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From: dmitry6243@my-deja.com
Subject: Re: Constructors/Destructors in Ada95
Date: 2000/10/25
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In article <39F6D201.73C006FA@acm.org>,
  Marin David Condic <mcondic.nospam@acm.org> wrote:
> Ray Blaak wrote:
>
> > I really would like to be able to do:
> >
> >   procedure ":="(target : in out T; source : in T);
> >
> > so as to have complete control over what is happening. I vaguely recall some
> > rationale from the Ada 9X discussions for why user-defined assignment per se
> > was not incorporated. Does anyone remember?
>
> Does a
> statement like: "X < Y ;" make sense in Ada?) As a procedure, you'd have to allow
> procedures to have symbol names - which opens up a whole can of worms. Further, it
> would mean allowing "infix procedures" which is hard to make sense of - or at least
> could make programs look really strange.

It would be nice (it is in my private Ada to-do list for a long time):

function "*" (Left, Right : Matrix) return Matrix;  -- Produces a new object
procedure "*" (Left : in out Matrix; Right : Matrix);  -- Multiplies Left to
Right "in-place"

So I would count it as an advantage. There is another. If you have

procedure ":="(target : in out T; source : in T);

then your assignment may take a look on the left-side object before its
destruction. Ada's Adjust is much more (IMO too much) specialized.

> I'm sure there are dozens of other reasons why it was decided not to provide a
> means of letting the user define assignment. I'd think it would require perverting,
> warping and twisting language concepts too much.

I think that the major reason was the decision to limit user defined
assigments by Controlled types. Doing so you must drop ":=" form, which
ontherwise would permanently remind you that actually all types might be
assigned (:-))

> (Look at the semantics of C++
> construction/destruction sometime - especially as it applies to function parameters
> - and see what an abomination that can become! :-)

So let's show them how to do it right (:-))

--
Regards,
Dmitry Kazakov


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