39. Mistakes and pain caused by careless overriding of virtual and non-virtual functions

#include <iostream>

class Base {
public:
    virtual void vf() const
    {
        std::cout << "Base::vf\n";
    }

    void f() const
    {
        std::cout << "Base::f\n";
    }
};//class Base

class Derived : public Base {
public:
    // Attempt to "override" virtual:
    virtual void vf()
    {
        std::cout << "Derived::vf\n";
    }

    // Override non-virtual:
    void f() const
    {
        std::cout << "Derived::f\n";
    }
};//class Derived

Functions in derived classes override virtual functions in base classes only if their type is the same .

Redefinition of non-virtual functions may also cause confusion,
especially in a mix-in of virtual and non-virtual functions in one interface.

int main()
{
    Derived d;
    Derived* dp = &d;

    Base* bp = &d;
    dp->vf(); // expected Derived::vf
    dp->f();  // expected Derived::f
    bp->vf(); // expected Derived::vf (it's virtual!)
    bp->f();  // expected Derived::f (confusing for virtual)
    return 0;
}
/*Program output:
Derived::vf
Derived::f
Base::vf
Base::f
*/