C++ Algorithm Library - is_heap_until() Function


The C++ function std::algorithm::is_heap_until() finds the first element from the sequence which violates the max heap condition. It uses binary function for comparison.


Following is the declaration for std::algorithm::is_heap_until() function form std::algorithm header.


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


  • first − Random access iterator to the initial position.

  • last − Random access iterator to the final position.

  • comp − A binary function which accepts two arguments and returns bool.

Return value

Returns an iterator to the first element which violates the max heap condition. If entire sequence is valid max heap then it returns last.


Throws exception if either the element comparison or an operation on an iterator throws exception.

Please note that invalid parameters cause undefined behavior.

Time complexity



The following example shows the usage of std::algorithm::is_heap_until() function.

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

bool ignore_case(char a, char b) {
   return (tolower(a) == tolower(b));

int main(void) {
   vector<char> v = {'E', 'd', 'C', 'b', 'A'};
   auto result = is_heap_until(v.begin(), v.end());

   cout << char(*result) << " is the first element which "
        << "violates the max heap." << endl;

   v = {5, 4, 3, 2, 1};

   result = is_heap_until(v.begin(), v.end(), ignore_case);

   if (result == end(v))
      cout << "Entire range is valid heap." << endl;

   return 0;

Let us compile and run the above program, this will produce the following result −

d is the first element which violates the max heap.
Entire range is valid heap.