queuing events with different kind of partners

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I have an event struct like that:

C++

struct Event
{
    public:
        double time;
        Ball partners[2];
};

I check collision times of pool balls. I'm adding all possible events to a list. If partners were only balls it would be easy. However i need to check edges of table. So i think i need something like that:

C++

Partner partners[2];

But how? If I inherit both ball and edge class from a partner class I loose information when transferring back.

Any suggestion?

Posted 10-Jul-13 1:06am

maxc900

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nv3 · Accepted Answer · 2013-07-10T01:34:00

Solution 1

The alternative is to make Event a base class and derive multiple types of events. The base class would just contain the time member and perhaps an event type. The derived classes bring in the rest.

Don't forget to make the destructor of that Event base class virtual!

Posted 10-Jul-13 1:34am

nv3

Comments

maxc900 10-Jul-13 8:17am

but can I add to my eventlist different types of events?

and this may be a silly question but why virtual destructor? i'm just a scientist away from c++ :)

nv3 10-Jul-13 8:38am

Yes, you can add multiple event types to your event list, as long as they are all derived from your base class Event. And all general code that just deals with arbitrary events from your event list would use virtual functions of the Event class to perform operations. For example, one of the virtual functions you might want to define is a GetEventName() function that returns some kind of type-specific name. You would then override this virtual function in each of your derived classes and return the desired name for that class. That whole concept is call polymorphy and is one of the key features of object oriented languages like C++.

The destructor of a class that is intended to be derived from, i.e. a base class, should have a virtual destructor. The reason is that all your derived classes will have different length and will need to do different things in their destructors. For example, your EventBalls class will need to call the destructor for the embedded Ball object, and EventTable's destructor would need to call some other destructors. Therefore, you should tell your base class that the destructor function is virtual and that the respective destructor of the derived class needs to be called.

maxc900 10-Jul-13 8:44am

This is gold for me. I appreciate your help. Thank you very much.

nv3 10-Jul-13 9:03am

You are welcome!

maxc900 10-Jul-13 10:44am

i'll disturb you once again.

i did what you told me. my ball_event and edge_event derive from event class. I made an eventlist (list<event>). I can add both ball_events and edge_events to that list however I can access only members of event class (type and time). I cannot determine which are partners.

nv3 10-Jul-13 11:42am

When you have pointer to an Event (baseclass) you can either use all its virtual functions (which is the normal way of using polymorphic objects) or you can add a member m_type to the baseclass, so that you can recognize, which type of event this is. You have to fill that m_type member by your own code.

Once you know, which type of event it is, you can cast the pointer to the derived type, for example:

EdgeEvent* pEdgeEvent = dynamic_cast<edgeevent*> (pEvent);

That dynamic cast will deliver a Null pointer when pEvent was not really pointing to an object of type EdgeEvent. This nice feature is only available when you enable runtime typing in your compiler options.

Alternatively you can use a simple cast and total rely on your m_type member:

EdgeEvent* pEdgeEvent = (EdgeEvent*) pEvent;

maxc900 10-Jul-13 11:47am

in addition to your solution, i was wondering in the web, saw something.

first i created a an event class using template for partners. then i'll add different instance of this class into a list (or set maybe) using boost::any.

what do you think?

nv3 10-Jul-13 12:09pm

No, templates will not help you much in this situation. You need true runtime polymorphy.

maxc900 10-Jul-13 13:16pm

that worked. thank you again.

nv3 10-Jul-13 15:22pm

Great, glad you got the hang on it. Once you done a case like that you have understood a great deal of object oriented programming and made a big step forward. You will find the same situation over and over again and now you have the right tool to solve those.