Re: 'SIZE attribute of a type

[elroy.jpl.nasa.gov!sdd.hp.com!caen!uvaarpa!software.org!blakemor@ames.arc .nasa.gov (Alex Blakemore)]

In article <1991Dec13.152340.20444@software.org> smithd@software.org (Doug Smit
h) writes:
> If I have understood this thread, the following
> statement should minimize the storage for variables of type b:
>     for b'Size use b'Size;

Interesting idea.

> So I used a Verdix compiler (old) to see if it would even compile the
> code--it does!  But for a Boolean type it still uses 8:

> How many compilers can compile this and then how many actually
> use one bit for this type?  Very interesting...but cryptic.

Hi Doug,
  I ran this through our VMS Ada compiler and got the following.
I think you could read the LRM more than one way here.  Is there
an AI about this ? LRM excerpt is at the end.

--------------- Pruned listing from VAX Ada V2.2-38 --------------
    1   with Text_IO;
    2
    3   procedure Test_B is
    4
    5       type B is new Boolean;
    6       for B'Size use B'Size;
................1...........2
%ADAC-I, (2) The representation of type B at line 5 is forced here
%ADAC-E, (1) The representation of type B at line 5 has already been forced at 
line 6 [LRM 13.1(6)]
    7
    8       B_variable : B;
    9   begin
   10       Text_IO.Put_Line ("Type B has size              " &
   11                         Integer'Image (B'Size));
   12       Text_IO.Put_Line ("Variables of type B have size" &
   13                         Integer'Image (B_variable'Size));
   14   end Test_B;
--------------- end VAX Ada V2.2-38 listing --------------

For those unfamiliar with VMS, the messages preceeded with %ADAC-I
are informational messages which may help shed light on the subject 
but are not errors or even warnings.  Errors are preceeded with %ADAC-E.

LRM 13.1(6)
... certain occurrences of its name imply that the representation of the type
must already have been determined. ... A forcing occurrence is any occurrence
other than in a type or subtype declaration, or a representation clause for the
type itself. In any case, an occurrence within an expression is always forcing.
-- 
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Alex Blakemore           blakemore@software.org        (703) 742-7125
Software Productivity Consortium  2214 Rock Hill Rd, Herndon VA 22070