Hi! The following piece of code works (finally, sigh), but I would like to hear some comments on whether and how one can do better. #include "boost/mpl/apply_if.hpp" #include "boost/mpl/identity.hpp" #include "boost/mpl/lower_bound.hpp" #include "boost/mpl/deref.hpp" #include "boost/mpl/find.hpp" #include "boost/mpl/vector.hpp" #include "boost/mpl/pair.hpp" #include "boost/mpl/bool.hpp" #include "boost/mpl/select1st.hpp" #include "boost/mpl/select2nd.hpp" #include "boost/mpl/lambda.hpp" #include "boost/type_traits/is_same.hpp" #include <typeinfo> #include <iostream> namespace mpl = boost::mpl; template class ComplicatedDecision { private: template <int i> struct C { // typedef probably_very_expensive_template type; }; // order matters typedef mpl::vector < mpl::pair, C<0> >, mpl::pair, C<1> >, mpl::pair, C<2> >, mpl::pair, C<3> >, mpl::pair, C<4> >

decision_sequence;

typedef typename mpl::find_if< decision_sequence, boost::is_samempl::_1, mpl::bool_<true> > > iter_; typedef typename iter_::type::type pair_; public: typedef typename mpl::select2nd::type type; }; int main() { std::cerr << typeid(ComplicatedDecision<1,1>::type).name() << std::endl; std::cerr << typeid(ComplicatedDecision<-1,1>::type).name() << std::endl; std::cerr << typeid(ComplicatedDecision<1,2>::type).name() << std::endl; std::cerr << typeid(ComplicatedDecision<-1,2>::type).name() << std::endl; std::cerr << typeid(ComplicatedDecision<2,3>::type).name() << std::endl; } best regards, Markus -- Compile time analytic differentiation? Yes, at http://daixtrose.sourceforge.net/