[Xotcl] Non-polymorphic methods (long post)

Kurt Stoll stoll at mac.com
Sun Feb 26 17:39:54 CET 2006

(Sorry for the long post)

Well, I've perused the archives and can't find anything on this  
topic.  Nor can I find it in the tutorial or reference guide.  So, I  
must conclude that I am missing something.  Either this is obvious,  
and everyone has already figured this out.  Or, there is no real need  
for this type of a construct and I'm thinking about this incorrectly.

Many OO languages provide a means to make some methods be polymorphic  
and some not.  For example, C++ uses the virtual keyword to make a  
function polymorphic; otherwise it's not.   XOTcl on the other hand  
appears to only support polymorphic methods.  A call to a method from  
a given level of the hierarchy starts from the class of the calling  
object and works its way up (assuming next is used).

For example, a class Base provides an interface, then Sub1 and Sub2  
(both subclasses of Base) provide a thin veneer on that interface.   
The methods have the same name.  And, in some cases, they rely on  
Base to provide some of the functionality.  But, Base provides its  
own functionality by composing some of its own methods into more  
complex methods.  Now, when a method from Sub1 is called, it calls  
that method in Base, and Base then calls another method within Base -  
except that when it makes this call, it gets the version from Sub1.   
In some cases, this may be the correct approach.  But, there are  
certainly cases where this is not desired.

We could try pushing some of the implementation from Base into Sub1,  
but Sub2 wants similar functionality.  This would mean duplicating code.

Another approach might be delegation, instead of inheritance.  This  
would work, but, all three classes (Base, Sub1, and Sub2) have  
instances that share a common interface.  Now, to add a new method to  
that interface requires changing two different class hierarchies.   
Again, this would work, but it just doesn't feel right.

I realize that this is a fairly abstract presentation of the issue.   
I have attempted to come up with a simple example - for example,  
geometric figures, with the base class rooting the centroid at the  
origin to provide easy rotations.  But, even here, this gets  
complicated.  and I'm afraid that the discussion will veer off to  
trying to solve some particular quirk of the example.  But, I have  
run into this issue several times, and have found it frustrating.

The only solution that I have found is to provide both an "internal"  
implementation and a public implementation of some methods (for  
example, prefix the internal method name with an underscore).  Then,  
the implementation can decide whether it is wants to use the  
polymorphic method or the non-polymorphic version.  But, this doesn't  
really solve the problem - I still have to be careful not to override  
the internal version (because, I don't really have a non-polymorphic  
method - I just have some conventions to avoid the polymorphism).   
This could get truly complicated with a large class hierarchy, but  
none of my constructs have reached the point (yet).

Actually, I can imagine a second potential solution, although I have  
not tried it.  I think it would work, but would be very ugly.   
Nevertheless, I bring it up because I believe it might form the basis  
of what I think would be an elegant solution to this problem.  I  
could temporarily change the class of the object to reflect where I  
want the method search to begin from.  Then, change it back when the  
method returns.  Artificially changing the class of an object makes  
me cringe - I don't really like this approach.

But, it seems to me that if XOTcl can provide a way to force the  
method search to begin at a particular class, this would fix the  
problem.  Maybe something like: [self as class] <method> <args>.  In  
fact, I think I can implement this as an Object method - simply copy  
the class, change it, make the call, destroy the copy, and return the  
result.  I think this would work, and may try it.  But, I still feel  
like I must be missing something.

Any  thoughts would be appreciated.

Kurt Stoll

More information about the Xotcl mailing list