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eieio: Inheritance

 
 3.1 Inheritance
 ===============
 
 “Inheritance” is a basic feature of an object-oriented language.  In
 EIEIO, a defined class specifies the super classes from which it
 inherits by using the second argument to ‘defclass’.  Here is an
 example:
 
      (defclass my-baseclass ()
         ((slot-A :initarg :slot-A)
          (slot-B :initarg :slot-B))
        "My Baseclass.")
 
 To subclass from ‘my-baseclass’, we specify it in the superclass list:
 
      (defclass my-subclass (my-baseclass)
         ((specific-slot-A :initarg specific-slot-A)
          )
         "My subclass of my-baseclass")
 
    Instances of ‘my-subclass’ will inherit ‘slot-A’ and ‘slot-B’, in
 addition to having ‘specific-slot-A’ from the declaration of
 ‘my-subclass’.
 
    EIEIO also supports multiple inheritance.  Suppose we define a second
 baseclass, perhaps an “interface” class, like this:
 
      (defclass my-interface ()
         ((interface-slot :initarg :interface-slot))
         "An interface to special behavior."
         :abstract t)
 
 The interface class defines a special ‘interface-slot’, and also
 specifies itself as abstract.  Abstract classes cannot be instantiated.
 It is not required to make interfaces abstract, but it is a good
 programming practice.
 
    We can now modify our definition of ‘my-subclass’ to use this
 interface class, together with our original base class:
 
      (defclass my-subclass (my-baseclass my-interface)
         ((specific-slot-A :initarg specific-slot-A)
          )
         "My subclass of my-baseclass")
 
 With this, ‘my-subclass’ also has ‘interface-slot’.
 
    If ‘my-baseclass’ and ‘my-interface’ had slots with the same name,
 then the superclass showing up in the list first defines the slot
 attributes.
 
    Inheritance in EIEIO is more than just combining different slots.  It
 is also important in method invocation.  Methods.
 
    If a method is called on an instance of ‘my-subclass’, and that
 method only has an implementation on ‘my-baseclass’, or perhaps
 ‘my-interface’, then the implementation for the baseclass is called.
 
    If there is a method implementation for ‘my-subclass’, and another in
 ‘my-baseclass’, the implementation for ‘my-subclass’ can call up to the
 superclass as well.
 

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