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Newsgroups: comp.lang.ada
Date: Wed, 30 Jul 2014 18:38:23 -0700 (PDT)
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Subject: Re: Quick question regarding limited type return syntax
From: NiGHTS <nights@unku.us>
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Xref: news.eternal-september.org comp.lang.ada:21364
Date: 2014-07-30T18:38:23-07:00
List-Id: <comp.lang.ada>

On Wednesday, July 30, 2014 9:29:10 PM UTC-4, Adam Beneschan wrote:
> On Wednesday, July 30, 2014 6:00:43 PM UTC-7, NiGHTS wrote:
>=20
> > On Wednesday, July 30, 2014 8:48:34 PM UTC-4, Shark8 wrote:
>=20
> >=20
>=20
> > > On 30-Jul-14 17:51, NiGHTS wrote:
>=20
> >=20
>=20
> > > >   return  Object (Connection_String'Access) do null; end return;
>=20
>=20
>=20
> > package body Test is
>=20
> >    function Create(...) return Object is
>=20
> >       ...
>=20
> >    begin
>=20
> >       return Object'(Connection_String'Access);
>=20
> >    end;
>=20
> >    ...
>=20
> > end Test;
>=20
>=20
>=20
> What you need to do is return an aggregate.  And to return an aggregate, =
you need to specify both fields--the Parameters discriminant and the Databa=
se.  This syntax:
>=20
>=20
>=20
>     Object'(Connection_String'Access)
>=20
>=20
>=20
> is just wrong.  It tells the compiler that the value is Connection_String=
'Access, and asks the compiler to treat it as an Object.  It can't do that,=
 because Object isn't an access type.  (This sort of syntax can be useful i=
n overloading cases, when the compiler can't determine the correct access t=
ype without help.)
>=20
>=20
>=20
> If you said
>=20
>=20
>=20
>     Object'(Parameters =3D> Connection_String'Access)
>=20
>=20
>=20
> you'd get an error that Database is missing.  So you need
>=20
>=20
>=20
>     Object'(Parameters =3D> Connection_String'Access, Database =3D> <>)
>=20
>=20
>=20
> or just
>=20
>=20
>=20
>     (Parameters =3D> Connection_String'Access, Database =3D> <>)
>=20
>=20
>=20
> or   =20
>=20
>=20
>=20
>     (Parameters =3D> Connection_String'Access, others =3D> <>)
>=20
>=20
>=20
> (It's OK for a function that returns a limited type to return an aggregat=
e like this.)
>=20
>=20
>=20
> The effect is that subcomponents of Database that have default initial va=
lues will be initialized, and subcomponents that don't have default initial=
 values will be initialized to garbage (except where the language says othe=
rwise).  That's really the same as your original example was trying to do. =
 I guess that explicitly putting "others =3D> <>" in an aggregate is enough=
 to convince GNAT not to think you were accidentally leaving something init=
ialized to garbage. =20
>=20
>=20
>=20
>                               -- Adam

You're a good man Adam. Thank you for your very clear explanation. This wor=
ked for me and I'm very pleased by how logical this ended up being.

And thank you as well Shark8 for your help.