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From: prichtmyer@yahoo.com (Peter Richtmyer)
Newsgroups: comp.lang.ada
Subject: Null Record is not always Null
Date: 10 Oct 2002 06:12:01 -0700
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Date: 2002-10-10T13:12:01+00:00
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This is a little on the light side, but is also 
frustrating to see in "portable Ada". Perhaps others
with different compilers will find even different
results.
-----------------------------------------------------
with Text_Io;
procedure nil is
    type nul is null record;
    for nul'size use 0;
    nil : nul;
    type aray is array (1 .. 1000) of nul;
--    pragma pack (aray);                    -- line 7
--    for aray'size use 0;                   -- line 8
   ary : aray;
   type dbl is array (1 .. 1000) of aray;
--   pragma pack (dbl);                     -- line 11
--   for dbl'size use 0;                    -- line 12
   dub : dbl;
begin    
   text_io.put_line ("Nil size = " & integer'image(nil'size));
   text_io.put_line ("Ary size = " & integer'image(ary'size));
   text_io.put_line ("Dub size = " & integer'image(dub'size));
   --------------------------------------------
   -- output on Gnat 3.14p (win2000):
   -- Nil size = 8
   -- Ary size = 8
   -- Dub size = 8
   --------------------------------------------
   -- Rational Apex Ada 95 v. 4.0.0b (Solaris)
   --    (would not compile until I commented out lines 8 & 12)
   -- 06:39:44 AM >>> Line 8: for Aray'Size use 0;
   -- 06:39:44 AM *** Specified size 0 is too small for array (1 .. 1000) of Nul
   -- 06:39:44 AM ++* nil.2.ada has failed to install in ...
   -- 06:39:50 AM +++ 1 unit was parsed
   -- 06:39:50 AM ++* 1 unit could not be installed
   --
   -- Rational output with lines 8 & 12 commented out:
   -- Nil size =  0
   -- Ary size =  1000
   -- Dub size =  1000000
   --------------------------------------------  
   -- Aonix ObjectAda 7.1.105 Professional Version (win2000)
   -- (Would not compile. Window said:)
   --    Program Error
   --      ADACOMP.exe has generated errors and will be closed by
   --      Windows. You will need to restart the program.
   --      An error log is being created.
   -- It did not close. I could not find an error log.
   -- In order to even start compiling, I had to comment lines 7,8,11,12
   -- Then output was:
   -- Nil size =  0
   -- Ary size =  0
   -- Dub size =  0
   -- which is what I wanted!
   ---------------------------------------------

end nil;

-----
Don't know if there is a "right" result, or is this one of 
those gray areas. Anybody?

Peter