11 Classes [class]

11.8 Member access control [class.access]

11.8.6 Access to virtual functions [class.access.virt]

The access rules ([class.access]) for a virtual function are determined by its declaration and are not affected by the rules for a function that later overrides it.
[Example 1: class B { public: virtual int f(); }; class D : public B { private: int f(); }; void f() { D d; B* pb = &d; D* pd = &d; pb->f(); // OK: B​::​f() is public, D​::​f() is invoked pd->f(); // error: D​::​f() is private } — end example]
Access is checked at the call point using the type of the expression used to denote the object for which the member function is called (B* in the example above).
The access of the member function in the class in which it was defined (D in the example above) is in general not known.