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

NAME
     std::is_scalar - std::is_scalar

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

	std::is_scalar is a UnaryTypeTrait.

	If  T  is  a scalar type, provides the member constant value equal true.
     For any other
	type, value is false.

	If   the   program   adds   specializations   for   std::is_scalar    or
     std::is_scalar_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_scalar_v = is_scalar<T>::value;

     Inherited from std::integral_constant

Member constants
	value	 true if T is a scalar 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
	Each  individual  memory location in the C++ memory model, including the
     hidden memory
	locations used by language features (e.g. virtual  table  pointer),  has
     scalar type
	(or is a sequence of adjacent bit-fields of non-zero length). Sequencing
     of
	side-effects in expression evaluation, inter-thread synchronization, and
     dependency
	ordering are all defined in terms of individual scalar objects.

Possible implementation
	template<class T>
	struct	  is_scalar    :   std::integral_constant<bool,   std::is_arith-
     metic<T>::value
						     || std::is_enum<T>::value
						     ||
     std::is_pointer<T>::value
						     || 	    std::is_mem-
     ber_pointer<T>::value
						     ||
     std::is_null_pointer<T>::value>
	{};

Example
     // Run this code

      #include <iostream>
      #include <type_traits>
      #include <typeinfo>
      #include <utility>

      template<typename Head, typename... Tail>
      void are_scalars(Head&& head, Tail&&... tail)
      {
	  using T = std::decay_t<decltype(head)>;

	  std::cout << typeid(T).name() << " is "
		    << (std::is_scalar_v<T> ? "" : "not ")
		    << "a scalar\n";

	  if constexpr (sizeof... (Tail))
	  {
	      are_scalars(std::forward<decltype(tail)>(tail)...);
	  }
      }

      int main()
      {
	  struct S { int m; } s;
	  int S::* mp = &S::m;
	  enum class E { e };

	  are_scalars(42, 3.14, E::e, "str", mp, nullptr, s);
      }

Possible output:
      int is a scalar
      double is a scalar
      main::E is a scalar
      char const* is a scalar
      int main::S::* is a scalar
      nullptr is a scalar
      main::S is not a scalar

See also
	is_arithmetic	  checks if a type is an arithmetic type
	(C++11) 	  (class template)
	is_enum 	  checks if a type is an enumeration type
	(C++11) 	  (class template)
	is_pointer	  checks if a type is a pointer type
	(C++11) 	  (class template)
	is_member_pointer  checks  if a type is a pointer to a non-static member
     function or
	(C++11) 	  object
			  (class template)

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