< 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

#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
    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';


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)
parallelized version of std::is_heap_until
(function template)