on Sat Jul 19 2008, Steven Watanabe <watanabesj-AT-gmail.com> wrote:
AMDG
David Abrahams wrote:
How?
In pseudocode using iterators:
deref(advance(begin(tuple)), n)
where advance is implemented as
iterator advance(iterator x, int n) { if(n == 0) { return(x); } else { return(advance(x, n - 1)); } }
Wow, looking at the tuples code it seems there's a lot of optimization we can do now! // -- proof of concept -- // doesn't even need partial specialization (in case we care) template <unsigned N> struct tuple_drop_front { template <class Tuple> // compute resultant tuple struct result_ { typedef typename tuple_drop_front<N-1>::template result_<Tuple>::type::tail_type type; }; template <class Tuple> typename result_<Tuple>::type const& operator()(Tuple const& x) const { return tuple_drop_front<N-1>()(x).tail; } template <class Tuple> typename result_<Tuple>::type& operator()(Tuple& x) const { return tuple_drop_front<N-1>()(x).tail; } }; template <> struct tuple_drop_front<0> { template <class Tuple> struct result_ { typedef Tuple type; }; template <class Tuple> Tuple const& operator()(Tuple const& x) const { return x; } template <class Tuple> Tuple& operator()(Tuple& x) const { return x; } }; template <unsigned N, class Tuple> typename tuple_drop_front<N>::template result_<Tuple>::type::head_type& get(Tuple& t) { return tuple_drop_front<N>()(t).head; } template <unsigned N, class Tuple> typename tuple_drop_front<N>::template result_<Tuple>::type::head_type const& get(Tuple const& t) { return tuple_drop_front<N>()(t).head; } struct null_type {}; template <class HT, class TT = null_type> struct cons { cons() : head(), tail() {} template <class T, class U> cons(T const& x, U const& y) : head(x), tail(y) {} template <class T, class U> cons(T& x, U& y) : head(x), tail(y) {} template <class T, class U> cons(T const& x, U& y) : head(x), tail(y) {} template <class T, class U> cons(T& x, U const& y) : head(x), tail(y) {} template <class T> cons(T const& x) : head(x), tail() {} typedef HT head_type; HT head; typedef TT tail_type; TT tail; }; int main() { assert( get<0>( cons<int>(42) ) == 42 ); assert( get<0>( cons<int, cons<long> >(42, 10) ) == 42 ); assert( get<1>( cons<int, cons<long> >(42, 10) ) == 10 ); assert( get<0>( cons<int, cons<long, cons<char> > >(42, 3) ) == 42 ); assert( get<1>( cons<int, cons<long, cons<char> > >(42, 3) ) == 3 ); assert( get<2>( cons<int, cons<long, cons<char> > >(42, 3) ) == 0 ); } -- Dave Abrahams BoostPro Computing http://www.boostpro.com