[01/18] windowscodecs: Do not assume that vtable is the first element of the object.

Jacek Caban jacek at codeweavers.com
Tue Jul 24 11:12:26 CDT 2012

On 07/24/12 17:47, Dmitry Timoshkov wrote:
> Vincent Povirk <madewokherd at gmail.com> wrote:
>>>> I disagree with many of the choices in this series (for example, I
>>>> think casting an interface pointer to an inherited interface is fine,
>>>> and replacing that with QueryInterface just adds unnecessary
>>>> additional logic), but I don't think any semantics have been changed
>>>> and overall it's a good cleanup.
>>> Please also consider that many of the objects may be implemented by
>>> a someone's else codec dll, and relying on an implementation detail
>>> is not acceptable in that case.
>> An implementation where the two approaches work differently would be
>> wrong. Every interface with IWICBitmapSource methods should implement
>> them in the same way. In cases where it matters we "should" match
>> native which could be doing either.
> Requesting somebody how he "should" implement things instead of calling
> QueryInterface() when appropriate and avoiding potentially harmful casts
> sounds like a not very nice stance. Arguing that native implementation
> may be doing it wrong doesn't help either.

Casts to inherited interfaces are the kind of casts that are not bad at
all. The child interface is perfectly good implementation of parent
interface by definition. It's just C that can't reflect it in its typing
system. So, while we want to avoid casts as much as possible, we need to
live with the fact that we sometimes need to cast inteface pointer to
its inherited parent.

BTW, changing them to QueryInterface call is not avoiding any cast. It
actually adds more of them in form of cast to void** in caller and
assignment to void* in QI implementation (not to mention runtime overhead).

Obviously many other casts you're fixing should indeed be fixed.


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