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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: discriminant questions
Date: Wed, 21 Sep 2011 22:05:45 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Date: 2011-09-21T22:05:45-05:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:pvjwyqljhfih.1d1hj4pze7bpl$.dlg@40tude.net...
...
> P.S. I also see little or no point in using "accessors". Primitive
> operations Set and Get would be as good/bad. They are also cleaner, much
> more efficient and, what is far more important, they are *primitive*.
> Accossor is a new type which means *contravariance*. The stuff is not
> composable under inheritance.

I agree with all of this except the part about "much more efficient". Since 
such routines require copying the entire element, they cannot be more 
efficient than a construct that does not require such copying. Get/Set 
(really Element/Replace_Element) are good, so long as copying the element is 
cheap or you only need to modify the entire element.

But if you have a large element where only a small part will be modified, 
Set simply isn't practical. All you can do is introduce an explicit access 
type, which is *exactly* what you don't want (because it forces the client 
to do all of the memory management).

You could of course just return a reference to the element in place instead, 
but that sacrifies all safety -- that might be OK in the C++ library, but it 
is unacceptable in Ada.

                                   Randy.