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Notes

[1] Priority queues are a standard concept, and can be implemented in many different ways; this implementation uses heaps. Priority queues are discussed in all algorithm books; see, for example, section 5.2.3 of Knuth. (D. E. Knuth, The Art of Computer Programming. Volume 3: Sorting and Searching. Addison-Wesley, 1975.)

[2] This restriction is the only reason for priority_queue to exist at all. If iteration through elements is important, you can either use a vector that is maintained in sorted order, or a set, or a vector that is maintained as a heap using make_heap, push_heap, and pop_heap. Priority_queue is, in fact, implemented as a random access container that is maintained as a heap. The only reason to use the container adaptor priority_queue , instead of performing the heap operations manually, is to make it clear that you are never performing any operations that might violate the heap invariant.

[3] One might wonder why pop() returns void, instead of value_type. That is, why must one use top() and pop() to examine and remove the element at the top of the priority_queue, instead of combining the two in a single member function? In fact, there is a good reason for this design. If pop() returned the top element, it would have to return by value rather than by reference: return by reference would create a dangling pointer. Return by value, however, is inefficient: it involves at least one redundant copy constructor call. Since it is impossible for pop() to return a value in such a way as to be both efficient and correct, it is more sensible for it to return no value at all and to require clients to use top() to inspect the value at the top of the priority_queue.


New members | Standard Template Library Programmer`s Guide | See also







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