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@lists.boost.org [mailto:boost-bounces@lists.boost.org] On Behalf Of Andreas Huber Sent: Monday, August 01, 2005 3:33 AM To: boost@lists.boost.org Subject: [boost] [mpl] as_sequence< X< Y > > instantiates typedefs in X< Y

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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

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