events

A generic event dispatcher and listener implemented in c++

What is this?

A proof of concept for generic type-safe event handling, intended to be an improvement over code like this:

class Foo : public EventListener {

  /* ... */

  void onEvent(const Event* evt) {
    switch (evt.getId()) {
      case A_EVENT:
       AEvent *aEvt = static_cast<AEvent*>(evt);
       // ...
       break;
      case B_EVENT:
       BEvent *bEvt = static_cast<BEvent*>(evt);
       // ...
       break;
       // etc
    }
  }
};

What's so bad about it?

• Switching on Event::getId virtual function is clunky and doesn't benefit from type checking.
• This style encourages a bloated onEvent function when handling many different event types.

The solution

Consider this code:

class Foo : public EventListener<AEvent, BEvent> {

  /* ... */

  void onEvent(const AEvent& evt) {
    // ...
  }

  void onEvent(const BEvent& evt) {
    // ...
  }
};

It's better because:

• The implementation doesn't use run-time type information (check out events.h) so there's no overhead.
• It's explicit and DRY.

It's also type safe -- it will generate friendly compiler errors if you try to misuse it.

Try it out

I built this using g++ 5.2.0 and the following command: g++ events.h example.cpp -std=c++11 -o example -fno-rtti. Convince yourself that this doesn't use RTTI by looking at the last option. ;)