std::tuple_element<std::tuple>

 

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Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) is_within_lifetime (C++26) initializer_list (C++11) source_location (C++20) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20)   bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17) to_chars_result (C++17) from_chars_result (C++17)

 

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)

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 <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>;
        using RemovedRefType = std::remove_reference_t<SelectedType>;
        std::cout << typeid(std::remove_cvref_t<SelectedType>).name();
        if constexpr (std::is_const_v<RemovedRefType>)
           std::cout << " const"; 
        if constexpr (std::is_volatile_v<RemovedRefType>)
           std::cout << " volatile";
        if constexpr (std::is_lvalue_reference_v<SelectedType>)
           std::cout << "&";
        if constexpr (std::is_rvalue_reference_v<SelectedType>)
           std::cout << "&&";
        std::cout << '\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&, 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> &&
    std::is_same_v<std::tuple_element_t<1, MyPair>, bool&&>
);
 
int main()
{
    printTypes<MyTuple>();
    printTypes<MyPair>();
}

Possible output:

# GCC/Clang output:
St5tupleIJiRlRKcObNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEV8MyStructEE
  The type at index 0 is: i
  The type at index 1 is: l&
  The type at index 2 is: c const&
  The type at index 3 is: b&&
  The type at index 4 is: NSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE
  The type at index 5 is: 8MyStruct volatile
St4pairIcObE
  The type at index 0 is: c
  The type at index 1 is: b&&
 
 
# MSVC output:
class std::tuple<int,long & __ptr64,char const & __ptr64,bool && __ptr64,⮠
class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,⮠
struct 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: class std::basic_string<char,struct std::char_traits<char>,⮠
class std::allocator<char> >
  The type at index 5 is: struct MyStruct volatile
struct std::pair<char,bool && __ptr64>
  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)