std::ranges::is_sorted

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 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>> 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.

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

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

#include <algorithm>
#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, " "));
    std::cout << ": is_sorted: " << std::boolalpha
              << ranges::is_sorted(digits) << '\n';
 
    ranges::sort(digits);
 
    ranges::copy(digits, std::ostream_iterator<int>(std::cout, " "));
    std::cout << ": is_sorted: "
              << ranges::is_sorted(ranges::begin(digits), 
                                   ranges::end(digits)) << '\n';
}

3 1 4 1 5 : is_sorted: false
1 1 3 4 5 : is_sorted: true

See also