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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 <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>				(since
								C++20)
	  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
       C++20)
	  empty(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 initializer list

Return value
	  true if the range doesn't have any element.

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: __cpp_lib_nonmember_container_access

Possible implementation
First version
	  template <class C>
	  [[nodiscard]]	  constexpr   auto   empty(const   C&	c)   ->	   de-
       cltype(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		  2022.07.31			 std::empty(3)

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