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

NAME
       std::is_const - std::is_const

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

	  std::is_const	is a UnaryTypeTrait.

	  If  T	is a const-qualified type (that	is, const, or const volatile),
       provides	the
	  member constant value	equal to true. For any other  type,  value  is
       false.

	  If   the   program   adds   specializations	for  std::is_const  or
       std::is_const_v,	the
	  behavior is undefined.

Template parameters
	  T - a	type to	check

	  Helper variable template

	  template<  class  T	>					(since
       C++17)
	  inline constexpr bool	is_const_v = is_const<T>::value;

       Inherited from std::integral_constant

Member constants
	  value	   true	if T is	a const-qualified type,	false otherwise
	  [static] (public static member constant)

Member functions
	  operator bool	converts the object to bool, returns value
			(public	member function)
	  operator()	returns	value
	  (C++14)	(public	member function)

Member types
	  Type	     Definition
	  value_type bool
	  type	     std::integral_constant<bool, value>

Notes
	  If  T	 is  a reference type then is_const<T>::value is always	false.
       The proper way to
	  check	a potentially-reference	type for const-ness is to  remove  the
       reference:
	  is_const<typename remove_reference<T>::type>.

Possible implementation
	  template<class T> struct is_const	     : std::false_type {};
	  template<class T> struct is_const<const T> : std::true_type {};

Example
       // Run this code

	#include <type_traits>

	static_assert(std::is_same_v<const int*, int const*>,
	    "Remember, const-ness binds	tightly	inside pointers.");
	static_assert(!std::is_const_v<int>);
	static_assert(std::is_const_v<const int>);
	static_assert(!std::is_const_v<int*>);
	static_assert(std::is_const_v<int* const>,
	    "Because  the  pointer itself can't	be changed but the int pointed
       at can.");
	static_assert(!std::is_const_v<const int*>,
	    "Because the pointer itself	can be changed but not the int pointed
       at.");
	static_assert(!std::is_const_v<const int&>);
	static_assert(std::is_const_v<std::remove_reference_t<const int&>>);

	struct S
	{
	    void foo() const {}
	    void bar() const {}
	};

	int main()
	{
	    // A const member function is const	in a different way:

	    static_assert(!std::is_const_v<decltype(&S::foo)>,
		"Because &S::foo is a pointer.");

	    using S_mem_fun_ptr	= void(S::*)() const;

	    S_mem_fun_ptr sfp =	&S::foo;
	    sfp	= &S::bar; // OK, can be re-pointed
	    static_assert(!std::is_const_v<decltype(sfp)>,
		"Because sfp is	the same pointer type  and  thus  can  be  re-
       pointed.");

	    const S_mem_fun_ptr	csfp = &S::foo;
	    // csfp = &S::bar; // Error
	    static_assert(std::is_const_v<decltype(csfp)>,
		"Because csfp cannot be	re-pointed.");
	}

See also
	  is_volatile checks if	a type is volatile-qualified
	  (C++11)     (class template)
	  as_const    obtains a	reference to const to its argument
	  (C++17)     (function	template)

http://cppreference.com		  2024.06.10		      std::is_const(3)

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