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# std::is_heap_until

From cppreference.com
< cpp‎ | algorithm

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)

Examines the range `[first, last)` and finds the largest range beginning at `first` which is a max heap. The first version of the function uses operator< to compare the elements, the second uses the given comparison function `comp`.

### [edit] Parameters

first, last | - | the range of elements to examine
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); The signature does not need to have const &, but the function object must not modify the objects passed to it. 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 `RandomAccessIterator`.

### [edit] Return value

The upper bound of the largest range beginning at `first` which is a max heap. That is, the last iterator `it` for which range `[first, it)` is a max heap.

### [edit] Complexity

Linear in the distance between `first` and `last`

### [edit] Notes

A max heap is a range of elements `[f,l)` that has the following properties:

  - *f is the largest element in the range
  - a new element can be added using std::push_heap()
  - the first element can be removed using std::pop_heap()

The actual arrangement of the elements is implementation defined.

### [edit] Example

Run this code

    #include <iostream>
    #include <algorithm>
    #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 (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

is_heap (C++11) | checks if the given range is a max heap (function template) [edit]
std::experimental::parallel::is_heap_until (parallelism TS) | parallelized version of `std::is_heap_until` (function template) [edit]

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