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From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Re: Ensuring postconditions in the face of exceptions
Date: Fri, 12 Mar 2010 03:08:58 -0800 (PST)
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Date: 2010-03-12T03:08:58-08:00
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On Mar 12, 10:29=A0am, Niklas Holsti <niklas.hol...@tidorum.invalid>
wrote:
> Ludovic Brenta wrote:
> > Consider the procedure:
>
> > type T is private; -- completion elided
>
> > generic
> > =A0 =A0with procedure Visit (Object : in out T);
> > procedure Refresh (Object : in out T; Dirty : in out T) is
> > begin
> > =A0 =A0if Dirty then
> > =A0 =A0 =A0 Visit (Object);
> > =A0 =A0 =A0 Dirty :=3D False;
> > =A0 =A0end if;
> > exception
> > =A0 =A0when others =3D>
> > =A0 =A0 =A0 Dirty :=3D True; -- warnings here
> > =A0 =A0 =A0 raise;
> > end Refresh;
>
> > GNAT says:
> > warning: assignment to pass-by-copy formal may have no effect
> > warning: "raise" statement may result in abnormal return (RM
> > 6.4.1(17))
>
> > The reason for the exception handler is to enforce a postcondition
> > that Dirty must be True if Visit raises an exception. However the
> > warnings suggest that the postcondition cannot be enforced this way.
> > How should I rewrite my code?
>
> Perhaps change the mode of Dirty to
>
> =A0 =A0 Dirty : access T;
>
> and assign
>
> =A0 =A0 Dirty.all :=3D True;
>
> Or use a named access type instead of the anonymous access, as you prefer=
.

Heh. I was kind of hoping you wouldn't say that :) I should have been
more specific; I think using an access type is ugly but I can't think
of a better solution.

Thanks.

--
Ludovic Brenta.