[Development] QtMultimedia BIC / header cleanliness issue

[Thiago Macieira]

On segunda-feira, 24 de dezembro de 2012 06.30.58, Sascha Cunz wrote:
> > Adding a new virtual implies changing the layout of the virtual table. If
> > we append the virtual, the order of the existing virtuals should not
> > change, so the calls should still make through.
> 
> Given the two classes:
> 
> class A {
> public: virtual void a();
> };
> 
> class B : public A {
> public: virtual void b();
> };
> 
> You're effectively saying here that adding a virtual to A doesn't change the
> v-table layout of A (which by itself is right), but it implicitly changes
> the v-table layout of B (B::b()'s address-slot moves one slot behind).

Correct.

> > Since the virtual destructor is missing, one can assume that objects of
> > those classes are never deleted through those specific classes anyway.
> > Moreover, it seems that those classes are often used in multiple
> > inheritance and are not in the primary inheritance path. That means the
> > addition of virtuals will not change the primary virtual table.
> 
> What is the effect to:
> 
> class UserClass : public QObject, WhereVTableWouldBeExpanded,
> AnotherInterface {}
> 
> I think to remember to see that compilers produce code that expects the
> vtable of subobjects at a fixed offset from the beginning of the overall
> vtable.

Correct too.

> But strictly spoken, I would rather say no: BC is BC no matter what silly
> mistakes it includes (I have actually had to deal with code that used the
> above mentioned inheritance layouts).
> OTOH, it's still very early in the life of released Qt5.

We can change the soname for the library. libQt5Multimedia.so.6.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center
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