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Yeah, there's a lot of evolution in Obj-C and the compilers here that I'm glossing over. It gets even more funky because Obj-C uses hash table dispatch in the "slow" path which is immune the offset issues. But there's also vtable-style optimizations which lead to issues like you see here, which C++ also shares.

And yes, data structure layout (ivar placement) make things tricky for C, C++ and Obj-C. I've seen some creative solutions to this, especially in C.