Virtual methods in C++

12-28-2008, 10:37 PM

o harro der

Well I've been working on a small game lately, nothing special. It's my first real project in C++ so it's pretty cliche and seemingly simple. Here's basically what I've been trying to accomplish:

You control a small spaceship. Your spaceship fires two kinds of weapons: linear beams and homing missiles fired respectively with the left and right mouse buttons. You battle an array of different enemies each with different characteristics.

Here's how I went about defining the enemies. I have a parent class called 'ship':

Code:

class ship
{
    public:
        // Some generic members go here like x,y position etc...
        virtual void draw(SDL_Surface* screen) {}
        virtual void loop(void) {}
        virtual void explode(void) {}
};

Now, there is one child class per enemy type:

Code:

class enemy1: public ship
{
    public:
        void draw(SDL_Surface* screen)
        {
            // Draw the enemy here
        }
        void loop(void)
        {
            // Do things with the concerned object.
        }
// etc..
};

I stack all of the enemies up in a 'ship' array called 'ships', run through the array in the main loop and do:

Code:

for(int i = 0;i<COUNT;i++)
{
    ships[i]->loop();
    ships[i]->draw(screen);
}

So when I remove one of the enemies, I have to refresh the stack. Here's what I thought if let's say an enemy was removed from the index 'id':

Code:

COUNT--;
*sh[id] = *sh[COUNT];
delete sh[COUNT];

But apparently, virtual methods don't like getting moved around much and so I was stuck with the virtual methods from one enemy being assigned to the characteristics of another... Any idea on how to fix this?

12-29-2008, 12:06 AM	#2
TheDamien	Code: sh[id] = sh[COUNT]; You shouldn't be dereferencing here. Try this instead: Code: sh[id] = sh[COUNT];

12-29-2008, 12:13 AM	#3
HINDYhat	No, by doing that, I'd mess it all up: Code: COUNT--; sh[id] = sh[COUNT]; delete sh[COUNT]; That would also delete sh[id], since by I'm only equating pointers. I tried without dereferencing and it crashed.

12-29-2008, 12:21 AM

TheDamien

You are right, I must have tunnel vision today.

It should probably look something more like this:

Code:

COUNT--;
delete sh[id];
sh[id] = sh[COUNT];

I can't think how this dereferencing alone could account for this problem but it is almost certainly not what you want to do.

EDIT: Now that I think about it, if the array is a ship* array and sizeof(enemy1) is different to sizeof(ship), then the problem may be that a partial copy results in the type data not being transferred.

12-29-2008, 12:26 AM	#5
HINDYhat	Hah! That worked! Thanks man I've been killing myself over this problem. I guess the solution was pretty simple after all. I was going looking over dynamic casts, RTTI and other stuff I don't understand (note I'm a beginner to this stuff). Anyway, thanks! :)

12-30-2008, 02:39 AM	#6
Deathcom3s	I had no idea we had a programming forum here, that's awesome! Wait, how new are you to C++? Programming a game as a beginner is pretty ambitious.

12-30-2008, 02:33 PM	#7
HINDYhat	Well I know a bunch of programming structure and some of the basics. I just need to apply them. Trial and error by making a game seems to work quite well. :P

12-30-2008, 06:47 PM	#8
Deathcom3s	Yes it does, it's how I got started. It wasn't until after i made my first tetris game that I looked back and actually read tutorials. :P And now i'm working with Craka_J as a C++ Programmer.