std::ranges::adjacent_find(3)

NAME
std::ranges::adjacent_find - std::ranges::adjacent_find

Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::forward_iterator I, std::sentinel_for<I> S, class
   Proj = std::identity,
   std::indirect_binary_predicate<
   std::projected<I, Proj>, (1) (since C++20)
   std::projected<I, Proj>> Pred = ranges::equal_to >
   constexpr I adjacent_find( I first, S last, Pred pred = {}, Proj
   proj = {} );
   template< ranges::forward_range R, class Proj = std::identity,
   std::indirect_binary_predicate<
   std::projected<ranges::iterator_t<R>, Proj>,
   std::projected<ranges::iterator_t<R>, Proj>> Pred = (2) (since C++20)
   ranges::equal_to >
   constexpr ranges::borrowed_iterator_t<R>
   adjacent_find( R&& r, Pred pred = {}, Proj proj = {} );
   Searches the range [first, last) for two consecutive equal elements.
   1) Elements are compared using pred (after projecting with the projection proj).
   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.
   The function-like entities described on this page are niebloids, that is:
     * Explicit template argument lists may not be specified when calling any of them.
     * None of them is visible to argument-dependent lookup.
     * When one of them is found by normal unqualified lookup for the name to the left
       of the function-call operator, it inhibits argument-dependent lookup.
   In practice, they may be implemented as function objects, or with special compiler
   extensions.

Parameters
   first, last - the range of elements to examine
   r - the range of the elements to examine
   pred - predicate to apply to the projected elements
   proj - projection to apply to the elements

Return value
   An iterator to the first of the first pair of identical elements, that is, the first
   iterator it such that bool(std::invoke(pred, std::invoke(proj1, *it),
   std::invoke(proj, *(it + 1)))) is true.
   If no such elements are found, an iterator equal to last is returned.

Complexity
   Exactly min((result-first)+1, (last-first)-1) applications of the predicate and
   projection where result is the return value.

Possible implementation
  struct adjacent_find_fn {
    template< std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
              std::indirect_binary_predicate<
                  std::projected<I, Proj>,
                  std::projected<I, Proj>> Pred = ranges::equal_to >
    constexpr I operator()( I first, S last, Pred pred = {}, Proj proj = {} ) const
    {
        if (first == last) {
            return first;
        }
        auto next = ranges::next(first);
        for (; next != last; ++next, ++first) {
            if (std::invoke(pred, std::invoke(proj, *first), std::invoke(proj, *next))) {
                return first;
            }
        }
        return first;
    }
    template< ranges::forward_range R, class Proj = std::identity,
              std::indirect_binary_predicate<
                  std::projected<ranges::iterator_t<R>, Proj>,
                  std::projected<ranges::iterator_t<R>, Proj>> Pred = ranges::equal_to >
    constexpr ranges::borrowed_iterator_t<R>
      operator()( R&& r, Pred pred = {}, Proj proj = {} ) const
    {
      return (*this)(ranges::begin(r), ranges::end(r), std::ref(pred), std::ref(proj));
    }
  };
  inline constexpr adjacent_find_fn adjacent_find;

Example
// Run this code
 #include <algorithm>
 #include <iostream>
 #include <vector>
 #include <functional>
 int main()
 {
     std::vector<int> v1{0, 1, 2, 3, 40, 40, 41, 41, 5};
     // ^^ ^^
     namespace ranges = std::ranges;
     auto i1 = ranges::adjacent_find(v1.begin(), v1.end());
     if (i1 == v1.end()) {
         std::cout << "No matching adjacent elements\n";
     } else {
         std::cout << "The first adjacent pair of equal elements is at ["
                   << ranges::distance(v1.begin(), i1) << "] == " << *i1 << '\n';
     }
     auto i2 = ranges::adjacent_find(v1, ranges::greater());
     if (i2 == v1.end()) {
         std::cout << "The entire vector is sorted in ascending order\n";
     } else {
         std::cout << "The last element in the non-decreasing subsequence is at ["
                   << ranges::distance(v1.begin(), i2) << "] == " << *i2 << '\n';
     }
 }

Output:
 The first adjacent pair of equal elements is at [4] == 40
 The last element in the non-decreasing subsequence is at [7] == 41

See also
   ranges::unique removes consecutive duplicate elements in a range
   (C++20) (niebloid)
                  finds the first two adjacent items that are equal (or satisfy a given
   adjacent_find predicate)
                  (function template)