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Newsgroups: comp.lang.ada
From: Anonymous Coward <spam@spam.com>
Subject: 'Size can mean value size OR object size, depending..
References: <slrndmloee.kq0.bogus_addy@tango.mindfuq.org>
 <meKdnXXIJd6tVPfeRVn-vw@comcast.com>
 <slrndmlus8.kq0.bogus_addy@tango.mindfuq.org> <uacgktuhd.fsf@acm.org>
 <slrndmoh1u.b6j.bogus_addy@tango.mindfuq.org> <uk6fn758i.fsf@acm.org>
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Date: Wed, 09 Nov 2005 02:12:47 GMT
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Date: 2005-11-09T02:12:47+00:00
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In article <uk6fn758i.fsf@acm.org>, Stephen Leake wrote:
> 
> Hmm. You seem to be saying "convention (C) doesn't work for
> me". That seems like a bug. Can you post some code that doesn't
> work? Or have you submitted a bug report?

I explored this issue a little more, and it looks like it was my
error.  I thought 'Size returned the object size, not the value size!
To top it off, I was using GNAT 2.8.1, which does not have a
-gnatR[1-3] switch.  If it did, this it would have clearly given me
this output for my enum:

   $ gcc -c -gnatR3 main.adb
    
   Representation information for unit Main (body)
   -----------------------------------------------
    
   for Entity_Type'Object_Size use 32;
   for Entity_Type'Value_Size use 2;
   for Entity_Type'Alignment use 4;

   $ gcc --version
   gcc (GCC) 3.2.2 20030222 (Red Hat Linux 3.2.2-5)

(as you can see, later versions of GNAT support 
 enum representation printing) 

So at work I was reliant on 'Size to tell me how large the enum is,
not knowing until now that 'Object_Size is really an attribute
operation.  I thought 'Object_Size was just gnats way of making the
distinction.  

So this code:

   with Ada.Text_Io;

   procedure Main is

      type Entity_Type is (Red, Green, Blue);

      pragma Convention (Convention => C, Entity => Entity_Type);

      --In this case, 'Size really means the *object* size!
      --
      --for Entity_Type'Size use 32; --not needed with convention C!

   begin

      Ada.Text_Io.Put_Line ("The object size is " & 
                            Integer'Image(Entity_Type'Object_Size));

      --ADA throws a curve ball here, by changing the meaning of 'Size
      --to actually indicate the *value* size in this case!
      --
      Ada.Text_Io.Put_Line ("The value size is "  & 
                            Integer'Image(Entity_Type'Size));

   end Main;

generates:

   The object size is  32
   The value size is  2

proving that pragma convention effectively works on enumeration
types.  So in the interest of writing clean code, I do not intend to
rep spec enum sizes anymore, because pragma Convention takes care of
this from a higher level.