Virtual Functions and Scope

We can now understand why virtual functions must have the same parameter list in the base and derived classes (§15.3, p. 605). If the base and derived members took arguments that differed from one another, there would be no way to call the derived version through a reference or pointer to the base class. For example:

class Base {
public:
    virtual int fcn();
};
class D1 : public Base {
public:
    // hides fcn in the base; this fcn is not virtual
    // D1 inherits the definition of Base::fcn()
    int fcn(int);      // parameter list differs from fcn in Base
    virtual void f2(); // new virtual function that does not exist in Base
};
class D2 : public D1 {
public:
    int fcn(int); // nonvirtual function hides D1::fcn(int)
    int fcn();    // overrides virtual fcn from Base
    void f2();    // overrides virtual f2 from D1
};

The fcn function in D1 does not override the virtual fcn from Base because they have different parameter lists. Instead, it hides fcn from the base. Effectively, D1 has two functions named fcn: D1 inherits a virtual named fcn from Base and defines its own, nonvirtual member named fcn that takes an int parameter.