An example would be non-fragile ABIs (ObjC has this, C++ doesn't) or typestate/built in state machines (so you can't call methods if the object is in an invalid state for them). There's encapsulation at least, it's a start.
Fragile ABI is basically a C++ problem. Java has had a stable OO ABI for 25 years now. The .NET ABI has gone through a few iterations, but is stable now. Of course Microsoft also had COM, which is/was a stable OO ABI accessible to many languages including C++ and was arguably just a subset of the C++ ABI without templates.
In the AOT compiled work Swift now has a stable OO ABI (sort of). Rust is interested in getting one. Lack of a stable ABI is not an OO problem but more a problem of languages that are designed to compile ahead of time to native code and use native formats as their primary artifact format.
Most features OO has reduce complexity and bugs, that's why people use them. There's no comparison between a clean OO API and the de-facto standard state before that, e.g. POSIX, Win32, Carbon.
C++ is more of a multi paradigm language where the designers (eg Alexandrescu) actually publish books telling you to focus on algorithms and generic programming rather than object trees. KDE/Qt reinvent messaging on top of the default OO, don’t they? There’s something about slots.
> There's no comparison between a clean OO API and the de-facto standard state before that, e.g. POSIX, Win32, Carbon.
Don’t know about Windows but POSIX and Carbon were as OO as they needed to be - a function where the first parameter is a “file” or “context” is being object oriented. There were actually several attempts to replace Carbon with more OO APIs, that used heavy implementation inheritance, like Taligent and MacApp and they failed because they were too complicated. Cocoa succeeded by using messaging and composition instead like Alan Kay wanted.
Why the past tense, when COM is the main way Windows APIs have been introduced since Windows Vista, as COM took over Longhorn OO ABI ideas?
Nowadays we just call it WinRT.
I would suggest that if you asked people to define the term, Kay’s definition would likely be the single most common (even after normalizing variations in wording).
It would still only be a plurality, sure, but the remaining majority doesn't share a common understanding, either.
