[Rusky]

The ABI discussed here is a bit higher level than that. Even with a fixed ABI for vtables, calling conventions, etc. you still have to care about what happens when you change a "vocabulary type" in the standard library- some changes are source-compatible but binary-breaking.

For example, C++11 broke ABI at this level by changing the representation of std::string.

[olliej]

Oof I was unaware of the string ABI break (just had to look it up) - that’s kind of gross :-/

That said in general ABI compatibility is expected of all changes, without very good reasons - security seems like a good one, performance alas not - I assume std::string got the small string optimization because they were already going to break ABI for the threading issues.

Of course it doesn’t help that c++ is still C like so implementations/use of data layout gets compiled directly into the client application :-/