Re: Ensuring postconditions in the face of exceptions

[Ludovic Brenta <ludovic@ludovic-brenta.org>]

On Mar 12, 10:29 am, Niklas Holsti <niklas.hol...@tidorum.invalid>
wrote:
> Ludovic Brenta wrote:
> > Consider the procedure:
>
> > type T is private; -- completion elided
>
> > generic
> >    with procedure Visit (Object : in out T);
> > procedure Refresh (Object : in out T; Dirty : in out T) is
> > begin
> >    if Dirty then
> >       Visit (Object);
> >       Dirty := False;
> >    end if;
> > exception
> >    when others =>
> >       Dirty := True; -- warnings here
> >       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
>
>     Dirty : access T;
>
> and assign
>
>     Dirty.all := 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.