std::ranges::is_sorted(3)

NAME
std::ranges::is_sorted - std::ranges::is_sorted

Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::forward_iterator I, std::sentinel_for<I> S,
             class Proj = std::identity,
             std::indirect_strict_weak_order<std::projected<I, Proj>> (1) (since C++20)
                 Comp = ranges::less >
   constexpr bool
       is_sorted( I first, S last, Comp comp = {}, Proj proj = {} );
   template< ranges::forward_range R, class Proj = std::identity,
             std::indirect_strict_weak_order<
                 std::projected<ranges::iterator_t<R>, Proj>> (2) (since C++20)
                     Comp = ranges::less >
   constexpr bool
       is_sorted( R&& r, Comp comp = {}, Proj proj = {} );
   Checks if the elements in range [first, last) are sorted in non-descending order.
   A sequence is sorted with respect to a comparator comp if for any iterator it
   pointing to the sequence and any non-negative integer n such that it + n is a valid
   iterator pointing to an element of the sequence, std::invoke(comp, std::invoke(proj,
   *(it + n)), std::invoke(proj, *it)) evaluates to false.
   1) Elements are compared using the given binary comparison function comp.
   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 - iterator-sentinel defining the range to check if it is sorted
   r - the range to check if it is sorted
   comp - comparison function to apply to the projected elements
   proj - projection to apply to the elements

Return value
   true if the elements in the range are sorted according to comp.

Complexity
   Linear in the distance between first and last.

Possible implementation
 struct is_sorted_fn
 {
     template<std::forward_iterator I, std::sentinel_for<I> S,
              class Proj = std::identity,
              std::indirect_strict_weak_order<std::projected<I, Proj>>
                  Comp = ranges::less>
     constexpr bool operator()(I first, S last, Comp comp = {}, Proj proj = {}) const
     {
         return ranges::is_sorted_until(first, last, comp, proj) == last;
     }
     template<ranges::forward_range R, class Proj = std::identity,
              std::indirect_strict_weak_order<
                  std::projected<ranges::iterator_t<R>, Proj>>
                      Comp = ranges::less>
     constexpr bool operator()(R&& r, Comp comp = {}, Proj proj = {}) const
     {
         return (*this)(ranges::begin(r), ranges::end(r), std::ref(comp), std::ref(proj));
     }
 };
 inline constexpr is_sorted_fn is_sorted;

Notes
   ranges::is_sorted returns true for empty ranges and ranges of length one.

Example
// Run this code
 #include <algorithm>
 #include <array>
 #include <functional>
 #include <iostream>
 #include <iterator>
 int main()
 {
     namespace ranges = std::ranges;
     std::array digits {3, 1, 4, 1, 5};
     ranges::copy(digits, std::ostream_iterator<int>(std::cout, " "));
     ranges::is_sorted(digits)
         ? std::cout << ": sorted\n"
         : std::cout << ": not sorted\n";
     ranges::sort(digits);
     ranges::copy(digits, std::ostream_iterator<int>(std::cout, " "));
     ranges::is_sorted(ranges::begin(digits), ranges::end(digits))
         ? std::cout << ": sorted\n"
         : std::cout << ": not sorted\n";
     ranges::reverse(digits);
     ranges::copy(digits, std::ostream_iterator<int>(std::cout, " "));
     ranges::is_sorted(digits, ranges::greater {})
         ? std::cout << ": sorted (with 'greater')\n"
         : std::cout << ": not sorted\n";
 }

Output:
 3 1 4 1 5 : not sorted
 1 1 3 4 5 : sorted
 5 4 3 1 1 : sorted (with 'greater')

See also
   ranges::is_sorted_until finds the largest sorted subrange
   (C++20) (niebloid)
   is_sorted checks whether a range is sorted into ascending order
   (C++11) (function template)