Why can I access a derived private member function via a base class pointer to a derived object?-Collection of common programming errors

Access modifiers, such as public, private and protected are only enforced during compilation. When you call the function through a pointer to the base class, the compiler doesn’t know that the pointer points to an instance of the derived class. According to the rules the compiler can infer from this expression, this call is valid.

It is usually a semantic error to reduce the visibility of a member in a derived class. Modern programming languages such as Java and C# refuse to compile such code, because a member that is visible in the base class is always accessible in the derived class through a base pointer.