# std::is_heap

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< cpp‎ | algorithm

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

Run this code

    #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

### See also

is_heap_until (C++11) | finds the largest subrange that is a max heap (function template)

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