# std::is_heap_until

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class RandomIt > RandomIt is_heap_until( RandomIt first, RandomIt last ); |  | (since C++11) (until C++20)
template< class RandomIt > constexpr RandomIt is_heap_until( RandomIt first, RandomIt last ); |  | (since C++20)
template< class ExecutionPolicy, class RandomIt > RandomIt is_heap_until( ExecutionPolicy&& policy, RandomIt first, RandomIt last ); | (2) | (since C++17)
| (3) |
template< class RandomIt, class Compare > RandomIt is_heap_until( RandomIt first, RandomIt last, Compare comp ); |  | (since C++11) (until C++20)
template< class RandomIt, class Compare > constexpr RandomIt is_heap_until( RandomIt first, RandomIt last, Compare comp ); |  | (since C++20)
template< class ExecutionPolicy, class RandomIt, class Compare > RandomIt is_heap_until( ExecutionPolicy&& policy, RandomIt first, RandomIt last, Compare comp ); | (4) | (since C++17)

Examines the range `[`first`,` last`)` and finds the largest range beginning at first which is a heap.

1) The heap property to be checked is with respect to operator<.
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 do not participate in overload resolution unless

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

### Return value

The last iterator it for which range `[`first`,` it`)` is a heap.

### Complexity

Linear in std::distance(first, 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 heap with respect to comp (max heap) is a random access range `[`first`,` last`)` that has the following properties:

- Given \(\scriptsize N\)N as last - first, for all integer i where \(\scriptsize 0 < i < N\)0 < i < N, bool(comp(first[(i - 1) / 2], first[i])) is false.
- A new element can be added using std::push_heap, in \(\scriptsize \mathcal{O}(\log N)\)𝓞(log N) time.
- *first can be removed using std::pop_heap, in \(\scriptsize \mathcal{O}(\log N)\)𝓞(log N) time.

### Example

Run this code

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

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

        std::make_heap(v.begin(), v.end());

        // probably mess up the heap
        v.push_back(2);
        v.push_back(6);

        auto heap_end = std::is_heap_until(v.begin(), v.end());

        std::cout << "all of v: ";
        for (auto i : v)
            std::cout << i << ' ';
        std::cout << '\n';

        std::cout << "only heap: ";
        for (auto i = v.begin(); i != heap_end; ++i)
            std::cout << *i << ' ';
        std::cout << '\n';
    }

Output:

    all of v:  9 5 4 1 1 3 2 6
    only heap: 9 5 4 1 1 3 2

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2166 | C++11 | the heap reqirement did not match the definition of max heap closely enough | requirement imporved

### See also

is_heap (C++11) | checks if the given range 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_until (C++20) | finds the largest subrange that is a max heap (niebloid)

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