std::tuple_size

< cpp‎ | utility

Defined in header `<tuple>`
Defined in header `<array>`
Defined in header `<utility>`
Defined in header `<ranges>`		(since C++20)
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< const volatile 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; };
const auto [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 `tuple` at compile time (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 number of components of a std::ranges::subrange (class template specialization)

All specializations of std::tuple_size satisfy UnaryTypeTrait with base characteristic std::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 > inline constexpr std::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

#include <array>
#include <cstddef>
#include <ranges>
#include <tuple>
#include <utility>
 
template<class T, std::size_t Size> struct Arr { T data[Size]; };
// a 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

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)