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Category: algorithms Component type: function


Is_heap is an overloaded name; there are actually two is_heap functions.

template <class RandomAccessIterator>
bool is_heap(RandomAccessIterator first, RandomAccessIterator last);

template <class RandomAccessIterator, class StrictWeakOrdering>
inline bool is_heap(RandomAccessIterator first, RandomAccessIterator last,
                    StrictWeakOrdering comp)


Is_heap returns true if the range [first, last) is a heap [1], and false otherwise. The two versions differ in how they define whether one element is less than another: the first version compares objects using operator<, and the second compares objects using a functors comp.


Defined in the standard header algorithm, and in the nonstandard backward-compatibility header algo.h. This function is an SGI extension; it is not part of the C++ standard.

Requirements on types

For the first version:

For the second version:

  • RandomAccessIterator is a model of RandomAccessIterator.
  • StrictWeakOrdering is a model of StrictWeakOrdering.
  • RandomAccessIterator's value type is convertible to StrictWeakOrdering's argument type.


  • [first, last) is a valid range.


Linear. Zero comparisons if [first, last) is an empty range, otherwise at most (last - first) - 1 comparisons.


int A[] = {1, 2, 3, 4, 5, 6, 7};
const int N = sizeof(A) / sizeof(int);

assert(!is_heap(A, A+N));
make_heap(A, A+N);
assert(is_heap(A, A+N));


[1] A heap is a particular way of ordering the elements in a range of RandomAccessIterator [f, l). The reason heaps are useful (especially for sorting, or as priority queues) is that they satisfy two important properties. First, *f is the largest element in the heap. Second, it is possible to add an element to a heap (using push_heap), or to remove *f, in logarithmic time. Internally, a heap is a tree represented as a sequential range. The tree is constructed so that that each node is less than or equal to its parent node.

See also

make_heap, push_heap, pop_heap, sort_heap

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