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    std::is_sorted

    From cppreference.com
     
    C++
     
    Algorithm library
    Constrained algorithms and algorithms on ranges (C++20)
    Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
    Non-modifying sequence operations    
    Batch operations
    (C++17)
    Search operations
    Modifying sequence operations
    Copy operations
    (C++11)
    (C++11)
    (C++11)
    Swap operations
    Transformation operations
    Generation operations
    Removing operations
    Order-changing operations
    (until C++17)(C++11)
    (C++20)(C++20)
    Sampling operations
    (C++17)

    Sorting and related operations
    Partitioning operations
    (C++11)    
    (C++11)

    Sorting operations
    Binary search operations
    (on partitioned ranges)
    Set operations (on sorted ranges)
    Merge operations (on sorted ranges)
    Heap operations
    Minimum/maximum operations
    (C++11)
    (C++17)
    Lexicographical comparison operations
    (C++20)
    Permutation operations


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

    Checks if all elements of the source range [first, last) are sorted.

    1) Checks if all elements are sorted with respect to operator<(until C++20)std::less{}(since C++20).
    2) Checks if all elements are sorted with respect to comp.
    3,4) Same as (1,2), but executed according to policy.
    These overloads participate in overload resolution only if the value of the following expression is true:

    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)

    Parameters

    first, last - the pair of iterators defining the source range
    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. ​

    policy - the execution policy to use
    Type requirements
    -
    ForwardIt must meet the requirements of LegacyForwardIterator.
    -
    Compare must meet the requirements of Compare.

    Return value

    true if all elements of the source range are sorted, false otherwise.

    Complexity

    Given N as std::distance(first, last):

    1,3) 𝓞(N) comparisons using operator<(until C++20)std::less{}(since C++20).
    2,4) 𝓞(N) applications of the comparator comp.

    Exceptions

    3,4) During the execution process:
    • If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
    • If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).

    Notes

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

    Possible implementation

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


    is_sorted (1) 
    template<class ForwardIt>
bool is_sorted(ForwardIt first, ForwardIt last)
{
    return std::is_sorted_until(first, last) == last;
}
    is_sorted (2) 
    template<class ForwardIt, class Compare>
bool is_sorted(ForwardIt first, ForwardIt last, Compare comp)
{
    return std::is_sorted_until(first, last, comp) == last;
}

    Example

    Run this code
    #include <algorithm>
#include <cassert>
#include <functional>
#include <iterator>
#include <vector>

int main()
{
    std::vector<int> v;
    assert(std::is_sorted(v.cbegin(), v.cend()) && "an empty range is always sorted");
    v.push_back(42);
    assert(std::is_sorted(v.cbegin(), v.cend()) && "a range of size 1 is always sorted");
    
    int data[] = {3, 1, 4, 1, 5};
    assert(not std::is_sorted(std::begin(data), std::end(data)));
    
    std::sort(std::begin(data), std::end(data));
    assert(std::is_sorted(std::begin(data), std::end(data)));
    assert(not std::is_sorted(std::begin(data), std::end(data), std::greater<>{}));
}

    See also

    (C++20)
    		 checks whether a range is sorted
    (algorithm function object)[edit]
    (C++11)
    		 finds the largest sorted subrange
    (function template & algorithm function object)[edit]
    (C++20)