Question

I have a priority_queue of some object:

typedef priority_queue<Object> Queue; Queue queue; 

From time to time, the priority of one of the objects may change – I need to be able to update the priority of that object in the queue in an efficient way. Currently I am using this method which works but seems inefficient:

Queue newQueue; while (!queue.empty()) {   Object obj=queue.top();   queue.pop();    if (priorityHasChanged(obj))     newQueue.push_back(Object(new_priority));   else     newQueue.push_back(obj); }  newQueue.swap(queue); // this only works because I actually subclassed the priority_queue                  // class and exposed a swap method that swaps in the container 

I implemented it this way because I was in kind of a hurry at the time and this was the quickest thing I could do that I could be sure it would work ok. There has to be a better way than this though. Really what I want is a way to either:

• extract out the instance with the changed priority and insert a new one with the new priority value
• update the instance with the changed priority and then update the queue so that it is correctly sorted

What is the best way to do this?

Answer 1

I think you are out of luck with standard priority queue because you can’t get at the underlying deque/vector/list or whatever. You need to implement your own – it’s not that hard.