std::tuple_element<std::tuple>

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros(C++20)

Language support library

Concepts library(C++20)

Diagnostics library

Memory management library

Metaprogramming library(C++11)

General utilities library

Containers library

Iterators library

Ranges library(C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library(C++17)

Concurrency support library(C++11)

Execution control library(C++26)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros(C++20) Program utilities Variadic functions initializer_list (C++11) is_constant_evaluated (C++20) is_within_lifetime (C++26) source_location (C++20) Coroutine support(C++20) Contract support(C++26) 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 Function objects Bit manipulation(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)

 

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 casetemplate<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 casetemplate<class Head, class... Tail>struct tuple_element<0, std::tuple<Head, Tail...>>{using type = Head;};   #else// C++26 implementation using pack indexingtemplate<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(){ifconstexpr(I ==0)std::cout<< boost::typeindex::type_id_with_cvr<TupleLike>()<<'\n';   ifconstexpr(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&, constchar&, 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)