Re: Ada 90 inheritance request?

[jgoodsen@treasure.radsoft.com (John Goodsen)]

In article <EACHUS.94Dec15184124@spectre.mitre.org> eachus@spectre.mitre.org (Robert I. Eachus) writes:

      Never being afraid to jump into the fray, I'll respond to John by
   taking his MI example and asking why the obvious Ada 95 version is not
   acceptable: 

       type Needs_Finalization is new Controlled with...;
       -- overriding of Initialize/Adjust/Finalize
       procedure Initialize (Obj : in out Needs_Finalization);
       procedure Adjust     (Obj : in out Needs_Finalization);
       procedure Finalize   (Obj : in out Needs_Finalization);

       type Ctrl_T is new T with record
	 NF:  Needs_Finalization;
       end record;


Maybe it's just me not seeing something here (which could be an indicator
of the usability issues involved with addressing MI in Ada95), but where 
is the polymorphic behavior in the MI lattice?  Wouldn't the above solution
require me to delegate methods to the NF member of Ctrl_T (by hand)?

       Now I can think of cases where this won't work, but they are
   precisely the cases where the "true" MI version won't work either.

   No, I take that back.  There are cases where true MI will fail, and
   the Ada 95 version will sail through.  For example, if someone
   modifies T so that it inherits directly from Controlled...

       In general where Ada 95 says "this is unsafe, so we will not allow
   it," most MI languages say "this is really neat, and if you are VERY
   careful, it will work, but never on large projects, because it doesn't
   scale well."

	I've been there, I've done that, I've seen the maintenance
   nightmares that can occur when more than one programmer depends on
   unsafe MI on the same project.

This isn't clear to me.  Can you give an example (besides the overused
diamond inheritance pattern) ?  What do you mean by "unsafe MI" ?

thanks
-- 
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John Goodsen                         Currently on-site at:
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