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std::tuple_size

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< cpp‎ | utility
 
 
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Common vocabulary types
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(C++17)
(C++17)
tuple_size
(C++11)
(C++17)
(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< 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)

Contents

[edit] Specializations

The standard library provides following specializations for standard library types:

obtains the size of

a tuple
(class template specialization) [edit]

obtains the size of a pair
(class template specialization) [edit]
obtains the size of an array
(class template specialization) [edit]
obtains the size of a std::ranges::subrange
(class template specialization) [edit]
obtains the size of a std::complex
(class template specialization) [edit]

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.

[edit] Helper variable template

Defined in header <tuple>
template< class T >
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>

[edit] 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>);
}

[edit] 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

[edit] See also

Structured binding (C++17) binds the specified names to sub-objects or tuple elements of the initializer[edit]
obtains the element types of a tuple-like type
(class template) [edit]
(C++11)
creates a tuple by concatenating any number of tuples
(function template) [edit]

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