# std::is_sorted

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class ForwardIt > bool is_sorted( ForwardIt first, ForwardIt last ); |  | (since C++11) (until C++20)
template< class ForwardIt > constexpr bool is_sorted( ForwardIt first, ForwardIt last ); |  | (since C++20)
template< class ExecutionPolicy, class ForwardIt > bool is_sorted( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last ); | (2) | (since C++17)
| (3) |
template< class ForwardIt, class Compare > bool is_sorted( ForwardIt first, ForwardIt last, Compare comp ); |  | (since C++11) (until C++20)
template< class ForwardIt, class Compare > constexpr bool is_sorted( ForwardIt first, ForwardIt last, Compare comp ); |  | (since C++20)
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);

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 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 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;
    }

### Notes

std::is_sorted and std::is_sorted_until both return 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 (C++11) | finds the largest sorted subrange (function template)

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