Re: priority_queue help

From:
"Bo Persson" <bop@gmb.dk>
Newsgroups:
comp.lang.c++
Date:
Tue, 14 Oct 2008 00:37:32 +0200
Message-ID:
<6li0t7FcghbsU1@mid.individual.net>
Joe, G.I. wrote:

Ok, yes, I have higher level language backgrounds,


Java, right?

but my c++ books
also use 'new' so I'm a bit confused here. Is everywhere I'm using
'new' here not correct? I've added more detail here to clarify what
I can and tried to make the priority_queue changes.


C++ also has a new, but it is more unusual. There is no garbage
collection in the language, so you should also match each new with a
delete.

This all compiles, but crashes so I know I've got something wrong
w/ the new queue.

App::App()
{
    pq = new EventQueue();
}


This looks very much like Java. In C++ you don't have to allocate
everything dynamically.

// Here I generate my Event classes w/ timestamps that will expire
soon. App::init()
{
       for (int i = 0; i < 10; i++) {
            Event *event = new Event();
            event->set_tstamp();

            pq->push_event(event);
        }
}

// Now I need to find the event w/ the smallest timestamp.
// And this actually is in a while loop of sorts.
void EventListener::check_queue()
{
    if (pq->count() > 0) {
        Event *evnt = pq->top_event();

        if (evnt->is_expired()) {
            app.do_this(evnt->id());

            pq->pop_event();


Here you will lose you object. The pointer is destroyed when popped,
but the pointed-to object is not.

        }
    }
}

// Here's what's now in my Event.h
class Event
{
    private:
          ...
        long _id;

    public:
        Event();
        ~Event();

        double _exec_time;

        long id();
        bool set_id(long idx);
        bool is_expired();
        bool set_tstamp();
};

struct EventPointerCompare {
    bool operator() ( Event* lhs, Event* rhs ) const {
        return ( lhs->_exec_time < rhs->_exec_time );
    }
};


I can't see that you are using this anywhere. The default for the
priority_queue is to sort elements stored - the pointers. the priority
will then be the address of the Events - random!

// EventQueue.h
// my priority_queue has to be checked for expired events and
// there are cases where I want to know what type of event it is ...
// so that is why it's in a wrapper. not ok?
class EventQueue
{
    private:
        priority_queue<Event *> pq;

    public:
        EventQueue();
        EventQueue(int queue_size);
        ~EventQueue();

        int count();
        bool push_event(Event *event);
        bool pop_event();
        Event* top_event();
        bool execute_event();
        bool check_queue();
        bool contains_typeA_events();
};

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