std::ranges::equal

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 > requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2> constexpr bool equal( I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );	(1)	(since C++20)
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 equal( R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );	(2)	(since C++20)

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 << 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

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Technical specifications
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External libraries

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