Re: NSSound Reimplementation

[Richard Frith-Macdonald]

On 16 Jul 2009, at 14:37, David Chisnall wrote:

> On 16 Jul 2009, at 14:23, Jamie Ramone wrote:
>
> > I'd like to chime in here and say that this approach IS actually a
> > good idea as :
> >
> > 1 ) it does solve the ABI change breakdown problem and
>
> Except that it doesn't, it just hides it.  Now people subclassing  
> and referencing variables in the superclass need to explicitly cast  
> a pointer to a structure.  If this structure changes, they need to  
> manually update their private copy of the ivars and if they don't  
> thing break in exciting ways.

No ... people subclassing NEVER reference any variables of the  
superclass, because the superclass only contains a single instance  
variable, and that's declared @private.  Because the superclass only  
ever contains the single instance variable, it never changes with  
impementation details, and the ABI doesn't change.

> > 2 ) it is actually much easier to read because it's a prime example  
> > of
> > encapsulation and abstraction, so complexity is hidden.
>
> It is not easier to read, because now you need a separate structure  
> definition, every ivar access has to go via a macro which will look  
> something like this:
>
> #define ivar (((struct private_ivars*)_private)->ivar)

Actually it's

#define private ((PrivateClass*)_private)

Then

   private->ivar = ...

Which can easily be converted to 'self->ivar' or just 'ivar' by a  
global replace when we want to change because we have non-fragile  
ivars available.