Basic polymorphism/event receiver

[Moo]

Hi I've tried to make a simple event receiver class to experience how the whole Irrlicht OOP system worked and to practice polymorphism. (this being my reacher). The whole thing compiles fine and I thought I had understood it all, but I get

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Unhandled exception at 0x1010af46 in Client.exe: 0xC0000005: Access violation reading location 0x00000004.

At runtime. Here is my code:

Event.h

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#include "Global.h"
#include <iostream>

class CGameEventReceiver : public IEventReceiver
{
	private:
		IrrlichtDevice* Device;
	public:
		CGameEventReceiver(IrrlichtDevice* device);
		virtual bool OnEvent(const SEvent& event);
};

IEventReceiver* createGameEventReceiver(IrrlichtDevice* device);

Event.cpp

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#include "Event.h"

bool CGameEventReceiver::OnEvent(const SEvent &event)
{
	if (event.EventType == EET_KEY_INPUT_EVENT && !event.KeyInput.PressedDown)
	{
		switch (event.KeyInput.Key)
		{
			case KEY_ESCAPE:
				Device->closeDevice();
			default:
				break;
		}
	}
	return false;
}

CGameEventReceiver::CGameEventReceiver(IrrlichtDevice* device)
{
	Device = device;
}

IEventReceiver* createGameEventReceiver(IrrlichtDevice* device)
{
	CGameEventReceiver er(device);
	return &er;
}

Main.cpp (calling it)

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	IEventReceiver* er = createGameEventReceiver(Device);
	Device->setEventReceiver(er);

Thanks and sorry for not knowing how to use a debugger

[Acki]

this is bad:

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IEventReceiver* createGameEventReceiver(IrrlichtDevice* device)
{
   CGameEventReceiver er(device);
   return &er;
} 

you're creating an event receiver (er) and return a pointer to it, but after the function returns to main er is no longer existant, because it's only local and so only existant inside this function !!!

you could define er as a global, probably not wanted...

or you can do it this way:

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IEventReceiver* createGameEventReceiver(IrrlichtDevice* device)
{
   CGameEventReceiver* er = new CGameEventReceiver(device);
   return er;
} 

but than you'll need to delete the returned pointer when it's no longer needed (usually at program end)...

or you can do it this way:

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IEventReceiver createGameEventReceiver(IrrlichtDevice* device)
{
   CGameEventReceiver er(device);
   return er;
} 

this way you return a copy of er and not a pointer to it...

but I'm not sure why you create this function just for this one creation call ???
why not creating the event receiver directly instead of calling this function ???

[vitek]

or you can do it this way:

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IEventReceiver createGameEventReceiver(IrrlichtDevice* device)
{
   CGameEventReceiver er(device);
   return er;
} 

This code is illegal and bad. It is illegal because you cannot instantiate an abstract class (you can't create an instance of IEventReceiver). It is bad because if this were legal, you'd be slicing the CGameReceiver object.

Travis

[Moo]

Yeah, well I used the new keyword. So here is my class so far:

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class CGameEventReceiver : public IEventReceiver
{
	private:
		CEVENT_STATE mouseStates[2];
		CEVENT_STATE keyStates[KEY_KEY_CODES_COUNT];
		CPROCESS_STATE processState;
		CMouseData mouse;

		IrrlichtDevice* Device;
	public:
		CGameEventReceiver(IrrlichtDevice* device);
		virtual bool OnEvent(const SEvent& event);
		CEVENT_STATE getKeyState(EKEY_CODE key);
		CEVENT_STATE getMouseState(CMOUSE_BUTTON button);
};

IEventReceiver* createGameEventReceiver(IrrlichtDevice* device);

Now I'm struggling with an odd (but probably noobish) problem. When creating my event receiver:

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	IEventReceiver* er = createGameEventReceiver(Device);
	Device->setEventReceiver(er);

I can't use the functions specific to CGameEventReceiver with the IEventReceiver pointer because I'm using it as a base class. So I can use

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er->OnEvent
er->~IEventReceiver

Because they are virtual methods of the abstract base class IEventReceiver, but I can't use

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er->getKeyState
er->getMouseState

because I'm using the IEventReceiver pointer.

So question: How do I use the custom functions of a derived pointer of an instance of an abstract base class? My goal is to use

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er->getKeyState
er->getMouseState

Thanks, and sorry if I'm not clear. I know it's confusing but IDK how to put it clearly.

[Acki]

This code is illegal and bad.

so now I'll get arrested !?!?!
yeah, you're right, I had normal vars in mind...

So question: How do I use the custom functions of a derived pointer of an instance of an abstract base class? My goal is to use

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er->getKeyState
er->getMouseState

you probably mean "from a pointer to an instance of a derived class"...

well, I think this is not what you want:

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IEventReceiver* createGameEventReceiver(IrrlichtDevice* device);

this will create/return the normal (standard) event receiver,

but as far as you want your custom event receiver you'll need this:

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CGameEventReceiver* createGameEventReceiver(IrrlichtDevice* device);

and of course this then:

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CGameEventReceiver* er = createGameEventReceiver(Device);

but if you create the custom receiver inside the function and just returning a pointer to the standard receiver, then you also can try a cast like this:

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((CGameEventReceiver*)er)->getKeyState

[Moo]

Surprisingly, the hackish cast seems to work. Any depreciation on thiscode? Any reason why I shouldn't use it? Looks too good to be true

[Acki]

Surprisingly, the hackish cast seems to work.

it's not surprisingly and not hackish, sometimes you'll have no other choice than doing such a cast...

Any reason why I shouldn't use it?

no, not in this case...
maybe if you do such a cast inside your main loop, because of performance reasons...
I did no benchmark tests, but I can imagine that such a cast is a bit more expensive than using the correct type directly...

[Moo]

Surprisingly, the hackish cast seems to work.

it's not surprisingly and not hackish, sometimes you'll have no other choice than doing such a cast...

Any reason why I shouldn't use it?

no, not in this case...
maybe if you do such a cast inside your main loop, because of performance reasons...
I did no benchmark tests, but I can imagine that such a cast is a bit more expensive than using the correct type directly...

The whole thing is about using it in the main loop. But maybe I could make it typecast only if an event occurs. But I don't know which parameter to use for the OnEvent bool. Example:

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	while(Device->run())
	{
		if(er->OnEvent(----------))
		{
			if(((CGameEventReceiver*)er)->getKeyState(KEY_ESCAPE) == RELEASED)
			{
				Device->closeDevice();
			}
		}
	}

Thanks again

[Acki]

no, you shouldn't call onEvent on your own, it's a callback for events that occures and uses an event as parameter...
I think there is no other way then doing the type cast, beside using the correct type directly...
you could add a flag that specifies if an event has happened, but inside the receiver it would cause the same type cast...
or make this flag global, that would be a workaround...

but I would use the correct type everywhere (changing all the IEventReceiver* to CGameEventReceiver*), it would be much more effective in this case...