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From: elcond@garnet.berkeley.edu (Gregory Dow)
Newsgroups: comp.sys.mac.programmer
Subject: Inheritance in The Crunched Shell
Keywords: Object-oriented programming
Message-ID: <12405@agate.BERKELEY.EDU>
Date: 22 Jul 88 07:09:09 GMT
References: <2328@pt.cs.cmu.edu> <12362@agate.BERKELEY.EDU> <60865@sun.uucp>
Sender: usenet@agate.BERKELEY.EDU
Organization: University of California, Berkeley
Lines: 33

In article <60865@sun.uucp> landman@sun.UUCP (Howard A. Landman) writes:

 [description of how single inheritance is achieved through nested #include's]

>Gee - this sounds like you can get multiple inheritance just by nesting
>2 or more include files.  So why "single only"?  (Assuming no conflicts.)
>Is it because you need to know which SuperClass to pass a message to if the
>local Class can't handle it, so the dispatcher would get more complicated?
>

The SuperClass ordering is one problem (depth-first or breadth-first search),
the other is more mundane.  Message names are implemented as an enumerated
type, which C automatically converts to integer values.  The root object
class has 3 messages, which get numbered as 0, 1, and 2.  If Class A,
a child of the root class, has 4 messages, they get numbered 3, 4, 5, and 6.
Similarly for a Class B which is also descended from the root class.  If
a Class X has both A and B as superclasses, it will not know how to handle
the message number 4.  Message 4 may be a FOO message to Class A, and
a BAR message to Class B, but the Class X has no way of knowing which one it
is.  The C preprocessor converts both FOO and BAR to 4.

Can anyone suggest an alternative?  I'd love it if I could easily implement
the "set" data structure from Pascal in C.  Then I could just take the
union of the messages of the superclasses of a class.  I'd really like
to find a clean solution to this problem, because standard C++ doesn't
support multiple inheritance either.



  Gregory Dow			ARPA:   elcond@garnet.berkeley.edu
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