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::begin(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 inhibited.
   In practice, they may be implemented as function objects, or with special 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 iterators 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 corresponding
   container are usually preferred.

Complexity
   At most min(last1 - first1, last2 - first2) applications of the predicate and
   corresponding projections.
   However, if S1 and S2 both model std::sized_sentinel_for their respective iterators,
   and last1 - first1 != last2 - first2 then no applications of the predicate 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_sentinel_for<S2, I2>)
          if (std::ranges::distance(first1, last1) != std::ranges::distance(first2, last2))
              return false;
      for (; first1 != last1; ++first1, (void)++first2)
          if (!std::invoke(pred, std::invoke(proj1, *first1), std::invoke(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::iterator_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 palindrome. // 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 specific criteria
   (C++20) (niebloid)
   (C++20)
   (C++20)
   ranges::lexicographical_compare returns true if one range is lexicographically 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)