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From: "Robert I. Eachus" <rieachus@comcast.net>
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Subject: Re: Deallocation & Tagged Types
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chris wrote:

 > I tried instantiating Unchecked_Deallocation on Vehicle and not
 > Vehicle'Class not knowing you could do that.

Sigh, I should have tried harder to get that notation fixed in Ada 83. 
Most of the time the Object name in an instantiation of 
Unchecked_Deallocation is a dummy parameter.

Yes, there are cases where the size of the object being deallocated 
cannot be determined from the access type name, and the subtype of the 
Object type can be used by the compiler.  But that only argues that 
there should be two versions of Unchecked_Deallocation, and the compiler 
should reject the one parameter version at compile time if it does need 
to use the designated subtype information.  But that would IMHO be 
better than allowing users to instantiate with the "wrong" constrained 
subtype thinking it doesn't matter.

Requiring the user to provide the "right" subtype when it doesn't 
matter--except that the compiler can reject a wrong value--is one of 
those "little tests" that Tucker used to talk about.

 > Yes the ADT abstraction was broken, but only after trying to get
 > around the problem of deallocating the access component in the
 > record.  The ugliness is (IMO) gone now.

It sounds like you are still doing more than you really have to.  But I
don't know understand your full design well enough to understand whether
  the explicit allocation and deallocation are necessary.

But I assume that your "Proxy" notation indicates that you do need to
use a pointer to the object rather than copying the object.


-- 
                                        Robert I. Eachus

"Quality is the Buddha. Quality is scientific reality. Quality is the
goal of Art. It remains to work these concepts into a practical,
down-to-earth context, and for this there is nothing more practical or
down-to-earth than what I have been talking about all along...the repair
of an old motorcycle."  -- from Zen and the Art of Motorcycle
Maintenance by Robert Pirsig