In the following code a non-virtual method is redefined, which is not allowed.

class Base
{
public: // Methods
    void Foo();
};

class Derived : public Base
{
public: // Methods
    void Foo(); // Not allowed !!!
};

The reasons can be found in "Effective C++" item 37 (see [Meyers]). When an inherited non-virtual method may not be redefined, it may also not be made virtual in the derived class, as in:

class Base
{
public: // Methods
    void Foo();
};

class Derived : public Base
{
public: // Methods
    virtual void Foo(); // Not allowed !!!
};

Exception: this rule doesn't hold for classes that have been derived privately from its base class because in such a case no harm can be done.