In JS for example, if you change the prototype of an object you have effectively changed it's data layout but of course everything is behind a pointer / an indi

In JS for example, if you change the prototype of an object you have effectively changed it's data layout but of course everything is behind a pointer / an indirection. When changing instance or class variables in Smalltalk, the system actually changing the object layout and goes and recompiles code using it. If you are interested in the details: Chapter 5 of [1] explains it a bit. [1] Hirschfeld et al. Assisting System Evolution: A Smalltalk Retrospective. http://soft.vub.ac.be/Publications/2002/vub-prog-tr-02-25.pdf http://soft.vub.ac.be/Publications/2002/vub-prog-tr-02-25.pd...