Path: utzoo!attcan!uunet!mcvax!enea!kth!luth!micke
From: micke@sm.luth.se (Mikael Degermark)
Newsgroups: comp.lang.eiffel
Subject: Multiple inheritance and dynamic binding.
Message-ID: <203@my2.luth.se>
Date: 11 Jan 89 13:09:38 GMT
Reply-To: Mikael Degermark <micke@my.sm.luth.se>
Organization: University of Lulea, Sweden
Lines: 62
UUCP-Path: {uunet,mcvax}!enea!my.sm.luth.se!micke

<greeting phrase>

I have recently read the Book by Meyer, and found it very interesting.
Eiffel seems to be a language worth trying, and I hope to be able to do so
in the near future.

When reading about multiple inheritance, there was something which I 
wasn`t able to understand at first, and after discussing it with my colleagues
(which didn`t have any solution either), I'm now asking You to help me.

The problem conserns the DRIVER, FRENCH_DRIVER, US_DRIVER, FRENCH_US_DRIVER
example given in the book (no, I haven't got the page reference),
here is a condensed version:

Assume there is a class A that declares and exports a feature f:INTEGER.
Assume there is two other classes B and C which each inherit A.
Assume a fourth class D that inherits both B and C and renames f:

    class D export f_b, f_c
        inherit B rename f as f_b;
        inherit C rename f as f_c;
    ...

Now, an instance of D has two distinct integer features f_b, f_c.

Consider the following fragment of eiffel: (I hope it is eiffel ;->)

    a:A;
    b:B;
    c:C;
    d:D;
    ...
    d.Create;
    
    a := d;   -- legal, A is an ancestor to D.
    b := d;   -- legal, B is an ancestor to D.
    c := d;   -- legal, C is an ancestor to D.

    if b.f = ...   -- this should be the same as d.f_b, right?
    if c.f = ...   -- this should be the same as d.f_c, right?
    if a.f = ...   -- here is the problem!

PROBLEM: Which of the features d.f_b and d.f_c is denoted by a.f?

The type-checking is supposed to prevent use of undefined features,
but there doesn't seem to be any sensible way to decide which of d.f_a and
d.f_b is denoted by a.f when the dynamic type of a is D.

So, is it d.f_a? or d.f_b? or neither? it couldn't be both, surely?

Actually, I suspect that there will be problems with b.f and c.f also,
consider the implementation.

Hopefully, those of You who have access to a compiler could try it and
tell me how it works. Would also like comments on how it *ought* to be.

Thanks!

     Mikael Degermark                    email: micke@sm.luth.se
     Dept. of CS
     U. of LULE] (read ] as an A with a ring)
     Sweden