Chapter 17. Boost.Heap

Boost.Heap could have also been called Boost.PriorityQueue since the library provides several priority queues. However, the priority queues in Boost.Heap differ from std::priority_queue by supporting more functions.

#include <boost/heap/priority_queue.hpp>
#include <iostream>

using namespace boost::heap;

int main()
{
  priority_queue<int> pq;
  pq.push(2);
  pq.push(3);
  pq.push(1);

  for (int i : pq)
    std::cout << i << '\n';

  priority_queue<int> pq2;
  pq2.push(4);
  std::cout << std::boolalpha << (pq > pq2) << '\n';
}

Example 17.1 uses the class boost::heap::priority_queue, which is defined in boost/heap/priority_queue.hpp. In general this class behaves like std::priority_queue, except it allows you to iterate over elements. The order of elements returned in the iteration is random.

Objects of type boost::heap::priority_queue can be compared with each other. The comparison in Example 17.1 returns true because pq has more elements than pq2. If both queues had the same number of elements, the elements would be compared in pairs.

#include <boost/heap/binomial_heap.hpp>
#include <iostream>

using namespace boost::heap;

int main()
{
  binomial_heap<int> bh;
  bh.push(2);
  bh.push(3);
  bh.push(1);

  binomial_heap<int> bh2;
  bh2.push(4);
  bh.merge(bh2);

  for (auto it = bh.ordered_begin(); it != bh.ordered_end(); ++it)
    std::cout << *it << '\n';
  std::cout << std::boolalpha << bh2.empty() << '\n';
}

Example 17.2 introduces the class boost::heap::binomial_heap. In addition to allowing you to iterate over elements in priority order, it also lets you merge priority queues. Elements from one queue can be added to another queue.

The example calls merge() on the queue bh. The queue bh2 is passed as a parameter. The call to merge() moves the number 4 from bh2 to bh. After the call, bh contains four numbers, and bh2 is empty.

The for loop calls ordered_begin() and ordered_end() on bh. ordered_begin() returns an iterator that iterates from high priority elements to low priority elements. Thus, Example 17.2 writes the numbers 4, 3, 2, and 1 in order to standard output.

#include <boost/heap/binomial_heap.hpp>
#include <iostream>

using namespace boost::heap;

int main()
{
  binomial_heap<int> bh;
  auto handle = bh.push(2);
  bh.push(3);
  bh.push(1);

  bh.update(handle, 4);

  std::cout << bh.top() << '\n';
}

boost::heap::binomial_heap lets you change elements after they have been added to the queue. Example 17.3 saves a handle returned by push(), making it possible to access the number 2 stored in bh.

update() is a member function of boost::heap::binomial_heap that can be called to change an element. Example 17.3 calls the member function to replace 2 with 4. Afterwards, the element with the highest priority, now 4, is fetched with top().

In addition to update(), boost::heap::binomial_heap provides other member functions to change elements. The member functions increase() or decrease() can be called if you know in advance whether a change will result in a higher or lower priority. In Example 17.3, the call to update() could be replaced with a call to increase() since the number is increased from 2 to 4.

Boost.Heap provides additional priority queues whose member functions mainly differ in their runtime complexity. For example, you can use the class boost::heap::fibonacci_heap if you want the member function push() to have a constant runtime complexity. The documentation on Boost.Heap provides a table with an overview of the runtime complexities of the various classes and functions.