457: Mainline

More generally, you define a virtual member function under the following circumstances:

• You have a pointer defined to point to an object belonging to a certain class. Suppose the class is named superclass.
• You assign the pointer to an object, introduced at run time, that belongs to a subclass of superclass. Suppose that the subclass is named subclass.
• You want C++ to pick a member function, say member_function, on the basis of the object's class, subclass.

Then, you must define a version of member_function in superclass, and you must mark that function with virtual. That member function and all versions defined in classes that are subclasses of superclass are said to be virtual member functions.