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std::ranges::equal(3)	       C++ Standard Libary	   std::ranges::equal(3)

NAME
     std::ranges::equal - std::ranges::equal

Synopsis
	Defined in header <algorithm>
	Call signature
	template< std::input_iterator I1, std::sentinel_for<I1> S1,

		  std::input_iterator I2, std::sentinel_for<I2> S2,
		  class Pred = ranges::equal_to,
		  class  Proj1	=  std::identity,  class Proj2 = std::identity >
     (1) (since
	requires  std::indirectly_comparable<I1,   I2,	 Pred,	 Proj1,   Proj2>
     C++20)
	constexpr bool
	    equal( I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {},

		   Proj1 proj1 = {}, Proj2 proj2 = {} );
	template< ranges::input_range R1, ranges::input_range R2,

		  class Pred = ranges::equal_to,
		  class Proj1 = std::identity, class Proj2 = std::identity >
	requires	      std::indirectly_comparable<ranges::iterator_t<R1>,
     (since
	ranges::iterator_t<R2>,
     (2) C++20)
					    Pred, Proj1, Proj2>
	constexpr bool

	    equal( R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2
	proj2 = {} );

	1) Returns true if the projected values of the range [first1, last1) are
     equal to
	the projected values of the range [first2, last2), and false otherwise.
	2) Same as (1), but uses r as the source range, as if using  ranges::be-
     gin(r) as
	first and ranges::end(r) as last.

	Two ranges are considered equal if they have the same number of elements
     and every
	pair  of  corresponding  projected  elements  satisfies  pred.	That is,
     std::invoke(pred,
	std::invoke(proj1, *first1), std::invoke(proj2, *first2))  returns  true
     for all pairs
	of corresponding elements in both ranges.

	The  function-like  entities  described on this page are niebloids, that
     is:

	  * Explicit template argument lists cannot be	specified  when  calling
     any of them.
	  * None of them are visible to argument-dependent lookup.
	  *  When any of them are found by normal unqualified lookup as the name
     to the left
	    of the function-call operator, argument-dependent lookup  is  inhib-
     ited.

	In  practice,  they may be implemented as function objects, or with spe-
     cial compiler
	extensions.

Parameters
	first1, last1 - an iterator-sentinel pair denoting the	first  range  of
     the elements
			to compare
	r1	      - the first range of the elements to compare
	first2,  last2	- an iterator-sentinel pair denoting the second range of
     the elements
			to compare
	r2	      - the second range of the elements to compare
	pred	      - predicate to apply to the projected elements
	proj1	      - projection to apply to the first range of elements
	proj2	      - projection to apply to the second range of elements

Return value
	If the length of the range [first1, last1) does not equal the length  of
     the range
	[first2, last2), returns false.

	If  the  elements  in the two ranges are equal after projection, returns
     true.

	Otherwise returns false.

Notes
	ranges::equal should not be used to compare the ranges formed by the it-
     erators from
	std::unordered_set, std::unordered_multiset, std::unordered_map, or
	std::unordered_multimap because the order  in  which  the  elements  are
     stored in those
	containers  may  be  different even if the two containers store the same
     elements.

	When comparing entire containers for equality, operator== for the corre-
     sponding
	container are usually preferred.

Complexity
	At most min(last1 - first1, last2 - first2) applications of  the  predi-
     cate and
	corresponding projections.

	However,  if  S1 and S2 both model std::sized_sentinel_for their respec-
     tive iterators,
	and last1 - first1 != last2 - first2 then no applications of the  predi-
     cate are made
	(size mismatch is detected without looking at any elements).

Possible implementation
     struct equal_fn {
       template<std::input_iterator I1, std::sentinel_for<I1> S1,
		std::input_iterator I2, std::sentinel_for<I2> S2,
		class Pred = ranges::equal_to,
		class Proj1 = std::identity, class Proj2 = std::identity>
       requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
       constexpr bool
	   operator()(I1 first1, S1 last1, I2 first2, S2 last2,
		      Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const
       {
	   if  constexpr  (std::sized_sentinel_for<S1,	I1>  and std::sized_sen-
     tinel_for<S2, I2>)
	       if  (std::ranges::distance(first1,  last1)  !=  std::ranges::dis-
     tance(first2, last2))
		   return false;

	   for (; first1 != last1; ++first1, (void)++first2)
	       if   (!std::invoke(pred,  std::invoke(proj1,  *first1),	std::in-
     voke(proj2, *first2)))
		   return false;
	   return true;
       }

       template<ranges::input_range R1, ranges::input_range R2,
		class Pred = ranges::equal_to,
		class Proj1 = std::identity, class Proj2 = std::identity>
       requires std::indirectly_comparable<ranges::iterator_t<R1>, ranges::iter-
     ator_t<R2>,
					   Pred, Proj1, Proj2>
       constexpr bool
	   operator()(R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {},  Proj2
     proj2 = {}) const
       {
	   return (*this)(ranges::begin(r1), ranges::end(r1),
			  ranges::begin(r2), ranges::end(r2),
			  std::ref(pred), std::ref(proj1), std::ref(proj2));
       } };

     inline constexpr equal_fn equal;

Example
	The  following	code  uses ranges::equal to test if a string is a palin-
     drome.

     // Run this code

      #include <algorithm>
      #include <iomanip>
      #include <iostream>
      #include <ranges>
      #include <string_view>

      constexpr bool is_palindrome(const std::string_view s)
      {
	  namespace views = std::views;
	  auto forward = s | views::take(s.size() / 2);
	  auto backward = s | views::reverse | views::take(s.size() / 2);
	  return std::ranges::equal(forward, backward);
      }

      void test(const std::string_view s)
      {
	  std::cout << std::quoted(s) << " is "
		    << (is_palindrome(s) ? "" : "not ")
		    << "a palindrome\n";
      }

      int main()
      {
	  test("radar");
	  test("hello");
	  static_assert(is_palindrome("ABBA") and not is_palindrome("AC/DC"));
      }

Output:
      "radar" is a palindrome
      "hello" is not a palindrome

See also
	ranges::find
	ranges::find_if
	ranges::find_if_not		finds the first element satisfying  spe-
     cific criteria
	(C++20) 			(niebloid)
	(C++20)
	(C++20)
	ranges::lexicographical_compare  returns  true	if  one range is lexico-
     graphically less
	(C++20) 			than another
					(niebloid)
	ranges::mismatch		 finds	the  first  position  where  two
     ranges differ
	(C++20) 			(niebloid)
	ranges::search			searches for a range of elements
	(C++20) 			(niebloid)
	ranges::equal_range		 returns  range  of  elements matching a
     specific key
	(C++20) 			(niebloid)
	equal_to			function object implementing x == y
					(class template)
	equal				determines if two sets of  elements  are
     the same
					(function template)

http://cppreference.com 	   2024.06.10		   std::ranges::equal(3)
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