take_whileは条件を満たす間、先頭部分を取り出すメタ関数です。
template <bool Cond, class Then, class Else> struct if_c; template <class Then, class Else> struct if_c<true, Then, Else> { typedef Then type; }; template <class Then, class Else> struct if_c<false, Then, Else> { typedef Else type; }; template <template <class T> class P, class Seq1, class Seq2> struct take_while_impl; template <template <class T> class P, class... Seq, class Head, class... Tail> struct take_while_impl<P, tuple<Seq...>, tuple<Head, Tail...>> { typedef typename if_c<P<Head>::value, typename take_while_impl<P, typename concat<tuple<Seq...>, tuple<Head>>::type, tuple<Tail...> >::type, tuple<Seq...> >::type type; }; template <template <class T> class P, class... Seq> struct take_while_impl<P, tuple<Seq...>, tuple<>> { typedef tuple<Seq...> type; }; template <template <class T> class P, class Seq> struct take_while; template <template <class T> class P, class... Seq> struct take_while<P, tuple<Seq...>> { typedef typename take_while_impl<P, tuple<>, tuple<Seq...>>::type type; };
take_while<is_integral, tuple<int, long, char, double>>::type → tuple<int, long, char>
