construction of extern objects vs. "normal ones"

[ssouffri]

I'm seeing some strange behavior I can't explain. I have the following code:

in eventq.hh

class EventQueue;       // forward declaration

extern EventQueue mainEventQueue;
...
class EventQueue : public Serializable
{
  private:
    std::string objName;
    Event *head;

    void insert(Event *event);
    void remove(Event *event);

  public:
    typedef std::vector<EventQueue *>  EventQueueSet;
    static EventQueueSet eventQueueSet;

    EventQueue(const std::string &n)
        : objName(n), head(NULL)
    {
    	eventQueueSet.push_back(this);
    	std::cout << "EventQueue(" << n << ") = " << this << "\n";
    }
...
};

in eventq.cc

...
EventQueue mainEventQueue("Main Event Queue");
...

printout from my main (all the other EventQueues are created dynamically)

EventQueue(Main Event Queue) = 0xde3f90
...
EventQueue(system_EventQueue) = 0x1341660
EventQueue(instruction-based event queue) = 0x138dff0
EventQueue(load-based event queue) = 0x138e070
...
iterating over eventQueueSet:
######## (EventQueue=0x1341660) != (mainEventQueue=0xde3f90), eventQueueSet.size()=3
######## (EventQueue=0x138dff0) != (mainEventQueue=0xde3f90), eventQueueSet.size()=3
######## (EventQueue=0x138e070) != (mainEventQueue=0xde3f90), eventQueueSet.size()=3

Now I would expect the code in the constructor to get executed. However when I test this I only see a printout "EventQueue(Main Event Queue) = 0xde3f90" but mainEventQueue doesn't seem to be added to eventQueueSet. But since I see the printout I would think the constructor got executed. So it appears as if I the eventQueueSet got overwritten with a new EventQueueSet after the mainEventQueue was constructed.

How do get the mainEventQueue pointer in the eventQueueSet.

Side note:
I also tried "typedef std::set<EventQueue *> EventQueueSet;" instead of std::vector but then I got a segmentation fault. Both issues probably have the same underlying problem.

[Lindley]

Hmm. You're relying on the set being initialized prior to the global EventQueue. I don't think C++ offers ordering guarantees on initialization of global objects.

[Paul McKenzie]

I'm seeing some strange behavior I can't explain.

extern EventQueue mainEventQueue;
//...
class EventQueue : public Serializable
{
  //...
static EventQueueSet eventQueueSet;
//...
};

Which one of the variables in red gets initialized first? The answer is that there is no answer -- the order of globals and static initialization is undefined, unless all of your variables are defined in one source unit (then the order is the order of declaration).

Since the "extern" variable is defined in another source unit, you're in big trouble. The static may not have been initialized yet, but you're trying to do a push_back() onto it, causing all sorts of problems.

Regards,

Paul McKenzie