std::is_sorted(3)

NAME
std::is_sorted - std::is_sorted

Synopsis
   Defined in header <algorithm>
   template< class ForwardIt > (since C++11)
   bool is_sorted( ForwardIt first, ForwardIt last (until C++20)
   );
   template< class ForwardIt >
   constexpr bool is_sorted( ForwardIt first, (since C++20)
   ForwardIt last );
   template< class ExecutionPolicy, class ForwardIt
   > (2) (since C++17)
   bool is_sorted( ExecutionPolicy&& policy,
   ForwardIt first, ForwardIt last );
   template< class ForwardIt, class Compare > (since C++11)
   bool is_sorted( ForwardIt first, ForwardIt last, (1) (until C++20)
   Compare comp );
   template< class ForwardIt, class Compare >
   constexpr bool is_sorted( ForwardIt first, (since C++20)
   ForwardIt last, Compare comp );
   template< class ExecutionPolicy, class (3)
   ForwardIt, class Compare >
   bool is_sorted( ExecutionPolicy&& policy, (4) (since C++17)
   ForwardIt first, ForwardIt last,
   Compare comp );
   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, comp(*(it + n), *it) evaluates to
   false.
   1) Elements are compared using operator<.
   3) Elements are compared using the given binary comparison function comp.
   2,4) Same as (1,3), but executed according to policy. These overloads do not
   participate in overload resolution unless
   std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
   (until C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
   (since C++20) is true.

Parameters
   first, last - the range of elements to examine.
   policy - the execution policy to use. See execution policy for details.
                 comparison function object (i.e. an object that satisfies the
                 requirements of Compare) which returns true if the first argument
                 is less than (i.e. is ordered before) the second.
                 The signature of the comparison function should be equivalent to the
                 following:
                 bool cmp(const Type1 &a, const Type2 &b);
   comp -
                 While the signature does not need to have const &, the function must
                 not modify the objects passed to it and must be able to accept all
                 values of type (possibly const) Type1 and Type2 regardless of value
                 category (thus, Type1 & is not allowed
                 , nor is Type1 unless for Type1 a move is equivalent to a copy
                 (since C++11)).
                 The types Type1 and Type2 must be such that an object of type
                 ForwardIt can be dereferenced and then implicitly converted to both of
                 them.

Type requirements
   -
   ForwardIt must meet the requirements of LegacyForwardIterator.

Return value
   true if the elements in the range are sorted in non-descending order.

Complexity
   Linear in the distance between first and last.

Exceptions
   The overloads with a template parameter named ExecutionPolicy report errors as
   follows:
     * If execution of a function invoked as part of the algorithm throws an exception
       and ExecutionPolicy is one of the standard policies, std::terminate is called.
       For any other ExecutionPolicy, the behavior is implementation-defined.
     * If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation
   See also the implementations in libstdc++ and libc++.

First version
   template<class ForwardIt>
   bool is_sorted(ForwardIt first, ForwardIt last)
   {
       return std::is_sorted_until(first, last) == last;
   }

Second version
   template<class ForwardIt, class Compare>
   bool is_sorted(ForwardIt first, ForwardIt last, Compare comp)
   {
       return std::is_sorted_until(first, last, comp) == last;
   }

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

Example
// Run this code
 #include <iostream>
 #include <algorithm>
 #include <iterator>
 int main()
 {
     int digits[] = {3, 1, 4, 1, 5};
     for (auto i : digits) std::cout << i << ' ';
     std::cout << ": is_sorted: " << std::boolalpha
               << std::is_sorted(std::begin(digits), std::end(digits)) << '\n';
     std::sort(std::begin(digits), std::end(digits));
     for (auto i : digits) std::cout << i << ' ';
     std::cout << ": is_sorted: "
               << std::is_sorted(std::begin(digits), std::end(digits)) << '\n';
 }

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

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