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Showing content from https://en.cppreference.com/w/cpp/algorithm/../ranges/../algorithm/is_heap_until.html below:

std::is_heap_until - cppreference.com

template< class RandomIt > RandomIt is_heap_until( RandomIt first, RandomIt last );

(1) (since C++11)
(constexpr since C++20) template< class ExecutionPolicy, class RandomIt > RandomIt is_heap_until( ExecutionPolicy&& policy, RandomIt first, RandomIt last ); (2) (since C++17)

template< class RandomIt, class Compare > RandomIt is_heap_until( RandomIt first, RandomIt last, Compare comp );

(3) (since C++11)
(constexpr since C++20) template< class ExecutionPolicy, class RandomIt, class Compare > RandomIt is_heap_until( ExecutionPolicy&& policy, RandomIt first, RandomIt last, Compare comp ); (4) (since C++17)

Examines the range [first, last) and finds the largest range beginning at first which is a heap.

1)

The heap property to be checked is with respect to

operator<(until C++20)std::less{}(since C++20)

3) The heap property to be checked is with respect to comp.

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

These overloads participate in overload resolution only if all following conditions are satisfied:

[edit] Parameters first, last - the pair of iterators defining the range of elements to examine policy - the execution policy to use 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.

Type requirements -RandomIt must meet the requirements of LegacyRandomAccessIterator. -Compare must meet the requirements of Compare. [edit] Return value

The last iterator it for which range [first, it) is a heap.

[edit] Complexity

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

1,2) \(\scriptsize O(N)\)O(N)

comparisons using

operator<(until C++20)std::less{}(since C++20)

3,4) \(\scriptsize O(N)\)O(N) applications of the comparison function comp.

[edit] 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.

[edit] Example

#include <algorithm>
#include <iostream>
#include <vector>
 
int main()
{
    std::vector<int> v{3, 1, 4, 1, 5, 9};
 
    std::make_heap(v.begin(), v.end());
 
    // probably mess up the heap
    v.push_back(2);
    v.push_back(6);
 
    auto heap_end = std::is_heap_until(v.begin(), v.end());
 
    std::cout << "all of v:  ";
    for (const auto& i : v)
        std::cout << i << ' ';
    std::cout << '\n';
 
    std::cout << "only heap: ";
    for (auto i = v.begin(); i != heap_end; ++i)
        std::cout << *i << ' ';
    std::cout << '\n';
}

Output:

all of v:  9 5 4 1 1 3 2 6
only heap: 9 5 4 1 1 3 2

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

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