Removing the first type of a std::tuple

Question

This seems to be a very simple question: How does one remove the first (the n-th) type in a std::tuple?

Example:

typedef std::tuple<int, short, double> tuple1; typedef std::tuple<short, double> tuple2; 

The operation described above would transform tuple1 into tuple2. Is it possible?

Answer 1

You can use a simple type function based on partial specialization of a class template:

#include <type_traits> #include <tuple>  using namespace std;  template<typename T> struct remove_first_type { };  template<typename T, typename... Ts> struct remove_first_type<tuple<T, Ts...>> {     typedef tuple<Ts...> type; };  int main() {     typedef tuple<int, bool, double> my_tuple;     typedef remove_first_type<my_tuple>::type my_tuple_wo_first_type;      static_assert(         is_same<my_tuple_wo_first_type, tuple<bool, double>>::value,          "Error!"         ); } 

Also, this solution can be easily generalized to remove the i-th type of a tuple:

#include <type_traits> #include <tuple>  using namespace std;  template<size_t I, typename T> struct remove_ith_type { };  template<typename T, typename... Ts> struct remove_ith_type<0, tuple<T, Ts...>> {     typedef tuple<Ts...> type; };  template<size_t I, typename T, typename... Ts> struct remove_ith_type<I, tuple<T, Ts...>> {     typedef decltype(         tuple_cat(             declval<tuple<T>>(),             declval<typename remove_ith_type<I - 1, tuple<Ts...>>::type>()             )         ) type; };  int main() {     typedef tuple<int, bool, double> my_tuple;     typedef remove_ith_type<1, my_tuple>::type my_tuple_wo_2nd_type;      static_assert(         is_same<my_tuple_wo_2nd_type, tuple<int, double>>::value,          "Error!"         ); }