Re: Gem 39 - compiler specific?

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Wed, 2 Sep 2009 18:20:36 -0500, Randy Brukardt wrote:

> "Maciej Sobczak" <see.my.homepage@gmail.com> wrote in message 
> news:9e0bbbcd-260f-48ed-8043-d6280c633e85@h3g2000yqa.googlegroups.com...
>> Consider:
>>
>> http://www.adacore.com/2008/06/09/gem-39/
>>
>> The example code performs Unchecked_Conversion between addresses of
>> two different array types.
>> As far as I understand, there is no standard provision for arrays to
>> occupy contiguous memory space (and not even for the alignment of
>> their components) and such a conversion relies heavily on the
>> assumption that arrays can be "overlaid" by plain address
>> reinterpretation.
>>
>> Still, this technique is quite attractive. What is the chance (in
>> practice) to hit the compiler that does not get it "right"?
> 
> This is horrible code; it should never be written in Ada 95 or newer! The 
> problem is that System.Address is not necessarily the same as a general 
> access type, so this Unchecked_Conversion may not work (or may not even 
> compile) on another implementation. (It won't on at least some versions of 
> Janus/Ada, for instance.)
> 
> Ada 95 provides System.Address_to_Access_Conversions for this purpose, so at 
> the very least the writer of the Rock (it surely isn't a "Gem"!!!) should 
> have used it:
> 
> procedure Write_Buffer
>       (Stream : not null access Ada.Streams.Root_Stream_Type'Class;
>        Item   : in Buffer)  is
>       Item_Size : constant Stream_Element_Offset :=
>                      Buffer'Object_Size / Stream_Element'Size;
> 
>      subtype SEA is Stream_Element_Array (1..Item_Size);
> 
>      package A2A is new System.Address_to_Access_Conversions (SEA);
> 
> begin
>       Ada.Streams.Write (Stream.all, A2A.To_Pointer (Item'Address).all);
> end Write_Buffer;
> 
> (As an aside, this technique could not work in Janus/Ada if Buffer was an 
> unconstrained array: 'Address of an unconstrained array points at the array 
> descriptor, not the data.

It must point to the first array element 13.3(14). BTW, it is interesting
where it should point when the array is empty.

> (That's in fact true for all objects in Janus/Ada: 
> there is a static part and an optional dynamic part, and all operations like 
> 'Address and 'Size point at the static part. That was decided long before 
> Ada 95 came around and tried to change these definitions; it's not practical 
> to change in Janus/Ada as address clauses could not be made to work - among 
> other things - if the static part is ignored.)

Skewed address is not a problem here. The problem is that
System.Address_To_Access_Conversions should take an access type as a
parameter rather than declare it new. This makes its use quite limited.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de