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

NAME
     std::empty - std::empty

Synopsis
	Defined in header <array>
	Defined in header <deque>
	Defined in header <flat_map>
	Defined in header <flat_set>
	Defined in header <forward_list>
	Defined in header <iterator>
	Defined in header <list>
	Defined in header <map>
	Defined in header <regex>
	Defined in header <set>
	Defined in header <span>
	Defined in header <string>
	Defined in header <string_view>
	Defined in header <unordered_map>
	Defined in header <unordered_set>
	Defined in header <vector>
	template< class C >					     (since
	constexpr auto empty( const C& c ) ->			     C++17)
	decltype(c.empty());					     (until
								     C++20)
	template< class C >					     (since
	[[nodiscard]] constexpr auto empty( const C& c ) ->	     C++20)
	decltype(c.empty());
	template<	  class 	T,	   std::size_t	      N        >
     (since
	constexpr     bool	empty(	    const      T      (&array)[N]      )
     C++17)
	noexcept;
     (until
							     (1)
     C++20)
	template<	  class 	T,	   std::size_t	      N        >
     (since
	[[nodiscard]]	   constexpr	   bool       empty(	   const       T
     C++20)
	(&array)[N] ) noexcept; 				 (2)
	template<		    class		    E		       >
     (since
	constexpr     bool     empty(	  std::initializer_list<E>     il      )
     C++17)
	noexcept;
     (until
								     (3)
     C++20)
	template<		    class		    E		       >
     (since
	[[nodiscard]]		  constexpr		bool		  empty(
     C++20)
	std::initializer_list<E> il ) noexcept;

	Returns whether the given range is empty.

	1) Returns c.empty().
	2) Returns false.
	3) Returns il.size() == 0.

Parameters
	c     - a container or view with an empty member function
	array - an array of arbitrary type
	il    - an std::initializer_list

Return value
	1) c.empty()
	2) false
	3) il.size() == 0

Exceptions
	1) May throw implementation-defined exceptions.

Notes
	The  overload for std::initializer_list is necessary because it does not
     have a
	member function empty.

		 Feature-test macro	      Value    Std		 Feature
	__cpp_lib_nonmember_container_access   201411L	 (C++17)    std::size(),
     std::data(), and
							     std::empty()

Possible implementation
First version
	template<class C>
	[[nodiscard]] constexpr auto empty(const C& c) -> decltype(c.empty())
	{
	    return c.empty();
	}

Second version
	template<class T, std::size_t N>
	[[nodiscard]] constexpr bool empty(const T (&array)[N]) noexcept
	{
	    return false;
	}
				     Third version
	template<class E>
	[[nodiscard]] constexpr bool empty(std::initializer_list<E> il) noexcept
	{
	    return il.size() == 0;
	}

Example
     // Run this code

      #include <iostream>
      #include <vector>

      template<class T>
      void print(const T& container)
      {
	  if (std::empty(container))
	      std::cout << "Empty\n";
	  else
	  {
	      std::cout << "Elements:";
	      for (const auto& element : container)
		  std::cout << ' ' << element;
	      std::cout << '\n';
	  }
      }

      int main()
      {
	  std::vector<int> c = {1, 2, 3};
	  print(c);
	  c.clear();
	  print(c);

	  int array[] = {4, 5, 6};
	  print(array);

	  auto il = {7, 8, 9};
	  print(il);
      }

Output:
      Elements: 1 2 3
      Empty
      Elements: 4 5 6
      Elements: 7 8 9

See also
	ranges::empty checks whether a range is empty
	(C++20)       (customization point object)

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