🚀 go-pugleaf

Graham:
>  I was wondering about a small implementation problem I am having.
>  Imagine I have two classes A and B defined as follows,
>  
>  	class A
>  		instance variables a, b, c;
>  	end class;
>  
>  	class B
>  		subclass of class A;
>  		instance variables e, f;
>  	end class;
>  
>  (by "instance variable" i mean that an instance of class A will
>  have three "fields"/"slots" (pick your terminology!) named a, b and
>  c).
>  
>  Now my problem is as follows. How do OO implentations handle the 
>  space allocation for an instance of class B? Does an instance of B
>  have to have space allocated for all the instance variables in B as 
>  well as all the instance variables in all the classes of which B is 
>  a sub-class (in this case class A). I ask this for three reasons :-
>
>  a) I have many examples where B is a subclass of A but one rarely
>  utilises this feature. All the time is spent computing with instances
>  of B as instances of the sub-class B and not as instances of the 
>  super-class A. So I do not want to allocate space for any instance 
>  variables (in this case those in class A) which I am rarely if ever 
>  going to use.

kanderso@bbn.com writes:
>But there must be some A behavior that B needs, otherwise B would not need
>to be a subclass of A.  It sounds like you want your B class to inherit
>from a class "essential A" which has the A behavior you need, without the slots.

No not really. The example I have in mind is a module and an abelian
group. Now a module *is* an abelian group, but one hardly ever computes 
with it as such - users are aware of the is-a relationship but hardly
ever make any use of it. They always want to compute with modules and
sub-types of it such as vector spaces, and are not at all interested
in its abelian group structure.

graham
>  b) I have example code where the representation of B is not the
>  same as A. That is mathematically B is a sub-type of A but for
>  various reasons the implementation of B does not have any of the 
>  slots of A.
>  
kanderso@bbn.com writes:
>You can separate the "mathematical subtypeness" from implementation using
>a level of indirection:
>class A
>       instance variables implementation;
>end class;
>class B
>       subclass of A;
>end class;
>A's and B's can forward to an object in instance variable implementation
>that implements the real behavior.

I am not sure I understand what you mean here. What do you mean by
"forward to an object in instance variable implementation"?

graham
--
Graham Matthews                 And it's true we are immune
Pure Math, Uni.Sydney, Oz       When fact is fiction and T.V. is reality
graham@maths.su.oz.au