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Re: Alternatives languages on the JVM: which one or is there no alternative?

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On 11/28/2013 5:41 PM, Andreas Leitgeb wrote:
> Joshua Cranmer 🐧 <Pidgeot18@verizon.invalid> wrote:
>> And you think multiple inheritance has no costs? The moment you allow
>> multiple inheritance of implementations, you permit the so-called
>> "diamond problem." The diamond problem has three solutions:
>> 1. There are multiple copies of some fields.
>> 2. Offsets to a particular named field are dependent on object
>> instances, not on classes/class hierarchies.
>> 3. Casting up or down a tree requires recomputing offsets on a
>> per-object instance basis, not a per-class basis.
>
> I think, the diamond problem w.r.t fields is over-rated.

Given your response below, I think you misunderstand the problem. Let me 
try to explain it better. There are two separate issues: name 
resolution, and class layout. Name resolution is relatively simple to 
solve; it's the layout that is the thorny solution.

When you layout a class in memory, it is easiest to think of it as an 
array of "slots" for actual field storage (think C structs if you're 
used to that). Class layout is typically done by laying out the 
superclass first and then using extra slots for the subclass. If done 
this way, this lets you guarantee that all objects of class A, slot 0 
always points to the same field, even if the object's dynamic type is 
really a subclass of A.

Multiple inheritance throws a wrinkle into this scheme because of the 
diamond problem. If D inherits from B and C, which both inherit from A, 
then the standard way of laying out the class yields two copies of A. If 
you cast D to type C, then a method of C that tries to access a field of 
type A would access C's copy of A instead of B (and presumably D's 
"primary" copy)'s copy. This is the first case I give. In this 
situation, accessing a field only requires knowing the static type of 
the object, not the dynamic type.

You can get around this by requiring knowledge about dynamic types. The 
option taken by C++'s virtual bases is to have the class layout 
mechanism create only one copy of A. In this case, there is no longer a 
guarantee that the field is always at the same offset for the static 
type of an object--you have to do a layer of indirection to find out 
where A's slots are and then use those. This is case #3 (note that the 
casting to use a superclass's fields is implicit here).

The final option is to refer to fields internally via a hashtable of 
(qualified) name to their values. This is generally what is done in most 
dynamic programming languages (Python, JS, etc.), which is why they can 
suffer multiple inheritance: all property accesses already go through 
the hashtable, so there is no extra cost. This is what I called case #2.

>> Multiple inheritance is a seductive call. There are very, very, very few
>> times where it is actually proper--recall that inheritance, at least as
>> Java is concerned, is meant to model an "is-a" relationship, not a
>> "has-a" relationship.
>
> So, if some entity "is-a"nother entity, but "is-also-a" yet another
> entity, then why is Java praised for forcing me to model an "is-also-a"
> relation as a "has-a"?

There are certainly cases where multiple inheritance is warranted; I am 
not disputing that. That said, there are also people who try to inherit 
for the wrong reasons, so probably most cases where people think they 
want to use multiple inheritance, they don't. (I'm speaking in 
generalities, though, and all generalities are false).

-- 
Beware of bugs in the above code; I have only proved it correct, not 
tried it. -- Donald E. Knuth

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Alternatives languages on the JVM: which one or is there no alternative? Saxo <jeti789@web.de> - 2013-11-25 03:09 -0800
  Re: Alternatives languages on the JVM: which one or is there no alternative? Silvio <silvio@internet.com> - 2013-11-25 12:46 +0100
    Re: Alternatives languages on the JVM: which one or is there no alternative? Saxo <jeti789@web.de> - 2013-11-25 05:20 -0800
    Re: Alternatives languages on the JVM: which one or is there no alternative? Robert Klemme <shortcutter@googlemail.com> - 2013-11-25 22:10 +0100
      Re: Alternatives languages on the JVM: which one or is there no alternative? Saxo <jeti789@web.de> - 2013-11-25 14:34 -0800
      Re: Alternatives languages on the JVM: which one or is there no alternative? Silvio <silvio@internet.com> - 2013-11-26 08:54 +0100
        Re: Alternatives languages on the JVM: which one or is there no alternative? Robert Klemme <shortcutter@googlemail.com> - 2013-11-26 00:29 -0800
          Re: Alternatives languages on the JVM: which one or is there no alternative? Silvio <silvio@internet.com> - 2013-11-26 15:29 +0100
            Re: Alternatives languages on the JVM: which one or is there no alternative? Arne Vajhøj <arne@vajhoej.dk> - 2013-11-26 21:55 -0500
              Re: Alternatives languages on the JVM: which one or is there no alternative? Silvio <silvio@internet.com> - 2013-11-27 08:59 +0100
                Re: Alternatives languages on the JVM: which one or is there no alternative? Joerg Meier <joergmmeier@arcor.de> - 2013-11-27 10:41 +0100
                Re: Alternatives languages on the JVM: which one or is there no alternative? Silvio <silvio@internet.com> - 2013-11-28 12:48 +0100
                Re: Alternatives languages on the JVM: which one or is there no alternative? leif.roar@invalid.com (Leif Roar Moldskred) - 2013-11-28 16:28 -0600
                Re: Alternatives languages on the JVM: which one or is there no alternative? Silvio <silvio@internet.com> - 2013-11-29 00:03 +0100
                Re: Alternatives languages on the JVM: which one or is there no alternative? Saxo <jeti789@web.de> - 2013-11-29 00:41 -0800
                Re: Alternatives languages on the JVM: which one or is there no alternative? Silvio <silvio@internet.com> - 2013-11-29 12:05 +0100
                Re: Alternatives languages on the JVM: which one or is there no alternative? Saxo <jeti789@web.de> - 2013-11-29 03:51 -0800
                Re: Alternatives languages on the JVM: which one or is there no alternative? Silvio <silvio@internet.com> - 2013-11-29 16:44 +0100
                Re: Alternatives languages on the JVM: which one or is there no alternative? Silvio <silvio@internet.com> - 2013-11-29 16:48 +0100
                Re: Alternatives languages on the JVM: which one or is there no alternative? Robert Klemme <shortcutter@googlemail.com> - 2013-12-02 06:33 -0800
                Re: Alternatives languages on the JVM: which one or is there no alternative? Silvio <silvio@internet.com> - 2013-12-02 15:50 +0100
                Re: Alternatives languages on the JVM: which one or is there no alternative? Silvio <silvio@internet.com> - 2013-12-02 16:26 +0100
                Re: Alternatives languages on the JVM: which one or is there no alternative? Robert Klemme <shortcutter@googlemail.com> - 2013-12-02 21:07 +0100
                Re: Alternatives languages on the JVM: which one or is there no alternative? Silvio <silvio@internet.com> - 2013-12-02 23:33 +0100
                Re: Alternatives languages on the JVM: which one or is there no alternative? markspace <markspace@nospam.nospam> - 2013-12-02 18:07 -0800
                Re: Alternatives languages on the JVM: which one or is there no alternative? Silvio <silvio@internet.com> - 2013-12-03 08:59 +0100
                Re: Alternatives languages on the JVM: which one or is there no alternative? Saxo <jeti789@web.de> - 2013-12-03 00:14 -0800
                Re: Alternatives languages on the JVM: which one or is there no alternative? Silvio <silvio@internet.com> - 2013-12-03 09:42 +0100
                Re: Alternatives languages on the JVM: which one or is there no alternative? Saxo <jeti789@web.de> - 2013-12-03 02:58 -0800
                Re: Alternatives languages on the JVM: which one or is there no alternative? Martin Gregorie <martin@address-in-sig.invalid> - 2013-12-03 20:55 +0000
                Re: Alternatives languages on the JVM: which one or is there no alternative? Saxo <jeti789@web.de> - 2013-12-04 00:12 -0800
                Re: Alternatives languages on the JVM: which one or is there no alternative? Arved Sandstrom <asandstrom2@eastlink.ca> - 2013-12-14 03:31 -0400
                Re: Alternatives languages on the JVM: which one or is there no alternative? Martin Gregorie <martin@address-in-sig.invalid> - 2013-12-14 10:11 +0000
                Re: Alternatives languages on the JVM: which one or is there no alternative? Robert Klemme <shortcutter@googlemail.com> - 2013-12-14 17:39 +0100
                Re: Alternatives languages on the JVM: which one or is there no alternative? "Chris Uppal" <chris.uppal@metagnostic.REMOVE-THIS.org> - 2013-12-14 16:52 +0000
                Re: Alternatives languages on the JVM: which one or is there no alternative? Arved Sandstrom <asandstrom2@eastlink.ca> - 2013-12-14 13:47 -0400
                Re: Alternatives languages on the JVM: which one or is there no alternative? Robert Klemme <shortcutter@googlemail.com> - 2013-12-14 23:55 +0100
                Re: Alternatives languages on the JVM: which one or is there no alternative? Sebastian <news@seyweiler.dyndns.org> - 2013-12-04 10:39 +0100
                Re: Alternatives languages on the JVM: which one or is there no alternative? Arved Sandstrom <asandstrom2@eastlink.ca> - 2013-12-14 03:56 -0400
  Re: Alternatives languages on the JVM: which one or is there no alternative? v_borchert@despammed.com (Volker Borchert) - 2013-11-26 21:28 +0000
    Re: Alternatives languages on the JVM: which one or is there no alternative? Robert Klemme <shortcutter@googlemail.com> - 2013-11-26 23:29 +0100
      Re: Alternatives languages on the JVM: which one or is there no alternative? v_borchert@despammed.com (Volker Borchert) - 2013-11-27 05:13 +0000
        Re: Alternatives languages on the JVM: which one or is there no alternative? Robert Klemme <shortcutter@googlemail.com> - 2013-11-27 22:57 +0100
          Re: Alternatives languages on the JVM: which one or is there no alternative? v_borchert@despammed.com (Volker Borchert) - 2013-11-29 23:22 +0000
            Re: Alternatives languages on the JVM: which one or is there no alternative? Robert Klemme <shortcutter@googlemail.com> - 2013-11-30 14:41 +0100
            Re: Alternatives languages on the JVM: which one or is there no alternative? Joshua Cranmer 🐧 <Pidgeot18@verizon.invalid> - 2013-11-30 16:49 -0600
        Re: Alternatives languages on the JVM: which one or is there no alternative? Joshua Cranmer 🐧 <Pidgeot18@verizon.invalid> - 2013-11-27 23:07 -0600
          Re: Alternatives languages on the JVM: which one or is there no alternative? Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2013-11-28 23:41 +0000
            Re: Alternatives languages on the JVM: which one or is there no alternative? Joshua Cranmer 🐧 <Pidgeot18@verizon.invalid> - 2013-11-29 00:08 -0600
              Re: Alternatives languages on the JVM: which one or is there no alternative? Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2013-11-29 16:49 +0000
                Re: Alternatives languages on the JVM: which one or is there no alternative? Joshua Cranmer 🐧 <Pidgeot18@verizon.invalid> - 2013-11-30 16:46 -0600
                Re: Alternatives languages on the JVM: which one or is there no alternative? Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2014-02-14 15:36 +0000
          Re: Alternatives languages on the JVM: which one or is there no alternative? v_borchert@despammed.com (Volker Borchert) - 2013-11-29 23:37 +0000
  Re: Alternatives languages on the JVM: which one or is there no alternative? Roedy Green <see_website@mindprod.com.invalid> - 2013-11-28 12:50 -0800

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