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.