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Polymorphism and Lists

Started by medevilenemy Jun 16, 2007 at 8:15 PM 8 replies 900+ views

Original Post

Jun 16, 2007 at 8:15 PM

I'm writing a system for handling ships in a new game of mine, and I've run into a bit of a problem -- mostly with my inexperience with using polymorphism. I have class Ship which inherits from a class Rotatable. Ship is meant to be a base class for more specialized ship classes (IE: Frigate, Corvette, etc). Whenever a new ship is spawned, it is placed into a list of ship objects (list allShips) and I use an iterator (list::iterator currentShip) to deal with each ship one by one. I then call class functions as needed (in the form currentShip->render() etc...). The problem is that when called, the function as defined in Ship is always called, not as defined in the subclass (Frigate, etc). Anyone know what I'm doing wrong? A bit of relevant code is below: Here is the list and ship handler function


list<Ship> Allships;
list<Ship>::iterator currentship;

void HandleShips()
{
    for(currentship = Allships.begin(); currentship != Allships.end(); currentship++)
    {
        currentship->RotateX(0.005); 
        currentship->Render();
    }
}

And here I create a couple of ship objects (of class TestShip) for testing purposes.


Allships.push_back(TestShip(100, 100, 0.0, 0.0, 0.0));
Allships.push_back(TestShip(100, 100, 0.0, 0.1, 0.0));

OrangyTang

Jun 16, 2007 at 8:25 PM

You can't store Ship classes by value and use derived types as well. You'll get all sorts of odd object slicing. Instead have a list of Ship* and explicitly new required type (and remember to delete things when you remove them too).

nobodynews

Jun 16, 2007 at 8:40 PM

If you use Boost Smart Pointers that whole 'keeping track of what to delete thing' becomes a little simpler. Or so I hear.

medevilenemy

Jun 16, 2007 at 9:52 PM

I've read that, but in that case I have to dereference the pointer differently (as in a modification to the handleships function). How do you suggest I do that? Perhaps create a temporary copy locally?

[edit] Oh, and deleting the ship objects shouldn't be a problem, I fully intended to manually remove them upon their destruction/game exit.
[edit] Without changing the means of dereferencing, I get the following error: "RotateX has not been declared" and "Request for member of non-aggregate type before '(' token"

[Edited by - medevilenemy on June 16, 2007 10:52:28 PM]

medevilenemy

Jun 17, 2007 at 12:12 AM

[update] I changed the HandleShips() function above to the following

void HandleShips(){    Ship *cship;    for(currentship = Allships.begin(); currentship != Allships.end(); currentship++)    {        cship = *currentship;        cship->RotateX(-dx/1000);        cship->Render();    }}

and it seems to work fine with no errors, warnings, or apparant bugs. So thats that I suppose.

[edit] strike that... it runs, but it is still calling the functions from Ship and not the overridden functions from TestShip.

My_Mind_Is_Going

Jun 17, 2007 at 2:31 AM

Maybe I'm being dumb and not understanding what's going on here because nobody has suggested this but have you declared the function in the base class as virtual? If you don't declare it virtual and then override in a derived class, the version of the function you'll end up calling will be determined by the type of pointer you're using although I don't really know that much about all this STL stuff so maybe that's not an issue here...

medevilenemy

Jun 17, 2007 at 7:41 AM

Good Idea, I'll try that, though I don't know if it will work properly with evreything (ie: render is defined inherited all the way from Rotatable and I dont know if virtual would have the same effect two generations later).

medevilenemy

Jun 17, 2007 at 7:43 AM

I stand thoroughly corrected, it works perfectly. Now I should be able to get specialized behavior type stuff working. Thanks again!

Captain P

Jun 17, 2007 at 5:13 PM

If you're calling delete on base class pointers, while you might be dealing with derived class objects, then make sure the base class destructor is also virtual - otherwise, only the base class destructor is called, resulting in a half-destructed object. Which likely falls into the realm of undefined behaviour, and that's not what you want. :)

medevilenemy

Jun 17, 2007 at 7:32 PM

Sure, I'll keep that in mind.

Kudos for another problem completely and effectively squashed [grin]. Gamedev++

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