[boost] Re: Re: Re: [typeof][result_of] Separated at Birth?

From: Arkadiy Vertleyb (vertleyb_at_[hidden])
Date: 2005-06-10 09:40:40

"Peder Holt" <peder.holt_at_[hidden]> wrote

> On 6/9/05, Arkadiy Vertleyb <vertleyb_at_[hidden]> wrote:
> > "jarvi" <jarvi_at_[hidden]> wrote
> >
> > > The main point was that the result_of template needs to
> > > detect whether the BOOST_TYPEOF will succeed (all relevant
> > > types/templates have been typeof-registered),
> > > and if it will not succeed,  then follow the  list of steps specified
> > > in the TR1.
> >
> > Well, I am afraid this might not be possible with current
> > BOOST_TYPEOF --Types/templates not registerd just result in compile
error
> > :-(
> >
> > Come to think about it, this doesn't have to stay this way.  I havn't
tried
> > it, but it seems possible to just put some well-known type in place of
> > not-registered one, for example, something like this:
> >
> > "std::pair<std::vector<boost::type_of::not_registered>, int>"
> >
> > or maybe just
> >
> > "boost::type_of::not_registered"
> >
> > This way the fact that types are not registered can be used in
> > meta-programs.  The drawback -- it's unclear what's not registered.  So
> > maybe we need some sort of combination of both.
> >
> > Again, *I think* it's possible, but I am not in a position to verify
this
> > until later in the evening...  Is it something that would be of
interest?
>
> I did this once for the vintage implementation. Here is the technique
> I used: (Except I used function overloads...)
>
> namespace boost {namespace type_of{ namespace {
>
> struct not_registered_vector {
>    typedef mpl::int_<1> item ## n; //For n=0 to BOOST_TYPEOF_LIMIT_SIZE
> };
>
> template<typename T>
> struct push_back<not_registered_vector,T>
> {
>     typedef not_registered_vector type;
> };
>
> //Default implementation of encode_type_impl
> template<typename V,typename T>
> struct encode_type_impl {
>   typedef uninitialized_type_vector type;
> };
>
> struct not_registered {};
> template<class Iter> struct decode_type_impl<mpl::size_t<1>, Iter>
> {
>     typedef not_registered type;
>     typedef Iter iter;
> };
>
> }}
>
> If we place this code in a separate header file, we can trigger
> compile errors on demand.

Actually what I meant is even simpler, something like:

given desired results:
1) "std::pair<std::vector<boost::type_of::not_registered>, int>"
2) "boost::type_of::not_registered"

namespace boost { namespace type_of {
    srtuct not_registered;
}}

BOOST_TYPEOF_REGISTER_TYPE(not_registered)

namespace boost { namespace type_of {
    template<class V, class T> struct encode_type_impl // default impl
    {
        typedef typename encode_type_impl<V, not_registered>::type type;
    };
}}

should achieve the result #1.  To achieve the result #2 there seem to be a
few alternatives.

But again, I would first like to know if this would be useful.

Regards,
Arkadiy