[cfe-dev] Flexible array members in ObjC classes

[David Chisnall via cfe-dev]

On 21 Sep 2017, at 02:51, John McCall via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> 
> Objective-C objects are allocated dynamically, and it's absolutely possible to allocate extra storage in an object and then access that via a trailing flexible or zero-length array.  It is, of course, somewhat perilous, but that's true of flexible/zero-length array members of structs, too.

This is a lot more perilous with the non-fragile ABI.  In classical Objective-C, classes were lowered to structs and so all of these tricks were used (indeed, allocating objects with extra space was explicitly supported by all of the common allocation mechanisms).  Adding a flexible array member at the end was not a problem, because anyone subclassing your class could see the structure layout in the header and know that subclassing was probably dangerous.

The only place where it should be allowed at all is in the last ivar in a class, and the only place it should be allowed in the non-fragile ABI is the last ivar of a class that cannot be subclassed (which, given that objc_subclassing_restricted doesn’t seem to be documented and is not supported by older compilers, is not something that’s easy to enforce).

David