C++98: Priority Queues

Priority Queues let items go to the head of the queue based on priority. Items with the same priority are not necessarily taken in order. Here is an example:

#include <iostream>
#include <queue>

enum Priority
{
  LOW,
  MEDIUM,
  HIGH
};

class Message
{
 public:
  Priority mPriority;
  int      mMessage;

  Message(int message)
  : mMessage(message), mPriority(MEDIUM)
  {}

  Message(int message, Priority priority)
  : mMessage(message), mPriority(priority)
  {}
};

struct PriorityLessThan
{
  bool operator()(Message lhs, Message rhs)
  {
    if ((int)lhs.mPriority < (int)rhs.mPriority)
    {
      return true;
    }
    else
    {
      return false;
    }
  }
};

int main()
{
  std::priority_queue<Message             ,
                      std::vector<Message>,
                      PriorityLessThan    > priorityQueue;

  priorityQueue.push(Message(1        )); // 4th or 5th
  priorityQueue.push(Message(2, HIGH  )); // 1st or 2nd or 3rd 
  priorityQueue.push(Message(3, LOW   )); // 6th or 7th
  priorityQueue.push(Message(4, HIGH  )); // 1st or 2nd or 3rd 
  priorityQueue.push(Message(5, LOW   )); // 6th or 7th
  priorityQueue.push(Message(6, MEDIUM)); // 4th or 5th
  priorityQueue.push(Message(7, HIGH  )); // 1st or 2nd or 3rd 

  for (int i = 0; i < 7; i++)
  {
    std::cout << priorityQueue.top().mMessage << " ";
    priorityQueue.pop();
  }
  std::cout << std::endl;
  return 0;
}
// Output: 2 7 4 6 1 3 5

Reference: Josuttis, Nicolai M., The C++ Standard Library: A Tutorial and Reference. New York: Addison-Wesley, 1999, pp. 453-9.