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Date: Sat, 27 Jun 2009 14:35:38 +0200
From: Damien Carbonne <damien.carbonne@free.fr>
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Subject: Re: Issue with GNAT GPL 2009 and GtkAda
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Date: 2009-06-27T14:35:38+02:00
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Damien Carbonne a �crit :
> Stephen Leake a �crit :
>>>>
>>>> ...
>>> If what you say is true, why does the following example work fine
>>> (using GNAT), without any compiler warning or execution error ?
>>
>> Because the object being passed is explicitly labeled "aliased"; that
>> means 'Access is allowed. See my other post from today.
>>
> In the example, there is only one place where an object is explicitly 
> declared and labeled "aliased" : in Main.
> In signatures of P[123]M, X is not aliased (only in, out or in out modes).
> There is no "aliased" key word in the bodies of corresponding procedures.
> 
> e.g.:
>    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); -- Here use of Access on a tyoe 
> that is implicitely aliased
>          X.P1A (Max_Depth - 1);
>       end if;
>    end P1M;
> Nowhere X is explicitely declared aliased. It is only an "in out" tagged 
> parameter. It is however possible to use X'Access, in my mind because X 
> is tagged (and so passed by reference).
> 
> Do you mean that the fact that X is labeled "aliased" in Main is 
> propagated everywhere it is used ? I would find this strange (and 
> certainly needing a special support from compiler).
> 
> I really don't see the difference with the initial problem, where all 
> types are tagged types or interfaces.

I would add that if declaration of X is changed to:
    type Base is null record; -- no more tagged

and that the object notation lines are removed, the compiler complains:

pack03.adb:15:15: prefix of "Access" attribute must be aliased
...

    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); -- line 15
       end if;
    end P1M;

This behaves as expected (by me).
So a tagged type is really and always aliased, whatever its mode (in, 
out, in out) is. At least, this is what understand from those examples.