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!"
        );
}