Question

I’m mainly a C++ guy. As C++ lacks an official ABI I always use a COM-like approach for component designs that support more than one compiler.

Recently I came across the question whether Objective-C would be a replacement for the COM-like approach. Obviously for Objective-C to be a replacement one would need a stable ABI, therefor I’d like to know if a stable ABI for Objective-C exists (on all major OSes [OSX, GNU/Linux, Windows]) and how easy it would be to use Objective-C(++) as “glue” between components created by different compilers.

EDIT:
As Nikolai Ruhe pointed out a short description of COM may be helpful. COM is essentially a “binary standard” that allows mixing binarys of different compilers (and in a variety of languages). The vehicle of COM are interfaces, which define methods (which map to C++’s virtual functions). Components implement a at least one interfaces and are distributed as DLLs. They can be located anywhere on the system (the position is specified in the Registry) and can be loaded by any COM-client via the ID of the interface they implement.

Answer 1

I can only speak for Apple’s implementation, as I have no experience with the GNU or other ports.

Objective-C relies on C’s ABI for the most part (like function calls and memory layout of structs).

It’s own ABI underwent a couple of changes in Apple’s implementation, like non-fragile instance variables introduced with the “Modern Runtime”, introduction of properties, faster exception handling, garbage collection, __weak support for ARC.

Some of the changes were backwards compatible, some not. But since the whole system and frameworks are provided by Apple and the changes were usually introduced with other non-compatible changes (switch to Intel, and LP64) this was without consequences to users.

Edit: One thing you should have in mind is that Objective-C does not only rely on a fixed ABI but also on a compatible runtime. That’s one more headache to care about for your purpose.