Question

I would appreciate help debugging some strange behavior by a multiset container. Occasionally, the container appears to stop sorting. This is an infrequent error, apparent in only some simulations after a long time, and I’m short on ideas. (I’m an amateur programmer–suggestions of all kinds are welcome.)

My container is a std::multiset that holds Event structs:

typedef std::multiset< Event, std::less< Event > > EventPQ;

with the Event structs sorted by their double time members:

struct Event {

 public:
explicit Event(double t) : time(t), eventID(), hostID(), s() {}
Event(double t, int eid, int hid, int stype) : time(t), eventID( eid ), hostID( hid ), s(stype) {}

  bool operator < ( const Event & rhs ) const {
    return ( time < rhs.time );
  }

  double time;
  ...
};

The program iterates through periods of adding events with unordered times to EventPQ currentEvents and then pulling off events in order. Rarely, after some events have been added (with perfectly ‘legal’ times), events start getting executed out of order.

What could make the events ever not get ordered properly? (Or what could mess up the iterator?) I have checked that all the added event times are legitimate (i.e., all exceed the current simulation time), and I have also confirmed that the error does not occur because two events happen to get scheduled for the same time.

I’d love suggestions on how to work through this.

The code for executing and adding events is below for the curious:

  double t = 0.0;
  double nextTimeStep = t + EPID_DELTA_T;
  EventPQ::iterator eventIter = currentEvents.begin();

while ( t < EPID_SIM_LENGTH ) {

     // Add some events to currentEvents

     while ( ( *eventIter ).time < nextTimeStep ) { 

         Event thisEvent = *eventIter;
     t = thisEvent.time;
     executeEvent( thisEvent );
     eventCtr++;
     currentEvents.erase( eventIter );
     eventIter = currentEvents.begin();

  }

  t = nextTimeStep;
  nextTimeStep += EPID_DELTA_T;
}


void Simulation::addEvent( double et, int eid, int hid, int s ) {
  assert( currentEvents.find( Event(et) ) == currentEvents.end() );

  Event thisEvent( et, eid, hid, s ); 
  currentEvents.insert( thisEvent );
}

---

I should add that occasionally an event, when executed, will delete other events from currentEvents. This is done with

double oldRecTime = 10.0; // gets defined legitimately in simulation
EventPQ::iterator epqItr = currentEvents.find( Event(oldRecTime) );
assert( currentEvents.count( Event(oldRecTime) ) == 1 );
currentEvents.erase( epqItr );

Even if this code looks okay, I’d like to know other ways I can examine what’s going on–I’m currently using a lot of asserts() and cout << checks.

Answer 1

In the simulation, where I have commented

// Add some events to currentEvents

events were being added to currentEvents. (Hope that was clear.) If an event was added that happened to belong at the top of the queue, I believe it messed up the iterator pointing to currentEvents.begin(). I reset the iterator immediately before the inner while loop, and things appear to work.

I will update this question if this turns out not to be the solution, or if there are other problems in what I have here.

Thanks to all those who commented; it helps me learn how I should be approaching these problems.