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 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
   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 next;
     }
     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 <functional>
 #include <iostream>
 #include <ranges>
 constexpr bool some_of(auto&& r, auto&& pred) // some but not all
 {
     return std::ranges::cend(r) != std::ranges::adjacent_find(r,
         [&pred](auto const& x, auto const& y)
         {
             return pred(x) != pred(y);
         });
 }
 // test some_of
 constexpr auto a = {0, 0, 0, 0}, b = {1, 1, 1, 0}, c = {1, 1, 1, 1};
 auto is_one = [](auto x){ return x == 1; };
 static_assert(!some_of(a, is_one) && some_of(b, is_one) && !some_of(c, is_one));
 int main()
 {
     const auto v = {0, 1, 2, 3, 40, 40, 41, 41, 5}; /*
                        ^^ ^^ */
     namespace ranges = std::ranges;
     if (auto it = ranges::adjacent_find(v.begin(), v.end()); it == v.end())
         std::cout << "No matching adjacent elements\n";
     else
         std::cout << "The first adjacent pair of equal elements is at ["
                   << ranges::distance(v.begin(), it) << "] == " << *it << '\n';
     if (auto it = ranges::adjacent_find(v, ranges::greater()); it == v.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(v.begin(), it) << "] == " << *it << '\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)