# std::tuple_element<std::tuple>

From cppreference.com
< cpp‎ | utility‎ | tuple

Defined in header `<tuple>` |  |
template< std::size_t I, class... Types > struct tuple_element< I, std::tuple <Types...> >; |  | (since C++11)

Provides compile-time indexed access to the types of the elements of the tuple.

### Member types

Type | Definition
type | the type of `I`th element of the tuple, where `I` is in `[`​0​`,` sizeof...(Types)`)`

### Possible implementation

    template<std::size_t I, class T>
    struct tuple_element;

    #ifndef __cpp_pack_indexing
    // recursive case
    template<std::size_t I, class Head, class... Tail>
    struct tuple_element<I, std::tuple<Head, Tail...>>
        : std::tuple_element<I - 1, std::tuple<Tail...>>
    { };

    // base case
    template<class Head, class... Tail>
    struct tuple_element<0, std::tuple<Head, Tail...>>
    {
        using type = Head;
    };

    #else
    // C++26 implementation using pack indexing
    template<std::size_t I, class... Ts>
    struct tuple_element<I, std::tuple<Ts...>>
    {
        using type = Ts...[I];
    };
    #endif

### Example

Run this code

    #include <boost/type_index.hpp>
    #include <cstddef>
    #include <iostream>
    #include <string>
    #include <tuple>
    #include <utility>

    template<typename TupleLike, std::size_t I = 0>
    void printTypes()
    {
        if constexpr (I == 0)
            std::cout << boost::typeindex::type_id_with_cvr<TupleLike>() << '\n';

        if constexpr (I < std::tuple_size_v<TupleLike>)
        {
            using SelectedType = std::tuple_element_t<I, TupleLike>;

            std::cout << "  The type at index " << I << " is: "
                      << boost::typeindex::type_id_with_cvr<SelectedType>() << '\n';
            printTypes<TupleLike, I + 1>();
        }
    }

    struct MyStruct {};

    using MyTuple = std::tuple<int, long&, const char&, bool&&,
                               std::string, volatile MyStruct>;

    using MyPair = std::pair<char, bool&&>;

    static_assert(std::is_same_v<std::tuple_element_t<0, MyPair>, char>);
    static_assert(std::is_same_v<std::tuple_element_t<1, MyPair>, bool&&>);

    int main()
    {
        printTypes<MyTuple>();
        printTypes<MyPair>();
    }

Possible output:

    std::tuple<int, long&, char const&, bool&&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, MyStruct volatile>
      The type at index 0 is: int
      The type at index 1 is: long&
      The type at index 2 is: char const&
      The type at index 3 is: bool&&
      The type at index 4 is: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >
      The type at index 5 is: MyStruct volatile
    std::pair<char, bool&&>
      The type at index 0 is: char
      The type at index 1 is: bool&&

### See also

Structured binding (C++17) | binds the specified names to sub-objects or tuple elements of the initializer
tuple_element (C++11) | obtains the element types of a tuple-like type (class template)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/utility/tuple/tuple_element&oldid=176749"