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    std::is_heap_until

    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 RandomIt >
RandomIt is_heap_until( RandomIt first, RandomIt last );
    		(1) (since C++11) 
    template< class RandomIt, class Compare >
RandomIt is_heap_until( RandomIt first, RandomIt last, Compare comp );
    		(2) (since C++11) 
    template< class ExecutionPolicy, class RandomIt >
RandomIt is_heap_until( ExecutionPolicy&& policy,
                        RandomIt first, RandomIt last );
    		(3) (since C++17) 
    template< class ExecutionPolicy, class RandomIt, class Compare >
RandomIt is_heap_until( ExecutionPolicy&& policy,
                        RandomIt first, RandomIt last, Compare comp );
    		(4) (since C++17)
    See More

    Examines the target range [first, last) and finds the largest range which begins at first and represents a heap.

    1) The heap property to be checked is with respect to operator<(until C++20)std::less{}(since C++20).
    2) The heap property to be checked is 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 target 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 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 RandomIt can be dereferenced and then implicitly converted to both of them.

    policy - the execution policy to use
    Type requirements
    -
    RandomIt must meet the requirements of LegacyRandomAccessIterator.
    -
    Compare must meet the requirements of Compare.

    Return value

    The past-the-end iterator of the largest range found.

    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).

    Example

    Run this code
    import std;

void print_tree(const char* remark, std::span<const int> tree)
{
    std::size_t w = (1 << (std::bit_width(tree.size()) - 1)) * 4;
    std::println("{:^{}}", remark, w);
    for (std::size_t n{}; n != tree.size(); ++n)
        if (std::print("{:^{}}", tree[n], w); std::has_single_bit(n + 2))
            w /= 2, std::println();
    std::println();
}

int main()
{
    std::vector tree{3, 1, 4, 1, 5};
    std::make_heap(tree.begin(), tree.end());

    // Mess up the heap
    tree.push_back(9);
    tree.push_back(2);

    const auto it_heap_end = std::is_heap_until(tree.begin(), tree.end());
    const std::span heap(tree.begin(), it_heap_end);

    print_tree("Full tree:", tree);
    print_tree("Heap:", heap);
}

    Output: 

       Full tree:
       5
   3       4
 1   1   9   2

     Heap:
       5
   3       4
 1   1

    See also

    (C++20)
    		 finds the largest subrange that is a max heap
    (algorithm function object)[edit]
    (C++11)
    		 checks if the given range is a max heap
    (function template & algorithm function object)[edit]
    (C++20)
    		creates a max heap out of a range of elements
    (function template & algorithm function object)[edit]
    (C++20)
    		adds an element to a max heap
    (function template & algorithm function object)[edit]
    (C++20)
    		removes the largest element from a max heap
    (function template & algorithm function object)[edit]
    (C++20)
    		turns a max heap into a range of elements sorted in ascending order
    (function template & algorithm function object)[edit]
    (C++20)