The guy was referring to the explicit case where bar is a member variable. The cases where it is in the local scope under scoping rules or the global scope are not really an issue, since you can check the function definition to find if it is local. In the case that it is not in the function definition, then it is in the global scope in C. If implicit this were not done in C++, that would also be the case in C++, provided you do the sane thing and use the std namespace for everything. Just thinking about the headaches namespaces could cause when looking for such definitions with cscope gives me yet another reason to stay away from C++ whenever possible.

It absolutely is allowed to be null:

    #include <iostream>
    struct Foo {
      void bar() {
        std::cout << this << std::endl;
      }
    };
    
    int main() {
      Foo *p = nullptr;
      p->bar();
    }

will print 0

Only in so much that this being null is UB, but in the real world it very much can be null and programs will sometimes crash deep inside methods called on nullptr.