Simulating FIFO behaviour with priority queues for equal priorities

In priority queue, the C++ specifications does not define the behaviour of the algorithms when equal-priority items are involved. It is assumed that the ordering is not important in case of a priority queue as long as the same priority items are in correct order as compared to other priorities. For a programmer though this can be important as he expects FIFO ordering for same priority items. To overcome the problem I mentioned in earlier program, we can add another item to get correct FIFO ordering for same priority items.

This is just one approach and there may be other approaches. Also notice that I have used a local variable which requires the order to be input. You can also use the static variable approach for this.

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
//Example of slightly advanced priority queue making sure that
//along with priorities, queue functionality of FIFO is maintained
#include <iostream>
#include <string>
#include <vector>
#include <queue>

using namespace std;

class PrioritizedWord
{
private:
  int m_prio;
  string m_word;
  int m_order; //To set FIFO operation

public:
  explicit PrioritizedWord()
  { }
  explicit PrioritizedWord(const std::string& word, const int priority = 0, const int order = 0): 
                                   m_word(word), m_prio(priority), m_order(order)
  { }
  //Operator overloading for priority comparison
  const bool operator <(const PrioritizedWord& pW) const
  {
    if(m_prio == pW.m_prio)
      return (m_order > pW.m_order);
    else
      return (m_prio < pW.m_prio);
  }
  //Operator overloading to print output 
  friend ostream& operator <<(ostream& out, const PrioritizedWord& pW)
  {
    out<<pW.m_word;
    return out;
  }
  int getPrio(void)
  {return m_prio;}
  string getWord(void)
  {return m_word;}
};

int main()
{
  priority_queue<PrioritizedWord> zahidQueue;

  //Note the final order will not be the same as the specs do not
  //define behaviour in case two numbers with equal priority exist
  zahidQueue.push(PrioritizedWord("First", 23, 1));
  zahidQueue.push(PrioritizedWord("Second", 23, 2));
  zahidQueue.push(PrioritizedWord("Third", 23, 3));
  zahidQueue.push(PrioritizedWord("Fourth", 51, 4));
  zahidQueue.push(PrioritizedWord("Fifth", -1, 5));

  cout<<"\n\nZahid Queue elements :"<<endl;
  while(!zahidQueue.empty())
  {
    cout<<zahidQueue.top()<<" "<<endl;
    zahidQueue.pop();
  }

  return 0;
}

The output is as follows: