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From: shang@corp.mot.com (David L. Shang)
Subject: Re: Multiple Inheritance (was Re: Has C++ had its day?)
Reply-To: shang@corp.mot.com
Organization: MOTOROLA 
Date: Fri, 7 Jul 1995 15:14:08 GMT
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In article <3ti7hd$6hu@steel.interlog.com> herbs@interlog.com (Herb Sutter)  
writes:
> I'd be happy to hear how other languages support mixins (mixing in both 
> interface and behaviour) without MI... do we have any examples?

I'll cross post this subject into some language groups in
order to get more suggestions and opinions from different
language people.

Consider what features should be inherited, there are three
kinds of multiple inheritance:

W1. MI on interface
W2. MI on interface and behavior
W3. MI on interface, behavior, and instance variable

Consider how features should be inherited, there are three
kinds of multiple inheritance too:

H1. Base is duplicated n times if it is inherited n times
H2. Base is duplicated exactly once
H3. Base is shared, never duplicated

H3 is dynamic inheritance, which is usually used by dynamic
languages that do not use the concept of class or do not use
class as an abstraction type. We do not consider H3 here.

C++ supports W1, W2, W3, H1, and H2. For H2, virtual base is
used, should the base be inherited repeatedly. Thus C++
chose H1 as default MI rule.

Eiffel supports W1, W2, W3, H1 and H2. For H1, renaming should
be used, should the base be inherited repeatedly. Thus Eiffel
chose H2 as default MI rule.

Java supports W1 and H2. Since methods defined in interface
are abstract (or pure virtual, i.e. no implementation), there
is no need to have multiple copies of an interface (note that
two interface with a same name but different signatures are
considered different interfaces).

H1 can always be simulated by composition as shown in Beta.
H1 can also be simulated by parameterization.

But H2 cannot be simulated. That is why H2 is the real value
of MI. Therefore, Eiffel's choice of H2 as default is clever
than C++.

However, chosing H2 (with W2 and W3) as default has penalty.
It might need indirect linking to the base (super) part even MI
is not used. I don't know how Effel's implementation solves
this problem. Can any people from Eiffel group address this
question?

Using H2 with only W1 as in Java has no penalty on SI.

So, the question is, are W2 and W3 really required? Or, is
W1 and H2 sufficient to support all the requirement of
multiple inheritance? If so, Java's decision is correct.

The major purpose of W2 and W3 is for reuse of implementation;
while the major purpose of W1 is for polymorphism.

I think we should have a way to use SI, composition and
parameterization, together with W1, to achieve the effects
of W2 and W3. Any examples are apprieciated.

David Shang