From: Damien Carbonne <damien.carbonne@free.fr>
Subject: Re: Issue with GNAT GPL 2009 and GtkAda
Date: Fri, 26 Jun 2009 23:03:30 +0200
Date: 2009-06-26T23:03:31+02:00 [thread overview]
Message-ID: <4a4537a3$0$441$426a34cc@news.free.fr> (raw)
In-Reply-To: <u1vp7l4tm.fsf@stephe-leake.org>
Stephen Leake a �crit :
> The problem is clear here.
>
> "D : in out Derived" is allowed to create a copy of the actual
> parameter. Thus the 'Access in Foreach could be on a stack copy of D,
> which will disappear eventually, leaving a dangling pointer in BB. So
> the accessibility error is correct.
>
> If you replace "in out" in Process with "access":
>
> procedure Process (L : access Listener) is abstract;
>
> it works fine (with GNAT 6.2.1 on Windows).
>
>
> Your original program had the same issue; I just missed it.
>
>> I find usage of 'Access quite annoying, because I never really know
>> when there is an exception whether it is becauise my code is incorrect
>> or because the compiler is incorrect.
>
> It is frustrating.
>
> It would be nice if the runtime error had some more info; in this case
> "taking 'Access of a stack copy due to 'in out'" would be nice :).
>
> Part of the problem is the implicit conversion from object to access
> in Process:
>
> procedure Process (D : in out Derived) is
> begin
> Ada.Text_IO.Put_Line ("Process");
> D.Foreach (2);
> end Process;
>
> The call to Foreach is effectively:
>
> Foreach (D'access, 2);
>
> If you had written it that way, it would have been clearer what was
> happening.
>
> The moral here: if the body of the program needs an access value, pass
> in an access value!
>
> My code almost never uses 'Access, because it's better to create the
> object via an allocator in the first place, and pass around access
> values instead.
>
> Thanks for this puzzle; it has helped me realize exactly why I do
> this!
>
>> The last simple example I wrote seems to indicate that GNAT has a
>> bug, but I'm not an Ada expert ! If you also consider that there is
>> a GNAT bug, I'll submit a bug to AdaCore.
>
> I now think this is not a compiler bug.
>
If what you say is true, why does the following example work fine (using
GNAT), without any compiler warning or execution error ?
As I told in a previous message, I don't want to change the "in out"
mode of Interface (representing the code I wrote and that is independant
of GtkAda or any graphical toolkit), because one day it could be used in
cunjunction with third party code that uses access !
Otherwise, it would almost certainly mean that one needs to use access
almost everywhere, even if it is meaningless.
The following example works as I expect. If it should not, I have to do
something (go back to school ?) ;-)
-------------------------------------------------------------------
package Pack02 is
type Base is tagged null record;
procedure P1M (X : in out Base; Max_Depth : Positive);
procedure P1A (X : access Base; Max_Depth : Positive);
procedure P2M (X : in Base; Max_Depth : Positive);
procedure P2A (X : access constant Base; Max_Depth : Positive);
procedure P3M (X : out Base; Max_Depth : Positive);
procedure P3A (X : access Base; Max_Depth : Positive);
procedure Main;
end Pack02;
-------------------------------------------------------------------
with Ada.Text_IO;
package body Pack02 is
function Indent (X : Positive) return String is
S : constant String (1 .. 6 - X) := (others => ' ');
begin
return S;
end Indent;
procedure P1M (X : in out Base; Max_Depth : Positive) is
begin
Ada.Text_IO.Put_Line (Indent (Max_Depth) & "P1M" & Positive'Image
(Max_Depth));
if Max_Depth > 1 then
P1A (X'Access, Max_Depth - 1);
X.P1A (Max_Depth - 1);
end if;
end P1M;
procedure P1A (X : access Base; Max_Depth : Positive) is
begin
Ada.Text_IO.Put_Line (Indent (Max_Depth) & "P1A" & Positive'Image
(Max_Depth));
if Max_Depth > 1 then
P1M (X.all, Max_Depth - 1);
X.P1M (Max_Depth - 1);
end if;
end P1A;
procedure P2M (X : in Base; Max_Depth : Positive) is
begin
Ada.Text_IO.Put_Line (Indent (Max_Depth) & "P2M" & Positive'Image
(Max_Depth));
if Max_Depth > 1 then
P2A (X'Access, Max_Depth - 1);
X.P2A (Max_Depth - 1);
end if;
end P2M;
procedure P2A (X : access constant Base; Max_Depth : Positive) is
begin
Ada.Text_IO.Put_Line (Indent (Max_Depth) & "P2A" & Positive'Image
(Max_Depth));
if Max_Depth > 1 then
P2M (X.all, Max_Depth - 1);
X.P2M (Max_Depth - 1);
end if;
end P2A;
procedure P3M (X : out Base; Max_Depth : Positive) is
begin
Ada.Text_IO.Put_Line (Indent (Max_Depth) & "P3M" & Positive'Image
(Max_Depth));
if Max_Depth > 1 then
P3A (X'Access, Max_Depth - 1);
X.P3A (Max_Depth - 1);
end if;
end P3M;
procedure P3A (X : access Base; Max_Depth : Positive) is
begin
Ada.Text_IO.Put_Line (Indent (Max_Depth) & "P3A" & Positive'Image
(Max_Depth));
if Max_Depth > 1 then
P3M (X.all, Max_Depth - 1);
X.P3M (Max_Depth - 1);
end if;
end P3A;
procedure main is
X : aliased Base;
begin
P1M (X, 3);
P2M (X, 3);
P3M (X, 3);
end main;
end Pack02;
-------------------------------------------------------------------
with Pack02;
procedure Main02 is
begin
Pack02.Main;
end Main02;
-- Output:
--
-- P1M 3
-- P1A 2
-- P1M 1
-- P1M 1
-- P1A 2
-- P1M 1
-- P1M 1
-- P2M 3
-- P2A 2
-- P2M 1
-- P2M 1
-- P2A 2
-- P2M 1
-- P2M 1
-- P3M 3
-- P3A 2
-- P3M 1
-- P3M 1
-- P3A 2
-- P3M 1
-- P3M 1
-------------------------------------------------------------------
next prev parent reply other threads:[~2009-06-26 21:03 UTC|newest]
Thread overview: 48+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-06-23 21:52 Issue with GNAT GPL 2009 and GtkAda Damien Carbonne
2009-06-24 7:40 ` Dmitry A. Kazakov
2009-06-24 10:15 ` Stephen Leake
2009-06-25 9:06 ` Stephen Leake
2009-06-25 9:39 ` Dmitry A. Kazakov
2009-06-25 19:02 ` Damien Carbonne
2009-06-26 9:31 ` Stephen Leake
2009-06-26 11:18 ` Niklas Holsti
2009-06-26 16:29 ` Damien Carbonne
2009-06-26 17:28 ` Dmitry A. Kazakov
2009-06-26 19:27 ` Damien Carbonne
2009-06-26 19:50 ` Dmitry A. Kazakov
2009-06-26 21:51 ` Randy Brukardt
2009-06-27 11:11 ` Stephen Leake
2009-06-27 17:04 ` Robert A Duff
2009-06-30 11:11 ` Stephen Leake
2009-06-30 18:10 ` Robert A Duff
2009-06-29 22:11 ` Randy Brukardt
2009-06-30 11:13 ` Stephen Leake
2009-06-30 15:26 ` Adam Beneschan
2009-06-30 15:59 ` Adam Beneschan
2009-06-30 23:11 ` Randy Brukardt
2009-06-27 9:56 ` Stephen Leake
2009-06-26 21:03 ` Damien Carbonne [this message]
2009-06-27 11:21 ` Stephen Leake
2009-06-27 12:25 ` Damien Carbonne
2009-06-27 12:35 ` Damien Carbonne
2009-06-29 22:15 ` Randy Brukardt
2009-07-01 19:22 ` Damien Carbonne
2009-06-30 0:48 ` Adam Beneschan
2009-06-30 11:18 ` Stephen Leake
2009-06-25 20:49 ` Randy Brukardt
2009-06-26 7:20 ` Dmitry A. Kazakov
2009-06-26 8:17 ` Georg Bauhaus
2009-06-26 8:52 ` Dmitry A. Kazakov
2009-06-26 21:38 ` Randy Brukardt
2009-06-27 7:47 ` Dmitry A. Kazakov
2009-06-29 21:59 ` Randy Brukardt
2009-06-30 8:31 ` Dmitry A. Kazakov
2009-06-26 21:31 ` Randy Brukardt
2009-06-27 7:53 ` Dmitry A. Kazakov
2009-06-26 8:39 ` Alex R. Mosteo
2009-06-26 9:07 ` Dmitry A. Kazakov
2009-06-27 9:53 ` Stephen Leake
2009-06-26 21:40 ` Randy Brukardt
2009-06-29 10:04 ` Alex R. Mosteo
2009-06-26 9:02 ` Stephen Leake
2009-06-26 9:14 ` Dmitry A. Kazakov
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