Re: Constructors/Destructors in Ada95

[Marin David Condic <mcondic.nospam@acm.org>]

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?

I don't know about the Ada 9X discussions specifically, but if you think about it,
there are a lot of obvious problems with trying to redefine ":=". The biggest
problem is that it is not a subprogram - it is a primitive feature of the language.
I believe this might be an example of Godel's Theorem. There are some features of
the language (any language) that cannot be expressed in the language itself. As a
"function" it would require that a) the left parameter be treated as "out" and b)
that the function need not return a result. (or that it could be ignored. 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.

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