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NNTP-Posting-Date: Tue, 15 Sep 2009 20:22:58 -0500
From: R.B. Love <rblove@airmail.net>
Newsgroups: comp.lang.ada
Date: Tue, 15 Sep 2009 20:22:58 -0500
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Subject: Rep Specing Variant Records
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Date: 2009-09-15T20:22:58-05:00
List-Id: <comp.lang.ada>

I want to understand how a variant record gets rep spec'd.  Let's say I 
have a sample of my record:

type stuff(has_extra:boolean) is
record
    foo: integer;
    bar: integer;
    case has_extra
        when true =>
            blah : float;
        when false =>
            null;
    end case;
end record;

for stuff use
record
    foo at 0 range 0..31;
    bar at 4 range 0..31;
end;

type normal_stuff is new stuff(has_extra=>false);
type good_stuff is new stuff(has_extra=>true);

for good_stuff use
record
    blah at 12 range 0..31;
end record;

for normal_stuff'size use 64;  -- 8 bytes
for good_stuff'size use 96;    -- 12 bytes

First, does the discriminant take up any size in the record?  When I 
tried this with a tagged record, I was told that my placement at 0 
bytes off set classed with the tag.  That's no good.

Second, does the layout of normal_stuff look like I expect it would, 
that is, the same as stuff?

Does the good_stuff record have a complete rep spec?

Thanks for any replies.  I see books covering the rep spec of plain 
records but none seem to cover variant records.