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Expertise: Beginner

Nov 29, 2001

Declaring a virtual member function private may be useful when you wish to limit the number of derived classes that can override the given function. In such a case, you declare all the derived classes, which will be able to override the private virtual member function as friends of the base class.

In the following example, only B can override the member function f().

 
class A {
  friend class B;
private:
  virtual int f() { return 1; }
};

class B : public A {
private:
  virtual int f() { return 2; }
};

class C : public A { // will use A::f(), cannot _
override it
};

class D : public B { // will use B::f(), cannot _
override it
};