std::tuple_element<std::tuple>

 

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std::tuple

Member functions

tuple::tuple

tuple::operator=

tuple::swap

Non-member functions

make_tuple

tie

forward_as_tuple

tuple_cat

operator==operator!=operator<operator<=operator>operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)

swap(std::tuple)

get(std::tuple)

Helper concepts

tuple-like (C++23)

Helper classes

tuple_size<std::tuple>

tuple_element<std::tuple>

uses_allocator<std::tuple>

basic_common_reference<std::tuple> (C++23)

common_type<std::tuple> (C++23)

formatter<std::tuple> (C++23)

ignore

Deduction guides(C++17)

 

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

See More

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

Member types

Type	Definition
type	the type of Ith 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)