What's wrong with this code?

What's wrong with this code?

[okellogg]

Dear colleagues:

I can't find what wrong with this code (see full declaration of type Alarm.)
Perhaps someone would like to comment?

Thanks alot,

Oliver Kellogg
DaimlerChrysler Aerospace
Sensor Systems, Dept. VGE15
D-89070 Ulm, Germany
e-mail: oliver.kellogg@vs.dasa.de

--
-- file: alarm_model.ads
package Alarm_Model is

    type Alarm is tagged limited private;
    type Alarm_P is access all Alarm'Class;

    procedure Norm (This : access Alarm'Class);

private

    type Alarm_State_Et is
       (Snafu, Data, Proposal);

    type Alarm_State (My_Alarm : access Alarm) is
       abstract tagged limited null record;
    type Alarm_State_P is access all Alarm_State'Class;

    type Alarm_State_Pool_At is
       array (Alarm_State_Et) of Alarm_State_P;

    procedure Enter (This : access Alarm_State);

    type Snafu_State is new Alarm_State with null record;
    type Snafu_State_P is access all Snafu_State'Class;

    procedure Request_Data (This : access Snafu_State);

    type Data_State is new Alarm_State with null record;
    type Data_State_P is access all Data_State'Class;

    type Proposal_State is new Alarm_State with null record;
    type Proposal_State_P is access all Proposal_State'Class;

    protected type Alarm_Semaphore (My_Alarm : access Alarm) is
        function Get return Alarm_State_P;
        procedure Set (New_State : in Alarm_State_Et);
    private
        Current_State : Alarm_State_Et := Snafu;
        ----------------------------- This is the workaround used
        -- Alarm_State_Pool : Alarm_State_Pool_At :=
        --    (Snafu => new Snafu_State (My_Alarm),
        --     Data => new Data_State (My_Alarm),
        --     Proposal => new Proposal_State (My_Alarm));
    end Alarm_Semaphore;

    type Alarm is tagged limited
        record
            -- This compiles alright:
            Semaphore : Alarm_Semaphore (Alarm'Access);

            -- But here, GNAT 3.11p says:
            --  "Access" attribute cannot be applied to type
            Alarm_State_Pool : Alarm_State_Pool_At :=
               (Snafu => new Snafu_State (Alarm'Access),
                Data => new Data_State (Alarm'Access),
                Proposal => new Proposal_State (Alarm'Access));

        end record;

end Alarm_Model;


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Re: What's wrong with this code?

[dennison]

In article <7fi85m$sb3$1@nnrp1.dejanews.com>,
  okellogg@my-dejanews.com wrote:
> Dear colleagues:
>
> I can't find what wrong with this code (see full declaration of type Alarm.)
> Perhaps someone would like to comment?

>     type Alarm is tagged limited
>         record
>             -- This compiles alright:
>             Semaphore : Alarm_Semaphore (Alarm'Access);
>
>             -- But here, GNAT 3.11p says:
>             --  "Access" attribute cannot be applied to type
>             Alarm_State_Pool : Alarm_State_Pool_At :=
>                (Snafu => new Snafu_State (Alarm'Access),
>                 Data => new Data_State (Alarm'Access),
>                 Proposal => new Proposal_State (Alarm'Access));
>
>         end record;
>
> end Alarm_Model;

That's probably because 'Access can't be applied to a type. :-)

What is it you are trying to accomplish with Alarm'Access? Surely you don't
want a pointer into the compiler's type table for "Alarm", which is about the
only thing this could mean. Perhaps you are hoping to automaticly get a
pointer to the *object* in question when you create objects of this type?

The contexts under which 'Access may be used are listed in Annex K of the LRM
( http://www.adahome.com/rm95/rm9x-K.html ). Its prefix may be a subprogram,
object, or label.

--
T.E.D.

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Re: What's wrong with this code?

[Matthew Heaney]

dennison@telepath.com writes:

> In article <7fi85m$sb3$1@nnrp1.dejanews.com>,
>   okellogg@my-dejanews.com wrote:
> > Dear colleagues:
> >
> > I can't find what wrong with this code (see full declaration of type 
> > Alarm.) Perhaps someone would like to comment?
> >
> >     type Alarm is tagged limited
> >         record
> >             -- This compiles alright:
> >             Semaphore : Alarm_Semaphore (Alarm'Access);
> >
> >             -- But here, GNAT 3.11p says:
> >             --  "Access" attribute cannot be applied to type
> >             Alarm_State_Pool : Alarm_State_Pool_At :=
> >                (Snafu => new Snafu_State (Alarm'Access),
> >                 Data => new Data_State (Alarm'Access),
> >                 Proposal => new Proposal_State (Alarm'Access));
> >
> >         end record;
> >
> > end Alarm_Model;
> 
> That's probably because 'Access can't be applied to a type. :-)

This statement is incorrect.

Inside the declaration of the Alarm type, Alarm'Access denotes the
"current instance" the type.  

 
> What is it you are trying to accomplish with Alarm'Access? Surely you
> don't want a pointer into the compiler's type table for "Alarm", which
> is about the only thing this could mean. Perhaps you are hoping to
> automaticly get a pointer to the *object* in question when you create
> objects of this type?

Yes.

Re: What's wrong with this code?

[dennison]

In article <7fiegr$43q$1@nnrp1.dejanews.com>,
  dennison@telepath.com wrote:
> In article <7fi85m$sb3$1@nnrp1.dejanews.com>,
>   okellogg@my-dejanews.com wrote:
> > Dear colleagues:
> >
> > I can't find what wrong with this code (see full declaration of type Alarm.)
> > Perhaps someone would like to comment?
>
> >     type Alarm is tagged limited
> >         record
> >             -- This compiles alright:
> >             Semaphore : Alarm_Semaphore (Alarm'Access);
> >
> >             -- But here, GNAT 3.11p says:
> >             --  "Access" attribute cannot be applied to type
> >             Alarm_State_Pool : Alarm_State_Pool_At :=
> >                (Snafu => new Snafu_State (Alarm'Access),
> >                 Data => new Data_State (Alarm'Access),
> >                 Proposal => new Proposal_State (Alarm'Access));
> >
> >         end record;
> >
> > end Alarm_Model;
>
> That's probably because 'Access can't be applied to a type. :-)

Drat.

Before anyone else flames me: yes, I had forgotten about the "current
instance" rule. From what I can see in the LRM its supposed to be valid
anywhere "within the declarative region of a type_declaration". So now I'm
also curious why the compiler doesn't like it.

I tried adding the following declarations:
    type Alarm_List is array (1..2) of Alarm_P;

    type Alarm is tagged limited
    ...
            Al : Alarm_List := (others => Alarm'Access);

And got this error (on the "Al" field) instead:
alarm_model.ads:52:43: non-local pointer cannot point to local object


--
T.E.D.

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Re: What's wrong with this code?

[Matthew Heaney]

dennison@telepath.com writes:

> I tried adding the following declarations:
>     type Alarm_List is array (1..2) of Alarm_P;
> 
>     type Alarm is tagged limited
>     ...
>             Al : Alarm_List := (others => Alarm'Access);
> 
> And got this error (on the "Al" field) instead:
> alarm_model.ads:52:43: non-local pointer cannot point to local object

What message do you get if you use 'Unchecked_Access instead of 'Access?

Re: What's wrong with this code?

[dennison]

In article <m3zp43ywi8.fsf@mheaney.ni.net>,
  Matthew Heaney <matthew_heaney@acm.org> wrote:
> dennison@telepath.com writes:
>
> > I tried adding the following declarations:
> >     type Alarm_List is array (1..2) of Alarm_P;
> >
> >     type Alarm is tagged limited
> >     ...
> >             Al : Alarm_List := (others => Alarm'Access);
> >
> > And got this error (on the "Al" field) instead:
> > alarm_model.ads:52:43: non-local pointer cannot point to local object
>
> What message do you get if you use 'Unchecked_Access instead of 'Access?


Just the same old ""Access" attribute cann be applied to type". Good call.

I tried compiling the same code (the original code, not my modified
version) using my GreenHills (Intermetrics-based) compiler, and got:

d:\test\tmp\alarm_model.ads: Error: line 53 col 26 LRM:3.10.2(28), The
accessibility level of the prefix to 'ACCESS shall not be statically deeper
than that of the expected type of the allocator, Continuing

on the same line I got the above Gnat error. Changing the three "'Access"es to
"'Unchecked_Access" caused the GrennHills compile to succeed. But in Gnat the
same code gave me

alarm_model.ads:53:43: "Unchecked_Access" attribute cannot be applied to type
alarm_model.ads:54:41: "Unchecked_Access" attribute cannot be applied to type
alarm_model.ads:55:49: "Unchecked_Access" attribute cannot be applied to type


So it looks like one compiler (gnat) thinks the whole construct is illegal,
while the other (GreenHills) thinks the only problem is accessability levels.

--
T.E.D.

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Re: What's wrong with this code?

[Stephen Leake]

okellogg@my-dejanews.com writes:

> package Alarm_Model is
> 
>     type Alarm is tagged limited private;
>     type Alarm_P is access all Alarm'Class;
> 
>     procedure Norm (This : access Alarm'Class);
> 
> private
> 
>     type Alarm_State_Et is
>        (Snafu, Data, Proposal);
> 
>     type Alarm_State (My_Alarm : access Alarm) is
>        abstract tagged limited null record;
>     type Alarm_State_P is access all Alarm_State'Class;
> 
>     type Alarm_State_Pool_At is
>        array (Alarm_State_Et) of Alarm_State_P;
> 
>     procedure Enter (This : access Alarm_State);
> 
>     type Snafu_State is new Alarm_State with null record;
>     type Snafu_State_P is access all Snafu_State'Class;
> 
>     procedure Request_Data (This : access Snafu_State);
> 
>     type Data_State is new Alarm_State with null record;
>     type Data_State_P is access all Data_State'Class;
> 
>     type Proposal_State is new Alarm_State with null record;
>     type Proposal_State_P is access all Proposal_State'Class;
> 
>     protected type Alarm_Semaphore (My_Alarm : access Alarm) is
>         function Get return Alarm_State_P;
>         procedure Set (New_State : in Alarm_State_Et);
>     private
>         Current_State : Alarm_State_Et := Snafu;
>         ----------------------------- This is the workaround used
>         -- Alarm_State_Pool : Alarm_State_Pool_At :=
>         --    (Snafu => new Snafu_State (My_Alarm),
>         --     Data => new Data_State (My_Alarm),
>         --     Proposal => new Proposal_State (My_Alarm));
>     end Alarm_Semaphore;
> 
>     type Alarm is tagged limited
>         record
>             -- This compiles alright:
>             Semaphore : Alarm_Semaphore (Alarm'Access);
> 
>             -- But here, GNAT 3.11p says:
>             --  "Access" attribute cannot be applied to type
>             Alarm_State_Pool : Alarm_State_Pool_At :=
>                (Snafu => new Snafu_State (Alarm'Access),
>                 Data => new Data_State (Alarm'Access),
>                 Proposal => new Proposal_State (Alarm'Access));
> 
>         end record;
> 
> end Alarm_Model;

ObjectAda gives the error message (for Alarm_State_Pool):

alarm_model.ads: Error: line 56 col 38 LRM:3.10.2(28), The
accessibility level of the prefix to 'ACCESS shall not be statically
deeper than that of the expected type of the allocator, Continuing

(The LRM reference says the same thing :)

Apparently using 'Access inside an allocator introduces an access
layer? I don't see why it should in this case. In a function call, it
might be a problem. And everything is at library level anyway. 


GNAT 3.11p gives the error:

alarm_model.ads:56:55: "Access" attribute cannot be applied to type

As others have pointed out, and as Semaphore demonstrates, it can too.
So this is clearly a bad error message - send it in to ACT.

I suspect both compilers are confused, not having encountered this
construct before.

As a work around, you could make Alarm a Controlled type, and create
the Alarm_State_Pool objects in Initialize.

-- Stephe