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Date: Fri, 9 Nov 2012 11:42:37 -0800 (PST)
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Subject: Re: Question[s] about aliased extended return.
From: Shark8 <onewingedshark@gmail.com>
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Date: 2012-11-09T11:42:37-08:00
List-Id: <comp.lang.ada>

On Thursday, November 8, 2012 5:43:18 PM UTC-7, Randy Brukardt wrote:
> "Shark8" <onewingedshark@gmail.com> wrote in message=20
>=20
> > There are a couple of times building the OpenGL binding where I would h=
ave=20
> > liked to do something like:
> > Function some_vector( [params] ) Return Vector_of_points is
> > begin
> >  Return Result : ALIASED Vector_of_points( 1..size_from_params ) do
> >    glFunctionPopulatingVector( glEnum_from_params, Result'access )
> >  end return;
> >end some_vector;
> >in order to populate the vector with the requisite data, but this is not=
=20
> >allowed.
>=20
> I presume you meant "aliased" rather than "access" in the return statemen=
t=20
> here.

Yes.

>=20
> IMHO, you get what you deserve when you use access parameters rather than=
=20
> "in out" parameters. There is no good reason to use an access parameter i=
n a=20
> routine like Populating_Vector (at least in the Ada part - you might have=
 to=20
> do all kinds of nasty things in the thin interface).

Ah that's kinda the point, this is WRT the implementation part [of my OpenG=
L binding] and therefore the interface between the thin/raw-imports and the=
 more useful/friendly Ada world. (By Vector I mean something like "Array (P=
ositive Range <>) of Float" [or whatever], not the Containers's Vector.)

As it is I have to do something like:
    Function Get_Elements(Item:Texture_Coordinate_Generation) Return Positi=
ve is
    begin
	case Item is
	when TEXTURE_GEN_MODE		=3D> Return 1;
	when OBJECT_PLANE | EYE_PLANE	=3D> Return 4;
	end case;
    end Get_Elements;

    Function GetTexGen (	coord : Texture_Coordinate_Type;
				pname : Texture_Coordinate_Generation)
				Return Double_Vector_Type is
    begin
	Return Result : Double_Vector_Type(1..Get_Elements( pname )) do
		glGetTexGendv(
			coord  =3D> Convert(coord),
			pname  =3D> Convert(pname),
			params =3D> To_Pointer( Result'Address )
		);
	End Return;
    end GetTexGen;

Where Convert is Unchecked_Conversion to GLEnum, and To_Pointer is Address/=
access conversion. {Aliased return statements would then allow me to be rid=
 of a whole dependency... oh well.}

>=20
> > Lastly, the GNAT compiler seems to reject the following, [..]=20
> >(3) Is this a bug, or am I just misreading everything?
>=20
> Looks like a bug to me.

Thank you, good to know.