Re: why can't we declare unconstrained objects ?

[Jeffrey Carter <spam@spam.com>]

Martin Dowie wrote:

> Michael Mounteney wrote:
> 
>>     type thing (what : Boolean) is
> 
> Simply add a default to the discriminant and you can change it all you 
> want at run-time (e.g. "what : Boolean := Boolean'First").

To be precise (and it sounds as if the OP needs this level of detail), 
if a discriminated record type has defaults for all discriminants, then 
you can declare an unconstrained object of the type by not specifying 
the discriminants:

type R (D : Boolean := False) is record ...

V : R;
...
V := R'(D => True, ...);

However, if you declare objects with discriminants specified:

X : R (D => False);

then the object is constrained and the discriminant cannot be changed.

Also, you can only change the discriminant by assigning the entire record.

V.D := True; -- illegal

If you're interested in interpreting the same bit pattern as 2 different 
types, this is not the mechanism Ada uses. Use Unchecked_Conversion instead.

-- 
Jeff Carter
"Nobody expects the Spanish Inquisition!"
Monty Python's Flying Circus
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