Obtaining access to protected object

Obtaining access to protected object

[Maciej Sobczak]

Consider 9.4-21/2:

"Within the declaration or body of a protected unit other than in an
access_definition, the name of the protected unit denotes the current
instance of the unit"

I understand that this allows me to obtain the access to the current
object (the "this" access) of the protected object.
Let's try:

   protected type P is
      procedure Foo;
   end P;

   protected body P is
      procedure Foo is
         Ptr : access P;   -- P denotes the type P
      begin
         Ptr := P'Access;    -- here P denotes the "this" instance of
P
      end;
   end P;

This is what GNAT has to say about it: ""Access" attribute cannot be
applied to type".
As far as I understand, this is in conflict with the AARM paragraph
mentioned above.

Am I doing something wrong or is it a compiler bug?

--
Maciej Sobczak * www.msobczak.com * www.inspirel.com

Re: Obtaining access to protected object

[Adam Beneschan]

On Jul 8, 1:41 pm, Maciej Sobczak <see.my.homep...@gmail.com> wrote:
> Consider 9.4-21/2:
>
> "Within the declaration or body of a protected unit other than in an
> access_definition, the name of the protected unit denotes the current
> instance of the unit"
>
> I understand that this allows me to obtain the access to the current
> object (the "this" access) of the protected object.
> Let's try:
>
>    protected type P is
>       procedure Foo;
>    end P;
>
>    protected body P is
>       procedure Foo is
>          Ptr : access P;   -- P denotes the type P
>       begin
>          Ptr := P'Access;    -- here P denotes the "this" instance of
> P
>       end;
>    end P;
>
> This is what GNAT has to say about it: ""Access" attribute cannot be
> applied to type".
> As far as I understand, this is in conflict with the AARM paragraph
> mentioned above.
>
> Am I doing something wrong or is it a compiler bug?

It's a compiler bug.

                        -- Adam

Re: Obtaining access to protected object

[Maciej Sobczak]

On 9 Lip, 17:03, Adam Beneschan <a...@irvine.com> wrote:

> It's a compiler bug.

Reported to bugzilla.
I cannot believe nobody [*] tried it before.

[*] What about ACATS? My code example looks like a unit test derived
directly from AARM.

--
Maciej Sobczak * www.msobczak.com * www.inspirel.com

Re: Obtaining access to protected object

[Adam Beneschan]

On Jul 9, 2:21 pm, Maciej Sobczak <see.my.homep...@gmail.com> wrote:
> On 9 Lip, 17:03, Adam Beneschan <a...@irvine.com> wrote:
>
> > It's a compiler bug.
>
> Reported to bugzilla.
> I cannot believe nobody [*] tried it before.
>
> [*] What about ACATS? My code example looks like a unit test derived
> directly from AARM.

There's plenty that ACATS can't test.  The ACATS tries to test every
rule in the RM, but it doesn't try to test every possible combination
of rules that could interact---that would be impossibly hard.  In
fact, I just tried this on a *task* type, rather than a protected
type, and it works fine.  So if ACATS has a test that tests the
"current instance" rule, that would be considered good enough; if a
compiler handles current instances correctly in some cases but not
others, the ACATS isn't designed to catch all of those problems.

                                  -- Adam

Re: Obtaining access to protected object

[Georg Bauhaus]

Maciej Sobczak wrote:
> On 9 Lip, 17:03, Adam Beneschan <a...@irvine.com> wrote:
> 
>> It's a compiler bug.
> 
> Reported to bugzilla.
> I cannot believe nobody [*] tried it before.
> 

Maybe Ada programmer have not tried creating a "this" pointer
inside a shared variable's procedure because they are much less
used to passing references around using pointers.  ;-)


-- 
Georg Bauhaus
Y A Time Drain  http://www.9toX.de

Re: Obtaining access to protected object

[Randy Brukardt]

"Maciej Sobczak" <see.my.homepage@gmail.com> wrote in message 
news:d234e827-c3e6-481a-a15e-94382c2fbf6c@d77g2000hsb.googlegroups.com...
...
> [*] What about ACATS? My code example looks like a unit test derived
> directly from AARM.

The Ada 95 ACATS writers made a rather fundamental mistake, in that they 
made no attempt to look at whether rules inherited from Ada 83 were tested 
properly. One result of that is that kinds of types added in Ada 95 
(modular, decimal, protected) types are not tested much with the basic rules 
of the language in the ACATS. (Another error that they made was to consider 
a paragraph covered if any rule in the paragraph is tested -- but there are 
a lot of paragraphs with multiple rules.)

The current ACATS work includes going back and restarting the coverage 
analysis from first principles. I'm finding that the coverage of the 
Standard in the ACATS is around 60% (far less than the Ada 95 contractors 
claimed). I doubt that given the current ACATS budget that this situation is 
going to change much -- I'm primarily emphasizing Ada 2005 changes for new 
tests.

Anyone can help, however, by submitting candidate ACATS tests. (I recommend 
doing so to complete untested test objectives where those have already been 
created for the full language, as that would increase the chances of a test 
being used in the ACATS.) Information on how to create tests can be found in 
the ACATS documentation - look at http://www.ada-auth.org/acats.html to find 
that. (I'm too busy at the moment to look up the exact link.) Feel free to 
contact me if you are interested and/or have questions. (Also, you can ask 
me for the most recent test objectives for a section of interest, as the 
ones available with the ACATS distribution are pretty stale.)

                                                       Randy.

Re: Obtaining access to protected object

[george.priv]

On Jul 8, 4:41 pm, Maciej Sobczak <see.my.homep...@gmail.com> wrote:
> Consider 9.4-21/2:
>
> "Within the declaration or body of a protected unit other than in an
> access_definition, the name of the protected unit denotes the current
> instance of the unit"
>
> I understand that this allows me to obtain the access to the current
> object (the "this" access) of the protected object.
> Let's try:
>
>    protected type P is
>       procedure Foo;
>    end P;
>
>    protected body P is
>       procedure Foo is
>          Ptr : access P;   -- P denotes the type P
>       begin
>          Ptr := P'Access;    -- here P denotes the "this" instance of
> P
>       end;
>    end P;
>
> This is what GNAT has to say about it: ""Access" attribute cannot be
> applied to type".
> As far as I understand, this is in conflict with the AARM paragraph
> mentioned above.
>
> Am I doing something wrong or is it a compiler bug?
>
> --
> Maciej Sobczak *www.msobczak.com*www.inspirel.com

The instance may not be aliased, and type itself is not enough to
determine that at compile time.

You can reference the protected type "this" instance though:

   protected type Xt is
      procedure Foo;
   end Xt;

   procedure Call_Foo (A : in out Xt);

   protected body Xt is

      procedure Foo is
      begin
         Call_Foo (Xt);
      end;

   end Xt;

Re: Obtaining access to protected object

[Adam Beneschan]

On Jul 9, 8:33 am, george.p...@gmail.com wrote:

> The instance may not be aliased, and type itself is not enough to
> determine that at compile time.

Wrong.  The current instance of a protected type is always considered
aliased, and 'Access is allowed.  3.10(9/2).

                             -- Adam

Re: Obtaining access to protected object

[george.priv]

On Jul 9, 3:27 pm, Adam Beneschan <a...@irvine.com> wrote:
> On Jul 9, 8:33 am, george.p...@gmail.com wrote:
>
> > The instance may not be aliased, and type itself is not enough to
> > determine that at compile time.
>
> Wrong.  The current instance of a protected type is always considered
> aliased, and 'Access is allowed.  3.10(9/2).
>
>                              -- Adam

I may be wrong, but it seems that Aliaseness is guaranteed only within
the protected body. So taking pointer and spilling it outside of the
scope (even if allowed) is bad idea IMHO.  If used inside the
protected body then what's advantage of using pointer vs. referring
directly?

George

Re: Obtaining access to protected object

[Adam Beneschan]

On Jul 9, 7:10 pm, george.p...@gmail.com wrote:
> On Jul 9, 3:27 pm, Adam Beneschan <a...@irvine.com> wrote:
>
> > On Jul 9, 8:33 am, george.p...@gmail.com wrote:
>
> > > The instance may not be aliased, and type itself is not enough to
> > > determine that at compile time.
>
> > Wrong.  The current instance of a protected type is always considered
> > aliased, and 'Access is allowed.  3.10(9/2).
>
> >                              -- Adam
>
> I may be wrong,

You're definitely wrong about the rules.  If there's a possibility
that the rules can allow a programmer to write a legal program that
creates a dangling reference (without involving certain dangerous
features), then the ARG needs to hear about it.  But I don't think
that's the case here.  The biggest problem, as far as I know, with
allowing something to access an unaliased object, is that the compiler
may not realize that something could point to it, and therefore could
optimize certain calculations so that the object doesn't necessarily
hold the right value when expected, which will then be a problem if
the program does a dereference of the access.  However, none of this
reasoning applies to protected types, or task types.

In fact, there would probably be no accessing problem at all if *all*
protected and task objects were considered aliased, even without an
explicit "aliased" keyword.  See AARM 3.10(9.l/2) [that's "nine" "dot"
"the lower-case letter L"]; the ARG actually considered doing this for
all by-reference types (which would include task and protected types,
as well as all tagged types and all limited record types).  They
decided not to do so because of some problems involving private parts.


> but it seems that Aliaseness is guaranteed only within
> the protected body. So taking pointer and spilling it outside of the
> scope (even if allowed) is bad idea IMHO.

See above; there's no danger in letting such pointers outside, even if
the object is not explicitly declared as "aliased".  Were it not for
some problems involving privacy breakage, the ARG saw no danger in
simply decreeing that all protected types, etc., are automatically
aliased.


> If used inside the
> protected body then what's advantage of using pointer vs. referring
> directly?

Because you could have an object of an access-to-protected type that
could, in some instances, be set to the same protected type, or could
be set to point to some other protected type.  Of course there's
little point in setting up a pointer that always refers to the same
object; the idea is that you could modify it to point to something
else.

                                 -- Adam