std:: tuple_element <std::tuple>

From cppreference.com
C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
std::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
(C++11)
obtains the element types of a tuple-like type
(class template)
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