Re: How pass a "null" as a parameter of anonymous access type

["Mark Lundquist" <mark@rational.com>]

Marc A. Criley <mcqada@earthlink.net> wrote in message
news:3ABF3A34.D961881E@earthlink.net...
> Freddy wrote:
> >
> > Hello !
> >
> > I want call a C-binding procedure which is defined as "procedure p (a :
> > access X; b : access Y);"
> > Both a or b may be a null pointer, but Ada say "null not allowed for
> > anonymous access type".
> > Any idea what to do ?
>
> Here's another approach, which worked with JGNAT both at compile-time
> and run-time:
>
>   type X_Access is access all X;
>   type Y_Access is access all Y;
>
>   Null_X : constant X_Access := null;
>   Null_Y : constant Y_Access := null;
>
>   ...
>
>   P(Null_X'access, Null_Y'access);

Hokey Smokes, Bullwinkle! :-)

>
>
> Even if this works (which it shouldn't),

No kidding... :-)

> you should submit a bug report
> to the providers of the binding

And to your Ada vendor...

Null_X is not aliased, so Null_X'Access is illegal.

Make it aliased, then you can take the 'Access... but then, Null_X'Access
should *not* yield a null value (the fact that Null_X itself *has* a null
value is irrelevant, the value of Null_X makes no difference).
Null_X'Access by definition yields an access value that denotes the object
Null_X (and a null value does not denote anything... that's why it's called
"null" :-).

And even if there is some compiler bug that would cause Null_X'Access to
yield a null value, that should cause Constraint_Error to be raised when
it's passed to P (it doesn't matter where the null value came from).

For that cr@p to compile *and* run would require an incredible conjunction
of bugs in the Ada implementation.

-- mark