std::tuple_size(3)

NAME
std::tuple_size - std::tuple_size

Synopsis
   Defined in header <utility>
   template< class T1, class T2 >
   struct tuple_size<std::pair<T1, T2>> (since C++11)
       : std::integral_constant<std::size_t, 2> { };
   The partial specialization of std::tuple_size for pairs provides a compile-time way
   to obtain the number of elements in a pair, which is always 2, using tuple-like
   syntax.

Member constants
   value the constant value 2
   [static] (public static member constant)

Member functions
   operator std::size_t converts the object to std::size_t, returns value
                        (public member function)
   operator() returns value
   (C++14) (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 <iostream>
 #include <tuple>
 #include <utility>
 template<class T>
 void test([[maybe_unused]]T t)
 {
     [[maybe_unused]]
     int a[std::tuple_size<T>::value]; // can be used at compile time
     std::cout << std::tuple_size<T>::value << '\n'; // or at run time
 }
 int main()
 {
     test(std::make_tuple(1, 2, 3.14));
     test(std::make_pair(1, 3.14));
 }

Output:
 3
 2
   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 2313 C++11 specializations for pair were not required to required
                       be derived from integral_constant

See also
   Structured binding (C++17) binds the specified names to sub-objects or
                        tuple elements of the initializer
   tuple_size obtains the number of elements of a
   (C++11) tuple-like type
                        (class template)
   std::tuple_size<std::array> obtains the size of an array
   (C++11) (class template specialization)
   std::tuple_size<std::tuple> obtains the size of tuple at compile time
   (C++11) (class template specialization)
   std::tuple_size<std::ranges::subrange> obtains the number of components of a
   (C++20) std::ranges::subrange
                        (class template specialization)
   std::tuple_element<std::pair> obtains the type of the elements of pair
   (C++11) (class template specialization)