Re: Maybe not exactly a variant record.

[dennison@telepath.com]

In article <371f1e6d.0@silver.truman.edu>,
  "Josh Highley" <joshhighley@hotmail.com> wrote:
>
> I guess I should clarify.  I'm not exactly sure the record is variant, at
> least not in the sense as some people have thought.  So, here's the private
> declaration of "type field(length : positive);"
>
> private
>     type field (length : positive ) is record
...
>          text            : string(1..length) := (others => ' ');
>          text_length  : natural := 0;

...
>     end record;
>
> So, the value of "length" doesn't change the record fields that are
> accessible, just the number of characters in a string.  Now that I've better
> described the situation, does this change the responses to declaring an
> array of type field with different lengths??

No it changes neither the responses, nor their rationale.

Assme you make an object of this type called "foo". If foo.length changes
from 4 to 5, suddenly foo.text(5) is available for you to access. If you don't
also assign a vaule to foo.text(5) when you assign a 5 to foo.length, then
there's uninitialized garbage sitting in foo.text(5).

What's worse, you could in theory try to hose the type of the .text_length
field by doing a foo.length := foo.length - 4. Depending on how your compiler
arranges the fields and implements the variant change, that could end up
putting the previous last 4 characters in foo.text into .text_length as an
integer!

Except of course this won't happen, because Ada won't allow you to change
foo.length w/o changing all of foo at the same time. :-)

> P.S.  For my future reference, is the record that I've declared above
> considered a variant record, or is it called something else since the
> available record fields don't actually change with the value of "length"?

Yes, it is called a variant record, with "length" as the variant. Also the
available record fields *do* change, after a fashion. Incrementing foo.length
gives you an extra value in foo.text

--
T.E.D.

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