Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!boulder!andreasg
From: andreasg@boulder.Colorado.EDU (Andreas Girgensohn)
Newsgroups: comp.object
Subject: Re: "Sender" construct in OOLs
Message-ID: <12545@boulder.Colorado.EDU>
Date: 9 Oct 89 21:44:53 GMT
References: <TIME.89Oct4131658@oxtrap.oxtrap.UUCP> <9450008@hplsla.HP.COM> <5677@portia.Stanford.EDU>
Sender: news@boulder.Colorado.EDU
Reply-To: andreasg@boulder.Colorado.EDU (Andreas Girgensohn)
Organization: University of Colorado, Boulder
Lines: 40

In article <5677@portia.Stanford.EDU> dchapman@Portia.Stanford.EDU (David Chapman) writes:
>In article <9450008@hplsla.HP.COM> jima@hplsla.HP.COM (Jim Adcock) writes:
>>You should be able to do this using function call overloading in C++.
>>I suspect its not trivial to figure out, however.
>
>After thinking about this some more, I cannot conceive of a way to
>implement the "sender" construct.  Why?  Because the "sender" is of 
>arbitrary type.  *Anything* can send an object a message.  That's the 
>whole point.  Maybe Smalltalk or Flavors could handle this by looking 
>up the type of the object, but it would be horrendous to do in a 
>compiled language like C++.
>
>Comments?  Remember, the original poster wanted mutual recursion - he
>wanted to send messages back to the caller.


In Lisp one could do it with a variable with dynamic scope:

(defvar sender :no-object)

;;; within every method:

(let ((sender self))
  ;; message body (actually, the previous value of sender is needed within
  ;; the body)
)

There are no variables with dynamic scope in C.  A solution could be
an array as a stack of senders (I don't no the C++ syntax).

object *senders[200];
int    senders_top = 0;

senders[0] = (object *)NULL;

/* within every method: */

senders[++senders_top] = self;
/* message body */
senders_top--;