< cpp‎ | algorithm
Algorithm library
Execution policies (C++17)
Non-modifying sequence operations
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations

Numeric operations
C library
Defined in header <algorithm>
template< class RandomIt >
RandomIt is_heap_until( RandomIt first, RandomIt last );
(1) (since C++11)
template< class ExecutionPolicy, class RandomIt >
RandomIt is_heap_until( ExecutionPolicy&& policy, RandomIt first, RandomIt last );
(2) (since C++17)
template< class RandomIt, class Compare >
RandomIt is_heap_until( RandomIt first, RandomIt last, Compare comp );
(3) (since C++11)
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 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


[edit] 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.

[edit] Return value

The upper bound of the largest range beginning at first which is a max heap. That is, the last iterator it for which range [first, it) is a max heap.

[edit] Complexity

Linear in the distance between first and last

[edit] 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,
  • if policy is std::parallel_vector_execution_policy, std::terminate is called
  • if policy is std::sequential_execution_policy or std::parallel_execution_policy, the algorithm exits with an std::exception_list containing all uncaught exceptions. If there was only one uncaught exception, the algorithm may rethrow it without wrapping in std::exception_list. It is unspecified how much work the algorithm will perform before returning after the first exception was encountered.
  • if policy is some other type, the behavior is implementation-defined
  • If the algorithm fails to allocate memory (either for itself or to construct an std::exception_list when handling a user exception), std::bad_alloc is thrown.

[edit] Notes

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

  • *f is the largest element in the range
  • a new element can be added using std::push_heap()
  • the first element can be removed using std::pop_heap()

The actual arrangement of the elements is implementation defined.

[edit] Example

#include <iostream>
#include <algorithm>
#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
    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';


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

[edit] See also

checks if the given range is a max heap
(function template) [edit]
parallelized version of std::is_heap_until
(function template) [edit]