Why do you need runtime polymorphismis inheritance necessary for it?

Answer 1

Runtime polymorphism is required because accessing the runtime type of an object is expensive. And yes, inheritance is required for it because polymorphism is enabled through virtual functions. It is not the programmer's responsibility to determine the runtime type of an object because that information should be provided by the object's own virtual table. Thus when you implicitly invoke a virtual method of the base class, you rightly expect most-derived method to be executed, even when you cannot know in advance the exact type of the derivative -- all you know is that it is a type of base class. And that's all you need for polymorphism to work. If you ever find yourself having to turn on the runtime type of an object in order to invoke one of its methods, then you need to seriously consider redesigning your classes. How can your code possibly cope with a runtime type it has no knowledge of, such as a derived object of a class created by a third-party? In those instances you would have no way of knowing what specific methods were available. But with virtual methods there is no need to know. You simply invoke the generic virtual methods of your base class and you automatically get specific behaviour; all your derivatives do whatever they've been implemented to do, whether it is to invoke the base class method explicitly, or provide an override, or indeed both!