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std::is_sorted

Defined in header `<algorithm>`
template< class ForwardIt > bool is_sorted( ForwardIt first, ForwardIt last );	(1)	(since C++11)
template< class ExecutionPolicy, class ForwardIt > bool is_sorted( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last );	(2)	(since C++17)
template< class ForwardIt, class Compare > bool is_sorted( ForwardIt first, ForwardIt last, Compare comp );	(3)	(since C++11)
template< class ExecutionPolicy, class ForwardIt, class Compare > bool is_sorted( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, Compare comp );	(4)	(since C++17)

Checks if the elements in range [first, last) are sorted in non-descending order.

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>> is true

Parameters

first, last	-	the range of elements to examine
policy	-	the execution policy to use. See execution policy for details.
comp	-	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);` The signature does not need to have `const &`, but the function object must not modify the objects passed to it. 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 `ForwardIterator`.

Return value

true if the elements in the range are sorted in ascending 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 three 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

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; }

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;
}

Example

#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