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Showing content from https://en.cppreference.com/w/cpp/language/../error/error_code/../../utility/tuple_size.html below:

std::tuple_size - cppreference.com

template< class T >
struct tuple_size; // not defined

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

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.

#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>);
}

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

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