抽象基类中私有虚函数的可见性

Question

Consider following code:

class A {
public:
  virtual void f() const = 0;
  void callf() const { f(); }
};

class B : public A {
  virtual void f() const { }
};

int main()
{
  B x;
  x.callf();

  return 0;
}

Class B derives from the abstract base class A , but "hides" the implemented method f() as private member. Still, the inherited member callf() is able to call f() , which was public in the base class. The code compiles with no warning on g++ 10.1.0 and clang++ 11.1.0.

Is this a legal code, ie, does the inherited callf() correctly see the private member f() ?

Alternatively, would it be possible for the derived class B to implement the purely virtual methods of the base class, such that they can only be called by B (and friends)?

Answer 1

Is this a legal code, ie, does the inherited callf() correctly see the private member f()?

Yes, it is legal code. From the compiler's point-of-view, the callf function is referencing the f function of its own class; the fact that that is a virtual function does not affect the scope (or accessibility) – only the implementation, which will be in the form of a call to a v-table entry. That v-table entry will be correctly interpreted at run time, when it is called via the derived class.