Each type of
Control has a different shape and appearance. Drop-down
ListBoxes look very different from
Buttons. Clearly, every subclass of
Control should implement its own
DrawControl( ) method—but so far, nothing in the
Control class enforces that they must do so. To require subclasses to implement a method of their base, you need to designate that method as abstract, rather than virtual.
An abstract method has no implementation. It creates a method name and signature that must be implemented in all derived classes. Furthermore, making at least one method of any class abstract has the side effect of making the entire class abstract.
Abstract classes establish a base for derived classes, but it is not legal to instantiate an object of an abstract class. Once you declare a method to be abstract, you prohibit the creation of any instances of that class.
Thus, if you were to designate
DrawControl( ) as an abstract method in the
Control class, the
Control class itself would become abstract. Then you could derive from
Control, but you could not create any
Control instances. That makes sense, because the
Control class is an abstraction—there is no such thing as a simple
Control object, only objects derived from
Control.DrawControl( ) abstract means that each class derived from
Control would have to implement its own
DrawControl( ) method. If the derived class failed to implement the abstract method, that derived class would also be abstract, and again no ...