On Mon, Mar 10, 2008 at 9:18 PM, Giel van Schijndel <[EMAIL PROTECTED]> wrote:
> > You will find that passing along a field belonging to
> > an object (psDroid->baseObject) and passing along an enlarged version
> > of an object ((BASE_OBJECT *)psDroid) are two very different things,
> > and to code to handle it has to be written differently. Eg BASE_OBJECT
> > *psTile->psObject cannot be cast back to DROID*, you need a pointer
> > back to the parent to do that, either by a void pointer or a union -
> > either way it gets ugly, IMHO.
>
> Actually in this example case you can just downcast baseObj to a DROID*
> without any problem (provided you checked the type properly), as both
> baseObj and droidObj start at the same address.
Just to nitpick a little bit. You cannot downcast, since the memory
address points to the child object. Take this example:
void objBar(BASE_OBJECT *psObj)
{
doBarify(psObj);
if (psObj->type == OBJ_DROID)
{
doBarForDroidToo((DROID *)psObj);
}
}
void droidBar(DROID *psDroid)
{
someOtherBarFunc(psDroid, SOME_SILLY_CONSTANT);
objBar((BASE_OBJECT *)psDroid);
}
If you would pass on psDroid->baseObj to objBar, then the original
memory address would be lost, unless you recorded it somewhere in
baseObj, breaking the abstraction. This reference to the higher level
object from a lower level one is often used in the code base. I think
the neatest way around that would be to keep a (object)->parent field
in every object, eg psDroid->baseObj->parent. This would be eerily
similar to how the tagfile structures work, too ;-)
Not that I would recommend doing this for reasons previously mentioned.
- Per
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