std::tuple_element<std::tuple>

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

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

Member types

Member 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
// 哋它亢++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 <cstddef>
#include <iostream>
#include <string>
#include <tuple>
#include <typeinfo>
#include <type_traits>
#include <utility>
 
namespace details
{
    template<class TupleLike, std::size_t I>
    void printTypeAtIndex()
    {
        std::cout << "  The type at index " << I << " is: ";
        using SelectedType = std::tuple_element_t<I, TupleLike>;
        std::cout << typeid(SelectedType).name() << '\n';
    }
}
 
template<typename TupleLike, std::size_t I = 0>
void printTypes()
{
    if constexpr (I == 0)
        std::cout << typeid(TupleLike).name() << '\n';
 
    if constexpr (I < std::tuple_size_v<TupleLike>)
    {
        details::printTypeAtIndex<TupleLike, I>();
        printTypes<TupleLike, I + 1>();
    }
}
 
struct MyStruct {};
 
using MyTuple = std::tuple<int, long, char, bool, std::string, MyStruct>;
 
using MyPair = std::pair<char, bool>;
 
static_assert
(
    std::is_same_v<std::tuple_element_t<0, MyPair>, char> &&
    std::is_same_v<std::tuple_element_t<1, MyPair>, bool>
);
 
int main()
{
    printTypes<MyTuple>();
    printTypes<MyPair>();
}

Possible output: 

St5tupleIJilcbNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE8MyStructEE
  The type at index 0 is: i
  The type at index 1 is: l
  The type at index 2 is: c
  The type at index 3 is: b
  The type at index 4 is: NSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE
  The type at index 5 is: 8MyStruct
St4pairIcbE
  The type at index 0 is: c
  The type at index 1 is: b

See also

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