tag::ClassType >::type
>    , typename BaseList = typename boost::parameter::value_type<
>            // ... COMMA_IF(COMPL(IS_EMPTY(DEFAULT(p)))) DEFAULT(p) >::type
>            Args123, tag::BaseList, bases<> >::type
>    , typename HeldType = typename boost::parameter::value_type<
>            Args123, tag::HeldType, ClassType >::type
>    , typename Copyable = typename boost::parameter::value_type<
>            Args123, tag::Copyable, void >::type
> >
> struct class_ : boost::python::class_<ClassType, BaseList, Heldtype,
> Copyable>
>
[...]

This is what I figured you had in mind, and I don't see a problem with it
technically.  This isn't something I'd encourage for typical use cases,
though, as I'd imagine it would make compiler error messages even more
unwieldy :)

- Jeff