std::tuple_size

 

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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)

 

Defined in header <array>
Defined in header <tuple>
Defined in header <utility>
Defined in header <ranges>		(since C++20)
Defined in header <complex>		(since C++26)
template<class T > struct tuple_size;// not defined	(1)	(since C++11)
template<class T > struct tuple_size<const T >     :std::integral_constant<std::size_t, std::tuple_size<T>::value>{};	(2)	(since C++11)
template<class T > struct tuple_size<volatile T >     :std::integral_constant<std::size_t, std::tuple_size<T>::value>{};	(3)	(since C++11) (deprecated in C++20)
template<class T > struct tuple_size<constvolatile T >     :std::integral_constant<std::size_t, std::tuple_size<T>::value>{};	(4)	(since C++11) (deprecated in C++20)

Provides access to the number of elements in a tuple-like type as a compile-time constant expression.

1) The primary template is not defined. An explicit (full) or partial specialization is required to make a type tuple-like.

2-4) Specializations for a cv-qualified types reuse the value from the corresponding cv-unqualified versions by default.

std::tuple_size interacts with the core language: it can provide structured binding support in the tuple-like case. (2-4) are SFINAE-friendly: if std::tuple_size<T>::value is ill-formed when treated as an unevaluated operand, they do not provide the member value. Access checking is performed as if in a context unrelated to tuple_size and T. Only the validity of the immediate context of the expression is considered. This allows #include <utility>   struct X {int a, b;};constauto[x, y]= X();// structured binding declaration first attempts// tuple_size<const X> which attempts to use tuple_size<X>::value,// then soft error encountered, binds to public data members

(since C++17)

Specializations

The standard library provides following specializations for standard library types:

std::tuple_size<std::tuple> (C++11)	obtains the size of a tuple (class template specialization)
std::tuple_size<std::pair> (C++11)	obtains the size of a pair (class template specialization)
std::tuple_size<std::array> (C++11)	obtains the size of an array (class template specialization)
std::tuple_size<std::ranges::subrange> (C++20)	obtains the size of a std::ranges::subrange (class template specialization)
std::tuple_size<std::complex> (C++26)	obtains the size of a std::complex (class template specialization)

All specializations of std::tuple_size satisfy UnaryTypeTrait with base characteristicstd::integral_constant<std::size_t, N> for some N.

Users may specialize std::tuple_size for program-defined types to make them tuple-like. Program-defined specializations must meet the requirements above.

Usually only specialization for cv-unqualified types are needed to be customized.

Helper variable template

Defined in header <tuple>
template<class T > constexprstd::size_t tuple_size_v = tuple_size<T>::value;		(since C++17)

Inherited from std::integral_constant

Member constants

value [static]

for a standard specialization, the number of elements in the tuple-like type T (public static member constant)

Member functions

operator std::size_t	converts the object to std::size_t, returns value (public member function)
operator() (C++14)	returns value (public member function)

Member types

Type	Definition
value_type	std::size_t
type	std::integral_constant<std::size_t, value>

Example

Run this code

#include <array>#include <cstddef>#include <ranges>#include <tuple>#include <utility>   template<class T, std::size_t Size>struct Arr { T data[Size];};   // Program-defined specialization of std::tuple_size:template<class T, std::size_t Size>struct std::tuple_size<Arr<T, Size>>:public integral_constant<std::size_t, Size>{};   int main(){using tuple1 =std::tuple<int, char, double>; static_assert(3== std::tuple_size_v<tuple1>);// uses using template (C++17)   using array3x4 =std::array<std::array<int, 3>, 4>; static_assert(4== std::tuple_size<array3x4>{});// uses operator std::size_t   using pair =std::pair<tuple1, array3x4>; static_assert(2== std::tuple_size<pair>());// uses operator()   using sub = std::ranges::subrange<char*, char*>; static_assert(2== std::tuple_size<sub>::value);   using Arr5 = Arr<int, 5>; static_assert(5== std::tuple_size_v<Arr5>);}

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2212	C++11	specializations for cv types were not required in some headers, which led to ambiguity	required

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)
tuple_cat (C++11)	creates a tuple by concatenating any number of tuples (function template)