Usefulness of classwide "out" parameter

[okellogg@my-dejanews.com]

Hi,

Has anyone ever used a classwide subprogram parameter with
mode "out" ?
For example,

    type Mytype is tagged null record;
    procedure Proc (Result : out Mytype'Class);

The problem I have with this is, I cannot assign another variable
to the out parameter (that would change the tag, which is not
possible.)

Am I correct in asserting that classwide parameters of mode "out"
are not terribly useful? What would a practical application of this
look like?

I am appending an example that shows the problem.

Thanks,

O. Kellogg
e-mail: oliver.kellogg@vs.dasa.de

-- base.ads
package Base is
   type Mytype is tagged null record;
   procedure Proc (Result : out Mytype'Class);
end Base;

-- base-derived1.ads
package Base.Derived1 is
   type Deriv1 is new Mytype with null record;
end Base.Derived1;

-- base-derived2.ads
package Base.Derived2 is
   type Deriv2 is new Mytype with null record;
end Base.Derived2;

-- base.adb
with Base.Derived1, Base.Derived2;
package body Base is
   V1 : Base.Derived1.Deriv1;
   V2 : Base.Derived2.Deriv2;
   Some_Flag : Boolean := False;
   procedure Proc (Result : out Mytype'Class) is
   begin
      if Some_Flag then
         Result := V1;   -- GNAT says:
      else
         Result := V2;   -- dynamically tagged expression required
      end if;
   end Proc;
end Base;

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