Section 11.4

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The previous section only provided the definition of a class Packet.

Section 11.12

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To get the overridden method, the parent method needs to be declared virtual (see section 11.18).

To:

To call the overridden method via a parent class object (p in the example), the method needs to be declared virtual (see section 11.18).

Section 11.13

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It is necessary to use super to access properties of a parent class when those properties are overridden by the derived class.

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Whereas superclasses are classes that the current class is extended from, beginning with the original base class.

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This is because the superclass must be initialized before the current class and if the user code doesn’t provide an initialization,

Section 11.14

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It is always legal to assign subclass variable to a variable of a class higher in the inheritance tree. It is never legal to directly assign a superclass variable to a variable of one of its subclasses. However, it may be legal to place the contents of the superclass handle in a subclass variable.

To check if the assignment is legal, the dynamic cast $cast() is used (see section 3.14).

When used with object handles, $cast() checks the hierarchy tree (super and subclasses) of the source_expr to see if it contains the class dest_var. If it does, $cast() does the assignment otherwise the error handling is as described in section 3.14.

Section 11.15

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The first, implicit action new() takes is to invoke the new() method of its superclass, and so on up the inheritance hierarchy.

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If the initialization method of the superclass requires arguments, one has two choices.

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A more general approach is to use the super keyword, to call the superclass constructor:

Section 11.18

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An abstract class may contain methods for which there is only a prototype and no implementation (i.e., an incomplete class). An abstract class cannot be instantiated, it can only be derived.

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However, if the subclass overrides the virtual method, then the new method must exactly match the superclass’s prototype.

Section 11.19

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Polymorphism allows one to use superclass variables to hold subclass objects, and to reference the methods of those subclasses directly from the superclass variable.