std::tuple_size - cppreference.com (original) (raw)

Defined in header
Defined in header
Defined in header
Defined in header (since C++20)
Defined in header (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   struct X { int a, b; }; const auto [x, y] = X(); // structured binding declaration first attempts // tuple_size which attempts to use tuple_size::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:

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

| | | | | --------------------------------------------------------------------------------------------------------------- | | ------------- | | template< class T > constexpr std::size_t tuple_size_v = tuple_size<T>::value; | | (since C++17) |

Inherited from std::integral_constant

Member constants

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

Member functions

| | converts the object to std::size_t, returns value (public member function) | | -------------------------------------------------------------------------------------------------------- | | | returns value (public member function) |

Member types

[edit] Example

#include #include #include #include #include   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); // uses using template (C++17)   using array3x4 = std::array<std::array<int, 3>, 4>; static_assert(4 == std::tuple_size{}); // uses operator std::size_t   using pair = std::pair<tuple1, array3x4>; static_assert(2 == std::tuple_size()); // uses operator()   using sub = std::ranges::subrange<char*, char*>; static_assert(2 == std::tuple_size::value);   using Arr5 = Arr<int, 5>; static_assert(5 == std::tuple_size_v); }

[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]
creates a tuple by concatenating any number of tuples (function template) [edit]