Unhandled Exception due to Class slicing - c++

Question

I'm getting an unhandled exception reading location 0x00000008 (reading NULL value) on the noted line below, relevant methods leading up to the error are included (continued below examples):
Event Methods:
Event::Event(Event::EVENTTYPE type) : eventType(type) { }
KeyEvent Methods:
class KeyboardKeyEvent : public Event {
public:
//...
int GetKey() const;
protected:
//...
};
int KeyboardKeyEvent::GetKey() const {
return this->_scancode; //Errors out here. "this" returns 0x000000
}
KeyboardKeyEvent::KeyboardKeyEvent(int key, Event::EVENTTYPE type) : Event(type), _scancode(key) { }
KeyDownEvent Methods:
KeyboardKeyDownEvent::KeyboardKeyDownEvent(int scancode) : KeyboardKeyEvent(scancode, Event::KEYBOARD_KEYDOWN) { }
Event Handler Methods:
bool EventHandler::EnqueueEvent(Event* event) {
if(event == NULL) return false;
try {
this->_eventQueue.push(event);
} catch (...) {
return false;
}
return true;
}
Event* EventHandler::DequeueEvent() {
if(this->_eventQueue.empty() == false) {
Event* result = new Event(*this->_eventQueue.front());
delete this->_eventQueue.front();
this->_eventQueue.pop();
return result;
}
return NULL;
}
Main Loop Sequence:
if(_eh->HasEvents()) {
Event* nxtEvent = _eh->DequeueEvent();
switch(nxtEvent->GetType()) {
case Event::KEYBOARD_KEYDOWN:
allegro_message("You pressed the %d key!", dynamic_cast<KeyboardKeyDownEvent*>(nxtEvent)->GetKey());
break;
default:
/* DO NOTHING */;
}
delete nxtEvent;
nxtEvent = NULL;
}
I know this is a slicing problem I just don't see why it's happening or how to fix it (Actually, now that I think about it, it's probably a "Can not convert to requested type" error). All throughout when I step through the program _scancode is the appropriate value, but the second the line dynamic_cast<KeyboardKeyDownEvent*>(nxtEvent)->GetKey() runs it throws the error. Double casting as dynamic_cast<KeyboardKeyDownEvent*>(dynamic_cast<KeyboardKeyEvent*>(nxtEvent))->GetKey() fails with the same error as well.
EDIT:
After some tweaking, this variant works perfectly:
if(_eh->HasEvents()) {
switch(_eh->PeekEvent()->GetType()) {
case Event::KEYBOARD_KEYDOWN:
allegro_message("You pressed the %s key!", scancode_to_name(dynamic_cast<KeyboardKeyDownEvent*>(_eh->PeekEvent())->GetKey()));
break;
case Event::MOUSE_BUTTONDOWN:{
Mouse::BUTTONS btn = dynamic_cast<MouseButtonDownEvent*>(_eh->PeekEvent())->GetButton();
if(btn == Mouse::BUTTON2) {
allegro_message("You pressed the %d button!", dynamic_cast<MouseButtonDownEvent*>(_eh->PeekEvent())->GetButton());
}
}
break;
default:
/* DO NOTHING */;
}
}

Answer 1

One solution to avoid slicing is to make the destructor of base class virtual, so in your case you can make ~Event() virtual:
class Event
{
public:
//...
virtual ~Event() {}
};
By the way, I'm wondering why you do the following:
//YOUR CODE : its causing the problem!
Event* EventHandler::DequeueEvent() {
if(this->_eventQueue.empty() == false) {
Event* result = new Event(*this->_eventQueue.front()); // WHY?
delete this->_eventQueue.front(); //WHY?
this->_eventQueue.pop();
return result;
}
return NULL;
}
Why don't you simply do this:
//Use it. Because it should not cause that probem
Event* EventHandler::DequeueEvent() {
if(this->_eventQueue.empty() == false) {
Event* result = this->_eventQueue.front();
this->_eventQueue.pop();
return result;
}
return NULL;
}

Answer 2

In Event* EventHandler::DequeueEvent() you have line
Event* result = new Event(*this->_eventQueue.front()); Here the slicing occurs.
You can do the following:
class Event {
public:
virtual Event* clone() {
// create a new instance and copy all the fields
}
}
Then override clone() in derived classes, e.g.
class KeyboardKeyEvent :public Event {
public:
...
virtual KeyboardKeyEvent* clone(); // note - it returns different type
}
Then change Event* EventHandler::DequeueEvent() :
Event* result = (*this->_eventQueue.front()).clone();

Answer 3

Your DequeueEvent method will always return an Event object, not any of the sub-classes that you are expecting.
Event* result = new Event(*this->_eventQueue.front());
Your Dequeue event should either return the actual reference it is caching, or your base Event class need to provide some sort of virtual copy operation that will provide a real clone.

Answer 4

Why are you copying the Event when you remove it from the queue? That's what is doing the slicing, since you're constructing the base class. Instead, return the pointer that was on the queue to the user.
As noted above, Event should have a virtual ~Event(), so that the recipient of the event can delete it properly. Otherwise, the concrete class destructor will not be properly run.
Event* EventHandler::DequeueEvent() {
if(this->_eventQueue.empty() == false) {
Event* result = this->_eventQueue.front();
this->_eventQueue.pop();
return result;
}
return NULL;
}