[Newbies] Demystifying "most Smalltalk’s don't directly support multiple inheritance"

Klaus D. Witzel klaus.witzel at cobss.com
Thu Jun 22 22:20:49 UTC 2006


haven't you heard that Smalltalk and Squeak do not directly support  
multiple inheritance, time and again? But that Perl, Python and Ruby do  
(to some extent)?
- http://www.google.com/search?q=python+ruby+multiple+inheritance

Well, that is not the case for Smalltalk since the time that VMs are  
implemented by using the specs of the Blue Book.

I have attached a small .st file which demonstrates that the *unchanged*  
VM *must* support multiple inheritance (for message sends), or else it  
won't work properly. It's a matter of view on what the VM does for you :-D

With the exception of support code for exceptions [pun intended] the  
attached examples work on VMs which implement the super send from the Blue  

The first example demonstrates that multiple inheritance works; in this  
example it is also checked that the requesting method qualifies (that the  
instances qualify is checked at fileIn time).

The second example does a message tally and shows that there is less than  
50% overhead when possible walkbacks are guarded at every multiple  
inheritance message send.

The third example employs the existing method cache of the VM (the  
original Squeak VM) for multiple inheritance message sends and thus  
minimizes the overhead to less than 5% (and the demo is written in .st  
code :-)

Again, this all exists *unchanged* for decades. It only had to be brought  
to the surface?


I would be very interested to see if my examples run on other platforms  
and VMs, please post your experience. Thank you.

It'd be equally interesting if the approach had been used before.

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