std::is_sorted_until() algorithm

since C++20

// (1)
template< class ForwardIt >
constexpr ForwardIt is_sorted_until( ForwardIt first, ForwardIt last );

// (2)
template< class ForwardIt, class Compare >
constexpr ForwardIt is_sorted_until( ForwardIt first, ForwardIt last, Compare comp );

// (3)
template< class ExecutionPolicy, class ForwardIt >
ForwardIt is_sorted_until( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last );

// (4)
template< class ExecutionPolicy, class ForwardIt, class Compare >
ForwardIt is_sorted_until( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, Compare comp );

since C++17

// (1)
template< class ForwardIt >
ForwardIt is_sorted_until( ForwardIt first, ForwardIt last );

// (2)
template< class ForwardIt, class Compare >
ForwardIt is_sorted_until( ForwardIt first, ForwardIt last, Compare comp );

// (3)
template< class ExecutionPolicy, class ForwardIt >
ForwardIt is_sorted_until( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last );

// (4)
template< class ExecutionPolicy, class ForwardIt, class Compare >
ForwardIt is_sorted_until( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, Compare comp );

since C++11

// (1)
template< class ForwardIt >
ForwardIt is_sorted_until( ForwardIt first, ForwardIt last );

// (2)
template< class ForwardIt, class Compare >
ForwardIt is_sorted_until( ForwardIt first, ForwardIt last, Compare comp );

Examines the range [first; last) and finds the largest range beginning at first in which the elements 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<.

• (2) Elements are compared using the given binary comparison function comp.

• (3 - 4 ) Same as (1 - 2), but executed according to policy.

  Overload Resolution

  These overloads participate in overload resolution only if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>  (until C++20) std::is_execution_policy_v<std::is_sorted_until_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.
p	Comparison function object (i.e. an object that satisfies the requirements of Compare), which returns true if the first argument is less than 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 must not modify arguments. Must accept all values of type (possibly const) Type and Type2, regardless of value category (so 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 RandomIt can be implicitly converted to both of them.

Type requirements

ForwardIt

LegacyForwardIterator

Return value

The upper bound of the largest range beginning at first in which the elements are sorted in ascending order. That is, the last iterator it for which range [first; it) is sorted.

Returns last for empty ranges and ranges of length one.

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

is_sorted_until (1)

template<class ForwardIt>
constexpr //< since C++20
ForwardIt is_sorted_until(ForwardIt first, ForwardIt last)
{
    return std::is_sorted_until(first, last, std::less<>());
}

is_sorted_until (2)

template<class ForwardIt, class Compare>
constexpr //< since C++20
ForwardIt is_sorted_until(ForwardIt first, ForwardIt last, Compare comp)
{
    if (first != last)
    {
        ForwardIt next = first;
        while (++next != last)
        {
            if (comp(*next, *first))
                return next;
            first = next;
        }
    }
    return last;
}

Examples

Main.cpp

#include <algorithm>
#include <cassert>
#include <iostream>
#include <iterator>
#include <random>
#include <string>

int main()
{
    std::random_device rd;
    std::mt19937 g(rd());
    const int N = 6;
    int nums[N] = {3, 1, 4, 1, 5, 9};

    const int min_sorted_size = 4;

    for (int sorted_size = 0; sorted_size < min_sorted_size;)
    {
        std::shuffle(nums, nums + N, g);
        int *const sorted_end = std::is_sorted_until(nums, nums + N);
        sorted_size = std::distance(nums, sorted_end);
        assert(sorted_size >= 1);

        for (auto i : nums)
            std::cout << i << ' ';
        std::cout << " : " << sorted_size << " initial sorted elements\n"
                  << std::string(sorted_size * 2 - 1, '^') << '\n';
    }
}

Possible output

4 1 9 5 1 3  : 1 initial sorted elements
^
4 5 9 3 1 1  : 3 initial sorted elements
^^^^^
9 3 1 4 5 1  : 1 initial sorted elements
^
1 3 5 4 1 9  : 3 initial sorted elements
^^^^^
5 9 1 1 3 4  : 2 initial sorted elements
^^^
4 9 1 5 1 3  : 2 initial sorted elements
^^^
1 1 4 9 5 3  : 4 initial sorted elements
^^^^^^^