# std::is_heap

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class RandomIt > bool is_heap( RandomIt first, RandomIt last ); |  | (since C++11) (until C++20)
template< class RandomIt > constexpr bool is_heap( RandomIt first, RandomIt last ); |  | (since C++20)
template< class ExecutionPolicy, class RandomIt > bool is_heap( ExecutionPolicy&& policy, RandomIt first, RandomIt last ); | (2) | (since C++17)
| (3) |
template< class RandomIt, class Compare > bool is_heap( RandomIt first, RandomIt last, Compare comp ); |  | (since C++11) (until C++20)
template< class RandomIt, class Compare > constexpr bool is_heap( RandomIt first, RandomIt last, Compare comp ); |  | (since C++20)
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);

While the signature does not need to have const &, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) `Type1` and `Type2` regardless of value category (thus, Type1 & is not allowed, nor is Type1 unless for `Type1` a move is equivalent to a copy (since C++11)).
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 LegacyRandomAccessIterator.

### 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 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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