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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 01:24:28 -0800 (PST)
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Date: 2010-03-12T01:24:28-08:00
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Ludovic Brenta wrote on comp.lang.ada:
> 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

Sorry, I meant:

procedure Refresh (Object : in out T; Dirty : in out Boolean) 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?
>
> --
> Ludovic Brenta.