I am not sure if this approach can be generalized to an arbitrary number of sequences. Perhaps it can be, by making the whole thing a recursive template instead of 3 separate ones. I'd try it just to see which version results in simpler code (gut feeling: the recursive template) and in faster compilation times (gut feeling: my current implementation) but customers and management are jointly snapping at my heels...
Against my better instincts I got drawn into this exercise... and I am now officially stumped. Since we have this thread, perhaps one of the MPL gurus would chime in. Here's the [entire!] code that uses the accumulator idiom: =================================================================== template< typename S, typename A> struct product_view_ex : public if_< typename empty<S>::type, typename push_front< S, A >::type, transform_view< typename front<S>::type, product_view_ex< typename pop_front<S>::type, push_front<A,_1> > > >::type { typedef product_view_ex type; }; template< typename S> struct product_view : public if_< typename empty<S>::type, S, product_view_ex< S, typename clear<S>::type > >::type { typedef product_view type; }; =================================================================== The problem is that "if_< typename empty<S>::type ...." appears to always evaluate its "else" clause. Or, in other words [I think] empty<S>::type never evaluates as false_, even when the type of S is clearly an empty sequence (vector0<>) according to compiler messages. I am definitely missing something, but what? ...Max...