std::ranges::equal() algorithm

since C++20

Simplified

// (1)
constexpr bool equal( I1 first1, S1 last1, I2 first2, S2 last2,
                      Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );

// (2)
constexpr bool equal( R1&& r1, R2&& r2, Pred pred = {},
                      Proj1 proj1 = {}, Proj2 proj2 = {} );

The type of arguments are generic and have the following constraints:

• I1, I2 - std::input_iterator
• S1, S2 - std::sentinel_for<I1>, std::sentinel_for<I2>
• R1, R2 - std::ranges::input_range
• Pred - (none)
• Proj1, Proj2 - (none)

The Proj1, Proj2 and Pred template arguments have the following default types: std::identity, ranges::less for all overloads.

Additionally, each overload has the following constraints:

• (1) - std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
• (2) - std::indirectly_comparable<ranges::iterator_t<R1>, ranges::iterator_t<R2>, Pred, Proj1, Proj2>

Detailed

// (1)
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 = {} );

// (2)
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 = {} );

• (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.

Parameters

first1 last1	The first range of elements to compare.
r1	The first range of elements to compare.
first2 last2	The second range of elements to compare.
r2	The first range of elements to compare.
pred	Predicate to apply to the projected elements.
proj1	Projection to apply to the elements of the first range.
proj2	Projection to apply to the elements of the first range.

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.

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).

Exceptions

(none)

Possible implementation

ranges::equal

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;

Examples

The following code uses ranges::equal to test if a string is a palindrome.

Main.cpp

#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