Re: [boost] [mpl] as_sequence< X< Y > > instantiates typedefs in X< Y >

From: Brian Braatz (brianb_at_[hidden])
Date: 2005-08-01 23:21:16

Suggestion:

is there some way in your design you can get away with deriving from the
incomplete type to "de-incomplete" it?

> -----Original Message-----
> From: boost-bounces_at_[hidden]
[mailto:boost-bounces_at_[hidden]]
> On Behalf Of Andreas Huber
> Sent: Monday, August 01, 2005 3:33 AM
> To: boost_at_[hidden]
> Subject: [boost] [mpl] as_sequence< X< Y > > instantiates typedefs in
X< Y
> >
> 
> Hi there
> 
> I'm trying to address a point raised by Jonathan Turkanis during the
> formal review of the statechart library. Basically, the problem boils
> down to the following:
> 
> #include <boost/mpl/list.hpp>
> #include <boost/mpl/size.hpp>
> #include <boost/mpl/as_sequence.hpp>
> 
> namespace mpl = boost::mpl;
> 
> // InnerInitial is either an mpl sequence of types or a single type
> template< class InnerInitial >
> struct state
> {
>   // stateArray_ must have a length equal to the number of types
>   // in InnerInitial
>   int stateArray_[
>     mpl::size<
>       typename mpl::as_sequence< InnerInitial >::type
>     >::type::value
>   ];
> };
> 
> // A is defined later, possibly in a different translation unit
> struct A;
> 
> template< class X >
> struct B
> {
>   typedef typename X::reactions reactions;
> };
> 
> int main()
> {
>   // fine: instantiation with an incomplete type
>   state< A > a;
>   // fine: instantiation with an mpl sequence containing B< A >
>   state< mpl::list< B< A > > > b;
>   // error: mpl::as_sequence triggers the instantiation of B's member
>   // typedef, which tries to access the member typedef of A. A
compile-
>   // time error results because A is still incomplete at this point.
>   state< B< A > > c; // *** here ***
> 
>   return 0;
> }
> 
> MSVC7.1 and GCC3.4 agree that the last state template instantiation is
> illegal (use of undefined type A). I'm wondering whether there is any
> way to make this legal (different as_sequence implementation,
implement
> the state template differently, etc.)?
> 
> Thanks & Regards,
> 
> --
> Andreas Huber
> 
> When replying by private email, please remove the words spam and trap
> from the address shown in the header.
> 
> 
> _______________________________________________
> Unsubscribe & other changes:
> http://lists.boost.org/mailman/listinfo.cgi/boost