Re: Clean ways to identify derived class types.

Stuart Golodetz <sgolodetz@NdOiSaPlA.pMiPpLeExA.ScEom>
Fri, 03 Jul 2009 00:25:47 +0100
JC wrote:

I'm designing an application that uses a simple event-based model for
passing messages between objects. It's a basic "observer" setup. I
have it set up something like this (I'm just typing these here,
leaving a lot out I know):

class Event {

class EventListener {
  virtual void onEvent (const Event &);

class EventSource {
  void addListener (EventListener *);
  void notifyListeners (const Event &);

Specific events (possibly with extra event-specific info) are derived
from Event. Note that an EventListener receives all types of events
that an EventSource generates -- it's all-or-nothing rather than
registering for specific types of events. This is important to me and
simplifies a lot of the logic throughout the application.

So, here is my question. In the various implementations of
EventListener::onEvent, some of the EventListeners handle a lot of
different event types, and the implementations end up looking rather
ugly, sort of like (again just typed here, pardon any errors):

void SomeEventListener::onEvent (const Event &e) {

  const AnEvent *a = dynamic_cast<const AnEvent *>(&e);
  if (a) {

  const OtherEvent *b = dynamic_cast<const OtherEvent *>(&e);
  if (b) {

  // and so on...


Are there other good ways to do this? I'm pretty much asking just out
of curiosity, as the above method actually does work adequately, even
though it's sort of painful to look at. Fortunately, in this
particular application, performance penalties of dynamic_cast are
negligible and not an issue, but what if performance did matter --
would there still be a way to keep the flexibility of EventListeners
not having to register for specific event types?

One obvious solution is to have type ID numbers, unique to each event
type, with a virtual int getType() or some such. However, I don't
think that's really a good solution here -- I think it will be a
maintenance problem in the future if new events are added, to ensure
uniqueness of the IDs (unless IDs are noted in a document somewhere,
which I guess is OK, or if they're all declared in some common header
or even assigned dynamically on first access, which works but breaks
encapsulation a bit). I could use ID strings with the same name as the
class to ensure uniqueness, but *if* the goal was performance, I'm not
sure if I'd be comfortable with string compares every time.


Sounds like double dispatch might be what you're after, in which case
look at:


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