Thinking in C++ Vol 2 - Practical Programming
Prev	Home	Next

Heap operations

A heap is an array-like data structure used to implement a priority queue, which is just a range that is organized in a way that accommodates retrieving elements by priority according to some comparison function. The heap operations in the standard library allow a sequence to be treated as a heap data structure, which always efficiently returns the element of highest priority, without fully ordering the entire sequence.

As with the sort operations, there are two versions of each function. The first uses the object s own operator< to perform the comparison; the second uses an additional StrictWeakOrdering object s operator( )(a, b) to compare two objects for a < b.

Turns an arbitrary range into a heap.

Adds the element *(last-1) to the heap determined by the range [first, last-1). In other words, it places the last element in its proper location in the heap.

Places the largest element (which is actually in *first, before the operation, because of the way heaps are defined) into the position *(last-1) and reorganizes the remaining range so that it s still in heap order. If you simply grabbed *first, the next element would not be the next-largest element; so you must use pop_heap( ) if you want to maintain the heap in its proper priority-queue order.

This could be thought of as the complement to make_heap( ). It takes a range that is in heap order and turns it into ordinary sorted order, so it is no longer a heap. That means that if you call sort_heap( ), you can no longer use push_heap( ) or pop_heap( ) on that range. (Rather, you can use those functions, but they won t do anything sensible.) This is not a stable sort.

Thinking in C++ Vol 2 - Practical Programming
Prev	Home	Next