The OP's title is a bit confused. Things like access control have little to do with the object model: that's a feature of the access control system which is a compile time feature. The sequencing of object construction is part of the object model. The key difference between C++ and Java is actually that Java is garbage collected, whereas C++ is not.
Another core differences is that Java uses archaic crud for polymorphism, whereas C++ uses a broken template concept: in C++ templates are compile time and don't impact the object model, in Java polymorphism is hacked around what in C++ is called a dynamic cast.
Another major core difference is that Java supports threads whereas in C++ you have to use a library of some kind.
There are lower level type system differences too: Java has minimal static typing but otherwise is basically a dynamic language. C++ on the other hand is almost entirely a statically typed language, with the exception of exceptions ..:)
