Re: Default expression only allowed for "in" parameters - why?

["Ben Brosgol" <brosgol@world.std.com>]

Robert A Duff <bobduff@world.std.com> wrote in message
news:wccvgohvgv7.fsf@world.std.com...
> "Smark" <not.provided@all.com> writes:
>
> > procedure Do_Something (Data   : in Some_Type;
> >                         Status : out Status_Type := Ok);
> >
> > It also seemed like a strange idea, because I never recalled seeing
> > it done.  Sure enough, it is prohibited by the LRM.  What is the
> > rationale for this?
>
> If defaults were allowed for 'out' params, I would think it ought to
> mean that this is the default variable to pass to the parameter, not a
> default initial value for it.

I agree with Bob.

> In Ada, a default applies if the
> parameter is not explicitly given at the call site.  Are you saying that
> Status should be initialized to OK if the actual parameter is specified?
>
> Defaults for 'out' params were allowed in Green.  I don't know why they
> were removed.  Perhaps the fact the you and I disagree on what they
> ought to mean is a hint.

If memory serves, the reason for the absence of defaults for 'out' and 'in
out' parameters was methodological.  A variable is passed as an [in] out
parameter in order to be modified, and the design team felt that this effect
should be explicit at the point of call, by the explicit presence of an
actual parameter.  In any event the capability (of default [in] out objects)
would probably not be too useful, since the default would need to be an
object visible at the procedure declaration whereas it is more typical for
an actual to be local to the scope in which the procedure is called.

Ben Brosgol
Ada Core Technologies
brosgol@gnat.com