[cfe-dev] Flexible array members in ObjC classes

[Nat! via cfe-dev]

David Chisnall schrieb:
> On 21 Sep 2017, at 10:07, Nat! via cfe-dev<cfe-dev at lists.llvm.org>  wrote:
>> My second reaction, was to think that there are class cluster classes with allocate a flexible amount of memory (say string classes) that aren't ususally subclassed. Who am I to say, that using a convenient flexible array there can't be done (anymore) ?
>
> This is more or less how a number of the GNUstep Foundation classes work, using the extra data allocated with class_createInstance() and doing some pointer arithmetic.  Flexible array members would possibly simplify the code, but they’re not needed for this to work.
>
>> I would think a warning would be nice for classes who try to subclass a class with a flexible array ivar if they add another property or ivar.
>
> The problem is that you can’t do this with the non-fragile ABI.  You can write the flexible array member in the @implementation of a class and the person writing the subclass won’t see it.
>
> To make this safe, the minimum set of constraints that you’d need would be:
>
>   - The ivar is the last one
>   - The ivar is declared in the @interface
>   - The class does not declare any ivars in the @implementation
>   - Subclassing a class whose @interface ends with a flexible member is an error
>

That'd mean you couldn't write a subclass that doesn't add an ivar. Or a 
subclass that overrides the flexible allocation methods but adds some 
ivars and does something completely else. That's why I think a warning 
would be nice, but not an error.

Obviously I am just speaking from my fragile-ABI only perspective :)

Ciao
    Nat!