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std::is_heap

Defined in header `<algorithm>`
template< class RandomIt > bool is_heap( RandomIt first, RandomIt last );	(1)	(since C++11)
template< class ExecutionPolicy, class RandomIt > bool is_heap( ExecutionPolicy&& policy, RandomIt first, RandomIt last );	(2)	(since C++17)
template< class RandomIt, class Compare > bool is_heap( RandomIt first, RandomIt last, Compare comp );	(3)	(since C++11)
template< class ExecutionPolicy, class RandomIt, class Compare > bool is_heap( ExecutionPolicy&& policy, RandomIt first, RandomIt last, Compare comp );	(4)	(since C++17)

Checks if the elements in range [first, last) are a max heap.

1) Elements are compared using operator<.

3) Elements are compared using the given binary comparison function comp.

2,4) Same as (1,3), but executed according to policy. These overloads do not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

Parameters

first, last	-	the range of elements to examine
policy	-	the execution policy to use. See execution policy for details.
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`.

Return value

true if the range is max heap, false otherwise.

Complexity

Linear in the distance between first and last.

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 three 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.

Notes

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

With N = l - f, for all 0 < i < N, f[floor( i-1 2 )] does not compare less than f[i].
a new element can be added using std::push_heap()
the first element can be removed using std::pop_heap()

Example

#include <iostream>
#include <algorithm>
#include <vector>
 
int main()
{
    std::vector<int> v { 3, 1, 4, 1, 5, 9 };
 
    std::cout << "initially, v: ";
    for (auto i : v) std::cout << i << ' ';
    std::cout << '\n';
 
    if (!std::is_heap(v.begin(), v.end())) {
        std::cout << "making heap...\n";
        std::make_heap(v.begin(), v.end());
    }
 
    std::cout << "after make_heap, v: ";
    for (auto i : v) std::cout << i << ' ';
    std::cout << '\n';
}

Output:

initially, v: 3 1 4 1 5 9 
making heap...
after make_heap, v: 9 5 4 1 1 3