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std::integral_constant(3)     C++ Standard Libary    std::integral_constant(3)

NAME
       std::integral_constant -	std::integral_constant

Synopsis
	  Defined in header <type_traits>
	  template< class T, T v >	   (since C++11)
	  struct integral_constant;

	  std::integral_constant wraps a static	constant of specified type. It
       is the base
	  class	for the	C++ type traits.

	  The  behavior	 of  a	program	 that  adds  specializations for inte-
       gral_constant is
	  undefined.

	 Helper	alias templates

	  A helper alias template std::bool_constant is	defined	for the	common
       case where T is
	  bool.

	  template <bool B>				     (since C++17)
	  using	bool_constant =	integral_constant<bool,	B>;

	  Feature-test macro: __cpp_lib_bool_constant

Specializations
	  Two typedefs for the common case where T is bool are provided:

	  Defined in header <type_traits>
	  Type	     Definition
	  true_type  std::integral_constant<bool, true>
	  false_type std::integral_constant<bool, false>

Member types
	  Type	     Definition
	  value_type T
	  type	     std::integral_constant<T,v>

Member constants
	  Name		    Value
	  constexpr T value static constant of type T with value v
	  [static]	    (public static member constant)

Member functions
	  operator value_type returns the wrapped value
			      (public member function)
	  operator()	      returns the wrapped value
	  (C++14)	      (public member function)

       std::integral_constant::operator	value_type

	  constexpr operator value_type() const	noexcept;

	  Conversion function. Returns the wrapped value.

       std::integral_constant::operator()

	  constexpr value_type operator()() const noexcept;  (since C++14)

	  Returns the wrapped value. This function enables  std::integral_con-
       stant to	serve as
	  a source of compile-time function objects.

	  Feature-test macro: __cpp_lib_integral_constant_callable

Possible implementation
	  template<class T, T v>
	  struct integral_constant {
	      static constexpr T value = v;
	      using value_type = T;
	      using type = integral_constant; // using injected-class-name
	      constexpr	operator value_type() const noexcept { return value; }
	      constexpr	value_type operator()()	const noexcept { return	value;
       } // since c++14
	  };

Example
       // Run this code

	#include <type_traits>

	int main()
	{
	    typedef std::integral_constant<int,	2> two_t;
	    typedef std::integral_constant<int,	4> four_t;

	//  static_assert(std::is_same<two_t, four_t>::value,
	//		  "two_t and four_t are	not equal!");
	//  error: static assertion failed: "two_t and four_t are not equal!"

	    static_assert(two_t::value*2 == four_t::value,
	       "2*2 != 4"
	    );

	    enum class my_e { e1, e2 };

	    typedef std::integral_constant<my_e, my_e::e1> my_e_e1;
	    typedef std::integral_constant<my_e, my_e::e2> my_e_e2;

	    static_assert(my_e_e1() == my_e::e1);

	//  static_assert(my_e_e1::value == my_e::e2,
	//		 "my_e_e1::value != my_e::e2");
	//  error: static assertion failed: "my_e_e1::value != my_e::e2"

	    static_assert(std::is_same<my_e_e2,	my_e_e2>::value,
			  "my_e_e2 != my_e_e2");
	}

http://cppreference.com		  2022.07.31	     std::integral_constant(3)

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