when should sends be virtual?

Rainer Blome rainer at physik3.gwdg.de
Wed Dec 7 16:51:10 UTC 1994


i am wondering whether there are any "fundamental" arguments in
favor of or against the convention in self that all sends to self
are 'virtual' by default.

what i mean is this:

imagine object x receives message foo and the lookup finds a
corresponding method in ancestor y.  now this method foo sends the
message bar to self.  who is self, where does the lookup start
(after searching the local (method) slots), who is the pareent of
the method?  this is a question, i am unable to determine that
decidedly from the manuals, i can't run self here.

i call the send 'virtual' when the lookup starts in x, being the
receiver of the last non-self send (e.g. foo).  this is the default
with self, right?

and it is 'non-virtual' when lookup starts in y, being the holder of
bar.  in self, this is called an 'undirected resend'.

since one can always deliberately decide which way to use, it might
just be a matter of convenience which way is to be the default.  but
it might be a matter of good programming style as well: shouldn't it
always have to be made explicit when the result of a lookup cannot
be determined by the sender?  dynamic inheritance set aside, a
method 'knows' the behavior (of its ancestors) it can rely on when
doing a non-virtual send.  it seems like good style to me to make it
explicit when one relies on things that are uncertain.

making non-virtual sends the default would also give an easy answer
for a question discussed here some weeks ago:  how to determine what
behavior some objects rely on their childs to supply it.  simply
look for virtual sends instead of having to check whether a send to
self is already implemented in the object or its ancestors.

there are more possibilities, but only for statically compiled
languages, i think: actually, one might not be able to deliberately
decide which way to use because it has been defined one way at the
time of class-declaration.  this is the case with c++ for example.

did anybody ever see a treatise about where and when it should be
specified that a message send should be virtual?  at the send site?
or at the declaration/implementation of a method?
ever seen one of these answered?

greetings,		rainer

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