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From: "Hyman Rosen" <hyman.rosen@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: tagged record child: override constructor?
Date: 16 Sep 2005 14:33:45 -0700
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Date: 2005-09-16T14:33:45-07:00
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Dmitry A. Kazakov wrote:
> Why do you think so? The program semantics should be independent from the
> parameter passing mode.

If tagged types were not passed by reference then the program
semantics would be severely affected. That's why the language
requires that passing mode. But in that case, it should be part
of the program, not a hidden requirement.

> You cannot have a reference to a specific type. Same with pointers.

True (for class types).

> Further, because
> T& is not an object, you cannot do such elementary thing as to copy a
> polymorphic object.

No, that's not why. It's purely the pragmatic implementation
complexity of arranging for the space and copying of an object
of runtime-determined size. Even in Ada objects of T'Class type
are somewhat half-baked. They can't live inside records, you
can't have arrays of them, etc.

> No. It is the class rooted in the type which do. A T'Class serves as a
> closure of all types derived from T.
> A view conversion is still a conversion.

When I view convert from a derived class type to a base class type,
and then back again, I have the same contents (and the same object)
that I started with. The extra derived stuff cannot magically appear
from nowhere; therefore it must have been there all along. Therefore
even though I have an object of some declared type I know that it may
be carrying along a bunch of extra data.