Everything OOP works well in Smalltalk (and most of it does in Python), but you pay dearly for that in efficiency - which is what this article as about.
The only part of my response that would have effected was the aside about Smalltalk-style “become”, not the main point about the problem being one of constructing an inheritance heirarchy based on relations of immutable entity and using it for mutable entities with mutation contracts that don’t observe the same is-a heirarchy.
I agree, but that's the thing: It is non idomatic to write immutable Shape-style classes in C++. Possible, yes. Idiomatic, no.
IIRC in Stroustroup's book (the one I read a decade ago, anyway), he concludes that Rect:Square and Circle:Ellipse can not inherit from each other in either direction, and did not suggest switching to immutable everything.
Everything OOP works well in Smalltalk (and most of it does in Python), but you pay dearly for that in efficiency - which is what this article as about.