Polymorphic objects on the stack?

Question

In Why is there no base class in C++?, I quoted Stroustrup on why a common Object class for all classes is problematic in c++. In that quote there is the statement:

Answer 1

Bjarne's statement is not correct.

Objects, that is instances of a class, become potentially polymorphic by adding at least one virtual method to their class declaration. Virtual methods add one level of indirection, allowing a call to be redirected to the actual implementation which might not be known to the caller.

For this it does not matter whether the instance is heap- or stack-allocated, as long as it is accessed through a reference or pointer (T& instance or T* instance).

One possible reason why this general assertion slipped onto Bjarne's web page might be that it is nonetheless extremely common to heap-allocate instances with polymorphic behavior. This is mainly because the actual implementation is indeed not known to the caller who obtained it through a factory function of some sort.