In C++, virtual functions provide polymorphism. This is because a polymorphic object (pointer or reference (or such parameter)) is assignment compatible with any object of a derived class. Is this polymorphism in itself? Objects can take on objects of different forms (the derived classes), but of what use is it? To make any difference, the differing forms must have some effect. In dynamically typed languages, polymorphic objects are passed messages and will respond in whatever way the object has defined (usually starting from its most derived class and working its way up). But for static objects, a virtual function is invoked. This is the stored method from the derived class that overrode the virtual method from its base class, providing specialized behavior for the polymorphic object; and hence, polymorphism. This common pure statically typed example is, of course, an example of inclusion polymorphism, subclass polymorphism to be more specific (see section 2.1). Pure statically typed subtype polymorphism, as provided in Emerald, can be implemented similarly [Black 86].
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