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Showing content from https://cplusplus.com/reference/algorithm/is_heap/ below:

function template

<algorithm>

std::is_heap default (1) 
template <class RandomAccessIterator>  bool is_heap (RandomAccessIterator first, RandomAccessIterator last);
custom (2) 
template <class RandomAccessIterator, class Compare>  bool is_heap (RandomAccessIterator first, RandomAccessIterator last,                Compare comp);

Test if range is heap

Returns true if the range [first,last) forms a heap, as if constructed with make_heap.

The elements are compared using operator< for the first version, and comp for the second.



Parameters
first, last
RandomAccess iterators to the initial and final positions of the sequence. The range checked is [first,last), which contains all the elements between first and last, including the element pointed by first but not the element pointed by last.
comp
Binary function that accepts two elements in the range as arguments, and returns a value convertible to bool. The value returned indicates whether the element passed as first argument is considered to go before the second in the specific strict weak ordering it defines.
The function shall not modify any of its arguments.
This can either be a function pointer or a function object.

Return valuetrue if the range [first,last) is a heap (as if constructed with make_heap), false otherwise.

If the range [first,last) contains less than two elements, the function always returns true.



Example 
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// is_heap example
#include <iostream>     // std::cout
#include <algorithm>    // std::is_heap, std::make_heap, std::pop_heap
#include <vector>       // std::vector

int main () {
  std::vector<int> foo {9,5,2,6,4,1,3,8,7};

  if (!std::is_heap(foo.begin(),foo.end()))
    std::make_heap(foo.begin(),foo.end());

  std::cout << "Popping out elements:";
  while (!foo.empty()) {
    std::pop_heap(foo.begin(),foo.end());   // moves largest element to back
    std::cout << ' ' << foo.back();         // prints back
    foo.pop_back();                         // pops element out of container
  }
  std::cout << '\n';

  return 0;
}

Output: 
Popping out elements: 9 8 7 6 5 4 3 2 1


Complexity Up to linear in one less than the distance between first and last: Compares pairs of elements until a mismatch is found.

Data races The objects in the range [first,last) are accessed.

Exceptions Throws if either an element comparison or an operation on an iterator throws.
Note that invalid arguments cause undefined behavior.

See also
is_heap_until
Find first element not in heap order (function template)
make_heap
Make heap from range (function template)
sort_heap
Sort elements of heap (function template)

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