For the purpose of forward declaration of class templates this read-only lightweight sequence might be useful. For example, one might declare class template Selector in fwd file: // empty_sequence.hpp is designed for inclusion into fwd files #include <boost/mpl/empty_sequence.hpp> template<class Variants = empty_sequence> class Selector; Another way to archieve this could be inclusion of <boost/mpl/vector_fwd.hpp> (why there is no such file in mpl?) and using mpl::vector<> instead. Although, it could extend a set of class templates with those modifying template parameter but a user would find it strange to include <boost/mpl/vector.hpp> if he only uses Selector<>. The empty_sequence definition is as simple as: namespace boost { namespace mpl { namespace aux { struct empty_sequence_iter { typedef forward_iterator_tag category; }; } struct empty_sequence { typedef aux::empty_sequence_iter begin; typedef aux::empty_sequence_iter end; }; }} This is not yet complete. I hope that support for some transformations will be added later (transformation doesn't change empty_sequence unless the size is changed) -- Alexander Nasonov Independent Developer and Consultant