# 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) (constexpr since C++20)
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) (constexpr 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 whether `[`first`,` last`)` is a heap.

1) The heap property to be checked is with respect to operator<(until C++20) std::less {}(since C++20).
3) The heap property to be checked is with respect to comp.
2,4) Same as (1,3), but executed according to policy.
These overloads participate in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v < std::decay_t <ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v < std::remove_cvref_t <ExecutionPolicy>> is true. | (since C++20)

### Parameters

first, last | - | the range to be checked
policy | - | the execution policy to use

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.
- `Compare` must meet the requirements of Compare.

### Return value

true if the range is a heap with respect to the corresponding comparator, false otherwise.

### Complexity

Given \(\scriptsize N\)N as std::distance (first, last):

1,2) \(\scriptsize O(N)\)O(N) comparisons using operator<(until C++20) std::less {}(since C++20).
3,4) \(\scriptsize O(N)\)O(N) applications of the comparison function comp.

### 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.

### Example

Run this code

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

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

        std::cout << "initially, v:\n";
        for (const 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:\n";
        for (auto t{1U}; const auto& i : v)
            std::cout << i << (std::has_single_bit(++t) ? " | " : " ");
        std::cout << '\n';
    }

Output:

    initially, v:
    3 1 4 1 5 9 2 6 5 3 5 8 9 7 9
    making heap...
    after make_heap, v:
    9 | 6 9 | 5 5 9 7 | 1 1 3 5 8 3 4 2 |

### See also

is_heap_until (C++11) | finds the largest subrange that is a max heap (function template)
make_heap | creates a max heap out of a range of elements (function template)
push_heap | adds an element to a max heap (function template)
pop_heap | removes the largest element from a max heap (function template)
sort_heap | turns a max heap into a range of elements sorted in ascending order (function template)
ranges::is_heap (C++20) | checks if the given range is a max heap (algorithm function object)

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