Re: 'Size can mean value size OR object size, depending..

[Anonymous Coward <spam@spam.com>]

In article <E0ecf.1856$Id6.503@newsread1.news.pas.earthlink.net>, 
Jeffrey R. Carter wrote:
> 
> Note that 'Object_Size is GNAT specific. The portable way to obtain
> this value is to use 'Size on an object:
> 
> X : T;
> 
> X'Size

That's interesting!  I would not have expected ADA to even allow
compilers to offer their own attributes like that.  It seems like a
bad thing.  Folks could unwittingly write GNAT only code, which would
break later on if a different compiler is chosen.

For the archives, here's a complete example that includes Jeff's
comment:

   with Ada.Text_Io;

   procedure Size_Example is

      type Color_Type is (Red, Green, Blue);

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

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

      Color_Object : Color_Type := Color_Type'First;

   begin

      --GNAT *uniquely* offers a 'Object_Size attribute,
      --so the following line is not ADA!
      --
      Ada.Text_Io.Put_Line ("The object size is " &
                            Integer'Image(Color_Type'Object_Size) &
                            " GNAT only!");

      --ADA does not provide an attribute to get the object size of a type.
      --You can only acquire this by doing a 'Size directly on an object:
      --
      Ada.Text_Io.Put_Line ("The object size is " &
                            Integer'Image(Color_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(Color_Type'Size));

   end Size_Example;

Output:

   The object size is  32 GNAT only!
   The object size is  32
   The value size is  2