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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: I/O - exception handling
Date: 28 May 2003 17:47:39 -0400
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2003-05-28T17:47:39-04:00
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Hyman Rosen <hyrosen@mail.com> writes:

> Sergey Koshcheyev wrote:
> > I myself miss constructors and destructors in Ada a lot.
> 
> Well, Ada has them, albeit in an inconvenient form. You need a
> type which inherits from Controlled, and that type must be
> declared at library level for some safety reason that Ada folks
> appreciate.
> 
> I don't know if there's a way to use access discriminants in
> combination with generics to get a "universal" cleanup object
> that could forward to a function. Ada experts?

The problem is that finalization code has to be at library level, so it
can't easily get its hands on local objects.  A solution/workaround is
to declare a Limited_Controlled type with an access discriminant, and
make that discrim point at the local object.  It's kind of ugly, but
it's exception-safe and abort-safe.  You have to make the local object
aliased.  And if the "local object" is a parameter (which it often is,
in my experience), then you have to make the type tagged so it will be
aliased, or you have to copy the param into a local aliased object.  And
you have to use 'Unchecked_Access, even though there is little danger of
creating dangling pointers.

I suppose you could wrap this "design pattern" in a generic, but I doubt
if it would be pretty.  Note that a generic instantiation must be at
library level if it contains a controlled type.

- Bob