Re: Bitmanipulation in Ada

["(see below)" <byaldnif@blueyonder.co.uk>]

On 18/8/04 10:56 pm, in article cg0j9n$bkq$1@sparta.btinternet.com, "Martin
Dowie" <martin.dowie@btopenworld.com> wrote:

...
>>> TYPE ov is record
>>> case boolean of
>>>   true : i : integer;
>>>   false  : byte_array;
>>> end;
>>> end;
>>> 
>>> Such overlay structures are not valid with Ada, so what do instead?
>> 
>> Such usages are not valid in (ISO Standard) Pascal either,
>> but you don't let that stop you. 8-)
> 
> Where does the standard deem this "not valid" in Ada?

I'm not sure who Martin is asking, nor what about; but:

If "this" means Pascal-style tagless variant records, see the RM, section
3.8.1 "Variant Parts and Discrete Choices", where no such syntax appears.

If "this" means assigning a value of type T1 to a field F1 of a variant V1
of a record structure R and retrieving a value of type T2 from a field F2 of
a variant V2, "R.F1 := T1_Val; ...; T2_var := R.F2" then:

 (a) If the variant has not been changed from V1 to V2, then a discriminant
     check on access to F2 will raise a Constraint_Error.
 (b) If the variant has been changed to V2, then F2 has been given a new
     value and so does not retain the value placed in F1.
 (c) It cannot even be assumed that F1 and F2 coincide, unless a
      representation specification can be used to that effect.

(I suspect Martin knows all this perfectly well, so I wonder why he asks.)
-- 
Bill Findlay
<surname><forename> chez blueyonder.co.uk